Molecular Dynamics of Glass-Forming Polymers, Plasticized Polymers, and Liquid Crystal Polymers: AN Electron Paramagnetic Resonance Study.

The relaxation behavior of polymers and polymer/plasticizer mixtures is of interest but is not yet fully understood. Molecular dynamics and ordering studies of side-chain liquid crystal polymers (SCLCPs) as a function of the concentration of side chains attached to polymer backbone have not been previously carried out. An electron paramagnetic resonance (EPR) spin probe study which is a non-destructive and non-perturbative method for gathering information concerning the molecular dynamics and ordering of these materials has been performed. For polymers and plasticized polymers with a cholestane probe, a discontinuity is observed in the Arrhenius plots of the rotational correlation time vs. 1/T near the temperature at which extrema separation equals to 50 gauss (T _{50G}). The spectra obtained near T_{50G} proved to be composite spectra reflecting multiple relaxations. With decreased concentration of side chain polymers (SCPs) in plasticizer, the discontinuity disappears for Poly(vinyl acetate) but not for Poly(butyl methacrylate). Similar effects have been observed in other two linear chain polymers. The retention of the discontinuity is associated with the bulkiness of the pendant group for SCPs. For linear chain polymers, it appears to be related to the bulkiness of the repeating unit between the flexible linkage. SCLCPs with different concentration of side chains (100%, cong50%, cong 30%), monotropic LC, and a siloxane copolymer were studied separately to understand the coupling strength between the polymer backbone and side chain via a spacer, (CH_2)_6. Results show that (i) the coupling strength is quite significant, (ii) the rotational motion about the perpendicular axis is slowed more than that about the parallel axis with increased concentration of the side chains, (iii) cong50%, cong30% LCPs have about the same viscosity, (iv) there is a possibility of the existence of tricritical point as the concentration of the side chains changes, (v) the activation energy in the isotropic phase is increasing with increasing concentration of the side chains, (vi) the cong30% LCP has much higher activation energy in the nematic phase than cong50% and 100% LCPs, which is strongly correlated with the slope of the order parameter vs. temperature in the nematic range, (vii) the activation energy in the isotropic phase is observed to be smaller than that in the nematic phase for cong30% LCP.