Synthesis and Characterization of In-Situ Nylon-6/Epoxy Blends

Synthesis and Characterization of

in-situ Nylon-6/Epoxy Blends

A thesis submitted to the Division of Research and Advanced Studies

University of Cincinnati

In partial fulfillment of the requirements for the degree of Master of Science

2016

In the Materials Science and Engineering Program, The Department of Mechanical and Materials Engineering By Anushree Deshpande

B.E Polymer, University of Pune, 2011

Committee Members: Dr. Jude O. Iroh (Chair) Dr. Relva C. Buchanan Dr. Raj M. Manglik

ABSTRACT

Epoxy is a thermosetting polymer known for its excellent adhesion, thermal stability, chemical resistance and mechanical properties. However, one of the major drawbacks of epoxies is its inherent brittleness. In order to overcome this drawback, incorporation of a thermoplastic as a second phase has proven to improve the impact strength without affecting the mechanical properties of epoxy.

Researchers in the past have studied polyamide/epoxy blends in terms of blend compatibility, thermo-mechanical properties and morphology via solution blending.

The current research effort employs in-situ polymerization to synthesize polyamide/epoxy blends. Blends of various compositions were synthesized by introducing Ɛ-Caprolactam (monomer of nylon-6) in the prepolymer of epoxy. All blend fractions were cured by exposing them to the same time and temperature conditions; and characterized using Dynamic Mechanical Analysis (DMA),

Fourier Transform Infrared Spectroscopy (FTIR), Brookfield Viscometry, Scanning Electron

Microscopy (SEM) and Thermogravimetric Analysis (TGA).

DMA results show an overall increase in glass transition temperature and storage modulus in the rubbery region. FTIR results reveal maximum epoxy curing up to 15 wt% monomer loading, beyond which the plateauing of the epoxy conversion is recognized. Shear viscosity measurements, along with FTIR and DMA results reveal increased interactions between Ɛ-Caprolactam and epoxy.

TGA results display improved thermal stability till 250oC after which the degradation onset of the in-situ nylon-6/epoxy blend shifts to lower temperatures with respect to the neat nylon-6 and neat

2 epoxy. SEM micrographs reveal absence of Nylon-6 agglomerates in the blend indicating improved dispersion of nylon-6 in epoxy.

Moreover, this study draws a comparison between in-situ and ex-situ blending of polyamide and epoxy, with respect to trends obtained in glass transition temperature (Tg), storage modulus and nature of dispersion. Nylon-6/epoxy blends processed via ex-situ blending possessed reduced storage modulus and glass transition temperature with good damping properties compared to neat epoxy.

On the other hand, reactive blends of nylon-6/epoxy processed via in-situ polymerization of Ɛ-

Caprolactam in presence of epoxy prepolymer resulted in a material with increased modulus and high glass transition temperature and improved dispersion of filler phase in the matrix phase due to reduced agglomeration of nylon-6.

ACKNOWLEDGEMENTS

First, I would like to thank my research advisor, Dr. Jude Iroh for his insightful comments and guidance all through my research and for allowing me the freedom to explore different ideas. I appreciate his patience and steadfast support which helped me overcome difficult times. I would like to thank Dr. Relva Buchanan and Dr. Raj Manglik for serving on my defense committee.

I would like to thank Dr. Aniket Vyas for training me on the synthesis and lab instruments, and providing useful inputs based on his experience in ex-situ blending during the course of my research.

Moreover, I am extremely thankful to my lab members- Shirley Peng, Dr. Patricia Okafor, Praveen

Balasubramani, Nathan Holliday, Yujie Zhang, Xueying He, Wajeeh Marashdeh, Jehan Kothari,

Caroline, Brent Huxel for their help and support and for making this journey through graduate school an enjoyable one.

I dedicate this work to my parents who have been a constant source of emotional strength and support and I thank them for encouraging me to pursue my dreams to study at the University of Cincinnati.

Lastly, I would like to thank Dr. Necati Kaval for allowing timely access to the Spectroscopy Lab and training me on Fourier Transform Infrared Spectroscopy instrument.

TABLE OF CONTENTS Abstract 2 Acknowledgements 5 List of Tables 8 List of Figures 10 Chapter 1: Introduction………………………………………………………………………….15 1.1. Synthesis routes for Polymer blends…………………………………………….….15 1.2. Incorporation of thermoplastic in Epoxy……………………………….………….16 1.3. Incorporation of Nylon in Epoxy……………………………………………………17 1.4. Significance of in-situ blending……………………………………………………...18 1.5. Epoxy Resin……………………………………………………………………..…....19 1.5.1. Types of Epoxy Resins………………………………………………...…...20 1.5.1a. Bisphenol A type resins…………………..……………….……...20 1.5.1b. Bisphenol F type resins…………………………………………...21 1.5.1c. Multifunctional epoxy resin……………………………..……….21 1.5.1d. Novolac Epoxy Resin………………………………………..……22 1.5.1e. Cycloaliphatic Epoxy resin…………………………………..…...23 1.6. Curing of Epoxy………………………………………………...... 23 1.7. Nylon-6………………………………………………...... 25 1.8. Synthesis Routes of Polyamide-6…………………………………………….……...25 1.9 Research Objective…………………………………………………………………...26 Chapter 2: Experimental………………………………………………………………….……...28 2.1. Materials……………………………………………………………………………...28 2.2. Synthesis of neat Nylon-6……………………………………………………………28

2.3. In-situ synthesis of Nylon-6/Epoxy reactive blends…………………………..……28 2.4. Sample preparation for neat Epoxy (control)……………………………….……..30 2.5. Cure Conditions………………………………………………………………...……30 2.6. Characterization Techniques…………………………………………………….….31 2.6.1. Fourier Transform and Infrared Spectroscopy - Attenuated Total Reflectance (FTIR-ATR)…………………………………………….………….. 31 2.6.2. Dynamic Mechanical Analysis (DMA)…………………………………....31 2.6.3. Scanning Electron Microscopy (SEM)………………..…………….…….32 2.6.4. Thermogravimetric Analysis (TGA)………………………………….…..33 2.6.5 Shear Viscometry…………………………………………………….……..33 Chapter 3: Results & Discussion………………………………………………………………...36 3.1. Reactive blends of Nylon-6 & epoxy……………………………………………..36 3.1.1 Fourier Transform Infrared (FTIR) Spectroscopy Analysis…………..36 3.1.1a. FTIR analysis of Ɛ-caprolactam…………………………………………36 3.1.1b. FTIR analysis of neat Epoxy……………………………………………..37 3.1.1c. FTIR analysis of Ɛ-caprolactam in Epoxy during in-situ synthesis……38 3.1.1d. FTIR Analysis of Nylon-6/Epoxy blends compositions after curing…..38 3.1.1e. FTIR Analysis of neat Nylon-6 synthesized at 80oC……………………39 3.1.1f. FTIR Analysis of Epoxy conversion……………………………………..40 3.2. Dynamic Mechanical Analysis (DMA) …………………………………………..41 3.2.1. Analysis of Tan delta peaks……………………………………………..…42 3.2.2. Analysis of Storage moduli…………………………………………..…….45 3.3. Shear Viscometry…………………………………………………………………47 3.3.1. Effect of Nylon-6 monomer loading on shear viscosity………………….48

3.3.2. Effect of change in temperature on shear viscosity……………………...48 3.3.3. Effect of increase in polymerization time on shear viscosity……………50 3.4. Thermogravimetric Analysis (TGA)……………………………………….……….51 3.5. Comparison between ex-situ and in-situ method of blending………..……………53 Chapter 4: Conclusion………………………………………………………………………...56 Chapter 5: Future Work and Suggestions……………………………………………..…….58 References………………………………………………………………………………..………..59 Figures………………………………………………………………………………………..…....65 Appendix……………………………………………………………………………………..…..105

LIST OF TABLES Table 1: Compositions for in-situ Nylon-6/Epoxy blends

Table 2: Cure conditions

Table 3: Absorbance peaks for Ɛ-caprolactam

Table 4: Absorbance peaks for neat Epoxy

Table 5: Absorbance peaks for Nylon-6

Table 6: Summary of Tan delta peak analysis

Table 7: Summary of Storage moduli analysis

Table 8: Summary of Activation energies for 2.5wt% and 30wt% NY-EP blends

Table 9: Degradation temperatures for Nylon-6 and Epoxy in the in-situ blends

LIST OF FIGURES Figure 1: Structure of an Oxirane Ring Figure 2: Structure of Bisphenol A oligomer Figure 3: Structure of Bisphenol F oligomer Figure 4: Triglycidyl derivative of p-amino phenol Figure 5: Tetraglycidylmethylene dianiline (TGDDM) Figure 6: Epoxy Novolac Resin Figure 7: Cycloaliphatic Epoxy Figure 8: Epoxy Curing mechanism by an amine Figure 9: Structure of Nylon-6 Figure 10: Schematic representation of Nylon-6 synthesis Figure 11: Schematic of a typical DMA graph Figure 12: Schematic of a TGA thermogram Figure 13: Structure of Ɛ-Caprolactam (monomer of Nylon-6) Figure 14: FTIR spectrum of Ɛ-caprolactam (monomer) showing presence of N-H stretch, hydrocarbon stretch, carbonyl (C=O) of the amide Figure 15: Structure of an Epoxy resin molecule Figure 16: FTIR spectrum of neat epoxy showing the presence of hydroxyl (O-H) at 3402 cm-1, (C-H) at 2927 cm-1, (C-C) of the benzene ring and oxirane ring at 914cm-1 Figure 17: FTIR spectrum at the start of synthesis where caprolactam was dispersed in neat epoxy at room temperature of 25oC Figure 18. FTIR spectrum during blend synthesis of caprolactam in neat epoxy at 65oC. Figure 19: FTIR spectrum of thermally cured 5 wt% in-situ nylon-6/epoxy blend showing the absence of (C=O) carbonyl bond of the amide group Figure 20. FTIR spectrum of thermally cured 10 wt% in-situ nylon-6/epoxy blend showing a slight shoulder at 1649 cm-1 attributed to the (C=O) carbonyl bond of the amide group

Figure.21 FTIR Spectrum of thermally cured 15 wt% in-situ nylon-6/epoxy blend showing increased absorption of the amide carbonyl (C=O) at 1643cm-1 Figure 22. FTIR spectrum of thermally cured 20 wt% in-situ nylon-6/epoxy blend showing diminishing amide carbonyl (C=O) peak at 1649cm-1 Figure.23 FTIR spectrum of thermally cured 25 wt% in-situ nylon-6/epoxy blend showing reappearance of the amide carbonyl (C=O) at 1661cm-1. Figure 24. FTIR spectrum of thermally cured 30 wt% in-situ nylon-6/ epoxy blend showing diminished amide carbonyl (C=O) peak at 1649 cm-1 Figure 25. FTIR Spectrum of thermally cured 35 wt% in-situ nylon-6/ epoxy blend showing the amide carbonyl (C=O) peak at 1649cm-1 Figure 26. Overlay of FTIR Spectra of in-situ nylon-6/epoxy blend fractions with neat epoxy showing the appearance of carbonyl (C=O) peak of the amide functional group between 1640cm-1 and 1660 cm-1 Figure 27. Hydroxyl peak shift towards low wavenumbers starting from thermally cured in- situ (a) neat epoxy, (b) 5 wt% nylon-6/epoxy (c) 10 wt% nylon-6/epoxy, (d) 15 wt% nylon- 6/epoxy (e) 20 wt% nylon-6/epoxy, (f) 30 wt% nylon-6/epoxy and (g) 35 wt% nylon-6/epoxy. Hydroxyl shift towards low wavenumbers suggests hydroxyl group (O-H) being surrounded by neighboring atoms in the blend Figure 28. Structure of Nylon-6 Figure 29. FTIR spectrum of neat nylon-6 synthesized at 80oC showing the presence of N-H , C-H, amide C=O, C=N stretch, skeletal motion of amide group (–CONH-) confirming the formation of Nylon-6 at 80oC via ring opening anionic polymerization of -caprolactam Figure 30. Oxirane peak at 914 cm-1 with increasing loading of -caprolactam (a) neat epoxy, (b) 5 wt% Nylon-6/Epoxy, (c)10 wt% Nylon-6/Epoxy, (d) 20 wt% Nylon-6/Epoxy (e) 30 wt% Nylon-6/Epoxy. Figure 31. Epoxy fractional conversion with increase in -caprolactam loading from 5 wt% -caprolactam to 35 wt% -caprolactam in epoxy matrix. Figure 32. Glass transition temperatures for all blend fractions synthesized by in-situ polymerization were obtained by plotting the tan delta peaks from dynamic mechanical measurements. Figure shows variation of tan delta with -caprolactam loading for (a.) neat epoxy, (b.) 5 wt% nylon-6/epoxy, (c.) 10 wt% nylon-6/epoxy, (d.) 15 wt% nylon-6/epoxy (e.) 20 wt% nylon-6/epoxy loading, (f.) 25 wt% nylon-6/epoxy (g.) 30 wt% nylon-6/epoxy (h.) 35 wt% nylon-6/epoxy. 15 wt% nylon-6epoxy blend fraction possesses the highest tg. 11

