Phonon Transport in Supramolecular Polymers Regulated by Hydrogen Bonds

Supramolecular polymers hold promise in thermal management applications due to their multistability, high responsiveness, and cost-effectiveness. In this work, we successfully regulate phonon transport at the molecular level in supramolecular polymers by adjusting the strength of intermolecular hydrogen bonding. We synthesized three supramolecular polymer fibers with thermal conductivity differences of up to 289% based on melamine (M) and three simple positional isomers of hydroxybenzoic acid. Differential Scanning Calorimetry (DSC) measurement revealed discrepancies in thermal stability of the polymers, where structures with higher stability exhibited enhanced thermal conductivity. Fourier Transform Infrared Spectroscopy (FTIR) measurement and Density Functional Theory (DFT) calculations indicate that these differences arise from variations in hydrogen-bonding strengths at different bonding sites. Higher hydrogen-bonding strength leads to more stable thermal pathways, reduces phonon scattering, and increases thermal conductivity. Our findings provide valuable insights into controlling the thermal conductivity of polymer fibers, paving the way for applications in phonon-based thermal devices.

Intrinsically thermally conductive polymers

Here, we describe the design features that lead to intrinsically thermally conductive polymers. Though polymers are conventionally assumed to be thermal insulators (<0.3 W m-1 K-1), significant efforts by the thermal transport community have shown that polymers can be intrinsically thermally conductive (>1.0 W m-1 K-1). However, these findings have not yet driven comprehensive synthetic efforts to expose how different macromolecular features impact thermal conductivity. Preliminary theoretical and experimental investigations have revealed that high k polymers can be realized by enhancing the alignment, crystallinity, and intermolecular interactions. While a holistic mechanistic framework does not yet exist for thermal transport in polymeric materials, contemporary literature suggests that phonon-like heat carriers may be operative in macromolecules that meet the abovementioned criteria. In this review, we offer a perspective on how high thermal conductivity polymers can be systematically engineered from this understanding. Reports for several classes of macromolecules, including linear polymers, network polymers, liquid-crystalline polymers, and two-dimensional polymers substantiate the design principles we propose. Throughout this work, we offer opportunities for continued fundamental and technological development of polymers with high thermal conductivity.

Cooling dynamics of droplets exposed to solid surface freezing and vitrification

Solid surface freezing or vitrification (SSF/SSV) can be done by depositing droplets of a sample, e.g., cells in a preservation solution, onto a pre-cooled metal surface. It is used to achieve higher cooling rates and concomitant higher cryosurvival rates compared to immersion of samples into liquid nitrogen. In this study, numerical simulations of SSF/SSV were conducted by modeling the cooling dynamics of droplets of cryoprotective agent (CPA) solutions. It was assumed that deposited droplets attain a cylindrical bottom part and half-ellipsoidal shaped upper part. Material properties for heat transfer simulations including density, heat capacity and thermal conductivity were obtained from the literature and extrapolated using polynomial fitting. The impact of CPA type, i.e., glycerol (GLY) and dimethyl sulfoxide (DMSO), CPA concentration, and droplet size on the cooling dynamics was simulated at different CPA mass fractions at temperatures ranging from -196 to 25 °C. Simulations show that glycerol solutions cool faster compared to DMSO solutions, and cooling rates increase with decreasing CPA concentration. However, we note that material property data for GLY and DMSO solutions were obtained in different temperature and concentration ranges under different conditions, which complicated making an accurate comparison. Experimental studies show that samples that freeze have a delayed cooling response early on, whereas equilibration times are similar compared to samples that vitrify. Finally, as proof of concept, droplets of human red blood cells (RBCs) were cryopreserved using SSV/SSF comparing the effect of GLY and DMSO on cryopreservation outcome. At 20% (w/w), similar hemolysis rates were found for GLY and DMSO, whereas at 40%, GLY outperformed DMSO.

