Polycaprolactone-Based Block Copolymers. 3. Mechanical Behavior of Diblock Copolymers of Styrene and ∊-Caprolactone

Poly(styrene-block-∊-caprolactone) copolymers (PS-PCL) exhibit a two-phase morphology that dictates their mechanical behavior. The dependence of the isochronous torsion modulus on the copolymer composition clearly shows that a phase inversion takes place at ca. 45 wt % PCL. Although the stress-strain curves are governed by the nature of the continuous phase, the minor component influences the values of stress and strain in each deformation region. At least at compositions close to the phase inversion, the minor component forms semicontinuous phases as supported by electron microscopy and by the mechanical hysteresis of a completely amorphous PS-polyester copolymer comprising 55 wt % polyester. Although brittle at 77 K, a PS-PCL copolymer containing 56 wt % PCL exhibits a ductile fracture when extended by 300%, i.e., when the PCL spherulites are transformed into a microfibrillar structure. © 1989, American Chemical Society. All rights reserved.