Polymer Relaxation Research Articles

Swelling and Diffusion during Methanol Sorption into Hydrated Nafion

Diffusion within polymer electrolyte membranes is often coincident with time-dependent processes such as swelling and polymer relaxation, which are factors that limit their ability to block molecular crossover during use. The solution-diffusion model of membrane permeation, which is the accepted theory for dense polymers, applies only to steady-state processes and does not address dynamic internal structural changes that can accompany permeation. To begin discovery of how such changes can be coupled to the permeation process, we have constructed a stochastic multiscale reaction-diffusion model that examines time-dependent methanol uptake into and swelling of hydrated Nafion. Several potential mechanisms of diffusion and polymer response are tested. The simulation predictions are compared to real-time Fourier transform infrared attenuated total reflectance spectroscopy (FTIR-ATR) absorbance reported in the literature [ Hallinan , D. T. , Jr. ; Elabd , Y. A. J. Phys. Chem. B 2007 , 111 , 13221 - 13230 ]. Of the proposed polymer response mechanisms, only one, a reaction-limited, local response to increasing methanol concentration that takes the entire experimental time frame of 600 s, produces simulated FTIR-ATR data consistent with experiment. The simulations show that water diffusion out of the membrane is minimal during methanol sorption and that changes in the measured infrared absorbances are due primarily to the increase in methanol concentration accompanied by dilution of water during swelling. Swelling involves densification of the polymer structure even as there is an overall volume expansion of the film. Potential connections between the polymer densification and molecular-level structural changes of Nafion in methanol are discussed. These results indicate that the interaction between methanol and Nafion serves to increase Nafion's capacity to accommodate large volumes of methanol-water solutions, facilitating increased permeation across the membrane relative to pure water.

Lubricity of High Water Content Aqueous Gels

Aqueous gels such as biopolymer gels, mucus, and high water content hydrogels are often qualitatively described as lubricious. In hydrogels, mesh size, ξ, has been found to be a controlling parameter in friction coefficient. In the tribology of aqueous gels, we suggest that the Weissenberg number (Wi) is a useful parameter to define different regimes, and following the original formulations in rheology, Wi is given by the polymer relaxation time (ηξ3/kBT) multiplied by the shear rate due to fluid shear through a single mesh (V/ξ): Wi = ηVξ2/kBT. At sliding speeds below a Weissenberg number of approximately 0.1, Wi < 0.1, the friction coefficient is velocity-independent and scales with mesh size to the − 1 power, µ ∝ ξ−1. De Gennes’ scaling concepts for elastic modulus, E, give a dependence on polymer mesh size to the − 3 power, E ∝ ξ−3, and following Hertzian contact analysis, the contact area is found to depend on the mesh size squared, A ∝ ξ2. Combining these concepts, the shear stress, τ, and therefore the lubricity of aqueous gels, is predicted to be highly dependent on the mesh size, τ ∝ ξ−3. Studies aimed at elucidating the fundamental mechanism of lubricity in biopolymer gels, mucus, and hydrogels have wrestled with comparisons across mesh size, which can be extremely difficult to accurately quantify. Using scaling concepts relating polymer mesh size to water content reveals that shear stress decreases rapidly with increasing water content, and plots of shear stress as a function of swollen water content are suggested as a useful method to compare aqueous gels of unknown mesh size. As a lower bound, these data are compared against estimates of fluid shear stress for free and bound water flowing through a mesh size estimated by the water content of the gels. The results indicate that the strong dependence on lubricity is likely due to a synergistic combination of a low viscosity solvent (water) coupled to a system that has a decreasing friction coefficient, modulus, and the resulting contact pressure with increasing water content. Although the permeability, K, of aqueous gels increases dramatically with water content (and mesh size), K ≅ ξ2/η, the stronger decrease of the elastic modulus and subsequent decrease in contact pressure due to an increase in the contact length, predicts that the draining time under contact, t, actually increases strongly with increasing water content and mesh size, t ∝ ξ2. Consistent with the finding of extremely high water content aqueous gels on the surfaces of biological tissues, these high water content gels are predicted to be optimal for lubrication as they are both highly lubricious and robust at resisting draining and sustaining hydration.

