The crystallization of a polyethylene with precise chlorine substitution on each and every 15th backbone carbon displays a drastic change in crystalline structure in a narrow interval of crystallization temperatures. The structural change occurs within one degree of undercooling and is accompanied by a sharp increase in melting temperature, a change in WAXD patterns, and a dramatic increase in TG conformers around the Cl substitution while the main CH2 sequence remains with the all-trans packing. These changes correlate with the formation of two different polymorphs characterized by a different packing and distribution of Cl atoms in the crystallites. Under fast crystallization kinetics, the chains assemble in an all-trans planar packing (form I) with a layered Cl distribution that presents some longitudinal disorder, while slower crystallization rates favor a more structured intermolecular halogen staggering consistent with a herringbone-like nonplanar structure (form II). The drastic change in morphology is enabled by the precise halogen placement in the chain and appears to be driven by the selection of the nucleus stem length in the initial stages of the crystallization. Exquisite kinetic control of the crystallization in novel polyolefins of this nature allows models for generating new materials based on nanostructures at the lamellar and sublamellar level not feasible in classical branched polyethylenes.

Chemical separations account for about 50% of costs and energy use associated with chemical and petrochemical manufacturing, corresponding to about 10% of all energy use in the U.S. Membrane separations are highly energy efficient, simple to operate, scalable, and portable. Broader use of membranes is limited by the selectivity of available membranes, mostly confined to the separation of species about an order of magnitude or more different in size in the liquid phase. This perspective focuses on new approaches for creating liquid filtration membranes that can perform more challenging separations. We first discuss the selectivity mechanisms of currently available membranes and compare them with the operation of biological systems that exhibit enhanced selectivity. Then, we review some approaches for creating isoporous membranes with narrow pore size distributions for enhanced size-based selectivity. We discuss biological systems that exhibit selectivity based on factors beyond size and how they can inspire the design of membranes capable of complex separations. After a review of approaches for creating membranes for separating similarly sized solutes, based on their charge, we discuss the development of membranes that can perform even more challenging separations, differentiating between solutes of similar size and charge based on other molecular criteria. This burgeoning area of research promises to transform chemical and pharmaceutical manufacturing if membranes with sufficient selectivity and permeability for realistic separations can be prepared using scalable manufacturing methods.

Fouling is likely the most important obstacle to the use of membranes in many applications, especially in those that the feed contains high concentrations of organics such as oil and biomacromolecules. Zwitterions, defined as molecules with equal numbers of positively and negatively charged functional groups, show excellent fouling resistance and hydrophilicity. These features can be incorporated into ultrafiltration (UF) membranes during their manufacture by blending a commodity polymer like polyvinylidene fluoride (PVDF) with a copolymer containing zwitterionic groups. This approach can be used directly in existing membrane production systems, with no need for post-processing. Research to date, however, does not provide any guidelines for designing or selecting a zwitterion-containing polymer for this purpose to achieve the best possible performance. In this work, we synthesized copolymers of methyl methacrylate (MMA), whose homopolymer is compatible with PVDF, with two different zwitterionic copolymers, sulfobetaine methacrylate (SBMA) and sulfobetaine-2-vinylpyridine (SB2VP). These copolymers were not previously investigated as surface segregating additives in membrane manufacture. We investigate the impact of different copolymer properties such as zwitterion chemistry, copolymer composition (i.e. zwitterionic/hydrophobic monomer ratio), and blend composition on the performance of membranes manufactured from their blends with PVDF. We report how changing these variables affect the morphology, selectivity, permeance and fouling resistance of membranes, and associate this data with design rules for selecting favorable copolymers. Our study showed that, in contrast to previous literature, increasing the hydrophilic/zwitterionic monomer amount in the additive copolymer does not always result in improved membrane performance. Instead, during membrane formation by non-solvent induced phase separation (NIPS), copolymer additives with high zwitterion content (51-52 wt%) undergo macrophase separation from PVDF, and the product membrane shows poor performance. On the other hand, with the appropriate copolymers that contain 18-19 wt% zwitterionic monomer, membranes with significantly higher permeance and remarkable fouling resistance can be attained even with very small amounts of additive copolymer. Zwitterionic additive contents as low as 5 wt% in PVDF can lead to membranes with doubled water flux (up to 99 L/m 2 .h.bar) and complete irreversible fouling resistance against oil suspensions and protein solutions. Only 10 wt% additive can yield membranes with even higher flux (up to 165 L/m 2 .h.bar), and complete resistance to irreversible fouling by an oil suspension in 24-hour dead-end fouling experiments. This degree of fouling resistance have not previously been reported for PVDF-based membranes, to our knowledge, and indicates the promise of this membrane modification approach for a wide range of applications.

