Lauren J. Abbott

Polymatic : a generalized simulated polymerization algorithm for amorphous polymers

This work presents a generalized structure generation methodology for amorphous polymers by a simulated polymerization technique and 21-step molecular dynamics equilibration, which is particularly effective for high-Tg polymers. The essential framework and parameters of the techniques and algorithms are described in detail, and example input scripts are provided for use with the freely available Polymatic simulated polymerization code and LAMMPS molecular dynamics package. The capabilities of the methods are examined through application to six linear, glassy polymers ranging in functionality, polarity, and rigidity. Validation of the methodology is provided by comparison of the simulations and experiments for a variety of structural, adsorption, and thermal properties, all of which showed excellent agreement with available experimental data.

Characterizing the Structure of Organic Molecules of Intrinsic Microporosity by Molecular Simulations and X-ray Scattering

The design of a new class of materials, called organic molecules of intrinsic microporosity (OMIMs), incorporates awkward, concave shapes to prevent efficient packing of molecules, resulting in microporosity. This work presents predictive molecular simulations and experimental wide-angle X-ray scattering (WAXS) for a series of biphenyl-core OMIMs with varying end-group geometries. Development of the utilized simulation protocol was based on comparison of several simulation methods to WAXS patterns. In addition, examination of the simulated structures has facilitated the assignment of WAXS features to specific intra- and intermolecular distances, making this a useful tool for characterizing the packing behavior of this class of materials. Analysis of the simulations suggested that OMIMs had greater microporosity when the molecules were the most shape-persistent, which required rigid structures and bulky end groups. The simulation protocol presented here allows for predictive, presynthesis screening of OMIMs and similar complex molecules to enhance understanding of their structures and aid in future design efforts.

Analysis of force fields and BET theory for polymers of intrinsic microporosity

A detailed force field analysis for polymers of intrinsic microporosity (PIMs) was carried out in this study. The generalised amber force field (GAFF) with united atom transferable potential for phase equilibria (TraPPE-UA), and the atomistic polymer consistent force field were evaluated. Analysis carried out with PIM-1 showed that the use of GAFF for bonded interactions and TraPPE-UA for non-bonded interactions yielded a simulated sample that compared best with available experimental data (wide-angle X-ray scattering and nitrogen adsorption at 77 K). In addition, Brunauer–Emmett–Teller surface areas, calculated from simulated nitrogen isotherms as pseudo-experimental data, showed that this common method failed to measure the geometric surface area of this type of material. These findings are expected to facilitate the predictive screening of different PIM functionalities.

Design principles for microporous organic solids from predictive computational screening

Predictive molecular simulations were utilized to screen an array of microporous organic solids in order to provide insight into important design principles for increasing porosity in these types of materials. The computational screening considered 22 organic molecules of intrinsic microporosity (OMIMs) incorporating varying core and end-group geometries. The simulations were validated for a subset of experimentally synthesized biphenyl-core OMIMs by excellent agreement with X-ray scattering data. Analysis of the simulations revealed the role of three structural design aspects for increasing porosity in these types of materials: (i) molecular rigidity, (ii) bulky end groups, and (iii) three-dimensionality. In particular, the porosity increased with molecular rigidity, and ideal simulations of rigid-body molecules resulted in surface areas larger by an order of magnitude. Increasing the bulkiness of the end groups, such as by the addition of awkward tert-butyl and adamantyl functional groups, also encouraged a more inefficient packing. Lastly, molecular shapes that were more three dimensional, instead of planar, provided greater porosity. While these design principles were examined here for OMIMs, they extend to similar classes of discrete organic materials, as well as microporous polymers.

