Large Surface Area Non-equilibrium Morphologies Produced via Sequential Thermal and Solvent Immersion Annealing of Block Copolymer Thin Films

Processing conditions like thermal annealing (TA), solvent vapor annealing (SVA), and direct solvent immersion annealing (DIA) expose the block copolymers (BCP) to very different swelling and diffusion environments (vacuum vs. vapor vs. liquid). These conditions significantly affect the type and orientation of microstructure achieved by the BCP. So, it can be fairly expected that adding more complexity to the processing environment, say, by combining two or more of these annealing techniques, can alter the BCP thin film's natural interactions, ordering kinetics, and morphology evolution. We explore such methods by combining DIA (liquid processed) and TA (heat processed) methods in a sequential manner. While TA slowly (hours) orders a disordered symmetric-BCP film into parallel lamellar layers on silicon substrates with large domain size (L_o,TA) owing to slow diffusion in the melt, DIA rapidly induces the parallel lamellar morphology by means of direct swelling in good solvents and with highly reduced domain size (e.g., L_o,DIA ≈ 0.5 L_o,TA), providing a way to distinguish between the two. Sequential DIA on TA-ordered lamellar structure produces highly irregular intermediate morphologies that can be trapped on solvent removal by stopping the process. These intermediate structures feature large surface areas with potential applications in filtration membranes, scaffolds for catalysis, and biomass interaction and generation. Controlling the degree of stratification for the first TA step and the duration of the second DIA step allows us to tune the magnitude and the quality of the surface roughness (peaks vs. terraces). In limits of long-time DIA, the rough transient morphology heals back into a regular lamellar morphology with L_o,DIA size domains. Considering the structural crossovers observed, a chain rearrangement mechanism for transition between the two distinct morphologies is proposed, and the underlying dynamics of this reversibility process are analyzed in terms of polymer chain length, layer swelling, diffusion, and in-plane vs. out-of-plane interfacial evolution. The various morphologies observed are plotted on an energy diagram to establish the relation between interfacial energy changes with processing time and conditions.