Mesoporous multi-shelled hollow resin nanospheres with ultralow thermal conductivity

Hollow nanostructures exhibit enclosed or semi-enclosed spaces inside and the consequent features of restricting molecular motion, which is crucial for intrinsic physicochemical properties. Herein, we developed a new configuration of hollow nanostructures with more than three layers of shells and simultaneously integrated mesopores on every shell. The novel interior configuration expresses the characteristics of periodic interfaces and abundant mesopores. Benefiting from the suppression of gas molecule convection by boundary scattering, the thermal conductivity of mesoporous multi-shelled hollow resin nanospheres reaches 0.013 W m-1 K-1 at 298 K. The designed interior mesostructural configuration of hollow nanostructures provides an ideal platform to clarify the influence of nanostructure design on intrinsic physicochemical properties and propels the development of hollow nanostructures.

Tuning the polymer thermal conductivity through structural modification induced by MoS2 bilayers

The present work refers to a physical and structural study of nanoconfined polymers in polymer-MoS₂ nanocomposites as a function of MoS₂-MoS₂ interlayer distance. We applied reverse nonequilibrium molecular dynamics (RNEMD) simulations to investigate the thermal conductivity (λ) of polyamide oligomers confined by MoS₂ bilayers. The results of this study indicate that thermal conductivity of polymer can be considerably enhanced when polymer chains are confined by MoS₂ sheets, this behavior is more pronounced by charged surfaces. The presence of MoS₂ surfaces leads to elongation as well as preferential alignment of polymer chains parallel to the MoS₂ surfaces, which in turn results in higher order and denser packing of polymer content and hence larger thermal conductivity in comparison to the bulk polymer. Additionally, the analysis of the number of hydrogen bonds (HBs) in confined polymer chains suggests that a combined effect of the mentioned structural modification and enlarged values of HBs may cooperatively contribute to high polymer thermal conductivity, facilitating phonon transport. The results reported here suggest a significant way to design confined polymer-MoS₂ composites for significantly improving thermal conductivity for a wide variety of applications.

Ice-Template Crosslinked PVA Aerogels Modified with Tannic Acid and Sodium Alginate

With the commitment to reducing environmental impact, bio-based and biodegradable aerogels may be one approach when looking for greener solutions with similar attributes to current foam-like materials. This study aimed to enhance the mechanical, thermal, and flame-retardant behavior of poly(vinyl alcohol) (PVA) aerogels by adding sodium alginate (SA) and tannic acid (TA). Aerogels were obtained by freeze-drying and post-ion crosslinking through calcium chloride (CaCl₂) and boric acid (H₃BO₃) solutions. The incorporation of TA and SA enhanced the PVA aerogel’s mechanical properties, as shown by their high compressive specific moduli, reaching up to a six-fold increase after crosslinking and drying. The PVA/TA/SA aerogels presented a thermal conductivity of 0.043 to 0.046 W/m·K, while crosslinked ones showed higher values (0.049 to 0.060 W/m·K). Under TGA pyrolytic conditions, char layer formation reduced the thermal degradation rate of samples. After crosslinking, a seven-fold decrease in the thermal degradation rate was observed, confirming the high thermal stability of the formed foams. Regarding flammability, aerogels were tested through cone calorimetry. PVA/TA/SA aerogels showed a significant drop in the main parameters, such as the heat release rate (HRR) and the fire growth (FIGRA). The ion crosslinking resulted in a further reduction, confirming the improvement in the fire resistance of the modified compositions.

Recent advances in thermally conductive polymer composites

Polymer matrix composites (PMCs) with high thermal conductivity (TC) play an important role in improving the heat dissipation capacity of a new generation of electronic devices, particularly for 5G and aviation applications. Over the last few decades, considerable efforts have been made in the fabrication of highly thermally conductive PMCs. Advances in the thermal conduction mechanism of polymer composites are induced to, and then commonly used thermally conductive fillers are presented. In the following, the factors affecting the TC of polymer composites are discussed in detail, including fillers, interfaces, polymer matrices and processing technologies. Special attention is paid to the thermally conductive fillers. Then, some application areas of thermally conductive polymer composites are introduced. Finally, the deficiencies and future development trends in this research field are put forward. It is expected that this review will provide some beneficial inspiration in improving the TC of PMCs.

