Tailored Chain Sequences of Brominated Syndiotactic Polystyrene Copolymers via Post-Polymerization Functionalization in the Heterogeneous Gel State
Noble, Kristen Felice
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This dissertation demonstrates the preparation of blocky brominated syndiotactic polystyrene (sPS-co-sPS-Br) copolymers with tailored chain sequences using a simple, post-polymerization functionalization method conducted in the heterogeneous gel state, and investigates the effect of sPS reaction state and sPS/solvent gel morphology on the copolymer microstructure and thermal properties. Gel-state (Blocky) brominated copolymers were prepared from a 10 w/v% sPS/carbon tetrachloride (CCl4) gel and a 10 w/v% sPS/chloroform (CHCl3) gel in a matched set containing 6−32 mol% p-bromostyrene (Br-Sty) units. For comparison, a matched set of randomly brominated copolymers was prepared using a homogeneous solution-state (Random) reaction method and a set of brominated copolymers was prepared using a heterogenous powder-state (Powder) reaction method. The degree of bromination was evaluated using 1H nuclear magnetic resonance (NMR) spectroscopy. Powder-state bromination produced copolymers with a limited degree of functionalization of up to 12 mol% Br and required a threefold longer reaction time than the gel-state method conducted on the sPS/CHCl3 gel, demonstrating that the powder-state method is time-consuming and the dense sPS powder is incapable of producing copolymers with high Br-content. Microstructural characterization provided by 13C NMR spectroscopy, showed that bromination of sPS produces multiple peaks in the quaternary carbon region of the NMR spectrum, signifying through-bond communication between neighboring styrene and Br-Sty monomers. This work provides the first high-resolution comonomer sequencing of brominated sPS copolymers. Characterization of the quaternary carbon spectrum, assisted by band selective gradient heteronuclear multiple bond correlation (bsgHMBC) spectroscopy, electronic structure calculations, and simulated statistically random copolymer chains, revealed that each resonance peak could be assigned to a styrene or Br-Sty unit that exists in the center of a unique sequence of five monomers (i.e., a pentad) along the copolymer chain (e.g., ssssb where s = styrene and b = brominated styrene). Our comonomer sequencing method demonstrated that the Blocky and Powder copolymers have block-like character. Remarkably, the Blocky copolymers exhibit notably higher degrees of blockiness and larger fractions of sssss and bbbbb pentads at low Br contents (i.e., 32 mol% Br), relative to the Powder copolymers, confirming their blocky microstructure. Quenched films of the Blocky copolymers, analyzed using ultra-small-angle (USAXS) and small-angle X ray scattering (SAXS), show micro-phase separated morphologies that are reminiscent of conventional block copolymer phase behavior, supporting that the Blocky copolymers contain distinct segments of pure sPS and segments of randomly brominated sPS. Crystallization behavior of the copolymers, examined using differential scanning calorimetry (DSC), demonstrates that the Blocky copolymers are more crystallizable and crystallize faster at lower supercooling compared to their Random analogs. Simulations of blocky copolymers were developed based on the semicrystalline gel morphology to rationalize the effect of gel-state functionalization on copolymer microstructure and crystallization behavior. The simulations confirm that restricting the accessibility of the brominating reagent to monomers well removed from the crystalline fraction of the gel network produces copolymers with a greater prevalence of long runs of pure sPS that is advantageous for preserving desired crystallizability of the resulting blocky copolymers. To investigate the effect of sPS/solvent gel morphology on copolymer microstructure and crystallization behavior, the sPS/CCl4 and sPS/CHCl3 copolymers were compared directly. Characterization of the sPS/solvent gels using USAXS/SAXS, revealed that the gels exhibit different morphologies and average lamella thicknesses. Microstructural analysis showed that the sPS/CHCl3 copolymers contain larger fractions of sssss pentad and a greater degree of blockiness. The sPS/CHCl3 copolymers contain larger phase domains, supporting that these copolymers contain longer distinct segments of pure sPS and randomly brominated sPS in a multiblock-like microstructure. In addition, the sPS/CHCl3 copolymers are more crystallizable during conditions of rapid cooling and crystallize faster at low supercooling relative to their sPS/CCl4 analogs. Simulated average chains of the Blocky copolymers, generated from the empirical pentad sequence distributions, provide strong evidence that the runs of pure sPS in the Blocky copolymers originate from the crystalline stems within the crystalline lamellae. Thus, the simulations support that semicrystalline blocky brominated copolymers with tailored chain sequences, phase behavior, and crystallization properties and can be prepared simply by changing the gelation solvent.
General Audience Abstract
Block copolymers are a class of large molecules (polymers) that are made up of two or more chains (blocks) of different smaller units (monomers) linked together at one of each of the chain ends. When the monomers that make up each block have distinctly different chemical properties, the blocks may be capable of self-assembling into well-ordered physical structures, which give the block copolymer unique material properties that are different, and often better than the properties of the individual blocks alone (homopolymers). Block cop olymers have thus received tremendous attention with respect to controlled preparation, tailored structure development, and customized physical properties, for their potential use in self-assembled, nanostructured materials. Nevertheless, the generally difficult procedures and conditions required to make (polymerize) block copolymers with controlled sequences limits the scope of their commercial application. As an alternative to conventional polymerization methods, this dissertation demonstrates a comparatively simple physical method to make copolymers that contain significantly non-random (blocky) monomer sequences, starting with a homopolymer and using a reagent to modify units along the polymer chain. This post-polymerization method is conducted in the homopolymer’s gel state, in which segments of the homopolymer chains are effectively shielded from the reagent. The homopolymer, syndiotactic polystyrene (sPS), was used as a model to conduct a fundamentical investigation into the effects of the polymer reaction state, i.e., gel, solution, or powder, and the gel structure (morphology) on the copolymer structure and properties. The gel-state was found to produce copolymers with a high degree of modification and a greater degree of blockiness than the solution-state and powder-state. Copolymers prepared from the gel state exhibited properties that are characteristic of conventional block copolymers. Furthermore, using the gel-state method, blocky copolymers with tailored chain sequences and properties were prepared by simply changing the gel morphology. Thus, reaction in the gel-state is demonstrated as a simple physical approach to polymer design and synthesis that will be useful in the development of next-generation functionalized materials through the modification of lowcost commodity polymers. As an advancement to the manner in which nanostructured materials are created, these tailored materials will greatly enhance the convenience of block copolymers for a wide variety of applications including structural and biomechanical materials, and polymeric membranes for energy conversion and water purification systems.
- Doctoral Dissertations