Chain Relaxation Research Articles

Progresses in the glass transition and molecular dynamics of thin polymer film

With the advances in nanotechnology, the glass transition and chain relaxation behavior for polymer molecules in a confined geometry have been attracted great attentions. Due to the nano-scale effects, the molecular dynamics of polymer thin film deviates from that in the bulk, and are a function of the dimension of films (i.e. thickness). The studies of the molecular dynamic in polymer thin film are of significant importance both in term of optimizing the physical properties of polymer nanomaterials and also in the developing our understanding of the nature of glass transition for amorphous materials. In this paper, some important results in the field of the dynamics of thin polymer film are reviewed. We summarized the physical mechanisms responsible for the deviation of chain dynamic from that in bulk, and discussed the distribution of chain mobility in polymer thin film, as well, some theoretical models describing the glass transition of thin polymer film was introduced. In the end, the problems in this field and prospects in the further studies were also discussed.

The segmental and chain relaxation modes in high-cis-polyisoprene as studied by thermally stimulated currents.

The technique of Thermally Stimulated Currents is used to study the slow molecular mobility in a series of poly (1,4-cis-isoprene) samples with different molecular weights, Mw, and low polydispersity. The technique revealed a high resolution power, particularly useful in the study of the lower molecular weight samples where the chain and the segmental relaxations strongly overlap. The dynamic crossover that is reported for the normal mode by varying the molecular weight is clearly revealed by the thermally stimulated depolarization currents results through the temperature location, TMn, of the normal mode peak, the values of the relaxation time at TMn, τ(TMn), and the value of the fragility index of the normal mode, mn. The kinetic features of the glass transition relaxation of polyisoprene have also been determined.

Synthesis and linear rheological property of comb-like styrene-based polymers with a high degree of branch chain

A series of model comb-like polymers (PSVS-g-PS) with high branching degree were synthesized by nucleophilic substitution reaction between iodinated copolymer of styrene (St) with 4-(vinylphenyl)-1-butene (VSt) (PSVSI, backbone) and living polystyrene lithium (PSLi, side chain). This method is a facile way for modular synthesis of graft copolymers based on living anionic polymerization and hydrozirconation reaction. The branching density of PSVS-g-PS (the number of branch chain per 100 repeat backbone units) ranged from 2.4 to 20.7%. The backbone length of PSVS-g-PS was larger than the entanglement molecular weight (Me) of PS, while the molecular weight of branch chain was designed to be below or above the Me of PS. The results from the linear rheological properties of the PSVS-g-PS showed hierarchical relaxation process. One appeared in the intermediate angular frequency region, corresponding to the relaxation of the side chain. The other is in the terminal region, which is related to the movement of the whole comb polymer. From scaling law of zero shear viscosity (η0) of PSVS-g-PS with versus molecular weight, it could be seen that the dynamical behavior of PSVS-g-PS was different from those of both poly(macromonomers) and entangled linear polymer chains, indicating that there is certain entanglement for PSVS-g-PS with long branch chain.

Physical Modeling of Chromosome Segregation in Escherichia coli Reveals Impact of Force and DNA Relaxation

The physical mechanism by which Escherichia coli segregates copies of its chromosome for partitioning into daughter cells is unknown, partly due to the difficulty in interpreting the complex dynamic behavior during segregation. Analysis of previous chromosome segregation measurements in E. coli demonstrates that the origin of replication exhibits processive motion with a mean displacement that scales as t0.32. In this work, we develop a model for segregation of chromosomal DNA as a Rouse polymer in a viscoelastic medium with a force applied to a single monomer. Our model demonstrates that the observed power-law scaling of the mean displacement and the behavior of the velocity autocorrelation function is captured by accounting for the relaxation of the polymer chain and the viscoelastic environment. We show that the ratio of the mean displacement to the variance of the displacement during segregation events is a critical metric that eliminates the compounding effects of polymer and medium dynamics and provides the segregation force. We calculate the force of oriC segregation in E. coli to be ∼0.49 pN.

