Abstract
The vertical depth distributions of individual additive components [cetyltrimethylammonium bromide (CTAB), deuterated pentaethylene glycol monododecyl ether (d25-C12E5), and deuterated glycerol (d-glycerol)] in PVA films have been isolated and explored by ion beam analysis techniques and neutron reflectometry. The additives display an unexpectedly rich variety of surface and interfacial behaviors in spin-cast films. In separate binary films with PVA, both d-glycerol and CTAB were evenly distributed, whereas d25-C12E5 showed clear evidence for surface and interfacial segregation. The behavior of each surfactant in PVA was reversed when the plasticizer (glycerol) was also incorporated into the films. With increasing plasticizer content, the surface activity of d25-C12E5 systematically decreased, but remarkably, when glycerol and CTAB were present in PVA, the surface and interfacial activities of CTAB increased dramatically in the presence of glycerol. Quantification of the surface excess by ion beam analysis revealed that, in many cases, the adsorbed quantity far exceeded what could reasonably be explained by a single layer, thus indicating a wetting transition of the small molecules at the surface or interface of the film. It appears that the surface and interfacial behaviors are partly driven by the relative surface energies of the components, but are also significantly augmented by the incompatibility of the components.
Highlights
Poly(vinyl alcohol) (PVA) is a semicrystalline synthetic polymer with excellent film-forming ability and optical transparency
The results are generally presented as composition versus depth profiles for individual components within PVA films that were derived from ion beam analysis and neutron reflectometry experiments
Note that ion beam analysis is sensitive to composition in terms of “atom” fraction, where molecules are represented by atoms of their average elemental composition. (The mass fractions of the components would be scaled by their relative densities.) The small discrepancy between the model fit and the experimental data at approximately 13.1 MeV indicates some roughness or variation in effective film thickness, which is not taken into account in the model composition profile
Summary
Poly(vinyl alcohol) (PVA) is a semicrystalline synthetic polymer with excellent film-forming ability and optical transparency. PVA is used primarily in food packaging, medical applications, and increasingly in the laundry industry because of its resistance to organic solvents, aqueous solubility, biodegradability, and low environmental impact. PVA cannot be synthesized directly from vinyl alcohol but is instead prepared by the hydrolysis of poly(vinyl acetate) (PVAc), so it is typically a copolymer of PVA with some residual PVAc. The degree of hydrolysis (DH) is a measure of the percentage of OC( O)CH3 acetate groups that have been converted to OH groups in the final polymer, and the number and positions of residual acetate groups largely determine the properties of the polymer, having the most significant effect on water solubility. The properties of pure PVA films are extensively customizable through alterations of these parameters, PVA alone is still generally too brittle and inflexible for many uses
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