Relaxation Processes In Polymers Research Articles

Roles of repeating-unit interactions in the stress relaxation process of bulk amorphous polymers: 2. nonlinear viscoelasticity

Stress relaxation manifests itself as the most fundamental mechanical response of bulk amorphous polymers. We characterized polymer repeating units with four-set interaction parameters separately representing the thermodynamic and kinetic aspects of local intrachain and interchain interactions, and evaluated their relative importance in stress relaxation with linear viscoelasticity at high temperatures by means of kinetic Monte Carlo simulations (DOI: 10.1016/j.polym.2021.123740). Hereby we continued to evaluate their relative importance in the initiation and the deviation extent of nonlinear viscoelasticity at low temperatures. We observed that thermodynamic interchain interactions seem no effective, while the other three local interactions retard the later stage of stress relaxation for nonlinear viscoelasticity, responsible for the KWW indexes of polymer rubbery states around 0.5. Both the kinetic aspects of intrachain and interchain interactions dominate the initiation temperatures of nonlinear viscoelasticity, and their high barriers suppress the deviation extent of nonlinear viscoelasticity towards the strong liquids like metals and ceramics, while their low barriers leave the fragile liquids to more thermodynamically rigid polymers. Our work facilitates a better understanding of structure-property relationship in nonlinear viscoelasticity of bulk amorphous polymers.

Nonlocal dual-phase-lag thermoviscoelastic response of a polymer microbeam incorporating modified couple stress and fractional viscoelastic theories

Ultra-slow relaxation process of polymers has the memory-dependent feature, integer-order thermoviscoelastic models may fail to describe the dynamic behaviors of viscoelastic structures accurately. Additionally, it is noticed that the small-scale effect of elastic deformation and heat conduction in a non-isothermal temperature environment is becoming significant due to the development of micro-devices. To better capture the memory-dependent effect and the small-scale effect of viscoelastic micro-structures in heat transfer environment, as a first attempt, present work focuses on developing a refined fractional Kelvin-Voigt thermoviscoelastic model by incorporating the nonlocal dual-phase-lag (NDPL) heat conduction model and the modified coupled stress theory (MCST). Then, the model is applied to investigating the transient response of a polymer microbeam subjected to a harmonic thermal loading. The governing equations involving the modified parameters are formulated and then solved by Laplace transform method. Some parametric results are demonstrated to display the impacts of the nonlocal thermal parameter, the material length-scale parameter and the fractional-order parameter on the considered physical quantities. The results show that the small-scale effect and the memory-dependent effects strongly depend on the polymer micro-structure characteristics in thermal environment.

Изучение процесса диффузии в пленках натриевая соль карбоксиметилцеллюлозы – лекарственное вещество

The transport properties of medicinal films based on sodium salt of carboxymethylcellulose and the antibiotic amikacin sulfate have been studied in this work. It has been shown that the process of sorption of water vapor by such films and the release of a drug from them proceeds in an abnormal diffusion mode, which is explained by the slowdown of relaxation processes in glassy polymers, which include the sodium salt of carboxymethylcellulose. An increase of the amount of the introduced drug is accompanied by a regular decrease in the diffusion coefficients of both the process of sorption of water vapor and the release of amikacin from the films. It is noted that the formed films of sodium salt of carboxymethylcellulose-amikacin sulfate dissolve in water during the day and do not provide a prolonged release of the drug. To reduce the solubility of the films in water, the surface modification of the polymer film with calcium chloride has been carried out. It has been found that the modification does not lead to a change in the diffusion mode, but is accompanied by a regular change in the diffusion coefficients – the longer the formed films were kept in a calcium chloride solution, the lower the diffusion coefficients of the sorption of water vapor by medicinal films and the diffusion coefficients of the release of the drug amikacin from the film. It is argued that the surface modification of polymer films based on the sodium salt of carboxymethylcellulose is an effective way of imparting to them the effect of prolonging the release of a drug.