Figure 33. Variation in Storage Modulus plotted against temperature with increasing - caprolactam loading for the following in-situ blends (a) neat epoxy, (b.) 5 wt% nylon-6/Epoxy, (c.) 10 wt% nylon-6/Epoxy, (d.) 15 wt% nylon-6/Epoxy (e.) 20 wt% nylon-6/Epoxy (f.) 25 wt% nylon-6/epoxy (g.) 30 wt% nylon-6/epoxy (h.) 35 wt% nylon-6/epoxy Figure 34. Plot of viscosity vs shear rate for 2.5 wt% nylon-6/epoxy blend fraction showing shear thinning behavior. Viscosity values were obtained at different spindle speeds at a temperature setting of 80oC. Figure 35. Plot of viscosity vs shear rate for 30 wt% nylon-6/epoxy blend fraction showing reduction in the extent of shear thinning compared to 2.5 wt% nylon-6/epoxy blend fraction in figure 33. Viscosity values were obtained at different spindle speeds at a temperature setting of 80oC Figure 36. Plot of viscosity vs shear rate for 30 wt% nylon-6/epoxy blend fraction showing shear thickening behavior. Viscosity values were obtained at different spindle speeds at a temperature setting of 25oC. Brookfield measurements were performed on the 30 wt% nylon- 6/epoxy blend fractions at different temperature settings to investigate the presence of residual nylon-6 which did not participate in crosslinking with epoxy. Figure 37. Plot of shear viscosity vs shear rate for 30 wt%nylon-6/epoxy blend fraction showing shear thinning behavior. Viscosity values were obtained at different spindle speeds at a temperature setting of 60oC. Figure 38. Plot of shear viscosity vs shear rate for 30 wt%nylon-6/epoxy blend fraction showing shear thinning behavior. Viscosity values were obtained at different spindle speeds at a temperature setting of 80oC Figure 39. Plot of shear viscosity vs shear rate for 30 wt%nylon-6/epoxy blend fraction showing shear thinning behavior. Viscosity values were obtained at different spindle speeds at a temperature setting of 100oC Figure 40. Shear viscosity measurements at room temperature of 30 wt% nylon-6/epoxy in order to study variation of viscosity with polymerization time (a) 5 min, (b) 15 min, (c) 25 min and (d) 30 min after adding initiator. Increase in viscosity with reduced shear thinning behavior with time indicates increased interactions between nylon-6 and epoxy Figure 41. TGA of thermally cured neat Epoxy showing the maximum mass loss and thermal degradation at 397oC

Figure 42. TGA of Nylon-6 synthesized at 80oC by anionic ring opening polymerization of - caprolactam showing thermal degradation occurring at 314oC and 425oC. Weight loss at 216oC is due to loss of unreacted -caprolactam Figure 43. Percent weight vs temperature and the corresponding derivative weight in region I for in-situ nylon-6/ epoxy blend fractions: (a) Neat epoxy, (b) 2.5 wt% nylon-6/epoxy, (c) 20 wt% nylon-6/epoxy (d) 30 wt%nylon-6/epoxy, (e ) 35 wt% nylon-6/epoxy. Nylon-6/epoxy blend fractions with higher -caprolactam loadings (c, d & e) show enhanced thermal stability than neat epoxy (a) till 220oC

Figure 45. Glass transition temperatures for all blend fractions synthesized by solution (or ex- situ) blending were obtained by plotting the tan delta peaks from dynamic mechanical measurements and variation of tan delta with temperature for ex-situ synthesized nylon-6 loadings in epoxy for (a.) neat epoxy (control), (b.) 5 wt% nylon-6/epoxy, (c.) 10 wt% nylon- 6/epoxy, (d.) 20 wt% nylon-6/epoxy, (e.) 30 wt% nylon-6/epoxy [15]. Figure 46. Comparison between variation in glass transition temperature with increase in nylon-6 loading in (a) ex-situ blends and -caprolactam (monomer of nylon-6) loading in (b) in-situ blends. Figure 47. Variation of Storage Modulus with temperature for the following ex-situ nylon- 6/epoxy blend fractions [(a.) neat epoxy (control), (b.) 5 wt% Nylon-6, (c.) 10 wt% Nylon-6 (d.) 20 wt% Nylon-6, (e.) 30 wt% Nylon-6 in epoxy matrix [15] Figure 48. Comparison between variation in storage modulus (glassy region) measured at 25oC with increase in nylon-6 loading in (a) ex-situ blends and -caprolactam (monomer of nylon-6) loading in (b) in-situ blends. Figure 49. Comparison of variation in storage modulus (rubbery plateau region) measured at 130oC with increase in nylon-6 loading in (a) ex-situ blends and -caprolactam (monomer of nylon-6) loading in (b) in-situ blends. Figure 50. Crosslink density variation for blends formed by in-situ polymerization with increasing -caprolactam (monomer of nylon-6) loading (wt%).

Figure 51. Crosslink density variation for blends formed by solution (ex-situ) polymerization with increasing Nylon-6 loading (wt%). Crosslink density for solution blended nylon-6/epoxy Figure 52. SEM Micrograph of a cross section of neat epoxy at 1000x and 2000x magnification Figure 53. SEM Micrographs of 20 wt% in-situ Nylon-6/Epoxy blend at 1000 X and 2000 X magnification. Figure 54. SEM Micrographs of 30 wt% in-situ Nylon-6/Epoxy blend at 1000 X and 2000X magnification Figure 55. SEM Micrographs of (a) 20 wt% Nylon-6/ Epoxy solution (ex-situ) blend[ ], (b) 20 wt% Nylon-6/ Epoxy in-situ blend show a comparison between the nature of dispersion of nylon-6 in solution and (b) in-situ polymerization methods of blending respectively Figure 56. SEM Micrograph of neat Nylon-6 synthesized at 80oC

CHAPTER 1

INTRODUCTION

Epoxy resins are thermosets that comprise the polymer matrix in adhesives and composite materials for aerospace, construction, coatings, and automotive industries. Their abundant use is dictated by their excellent chemical resistance, physical and mechanical properties. The microelectronics industry also uses epoxy in packaging applications. However, epoxy has been known to suffer from inherent brittleness and low fracture toughness toughness due to high crosslink density. Various attempts have been made to eradicate these drawbacks by blending impact modifiers such as elastomers and thermoplastics as the second phase in epoxy matrix.

1.1. Synthesis routes for polymer blends:

Polymer blends are synthesized by a variety of methods, namely; melt blending, solution blending and in-situ polymerization [1]. Melt blending is known to be the most practical and commonly used method in industry due to its simplicity. In this method, the thermoplastics are mixed with fillers or polymers in an extruder via shear mixing above their glass transition temperature (Tg).

However, this method is only restricted to thermoplastics [2].

High performance polymers which cannot be melt blended due to their high glass transition temperatures, are synthesized by solution blending. This technique involves mixing polymers with a second phase, dispersed in a suitable solvent. On careful removal of this solvent, a composite

15 structure is created. Most of the research focused on nylon/epoxy blends uses the solution blending method.

Lastly, in-situ polymerization is widely used to synthesize polymer nanocomposites and blends, and has attracted attention off late. For polymer blends, in-situ polymerization is a type of reactive blending of a monomer in presence of a polymer; accompanied by a chemical reaction between the functional groups of the individual polymer components in the blends [3]. In some cases

[4], this method leads to the formation of interpenetrated polymer networks (IPNs).

1.2. Incorporation of thermoplastic in Epoxy:

The idea of incorporating thermoplastics and elastomers in epoxy has been known to improve the damping properties of inherently brittle epoxy. When a thermoplastic or elastomer is introduced in epoxy matrix, the former precipitates out on curing, leading to a multiphase morphology; where the filler phase prevents crack propagation by absorbing the impact forces and thereby increasing the strength [5]. However, Pearson and Yee [6] pointed out some major drawbacks of using elastomers as impact modifiers, claiming that the rubbery particles decrease elastic modulus and fail to impact highly cross-linked systems. Another drawback of elastomers is the presence of unsaturation, which make the blend susceptible to thermal or oxidative degradation [7]. According to Hodgkin, inclusion of thermoplastics does not cause decrease in modulus and tensile strength of epoxy. Conclusively, thermoplastics have been identified as better impact modifiers than the elastomeric ones. Barone et al. [8] studied the thermo-mechanical properties of epoxy/poly(ε-

16 caprolactone) (PCL) blends by varying the amount of poly (ε-caprolactone). The toughness improved with increase in PCL content, accompanied by reduction of glass transition temperature.

In order to increase the glass transition temperature, a tri-functional resin was added. Blanco et al

[9] investigated the behavior of co-polyethersulphone-epoxy blends at various loadings of co- polyethersulphone, and noticed an absence of phase separation and increase in glass transition temperature for all loadings. Based on this observation, the study [9] concluded the formation of an inter-penetrating polymer network (IPN), which was responsible for toughening.

1.3. Incorporation of Nylon in Epoxy:

A few researchers in the past have studied nylon/epoxy blends with respect to blend compatibility, cure kinetics, morphology and thermal characterization. The advantages offered by blending nylons are good adhesion with epoxy; and curing of epoxy without blushing. By virtue of excellent adhesion with epoxy, nylons can be blended with epoxy to produce adhesives with high peel strength [10,11]. Motivated by the idea, Gorton [12] studied adhesive blends of nylon-epoxy and investigated the interaction of nylon with epoxy by i) dissolving a commercially available nylon grade with hot ethanol, and ii) blending with epoxy resin and curing agent. On the basis of swelling tests, occurrence of cross-linking between epoxy rings and hydrogen of the amide group in nylon has been reported [12]. Based on DMA and differential scanning calorimetry (DSC) tests, Wang et al. [13] used the similar approach of blend preparation as Gorton et al [12], and studied the blend compatibility of nylon-epoxy resins by varying the epoxy loading in nylon. A reduction in

17 crystallinity of nylon phase with increase in epoxy was detected, whereas the tensile strength of the blend varied linearly with epoxy loading in nylon. Wang et al [13] identified the presence of three phases; crystalline nylon, cross-linked nylon-epoxy and cross-linked epoxy phase for more than 10 wt% epoxy resin in nylon. Zhong et al [14] scrutinized the miscibility and cure kinetics of blends; prepared by dissolving nylon in alcohol, adding epoxy to nylon, and finally solution casting. Zhong et al [14] reported the presence of a single amorphous phase in cured nylon-epoxy reactive blends and two phases in the uncured nylon-epoxy system; thus establishing the fact that crystallinity of nylon is reduced due to curing. Vyas and Iroh [15] studied the effect of ex-situ blending of nylon-

6/epoxy on thermal and mechanical properties; demonstrating that nylon-6 participated in epoxy curing; eventually leading to higher rubbery storage modulus than neat epoxy. However, a dip in rubbery storage modulus and Tg was witnessed with further increase in nylon-6 loading due to plasticizing effect of nylon-6 crystals in epoxy. The nylon was found to crystallize in the cracks and crevices of epoxy thereby leading to a toughening effect. Vyas and Iroh [15] explained the rise in storage modulus from neat epoxy to 5 wt% nylon-6 loading, which was attributed to increased crosslinking density.

1.4. Significance of In-situ Blending

Prior work caters to ex-situ blending of nylon in epoxy, where epoxy loading was varied in nylon and observed a dip in Tg at higher nylon loadings with an increase in cross-linking at higher epoxy loadings. In-situ polymerization of nylon-6 in epoxy has been unexplored. In-situ

18 polymerization method is a reactive blending route, where a monomer is polymerized in presence of another polymer; accompanied by chemical reactions of functional groups of the polymer blend components [3]. Landry et.al [16] have demonstrated that in-situ polymerization of tetraethoxysilane in poly (methly methacrylate) (PMMA) led to increased plateau modulus, good dispersion of silicon dioxide (SiO2) and strong interaction between the phases. The in-situ method has been extensively used in synthesizing polymer nanocomposites. Ou et al [7] reported enhanced mechanical properties of nylon-6/silica composites prepared via in-situ polymerization of nylon-6 in presence of silica nanoparticles up to 5 wt% silica in the epoxy matrix. Scanning electron microscopy (SEM) results reveal that this enhancement was due to homogeneous distribution of the filler in nylon matrix. The in-situ method is known to improve the nanofiller dispersion in the matrix, and increase the interaction between matrix phase and reinforcing phase. The initial low viscosity allows uniform distribution of the filler phase in the polymer matrix [18]. Other in-situ polymerization blends studies include PMMA/styrene acrylonitrile [19], poly (phenyl ether)/ high impact polystyrene [20], PCL/

PMMA [21].

1.5. Epoxy Resin

Epoxy prepolymers are predominantly ethers obtained as a result of the reaction between bisphenol A and epichlorohydrin [22,23]. These low molecular weight prepolymers can be cured to form three-dimensional networks by subjecting them to heat or adding curing agents such as amines, polyamides and polysulphides [22].

Epoxy prepolymers comprise a three membered ring, known as the oxirane ring as shown in figure (1) below, which is a highly unstable structure owing to its strained nature. The oxygen atom in the oxirane ring, which has an electron withdrawing nature, attracts the electrons from carbon atoms towards itself, thus leading to a strained structure and making it prone to cleavage in presence of a curing agent [24].

Figure.1 Structure of an Oxirane Ring

1.5.1 Types of epoxy resins There is a plethora of epoxies available in the market. The major types of epoxy resins have been introduced below [25].

1.5.1a. Bisphenol A type resins

Amongst the Bisphenol A type resins, diglycidyl ether of bisphenol A (DGEBA) is the most commonly used resin. It is formed by reacting bisphenol A with epichlorohydrin. DGEBA resins are bifunctional resins and contain oligomers of different molecular weights. Owing to the presence of

–OH along the chain backbone, DGEBA resins possess excellent adhesion, chemical resistance and thermal stability. Therefore, bisphenol A resins are widely used in automotives, electronics, adhesives and coatings [26].

