Phase separation and rheological behavior during curing of an epoxy resin modified with syndiotactic polystyrene.
The applications of cross-linked thermoset polymers are limited by their brittleness. To increase their toughness, different methods have been developed. For instance, thermosetting resins have been modified with a variety of thermoplastic polymers and rubbers in order to improve their performances. However, in only a few studies have semicrystalline thermoplastics been used as modifiers [1-3].
On the one hand, many researchers have focused their work on blends of syndiotactic polystyrene (sPS) with other polymers [4-7], due to the very promising properties of this polymer, and especially because of its crystalline polymorphism and its relatively fast crystallization rate. Additionally, sPS is a semicrystalline, engineering polymer with a melting point of 265.5[degrees]C and a glass transition temperature of 97.5[degrees]C as measured by differential scanning calorimetry, similar to that of atactic polystyrene (aPS). The high heat resistance and modulus of elasticity, low dielectric constant, very good resistance to chemicals because of its stereoregularity, and relatively fast crystallization rate make sPS a potential thermoplastic for a large number of applications in automotive and electronic industries [8-10].
On the other hand, it is known that DGEBA is a good solvent for many thermoplastics such as poly(ether sulfone) (PES) [11-13], poly(methyl methacrylate) (PMMA) [14, 15], or poly([epsilon]-caprolactone) , among others. Schut et al.  found that DGEBA is also a good solvent for syndiotactic polystyrene at 220[degrees]C and that the most suitable hardener for this system is MCDEA. This is so because the curing rate is not too fast at this temperature, thus allowing some time for the manipulation of samples before curing. Salmon et al.  studied the curing kinetics of sPS blends by using differential scanning calorimetry (DSC) and near infrared spectroscopy (NIR). They concluded that sPS has a slight influence on the curing behavior of the DGEBA/MCDEA system. Initially, a slight delay on the curing reactions was observed, but after 5-8 min the reaction rate of thermoset/thermoplastic blends became faster than that for the neat DGEBA/MCDEA system. This particular effect in their opinion could be attributed to phase separation, which cancels the dilution effect in the blends and thus modifies their curing kinetics. The sPS-(DGEBA/MCDEA) systems have also been characterized by Zafeiropoulos et al.  by X-ray scattering techniques. From these investigations it is well established that the relative degree of crystallinity increases with decreasing the sPS content in sPS-(DGEBA/MCDEA) blends. Moreover, small angle X-ray scattering (SAXS) investigations revealed that the long period is shifted towards higher values with increasing the DGEBA content in the sPS-(DGEBA/MCDEA) systems. These results might be attributed to a possible change in chain conformation at different sPS concentrations. Korenberg et al.  studied the toughness of sPS/epoxy blends. From their results, they concluded that the very significant increase in toughness observed for the 3 wt% sPS-(DGEBA/MCDEA) system could be attributed to the different mechanisms of phase separation that take place over the range of sPS concentrations used. They did not find any evidence of plastic yielding on the sPS phase, which might readily explain the significant increase in toughness seen in the sPS blends.
[FIGURE 1 OMITTED]
Taking this in consideration, it seems very interesting to study how the crystallization of sPS on the sPS-(DGEBA/MCDEA) system influences the morphology and rheological behavior of the reactive system during the curing process, which would allow to gain a deeper understanding of this semicrystalline thermoplastic/thermoset system during processing.
The final morphology in the semicrystalline thermoplastic/epoxy system is the consequence of the competition between the driving forces for crystallization and for microphase separation. This means that the final morphology strongly depends, among other factors, on the crystallization temperatures, glass transition temperature of the amorphous phase as well as the crystallization rate [20, 21].
On curing, the initial homogenous mixture consisting of a thermoplastic dissolved in an epoxy precursor starts to increase its molecular mass. This implies a decrease in the conformational entropy of mixing leading to phase separation between the thermoplastic and the forming epoxy network. This phase separation is called "reaction-induced phase separation" (RIPS) or "chemically-induced phase separation" [22-27] and may occur by spinodal decomposition (SD) for thermoplastic concentrations near the critical composition  or by a nucleation and growth (NG) mechanism for off-critical compositions. Additionally, when the thermoplastic is semicrystalline, the crystallization of the thermoplastic gives an alternative pathway to RIPS. This process is called crystallization-induced phase separation (CIPS) [20, 21].
