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[摘要]：In-situ flow alignment kinetics of a self-assembled lamellar phase polystyrene-block-polyisoprene (PS-b-PI, M(w) = 26 500 g/mol, f(PS) = 51%) diblock copolymer melt has been investigated in detail under mechanical large amplitude oscillatory shear (LAOS) utilizing a unique Rheo-SAXS combination developed in cooperation with the German Electron Synchrotron (DESY) in Hamburg. This marks the first time that the strain and time dependence of the shear-induced macroscopic perpendicular orientation of the lamellar microstructure could be monitored with a time resolution of 10 s per frame. Two mechanical parameters were used to compare the structural evolution and dynamics with the mechanical response of the sample. The mechanical loss modulus G '', which was directly obtained from the in situ Rheo-SAXS experiments performed with a stress controlled rheometer, and the nonlinear parameter I(3/1), which was calculated by Fourier-transform-rheology (FT-rheology) from the raw stress data obtained from a strain controlled rheometer. Significant correlations between the mechanical response and the structural changes of the sample were detected. For example, the orientation times tau calculated from both the X-ray and the mechanical measurements showed a power law dependence with tau similar to gamma(-1.6)(0) (in situ SAXS) and gamma(-2)(0) (FT-rheology). Furthermore, the quality of the macroscopic orientation at large shear amplitudes (gamma(0) = 2 and gamma(0) = 3) was found to be a function of the mechanical excitation time. A better macroscopic orientation for shorter mechanical excitation times was achieved, while longer experimental times caused an unexpected reduction in the degree of orientation. In these situations, ex-situ SAXS and TEM studies indicated that a stable biaxial distribution of the lamellar microstructure that was preferentially orientated both parallel and perpendicular was formed, causing a drastic change in the response of both the mechanical quantities G ''(t) and I(3/1)(t). |
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