Mechanically Strong Polymer Sheets from Aligned Ultrahigh

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Mechanically Strong Polymer Sheets From Aligned UltraHigh Molecular Weight Polyethylene Nanocomposites Zhuolei Zhang, Santosh Mogurampelly, Simona Percec, Yong Hu, Giacomo Fiorin, Michael L. Klein, and Shenqiang Ren J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00790 • Publication Date (Web): 27 Apr 2018 Downloaded from http://pubs.acs.org on April 30, 2018

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The Journal of Physical Chemistry Letters

Mechanically Strong Polymer Sheets From Aligned Ultra-high

Molecular

Weight

Polyethylene

Nanocomposites Zhuolei Zhang⊥ ‖, Santosh Mogurampelly‡⊥ ‖, Simona Percec‡⊥ , Yong Hu†, Giacomo Fiorin‡⊥ *, Michael L Klein‡⊥ *, and Shenqiang Ren†⊥ * † Department of Mechanical and Aerospace Engineering, Research and Education in Energy, Environment & Water (RENEW) Institute, University at Buffalo, The State University of New York, Buffalo, NY 14260 ‡ Institute for Computational Molecular Science, and Center for the Computational Design of Functional Layered Materials, Temple University, Philadelphia, PA 19122, USA ⊥ Temple Materials Institute, Temple University, Philadelphia, PA 19122, USA

Corresponding Author [email protected]; [email protected]; [email protected]

ACS Paragon Plus Environment

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ABSTRACT: Ultra-high molecular weight polyethylene (UHMWPE) is of great interest as a nextgeneration body armor material due to its superior mechanical properties. However, such unique properties depend critically on its microscopic structure characteristics, including the degree of crystallinity, chain alignment and morphology. Here we present a highly aligned UHMWPE and of its composite sheets containing uniformly dispersed boron nitride (BN) nanosheets. The dispersion of BN nanosheets into the UHMWPE matrix increases its mechanical properties over a broad temperature range. Experiments and simulation confirm that the alignment of chain segments in the composite matrix increases with temperature, leading to an improvement in mechanical properties at high temperature. Together with the large thermal conductivity of UHMWPE and BN, our findings serve in expanding the application spectrum of highly aligned polymer nanocomposite materials for ballistic panels and body armor over a broad range of temperatures. TOC


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Ultra-high molecular weight polyethylene (UHMWPE) is a linear homo-polymer bearing (CH2-CH2-)n - as the repeat unit and having an average molecular weight in the range of 3,500,000 -7,500,000 g/mol (n ≈100,000 monomeric units).1-2 Its superior mechanical properties derive from the enormous number of covalently linked monomeric units giving rise to UHMWPE. Despite weak van der Waals interactions between polymer chains, the presence of a large amount of aligned overlaps between neighboring chains can lead collectively to high intermolecular strength.3-4 Due to its excellent mechanical properties, chemical stability and effective impact load damping, UHMWPE derived materials have been extensively used in military armor, 5-6 orthopedic bearing materials, 7 and additive manufacturing, 8 to name just three examples. The mechanical and thermal properties of UHMWPE materials are inextricably linked to their crystalline organization. Bulk UHMWPE is primarily comprised of crystalline domains, which are bridged by nanoscale amorphous. The crystalline lamellae consist of several rows of tightly packed -CH2-CH2- monomeric units, while the surrounding amorphous regions consist of randomly oriented and entangled polymer chains traversed by tie molecules to interconnect lamellae. Thus, the non-homogeneous nature of bulk UHMWPE accommodates abundant defects, which act as the stress concentration and phonon scattering sites, leading to relatively poor mechanical strength and thermal conductivity.9-10 A variety of strategies have been pursued in the quest for improving the mechanical and thermal properties of UHMWPE materials.11-17 For example, manipulating the polymer chain alignment to form the ordered UHMWPE fibers has shown improved mechanical strength and thermal conductivity.11-12 The high crystallinity in the aligned UHMWPE enables long-range intermolecular order and a decrease in density of defects. As a result, it can simultaneously increase


