Thermal Conductivity of Ultra High Molecular Weight Polyethylene: From Fibers to Fabrics

Unique combinations of properties such as mechanical compliance and chemical stability make polymers attractive for many applications. However, the intrinsic low thermal conductivity of bulk polymers has generally limited their potential for heat dissipation applications, and in fact they are widely used as thermal insulators. But in recent years, gel-spun, ultraoriented fibers made of ultrahigh molecular weight polyethylene (UHMW-PE) have sparked interest in the thermal management community due to their exceptionally high thermal conductivity. These fibers are typically used in commercially produced protective gear such as motorcycle jackets and ballistic vests due to their high mechanical strength, but they have not been widely utilized for heat spreading and thermal management applications. While recent studies have characterized individual fibers and ultradrawn films, the thermal properties of fabrics constructed from these materials remain virtually unexplored. Here, we synthesize plain-weave fabrics from yarns of commercially available gel-spun UHMW-PE and measure the thermal properties of the individual microfibers, yarns, and woven fabrics using an in-house thermal characterization technique based on infrared microscopy. For the woven fabric, we report an effective in-plane thermal conductivity of ∼10 W m−1 K−1 in the direction aligned with the weft yarns, which is 2−3 orders of magnitude higher than conventional textile materials. This work reveals the high thermal conductivity of UHMW-PE fabrics that can be realized by using a scalable textile manufacturing platform and lays the foundation for exploiting their unique thermomechanical properties for heat spreading functions in flexible/wearable devices

Thermal and mechanical characterization of high-performance polymer fabrics for applications in wearable devices

With advances in fexible and wearable device technology, thermal regulation will become increasingly important. Fabrics and substrates used for such applications will be required to efectively spread any heat generated in the devices to ensure user comfort and safety, while also preventing overheating of the electronic components. Commercial fabrics consisting of ultra-high molecular weight polyethylene (UHMW-PE) fbers are currently used in personal body armor and sports gear owing to their high strength, durability, and abrasion resistance. In addition to superior mechanical properties, UHMW-PE fbers exhibit very high axial thermal conductivity due to a high degree of polymer chain orientation. However, these materials have not been widely explored for thermal management applications in fexible and wearable devices. Assessment of their suitability for such applications requires characterization of the thermal and mechanical properties of UHMW-PE in the fabric form that will ultimately be used to construct heat spreading materials. Here, we use advanced techniques to characterize the thermal and mechanical properties of UHMW-PE fabrics, as well as other conventional fexible materials and fabrics. An infrared microscopy-based approach measures the efective in-plane thermal conductivity, while an ASTM-based bend testing method quantifes the bending stifness. We also characterize the efective thermal behavior of fabrics when subjected to creasing and thermal annealing to assess their reliability for relevant practical engineering applications. Fabrics consisting of UHMW-PE fbers have signifcantly higher thermal conductivities than the benchmark conventional materials while possessing good mechanical fexibility, thereby showcasing great potential as substrates for fexible and wearable heat spreading application

Thermal Metrology and Characterization of High Thermal Conductivity Polymer Fibers and Fabrics

