Crystallization and morphology of polymerized cyclic butylene terephthalate

The crystallization behavior and morphology of polymerized cyclic butylene terephthalate (pCBT) were investigated by thermal differential scanning calorimetry (DSC) and polarized light microscopy (PLM). The spherulite growth rate was analyzed based on the Hoffman and Lauritzen theory to better understand the crystallization behavior. We found four typical morphologic features of pCBT corresponding to the crystallization temperature spectrum: usual negative spherulite, unusual spherulite, mixed birefringence spherulite coexisting with boundary crystals, and highly disordered spherulitic crystallites. The Avrami crystallization kinetics confirmed the occurrence of combined heterogeneous nucleation accompanied by a change in the spherulitic shape of pCBT, which also agreed with the PLM results. The equilibrium melting temperature and glass transition temperature of pCBT were 257.8 °C and 41.1 °C, respectively. A regime II–III transition occurred at 200.9 °C, which was 10 °C lower than that reported for poly(butylene terephthalate) (PBT). Coinciding with and attributed to the regime transition, the boundary crystal disappeared at temperatures above 200 °C and the morphology changed from the mixed type to highly disordered spherulitic crystallites. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1127–1134, 2010     

Polymorphism in polymers. I. The equilibrium melting temperature of two crystalline modifications of trans-1,4-polyisoprene

Dilatometric, calorimetric, and dissolution studies have been made of two crystalline modifications of trans-1,4-polyisoprene in order to determine their equilibrium melting temperatures. This parameter is of fundamental importance in the formal treatment of polymorphism in crystalline polymers. A consistent set of thermodynamic parameters has been derived for both crystalline modifications. The equilibrium melting temperature of the polymorph, which was previously observed to melt from carefully crystallized bulk material at 64°C, was calculated to be at least 82.4°C. The other form, which melts from the bulk at 74°C, has an equilibrium melting temperature of 79.5 ± 0.5°C. The trans-1,4-polyisoprene, crystallized by stirring n-butyl acetate solutions at 49°C, was found by x-ray diffraction to be in the first form and melts at 81.2 ± 0.5°C when very slow heating rates are applied. This melting temperature is very close to the independently derived equilibrium melting temperature and lends support to the possibility that extended chain crystals are present in these solution crystallized crystals. Using the newly found melting temperatures of the two crystalline modifications it can be derived from the free energies of fusion that the first crystalline form is more stable at temperatures above approximately 66°C, whereas the other form is more stable below this temperature.     

Plastic deformation of polypropylene. IV. Wide-angle x-ray scattering in the neck region

Wide-angle x-ray scattering (WAXS) patterns of two polypropylene samples, a quenched sample drawn at 21°C and an annealed sample drawn at 100°C, were investigated in a range of values of draw ratio λ very closely spaced through the neck region. In both cases, a range of small λ where deformation occurred by spherulite deformation was followed by one of higher λ where microfibrils were formed. The contribution to the WAXS pattern of microfibrils could be clearly distinguished from that of deformed spherulites because of the better orientation parallel to the draw direction of the former as compared to the latter. Additionally, for a drawing temperature of 21°C, microfibrils crystallize in the “smectic” phase as compared to the monoclinic phase for the initial sample and deformed spherulites. At this temperature, plastic deformation proceeds through the spherulite deformation mechanism up to λ = 1.4 accompanied by an increase in chain orientation with increasing λ. For λ > 1.4 plastic deformation appears to occur exclusively through microfibril formation. For drawing at 100°C, spherulite deformation is accompanied by very little change in chain orientation up to λ = 2, where microfibril formation begins. For λ > 2 (T_d = 100°C) plastic deformation is accompanied by both microfibril formation and some spherulite deformation as reflected by changes in both orientation and crystallite size. At this temperature the lateral crystallite size in the microfibrils is related to the long period according to the “equilibrium crystallite shape” previously found for annealed polypropylene.     

Study of crystallization kinetics of trans-1,4-polyisoprene

The concentrations and the growth rates of high- and low-melting type spherulites of trans-1,4-polyisoprene were measured in the temperature range 39–49°C. It was shown that above about 40°C., the crystallization rate of trans-1,4-polyisoprene is determined primarily by the radial growth rate of high-melting form (HMF) spherulites, whereas the predominance of the low-melting form (LMF) crystals below 40°C. can be attributed to the high rate of formation of LMF primary nuclei at lower crystallization temperatures. Temperature-independent rate parameters were calculated from optical and dilatometric measurements and were found to be in good agreement. Both the change in nucleation habit and spherulite growth rate with temperature can be explained on the basis of a lower end surface free energy of LMF crystals of trans-1,4-polyisoprene compared to that of the HMF crystals.     

