Spontaneous Co-Assembly of Cellulose Nanocrystals and TiO2 Nanorods Followed by Calcination to Form Cholesteric Inorganic Nanostructures.

Chiral nanomaterials possess unique electronic, magnetic, and optical properties that are relevant to a wide range of applications including photocatalysis, chiral photonics, and biosensing. A simple, bottom-up method to create chiral, inorganic structures is introduced that involves the co-assembly of TiO2 nanorods with cellulose nanocrystals (CNCs) in water. To guide experimental efforts, a phase diagram was constructed to describe how phase behavior depends on the CNCs/TiO2/H2O composition. A lyotropic cholesteric mesophase was observed to extend over a wide composition range as high as 50 wt % TiO2 nanorods, far exceeding other examples of inorganic nanorods/CNCs co-assembly. Such a high loading enables the fabrication of inorganic, free-standing chiral films through removal of water and calcination. Distinct from the traditional templating method using CNCs, this new approach separates sol-gel synthesis from particle self-assembly using low-cost nanorods.

Modulation of Interfacial Adhesion Using Semicrystalline Shape-Memory Polymers.

Semicrystalline shape-memory elastomers are molded into deformable geometrical features to control adhesive interactions between elastomers and a glass substrate. By mechanically and thermally controlling the deformation and phase-behavior of molded features, we can control the interfacial contact area and the interfacial adhesive force. Results indicate that elastic energy is stored in the semicrystalline state of deformed features and can be released to break attractive interfacial forces, automatically separating the glass substrate from the elastomer. Our findings suggest that the shape-memory elastomers can be applied in various contact printing applications to control adhesive forces and delamination mechanics during ink pickup and transfer.

Porous polymers by controlling phase separation during vapor deposition polymerization.

A template-free method is described to fabricate continuous-phase, porous polymer films by simultaneous phase separation during vapor deposition polymerization. The technique involves concurrent polymerization, crosslinking, and phase separation of condensed species and reaction products. Deposited films form open-cell, macroporous structures consisting of crosslinked and glassy poly(glycidyl methacrylate). By limiting phase separation during vapor phase deposition, spatially dependent morphologies, such as layered morphologies, can be grown. Results show that combining vapor deposition polymerization with phase separation establishes morphological control, which may be applied to applications including cellular scaffolds, thin cushions and vibration dampers, and membranes for separations.

Condensation and polymerization of supersaturated monomer vapor.

Initiated chemical vapor deposition (iCVD) of poly(glycidyl methacrylate) from supersaturated monomer vapor is reported. Rapid film growth rates, up to 600 nm/min, were observed. Films grown from supersaturated monomer exhibited distinct surface undulations. The temporal evolution of surface features during film growth was studied and is explained by monomer condensation followed by droplet coalescence and film growth. High droplet densities were observed at the early times and are attributed to rapid polymerization of monomer within condensed liquid nuclei. Droplet nucleation resulting in surface undulations can be avoided by first depositing a thin, cross-linked film from ethylene glycol diacrylate monomer followed by deposition of supersaturated monomer vapors.

Shape Actuation via Internal Stress-Induced Crystallization of Dual-Cure Networks.

We demonstrate a single-phase, two-way shape actuator that, in the absence of an external load, elongates upon cooling and reversibly contracts upon heating. In a simple and straightforward process, a partially cross-linked, semicrystalline poly(ε-caprolactone) (PCL) network is melted, stretched to several hundred percent strain, and further cross-linked. Upon removal of the applied load, the elastic double network adopts a "state-of-ease" that retains part of its former strain. When cooled, internal stress-induced crystallization causes further elongation of configurationally biased chains. When heated, crystallites melt, and the sample returns to its equilibrium state-of-ease. Under optimized conditions, reversible actuation >15% strain can be reproducibly achieved, and samples can be cycled multiple times with highly uniform actuation with no observable creep. The mechanism behind such actuation was further confirmed via calorimetry and X-ray scattering.

Gradient-index materials based on thiol-ene networks.

Gradient-index (GRIN) optics offer spatially varied refractive indexes that can enhance current imaging technologies. Current methods to fabricate GRIN optics are highly complex and costly. Here we report a simple and efficient method that utilizes commercially available reagents to fabricate polymeric GRIN optics with significant refractive index differences (Δn = 0.04). First, two different mixtures of network precursors are layered and time allotted for molecular diffusion in the liquid state, prior to curing. The resulting, partially mixed layers are UV-cured to yield clear, glassy molecular networks with fixed refractive index gradients. The fully cured network resins exhibit smoothly varying composition and refractive index over centimeter length scales, confirmed by spectroscopy and interferometry.

Photoinduced diffusion through polymer networks.

Photomediated addition-fragmentation chemistry is applied to demonstrate the precisely controlled diffusion of chemical species through polymer networks. Fluorescent groups connected to polymer networks by allyl sulfide moieties become mobile upon irradiation with UV light due to radical-mediated addition-fragmentation bond exchange. Photoinduced transport through the bulk, into solution, and across film interfaces is demonstrated.