From modeling to implementation of a method for restraining back relaxation in ionic polymer–metal composite soft actuators

Ionic polymer–metal composites are an emerging kind of electroactive polymer actuators, which can bend in response to a relatively low driving voltage. However, to enhance the actuation performance of ionic polymer–metal composites, some of their drawbacks should be considered. One of the most important drawbacks is “back relaxation.” The so-called back relaxation effect means, when a step input voltage is applied to the ionic polymer–metal composite, the conventional bending displacement toward the anode is followed by an unwanted and slow back relaxation toward the cathode. Control-based methods for restraining the ionic polymer–metal composite back relaxation effect are feedback-based schemes which apply significant constraints to dominant applications of ionic polymer–metal composite actuators especially in biomedical applications. In this article, we present an entirely scientific-based mathematical modeling to achieve a practical method for restraining the back relaxation effect in Nafion-based ionic polymer–metal composites, relying on creating a specific pattern on Pt layers of the ionic polymer–metal composites and applying a local Gaussian disturbance to this patterned ionic polymer–metal composites.

Formulation and optimization of clotrimazole oromucosal emulgels

Clotrimazole is an antifungal agent of choice for the management of oral candidiasis. In order to improve its aqueous release and enhance its residence time at the target site of oral mucosa, emulgel formulations of clotrimazole were investigated. Different oils and Tween 80/ethanol combinations were examined as drug carrier systems and central composite design was used for the optimization of the emulgels. The optimized emulgels were characterized for their globule size, robustness, release kinetics, and the effect of storage conditions on their release profiles. All the prepared formulations showed a shear-thinning rheological behaviour indicating structural breakdown of intermolecular interaction between polymeric chains and were homogeneous with pH very close to that of normal oral cavity. The optimized formulations containing 30% Tween 80/ethanol solution (2:1 ratio), 12.4% olive oil/oleic acid combination (1:1 ratio), and 4% sodium carboxymethyl cellulose released clotrimazole in vitro with approximately zero order release kinetics as a result of polymer relaxation and erosion in the aqueous media. They also showed superior residence time and spreadability in comparison to the marketed oromucosal gels with less than 200 nm globule size and nearly the same release profile over three months of storage at real and accelerated time conditions. Keywords: clotrimazole, oromucosal emulgel, optimization, central composite design, drug release

Surprising Temperature Scaling of Viscoelastic Properties in Polymers

We present detailed studies of segmental dynamics and viscosity in polystyrene (PS) and poly(2-vinylpyridine) (P2VP) with different molecular weights (MW). Analysis reveals that the molecular weight dependence exhibits very different temperature scaling for segmental and chain dynamics: while segmental relaxation in samples with different MW forms a master curve when presented vs T – Tg, the viscosity of the same samples falls on a master curve when presented vs Tg/T (here Tg is the glass transition temperature). This result indicates significant difference in the friction mechanisms for chain and segmental dynamics. Even more puzzling is that the absolute values of viscosity appear to be essentially MW independent when presented vs Tg/T up to surprisingly high molecular weight. In particular, viscosity at Tg does not show any appreciable molecular weight dependence up to MW ≈ 30 000 g/mol. We speculate that molecular scale relaxation (chain dynamics) in polymers behaves similar to the structural relaxati...

Dynamically Heterogeneous Relaxation of Entangled Polymer Chains.

Stress relaxation following deformation of an entangled polymeric liquid is thought to be affected by transient reforming of chain entanglements. In this work, we use single molecule techniques to study the relaxation of individual polymers in the transition regime from unentangled to entangled solutions. Our results reveal the emergence of dynamic heterogeneity underlying polymer relaxation behavior, including distinct molecular subpopulations described by a single-mode and a double-mode exponential relaxation process. The slower double-mode timescale τ_{d,2} is consistent with a characteristic reptation time, whereas the single-mode timescale τ_{s} and the fast double-mode timescale τ_{d,1} are attributed to local regions of transient disentanglement due to deformation.