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mechanical properties and decrease the phonon scattering to facilitate efficient heat transfer. Besides tuning the alignment of polymer chains, doping UHMWPE with nanoparticles has also garnered significant attention. Specifically, the improved mechanical properties of UHMWPE composites have been shown through doping of nanoparticles, such as silicon nitride (Si3N4),13-14 silicon carbide (SiC),15 boron nitride (BN), 16 and aluminum nitride (AlN).17 However, it is still a significant challenge to produce polymer composites that simultaneously possess high mechanical strength and thermal conductivity. Towards the above objectives, we explored use of hexagonal boron nitride (h-BN) as dopant into the UHMWPE matrix based on its high thermal conductivity, excellent mechanical properties, low coefficient of thermal expansion (CTE), nontoxicity, and high electrical resistivity over a wide range of operating temperatures. 18 This nanocomposite of highly aligned UHMWPE chains and BN nanosheets possesses enhanced mechanical properties at elevated temperatures. The improved mechanical properties at high temperature result from changes of segmental order parameters in monomeric to monomeric units. The mechanical properties and thermal conductivity of BNUHMWPE composite are determined by the number of layers and loading concentration of BN sheets as well as the chain alignment in the UHMWPE matrix. To facilitate the uniform mixing between BN and UHMWPE, both components were mixed at an elevated temperature to form a composite gel. Then, a composite sheet was mechanically aligned at high temperature to increase its crystallinity. Accordingly, a solution-based processing technique was applied to fabricate a BN-UHMWPE composite (Figure 1a-d),

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in which the

liquid-exfoliated BN layers are uniformly mixed with the UHMWPE matrix in 1,2dicholorobenzene at 493 K to form a gel. Bulk BN transforms to ultrathin nanosheets with an average thickness of about 3.2 nm after exfoliation (Figure 1e, 1f and Figure S1). The gel is then


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coated onto a glass substrate to form thin film composites, which are subsequently peeled off to generate free-standing BN-UHMWPE sheets (Figure 1a). The free-standing sheets are further processed under thermo-mechanical stretching (Figure 1b-c) in a temperature range of 413 K to 473 K with a controlled speed. The aligned BN-UHMWPE sheets show a wrinkle-like surface morphology due to the alignment of polymer chain segments (Figure 1g), while the untreated sheet exhibits isotropic structure (the inset of Figure 1g). The aligned BN-UHMWPE composite maintains its flexibility and semitransparency after the alignment (Figure 1h). The morphological changes induced by the thermo-mechanical treatment were probed by using atomic force microscopy (AFM).22-23 The results revealed in Figure 1i show highly oriented polymer fibers with respect to the isotropic structure before alignment (the inset of Figure 1i). The X-ray diffraction pattern of the pristine sheets shown in Figure 1j displays two distinct peaks at 21.44o and 23.99o, corresponding to the (110) and (200) planes, respectively. 24 The positions of these peaks are unchanged after the alignment of UHMWPE composite sheets. However, the intensity of (200) peak increases significantly after the alignment, indicating the enhancement of crystallinity. In addition, the full width at half maximum (FWHM) of the (002) peak of BN phase in the X-ray diffraction pattern increases while its intensity decreases, suggesting reduced van der Waals interactions between BN layers within the UHMWPE matrix (Figure 1j).


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Figure 1. Morphology and structure of BN-UHMWPE composite sheets. (a-c) Schematic illustration of the alignment procedure of BN-UHMWPE composite sheets. (d) Chemical structure of UHMWPE and BN. e) TEM image of exfoliated BN nanosheets. (f) AFM image of exfoliated BN nanosheets. Inset: Corresponding 3D AFM image. (g) SEM image of aligned BN-UHMWPE composite sheets. Inset: SEM image of BN-UHMWPE composite sheets. (h) Photographs of a 2 µm thick aligned BN-UHMWPE composite sheet wrapped around a glass tube. Bottom: Semitransparency of a 2 µm thick aligned BN-UHMWPE composite sheet. (i) AFM image of aligned BN-UHMWPE composite sheets. Inset: AFM image of BN-UHMWPE composite sheets. (j) XRD patterns of bulk BN, exfoliated BN, bulk UHMWPE, BN-UHMWPE melt and aligned BN-UHMWPE sheets.


