Tg Relaxation Research Articles

Fast crystallization below the glass transition temperature in hyperquenched systems.

Many phase change materials (PCMs) are found to crystallize without exhibiting a glass transition endotherm upon reheating. In this paper, we review experimental evidence revealing that these PCMs and likely other hyperquenched molecular and metallic systems can crystallize from the glassy state when reheated at a standard rate. Among these evidences, PCMs annealed below the glass transition temperature Tg exhibit slower crystallization kinetics despite an increase in the number of sub-critical nuclei that should promote the crystallization speed. Flash calorimetry uncovers the glass transition endotherm hidden by crystallization and reveals a distinct change in kinetics when crystallization switches from the glassy to the supercooled liquid state. The resulting Tg value also rationalizes the presence of the pre-Tg relaxation exotherm ubiquitous of hyperquenched systems. Finally, the shift in crystallization temperature during annealing exhibits a non-exponential decay that is characteristic of structural relaxation in the glass. Modeling using a modified Turnbull equation for nucleation rate supports the existence of sub-Tg fast crystallization and emphasizes the benefit of a fragile-to-strong transition for PCM applications due to a reduction in crystallization at low temperature (improved data retention) and increasing its speed at high temperature (faster computing).

Effect of Different Phosphate Glass Compositions on the Process-Induced Macromolecular Dynamics of Polyamide 66.

The present study provides a fundamental understanding of the mechanism of action of special new phosphate glass (P-glass) systems, having different glass transition temperatures (Tg), in polyamide 66 (PA66). Dynamic mechanical analysis (DMA) revealed that the Tg of PA66/low Tg P-glass (ILT-1) was significantly shifted to a lower Tg (65 °C), and another transition appeared at high temperature (166 °C). This was supported by a drop in the melting point and the crystallinity of the PA66/ILT-1 hybrid material as detected by differential scanning calorimetry (DSC). The dielectric spectroscopic investigation on the networks’ molecular level structural variations (Tg and sub-Tg relaxations) agreed very well with the DMA and DSC findings. Contrary to intermediate Tg(IIT-3) and high Tg P-glass (IHT-1) based materials, the PA66/ILT-1 hybrid material showed an evidence of splitting the PA66 Tg relaxations into two peaks, thus confirming a strong interaction between PA66 and ILT-1 (low Tg P-glass). Nevertheless, the three different P-glass compositions did not show any effect on the PA66 sub-Tg relaxations (related to the –NH2 and –OH chain end groups’ motion).

Inkjet printing of 3D nano-composites formed by photopolymerization of an acrylate monomer

This paper considers inkjet printing of optical quality 3D nano-composites from diethylene-glycol diacrylate monomer (DEGDA) containing ZrO2 nanoparticles at varying concentrations. One application for the composites is for gradient refractive index (GRIN) lenses. The process involves printing of a nanoparticle loaded monomer “ink” onto a substrate and then photopolymerizing the monomer layer by layer using UV light. The results of the study confirm that the presence of nanoparticles favorably affects the reaction kinetics. The reaction rate and chemical conversion are enhanced considerably by the nanoparticles. A seamless interface between the layers, which are 20μm thick, may be achieved if the conversion level of the layer onto which ink is deposited is limited at 60 to 80%. Dynamic mechanical analysis (DMA) data indicate that both the glass transition and sub Tg viscoelastic properties are influenced by the nanoparticles. When nanoparticles are introduced the Tg relaxation shifts to lower temperatures and a sub Tg relaxation appears whose intensity increases with particle concentration. These results are consistent with a molecular confinement model involving a lower crosslink density rubbery layer at the polymer–particle interfaces.

Free volumes and structural relaxations in diglycidyl ether of bisphenol-A based epoxy–polyether amine networks

