Molecular dynamics simulation of water diffusion in atactic and amorphous isotactic polypropylene

Evaluation of polymer-preservative interactions for preservation efficacy: molecular dynamics simulation and QSAR approaches

Preservatives are critical ingredients in various pharmaceutical and consumer products. In particular, a high efficacy preservative system is essential in enhancing the shelf-life and safety of these products. However, the development of such a preservative system heavily relies on experimental approaches. In this study, molecular dynamics (MD) simulation was complemented with quantitative structure-activity relationship (QSAR) modelling to comprehensively evaluate polymer-preservative interactions between three different polymers (polyethylene terephthalate, PET; polypropylene, PP; and cellulose) and a series of preservatives from the classes of aliphatic, aromatic, and organic acids. First, adsorption of preservatives onto polymer surfaces was simulated in an aqueous environment. The preservatives did not adhere to hydrophilic cellulose, but most preservatives were adsorbed by PET and PP in distinct configurations. Interaction energies (IEs) between the preservatives and the polymers generally increase from cellulose to PP and PET. The diffusion coefficients of preservatives are dependent on polymer nature, preservative structure, and their resulting molecular interactions. Linear and low molecular weight preservatives exhibit higher diffusion coefficients in polymers. For a particular preservative, diffusion coefficients increased in the order of cellulose < PET < PP. Finally, using MD properties and molecular descriptors of preservatives, QSAR models were developed to identify key descriptors of preservatives and predict their IEs and diffusion coefficients in polymers. This study demonstrates a computational approach for identifying critical materials properties, and predicting polymer-preservative molecular interactions in water. Such an approach streamlines the rational selection and design of high efficacy preservative systems for various pharmaceutical, food and cosmetic products. Furthermore, the integrated computational strategy also reduces trial-and-error experimental efforts, thereby accelerating the development of high efficacy preservative systems.

Hop, Skip, and Jump: Hydrogen Molecular Transport through Amorphous Polyethylene Matrices Studied via Molecular Dynamics Simulations

In the pursuit of advancing and diversifying energy technologies for a more sustainable future, the possibilities of hydrogen (H2) usage will broaden, as will our understanding of its containment materials. Polyethylene (PE) has a vast assortment of uses and applications, which are growing with demands for alternative energy possibilities. One use of PE liner is as a prime candidate for nonmetallic piping and pressurized type IV storage devices. Such applications require PE to effectively prevent H2 transport through containment systems. To study the molecular transport mechanism of hydrogen through polymeric barriers, a system containing hydrogen molecules absorbed within amorphous PE is modeled here using molecular dynamics simulations. The simulations are conducted within a range of temperatures that span the glass transition temperature of amorphous PE. The simulated PE displays bulk density, radius of gyration, and self-diffusion coefficient that are consistent with experimental data. The simulated trajectories are interrogated to study the movement of the guest gas molecules. The results show that the diffusion coefficients increase with temperature, as expected. However, the mobility of the PE chains is found to affect the mobility of absorbed H2 molecules to a much lower extent than it affects that of CH4 molecules because of the much smaller size of the former than of the latter guest. From a molecular perspective, a "hopping" mechanism is observed, according to which H2 molecules hop between one vacant free volume space to another within the polymer matrix, in combination with longer, straight, undisturbed "jumps" or "skips" along directions aligned with regions of ordered PE chains. This suggests that the orientation of the crystallites within the semicrystalline PE matrix affects the H2 containment. Implications of these findings toward PE usage as containment material are discussed.

Molecular dynamics simulation of polypropylene: diffusion and sorption of H2O, H2O2, H2, O2 and determination of the glass transition temperature

Molecular dynamics (MD) simulations in the canonical (NVT) and the isothermal-isobaric (NPT) ensemble using COMPASS III molecular force fields were performed to study the penetrant diffusion of water (H2O), hydrogen peroxide (H2O2) and oxygen (O2) in isotactic polypropylene (iPP) and hydrogen (H2) in iPP and atactic polypropylene (aPP) for time intervals up to 11 ns and in the case of H2O2 up to 22 ns. We found robust cluster formation in the case of H2O and H2O2. Further, the diffusion coefficients for all these systems were estimated by mean-square displacement analysis. Our results are consistent with previously published experimental and computational data except for the diffusion of H2 in polypropylene where our results are one and two orders of magnitude higher, respectively. Grand Canonical Monte Carlo (GCMC) simulations were used to determine the sorption loading and saturation concentration of H2O, O2 and H2 in iPP, where we find good agreement for H2O with experimental results. By means of MD simulation the glass transition temperature (Tg) of iPP was estimated to 273.66 ± 4.21 K which is consistent with previously published experimental results.

Characterization of entanglements in glassy polymeric ensembles using the Gaussian linking number

We propose a method for enumerating entanglements between long chained, linear polymers that is based on the Gaussian linking number. The linking number is calculated between closely approaching segments of the macromolecular chains. Topological features of an entanglement, i.e., the extent to which one open segment winds around another, are reflected by the linking number. We show that using this measure, we can track disentanglement events through a deformation history and gain insights into how large scale disentanglements lead to failure. Incorporating an additional step where the topological entanglements identified along each chain are optimally clustered using standard clustering algorithms, we can also obtain a measure of the average number of rheological constraints that exist along each chain in an ensemble. Comparisons with other methods of enumerating entanglements, especially the primitive path analysis, are also made. Our results indicate that the linking number between two entangled segments in the undeformed state is a good indicator of the strength of the entanglement. Also, disentanglements occurring overwhelmingly around chain ends are an important cause of failure when a triaxial stress state exists in the polymer.

Prediction and validation of diffusion coefficients in a model drug delivery system using microsecond atomistic molecular dynamics simulation and vapour sorption analysis

Diffusion of small to medium sized molecules in polymeric medical device materials underlies a broad range of public health concerns related to unintended leaching from or uptake into implantable medical devices. However, obtaining accurate diffusion coefficients for such systems at physiological temperature represents a formidable challenge, both experimentally and computationally. While molecular dynamics simulation has been used to accurately predict the diffusion coefficients, D, of a handful of gases in various polymers, this success has not been extended to molecules larger than gases, e.g., condensable vapours, liquids, and drugs. We present atomistic molecular dynamics simulation predictions of diffusion in a model drug eluting system that represent a dramatic improvement in accuracy compared to previous simulation predictions for comparable systems. We find that, for simulations of insufficient duration, sub-diffusive dynamics can lead to dramatic over-prediction of D. We present useful metrics for monitoring the extent of sub-diffusive dynamics and explore how these metrics correlate to error in D. We also identify a relationship between diffusion and fast dynamics in our system, which may serve as a means to more rapidly predict diffusion in slowly diffusing systems. Our work provides important precedent and essential insights for utilizing atomistic molecular dynamics simulations to predict diffusion coefficients of small to medium sized molecules in condensed soft matter systems.

Study on the Bleeding Mechanism of Slip Agents in a Polypropylene Film using Molecular Dynamics

The bleeding (internal transport) process of additives in a polypropylene film under atmospheric pressure was investigated. The experimental results were explained more precisely by a new model assuming the two-step transport between the amorphous and crystalline regions. The diffusion coefficient of a higher fatty acid such as behenic acid (docosanoic acid) in isotactic polypropylene film and that of higher fatty acid amides such as erucamaide (13-cis-docosenamide) in ethylene copolymerized polypropylene film were determined at 40 and 50°C respectively. The difference between the diffusion coefficients of three slip agents in a polypropylene film at 50°C was explained using a molecular dynamics simulation in which self-association of the slip agent molecules by hydrogen bonding was considered.