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Synthesis and Characterization of High Performance Interpenetrating Polymer Networks With Polyurethane and Poly(methyl methacrylate)
CHAPTER
SYNTHESIS AND CHARACTERIZATION OF HIGH PERFORMANCE INTERPENETRATING POLYMER NETWORKS WITH POLYURETHANE AND POLY(METHYL METHACRYLATE)
11
Nima Alizadeh1,2, Samantha A. Bird1, Ricardo Ballestero Mendez1,2, Keilash C. Jajam3, Andrea C. Alexander1, Hareesh V. Tippur3 and Maria L. Auad1,2 1
Center for Polymers and Advanced Composites, Auburn University, Auburn, AL, United States 2Department of Chemical Engineering, Auburn University, Auburn, AL, United States 3Department of Mechanical Engineering, Auburn University, Auburn, AL, United States
11.1 INTRODUCTION Many scenarios, whether in military applications or in domestic life, call for a transparent material that can withstand high impacts and offer protection to individuals. Until now, polycarbonate (PC) and poly(methyl methacrylate) (PMMA) have remained the most widely used lightweight transparent materials for explosion and shock protection in applications such as personnel eyewear and face shields. The optical and mechanical performances at elevated temperatures and chemical exposure have become an area of concern for some of the currently fielded polymer armors and protective materials, which need further attention [1]. Experimentation with interpenetrating polymer networks (IPNs) began almost a century ago, and they are also widely studied today [24]. What makes these networks so popular is their inherent ability to combine two or more materials and create a synergistic, molecular composite. While two or more polymers may reject one another within one system, creating an IPN offers the chance for these polymers to be in close proximity with each other with minimal phase separation. Physical mixtures of polymers have been explored in the past [5,6], but having materials integrated at such a high level allows polymers of opposing characteristics such as PMMA and PUR to coexist as one material. Extensive research works on creating various types of IPNs with PMMA and PUR have been explored by the authors. In these previous studies [713], both sequential and simultaneous Unsaturated Polyester Resins. DOI: https://doi.org/10.1016/B978-0-12-816129-6.00011-9 © 2019 Elsevier Inc. All rights reserved.
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CHAPTER 11 SYNTHESIS AND CHARACTERIZATION
full-IPNs were investigated with different parameters such as the ratio of PUR to PMMA, curing profile, inclusion of aromatic and aliphatic isocyanates for the PUR phase, and the use of an inhibitor in the PMMA phase. The findings suggested that samples containing a diol with a molecular weight of 650 g/mol, 80 wt.% PMMA, an aliphatic isocyanate, and an inhibitor with the MMA monomer resulted in favorable thermomechanical and optical properties. The developed, transparent IPNs were formed by a soft phase constructed from a PUR network, which imparted flexibility and high impact resistance [12,13]. The hard phase was formed by an acrylic network, and this imparted high strength and excellent chemical resistance as required for these materials. The current study was focused on manipulating the cross-link density of the PUR network by adjusting its flexibility and size. This was attempted by increasing the molecular weight of the diol, and consequently, changing the molecular weight between cross-links or the chain length. The purpose of this research was to determine the effect of changing the “mesh size” of the PUR phase on the final performance of the IPN.
11.2 EXPERIMENTAL AND TESTING PROCEDURES 11.2.1 MATERIALS The PUR phase consisted of two different polyols, 1,1,1-tris(hydroxymethyl) propane (TRIOL) from Acros Organics and poly(tetramethylene ether) glycol (PTMG) from Sigma Aldrich. Since part of the objective of this research was determining the change in the thermomechanical properties of the IPNs due to different chain lengths between cross-links, several PTMGs of varying masses were incorporated, namely 650, 1400, 2000, and 2900 g/mol. Each diol was combined beforehand with the triol, and the mixture was melted in an oven under a strong vacuum to remove residual moisture. The isocyanate utilized for this study was 1,6-diisocyanatohexane 99 1 % (DCH) from Acros Organics (U.S.), and the catalyst used was dibutyltin dilaurate 98% (DD) distributed by Pfaltz & Bauer (U.S.). Ethyl acetate was used as an analogue for the DD. The PMMA phase consisted of methyl methacrylate 99% stabilized (MMA) from Acros Organics and trimethylolpropane trimethacrylate (TRIM) from Sigma Aldrich, another methacrylate utilized as a cross-linker to create a 3D network. The initiator was 2,20 -azobis(2-methylpropionitrile) 98% (AIBN) from Sigma Aldrich.
11.2.2 PROCEDURE The IPN reaction proceeded as a one-step, bulk polymerization with all reactants mixed together at room temperature following the procedure reported by the authors [12,13]. The PMMA and PUR phases were initially prepared separately. Once the PMMA system was prepared by mixing the MMA, TRIM, and AIBN, and the PUR phase was prepared with the PTMG/TRIOL mixture and the DCH, these precursor solutions were combined. After thorough stirring, DD was added to catalyze the PUR system. These IPN sample precursors were then cured at 60 C for 24 hours and again at 80 C for another 24 hours. The ratios used were: 5.3:1.1 by mass of PTMG to TRIOL, 1.8571:1
11.2 EXPERIMENTAL AND TESTING PROCEDURES
245
by mass of PTMG/TRIOL to DCH, 1 g:15.4 μL of PTMG/TRIOL to DD, 95:5 by mass of PMMA to TRIM, and 1.3 mL of AIBN (with ethyl acetate as an analogue) for every 123.5 g of MMA. From this method, several IPNs of varying PMMA/PUR contents were synthesized, namely 80:20 and 70:30. Samples with varying PUR chain flexibilities and PUR chain lengths were synthesized in the same manner with the only difference being the PTMG/TRIOL mixture utilized.
