Effect of cooling rate and frequency on the calorimetric measurement of the glass transition

The glass transition of thermoplastics of different polydispersity and thermosets of different network structure has been studied by conventional differential scanning calorimetry (DSC) and temperature modulated DSC (TMDSC). The cooling rate dependence of the thermal glass transition temperature T_g measured by DSC, and the frequency dependence of the dynamic glass transition temperature T measured by TMDSC have been investigated. The relation between the cooling rate and the frequency necessary to achieve the same glass transition temperature has been quantified in terms of a logarithmic difference Δ=log₁₀[|q|]−log₁₀(ω), where |q| is the absolute value of the cooling rate in K s⁻¹ and ω is the angular frequency in rad s⁻¹ necessary to obtain T_g(q)=T(ω). The values of Δ obtained for various polymers at a modulation period of 120 s (frequency of 8.3 mHz) are between 0.14 and 0.81. These values agree reasonably well with the theoretical prediction [Hutchinson JM, Montserrat S. Thermochim Acta 2001;377:63 [6]] based on the model of Tool–Narayanaswamy–Moynihan with a distribution of relaxation times. The results are discussed and compared with those obtained by other authors in polymeric and other glass-forming systems.     

Entropic model for the relaxation in vitreous systems. Estimation of uncertainty in the calculation of the conformational relaxation times

Differential scanning calorimetry experiments were conducted on a thermotropic polyester. The experimental results consists of a series of thermograms measured on heating after different thermal histories that contained (or not) an isothermal stage at a temperature T_a below the glass transition with a duration t_a. The thermogram showed the characteristic peak in the region of the glass transition. The calculation of the relaxation times of the co-operative conformational rearrangements related to the glass transition and structural relaxation phenomena has been conducted by curve fitting using a phenomenological model. The curve fitting procedure was conducted simultaneously on a fixed number n of experimental thermograms. The aim of the paper is to present a method to estimate the uncertainty in the calculation of the model parameter. The results show how the computer simulated thermograms agree quite well to the experimental data. The uncertainty in the model parameters and through them the uncertainty in the calculated relaxation times is quite important when the number of experimental curves n is small but rapidly decreases as n increases and if more than five curves are used simultaneously in the fitting routine both the values of the model parameters and their uncertainty become independent on the number and thermal histories of the experimental thermograms.     

Thermal and dielectric properties of powder coatings based on carboxylated polyester and β-hydroxyalkylamide

Thermal transitions, relaxations and the crosslinking reaction in varnishes and pigmented paints based on carboxylated polyester and β-hydroxyalkylamide (Primid) were studied by differential scanning calorimetry (DSC), temperature modulated DSC (TMDSC), thermogravimetry (TGA) and dielectric analysis (DEA). The effect of the Primid content on the glass transition of the powder coatings before and after crosslinking was analysed. The melting and flow regions were studied by TMDSC and correlated with the loss factor, which was measured by DEA. A comparison of the DSC and TGA scans showed that the crosslinking reaction was masked by the vaporisation of water formed during esterification. The resulting endothermic reaction made it difficult to analyse the crosslinking by DSC. Nevertheless, it was possible to detect the crosslinking reaction by the dielectric measurement of the loss factor and the conductivity.     

A comparison of concentration-glasses and temperature-hyperquenched glasses: CO2-formed glass versus temperature-formed glass

Prior work from this laboratory has reported anomalous differences in the viscoelastic responses between temperature-jump formed glasses and carbon dioxide pressure-jump or relative humidity formed glasses. In the present work, we investigate the anomalous behaviour further by examining the structural response of an epoxy resin after pre-annealing treatments. In particular, we have measured the volume change of amine-cured diglycidyl ether of bisphenol A after thermal and carbon dioxide pressure (PCO₂) treatments. Our results show that contrary to prior interpretations in the literature, a plasticizer quench is different from a temperature hyperquench. Consistent with our prior work, the CO₂-formed glass is more stable than the temperature-formed glass in spite of the former having a higher excess volume. Our new results show that the stability persists to above the nominal glass temperature, contrary to what happens in a temperature hyperquench.     

