Laura Clarke

This proposal seeks to understand the fundamental science underlying electrospinning from an unconfined sheet of molten polymer. This method is "green" (solvent-free and compatible with recyclable plastics), should result in meso-fibers having dramatically improved mechanical properties, and will allow manipulation of the electrospinning process to create smaller fiber diameters than typically achievable under traditional needle-based approaches. The work will involve unconfined melt electrospinning of the three common commercial thermoplastics: polyethylene, polypropylene and polyethylene terephthalate. The research focuses on the roles of flow rate and melt conductivity, which have not been previously explored due to severe experimental challenges. Conductivity will be altered by adding conductive compounds (Aim 1) or by generating a localized electrical discharge within or near the fluid (Aim 2).

The ability to controllably trigger breaking of chemical bonds is a crucial step in reducing environment waste and pollution due to plastics. Discarded plastic material often detrimentally resides within the environment for many years, leading to myriad problems: sickening or killing a wide range of life forms from microbes to large animals, effecting both marine and terrestrial mammals and birds, clogging drainage and water processing systems, contaminating landfills with deleterious chemicals, or resulting in sterile soil and accumulation of toxins within specific ecosystems. A key issue underlying the ability intentionally initiate degradation of polymeric materials is implementing an approach to start breaking of chemical bonds - enabling a substance that has robust material properties during use, which can then be re-worked or deteriorated upon command. Proving this proposal??????????????????s hypothesis that degradation (i.e., bond-breaking) could be controllably triggered and/or driven via photothermal heating would result in a powerful tool that addresses this complex problem on multiple-levels: for instance, providing an opportunity for one-time treatment to begin a long degradation process (a consumer shines a green light on a plastic object upon discarding), generating visible light sensitivity that would enhance deterioration using sunlight, or resulting in thermally triggered re-workable adhesives/epoxies. The success of even a single proposed modality would have a significant effect in mediating plastic environmental pollution. This proposal tests the hypothesis that the strongly inhomogeneous temperature gradients created in the interior of polymers upon photothermal heating of embedded nanoparticles can be utilized to efficiently trigger thermally-induced bond breaking in polymeric materials via exposure to near infrared or visible light. Such heat generated within the material can impel a degrading or cross-link-breaking chemical reaction either by instigating a self-sustaining process, or when applied over a longer time by continuously enhancing such reaction rates. Thus, the materials developed due to this scientific knowledge would have an additional functionality -- the ability to degrade upon command. Previous work has shown that the inhomogeneous temperature profile created within polymers undergoing photothermal heating, which creates intense local heat at isolated nanoparticle locations embedded within the sample, results in a very different material response (for instance, far more rapid crystallization rates) than when utilizing conventional heating methods, where the entire sample is slowly, uniformly warmed from the outside to the inside. This internal placement of nanoscale heaters leads to triggering of chemical reaction fronts that spread outwards and natural segregating of the material into smaller pieces. Rather than degrading from the surface inwards, which always retains an intact inner core of material, here the material is homogeneously fragmented from within and the chemistry reactions occur from the inside outward.

We will investigate the use of the photothermal effect of metal nanoparticles as a tool for materials processing and actuation. This effect converts light energy into heat via coupling with the nanoparticle surface plasmon resonance; thus, metal nanoparticles can act as externally-driven, nano-sized thermal sources from within a material, providing efficient, selective, and remotely-controlled localized heating when the material (doped with a small fraction of nanoparticles) is irradiated with relatively weak light. Our previous research has recently demonstrated that using relatively weak irradiation intensity, nanofibrous mats with a low concentration of metal nanoparticles can be facilely heated to temperatures sufficient to melt the polymer matrix; this effect can also be usefully employed in bulk materials. Such efficient and controllable localized heating represents a powerful new tool for in situ processing and actuation. Instead of requiring development of specific light-sensitive polymeric materials, any material that can be thermally processed, activated, or actuated could now be controlled in-situ by a light field. The spatial and wavelength specificity of the heating offers several implicit advantages: 1) Heat is generated from within a medium, as opposed to conventional methods where the surface of the material is the first to warm; thus, for instance, the interior of a structure could be processed to a higher temperature than the surface. 2) Heat is generated only in the immediate region of the nanoparticles; thus by placing nanoparticles in particular regions of the sample, selective in situ processing can be accomplished. 3) The heat-generating illumination source can be optimally tuned for the given system; as the nanoparticle absorption wavelength is characteristic of the particle type, one can select a particle resonant with a preferred wavelength in order to obtain maximum light penetration into a particular material. 4) Different nanoparticle heater types (resonant at different light frequencies) can be incorporated into different regions of the same sample allowing heating of different regions at will. We will address three specific aims: 1) To develop techniques to measure temperature at the nanometer-size scale in order to experimentally determine the maximum temperature and the temperature gradient within the medium. Towards this activity we have utilized distinct, temperature-dependent changes in the emission spectrum of fluorophores such that they act as remote-readout nanoscale thermometers. 2) To utilize metal nanoparticles to actuate shape-memory polymers and explore multistage shape memory schemes. 3) To develop hybrid nanofiber materials (core-sheath structures with thermoset-precursor cores) that are flexible when fabricated and can be positioned, deformed, or templated on a three-dimensional structure and subsequently "stiffened" from within, dramatically increasing their modulus, by photothermal curing of the thermoset core to form a rigid assembly.

