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11-3-2016 Synthesis and Characterization of Novel Polyurethanes and Polyimides Kenneth Kull University of South Florida, [email protected]
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Synthesis and Characterization of Novel Polyurethanes and Polyimides
by
Kenneth Kull
A dissertation submitted in the partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Chemistry College of Arts and Sciences University of South Florida
Major Professor: Julie P. Harmon, Ph.D. Abdul Malik, Ph.D. Shengqian Ma, Ph.D. Jianfeng Cai, Ph.D.
Date of Approval: September 9, 2016
Keywords: Polyetherdiamine, Polycarbonate Polyol, Soft Thermoplastic Urethane, Phase Seperation
Copyright © 2016, Kenneth Kull
Dedication
This dissertation is dedicated to my family. In particular, to my children Kaleb and Kara. I dedicate this dissertation as a reminder that all dreams and goals are possible. That with hard work, dedication and perseverance, all you set out to accomplish can be achieved.
Acknowledgments
I would like to thank my Advisor, Dr. Harmon for guidance and tutelage. I appreciate her sticking with me throughout this long and drawn out process. My committee members: Dr.
Abdul Malik, Dr. Shengian Ma and Dr. Jianfeng Cai. I would like to thank the Department of
Chemistry at USF and all those that helped with the paperwork and deadlines.
I would especially like to thank the late Dr. Ralph Moore for being the one who guided, mentored and pushed me to start this journey. Ralph was a trusted friend who will forever be missed.
I would like to thank the Klingel family for their support and understanding while working full time and pursuing my degree. I would like to thank my current employer Brighvolt for allowing me the time and providing many resources as I finished my degree. Many thanks to
Dr. Anaba Anani for his encouragement, guidance as well as his ability to help others understand what it will take to finish my degree. Imalka Marasinghe Arachchilage
I would like to thanks all my past and present lab colleagues: Dr. Timofey Gerasimov,
Lanetra Clayton, Kadine Mohomed, Butch Knudsen, Garrett Craft, Alejandro Rivera-Nicholls,
Tamalia Julienne and Imalka Marasinghe Arachchilage.
Table of Contents
List of Tables ...... iv
List of Figures ...... v
List of Equations ...... viii
Abstract ...... ix
Chapter 1: Introduction ...... 1 Polyurethanes ...... 1 Polyimides...... 5
Chapter 2: New Generation of Ultrasoft Non-Blocking Polyurethanes with High Mechanical Properties for Biomedical Applications ...... 8 Polyurethane Calculations ...... 9 One Shot Method ...... 13 Two Shot Method ...... 13 Particle Size Reduction ...... 14 Testing ...... 14 Instruments ...... 15 Results and Discussion ...... 17 Hardness ...... 17 Tensile/Elongation ...... 18 Molecular Weight Determination (GPC) ...... 21 FTIR ...... 23 Small Angle Xray (SAXS) ...... 29 Dynamic Mechanical Analysis, DMA ...... 30 Thermongravimetric Analysis (TGA) ...... 35 Differential Scanning Calorimetry, DSC ...... 40 Modulated Differential Scanning Calorimetry (MDSC) ...... 47 Atomic Force Microscopy (AFM) ...... 52 Scanning Electron Microscopy (SEM) ...... 53 USP Class VI Testing ...... 54 Conclusions ...... 55
Chapter 3: Synthesis and Characterization of Novel Melt Processable Polyimide ...... 57 Materials and Methods ...... 57 TMMDA Synthesis and Characterization ...... 57 FTIR ...... 58
i
Titration...... 58 NMR ...... 59 TMMDA Mechanism ...... 62 General Procedure for Polymerization ...... 62 FT-IR...... 65 Rheology ...... 65 Thermogravimetric Analysis (TGA) ...... 65 Tensile Test ...... 65 Microhardness Test ...... 66 Results ...... 66 Rheology ...... 66 Thermogravimetric Analysis ...... 68 Gel Permeation Chromatography ...... 69 FTIR ...... 70 Vickers Hardness ...... 70 Discussions ...... 71 Gel Permeation Chromatography ...... 71 Rheology ...... 71 Thermogravimetric Analysis ...... 72 Infrared Spectroscopy ...... 72 Microhardness ...... 73 Conclusions ...... 74
Chapter 4: Future Work ...... 76 Polyurethane and Nanoparticles ...... 76 Current Target Areas ...... 79 Threading the Nanoballs with Linear Polyurethanes ...... 79 Short Term Objectives ...... 81 Research Plan ...... 81 Polymerize Baseline Material ...... 81 Determine Solubility of Nanoballs ...... 81 Cap Hydoxylated Nanoballs ...... 82 Fabrication of Polyurethane Capped Nanoball Composites ...... 83 Thermoplastic Polyurethane Threaded Through the Capped Nanoballs ...... 83 A Millable Polyurethane Rubber Threaded Through the Capped Nanoballs ...... 83 Thermoplastic Polyurethane Threaded Through the Capped Nanoballs ...... 84 Fabrication of Cross-Linked Polyurethane Nanoball Composites ...... 84 Thermoplastic Polyurethane Cross-Linked By the Nanoballs ...... 84 Cast Polyurethane Cross-Linked By the Nanoballs ...... 85 Millable Polyurethane Rubber Cross-Linked By the Nanoballs ...... 85 Testing of the Urethane/Nanoball Composites ...... 86 Differential Scanning Calorimetry (DSC) ...... 86 Dynamic Mechanical Analysis (DMA) ...... 86 Thermal Gravimetric Analysis (TGA) ...... 86 UV Visible Spectroscopy ...... 87 Atomic Force Microscopy ...... 87
ii
Tensile Testing ...... 87 Abrasion Testing ...... 87 Polyimide ...... 88
References ...... 90
About the Author ...... End Page
iii
List of Tables
Table 1. Thermoplastic Polyurethane Physical Properties ...... 19
Table 2. Thermoplastic Polyurethane Physical Properties ...... 20
Table 3. Molecular Weight of TPU's ...... 22
Table 4. Nomenclature, Composition and Process Type ...... 41
Table 5. Glass Transition and Melting Temperatures Observed from DSC Thermograms ...... 41
Table 6. Feedstock Stoichiometry of Polyimide ...... 63
Table 7. Onset and Temperature at 97% Starting Weight for the Polyimides Studied ...... 67
Table 8. TGA Onset and Temperature at 97% Starting Weight for the Polyimides Studied ...... 68
Table 9. Mw, Mn and Polydispersity by GPC ...... 69
Table 10. Vickers Hardness Values ...... 70
iv
List of Figures
Figure 1: Urethane Reaction Scheme ...... 1
Figure 2: Segmented Polyurethane ...... 3
Figure 3: Formation of Polycarbonate Copolymer ...... 8
Figure 4: Urethane Polymerization Scheme ...... 12
Figure 5: Molecular Weight Graph ...... 22
Figure 6: FTIR Hydrogen bonding Depiction ...... 23
Figure 7: FTIR Hydrogen Bond N-H Stretch and Non Hydrogen Bonded N-H Stretch ...... 27
Figure 8: FTIR Hydrogen Bonded Carbonyl Stretch and Non Hydrogen Bonded Carbonyl Stretch ...... 28
Figure 9: SAXS of PCPU 7 Days after Molding ...... 29
Figure 10: Dynamic Mechancal Analysis of PCPU ...... 31
Figure 11: Apparent Activation Energy of the Beta Transition ...... 32
Figure 12: WLF/Time Temperature Superposition fit at Tg from DMA data ...... 33
Figure 13: TGA XP-2816 ...... 36
Figure 14: TGA XP-28203 ...... 37
Figure 15: TGA XP-28062 ...... 38
Figure 16: TGA XP-28221 ...... 39
Figure 17: DSC XP-162 1st Heat Cycle ...... 42
Figure 18: DSC XP-28162 2nd Heat Cycle ...... 42
Figure 19: DSC XP-28203 1st Heat Cycle ...... 43
v
Figure 20: DSC XP-28203 2nd Heat Cycle ...... 43
Figure 21: DSC XP-28062 1st Heat Cycle ...... 44
Figure 22: DSC XP-28062 2nd Heat Cycle ...... 44
Figure 23: DSC XP-28221 1st Heat Cycle ...... 45
Figure 24: DSC XP-28221 2nd Heat Cycle ...... 45
Figure 25: MDSC XP-28062 1st Heat Cycle ...... 48
Figure 26: MDSC XP-28062 2nd Heat Cycle ...... 48
Figure 27: MDSC XP-28203 2nd Heat Cycle ...... 49
Figure 28: MDSC XP-28203 2nd Heat Cycle ...... 49
Figure 29: MDSC XP-28162 1st Heat Cycle ...... 50
Figure 30: MDSC XP-28162 2nd Heat Cycle ...... 50
Figure 31: MDSC XP-28221 1st Heat Cycle ...... 51
Figure 32: MDSC XP-28221 2nd Heat Cycle ...... 51
Figure 33: AFM Tapping Mode Phase Images ...... 53
Figure 34: SEM Image x1000 ...... 54
Figure 35: SEM Image x25000 ...... 54
Figure 36: TMMDA FTIR...... 58
Figure 37: Carbon NMR ...... 60
Figure 38: Hydrogen NMR ...... 61
Figure 39: TMMDA Mechanism ...... 62
Figure 40: Synthetic Procedure Outline for the Formation of the Polyether Polyimide ...... 64
Figure 41: Rheology Temperature Sweeps ...... 66
Figure 42: Rheology Temperature Sweeps ...... 66
vi
Figure 43: TGA Thermograms for the Polyimides ...... 68
Figure 44: Gel Permeation Chromatograms for the Polyimides ...... 69
Figure 45: FTIR Sprectrum of Polyimide ...... 70
Figure 46: Calculated Nanoball Window Sizes (Angstroms) ...... 80
Figure 47: Mono Functional Isocyanates ...... 82
vii
List of Equations
Equation 1: Urethane Calculation ...... 10
Equation 2: Prepolymer Calculation ...... 10
Equation 3: Percent Hardsegment ...... 11
Equation 4: Bragg's Law ...... 29
Equation 5: Activation Energy ...... 31
Equation 6: WLF of Alpha Tranisitions ...... 34
Equation 7: Activation Energy for the Glass Transition ...... 34
Equation 8: TMMDA % Purity ...... 59
viii
Abstract
Four novel high performance soft thermoplastic polyurethane elastomers utilizing methylene bis(4-cyclohexylisocyanate) as a hard segment, 1,4 butanediol as a chain extender and modified low crystallinity carbonate copolymer as a soft segment were synthesized. The samples were characterized by infrared spectroscopy (FTIR), tensile, elongation, hardness, abrasion resistance and atomic force microscopy (AFM). SAXS data shows evidence of an interdomain
"center-to-center" distance of 45Å. DSC traces show evidence of one glass transition temperature and a weak melting region. DMA analysis reveals a low temperature secondary relaxation and the glass to rubber transition followed by a rubbery plateau. All samples demonstrated the ability to maintain excellent physical and mechanical properties in hardness below 70 Shore A.
Thermoplastic polyurethanes in this study do not possess surface tackiness usually observed in soft polyurethanes. Biocompatability testing showed no toxicity of these samples as indicated by
USP Class VI, MEM Elution Cytotoxicity and Hemolysis toxicology reports. This novel type of polyurethane material targets growing markets of biocompatible polymers and can be utilized as peristaltic pump tubing, balloon catheters, enteral feeding tubes and medical equipment gaskets and seals.
Polyimides are a family of engineering polymers with temperature stability, high polarity and solvent resistance. These high-performance materials are used in aerospace applications, in the production of semi-dry battery binders, and in a host of other high temperature demanding situations. However, their glass transition and melt temperatures are characteristically very high
ix and close to one another, making them difficult to melt process and limiting them to thin film formulations from their polyamic acid precursors. Here, a new series of thermoplastic polyether- polyimides (PE-PIs) are synthesized by incorporating a polyetherdiamine monomer to reduce rigidity and break up an otherwise fully aromatic backbone as seen with most conventional polyimides. It will be shown that control of the stoichiometric ratio between the aromatic 4,4'- methylenebis(2,6-dimethylaniline) and aliphatic polyetherdiamines relative to PMDA
(pyromellitic dianhydride), along with the molecular weight of the polyetheramine, can be used to tune the Tg to best balance between temperature performance and processability.
x
Chapter 1:
Introduction
Polyurethanes
Polymers have become widely used in many different industries for many different applications. Two different types of polymers, a polyurethane and a polyimide will be discussed regarding the synthesis, properties, processing and applications. Discussed will be specific target end uses and the potential benefits of the novel polymers developed in this study.
Polyurethane polymers (can also be called carbamates) are formed when you react an isocyanate (sometimes called the “A” side of the formula) and a hydroxyl group (called the “B” side) as demonstrated in the below figure.
RNCO ROH
ISOCYAN POLYO
RNHCOOR
URETHANE Figure 1: Urethane Reaction Scheme
1
An isocyanate contains a nitrogen atom (N), a carbon atom (C), and an oxygen atom (O) and is represented as –N=C=O. The common “B” side is generally referred to as a polyol or a diol and contains a hydroxyl group (-OH). In general, polyurethanes are produced using diisocyanates and polyols containing two or more hydroxyl groups. There are two types of hydroxyl containing groups. The polyol referred to above is generally a long chain molecule with a molecular weight range between 700 – 4000 amu. These molecules are known for their ability to flex and stretch. There are also diols called chain extenders which are much lower in molecular weight. An example would be 1, 4 butanediol with a MW of 90 amu. Chain extenders, as their name suggests link long, flexible portions of the urethane molecule and impart rigidity to the polymer. The chain extender is part of the total “B” side but is a much smaller portion than the long chain polyol. It is well known and has been extensively studied that thermoplastic polyurethanes are linear segmented block copolymers made up of a hard segment (HS) and a soft segment (SS). The hard segment is made from the diisocyanate and the short chain diol called the chain extender. The soft segment consists of a long, flexible diol called a polyol and can be a polyether, polyester or in our case a polycarbonate. The hard segment directly impacts the final hardness of the polymer as well as imparts the excellent physical and mechanical properties. The soft segment accounts for the elastic and flexible nature of the final polymer. The figure below shows a representation of the hard and soft segments.
