Molecular dynamics of amorphous/crystalline polymer blends studied by broadband dielectric spectroscopy

The molecular dynamics of poly(vinyl acetate), PVAc, and poly(hydroxy butyrate), PHB, as an amorphous/crystalline polymer blend has been investigated using broadband dielectric spectroscopy over wide ranges of frequency (10⁻² to 10⁵ Hz), temperature, and blend composition. Two dielectric relaxation processes were detected for pure PHB at high and low frequency ranges at a given constant temperature above the T_g. These two relaxation peaks are related to the α and α′ of the amorphous and rigid amorphous regions in the sample, respectively. The α′-relaxation process was found to be temperature and composition dependent and related to the constrained amorphous region located between adjacent lamellae inside the lamellar stacks. In addition, the α′-relaxation process behaves as a typical glass relaxation process, i.e., originated from the micro-Brownian cooperative reorientation of highly constraints polymeric segments. The α-relaxation process is related to the amorphous regions located between the lamellar crystals stacks. In the PHB/PVAc blends, only one α-relaxation process has been observed for all measured blends located in the temperature ranges between the T_g’s of the pure components. This last finding suggested that the relaxation processes of the two components are coupled together due to the small difference in the T_g’s (ΔT_g = 35 °C) and the favorable thermodynamics interaction between the two polymer components and consequently less dynamic heterogeneity in the blends. The T_g’s of the blends measured by DSC were followed a linear behavior with composition indicating that the two components are miscible over the entire range of composition. The α′-relaxation process was also observed in the blends of rich PHB content up to 30 wt% PHB. The molecular dynamics of α and α′-relaxation processes were found to be greatly influenced by blending, i.e., the dielectric strength, the peak broadness, and the dielectric loss peak maximum were found to be composition dependent. The dielectric measurements also confirmed the slowing down of the crystallization process of PHB in the blends.     

Dielectric relaxation of poly(phenylene sulfide) containing a fraction of rigid amorphous phase

The dielectric relaxation behavior of poly(phenylene sulfide), PPS, has been investigated from room temperature to 180°C. This study was undertaken to examine the mobility of the amorphous phase through the glass transition region, to determine the contribution that rigid amorphous phase material makes to the relaxation process. Semicrystalline samples contain a fraction of the rigid amorphous phase, which was determined from the heat capacity increment at the glass transition, using degree of crystallinity determined from x-ray scattering. In the dielectric experiment, we measured the temperature and frequency dependence of the real and imaginary parts of the dielectric function. ε″ vs. ε′ was used to determine the dielectric relaxation intensity, δε = ε_s–ε∞, at temperatures above the glass transition. For amorphous PPS, δε decreases as temperature increases, while for all semicrystalline PPS, δε increases with temperature. The ratio of semicrystalline intensity to amorphous intensity determines the total fraction of dipoles which are already relaxed at a given temperature. Results indicate that more and more rigid amorphous phase material relaxes as the temperature is increased. This provides the first evidence that rigid amorphous phase material in PPS contains chains that possess different levels of molecular mobility. Finally, to the temperature of the loss peak maximum, at a given frequency, we assign the value of the dielectric T_g. For both melt and cold crystallization, the dielectric T_g systematically decreases as the crystallization temperature increases, and as the fraction of rigid amorphous phase decreases.     

Study of poly(bisphenol A carbonate) relaxation kinetics at the glass transition temperature

In this work, the variations of the relaxation times are investigated above and below the glass transition temperature of a model amorphous polymer, the polycarbonate. Three different techniques (calorimetric, dielectric and thermostimulated currents) are used to achieve this goal. The relaxation time at the glass transition temperature was determined at the temperature dependence convergence of the relaxation times calculated with dynamic dielectric spectroscopy (DDS) for the liquid state and thermostimulated depolarisation currents (TSDC) for the vitreous state. We find a value of τ(T_g) = 110 s for PC samples. The knowledge of the temperature dependence, τ(T), and the value τ(T_g) enables to determine the glass-forming liquid fragility index, m. We find m = 178 ± 5.     

