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.

Progress in Multiscale Modeling of Silk Materials

As a result of their hierarchical structure and biological processing, silk fibers rank among nature's most remarkable materials. The biocompatibility of silk-based materials and the exceptional mechanical properties of certain fibers has inspired the use of silk in numerous technical and medical applications. In recent years, computational modeling has clarified the relationship between the molecular architecture and emergent properties of silk fibers and has demonstrated predictive power in studies on novel biomaterials. Here, we review advances in modeling the structure and properties of natural and synthetic silk-based materials, from early structural studies of silkworm cocoon fibers to cutting-edge atomistic simulations of spider silk nanofibrils and the recent use of machine learning models. We explore applications of modeling across length scales: from quantum mechanical studies on model peptides, to atomistic and coarse-grained molecular dynamics simulations of silk proteins, to finite element analysis of spider webs. As computational power and algorithmic efficiency continue to advance, we expect multiscale modeling to become an indispensable tool for understanding nature's most impressive fibers and developing bioinspired functional materials.

Exploring keratin composition variability for sustainable thermal insulator design

In pursuing sustainable thermal insulation solutions, this study explores the integration of human hair and feather keratin with alginate. The aim is to assess its potential in thermal insulation materials, focusing on the resultant composites' thermal and mechanical characteristics. The investigation uncovers that the type and proportion of keratin significantly influence the composites' porosity and thermal conductivity. Specifically, higher feather keratin content is associated with lesser sulfur and reduced crosslinking due to shorter amino acids, leading to increased porosity and pore sizes. This, in turn, results in a decrease in β-structured hydrogen bond networks, raising non-ordered protein structures and diminishing thermal conductivity from 0.044 W/(m·K) for pure alginate matrices to between 0.033 and 0.038 W/(m·K) for keratin-alginate composites, contingent upon the specific ratio of feather to hair keratin used. Mechanical evaluations further indicate that composites with a higher ratio of hair keratin exhibit an enhanced compressive modulus, ranging from 60 to 77 kPa, demonstrating the potential for tailored mechanical properties to suit various applications. The research underscores the critical role of sulfur content and the crosslinking index within keratin's structures, significantly impacting the thermal and mechanical properties of the matrices. The findings position keratin-based composites as environmentally friendly alternatives to traditional insulation materials.

Tunable thermal diffusivity of silk protein assemblies based on their structural control and photo-induced chemical cross-linking

Silk, which has excellent mechanical properties and is lightweight, serves as a structural material in natural systems. However, the structural and functional applications of silk in artificial systems have been limited due to the difficulty in controlling its properties. In this study, we demonstrate the tunable thermal diffusivity of silk-based assemblies (films) based on secondary structural control and subsequent cross-linking. We found that the thermal diffusivity of the silk film is increased by the formation of β-sheet structures and dityrosine between Tyr residues adjacent to the β-sheet structures. Our results demonstrate the applicability of silk proteins as material components for thermally conductive biopolymer-based materials.

The hydrogen-bond configuration modulates the energy transfer efficiency in helical protein nanotubes

Energy transport in proteins is critical to a variety of physical, chemical, and biological processes in living organisms. While strenuous efforts have been made to study vibrational energy transport in proteins, thermal transport processes across the most fundamental building blocks of proteins, i.e. helices, are not well understood. This work studies energy transport in a group of "isomer" helices. The π-helix is shown to have the highest thermal conductivity, 110% higher than that of the α-helix and 207% higher than that of the 310-helix. The H-bond connectivity is found to govern thermal transport mechanisms including the phonon spectral energy density, dispersion, mode-specific transport, group velocity, and relaxation time. The energy transport is strongly correlated with the H-bond strength which is also modulated by the H-bond connectivity. These fundamental insights provide a novel perspective for understanding energy transfer in proteins and guiding a rational molecule-level design of novel materials with configurable H-bonds.

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.

Effect of Different Additives in Diets on Secondary Structure, Thermal and Mechanical Properties of Silkworm Silk

In this study, eight types of materials including nanoparticles (Cu and CaCO₃), metallic ions (Ca2+ and Cu2+), and amino acid substances (serine, tyrosine, sericin amino acid, and fibroin amino acid) were used as additives in silkworm diets to obtain in-situ modified silk fiber composites. The results indicate that tyrosine and fibroin amino acids significantly increase potassium content in silk fibers and induce the transformation of α-helices and random coils to β-sheet structures, resulting in higher crystallinities and better mechanical properties. However, the other additives-modified silk fibers show a decrease in β-sheet contents and a slight increase or even decrease in tensile strengths. This finding provides a green and effective approach to produce mechanically enhanced silk fibers with high crystallinity on a large scale. Moreover, the modification mechanisms of these additives were discussed in this study, which could offer new insights into the design and regulation of modified fibers or composites with desirable properties and functions.

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.

Orientational Mapping Augmented Sub-Wavelength Hyper-Spectral Imaging of Silk

Molecular alignment underpins optical, mechanical, and thermal properties of materials, however, its direct measurement from volumes with micrometer dimensions is not accessible, especially, for structurally complex bio-materials. How the molecular alignment is linked to extraordinary properties of silk and its amorphous-crystalline composition has to be accessed by a direct measurement from a single silk fiber. Here, we show orientation mapping of the internal silk fiber structure via polarisation-dependent IR absorbance at high spatial resolution of 4.2 μm and 1.9 μm in a hyper-spectral IR imaging by attenuated total reflection using synchrotron radiation in the spectral fingerprint region around 6 μm wavelength. Free-standing longitudinal micro-slices of silk fibers, thinner than the fiber cross section, were prepared by microtome for the four polarization method to directly measure the orientational sensitivity of absorbance in the molecular fingerprint spectral window of the amide bands of β-sheet polypeptides of silk. Microtomed lateral slices of silk fibers, which may avoid possible artefacts that affect spectroscopic measurements with fibers of an elliptical cross sections were used in the study. Amorphisation of silk by ultra-short laser single-pulse exposure is demonstrated.

Tuning thermal conductivity of crystalline polymer nanofibers by interchain hydrogen bonding

Interchain hydrogen bonds enhance thermal conduction in crystalline polymer nanofibers by confining torsional motion of polymer chains and by increasing the group velocity of phonons.