Relaxation Processes In Polymers Research Articles

Micromorphology and Relaxation Processes of Low Density Polyethylene Probed by Fluorescence Spectroscopys

Fluorescence spectra of molecular guests at several temperatures are useful technique to study several types of polymer properties. In particular, it has been often employed to study polymer relaxation processes either in static or in dynamic conditions. In this work some applications of the steady-state fluorescence spectroscopy of different guests in polyethylene and in particular reports the photophysical behavior of 1,3-di(1-pyrenyl)propane (1Py(3)1Py) sorbed in low density polyethylene are shown. This molecular probe differs of pyrene by its ability to form intramolecular excimer species and we discuss the temperature dependence of this formation. In this study we recorded simultaneously the fluorescence spectra and the differential scanning calorimetry (dsc) traces. The relative dependence of fluorescence intensities on temperature of the higher energy vibronic band at 367 nm, of the isolated choromophore emission and of the excimer emission were discussed and associated with the polyethylene polymer relaxation processes. The influence of the macroscopic stretching upon the polymer relaxation processes and the fluorescence emission was also investigated and compared with previous results.

Cooperative Character of the Relaxation Processes in a Side-Chain Liquid Crystalline Polymer

Side-chain liquid crystalline polymers (SCLCP) are complex materials that exhibit peculiar relaxation processes. Particularly, the assignment of the α-relaxation has been controversial. In this work, its attribution to the segmental dynamics within the backbone is strengthened, despite possibly being highly influenced by special modes of motions within the mesogenic groups in the sidechains. The discussion is based on results obtained by dielectric relaxation spectroscopy, TSDC, DSC, DMA, and TMA. It was found that the motion within the spacers may influence the α-process dynamics, broadening the distribution of relaxation times. The analysis of the fragility locates the α-process in the fragile region of the fragile/ strong Angell scale. The δ-relaxation of SCLCPs is cooperative and is found to have similar features to the α c -relaxation seen in flexible semicrystalline polymers.

Variable-temperature UV and Raman study of complicated thermochromic phase transition of order-disorder type in poly(di-n-decylsilane) [(n-C10H21)2Si

A complicated thermochromic phase transition (PT) of order—disorder type in poly(di-n-decylsilane) has been studied in detail by variable-temperature UV and Raman spectroscopy. Not less than five polymer modifications were shown to participate in this PT. Above the equilibrium PT temperature Tc (∼60 °C), the polymer exists as a hexagonal columnar mesophase (HCM) with a disordered silicon backbone and disordered side chains. PT proceeds not abruptly but over a temperature range 60—5 °C and does not reach completion, being stopped by glassification. At room temperature, a coexistence in the polymer of HCM and a few crystalline phases with ordered backbone is observed. The latter are separated in space, what was evidenced by Raman micromapping of a polymer film. The temperature intervals of existence of different ordered phases also do not coincide, this enables one to identify in the UV and Raman spectra the components, corresponding to the modifications possessing trans (anti) and, supposedly, AD+AD– main chain conformations. As the relaxation processes in this comb-like polymer occur slowly, the quantitative ratio of different phases is not a single-valued function of temperature but depends on the thermal prehistory of the sample. Ordering of long side chains on cooling was demonstrated by the Raman method.

Broadening of the fluorescence spectra of hydrocarbons in ethylene-vinyl acetate copolymers and the dynamics of the glass transition

The steady-state fluorescence of two fluorescent condensed aromatic hydrocarbons (anthracene and pyrene) with different fluorescence decay rates (a few nanoseconds and hundreds of nanoseconds, respectively) were employed to investigate the relaxation processes of several random ethylene-co-vinyl acetate copolymers (EVA). Copolymers with various comonomer compositions (EVA-9, EVA-18, EVA-25 and EVA-33) were studied. Data for the EVAs were compared with the same aromatic molecules in low-density polyethylene (LDPE), high-density polyethylene (HDPE) and poly(vinyl acetate) (PVAc) homopolymer models. The fluorescence rate constants were measured by single photon counting at room temperature. The wavelength dependence of the emission spectra (the edge-excitation red-shift (EERS)) were studied at room temperature and at 77 K and the data were interpreted based on the correlation of the differences of decay rates for both molecules and the time correlation of the polymer relaxation processes. The spectral broadenings (full-width at half-maximum (FWHM)) of the fluorescence spectra were studied from 20 to 400 K, which included temperature ranges above and below the glass transition, and these broadenings are compared.

