Microstructure and modeling of edge dislocations and grain boundaries in polydiacytylenes.

Abstract

The objective of this research has been to investigate the structure of dislocations and grain boundary defects in polydiacetylene crystals. The questions which were addressed concerned the formation of these defects, their structure, their influence on polymerization, and their predicted influence on macroscopic transport and mechanical properties. Experiments and theoretical modeling were performed in three general areas: (1) the influence of curved surfaces on morphology of polydiacetylene crystals, (2) the structure of individual dislocations in polydiacetylenes and governing elasticity theories, and (3) the structure of grain boundaries and geometrical theories. Droplets on substrates and spheres were formed from dilute solution to study curved surfaces. Both droplets and spheres were found to be crystallographically textured with a preferred orientation of the chain axis. High resolution electron microscopy (HREM) revealed that the polymer crystal lattice bent to incorporate the curvature of the surfaces. Etching experiments of polymerized droplets showed that areas of unreacted monomer were present where the lattice had curved. The strain fields around individual dislocations in polydiacetylenes were investigated using HREM, revealing the localized strain near the dislocation core. Anisotropic linear elastic dislocation theory was compared with columnar dislocation theory. It showed that as the anisotropy of the elastic modulus increased due to covalent bonding in the chain direction, the predicted distortions near the defect approached that expected from the columnar liquid crystal solution. Development of O-lattice theory for grain boundary structures in monoclinic diacetylene bicrystals predicted that the macroscopic properties of these materials are very sensitive to misorientations of only a few degrees (3$\sp\circ$-5$\sp\circ$). The modified O-lattice theory was fitted to experimental data of the tensile strength and photoconduction through grain boundaries. Mechanisms of high resolution image formation using electron microscopy were discussed and dynamic HREM theory was compared to experimental results on polymer crystals as a function of thickness. Dynamical theory was found to qualitatively predict the appearance of Pendellosung fringes in wedge shaped samples. Scanning probe microscopy measurements of wedge thicknesses were compared to thicknesses predicted from the HREM image experimental contrast and simulations obtained from dynamical theory.