A three-phase microstructural model to explain the mechanical relaxations of branched polyethylene: a DSC,WAXD and DMTA combined study

The thermal transitions of well-characterised single-site catalysed polyethylenes having various degrees of short chain branching have been studied by differential scanning calorimetry, X-ray diffraction and dynamic mechanical thermal analysis. A critical discussion based on the results obtained by means of the different techniques is presented. The results suggest that the γ transition is independent of the branching content and degree of crystallinity, pointing towards a sub-glass local relaxation mechanism related to both amorphous and crystalline fractions. The temperature of the β transition, T _β from dynamic mechanical measurements, is in agreement with the glass transition temperature obtained by calorimetry, T _g. Moreover, T _γ, and also T _β are directly related to a change in the thermal expansion coefficient of the amorphous phase observed by X-ray scattering. It is found that the corresponding scattering distance of the amorphous halo depends on crystallinity. In addition, the calorimetric heat capacity values at T _β do not account for the total amorphous fraction determined for each material. The relaxation motions assigned to the amorphous phase glass transition seems to parallel the subsequent melting of the crystalline structure, suggesting a hierarchical motion of different structures as temperature increases. Dynamic mechanical thermal analysis supports these observations, showing a broad transition in the phase angle involving first the relaxation of amorphous phase, then the (presumable) more rigid intermediate phase, and finally the crystalline phase, as the temperature increases.     

Dehydration-induced structural relaxation effects in poly(methyl methacrylate)

The Thermally Stimulated Depolarization (TSD) dielectric technique and Dielectric Relaxation Spectroscopy (DRS) have been used in order to investigate aging phenomena in poly(methyl methacrylate) (PMMA). Earlier TSD studies on amorphous PMMA report peculiar dielectric relaxation signals within the range of the glass transition (at ∼378 K) and the secondary relaxation (∼230 K). In the present study, an intense TSD current relaxation band maximizing around 310 K is tentatively attributed to the molecular mobility due to a residual free volume below the glass transition temperature, T_g, that allows structural recovery at the free volume released from the desorption of H₂O molecules during evacuation. Limited motions in the main backbone provoke dipole (re)orientation of the ester carbonyl pendant groups with an activation energy E=0.85±0.05 eV, being responsible for the latter dielectric relaxation effect. Alternative attributions based on the short-range jump relaxation of electric charges and boundary effects are also discussed.     

Molecular relaxation of isotactic polystyrene: Real‐time dielectric spectroscopy and X‐ray scattering studies*

The molecular relaxation processes and structure of isotactic polystyrene (iPS) films were investigated with real‐time dielectric spectroscopy and simultaneous wide‐ and small‐angle X‐ray scattering. The purpose of this work was to explore the restrictions imposed on molecular mobility in the vicinity of the α relaxation (glass transition) for crystallized iPS. Isothermal cold crystallization at temperatures of T_c = 140 or 170 °C resulted in a sigmoidal increase of crystallinity with crystallization time. The glass‐transition temperature (T_g), determined calorimetrically, exhibited almost no increase during the first stage of crystal growth before impingement of spherulites. After impingement, the calorimetric T_g increased, suggesting that confinement effects occur in the latter stages of crystallization. For well‐crystallized samples, the radius of the cooperativity region decreased substantially as compared with the purely amorphous sample but was always smaller than the layer thickness of the mobile amorphous fraction. Dielectric experiments directly probed changes in the amorphous dipole mobility. The real‐time dielectric data were fitted to a Havriliak–Negami model, and the time dependence of the parameters describing the distribution of relaxation times and dielectric strength was obtained. The central dipolar relaxation time showed little variation before spherulite impingement but increased sharply during the second stage of crystal growth as confinement occurred. Vogel–Fulcher–Tammann analysis demonstrated that the dielectric reference temperature, corresponding to the onset of calorimetric T_g, did not vary for well‐crystallized samples. This observation agreed with a model in which constraints affect primarily the modes having longer relaxation times and thus broaden the glass‐transition relaxation process on the higher temperature side. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 777–789, 2004     

Rigid amorphous fraction in poly(ethylene terephthalate) determined by dilatometry

Volumetric thermal analysis of semicrystalline poly(ethylene terephthalate), PET, with different content of crystalline phase was carried out using mercury-in-glass dilatometry. The effect of crystals on the thermal properties of amorphous phase (glass transition temperature, T _g, thermal expansion coefficients, α) were determined. At cold-crystallization (106°C, up to 4 h), crystalline content of 2.4–25.3 vol.% was achieved. Increasing content of crystalline phase broadens the glass transition region and increases T _g. The change of thermal expansion coefficient during glass transition is lower than that predicted by the two-phase model, which indicates the presence of a third fraction — rigid amorphous fraction (RAF), whose content steadily increases during crystallization. However, its relative portion (specific RAF) is significantly reduced. Further significant decrease in specific RAF appears after annealing at a higher temperature.     

