StepScan DSC

StepScan DSC technique was used for the study of the glass transition phenomenon. This method allows relatively good monitoring of reversing and non-reversing processes, and thus is very useful tool for glass transition studies of wide type of glass-forming materials, including inorganic glasses as well as organic polymers. In this work, experience with the StepScan DSC technique is summarized. Some interesting results of its application are presented, such as determination of glass transition temperature independent on thermal history of glass, discovered relationship between the slope of temperature dependence of C _p in T _g, and Angel’s index of fragility and estimation of viscosity glass transition temperature, T _g,η.     

Preparation and properties of waterborne polyurethane–urea anionomers—influences of the type of neutralizing agent and chain extender

Waterborne polyurethane–urea anionomers were prepared by polyaddition reaction using isophorone diisocyanate (IPDI), poly(tetramethylene ether) glycol (PTMG, Mn=1000), dimethylolbutanoic acid (DMBA), and hydrazine monohydrate (HD), ethylene diamine (EDA), 1,4-butane diamine (BDA) as a chain extender, followed by neutralization of pendant COOH groups by NH₄OH/Cu(OH)₂ or triethylamine (TEA) as a neutralizing agent. The effects of the types of neutralizing agent such as NH₄OH/Cu(OH)₂ and TEA with various chain extenders on the properties of waterborne polyurethane–urea ionomers were investigated. Two loss modulus peaks for all samples are observed owing to the glass transition temperature of soft segments (Tg_s) and the glass transition temperature of hard segments (Tg_h). The conductivity, Tg_h, Tg, and tensile strength/modulus of TEA-based samples increased in the order of BDA>EDA>HD; however, those of NH₄OH/Cu(OH)₂-based samples increased in the order of HD>EDA>BDA. TEA-based film samples were found to have higher thermal stability, Tg_h, Tg, tensile strength/modulus, and storage modulus than NH₄OH/Cu(OH)₂-based ones at the same chain extender. On the other hand NH₄OH/Cu(OH)₂-based samples had higher conductivity and stronger antibacterial halo than TEA-based samples.     

TMDSC analysis of single-site copolymer blends after thermal fractionation

Temperature-modulated differential scanning calorimetry (TMDSC) has been used to study the melting of a series of blends containing linear low-density polyethylene (LLDPE) and very low-density polyethylenes (VLDPE) with long chain branches. After the blends were subjected to different thermal histories including thermal fractionation by stepwise isothermal cooling, they were examined by TMDSC. TMDSC curves have been interpreted in terms of a combination of the reversing and non-reversing specific heats that result from reversible and irreversible events at the time and temperature, which they are detected, respectively. It was found that crystals formed at different crystallisation conditions had different internal order; hence they showed different amounts of reversing and non-reversing contributions. There is no exothermic activity seen in the non-reversing signal for the thermally fractionated polymers and their blends suggesting formation of crystals approaching equilibrium. In contrast, polymers and blends cooled at 10°C min^-1 cooling rate showed large exothermic contributions corresponding to irreversible effects. In addition, a true reversible melting contribution is also detected for both fast-cooled and thermally-fractionated samples during the quasi-isothermal measurements. This revised version was published online in July 2006 with corrections to the Cover Date.     

A Modulated Differential Scanning Calorimetric study

Modulated Differential Scanning Calorimetry^? has been applied to frozen sucrose solutions in the concentration range 10–80% w/w. The results from this study present, for the first time, information on the reversing and non-reversing nature of events that occur in these solutions. The study demonstrates the potential benefits of this new technique to help separate complex transitions that can occur in the total heat-flow curves obtained using traditional differential scanning calorimetry. The results illustrate how this new technique can separate the different enthalpic events, which relate to the glass transition and the onset of ice dissolution that occurs during the heating of these frozen systems, by nature of their “reversing” and “non-reversing” contributions to the total heat flow.     

Use of thermal analysis techniques for examining blow-molded pet bottle specimens

Eight samples from different areas of stretch-blow-molded poly(ethylene terephthalate) [PET] bottles, including a PET resin control, were tested by differential scanning calorimetry (DSC) and thermomechanical analysis (TMA). The glass transition temperature (T _g) was found to linearly decrease about 6C from zero to 45 percent initial crystallinity. Measurements ofT _c (crystallization temperature, DSC) and film tension modulus (TMA) were related to crystallization rate during stretch-blow-molding. The TMA linear coefficients of thermal expansion and shrinkage were shown to be important for blow-molding temperature control.     

