Relaxation Processes In Polymers Research Articles

Highly fluorinated poly(1,3,4-oxadiazole-ether)s. structural, optical and dielectric characteristics

New fluorinated poly(1,3,4-oxadiazole-ether)s were prepared by nucleophilic aromatic substitution technique of an activated aromatic difluorinated compound with a bisphenol containing three trifluoromethyl groups, namely 1,1-bis(4′-hydroxyphenyl)-1-(3′,5′-ditrifluoromethylphenyl)-2,2,2-trifluoroethane, 1, or with a mixture of 1 and 9,9-bis(4-hydroxyphenyl)fluorene. FTIR and NMR spectroscopy were used to confirm the structure of the polymers. The polymers were easily soluble in organic solvents and could be processed in flexible thin films. The film surfaces exhibited hydrophobic characteristics, as it was determined by contact angle measurements. The polymers had high thermal stability, up to 440 °C, and glass transition temperature in the range of 202–237 °C. They showed fluorescence in the blue region, both in solution and in solid state. The change in electronic absorption and emission spectra of one of the polymer in neat and binary solvent mixtures has been studied. Broadband dielectric spectroscopy was used to investigate the relaxation processes in polymers. The dielectric constant of the polymers was in the range of 2.84–2.96, at 100 °C and 10 kHz, and decreased with the increase of fluorine content.

Study on enthalpy relaxation of glassy polystyrene using a structure-dependent Kohlrausch stretch exponent combined with coupling model.

In this paper the enthalpy relaxation of polystyrene (PS) was restudied using a structure-dependent Kohlrausch stretch exponent β with incorporation of a coupling model (CM). The structure dependence of β is described in 3 semi-phenomenological equations. The temperature and structure dependence of the relaxation time of the Johari-Goldstein (JG) relaxation (τ JG) is presented using the traditional Tools-Narayanaswamy-Moynihan (TNM) and Adam-Gibbs-Vogel (AGV) equations. The fitting results of heat capacity data are much better than the conventional TNM and Adam-Gibbs (AG) models when the structure dependence of β is described using an exponential equation and τ JG is calculated using the AGV equation, although there are one fewer fitting parameters in the new model. The results indicate that both the structure dependence of β and the CM model may play considerable roles in the investigation on the structure relaxation process in polymers around the glass transition.

Modeling attempt of shape memory performance of epoxy resin with experimental methods

Epoxy resin samples with low cross-linking density were prepared, and their shape recovery rate and glass transition were studied. The results showed that the shape fixity ratio was over 99 % for all of the samples. Without constraint, the final recovery ratio was approximate to 100 % for all of the samples. The temperature with rapid recovery rate for different samples changed in accordance with Tg. Under a constant temperature, the folding angle for the samples of EP80 decreased with the increase in time rapidly, at first, and then tended to level off. Curves can be fitted with the formula of \( y + A_{0} = A/(1 + \exp ((t - t_{0} )/\tau )) \) with R2 higher than 99.9 %. The fitting results demonstrated that the value of τ decreased from 15.1 to 6.39 when the temperature increased from 88 to 98 °C for EP80. The recovery rate decreased a little by extending the holding time from 10 to 60 s. By keeping the testing temperature constant, glass transition temperature (Tg) decreased with the increase in curing agents, and the value of τ reduced with the decrease in Tg. Usually, when temperature was close to Tg, segments of macromolecules were idle to move, and then, the relaxation process, τ, was lengthened and the shape recovery rate decreased accordingly. In a word, τ showed the similar change rules with that of relaxation process of polymers; therefore, the shape recovering process could be predicted with the model of relaxation time and modulus according to relaxation formulas.

Study of polarization parameters effect on dipolar relaxation in epoxy-based polymer using thermally stimulated depolarization current

In this paper, dipolar relaxation processes in epoxy-based polymer were investigated under various polarization conditions, using thermally stimulated depolarization current (TSDC) technique. The TSDC spectrum of un-poled epoxy polymer reveals an unusual negative current, associated with a thermally generated charge carriers, which were due to impurities ionization and structural changes mainly at high temperature (above T g ). β and α dipolar relaxations were detected respectively around 115 °C and 154 °C in conventionally poled sample at T P =110 °C and E P = 3 kV/mm. Dipolar relaxation parameters were evaluated using two techniques: (1) theoretical decomposition of global complex TSDC spectrum, using Bucci-Fieschi expression based on single Debye process, (2) experimental procedure based on selective polarization, commonly known as windowing polarization (WP). The obtained values are T P dependent: relaxation time decreases and activation energy increases when polarization temperature increases. An ω relaxation peak due to water molecule departure was detected around 135 °C, using WP techniques for T P =100 °C Unlike dipolar relaxations, its position is independent of polarization temperature. Complementary thermal analysis investigations by differential scanning calorimetric (DSC) and thermogravimetric analysis (TGA), were made to support some given conclusions.