Roles of repeating-unit interactions in the stress relaxation process of bulk amorphous polymers

How the repeating-unit chemistry dominates polymer viscoelasticity remains as unclear. We employed four-set interaction parameters to represent thermodynamic and kinetic aspects of local intrachain and interchain interactions between monomers, which characterize polymer repeating-unit chemistry. We performed kinetic Monte Carlo simulations of Debye stress relaxation in stretched and parallel-aligned bulk amorphous polymers and evaluated the relative importance of each interactions in the diffusion energy derived from their Arrhenius fluid behaviors. The results demonstrated that, under the same strengths, the kinetic aspects of intrachain and interchain interactions raise much higher diffusion energies than their thermodynamic aspects. We discussed the example case of polyethylene. Our work paves the way towards the prediction of the structure-property relationship of the mechanical properties in bulk amorphous polymers.

Relaxation behavior of poly(diisopropyl fumarate) including no methylene spacer in the main chain

Poly(diisopropyl fumarate) (PDiPF) has dynamic mechanical properties different from the relaxation processes of conventional vinyl polymers, due to the absence of a methylene spacer in the main chain. In this study, we investigated the relaxation behavior of PDiPF using dielectric spectroscopy (DS), differential scanning calorimetry (DSC), wide angle X-ray scattering (WAXS), and dynamic mechanical analysis (DMA). We have demonstrated that the β relaxation of PDiPF has unusual characteristics originating from the absence of a methylene spacer; for example, the β relaxation of PDiPF follows the Vogel–Fulcher–Tammann (VFT) equation, causes a clear step in the DSC curve, and enhances the microscopic packing of the polymer chains in the solid state. These β relaxation behaviors of PDiPF reflect the enlarged cooperative rearranging region (CRR).

Theoretical Analysis of Relaxation Processes in Acrylate Latex Polymers

Dissipative processes in highly elastic latex polymers have been analyzed based on a theoretical calculation of the relaxation microheterogeneity of polymer structures. The microheterogeneity is evaluated graphically from the dependence of the dissipation intensity ratio λ/λmax on the temperature ratio (Т – Тmax)/Тmax. The α-relaxation intensity is determined using the internal friction spectrum recorded by dynamic relaxation spectroscopy.

A Study on Molecular Motions through the Glass Transition of Styrene-Butadiene Rubber/Carbon Nanotubes Nanocomposites

To fully exhibit the development of carbon nanotubes (CNTs) on the glass transition and molecular motions in styrene-butadiene copolymers, styrene-butadiene rubber/carbon nanotubes (SBR/CNTs) nanocomposites are prepared by latex mixing. The uniform dispersion of CNTs in matrix is guaranteed by this approach, which provides necessary conditions for studying the influence of CNTs on the motions of molecular chains. With increase of the CNTs loading, the storage modulus E ' of nanocomposites is correspondingly increased whereas tan δ is reduced compared with pure SBR. Furthermore, the glass transition is also moving towards higher temperature directly because the network structure is formed and endows stronger interaction between the fillers. Relaxation processes in polymers have been studied by dielectric spectroscopy. With the increase of CNTs loading in SBR matrix, the density of the packing network increases, and the free volume of the cavity would be smaller, which leads to the relaxation time of SBR/CNTs nanocomposites being longer than the pure SBR. It is found that the medium loading of CNTs are remarkably effective in improving the tensile strength of SBR.

Drug release and swelling behavior of magnetic iron oxide nanocomposite hydrogels based on poly(acrylic acid) grafted onto sodium alginate

In this paper, a novel iron oxide nanocomposite hydrogel was prepared by simultaneous formation of superparamagnetic iron oxide nanoparticles and cross-linking of poly(acrylic acid) grafted onto sodium alginate polysaccharide. The prepared optimized hydrogel was characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, scanning electron microscopy with energy-dispersive X-ray analysis, atomic force microscopy, and transmission electron microscopy. Moreover, swelling capacity of the obtained hydrogel was measured at different temperatures, different pHs, absorption under load, and magnetic field to assess the sensitivity of ION–PAA-g-NaAlg hydrogel. This hydrogel was also examined as a controlled drug delivery system. Doxorubicin and tetracycline (as model drugs) release was investigated at different pHs, different temperatures, and magnetic field. The release curves were nicely fitted by the Korsmeyer–Peppas equation, and the release is controlled by polymer relaxation process.

