No abstract

Poly(methyl methacrylate)-b-poly(n-butyl acrylate)-b-poly(methyl methacrylate) triblock copolymers have been prepared by ligated anionic polymerization (LAP; 8K-50K-8K) and atom transfer radical polymerization (ATRP; 9K-51K-9K). Size exclusion chromatography, nuclear magnetic resonance, and differential scanning calorimetry have confirmed that the molecular structure of the two triblock copolymers is essentially identical. However, important differences are found in dynamic mechanical properties, viscoelastic properties, and stress−strain behavior. Indeed, the ATRP copolymer has low storage modulus, high complex viscosity, high order−disorder transition temperature, and poor ultimate tensile strength and elongation at break, compared to those of the LAP analogue. Marked differences also observed by tapping mode atomic force microscopy in the microscopic morphology of thin films of these copolymers. All these observations can be explained by the slow initiation of MMA by the poly(n-butyl acrylate) macroinitiator used in ATRP in contrast to what happens when MMA is added to living poly(tert-butyl acrylate) anions. As a result, the polydispersity of the short poly(methyl methacrylate) (PMMA) outer blocks is much broader in the ATRP copolymer, although the polydispersity index of the triblock is only 1.15. This heterogeneous structure of the ATRP triblock is also supported by the comparison of homo-PMMAs prepared by LAP and ATRP.

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).

A series of well-defined poly(methylmethacrylate) (PMMA)-b-poly(alkylacrylate)-b-PMMA triblock copolymers (MAM) has been synthesized by transalcoholysis of PMMA-b-poly(tert-butylacrylate)-b-PMMA precursors by alkyl alcohols. The molecular weight (MW) of the outer PMMA blocks is in the 10,000-50,000 range, compared to 50,000-200,000 for the inner poly(alkylacrylate) block. Phase separation, as studied in direct space by atomic force microscopy, is observed for all the investigated triblock copolymers, except for the PMMA-b-poly(ethylacrylate)-b-PMMA and the PMMA-b-poly(n-propylacrylate)-b-PMMA triblocks of 10,000-50,000-10,000 MW. The ultimate tensile strength measured for the MAM triblocks is strongly dependent on the MW between chain entanglements for the central block. The tensile behavior is however affected by the partial miscibility of the outer and inner blocks when the PMMA MW is low. When this situation prevails, it makes the melt processing possible at temperatures lower than 200°C.