Curing behavior and structure of an epoxy/clay nanocomposite system.
A method to improve the physical properties of a polymer is the addition of reinforcing particles or fibers to the polymer to form composite materials. Owing to the development of high-strength and light-weight fibers, such as carbon fiber and silicon carbide fiber, the fiber-reinforced composites have been widely used. The recent development of polymer nanocomposites containing nanometer scale fillers, such as layered silicates or carbon nanotubes, provides a new promising opportunity in composite materials [1-3].
Layered materials are well suited for the design of hybrid composites, because their lamellar elements have high in-plane strength and a high aspect ratio . The smectite clays (e.g. montmorillonite) and related layered silicates are the materials of choice for polymer nanocomposite design, because they can not only be obtained easily from nature at low cost but can also be modified chemically to make themselves compatible with organic polymers. Each silicate layer has a lateral dimension of 200-2000 nm and a thickness of about 1 nm. The stacking of silicate layers forms tactoids that are typically 100-1000 nm thick. The cation-exchange property of smectite clays is an important aspect of their use in nanocomposite formation .
The overall properties of a polymer/clay nanocomposite material are determined not only by the parent components but also by the composite phase morphology and interfacial properties. Nanocomposites usually exhibit improved performances compared to conventional microcomposites, owing to their unique phase morphology and improved interfacial properties [4-9]. Major differences between conventional microcomposites and nanocomposites result from the fact that the latter has much larger surface (or interface) area per unit volume. Since many important chemical and physical interactions are governed by surfaces, a nanostructured composite can have substantially different properties from a microstructured composite of the same composition [10, 11].
Two types of polymer/clay nanocomposite are possible. Intercalated nanocomposites are formed when some organic molecules are inserted between the silicate layers. Exfoliated nanocomposites are formed when the silicate layers are individually dispersed in the polymer matrix. Exfoliated nanocomposites show greater phase homogeneity than do intercalated nanocomposites. This structural distinction is the primary reason why the exfoliated structure is especially effective in improving the reinforcement and other performances of polymer/clay nanocomposite materials . The ability of smectite clays to improve the mechanical properties of an engineering plastic (Nylon 6) was first demonstrated by Toyota researchers [12-14]. The key to this extraordinary performance of Nylon 6/clay hybrids was the complete dispersal exfoliation of the silicate layers in the polymer matrix.
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The nanocomposite chemistry developed first for thermoplastics has been extended in recent years to thermosets. The dimensional stability, thermal stability, and solvent resistance of an epoxy resin, one of the most common thermosets, can be improved when the silicate layers are present [15-18]. Epoxy resins can be used in the adhesive, coating, electronic, and aerospace industries, because they have excellent thermo-mechanical and chemical properties . The inherent excellent properties of epoxy resins are a result of the curing process, in which a low-molecular-weight resin is transformed into an infinite molecular weight polymer with a three-dimensional network structure . The curing process of an epoxy resin would be affected by the incorporation of a smectite clay. Therefore, in this study, an epoxy/clay nanocomposite system has been prepared by mixing an organically modified smectite clay with an epoxy resin, and their curing behavior has been investigated to see the effect of the organoclay on the curing behavior of the epoxy resin. Thermomechanical properties of the nanocomposite system and the structure of the nanocomposite system were investigated, to confirm the formation of intercalated and/or exfoliated nanostructure in the epoxy/clay nanocomposite system.
The epoxy resin was a bifunctional diglycidyl ether of bisphenol-A (YD-128 from Kuk Do Chem., Seoul, Korea), and the curing agent was 4,4'-methylene dianiline (Kuk Do Chem.). The epoxy equivalent weight of the epoxy resin was 187 g/mol, and the viscosity of the resin was about 12,000 cP at 25[degrees]C. The organophilic smectite clay (Cloisite 30B, a chemically modified montmorillonite) was supplied by Southern Clay Products, USA. The organic modifier used to modify pristine clay ([Na.sup.+] montmorillonite) was methyl tallow bis-2-hydroxyethyl quaternary ammonium. Tallow means, predominantly, an octadecyl chain with smaller amounts of lower homologues (approximate composition: ~65% CI8; ~30% CI6; ~5% C14). It is noteworthy that the organic modifier has two hydroxy 1 groups, which may affect the curing behavior of the epoxy resin. The modifier concentration in the organoclay was 90 mequiv/100 g clay, and the interlayer spacing of silicate layers was 1.86 nm. Figure 1 shows the structures of the materials used in this study.