Mechanical and Electrical Properties of Styrene‐Isoprene‐Styrene Copolymer Doped with Expanded Graphite Nanoplatelets

The molecular dynamics of a triblock copolymer and of expanded graphite nanoplatelets were investigated. Composites were prepared using the solution technique. The effects of filler addition and of filler‐matrix interactions were investigated using dielectric relaxation spectroscopy (DRS) and dynamic mechanical analysis (DMA). Only one relaxation was observed by DRS, which was associated with the relaxation of the main polymer chain. Both DRS and DMA demonstrated that the addition of the filler does not cause a significant change in either the temperature of the relaxation or its activation energy, which suggests the presence of weak interactions between the filler and matrix. The storage modulus of the composites increased with increasing filler content. The composite containing 8% filler exhibited a storage modulus increase of approximately 394% in the rubber area. Using the DC electrical conductivity measurements, the electrical percolation threshold was determined to be approximately 5%. The dielectric permittivity and conductivity in the microwave region were determined, confirming that percolating behavior and the critical threshold concentration.

The chain conformation and relaxation dynamics of poly(acrylic acid)-graft-poly(ethylene oxide)-graft-dodecyl in water: effect of side-chains and distribution of counterions.

We present a study on the dielectric behavior of an aqueous solution of an amphiphilic copolymer, poly(acrylic acid)-graft-poly(ethylene oxide)-graft-dodecyl (PAA-g-PEO-g-dodecyl), in the frequency range of 40 Hz to 110 MHz at varying concentrations and temperatures. After eliminating the electrode polarization at low-frequency, three dielectric relaxation processes were observed at about 1.2 MHz, 150 kHz and 30 kHz, whose mechanisms were proved to originate from the fluctuations of free counterions, the fluctuation of condensed counterions, and the rotation of intramolecular aggregates, respectively. The concentration dependence of the dielectric increment Δε and relaxation time τ for these three relaxations presents an abrupt change at 0.15 mg ml(-1), indicating that PAA-g-PEO-g-dodecyl molecules undergo a conformational transition from intramolecular aggregates to intermolecular aggregates. Moreover, both Δε and τ show a clear transition at about 317 K, suggesting a partial collapse of the aggregates. The correlation length and the contour length of the PAA-g-PEO-g-dodecyl chain were estimated according to Ito's theory of counterion fluctuation. It was found that the hydrophobic/hydrophilic side-chains affected the microscopic conformation of PAA, and the hydrogen-bond interactions greatly influenced the conformation. Additionally, the activation energy of these three relaxations was calculated and the process of ionic conduction was studied and the results were used to discuss counterion distribution and ion conduction.

Rouse mode analysis of chain relaxation in polymer nanocomposites.

Large-scale molecular dynamics simulations are used to study the internal relaxations of chains in nanoparticle (NP)/polymer composites. We examine the Rouse modes of the chains, a quantity that is closest in spirit to the self-intermediate scattering function, typically determined in an (incoherent) inelastic neutron scattering experiment. Our simulations show that for weakly interacting mixtures of NPs and polymers, the effective monomeric relaxation rates are faster than in a neat melt when the NPs are smaller than the entanglement mesh size. In this case, the NPs serve to reduce both the monomeric friction and the entanglements in the polymer melt, as in the case of a polymer-solvent system. However, for NPs larger than half the entanglement mesh size, the effective monomer relaxation is essentially unaffected for low NP concentrations. Even in this case, we observe a strong reduction in chain entanglements for larger NP loadings. Thus, the role of NPs is to always reduce the number of entanglements, with this effect only becoming pronounced for small NPs or for high concentrations of large NPs. Our studies of the relaxation of single chains resonate with recent neutron spin echo (NSE) experiments, which deduce a similar entanglement dilution effect.