11.2.3 CHARACTERIZATION TECHNIQUES AND EQUIPMENT A Zeiss EM 10C 10CR Transmission Electron Microscope (TEM) was used to study the morphology of the IPNs. The TEM samples preparation procedure was based on Kato’s osmium tetroxide (OsO4) sataining method [14]. The specimens were allowed to sit in the dye for at least a week to ensure a sufficient amount would penetrate the materials. Afterwards, the samples were microtomed. The thermal properties of pure PURs and IPNs were also observed using a TA Instruments Q2000 Modulated Differential Scanning Calorimeter (DSC). The procedure used to analyze the samples was to: equilibrate at 80 C, modulate by 6 1.00 C every 60 seconds, isothermal for 5 minutes, ramp by 10.00 C/min to 250 C, ramp by 10.00 C/min to 80 C, equilibrate at 80 C, isothermal for 5 minutes, ramp by 10.00 C/min to 250 C, ramp by 10.00 C/min to 80 C. The transparency of the IPNs was measured using a UV-visible 2450 Spectrophotometer from Shimadzu Scientific Instruments. Thermomechanical properties were studied through tensile testing and 3-point bending tests were performed on a TA Instruments RSAIII Dynamic Mechanical Analyzer (DMA) with a frequency of 1 Hz. For studying the density of the IPNs and pure PURs, a pycnometer was utilized at room temperature with distilled water. For density experiments, the samples were dried in an oven at approximately 80 C for a couple hours to remove any residual moisture. In order to characterize the fracture toughness of the IPNs in terms of the critical stress intensity factor, KIc, quasi-static tests were performed in accordance with ASTM D5045-96 guidelines [15]. Single edge notched 3-point bend specimens (Fig. 11.1) were loaded in displacement control mode with a testing speed of 0.25 mm/min using a Instron 4465 testing machine. The load versus deflection data were recorded up to crack initiation and during stable crack growth, if any, and the crack initiation toughness, KIc, was calculated using the load (P) at crack initiation. For each IPN category at least four sets of experiments were performed. The mode-I stress intensity factor for a single edge notched bend (SENB) specimen loaded in 3-point bending using linear elastic fracture mechanics is given by Eqs. (11.1) and (11.2) [16], f
pffiffiffiffi a a 2
3 WS Wa a a 1:99 2 5 1 2 2:15 2 3:93 1 2:7 3=2 W W W W W 2 1 1 2 Wa 12 Wa a Pf ffiffiffiffiffi KIc 5 pW B W
a
(11.1)
(11.2)
For these equations, P is the load at fracture, S is the span, B is the thickness, W is the width, and a is the crack length of the material.
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CHAPTER 11 SYNTHESIS AND CHARACTERIZATION
P B
W
S = 80 mm W = 20 mm B = 8 mm a /W = 0.3 a
S FIGURE 11.1 Experimental setup for quasi-static fracture analysis.
11.2.4 DETERMINATION OF MOLECULAR WEIGHT BETWEEN CROSS-LINKING POINTS With the assumption of an ideal rubbery material, the molecular weight between cross-linking points (Mc) can be calculated based on the rubber elasticity theory (Eq. 11.3) [17]. G5
E [ρRT 5 3 Mc
(11.3)
where G and E are the shear and tensile moduli, respectively. T is the temperature, R is the gas constant, ρ is the density of the material, φ is the correction factor defined as the ratio between the mean square end-to-end distance of a chain in the network and the length of a randomly coiled chain [18]. During this calculation, the value of E was calculated as the storage modulus of the network that corresponds to the temperature Tg 1 50 C in the rubbery state. The density values, ρ, were calculated using a pycnometer at room conditions. The correction value φ is assumed to be equal to 1 as suggested in the literature [18].
11.3 RESULTS AND DISCUSSION 11.3.1 MORPHOLOGY AND TRANSPARENCY CHARACTERIZATION The IPNs were formed by a sequential process; where the PUR network was polymerized first at 60 C, while the monomer and initiator for the acrylic copolymerization were swollen into the incipient PUR network and polymerized in situ during the second stage of the reaction at 80 C. IPNs with different molecular structures based on the molecular weight of the diol used for the formulation of the PUR system were prepared. Fig. 11.2 shows the differences in phase morphology when studied under TEM. A previous study [12,13,15] revealed that when comparing the two
11.3 RESULTS AND DISCUSSION
247
FIGURE 11.2 TEM photos of IPNs (PMMA:PUR) with diols of varying molecular weights: (A) 650 g/mol (80:20); (B) 650 g/ mol (70:30); (C) 1400 g/mol (80:20); (D) 1400 g/mol (70:30); (E) 2000g/mol (80:20); (F) 2000g/mol (70:30); (G) 2900 g/mol (80:20); and (H) 2900 g/mol (70:30).
polymer systems, PUR absorbed the dye, thus becoming black, while PMMA remained unstained [14]. The distinction between the two phases has enabled the study of the domain formation and phase separation processes for IPN systems. Other research groups were also able to study the morphologies of IPN systems based on this technique [1921].
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CHAPTER 11 SYNTHESIS AND CHARACTERIZATION
TEM images show that when the molecular weight of the diol is increase, the domain size of the PMMA phase is also considerably enlarged. A similar trend was also observed for similar IPN systems with different PMMA to PUR ratios. From the TEM pictures, major differences in morphology can be observed between the 80:20 and 70:30 PMMA:PUR ratio samples for each different molecular weight diol. As the amount of PUR increased, the smaller the PMMA domains became. For these samples, since more PUR is present, the PUR phase occupies a greater area than the PMMA domains, thus making the PMMA domains smaller, but at the same time more numerous. In these pictures, it is possible to observe clear round PMMA domains surrounded by dark PUR areas. The change in the samples’ compositions from 80:20 to 70:30 generated a clear decrease of the PMMA domains with order of magnitude differences in the sizes. It is clear from the pictures that during the polymerization the PUR phase that reacts first is the continuous matrix (black areas in the TEM micrographs), while the PMMA phase, which polymerizes during the second part of the reaction, generates fairly round zones (white spherical domains) packed inside the continuous phase. Allan et al. [22,23] observed a similar tendency in their work. They prepared samples with two diols with molecular weights of 2000 and 3000 D, respectively, and they clearly observed phase separated domains similar to those observed here. The same authors [24] also observed that the greatest factor affecting the domain size is on the duration of the post gelation period before the beginning of the MMA curing process. Similar trends were observed in this work. Samples that were not able to reach the gel time during the first part of the polymerization show macroscopic phase separation (whitening process). Allan et al. [24] concluded that there is a competing kinetic mechanism between the polymerization of the PUR phase and the vinyl monomer (MMA). Results of UV-vis analysis of commercial PMMA along with IPNs with 80 and 70 wt.% PMMA with diols of varying molecular weights are presented in Fig. 11.3A and B. The commercial PMMA almost reached 100% transparency, while the synthesized IPNs exhibited lower, but still relatively high, transparency values. From Fig. 11.3, it is clear that in most cases, samples with 70 wt.% PMMA (Fig. 11.3A) showed higher transparency tendencies than their 80 wt.% counterparts (Fig. 11.3B), especially for samples with diols with molecular weights of 2000 and 2900 g/mol. The nature of the phase separation process contributed to the low transparency of the samples. Similar phenomena were observed for the different molecular weight diols. As the molecular weight of the diol increased, the phase separation process seems to occur at lower percentages of PMMA. For instance, for IPNs with 1400, 2000, and 2900 g/mol molecular weight diols, PTMG phase separation occurs with a 80:20 wt.% ratio (PMMA:PUR). In addition, the crystallization of the diol also contributed to the lower transparency values. Dadbin and Frounchi [25] reported similar results in their work with simultaneous IPNs consisting of PUR and poly(allyl diglycol carbonate). They observed similarities in the refractive indices of the phases [25]. This trend was also observed in the TEM microscopy; samples with 80 wt.% PMMA showed irregular domains, particularly for the 2900 g/mol molecular weight diol. On the other hand, samples with 70 wt.% PMMA displayed domains homogeneously distributed. This assembly at the molecular level may be the reason for the impact on the transparency of the samples.