The effect of temperature gradients on the parameters of Adam–Gibbs model

The most commonly used method for studying relaxation processes in polymers is monitoring the enthalpy recovery by measuring differential scanning calorimetry (DSC) heat capacity data. Because of its great influence on heat capacity data, the effect of temperature gradients in the sample on relaxation dynamics study should be investigated. A model for calculating temperature profile in a DSC sample was proposed previously by combining Fourier's law of heat conduction with the Tool–Narayanaswamy–Moynihan (TNM) model. This model was examined using PS samples with different thicknesses. It was found that the temperature gradient influenced optimized values of the kinetic parameters. The thermal history dependence of model parameters is still prominent, however. Here the Scherer–Hodge (SH) equation (based on the Adam–Gibbs model) instead of the TNM equation, is used for describing the relaxation time–structure–temperature relationship in the temperature profile calculation. It is found that the influence of temperature gradients on the thermal history dependence of the Adam–Gibbs (AG) model parameters is greater than that of the TNM model parameters. But the thermal history dependence of the AG model parameters still exists when the thermal lag effect was incorporated.

Heterogeneity of spiral wear patterns produced by local heating on amorphous polymers

We report on spiral wear patterns produced at constant angular velocity by hot tip atomic force microscopy (HT-AFM) on surfaces of two common amorphous polymers: polystyrene (PS) and polymethylmethacrylate (PMMA). Topography of these patterns is obtained with regular AFM cantilevers. Topography cross-sections taken from a center of each spiral at a given azimuthal angle Θ relate changes of surface corrugation hcorr with tangential velocity v of a thermal cantilever. Polymer wear is characterized by a power law hcorr(v) = α(v/vmax)−β, which yields a pre-factor α and an exponent β. Below the glass transition temperature Tg, α is polymer specific and β varies weakly between similar conditions and samples. Variations of β are hypothesized to reflect polymer relaxation processes, which are expected to vary only weakly between amorphous polymers. At and above Tg, α approaches initial thermal tip indentation depth within a polymer, β plummets, and a power law relation of hcorr with v diverges. These results are explained by heterogeneous wear around Tg due to a local nature of glass transition. At all studied temperatures, additional wear heterogeneities are found as due to position on the polymer and Θ. Variations of α and β with position on the polymer are found to be only marginally larger then uncertainties of the thermal tip–polymer interface temperature. Variations of α and β with Θ are found to be largely influenced by buckling of thermal cantilevers traveling in a spiral pattern.

The water vapour adsorption–desorption behaviour of naturally aged Tilia cordata Mill. wood

The effect caused by the natural ageing of wood on the sorption properties during two consecutive sorption cycles of historical Tilia cordata Mill. wood has been investigated and compared with a reference sample. Differences were found in the sorption isotherm between the first and second sorption cycles for most of the samples and also between the reference and historical wood.All samples exhibited sorption hysteresis, but with important differences in behaviour observed with the historic wood samples. It was interesting to note that the oldest historic wood sample did not display any discernible difference between the first and second sorption cycle throughout much of the hygroscopic range and also exhibited absolute hysteresis values essentially identical with the reference sample. These results indicate that the sorption behaviour of wood is dependent upon the previous exposure of the wood to atmospheric relative humidity. The sorption kinetics was also analysed in terms of the parallel exponential kinetics (PEK) model, and excellent fits to the data were obtained. The PEK model describes the dynamic sorption behaviour in terms of a fast and slow kinetic process and important differences in behaviour for these two processes were found. It is thought that the fast process is associated with a physical diffusion phenomenon, while the slow kinetics process is considered to be associated with cell wall matrix polymer relaxation processes.