Setting the Stage for Fabrication of Self-Assembled Structures in Compact Geometries: Inside-Out Isoporous Hollow Fiber Membranes.

Fabrication of evaporation-induced self-assembled structures on easily accessible surfaces is an established strategy, while achieving such microphase-separated structures in compact geometries has been a long-standing goal. The requirement of comparatively less concentrated block copolymer (BCP) solution to pass through the compact geometries significantly reduces the stimulations required for self-assembly. The high polymer relaxation rates and decreased thermodynamic driving forces, as well as high capillary suction of dilute solutions in the porous substrates, complicates the BCP self-assembly and fabrication of the uniform coated layer, respectively. In this study, highly permeable robust poly(ether sulfone) hollow fiber membranes (PES HFM) with an inner diameter of approximately 1 mm are selected as compact geometries, and the isoporous structures are developed on top of ≤10 μm thin coated layer. This fabrication process introduces a technologically favored inside-out configuration for isoporous composite HFM with large bore diameters.

Globally Suppressed Dynamics in Ion-Doped Polymers.

We investigate how ion-polymer complexation suppresses molecular motion in conventional polymer electrolytes using molecular dynamics (MD) simulations of lithium hexafluorophosphate in poly(ethylene oxide) and a modified Rouse model. The employed model utilizes an inhomogeneous friction distribution to describe ion-polymer interactions and provides an effective way to examine how ion-polymer interactions affect polymer motion. By characterizing the subdiffusive Li+ transport and polymer relaxation times at several salt concentrations, we observe that increases in local friction due to ion-polymer complexation are significantly smaller than previously assumed. We find that a Rouse-based model that only considers local increases in friction cannot simultaneously capture the magnitude of increased polymer relaxation times and the apparent power-law exponent for Li+ subdiffusion observed in MD simulations. This incompatibility is reconciled by augmenting the modified Rouse model with a term that increases the global friction with the square of the salt concentration; this significantly improves the agreement between the model and MD, indicating the importance of ion-ion interactions and distributions on ion/polymer mobility.

Flexoelectric effect in PVDF-based copolymers and terpolymers

Compared with their inorganic counterparts, the flexoelectric effect of polymers is much less studied, and the mechanisms underlying this gradient electromechanical coupling effect have rarely been explored. In this work, the temperature dependence of the flexoelectric coefficients of PVDF-based copolymers and terpolymers was measured, and the mechanisms that affect the flexoelectric response of the polymers were investigated. For these polymers, the flexoelectric response decreases abruptly in the temperature range where the peaks of dielectric loss or mechanical loss are observed. These loss peaks originate from the phase transition or polymer chain relaxation processes. Based on this observation, we propose that the temperature dependence of the flexoelectric response of the polymers is closely related to polymer chain mobility and relaxation processes. Our study provides an in-depth understanding of the flexoelectric effect in polymers.

Crystalline Ethylene Oxide and Propylene Oxide Triblock Copolymer Solid Dispersion Enhance Solubility, Stability and Promoting Time- Controllable Release of Curcumin.