Interrelation between side chain crystallization and dynamic glass transitions in higher poly(n-alkyl methacrylates)

Calorimetric and dielectric results for crystallizable poly(n-alkyl methacrylates) (PnAMA) with C=12, 16 and 18 alkyl carbons per side chain are presented. Degree of crystallization D_cal and melting peak temperature T_M are estimated from conventional DSC measurements. For poly(n-hexadecyl methacrylate) (C=16) the influence of isothermal crystallization is studied by DSC as well as TMDSC. Changes in dielectric relaxation strength Δε and α peak shape during crystallization are investigated. Effects of side chain crystallization on the complex dynamics of PnAMA are discussed. The results are related to the relaxation behavior of lower nanophase-separated PnAMA with two co-existing glass transitions, the conventional glass transition (a or α) and the polyethylene-like glass transition (α_PE) within alkyl nanodomains formed by aggregated alkyl rests. It is shown that amorphous as well as semicrystalline PnAMA can be understood as nanophase-separated polymers with alkyl nanodomains having a typical dimension of 1-2 nm. The results are compared with the predictions of simple morphological pictures for side chain polymers. X-ray scattering data for the amorphous and semicrystalline PnAMA are included in the discussion. Common aspects of nanophase-separated systems in both states as well as differences caused by crystallization are discussed. Indications for the existence of rigid amorphous regions are compiled. Different approaches to explain a similar increase of T_g(α_PE)—the glass temperature of the amorphous alkyl nanodomains—and T_M—the melting temperature of crystalline alkyl nanodomains—with side chain length are considered. Pros and cons of both approaches, based on increasing order within the alkyl nanodomains and confinement effects in nanophase-separated systems, are discussed. Main trends concerning crystallization and cooperative dynamics are compared with those in other systems with self-assembled nanometer confinements like microphase-separated blockcopolymers or semicrystalline main chain polymers.     

Influence of the crystalline phase on the molecular mobility of PVDF

Dielectric relaxation and pyrocurrent of PVDF were studied by thermostimulated current spectroscopy. The transition spectrum of the material was investigated by differential scanning calorimetry. Two well-resolved relaxation peaks have been observed in the temperature range [?100–100°C]. The molecular mechanisms of these phenomena have been discussed, based on a comparative study of α-PVDF. and β-PVDF. The β relaxation mode is located at ?41°C in α-PVDF and is slightly shifted toward higher temperatures in the stretched material. This mode has been ascribed to the dielectric manifestation of the glass transition (T_g) of PVDF. It is comprised of two components corresponding to the free and constrained amorphous phases, respectively, in the order of increasing temperatures. The α_c transition/relaxation has been associated with molecular motions in the crystalline/amorphous interphase. At higher temperatures, a compensation phenomenon corresponding to cooperative movements liberated at the Curie transition has been observed in β-PVDF. © 1993 John Wiley & Sons, Inc.     

Rotating frame proton spin-lattice relaxation measurements of poly (ethylene oxides)

Measurements of T_lρ as a function of temperature have been made on two polyethylene oxides (PEO) with molecular masses of 5,000 and 30,000. The T_1ρ measurements show biexponential behavior of the relaxation function in the temperature range from 170 K to 350 K. The intensities of the components of the relaxation function are constant over this temperature range in agreement with the crystallinities of the samples. The two relaxation times can be associated with the crystalline and amorphous component; the relaxation time minima describe the α relaxation in the crystalline regions of PEO and the glass transition in amorphous PEO.     

