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Grain growth kinetics in microphase separated An/Bn star block copolymers
Currently, there is tremendous interest in using block copolymers to generate nanostructures where a critical issue is the control of long range order. This dissertation focuses on how molecular architecture of block copolymers influences the long range order. Using a series of AnBn star block copolymers with different numbers of arms (n = 1, 2, 4 and 16), the effect of molecular architecture on the grain growth kinetics is investigated by both thermal annealing and annealing in supercritical carbon dioxide (CO 2). Across this entire series of materials, all the A arms are polystyrene (PS) blocks from the same anionically synthesized batch, and all the B arms are polyisoprene (PI) blocks from the same anionically synthesized batch. Thus, all the star block copolymers employed in this study are composed of the same A and B arms linked together in symmetric numbers and the only difference within this series is the number of arms, n. The grain growth kinetics of these AnBn materials is then monitored in real space by transmission electron microscopy (TEM), followed by subsequent micrograph image analysis. It is found that the molecular architecture influences the grain growth kinetics of these An Bn star block copolymers significantly under both thermal and supercritical CO2 annealing. Their grain growth kinetics shows a strong dependence on the number of arms. Also, the grain coarsening kinetics followed a scaling law as V ∼ tβ, where V is the characteristic grain volume and t is annealing time. Under simple thermal annealing, the exponent, β, is found to be about 0.2 for the A1B1 diblock copolymer (AnBn with n = 1) and 0.4 for all three star block copolymers with n = 2, 4 and 16. Meanwhile, under supercritical CO2 annealing, the grain growth dynamics of these AnB n stars with n = 2, 4, and 16 is found to be the same as that of the same AnBn materials under thermal annealing. However, the grain growth kinetics of the A1B1 diblock is dramatically enhanced in supercritical CO2 relative to thermal annealing. Comparison of the scaling relationships strongly suggests that the difference in grain growth between the A1B1 diblock and the AnB n star block copolymers can be attributed to the difference in chain entanglements and to the thermodynamic barrier to diffusion perpendicular to the lamellar layers.
Hu, Xiaochuan, "Grain growth kinetics in microphase separated An/Bn star block copolymers" (2006). Doctoral Dissertations Available from Proquest. AAI3212733.