Phase transitions and relaxation processes in ethylene-vinyl acetate copolymers probed by fluorescence spectroscopy

The steady-state fluorescence of pyrene and anthracene are used to investigate the relaxation processes of several random ethylene-co-vinyl acetate copolymers, EVA, with defined comonomer compositions (EVA-9, EVA-18, EVA-25, EVA-33 and EVA-40). The temperature of the relaxation processes are compared with those of low-density polyethylene (LDPE) and poly(vinyl acetate) (PVAc). The polymer relaxation processes are assigned to T g=300–310 K (glass transition temperature of the PVAc); T α =270–300 K (relaxation processes of the ethylene units present in LDPE and EVA); T g=220–250 K (glass transition of the LDPE and of the EVA); T γ or T β =160–190 K (relaxation processes of interfacial defects of methylenic chains of LDPE and rotation of the acetate group of the PVAc and the EVA); and T γ =90–130 K (relaxation processes of small sequences of methylene units of LDPE and end groups of PVAc). An Arrhenius-type function was employed as an attempt to represent the experimental data of fluorescence intensity versus temperature above the γ-relaxation temperature. As obtained with other techniques, there is not a simple relationship between the polymer relaxation processes and the vinyl acetate content that can be explained by the morphology in these copolymers.

Epifluorescence studies and secondary relaxation processes in immiscible blends of polybutadiene‐poly(2‐vinyl naphthalene) by fluorescence spectroscopy

AbstractThe development of the morphology of polybutadiene/poly(2‐vinyl naphthalene) blends in five proportions by mass (5, 10, 50, 90, and 95%, w/w) is studied by epifluorescence and scanning electron microscopy (SEM) techniques. The phase separation process of these immiscible polymers produces a primary morphology that is formed by dispersed droplets in a continuous matrix. In the sequence a secondary phase separation inside the primary domains is detected by epifluorescence microscopy of the intrinsically fluorescent domains. Secondary phase separation is confirmed by SEM fracture surface analysis. The relative size of the droplets and the matrix composition depend on the proportion of the components of the blends. The mechanism of the phase separation process is preferentially by nucleation growth for either primary or secondary phase separation processes. Secondary relaxation processes involving the poly(2‐vinyl naphthalene) phase are studied by fluorescence spectroscopy. The profile of the steady‐state excimer fluorescence of poly(2‐vinyl naphthalene) with the temperature in the blend differs from that of the isolated homopolymer and is explained by the contribution from the interface to the radiationless deactivation. The Arrhenius plot for the temperature dependence exhibits slope changes that are related to the polymer relaxation processes. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 1637–1649, 2002; DOI 10.1002/app.10389

Fine Structure and Thermorheological Complexity of the Softening Dispersion in Styrene-Based Copolymers

The segmental and terminal relaxation processes of polystyrene, styrene−acrylonitrile, and α-methylstyrene−acrylonitrile copolymers have been investigated by means of both dynamic-mechanical and dielectric spectroscopy in the linear response region. The temperature dependence of the average relaxation time τ of the two processes follows a Vogel−Tamman−Fulcher (VTF) equation: τ ∝ exp[B/(T − T∞)]. Nevertheless, the segmental and terminal relaxations exhibit appreciably different VTF parameters. This vitiates time−temperature superpositioning in the segmental relaxation temperature region, giving rise to complex thermorheological behavior. As first shown by Plazek et al., this finding further confirms the Donth and Ngai models. Peculiar relationships between the VTF parameters of the segmental and terminal relaxation of the same polymer and of the same relaxation process of different polymers are pointed out. These relationships reveal general features of the VTF equation. A comparison between dynamic-mecha...

