1.3 Properties of Polyetherimide

Home , Polyetherimide

Telechelic Polyetherimides with Functionalized End Groups for Enhancement of Mechanical Strength, Flame Retardancy, and Optical Properties

Ke Cao

Thesis submitted to the faculty of the Virginia Polytechnic Institute and State University in

partial fulfillment of the requirements for the degree of

Master of Science In Macromolecular Science and Engineering

Guoliang (Greg) Liu (Chair) Timothy E. Long Robert B. Moore

July 16, 2018 Blacksburg, VA

Keywords: polyetherimide, mechanical properties, yellowness, flame retardancy, telechelic oligomer Telechelic Polyetherimides with Functionalized End Groups for Enhancement of Mechanical Strength, Flame Retardancy, and Optical Properties

Ke Cao

Abstract

This thesis focuses on understanding the factors that affect the properties of polyetherimide (PEI) and improving the properties. As a high-performance thermoplastic resin, the first challenge in PEI application is its high processing temperature and viscosity.

Therefore, two supramolecular strategies were applied to not only solve the problem of high processing temperature or viscosity but also enhance the mechanical and flame retardancy. In addition, the yellow to amber color of PEIs limits its applications in high-tech fields such as microelectronics and optoelectronics. Thus, a fundamental study of how end group and molecular weight affect the optical properties of PEIs provides a better knowledge of the mechanism and an effective strategy for designing PEIs.

To lower the processing viscosity while maintaining or even improving the mechanical properties of PEI, the first strategy was to synthesize PEI oligomers, and incorporate self-complementary quadruple hydrogen bonding ureidopyrimidinone (UPy) units at the

chain ends. Surprisingly, the UPy imparted PEI with a Mn as low as 8 kDa (8k-PEI) with great film formability. Excitingly, 8k-PEI-UPy exhibited an outstanding Young’s modulus

higher than those of state-of-the-art high-molecular-weight (high-MW) commercial PEIs.

Therefore, the incorporation of UPy was proved to be an effective method to synthesize low-molecular-weight, high-mechanical-strength PEIs.

Although low-molecular-weight PEI-UPy had high mechanical properties, its limited thermal stability and potentially low flame retardancy, however, restricted its applications in areas such as aerospace and aircrafts. Hence in another strategy, which utilize the phosphonium ionic groups were incorporated into PEI oligomers targeting at achieving high thermal stability, flame retardancy, and mechanical properties simultaneously.

Functionalization of dianhydride-terminated PEI by tetraphenylphosphonium bromide

afforded the synthesis of phosphonium bromide terminated PEI (PEI-PhPPh3Br), which simultaneously exhibited excellent thermal stability up to ~400 ˚C, outstanding flame retardancy evidenced by high char yield and extremely high limiting oxygen index, and a very high mechanical strength. The study thus provides an efficient strategy to simultaneously enhance the thermal and mechanical properties as well as flame retardancy.

Furthermore, the low-molecular-weight PEI-PhPPh3Br had good processability due to its strong shear thinning.

In addition to the thermal and mechanical properties and flame retardancy, the end groups affect the optical properties, especially the yellowness, of PEIs. Understanding how end group and molecular weight affect the yellowness, of PEIs is critical for their applications in fields including optoelectronics and microelectronics. Thereby, PEIs with

different Mn and various end groups including electron-withdrawing and electron-donating were prepared and characterized. Electron-withdrawing end groups reduced the

yellowness and increased the transparency of PEI, regardless of the Mn. Electron-donating

end groups increased the yellowness of PEIs with dependence on the Mn. The Mn affected the yellowness of PEIs by changing end group density and the probability of charge-transfer complex formation. The systematic study reveals the correlations among yellowness, end group, and molecular weight of PEIs. Telechelic Polyetherimides with Functionalized End Groups for Enhancement of Mechanical Strength, Flame Retardancy, and Optical Properties

Ke Cao

General Audience Abstract

One small step for end groups, one giant leap for properties. Simply tuning the repeating units at the polymer chain ends drastically changes the properties of the polymers.

This thesis focuses on the modification of the end groups in low-molecular-weight polyetherimides, a class of high-temperature high-performance engineering thermoplastics, to achieve improved and tunable properties, such as mechanical strength, flame retardancy, and optical properties.

On one hand, low-molecular-weight polyetherimides enabled low processing temperatures to decrease the processing cost. On the other hand, the incorporation of noncovalent hydrogen bonding interactions improved the mechanical strength of low-molecular-weight polyetherimides and maintained their thermal stability. This study for the first time showed the incorporation of multiple hydrogen bonds was effective to generate low-molecular weight but high-mechanical-strength polyetherimides.

Although multiple hydrogen bonds improved the mechanical properties of polyetherimides, the thermal stability was inadequate for industrial melt processing at

elevated temperatures. Alternatively, by incorporating noncovalent electrostatic interaction groups, the polyetherimides showed not only improved mechanical properties but also high thermal stability. Excitingly, their flame retardancy and melt processability were also significantly improved. This polyetherimide has great potential for applications such as aircrafts and aerospace.

The end groups affected not only the thermal, mechanical, and rheological properties, but also the optical properties of polyetherimide. Polyetherimide has an intrinsic yellow color originated from the charge transfer complexes that are formed between electron-rich and electron-deficient moieties in the polymer chains. By tuning the concentrations of the different end groups, we controlled the strength of the charge transfer complexes and thus the yellowness of the films. Through a systematic study, a 3D contour was constructed and revealed the relations among the yellowness, the end group, and the molecular weight of polyetherimides. The 3D contour provides guidelines for designing polyetherimides with suitable molecular weights and adequately low yellowness. Acknowledgments

I sincerely thank my advisor, Prof. Guoliang (Greg) Liu for his intellectual support and continual encouragement through my studies. This thesis was made possible by his patience and persistence. Prof. Liu has very rich knowledge and experience in teaching me to conduct scientific research. I was deeply inspired by his critical thinking and rigorous spirit in scientific research.

I thank Prof. Timothy E. Long for his support in the first year of research as he was willing to have me as the co-advised student to learn the polymer synthesis and the various characterization techniques. I would like to thank Prof. Robert B. Moore for his excellence in teaching me profound knowledge in polymer. Both Prof. Long and Prof. Moore devoted to the further development of Macromolecular Innovation Institute (MII) and I feel excited to be a student in the Macromolecular Science and Engineering under MII.

I also wish to thank my previous and current lab mates, including Dr. Joseph M. Dennis who mentored me at early stage for polyetherimide synthesis, Liu group members, especially Dr. Zhengping Zhou and Mr. Assad U. Khan who helped in daily communication and discussion in research, Mr. Ryan Mondschein, Mr. Josh Wolfgang, Mr.

Clay Arrington for their discussions and assistances in the SABIC projects. I would like to appreciate Mingxuan Zhang, the undergraduate who played a key role in the synthesis of

vii phosphonium end capper, along with other significant work.

I am also thankful to Dr. Charles Carfagna for assistance in using the instruments of

Macromolecular Materials Discovery Center (MMDC).

I want to thank my wife, Xueyu Wang, my parents, Mr. Yangshen Cao and Ms. Yuzhen

Luo, and my sister Yan Cao, for their support of my studying abroad. Their encouragement and support drive me forward all the time.

Finally, full support for the project from SABIC is gratefully acknowledged.

viii Table of Contents

Acknowledgments ...... vii

CHAPTER 1: Introduction of Polyetherimide ...... 1

1.1 History ...... 1

1.2 Synthesis of Polyetherimide ...... 1

1.3 Properties of Polyetherimide ...... 4

1.3.1 Processability ...... 5

1.3.2 Mechanical Properties ...... 6

1.3.3 Optical Properties ...... 7

1.3.4 Stability ...... 8

1.3.5 Flame Retardancy ...... 11

1.3.6 Electrical Properties ...... 12

1.4 Applications ...... 13

1.5 Thesis Overview ...... 14

1.6 References ...... 15

CHAPTER 2: Low-Molecular-Weight, High-Mechanical-Strength, and

Solution-Processable Telechelic Poly(ether imide) End-Capped with Ureidopyrimidinone

...... 22

2.1 Abstract ...... 22

2.2 Introduction ...... 23

2.3 Experimental ...... 27

2.4 Results and Discussion ...... 30

2.5 Conclusions ...... 46

2.6 Supplementary Information ...... 47

2.7 References ...... 55

CHAPTER 3: Highly Mechanically Strong, Thermally Stable, and Flame Retardant

Polyetherimide Terminated with Phosphonium Bromide ...... 61

3.1 Abstract ...... 61

3.2 Introduction ...... 62

3.3 Experimental ...... 64

3.4 Results and Discussion ...... 70

3.5 Conclusions ...... 85

3.6 Supplementary Information ...... 86

3.7 References ...... 93

CHAPTER 4: The Effect of End Group and Molecular Weight on the Yellowness of

Polyetherimide ...... 99

4.1 Abstract ...... 99

4.2 Introduction ...... 100

4.3 Experimental ...... 102

4.4 Results and Discussion ...... 108

4.5 Conclusions ...... 116

4.6 Supplementary Information ...... 117

4.7 References ...... 120

CHAPTER 5: Suggested Future Work ...... 125

5.1 High-Mechanical-Strength, Halogen-Free, and Flame-Retardant PEIs ...... 125

5.2 PEI Chelate Membranes for Heavy Metal Removal ...... 127

5.3 PEI-g-PEKK as a Sacrificial Scaffold for 3D Printing ...... 130

5.4 References ...... 132

xii

CHAPTER 1: Introduction of Polyetherimide

1.1 History

Polyimides (PIs) are a class of high-temperature engineering polymers synthesized from diamines and dianhydrides through polycondensation reactions. Different from common thermoset PIs such as Kapton (Scheme 1), polyetherimide (PEI) is a thermoplastic and can be processed. The best known PEI is Ultem 1000 (Scheme 1) that was first developed by Darrell R. Heath and Joseph G. Wirth1 in early 1976 in General

Electric Company, whose plastics part was later acquired by SABIC in 2007.

Scheme 1. Chemical structure of Ultem 1000 and Kapton

1.2 Synthesis of Polyetherimide

Typical synthesis of thermoset PI (e.g. Kapton) from dianhydride and diamines is a two-step reaction (Scheme 2). The first step occurs in a polar aprotic solvent, such as NMP,

where a difunctional dianhydride reacts with a difunctional diamine and forms the poly(amic acid) (PAA). This step is a reversible nucleophilic acyl substitution. The second step is usually thermal imidization, where a poly(amic acid) solution was cast onto a substrate, and then subjected to gradual heating in a programmed time and temperature cycle to remove solvent and complete the cyclodehydration reaction. After the two-step reactions, PI is produced in a film form. Thus, the thermal imidization is also known as the

“bulk imidization”.

Scheme 2. Synthesis of a typical polyimide (Kapton) via a two-step reaction

Unlike the synthesis of PIs where two-step procedures were applied, the synthesis of

PEIs is one-step and one-pot, which involves the synthesis from dianhydride and diamine in solvents with high boiling point (e.g. N-Methyl-2-pyrrolidone (NMP),

Dimethylacetamide (DMAc)) as the reaction solvent and an azeotropic reagent (e.g. o-dichlorobenzene, toluene, xylene) to remove the water formed during the reaction through distillation into a Dean-Stark trap. In the synthesis of Ultem (Scheme 3), however, only o-dichlorobenzene (oDCB) is used throughout the reaction. This is because oDCB is not only a solvent with a high boiling point (b.p. =180 ˚C), but also an azeotropic reagent.

Compared to the PIs synthesized via a two-step reaction, the advantages of one-step synthesized PEIs include the following:

1) The reaction is simple because it only requires one-step homogeneous solution

imidization. It also avoids handling and processing of PAA. Thus, the issues related to

PAA such as hydrolysis are solved.

2) Un-imidized amic acid groups are usually fewer than 1%. Typically higher

temperatures (~380 ˚C) are applied in order to further increase the imidization degree

and remove oDCB.

3) As a thermoplastic, PEIs can be re-dissolved in solvents for solution casting, or re-melt

for melt processing. In contrast, common PIs are thermosets and cannot be

re-processed upon full imidization.

4) Although being a thermoplastic resin, PEIs possess high mechanical properties and

thermal stabilities, and thus they belong to engineering plastics. Compared to typical

PIs such as Kapton with a tensile strength of ~160 MPa, PEIs like Ultem maintain

strong tensile strength of ~120 MPa and possess melt processability that Kapton lacks.

Scheme 3. Synthesis of a polyetherimide (Ultem) via a one-step reaction

1.3 Properties of Polyetherimide

Compared to thermoset PIs such as Kapton that has no glass transition temperature

(Tg), Ultem has a Tg of 217 ˚C, which makes PEI an engineering thermoplastic. The processability results from the flexible groups and kinked linkages. Therefore, with the

appropriate balance, Ultem retains the merits of traditional PIs including high mechanical toughness, good heat resistance, and high flame retardancy while gains melt processability.

1.3.1 Processability

The flexible groups from BPADA moieties, i.e., ether (−O−)2 and isopropylidene

3 4 [−C(CH3)2−], and the kinked linkages from mPD moieties, impart Ultem with melt processability. In manufacturing, Ultem can be extruded to fibers and filaments,5 which are chopped for sale. Customers can melt process Ultem at temperatures of 330-400 ˚C, with injection molding6, 7 or extrusion blow molding7, into parts. Besides the common processing method, Ultem is welded to match the contours of joint interface with metals.8

Although PEI can be melt processed, the processing temperature and viscosity are still high.9 Therefore, researchers have devoted to lower the processing cost. For instance, resorcinol bis(diphenyl phosphate) was added to achieve the antiplasticization of PEI

10 which exhibited lower Tg, higher mechanical properties. The high viscosity could also be lowered through addition of liquid crystalline polymers to make blends, while the mechanical strength was retained.6, 11-16 The lowered viscosity resulted from the decreased glass transition temperature of PEI phase.17

Ultem not only is melt processable, but also can dissolve in some organic solvents for casting or spinning,18-20 owing to the flexible linkages. Commonly used are aprotic polar 5

solvents,21 such as DMAc,22-27 DMF,28 and NMP,25, 28-32 with which Ultem can be dissolved and cast into membranes for gas separation, liquid separation, and dehydration of alcohol, or spin into fibers for ultrafiltration.

1.3.2 Mechanical Properties

The unreinforced Ultem, for example, Ultem 1000, possesses outstanding mechanical properties, including as a tensile stress of 110 MPa and tensile modulus of 3580 MPa at ambient temperature. A typical tensile stress-strain profile of Ultem 1000 at room temperature is as follows (Figure 1).

Figure 1. Typical tensile stress-strain curve of Ultem 1000.

The four stages are elastic response, yielding (strain softening), and strain hardening,

and rupture. At elevated temperature, the strain softening becomes less significant.33 Due to the high mechanical strength, it was widely used as toughening agent to improve the toughness and mechanical properties of epoxy resins34-38 or carbon/epoxy composites.39

With different molecular weight of PEI, the morphologies of the blends varied from a PEI particle dispersion to continuous phases.40

1.3.3 Optical Properties

The pristine PEIs have yellow to amber color derived from the charge-transfer complex

(CTC) between the electron-rich diamine moieties and electron-deficient dianhydride moieties, both intrachain and interchain (Scheme 4).41-43 Lowering the yellowness and increasing the transparency is of interest to researchers since colorless and transparent PEIs have potential to be used as flexible displays,44 solar radiation protectors,45, 46 and optical waveguides.47-49 With the incorporation of phenoxy groups, researchers at NASA achieved lightly colored to colorless PEI films that were 2.2 to 2.6 times more transparent than commercial PI film of the same thickness.50 More recent studies to lower the yellowness and increase the transparency of PEI/PI films were through introducing electron-withdrawing groups such as fluorinated groups.51 Besides the compositional factors, processing conditions such as injection speed, melt temperature, residence time, and mold temperature can also affect the yellow index as well as other optical properties

such as birefringence and haze.52 PEI has very high refraction indices, ranging from 1.52 to

1.80 in the wavelength range of 105 nm ~ 40 μm.53 Combining with its thermal stability at welding temperature, it has potential applications in printed circuit boards and packaging for microelectronic circuit.

Scheme 4. Charge transfer complex (CTC) in PEIs

1.3.4 Stability

A typical thermogravimetric analysis (TGA) of Ultem 1000 degradation is illustrated in

Figure 2. Ultem is thermally stable up to ~540 ˚C, which is much higher than other thermoplastic materials. The thermal degradation of Ultem mechanism was studied by different researchers with different techniques. Using infrared (IR), elemental analysis, and

gas chromatography–mass spectrometry (GC-MS) to analyze the pyrolysis products of

Ultem, Huang et al. tentatively proposed that PEI is thermally degraded by two mechanisms, initiated by either ether-bond breaking (Scheme 5a) or carboxyl-induced chain breaking.54 The carboxyl formed as the product of hydrolysis of imide ring at high temperature (Scheme 5b). With a more advanced technique of direct pyrolysis mass spectrometry, Carroccio et al.55 proposed that the degradation of Ultem occurred first by the scission of the weakest bonds, including the isopropylidene bridge of bisphenol-A, the oxygen-phthalimide bond, and the phenyl-phthalimide bond (Scheme 6). Subsequent reactions leaded to crosslinking of the residues and resulted in high char yields. In addition to the experimental studies, theoretical models were developed to explain the degradation processes and achieve kinetic parameters such as critical temperature, energy barrier, and reaction-order.56

Figure 2. Typical degradation behavior of Ultem 1000 under N2.

Scheme 5. Thermal degradation process of Ultem proposed by Huang et al.,54 by (a) ether-bond breaking or carboxyl formed from (b) hydrolysis of imide rings.

Scheme 6. Thermal degradation process of Ultem proposed by Carroccio et al., initiated by breaking of the isopropylidene bridge of bisphenol-A, the oxygen-phthalimide bond, and the phenyl-phthalimide bond.

Besides thermal stability, the stability of PEIs in severe environment such as UV radiation is of interest to researchers, as PEI is used as an aircraft coating material. Under

UV irradiation, PEI can be photo-oxidized, mainly through the scission of chains at the isopropylidene bridges and at the phthalimide units.57 High electron radiation doses led to dehydrogenation of methyl groups, rupture of ether linkages, and crosslinking by weaker bonds, which resulted in embrittlement of PEIs.58 Kiefer et al. mixed bis(triphenyltin) oxide with PEIs, which showed less mass loss in the oxygen atom corrosion compared to the pristine PEIs.59

1.3.5 Flame Retardancy

With a high carbon content and high aromaticity,60, 61 Ultem has outstanding flame characteristics, including V-0 grade in UL-94 rating, and a high limiting oxygen index

(LOI) of 47%. Therefore, it has been used to boost the flame retardancy though copolymers

such as siloxane-PEI copolymer,62 or polymer blends such as with polycarbonate.63 The high char yields of Ultem makes it a superior precursor for carbon materials. The chars from decomposition of PEIs at 950 ˚C were microporous which showed exfoliation of the char layers.64 Although the pristine PEI has superb flame retardancy, the shape stability is not the optimal. Researchers fabricated intercalated silicate-PEI composites which had a higher char yield and better shape stability after burning, compared to the pristine PEI.65

1.3.6 Electrical Properties

PEI is generally a good insulator. The activation energy for electrical conduction is

0.74 eV, and the increase of current density in PEI was achieved at elevated temperatures.66 The dielectric constant of Ultem 1000 is 3.15 at 100 Hz. To lower the dielectric constant for potential applications as high-performance micro-electronics, less efficient chain packing and larger free volume are needed.67 For instance, Vora et al. synthesized fluoro-containing PEIs, one of which exhibited reduced dielectric constants of 2.65 at 10 MHz compared to Ultem 1000.51 Similarly, Chen et al. synthesized PEIs from fluorinated dianhydride and phenylene ether diamines, which had dielectric constant of 2.78 at 1 MHz and maintained high thermal stability and good mechanical properties.68

Another strategy is the incorporation of inorganic particles. For instance, Chen et al.

prepared PEI-SiO2 nanocomposites by sol-gel method, which not only exhibited

decreased dielectric constant, but also enhanced thermal stability.67, 69

On the other hand, researchers have also been studying the enhancement of dielectric constants for energy-storage devices by incorporating high dielectric particles. For

example, Choudhury prepared PEI/barium titanate (BaTiO3) nanocomposite films

through polymerization the mixture of BaTiO3 with the poly(amic acid) solution, which exhibited a dielectric constant of 37 at 1 kHz.70

1.4 Applications

PEIs are used in reflectors, seats, lighting, and wiring in the field of automotive and aircrafts due to its good flame retardancy, and as appliances in microwaves.71 In addition, the strong adhesion of PEI to copper was achieved with various techniques,72-75 which provided value to the manufacture of molded circuit devices. PEI combining with photosensitive compounds could generate positive-tone behavior by UV-irradiation followed by development in solution. The patterning was caused by main-chain scission from the amine generated by the photosensitive compounds. This has applications in UV curing inks, information recording, and microelectronics.76 A small form factor optical disc of PEI was injection-compression molded with better mechanical properties and comparably smooth surface to polycarbonate, which was promising substrate for next-generation optical storage.77 13

Besides the traditional applications, researchers have been devoted to fabricate PEI hollow fiber membranes for gas separation,28, 78 oil-water separation.79 In addition, as PEI has a high char yield, it was used as the precursors for carbon materials.80 For example,

Sedigh et al. carbonized PEIs to make carbon molecular sieve membranes, which showed better operational stability and good mechanical strength for gas separation.81

1.5 Thesis Overview

In this thesis, chapter 2 describes the synthesis and characterization of low-molecular-weight, high-mechanical-strength, and solution-processable telechelic PEI end-capped with ureidopyrimidinone (UPy). The resulting UPy terminated PEI with a low molecular weight exhibited higher Young’s modulus compared to the high-molecular-weight PEIs. Chapter 3 presents the synthesis and performance of phosphonium bromide terminated polyetherimide with high tensile properties, easy processability, and enhanced flame retardancy. With the phosphonium bromide end groups, the PEI oligomers gain thermal stability, flame retardancy, mechanical properties, and melt processability simultaneously. Chapter 4 studies the effect of end group and molecular weight on the yellowness of polyetherimide. The correlations among the yellowness, molecular weight, and end group are constructed to better understand how different factors together affect the yellowness of PEIs, Chapter 5 suggests future research directions.