Estimating the reversing and non‐reversing heat flow from standard DSC curves in the glass transition region

A mathematical model for the total heat flow obtained in differential scanning calorimetry (DSC) experiments from polymers with enthalpic relaxation is proposed. It is limited to the glass transition and enthalpic relaxation range of temperature and to the cases where the enthalpic relaxation is the only non‐reversing process taking place. The model consists of a mixture of functions representing the heat capacity heat flow of the glassy and non‐glassy fractions, the glass transition progress and the enthalpic relaxation heat flow. Optimal fittings of the model were performed on the experimental total heat flow data, obtained from two thermoplastics with different aging times. Considering which functions of the mixture represent reversing and non‐reversing processes, the reversing and non‐reversing heat flows were also estimated. The estimated reversing and non‐reversing signals were compared with the ones obtained by modulation. On the whole a good match was found, which was even better considering that the estimates are not affected by the frequency effect of the modulated temperature DSC (MTDSC) measurements. The model assumes linear trends for the heat capacity heat flow of the glassy and non‐glassy structures. The glass transition progress is represented by a generalized logistic function and the enthalpic relaxation heat flow by the first derivative of another generalized logistic. It brings about a new approach to these phenomena, where the parameters of these functions represent the temperature at which each event is centered, the change of heat capacity (C_p) at the glass transition and the energy involved in the enthalpic recovery. Copyright © 2011 John Wiley & Sons, Ltd.     

Study of the crystallization and melting region of PET and PEN and their blends by TMDSC

The cold crystallization and melting of poly(ethylene therephthalate) (PET), poly(ethylene 2,6-naphthalene dicarboxylate) (PEN) and their blends were studied using temperature modulated differential scanning calorimetry (TMDSC) at underlying heating rates of between 1 and 3 K min^-1 and periods ranging from 30 to 90 s. The amplitude of modulation was selected in order to give an instantaneous heating rate β≥0. Heat flow is analyzed by the total heat flow signal o, which is equivalent to the conventional DSC signal, and the reversing heat flow o_REV, which only detects the glass transition and the melting processes. The dependence of the melting region in the reversing heat flow on the frequency of modulation is analyzed. The use of the so-called non-reversing heat flow o_NREV (=o-o_REV)) and the effect of frequency and amplitude on the complex heat capacity are also studied. The results show the complexity of these magnitudes. This revised version was published online in July 2006 with corrections to the Cover Date.     

Enthalpic relaxation in frozen aqueous trehalose solutions

Our object was to investigate the effect of annealing on the glass transition temperatures and enthalpic recovery of frozen aqueous solutions of trehalose. Trehalose solutions were subjected to differential scanning calorimetry wherein they were first cooled from room temperature to −60 °C, and heated to the annealing temperature, which ranged between −34 and −48 °C. Following isothermal annealing for the desired time period, the glass transition temperatures and the enthalpic recovery were determined in the final heating scan. Frozen unannealed trehalose solutions were characterized by two glass transition events. At a heating rate of 2 °C/min, the observed Tg₁′ and Tg₂′ were ∼−45 and −31 °C, respectively. Annealing resulted in an increase in the lower transition temperature, Tg₁′, while the higher transition temperature, Tg₂′, was unaffected. Enthalpic recovery due to annealing was associated only with Tg₂′. Annealing at −36 °C resulted in the highest value of Tg₁′ and the maximum enthalpic recovery. Irrespective of the heating rates, the magnitude of enthalpic recovery and Tg₁′ showed a similar trend (first an increase, followed by a decrease) as a function of annealing temperature. This suggests that annealing led to crystallization of ice and subsequently the system became maximally freeze-concentrated. Annealing, at temperatures higher than −36 °C, led to a reduction in enthalpic recovery associated with Tg₂′ and a lowering of Tg₁′. These observations are consistent with the hypothesis that the higher transition temperature coincides with the onset of ice melting. We have attempted to bridge two conflicting schools of thought regarding the origin of multiple glass transitions in frozen aqueous sugar solutions. The two glass transitions are attributed to the formation of two “populations” in the freeze-concentrated phase during “non-equilibrium” or rapid cooling—one, that is maximally freeze-concentrated and the other that contains a higher amount of water. The higher transition temperature also overlaps with the onset of ice melting.     

