Showing papers on "Differential scanning calorimetry published in 1995"

Molecular mobility of amorphous pharmaceutical solids below their glass transition temperatures.

Abstract: Purpose. To measure the molecular mobility of amorphous pharmaceutical solids below their glass transition temperatures (Tg), using indomethacin, poly (vinyl pyrrolidone) (PVP) and sucrose as model compounds. Methods. Differential scanning calorimetry (DSC) was used to measure enthalpic relaxation of the amorphous samples after storage at temperatures 16-47 K below Tg for various time periods. The measured enthalpy changes were used to calculate molecular relaxation time parameters. Analogous changes in specimen dimensions were measured for PVP films using thermomechanical analysis. Results. For all the model materials it was necessary to cool to at least 50 K below the experimental Tg before the molecular motions detected by DSC could be considered to be negligible over the lifetime of a typical pharmaceutical product. In each case the temperature dependence of the molecular motions below Tg was less than that typically reported above Tg and was rapidly changing. Conclusions. In the temperature range studied the model amorphous solids were in a transition zone between regions of very high molecular mobility above Tg and very low molecular mobility much further below Tg. In general glassy pharmaceutical solids should be expected to experience significant molecular mobility at temperatures up to fifty degrees below their glass transition temperature.

Formation of Ti–Zr–Cu–Ni bulk metallic glasses

Abstract: Formation of bulk metallic glass in quaternary Ti–Zr–Cu–Ni alloys by relatively slow cooling from the melt is reported. Thick strips of metallic glass were obtained by the method of metal mold casting. The glass forming ability of the quaternary alloys exceeds that of binary or ternary alloys containing the same elements due to the complexity of the system. The best glass forming alloys such as Ti34Zr11Cu47Ni8 can be cast to at least 4-mm-thick amorphous strips. The critical cooling rate for glass formation is of the order of 250 K/s or less, at least two orders of magnitude lower than that of the best ternary alloys. The glass transition, crystallization, and melting behavior of the alloys were studied by differential scanning calorimetry. The amorphous alloys exhibit a significant undercooled liquid region between the glass transition and first crystallization event. The glass forming ability of these alloys, as determined by the critical cooling rate, exceeds what is expected based on the reduced glass transition temperature. It is also found that the glass forming ability for alloys of similar reduced glass transition temperature can differ by two orders of magnitude as defined by critical cooling rates. The origins of the difference in glass forming ability of the alloys are discussed. It is found that when large composition redistribution accompanies crystallization, glass formation is enhanced. The excellent glass forming ability of alloys such as Ti34Zr11Cu47Ni8 is a result of simultaneously minimizing the nucleation rate of the competing crystalline phases. The ternary/quaternary Laves phase (MgZn2 type) shows the greatest ease of nucleation and plays a key role in determining the optimum compositions for glass formation.

Isolation of predominantly submicron-sized UHMWPE wear particles from periprosthetic tissues

Abstract: A method of tissue digestion using sodium hydroxide was applied to the isolation and recovery of ultra-high-molecular-weight polyethylene (UHMWPE) particles from tissues around failed total hip replacements. Density gradient ultracentrifugation of the digested tissues was performed to separate the UHMWPE from cell debris and other particulates. Fourier transform infrared spectroscopy and differential scanning calorimetry (DSC) verified that the recovered particles were UHMWPE. When viewed by scaning electron microscopy, individual particles were clearly observed and were either rounded or elongated. The majority were submicron in size. The application of this method to the study or particles from periprosthetic tissues may elucidate aspects of biomaterial particles size and shape that are important to the biologic response to, and clinical outcome of, total joint replacement. © 1995 John wiley & Sons, Inc.

Temperature Effects on Structural Properties of Pluronic P104 and F108 PEO-PPO-PEO Block Copolymer Solutions

Abstract: The effects of temperature on the micellization properties and the structure of the micelles for two poly(ethylene-oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-PPO-PEO) copolymers, having similar size hydrophobic (PPO) blocks and different size hydrophilic (PEO) blocks, are reported. Critical micellization concentration (CMC) and temperature (CMT) values for aqueous copolymer solutions were obtained from a dye solubilization method and corroborated with differential scanning calorimetry, surface tension, density, light scattering intensity, and fluorescence spectroscopy experiments; very good agreement among the different techniques is observed. The extraction of CMC and CMT values from the characteristic features of the data obtained from the various techniques is discussed. The hydrodynamic radii of the copolymer micelles, determined using dynamic light scattering, remained constant over the temperature range investigated; the polydispersity of the micelle size decreased with temperature. The micropolarity in aqueous copolymer solutions was probed as a function of temperature using the I{sub 1}/I{sub 3} intensity ratio of the pyrene vibrational fine structure recorded in fluorescence emission spectra. The decreasing values for the intensity ratio, following the formation of micelles, were considered to be a linear combination of the temperature effects on polarity observed in bulk PEO and PPO homopolymers, and in water. 59 refs., 10 figs., 2 tabs.

