Dynamic light scattering in solid polymers

The viscosity of an amorphous polymeric solid above its glass transition [T _g (T,P)] increases as the temperature of the solid is decreased or the pressure is increased. Under changes in temperature or pressure, molecular subunits in the polymeric solid undergo configurational changes. Such changes or relaxations have a distribution of relaxation strengths and times. As the solid is cooled or as the hydrostatic pressure on the solid is increased, the relaxation strengths increase and the relaxation times increase. These changes in relaxation or dynamic properties are very dramatic as the empirical T _g is approached. Near T _g the polymeric solid is no longer in volume equilibrium; continued cooling or pressuring at a time rate faster than the average relaxation time will produce a polymeric glass. This glass is a nonequilibrium, amorphous solid. If the glass is held at a fixed temperature and pressure very close to, but below, T _g , the volume of the glass will be observed to relax to its equilibrium value. For temperatures and pressures well below T _g , equilibrium is a much more conjectural concept since the relaxation times become extremely long. It has been proposed^1,2 that there is a characteristic temperature T _g at which an amorphous polymer undergoes a second-order transition to an equilibrium glass with zero configurational entropy (i.e., a noncrystallizable solid).     

Enthalpy Relaxation Study of Poly(Vinyl Chloride) Films Based on the Time-Temperature Superposition Principle

Abstract

In the enthalpy relaxation of poly(vinyl chloride), a decrease in enthalpy upon the isothermal ageing was measured using the differential scanning calorimetry method as a function of ageing time (t_A) and ageing temperature. The range of the ageing temperature was from 56?°C (T_g ? 25?°C) to 72?°C (T_g ? 9?°C) where T_g denotes the glass transition temperature. The limiting value of the decrease in enthalpy was determined by applying a stretched exponential function to the measured enthalpy data. The relaxation function (?) was derived from the measured enthalpy and the construction of a master curve was tried by shifting the ? ? t_A curves of the respective ageing temperatures horizontally. Although there was no agreement between the shift factors (a_T) and the relaxation times of the ? ? t_A curves, the superposition was successfully constructed and the a_T values obtained for the poly(vinyl chloride) sample were found to be comparable to those reported for viscoelastic experiments over a broad temperature range above and below T_g carried out for different polymers. The origin of the decrease in enthalpy was briefly discussed in terms of the chain dynamics in the isothermal condition.     

A mechanism for spontaneous relaxation of glass at room temperature

Silicate glasses kept at ambient temperature densify spontaneously, and their properties change with time. This is known as ageing of a glass. The spontaneous change occurs according to first-order rate kinetics, with a characteristic (relaxation) time much shorter than that of the α-relaxation process. According to the mechanism proposed here, there are local regions in the network structure of a silicate glass which collapse spontaneously with time by simultaneous motions of atoms seen as β or the Johari-Goldstein relaxation. The collapse causes the Si-O-Si bond angles in the immediate surroundings to change, and these angles become elastically strained. This strain biases the potential energy in a two-site model for oxygen-atom displacement, which occurs at a faster rate and rapidly dissipates the strain. Thus the glass densifies homogeneously on ageing, by two processes: process I, spontaneous collapse of local regions; process II, subsequent dissipation of strain energy resulting from the collapse. The first process, which is much slower, determines the kinetics of ageing. A fictive temperature T_f,β corresponding to the freezing out of the localized motions of the Johari-Goldstein relaxation process is proposed. For silicate glasses, T_f,β is 40-45% of the usual fictive temperature for the α-relaxation process. The volume lost during ageing may be recovered on heating the aged glass to a temperature far below T _g in a time- and temperature-dependent manner, as has already been found from zero-point measurements of a glass thermometer.     

Ageing exploration of the energy landscape of a glass by the TSDC technique

Different thermal treatments were used in order to follow the structural relaxation of m-toluidine by thermally stimulated depolarization currents (TSDC) in the sub-T _g temperature region. In some of these thermal treatments, the ageing time, t _a, and the probing (or polarization) time, t _P, are varied independently in order to allow a clear understanding of the exploration of the energy landscape which occurs as a consequence of the structural relaxation. The TSDC results show that ageing induces a shift to higher temperatures of the TSDC peaks, in a way similar to that observed in the calorimetric heat capacity overshoot. More importantly, TSDC results show that ageing induces a decrease of the activation energies with regard to the window defined by the probing time and a sharpening of the energy distribution of the modes of motion that are present in the glass, in the studied sub-T _g temperature region.     

