Molecular motions of liquid crystalline polymers with various spacer lengths in the glass and nematic states

Abstract

A side-chain liquid crystalline polymer (LCP) based on the 4-hydroxy-4′-methoxy- α -methylstilbene mesogen attached through a flexible spacer of eight methylenic units to a poly(methyl acrylate) backbone is studied via solid-state 13 C nuclear magnetic resonance (n.m.r.) spectroscopy. The molecular dynamics in the MHz frequency regime are characterized as a function of temperature by the spin—lattice relaxation time constant T 1 . Rotational correlation times ( τ c s) and activation energies ( E a S) are calculated for motions at various local sites in the glass and nematic states. Rapid spinning of the methyl carbons occurs on the fast side of the T 1 minimum, and the activation energy does not change at the phase transition. The motional activation energies of all three sites in the mesogen are nearly equal, indicating that the motion is collective. The activation energy decreases for the mesogen carbons in the nematic state by a factor of six, as the LCP is no longer frozen. In the glassy state the activation energy of the α spacer carbon is nearly equal to that of the mesogen, while the β spacer carbon is 30 times more mobile. The spacer begins to function as a flexible free spacer at the β position. Further flexibility is introduced at the γ and δ positions. Likewise, in the nematic state the mobility increases in the spacer from the α position inward. The activation energy of the backbone increases in the nematic state, with the backbone acting as a viscous drag to the motion of the mesogen. A comparison is made between the LCP with a spacer of eight methylenic units and an analogous LCP with a spacer of three methylenic units. In the glass state, E a is larger for the motion of the mesogen, the α spacer carbon and the β spacer carbon, for the LCP with eight methylenic units.