Hierarchical Dynamics and Phase Transitions in Wedge-Shaped Side-Chain Liquid Crystalline Polymers with Varying Spacer Lengths

Abstract As an important class of graft polymers, side-chain liquid crystalline polymers (SCLCPs) containing anisotropic mesogenic groups exhibit complex molecular dynamics that are closely coupled with hierarchical structural evolution. In this study, using various characterization techniques, we investigate the phase structures and viscoelastic behavior of a series of wedge-shaped SCLCPs, denoted as CxBP (x = 1, 3, 7), where x is the number of methylene units in the spacer linking the wedge-shaped mesogenic side group to the polynorbornene backbone. CxBPs form a hexagonal columnar phase (ColH), with multichain supramolecular columns serving as the basic building block arising from microphase separation of the main and side chains. Upon cooling from the isotropic state, CxBPs show unique multistep increases in modulus through four distinct stages (I–IV) with shear moduli of 104, 105, 107, and 108 Pa, respectively, separated by three relaxation/transition processes (α, β, γ). When approaching to –100 °C, a δ-relaxation further raises the modulus to ∼109 Pa. The α- and γ-processes correspond to first-order transitions: the isotropic-to-ColH transition at ∼90 °C and partial ordering of the side-chain alkyl tails at ∼0 °C, respectively. Notably, the β-process is a predominantly noncooperative relaxation following Arrhenius behavior with an activation energy of ∼300 kJ/mol, attributed to the motion of mesogens on the side chains. Freezing of this motion transforms the ColH from a “mobile” to a “mesogen-arrested” state, increasing the shear modulus by 2 orders of magnitude from stage II to stage III. We find that a longer spacer lowers the β-relaxation temperature. With a single methylene unit as the spacer, C1BP shows β-relaxation partially overlapping with the α-process. Extending the spacer length to three or more methylene units (C3BP, C7BP) enables sufficient dynamic decoupling of the main and side chains, yielding a wider stage II with a shear modulus of 105 Pa.