Relaxation Dynamics Of Poly Research Articles

Regulating Dynamics of Polyether-Based Triblock Copolymer Hydrogels by End-Block Hydrophobicity

The relaxation dynamics of poly(ethyl glycidyl ether-co-isopropyl glycidyl ether-b-ethylene oxide-b-ethyl glycidyl ether-co-isopropyl glycidyl ether) triblock copolymer hydrogels were investigated as a function of the end-block hydrophobicity and temperature primarily using the oscillatory rheometer, which is crucial to understand the unique viscoelastic behaviors including injectability and self-healing property of the self-assembled hydrogels. Lower critical solution temperature behavior of the poly(alkyl glycidyl ether) end-blocks was harnessed by random copolymerization of poly(ethyl glycidyl ether) (EGE) and poly(isopropyl glycidyl ether) (iPGE), resulting in the remarkable temperature responsiveness of the hydrogel. As the fraction of hydrophilic monomer (i.e., EGE) increases, the sol-to-gel transition occurs at higher temperature and higher polymer concentration. The hydrogel relaxation measured by the oscillatory rheometer becomes faster with decreasing temperature and increasing fraction of hydrophilic monomers. In particular, we observed that a small increment of the hydrophilic monomer fraction significantly reduces the unfavorable interaction between the end-block and aqueous media, resulting in faster hydrogel relaxation dynamics. All polyether-based hydrogels showed biocompatibility and injectability, indicating promising soft materials for biological and biomedical applications. The results are discussed in terms of the current understanding of self-assembled triblock copolymer hydrogels, and particular attention is paid to the issue of chain dynamics.

Dynamics of hyperbranched polymers derived from acrylated epoxidized soybean oil

In this article we demonstrate the role of chain architecture on the relaxation dynamics of poly(acrylated epoxidized soybean oil) (PAESO), a hyperbranched polymer, with a particular focus on the role played by intramolecular loops. PAESO is produced by reversible addition-fragmentation chain transfer (RAFT) polymerization from acrylated epoxidized soybean oil (AESO) containing an average of 2.6 polymerizable acrylic moieties per trigylceride. By adjusting the relative concentrations of solvent, monomer, and RAFT chain transfer agent, PAESO undergoes a series of both inter-molecular and intra-molecular reactions such that both molecular weight and chain architecture vary throughout the reaction. While intermolecular reactions are responsible for branching, intramolecular reactions introduce loops to the structure with altogether different consequences on chain conformations and segmental dynamics. Using dynamic shear rheology in the linear viscoelastic regime we find that Rouse/Zimm-like dynamics persist to molecular weights as high as 390 kDa and that the primary influence of looping is to strongly suppress the number of Zimm relaxation modes.

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.

Relaxation dynamics and cold crystallization of poly(pentamethylene terephthalate) as revealed by dielectric spectroscopy

The relaxation dynamics of poly(pentamethylene terephthalate) has been investigated by means of dielectric spectroscopy. The sub-glass dynamics is characterized by the existence of a bimodal β process whose faster and slower components have been assigned to the relaxation of the bond between the ester oxygen and the aliphatic carbon and to the link between the aromatic ring carbon and the ester carbon, respectively. By comparison with other closely related aromatic polyesters it is shown that the faster component strongly depends on the amount of methylene groups while the slower one is not considerably affected by the nature of the glycol subunit. The changes in the α process associated to the segmental relaxation during cold crystallization reveal the formation of a rigid amorphous phase fraction. Combination of dielectric experiments with X-ray scattering ones suggests that during cold crystallization PPT crystal lamellae tend to fill the space homogeneously.

Relaxation dynamics of poly(vinylidene fluoride) studied by dynamical mechanical measurements and dielectric spectroscopy

The aim of this study is to analyze the mobility of polymer chains in semicrystalline poly(vinylidene fluoride) (PVDF). PVDF crystallizes from the melt in the α crystalline phase. The transformation from the α phase to the electroactive β phase can be induced by stretching at temperatures in the range between 80 and 140 °C. The spherulitic structure of the crystalline phase is deformed during stretching to form fibrils oriented in the direction of the strain. The amorphous phase confined among the crystalline lamellae is distorted as well and some degree of orientation of the polymer chains is expected. Dynamic-mechanical and dielectric spectroscopy measurements were performed in PVDF films stretched to strain ratios up to 5 at temperatures between 80 and 140 °C. Dynamic-mechanical measurements were conducted between -60 °C and melting and in this temperature range the relaxation spectra show the main relaxation of the amorphous phase (called β-relaxation) and at higher temperatures a relaxation related to crystallites motions (α (c)-relaxation). Although the mean relaxation times of the β-relaxation are nearly equal in PVDF before and after crystal phase transformation, a significant change of shape of the relaxation spectrum proves the effect of chain distortion due to crystal reorganization. In stretched PVDF the elastic modulus of the polymer in the direction of deformation is significantly higher than in the transversal one, as expected by chain and crystals fibril orientation. The recovery of the deformation when the sample is heated is related with the appearance of the α (c)-relaxation. Dielectric spectroscopy spectrum shows the main relaxation of the amorphous phase and a secondary process (γ-relaxation) at lower temperatures. Stretching produces significant changes in the relaxation processes, mainly in the strength and shape of the main relaxation β. The Havriliak-Negami function has been applied to analyze the dielectric response.