1.6 References

1. Heath, D. R.; Wirth, J. G., 4, 4'-Isopropylidene-bis(3- and 4-phenyleneoxyphthalic anhydride). US Patent 3972902 A: 1976. 2. Hsiao, S.-H.; Chung, C.-L.; Lee, M.-L. Synthesis and characterization of soluble polyimides derived from 2',5'-bis(3,4-dicarboxyphenoxy)-p-terphenyl dianhydride. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 1008-1017. 3. Kute, V.; Banerjee, S. Novel semi-fluorinated poly(ether imide)s derived from 4-(p-aminophenoxy)-3-trifluoromethyl-4'-aminobiphenyl. Macromol. Chem. Phys. 2003, 204, 2105-2112. 4. Chen, B.-K.; Fang, Y.-T.; Cheng, J.-R.; Tsay, S.-Y. Effects of meta and para diamines on the properties of polyetherimide nanocomposite films prepared by the sol-gel process. J. Appl. Polym. Sci. 2007, 105, 1093-1100. 5. Böhringer, B.; Schilo, D.; Birkenfeld, W.; Odenthal, W. New filaments and fibres of polyetherimide. Makromol. Chem. Macromol. Symp. 1991, 50, 31-39. 6. de Souza, J. P.; Baird, D. G. In situ composites based on blends of a poly(ether imide) and thermotropic liquid crystalline polymers under injection moulding conditions. Polymer 1996, 37, 1985-1997. 7. de Souza, J. P.; Baird, D. G. In situ composites based on blends of a polyetherimide and thermotropic liquid crystalline polymers subjected to shearfree deformations. Polym. Compos. 1996, 17, 578-595. 8. Stokes, V. K. A phenomenological study of the hot-tool welding of thermoplastics Part 3. Polyetherimide. Polymer 2001, 42, 775-792. 9. Sandeep, K.; Bin, L.; Santiago, C.; Russ, G. M.; Wei-Hong, Z. Dramatic property enhancement in polyetherimide using low-cost commercially functionalized multi-walled carbon nanotubes via a facile solution processing method. Nanotechnology 2009, 20, 465708. 10. Alexandrino, E. M.; Conceição, T. F.; Felisberti, M. I. Improvement of processing and mechanical properties of polyetherimide by antiplasticization with resorcinol bis(diphenyl phosphate). J. Appl. Polym. Sci. 2014, 131. 11. Carfagna, C.; Amendola, E.; Nicolais, L.; Acierno, D.; Francescangeli, O.; Yang, B.; Rustichelli, F. Blends of a polyetherimide and a liquid crystalline polymer: Fiber orientation and mechanical properties. J. Appl. Polym. Sci. 1991, 43, 839-844. 15

12. Rose, W.; Yin, L. Z.; Pfandl, W.; Meurer, C. Structure/property relationships in LCP/PEI blends revealed with a new selective etching process. Colloid. Polym. Sci. 1992, 270, 97-105. 13. Incarnato, L.; Nobile, M. R.; Acierno, D. Morphology-rheology relationships in incompatible blends based on thermotropic liquid crystalline polymers. Makromol. Chem. Macromol. Symp. 1993, 68, 277-290. 14. Baird, D. G.; Bafna, S. S.; de Souza, J. P.; Sun, T. Mechanical properties of in situ composites based on partially miscible blends of polyetherimide and liquid crystalline polymers. Polym. Compos. 1993, 14, 214-223. 15. Ryu, C.; Seo, Y.; Hwang, S. S.; Hong, S. M.; Park, T. S.; Kim, K. U. Miscibility and mechanical properties of poly(ether imide)/liquid crystalline poly(ester imide) blends. Int. Polym. Proc. 1994, 9, 266-272. 16. Bretas, R. E. S.; Collias, D.; Baird, D. G. Dynamic rheological properties of polyetherimide/polyetheretherketone/liquid crystalline polymer ternary blends. Polym. Eng. Sci. 1994, 34, 1492-1496. 17. Bafna, S. S.; Sun, T.; Baird, D. G. The role of partial miscibility on the properties of blends of a polyetherimide and two liquid crystalline polymers. Polymer 1993, 34, 708-715. 18. Scarlet, R.; Manea, L. R.; Sandu, I.; Martinova, L.; Cramariuc, O.; Sandu, I. G. Study on the solubility of polyetherimide for nanostructural electrospinning. Rev. Chim. 2012, 63, 688-692. 19. Manea, L. R.; Scarlet, R.; Leon, A. L.; Sandu, I. Control of nanofibers production process through electrospinning. Rev. Chim. 2015, 66, 640-644. 20. Manea, L. R.; Nechita, E.; Sandu, I. Electrospinning of Polyetherimide (PEI) Solution Effect of nozzle sizes on the diameter of the fiber. Rev. Chim. 2015, 66, 1841-1845. 21. Wang, D.; Li, K.; Teo, W. K. Phase separation in polyetherimide/solvent/nonsolvent systems and membrane formation. J. Appl. Polym. Sci. 1999, 71, 1789-1796. 22. Huang, R. Y. M.; Feng, X. Pervaporation of water/ethanol mixtures by an aromatic polyetherimide membrane. Sep. Sci. Technol. 1992, 27, 1583-1597. 23. Huang, R. Y. M.; Feng, X. Dehydration of isopropanol by pervaporation using aromatic polyetherimide membranes. Sep. Sci. Technol. 1993, 28, 2035-2048. 24. Deng, S.; Liu, T.; Sourirajan, S.; Matsuura, T., A Study of Volatile Hydrocarbon Emission Control by Polyetherimide Hollow Fiber Membranes. In J. Polym. Eng., 1995;

Vol. 14, p 219. 25. Huang, R. Y. M.; Feng, X. Studies on solvent evaporation and polymer precipitation pertinent to the formation of asymmetric polyetherimide membranes. J. Appl. Polym. Sci. 1995, 57, 613-621. 26. Feng, X.; Huang, R. Y. M. Preparation and performance of asymmetric polyetherimide membranes for isopropanol dehydration by pervaporation. J. Membr. Sci. 1996, 109, 165-172. 27. Shen, L.-Q.; Xu, Z.-K.; Liu, Z.-M.; Xu, Y.-Y. Ultrafiltration hollow fiber membranes of sulfonated polyetherimide/polyetherimide blends: preparation, morphologies and anti-fouling properties. J. Membr. Sci. 2003, 218, 279-293. 28. Kneifel, K.; Peinemann, K. V. Preparation of hollow fiber membranes from polyetherimide for gas separation. J. Membr. Sci. 1992, 65, 295-307. 29. Fouda, A.; Bai, J.; Zhang, S. Q.; Kutowy, O.; Matsuura, T. Membrane separation of low volatile organic compounds by pervaporation and vapor permeation. Desalination 1993, 90, 209-233. 30. Park, Y.-I.; Lee, K.-H. The permeation of CO2 and N2 gases through asymmetric polyetherimide membrane. Energy Convers. Manage. 1995, 36, 423-426. 31. Feng, C. Y.; Khulbe, K. C.; Chowdhury, G.; Matsuura, T.; Sapkal, V. C. Structural and performance study of microporous polyetherimide hollow fiber membranes made by solvent-spinning method. J. Membr. Sci. 2001, 189, 193-203. 32. SungCheal, M.; JaeKon, C.; J., F. R. Preparation of aligned polyetherimide fiber by electrospinning. J. Appl. Polym. Sci. 2008, 109, 691-694. 33. Nied, H. F.; Stokes, V. K.; Ysseldyke, D. A. High-Temperature large-strain behavior of polycarbonate, polyetherimide and poly(butylene terephthalate). Polym. Eng. Sci. 1987, 27, 101-107. 34. Bucknall, C. B.; Gilbert, A. H. Toughening tetrafunctional epoxy resins using polyetherimide. Polymer 1989, 30, 213-217. 35. Jang, J.; Lee, W. Polyetherimide-modified high performance epoxy resin. Polym. J. 1994, 26, 513. 36. Hourston, D. J.; Lane, J. M. The toughening of epoxy resins with thermoplastics: 1. Trifunctional epoxy resin-polyetherimide blends. Polymer 1992, 33, 1379-1383. 37. Hourston, D. J.; Lane, J. M.; Macbeath, N. A. Toughening of epoxy resins with

thermoplastics. Ii. Tetrafunctional epoxy resin‐polyetherimide blends. Polym. Int. 1991, 26, 17-21. 38. Murakami, A.; Saunders, D.; Ooishi, K.; Yoshiki, T.; Saitoo, M.; Watanabe, O.; Takezawa, M. Fracture behaviour of thermoplastic modified epoxy resins. J. Adhes. 1992, 39, 227-242. 39. Shin, S.; Jang, J. The effect of thermoplastic coating on the mechanical properties of woven fabric carbon/epoxy composites. J. Mater. Sci. 2000, 35, 2047-2054. 40. Jun, C.; Yinfeng, Y.; Wenjie, C.; Shanjun, L. Studies on the phase separation of polyetherimide-modified epoxy resin, 2. Effect of molecular weight of PEI on the structure formation. Macromol. Chem. Phys. 1997, 198, 3267-3276. 41. Ando, S.; Matsuura, T.; Sasaki, S. Coloration of aromatic polyimides and electronic properties of their source materials. Polym. J. 1997, 29, 69-76. 42. Hasegawa, M.; Horie, K. Photophysics, photochemistry, and optical properties of polyimides. Prog. Polym. Sci. 2001, 26, 259-335. 43. Ke, F.; Song, N.; Liang, D.; Xu, H. A method to break charge transfer complex of polyimide: A study on solution behavior. J. Appl. Polym. Sci. 2013, 127, 797-803. 44. Lim, H.; Cho, W. J.; Ha, C. S.; Ando, S.; Kim, Y. K.; Park, C. H.; Lee, K. Flexible organic electroluminescent devices based on fluorine-containing colorless polyimide substrates. Adv. Mater. 2002, 14, 1275-1279. 45. Du Pont, P. S.; Bilow, N. Polyimide composition and method for protecting photoreactive cells. 4592925 A, 1986. 46. Landis, A. L.; Naselow, A. B., Method of preparing high molecular weight polyimide, product and use. US Patent 4645824 A: 1987. 47. Matsuura, T.; Ando, S.; Matsui, S.; Sasaki, S.; Yamamoto, F. Heat-resistant singlemode optical wave-guides using fluorinated polyimides. Electron. Lett 1993, 29, 2107-2109. 48. Ando, S.; Sawada, T.; Inoue, Y. Thin, flexible waveplate of fluorinated polyimide. Electron. Lett 1993, 29, 2143-2145. 49. Gao, H.; Wang, D.; Guan, S.; Jiang, W.; Jiang, Z.; Gao, W.; Zhang, D. Fluorinated hyperbranched polyimide for optical waveguides. Macromol. Rapid Commun. 2007, 28, 252-259. 50. Stclair, A. K.; Slemp, W. S., Evaluation of optically transparent polyetherimide films

for applications in space. 1991; Vol. 23, p 817-830. 51. Vora, R. H.; Goh, S. H.; Chung, T.-S. Synthesis and properties of fluoro-polyetherimides. Polym. Eng. Sci. 2000, 40, 1318-1329. 52. Zhao, W.; Wall, C.; Maddikeri, R.; May, A., Effect of Injection Molding Processing Conditions on Optical Properties of Polyetherimide. In Current Developments in Lens Design and Optical Engineering Xv, Johnson, R. B., Mahajan, V. N., Thibault, S., Eds. 2014; Vol. 9192. 53. Philipp, H. R.; Le Grand, D. G.; Cole, H. S.; Liu, Y. S. The optical properties of a polyetherimide. Polym. Eng. Sci. 1989, 29, 1574-1578. 54. Huang, F.; Wang, X.; Li, S. The thermal stability of polyetherimide. Polym. Degrad. Stab. 1987, 18, 247-259. 55. Carroccio, S.; Puglisi, C.; Montaudo, G. Thermal degradation mechanisms of polyetherimide investigated by direct pyrolysis mass spectrometry. Macromol. Chem. Phys. 1999, 200, 2345-2355. 56. Sonia, Z.; Ana, Á.; Begoña, Á.; Jorge, L.-B.; Salvador, N.; Patricia, F.; Ramón, A. Thermogravimetric study of thermal degradation of polyetherimide. J. Appl. Polym. Sci. 2015, 132. 57. Carroccio, S.; Puglisi, C.; Montaudo, G. Photo-oxidation products of polyetherimide ULTEM determined by MALDI-TOF-MS. Kinetics and mechanisms. Polym. Degrad. Stab. 2003, 80, 459-476. 58. Long, S. A. T.; Long, E. R. Effects of Intermediate-Energy Electrons on Mechanical and Molecular Properties of a Polyetherimide. IEEE Trans. Nucl. Sci. 1984, 31, 1293-1298. 59. Kiefer, R. L.; Orwoll, R. A.; Aquino, E. C.; Pierce, A. C.; Glasgow, M. B.; Thibeault, S. A. The effects of atomic oxygen on polymer films containing bis(triphenyltin) oxide. Polym. Degrad. Stab. 1997, 57, 219-226. 60. Ghosh, M. K.; Mittal, K. L., Polyimides: fundamentals and applications. Marcel Dekker, Inc.: New York, 1996. 61. Mittal, K. L., Polyimides: synthesis, characterization, and applications. Plenum Press: New York, 1984. 62. Rock, J. A.; Male, L. J.; Durfee Jr, N. E., Flame resistant polyetherimide resin blends. US Patent 5051483 A: 1992.

63. Rock, J. A., Flame retardant polyetherimide-polycarbonate blends. US Patent 4629759 A: 1986. 64. Li, J.; Stoliarov, S. I. Measurement of kinetics and thermodynamics of the thermal degradation for charring polymers. Polym. Degrad. Stab. 2014, 106, 2-15. 65. Lee, J.; Takekoshi, T.; Giannelis, E. P., Fire retardant polyetherimide nanocomposites. In Nanophase and Nanocomposite Materials Ii, Komarneni, S., Parker, J. C., Wollenberger, H. J., Eds. 1997; Vol. 457, pp 513-518. 66. Suh, K. S.; Nam, J. H.; Lim, K. J. Electrical conduction in polyetherimide. J. Appl. Phys. 1996, 80, 6333-6335. 67. Chen, B. K.; Du, J. U.; Hou, C. W. The effects of chemical structure on the dielectric properties of polyetherimide and nanocomposites. IEEE Trans. Dielectr. Electr. Insul. 2008, 15, 127-133. 68. Chen, B.-K.; Fang, Y.-T.; Cheng, J.-R. Synthesis of low dielectric constant polyetherimide films. Macromol. Symp. 2006, 242, 34-39. 69. Chen, B.-K.; Su, C.-T.; Tseng, M.-C.; Tsay, S.-Y. Preparation of Polyetherimide Nanocomposites with Improved Thermal, Mechanical and Dielectric Properties. Polym. Bull. 2006, 57, 671-681. 70. Choudhury, A. Dielectric and piezoelectric properties of polyetherimide/BaTiO3 nanocomposites. Mater. Chem. Phys. 2010, 121, 280-285. 71. Kirby, A. J., Polyimides: materials, processing and applications. iSmithers Rapra Publishing: 1992; Vol. 59. 72. Foust, D. F.; Dumas, W. V., Polyetherimide Surfaces Chemically Treated To Improve Adhesion to Electroless Copper. In Metallization of Polymers, American Chemical Society: 1990; Vol. 440, pp 485-499. 73. Karas, B. R.; Foust, D. F.; Dumas, W. V.; Lamby, E. J. Aqueous pretreatments of polyetherimide to facilitate the bonding of electrolessly deposited copper. J. Adhes. Sci. Technol. 1992, 6, 815-828. 74. Karas, B. R.; Foust, D. F.; Dumas, W. V. Aqueous pretreatment of polyetherimide to facilitate the bonding of electrolessly deposited metals. Part 2. Solubilizer-free systems. J. Adhes. Sci. Technol. 1992, 6, 1205-1219. 75. Burrell, M. C.; Porta, G. M.; Karas, B. R.; Foust, D. F.; Chera, J. J. Copper deposition onto polyetherimide: Interface composition and adhesion. Journal of Vacuum Science & Technology A 1992, 10, 2752-2757.

76. Fukushima, T.; Kawakami, Y.; Oyama, T.; Tomoi, M. Photosensitive polyetherimide (Ultem) based on reaction development patterning (RDP). J. Photopolym. Sci. Technol. 2002, 15, 191-196. 77. Kim, J.-H. Polyetherimide substrates for future high density optical data storage. Polym. Eng. Sci. 2008, 48, 97-101. 78. Deng, S.; Lang, K.; Matsuura, T.; Tremblay, A. Polyetherimide hollow-fiber module for the removal of volatile organic compounds from air: Experimental results and computer simulation. Sep. Sci. Technol. 2000, 35, 2227-2242. 79. Zhen-Liang, X.; Tai-Shung, C.; Yu, H. Effect of polyvinylpyrrolidone molecular weights on morphology, oil/water separation, mechanical and thermal properties of polyetherimide/polyvinylpyrrolidone hollow fiber membranes. J. Appl. Polym. Sci. 1999, 74, 2220-2233. 80. Fuertes, A. B.; Centeno, T. A. Carbon molecular sieve membranes from polyetherimide. Microporous Mesoporous Mater. 1998, 26, 23-26. 81. Sedigh, M. G.; Xu, L.; Tsotsis, T. T.; Sahimi, M. Transport and morphological characteristics of polyetherimide-based carbon molecular sieve membranes. Ind. Eng. Chem. Res. 1999, 38, 3367-3380.

CHAPTER 2: Low-Molecular-Weight,

High-Mechanical-Strength, and Solution-Processable

Telechelic Poly(ether imide) End-Capped with

Ureidopyrimidinone

(Published in Macromolecules 2017, 50, 2016)

2.1 Abstract

Solution-processable poly(ether imide)s (PEIs) with ureidopyrimidinone (UPy) end groups were prepared by incorporating monoisocyanato-6-methylisocytosine into amine-terminated PEI oligomers. After functionalization with UPy end groups, PEI with a molecular weight as low as 8 kDa (8k-PEI-UPy) can be solution-cast to form films. Tensile tests revealed that 8k-PEI-UPy had an outstanding Young’s modulus higher than those of state-of-the-art high-molecular-weight commercial PEIs. The tensile strength, maximum elongation, and Young’s modulus of 8k-PEI-UPy were 87.2 ± 10.8 MPa, 3.10 ± 0.39%, and

(3.20 ± 0.14) × 103 MPa, respectively. The discovery herein significantly advances the chemistry of high-temperature PEI resins. UPy-based supramolecular chemistry is an effective and general strategy to achieve outstanding mechanical properties for PEI

oligomers.

2.2 Introduction

Poly(ether imide) (PEI) is a high-temperature engineering thermoplastic with outstanding mechanical properties, thermal stability, and chemical resistance.1-4 Due to the excellent properties, PEI is widely used as matrix resins,5, 6 adhesives,7, 8 and coatings9, 10 in fields such as aerospace and microelectronics.11-17 High-molecular-weight

2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl] propane dianhydride (BPADA) based PEI can be processed at ~340 ˚C due to the flexible linkages in PEI backbones, i.e., ether (−O−)18

19 and isopropylidene [−C(CH3)2−]. However, the required processing temperature is high, and as a result, the process is energy-inefficient. Furthermore, it can lead to slow thermal degradation when the polymer is processed in air.2 To overcome these drawbacks possessed by high-molecular-weight PEIs, our strategy herein is to synthesize PEI oligomers that allow for lower processing temperatures and alternative methods such as solution-casting. To ensure that the PEI oligomers can be linked to form PEIs with high molecular weights and maintain the mechanical strength, we have introduced interactive end groups. Specifically, by end-capping PEI with ureidopyrimidinone (UPy) that enables interactive hydrogen bonding, PEI can form supramolecular polymers with excellent mechanical properties.

Hydrogen-bonded supramolecular polymers, especially those based on UPy quadruple hydrogen bonding, have been studied extensively. The first report by Meijer and co-workers20 showed that UPy-based telechelic oligomers formed linear supramolecular polymers. The complementary quadruple hydrogen bonds between UPy groups have a

7 -1 21, 22 dimerization constant of Kdim ~ 10 M in chloroform (CHCl3). Since then, many have prepared UPy-based supramolecular polymers to improve mechanical properties and increase functionalities of materials. For example, Stang and co-workers have coupled

UPy with other orthogonal interactions such as coordination-driven self-assembly and host−guest interactions to achieve supramolecular polymers with high structural ordering23 and complex functionalities.24 Guan and co-workers utilized UPy cross-linker to increase the stiffness of elastomers without sacrificing extensibility.25 Wang and co-workers combined UPy hydrogen bonding and host−guest interactions to prepare multiple hydrogen-bonding interlocked catenanes.26 The more widely-used method is to modify the end groups of nucleophilic hydroxyl- or amine-terminated oligomers with electrophilic monoisocyanato 6-methylisocytosine (UPy-synthon) to create UPy-functionalized supramolecules with increased mechanical strength while inheriting the processability of oligomers.27-30 Meijer and co-workers attached UPy to oligomers of polydimethylsiloxanes, poly (ethylene oxide)s, polyethers, and polycarbonates.27, 31, 32 By incorporating UPy into low-molecular-weight polyesters, Long and coworkers successfully lowered the melt 24

viscosity but maintained strong mechanical properties of polyesters.33 Similarly, they used

UPy to increase the Young’s moduli of poly(ethylene-co-propylene)s.34 Nasseri and coworkers enhanced the storage moduli of poly(ethylene-co-vinyl alcohol)s by grafting

UPy onto the side chains.35 Weder and co-workers showed that UPy-functionalized side chains increased mechanical properties of poly(methacrylamide).36 All these studies

focused on incorporating UPy into polymers whose glass transition temperatures (Tg) are

lower than 100 ˚C due to the concerns of thermal stability of UPy. With Tg above 200 ˚C,

PEIs were never functionalized by UPy but simple hydrogen bonding end groups such as uracil37 and benzimidazole,38, 39 yielding mild improvement in mechanical properties of

PEI fibers.40, 41 In model product studies, however, Armstrong and Buggy reported that

UPy did not degrade until ~240 ˚C,42, 43 suggesting that UPy is applicable to polymers with

high Tgs such as PEI.

In this work, we describe the synthesis of thermally stable and solution-processable

UPy-terminated PEI oligomers by functionalizing amine-terminated PEIs (PEI-NH2,

Scheme 1) with UPy-synthon (Schemes 2 and 3). Stoichiometric imbalance of BPADA and m-phenylenediamine (mPD) ensured the synthesis of amine-terminated PEI, which could react with UPy-synthon for PEI functionalization. Tuning the molecular weight of PEI oligomers allowed for synthesis of UPy-terminated poly(ether imide) (PEI-UPy) with

various molecular weights and enabled the determination of the lowest molecular weight for PEI film formation. By comparing the mechanical properties of PEI-UPy with benchmark commercial PEIs, we showed that UPy incorporated PEI oligomers with a molecular weight as low as 8 kDa possessed outstanding mechanical properties comparable to state-of-the-art high-molecular-weight PEIs. Surprisingly, although thermally unstable by itself, the UPy end group exhibited excellent thermal stability after being incorporated into PEI oligomers.