Micro-thermal analysis of polyester coatings

The application and suitability of micro-thermal analysis to detect changes in the chemical and physical properties of coating due to ageing and especially photo-degradation is demonstrated using a model polyester coating based on neopentyl glycol isophthalic acid. The changes in chemical structure like chain scission and cross-linking are manifested by a shift of the LTA detectable T_g and by a change of the slope of the part of the LTA graph responsible for the penetration of the hot sensor into the material after passing the glass transition temperature. As such LTA is a valuable tool to have a quick look into coating surfaces and especially their ageing. The photo-degradation of polyester in air leads to the formation of a cross-linked network at a surface layer of about 3-4 μm coupled with an increase in hardness and of the glass transition temperature by ∼90 K, the effect is less drastic for a photo-degradation in a nitrogen environment. Moreover, the presence of a non-equilibrium dense surface layer with a higher T_g formed during the drying of the coating formulation and the film solidification can be shown.     

Is the configurational entropic model able to predict the final equilibrium state reached by Se glasses after very long ageing durations?

This work compares the equilibrium states reached by glassy selenium (g-Se) after several ageing durations at temperatures lower than the glass transition temperature T _g, with the forecasts given by the configurational entropic model. The comparison is carried out through experimental data collected both on glassy samples after short-term ageing and on glassy samples older than 20?years, kept at room temperature. It is shown that the configurational entropic model is not able to describe the behaviour of g-Se, both at short- and long-ageing terms with the same set of fitting parameters. For short-term ageing, the forecasts given by the entropic model are in good agreement with the experimental data; the hypothesis that the heat capacities corresponding to the equilibrium state and to the state extrapolated from the liquid differ by an amount δ is therefore properly justified. For long-term ageing, the amount δ vanishes and the assumption does not hold any longer. Indeed, experimental results support the idea that at least two separate mechanisms of relaxation coexist in the physical ageing of g-Se, one with very fast kinetics and the other with much slower kinetics.     

Behavior of the glass transition and crystallization temperatures of the oxide glass MgO-Al2O3-SiO2 under external influences

The kinetics of the glass transition Tg and crystallization Tcr temperatures of the glass MgO-Al₂O₃-SiO₂ is studied under conditions of annealing in the glass transition interval and after treatment with a pulsed magnetic field (PMF). It is found that all the parameters studied exhibit nonmonotonic and intercorrelated variations in time. These temporal variations are due to structural relaxation (SR) in the glass. It is observed that a relative decrease of T _g and T _cr occurs in the annealed or PMF-treated glass. This effect is important from the standpoint of modern ideas about the mechanisms of structural relaxation and is due to the concentrational redistribution of chemical bonds. The condition for externally induced anomalous behavior of the temperatures investigated is determined. Zh. Tekh. Fiz. 67, 30–34 (October 1997)     

Segmental Dynamics in net -poly(methyl methacrylate)- co -poly(n-butyl acrylate) Copolymer Networks

Dynamic mechanical spectroscopy and differential scanning calorimetry investigations of segmental dynamics are reported for net-poly(methyl methacrylate)-co-poly(n-butyl acrylate) copolymer networks. Three characteristic temperatures, namely, Vogel (T_∞), glass transition (T _g ), and crossover (T _c ), were used to define cooperativity range and a new reduced temperature parameter (Solidness, S). The results showed that broadness of the α -dispersion (glass transition) and cooperativity length scale at the glass transition temperature decreased with increasing butyl acrylate content and T _g -scaled temperature dependence of the relaxation time (fragility). However, the cooperativity range (T _c –T_∞), decreased with increasing fragility index. Furthermore, the solidness at T _g (S(T _g )) was nearly independent of chemical structure of the samples. Finally, a correlation was found between two measures of cooperativity length scale in the glass transition region, namely, average volume of cooperatively rearranging regions, V _CRR , and the number of basic units in an act of rearrangement in the glass transition region, Z(T _g ), determined from two completely independent experimental techniques.     

