Evaluation of the physical stability and local crystallization of amorphous terfenadine using XRD-DSC and micro-TA

It is very difficult to follow rapid changes in polymorphic transformation and crystallization and to estimate the species recrystallized from the amorphous form. The aim of this study was to clarify the structural changes of amorphous terfenadine and to evaluate the polymorphs crystallized from amorphous samples using XRD-DSC and an atomic force microscope with a thermal probe (micro-TA). Amorphous samples were prepared by grinding or rapid cooling of the melt. The rapid structural transitions of samples were followed by the XRD-DSC system. On the DSC trace of the quenched terfenadine, two exotherms were observed, while only one exothermic peak was observed in the DSC scan of a ground sample. From the in situ data obtained by the XRD-DSC system, the stable form of terfenadine was recrystallized during heating of the ground amorphous sample, whereas the metastable form was recrystallized from the quenched amorphous sample and the crystallized polymorph changed to the stable form. Obtained data suggested that recrystallized species could be related to the homogeneity of samples. When the stored sample surface was scanned by atomic force microscopy (AFM), heterogeneous crystallization was observed. By using micro-TA, melting temperatures at various points were measured, and polymorph forms I and II were crystallized in each region. The percentages of the crystallized form I stored at 120 and 135 °C were 47 and 79%, respectively. This result suggested that increasing the storage temperature increased the crystallization of form I, the stable form, confirming the temperature dependency of the crystallized form. The crystallization behavior of amorphous drug was affected by the annealing temperature. Micro-TA would be useful for detecting the inhomogeneities in polymorphs crystallized from amorphous drug.     

Thermal degradation kinetics of the biodegradable aliphatic polyester, poly(propylene succinate)

The preparation of the biodegradable aliphatic polyester poly(propylene succinate) (PPSu) using 1,3-propanediol and succinic acid is presented. Its synthesis was performed by two-stage melt polycondensation in a glass batch reactor. The polyester was characterized by gel permeation chromatography, ¹H NMR spectroscopy and differential scanning calorimetry (DSC). It has a number average molecular weight 6880 g/mol, peak temperature of melting at 44 °C for heating rate 20 °C/min and glass transition temperature at −36 °C. After melt quenching it can be made completely amorphous due to its low crystallization rate. According to thermogravimetric measurements, PPSu shows a very high thermal stability as its major decomposition rate is at 404 °C (heating rate 10 °C/min). This is very high compared with aliphatic polyesters and can be compared to the decomposition temperature of aromatic polyesters. TG and Differential TG (DTG) thermograms revealed that PPSu degradation takes place in two stages, the first being at low temperatures that corresponds to a very small mass loss of about 7%, the second at elevated temperatures being the main degradation stage. Both stages are attributed to different decomposition mechanisms as is verified from activation energy determined with isoconversional methods of Ozawa, Flyn, Wall and Friedman. The first mechanism that takes place at low temperatures is auto-catalysis with activation energy E = 157 kJ/mol while the second mechanism is a first-order reaction with E = 221 kJ/mol, as calculated by the fitting of experimental measurements.     

Thermal properties of drug polymorphs: A case study with felodipine form I and form IV

Thermal decomposition kinetics and solution thermodynamics of two polymorphs, felodipine form I and form IV, were investigated. The thermal decomposition kinetics of the polymorphs, containing thermal decomposing mechanism and the kinetic parameters were studied under non-isothermal conditions using Popescu method, and the shelf life was simply calculated. The results showed that nucleation and growth (n = 3/4) of Avremi–Erofeev equation is the most probable mechanism function for form I, and the integral form is G(α) = [−ln(1 − α)]^3/4; while the Mample Powel law (n = 1) is the most probable mechanism function for form IV, corresponding to G(α) = α. Notably, the individual amorphous phases of crystal felodipine form I and IV were obtained after the heating–cooling cycle of DSC tests, which were identified by TMDSC and FT-IR measurements. As the interim products before the collapse, it can be inferred that different amorphous intermediates may be the determinant for different thermal decomposition mechanisms of crystal forms I and IV. The solubility data and solution thermodynamic parameters, including enthalpy, entropy and Gibbs free energy have also been calculated by Van’t Hoff equation in ethanol aqueous. The results illustrated the polymorphic pair is enantiotropic with the transition temperature of 322.23 K and the conversion is driven by entropy.     