Scheme 1. Synthesis of Amine-Terminated Poly(ether imide) Oligomer (PEI-NH2)

Scheme 2. Synthesis of Monoisocyanato-6-methylisocytosine (UPy-synthon)

Scheme 3. Synthesis of UPy-Terminated Poly(ether imide) (PEI-UPy)

2.3 Experimental

Materials. 2,2-Bis[4-(3,4-dicarboxyphenoxy) phenyl] propane dianhydride (BPADA) was supplied by SABIC and subjected to a heating−cooling cycle to remove any residual moisture before use. Phthalic anhydride (PA) was provided by SABIC and used as received. m-Phenylenediamine (mPD, 99%) was purchased from Sigma-Aldrich and purified by sublimation before use. 6-Methylisocytosine (MIC, 98%), hexamethylene diisocyanate

(HMDI, 99%), o-dichlorobenzene (oDCB), dibutyltin dilaurate (95%), tetrahydrofuran

(THF, 99.9%) and silica gel were purchased from Sigma-Aldrich and used as received.

Hexyl isocyanate (98%) was purchased from TCI chemicals and used as received. Hexanes

was purchased from Fisher Chemical. CHCl3 was obtained from Spectrum Chemical.

Methanol (MeOH) was obtained from Pharmco-AAPER. All deuterated solvents were purchased from Cambridge Isotope Laboratories, Inc. All solvents were used as received.

Analytical Methods. Proton nuclear magnetic resonance (1H NMR) spectroscopy characterization was performed on a Varian Unity 400 at 399.98 MHz in deuterated chloroform. Thermogravimetric analysis (TGA) was performed by heating the samples to

600 ˚C under a nitrogen flush of 60 mL/min on a TA Instruments Q500 TGA. Differential scanning calorimetry (DSC) was performed under a nitrogen flush of 50 mL/min at a heating rate of 10 °C/min on a TA Instruments Q1000 DSC, which was calibrated using

indium (melting point (mp) = 156.60 °C) and zinc (mp = 419.47 °C) standards. Tg was measured as the midpoint of the transition in the second heating ramp. Polymers were

dissolved in CHCl3 and cast into a Teflon Petri dish, followed by slow evaporation of the solvent and drying the film at 180 ˚C in vacuo. Chloroform size exclusion chromatography

(SEC) provided absolute molecular weights using a Waters 1515 isocratic HPLC pump and a Waters 717plus autosampler with Waters 2414 refractive index and Wyatt MiniDAWN

MALLS detectors (flow rate 1.0 mL/min). The column set consisted of two Shodex

KF-801 columns and a guard column with the same stationary phase of cross-linked polystyrene. The columns and detectors were maintained at 35 ˚C. Tensile testing was

performed on a 5500R Instron universal testing at a cross-head speed of 5 mm/min; the tensile strength, maximum elongation, and Young's moduli are reported based on an average of five specimens. The viscosity measurements were carried out using a

Brookfield viscometer (LVDV-E model) at room temperature.

Synthesis of UPy-Synthon. UPy-synthon was synthesized following a method described in a previous report (Scheme 2).44 A suspension of MIC (12 g, 95.92 mmol) and

6.2-fold excess of HMDI (100 g, 595 mmol) was stirred at 100 ˚C for 24 h. The white product was precipitated in hexanes, washed with hexanes three times, filtered, and then

1 dried at 40 ˚C in vacuo. H NMR (400 MHz, CDCl3, δ): 13.1 ppm (s, 1H, −NH−C(CH3)=

(a)), 11.9 ppm (s, 1H, −NH−CO−NH−CH2− (b)), 10.2 ppm (s, 1H, −NH−CO−NH−CH2−

(c)), 5.8 ppm (s, 1H, −CH=C(CH3) (d)), 3.3 ppm (m, 4H, −NH−CO−NH−CH2−,

−CH2NCO− (e)), 2.2 ppm (s, 3H, −NHC(CH3)=CH−CO− (f)), 1.3−1.7 ppm (m, 8H,

+ −(CH2)4− (g)). ESI-MS calcd: M = 293.1 g/mol; found: m/z 294.2, [M+H] , 316.1

[M+Na]+, 587.3 [2M+H]+, 609.3 [2M+Na]+.

Synthesis of PEI-NH2. Scheme 1 illustrates the general synthetic procedure and reaction conditions for an amine-terminated poly(ether imide) from BPADA and mPD with a stoichiometric imbalance. An exemplary synthesis of a 2500 g/mol PEI is described. A three-neck 500 mL round-bottomed flask, equipped with an overhead stirring rod, a

Dean−Stark trap, and a nitrogen inlet, was charged with BPADA (16.800 g, 32.28 mmol),

mPD (4.363 g, 40.35 mmol), and 60 mL of oDCB and then purged with N2. Subsequently, the slurry was heated to 180 ˚C and stirred for 12 h and then heated to 380 ˚C in a metal

bath for 30 min. The entire reaction was conducted in a constant N2 stream. The oligomer

was recovered by dissolution in CHCl3 and precipitation into MeOH. The precipitate was filtered and washed with MeOH three times and dried in vacuo at 180 ˚C for 8 h.

Synthesis of PEI-UPy. PEI-UPy was prepared by reacting PEI-NH2 oligomers with an

excess of the UPy-synthon in CHCl3. A typical synthesis is described as follows. A

flame-dried, 100 mL round-bottomed flask was charged with PEI-NH2 (molecular weight

(Mn) = 3.1 kg/mol, 3.100 g, 1 mmol), UPy-synthon (1.173 g, 4 mmol) and CHCl3 (60 mL),

and then purged with N2. The reaction mixture was stirred at 60 ˚C for 24 h. Subsequently,

2.346 g of silica gel and 0.5 mL of 3 wt% dibutyltin dilaurate in THF (3 wt%) were added to the mixture and allowed to react at 60 ˚C for 1 h. The suspension was diluted by 120 mL

of CHCl3 and filtered through Celite. The filtrate was precipitated in MeOH. The precipitate was filtered and washed with MeOH three times and dried in vacuo at 100 ˚C for 24 h.

2.4 Results and Discussion

Table 1. Summary of Mn, Td,5%, and Tg of PEI-NH2 Oligomers

a b theor Mn NMR Mn SEC Mn Td,5% sample Tg (˚C) (kDa) (kDa) (kDa) (˚C)

8k-PEI-NH2 7.8 8.0 8.4 520 ± 12 205 ± 0

6k-PEI-NH2 6.0 5.6 7.1 511 ± 20 199 ± 0

4k-PEI-NH2 4.3 4.5 5.8 513 ± 17 195 ± 0

2k-PEI-NH2 2.5 3.1 3.1 500 ± 24 186 ± 1 a The Mn of PA-terminated PEI (PEI-PA) was first determined by SEC. The Mn of PEI-NH2

was then estimated by taking consideration of the molecular difference between PEI-NH2 and PEI-PA. See the Supporting Information for more details. bThe TGA analyses were based on five repeating measurements.

PEI-NH2 Synthesis. A stoichiometric imbalance of the monomers in step-growth polymerization can yield telechelic polymers with specific end groups.45-47 By adding excess mPD into the reaction mixture, we synthesized PEIs with amine end groups

(Scheme 1). 1H NMR spectroscopy confirmed the amine end groups (Figure 1) and allowed for quantification of the degree of polymerization and molecular weight of

1 PEI-NH2 (Table 1 and Figure S1). The molecular weights of PEI-NH2 calculated via H

NMR were further confirmed by determining the molecular weights of phthalic anhydride

(PA)-terminated PEIs using SEC (Table 1). Since PEI-NH2 cannot be directly characterized by SEC due to the strong interactions between amine end groups and our

SEC columns, the amine-terminated PEIs were end-capped with PA (Scheme S1 and

Figure S2) to allow for SEC characterization.

1 Figure 1. H NMR spectra of (a) 8k-PEI-NH2, (b) 6k-PEI-NH2, (c) 4k-PEI-NH2, and (d)

2k-PEI-NH2 in CDCl3. Peak intensities are normalized to peak i. Peaks f, g, and h are signals of the repeating units, and peaks f’, g’, and h’ are signals of the end group.

Thermal Analysis of PEI-NH2. TGA and DSC were used to analyze the thermal

properties of PEI-NH2. TGA revealed that PEI-NH2 oligomers had outstanding thermal

stability, which is attributed to the strong imide rings in the PEI backbones. All PEI-NH2 oligomers degraded through a single degradation step in the range of 400−600 ˚C (Figure

2). The decomposition temperatures at 5% weight loss (Td,5%) were about 500−520 ˚C

(Table 1).

Figure 2. TGA thermograms display high thermal stability of PEI-NH2 oligomers. The inlet shows the zoomed-in curves around 5% weight loss.

DSC was used to determine the glass transition temperatures of the PEI-NH2 oligomers

(Figure 3a). As shown by the DSC traces, Tg increased with Mn following the Flory−Fox equation:48

퐾 푇g = 푇g,∞ − (1) 푀n

The linear fitting (Figure 3b) had an R2 value of 0.9985 and suggested an intrinsic glass

transition temperature (Tg, ∞) of 217 ˚C, which is in excellent agreement with the Tg of

49 state-of-the-art high-molecular-weight PEIs (Tg = 217 ˚C).

Figure 3. (a) DSC traces of PEI-NH2 oligomers. The Tg increased with Mn. (b) Fitting of Tg

to the Flory-Fox equation. The solid line is a linear fit of Tg with respect to 1/Mn (eq 1). K =

−1 100 ˚C kg mol ; Tg, ∞ = 217 ˚C.

PEI-UPy Synthesis. Reacting PEI-NH2 with excess UPy-synthon at a ratio of

[NH2]:[NCO] = 1:2 transformed PEI-NH2 into PEI-UPy. Noticeably, the viscosity of the products increased significantly compared to the precursors. The 1H NMR spectra of

PEI-UPy had characteristic downfield signals of UPy groups (9.9, 11.7, and 13.2 ppm;

Figure 4 inset), confirming the successful incorporation of UPy moieties into the PEI oligomers. However, the three peaks shifted slightly in comparison with the original positions in the 1H NMR spectrum of UPy-synthon (Figure S3). Since there was excessive

UPy-synthon in the reaction, adding silica gel removed the residual UPy-synthon in the reactor,27 as confirmed by 1H NMR spectroscopy (Figure S3, panels d and e). By

1 determining the amount of PEI-NH2 in the product using H NMR, the conversion of

PEI-NH2 to PEI-UPy was calculated (see Supporting Information). Although we have optimized the reaction conditions including solvent, temperature, and amount of

UPy-synthon, the conversion of PEI-NH2 to PEI-UPy was not more than 90% (Figure S4

and Table 2). The degrees of UPy-functionalization of 2k-PEI-NH2 were 88%, 82%, 50%, and 78% in chloroform, DMF, DMSO, and NMP, respectively. The over 85% conversion in chloroform is at an acceptable level of functionalization,33-35 particularly for the purpose of

this work. The incompleteness of the PEI-NH2 to PEI-UPy conversion is probably due to the low solubility of UPy-synthon in chloroform and the high viscosity of the reaction

mixtures. The viscosity can be potentially reduced by reacting PEI-NH2 with a mixture of 35

UPy-synthon and ethylene glycol-UPy. As reported previously by Scherman et al.,50

UPy-synthon can react with PEI-NH2 while ethylene glycol-UPy can lower the virtual molecular weight and hence the viscosity.

Figure 4. 1H NMR spectra of (a) 8k-PEI-UPy, (b) 6k-PEI-UPy, (c) 4k-PEI-UPy, and (d)

2k-PEI-UPy in CDCl3. Peak intensities are normalized to peak a for comparison.

Thermal Analysis of PEI-UPy. PEI-UPy exhibited two stages of weight loss, as

shown by TGA (Figure 5). The first stage at ~240 ˚C was due to the decomposition of

6-methylisocytosine (MIC) moieties in PEI-UPy, which is suggested by the same onset degradation temperatures of PEI-UPy and pure MIC. The decomposition temperature of

MIC moieties in PEI-UPy agreed well with the previous reports whereas the urea bonds between alkyl chains and isocytosine rings were cleaved at ~240 ˚C.42, 43It is noteworthy that the first degradation temperature of PEI-UPy (~240 ˚C) was much higher than that of

UPy-synthon (~160 ˚C), which is due to the configurational change of −NCO after being

incorporated into PEIs. In pure UPy-synthon, −CH2NCO degraded first at ~160 ˚C corresponding to a weight loss of 19%, followed by the cleavage of urea bonds at ~240 ˚C, as shown by TGA (Figure 5). Once the −NCO group in UPy-synthon was attached to PEIs, it changed to a urea bond that linked the PEI backbone with the hexyl chain. The TGA analyses showed that at ~240 ˚C PEI-UPy had weight losses matching with the weight percentage of MIC in PEI-UPy oligomers of all molecular weights (Table 2). After cleaving the urea bond closest to the chain ends, the remaining polymer chains may react with one another to form PEIs of larger molecular weights, i.e., PEI-urea-hexyl-urea-PEI, similar to the formation of N,N-di-n-butylurea during the pyrolysis of

N-[(butylamino)carbonyl]-6-methylisocytosine in a previous report by Armstrong and

Buggy.42 The weight loss at the second stage was mainly caused by the degradation of PEI

backbones, similar to that of PEI-NH2. DSC measurements revealed that Tg of PEI-UPy 37

increased with molecular weight and there was a significant change in Tg after

incorporating UPy into PEI-NH2 (Figure 6 and Table 2). In comparison with PEI-NH2, Tg of PEI-UPy increased by 8, 10, 13, and 19 ˚C for 8k-, 6k-, 4k-, and 2k-PEIs, respectively.

The changes in Tg are more significant for the oligomers with lower molecular weights.

Figure 5. TGA thermograms exhibit thermal stability of PEI-UPy oligomers, UPy-synthon, and MIC.

Table 2. Degree of UPy-Functionalization, MIC Weight Percent in PEI-UPy, Weight

Loss of PEI-UPy at the First Degradation Stage, and Tg of PEI-UPy

degree of UPy MIC weight loss at sample Tg (˚C) functionalization (%) (wt % 400 ˚C (%) ) 8k-PEI-UPy 82 2.9 2.9 213±1 6k-PEI-UPy 83 4.0 4.5 209±1 4k-PEI-UPy 87 4.9 6.8 208±1 2k-PEI-UPy 88 6.8 6.7 204±1

Figure 6. DSC traces of PEI-UPy series.

Figure 7. Solution-cast (a) 8k-PEI-NH2, (b) 6k-PEI-NH2, (c) 4k-PEI-NH2, (d) 2k-PEI-NH2,

(e) 8k-PEI-UPy, (f) 6k-PEI-UPy, (g) 4k-PEI-UPy, and (h) 2k-PEI-UPy. Only 8k-PEI-NH2 and 8k-PEI-UPy formed films.

Mechanical Properties. To test the mechanical properties of PEI-NH2 and PEI-UPy

oligomers, we have solution-cast films using CHCl3. Among all the oligomers, only

8k-PEI-UPy and 8k-PEI-NH2 formed intact films (Figure 7). The film-forming ability of

PEI seemed to mainly depend on the molecular weight, and the end group had a minor effect, in agreement with the common understanding that molecular weight is the dominant factor for controlling polymer physical properties.51 The inability to form films prohibited

comparing mechanical properties of PEI-NH2 and PEI-UPy with Mn lower than 6 kDa.

Figure 8. Comparison of solution-cast (a) 8k-PEI-NH2 and (b, c) 8k-PEI-UPy films. The

8k-PEI-UPy film was flexible and could be cut into dumbbell-shaped specimens, while the

8k-PEI-NH2 film cracked.

Nevertheless, the comparison between 8k-PEI-UPy and 8k-PEI-NH2 highlighted the

improvement in mechanical properties after functionalization of PEI-NH2 with UPy.

Although both 8k-PEI-NH2 and 8k-PEI-UPy showed film-forming ability, the

solution-cast 8k-PEI-NH2 film was fragile and could not be cut into dumbbell structures

(Figure 8a). In contrast, 8k-PEI-UPy could be cut and the resulting dumbbell structures showed great bending flexibility (Figure 8, panels b and c). The sharp difference in film flexibility shows that the incorporation of UPy significantly improved the mechanical properties of the 8k-PEI oligomer.

Table 3. Mechanical Properties of Solution-Cast 8k-PEI-UPy and State-of-the-Art

Commercial PEIs: PEI(1) and PEI(2)

sample tensile strength maximum Young’s modulus (MPa) elongation (%) (MPa)

8k-PEI-UPy 87.2 ± 10.8 3.10 ± 0.39 (3.20 ± 0.14)×103 PEI(1): 16.9k-PEI 98.8 ± 1.1 4.98 ± 0.54 (2.79 ± 0.09)×103 PEI(2): 24.5k-PEI 99.0 ± 1.3 5.02 ± 0.40 (2.94 ± 0.12)×103

We further compared the tensile properties of the solution-cast 8k-PEI-UPy film with standard PEI films including PEI(1) and PEI(2) (Figure 9, Table 3). Surprisingly, the

8k-PEI-UPy film, with a molecular weight of only 8.7 kDa (calculated by assuming full

conversion from 8k-PEI-NH2 to 8k-PEI-UPy), showed higher Young’s modulus than

PEI(1) (Mn, SEC = 16.9 kDa) and PEI(2) (Mn, SEC = 24.5 kDa), while its tensile strength and maximum elongation were comparable to those of PEI(1) and PEI(2) films. These results showed that the incorporation of UPy end groups is an effective approach to improve the mechanical properties of low-molecular-weight PEIs.

Figure 9. Stress−strain curves obtained from tensile tests of (a) 8k-PEI-UPy, (b) PEI(1), and (c) PEI(2) films.

Origin of Changes in Tg and Improved Mechanical Strength of PEI-UPy. To verify

whether the increase of Tg results from the urea linkage or the end UPy moiety, PEIs terminated with urea bonds and flexible hexyl groups (PEI-UHex, Scheme 4) were synthesized as a set of reference polymers for comparison (Scheme S2). PEI-UHex differs

from PEI-NH2 by having a hexyl group at each end of the polymer chain linked by urea;

PEI-UPy differs from PEI-UHex by having a UPy end group at each end of the polymer

chain linked by another urea. Surprisingly, after incorporation of UHex, Tg decreased or

did not change significantly compared with PEI-NH2 (Table 4 and Figure S5), which is attributed to the two competing factors: hydrogen bonding and flexible alkyl chains. Being

locked in hydrogen bonds, polymer chain ends tend to be less free and an increase of Tg is expected. On the other hand, short flexible alkyl chains are known to generate free volume

52 at polymer chain ends, and thus a decrease of Tg is anticipated. Combining these two factors, it seems that the effect of flexible alkyl group outweighs that of the hydrogen

bonding, leading to a decrease of Tg in PEI-UHex. In contrast, the combining hydrogen bonding effects from urea linkage and UPy in PEI-UPy prevail over the flexible alkyl chain

effect, resulting in increased Tg compared to PEI-NH2. It is interesting that, although UPy multiple hydrogen bonding dissociates at above 80 ˚C,53 UPy still played a role in the

increase of the relatively high Tg of PEI. In addition, all PEI-UHex and PEI-UPy oligomers started to degrade at ~240 ˚C by cleaving the polymer chains at the outmost urea bond

(Figure 5 and Figure S6). 44

Scheme 4. Structure of PEI-UHexa

aPEI-UHex differs from PEI-UPy by having no UPy end groups.

Table 4. Comparison of Tg among PEI-NH2, PEI-UHex, and PEI-UPy.

a b Tg of PEI-NH2 Tg of PEI-UHex Tg of PEI-UPy mol wt (kDa) (˚C) (˚C) (˚C)

8 205 200 (−5) 213 (+ 8) 6 199 189 (−10) 209 (+10) 4 195 195 (+0) 208 (+13) 2 185 185 (+0) 204 (+19) a b Values in parentheses were the difference of Tg between PEI-UHex and PEI-NH2. Values

in parentheses were the difference of Tg between PEI-UPy and PEI-NH2.

Similarly, to determine whether the improved mechanical strength of 8k-PEI-UPy stems from the UPy moieties or the urea linkage, we utilized the reference polymers

PEI-UHex to compare with PEI-UPy. The solution viscosities (η) of the 8k-PEI-UPy,

8k-PEI-UHex, and 8k-PEI-NH2 were measured. Given the same polymer concentration

(200 mg/mL, in CHCl3), the solution viscosity of 8k-PEI-UHex was only 1.8 times that of

8k-PEI-NH2, while the solution viscosity of 8k-PEI-UPy was 87 times larger than that of

the 8k-PEI-NH2. The significant increase in viscosity shows strong chain interactions resulting from the UPy moieties (Table 5), which ultimately improve the Young’s modulus of PEI-UPy.

Table 5. Solution Viscosities of 8k-PEI-UPy, 8k-PEI-UHex, and 8k-PEI-NH2 at a

Concentration of 200 mg/mL

sample η (cP) 8k-PEI-UPy 2058 8k-PEI-UHex 41.99

8k-PEI-NH2 23.68

2.5 Conclusions

Step-growth polymerization of BPADA and mPD at stoichiometric imbalance afforded

the synthesis of amine-terminated telechelic PEIs. The molecular structures and Mn were confirmed by 1H NMR spectroscopy. Despite the low molecular weights, TGA revealed

excellent thermal stability of PEI-NH2 oligomers. DSC illustrated that Tg of PEI-NH2 increased following the classic Flory−Fox equation. UPy-terminated telechelic PEIs were synthesized by functionalizing amine-terminated telechelic PEIs with UPy-synthon. After

incorporating UPy, PEI had two-stage degradation behavior and the Tg increased. Effects of molecular weights and UPy incorporation on mechanical properties of PEI films were

systematically studied. Only PEIs with Mn higher than 6 kDa showed film-forming capability regardless of the end groups. Incorporation of UPy end group significantly

enhanced the flexibility of PEI. Importantly, PEI-UPy oligomers with Mn of only 8 kDa was solution-processable and exhibited outstanding mechanical strength comparable to high-molecular-weight state-of-the-art commercial PEIs. The findings herein provide insight into the design of low-molecular-weight PEIs with solution processability and outstanding mechanical properties, and thus, UPy-based supramolecular chemistry represents an effective and general approach to enhancing mechanical properties of high-temperature PEI oligomers.

2.6 Supplementary Information

1 Calculation of Mn from H NMR of PEI-NH2. The number average molecular weight

(Mn) and the degree of polymerization (n) of each PEI-NH2 were calculated using the following equations based on the integral areas of peaks i and j (Figure S1).

1 Figure S1. H NMR spectrum of 2k-PEI-NH2 in CDCl3.

# 표푓 푟푒푝푒푎푡𝑖푛푔 푢푛𝑖푡푠 퐴푟푒푎 (퐣)/6 n = = # 표푓 푐ℎ푎𝑖푛푠 퐴푟푒푎 (퐢)/4

−1 푀푛 = 푀푟푒푝푒푎푡푖푛푔 푢푛푖푡 ∙ n + 푀푒푛푑 푔푟표푢푝 = (592.61푛 + 108.14) g ∙ mol where the number of repeating units was calculated by normalizing peak j by a factor of 6 since each repeating unit consists 6H in the methyl groups of BPADA. Similarly, the number of chains was normalized by a factor of 4 since each chain has 4H in the amine end groups.

