Possible Way to Study Cononsolvency in Confinement: A Lattice Density Functional Theory Approach

Using molecular dynamics simulations, we study the response of a polymer brush exposed to co-nonsolvent (CNS), which acts as a preferential solvent for the polymer. We investigate a broad range of attractions between CNS and monomers and of grafting densities over the full range of cosolvent volume fractions. We compare our simulation results with the recently proposed adsorption−attraction model for co-nonsolvency in polymer brushes. The brush layer collapses with increasing CNS concentration into a more compact layer and followed by a reswelling toward sufficiently high CNS concentrations. As the strength of attraction of CNS toward the brush increases, the collapse transition becomes discontinuous. Increasing the grafting density leads to higher sensitivity with respect to CNS but also to a weaker collapse behavior as predicted from the analytical model. In the narrow collapse region, two states of the brush layer coexist, as can be expected from the type-II phase-transition behavior. The coexistence states are identified by analyzing the density profiles. Here, a dense layer is formed at the substrate, which is covered by a swollen layer. Both collapse and coexistence behaviors are even more pronounced in polydisperse brushes, indicating the robust nature of the co-nonsolvency effect in polymer brushes.

Section: Methodsmentioning

confidence: 99%

Co-Nonsolvency Response of a Polymer Brush: A Molecular Dynamics Study

Galuschko

Sommer

2019

“…Significant efforts have been devoted to explaining these phenomena, and currently, there are two main categories of opinions on the molecular driving forces. The first category regards the collapse of a single homopolymer chain or phase separation of homopolymer solutions to the preferential adsorption of one type of solvent over the other to the polymer backbone. ,− Specifically, Tanaka and co-workers attributed the co-nonsolvency effect of a PNIPAM chain in the mixed solvent of water and methanol to the competitive hydrogen bonding by water and methanol molecules onto the polymer chain. − This explanation was supported by the subsequent experiment by Wang et al The coarse-grained molecular dynamics study by Mukherji et al suggests that the co-nonsolvency effect is a generic phenomenon due to the preferential attraction of the better solvent to monomers. ,, By combining the preference attraction concept with the Alexander–de Gennes brush theory, , Sommer developed an adsorption–attraction mean-field model for polymer brushes and found that this preferential attraction can also lead to a swell–collapse–reswell transition in homopolymer brushes in mixtures of two good solvents . He further generalized this theory to homopolymer solutions and found that the preference attraction also results in a re-entrance phase separation in mixtures of binary good solvents .…”

Section: Introductionmentioning

confidence: 98%

Phase Separation of Homopolymers in Binary Mixed Solvents─The Co-Nonsolvency Effect

Zhang

2024

As one of the most intriguing phenomena in polymer solutions, the co-nonsolvency effect refers to the collapse of a homopolymer chain or phase separation of homopolymer solutions in mixtures of two good solvents. Despite decades of research, the molecular mechanism underlying the co-nonsolvency effect is still under debate. In this work, by using the ternary Flory− Huggins theory, we report the first complete mean-field phase behaviors of homopolymer solutions in binary mixtures of two miscible good solvents. We construct the binodal, spinodal, and critical points for several representative systems. Our calculations show that a homopolymer solution can separate into coexisting phases either when the quality difference between the two solvents is sufficiently large or in the presence of strong attractions between the two solvents. For both cases, the binodal and spinodal curves in the concentration plane of the polymer and the better solvent exhibit closed-loop features along with two critical points. However, the slope of the tie lines has the opposite sign in these two cases: it is positive in the former and negative in the latter. This suggests a convenient and decisive means to elucidate the physical mechanism of the co-nonsolvency effect. Moreover, we find that a phaseseparated solution may undergo a re-entrant phenomenon while increasing the attractive strength between the two solvents.

“…It refers to the collapse of a single chain/gel or phase separation of polymer solution in a good solvent A when adding a certain amount of a second good solvent B that is miscible with A ; with further addition of solvent B , the chain reswells gradually. Co-nonsolvency was observed in several polymeric systems, such as poly( N -isopropylacrylamide) in a water/methanol mixture ,,− and elastin-like polypeptide in a mixture of water and ethanol, and has many applications such as block copolymer self-assembly, , nanoparticle fabrication, and smart brush design. − Although co-nonsolvency has been known for several decades and many different opinions have been proposed to explain this phenomenon, ,,,,,− a consensus on the underlying microscopic mechanism is still lacking. Briefly, there are two main categories of opinions on the molecular driving forces.…”

Section: Introductionmentioning

confidence: 99%

Conformation Transition of a Homopolymer Chain in Binary Mixed Solvents

Zhang

Wang

2022

On the basis of a minimal lattice model, we apply the Wang−Landau Monte Carlo (MC) algorithm and a Flory-type mean-field theory to investigate the conformation of a homopolymer chain in a mixture of binary solvents A and B. This MC method enables us to accurately locate the conformation transitions and to determine their nature over a large parameter space. We find that in a good solvent A, adding a small amount of better solvent B causes a continuous collapse transition when the difference in the solvent quality is large. Increasing the fraction of solvent B results in a smooth reswelling of the chain. We explain these findings by the delicate interplay among the mixing entropy of binary solvents, the chain conformational entropy, and the competition in the interactions between the monomer and the two solvents.