Insights into the microphase separation of polystyrene-b-polyproline: Role of polyprolines' secondary structure

Understanding the role of polymers' secondary structure on self-assembly provides a deeper insight into structure-property relationships and is also imperative for designing functional materials. This study investigates the self-assembly of a multi-structured rod-coil diblock copolymer consisting of poly(L-proline) and polystyrene (PS) segments. The unique feature of these diblock copolymers is that the secondary structure of polyproline (PLP) blocks can independently be tuned into two distinct conformations (PPI and PPII). Furthermore, a high energy barrier (81 KJ/mole) associated with the conformational transition between PPI and PPII structures makes it attractive to study the influence of the secondary structure on self-assembly under thermodynamic equilibrium. Thus, the self-assembly of this block copolymer was investigated in terms of the molecular weight and composition of the individual blocks and their secondary structures. A pair of diblock copolymers (PSm-b-PLPn) with differing polyproline block lengths have been synthesized. It has been found that irrespective of the length of the PLP block studied here, the diblock copolymer with trans-polyproline (PPII) self-assembled and microphase separated into a lamellar morphology. However, the diblock copolymer with a cis-polyproline (PPI) segment does not exhibit a well-defined microphase-separated structure despite having identical compositions and molecular weights. As a result, secondary structures are shown to play a role in self-assembly.