Effect of silicone dioxide and poly(ethylene glycol) on the conductivity and relaxation dynamics of poly(ethylene oxide)–silver triflate solid polymer electrolyte

AbstractThe conducting and relaxation dynamics of Ag+ ions in poly(ethylene oxide) (PEO)–silver triflate (AgCF3SO3) solid polymer electrolytes (SPEs) containing nanosize SiO2 filler and poly(ethylene glycol) (PEG) as a plasticizer were studied in the frequency range 10 Hz to 10 MHz and in the temperature range 303–328 K. The comparatively lower conductivity of the plasticized (PEG) PEO–AgCF3SO3–SiO2 nanocomposite electrolyte system was examined by analysis of the Fourier transform infrared (FTIR) spectroscopy and conductivity data. The electric modulus (M″) properties of the SPE systems were investigated. A shift of the M″ peak spectra with frequency was found to depend on the translation ion dynamics and the conductivity relaxation of the mobile ions. The value of the conductivity relaxation time was observed to be lower for the PEO–AgCF3SO3 system only with nanofiller SiO2. The scaling behavior of the M″ spectra showed that the dynamical relaxation processes was temperature‐independent in the PEO–AgCF3SO3 and PEO–AgCF3SO3–SiO2–PEG polymer systems, whereas they were temperature‐dependent for the PEO–AgCF3SO3–SiO2 system. However, the relaxation processes of all of theses systems were found to be dependent on their respective compositions. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Excited-State Self-Trapping and Ground-State Relaxation Dynamics in Poly(3-hexylthiophene) Resolved with Broadband Pump–Dump–Probe Spectroscopy

Broadband femtosecond transient absorption spectroscopy is used to explore the mechanisms underlying excited-state and ground-state exciton relaxation in poly(3-hexylthiophene) (P3HT) solution. We focus on the picosecond spectral shifts in the ground and excited states of P3HT, using pump–probe (PP) and pump–dump–probe (PDP) techniques to investigate exciton relaxation mechanisms. Excited-state PP signals resolved a dynamic stimulated emission Stokes shift and ground-state reorganization; PDP signals resolved a blue-shifting nonequilibrium ground-state bleach. Initial structural reorganization is shown to be faster in the excited state. Ground-state reorganization is shown to be dependent on dump time, with later times resulting in relatively more population undergoing slow (∼20 ps) reorganization. These observations are discussed in the context of structural relaxation involving small-scale ( 1 ps) planarization of thiophene groups following photoexcitation. Excited-state and grou...

Relaxation Dynamics of Poly(methyl acrylate) at Elevated Pressure

Dielectric relaxation and complex heat capacity were carried out on high molecular weight poly(methyl acrylate) (PMA). The temperature was found to exert a stronger effect than density on the segme...

Second harmonic generation studies of intrinsic and extrinsic relaxation dynamics in poly(methy1 methacrylate)

Second harmonic generation (SHG) is used to monitor the reorientation a dopant chromophore in slightly entangled poly(methy1 methacrylate) (PMMA). The effect of charge and temperature on both the decay and the much less studied onset modes of SHG signal at temperatures above the glass transition has been examined. At variance with the theoretical predictions, it is shown that the onset and the decay times are not coincident. An isothermal experiment above the glass transition shows a lengthening of relaxation time of the decay mode due to successive poling process, which is ascribed to charge memory effects. In contrast, the latter do not affect the onset characteristic time. The effect of temperature above the glass transition on dopant rotation and polymer relaxations has been also examined. As temperature increases the relaxation times of both the onset and the decay modes decrease. If the surface charge and the charge memory effect are erased, the decay time compares quite well with the structural relaxation time. Differently, the onset time exhibits a partial decoupling.

