Structural Relaxation Of Poly Research Articles

Relaxation‐Induced Significant Room‐Temperature Dielectric Pulsing Effects

AbstractThermo‐responsive dielectric materials are in urgent demand owing to the rapid development of smart electronic/electrical systems. Although different types and structures of thermally responsive dielectric materials have been continuously reported, their dielectric response behaviors all originate from thermodynamic phase transitions. Herein, it is demonstrated that structural relaxation in poly(vinylidene fluoride) (PVDF), a non‐thermodynamic phase transition, can induce a significant thermal dielectric pulse at room temperature. The dielectric pulse strength of up to 6.3 × 105 at 20 Hz, with a dielectric pulsing temperature of 24 °C, is achieved from polyethylene glycol (PEG)‐PVDF coaxial nanofibrous films (PVDF@PEG), fabricated via a continuous blow spinning method. Moreover, the films exhibit excellent flexibility, adjustable strength and toughness, switchable hydrophilicity/hydrophobicity, and effective thermal management capability. The relaxation‐induced dielectric pulsing effect, outstanding multifunctionality, and simple preparation combine to promote further scalability and prospects of PVDF@PEG. In particular, the work contributes to the discovery of the relaxation‐induced dielectric response mechanism, which provides a new strategy for the generation of thermo‐responsive dielectric materials.

Monitoring the structural relaxation in poly(ethylene terephthalate) fibers: Dye sorption and FTIR considerations

This work is aimed to monitor the physical aging (structural relaxation) of polyester fibers in terms of changes in the number of dye molecules absorbed by aged specimens as a new method and to make some aspects of this complex phenomenon more clear which rarely considered in the literature. Specimens, aged thermally and hygrothermally at the specified temperature and periods, are, thus, conducted to the dye sorption experiments to monitor their physical aging. Both purified and commercial dyes are employed. Independent from dye purities, the number of absorbed dye molecules reduces for both aged thermally and hygrothermally specimens with increasing the aging time because of decreasing of the free volume as well as increasing the crystallinity, revealed by WAXD measurement. However, the rate of aging of hygrothermally aged specimen is faster than that of the thermal one due to the role of water as a plasticizer in the polymeric bulk. The non-exponential character of the relaxation is exhibited by fitting of KWW (Kohlrausch-Williams-Watts) stretched exponential function and two additive exponential models into the relaxation data, constructed from the measuring of the changes in the number of absorbed dye molecules during aging. Using FTIR spectroscopy a correlation between the number of absorbed dye molecules and the amount of trans conformers for the sample upon thermal aging is found which shows the method yields reliable information on physical aging of the polymer.

Effect on structural relaxation of the poly(methyl‐methacrylate) copolymers chain flexibility

AbstractThe structural relaxation of poly(methyl‐methacrylate) (PMMA)‐based copolymers with different chain flexibility has been studied by DSC with the classical procedure of the isothermal and dynamical approach. Modified PMMA with different chain flexibility have been prepared by free radical polymerization in solution using a mixture of monomers containing 10 mol % of alkyl methacrylate (i.e., ethyl, buthyl, and hexyl methacrylate). The molecular characteristics of all the prepared copolymers have been performed by a multiangle laser light scattering (MALS) photometer on‐line to a size exclusion chromatography (SEC) system (SEC‐MALS) after and before the thermal treatments, NMR (1H and 13C) and MALDI‐TOF mass spectrometry. A comparison of the apparent relaxation rate (RH) was appraised from the enthalpy loss by annealing the different samples at the same level of undercooling (Ta = Tg − 18 °C). It was found an increase of RH increasing the chain flexibility in the copolymers. Dynamical tests, performed at different cooling rates, have been used to estimate the apparent activation energy of the relaxation process. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 596–607, 2009

Heterogeneous dynamics and dynamic heterogeneities at the glass transition probed with single molecule spectroscopy

We studied the temperature dependence of the structural relaxation in poly(vinyl acetate) near the glass transition temperature with single molecule spectroscopy from Tg-1 K to Tg+12 K. The temperature dependence of the observed relaxation times matches results from bulk experiments; the observed relaxation times are, however, 80-fold slower than those from bulk experiments at the same temperature. We attribute this factor to the size of the probe molecule. The individual relaxation times of the single molecule environments are distributed normally on a logarithmic time scale, confirming that the dynamics in poly(vinyl acetate) is heterogeneous. The width of the distribution of individual relaxation times is essentially independent of temperature. The observed full width at half maximum (FWHM) on a logarithmic time axis is approximately 0.7, corresponding to a factor of about 5-fold, significantly narrower than the dielectric spectrum of the same material with a FWHM of about 2.0 on a logarithmic time axis, corresponding to a factor of about 100-fold. We explain this narrow width as the effect of temporal averaging of single molecule fluorescence signals over numerous environments due to a limited lifetime of the probed heterogeneities, indicating that heterogeneities are dynamic. We determine a loose upper limit for the ratio of the structural relaxation time to the lifetime of the heterogeneities (the rate memory parameter) of Q<80 for the range of investigated temperatures.

