Differential Polymorphic Transformation Behavior of Polybutene-1 with Multiple Isotactic Sequences

For the solid-solid transformation from form Ⅱ to form Ⅰ of isotactic polybutene-1(iPB),the temperature dependence of form Ⅰ nucleation and growth was deemed to control the transformation process.However,the relationship between formⅠ formation and form Ⅱ disappearance in the transformation process is not clear.In this work,the spontaneous crystal transformation from form Ⅱ to Ⅰ of iPB with 81 mol%mmmm sequence concentration is studied firstly by tracking the two processes,the decay of form Ⅱ and the yielding of form Ⅰ in a wide range of temperature spanning from 0℃ to 50℃ and in a long transformation time ranging from 5 min to 65 days with in situ FTIR and WAXD.Unlike the literature reports,the decay rate of form Ⅱ is firstly found to be lower than the yielding rate of form Ⅰ at all studied temperatures,especially at low transition temperature.This is attributed to the amorphous chains which locate near crystal lamella participating into the nucleation of form Ⅱ.The regular chain folding and growth of i PB form Ⅰ from amorphous chains containing short isotactic sequences also lead to an increase in crystallinity of form Ⅰ compared with that of initial form Ⅱ crystallized at 60℃.An increase in the annealing temperature results in decrease in crystallinity and increase in lamellae thickness of i PB formⅠ.     

Thermally-Induced Phase Transitions in the Uniaxially-Oriented δ Form of Syndiotactic Polystyrene

The empty δ (δ_e) form of uniaxially-oriented syndiotactic polystyrene (sPS) samples were obtained by extracting the solvent molecules from the δ form of sPS and solvent complex in acetone and methanol. Temperature dependence of the X-ray fiber diagrams starting from the uniaxially-oriented δ_e and δ form has been measured successfully at various temperatures for the first time. The transition behavior was traced clearly by separating the equatorial and layer line reflections. The δ_e form transformed to the γ form via an intermediate form. The intermediate form is speculated to take disordered structure due to the empty cavities present in the δ_e form. Calorimetric studies showed an endotherm followed by an exotherm during this phase transition, which is consistant with such a speculation. On the other hand the δ form transformed to the γ form directly without passing through the intermediate form or δ_e form. During the δ to γ phase transition the solvent molecules evaporate through the columnar structure in a broad range of temperature, allowing the transition to occur smoothly.     

Effect of Pre-Existed Form I' on the Phase Transition of Isotactic Poly-1-butene Form III under High Pressure CO2

The CO₂ plasticization effect on the isotactic poly-1-butene (iPB-1) form III with pre-existed minority form I' was investigated by using high-pressure differential scanning calorimetry (DSC). The results showed that the form III melting peak moved to a lower temperature and the melting peak area of form II generated during heating decreased with the increased CO₂ pressure. The solid-solid transition of form III to I' in the iPB-1 was mainly studied by fourier transform infrared spectroscopy (FTIR). It was claimed that the phase transition was a thermodynamic process and the pre-existed form I' in form III inhibit the phase transition of form III into I'. The influence of the pre-existed form I' on the melting behavior of the iPB-1 was also studied by DSC. It was found that the form I' also suppressed the recrystallization of form II.     

间规聚苯乙烯同质多晶现象的研究

间规聚苯乙烯(ｓＰＳ)是由日本出光公司在1985年首先合成,由于其优良的耐热性(熔点高达270℃),立刻被认为有可能或为下一世纪最重要的工程塑料.独特的分子结构,使间规聚苯乙烯具有密度低、尺寸稳定、耐热性、耐溶剂性好等诸多优点,因而引起研究人员的广泛重视.同时,间规聚苯乙烯存在复杂的同质多晶现象,共有α、β、γ和δ四种晶型.以不同的条件从熔体结晶,间规聚苯乙烯可形成平面锯齿结构的α晶和β晶.其中,α晶属六方晶系,晶胞参数为:ａ=263ｎｍ,ｃ=078ｎｍ;β晶是斜方晶,晶胞参数为:ａ=088ｎｍ,ｂ=263ｎｍ,ｃ=078ｎｍ.与传统的全同聚丙烯…     

Capturing a metastable chiral polymorph of an achiral molecule--hexa-O-benzoyl-myo-inositol

myo-Inositol hexabenzoate having meso configuration produces chiral polymorph (form I) when crystallized rapidly but yields achiral polymorph (form II) when allowed to crystallize slowly; in the mother liquor form I slowly but completely disappears to give form II.     

