Abstract
The blooming of sodium dodecyl sulfate (SDS) and the influence of plasticizer (glycerol) on the surfactant distribution in poly(vinyl alcohol) (PVA) films have been explored by neutron reflectometry (NR) and ion beam analysis techniques. When in binary films with PVA, deuterated SDS (d25-SDS) forms a surface excess corresponding to a wetting layer of the surfactant molecules at the film surface. The magnitude of this surface excess increased significantly in the presence of the plasticizer, and the surfactant was largely excluded from the PVA subphase. NR revealed smectic nanostructures for both SDS and glycerol components within this surface excess in plasticized films. This combined layer comprises surfactant lamellae, separated by interstitial glycerol-rich layers, which is only formed in the plasticized films and persists throughout the surface excess. Atomic force microscopy micrographs of the film surfaces revealed platelike structures in the plasticized PVA, which were consistent with the rigid defects in the surfactant-rich lamellae. The formation of these structures arises from the synergistic surface segregation of SDS and glycerol, evidenced by surface tensiometry. Cloud point analysis of bulk samples indicates a transition at ∼55% water content, below which phase separation occurs in ternary films. This transition is likely to be necessary to form the thick wetting layer observed and therefore indicates that film components remain mobile beyond this point in the drying process.
Highlights
Poly(vinyl alcohol) (PVA) is a semicrystalline synthetic polymer that has been widely exploited for its ability to form high-quality optically transparent films
The results are presented as composition versus depth profiles for individual components within PVA films
Using neutron reflectometry (NR) and Ion beam analysis (IBA), we have previously established that glycerol exhibits a homogeneous vertical composition profile in PVA,[6] which is consistent with previous reports indicating its low surface activity.[3]
Summary
Poly(vinyl alcohol) (PVA) is a semicrystalline synthetic polymer that has been widely exploited for its ability to form high-quality optically transparent films. Characteristics such as degree of hydrolysis (DH) and degree of polymerization must be carefully controlled to provide optimal physical properties such as strength and solubility of PVA films.[1,2] For many industrial applications, pure PVA films are too brittle and inflexible; plasticizers are introduced into the system. Segregation of surfactants has been shown to be very important in the formation and physical properties of latexes.[4,5] Our inspiration to study the surfactant behavior in these films derives from the number of applications in which plasticized PVA films encounter surfactant-rich environments. The presence and migration of surfactants have potential to interfere with processes such as aging in polymer films, which, in turn, may have a profound impact on product performance and lifetime
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