AbstractOne‐to‐one block polymers of styrene and acrylonitrile were prepared by a two step process: (1) styrene was polymerized in the presence of a dialkylphosphine and (2) the polystyrene was used to initiate the polymerization of acrylonitrile. The blocks contained 14–76 acrylonitrile and were 60–100 pure as judged by extraction with toluene at 110°C. Phase and electron microscopy showed that the block polymers consisted of a continuous polystyrene phase when the acrylonitrile content was less than 70 by weight and of a continuous acrylonitrile phase when the acrylonitrile content was above 70 by weight. The polyacrylonitrile dispersed phase was regular and almost spherical. Its measured size equaled the calculated size, assuming that the spheres had a diameter of twice the length of the fully extended polyacrylonitrile chains. Polyblends of homopolymers were similar to the block polymers in structure, except that the dispersed phase was very irregular in size and shape. The physical properties of blocks and polyblends were similar and were inferior to those of polystyrene homopolymers and styrene–acrylonitrile random copolymer except for modulus and for maintenance of modulus over a temperature range. The random copolymer had the highest strength properties, which fact was attributed to the absence of a two phase system and the presence of psuedo‐crosslinks. The relatively poor properties of the blocks and polyblends were ascribed to the inability of bulk polyacrylonitrile to absorb energy and to the buildup of stress concentrations at the phas
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