Multiple melting in nylon 66

Either of the two endothermic melting peaks found by differential thermal analysis of nylon 66 may be converted to the other by appropriate choice of annealing conditions. The two peaks are considered due to the melting of two morphological species, forms I and II. Form I is relatively fixed in melting temperature, while the form II melting temperature varies with annealing conditions and can be either above or below form I. The two forms can be distinguished by whether or not the conversion I → II takes place; if the sample is in form II no change in the thermogram is observed under suitable conversion conditions. The conversion of form I to form II also takes place during cold drawing. It has been previously shown that form I results from rapid cooling from the melt, and form II results from slow cooling. Form I appears to be kinetically favored, while form II is thermodynamically preferred. The variability in the form II melting point is attributed to variable crystal size and/or perfection.     

Stability and phase transformations of the mesomorphic form of isotactic polypropylene in stereodefective polypropylene

A study of the thermodynamic stability and the related polymorphic transformations induced by thermal treatments of the mesomorphic form that crystallizes in stereodefective metallocene isotactic polypropylene (iPP) is presented. We show that the mesomorphic form of the more isotactic samples is stable at room temperature, whereas the mesomorphic form crystallizing in the more stereoirregular sample is unstable and crystallizes at room temperature in the crystalline α form. In any case, the mesomorphic form transforms during heating or by annealing at temperatures higher than 60–80 °C always in the α form, regardless of the stereoregularity, even in the case of stereoirregular samples generally crystallizing from the melt in the γ form. These data confirm the proposed model of structure of the mesomorphic form as small aggregates of chains in three-fold helical conformation packed with lateral correlations similar to the α form of iPP.     

Polymorphic transformations of Bi2MoO6

The polymorphism of Bi₂MoO₆ has been studied by differential thermal analysis, differential dilatometry, and differential scanning calorimetry with γ form specimens having the koechlinite structure prepared by sintering the oxides Bi₂O₃ and MoO₃. Two stable γ and γ′ forms and one metastable γ″ form were observed. The relative thermal stability of the γ form compared with the γ′ form has been examined by isothermal heating of a mixture of the two forms under hydrothermal conditions. Thus the low-temperature stable γ form transformed reversibly to the γ″ form at 604 ± 3°C, and on subsequent heating, the γ″ form transformed irreversibly to the high-temperature stable γ′ form in the range 640 to 670°C, depending on heating rates; however, an isothermal treatment at a temperature above 604 ± 3°C brought the gradual transition of the γ″ form into the γ′ form.     

Mechanisms of electron-transfer oxidation of NADH analogues and chemiluminescence. Detection of the keto and enol radical cations

