Molecular Dynamics Simulation on the Heat Transfer in the Cross-Linked Poly(dimethylsiloxane)

In this work, the effect of cross-linking degree and stretching on the thermal conductivity of poly(dimethylsiloxane) (PDMS) is explored by performing a molecular dynamics simulation. Our results demonstrate that the thermal conductivity of PDMS exhibits a monotonous rise with an increase in the cross-linking degree. By decomposing the total heat flux into three microscopic heat transfer modes, the high cross-linking degree improves the contribution from bonding interactions to the heat transfer more than that from the nonbonding interactions. An analysis of the vibrational density of states shows a blue-shift of the vibrational modes at low frequencies, indicating a large phonon group velocity due to the strong interchain bonding interaction. From the spectral distribution of heat flux, the spectral contributions are shifted toward the higher frequencies with the increasing cross-linking degree, which reflects more contribution from the high-frequency modes to the heat transfer. Stretching can improve the thermal conductivity parallel to the tensile direction with the increase in strain. This is mainly due to the further improved contribution of bonding interactions or high-frequency modes to heat transfer. Interestingly, the anisotropy of the thermal conductivity first decreases and then increases with the increasing cross-linking degree. Our study conducts a detailed investigation of the thermal conductivity of cross-linked PDMS, providing guidance on the application of thermal interface materials.

Tuning the Thermal Conductivity of the Amorphous PAA Polymer via Vapor-Phase Infiltration

The thermal properties of the polymer, together with mechanical stability, have been one of the key engineering factors to be considered for various applications. Here, we engineered the thermal conductivity of the amorphous poly(acrylic acid) (PAA) polymer by vapor-phase infiltration (VPI), which has usually occurred during the atomic layer deposition process. We observed that the VPI causes metal infiltration (e.g., Al and Zn) into the amorphous PAA polymer, which noticeably increases the thermal conductivity of the PAA polymer. From spectroscopy analysis and density functional theory simulations, we found that the carboxyl groups (-COOH) in PAA are notably modified and the bonding states of carbon and oxygen are significantly altered by the infiltrated metal. The newly formed Al-mediated bonds likely provide continuous phonon propagation pathways, thereby enhancing the thermal conductance. We believe that VPI could be a simple and useful way to engineer the thermal properties of various polymeric materials.

Thermal Conductivities of Crosslinked Polyisoprene and Polybutadiene from Molecular Dynamics Simulations

For the first time, the thermal conductivities of vulcanized polybutadiene and polyisoprene have been investigated according to their degree of crosslinking. The C-C and C-S-S-C crosslink bridges, which can be obtained via vulcanization processes using peroxides and sulfur, respectively, are considered. The temperature dependence of the thermal conductivity of soft rubber derived from molecular dynamics (MD) simulations is in very good agreement with the experimental results. The contributions of bonded and non-bonded interactions in the MD simulations and their influence on the thermal conductivities of polyisoprene and polybutadiene are presented. The details are discussed in this paper.

Solvent-induced changes in the reactivity of tricyanate esters undergoing thermal polymerization

The mechanism of thermally stimulated polymerization of tricyanate ester remains the same in solution as in the melt, but Arrhenius parameters of the rate-limiting reaction are significantly affected by solvation.

Designing high thermal conductivity of cross-linked epoxy resin via molecular dynamics simulations

Coarse-grained (CG) non-equilibrium molecular dynamics simulation was used to study the thermal conductivity of a cross-linked network composed of epoxy resin (E51) and polyether amine (PEA). By probing the mechanism of heat transfer in the cross-linked epoxy resin, we systematically explored the effects of the crosslinking degree, chain length and multi-functional groups of the curing agent on the thermal conductivity behavior. Our results indicate that the thermal conductivity is mainly dependent on the chain length and the functional groups of the curing agent. A shorter chain length and a curing agent with more functional groups contribute to higher thermal conductivity, while the crosslinking degree has a negligible effect. Moreover, it is revealed that the thermal conductivity is manipulated by the non-bonding interaction energy (E_pair) and the vibrational density. In general, our work could provide some guidelines for the design and fabrication of a cross-linked epoxy network with high thermal conductivity.

Thermal Conductivity of Polyisoprene and Polybutadiene from Molecular Dynamics Simulations and Transient Measurements

The thermal conductivities of untreated polyisoprene and polybutadiene were calculated by molecular dynamics (MD) simulations using a Green-Kubo approach between -10 °C and 50 °C at atmospheric pressure. For comparison, the thermal conductivities of untreated polyisoprene with a molecular weight of 54,000 g/mol and untreated polybutadiene with a molecular weight of 45,000 g/mol were measured by the transient hot wire method in similar conditions. The simulation results of both polymers are in good agreement with the experimental data. We observed that the MD simulations slightly overestimate the thermal conductivity due to the chosen force field description. Details are discussed in the paper.

Increasing the thermal conductivity of styrene butadiene rubber: insights from molecular dynamics simulation

It is very important to improve the thermal conductivity of styrene butadiene rubber (SBR) which can widen its application.

Designing the Slide-Ring Polymer Network with both Good Mechanical and Damping Properties via Molecular Dynamics Simulation

Through coarse-grained molecular dynamics simulation, we have successfully designed the chemically cross-linked (fixed junction) and the slide-ring (SR) systems. Firstly, we examine the dynamic properties such as the mean-square displacement, the bond, and the end-to-end autocorrelation functions as a function of the cross-linking density, consistently pointing out that the SR system exhibits much lower mobility compared with the fixed junction one at the same cross-linking density. This is further validated by a relatively higher glass transition temperature for the SR system compared with that of the fixed junction one. Then, we calculated the effect of the cross-linking density on the stretch-recovery behavior for the SR and fixed junction systems. Although the chain orientation of the SR system is higher than that of the fixed-junction system, the tensile stress is smaller than the latter. We infer that much greater chain sliding can occur during the stretch, because the movable ring structure homogeneously sustains the external force of the SR system, which, therefore, leads to much larger permanent set and higher hysteresis during the recovery process compared with the fixed-junction one. Based on the stretch-recovery behavior for various cross-linking densities, we obtain the change of the hysteresis loss, which is larger for the SR system than that of the fixed junction system. Lastly, we note that the relatively bigger compressive stress for the SR system results from the aggregation of the rigid rings compared with the fixed junction system. In general, compared with the traditionally cross-linked system, a deep molecular-level insight into the slide-ring polymer network is offered and thus is believed to provide some guidance to the design and preparation of the slide-ring polymer network with both good mechanical and damping properties.