Quantitation of amorphicity by microcalorimetry

The amorphous state of solids is characterized by a higher chemical and physical reactivity and a hygroscopic behaviour. Furthermore processing of amorphous powders is often difficult, because of the instability. Fast crystallizations, precipitations and milling favour the formation of the amorphous state. Galenical processes like granulation, drying, lyophilization, mixing, may also induce amorphous regions in the drug products.X-ray diffraction techniques can be used for the determination of the amorphicity of drug raw materials or drug products. Unfortunately, 10% is the detection limit, which in normal cases can be attained. Amorphous substances undergo an exothermic crystallization at temperatures above the glass transition point. Water which is a plasticizer decreases the temperature of the glass transition point, allowing the crystallization to occur at lower temperatures. The crystallization energy is measure of by microcalorimetry.Examples show the influence of the choice of the experimental conditions, especially the influence of the amorphicity on the kinetic of the reaction. Critical steps are discussed for three different drug substances. Limits of detection in the magnitude of 1 % are possible using microcalorimetry.     

Heat capacity, enthalpy fluctuations, and configurational entropy in broken ergodic systems

A common assumption in the glass science community is that the entropy of a glass can be calculated by integration of measured heat capacity curves through the glass transition. Such integration assumes that glass is an equilibrium material and that the glass transition is a reversible process. However, as a nonequilibrium and nonergodic material, the equations from equilibrium thermodynamics are not directly applicable to the glassy state. Here we investigate the connection between heat capacity and configurational entropy in broken ergodic systems such as glass. We show that it is not possible, in general, to calculate the entropy of a glass from heat capacity curves alone, since additional information must be known related to the details of microscopic fluctuations. Our analysis demonstrates that a time-average formalism is essential to account correctly for the experimentally observed dependence of thermodynamic properties on observation time, e.g., in specific heat spectroscopy. This result serves as experimental and theoretical proof for the nonexistence of residual glass entropy at absolute zero temperature. Example measurements are shown for Corning code 7059 glass.     

无定形高分子的结构与性能关系的研究(Ⅱ)-热处理对聚醚砜的热性能、力学性能和形态结构的影响

研究了热处理对无定形聚醚砜的结构和性能的关系。结果表明,主链含有苯环的刚性链聚醚砜经热处理后,其热性能和力学性能的变化与柔性链高分子的某些规律不同。同时电镜的形态结构分析表明,经热处理后其聚集态确实形成了某种局部有序结构。利用这一事实可使上述热性能和力学性能的变化规律得到较为满意的解释,也为Yeh等人的观点提供了一个实验依据。     

Diffusion in atactic polystyrene above the glass transition point

Diffusivities and solubilities in the system n-pentane-polystyrene were measured for low penetrant concentrations at several temperatures above the glass transition temperature. The relatively sharp changes in the activation energy of diffusion and the heat of solution near 150°C. are interpreted tentatively as indicating that a second-order transition exists in atactic polystyrene above the glass transition temperature. Evidence indicating the existence of this transition has been obtained by other techniques and the effects observed in this study should also be present in the diffusion of other penetrants in polystyrene. Correlation of these and other available data for diffusion in polystyrene as a function of the molecular size of the penetrant indicates that specific thermodynamic interactions between polymer and penetrant have little influence on the diffusion process.     

Calorimetric study and modeling of molecular mobility in amorphous organic pharmaceutical compounds using a modified Adam-Gibbs approach

The purpose of this study is to provide a quantitative characterization of the thermal behavior of amorphous organic pharmaceutical compounds across their glass transition temperature, and to assess their molecular mobility as a function of temperature and time by combining theoretical simulations with experimental measurements using differential scanning calorimetry. A computational approach built on the Boltzmann superposition principle of nonexponential decay and the Adam-Gibbs theory of entropic-dependent structural relaxation is presented. The heat capacities of the crystalline and amorphous forms are incorporated into the simulation in order to accurately assess the entropic fictive temperature as functions of temperature and time under any arbitrary set of experimental conditions. Using this method, we evaluated properties of the glass former, D and T0, and the nonexponentiality index beta, for amorphous salicin, felodipine, and nifedipine, by fitting the simulated glass transition profile with the experimentally determined heat capacity across the glass transition region. From this fit, the evolution of the relaxation time of the model compounds following any thermal cycle, including heating, cooling, and isothermal holds can then be estimated a priori. This study reveals the profound and inextricable effect of thermal history on the molecular mobility of the amorphous materials, and the ability of the glass to undergo fast changes in its molecular motions over an aging process even at low temperatures.     