The effect of thermal treatment on the structure and relaxation processes of olefinic polymers synthesized with metallocene catalysts

The relationships between the structure and properties have been established in several copolymers of ethylene and 1-hexene, synthesized by a metallocene catalyst. A dependency with composition and thermal history has been found. The branches cannot be incorporated into the orthorhombic crystal lattice, and consequently, structural parameters such as crystallinity and crystal lattice constants are considerably affected as 1-hexene content increases in the copolymer. The β-relaxation does not appear in the homopolymer. In the copolymers, it is shifted to lower temperature and its intensity is increased as 1-hexene content raises. On the other hand, the α-mechanism associated to motion within the crystalline regions is also moved to lower temperatures but its intensity is diminished as 1-hexene content increases in the copolymer.

Photostability of xanthene molecules trapped in poly(vinyl alcohol) (PVA) matrices

Photobleaching reactions of fluorescein and Rose Bengal molecules dissolved in PVA matrices were studied as a function of the temperature. We used and compared two theoretical approaches to describe these photobleaching kinetics. Photobleaching quantum yield calculations were done using an adapted model (B-model) for polychromatic excitation that allowed us to estimate the average number of photons absorbed by each dye molecule before its bleaching. The other model uses exponential functions allowing us to calculate rate constants and activation energy. A slower photobleaching process was obtained for Rose Bengal sorbed in PVA matrices compared with fluorescein. We also obtained that the Rose Bengal photobleaching process is always single-exponential kinetic independent of the polymer relaxation process. Nevertheless, fluorescein photobleaching reaction is bi-exponential process at higher concentrations and it is strongly dependent on the polymer relaxation processes. These results were interpreted in terms of the different efficiencies among the processes involving quenching of the lowest triplet state.

Temperature-domain analysis of primary and secondary dielectric relaxation phenomena in a nonlinear optical side-chain polymer

The investigation of dipole relaxation processes in nonlinear optical (NLO) polymers containing chromophore dipoles with large hyperpolarizabilities is important for optimizing the poling process and for predicting the long-term stability of the poling-induced order. The primary or α relaxation is difficult to assess by dielectric spectroscopy in polymers with high glass transition temperature due to thermally induced chromophore degradation. A fast experimental procedure is developed for the investigation of dielectric relaxation processes in NLO polymers, without severely inducing chromophore degradation. The procedure is based on the measurement of the dielectric function ε̃(T)=ε′(T)−iε″(T) at a few frequencies from 30 Hz to 30 kHz, while heating the polymer at a constant rate. The complex plane representation of the temperature-dependent dielectric function is used to determine the distribution of relaxation times, while the temperature-dependent mean relaxation time τα(t) is numerically determined from the dielectric loss ε″(T). With only three decades in frequency, information on five decades in time or 90 K in temperature is gained above the glass transition temperature. The strong α and the weak β relaxation below the glass transition are separately investigated by thermally stimulated depolarization after a suitable two-step poling procedure. The method has been applied to a typical polyimidelike side-chain nonlinear optical polymer with modified Disperse Red 1 chromophores. Even below room temperature, a γ-relaxation process is observed, demonstrating significant mobility of the chromophores much below the glass transition. From the results of thermally stimulated depolarization it is concluded that the initial fast decay of the electro-optical response to a temporally stable value is related to the partial depolarization caused by the β relaxation.

Translocation of fluorescent probes upon stretching low-density polyethylene films. Comparison between ‘free’ and covalently-attached anthryl groups

Fluorescence from (9-anthryl)methyl groups (AN-C) attached covalently to polymer chains has been used to study secondary α-, β- and γ-relaxation processes in two types of stretched and unstretched low-density polyethylene (LDPE) with different degrees of crystallinity and frequencies of branches in the temperature range 55–400 K. Plots of total fluorescence intensity or fluorescence intensity ratios at different wavelengths versus temperature indicate that the efficiency of radiationless deactivation processes is very sensitive to film stretching (i.e. micromorphological changes induced by macroscopically applied stress) and to secondary relaxation processes of LDPE (i.e. micromorphological changes induced by temperature). Also, since the efficiencies of intermolecular radiative energy transfer processes in LDPE suffered by AN-C and ‘free’ anthracene (ANH), doped into native films, are much different, the mode of ‘reporter’ incorporation into a film and its treatment thereafter can establish differing local concentration gradients. The results suggest that, although stretching reduces, on average, the amount of free volume available at guest sites and partially orients both AN-C and ANH probes, the AN-C (due to their being constrained to their original positions of incorporation) are more influenced by polymer relaxation processes, and the excited singlet states of both probes are more sensitive to temperature in stretched films than in unstretched ones. Overall, comparisons between the behaviour of the covalently-attached and free lumophores provide insights into how external perturbations, such as stress and temperature changes, affect specific aspects of polymer microstructure and mobility and what is the influence exerted by subtly differing environments on the photophysical properties of the probe.