Glass transition behavior and crystallization kinetics of Cu0.3(SSe20)0.7 chalcogenide glass

The glass transition behavior and crystallization kinetics of Cu_0.3(SSe₂₀)_0.7 chalcogenide glass were investigated using differential scanning calorimetry (DSC), X-ray diffraction (XRD). Two crystalline phases (SSe₂₀ and Cu₂Se) were identified after annealing the glass at 773 K for 24 h. The activation energy of the glass transition (E_g), the activation energy of crystallization (E_c), the Avrami exponent (n) and the dimensionality of growth (m) were determined. Results indicate that this glass crystallizes by a two-stage bulk crystallization process upon heating. The first transformation, in which SSe₂₀ precipitates from the amorphous matrix with a three-dimensional crystal growth. The second transformation, in which the residual amorphous phase transforms into Cu₂Se compound with a two-dimensional crystal growth.     

Kinetics of thermal degradation and estimation of lifetime for polypropylene particles: Effects of particle size

The thermal stability and degradation behavior of polypropylene (PP) particles having diameter varying from few micrometers to nanometers were studied by thermogravimetric analysis (TGA). The PP particles of average diameter ∼20 μm, ∼10 μm, ∼5 μm, ∼1 μm and <500 nm were studied over a range of temperature from 25 to 600 °C in N₂ atmosphere and heating rates of 5, 10 and 15 °C/min. Thermal stability of PP particles initially decreases and then increases as particle size further decreases to nanometer scale. The five single heating rate techniques such as Friedman, Freeman-Carroll, Chang, Coats-Redfern and second Kissinger; and three multiple heating rate techniques such as the first Kissinger, Kim-Park and Flynn-Wall were used to compute the kinetic parameters of degradation reaction, e.g., activation energy (E_a), order of reaction (n) and frequency factor [ln(Z)]. The lifetime of macro-, micro- and nanosized PP particles was also estimated by a method proposed by Toop. It was found that the activation energy and lifetime of nanosized PP particles are moderately high compared to the microsized PP particles. Moreover, the decomposition temperature, order of reaction (n), frequency factor [ln(Z)] not only depend on the heating rate and calculation technique but also on the particle size of polymer. The results are compared with macrosized PP.     

Crystallization kinetics of PHB/PVAc blends using time resolved dielectric spectroscopy

Crystallization kinetics of poly(hydroxy butyrate), PHB, and its blends with poly(vinyl acetate), PVAc, have been thoroughly investigated using broadband dielectric technique over a wide range of frequencies (10⁻²-10⁵ Hz) as functions of crystallization temperature and blend composition. The dielectric strength of the amorphous segments, Δε, which is directly proportional to the volume fraction of the mobile amorphous phase in the blend decreases exponentially with increasing the crystallization time. However, on the other hand, the dielectric strength of the rigid amorphous segments, Δε_α′, which is related to the percentage of crystallinity in the blend increases dramatically with increasing crystallization time. A great variation in the dynamical constraints of relaxation segments with increasing crystallization time has been observed as a result of different environments, which would lead to a variation in the consistency of the cooperative regions. The value of the dielectric constant, ε′, decreases dramatically with increasing crystallization time, after that it reaches an equilibrium value at the end of the crystallization process. This dramatic decrease in the value of ε′ as a result of crystallization at a given crystallization temperature, was taken as an accurate evaluation for the amount of the amorphous phase that has undergone crystallization considering the theoretical approach of Avrami. The Avrami exponent, n, was found to be crystallization temperature, T_c, independent (n ∼ 3) indicating a three-dimensional crystal growth for pure PHB. The crystallization rate constant, k, increases greatly with increasing T_c due to the high crystallization rate. In the blend the value of n was found to be concentration dependent (n ∼ 1.8-3.2). The different values of n indicate that the shapes of the growing crystals are affected by blend concentration. For n ∼ 1.8, the crystals can either grow sporadically as rods or instantaneously as disks, while for n ∼ 3 a three-dimensional crystal growth takes place.     

