Preparation and Stress-Strain Properties of ABC Triblock Copolymers of a-Methylstyrene, Bu tadiene, and Styrene
A few descriptions of ABA triblock copolymers of a-methylstyrene (monomer A) and butadiene or isoprene have appeared in the literature.14 Because of the high Tg of the poly(a-methylstyrene) end blocks (172"C), they retain some tensile strength at elevated temperatures (up to at least 100°C)
where similar copolymers with polystyrene end blocks lose all strength.
It has been shown that block copolymers of styrene and a-methylstyrene exhibit a single Tgr which is intermediate between that of the two homopolymers (100°C for p~lystyrene)."-'~~ These Tg's ranged from 127" to 155"C, depending to some extent on copolymer composition and molecular weight. This was taken as evidence that the blocks were compatible and formed a single phase. Blends of the homopolymers, or block copolymers with incompatible blocks, form two phases which exhibit their respective Tg's.
This suggested that it might be possible to prepare a triblock thermoplastic elastomer with an ABC block structure, in which the A and C blocks would be polystyrene and poly(a-methylstyrene).
If these were compatible, they could form a single species of glassy domains rather than separate A and C domains. Such species should have a Tg higher than that of polystyrene; if so, the ABC copolymers ought to have better retention of tensile strength at elevated temperatures (>-7O0C) than ABA copolymers with polystyrene end blocks.
A patent described "pure" ABC block copolymers, in which "living" polystyrene was used to initiate butadiene; after the latter had polymerized, temperature was lowered below 15°C and a-methylstyrene, together with a strong polar modifier as dimethoxyethane, as added." These ABC copolymers were shown to have fairly good tensile strengths at 80°C; no evidence was shown of possible blending of the polystyrene and poly(a-methylstyrene) end blocks.
This paper reports a few "tapered" ABC copolymers, prepared by initiating a butadiene-styrene mixture with "living" poly(a-methylstyrene). Previously it was shown that the end blocks were compatible and formed a composite glassy domain with a Tg of -150"C.12 The syntheses and stress-strain properties of these polymers are discussed herein.