Chain Relaxation Research Articles

Influence of cross-linkers on the properties of cotton grafted poly (acrylamide-co-acrylic acid) hydrogel composite: swelling and drug release kinetics

Cross-linker plays a crucial role in monitoring water holding and drug release properties of a hydrogel system, an essential requirement for smart wound dressings. Present study is focused on the influence of cross-linkers poly ethylene glycol (PEG) and N,Nʹ-methylene bisacrylamide (MBAAm) on the properties of poly (acrylamide-co-acrylic acid) hydrogel grafted over the cotton fabric to form composite for medicated dressings. Fourier transform infrared spectroscopy (FTIR) confirms the grafting of hydrogel on the cotton fabric. Uniform hydrogel layer on the cotton surface is obtained under scanning electron microscopy (SEM). Swelling of the composite prepared using PEG follow first-order kinetics at acidic and neutral pH whereas second-order kinetic model at pH 8.5 while that prepared using MBAAm follow second-order kinetic equation at all the pHs studied. The swelling kinetics is also governed by Peppas model at all pHs. Release of gentamicin sulphate from both the composites are studied in phosphate buffers having pH 4.5, 7 and 8.5 at 37 ± 0.1 °C and observed that it is fastest in phosphate buffer having pH 7. On fitting drug release data into Peppas model, first and second-order kinetic equations, it is found that drug release is diffusion controlled and follows Fickian diffusion mechanism in case of the composite prepared by using PEG as cross-linker, whereas it is controlled by diffusion as well as chain relaxation in case of the composite prepared by using MBAAm. Mechanical testing using universal testing machine supports a higher mechanical strength of the hydrogel composite as compared to its film.

Effect of SiO2 Particles on the Relaxation Dynamics of Epoxidized Natural Rubber (ENR) in the Melt State by Time-Resolved Mechanical Spectroscopy.

The rheological behavior of an epoxidized natural rubber (ENR) nanocomposite containing 10 wt.% of silica particles was examined by time-resolved mechanical spectroscopy (TRMS), exploiting the unique capability of this technique for monitoring the time-dependent characteristics of unstable polymer melts. The resulting storage modulus curve has revealed a progressive evolution of the elastic component of the composite, associated with slower relaxations of the ENR macromolecular chains. Two major events were identified and quantified: one is associated with the absorption of the epoxidized rubber macromolecules onto the silica surface, which imposes further restrictions on the motions of the chains within the polymer phase; the second is related to gelation and the subsequent changes in rheological behavior resulting from the simultaneous occurrence cross-linking and chain scission reactions within the ENR matrix. These were quantified using two parameters related to changes in the storage and loss modulus components.

The relaxation dynamics of single flow-stretched polymers in semidilute to concentrated solutions.

Recent experiments on the return to equilibrium of solutions of entangled polymers stretched by extensional flows [Zhou and Schroeder, Phys. Rev. Lett. 120, 267801 (2018)] have highlighted the possible role of the tube model's two-step mechanism in the process of chain relaxation. In this paper, motivated by these findings, we use a generalized Langevin equation (GLE) to study the time evolution, under linear mixed flow, of the linear dimensions of a single finitely extensible Rouse polymer in a solution of other polymers. Approximating the memory function of the GLE, which contains the details of the interactions of the Rouse polymer with its surroundings, by a power law defined by two parameters, we show that the decay of the chain's fractional extension in the steady state can be expressed in terms of a linear combination of Mittag-Leffler and generalized Mittag-Leffler functions. For the special cases of elongational flow and steady shear flow, and after adjustment of the parameters in the memory function, our calculated decay curves provide satisfactory fits to the experimental decay curves from the work of Zhou and Schroeder and earlier work of Teixeira et al. [Macromolecules 40, 2461 (2007)]. The non-exponential character of the Mittag-Leffler functions and the consequent absence of characteristic decay constants suggest that melt relaxation may proceed by a sequence of steps with an essentially continuous, rather than discrete, spectrum of timescales.