Rouse mode analysis of chain relaxation in polymer nanocomposites

Rouse mode analysis of chain relaxation in polymer nanocomposites

Long-term controlled release of PLGA microparticles containing antidepressant mirtazapine

The aim of the study was to prepare PLGA microparticles for prolonged release of mirtazapine by o/w solvent evaporation method and to evaluate effects of PVA concentration and organic solvent choice on microparticles characteristics (encapsulation efficiency, drug loading, burst effect, microparticle morphology). Also in vitro drug release tests were performed and the results were correlated with kinetic model equations to approximate drug release mechanism. It was found that dichloromethane provided microparticles with better qualities (encapsulation efficiency 64.2%, yield 79.7%). Interaction between organic solvent effect and effect of PVA concentration was revealed. The prepared samples released the drug for 5 days with kinetics very close to that of zero order (R2 = 0.9549 – 0.9816). According to the correlations, the drug was probably released by a combination of diffusion and surface erosion, enhanced by polymer swelling and chain relaxation.

Temperature-sensitive polyethylenglycol/N-isopropylacrylamide hydrogels: Impact of material parameters on swelling and drug release

In this study, we fabricated Temprature-sensitive polyethyleneglycol/N-isopropylacrylamide (PEG/NIPAAm) hydrogels by a free-radical polymerisation method with variation in the content of monomer, polymer and cross-linking agent. Swelling was performed in USP phosphate buffer solutions of pH 7.4 for selected samples. It was observed that swelling and drug release from hydrogels can be modified by changing composition and degree of cross-linking of the hydrogels under investigation. As expected LCST increased from 34 to 39C as PEG content in copolymers increased from 2 to 8% (w/w). The presence of polyethyleneglycol in the hydrogel formulation resulted the higher mechanical strength and swelling. Network structure was evaluated by different parameters and FTIR confirmed the formation of cross-linked hydrogels. The percent swelling, equilibrium swelling, diffusion constant values are evaluated for PEG/NIPAAm hydrogels at 1% of Insulin solution at room temperature. Drug release increased by increasing PEG contents in hydrogels while increasing the concentration of cross-linking agent had the opposite effect. Based on the release kinetic of the Insulin drug, the hydrogels displayed a non-Fickian diffusion mechanism. According the diffusion kinetic data in hydrogels became clear that diffusion kinetic data were best described by Higuchi model. Permeation from PEG/NIPAAm hydrogels followed a Super Case II transport mechanism, most likely driven by macro molecular chain relaxation and swelling of hydrophilic polymers.

Breakdown of time-temperature superposition in a bead-spring polymer melt near the glass transition temperature.

The breakdown of the time−temperature superposition (TTS) near its glass transition temperature (T(g)) in simple bead−spring polymer melts with and without the chain angle potential was numerically investigated. The stress relaxation modulus at different temperatures G(t,T) was calculated by the Green−Kubo relation. The TTS of G (t,T) of bead−spring polymer melts worked well at temperatures sufficiently higher than its T(g). However, when the system temperature is approaching the glass transition regime, the breakdown of TTS is observed. At temperatures near the Tg,the temperature dependence of the shift factor (a(TB)), which is defined on the time scale between the bond relaxation and the chain relaxation regimes of a G(t) function, is significantly stronger than ones (a(TA)) defined by the time scale of the chain relaxation modes. The analysis of the van Hove function G(s)(r,t) and non-Gaussian parameter, α(2)(t), of the bead motions strongly suggests that the TTS breakdown is concerned with the dynamic heterogeneity. The effect of the chain stiffness on the temperature dependence of the shift factors was also investigated in this study. The stiffer chains melt has a stronger temperature dependence of the shift factors than the ones of the flexible chains melt. However, regardless of the chain stiffness, the stress relaxation modulus functions of the bead−spring polymer melts will begin to break down the TTS at a similar T(g)-normalized temperature (T/T(g)).

Conductivity scaling and near-constant loss behavior in ion conducting polymer blend

Literature lacks unanimity of opinion on near-constant loss (NCL) behavior in disordered solids of technological importance. Earlier reports attributed NCL in disordered glass and ceramics to correlated localized displacement of charged entities. By contrast, in pure polymer/ion conducting polymer electrolytes, no conclusive reason has yet emerged. We report novel results with convincing evidences for relaxation, scaling and NCL behavior in ion conducting polymer blends perhaps for the first time. NCL effect, noticed predominantly at higher frequencies and low temperatures close to polymer blend glass transition (Tg), originates from coupled ion and polymer chain relaxation due to frozen mobility near Tg.