Thermodynamics and kinetics of the undercooled liquid and the glass transition of the Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 alloy

Abstract: Differential scanning calorimetry (DSC) was used to determine the thermodynamic functions of the undercooled liquid and the amorphous phase with respect to the crystalline state of the Zr412Ti138Cu125Ni100Be225bulk metallic glass forming alloy The specific heat capacities of this alloy in the undercooled liquid, the amorphous state and the crystal were determined The differences in enthalpy, ∆H, entropy, ∆S, and Gibbs free energy, ∆G, between crystal and the undercooled liquid were calculated using the measured specific heat capacity data as well as the heat of fusion The results indicate that the Gibbs free energy difference between metastable undercooled liquid and crystalline solid, ∆G, stays small compared to conventional metallic glass forming alloys even for large undercoolings Furthermore, the Kauzmann temperature, TK, where the entropy of the undercooled liquid equals to that of the crystal, was determined to be 560 K The Kauzmann temperature is compared with the experimentally observed rate-dependent glass transition temperature, Tg Both onset and end temperatures of the glass transition depend linearly on the logarithm of the heating rate based on the DSC experiments Those characteristic temperatures for the kinetically observed glass transition become equal close to the Kauzmann temperature in this alloy, which suggests an underlying thermodynamic glass transition as a lower bound for the kinetically observed freezing process

Synthesis and Characterization of Thermo- and Chemomechanically Responsive Poly(N-isopropylacrylamide-co-methacrylic acid) Hydrogels

Abstract: Random copolymer hydrogels of methacrylic acid (MAA) and N-isopropylacrylamide (NIPAAm) were synthesized by free-radical polymerization in the presence of a cross-linking agent. The gels were characterized for their temperature- and pH-responsive behavior by equilibrium swelling experiments, differential scanning calorimetry, and thermal mechanical analysis. Depending upon composition, the gels showed sharp swelling transitions with small changes in temperature or pH, with initial time-dependent response reaching equilibrium on a time scale of hours.

Novel Side-Chain Liquid Crystalline Polyester Architecture for Reversible Optical Storage

Abstract: New side-chain liquid crystalline polyesters have been prepared by melt transesterification of diphenyl tetradecanedioate and a series of mesogenic 2-[ω-[4-[(4-cyanophenyl)azo]phenoxy]alkyl]-1,3-propanediols, where the alkyl spacer is hexa-, octa-, and decamethylene in turn. The polyesters have molecular masses in the range 5000-89 000. Solution 13 C NMR spectroscopy has been employed to identify carbons of polyester repeat units and of both types of end groups. Polyester phases and phase transitions have been investigated in detail by polarizing optical microscopy and differential scanning calorimetry for the hexamethylene spacer architecture with different molecular masses. Using FTIR polarization spectroscopy, the segmental orientation in unoriented polyester films induced by argon ion laser irradiation has been followed and an irradiation-dependent order parameter for the cyanoazobenzene mesogens calculated. FTIR is also utilized to follow the temperature-dependent erasure of the induced orientation. Optical storage properties of thin unoriented polyester films are examined through measurements of polarization anisotropy and holography. A resolution of over 5000 lines/mm and diffraction efficiencies of about 40% have been achieved. Lifetimes greater than 30 months for information stored have been obtained, even though the glass transition temperatures are about 20 °C. Complete erasure of the information can be obtained by heating the films to about 80 °C, and the films can be reused many times without fatigue.

Process-Induced Crystallinity Changes in Albuterol Sulfate and Its Effect on Powder Physical Stability

Abstract: Pharmaceutical powders are often milled to achieve the optimum particle size. These size reduction processes can introduce dislocations and/or defects onto particle surfaces affecting the overall crystallinity of the powder. If enough energy is imparted, amorphous regions on the particle surfaces may be produced. These amorphous regions have the propensity to absorb significant quantities of water. In this study the effect of sorbed water on the physical characteristics of albuterol sulfate is investigated. Physical properties of this compound are studied in both micronized and unmicronized states using scanning electron microscopy, differential scanning calorimetry, powder x-ray diffraction, solution microcalorimetry, laser diffraction particle size analysis and water vapor sorption analysis. Subtle differences in crystallinity induced by air jet micronization are detected by several analytical methods. Amorphous to crystalline conversions are observed, the kinetics of which are found to be both temperature and relative humidity dependent. These experiments show the dynamic nature of micronized albuterol sulfate and aid in the determination of the actual physical state of this pharmaceutical powder.

