Free Volume in Polyimides: Positron Annihilation Experiments and Molecular Modeling

LUNAR: Automated Input Generation and Analysis for Reactive LAMMPS Simulations

Generating simulation-ready molecular models for the LAMMPS molecular dynamics (MD) simulation software package is a difficult task and impedes the more widespread and efficient use of MD in materials design and development. Fixed-bond force fields generally require manual assignment of atom types, bonded interactions, charges, and simulation domain sizes. A new LAMMPS pre- and postprocessing toolkit (LUNAR) is presented that efficiently builds molecular systems for LAMMPS. LUNAR automatically assigns atom types, generates bonded interactions, assigns charges, and provides initial configuration methods to generate large molecular systems. LUNAR can also incorporate chemical reactivity into simulations by facilitating the use of the REACTER protocol. Additionally, LUNAR provides postprocessing for free volume calculations, cure characterization calculations, and property predictions from LAMMPS thermodynamic outputs. LUNAR has been validated via building and simulation of pure epoxy and cyanate ester polymer systems with a comparison of the corresponding predicted structures and properties to benchmark values, including experimental results from the literature. LUNAR provides the tools for the computationally driven development of next-generation composite materials in the Integrated Computational Materials Engineering (ICME) and Materials Genome Initiative (MGI) frameworks. LUNAR is written in Python with the usage of NumPy and can be used via a graphical user interface, a command line interface, or an integrated design environment. LUNAR is freely available via GitHub.

Modelling Sorption and Transport of Gases in Polymeric Membranes across Different Scales: A Review

Professor Giulio C. Sarti has provided outstanding contributions to the modelling of fluid sorption and transport in polymeric materials, with a special eye on industrial applications such as membrane separation, due to his Chemical Engineering background. He was the co-creator of innovative theories such as the Non-Equilibrium Theory for Glassy Polymers (NET-GP), a flexible tool to estimate the solubility of pure and mixed fluids in a wide range of polymers, and of the Standard Transport Model (STM) for estimating membrane permeability and selectivity. In this review, inspired by his rigorous and original approach to representing membrane fundamentals, we provide an overview of the most significant and up-to-date modeling tools available to estimate the main properties governing polymeric membranes in fluid separation, namely solubility and diffusivity. The paper is not meant to be comprehensive, but it focuses on those contributions that are most relevant or that show the potential to be relevant in the future. We do not restrict our view to the field of macroscopic modelling, which was the main playground of professor Sarti, but also devote our attention to Molecular and Multiscale Hierarchical Modeling. This work proposes a critical evaluation of the different approaches considered, along with their limitations and potentiality.

Defining the optimal criterion for separating gases using polymeric membranes

Polymeric membranes are efficient in separating gas mixtures, typically by exploiting the sieving mechanism. What controls the sieve size of a given polymer matrix is unclear, although one line of thought implies that the local cage size, defined by the dynamic motions of the glassy polymer matrix, is the relevant metric. Here, we use coarse-grained molecular dynamics simulations and show that the sieve size is defined by a static cavity size controlled by polymer chain stiffness (a packing-driven metric) combined with the local cage-like motions of the polymer host. The best separation performance for a pair of gases is when this combined metric is roughly half way between the diameters of the gases in question, with the static and dynamic quantities contributing roughly equally. For the various models simulated we find the existence of an upper bound correlation which passes through this optimal point and has a slope expected from the Freeman model, namely , where the d's correspond to the kinetic diameters of the gases in question. Our results thus demonstrate that the relevant free volume size that affects gas transport in these condensed phases is defined by both static and dynamic measures.

Soluble Perfluorocyclobutyl Aryl Ether-Based Polyimide for High-Performance Dielectric Material

High durability of low-k value is a desired property for dielectrics serving under humid conditions, because absorbing a small amount of moisture by the material can considerably increase the k value so as to result in function deterioration. Aiming to develop a dielectric polymer with superior durability of low-k value and high thermal stability, a perfluorocyclobutyl (PFCB) biphenyl ether-based polyimide, PFCBBPPI, was synthesized. This polymer possesses a T_g of 310.3 °C and a 5% weight loss temperature of 510.5 °C. PFCBBBPPI exhibited an extremely low water uptake of 0.065 ± 0.018%, representing the best water resistance in polyimides. The increasing percentage in k value was below 2% for PFCBBPPI film exposed to moisture under various humidity conditions for 6 h. PFCBBPPI film equilibrated at 75% R.H. for 2 weeks still kept its k value below 2.50, remarkably outperforming the Kapton film. The remarkable water resistance and resulting high durability of low-k property displayed by PFCBBPPI are originated from the hydrophobic nature and small free volume fraction of the polymer, as confirmed by contact angle test and positron annihilation lifetime spectroscopy results. The outstanding moisture resistance and overall performance of PFCBBPPI make it a suitable candidate for dielectric applications under both dry and humid conditions.

Molecular dynamic simulations of pressure-driven water transport through polyamide nanofiltration membranes at different membrane densities

The amorphous aromatic polyamide membranes with different membrane densities were modeled to study the porous structure of free-volume pores and the pressure-driven water transport by using molecular dynamics simulations.

