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Electrospinning has been widely used to prepare polymerfiberswith diameters ranging from a few nanometers to micrometers. Whilemost studies focus on controlling the sizes and morphologies of electrospunpolymer fibers by changing electrospinning conditions, the effectof post-treatments such as thermal annealing on the properties ofelectrospun polymer fibers has been less studied. Here, we investigatethe effect of thermal annealing on the morphology changes of electrospunpolystyrene (PS) fibers on substrates. Different from annealing thefibers in a uniform environment, annealing the fibers on substratesresults in a substrate-dependent morphology transformation. When theelectrospun PS fibers are annealed on a glass substrate, wetting ofthe fibers on the glass substrate occurs. When the electrospun PSfibers are annealed on a poly(methyl methacrylate) (PMMA)-coated substrate,a Rayleigh-instability-driven morphology transformation is observed.The polymer fibers transform into hemispherical polymer particlescaused by the lower surface tension of PS than that of PMMA and theinterfacial tension between PS and PMMA. This transformation processis influenced by the annealing time and temperature. The characteristictime of the transformation process is shorter when the sample is annealedat a higher temperature because of the lower polymer viscosity. Thesize of the polymer particles fits well with the theoretical prediction,which is dependent on the initial fiber diameter and is independentof the annealing temperature. [ABSTRACT FROM AUTHOR]

Large, characteristically tear- or finger-shaped voids found in polymer hollow fiber membranes, termed macrovoids, have been known since the early development of asymmetric membranes. These voids are undesirable as they decrease the mechanical integrity of the hollow fiber membrane, limiting the use of high pressure feeds for separations. Moreover, if such macrovoids penetrate the selective layer they become and even more serious problem for membrane performance. With current membrane technology moving toward a mixed-matrix hybrid format with inorganic and carbon molecular sieves embedded in a polymer matrix, additional complications caused by the presence of particulates within the spinning dope toward the formation of macrovoids must be considered. While numerous hypotheses have been suggested regarding the formation of macrovoids formed in polymer-only membranes, no mention so far has been made of such macrovoids initiated in hollow fiber membranes by particles present in the spinning dope. This paper provides evidence for the presence of such macrovoids for such a spinning dope and presents a hypothesis for their formation. This hypothesis is presented in terms of a similar basic mechanism for macrovoid formation in mixed-matrix membranes that benefits from an understanding of macrovoid formation in conventional polymer-only asymmetric membranes. Moreover, decreasing the sieve particle size tends to suppress macrovoid formation tendency, which is consistent with the proposed mechanism. [ABSTRACT FROM AUTHOR]

The article describes the development of nitric oxide (NO)-releasing diazeniumdiolated derivatives of poly(acrylonitrile) (PAN) and a variety of related materials that can form the basis of NO-releasing textiles, rubbers and plastics. According to the authors, the polymers are stable at room temperature and represent a broad class of commercially viable materials to be developed as NO donors. The authors add that PAN is easily spun into fibers for fabric production and can form graft copolymers.