Formation and Thermodynamics of Heteropolymers with Adjustable Monomer Sequence Distribution

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Title: Formation and Thermodynamics of Heteropolymers with Adjustable Monomer Sequence Distribution
Author: Jhon, Young Kuk
Advisors: Jan Genzer, Committee Chair
Richard J. Spontak, Committee Member
Christopher B. Gorman, Committee Member
Orlando Rojas, Committee Member
Abstract: Over the past few decades, multiple experimental and theoretical studies have reported on the formation and thermodynamics of copolymers with ordered sequences. Because of challenges associated with synthesizing random copolymers (RCPs) with tunable co-monomer sequences and analyzing their co-monomer sequence distribution, only a few computer simulations and sophisticated theoretical approaches have been employed that provided insight in the thermodynamical behavior of such RCPs. Recently, we developed a methodology facilitating the formation of RCPs with tunable co-monomer sequence distributions by invoking the “coloring scheme†suggested Khokhlov and coworkers in their computer simulations. The RCPs considered in this work are prepared by bromination of polystyrene (PS) with bromine leading to poly(styrene-co-4-bromostyrene) (PBrxS) RCPs, where x is the mole fraction of the 4-bromostyrene (4-BrS) segments in PBrxS. In this Thesis, we demonstrate that the co-monomer sequence distribution in PBrxS can be adjusted by carrying out the bromination reaction in solutions of various solvent quality and reaction geometry. By adjusting the solvent quality during the bromination reaction, either random or random blocky PBrxS, (r-PBrxS or b-PBrxS, respectively), are synthesized. We find that: 1) the bulk bromination of PS follows the second-order kinetic in bromine, 2) the reaction rate increases with increasing the solvent dielectric constant, and 3) decreasing the solvent quality decreases the bromination reaction rate. In addition, we report that the reaction rates for brominating PS brushes are much smaller than those of free PS chains in solution. We attribute this latter behavior to steric hindrance due to PS confinement on the substrate. We also report on the effect of chemical composition, co-monomer distribution, and H/D isotopic substitution in the RCP and the solvent on the phase behavior of PBrxS in cyclohexane (CH). We use turbidity and small angle neutron scattering measurements to assess the temperature-dependence of phase behavior of PBrxS with various x as a function of polymer concentration in CH. Our results reveal that while r-PBrxS chains collapse as individual coils during the coil-to-globule transition, the 1-phase to 2-phase transition in b PBrxS is very complex. Specifically, we report that larger macromolecular aggregates comprising multiple b-PBrxS chains form in solution well above the coil-to-globule transition observed in r-PBrxS having the same x, and that this assembly acts as a precursor to the “true phase transition†. This complex phase behavior of b PBrxS is associated with the existence of inter- and intra-chain contacts acting among styrene and 4 BrS units (“pattern recognition†) leading possibly to “flower-like†micelle formation. Finally, we report on the adsorption kinetics of PBrxS on flat substrates by monitoring the adsorption of PBrxS from various solvents on flat silica surfaces. The PBrxS adsorption is driven by the strong affinity of the 4-BrS units towards silica while the interaction between styrene and the surface is nearly athermal. For a given solvent the amount of PBrxS adsorbed onto the surface increases with increasing the 4-BrS content and the blockiness in the monomer distribution in the RCP. Concurrently, the amount of PBrxS on the substrate also increases with decreasing the quality of the solvent, from which the copolymer is adsorbed. We provide theoretical insight into the various molecular phenomena that govern both the kinetics and the equilibrium amount of the RCPs on the surface as a function of the co monomer distribution in the RCPs.
Date: 2008-12-19
Degree: PhD
Discipline: Chemical Engineering

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