Formulation and evaluation of sertaconazole nitrate loaded nanosponges for topical application

The aim of present research work is to formulate sertaconazole nitrate an antifungal drug into nanosponges loaded topical hydrogel using simple and cost effective method. Nanosponges loaded hydrogels were prepared by “Emulsion solvent diffusion method” using different polymers like ethyl cellulose and polymethyl methacrylate, a surfactant named polyvinyl alcohol and a crosslinking agent dichloromethane. Taguchi experimental design (L8) was applied for screening nanosponges by taking independent variables as type of polymers (X1), concentration of surfactant (X2) and amount of crosslinking agent (X3) while percentage entrapment efficiency (Y1) is dependent variable. Further, they were evaluated for particle size, PDI, zeta potential, percentage yield, entrapment efficiency, in-vitro and ex-vivo diffusion studies. Formulation F2 was screened based on its percentage drug release from in-vitro diffusion studies. Ex-vivo studies were conducted for screened formulation, marketed formulation (Onabet), pure drug dispersion and control gel to determine there percentage drug release and skin retention. F2 formulation followed zero order kinetics with Higuchi release mechanism, which indicates the rate of drug release through the mode of diffusion. The value of ‘n’ from korsemeyerpeppas indicates the anomalous transport concluding the drug release both by diffusion and relaxation of polymer chains. F2 formulation have shown 70.40% skin retention which was more when compared with marketed formulation, pure drug dispersion and control gel. The study concluded that sertaconazole nitrate loaded nanosponges have control release effect for prolonged period of time in the skin and it has been reported to show site targeted effect with reduced incidence of side effects and dosing frequency for the treatment of skin disorders like athelete's foot, dermatophytosis and candidiasis.

Kinetics, absorption and diffusion mechanism of crosslinked Chitosan Hydrogels

Green polymers are extremely useful for various environmental applications. One such biopolymer is Chitosan. In this study, crosslinked and physical Chitosan hydrogels were synthesized. The swelling of disc-shaped hydrogels crosslinked using different concentrations of Glutaraldehyde were compared with physical film and bead shaped hydrogels. Best swelling of around 3000% was observed in case of square film shaped hydrogels but they lacked rigidity and dissolved in mild acids. In case of crosslinked hydrogels, as the crosslinker concentration increased, the hydrogels entrapped less water but gained better mechanical strength. Characterization of synthesized crosslinked hydrogels was carried out using FTIR, TGA and DSC. Equilibrium swelling results indicated more water absorption at acidic pH (2.5). Simultaneously, increase in temperature led to enhancement of swelling degree. The hydrogels trapped more water leading to increased swelling, in case of lower molar salt concentrations. Second order kinetics was followed due to stress relaxation of polymeric chain. Diffusion was found to be anomalous since exponent values lied between 0.5 and 1. Peleg‟s and Exponential association model were used to carry out absorption modeling. The data was found to fit the Peleg‟s absorption model. Degree of swelling is a major factor for deciding a hydrogels utility. Swelling ability, biocompatibility and availability of lone pairs of oxygen and nitrogen on the surface of CS makes it ideal for applications in drug delivery, controlled release of fertilizers and adsorption of environmental contaminants.

Rouse Mode Analysis of Relaxation in Polymer Blends

AbstractThe relaxation spectra of the components in polymer blends determine the dynamic mechanical properties of the corresponding composite materials, which, in thermo‐rheological studies, show a delicate dependence on their chemical and physical properties as well as its bulk composition. A molecular dynamics simulation based Rouse mode analysis is performed to detect the relaxation spectra of the component chains on all length scales ranging from one monomer to the whole chain size, indicating that the bulk composition dominates the relaxation dynamics of the components in miscible polymer blends. The relaxation of component chains accelerates on all length scales as increasing the free volume available to monomers to move, with the dependence of mobility shift on composition being quantified by a Williams–Landel–Ferry like function governing the time‐composition superposition. Role of the chain connectivity induced self‐concentration effect on chain relaxation in polymer blends is discussed and proved to be limited, which leads to a visible but subtle length‐scale dependent rheological complexity as the composition is varied.