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In order to quantify the effect of doping and alignment on the mechanical properties of UHMWPE and BN-UHMWPE composite sheets, nanoindentation measurements were performed by applying an external load through a nanoindenter tip at the sheet surface. The Young’s modulus, E, of UHMWPE and BN-UHMWPE composite can be obtained from the force-displacement behavior as follows: 1 (1  v 2 ) (1  vi2 ) ,  S , (1)   Er  Er E Ei 2 A

where A is the contact surface area at peak load, S is the initial unloading contact stiffness calculated using

S

dP , dh

β is an indenter geometry dependent dimensionless parameter25, Er is the

reduced modulus, Ei is the Young’s modulus of the indenter, ν, νi are the Poisson’s ratio of the specimen and the indenter, respectively.25 The measurements are conducted in the force-control mode in which the maximum force applied on the BN-UHMWPE composite sheet is 100 µN. Figure 2a displays the Young’s modulus of UHMWPE, BN-UHMWPE and bulk BN doped UHMWPE sheets, indicating that the BN-UHMWPE sheets exhibit the highest elastic modulus.26 For the bulk BN doped UHMWPE sheets, the force-displacement curves exhibit discrete steps, indicating a non-uniform distribution of BN particles (Figure S2a). In contrast, the uniform forcedisplacement curves of BN-UHMWPE sheets suggest homogeneously distributed stress relaxation throughout the material (Figure S2b). This is facilitated by the large contact area between exfoliated BN nanosheets and the polymer chains leading to an optimum elastic modulus. Without exfoliation, the bulk BN particles (white dots, the inset of Figure S1) tend to aggregate within the UHMWPE matrix in contrast to the homogeneously distributed exfoliated BN nanosheets. As a result, the Young’s modulus of the bulk BN doped UHMWPE sheets fluctuates largely (>50%), as compared to the small fluctuations observed for the BN-UHMWPE nanosheets.


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Figure 2. Alignment effect on mechanical properties of BN-UHMWPE composite film. (a) Compressive Young’s modulus of UHMWPE film, BN-UHMWPE composite sheets and bulk BN-doped UHMWPE sheets. (b) Force-displacement curves of BN-UHMWPE composite sheets aligned at different temperatures from 373 K to 473 K. (c) Young’s modulus of BN-UHMWPE composite sheets at different alignment degrees. (d-e) XPM images of BN-UHMWPE composite sheets and aligned BN-UHMWPE composite sheets. (f) Calculated average Young’s modulus and error bars of BN-UHMWPE composite sheets and aligned BN-UHMWPE composite sheets. The force-displacement curves are shown in Figure 2b for thermo-mechanically treated BNUHMWPE sheets containing a large proportion of aligned polymer chains under different processing temperatures. An applied tensile stress along the aligned BN-UHMWPE composite sheets causes the covalent bonds within the chain to stretch. The thermally-treated BN-UHMWPE polymer sheets are characterized by different degrees of alignment depending on the processing temperature (Figure S3). The stretching behavior is elastic and the polymer chains of UHMWPE


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move back to their original position after returning to low temperature. It can be speculated that a higher temperature facilitates the rearrangement of the orientation of UHMWPE chains under external tensile strain, increasing the material’s stiffness. Therefore, the Young’s modulus of BNUHMWPE nanocomposite increases with the degree of alignment (Figure 2c), as indicated by the force-displacement curves. To confirm the validity of above considerations, we use ultra-fast extreme property mapping (XPM) of BN-UHMWPE and aligned BN-UHMWPE composite sheets to obtain spatial distribution maps for the elastic modulus (Figure 2d-e). The measurement is applied on the surface of BN-UHMWPE sheets, spanning an area of 5×5 µm2 with a 5×5 grid. The corresponding XPM images of the sheets before and after alignement are shown in Figure 2d and 2e, respectively. The aligned BN-UHMWPE sheets exhibit an average modulus E = ~4.1 GPa, which is larger than that of the isotropic sheet at ~3.0 GPa. The calculated average modulus of these sheets is presented in Figure 2f, where both the aligned and isotropic sheets confirm the enhancement of the mechanical properties after the polymer alignment. In addition to the polymer chain alignment effect, we investigate also the effect of BN concentration on the elastic modulus of aligned BN-UHMWPE sheets (Figure S6). Specifically, the BN loading concentration from 0 to 40 wt% were used and both the compressive and tensile modulus of aligned BN-UHMWPE sheets were determined (Figure 3a). The compressive Young’s modulus of aligned BN-UHMWPE composite increases from 2.7 GPa at 0 wt% of BN to 3.9 GPa at 40 wt% of BN content. Furthermore, the ultra-fast extreme property mapping measurements (shown in Figure S5) reveals a uniformly-distributed elastic modulus with small variations of ± 0.3 GPa. Moreover, the average Young’s modulus of BN-UHMWPE composite also increases with the increasing the concentration of BN, due to higher rigidity of BN nanosheets compared to the polymer matrix.