Polyurethanes were discovered in 1937 by Otto Bayer and were produced on a commercial scale in the 1940’s. The first thermoplastic polyurethanes (TPU) became available in the 1950’s, and by the 1960’s many TPU’s were available from companies such as DuPont
(Lycra®), B. F. Goodrich (Estane®), Mobay (Texin®), Upjohn (Pellethane®), Bayer
2
LONG CHAIN DIOL (HIGH MOLECULAR WEIGHT) (SOFT SEGMENT)
SHORT CHAIN (LOW MOLECULAR WEIGHT) (CHAIN EXTENDER/CROSSLINKER)
REST OF THE DIISOCYANATE
URETHANE GROUP
HARD SEGMENT
Figure 2: Segmented Polyurethane
(Desmopan®), and Elastogran (Elastollan®) (Meckel 1987) (Hepburn 1987). Thermoplastic polyurethanes are linear segmented block copolymers. Their unique mechanical properties result from two phase morphology: the separation of hard segments made of the diisocyanate/chain extender from the soft segment made of aliphatic oligomeric diol. The hard segments serve both as reinforcement sites and physical crosslinks greatly influencing modulus, hardness and tear strength of polyurethane. The soft segments contribute to both the elastic and mechanical properties to polyurethanes (Kull, et al. 2015) (Meckel 1987) (Hepburn 1987) (Velankar 1998)
(Frick and Rochman 2004) (Culin, et al. 2004). A combination of properties such as high tensile strength and ultimate elongation, excellent toughness, abrasion and tear resistance, low compression and tensile set, low temperature performance, and resistance to oil have allowed polyurethanes to be used in many demanding applications including: automotive, industrial and printing rolls, cables and wires, sealants and adhesives (Kull, et al. 2015) (Meckel 1987)
(Hepburn 1987) (Wirpsza 1993). The polyaddition reaction can be easily adapted to produce a wide variety of desired properties by adjusting hard and soft segment length and composition
(Wirpsza 1993) (Liaw 1997) (Sanches- Adsuar 2000)
3
Excellent biocompatibility and biostability of TPU combined with their softness without the use of potentially extractable plasticizers have made them an important part of the medical device market. Beginning in the 1950’s they have been used in applications as diverse as medical tubing, catheters, prosthetic valve leaflets, arteriovenous access grafts and electrical insulation on pacemaker electrodes (Kull, et al. 2015) (Hsu and Lin 2004) (Ioan and Stanciu 2001) (Howard
2002) (Khan, et al. 2005) (Christenson, et al. 2004) (Wiggins, et al. 2003) (Tanzi, et al. 1996)
(Puskas and Chen 2004) (Hsu and Lin 2004). The first generation polyurethanes used in medical industry consisted mostly of poly(ester urethanes) (Ioan and Stanciu 2001) (Howard 2002)
(Khan, et al. 2005). However, rapid hydrolysis of polyester soft segment made them useless for long-term use. Second generation of biomedical polyurethanes is made of poly(ether urethanes) and was extensively used in medical industry for the last 20 years (Kull, et al. 2015)
(Christenson, et al. 2004) (Wiggins, et al. 2003) (Tanzi, et al. 1996). TPU containing ether linkage possess excellent hydrolytic stability but susceptible to oxidative degradation leading to chain scission and crosslinking (Kull, et al. 2015) (Ioan and Stanciu 2001) (Christenson, et al.
2004) (Wiggins, et al. 2003) (Tanzi, et al. 1996). Poly(carbonate urethanes), a new class of polyurethane elastomers that does not possess ether linkages have recently gained significant interest as biomedical materials. Several studies have shown that poly(carbonate urethanes) are more biostable than compatible poly(ether urethanes) while exhibiting good mechanical and surface properties (Howard 2002) (Khan, et al. 2005) (Christenson, et al. 2004) (Wiggins, et al.
2003) (Tanzi, et al. 1996) (Puskas and Chen 2004) (Hsu and Lin 2004).
One of the barriers to wider use of poly(carbonate urethanes) has been the commercial unavailability of soft grades (below 75 Shore A). This study focuses on the development of ultra soft poly(carbonate urethane) using a novel soft segment (Moore, et al. 2003). A typical
4 polycarbonate polyol is a crystalline solid at room temperature. These types of polycarbonates yield polyurethane elastomers that are tough, but also stiff; this is caused by the tendency of the soft segment to crystallize. By the use of modified low crystallinity polycarbonate copolymer diol depicted in Figure 1 we have been able to demonstrate the ability to maintain excellent physical and mechanical properties in hardness ranges between 60 and 70 Shore A. The low crystallinity of these novel polycarbonate copolymers allows for their use in systems that are liquid at room temperature and prevents cold hardening of the resulting polyurethane. The structure/property relationships of thermoplastic polyurethane elastomers based on polycarbonate copolymer diols with 2,000 molecular weight were studied.
Polyimides
Polyimides are a class of high temperature resistant polymers that are most frequently used for structural engineering purposes. The first description of araomatic polyimides was first described by Endrey in 1962 and subsequently in several patents by Dupont (Endrey 1962) (A. L.
Endrey 1965). Indeed they are mechanically, chemically and also thermally resistant. Polyimides are a class of thermally stable polymers that are often based on stiff aromatic backbones. They are most frequently used because of their thermal stability and good mechanical properties. Polyimides offer a mean to a higher dielectric constant material by the introduction of a polar group in the polymer backbone and are thermally stable at temperatures exceeding °C. Their resistance is so great that these materials often replace glass and metals like steel in highly demanding industrial applications. Polyimides are also used in some everyday life applications. They are used for the reinforcements and chassis in some cars, as well as for some parts located under the hood because they support intense heat, corrosive lubricants, fuel, cooling liquid that requires a car. They are also used in the construction of many appliances, as well as for some baking dishes for microwave
5 and for food packaging because of their thermal stability, resistance to oils, greases, fats and transparent to microwave radiation. They can also be used for the plates of integrated circuits, insulation, protective clothing fibers, composites and adhesives (Regnier and Guibe 1997) (Li, et al. 1992) (Lua and Su 2006) (Yang, et al. 2012) (Ghosh 1996).
The chemistry of polyimides is in itself a vast area with a large variety of monomers available and several methodologies available for synthesis. However, there has been considerable debate on the various reaction mechanisms involved in different synthesis methods (Xia, et al.
2013) (Hsiao, Hsiao and Kung 2016) (Han, Fang and Zuo 2010). Although polymerization of a dianhydride and a diamine to create the polyamic acid intermediate is generally simple, certain monomer choices do not react well causing either a low molecular weight or the need for catalysts and special reaction conditions to obtain product with measurable molecular weight (Han, Fang and Zuo 2010) (Myung, Kim and Yoon 2002). They also have rather unique properties for small ion or molecule diffusive transport. We have an interest in lithium ion transport in a conductive separator matrix for batteries without short circuiting since these materials have low electrical conductivities (Baldwin, et al. 2013) (Hsiao, Hsiao and Kung 2016). Also possible are materials for selective gas transport and fuel cell membranes (Qi, et al. 2015) (Kiatkittikul, Nohira and
Hagiwara 2015). Although polyimides are relatively expensive compared to most polymer classes, they can be tailored at the molecular level with a much broader combination of controllable properties than any other class. This makes them very useful when a unique and specific target is required. This review however, covers only the important fundamentals regarding the polyimide synthesis. This review will discuss ‘aromatic’ polyimides as they constitute the major category of such materials. The nature of these types of polyimides makes them very rigid with a very high melting point which limits the processing options (Pei, et al. 2013) (Lua and Su 2006) (Li, et al.
6
2004) (Takassi, et al. 2015) (Regnier and Guibe 1997). We have designed polyimides that will have a lower melting point using a range of molecular weight aliphatic diamines. A common dianhydride, pyromellitic dianhydride (PMDA) is reacted with various aliphatic diamines to produce lower melting, flexible polymers. Secondly, the properties of these polyimides can be dramatically altered by minor variations in the structure. The subtle variations in the structures of the dianhydride and diamine components have a tremendous effect on the properties of the final polyimide. We have designed a series of polyimides that will have a lower melting point using a range of molecular weight aliphatic diamines (Jeffamines D230, D400, D2000 and D4000). The polyimide is composed of aromatic and aliphatic diamines to obtain the flexibility and rigidity optimal for processing on conventional thermoplastic equipment such as injection molding or extrusion. The stability seen in the conventional fully aromatic polyimides, however, comes at the price of difficulty in processing for use in their myriad high-performance roles due to highly elevated glass transitions or melt temperatures (Pei, et al. 2013) (Lua and Su 2006) (Li, et al. 2004)
(Takassi, et al. 2015). Typically, these conventional PIs are first synthesized as polyamic acids and coated as thin films on a surface to be imidized ad hoc by heat treatment to form the finalized polyimide (Noda, et al. 2014) (Takabayashi and Murakami 2014). To circumvent this shortcoming we are reporting here the successful incorporation of polyetheramine backbone linkers which provide flexibility and break up the otherwise rigid aromatic main chain of conventional polyimides. The polyetheramine-polyimides synthesized in this work yield a functional thermoplastic material which can be molded or extruded into shapes necessary for diverse uses, thus negating the need for the highly limiting thin-film casting methodology of conventional PIs.
The glass transition temperatures can be controlled both with the stoichiometric ratio between
TMMDA and polyetheramine, along with the molecular weight of the polyetheramine itself.
7
Chapter 2
New Generation of Ultrasoft Non-Blocking Polyurethanes with High Mechanical
Properties for Biomedical Applications
Five thermoplastic polyurethane elastomers were synthesized and compared. The isocyanate portion of the hard segment was methylene bis(4-cyclohexylisocyanate) (H12MDI or
Desmodur W) purchased from Bayer. Polyurethane elastomers based on H12MDI are known for their excellent light stability and hydrolysis resistance (19). The chain extender was 1,4 butanediol purchased from Dupont.
Figure 3: Formation of Polycarbonate Copolymer. R is either x or y structure
8
The modified polycarbonate copolymer PES EX-619 (purchased from Hodogaya
Chemical Co., Ltd.) was used as soft segment. Reaction components were dried to less than
500ppm of water prior to mixing. No further purification techniques were used (Kull, et al.
2015).
Polyurethane Calculations
Polyurethane chemistry and the calculations used are not like other typical chemistry calculations in that most chemical reactions are calculated using moles whereas urethane chemical reactions use equivalence. When the molecular weight of a material is divided by the number of places or functional groups on the molecule where reactions or bonding can take place, this is known as the equivalent weight of the material. The equivalent weight tells you how many grams of a material you need to have one equivalent of reactive groups. One equivalent weight of isocyanate (-N=C=O) will always react with one equivalent weight of hydroxyl (-OH). The functionality refers to the number of reactive sites per molecule. The functionality is an average calculated using the molecular weight and the weight percent of each material. The following formulas will demonstrate the process by which the required amounts of diisocyanate and diol are determined. A typical urethane consists of a diisocyanate, one or more long chain polyols and a short chain diol often referred to as the chain extender. Polyurethanes can be prepared by two different paths, one called the one shot method and the other called the two shot method.
In the one shot method, the isocyanate is used as received from the supplier and the polyol parts by weight will include the chain extender. When the number of hydroxyl groups equals the number of isocyanate groups you have a stoichiometric ratio of 1.0. It is typical to
9 over index the isocyanate to insure that all the hydroxyl groups get reacted. So in the formula, there is a component to set the isocyanate at a ratio greater than one. It is typical to use an index of 1.03
ONE SHOT FORMULA
Total Weight of Isocyanate required =
푝푏푤 푝표푙푦표푙 퐴 푝푏푤 푝표푙푦표푙 퐵 푝푏푤 푝표푙푦표푙 푁 푝푏푤 퐻2푂 (index)(Isocyanate eq. wt.) [ + + ⋯ + + ] 퐸푞.푤푡.푝표푙푦표푙 퐴 퐸푞.푤푡.푝표푙푦표푙 퐵 퐸푞.푤푡.푝표푙푦표푙 푁 퐸푞.푤푡.퐻2푂 Equation 1: Urethane Calculation
In the two shot method, the isocyanate percent is known and will be used to make a prepolymer. If the isocyanate is 33% NCO and a 20% prepolymer is desired the calculation below will yield the necessary amount of long chain diol needed to form the prepolymer.
Total Weight of H12MDI required to make a 20% prepolymer
푁(푋+푌) H12MDI = X + 42 ( )−푁 푋 Equation 2: Prepolymer Calculation
Where: N = desired NCO of the prepolymer (expressed as fraction) X = equivalent weight of the isocyanate Y = eq. wt. of the polyol (or average equivalent weight of the polyol blend)
0.2(132+1000) Total H12MDI needed = 132 + 42 = 2050.6 pbw H12MDI ( )−0.2 132
So, 1000 x 100 = 32.8% Carbonate diol 1000+2050.6
10
2050.6 And x 100 =67.2% H12MDI 1000+2050.6
Once the prepolymer is made, formula 1 is used to calculate the remaining amounts of diols necessary to make the urethane. The key to making a soft thermoplastic polyurethane is to control the hardsegment. As mentioned earlier, urethanes are segmented polymers consisting of the hardsegment and the softsegment. It is important to know the amount of hardsegment in each polymer as it directly affects the overall hardness of the final polymer. Using the calculation below we are able to determine the percent hardsegment knowing it is comprised of the diisocyanate and the short chain diol.
Calculate % Hard Segment Assume 1000g batch 297.3 g prepolymer of which 199.79 is H12MDI 671.1 g polyol 31.6 g BDO
199.79 + 31.6 =231.39
% Hardsegment = 231.39 = .23139 x 100 = 23.14 1000 Equation 3: Percent Hardsegment
The thermoplastic polyurethanes were prepared either via a one shot reaction or by preparing an isocyanate terminated prepolymer (the two shot method). The one shot method consists of reacting the –OH or hydroxyl groups of the polycarbonate copolymer and chain extender with the –NCO groups of the diisocyanate in the appropriate equivalent weight ratio in one step. The two shot method consists of first making a prepolymer by reacting some of the –
11
OH or hydroxyl groups of the polycarbonate copolymer with the –NCO or diisocyanate groups of the isocyanate to generate an –NCO terminated prepolymer having a certain % NCO. Then, the remaining –OH or hydroxyl groups of the polycarbonate copolymer and chain extender are reacted with the –NCO terminated prepolymer at the appropriate equivalent weight ratio. The reaction scheme is shown in Figure 4.
