Ind. Eng. Chem. Res. 2000, 39, 955-959
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Effect of Phase Separation on Rheological Properties during the Isothermal Curing of Epoxy Toughened with Thermoplastic Polymer Hongkyeong Kim and Kookheon Char* School of Chemical Engineering, Seoul National University, San 56-1, Shinlim-dong, Kwanak-gu, Seoul 151-742, Korea
Rheological behavior of thermoset/thermoplastic blends of epoxy/poly(ether sulfone) (PES) was monitored during the curing of epoxy resin. During isothermal curing of the mixture, a fluctuation in viscosity before the abrupt viscosity increase was observed. This fluctuation is believed to be due to the phase separation of PES from the matrix epoxy resin during curing and confirmed by small-angle light scattering (SALS) results. The viscosity behavior is classified into three regions: the initial low viscosity of the mixture gradually increases with time during the early stage of curing and then it reaches a local maximum. After that, the viscosity slightly decreases because of a decrease of the PES concentration in an epoxy-rich medium upon phase separation and finally increases abruptly because of the extensive network formation of the epoxy matrix. The effects of the domain viscoelasticity and the change of the PES composition in the medium due to the phase separation on rheological properties of the blends are considered, verifying that the change of the PES composition in the medium upon phase separation is known to be the key to interpret the observed viscosity fluctuation. We also observed that the initially negligible storage modulus abruptly increases upon phase separation because of the elastic contribution of high molecular weight PES domains. The study on the mechanism of simultaneous curing and phase separation has been a topic of many researches for many years1 because of the fact that toughened thermosets have been widely used in many applications, but the mechanism is still complicated. Many studies on the toughened thermosets have mainly focused on the structure-property relationship and the toughening mechanism1,2 and some on the phaseseparation kinetics.2-8 In the reaction-induced phase separation of a thermoset/thermoplastic polymer blend system, the change in the characteristic length of the concentration fluctuation is mainly attributed to two factors: diffusion and reaction constant. The fluctuation length can thus be either increased or decreased according to the competition between these two factors.6-8 The present study focuses on the effect of phase separation on rheological properties of a thermoplastic toughened epoxy system before gelation, and this system is thought to be dominated by diffusion rather than reaction constant. The toughened epoxy system employed in this study consisted of an epoxy (diglycidyl ether of Bisphenol A (DGEBA), YD128, Kukdo Chemical Co.), a curing agent (4,4′-diaminodiphenylmethane (DDM), Aldrich), and a thermoplastic modifier [poly(ether sulfone) (PES), Ultrason E6020, BASF]. The epoxy equivalent weight of YD128 was 184-190. The amounts of YD128 and DDM were determined to ensure a stoichiometric mixture. PES was purified by reprecipitation before use, and its weight-average molecular weight was 58 000. The change of complex viscosity of the mixture during isothermal curing was monitored by rheometrics me* To whom all correspondence should be addressed. Email:
[email protected]. Fax: +82-2-873-7523. Tel: +82-2-880-7431.
Figure 1. Complex viscosity profile of a DEGBA/DDM stoichiometric mixture with 20 wt % PES at an isothermal curing temperature of 90 °C.
chanical spectroscopy (RMS 800, Rheometrics Inc.) with disposable parallel stainless steel plates of 10 mm diameter, and the gap height between parallel plates was set to 0.5 mm. To determine optimum experimental conditions, preliminary frequency and strain sweeps were carried out. The curing experiments were performed using a frequency of 20 rad/s and a strain of 25%, and the initial mixtures showed the Newtonian behavior with this condition. As shown in Figure 1, which is the time change of complex viscosity of a DGEBA/DDM stoichiometric mixture with 20 wt % PES at an isothermal curing temperature of 90 °C, the initially low complex viscosity increases abruptly during curing, which is the typical viscosity profile during the curing of toughened epoxy systems.1 We also note that there is a fluctuation in viscosity just before the abrupt
10.1021/ie990536x CCC: $19.00 © 2000 American Chemical Society Published on Web 03/07/2000
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Figure 2. Changes of the complex viscosity of the DGEBA/DDM/ PES mixture with 10 wt % PES at different isothermal curing temperatures as a function of curing time.