Solid and melt-state 1H NMR studies of relaxation processes in isotactic polypropylenes

Relaxation processes in metallocene and Ziegler–Natta isotactic polypropylenes were studied using 1H MAS NMR spectroscopy. 1H MAS NMR spectra and laboratory-frame spin-lattice relaxation times T1(1H) were measured within the temperature range 30–190 °C, which covers the glass transition relaxation and melting processes of both polymers. Splitting of the peak related to protons in amorphous regions of the studied samples into three sharp peaks at elevated temperatures made it possible to determine the spin-lattice relaxation times T1(1H) for particular iPP proton groups. The melt-state NMR spectra of ZN-iPP display three sharp peaks with three additional weak peaks positioned on the less shielded side. Entanglements of ZN-iPP chains are suggested as a possible source of these additional peaks. The spectra of m-iPP indicate substantially fewer entanglements due to its lower molecular weight compared with that of ZN-iPP. The temperature dependences of the relaxation times T1(1H) relating to specific groups of ZN-iPP were shown to reach minima associated with the motions of amorphous chain segments (glass transition relaxation), which are very close to the melting temperature and minima associated with the melting process. Each of the T1(1H) temperature dependences for m-iPP shows only one minimum associated with the melting process. When the particular relaxation times T1, min relating to the minima that occur above the melting temperatures were considered, and these relaxation times were compared for the same proton groups in different samples, significant differences in the relaxation times were observed between samples. Polymer chain motion was more restricted in melted ZN-iPP than in m-iPP, as inferred from the T1, min values.

Formation and relaxation of the elastic strain generated by photocuring in polymer blends monitored by Mach–Zehnder interferometry

The deformation associated with the photocuring reaction of an anthracene-labeled polystyrene (PSA) in a miscible blend with poly(vinyl methyl ether) (PVME) was in situ monitored at ambient temperature by using Mach–Zehnder interferometry (MZI).The curing kinetics of a PSA/PVME (30/70) blend was followed by observing the decrease in the absorption of the anthracene moieties upon irradiation using UV–visible spectroscopy. It was found that both the kinetics of the curing and deformation processes can be well described by the modified Kohlrausch–Williams–Watts (KWW) empirical function. The experimental results reveal a strong correlation between the photocuring and deformation processes of the blend. From the dynamic mechanical measurements performed for samples with different irradiation times, it was also found that the deformation process of the blend observed by MZI is controlled by the on-going cross-link-induced glassification process of the mixtures. These experimental results suggest that MZI is a useful technique to monitor in situ the local deformation and relaxation processes in photoreactive polymers.

Synthesis, photophysical properties and thermal relaxation processes of carbazolyl-labeled polysiloxanes

The photophysical properties of a fluorescent silicone prepared by a hydrosilylation reaction from poly(dimethylsiloxane-co-methylhydrogensiloxane) terminated by dimethylhydrogensilyloxy groups, poly(dimethylsiloxane-co-methylvinylsiloxane) terminated by dimethylvinylsilyloxy groups and 9-vinylcarbazole, as the luminescent group, have been studied under steady-state fluorescence and time-resolved conditions. At low temperature both steady-state fluorescence and phosphorescence spectra of isolated species are observed. Phosphorescence is quenched at temperatures higher than 150K indicating that the environments where the carbazolyl moieties are located are very flexible, as expected for silicones. Fluorescence decays and phosphorescence (at 77K) showed typical carbazolyl lifetimes and no rise-time was observed, which is consistent with the absence of excimer emission. The temperature dependence of the emission can be used to determine the polymer relaxation processes and transitions (Tγ=80–100K, Tg=130–150K and Tm=220K), these last two comparable with those observed by differential scanning calorimetry.

Relaxation processes in polymers filled with nanoparticles

We report on the influence of filling poly( n-alkyl methacrylates) with 12 nm in diameter Aerosil nanoparticles on relaxation processes and glass transitions, as investigated by dielectric spectroscopy and differential scanning calorimetry. The agglomeration of Aerosil particles in polymer formed a three-dimensional network dividing the polymer into random domains. The filling of polymers with nanoparticles had at least two effects: random field effects imposed by the network and a large particle surface area that imposed an interfacial effect on the polymer. In order to study the role of interfacial effects on the nanoparticle–polymer interface we used two types of Aerosil particles: with hydrophilic and hydrophobic surfaces. We found that the relaxation times of α-relaxation process (sensitive to glass transition) in polymers filled with both hydrophilic and hydrophobic particles were faster than those of bulk polymers measured at the same temperatures. This might be interpreted as reduction of the glass transition temperature in the filled polymers. Semi quantitatively this reduction is in accordance with the Vogel–Fulcher data analysis of the temperature dependencies of the α-process relaxation times. The β-relaxation process was slower for bulk than for filled polymers, but the activation energies of this process were about the same for pure and filled polymers.

