Polymer Dynamics in Polyurethane/clay Nanocomposites Studied by Dielectric and Thermal Techniques

Differential scanning calorimetry (DSC), broadband dielectric relaxation spectroscopy (DRS), and thermally stimulated depolarization current (TSDC) techniques were employed to investigate glass transition and polymer dynamics in nanocomposites of polyurethane (PU) and organically modified montmorillonite (MMT) (weight fraction 0%–15%) prepared by solution casting. The PU matrix was obtained from oligo(oxytetramethylene glycol) of molar mass 1000 g/mol, 4,4′-diphenylmethane diisocyanate and 1,1-dimethylhydrazine as chain extender. Wide-angle X-ray scattering confirmed the formation of partly exfoliated structures at low MMT content. DSC, DRS, and TSDC show, in agreement with each other, that a fraction of polymer makes no contribution to the glass transition and to the corresponding α relaxation, whereas the rest exhibits similar glass transition dynamics as the pure matrix. This fraction of immobilized polymer reaches a maximum at about 5 wt% MMT. Effects of MMT on the microphase-separated structure of PU are negligible, as indicated by the study of glass transition and interfacial dielectric polarization/relaxation. No effects of MMT on the local, secondary γ and β relaxations were observed. Mechanical properties show a maximum improvement at about 5 wt% MMT, in good correlation with morphology and dynamics.     

The Slow Molecular Mobility in Semicrystalline Poly(Ethylene Oxide)

The thermally stimulated depolarization current (TSDC) technique has been used to study the slow molecular mobility in the amorphous part of the semicrystalline polymer, poly(ethylene oxide) (PEO). Experiments were carried out in the temperature range that includes the glassy state, the glass transformation region and the rubber state. The dipole moments in the polymeric main chain originated a broad and low intensity secondary relaxation in the temperature region from ?130°C up to the glass transition region; the activation energy of the motional modes of this secondary relaxation was in the range between 35 and 100 kJ mol^?1. The glass transition temperature of the PEO, provided by the TSDC technique, was T_g = ?53°C, and the fragility index was found to be m = 43. A strong relaxation above T_g was observed, whose molecular origin was discussed. The thermal behavior of the PEO was also characterized by differential scanning calorimetry.     

Role of Bi incorporation on glass transition kinetics in glassy Se78Te20Sn2 alloy

In this communication, we report the results of calorimetric measurements on the samples of recently synthesized multi-component glassy alloys of Se_78?xTe₂₀Sn₂Bi_x (0 ≤ x ≤ 6) system. For calorimetric study of glass transition kinetics, differential scanning calorimetry (DSC) technique has been used in non-isothermal mode. Peak glass transition temperature (T_g) is determined using the DSC scans. Kinetic parameters A and B of glass transition are determined using heating rate dependence of T_g. Activation energy of glass transition (E_g) has been calculated using Moynihan and Kissinger methods. Glass-forming ability and thermal stability are also determined using Hurby and Saad–Poulin relations, respectively.     

Glass transition,thermal stability and glass-forming ability of Se90In10− x Sb x (x = 0, 2, 4, 6, 8, 10) chalcogenide glasses

Differential scanning calorimetry (DSC) has been employed to investigate the glass transition activation energy E _g, thermal stability and glass-forming ability (GFA) of Se₉₀In_{10? x} Sb _x (x = 0, 2, 4, 6, 8, 10) chalcogenide glasses. DSC runs were performed at six different heating rates. Well-defined endothermic and exothermic peaks were obtained at glass transition and crystallization temperature. The dependence of glass transition temperature T _g on heating rate (α), as well as composition of Sb, has been studied. From the dependence of glass transition temperature on heating rate, the E _g has been calculated on the basis of the Kissinger [Anal. Chem. 29 (1957) p.1702] and Moynihan [J. Phys. Chem. 78 (1974) p.267] models. Thermal stability has been monitored through the calculation of temperature differences T _c–T _g, the stability parameter S, and the enthalpy released during crystallization H _c. The GFA has been investigated on the basis of the Hruby parameter H _r, which is strong indicator of GFA. Results for GFA are in good agreement with fragility index F _i calculations, indicating that Se₉₀In₆Sb₄ is an excellent glass-former.     