viscosity increase, which is believed to be due to the phase separation of the mixture induced by the curing reaction. These fluctuations in viscosity are magnified by using semilog plots as shown in Figure 2 for different isothermal curing temperatures. We note from Figure 2 that the initial viscosity is lower and the time for the incipient viscosity drop, presumably due to the phase separation of PES from the epoxy matrix, is shorter for higher curing temperature. This type of viscosity behavior has also been noticed in the results of other researchers1 but was not investigated in detail. The phase separation induced by the curing reaction was observed by using both small-angle light scattering (SALS) and scanning electron microscopy (SEM). A 30 mW He-Ne laser with wavelength λ ) 633 nm was used as an incident light source for the SALS experiment. The scattered light intensity was measured with a photomultiplier tube connected to an optical fiber rotating with a stepping motor at measuring angles ranging from 1° to 40° within 10 s, which is sufficiently short to be regarded as an instant snapshot. The incident and transmitted light intensities were measured with photodiodes to correct the observed scattered light intensity. Figure 3a shows one example of a time evolution of scattered light intensity profiles at a curing temperature of 110 °C. The angle at the maximum scattered intensity initially moves to a smaller angle as phase separation proceeds and then becomes almost constant when an extensive network of the matrix epoxy resin is formed. The domain correlation length of the phase-separated PES domains (Λ) in Figure 3b, which is determined from the angle at the maximum scattered intensity as Λ ) λ/[2 sin(θmax/2)], is known to be proportional to the domain size,3-8 and correct domain sizes were obtained by comparing the domain correlation lengths obtained from SALS with the SEM photographs of samples in a fully cured state and used to estimate the domain volume fraction (φ) which will be explained later. During the phase separation, the domain correlation length or domain size is known to increase with time, obeying the power law (Λ ∼ tR). SEM photographs in Figure 4 show that there is a factor of 4-6 difference between the actual average domain size and Λ, which is known to be typical for other systems.9 At an isothermal curing temperature, as shown in Figure 2, viscosity gradually increases with time because of the increase in molecular weight of the epoxy matrix, and it suddenly decreases when the homoge-
Figure 3. (a) Time evolution of the scattered light intensity profiles of the DGEBA/DDM/PES mixture with 10 wt % PES at an isothermal curing temperature of 110 °C. (b) Changes of the domain correlation length of the DGEBA/DDM/PES mixture with 10 wt % PES at different isothermal curing temperatures as a function of curing time.
neous mixture phase separates into epoxy-rich and PESrich phases. It is also worthwhile to note that the start and the end points of the domain growth are comparable to the fluctuations in viscosity profiles, implying that the domain growth due to phase separation has a profound effect on the viscosity of the mixture. The viscosity then abruptly increases because of the extensive network formation of the epoxy matrix. During the curing of the toughened epoxy system, the initially homogeneous epoxy mixture usually separates into two phases, epoxy-rich and epoxy-lean phases, because of the increase in the molecular weight of the epoxy. In this study, the curing of the DGEBA/DDM/PES system goes through three steps as schematically depicted in Figure 1; first, epoxy molecules combine with one another (curing) in a homogeneous mixture (step I) before the onset of phase separation due to the molecular weight increase of the matrix epoxy (step II). After completion of phase separation, the epoxy molecules still grow to an infinite network (step III). To more quantitatively analyze the viscosity fluctuation during the phase separation, the phase separation kinetics was first examined. Assuming for simplicity that all of the domains are perfect spheres consisting of pure PES, that the morphology is developed via the nucleation and growth mechanism, and that the number of domains remains constant throughout the phase separation once the mixture is phase separated, the domain volume fraction of the epoxy/PES mixtures (φ)
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Figure 4. Scanning electron micrographs (SEM) of phase-separated epoxy/PES mixtures fully cured at isothermal curing temperatures of 90, 100, and 110 °C.
mixture during phase separation: one is the change of composition in the epoxy-rich matrix and the other is the variation of viscoelastic behavior of the phaseseparated blend. To explain the viscoelastic behavior of a heterogeneous blend, many models have been suggested.10-14 In this study, a simple two-phase suspension model proposed by Palierne12 was employed to test the effect of viscoelastic behavior of the phase-separated mixture:
G*(ω) ) G/med(ω)
1 + 3φH(ω) ) iωη*(ω) 1 - 2φH(ω)
(1)
where H)
(Rr )(2G R 40( )(G r 4
/ med
/ med
Figure 5. Changes of the domain volume fraction (φ) and the PES weight fraction in the epoxy-rich matrix (wPES) at different isothermal curing temperatures as a function of curing time.
