Glassy Polymer Matrix Research Articles

Construction of Elastomeric Nanobelts in a Glassy Polymer Matrix: Toward Supertoughened Nanoalloys with Significantly Depressed Yielding Behaviors

Construction of Elastomeric Nanobelts in a Glassy Polymer Matrix: Toward Supertoughened Nanoalloys with Significantly Depressed Yielding Behaviors

Superionic Bifunctional Polymer Electrolytes for Solid-State Energy Storage and Conversion.

Achieving superionic conductivity from solid-state polymer electrolytes is an important task in the development of future energy storage and conversion technologies. Herein, a platform for innovative electrolyte technologies based on a bifunctional polymer, poly(3-hydroxy-4-sulfonated styrene) (PS-3H4S), is presented. By incorporating OH and SO3 H functional groups at adjacent positions in the styrene repeating unit, "intra-monomer" hydrogen bonds are formed to effectively weaken the electrostatic interactions of the SO3 - moieties in the polymer matrix with embedded ions, promoting rich structural and dynamic heterogeneity in the PS-3H4S electrolyte. Upon the incorporation of an ionic liquid, interconnected rod-like ion channels, which allow the decoupling of ion relaxation from polymer relaxation, are formed in the stiff motif of the polymeric domains passivated by interfacial ionic layers. This results in accelerated proton hopping through the glassy polymer matrix, and proton hopping becomes more pronounced at cryogenic temperatures down to -35 °C. The PS-3H4S/ionic liquid composite electrolytes exhibit a high ionic conductivity of 10-3 S cm-1 and high storage modulus of ≈100MPa at 25 °C, and can be successfully applied in soft actuators and lithium-metal batteries.

Hyper Cross-Linked Polymers as Additives for Preventing Aging of PIM-1 Membranes.

Mixed-matrix membranes (MMMs) are membranes that are composed of polymers embedded with inorganic particles. By combining the polymers with the inorganic fillers, improvements can be made to the permeability compared to the pure polymer membranes due to new pathways for gas transport. However, the fillers, such as hyper cross-linked polymers (HCP), can also help to reduce the physical aging of the MMMs composed of a glassy polymer matrix. Here we report the synthesis of two novel HCP fillers, based on the Friedel–Crafts reaction between a tetraphenyl methane monomer and a bromomethyl benzene monomer. According to the temperature and the solvent used during the reaction (dichloromethane (DCM) or dichloroethane (DCE)), two different particle sizes have been obtained, 498 nm with DCM and 120 nm with DCE. The change in the reaction process also induces a change in the surface area and pore volumes. Several MMMs have been developed with PIM-1 as matrix and HCPs as fillers at 3% and 10wt % loading. Their permeation performances have been studied over the course of two years in order to explore physical aging effects over time. Without filler, PIM-1 exhibits the classical aging behavior of polymers of intrinsic microporosity, namely, a progressive decline in gas permeation, up to 90% for CO2 permeability. On the contrary, with HCPs, the physical aging at longer terms in PIM-1 is moderated with a decrease of 60% for CO2 permeability. 13C spin-lattice relaxation times (T1) indicates that this slowdown is related to the interactions between HCPs and PIM-1.

Molecular Modeling Investigations of Sorption and Diffusion of Small Molecules in Glassy Polymers.

With a wide range of applications, from energy and environmental engineering, such as in gas separations and water purification, to biomedical engineering and packaging, glassy polymeric materials remain in the core of novel membrane and state-of the art barrier technologies. This review focuses on molecular simulation methodologies implemented for the study of sorption and diffusion of small molecules in dense glassy polymeric systems. Basic concepts are introduced and systematic methods for the generation of realistic polymer configurations are briefly presented. Challenges related to the long length and time scale phenomena that govern the permeation process in the glassy polymer matrix are described and molecular simulation approaches developed to address the multiscale problem at hand are discussed.

