Mechanisms of liquid crystal and biopolymer alignment on highly-oriented polymer thin films

Abstract

Molecular order can strongly enhance material properties, or produce materials which perform advanced functions. Many materials, from small crystals to large macromolecules, may be aligned on highly-oriented poly(tetrafluoroethylene) (PTFE) or high-density polyethylene (HDPE) thin films, prepared by a simple shear deposition procedure. Here, processes by which these films produce order are examined, first in a well-characterized liquid crystal, then in two more complex polymer liquid crystals, and finally in an adsorbed motor protein system.Optical second harmonic generation (SHG) was used to study surface molecular order in the liquid crystal 4$\sp\prime$-n-octyl-4-cyano-biphenyl (8CB) on PTFE and HDPE films. In nematic 8CB cells with bulk alignment along the polymer orientation axis, the surface monolayers of 8CB were also aligned, and showed $C\sb{2\nu}$ symmetry. In the isotropic phase, the surface monolayer alignment was lost. Monolayers of 8CB evaporated onto either polymer showed little or no alignment. The bulk 8CB alignment appears to be primarily caused by surface ridges through an elastic, bulk-mediated mechanism, unlike the epitaxy-like alignment found on some cloth-rubbed polymer surfaces.For the polymer liquid crystal poly-$\gamma$-benzyl-glutamate (PBG), uniform homogeneous surface alignment was observed on PTFE films; this is the first report of PBG surface alignment. However, liquid crystalline samples of microtubules were not aligned. PTFE films show promise for aligning some other polymer liquid crystals via elastic interactions. The motor protein kinesin, adsorbed to PTFE films, transported fluorescently labeled microtubules predominantly in straight lines along the films' orientation axis, not in random directions as observed on glass surfaces. As the kinesin surface density was increased, the degree of alignment peaked and then declined. The results indicate that directed motion occurs because active kinesin preferentially adsorbs to surface sites along linear tracks on the film. This suggests that kinesin may be harnessed for nanoscale surface transport along predetermined tracks.Improvements in SHG methods are also reported. Fresnel factors for buried interfaces are corrected and extended to uniaxial materials. For isotropic interfaces, a simple method is presented to measure the complete second-order susceptibility, up to an overall phase factor, without using a reference phase. This method is illustrated using 8CB monolayers.