Effect of chemical structure of acrylic pressure‐sensitive adhesives for polarizing film on light leakage phenomenon in TFT‐LCDs

Increasing image quality in thin‐film transistor liquid crystal displays (TFT‐LCD) is a recognized challenge for electronic companies and specialists working in this area. One of the main problems in TFT‐LCDs is a phenomenon called “light leakage”, affecting black–white contrast and color brightness. It occurs because of a heat‐induced shrinkage and disorientation of the polarizing film of TFT‐LCD, which controls the intensity of the light from the backlight unit. Improvement of the light leakage can be achieved through using a pressure‐sensitive adhesive (PSA) used for assembling the polarizing film onto the TFT‐LCD panel. In this paper, eight acrylic/methacrylic monomers with high glass transition temperature (Tg) were employed for synthesis of the polymers for the adhesive. Effect of structure, Tg, and elasticity modulus of the synthesized polymers on the light leakage was investigated simultaneously for 2.5‐ and 7.0‐in. size samples. We demonstrated that the light leakage can be minimized through two different mechanisms—high stress relaxation of the polymers with low Tg and low modulus and high shrinkage resistance of the polymers with high Tg and high modulus. The results of this work indicate a possibility to develop a universal PSA for polarizing film in TFT‐LCDs of different sizes that will have a positive effect on manufacturing productivity and lowering prices of digital devices. Copyright © 2011 John Wiley & Sons, Ltd.     

Surface thermomechanical and glass transition temperature measurements of polymeric solids

The surface molecular motion of polymeric solids was investigated on the basis of scanning force microscopic and temperature-dependent X-ray photoelectron spectroscopic measurements. The surface of the monodisperse polystyrene films was in a glass-rubber transition state even at 293 K in the case of number-average molecular weight less than ca. 30k. The surface glass transition temperature, Tgs for the symmetric poly(styrene-block-methyl methacrylate) diblock copolymer films were much lower than those for the bulk samples. A remarkable depression of Tg at the air-polymer interface was explained by the surface localization of chain end groups.     

Effect of soft segment length on the thermal behaviors of fluorinated polyurethanes

Differential scanning calorimetry (DSC) and temperature modulated DSC (MDSC) have been applied to investigate the thermal behaviors of fluorinated polyurethanes (FPU), which were obtained using 2,2,3,3-tetrafluoro-1, 4-butanediol as the chain extender and based on various soft segments—polytetramethyl oxides (PTMO) with molecular weights of 650, 1000, 1400 and 2000. An exothermic peak and/or multiple melting endotherms were observed during the heating to melting temperature of soft and hard segments. Attributed to the simultaneous recrystallization and melting processes during heating, these features have been confirmed via MDSC, where an endotherm and an exotherm were noted in reversing and non-reversing components of the heat flow. Separating the non-reversing components from the reversing curves, the dependencies of polyurethane morphology on the length of the soft segment could be clarified using MDSC analysis. Soft segment lengthening significantly influences the morphology of soft segment domains in FPUs. The phase separation and crystallinity of the soft segment increased with its length. However, soft segment length exerted a minor influence on the dissociation temperature of the short-range ordered hard segment domain and on the melting temperature of hard segment crystals. Examination of the heats of melting based on the quasi-isothermal MDSC experiments indicated that the crystallinity of hard segment domains declined with increasing soft segment length.     

Ellipsometric study of the glass transition and thermal expansion coefficients of thin polymer films

The glass transition (T_g) of thin polystyrene films (ca. 3000 A?) cast on silicon wafers was determined by a new technique. An ellipsometer was used to determine the refractive index and thickness of the polystyrene films. T_g was determined by measuring the temperature dependence of the refractive index. The change in thickness with temperature was used to calculate the linear and bulk thermal expansion coefficients of the material. A significant shift in T_g, possibly due to strains induced in the cooled films, was observed between heating and cooling for polystyrene films. © 1993 John Wiley & Sons, Inc.     