Preparation of Epoxy/Clay Nanocomposites
A certain amount (1, 3, and 5 phr (parts per hundred of epoxy resin)) of the organoclay was mixed with the epoxy resin, and stirred using a mechanical stirrer at room temperature. The mixing time was changed up to 12 h. to see the effect of mixing time on the structure and properties of the epoxy/clay nanocomposite system. And then, the binary mixture of the epoxy resin and the organoclay was mixed again with the curing agent by stoichiometry, and stirred for 5 min. After degassing the ternary mixture in a vacuum oven at 60[degrees]C for 5 min to make an epoxy/clay nanocomposite sample, for mechanical tests and structure analyses, it was poured into a silicon rubber mold (35 mm x 13 mm X3.2 mm) and then cured in a hot press at 170[degrees]C for 2 h, and finally postcured at 200[degrees]C for 30 min.
Differential Scanning Calorimetry. In order to investigate the curing behavior of the epoxy/clay nanocomposite system, the DSC 2910 (TA Instruments, New Castle, DE) was used. About 10 mg of the degassed ternary mixture before curing was placed in a hermetic aluminum liquid sample pan, and the sample pan was tested immediately after sealing and positioning it right on the differential scanning calorimetry (DSC) sample cell. Each sample was cured dynamically at different scanning rates of 5, 10, and 20[degrees]C/min, respectively. The dynamic DSC scanning temperature range was from 10 to 200[degrees]C under a nitrogen gas flow (65 ml/min). Dynamic DSC second scans were performed at a scanning rate of 10[degrees]C/min, to investigate the glass transition behavior of the nanocomposites.
Dynamic Mechanical Analysis. Thermomechanical properties of the fully cured epoxy/clay nanocomposites were investigated using the DMA 2940 (TA Instruments) mounted with a single cantilever. The frequency was 1 Hz, and the scanning rate was 5[degrees]C/min. The scanning temperature range was from room temperature to 300[degrees]C.
X-ray Diffractometer. X-ray diffractometer (XRD; SCINTAG XDS 2000, Scintag. Cupertino, CA) was used to analyze the nanostructures of the epoxy/clay nanocomposites. The XRD instrument was equipped with Cu K[alpha] radiation (wavelength = 0.15418 nm). The scanning was carried out from 1.5[degrees] to 10[degrees] at a rate of 0.6[degrees]/min.
Transmission Electron Microscope. Transmission electron microscope (TEM) photographs of the cured epoxy/clay nanocomposites were obtained using the JEM-2020 (JEOL, Tokyo, Japan). To make specimens for TEM analyses, the epoxy/clay nanocomposites were microtomed by Leica Ultracut-R into about 80-nm thick slices. A carbon layer was deposited on these slices, followed by placing them on a 400-mesh copper grid for TEM imaging.
RESULTS AND DISCUSSION
The curing reaction of a thermoset resin system results in an increase of molecular weight with curing lime. The polymerization process of the thermoset resin system can be characterized by gelation and vitrification. The attainment of a gel formation threshold corresponds to the formation of a macroscopic crosslinked structure that constrains the mobility of chains, supposing an increasing loss of fluidity until viscosity rises toward an infinite value. The vitrification usually appears when the glass transition temperature of the reacting medium exceeds the reaction temperature of the system during polymerization. So the vitrification can be observed during a step-by-step isothermal polymerization procedure, or during a dynamic polymerization at a very low heating rate. But the curing reaction, which has been stopped temporarily by vitrification, can be resumed upon subsequent heating of the reacting medium above its glass transition temperature.
There have been several ways to investigate the curing kinetics of a thermoset resin system. One of them is to measure the change of a specific physical property that can be related to the chemical conversion during polymerization. These properties include rheological properties, electrical properties, thermomechanical properties, and heat evolution by reaction. Among these methods, the thermal analysis by DSC [21, 22], measuring heat evolution by reaction, has an advantage of simultaneously providing the kinetic and thermal data on the resin system.
The epoxy curing reactions by amines are exothermic and analyzed by somewhat different order kinetics, because they indicate different curing characteristics to each other. Shechter et al.  investigated the chemistry about the curing of bifunctional epoxy resins by amine hardeners and found that a combination of an epoxide and a primary amine leads to two principal reactions; (1) the addition reaction of a primary amine hydrogen to an epoxy group to form a secondary amine and (2) the addition reaction of an amine hydrogen in the secondary amine formed by the reaction (1) to another epoxy group to create a tertiary amine. These epoxy curing reactions by amines are known to be autocatalytic, because the OH groups formed during the reaction helps in the ring opening of epoxy groups .
