Characteristic Glass Transition Temperature Research Articles

Water sorption isotherms and phase transitions in kiwifruit

Adsorption and desorption isotherms were determined in entire and homogenized kiwifruit tissue. Fresh samples (desorption process) and freeze-dried samples (adsorption process), were conditioned at various water activities (0–0.675) at 30 °C and, at equilibrium, had attained different water contents. In each sample, glass transition was analysed by differential scanning calorimetry (DSC). BET and GAB models were fitted to sorption data and the Gordon and Taylor equation was used to model the water plasticization effect. Results showed that the different pretreatments applied did not imply differences in those relationships. The mean values for the parameters of the fitted models were: w 0 = 0.057 g water/g dry product and C = 7.9 (BET model); w 0 = 0.046 g water/g dry product, C = 10.6 and K = 1.20 (GAB model); k = 4.88 and T g(as) = 40.3 °C (Gordon and Taylor model). The state diagram of the kiwifruit liquid phase was obtained including the characteristic glass transition temperature of the maximally cryoconcentrated matrix (m.c.m.) T g ′ = - 52.0 ± 0.4 °C, the melting temperature of ice crystals surrounding the m.c.m. T m ′ = - 40.4 ± 0.4 °C, and the amount of non-freezable water content W g ′ = 0.186 g water/g m.c.m.

Miscibility as a function of copolymer composition in blends of a random copolymer and a homopolymer

Random copolymers of n-butyl acrylate (BA) and cyclohexyl acrylate (CHA) were synthesized by solution polymerization in cyclohexane. Blends of polystyrene with the poly(CHA-stat-BA) copolymers were prepared by solvent casting and coprecipitation. The miscibility of the blends was characterized by means of differential scanning calorimetry. While blends with a low content of CHA in the copolymer showed two characteristic glass-transition temperatures of the corresponding blend components, those with a CHA content higher than 70% presented good compatibility. Phase separation of the miscible blends took place after annealing at 200 °C for 1 h, which implies an upper miscibility gap (lower critical solution temperature).

Macroscopic phase separation of Se-rich (x < 1/3)ternary Agy(GexSe1−x)1−y glasses

Temperature modulated differential scanning calorimetry measurements on Agy(GexSe1−x)1−yglasses provide evidence for bimodal glass transition temperatures in Se-rich glasses (x < 1/3). Atx = 0.20and 0.25, thermal measurements performed as a function of Ag content in the 0 ≤ y ≤ 0.25range reveal that the additive (Ag) segregates into an Ag2Se-rich glass phase,possessing a characteristic glass transition temperature Tga = 230° C,while the remaining base glass displays a second glass transition temperatureTgbthat systematically increases as its Se content is depleted. The present thermalresults are in harmony with Raman scattering, neutron diffraction, dielectricspectroscopy and optical microscopy measurements and suggest that Se-richglasses in the Ag–Ge–Se ternary system intrinsically phase separate on amacroscopic scale.

Effect of nickel ions on electron paramagnetic resonance, DC conductivity and thermal behavior in vanadyl doped NiO·Li2O·B2O3 glasses

Electron paramagnetic resonance (EPR), DC conductivity and differential scanning calorimetry (DSC) of the xNiO·(0.3−x)Li2O·0.7B2O3(0.0≤x≤0.15) containing 1.0 and 2.0mol% of V2O5 have been studied. Spin Hamiltonian parameters, dipolar hyperfine coupling parameter, P, and Fermi contact interaction parameter, K, have been calculated. It is found that the size of the 3dxy orbit of unpaired electron in the vanadium ion is increased with NiO contents in the sample. The DC conductivity decreases with increase in the NiO:Li2O ratio. Characteristic glass transition temperature, Tg, is found to increase with increase in NiO content. Theoretical optical basicity of the samples is also calculated.