Effects of Asymmetric Molecular Architecture on Chain Stretching and Dynamics in Miktoarm Star Copolymers

We use broad-band dielectric spectroscopy (BDS) and small-angle X-ray scattering (SAXS) to investigate the impact of architectural asymmetry in the miktoarm star copolymer on the dielectric polymer relaxations. The miktoarm copolymers studied contain one or two polystyrene (PS) chains of constant molecular weight and two identical poly(cis-1,4-isoprene) (PI) chains with varied molecular weights. Using the chains in the PI block as dielectric probes, we find that the architecturally asymmetric miktoarm star copolymer systems (PSPI2) feature distributions in chain relaxation times and dielectric relaxation strengths that are not dependent on molecular weight or morphology, in stark contrast to the effects of morphological confinement observed for symmetric diblock systems (PS-b-PI or PS2PI2). Along with evidence from the SAXS measurements regarding phase separation, these results are attributed to influences from chain stretching within the framework of the Gaussian chain model for block copolymer systems. As such, the molecular architecture in block copolymers happens to be a versatile handle to control polymer chain dynamics and, ultimately, the macroscopic physicochemical properties in architecturally complex polymers.

Water governs the mechanical properties of poly(vinyl alcohol)

The presence of water molecules can significantly determine the macroscopic mechanical property and microscopic chain dynamics of poly(vinyl alcohol) (PVA). However, to date it has remained fully understood the molecular mechanism behind experimentally and theoretically. Herein, we systematically examine the effect of water contents on the mechanical property, glass transition, free volume, and intermolecular interactions by combining experimental and MD simulation. Our results show that the presence of water significantly reduces the mechanical strength of PVA, and only 1.8 wt% of water reduces the tensile strength by ~32% but notably increases the plasticity of PVA by ~2.5 times. Meanwhile, the inclusion of water remarkably lowers the glass transition temperature, increases free volume, and promotes the relaxation and mobility of PVA chains. This is mainly because the presence of water gives rise to both the plasticization and even lubricating effect on the PVA chains. This work unravels how water governs the mechanical performances of the PVA.

Drug-Eluting Biodegradable Implants for the Sustained Release of Bisphosphonates.

Despite being one of the first-line treatments for osteoporosis, the bisphosphonate drug class exhibits an extremely low oral bioavailability (<1%) due to poor absorption from the gastrointestinal tract. To overcome this, and to explore the potential for sustained drug release, bioerodible poly(lactic acid) (PLA) and poly(D,L-lactide-co-glycolide) (PLGA) implants loaded with the bisphosphonate alendronate sodium (ALN) were prepared via hot-melt extrusion. The rate of drug release in vitro was modulated by tailoring the ratio of lactide to glycolide in the polymer and by altering the ALN-loading of the implants. All investigated implants exhibited sustained ALN release in vitro between 25 to 130 days, where implants of greater glycolide composition and higher ALN-loadings released ALN more rapidly. All PLGA implants demonstrated a sigmoidal release profile, characterised by an initial surface dissolution phase, followed by a period of zero-order drug diffusion, then relaxation or erosion of the polymer chains that caused accelerated release over the subsequent days. Contrastingly, the PLA implants demonstrated a logarithmic release profile, characterised by a gradual decrease in ALN release over time.

Effect of ibuprofen entrapment procedure on physicochemical and controlled drug release performances of chitosan/xanthan gum polyelectrolyte complexes

The effect of the entrapment procedure of a poorly water soluble drug (ibuprofen) on physicochemical and drug release performances of chitosan/xanthan polyelectrolyte complexes (PECs) was investigated to achieve controlled drug release as the ultimate goal. The formation of PECs for two drug entrapment procedures (before or after the mixing of polymers) at pH 4.6 and 5.6 and three chitosan-to-xanthan mass ratios (1:1, 1:2 and 1:3) was observed by continuous decrease in conductivity during the PECs formation and increased apparent viscosity and hysteresis values. The most extensive crosslinking was observed with ibuprofen added before the PECs formation at pH 4.6 and chitosan-to-xanthan mass ratio 1:1. The PECs prepared at polymers' mass ratios 1:2 and 1:3 had higher yield and drug entrapment efficiency. DSC and FT-IR analysis confirmed ibuprofen entrapment in PECs and the partial disruption of its crystallinity. All ibuprofen release profiles were similar, with 60–70% of drug released after 12 h, mainly by diffusion, but erosion and polymer chain relaxation were also included. Potentially optimal can be considered the PEC prepared at pH 4.6, ibuprofen entrapped before the mixing of polymers at chitosan-to-xanthan mass ratio 1:2, which provided controlled drug release by zero-order kinetics, high yield, and drug entrapment efficiency.

