Calorimetric Relaxation Times Research Articles

Dynamics in Polysaccharide Glasses and Their Impact on the Stability of Encapsulated Flavors

The aim of this work is to examine the correlation between measured instability of model flavor compounds in glassy matrices with the calorimetric relaxation times of the matrices. Spray-dried carbohydrate matrices were chosen as the model compounds for this study. Enthalpy relaxation times were determined for spray-dried carbohydrate matrices using differential and isothermal calorimetric methods. The losses of the volatile methyl acetate, ethyl acetate and limonene, as well as formation of limonene oxidation products, were measured by gas chromatography. Storage conditions were 30 and 40 °C, with samples equilibrated with 11, 23, 33 and 43 % RH at each temperature. A comparison of the relaxation times for temperatures below Tg was made using Modulated DSC (MDSC) and a Thermal Activity Monitor (TAM). TAM yields significantly lower values for relaxation times implying that it is capturing some of the faster dynamics as well as dynamics that are activated near Tg. However, plots of relaxation times as determined by both techniques versus temperature appear to converge at Tg. An increase in the relative humidity results in moderately higher loss of volatiles (methyl acetate, ethyl acetate and limonene) and greater oxidation rates. In general, there is a good correlation between relaxation time and stability, with greater enthalpy relaxation time associated with better stability. Enthalpy relaxation time appears to be a useful predictor of stability for both loss of volatiles and oxidation of limonene.

Crystallization Kinetics of Ultraviscous Acetaminophen by Heat Capacity and Enthalpy Measurements and Diffusion Control

Crystallization kinetics of ultraviscous acetaminophen has been studied at 308.2, 318.2, and 328.2 K by measuring the heat capacity, C(p), and the heat release in real time up to a period of 2.5 days. C(p) decreases according to an inverted sigmoid-shape curve, and the heat released increases according to a similar shape. The extent of crystallization determined from the two measurements differs, thus indicating that the interfacial liquid's C(p) may be slightly different from that of the bulk liquid. Both the excess C(p) of the liquid over the crystal phase and the corresponding excess enthalpy decrease with decrease in the temperature. The kinetics of crystallization follows the Kolmogorov-Johnson-Mehl-Avrami relation, alpha(cryst)(t) = 1 - exp(-kt(m)). The logarithm of the rate constant, ln k, increases from -40.78 at 308.2 K to -32.85 at 328.2 K, and m from 3.60 to 3.83. The product of k and the calorimetric relaxation time remains constant with changing temperature thus showing that the two are inversely related. This shows that crystallization may be diffusion-controlled in the ultraviscous melt. The C(p) data indicate that slow crystallization of melt produces acetaminophen's (monoclinic) form I. Several effects usually overlooked in the crystallized kinetics formalisms have been described.

On the use of relaxation times for comparing ultraviscous liquid dynamics

In studies of ultraviscous liquids and glasses, (i) the relaxation time and its temperature dependence are generally compared without regard to its asymmetric distribution and (ii) the calorimetric relaxation time at T(g,DSC), the onset temperature of the specific heat rise in a heating scan at 20 K/min, is arbitrarily chosen as 100 s and compared against the relaxation time determined by spectroscopy. We propose that the relaxation time and its derivatives should be compared in a fixed frame of reference, preferably by using its average value that takes into account the asymmetric distribution, and we show that the relaxation time at T(g,DSC) is a material characteristic. It varies with the extent of nonexponential and nonlinear relaxation and the activation energy.