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Figure 3. Effects of BN concentration on mechanical properties of aligned BN-UHMWPE composite sheets. (a) Compressive stress-strain curves of aligned BN-UHMWPE composite sheet with various BN concentrations ranging between 0 and 40 wt%. (b) Compressive Young’s modulus of aligned BN-UHMWPE composite sheets as a function of BN concentration. (c) Tensile stress-strain curves of aligned BN-UHMWPE composite sheets at different BN concentrations. (d) Tensile Young’s modulus and elongation at break of aligned BN-UHMWPE composite sheets as a function of BN concentration. The stress-strain curves characterizing the tensile mechanical properties of BN-UHMWPE are presented in Figure 3c. The stress-strain behavior reveals similar trends of initial linear elasticity, non-linear transition and collapse stages.27 The linear portion of the curve is the elastic region. The


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Young's modulus could be calculated by the slope (the ratio of stress/strain) in the elastic region and the elongation is obtained from the breaking point. There is a significant increase of tensile Young’s modulus with the incorporation of BN nanosheets for loadings below 20 wt%. The above results suggest that the exfoliated BN nanosheets can contribute considerably to the overall mechanical properties, in spite of destabilizing the polymer-polymer alignment. On the other hand, the alignment of polymer chains on the surface of BN nanosheets enables the transfer of mechanical load across the interface between BN nanosheets and UHMWPE matrix, which contributes to the increase of Young’s modulus of the composite. Not surprisingly, further increase of BN loading beyond 20 wt% is seen to have no significant effect on the mechanical properties. The above result can be understood as a consequence of the disrupting effect of BN on polymerpolymer interactions. Unlike the elastic modulus, the elongation at break of BN-UHMWPE slightly increases with BN content up to 5 wt% and significantly decreases with further increase of BN (Figures 3c-d). Below 5 wt% BN, the strain for breaking increases slightly due to the interaction between BN and UHMWPE matrix. While the strain for the breaking decreases, as the incorporation of more than 5 wt% BN nanosheets can act as the crack terminator leading to small elongation of the composite sheets at the breaking point.


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Figure 4. Classical MD simulations on aligned BN/UHMWPE composite. (a-b) Initial configurations for MD simulations of pre-aligned pure UHMWPE and BN dispersed UHMWPE. The system contains 47,916 monomeric units; for the BN-UHMWPE composite, six BN sheets (equalling a 20 wt% concentration) are uniformly distributed within the polyethylene sheets. (c) Monomeric unit – monomeric unit separations and (d) angle distributions of polyethylene chains of the equilibrium structures demonstrating a high degree of polymer alignment, representatives of experimentally investigated pure and composite films (Inset: Equilibrated configurations of pure UHMWPE and BN dispersed UHMWPE at room temperature). (e) Elastic moduli of the aligned


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films along the X-axis, Y-axis and Z-axis at room temperature (chains are aligned along Z and sheets are parallel to the Y-Z plane). The results discussed above demonstrate that the incorporation of BN nanosheets into UHMWPE enhances its mechanical properties, with a maximum observed at a loading of 20 wt%. In order to understand further the BN doping effect on the mechanical properties of highly aligned UHMWPE sheets, molecular dynamics (MD) simulations28-30 of pure UHMWPE and composite sheets (with a loading of 20 wt%) were performed. For the purpose of modeling highly aligned polymer chains, we build a pure UHMWPE crystal consisting of 47,916 monomeric units (with a molar mass of 1,344,523 g/mol) and an aligned BN-UHMWPE composite as shown in Figure 4ab. Specifically, covalently attached ethylene monomeric units are systematically constructed on a 2D lattice to form a large sheet (Inset of Figure 4a). We refer in the following to the axis parallel to the polymer chains in this initial configuration as the director axis, n. Such 2D sheets are replicated in the orthogonal dimension, and covalent linkages are created between monomers of the sheets to form a 3D mesh as shown in Figure 4a. For the composite structures, six BN sheets dimensions of