Structure and relaxation dynamics of poly(amide urethane)s with bioactive transition metal acetyl acetonates in hard blocks

Structural characteristics, thermal transitions and molecular dynamics of selected poly(amide urethane)s with transition metal acetyl acetonates Me(AcAc)(2) (Me = Sn(4+), Zn(2+), Cu(2+), Pb(2+)) as chain extenders, were comparatively investigated using small- and wide-angle X-ray scattering (SAXS, WAXS), differential scanning calorimetry (DSC), and dielectric techniques (dielectric relaxation spectroscopy, DRS; thermally stimulated currents, TSC). We studied the influence of metal chelates on the mixing of the soft-segment (SS) and hard-segment (HS) domains and the related degree of microphase separation (DMS). The reactivity of Me(AcAc)(2) with macrodiisocyanate was found to decrease in the order Sn(AcAc)(2)Cl(2) > Cu(AcAc)(2) > Zn(AcAc)(2) > Pb(AcAc)(2). While Pb(AcAc)(2) shows a higher tendency for crystallisation, both the dielectric and calorimetric results suggest that the corresponding polyurethane has comparatively low DMS. The type of the transition metal has moderate effect on the glass transition temperature and no influence on the shape of the dielectric alpha relaxation signal, indicating weak interactions between metal ions and SS domains. In contrast, structural parameters and the dielectric behaviour of the beta relaxation suggest preference for hydrogen-bonding interactions between Sn(4+) and Cu(2+) metal-chelates and HS domains. The temperature dependence of dc conductivity sigma(dc) is described by the Vogel-Tammann-Fulcher equation and signifies the coupling between the mobility of polymeric chains and charges' motion. It may be expected that the present combination of techniques and particular results with respect to DMS will contribute to the development and testing of novel biodegradation-resistant and antibacterial metal-polyurethanes for biotechnological and industrial applications.

The effect of molecular weight on the dynamics of adsorbed poly(N-isopropylacrylamide) at particle surfaces in water

The relaxation dynamics of poly(N-isopropylacrylamide) (PNIPAM) at surfaces of poly(N-tert-butylacrylamide) latex particles was studied to determine the influence of molecular weight on the relaxation process of interfacial PNIPAM chains. The relaxation dynamics of adsorbed PNIPAM layers under the present experimental conditions was postulated to include the extended-to-loopily adsorbed state transition of tightly adsorbed chains and the diffusion of tenuously adsorbed or dangling chains, combined with a slow conformational equilibrium. The dynamics was described by a nonexponential rate process. The relaxation times calculated were found to show a weak dependence on molecular weight. The kinetic constraints for the relaxation in the present system are believed to be stronger than those in the normal polymer adsorption from solutions.

The effect of molecular weight on the dynamics of adsorbed poly( N ‐isopropylacrylamide) at particle surfaces in water

The relaxation dynamics of poly(N-isopropylacrylamide) (PNIPAM) at surfaces of poly(N-tert-butylacrylamide) latex particles was studied to determine the influence of molecular weight on the relaxation process of interfacial PNIPAM chains. The relaxation dynamics of adsorbed PNIPAM layers under the present experimental conditions was postulated to include the extended-to-loopily adsorbed state transition of tightly adsorbed chains and the diffusion of tenuously adsorbed or dangling chains, combined with a slow conformational equilibrium. The dynamics was described by a nonexponential rate process. The relaxation times calculated were found to show a weak dependence on molecular weight. The kinetic constraints for the relaxation in the present system are believed to be stronger than those in the normal polymer adsorption from solutions.

Relaxation dynamics in poly(methylphenylsiloxane), 1,1-bis(p-methoxyphenyl)cyclohexane, and their mixtures

Mixtures of the small-molecule glass former 1,l-bis@-methoxypheny1)cyclohexane (BMC) with poly(methylphenylsi1oxane) (PMPS) are especially interesting because the glass transition temperatures of the components are nearly equal.' Moreover, these Tg differences can be systematically varied by varying the molecular weight of the polymer. Dynamic mechanical measurements of the glass transition dispersion have been carried out on the neat liquids and their mixtures. The former exhibit normal segmental relaxation behavior, including a correlation of time and temperature dependences. While the behavior of the polymer- rich mixtures is unexceptional, the BMC dynamics are modified in a very interesting way by the presence of the polymer. In particular, the introduction of a small quantity of higher Tg PMPS is found to reduce the reorientational relaxation time of the BMC; that is, the relaxation speeds up. A similar anomaly has been observed in polychlorinated biphenyl containing a few percent of poly(vinylethylene).z~3 This behavior is at odds with a simple free volume approach to relaxation in the vicinity of T,; in fact, the excess volume is negative for BMC mixed with 10% PMPS. An interpretation for these results based on the coupling model of relaxation is suggested.