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.     

Thermotropic phase behaviour of sodium oleate as studied by FT-Raman spectroscopy and X-ray diffraction

The thermotropic behaviour of sodium oleate (NaOl) has been studied in the temperature range 10–125°C by using Fourier transform-Raman spectroscopy, X-ray diffraction and differential scanning calorimetry (DSC). The temperature dependence of conformationally sensitive bands in the CH₂ stretching (2800–2900 cm⁻¹), C–C stretching (1050–1150 cm⁻¹) and CH₃ rocking region (830–900 cm⁻¹) has been used to characterize the order/disorder behaviour of alkyl chains. It is found that in phase I, NaOl exhibits the crystalline ordered lamellar structure with a repeat period of 4.51 nm. The first broad peak in the DSC trace is due to superposition of two transitions (phase I to phase II and phase II to phase III), therefore, it is not possible to determine the lamellar structure of phase II. This broad transition from phase I to phase III is associated with the melting of methyl-sided chains and increase in gauche conformers in carboxylate-sided chains. Finally, NaOl undergoes a transition from crystalline to a liquid crystalline phase IV, which is associated with the melting of the carboxylate-sided chain.     

Structure and phase transitions in poly[bis-(2,2,3,3-tetrafluoropropoxy) phosphazene]

The structure and phase transitions in poly[bis-(2,2,3,3-tetrafluoropropoxy)phosphazene] have been studied by differential scanning calorimetry (DSC) and x-ray diffraction. Two crystalline phases and one mesomorphic phase are found, denoted I, II, and III, respectively. These phases convert reversibly one into the other on heating and cooling. The Phase I–Phase II transition occurs in a temperature range from 5 to 30°C whereas the Phase II mesophase (Phase III) transition proceeds above 80°C. Heats of transitions are measured to be about 29.0 J/g and 3.6 J/g, respectively. Crystalline Phase I is characterized by a monoclinic unit cell with the parameters: α = 24.4 Å, b = 9.96 Å, c = 4.96 Å, γ = 123°. The axes of both chains, traversing the unit cell, are directed along the “c” axis, the main chains having cis-trans conformation. Phase I is the common crystalline structure for the main chain and side chains. The structure of Phase II is controlled mainly by packing of the side chains. Transition of Phase II into mesomorphic Phase III is accompanied with distortion of packing of the side chains. Only regular packing of the main chains of macromolecules in the plane perpendicular to their axes exists in Phase III. Mesomorphic phase III is stable up to the degradation temperature of the polymer. A significant effect of stress on the Phase II–III transition in oriented samples was found.     

Ferroelectric properties of smectic C liquid crystals with induced helical structure

Ferroelectricity has been observed in liquid crystalline mixtures of chiral smectic C type with a helical structure induced by chiral guest molecules (pitch p ≈ 100 μm). Samples of thickness d?p form monodomains with finite zero-field electric polarization because the spontaneous polarization of the smectic layers is not compensated by the helical twist. The temperature dependence of the spontaneous polarization has been studied near the phase transition smectic C→ smectic A.     

Higher order structural analysis of stereocomplex‐type poly(lactic acid) melt‐spun fibers

The higher order structure of stereocomplex‐type poly(lactic acid) melt‐spun fibers of an equimolar blend of poly(L ‐lactic acid) and poly(D ‐lactic acid) was analyzed with wide‐angle X‐ray diffraction (WAXD) and birefringence measurements. Two different crystalline structures were observed in the fibers: α‐form homocrystals and stereocomplex crystals. The weight fractions of the two crystals were estimated with the WAXD integrated intensity data. The crystalline orientation factors were obtained from the WAXD measurements. Well‐oriented homocrystals formed during a drawing process at the crystallization temperature of the homocrystal. Drawing above this temperature caused the stereocomplex crystal to be formed. The crystalline orientation tended to be lower with increasing drawing temperatures. Through the combination of the intrinsic birefringence and the fractions of the α‐form homocrystals and stereocomplex crystals, the birefringence of the amorphous phase was evaluated. The amorphous birefringence stayed positive and decreased with increasing drawing temperature. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 218–228, 2007     

Mechanism of polymorph selection during crystallization of random butene-1/ethylene copolymer

Starting from an initial sample of butene-1/ethylene copolymer with stable form I’, we examined the nucleation of different crystalline polymorphs (here metastable form II and stable form I’) at different isothermal crystallization temperatures after being melted at different melt temperature (T _melt). When T _melt was just above the melting temperature (T _m) of the crystallites, self-seeding took place. There, residue crystallites served as nuclei leading to the crystallization of the same crystalline phase. When T _melt was a few degrees above the T _m, self-seeding was disabled due to complete melting of the initial crystals. Upon crystallization, the selection of the different polymorphs in this random copolymer was found to depend on an interplay between the domain size of segregated long crystallizable sequences and the size and energy barrier of the critical nucleus of the respective crystalline forms. Our results provide a clear understanding of the polymorphs selection during crystallization of a random copolymer as well as homo-polymers under confinement.     