Phase transformations in the systems Ti<Subscript>2</Subscript>Fe–H<Subscript>2</Subscript> and Ti<Subscript>2</Subscript>Fe–NH<Subscript>3</Subscript>

The interactions of two-phase alloy Ti₂Fe with hydrogen and ammonia at 100–500°C were studied, the compositions of the products were found, and the conditions for producing hydride and nitride phases were determined. The potential of using the two-phase alloy in a metal hydride hydrogen accumulator operating at 20–600°C was considered.     

Polymeric nature of selenium crystallization. II. Crystallization kinetics and secondary crystallization

The crystallization of elemental selenium has been studied in light of present concepts of crystallization in organic polymers. Bulk-crystallization kinetic data as measured by a dynamic density technique and spherulite growth-rate data as measured by optical microscopy are presented for the temperature range 70°C to 160°C. Plots of extent of isothermal crystallization versus time were sigmoidal in shape. Spherulite growth rates were constant for a given temperature and reached a maximum at approximately 130°C. Evidence is presented for secondary crystallization in selenium, and a model is proposed for destruction of chain folds with interlamellar crystallization during the spherulitic-to-“metallic” transformation above 100°C.     

Molecular and crystalline structure of pyrocatechol and hydroquinone dimethacrylates and their reactivity in melts

X-ray diffraction analysis of pyrocatechol and hydroquinone dimethacrylates (T _m = 18 and 86–88°C, respectively) shows that the oligomer molecules within crystals are packed in stacks where the methacrylate fragments of neighboring molecules are parallel to each other. The minimum distances between the centers of double bonds of adjacent methacrylate fragments in crystals of pyrocatechol and hydroquinone dimethacrylates are 4.621(3) and 4.269(4) Å. The curves showing the reduced rate of photopolymerization of oligomer melts versus conversion (9,10-phenanthrenequinone used as the initiator) display a maximum at conversions of 1.5–3.0%. The limiting conversion in photopolymerization of molten pyrocatechol dimethacrylate at 25 and 40°C is 20%; for hydroquinone dimethacrylate at 95°C, it is approximately 10%. As the temperature rises from 25 to 40°C, the maximum reduced rate of photopolymerization of pyrocatechol dimethacrylate increases by a factor of 1.4.     

New understanding on the memory effect of crystallized iPP

In this study, recovery processes of isotactic polypropylene(iPP) melted spherulites at 135 °C after melting at higher temperatures(170 °C–176 °C) were investigated with polarized optical microscopy and Fourier transform infrared spectroscopy. The recovery temperature was fixed to exclude the interference from heterogeneous nuclei. After melting at temperatures between 170 °C and 174 °C, the melted spherulite could recover back to the origin spherulite at low temperatures. Interestingly, a distinct infrared spectrum from iPP melt and crystal was observed in the early stage of recovery process after melting at low temperatures, where only IR bands resulting from short helices with 12 monomers or less can be seen, which indicates that the presence of crystal residues is not the necessary condition for the polymer memory effect. Avrami analysis further indicated that crystallization mainly took place in melted lamellae. After melting at higher temperatures, melted spherulite cannot recover. Based on above findings, it is proposed that the memory effect can be mainly ascribed to melted lamellae, during which crystalline order is lost but conformational order still exists. These conformational ordered segments formed aggregates, which can play as nucleation precursors at low temperatures.     

Thermal stability of shear-induced precursor structures in isotactic polypropylene by rheo-X-ray techniques with couette flow geometry