A simple n-th order reaction kinetic model can be expressed as follows:
d[alpha]/dt = k(1 - [alpha])[.sup.n] (1)
where k is the reaction rate constant and n is the reaction order. This simple n-th order reaction kinetics is generally used for a polyurethane system. This model assumes a maximum initial reaction rate and, consequently, is not capable of describing an epoxy curing reaction by an amine hardener, which exhibits a maximum reaction rate during isothermal curing because of the autocatalytic effect by OH groups. Kamal and Sourour  proposed the following semi-empirical reaction kinetic model that could successfully describe the autocatalytic reaction mechanism of an epoxy curing reaction by an amine hardener,
d[alpha]/dt = ([k.sub.1] + [k.sub.2][[alpha].sup.m])(1 - [alpha])[.sup.n], (2)
The reaction rate constants, [k.sub.1] and [k.sub.2], are usually assumed to have an Arrhenius temperature dependence. This autocatalytic kinetic equation has been found to describe well the epoxy curing reactions by amine hardeners.
Mechanistic models are obtained from the balances of reacting species. Thus, a good understanding of a reaction mechanism is required. Therefore, mechanistic models are better than the phenomenological ones in terms of prediction and interpretation of curing reaction kinetics of a thermosetting system. However, because thermosetting reactions are rather complex, mechanistic models are not always feasible. Therefore, in this study, the phenomenological model expressed as Eq. 2 was used to analyze the reaction kinetics of the epoxy/clay nanocomposite system. The overall reaction order was assumed to be 2, in this study, not only to make the kinetic model have some mechanistic aspect but also because this assumption was reasonable for other thermosetting resin systems similar to the system of this work . The rate constants, [k.sub.1] and [k.sub.2], can be described as follows, using an Arrhenius temperature dependence:
[k.sub.1] = [k.sub.11] exp([-E.sub.1]/RT) (3)
[k.sub.2] = [k.sub.22] exp([-E.sub.2]/RT) (4)
where [k.sub.11] and [k.sub.22] are frequency factors, [E.sub.1] and [E.sub.2] are activation energies, and R is the ideal gas constant.
The following autocatalytic reaction kinetic equation was obtained by combining the aforementioned equations, Eq. 2-4, and introducing the scanning rate term ([S.sub.r]) to analyze directly the reaction kinetic data obtained from the dynamic DSC experiments.
d[alpha]/dT = 1/[S.sub.r]([k.sub.11] exp([-E.sub.1]/RT) + [k.sub.22] exp([-E.sub.2]/RT)[[alpha].sup.n])(1 - [alpha])[.sup.2-n]. (5)
The dynamic DSC experimental technique was used to obtain reaction kinetic data of the epoxy/clay nanocomposite system. Figure 2a shows the dynamic DSC thermograms of the pure epoxy resin system for various scanning rates. The peak temperature showing a maximum heat evolution shifted to a higher temperature region with increasing scanning rate. This peak-shifting phenomenon caused by increasing the scanning rate depends on the activation energy associated with each reaction. Based on this peak-shifting phenomenon, there have been two methods discussed in the literature to calculate the activation energy associated with each reaction. They are Kissinger's method  and the method suggested by Ozawa  and Flynn . But these methods are not sufficient in analyzing the reaction kinetics of a thermosetting system accurately, for a whole range of the reaction, because they use only limited information from the DSC thermograms. So a numerical fitting method, which uses all the experimental reaction kinetic data calculated from the dynamic DSC thermograms for a whole range of the reaction, was used to analyze the reaction kinetics of the epoxy/clay nanocomposite system in this study. To obtain conversion data from the dynamic DSC thermograms, the conversion was assumed to be the ratio of the reaction heat generated until a certain temperature, HT, to the overall heat of reaction at complete conversion, [H.sub.rxn]. The overall heat of reaction. [H.sub.rxn], for each epoxy/clay nanocomposite system was obtained by integrating each dynamic DSC thermogram, respectively.
[FIGURE 2 OMITTED]
The reaction kinetic parameters of the kinetic equation were determined by fitting the dynamic DSC conversion data to the kinetic equation, Eq. 5, using the Marquardt's multivariable nonlinear regression method and Runge-Kutta integration technique . Three sets of experimental conversion data obtained from the three dynamic DSC thermograms, respectively, were fitted simultaneously by the kinetic equation, to determine accurate and reasonable values of the reaction kinetic parameters. The values of the reaction kinetic parameters, [k.sub.1], [k.sub.2], [E.sub.1], [E.sub.2], and n, determined by the fitting process are listed in Table 1 for each epoxy/clay nanocomposite system together with the pure epoxy resin system. Figure 2b shows that the conversion data obtained from the dynamic DSC thermograms agree well with the conversion curves calculated from the reaction kinetic equation for the pure epoxy resin system. The second order autocatalytic reaction kinetics could describe well the reaction kinetics of the pure epoxy resin system.