Evolution of structural features and mechanical properties during the conversion of poly[(methylamino)borazine] fibers into boron nitride fibers

Poly[(methylamino)borazine] (PolyMAB) green fibers of a mean diameter of 15 μm have been pyrolyzed under ammonia up to 1000°C and heat treated under nitrogen up to 2000°C to prepare boron nitride (BN) fibers. During the polymer-to-ceramic conversion, the mechanical properties of the green fibers increase within the 25-400°C temperature range owing to the formation of a preceramic material and remain almost constant up to 1000°C. Both the crystallinity and the mechanical properties slightly increase within the 1000-1400°C range, in association with the consolidation of the fused-B₃N₃ basal planes. A rapid increase in tensile strength (σ^R) and elastic modulus (Young's modulus E) is observed in relation with crystallization of the BN phase for fibers treated between 1400°C and 1800°C. At 2000°C, “meso-hexagonal” BN fibers of 7.5 μm in diameter are finally obtained, displaying values of σ^R=1.480 GPa and E=365 GPa. The obtention of both high mechanical properties and fine diameter for the as-prepared BN fibers is a consequence of the stretching of the green fibers on a spool which is used during their conversion into ceramic.     

Free radical copolymerization of methyl substituted stilbenes with maleic anhydride

Stilbene-maleic anhydride is a well-known donor-acceptor comonomer pair which undergoes free radical copolymerization to form an alternating copolymer. A series of methyl substituted stilbenes were synthesized and copolymerized with maleic anhydride. A conversion versus time study was undertaken to understand the methyl substituent effect on copolymerization rates. Methyl substituents on the phenyl ring of stilbene can change the reactivity of stilbene by changing the resonance stability of the propagating radical and steric hindrance in the propagation step and thereby change the copolymerization rate. Methyl substituted stilbene-maleic anhydride copolymers were determined by quantitative ¹³C 1D NMR to be alternating copolymers. Size exclusion chromatography (SEC) measurements showed that the weight-average molecular weights of these copolymers varied from 3000 to over 1,000,000 g/mol. Interchain aggregation was observed in poly((E)-4-methylstilbene-alt-maleic anhydride) by dynamic light scattering (DLS). The SEC trace for poly((E)-4-methylstilbene-alt-maleic anhydride) exhibited bimodal peaks. No glass transition temperature or crystalline melting temperature was observed between 0 °C and 250 °C by differential scanning calorimetry (DSC). Thermogravimetric analysis (TGA) showed that these polymers have 5% weight loss around 290 °C.     

Thermo-mechanical behavior of electrospun thermoplastic polyurethane nanofibers

Analysis of the thermo-mechanical behavior of electrospun thermoplastic polyurethane (TPU) block co-polymer nanofibers (glass transition temperature ∼−50 °C) is presented. Upon heating, nanofibers began to massively contract, at ∼70 °C, whereas TPU cast films started to expand. Radial wide-angle X-ray scattering (WAXS) profiles of the nanofibers and the films showed no diffraction peaks related to crystals, whereas their amorphous halo had an asymmetric shape, which can be approximated by two components, associated with hard and soft segments. During heating, noticeable changes in the contribution of these components were only observed in nanofibers. These changes, which were accompanied with an endothermic DSC peak, coinciding with the start of the nanofibers contraction, can be attributed to relaxation of an oriented stretched amorphous phase created during electrospinning. Such structure relaxation becomes possible when a portion of the hard segment clusters, forming an effective physical network, is destroyed upon heating.     