Aquisição de tempos de vida de fosforescência com resolução de fase e sua aplicação ao estudo de relaxações em polímeros

We describe in this work a simple experimental set up to perform time dependent luminescence experiments in time scales from mili-seconds to seconds, based on the phase resolution of the emission signal. This system is composed by modulation of a continuous light source with an external chopper controlled by a lock-in amplifier. We exemplified the utility of the system in studies of phosphorecence emission using benzophenone dissolved in polystyrene and since the phosphorescence intensity and lifetime are temperature-dependent processes, we also studied polymer relaxation processes in the temperature range from 20 to 400K. A software that drives the spectrofluorimeter and controls both the cryosystem and the lock-in amplifier was developed.

Relaxation Processes in Nonlinear Optical Polymers: A Comparative Study

Relaxation processes in nonlinear optical (NLO) polymers with glass transition temperatures in the range of 125 °C < Tg < 176 °C have been studied. The relaxational mechanisms of these side- and main-chain polymers have been investigated above and below the glass transition by second-harmonic decay, dielectric relaxation, and differential scanning calorimetry measurements, and the results obtained have been compared with a variety of nonlinear optical polymer systems cited in the literature. The nonexponential relaxation in both the time and frequency domain was modeled by the Kohlrausch−Williams−Watts function whereas the nonlinear relaxational behavior of these polymers was modeled in terms of the Tool−Narayanaswamy description of the glassy state using the Adam−Gibbs expression for the relaxation time. This procedure allows for the nonlinear extension of the liquid equilibrium state behavior into and below the glass transition region with an accurate prediction of the relaxation times over more than 15 orders of magnitude in time. Time−temperature scaling of the relaxation times with respect to Tg/T as the relevant scaling parameter is observed below the glass transition. The effect of annealing was investigated using differential scanning calorimetry with the result that a single set of parameters is sufficient to describe a wide range of thermal histories with as well as without annealing. Optimum annealing temperatures/annealing times for best orientational stability in NLO polymers were calculated according to the same relaxation scheme.

Atomistic modelling study of relaxation processes in polymers: the β-relaxation in polyvinylchloride

Activation energies related to the β-relaxation process in polyvinylchloride were calculated from atomistic simulations. Pure Boyer and Schatzky crank-shaft type motion seem idealised situations which rarely contribute to relaxation in amorphous polymers. However, the activation energies accompanying other types of local conformational transitions were found in the appropriate range to account for the β-relaxation. Moreover, the present results suggest that local conformational transitions may account for both the β-relaxation as well as the α-or glass transition in polyvinylchloride, whereas motions related to the glass transition involve considerable distortion of the environment next to the conformational transition. The results obtained point out possible answers to long-standing questions concerning the details at the molecular level of the glass-transition and the secondary relaxation mechanism in an amorphous polymer.

ガラス状高分子の非晶構造と緩和 ガラス形成高分子の緩和過程と不均一性

ガラス状高分子の非晶構造と緩和 ガラス形成高分子の緩和過程と不均一性

Dynamic structure factors due to relaxation processes in glass-forming polymers

We report neutron spin echo experiments on the momentum ( Q) and time ( t)-dependent dynamic structure factor S( Q, t) from the glass-forming polymers polyisobutylene (PIB) and polybutadiene (PB). Performing measurements in a Q-range encompassing the first and second structure factor peaks, we are able to separate inter and intrachain relaxations and to assign the secondary Johari-Goldstein β-relaxation to an intrachain relaxation. While for PB this process exhibits average motional amplitudes of about 1.5 Å, the underlying jump distances for PIB are much smaller (0.6 Å). The structural α-relaxation is characterized as an interchain process displaying anomalous diffusion. Its temperature dependence thereby agrees with that of the viscous flow. The data bear evidence that a polymer segment undergoes primary and secondary relaxation in a statistically independent way.