Thermodynamic properties of dendrimers compared with linear polymers: General observations

The pressure‐volume‐temperature and thermal properties of dendrimers based on benzyl ether were measured and compared with literature values for monodisperse, linear polystyrenes. In addition, property measurements are presented for an exact linear analogue to the fifth‐generation dendrimer. The thermodynamic properties' molecular weight behavior for the dendrimers is unique when compared with that of linear polystyrene. All of the evidence presented in this work suggests that some form of structural transition occurs in the bulk at a molecular mass near that for the fourth‐generation dendrimer. No such transition is seen for polystyrene. Dendrimers exhibit an increased packing efficiency as evidenced by a decreased specific volume (increased density) as compared with an exact linear analogue of the fifth‐generation dendrimer analogue, and the dendrimer highlights the entropic differences between the two molecules. In addition, differences in the change in heat capacity with temperature for the two systems further allude to their entropic differences. A crystalline state can be formed for the lower generation dendrimer and linear analogue. This crystalline state is not seen in dendrimers above the third generation. These behaviors compiled with the difference in the glass‐transition temperature for the linear analogue suggest that the dendrimers' microstructure has a significant influence on the bulk thermodynamic state of the material. The Tait equation was fitted to the volume data for comparative purposes; the Tait equation has known limitations but was selected because of its widespread application. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1766–1777, 2001     

Structure of glassy polymers. V. Small-angle x-ray scattering from polystyrene

Small-angle x-ray scattering (SAXS) from glassy atactic polystyrene has been measured using a Bonse–Hart system. After correcting for absorption, background, and beam divergence, the scattering has been placed on an absolute basis using a standard silica suspension as a reference.The desmeared absolute intensity decreases strongly with increasing scattering angle over the range between 20 sec and 20 min. At larger angles, the intensity decreases much more slowly with increasing angle and approaches the value expected for density fluctuations frozen-in at the glass transition. The angular variation of intensity is well described by the scattering from heterogeneities of various sizes and concentrations superimposed on the scattering from thermal density fluctuations. These heterogeneities range in radius from 10 to 4000 Å. The present SAXS results on glassy polystyrene seem inconsistent with the presence of nodular features as representative of the bulk polymer.     

Recent developments in thermal analysis of polymers: Calorimetry in the limit of slow and fast heating rates

This review focuses on new insights into the crystal melting transition and the amorphous glass transition of polymers that have been gained through recent advances in thermoanalytical methods. The specific heat capacity can now be studied under two extreme limits, that is, under quasi‐isothermal conditions (limit of zero heating rate) and, at the other end of the scale, under rapid heating conditions (heating rates on the order of thousands of degrees per second), made possible through nanocalorimetry. The reversible melting, and multiple reversible melting, of semicrystalline polymers is explored using quasi‐isothermal temperature modulated differential scanning calorimetry, TMDSC. The excess reversing heat capacity, above the baseline, measured under nearly isothermal conditions is attributed to locally reversible surface melting and crystallization processes that do not require molecular nucleation. Observations of double reversible melting endotherms in isotactic polystyrene suggest existence of two distinct populations of crystals, each showing locally reversible surface melting. The second subject of the review, nanocalorimetry, is utilized to study samples of small mass under conditions of very fast heating and cooling. The glass transition properties of thin amorphous polymer films are observed under adiabatic conditions. The glass transition temperature appears to be independent of film thickness, and is observed even in ultra‐thin films. Recrystallization and reorganization during rapid heating are studied by nanocalorimetry of semicrystalline polymers. The uppermost endotherm seen under normal DSC scanning of poly(ethylene terephthalate) is caused by reorganization, and vanishes under the rapid heating conditions (3000K/s) provided by nanocalorimetry. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 629–636, 2005     

Temperature modulated calorimetry and dielectric spectroscopy in the glass transition region of polymers

The results from temperature modulated DSC in the glass transition region of amorphous and semicrystalline polymers are described with the linear response approach. The real and the imaginary part of the complex heat capacity are discussed. The findings are compared with those of dielectric spectroscopy. The frequency dependent glass transition temperature can be fitted with a VFT-equation. The transition frequencies are decreased by 0.5 to 1 orders of magnitude compared to dielectric measurements. Cooling rates from standard DSC are transformed into frequencies. The glass transition temperatures are also approximated by the VFT-fit from the temperature modulated measurements. The differences in the shape of the curves from amorphous and semicrystalline samples are discussed.Dedicated to Professor Bernhard Wunderlich on the occasion of his 65th birthday     

Detection of obscured glass transitions by QiDSC

Determination of compatibility in the amorphous phase for a two component blend is usually accomplished by analyzing for whether one notes one or two glass transitions. This can be complicated when one of the components is semicrystalline and its melting peak obscures the second glass transition. Quasi-isothermal differential scanning calorimetry (QiDSC) can be used to detect an obscured glass transition by allowing the semicrystalline component to melt and relax revealing the underlying glass transition of the other component. QiDSC is accomplished by performing a modulated temperature DSC experiment at a particular temperature and step ramping through the transitions of interest. For this study two systems are investigated. The first system is a model system based on a blend of polystyrene (PS) and a copolymer of vinylidene fluoride and hexafluoropropylene, P(VF₂/HFP). The glass transition for the PS occurs at the same temperature as the melting point for the fluoro-copolymer. The second system is a fluoro-copolymer/acrylic dried latex. In both cases the hidden glass transition can be noted in the reversing heat capacity of the QiDSC analysis.     