Structural relaxation in poly(ethylene terephthalate) studied by positron annihilation lifetime spectroscopy

AbstractPositron lifetime technique was used to study the physical ageing process in poly(ethylene terephthalate) (PET). Positron lifetime results show that the structural relaxation processes in PET encompass two different time regimes, one short and the other long. The relaxation function constructed from the measured o‐Ps intensity I3 exhibits non‐exponential character, which can be best fitted with two additive exponentials. The Narayanaswamy model (Kohlraush‐William Watt (KWW) function) is invoked to extract the stretching parameter β indicating the extent of deviation from exponential relaxation. Based on the relaxation times, the activation energies calculated seem to label the different kinetic units of PET structure participating in the relaxation process.© 2002 Society of Chemical Industry

Structural Relaxation in Poly (ethylene terephthalate) Glasses Annealed at Temperatures above the Glass Transition.

Isothermal volume contraction due to physical aging was measured for linear poly(ethylene terephthalate)(PET) glasses and for branched PET glasses of ethylene terephthalate-trimellitic(TMA) copolyesters. Materials were subjected to various thermal annealing treatments at temperatures above their glass transition temperatures (Tg), and then aged at temperatures ranging from Tg-10 °C to Tg with aging time ranging from 10 min to 7 days. Over the range of temperatures studied, normalized volume data for the annealed glasses could be superimposed using aging time-temperature superposition. The shape of reduced curves not to be described by a stretched exponential function was drastically influenced by thermal annealing conditions. It was found that a change of amorphous structures in the linear and lightly branched PET glasses due to annealing resulted in significant acceleration of the rate of volume relaxation. Furthermore, the correlation between the local rearrangement of amorphous network chains and the volume relaxation process of polymer glasses was discussed.

Structural relaxation in poly(ethylene oxide)-salt solutions

We present the results of molecular dynamics simulations of PEOxNaI polymer electrolytes, performed to study the influence of NaI on the dynamics of PEO. By calculating the mean square displacement of the CH2 groups we observe that the Rouse-like motion, typical for short polymer chains, is slowed down under the influence of NaI. This slowing down is also observed in the self part of the intermediate scattering function Fs(k,t), as an increase in the relaxation time. By using the Gaussian approximation it is shown that the stretched exponential relaxation in the small k-limit of Fs(k,t) is a result of the Rouse-like motion of the polymers. We find that our results are in good agreement with neutron spin-echo experiments.

Long-time structural relaxation in poly(vinyl-chloride) below glass transition

We have studied the plasticized poly(vinyl-chloride) in the bulk by polarized neutrons. The long heat treatment (∼10 6 s) at T=45°C< T G=60°C below glass transition induces molecular ordering at distances larger than coil size. The annealing at 180°C< T M=230°C below melting temperature of crystallites breaks the structure to segmental scale fluctuation for time ∼10 3 s. The reptation mechanism of ordering is discussed.

Influence of Physical Aging on the Molecular Motion and Structural Relaxation in Poly(ethylene terephthalate) and Related Polyesters

A combination of dielectric relaxation, dynamic mechanical thermal analysis, and positron annihilation measurements is reported on melt-cast films of poly(ethylene terephthalate), poly(ethylene naphthalate), and their copolymers. The effects of change in the chemical structure of the polyesters are rationalized in terms of increased restriction of the mobility of the polar segments of the chain by the incorporation of the bulky naphthalate structure. All the quenched samples exhibit physical aging when raised to elevated temperatures, the rate depending on the degree of undercooling used in the aging experiments. The free volume surprisingly does not change significantly with temperature and leads to the suggestion that the reduction in the dielectric permittivity is a consequence of a reduction in the mobility of the local segments. The increased storage modulus is also consistent with a reduction in mobility. This proposal is further confirmed by the observation of a good correlation between the rates and extents of the physical aging as detected by dynamic mechanical and dielectric relaxation measurements. Similarities in the activation energies of the β relaxation process for all the polymers investigated indicate that the dipole relaxation processes have a common origin and can be ascribed to motion of the linking polar entity. This study implies that aging is accompanied by an increase in ordering within these polyesters.