Chitosan-based hydrogels for the sorption of metals and dyes in water: isothermal, kinetic, and thermodynamic evaluations

The aim of the present study was to evaluate a chitosan-based hydrogel and chitosan/magnetite-based composite hydrogel as potential matrices for water treatment through sorption experiments involving metals and dyes. Cadmium and methylene blue were used as model pollutants. The best isotherm fits were found using three-parameter isotherms, indicating both the formation of a monolayer and multi-site interactions in the hydrogel networks due to the diffusion of the solutes and macromolecular relaxation of the polymer chains. Maximum methylene blue sorption capacities of the chitosan-based hydrogel and chitosan/magnetite-based composite hydrogel were 23.389 and 23.478 mg g−1, respectively. These values were respectively 90.038 and 80.383 mg g−1 for cadmium sorption. The best kinetic fit for the interaction of methylene blue and cadmium to the chitosan-based hydrogel was the nonlinear pseudo-second-order kinetic model, indicating that a chemical reaction controls the adsorption rate between the hydrogel and pollutant. The opposite was found for interaction with the chitosan/magnetite-based composite hydrogel, suggesting that the active-site occupation rate is proportional to the number of non-occupied active sites. The thermodynamic results revealed that the sorption processes were favorable and endothermic, with the possible occurrence of physical interactions. However, hydrogel swelling can alter the sorption isotherm, kinetics, and thermodynamics. The interaction of methylene blue and cadmium with both hydrogels was confirmed by Fourier-transform infrared spectroscopy and thermogravimetric analyses.

Development of Coarse-Grained Models for Poly(4-vinylphenol) and Poly(2-vinylpyridine): Polymer Chemistries with Hydrogen Bonding.

In this paper, we identify the modifications needed in a recently developed generic coarse-grained (CG) model that captured directional interactions in polymers to specifically represent two exemplary hydrogen bonding polymer chemistries—poly(4-vinylphenol) and poly(2-vinylpyridine). We use atomistically observed monomer-level structures (e.g., bond, angle and torsion distribution) and chain structures (e.g., end-to-end distance distribution and persistence length) of poly(4-vinylphenol) and poly(2-vinylpyridine) in an explicitly represented good solvent (tetrahydrofuran) to identify the appropriate modifications in the generic CG model in implicit solvent. For both chemistries, the modified CG model is developed based on atomistic simulations of a single 24-mer chain. This modified CG model is then used to simulate longer (36-mer) and shorter (18-mer and 12-mer) chain lengths and compared against the corresponding atomistic simulation results. We find that with one to two simple modifications (e.g., incorporating intra-chain attraction, torsional constraint) to the generic CG model, we are able to reproduce atomistically observed bond, angle and torsion distributions, persistence length, and end-to-end distance distribution for chain lengths ranging from 12 to 36 monomers. We also show that this modified CG model, meant to reproduce atomistic structure, does not reproduce atomistically observed chain relaxation and hydrogen bond dynamics, as expected. Simulations with the modified CG model have significantly faster chain relaxation than atomistic simulations and slower decorrelation of formed hydrogen bonds than in atomistic simulations, with no apparent dependence on chain length.

Approximate solution techniques for the sorption of a finite amount of swelling solvent in a glassy polymer

This study examines a one-dimensional Stefan problem describing the sorption of a finite amount of swelling solvent in a glassy polymer. The polymer is initially in a dry non-swollen state, where the polymer network is dense. The polymer is then injected with a critical concentration of a swelling solvent, causing polymer chain relaxation to occur. A moving boundary separating the swollen rubbery polymer from the dry glassy polymer is created, whose speed is defined by a kinetic law. The form of the kinetic law is typically assumed to be linear, but this is a nonphysical restriction, and thus we consider the case for a general exponent. We present formal asymptotic expansions, for both small and large times as well as for small and large values of the control parameter, as well as considering the heat balance integral method. These approximations are compared with a numerical scheme, which uses a boundary immobilisation technique and correctly identifies the appropriate starting solution.

