Block Copolymer Self-Assembly, Hierarchical Assembly, and Application

Abstract

This thesis addresses three issues. These are the self-assembly of block copolymer in selective solvents, hierarchical assembly of micelles or crosslinked micelles of block copolymers, and the application of block copolymers as solid state compatibilizers in polymer-based photovoltaic cells. Poly(acrylic acid)-block-(2-cinnamoyloxylethyl methacrylate)-block-poly(perfluorooctylethyl methacrylate) or PAA-b-PCEMA-b-PFOEMA self-assembles in solvent mixtures of α,α,α-trifluorotoluene (TFT) and methanol, which are selective towards PAA. At TFT volume fraction (fTFT) of 40 %, the copolymer forms vesicles at 70 oC and cylinders at 21 oC. These two structures inter-convert via meta-stable intermediates including jellyfish-like, tethered vesicular, and bilayer sheet-like structures. These structures occur in kinetic experiments involving quick temperature swing from 21 to 70 oC or vice versa and also in experiments involving annealing samples long at temperatures between 21 to 70 oC. Thus, they are meta-stable and point to complex pathways for the morphological transition. At fTFT = 10 %, the polymer forms vesicles with bumpy surface at 70 oC and toroids with sharp angles at 21 oC. Closely examined is how the liquid crystalline nature of the PFOEMA block affects the formation of these unique morphologies and their morphological transitions. Two types of hierarchical assembly of cylindrical micelles (cylinders) or crosslinked cylindrical micelles (fibers) of block copolymers are examined. First, carboxyl-bearing nanofibers of PAA-b-PCEMA and amino-bearing nanocylinders from poly(tert-butyl acrylate)-block-poly(2-cinnamoyloxyethyl methacrylate)-block-poly(2-dimethylamino-ethylmethacrylate), PtBA-b-PCEMA-b-PDMAEMA, are mixed in solvent. The two species firstly aggregate via electrostatic interaction. Upon heating and aging, the cylinders dissociate on the fibers and eventually evolve into composite multilayered cylindrical structures. Second, layer-by-layer (LBL) deposition of carboxyl- and amine-bearing nanofibers yielded multilayer films. These films detached from a substrate separate nanospheres based on their size and surface charge differences. Diblock copolymers poly(3-hexylthiophene)-block-poly(2-cinnamoyloxyethyl methacrylate-random-2-[6,6]-phenyl-C61-butyroyoxyethyl methacrylate) (T-C60C) and poly(3-hexylthiophene)-block-poly(2-acetoxyethyl methacrylate-random-2-[6,6]-phenyl-C61-butyroyoxyethyl methacrylate) (T-C60A) are synthesized and used as compatibilizers for polymer-based photovoltaic cells containing poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Both copolymers can stabilize the morphology of the active layer and thus the device performance. T-C60A in the active layer yields longer life-times and better initial performance of the devices, due to the matching of surface tensions between C60A and PCBM.