Figure 2. Structure of a Bisphenol A oligomer [26]

1.5.1b. Bisphenol F type resins

Bisphenol F resins are formed by reacting bisphenol F with a formaldehyde. Figure (3) shows the molecular structure of bisphenol-F resin. Due to the absence of methyl groups between the two benzene rings of epoxy, the viscosity of these resins is lower than that of bisphenol A. Additionally, owing to higher functionality than the bisphenol A type, these resins have a tighter cross-linking due to availability of more reaction sites [28].

Figure 3. Structure of Bisphenol F oligomer [27]

1.5.1c. Multifunctional epoxy resins Multifunctional epoxy resins have more than two oxirane rings in their structure and are well suited for high temperature applications in adhesives, and electronic circuit components [29]. Some of these include triglycidyl derivative of p-aminophenol (trifunctional epoxy resin) and tetraglycidylmethylenedianiline (TGDDM) shown in figure (4) and (5) respectively 21

Figure 4. Triglycidyl derivative of p-aminophenol [30]

Figure 5. Tetraglycidylmethylenedianiline (TGDDM) [31]

1.5.1d. Novolac Epoxy Resins Owing to higher functionality than bisphenol A and bisphenol F, novolac epoxy resins have a tighter cross-linking thus making the resin more brittle than bisphenol A and bisphenol F. Also, due to high funtionality, and presence of aromatic rings, these resins are highly chemical and heat resistant [28]. Figure (6) below shows the typical structure of an epoxy novolac resin.

Figure 6. Epoxy Novolac resin [32]

1.5.1e. Cycloaliphatic epoxy resins Cycloaliphatic epoxies are short molecules with an absence of aromatic rings and repeat unit.

These epoxies have good weathering properties due to lack of aromatic unsaturation and possess low viscosity [33]. The structure of a cycloaliphatic epoxy is illustrated by figure (7) below

Figure 7. Cycloaliphatic epoxy [33]

1.6. Curing of Epoxy

Amines, polyamides, carboxylics and anhydrides are some of the hardeners used to cure epoxies [34, 35]. Curing agents or ‘hardeners’ are responsible for the opening of the oxirane ring to

23 form a three dimensional structure. The reactive groups such as amine (N-H), carboxyl (COOH) present on the hardeners open up the oxirane ring by chain scission. Consequently, a hydroxyl (O-

H) group is generated which reacts with other epoxy molecule leading to curing of epoxies by homopolymerization [36]. Figure (8) below illustrates the curing mechanism of epoxy by an amine

[37].

III

Figure 8. Epoxy curing mechanism by an amine [37]

Curing of epoxy resins using anhydride curing agents involves opening of the anhydride ring to form a monoester. The carboxylic groups on the monoester react with the oxirane ring, thus generating

24 hydroxyl groups [38]. Anhydride curing agents produce epoxies with excellent thermal stability and high glass transition temperatures [39, 40].

1.7. Nylon 6

Nylon 6 is classified as a synthetic aliphatic polyamide which is an extensively used engineering thermoplastic. The characteristic highly polar amide group (CO-NH) imparts strong interchain attraction between the polymer chains and is also responsible for its hygroscopic character

[41]. Figure (9) below shows the structural formula of nylon-6.

Figure 9. Nylon-6

The intermolecular cohesive forces due to presence of polar groups and hydrogen bonding between the polymer chains in nylons impart good chemical resistance [42, 43]. However, the presence of polar groups makes nylons susceptible to moisture absorption.

1.8. Synthesis routes of Polyamide 6 Polyamide 6 is typically synthesized via ring opening accompanied by chain polymerization reaction of the Ɛ-caprolactam. Owing to its ring strain and presence of heteroatom, the seven membered ring tends to polymerize to relieve itself from the strain in the presence of a catalyst and initiator [44]. Chain polymerization is classified into radical, cationic and anionic polymerization which involves addition of the monomer to growing chain ends which is in the form of a free radical, 25 anion or cation [45]. Cationic polymerization [46] involves generation of a caprolactam cation in an acidic medium which adds itself to the monomers and chain propagates to form nylon-6. However, nylon-6 synthesis via cationic polymerization is carried out at high reaction temperatures. In the current study, anionic ring opening addition of caprolactam has been employed to prepare the reactive blend. The mechanism [47, 48] for anionic polymerization of nylon-6 is illustrated below in Figure (10).

Figure 10. Schematic representation of Nylon-6 synthesis

In the current work, the idea was to use ε-caprolactam; a monomer of nylon-6, which would polymerize and cure epoxy, leading to the formation of a reactive in-situ blend of nylon-6/epoxy.

1.9. Research Objective

As discussed previously, solution blending (ex-situ blending) of nylon-6 in epoxy, by Vyas et al. led to the toughening of epoxy as a result of crystallization of nylon-6 in the cracks and crevices of epoxy. However, it also led to a reduction in glass transition temperature at higher nylon loadings

26 which has been attributed to plasticizing effect of nylon-6 crystals; indicative of poor dispersion of nylon-6 in epoxy. The aim of the current work is to fabricate and characterize a blend of nylon-6 and epoxy with improved dispersion of Nylon-6 (thermoplastic phase) in epoxy (thermoset phase).

(i) This study will employ low temperature synthesis of nylon-6 by anionic ring opening

polymerization of ε-caprolactam in presence of epoxy prepolymer. Consequently, the

polymerization of nylon-6 and curing of epoxy will occur simultaneously to form a

reactive blend. The proposed method draws its inspiration from the in-situ synthesis of

polymer-clay nanocomposites, which results in better dispersion of the filler in matrix.

(ii) The ε-caprolactam loading will be varied from low to high weight percent with respect

to solid content in the epoxy resin to study the effect of caprolactam loading in epoxy on

the thermal and thermo-mechanical properties of the blends

(iii) The thermo-mechanical properties will be correlated with the interactions between

monomeric nylon-6 and epoxy.

(iv) Consequently, the effect on properties obtained via in-situ blending will be compared to

the solution blending (ex-situ) method.

CHAPTER 2

EXPERIMENTAL

2.1. Materials

DGEBA epoxy resin EPIREZ 5522-WY-55 dispersion was procured from Hexion Inc.

Columbus, USA. Ɛ-Caprolactam (monomer), sodium hydride NaH 95% pure (catalyst), N- acetylcaprolactam (initiator) and 1-Methyl-2-Pyrrolidinone anhydrous 99.5% pure (NMP solvent) were purchased from Sigma-Aldrich, USA.

2.2. Synthesis of Neat Nylon-6

Ɛ-caprolactam was dissolved in 25ml NMP in a 500 ml three-neck round bottom flask immersed in a water bath. Nitrogen blanket was introduced and this was followed by the addition of sodium hydride (NaH) catalyst. The solution was stirred using a mechanical stirrer and heated gradually till 80oC. When the temperature of the solution reached 60oC, N-acetyl Caprolactam

(initiator) was added to it. When the temperature reached 80oC in approximately 2 hours, the nitrogen blanket was removed and the solution was stirred for 30 mins in order to terminate the reaction by air.

2.3. In-situ synthesis of Nylon 6/ Epoxy reactive blend

The solution blend was synthesized using a 500 ml three-neck round bottom flask immersed in a water bath. 25 ml EPIREZ 5522-WY-55 resin was dispersed in 40 ml NMP and charged into the three-neck round bottom flask. Monomer, Ɛ-caprolactam, was weighed with respect to solid

28 content epoxy resin and dissolved in 10 ml NMP. This solution was slowly added to the dispersed epoxy resin, followed by addition of NaH. The solution was subjected to mechanical stirring till it formed a homogeneous mixture. The three-neck flask was then lowered into the water bath with the introduction of nitrogen blanket and temperature was raised to 80oC. N-acetylcaprolactam (initiator) was added when the solution temperature had reached 60oC. When the solution temperature reached

80oC, the nitrogen supply was ceased and solution polymerization was terminated by air for 30 minutes. In the course of polymerization, various color transitions were observed from white to blue and from blue to green. The solution blend was then poured into a Teflon mold till half the depth of the mold cavity.

The compositions of the various in-situ blend fractions prepared have been summarized in table.1 below.

Table.1 Compositions for in-situ Nylon-6/Epoxy blends

Blend Epoxy Ɛ- (% Ɛ NMP EPIREZ Ɛ-Caprolactam Epoxy (solids) caprolactam Caprolactam ml Resin (ml) (%wt) (%wt) w.r.t. Epoxy g g solids)

Neat Epoxy 20 10 5 0 0 100

5% NY-EP 50 25 12.5 0.7389 5.58 94.41

10% NY-EP 100 50 25 2.95587 10.57 89.42

15% NY-EP 50 25 12.5 2.2169 15.06 84.93

20% NY-EP 50 25 12.5 2.955 19.12 80.87

25% NY-EP 50 25 12.5 3.6948 22.81 77.18

30%NY-EP 50 25 12.5 4.433 26.18 73.82

35%NY-EP 50 25 12.5 5.09872 28.97 71.03

2.4. Sample preparation for neat Epoxy (control):

25 ml EPIREZ 5522-WY-55 resin was dispersed in 50 ml NMP. The solution was charged in a three neck round bottom flask and heated till 80oC and stirred for 30 minutes at 80oC. It was then poured into the teflon mold cavity and subjected to the same cure conditions discussed in the following section (section 2.4).

2.5. Cure Conditions:

The synthesized blend fractions were cured step-wise in a vacuum oven using the following conditions of temperature and time of exposure:

Table 2. Cure conditions

Temperature Time of exposure

60oC 14 hours

100oC 4 hours

120oC 2 hours

150oC 2 hours

165oC 1 hour

180oC 2 hours

190oC 1 hour

200oC 1 hour

2.6. Characterization Techniques

2.6.1. Fourier Transform and Infrared Spectroscopy Attenuated Total Reflectance (FTIR-

ATR):

FTIR-ATR was performed on the test samples using NICOLET 6700 accessorized with a diamond crystal to observe the behavior of the oxirane and carbonyl peak of amide group and behavior of the hydroxyl peak in order to study hydrogen bonding. The range of wavenumbers used to collect the spectra was 4000cm-1 – 500cm-1. A total of 32 scans were collected for each sample.

2.6.2. Dynamic Mechanical Analysis (DMA):

DMS 6000, purchased from Seiko Instrument Inc. was used to study the viscoelastic properties and determine the glass transition temperature (Tg) of the in-situ nylon 6/ epoxy blends of varying monomer content, in tension mode, at a frequency of 1Hz using a temperature range of

25oC - 200oC. Figure (11) below shows a typical DMA curve where the storage modulus is plotted

31 against temperature, with a corresponding tan delta peak in the glass transition region denoting the transition temperature from the glassy region to the rubbery region.

Figure 11. Schematic of a typical DMA graph

2.6.3. Scanning Electron Microscopy (SEM)

Scanning electron microscopy (SEM) was conducted with an FEI Phillips Electroscan XL30

ESEM FEG microscope using an acceleration voltage of 25 kV. 1mm thickness samples of Neat nylon, neat epoxy, 20% nylon-epoxy and 30% nylon-epoxy were cryo-treated prior to fracture. The fractured specimens were vertically mounted on individual specimen holders with the cross-section facing upwards, using conductive double-sided carbon tape. To ensure the samples are conductive, the top surface was coated with a thin Au-Pd layer in inert atmosphere for 45 seconds.

2.6.4. Thermogravimetric Analysis (TGA)

Thermogravimetric Analysis was performed using TA Instrument TGA Q50 V20.10, at a heating rate of 10oC per min from 23oC to 600oC under nitrogen atmosphere. The sample mass ranged from 6 mg to 18 mg. Neat epoxy, neat nylon-6, and in-situ blends of nylon-6/epoxy were evaluated for thermal stability and composition of the blends.

Figure 12. Schematic of a TGA thermogram [49]

2.6.5. Shear Viscometry:

Shear viscosity measurements were carried out at room temperature, using the Brookfield

DV-I + Viscometer using 10 ml solution blend of 2.5% and 30% nylon-6/epoxy (NY-EP) fractions. 33

An S31 spindle of diameter 11.88mm and a container of diameter 18.84 mm were used for measurement. Viscosity readings were recorded at spindle speeds varying from 5 rpm to 100 rpm.

Shear rate values were calculated using the following equations [50]

2 2 2휔푅푐 푅푏 훾′ = 2 2 2 푥 (푅푐 − 푅푏)

2휋 휔 = 푅푃푀 60

Where γ’ is the shear rate (sec-1), ω is the angular velocity of the spindle (rad/sec), is the inner radius of the container (cm) , is the radius of the spindle (cm) and x is the radius at which shear rate is being calculated.

The temperature of the Brookfield Viscometer jar was varied from 25oC to 80oC and viscosity readings were taken at 25oC, 60oC, and 80oC at spindle speeds ranging from 5 rpm to 100 rpm. Activation energies were evaluated for the 2.5 wt% NYEP and 30% wt% NYEP in-situ blends at spindle speeds of 10, 20, 50 and 100rpm at 25oC, 40oC and 80oC.

For time-based viscosity measurements, the synthesis process was altered to carry out viscosity measurements at a constant polymerization temperature of 80oC. Thus, the initiator was added at 80oC instead of 60oC, unlike the synthesis method described in section (2.2). The solution was subjected to mechanical mixing for two minutes for the initiator to disperse well. Stirring was ceased and the solution was polymerization for 30 minutes. During the course of polymerization,

34 from the point of adding the initiator, 10-12 ml aliquots were taken from the flask after 5, 15, 25 and

30 minutes to carry out shear viscosity measurements using Brookfield Viscometer in order to study the variation of viscosity with time.