Kinetic stabilization of cellulose nanocrystals in a photocurable prepolymer for application as an adhesion promoter in UV-curable coatings

Cellulose nanocrystals (CNC) at low loading levels were shown to reinforce a photocurable coating resulting in improved adhesion. A polyether polyol containing CNC at loading levels of up to 1.8 wt% was grafted with 3-isopropenyl-α,α-dimethylbenzyl isocyanate to functionalize it with a photocurable group. The nanoparticles were kinetically stabilized in the rapidly forming prepolymer of high viscosity. Photoinitiators and a difunctional reactive diluent were added to produce optically transparent coatings and free films upon irradiation by ultraviolet (UV) light. This allowed evaluation of the effects of CNC at low loading levels in a glassy polymer matrix obtained through a rapid cure system. Incorporation of CNC nanoparticles in the polymer matrix resulted in an average improvement in adhesive strength of 154% while enhancing tensile strength by an average of 16%. The technique described could be used as a new approach to reduce adhesive failure in UV-curable coatings without sacrificing their mechanical strength.

Explicit finite element analysis of failure behaviors of thermoplastic composites under transverse tension and shear

This paper studies failure behaviors of glass fiber/PC resin thermoplastic composites under transverse tension and shear loads using explicit finite element method. Microscopic long fiber/matrix composite representative volume elements with randomly distributed fibers and periodic boundary conditions are established by ABAQUS-PYTHON script language. The finite-deformation viscoplastic Gurson model is used for predicting the plastic deformation and void growth of thermoplastic glassy polymer matrix, which is implemented using ABAQUS-VUMAT explicit subroutine. The bilinear cohesive model with finite-thickness interface element is used to simulate the fiber/matrix debonding. Three important issues are explored: first, effects of the cohesive interface strength on the plastic deformation and void growth of composites are studied using three mesh models and three fiber distributions. Second, failure surface under different tensile and shear displacement ratios is obtained, compared with the predictions using the Hashin and Puck failure criteria developed mainly for thermoset composites. Finally, the influence of the plastic softening effect of matrix on the strain concentration or localization behaviors is discussed. Results show the tension/shear load ratio and the cohesive interface strength produce large effects on the void growth and failure surface of composites, but fiber distribution shows a small effect on transverse load-bearing ability of composites.

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.

Elementary prediction of gas permeability in glassy polymers

The transport model proposed by Minelli and Sarti for the representation of gas and vapor permeability in glassy polymers has been extensively applied to various systems, and the model results are thoroughly analyzed. The approach is based on fundamental theory for the diffusion of low penetrant species in polymers, in which the diffusivity is considered as the product of the molecular mobility, and a thermodynamic coefficient, accounting for the concentration dependence of the chemical potential. The model relies on the thermodynamic description of the penetrant/polymer systems provided by the NonEquilibrium Thermodynamics for Glassy Polymers (NET-GP) approach. The penetrant mobility is assumed to depend exponentially on penetrant concentration, and the model contains two parameters only: mobility coefficient at infinite dilution and plasticization factor.The model parameters obtained from the analysis of the permeability behaviors of various systems have been examined and general correlations are derived. The mobility coefficient is indeed correlated to the properties of the pure penetrants (penetrant molecular size) and pure polymer (fractional free volume and characteristic energy). This allows the derivation of a simple and general expression for the prediction of the permeability of any penetrant species in glassy polymers in the range of low penetrant pressures, as well as the selectivity of any gas pair. Remarkably, the model predictions are able to represent quite accurately the experimental data available in the literature. Furthermore, the plasticization factor is correlated to the swelling produced by the penetrant into the glassy polymer matrix, obtaining thus a reliable tool for the estimation of the pressure dependence of gas permeability on upstream pressure.

Simulation of carbon nanotubes as macromolecular coils in nanocomposites with glassy polymer matrix

Ring-shaped carbon nanotubes have been simulated as macromolecular coils in the context of the fractal physical chemistry of polymer solutions k]The dependence of the interfacial adhesion level in polymer/carbon nanotube nanocomposites on the structure of the above composites k]characterized by its fractal dimension k]has been shown k]The validity of this interpretation has been confirmed via a description of the degree of reinforcement of nanocomposites within the reinforcement molecular model.

Macromol. Chem. Phys. 6/2014

Front Cover: The preparation of different styrene-based polymer films containing small amounts of tetraphenylethylene (TPE) and the evaluation of their photoluminescent behavior are reported. TPE molecular rotors show brilliant blue emission in a poor solvent or in a glassy styrene-based polymer matrix, where the intramolecular rotations of its aryls result as being completely arrested, but the fluorescence is weakened to pale green and eventually to a faint signal when good solvents or viscous but not glassy polymer matrices are used. This behavior suggests potential benefits in applications such as polymer traceability. Further details can be found in the article by G. Iasilli, A. Battisti, F. Tantussi, F. Fuso, M. Allegrini, G. Ruggeri, and A. Pucci* on page 499.