Although the behavior of the reactive systems during the curing process is very complicated and needs to be studied carefully, in this paper the results about the influence of crystallization of sPS on the phase separation during the curing process of sPS-(DGEBA/MCDEA) reactive blends are reported. The main objective of this work, which is part of a common research performed by several European laboratories aiming to characterize the sPS-(DGEBA/MCDEA) system from a multidisciplinary point of view and to extend the possible applications of sPS, is to study the behavior of sPS/epoxy blends during curing. Results on the crystal development as well as on the morphology resulting from phase separation are presented. Results are analyzed mainly by differential scanning calorimetry (DSC), transmission optical microscopy (TOM), atomic force microscopy (AFM), and also by rheology.
[FIGURE 2 OMITTED]
Materials and Sample Preparation
A diglicydylether of bisphenol-A epoxy resin was used as reactive solvent (Dow DER 330 from Dow Chemical). It has an epoxy equivalent weight of 191.5 g [mol.sup.-1]. This epoxy resin was cured with a stoichiometric amount of an aromatic amine hardener, 4,4'-methylene bis (3-chloro-2,6-diethylaniline) (MCDEA), kindly supplied by Lonzacure (Lonza). The semicrystalline thermoplastic modifier used was sPS, provided by Dow Chemical Company (DCG Buna Sow Luena Olefinverbund GmbH), known under the trade name Questra QA 101. Its number average molar mass, [M.sub.n], is 94,000 g [mol.sup.-1] and its weight average molar mass, [M.sub.w], is 192,000 g [mol.sup.-1]. For some measurements, atactic polystyrene (aPS) provided by BASF under the trade name Polystyrol 144C, and having a molecular weight similar to the sPS ([M.sub.n] = 80,000 g [mol.sup.-1]), was also used.
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
sPS-(DGEBA/MCDEA) blends were prepared by following the procedure recommended by Schut et al. . First, sPS pellets were powdered and dried overnight at 80[degrees]C in an air oven. Then, sPS-DGEBA mixtures containing different sPS weight percentages (2.5, 5, 7.5, 10, 15, and 20 wt%) were prepared by dispersing for 10 min a weighed amount of sPS in DGEBA at 220[degrees]C in an oil bath under continuous stirring. Following, the mixture was moved to a Wood's metal bath at 290[degrees]C and stirred for 10 min until complete dissolution of sPS. Simultaneously, MCDEA was melted in another test tube at 220[degrees]C for 5 min. Finally, the molten MCDEA was added to the sPS/DGEBA mixtures cooled down to 220[degrees]C and stirred for 30 s in order to start the curing reaction. A clear, homogeneous reacting mixture was obtained. Moreover, in order to obtain cured blends, the samples were kept for 2 h at 220[degrees]C.
The curing behavior was analyzed on a Mettler Toledo DSC 822 differential scanning calorimeter equipped with a Sample Robot TSO 801 RO. Nitrogen was used as a purge gas (10 mL/min). Temperature and enthalpy were calibrated by using an indium standard. Curing was performed in sealed aluminum pans containing a sample weight of around 7 mg. The curing behavior of the DGEBA/MCDEA systems modified with sPS or in some cases with aPS was analyzed in isothermal experiments performed at 220[degrees]C for 2 h. In order to ascertain whether RIPS or CIPS has taken place, after the isothermal test, samples were heated to 290[degrees]C at 10[degrees]C [min.sup.-1] and then cooled to room temperature at 10[degrees]C [min.sup.-1]. All samples were tested immediately after preparation.
[FIGURE 5 OMITTED]
A transmission optical microscope (Nikon Eclipse E600) equipped with a hot stage (Mettler FP 82HT) was used to study the phase separation and crystallization during isothermal curing. A drop of the uncured reacting sample was kept between two microscope slides at 220[degrees]C for 2 h. Phase separation and crystallization processes were monitored simultaneously using a 20X objective lens. The eyepiece magnification was 10X. Micrographs were captured with a Color View 12 camera by using the AnalySIS Auto 3.2 software (Soft Imaging System GmbH).
[FIGURE 6 OMITTED]
AFM topography images of the cryogenic fracture surfaces of cured blends were recorded in tapping mode at room temperature by using a scanning probe microscope (SPM) (Nanoscope IIIa, Multimode[TM] from Digital Instruments). Etched single-beam cantilever (225 [micro]m length) silicon nitride probes having a tip's nominal radius of curvature of 5-10 nm were used. Scan rates ranged from 0.8 to 1.6 Hz/s. The sample line was 512 and the target amplitude was around 2 V. Height and phase images were recorded simultaneously during scanning. In order to obtain repeatable results of the blend morphology, different regions of the specimens were scanned. Similar images were obtained, thus demonstrating the reproducibility of the results.