11.3 RESULTS AND DISCUSSION
249
70:30 (PMMA:PU) IPNs with different MW diols (PTMG)
(A) 100 90 80 Transmittance (%)
70 60 50 40 PMMA 650 g/mol 1400 g/mol 2000 g/mol 2900 g/mol
30 20 10 0 400
500
600 Wavelength (nm)
700
800
80:20 (PMMA:PU) IPNs with different MW diols (PTMG)
(B) 100 90
Transmittance (%)
80 70 60 50 40 PMMA 650 g/mol 1400 g/mol 2000 g/mol 2900 g/mol
30 20 10 0 400
500
600 Wavelength (nm)
700
800
FIGURE 11.3 UV-vis analysis of (A) 70:30 and (B) 80:20 (PMMA:PU) IPN samples consisting of different molecular weight diols.
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CHAPTER 11 SYNTHESIS AND CHARACTERIZATION
11.3.2 THERMAL AND THERMOMECHANICAL ANALYSIS Table 11.1 summarizes the differential scanning calorimetry (DSC) results for pure and crosslinked PUR samples and IPNs with different molecular weight PTMGs Crystallinity was present for all of the linear and cross-linked PUR and IPN samples with diols of 2000 and 2900 g/mol molecular weights. While no crystallization was detected by DSC for samples with 650 and 1400 g/mol molecular weight diols. These results support the globular structure observed in the TEM micrograph results. In the case of IPNs with high molecular weight diols, the MMA phase present inhibited the PUR phase from forming a substantial network system capable of accommodating PMMA domains; instead of the MMA swelling the PUR phase, the PUR reacted separately and independently of the PMMA phase, followed by the synthesis of the PMMA phase outside of the PUR network. Li et al. [26] worked with segmented polyurethanes (PURs). They noticed that samples consisting of low molecular weight diols did not show crystallinity, probably suppressed due to hydrogen bond interaction with other segments in the same molecule. On the other hand, the same authors prepared samples from high molecular weight diols and observed that crystallinity did become evident. They also observed that the ability of the diols to crystallize was significantly depressed due to the hydrogen bond interaction with other segments in the same molecule. Table 11.2 summarizes the dynamic mechanical characteristic (storage modulus, E0 , and glass transition, Tg) values of the synthesized samples. The pure components (PURs and cross-linked PMMA) are also listed for comparison.
Table 11.1 DSC Results for Pure and Cross-linked PUR Samples and IPNs With Different Molecular Weight PTMGs Sample (g/mol)
Composition (PMMA:PUR)
Crystallinity ( C)
ΔHc (J/g)
Melting ( C)
ΔHm (J/g)
IPN 650 IPN 650 Cross-linked PUR Linear PUR 650 IPN 1400 IPN 1400 Cross-linked PUR Linear PUR 1400 IPN 2000 IPN 2000 Cross-linked PUR Linear PUR 2000 IPN 2900 IPN 2900 Linear PUR 2900 Cross-linked PUR
80:20 70:30 0:100 0:100 80:20 70:30 0:100 0:100 80:20 70:30 0:100 0:100 80:20 70:30 0:100 0:100
13.75 23.29 35.34 38.56 32.38 21.32 19.36 19.37 7.54 14.24
25.92 34.34 4.72 2.59 26.68 43.10 7.92 11.01 53.04 38.28
40.97 18.26 13.03 11.51 8.53 20.8 18.23 17.24 22.14 12.51
15.75 39.52 7.83 4.51 30.74 70.65 7.97 7.70 54.86 36.27
650
1400
2000
2900
11.3 RESULTS AND DISCUSSION
251
Table 11.2 Glass Transition (Tg) and Storage Modulus (E0 ) Values of Synthesized Samples Tg ( C)
Storage Modulus, E0 , at 30 C (Pa)
Storage Modulus, E0 , at Tg 1 50 C (Pa)
80:20 70:30 0:100
121.9 6 14.3 102.6 6 15.4 11.2 6 0.25
6.93E 1 08 6 8.42E 1 08 3.77E 1 06 6 5.62E 1 05 2.77E 1 06 6 1.64E 1 06
5.91E 1 06 6 6.47E 1 06 3.77E 1 06 6 5.62E 1 05 NA
0:100
11.2 6 0.25
2.83 6 1.64
NA
80:20 70:30 0:100
129.7 6 3.15 110.0 6 1.0 45.4 6 1.3
7.28E 1 08 6 1.11E 1 08 3.65E 1 08 6 1.43E 1 08 1.94E 1 07 6 2.44E 1 07
2.14E 1 06 6 3.68E 1 05 2.62E 1 06 6 1.41E 1 05 NA
0:100
46.8 6 0.7
4.60 6 0.25
NA
80:20 70:30 0:100
133.2 6 2.9 121.0 6 9.2 62.3 6 0.6
8.46E 1 08 6 3.06E 1 08 3.56E 1 08 6 1.70E 1 08 3.59E 1 06 6 2.26E 1 05
1.86E 1 06 6 8.65E 1 0.5 2.51E 1 06 6 5.66E 1 05 NA
0:100
64.6 6 1.3
8.24 6 2.99
80:20 70:30 0:100
143.8 6 1.5 124.1 6 3.3 68.6 6 0.4
8.16E 1 08 6 2.32E 1 08 4.38E 1 08 6 1.37E 1 07 18.72 6 4.43
2.95E 1 06 6 2.56E 1 05 1.95E 1 06 6 7.52E 1 05
NA
NA
Sample (g/mol)
Composition PMMA:PUR
IPN 650 IPN 650 Cross-linked PUR 650 Linear PUR 650 IPN 1400 IPN 1400 Cross-linked PUR 1400 Linear PUR 1400 IPN 2000 IPN 2000 Cross-linked PUR 2000 Linear PUR 2000 IPN 2900 IPN 2900 Cross-linked PUR 2900 Linear PUR 2900
0:100