The design and development of an effective medicine are, however, often faced with a number of challenges. One of them is the close relationship of drug's bioavailability with solubility, dissolution rate and permeability. The use of curcumin's (CUR) therapeutic potential is limited by its poor water solubility and low chemical stability. The purpose was to evaluate the effect of polymer and solid dispersion (SD) preparation techniques to enhance the aqueous solubility, dissolution rate and stability of the CUR. The recent patents on curcumin SD were reported as (i) curcumin with polyvinylpyrrolidone (CN20071 32500 20071214, WO2006022012 and CN20151414227 20150715), (ii) curcumin-zinc/polyvinylpyrrolidone (CN20151414227 20150715), (iii) curcumin-poloxamer 188 (CN2008171177 20080605), (iv) curcumin SD prepared by melting method (CN20161626746-20160801). SD obtained by co-preciptation or microwave fusion and the physical mixture of CUR with Poloxamer-407 (P-407), Hydroxypropylmetylcellulose-K4M (HPMC K4M) and Polyvinylpyrrolidone-K30 (PVP-K30) were prepared at the ratios of 1:2; 1:1 and 2:1. The samples were evaluated by solubility, stability, dissolution rate and characterized by SEM, PXRD, DSC and FTIR. The solubility, stability (pH 7.0) and dissolution rate were significantly greater for SD (CUR:P-407 1:2). The PXRD,SEM and DSC indicated a change in the crystalline state of CUR. The enhancement of solubility was dependent on a combination of factors including the weight ratio, preparation techniques and carrier properties. The drug release data fitted well with the Weibull equation, indicating that the drug release was controlled by diffusion, polymer relaxation and erosion occurring simultaneously. Thus, these SDs, specifically CUR:P-407 1:2 w/w, can overcome the barriers of poor bioavailability to reap many beneficial properties.

An Ambient Temperature Electrolyte with Superior Lithium Ion Conductivity based on a Self-Assembled Block Copolymer.

In searching for polymer-based electrolytes with improved performance for lithium ion and lithium metal batteries, we studied block copolymer electrolytes with high amounts of bis(trifluoromethane)sulfonimide lithium obtained by macromolecular co-assembly of a poly(isoprene)-block-poly(styrene)-block-poly(ethylene oxide) and the salt from tetrahydrofuran. Particularly, an ultra-short poly(ethylene oxide) block of 2100 g mol-1 was applied, giving rise to 2D continuous lamellar microstructures. The macroscopic stability was ensured with major blocks from poly(isoprene) and poly(styrene), which separated the ionic conductive PEO/salt lamellae. Thermal annealing led to high ionic conductivities of 1.4 mS cm-1 at 20 °C with low activation energy and a superior lithium ion transference number of 0.7, accompanied by an improved mechanical stability (storage modulus of up to 107 Pa). With high Li:O ratios >1, we show a viable concept to achieve fast Li+ transport in block copolymers (BCP), decoupled from slow polymer relaxation.

Hydration-Dependent Dynamical Modes in Xyloglucan from Molecular Dynamics Simulation of 13C NMR Relaxation Times and Their Distributions.

Macromolecular dynamics in biological systems, which play a crucial role for biomolecular function and activity at ambient temperature, depend strongly on moisture content. Yet, a generally accepted quantitative model of hydration-dependent phenomena based on local relaxation and diffusive dynamics of both polymer and its adsorbed water is still missing. In this work, atomistic-scale spatial distributions of motional modes are calculated using molecular dynamics simulations of hydrated xyloglucan (XG). These are shown to reproduce experimental hydration-dependent 13C NMR longitudinal relaxation times ( T1) at room temperature, and relevant features of their broad distributions, which are indicative of locally heterogeneous polymer reorientational dynamics. At low hydration, the self-diffusion behavior of water shows that water molecules are confined to particular locations in the randomly aggregated XG network while the average polymer segmental mobility remains low. Upon increasing water content, the hydration network becomes mobile and fully accessible for individual water molecules, and the motion of hydrated XG segments becomes faster. Yet, the polymer network retains a heterogeneous gel-like structure even at the highest level of hydration. We show that the observed distribution of relaxations times arises from the spatial heterogeneity of chain mobility that in turn is a result of heterogeneous distribution of water-chain and chain-chain interactions. Our findings contribute to the picture of hydration-dependent dynamics in other macromolecules such as proteins, DNA, and synthetic polymers, and hold important implications for the mechanical properties of polysaccharide matrixes in plants and plant-based materials.