Short‐term and long‐term performance of thermosets exposed to water at elevated temperatures

AbstractThe water‐transport, mechanical, and chemical‐structure changes in various vinyl ester, novolac, and urethane‐modified vinyl ester thermosets exposed to water at 50 to 95°C for times up to 1000 days have been studied within the framework of a larger study of osmotic blistering in fiber reinforced plastics (FRP) process components. The water sorption saturation concentration did not reach a steady‐state value but gradually increased in many cases upon long‐term exposure. The diffusion coefficient was not significantly affected. Infrared spectroscopy and gas chromatography‐mass spectrometry indicated that the net mass loss from the thermosets on immersion in water was due to the leaching of non‐reacted styrene, monomer, and additives. It is suggested that this, together with polymer relaxation processes (as measured on specimens under tension in water at 80°C), is the primary reason for the time‐dependent increase in the water saturation concentration. Infrared spectroscopy indicated that, even at the highest temperatures, hydrolysis of the polymer ester groups was small. Correlations were found between the styrene content in the uncured thermosets, the estimated solubility parameters, and the sorption and diffusion coefficients. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

A Multitechnique Study of Structure and Dynamics of Polyfluorene Cast Films and the Influence on Their Photoluminescence

This article describes the microstructure and dynamics in the solid state of polyfluorene-based polymers, poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO), a semicrystalline polymer, and poly[(9,9-dioctyl-2,7-divinylene-fluorenylene)-alt-co-{2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylene vinylene}, a copolymer with mesomorphic phase properties. These structures were determined by wide-angle X-ray scattering (WAXS) measurements. Assuming a packing model for the copolymer structure, where the planes of the phenyl rings are stacked and separated by an average distance of approximately 4.5 A and laterally spaced by about approximately 16 A, we followed the evolution of these distances as a function of temperature using WAXS and associated the changes observed to the polymer relaxation processes identified by dynamical mechanical thermal analysis. Specific molecular motions were studied by solid-state nuclear magnetic resonance. The onset of the side-chain motion at about 213 K (beta-relaxation) produced a small increase in the lateral spacing and in the stacking distance of the phenyl rings in the aggregated structures. Besides, at about 383 K (alpha-relaxation) there occurs a significant increase in the amplitude of the torsion motion in the backbone, producing a greater increase in the stacking distance of the phenyl rings. Similar results were observed in the semicrystalline phase of PFO, but in this case the presence of the crystalline structure affects considerably the overall dynamics, which tends to be more hindered. Put together, our data explain many features of the temperature dependence of the photoluminescence of these two polymers.

Dielectric relaxation in amorphous linear aliphatic copolyesters

In order to contribute to a more extensive body of experimental knowledge concerning the relaxation processes in glass-forming amorphous polymers, broad-band frequency dielectric measurements have been carried out on two examples of a model amorphous polar polymer system. The system consists of a linear random copolyester with one-to-one mole ratios of adipic acid and succinic acid coupled with glycol units. In one example, ethylene glycol is the coupling unit and in the other it is trimethylene glycol The subglass β relaxations as well as the primary α regions were characterized and emphasis was placed on phenomenological description of the coalescence of the two regions with increasing temperature. Each subglass process was found to be complex in the sense of exhibiting two partially resolved components. One component is a typical extremely broad-in-frequency subglass relaxation and the other component is weaker, narrower in frequency and occurs at lower frequency isothermally or higher temperature isochronally. Tentative assignments of the source of the β process components are made. Phenomenological analysis of the α– β coalescence region invoking the Havriliak–Negami and Cole–Cole functions and utilizing numerical regression indicates that the merging process is optimally described as the β process losing relaxation strength and its presence being expressed as increased skewing in the primary relaxation.

Poly(Vinyl Ferrocene) Redox Behavior in Ionic Liquids

We describe in this report a systematic electrochemical characterization of the ion-solvent coupling mechanisms of poly(vinyl ferrocene) (PVF) in pure ionic liquid (IL) and IL aqueous solutions. Our study showed that the unique solvation and ionic properties of ILs significantly affected the break-in process and the ion-solvent transport mechanisms of PVF redox switching. A square model that emphasized both faradaic and nonfaradaic processes of PVF was used to explain the unique irreversible break-in effect in the pure ILs. The electrochemical quartz crystal microbalance technique was used to characterize the PVF redox processes in 1-butyl-3-methyl imidazolium tetrafluoroborate and methanesulfonate ILs in which an obvious difference of cyclic voltammogram was observed. Our results suggested the existence of strong IL-polymer interaction in methanesulfonate IL solutions, i.e., not only the anions but also the IL molecules interacted with the PVF matrix. The cations were later removed from the PVF matrix to balance the excessive positive charge in PVF oxidation. Our study confirmed that IL was not only an electrolyte but also a solvent in PVF redox switching processes. Various types of interactions between PVF and the IL, including dispersion, dipole induction, dipole orientation, hydrogen-bonding, or ionic/charge-charge interactions, could significantly change the PVF redox dynamics. Thus, IL tremendous diversity in structural and chemical properties and their distinctive properties offer us an excellent opportunity to explore IL-polymer interactions and to dynamically control the conductive polymer relaxation processes and their redox switching mechanism for various applications.

