Relaxation Of Poly Research Articles

Quasielastic scattering and fast localized relaxation in poly(methyl methacrylate) glass

Quasielastic scattering and fast localized relaxation in poly(methyl methacrylate) glass

Enthalpy relaxation in glassy polystyrenes Part 3Stress and enthalpy relaxation in poly(4-methylstyrene) and poly(4-chlorostyrene)

Structural recovery in poly(4-chlorostyrene) and poly(4-methylstyrene) at temperatures below their glass transitions has been examined by determining the time dependent stress relaxation modulus, G(t), as a function of the ageing time. Time-ageing time superposition of the modulus curves gives a set of ageing time shift factors which characterise the structural state of the glassy polymer in terms of its mechanical response. The mechanical ageing rate of poly(4-methylstyrene) decreases with increasing temperature, like that of polystyrene, but that of poly(4-chlorostyrene) increases with decreasing temperature. Free volume arguments cannot adequately explain this behaviour and it is proposed that dipole interactions, which appear not to affect enthalpic ageing behaviour in these polymers, become active under the mechanical probe. The ratios of enthalpic and mechanical ageing rates are compared and found to be constant over the range of ageing times examined.

Dynamic mechanical and dielectric relaxations in poly(monoethylphenyl itaconate)

Dynamic mechanical and dielectric relaxational behavior of poly(monoethylphenyl itaconate) at different frequencies and temperatures was studied. Three relaxation zones are found. The dynamic mechanical response is dominated by a relaxation peak at room temperature, labeled β relaxation. Two prominent shoulders labelled as γ and α relaxations are observed. Because of the overlapping of the α and γ with the β relaxation, a deconvolution method to improve the understanding of these phenomena is proposed. In spite of the complexity of the experimental spectra, the proposed deconvolution method seems to be a convenient approach to interpret the relaxational behavior of this polymer. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2749–2756, 1997

Crystallization of poly(aryl ether ketone) polymers as revealed by time domain dielectric spectroscopy

Time domain dielectric methods have been used to compare the dynamics of the α relaxation of poly(aryl ether ketone) polymers. The real time variation of the macroscopic dielectric decay functions with crystallization time has been interpreted assuming an increase in the intermolecular cooperativity of the α relaxation as the polymer chains are obliged to move between crystalline regions.

Dynamic mechanical characterization of relaxations in poly(oxymethylene), miscible blends, and oriented filaments

Multifrequency dynamic mechanical analysis (DMA) data were obtained for molded poly(oxymethylene) (POM) and its blends from-150°C to 150°C. Because of the high crystallinity, the assignment of the glass transition in POM has been controversial in the literature. Low and high glass transition temperature (T g) phenolated compounds, including poly(vinyl phenol), were found to be miscible with POM. The shift of the β transition in the POM blends favors an assignment of the β transition detected at −3°C(1 Hz), not the −80°C γ transition, as the T g in semicrystalline POM because the latter is invariant with diluent. The peak at the β transition in pure POM is weak and can only be seen clearly by DMA measurements on samples that have not “aged” at ambient temperature. This is further evidence that the β transition arises from a cooperative glass-transition-like motion. The γ transition is not influenced by aging because it is due to a concerted localized main chain motion. The β transition of an oriented POM filament can be seen in the DMA flexural loss spectrum at-18°C (1 Hz), but not in a tensile loss spectrum. The broad a relaxation was detected at about 110°C (1 Hz) in molded POM and its blends, while it was shifted to about 135°C in the higher crystallinity, oriented system. The α peak is also independent of diluent, consistent with a crystalline origin for this transition, as was proposed earlier.

Relaxations in poly (tetra methylene terephthaloyl-bis-4-oxybenzoate): effect of substitution in the mesogenic unit and in the flexible spacer

The relaxation behaviour of a thermotropic polymer liquid crystal, poly(tetramethylene terephthaloyl-bis-4-oxybenzoate), and its derivatives with substitution in the mesogen and/or in the spacer, have been analysed by dynamic mechanical analysis and differential scanning calorimetry (d.s.c.). By comparing the relaxation spectra of the four polymers some conclusions can be drawn. They all show a β relaxation at low temperature and an α relaxation above room temperature. The β relaxation has been associated with local motions of the mesogenic unit. The substitution of chlorine in the mesogen increases the temperature of this relaxation by 60°C, but it is not affected by substitution in the spacer. The a relaxation has been associated with the glass transition of the amorphous phase. It increases by approximately 15°C with the introduction of lateral methyls in 1,4 position in the spacer, independently of substitution in the mesogen. The presence of crystalline order, besides the nematic mesophase, broadens this relaxation.

