Synthesis of poly(methyl methacrylate)-b-poly(n-butyl acrylate)-b-poly(methyl methacrylate) triblocks and their potential as thermoplastic elastomers

A series of well defined poly(methyl methacrylate) (PMMA)-b-poly(n-butyl ac rylate) (PnBA)-b-PMMA triblock copolymers (MnBM) has been synthesized by tr ansalcoholysis of PMMA-b-poly(tert-butylacrylate) (PtBA)-b-PMMA precursors (MTM) by n-butanol. Phase separation is observed for all the investigated t riblock copolymers, which contain PMMA outer blocks in the 5000-50 000 mole cular weight (MW) range and PnBA inner blocks with MW in the 100 000-200 00 0 range. Although the ultimate tensile properties of these MnBM triblock co polymers are poor compared to traditional diene-based TPEs (SBS and SIS), t hey are much better than those ones reported for PMMA-b-poly(isooctyl acryl ate) (PIOA)-b-PMMA triblocks (MTM). A reasonable explanation for this obser vation is found in the average molecular weight between chain entanglements (M-e) that has been estimated to be 28 000 for the central PnBA rubbery bl ock, which is consistently much smaller than for PIOA (59 000) and substant ially higher than M-e for polybutadiene (1700) and polyisoprene (6100). The tensile behavior of MnBM copolymers cannot be fitted by either a simple el astomer model free from chain entanglements (suitable to MIM) or by a "fill er" modified rubber model (suitable for diene-based TPEs), supporting the h ypothesis that the mechanical properties of the investigated (meth)acrylate thermoplastic elastomers are significantly affected by any change in M-e o f the central acrylate block. Viscoelastic analysis shows that MnBM tribloc ks are of higher complex viscosity than the SBS and SIS analogs, leading to a shift in the order-disorder transition temperature to much higher temper ature, unless the outer PMMA blocks are of very low molecular weight (5000) . (C) 1999 Elsevier Science Ltd, All rights reserved.