Chain Architecture Effects on Deformation and Fracture of Block Copolymers with Unentangled Matrices

Abstract

We investigate the influence of chain architecture on the microscopic deformation and fracture mechanisms of poly(vinylcyclohexane)−poly(ethylene) (PCHE−PE) block copolymer thin films. To investigate the correlation between the mechanical and fracture properties of the polymer and its chain architecture, a “metal grid technique” was employed to apply tensile tension on the thin films of PCHE homopolymer (M̄w = 283 000 g/mol), ordered PCHE−PE−PCHE triblock (CEC; M̄w = 107 000 g/mol and fPE = 0.29 by weight), and PCHE−PE−PCHE−PE−PCHE pentablock (CECEC; M̄w = 110 000 g/mol and fPE = 0.30 by weight) copolymers. We observe that both PCHE and CEC deform plastically by crazing. The median strain for crazing of the CEC is 1.3%, whereas that for PCHE is 0.7%. The extension ratio of the crazes λcraze also decreases from 8.4 (PCHE) to 4.3 (CEC), indicating that the PE midblock dramatically decreases λcraze. Both PCHE and CEC crazes, however, eventually break down to form cracks at relatively small strains. The mechanism of deformation and fracture changes dramatically for the pentablock copolymer, CECEC. This pentablock deforms primarily by the formation of shear deformation zones at a median strain of about 2.1%, but crazing competes with shear deformation and crazes with tips blunted by shear deformation zones are frequently observed. We do not observe any significant craze or deformation zone breakdown in the CECEC even at strains up to 23%. Therefore, while maintaining the total Mw and fPE nearly unchanged, a “brittle-to-ductile” transition is caused by changing the chain architecture from triblock to pentablock. Because the PCHE midblock chains in CECEC can form bridging chains between highly entangled PE domains, we attribute the ductility of CECEC primarily to the increase in the network density that disfavors both craze formation and premature craze breakdown.