Electron paramagnetic resonance and thermal behaviour of lithium potassium borate glasses containing Cu 2+ ions

The structure and thermal behaviour of lithium potassium borate glasses, containing 2 mol% Cu 2+ ions, have been studied by means of electron paramagnetic resonance (EPR) and differential scanning calorimetry (DSC). From the observed EPR spectra, the spin-Hamiltonian parameters have been evaluated which are independent of the temperature from room temperature to liquid helium temperature and it is observed that the spin-Hamiltonian parameters (SHP) are dependent on the concentration of alkali ions present in the glass system. The observed SHP shows that Cu 2+ ions in lithium potassium borate glasses are present in a distorted octahedral environment. Characteristic glass transition temperatures, T g, have also been measured for these borate glasses and it is found that the T g decreases with increasing lithium content upto 10 mol% and then increases for further lithium content. The theoretical optical basicity, Λ th, of the glasses has also been evaluated and it is observed that the changes in optical basicity values are independent of the changes in SHP.

Physical Factors Affecting the Storage Stability of Freeze-Dried Interleukin-1 Receptor Antagonist: Glass Transition and Protein Conformation

The effects of glass transition of, and protein conformation in, the dried solid on the storage stability of freeze-dried recombinant human interleukin-1 receptor antagonist (rhIL-1ra) were examined. Glass transition is a temperature-dependent phenomenon. Amorphous materials become hard and brittle at temperatures below their characteristic glass transition temperatures (Tg) such that diffusion of molecules along the matrix is not sufficient to cause large-scale structural changes. To ascertain the importance of the glass transition in protein storage stability, we compared 10 different lyophilized rhIL-1ra formulations, with Tgs ranging from 20 to 56°C, during several weeks of storage at temperatures above and below the samples’ Tgs. Protein degradation, both deamidation and aggregation, was greatly accelerated at temperatures above Tg, but for some formulations also arose below Tg. Thus, storage of dried proteins below the Tg is necessary but not sufficient to ensure long-term stability. To examine the effects of protein structure in the dried solid, we prepared formulations with various sucrose concentrations, all of which had a Tg = 66 ± 2.5°C. With infrared spectroscopy, we determined that the protein lyophilized with ≤1% sucrose was unfolded in the initial dried solid. In contrast, in those formulations with ≥5% sucrose, conformational change was inhibited during lyophilization. When stored at 50°C, degradation of the freeze-dried protein varied inversely with sucrose concentration. These results indicate that structural changes arising during the lyophilization process led to damage during subsequent storage, even if the storage temperature was less than the Tg. Together the results of these studies document that to obtain optimum stability of dried rhIL-1ra it was necessary to inhibit conformational change during lyophilization and to store at temperatures below the Tg of the dried formulation.

Cure behaviour of visible light activated dental composites

During curing of multifunctional methacrylate-based composites for dental restorations, strong structural changes, significantly affecting the final properties of the materials, are observed. In fact, the polymerization of thermoset matrices involves the transformation of a viscous liquid in a glassy network. The final glass transition temperature of the composite matrix may be considered a relevant parameter for the durability of a restoration. In this study, the complex cure behaviour of a commercial dental composite, activated by visible-light, is analysed by differential scanning calorimetry (DSC). The maximum degree of reaction of the crosslinked resin and the characteristic glass transition temperature are quantitatively related to the cure temperature. Furthermore, a kinetic model, accounting also for the diffusion control effects associated with vitrification, is presented. Finally, the cure behaviour expected during practical application of these materials in the oral cavity for dental restorations, is discussed.

Transesterification in blends of poly(butylene terephthalate) and bisphenol‐A‐polycarbonate

AbstractBy means of differential calorimetric and X‐ray examinations it was shown that poly(butylene terephthalate) (PBT) and bisphenol‐A‐polycarbonate (PC) are only compatible in the melt if they stay in the melt for a short period of time. During mixing in the melt, transesterification begins and block copolymers are formed. A characteristic glass transition temperature is observed for each individual blend. Butylene terephthalate (BT)‐blocks are crystallizable; however, with an increasing share of bisphenol‐A‐carbonate (BPAC) the specific melting enthalpy of the BT‐blocks decreases. Within copolymers with a high BPAC content, the BPAC segments crystallize considerably faster than in pure PC. By means of diffraction measurements with synchrotron radiation, the mechanism of stress‐induced α‐β‐modification change in pure PBT and in block copolymers with high BT‐contents could be explored.