Changes in the thermomechanical behavior of epoxy glasses under the state of strain aging

Changes in thermomechanical behavior with structural relaxation taking place in epoxy glasses were studied. Differential scanning calorimetry measurements and thermostimulated strain recovery tests were performed for specimens deformed and then aged under fixed strain. In the course of heating, the specimens started to absorb thermal energy, whereas plastic strain was still stable. At higher temperatures, plastic strain started recovery, which was accompanied by exothermic behavior of the specimen. With an increase in the aging duration, the endothermic peak signified and moved to a higher temperature. These results indicated that the longer the aging duration was, the harder the plastic strain and strain energy were frozen in the glassy structure. This freeze‐strain phenomenon was observed for crosslinked epoxy glass, as well as polymeric glasses with linear molecular structures, aged under strain. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Crystallization behaviour of biodegradable poly(ethylene succinate) from the amorphous state

Nonisothermal and isothermal crystallization kinetics of biodegradable poly(ethylene succinate) (PES) from the amorphous state were studied by differential scanning calorimetry (DSC). For the nonisothermal crystallization, there were two crystallization exotherms upon heating from the amorphous state. One major crystallization exotherm located at low temperature corresponded to the real cold crystallization of PES, while the other minor one located at high temperature may correspond to the melt-recrystallization of the unstable crystals formed during the nonisothermal crystallization earlier. Several methods, such as Avrami equation, Tobin equation and Ozawa equation, were applied to describe the nonisothermal crystallization process of PES. Meanwhile, Avrami equation was also employed to study the isothermal crystallization of PES from the amorphous state. Similar to the nonisothermal crystallization the minor crystallization exotherm was also found in the DSC trace upon heating to the melt after the isothermal cold crystallization finished completely, and was attributed to the melt-recrystallization of the unstable crystals formed during the isothermal cold crystallization. Temperature modulated differential scanning calorimetry (TMDSC) was used in this work to investigate the origin of the minor crystallization exotherm located at high temperature, and the TMDSC experiments gave a direct evidence that the origin of the minor crystallization exotherm was from the melt-recrystallization of the originally existed unstable crystals formed through previous crystallization.     

Glass transition and structural relaxation in semi-crystalline poly(ethylene terephthalate): a DSC study

The aim of this work is to determine the relaxation times of the cooperative conformational rearrangements of the amorphous phase in semi-crystalline poly(ethylene terephthalate) (PET) and compare them with those calculated in amorphous PET. Samples of nearly amorphous polymer were prepared by quenching and samples with different crystallinity fractions were prepared from the amorphous one using cold crystallisation to different temperatures. The differential scanning calorimetry (DSC) thermograms measured on samples rapidly cooled from temperatures immediately above the glass transition show a single glass transition which is much broader in the case of high-crystallinity samples than in the amorphous or low-crystallinity PET. To clarify this behaviour, the samples were subjected to annealing at different temperatures and for different periods prior to the DSC measuring heating scan. The thermograms measured in samples with low crystallinity clearly show the existence of two amorphous phases with different conformational mobility, these are called Phases I and II. Phase I contains polymer chains with a mobility similar to that in the purely amorphous polymer, while Phase II shows a much more restricted mobility, probably corresponding to conformational changes within the intraspherulitic regions. The model simulation allows to determine the temperature dependence of Phase II relaxation times, which are independent from the crystallinity fraction in the sample and around two decades longer than those of the amorphous polymer at the same temperature.     

Glass-transition temperatures for soft-contact-lens materials. Dependence on water content

Glass-transition temperatures for three soft-contact-lens (SCL) materials are measured by modulated differential scanning calorimetry as a function of SCL hydration, as determined by thermogravimetric analysis. The SCL materials are: a conventional hydrogel (SofLens^® 38, polymacon) with a low water content at saturation; a conventional hydrogel (SofLens™ One Day, hilafilcon A) with a high water content at saturation; and a siloxane-hydrogel (PureVision™, balafilcon A).Polymacon, hilafilcon A, and balafilcon A turn glassy at 35 °C when their water contents drop below 10.4, 13.5, and 6 wt%, respectively. These water contents correspond to the equilibrium water uptake at 35 °C for polymacon, hilafilcon A, and balafilcon A at relative humidities, RH, of 74, 64, and 57%, respectively. Our results suggest that the outer surface of a soft contact lens worn on the eye may develop a glassy skin when exposed to air at low relative humidity. This glassy skin may alter fluid transport through the soft contact lens, and influence SCL-wear comfort.     

Comparison of the enthalpic relaxation of poly(lactide–co‐glycolide) 50:50 nanospheres and raw polymer

The phenomenon of enthalpic relaxation was evaluated for poly(lactide‐co‐glycolide) (PLGA, 50:50), in terms of storage of nanospheres for use as a controlled drug delivery system. Samples were stored for different times and temperatures below the glass transition temperature (T_g). Relaxation occurred at a significant rate up to 15 degrees below the T_g of 39.2°C. The effect of polymer morphology was considered by comparing the relaxation kinetics of the raw polymer with that of nanospheres formed using a novel technique. The nanospheres were shown to have a larger change in heat capacity at the glass transition and a longer average relaxation time than that of the raw polymer, and the relationship between these two parameters was discussed. For both the raw polymer and the nanospheres, relaxation was found to occur at a significant rate at room temperature. The storage of this system at subambient temperatures was therefore deemed important for maintaining the physicochemical properties of the system. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1868–1872, 2002     