Guar gum–poly(N-isopropylacrylamide) smart hydrogels for sustained delivery of 5-fluorouracil

Smart hydrogels comprising guar gum (GG) and poly(N-isopropylacrylamide) (PNIPAAm) cross-linked using a non-toxic cross-linker, tetraethyl orthosilicate, have been designed to explore the sustained release of an anticancer drug, 5-fluorouracil (5FU). Hydrogels containing preformed solid inclusion complexes (IC) of 5FU in β-cyclodextrin (β-CD) were designed to regulate the drug delivery. The formation of true 5FU/β-CD ICs was affirmed from different spectroscopic techniques. The DSC studies of the hydrogels revealed that the thermosensitivity of PNIPAAm is retained in the resultant hydrogels, also corroborated from the swelling measurements. The rate of 5FU release from the hydrogels containing the IC was significantly prolonged in comparison with those containing the free drug. Higher amounts of GG in the matrix led to slower release of drug. The preliminary kinetics of drug release indicated the collective influence of β-CD and GG content on the polymer relaxation process which was key in sustaining the drug delivery. Release studies in simulated gastric (SGF) and intestinal (SIF) fluids showed less than 10% of 5FU release during the initial 2 h in SGF and an increased release in SIF. Thus, the developed smart hydrogels can be utilized as potential candidates for sustained oral delivery of 5FU to the intestine.

Broadband infrared Mueller-matrix ellipsometry for studies of structured surfaces and thin films.

We present a high-optical-throughput infrared Mueller-matrix (MM) ellipsometer for the characterization of structured surfaces and ultrathin films. Its unprecedented sensitivity of about 10-4 in the normalized MM elements enables studies of the complex vibrational fingerprint of thin organic films under different ambient conditions. The ellipsometer acquires quadruples of MM elements within a few 10 s to min, rendering it interesting for process and in-line monitoring. It uses retractable achromatic retarders for increased signal to noise, and tandem wire-grid polarizers for improved polarization control. We demonstrate several scientific and industry-related applications. First, we determine the 3D profile of μm-sized trapezoidal SiO2 gratings on Si from azimuth-dependent MM measurements. Data modeling based on rigorous coupled-wave analysis is employed to quantify grating structure and orientation. We then monitor polymer relaxation processes with a time resolution of 47 s. Measurements of polymer films as thin as 7.7 nm illustrate the sensitivity of the device. We finally couple a liquid flow cell to the ellipsometer, highlighting the prospects for in situ infrared MM studies of thin films at solid-liquid interfaces.

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.

Unusual effects in single molecule tautomerization: hemiporphycene.

Parent hemiporphycene, a recently obtained constitutional isomer of porphyrin, exists in room temperature solutions and polymer matrices in the form of two trans tautomers interconverting via double hydrogen transfer. Using confocal fluorescence microscopy, it was possible to monitor tautomerization in single hemiporphycene molecules embedded in a PMMA film by monitoring the spectral and temporal evolution of their fluorescence spectra. The emission spectra of the two tautomeric forms are similar to those obtained from ensemble studies. However, the analysis of temporal spectral evolution reveals effects not detected in the bulk. For some single molecules, a large decrease of tautomerization rate was observed. This is interpreted as an indication of multidimensional character of the tautomerization coordinate and coupling of the reaction with the polymer relaxation processes. In addition, fluorescence lifetimes obtained for single molecules are significantly shorter than those measured for the bulk. It is proposed that the shortening is caused by environment-induced distortion of the molecule, which enhances the S0← S1 internal conversion rate by lowering the barrier to excited state single hydrogen transfer. This effect seems to reflect the specific morphology of thin (30 nm) polymer samples, because it is not observed in ensemble studies carried out using thick (tens of micrometers or more) PMMA films.

Viscoelastic changes in chlorinated butyl rubber modified with graphene oxide

The glass–rubber transition region in multiple component systems is significant for studying the slow relaxation processes in amorphous polymers. It is the first time that graphene oxide (GO) is added into chlorinated butyl rubber (CIIR) to study the effect of GO on different relaxation processes of CIIR. We aimed to give a possible insight to the molecular relaxation behaviors of CIIR/GO nanocomposites. In this study, GO was synthesized by a revised Hummers method, and it was incorporated with CIIR at different contents of 0, 1, 2, 3 and 5 phr (parts per hundred rubber). The structure of GO and CIIR/GO nanocomposites was studied by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), attenuated total reflectance-Fourier transform infrared (ATR-FTIR), scanning electron microscopy (SEM) and transmission electron microscope (TEM).Bound rubber was adopted to study the interfacial interaction between GO and CIIR. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were also performed. Since there were many conflicting results on the effect of nanoparticles in relation to the glass transition temperature (T g) of polymer matrixes in correlative literature, we investigated the effect of GO on that of CIIR. The T g determined by DSC shows slight shifting. However, the maximum and the shoulder of tan δ both shift to low temperatures. In addition, GO increases the coupling effect of CIIR, resulting the shoulder merged with the maximum. A mechanism, though still needs to be further refined, has been proposed to interpret the contradictory results in our case.