Figure 3a shows the theromograms of the epoxy/clay nanocomposite system for various clay contents obtained at a scanning rate of 10[degrees]C/min. The peak temperature shifted to a lower temperature region with increasing clay content: 169.0[degrees]C for 0 phr, 166.7[degrees]C for 1 phr, 165.6[degrees]C for 3 phr. and 164.5[degrees]C for 5 phr. This result shows that the curing rate of the nanocomposite system increased as the clay content was increased. This increase in curing rate with increasing clay content was considered to be due to the OH groups of the organic modifier of the clay, which could accelerate the epoxy curing reaction. Figure 3b shows that the conversion data obtained from the dynamic DSC thermograms agree well with the conversion curves calculated from the aforementioned reaction kinetic equation for the epoxy/clay nanocomposite system. The second order autocatalytic reaction kinetics could describe well the reaction kinetics of not only the pure epoxy resin system but also the epoxy/clay nanocomposite system.
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Dynamic Mechanical Properties
Dynamic mechanical analysis (DMA), which measures the modulus and energy dissipation properties of a material, was carried out to determine both the glass transition temperatures and thermomechanical properties of the cured epoxy/clay nanocomposites. Two different moduli of the nanocomposites, a storage modulus (E') which is related to the ability of the material to return or store energy, and a loss modulus (E") which is related to the ability of the material to dissipate energy, were determined as a function of temperature. The temperature dependence of the ratio, E'/E", which is called tan delta (tan 8) of the material, was also determined as a function of temperature.
Figure 4a shows the dependence of the storage modulus of the epoxy/clay nanocomposites containing various clay contents on temperature. Compared to the pure epoxy resin system, the storage moduli of the nanocomposites were very slightly increased. The glass transition temperature of the epoxy/clay nanocomposite system, which can be determined by taking the temperature of the most drastic decrease in the storage modulus, increased with increasing clay content. The increase in the glass transition temperature of the nanocomposite system was considered to be due to the dispersion of the silicate layers of the clay and their ability to hinder the motion of the molecular chains and network junctions.
Figure 4b shows the dependence of the storage modulus of the epoxy/clay nanocomposite containing the clay 3 phr on temperature as well as the mixing time during the nanocomposite sample preparation step. With increasing mixing time, the storage modulus and the glass transition temperature of the epoxy/clay nanocomposite increased first, and then almost unchanged after that, because a sufficient mixing was already attained. The initial increase with the mixing time in the storage modulus and the glass transition temperature of the nanocomposite was considered to be due to the structure change of the nanocomposite with the mixing time. The nanocomposite would have more exfoliated and more intercalated clay structure with increasing mixing time, and this structure change would result in the increase in the storage modulus and the glass transition temperature. The structure change with the mixing time was investigated by XRD and TEM, and the results are shown in the following section. The mixing time of 12 h was considered to be sufficient for the epoxy/clay nanocomposite system, because a mixing time more than 12 h showed a negligible effect on the thermomechanical properties of the nanocomposites. The DMA data shown in Fig. 4a are for the epoxy/clay nanocomposites prepared by the mixing time of 12 h.
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Structure and Morphology
The XRD patterns of the clay and the epoxy/clay nanocomposites containing different amounts of clay are shown in Fig. 5a. The peaks for the epoxy/clay nanocomposites were observed at a lower diffraction angle region compared to the peak for the clay, and their positions were almost same regardless of clay content. The peak shift to a lower angle region means that the interlayer spacing of the clay was increased by the intercalation of organic molecules between the silicate layers of the clay. But the fact that the peak positions of the nanocomposites were negligibly affected by clay content means that the interlayer spacing was determined predominantly by chemical and physical interactions between the clay and the epoxy resin system. This consideration could also be supported by the XRD patterns shown in Fig. 5b, demonstrating the effect of mixing time on the structure of the nanocomposite containing the clay 3 phr. With the increase in the mixing time the peak height was decreased, because the portion of the clay exfoliated into the epoxy matrix, among the total amount (3 phr) of the clay incorporated in the epoxy resin system, would be increased gradually with the increase in the mixing time. Just a negligible XRD peak shift to a lower angle region was observed with increasing the mixing time, even though the peak height was considerably decreased. By the natural chemical and physical interactions between the clay and the epoxy resin system, the intercalation process was found to progress quite rapidly, because the XRD-peak shift to a lower angle region due to the intercalation of the epoxy resin system was almost completed in 10 min, as shown in Fig. 5b.
