@article{stevens_skau_downen_roman_clarke_2011, title={Finite-size effects in nanocomposite thin films and fibers}, volume={84}, ISSN={["1550-2376"]}, DOI={10.1103/physreve.84.021126}, abstractNote={Monte Carlo simulations of finite-size effects for continuum percolation in three-dimensional, rectangular sample spaces filled with spherical particles were performed. For samples with any dimension less than 10--20 times the particle diameter, finite-size effects were observed. For thin films in the finite-size regime, percolation across the thin direction of the film gave critical volume fraction (${p}_{c}$) values that differed from those along the plane of the film. Simulations perpendicular to the film for very thin samples resulted in ${p}_{c}$ values lower than the classical limit of \ensuremath{\sim}29% (for spheres in a three-dimensional matrix) which increased with film thickness. For percolation along thin films, while holding film thickness constant, ${p}_{c}$ increased with increasing sample size, which is a modification of the finite-sized scaling effect for cubic samples. For samples with a large aspect ratio (fibers) and a finite-sized cross-sectional area, the critical volume fraction increased with sample length, as the sample became quasi-one-dimensional. The results are discussed in the context of adding volume along or perpendicular to the percolation direction. From an experimental perspective, these findings indicate that sample shape, as well as relative size, influences percolation in the finite-size regime.}, number={2}, journal={PHYSICAL REVIEW E}, author={Stevens, D. R. and Skau, E. W. and Downen, L. N. and Roman, M. P. and Clarke, L. I.}, year={2011}, month={Aug} }
 @article{crowe-willoughby_stevens_genzer_clarke_2010, title={Investigating the Molecular Origins of Responsiveness in Functional Silicone Elastomer Networks}, volume={43}, ISSN={["1520-5835"]}, DOI={10.1021/ma100470w}, abstractNote={Dielectric, calorimetric, and dynamic mechanical measurements were performed to delineate the types and dynamic rates of molecular scale motion in modified poly(vinylmethyl siloxane) (PVMS) stimuli-responsive networks, where pendent groups of the form −S−(CH2)n−OH were chemically attached to the vinyl moiety of PVMS. The glass transition temperature (Tg) for the unsubstituted PVMS network matches that previously reported for linear PVMS indicating that the flexibility of the polymer chains is unaffected by the network cross-linking. In contrast, Tg increases with the introduction of pendent groups of the type −S−(CH2)n−CH3 or −S−(CH2)n−OH, where n is 2, 6, or 11, as the different groups constrain the siloxane backbone to differing degrees. The macroscopic response time and amplitude, as previously measured by dynamic contact angle, are correlated with the observed glass transition temperatures. One conclusion is that the flexibility of the network and the interactions between pendent groups affect responsiveness.}, number={11}, journal={MACROMOLECULES}, author={Crowe-Willoughby, Julie A. and Stevens, Derrick R. and Genzer, Jan and Clarke, Laura I.}, year={2010}, month={Jun}, pages={5043–5051} }
 @article{ojha_stevens_stano_hoffman_clarke_gorga_2008, title={Characterization of electrical and mechanical properties for coaxial nanofibers with poly(ethylene oxide) (PEO) core and multiwalled carbon nanotube/PEO sheath}, volume={41}, ISSN={["1520-5835"]}, DOI={10.1021/ma702634a}, abstractNote={The present work focuses on the electrical and mechanical characterization of nanocomposite fibers having core−sheath (or bicomponent) morphologies. Owing to their unique mechanical and electrical properties, multiwalled carbon nanotubes (MWNTs) have been utilized in the nanocomposite construction. Submicron diameter nanofibers (200–300 nm) with core−sheath morphology were fabricated from a polymer/MWNT solution and collected in random mats. By constraining the MWNTs to the sheath, significant increases in the mechanical properties were observed at lower MWNT concentrations when compared to mats made from single-layer fibers. The electrical properties of the core−sheath mats showed similar gains, having a critical weight percent more than 10 times lower than that of the single-layer mats.}, number={7}, journal={MACROMOLECULES}, author={Ojha, Satyajeet S. and Stevens, Derrick R. and Stano, Kelly and Hoffman, Torissa and Clarke, Laura I. and Gorga, Russell E.}, year={2008}, month={Apr}, pages={2509–2513} }