Chain Dynamics on Crossing the Glass Transition: Nonequilibrium Effects and Recovery of the Temperature Dependence of the Structural Relaxation.

In this paper we report thermally stimulated depolarization current results on the chain and segmental dynamics of two monodisperse polyisoprenes accessing both dynamics at ultralow frequency range and exploring the relationship between segmental and chain time scales when crossing the glass transition. In this range, we have recorded experimental evidence of nonequilibrium effects on the slowest chain mode dynamics. The nonequilibrium effects seem to occur simultaneously for both chain and α-relaxation. Moreover, detailed analysis strongly indicates the recovery of an even T-dependence for the chain and α-relaxation dynamics on crossing glass transition and in the glassy state. The obtained results can be understood taking into account the different temperature dependences of the length scales involved in the segmental and chain relaxations.

Efficient Use of Semidefinite Programming for Selection of Rotamers in Protein Conformations

Determination of a protein's structure can facilitate an understanding of how the structure changes when that protein combines with other proteins or smaller molecules. In this paper we study a semidefinite programming (SDP) relaxation of the (NP-hard) side chain positioning problem presented in Chazelle et al [Chazelle B, Kingsford C, Singh M (2004) A semidefinite programming approach to side chain positioning with new rounding strategies. INFORMS J. Comput. 16:380-392]. We show that the Slater constraint qualification (strict feasibility) fails for the SDP relaxation. We then show the advantages of using facial reduction to regularize the SDP. In fact, after applying facial reduction, we have a smaller problem that is more stable both in theory and in practice. We include cutting planes to improve the rounded SDP approximate solutions.

Crystallization, recrystallization, and melting lines in syndiotactic polypropylene crystallized from quiescent melt and semicrystalline state due to stress-induced localized melting and recrystallization.

Crystalline lamellar thickness in syndiotactic polypropylene (sPP) during crystallization from either isothermal molten or stretching induced localized melt states and during subsequent heating was investigated by means of temperature dependent small-angle X-ray scattering techniques. Well-defined crystallization lines where the reciprocal lamellar thickness is linearly dependent on crystallization temperature were observed. Unlike in the case of polybutene-1 where stretching crystallization line was shifted to direction of much smaller lamellar thickness (Macromolecules 2013, 46, 7874), the stretching induced crystallization line for sPP deviates from its corresponding isothermal crystallization line only slightly. Such phenomenon could be attributed to the fact that both crystallization processes from quiescent melt and stress induced localized melt are mediated in a mesomorphic phase in sPP. Subsequent heating of sPP after crystallization revealed the same melting behavior in both systems for the two kinds of crystallites obtained from either quiescent melt or stretching induced localized melt. Both of them underwent melting and recrystallization when the lamellar thickness was smaller than a critical value and melting directly without changing in thickness when the lamellar thickness was larger than the critical value. The melting behavior in sPP systems can be understood by considering the chain relaxation ability within crystalline phase and also can be used as evidence that the crystallization from molten state and stress-induced crystallization passed through the intermediate phase before forming crystallites.

Interphase vs confinement in starch-clay bionanocomposites.

Starch-clay bionanocomposites containing 1-10% of natural montmorillonite were elaborated by melt processing in the presence of water. A complex macromolecular dynamics behavior was observed: depending on the clay content, an increase of the glass transition temperature and/or the presence of two overlapped α relaxation peaks were detected. Thanks to a model allowing the prediction of the average interparticle distance, and its comparison with the average size of starch macromolecules, it was possible to associate these phenomena to different populations of macromolecules. In particular, it seems that for high clay content (10%), the slowdown of segmental relaxation due to confinement of the starch macromolecules between the clay tactoïds is the predominant phenomenon. While for lower clay contents (3-5%), a significant modification of chain relaxation seems to occur, due to the formation of an interphase by the starch macromolecules in the vicinity of clay nanoparticles coexisting with the bulk polymer.

