Thrust 2 – Cooperative Assemblies of Functional Materials
Order-disorder transitions in novel block copolymers. Order-disorder transitions in novel block copolymers were studied by a combination of birefringence, small angle X-ray scattering, and rheology. As a strategy to direct the self-assembly of block copolymers, promote phase separation at low block lengths for fine feature sizes, and to create sharp interfaces, Balsara, Hedrick, and Waymouth introduced self-complementary hydrogen bonding arrays at the block junction. To enable facile modification of the block copolymer structure, molecular weight, and volume fractions, a general platform based on bifunctional initiators was developed.
The groups for directed self-assembly should have sufficiently high association constants to ensure persistence of the self-assembled structure throughout the thickness of the film by acting as a sharp boundary between the two phases. To tailor the strength of the supramolecular interaction at the block junction, they have designed and synthesized a range of initiators having groups suitable for self-recognition based on (thio)urea and bis-(thio)urea self-assembly that were used for the synthesis of block copolymers (Scheme 2).
Initial studies were done with poly(styrene-block-lactide) copolymers with different connectors capable of forming hydrogen bonding at the junction of the two blocks. Scheme 2 illustrates the connector with no hydrogen bonding capabilities (PS-PLA) and connectors with urea (UR), thiourea (TU), and two thioureas (TUTU) used for the study. Small angle x-ray scattering (SAXS) measurements suggest that the presence of hydrogen bonding increases domain size. However, polylactide-containing block copolymers are difficult to process and readily degrade upon sample preparation. Two new systems have been designed with the same block junctions and address the drawbacks: poly(styrene-block-carbosilane) (PS-PCS) and poly(styrene-block-trimethylene carbonate) (PS-PTMC). Planned SAXS measurements will be confirmed with birefringence measurements. Preliminary studies indicate that the new materials are easy to process and more resilient to degradation. Moreover, thermal analysis measurements show that microphase-separated morphologies were obtained for the low molecular weight systems, but only with the molecular recognition arrays at the block junction.
Scheme 2. Connector linkages for PS-PLA diblocks.