 @article{scott_stevens_bochinski_clarke_2008, title={Dynamics within Alkylsiloxane Self-Assembled Monolayers Studied by Sensitive Dielectric Spectroscopy}, volume={2}, ISSN={["1936-0851"]}, DOI={10.1021/nn800543j}, abstractNote={Self-assembled monolayers are a ubiquitous laboratory tool and have been the subject of many experimental investigations which have primarily focused on static properties of full coverage monolayers, with the maximum density and ordering possible. In this work, dynamics within low density, planar siloxane self-assembled monolayers are studied utilizing highly sensitive dielectric spectroscopy. Dilute, disordered films were intentionally fabricated in order to study the widest range of possible motions. At low coverage, an interacting relaxation is observed, which has similar dynamics to polyethylene-like glass transitions observed in phase-segregated side-chain polymers, despite the rigidity of the substrate and the constraint of ethyl groups in relatively short chains. As density is increased, a second local relaxation, previously observed in three-dimensional SAMs and associated with rotation within a small segment of the alkyl chain, is also observed.}, number={11}, journal={ACS NANO}, author={Scott, Mary C. and Stevens, Derrick R. and Bochinski, Jason R. and Clarke, Laura I.}, year={2008}, month={Nov}, pages={2392–2400} }
 @article{ojha_stevens_hoffman_stano_klossner_scott_krause_clarke_gorga_2008, title={Fabrication and characterization of electrospun chitosan nanofibers formed via templating with polyethylene oxide}, volume={9}, ISSN={["1526-4602"]}, DOI={10.1021/bm800551q}, abstractNote={Chitosan is an abundantly common, naturally occurring, polysaccharide biopolymer. Its biocompatible, biodegradable, and antimicrobial properties have led to significant research toward biological applications such as drug delivery, artificial tissue scaffolds for functional tissue engineering, and wound-healing dressings. For applications such as tissue scaffolding, formation of highly porous mats of nanometer-sized fibers, such as those fabricated via electrospinning, may be quite important. Previously, strong acidic solvents and blending with synthetic polymers have been used to achieve electrospun nanofibers containing chitosan. As an alternative approach, in this work, polyethylene oxide (PEO) has been used as a template to fabricate chitosan nanofibers by electrospinning in a core-sheath geometry, with the PEO sheath serving as a template for the chitosan core. Solutions of 3 wt % chitosan (in acetic acid) and 4 wt % PEO (in water) were found to have matching rheological properties that enabled efficient core-sheath fiber formation. After removing the PEO sheath by washing with deionized water, chitosan nanofibers were obtained. Electron microscopy confirmed nanofibers of ∼250 nm diameter with a clear core-sheath geometry before sheath removal, and chitosan nanofibers of ∼100 nm diameter after washing. The resultant fibers were characterized with IR spectroscopy and X-ray diffraction, and the mechanical and electrical properties were evaluated.}, number={9}, journal={BIOMACROMOLECULES}, author={Ojha, Satyajeet S. and Stevens, Derrick R. and Hoffman, Torissa J. and Stano, Kelly and Klossner, Rebecca and Scott, Mary C. and Krause, Wendy and Clarke, Laura I. and Gorga, Russell E.}, year={2008}, month={Sep}, pages={2523–2529} }
 @article{stevens_downen_clarke_2008, title={Percolation in nanocomposites with complex geometries: Experimental and Monte Carlo simulation studies}, volume={78}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.78.235425}, abstractNote={The development of nanocomposites (a matrix, often polymeric, enhanced by a particle with a nanometer-sized dimension) has expanded dramatically in recent years with a particular focus on materials with complex microstructure and nanostructure. Such composites rely on formation of a connected network of particles throughout the sample volume in order to enhance the polymer's mechanical and electrical properties. From a fundamental perspective, this network formation will be governed by a percolation process within the constrained geometry of the particular microstructure. In this paper, the percolation process within a particular complex nanostructure, namely, a mat of electrospun nanofibers with fiber size of $\ensuremath{\approx}100\text{ }\text{nm}$ and high porosity, is studied via continuum Monte Carlo simulations, where the sample geometry (fiber and particle sizes, orientation, and sample porosity) is matched to the mats utilized in our previous experimental work. A good agreement between experimental and computational results is observed. Simulations of spherical dopant in uniform samples, with zero, one, or two sample dimensions similar in size to the particle, were completed to explore the effects of confinement, in particular within a single fiber. These results were compared and contrasted with those from porous fibrous mats to determine the influence of porosity on the critical volume fraction. The results indicate that percolation in fibrous mats occurs via pathways that include sections of many fibers rather than being contained within single fibers which span the sample. The detailed dependence of critical volume fraction on porosity and the sensitivity to fiber number and width is discussed.}, number={23}, journal={PHYSICAL REVIEW B}, author={Stevens, D. R. and Downen, L. N. and Clarke, L. I.}, year={2008}, month={Dec} }
