A simulation study on the glass transition behavior and relevant segmental dynamics in free-standing polymer nanocomposite films

Role of additive size in the segmental dynamics and mechanical properties of cross-linked polymers

Thermoset materials often involve the addition of molecular and nanoparticle additives to alter various chemo-physical properties of importance in their ultimate applications. The resulting compositional heterogeneities can lead to either enhancement or degradation of thermoset properties, depending on the additive chemical structure and concentration. We tentatively explore this complex physical phenomenon through the consideration of a model polymeric additive to our coarse-grained (CG) thermoset investigated in previous works by simply varying the size of additive segments compared to those of polymer melt. We find that the additive modified thermoset material becomes chemically heterogeneous from additive aggregation when the additive segments become much smaller than those of the thermoset molecules, and a clear evidence is observed in the spatial distribution of local molecular stiffness estimated from Debye-Waller factor 〈u2〉. Despite the non-monotonic variation trends observed in dynamical and mechanical properties with decreasing additive segmental size, both the structural relaxation time and moduli (i.e., shear modulus and bulk modulus) exhibit scaling laws with 〈u2〉. The present work highlights the complex role of additive size played in the dynamical and mechanical properties of thermoset polymers, which should provide a better understanding for the glass formation process of cross-linked polymer composites.

Removal and reuse of heavy metal ions on mildly oxidized Ti3C2 @BF membrane via synergistic photocatalytic-photothermal approach

The pollution of heavy metal ions in water seriously affects the ecosystem and human health. Here, an efficient synergetic photocatalytic-photothermal system is designed by combining a mildly oxidized Ti3C2 (mo-Ti3C2) with a super hydrophilic bamboo fiber (BF) membrane. The mo-Ti3C2 heterojunction promotes the transfer and separation of photoinduced charges and thus enhances the photocatalytic reduction of heavy metal ions (Co2+, Pb2+, Zn2+, Mn2+ and Cu2+). The photoreduced metal nanoparticles with high conductivity and LSPR effect further accelerate the transfer and separation of photoinduced charges, and improve photothermal and evaporative performance. The mo-Ti3C2-2.4 @BF membrane in Co(NO3)2 solution can achieve an excellent evaporation rate of 4.6 kg·m-2·h-1 and a high solar-vapor efficiency of up to 97.5% under the light intensity of 2.44 kW·m-2, which are 27.8% and 19.6% higher than those in H2O, respectively, demonstrating the reuse of photoreduced Co nanoparticles. No heavy metal ions are detected in any of the condensed water, and the Co2+ removal rate in the concentrated Co(NO3)2 solution is up to 80.4%. The synergetic photocatalytic-photothermal approach on the mo-Ti3C2 @BF membrane provides a new scope for the continuous removal and reuse of heavy metal ions, as well as for obtaining clean water.

Atomistic Construction of Silicon Nitride Ceramic Fiber Molecular Model and Investigation of Its Mechanical Properties Based on Molecular Dynamics Simulations

Molecular simulations are currently receiving significant attention for their ability to offer a microscopic perspective that explains macroscopic phenomena. An essential aspect is the accurate characterization of molecular structural parameters and the development of realistic numerical models. This study investigates the surface morphology and elemental distribution of silicon nitride fibers through TEM and EDS, and SEM and EDS analyses. Utilizing a customized molecular dynamics approach, molecular models of amorphous and multi-interface silicon nitride fibers with complex structures were constructed. Tensile simulations were conducted to explore correlations between performance and molecular structural composition. The results demonstrate successful construction of molecular models with amorphous, amorphous-crystalline interface, and mixed crystalline structures. Mechanical property characterization reveal the following findings: (1) The nonuniform and irregular amorphous structure causes stress concentration and crack formation under applied stress. Increased density enhances material strength but leads to higher crack sensitivity. (2) Incorporating a crystalline reinforcement phase without interfacial crosslinking increases free volume and relative tensile strength, improving toughness and reducing crack susceptibility. (3) Crosslinked interfaces effectively enhance load transfer in transitional regions, strengthening the material's tensile strength, while increased density simultaneously reduces crack propagation.

