Flow-induced alignment in intrinsically anisotropic polymeric materials

Citation

Kannan, Rangaramanujam Mudumbai (1995) Flow-induced alignment in intrinsically anisotropic polymeric materials. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/ZDCX-2W89. https://resolver.caltech.edu/CaltechETD:etd-10112007-104204

Abstract

To understand the processing behavior of complex polymeric materials, it is imperative that we understand the distinct underlying molecular and microstructural contributions to macroscopic mechanical response. We have developed rheo-optical methods to isolate the dynamics at such length scales, by combining conventional mechanical testing with polarimetry. The rheology and flow-induced alignment in side-group liquid-crystalline polymers (SG-LCPs) and lamellar diblock copolymers (BCPs) have been investigated using the rheo-optical apparatus. The rheology of SG-LCPs arises from a combination of the polymeric nature of the backbone, the ordering tendency of the mesogens, and the coupling between the two introduced by the spacer. Contrary to previous studies, we find that the nematic order can significantly affects the macroscopic viscoelastic response in SG-LCP melts. In the nematic state, we have discovered that oscillatory shear is effective in inducing macroscopic orientation of the sample. Shear flow may prove to be an economical, efficient way to obtain macroscopically aligned SGLCPs, which have potential as optical data storage media and nonlinear optical materials. In contrast to magnetic field induced alignment, the mechanism of shear induced alignment appears to bedriven by the distortion of the polymer backbone. We study the mechanisms of flow-induced alignment in a poly(ethylenepropylene)-poly(ethylethylene) lamellar diblock copolymer, which produce macroscopic alignment with lamellae normal along the shear gradient direction ('parallel' alignment), or along the vorticity direction ('perpendicular' alignment). We accomplish this by monitoring the evolution of the microstructure and viscoelasticity in situ during the alignment process. Our results suggest that 'parallel' alignment occurs due to irreversible 'rocking' of grains which tends to align the grains along the shear direction, whereas 'perpendicular' alignment occurs at conditions where the microscale deformation is nearly homogeneous.

Thesis Files