Thermal diffusion near glass transition in Ge-Se glasses measured by photoacoustics

The glass transition in Ge _x Se_1−x ) (0·1 ⩽x ⩽ 0·25) glasses has been investigated using the photoacoustic (PA) technique. It is found that the PA amplitude and phase undergo anomalous changes at the glass transition temperatureT _g. The amplitude has critical minimum and phase has maximum values atT _g. The variation of the thermal diffusivity, determined by measuring the frequency dependence of the PA amplitude and phase, with temperature shows sharp decrease near the glass transition temperature. The temperature dependence of the optical energy gap also has been measured and it shows a decrease with temperature for all compositions, the rate of decrease being higher for temperatures greater thanT _g.     

Calorimetric study of Te15(Se100− x Bi x )85 glassy alloys using differential thermal analysis

A calorimetric study of Te₁₅(Se_{100? x} Bi _x )₈₅ glassy alloys (x = 0, 1, 2, 3 and 4 at. %) is reported. Differential thermal analysis (DTA) was performed at heating rates of 10, 15, 20 and 25 K/min. The spectra were used to determine the glass transition temperature, T_g , the crystallisation temperature, T_c and the melting temperature, T_m . All these parameters shift to higher values with increasing heating rate, β. The glass transition temperature and the melting temperature increase, and the crystallisation temperature decreases, with increase in the Bi content, x. The activation energy of the glass transition, E_g , was evaluated using the Moynihan and Kissinger methods. The activation energy of crystallisation, E_c , was calculated using modified Kissinger and Matusita approaches. The thermal stability of these glasses has been studied and found to decrease with increase in Bi content. The results obtained are explained on the basis of a chemically ordered network model and an average coordination number.     

Study on the Molecular Mobility in the Polyamide/SrFeO Composites by In Situ Infrared Spectroscopy

Polyamide 6(PA6) and its composite with strontium ferrite (SrFeO) magnetic powders at different concentration levels (33%, 80%) are in situ investigated by temperature-dependent Fourier transform infrared spectroscopy (FT-IR). It is found that the intensity changes of the “free” (nonhydrogen bonded) N?H stretching vibration as a function of temperature can be used to determine the glass transition (T _g) and melting temperature (T _m); whereas the hydrogen-bonded N?H stretching vibration is only sensitive to T _m. Our results reveal that high SrFeO contents increase the T _g and T _m of PA6. These observations suggest that the molecular chains of PA6 are confined with increasing content of SrFeO.     

Metal-induced effects on the glass transition kinetics of glassy Se70Te30 alloy

The calorimetric measurements have been made in glassy Se₇₀Te₃₀ and Se₇₀Te₂₈M₂ (M?=?Ag, Cd, and Zn) alloys using non-isothermal differential scanning calorimetry technique to see the effects of Ag, Cd, and Zn additives on the glass transition kinetics of binary Se₇₀Te₃₀. From the heating rate dependence of glass transition temperature, T _g, different kinetic parameters of glass transition have been evaluated. The composition dependence of glass transition temperature T _g and the related activation energy (E_t ) is also discussed.     

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.     

Thermal behaviour of two anti-inflammatory drugs (celecoxib and rofecoxib) and slow relaxation dynamics in their amorphous solid state. Comparison between the dynamic fragility obtained by dielectric spectroscopy and by thermostimulated currents

This study of the thermal behaviour of two glass formers showed that rofecoxib has a good ability to vitrify while celecoxib displays a moderate glass forming ability. On the other hand, celecoxib forms an instable glass while rofecoxib displays moderate glass stability. The α-relaxation of the two amorphous drugs was studied by differential scanning calorimetry (DSC) and thermally stimulated depolarization currents (TSDC). The values of the dynamic fragility obtained by the two techniques are in good mutual agreement and suggest that both drugs are moderately fragile glass formers. These values are however considerably lower than those obtained by dielectric relaxation spectroscopy (DRS) and published in the literature. In the present work we will compare the values of the dynamic fragility obtained by TSDC and DRS for a set of ~30 glass-forming liquids, in order to evaluate their relative suitability for the determination of the m-index. The observed differences are discussed and interpreted. Finally, secondary mobility modes were detected by TSDC in both celecoxib and rofecoxib. The kinetic parameters are identical in both cases and similar to those found by DRS for the slow-b relaxation of celecoxib. These secondary relaxations, that are precursors of the main relaxation on its lower temperature side, are believed to be Johari-Goldstein relaxations.     