The radical cation of an NADH analogue (BNAH: 1-benzyl-1,4-dihydronicotinamide) has been successfully detected as the transient absorption and ESR spectra in the thermal electron transfer from BNAH to Fe(bpy)(3)(3+) (bpy = 2,2'-bipyridine) and Ru(bpy)(3)(3+). The ESR spectra of the radical cations of BNAH and the dideuterated compound (BNAH-4,4'-d(2)) indicate that the observed radical cation is the keto form rather than the enol form in the tautomerization. The deprotonation rate and the kinetic isotope effects of the keto form of BNAH(*)(+) were determined from the kinetic analysis of the electron-transfer reactions. In the case of electron transfer from BNAH to Ru(bpy)(3)(3+), the chemiluminescence due to Ru(bpy)(3)(2+) was observed in the second electron-transfer step from BNA(*), produced by the deprotonation of the keto form of BNAH(*)(+), to Ru(bpy)(3)(3+). The observation of chemiluminescence due to Ru(bpy)(3)(2+) provides compelling evidence that the Marcus inverted region is observed even for such an intermolecular electron-transfer reaction. When BNAH is replaced by 4-tert-butylated BNAH (4-t-BuBNAH), no chemiluminescence due to Ru(bpy)(3)(2+) has been observed in the electron transfer from 4-t-BuBNAH to Ru(bpy)(3)(3+). This is ascribed to the facile C-C bond cleavage in 4-t-BuBNAH(*)(+). In the laser flash photolysis of a deaerated MeCN solution of BNAH and CHBr(3), the transient absorption spectrum of the enol form of BNAH(*)(+) was detected instead of the keto form of BNAH(*)(+), and the enol form was tautomerized to the keto form. The rate of intramolecular proton transfer in the enol form to produce the keto form of BNAH(*)(+) was determined from the decay of the absorption band due to the enol form and the rise in the absorption band due to the keto form. The kinetic isotope effects were observed for the intramolecular proton-transfer process in the keto form to produce the enol form.     

Effect of cosolvent on solid phase transitions of α‐ to β‐form crystal of syndiotactic polystyrene occurred in supercritical Co2 containing solvent

The solid phase transition mechanism of α‐ to β‐form crystal upon specific treating with supercritical CO₂ + cosolvent on original pure α and mixed (α+β) form syndiotactic polystyrene (sPS) was investigated, using wide angle X‐ray diffraction and differential scanning calorimetry measurements as a function of temperature, pressure, and cosolvent content. As in the supercritical CO₂, sPS in supercritical CO₂ + cosolvent underwent solid phase transitions from α‐ to β‐form, and higher temperature or higher pressure favored this transformation. Due to the higher dipole moment of acetone, small amounts of acetone used as cosolvent with CO₂ made the transition of α‐ to β‐form occur at lower temperature and pressure than in supercritical CO₂, and made the α‐form crystal completely transform to β‐form in the original mixed (α+β) form, whereas ethanol did not. The original β‐form crystal in the original mixed (α+β) form sample acted as the nucleus of new β‐form crystal in the presence of cosolvent as it did in supercritical CO₂, when compared with the original pure α‐form sample. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1625–1636, 2007     

Melting and crystallization of poly(vinylidene fluride) under high pressure

Precise melting and crystallization temperatures of extended-chain and folded-chain crystals of form I and folded-chain crystals of form II poly(vinylidene fluoride) under high pressure have been obtained by microdifferential thermal analysis (DTA). Upon heating at pressures above 4000 kg/cm², the micro-DTA thermogram of form II crystallized from the melt at atmospheric pressure shows melting of the form II structure and the melting of the folded-chain and extended-chain crystals of form I, formed through recrystallization processes. These features were clarified by supplemental methods. The bandwidth seen in electron micrographs of the extended-chain crystal of form I obtained by crystallization under high pressure was in the range of 1500 to 2000 Å. At atmospheric pressure, the extended-chain crystal of form I melted at 207°C, approximately 17°C higher than the folded-chain crystal of form I and 31°C higher than the folded-chain crystal of form II.     

Electronic and molecular structures of HCNn polymers: (HCN)3n

The CNDO/2 method using the tight-binding approximation for polymers was applied to several forms of (HCN)_3n polymers. From these calculations, we conclude that the polymer exists in the helical form rather than the planar trans zigzag, TTT, form. It was found that, from comparison of the TTT form with the polyglycine II type (Gly-II) form, the intrasegment energy of the TTT form is lower than for the Gly-II type form while the intersegment energy of the Gly-II form is lower than for the TTT form. It was also revealed that, in the typical α-helix type structures, hydrogen bonding between the central and the fourth nearest neighbour segments may play an important stabilizing role.     

