Computer Simulation of Protein-like Copolymers (PLCs).

摘要

This thesis describes a computational investigation, using discontinuous molecular dynamics and kinetic Monte Carlo simulation, aimed at the development of protein-like copolymers (PLCs) as compatibilizing agents for polymer blends, and as "drug" delivery agents.;Protein-like copolymers (PLCs) represent a new type of functional copolymer, that exhibit large-scale compositional heterogeneities and long-range statistical correlations along the co-monomer sequence. The concept of PLCs was first introduced by Khokhlov and coworkers who demonstrated using computer simulations that random copolymers with tunable monomer sequences (RCPs) could be generated by adjusting the compactness of a parent homopolymer composed of component A, and then converting exposed segments on the exterior of the coil into B segments by reacting them with specific chemical species in the surrounding solution.;We employed two computer simulation methods to investigate the role of PLCs as interfacial compatibilizers for a polymer blend containing two immiscible homopolymers. We used large-scale equilibrium discontinuous molecular dynamics (DMD) simulations to explore the effect of compatibilizer co-monomer sequence distribution on miscibility and interfacial characteristics of two incompatible homopolymers. Kinetic Monte Carlo (MC) simulations were used to study the phase separation dynamics of immiscible polymer blends compatibilized by PLCs. The effectiveness of PLCs to act as compatibilizers was compared with those of diblock, simple linear gradient, random, and alternating copolymers. The simulations indicated that for the chain lengths considered PLCs were better compatibilizers than alternating and random copolymers, acted at par with simple linear gradient copolymers, but were not as good as diblocks.;We employed kinetic Monte Carlo to investigate how adding ≈4.92% PLCs composed of segments of type C and D to an immiscible asymmetric A/B binary polymer blend containing 80% homopolymers of type A and 20% homopolymers of type B affected the phase separation dynamics. The ability of PLCs to slow down the phase separation process depended sensitively on the interaction energy between the PLCs and homopolymers, the PLC chain length, and the PLC chemical composition. PLCs compatibilized the binary blend more effectively as the attractive interaction between the PLC blocks and homopolymers increased. Nominal improvement in compatibilization of the binary blend was achieved with increasing PLC chain length. For a given interaction energy and chain length C/D PLCs with composition around 0.3 ≤ xC ≤ 0.5 (where xC is the mole fraction of the C component in the PLC) acted as the most effective compatibilizers. The growth of phase-separated domains followed a dynamical scaling law for both the A/B binary blend and PLC compatibilized A/C-D/B ternary blend in the late stages of phase separation. The universal scaling functions were nearly independent of the interaction energy, PLC chain length, and PLC chemical composition. The phase-separated domains grew with dynamical self-similarity irrespective of the type of PLC added to the binary blend, although the type of PLC significantly altered the growth rate of the phase-separated domains.;We also used kinetic Monte Carlo simulation to explore the role of PLCs as "drug" carriers. We examined the assembly of PLCs and delineated the conditions conducive to drug encapsulation. The effects of changes in the system volume fraction, PLC composition, and the strength and range of the interaction between the hydrophobic component of the PLC and the drug on the encapsulation efficiency was explored by performing cluster analysis and evaluating the density profiles of the PLC copolymer segments and those of the drugs. The presence of drug facilitated easier coil-to-globule transition for the PLCs. The interaction strength between the hydrophobic component of the PLC and the drug acted as a coupling parameter that determined whether the system encapsulated or whether the PLCs and drugs aggregated separately.