Calculation of Mn from phthalic anhydride capped PEI (PEI-PA). Because amines

can stick to chromatography columns, the amine-terminated PEI was end-capped with phthalic anhydride (PA) (Scheme S1) to allow for SEC analysis and confirmation of the

1 molecular weights characterized by H NMR. An exemplary reaction of 2k-PEI-NH2 with

PA is described as follows. A 500 mL three-neck round-bottomed flask, equipped with an overhead stirring-rod, a Dean-Stark trap, and a nitrogen inlet, was charged with

2k-PEI-NH2 (5.700 g, 1.839 mmol), PA (0.654 g, 4.41 mmol), and 60 mL

o-dichlorobenzene (oDCB) and purged with N2. The subsequent slurry was first heated at

180 ˚C for 6 h with constant stirring, and then heated at 380 ˚C for 0.5 h under constant N2

purge in a metal bath without stirring. The product was dissolved in CHCl3 and precipitated into MeOH. The precipitate was filtered, washed with MeOH three times, and dried in vacuo at 180 ˚C for 8 h.

Scheme S1. Synthesis of PA-terminated PEI by end-capping PEI-NH2 with PA

As evidenced in the 1H NMR spectra, the peaks representing the amine end groups in

PEI-NH2 disappeared after end capping with PA. Furthermore, new peaks characteristic of

PA (labeled as p and q) appeared and had integral areas close to that of peak i in PEI-NH2.

The reaction between PEI and PA, as well as the intensities of peaks p and q, confirmed that the oligomers were indeed amine-terminated at both chain ends.

1 Figure S2. H NMR spectra of 2k-PEI-NH2 and 2k-PEI-PA. Resonance peaks i, f’, and h’,

disappeared while new peaks p and q appeared after reacting PEI-NH2 with PA.

Evolution of UPy characteristic peaks during reactions. The reaction of

4k-PEI-NH2 and UPy-synthon was chosen as an example to demonstrate the evolution

1 from PEI-NH2 to PEI-UPy. H NMR confirmed the components in each step (Figure S3). It

is noteworthy that the three characteristic UPy peaks at 13.4-9.8 ppm shifted after incorporation of UPy. More importantly, no free UPy-synthon existed after silica gel

addition and precipitation into MeOH, showing the conversion from PEI-NH2 to PEI-UPy.

1 Figure S3. H NMR spectra of (a) 4k-PEI-NH2; (b) UPy-synthon; (c) 4k-PEI-UPy after

reacting 4k-PEI-NH2 with excessive UPy-synthon; (d) 4k-PEI-UPy after adding silica gel to remove residual UPy-synthon; (e) 4k-PEI-UPy after precipitation in MeOH.

Calculation of the conversion from PEI-NH2 to PEI-UPy. Due to the high viscosity

of reaction slurry during the reaction of PEI-NH2 with UPy-synthon, the conversion of

1 -NH2 to UPy was not complete, indicated by the residual -NH2 peaks in H NMR spectra

(Figure S4). Thus, the conversion could be calculated by the following equation, which

utilized the integral of amine peak (i) areas in PEI-UPy spectra and PEI-NH2 spectra, respectively, after all spectra were normalized to the methyl groups in PEI backbone (peak j).

퐴푟푒푎 (퐢, PEI-UPy) Conversion (NH2 to UPy) = [1 − ] × 100% 퐴푟푒푎 (퐢, PEI-NH2)

1 Figure S4. H NMR spectra of (a) 2k-PEI-NH2 and (b) 2k-PEI-UPy. Resonance peaks i

(normalized to j) are used to calculate the NH2 to UPy conversion.

Synthesis of PEI-UHex. PEI-UHex was prepared by reacting PEI-NH2 oligomers with excess hexyl isocyanate (Scheme S2). A typical synthesis procedure is as follows. A

flame dried, N2 purged 100 mL round-bottomed flask was charged with PEI-NH2 (Mn =

3,100 g/mol, 3.100 g, 1 mmol), hexyl isocyanate (0.509 g, 4 mmol) and CHCl3 (30 mL),

and then purged with N2. The synthesis reaction was conducted by stirring the mixture at

60 ˚C for 24 h. After the reaction, the product was precipitated in MeOH. The precipitate was filtered and washed with MeOH three times and dried in vacuo at 180 ˚C for 6 h.

Scheme S2. Synthesis of PEI-UHex by end-capping PEI-NH2 with hexyl isocyanate.

Thermal properties of PEI-UHex series. The DSC traces of PEI-UHex series are shown in Figure S5. TGA thermograms revealed that PEI-UHex series started degradation at ~ 240 ˚C (Figure S6). The degradation temperature was similar to the first degradation

temperature of PEI-UPy (Figure 5).

Figure S5. DSC traces of PEI-UHex oligomers.

Figure S6. TGA thermograms show that the PEI-UHex oligomers started degradation at around 240 ˚C.

2.7 References

1. Bessonov, M. I.; Koton, M. M.; Kudryavtsev, V. V.; Laius, L. A., Polyimides--thermally stable polymers. Consultants Bureau: New York, 1987. 2. Ghosh, M. K.; Mittal, K. L., Polyimides: fundamentals and applications. CRC Press: New York, 1996. 3. Wilson, D.; Stenzenberger, H. D.; Hergenrother, P. M., Polyimides. Springer: London, 1990. 4. Sroog, C. E. Polyimides. Prog. Polym. Sci. 1991, 16, 561-694. 5. Jang, B. Z.; Pater, R. H.; Soucek, M. D.; Hinkley, J. A. Plastic deformation mechanisms in polyimide resins and their semi-interpenetrating networks. J. Polym. Sci., Part B: Polym. Phys. 1992, 30, 643-654.

6. McDonald, W. F.; Urban, M. W. Chemical structures at the Nextel fibre/polyimide matrix interface detected by photoacoustic FTIR spectroscopy. Composites 1991, 22, 307-318. 7. Progar, D. J.; Clair, T. L. S. Evaluation of a novel thermoplastic polyimide for bonding titanium. Int. J. Adhes. Adhes. 1986, 6, 25-30. 8. Progar, D. J.; Clair, T. L. S. A new flexible backbone polyimide adhesive. J. Adhes. Sci. Technol. 1990, 4, 527-549. 9. Maggioni, G.; Carturan, S.; Rigato, V.; Della Mea, G. Glow discharge vapour deposition polymerisation of polyimide thin coatings. Surf. Coat. Technol. 2001, 142–144, 156-162. 10. He, S.; Zhang, S.; Lu, C.; Wu, G.; Yang, Y.; An, F.; Guo, J.; Li, H. Polyimide nano-coating on carbon fibers by electrophoretic deposition. Colloids Surf., A 2011, 381, 118-122. 11. Yu, W.; Ko, T.-M. Surface characterizations of potassium-hydroxide-modified Upilex-S® polyimide at an elevated temperature. Eur. Polym. J. 2001, 37, 1791-1799. 12. Hilado, C. J., Reinforced Phenolic, Polyester, Polyimide, and Polystyrene Systems. Technomic Publishing Company: Westport, CT, 1974; Vol. 5. 13. Mittal, K. L., Polyimides: synthesis, characterization, and applications. Plenum Press: New York, 1984. 14. Kricheldorf, H. R.; de Abajo, J., Progress in polyimide chemistry, advances in polymer science. Springer: Berlin, 1999. 15. Xie, K.; Liu, J. G.; Zhou, H. W.; Zhang, S. Y.; He, M. H.; Yang, S. Y. Soluble fluoro-polyimides derived from 1,3-bis(4-amino-2-trifluoromethyl-phenoxy) benzene and dianhydrides. Polymer 2001, 42, 7267-7274. 16. Wu, J.; Yang, S.; Gao, S.; Hu, A.; Liu, J.; Fan, L. Preparation, morphology and properties of nano-sized Al2O3/polyimide hybrid films. Eur. Polym. J. 2005, 41, 73-81. 17. Zhai, F.; Guo, X.; Fang, J.; Xu, H. Synthesis and properties of novel sulfonated polyimide membranes for direct methanol fuel cell application. J. Membr. Sci. 2007, 296, 102-109. 18. Hsiao, S.-H.; Chung, C.-L.; Lee, M.-L. Synthesis and characterization of soluble polyimides derived from 2',5'-bis(3,4-dicarboxyphenoxy)-p-terphenyl dianhydride. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 1008-1017.

19. Kute, V.; Banerjee, S. Novel semi-fluorinated poly(ether imide)s derived from 4-(p-aminophenoxy)-3-trifluoromethyl-4'-aminobiphenyl. Macromol. Chem. Phys. 2003, 204, 2105-2112. 20. Sijbesma, R. P.; Beijer, F. H.; Brunsveld, L.; Folmer, B. J. B.; Hirschberg, J. H. K. K.; Lange, R. F. M.; Lowe, J. K. L.; Meijer, E. W. Reversible polymers formed from self-complementary monomers using quadruple hydrogen bonding. Science 1997, 278, 1601-1604. 21. Beijer, F. H.; Sijbesma, R. P.; Kooijman, H.; Spek, A. L.; Meijer, E. W. Strong dimerization of ureidopyrimidones via quadruple hydrogen bonding. J. Am. Chem. Soc. 1998, 120, 6761-6769. 22. Söntjens, S. H. M.; Sijbesma, R. P.; van Genderen, M. H. P.; Meijer, E. W. Stability and lifetime of quadruply hydrogen bonded 2-ureido-4[1h]-pyrimidinone dimers. J. Am. Chem. Soc. 2000, 122, 7487-7493. 23. Yan, X.; Jiang, B.; Cook, T. R.; Zhang, Y.; Li, J.; Yu, Y.; Huang, F.; Yang, H.-B.; Stang, P. J. Dendronized organoplatinum(II) metallacyclic polymers constructed by hierarchical coordination-driven self-assembly and hydrogen-bonding interfaces. J. Am. Chem. Soc. 2013, 135, 16813-16816. 24. Zhou, Z.; Yan, X.; Cook, T. R.; Saha, M. L.; Stang, P. J. Engineering functionalization in a supramolecular polymer: Hierarchical self-organization of triply orthogonal non-covalent interactions on a supramolecular coordination complex platform. J. Am. Chem. Soc. 2016, 138, 806-809. 25. Kushner, A. M.; Gabuchian, V.; Johnson, E. G.; Guan, Z. Biomimetic design of reversibly unfolding cross-linker to enhance mechanical properties of 3D network polymers. J. Am. Chem. Soc. 2007, 129, 14110-14111. 26. Xiao, T.; Li, S.-L.; Zhang, Y.; Lin, C.; Hu, B.; Guan, X.; Yu, Y.; Jiang, J.; Wang, L. Novel self-assembled dynamic [2]catenanes interlocked by the quadruple hydrogen bonding ureidopyrimidinone motif. Chem. Sci. 2012, 3, 1417-1421. 27. Folmer, B. J. B.; Sijbesma, R. P.; Versteegen, R. M.; van der Rijt, J. A. J.; Meijer, E. W. Supramolecular polymer materials: Chain extension of telechelic polymers using a reactive hydrogen-bonding synthon. Adv. Mater. 2000, 12, 874-878. 28. Foster, E. J.; Berda, E. B.; Meijer, E. W. Metastable supramolecular polymer nanoparticles via intramolecular collapse of single polymer chains. J. Am. Chem. Soc. 2009, 131, 6964-6966.

29. Guo, M.; Pitet, L. M.; Wyss, H. M.; Vos, M.; Dankers, P. Y. W.; Meijer, E. W. Tough Stimuli-Responsive Supramolecular Hydrogels with Hydrogen-Bonding Network Junctions. J. Am. Chem. Soc. 2014, 136, 6969-6977. 30. Hosono, N.; Kushner, A. M.; Chung, J.; Palmans, A. R. A.; Guan, Z.; Meijer, E. W. Forced unfolding of single-chain polymeric nanoparticles. J. Am. Chem. Soc. 2015, 137, 6880-6888. 31. Hirschberg, J. H. K. K.; Beijer, F. H.; van Aert, H. A.; Magusin, P. C. M. M.; Sijbesma, R. P.; Meijer, E. W. Supramolecular polymers from linear telechelic siloxanes with quadruple-hydrogen-bonded units. Macromolecules 1999, 32, 2696-2705. 32. Lange, R. F. M.; Van Gurp, M.; Meijer, E. W. Hydrogen-bonded supramolecular polymer networks. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 3657-3670. 33. Yamauchi, K.; Kanomata, A.; Inoue, T.; Long, T. E. Thermoreversible polyesters consisting of multiple hydrogen bonding (MHB). Macromolecules 2004, 37, 3519-3522. 34. Elkins, C. L.; Viswanathan, K.; Long, T. E. Synthesis and characterization of star-shaped poly(ethylene-co-propylene) polymers bearing terminal self-complementary multiple hydrogen-bonding sites. Macromolecules 2006, 39, 3132-3139. 35. Jangizehi, A.; Ghaffarian, S. R.; Kowsari, E.; Nasseri, R. Supramolecular polymer based on poly (ethylene-co-vinyl alcohol)-g-ureidopyrimidinone: Self-assembly and thermo-reversibility. J. Macromol. Sci., Part B: Phys 2014, 53, 848-860. 36. Heinzmann, C.; Lamparth, I.; Rist, K.; Moszner, N.; Fiore, G. L.; Weder, C. Supramolecular polymer networks made by solvent-free copolymerization of a liquid 2-ureido-4[1H]-pyrimidinone methacrylamide. Macromolecules 2015, 48, 8128-8136. 37. Ye, Y.-S.; Huang, Y.-J.; Cheng, C.-C.; Chang, F.-C. A new supramolecular sulfonated polyimide for use in proton exchange membranes for fuel cells. Chem. Commun. 2010, 46, 7554-7556. 38. Musto, P.; Karasz, F. E.; MacKnight, W. J. Hydrogen bonding in polybenzimidazole/polyimide systems: a Fourier-transform infra-red investigation using low-molecular-weight monofunctional probes. Polymer 1989, 30, 1012-1021. 39. Ahn, T.-K.; Kim, M.; Choe, S. Hydrogen-bonding strength in the blends of polybenzimidazole with BTDA- and DSDA-based polyimides. Macromolecules 1997, 30, 3369-3374. 40. Liu, X.; Gao, G.; Dong, L.; Ye, G.; Gu, Y. Correlation between hydrogen-bonding interaction and mechanical properties of polyimide fibers. Polym. Adv. Technol. 2009, 20,

362-366. 41. Dong, J.; Yin, C.; Zhang, Z.; Wang, X.; Li, H.; Zhang, Q. Hydrogen-bonding interactions and molecular packing in polyimide fibers containing benzimidazole units. Macromol. Mater. Eng. 2014, 299, 1170-1179. 42. Armstrong, G.; Buggy, M. Thermal stability of a ureidopyrimidinone model compound. Mater. Sci. Eng., C 2001, 18, 45-49. 43. Armstrong, G.; Buggy, M. Thermal stability of some self-assembling hydrogen-bonded polymers and related model complexes. Polym. Int. 2002, 51, 1219-1224. 44. Keizer, H. M.; Sijbesma, R. P.; Jansen, J. F. G. A.; Pasternack, G.; Meijer, E. W. Polymerization-induced phase separation using hydrogen-bonded supramolecular polymers. Macromolecules 2003, 36, 5602-5606. 45. Lee, H.-S.; Badami, A. S.; Roy, A.; McGrath, J. E. Segmented sulfonated poly(arylene ether sulfone)-b-polyimide copolymers for proton exchange membrane fuel cells. I. Copolymer synthesis and fundamental properties. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 4879-4890. 46. Hedrick, J. L.; Hawker, C. J.; DiPietro, R.; Jerome, R.; Charlier, Y. The use of styrenic copolymers to generate polyimide nanofoams. Polymer 1995, 36, 4855-4866. 47. Carter, K. R.; DiPietro, R. A.; Sanchez, M. I.; Swanson, S. A. Nanoporous Polyimides Derived from Highly Fluorinated Polyimide/Poly(propylene Oxide) Copolymers. Chem. Mater. 2001, 13, 213-221. 48. Fox, T. G., Jr.; Flory, P. J. Second‐order transition temperatures and related properties of polystyrene. I. Influence of molecular weight. J. Appl. Phys. 1950, 21, 581-591. 49. Belana, J.; Cañadas, J. C.; Diego, J. A.; Mudarra, M.; Díaz, R.; Friederichs, S.; Jaimes, C.; Sanchis, M. J. Physical ageing studies in polyetherimide ULTEM 1000. Polym. Int. 1998, 46, 29-32. 50. Scherman, O. A.; Ligthart, G. B. W. L.; Ohkawa, H.; Sijbesma, R. P.; Meijer, E. W. Olefin metathesis and quadruple hydrogen bonding: A powerful combination in multistep supramolecular synthesis. Proceedings of the National Academy of Sciences 2006, 103, 11850-11855. 51. Odian, G., Principles of polymerization. Wiley & Sons, Inc.: New York, 2004. 52. Blackley, D. C., Polymer Latices: Science and Technology Chapman & Hall:: London, 1997.

53. Yamauchi, K.; Lizotte, J. R.; Hercules, D. M.; Vergne, M. J.; Long, T. E. Combinations of microphase separation and terminal multiple hydrogen bonding in novel macromolecules. J. Am. Chem. Soc. 2002, 124, 8599-8604.

CHAPTER 3: Highly Mechanically Strong, Thermally

Stable, and Flame Retardant Polyetherimide

Terminated with Phosphonium Bromide

3.1 Abstract

High-performance polymers must possess high mechanical strength, thermal stability, and flame retardancy to be suitable for aerospace applications, but most polymers cannot meet the three criteria simultaneously. Phosphonium bromide terminated polyetherimides

(PEI-PhPPh3Br) were synthesized by functionalizing dianhydride terminated polyetherimides (PEI-DA) with triphenyl-4-aminophenylphosphonium bromide. Thanks

to the judiciously designed functional end group, PEI-PhPPh3Br exhibited excellent

thermal stability, tensile properties, and flame retardancy. PEI-PhPPh3Br with a molecular

weight of 12 kDa (12k-PEI-PhPPh3Br) showed a tensile strength of 109 ± 4 MPa and and a

Young's modulus of 2.75 ± 0.12 GPa, much higher than those of the PEI-DA prepolymer

and the noncharged polyetherimide analogue. Importantly, 12k-PEI-PhPPh3Br showed outstanding flame retardancy, better than the state-of-the-art commercial PEIs, as evidenced by the high limiting oxygen index of 51% and the char yield of 60% at 980 ˚C.

The study herein provides a highly effective strategy to simultaneously improve thermal

stability, mechanical strength, and flame retardancy, which are three important properties rarely possessed by most polymers.

3.2 Introduction

High mechanical strength, thermal stability and flame retardancy are three important criteria for the use of polyetherimide (PEI), an amorphous thermoplastic resin, in aviation and aerospace.1-4 Most polymers, however, cannot simultaneously provide all three properties. To enhance the mechanical properties, multiple hydrogen bonding5-8 groups have been introduced to polymers to induce noncovalent interactions. Particularly, quadruple hydrogen bonding ureidopyrimidinone (UPy) has been incorporated into PEI oligomers so that they form supramolecular PEIs with enhanced tensile properties.9

Nonetheless, their thermal stability was reduced due to the UPy group. Although high-molecular-weight (high-MW) PEIs inherently possess flame retardancy because of the high carbon content and high aromaticity,10, 11 the flame retardancy of low-molecular-weight PEIs is unknown and can be limited. To simultaneously achieve high tensile strength, thermal stability, and flame retardancy, our strategy in this work is to incorporate thermally stable quaternary tetraphenylphosphonium bromide end groups into telechelic PEI oligomers.

Phosphonium is a promising candidate to impart PEIs with simultaneously enhanced 62

thermal stability, flame retardancy, and mechanical strength. First, phosphonium is thermally stable at ~300 ˚C.12, 13 Particularly, the large phenyl groups in tetraphenylphosphonium provide strong steric hindrance around phosphorous and thus inhibit the decomposition reactions.12 Thus, phosphonium has been either physically blended in inorganic materials as modifiers14-16 or chemically incorporated into polymers.17, 18 For instance, Calderon et al. prepared phosphonium-modified montmorillonite which exhibited degradation temperatures higher than the ammonium-modified montmorillonite.14 Long and co-workers showed that the phosphonium-containing polymerized ionic liquids had thermal stability up to ~370 ˚C, much higher than the ammonium-containing counterparts that were stable up to ~220 ˚C.18

Second, phosphonium has been mixed with polymers such as polystyrene,19 polycarbonate,20 and polylactide,21 to improve their flame retardancy.22 For example, Hou et al. mixed phosphonium sulfonates with polycarbonate and significantly increased the limiting oxygen indices (LOIs) by 5-9%.20 Similarly, Gui et al. blended polylactides with phosphonium-containing ionic liquids, which increased the char yields and the LOIs. 21

Lastly, phosphonium is known to improve the mechanical properties once mixed with polymer-clay composites23-26 or incorporated into polymers27, 28. The enhanced mechanical properties root from the ionic interactions among the phosphonium groups. For example,

Long and co-workers synthesized phosphonium-containing triblock copolymers, which 63

exhibited high tensile modulus at high ionic concentrations.27 Wathier et al. constructed a phosphonium-containing poly(acrylic acid) network, which possessed outstanding mechanical properties.28 All these examples suggest that it is possible to obtain all the three properties via the incorporation of phosphonium groups.

Herein, we present the preparation of phosphonium bromide terminated PEIs

(PEI-PhPPh3Br) to study how phosphonium end groups affect the thermal stability, flame

retardancy, and mechanical properties of PEIs. PEI-PhPPh3Br was synthesized by reacting dianhydride-terminated PEIs (PEI-DA) with triphenyl-4-aminophenylphosphonium

bromide ([Ph3P(C6H4-4-NH2)]Br) (Scheme 1). Phenyl end-capped PEIs (PEI-Ph) were synthesized by reacting PEI-DA with aniline to serve as analogues for comparison. We

have found that, after incorporating the phosphonium end groups, 12k-PEI-PhPPh3Br was thermally stable and exhibited higher flame retardancy than the state-of-the-art commercial high-MW PEIs (PEI-1 and PEI-2). Furthermore, the tensile strength and Young’s modulus

of 12k-PEI-PhPPh3Br were drastically improved compared to 12k-PEI-Ph and were

comparable to those of high-MW PEIs. Lastly, 12k-PEI-PhPPh3Br exhibited superior processability compared to both 12k-PEI-Ph and high-MW PEIs.