Controlled radical polymerization of cholesteryl acrylate and its block copolymer with styrene via the RAFT process

Reversible addition fragmentation chain transfer (RAFT) polymerization of cholesteryl acrylate (ChA) was conducted using S-1-dodecyl-S′-(α,α′-dimethyl-α′′-acetic acid)trithiocarbonate as CTA and AIBN as initiator in toluene at 80 °C. The polymerization was investigated at two different CTA concentrations (0.025 and 0.040 M). Polymerization of ChA with CTA concentration of 0.040 M proceeds in a controlled/living manner as evidenced by linear increase of the molecular weight with conversion and narrow polymer polydispersity (PDI < 1.2). With lower initial CTA concentration, namely 0.025 M, although poly(cholesteryl acrylate) (PChA) exhibiting narrow molecular weight distributions could be synthesized, the polymerization showed relatively low control with many termination products. Chain extension polymerizations were performed starting from either the PChA or the polystyrene (PS) block, and well-defined copolymers based on ChA and styrene were prepared. Thermal properties of PChA and PS-b-PChA copolymer were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), and the results showed that both PChA and PS-b-PChA are amorphous polymers. PChA begins to decompose at ca. 218 °C with maximum weight loss rate at 351 °C, while PS-b-PChA shows double weight loss rate peaks located at 345 and 415 °C, respectively.     

Lifetime predictions for semi-crystalline cable insulation materials: I. Mechanical properties and oxygen consumption measurements on EPR materials

Long-term accelerated aging studies (up to 7 years of aging) were conducted on four typical EPR materials used as cable insulation in nuclear power plant safety applications with the goal of establishing lifetime estimates at typical aging conditions of ∼50 °C. The four materials showed slow to moderate changes in mechanical properties (tensile elongation) until just before failure where abrupt changes occurred (so-called “induction-time” behavior). Time-temperature superposition was applied to derive shift factors and probe for Arrhenius behavior. Three of the materials showed reasonable time-temperature superposition with the empirically derived shift factors yielding an approximate Arrhenius dependence on temperature. Since the elongation results for the fourth material could not be successfully superposed, consistency with Arrhenius assumptions was impossible. For this material the early part of the mechanical degradation appeared to have an Arrhenius activation energy E_a of ∼100 kJ/mol (24 kcal/mol) whereas the post-induction degradation data had an E_a of ∼128 kJ/mol. Oxygen consumption measurements were used to confirm the 100 kJ/mol E_a found from early-time elongation results and to show that the chemistry responsible before the induction time is likely to remain unchanged down to 50 °C. Reasonable extrapolations of the induction-time results indicated 50 °C lifetimes exceeding 300 years for all four materials.     

Rapid Ti(Oi-Pr)4 facilitated synthesis of α,α,α-trisubstituted primary amines by the addition of Grignard reagents to nitriles under microwave heating conditions

A series of carbinamines (α,α,α-trisubstituted amines) have been prepared in a simple and efficient one-pot procedure by the addition of Grignard reagents to a series of aliphatic, aromatic and heteroaromatic nitriles. The resulting magnesium imines are subsequently converted to the desired amine after treatment with Ti(Oi-Pr)₄ and additional microwave heating. Key to this procedure is the use of microwave heating for both steps of the reaction protocol, which significantly improves both reaction yields and reduces reaction times. In general, the Grignard addition reaction is complete within 5-10 min at 100 °C followed by conversion with Ti(Oi-Pr)₄ and additional microwave heating to give the target amines in good yields.     

Effect of lithium-sodium mixed-alkali on phase transformation kinetics in Er/Yb co-doped aluminophosphate glasses

Er³⁺ doped aluminophosphate glasses with various Na₂O/Li₂O ratios were prepared at 1250 °C using a silica crucible to study mixed alkali effect (MAE). The effect of relative alkali content on glass transition temperature, crystallization temperature and thermal stability were investigated using differential scanning calorimetry (DSC). In addition, apparent activation energies for crystallization, E_c, were determined employing the Kissinger equation. The effect of Al₂O₃ content on the magnitude of MAE was also discussed. No mixed-alkali effect is observed on crystallization temperature.     