Solvent-assisted dewetting during chemical vapor deposition.

This study examines the use of a nonreactive solvent vapor, tert-butanol, during initiated chemical vapor deposition (iCVD) to promote polymer film dewetting. iCVD is a solventless technique to grow polymer thin films directly from gas phase feeds. Using a custom-built axisymmetric hot-zone reactor, smooth poly(methyl methacrylate) films are grown from methyl methacrylate (MMA) and tert-butyl peroxide (TBPO). When solvent vapor is used, nonequilibrium dewetted structures comprising of randomly distributed polymer droplets are observed. The length scale of observed topographies, determined using power spectral density (PSD) analysis, ranges from 5 to 100 microm and is influenced by deposition conditions, especially the carrier gas and solvent vapor flow rates. The use of a carrier gas leads to faster deposition rates and suppresses thin film dewetting. The use of solvent vapor promotes dewetting and leads to larger length scales of the dewetted features. Control over lateral length scale is demonstrated by preparation of hierarchal "bump on bump" topographies. Vapor-induced dewetting is demonstrated on silicon wafer substrate with a native oxide layer and also on hydrophobically modified substrate prepared using silane coupling. Autophobic dewetting of PMMA from SiOx/Si during iCVD is attributed to a thin film instability driven by both long-range van der Waals forces and short-range polar interactions.

Development of a clothing iron safety device.

Contact burns from clothing irons are a common injury seen in children. These injuries occur when an unattended iron is within reach of toddlers in its upright position. In a previous study, the authors have shown that the surface of an iron takes 90 minutes to cool below the epidermal injury threshold of 49 degrees C. The authors have constructed an "iron shoe" to shield the iron surface from young hands during cooling. The device is intended to set the cooling iron in its down position providing additional protection. The device will insulate the iron surface to avoid the fire hazard when positioned in this manner. A silicone polymer was used to create an "iron shoe." This polymer is stable at temperatures up to 370 degrees C. The device included sidewalls to shield the edges from contact. Thermal analysis of the device was conducted using an inexpensive and expensive iron. Thermocouples were placed on the iron surface and below the iron shoe. The irons were heated to its maximum temperature, placed in the shoe and then unplugged. Temperature cooling curves were obtained from the thermocouples. The experiment was repeated by measuring the temperature difference between the iron edge and the shoe sidewalls. The surface of both expensive and inexpensive irons reached a maximum of 205 degrees C. The temperature below the iron shoe reached a maximum of 49 degrees C (inexpensive) and 60 degrees C (expensive). The iron edge temperature reached a maximum of 188 degrees C (inexpensive) and 154 degrees C (expensive). The shoe sidewall temperature achieved a maximum of 52 degrees C (inexpensive) and 49 degrees C (expensive). Both expensive and inexpensive irons reach temperatures over 200 degrees C. The silicone "iron shoe" effectively shielded the surface and edge of both irons and approached the epidermal injury threshold of 49 degrees C. The temperature beneath the expensive iron did exceed 49 degrees C, but because the intention of the device is to place the iron in the down position, the surface will be out of reach from children. This device prototype offers a solution to protect toddler's hands from contact with cooling irons. Further design modifications will be tested to reduce the cost of the device without impairing its effectiveness.

Controlling surface roughness in vapor-deposited poly(amic acid) films by solvent-vapor exposure.

A series of vapor-deposited poly(amic acid) (PAA) films were exposed to dimethyl sulfoxide (DMSO) vapors to investigate sorption kinetics and surface smoothing phenomena. Gravimetric sorption and secondary-ion mass spectrometry (SIMS) results are both consistent with frontal (case II) diffusion. These experiments suggest that the solvent front is defined by a sharp interface that delineates the swollen material from the unswollen material. Solvent-vapor smoothing was studied by first depositing PAA onto rough aluminum surfaces, and then, during solvent-vapor exposure, the surface topology was continuously monitored using a light interference microscope. The resulting time-dependent power spectra indicate that high-frequency defects smooth faster than low-frequency defects. This frequency dependence was further investigated by depositing PAA onto a series of sinusoidal surfaces and exposing them to solvent vapor inside a flow channel. The sinusoidal amplitudes decay exponentially with time, with decay constants that are proportional to the surface frequency. To explain the physics of surface smoothing, a two-parameter model is presented and agrees qualitatively with experimental data.

Characterization of an insoluble polyimide oligomer by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry.

In the past two years, papers have appeared in the literature which demonstrate that matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectra can be obtained from matrix-analyte preparations which have been produced by grinding the two materials together until a powder of small particle size is obtained. In the present study that methodology was modified and applied to an insoluble polyimide oligomer, poly(4,4'-oxydiphenylenepyromellitimide) (POPM). Two matrix materials were employed in this analysis, 1,8 dihydroxyanthrone (dithranol) and 3-aminoquinoline, with and without an additional cationizing agent. The spectra obtained by this method are shown to be sensitive to the matrix employed in the analysis as well as the quantity of cationizing agent combined with the matrix.