Synthesis,characterization, and swelling responses of poly(N‐isopropylacrylamide)‐ and hydroxypropyl cellulose‐based environmentally sensitive biphasic hydrogels

Thermosensitive networks based on hydroxypropyl cellulose and N‐isopropylacrylamide crosslinked with N,N‐methylene bisacrylamide were synthesized by a simultaneous gamma radiation technique. The network yield was optimized by the variation of reaction parameters such as the total radiation dose, concentration of crosslinker and monomer, and amount of water. The hydrogels had a biphasic structure and good mechanical strength, even in the fully swollen state, and could be synthesized in any shape and size. Volume transitions as a function of time and temperature were studied for these hydrogels in water, and the effects on swelling in different media such as 0.5N NaOH, 0.5N HCl, and 5% NaCl at the optimum time and temperature were also studied. The response of the hydrogels to these diverse changes in the swelling media was observed, and the volume transitions due to environmental changes in the hydrogels were not sharp and discontinuous as a maximum volume collapse occurred at a temperature higher than the reported lower critical solution temperature of 32.5°C for N‐isopropylacrylamide. These hydrogels were environmentally sensitive and responded to changes in their thermal and ionic environment and have potential applications in diverse fields such as drug delivery, enzyme technology, and environmental management. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 479–488, 2004     

Synthesis and characterization of graft copolymers of hydroxypropyl cellulose with acrylamide and some comonomers

To obtain new polymeric materials for environmental management, we used pine needles from the huge forest of the Western Himalayas as a source of cellulose. Cellulose was derivatized to hydroxypropyl cellulose (HPC), a useful water‐soluble cellulose ether. HPC was graft‐copolymerized with acrylamide (AAm) with benzoyl peroxide as the initiator. At optimum grafting conditions, five different concentrations of the comonomers glycidyl methacrylate, acrylic acid, 2‐hydroxyethyl methacrylate, and acrylonitrile were grafted with AAm. Networks of HPC and AAm were also synthesized by crosslinking reactions with glutaraldehyde as a crosslinker over a range of four different concentrations of crosslinker under acidic conditions. Crosslinked networks of HPC with AAm and a comonomer at one comonomer concentration were also synthesized. Graft copolymers were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, and swelling behavior. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 545–555, 2004     

A new interpretation of interferometric fringe patterns

The interferometric method of Nisida and Saito, the absolute retardation method of Post and the double-exposure holo-interferometric method of Hovanesian, Brcic and Powell are correlated and shown to give identical fringe patterns as regards intensity. Various discrepancies in Nisida and Saito's interpretation of their fringe patterns are pointed out, and the simultaneous presence and relative predominance of isochromatic and isopachic half-tones are discussed. The principles of photography and the concept of half-tones are used to explain the formation of concomitant isochromatics and isopachics and to give a new interpretation to these fringe patterns.     

An energy conservation approach to adsorbate-induced surface stress and the extraction of binding energy using nanomechanics

Surface stress induced by molecular adsorption in three different binding processes has been studied experimentally using a microcantilever sensor. A comprehensive free-energy analysis based on an energy conservation approach is proposed to explain the experimental observations. We show that when guest molecules bind to atoms/molecules on a microcantilever surface, the released binding energy is retained in the host surface, leading to a metastable state where the excess energy on the surface is manifested as an increase in surface stress leading to the bending of the microcantilever. The released binding energy appears to be almost exclusively channeled to the surface energy, and energy distribution to other channels, including heat, appears to be inactive for this micromechanical system. When this excess surface energy is released, the microcantilever relaxes back to the original state, and the relaxation time depends on the particular binding process involved. Such vapor phase experiments were conducted for three binding processes: physisorption, hydrogen bonding, and chemisorption. Binding energies for these three processes were also estimated.     

Profiling the near field of a plasmonic nanoparticle with Raman-based molecular rulers

The enhanced local optical fields at the surface of illuminated metallic nanoparticles and nanostructures are of intense fundamental and technological interest. Here we report a self-consistent measurement of the spatial extent of the fringing field above a plasmonic nanoparticle surface. Bifunctional DNA-based adsorbate molecules are used as nanoscale optical rulers, providing two distinct surface enhanced Raman scattering signals that vary independently in intensity as a function of distance from the nanoparticle surface. While the measurement technique is calibrated on gold nanoshell surfaces with controlled and predictable electromagnetic nanoenvironments, this approach is broadly adaptable to a wide range of plasmonic geometries.     

