Factors Responsible for the Aggregation of Poly(vinyl alcohol) in Aqueous Solution as Revealed by Molecular Dynamics Simulations

Abstract

Molecular dynamics simulations are used to study the structure and dynamics of poly­(vinyl alcohol) and water in aqueous solution as a function of concentration at different temperatures in the range of 278–338 K. Simulations were performed using multiple oligomeric chains for facilitating interchain interactions as well as a direct comparison with experimental data. PVA chains fold and bundle up to form an aggregate in solution. The intermolecular spatial distributions show the structure of aggregate to be ordered. PVA chains show a high tendency to form intrachain hydrogen bonds between adjacent repeating units, instead of interchain H-bonds, indicating hydrophobic effect as the major driving force for aggregate formation. At all temperatures, the conformations of a single PVA chain by itself in solution are unstable, going back and forth between extended and folded states. However, interchain interactions among PVA chains in the aggregate stabilize the folded conformation. An increase in temperature results in faster motions and an increase in concentration results in slower dynamics. At higher concentration, the chains adopt a single folded state independent of temperature so that there is an insignificant effect on R g. The competition between the formation of various hydrogen bonds such as intrachain, interchain, and PVA–water is the key to understand the solvation behavior of PVA. The activation energy for the conformational transition between the trans and gauche states of backbone dihedrals obtained from the simulations is 15.73 kJ/mol, which is close to the value of 13.4 kJ/mol obtained from experiments for 15 wt % PVA solution. The hydrophobic effect rather than interchain PVA hydrogen bonding is the major driving force for the aggregation of PVA in water.

Unidirectional Freezing of Polymer Solution Droplets

“…, where M w , N A , and <R g 2 > are the molecular weight, Avogadro's number, and the mean square radius of gyration of the PVA chains, respectively. Using an average molecular weight of 22,000 Da provided by the manufacturer, and values for the radius of gyration between 1.5 and 2.5 nm, 40 we find that theoretically C* ≈ 1.57 wt %, in close agreement with our experimental results (Figure 6c) and the literature. 39,41−44 To gain further insights into the effect of polymer concentration on the tip formation, we analyzed the micrographs of each frozen water−polymer mixture droplet and extracted the tip angles α, as shown in Figure 6a.…”

“…We calculated and measured C * = 3 M w 4 π N A < R g 2 > 3 / 2 , where M w , N A , and < R g 2 > are the molecular weight, Avogadro’s number, and the mean square radius of gyration of the PVA chains, respectively. Using an average molecular weight of 22,000 Da provided by the manufacturer, and values for the radius of gyration between 1.5 and 2.5 nm, we find that theoretically C * ≈ 1.57 wt %, in close agreement with our experimental results (Figure c) and the literature. ,− …”

A Systematic Approach to the Development of Cilostazol Nanosuspension by Liquid Antisolvent Precipitation (LASP) and Its Combination with Ultrasound

“…In the current study, the advantage of PVA (9-10 kDa, 80% hydrolyzed) over other tested stabilizers is evident. A likely explanation for this affinity to cilostazol is favorable hydrogen bonding, since CIL molecule has 5 acceptor sites and 1 donor site, and most hydroxyl groups in PVA are donors [35,36]. On the other hand, other tested stabilizers either have only acceptor sites (PVP, PX, SDS, DOSS) or potential for both accepting and donating hydrogen atoms (HPMC, HPC) [37].…”

Structural and Transport Properties of Norbornene-Functionalized Poly(vinyl alcohol) “Click” Hydrogel: A Molecular Dynamics Study

“…The relationship of H–bond evolution and stability with temperature can be adequately mapped through molecular trajectories. – Intramolecular structural distribution leads to the aggregation behavior of the PVA gel network. The temperature dependency of such behavior is reported in the literature . The MD simulation investigates the composition of nonfreezing water, free water, and freezing water within an irradiation cross–linked PVA hydrogel.…”

“…The temperature dependency of such behavior is reported in the literature. 30 The MD simulation investigates the composition of nonfreezing water, free water, and freezing water within an irradiation cross− linked PVA hydrogel. This study explores the dependencies between these water components and the cross−linking process of the hydrogel.…”