Hydrostatic pressure influence on electric relaxation response of bismuth manganite ceramics

Electrical impedance of bismuth manganite ceramics was studied under ambient and high hydrostatic pressure. Local disorder of crystal lattice was confirmed using XRD and DSC. Two relaxation processes were discerned. One was attributed to small polarons, which showed a changeover from variable range hopping features related to Fermi glass and structural disorder to the nearest neighbor hopping behavior in higher temperature range related to uniform distribution of energy levels. Hydrostatic pressure shortened relaxation times that would be interesting for applications. The second process, which was assigned to the nearest neighbor hopping of polarons, also exhibited a changeover to glassy features in a high-temperature range.     

Enthalpy relaxation studies in polymethyl methacrylate networks with different crosslinking degrees

Structural relaxation of PMMA networks with distinct crosslink density has been studied by differential scanning calorimetry (DSC). The crosslinking agent used was ethylene glycol dimethacrylate (EGDMA). The experiments were carried out on heating after the samples have been subjected to distinct thermal histories, namely isothermal stages at different temperatures below the glass transition temperature for distinct times and cooling at different rates. These studies revealed a broadening of the glass transition with increasing crosslinking degree due to the constraints imposed by the crosslinks and suggested the presence of crosslink heterogeneity in the networks. A phenomenological model based on the configurational entropy concept was used to simulate the structural relaxation phenomenon and to evaluate the temperature dependence and distribution of the relaxation times of the conformational rearrangements for these networks. The agreement between the experimental results and the simulated thermograms was quite satisfactory.In addition, the kinetic fragility of the networks was evaluated from the results corresponding to the thermal treatments at distinct cooling rates. It was found an increase of the fragility index m with increasing crosslinking degree.     

Thermal properties and enthalpy relaxation of tyrosine-derived polyarylates

Sixteen degradable, tyrosine-derived polyarylates with well-defined chemical structures were used to study the effect of polymer structure on the glass transition temperature and enthalpy relaxation kinetics (physical aging). These polyarylates compose a model system where the number of methylene groups present in either the pendent chain or the polymer backbone can be altered independently and in a systematic fashion. Quantitative differential scanning calorimetry was employed to measure the glass transition temperature and the enthalpy relaxation kinetics. Correlations between these material properties and the polymer structure were established. The glass transition temperature of this family of polymers ranged from 13 to 78°C. The addition of methylene groups to either the pendent chain or the polymer backbone made a fairly constant contribution to lowering the glass transition temperature. The rate of enthalpy relaxation increased with an increasing number of methylene groups in the polymer backbone, but was independent of the number of methylene groups in the pendent chain. This observation indicated that the rate of enthalpy relaxation in these polymers was limited by the mobility of the polymer backbone. The enthalpy relaxation data was fitted to the Cowie-Ferguson model and the relaxation times obtained ranged from 44 min to about 100 min. Although these structure-property correlations facilitate the design of new materials with predictable thermal properties, they are rarely investigated for biomedical polymers. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 1441–1448, 1997     

Thermal memory of polyethylenes analyzed by temperature modulated differential scanning calorimetry

Temperature modulated differential scanning calorimetry (TMDSC) was employed to study the melting and crystallization behavior of various polyethylenes (PEs). Samples of high density PE (HDPE), low density PE (LDPE), linear low density PE (LLDPE), and very low density PE (VLDPE) with different crystal structures and morphologies were prepared by various thermal treatments (isothermal crystallization and slow, fast, and dynamic cooling). The reversing and nonreversing contributions, measured on the experimental time scale, were varied, depending on the crystal stability. A relatively large reversing melt contribution occurs for unstable crystals formed by fast cooling compared to those from slow cooling treatments. All samples of highly branched LDPE, LLDPE, and VLDPE showed a broad exotherm before the main melting peak in the nonreversing curve, suggesting crystallization and annealing of crystals to more stable forms. Other samples of HDPE, except when cooled quickly, did not show any significant crystallization and annealing before melting. The crystallinity indicated that dynamically cooled polymers were much more crystalline, which can be attributed to crystal perfection at the lamellar surface. A reversible melting component was also detected during the quasiisothermal TMDSC measurements. Melting is often accompanied by large irreversible effects, such as crystallization and annealing, where the crystals are not at equilibrium. Such phenomena during a TMDSC scan provide information on the polymer thermal history. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 681–692, 2003     