Poly(acrylonitrile-butadiene-styrene) Modified Epoxy–Amine Systems Analyzed by FTIR and Modulated DSC

Epoxy networks based on diglycidyl ether of bisphenol A (DGEBA) and diamino diphenyl sulfone (DDS) as curing agent were prepared in the presence of poly(acrylonitrile-butadiene-styrene) (ABS). Blends containing 3.6, 6.9, 10, and 12.9 wt% ABS were obtained by a melt mixing technique. Infrared spectroscopy was employed to follow the curing progress by determining the decrease of the band intensity caused by the epoxide groups. The evolution of glass transition temperatures (T _g) and heat of reaction (ΔH) of the blends cured for different times were studied in detail by modulated DSC and used for evaluation of the overall curing degree. The increase in the epoxy network density was detected from the magnitude of the decrease in heat capacity change at T _g, which reduces with conversion. The addition of ABS decreases the degree of cure and the epoxy network density due to the dilution of the reactants with the thermoplastic.     

Ionic noise measurement in polymer electrolytes

Electrical noise associated with ion transport (termed as “ionic noise”) has been measured at different temperatures, using a lock-in amplifier and dynamic signal analyzer for a polymer electrolyte PEO:NH₄I and its CdS dispersed composite. The ionic noise suddenly increases as the polymer passes through its phase transition at T _g and T _m. The T _g-peak in the noise measurement appears more clearly than what it does in DTA/DSC or conductivity measurements. Therefore, we suggest the noise technique as a good probe for studying phase transitions in ion conducting solid electrolytes. Further, the present noise measurements also confirm the known results of DTA/DSC studies that both T _g and T _m of polymer electrolytes shift on the formation of composites.     

The Evolution of Interfacial Strength as a Way to Characterize a Low-Temperature Limit of the Surface Glass Transition of an Amorphous Polymer

The evolution of autoadhesive strength, σ, with healing temperature, T _h, at the symmetric amorphous polystyrene (PS)?PS interfaces of the samples with vitrified bulk has been used to characterize a low-temperature limit of the surface glass transition temperature T _g ^surface(low). The existence of a linear relationship between the square root of σ and T _h has been found for both polydisperse and monodisperse polymers. By the extrapolation of straight lines σ ^1/2 ? T _h to σ ^1/2 = 0, the values of T _g ^surface(low) have been determined and compared with those of a high-temperature limit of T _g ^surface, T _g ^surface(high), measured earlier. The differences between T _g ^surface(low) and T _g ^surface(high) have been found to be insignificant, 10–20°C. Using an average value of the shift of T _g ^surface(low) with healing time, t _h, the quasi-equilibrium value of the surface glass transition temperature of amorphous PS T _∞ ^surface has been estimated to be 10–15°C.     

Crystallization kinetics of the TeO2–BaO glass system

Tellurite glasses of the system (100–x)TeO₂–xBaO, with x = 05, 10, 15 and 20 wt%, have been prepared and studied by differential scanning calorimetry (DSC). The crystallization kinetics of the glasses were investigated under non-isothermal conditions, applying the formal theory of transformations for heterogeneous nucleation to the experimental data obtained by DSC, using continuous-heating techniques. In addition, from the dependence of the glass-transition temperature (T _g) on heating rate, the activation energy for the glass transition was derived. Similarly, the activation energy of the crystallization process was determined and the crystallization mechanism characterized. The thermal stability of these glasses are considered in terms of the characteristic temperatures, T _g and T _in (the onset temperature of crystallization), via ΔT = T _in?T _g and a kinetic parameter K(T _g). The results confirm that thermal stability decreases with increasing BaO content. The phases into which the glass crystallizes have been identified by X-ray diffraction. Diffractograms of the transformed material indicate the presence of microcrystallites of α-TeO₂, γ-TeO₂ and BaTeO₃ in the remaining amorphous matrix.     

