Design and Interfacial Activity of Copolymers with Controlled Monomer Sequence Distributions
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Date
2004-01-13
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Abstract
We study the bulk and interfacial behavior of A-B copolymers. Emphasis is placed upon addressing the role of the monomer sequence distribution in A-B copolymers as it pertains to the copolymer's mobility in confined geometries and its ability to recognize chemical patterns on surfaces.
Monte Carlo simulations are used to study the ability of block (A-b-B) and alternating (A-alt-B) copolymers to recognize chemical patterns on flat, impenetrable surfaces comprising two distinct chemical sites, C and D. The copolymer adsorption is driven by the repulsion between A and B segments along the copolymer chain and the attraction between B segments and D sites on the surface. The principle parameters that govern the ability of A-b-B and A-alt-B copolymers to recognize surface patterns are: the strength of the interaction between B segments and D surface sites, the A-B monomer sequence distribution, and the size and spatial distribution of adsorbing D sites. Our simulations reveal that both A-b-B and A-alt-B copolymers are capable of recognizing surface patterns and increasing the B-D attraction enhances the partitioning of A and B segments at the surface.
Commensurability between the copolymer's monomer sequence distribution and the size and spatial distribution of the surface heterogeneities is also found to affect the ability of A-b-B and A-alt-B copolymers to recognize surface chemical patterns. When the adsorbing domain size exceeds the size of the copolymer's parallel component to the radius of gyration, A-b-B copolymers are found to transfer the surface pattern into the bulk with high fidelity. A-alt-B copolymers, however, are able to replicate the surface pattern into the bulk material when heterogeneous domain sizes are much smaller.
We introduce a novel 'coloring' scheme to synthesize polystyrene-polybromostyrene (PS-co-PBrS) copolymers with statistically random (r-(PS-co-PBrS)) and random-blocky (b-(PS-co-PBrS)) monomer sequence distributions. Our results show that r-(PS-co-PBrS) and b-(PS-co-PBrS) copolymers with equivalent bromine content possess different intrinsic viscosities and radii of gyration. We attribute this behavior to the ability of b-(PS-co-PBrS) coils to form globular structures in toluene where PBrS forms a dense core and PS remains predominantly in a loose corona. This behavior is in contrast to that of r (PS co-PBrS) coils where both the PBrS and PS are homogeneously distributed. The interfacial behavior of the random and blocky copolymers is also found to differ. Specifically, thin films of r-(PS-co-PBrS) deposited on top of flat silica substrates covered with a semifluorinated self-assembled monolayer are found to dewet at a faster rate than b-(PS-co-PBrS) of comparable thickness at the same T−Tg, where Tg is the bulk glass transition temperature of the PS-co-PBrS copolymer. To our knowledge, this is the first experimental evidence that supports claims from computational studies arguing that the sequence distribution of random copolymers affects the chain's mobility on a surface.
Molecular insights into the 'coloring' reaction are provided by Monte Carlo simulations of the experimental reaction scheme. The probability of chemically altering expanded homopolymer coils is found to be equal for all units along the length of the chain. In contrast, 'coloring' of collapsed homopolymer coils reveals that the probability of modification is widely distributed. These results further support our claim that copolymers with random and random-blocky monomer sequence distributions can be synthesized by 'coloring' expanded and collapsed homopolymer coils, respectively.
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copolymer, monomer sequence distribution, polystyrene, polybromostyrene, pattern recognition, coloring reaction
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Degree
PhD
Discipline
Chemical Engineering