Figure 4: Urethane Polymerization Scheme
The hard segment to soft segment ratios were varied from 20.4 to 27.6 % by weight in the one shot method and the results compared. We also compare the one shot method verse the two shot method while holding the hard segment content constant.
12
One Shot Method
Into a 2L reactor equipped with constant nitrogen blanketing and a heating mantle with controlled temperature and mixing, the polycarbonate copolymer is charged and the temperature is maintained between 60 and 80°C. The low molecular weight chain extender is then added and mixed. The processing aids and stabilizers are added and the mixture is mixed until homogenous. A small amount of catalyst (typically stannous octoate) is added. Once the mixture is homogeneous, a slight stoichiometric excess of isocyanate is added to the mixture while mixing. The resulting thermoplastic urethane is then placed in an oven at 107°C for two hours for curing and then the temperature is dropped to 93° C for several days while post curing.
The post curing is continued until the absence of isocyanate groups absorption at 2264 cm-1 was confirmed by FTIR spectrometry. All samples were aged seven days before continuing (Kull, et al. 2015).
Two Shot Method
Step 1 - The isocyanate is charged into a 2L reactor equipped with constant nitrogen
blanketing and a heating mantle with controlled temperature and mixing. The
polycarbonate copolymer is then added to the isocyanate targeting a specific % NCO. A
small amount of catalyst (typically stannous octoate) is added. The mixture is heated to
100° C and allowed to react for several hours. The result is an isocyanate terminated
prepolymer.
Step 2 - The polycarbonate copolymer is charged into a 2L reactor equipped with
constant nitrogen blanketing and a heating mantle with controlled temperature and
mixing, and the temperature is maintained between 60-80°C. The low molecular weight
chain extender is then added and mixed. The processing aids and stabilizers are added
13
and the mixture is stirred until homogenous. A small amount of catalyst (typically
stannous octoate) is added. The prepolymer is then added to this mixture in a slight
stoichiometric excess and mixed thoroughly. The resulting thermoplastic urethane is
them placed in an oven at 107°C for two hours for curing and then the temperature is
dropped to 93°C for several days while post curing. The post curing is continued until
the absence of isocyanate groups absorption at 2264 cm-1 was confirmed by FTIR
spectrometry. All samples were aged seven days before continuing (Kull, et al. 2015).
Particle Size Reduction
The TPU made by both processes was guillotined into small chunks and frozen at-18° C.
The frozen chunks were then ground and dried in a vacuum oven at 55°C for 12 hours. Tensile sheets and compression buttons were produced by injection molding. The molded samples were aged for seven days before testing.
Testing
Standard physical property tests were performed using the ASTM methods common in the US:
- Hardness: ASTM D2240 Shore A hardness
- Tensile stress/strain properties at 23° C: ASTM D412C, tensile strength at break, tensile strength at 100%, 200% and 300% elongation, elongation at break and tensile set at break.
- Tear Strength: ASTM 624 - 00
- Melt Flow ASTM D123 8
- Density ASTM 274
14
- Abrasion Resistance (Rotary Drum Abrader): ASTM D5 963-97a, volume loss in mm3
- GPC – Molecular Weight
- Small Angle Xray (SAX) Rigaku MicroMax-002+ generator and Cu anode tube.
- FTIR – Fourier Transform Infrared Spectroscopy
- Differential Scanning Calorimetry (DSC): ASTM D3418-03
- Modulated Differential Scanning Calorimetry MDSC Q200
- Thermogravimetric analyzer TGA Q500/50 –
- Dynamic Mechanical Analyzer (TA Instruments) -2980
- Hemolysis – Rabbit Blood: ISO 10993-4,2002
- MEM Elution Cyytotoxicity USP 27, NF 22, 2004, ISO 17025, 1999
- Class VI Plastics – USP 27, NF 22, 200
Instruments
Hardness measurements were done using Pacific Tranducer Corp. Durometer Model
470.
A Tensiometer measured tear strength, tensile, elongation and modulus and were all done with Alpha technologies tensiometer 10K.
Melt Flow measurements were done with Tinus-Olsen Extrusion Plasto Meter Model UE-
4-78.
15
Abrasion resistance was measured with Hampden Din Abrator.
Gel Permeation Chromatagraphy (GPC) was determined using a PerkinElmer LC
200 with using THF as the Eluent and standard polystyrene as the reference
Infrared spectroscopy scans were collected with a PerkinElmer Spectrum 2000 using an
ATR cell to scan thin film samples from 4000 to 600 cm-1 at a resolution of 4 cm-1.
SAXS data was collected on a Rigaku with a MicroMax-002+ generator, a Cu anode tube and a 120mmDET detector and a wavelength of 1.45Å. Two traces were recorded. One trace was recorded 7 days after compression molding; a second trace was recorded 32 days after after molding.
A 2980 Dynamic Mechanical Analyzer (TA Instruments) was used for the dynamic mechanical analysis (DMA). The loss and storage moduli, E” and E' respectively were probed.
The sample was run in tension with 5mm displacement at frequencies between 1 and 100Hz at a temperature range of -150 to 150°C. The frequency sweep was conducted at 5 degree increments.
The sample size was 12.9mm length, 5.3mm width, 1.2mm thick.
The Q500/50 TGA is a research grade thermogravimetric analyzer, whose leading performance arises from a responsive low-mass furnace; sensitive thermobalance, and efficient horizontal purge gas system (with mass flow control).
The Q2000 DSC is a research-grade with unmatched performance in baseline flatness, precision, sensitivity, and resolution. Tzero™ and Modulated DSC® technologies, a reliable 50- position autosampler, and multiple new hardware and software features
16
The DSC 2920 differential scanning calorimeter (TA Instruments) was used to characterize the thermal behavior of the samples. Dry nitrogen gas with a flow rate of 75 ml/min was purged through the sample cell. Cooling was accomplished with the liquid nitrogen cooling accessory (LNCA). Indium was used for temperature calibration. For all DSC experiments a temperature ramp of 5C/min was used. Experimental data was recorded from the second heat to erase previous thermal history of the samples.
Atomic Force Microscopy Surface studies were performed with Digital Instruments atomic force microscope using tapping mode and phase imaging. The images were acquired under ambient conditions with standard silicon tapping tip on a beam cantilever. Thin films used for AFM studies were prepared by casting from 1% bwt. THF solutions. Several drops of polymer solution were placed on glass slides and dried in a vacuum oven at 60ºC for 24 hours.
Results and Discussion
Hardness
The Shore A hardness of the PCPU is 64. This is an unusually low value considering the fact that the polymer did not exhibit blocking as described below. One of the barriers to wider use of PCPUs has been the availability of soft grades (below 75 Shore A). Our research focuses on bridging this gap by synthesizing a soft PCPU with a <70 Shore A value by utilizing the soft segment derived from the modified poly(carbonate) polyol [17] combined with effects of incorporating 4,40-diisocyanate dicyclohexylmethane into the hard segment. By using the modified, low crystallinity polycarbonate copolymer diol we have been able to demonstrate the ability to maintain excellent physical and mechanical properties in hardness ranges between 60 and 70 Shore A. The low crystallinity of these novel polycarbonate diols allows for their use in
17 systems that are liquid at room temperature and prevents cold hardening of the resulting polyurethane. Kultys et al. [80] summarized hardness properties of PCPUs as they relate to structure. Soft PCPUs with low moduli of elasticity are obtained in commercially aliphatic
H12MDI/butane diol hard segment polymers. The ChronoFlex_ polymers referenced however still have Shore A hardness values that range from 75 to 80. Despite the relative softness our sample did not show the typical tendency to fuse together usually referred to as blocking when virgin surfaces were merged. TPU’s which are made in soft grades (75 Shore A or less) have a tendency to permanently fuse or block to themselves. Our results for this test determined that the surface force for the TPU is <0.44 N, the minimum force resolution of the instrument, showing that there is no tack between the surfaces. This is due to the fact that high surface free energy hydrogen bonding sites move into the matrix after sample preparation as described above.
Tensile/Elongation
Thermoplastic polyurethanes studied in this paper demonstrate softness with Shore A values of 70 or lower. These materials are the first commercially available TPU’s that have shown no tendency to fuse together, usually referred to as blocking. The grades discussed in this paper do not possess the very high degree of surface tackiness observed in previous soft polyurethanes. The standard test procedure for testing surface tackiness is the BFGoodrich tack test. This procedure requires the surfaces of the sample be forced together with the contact area controlled. The applied force is held constant for a given dwell period. After the dwell period the surfaces are separated at a controlled rate and this separation force is recorded. The minimum force resolution is 0.1 lbf. An independent testing lab has determined that the TPU is
<0.1 lbf and concluded that there is no tack between the surfaces. Despite their softness, however, the thermoplastic polyurethanes described in this paper retained excellent thermal and
18 mechanical properties (Abraham, Frontini and Cuadrado 1997) (Gorman, et al. 1997) which are summarized in Table 3.
Table 1: Thermoplastic Polyurethane Physical Properties
Units Experiment number 28062 28162 28172 28203 28221
PES
Composition: polyol PES EX619 PES EX619 EX2000 PES EX619 PES EX619
isocyanate DES W DES W DES W DES W DES W
% hard segment 25.7 20.4 23 23 23
Process type One shot One shot One shot Two shot One shot
Shore A Durometer 70 60 65 64 66
Psi Tensile Strength 1792 1497 1866 3067 1574
Psi 100% Modulus 380 283 330 357 328
Psi 200% Modulus 579 349 468 534 475
psi 300% Modulus 862 443 653 813 680
19
Table 2: Thermoplastic Polyurethane Physical Properties
Experiment number Units 28062 28162 28172 28203 28221
Elongation at break % 452 549 576 492 506
Glass Transition (Tg), °C -29.2 -24.1 -29.9 -23.2 -24.6
Tensile Set, % 97 96 98 97 97
Tear Strength, Die C, pli 155 176 218 208 214
Specific Gravity Ratio 1.12 1.12 1.15 1.13 1.14
Abrasion Resistance, mm3 loss 38.0 32.4 32.5 36.0 32.8
Melt Flow Rate, Condition 228/2.16 g/10min 10.8 9.98 9.10 10.9 10.6
Despite their softness, however, the urethanes described in this paper retained excellent thermal and mechanical properties which can be varied by adjusting the concentration of hard segment, soft block composition, and processing techniques to finely tune to the desired properties of the end product. As was expected, an increase in hard segment concentration results in an increase in tensile strength of corresponding thermoplastic polyurethanes. In addition, polyurethane produced by a two-shot method yields higher tensile strength value due to the more even distribution of hard segment within the polymer matrix. When comparing the two shot method verse the one shot method while holding the percent hard segment constant we see a two-fold increase in tensile strength while maintaining nearly the same hardness and elongation.
20
The tensile strength at intermediate elongation values such as 100 or 300% (often referred to as modulus) was measured for all samples. For polymers with the same composition of soft block the modulus depended directly on the concentration of hard segment. The two-shot sample exhibited higher modulus, but this is a consequence of the sample’s high tensile strength.
The EX2000 sample showed slightly lower modulus than that of EX619 sample with the same hard segment concentration, even though the ultimate tensile strength for EX2000 sample is significantly higher.
Tensile set is the primary tool used to determine a polymer’s elastic properties. All samples showed excellent recovery to their original dimensions after break with tensile set values ranging from 96 to 98%. The most important mechanical property is the abrasion resistance, which measures the ability of polymer to work under friction for prolonged time periods and becomes especially vital for soft polyurethanes used in medical tubing. The abrasion resistance numbers (in mm3) range from 32 to 38. These abrasion resistance values approach values that are equal to or better than other urethanes that are much harder.
It can be seen that in all examples, the 300% modulus is less than 50% of the ultimate tensile strength and some samples approach 25%. Due to the high resistance to abrasion and other superior properties, such as high burst strength, it may be possible to make tubes with much thinner walls without sacrificing performance.
Molecular Weight Determination (GPC)
A series of thermoplastic polyurethanes having a weight percent hardsegment ranging between 20 and 25% were synthesized using hydrogenated MDI and a 2000 molecular weight polycarbonatediol. The molecular weights were determined and are listed in the following table.
21
Table 3: Molecular Weight of TPU's
Sample ID Mw Polydispersity Mw/Mn
28162 256507 1.86
28172 203239 1.94
28221 172801 1.78
28203 268264 2.16
28062 180223 1.66
Figure 5: Molecular Weight Graph
22
All polymers were cured in an oven at 190°C until no additional molecular weight growth was seen via gel permeation chromatography technique described.
FTIR
Excellent mechanical properties may partially result from the strong hydrogen bonding in polyurethane molecules. In polycarbonate-polyurethane segmented copolymers, the urethane N-
H can bond to urethane C=O groups or to –C=O and –O- groups of polycarbonate as depicted in the following illustration (Yen and Hong 1997) (Furukawa, Shiiba and Murata 1999) (Wilhelm and Gardette 1998) (Furukawa and Wakiyama 1999).
Figure 6: FTIR Hydrogen Bonding Depiction
23
This creates physical cross-links between both hard segment molecules and hard segment-soft segment molecules increasing the overall performance of polymer. Hydrogen bonding in polyurethanes is observed by means of IR spectroscopy which exhibits separation of free and bonded –N-H and –C=O groups (Yen and Hong 1997) (Furukawa, Shiiba and Murata
1999) (Wilhelm and Gardette 1998) (Furukawa and Wakiyama 1999). An IR study of five polyurethane samples presented in this paper revealed free and bonded –N-H stretch absorptions at 3379 cm-1 and 3305 cm-1, respectively. Carbonyl absorptions also showed a separation into free and bonded state at 1739 cm-1 and 1710 cm-1, respectively. Unfortunately, it is not possible to separate urethane and carbonate –C=O absorptions. However, DSC data suggests partial mixing of hard and soft segments which in turn will open possibility for hydrogen bonding between molecules of different segments.