Crystal Structures of Lignocellulosic Furfuryl Biobased Polydiacetylenes with Hydrogen-Bond Networks: Influencing the Direction of Solid-State Polymerization through Modification of the Spacer Length

We present the topochemical polymerization of two lignocellulosic biobased diacetylenes (DAs) that only differ by an alkyl spacer length of 1 methylene (n = 1) or 3 methylene units (n = 3) between the diyne and carbamate functionalities. Their crystalline molecular organizations have the distinctive feature of being suitable for polymerization in two potential directions, either parallel or skewed to the hydrogen-bonded (HB) network. However, single-crystal structures of the final polydiacetylenes (PDAs) demonstrate that the resulting orientation of the conjugated backbones is different for these two derivatives, which lead to HB supramolecular polymer networks (2D nanosheets) for n = 1 and to independent linear PDA chains with intramolecular HBs for n = 3. Thus, spacer length modification can be considered a new strategy to influence the molecular orientation of conjugated polymer chains, which is crucial for developing the next generation of materials with optimal mechanical and optoelectronic properties. Calculations were performed on model oligodiacetylenes to evaluate the cooperativity effect of HBs in the different crystalline supramolecular packing motifs and the energy profile related to the torsion of the conjugated backbone of a PDA chain (i.e., its ability to adopt planar or helical conformations).

Molecular dynamics simulation of the formation mechanism of the thermal conductive filler network of polymer nanocomposites

In this work, the thermal transfer capabilities of spherical and laminar/spherical filled polymer nanocomposites (PNCs) were systematically investigated by using molecular dynamics (MD) simulation. The effects of various factors such...

Noncured Graphene Thermal Interface Materials for High-Power Electronics: Minimizing the Thermal Contact Resistance

We report on experimental investigation of thermal contact resistance, RC, of the noncuring graphene thermal interface materials with the surfaces characterized by different degree of roughness, Sq. It is found that the thermal contact resistance depends on the graphene loading, ξ, non-monotonically, achieving its minimum at the loading fraction of ξ ~15 wt%. Decreasing the surface roughness by Sq~1 μm results in approximately the factor of ×2 decrease in the thermal contact resistance for this graphene loading. The obtained dependences of the thermal conductivity, KTIM, thermal contact resistance, RC, and the total thermal resistance of the thermal interface material layer on ξ and Sq can be utilized for optimization of the loading fraction of graphene for specific materials and roughness of the connecting surfaces. Our results are important for the thermal management of high-power-density electronics implemented with diamond and other wide-band-gap semiconductors.

Thermal interface materials with graphene fillers: review of the state of the art and outlook for future applications

We review the current state-of-the-art graphene-enhanced thermal interface materials for the management of heat in the next generation of electronics. Increased integration densities, speed and power of electronic and optoelectronic devices require thermal interface materials with substantially higher thermal conductivity, improved reliability, and lower cost. Graphene has emerged as a promising filler material that can meet the demands of future high-speed and high-powered electronics. This review describes the use of graphene as a filler in curing and non-curing polymer matrices. Special attention is given to strategies for achieving the thermal percolation threshold with its corresponding characteristic increase in the overall thermal conductivity. Many applications require high thermal conductivity of composites, while simultaneously preserving electrical insulation. A hybrid filler approach, using graphene and boron nitride, is presented as a possible technology providing for the independent control of electrical and thermal conduction. The reliability and lifespan performance of thermal interface materials is an important consideration towards the determination of appropriate practical applications. The present review addresses these issues in detail, demonstrating the promise of graphene-enhanced thermal interface materials compared to alternative technologies.