玻璃成分分析标准物质的研究及应用

玻璃标准物质是用于比较的基准物质,在玻璃的生产和质量控制中是保证数值有效可靠的基础.分析了国内外玻璃标准物质的研究进展情况,论述了玻璃标准物质的作用,同时指出了现阶段玻璃标准物质研究中存在的问题,并指出了其未来的发展方向.     

Quantification of amorphous content in maltitol by

A method was developed for the quantification of low levels of amorphous content in maltitol with StepScan DSC. The method was based on the fact that the change of specific heat at the glass transition is linearly proportional to the amorphous content. The influence of different measurement parameters of StepScan DSC was evaluated and two different calibration heating rates were tested. Synthetic mixtures with various proportions of crystalline and amorphous maltitol were prepared. Two different measurement methods were compared and the linear regression between ΔC_p and amorphous content was obtained. The limit of detection (LOD) and the limit of quantification (LOQ) values were for the fictive temperature 0.24% (amorphous content) and 0.81% and for the half point temperature 0.27 and 0.92%, respectively, (method 1) and for the fictive temperature 0.18 (amorphous content) and 0.61% and for the half point temperature 0.16 and 0.52%, respectively (method 2). Very low determination limits for the quantification of amorphous content could be attained with the StepScan DSC method. However, the realistic limit of quantification was somewhat higher (about 3%) because of noise in the StepScan measurement. The main advantage of the StepScan DSC method for quantification of amorphous content was that the glass transition and relaxation peaks are separated into different curves and the interpretation becomes easier.     

Uncertainty calculations in the certification of reference materials 4. Characterisation and certification

The main objective of the certification of a reference material is to determine its property values including their uncertainty. In the previous parts of this series, the basis for the evaluation of measurement uncertainty of the property values has been examined, including the conversion of homogeneity and stability study data into standard uncertainties. In this final part, the determination of the property values and the modelling of the certification process is discussed. It is noted that the characterisation of a reference material can be modelled in some cases using analysis of variance statistics, but a more generally applicable model can be developed based on χ²-fitting. Furthermore, it is concluded that there is an advantage in using absolute standard uncertainties instead of relative ones when modelling the certification process. Received: 14 October 2000 Accepted: 21 January 2001     

Differential AC-chip calorimeter for glass transition measurements in ultrathin films

A differential AC-chip calorimeter capable of measuring the step in heat capacity at the glass transition in nanometer-thin films is described. Because of the differential setup, pJ/K sensitivity is achieved. Heat capacity can be measured for sample masses below 1 ng in broad temperature range as needed for the study of the glass transition in nanometer-thin polymeric films. Relative accuracy is sufficient to investigate the changes in heat capacity as the step at the glass transition of polystyrene. The step is about 25% of the total heat capacity of polystyrene. The calorimeter allows for the frequency dependent measurement of complex heat capacity in the frequency range from 1 Hz to 1 kHz. The glass transition in thin polystyrene films (50–4 nm) was determined at well-defined experimental time scales. No thickness dependency of the glass transition temperature was observed within the error limits (±3 K)—neither at constant frequency (40 Hz) nor for the trace in the activation diagram (1 Hz–1 kHz). © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2996–3005, 2006     

The glass transition temperature of polystyrene

A round robin test was performed to determine the reliability of values for the glass transition temperatureT _g as determined by DTA on polymers. Ten different instruments were involved. The test material was high molecular weight polystyrene. Values forT _g (midpoint) were reported in the range 107°C±2 K. The respective heat flow curves differed considerably in shape. In the literature aT _g of 100°C is often given for polystyrene. The discrepancy between this value and the value of 107°C found in the round robin test is due to three differences: the thermal history of the sample, the evaluation of the heat flow curves, and the effect of finite sample size.     

Ultrasonic characterization of polymer viscoelasticity. Application to polystyrene

Ultrasonics is presented as a powerful method for the thermomechanical analysis of polymers. An experiment is described where the specific volume of a polymer is measured in simultaneity with the acoustic parameters. The technique allows full control of the thermal history in the temperature range from — 150°C to 300°C, where the material goes from a glasslike to a liquidlike state. Moreover the technique allows measurements to be performed at different pressures, up to 2 kbar. Results obtained on a typical amorphous polymer (polystyrene) are reported and the propagation of sound is discussed in regards to the well defined thermodynamic state for the material. Also, the results discriminate between the thermodynamic and the relaxational aspects of the glass transition.     

Calorimetric glass transitions in the amorphous forms of water: a comparison

The calorimetric glass transition behaviour in the amorphous forms of water is reviewed: for a heating rate of 30 K min⁻¹ the onset temperature, or T_g, of the glass transition is 136±1 K for hyperquenched glassy water and annealed vapour-deposited amorphous solid water, and 129±1 K for the low-density form of pressure-amorphized hexagonal ice. The increase in heat capacity in the glass transition region is between 1.6–2 J K⁻ mol⁻ for the three amorphous forms. Annealing of the samples a few degrees below T_g or heating several degrees above the glass transition region has no influence on the onset temperatures at 136 K and 129 K respectively, which is contrary to ‘normal’ behaviour. The results are discussed with respect to the ‘structure’ of the three amorphous forms of water below the glass transition region and a “gel-like” state of water above T_g.