is calculated from the measured domain size and the number of domains per unit volume (N) with φ ) 4πr3N/3. Figure 5 shows the domain volume fraction after the onset of PES phase separation and the PES weight fraction (wPES) remaining in the epoxy-rich matrix, which is calculated from the estimated domain volume fraction (φ), densities of epoxy and PES (FEpoxy ) 1.17 and FPES ) 1.37), and the initial volume fraction of epoxy and PES (φEpoxy,0 and φPES,0) through the relation wPES ) [FPES(φPES,0 - φ)]/[FEpoxyφEpoxy,0 + FPES(φPES,0 - φ)], as a function of curing time for isothermal curing temperatures of 90, 100, and 110 °C. We note from the figure that the domain volume fraction and the PES weight fraction remaining in the epoxyrich medium approach more or less the same value regardless of the curing temperature when the phase separation is completed. This is to say that the effect of the curing temperature is almost negligible in this situation because the increase in the domain size nearly compensates the decrease in the total number of domains as the isothermal curing temperature is increased. The domain phase separation will affect rheological properties of the epoxy/PES mixture in the sense that the mixture evolves from a homogeneous state to a heterogeneous one and that the compositions of PES and epoxy vary throughout the phase separation. There are mainly two factors affecting rheological properties of the
+ 5G/PES) + (G/PES - G/med)(16G/med + 19G/PES) + G/PES) + (2G/PES + 3G/med)(16G/med + 19G/PES)
ω is the angular frequency of oscillatory shear, i is x-1, φ is the domain volume fraction in the phaseseparated mixture, r is the average domain radius, and G/med and G/PES are the complex shear modulus of the medium and PES, respectively. R is the interfacial tension between the epoxy-rich matrix and the PESrich domain which is assumed to be independent of local shear and interfacial area change. Typical values of the interfacial tension are 10-3-10-2 N/m for blends of molten polymers.12 For the sake of simplicity, we first assume that there is no appreciable elastic effect in the rheological behaviors of both matrix and domain. Because the order of magnitude of the viscosity of the epoxy-rich medium is 0 whereas the PES viscosity is too high to measure at experimental temperatures that are far below the glass transition temperature of PES, the viscosity ratio of the PES domain to the epoxy-rich medium (ηPES/ηmed) is large enough that the zero-shear viscosity of the mixture from eq 1 is reduced to the well-known Einstein relation:
5 η* ) η0 ≈ ηmed 1 + φ 2
(
)
(2)
With this equation, the viscosity of the epoxy-rich medium (ηmed) can be estimated from the measured complex viscosity shown in Figure 2 and the domain volume fraction given in Figure 5. As can be noticed from Figure 6, the calculated ηmed nearly overlaps the measured complex viscosity because the domain volume fraction is small enough to ignore (i.e., φ < 0.03).
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Figure 6. Measured complex viscosity (line) and the viscosity of the epoxy-rich medium (symbol) calculated with the suspension model explained in the text.
Once PES domains are formed, they are likely to have a viscoelastic effect on rheological properties because these domains mainly consist of PES with high molecular weight. To take into account the viscoelastic behavior of PES, we also employed a simple Maxwell model:
G/PES )
iωηPES 1 + iωτPES
(3)
where τPES is the characteristic relaxation time for the PES domain and ω is the angular frequency of oscillatory shear. With eqs 1 and 3, the shear and interfacial tension effects on the complex viscosity were tested with two extreme cases of oscillatory shear (ωτPES ) 0 and ∞) and three values of interfacial tension in eq 1 (R ) 0, 10-3, and 1 N/m), and it was noted that the viscosity profiles of the epoxy-rich medium calculated with different frequencies and interfacial tensions almost overlap the profile shown in Figure 6 within the symbol size of the figure, implying that the viscoelastic effect of the PES domain and the effects of shear and interfacial tension are almost negligible in our system. This is reasonable because the viscosity of the mixture still remains quite low (