Scaling Behavior of Nafion with Different Model Parameterizations in Dissipative Particle Dynamics Simulations

AbstractThe scaling behavior of the conformation and dynamics of mesoscopic models of a polymer melt, Nafion, and hydrated Nafion are investigated with dissipative particle dynamics (DPD) simulations. The scaling behavior of all studied systems generally follows the Rouse theory. For the polymer melt and dry Nafion, increase in chain stiffness slightly shrinks the polymer and leads to slower dynamics with longer polymer relaxation time. The addition of a stiff constraint to five different DPD parameterizations of hydrated Nafion expands the chain conformation, while the repulsion parameter effect on size is secondary. In contrast, softer repulsion predominately dictates faster dynamics of both the polymer and water, and stiffer chains also lead to improved water diffusion to a much lesser extent. Sufficiently long Nafion chains are required to realistically represent the relatively immobile Nafion framework and mobile proton conducting water domain. Direct quantitative cluster size analysis is advocated to characterize the water channel connectivity.

Molecular dynamics modeling framework for overcoming nanoshape retention limits of imprint lithography

Complex nanoshaped structures (nanoshape structures here are defined as shapes enabled by sharp corners with radius of curvature <5 nm) have been shown to enable emerging nanoscale applications in energy, electronics, optics, and medicine. This nanoshaped fabrication at high throughput is well beyond the capabilities of advanced optical lithography. While the highest-resolution e-beam processes (Gaussian beam tools with non-chemically amplified resists) can achieve <5 nm resolution, this is only available at very low throughputs. Large-area e-beam processes, needed for photomasks and imprint templates, are limited to ~18 nm half-pitch lines and spaces and ~20 nm half-pitch hole patterns. Using nanoimprint lithography, we have previously demonstrated the ability to fabricate precise diamond-like nanoshapes with ~3 nm radius corners over large areas. An exemplary shaped silicon nanowire ultracapacitor device was fabricated with these nanoshaped structures, wherein the half-pitch was 100 nm. The device significantly exceeded standard nanowire capacitor performance (by 90%) due to relative increase in surface area per unit projected area, enabled by the nanoshape. Going beyond the previous work, in this paper we explore the scaling of these nanoshaped structures to 10 nm half-pitch and below. At these scales a new “shape retention” resolution limit is observed due to polymer relaxation in imprint resists, which cannot be predicted with a linear elastic continuum model. An all-atom molecular dynamics model of the nanoshape structure was developed here to study this shape retention phenomenon and accurately predict the polymer relaxation. The atomistic framework is an essential modeling and design tool to extend the capability of imprint lithography to sub-10 nm nanoshapes. This framework has been used here to propose process refinements that maximize shape retention, and design template assist features (design for nanoshape retention) to achieve targeted nanoshapes.

Viscoelastic properties and reinforcement of non-aggregated and aggregated nanocomposites

We investigate the properties of polymer nanocomposites consisting of narrow disperse spherical nanoparticles that are coated with narrow disperse polymers. These nanoparticles can be homogeneously dispersed in a polymer matrix. The structure of the polymer-coated nanoparticles was examined by using small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM), and was related to the viscoelastic properties of the nanocomposites. We observe a systematic retardation of the polymer relaxation dynamics with increasing particle loading. Nanoparticle aggregation leading to the formation of chain-like structures causes a strong reinforcement effect.

Biopolymer-based strategies in the design of smart medical devices and artificial organs.

Advances in regenerative medicine and in modern biomedical therapies are fast evolving and set goals causing an upheaval in the field of materials science. This review discusses recent developments involving the use of biopolymers as smart materials, in terms of material properties and stimulus-responsive behavior, in the presence of environmental physico-chemical changes. An overview on the transformations that can be triggered in natural-based polymeric systems (sol–gel transition, polymer relaxation, cross-linking, and swelling) is presented, with specific focus on the benefits these materials can provide in biomedical applications.