Two types of polyether diamines were used to prepare model rubbery and glassy epoxy–amine networks with diglycidyl ether of bisphenol-A; α,ω-diamino terminated polyoxypropylene (POP) diamines and α,ω-diamino terminated poly(oxypropylene)-block-poly(oxyethylene)-block-poly(oxypropylene)s (POP-POE-POP). The structural relaxations in the glassy and rubbery epoxy–amine networks at segmental (α relaxation) and local (β relaxation) levels were investigated by modulated differential scanning calorimetry (MDSC) and dynamic mechanical analysis (DMA). The characteristic length of glass transition of the networks ξ(Tg) was evaluated from MDSC using Donth's thermal fluctuation approach. While the POP diamine networks showed ξ(Tg) values of 2.0 and 2.07 nm, for POP diamine molecular weights of 230 and 400, respectively, the corresponding values for POP-POE-POP diamine networks were found to be 1.41 and 1.58 nm for POP-POE-POP diamine molecular weights of 600 and 900. This implied diminishing size of the cooperatively rearranging regions with decreasing crosslink density. DMA measurements were used to evaluate the crosslink density of the networks, characteristic features of the α and β transitions in terms of the width, intensity of transitions, and activation energy of the β relaxation. The studies revealed highly cooperative sub-Tg β relaxations for the glassy networks and a truncated but pronounced β relaxation for the rubbery networks. Positron annihilation lifetime spectroscopy (PALS) was used to characterize the molecular topology of the networks in terms of the free volume nanohole sizes and their distribution. The difference of the average distance between crosslink points and the free volume nanohole size was seen to increase with the chain length of the diamines, indicating the fluctuational nature of the networks influenced by the sub Tg relaxation.

Synergistic temperature and pH effects on glass (Tg) and stimuli-responsive (TSR) transitions in poly(N-acryloyl-N′-propylpiperazine-co-2-ethoxyethyl methacrylate) copolymers

N-Acryloyl-N′-propylpiperazine (AcrNPP) and 2-ethoxyethyl methacrylate (EEMA) monomers were copolymerized to form random stimuli-responsive p(AcrNPP/EEMA) copolymers in a form of colloidal dispersions which upon coalesce form uniform films. The presence of AcrNPP units facilitates temperature and pH responsiveness, thus resulting in composition-dependent and pH–temperature sensitive endothermic transitions: the glass (Tg) and stimuli-responsive (TSR) transitions. These studies show that the relationship between the newly discovered TSR and known Tg relaxations can be predicted by the following formula: 1/TSR = [Tg1 × Tg2 × (Tbinary − T)]/[Tbinary × T × (Tg1 − Tg2) × Tg] + (Tg1 × T − Tbinary × Tg2)/[Tbinary × T × (Tg1 − Tg2)], where TSR is the stimuli-responsive transition temperature, Tg is the glass transition temperature of the copolymer; Tbinary is the temperature of stimuli-responsive homopolymer in a binary polymer–water equilibrium, Tg1 and Tg2 are the glass transition temperatures of stimuli-responsive and non-stimuli-responsive homopolymers, respectively, and T is the film formation temperature. Experimental spectroscopic and differential scanning calorimetry (DSC) evidence showed that dipole–dipole interactions are responsible for the molecular changes at the TSR for a non-protonated state, and the shift of the TSR under protonated conditions is attributed to the synergistic pH and temperature effects associated with H-bonding and conformational backbone and side chain rearrangements. Computer simulations also showed that the buckling of the copolymer backbone and collapse of propylpiperazine groups occur above TSR. The total energies (ΔEtotal) of the TSR transitions for protonated and non-protonated states are 159 and 132 kcal mol−1, respectively, and are in good agreement with the energy values determined experimentally (DSC).

New Silicone Grafted Copoly(ether imide) from 4,4′-(hexafluoro-isopropylidene)diphthalic Anhydride

A new copoly(ether imide) 1a was synthesized from 4,4′-(hexafluoro-isopropylidene)diphthalic anhydride (6FDA) and 4,4′- bis [3′-trifluoromethyl-p-aminobiphenyl ether)biphenyl (TFBB) with 20 wt% loading of 3,5-diaminobenzoate terminated polydimethylsiloxane (DBPDMS) by solution imidization technique. The copoly(ether imide) showed a glass transition temperature of 272°C by differential scanning calorimetry and of 300°C by dynamic mechanical analysis, respectively. The polymer showed thermal stability up to 371°C for 5% wt loss in synthetic air. Dynamic mechanical analysis showed storage modulus as high as 1.62 GPa at 40°C. In the DMA plot a distinct sub Tg relaxation was observed at 130°C. The copolyimide formed tough transparent film, with tensile strength up to 47 MPa, a modulus of elasticity up to 1.83 GPa and elongation at break up to 65%. Thermal, mechanical and surface properties of the copoly(ether imide) have been evaluated and compared with the already reported analogous random copoly(ether imide) containing siloxane unit in the main chain and with the homopolyimide without siloxane unit.

