HA (Hydrophobic/Amphiphilic) Copolymer Model:  Coil−Globule Transition versus Aggregation

Abstract

For simulating hydrophobic−amphiphilic (HA) copolymers, we have developed a “side-chain” HA model in which hydrophilic (P) interaction sites are attached to hydrophobic (H) main chain, thereby forming amphiphilic (A) monomer units, each with dualistic (hydrophobic/hydrophilic) properties. Using this coarse-grained model, we performed molecular dynamics simulations of the hydrophobically driven self-assembly in a selective solvent, for both single-chain and multichain systems. The focus is on the regime in which H and P interaction sites are strongly segregated. Single-chain simulations are performed for copolymers with the same HA composition but with different distribution of H and A monomer units along the hydrophobic backbone, including regular copolymers comprising H and A units in alternating sequence, (HA) x , regular multiblock copolymers (H L A L ) x composed of H and A blocks of equal lengths L = 3, and quasi-random proteinlike copolymers having quenched primary structure. In a solvent selectively poor for H sites, the proteinlike polyamphiphiles can readily adopt spherical-shaped compact conformations with the hydrophobic chain sections clustered at the globular core and the hydrophilic groups forming the envelope of this core and buffering it from solvent. Because of the fact that these globules are size- and shape-persistent objects, they maintain their morphological integrity even in rather concentrated solutions where no large-scale aggregation is observed. Moreover, we find that the population of aggregates generally decreases with worsening solvent quality. The compact conformations of long regular copolymers tend to be strongly elongated in one direction.

Small Angle Neutron Scattering Study of Conformation of Oligo(ethylene glycol)-Grafted Polystyrene in Dilute Solutions: Effect of the Backbone Length

“…This conclusion is not affected by the finite concentration effect because it tends to overestimate the dimensions of the polymers in D 2 O and underestimate the dimensions in toluene and methanol. Further collapse of this polymer in D 2 O is expected at higher temperatures, 2 which is consistent with the proposed multistep chain collapse during the coil-to-globule transiton, 27,33 whereas for DP ) 47 and 40, the resulted dimension of the polymers in D 2 O is comparable to or larger than that in toluene and methanol, which is due to finite interactions in the solutions and possible aggregation of a fraction of polymers in D 2 O due to strong end-group attractions. 27 As will be demonstrated below, the end-group interactions facilitate the formation of the polymer dimers in aqueous solutions of short polymer chains with DP ) 8.…”

“…Further collapse of this polymer in D 2 O is expected at higher temperatures, 2 which is consistent with the proposed multistep chain collapse during the coil-to-globule transiton, 27,33 whereas for DP ) 47 and 40, the resulted dimension of the polymers in D 2 O is comparable to or larger than that in toluene and methanol, which is due to finite interactions in the solutions and possible aggregation of a fraction of polymers in D 2 O due to strong end-group attractions. 27 As will be demonstrated below, the end-group interactions facilitate the formation of the polymer dimers in aqueous solutions of short polymer chains with DP ) 8. We note that there are some differences between the polydispersity of the cross-section radius of the polymers in toluene, methanol, and in D 2 O, which can be explained as being due to the relatively sharp interface between the polymer and the solvent in D 2 O 27 as opposed to a rather diffuse interface in toluene and methanol solutions.…”

“…Individual homopolymers such as PS are known to collapse into globules in poor solvents, and the globules tend to form clusters because of attractive interactions between polymer segments. 27,28 The absence of clusters in the aqueous solution of the PTrEGS (DP ) 85) is probably due to the availability of the water-soluble EG side chains, which stabilize the solubility of PTrEGS in D 2 O. 27 However, with the decrease in the DP, as will be discussed later, other factors start to dominate the clusterization.…”

“…27,28 The absence of clusters in the aqueous solution of the PTrEGS (DP ) 85) is probably due to the availability of the water-soluble EG side chains, which stabilize the solubility of PTrEGS in D 2 O. 27 However, with the decrease in the DP, as will be discussed later, other factors start to dominate the clusterization. The nearly zero A 2 suggests that the solubility of the polymers in methanol is between toluene and D 2 O.…”

Computer modeling of radical copolymerization under unusual conditions

“…The heteropolymer obtained as a result of such a simple one-step procedure can self-assemble into a segregated core-shell microstructure, thus resembling some of the basic properties of globular proteins, particularly their solubility in water. 21,22 Govorun et al 17 has shown that the corresponding chemical sequence is nonalternating and demonstrated the specific long-range correlations (LRCs), which can be described by the statistics of the Lévy-flight type. 23 These correlations are directly related to the CDSD scheme.…”