In situ rheodielectric, ex situ 2D-SAXS, and Fourier transform rheology investigations of the shear-induced alignment of poly(styrene-b-1,4-isoprene) diblock copolymer melts

摘要

A highly sensitive rheodielectric experimental setup was used to investigate the macroscopic alignment of symmetric poly(styrene-b-1,4-isoprene) (SI) diblock copolymers under large-amplitude oscillatory shear (LAOS). The dielectric normal-mode of the 1,4-cis-polyisoprene chains in the diblock copolymer was chosen to probe in situ the macroscopic orientation process. It was shown that the development of the overall orientation of the lamellar microstructure can be followed in situ using the time progression of the dielectric loss modulus ε" (t). The dielectric loss ε"(t) correlates directly with the nonlinear mechanical response I _(3/1)(t) of the sample as determined via Fourier transform rheology (FT-rheology). In addition to these two dynamic methods, small-angle X-ray scattering was used to ascertain the degree and type of the macroscopic orientation as a function of the applied shear conditions. Evidence presented here showed that a decrease in ε"(t) relative to the initial value of ε"(t = 0 s) for a macroscopically isotropic sample melt was indicative of a macroscopic parallel orientation while an increase in ε"(t) corresponded to an overall perpendicular alignment. These phenomena are explained on a molecular level by the anisotropic diffusion of the confined polymer chains, resulting in a higher mobility of the dielectrically active end-to-end vector parallel to the interface, which can be detected via dielectric spectroscopy.