Frustrated percolation,spin glasses and glasses

Summary The static and dynamic properties of the frustrated percolation model are investigated. This model, which contains frustration as an essential ingredient, exhibits two transitions: a percolation transition at a temperatureT _p with critical exponents of the ferromagnetic (s=1/2)-state Potts model, and a second transition at a lower temperatureT _g in the same universality class of the Ising spin glass model. AboveT _p the time-dependent autocorrelation function is characterized by a single exponential, while forT _p>T>T _g preliminary numerical results show a broad shoulder or plateau typical of a structural glass transition. BelowT _g the system is in glassy state with an infinitely long relaxation time. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Confinement and processing effects on glass transition temperature and physical aging in ultrathin polymer films: Novel fluorescence measurements

Fluorescence intensity measurements of chromophore-doped or -labeled polymers have been used for the first time to determine the effects of decreasing film thickness on glass transition temperature, T _g, the relative strength of the glass transition, and the relative rate of physical aging below T _g in supported, ultrathin polymer films. The temperature dependence of fluorescence intensity measured in the glassy state of thin and ultrathin films of pyrene-doped polystyrene (PS), poly(isobutyl methacrylate) (PiBMA), and poly(2-vinylpyridine) (P2VP) differs from that in the rubbery state with a transition at T _g. Positive deviations from bulk T _g are observed in ultrathin PiBMA and P2VP films on silica substrates while substantial negative deviations from bulk T _g are observed in ultrathin PS films on silica substrates. The relative difference in the temperature dependences of fluorescence intensity in the rubbery and glassy states is usually reduced with decreasing film thickness, indicating that the strength of the glass transition is reduced in thinner films. The temperature dependence of fluorescence intensity also provides useful information on effects of processing history as well as on the degree of polymer-substrate interaction. In addition, when used as a polymer label, a mobility-sensitive rotor chromophore is demonstrated to be useful in measuring relative rates of physical aging in films as thin as 10 nm. Received 21 August 2001     

The determination of the glass transition temperature by thermally stimulated depolarization currents. Comparison with the performance of other techniques

Several experimental techniques are currently used for the determination of the glass transition temperature, T_g. Thermally stimulated depolarization currents (TSDC) is a thermal analysis technique whose experimental results display a very clean glass transition signature and that, nevertheless, is seldom used as a technique for T_g determination. In the present work we explain how to get the glass transition temperature from TSDC data, and we compare the values obtained for a vast number of glass forming systems (with T_gs in a wide range between ?145 and +180 °C and fragilities between m = 15 and m = 100), with the values obtained by differential scanning calorimetry (DSC) and dielectric relaxation spectroscopy (DRS). We conclude that the T_g determination by TSDC is direct, accurate and reproducible and that the obtained values correlate very well with those obtained by DSC and DRS. This general survey thus suggests TSDC as a valuable alternative technique for determining T_g.     

Physical ageing in Se94Sn6 glass induced by gamma irradiation

A differential scanning calorimeter (DSC) has been used to monitor the influence of high-energy Co⁶⁰ gamma-irradiation on physical ageing in Se₉₆Sn₄ glass. It is observed that the relaxation process in the studied glass is slow after 1 and 3 years of natural storage, and can be highly accelerated by γ-irradiation. The glass transition temperature T_g and the endothermic peak area, which is directly related to the relaxation enthalpy Δh, were found to exhibit a remarkable change after irradiation and also during an additional natural storage for 6 months and 1 year of the irradiated Se₉₆Sn₄ glass. T_g value of γ-irradiated glass increases by 12 K; meanwhile, that of additional 6-months- and 1-year-stored glass T_g increases by 19 and 20 K in comparison to its counterpart of 1-year-aged non-irradiated sample. The same trend is also observed for Δh, which becomes 2.33, 3.61 and 3.65 times that of the 1-year-aged non-irradiated case, respectively, for γ-irradiated, additional 6-months- and 1-year-stored glass. These indications reveal that γ-irradiation activates the relaxation of the glass towards a state thermodynamically close to the equilibrium super-cooled liquid state.     

