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RESEARCH PRODUCT

The glass transition of mixtures of polystyrene with alkyl-terminated oligostyrene — experimental evidence for microphase separation in a polymer blend

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Mixtures of high molecular weight polystyrene (P n = 961.5 ; T g = 373 K) and sec-butyl-terminated oligostyrene (P n = 10.6 ; T g = 302 K) show a dependence of the glass transition on the composition (i.e., number-average molecular weight) which cannot be described by the classic equations of Fox & Flory or Kanig & Ueberreiter. Addition of small amounts of the oligostyrene strongly decreases the glass transition, and above an oligomer content of 40 wt.-% the glass transition remains constant within experimental error. This behavior can be attributed to the unfavorable interactions between the alkyl-terminated oligomer and the high molecular weight PS. As a result the oligomer is a bad solvent but - according to the requirements defined by Kanig - an efficient plasticizer. This behaviour is even more pronounced in mixtures of the alkyl-terminated oligostyrene with a poly(styrene-co-maleic anhydride) of low maleic anhydride content. In this case, ESR spectra of the spin-labelled styrene copolymer demonstrate the existence of micro-heterogeneities on the 2-5 nm scale. The findings are in qualitative agreement with recent theoretical work of Khokhlov & Erukhimovich on entropy-induced microphase separation in blends.