Search

[Display omitted] • Progress and promises of predictive gas transport models for polymers are surveyed. • Agreement between experimental and predicted properties is highlighted. • Developments in handling the extensive polymer database is emphasized. • Advantages and challenges of models to identify novel structures are addressed. Membrane-based gas separation is a commercially practiced technology dominated by polymeric materials. Nevertheless, as established through the accumulation of large datasets of various polymers tested for decades, polymeric membranes suffer from an inherent permeability-selectivity trade-off. The natural culmination of such trade-off behavior has been the construction of chemical/molecular structure-transport property relationships, fueling an ongoing search for new and improved polymers. Yet, considering the time and financial costs of experimental research, it seems hard to fully harness the potential of polymer technology for developing membranes unless we switch towards data-driven prediction as a mainstream approach. Particularly the predictive models capable of estimating polymer permeation properties with high accuracy could propel the field. However, the data presence and accessibility issues hamper such a transition. Here, we provide a historical overview of the predictive models, highlighting the main incentive behind: Facilitating advanced membrane research by identifying chemical structures not studied or synthesized yet. To this end, we specifically focus on the gas transport properties of existing polymers and provide insights into their use and further development. Then, we discuss the establishment of predictive methods, which are mainly based on the representation of structural fragments constituting polymers, analysis of existing transport data, and estimation of increments for corresponding fragments. Within these predictive methods, the models based on the concept of additive contributions and machine learning approaches are particularly instrumental for handling extensive polymer databases. Still, since they complement the semi-empirical models, we also briefly touched upon non-equilibrium thermodynamics-based models for glassy polymers in our analysis. Overall, we address the advantages and challenges of using these models as a tool to identify novel polymer structures for designing high-performance membranes. We hope this review will help initiate new collaborations between membrane scientists/technologists and polymer informaticians. [ABSTRACT FROM AUTHOR]

Poly(N -isopropylacrylamide) (PNIPAM) is a highly versatile thermo-responsive hydrogel with immense potential for applications in biomedicine and tissue engineering. While PNIPAM has been extensively researched, there remains a significant gap in understanding its mechanical behavior at the atomistic scale. To address this challenge, this study employs molecular dynamics (MD) simulations to delve into the mechanical response of cross-linked PNIPAM hydrogels. The uniqueness of this research lies in our emphasis on providing atomic-level insights into the influence of chemical cross-linking and physical entanglements, as well as quantifying their effects during the application of strain. This represents a novel contribution to the study of hydrogels. MD simulations enable us to scrutinize the behavior of individual polymer chains and their intricate interactions in unprecedented detail, shedding light on microscale phenomena that are often inaccessible through experiments alone. To tackle the complexities associated with atomistic simulations of cross-linked hydrogels, we introduce a dynamic cross-linking algorithm. This innovative approach incorporates a nonreactive force field to construct realistic three-dimensional hydrogel microstructures, employing a stepwise bond formation strategy. Our findings underscore the remarkable impact of increasing the Degree of Cross-linking (DoC) and/or the Degree of Polymerization (DoP) on enhancing the robustness and elastic modulus of PNIPAM hydrogels. Additionally, we uncover the stochastic nature of chemical cross-linking, leading to the formation of anisotropic super-fragments when DoC exceeds a critical threshold. Furthermore, this research highlights the pivotal role of low strain rates on the order of 106 s−1 in accurately modeling the mechanical properties of hydrogels using MD simulations. This approach enables us to obtain meaningful quantitative mechanical properties that align with experimental results reported in the existing literature. In summary, this paper offers an unprecedented level of insight into the intricate interplay of chemical cross-linking and physical entanglements, providing a foundation for future research in this domain. [Display omitted] • Presented a dynamic crosslinking algorithm for MD simulations of hydrogels and polymers. • Explored the effect of chain length and cross-linking on mechanical properties of PNIPAM. • Identified the role of fragmentation on mechanics of PNIPAM hydrogel. • Quantified chain entanglements in cross-linked hydrogel during application of strain. • Clarified the crucial role of low strain rates in MD mechanical tests on polymers. [ABSTRACT FROM AUTHOR]

Digital image correlation (DIC) techniques have been widely used for experimentally characterizing the mechanical behavior of polymer materials. However, practical applications of DIC for tensile testing of polymers often face serious image decorrelation caused by crazing and excessive deformation. In this work, a fluorescent three-dimensional digital image correlation (3D-DIC) with an adaptive incremental calculation strategy is proposed to realize reliable large deformation measurement of polymers by eliminating or mitigating the image decorrelation caused by these issues. The proposed technique first utilizes fluorescent 3D-DIC to eliminate image decorrelation caused by the crazing effect. Then, an adaptive incremental calculation strategy that can automatically update reference images is adopted to mitigate image decorrelation due to excessive deformation. The effectiveness and practicality of the proposed technique were demonstrated by measuring the full-field deformation in chloroprene rubber samples subjected to large tensile deformation. Also, the proposed method was applied to investigate the mechanical behavior of a specific waterproof coating material. Experimental results indicate that the combination of fluorescent 3D-DIC and adaptive incremental calculation strategy can effectively address image decorrelation problems in large deformations of polymers, yielding high-accuracy displacement and strain measurements. This technique holds potential for broader applications in studying the mechanical behavior of other materials or structures undergoing large or super-large deformations. • Propose a fluorescent 3D-DIC with an adaptive incremental calculation strategy to measure large deformations in polymers. • Fluorescent stereovision imaging effectively avoids decorrelation problem caused by "crazing" in strained polymers. • Adaptive incremental calculation strategy mitigates image decorrelation problem caused by large deformation. [ABSTRACT FROM AUTHOR]

• Highest nanoindentation strain rate reported on a polymer: 103 s−1. • Nanoindentation and micropillar compression of SU-8 polymer from 10-3 to 103 s−1. • Novel nanoindentation protocol to obtain reliable modulus and hardness at 103 s−1. • High strain rate properties of SU-8 reported for first time. In situ micropillar compression and nanoindentation were performed on photolithographically fabricated SU-8 polymer inside a scanning electron microscope (SEM), covering seven orders of strain rates from 10-3 to 103 s−1. The extracted mechanical properties – modulus, hardness, yield strength, strain rate sensitivity (SRS) exponent – were systematically compared from both microscale tests and showed excellent agreement. No change in deformation mechanism was observed at high strain rates. We report a novel experimental protocol for high strain rate nanoindentation, comprising of input profile smoothing, that minimizes resonance amplitude during unloading and allows reliable extraction of modulus and hardness using standard Oliver-Pharr analysis. To the best of our knowledge, this is the first report of mechanical properties of SU-8 beyond 1 s−1 strain rate using nanoindentation based tests. [ABSTRACT FROM AUTHOR]