No abstract

The absolute density, thermal expansion coefficient a, and thermal pressure coefficient y have bcen determined for atactic polystyrene in the non-glassy liquid (or amorphous) state. Dilatometl-ic measurements on the pure polymer furnishcd a over the rangc 100" to 230°C. Thermal pressure coefficients were determined by extrapolation from measurements on concentrated soluticns of the polymer in benzene, from 24" to 100°C. No evidence was found for a second order transition above 100°C. A small change in dcc/dT was observed near 170°C. This and other observations suggest a minor departure from equilibrium below this temperature.

SYNOPSISReactive extrusion is the term used to denote a plastics processing method in which an extruder is used as a reactor for the continuous synthesis and modification of polymers. This paper describes the manufacture of polystyrene and styrene-isoprene copolymers by a reactive extrusion process: anionic "living" s-butyllithium-initiated bulk polymerization was performed in a co-rotating closely intermeshing twin-screw extruder. The results of the process analysis show that living polymerization of styrene can be performed in a screwtype reactor, despite the high reaction temperatures of over 200°C. The polystyrene melt can be modified in bulk with comonomers or coupling reagents immediately after polymer synthesis. Depending on the raw material, formulation, and process parameters, the process variants developed and analyzed at the Institut fur Kunststoffverarbeitung for homopolymerization of styrene, copolymerization of styrene-isoprene mixtures, and sequential polymerization of styrene and isoprene resulted in styrene polymers with widely differing structural characteristics and properties. For example, the copolymerization of styreneisoprene monomer mixtures produced poly [ isoprene-co-styrene) 4-styrene] . The sequential polymerization of styrene and isoprene led to poly ( styrene-b-isoprene) contaminated with partly cross-linked low molecular weight polyisoprene. The polyisoprene content is presumably formed by side reactions due to the high reaction temperatures. 0 1993

The excess volumes VE of mixtures of polystyrene with cyclohexane at 25°C are negativc ; at a segment fraction of d2 = 0.5, V E amounts to ca. 0.14 % of the total volume. This result in conjunction with chemical potentials and excess enthalpies determined by Palmen, Schmoll and Jenckel, by Krigbaum and Geymer, and by Scholte are interrelated here by the statistical thermodynamic theory employed in the preceding papers. On assignment of XI2 = 42 J ~r n -~ and Qlz = 0.023 J ~r n -~ deg-I the excess volume P, the reduced residual chemical potential x, and the reduced partial molar excess enthalpy XH are well represented at all concentrations. The theory also predicts a lower critical solution temperature in agreement with experiments of Allen et al. and of Tager et al. indicating an LCST in the range 180°C to 210°C for this system. Cyclohexane (CH) is a poor solvent for polystyrene at ordinary temperatures. In this respect it resembles methyl ethyl ketone (MEK). However, the two solvents differ notably in their interactions with polystyrene when the properties of the respective solutions are examined in greater detail. Whereas solutions of the polymer in MEK are very nearly atherma1,l the heat of dilution for the PS-CH system is positive and large according to osmotic pressure measurements by Schick, Doty and Zimm,, by Krigbaum and Geyme~,~ and by Palmen and Rehage.4 Krigbaum and Geymer also carried out comprehensive measurements of activities by the method of isothermal distillation at intermediate concentrations and by vapour pressure measurements at higher concentrations which yielded enthalpies of dilution covering virtually the entire range of composition. These various studies have been confirmed recently by two interrelated investigations on this system carried out by S~h o l t e .~~ In one of these investigations the derivative of the chemical potential with respect to composition was measured by the method of equilibrium ultracentrifugation at weight fractions w, from 0 to 0.80 and at temperatures of 30°, 45" and 65°C. Light scattering methods were employed in the second investigation ; measurements were conducted at the same temperatures and at concentrations from w 2 = 0 to 0.30.Owing to the large positive heat of dilution, the solvent action of CH changes rapidly with temperature. Thus, the PS-CH system exhibits a @-point associated with upper critical solubility (UCST) at 34"C, as determined by Fox and Flory and their co-workers,* and abundantly confirmed in numerous subsequent investigations. Recently, Koningsveld, Kleintjens and Shultz have deduced the chemical potential and its dependence on composition and temperature through analysis of liquid-liquid phase equilibria in the same system, polymers of various molecular weights and

The excess volumes V E of mixtures of polystyrene with methyl ethyl ketone determined at 25°C we negative. At a segment fraction & = 0.5 of polymer, VE is ca. 0.9 % of the total volume.Osmotic pressures have been measured from = 0.10 to 0.35 at 10" and at 50°C. The temperature coefficient of the reduced residual chemical potential x indicates a very small positive heat of mixing in accord with vapour pressure measurements of Bawn, Freeman and Kamaliddin at higher concentrations. The recent statistical thermodynamic theory of solutions which relatcs their properties to characteristics of the pure liquids manifested in the equation-of-state parameters succeeds in accounting for the principal features of this system. This theory correctly predicts the sign of the excess volume, although it underestimates its magnitude. It also accounts for the observed increase of x with concentration, and for the small negative entropy of dilution at low concentrations, with change of sign at ca. 42 = 0.05.Thermal expansivities and thermal pressure coefficients for methyl ethyl ketone have been measured from 20" to 60°C with an accuracy of &l % or better.