RAFT polymerization kinetics and polymer characterization of P3HT rod-coil block copolymers and their uses to prepare hybrid nanocomposites

Abstract

The purpose of this work is to provide well-defined P3HT rod-coil block copolymer systems that can form self-assembled structures and be used as templates for the preparation of P3HT hybrid nanocomposites. To this end, a new P3HT macroRAFT agent was designed such that the P3HT was incorporated into the R group rather than the Z group which, in addition to providing a useful end group functionality, allows for a true 'grafting from' approach when preparing rod-coil block copolymers. The prepared P3HT macroRAFT agent is extensively characterized by proton nuclear magnetic resonance (1H NMR) spectroscopy, gel permeation chromatography (GPC) and matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) with a focus on an understanding of the limitations of each technique for molecular weight characterization and gaining insight into the efficiency of end-group reactions as both of these factors affect the composition of the subsequently prepared block copolymers. The RAFT polymerization kinetics of the coil blocks, namely poly(styrene) and poly(tert-butylacrylate) were followed in order to demonstrate the effectiveness of the P3HT macroRAFT agent and gain insight into the polymer composition. Direct quantification of the RAFT polymerization is necessary to obtain reproducible block copolymers with predictable molecular weights and narrow molecular weight distributions and yet an in-depth analysis of the RAFT polymerization kinetics is lacking in prior reports. Again, comprehensive characterization of the block copolymers is used to present a more realistic representation of the block copolymer sample. It is purposed that 1H NMR provides the most accurate molecular weight and composition data, although there is a slight overestimation due to inefficient end-capping reactions. However, by combining MALDI-TOF end-group analysis with 1H NMR data, some understanding of the amount of overestimation is achieved. This is particularly important as inefficient end group reactions result in P3HT homopolymer in the final sample. Therefore accurate characterization requires molecular weight determination of the block copolymer and quantification of the sample composition. Also presented in this work is the synthesis of amphiphilic rod-coil block copolymers of P3HT with poly(4-vinyl pyridine) (4VP) and poly(acrylic acid) (AA). Here, P3HT-b-PAA is prepared by the direct synthesis of acrylic acid with the macroRAFT agent using a binary solvent system of trichlorobenzene and dioxane to maintain polymer solubility throughout the polymerization. The micelle formation of the resulting block copolymer is detailed as the transparent micelle solution of P3HT-b-PAA exhibits the optical behavior of solid-state P3HT. Finally, the preparation of various nanocomposites from the synthesized P3HT homopolymer and block copolymers is presented. In the first method, the RAFT end group of the P3HT macroRAFT agent and the P3HT block copolymers is reduced to a thiol to allow for attachment to Au nanoparticles. While Au nanoparticles are not useful for photovoltaic applications, the surface modification demonstrates the utility of the P3HT macroRAFT agent, which is used to modify the surface with P3HT homopolymer and P3HT block copolymers. The second method to prepare P3HT hybrid nanocomposites is in-situ growth of CdS in P3HT-b-P4VP. With analysis by dynamic light scattering (DLS), Fourier-transform infrared (FT-IR) spectroscopy and transmission electron microscopy (TEM), it is concluded that a majority of the CdS growth in the P3HT-b-P4VP was confined to the P4VP block.