3.3 Experimental

Materials. 2,2-Bis[4-(3,4-dicarboxyphenoxy) phenyl] propane dianhydride (BPADA) 64

was provided by SABIC and purified by heating to ~250 ˚C to remove any moisture before use. m-phenylenediamine (mPD, 99%), o-dichlorobenzene (oDCB, 99%),

triphenylphosphine (PPh3, 99%), aniline (99.5%), 4-bromoaniline (97%), palladium(II)

acetate (Pd(OAc)2, 98%) were purchased from Sigma-Aldrich. mPD was purified by

sublimation. Chloroform (CHCl3) and acetonitrile (CH3CN) were purchased from

Spectrum Chemical. Methanol (MeOH) and acetone were obtained from Pharmco-AAPER.

All chemicals were used as received until otherwise noted.

Synthesis of triphenyl-4-aminophenylphosphonium bromide

([Ph3P(C6H4-4-NH2)]Br). [Ph3P(C6H4-4-NH2)]Br was synthesized following a previous

29 report with slight modification (Scheme S1). PPh3 (2.649 g, 10.00 mmol),

4-bromoaniline (1.773 g, 10.00 mmol), and CH3CN (10 mL) were charged to a 100-mL

two-neck round-bottom flask with a stirring bar. Pd(OAc)2 (0.023 g, 1.0 mmol) was added as the catalyst. The mixture was refluxed until a white precipitate started hindering the stirring. The precipitate was filtered and washed with acetone until the filter cake was completely white. The white powder was then collected and dried in vacuo at 100 ˚C.

[Ph3P(C6H4-4-NH2)]Br: 30% yield; melting point (mp) ~340 ˚C (reported mp values are

29 1 334–337 ˚C). H NMR (400 MHz, CDCl3, δ): 7.87–7.82 ppm (m, 3H, C6H5P-), 7.72–7.67

ppm (m, 6H, C6H5P-), 7.65–7.55 ppm (m, 6H, C6H5P-), 7.20–7.09 ppm (m, 4H,

31 4-NH2C6H4P-). P NMR (162 MHz, CDCl3, δ): 22.1.

Scheme 1. Synthesis of dianhydride terminated PEI (PEI-DA), phenyl terminated

PEI (PEI-Ph), and phosphonium bromide terminated PEI (PEI-PhPPh3Br)

Synthesis of dianhydride-terminated PEI (PEI-DA). The synthesis of PEI-DA and

PEI-Ph followed a previous report.30 For example, to synthesize PEI-DA with a targeted

Mn of 8 kDa (8k-PEI-DA), BPADA (17.389 g, 33.409 mmol), mPD (3.335 g, 30.84 mmol), and oDCB (80 mL) were added to a three-neck round-bottom flask equipped with a

mechanical stirrer, an N2 inlet, and a Dean-Stark trap. The slurry was heated at 180 ˚C for

12 h under constant stirring and then at 380 ˚C for 0.5 h without stirring. The reaction was

fully conducted in a constant N2 flow. Most of the resulting PEI-DA was stored in an N2

atmosphere in the flask, and a portion was collected, dissolved in CHCl3, and precipitated into acetone. The precipitate was filtered, washed with acetone, and dried in vacuo at 180

˚C.

Synthesis of phenyl-terminated PEI (PEI-Ph). The synthesis of PEI-Ph also followed our previous report.30 For example, aniline (0.48 mL, 5.3 mmol) and oDCB (60 mL) were added to the three-neck round-bottom flask that contained the as-synthesized

8k-PEI-DA. The flask was equipped with a mechanical stirrer, a Dean-Stark trap, and an

N2 inlet with a constant N2 flow. The mixture was heated at 180 ˚C for 6 h under constant stirring and then heated at 300 ˚C for 0.5 h without stirring. The resulting polymer was

dissolved in CHCl3 and precipitated into MeOH. The precipitate was filtered, washed with

MeOH, and dried in vacuo at 180 ˚C.

Synthesis of phosphonium bromide terminated PEI (PEI-PhPPh3Br). The

synthesis of PEI-PhPPh3Br was fulfilled by reacting PEI-DA with [Ph3P(C6H4-4-NH2)]Br

(Scheme 1). For example, the synthesis of PEI-PhPPh3Br with a targeted Mn of 8 kDa

(8k-PEI-PhPPh3Br) is described as follows. [Ph3P(C6H4-4-NH2)]Br (2.448 g, 5.637 mmol) was added to the three-neck round-bottom flask that contained the as-synthesized

8k-PEI-DA. The flask was equipped with a mechanical stirrer, an N2 inlet, and a

Dean-Stark trap. The mixture was heated at 180 ˚C for 12 h under constant stirring.

Afterwards the mixture was heated at 300 ˚C for 0.5 h without stirring. The reaction was

conducted with a constant N2 flow. After the reaction, the product was dissolved in CHCl3

and precipitated into MeOH. The precipitate was filtered, washed with MeOH, and dried in vacuo at 180 ˚C.

Characterization. Proton nuclear magnetic resonance (1H NMR) spectroscopy was

carried out on a Varian Unity 400 at 399.98 MHz in CDCl3. The number average molecular

weight (Mn), weight average molecular weight (Mw), and polydispersity (PDI) were measured by size-exclusion chromatography (SEC, EcoSECHLC-8320, Tosoh Bioscience) equipped with a Wyatt MiniDAWN TREOS multi-angle light scattering detector and a differential refractive index detector. DMF was used as the solvent. The flow rate was 0.5 mL/min. The column set was consisted of a SuperH-H guard column (4.6 mm ID × 3.5 cm,

4 μm), a SuperH-H guard column (6.0 mm ID × 15 cm, 4 μm), and two SuperH-H guard columns (6.0 mm ID × 15 cm, 4 μm). All columns and detectors were kept at 50 ˚C.

Thermogravimetric analysis (TGA) was performed on a Discovery TGA5500 (TA

Instruments) by heating the samples to 1000 ˚C at a heating rate of 20 ˚C/min in a stream of nitrogen (25 mL/min). The infrared (IR) spectra of the gas phase were collected by a

Nicolet iS10 FT-IR Spectrometer (Thermo Fisher Scientific). Differential scanning calorimetry (DSC) was performed on a Discovery DSC2500 (TA Instruments), which was calibrated using indium (mp = 156.60 °C) and zinc (mp = 419.47 °C) standards. The samples were heated at a rate of 10 °C/min in a stream of nitrogen (25 mL/min). Glass

transition temperature (Tg) was determined as the midpoint of the transition in the second heating ramp. Dynamic mechanical analysis (DMA) was conducted on a DMA Q800 (TA

Instruments) at a heating rate of at 3 ˚C/min and an oscillatory frequency of 1 Hz in tension mode. The polymers were hot-pressed between two Kapton sheets at 300 ˚C using two

0.1-inch-thick shims to control the film thickness. A mold release agent (provided by

REXCO) was applied on the Kapton sheets to prevent the polymers from sticking. The polymer films were stepwise hot-pressed at a force of 1, 5, 7, 10, and 10 tons for one minute and released after each pressing (five press-release cycles in total). Tensile tests were performed on an Instron 5500R at a cross-head speed of 5 mm/min. The tensile strength, maximum elongation, and Young's modulus were averaged over five specimens.

Melt rheological studies were performed on an AR-G2 rheometer (TA Instruments) using two 25-mm-diameter parallel plates. Time sweep tests (1% oscillatory strain, 1 Hz, 1 h, under air) were used to evaluate the changes in the storage moduli, loss moduli, and complex viscosities of the polymer melts at constant temperatures of 320 and 340 ˚C. The storage moduli, loss moduli, and viscosities over a range of frequencies (1-100 rad/s) were collected at temperatures from 340 to 250 ˚C (decrement of 10 ˚C) under an oscillatory strain of 1%. The master curves were generated by shifting the curves using TRIOS (TA

Instruments). The flame retardancy was characterized by the UL-94 vertical burning test following ASTM D3801 standard. In each test, a flame was applied to the sample for 10 s 69

and then removed. The self-extinguishing time was recorded as t1. The flame was then

re-applied for another 10 s and the self-extinguishing time was recorded as t2. In addition, a piece of cotton was placed beneath the sample to determine if the sample generated any drips, which may cause the cotton to burn during the burning tests. Limiting oxygen index

(LOI) test was performed according to ASTM D2863.

3.4 Results and Discussion

Synthesis of PEI-DA, PEI-Ph, and PEI-PhPPh3Br. PEI-DA was synthesized by charging BPADA in excess during the polycondensation reaction following our previous report.30 PEI-Ph was prepared by reacting the as-synthesized PEI-DA with aniline (Scheme

1). Proton nuclear resonance (1H NMR) spectra (Figure 1) confirmed the structures of

PEI-DA and PEI-Ph. Since PEI-DA has unique end group protons (a'), the number average

molecular weight (Mn) were calculated based on the signals from the end groups and the repeating units, similar to that in our previous report.30 In addition, number average

molecular weight (Mn), weight average molecular weight (Mw), and polydispersity index

(PDI) of PEI-DA and PEI-Ph were confirmed by size-exclusion chromatography (SEC)

(Table S1).

1 Figure 1. H NMR spectra of [Ph3P(C6H4-4-NH2)]Br, PEI-DA, PEI-Ph, and PEI-PhPPh3Br.

The resonance peaks of the end group protons (p, k, m, and n) in PEI-PhPPh3Br shift

downfield compared to those in [Ph3P(C6H4-4-NH2)]Br. Protons q in PEI-PhPPh3Br are indistinguishable due to overlap.

Table 1. Conversion, Tg, and char yield of PEI-Ph, PEI-PhPPh3Br, PEI-1, and PEI-2

Conv Tg Char Yield at Sample (Mn) .* (˚C) 980 ˚C (%)

PEI-Ph ( 8k) - 205 49 ± 1

PEI-Ph (10k) - 210 50 ± 1

PEI-Ph (12k) - 214 50 ± 1

PEI-PhPPh3Br ( 8k) 90% 224 59 ± 1

PEI-PhPPh3Br (10k) 91% 223 60 ± 0

PEI-PhPPh3Br (12k) 93% 224 60 ± 1

PEI-1 (16.9k)** - 217 52 ± 0

PEI-2 (24.5k)** - 217 54 ± 0

*The conversions of PEI-Ph were not calculated because the end group peaks could not be identified in 1H NMR spectra.

**Commercially available high-MW PEI-1 and PEI-2 were used as benchmarks for comparison in this work.

Similar to the preparation of PEI-Ph, the functionalization of PEI-DA with

[Ph3P(C6H4-4-NH2)]Br yielded PEI-PhPPh3Br. After the functionalization reaction, the

chemical shifts of the protons in [Ph3P(C6H4-4-NH2)]Br changed significantly. The

signature peak j corresponding to -NH2 in [Ph3P(C6H4-4-NH2)]Br was not observed in the

final product PEI-PhPPh3Br, indicating that the end cappers fully reacted with PEI-DA, or if there were any unreacted residues, they were washed away by methanol during

precipitation. In addition, because the -NH2 group was converted to electron-withdrawing imide,30 the peaks p, q, k, m, and n are expected to shift downfield in the spectrum of

PEI-PhPPh3Br. However, only the peaks p, k, m, and n were identified. Peaks q became indistinguishable due to the potential overlap with peaks g and f. Since peaks k overlapped partially with peaks a and a', the disappearance of peak a' cannot be used to calculate the

conversion of PEI-DA to PEI-PhPPh3Br. Instead, the conversions were estimated to be >90% based on the integral areas of peaks n and b (see Supporting Information), confirming the

successful synthesis of PEI-PhPPh3Br from PEI-DA (Table 1).

PEI-Ph PEI-PhPPh Br

Relative Heat Flow (a.u.) Flow Heat Relative 3 8k- 10k- endo up 12k- 150 175 200 225 250 Temperature (°C)

Figure 2. DSC traces of PEI-Ph and PEI-PhPPh3Br with Mn = 8, 10, and 12 kDa. Tg increased after the incorporation of phosphonium bromide.

Figure 3. (a) TGA showed different degradation behaviours for PEI-Ph, PEI-PhPPh3Br,

[Ph3P(C6H4-4-NH2)]Br, PEI-1, and PEI-2 at a heating rate of 20 ˚C/min. (b) IR spectra of

the gas phases of PEI-Ph, PEI-PhPPh3Br, and [Ph3P(C6H4-4-NH2)]Br upon degradation at

400 ˚C. The degraded species from PEI-PhPPh3Br and PPh4NH2Br were similar and their

IR spectra showed aromatic C-H stretching at 3073 cm-1, suggesting the release of benzene rings.

Thermal Properties. The thermal properties of PEI-Ph and PEI-PhPPh3Br were evaluated using differential scanning calorimetry (DSC). The glass transition temperature

(Tg) of PEI-PhPPh3Br was at least 10 ˚C higher than that of the noncharged PEI-Ph with the

same Mn (Figure 2, Table 1), which is attributed to the strong ionic interactions among the

charged end groups in PEI-PhPPh3Br. The increase in Tg was more significant for PEIs with lower molecular weights due to the higher ionic concentration, similar to the PEIs terminated with interactive hydrogen bonds in our previous report.9

Thermogravimetric analysis (TGA) further revealed their degradation behaviors. The

degradation of PEI-PhPPh3Br differed drastically from that of PEI-Ph (Figure 3a). PEI-Ph

exhibited single step degradation of the backbone. In contrast, PEI-PhPPh3Br showed

two-step degradation. The first thermal degradation of PEI-PhPPh3Br was at ~400 ˚C,

which coincided with that of the end capper [Ph3P(C6H4-4-NH2)]Br. The coincidence suggests that the first degradation occurred at the phosphonium end groups of

PEI-PhPPh3Br. To probe the degradation mechanism, IR was coupled with TGA to analyse

the gas phase upon degradation at 400 ˚C. The gas phases released by both PEI-PhPPh3Br

and [Ph3P(C6H4-4-NH2)]Br upon heating at 400 ˚C contained benzene rings, as shown by the peaks at 3073 cm-1 in the IR spectra (Figure 3b). In contrast, no peak was observed at

~3073 cm-1 upon heating PEI-Ph at ~400 ˚C. The IR spectra suggest the followings: 1) the

first degradation of PEI-PhPPh3Br occurred at the end groups; 2) PEI-PhPPh3Br most likely lost some phenyl groups from the chain ends upon degradation at 400 ˚C. In addition, the weight loss percentage at 400 ˚C matched well the weight percentage of two phenyl and

two bromine groups in PEI-PhPPh3Br, indicating that PEI-PhPPh3Br lost one phenyl and one bromine groups from each chain end. These degradation analyses agree with the mechanism proposed in the study of quaternary phosphonium modified

12 montmorillonites. Although PEI-PhPPh3Br started to degrade at ~400 ˚C, the thermal stability was better than that of the ureidopyrimidinone end capped PEIs,9 and thus

PEI-PhPPh3Br offered a much larger temperature window for melt processing. Noteworthy,

the loss of bromine groups at ~400 ˚C made PEI-PhPPh3Br both a good solid phase flame retardant and a good gas phase flame retardant (vide infra). The second thermal degradation step corresponded to the degradation of the PEI backbones, similar to that of

PEI-Ph. The decomposition temperatures of PEI-PhPPh3Br decreased to ~360 ˚C when the heating rate was decreased to 3 ˚C/min (Figure S1).

Processability. High thermo-oxidative stability is crucial for PEIs to be melt-processable. Rheological time sweep tests at 320 and 340 ˚C showed that

12k-PEI-PhPPh3Br had excellent thermo-oxidative stability, as evidenced by the almost constant complex viscosities and the non-crossover of G' and G'' (Figure S2). Besides the

thermo-oxidative stability, the flow characteristics are also important to evaluate the melt processability of PEIs. To examine the impact of the different end groups on the flow characteristics, we have characterized the rheological properties of 12k-PEI-Ph,

12k-PEI-PhPPh3Br, PEI-1, and PEI-2 (Figure 4 and 5). The relaxation of the polymer chains, indicated by the crossover of storage modulus (G') and loss modulus (G"), occurred

at lower frequencies for 12k-PEI-PhPPh3Br than that for 12k-PEI-Ph (Figure 4),

confirming that 12k-PEI-PhPPh3Br possessed strong ionic interactions among the

-PhPPh3Br end groups. In addition, the relaxation of 12k-PEI-PhPPh3Br occurred at frequencies similar to that of the high-MW PEIs, indicating that the ionic interactions were

strong and 12k-PEI-PhPPh3Br could possess tensile properties comparable to the high-MW PEIs. In the terminal flow region after relaxation, G' and G" of all polymers exhibited similar slopes of 1.60 and 0.96, approximating the expected slopes of 2.0 and 1.0, respectively, for typical polymer melts.

Figure 4. G' and G" master curves of 12k-PEI-Ph, 12k-PEI-PhPPh3Br, PEI-1 and PEI-2 at

Tr = 340 ˚C.

Time-temperature superposition (TTS) master curves revealed their viscosities at a

reference temperature of Tr = 340 ˚C (Figure 5), the lowest melt processing temperature commonly used for PEIs in the industry. Following the Cox-Merz rule,31 the steady shear viscosity at a given shear rate is empirically equal to the dynamic viscosity at the same

vibration frequency. Among the PEIs, PEI-PhPPh3Br exhibited the fastest decrease in complex viscosity and the strongest shear thinning behaviour. Its complex viscosity was the lowest at typical processing shear rates (>1000 rad/s), suggesting the best processability among all PEIs. In the low angular frequency region, the viscosities of

12k-PEI-PhPPh3Br were higher than that of 12k-PEI-Ph and comparable to those of PEI-1

and PEI-2, which was attributed to the ionic interactions among the chain ends of

12k-PEI-PhPPh3Br.

Figure 5. Complex viscosity master curves of 12k-PEI-Ph, 12k-PEI-PhPPh3Br, PEI-1 and

PEI-2 at Tr = 340 ˚C.

Table 2. Tensile properties of melt-pressed 12k-PEI-Ph, 12k-PEI-PhPPh3Br, PEI-1, and PEI-2

Tensile Young’s

Sample (Mn) strength modulus (MPa) (GPa)

PEI-Ph (12k) 104 ± 3 2.57 ± 0.07

PEI-PhPPh3Br (12k) 109 ± 4 2.75 ± 0.12

PEI-1 (16.9k) 109 ± 2 2.82 ± 0.08

PEI-2 (24.5k) 107 ± 1 2.79 ± 0.08

Figure 6. Representative tensile stress-strain curves of 12k-PEI-Ph, 12k-PEI-PhPPh3Br,

PEI-1, and PEI-2.

Tensile Properties. All PEI samples were melt-pressed into thin films to evaluate the

tensile properties. Among the PEIs, 8k-PEI-PhPPh3Br formed a film, but 8k-PEI-Ph could not (Figure S3), indicating that the phosphonium end groups endowed the PEI with better film formability. To characterize the tensile properties, the films were cut into

dumbbell-shaped specimens. Only 12k-PEI-Ph and 12k-PEI-PhPPh3Br generated intact

specimens. The 12k-PEI-PhPPh3Br exhibited a tensile strength of 109 ± 4 MPa and a

Young’s modulus of 2.75 ± 0.12 GPa. The tensile strength and the Young's modulus of

12k-PEI-PhPPh3Br were 5% and 7%, respectively, higher than those of 12k-PEI-Ph (Table

2 and Figure 6), owing to the strong ionic interactions by the phosphonium end groups. For

comparison, high-MW PEI-1 and PEI-2 were melt-pressed and cut into dumbbell-shaped

specimens. The tensile strength and Young’s modulus of 12k-PEI-PhPPh3Br were comparable to those of PEI-1 and PEI-2. Therefore, the phosphonium end groups impart

PEI with excellent film formability at a Mn as low as 8 kDa and outstanding tensile strength

at a Mn as low as 12 kDa. In addition, dynamic mechanical analysis (DMA) of

12k-PEI-PhPPh3Br showed almost similar characteristics to the high-MW PEI-1 and PEI-2, suggesting the great mechanical strength endowed by the strong ionic interactions among the phosphonium end groups (Figure S4).

Flame retardancy. The flame retardancy of the PEIs were evaluated in three aspects: the UL-94 rating, the char yield, and the limiting oxygen index (LOI). In general, PEI is flame retardant due to the high content of aromatic rings in the polymer backbone.32-34

PEI-Ph, PEI-PhPPh3Br, PEI-1, and PEI-2 all passed the stringent UL-94 V-0 rating (Table

3 and S2). In the UL-94 tests, PEI-1 and PEI-2 bars showed slight dripping, although the drips did not ignite the cotton underneath. In contrast, the PEI-Ph bar only deformed

slightly and there was no dripping. More excitingly, the PEI-PhPPh3Br bar did not even deform and the shape remained unchanged throughout the burning test.

Table 3. Flame retardancy of 12k-PEI-Ph, 12k-PEI-PhPPh3Br, PEI-1, and PEI-2

Sample (Mn) LOI (%) UL-94 grade Dripping (Y/N) PEI-Ph (12k) 44.5 ± 0.5 V-0 N

PEI-PhPPh3Br (12k) 51.0 ± 0.5 V-0 N PEI-1 (16.9k) 42.0 ± 0.5 V-0 Y PEI-2 (24.5k) 45.0 ± 0.5 V-0 Y

To reveal the cause of anti-dripping, rheological frequency sweep tests were conducted at 320 ˚C. The storage moduli (G') of all PEIs were plotted against the frequency (Figure 7) to elucidate the changes in the elasticities of the polymer melts. PEI-1 and PEI-2 displayed the usual storage modulus response of entangled polymer melts at terminal flow. The G' of

PEI-1 and PEI-2 decreased as the frequency was reduced. The decreasing rates were nearly

constant in the range of 0.01 to 100 Hz. In contrast, PEI-Ph and PEI-PhPPh3Br showed much slower decreasing rates of G' at low frequencies than at high frequencies. The weak dependence of G' on frequency in the frequency range of <0.1 Hz implies that PEI-Ph and

PEI-PhPPh3Br gained an enhanced melt elasticity from the transition from a liquid-like

35-38 state to a pseudo-solid state. The enhanced melt elasticity of PEI-PhPPh3Br is ascribed to the ionic interactions among the phosphonium end groups, while that of PEI-Ph possibly stems from the π-π stacking of the phenyl end groups. At low frequencies, the G' values of

PEI-PhPPh3Br were about three times those of PEI-Ph due to the strong ionic interactions

of the phosphonium end groups. The larger G' values of PEI-PhPPh3Br than PEI-Ph are in

consistence with the better melt shape stability of PEI-PhPPh3Br than PEI-Ph in the flame tests.

Figure 7. The storage modulus response to frequency during the rheological frequency sweep at 320 ˚C across the frequency range of 0.01 to 100 Hz.

In addition to enhancing the melt elasticity, the phosphonium end groups bestowed better char formability on PEIs. The char yields of the PEIs at 980 ˚C were extracted from the TGA curves (Figure 3a) and summarized in Table 1. PEI-1 and PEI-2 showed a char yield of 52% and 54%, respectively. All PEI-Ph showed char yields of ~50%. In stark

contrast, after incorporating the phosphonium end groups, the PEI-PhPPh3Br showed char yields of ~60%, which were ~10% higher than their PEI-Ph counterparts. The increased

char yields of PEI-PhPPh3Br are attributed to PEI cross-linking induced by the

decomposed products from the phosphonium bromide end groups (e.g., phosphorous acids and anhydrides39, 40). The large amount of char reduced the conversion of PEI into combustible gases, and thereby it further enhanced the flame retardancy of PEI.