Hydrogen bonding in polyurethanes has been thoroughly studied via FTIR in the C=O and N-H stretching regions (Hwang, et al. 1984) (Tanaka, Yokoyama and Yamaguchi 1968)
(Seymour, Estes and Cooper 1970) (Martin, et al. 1996) (Tsai, Yu and Teng 1998) (Kim, et al.
1999). IR absorption of hydrogen bonded carbonyl groups occurs at lower frequencies than that of free urethane carbonyl groups. Hydrogen bonded carbonyl groups in PUs absorb from 1695 to
1719 cm_1, whereas free carbonyl groups absorb from 1731 to 1733 cm_1 (Tsai, Yu and Teng
1998). N-H hydrogen-bonded-stretching occurs at 3329–3324 cm_1, while stretching in free N-H occurs at 3446–3441 cm_1 (Tsai, Yu and Teng 1998). In PCPUs the carbonate C=O vibrations occur at 1737–40 cm_1; this masks free carbonyl group stretching in the urethane groups (Fare, et al. 1999) (Eceiza, et al. 2008) (Ma, et al. 2011) (Kultys, Rogulska and Pikus 2012). Kultys et al. formulated PCPUs containing poly(hexane-1,6-diyl carbonate) diol (PHCD), 4,40 diphenylmethane diisocyanate and novel methylenebis(1,4-phenylenemethylenethio)dialcanol
24 chain extenders (Kultys, Rogulska and Pikus 2012). At 20% PHCD, they observed non- hydrogen-bonded urethane carbonyl stretching at 1734–1731 cm_1 and hydrogen bonded urethane carbonyl stretching at 1706 cm_1. Carbonate carbonyl stretching was negligible. At higher carbonate concentrations the absorption of carbonate carbonyl at 1743–39 cm_1 masked the non-hydrogen-bonded carbonyl stretching.
Hydrogen-bonded urethane carbonyl stretching diminished as the polycarbonate diol increased. The diminishing hydrogen bonding was attributed to the fact that the polycarbonate diol exhibits a low degree of phase separation in the urethane as compared to similar formulations made with poly(oxytetramethylene) diol (Eceiza, et al. 2008). A synthesized
PCPUs containing 4,4-diphenylmethane diisocyanate, 1,4-butanediol chain extender with polyhexamethylene carbonate diols and polyhexamethylene–pentamethylene carbonate diols
(Eceiza, et al. 2008). Here, again, the free carbonyl stretching is masked by the polycarbonate stretching at 1737 cm_1. Disordered hard segment domains are accompanied by carbonyl stretching vibrations observed at 1718 cm_1 in polymers containing polyhexamethylene carbonate soft segments. These are thought to be due to hydrogen bonds between carbonate carbonyl groups near the hard segments and -NH groups in the hard segments. The number of these hydrogen bonded group’s increases as the molar mass of the soft segment decreases. The ability of the carbonate carbonyl groups to hydrogen bond with the -NH groups is much higher in formulations containing polyhexamethylene carbonate soft segments than in those containing poly hexamethylene–pentamethylene carbonate soft segments. This is attributed to the fact that there is a better fit between carbonate carbonyl groups and –NH groups in polymers with even numbers of carbon atoms in both the soft and hard segments. Indeed, Pongkitwitoon et al., report larger interdomain spacings in PCPUs containing even numbers of carbon atoms (140 Å) that we
25 report for our PCPU containing odd–even numbers of carbon atoms (45 Å) (Pongkitwitoon, et al.
2009). Fig. 7 show FTIR data for the -NH and AC=O absorptions. As observed in the research described above, carbonate carbonyl absorption is noted at 1740 cm_1 while the absorption peak at 1718 cm_1, due to associated groups in somewhat disordered regions can be interpreted, in light of Ref. (Eceiza, et al. 2008)to be due to hydrogen bonding between -NH groups in the hard segments and nearby carbonate carbonyl groups. The polyol used in this research contains an odd number of methylene groups; however, there is still evidence of hydrogen bonding with the hard segment -NH. N-H hydrogen-bonded-stretching occurs at 3340 cm_1, while stretching in free N-H occurs at 3380 cm_1. A shifting of N-H bonded stretching to higher wavenumbers indicates a disruption in H-bonding in polycarbonate containing urethanes as compared to nonpolycarbonate systems (Tsai, Yu and Teng 1998). All of this reflects the fact that hydrogen bonding occurs between the soft and hard segments and that phase separation is diminished in
PCPU structures at low hard segment contents. The domain structure of 45 Å observed in SAXS is likely a hybrid due to partial hard and soft segment mixing. This ultrasoft PCPU may undergo segmental diffusion and hydrogen bond formation during the self-healing process. The lack of well-defined crystalline domains and ample mobility may be responsible for the partial self- healing.
26
Figure 7: FTIR Hydrogen Bond N-H Stretch and Non Hydrogen Bonded N-H stretch
27
Figure 8: FTIR Hydrogen Bonded Carbonyl Stretch and Non Hydrogen Bonded Carbonyl
Stretch
28
Small Angle Xray (SAXS)
1
sec
- 2
0.1 I counts/m I
0.01 0 0.05 0.1 0.15 0.2 0.25 0.3 q (A-1)
Figure 9: SAXS of PCPU 7 Days after Molding
The SAXS data (Figure 9) revealed a peak at 0.14 Å-1. According to the Bragg’s Law,
2dsinθ = nλ
Equation 4: Bragg's Law
and then d = 2π/q where d is the estimated interdomain spacing and q is the scattering vector.
When q=0.14 Å-1, d=44.88 Å. The data for the sample aged 32 days exhibited at peak at 0.14 Å-1 as well. However the intensity increased slightly from 0.06476 intensity units to 0.06832 upon aging for 32 days. The SAXS data evidences an interdomain "center-to-center" distance of 45Å.
Interdomain spacings in polyurethanes are usually of the order of 100Å. For example,
Pongkitwitoon et al, studied polymers made from 4,4-methylenediphenyl diisocyanate and 1,4- butanediol, and soft segments from an aliphatic polycarbonate [poly(1,6-hexyl 1,2-ethyl carbonate)] (Pongkitwitoon, et al. 2009). They reported interdomain spacings of about 140 Å near room temperature. It is known that polyurethanes, in general, have microphases containing
29 mixed hard and soft segments (Eceiza, et al. 2008). This is especially true of PCPUs which often exhibit a high degree of mixing in the phases due to hydrogen bonding between urethane groups and carbonate groups in the soft segments (Gunatillake, et al. 1998) (Trovati, et al. 2010)
Dynamic Mechanical Analysis, DMA
DMA was used to characterize the viscoelastic properties of the polymer. Figure 7 depicts traces of tan δ, E’ and E” versus temperature. Two transitions which are identified by peaks in the tan δ and E” traces. Note that the low temperature moduli exceed 1010 Pascals.
This is uncommon in organic polymers and is likely due to machine compliance problems encountered at low temperatures. The data does depict reasonable transition data and is not intended for an accurate determination of moduli in this low temperature region. Fig. 7. E',
E” and tan delta vs. temperature. The higher temperature relaxation due to large scale chain slippage occurs at the glass transition region [61]. At lower temperatures a secondary relaxation is noted as well. McCrum discusses relaxations noted in polyurethanes (McCrum, Read and
Willeams 1967). Three relaxations are noted, a relaxation and two secondary relaxations, b and g. The higher temperature b relaxation at -100 to -50 °C is attributed to absorbed water molecules and was not observed in our studies. A secondary relaxation is noted in figure 7 near -
120 °C in the E” and tan(delta) traces. Time-temperature superposition software was used to plot shift factor versus temperature and the resulting linear graph in figure 8 is evidence of Arrhenius behavior. Equation 3 was used by the software to calculate activation energy where log(aT) is the shift factor, Ea is the activation energy, R is the universal gas constant, To is initial temperature in Kelvin and T is temperature in Kelvin.
30
Figure 10: Dynamic Mechancal Analysis of PCPU
2.303 log (aT) = -Ea/R [1/T0 – 1/T]
Equation 5: Activation Energy
The activation energy is 51.5 kJ/mol (12.4 kcal/mol). The g relaxations in polyurethanes described by McCrum occur near -120°C and are thought to arise from local motion in the carbon chains of the soft segments (McCrum, Read and Willeams 1967). The reported observed activation energies for this local motion are from 50-63 kJ/mol (12-15 kcal/mol) and in the range our value, 51.5 kJ/mol (12.3 kcal/mol), figure 11.
31
Figure 11: Apparent Activation Energy of the Beta Transition
32
Figure 12: WLF/Time Temperature Superposition fit at Tg from DMA data
33
Alpha transitions that follow WLF behavior are traditionally analyzed via equation 4 (Wilkes
1975).
− C1(T − T g ) log(aT )= C 2+ (T − T g)
Equation 5: WLF of Alpha Tranisitions
Where C1 and C2 are material constants, aT is the shift factor, R is the universal gas constant, T is temperature and Tg is the glass transition temperature, both in Kelvin. Here, the reference peak is Tg, and experimental values for C1 and C2 (18.6 and 81.4 respectively) were found by plotting the log(aT) vs. temperature (figure 12) using a time-temperature superposition program. The apparent activation energy for the glass transition, ΔEa, is calculated from Eq 6:
C1 2 Δ Ea= (− 2.303)( )RT C 2
Equation 6: Activation Energy for the Glass Transition
The Tg showed WLF behavior and from this the apparent activation energy of the alpha transition was calculated using equation 4 is 255 kJ/mol (61 kcal/mol) and is indicative of the energy required to induce large segmental slippage associated with the glass transition and similar to that reported for a series of polyurethanes made from vegetable oil polyols, where the apparent activation energies varied from 179-209kJ/mol (Ma, et al. 2011). It is significant to note that the storage modulus ranged from 1.5 X106 – 5.6 106 Pa at temperatures up to 423K (150°C) depending on the frequency. The tan delta remained relatively constant in this region and this indicates that the material was still a solid network physically cross-linked by hydrogen bonding
34 within and possibly reinforced by the weakly ordered moieties. Additionally, DMA data indicates that rehealing studies presented below take place in a matrix with minimal phase separation and that it is possible that physical crosslinks can break with stress and reform.
(Hwang, et al. 1984) (Martin, et al. 1996) (Tsai, Yu and Teng 1998) (Kim, et al. 1999)
Thermongravimetric Analysis (TGA)
TGA is an analytical method where the mass of a substance is observed and data recorded as a function of temperature or time as the sample is subjected to a programmed temperature profile while in a controlled environment. TGA simply records a samples weight as it is subjected to temperature variations. This allows for the quantification of such properties as water loss, pyrolysis, oxidation and decomposition.
Thermogravimetric Analysis of the decomposition temperature will aid in determining the test temperature’s in other thermal techniques such as DSC and Rheometry. The following graphs shows the results from TGA is a thermal analysis technique used to determine chemical and physical changes in a materials property. All samples were tested by increasing the temperature at 10°C/Min from ambient to 650°C which helps determine the decomposition temperature. The determination of the decomposition temperature aids in the testing conditions of other techniques such DSC and Rheology.
35
Sample: XP-28162 Size: 8.7520 mg
120 2.0 346.55°C
314.48°C 100 192.26°C 1.5
80
1.0 60 102.2% (8.948mg)
Weight (%) 40 0.5
Deriv. Weight (%/°C)Deriv. Weight
20
0.0
0 466.34°C
-20 -0.5 0 100 200 300 400 500 600 Temperature (°C) Universal V4.3A TA Instruments Figure 13: TGA XP-2816
36
Sample: XP-28203 Size: 8.8310 mg
120 2.0 345.92°C
317.38°C 100 196.19°C 1.5
80
1.0
60
Weight (%) 97.39% 0.5 (8.600mg)
Deriv. Weight (%/°C)Deriv. Weight 40
0.0 20
435.29°C 0 -0.5 0 100 200 300 400 500 600 Temperature (°C) Universal V4.3A TA Instruments Figure 14: TGA XP-28203
37
Sample: XP-28062 Size: 8.3070 mg
120 2.0 340.72°C
306.52°C 100 204.17°C 1.5
80
1.0 60 103.2% (8.571mg)
Weight (%) 40 0.5
Deriv. Weight (%/°C)Deriv. Weight
20
0.0
0
500.07°C
-20 -0.5 0 100 200 300 400 500 600 700 Temperature (°C) Universal V4.3A TA Instruments Figure 15: TGA XP-28062
38
Sample: XP-28221 Size: 9.4590 mg
120 2.0
342.94°C
312.79°C 100 200.22°C 1.5
80
1.0
60
93.81%
Weight (%) (8.873mg) 0.5
Deriv. Weight (%/°C)Deriv. Weight 40
0.0 20
449.85°C 0 -0.5 0 100 200 300 400 500 600 Temperature (°C) Universal V4.3A TA Instruments Figure 16: TGA XP-28221
39
Differential Scanning Calorimetry, DSC
Four poly(carbonate)urethanes were synthesized by traditional one- and two-shot techniques. The composition and nomenclature of these samples is summarized in Table 1. The differential scanning calorimetry has been carried out. DSC thermograms are shown on Figure 3.
The glass transition temperatures of the soft segments as well as the melting temperatures of the hard segments observed on these thermograms are tabulated in Table 2. The soft segments exhibit well defined glass transition temperatures around -25C. The variations in soft segment
Tg due to the change in hard segment content are minimal indicating good phase separation in the samples (Frick and Rochman 2004) (Ioan and Stanciu 2001) (Gunatillake, et al. 1998) (Fujiwara and Wynne 2004). However, the glass transition temperatures of polyurethane samples are about
25C higher than that of pure carbonate polyol (-50C) recorded at the same experimental conditions. This indicates that there is some mixing of hard segment in soft segment microphase
(Gunatillake, et al. 1998) (Fujiwara and Wynne 2004). The temperatures of well pronounced endothermic peaks at about 65-80C are assigned to the melting point of the hard segment domain and serves as an indicator of a high crystalline order material. The position change for hard segment melting peak does not correlate directly to the hard segment content and can be attributed to differences in hard segment sequence length (Li, et al. 1992). Upon heating above the Tm hard segment dissolves completely in the soft segment matrix resulting in an amorphous melt (Li, et al. 1992) (Ryan, Macasko and Bras 1992) (Brunette, et al. 1981). Fast cooling of the amorphous melt with liquid nitrogen freezes the structure in its state resulting in the disappearance of hard segment melting peaks from the second heating curves of DSC (Li, et al.