Confinement effects on the mechanical heterogeneity of polymer fiber glasses.

Both polymer fiber glasses and bulk polymer glasses exhibit nonlinear mechanical responses under uniaxial deformation. In polymer fibers, however, polymer chains are confined strongly and the surface area is relatively large compared to their volume. The confinement and the surface may lead to the spatially heterogeneous relaxation of chains in polymer fibers. In this work we perform molecular dynamics simulations and investigate the relation between the heterogeneous dynamics and the nonlinear mechanical responses at a molecular level. Our molecular simulations capture successfully not only the nonlinear mechanical response but also the dependence of mechanical properties on the strain rate of typical polymer glasses as in experiments. We find that the local elastic modulus and the nonaffine displacement are spatially heterogeneous in the pre-yield regime, which results in a lower elastic modulus for polymer fibers than bulk polymer glasses. In the post-yield regime, those mechanical properties become relatively homogeneous. Monomers with large nonaffine displacement are localized mainly at the interfacial region in the pre-yield regime while highly nonaffine monomers are distributed throughout the fibers in the post-yield regime. We show that the nonaffine displacement during deformation relates closely to the mechanical response of the polymer fibers. We also find that in the strain-hardening regime there is a significant difference in the energetic contribution to the stress between polymer fibers and bulk polymers, for which the modulus of the strain-hardening regime of the polymer fibers is smaller than that of bulk polymers.

Stress relaxation of entangled ring polymer chains in a linear matrix

We theoretically study the stress relaxation process of small amounts of entangled ring polymer chains in a linear matrix by combining several important physical mechanisms: (1) Rouse relaxation, (2) longitudinal stress relaxation along the tube, and (3) constraint release. The contribution to stress relaxation of entangled rings by longitudinal modes is calculated analytically, and a compact expression of the relaxation modulus is given.

Selective CO2 capture through microporous Tb(BTC)(H2O).(DMF)1.1 MOF as an additive in novel MMMs fabricated from Matrimid® 5218

Mixed matrix membranes (MMMs) fabricated with porous metal organic frame works have enhanced the separation performance of polymer membranes. In this context microporous 3D Tb(BTC)(H2O).(DMF)1.1 MOF was incorporated into pristine Matrimid® with loadings of 10, 20 and 30 weight percentages. SEM micrographs indicated proper distribution of filler in the Matrimid and no interfacial voids were observed. Gas permeation studies evidenced the CO2 permeability to be 13.2 (82.32%) and 18.34 (153.31%) and 25.86 Barrer for 10, 20 and 30 wt% MMMs respectively. The 257.18% increase in CO2 permeability of 30 wt% MMM than methane was attributed to polar nature of CO2, its smaller kinetic diameter, condensability, and larger solubility within the Matrimid matrix than non – polar and larger CH4 molecules.Addition of filler influenced the pure gas selectivity of all MMMs positively. So, 30 wt% MMM exhibited the highest 58.04% increase in selectivity that was attributed to the molecular sieving property of the filler and the size exclusion phenomena as followed by CH4 and CO2. The high values of mixed and pure gas selectivity were obtained upon increasing filler concentration. The commercial applicability of these MMMs was tested by checking their selectivity under increased feed concentrations of CO2 and checking permeability and selectivities at high temperatures. The study depicted that, competitive sorption of gases, prevalence of size exclusion phenomena and polymer chains relaxation at higher temperature were responsible for low gas selectivity. MMM with 30 wt% of MOF lied close to Robson’s Upper bound 2008 that indicated its good separation potential.

Modeling liquid absorption of highly cross-linked epoxy resins in aqueous electrolyte solutions

The solvent absorption of an epoxy o-cresol novolac resin composite has been measured in different aqueous electrolyte solutions (NaCl, CaCl2 and MgCl2) at different salt concentrations from 0.1 to 0.3 mg/l. Next to the total solvent uptake, which was measured by a gravimetric measurement, the absorption of ions was determined by ion chromatography and by atomic absorption spectroscopy. The measured solvent absorption in equilibrium was calculated by combining the ePC-SAFT equation of state with a network term, which takes into account elastic forces in the polymer network counteracting a further solvent absorption. In order to model the solvent absorption kinetics, the equation of state was combined with a Maxwell-Stefan diffusion approach and the viscoelastic Kelvin-Voigt model for chain relaxation. The model parameters were only fitted to the absorption in pure water, what was only possible because the epoxy resin absorbed a neglectable amount of ions. The fully predictively calculated values for the absorption in electrolyte solutions are in qualitative agreement to the measured data.