The limiting oxygen index (LOI) was measured to further illustrate the enhanced flame

retardancy of PEI-PhPPh3Br (Table 3). The LOI of PEI-PhPPh3Br reached 51%, while those of high-MW commercial PEIs and PEI-Ph were <45%. The high LOI of

PEI-PhPPh3Br is attributed to the following two factors: 1) The phosphorous portion of the end group facilitates the char formation through generating phosphoric acid during burning.41, 42 The resulting char acts as a separating layer between the flame and the unburned PEI. The separating layer slows down the heat transfer, which further retards the ignition of the unburned parts. 2) Upon burning and degradation, the phosphonium bromide end group generates bromine radicals, which act as radical trappers in the gas phase. In general, radical oxidation reactions dominate in the burning process and they propagate through H* and HO* radicals.43-45 Bromine radicals, however, quench the H* and HO* radicals and attenuate the propagation of the oxidation reaction.46 Therefore, the presence of Br in the end group reduces the potential to burn. In short, the incorporation of phosphonium bromide end groups has enhanced the flame retardancy of PEI, as evidenced by the anti-dripping effect, the high char yield in the condense phase, and the

radical-trapping bromine in the gas phase.

3.5 Conclusions

Direct functionalization of PEI-DA with [Ph3P(C6H4-4-NH2)]Br affords the synthesis

of PEI-PhPPh3Br with high conversions. All PEI-PhPPh3Br show two-step degradation.

The degradation mechanism is elucidated by TGA coupled with IR. PEI-PhPPh3Br

exhibits Tg higher than the corresponding PEI-Ph as revealed by DSC, but the

PEI-PhPPh3Br with a Mn as low as 12 kDa has the strongest shear thinning characteristics

and thus great melt processability. Most importantly, 12k-PEI-PhPPh3Br possesses not only tensile properties comparable to but also flame retardancy superior to the high-MW

state-of-the-art commercial PEIs. The higher char yield and LOI of 12k-PEI-PhPPh3Br than the high-MW PEIs are due to the phosphonium and bromide at the chain ends, which act as superb flame retardants in the condensed phase and in the gas phase, respectively.

This work presents a method for synthesizing high-performance polymers with excellent flame retardancy and mechanical strength that are suitable for applications such as aerospace and aircrafts.1-4 In addition, phosphonium-functional polymers have great potentials to be used as polyelectrolytes,47-49 anion exchange membranes,50, 51 antimicrobial coatings,52-59 and polymer supports for phase-transfer catalysts.60, 61

3.6 Supplementary Information

Synthesis of triphenyl-4-aminophenylphosphonium bromide

([Ph3P(C6H4-4-NH2)]Br). [Ph3P(C6H4-4-NH2)]Br was prepared by reacting

triphenylphosphine (PPh3) with 4-bromoaniline, catalyzed by palladium(II) acetate

29 (Pd(OAc)2) in acetonitrile (CH3CN), following a previous report with slight modifications.

Scheme S1. Synthesis of triphenyl-4-aminophenylphosphonium bromide

([Ph3P(C6H4-4-NH2)]Br)

Determination of the molecular weights of PEI-DA and PEI-Ph. The number

average molecular weight (Mn) of PEI-DA was calculated based on the signals from the end groups and the repeating units using the following equations, similar to our previous report.30

# 표푓 푟푒푝푒푎푡𝑖푛푔 푢푛𝑖푡푠 퐴푟푒푎 (퐚)/2 퐴푟푒푎 (퐚) n = = = # 표푓 푐ℎ푎𝑖푛푠 퐴푟푒푎 (퐚′)/2 퐴푟푒푎 (퐚′)

−1 푀푛 = 푀푟푒푝푒푎푡푖푛푔 푢푛푖푡 ∙ n + 푀푒푛푑 푔푟표푢푝 = (592.61푛 + 520.49) g ∙ mol

Additionally, Mn, weight average molecular weight (Mw), and polydispersity (PDI) of all PEI-DA and PEI-Ph were estimated by size-exclusion chromatography (Table S1).

Table S1. Mn, Mw, and PDI of PEIs with different end groups.

Mn, NMR Mn, SEC Mw, SEC b Sample Mn, theo (kDa) PDI (kDa)a (kDa)b (kDa)b

8k-PEI-DA 7.6 8.5 8.4 18.7 2.23

10k-PEI-DA 9.4 11.6 9.7 22.7 2.34

12k-PEI-DA 11.8 13.7 11.7 27.7 2.36

8k-PEI-Ph 7.8 - 9.3 22.9 2.46

10k-PEI-Ph 9.6 - 9.7 26.7 2.74

12k-PEI-Ph 11.9 - 15.4 33.7 2.19 a Mn measured in CDCl3 under ambient conditions and calculated by the end group analysis. bSEC characterization was carried out in DMF with 0.05 M LiBr. The molecular weights were based on refractive index signals using polystyrene standards.

Calculation of conversion from PEI-DA to PEI-PhPPh3Br. The conversion from

PEI-DA to PEI-PhPPh3Br can be calculated based on the integral areas of the end groups n and the normalized repeating units b in 1H NMR (Figure 1, main text). The procedure is as follows. First, the integral areas of end groups a' were normalized to 2 since each PEI-DA

chain has two end groups. Then, the integral areas of peaks b in PEI-PhPPh3Br were

normalized to the same values of peaks b in PEI-DA, as the repeating units remained unchanged before and after the reaction. Finally, the conversion is calculated using the following equation, in which the denominator 12 is the number of protons n in

PEI-PhPPh3Br with full conversion.

퐴푟푒푎 (퐧, PEI-PhPPh Br) Conversion (PEI-DA to PEI-PhPPh Br) = [ 3 ] × 100% 3 12

Effect of heating rate on the thermal stability of PEI-PhPPh3Br. The effect of

heating rate on the thermal stability of PEI-PhPPh3Br was investigated by heating the

polymers at 3 and 20 ˚C/min (Figure S1). At a low heating rate of 3 ˚C/min, PEI-PhPPh3Br of various molecular weights exhibited thermal stability up to ~360 ˚C, while at a high heating rate of 20 ˚C/min, the polymers were stable up to ~400 ˚C.

Figure S1. TGA thermograms of (a) 8k-, (b) 10k-, and (c) 12k-PEI-PhPPh3Br at heating rates of 3 and 20 ˚C/min.

Thermo-oxidative stability of 12k-PEI-PhPPh3Br. Time sweep tests revealed the

thermos-oxidative stability of PEI-PhPPh3Br. For example, 12k-PEI-PhPPh3Br was subjected to rheological time sweep tests at 320 and 340 ˚C (Figure S2). At 320 ˚C (Figure

S2a), the storage modulus (G') did not crossover G" during a testing time period of 1 h, and the complex viscosity (η*) remained nearly constant, showing great thermo-oxidative stability. At 340 ˚C (Figure S2b), G' did not cross over G" within ~2800 s, and η* did not

change much throughout the testing period, indicating that 12k-PEI-PhPPh3Br remained thermo-oxidatively stable and there was no significant structural change in the polymer.

Figure S2. Time sweep (1% oscillatory strain, 1 Hz, 1 h) of 12k-PEI-PhPPh3Br at constant temperatures of (a) 320 ˚C, and (b) 340 ˚C.

Melt-press of the PEI films. The PEI-Ph and PEI-PhPPh3Br with all Mn were melt-pressed into thin films (Figure S3) and then cut into dumbbell-shaped specimens for comparison of their tensile properties. Owing to the ionic interactions at the chain ends, the

film formability or tensile properties of PEI-PhPPh3Br increased, in comparison with the

noncharged PEIs including PEI-DA and PEI-Ph. For instance, among all PEIs with a

molecular weight of 8 kDa, only 8k-PEI-PhPPh3Br formed an intact film while 8k-PEI-Ph

and 8k-PEI-DA could not. However, 8k-PEI-PhPPh3Br was not strong enough to be cut

into intact dumbbell-shaped specimens for tensile tests. As the Mn was increased to 10 kDa,

both 10k-PEI-PhPPh3Br and 10k-PEI-Ph were rigid enough to form intact films, but they were still not strong enough to generate intact tensile bars. With molecular weights of 12

kDa, both 12k-PEI-Ph and 12k-PEI-PhPPh3Br could be cut to intact dumbbell-shaped specimens for tensile tests.

Figure S3. Melt pressed films of PEI films with different Mn including (a, d) 8 kDa, (b, e)

10 kDa, and (c, f) 12 kDa, and end groups (a, b, c) -Ph, and (d, e, f) -PhPPh3Br. Inlets show

(b, d, e) cracked and (c, f) intact dumbbell-shaped specimens after being cut.

Dynamic mechanical analysis of the PEIs. Figure S4 shows the storage modulus and

tan delta responses of 12k-PEI-Ph, 12k-PEI-PhPPh3Br, PEI-1, and PEI-2 in the dynamic

mechanical analysis (DMA). 12k-PEI-PhPPh3Br showed a Tg of 221 ˚C, which was close

to those of PEI-1 (Tg = 221 ˚C) and PEI-2 (Tg = 223 ˚C). The high Tg indicates that the ionic

interactions are strong among the chains of 12k-PEI-PhPPh3Br. However, above Tg, the

ionic interactions became weak and thus 12k-PEI-PhPPh3Br showed similar storage moduli to those of PEI-1 and PEI-2. Noticeably, 12k-PEI-Ph showed limited mechanical

strength and its storage modulus decreased at a temperature lower than 12k-PEI-PhPPh3Br,

PEI-1, and PEI-2.

Figure S4. Tension mode dynamic mechanical analysis (DMA) of 12k-PEI-Ph,

12k-PEI-PhPPh3Br, PEI-1, and PEI-2 at a heating rate of 3 ˚C/min and a frequency of 1 Hz.

The solid lines are storage moduli, and the dashed lines are tan δ.

Effect of sweeping frequency on the response of storage modulus. To evaluate how

frequency sweep affects the characteristics of the storage moduli, two frequency sweep

tests were conducted on PEI-1 and 12k-PEI-PhPPh3Br by sweeping the frequency from 0.1 to 100 Hz and from 100 to 0.1 Hz. The results showed nearly identical frequency sweep

profiles, suggesting that the storage modulus responses of PEI-1 and 12k-PEI-PhPPh3Br did not depend on the frequency sweep direction (Figure S5).

Figure S5. Frequency sweep tests of a) PEI-1 and b) 12k-PEI-PhPPh3Br with different frequency sweep directions: from 0.1 to 100 Hz (green) and from 100 to 0.1 Hz (red).

UL-94 vertical burning test. To examine the self-extinguishing ability, 12k-PEI-Ph,

12k-PEI-PhPPh3Br, PEI-1, and PEI-2 were subjected to UL-94 vertical burning test (Table

S2). According the ASTM D3801, all the examined samples exhibited outstanding V-0

rating. PEI-1 and PEI-2 exhibited dripping while 12k-PEI-Ph and 12k-PEI-PhPPh3Br did not.

Table S2. Results of UL-94 tests of 12k-PEI-Ph, 12-PEI-PhPPh3Br, PEI-1, and PEI-2

Cotton ignition Sample t1 (s) t2 (s) Dripping (Y/N) Rating (Y/N)

12k-PEI-Ph 1 1 N N V-0

12k-PEI-PhPPh3Br 1 1 N N V-0

PEI-1 2 2 Y N V-0

PEI-2 1 1 Y N V-0

3.7 References

1. Williams III, F. J.; Donahue, P. E. Novel polyetherimides. US Patent 3983093 A, 1976. 2. Pitchan, M. K.; Bhowmik, S.; Balachandran, M.; Abraham, M. Process optimization of functionalized MWCNT/polyetherimide nanocomposites for aerospace application. Mater. Des. 2017, 127, 193-203. 3. Rock, J. A.; Male, L. J.; Durfee Jr, N. E., Flame resistant polyetherimide resin blends. US Patent 5051483 A: 1992. 4. Amancio-Filho, S. T.; Roeder, J.; Nunes, S. P.; dos Santos, J. F.; Beckmann, F. Thermal degradation of polyetherimide joined by friction riveting (FricRiveting). Part I: Influence of rotation speed. Polym. Degrad. Stab. 2008, 93, 1529-1538. 5. Brunsveld, L.; Folmer, B. J. B.; Meijer, E. W.; Sijbesma, R. P. Supramolecular polymers. Chem. Rev. 2001, 101, 4071-4098. 6. Foster, E. J.; Berda, E. B.; Meijer, E. W. Metastable supramolecular polymer nanoparticles via intramolecular collapse of single polymer chains. J. Am. Chem. Soc. 2009, 131, 6964-6966. 7. Guo, M.; Pitet, L. M.; Wyss, H. M.; Vos, M.; Dankers, P. Y. W.; Meijer, E. W. Tough stimuli-responsive supramolecular hydrogels with hydrogen-bonding network junctions. J. Am. Chem. Soc. 2014, 136, 6969-6977. 8. Hosono, N.; Kushner, A. M.; Chung, J.; Palmans, A. R. A.; Guan, Z.; Meijer, E. W. Forced unfolding of single-chain polymeric nanoparticles. J. Am. Chem. Soc. 2015, 137, 93

6880-6888. 9. Cao, K.; Liu, G. Low-molecular-weight, high-mechanical-strength, and solution-processable telechelic poly(ether imide) end-capped with ureidopyrimidinone. Macromolecules 2017, 50, 2016-2023. 10. Ghosh, M. K.; Mittal, K. L., Polyimides: fundamentals and applications. Marcel Dekker, Inc.: New York, 1996. 11. Mittal, K. L., Polyimides: synthesis, characterization, and applications. Plenum Press: New York, 1984. 12. Xie, W.; Xie, R.; Pan, W.-P.; Hunter, D.; Koene, B.; Tan, L.-S.; Vaia, R. Thermal stability of quaternary phosphonium modified montmorillonites. Chem. Mater. 2002, 14, 4837-4845. 13. Ganguly, S.; Dana, K.; Mukhopadhyay, T. K.; Ghatak, S. Thermal degradation of alkyl triphenyl phosphonium intercalated montmorillonites. J. Therm. Anal. Calorim. 2011, 105, 199-209. 14. Calderon, J. U.; Lennox, B.; Kamal, M. R. Thermally stable phosphonium-montmorillonite organoclays. Appl. Clay Sci. 2008, 40, 90-98. 15. Patel, H. A.; Somani, R. S.; Bajaj, H. C.; Jasra, R. V. Preparation and characterization of phosphonium montmorillonite with enhanced thermal stability. Appl. Clay Sci. 2007, 35, 194-200. 16. Prahsarn, C.; Roungpaisan, N.; Suwannamek, N.; Klinsukhon, W.; Hayashi, H.; Kawasaki, K.; Ebina, T. Influence of molecular structure of quaternary phosphonium salts on Thai bentonite intercalation. Clays Clay Miner. 2014, 62, 13-19. 17. Tsunashima, K.; Niwa, E.; Kodama, S.; Sugiya, M.; Ono, Y. Thermal and transport properties of ionic liquids based on benzyl-substituted phosphonium cations. J. Phys. Chem. B 2009, 113, 15870-15874. 18. Hemp, S. T.; Zhang, M.; Allen, M. H.; Cheng, S.; Moore, R. B.; Long, T. E. Comparing ammonium and phosphonium polymerized ionic liquids: Thermal analysis, conductivity, and morphology. Macromol. Chem. Phys. 2013, 214, 2099-2107. 19. Granzow, A.; Savides, C. Flame retardancy of polypropylene and impact polystyrene: Phosphonium bromide/ammonium polyphosphate system. J. Appl. Polym. Sci. 1980, 25, 2195-2204. 20. Hou, S.; Zhang, Y. J.; Jiang, P. Phosphonium sulfonates as flame retardants for polycarbonate. Polym. Degrad. Stab. 2016, 130, 165-172.

21. Gui, H.; Xu, P.; Hu, Y.; Wang, J.; Yang, X.; Bahader, A.; Ding, Y. Synergistic effect of graphene and an ionic liquid containing phosphonium on the thermal stability and flame retardancy of polylactide. RSC Advances 2015, 5, 27814-27822. 22. van der Veen, I.; de Boer, J. Phosphorus flame retardants: Properties, production, environmental occurrence, toxicity and analysis. Chemosphere 2012, 88, 1119-1153. 23. Patro, T. U.; Khakhar, D. V.; Misra, A. Phosphonium-based layered silicate—Poly(ethylene terephthalate) nanocomposites: Stability, thermal and mechanical properties. J. Appl. Polym. Sci. 2009, 113, 1720-1732. 24. Abdallah, W.; Yilmazer, U. Preparation and characterization of thermally stable phosphonium organoclays and their use in poly(ethylene terephthalate) nanocomposites. J. Appl. Polym. Sci. 2013, 128, 4283-4293. 25. Abdallah, W.; Yilmazer, U. Polyamide 66 nanocomposites based on organoclays treated with thermally stable phosphonium salts. J. Appl. Polym. Sci. 2013, 127, 772-783. 26. Zeng, X.; Cai, D.; Lin, Z.; Cai, X.; Zhang, X.; Tan, S.; Xu, Y. Morphology and thermal and mechanical properties of phosphonium vermiculite filled poly(ethylene terephthalate) composites. J. Appl. Polym. Sci. 2012, 126, 601-607. 27. Cheng, S.; Beyer, F. L.; Mather, B. D.; Moore, R. B.; Long, T. E. Phosphonium-containing ABA triblock copolymers: Controlled free radical polymerization of phosphonium ionic liquids. Macromolecules 2011, 44, 6509-6517. 28. Wathier, M.; Grinstaff, M. W. Synthesis and creep-recovery behavior of a neat viscoelastic polymeric network formed through electrostatic interactions. Macromolecules 2010, 43, 9529-9533. 29. Ziegler, C. B.; Heck, R. F. Palladium-catalyzed vinylic substitution with highly activated aryl halides. J. Org. Chem. 1978, 43, 2941-2946. 30. Cao, K.; Zhang, M.; Liu, G. The effect of end group and molecular weight on the yellowness of polyetherimide. Macromol. Rapid Commun. 2018, 39, 1800045. 31. Cox, W. P.; Merz, E. H. Correlation of dynamic and steady flow viscosities. J. Polym. Sci. 1958, 28, 619-622. 32. Böhringer, B.; Schilo, D.; Birkenfeld, W.; Odenthal, W. New filaments and fibres of polyetherimide. Makromol. Chem. Macromol. Symp. 1991, 50, 31-39. 33. Lee, J.; Takekoshi, T.; Giannelis, E. P., Fire retardant polyetherimide nanocomposites. In Nanophase and Nanocomposite Materials II, Komarneni, S., Parker, J. C., Wollenberger, H. J., Eds. 1997; Vol. 457, pp 513-518.

34. Sepe, M. P., Thermal analysis of polymers. iSmithers Rapra Publishing: 1997; Vol. 95. 35. Kashiwagi, T.; Du, F.; Douglas, J. F.; Winey, K. I.; Harris Jr, R. H.; Shields, J. R. Nanoparticle networks reduce the flammability of polymer nanocomposites. Nat. Mater. 2005, 4, 928. 36. Guo, Y.; Chang, C.-C.; Halada, G.; Cuiffo, M. A.; Xue, Y.; Zuo, X.; Pack, S.; Zhang, L.; He, S.; Weil, E.; Rafailovich, M. H. Engineering flame retardant biodegradable polymer nanocomposites and their application in 3D printing. Polym. Degrad. Stab. 2017, 137, 205-215. 37. Guo, Y.; Xue, Y.; Zuo, X.; Zhang, L.; Yang, Z.; Zhou, Y.; Marmorat, C.; He, S.; Rafailovich, M. Capitalizing on the molybdenum disulfide/graphene synergy to produce mechanical enhanced flame retardant ethylene-vinyl acetate composites with low aluminum hydroxide loading. Polym. Degrad. Stab. 2017, 144, 155-166. 38. Li, S.; Yuan, H.; Yu, T.; Yuan, W.; Ren, J. Flame-retardancy and anti-dripping effects of intumescent flame retardant incorporating montmorillonite on poly(lactic acid). Polym. Adv. Technol. 2009, 20, 1114-1120. 39. Levchik, S. V.; Weil, E. D. A Review of Recent Progress in Phosphorus-based Flame Retardants. J. Fire Sci. 2006, 24, 345-364. 40. Schartel, B. Phosphorus-based flame retardancy mechanisms—old hat or a starting point for future development? Mater. 2010, 3, 4710. 41. Stewart, B.; Harriman, A.; Higham, L. J. Predicting the Air Stability of Phosphines. Organometallics 2011, 30, 5338-5343. 42. Grand, A. F.; Wilkie, C. A., Fire retardancy of polymeric materials. CRC Press: 2000. 43. Bisschoff, K.; Focke, W. W., Oxygenated hydrocarbon compounds as flame retardants for polyester fabric. In Abstracts of Papers of the American Chemical Society, 2000; Vol. 220, pp U342-U342. 44. Avondo, G.; Vovelle, C.; Delbourgo, R. The role of phosphorus and bromine in flame retardancy. Combust. Flame 1978, 31, 7-16. 45. Morgan, A. B.; Gilman, J. W. An overview of flame retardancy of polymeric materials: application, technology, and future directions. Fire Mater. 2013, 37, 259-279. 46. Horrocks, A. R.; Price, D., Fire retardant materials. Woodhead Publishing: 2001. 47. Tennyson, E. G.; He, S.; Osti, N. C.; Perahia, D.; Smith, R. C. Luminescent phosphonium polyelectrolyte prepared from a diphosphine chromophore: synthesis,

photophysics, and layer-by-layer assembly. J. Mater. Chem. 2010, 20, 7984-7989. 48. Kristufek, S. L.; Maltais, T. R.; Tennyson, E. G.; Osti, N. C.; Perahia, D.; Tennyson, A. G.; Smith, R. C. Bipyridyl-modified phosphonium polyelectrolytes: synthesis, photophysics, metal ion coordination and layer-by-layer assembly with anionic conjugated polymers. Polym. Chem. 2013, 4, 5387-5394. 49. Conrad, C. A.; Bedford, M. S.; Buelt, A. A.; Galabura, Y.; Luzinov, I.; Smith, R. C. Phosphonium polyelectrolytes: influence of diphosphine spacer on layer-by-layer assembly with anionic conjugated polymers. Polym. Int. 2015, 64, 1381-1388. 50. Gu, S.; Cai, R.; Luo, T.; Jensen, K.; Contreras, C.; Yan, Y. Quaternary phosphonium-based polymers as hydroxide exchange membranes. ChemSusChem 2010, 3, 555-558. 51. Wang, K.; Zeng, Y.; He, L.; Yao, J.; Suresh, A. K.; Bellare, J.; Sridhar, T.; Wang, H. Evaluation of quaternary phosphonium-based polymer membranes for desalination application. Desalination 2012, 292, 119-123. 52. Popa, A.; Ilia, G.; Iliescu, S.; Dehelean, G.; Pascariu, A.; Bora, A.; Davidescu, C. M. Mixed quaternary ammonium and phosphonium salts bound to macromolecular supports for removal bacteria from water. Mol. Cryst. Liq. Cryst. 2004, 418, 195-203. 53. Xue, Y.; Xiao, H. N.; Zhang, Y. Antimicrobial polymeric materials with quaternary ammonium and phosphonium salts. Int. J. Mol. Sci. 2015, 16, 3626-3655. 54. Kenawy, E. R.; El-Shanshoury, A.; Shaker, N. O.; El-Sadek, B. M.; Khattab, A. H. B.; Elzatahry, A. Synthesis and biocide activity of polymers based on poly(hydroxy styrene) and poly(hydroxy styrene-co-2-hydroxyethyl methacrylate). Main Group Chem. 2013, 12, 293-306. 55. Kenawy, E.-R.; Abdel-Hay, F. I.; El-Shanshoury, A. E.-R. R.; El-Newehy, M. H. Biologically active polymers: synthesis and antimicrobial activity of modified glycidyl methacrylate polymers having a quaternary ammonium and phosphonium groups. J. Controlled Release 1998, 50, 145-152. 56. Li, L.; Ke, Z.; Yan, G.; Wu, J. Polyimide films with antibacterial surfaces from surface-initiated atom-transfer radical polymerization. Polym. Int. 2008, 57, 1275-1280. 57. Popa, A.; Davidescu, C. M.; Ilia, G.; Iliescu, S.; Dehelean, G.; Trif, R.; Pacureanu, L.; Macarie, L. Synthesis, characterisation and antibacterial activity of quaternary phosphonium salts bonded on poly(oxyethylene)s. Rev. Roum. Chim. 2003, 48, 45-52. 58. Takamasa, N.; Li, H.; Tomonari, O.; Seiji, K. Synthesis of water-soluble

thermosensitive polymers having phosphonium groups from methacryloyloxyethyl trialkyl phosphonium chlorides-N-isopropylacrylamide copolymers and their functions. J. Appl. Polym. Sci. 2003, 87, 386-393. 59. Akihiko, K.; Tomiki, I.; Takeshi, E. Polymeric phosphonium salts as a novel class of cationic biocides. IV. Synthesis and antibacterial activity of polymers with phosphonium salts in the main chain. J. Polym. Sci., Part A: Polym. Chem. 1993, 31, 3031-3038. 60. Molinari, H.; Montanari, F.; Quici, S.; Tundo, P. Polymer-supported phase-transfer catalysts. High catalytic activity of ammonium and phosphonium quaternary salts bonded to a polystyrene matrix. J. Am. Chem. Soc. 1979, 101, 3920-3927. 61. Tomoi, M.; Kori, N.; Kakiuchi, H. A novel one-pot synthesis of spacer-modified polymer supports and phase-transfer catalytic activity of phosphonium salts bound to the polymer supports. React. Funct. Polym. 1985, 3, 341-349.