Plastic and glassy crystal states of caffeine

The present paper focuses on the high temperature form I of caffeine and on its low temperature metastable form. Structural, dynamic, and kinetic information has been obtained by X-ray, dielectric, and calorimetric investigations. This study shows the following features: (1) The high temperature phase (I) of caffeine is in a state of dynamically orientationally disordered crystalline state (so-called "plastic, or rotator, phase"). (2) This high-symmetry hexagonal phase can be maintained at low temperature in a metastable situation. (3) Under deep undercooling of form I a glass transition occurs in the disordered crystalline state near room temperature. It is associated with the orientational freezing in of the molecular motions. Otherwise stated, the metastable state I enters into a nonergodic unstable state, so-called "glassy crystal" state. These findings rationalize the difficulties seen with caffeine in pharmaceutical science.     

Dynamical observation of structural transition of polymers using an X‐ray diffraction system with imaging plates. II. Crystalline transition of poly(butylene terephthalate)

Poly(butylene terephthalate) has two crystalline forms of α and β. The α form is more stable and is transformed into the β form by stretching. The α↔β transition stress was measured at various temperatures with the use of an X‐ray diffraction system with imaging plates, and the relationship between the transition stress and temperature was obtained. Further, using the X‐ray diffraction system, the drawing behavior of the nearly amorphous, undrawn polymer film was observed by taking up dynamically a series of X‐ray diffraction patterns while a film was being drawn. The α form never occurred during drawing at any temperature because the stress to draw the film exceeded the α→β transition stress. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 838–845, 2000     

拉伸对β晶型聚丙烯结构的影响

用DSC法研究了不同拉伸条件对β晶型聚丙烯(β-PP)结构的影响,结合X-射线衍射法探讨了拉伸比,拉伸温度和拉伸速率等对β→α转变及其结晶结构的变化所起的作用。 结果表明提高拉伸温度和拉仲比有利于β→α转变及结晶度的提高。并讨论了拉伸过程中β→α转变机理及影响结晶结构的因素。     

不同后处理条件下聚萘二甲酸乙二酯纤维的结构变化

对不同纺速下制得的聚萘二甲酸乙二酯 (PEN)初生纤维进行了冷拉伸、定长热处理和热拉伸等后处理 .通过WAXS、DSC等测试研究了纤维中结构变化与后处理条件之间的关系 .结果表明 ,较低纺速下所制得的无定形初生纤维在低于Tg 温度下的冷拉伸时发生了应力诱导结晶 ,纤维中生成了α晶体 .同样的初生纤维在定长热处理过程中直至 2 0 0℃仍保持无定形结构 .这些结果说明施加应力相对于升高温度对于α晶体的生成更为重要 .而热拉伸样品中结晶结构的形成与初生纤维的结构有很大关系 ,低纺速下无定形初生纤维在热拉伸后形成α晶体 ,而高纺速下主要含有 β晶体的初生纤维经热拉伸后 β晶体会部分转变为α晶体 ,且 β晶转变为α晶的难易程度取决于初生纤维中 β晶的完善程度 ,初生纤维中 β晶越完善 ,热拉伸时 β晶体越不易转变为α晶体     

Crystalline transition in Nylon 10 10

Variable‐temperature X‐ray diffraction was used to monitor the crystalline transition of Nylon 10 10. It could be found that the α‐phase of the sample transforms into a γ‐phase at about 135°C, if the sample is heated from room temperature to a high temperature, which is the so‐called Brill transition of Nylon 10 10. In addition, Nylon 10 10 was found to crystallize directly in a kind of α‐phase from the melt at high temperature, which is much different from the behavior of Nylon 66 and Nylon 10 12. Upon further cooling to room temperature, Nylon 10 10 preserved the α‐phase revealing two peaks in its XRD patterns. However, if the Nylon 10 10 sample with γ‐form was not melted, but immediately cooled from a temperature between T_B and T_m, the reverse transition from γ‐form to α‐form could be observed at about 130°C, indicating reversible Brill transition of Nylon 10 10.     