Concurrent Solution‐Like Decoloration Rate and High Mechanical Strength from Polymer‐Dispersed Photochromic Organogel

To achieve a fast photochromic response in solid matrix, photochromic molecules/segments have been either dispersed into elastomers via physical doping or linked to glassy polymers by soft units through covalent bonding. However, the former is lack of high mechanical strength and the latter owes the drawback of time-consumption of synthesis. Here, we propose a facile strategy of co-solvent evaporation to prepare polymer-dispersed photochromic organogel where both high mechanical strength of the glassy polymer matrix and solution-like fast photochromism of the photochromic molecule within organogel can be retained concurrently. Glassy PVA matrix and dispersed organogel of 1,3:2,4-di-O-benzylidene-d-sorbitol/poly(propylene glycol) (DBS/PPG) provide high mechanical strength and sufficient free volume for intramolecular rotation of photochromic spiropyran (SP), respectively. Interestingly, these thin films behave a solution-like decoloration the decay rate of which is 65-70 fold faster than that in the SP-directly doped PVA film and only slightly slower than those in their corresponding PPG solutions.

Modeling mechanophore activation within a crosslinked glassy matrix

Mechanically induced reactivity is a promising means for designing self-reporting materials. Mechanically sensitive chemical groups called mechanophores are covalently linked into polymers in order to trigger specific chemical reactions upon mechanical loading. These mechanophores can be linked either within the backbone or as crosslinks between backbone segments. Mechanophore response is sensitive to both the matrix properties and placement within the matrix, providing two avenues for material design. A model framework is developed to describe reactivity of mechanophores located as crosslinks in a glassy polymer matrix. Simulations are conducted at the molecular and macromolecular scales in order to develop macroscale constitutive relations. The model is developed specifically for the case of spiropyran (SP) in lightly crosslinked polymethylmethacrylate (PMMA). This optically trackable mechanophore (fluorescent when activated) allows the model to be assessed in terms of observed experimental behavior. The force modified potential energy surface (FMPES) framework is used in conjunction with ab initio steered molecular dynamics (MD) simulations of SP to determine the mechanophore kinetics. MD simulations of the crosslinked PMMA structure under shear deformation are used to determine the relationship between macroscale stress and local force on the crosslinks. A continuum model implemented in a finite element framework synthesizes these mechanochemical relations with the mechanical behavior. The continuum model with parameters taken directly from the FMPES and MD analyses under predicts stress-driven activation relative to experimental data. The continuum model, with the physically motivated modification of force fluctuations, provides an accurate prediction for monotonic loading across three decades of strain rate and creep loading, suggesting that the fundamental physics are captured.

Room Temperature Phosphorescence of Metal-Free Organic Materials in Amorphous Polymer Matrices

Developing metal-free organic phosphorescent materials is promising but challenging because achieving emissive triplet relaxation that outcompetes the vibrational loss of triplets, a key process to achieving phosphorescence, is difficult without heavy metal atoms. While recent studies reveal that bright room temperature phosphorescence can be realized in purely organic crystalline materials through directed halogen bonding, these organic phosphors still have limitations to practical applications due to the stringent requirement of high quality crystal formation. Here we report bright room temperature phosphorescence by embedding a purely organic phosphor into an amorphous glassy polymer matrix. Our study implies that the reduced beta (β)-relaxation of isotactic PMMA most efficiently suppresses vibrational triplet decay and allows the embedded organic phosphors to achieve a bright 7.5% phosphorescence quantum yield. We also demonstrate a microfluidic device integrated with a novel temperature sensor based on the metal-free purely organic phosphors in the temperature-sensitive polymer matrix. This unique system has many advantages: (i) simple device structures without feeding additional temperature sensing agents, (ii) bright phosphorescence emission, (iii) a reversible thermal response, and (iv) tunable temperature sensing ranges by using different polymers.