1992) (Ryan, Macasko and Bras 1992) (Brunette, et al. 1981). Upon careful examination, a weak endotherm at about 20 °C between the Tg of the soft segment and the Tm of the hard
40 segment can be noticed. It is usually attributed to the partial mixing of the short-range ordered hard segment (Ryan, Macasko and Bras 1992) (Chen, Shieh and Chui 1998) (Brunette, et al.
1981) (Yoon and Han 2000).
Table 4: Nomenclature, Composition and Process Type
Units 28162 28221 28203 28062
Polyol PES EX619 PES EX619 PES EX619 PES EX619
Diisocyanate Desmodur W Desmodur W Desmodur W Desmodur W
% Hard segment 20.4 23.0 23.0 25.7
Process type One shot One shot Two shot One shot
Table 5: Glass Transition and Melting Temperatures Observed from DSC Thermograms
Sample 28162 28221 28203 28062
Tg soft segment (C) -26.2 -26.0 -26.3 -27.2
Tm hard segment (C) 74.8 73.9 75.6 67.0
41
Sample: XP-28162 st Size: 9.0400 mg DSC Analysis of XP 28162: 1 Heat Cycle
0 .0
F irs t H e a t
-0 .2 -3 0 .8 8 °C
-26.18°C(I)
6 0 .1 5 °C -2 0 .5 0 °C 3 .9 7 9 J /g
Heat Flow (W/g)
-0 .4 7 4 .8 0 °C
-0 .6 -9 0 -4 0 10 60 110 160 210
E x o U p Temperature (°C) Universal V4.3A TA Instrum ents
Figure 17: DSC XP-162 1st Heat Cycle
Sample: XP-28162 nd Size: 9.0400 mg DSC Analysis of XP 28162: 2 Heat Cycle
0.0 Second Heat
-0.2 -30.22°C
-25.82°C(I)
-18.43°C 56.67°C 0.9809J/g
Heat Flow (W/g) Heat Flow 81.00°C -0.4
-0.6 -90 -40 10 60 110 160 210 Exo Up Temperature (°C) Universal V4.3A TA Instruments
Figure 18: DSC XP-28162 2nd Heat Cycle
42
Sample: XP-28203 Size: 8.9300 mg st DSC Analysis of XP 28203: 1 Heat Cycle -0.1
First Heat
-31.61°C
-26.32°C(I)
-20.71°C 61.32°C 4.606J/g -0.3
Heat Flow (W/g) Heat Flow
75.56°C
-0.5 -90 -40 10 60 110 160 Exo Up Temperature (°C) Universal V4.3A TA Instruments
Figure 19: DSC XP-28203 1st Heat Cycle
nd DSC Analysis of XP 28203: 2 Heat Cycle
Figure 20: DSC XP-28203 2nd Heat Cycle
43
Sample: XP 28062 Size: 5.5800 mg st DSC Analysis of XP 28062: 1 Heat Cycle
0.05 0.05 First Heat Enthalpic relaxation
52.96°C 4.404J/g 0.00 0.00 0.00
66.61°C
Crystalline melt
-0.05 Glass transition -0.05 -0.05
-27.15°C(I)
Heat Flow (W/g) Heat Flow
51.13°C Flow (W/g) Rev Heat 5.592J/g (W/g) Heat Flow Nonrev
-25.84°C(I) -0.10 -0.10 -0.10
67.00°C
-0.15 -0.15 -90 -40 10 60 110 Exo Up Temperature (°C) Universal V4.3A TA Instruments
Figure 21: DSC XP-28062 1st Heat Cycle
Sample: XP-28062 Size: 8.7800 mg nd DSC Analysis of XP 28062: 2 Heat Cycle
0.0 Second Heat
-0.1
-31.02°C -0.2
-24.90°C(I)
-0.3 58.95°C
Heat Flow (W/g) Heat Flow -16.12°C 0.8507J/g
78.24°C
-0.4
-0.5 -90 -40 10 60 110 160 210 Exo Up Temperature (°C) Universal V4.3A TA Instruments
Figure 22: DSC XP-28062 2nd Heat Cycle
44
Sample: XP-28221 Size: 9.1400 mg st DSC Analysis of XP 28221: 1 Heat Cycle
0.0 First Heat
-0.1
-32.19°C -0.2 -26.02°C(I)
-21.18°C 59.58°C 4.805J/g -0.3
Heat Flow (W/g) Heat Flow
73.90°C
-0.4 Anomaly not related to sample. Possibly a vibration from the bench top
-0.5 -100 -50 0 50 100 150 200 250 Exo Up Temperature (°C) Universal V4.3A TA Instruments
Figure 23: DSC XP-28221 1st Heat Cycle
Sample: XP-28221 Size: 9.1400 mg nd DSC Analysis of XP 28221: 2 Heat Cycle -0.1
Second Heat
-0.2 -31.47°C
-25.84°C(I)
-16.78°C -0.3 61.27°C 0.6021J/g
Heat Flow (W/g) Heat Flow 79.46°C
-0.4
-0.5 -90 -40 10 60 110 160 210 Exo Up Temperature (°C) Universal V4.3A TA Instruments
Figure 24: DSC XP-28221 2nd Heat Cycle
45
The PCPU with 23.0% hard segment was produced by the described two-shot method and it is thought that this method produces material with more random structure leading to less microheterogeneity (Peebles 1974). DSC results are shown. We did not observe two glass transition temperatures when the sample was scanned to 165°C. An additional run was taken to
200 °C and this verified that there was not a second glass transition temperature above 165 °C.
The glass transition temperature of the PCPU decreased slightly from -26.3°C to – 25.6°C from the first to second heating cycle. The presence of one glass transition temperature indicates that there is some mixing of hard segments within the soft segment microphase (Gunatillake, et al.
1998) (Li, et al. 1992) (Liu, et al. 2003) (Chen, Shieh and Chui 1998) (Wilkes 1975) (Aklonis,
MacKnight and Shen 1972) (McCrum, Read and Willeams 1967) (Ma, et al. 2011) (Xu and
Zhang 2007) (Wu, Meure and Dolomon 2008) (Jud and Kausch 1979) (Boiko, Guerin and
Marikhin 2001). The high degree of mixing in some PCPUs has been attributed to the hydrogen bonding between urethane groups in the hard segment and carbonate groups in the polyol
(Gunatillake, et al. 1998). Further, Baer reported that when phase separations are of the order of a few tens of Angstroms, separate glass transition temperatures are not evident (Liu, et al. 2003).
The Tg of the PCPU sample is higher than that of pure carbonate polyol (-50°C) recorded under the same ramp rate. This is thought to be due to both the presence of hard segments in the soft phase and to inhibited movement caused by hard segments where the ends of the soft segments are pinned (Pongkitwitoon, et al. 2009). Additionally, one diffuse melt region was noted around
80 °C. Fast cooling of the sample with liquid nitrogen resulted in a significant peak area decrease in the melt in the second heating curve. The endotherm is barely visible and occurs in a lower temperature range in the second run. This has been observed in polyurethane studies in the past and is attributed to the partial mixing of the short-range ordered hard segment (Li, et al. 1992)
46
(Chen, Shieh and Chui 1998). This indicates that aging and processing conditions influence the microstructure of the polymer. This is well known and documented in the literature (Wilkes
1975). Additionally, we did not observe endotherm increases in DSC traces after samples were deformed. The fact that ordered structure was observed in the deformed structure in XRD, figure
4, but not easily detected by DSC indicates that our polymer does not contain well ordered, distinct hard segment regions. SAXS data did clearly depict an interdomain structure (45Å) that is attributed to hard segment-soft segment association.
Modulated Differential Scanning Calorimetry (MDSC)
Is a technique similar to DSC but has the advantages of allowing for the optimization of both sensitivity and resolution in a single test by allowing for a slow average heating rate
(optimizing resolution) and a fast changing heating rate (optimizing sensitivity). A sinusoidal modulation is overlaid on the conventional linear temperature ramp. This yields a heating profile which is continuously increasing with time, but in an alternating heating/cooling program. This allows for the Separation of complex transitions into individual components. It increases the sensitivity for weak transitions. It increases the resolution without loss of sensitivity and gives a direct measurement of heat capacity. In addition, Modulated DSC shows Enthalpy Relaxation
Endotherm or the physical ageing of a material. Amorphous materials can age or relax over time. The physical properties of amorphous materials can change with time as the sample relaxes. Aged materials show decreased physical and chemical reactivity compared to unaged materials. As a material ages, its density increases while the free volume decreases. Blue is reversible and Red in non-reversible. Green is Heat capacity (Cp). Heat capacity is the amount of joules energy needed to increase the temperature of 1 gram of material by 1°C. These materials have two phases: amorphous and crystalline. The two phases are clearly seen in the first heat, no
47 recrystallization is observed in the cool cycle. A glass transition and very little crystallinity is observed in the second heat. It can be seen that the enthalpic relaxation (physical ageing) is removed after the first heat cycle.
Sample: XP 28062 Size: 5.5800 mg st MDSC Analysis of XP 28062: 1 Heat Cycle 0.05 0.05 First Heat Enthalpic relaxation
52.96°C 4.404J/g 0.00 0.00 0.00
66.61°C
Crystalline melt
-0.05 Glass transition -0.05 -0.05
-27.15°C(I)
Heat Flow (W/g) Heat Flow
51.13°C Flow (W/g) Rev Heat 5.592J/g (W/g) Heat Flow Nonrev
-25.84°C(I) -0.10 -0.10 -0.10
67.00°C
-0.15 -0.15 -90 -40 10 60 110 Exo Up Universal V4.3A TA Instruments Temperature (°C)
Figure 25: MDSC XP-28062 1st Heat Cycle
nd Sample: XP 28062 MDSC Analysis of XP 28062: 2 Heat Cycle Size: 5.5800 mg
0.02 0.00
Second Heat
0.00 Enthalpic recovery -0.02 0.006
-0.02 -0.04
0.004 -0.04 Very little crystalline melt is observed in the second heat -30.68°C -0.06
-33.81°C -23.74°C(I) -0.06 59.74°C 0.002 Heat Flow (W/g) Heat Flow 0.2074J/g
-21.88°C(I) Flow (W/g) Rev Heat -14.63°C (W/g) Heat Flow Nonrev -0.08 -0.08 -13.26°C 75.11°C 0.000 -0.10 -0.10 57.32°C 0.7089J/g 75.11°C
-0.12 -0.12 -80 -30 20 70 120 Exo Up Temperature (°C) Universal V4.3A TA Instruments
Figure 26: MDSC XP-28062 2nd Heat Cycle
48
Sample:XP28 XP203 28221 Size: 6.0500 mg st MDSC Analysis of XP 28203: 1 Heat Cycle
0.02 0.00
First Heat 53.15°C 4.576J/g 0.00 0.002 -0.02
-0.02 0.000
-0.04
-0.04 -0.002
-0.06 Heat Flow (W/g) Heat Flow -25.66°C(I)
-28.48°C(I) Flow (W/g) Rev Heat -0.06 (W/g) Heat Flow Nonrev -0.004
67.61°C -0.08 -0.08 -0.006 51.81°C 6.312J/g
67.86°C -0.10 -0.10 -90 -40 10 60 110 Exo Up Temperature (°C) Universal V4.3A TA Instruments
Figure 27: MDSC XP-28203 2nd Heat Cycle
Sample: XP 28203 Size: 5.9200 mg nd MDSC Analysis of XP 28203: 2 Heat Cycle
0.02 0.00
Second Heat
0.00 -0.02 1.5
-0.02
-0.04 -25.97°C(I)
-0.04 1.0
-23.79°C(I) -0.06
Heat Flow (W/g) Heat Flow
-25.97°C(I) Flow (W/g) Rev Heat -0.06 (J/(g·°C)) Heat Capacity
0.5 56.42°C 0.6180J/g -0.08 -0.08
74.15°C
-0.10 -0.10 -80 -30 20 70 120 Exo Up Temperature (°C) Universal V4.3A TA Instruments
Figure 28: MDSC XP-28203 2nd Heat Cycle
49
Sample: XP 28162 Size: 6.2500 mg st MDSC Analysis of XP 28162: 1 Heat Cycle 0.00 0.00
First Heat
-0.02 -0.02 54.61°C 2.912J/g 0.005
-0.04 -0.04
0.000 -28.34°C(I) -0.06 -0.06
Heat Flow (W/g) Heat Flow
Rev Heat Flow (W/g) Rev Heat
Nonrev Heat Flow (W/g) Heat Flow Nonrev 54.11°C 68.19°C 3.804J/g -0.005 -0.08 -0.08
68.96°C
-0.10 -0.10 -80 -30 20 70 120 Exo Up Temperature (°C) Universal V4.3A TA Instruments
Figure 29: MDSC XP-28162 1st Heat Cycle
Sample: XP 28162 nd Size: 6.2500 mg MDSC Analysis of XP 28162: 2 Heat Cycle
0.00 0.00
-0.02 -0.02 1.5
-0.04 -0.04
1.0
-24.91°C(I) -0.06 -0.06
Heat Flow (W/g) Heat Flow -23.54°C(I)
Rev Heat Flow (W/g) Rev Heat
Heat Capacity (J/(g·°C)) Heat Capacity 51.63°C 0.8162J/g 0.5 -0.08 -0.08 74.10°C
-0.10 -0.10 -80 -30 20 70 120 Exo Up Temperature (°C) Universal V4.3A TA Instruments
Figure 30: MDSC XP-28162 2nd Heat Cycle
50
Sample: XP 28221 Size: 6.0500 mg st MDSC Analysis of XP 28221: 1 Heat Cycle 0.02 0.00
53.15°C 4.551J/g 0.00 0.002 -0.02
-0.02 0.000
-0.04
-0.04 -0.002
-0.06
Heat Flow (W/g) Heat Flow -25.72°C(I)
Rev Heat Flow (W/g) Rev Heat
-0.06 -28.58°C(I) (W/g) Heat Flow Nonrev -0.004
-0.08 -0.08 -0.006 67.84°C 52.81°C 4.216J/g
67.61°C -0.10 -0.10 -80 -30 20 70 120 Exo Up Temperature (°C) Universal V4.3A TA Instruments
Figure 31: MDSC XP-28221 1st Heat Cycle
Sample: XP 28221 Size: 6.0500 mg nd MDSC Analysis of XP 28221: 2 Heat Cycle 0.02 0.00 Second Heat
0.00 -0.02 1.5
-0.02
-0.04 -25.27°C(I)
-0.04 1.0
-0.06
Heat Flow (W/g) Heat Flow -23.44°C(I)
Rev Heat Flow (W/g) Rev Heat
-0.06 (J/(g·°C)) Heat Capacity -25.27°C(I) 0.5 59.19°C -0.08 0.5001J/g -0.08
74.14°C
-0.10 -0.10 -80 -30 20 70 120 Exo Up Temperature (°C) Universal V4.3A TA Instruments
Figure 32: MDSC XP-28221 2nd Heat Cycle
51
Atomic Force Microscopy (AFM)
Free surface AFM images of the polyurethane with highest hard-segment content we acquired to further investigate the phase separation. It is believed that surface of the polyurethane is dominated by more hydrophilic soft segment (Garrett, Siedlecki and J. 2001)
(Aneja and Wilkes 2003) (McLean and Sauer 1997) (Tan, et al. 2004) (Revenko, Tang and
Santerre 2001). Thus, simple topography imaging usually yields featureless image of smooth surface. However, by using tapping mode imaging with increased tapping force (A/A0 = 0.7) it is possible to obtain a phase-separated phase image reflecting the presence of near-surface hard domains. AFM images indicate that sample’s morphology consists of randomly oriented domains with lateral diminutions of about 10nm. This data is compatible with domain sizes reported for solution cast thin urethane films (Garrett, Siedlecki and J. 2001) (Aneja and Wilkes 2003)
(McLean and Sauer 1997) (Tan, et al. 2004) (Revenko, Tang and Santerre 2001). The images in
Figure 35 are 500x500 nm (top) and 100x100 nm (bottom). The phase scale is 20° for the
500nm image and 10° for the 100nm image.
