Chain Architecture Effect on Fracture of Semicrystalline-Glassy Block Copolymers

摘要

Many glassy polymers with otherwise desirable characteristics are very brittle even at rather large molecular weights. Often this brittleness can be traced to a low entangled strand density (high entanglement molecular weight M_e), which predisposes the glassy polymer to form weak crazes that break down readily to form cracks. One such polymer is poly(vinyl cyclohexane) (PCHE) which has excellent clarity, excellent UV stability, low birefringence and a high glass transition temperature T_g of about 145°C. Its large M_e of ～40,000 g/mol~3 means that PCHE is also extremely brittle. One method of relieving this brittleness while retaining a relatively high modulus is to use PCHE and poly(ethylene) (PE) as blocks in a block copolymer with a majority of the PCHE glassy domain. Surprisingly these materials are optically clear and can exhibit practical toughness at relatively low molecular weight, making them easy to process. In this paper we investigate the influence of chain architecture on the microscopic deformation and fracture mechanisms of thin films of these poly(vinyl cyclohexane)-poly(ethylene) (PCHE-PE) block copolymers. We compare the deformation of ordered triblock copolymers PCHE-PE-PCHE [CEC] and pentablock PCHE-PE-PCHE-PE-PCHE [CECEC] copolymers with that of the PCHE homopolymer.