Glass transition temperature of a polymer thin film: Statistical and fitting uncertainties

Understanding the thermomechanical behavior of graphene-reinforced conjugated polymer nanocomposites via coarse-grained modeling

Graphene-reinforced conjugated polymer (CP) nanocomposites are attractive for flexible and electronic devices, but their mechanical properties have been less explored at a fundamental level. Here, we present a predictive multiscale modeling framework for graphene-reinforced poly(3-alkylthiophene) (P3AT) nanocomposites via atomistically informed coarse-grained molecular dynamics simulations to investigate temperature-dependent thermomechanical properties at a molecular level. Our results reveal reduced graphene dispersion with increasing graphene loading. Nanocomposites with shorter P3AT side chains, lower temperatures, and higher graphene content exhibit stronger mechanical responses, which correlates with polymer dynamics. The elastic modulus increases linearly with the graphene content, which slightly deviates from the "Halpin-Tsai" micromechanical model prediction. Local stiffness analysis shows that graphene possesses the highest stiffness, followed by the P3AT backbone and side chains. Deformation-induced stronger chain alignment of the P3AT backbone compared to graphene may further promote conductive behavior. Our findings provide insights into the dynamical heterogeneity of nanocomposites, paving the way for understanding and predicting their thermomechanical properties.

Critical Thickness of Free-Standing Nanothin Films Made of Melted Polyethylene Chains via Molecular Dynamics

The mechanical stability of nanothin free-standing films made of melted polyethylene chains was predicted via molecular dynamics simulations in the range of 373.15-673.15 K. The predicted critical thickness, tc, increased with the square of the temperature, T, with additional chains needed as T increased. From T = 373.15 K up to the thermal limit of stability for polyethylene, tc values were in the range of nanothin thicknesses (3.42-5.63 nm), which approximately corresponds to 44-55 chains per 100 nm2. The density at the center of the layer and the interfacial properties studied (density profiles, interfacial thickness, and radius of gyration) showed independence from the film thickness at the same T. The polyethylene layer at its tc showed a lower melting T (<373.15 K) than bulk polyethylene.

Preparation, characterization, and in vivo evaluation of levonorgestrel-loaded thermostable microneedles

To facilitate the storage and use of poly (lactic-co-glycolic acid) (PLGA)-based microneedles (MNs) in hot seasons and regions, thermally stable MNs loaded with levonorgestrel (LNG) were developed. Due to its good biocompatibility and high glass transition temperature (T_g), Hydroxypropyl methylcellulose (HPMC) was added to the PLGA-based MNs to increase thermal stability. MNs with HPMC exhibited excellent thermal stability at high temperatures. After the MNs has been applied to the skin for 10 min, the backing layer of the MNs was dissolved by contact with the interstitial fluid of skin, which resulted in the separation of the MN tips from the backing layer. The MN tips were implanted intradermally and sustained-release LNG. Biodegradable polymers were used to encapsulate the LNG, providing long-acting contraception. The in vitro release rate of LNG from the MNs reached 72.78%-83.76% within 21 days. In rats, the MNs maintained plasma concentrations of LNG above the human contraceptive level for 8-12 days. In mice, the time required for complete degradation of the MN tips was 12-16 days. MNs have excellent medication adherence due to the advantages of painlessness, minimally invasive, and self-administered. MNs can make long-acting contraceptives more readily available to humans.

Unraveling Depth-Specific Ionic Conduction and Stiffness Behavior across Ionomer Thin Films and Bulk Membranes

Interfacial behavior of submicron thick polymer films critically controls the performance of electrochemical devices. We developed a robust, everyday-accessible, fluorescence confocal laser scanning microscopy (CLSM)-based strategy that can probe the distribution of mobility, ion conduction, and other properties across ionomer samples. When fluorescent photoacid probe 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS) was incorporated into <1 μm thick Nafion films on substrates, the depth-profile images showed thickness- and interface-dependent proton conduction behavior. In these films, proton conduction was weak over a region next to substrate interface, then gradually increased until air interface at 88% RH. Conversely, consistently high proton conduction with no interface dependence was observed across 35-50 μm thick bulk, free-standing Nafion membranes. A hump-like mobility/stiffness distribution was observed across Nafion films containing mobility-sensitive probe (9-(2-carboxy-2-cyanovinyl)julolidine) (CCVJ). The proton conduction and mobility distribution were rationalized as a combinatorial effect of interfacial interaction, ionomer chain orientation, chain density, and ionic domain characteristics.

Modified Group Contribution Scheme to Predict the Glass-Transition Temperature of Homopolymers through a Limiting Property Dataset

Previous studies on glass-transition temperature (T _g) prediction mainly focus on developing diverse methods with higher regression accuracy, but very little attention has been paid to the dataset. Generally, a large range of T _g values of a specified polymer could be found in the literature but which one should be selected into a dataset merely depends on the implicit preference rather than a recognized and clear criterion. In this paper, limiting glass-transition temperature (T _g(∞)), a constant value obtained at the infinite number-average molecular weight M _n, was validated to be an adequate bridge index in the T _g prediction models. Furthermore, a new dataset containing 198 polymers was established to predict T _g(∞) using the improved group contribution method and it showed a good correlation (R ² = 0.9925, adjusted R ² = 0.9894). The method could also generate T _g-M _n curves by introducing the T _g(∞) function and provide more information to polymer scientists and engineers for material selection, product design, and synthesis.