Synthesis of tri-aryl ether epoxy resin isomers and their cure with diamino diphenyl sulphone

The synthesis of bi- and tetra-functional tri-aryl ether epoxy resin isomers and their subsequent cure with 44 diamino diphenyl sulphone (DDS) is presented here. The effect of varying aromatic substitution and cross-link density on the structure, property, and processing relationships is explored for 1,3 bis(3-glycidyloxyphenoxy)benzene (133 BGOPB), 1,4 bis(4-glycidyloxyphenoxy)benzene (144 BGOPB), N,N,N,N-tetraglycidyl 1,3-bis (3-aminophenoxy) benzene (133 TGAPB), and N,N,N,N-tetraglycidyl 1,4-bis (4-aminophenoxy) benzene (144 TGAPB). Meta substitution to the aromatic ring reduces the rate of reaction, glass transition temperature, yield strain and crosslink density, coefficient of thermal expansion, and side reactions, while increasing strain softening, compressive modulus and strength, and methyl ethyl ketone ingress. Increasing crosslink density increases the glass transition temperature, promotes side reactions during cure, and increases compressive modulus, strength, and yield strain, while reducing coefficients of thermal expansion, methyl ethyl ketone ingress, and density. The results are discussed in terms of packing efficiency of the meta-substituted epoxy resins and the role of short range molecular mobility caused by the lack of an aromatic axis of rotation.     

Influence of fluorine on thermal properties of lead oxyfluoride glass

Oxyfluoride glasses are the basic materials for obtaining transparent glass–ceramic (TGC) which can be used in a wide range of optoelectronics devices such as: amplifiers, up-conversion, telescopes, laser sources. Oxyfluoride TGC is obtained by the control heat treatment of the parent glass due to low phonon nanocrystalline phases. The oxyfluoride glasses from the sodium–lead–silica system were the object of investigation. The influence of fluoride content on the thermal properties of glasses was analyzed. Thermal characteristics of glasses like the transition temperature T _g, the temperature for the crystallization onset T _x, and the maximum crystallization temperature T _c, thermal stability parameter were determined by DTA/DSC method. The linear expansion coefficients of oxyfluoride glasses as a function of temperature were measured using a thermo-mechanical analyzer (TMA 7 Perkin-Elmer). The effect of crystallization on the thermal expansion coefficient and softening temperature T _s was found.     

Comparison of the transient stress–strain response of rubber to its linear dynamic behavior

Master curves of the small strain and dynamic shear modulus are compared with the transient mechanical response of rubbers stretched at ambient temperature over a seven‐decade range of strain rates (10^?4 to 10³ s^?1). The experiments were carried out on 1,4‐ and 1,2‐polybutadienes and a styrene–butadiene copolymer. These rubbers have respective glass transition temperatures, T_g, equal to ?93.0, 0.5, and 4.1 °C, so that the room temperature measurements probed the rubbery plateau, the glass transition zone, and the onset of the glassy state. For the 1,4‐polybutadiene, in accord with previous results, strain and strain rate effects were decoupled (additive). For the other two materials, encroachment of the segmental dynamics precluded separation of the effects of strain and rate. These results show that for rubbery polymers near T_g the use of linear dynamic data to predict stresses, strain energies, and other mechanical properties at higher strain rates entails large error. For example, the strain rate associated with an upturn in the modulus due to onset of the glass transition was three orders of magnitude higher for large tensile strains than for linear oscillatory shear strains. © 2011 Wiley Periodicals, Inc.* J Polym Sci Part B: Polym Phys, 2011     

Significance of glass transition temperature to polymer latex stabilisation by nonionic surfactants

Equilibrium colloid stability measurements with nonionic surfactant (C₁₂E₈) stabilised polybutyl methacrylate (PBMA) latex dispersions indicate a sudden temperature induced destabilisation coinciding with the glass transition temperature,Tg, of the polymer. In control experiments with polystyrene latex particles of similar size, for whichTg was not approached, the flocculation temperature was significantly higher. The effect is interpreted in terms of a reduced adsorbed layer thickness aboveTg caused by mixing of part of the surfactant molecule with the polymer. This interpretation is supported by DSC, elastic modulus and mechanical damping measurements on films made from dispersions of the same latex containing commercial nonionic surfactants. These measurements indicate a shift inTg in the presence of surfactant consistent with partial penetration of the polymer surface by the surfactant. In addition, C₁₂E₈ adsorption measurements show increased adsorption (or absorption) onto PBMA aboveTg which is irreversible on both dilution and temperature reduction.     