Lactone polymers. I. Glass transition temperature of poly-ε-caprolactone by means on compatible polymer mixtures

Dynamic mechanical properties determined with a torsion pendulum were used to ascertain the glass transition temperature T_g of poly-ε-caprolactone. By measurements on compatible blends of poly-ε-caprolactone and poly(vinyl chloride), the T_g of amorphous poly-ε-caprolactone was shown to be 202°K at about 1 cps. This is 16°K lower than the T_g of annealed, crystalline polymer. The blend transition data were well fitted by both the Fox and the Gordon-Taylor expressions. The Fox expression was also used to describe the decrease from 233°K of the secondary low-temperature relaxation due to poly(vinyl chloride) by assuming the low temperature relaxation of poly-ε-caprolactone, 138°K, was responsible for the decrease in the blends. The 138°K relaxation due to poly-ε-caprolactone was decreased when more than 50% poly(vinyl chloride) was present.     

Low-temperature behaviour of water in nafion membranes

Proton NMR measurements of T₁, T₁π and T₂ in “Nafion” perfluorosulphonate membranes, together with neutron- scattering and dielectric data, show that the aqueous phase in Nafion solidifies at a glass transformation whose temperature T_g is 168 K in water-saturated acid membranes. T_g is higher for the salt forms. Mo¨ssbauer measurements on Eu³⁺ Nafion confirm that the cations are present in an aqueous phase with T_g ≈ 220 K.     

Slow molecular mobility in the amorphous thermoplastic polysulfone

The thermally stimulated depolarization current (TSDC) technique has been used to study the slow molecular mobility of polysulfone in the glassy state and in the glass transformation region, i.e., in the temperature ranging from ?155 to 183 °C. Since the polysulfone is a rigid polymer without polar side-groups, a broad and low-intensity secondary relaxation was detected in the temperature region from ?120 °C up to the glass transition; the activation energy of the motional modes of this secondary relaxation is in the range between 35 and 100 kJ mol^?1. The glass transition temperature of polysulfone provided by the TSDC technique is T _M = T _g = 176 °C (at 4 °C min^?1). The relaxation time at this temperature is τ(T _g) = 33 s and the fragility index was found to be m = 91. Our results are compared with literature values obtained by dynamic mechanical analysis and by dielectric relaxation spectroscopy. The amorphous polysulfone was also characterized by DSC; a glass transition signal with an onset at T _on = 185.5 ± 0.3 °C (heating rate 10 °C min^?1) was detected, with ΔC _p = 0.21 ± 0.01 J g^?1 °C^?1.     

The temperature dependence of the local free volume in polyethylene and polytetrafluoroethylene: A positron lifetime study

Positron lifetime measurements, performed in the temperature range 80–300 K, are reported for polyethylene (PE) and polytetrafluoroethylene (PTFE). The lifetime spectra have been analyzed using the data processing routines LIFSPECFIT and MELT. Two long-lived components appear, which are attributed to pick-off annihilation of ortho-positronium in crystalline regions and at holes in the amorphous phase. The ortho-positronium lifetimes, τ₃ and τ₄, are used to estimate the crystalline packing density and the size of local free volumes in the crystalline and amorphous phases. The interstitial free volume in the crystals exhibits a weak linear increase with the temperature which is attributed to thermal expansion of the crystal unit cell. In the amorphous phase, the hole volume varies between 0.053 and 0.188 nm³ (PE) and between 0.152 and 0.372 nm³ (PTFE). Its temperature variation may be fitted by two straight lines, the intersection of which is used to estimate a glass transition temperature of T_g = 195 K for both PE and PTFE. The slopes of the free volume in the glassy and crystalline phases with the temperature correlate well with each other. The coefficients of thermal expansion of the hole volume are compared with the macroscopic volume change below and above the glass transition. From this comparison a fractional hole volume at T_g of 4.5 (PE) and 5.7% (PTFE) and a number of 0.73 (PE) and 0.36 (PTFE) × 10²⁷ holes/m³ is estimated. Finally, it is found that the intensity of o-Ps annihilation in crystals shows a different temperature dependence to that in the amorphous phase. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1513–1528, 1998     

Boson peak and fast relaxation process near the glass transition in polystyrene

Atactic polystyrene, both side group and main chain deuterated, was investigated by inelastic neutron scattering in a wide temperature range around the glass transition from 2 to 450 K. In the glass the Boson peak position is only very weakly influenced by the deuteration of the phenyl group. In the neighborhood of the glass transition temperatureT _g we find a fast relaxation process similar to other glasses. The onset of the fast relaxation in polystyrene, however, is observed already at temperaturesT _g — 200 K. Results from partially deuterated polystyrene suggest a change of the phenyl ring dynamics already far belowT _g.     