Combined in situ rheo-SAXS (small-angle X-ray scattering) and -WAXD (wide-angle X-ray diffraction) studies using couette flow geometry were carried out to probe thermal stabilty of shear-induced oriented precursor structure in isotactic polypropylene (iPP) at around its normal melting point (162 °C). Although SAXS results corroborated the emerging consensus about the formation of “long-living” metastable mesomorphic precursor structures in sheared iPP melts, these are the first quantitative measures of the limiting temperature at which no oriented structures survive. At the applied shear, rate = 60 s⁻¹ and duration t_s = 5 s, the oriented iPP structures survived a temperature of 185 °C for 1 h after shear, while no stable structures were detected at and above 195 °C. Following Keller's concepts of chain orientation in flow, it is proposed that the chains with highly oriented high molecular weight fraction are primarily responsible for their stability at high temperatures. Furthermore, the effects of flow condition, specifically the shear temperature, on the distributions of oriented and unoriented crystals were determined from rheo-WAXD results. As expected, at a constant flow intensity (i.e., rate = 30 s⁻¹ and duration, t_s = 5 s), the oriented crystal fraction decreased with the increase in temperature above 155 °C, below which the oriented fraction decreased with the decrease in temperature. As a result, a crystallinty “phase” diagram, i.e., temperature versus crystal fraction ratio, exhibited a peculiar “hourglass” shape, similar to that found in many two-phase polymer–polymer blends. This can be explained by the competition between the oriented and unoriented crystals in the available crystallizable species. Below the shear temperature (155 °C), the unoriented crystals crystallized so rapidly that they overwhelmed the crystallization of the oriented crystals, thus depleting a major portion of the crystallizable species and increasing their contribution in the final total crystalline phase. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3553–3570, 2006     

Diagramme de phases du système Ag2Te-In2Te3

The phase diagram of the Ag₂Te-In₂Te₃ quasibinary system was established by differential thermal analysis and X-ray diffraction, in particular by the Guinier — Lenné method. This study confirms the existence of three intermediate ternary phases: AgInTe₂, which crystallizes in a chalcopyrite-type structure and possesses a homogeneity range, undergoes a peritectic decomposition at 650 °C; AgIn₅Te₈, which possesses a large homogeneity range, shows a phase transition at 699 °C and congruently melts at 725 °C; Ag₃In₉₇Te₁₄₇, which crystallizes in a cubic structure, incongruently melts at 672 °C.     

Infrared crystal spectra of heterocyclic compounds. 1 . IR spectra of solid modifications of thiophene

The infrared spectra of the four solid modifications of thiophene have been studied. Absence of appreciable band shifts and solid state splitting, in going from the liquid to the solid phases, seems to indicate that the crystals of thiophene are disordered at least until the 4th solid modification becomes stable. Polarization measurements, carried out on the 4th phase, allow a tentative determination of the crystal structure at 100°K.     

Crystallization of polydioxolan. III. Microscopy and dilatometric analysis of the crystallization kinetics

Crystallization kinetics has been studied for a polydioxolan (PDOL) sample, over a wide temperature range, by dilatometry and microscopy. The dilatometry results can be analyzed using the Avrami equation. At temperatures higher than 22°C, the crystallization data must be analyzed in two steps: the first part of the curve corresponds to PDOL with a very disordered morphology (Phase I) while the second part of the crystallization curve is related to a spherulitic morphology (Phase II). The passage from the low to the high crystallization temperature region is associated with a change in the Avrami exponent from 3 to 4. The crystal surface free-energy product σσ_e was found to be 18 × 10² erg²/cm⁴, very close to that of polyoxymethylene. The crystallization kinetics was studied by microscopy over the temperature range?18 to 35°C. Growth and nucleation rates were recorded. Two phases are found only at temperatures higher than 22°C. The appearance of Phase II is related to a decrease in the growth rate of the sample. From the growth rates, the crystal surface free-energy product σσ_e was found equal to 17 × 10² erg²/cm⁴. The detailed analysis of the crystallization of the two phases reveals a complicated process which can be divided into four different steps: (a) growth of a disordered phase, Phase I; (b) nucleation of a higher birefringence structure; (c) propagation of a high birefringence phase; and (d) spherulitic growth, Phase II. The analysis of PDOL crystallization strongly suggests the presence of a hedrite → oval → spherulite transition: the hedrite formation corresponds to step (a), the oval formation to steps (b) and (c), and the spherulite formation to step (d).     

Multiphase structure of segmented polyurethanes: Its relation with spherulite structure

Small-angle light-scattering (SALS), Polarized light microscopy (PLM), differntial scanning calorimetry (DSC), and small-angle x-ray scattering (SAXS) were used to study morphological changes in segmented polyurethanes with 4,4′-diphenylmethane diisocyanate (MDI) and 1,4-butanediol (BD) as the hard segment. It was found. for the first time, that spherulites could form from the melt by quenching the polyurethanes in the melt state to annealing temperatures between 120°C and T_h, the highest annealing temperature for spherulite formation. T_h ranged from 140°C to ca. 170°C and depended upon the hard-and soft-segment compatibility. Within the range 120°C to T_h, the radius of the spherulite increased with increasing hard-segment content at each fixed annealing temperature. Annealing at 135–140°C gave rise to the largest spherulites. SAXS was used to investigate the phase-separated structures corresponding to the spherulite formation. The interdomain spacing increased with increasing hard-segment content and with increasing annealing temperature.The degree of phase separation first increased with increasing annealing temperature from room temperatures (ca. 25°C), reached a maximum at ca. 107°C, and then decreased with further increase in the annealing temperature. On the basis of these observations, the mechanisms of phase separation, crystallization, and spherulite formation are discussed. © 1993 John Wiley & Sons, Inc.     