 @article{mccullen_stevens_roberts_clarke_bernacki_gorga_loboa_2007, title={Characterization of electrospun nanocomposite scaffolds and biocompatibility with adipose-derived human mesenchymal stem cells}, volume={2}, number={2}, journal={International Journal of Nanomedicine}, author={McCullen, S. D. and Stevens, D. R. and Roberts, W. A. and Clarke, L. I. and Bernacki, S. H. and Gorga, R. E. and Loboa, E. G.}, year={2007}, pages={253–263} }
 @article{mccullen_stano_stevens_roberts_monteiro-riviere_clarke_gorga_2007, title={Development, optimization, and characterization of electrospun poly(lactic acid) nanofibers containing multi-walled carbon nanotubes}, volume={105}, ISSN={["1097-4628"]}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000247576000079&KeyUID=WOS:000247576000079}, DOI={10.1002/app.26288}, abstractNote={Abstract Electrospinning of poly ( L ‐ D ‐lactic acid) (PLA) was investigated with the addition of multi‐walled carbon nanotubes (MWNT) for development of a scaffold for tissue engineering. Through this experiment, it was determined that the optimal concentration of PLA with weight average molecular weight ( M w ) 250,000 g/mol is ∼20 wt % as indicated by scanning electron microscopy. This concentration produces fibers with no beading or film formation. The preferred solvent system is a combination of chloroform and dimethyl formamide to alleviate the volatile action of chloroform. The optimum processing parameters for PLA are an electric field of 1 kV/cm which was determined by a surface response plot to minimize fiber diameter based on the applied voltage, working distance, and addition of MWNT. Fourier Transform infrared spectroscopy has indicated the removal of the solvent system. With the addition of MWNT, the fiber diameter was drastically reduced by 70% to form fibers with a mean diameter of 700 nm. This is believed to be due to an increased surface charge density for the MWNT/polymer solution. Transmission electron microscopy validated the alignment of the MWNT within the fibers. MWNT loading exhibited an increase in the conductance of the scaffold and the tensile modulus at an optimal loading level of 0.25 wt %. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007}, number={3}, journal={JOURNAL OF APPLIED POLYMER SCIENCE}, author={McCullen, Seth D. and Stano, Kelly L. and Stevens, Derrick R. and Roberts, Wesley A. and Monteiro-Riviere, Nancy A. and Clarke, Laura I. and Gorga, Russell E.}, year={2007}, month={Aug}, pages={1668–1678} }
 @article{mccullen_stevens_roberts_ojha_clarke_gorga_2007, title={Morphological, electrical, and mechanical characterization of electrospun nanofiber mats containing multiwalled carbon nanotubes}, volume={40}, ISSN={["1520-5835"]}, DOI={10.1021/ma061735c}, abstractNote={This work focuses on the development of electrically conducting porous nanocomposite structures by the incorporation of multiwalled carbon nanotubes (MWNT) into electrospun poly(ethylene oxide) (PEO) nanofibers. Electron microscopy confirmed the presence of individual aligned MWNT encapsulated within the fibers and showed fiber morphologies with diameters of 100−200 nm. Electrical conductance measurements of the random nanofiber mats showed that by increasing the concentration of MWNT we were able to produce porous nanocomposite structures with dramatically improved electrical conductivity. Above a percolation threshold of 0.365 ± 0.09 MWNT weight percent (wt %) in PEO the conductance increased by a factor of 1012 and then became approximately constant as the concentration of MWNT was further increased. Because of this percolation threshold, for a 1 wt % loading of MWNT, the conductivity is essentially maximized. Mechanical testing confirmed that the tensile strength did not change, and there was a 3-fold increase in the Young's modulus when comparing a 1 wt % MWNT loading to the pure electrospun PEO. Thus, the optimal MWNT concentration for PEO nanofiber mats with enhanced mechanical and electrical properties is ∼1 wt %.}, number={4}, journal={MACROMOLECULES}, author={McCullen, Seth D. and Stevens, Derrick R. and Roberts, Wesley A. and Ojha, Satyajeet S. and Clarke, Laura I. and Gorga, Russell E.}, year={2007}, month={Feb}, pages={997–1003} }