Dielectric and molecular mechanics study of the γ relaxations of poly(chloroethyl methacrylate) and poly(chloropropyl methacrylate)

Cryogenic dielectric measurements corresponding to the γ-relaxation zone have been made for poly(chloroethyl methacrylate) and poly(chloropropyl methacrylate). Molecular mechanics calculations using the Allinger MM2 force field have been carried out on monomer and trimer model compounds in an attempt to gain some insight into the possible origin of the γ-relaxation process which occurs in these polymers at low temperatures. In both cases, we found that the molecular mechanism of oxylcarbonyl side group rotation combined with reasonable constraints on movements of the neighbouring side groups gave theoretical activation energy requirements which were in good agreement with those determined from the experiment.

Mössbauer Study of FeCl3-Doped Polythiophene and Poly(3-Octylthiophene)

We have specified the chemical species of the dopants FeCl 3 -doped polythiophene and poly(3-octylthiophene) through 57 Fe-Mossbauer study as FeCl 4 - with slightly distorted tetrahedral structure. Spectra with a quadrupole-split doublet were observed for polythiophene, while those for poly(3-octylthiophene) were broadened by magnetic relaxation. Application of a magnetic field induced magnetically split spectra in both polymers due to a suppression of the relaxation frequency. In these conducting polymers there exist two kinds of chemical species with different relaxation frequency. The relaxation in poly(3-octylthiophene) is considerably slower than in polythiophene. The relaxation mechanism is attributable to the interaction between dopant spins and carrier spins.

Pressure-Dependent Photon Correlation Spectroscopic Investigation of Poly(propylene oxide) near the Glass Transition

The effect of pressure on structural relaxations in poly(propylene oxide) (PPO; MW = 4000) near the glass transition has been determined using photon correlation spectroscopy. Experimental results are reported for pressures ranging from 2.5 to 3.9 kbar at 253 K. The structural relaxation exhibits two distinct processes separated by ∼3 orders of magnitude in time. This observation is consistent with results of dielectric relaxation studies of PPO. The faster relaxation is associated with local segmental motions important in the glass−rubber transition. The mean relaxation time for the faster process exhibits an exponential dependence on pressure.

Femtosecond site-selective probing of energy relaxing excitons in poly(phenylenevinylene): Luminescence dynamics and lifetime spectra

Exciton relaxation in poly(phenylenevinylene), PPV, has been probed by femtosecond luminescence-up-conversion. We report on excitonic luminescence profiles that depend on the spectral position of the detection window (ε̂=2.7,2.6,2.5, and 2.4 eV, respectively). In an attempt to reveal the transient steps implied in fs relaxation, we present a quantitative forward reconvolution fit procedure that is based on a microscopic incoherent transport model, including diagonal disorder, dipolar intersite coupling, and a density-of-states (DOS) of molecular site excitations. Special emphasis has been placed (i) on the analysis of luminescence lifetime distributions 〈φ(τ;ε̂,τ0)〉 which directly map out the spectra of hopping modes of energy-cascading neutral excitations, and (ii) on the rigorous evaluation of (radiationless) transfer population from high-energy subensembles to low-energy tail states of the DOS. We quantitatively show that the absence of significant rise terms in the S0ν=0←S1ν=0 luminescence transition of low-lying tail states is due to the spectral superposition of the S0ν=1←S1ν=0 luminescence decay from states near the center of the DOS which, owing to the predominantly positive amplitudes of their ultrafast decay channels, destructively interfere with the negative amplitudes (luminescence rise terms) of migratively prepared bottom states. Finally, the asymptotic branch of the luminescence lifetime distribution 〈φ(τ;ε̂=2.4 eV,τ0)〉 has been reconstructed in an unbiased fashion by inverting the high-precision, picosecond, single-photon counting luminescence data at ε̂=2.4 eV with the help of a regularized exponential series methodology. The measurements combined by numerical computing are in accord with the molecular picture of exciton relaxation in PPV in which neutral, localized excitations undergo predominantly incoherent excitation energy transfer among sites of fluctuating self-energies related to segmental subunits with distributed π-bond conjugation lengths.

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.

Space charge formation and relaxation in ion-bombarded poly(imide) Kapton films

The space charge formation and relaxation in poly(imide) Kapton thin films, deep-implanted with heavy ions in the MeV amu −1 range, have been studied by on-line measurements, with the pressure wave propagation method. The measured space charge depth profiles correspond to the damage depth profiles due to the nuclear collisions. The space charge buildup in the bulk thus seems to be linked to some end-of-range defects acting as trapping sites. Ionization-induced defects seem to play a negligible role in the formation of the bulk space charge.