聚合物DSC测试结果的影响因素探讨

以聚酯切片为例,对差示扫描量热法(DSC)测定聚合物的热性能的影响因素进行探讨。结果表明,盛放试样的容器可用国产铝坩埚;进行二次升温时升温速率宜低;根据试样的特性,用氮气保护,氮气流量以20mL/min为宜;试样量为5～10mg,粒径小于0.5 mm,可以降低测试成本和提高结果的准确度。     

Understanding the effect of chain entanglement on the glass transition of a hydrophilic polymer

In this article, we report an interesting phenomenon of the glass transition temperature (T_g) deviation of a hydrophilic polymer. Polyacrylamide (PAL) samples with different extents of chain entanglement were prepared by spray drying and solution casting. We found that the glass transition temperature increases as the extent of chain entanglement decreases upon the sub‐T_g annealing. The water content in the PAL matrix is found with no direct correlation to T_g. However, the observation of a faster diffusion process of water in the disentangled PAL matrix offers an evidence of a faster relaxation process of disentangled PAL molecules. The T_g increase of the disentangled PAL samples is believed to be associated with the increased molecular interaction during the chain relaxation process upon the sub‐T_g annealing. A macroscopic evidence is the fact that the density of the hot‐laminated samples increases as the extent of chain entanglement decreases. A thermodynamic analysis suggests that enthalpy more than entropy favors an elevated T_g of a disentangled hydrophilic polymer upon the sub‐T_g annealing. We believe that this research provides new understanding of T_g of the hydrophilic polymers, which are being extensively used in bio‐related studies. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Glass transition,topology, and elastic models of Se-based glasses

Features intrinsic to disorder and network aspects are ubiquitous in structural glasses. Among this important class of materials, chalcogenide glasses are special—they are built of short-range covalent forces, making them simpler than silicate glasses that possess mixed ionic and covalent forces. Selenium-based glasses also display complex elastic phase transitions that have been described from various models, including mean-field approaches to molecular simulations. These point to the presence of two sharply defined elastic phase transitions, a rigidity and stress transition that are non-mean-field in character, and separate the three distinct topological phases of flexible, isostatically rigid, and stressed-rigid. This article reviews the physics of these glassy networks. The elastic phases and glass transition temperature are explained on a molecular level in terms of topological constraint theory (TCT), connectivity, and the open degrees of freedom. The broader aspects of TCT in relation to phase change materials, high-k dielectrics, and cements are also commented upon.     

Vitrification,devitrification, and dielectric relaxations during the non‐isothermal curing of diepoxy‐cycloaliphatic diamine

The curing of an epoxy resin based on diglycidyl ether of bisphenol A (DGEBA) with a diamine based on 4,4′‐diamino‐3,3′‐dimethyldicyclohexylmethane (3DCM) was analyzed by dielectric relaxation spectroscopy (DRS) between ?100 and 220°C, at heating rates ranging from 0.1 to 2 K min^?1. The permittivity, ε′, and the loss factor, ε″, were measured by DRS in the frequency range between 1 and 100 kHz. The dielectric relaxations were correlated with the relaxations observed previously by temperature modulated differential scanning calorimetry (TMDSC) at the same heating rates and in modulation conditions of amplitude 0.2 K and a period of 60 s, which is equivalent to a measuring frequency of 16.7 mHz. The dielectric measurements showed three frequency‐dependent dipolar relaxations and one ionic relaxation, which was independent of the frequency. The dipolar relaxations were associated with the glass transition of the unreacted system and the vitrification and the devitrification processes of the system during the crosslinking reaction, and the ionic relaxation was associated with the beginning of the crosslinking reaction. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 558–563, 2006     

Study of the dielectric relaxation of poly(phenylpropyl acrylate) and poly(phenylpropyl methacrylate): effect of slight differences in chemical structure

A comparative study of the dielectric relaxational behaviour of two structurally close polymers, containing aromatic side groups, was carried out in order to analyse how slight differences in the chemical structure affect the molecular responses to perturbation field. Specifically, poly(phenylpropyl acrylate) (P3Ph1PA) and poly(phenylpropyl methacrylate) (P3Ph1PM) were studied using differential scanning calorimetry and dielectric relaxation spectroscopy in the frequency range 10^?2–10⁶ Hz and temperature window of ?80 to 120 °C. Both techniques show one glass–rubber transition for P3Ph1PA and two for P3Ph1PM, which evidence the great effect of the methyl groups on the segmental motions of the polymer. Phenomenological analysis of the data was carried out in order to establish the strength, width and fragility parameters of the glass–rubber transitions. In the case of P3Ph1PA, the strength is found to be larger than for P3Ph1PM, pointing out that the methyl group disturbs the mobility. Conductive processes dominate the dielectric spectra at high temperatures and low frequencies. © 2015 Society of Chemical Industry