Dynamic oscillatory shear measurements were carried out at 0.5, 1, 2, 3, or 5 Hz and 100% strain in a Rheometrics Advanced Expansion System (ARES) rheometer equipped with two 25-mm-diameter parallel plates. Sample thickness was about 1.2 mm. The transducer operating range was set to 0.2-200 g.cm or 0.2-2000 g.cm depending on the measured torque values. The parallel plates were heated up to 220[degrees]C before the liquid uncured sample was loaded, and the experiment started immediately after. Data were collected and analyzed using Rhios Rheometrics Software.
RESULTS AND DISCUSSION
Evolved heat vs. curing time plots during isothermal tests at 220[degrees]C for sPS-(DGEBA/MCDEA) and for aPS-(DGEBA/MCDEA) systems with different thermoplastic contents are shown in Figs. 1 and 2, respectively.
[FIGURE 7 OMITTED]
In Fig. 1, it can be observed that for samples containing 10, 15, and 20 wt% sPS, in addition to the exothermic peak of curing, also displayed by the neat and 5 wt% sPS samples, another additional exothermic peak whose position slightly depended on sPS content could be detected. This peak appeared at times of maximum exothermicity of 620 s, 672 s, and 658 s, respectively. For the 5 wt% sPS system, this additional peak could not be detected because of the small sPS content. After the isothermal test, the temperature was increased up to 290[degrees]C and kept for 10 min in order to melt the sPS crystals and to destroy the thermal history. Following, a cooling scan was performed. On cooling, as shown in Fig. 3, a single exothermic peak was recorded which is attributed to the crystallization of sPS from the melt under dynamic thermal conditions.
On the other hand, blends containing aPS also showed a shoulder, for 10 wt% aPS, or a small peak, for 15 and 20 wt% aPS, in addition of the main exothermic curing peak (Fig. 2). However, the enthalpies of these peaks are much lower and the time at which the exothermic peak appeared was longer, 830 s for the 15 wt% and 920 s for the 20 wt% containing aPS blend, than for their sPS counterparts. In all cases the measured enthalpies were lower than 1 J.[g.sup.-1]. In epoxy systems modified with amorphous thermoplastics, this peak or shoulder has been attributed to reaction induced phase separation, and it is mostly due to an increase of the concentration of reactive groups in the epoxy rich phase because of liquid-liquid phase separation.
The enthalpy associated to the additional exothermic peak on isothermal test, together with the enthalpy of the crystallization peak, as measured on the cooling scan, for sPS-(DGEBA/MCDEA) mixtures are collected in Table 1. The recorded enthalpy for the additional exothermic peak and that for crystallization are quite similar, and their difference is small, around 1 J.[g.sup.-1]. Thus, it can be concluded that the main phenomenon associated to the additional exothermic peaks during curing of sPS-(DGEBA/MCDEA) systems is crystallization of sPS. But, it should also be reminded that, as seen in aPS-(DGEBA/MCDEA) systems, reaction induced phase separation takes place at similar times.
As a conclusion, by comparing the thermograms for DGEBA/MCDEA systems containing sPS or aPS it cannot be clearly ascertained whether in sPS-(DGEBA/MCDEA) systems curing at 220[degrees]C RIPS or CIPS takes place earlier, nevertheless DSC reveals that both phase separation and crystallization take place at similar times. Temperatures higher than 220[degrees]C were not studied because the curing reaction proceeded too quickly  and the results were not accurate. On the other hand, at lower temperatures the fast crystallization rate of sPS made the experiments impossible.
Phase Separation and Crystallization During Curing
As is well known, phase separation in amorphous thermoplastic/thermoset systems is induced by the curing reaction, i.e., RIPS. Nevertheless, in a semicrystalline thermoplastic/thermoset system, phase separation can also be induced by crystallization of the thermoplastic. This is known as CIPS [20, 21]. For these last systems, both RIPS and CIPS may occur at different times during curing. In a try to clarify whether RIPS, CIPS, or both are responsible for phase separation and even more, which one takes place firstly, the morphology evolution during curing for DGEBA/MCDEA systems modified with amorphous aPS or with semicrystalline sPS was investigated by transmission optical microscopy.
For aPS-(DGEBA/MCDEA) systems, the only possible mechanism for phase separation is RIPS. For the 5 wt% aPS containing aPS-(DGEBA/MCDEA) system, shown in Fig. 4, up to 610 s the curing mixture was homogeneous. By 640 s, RIPS had started, and the final morphology was attained by 1200 s.