Local relaxation of polymers in dense media: Cooperative kinematics theory and applications

AbstractCooperative kinematics (CK) theory and its recent applications are presented. CK theory has been developed as an efficient approach for predicting the mechanism of segmental relaxation processes in bulk polymers. The theory aims at determining the most probable changes in atomic coordinates, occurring collectively in response to a given, external or localized, structural perturbation. The basic postulate is the minimization of the energy change involved in the overall conformational motion, which naturally yields the optimal pathway of cooperative relaxation. Attention has been confined here to the collective motions accompanying the rotational transitions of backbone bonds in polyethylene (PE) and polybutadiene (PB). The strong dependence of the mechanism of motions on the geometry of the repeat unit and on chain connectivity is emphasized. The differences in the types of correlated transitions operating in different structures, the effective conformational energy changes triggered by bond rotational jumps, and the correlation lengths for particular bond isomerizations are analyzed. The reorientations of CH bond vectors in cis‐ and trans‐PB are also examined to explain the shorter correlation time of cis units, compared to trans, detected by NMR. A good agreement between various CK predictions and results from molecular dynamics (MD) simulations is obtained. The fact that CK calculations are at least two orders of magnitude faster than MD simulations invites attention to the utility of the CK method as an efficient tool for elucidating the pathway of motion in complex systems.

Cooperative relaxation processes in polymers

The basic mode of relaxation in polymer molecules involves the rotation of a conformer, with a time scale of the order of picoseconds. This fast relaxation process, however, cannot take place easily in the condensed state crowded by densely packed conformers, necessitating the intermolecular cooperativity among them. The domain of cooperativity grows at lower temperatures, towards the infinite size at the Kauzman zero entropy temperature, though the system deviates from the equilibrium as the glass transition intervenes at about 50°C above that temperature. From the temperature dependence of the domain size, the well-known Vogel equation is derived, which we consider is the basic origin of the empirical WLF and free volume equations. The molar volume is a crucial factor in determining molar free volume. The molecular weight of a conformer with a density correction, therefore, can be used as a parameter in determining the T g of liquids and amorphous polymers. A larger size conformer means a higher glass transition temperature. A conformer at the chain end, on the other hand, has a higher enthalpy, i.e., a smaller effective size for that conformer. If a conformer is reacted trifunctionally, the resulting conformer is a combination of the two conformers and T g increases, but a further addition of another conformer to that branch point reduces the average size of the conformers, so T g decreases. The model for cooperative relaxation can be directly applied to predicting T g S from the chemical structure of polymers, the kinetics and T g S of thermosets during the crosslinking reaction, the distribution of relaxation times from the domain size distribution at a given temperature, the dynamics of the physical aging process, and other complex behaviors of polymers and liquids near the glass transition temperature.

Photo-induced ultrafast relaxation process in polymers: effect of lattice fluctuation

Our simulation shows that the lattice fluctuation has an essential effect on the photo-induced ultrafast relaxation process in a polymer and the relaxation process deviates significantly from that given in previous works which overlooked the lattice fluctuation.

Investigation of Dipolar Relaxation Processes in a Side-Chain Nonlinear Optical Polymer

AbstractAmorphous thin films of a typical side-chain nonlinear optical polymer were investigated by means of dielectric spectroscopy and thermally stimulated depolarization current (TSDC) within the temperature range -40°C to 230°C. Three distinct relaxation processes, labelled α, β and γ were observed. The α-relaxation, associated with the glass transition, is well described by the Vogel-Fulcher relation, while the sub glass transition β and γ-relaxation follow Arrhenius relations. The advantage of TSDC over dielectric spectroscopy is demonstrated for the investigation of the weak β-relaxation in the side-chain polymer. Furthermore, the β -relaxation is responsible for the initial, fast decay of the electro-optical response of the side-chain polymer at room temperature.