Triblock thermoplastic elastomers with poly(lauryl methacrylate) as the center block and poly(methyl methacrylate) or poly(tert-butyl methacrylate) as end blocks. Morphology and thermomechanical properties

摘要

The morphology, dynamic mechanical, and tensile properties of poly(methyl methacrylate)-b-poly(lauryl methacrylate)-b-poly(methyl methacrylate) triblock copolymers have been studied. Atomic force microscopy reveals that the polymer phase-separates with the PMMA phase appearing as microdomains in a continuous rubbery PLMA matrix for PMMA concentrations ranging between 16.6 and 45% by weight. The domains are spheres for 16.6% PMMA and cylinders, some lying flat on the surface and others buried in the sample with their apexes on the surface, for PMMA concentrations between 26 and 45% PMMA. As the PMMA concentration is increased to 52%, a short lameller morphology is seen. The dynamic mechanical measurements reveal two glass transitions and a rubbery plateau between them. The T-g values indicate some degree of mixing between phases. The tensile properties are poor. For example, for the copolymer with 26% PMMA the elongation at break is 80% and the ultimate tensile strength is 3 MPa. The M-e of PLMA has been measured to be 225000 which confirms the absence of entanglements in the PLMA blocks. Accordingly, the initial tensile behavior fits into the simple elastomer model which gives a surprisingly low value of 3600 for M-c. In the absence of entanglements the expected value of M-c is equal to the M-n of PLMA. The low M-c value has been attributed to shear induced side chain crystallization of PLMA which is not known to crystallize. Evidence of crystallization in the stretched sample has been obtained from X-ray diffraction studies. A poly(tert-butyl methacrylate-b-LMA-b-tert-butyl methacrylate)(tBLtB) copolymer prepared in one pot gives a substantially greater elongation at break. The PtBMA block, however, exhibits greater mixing with PLMA presumably due to its being a copolymer containing a small percentage of LMA residues. The initial tensile behavior of this triblock copolymer fitted into the simple elastomer model gives M-c = 29 000 which is still lower than the M-n of the PLMA center block. No crystallization of PLMA in a stretched (200%) sample of this triblock copolymer is evident from XRD studies. Apparently, the higher degree of mixing of PLMA with PtBMA prevents crystallization of the former. The lack of crystallization also helps to get larger elongation of this block copolymer.