CHAPTER 3

RESULTS & DISCUSSION

3.1. Reactive Blends of Nylon-6 and Epoxy 3.1.1. Fourier Transform Infrared (FTIR) Spectroscopy Analysis

In order to synthesize in-situ blends of nylon-6/ epoxy (NY-EP), ε-caprolactam (monomer) was added to epoxy pre-polymer, followed by addition of initiator and catalyst for polymerization of nylon-6 in presence of epoxy matrix.

3.1.1a. FTIR Analysis of ε-Caprolactam

ε- caprolactam is a seven membered ring which contains: secondary amine group, carbonyl group and methyl groups, as shown in figure (13) below.

Figure 13. Structure of ε- caprolactam

Figure (14) shows the absorbance spectrum of the monomer, ε-caprolactam. The peaks at

3292 cm-1, 1653.7 cm-1 and 2926 cm -1 are assigned to secondary amide N-H, C=O (carbonyl) of

36 amide and –CH2 (methyl) groups respectively. The characteristic peak attribution in the monomer is tabulated below,

Table.3 Absorbance peaks for ε-caprolactam [51]

Functional Group Absorbance peak (cm-1) N-H 3292 C=O 1653.7 C-H 2926

3.1.1b. FTIR Analysis of neat Epoxy Epoxy molecule, shown in figure (15), consists of the highly reactive oxirane rings, - OH groups, benzene rings, and methyl groups.

Figure 15. Structure of an Epoxy resin molecule [27]

Figure (16) shows the absorbance spectrum of neat epoxy. Peaks at 3402 cm-1, 1506 cm-1,

914 cm-1 and 862 cm-1 are attributed to the O-H (hydroxyl), C-C of benzene ring and unreacted oxirane ring respectively [52] [15].

Table. 4 Absorbance peaks for neat Epoxy [52]

Functional group Absorbance peak (cm-1)

O-H 3402

C-C 1506

914, 862

3.1.1c. FTIR Analysis of ε-caprolactam in Epoxy during in-situ synthesis

Dispersion of ε-caprolactam in epoxy pre-polymer gives rise to the absorbance spectrum in figure (17). At room temperature, the peak at 3403 cm-1 had a shoulder. When the mixture was heated up to 65oC, the N-H peak from the caprolactam and O-H peak from the epoxy merged to form a single peak at 3400 cm-1 as seen in figure (18). The sharp peak at 1665 cm-1 is due to the presence of the conjugated C=O group of the amide [53].

3.1.1d. FTIR Analysis of Nylon-6/Epoxy blend compositions after curing

2.5, 5%, 10%, 15% 20%, 25%, 30% and 35% monomer was weighed with respect to solid epoxy and in-situ blend fractions were prepared at a reaction temperature of 80oC. All blend solutions were cured at the same time and temperature. Figure (19-25) show the FTIR spectra for

5%, 10%, 15%, 20%, 25%, 30% and 35% monomeric nylon-6/ epoxy blends respectively and figure

(26) shows the overlay of FTIR spectra for all the in-situ blend fractions. The effect of increasing 38 nylon-6 loading on O-H groups is seen in figure (27). The O-H peak is seen to shift to low wavenumbers with increase in nylon-6 loading. Peak shift to lower wavenumbers suggests O-H peak being surrounded by neighboring atoms [54]. There could be a possibility of the O-H group forming hydrogen bonds with oxygen atoms of C=O group of the amide and hydrogen atoms of other –CH2 and –NH groups of the caprolactam.

3.1.1e. FTIR Analysis of neat Nylon-6 synthesized at 80oC

Neat nylon-6 was synthesized using NaH as a catalyst and N-acetyl caprolactam as initiator at 80oC using anionic ring opening polymerization. Nylon-6 consists of an amide and methyl groups as shown in figure (28) below

Figure 28. Structure of Nylon-6

Figure (29) shows the FTIR spectrum of neat nylon. The nylon formed shows a hydrogen bonded secondary amine peak at 3290 cm-1[55, 56]. The peak at 1676 cm-1 and 1155 cm-1 has been assigned to the C=O bond of the amide group and –CONH skeletal motion, although literature reports –CONH skeletal motion peak to be at 1160 cm-1 [56]. Absorption peak attribution for nylon-

6 has been summarized in Table.5 below. 39

Table.5 Absorbance peaks for Nylon-6

Functional Groups Absorbance peak (cm-1)

Secondary Amine (R2NH) 3290

Carbonyl (C=O) 1676

Amide (-CONH-) 1155

3.1.1f. FTIR Analysis of Epoxy conversion

Incorporation of caprolactam in epoxy resin, followed by addition of catalyst and initiator and heating the mixture up to 80oC would give rise to two competing processes: i) anionic ring opening polymerization of caprolactam to form nylon-6 and ii) curing of epoxy. The three membered oxirane ring in epoxy is the most reactive site and is responsible for the curing of epoxy. The oxirane peak behavior was monitored using FTIR-ATR mode by varying the monomer loading in epoxy.

Figure (16) shows the FTIR spectra of neat epoxy having an oxirane peak at 914 cm-1. Figure (30) shows an overlay of the oxirane peaks for various monomer loadings ranging from 2.5% to 30% by weight of solid epoxy. The fractional conversion of epoxy was calculated using the following equation below [15].

퐴914,표푥푖푟푎푛푒 퐴 α = ( 1506,푏푒푛푧푒푛푒 ) 퐴914,표푥푖푟푎푛푒 퐴1506,푏푒푛푧푒푛푒 40

In order to quantify the change in the area, the stretching vibration of C=C at 1506 cm-1 was chosen as an internal standard to normalize the variation. As seen in figure (31), amongst the in-situ nylon-6/epoxy blend fractions, the epoxy fractional conversion was the least at 2.5% after neat epoxy and increased drastically as the monomer content was increased up to 15%. The conversion has been seen to almost reach a plateau after reaching 15% monomer loading in epoxy. This indicates maximum consumption of the oxirane rings.

Also, the C=O peak of the amide seen at 1653 cm-1 in figure (14) for the caprolactam is seen to have vanished completely in figure (19) and figure (20) suggesting that a C=O group of the amide in ε-caprolactam participated in curing. In figure (21), when monomer loading is increased to 15%, the reappearance of the C=O absorbance at 1643 cm-1 is noted. This peak is clearly visible in figure

(22), figure (23), figure (24) and figure (25) with further increase in monomer loading. The reappearance of the amide carbonyl may be due to the formation of small residual nylon-6 crystals which have not participated in cross-linking. There could be a possibility of polymerized nylon-6 particle inclusions in the cross-linked nylon-6/epoxy matrix.

3.2. Dynamic Mechanical Analysis (DMA)

Dynamic Mechanical Analysis involves subjecting the test sample to an oscillatory force; and recording the response of the material in the form of storage modulus, loss modulus and tan delta as a function of temperature, at a specified frequency. Dynamic Mechanical Analyzer is a useful polymer characterization tool to study and obtain information pertaining to thermal and

41 viscoelastic properties like storage modulus, loss modulus and glass transition temperature as a function of temperature or frequency. In addition to the thermo-mechanical properties, cross-link densities were evaluated for all compositions and a qualitative analysis of morphology has been discussed further.

In-situ polymer blends of nylon-6/epoxy with varying nylon-6 monomer content were synthesized. Samples were cast in a Teflon cavity and exposed to the same curing time and temperature conditions. Rectangular strips of the samples were cut and clamped onto the sample holder. The samples were subjected to an oscillatory tensile force at a frequency of 1 Hz and temperature sweep from 23oC-200oC. Storage modulus, loss modulus and tan delta values were recorded for each nylon-6/ epoxy fraction.

3.2.1. Analysis of Tan δ peaks

Figure (32) & figure (33) shows the DMA thermograms for neat epoxy and seven blend fractions: 5 wt% NY-EP, 10 wt% NY-EP, 15 wt% NY-EP, 20 wt% NY-EP, 25 wt% NY-EP ,30 wt% NYEP and 35 wt% NY-EP; displaying the variation in tan delta and storage modulus plotted against temperature respectively. Glass transition temperatures were obtained from temperature values on the X-axis corresponding to the maxima of the tan delta peak values. The Tg, tan delta peak height and area under the tan delta peak have been tabulated below in Table 6.

Table.6 Summary to Tan delta peak analysis

Nylon-6/epoxy Glass Transition Tan delta Peak Area under Full width fraction temperature o C maximum Tan delta peak Half Maximum

Neat epoxy 74 1.04 22.2 16.9

5% Nylon-6/epoxy 68 ± 2.7 1.12 25.6 18.0

10% Nylon-6/epoxy 73 ± 4.4 1.00 23.3 19.1

15% Nylon-6/epoxy 86 ± 1.45 0.9 18.9 17.2

20% Nylon-6/ epoxy 82 ± 0.46 1.19 19.4 13.7

25% Nylon-6/epoxy 85 1.03 23.1 17.7

30% Nylon-6/ epoxy 78 ± 3.71 1.05 26 19.6

35% Nylon-6/epoxy 81 ± 0.09 0.93 17.30 14.6

The area, width and height of the tan delta peaks were determined using Origin Pro 8.5, to study the effect of monomer loading on the damping properties. Tan delta is defined as the ratio of loss modulus to storage modulus. Higher the magnitude of tan delta, higher is the energy loss and viscous character in the viscoelastic material. Therefore, information pertaining to polymer chain mobility can be obtained from the magnitude of tan delta peak [57, 58]. Peak width and area under the tan delta peaks reveal presence of heterogeneity in the samples and damping behavior [58].

As seen in figure (32), all compositions of the in-situ blends of nylon-6/epoxy show a single tan delta peak which indicates the formation of a homogeneous blend morphology. This could be attributed to the formation of interpenetrating polymer network due to reaction of monomeric nylon

6 with epoxy pre-polymer. Tan delta peak heights for 5% and 20% NY-EP samples are higher than that of neat epoxy. On close observation, the peak height reduces with further addition of monomer content for 15% and 35% NY-EP samples. The reduction in peak height is attributed to restricted molecular motion with increase in reactive sites. Interestingly, peak broadening occurred at 5%,

10%, 25% and 30% NY-EP blends.

With the introduction of nylon-6 monomer up to 5% by weight of solid epoxy, the Tg falls

o o from 74 C to 68 C accompanied by sudden increase in peak area. The initial plunge in the Tg can be ascribed to the plasticizing effect of the low molecular weight monomer in an epoxy matrix. With further incorporation of the monomer, there is increase in glass transition temperature from 68oC to

86oC at 15 wt% loading. Xiong et al. [59] have reported a similar behavior where the tan delta peaks shifted to higher temperatures with increase in titania filler in acrylic resin. The rise in Tg has been attributed to reduced segmental chain mobility due to enmeshed polymer chains in the inorganic network [59]. It can also be noted that the tan delta peak area, height and width decrease with an increase in monomer content from 5 wt% to 15 wt% implying that polymer chain mobility is restricted. This could be due to increased concentration of polar amide groups leading to an increase in the number of reactive sites for crosslinking. At 30 wt% monomer loading, the Tg reduces to

75.4oC. The dip in Tg from 86oC to 78oC is accompanied with an increase in tan delta peak area and 44 width. Broadening of tan delta peak suggests a presence of slight micro-heterogeneity in the blend as reported by Park et al. [58].

3.2.2. Analysis of Storage Moduli

Figure (33) shows the storage modulus plotted against temperature with varying monomer content for neat epoxy, and 5wt%, 10 wt%, 15 wt%, 20 wt%, 25wt%, 30 wt%, 35 wt% NY-EP blend compositions. The values of storage modulus in the glassy and rubbery regions, along with crosslinking densities are summarized below.

Table.7 Summary of Storage moduli analysis

Nylon-6/Epoxy Glassy Storage Rubbery Storage Crosslink blend fraction Modulus at 23oC Modulus at 130oC density (Pa) (Pa) computed at 130oC 3 (nv/m ) Neat epoxy 2.19E+09 9.07E+06 889

5% NY-EP 1.98E+09±6.29E+08 8.29E+06±4.31E+06 825±429

10% NY-EP 2.30E+09 ± 5.66+07 9.90E+06±1.22E+06 985±121

15% NY-EP 2.82E+09± 5.66+07 2.07E+07±4.59E+06 2055±456

20% NY-EP 2.54E+09± 1.50E+08 1.26E+07±8.01E+05 1264±78

25% NY-EP 1.71E+09 7.47E+06 726

30% NY-EP 3.07E+09±1.54E+09 9.65E+06±5.70+06 973±571

35% NY-EP 2.53E+09±1.84E+08 2.85E+07±5.34E+06 2838±531

Nair and coworkers [60] have stated that storage modulus in the glassy region is an outcome of intermolecular interactions and crystalline materials have displayed a higher storage modulus in the glassy region. As seen in figure (33), storage moduli for majority of blend compositions in both glassy and rubbery region are higher than neat epoxy. On close observation, storage modulus in the glassy region increases with an increase in monomer loading except for 5wt% and 25 wt%. However, in the rubbery region, it increases up to 15 wt% monomer loading, after which it shows a dip at 25 wt% monomer loading. The storage modulus in the rubbery region provides information on the crosslinking density. Cross-linking density, is defined as the number of crosslinking sites per unit volume and was calculated using storage moduli values falling in the rubbery plateau region at 130oC using the equation below [61] [62] [63]

퐸′ 푣 = 푒 3푅푇

3 where 푣푒 is the cross-link density in number of cross-links per unit volume (nv/m ), E’ is the storage modulus obtained from the rubbery plateau, R is the universal gas constant (8.314 m3PaK-1mol-1) and T is the temperature in Kelvin at which the storage modulus in the rubbery region is measured.