Research on the Thermal Oxidation and Ultraviolet Radiation Aging of Nano-ZnO/NR Composite

To study the effection of nano-ZnO on thermal oxidative aging and ultraviolet radiation aging of NR, and the mechanisms of thermal oxidation aging and ultraviolet radiation aging of nano ZnO in NR were explained by TG and stress relaxation and ultraviolet analysis. The results revealed that nano-ZnO can improve the anti-thermal oxidation aging and anti-ultraviolet radiation aging of NR, and the protective effect improves with the increase dose of nano-ZnO.

High-Tg GeS2 based glasses for chemical sensing applications

Chalcogenide glasses based on the GeS2–Ga2S3–CsI system have been engineered for high temperature optical fibres operating above 150 °C in chemical process environments. The glass structure has been modified to enhance the glass transition temperatures (Tg) from 350 to 450 °C, leaving a clear margin of 200 °C at which the structural relaxation is likely to be insignificantly small for contributing to structural damage to the fibre. We have systematically carried out the Raman spectroscopy at 514 nm for analysis of the structural changes as a function of composition. The Raman spectroscopic analysis is also complemented by the Infrared (IR) transmission spectroscopy for the characterisation of likely vibrational modes. The glass compositions were also analysed for their devitrification and near Tg relaxation behaviour. On the basis of the devitrification characteristics we have also characterised the thermal stabilities of various GeS2–Ga2S3–CsI modified glasses. The structural relaxation behaviour above the Tg was also characterised using a differential mechanical analyser by measuring the viscosity (η).

A note on ‘sub- Tg relaxation processes in poly(cyclohexylmethacrylate)’

We have critically examined the various relaxation processes occurring in poly(cyclohexylmethacrylate) using dielectric spectroscopy. In addition to the α- and γ-processes found earlier by other workers, we have detected a secondary (β-)process in the temperature range of 293–353 K with an activation energy of about 73 ± 5 kJ/mol.

Dielectric spectroscopy measurements of the sub- Tg relaxations in amorphous ethyl cellulose: A relaxation magnitude study

Amorphous ethyl cellulose exhibits three secondary relaxations at temperatures below its glass transition. The fitted parameters that describe these processes and the comparison with other polysaccharides allow to ascribe the relaxations to lateral groups on one hand and to local main chain motion on the other hand. Their contributions to the dielectric constant overlap and induce a broad dielectric losses peak. The amplitude of one of these relaxations is found to decrease regularly with time. It is believed it comes from a gradual change of the polar groups chemical environment that constrains their motion.

Order in amorphous di‐n‐alkyl itaconate polymers, copolymers, and blends

AbstractThe presence of a main‐chain correlation distance (dII) in the poly(di‐n‐alkyl itaconate)s was confirmed with small‐angle X‐ray scattering/wide‐angle X‐ray scattering measurements taken over the temperature range of 293–478 K. Data for a series of alkyl acrylate polymers were also obtained for comparison. The intensity of the itaconate dII peak was significant and indicated a greater level of nanophase formation than in analogous systems. In the lower members of the series, nanophase formation appeared to be further enhanced in the temperature range above the glass‐transition temperature (Tg). This was ascribed to the rapidly increasing main‐chain mobility in this region. Macroscopically phase‐separated itaconate blends displayed the individual dII nanospacings of each homopolymer component. Copolymers, on the other hand, showed more interesting behavior. Poly(methyl‐co‐di‐n‐butyl itaconate) followed an average behavior in which the dII spacing and Tg changed progressively with the comonomer content. In contrast, the side‐chain pairing in poly(methyl‐co‐di‐n‐octyl itaconate) generated dII spacings characteristic of separate methyl and octyl nanodomains. The observation of the dioctyl nanodomains, along with the dioctyl side‐chain lower Tg relaxation event, confirmed the concept of independent side‐chain‐domain relaxation in these polymers. The temperature behavior of the poly(methyl‐co‐di‐n‐octyl itaconate) small‐angle X‐ray scattering profiles and scattering correlation lengths indicated that the two nanodomains were not completely structurally independent. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4000–4016, 2004

The sub- Tg relaxations in pure and antiplasticized model epoxy networks as studied by high resolution creep rate spectroscopy