Statistical mechanics of chromosomes: in vivo and in silico approaches reveal high-level organization and structure arise exclusively through mechanical feedback between loop extruders and chromatin substrate properties.

The revolution in understanding higher order chromosome dynamics and organization derives from treating the chromosome as a chain polymer and adapting appropriate polymer-based physical principles. Using basic principles, such as entropic fluctuations and timescales of relaxation of Rouse polymer chains, one can recapitulate the dominant features of chromatin motion observed in vivo. An emerging challenge is to relate the mechanical properties of chromatin to more nuanced organizational principles such as ubiquitous DNA loops. Toward this goal, we introduce a real-time numerical simulation model of a long chain polymer in the presence of histones and condensin, encoding physical principles of chromosome dynamics with coupled histone and condensin sources of transient loop generation. An exact experimental correlate of the model was obtained through analysis of a model-matching fluorescently labeled circular chromosome in live yeast cells. We show that experimentally observed chromosome compaction and variance in compaction are reproduced only with tandem interactions between histone and condensin, not from either individually. The hierarchical loop structures that emerge upon incorporation of histone and condensin activities significantly impact the dynamic and structural properties of chromatin. Moreover, simulations reveal that tandem condensin–histone activity is responsible for higher order chromosomal structures, including recently observed Z-loops.

100th Anniversary of Macromolecular Science Viewpoint: Block Copolymers with Tethered Acid Groups: Challenges and Opportunities.

Scientific research on advanced polymer electrolytes has led to the emergence of all-solid-state energy storage/transfer systems. Early research began with acid-tethered polymers half a century ago, and research interest has gradually shifted to high-precision polymers with controllable acid functional groups and nanoscale morphologies. Consequently, various self-assembled acid-tethered block polymer morphologies have been produced. Their ion properties are profoundly affected by the multiscale intermolecular interactions in confinements. The creation of hierarchically organized ion/dipole arrangements inside the block copolymer nanostructures has been highlighted as a future method for developing advanced single-ion polymers with decoupled ion dynamics and polymer chain relaxation. Several emerging practical applications of the acid-tethered block copolymers have been explored to draw attention to the challenges and opportunities in developing state-of-the-art electrochemical systems.

Development of a modified drug delivery system through the incorporation of furosemide into sericin and alginate matrix using the experimental design approach

AbstractBACKGROUNDMost drugs are designed for immediate release via oral administration, releasing the dosage rapidly. Those administration forms can present a wide variation in drug concentrations, which can be harmful to a patient, when the amount of drug is either below or above the therapeutic range. Modified‐release formulations promise to alleviate the problems inherent to immediate‐release drugs by modifying their form of encapsulation. A sericin/alginate blend was applied as a matrix to modify the release of furosemide. The influence of furosemide and alginate concentration was evaluated in the incorporation efficiency and release time of 85% of drug content using a 22 full‐factorial experimental design. Various analytical techniques were employed to characterize the produced drug particles.RESULTSThe incorporation efficiencies ranged from 73.4 ± 2.1% to 84.7 ± 2.9% and the release could be classified as delayed and sustained due to gastroresistance of the particles and a longer release time compared to the commercial drug. Release mechanism was controlled by the relaxation of macromolecular chains due to absorption of water by the particles and the erosion of the matrix. The presence of furosemide crystals was confirmed. Furosemide thermal stability was not altered with its incorporation into the sericin/alginate blend and mean diameter was small enough that the particles could be applied to multiparticulate systems.CONCLUSIONSSericin/alginate blend could serve as a biodegradable and biocompatible matrix for furosemide incorporation for delayed and sustained drug delivery. © 2020 Society of Chemical Industry (SCI)