In summary, after considering the plots of tan delta and storage modulus vs. temperature, rise in glass transition temperature up to 20 wt% is due to an increase in cross-linking density. The cross-links impede polymer chain mobility, which is reflected from the drop in tan delta peak height,

46 reduction in area under the peak and peak width. The glass transition temperature dropped on incorporation of monomer up to 30 wt%. This drop can be attributed to decreased cross-link density.

Broadening of tan delta peak; accompanied by an increase in area under the peak suggests improved damping ability at higher monomer loadings and increased chain mobility. Moreover, this can also be attributed to presence of micro-heterogeneities. The heterogeneous blend may be composed of regions of cross-linked nylon-6/ epoxy and residual nylon-6. The residual nylon-6 may be responsible for increasing the crystallinity in the 30 wt % nylon-6/ epoxy blend fraction [60]. Hence the 30 wt % fraction possessed the highest storage modulus below Tg. When the temperature crossed

Tg, the crystalline inclusions of nylon-6 between the crosslinks softened, causing chain slippage, thereby reducing the storage modulus in the rubbery region.

3.3. Shear Viscometry Knowledge of rheological behavior is crucial to understand processability and product performance of polymers. Polymer viscosity is an important rheological property. Viscosity measurement is a convenient method to detect structural changes occurring in the polymer due to modifiers, additives and polymer blending. In the current work, the viscosity and flow behavior of nylon-6/epoxy blends has been studied with respect to nylon-6 monomer composition, temperature and polymerization time using the Brookfield viscometer.

3.3.1. Effect of Nylon-6 monomer loading on shear viscosity

Shear viscosity measurements were carried out for 12 ml solutions of 2.5 wt% and 30 wt % nylon-6/ epoxy blend at a temperature of 80oC using the Brookfield DV Pro I viscometer. Figure

(34) and figure (35) show plots of viscosity vs. shear rate for 2.5 wt% and 30 wt% nylon-6 epoxy compositions recorded at 80oC. Both blend fractions exhibit pseudoplastic or shear thinning behavior. It can be noted that viscosity of the 30 wt% nylon-6/epoxy blend fraction lies in the higher range than 2.5 wt% nylon-6/epoxy. This could be attributed to increased interaction between nylon-

6 and epoxy.

3.3.2. Effect of change in temperature on shear viscosity

Viscosity measurements were carried out for the 30 wt% nylon-6/epoxy blend fraction to study flow behavior with variation in temperature. Figure (36), (37), (38) and (39) show plots of viscosity vs. shear rate at 25oC, 60oC, 80oC and 100oC. It can be seen that; at room temperature of

25oC, the blend shows a sharp dip in viscosity at low shear rates, and exhibits shear thickening behavior at high shear rates. This suggests occurrence of shear induced crystallization and inter- particle interaction of nylon-6. Vyas et al. [15] observed a similar behavior for ex-situ blends of nylon-6/epoxy at room temperature and have attributed shear thickening to crystallization of nylon-

6 under shear. As the temperature crosses the Tg of nylon-6, the flow behavior is pseudoplastic. This suggests increased mobility of the residual nylon-6 chains, leading to a reduction in viscosity at high shear rates. This proves the presence of heterogeneities in the 30 wt% nylon-6/epoxy fraction. This

48 also explains the reduction of crosslink density in the plateau region for the 30 wt% blend fraction above Tg in figure (31).

Shear viscosity measurements were performed on the 2.5 wt% NYEP blend at 25oC, 40oC and 80oC. Activation energies were calculated using the Arrhenius equation given below [64]

퐸푎 =퐴푒푅푇

Where  is the viscosity, A is the Arrhenius constant, Ea is the Activation energy, R is the gas constant per mole and T is the absolute temperature in kelvin. Table.8 below summarizes the activation energy variation evaluated for 2.5% nylon-6/epoxy and 30% nylon-6/epoxy in-situ blends at spindle speeds of 10, 20, 50 and 100rpm at 25oC, 40oC and 80oC.

Table 8. Summary of Activation energies for 2.5wt% and 30wt% NY-EP blends

2.5% Nylon-6/Epoxy 30% Nylon-6/Epoxy

Activation Energy Ea(kJ/mol) Activation Energy Ea(kJ/mol) Temperature (C) / Spindle 25 40 80 25 40 80 Speed (rpm) 10 35.83 35.83 42.44 47.37 47.37 53.42 20 35.22 39.44 40.41 47.60 48.42 52.58 50 33.56 39.29 45.67 47.82 48.90 53.11 100 34.56 45.43 38.38 47.90 49.04 53.19

In the above table, it seen that the activation energy values for the 30% in-situ nylon-6/epoxy blend are higher than those of the 2.5% in-situ nylon-6/epoxy blend. Activation energy (Ea) is a measure of ease of moving a polymer chain from one position to another [64]. Polymer inter-chain interactions and presence of bulky side chains are the main factors governing the the value of Ea.

Hence we conclude that the interactions between the in-situ nylon-6 and epoxy are responsible for a rise in Ea.

3.3.3. Effect of increase in polymerization time on shear viscosity

Lastly, viscosity behavior was studied with respect to polymerization time. After adding the initiator at 80oC, a fixed volume of the blend was withdrawn from the round bottom flask after 5,

15, 25 and 30 minutes to carry out Brookfield measurements during the course of the reaction. Figure

(40) shows the plot of viscosity vs. shear rate at different time stamps. The increase in viscosity with time can be attributed to coexistence of crosslinking of epoxy and polymerization of nylon-6 in epoxy. Shear thinning behavior was observed 5 min after adding the initiator. This suggests a broad molecular weight distribution in the blend, i.e, low molecular weight monomer and cross-linked epoxy chains of different lengths. Lai et al. [65] explored polyolefin synthesis using heterogeneous

Ziegler Natta catalyst and have reported shear thinning behavior, which has been ascribed to a broadening of molecular weight distribution. As time progressed, the extent of shear thinning reduced, thus suggesting formation of networks responsible for holding the polymer chains together leading to reduced shear thinning and increased viscosity after 30 min of polymerization.

3.4. Thermogravimetric Analysis (TGA)

Thermogravimetric analysis (TGA) is an important technique to evaluate the thermal stability of materials and to investigate the species present in the material. TGA was performed on neat epoxy, neat nylon and in-situ blends of nylon-6/epoxy. TGA thermograms in figure (41) & figure (42) reveal significant weight loss of neat epoxy at 397oC and a multistep weight loss at 314oC and 425oC for neat nylon-6 respectively. Literature reports melting temperatures of nylon-6 to range from 220oC -

230oC, thermal stability upto 400oC and thermal degradation occurs in the range of 428oC - 470oC.

[66] [67]. Mass loss due to unreacted Ɛ-caprolactam from 100oC to 300oC has been reported in literature [68]. Figure (41) reveals a mass loss at 216oC, which is attributed to the loss of unreacted

Ɛ-caprolactam. This observation complies with the DMA results and confirms the plasticizing effect of unreacted caprolactam in the 5% NY-EP blend.

Figure (43) shows the TGA thermogram for a temperature range of 28oC to 250oC in region I. Neat epoxy and 2.5% NY-EP exhibit mass loss at 147oC and 133oC respectively whereas blends with higher loadings of nylon-6 monomer showed no loss in mass. This could be attributed to higher crosslinking densities achieved at higher loadings of nylon-6. Thus, the cross-linked networks would prevent small molecules and gases from escaping at low temperatures.

Figure (44) shows TGA thermograms for region II and region III. The decomposition occurring in region II for a temperature range of 310oC to 425oC is due to degradation of nylon-6 and epoxy. The

51 degradation temperatures for nylon-6, epoxy and in-situ blends of nylon-6/epoxy are summarized in

Table.9 below.

Table.9 Degradation temperatures for Nylon-6 and Epoxy in the in-situ blends

In-situ Nylon- Residual solvent Nylon-6 Nylon-6 Epoxy

6/Epoxy blend /Ɛ-Caprolactam degradation oC degradation oC degradation oC

fraction loss (Peak 1) (Peak 2)

neat Epoxy 147 - - 397

neat Nylon-6 216 314 425 -

2.5% NY-EP 133 345 409 385

5% NY-EP 185 352 406 388

20% NY-EP - 326 401 389

30% NY-EP - 317 374 363

35% NY-EP - 318 378 356

Although an addition of caprolactam increases the cross-link density, it also leads to an increase in

C-N bond formation due to reaction between the amide group of the caprolactam and nylon-6 with the oxirane ring present in the epoxy molecule [69]. Literature reports that C-N bonds formed as a 52 result of epoxy-amine reaction are chemically stable however, they form less thermally stable epoxy resins owing to the basic character of amines [69]. In the above table and also in figure (44), the thermal degradation temperatures of nylon-6 and epoxy in the in-situ blends shift to lower temperatures. This could be attributed to the scission of increase number of C-N bonds at higher loadings of caprolactam.

3.5. Comparison between solution (ex-situ) and in-situ blending This section pertains to a comparison between the current work (in-situ blending) and the ex-situ solution blending of nylon-6/ epoxy, carried out by Vyas and Iroh. Ex-situ blending was a two-step process: i) polymerizing nylon-6 separately, and ii) solution blending it with neat epoxy using mechanical stirring [15]. On the other hand, in-situ blending was a one-pot synthesis involving two simultaneous operations: i) the polymerization of nylon-6 and ii) curing of neat epoxy.

Vyas et al [15] observed a significant fall in glass transition temperature for all nylon-6/ epoxy formulations in comparison to neat epoxy as seen in figure (45). In our current work, we noticed a rise in Tg at 15 wt%, 20 wt%, 25 wt%, 30wt% and 35 wt% nylon-6 loadings when compared to neat epoxy as seen in figure (32). A comparison between the trends obtained in the two approaches has been shown in figure (46).

From the storage modulus vs. temperature plots as seen in figure (47), it is evident that ex- situ blending lead to a fall in storage modulus in both glassy and rubbery regions, with increasing concentration of nylon-6 in the blend system [15]. In the current work (in-situ blending), the storage

53 modulus and crosslink density increased with Ɛ-caprolactam loadings in comparison to neat epoxy.

Even the lower value observed at 5 wt%, 25 wt% and 30 wt% loadings are comparable to the storage modulus of neat epoxy. The storage modulus in the glassy region was also found to increase with nylon-6 loading for a majority of blend fractions. Figure (48) & figure (49) summarize the comparison between the two approaches in terms of variation of storage modulus in the glassy region and the rubbery region respectively, with nylon-6 loading in epoxy. Figure (50) and figure (51) show the trend in cross-link density obtained with nylon-6 loading for in-situ and ex-situ blending respectively.

Nature of dispersion of nylon-6 could be one of the factors responsible for the difference in the observed trends for ex-situ and in-situ blending of nylon-6. In the current work, since the nylon-

6 was polymerized in presence of epoxy monomer, it created a higher number of reactive sites for the epoxy. As mentioned earlier, there was a simultaneous occurrence of two competing reactions;

(i) curing of epoxy via ring opening mechanism of the oxirane by the amide groups, and (ii) anionic ring opening of the caprolactam to form nylon-6. The increase in shear viscosity accompanied by a reduction in the extent of shear thinning with time suggests the formation of a narrower molecular weight distribution. This would prevent the residual nylon-6 formed from diffusing through the crosslinked epoxy network, consequently reducing the formation of agglomerates. This phenomenon was also reflected in the SEM micrographs of in-situ NY-EP blends. Figures (52), (53), (54) show

SEM migrographs of the cryofractured surfaces of neat epoxy, 20 wt% and 30wt% in-situ nylon-6/

54 epoxy blends respectively. SEM reveals an absence of agglomeration at higher loadings of monomeric nylon-6.

A comparison between the resulting morphologies by ex-situ and in-situ blending was observed under SEM for the 20 wt% blend fraction shown in figure (55). Figure (56) show the SEM micrograph of neat nylon-6. Ex-situ blending lead to plasticization of nylon-6 in the amorphous epoxy matrix [15]. Vyas et al. pointed out the agglomeration of nylon-6 with increase in nylon-6 content in the ex-situ blend. This agglomeration could be due to the dominance of cohesive forces within the relatively high molecular weight nylon-6, compared to the adhesive forces between nylon-

6 and epoxy. Moreover, in the ex-situ process, since the nylon-6 was polymerized prior to blending, the chains were terminated before blending, leading to lesser number of reactive groups in nylon-6

[70].

CHAPTER 4:

CONCLUSION

In-situ polymerization method of blending of nylon-6 with epoxy was explored and studied. An array of nylon-6/epoxy blend fractions were synthesized by varying the loading of Ɛ-caprolacatam via in-situ polymerization, characterized and analyzed for their morphological, thermal, mechanical, rheological and structural properties. The observed trends were then compared with those synthesized by the ex-situ blending method by Vyas et al.

Incorporation of monomeric nylon-6 led to improved thermal and mechanical properties.

