Order in side-chain liquid crystalline diblock copolymers

Abstract

The architecture of side-chain liquid crystalline diblock copolymers (SCLCBC's) involves order on two different but interdependent length scales. Through molecular-scale interactions, liquid crystalline moieties self-assemble into a variety of mesophases, and on a larger length scale, chemical differences between the two blocks can result in phase-segregated block copolymer microstructures. This combination of organizational tendencies offers experimentally and theoretically challenging problems that precede a wide array of applications. Using sequential anionic polymerization we synthesized a large number of well-defined SCLCBC's. These materials consist of an amorphous, polystyrene (PS) block and a methacrylate-based, LC block with side-chain mesogens that exhibit the Sc* mesophase. The samples' block copolymer morphology was evaluated using a combination of small angle X-ray scattering (SAXS) and electron microscopy, and thermal transitions were identified using polarized microscopy, calorimetry, and elevated temperature-SAXS. We found the formation of micron-sized domains and focal-conic superstructures to depend on the length of the PS block and the overall molecular weight. A morphological phase diagram was constructed based on LC volume fraction for molecular weights up to 40,000 Daltons. A broad lamellar regime was identified that extends to unusually low LC compositions. At intermediate LC fractions, just over 50 %-wt. LC, where one would expect lamellar morphologies for analogous coil-b-coil diblocks, morphologies are predominately lamellar, but exhibit perforated defects that connect the lamellar LC microdomains.

(cont.) A morphology consisting of hexagonally-packed PS cylinders was observed at 79 %-wt. LC, and at very high LC volume fractions (85 %-wt.) an unusual layered morphology was observed in both bulk and thin film studies. For this sample, the microstructure has a periodicity of -70 Angstroms and forms highly ordered micron-size monodomains. SAXS and TEM data suggest that the LC domains consist of smectic bilayers, and the amorphous polystyrene domains are highly oblate spheres that arrange hexagonally between smectic bilayers. We investigated the interdependence of block copolymer morphology and LC thermotropic phase behavior using elevated temperature SAXS. Order-disorder and order-order block copolymer transitions were located and compared to the LC isotropization temperature. For materials with low LC volume fractions, and low overall molecular weight, LC isotropization is shown to trigger morphological order-disorder transitions (ODT's). In other samples the LC clearing point precedes the ODT temperature, and this temperature difference largely depends on the length of the LC block. One sample, with 58 wt-% LC, undergoes a thermal-reversible LC triggered order-order transition between a predominately lamellar morphology with cylindrical defects and a completely lamellar morphology. These and other observations are explained on the basis of conformational asymmetry. Our analysis indicates that the length of the LC block and the related block copolymer superstructure are key parameters to controlling both LC mesophase and morphology. A simple free energy model was developed to capture the interplay between liquid crystalline and block copolymer morphology ....