An original laser-interferometric creep rate spectroscopy method was used for studying the molecular mobility in the glassy state of a series of ‘model’ epoxy-amine networks varying in cross-link density and rigidity. The experiments were carried out over the temperature range 115–300 K, which covers the regions where the secondary relaxations are expected to occur. The creep rate spectra (CRS) obtained at low compressive stresses exhibited systematically multiple creep rate ε ̇ peaks, irrespective of the applied stress and the experimental time in the ranges under consideration, namely 20–40 MPa and 10–40 s, respectively. Data analysis was based on the inspection of the major peaks of the CRS without entering much into the details of the fine structure. The peaks assigned to the β relaxation were shown to depend on the changes in network cross-link density and to the addition of antiplasticizing molecules in the same way as reported earlier from dynamic mechanical analysis (DMA) and 13C nuclear magnetic resonance (NMR) studies. In the case of the resins cured with aromatic amines, for which no γ relaxation is evidenced by DMA, creep rate spectroscopy allowed the detection of an additional relaxation, so called γ′, at temperatures well below the β process. It was tentatively assigned to DMA-inactive motions of the aromatic amine residues, i.e. to the phenyl ring flips of the diphenylmethane units.

Characterisation of the glass transition of an amorphous drug using modulated DSC.

The use of modulated differential scanning calorimetry (MDSC) as a novel means of characterising the glass transition of amorphous drugs has been investigated, using the protease inhibitor saquinavir as a model compound. In particular, the effects of measuring variables (temperature cycling, scanning period, heating mode) have been examined. Saquinavir samples of known moisture content were examined using a TA Instruments 2920 MDSC at a heating rate of 2 degrees C/min and an amplitude of +/-0.159 degrees C with a period of 30 seconds. These conditions were used to examine the effects of cycling between - 50 degrees C and 150 degrees C. A range of periods between 20 and 50 seconds were then studied. Isothermal measurements were carried out between 85 degrees C and 120 degrees C using an amplitude of +/-0.159 degrees C with a period of 30 seconds. MDSC showed the glass transition of saquinavir (0.98 +/- 0.05%w/w moisture content) in isolation from the relaxation endotherm to give an apparent glass transition temperature of 107.0 degrees C +/- 0.4 degrees C. Subsequent temperature cycling gave reproducible glass transition temperatures of approximately 105 degrees C for both cooling and heating cycles. The enthalpic relaxation peak observed in the initial heating cycle had an additional contribution from a Tg "shift" effect brought about by the difference in response to the glass transition of the total and reversing heat flow signals. Isothermal studies yield a glass transition at 105.9 degrees C +/- 0.1 degrees C. MDSC has been shown to be capable of separating the glass transition of saquinavir from the relaxation endotherm, thereby facilitating measurement of this parameter without the need for temperature cycling. However, the Tg "shift" effect and the number of modulations through the transition should be taken into account to avoid drawing erroneous conclusions from the experimental data. MDSC has been shown to be an effective method of characterising the glass transition of an amorphous drug, allowing the separate characterisation of the Tg and endothermic relaxation in the first heating cycle.

The molecular relaxation mechanisms in cork as studied by thermally stimulated discharge currents

The dielectric relaxation mechanisms present in cork have been investigated in the temperature range -100 to 100 °C using the technique of thermally stimulated discharge currents. A relaxation mechanism was detected which showed a compensation behaviour as observed for the α-relaxation (or glass transition relaxation) of synthetic polymers and which lead us to attribute to cork a glass transition-like temperature of 18 °C. One lower temperature mechanism was also observed, with low activation enthalpy and entropy, which is presumably originated by local motions (internal rotations) of polar molecular groups. An upper Tg relaxation of higher intensity was also detected which was attributed to large-scale non-cooperative motions of polymeric segments.

Aging and sub- Tg relaxation in arsenic selenide glass fibers

The strengths of arsenic selenide glass fibers have a complex time dependence. A large degradation in strength occurs after fiberization at the beginning of aging: approximately 30 to 40% of freshly drawn strength can be lost during the first several days of aging. The same pattern of strength degradation during aging was observed in fibers that were stored under vacuum, which suggests that the strength degradation process is mainly a physical process of relaxation unrelated to the storing atmosphere. Sub- T g stress and structural relaxations can take place in arsenic selenide glass fibers at temperatures substantially below T g . The characterization of the relaxation function for these materials requires more than a single exponential function.

