22 Physical Properties of Blends of Polystyrene with Poly(methyl Methacrylate) and Styrene/ (methyl Methacrylate) Copolymers DENNIS J. MASSA Research Laboratories, Eastman Kodak Co., Rochester, NY 14650
The effect of polystyrene (PS) molecular weight (M) on compatibility and physical properties of blends of PS with poly(methyl methacrylate) (PMMA) and PS with styrene/ (methyl methacrylate), PMMA, and PS with styrene/ (methyl methacrylate) copolymers has been investigated. The polystyrene M ranged from 600 to 110,000, and the styrene fraction in the copolymers varied from 0 to 0.8. The conclusions of this work are as follows: (i) PS is compatible at low M with PMMA over a range of PS concentrations, up to nearly 40% for M = 600; (ii) increasing the styrene content in the styrene/(methyl methacrylate) copolymers increases compatibility with PS; (iii) molecular weight is important in all cases; compatibility increases with decreasing M; and (iv) the Flory-Huggins-Scott solubility parameter approach is consistent with qualitative trends in the results.
immense importance to their use i n mixtures and blends. The phase morphology and resulting physical properties of polymer blends are critically dependent upon the equilibrium thermodynamic interactions among polymers as well as upon the nonequilibrium effects that the thermal and physical histories can impress on the blend during preparation. Although the prediction of polymer-polymer miscibility based o n chemical structure and/or equilibrium thermodynamic properties has been a goal of polymer chemists for decades (1,2), the complexity of 0-8412-0457-8/79/33-176-433$05.00/0 © 1979 American Chemical Society
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MULTIPHASE
POLYMERS
the problem has limited successful efforts to relatively few cases. Instead, compatible polymer pairs are often discovered empirically and only thereafter studied intensively. Nonetheless, despite difficulties with quantitative predictions, the way i n which molecular variables affect polymer-polymer miscibility is understood well enough to allow qualitative judgements of their influence on polymer blend properties. F o r example, the Flory-Huggins-Scott (3-8) expression for the free energy of mixing, shown below i n the form given b y Krause ( J ) , where G
mis
-
^ ~
lnV + | ^ l n F + X A B F F ] , B
A
A
B
V is the total volume of the mixture; V is the reference volume, the volume of polymer repeat unit; V and V are the volume fractions of polymers A and B ; X and X are the degrees of polymerization of A and B ; X is related to the enthalpy of interaction of the polymer repeat units, each of molar volume V indicates that whereas at high degrees of polymerization ( X and X ) the value of the interaction parameter X would have to be nearly zero or negative to result i n a favorable (negative) free energy of mixing (9), even fairly dissimilar polymers might be compatible at low molecular weights ( small X and X ). The relatively few studies of compatibility of low-molecular-weight polymers reported i n the literature, an example being the work of Allen et al. (10), confirm the latter prediction. A recent treatment of molecular weight effects i n polymer compatibility is contained i n the paper of Casper and Morbitzer (11). r
A
A
A
B
B
B
T>
A
B
A B
A
B
The extent of incompatibility of high-molecular-weight polymers has been elegantly shown by Yuen and Kinsinger (12) i n their work on light scattering from blends of polystyrene, PS, and poly(methyl methacrylate), P M M A . F o r these high-molecular-weight blends, incompatibility was detected when the PS concentration exceeded 0.008 w t % . The effect of molecular weight, M , was not investigated by Yuen and Kinsinger. In an effort to define more clearly the effects of molecular weight and composition on polymer compatibility, a study was undertaken on the effect of PS molecular weight on compatibility and physical properties of blends of PS with P M M A and PS with styrene( S ) /methyl methac r y l a t e ( M M A ) copolymers. The range i n M investigated for PS was 600-110,000, and the S / M M A ratio was varied from 0 to 4.0. The results of this study are reported below. Experimental The polymers studied are listed i n Table I along with their sources and characterizations. Bulk samples were prepared i n two steps. First, a thin film ( 2-10 mils ) was obtained by casting onto glass from a solution
22.
MASSA
Table I. Polymer PMMA S - M M A - 1 0 (9.8) * S - M M A - 2 5 (26.6) * S - M M A - 6 0 (62.6) * S - M M A - 8 0 (80.0) * PS-600* PS-2100 PS-4000 PS-10,000 PS-20,400 PS-37,000 PS-110,000 0
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Properties of Blends Materials and Characterization
τ„ co rn 109 106 -15 60 78 96
M /M w
n
1.77
0.34 2.19 1.10 1.18 1.19
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