CHAPTER 4: The Effect of End Group and Molecular

Weight on the Yellowness of Polyetherimide

(Published in Macromolecular Rapid Communication 2018, 39, 1800045)

4.1 Abstract

The effect of end group and molecular weight on the yellowness of telechelic polyetherimide (PEI) was investigated. Electron-withdrawing dianhydride end groups reduced the yellowness of PEI, which showed high transparency and low yellowness regardless of the molecular weight. Electron-donating phenyl, amine, and phthalic end groups increased the yellowness of PEI but the effect depended on the end group density.

As the molecular weight was increased, the yellowness of PEIs with electron-donating end groups initially decreased due to a decreasing end group density and then increased due to an increasing probability of charge-transfer complex formation. The systematic study reveals the correlations among yellowness, end group, and molecular weight of PEIs. The correlations can be used for designing highly transparent PEIs and other high-performance polymers.

4.2 Introduction

As high-performance thermoplastics that possess excellent chemical resistance, outstanding thermal stability, and superb mechanical strength,1, 2 polyetherimides (PEIs) are widely used in automotive,3 aviation,4 aerospace,5 optoelectronics,6 and microelectronics.7-13 Besides the chemical, thermal, and mechanical properties,8, 14 researchers have focused on improving the optical properties of PEIs. Certain applications, for example, portable electronics,15 flexible displays,16 solar radiation protectors,17, 18 and optical waveguides,19-21 require optically clear PEIs for cosmetic satisfaction to end users and high transmittance of light.

Charge-transfer complex (CTC)22-24 is mainly responsible for the yellowness of PEIs and other polyimides. In PEIs synthesized from 2,2-bis[4-(3,4-dicarboxyphenoxy) phenyl] propane dianhydride (BPADA) and m-phenylenediamine (mPD), the electron-rich diamine moieties serve as electron donors and the electron-deficient dianhydride moieties as electron acceptors. When adjacent polymer chains come close to one another, interchain

CTC forms and diminishes the optical clarity of PEIs.23, 25 Generally, there are two strategies to decrease the yellowness: 1) to modify PEIs with electronegative components

(e.g., fluorinated groups),26, 27 which reduce the electron density in the polymer chains and suppress CTC formation, and 2) to incorporate bulky groups that introduce steric hindrance

100

to interchain CTC formation.28-30 In the former approach, fluorine, as the most electronegative element, significantly reduces the electron density in the PEI backbone, resulting in PEIs with little yellowness.22 In the latter strategy, bulky groups such as phenyl,28 t-butyl,29 norbornanyl,30 and trifluoromethyl31 are often utilized, which induce steric hindrance to interchain CTC formation and thus decrease the yellowness. Since fluorinated groups are often bulky, the two strategies are often combined to achieve PEIs with improved optical clarity and low yellowness.25, 32-35 However, several drawbacks limit the application of the above two strategies. Fluorinated monomers were usually synthesized from toxic and corrosive chemicals under harsh synthetic conditions.25 In addition, due to the strong electron-withdrawing effect of fluorine, the fluorinated diamines have lower reactivity than the non-fluorinated ones in the polycondensation reaction,35 resulting in uncontrolled distribution of fluorinated groups in the PEI chains.

Furthermore, the bulky group also affects the macromolecular packing, and thus deteriorates the thermal stability and mechanical strength.30, 36 Therefore, there is a strong need for developing a rationale of how many electron-withdrawing or bulky groups are required to achieve satisfactory optical clarity but maintain the thermal stability and mechanical strength of PEIs.

Instead of randomly incorporating functional groups into PEIs, we systematically

101

utilize the end groups of telechelic PEIs to identify the maximum distance between two electron-donating or electron-withdrawing groups in a PEI chain to maintain low yellowness and optical clarity. By measuring the optical properties (UV-Vis transmittance, yellowness, and haze) of telechelic PEIs with anhydride, phenyl, amine, and phthalic end

groups (termed PEI-DA, PEI-Ph, PEI-NH2, and PEI-PA, respectively) and various molecular weights (8, 10, 12, and 16.9 kDa), we have derived rational correlations among yellowness, end group electronegativity, and end group density. We have found that all

PEI-DA possessed high transparency, low yellowness, and low haze index. After

incorporating the electron-donating groups, PEI-Ph, PEI-PA, and PEI-NH2 showed higher yellowness than PEI-DA. Surprisingly, the PEIs with electron-donating groups exhibited non-monotonic increase in yellowness with molecular weight.

4.3 Experimental

Materials. 2,2-Bis[4-(3,4-dicarboxyphenoxy) phenyl] propane dianhydride (BPADA)

was heated to ~250 ˚C and cooled to room temperature in an N2 atmosphere to remove any moisture before use. m-Phenylenediamine (mPD, 99%, Sigma-Aldrich) was purified by

sublimation before use. Aniline (99%, Sigma-Aldrich) was protected under N2 in a

refrigerator. o-dichlorobenzene (oDCB) and chloroform (CHCl3) were purchased from

Sigma-Aldrich and used as received. Acetone (99.5%) was obtained from

102

Pharmco-AAPER and used as received. N,N-Dimethylformamide (DMF, ≥99.7%) and anhydrous lithium bromide (LiBr, 99%) were purchased from Alfa Aesar. Deuterated

chloroform (CDCl3) was supplied by Cambridge Isotope Laboratories, Inc.

Scheme 1. Synthesis of dianhydride-terminated polyetherimide (PEI-DA) and phenyl-terminated polyetherimide (PEI-Ph)

Synthesis of dianhydride-terminated PEI (PEI-DA). The synthesis of PEI-DA from

BPADA and mPD with a stoichiometric imbalance is shown in Scheme 1. For example, the

synthesis procedure for PEI-DA with a target number average molecular weight (Mn) of 8 kDa (8k-PEI-DA) is described as follows. A 500-mL three-neck round-bottom flask was

103

equipped with a stirring-rod, a Dean-Stark trap, and a nitrogen inlet. BPADA (17.389 g,

33.409 mmol), mPD (3.335 g, 30.84 mmol), and oDCB (80 mL) were charged to the flask.

The flask was constantly purged with N2 and kept in an oil bath at 180 ˚C for 12 h with constant stirring, and then in an alloy bath at 380 ˚C for 0.5 h without stirring. The product

was dissolved in CHCl3 and precipitated into acetone. The precipitate was collected by filtration, washed with acetone, and dried in vacuo at 180 ˚C.

Synthesis of phenyl-terminated PEI (PEI-Ph). PEI-Ph was synthesized by end-capping PEI-DA with aniline (Scheme 1). For example, to end-cap 8k-PEI-DA into

8k-PEI-Ph, aniline (0.48 mL, 0.49 g, 5.26 mmol) and oDCB (60 mL) were added to the

flask that contained as-synthesized 8k-PEI-DA (before being dissolved in CHCl3). The mixture was kept in an oil bath at 180 ˚C for 6 h with constant stirring and then in an alloy

bath at 380 ˚C for 0.5 h without stirring. N2 was purged throughout the reaction. The

product was dissolved in CHCl3 and precipitated into acetone. The precipitate was collected by filtration, washed with acetone, and dried in vacuo at 180 ˚C.

Synthesis of amine-terminated PEI (PEI-NH2) and phthalic-terminated PEI

(PEI-PA). The synthesis of PEI-NH2 and PEI-PA followed the same procedures in our

14 previous report, as shown in Scheme 2. For example, to synthesize 8k-PEI-NH2, BPADA

(17.323g, 33.282 mmol), mPD (3.876 g, 35.84 mmol) and oDCB (80 mL) were charged

104

into a three-neck round bottom flask equipped with a nitrogen inlet, an overhead stirring rod, and a Dean-Stark trap. The mixture was placed in an oil bath at 180 ˚C for 12 h with constant stirring, and then in an alloy bath at 380 ˚C without stirring. The whole reaction

was carried out under constant N2 flow. Subsequently, the product was dissolved in CHCl3 and then precipitated into acetone. The solid product was filtrated, washed with acetone, and then dried in vacuo at 180 ˚C.

Scheme 2. Synthesis of amine-terminated polyetherimide (PEI-NH2) and phthalic-terminated polyetherimide (PEI-PA).

105

To synthesize 8k-PEI-PA, phthalic anhydride (0.356 g, 2.40 mmol) and oDCB (60 mL)

were added to the dried 8k-PEI-NH2 (Mn = 7.8 kg/mol, 7.8 g, 1.0 mmol) in a three-neck round bottom flask equipped with a nitrogen inlet, an overhead stirring rod, and a

Dean-Stark trap. The mixture was placed in an oil bath at 180 ˚C for 12 h with constant stirring, and subsequently in an alloy bath at 380 ˚C without stirring. The whole reaction

was carried out under constant N2 flow. Afterwards, the product was dissolved in CHCl3 and then precipitated into a mixture of acetone and methanol (v:v = 1:1). The solid product was filtrated, washed with methanol, and then dried in vacuo at 180 ˚C.

Characterization. Proton nuclear magnetic resonance (1H NMR) spectroscopy was

performed on a Varian Unity 400 at 399.98 MHz in CDCl3. The PEIs were hot-pressed between two pieces of Kapton sheets at 315 ˚C and the thickness was controlled by placing two 0.051 mm-thick shims in between. No mold releasing agent was applied during hot-pressing to avoid any chemical contamination or yellowness distortion of the films.

The hot-pressing was conducted by five press-release cycles at forces of 1, 5, 7, 10, and 10 tons with a duration of 1 minute each. After the final press-release cycle, the PEI films were immediately quenched to room temperature so that they can be peeled off from the

Kapton sheets. The molecular weights were measured by size-exclusion chromatography

(SEC, EcoSECHLC-8320, Tosoh Bioscience) equipped with a Wyatt MiniDAWN TREOS

106

multi-angle light scattering detector and a differential refractive index detector (DRI).

DMF was used as the solvent. The flow rate was 0.5 mL/min. The column set was consisted of a SuperH-H guard column (4.6 mm ID × 3.5 cm, 4 μm), a SuperH-H guard column (6.0 mm ID × 15 cm, 4 μm), and two SuperH-H guard columns (6.0 mm ID × 15 cm, 4 μm). All columns and detectors were kept at 50 ˚C. The UV-Vis spectra were collected on an Agilent Cary 5000 UV-Vis-NIR spectrophotometer. The cutoff wavelength was defined as the wavelength-intercept of a linear fitting to the fastest dropping segment of the transmittance (T) spectrum (T = ~12−50 %). Transmittance haze values were measured using a BYK Gardner haze-gard plus hazemeter and averaged over five measurements. Using paper as a standard, the red (R), green (G), and blue (B) color values were extracted from the optical images of the PEI films, which were then used to calculate the yellowness following the equations as reported previously.37-39

푅 − 퐺 푅 + 퐺 − 2퐵 푢 = atan2 [ , ] √2 √6

푉 = max(푅, 퐺, 퐵)

푣 = min(푅, 퐺, 퐵)

2(푉 − 푣) 푆 = 1 + |푉 − 0.5| + |푣 − 0.5|

Yellowness = S cos(푢)

107

4.4 Results and Discussion

Synthesis of PEIs. The stoichiometric imbalance of monomers in polycondensation reactions determines the polymer end group and the number average molecular weight

14, 40-42 (Mn). Following this rationale, by charging BPADA and mPD in excess, we have

synthesized PEI-DA (Scheme 1) and PEI-NH2 (Scheme 2), respectively. PEI-DA and

PEI-NH2 can further react with aniline and phthalic anhydride to form PEI-Ph (Scheme 1) and PEI-PA (Scheme 2), respectively. All PEIs in this work were synthesized in o-dichlorobenzene (oDCB) instead of N-methyl-2-pyrrolidone (NMP) because NMP degrades at high temperatures and leads to deep colors of PEIs.43 Proton nuclear magnetic resonance (1H NMR) spectroscopy confirmed the chemical structures and revealed the

different end groups in PEIs, including anhydride in PEI-DA (peaks a'), amine in PEI-NH2

(peaks f', g', h', and j), and the phthalic end group in PEI-PA (peaks p and q). Despite the indistinguishable signature peaks of the phenyl group (A) in PEI-Ph, which overlap those of the benzene rings in the repeating units, the disappearance of the peaks a' suggests the high conversion of PEI-DA to PEI-Ph. Similarly, the signature peaks of the amine end groups disappeared while the peaks p and q showed up, indicating a high conversion from

1 PEI-NH2 to PEI-PA. H NMR spectra, along with SEC, were used to estimate the molecular weights of the PEIs (Table S1, Figure S1).

108

Figure 1. The chemical structures and the corresponding 1H NMR spectra of

representative PEI-DA, PEI-Ph, PEI-PA, and PEI-NH2 with a Mn of 8 kDa. The magnified peaks highlight the changes after reacting PEI-DA with aniline to form PEI-Ph, as well as

109

after reacting PEI-NH2 with phthalic anhydride to form PEI-PA. Peaks a' and a denote the protons from the end groups and the repeating units, respectively. Peaks a' disappeared after converting PEI-DA to PEI-Ph. Protons from the end group (A) in PEI-Ph are

indistinguishable and overlap the peaks from the repeating units. After reacting PEI-NH2 with phthalic anhydride, the peaks f', g', h', and j corresponding to the end group protons

in PEI-NH2 disappeared, while the peaks p and q corresponding to the end group protons in

PEI-PA appeared. All peaks were roughly normalized to peak i.

Figure 2. Optical photographs of the PEI films with various end groups and Mn. The yellowness of the films changes as the end group changes from electron withdrawing

(EWG) to electron donating (EDG), or as the concentration of the end group increases.

110

Yellowness of the PEI films. All PEIs were hot-pressed into thin films with a constant thickness of ~0.051 mm. A photograph of all the hot-pressed PEI films was taken by using a piece of white paper as the background. The yellowness of the PEI films can be compared visually (Figure 2). As the end group was changed from the most electron-withdrawing to

the most electron-donating, i.e. from -DA to -Ph, to -PA, and to -NH2 in sequence, the PEI

films became increasingly yellowish. The changes in yellowness was unapparent when Mn was increased. To quantify the degree of yellowness, red (R), green (G), and blue (B) color values were extracted from the photograph to calculate the yellowness following the previous reports (Supporting Information).37-39 As shown in Figure 3a, PEIs of the same

Mn exhibited an increasing yellowness as the end group was changed from -DA to -Ph, to

-PA, and to -NH2, in agreement with the visual observations. With the same end group,

however, PEIs showed different trends in yellowness as Mn was increased (Figure 3b). All

PEI-DAs showed low yellowness regardless of Mn. Different from PEI-DA, the yellowness

of PEI-Ph, PEI-PA, and PEI-NH2 decreased as Mn was changed from 8 to 10 kDa and then

increased gradually as Mn was further increased to 16.9 kDa, resulting in a valley-shaped

3D contour of PEI yellowness as a function of Mn and end group (Figure 3c).

111

Figure 3. Yellowness of the PEI films as a function of (a) end group and (b) Mn. (c)

Contour plot of the yellowness as a function of Mn and end group.

112

Figure 4. (a-d) UV-Vis spectra, (e) cut-off wavelengths, and (f) haze indices of the PEI

films with various Mn and end groups.

113

Transmittance and haze of the PEI films. All PEI films were transparent and the

visible light transmittance was >70% on average (Figure 4a-d). Regardless of the Mn,

PEI-DAs exhibited the highest visible light transmittance (>80%) among all PEIs.

Typically, the cutoff wavelength is related to the yellowness of polymers. The cutoff wavelength of PEIs increased when the end group was changed from -DA to -Ph, to -PA,

and to -NH2, following a trend similar to the yellowness (Figure 4e).

Haze index is another metric to evaluate the wide-angle light scattering and optical properties of polymers. In typical optical applications, the haze index should be <5% and

<3% for ordinary and extraordinary films, respectively.44 PEI-DA showed low haze values

of <5%, while PEI-NH2 had values of ~5% (Figure 4f). PEI-Ph and PEI-PA, however, possessed high haze values of >10%, probably due to the additional end-capping synthesis steps and the processing conditions.

Discussion. The formation of CTC, which is responsible for the yellowness of PEI, is governed by the following factors: (i) the type of the end group (electron-withdrawing

versus electron-donating), (ii) the end group concentration, which decreases with Mn, and

(iii) the probability of CTC formation, which increases with Mn. First, electron-donating groups facilitate, while electron-withdrawing groups suppress, the formation of CTC in

22-24 PEIs. Given the same Mn, the yellowness of PEI increases as the end group becomes

114

more electron-donating (-DA < -Ph < -PA < -NH2). The trend becomes less significant as

Mn increases because the end group concentration is reduced. In particular, all PEI-DA films showed the lowest yellowness, the highest transparency, and the lowest haze index

regardless of the Mn, indicating that strong electron-withdrawing groups such as -DA could effectively reduce the formation of CTC.

Second, for PEIs with end groups that are less electron-withdrawing but more

electron-donating, the end group concentration decreases with Mn and therefore the yellowness should decrease as well. On the other hand, the probability of CTC formation

increases with Mn and thus the yellowness should increase. Therefore, the end group concentration and the probability of CTC formation become competing factors, resulting in a valley in the contour plot. The competition is the most significant for the end group

with the strongest electron-donating effect (-NH2). As the end-group becomes less electron-donating, the valley becomes shallower. Therefore, the yellowness of PEI-Ph,

PEI-PA, and PEI-NH2 all decreased as the Mn increased from 8 to 10 kDa and then

increased from 10 to 16.9 kDa. The depth of the valley decreased in the sequence of -NH2,

-PA, and -Ph.

The fundamental understanding of how the end groups and Mn affect the yellowness of PEIs have practical implications and can be used as design principle for future synthesis,

115

which was unavailable in previous reports.26, 27, 31, 45 The 3D contour provides insights into

the design of PEIs with specific end group and Mn to achieve low yellowness. In comparison to the conventional synthesis of colorless PEI using fluorinated monomers, our results herein suggest that one can simply utilize end groups with inherent electron-donating/withdrawing properties to design colorless PEIs with mild synthetic conditions, in the absence of any harsh conditions or complicated procedures.

4.5 Conclusions

We have revealed the correlations among the yellowness, the end group electronegativity, and the end group density of PEIs. The end groups investigated herein

include -DA, -Ph, -PA, and -NH2, and the end group concentration is systematically

controlled by Mn, which ranges from 8 to 16.9 kDa. PEIs terminated with strong electron withdrawing groups such as -DA show the lowest yellowness and the highest transparency

regardless of Mn, indicating that modifying PEIs with strong electron-withdrawing end groups can effectively reduce the yellowness. With end groups that are not as

electron-withdrawing (e.g., -Ph, -PA, and -NH2), the yellowness shows a valley-shaped

change with Mn—the yellowness first decreases and then increases with Mn. The study provides guidelines for designing transparent PEIs with low yellowness. The fundamental understanding of the optical properties can be integrated with other strategies such as

116

H-bonding14 and ionic crosslinking46 to obtain PEIs with high transparency, high mechanical strength, and high thermal stability for applications including portable electronics,15 flexible displays,16 solar radiation protectors,17, 18 and optical waveguides.19-21

4.6 Supplementary Information

1 Calculation of Mn from H NMR spectrum. The number average molecular weight

(Mn) and the degree of polymerization (n) of dianhydride-terminated polyetherimide

(PEI-DA) were estimated using the following equations based on the integral areas of peaks a and a' (Figure S1).

