Tear force of physically crosslinked poly(vinyl alcohol) gels with different submicrometer‐scale network structures

Abstract

Poly(vinyl alcohol) (PVA) gels can be easily prepared by either the freeze‐thawing (FT gel) method or by the cast‐drying (CD gel) method. Although the resulting nanostructured networks of the FT and CD gels are similar, their physical properties are quite different; while CD gels are transparent and elastic, FT gels are opaque and less elastic. Moreover, the tear energy of the FT gels is much greater than that of the CD gels, which is a direct result of micrometer‐scale differences in their network structures. In order to control the distribution of microcrystallites on nano‐ and micrometer scales, FT gels were prepared from PVA solutions with different water contents. As a result, the gel gradually became more transparent as the initial water content was decreased; and accordingly, the tear energy decreased. Tear resistance was improved in the case of FT gels by repeating the number of FT cycles, whereas with CD gels, this was achieved by increasing the gelation temperature. These results indicate that the microscopic network structures are characterized by a micrometer‐scale bundled‐polymer (fibril), which determines the tear energy of FT gels. Simple methods to control the fibril network structure of FT gels using a unidirectional freezing method are presented herein, with the swelling and mechanical properties of modified FT gels discussed in terms of their multiple‐scale network structures. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41356.

Morphology, flammability, and properties of graphite reinforced polymer composites. Systematic review

“…Graphene nanoplatelet is defined as a flat monolayer of carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice. Thermally conductive fillers used with graphite (in this case, graphite nanoplatelets (GNP) and expanded graphite to form synergistic composites include carbon nanotubes, silicon carbide microparticles, and carbon fiber (CF) [67,68,79,80]. The development of synergistic polymer composites between CNTs and graphene has been conducted because of their higher thermal conductivity of 3000-6000 and 100-400 W/m.K, respectively.…”

“…The development of synergistic polymer composites between CNTs and graphene has been conducted because of their higher thermal conductivity of 3000-6000 and 100-400 W/m.K, respectively. It was reported by Yu et al [80] and Pak et al [81] that incorporating a small amount of carbon nanotubes (CNTs) with a high aspect ratio induced a synergistic enhancement in the thermal conductivity of composites by forming an effective 3D thermally conductive network. For instance, some researchers reported that the thermal conductivity of polycarbonate (PC) composite filled with 9.9 wt% expanded graphite and 0.1 wt% multi-walled carbon nanotubes (MWCNT) fillers was synergistically improved 49% when compared with pristine PC composite filled with 10 wt% EG alone [79].…”

“…Expanded graphite possessing a high aspect ratio and an excellent electrical conductivity is normally produced by exfoliating GICs through rapid heating in a furnace. In most studies [80][81][82], expanded graphite and graphite nanoplatelets are mostly employed. Graphite continues to attract considerable attention because of its outstanding electrical properties as well as higher aspect ratio, which makes it ideal filler for developing functional and structural graphene-reinforced composites.…”

“…Graphite continues to attract considerable attention because of its outstanding electrical properties as well as higher aspect ratio, which makes it ideal filler for developing functional and structural graphene-reinforced composites. The significant improvement in electrical conductivity in the graphite content was observed for most composites, and it was attributed to the percolation transition of the conductive network formation [20,[80][81][82][83][84]. The addition of conductive particles to insulating polymers can result in an electrically conductive composite.…”

Low-cost and cleanroom-free fabrication of microneedles

“…PVA is a water-soluble polymer with wide practical application in a variety of fields, including wound dressing, implants, cell encapsulation, drug-delivery systems, soft contact lenses and dental applications, due to its excellent chemical resistance and physical properties, low toxicity and high biocompatibility [14][15][16] . An aqueous solution of PVA can form hydrogels through cast-drying process, where PVA physically cross links during the dehydration process 17 . In summary, the proposed approach facilitates facile, ondemand and cleanroom-free fabrication of microneedles with a desired geometry.…”