Influence of carbon nanotube dispersion on the mechanical properties of phenolic resin composites

Despite the much touted mechanical properties of carbon nanotubes, composites reinforced with nanotubes have failed to achieve mechanical properties which rival those present in conventional fiber reinforced polymer composites. This article describes an attempt to bridge this gap. Multi‐walled carbon nanotubes (MWCNT) were synthesized using a chemical vapor deposition method and were dispersed in phenolic resin by both the wet and dry dispersion techniques before molding into composite bars (50 × 5 × 3 mm³). Although no improvement in the mechanical properties of the MWCNT/phenolic composites was observed over the neat resin value when wet mixing dispersion was employed, an improvement of nearly 158% (160 MPa as compared with 62 MPa for neat resin) was achieved in 5 vol% MWCNT containing phenolic resin prepared by the dry mixing. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

Imprinting localized plasmons for enhanced solar cells

Imprinted silver nanovoid arrays are investigated via angle-resolved reflectometry to demonstrate their suitability for plasmonic light trapping. Both wavelength-?and subwavelength-scale nanovoids are imprinted into standard solar cell architectures to achieve nanostructured metallic electrodes which provide enhanced absorption for improving solar cell performance. The technique is versatile, low-cost and scalable and can be applied to a wide range of organic semiconductors. Absorption features which are independent of incident polarization and weakly dependent on incident angle reveal localized plasmonic modes at the structured interface. Metallic nanostructure-PCPDTBT:PCBM samples demonstrate absorption enhancements of up to 40%. The structured interface provides light trapping, which boosts absorption at wavelengths where the semiconductors absorb poorly.     

Random preferences towards bioenergy environmental externalities: A case study of woody biomass based electricity in the Southern United States

This paper contrasts alternate methodological approaches of investigating public preferences, the random parameter logit (RPL) where tastes and preferences of respondents are assumed to be heterogeneous and the conditional logit (CL) approach where tastes and preferences remain fixed for individuals. We conducted a choice experiment to assess preferences for woody biomass based electricity in Arkansas, Florida, and Virginia. Reduction of CO₂ emissions and improvement of forest habitat by decreasing risk of wildfires and pest outbreaks were presented to respondents as attributes of using green electricity. The results indicate that heterogeneous preferences might be a better fit for assessing preferences for green electricity. All levels of both attributes were positive contributors to welfare but they were no statistically significant. Respondents expressed a positive mean marginal willingness to pay (WTP) for each attribute level. The total WTP for green electricity per kilowatt hour was $0.049 kWh or $40.5 per capita year^{− 1} when converted into future total annual expenditures.     

High-efficiency ordered silicon nano-conical-frustum array solar cells by self-powered parallel electron lithography

Nanostructured silicon thin film solar cells are promising, due to the strongly enhanced light trapping, high carrier collection efficiency, and potential low cost. Ordered nanostructure arrays, with large-area controllable spacing, orientation, and size, are critical for reliable light-trapping and high-efficiency solar cells. Available top-down lithography approaches to fabricate large-area ordered nanostructure arrays are challenging due to the requirement of both high lithography resolution and high throughput. Here, a novel ordered silicon nano-conical-frustum array structure, exhibiting an impressive absorbance of 99% (upper bound) over wavelengths 400-1100 nm by a thickness of only 5 μm, is realized by our recently reported technique self-powered parallel electron lithography that has high-throughput and sub-35-nm high resolution. Moreover, high-efficiency (up to 10.8%) solar cells are demonstrated, using these ordered ultrathin silicon nano-conical-frustum arrays. These related fabrication techniques can also be transferred to low-cost substrate solar energy harvesting device applications.     

Nanoparticle surface as activation site

The immense surface-to-volume (S/V) ratio in nanoparticles leads to large surface energy density. These high densities play the role of sites for activities that are not triggered in bulk materials. Here we present some examples of such distinctive activities taking place at nanoparticle surfaces. Our first example involves the morphological changes in silkworm (Bombyx mori L.) nuclear polyhedrosis virus (BmNPV) brought about by lipophilic amorphous silica nanoparticles (LASN). Microscopy studies show that nanoparticles severely alter the structure of the virus envelope by a 'deflation' of the viral polyhedron and formation of elongated structures. The second example shows the spatial variation in aggregation potential with temperature, for dodecanethiol-capped Au nanoparticles on an amorphous polystyrene film surface. We find that on increasing the temperature from 32 degrees C to 50 degrees C the aggregating potential becomes almost completely confined to the film surface, whereas going over to 100 degrees C the confining potential is overcome and out-of-plane growth takes place. A tentative and qualitative explanation has been attempted.