Nitin Mehra

Expedited Phonon Transfer in Interfacially Constrained Polymer Chain along Self-Organized Amino Acid Crystals

In this work, poly(vinyl alcohol) (PVA)/amino acid (AA) composites were prepared by a self-organized crystallization process. Five different AAs (cysteine, aspartic acid, glutamic acid, ornithine, and lysine) were selected based on their similar functional groups but different molecular structures. The different PVA-AA interactions in the five PVA/AA composites lead to two crystal patterns, i.e., continuous network (cysteine and lysine) and discrete particles (glutamic acid, ornithine, and aspartic acid). Scanning thermal microscopy is then applied to map the distribution of thermal conduction in these composites. It is found that the interface surrounding the crystals plays a dominating role in phonon transport where the polymer chains are greatly restrained by the interfacial confinement effect. Continuous crystal network builds up a continuous interface that facilitates phonon transfer while phonon scattering occurs in discrete crystalline structures. Significantly improved thermal conductivity of ∼0.7 W/m·K is observed in PVA/cysteine composite with AA loading of 8.4 wt %, which corresponds to a 170% enhancement as compared to pure PVA. The strong PVA-AA molecular interaction and self-organized crystal structure are considered the major reasons for the unique interface property and superior thermal conductivity.

A review on the role of interface in mechanical, thermal, and electrical properties of polymer composites

AbstractComposite materials and especially polymer composites are widely used in daily life and different industries due to their vastly different properties and design flexibility. It is known that the properties of the composites are strongly related to the properties of its constituents. However, it has been reported in many studies, experimentally and by simulations, that the characteristics of the composites do not follow the rule of mixing. It means that in addition to properties of the constituents, there are other parameters affecting the final physicochemical properties of composites. The interfacial interactions between fillers and host is one of the factors which can strongly affect the properties of the composite. In this review, we summarized the type of interactions between the constituents, their improvement techniques, interaction measurement methods, and the effects of interfacial interactions on thermal, mechanical, and electrical properties of composites. Graphical abstractThe interface of composites especially nanocomposites greatly determines material behavior including but not limited to mechanical, thermal, and electrical properties.

Realizing the nanoscale quantitative thermal mapping of scanning thermal microscopy by resilient tip–surface contact resistance models

Quantitative assessment of thermal properties by scanning thermal microscopy (SThM) is a demanded technology, but still not yet available due to the presence of unpredictable thermal contact resistance (TCR) at the tip/substrate interface. The TCR is mainly affected by three major interfacial characteristics including surface roughness, hardness and contacting force. In this work, the TCR is mathematically derived into linear and non-linear models based on the interfacial micro-characteristics. The models have the capability to predict the TCR for both rough and smooth surfaces with satisfactory accuracy. With a predictable TCR, the heat transport across the tip/substrate nanointerface can be precisely described and thus quantitative thermal properties can be predicted from SThM measurements. The models are tested in three polymeric material systems, PDMS, epoxy and PVA. The thermal conductivity from the model prediction matches very well (<10% error) with the measured values from bulk polymer samples. Such models use general surface features as inputs, so they have wide applicability to other similar materials, especially polymers. Moreover, the models have been shown to be valid in doped PVA samples when extrapolated to predict thermal conductivity beyond the range of model development. This work extends the capability of SThM in quantitative measurement and enables a unique platform for thermal conductivity measurement at nanometer spatial resolution.

Permselective H2/CO2 Separation and Desalination of Hybrid GO/rGO Membranes with Controlled Pre-crosslinking

Covalent bonding is widely adopted in graphene oxide (GO) membrane to improve structural integrity and restrict swelling, while it comes with a price of enlarged d-spacing and sacrifices membrane selectivity. This work offers a facile strategy to break the trade-off between membrane stability and selectivity. Specifically, graphene oxide (GO)/reduced graphene oxide (rGO) hybrid membranes were fabricated by a controlled pre-cross-linking method. With this method, restricted swelling by cross-linking and reduced d-spacing by GO reduction can be achieved simultaneously by controlling reaction time. Membranes were prepared on porous alumina support by vacuum filtration method. Two different d-spacing values (∼12.0 and ∼7.5 Å) were found in the hybrid membrane, representing the layer structures of expanded GO interspacing with inserted cross-linker and reduced layer spacing after GO reduction. The presence of such mixed layer structures enables restricted swelling, excellent mechanical strength, and unique separation property. The hybrid membrane shows excellent permselective H2/CO2 separation with a separation factor of 22.93 ± 1.57 and H2 permeance of 2.46 ± 0.01× 10-8 mol m-2 s-1 Pa-1. In desalination test with 3.5 wt % sea salt solution, the hybrid membrane shows high ion (Na+, K+, Mg2+, Cl-, and SO42-) rejection rate of above 99%, as well as excellent durability.