Simulated glass-forming polymer melts: Glass transition temperature and elastic constants of the glassy state

By means of molecular-dynamics simulation we study a flexible and a semiflexible bead-spring model for a polymer melt on cooling through the glass transition. Results for the glass transition temperature T _g and for the elastic properties of the glassy state are presented. We find that T _g increases with chain length N and is for all N larger for the semiflexible model. The N dependence of T _g is compared to experimental results from the literature. Furthermore, we characterize the polymer glass below T _g via its elastic properties, i.e., via the Lamé coefficients λ and μ. The Lamé coefficients are determined from the fluctuation formalism which allows to split λ and μ into affine (Born term) and nonaffine (fluctuation term) contributions. We find that the fluctuation term represents a substantial correction to the Born term. Since the Born terms for λ and μ are identical, the fluctuation terms are responsible for the different temperature dependence of the Lamé coefficients. While λ decreases linearly on approaching T _g from below, the shear modulus μ displays a much stronger decrease near T _g. From the present simulation data it is not possible to decide whether μ takes a finite value at T _g, as would be expected from mode-coupling theory, or vanishes continuously, as suggested by recent work from replica theory.     

Effect of structural relaxation on low energy excitations in amorphous Zr x Cu1−x

We report on an investigation of the liquid-quenched metallic glass Zr _x Cu_1?x (0.6≦x≦0.74) subjected to heat treatments below the glass transition temperatureT _g. Annealing temperatures up to 200°C (<0.8T _g) were chosen as to achieve topological relaxation only. The superconducting transition temperaturesT _c are lowered, as already observed for other metallic glasses. Low temperature measurements of the thermal conductivity (0.5 K≦T≦15 K) and of the specific heat (0.1 K≦T≦3 K) were carried out in order to determine the effect of structural relaxation on the low energy configurational excitations characteristic of the amorphous state. The annealed samples show no detectable (<20%) change in the specific heat forT?T _c, but an increase of the thermal conductivity by a factor of 2 forT?T _c is observed. Within the tunneling model of two level systems (TLS) for the low energy excitations, this behavior can be qualitatively understood in terms of a change of the TLS relaxation time distribution upon annealing. This distribution differs from that of the commonly used standard tunneling model. The change of the phonon scattering by TLS directly observed forT?T_c is largely responsible for the enhancement of the thermal conductivity found also aboveT _c.     

The kinetics of the structural relaxation process in PHEMA-silica nanocomposites based on an equation for the configurational entropy

The enthalpy relaxation of polymer-silica nanocomposites prepared by simultaneous polymerization of poly(2-hydroxyethyl methacrylate) (PHEMA) and tetraethyloxysilane, TEOS, a silica precursor, is investigated. Both the glass transition temperature, T_g, and the temperature interval of the glass transition, ΔT _g , increase as the silica content in the sample does. Structural relaxation experiments show that the temperature interval in which conformational motions take place broadens as the silica content in the hybrid increases. A phenomenological model based on the evolution of the configurational entropy during the structural relaxation process, the SC model, has been used for determining the temperature dependence of the relaxation times during the process. The results show an increase of the fragility of the polymer as the silica content increases, a feature that can be related to the broadening of the distribution of relaxation times characterized by the β parameter of the stretched exponential distribution. On another hand the silica content increase produces a significant change of the relaxation times in the glassy state.     

Mechanical spectroscopy of laser deposited polymers

Pulsed laser deposition (PLD) at 248 nm in ultra high vacuum was used to produce thin poly(methyl methacrylate) (PMMA) and poly(ethyl methacrylate) (PEMA) films. The ablation and deposition mechanisms were found to be similar in both systems. Having the same backbone, these polymers differ in the size of their polar side groups leading to changes in their dynamics. Studies of the relaxation processes were performed using mechanical torsion and bending spectroscopy by means of a double-paddle oscillator (DPO) and an in-situ plasma plume excited reed (PPXR), respectively. A strong increase of the mechanical damping was observed during annealing of the polymer films well above the glass transition temperature T _g, while in-situ X-ray measurements did not reveal any structural changes. For PEMA, the glass transition temperature T _g=335 K and the main absorption maximum appear at lower temperatures compared to PMMA (T _g=380 K), allowing one to measure the mechanical properties in a much wider range above T _g.     