Computational homogenization of elastic–plastic composites

This work describes a computational homogenization methodology to estimate the effective elastic–plastic response of random two-phase composite media. It is based on finite element simulations using three-dimensional cubic cells of different size but smaller than the deterministic representative volume element (DRVE) of the microstructure. We propose to extend the approach developed in the case of elastic heterogeneous media by Drugan and Willis (1996) and Kanit et al. (2003) to elastic–plastic composites. A specific polymer blend, made of two phases with highly contrasted properties, is selected to illustrate this approach; it consists of a random dispersion of elastic rubber spheres in an elastic–plastic glassy polymer matrix. It is found that the effective elastic–plastic response of this particulate composite can be accurately determined by computing a sufficient number of small subvolumes of fixed size extracted from the DRVE and containing different realizations of the random microstructure. In addition, the response of an individual subvolume is found anisotropic whereas the average of all subvolumes leads to recover the isotropic character of the DRVE. The necessary realization number to reach acceptable precision is given for two examples of particle volume fractions.

Mechanical Properties of Thin Glassy Polymer Films Filled with Spherical Polymer-Grafted Nanoparticles

It is commonly accepted that the addition of spherical nanoparticles (NPs) cannot simultaneously improve the elastic modulus, the yield stress, and the ductility of an amorphous glassy polymer matrix. In contrast to this conventional wisdom, we show that ductility can be substantially increased, while maintaining gains in the elastic modulus and yield stress, in glassy nanocomposite films composed of spherical silica NPs grafted with polystyrene (PS) chains in a PS matrix. The key to these improvements are (i) uniform NP spatial dispersion and (ii) strong interfacial binding between NPs and the matrix, by making the grafted chains sufficiently long relative to the matrix. Strikingly, the optimal conditions for the mechanical reinforcement of the same nanocomposite material in the melt state is completely different, requiring the presence of spatially extended NP clusters. Evidently, NP spatial dispersions that optimize material properties are crucially sensitive to the state (melt versus glass) of the polymeric material.

Dynamic heterogeneities in glassy polymers

The photoinduced isomerization of molecules incorporated in a glassy polymer matrix exhibits a wide spectrum of quantum yields. The source of the spectrum is matrix heterogeneities. The kinetics of the photoisomerization of 1-naphthyl-p-azomethoxybenzene in poly(ethyl methacrylate) and poly(n-butyl methacrylate) films is first used to study the rearrangement of the environments of photochromic molecules. The nonequilibrium distribution of cis molecules over the spectrum is obtained via conversion of trans molecules with the highest quantum yield into the cis form with the use of 405-nm light. The kinetics of attainment of the photostationary ratio for concentrations of cis and trans isomers under the action of light with a wavelength of 546 nm is studied through variation in the pause between the conversion of molecules into the cis form and the beginning of the studied process. It is shown that reversible changes in the structure of polymer matrices occur at a high rate at temperatures much lower than the glasstransition temperature.

Transport Processes in Vaginal Films that Release Anti-HIV Microbicide Molecules

A promising method for blocking sexual transmission of HIV is application of topical microbicide molecules to mucosal surfaces and the fluids contacting them. There is widespread agreement that more effective and diverse drug delivery vehicles, as well as better active ingredients, must be developed to increase microbicide efficacy. There is now great interest in developing a variety of delivery vehicles to suit the preferences of a diverse group of users. These include intravaginal rings and dissolving films, as well as the original microbicide gels. Here, we develop a mechanistic mathematical model of the dissolution of a microbicide-bearing polymer film, and subsequent distribution of its active drug throughout the vaginal lumen. The film dissolves by first imbibing (or taking up) solvent (vaginal fluid), whereupon its material structure changes in a way that frees individual polymer molecules in the film to move. In the model, the polymer structural relaxation via water uptake forms a two-phase, glassy-rubbery system in which interfacial movement follows Fickian (i.e. diffusion dominated) dynamics. However, at some intermediate time the interfacial motion ceases to follow Fickian dynamics, due to viscoelastic stresses in the polymer. As the glassy polymer transforms into a rubbery polymer, drugs carried within the film are freed to move throughout the relaxed network, and begin to diffuse through the vaginal lumen in all directions, especially laterally between the apposed vaginal walls. Therefore, as the initially fully glassy polymer matrix relaxes to a two-phase, glassy-rubbery system, the governing diffusion equation is simultaneously solved to track the motion of released drug molecules in the vaginal lumen and model the drug delivery. This is the first mechanistic model of how a vaginal film releases microbicidal molecules to neutralize HIV virions and inhibit transmission. [Supported by NIH AI077289]

