Rheology and Dynamics of Side-Group Liquid Crystalline Polymers in Nematic Solvents (2004)

To form a liquid crystalline (LC) gel that retains the ability to respond rapidly to applied fields, it is necessary to work with low polymer concentrations. In turn, to form a dilute polymer network it is necessary to use very long polymers that are soluble in the small molecule LC. This research focuses on the synthesis of ultra-long side-group liquid crystalline polymers (SGLCPs), their properties when dissolved in nematic hosts, and the self-assembly of a nematic gel using an ABA triblock with an SGLCP midblock and LC-phobic end-blocks. Typically, LCs are made from small molecules that can be quickly reoriented. In applications such as artificial muscles, flexible displays, or compensating films, a more robust LC gel is desired. Prior routes to LC gels, typically using in situ polymerization, suffer from director misorientation, lack of control over cross-link density, polymer network inhomogeneity, undesired phase separation, and slow responses to applied fields. The present research (at the intersection of block copolymers, gels, and LCs) has demonstrated that an optically uniform LC gel with fast reorientational response can be achieved using a self-assembling ABA triblock copolymer. To provide the fundamental underpinning for the design of a self-assembling gel, we first advanced the synthesis of model SGLCPs that have well-defined length even at high degrees of polymerization. The polymerization method must provide narrow length distribution, be applicable to block copolymers, and preferably enable chains of varied side-group structure to be prepared. These requirements were met by starting from an anionically produced prepolymer and attaching the mesogen in a second step (a ?polymer analogous? approach). Homopolymers were made and characterized to determine how polymer structure affects solubility, rheological response, electro-optic response and chain conformation. This showed that cyanobiphenyl (CB) side-groups provide excellent solubility in CB-based small molecule LCs even for SGLCPs an order of magnitude longer than those investigated in solution previously; that ultra-long SGLCPs have unprecedented effects on the flow behavior of LC solutions, and that the anisotropy (R^/R?1.6) is insensitive to spacer length and degree of polymerization. The size of a polymer is related to the concentration necessary to form a gel network; however, there have been few studies of SGLCP dimensions in LC solvents. Since it is the polymer backbone conformation that is of interest, researchers use polymers labeled on the backbone to avoid scattering from the side groups. Unfortunately in a dilute solution this provides unacceptably low scattered intensity. Therefore, we demonstrate a method for measuring the dimensions of an unlabeled SGLCP in a perdeuterated nematic solvent, in which scattering originates from both the backbone and the pendant side groups. Since it is the backbone conformation that is of interest, we developed a method to mathematically account for scattering due to the side groups. Information gained from homopolymer studies guided the design of ABA block copolymers for nematic gels. We demonstrated that an optically uniform nematic gel can form in a small molecule LC even at low polymer concentrations using a triblock composed of an SGLCP center block and end-blocks that microphase separate to form physical cross-links. The key to making a dilute gel was using well-solvated, very long SGLCP midblocks. The necessity of cross-linking very dilute chain ends is prohibitive using covalent linking, but facile using end-blocks that spontaneously aggregate. In contrast to prior LC gels, these dilute gels maintain both the optical uniformity and fast reorientational responses of the small molecule LC host.