Origin of dielectric relaxor behavior in PVDF-based copolymer and terpolymer films

Relaxor ferroelectrics exhibit frequency-dispersion of their dielectric permittivity peak as a function of temperature, the origin of which has been widely debated. Microscopic understanding of such behavior for polymeric ferroelectrics has presented new challenges since unlike traditional ceramic ferroelectrics, dielectric relaxation in polymers is a consequence of short-range molecular dynamics that are difficult to measure directly. Here, through careful analysis of atomic-level H-atom dynamics as determined by Quasi-elastic Neutron Scattering (QENS), we show that short-range molecular dynamics within crystalline domains cannot explain the macroscopic frequency-dispersion of dielectric properties observed in prototypical polyvinylidene-fluoride (PVDF)-based relaxor ferroelectrics. Instead, from multiscale quantitative microstructural characterization, a clear correlation between the amount of crystalline-amorphous interfaces and dielectric relaxation is observed, which indicates that such interfaces play a central role. These results provide critical insights into the role of atomic and microscopic structures towards relaxor behavior in ferroelectric polymers, which will be important for their future design.

Chitosan/pvp-based mucoadhesive membranes as a promising delivery system of betamethasone-17-valerate for aphthous stomatitis

Mucoadhesive membranes were proposed in this study as drug delivery system for betamethasone-17-valerate (BMV) in the treatment of recurrent aphthous stomatitis (RAS). The membranes were obtained by using the polymers chitosan (CHI) in both presence and absence of polyvinilpyrrolidone (PVP), following the solvent evaporation method. The presence of PVP in the membranes causes significant modifications in its thermal properties. Changes in the thermal events at 114 and 193 °C (related to BMV melting point), and losses in mass (39.38 and 30.68% for CH:PVP and CH:PVP-B, respectively), suggests the incorporation of BMV in these membranes. However, the morphological aspects of the membranes do not change after adding PVP and BMV. PVP causes changes in swelling ratios (>80%) of the membranes, and it is suggested that the reorganization of the polymer mesh was highlighted by the chemical interactions between the polymers leading to different percentages of BMV released ∼40% and ∼80% from CH-B and CH:PVP-B. BMV release profile follows Korsmeyer and Peppas model (n > 0.89) which suggests that the diffusion of the drug in the swollen matrix is driven by polymer relaxation. In addition, the membranes containing PVP (higher swelling ability) present high rates of tensile strength, and therefore, higher mucoadhesion. Moreover, given the results presented, the developed mucoadhesive membranes are a promising system to deliver BMV for the treatment of RAS.

Coupling of Nanoparticle Dynamics to Polymer Center-of-Mass Motion in Semidilute Polymer Solutions

We investigate the dynamics of nanoparticles in semidilute polymer solutions when the nanoparticles are comparably sized to the polymer coils using explicit- and implicit-solvent simulation methods. The nanoparticle dynamics are subdiffusive on short time scales before transitioning to diffusive motion on long time scales. The long-time diffusivities scale according to theoretical predictions based on full dynamic coupling to the polymer segmental relaxations. In agreement with our recent experiments, however, we observe that the nanoparticle subdiffusive exponents are significantly larger than predicted by the coupling theory over a broad range of polymer concentrations. We attribute this discrepancy in the subdiffusive regime to the presence of an additional coupling mechanism between the nanoparticle dynamics and the polymer center-of-mass motion, which differs from the polymer relaxations that control the long-time diffusion. This coupling is retained even in the absence of many-body hydrodynamic interactions when the long-time dynamics of the colloids and polymers are matched.

Nature of Large Temporal Fluctuations of Hydrogen Transfer Rates in Single Molecules.

Double hydrogen transfer was monitored in single molecules of parent porphycene and its tetra- t-butyl derivative using confocal fluorescence microscopy. The molecules have been embedded in a polymer matrix. Under such conditions, a significant fraction of the population reveals a huge decrease of the tautomerization rate with respect to the value obtained from ensemble studies in solution. This effect is explained by a model that assumes that the rate is determined by the reorganization coordinate that involves slow relaxation of the polymer matrix. The model provides indirect evidence for the dominant role of tunneling. It is proposed that tautomerization in single molecules of the porphycene family can be used to probe polymer relaxation dynamics on the time scale ranging from picoseconds to minutes.