The storage modulus values were not significantly affected by the molecular weight of the PTMG used for the PUR system. The crystallinity developed in some of the IPN systems probably acted as physical reinforcement of the network. Also, a clear trend was observed for 80:20 (PMMA:PUR) and 70:30 (PMMA:PUR) ratio IPN samples, for all the utilized diols; samples with 80 wt.% PMMA had a higher storage modulus than those with 70 wt.% PMMA. Regardless of the existence of crystalline assemblies, the stiffness of the PMMA phase overpowered the effect of the crystalline structures. Regarding the glass transition temperatures, the values increased as the molecular weight of the diols increased as expected for longer and more flexible chains. The amount of energy required to active these chains is the reason for the higher glass transition temperatures. The reported trends were not followed by the pure cross-linked PUR samples. In this case, the higher molecular weight of the utilized diol increased the storage modulus while decreasing the glass transition temperature. In this case, the effect of crystallinity is probably less dominant compared to in the IPNs.
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CHAPTER 11 SYNTHESIS AND CHARACTERIZATION
11.3.3 MOLECULAR WEIGHTS BETWEEN CROSS-LINKING POINTS The molecular weight between cross-linking points (Mc) was calculated by substituting the rubbery modulus obtained using DMA (Table 11.2) into Eq. (11.3). Table 11.3 summarizes the results. For the IPNs, there are two factors associated to the variation in Mc values. First, the effect of the diol utilized. In this case, increasing the value of the molecular weight of the diol from 650 to 2900 g/mol decreases the value of the molecular weight between the cross-linking points of the PUR networks. Table 11.3 clearly shows this tendency in the cross-linked PUR samples. The second effect is associated to the interpenetrating polymerization process. During this two-step sequential process, the samples that do not show a prominent phase separation process (650 and 200 g/ mol) interpenetrate and the Mc shows a tendency that increases with the amount of MMA in the network, on the contrary, the samples with phase separation show a clear crystallization process (melting temperature, Tm) in the calorimetric analysis. In this case, the highest Mc corresponds to the network without MMA. These results confirm the TEM microscopy results and the globular structure obtained in this case.
11.3.4 FRACTURE PROPERTIES In this work, quasi-static fracture was utilized to study the fracture toughness of the IPNs with different molecular weight diols. Table 11.4 summarizes these results. Intenerating networks with the 2000 g/mol molecular weight diol showed the highest KIc value. Increasing the molecular weight of the diols used for the IPN systems increased the maximum load that the samples were able to withstand; this was made possible by the creation of more flexible PUR networks. This process allowed the IPN samples to absorb more energy during the fracture experiments and in this way to increase the fracture toughness parameter. Samples with greater amounts of PMMA displayed higher stiffness, while samples with higher weight percentages of PUR exhibited longer extension at brake and considerable plastic deformation. Table 11.3 IPNs Molecular Weigh Between Cross-linking Points Sample
Composition (PMMA:PU)
Density (g/mL)
Mc (g/mol)
IPN 650 IPN 650 Cross-linked IPN 1400 IPN 1400 Cross-linked IPN 2000 IPN 2000 Cross-linked IPN 2900 IPN 2900 Cross-linked
80:20 70:30 0:100 80:20 70:30 0:100 80:20 70:30 0:100 80:20 70:30 0:100
1.02 6 0.204 1.09 6 0.049 1.04 6 0.044 1.15 6 0.077 1.13 6 0.049 1.04 6 0.041 1.09 6 0.087 1.15 6 0.031 1.06 6 0.053 1.17 6 0.035 1.11 6 0.048 1.00 6 0.039
2029.24 3419.14 2848.49 6353.18 5081.48 4045.75 6888.20 5389.30 2222.66 4698.94 6697.95 NA
PUR 650
PUR 1400
PUR 2000
PUR 2900
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Table 11.4 Fracture Toughness of 70:30 (PMMA:PUR) IPNs With Different Molecular Weight Diols Molecular Weight 70:30 (PMMA:PUR) (g/mol)
Quasi-Static Fracture Toughness (KIc) (MPa m1/2)
650 1400 2000 2900
1.08 6 0.05 0.92 6 0.01 1.31 6 0.04 0.74 6 0.01
11.4 CONCLUSION In this study, the IPN samples were further investigated by manipulating the structure of the PUR network using diols of varying molecular weights, which in turn affected the chain length between cross-links. The morphology did change, evidenced by TEM images. Although there was little change in storage modulus values, this still leaves room for improving impact resistance while maintaining high values of stiffness and rigidity. The presence of crystallinity played a role as physical reinforcement increasing the storage modulus values. Improvement in the fracture toughness of the samples showed promising results. The 70:30 (PMMA:PUR) ratio IPN with 650 g/mol molecular weight diol exhibited the capability to withstand a high load, but plastic deformation at low extensions. Samples with higher molecular weight diols displayed increased plastic deformation while also showing relatively high KIc values. Finally, the transparency was relatively high, although samples with diols of higher molecular weight showed increased phase separation, as shown through both TEM photos and UV-vis analysis.