Untersuchungen zum System SmOCl/SeO2

Investigations on the System SmOCl/SeO₂ The existence of three ternary phases SmSeO₃Cl, SmSe₂₋O₅Cl, and SmSe₃O₇Cl is shown on the quasibinary line SmOClSeO₂. The phase barogram and the phase diagram are determined by total pressure measurements and DTA, respectively. SmSe₃O₇Cl melts peritectical at 340 ± 10 °C, SmSe₂O₅Cl melts peritectical at 560 ± 10 °C while SmSeO₃Cl shows no thermal effects up to 900 °C The data of enthalpies and entropies of the phases are evaluated from the total pressure measurements (Data see „Inhaltsübersicht”︁)     

Theoretical approach on ceria-based two-phase electrolytes for low temperature (300–600 °C) solid oxide fuel cells

Recently, extensive studies on the ceria-based two-phase composites as functional electrolytes have created excellent 300–600 °C fuel cell technology. There is an emergence need to deepen the knowledge and to develop theoretical methodologies in this field. The feasibility to design and develop two-phase materials as superionic conductors for 300–600 °C solid oxide fuel cells (LTSOFCs) is reported. The superionic conductivity at 300–600 °C in two-phase materials where the interfaces between the constituent phases are constructed as “superionic highways” resulting in interfacial high ionic conduction. The material architecture and design presented in this report thus reaches beyond the conventional molecular way to synthesize new compounds.     

Crystallization behavior of a propylene-1-butene random copolymer in its α and γ modifications

A random propylene-based copolymer containing 1.0 mol% 1-butene as co-unit, synthesized with Ziegler-Natta catalyst and then fractionated to make the sample having a uniform in molecular microstructure, was investigated by differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXD), and atomic force microscopy (AFM). In the DSC curves, one can see clearly the endothermic peaks corresponding to the melting of α-iPP crystals and a group of broad endothermic peaks associated to the melting of the γ-iPP crystals. Wide-angle X-ray diffraction results indicate that both the α and γ modifications can be formed in the copolymer in a wide temperature range. The γ fraction increases first with increasing the crystallization temperature at the expense of its α component, which has been explained according to crystalline structures of iPP in its α and γ forms, and then decreases with increasing crystallization temperature as the crystallization of iPP in its γ phase has been suppressed at high temperatures. The γ-iPP content in the copolymer reaches maximum at the temperature of 130 °C. The in situ X-ray diffraction characterization on the isothermal crystallization process at 130 °C indicates that, as long as the γ-iPP can be detected, it takes always ca. 25% of the overall crystallinity. This leads to the conclusion that α- and γ-iPP crystals grow simultaneously during the crystallization process. The fact that the α and γ phases cannot be distinguished by morphological observation leads to the conclusion that they may intermix within one spherulite.     

Analysis of Polymorphism and Dual Crystalline Morphologies in Poly(hexamethylene terephthalate)

Summary: The polymorphisms in poly(hexamethylene terephthalate) (PHT), along with their associated melting and spherulite morphologies, were examined by differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction (WAXD), and polarized‐light microscopy (PLM). The morphology and crystal cells were dependent on the temperature of crystallization. When melt‐crystallized at low temperatures (90–135 °C), PHT showed at least five melting peaks and two re‐crystallization peaks upon DSC scanning, and the samples displayed various fractions of both α and β crystals. However, only a single melting peak was obtained in PHT melt‐crystallized at 140 °C or above, which displayed a single type of β crystal. In addition, two different forms of spherulites were identified in melt‐crystallized PHT, with one being a typical Maltese‐cross spherulite containing the α crystal, and the other being a dendrite‐type packed mainly with the β crystal. This study provides timely evidence for a critical interpretation of the relationship between multiple melting and polymorphisms (unit cells and spherulites) in polymers, including semi‐crystalline polyesters.