Development of Coarse-Grained Models for Poly(4-vinylphenol) and Poly(2-vinylpyridine): Polymer Chemistries with Hydrogen Bonding

In this paper, we identify the modifications needed in a recently developed generic coarse-grained (CG) model that captured directional interactions in polymers to specifically represent two exemplary hydrogen bonding polymer chemistries-poly(4-vinylphenol) and poly(2-vinylpyridine). We use atomistically observed monomer-level structures (e.g., bond, angle and torsion distribution) and chain structures (e.g., end-to-end distance distribution and persistence length) of poly(4-vinylphenol) and poly(2-vinylpyridine) in an explicitly represented good solvent (tetrahydrofuran) to identify the appropriate modifications in the generic CG model in implicit solvent. For both chemistries, the modified CG model is developed based on atomistic simulations of a single 24-mer chain. This modified CG model is then used to simulate longer (36-mer) and shorter (18-mer and 12-mer) chain lengths and compared against the corresponding atomistic simulation results. We find that with one to two simple modifications (e.g., incorporating intra-chain attraction, torsional constraint) to the generic CG model, we are able to reproduce atomistically observed bond, angle and torsion distributions, persistence length, and end-to-end distance distribution for chain lengths ranging from 12 to 36 monomers. We also show that this modified CG model, meant to reproduce atomistic structure, does not reproduce atomistically observed chain relaxation and hydrogen bond dynamics, as expected. Simulations with the modified CG model have significantly faster chain relaxation than atomistic simulations and slower decorrelation of formed hydrogen bonds than in atomistic simulations, with no apparent dependence on chain length.

Influence of chain interaction and ordered structures in polymer dispersed liquid crystalline membranes on thermal conductivity

Polymer dispersed liquid crystalline (PDLC) membrane with intrinsic thermal conductivity was prepared by dispersing liquid crystalline polysiloxane containing crosslinked structure (liquid crystalline polysiloxane elastomer (LCPE)) into polyvinyl alcohol (PVA). Chemical structures were characterized by Fourier transform infrared (FT-IR) and 1H-NMR, and microscopic structures were analyzed by polarizing optical microscope (POM), scanning electron microscope (SEM) and X-ray diffraction (XRD). The thermal conductivity of PDLC membrane was characterized by hot disk thermal constants analyzer, and the tensile properties were measured by tensile testing machine. Thermal properties were characterized by differential scanning calorimeter (DSC) and thermal gravimetric analyzer (TGA). The results show that LCPE was dispersed in PVA uniformly, and the mesogenic monomer of LCPE formed microscopic ordered structures in PDLC membrane. Meanwhile, hydrogen-bond interaction was formed between LCPE and PVA chain. Both microscopic-ordered structure and the hydrogen-bond interaction improved the phonon transmission path, and the thermal conductivity of PDLC membrane was up to 0.74 W/m⋅K, which was 6 times higher than that of pure PVA film. PDLC membrane possessed proper tensile strength and elongation at break, respectively 5.18 MPa and 338%. As a result, PDLC membrane can be used as thermal conductive membrane in electronic packaging and other related fields.

Crosslinking Effect on Thermal Conductivity of Electrospun Poly(acrylic acid) Nanofibers

The thermal conductivity (k) of poly(acrylic acid) (PAA) nanofibers, which were electrospun at various electrospinning voltages, was measured using suspended microdevices. While the thermal conductivities of the as-spun PAA nanofibers varied depending on the electrospinning voltages, the most pronounced 3.1-fold increase in thermal conductivity in comparison to that of bulk PAA was observed at the electrospinning voltage of 14 kV. On the other hand, a reduction in the thermal conductivity of the nanofibers was observed when the as-spun nanofibers were either thermally annealed at the glass transition temperature of PAA or thermally crosslinked. It is notable that the thermal conductivity of crosslinked PAA nanofibers was comparable to that of crosslinked bulk PAA. Polarized Raman spectroscopy and Fourier transform infrared spectroscopy verified that the k enhancement via electrospinning and the k reduction by the thermal treatments could be attributed to the conformational changes between gauche and trans states, which may be further related to the orientation of molecular chains. In contrast, hydrogen bonds did not contribute significantly to the k enhancement. Additionally, the suppression of k observed for the crosslinked PAA nanofibers might result from the shortening of single molecular chains via crosslinking.