ACKNOWLEDGMENT The support of the US Army Research Office through grant W911NF-12-1-0317 is gratefully acknowledged.
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SYNTHESIS AND CHARACTERIZATION OF HIGH PERFORMANCE INTERPENETRATING POLYMER NETWORKS WITH POLYURETHANE AND POLY(METHYL METHACRYLATE)
11
Nima Alizadeh1,2, Samantha A. Bird1, Ricardo Ballestero Mendez1,2, Keilash C. Jajam3, Andrea C. Alexander1, Hareesh V. Tippur3 and Maria L. Auad1,2 1
Center for Polymers and Advanced Composites, Auburn University, Auburn, AL, United States 2Department of Chemical Engineering, Auburn University, Auburn, AL, United States 3Department of Mechanical Engineering, Auburn University, Auburn, AL, United States
11.1 INTRODUCTION Many scenarios, whether in military applications or in domestic life, call for a transparent material that can withstand high impacts and offer protection to individuals. Until now, polycarbonate (PC) and poly(methyl methacrylate) (PMMA) have remained the most widely used lightweight transparent materials for explosion and shock protection in applications such as personnel eyewear and face shields. The optical and mechanical performances at elevated temperatures and chemical exposure have become an area of concern for some of the currently fielded polymer armors and protective materials, which need further attention [1]. Experimentation with interpenetrating polymer networks (IPNs) began almost a century ago, and they are also widely studied today [24]. What makes these networks so popular is their inherent ability to combine two or more materials and create a synergistic, molecular composite. While two or more polymers may reject one another within one system, creating an IPN offers the chance for these polymers to be in close proximity with each other with minimal phase separation. Physical mixtures of polymers have been explored in the past [5,6], but having materials integrated at such a high level allows polymers of opposing characteristics such as PMMA and PUR to coexist as one material. Extensive research works on creating various types of IPNs with PMMA and PUR have been explored by the authors. In these previous studies [713], both sequential and simultaneous Unsaturated Polyester Resins. DOI: https://doi.org/10.1016/B978-0-12-816129-6.00011-9 © 2019 Elsevier Inc. All rights reserved.
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full-IPNs were investigated with different parameters such as the ratio of PUR to PMMA, curing profile, inclusion of aromatic and aliphatic isocyanates for the PUR phase, and the use of an inhibitor in the PMMA phase. The findings suggested that samples containing a diol with a molecular weight of 650 g/mol, 80 wt.% PMMA, an aliphatic isocyanate, and an inhibitor with the MMA monomer resulted in favorable thermomechanical and optical properties. The developed, transparent IPNs were formed by a soft phase constructed from a PUR network, which imparted flexibility and high impact resistance [12,13]. The hard phase was formed by an acrylic network, and this imparted high strength and excellent chemical resistance as required for these materials. The current study was focused on manipulating the cross-link density of the PUR network by adjusting its flexibility and size. This was attempted by increasing the molecular weight of the diol, and consequently, changing the molecular weight between cross-links or the chain length. The purpose of this research was to determine the effect of changing the “mesh size” of the PUR phase on the final performance of the IPN.
11.2 EXPERIMENTAL AND TESTING PROCEDURES 11.2.1 MATERIALS The PUR phase consisted of two different polyols, 1,1,1-tris(hydroxymethyl) propane (TRIOL) from Acros Organics and poly(tetramethylene ether) glycol (PTMG) from Sigma Aldrich. Since part of the objective of this research was determining the change in the thermomechanical properties of the IPNs due to different chain lengths between cross-links, several PTMGs of varying masses were incorporated, namely 650, 1400, 2000, and 2900 g/mol. Each diol was combined beforehand with the triol, and the mixture was melted in an oven under a strong vacuum to remove residual moisture. The isocyanate utilized for this study was 1,6-diisocyanatohexane 99 1 % (DCH) from Acros Organics (U.S.), and the catalyst used was dibutyltin dilaurate 98% (DD) distributed by Pfaltz & Bauer (U.S.). Ethyl acetate was used as an analogue for the DD. The PMMA phase consisted of methyl methacrylate 99% stabilized (MMA) from Acros Organics and trimethylolpropane trimethacrylate (TRIM) from Sigma Aldrich, another methacrylate utilized as a cross-linker to create a 3D network. The initiator was 2,20 -azobis(2-methylpropionitrile) 98% (AIBN) from Sigma Aldrich.
11.2.2 PROCEDURE The IPN reaction proceeded as a one-step, bulk polymerization with all reactants mixed together at room temperature following the procedure reported by the authors [12,13]. The PMMA and PUR phases were initially prepared separately. Once the PMMA system was prepared by mixing the MMA, TRIM, and AIBN, and the PUR phase was prepared with the PTMG/TRIOL mixture and the DCH, these precursor solutions were combined. After thorough stirring, DD was added to catalyze the PUR system. These IPN sample precursors were then cured at 60 C for 24 hours and again at 80 C for another 24 hours. The ratios used were: 5.3:1.1 by mass of PTMG to TRIOL, 1.8571:1
11.2 EXPERIMENTAL AND TESTING PROCEDURES
245
by mass of PTMG/TRIOL to DCH, 1 g:15.4 μL of PTMG/TRIOL to DD, 95:5 by mass of PMMA to TRIM, and 1.3 mL of AIBN (with ethyl acetate as an analogue) for every 123.5 g of MMA. From this method, several IPNs of varying PMMA/PUR contents were synthesized, namely 80:20 and 70:30. Samples with varying PUR chain flexibilities and PUR chain lengths were synthesized in the same manner with the only difference being the PTMG/TRIOL mixture utilized.
