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Upper Critical Solution Temperature Behavior in Polystyrene/Poly(Methyl Methacrylate) Mixture

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Upper Critical Solution Temperature Behavior in Polystyrene/Poly(Methyl Methacrylate) Mixture Polymer Journal, Vol. 25, No. 12, pp 1315-1318 (1993) NOTES Upper Critical Solution Temperature Behavior in Polystyrene/Poly(methyl methacrylate) Mixture Toshiaki OuGIZAWAt and David J. WALSH* Polymer Processing Laboratory, National Institute of Materials and Chemical Research, Tsukuba, Ibaraki 305, Japan * Du Pont Polymers, Sabine Research Laboratory, PO Box 1089, Orange, Texas 77631-1089, U.S.A. (Received April 2, 1993) KEY WORDS Phase Diagram I Cloud-Point Curve I Upper Critical Solution Temperature I Polystyrene I Poly(methyl methacrylate) I Oligomer Mixture I Equation of State Theory I Interaction Parameter I It is well-known that most paus of high are immiscible except for the extreme ends of molar mass polymers are immiscible. This is composition. This behavior can be interpreted so because the combinatorial entropy of mix­ by both the Flory-Huggins expression and the ing of two polymers is dramatically less than equation of state theories. that for two low molar mass compounds. 1 It is also well-known that PS and poly­ The enthalpy of mixing, on the other hand, is (methyl methacrylate) (PMMA) are immiscible often a positive quantity. Therefore dissimi­ but the interaction parameter between these lar polymers are only miscible if there are polymers is not so large. It has been reported favorable specific interactions between them that, at ambient temperature, lower molecular leading to a negative contribution to the Gibbs weight PS samples could be miscible with the free energy of mixing. Miscible polymers tend PMMA samples within limited composition to phase separate at elevated temperatures. ranges. 6 However, an actual UCST type phase This lower critical solution temperature diagram has not been reported yet. The (LCST) behavior is typical for miscible polymer measurements of the Flory-Huggins interac­ blends. The LCST behavior is interpreted in tion parameter x were also carried out by using terms of the equation of state or free volume a number of different methods, for example, contribution. 2 •3 Some miscible polymers also the measurements of interfacial thickness in the exhibit phase separation on cooling tempera­ two phase state, 7 light scattering on ternary tures. This upper critical solution temperature polymer solutions including solvent8 and (UCST) behavior has been observed even for neutron scattering from the homogeneous state the polymer mixtures with high molar mass, in a block copolymer of styrene and methyl but generally when one or two components are methacrylate. 9 The values of the x, themselves low molar mass, i.e., oligomers.4·5 A typical not so large, exhibit some scatter depending on example of UCST behavior was obtained the techniques, but are positive and decrease for polystyrene(PS)/polybutadiene(PB) mix­ with increasing temperature. One can predict tures,4 but the mixtures for high molar mass that the UCST phase diagram exists for an t To whom correspondence should be addressed. 1315 T. 0UGIZAWA and D. J. WALSH Table I. Specifications of materials and their equation of state parameters Polymer Source Mw Mw/Mn P*/MPa v,p*/cm3 g- 1 T*/K Ss/SMMA PS0.8k Aldrich 794 1.13 470 0.8457 7212 0.83 3 PMMA6k Polysciences 6350 1.06 600b 0.7224b 7748b PMMAI2k Polysciences 12900 1.04 600 0.7224 7748 • Calculated by Bondi's group contribution data. 17 b Since the values of characteristic parameters are almost constant for PMMA which has a molecular weight more than about five thousand, we used the same values as those of PMMAI2k for PMMA6k. oligomer mixture of PS/PMMA as well as for a PS/PB mixture. Here we describe results of UCST behavior for a PS/PMMA oligomer mixture. To our knowledge, this is the first observation of a UCST phase diagram in a PS/PMMA mixture. ··· .. T r ./ ., ·/ :. /: On the basis of the phase diagrams, we simulate . %' ' .2 I ···11 /; ·: 0 spinodal and binodal curves for this system by I '-- \ "•!. ' <1> o._ 0.8k/6k using the equation of state theory and estimate \\/' . E /\ ' the value of the interaction parameter x12 (not <1> /. \ 1- \ the x parameter). \ The characteristics of polymer samples used in this study are shown in Table I. PS and PMMA were dissolved at 5 wt% of total polymer in tetrahydrofuran. The solution was 0 0.5 10 cast onto a cover glass. A quick evaporation Wt. fr. of PMMA of solvent makes the sample film transparent Figure 1. Cloud point curves (dotted) and simulated but a slow one makes it opaque. The light binodal (--)and spinodal(------) curves of PS/PMMA scattering profile of the opaque film by a mixtures. Experimental cloud point data are shown by solid bars and arrows indicate data obtained from only goniometer trace often exhibits a peak; heating measurement. T8 for one phase mixture calculated furthermore the modulated structure was by Fox equation12 (--·-)are also shown. observed in the optical microscope, as in the PB/poly(styrene-ca-butadiene) system by cast­ in the intensity of the scattered light from ing from toluene solution. 