FTIR results demonstrate maximum epoxy conversion at 15 wt% Ɛ-caprolactam loading after which the fractional conversion remained fairly constant. Moreover, a shift in the O-H peak towards lower wavenumbers with increasing Ɛ-caprolactam loading indicates increased hydrogen bonding of the

O-H in the blend. Dynamic Mechanical Analysis displayed an increase in glass transition temperature and storage modulus in the rubbery plateau region. A drop in glass transition temperature and modulus at higher caprolactam loadings are attributed to the formation of residual nylon-6 finely dispersed in the epoxy matrix. Time based shear viscosity measurements reveal an increase in viscosity and decrease in shear thinning behavior suggesting a building up of a crosslinked network with time thereby narrowing the molecular weight distribution. Temperature based viscosity measurements for 30 wt% blend fraction show dilatant behavior at room temperature and pseudoplastic behavior at higher temperatures confirming the presence of residual nylon-6

56 chains finely distributed in the epoxy matrix. Thus, the thermoplastic character is pronounced at higher caprolactam loadings. Thermal stability was improved across a temperature range below the melting point of nylon-6 which has been attributed to increase cross-link density. However, the onset of degradation for the in-situ nylon-6/epoxy blends was shifted towards lower temperatures.

This is due to an increase in aliphatic chains of nylon-6 which are known to have lower degradation onset compared to aromatic rings present in epoxy.

Conclusively, while ex-situ method of blending of nylon-6 in epoxy led to a material with higher damping properties with low glass transition temperatures and storage modulus, the in-situ method of blending led to a material possessing high glass transition temperatures and storage modulus. This has been ascribed to an improved dispersion of the thermoplastic phase (nylon-6) in the thermoset phase (epoxy).

CHAPTER-5

FUTURE WORK & SUGGESTIONS

The aim of this research was to characterize a reactive blend of nylon-6 and epoxy via in- situ polymerization method of blending. However, the cure mechanism and kinetics for the in-situ nylon-6/epoxy system needs further investigation. Although the purpose of the research was fulfilled, the applicability of this novel blend has not been explored. By virtue of its enhanced strength and thermal properties, this system could have a potential application as a coating for 3-D printing substrates.

Reactive blends of nylon-6/ epoxy were synthesized using diglycidyl ether of bisphenol A

(DGEBA) epoxy resin, which is a di functional resin. Curing of the blend involved the use of high temperatures and a longer duration. With an increasing demand for isocyanate-free coatings possessing room temperature cure, the reactive blending approach used in the current research can be extended to polyfunctional epoxies to reduce the cure time.

Additionally, replacing N-methyl pyrrolidone as solvent for solution polymerization of the blend could be replaced with similar solvents of lower toxicity to produce environmental-friendly coatings.

REFERENCES [1] Sundarram, S., Y.-H. Kim, and W. Li. "Preparation and Characterization of Poly(ether Imide) Nanocomposites and Nanocomposite Foams. "Manufacturing of Nanocomposites with Engineering Plastics (2015): 61-85. [2] Coleman, Jonathan N., Umar Khan, Werner J. Blau, and Yurii K. Gun’Ko. "Small but Strong: A Review of the Mechanical Properties of Carbon Nanotube–polymer Composites." Carbon 44.9 (2006): 1624-1652. [3] Jones, Richard G., Edward S. Wilks, John W. Jost, W. V. Metanomski, Jaroslav Kahovec, Michael Hess, Robert Stepto, Tatsuki Kitayama, A.D Jenkins, Pavel Kratochvil, and IUPAC (International. "Chapter 12: Definitions of Terms Relating to Reactions of Polymers and to Functional Polymeric Materials." Compendium of Polymer Terminology and Nomenclature: IUPAC Recommendations 2008. Cambridge, UK: RSC, 2009. 241. [4] Tamaki, Ryo, and Yoshiki Chujo. "Synthesis of IPN Polymer Hybrids of Polystyrene Gel and Silica Gel by an In-situ Radical Polymerization Method." Journal of Materials Chemistry J. Mater. Chem. 8.5 (1998): 1113-115. [5] Hodgkin, J. H., G. P. Simon, and R. J. Varley. "Thermoplastic Toughening of Epoxy Resins: A Critical Review." Polym. Adv. Technol. Polymers for Advanced Technologies 9.1 (1998): 3-10 [6] Pearson, Raymond A., and Albert F. Yee. "Toughening Mechanisms in Thermoplastic-modified Epoxies: 1. Modification Using Poly(phenylene Oxide)." Polymer 34.17 (1993): 3658-670. [7] Ratna, D. "Phase Separation in Liquid Rubber Modified Epoxy Mixture. Relationship between Curing Conditions, Morphology and Ultimate Behavior." Polymer 42.9 (2001): 4209-218 [8] Barone, L., S. Carciotto, G. Cicala, and A. Recca. "Thermomechanical Properties of Epoxy/poly(ɛ-caprolactone) Blends." Polym. Eng. Sci. Polymer Engineering & Science 46.11 (2006): 1576-582 [9] Blanco, I., G. Cicala, C. Lo Faro, and A. Recca. "Improvement of Thermomechanical Properties of a DGEBS/DDS System Blended with a Novel Thermoplastic Copolymer by Realization of a Semi-IPN Network." Journal of Applied Polymer Science J. Appl. Polym. Sci. 88.13 (2003): 3021- 025. [10] Silva, Lucas Filipe Martins Da, Andreas Öchsner, and Robert D. Adams. "Classification of Adhesives and Sealant Materials." Handbook of Adhesion Technology. Heidelberg: Springer, 2011. 278.

[11] Skeist, Irving. Handbook of Adhesives. New York: Van Nostrand Reinhold, 1977. [12] Gorton, B. S. "Interaction of Nylon Polymers with Epoxy Resins in Adhesive Blends." Journal of Applied Polymer Science J. Appl. Polym. Sci. 8.3 (1964): 1287-295 [13] Wang, Yung-Yun, and Show-An Chen. "Polymer Compatibility: Nylon-epoxy Resin Blends." Polym. Eng. Sci. Polymer Engineering and Science20.12 (1980): 823-29. [14] Zhong, Zhikai, and Qipeng Guo. "Miscibility and Cure Kinetics of Nylon/epoxy Resin Reactive Blends." Polymer 39.15 (1998): 3451-458. [15] Vyas, Aniket, and Jude O. Iroh. "In Situ Growth of Multilayered Crystals in Amorphous Matrix: Thermal, Dynamic Mechanical, and Morphological Analysis of Nylon-6/epoxy Composites." Journal of Applied Polymer Science J. Appl. Polym. Sci. 130.5 (2013): 3319-327. [16] Landry, Christine J.t, Bradley K. Coltrain, and Brian K. Brady. "In Situ Polymerization of Tetraethoxysilane in Poly(methyl Methacrylate): Morphology and Dynamic Mechanical Properties." Polymer 33.7 (1992): 1486-495. [17] Ou, Yuchun, Feng Yang, and Zhong‐Zhen Yu. "A New Conception on the Toughness of Nylon 6/silica Nanocomposite Prepared via in Situ Polymerization." J. Polym. Sci. B Polym. Phys. Journal of Polymer Science Part B: Polymer Physics 36.5 (1998): 789-95. [18] Mamuya, Yevgen, Gisele Boiteux, and Eugene Lebedev. "Chapter 9: Thermoplastic Polymers Containing Nanofillers." Advances in Progressive Thermoplastic and Thermosetting Polymers, Perspectives and Applications. By Volodymyr Levchenko. 348. [19] Carvalho, Fabiana Pires De, Maria Do Carmo Gonçalves, and Maria Isabel Felisberti. "Effect of in Situ Polymerization Conditions of Methyl Methacrylate on the Structural and Morphological Properties of Poly(methyl Methacrylate)/poly(acrylonitrile-g-(ethylene-co-propylene-co-diene)-g- styrene) PMMA/AES Blends." Journal of Applied Polymer Science J. Appl. Polym. Sci. 124.4 (2011): 2846-856. [20] Nelissen, L., E.w. Meijer, and P.j. Lemstra. "In Situ Preparation of Blends of Polystyrene and Poly (2,6-dimethyl-1,4-phenylene Ether)." Polymer33.17 (1992): 3734-738. [21] Martuscelli, E., M. Pracella, and Ping Yue Wang. "Properties of Poly(Ethylene Oxide) / Poly(Methylmethacrylate) Blends: Morphology, Crystallization and Melting Behaviour." Polymer Blends (1984): 73-83. [22] Gowariker, V. R., N. V. Viswanathan, and Jayadev Sreedhar. "Individual Polymers/ Epoxy Polymers." Polymer Science. New York: Wiley, 1986. 253-54. 60

[23] Pascault, Jean-Pierre, and Roberto J. J. Williams. "General Concepts about Epoxy Polymers." Epoxy Polymers New Materials and Innovations (2010): 1-12. [24] Brydson, J. A. "Chapter 26: Epoxide Resins." Plastic Materials. Sixth ed. London: Butterworth- Heinemann, 1966. 722. [25] Kanitkar, S. "Chapter 8: High Performance Epoxies and Solvent-less Epoxies for Corrosion Protection/Types of Epoxies." High-performance Organic Coatings. Cambridge, England: Woodhead, 2008. 148. [26] Patel, V. S., R. G. Patel, and R. H. Patel. "Chapter 7: Novel Multifunctional Epoxy Resins." Handbook of Polymer Blends and Composites. Ed. A. K. Kulshreshtha and C. Vasile. Shrewsbury: RAPRA Technology, 2002. 299. [27] Tack, Jeremy L., and David M. Ford. "Thermodynamic and Mechanical Properties of Epoxy Resin DGEBF Crosslinked with DETDA by Molecular Dynamics." Journal of Molecular Graphics and Modelling 26.8 (2008): 1269-275. [28] Durig, John D. "Comparisons of epoxy technology for protective coatings and linings in wastewater facilities." Journal of Protective Coatings & Linings 17.5 (2000): 49-54. [29] Dufton, P. W. Lightweight Thermoset Composites: Materials in Use, Their Processing and Applications; a Report from Rapra's Industry Analysis and Publishing Group. Shrewsbury: Rapra Technology Limited, 2000. 11. [30] Patel, V. S., R. G. Patel, and R. H. Patel. "Novel Multifunctional Epoxy Resins/Trifunctional Epoxy Resins." Handbook of Polymer Blends and Composites. Vol. 1. Shawbury, Shewsbury, Shropshire: Rapra Technology Lt., 2002. 300. [31] Patel, V. S., R. G. Patel, and R. H. Patel. "Novel Multifunctional Epoxy Resins/Tetrafunctional Epoxy Resins." Handbook of Polymer Blends and Composites. Vol. 1. Shawbury, Shewsbury, Shropshire: Rapra Technology Lt., 2002. 302 [32] Patel, V. S., R. G. Patel, and R. H. Patel. "Novel Multifunctional Epoxy Resins/Epoxy Novolac Resins." Handbook of Polymer Blends and Composites. Vol. 1. Shawbury, Shewsbury, Shropshire: Rapra Technology Lt., 2002. 311 [33] Tess, Roy W. "Chapter 8: Epoxy Resin Coatings/ Formulations of Heavy-Duty Surfacings." Epoxy Resins: Chemistry and Technology. Second ed. New York: Marcel Dekker, 1988. 762-4. [34] Brock, Thomas, Michael Groteklaes, and Peter Mischke. "Raw Materials for Coatings." European Coatings Handbook. Hannover: Vincentz, 2000. 76. 61

[35] Strong, A. Brent. "Epoxies." Fundamentals of Composites Manufacturing, Second Edition: Materials, Methods and Applications. Second ed. Society of Manufacturing Engineers, 1989. 97. [36] Strong, A. Brent. "Epoxies." Fundamentals of Composites Manufacturing, Second Edition: Materials, Methods and Applications. Second ed. Society of Manufacturing Engineers, 1989. 96. [37] Hamerton, Ian. "Cure Reactions of Epoxy Resins." Recent Developments in Epoxy Resins. Vol. 8. 26. Rapra Technology Limited, 1996. 27 Aug. 2016. [38] Weiss, Hans. "Anhydride Curing Agents for Epoxy Resins." Industrial & Engineering Chemistry 49.7 (1957): 1089-090. [39] Harper, Charles A., and Edward M. Petrie. Plastics Materials and Processes: A Concise Encyclopedia. Hoboken, NJ: Wiley-Interscience, 2003. 166. [40] Puckett, Mac, and Mitch Petervary. "Materials/ Epoxy Systems." Resin Transfer Moulding for Aerospace Structures. Dordrecht: Kluwer Academic, 1998. 58. [41] Chanda, Manas, and Salil K. Roy. "Industrial Polymers/Aliphatic Polyamides." Industrial Polymers, Specialty Polymers, and Their Applications. Boca Raton: CRC, 2009. 1-74. [42] Campo, E. Alfredo. "Chapter 6: Microbial, Weather and Chemical Resistance of Polymeric Materials/ Chemical Resistance Testing of Polymeric Materials." Selection of Polymeric Materials: How to Select Design Properties from Different Standards. Norwich, NY: William Andrew, 2008. 218. [43] Shah, Vishu. "Chapter 9: Chemical Properties." Handbook of Plastics Testing and Failure Analysis. 3rd ed. Hoboken, New Jersey: John Wiley & Sons, 2007. 251. [44] Odian, George G. "Ring Opening Polymerization/ General Characteristics." Principles of Polymerization. 4th ed. New York: McGraw-Hill, 1970. 546. [45] Nuyken, Oskar, and Stephen Pask. "Ring-Opening Polymerization—An Introductory Review." Polymers 5.2 (2013): 361-403. [46] Sadhir, Rajender K., and Russell M. Luck. "Chapter 5/ Kinetics and Mechanism of Polymerization of Expandable Monomers." Expanding Monomers: Synthesis, Characterization, and Applications. Boca Raton, FL: CRC, 1992. [47] Feldman, Dorel, and Alla Barbalata. "Chapter 5/ Polyamides and Polyimides/Nylons." Synthetic Polymers: Technology, Properties, Applications. 1st ed. London: Chapman & Hall, 1996. 225-27.