Recent technological advances in the field of electronics and the accompanying trend of device miniaturization with enhanced functionality has led to growing interest in new methods of electronic device integration. As a result, flexible, wearable, and portable electronic devices have emerged as a way of providing a multifunctional infrastructure to facilitate various consumer needs, creating new challenges for materials development. Polymers possess a unique combination of desirable properties such as mechanical compliance, durability, low density and chemical stability which makes them ideally suitable as substrate materials to cater to such diverse applications. However, the low thermal conductivity of polymers hinders their heat spreading capability in thermal management applications for flexible and wearable devices. In recent years, there has been a growing interest in ultra-high molecular weight polyethylene (UHMW-PE) materials with aligned polymer chains due to their remarkably high thermal conductivity that is similar to some metals. These are commercially manufactured in large volumes as fibers using gel-spinning and ultra-drawing processes that impart a high degree of crystallinity and orientation to the polymer chains. As a result, these materials develop exceptionally high mechanical strength, elastic modulus, and thermal conductivity compared to conventional polymers. Therefore, UHMW-PE materials have found applications in commercial products like motorcycle gear and ballistic vests, but have not been commercially deployed for heat spreading and thermal management applications. While there has been much fundamental work on the development of high thermal conductivity fibers, effective translation of the high conductivity from individual fibers to macroscale (wearable) flexible fabrics has not been previously explored. The objective of this thesis is to obtain a fundamental understanding of the thermal transport properties of fabric materials constructed from the high conductivity polymer fibers, and assess their applicability for potential heat spreading applications. In the present work, commercially available high thermal conductivity fibers made of UHMW-PE are utilized to fabricate plain-weave fabrics prototypes, and the thermal properties of individual fibers, yarns, and woven fabrics are measured using a novel in-plane thermal measurement method. The characterization technique leverages infrared (IR) microscopy for a non-contact temperature sensing and is generally scalable for thermal characterization of the inplane thermal-conductivity of materials across different length scales. Effective thermal conductivities on the order of ~10 Wm-1K-1 are achieved along the in-plane dominant heat transport direction of the woven fabric, which is exceptionally high (~2-3 orders of magnitude) compared to conventional clothing and textile-based materials. The thermal conductivity and mechanical flexibility of the UHMW-PE fabrics are benchmarked with respect to conventional materials and the effect of bend-stressing and thermal annealing of the fabrics is characterization using the developed metrology. Additionally, a laser-based IR thermal metrology technique leveraging both non-contact heating and temperature sensing is conceptualized and validated using a numerical thermal modeling approach. The proposed technique provides an approach to estimate the in-plane heat spreading properties of anisotropic materials with direction-depended thermal properties based on quantifying the surface temperature map of a sample subjected to periodic heating. Numerical simulations are leveraged to demonstrate the applicability of this method to enable measurement of a wide range of thermal properties indicating great potential to develop this further as a standardized robust method for in-plane anisotropic thermal characterization of materials such as fabrics and films. This work sheds light on the high thermal conductivity of UHMW-PE materials that can be achieved using a scalable manufacturing process and describes the thermal metrology approaches to enable their characterization, thereby providing a foundation for the conceptualization and design of flexible substrate based thermal solutions in future wearable/flexible electronic devices

Neuroprotective Effects of Fingolimod in a Cellular Model of Optic Neuritis

Visual dysfunction resulting from optic neuritis (ON) is one of the most common clinical manifestations of multiple sclerosis (MS), characterized by loss of retinal ganglion cells, thinning of the nerve fiber layer, and inflammation to the optic nerve. Current treatments available for ON or MS are only partially effective, specifically target the inflammatory phase, and have limited effects on long-term disability. Fingolimod (FTY) is an FDA-approved immunomodulatory agent for MS therapy. The objective of the current study was to evaluate the neuroprotective properties of FTY in the cellular model of ON-associated neuronal damage. R28 retinal neuronal cell damage was induced through treatment with tumor necrosis factor-α (TNFα). In our cell viability analysis, FTY treatment showed significantly reduced TNFα-induced neuronal death. Treatment with FTY attenuated the TNFα-induced changes in cell survival and cell stress signaling molecules. Furthermore, immunofluorescence studies performed using various markers indicated that FTY treatment protects the R28 cells against the TNFα-induced neurodegenerative changes by suppressing reactive oxygen species generation and promoting the expression of neuronal markers. In conclusion, our study suggests neuroprotective effects of FTY in an in vitro model of optic neuritis

Process optimization of graphene growth in a roll-to-roll plasma CVD system

A systematic approach to mass-production of graphene and other 2D materials is essential for current and future technological applications. By combining a sequential statistical design of experiments with in-situ process monitoring, we demonstrate a method to optimize graphene growth on copper foil in a roll-to-roll rf plasma chemical vapor deposition system. Data-driven predictive models show that gas pressure, nitrogen, oxygen, and plasma power are the main process parameters affecting the quality of graphene. Furthermore, results from in-situ optical emission spectroscopy reveal a positive correlation of CH radical to high quality of graphene, whereas O and H atoms, Ar+ ion, and C2 and CN radicals negatively correlate to quality. This work demonstrates the deposition of graphene on copper foil at 1 m/min, a scale suitable for large-scale production. The techniques described here can be extended to other 2D materials and roll-to-roll manufacturing processes

Integrating Electrical and Mechanical Design and Process Planning

This paper reports on the development of the process-planning module for EDAPS, an integrated system for designing and planning the manufacture of microwave modules. Microwave modules are complex devices having both electrical and mechanical properties, and EDAPS integrates electrical design, mechanical design, and process planning for both the mechanical and electrical domains. EDAPS's process planning module provides an integrated approach to process planning in both the electronic and mechanical domains, specifically in the manufacture of microwave transmit-receive (T/R) modules. It enables EDAPS to generate process plans concurrently with design---and we are developing ways for EDAPS to use the process planning information provide feedback to designers about manufacturability, cost, and lead time for manufacturing their designs. The planning module is based on a modified version of an AI planning methodology called Hierarchical Task Network (HTN) planning. We provide an overview of..