Cryostructuring of polymer systems. XXX. Poly(vinyl alcohol)‐based composite cryogels filled with small disperse oil droplets: A gel system capable of mechanically induced releasing of the lipophilic constituents

Abstract

Composite heterophase poly(vinyl alcohol) (PVA) cryogels containing entrapped small droplets of Vaseline oil have been prepared and studied. Such oilfilled cryogels were formed via freeze-thaw treatment of freshly prepared oil-in-water emulsions containing varied volume fraction of lipophilic phase, and the influence of the amount of this phase, as well as the effects of freezing conditions on the physicomechanical (shear moduli) and thermal (gel fusion temperature and fusion enthalpy) characteristics of resulting composites have been explored. It was shown that over certain range of PVA concentrations in aqueous phase and a range of volume fraction of the hydrophobic phase its microdroplets performed as ''active'' fillers causing an increase in both the gel strength and the heat endurance of composites. The light microscopy data on the morphological features of such filled PVA cryogels revealed the effect of diminution in size of oil droplets entrapped in the gel matrix as compared with the initial emulsions. This effect can be explained by the disintegrating action of crushing and shear stresses arising upon the system freezing and growth of ice crystals. The oil-filled PVA cryogels were found to be capable of gradually releasing the lipophilic constituents (the Rose hips oil, in this case) in response to the cyclic mechanical compression.

Study of cryostructuring of polymer systems. 32. Morphology and physicochemical properties of composite poly(vinyl alcohol) cryogels filled with hydrophobic liquid microdroplets

“…As was shown previously [35], emulsification of Vaseline oil, i.e., a mixture of hydrocarbons, in an aqueous solution of highly deacylated PVA caused polymer coagulation at a certain critical content of the hydrophobic component, which depended on PVA concentration in the dispersion medium. In other words, a relatively stable emulsion could only be pre pared, provided that the volume fraction ϕ of the oil did not exceed some limiting value, above which the phase separation took place in the system.…”

“…The fusion temperatures T f of PVACGs were mea sured using the usual procedure [25,27,35,40]. For this purpose, a sealed PE test tube filled with a cryogel, the lower part of which contained the metal ball, was placed upside down into a water bath equipped with a stirrer.…”

“…For example, it was found that the influ ence of low molecular mass electrolytes is mainly related to the lyotropic properties of the latter [16][17][18][19][20], while, in the presence of polymer additives, phase separation processes take place due to the thermody namic incompatibility of macromolecules with differ ent chemical structures in a common solvent [6,7,21,22]. When any dispersed fillers (solid particles [23][24][25][26], viscoelastic gels [25,27,28], microorganism cells [8,23,29,30], gas bubbles [31][32][33], microdroplets of liquids immiscible with PVA solutions [34,35], or nanoparticles [36][37][38][39]) are incorporated into an initial PVA solution, subsequent cryogenic treatment of such suspensions, foams, or emulsions results in the forma tion of composite cryogels, the physical characteristics and structures of which are governed by a large number of factors. The nature and aggregate state of a filler, the size and concentration of particles in a composite, adhesiveness of the surface of a dispersed filler and its permeability for macromolecules of the gel forming compound (PVA), the presence or absence of charges on particle surface, etc., are among these factors.…”

“…Therefore, the preparation of uniform suspen sions encounters technical difficulties and their cryo genic treatment yields PVACGs in which filler parti cles are nonuniformly distributed [25]. In this respect, hydrophobic liquids, such as various oils, are more "manufacturable," because rather stable emulsions can be prepared from them and aqueous PVA solu tions, and freezing-thawing of these emulsions gives rise to the formation of oil filled PVACGs with rather uniform distributions of the dispersed phases (oil microdroplets) in the continuous phase of porous hydrogels [34,35]. It is of interest that the incorpora tion of a dispersion of Vaseline oil as a hydrophobic filler in amounts of 20-50 vol % (depending on PVA concentration in the continuous phase) into PVACG matrices yielded composites with higher rigidity and heat endurance than those of unfilled cryogels with the same polymer concentration [35].…”

“…In this respect, hydrophobic liquids, such as various oils, are more "manufacturable," because rather stable emulsions can be prepared from them and aqueous PVA solu tions, and freezing-thawing of these emulsions gives rise to the formation of oil filled PVACGs with rather uniform distributions of the dispersed phases (oil microdroplets) in the continuous phase of porous hydrogels [34,35]. It is of interest that the incorpora tion of a dispersion of Vaseline oil as a hydrophobic filler in amounts of 20-50 vol % (depending on PVA concentration in the continuous phase) into PVACG matrices yielded composites with higher rigidity and heat endurance than those of unfilled cryogels with the same polymer concentration [35]. Moreover, the freezing-thawing of an initial oil emulsion in an aque ous PVA solution led to a reduction in oil microdroplet sizes, thus increasing the total surface area of the inter face between the hydrophilic and hydrophobic phases.…”