Glass transition of ion-containing polymer melts in bulk and thin films

Ion-containing polymers often are good glass formers, and the glass transition temperature is an important parameter to consider for practical applications, which prescribes the working temperature range for different mechanical and dynamic properties. In this work, we present a systematic molecular dynamics simulation study on the coupling of ionic correlations with the glass transition, based on a generic coarse-grained model of ionic polymers. The variation of the glass transition temperature is examined concerning the influence of the electrostatic interaction strength, charge fraction, and charge sequence. The interplay with the film thickness effect is also discussed. Our results reveal a few typical features about the glass transition process that are in qualitative agreement with previous studies, further highlighting the effects of counterion entropy at weak ionic correlations and physical crosslinking of ionic aggregates at strong ionic correlations. Detailed parametric dependencies are displayed, which demonstrate that introducing strong ionic correlations promotes vitrification while adopting a precise charge sequence and applying strong confinement with weak surface affinity reduce the glass transition temperature. Overall, our investigation provides an improved picture towards a comprehensive understanding of the glass transition in ion-containing polymeric systems from a molecular simulation perspective.

Supercooled melt structure and dynamics of single-chain nanoparticles: A computer simulation study

By using coarse-grained molecular dynamics simulations, we have investigated the structure and dynamics of supercooled single-chain cross-linked nanoparticle (SCNP) melts having a range of cross-linking degrees ϕ. We find a nearly linear increase in glass-transition temperature (T_g) with increasing ϕ. Correspondingly, we have also experimentally synthesized a series of polystyrene-based SCNPs and have found that the measured T_g estimated from differential scanning calorimetry is qualitatively consistent with the trend predicted by our simulation estimates. Experimentally, an increase in T_g as large as ΔT_g = 61 K for ϕ = 0.36 is found compared with their linear chain counterparts, indicating that the changes in dynamics with cross-links are quite appreciable. We attribute the increase in T_g to the enlarged effective hard-core volume and the corresponding reduction in the free volume of the polymer segments. Topological constraints evidently frustrate the local packing. In addition, the introduction of intra-molecular cross-linking bonds slows down the structural relaxation and simultaneously enhances the local coupling motion on the length scales within SCNPs. Consequently, a more pronounced dynamical heterogeneity (DH) is observed for larger ϕ, as quantified by measuring the dynamical correlation length through the four-point susceptibility parameter, χ₄. The increase in DH is directly related to the enhanced local cooperative motion derived from intra-molecular cross-linking bonds and structural heterogeneity derived from the cross-linking process. These results shed new light on the influence of intra-molecular topological constraints on the segmental dynamics of polymer melts.

Dynamic behaviour of water molecules in heterogeneous free space formed in an epoxy resin

Although an epoxy resin is a stable material, it absorbs moisture over a long period of time, causing deterioration of its material properties. We here applied a full-atomistic molecular dynamics (MD) simulation to study where water molecules exist in an epoxy resin and how they dynamically behave. First, the curing reaction was simulated to obtain a network structure so that the time course of the density, and thereby the free space, in the resin were obtained. The results made it possible to discuss the formation and size distribution of the free spaces which were not connected to each other. Then, a few percent of water were inserted into the free space of the cured epoxy resin to examine the location and dynamics of their molecules. We found that several water molecules were clustered at a preferred site, where hydrogen bonds can be formed with hydroxy, ether and amino groups of the network, in the free space, and they heterogeneously moved from there to other sites.

A comparative study on the dynamic heterogeneity of supercooled polymers under nanoconfinement

Dynamic heterogeneity (DH) is a universal property of glass transition phenomena. In this work, we perform a comparative analysis of DH for pure polymer and polymer/nanoparticle composite systems in both film and bulk states via molecular dynamics simulations. We find that the dynamic gradient and the faster average dynamics due to the presence of a free surface are two leading factors, resulting from a nanoconfinement effect, which influence different parts of DH in a film system. The dynamic gradient results from differences in dynamics at different distances from the mobile surface, which induces a large deviation from the Gaussian distribution for the displacement distribution in the film. At the same time, the maximum string size which describes the region size for cooperative motion (dynamic correlation) can also be influenced by the dynamic gradient, although this influence is much weaker than that on the displacement distribution. On the other hand, reflecting temporal fluctuations of dynamics or temporal parts of DH, characteristic peak times of the non-Gaussian parameter and string size, and the ratio between persistent times and exchange times which describe the dynamic exchange properties, are mainly influenced by the faster dynamics on average. Our results demonstrate that measuring different properties (dynamic distribution, dynamic correlation or dynamic exchange) place an emphasis on distinct temporal and spatial parts of DH. It is necessary to use combinational measurements of these properties to give a complete picture of DH in nanoconfinement environments.