Effect of the molding temperature and cooling time on the residual stresses of crystal polystyrene

The use of crystal polystyrene for high performance components requires knowledge of the distribution of residual stresses. The aim of this research was to analyze the influence of the molding temperature and cooling time on the residual stresses present in parts of two types of crystal polystyrene PS1 and PS2, processed by injection molding. The results obtained using photoelasticity showed that at low temperatures the residual stresses increase due to the processes of formation and destruction of intermolecular forces. Internal stresses were reduced in the polymer specimens with greater thickness because the molecular relaxation of chains of polystyrene is facilitated by the space increase between the walls of the mold. It was concluded that the photoelasticity technique can be applied effectively in the measurement of residual stresses in injection molded crystal polystyrene parts.

Dynamic Dilution Effect in Binary Blends of Linear Polymers with Well-Separated Molecular Weights

We investigate and model the viscoelastic properties of binary blends composed of linear chains. These systems are indeed very suitable to test the validity and the limit of the constraint release (CR) process and dynamic tube dilution and to determine the value of the dynamic dilution exponent α. We first focus on binary blends containing barely entangled short chains. In such a case, we show that the tension re-equilibration process previously observed [van Ruymbeke Macromolecules 2012, 45, 2085] can be correctly described as a CR-activated contour length fluctuation (CLF) process, which takes place along the fully dilated tube and is governed by the intrinsic Rouse time of a long–long entanglement segment. In addition, reptation is considered to take place along the fully dilated tube. We also show that this CR-activated CLF process speeds up the relaxation of the long chains, which naturally leads to an effective dilution exponent α equal to 4/3, despite the fact that the modeling is based on α = 1. This result is in agreement with the experimental data. Then, we analyze the rheological behavior of binary blends containing entangled short chains. In such a case, we show that the CR-activated CLF also takes place, but with a delay time being necessary for the long chain to explore the dilated tube. This approach is tested for several binary blends, showing an improved quality of the predictions compared to previous tube modeling on the same blends. Indeed, by considering the chain motions on two different length scales, a larger fraction of the long chains relax at intermediate frequencies (through the tension re-equilibration) before their reptation, leading to an effective dilution exponent larger than 1 as determined from the low-frequency plateau modulus.

A Study of Linear versus Kinked Bulky Polymers for Proton Exchange Membranes Using Sulfonated Polyimides

Linear and kinked bulky monomers were incorporated into the main chain of a polyimide in order to investigate the effect of kinked versus linear polymers on membrane properties such as water uptake and proton conductivity. Polymers prepared using linear 1,4-bis (4-aminophenoxy)-naphthyl-2,7-disulfonic acid (BAPNDS), SPI-N, and using kinked 2,2’-bis (p-aminophenoxy)-1,1’-binaphthyl-6,6’-disulfonic acid (BNDADS), SPI-BN, were cast into membranes. All the copolymers showed excellent solubility and good film-forming capability. Membranes are thermally stable up to 300 °C under air. For SPI-BN, the nonplanar binaphthyl group result in polymer chain relaxation and produce large water uptake. However, the conductivity of kinked, SPI-BN membranes is lower than those prepared from SPI-N for a given IEC but water uptakes are higher. This might be related to substitution position of the sulfonic acid groups and the microstructure. Sulfonic acid group were located at the same side of the main chain will be favorable for forming hydrophilic clusters, thus better proton conductivity performance would be achieved.

Rouse Mode Analysis of Chain Relaxation in Homopolymer Melts.

We use molecular dynamics simulations of the Kremer–Grest (KG) bead–spring model of polymer chains of length between 10 and 500, and a closely related analogue that allows for chain crossing, to clearly delineate the effects of entanglements on the length-scale-dependent chain relaxation in polymer melts. We analyze the resulting trajectories using the Rouse modes of the chains and find that entanglements strongly affect these modes. The relaxation rates of the chains show two limiting effective monomeric frictions, with the local modes experiencing much lower effective friction than the longer modes. The monomeric relaxation rates of longer modes vary approximately inversely with chain length due to kinetic confinement effects. The time-dependent relaxation of Rouse modes has a stretched exponential character with a minimum of stretching exponent in the vicinity of the entanglement chain length. None of these trends are found in models that allow for chain crossing. These facts, in combination, argue for the confined motion of chains for time scales between the entanglement time and their ultimate free diffusion.