Surface properties, thermal behavior, and molecular simulations of azo-polysiloxanes under light stimuli. Insight into the relaxation

Light-sensitive materials as azo-polymers have interesting applications in microelectronics, biology, or energy storage, due to their capacity to change their supramolecular ordering. Light-induced modifications result in i) changes in the azobenzene-groups dipole moment due to the trans-cis isomerization, ii) thermal backward cistrans relaxation, and iii) re-ordering of the polymer chain conformation. Conjugation of these phenomena leads to mass transport in the solid state, which is still not well understood. Changes in the surface properties of non-substituted and p-CF3,-CN,-NO2-substituted azophenoxy-polysiloxane films, before and after photo-excitation, were examined. Molecular simulations were performed to evaluate the dipole-moments of the trans and cis p-substituted azobenzene groups and provide a general vision of the spatial arrangement of polymer chains. The role of trans-cis back relaxation and that of the polymer matrix relaxation on the surface and bulk of the materials was discussed. Surface free energy determination of the non-substituted azophenoxy-polysiloxane suggested that the relaxation process is dominated by the thermal slow back-isomerization of the cis fraction and not by the motion of a glassy polymer matrix involving both the surface and bulk. A less clear situation was presented by the semi-crystalline polysiloxanes with electron-withdrawing groups substituted in the para-position. This was attributed mainly to the similar values of the dipole-moment of trans and cis azo-group, the former being slightly more dipolar than the latter.

Enhancing the light driven modulation of the refractive index in organic photochromic materials: A quantum chemical strategy

A theoretical investigation based on DFT predictions on 1,2-dithienylperfluorocyclopentene derivatives is presented. Molecules of this class have been demonstrated to show efficient thermally irreversible photochromism for developing optical switches. In particular, by doping a glassy polymer matrix with 1,2-dithienylperfluorocyclopentene derivatives, an optical element with a light induced modulation of the refractive index can be obtained. Here, a simple model is proposed allowing the correlation between the change of the molecular polarizability, upon photoisomerization, to the modulation of the refractive index of the bulk material. Based on this model and on DFT simulations, we suggest criteria for the optimization and design of the bulk material property (i.e. refractive index modulation) by modifying, with suitable chemical substitutions, the molecular and electronic structure of the photochromic species (that is by changing the molecular property, i.e. the molecular polarizability). A systematic analysis of the polarizability changes upon photoisomerization (from the uncolored to the colored form) is carried out for 24 different molecules and correlations with the changes of the electronic and molecular structure are proposed and discussed.

A physically-based constitutive model for anisotropic damage in rubber-toughened glassy polymers during finite deformation

The present work focuses on the development of a physically-based model for large deformation stress–strain response and anisotropic damage in rubber-toughened glassy polymers. The main features leading to a microstructural evolution (regarding cavitation, void aspect ratio, matrix plastic anisotropy and rubbery phase deformation) in rubber-toughened glassy polymers are introduced in the proposed constitutive model. The constitutive response of the glassy polymer matrix is modelled using the hyperelastic–viscoplastic model of Boyce et al. (1988, 2000). The deformation mechanisms of the matrix material are accounted for by two resistances: an elastic–viscoplastic isotropic intermolecular resistance acting in parallel with a visco-hyperelastic anisotropic network resistance, each resistance being modified to account for damage effects by void growth with a variation of the void aspect ratio. The effective contribution of the hyperelastic particles to the overall composite behaviour is taken into account by treating the overall system in a composite scheme framework. The capabilities of the proposed constitutive model are checked by comparing experimental data with numerical simulations. The deformation behaviour of rubber-toughened poly(methyl methacrylate) was investigated experimentally in tension at a temperature of 80 °C and for different constant true strain rates monitored by a video-controlled technique. The reinforcing phase is of the soft core–hard shell type and its diameter is of the order of one hundred nanometers. The particle volume fraction was adjusted from 15% to 45% by increments of 5%. The stress–strain response and the inelastic volumetric strain are found to depend markedly on particle volume fraction. For a wide range of rubber volume fractions, the model simulations are in good agreement with the experimental results. Finally, a parametric analysis demonstrates the importance of accounting for void shape, matrix plastic anisotropy and rubber content.