[48] Dubois, Philippe, Olivier Coulembier, and Jean-Marie Raquez. "Chapter 7/ Mechanism of the Anionic Polymerization of Lactams." Handbook of Ring-opening Polymerization. Weinheim: Wiley-VCH, 2009. 167-68. [49] Elen, K., and A. Kelchtermans. "Comparison of Two Novel Solution-Based Routes for the Synthesis of Equiaxed ZnO Nanoparticles : Figure 4." Hindawi Publishing Corporation, 2 May 2011. [50] "Chapter 5: Data Analysis." More Solutions to Sticky Problems: A Guide to Getting More from Your Brookfield Viscometer. Stoughton, Mass: Laboratory, 1995. 21. [51] Stuart, Barbara H. "Infrared Spectroscopy: Fundamentals and Applications." Wiley: John Wiley and Sons Ltd. [52] Djouani, F., Y. Zahra, B. Fayolle, M. Kuntz, and J. Verdu. "Degradation of Epoxy Coatings under Gamma Irradiation." Radiation Physics and Chemistry 82 (2013): 54-62. [53] Whittaker, David. "Chapter 1: Infrared Spectroscopy." Interpreting Organic Spectra. Cambridge, UK: Royal Society of Chemistry, 2000. 7. [54] Koenig, Jack L. "Chapter 6: The Application of Molecular Spectroscopy." Physical Properties of Polymers. Third ed. Washington, D.C.: American Chemical Society, 2003. 387. [55] Silverstein, Robert M., G. Clayton Bassler, and Terence C. Morrill. Spectrometric Identification of Organic Compounds. Seventh ed. New York: Wiley, 2005. 101. [56] Robinson, James W., Eileen M. Skelly Frame, and George M. Frame. "Chapter 4: Infrared, Near-Infrared and Raman Spectroscopy/ Nitrogen Containing Organic Compounds." Undergraduate Instrumental Analysis. Seventh ed. New York: M. Dekker, 2014. 302-03 [57] Mokhothu, T.H., A.S. Luyt, and M. Messori. "Reinforcement of EPDM Rubber with in Situ Generated Silica Particles in the Presence of a Coupling Agent via a Sol–gel Route." Polymer Testing 33 (2014): 97-106. [58] Park, Jonggu, John Eslick, Qiang Ye, Anil Misra, and Paulette Spencer. "The Influence of Chemical Structure on the Properties in Methacrylate-based Dentin Adhesives." Dental Materials 27.11 (2011): 1086-093. [59] Xiong, Mingna, Shuxue Zhou, Bo You, Guangxin Gu, and Limin Wu. "Effect of Preparation of Titania Sol on the Structure and Properties of Acrylic Resin/titania Hybrid Materials." J. Polym. Sci. B Polym. Phys. Journal of Polymer Science Part B: Polymer Physics 42.20 (2004): 3682-694.

[60] Nair, T. Muraleedharan, M. G. Kumaran, G. Unnikrishnan, and V. B. Pillai. "Dynamic Mechanical Analysis of Ethylene-propylene-diene Monomer Rubber and Styrene-butadiene Rubber Blends." Journal of Applied Polymer Science J. Appl. Polym. Sci. 112.1 (2009): 72-81. [61] Hill, Loren W. "Chapter 46: Dynamic Mechanical and Tensile Properties."Paint and Coating Testing Manual: Fourteenth Edition of the Gardner-Sward Handbook. Fourteenth ed. Philadelphia, PA: ASTM, 1995. 541. [62] Hajighasem, A., and K. Kabiri. "Cationic Highly Alcohol-swellable Gels: Synthesis and Characterization." Journal of Polymer Research J Polym Res 20.8 (2013): 1-9. [63] Jiang, H., W. Su, P.t. Mather, and T.j. Bunning. "Rheology of Highly Swollen Chitosan/polyacrylate Hydrogels." Polymer 40.16 (1999): 4593-602. [64] Budtova, Tatiana, and Patrick Navard. "Viscosity-temperature Dependence and Activation Energy of Cellulose Solutions." Nordic Pulp and Paper Research Journal 30 (1) 2015 30.01 (2015): 099-104. [65] Soares, Joao B.P., and Archie E. Hamielec. "Metallocene Catalysts in Dispersed Media." Polymeric Dispersions: Principles and Applications. Dordrecht: Kluwer Academic, 1997. 163. [66] Ray, Sudip, and Ralph P. Cooney. "Chapter 7: Thermal Degradation of Polymer and Polymer Composites." Handbook of Environmental Degradation of Materials. By Myer Kutz. Second ed. Norwich, NY: William Andrew Pub., 2012. [67] Rusu, GH, and E. Rusu. "Nylon-6/copper Composites by In-situ Polymerization." Journal of Optoelectronics and Advanced Materials 11.5 (2009): 673-80. [68] Zhang, Cai-Liang, Lian-Fang Feng, and Guo-Hua Hu. "Anionic Polymerization of Lactams: A Comparative Study on Various Methods of Measuring the Conversion of ɛ-caprolactam to Polyamide 6." Journal of Applied Polymer Science J. Appl. Polym. Sci. 101.3 (2006): 1972-981. [69] Macan, Jelena, and Marica Ivankovic. "Influence of Epoxy-Amine Ratio on Degradation of Epoxy-Silica Hybrid Materials." 209-31. Progress in Polymer Degradation and Stability Research. Nova Science Publishers, 2008. [70] Vyas, Aniket, and Jude O. Iroh. "Morphology and Structure of Nylon-6 Crystallized in Epoxy Resin Matrix." Polym Eng Sci Polymer Engineering & Science 54.4 (2013): 858-66.

FIGURES

O H 1 6 5 C-N 3 CH CH 2 2 CH amide C=O CH2 2 CH 2

C-H

2 N-H 9 Absorbance (a.u.) Absorbance 2 6 3 2 9 2

4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber (cm ) Figure 14. FTIR spectrum of Ɛ-caprolactam (monomer) showing presence of N-H stretch, hydrocarbon stretch, carbonyl (C=O) of the amide

Figure 16. FTIR spectrum of neat epoxy showing the presence of hydroxyl (O-H) at 3402 cm-1, (C-H) at 2927 cm-1, (C-C) of the benzene ring and oxirane ring at 914cm-1

1 6 4 9 C=O of amide

O-H

3 4 0 2

Absorbance 3 9 2 C-H 7

4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber (cm )

Figure 17. FTIR spectrum at the start of synthesis where caprolactam was dispersed in neat epoxy at room temperature of 25oC

1 6 6 5

C=O of amide Absorbance (a.u.) Absorbance

N-H merged with O-H 2 C-H 9 3 2 4 7 0 0

4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber (cm ) Figure 18. FTIR spectrum during blend synthesis of caprolactam in neat epoxy at 65oC. An aliquot for FTIR was taken before adding the initiator

C-C 1 O 5 0 epoxide ring C-C of benzene 6

C-H stretch

O-H stretch 3 4 9 0 1 9 4

Absorbance (a.u) Absorbance

4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber(cm )

Figure 19. FTIR spectrum of thermally cured 5 wt% in-situ nylon-6/epoxy blend showing the absence of (C=O) carbonyl bond of the amide group

1 5 0 6

C-C of benzene Absorbance (a.u.) Absorbance C-H stretch

O-H stretch 3 amide C=O 3 9 2

4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber (cm ) Figure 20. FTIR spectrum of thermally cured 10 wt% in-situ nylon-6/epoxy blend showing a slight shoulder at 1649 cm-1 attributed to the (C=O) carbonyl bond of the amide group

0.30

0.25 C-C of benzene

0.20

0.15 2 C-H 1 9

Absorbance (a.u.) Absorbance 6 2 4 3 5 amide C=O 0.10 3 3 O-H 6 5

0.05

0.00 4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber (cm )

Figure.21 FTIR Spectrum of thermally cured 15 wt% in-situ nylon-6/epoxy blend showing increased absorption of the amide carbonyl (C=O) at 1643cm-1

1 5 0 6 C-C of benzene

C-H Absorbance (a.u.) Absorbance amide C=O 3 1 O-H 3 6 8 4 7 9

4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber (cm ) Figure 22. FTIR spectrum of thermally cured 20 wt% in-situ nylon-6/epoxy blend showing diminishing amide carbonyl (C=O) peak at 1649cm-1

C-C of benzene

C-H 1 6

Absorbance(a.u.) amide C=O 6 1

3 O-H 3 6 0

4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber (cm ) Figure.23 FTIR spectrum of thermally cured 25 wt% in-situ nylon-6/epoxy blend showing reappearance of the amide carbonyl (C=O) at 1661cm-1.

C-C of benzene

Absorbance (a.u.) Absorbance C-H stretch O-H stretch 3 amide C=O 3 7 1 5 6 4 9

4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber (cm )

Figure 24. FTIR spectrum of thermally cured 30 wt% in-situ nylon-6/ epoxy blend showing diminished amide carbonyl (C=O) peak at 1649 cm-1

1 5 C-C of benzene 0 6

C-H stretch 2 9 2 6

Absorbance (a.u.) Absorbance 3 O-H stretch 3 1 7 amide C=O 6 2 4 9

4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber (cm )

Figure 25. FTIR Spectrum of thermally cured 35 wt% in-situ nylon-6/ epoxy blend showing the amide carbonyl (C=O) peak at 1649cm-1

Figure 26. Overlay of FTIR Spectra of in-situ nylon-6/epoxy blend fractions with neat epoxy showing the appearance of carbonyl (C=O) peak of the amide functional group between 1640cm-1 and 1660 cm-1

Figure 27. Hydroxyl peak shift towards low wavenumbers starting from thermally cured in- situ (a) neat epoxy, (b) 5 wt% nylon-6/epoxy (c) 10 wt% nylon-6/epoxy, (d) 15 wt% nylon- 6/epoxy (e) 20 wt% nylon-6/epoxy, (f) 30 wt% nylon-6/epoxy and (g) 35 wt% nylon-6/epoxy. Hydroxyl shift towards low wavenumbers suggests hydroxyl group (O-H) being surrounded by neighboring atoms in the blend

H O

-[N-C-(CH ) ]n 2 5 C-N stretch

O H amide C=O C N 1 6 Skeletal motion 7 6 C-H stretch

2 Absorbance (a.u.) Absorbance 9 6 6 3 1 N-H 2 1 9 5 0 5

4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber (cm ) Figure 29. FTIR spectrum of neat nylon-6 synthesized at 80oC showing the presence of N-H , C-H, amide C=O, C=N stretch, skeletal motion of amide group (–CONH-) confirming the formation of Nylon-6 at 80oC via ring opening anionic polymerization of -caprolactam

Figure 30. Oxirane peak at 914 cm-1 with increasing loading of -caprolactam (a) neat epoxy, (b) 5 wt% Nylon-6/Epoxy, (c)10 wt% Nylon-6/Epoxy, (d) 20 wt% Nylon-6/Epoxy (e) 30 wt% Nylon-6/Epoxy.

Figure 31. Epoxy fractional conversion with increase in -caprolactam loading from 5 wt% - caprolactam to 35 wt% -caprolactam in epoxy matrix. Epoxy conversion remained constant above 15 wt% -caprolactam

Figure 32. Glass transition temperatures for all blend fractions synthesized by in-situ polymerization were obtained by plotting the tan delta peaks from dynamic mechanical measurements. Figure shows variation of tan delta with -caprolactam loading for (a.) neat epoxy, (b.) 5 wt% nylon-6/epoxy, (c.) 10 wt% nylon-6/epoxy, (d.) 15 wt% nylon-6/epoxy (e.) 20 wt% nylon-6/epoxy loading, (f.) 25 wt% nylon-6/epoxy (g.) 30 wt% nylon-6/epoxy (h.) 35 wt% nylon-6/epoxy. 15 wt% nylon-6epoxy blend fraction possesses the highest tg.

Figure 33. Variation in Storage Modulus plotted against temperature with increasing - caprolactam content for the following in-situ blends (a) neat epoxy, (b.) 5 wt% nylon-6/Epoxy, (c.) 10 wt% nylon-6/Epoxy, (d.) 15 wt% nylon-6/Epoxy (e.) 20 wt% nylon-6/Epoxy (f.) 25 wt% nylon-6/epoxy (g.) 30 wt% nylon-6/epoxy (h.) 35 wt% nylon-6/epoxy

1.0

0.8

0.6

0.4 Viscosity (Poise) Viscosity 0.2

0.0 0 1 10 10 -1 Shear rate sec

Figure 34. Plot of viscosity vs shear rate for 2.5 wt% nylon-6/epoxy blend fraction showing shear thinning behavior. Viscosity values were obtained at different spindle speeds at a temperature setting of 80oC.