52
Figure 33: AFM Tapping Mode Phase Images
Scanning Electron Microscopy (SEM)
The scanning electron micrographs of the fractured surface of the two shot soft thermoplastic urethane is shown below. It is well known that thermoplastic urethanes exhibit a two phase morphology. Moreover the separation of the soft segment and the hard segment is known to occur (Paul, et al. 1999). Our SEM studies show no evidence of phase separation most likely due to extremely small segment size and surface domination by the hydrophilic soft segment.
53
Figure 34: SEM Image x1000
Figure 35: SEM Image x25000
USP Class VI Testing
The Class VI is designed to determine the biological response of animals to direct and indirect contact with the test article or injection of the test article extract. The study conformed to all applicable laws and regulations. Specific regulatory requirements included the current
FDA, 21 CFR, Part 58 – Good Laboratory Practice for Non clinical Laboratory Studies;
AAALAC, International; “Guide for the Care and Use of Laboratory Animals, “National
54
Research council, 1996. (NIH) (OLAW), “Public Health Service Policy on Humane Care and
Use of Laboratory Animals, “Health Research Extension Act of 1985 (Public Law 99-158
November 20, 1985, Reprinted 1996; USDA, Department of Agriculture, Animal and Plant
Health Inspection Service, 9 CFR Ch. 1 (1/1/95 edition), Subchapter A – Animal Welfare.
The 0.9% USP Sodium Chloride for injection (NaCl), Cottonseed Oil, 1 in 20 dilution of ethanol in NaCl and Polyethylene Glycol 400 (PEG) extracts of the test article, and the test article, XP28221 did not produce a biological response following intramuscular implantation in rabbits, intracutaneous injection in rabbits, or systemic injection in mice. Therefore, the test article meets the requirements of USP 27, NF 22, 2004, for Class VI Plastics – 70°C. Due to the cost of this evaluation only one of the samples were tested.
Conclusions
A set of novel ultra-soft thermoplastic polyurethanes using low crystallinity carbonate polyol was synthesized and characterized. All samples showed softness on or below 70 Shore A while at the same time retaining excellent mechanical properties. This includes excellent tensile strength and elongation properties and superior tensile set at break with recovery of 97%. All samples exhibited excellent abrasion resistance of 32-38 mm3 loss which is attributed to the high degree of hydrogen bonding both within hard segment and between segments. In addition tack testing for all of the samples showed no indication of sample surfaces fusing together. Testing showed high biocompatibility and low toxicity of polyurethanes discussed in this paper with no signs of biological reactivity or toxic reaction as indicated by USP Class VI, MEM Elution
Cytotoxicity and Hemolysis toxicology reports. These novel polyurethane materials are currently being tested as performance enhancing substitutes for many high demand biomedical
55 applications including but not limited to peristaltic pump tubing, balloon catheters, enteral feeding tubes and medical equipment gaskets and seals.
56
Chapter 3
Synthesis and Characterization of Novel Melt Processable Polyimide
Materials and Methods
Pyromellitic dianhydride (PMDA), 4,4’-Methylenebis (2,6-dimethylaniline) (TMMDA), gamma- butyrolactone (GBL) were used as received, and kindly donated from BrightVolt, Inc.
(Lakeland, FL). Jeffamine® D230, D400, D2000 and D4000 were provided by Huntsman, Inc.
(The Woodlands, Texas). Tin (II) ethyl hexanoate (catalyst), Sigma Aldrich.
TMMDA Synthesis and Characterization
Tetramethymethyene Dianaline was synthesized by the author in house. A purity titration, FTIR and carbon and hydrogen NMR were used to verify structure and purity. The reaction procedure is typically 2,6-dimethylaniline hydrochloride dissolved in warm 2M hydrochloric acid solution into which ½ equivalent of formalin is added slowly. Some heat of reaction is evolved and the mixture crystallizes out the hydrochloride salt of the product. A thick slurry is obtained after about four hours at 80C. The hydrochloride salt is filtered first to remove the excess acid and unreacted material. The salt will be almost white. The hydrochloride salt is suspended in a large volume of water and then 1.2 equivalents of NH4OH per hydrochloride to convert the product to the free amine. The diamine is essentially insoluble in 1 M ammonia solution. Filter to recover. The recrystallization is accomplished with ethanol/water containing some ammonia. The final crystals of diamine must be vacuum dried to be completely dry (Oleinik, et al. 2009).
57
FTIR
FTIR is the instrumental technique most often applied to structure determination of organic compounds. Although it is not as revealing of the structure as NMR spectroscopy, it is useful in identifying the presence of certain functional groups. The FT-IR spectrum below was used to confirm the TMMDA structure (Figure 36). The Tetramethyl Methylene Dianaline
(TMMDA) exhibited the characteristic N-H aromatic stretch, the primary amines produce two
-1 N-H stretch absorptions at 3392 and 3461 cm . NH2 scissoring and deformation vibrations are seen at 1625 cm−1. There are characteristic bands of aromatic rings at 1489 cm−1 for C-C stretching vibrations. Alkyl C-H Stretch vibration are seen ranging from 2851 and 3052 cm−1
(Rouchaud, et al. 1989) (NIST Chemistry Webbook 2016)
Figure 36: TMMDA FTIR
Titration
The total amount of functional end groups as diamines is titrated with standardized 0.1N perchloric acid while the milivolt reading is recorded. The TMMDA must be dried to below 500 ppm prior to performing the titration. Dissolve approximately 500 mg into acetic acid using a
58 four place analytical balance. While mixing, immerse the probe in the solution and begin titrating using the 0.1N perchloric acid recording the millivolt readings according to the following
a. 0 ml – 20 ml in 5 ml increments b. 20 ml - 30 ml in 2 ml increments c. 30 ml – 35 ml in 1 ml increments d. 35 ml – 40 ml in 0.5 ml increments e. 40 ml – 45 ml in 1 ml increments
Enter results into the table in the excel spreadsheet named TMMDA to determine the end point making an ordinary titration curve. The first derivative is then computed. The first derivative of the graph represents the slope of the graph at any particular point. The equivalence point of the titration is the point of highest slope on the titrations curve, or where the curve changes from concave to convex. The slope can be approximated by the discreet rise over run values calculated from each pair of data points. So the endpoint volume is equal to the equivalence point and N is the normality of the standardized perchloric acid.
Titratione ndpo int Volume ( ml ) TMMDA % purity N 100 Wt .( g ) Sample
Equation 7: TMMDA % Purity
NMR
The 13C-NMR of TMMDA shows 6 individual product peaks for the TMMDA (due to symmetry) as well as a reference peak from the deuterated chloroform solvent. In the aromatic region, 4 distinct peaks are seen (140 ppm, 131ppm, 128 ppm, 121 ppm) displaying the symmetry within the aromatic ring and the symmetry of TMMDA itself. The peak at 40 ppm corresponds to the methylene bridge, and the right most peak at 17.68 ppm is the combination of 4 methyl signals on the two anilines.
59
The 1H-NMR of 4,4'-methylenebis(2,6-dimethylaniline), or TMMDA further confirms the structure, showing only 4 distinct peaks due to the symmetry of TMMDA. The aromatic region (furthest left) is condensed into a very finely coupled set of peaks (a) corresponding to 4 proton signals between the two aniline rings. The second peak from the left, at 3.73, corresponds to the methylene bridge between the two anilines. The broad singlet seen at 3.46ppm integrates for 4 protons and matches with the typical shift in literature for the NH2 on an aniline ring. The furthest peak to the right, and the largest peak, is standard for a set of 4 equivalent methyl groups on the TMMDA, integrating for 12 protons in total. This NMR matches that of previously published characterization of TMMDA 4,4'-
Methylenebis(2,6-dimethylaniline) (3k): Yellow oil. 1H NMR (CDCl3, 400 MHz) δ 6.87 (s, 4H), 3.79 (s,
2H), 3.50 (s, br, 4H), 2.23 (s, 12H). 13C NMR (CDCl3, 100 MHz) δ 140.6, 131.7, 128.7, 121.9, 40.4,
17.7 (Barluenga, et al. 1980).
Figure 37: Carbon NMR
60
Figure 38: Hydrogen NMR
61
TMMDA Mechanism
Methods
Figure 39: TMMDA Mechanism
General Procedure for Polymerization
A 1L three necked glass reactor equipped with Teflon® blades as a mechanical mixer and nitrogen purge was charged with Jeffamine®. TMMDA is then dissolved in GBL and charged
62 into the glass reactor. The diamine solution is warmed to 30ºC and homogenized for 20 minutes with mechanical stirring, with Tin (II) ethyl hexanoate added to catalyze the polymerization.
PMDA is then separately dissolved in GBL and charged into the glass reactor. The reaction is kept under an inert nitrogen atmosphere for 4 hours before the temperature is increased to 80°C and a vacuum is strip off the GBL and water resulting from imidization of the polyamic acid backbone. The reaction remains under vacuum and 80°C for 24 hours. Subsequently, the wet polyimide is placed in a vacuum oven at 100ºC to further dry the polymer and ensuring that imidization is complete. Table 6 shows the varying amounts of diamine used in each formula.
Table 6: Feedstock Stoichiometry of Polyimide
TMMDA PMDA mol D230 mol D400 mol
Name mol% % % % D2000 mol %
PI-1 11.0% 50.0% 38.8% 0.0% 0.2%
PI-2 11.0% 50.0% 35.4% 3.0% 1.4%
PI-3 11.0% 50.0% 39.4% 0.0% 0.0%
PI-4 11.0% 50.0% 0.0% 39.4% 0.0%
P-I5 25.0% 50.0% 0.0% 23.8% 1.4%
63
Figure 40: Synthetic Procedure Outline for the Formation of the Polyether Polyimide
64
FT-IR
Thin film samples were prepared from a heated Carver hydraulic press. The thin films were analyzed using a Perking Elmer Spectrum One FT-IR equipped with ATR. The resulting data was analyzed using Spectrum software.
Rheology
A rectangular solid sample measuring 50mm x 10mm x 3mm was molded in a heated
Carver hydraulic press at 3 metric tons to temperatures between 150 and 170°C are reached, with quick cooling under 5 metric tons until room temperature. Isothermal strain sweeps were performed on the rectangular samples to determine the regions of linear viscoelasticity (LVE) with an AR2000 rheometer from TA instruments at three temperatures (-50ºC, 25ºC and 100ºC).
The highest strain percent within the measured LVE common to the three temperatures was chosen to characterize the sample with a temperature ramp under that controlled strain from -
50ºC to 125ºC at 5ºC/min with liquid nitrogen used for active cooling. Resulting modulus data were analyzed with software available from TA Instruments.
Thermogravimetric Analysis (TGA)
A 10mg sample from each polymer formulation was assayed for temperature stability with a TGA Q50 from TA instruments. The samples was heated from 25ºC to 800ºC with a ramp rate of 10ºC per minute. The results were analyzed using a TA Data Analysis software.
Tensile Test
A sample of 120mm by 30mm by 3mm was molded in a heated press. Then carved using a FREEMAN die STYLE D638 TYPE 4. The samples were pulled at 2.50cm per minute in a
Shimatdzu® AGS-J and analyzed using a TrapeziumX software.
65
Microhardness Test
Rectangular solid samples were prepared in a matter analogous to those used in rheology.
Ten replicate 15 second indentations were obtained (5 per side of rectangular sample) under 500 gf using a Leica VMHT equipped with Vickers diamond indenter, with averages and their standard deviations computed.