Hierarchically hydrogen-bonded graphene/polymer interfaces with drastically enhanced interfacial thermal conductance

Interfacial thermal transport is a critical physical process determining the performance of many material systems with small-scale features. Recently, self-assembled monolayers and polymer brushes have been widely used to engineer material interfaces presenting unprecedented properties. Here, we demonstrate that poly(vinyl alcohol) (PVA) monolayers with hierarchically arranged hydrogen bonds drastically enhance interfacial thermal conductance by a factor of 6.22 across the interface between graphene and poly(methyl methacrylate) (PMMA). The enhancement is tunable by varying the number of grafted chains and the density of hydrogen bonds in the unique hierarchical hydrogen bond network. The extraordinary enhancement results from a synergy of hydrogen bonds and other structural and thermal factors including molecular morphology, chain orientation, interfacial vibrational coupling and heat exchange. Two types of hydrogen bonds, i.e. PVA-PMMA hydrogen bonds and PVA-PVA hydrogen bonds, are analyzed and their effects on various structural and thermal properties are systematically investigated. These results are expected to provide new physical insights for interface engineering to achieve tunable thermal management and energy efficiency in a wide variety of systems involving polymers and biomaterials.

Polymeric Self-Assembled Monolayers Anomalously Improve Thermal Transport across Graphene/Polymer Interfaces

Ultralow thermal conductivities of bulk polymers greatly limit their applications in areas demanding fast heat dissipation, such as flexible electronics and microelectronics. Therefore, polymeric composites incorporating highly thermally conductive filler materials (e.g., graphene and carbon nanotubes) have been produced to address the issue. However, despite some enhancement, thermal conductivities of these materials are still far below theoretical predictions, mainly due to the inefficient thermal transport across material interfaces. Here, using molecular dynamics simulations, we demonstrate that polyethylene (PE) self-assembled monolayer (SAM) functionalized graphene surfaces at a high grafting density can drastically improve interfacial thermal conduction between graphene and the matrix of poly(methyl methacrylate) (PMMA). In contrast to abrupt temperature drop across pristine graphene/PMMA interfaces, temperature field in the vicinity of a PE-grafted graphene/PMMA interface is continuous with a smoother transition and higher thermal conductance. This anomalous improvement is attributed to three factors that closely relate to the grafting density of the SAM of PE. First, the SAM with high grafting densities features highly extended chains that enhance along-chain thermal conduction. Second, the strong covalent bonding between the SAM and the graphene facilitates heat transfer at their joints. Third, the SAM and the PMMA matrix are both organic materials, leading to enhanced interfacial vibrational coupling. Molecular mechanisms underpinning these phenomena are systematically elucidated by analyzing the temperature field, density distribution, Herman's orientation factor, the vibrational density of states, cumulative correlation factor, the integrated autocorrelation of interfacial heat power, and interfacial adhesion. All results suggest the incorporation of SAMs at a high grafting density or extremely extended polymer brushes for drastically improved interfacial thermal transport between hard and soft materials toward a wide range of applications.

Vibrational Response of Methylammonium Lead Iodide: From Cation Dynamics to Phonon-Phonon Interactions

The dynamic evolution of the vibrational interactions in the prototypical CH₃ NH₃ PbI₃ was studied through a comprehensive experimental and theoretical investigation with a focus on the interactions between the organic cations and the inorganic cage. To date, no clear picture has emerged on the critical and fundamental interactions between the two perovskite components, despite the relevance of phonons to the electronic properties of several classes of perovskites. For the first time, we have monitored the IR and nonresonant Raman response in the broad frequency range 30-3400 cm^-1 and in the temperature interval 80-360 K. Strong changes in the energies of different vibrational modes with temperature are observed and examined in the framework of phonon-phonon interactions considering a significant anharmonic contribution to the phonon relaxation process. The vibrational relaxation of the bending modes and their reorientation activation energies identify that such mechanisms are governed by medium-to-strong hydrogen bonds in the orthorhombic phase; however, any ferroelectric ordering in the orthorhombic phase is governed mostly by dipole interactions. These changes imply that charge localization mechanisms play a primary role, and our study enriches the fundamental knowledge of phonon interactions and charge transport in CH₃ NH₃ PbI₃ for the further development of optoelectronic applications.