Thermal analyses of graft copolymers of poly (methyl methacrylate) main chain with poly (propylene oxide‐b‐ethylene oxide) graft chain

AbstractPoly(methyl methacrylate‐g‐(propylene oxide‐b‐ethylene oxide)) and poly (methyl methacrylate‐g‐(ethylene oxide‐b‐propylene oxide)), comprising chemically dissimilar sequences, exhibit intramolecular phase separation. These compositions have applications in coatings and as surface‐tension modifiers. This paper presents the thermal behavior of these graft copolymers: separate samples of the homopolymer and of the grafts were also analyzed to provide comparisons. The phase behavior has been analyzed by differential scanning calorimetry and by dynamic‐mechanical thermal measurements. Two glass transitions (Tg) are observed, caused by the partial incompatibility within the copolymers. The activation energy of the Tg relaxation process of the main chain is decreased by the graft chain. The influence of poly(propylene oxide‐b‐ethylene oxide) grafts on the thermal degradation of the poly(methyl methacrylate) (PMMA) main chain was studied by using thermogravimetric analysis. Prolysis of the graft copolymers occurs in three stages and begins on the graft chain and at a lower temperature than the pyrolysis of pure PMMA. Both the phase behavior and the thermal stability are found to depend sensitively on the composition of the copolymer. © 1993 John Wiley & Sons, Inc.

Effect of orientation on the Tg relaxation of polymers

Effect of orientation on the Tg relaxation of polymers

Gas transport and sub- Tg relaxations in unmodified and nitrated polyarylethersulfones

Aromatic nitration has been used to modify the polymer main chain of polysulfone and polyethersulfone. Alteration in the gas separation characteristics of homogeneous membrane made out of these polymers could be observed upon nitration. Generally, separation factors for gas mixtures of CO 2/CH 4 increased while the permeability decreased. Remarkably, a decreasing diffusivity for both gases upon nitration could not be correlated with an increasing free-volume but was interpreted as an drastically decreased main-chain flexibility. The decrease of main-chain segmental mobility could be verified experimentally by a diminished amount of β-relaxations determined by torsional braid analysis.

Application of compensation phenomena to the thermal analysis characterization of polymers: Introducing the degree of disorder (DOD) number:

AbstractThe objective of this article is to show that a deconvolution of the global Tg relaxation peak obtained by thermal stimulated current characterization technique leads to a better understanding of the coupling characteristics between the relaxation modes. The degree of coordination, i.e., coupling, is function of the state of the system and is revealed by the compensation phenomenon characteristics. In a compensation phenomenon, Arrhenius lines gather into packs of lines which converge to a single point, “the compensation point”. The coordinates of the compensation point are dependent on the actual state of the glass. We find that plots of the thermo‐kinetic variables against the polarization temperature Tp, when Tp varies, leads to characteristic results of the Relaxation Map: Tg is characterized by a maximum of the Entropy of activation (and of the enthalpy of activation), and also by the existence of two compensation lines, one positive one negative, which intercept for Tp = Tg. The relaxation map looks like a “Z” across Tg, as the Arrhenius relaxation lines gather into either a positive or negative compensation. We introduce a new method to characterize Tg, and to determine, from the Z‐structure, the activation entropy ΔSg, and the activation enthalpy ΔHg at Tg.

Ionic Conductivity of Bulk, Gels and Solutions of Perfluorinated Ionomer Membranes

Perfluorinated ionomer membranes either sulfonated or carboxylated have been extensively studied in order to understand the relations between their structure and properties. In addition to their uses as separator in the Cl2/NaOH industry large potential applications exist in different fields like sensors, batteries, fuel cells, electrochromic displays, etc. Some of these applications are due to the possibility of getting solutions, gels from these materials in different solvents and of reconstructing membranes of different thicknesses. The aim of this presentation is to present conductivity measurements of solutions, gels and bulk membranes, which measurements are of primary interest for potential applications in electrochemical devices. Room temperature measurements of conductivity of solutions/gels: First of all we recall the results already published which show the high conductivity of these gels associated with a transport number t+=1 since the COO− or SO3− anions are covalently bound to the rigid perfluorinated rod like structure. Low temperature measurements: Polymers having ionic conductivity usually show drastic changes upon temperature associated to transition (Tg, Tm or sub Tg relaxations). Measurements of conductivity of NAFION® 1100 solutions in propylene carbonate or dimethoxyethane over a temperature range between +20°C and −40°C have shown only small changes mainly due to viscosity modifications of the solvent as evidenced from the calculated activation energies. Conductivity/swelling measurements: We have recently developed an experiment which permits to obtain both the conductivity and the swelling of ion exchange membranes. Equilibrium experiments with solvent mixtures permit to check the conductivity and swelling changes versus different parameters (dielectric constant of solvent, ionic radius of the counterion, etc.). Diffusion coefficients are also obtained from the time lag when immersing the membrane in the solvent.