are intercalated within the PE sheets resulting in 20 wt% BN loading in

the polymer nanocomposite (Figure 4b). The above models of UHMWPE and aligned BNUHMWPE are then subjected to an equilibration protocol described in the methods section, and 50 ns long constant pressure-temperature (NPT) simulations is performed at 1 atm pressure and room temperature for each combination of model system and temperature. The equilibrium densities at room temperature of the UHMWPE and the aligned BN-UHMWPE are found to be 0.984 and 1.052 g/cc, respectively. The states equilibrated at room temperature (shown in the inset of Figure 4d) display a high degree of polymer alignment with sparsely distributed amorphous regions. Specifically, the tangent vector, t|𝑡̂| representing the monomer-monomer alignment is seen


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to align along the director axis, n. The distribution of |t̂| is peaks at 2.50 Å (Inset of Figure 4c), in close comparison with perfect crystalline monomeric unit to monomeric unit distance of 2.54 Å (Figure 4c). Similarly, the distribution of angles between tangent vectors and the director axis, , reveals that for pure UHMWPE maintains a very high degree of alignment as shown in the inset of Figure 4d. Interestingly, the loading of BN into pure UHMWPE induces misalignment of tangent vectors away from the director axis. The calculated order parameters

of UHMWPE and

BN dispersed UHMWPE are 0.867 and 0.668, respectively. The equilibrium polymer models are then subjected to external loading as outlined in the inset of Figure 4e. Independent simulations are carried out to apply mechanical stress along all three directions. The computed elastic moduli along x, y and z directions for both the aligned matrices are presented in Figure 4e. Despite a lower order parameter for BN dispersed UHMWPE composite, the elastic modulus is found to be higher than that of the UHMWPE matrix. The above results indicate that the contribution of BN sheets to the elastic modulus of the composite involves a significant structural change. The mechanical properties at increasing temperature are studied through in-situ nanoindentation and MD simulations. The Young’s modulus increases with the increase of temperature for both highly aligned UHMWPE and BN-UHMWPE sheets (Figure 5a-b). To understand the mechanistic origins of the increase in modulus at high temperature, it is useful to examine how different molecular segments of polymer chains respond to the temperature. Therefore, we carried out a heating simulation of both systems under NPT conditions and analyse the polymer segmental order parameter. Results of the normalized order parameter as a function of temperature calculated from united atom simulations are displayed in Figure 5c (Figure S3 represents the order parameter

as a function of temperature). Surprisingly, we find that

increases with temperature for both the pure UHMWPE and BN sheet dispersed UHMWPE


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system. However, the dispersion of BN nanosheets into the polymer matrix is seen to decrease compared to the pure UHMWPE. The introduction of BN sheets into a highly ordered polymer matrix is expected to influence (mostly, break) the polymer chain order around its surface. Accordingly, the computed polymer segment-segment coordination number changes dramatically near the BN surface leading to decreasing values of

⟨S2⟩ at any given temperature. The counter-

intuitive behavior of temperature effects on order parameter (and consequently the mechanical properties) can be understood to be a result of the changes in local ordering of polymer segments. It appears that an increase in temperature helps in overcoming local free energy barriers in a complex polymer network. In this case, C-C bond vectors defining the polymer segments are more likely to reach a higher degree of alignment. Consequently, the order parameter defining the morphology in each system (UHMWPE and BN-UHMWE) increases at higher temperature. Moreover, the increased degree of freedom of polymer segments facilitates the formation of selfassembled crystalline regimes in higher proportions. Collectively, the above discussed mechanisms give rise to enhanced mechanical properties of pure UHMWPE and BN-UHMWPE at higher temperature, rationalizing our experimental findings. The thermal diffusion abilities of UHMWPE and aligned BN-UHMWPE sheets of 20 wt% loading are captured by using the nearinfrared camera. This shows the similar aberration chromatique throughout the image at initial stage, indicating the same starting temperature. The sheets are then naturally cooled to room temperature, and the difference in aberration chromatique becomes larger between the two samples with the extension of the cooling time (Figure 5d and S7). While the aligned BN-UHMWPE composite sheets exhibit higher thermal conductivity, their heat diffusion is much faster than that of the pure UHMWPE. Therefore, the temperature of aligned BN-UHMWPE composite sheets would be lower than that of the pure UHMWPE. The temperature variation can be resolved from