Free-volume hole relaxation in molecularly oriented glassy polymers

The free-volume hole relaxation in polycarbonate and poly(methyl methacrylate) with different levels of molecular orientation was studied by positron annihilation lifetime spectroscopy at variable pressures. The molecular orientation was achieved through a simple shear process performed at different temperatures and extrusion rates. It has been demonstrated that the β relaxation is largely responsible for the free-volume hole anisotropy after simple shear orientation. Upon the removal of mechanical force, the deformation of the free volume is mostly reversible at temperatures much lower than the glass transition. No strong correlation between macroscopic deformation and the free-volume hole deformation was found regardless of molecular orientation.

Correlation of Gas Permeability in a Metal-Organic Framework MIL-101(Cr)-Polysulfone Mixed-Matrix Membrane with Free Volume Measurements by Positron Annihilation Lifetime Spectroscopy (PALS)

Hydrothermally stable particles of the metal-organic framework MIL-101(Cr) were incorporated into a polysulfone (PSF) matrix to produce mixed-matrix or composite membranes with excellent dispersion of MIL-101 particles and good adhesion within the polymer matrix. Pure gas (O₂, N₂, CO₂ and CH₄) permeation tests showed a significant increase of gas permeabilities of the mixed-matrix membranes without any loss in selectivity. Positron annihilation lifetime spectroscopy (PALS) indicated that the increased gas permeability is due to the free volume in the PSF polymer and the added large free volume inside the MIL-101 particles. The trend of the gas transport properties of the composite membranes could be reproduced by a Maxwell model.

Open volume in bioadhesive detected by positron annihilation lifetime spectroscopy

Barnacles attach to a wide variety of surfaces underwater and show substrate-specific adhesion mechanisms. Investigating and understanding these mechanisms is a key for developing new technical adhesives. We expected open volume (porosity) at the sub-nanometre scale to occur in barnacle adhesive. With positron annihilation lifetime spectroscopy (PALS) it is possible to detect porosity at the nanometre scale by determining the lifetime of positrons. This method has not been applied to bioadhesives so far. We showed that PALS is a suitable technique for the investigation of the barnacle base and its adhesive with respect to open volume. The results were interpreted using a standard model adapted from polymers. We thereby estimated pore sizes of 0.5 nm.

Effect of UV-ozone treatment on poly(dimethylsiloxane) membranes: surface characterization and gas separation performance

A thin SiO(x) selective surface layer was formed on a series of cross-linked poly(dimethylsiloxane) (PDMS) membranes by exposure to ultraviolet light at room temperature in the presence of ozone. The conversion of the cross-linked polysiloxane to SiO(x) was monitored by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray (EDX) microanalysis, contact angle analysis, and atomic force microscopy (AFM). The conversion of the cross-linked polysiloxane to SiO(x) increased with UV-ozone exposure time and cross-linking agent content, and the surface possesses highest conversion. The formation of a SiO(x) layer increased surface roughness, but it decreased water contact angle. Gas permeation measurements on the UV-ozone exposure PDMS membranes documented interesting gas separation properties: the O(2) permeability of the cross-linked PDMS membrane before UV-ozone exposure was 777 barrer, and the O(2)/N(2) selectivity was 1.9; after UV-ozone exposure, the permeability decreased to 127 barrer while the selectivity increased to 5.4. The free volume depth profile of the SiO(x) layer was investigated by novel slow positron beam. The results show that free volume size increased with the depth, yet the degree of siloxane conversion to SiO(x) does not affect the amount of free volume.

Temperature dependence of the free volume from positron lifetime experiments and its relation to structural dynamics: phenylphthalein-dimethylether

Positron annihilation lifetime spectroscopy (PALS) was used to study the microstructure of the free volume in the temperature range between 103 K and 393 K in phenylphthalein-dimethylether (PDE), a low-molecular-weight glass former. Using the routine LIFETIME9.0, the ortho-positronium (o-Ps) lifetime distribution was analyzed, and from this, the volume distribution gn(vh) of subnanometer-size holes was calculated. From a comparison of PALS and specific volume data, the number density and the volume fraction of holes were estimated. These free-volume data, as a function of temperature, were used to test the validity of the Cohen-Turnbull (CT) free-volume theory. It was found that the structural relaxation from dielectric spectroscopy can be described by the CT theory after introducing a corrected free volume (Vf-DeltaV), where DeltaV=0.014 cm3/g. The extended free-volume theory of Cohen and Grest can be fitted to the dielectric-relaxation and free-volume data, but the parameters of both fits are not consistent. PDE shows some peculiar features. The "knee" in the o -Ps lifetime expansion and crossover in temperature dependence of the frequency of the primary dielectric relaxation process occur at different temperatures. In addition, the change in the Vogel-Fulcher-Tammann parameters at TB/Tg=1.1 has no observable effect on the mean free volume vh (or Vf). The size of the smallest representative freely fluctuating subsystem, VSV estimated from the standard deviation sigmah of gn(vh), decreases from 4.1 nm3 to 2.6 nm3 when the temperature increases from T/Tg=1.0 to 1.15. Correspondingly, the length of dynamic heterogeneity, xi=VVS1/3, decreases from 1.6 nm to 1.4 nm. It is concluded that at T/Tg approximately 1.10=TB/Tg the system transforms from a heterogeneous to a homogeneous (true) liquid.