10 This implies that samples. 11 Next the transparent samples at the phase separation takes place by the high temperature were cooled with a constant spinodal decomposition during casting. The rate of 1 K min- 1 and the temperature at which cast film was further dried under vacuum. Then phase separation occurs was recorded by an the film on the cover glass was inserted in a increase in the intensity of the scattered light. heating stage setting horizontally on the light These temperatures correspond to cloud points scattering stage. 10 A He-Ne gas laser beam of and are indicated as solid bars in Figure 1. The 632.8 nm wavelength was applied vertically to upper end of the bar refers to phase dissolution the film specimen. The phase separated opaque temperature obtained on heating, and the lower samples were heated with a constant rate of end to phase separation temperature obtained 1 K min- 1 and the temperature at which phase on cooling.We also checked the reversibility of dissolution occurs was recorded by a decrease the transparent-opaque transition correspond- 1316 Polym. J., Vol. 25, No. 12, 1993 UCST Behavior in PS/PMMA Mixture ing to the one-two phase in every case except Table II. Values of interaction parameter for the composition in both the vicinity of the Mixture glass transition temperature (Tg) of the blends and the extreme end of the phase diagram. The PS0.8k/PPMA6k 3.77 difference between the cloud-point tempera­ PS0.8k/PPMA 12k 3.45 tures for heating and cooling measurements is large in end compositions of phase diagram. In Figure 1, the cloud point curves corre­ where r; is the number of segments per sponding to the binodal curve are drawn molecule. The first two terms in eq 3 represent somewhat arbitrarily (dotted curves). Thus the contribution of the combinatorial entropy, we have found a UCST phase diagram for the last term containing x12 represents the PS/PMMA systems. contribution of the interactions to the chemical We simulate the binodal and spinodal curves potential change, and the other terms represent in the phase diagram of PS/PMMA mixtures the contribution of free volume. by using the equation of state theory of Flory, From eq 3 one can calculate the phase Orwoll and Vrij (FOV)Y Since the detail of diagram for binary systems. To do this, the the formulae were described in previous characteristic parameters of pure components papers/4 •15 we indicate only the main equa­ and X 12 are necessary. The characteristic tions. The FOV equation of state is given by parameters of pure components can be obtained from the fitting pressure-volume­ PV/T= V113(V 113 -l)-l/(f'V) (1) temperature (PVT) data to the FOV equation where P= P/P*, V=vjv*, and T= T/T* are the of state as described in a previous paper. 16 The reduced pressure, volume and temperature, P, value of X 12 for PS/PMMA system has not v and Tare the actual values, and P*, v*, and been known. On the other hand, we can T* are the hard-core values (characteristic estimate the values of X 12 required to locate parameters). When calculating the free energy the phase diagaram in the correct temperature change on mixing of two substances to produce range from simulation by using eq 3. a homogeneous blend, one assumes that the The values of the parameters used in this mixture obeys the same equation of state as simulation are shown in Table I. S;/ Si is the the pure components, that the hard-core vol­ ratio of the surface area per unit volume and umes are additive, and that the interatomic used in order to obtain the value of e;. interactions can be assumed in such a way that Calculated binodal and spinodal curves are hard-core pressure of the mixture is given by shown in Figure 1 and the values of X12 used to simulate the phase diagrams are shown in P*=ifJ1P1*+¢zPz*-¢1e2x12 (2) Table II. These values of X 12 are small in where ¢; and ();is a hard-core volume fraction comparison with those of PSjPB (X12 = 7.0 and a site fraction of component i, respectively, J em- 3 ) and methoxylated poly(ethylene and X 12 is an interaction parameter and glycol)jpoly (propylene glycol) (X12 = 10--13 different from the Flory-Huggins x parameter. J em- 3), 18 as predicted. The shapes of the The resulting expression for the chemical simulated phase diagrams are not identical with potential of the mixture is that of the measured cloud point curves but the compositions at the maximum of the phase AJ.1 1 /RT=1n ¢ +(l-rtfr )¢ 1 2 2 diagrams are close in both mixtures.
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