Measuring the Glass Transition of Thermosets by Alternating Differential Scanning Calorimetry

The glass transition temperature of thermosets is determined by alternating differential scanning calorimetry (ADSC), which is a temperature modulated DSC technique. The different values of the glass transition obtained from heat flow measurements (total and reversible) and heat capacity (modulus of the complex heat capacity) are analysed and compared with the values obtained by conventional DSC. The effect of the sample mass on the values of T_g, heat capacity and phase angle has been analysed. The effect of the thermal contact between sample and pan has been studied using samples cured directly inside the pan and disc-shaped samples of different thickness. The results obtained for the thermal properties and the phase angle are compared and analysed. The modulus of the complex heat capacity enables the determination of the dynamic glass transition, T_g, which is frequency dependent. The apparent activation energy ofthe relaxation process associated with the glass transition has been evaluated from the dependence of T_g on the period of the modulation.This revised version was published online in November 2005 with corrections to the Cover Date.     

Peel adhesion and viscoelasticity of rubber–resin blends

Mixtures in various proportions of natural rubber (NR) and each of two tackifier resins, a poly-β-pinene and a modified pentaerythritol rosin ester, were used as the adhesive layer in joining a flexible polyester strip to a plane glass substrate. Measurements of the force required to peel the strip from the glass at a 90° angle were made over a range of pulling rates at several temperatures. Application of time-temperature superposition enabled a master curve of (reduced) peel force versus (log) pulling rate at a standard temperature (296 K) to be obtained for each adhesive composition. The master curves showed, in increasing order of pulling rate, some or all of four different modes of peeling: (i) peeling with viscous adhesive response, (ii) peeling with rubbery response, (iii) oscillatory or slip-stick peeling, and (iv) peeling with glassy adhesive response. In general, transitions between the different peeling modes were quite abrupt. Increase in concentration of tackifier resin caused displacement of the master curve toward lower pulling rates [an effect interpreted in terms of an increasing adhesive glass temperature (T_g)], and a superimposed displacement of the transition between peeling modes (i) and (ii) toward higher pulling rates-an effect attributed to reduction in adhesive average molecular weight. The influence of the tackifier resin in modifying the viscoelastic characteristics of the adhesive was further demonstrated in a comparison of the peel force master curves with corresponding master curves of dynamic storage modulus.     

A comparison of different evaluation methods in modulated temperature DSC

Modulated temperature-DSC is a new method for measuring the thermal behaviour of materials. In this method, the response of the sample to a time-dependent signal (sinusoidal temperature change) is measured. Two different methods are known for the evaluation of the measured data. The first is the separation of the measured data into reversing and non-reversing components of heat flow. The second is based on the linear response theory and yields a complex heat capacity with a real part (storage heat capacity) and an imaginary part (loss heat capacity).

The theoretical basis and the possibilities of interpretation of both evaluation methods are investigated. The results of both methods are compared theoretically for the case of simple time-dependent processes. Experimental results are given for the glass transition process.     

PVT properties of dodecane/polystyrene systems

The PVT properties of crosslinked polystyrene samples containing various amounts of dodecane were measured. The Tait equation was used to describe the PVT behavior of each system in both the glassy and rubbery regions. The glass transition temperature was determined from the abrupt change of the thermal expansion coefficient. Increase in the dodecane content in the samples resulted in a significant decrease of the difference between the expansion coefficients in the glassy and rubbery regions. Addition of dodecane lowered the glass transition temperature linearly. However, the dependence of the glass transition temperature on pressure was not affected by the presence of dodecane in the polymer samples. Above the glass transition temperature, the volume of the swollen polymer, V_m, could be determined by simple addition of the volumes of the pure components at the appropriate temperature and pressure; the volume change of mixing, δV_m, was independent of temperature and pressure. Below the glass transition temperature, volume additivity of the two components was also applicable after appropriate adjustment of the glass transition temperature of the polymer to that of the dodecane/polymer samples. © 1994 John Wiley & Sons, Inc.