Molecular Mobility, Thermodynamics and Stability of Griseofulvin’s Ultraviscous and Glassy States from Dynamic Heat Capacity

To determine the calorimetric relaxation time needed for modeling griseofulvin's stability against crystallization during storage. Both temperature-modulated and unmodulated scanning calorimetry have been used to determine the heat capacity of griseofulvin in the glassy and melt state. The calorimetric relaxation time, tau cal, of its melt varies with the temperature T according to the relation, tau cal [s] = 10(-13.3) exp [2, 292 /(T[K] - 289.5)] , and the distribution of relaxation times parameter is 0.67. The unrelaxed heat capacity of the griseofulvin melt is equal to its vibrational heat capacity. Griseofulvin neither crystallizes on heating to 373 K at 1 K/h rate, nor on cooling. Molecular mobility and vibrational heat capacity measured here are more reliable for modeling a pharmaceutical's stability against crystallization than the currently used kinetics-thermodynamics relations, and molecular mobility in the (fixed structure) glassy state is much greater than the usual extrapolation from the melt state yields. Molecular relaxation time of the glassy state of griseofulvin is about 2 months at 298 K, and longer at lower temperatures. It would spontaneously increase with time. If the long-range motions alone were needed for crystallization, griseofulvin would become more stable against crystallization during storage.

Calorimetric study of plastically crystalline o- and m-carboranes

The heat capacities of o- and m-carboranes were measured with an adiabatic calorimeter in the temperature range 5–310 K. m-carborane underwent two phase transitions at 170 K and 284 K with transition entropies of 12.70 and 15.52 J K−1 mol−1. The total transition entropy agreed well with the theoretical value, indicating that m-carborane is ordered at 0 K. o-carborane also exhibited two phase transitions at 160 K and 275 K with transition entropies of 3.72 and 13.72 J K−1 mol−1, and additionally a glass transition at 120 K. This glass transition does occur not in the supercooled intermediate-temperature phase but in the low-temperature phase of o-carborane. The high- and intermediate-temperature phases of o-carborane could not be quenched at the maximum cooling rate (10 K min−1) of this experiment. The total transition entropy of o-carborane was smaller than that of m-carborane by 10.8 J K−1 mol−1, suggesting that o-carborane has residual orientational disorder at 0 K. The enthalpy relaxation was examined by the temperature jump method around the glass transition of o-carborane. The results were reproduced well by the KWW function. The calorimetric relaxation time and nonexponential parameter agreed with dielectric ones at the glass transition temperature. This indicates that the glass transition of o-carborane is due to the freezing of molecular reorientation.

The application of a new configurational entropy model to the structural relaxation in an epoxy resin

The aim of this work is to model the structural relaxation of a fully cured epoxy resin. The samples were subjected to thermal treatments involving isothermal annealing at temperatures T a for times t a before the measuring scan carried out on heating in the DSC. The values of T a ranged between T g −8 and T g −50°C and those of t a between 0 and 16000 min. A new model based on the evolution of the configurational entropy was used to simulate the experimental data, obtained previously by one of the authors. A curve of calorimetric relaxation times as a function of reciprocal temperature and the β parameter of the KWW equation were determined from a comparison between experimental and calculated thermograms. Using the peak shift method, the Narayanaswamy parameter x was determined from both the experimental and simulated scans. The calorimetric relaxation times are compared with those found for dielectric and dynamic-mechanical relaxation.

Calorimetric relaxation and glass transition in poly(propylene glycols) and its monomer

AbstractThe glass transition temperature Tg of propylene glycol (PG) and poly(propylene glycols) (PPGs) of molecular weight up to 4000 has been measured by differential scanning calorimetry, and the activation energy and change in heat capacity ΔCp have been determined in the glass transition range. The activation energy increases with an increase in the molecular weight of the polymer, and ΔCp measured at a fixed heating rate decreases. The increase in Tg with molecular weight is remarkably more rapid for poly(propylene glycols) than for other polymers, and a limiting value of Tg is reached for a chain containing 20 monomer units. These results are discussed in terms of the Fox‐Flory and the entropy theories. The calorimetric relaxation times are comparable with the extrapolated dielectric relaxation times. The initial increase of ΔCp from PG to PPG 200 is attributed to the decrease of H‐bonding sites from 12 in 3 monomers to 4 on polymerization to PPG 200 and further decrease with increase in molecular weight to an increasingly large amplitude of the β‐process at T < Tg.