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

Abstract

A series of well defined poly(methyl methacrylate) (PMMA)-b-poly(n-butyl acrylate) (PnBA)-b-PMMA triblock copolymers (MnBM) has been synthesized by transalcoholysis of PMMA-b-poly(tert-butylacrylate) (PtBA)-b-PMMA precursors (MTM) by n-butanol. Phase separation is observed for all the investigated triblock copolymers, which contain PMMA outer blocks in the 5000-50 000 molecular weight (MW) range and PnBA inner blocks with MW in the 100 000-200 000 range. Although the ultimate tensile properties of these MnBM triblock copolymers are poor compared to traditional diene-based TPEs (SBS and SIS), they are much better than those ones reported for PMMA-b-poly(isooctyl acrylate) (PIOA)-b-PMMA triblocks (MIM). A reasonable explanation for this observation 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 block, which is consistently much smaller than for PIOA (59 000) and substantially higher than M e for polybutadiene (1700) and polyisoprene (6100). The tensile behavior of MnBM copolymers cannot be fitted by either a simple elastomer model free from chain entanglements (suitable to MIM) or by a "filler" modified rubber model (suitable for diene-based TPEs), supporting the hypothesis that the mechanical properties of the investigated (meth)acrylate thermoplastic elastomers are significantly affected by any change in M e of the central acrylate block. Viscoelastic analysis shows that MnBM triblocks are of higher complex viscosity than the SBS and SIS analogs, leading to a shift in the order-disorder transition temperature to much higher temperature, unless the outer PMMA blocks are of very low molecular weight (5000).

Morphology and mechanical properties of poly(methylmethacrylate)-b-poly(alkylacrylate)-b-poly(methylmethacrylate)

“…Previous observations have shown that the phase separation of the PMMA-PIOA-PMMA triblocks (MIM) is sharp (e.g. T g of PIOA is independent of the PMMA content and MW), to the point where the "sphere in a matrix" morphology is observed with AFM even when PMMA MW is as low as 3500 (PIOA MW:100,000) [13].…”

“…The high performances of these materials compared to the unreinforced vulcanizates of SBR rubbers are explained by the reinforcement of the rubber by the hard polystyrene microdomains and by the slippage of the entangled rubbery chains which contributes to delay the ductile failure of the PS microdomains [38][39][40]. We have previously reported that the mechanical properties of MIM and MnBM triblocks are usually poor when compared to the traditional dienebased TPEs [13]. This disappointing observation was tentatively explained by the partial miscibility of the PMMA and PIOA (PnBA) blocks particularly when the PMMA MW and content are low (i.e.…”

“…Although the methods proposed by these authors differ to some extent, the basic idea is that the rheological properties of the block copolymer must change dramatically at T ODT . Table 4 lists the T ODT determined by the Han's method for comparable MAM triblocks (examples of determination were reported elsewhere [13]). This transition occurs at 170°C in the case of central poly(alkyl acrylate) block with short alkyl substituents (ethyl and n-propyl), although it exceeds 200°C for longer alkyl groups (n-butyl and isooctyl).…”

“…Moreover, the rheological properties of the fully (meth)acrylic triblocks have not been studied in detail, although this issue is of prime importance for the melt processing of these potential TPEs. All what we know is that the average MW between chain entanglements (M e ) for the central block dominates the elastomeric properties, whereas M e for the outer blocks is thought to have an important effect on the rheological behavior of the MAM triblocks [13].…”

Thermoplastic Elastomers Based on Block, Graft, and Star Copolymers

“…By transalcoholysis with n-butanol of the precursor, PMMA-poly(n-butyl acrylate)-PMMA triblock copolymer was prepared with PMMA as the rigid domain and poly(n-butyl acrylate) (PnBA) as the rubbery matrix [29,30].…”