Effects of pressure on structure and dynamics of model elastomers: A molecular dynamics study

Impact of Sacrificial Hydrogen Bonds on the Structure and Properties of Rubber Materials: Insights from All-Atom Molecular Dynamics Simulations

The inclusion of sacrificial hydrogen bonds is crucial for advancing high-performance rubber materials. However, the molecular mechanisms governing the impact of these bonds on material properties remain unclear, hindering progress in advanced rubber material research. This study employed all-atom molecular dynamics simulations to thoroughly investigate how hydrogen bonds affect the structure, dynamics, mechanics, and linear viscoelasticity of rubber materials. As the modified repeating unit ratio (β) increased, both interchain and intrachain hydrogen bond content rose, with interchain bonds playing a predominant role. This physical cross-linking network formed through interchain hydrogen bonds restricts molecular chain movement and relaxation and raises the glass transition temperature of rubber. Within a certain content of hydrogen bonds, the mechanical strength increases with increasing β. However, further increasing β leads to a subsequent decrease in the mechanical performance. Optimal mechanical properties were observed at β = 6%. On the other hand, a higher β value yields an elevated stress relaxation modulus and an extended stress relaxation plateau, signifying a more complex hydrogen-bond cross-linking network. Additionally, higher β increases the storage modulus, loss modulus, and complex viscosity while reducing the loss factor. In summary, this study successfully established the relationship between the structure and properties of natural rubber containing hydrogen bonds, providing a scientific foundation for the design of high-performance rubber materials.

Through Diffusion Measurements of Molecules to a Numerical Model for Protein Crystallization in Viscous Polyethylene Glycol Solution

Protein crystallography has become a popular method for biochemists, but obtaining high-quality protein crystals for precise structural analysis and larger ones for neutron analysis requires further technical progress. Many studies have noted the importance of solvent viscosity for the probability of crystal nucleation and for mass transportation; therefore, in this paper, we have reported on experimental results and simulation studies regarding the use of viscous polyethylene glycol (PEG) solvents for protein crystals. We investigated the diffusion rates of proteins, peptides, and small molecules in viscous PEG solvents using fluorescence correlation spectroscopy. In high-molecular-weight PEG solutions (molecular weights: 10,000 and 20,000), solute diffusion showed deviations, with a faster diffusion than that estimated by the Stokes–Einstein equation. We showed that the extent of the deviation depends on the difference between the molecular sizes of the solute and PEG solvent, and succeeded in creating equations to predict diffusion coefficients in viscous PEG solutions. Using these equations, we have developed a new numerical model of 1D diffusion processes of proteins and precipitants in a counter-diffusion chamber during crystallization processes. Examples of the application of anomalous diffusion in counter-diffusion crystallization are shown by the growth of lysozyme crystals.

Mechanisms of reinforcement in polymer nanocomposites

Qualitatively different stress–strain responses of polymer nanocomposites are shown to result from the dynamical evolution of three principal molecular structural motifs in the polymer–filler network.

Detailed simulation of the role of functionalized polymer chains on the structural, dynamic and mechanical properties of polymer nanocomposites

To systematically study the effect of functionalized chain groups on polymer nanocomposites, we perform our simulation work in the following two ways. In the case of dilute loading of nanoparticles (NPs) with different geometries (spherical, sheet-like, rod-like NPs), we adopt coarse-grained molecular dynamics simulation to study the structural, dynamic and mechanical properties of polymer nanocomposites influenced by the terminal groups of linear polymer chains. We observe that the terminal groups have more probability to be adsorbed onto the surface of NPs with decreasing temperature, chain molecular weight and increasing chain stiffness. For all NPs with different geometries, more terminal groups segregate into the surface of NPs with increase in the interaction energy εf-n between the terminal groups and the NPs. We also notice that the attractive interaction between the terminal groups and the sheet-like NPs induces the appearance of a gradient of translational dynamics of polymer chains, and the relaxation at the chain length scale is evidently different for various adsorbed layers, whereas the segmental relaxation only becomes slightly slower nearby the sheet-like NPs. For both pure and filled systems with spherical NPs, it is found that the stress-strain curves and bond orientations are significantly enhanced with increase in the interaction strength between the terminal groups as well as terminal groups and NPs. In the case of concentrated loading of NPs, we construct the atomistic models of C60, CNT and graphene to accurately account for the "many body effect." We explore the influence of the functionalization position along the chain backbone on the dispersion kinetics, realizing that the end-functionalization is more effective. The end-groups effect on the chain configuration, chain packing and graphene equilibrium dispersibility is examined. The translational and rotational (segmental and terminal relaxation) dynamics influenced by the interactions between the end groups and graphene are probed by tuning εf-n and the volume fraction of graphene ϕ. Moreover, the shift in the glass transition temperature influenced by εf-n and ϕ is quantitatively estimated by fitting the temperature dependence of the relaxation time using the Vogel-Fulcher-Tammann (VFT) equation. This work is hoped to provide a deep understanding of the polymer nanocomposites with functionalized polymer chains.

Distinct mechanical properties of nanoparticle-tethering polymers

Nanoparticle-tethering polymers exhibit enhanced mechanical properties relative to neat polymers and nanoparticle/polymer blends.

Thermomechanical properties and deformation of coarse-grained models of hard-soft block copolymers

In this paper, we investigate the enhancement mechanism of the mechanical properties for hard-soft block copolymers by using molecular dynamics simulations at various temperatures. A coarse-grained approach is adopted to study sufficiently generic models. Our numerical experiments demonstrate that the nonbond potential plays a more significant role in mechanical properties compared to the bond potential. This finding serves as a cornerstone to understand the hard-soft materials. To explore the effect of hard segments, four copolymers with different concentrations and energy factors that describe the interaction between hard beads are conducted. Simulation results show that the mechanical performances of the system with large attractive force and small concentration of hard segments could be improved dramatically in conjunction with a moderate increment of the glass transition temperature. In particular, the energy factor shows a substantial influence in determining the microphase separation as well as the morphology of hard domains. These observations are believed to provide design guidelines for polymeric materials in engineering practice.

Calorimetric study of the Paranematic-to-Nematic transition of polydomain side-chain liquid-crystalline elastomers with different mesogen composition

The phase transition behaviour of various nematic side-chain liquid-crystalline elastomers with different mesogen composition has been explored by means of high-resolution ac calorimetry. Polydomain samples of the same crosslinking density and different type of mesogens have been investigated. The results show a strong dependence of the phase transition features upon the composition of the mesogen. The distance from the critical point, reflected in the sharpness of the heat capacity anomalies, increases when adding a shorter-length mesogen. The results provide new insight for the impact of mesogens on the thermodynamic behaviour and, thus, on the thermomechanical response of nematic liquid-crystalline elastomers.