Nanotechnology

Shape memory polymers (SMPs) are functional polymers, which find applications in a broad range of temperature sensing elements and biological micro-electro-mechanical systems (MEMS). These polymers are capable of fixing a transient shape and recovering to their original shape after a series of thermo-mechanical treatments. However in some applications, SMPs may not generate enough recovery force to be useful. In this work we explore a completely new approach to increase the recovery force of the shape memory polymer through the incorporation of cellulose nanocrystals and carbon nanotubes as nanofillers.

In this paper, shape memory polyurethane fibers (SMPF) reinforced with cellulose nanocrystals and carbon nanotubes were prepared by extrusion. The thermal and mechanical performance, (especially shape memory properties) of neat polyurethane fibers and resulting composites were characterized and compared through tensile and thermal cyclic tensile test. 

The protective clothing market covers a wide range of applications and products. The applications can be segmented into military, professional, industrial and consumer markets. Each of these markets utilizes a protective layer that must also be breathable. These protective layers can be coatings, films, Nonwovens and membranes. Each material has a different level of protection and breathability and is used in different applications. The figure below briefly outlines the breathability vs. water repellency of several different protective materials.  

This paper will evaluate the advantages and disadvantages of each material or combinations of materials and compare the performance that can be achieved and markets where they can be used.

Electrospinning is a simple, convenient, and versatile technique for generating long fibers with diameters on the micro- and nanoscale. There has been much interest recently in extending this technique to fabricate fibers of novel compositions, morphologies and arrangements. By modifying the collector, the spinneret, and the solution composition our group has been able to bring about several innovations in the field of electrospinning.

Recently it was demonstrated by our group that the introduction of an insulating gap in the collector electrode (either by introducing a void into the collector or by patterning an insulating substrate with gold electrodes) nanofibers could be aligned uniaxially across the gap. Subsequently deposited fibers tend to align themselves during the electrospinning process by electrostatic repulsion, thus the degree of uniaxial alignment increases directly with collection time. This method is quite promising in that it provides a simple and convenient method to align fibers for the production of nanoscale devices. By using patterned collectors and alternating the grounding of the collector electrodes, fiber meshes can be collected as well. Transfer of the fibers to alternative substrates is facile, as the fibers can be collected across a gap and the substrate can be passed through this gap.

We have also modified the collector in order to fabricate porous fibers. By immersing the collector in a bath of liquid nitrogen, porous fibers can be obtained from polymer solutions as a result of thermally induced phase separation (TIPS) between solvent-rich and solvent-poor regions in the fiber, followed by removal of solvent in vacuo. This method is versatile in that it can be readily used with non-volatile solvents and does not require selective dissolution of phase-separated polymers. The porosity and resultant morphology of the fibers can be controlled by altering the collection distance and freeze-dying protocol. As this method is dependent on the presence of residual solvent to induce phase separation, it is also applicable to electrospray in order to generate hollow and/or porous colloids in a simple and inexpensive fashion. In addition, the fibers are porous throughout, thus making them suitable for encapsulation of active substances or catalysts. By altering the surface properties of the fibers, it is possible to control the adhesion and growth of cells to the fibers, thus enhancing the properties of nanofiber assemblies for applications such as tissue engineering. In addition, the increased surface area of the fibers can be used to create superhydrophobic coatings.

Using a coaxial spinneret, we have been able to manufacture porous, hollow, and core-sheath nanofibers and control the surface chemistry of those resultant fibers by tuning the composition of the core and sheath solutions. Our coaxial methods have enhanced the utility of electrospinning as the spinneret can be loaded with a carrier polymer that is amenable to electrospinning and doped with a polymer that can not be electrospun individually. Following electrospinning, the carrier polymer can be removed by selective dissolution.

In order to fabricate phase change fibers composed of long-chain paraffin cores and composite sheaths, we have developed a melt coaxial electrospinning method. This method combines melt electrospinning with a coaxial spinneret, and allows for nonpolar solids such as paraffins to be encapsulated and electrospun in one step. Shape-stabilized phase change fibers have many potential applications as they are able to absorb, hold, and emit large amounts of thermal energy over a certain temperature range by taking advantage of the large heat of fusion of long-chain alkanes. Compounds with melting points near room temperature (octadecane) and body temperature (eicosane) were chosen as these temperature ranges are the most useful. We have created thermally-stable phase change materials up to 45 wt% octadecane, as measured by differential scanning calorimetry. In addition, the resultant fibers display novel segmented core morphologies due to the rapid solidification of the alkanes driven by evaporative cooling of the carrier solution. Aside from the fabrication of phase change fibers, the melt coaxial method is promising for applications involving encapsulation and controlled release.