Using the Bragg equation, 2d sin[theta] = n[lambda], where d denotes the lattice spacing of the clay, 2[theta] the angle of the XRD peak, n a positive number (generally 1), and [lambda] is the wavelength (0.15418 nm) of the X-ray, the interlayer spacing of the silicate layers of the clay could be calculated. The interlayer spacing of the silicate layers of the clay was calculated to be 1.86 nm from the XRD peak position of the clay (2[theta] = 4.75[degrees]). The interlayer spacing of the silicate layers of the clay in the nanocomposites was calculated to be about 3.68 nm from the XRD peak positions (20 = 2.40[degrees]) of the nanocomposites, as shown in Fig. 5a. The interlayer spacing of the silicate layers of the clay in the nanocomposites was increased by two, by the intercalation of the epoxy resin system compared to the clay itself. The interlayer spacing of the silicate layers of the clay in the nanocomposite containing the clay 3 phr was 3.53 and 3.68 nm according to the XRD peak positions (2[theta] = 2.50[degrees] and 2.40[degrees], respectively) of the nanocomposite for two different mixing times of 10 min and 12 h. respectively. The increase in the interlayer spacing of the silicate layers of the clay was negligible with increasing mixing time, because the intercalation process progressed rapidly by the natural chemical and physical interactions between the clay and the epoxy resin system. Figure 5b indicated that with increasing the mixing time, the silicate layers of the clay were readily intercalated at the early stage of mixing, and then the exfoliation of the silicate layers of the clay progressed gradually.
Although the XRD patterns gave some information on the interlayer spacing of the silicate layers of the clay in the epoxy/clay nanocomposites, information on the morphology and the exfoliation structure of the nanocomposites is not sufficient. Therefore. TEM was also used to visually evaluate the degree of intercalation and exfoliation of the clay, the amount of aggregation of clay clusters, and the morphology of the nanocomposites. TEM photographs of the epoxy/clay nanocomposite containing the clay 3 phr are shown in Fig. 6 for two different mixing times. Compared to the TEM image for the mixing time of 10 min, the TEM image for the mixing time of 12 h showed a little bit more intercalated and exfoliated structure. The findings from the TEM photographs agreed well with the findings from the XRD patterns.
[FIGURE 6 OMITTED]
The curing behavior, thermomechanical properties, and structures of the epoxy/clay nanocomposite system were studied in this work. The curing rate of the epoxy/clay nanocomposite system increased slightly with increasing clay content. The reaction kinetic parameters of the kinetic equation were determined by fitting the dynamic DSC conversion data to the kinetic equation, using the Marquardt's multivariable nonlinear regression and Runge-Kutta integration techniques. The fitting results showed that the reaction kinetics of the nanocomposite system could be described well by the autocatalytic second order reaction kinetic equation. The glass transition temperature of the epoxy/clay nanocomposite system was higher than the pure epoxy resin system, and increased slightly with increasing clay content. The storage modulus of the epoxy/clay nanocomposite system was slightly higher than the pure epoxy resin system, and increased slightly with increasing clay content. The XRD patterns and TEM photographs indicated the formation of dominant intercalated structures in the epoxy/clay nanocomposites together with some exfoliated structures.
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Kang Sick Seo, Dae Su Kim
Department of Chemical Engineering, Chungbuk National University, Cheongju, Chungbuk 361-763, Korea
Correspondence to: D.S. Kim: e-mail: firstname.lastname@example.org
Contract grant sponsor: Chungbuk National University.
TABLE 1. The values of the reaction kinetic parameters of the kinetic equation for the epoxy/clay nanocomposite system. Kinetic parameter Clay [k.sub.11] [E.sub.1] [k.sub.22] [E.sub.2] content ([10.sup.5] ([10.sup.4] ([10.sup.5] ([10.sup.4] (phr) see - 1) cal/mol) see - 1) cal/mol) n 0 4.75 1.35 3.09 1.08 1.25 1 3.37 1.32 2.83 1.06 1.25 3 2.91 1.28 2.35 1.06 1.25 5 2.16 1.29 2.25 1.07 1.25