Molecular Simulation of Ionic Polyimides and Composites with Ionic Liquids as Gas-Separation Membranes.

Polyimides are at the forefront of advanced membrane materials for CO2 capture and gas-purification processes. Recently, ionic polyimides (i-PIs) have been reported as a new class of condensation polymers that combine structural components of both ionic liquids (ILs) and polyimides through covalent linkages. In this study, we report CO2 and CH4 adsorption and structural analyses of an i-PI and an i-PI + IL composite containing [C4mim][Tf2N]. The combination of molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) simulations is used to compute the gas solubility and the adsorption performance with respect to the density, fractional free volume (FFV), and surface area of the materials. Our results highlight the polymer relaxation process and its correlation to the gas solubility. In particular, the surface area can provide meaningful guidance with respect to the gas solubility, and it tends to be a more sensitive indicator of the adsorption behavior versus only considering the system density and FFV. For instance, as the polymer continues to relax, the density, FFV, and pore-size distribution remain constant while the surface area can continue to increase, enabling more adsorption. Structural analyses are also conducted to identify the nature of the gas adsorption once the ionic liquid is added to the polymer. The presence of the IL significantly displaces the CO2 molecules from the ligand nitrogen sites in the neat i-PI to the imidazolium rings in the i-PI + IL composite. However, the CH4 molecules move from the imidazolium ring sites in the neat i-PI to the ligand nitrogen atoms in the i-PI + IL composite. These molecular details can provide critical information for the experimental design of highly selective i-PI materials as well as provide additional guidance for the interpretation of the simulated adsorption systems.

Disentanglement effects on welding behaviour of polymer melts during the fused-filament-fabrication method for additive manufacturing

Although 3D printing has the potential to transform manufacturing processes, the strength of printed parts often does not rival that of traditionally-manufactured parts. The fused-filament fabrication method involves melting a thermoplastic, followed by layer-by-layer extrusion of the molten viscoelastic material to fabricate a three-dimensional object. The strength of the welds between layers is controlled by interdiffusion and entanglement of the melt across the interface. However, diffusion slows down as the printed layer cools towards the glass transition temperature. Diffusion is also affected by high shear rates in the nozzle, which significantly deform and disentangle the polymer microstructure prior to welding. In this paper, we model non-isothermal polymer relaxation, entanglement recovery, and diffusion processes that occur post-extrusion to investigate the effects that typical printing conditions and amorphous (non-crystalline) polymer rheology have on the ultimate weld structure. Although we find the weld thickness to be of the order of the polymer size, the structure of the weld is anisotropic and relatively disentangled; reduced mechanical strength at the weld is attributed to this lower degree of entanglement.

Combined Mechanical and Electrical Study of Polymers of Biological Origin

Thermally Simulated Depolarization Current measurement is an excellent but not widely used method for identifying relaxation processes in polymers. The DMA method is used here to analyze the mechanical changes depend on temperature in biopolymers. The two techniques take advantage of the energy changes involved in the various phase transitions of certain polymer molecules. This allows for several properties of the material to be ascertained; melting points, enthalpies of melting, crystallization temperatures, glass transition temperatures and degradation temperatures. The examined biopolymer films are made from biological materials such as proteins and polysaccharides. These materials have gained wide usage in pharmaceutical, medical and food areas. The uses of biopolymer films depend on their structure and mechanical properties. This work is based on pectin and gelatin films. The films were prepared by casting. The casting technique used aqueous solutions in each case of sample preparation. The manufacturing process of the pectin and gelatin films was a single stage solving process.

Heterogeneity during Plasticization of Poly(vinylpyrrolidone): Insights from Reorientational Mobility of Single Fluorescent Probes.