Thermal, phase transition, and thermal kinetics studies of carbamazepine

The thermal, phase transition of carbamazepine dihydrate and the solid-state transformation of carbamazepine from form III to form I were performed by Differential scanning calorimetry (DSC), Thermo gravimetry (TG–DTA), and X-ray powder diffraction.The non-thermal kinetic analysis of carbamazepine dihydrate and form III was carried out by DSC at different heating rates in dynamic nitrogen atmosphere. The model-free model, the Kissinger method, was used to give the Arrhenius parameters. Arrhenius plots from the kinetic model yielded activation energies corresponding to dehydration of dihydrate and melting of anhydrate CBZ form I were 95.28, 966.06 kJ mol^?1, the pre-exponential factors were 8.34E+11 and 1.41E+149, respectively. For the transformation of carbamazepine from form III to form I, activation energies corresponding to the melting of CBZ form III, recrystallization of form I, and melting of form I were 1160.81, 710.89, 1265.89 kJ mol^?1, the pre-exponential factors were 2.29E+144, 4.43E+91, and 1.61E+151, respectively. As a comparison, Ozawa method was used to verify the activation energy values obtained by Kissinger method. The result shows a close activation energy values between two methods.     

Quantification of pharmaceutical polymorphs and prediction of dissolution rate using theophylline tablet by terahertz spectroscopy

Theophylline has an anhydrous form and a monohydrated form, and the dissolution rate of the anhydrous form is higher than that of the monohydrated form. Terahertz (THz) spectra of theophylline tablet containing the theophylline anhydrous form, monohydrated form, microcrystalline cellulose and magnesium stearate exhibited a specific absorption peak at 0.96 THz, where the theophylline anhydrous form demonstrated an absorption peak. Additionally, the intensity of the peak at 0.96 THz gradually decreased as the proportion of the anhydrous form decreased. The multivariate data analysis was performed to correlate the THz spectra of theophylline tablets with the ratio of the theophylline anhydrous form. The calibration model used to predict the mixing ratio of the theophylline anhydrous form from the THz spectra achieved root-mean-squared errors of cross-validation (RMSECV) of 2.89%, a slope of 0.9934 and an R(2) of 0.9927. In addition, there were intentions to develop a prediction model for the dissolution rate of theophylline from the drug product. The dissolution rate of theophylline tablet was gradually delayed as the proportion of the anhydrous form was decreased. The multivariate data analysis was performed to correlate the THz spectra of theophylline tablets with the dissolution rate. The calibration model used to predict the percentage of theophylline dissolved in 45 min from the THz spectra achieved an RMSECV of 3.29%, a slope of 0.9260 and an R(2) of 0.9423. Furthermore, there were no significant differences between the predicted and measured percentages of theophylline dissolved in 45 min in the theophylline tablets that were stored at 84% relative humidity (RH) and 25 °C for 12 h or 3 d.     

Light scattering data evaluation using legendre polynomials

Calculation of the masses and the radii of gyration from static light scattering experiments were performed by approximating the measured angle distributions by Legendre polynomials. The method allows a very good fit to the scattering angle 0°. In this paper the method is discussed applied to discretised theoretical form factors of several geometries as well as measured data of latex particles. If the sample is polydisperse, the resulting form factor is the sum of the form factors of the individual particles. An iterative method was developed, that allows the deconvolution of form factors of spherical particles from the measured form factor to give information about the polydispersity of the sample. This form factor analysis will be discussed for experiments using asymmetrical Flow‐Field‐Flow‐Fractionation (a‐FFFF) of complex plant extracts to interpret the measured form factor as the superimposition of two possible elution modes of the a‐FFF.     

The structure of hypericin in solution. Searching for hypericin's 1,6 tautomer

Hypericin in organic solvents displays two types of electronic spectra: one type which shows a distinct solvatochromic effect, the stable form, and the other, the unstable form, which lacks this property. The latter type is formed in dry nonprotic solvents (e.g. tetrahydrofuran, EtOAc) and can be converted to the stable form on addition of protic solvents. In order to establish the tautomeric structure of the unstable form we applied conventional nuclear magnetic resonance techniques as well as two-dimensional gradient-enhanced heteronuclear multiple-quantum correlation, gradient-enhanced ROESY and one-dimensional nuclear Overhauser effect difference experiments. All these techniques pointed to the fact that the unstable form has the 7,14-diketo tautomeric structure, like the stable form, and not the 1,6-diketo structure. Electronic spectroscopy indicated that the unstable form has acidic properties and therefore possesses two free OH groups at C3 and C4 at the bay region of the molecule.     