A comparative molecular dynamics study of crystalline,paracrystalline and amorphous states of cellulose

The quintessential form of cellulose in wood consists of microfibrils that have high aspect ratio crystalline domains embedded within an amorphous cellulose domain. In this study, we apply united-atom molecular dynamics simulations to quantify changes in different morphologies of cellulose. We compare the structure of crystalline cellulose with paracrystalline and amorphous phases that are both obtained by high temperature equilibration followed by quenching at room temperature. Our study reveals that the paracrystalline phase may be an intermediate, kinetically arrested phase formed upon amorphisation of crystalline cellulose. The quenched structures yield isotropic amorphous polymer domains consistent with experimental results, thereby validating a new computational protocol for achieving amorphous cellulose structure. The non-crystalline cellulose compared to crystalline structure is characterized by a dramatic decrease in elastic modulus, thermal expansion coefficient, bond energies, and number of hydrogen bonds. Analysis of the lattice parameters shows that Iβ cellulose undergoes a phase transition into high-temperature phase in the range of 450–550 K. The mechanisms of the phase transition elucidated here present an atomistic view of the temperature dependent dynamic structure and mechanical properties of cellulose. The paracrystalline state of cellulose exhibits intermediate mechanical properties, between crystalline and amorphous phases, that can be assigned to the physical properties of the interphase regions between crystalline and amorphous cellulose in wood microfibrils. Our results suggest an atomistic structural view of amorphous cellulose which is consistent with experimental data available up to date and provide a basis for future multi-scale models for wood microfibrils and all-cellulose nanocomposites.     

Nano size crystals of goethite, α-FeOOH: Synthesis and thermal transformation

An aqueous suspension of amorphous iron(III) hydroxide was kept at room temperature (298 K) for 23 years. During this period of time the pH of the liquid phase changed from 4.3 to 2.85, and nano size crystals of goethite, α-FeOOH crystallised from the amorphous iron(III) hydroxide. Transmission electron microscopy (TEM) investigations, Mössbauer spectra, and powder X-ray diffraction using Co Kα radiation showed that the only iron containing crystalline phase present in the recovered product was α-FeOOH. The size of these nano particles range from 10 to 100 nm measured by TEM. The thermal decomposition of α-FeOOH was investigated by time-resolved in situ synchrotron radiation powder X-ray diffraction and the data showed that the sample of α-FeOOH transformed to α-Fe₂O₃ in the temperature range 444-584 K. A quantitative phase analysis shows the increase in scattered X-ray intensity from α-Fe₂O₃ to follow the decrease of intensity from α-FeOOH in agreement with the topotactic phase transition.     

Poly(p-benzamide). II. Thermal behavior and crystal transformations

The thermal behavior and crystal transformation of poly(p-benzamide) are reported. Modification I contains both solvent (N,N-dimethylacetamide) and LiCl in its crystal lattice. Modification II is likely to form a complex crystal with LiCl, whereas crystalline modification III contains neither solvent nor LiCl in its unit cell. The crystal transformation temperature from modification I to II is 214°C, and the crystal–nematic transition temperatures of modification II and III are 475 and 544°C, respectively. Modification III can be obtained from II by heating above 475°C and cooling, or by washing with water. It can also be obtained from modification I by washing with water and annealing.     

尼龙1010的拉伸结晶行为及Brill转变

摘要 :本文通过示差扫描热分析,广角X射线衍射分析及二维广角X射线散射等手段研究和讨论了尼龙1010在不同的拉伸比率(λ)及拉伸温度（Td）下拉伸诱导的晶体转变及结晶取向行为。实验表明,在拉伸条件下,尼龙1010易于从α晶型向δˊ晶型转变,而拉伸比率的提高,有利于促进这种Brill转变。而在相同的拉伸比率下,随着拉伸温度的提高,发现了尼龙1010从δˊ晶型向α晶型的独特转变,而这种过程刚好与无拉伸状态下的尼龙1010晶型和温度的相互关系截然相反。二维广角X射线散射实验研究结果表明尼龙1010的晶体取向度主要和拉伸比有关,并且（002）的晶面间距随着拉伸比率的提高而增大,表明了长轴尺寸的增大对拉伸取向的依赖关系。     

Dielectric study of the slow motional processes in the polymorphic States of anhydrous caffeine

Molecular mobility in crystalline anhydrous caffeine was studied by the dielectric technique of thermally stimulated depolarization currents (TSDC). Two relaxational processes were found, one appearing at approximately -10 degrees C that is ascribed to a reorientational glass transition, and a higher temperature one that probably arises from local molecular motions that are precursors of diffusion and sublimation. The experimental results suggest that both crystalline phases II and I of caffeine, that have distinct crystal structures, are solid rotator phases. Furthermore, this dynamic reorientational disorder shows a reorientational glass transition at the same temperature in phase II and in metastable phase I.     