By electrospinning onto a drum, we have generated controllable tubular arrays of electrospun nanofibers, by modifying the conductivity and patterning of the drum can arrange the nanofibers along the long axis of the drum. These fibers can be released from the drum template using an acidic etching. Drum spinning also allows for the wounding of the drum collector with conductive wires around the drum, which will thereby generate fibers that are fully aligned in the direction of the metal coils. We have been able to achieve longitudinal and helical arrangement of the fibers by controlling the patterning of the tubular collector. We have also observed that by using a nonconductive rod as the drum with a conductive wire wound around the drum, the nanofibers could be aligned uniaxially across the gap with greater ease and a higher degree of alignment than with a conductive drum. The resultant assemblies have potential applications in vascular and neural tissue engineering.

The end-use application of nanofiber in filtration has been well established. However, the use of nanofiber webs as liner materials in protective ensembles needs further exploration. There are issues with regard to breathability, protection capabilities and washability of nanofiber embedded composite fabrics. The compromise between the protection and breathability of nanofiber composite fabrics is an important issue. Results from an ongoing research activity to develop breathable and self-cleaning composite nanofiber fabrics will be presented in this paper. Based on the optimization of electrospinning parameters, polyethylene oxide nanofibers with and without titanium dioxide have been electrospun on cotton fabrics. The arrangement of the nanofiber layers on the base cotton fabrics were examined using scanning electron and transmission emission microscopes. This study involved elaborate characterization of: 1) breathability characteristics and 2) protection capabilities. The breathability of nanofiber composite fabrics was quantified using Moisture Vapor Transmission (MVTR) properties. Rotating dish method was used for the objective quantification of MVTR. The protection capability was quantified using the instantaneous adsorption of toxic industrial chemicals. This was carried out on a dynamic basis, i.e., measuring adsorption at equal intervals of time as a function of weight gain. Both the MVTR and adsorption experiments required careful attention and fine tuning of the standard testing set-ups due to the nanofibers adhered to cotton base substrates. This is because layers of nanofiber are only held by electrostatic charge attraction on to the cotton base fabrics. Even though the nanofiber layers were extremely thin, it had influence on MVTR values. The presence of nanofibers inhibited the transmission of water vapor through the composite fabrics. Results indicated that addition of nanofiber layers reduced the amount of water vapor pass through giving lower MVTR for nanofiber composite fabrics. Thermogravimetric isotherms for the adsorption values showed a reverse trend. Plain cotton nonwoven fabrics have no adsorption which resulted in a more or less straight line adsorption curve. However polyethylene oxide nanofibers with and without titanium dioxide were able to have instantaneous adsorption of toxic industrial chemicals such as toluene. The composite cotton fabrics with nano layers had no adverse effect with regard to their adsorption capabilities. This result again verified the effect of enhanced surface area for improved adsorption and protection. Additional experiments on UV protection capabilities were conducted on the base cotton fabric and the nanofiber composite fabrics. Nanofiber fabrics without metal oxides did not have any significant reduction in UV transmission. Whereas, metal oxide embedded nanofiber composite fabrics provided more resistance to UV penetration. The base cotton fabric offered least resistance to UV transmission. The general conclusion from this study is that the addition of nano layers will enhance adsorption and will inhibit the transmission of water vapor offering less breathable fabrics.

The fibrous nonwoven materials formed from uni-axially stretched polyethylene blown films and sequentially stretched PTFE films have been used in filtration industry for two decades.   In air filtration markets, usage of these two fibrous materials has been rapidly growing due to the capability of highly electrically charged or of making nano size filaments in order to increase air filtration efficiency without sacrificing high pressure drop.  Other materials have been stretched and formed the fibrous materials as well.  Filtration performance of filters made with these materials was measured and discussed along with electric charging characteristics of these materials.