11.2.3 CHARACTERIZATION TECHNIQUES AND EQUIPMENT A Zeiss EM 10C 10CR Transmission Electron Microscope (TEM) was used to study the morphology of the IPNs. The TEM samples preparation procedure was based on Kato’s osmium tetroxide (OsO4) sataining method [14]. The specimens were allowed to sit in the dye for at least a week to ensure a sufficient amount would penetrate the materials. Afterwards, the samples were microtomed. The thermal properties of pure PURs and IPNs were also observed using a TA Instruments Q2000 Modulated Differential Scanning Calorimeter (DSC). The procedure used to analyze the samples was to: equilibrate at 80 C, modulate by 6 1.00 C every 60 seconds, isothermal for 5 minutes, ramp by 10.00 C/min to 250 C, ramp by 10.00 C/min to 80 C, equilibrate at 80 C, isothermal for 5 minutes, ramp by 10.00 C/min to 250 C, ramp by 10.00 C/min to 80 C. The transparency of the IPNs was measured using a UV-visible 2450 Spectrophotometer from Shimadzu Scientific Instruments. Thermomechanical properties were studied through tensile testing and 3-point bending tests were performed on a TA Instruments RSAIII Dynamic Mechanical Analyzer (DMA) with a frequency of 1 Hz. For studying the density of the IPNs and pure PURs, a pycnometer was utilized at room temperature with distilled water. For density experiments, the samples were dried in an oven at approximately 80 C for a couple hours to remove any residual moisture. In order to characterize the fracture toughness of the IPNs in terms of the critical stress intensity factor, KIc, quasi-static tests were performed in accordance with ASTM D5045-96 guidelines [15]. Single edge notched 3-point bend specimens (Fig. 11.1) were loaded in displacement control mode with a testing speed of 0.25 mm/min using a Instron 4465 testing machine. The load versus deflection data were recorded up to crack initiation and during stable crack growth, if any, and the crack initiation toughness, KIc, was calculated using the load (P) at crack initiation. For each IPN category at least four sets of experiments were performed. The mode-I stress intensity factor for a single edge notched bend (SENB) specimen loaded in 3-point bending using linear elastic fracture mechanics is given by Eqs. (11.1) and (11.2) [16], f
pffiffiffiffi a a 2
3 WS Wa a a 1:99 2 5 1 2 2:15 2 3:93 1 2:7 3=2 W W W W W 2 1 1 2 Wa 12 Wa a Pf ffiffiffiffiffi KIc 5 pW B W
a
(11.1)
(11.2)
For these equations, P is the load at fracture, S is the span, B is the thickness, W is the width, and a is the crack length of the material.
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CHAPTER 11 SYNTHESIS AND CHARACTERIZATION
P B
W
S = 80 mm W = 20 mm B = 8 mm a /W = 0.3 a
S FIGURE 11.1 Experimental setup for quasi-static fracture analysis.
11.2.4 DETERMINATION OF MOLECULAR WEIGHT BETWEEN CROSS-LINKING POINTS With the assumption of an ideal rubbery material, the molecular weight between cross-linking points (Mc) can be calculated based on the rubber elasticity theory (Eq. 11.3) [17]. G5
E [ρRT 5 3 Mc
(11.3)
where G and E are the shear and tensile moduli, respectively. T is the temperature, R is the gas constant, ρ is the density of the material, φ is the correction factor defined as the ratio between the mean square end-to-end distance of a chain in the network and the length of a randomly coiled chain [18]. During this calculation, the value of E was calculated as the storage modulus of the network that corresponds to the temperature Tg 1 50 C in the rubbery state. The density values, ρ, were calculated using a pycnometer at room conditions. The correction value φ is assumed to be equal to 1 as suggested in the literature [18].
11.3 RESULTS AND DISCUSSION 11.3.1 MORPHOLOGY AND TRANSPARENCY CHARACTERIZATION The IPNs were formed by a sequential process; where the PUR network was polymerized first at 60 C, while the monomer and initiator for the acrylic copolymerization were swollen into the incipient PUR network and polymerized in situ during the second stage of the reaction at 80 C. IPNs with different molecular structures based on the molecular weight of the diol used for the formulation of the PUR system were prepared. Fig. 11.2 shows the differences in phase morphology when studied under TEM. A previous study [12,13,15] revealed that when comparing the two
11.3 RESULTS AND DISCUSSION
247
FIGURE 11.2 TEM photos of IPNs (PMMA:PUR) with diols of varying molecular weights: (A) 650 g/mol (80:20); (B) 650 g/ mol (70:30); (C) 1400 g/mol (80:20); (D) 1400 g/mol (70:30); (E) 2000g/mol (80:20); (F) 2000g/mol (70:30); (G) 2900 g/mol (80:20); and (H) 2900 g/mol (70:30).
polymer systems, PUR absorbed the dye, thus becoming black, while PMMA remained unstained [14]. The distinction between the two phases has enabled the study of the domain formation and phase separation processes for IPN systems. Other research groups were also able to study the morphologies of IPN systems based on this technique [1921].
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CHAPTER 11 SYNTHESIS AND CHARACTERIZATION
TEM images show that when the molecular weight of the diol is increase, the domain size of the PMMA phase is also considerably enlarged. A similar trend was also observed for similar IPN systems with different PMMA to PUR ratios. From the TEM pictures, major differences in morphology can be observed between the 80:20 and 70:30 PMMA:PUR ratio samples for each different molecular weight diol. As the amount of PUR increased, the smaller the PMMA domains became. For these samples, since more PUR is present, the PUR phase occupies a greater area than the PMMA domains, thus making the PMMA domains smaller, but at the same time more numerous. In these pictures, it is possible to observe clear round PMMA domains surrounded by dark PUR areas. The change in the samples’ compositions from 80:20 to 70:30 generated a clear decrease of the PMMA domains with order of magnitude differences in the sizes. It is clear from the pictures that during the polymerization the PUR phase that reacts first is the continuous matrix (black areas in the TEM micrographs), while the PMMA phase, which polymerizes during the second part of the reaction, generates fairly round zones (white spherical domains) packed inside the continuous phase. Allan et al. [22,23] observed a similar tendency in their work. They prepared samples with two diols with molecular weights of 2000 and 3000 D, respectively, and they clearly observed phase separated domains similar to those observed here. The same authors [24] also observed that the greatest factor affecting the domain size is on the duration of the post gelation period before the beginning of the MMA curing process. Similar trends were observed in this work. Samples that were not able to reach the gel time during the first part of the polymerization show macroscopic phase separation (whitening process). Allan et al. [24] concluded that there is a competing kinetic mechanism between the polymerization of the PUR phase and the vinyl monomer (MMA). Results of UV-vis analysis of commercial PMMA along with IPNs with 80 and 70 wt.% PMMA with diols of varying molecular weights are presented in Fig. 11.3A and B. The commercial PMMA almost reached 100% transparency, while the synthesized IPNs exhibited lower, but still relatively high, transparency values. From Fig. 11.3, it is clear that in most cases, samples with 70 wt.% PMMA (Fig. 11.3A) showed higher transparency tendencies than their 80 wt.% counterparts (Fig. 11.3B), especially for samples with diols with molecular weights of 2000 and 2900 g/mol. The nature of the phase separation process contributed to the low transparency of the samples. Similar phenomena were observed for the different molecular weight diols. As the molecular weight of the diol increased, the phase separation process seems to occur at lower percentages of PMMA. For instance, for IPNs with 1400, 2000, and 2900 g/mol molecular weight diols, PTMG phase separation occurs with a 80:20 wt.% ratio (PMMA:PUR). In addition, the crystallization of the diol also contributed to the lower transparency values. Dadbin and Frounchi [25] reported similar results in their work with simultaneous IPNs consisting of PUR and poly(allyl diglycol carbonate). They observed similarities in the refractive indices of the phases [25]. This trend was also observed in the TEM microscopy; samples with 80 wt.% PMMA showed irregular domains, particularly for the 2900 g/mol molecular weight diol. On the other hand, samples with 70 wt.% PMMA displayed domains homogeneously distributed. This assembly at the molecular level may be the reason for the impact on the transparency of the samples.