While dynamics of single-molecule (SM) fluorescent probes have been used to investigate the structure and relaxation processes in polymers near the glass transition temperature (Tg), it is difficult to perform SM imaging at elevated temperatures which restricts such studies to a limited number of polymers for which Tg is close to room temperature (RT). Plasticization, solvent (or additive) induced lowering of Tg, offers an alternate avenue to access various effective temperatures in the glassy and rubbery phases of polymers under ambient conditions. By investigation of the reorientational propensity of individual Rhodamine 6G (Rh6G) probes, which is governed by rigidity/dynamics of the polymer cavities, we have explored the extent of spatiotemporal heterogeneity during moisture induced plasticization of poly(vinylpyrrolidone) (PVP), far below and near (below and above) bulk Tg. Lack of any probe reorientation suggests that the matrix remains extremely rigid up to a certain level of hydration, as expected for probes buried deep within the glassy state. At intermediate levels of hydration, SMs undergo a wide variety of rotational dynamics ranging from being static/wobbling motion to slow, hindered large-angle reorientation, as well as facile, intermittently hindered fast rotation, which reflects that swelling/softening of network cavities is spatiotemporally extremely diverse as the effective Tg approaches RT. SM probes exhibit temporally nonuniform rotational mobility even at relatively high moisture contents of the matrix beyond which probes can undergo translational motion, which indicates that relatively slow time scale polymer segmental motion can be operational for plasticized PVP (in the rubbery state). Our inferences are supported by the non-Gaussian nature of angular jump distributions for dipolar reorientation, similar to those reported for translational diffusion of SM tracers in polymers and cellular media, suggesting the existence of slow time-varying local environmental changes around individual probe molecules during plasticization.

Gamma irradiation induced in situ synthesis of lead sulfide nanoparticles in poly(vinyl alcohol) hydrogel

In this study, the nanocomposites based on semiconductor lead sulfide (PbS) nanoparticles and poly(vinyl alcohol) (PVA) were investigated. The gamma irradiation induced in situ incorporation of PbS nanoparticles in crosslinked polymer network i.e. PVA hydrogel was performed. PVA hydrogel was previously obtained also under the influence of gamma irradiation. UV–Vis absorption and X-ray diffraction measurements were employed to investigate optical and structural properties of PbS nanoparticles, respectively, and obtained results indicates the presence of nanoparticles with approximately 6nm in diameter and face centered cubic rock-salt crystal structure. The porous morphology was confirmed by scanning electron microscopy. Swelling data revealed that investigated hydrogels (PVA and PbS-PVA nanocomposite) shows non-Fickian diffusion, indicating that both diffusion and polymer relaxation processes controlled the fluid transport. The values of diffusion coefficients have an order of magnitude 10−9cm2/s (typical values for water diffusion in polymers) and the best fit with the experimental results showed the Etters approximation. Comparing the thermal properties of PbS-PVA xerogel nanocomposite with PVA xerogel it was observed that incorporation of PbS nanoparticles in crosslinked PVA matrix just slightly enhanced the thermal stability of nanocomposite.

Dielectric properties of liquid-crystal azomethine polymer with alkyl-substituted side chain

Relaxation processes in side-chain liquid-crystal azomethine polymer have been studied using dielectric spectroscopy in wide ranges of temperatures and frequencies. Two low-temperature relaxation processes (β1 and β2) caused by low-amplitude noncooperative motion of kinetic units in side groups and a high-temperature α-process caused by the large-scale cooperative motion of kinetic units in the main polymer chain have been revealed.

Dielectric α-relaxation of 1,4-polybutadiene confined between graphite walls : Molecular dynamics investigations through numerical simulations of polymer molecules relaxation.

We present results of Molecular Dynamics (MD) simulations of a chemically realistic model of 1,4-polybutadiene confined by crystalline graphite walls. The simulations cover a large range of temperatures from T ≈ 2T g to T ≈ 1.15T g, where relevant time scales are accessible using such computational methods. We investigate the dielectric relaxation close to the walls in comparison to the one in the center of the film, and study the latter as a function of the film thickness from the walls. The segmental dynamics in the film is slowed down close to the walls, in comparison to the bulk. In addition to the α-process, the relaxation exhibits an additional long time decay, the so-called wall desorption process. We focus here on the α-process and find no significant shift of the dielectric T g as a function of layer thickness, in agreement with recent dielectric experiments. These findings can be correlated with the importance of the dihedral dynamics for all relaxation processes in polymers, which is unaltered except for the first nanometer next to the walls.