Results
Rheology Overlay
Figure 41: Rheology Temperature Sweeps Overlay
Figure 42: Rheology Temperature Sweeps
66
Table 7: Onset and Temperature at 97% Starting Weight for the Polyimides Studied
Sample Tan δ Tg (at 10Hz) G'' Tg (at 10Hz)
PI-1 120.10 80.03
PI-2 105.06 65.08 PI-3 125.02 100.03
PI-4 40.08 20.06
PI-5 105.04 N/A
Storage, loss and damping moduli vs temperature are shown. The upper rheology trace shows the shift in glass transition temperature with varying amounts of Jeffamine
(polyetherdiamine lengths. The lower rheology trace shows the shift in glass transition temperature with varying the amounts of aromatic diamine (TMMDA). Temperature sweeps under constant strain within the LVE (determined from strain sweeps) illustrate beta and alpha transitions. The alpha (glass) transition can be seen as a drop in storage modulus (G’), an increase in dampening (Tan δ) and a peak in the loss modulus (G’’). Figure 41 shows an increase in the glass transition temperature with a decrease in the molecular weight of the polyetherdiamine linker. Figure 42 shows an increase in glass transition temperature with increase in the molar feedstock of the aromatic diamine (TMMDA).
67
Thermogravimetric Analysis
120 ––––––– PI-1 ––––––– PI-2 ––––––– PI-3 ––––––– PI-4 ––––––– PI-5 100
80
60
Weight (%) Weight
40
20
0 0 200 400 600 800 Temperature (°C) Universal V4.5A TA Instruments
Figure 43 TGA Thermograms for the Polyimides
Table 8 TGA Onset and Temperature at 97% Starting Weight for the Polyimides Studied
Sample Onset Temperature (ºC) Temperature (ºC) at 97% wt. PI-1 366.97 349.01 PI-2 363.02 342.86 PI-3 377.95 305.35 PI-4 361.99 352.76 PI-5 362.98 347.16
68
Gel Permeation Chromatography
Figure 44 Gel Permeation Chromatograms for the Polyimides
Table 9 Mw, Mn and Polydispersity by GPC
Sample Mw (g/mol) Mn (g/mol) Polydispersity (PD)
PI-1 31062 13026 2.3846
33301 14428 2.3081 PI-2 PI-3 28553 11992 2.3793
PI-4 35182 16310 2.2145
PI-5 N/A N/A N/A
69
Date: Friday, May 20, 2016 FTIR
105.0 100
90
761.32 80 2972.20 2871.69
70 1456.92 1771.05 917.53
60 %T
50 828.02 1039.30
40 1372.84
30 1095.84
1354.40 730.65 1714.35 17.0 4000.0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 650.0 cm-1 Figure 45 FTIR Sprectrum of Polyimide
Vickers Hardness
Table 10 Vickers Hardness Values
Samples HV Standard Deviation (HV) MPa
PI-1 17.6 0.95 172.6
PI-2 14.4 1.25 141.2
PI-3 17.6 1.37 172.6
PI-4 N/A N/A N/A
PI-5 9.8 0.85 96.11
70
Discussions
Gel Permeation Chromatography
Gel Permeation Chromatography (GPC) data (Figure 44) was obtained and yielded weight-averaged molecular weights (Mw) ranging from approximately 28500 to 3500Da, number-averaged molecular weights (Mn) ranging from approximately 12000 to 15900 Da, and polydispersity indices (Mw/Mn) ranging from 2.21 to 2.38, as summarized in Table 9.
Rheology
Rheological analysis was performed on rectangular solid samples under linear viscoelasticity (LVE) strains determined from previous isothermal strain sweeps as described in the Methods section. Temperature sweep data under these LVE-determined strains yielded alpha and beta transitions, with glass (alpha) transitions showing as precipitous drops in the storage modulus (G’), and peaks in both the loss modulus (G’’) and dampening (Tan δ). It can be seen that the storage modulus, representing the elastic properties of the material and its recoverable energy, drops as the sample nears its alpha transition and becomes more rubbery (viscous). The concomitant increase in the loss modulus (G’) is due to a shift in the material’s elastic (storage) modulus to a more viscous, rubbery material, as the main chains obtain enough degrees of freedom and thermal energy to exhibit segmental slippage seen at the glass transition. The ratio of the loss to storage modulus (known as dampening, or Tan δ) will be used to compare the glass transition temperatures.
71
Thermogravimetric Analysis
The thermal stability and dryness of the polyimide were evaluated by TGA measurements under nitrogen atmosphere. The thermograms of the polyetheramine-polyimides are shown in
Figure 43. The corresponding temperature at 3% weight loss and the onset temperature are reported in Table 8. Polyimides typically have higher temperature stability (Takassi, et al. 2015)
(Li, et al. 2004) (Lua and Su 2006) as their backbone is linear, well-ordered, rigid and aromatic.
The polyetheramine-polyimides synthesized in this paper show thermoplastic properties at the expense of relatively degraded thermal stability. The stability at the onset temperature represents a relation between the lengths of the different aliphatic diamines in the repeat unit vs. the temperature (Baldwin, et al. 2013) (Li, et al. 2004). As the aliphatic linkers increase in length, the temperature stability decreases due to the presence of longer aliphatic linkages that makes it more flexible and less heat resistant. Even though there is a decrease in thermal stability the difference is not large enough between the larger links in PI-4 and the shorter links in PI-3.
Further, the thermal stability of the polyetheramine-polyimide is conserved. The results provided by the TGA show that the majority of the samples have conserved a 97% of their mass up to
300ºC. The diamine monomers and the lactone solvent both have boiling points around 205ºC which can explain the minimal loss of mass around that temperature. The onset temperature was above 360ºC for each formulation. The information provided by TGA testing also allows the determination of appropriate temperature regimes for rheology.
Infrared Spectroscopy
FT-IR spectroscopy was used to confirm polyimide formation and complete ring closure
(imidization) from the polyamic acid backbone. The novel polyetheramine-polyimides studied here exhibited characteristic imide group absorptions around 1771 cm-1 and 1716 cm-1 typical of
72 imide carbonyl asymmetrical and symmetrical stretching arising from the anhydride ring of
PMDA, with 1355 - 1373 cm-1 being the characteristic C-N aromatic stretch. The disappearance of amide carboxyl absorbencies around 1540 cm-1 (–NHCO- stretch) indicated a virtually complete conversion of the polyamic acid (PAA) precursor into polyimide. NH2 symmetric and asymmetric stretching and deformation vibrations are seen at 3474 cm−1. There are characteristic bands of aromatic rings at 1565 cm−1 for C-C stretching vibrations. Alkyl C-H Stretch vibration are seen at 2871 and 2972 cm−1 (Georgiev, et al. 2014) (Hsiao, Hsiao and Kung 2016).
Microhardness
During the micro hardness test, the holding time for the indentation was set as 15s with an indentation load of 500 gf. The indentations were measured with a Leica VMHT apparatus.
Multiple indentations were done on different side of the sample to understand the entire composition of the polymer and to account for anisotropy. The random conformations of the polymer does not show different regions of varying hardness, implying an isotropic composition.
Four polymers of the series were tested for micro hardness, as PI-4 was too soft for the Vickers scale. The results of the indentations are presented in Table 10. Overall, those samples with larger amounts of Jeffamine D230 show higher hardness, ranging from 17.6 Hv to 14.4 Hv (PI-1 through PI-3), as compared to the sample with a large amount of Jeffamine D400 (PI-5) with a rather low hardness of 9.8 HV. Particularly, PI-1 and PI-3 both composed of the shorter
Jeffamine D230, along with it being the major diamine of those polymers, appear to have similar
Vickers Hardness values of 17.6 Hv. However, once the longer diamine is added the value decreases to 14.4 Hv as was seen in PI-2. Thus, the hardness appears to be governed by the aliphatic diamine size and concentration, and is also correlated with Tg due to polymer’s degrees of freedom and segmental slippage with applied force (Chandler 1999). This hardness-Tg
73 relationship is seen in the Tg of PI-1 being 120.1ºC and its hardness of 17.6Hv. While in PI-2, its Tg is 105.06ºC and hardness of 14.4Hv. PI-3 has a Tg of 125.02 ºC and a hardness of 17.6Hv.
The results for PI-4 and PI-5, in particular, are governed by the large size of the aliphatic
Jeffamine D400. In this chase, PI-4 was too soft to be measured using a Vickers indenter and when related to the Tg the value drops from 105.04ºC in PI-5 to 40.08ºC in PI-4.
Conclusions
The synthesis and subsequent characterization of the polyetheramine-polyimides described in this work serve as a proof of concept for their formulation to yield the desirable properties. Fully aromatic polyimides are well known to exhibit high thermal stability and resistance to solvents, and as a consequence they are typically unable to be processed with conventional thermoplastic equipment. By using long chain aliphatic polyether diamines we were able to impart into the backbone flexible components making our polyimides melt processable using typical thermoplastic techniques such as compression molding, injection molding and extrusion. Thermal and mechanical properties such as glass transition and decomposition temperatures were also characterized. The molecular weight and polydispersities were elucidated from gel permeation data and low in comparison to typical polymers high performance polymers such as the polyurethanes reported in chapter 2.
Rheological analysis of rectangular solid samples showed glass transition temperatures
(Tg) ranging from approximately 40°C to 125°C according to tan δ peaks at 10Hz. It can be seen from Figure 41 and 42 and Table 6 and 7 that with approximately equivalent amounts of
Jeffamine D230 that the glass transitions can be tuned by the addition of varying sizes of the aliphatic polyetherdiamines, with larger, higher MW aliphatic diamines reducing the glass
74 transition temperature. Conversely, the aromatic diamine shows a significant influence on glass transition temperature.
75
Chapter 4
Future Work
Polyurethane and Nanoparticles
The future research proposal will be to develop novel polyurethane nanocomposites with increased heat resistance and improved mechanical properties using the novel polycarbonate diol in chapter one. This work encompasses the understanding, design and testing polyurethane elastomer nanocomposites. The effect of nanoparticles on the properties of polyurethanes has attracted significant attention lately and the definition of nanocomposite materials has broadened to encompass a large variety of systems. This future work will focus on a specific class of nanoparticles, metalorganics. Metalorganic nanoballs will be processed with polyurethanes to yield nanocomposites with improved thermal and mechanical properties, chemical resistance, optical clarity, and optimum gas barrier properties as compared to those of unmodified polymers.
The incorporation of self-assembled nanoparticles like nanoballs into linear polyurethane chains will give rise to a mechanically cross-linked network resulting in a novel high performance polyurethane.
It is proposed that a novel class of polyurethane composite materials will be developed.
We will investigate both thermoset and thermoplastic types. These polymers will be synthesized via polyaddition polymerization. We will investigate both aliphatic and aromatic hard segments
76 during the polymerization process. The ultimate goal will be to develop high performance polyurethane composite materials that will outperform those currently available to industry.
Polyurethane elastomers of the TPU and rubber type have always had degradation problems at elevated temperatures. The intellectual merit of this work will be based on the premise that the addition of nano size structures will improve their physical properties by incorporating nanoballs linked via threading by the linear urethane or by crosslinking the hydroxylated nanoballs with the urethane thus increasing the thermal stability of these classes of materials. The interest given to this technology will allow entrance to industrial markets previously off limits due to the thermal degradation. The primary objective is to advance the knowledge of combining polyurethanes with nano structured materials with the ultimate goal of providing materials that can be used in real world applications.
The broader impact of this work represents the opportunity to develop a polymer that may be used in military tracked vehicles. The new tanks are heavier and faster than their predecessors, weighing over 54,400kg and traveling at speeds of up to 72km/hr. The track pad and bushings have a high rate of wear and failure and thus the military is constantly looking for materials to improve the lifetime of these components (Dwight and Lawrence 1987). Due to the military’s increased presence in desert climates, a thermally stable polyurethane would be of great interest and cost savings. There are also other industrial applications that could benefit such as automobile tires, high speed rollers and wear resistant tennis shoes.
Just in the last few years, nanostructured materials and nanotechnologies have received much attention from the scientific community owing to the novel properties that arise in ordinary matter when its size is reduced to a nanometric scale range (Carotenuto 2002). In 1960, Nobel
Laureate Richard Feynman, predicted that by the year 2000 products would be built one
77 molecule or one atom at a time. This shift is referred to today as the “nanotechnology revolution” and many people consider Dr. Feynman’s quote the birth of nanotechnology. The
National Science Foundation predicts that by 2010 nanotechnology will pervade virtually every corner of the economy and represent $1 trillion in goods and services. The term
“nanotechnology” is based on the root nanos, meaning one billionth. It refers to technology that uses components or features that measure 100 nanometers or less. The definition of nano- composite material has broadened significantly to encompass a large variety of systems such as one-dimensional, two-dimensional, three-dimensional and amorphous materials, made of distinctly dissimilar components and mixed at the nanometer scale. The general class of nanocomposite organic/inorganic materials is a fast growing area of research. Just in the last couple of years, nano-sized materials and the technologies associated have received a great deal of attention. These systems show great promise and have generated significant industrial interest.
The interest being given to this technology is owed to the improvement in thermal, mechanical, chemical resistance, optical clarity and gas barrier properties compared to those of pristine polymers. It is important to recognize that nanoparticle loading adds significant improvements at very low levels whereas traditional microparticle additives require much higher loading levels to achieve similar performance, such as weight reduction, greater strength for similar structural dimensions and improved barrier performance for materials of a similar thickness. There are a wide variety of nanoparticles available. A significant amount of research has been devoted to carbon nanotubes which can be single walled or multi-walled (Harada 1998) (Gong, Ji, et al.
1998) (Auten and Petrovic 2002) (Moulton, et al. 2001) (Abourahma, et al. 2001) (Srikanth, et al. 2003) (McManus, Wang and Zaworotko 2004) (Mohomed, Abourahma, et al. 2005).
Recently we shifted our focus to include novel metalorganic nanoballs (Mohomed, Gerasimov, et
78 al. 2005) (Moulton, et al. 2001) (Abourahma, et al. 2001) (Srikanth, et al. 2003) (McManus,
Wang and Zaworotko 2004) (Mohomed, Abourahma, et al. 2005). This proposal will focus on developing high performance polyurethane elastomers using self-assembled hydroxylated nanoballs as fillers.
Current Target Areas
The properties of these novel polyurethane nanocomposites can be tailored to fulfill the requirements of different applications like flex foam for cushions and bedding, rigid foam for insulation, urethane coating for auto exterior, resilient elastomers for skate wheels or industrial rollers, films or fibers and automotive molded seats and window encapsulates, medical device and military tank tracks. Polyurethane is a very general term which is utilized for a significant number of different polymers. Polyurethanes are formed by polyaddition polymerization.