Virtual Synthesis of Thermally Cross-Linked Copolymers from a Novel Implementation of Polymatic

Because of the complex connectivity of cross-linked polymers, generating structures for molecular simulations is a nontrivial task. In this work, a general methodology is presented for constructing post-cross-linked polymers by a new two-stage implementation of the Polymatic simulated polymerization algorithm, where linear polymers are first polymerized and then cross-linked. It is illustrated here for an example system of thermally cross-linked octene-styrene-divinylbenzene (OS-DVB) copolymers. In the molecular models, the degree of cross-linking is ranged from 0 to 100%, and the resulting structural and thermal properties are examined. The simulations reveal an increase in the free volume with higher cross-linking degrees. Shifts in the peaks of the structure factors, which are assigned to contributions from the backbone and side-chain atoms, correspond to the formation of larger free volume elements. Furthermore, the glass transition temperatures increase with higher degrees of cross-linking, while the thermal expansivity decreases. Comparisons with experimental results for similar systems are made when available. As demonstrated here, the presented methodology will provide an effective route to simulating post-cross-linked polymers for a variety of applications, which will enable an improved understanding of their structure-property relationships.

Single Chain Structure of a Poly(N-isopropylacrylamide) Surfactant in Water

We present atomistic simulations of a single PNIPAM-alkyl copolymer surfactant in aqueous solution at temperatures below and above the LCST of PNIPAM. We compare properties of the surfactant with pure PNIPAM oligomers of similar lengths, such as the radius of gyration and solvent accessible surface area, to determine the differences in their structures and transition behavior. We also explore changes in polymer-polymer and polymer-water interactions, including hydrogen bond formation. The expected behavior is observed in the pure PNIPAM oligomers, where the backbone folds onto itself above the LCST in order to shield the hydrophobic groups from water. The surfactant, on the other hand, does not show much conformational change as a function of temperature, but instead folds to bring the hydrophobic alkyl tail and PNIPAM headgroup together at all temperatures. The atomic detail available from these simulations offers important insight into understanding how the transition behavior is changed in PNIPAM-based systems.

A temperature-dependent coarse-grained model for the thermoresponsive polymer poly(N-isopropylacrylamide)

A coarse-grained (CG) model is developed for the thermoresponsive polymer poly(N-isopropylacrylamide) (PNIPAM), using a hybrid top-down and bottom-up approach. Nonbonded parameters are fit to experimental thermodynamic data following the procedures of the SDK (Shinoda, DeVane, and Klein) CG force field, with minor adjustments to provide better agreement with radial distribution functions from atomistic simulations. Bonded parameters are fit to probability distributions from atomistic simulations using multi-centered Gaussian-based potentials. The temperature-dependent potentials derived for the PNIPAM CG model in this work properly capture the coil-globule transition of PNIPAM single chains and yield a chain-length dependence consistent with atomistic simulations.

Morphology and molecular bridging in comb- and star-shaped diblock copolymers

Block copolymers spontaneously self-assemble into nanostructured morphologies with industrially attractive properties; however, the relationships between polymer architecture and self-assembled morphology are difficult to tailor for copolymers with increased conformational restrictions. Using Dissipative Particle Dynamics, the self-assembled morphology of comb- and star-shaped diblock copolymers was simulated as a function of the number of arms, arm length, weight fraction, and A-B incompatibility. As the number of arms on the star, or grafting points for the comb, was increased from three to four to six, the ability to self-assemble into ordered morphologies was restricted. The molecular bridging between adjacent ordered domains was observed for both comb- and star-shaped copolymers, which was found to be enhanced with increasing number of arms. This study illustrates that comb- and star-shaped copolymers are viable alternatives for applications that would benefit from highly bridged nanostructural domains.

Modeling Amorphous Microporous Polymers for CO2 Capture and Separations

This review concentrates on the advances of atomistic molecular simulations to design and evaluate amorphous microporous polymeric materials for CO2 capture and separations. A description of atomistic molecular simulations is provided, including simulation techniques, structural generation approaches, relaxation and equilibration methodologies, and considerations needed for validation of simulated samples. The review provides general guidelines and a comprehensive update of the recent literature (since 2007) to promote the acceleration of the discovery and screening of amorphous microporous polymers for CO2 capture and separation processes.