Different dynamic filters constructed from multidimensional NMR experiments

The design of specific dynamic filters on the basis of different multi-time correlation functions as measured by multidimensional solid-state nuclear magnetic resonance spectroscopy is discussed. Supplementary to previous applications of this type of experiment, not only slow dynamics but also fast relaxation processes are selected and their dynamic properties are analysed. The experiment is applied to the non-exponential structural relaxation of poly(vinyl acetate). Dynamic heterogeneities are shown to dominate the relaxation. Equilibration of both fast and slow subensembles is observed on the same time scale.

Low‐temperature structural relaxation in poly(alkyl methacrylate)s and related polymers probed by photochemical hole burning

AbstractExcursion temperature dependences of spectral hole profiles in photochemical hole burning for free‐base systems porphyrin/poly(alkyl methacrylate)s are investigated to see the relationship between relaxation properties and chemical structure of the polymers. The irreversible change in hole width is a characteristic of each polymer and is very similar to the relaxation behavior measured mechanically and dielectrically. Poly(methyl methacrylate) (PMMA) shows the highest thermal stability of hole profiles among the polymers presently studied with no marked appearance of local relaxation at 20–100 K. Poly(ethyl methacrylate) (PEMA) and poly(isopropyl methacrylate) (PiPMA) show an increase in hole width even at 20 K with a plateau region at 50–70 K, which is attributed to the rotation of the ethyl or isopropyl group around the CO bond. Polymers which contain more than two serial methylene groups in the side chains (poly(propyl methacrylate), poly(butyl methacrylate), poly(isobutyl methacrylate)) show a similar relaxation behavior around 50–60 K, suggesting the onset of the rotation of the terminal ethyl or isopropyl group in the ester side chain around the CC bond. Low‐density polyethylene (LDPE) shows a similar excursion‐temperature dependence probably due to the existence of branched oligomethylene side chains. The relaxation behavior is independent of the nature of chromophores, which indicates that irreversible hole broadening is controlled mainly by the local motion of the host matrices.

Changes of thermodynamic and dynamic mechanical properties of poly(methyl methacrylate) due to structural relaxation: low-temperature ageing and modelling

Structural relaxation of poly(methyl methacrylate) has been studied by differential scanning calorimetric (d.s.c.) and dynamic mechanical techniques, and the effect of temperature of ageing has been investigated. Isothermal ageings at temperatures between T g −30 K and T g −100 K lead to d.s.c. pre-peaks and a diminution of the mechanical loss tangent. The β relaxation is not affected by ageing and the α relaxation exhibits a low-temperature tail within the temperature domain of the β relaxation. A physical model based upon the concept of annihilation of quasi-point defects through diffusion is used to analyse the results. Diffusion is thought to occur by means of hierarchically correlated movements characterized by a correlation factor b. Simulation of the calorimetric data is satisfactory and comparison with the dynamic mechanical results underlines a specific mechanism for ageing temperatures far below T g

Structural relaxation in poly(ethyl methacrylate)/nylon fibre composites

Differential scanning calorimetry (d.s.c.) of composite samples made out of poly(ethyl methacrylate) (PEMA) and nylon fibres with a wide range of fibre volume fractions reveals several interesting features. A decrease in the glass transition temperature of the series as the fibre fraction increases, as well as a decrease of the specific heat increment at the transition are clearly detected; the glass transition (as determined by d.s.c.) even disappears in samples with high nylon content. The effect on the response thermograms of different ageing thermal histories is analysed with the help of a four-parameter model which predicts the qualitative features of the structural relaxation process. The parameters delivered by the fitting of the model to the observed behaviour suggest that the modified response of PEMA when acting as a matrix (when compared to its behaviour as a pure polymer) is caused by a shift of the equilibrium relaxation times with no accompanying widening of the relaxation time distribution. This supports an interpretation of the observed changes in terms of a bulk modification of PEMA in the composites, probably as a consequence of interactions of its molecules with nylon in the process of forming the composites.

Structural relaxation in poly(vinyl acetate)

AbstractStructural relaxation in poly(vinyl acetate) (PVAc) in and slightly above the glass‐transition region has been studied by monitoring the time dependence of enthalpy using differential scanning calorimetry and the frequency dependence of electric polarization by dielectric loss measurements. The results have been analyzed to yield the kinetic parameters characterizing the structural relaxation and are compared with similar analyses of previously published shear compliance and volume relaxation experiments. Relaxation of enthalpy, electric polarization, volume, and shear stress in PVAc all appear to be characterized by somewhat different relaxation times. The difference between the volume and enthalpy relaxation times, coupled with the fact that PVAc exhibits a Prigogine–Defay ratio greater than unity, is evidence for a previously proposed connection between the thermodynamics and kinetics of structural relaxation in terms of an order parameter model.