Preparation and characterization of poly(methyl‐3,3,3‐trifluoropropylsiloxane‐co‐dimethylsiloxane)

AbstractMethyl‐3,3,3‐trifluoropropylsiloxane (F)‐dimethylsiloxane (D) random and block copolymers were prepared. The random copolymers were prepared by equilibrium copolymerization starting from a mixture of cyclic F and D siloxanes with potassium silanolate as the catalyst. The F‐D block copolymer was prepared by sequential anionic living polymerization of strained cyclic trisiloxanes using butyllithium as initiator, first polymerizing D3 then adding F3 after consumption of D3. The copolymer microstructure was established by means of 29Si NMR, differential scanning calorimetry (DSC), and gel‐permeation chromatography (GPC). Characteristic glass transition temperature (Tg) shifts were observed depending on the F:D ratio of the random copolymers. It was demonstrated that the tensile strength of the poly(methyl‐3,3,3‐trifluoropropylsiloxane)‐poly(dimethylsiloxane) (PTFPMS‐PDMS) blend system was improved when either of the copolymers was added.

A polymer physico-chemical approach to the study of commercial starch hydrolysis products (SHPs)

The behavior of 55 commercial SHPs, of dextrose equivalent (DE) 0·3–100, was studied by a low-temperature differential scanning calorimetry technique. The method, based on derivative thermograms, is used to measure the characteristic sub-zero glass transition temperature, T′ g , of a maximally freeze-concentrated aqueous solution. An inverse linear correlation exists between T′ g and DE. A plot of T′ g vs. number-average molecular weight ( M n ) demonstrated the classical behavior of SHPs as a homologous series of amorphous glucose polymers, and revealed an ‘entanglement coupling’ capability for SHPs of ≤ 6 DE and T′ g ≥ −8°C. The possible relationship between intermolecular entanglement (leading to network formation) and SHP functional behavior as a food ingredient in applications involving gelation, encapsulation, frozen-storage stabilization, thermomechanical stabilization or facilitation of drying processes is discussed. The utility of low-DE SHPs for inhibiting various ‘collapse’-related phenomena, which affect the processing/storage stability of many foods, is described and explained. An example of the inhibition of enzymic activity in a frozen system, stabilized with low-DE maltodextrin, is cited. A generalized mechanism for collapse is proposed, based on the occurrence of a critical structural relaxation transition at T′ g , followed by viscous flow in the rubbery liquid state. The mechanism is derived from Williams-Landel-Ferry free volume theory for amorphous polymers, and leads to a conclusion of the fundamental equivalence of T′ g (or other relevant glass transition temperature, T g ) and the observed collapse temperature and the temperature of recrystallization.

Equation of state for linear polymer melts: the chemical structure parameter and its relationship to the characteristic volume, packing densities, characteristic bulk modulus and glass transition temperature

AbstractIt is shown that the three‐parameter equation of state for linear polymer melts, although consistent with the theorem of corresponding states, contains no information about the chemical structure of the polymer chain. This information is now included in the equation via a fourth parameter representing the structure. The relations between the chemical structure parameter and the characteristic volume of the polymer, its dense packing, characteristic bulk modulus, and glass transition temperature are deduced. It is demonstrated that the properties of the polymer are influenced by the compressibility of the polymer chain, which was not taken into account in the mechanical‐statistical approaches to the equation of state for liquid polymers. It is also shown that the dense packing of a polymer is almost equal to the close packing of spheres. The determination of the parameters of the four‐parameter equation requires the knowledge of the van der Waals volume of the constitutional repeating unit and p V T data for the liquid polymer, including the glassy transition region. This region should be investigated under cooling conditions considered as isochronal, the time of viscoelastic relaxations assumed to be zero.

Glass transition temperatures of poly(alkyl α‐cyanoacrylates)

AbstractThe glass transition temperatures of the poly(alkyl α‐cyanocrylates) were determined by the dilatometric technique, and some of the values were checked by differential thermal analysis. The data indicate that the Tg's appear to decrease with increase in the size of the alkyl group, for a given molecular weight range. It was also found that the Tg of poly(methyl or butyl α‐cyanoacrylate) increased with molecular weight. All cyanoacrylates, excepting methyl and ethyl esters, formed only low molecular weight polymers in aqueous surroundings. Therefore, they have characteristic low glass transition temperatures, causing coalescence at low temperatures.