11.3 RESULTS AND DISCUSSION
249
70:30 (PMMA:PU) IPNs with different MW diols (PTMG)
(A) 100 90 80 Transmittance (%)
70 60 50 40 PMMA 650 g/mol 1400 g/mol 2000 g/mol 2900 g/mol
30 20 10 0 400
500
600 Wavelength (nm)
700
800
80:20 (PMMA:PU) IPNs with different MW diols (PTMG)
(B) 100 90
Transmittance (%)
80 70 60 50 40 PMMA 650 g/mol 1400 g/mol 2000 g/mol 2900 g/mol
30 20 10 0 400
500
600 Wavelength (nm)
700
800
FIGURE 11.3 UV-vis analysis of (A) 70:30 and (B) 80:20 (PMMA:PU) IPN samples consisting of different molecular weight diols.
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CHAPTER 11 SYNTHESIS AND CHARACTERIZATION
11.3.2 THERMAL AND THERMOMECHANICAL ANALYSIS Table 11.1 summarizes the differential scanning calorimetry (DSC) results for pure and crosslinked PUR samples and IPNs with different molecular weight PTMGs Crystallinity was present for all of the linear and cross-linked PUR and IPN samples with diols of 2000 and 2900 g/mol molecular weights. While no crystallization was detected by DSC for samples with 650 and 1400 g/mol molecular weight diols. These results support the globular structure observed in the TEM micrograph results. In the case of IPNs with high molecular weight diols, the MMA phase present inhibited the PUR phase from forming a substantial network system capable of accommodating PMMA domains; instead of the MMA swelling the PUR phase, the PUR reacted separately and independently of the PMMA phase, followed by the synthesis of the PMMA phase outside of the PUR network. Li et al. [26] worked with segmented polyurethanes (PURs). They noticed that samples consisting of low molecular weight diols did not show crystallinity, probably suppressed due to hydrogen bond interaction with other segments in the same molecule. On the other hand, the same authors prepared samples from high molecular weight diols and observed that crystallinity did become evident. They also observed that the ability of the diols to crystallize was significantly depressed due to the hydrogen bond interaction with other segments in the same molecule. Table 11.2 summarizes the dynamic mechanical characteristic (storage modulus, E0 , and glass transition, Tg) values of the synthesized samples. The pure components (PURs and cross-linked PMMA) are also listed for comparison.
Table 11.1 DSC Results for Pure and Cross-linked PUR Samples and IPNs With Different Molecular Weight PTMGs Sample (g/mol)
Composition (PMMA:PUR)
Crystallinity ( C)
ΔHc (J/g)
Melting ( C)
ΔHm (J/g)
IPN 650 IPN 650 Cross-linked PUR Linear PUR 650 IPN 1400 IPN 1400 Cross-linked PUR Linear PUR 1400 IPN 2000 IPN 2000 Cross-linked PUR Linear PUR 2000 IPN 2900 IPN 2900 Linear PUR 2900 Cross-linked PUR
80:20 70:30 0:100 0:100 80:20 70:30 0:100 0:100 80:20 70:30 0:100 0:100 80:20 70:30 0:100 0:100
13.75 23.29 35.34 38.56 32.38 21.32 19.36 19.37 7.54 14.24
25.92 34.34 4.72 2.59 26.68 43.10 7.92 11.01 53.04 38.28
40.97 18.26 13.03 11.51 8.53 20.8 18.23 17.24 22.14 12.51
15.75 39.52 7.83 4.51 30.74 70.65 7.97 7.70 54.86 36.27
650
1400
2000
2900
11.3 RESULTS AND DISCUSSION
251
Table 11.2 Glass Transition (Tg) and Storage Modulus (E0 ) Values of Synthesized Samples Tg ( C)
Storage Modulus, E0 , at 30 C (Pa)
Storage Modulus, E0 , at Tg 1 50 C (Pa)
80:20 70:30 0:100
121.9 6 14.3 102.6 6 15.4 11.2 6 0.25
6.93E 1 08 6 8.42E 1 08 3.77E 1 06 6 5.62E 1 05 2.77E 1 06 6 1.64E 1 06
5.91E 1 06 6 6.47E 1 06 3.77E 1 06 6 5.62E 1 05 NA
0:100
11.2 6 0.25
2.83 6 1.64
NA
80:20 70:30 0:100
129.7 6 3.15 110.0 6 1.0 45.4 6 1.3
7.28E 1 08 6 1.11E 1 08 3.65E 1 08 6 1.43E 1 08 1.94E 1 07 6 2.44E 1 07
2.14E 1 06 6 3.68E 1 05 2.62E 1 06 6 1.41E 1 05 NA
0:100
46.8 6 0.7
4.60 6 0.25
NA
80:20 70:30 0:100
133.2 6 2.9 121.0 6 9.2 62.3 6 0.6
8.46E 1 08 6 3.06E 1 08 3.56E 1 08 6 1.70E 1 08 3.59E 1 06 6 2.26E 1 05
1.86E 1 06 6 8.65E 1 0.5 2.51E 1 06 6 5.66E 1 05 NA
0:100
64.6 6 1.3
8.24 6 2.99
80:20 70:30 0:100
143.8 6 1.5 124.1 6 3.3 68.6 6 0.4
8.16E 1 08 6 2.32E 1 08 4.38E 1 08 6 1.37E 1 07 18.72 6 4.43
2.95E 1 06 6 2.56E 1 05 1.95E 1 06 6 7.52E 1 05
NA
NA
Sample (g/mol)
Composition PMMA:PUR
IPN 650 IPN 650 Cross-linked PUR 650 Linear PUR 650 IPN 1400 IPN 1400 Cross-linked PUR 1400 Linear PUR 1400 IPN 2000 IPN 2000 Cross-linked PUR 2000 Linear PUR 2000 IPN 2900 IPN 2900 Cross-linked PUR 2900 Linear PUR 2900
0:100
The storage modulus values were not significantly affected by the molecular weight of the PTMG used for the PUR system. The crystallinity developed in some of the IPN systems probably acted as physical reinforcement of the network. Also, a clear trend was observed for 80:20 (PMMA:PUR) and 70:30 (PMMA:PUR) ratio IPN samples, for all the utilized diols; samples with 80 wt.% PMMA had a higher storage modulus than those with 70 wt.% PMMA. Regardless of the existence of crystalline assemblies, the stiffness of the PMMA phase overpowered the effect of the crystalline structures. Regarding the glass transition temperatures, the values increased as the molecular weight of the diols increased as expected for longer and more flexible chains. The amount of energy required to active these chains is the reason for the higher glass transition temperatures. The reported trends were not followed by the pure cross-linked PUR samples. In this case, the higher molecular weight of the utilized diol increased the storage modulus while decreasing the glass transition temperature. In this case, the effect of crystallinity is probably less dominant compared to in the IPNs.