TMDSC phase angle for a better nanocomposite interphase identification

The present study suggests a new approach, based on the utilization of temperature modulated differential scanning calorimetry (TMDSC) technique, for identifying and characterizing the organic?Cinorganic interphase of two materials: an epoxy?Cfumed silica nanocomposite and a thermoplastic polyurethane (TPU)?Cmultiwalled nanotube (MWNT) composite. The approach used here makes use of TMDSC data and basically consists of using the phase angle or the derivative of the reversing heat flow instead of the reversing heat flow curve itself. In the case of epoxy?Cfumed silica composites, two glass transition regions were identified. The glass transition temperature (T _g) of the composite was observed to vary as a consequence of the filler content. This study shows that the T _g variation is due to the formation of an organic?Cinorganic interphase, with its own glass transition temperature, which is different from the epoxy matrix T _g. In the case of TPU?CMWNT composites, two relaxations and an additional first order transition were observed: the first relaxation corresponds to the hard segment, the second is related to an interaction between filler and matrix and the third process may be connected to the partial melting of the hard segment. The addition of 0.5?wt% MWNT causes a small reduction in T _g of the TPU. A major nanotube addition, 10?wt%, induces the appearance of a new relaxation that may be associated with the existence of an interface. In general, a better separation between the matrix and interphase glass transitions was obtained by the TMDSC phase angle signal.     

Electrical properties of a novel fluorinated polycarbonate

The relative permittivity, loss and dielectric strength have been measured for a polycarbonate-based material, tetraaryl bisphenol A polycarbonate, that has been fluorine substituted (DiF p-TABPA-PC). The new material has a glass transition temperature, T_g = 489 K, that is higher than that for either conventional bisphenol A polycarbonate (BPA-PC) for which T_g = 421 K or for a copolymer of tetraaryl bisphenol A (TABPA) and bisphenol A (BPA) (TABPA-BPA-PC) for which T_g = 464 K. In addition, the dielectric strength of DiF p-TABPA-PC is almost identical to that for purified BPA-PC and slightly larger than the value for TABPA-BPA-PC. The relative permittivity and loss measurements were carried out from 10 to 10⁵ Hz over a wide temperature range and at pressures up to 0.25 GPa. Variable temperature results for the α relaxation and both temperature and pressure results for the γ relaxation regions are reported. The α relaxation exhibits standard behavior. The γ relaxation exhibits unusual characteristics such as a strong increase in peak height as temperature increases and a strong decrease in peak height with increasing pressure. The data for the γ relaxation have been analyzed using several formulations. Expressions for the peak height and peak position based on a two state (inequivalent well) model and the resulting parameters are discussed in terms of the insight they provide into the molecular mechanisms responsible for the sub-T_g relaxation. Ab initio SCF results for a related model compound are presented. Finally, the real part of the relative permittivity for the new polymer is about 10% higher than for BPA-PC.     