Molecular Mobility in Para-Substituted Polyaryls. 3. Low-Temperature Dynamics

The low-temperature mechanical relaxations in poly(oxy-1,4-phenyleneoxy-1,4-phenylenecarbonyl-1,4-phenylene [poly(aryl ether ether ketone, PEEK], poly(thio-1,4-phenylene) [poly(phenylene sulfide), ...

Segmental Relaxation in Poly(dimethylsiloxane)

Segmental Relaxation in Poly(dimethylsiloxane)

Excited state dynamics and relaxation of poly(methylphenylsilane) film

We report here measurements of the photo-luminescence spectra and the time response of the σ-conjugated polymer, poly(methylphenylsilane), film at 2 K. The site-selective luminescence and the time response are discussed in terms of the exciton band model and localized states in disordered system.

Piezoelectric relaxation of P(VDF-TRFE) copolymer

Abstract The dielectric and piezoelectric relaxations of poly(vinylidene fluoride-trifluoroethylene)(75/25) copolymer over the temperature interval of-100 to 60 °C are reported in this paper. A two-dimension model is proposed to explain the experimental results. The expressions of piezoelectric d constants from the direct and the converse effects, dielectric constant and the elastic compliances are derived as functions of the physical properties of crystalline and amorphous regions. The numerical calculations of the model are in good agreement with the experimental results.

Molecular Nature of the .beta. Relaxation in Poly(methyl methacrylate) Investigated by Multidimensional NMR

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTMolecular Nature of the .beta. Relaxation in Poly(methyl methacrylate) Investigated by Multidimensional NMRK. Schmidt-Rohr, A. S. Kulik, H. W. Beckham, A. Ohlemacher, U. Pawelzik, C. Boeffel, and H. W. SpiessCite this: Macromolecules 1994, 27, 17, 4733–4745Publication Date (Print):August 1, 1994Publication History Published online1 May 2002Published inissue 1 August 1994https://doi.org/10.1021/ma00095a014RIGHTS & PERMISSIONSArticle Views1295Altmetric-Citations260LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (3 MB) Get e-Alerts Get e-Alerts

Electron spin resonance study of γ‐irradiated poly(methacrylic acid)

AbstractLow temperature relaxations in poly(methacrylic acid) (PMAA) have been studied by electron spin resonance (ESR) spectroscopy. The observed 8 line ESR spectra of irradiated PMAA in the temperature range 77‐300K (LNT‐RT) is attributed to the free radicals of the type ∼ CH2CHCH3. Assignment of ESR spectra to free radicals has been made on the basis of magnetic parameters employed to simulate ESR spectra at different temperatures. Further, ESR spectra below LNT have been simulated, using the set of parameters employed to simulate the experimental spectrum at LNT. Magnetic parameters of the ESR spectra at LNT and below LNT indicate γ‐ and δ‐relaxations of PMMA chains. © 1994 John Wiley & Sons, Inc.

α Mechanical relaxation in poly(vinyl chloride): effect of ageing and crosslinking

Two types of poly(vinyl chloride) (PVC) samples are studied by mechanical spectroscopy, namely (i) a commercially available PVC that is submitted to different heat treatments and (ii) the same PVC but slightly crosslinked in order to compare the effect of this chemical crosslinking with the effect of the physical crosslinking obtained after ageing at T> T g. The comparison is made by means of a theoretical approach for the non-elastic deformation of polymers.

Structural Relaxation of Poly(methyl methacrylate)

The kinetics of structural, relaxation of poly(methyl methacrylate) (PMMA) is studied and it is shown that the activation energy of the process depends on the structural temperature Tf. A new method is proposed allowing the measurement experimentally of how the activation energy decreases with increasing Tf. The dependence of the glass transition temperature Tg on the heating rate q+ is investigated for samples that were previously cooled down at a rate q = 12.5 K/min. It is found that the activation energy for structural relaxation is E = (250 ± 10%) kJ/mol at T=386K. On the other hand, the value of the apparent ”activation energy” obtained from the dependence of the structural temperature Tf on the cooling rate q- is considerably higher. This is an indication that the activation energy depends on Tf. We found that the temperature dependence of the activation energy of PMMA in the vitrification interval is E(T) = 250–670* (386/T-1) kJ/mol.