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CHAPTER 11 SYNTHESIS AND CHARACTERIZATION
11.3.3 MOLECULAR WEIGHTS BETWEEN CROSS-LINKING POINTS The molecular weight between cross-linking points (Mc) was calculated by substituting the rubbery modulus obtained using DMA (Table 11.2) into Eq. (11.3). Table 11.3 summarizes the results. For the IPNs, there are two factors associated to the variation in Mc values. First, the effect of the diol utilized. In this case, increasing the value of the molecular weight of the diol from 650 to 2900 g/mol decreases the value of the molecular weight between the cross-linking points of the PUR networks. Table 11.3 clearly shows this tendency in the cross-linked PUR samples. The second effect is associated to the interpenetrating polymerization process. During this two-step sequential process, the samples that do not show a prominent phase separation process (650 and 200 g/ mol) interpenetrate and the Mc shows a tendency that increases with the amount of MMA in the network, on the contrary, the samples with phase separation show a clear crystallization process (melting temperature, Tm) in the calorimetric analysis. In this case, the highest Mc corresponds to the network without MMA. These results confirm the TEM microscopy results and the globular structure obtained in this case.
11.3.4 FRACTURE PROPERTIES In this work, quasi-static fracture was utilized to study the fracture toughness of the IPNs with different molecular weight diols. Table 11.4 summarizes these results. Intenerating networks with the 2000 g/mol molecular weight diol showed the highest KIc value. Increasing the molecular weight of the diols used for the IPN systems increased the maximum load that the samples were able to withstand; this was made possible by the creation of more flexible PUR networks. This process allowed the IPN samples to absorb more energy during the fracture experiments and in this way to increase the fracture toughness parameter. Samples with greater amounts of PMMA displayed higher stiffness, while samples with higher weight percentages of PUR exhibited longer extension at brake and considerable plastic deformation. Table 11.3 IPNs Molecular Weigh Between Cross-linking Points Sample
Composition (PMMA:PU)
Density (g/mL)
Mc (g/mol)
IPN 650 IPN 650 Cross-linked IPN 1400 IPN 1400 Cross-linked IPN 2000 IPN 2000 Cross-linked IPN 2900 IPN 2900 Cross-linked
80:20 70:30 0:100 80:20 70:30 0:100 80:20 70:30 0:100 80:20 70:30 0:100
1.02 6 0.204 1.09 6 0.049 1.04 6 0.044 1.15 6 0.077 1.13 6 0.049 1.04 6 0.041 1.09 6 0.087 1.15 6 0.031 1.06 6 0.053 1.17 6 0.035 1.11 6 0.048 1.00 6 0.039
2029.24 3419.14 2848.49 6353.18 5081.48 4045.75 6888.20 5389.30 2222.66 4698.94 6697.95 NA
PUR 650
PUR 1400
PUR 2000
PUR 2900
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Table 11.4 Fracture Toughness of 70:30 (PMMA:PUR) IPNs With Different Molecular Weight Diols Molecular Weight 70:30 (PMMA:PUR) (g/mol)
Quasi-Static Fracture Toughness (KIc) (MPa m1/2)
650 1400 2000 2900
1.08 6 0.05 0.92 6 0.01 1.31 6 0.04 0.74 6 0.01
11.4 CONCLUSION In this study, the IPN samples were further investigated by manipulating the structure of the PUR network using diols of varying molecular weights, which in turn affected the chain length between cross-links. The morphology did change, evidenced by TEM images. Although there was little change in storage modulus values, this still leaves room for improving impact resistance while maintaining high values of stiffness and rigidity. The presence of crystallinity played a role as physical reinforcement increasing the storage modulus values. Improvement in the fracture toughness of the samples showed promising results. The 70:30 (PMMA:PUR) ratio IPN with 650 g/mol molecular weight diol exhibited the capability to withstand a high load, but plastic deformation at low extensions. Samples with higher molecular weight diols displayed increased plastic deformation while also showing relatively high KIc values. Finally, the transparency was relatively high, although samples with diols of higher molecular weight showed increased phase separation, as shown through both TEM photos and UV-vis analysis.
ACKNOWLEDGMENT The support of the US Army Research Office through grant W911NF-12-1-0317 is gratefully acknowledged.
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