Dynamic Tube Dilation in Branched Polymers

In the tube model for entangled polymer chains, the topological constraint for a given probe chain imposed by surrounding matrix chains is represented as a tube confining the probe. Most of current tube models further assume that the relaxed portions of the probe and matrix chains are equivalent to a simple solvent and thus the tube dilates as the relaxation proceeds with time. for polymers of various architectures, this molecular picture of full dynamic tube dilation (full-DTD) can describe the linear viscoelastic relaxation modulus considerably well. However, recent experiments for polymers having so-called type-A dipoles indicate that the full-DTD picture cannot consistently describe the viscoelastic and dielectric relaxation processes of star-branched and multi-branched polymers because of inconsistent coarse-graining of length and time scales incorporated in this picture. This article presents a brief review of these experimental results and explains the molecular picture of partial-DTD in which the length and time scales are consistently coarse-grained on the basis of the constraint release (CR) mechanism.

NMR Molecular Dynamic and Thermal Analysis Studies of Amescla Resin

Natural resins have been a subject of the authors' research group since 2000 and a study involves a methodology to better analyze this type of sample was already published [1]. That study recommended that the first study should be done in the solid state. And in this case the authors decided to first investigate the molecular dynamic, using nuclear magnetic resonance from the measurements of proton spin–lattice and spin-spin relaxation time and the thermal parameters. The polymer relaxation processes occur over a wide range of time and length scales. A molecular-level understanding of such processes can lead to improved production methods and mechanical properties of the polymeric materials. The thermal motions, such as the glass transitions (Tg), occur at ∼ 100 Å. This work has demonstrated that the molecular dynamic of natural resins, as amescla resin is very well characterized by the relaxation parameters, using both high and low field NMR techniques. The thermal measurements corroborate the NMR data.

Operating temperature effects on the plasticization of polyimide gas separation membranes

Membrane plasticization is the process whereby penetrant dissolution causes membrane swelling or dilation, which in turn, can increase membrane diffusivity and solubility and lead to long time frame polymer relaxation processes. In this work, the effect of temperature upon the plasticization of a rigid polyimide, poly(4,4′-hexafluoroisopropylidene diphthalic anhydride–2,3,5,6-tetramethyl-1,4-phenylenediamine) (6FDA-TMPDA), by carbon dioxide is investigated. It is found that across the full range of temperatures studied, plasticization has little effect on carbon dioxide solubility as all results can be characterized by a standard dual mode sorption model. However, the effect upon diffusivity is significant and this can be described by both an exponential relationship with penetrant concentration and an Arrhenius relationship with temperature. The polymer relaxation processes induced by plasticization are also temperature dependent. However, the total proportion of penetrant sorption associated with such relaxation processes is relatively unaffected by temperature. This paper shows that plasticization effects are dominated by Henry's law dissolution. Conversely, while Henry's law species contribute most to diffusion at high temperatures, at lower temperatures the movement of Langmuir component species also contributes to the total diffusion coefficient.

Simulating Stress Relaxation Effects for Sequential Biaxial Stretching of Optical Films in a Tenter

Control of the sequential orientation (tenter) process for optical films is one of the most important factors in developing the retardation films which are needed for large sized liquid crystal displays (LCD). Analysis to predict the thickness distributions and bowing phenomenon during the tentering process is performed by finite element analysis (FEA). In addition, this study provides a new calculation method to predict the optical axes of stretched films from elastoplastic FEA results. The optical axes have two components. One is a stress-dependent term and the other is a strain-dependent term. To predict the molecular orientation, time-dependent effects are concerned with both. Moreover, polymer relaxation processes are examined. The FEA and optical property calculation are independent in the methods. This method enables one to easily calculate optical axes of films during the tentering process without a change of the conventional FEA calculation process.

On the Physical Parameters and Relaxation Processes in Nematic Azobenzene Polymers for Optical Nanowriting

AbstractThe possibility of achieving devices for rewritable optical storage is provided by liquid‐crystalline polymers with azobenzene side groups, because they undergo photochemically induced trans‐cis reversible isomerization. In this work we overview our investigations into a nematic azobenzene‐polymethacrylate (PMA4) as a suitable material for optical nanowriting. Relaxation processes over different time‐length scales are discussed with particular reference to understanding the interplay between homogeneity at the molecular level, bit stability and working temperature range.