Time-resolved synchrotron X-ray study of crystalline phase transition in poly(aryl ether ketone ketone) containing alternated terephthalic/isophthalic moieties

Unique crystallization and melting behavior in poly(aryl ether ketone ketone) containing alternated terephthalic and isophthalic moieties were studied by time-resolved synchrotron x-ray methods. Recently, this material has been shown to exhibit three polymorphs (forms I, II, and III). In this work, we further investigated their distinctive thermal properties and found that form I is the dominating and the most thermally stable phase while form II is favored by fast nucleation conditions and is the least stable phase. On the other hand, form III represents a minor intermediate phase that usually coexists with form I and can be transferred from form II and to form I. Structural and morphological changes in form I have been followed by simultaneous wide-angle x-ray diffraction (WAXD)/small-angle x-ray scattering (SAXS) measurements during cold- or melt-crystallization and subsequent melting. In all cases, a larger dimensional change was found in the crystallographic a-axis than the b-axis during heating and cooling. This may be due to the greater lateral stress variation with respect to temperature along the a direction of the primary lamellae which is induced by either the formation of secondary lamellae or the preferential chain-folding direction in poly(aryl ether ketone ketone)s. During the phase transitions of form II ← III in the cold-crystallized specimen and form III ← I in the melt-crystallized samples, lamellar variables (long period, lamellar thickness, and invariant) obtained from SAXS remain almost constant. This indicates that the density distribution in the long spacing is independent of the melting in form II or III. For melt-crystallization, the corresponding changes in unit-cell dimensions and lamellar morphology during the annealing-induced low endotherm are most consistent with the argument that these changes are due to the melting of thin lamellar population. © 1995 John Wiley & Sons, Inc.     

Thermal and structural characterization of two polymorphs of Atovaquone and of its chloro derivative

Atovaquone, 2-[4-(4-chlorophenyl)cyclohexyl]-3-hydroxy-1,4-naphthoquinone, is an antimicrobial medicament used to treat or prevent pneumocystis carinii pneumonia, toxoplasmosis and malaria. Two polymorphs of Atovaquone (crystal phases I and III) were isolated and their crystal and molecular structures were determined by single crystal X-ray analysis. In both crystal phases, strong hydrogen bond interactions link adjacent molecules in centrosymmetric dimers. The existence of the different polymorphs is determined by the different orientation of the dimers in the crystal packing. In addition, a crystalline phase of the 2-chloro substituted derivative, which is not stabilized by intermolecular H-bond interactions, was also studied, and compared with those of the pristine (hydroxylic) species. DSC measurements and thermodiffractometry analyses on polycrystalline batches witnessed the 100% purity of the isolated materials and disclosed the crystal-to-crystal interconversion of phase I to phase III upon heating at 210 °C.     

Self-assembly and morphology of gel networks in l,3:2,4-bis-O-(p-methylbenzylidene)-D-sorbitol/n-dibutylphthalate

We investigated the self-assembling structure of the 1,3:2,4-bis-O-(p-methylbenzylidene)-D-sorbitol (PDTS)/n-dibutylphthalate (DBP) system in the parameter space of temperature T and solute concentration Phi(PDTS). Optical microscopic studies revealed that the phase diagram can be divided into four regions. In region I at high T the system is a homogeneous solution. In region II at lower T and low Phi(PDTS), the system still has fluidity but has microgels having spherulitic texture of PDTS crystallites. Regions III and IV at even lower T but higher Phi(PDTS) are in a gel state. In region III, the PDTS forms volume-filling spherulites due to the solid-liquid phase transition of the saturated PDTS solutions. In region IV at the lowest T, both the liquid-liquid phase-separation process and the solid-liquid transition of the PDTS are involved in the self-assembling processes. In this region a bicontinuous phase-separated structure is first formed by liquid-liquid phase separation via spinodal decomposition (SD). The subsequent solid-liquid transition of the PDTS in the PDTS-rich region forms percolating crystalline fibrils rather than spherulites. The formation of the crystalline fibrils pins further growth of the bicontinuous structure via SD.     

Molecular motions and λ phase transition: Raman and far-ir studies of neat and isotopic mixed hexamethylbenzene crystal

Raman and far-IR studies of hexamethylbenzene (HMB) low temperature crystalline phases (neat and isotopic mixed) are presented. The Raman phonon spectrum changes drastically during the λ-phase transition. Site splittings on two intramolecular modes of HMB-h₁₈ (but on only one corresponding HMB-d₁₈ mode) are observed in the room temperature phase (phase II) spectrum. These splittings disappear in the lower temperature phase (phase III). The methyl torsional bands are identified and a significant shift is observed for them during the phase transition. Also, while the λ-phase transition takes place at 113°K (± 2°K) for HMB-h₁₈ crystal, the transition temperature is 133°K (± 2°K) for HMB-d₁₈. Our results suggest that (for phonon interactions) the symmetry in phase Ill is close to D_3d and reduces to C_i in phase II. Furthermore, the results support the mechanism of phase transition which involves a tilting of the methyl groups out of the benzene ring.