Threading the Nanoballs with Linear Polyurethanes
These high performance polyurethanes will incorporate carbon nanoballs by threading the linear urethane backbone though the nanoball which are termed rotaxanes (Gong, Ji, et al. 1998) (Gong and
Gibson, Controlling Polymer Topology by Polymerization Conditions Mechanical Linked network and
Branched Polyurethane Rotaxanes with Controllable Polydisperity 1997) (Nagapudi, et al. 1999) (Gong,
Glass and Gibson 1998) (Shen and Gibson 1992). High molecular weight polyol – a polar and highly flexible molecule is expected to thread through the nanoball windows. Estimated diameter for linear poly tetrahydrofuran polyol which is a standard component of modern high performance urethanes is about 0.3 nm (Gong, Ji, et al. 1998) (Gong and Gibson, Controlling Polymer Topology by Polymerization
Conditions Mechanical Linked network and Branched Polyurethane Rotaxanes with Controllable
Polydisperity 1997) (Nagapudi, et al. 1999) (Gong, Glass and Gibson 1998) (Shen and Gibson 1992).
Hydroxylated nanoballs created in Dr. Zaworotko’s laboratory are rhombi hexahedral in shape and possess functional hydroxyl groups on the surface. They have average sizes of 2.6 to 3.1 nm window
79 sizes ranging from 0.9 to 1.2 nm as measured by X-ray crystallography and shown in Figure 2. thus, allowing polyol molecules to enter the interior cavity of the nanoball efficiently creating a threaded mechanical link.
Figure 46 Calculated nanoball window sizes
(angstroms)
A true polyrotaxane has the threaded molecule confined between two blocking groups preventing the threaded molecule from slipping off. Upon formation of polyurethane a rigid MDI-based hard segment is added to flexible polyol molecule which is threaded through the nanoball. MDI-based hard segment is a less polar unit with significantly bigger diameter. Both factors should prevent the nanoball from slipping off the polyurethane molecule. Since the nanoball is confined between two blocking groups, the two components are mechanically linked thus making it thermally stable. Since the hydroxyl group is half of the urethane reaction it will be necessary to either cap them or incorporate them into the reaction as a cross link function. In this proposal we will consider both approaches.
80
Short Term Objectives
The short term research objectives are listed below in chronological order:
a. Determine base polyurethane polymers
b. Determine the solubility of nanoballs.
c. Cap hydoxylated nanoballs.
d. Incorporate the polyurethane polymers and nanoballs
e. Test the polyurethane nanoball composites to obtain the mechanical, thermal
and chemical properties.
Research Plan
Polymerize Baseline Material
We will carry out preliminary experiments by polymerizing and testing model thermoplastic polyurethane and millable polyurethane rubber. The formulations are similar to those presented in chapter
1.
Determine Solubility of Nanoballs
We will test the solubility of the untreated nanoballs in various solvent systems to establish best conditions for capping the nanoballs. In a separate study we will test the solubility of nanoballs in polyols so that threading or crosslinking reaction can be done in the absence of a solvent. Our experience in formulating polyurethanes leads us to believe that the selected materials will function to produce compatible systems.
81
Cap Hydoxylated Nanoballs
The hydroxyl units on the surface of the nanoballs are reactive with isocyanate groups and would thus interfere with the linear urethane reaction desired. We will cap the hydroxyl groups with a monoisocyanate. Typical examples would be the following
Figure 47 Mono functional Isocyanates
82
Untreated hydroxylated nanoballs are dissolved in optimized solvent system and placed into a 100 ml round bottom flask equipped with stirrer, heater and nitrogen blanket. A calculated equivalent amount of monoisocyanate is added drop wise under stirring. The equivalent weight tells you how many grams of a product you need to have one equaivalent of reactive groups. For an isocyanate, the reactive group is
N=C=O (NCO), and its concentration is measured by weight percent NCO.
Example: Isocyanate Equivalent Weight = 4,200 ÷ %NCO (g/eq)
Necessity of catalysts will be determined experimentally for each isocyanate used. Extent of reaction will be monitored by either FTIR spectroscopy or hydroxyl group titration.
Fabrication of Polyurethane Capped Nanoball Composites
Thermoplastic Polyurethane Threaded Through the Capped Nanoballs
Into a 200 ml round bottom flask equipped with constant nitrogen blanketing and a heating mantel with controlled temperature and stirring, the mixture of monoisocyanate-capped nanoballs in 50 ml of THF is added. A pre-determined amount of polytetrahydrofuran is added to the mixture which is stirred overnight at 70°C to allow threading between nanoballs and linear species. At the end of the threading period, calculated amounts of chain extender and butanediol are added and mixed. A small amount of catalyst is added (typically stannous octoate), and the mixture is mixed until homogeneous.
Once homogeneous, in case of the thermoplastic polyurethane a slight stoiciometric excess of methylene bis(diphenyldiisocyanate) is added to the mixture while mixing.
A Millable Polyurethane Rubber Threaded Through the Capped Nanoballs
Into a 200 ml round bottom flask equipped with constant nitrogen blanketing and a heating mantel with controlled temperature and stirring, the mixture of monoisocyanate-capped nanoballs in 50 ml of THF is added. A pre-determined amount of polytetrahydrofuran is added to the mixture which is stirred overnight at 70°C to allow threading between nanoballs and linear species. At the end of the
83 threading period, calculated amounts of chain extender and butanediol are added and mixed. A small amount of catalyst is added (typically stannous octoate), and the mixture is mixed until homogeneous.
Once homogeneous, in the case of the polyurethane rubber a slight stoiciometric deficiency of methylene bis(diphenyldiisocyanate) is added to the mixture while mixing.
Thermoplastic Polyurethane Threaded Through the Capped Nanoballs
Into a 200 ml round bottom flask equipped with constant nitrogen blanketing and a heating mantel with controlled temperature and stirring, the mixture of monoisocyanate-capped nanoballs in 50 ml of THF is added. A pre-determined amount of polytetrahydrofuran is added to the mixture which is stirred overnight at 70°C to allow threading between nanoballs and linear species. At the end of the threading period, calculated amounts of chain extender and butanediol are added and mixed. A small amount of catalyst is added (typically stannous octoate), and the mixture is mixed until homogeneous.
Once homogeneous, in the case of the cast polyurethane a slight stoiciometric excess of methylene bis(diphenyldiisocyanate) is added to the mixture while mixing.
Fabrication of Cross-Linked Polyurethane Nanoball Composites
Thermoplastic Polyurethane Cross-Linked by the Nanoballs
Into a 200 ml round bottom flask equipped with constant nitrogen blanketing and a heating mantel with controlled temperature and stirring, a pre-determined amount of polytetrahydrofuran is added. A calculated amount of untreated nanoballs in a minimum amount of THF is added to the reaction mixture which is stirred until homogenous. In this reaction hydroxylated nanoballs act as both crosslinkers and chain extenders. A small amount of catalyst is added (typically stannous octoate) and the mixture is mixed. Once homogeneous, a slight stoiciometric excess (based on pre-determined hydroxyl content of both polyol and nanoballs) of methylene bis(diphenyldiisocyanate) is added to the mixture while mixing. When all the components are added and sufficiently mixed stirring is stopped and reaction
84 temperature is maintained at 80° C for another 48 hours. Extent of the polymerization will be monitored by FTIR spectroscopy.
Cast Polyurethane Cross-Linked by the Nanoballs
Into a 200 ml round bottom flask equipped with constant nitrogen blanketing and a heating mantel with controlled temperature and stirring, a pre-determined amount of polytetrahydrofuran is added. A calculated amount of untreated nanoballs in a minimum amount of THF is added to the reaction mixture which is stirred until homogenous. In this reaction hydroxylated nanoballs act as both crosslinkers and chain extenders. A small amount of catalyst is added (typically stannous octoate) and the mixture is mixed. Once homogeneous, a slight stoiciometric excess (based on pre-determined hydroxyl content of both polyol and nanoballs) of methylene bis(diphenyldiisocyanate) is added to the mixture while mixing. When all the components are added and sufficiently mixed stirring is stopped and reaction temperature is maintained at 80° C for another 48 hours. Extent of the polymerization will be monitored by FTIR spectroscopy.
Millable Polyurethane Rubber Cross-Linked by the Nanoballs
Into a 200 ml round bottom flask equipped with constant nitrogen blanketing and a heating mantel with controlled temperature and stirring, a pre-determined amount of polytetrahydrofuran is added. A calculated amount of untreated nanoballs in a minimum amount of THF is added to the reaction mixture which is stirred until homogenous. In this reaction hydroxylated nanoballs act as both crosslinkers and chain extenders. A small amount of catalyst is added (typically stannous octoate) and the mixture is mixed. Once homogeneous, a slight stoiciometric excess (based on pre-determined hydroxyl content of both polyol and nanoballs) of methylene bis(diphenyldiisocyanate) is added to the mixture while mixing. When all the components are added and sufficiently mixed stirring is stopped and reaction temperature is maintained at 80° C for another 48 hours. Extent of the polymerization will be monitored by FTIR spectroscopy.
85
Testing of the Urethane/Nanoball Composites
Differential Scanning Calorimetry (DSC)
A TA Instruments 2920 DSC will be used to determine glass transition, Tg, and melt temperature, Tm. After calibration with an indium standard the second heating of each sample will used in order to earase any previous thermal history. Each sample will be heated at 3 C
/min with dry nitrogen gas flowing at a rate of 75 ml/min purged through the sample cell.
Cooling will be accomplished with the liquid nitrogen cooling accessory (LNCA), provided with the 2920.
Dynamic Mechanical Analysis (DMA)
Rectangular samples with dimensions of 30mm × 5mm × 1mm will be compression molded using 20 ton Carver press. TA Instruments DMA 2980 will be used to find tensile moduli for different samples. Data will be recorded at different frequencies from –150 C to temperatures at which the samples are unable to bear loads. This will define the useable temperature for the materials.
Thermal Gravimetric Analysis (TGA)
5 mg samples will be scanned on a TA Instruments QA 50 TGA at a scanning rate of 5
C/min under nitrogen and air. Thermal stability will be determined based on weight loss. The samples was heated from 25ºC to 800ºC with a ramp rate of 10ºC per minute. The results were analyzed using a TA Data Analysis software.
86
UV Visible Spectroscopy
Samples will be compression molded in 10mm diameter disk molds with a thickness of 5mm.
Transmission spectra will be recorded with an 8452A Hewlett-Packard UV/Visible Spectrophotometer.
These tests will show that nanoballs may have on transparency properties of polyurethanes.
Atomic Force Microscopy
Surface studies will be performed with Digital Instruments atomic force microscope using tapping mode and phase imaging. The images will be acquired under ambient conditions with standard silicon tapping tip on a beam cantilever. Thin films used for AFM studies will be prepared by casting from 1% bwt. THF solutions. Several drops of polymer solution will be placed on glass slides and dried in a vacuum oven at 60ºC for 24 hours.
Tensile Testing
Tensile Modulus and Tensile Strength: Dog-bone shaped samples will be compression molded. An Instron 2980 will be used to determine the modulus and strength of the samples.
Samples will be deformed at a cross head speed of 0.5 inch/min.
Abrasion Testing
Abrasion testers use an abradant to be applied to the surface of a test specimen. Test compounds are usually compared on a volume loss basis which is calculated from the weight loss and density of the material. ISO 4649 refers to the DIN Abrader. The test specimen is put in a holder that traverses a rotating cylinder covered with the specified abradant paper. By allowing the sample holder to move the test piece across the drum as it rotates, there is less chance of material buildup on the abradant paper (Gong, Ji, et al. 1998).
87
Polyimide
A polymers molecular weight and polydispersity are important properties and are measured using a technique known as gel permeation chromatography or size exclusion chromatography. Since all polymers are polydisperse it is better to refer to it as the average molecular weight. It would be more accurate to call it “average relative molecular weight” as a standard such as polystyrene with known molecular weights is used. A series of varying molecular weight polystyrene samples are used and their elution then determine the average molecular weight of the polymer sample being analyzed. The average molecular weight of a polymer directly affects the physical and mechanical properties and solubility and brittleness of the polymer. All the polyurethane polymers studied in chapter two had average molecular weights in the range of two hundred thousand Daltons whereas the polyimide polymers studied in chapter three had average molecular weight in the range of thirty thousand daltons. There will be an attempt to increase the average molecular weight of the polyimide formulas either by adjusting the reaction conditions such as temperature and time or chemically by adding in short chain extenders or increasing/changing the type of catalyst used. Once the molecular weights have been increased significantly the properties will be studied and compared to those polymers discussed in chapter three.
It is well known that 13C-NMR spectroscopy is a promising analytical and identification tool. H-NMR and C-NMR should be studied to confirm the structure of the polyimide. As well an analytical method should be developed for the spectral analysis of polyamic acids and polyamides. Developing a method to determine the full completion of ring closure would help confirm the finished polyimide has no remaining open rings.
88
Polyimides are a class of high temperature resistant polymers that are most frequently used for structural engineering purposes. They also have rather unique properties for small ion or molecule diffusive transport. This is the area that industry can most effectively serve. We have an interest in lithium ion transport in a conductive separator matrix for batteries. Also possible are materials for selective gas transport and fuel cell membranes. Although polyimides are relatively expensive compared to most polymer classes, they can be tailored at the molecular level with a much broader combination of controllable properties than any other class. This makes them very useful when a unique and specific target is required. Using the polyimides discussed in chapter 3 as a separator layer will demonstrate a useful application.
89
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About the Author
Kenneth Kull received a B.A. Degree in Chemistry from Wayne State University in
Detroit Michigan in 1995. He was admitted to the graduate degree program in the spring of 2001 at The University of South Florida and immediately joined Dr. Julie Harmon’s polymer materials research lab.
Kenneth has worked fulltime in industry since receiving his degree from Wayne State in1995, working for companies such as BASF, TSE Industries, Saint Gobain and currently at
Brightvolt. He took seven coursed earning 21 GPA hours for an overall GPA of 3.23. Kenneth has authored two publications and coauthored two publications in various peer-reviewed scientific journals. Ken has also presented at several subject matter relevant conferences such as
FAME, TOPCON, HEXAGON and The CASTLE CONFERENCE.