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the cooling evolution of near-infrared images. The temperature difference can reach up to 2.2 K after 90 s, indicating a faster thermal conductivity of the aligned BN-UHMWPE composite material (Figure 5d).

Figure 5. Temperature effects on thermo-mechanical properties of aligned UHMWPE and BNUHMWPE composite sheets. (a) Experimentally measured elastic moduli of UHMWPE and aligned BN-UHMWPE composite sheets with increasing temperature. (b) Elastic moduli of the aligned films along X-axis, Y-axis and Z-axis (inset) at various temperatures from MD simulations. (c) The variation of normalized order parameter with temperature, characterizing the monomer-monomer alignment of polyethylene with respect to the film direction. (d) Cooling curve


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of UHMWPE film and aligned BN-UHMWPE composite sheet at the same conditions. Inset: Near-infrared images of UHMWPE film (right) and aligned BN-UHMWPE composite sheets (left) at the initial and cooled stage. We have investigated BN nanosheets-doped UHMWPE composites with high degree of alignment through a thermal-mechanical tension process. The BN nanosheets play an important role in uniformly improving the mechanical properties of UHMWPE, compared to the bulk BN loading. This is due to the strong interfacial alignment of polymer chains on the surface of BN sheets. Furthermore, the aligned UHMWPE sheets exhibit higher mechanical modulus than that of UHMWPE in which the polymer chains are randomly oriented with respect to each other. In-situ nanoindentation also shows an increase of mechanical properties with temperature. Based on the results of MD simulations, we suggest that at elevated temperature the local free energy barriers in the polymer network, are overcome and the polymer segments are more likely to reach a higher degree of alignment. Given that the temperatures used are well below melting, the increased degree of freedom of polymer segments facilitates the formation of the self-assembled crystals. These effects collectively give rise to enhanced tensile and compressive properties of UHMWPE and BN-UHMWPE at high temperatures. We have not addressed the ultimate strength of BNUHMWPE in response to shear strain parallel to the films’ plane: it is likely that any losses in this direction are mitigated by the finite size of the BN sheets and their deviations from the ideal orientation. Furthermore, the aligned UHMWPE sheets show fast thermal diffusion and higher thermal conductivity, which will be useful in expanding the temperature range of UHMWPE-based materials for protective purposes.


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AUTHOR INFORMATION Notes ‖ Z.Z. and S. M. contributed equally to this paper. The authors declare no competing financial interest. ACKNOWLEDGMENT This research was supported by the Army Research Laboratory under contract number W911NF16-2-0189 to Temple University. Work (S.R) was supported by the Army Research Office - Young Investigator Program (W911NF-15-1-0610). All authors discussed the results and commented on the manuscript. Z.Z. carried out experiments and drafted the paper. S.M. performed the calculation and drafted the paper. S.P., G.F., M.K., and S.R. revised the paper. G.F., M.K., S.R. designed, guided and directed the project. Supporting Information Available: 2D height and 3D AFM image of the exfoliated BN nanosheets. Force-displacement curves of few layered BN doped UHMWPE melt and the bulk BN doped UHMWPE melt. XRD patterns of BN-UHMWPE composite with different alignment degree.

The DSC measurements on BN-UHMWPE composite with different BN doping

concentration. XPM images of few layered aligned BN doped UHMWPE composite with different BN content. The variation of order parameter with temperature, characterizing the monomermonomer alignment of polyethylene with respect to the film direction. Near infrared evolution images during the cooling process of BN-UHMWPE composite and pure UHMWPE matrix.


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