Mechanistic investigation, kinetic modeling and analysis parameters of poly(3-hexylthiophene) degradation to fullerenes

Thermal degradation of Poly (3-hexylthiopene) (P3HT) was studied under nitrogen environment. Kinetic parameters of thermal degradation were determined using Vyazovkin model free method and model fitting method. Vyazovkin model free kinetic analysis is carried out to understand the variation of activation energy (E_α) required for degradation of polymer with conversion (α). Various reaction models have been tested for probable reaction mechanism using hybrid genetic algorithm (HGA). Diffusion model and nucleation & growth with n = 2/3 has prominent role in thermal decomposition of P3HT. A plausible degradation route is proposed based on the experimental details acquired from gas chromatography (GC), Raman spectroscopy, FTIR spectroscopy, powder X-ray diffractometry (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Degradation of P3HT starts at around 195 °C with release of lighter units like CS. Further increase in the temperature results in detachment of the hexyl chain from P3HT and the residue obtained at 1050 °C contains fullerenes mixed with amorphous carbon.     

Simple and highly efficient synthesis of oxaziridines by tetrabutylammonium Oxone

Oxygenation of various aldimines with tetrabutylammonium monoperoxysulfate produced the corresponding E- or a mixture of E- and Z-oxaziridines with very high yields (?90%) and good to excellent selectivities (75-100%) within 20 min to 10 h in CH₃CN at room temperature (∼25 °C). The E/Z isomer ratio critically depends on the stereo-electronic nature of the substituents in the oxaziridines, solvent, and the presence of Lewis acids and bases.     

Crystallization kinetics of amorphous Fe75−xAgxSi9B16

The influence of Ag admixtures on the crystallization process of amorphous Fe-Si-B alloys of the series Fe_75−xAg_xSi₉B₁₆ (x=0, 1, 2, 3 and 4) was studied by differential scanning calorimetry (DSC) measurements at different heating rates. Two exothermic peaks were observed in the DSC trace when the Ag content was 2-4 at.% and one when it was 1 at.%, respectively. The activation energy was calculated both with Kissinger’s and isoconversional Flynn, Wall and Ozawa methods. The possibility of analysis with the JMA model using the Avrami plot was investigated for the third stage of crystallization.     

On α-β phase transition in cristobalite-type Al1−xGaxPO4 (0.00?x?1.00)

The results of variable temperature powder X-ray diffraction and differential thermal analysis (DTA) studies on the orthorhombic (α) low-cristobalite to cubic (β) high-cristobalite phase transition for Al_1−xGa_xPO₄, (0.00?x?1.00) are presented. These studies reveal that all these compositions undergo reversible phase transitions from orthorhombic to cubic form at higher temperature. The high-temperature behavior of GaPO₄ is observed to have a different behavior compared to all other compositions in this series. Orthorhombic low-cristobalite-type GaPO₄ transforms to cubic high-cristobalite form at ∼605 °C. Above ∼700 °C, the cubic high-cristobalite-type GaPO₄ slowly transforms to trigonal quartz type structure. At about 960 °C, the quartz type GaPO₄ transforms back to the cubic high-cristobalite form. During cooling cycles the cubic phase of GaPO₄ reverts to trigonal quartz type phase. However, annealing of GaPO₄ at higher temperatures for longer duration can stabilize the orthorhombic low cristobalite phase. The phase transition temperatures and associated enthalpies are related to the change in unit cell volume and the orthorhombicity of the respective low cristobalite lattice.     

Noncrystalline blue-emitting 9,10-diphenylanthracene end-capped with triphenylamine-substituted fluorene

Two blue-emitting oligomers, namely FDPA1 and FDPA2 containing 9,10-diphenylanthracene core end-capped with triphenylamine-substituted fluorene has been synthesized and characterized. The spiro-configuration end-capping groups imparts two compounds with pronounced morphological stability (T_g > 185 °C, T_d > 420 °C) and excellent hole injection ability (E_HOMO > −5.27 eV) with the advantageous optical characteristics of corresponding core. Scanning electron microscope (SEM) and X-ray diffraction (XRD) reveal that the two oligomers form excellent amorphous films and possess good morphological stability after annealing.