Internal stresses of deformation origin in bulk metallic glasses of the Pd-Cu-Ni-P system

The effective internal shear stresses arising during unloading of bulk Pd-Cu-Ni-P metallic glasses deformed under conditions of homogeneous flow below the glass transition temperature T _g have been determined using measurements of the torque relaxation. It has been shown that the level of effective internal stresses gradually decreases with increasing temperature and approaches zero at T ?? T _g . Possible mechanisms of the formation of internal stresses with variations in the experimental conditions have been discussed.     

Revisiting the Dielectric Relaxation of Ethylene-Vinylacetate Copolymers: Influence of Microstructure

The dielectric relaxation behavior of a series of ethylene-vinylacetate (EVA) copolymers was investigated by measuring the complex dielectric permittivity in a broad frequency and temperature range. Crystallinity of EVA copolymers was estimated by differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS). The shape of the higher temperature relaxation, appearing above the glass transition temperature T _g depends on the VA content. It was found that this relaxation was asymmetric for VA concentrations higher than 40 wt% and changed to a symmetric shape at lower VA values. Concurrently, as the VA content decreased, a major broadening of the relaxation over a wide frequency range was observed. It is found that the dielectric relaxation was preserved on going through the melting range of the semicrystalline samples, although it exhibited changes of its characteristic parameters that are typical for segmental relaxation appearing at T _g. This finding allows one to associate this relaxation to the segmental motions at T _g in the amorphous phase and not to the existence of interfacial regions.     

Molecular dynamics in thin films of isotactic poly(methyl methacrylate)

The molecular dynamics in thin films (18 nm-137 nm) of isotactic poly(methyl methacrylate) (i-PMMA) of two molecular weights embedded between aluminium electrodes are measured by means of dielectric spectroscopy in the frequency range from 50 mHz to 10 MHz at temperatures between 273 K and 392 K. The observed dynamics is characterized by two relaxation processes: the dynamic glass transition (α-relaxation) and a (local) secondary β-relaxation. While the latter does not depend on the dimensions of the sample, the dynamic glass transition becomes faster (≤2 decades) with decreasing film thickness. This results in a shift of the glass transition temperature T _g to lower values compared to the bulk. With decreasing film thickness a broadening of the relaxation time distribution and a decrease of the dielectric strength is observed for the α-relaxation. This enables to deduce a model based on immobilized boundary layers and on a region displaying a dynamics faster than in the bulk. Additionally, T _g was determined by temperature-dependent ellipsometric measurements of the thickness of films prepared on silica. These measurements yield a gradual increase of T _g with decreasing film thickness. The findings concerning the different thickness dependences of T _g are explained by changes of the interaction between the polymer and the substrates. A quantitative analysis of the T _g shifts incorporates recently developed models to describe the glass transition in thin polymer films. Received 12 August 2001 and Received in final form 16 November 2001     

Measuring surface and bulk relaxation in glassy polymers

We present a comprehensive study of gold nanoparticle embedding into polystyrene (PS) surfaces at temperatures ranging from T _g + 8 K to T _g − 83 K and times as long as 10⁵ minutes. This range in times and temperatures allows the first concurrent observation of and differentiation between surface and bulk behavior in the 20nm region nearest the free surface of the polymer film. Of particular importance is the temperature region near the bulk glass transition temperature where both surface and bulk processes can be measured. The results indicate that for the case of PS, enhanced surface mobility only exists at temperatures near or below the bulk T _g value. The surface relaxation times are only weakly temperature dependent and near T _g , the enhanced mobility extends less than 10nm into the bulk of the film. The results suggest that both the concept of a “surface glass transition” and the use of glass transition temperatures to measure local mobility near interfaces may not universally apply to all polymers. The results can also be used to make a quantitative connection to molecular dynamics simulations of polymer films and surfaces.