1.0

0.8

0.6

0.4 Viscosity (Poise)

0.2

0 1 10 -1 10 Shear rate (sec )

Figure 35. Plot of viscosity vs shear rate for 30 wt% nylon-6/epoxy blend fraction showing reduction in the extent of shear thinning compared to 2.5 wt% nylon-6/epoxy blend fraction in figure 33. Viscosity values were obtained at different spindle speeds at a temperature setting of 80oC

2.0

1.5

1.0 Viscosity (Poise) Viscosity

0.5

-1 0 10 10 -1 Shear rate (sec )

Figure 36. Plot of viscosity vs shear rate for 30 wt% nylon-6/epoxy blend fraction showing shear thickening behavior. Viscosity values were obtained at different spindle speeds at a temperature setting of 25oC. Brookfield measurements were performed on the 30 wt% nylon- 6/epoxy blend fractions at different temperature settings to investigate the presence of residual nylon-6 which did not participate in crosslinking with epoxy.

2.0

1.5

1.0 Viscosity (Poise) Viscosity

0.5

-1 0 10 10 -1 Shear rate (cm ) Figure 37. Plot of shear viscosity vs shear rate for 30 wt%nylon-6/epoxy blend fraction showing shear thinning behavior. Viscosity values were obtained at different spindle speeds at a temperature setting of 60oC.

2.0

1.5

1.0 Viscosity (Poise) Viscosity

0.5

-1 0 10 10 -1 Shear rate (sec )

Figure 38. Plot of shear viscosity vs shear rate for 30 wt%nylon-6/epoxy blend fraction showing shear thinning behavior. Viscosity values were obtained at different spindle speeds at a temperature setting of 80oC

2.0

1.5

1.0 Viscosity (Poise) Viscosity

0.5

-1 0 10 10 -1 Shear rate (sec )

Figure 39. Plot of shear viscosity vs shear rate for 30 wt%nylon-6/epoxy blend fraction showing shear thinning behavior. Viscosity values were obtained at different spindle speeds at a temperature setting of 100oC

1 10

(d)

(c)

Viscosity (Poise) Viscosity (b)

0 10

(a)

0 1 10 -1 10 Shear rate (sec)

Figure 40. Shear viscosity measurements at room temperature of 30 wt% nylon-6/epoxy in order to study variation of viscosity with polymerization time (a) 5 min, (b) 15 min, (c) 25 min and (d) 30 min after adding initiator. Increase in viscosity with reduced shear thinning behavior with time indicates increased interactions between nylon-6 and epoxy

100 3.5

90 3.0

80 Epoxy degradation

2.5 derivativeweight (%/°C) 70 397

60 2.0

Weight % 1.5 40

30 1.0

20 0.5

10 147 0.0 100 200 300 400 500 600 Temperature ºC Figure 41. TGA of thermally cured neat Epoxy showing the maximum mass loss and thermal degradation at 397oC

100 3.5

95 3.0 90

85 derivativeweight (%/°C) 2.5 80

75 2.0

70 1.5 Weight% 65 Caprolactam loss 60 1.0 55 Nylon-6 degradation

50 0.5 314 425 216 45 33

100 200 300 400 500 600 Temperature °C Figure 42. TGA of Nylon-6 synthesized at 80oC by anionic ring opening polymerization of - caprolactam showing thermal degradation occurring at 314oC and 425oC. Weight loss at 216oC is due to loss of unreacted -caprolactam

Figure 43. Percent weight vs temperature and the corresponding derivative weight in region I for in-situ nylon-6/ epoxy blend fractions: (a) Neat epoxy, (b) 2.5 wt% nylon-6/epoxy, (c) 20 wt% nylon-6/epoxy (d) 30 wt%nylon-6/epoxy, (e ) 35 wt% nylon-6/epoxy. Nylon-6/epoxy blend fractions with higher -caprolactam loadings (c, d & e) show enhanced thermal stability than neat epoxy (a) till 220oC 92

Figure 44. Percent weight loss vs temperature and the corresponding derivative weight in region II for in-situ nylon epoxy blend fractions: (a) Neat epoxy, (b) 2.5 wt% nylon-6/epoxy, (c) 20 wt% nylon-6/epoxy (d) 30 wt%nylon-6/epoxy, (e ) 35 wt% nylon-6/epoxy, (f) neat nylon- 6. Figure 43 shows nylon-6/epoxy blends with higher -caprolactam loadings (d & e) undergoing early thermal degradation. This is attributed to the scission of increased number of C-N bonds formed during epoxy amine reaction at higher caprolactam loadings. 93

(b)

C o

(a) Glass Transition Temperature (Tg) (Tg) GlassTransition Temperature 20

0 0 10 20 30 40 Nylon-6 loading (wt%)

Figure 46. Comparison between variation in glass transition temperature with increase in nylon-6 loading in (a) ex-situ blends and -caprolactam (monomer of nylon-6) loading in (b) in-situ blends. Ex-situ blending of fully formed nylon-6 in epoxy led to a reduction in glass transition temperature, which is attributed to plasticization effect of nylon-6. In-situ polymerization of -caprolactam in epoxy led to increased availability of reactive sites for epoxy cur

Figure 47. Variation of Storage Modulus with temperature for the following ex-situ nylon- 6/epoxy blend fractions [(a.) neat epoxy (control), (b.) 5 wt% Nylon-6, (c.) 10 wt% Nylon-6 (d.) 20 wt% Nylon-6, (e.) 30 wt% Nylon-6 in epoxy matrix [15]

(b) 9 10

(a) 8 10

7 10

6 10

10 Storage Modulus E' Storage (Glassy Region) (Pa)

4 10 0 5 10 15 20 25 30 35

Nylon-6 loading (wt%) Figure 48. Comparison between variation in storage modulus (glassy region) measured at 25oC with increase in nylon-6 loading in (a) ex-situ blends and -caprolactam (monomer of nylon-6) loading in (b) in-situ blends.

(b)

7 10

6 10

10 (a) Storage Modulus E' (Pa) Storage Region) (Rubbery

0 5 10 15 20 25 30 35 40

Nylon-6 loading (wt%) Figure 49. Comparison of variation in storage modulus (rubbery plateau region) measured at 130oC with increase in nylon-6 loading in (a) ex-situ blends and -caprolactam (monomer of nylon-6) loading in (b) in-situ blends. Storage Modulus in the rubbery plateau region is directly proportional to crosslink density

3500

) 3000

/m v 2500

2000

1500 Crosslink density (n

1000

500

0 5 10 15 20 25 30 35 40 Nylon-6 loading (wt%)

Figure 50. Crosslink density variation for blends formed by in-situ polymerization with increasing -caprolactam (monomer of nylon-6) loading (wt%). No significant reduction in crosslink density was observed at higher -caprolactam loadings due to increased availability of number of reactive sites for crosslinking with epoxy.

15 )

3 14

/m v

12 Crosslink density (n 11

0 5 10 15 20 25 30 Nylon-6 loading (wt%)

Figure 51. Crosslink density variation for blends formed by solution (ex-situ) polymerization with increasing Nylon-6 loading (wt%). Crosslink density for solution blended nylon-6/epoxy was higher than neat epoxy and decreased at higher loadings of Nylon-6 due to agglomeration of nylon-6 in epoxy matrix thus having a plasticizing effect.

100

Figure 52. SEM Micrograph of a cross section of neat epoxy at 1000x and 2000x magnification

Figure 53. SEM Micrographs of 20 wt% in-situ Nylon-6/Epoxy blend at 1000 X and 2000 X magnification.

101

Figure 54. SEM Micrographs of 30 wt% in-situ Nylon-6/Epoxy blend at 1000 X and 2000X magnification

102

Figure 55. SEM Micrographs of (a) 20 wt% Nylon-6/ Epoxy solution (ex-situ) blend[ ], (b) 20 wt% Nylon-6/ Epoxy in-situ blend show a comparison between the nature of dispersion of nylon-6 in solution and (b) in-situ polymerization methods of blending respectively

103

Figure 56. SEM Micrograph of neat Nylon-6 synthesized at 80oC

104

APPENDIX

I. FTIR

0.7

1 6 3 0.6 5 C=O of amide

H O 0.5 N-C skeletal motion

0.4 N-H C-H

0.3 1 1 Absorbance (a.u.) Absorbance 3 6 2 8 0.2 9 6

0.1

0.0 4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber (cm ) Figure 1. Neat Nylon-6 synthesized at 150oC for 1 hour by solution polymerization

105

II. DYNAMIC MECHANICAL ANALYSIS (DMA):

1.0

9 10 0.8

0.6 Delta Tan

8 10

0.4 Storage Modulus E' Storage Pa

0.2

7 10

50 100 150 200 250 300 350 o Temperature C

Figure 1. DMA of thermally cured neat epoxy till 200oC

106

T-1 1.2 9 10 T-2 1.0

0.8 Tan delta Tan

8 10 0.6

Storage Modulus Storage E' Pa S-1 0.4

7 0.2 10 S-2

0.0 50 100 150 200 o Temperature C

Figure 2. DMA of 5 wt% in-situ nylon-6/Epoxy. Curves S-1, T-1, S-2, T-2 are iterations 1 and 2 for Storage Modulus (S) and Tan delta (T) respectively.

107

1.2 T-1

9 10 1.0

0.8 Tan Delta Tan T-2 0.6 8 10

Storage Modulus E' Storage Pa 0.4

S-1 0.2 S-2 7 10

50 100 150 200 o Temperature C Figure 3. DMA of in-situ 10 wt% nylon-6 /epoxy. Curves S-1, T-1, S-2, T-2 are iterations 1 and 2 for Storage Modulus (S) and Tan delta (T) respectively.

108

T-1

9 10 0.8

T-2

0.6 Tan delta Tan

10 0.4 Storage Modulus E'Storage Pa

S-1 S-2 0.2

7 10 0.0 50 100 150 200 o Temperature C

Figure 4. DMA of 15 wt% in-situ nylon-6/epoxy. Curves S-1, T-1, S-2, T-2 are iterations 1 and 2 for Storage Modulus (S) and Tan delta (T) respectively.

109

T-1

9 1.0 10

T-2

0.8 Tan delta Tan 0.6

10 Storage Modulus E' Storage Pa 0.4

S-1

S-2 0.2

7 10 50 100 150 200 o Temperature C Figure 5. DMA of 20 wt% in-situ nylon-6/epoxy. Curves S-1, T-1, S-2, T-2 are iterations 1 and 2 for Storage Modulus (S) and Tan delta (T) respectively.

110

9 10

6 Tan delta Tan 8 10 S-1 4 S-2 S-3

Storage Modulus E' Storage Pa 7 10 T-1 T-2 2 T-3

6 10 0

50 100 o 150 200 Temperature C Figure 6. DMA of 30 wt% in-situ nylon-6/epoxy. Curves S-1, T-1, S-2, T-2, S-3, T-3 are iterations 1, 2 and 3 for Storage Modulus (S) and Tan delta (T) respectively.

111

1.0 T-1

9 10 0.8 T-2

0.6 Tan delta Tan

0.4 8

Storage Modulus E' Storage Pa 10

S-1 0.2 S-2

0.0 50 100 150 200 o Temperature C Figure 7. DMA of 35 wt% in-situ nylon-6/epoxy. Curves S-1, T-1, S-2, T-2 are iterations 1 and 2 for Storage Modulus (S) and Tan delta (T) respectively.

112

III. Thermogravimetric Analysis:

100 3.5

90 3.0 80

2.5 70 derivativeweight (%/°C)

60 2.0

Weight% 1.5 40 Epoxy NY-6 Peak 2

30 409 1.0 385 20 NY-6 Peak 1 513 345 0.5 10 133 0 0.0 50 100 150 200 250 300 350 400 450 500 550 600 Temperature (°C) Figure 1. TGA thermogram for thermally cured 2.5 wt% in-situ nylon-6/epoxy Blend

113

100 3.5

90 3.0 80 2.5

70 derivativeweight (%/°C)

60 2.0 50 Epoxy

Weight% 1.5 40 388 NY-6 Peak 2

30 406 1.0

20 NY-6 Peak 1 Caprolactam 515 0.5 10 352 185 0 0.0 100 200 300 400 500 600 Temperature (ºC) Figure 2. TGA thermogram for thermally cured 5 wt% in-situ nylon-6 /epoxy Blend

114

100 3.5

90 3.0 80 2.5

70 derivativeweight (%/°C)

60 2.0 50 Epoxy

Weight % 1.5 40 388 NY-6 Peak 2

30 406 1.0

20 Caprolactam NY-6 Peak 1 515 0.5 10 352 185 0 0.0 100 200 300 400 500 600 Temperature (ºC) Figure 3. TGA thermogram for thermally cured 20 wt% in-situ nylon-6/epoxy Blend

115

100 3.5

90 3.0 80

2.5

70 derivativeweight (%/ºC)

60 2.0 Epoxy 50 NY-6 Peak 2

Weight% 1.5 40 363

374 30 1.0 479 20 NY-6 Peak 1 0.5 10 317

0 0.0 50 100 150 200 250 300 350 400 450 500 550 600 Temperature °C

Figure 4. TGA thermogram for thermally cured 30 wt% in-situ nylon-6/epoxy

116

100 3.5

90 3.0 80 2.5

70 derivativeweight (%/ºC)

60 2.0

50 Epoxy NY-6 Peak 2

Weight% 1.5 40 379 357

30 1.0 481 20 NY-6 Peak 1 0.5 10 319

0.0 50 100 150 200 250 300 350 400 450 500 550 600 Temperature ºC Figure 5. TGA thermogram for thermally cured 35% in-situ nylon-6/epoxy

117

Figure 6. Derivative weight vs Temperature plot showing mass loss at 147oC for (a) neat epoxy, at 185oC for (b) 5% nylon-6/epoxy and at 216oC for (c) neat nylon-6

118