For sPS-(DGEBA/MCDEA) systems, both RIPS and CIPS may lead to phase separation. Changes in morphology for the 5 wt% sPS containing sPS-(DGEBA/MCDEA) system during curing are presented in Fig. 5 and for this same system observed between crossed polarizers in Fig. 6. As can be seen in Fig. 5, up to 540 s the system was homogeneous, but then some sPS nuclei started growing and at 560 s some small spherulites were already visible. Almost simultaneously, RIPS began, which was visible as a texture starting to appear over all the observed surface. This was seen more clearly at 600 s.
When observed between crossed polarizers (Fig. 6), sPS spherulites appeared bright on a black field. However, the newly reaction induced separated phases were not seen. In this way, by comparing micrographs taken with or without polarizers, spherulites and phases separated by RIPS could be undoubtedly distinguished. By 910 s, growing of spherulites has ended, since no change in their size was measured between 910 and 3020 s. Thus, for the 5 wt% sPS containing the sPS-(DGEBA/MCDEA) system at 220[degrees]C, roughly 300 s were available for the spherulites to grow. However, phase separation of an amorphous phase still increased until it was finally arrested by network formation, since some phase evolution could still be detected between 870 and 3100 s, as seen in Fig. 5.
Although not presented here, similar results were also found for sPS-(DGEBA/MCDEA) systems containing 2.5 and 7.5 wt% sPS. It was observed that at increasing sPS contents the number of sPS nuclei also increased and as a result smaller sPS spherulites were formed. But for systems containing 10 wt% sPS (Fig. 7) and more, RIPS as well as nucleation and growth of sPS spherulites occurred very quickly once started. Many nuclei were formed in a short time and started growing simultaneously, but their growth was limited because of impingement. Almost at the same time, RIPS took place. In this way, the analysis of the process was complicated. Moreover, the beginning of CIPS and RIPS were slightly delayed compared to systems containing less sPS, as can be seen by comparing the micrograph taken at 560 s in Fig. 5 to that taken at 570 s in Fig. 7.
[FIGURE 8 OMITTED]
Thus, the analysis by optical microscopy has clearly evidenced that both reaction induced phase separation and crystallization of sPS take place in this sPS-(DGEBA/MCDEA) system curing at 220[degrees]C. Both events occurred almost at the similar times but based on the available results, it is not possible to discern whether crystallization of sPS takes place from a sPS-rich phase produced by RIPS, or sPS starts crystallizing from a homogeneous sPS-(DGEBA/MCDEA) mixture before RIPS had begun.
[FIGURE 9 OMITTED]
Salmon et al.  studied the curing kinetics and phase separation for blends containing similar amounts of sPS than in his work but samples were prepared differently. In their work, samples were quenched and stored before any analysis. In this way sPS might crystallize during subsequent heating at temperatures higher than [T.sub.g] at early stages of curing.
Morphology of Cured Samples and Crystallization Process
The final morphology of the cured samples was observed by optical microscopy. In Fig. 8, the cryogenic fractured surface for 5 wt% sPS containing sPS-(DGEBA/MCDEA) is shown. Broken spherical sPS spherulites embedded in a DGEBA/MCDEA matrix and distributed over all the sample surface could easily be observed (Fig. 8a). Their average diameter was about 50 [micro]m. These spherulites have probably been formed by CIPS, growing from a miscible sPS-(DGEBA/MCDEA) reacting medium. As sPS crystallization progressed, the concentration of sPS dissolved in the DGEBA/MCDEA progressively decreased. Moreover, when observed in more detail (Fig. 8b), very small nodules, with narrow size distribution and having a diameter of around 1 [micro]m, could be seen inside the DGEBA/MCDEA matrix. Because of its higher spatial resolution, AFM was used to complementary analyze the DGEBA/MCDEA matrix and nodules. By AFM, the nodules appeared homogeneously distributed in a DGEBA/MCDEA-rich matrix (Fig. 9a and b). These rich-sPS amorphous nodules are probably the result of a RIPS process. In this way, the occurrence of both CIPS and RIPS in the sPS-(DGEBA/MCDEA) mixture has been clearly demonstrated.
[FIGURE 10 OMITTED]
For aPS-(DGEBA/MCDEA) blends, only aSP-rich nodules in a DGEBA/MCDEA matrix were seen (Fig. 10). Compared to sPS-containing mixtures, the nodules were bigger and the size distribution was broader. The bigger size of the nodules is attributed to the fact that for aPS-(DGEBA/MCDEA) blends, phase separation proceeded only by RIPS. In this way, when phase separation took place, a greater amount of aPS was dissolved on the DGEBA/MCDEA mixture than for sPS-containing mixtures, since no decrease in the thermoplastic concentration by crystallization is possible for aPS. Also, the coalescence of aPS particles, which are liquid at 220[degrees]C, may lead to bigger particles. For sPS particles, this mechanism would not be possible because the crystallization of sPS inhibits the coalescence process and as a result particles are smaller.
