Controlling molecular and microstructural alignment in anisotropic polymer systems

Citation

Gupta, Vinay Kumar (1996) Controlling molecular and microstructural alignment in anisotropic polymer systems. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/xb3f-3a10. https://resolver.caltech.edu/CaltechETD:etd-12132007-085351

Abstract

Molecular and microstructural orientation plays a key role in determining the useful mechanical, optical and electrical properties of a large class of polymeric materials. Realization of the technological promise of these materials requires rational control of the microstructure and understanding of the processing-structure-property relationships. Towards this end, we focus on the material properties and dynamical processes that affect molecular order of two complex polymeric systems (a) Ultra-thin Langmuir-Blodgett films of a hairy-rodlike polymer, and (b) Lamellar diblock copolymer melts of polystyrene and polyisoprene (PS-PI).

We accomplish our objective by improving and tying together existing analytic techniques into instruments extremely effective for measuring molecular orientation and order. For fundamental insights into the nature of alignment in ultra-thin (~ few nanometers thick) LB films, we integrate laser scanning microscopy (LSM) with polarization-modulation (PM) polarimetry. PM-LSM possesses high spatial resolution, sensitivity and speed, and allows us to image the anisotropy in films as thin as two molecular layers. To monitor the dynamics of flow alignment in diblock copolymers, in situ and in real time, we combine polarization-modulation polarimetry with rheometry. This rheo-optical technique enables us to monitor the evolution of microstructure by measuring the optical anisotropy, and correlate it with changes in macroscopic mechanical observables, such as stress and strain.

Using PM-LSM we have investigated the interplay of molecular weight, layer thickness and thermal annealing in controlling molecular order in rodlike polymer LB films. Upon investigating two different molecular weights of the polymer, we find lowering of molecular order in deposited films with increase in molecular weight. Furthermore, thermal annealing improves alignment only for films of shorter rodlike polymers (~ lower molecular weight). We believe that the underlying cause for these results is a difference in the structure of the deposited films for the longer polymer because of the relative changes in timescales for the macromolecular relaxation and the monolayer deformation during deposition. We also find that for both molecular weights, the substrate exerts an anchoring effect on the first two layers adjacent to it and suppresses improvement in alignment on annealing. This inability to improve alignment has important consequences for applications such as molecular electronics where monolayers or bilayers having a high degree of orientation are required.

In-situ measurement of flow birefringence during oscillatory shear alignment, clarifies the evolution of the lamellar orientation distribution for a diblock copolymer melt. Shearing results in "parallel" or "perpendicular" alignment i. e. layers normal to either the velocity gradient or the vorticity axis, respectively (Figure 1.1). Both states of alignment occur via an initial "fast" process followed by a "slow" one. The fast process is dominated by the depletion of the projection of the orientation distribution along either the perpendicular direction or the "transverse" direction (layers normal to the flow). This resulting biaxial distribution is then transformed into a well-aligned uniaxial one during the slow process. Surprisingly, at particular frequencies, the projection along the perpendicular direction can disappear faster than the projection along the transverse direction. In both the fast and the slow processes the time evolution of birefringence follows a highly non-linear scaling with strain; the scaling being different for the fast and slow processes. A systematic study of the effects of strain on shear alignment in block copolymers led to the discovery that strain affects not only the dynamics of alignment, but also the direction. This phenomenon expands the range of parameters that can be used to flip the direction of alignment. In relation to previous studies that examined the regimes of alignment only in terms of temperature and frequency, this phenomenon necessitates consideration of a third dimension - strain.

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