Thermal stability of nanocrystalline Ni

Abstract: Thermal stability of electroplated nanocrystalline Ni of 10 and 20 nm grain size was investigated by differential scanning calorimetry (DSC). The temperature dependence and heat release ΔH during grain growth have been determined by linear anisothermal measurements (linear heating at 10 K min −1 ). The corresponding change in microstructure has been monitored in the temperature range between 373 K and 693 K using transmission electron microscopy (TEM). The TEM and DSC studies identified three exothermic reactions: “nucleation” and abnormal grain growth (353–562 K), normal grain growth (562–593 K) and growth towards equilibrium (643–773 K). The grain growth behaviour, and the similar heat releases ΔH = 18 J g −1 , and 16 J g −1 measured for the 10 nm and 20 nm Ni nanocrystals respectively in the DSC experiments may be related to the observed sulphur segregation at grain boundaries and triple junctions.

Screening potato starch for novel properties using differential scanning calorimetry.

Abstract: Thermal and other physicochemical properties of starch from 42 potato genotypes were studied to find those with unique properties for food use, and to analyze relationships between thermal and other physicochemical properties. Onset and peak transition temperatures and gelatinization enthalpy intercorrelated. Transition temperatures intercorrelated with pasting temperature using a Brabender Visco-amylograph. Gelatinization entbalpy correlated with Brabender pasting temperature and peak paste viscosity, and onset temperature correlated with phosphorus content. Genotype E55–3.5 with highest onset and peak transition temperatures also had highest phosphorus and peak Brabender viscosity. DSC might be useful for rapidly screening samples of <1g starch for such. Potato starch DSC characteristics did not correlate with amylose, intrinsic viscosity, or water-binding. For 10 genotypes from successive years, correlations were observed for pasting temperature (r = 0.83), phosphorus content (r = 0.80), and stability ratio (r = 0.66). Direct comparison between samples from consecutive years showed good reproducibility for amylose, but not for phosphorus or pasting.

Catalyzing the curing reaction of a new benzoxazine‐based phenolic resin

Abstract: The effect of potential catalysts on the curing reaction of a new type of phenolic resins obtained from benzoxazine precursors is studied. These novel resins solve the shortcomings of traditional phenolics because they cure by a ring-opening mechanism that avoids the release of volatiles. Isothermal and nonisothermal differential scanning calorimetry (DSC) data is used to determine the influence of the catalysts on the curing kinetics. Fourier transform infrared (FTIR) spectroscopy is also applied. The benzoxazine chosen for this study is a purified benzoxazine monomer based on bisphenol-A, formaldehyde, and aniline. The as-synthesized benzoxazine precursor is also studied to determine the influence of the dimers and higher oligomers in the curing mechanism. The presence of these structures seems to catalyze the curing reactions. The activation energy and overall reaction order of the as-synthesized precursor are determined. Among the catalysts tested, adipic acid shows the most promising results. For all the cases studied the curing reaction is autocatalyzed up to a diffusion-controlled stage. © 1995 John Wiley & Sons, Inc.

Crystallization of very low density copolymers of ethylene with α-olefins

Abstract: Differential scanning calorimetry (DSC) was used to analyze the crystal distribution in homogeneous ethylene–octene copolymers polymerized by the constrained geometry catalyst technology (CGCT). To minimize ambiguities from thermal history effects, copolymers were isothermally annealed at temperatures within the melting range. The cumulative crystallinity was related to the crystal distribution by the Gibbs–Thomson equation. The results provided a clear distinction between Type I copolymers (density less than 0.89 g/cc) and Type II copolymers (densities between 0.89 and 0.91 g/cc). The former had a singlecrystal population that was identified with the bundled crystals seen in transmission electron micrographs. In comparison, the latter had two crystal populations that correlated with lamellar crystals and bundled crystals. © 1995 John Wiley & Sons, Inc.

Thermal Properties of Polyaniline and Poly(aniline-co-o-ethylaniline)

Abstract: Block copolymers of aniline (r 1 1) can readily be formed by acid-catalyzed oxidation. These copolymers have enhanced solubility in common solvents, compared to polyaniline. For example, while a 47 mol % o-ethylaniline copolymer is 2-7 times more soluble in ethanol, methanol, and acetone than polyaniline, the solubility of the copolymer in tetrahydrofuran increases by more than 20 times. Thermal effects on the properties of polyaniline and poly(aniline-co-o-ethylaniline) in their emeraldine base forms have been studied by means of thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), electron paramagnetic resonance (EPR), four-probe conductivity, UV-vis, and FT-IR spectroscopies. Upon heating, the number of free electrons in the polymer, as determined by EPR, increases by up to 10 times. An irreversible exothermic transition was observed by DSC, which is believed to be an indication of cross-linking. The electron-donating character of the ethyl group appears to help stabilize the intermediates during the cross-linking reaction. Complete characterization of the copolymer is given.