The rheological behavior of sPS-(DGEBA/MCDEA) blends containing 5 wt% sPS during curing at 220[degrees]C was analyzed by dynamic oscillatory shear measurements at 1 Hz, and this behavior was related to the main events previously observed by optical microscopy, which are RIPS and CIPS. Typical curves are shown in Fig. 11, in which the storage modulus, G', loss modulus, G", and complex viscosity, [eta]*, are plotted vs. curing time. As a reference, similar plots for aPS-(DGEBA/MCDEA) blends containing 5 wt% aPS and for neat DGEBA/MCDEA are shown in Figs. 12 and 13, respectively.
[FIGURE 11 OMITTED]
As can be seen, for sPS-(DGEBA/MCDEA) systems at curing times slightly shorter than 400 s, G' and G" start increasing and reach a maximum at 520 s. Later, the complex viscosity abruptly increases when the systems reaches the gel point. On the other hand, neither for aPS-(DGEBA/MCDEA) blends nor for neat DGEBA/MCDEA such maximum was observed in this region of times. For these last systems, the initially low complex viscosity abruptly increases during curing because of gelation at times longer than about 700 s. This is the typical viscosity profile during the curing of epoxy systems.
The initial maximum observed for sPS-(DGEBA/MCDEA) systems arises at times similar to those at which phase separation of sPS or aPS by RIPS, and crystallization of sPS was observed by TOM. Thus, the appearance of this maximum could be attributed either to RIPS or CIPS. However, since no maximum is seen for aPS-(DGEBA/MCDEA) blends, where only RIPS takes place, this maximum is attributed to crystallization of sPS.
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[FIGURE 13 OMITTED]
The crystallization and phase separation processes in the reactive DGEBA/MCDEA system modified with semicrystalline sPS were studied by DSC, TOM, AFM, and rheology. Good correlation was obtained between the results given by the different techniques. Two different mechanisms are responsible for phase separation of sPS during curing, RIPS and CIPS. Experimental results showed that when sPS concentration is lower than 7.5-10 wt% both reaction induced phase separation and crystallization of sPS take place in this sPS-(DGEBA/MCDEA) system curing at 220[degrees]C. Both events occur at almost similar times but based on the available results, it is not possible to discern whether crystallization of sPS takes place from a sPS-rich phase produced by RIPS, or sPS starts crystallizing from a homogeneous sPS-(DGEBA/MCDEA) mixture before RIPS has begun. Additional results, such as those obtained by real time SAXS and wide angle X-ray scattering (WAXS), would be useful to clarify this aspect. The resulting blend morphology consists of a cured epoxy/amine-rich phase in which sPS crystals, in the form of spherulites produced by CIPS, are embedded together with smaller sPS-rich nodules homogeneously distributed on the matrix, which are produced by RIPS.
The viscoelastic behavior for the sPS-(DGEBA/MCDEA) system is in good agreement with the behavior of these blends as observed by TOM. A first maximum in the complex viscosity appears while crystallization induced phase separation of sPS is being produced. Later, the complex viscosity greatly increases because of gelation.
TABLE 1. Enthalpy of the additional exothermic peak during curing at 220[degrees]C for sPS-(DGEBA/MCDEA) mixtures and enthalpy of crystallization for sPS in sPS-(DGEBA/MCDEA) mixtures. sPS content Enthalpy of the additional Enthalpy of crystallization (wt %) exothermic peak (J.[g.sup.-1]) (J.[g.sup.-1] 5 -- -1.5 10 -3.5 -2.7 15 -6.8 -5.8 20 -7.9 -6.7
Contract grant sponsor: European Commission, Research Training Network (RTN) Program, POLYNETSET; contract grant number: HPRN-CT-2000-00146; contract grant sponsor: Ministerio de Ciencia y Tecnologia, Spain; contract grant number: MAT 2000-0293.
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A. Tercjak, P.M. Remiro, I. Mondragon
Escuela Universitaria Politecnica, Departamento Ingenieria Quimica y Medio Ambiente, Universidad del Pais Vasco/Euskal Herriko Unibertsitatea, Plaza de Europa, 1, 20018 Donostia/San Sebastian, Spain
Correspondence to: I. Mondragon; e-mail: email@example.com
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|Author:||Tercjak, A.; Remiro, P.M.; Mondragon, I.|
|Publication:||Polymer Engineering and Science|
|Date:||Mar 1, 2005|
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