This work explores functional, fundamental and applied aspects of naturally harvested spider silk fibers. Natural silk is a protein polymer where different amino acids control the physical properties of fibroin bundles, producing, for example, combinations of β-sheet (crystalline) and amorphous (helical) structural regions. This complexity presents opportunities for functional modification to obtain new types of material properties. Electrical conductivity is the starting point of this investigation, where the insulating nature of neat silk under ambient conditions is described first. Modification of the conductivity by humidity, exposure to polar solvents, iodine doping, pyrolization and deposition of a thin metallic film are explored next. The conductivity increases exponentially with relative humidity and/or solvent, whereas only an incremental increase occurs after iodine doping. In contrast, iodine doping, optimal at 70• C, has a strong effect on the morphology of silk bundles (increasing their size), on the process of pyrolization (suppressing mass loss rates) and on the resulting carbonized fiber structure (that becomes more robust against bending and strain). The effects of iodine doping and other functional parameters (vacuum and thin film coating) motivated an investigation with magic angle spinning nuclear magnetic resonance (MAS-NMR) to monitor doping-induced changes in the amino acid-protein backbone signature. MAS-NMR revealed a moderate effect of iodine on the helical and β-sheet structures, and a lesser effect of gold sputtering. The effects of iodine doping were further probed by Fourier transform infrared (FTIR) spectroscopy, revealing a partial transformation of β-sheet-to-amorphous constituency. A model is proposed, based on the findings from the MAS-NMR and FTIR, which involves iodine-induced changes in the silk fibroin bundle environment that can account for the altered physical properties. Finally, proof-of-concept applications of functionalized spider silk are presented for thermoelectric (Seebeck) effects and incandescence in iodine-doped pyrolized 1468-6996/11/055002+13$33.00 1 © 2011 National Institute for Materials Science Printed in the UK Sci. Technol. Adv. Mater. 12 (2011) 055002 E Steven et al silk fibers, and metallic conductivity and flexibility of micron-sized gold-sputtered silk fibers. Inthelattercase,wedemonstratetheapplicationofgold-sputteredneatspidersilktomake four-terminal, flexible, ohmic contacts to organic superconductor samples.

A porous material that is both hydrophobic and fouling-resistant is needed in many applications, such as water purification by membrane distillation. In this work, we take a novel approach to fabricating such membranes. Using the zwitterionic amphiphilic copolymer poly(trifluoroethyl methacrylate- random-sulfobetaine methacrylate), we electrospin nonwoven, porous membranes that combine high hydrophobicity with resistance to protein adsorption. By changing the electrospinning parameters and the solution composition, membranes can be prepared with a wide range of fiber morphologies including beaded, bead-free, wrinkly, and ribbonlike fibers, with diameters ranging between ∼150 nm and 1.5 μm. The addition of LiCl to the spinning solution not only helps control the fiber morphology but also increases the segregation of zwitterionic groups on the membrane surface. The resultant electrospun membranes are highly porous and very hydrophobic, yet resist the adsorption of proteins and retain a high contact angle (∼140°) even after exposure to a protein solution. This makes these materials promising candidates for the membrane distillation of contaminated wastewater streams and as self-cleaning materials.