Role of copper ions in dielectric and structural investigations of lead– borate glasses

Glasses in the system 0.1CuO-(x-0.1)PbO-(1-x)B₂O₃ (0.3≤ x ≤ 0.7) were synthesized by using the melt quench technique. A number of studies such as X-ray diffraction (XRD), differential scanning calorimetry (DSC), fourier-transform infrared (FTIR) and Raman spectroscopy, electron paramagnetic resonance (EPR) and dielectric properties (viz., dielectric constant ??, dielectric loss and ac conductivity σ_ac) are employed to characterize the glasses. The amorphous nature of the glasses was confirmed using XRD while the glass transition temperature (T_g) of glass samples have been estimated from DSC investigation and found that the T_g decreases with increasing PbO content. Raman and FTIR spectroscopy reveals that when increasing lead ions, the tetrahedral [BO₄] units are gradually replaced by trigonal [BO₃] units. The EPR study leads to determine the local site of Cu²⁺ ions and its transformation with the Pb content in the studied glasses.     

Activation Energy for the Glass Transition of a Confined Elastomer in HDPE/PP Blends

Blends of two highly crystalline polymers containing an elastomer were prepared to study the glass transition of the confined elastomer. The polymers chosen were high density poly ethylene (HDPE), polypropylene (PP), and two elastomers of a different nature: natural number (NR) and EPDM. The dynamic mechanical analyzer (DMA) technique was used to analyze the storage modulus of blends with elastomer content from 0% to 30% by weight, with the remainder made up of equal amounts of HDPE and PP, and blends with 10% of the elastomer, but varied ratios of polyolefins. We used the differentiation modification of the Arrhenius method in the kinetic analysis assuming an n‐order relaxation mechanism, which allowed detecting the percolation threshold of NR. Results indicate that both temperature and activation energy for glass transition (T _g ) are dependent on the types of polymers in the blend and blend composition. The T _g and E values of the unblended elastomers are higher than those in blends; this behavior is associated with the elastomer confinement and blend morphology.     

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.     

Behaviour of ultrasonic velocities in amorphous Se90Ge10 and Se85Ge15 alloys near their glass transition

Precise measurements of 10 MHz frequency longitudinal and shear wave velocities are reported in amorphous SeGe alloys near their glass transition temperature T _g . There is a sharp decrease of the velocities near T _g , but the reduction in velocities appears smaller than expected.     

The glassy state of the amorphous V2O5–NiO–TeO2 samples

The glass transition temperature dependence to heating rate and therefore the activation energy (ΔH^?) of the glass transition of (60-x)V₂O₅–xNiO–40TeO₂ oxide glasses with 0≤x≤20 (in mol%) were investigated at heating rates φ (=3 6, 9, 10 and 12 K/min) using differential scanning calorimetry (DSC). The heating rate dependence of T_g was used to investigate the applicability of different theoretical models describing the glass transition. Using the application of Moynihan and Kissinger et al. models to the present data, different values of (ΔH^?) at each different heating-rate regions were obtained. The fragility parameter (m=ΔH^?/R T_g) was ∼24.98 for x=10 mol%, suggesting that this glass may be considered as a rather strong glass (fragility index m∼>20 is an indication of fragile glass). Also the compositional dependence of T_g and ΔH^? was investigated.     

Thermal analysis of specific heat measurements in glassy Se80−xTe20Sbx alloys in glass transition region

In the present report, we have done specific heat measurements in glassy Se_80?xTe₂₀Sb_x (0 ≤ x ≤ 15) alloys in glass transition region. Differential scanning calorimetry (DSC) technique is used for this purpose. We have observed a tremendously huge increase in the specific heat (C_p) values at the glass transition temperature. The thermal analysis shows that the values of C_p below glass transition temperature and the difference of C_p values before and after glass transition (?C_p) are highly composition-dependent.