Cu(OAc)2.H2O与1-苯基-3-甲基-4-苯甲酰基吡唑啉酮-5(HPMBP)的固相配位化学反应

在低加热条件下(<100℃), 研究了Cu(OAc)2.H2O与1-苯基-3-甲基-4-苯甲酰基吡唑啉酮-5-(HPMBP)两种异构体(烯醇式与酮式)的固相配位化学反应, 结果表明两种异构体与Cu(OAc)2.H2O固相化学反应活性并不相同。通过IR, UV测定, 发现酮式异构体在与Cu(OAc)2.H2O的固相反应过程中, 其自身经过了一个由酮式到烯醇式的固相异构化。     

Ultrafast excited-state dynamics in photochromic N-salicylideneaniline studied by femtosecond time-resolved REMPI spectroscopy

Ultrafast processes in photoexcited N-salicylideneaniline have been investigated with femtosecond time-resolved resonance-enhanced multiphoton ionization spectroscopy. The ion signals via the S(1)(n,pi( *)) state of the enol form as well as the proton-transferred cis-keto form emerge within a few hundred femtoseconds after photoexcitation to the first S(1)(pi,pi( *)) state of the enol form. This reveals that two ultrafast processes, excited-state intramolecular proton transfer (ESIPT) reaction and an internal conversion (IC) to the S(1)(n,pi( *)) state, occur on a time scale less than a few hundred femtoseconds from the S(1)(pi,pi( *)) state of the enol form. The rise time of the transient corresponding to the production of the proton-transferred cis-keto form is within 750 fs when near the red edge of the absorption is excited, indicating that the ESIPT reaction occurs within 750 fs. The decay time of the S(1)(pi,pi( *)) state of the cis-keto form is 8.9 ps by exciting the enol form at 370 nm, but it dramatically decreases to be 1.5-1.6 ps for the excitation at 365-320 nm. The decrease in the decay time has been attributed to the opening of an efficient nonradiative channel; an IC from S(1)(pi,pi( *)) to S(1)(n,pi( *)) of the cis-keto form promotes the production of the trans-keto form as the final photochromic products. The two IC processes may provide opposite effect on the quantum yield of photochromic products: IC in the enol form may substantially reduce the quantum yield, but IC in the cis-keto form increase it.     

Thermal history and melting behavior of poly(ethylene terephthalate)

Low molecular weight poly(ethylene terephthalate) samples were crystallized isothermally at 120–245°C from both the amorphous state and the melt. Isothermal annealing of these polymers at 215°C provided polymers which exhibited multiple melting peaks in thermal analysis, referred to as form I and form II, as assigned by Bell and Dumbleton. In these samples the peak temperature of the form II melting endotherm and the average crystallite size are dependent on the temperature of initial crystallization. This result requires a mechanism for retaining some structural feature during the conversion from morphological form I to form II. DSC thermograms obtained at varying heating rates on samples showing only form II endotherms support the assignment of superheating as the cause of the shift to higher peak temperatures with increasing heating rate.     

Thermochromic and solvatochromic zinc biladienones: dynamic equilibria of a metal complex having a flexible framework sensitive to environment

Dehydration of zinc biladienone was catalyzed by zinc acetate, while the reverse reaction was catalyzed by triethylamine. The transformation can be performed thermally without catalyst, and the ratio of the hydrated form to the dehydrated form depended on the solvents: the dehydrated form is favored in CHCl₃ and CH₂Cl₂, while the hydrated form is favored in hexane, pyridine, and DMF. Kinetic studies on the thermal transformation of zinc biladienone from its hydrated blue form to the dehydrated yellow-brown form were performed in toluene and THF. The rate law was half order with respect to the zinc biladienone concentration in toluene, while first order in THF, leading to a slow transformation in concentrated solution in toluene.