This fundamental research will enable the production of nanofibrous substrates for use in a wide-range of applications, such as filtration, sensors, fuel cells, and tissue engineering. Our approach is to make a paradigm shift from fiber growth from a droplet suspended from a needle to fiber growth from a large concentration of droplets on a surface. Such a surface can easily be patterned to provide literally thousands or tens of thousands (25 million sites for a 10 um square "patch" of hydrophobic/hydrophillic material on a 0.1 m square plate) of possible spinning sites. The ability to efficiently and continuously supply these "spinning sites" with solution, to determine where spinning originates, and to control droplet and fiber size are primary objectives of this research. We propose that much smaller diameter fibers (~ 50 nm or less) can be obtained by controlled design of the charged plate (where key parameters include interfacial tension and surface architecture). Morphology, mechanical, and electrical properties will be studied to develop a mechanistic understanding of the processing-property relationships as the primary fundamental outcome.

One research need in Clarke group involves non-contact temperature measurement of complex polymeric structures which have length scales of ~100 nm. To achieve this, we currently utilize a novel fluorescence technique which measures the average temperature by combining signals from embedded dye molecules throughout the sample. Specifically, this method enables observation of samples that are heated by the photothermal effect: where relatively low intensity visible light interacts strongly with the surface plasmon of metal nanoparticles, converting this light energy into heat in the interior of the sample. In these samples, the local temperature will vary widely (potentially by hundreds of degrees ??A?C) depending on the distance a dye molecule ?thermometer? resides from a nanoparticle "heater"; since the dye molecules are randomly placed, the average material temperature, as reported by the full sample fluorescence, smooths over some of the most interesting and important physics. There are few available experimental techniques to easily measure temperature on the shorter length scales needed to reveal the temperature gradients within these complex structures. The temperature of the hottest points in the sample, in the region just around each nanoparticle, is particularly important. Here we propose to develop a spatially-selective technique to determine the temperature of only the hottest points in the sample when undergoing photothermal heating. It's an elegant, schematically simple, modification of the existing approach. We are one of very few groups utilizing photothermal heating in solids, and the only one (to date) exploring polymer processing. This additional technique to better spatially quantify the heating provides an opportunity for us to solidify our positions as leaders in this emerging field. This novel research tool will enable us to more effectively compete for large group grants focused on the myriad fundamental and applications-oriented questions related to photothermal heating in solids.

Funding of this FRPD proposal will enable the Clarke group to expand into a new research area, sensors - specifically, the development of novel optical temperature sensing. The particular optical sensing approach we propose has not been previously reported and would serve as an important tool for our group and others working in the expanding area of nanoscale heating. Nanoscale heating is a rapid growth area in material science, and we believe that this project will have high future impact. The goal of this seed funding is to experimentally realize the proposed idea and enable generation of preliminary proof-of-concept data necessary to seek more extensive federal funding.

In this Small Grant for Exploratory Research proposal, we explore the possibility of utilizing the innate properties of nano-sized particles to process a material "from the inside". Such true "nanoprocessing" - processing applied at particular nanometer-sized locations within a material - could alter the way we think about composite materials. In particular, "nanoprocessing" could enable composite components (such as nanoparticles) to have additional functions, that is to be used to form or re-form material before or after use, in addition to serving as a functional element during the product lifetime. Two hypotheses are explored: 1) That the plasmon resonance of metal nanoparticles (excited by exposure to visible light) can be used as a source of localized heat (local changes of up to 100s ?aC) for processing purposes. Such heating could ultimately be used for tasks such as bonding of nanofibers or activating actuator motion in temperature-sensitive shape memory polymers. Importantly, the heating would be specific to the nanoparticle and localized, so the bulk of the polymer would not melt. 2) That an AC electric field can be used to induce nanoparticle electromigration such that composite electrical properties are improved (increase in conductivity or decrease in critical volume fraction). In other words, one could post-process a sample by locally heating each nanoparticle (via the above effect) and then apply an AC electric field to induce minute changes in the nanoparticle positions such that the net composite conductivity increased. We will explore these hypotheses by post-processing random mats of electrospun nanoparticle/polymer nanofibers. This proposal has three specific aims: 1) To determine if plasmon-mediated heating from gold nanoparticles embedded in polyethylene oxide nanofibers can produce sufficient heating to result in changes in fiber morphology. 2) To swell the polymer mat with water or methanol and utilize an applied AC electric field to induce electromigration. DC conductivity measurements will take place before and after the electromigration process. 3) To combine specific aims 1 and 2: softening the polymer matrix with plasmon-mediated heating and then applying an AC electric field to induce electromigration.

The proposed project will address the interplay between strength and toughness as a result of how the nanotube/polymer interface is engineered. For example, the interface between polymer and nanoparticle controls the composite?s strength and toughness. High interfacial strength is desirable to maximize the composite?s strength, while slip between the nanoparticle and matrix (low interfacial strength) is required for maximum toughness. For well-dispersed, oriented nanocomposites, we will quantify: ? The strength of the nanocomposite as a function of interfacial strength; ? The toughness of the nanocomposite as a function of interfacial strength; and ? The optimization of both strength and toughness for the nanocomposite. In this study, we will fabricate carbon nanotube/nylon fibers and films to study the molecular motion, nano- and macroscopic mechanical, electrical, rheological, and processing properties of the composite system of interest. We will combine many analytical techniques to develop a mechanistic understanding of the factors that influence strength and toughness at the nanotube/nylon interface.