# 표푓 푟푒푝푒푎푡𝑖푛푔 푢푛𝑖푡푠 퐴푟푒푎 (퐚)/2 퐴푟푒푎 (퐚) n = = = # 표푓 푐ℎ푎𝑖푛푠 퐴푟푒푎 (퐚′)/2 퐴푟푒푎 (퐚′)

−1 푀푛 = 푀푟푒푝푒푎푡푖푛푔 푢푛푖푡 ∙ n + 푀푒푛푑 푔푟표푢푝 = (592.61푛 + 520.49) g ∙ mol where the number of repeating units was calculated by normalizing peak (a) by a factor of

2 since each repeating unit has two Type (a) protons in BPADA. Similarly, the number of chains was calculated by dividing the peak area of (a') by a factor of 2 because each chain

has two Type (a') protons in the dianhydride end groups. The calculation of Mn and n of

14 PEI-NH2 and PEI-PA were both elaborated in our previous report.

The end-group analysis of Mn becomes less reliable as Mn increases because of a

117

decreasing concentration of the end group. More accurate characterization of Mn can be complemented by size exclusion chromatography.

1 Figure S1. H NMR spectrum of 8k-PEI-DA in CDCl3.

Calculation of Mn and Mw from SEC of PEIs. Since -DA can potentially be

hydrolyzed by moisture and the end group concentration is small in high-MW PEIs, the Mn determined by 1H NMR end group analysis can be imprecise. Therefore, the molecular weights of PEIs were further characterized by SEC in DMF (Table S1), similar to our previous report.14 For most PEIs except 16.9k-PEIs, the molecular weights from SEC agreed well with those from 1H NMR end group analysis, confirming the successful synthesis of the PEIs. The large deviation of the molecular weights of 16.9k-PEIs is due to

118

the imprecise NMR end group analysis and the incomplete dissolution of high-MW PEIs in

DMF.

Table S1. Mn, Mw, and PDI of PEIs with different end groups

Mn, NMR Mn, SEC Mw, SEC b Sample Mn, theo (kDa) PDI (kDa)a (kDa)b (kDa)b

8k-PEI-DA 7.6 8.5 8.4 18.7 2.23

10k-PEI-DA 9.4 11.6 9.7 22.7 2.34

12k-PEI-DA 11.8 13.7 11.7 27.7 2.36

16.9k-PEI-DA 16.9 29.3 8.6 20.0 2.33

8k-PEI-Ph 7.8 - 9.3 22.9 2.46

10k-PEI-Ph 9.6 - 9.7 26.7 2.74

12k-PEI-Ph 11.9 - 15.4 33.7 2.19

16.9k-PEI-Ph 17.1 - 10.3 21.2 2.06

8k-PEI-NH2 7.8 8.0 - - -

10k-PEI-NH2 9.6 11.7 - - -

12k-PEI-NH2 12.0 13.3 - - -

16.9k-PEI-NH2 16.9 15.1 - - -

8k-PEI-PA 8.1 8.8 9.4 20.0 2.13

10k-PEI-PA 9.9 12.7 9.4 18.8 2.01

12k-PEI-PA 12.3 14.0 10.3 20.3 1.98

16.9k-PEI-PA 17.2 20.0 10.3 20.9 2.03

119

a Mn measured in CDCl3 under ambient conditions and calculated by the end group analysis. bSEC characterization was carried out in DMF with 0.05 M LiBr. The molecular weights were based on polystyrene standards.

4.7 References

1. Ghosh, M. K.; Mittal, K. L., Polyimides: fundamentals and applications. Marcel Dekker, Inc.: New York, 1996. 2. Wilson, D.; Stenzenberger, H. D.; Hergenrother, P. M., Polyimides. Springer: London, 1990. 3. Greer, W. K. In Ultem Polyetherimide Resin and its Automotive Uses, Plast. Eng., 1983; Soc Plastics Eng Inc., Brookfield, CT: 1983; pp 34-34. 4. Williams III, F. J.; Donahue, P. E. Novel polyetherimides. 3983093 A, 1976. 5. Pitchan, M. K.; Bhowmik, S.; Balachandran, M.; Abraham, M. Process optimization of functionalized MWCNT/polyetherimide nanocomposites for aerospace application. Mater. Des. 2017, 127, 193-203. 6. Flaim, T. D.; Wang, Y.; Mercado, R. In High-refractive-index polymer coatings for optoelectronics applications, Proceedings of SPIE, 2003; 2003; pp 423-434. 7. Hilado, C. J., Reinforced Phenolic, Polyester, Polyimide, and Polystyrene Systems. Technomic Publishing Company: Westport, CT, 1974; Vol. 5. 8. Mittal, K. L., Polyimides: synthesis, characterization, and applications. Plenum Press: New York, 1984. 9. Kricheldorf, H. R.; de Abajo, J., Progress in polyimide chemistry, advances in polymer science. Springer: Berlin, 1999. 10. Xie, K.; Liu, J. G.; Zhou, H. W.; Zhang, S. Y.; He, M. H.; Yang, S. Y. Soluble fluoro-polyimides derived from 1,3-bis(4-amino-2-trifluoromethyl-phenoxy) benzene and dianhydrides. Polymer 2001, 42, 7267-7274. 11. Wu, J.; Yang, S.; Gao, S.; Hu, A.; Liu, J.; Fan, L. Preparation, morphology and properties of nano-sized Al2O3/polyimide hybrid films. Eur. Polym. J. 2005, 41, 73-81.

120

12. Zhai, F.; Guo, X.; Fang, J.; Xu, H. Synthesis and properties of novel sulfonated polyimide membranes for direct methanol fuel cell application. J. Membr. Sci. 2007, 296, 102-109. 13. Calil, V. L.; Legnani, C.; Moreira, G. F.; Vilani, C.; Teixeira, K. C.; Quirino, W. G.; Machado, R.; Achete, C. A.; Cremona, M. Transparent thermally stable poly(etherimide) film as flexible substrate for OLEDs. Thin Solid Films 2009, 518, 1419-1423. 14. Cao, K.; Liu, G. Low-molecular-weight, high-mechanical-strength, and solution-processable telechelic poly(ether imide) end-capped with ureidopyrimidinone. Macromolecules 2017, 50, 2016-2023. 15. Rothkopf, F. R.; Zadesky, S. P.; Dinh, R. H. M. Touch sensor structures for displays. 20110012845 A1, 2009. 16. Lim, H.; Cho, W. J.; Ha, C. S.; Ando, S.; Kim, Y. K.; Park, C. H.; Lee, K. Flexible organic electroluminescent devices based on fluorine-containing colorless polyimide substrates. Adv. Mater. 2002, 14, 1275-1279. 17. Du Pont, P. S.; Bilow, N. Polyimide composition and method for protecting photoreactive cells. 4592925 A, 1986. 18. Landis, A. L.; Naselow, A. B., Method of preparing high molecular weight polyimide, product and use. US Patent 4645824 A: 1987. 19. Matsuura, T.; Ando, S.; Matsui, S.; Sasaki, S.; Yamamoto, F. Heat-resistant singlemode optical wave-guides using fluorinated polyimides. Electron. Lett 1993, 29, 2107-2109. 20. Ando, S.; Sawada, T.; Inoue, Y. Thin, flexible waveplate of fluorinated polyimide. Electron. Lett 1993, 29, 2143-2145. 21. Gao, H.; Wang, D.; Guan, S.; Jiang, W.; Jiang, Z.; Gao, W.; Zhang, D. Fluorinated hyperbranched polyimide for optical waveguides. Macromol. Rapid Commun. 2007, 28, 252-259. 22. Ando, S.; Matsuura, T.; Sasaki, S. Coloration of aromatic polyimides and electronic properties of their source materials. Polym. J. 1997, 29, 69-76. 23. Hasegawa, M.; Horie, K. Photophysics, photochemistry, and optical properties of polyimides. Prog. Polym. Sci. 2001, 26, 259-335. 24. Ke, F.; Song, N.; Liang, D.; Xu, H. A method to break charge transfer complex of polyimide: A study on solution behavior. J. Appl. Polym. Sci. 2013, 127, 797-803.

121

25. Liu, Y.; Xing, Y.; Zhang, Y.; Guan, S.; Zhang, H.; Wang, Y.; Wang, Y.; Jiang, Z. Novel soluble fluorinated poly(ether imide)s with different pendant groups: Synthesis, thermal, dielectric, and optical properties. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 3281-3289. 26. Guggenheim, T. L.; Odle, R. R.; Venkataraman, K. Production of low color polyetherimides. 8080671 B2, 2011. 27. Chung, C.-L.; Yang, C.-P.; Hsiao, S.-H. Organosoluble and colorless fluorinated poly(ether imide)s from 1,2-bis(3,4-dicarboxyphenoxy)benzene dianhydride and trifluoromethyl-substituted aromatic bis(ether amine)s. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 3092-3102. 28. Hsiao, S.-H.; Yang, C.-P.; Chen, Y.-C.; Wang, H.-M.; Guo, W. Synthesis and properties of poly(ether imide)s derived from 2,5-bis(3,4-dicarboxyphenoxy)biphenyl dianhydride and aromatic ether–diamines. J. Appl. Polym. Sci. 2009, 113, 3993-4002. 29. Yang, C.-P.; Hsiao, S.-H.; Yang, H.-W. Organosoluble optically transparent poly(ether imide)s based on a tert-butylhydroquinone bis(ether anhydride). Macromol. Chem. Phys. 2000, 201, 409-418. 30. Chen, Y.-Y.; Yang, C.-P.; Hsiao, S.-H. Soluble and colorless poly(ether-imide)s based on a benzonorbornane bis(ether anhydride) and trifluoromethyl-substituted aromatic bis(ether-amine)s. Macromol. Chem. Phys. 2006, 207, 1888-1898. 31. Reddy, D. S.; Shu, C.-F.; Wu, F.-I. Synthesis and characterization of soluble polyimides derived from 2,2′-Bis(3,4-dicarboxyphenoxy)-9,9′-spirobifluorene dianhydride. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 262-268. 32. Liu, Y.; Zhang, Y.; Guan, S.; Li, L.; Jiang, Z. Synthesis and properties of soluble fluorinated poly(ether imide)s with different pendant groups. Polymer 2008, 49, 5439-5445. 33. Wang, X.-L.; Li, Y.-F.; Gong, C.-L.; Ma, T.; Yang, F.-C. Synthesis and properties of new pyridine-bridged poly(ether-imide)s based on 4-(4-trifluoromethylphenyl)-2,6-bis[4-(4-aminophenoxy)phenyl]pyridine. J. Fluorine Chem. 2008, 129, 56-63. 34. Yang, C.-P.; Chen, Y.-C.; Hsiao, S.-H.; Guo, W.; Wang, H.-M. Optically transparent and colorless poly(ether-imide)s derived from a phenylhydroquinone bis(ether anhydride) and various trifluoromethyl-substituted bis(ether amine)s. Journal of Polymer Research 2010, 17, 779-788.

122

35. Tao, L.; Yang, H.; Liu, J.; Fan, L.; Yang, S. Synthesis and characterization of highly optical transparent and low dielectric constant fluorinated polyimides. Polymer 2009, 50, 6009-6018. 36. Ni, H.-J.; Liu, J.-G.; Wang, Z.-H.; Yang, S.-Y. A review on colorless and optically transparent polyimide films: Chemistry, process and engineering applications. Journal of Industrial and Engineering Chemistry 2015, 28, 16-27. 37. Pătraşcu, V., Fuzzy Image Segmentation Based on Triangular Function and Its n-dimensional Extension. In Soft Computing in Image Processing: Recent Advances, Nachtegael, M., Van der Weken, D., Kerre, E. E., Philips, W., Eds. Springer: Berlin, Heidelberg, 2007; pp 187-207. 38. Pătraşcu, V., New Framework of HSL System Based Color Clustering Algorithm. In Proceedings of the 8th Conference of the European Society for Fuzzy Logic and Technology, Pasi, G., Montero, J., Ciucci, D., Eds. Atlantis Press: Paris, 2013; Vol. 32, pp 416-423. 39. Pătraşcu, V., New Fuzzy Color Clustering Algorithm Based on hsl Similarity. In Proceedings of the Joint 2009 International Fuzzy Systems Association World Congress and 2009 European Society of Fuzzy Logic and Technology Conference, 2009; pp 48-52. 40. Lee, H.-S.; Badami, A. S.; Roy, A.; McGrath, J. E. Segmented sulfonated poly(arylene ether sulfone)-b-polyimide copolymers for proton exchange membrane fuel cells. I. Copolymer synthesis and fundamental properties. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 4879-4890. 41. Carter, K. R.; DiPietro, R. A.; Sanchez, M. I.; Swanson, S. A. Nanoporous Polyimides Derived from Highly Fluorinated Polyimide/Poly(propylene Oxide) Copolymers. Chem. Mater. 2001, 13, 213-221. 42. Hedrick, J. L.; Hawker, C. J.; DiPietro, R.; Jerome, R.; Charlier, Y. The use of styrenic copolymers to generate polyimide nanofoams. Polymer 1995, 36, 4855-4866. 43. Berrueco, C.; Álvarez, P.; Venditti, S.; Morgan, T. J.; Herod, A. A.; Millan, M.; Kandiyoti, R. Sample Contamination with NMP-oxidation Products and Byproduct-free NMP Removal from Sample Solutions. Energy Fuels 2009, 23, 3008-3015. 44. Jang, W.; Shin, D.; Choi, S.; Park, S.; Han, H. Effects of internal linkage groups of fluorinated diamine on the optical and dielectric properties of polyimide thin films. Polymer 2007, 48, 2130-2143. 45. Matsuura, T.; Ando, S.; Sasaki, S.; Yamamoto, F. Polyimides Derived from

123

2,2'-Bis(trifluoromethyl)-4,4'-diaminobiphenyl. 4. Optical Properties of Fluorinated Polyimides for Optoelectronic Components. Macromolecules 1994, 27, 6665-6670. 46. Kammakakam, I.; Nam, S.; Kim, T.-H. Ionic group-mediated crosslinked polyimide membranes for enhanced CO2 separation. RSC Advances 2015, 5, 69907-69914.

124

CHAPTER 5: Suggested Future Work

5.1 High-Mechanical-Strength, Halogen-Free, and

Flame-Retardant PEIs

The phosphonium bromide terminated PEI (PEI-PhPPh3Br) oligomers synthesized in

Chapter 4 exhibited excellent flame retardancy, as well as high mechanical strength, great thermal stability, and superb processability. Particularly, we have observed the loss of bromine atom and phenyl groups from the chain ends which acts as the radical trapper.

However, regulations have been carried out to restrict the use of some bromine-containing flame retardants, due to the fear of toxicity and environmental impact.1 As a result, halogen-free PEI oligomers with the above-mentioned excellent properties are in urgent need. To get rid of the bromine element in the oligomers while maintaining the above properties, a new strategy is to blend sulfonate salt terminated PEIs (PEI-SAA-M, M = Li,

Na, K) with PEI-PhPPh3Br at stoichiometric ratio and remove the bromide salt by nonsolvent.

125

Scheme 1. Synthesis of PEI-SAA-M from PEI-DA and SAA-M (M = Li, Na, K)

Scheme 2. Halogen-free PEI blends from PEI-SAA-M and PEI-PhPPh3Br

Sulfonate salt terminated PEIs (PEI-SAA-M) can be synthesized from PEI-DA and

126

respect sulfanilate salts (SAA-M) (Scheme 1). After mixing PEI-SAA-M and

PEI-PhPPh3Br at stoichiometric ratio, the blends can be washed with nonsolvent (e.g. H2O, methanol) which can dissolve M-Br salt but not the PEI backbones (Scheme 2). After the above steps, Halogen-free PEI blends with positively and negatively charged terminal groups can be achieved. Furthermore, they can potentially be strong in mechanical properties because of strong electrostatic interactions between chains with phosphonium and those with sulfonate end groups.

5.2 PEI Chelate Membranes for Heavy Metal Removal

Heavy metal contamination is a vital environmental issue due to their hazard to human beings, animals, and plants. Of various available techniques for heavy metal removal or recovery, membrane processes are the most promising method due to energy efficiency and capability for heavy metal recovery and membrane reuse.2 However, most current membranes, such as polystyrene,3 polycarbonate,4 and polyethylene-graft-polystyrene,5 lack high mechanical strength for high permeate fluxes. In addition, although some membranes have amine-containing units, the binding force between the ligands and heavy metal ions is not strong enough for high efficiency of heavy metal removal. Therefore, a high-mechanical-strength membrane that can bind extremely strongly with heavy metal ions is in urgent need to solve the challenge. The polyetherimide (PEI) with bipyridine

127

moieties, is a promising candidate due to the following advantages. First, PEI has ultrahigh mechanical properties compared to most membranes.3-5 Second, bipyridine-containing moieties, including bipyridine and phenanthroline ligands, can chelate strongly with metal ions such as Zn2+ or Cu2+ with a large equilibrium constant (Table 1). For instance, various studies showed high efficiency of heavy metal removal by bipyridine units.3

Table 1. Examples of building ligand-metal interactions.

Ligand-Metal Complex Log Ka Ref.

9 6

17 7

13 7

128

Scheme 3. Synthesis of bipyridine ligands terminated PEIs

The synthesis of bipyridine-containing PEIs is proposed as in Scheme 3.

4-Phenylethynylphthalicanhydride (PEPA) reacts with BPADA and mPD to form ethynyl-terminated PEIs.8 Further end group crosslinking utilizing thiol-ene click-reaction with 5,5′-bis(mercaptomethyl)-2,2′-bipyridine9 would afford the synthesis of

129

bipyridine-containing PEIs with crosslinked network. The bipyridine-containing PEIs are expected to have even higher mechanical strength compared to linear PEIs with similar molecular weights due to the crosslinking, and thus to withstand high fluxes. The bipyridine-rich segments can bind with heavy metal ions efficiently due to the strong coordination effects, and thus the efficiency of heavy metal removal is expected high to be very high. Furthermore, the strong binding force between the bipyridine moieties and heavy metals makes the bipyridine-containing PEIs promising for recycling of heavy metals with high purity in electroplating industry.

5.3 PEI-g-PEKK as a Sacrificial Scaffold for 3D Printing

3D printing has emerged as an advanced technique in fabrication of sacrificial scaffolds for applications such as tissue engineering10 and microfluidic device.11, 12 The sacrificial scaffold materials are water-soluble or solvent-dissolvable and thus easily removed. Some removed scaffold materials can be recycled and reused. However, for large scale tissue engineering and complex scaffold building, current available sacrificial scaffold materials, such as poly(vinyl alcohol) (PVA)10, 12 and acrylonitrile-butadiene-styrene (ABS),11 do not have strong enough mechanical strength and thus cannot fulfil the requirements. Therefore, a mechanically strong, water soluble/crackable material is in urgent need for the fabrication of sacrificial scaffolds. A polyetherimide-graft-polyetherketoneketone

130

(PEI-g-PEKK) copolymer is a promising candidate due to the following. First, both PEI and PEKK have very high tensile strengths twice higher than PVA and ABS, which fulfil the mechanical requirements as strong scaffold materials. Second, although neither PEI nor

PEKK is water-soluble, careful design of linkages that can be relatively easily hydrolyzed affords the cracking of scaffold after its function, making the copolymers “sacrificial”.

The synthesis of PEI-g-PEKK is described as follows. The amine groups in amine-terminated PEIs in Chapter 2 can react with ketone bonds, forming imine linkages and losing water molecules.13 Therefore, amine-terminated PEI oligomers can blend with ketone-containing oligomers such as poly(ether ketone ketone) (PEKK) to form materials for scaffold (Scheme 4). At the stage of scaffold removal, dilute aqueous acid solution can be used to wash the scaffold. The imine bonds are susceptible to acid and will break.14 Thus, the scaffold will significantly lose strength by bond-breaking and crack. Both PEI and

PEKK oligomers can be removed easily as the scaffold is expected to crack into small pieces or even powders. Another merit of this novel strategy is that the materials can be recycled and reused as both polymer backbones are thermally stable.

131

Scheme 4. Synthesis and breaking of ketimine bonds

5.4 References

1. Lu, S.-Y.; Hamerton, I. Recent developments in the chemistry of halogen-free flame retardant polymers. Prog. Polym. Sci. 2002, 27, 1661-1712. 2. Fu, F.; Wang, Q. Removal of heavy metal ions from wastewaters: A review. Journal of Environmental Management 2011, 92, 407-418. 3. Card, R. J.; Neckers, D. C. Preparation of polymer-bound bipyridine and some of its transition metal complexes. J. Am. Chem. Soc. 1977, 99, 7733-7734. 4. Danis, U.; Aydiner, C. Investigation of process performance and fouling mechanisms in micellar-enhanced ultrafiltration of nickel-contaminated waters. J. Hazard. Mater. 2009, 162, 577-587. 5. Nasef, M. M.; Saidi, H.; Ujang, Z.; Dahlan, K. Z. M. Removal of metal ions from aqueous solutions using crosslinked polyethylene-graft-polystyrene sulfonic acid adsorbent prepared by radiation grafting. Journal of the Chilean Chemical Society 2010, 55, 421-427. 6. Delwar, H. M.; Takashi, S.; Masayoshi, H. A green copper-based metallo-supramolecular polymer: Synthesis, structure, and electrochromic properties. Chem. Asian J. 2013, 8, 76-79. 7. Yamasaki, K.; Yasuda, M. Stability of zinc and cadmium complexes with 2,2'-bipyridine and 1,10-phenanthroline. J. Am. Chem. Soc. 1956, 78, 1324-1324. 8. Meyer, G. W.; Glass, T. E.; Grubbs, H. J.; McGrath, J. E. Synthesis and characterization of polyimides endcapped with phenylethynylphthalic anhydride. J. Polym. 132

Sci., Part A: Polym. Chem. 1995, 33, 2141-2149. 9. Yao, B.; Mei, J.; Li, J.; Wang, J.; Wu, H.; Sun, J. Z.; Qin, A.; Tang, B. Z. Catalyst-free thiol–yne click polymerization: A powerful and facile tool for preparation of functional poly(vinylene sulfide)s. Macromolecules 2014, 47, 1325-1333. 10. Mohanty, S.; Larsen, L. B.; Trifol, J.; Szabo, P.; Burri, H. V. R.; Canali, C.; Dufva, M.; Emnéus, J.; Wolff, A. Fabrication of scalable and structured tissue engineering scaffolds using water dissolvable sacrificial 3D printed moulds. Materials Science and Engineering: C 2015, 55, 569-578. 11. Vittorio, S.; H., V. A. Simple 3d printed scaffold-removal method for the fabrication of intricate microfluidic devices. Advanced Science 2015, 2, 1500125. 12. Dahlberg, T.; Stangner, T.; Zhang, H.; Wiklund, K.; Lundberg, P.; Edman, L.; Andersson, M. 3D printed water-soluble scaffolds for rapid production of PDMS micro-fluidic flow chambers. Scientific Reports 2018, 8, 3372. 13. Brandom, D. K.; Desouza, J. P.; Baird, D. G.; Wilkes, G. L. New method for producing high-performance thermoplastic polymeric foams. J. Appl. Polym. Sci. 1997, 66, 1543-1550. 14. Meyer, C. D.; Joiner, C. S.; Stoddart, J. F. Template-directed synthesis employing reversible imine bond formation. Chem. Soc. Rev. 2007, 36, 1705-1723.

133