Thermal conductivity (k) of polymers is usually limited to low values of ~0.5 Wm−1K−1 in comparison to metals (>20 Wm−1K−1). The goal of this work is to enhance thermal conductivity (k) of polyethylene–graphene nanocomposites through simultaneous alignment of polyethylene (PE) lamellae and graphene nanoplatelets (GnP). Alignment is achieved through the application of strain. Measured values are compared with predictions from effective medium theory. A twin conical screw micro compounder is used to prepare polyethylene–graphene nanoplatelet (PE-GnP) composites. Enhancement in k value is studied for two different compositions with GnP content of 9 wt% and 13 wt% and for applied strains ranging from 0% to 300%. Aligned PE-GnP composites with 13 wt% GnP displays ~1000% enhancement in k at an applied strain of 300%, relative to k of pristine unstrained polymer. Laser Scanning Confocal Microscopy (LSCM) is used to quantitatively characterize the alignment of GnP flakes in strained composites; this measured orientation is used as an input for effective medium predictions. These results have important implications for thermal management applications.
In this work, we demonstrate that edge oxidation of graphene can enable larger enhancement in thermal conductivity (k) of graphene nanoplatelet (GnP)/polyetherimide (PEI) composites relative to oxidation of the basal plane of graphene. Edge oxidation offers the advantage of leaving the basal plane of graphene intact, preserving its high in-plane thermal conductivity (k in > 2000 W m −1 K −1 ), while, simultaneously, the oxygen groups introduced on the graphene edge enhance interfacial thermal conductance through hydrogen bonding with oxygen groups of PEI, enhancing the overall polymer composite thermal conductivity. Edge oxidation is achieved in this work by oxidizing graphene in the presence of sodium chlorate and hydrogen peroxide, thereby introducing an excess of carboxyl groups on the edge of graphene. Basal plane oxidation of graphene, on the other hand, is achieved through the Hummers method, which distorts the sp 2 carbon−carbon network of graphene, dramatically lowering its intrinsic thermal conductivity, causing the BGO/PEI (BGO = basal-plane oxidized graphene or basal-plane-functionalized graphene oxide) composite's k value to be even lower than pristine GnP/PEI composite's k value. The resulting thermal conductivity of the EGO/PEI (EGO = edge-oxidized graphene or edge-functionalized graphene oxide) composite is found to be enhanced by 18%, whereas that of the BGO/PEI composite is diminished by 57%, with respect to the pristine GnP/PEI composite with 10 wt % GnP content. Two-dimensional Raman mapping of GnPs is used to confirm and distinguish the location of oxygen functional groups on graphene. The superior effect of edge bonding presented in this work can lead to fundamentally novel pathways for achieving high thermal conductivity polymer composites.
To achieve high thermal conductivity (k) of polymer graphene nanocomposites, it is critically important to achieve efficient thermal coupling between graphene and its surrounding polymers through effective functionalization schemes. In...
We demonstrate in this work that expanded graphite (EG) can lead to a very large enhancement in thermal conductivity of polyetherimide−graphene and epoxy−graphene nanocomposites prepared via solvent casting technique. A k value of 6.6 W⋅m−1⋅K−1 is achieved for 10 wt% composition sample, representing an enhancement of ~2770% over pristine polyetherimide (k~0.23 W⋅m−1⋅K−1). This extraordinary enhancement in thermal conductivity is shown to be due to a network of continuous graphene sheets over long−length scales, resulting in low thermal contact resistance at bends/turns due to the graphene sheets being covalently bonded at such junctions. Solvent casting offers the advantage of preserving the porous structure of expanded graphite in the composite, resulting in the above highly thermally conductive interpenetrating network of graphene and polymer. Solvent casting also does not break down the expanded graphite particles due to minimal forces involved, allowing for efficient heat transfer over long−length scales, further enhancing overall composite thermal conductivity. Comparisons with a recently introduced effective medium model show a very high value of predicted particle–particle interfacial conductance, providing evidence for efficient interfacial thermal transport in expanded graphite composites. Field emission environmental scanning electron microscopy (FE−ESEM) is used to provide a detailed understanding of the interpenetrating graphene−polymer structure in the expanded graphite composite. These results open up novel avenues for achieving high thermal conductivity polymer composites.
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