Correlation of lattice deformation with macroscopic strain for the hexagonal-packed cylinder phase of a triblock copolymer

The deformation of a lattice of hexagonal-packed cylinders has been correla ted to the applied oscillatory strain via time-resolved simultaneous small- angle X-ray scattering (SAXS) and rheology experiments. Kraton-type poly(st yrene)-poly(ethylene-co-butylene) poly(styrene) (PS-PEB-PS) triblocks were pre-aligned in the hexagonal-packed cylinder phase formed in the melt. Spec imens were then subjected to oscillatory strain (7%, i.e. small but in the non-linear viscoelastic regime) with the cylinder axis parallel or perpendi cular to the shear axis at room temperature (where PS is glassy). At the sa me time, SAXS was used to probe the change in domain spacing during one cyc le of shear with a time resolution down to 2 ms. It was found that the latt ice deforms in phase with the applied strain (although the stress is out-of -phase). However, the deformation is not affine. The observed change in dom ain spacing for the cylinders was significantly smaller than the applied st rain for both the perpendicular and parallel orientation. The deviation fro m affine behaviour was greatest for the parallel orientation. This non-affi ne behaviour is ascribed to the take-up of strain by initially poorly orien ted grains. Rheology data obtained concurrently with the SAXS indicates a t wo-step relaxation of the dynamic shear modulus, The initial fast process m ay be due to the realignment of grains, whereas the slower process may be d ue to the perfection of lattice order resulting from non-linear viscoelasti c strains. (C) 1999 Elsevier Science Ltd. All rights reserved.