Succinic or glutaric anhydride modified linear unsaturated (epoxy) polyesters

A series of N-alkyl-N-alkyl′-pyrrolidinium-bis(trifluoromethanesulfonyl) imide (TFSI⁻) room temperature ionic liquids (RTILs) has been investigated by means of thermogravimetric analysis (TG), differential scanning calorimetry, FT-IR spectroscopy, and X-ray diffraction analysis. These compounds exhibit a thermal stability up to 548–573 K. The mass loss starting temperature, T _ml, falls in a narrow range of temperatures: 578–594 K. FT-IR spectra, performed before and after 24 h isothermal experiments at 553 and 573 K, have confirmed their great thermal stability. Below the ambient temperature, these compounds exhibit a complex behavior. N-methyl-N-propyl-pyrrolidinium-TFSI is the sole liquid which crystallizes without forming any amorphous phase even after quenching in liquid nitrogen. Its crystalline phase has a melting point, T _m, of 283 ± 1 K. When the amorphous solid is heated, the N-butyl-N-ethyl-pyrrolidinium-TFSI presents a glass transition temperature, T _g, at 186 K followed by a cold crystallization, T _cc, at 225 K, and a final T _m at 262 K. The N-butyl-N-methyl-pyrrolidinium-TFSI exhibits a T _g between 186 and 181 K, its cold crystallization leading to two different solid phases. Solid phase I has a melting point T _I,m = 252 K and phase II, T _II,m = 262 K. When the amorphous phase is obtained at a cooling rate of 10 K/min, its T _cc is 204 K, and a metastable solid phase (III) is obtained which transforms into the phase II at 226 K. However, when the sample is quenched, the amorphous phase transforms into phase II at T _cc = 217 K and phase I at 239 K. P₁₅-TFSI exhibits the most complicated pattern as, on cooling, it leads to both a crystallized phase at 237 K and an amorphous phase at 191 K. On heating, after a T _g at 186 K and a T _cc at 217 K, two solid–solid phase transitions are observed at 239 K and 270 K, the final T _m being 279 K.     

The chemical and dielectric properties of epoxy/(Ba0.8Sr0.2)(Ti0.9Zr0.1)O3 composites for embedded capacitor application

The characteristics of epoxy/(Ba_0.8Sr_0.2)(Ti_0.9Zr_0.1)O₃ (BSTZ) composites are investigated for the further application in embedded capacitor device. The effects of BSTZ ceramic powder filler ratio on the chemical, physical and dielectric properties of epoxy/BSTZ composites are studied. Differential scanning calorimeter (DSC) thermal analysis is conducted to determine the optimum values of hardener agent, curing temperature, reaction heat, and glass transition temperature (T_g). The hardener reaction process starts at about 115 °C and completes at about 200 °C, for that it is appropriate to process of epoxy/BSTZ composites in the range of temperature. The highest glass transition temperature (T_g) of 155 °C is obtained at one equivalent weight ratio (hardener/epoxy). Only the BSTZ phase can be detected in the XRD patterns of epoxy/BSTZ composites. The more BSTZ ceramic powder is mixed with epoxy, the higher crystalline intensity of tetragonal BSTZ phase are revealed in the XRD patterns. The dielectric constant measured at 1 MHz increases from 5.8 to 23.6 as the content of BSTZ ceramic powder in the epoxy/BSTZ composites increases from 10 to 70 wt%. The loss tangents of the epoxy/BSTZ composites slightly increase with the increase of measurement frequency.     

Viscoelastic properties and phase behavior of 12‐tert‐butyl ester dendrimer/poly(methyl methacrylate) blends

This study used refractometry, ultraviolet–visible spectroscopy, Fourier transform infrared spectroscopy, differential scanning calorimetry, and dielectric analysis to assess the viscoelastic properties and phase behavior of blends containing 0–20% (w/w) 12‐tert‐butyl ester dendrimer in poly(methyl methacrylate) (PMMA). Dendritic blends were miscible up through 12%, exhibiting an intermediate glass‐transition temperature (T_g; α) between those of the two pure components. Interactions of PMMA C?O groups and dendrimer N? H groups contributed to miscibility. T_g decreased with increasing dendrimer content before phase separation. The dendrimer exhibited phase separation at 15%, as revealed by Rayleigh scattering in ultraviolet–visible spectra and the emergence of a second T_g in dielectric studies. Before phase separation, clear, secondary β relaxations for PMMA were observed at low frequencies via dielectric analysis. Apparent activation energies were obtained through Arrhenius characterization. A merged αβ process for PMMA occurred at higher frequencies and temperatures in the blends. Dielectric data for the phase‐separated dendrimer relaxation (α_D) in the 20% blend conformed to Williams–Landel–Ferry behavior, which allowed the calculation of the apparent activation energy. The α_D relaxation data, analyzed both before and after treatment with the electric modulus, compared well with neat dendrimer data, which confirmed that this relaxation was due to an isolated dendrimer phase. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1381–1393, 2001