WAXD diffractograms for PHT melt‐crystallized at 140 °C, revealing a single type of β‐crystal cell.     

Existiert eine Wurtzit‐Modifikation von Lithiumbromid? — Untersuchungen im System LiBr/LiI —

Is there a Wurtzite‐Modification of Lithium Bromide? — Studies on the System LiBr/LiI — Deposition of mixtures of LiBr/LiI (ratio: LiBr/LiI = 3:1, 2:1, 1:1, 1:2, 1:3, 1:4) and of pure LiI and LiBr from the gas phase onto a sapphire substrate at ‐196 °C in a high vacuum chamber were investigated by means of temperature‐dependent X‐ray diffraction. Below 0 °C LiI crystallizes in the hexagonal Wurtzite‐modification (β‐LiI) with a = 451.4(1) und c = 731.1(2) pm, which transforms into the cubic rock salt modification (α‐LiI, a = 602.57(3) pm) by heating up to room temperature. Co‐depositions of LiBr/LiI formed solid LiBr_1‐xI_x solutions that also crystallize in the Wurtzite‐modification, below room temperature. Compared to β‐LiI, these solid solutions are more stable and transform into the cubic phase at the significantly higher temperature of 80 °C. The lattice constants of LiBr_1‐xI_x with x ≈ 0.7 are a = 445.48(7), c = 719.1(1) pm and with x ≈ 0.4 are a = 431.50(5), c = 691.7(1) pm. The hexagonal phase LiBr_1‐xI_x is observed for the complete series of mixed crystals with 0.25 ≤ x ≤ 0.8. Both cubic phases, α‐LiI and LiBr, show solubilities of up to ca. 10 % of the respective other compound. In case of pure LiBr only the cubic modification (a = 551.54(2) pm, 25 °C) was observed in the complete temperature range (‐196 °C to 25 °C).     

Structure,magnetic, and electrical properties of bismuth niobates doped with d-elements: VIII. Phase transitions and electrophysical properties of bismuth niobate

Reversible phase transitions of bismuth niobate Bi₃NbO₇ have been observed: from the lowtemperature cubic modification to the tetragonal one at 860°C and from the tetragonal modification to the hightemperature cubic one at 950–980°C. With increasing temperature of the cubic modification preparation, the unit cell parameter has been decreasing. Electrophysical properties of the cubic phases have matched at the frequencies below 1 kHz (up to 650 K). The specific conductivity of the tetragonal phase exceeds that of the cubic phase by no more than an order of magnitude (below 1000 K).     

Longitudinal growth of polymer crystals from flowing solutions. VI. Melting behavior of continuous fibrillar polyethylene crystals

The melting behavior of continuous fibrillar crystals of high-molecular-weight polyethylene has been investigated. The macrofibers were grown from dilute solutions in xylene subjected to Couette flow in the temperature range between 103 and 118.5°C. The thermograms, as determined by differential scanning calorimetry, exhibit three melting endotherms with peak temperatures at 141, 150.5, and 159.5°C after extrapolation to zero scan speed. All peaks were found to be strongly superheatable. Reduction of fiber length, in particular by etching with fuming nitric acid, led to the disappearance of the melting peaks at 150.5 and 159.5°C. The remaining peak at 136°C appeared not to be superheatable. The heat of fusion of the fragmented fibers was 69.8 cal/g. Wide-angle x-ray diffractograms taken on a macrofiber while gradually heated at a rate of 0.35°C/min at constant length showed that the triclinic phase present in the fiber disappeared at 130°C and that the orthorhombic cell transformed into the hexagonal modification at 150°C. This hexagonal phase was still observable at 180°C. The retractive force developed on heating at constant length displays first a slight decrease followed by a maximum at 150°C. Beyond the latter temperature the stress decays abruptly corresponding to the temperature at which fracture of the fiber could be observed visually. From all these observations it is inferred that the first melting endotherm in the differential scanning calorimeter (DSC) thermograms arises from the melting of unconstrained fibrillar crystal regions which are able to shrink during fusion. Moreover, the melting of lamellar overgrowths on the elementary fibrils on shish-kebab type may contribute to this endotherm. The second melting endotherm at about 150°C is associated with the transformation of the orthorhombic into the hexagonal lattice in constrained parts of the sample. This latter “rotator” phase allows slippage of the polymer chains past each other, giving rise to stress relaxation. The third endotherm arises from melting of this hexagonal phase and the heat take-up connected with the formation of higher energy gauche states upon randomization of the chains in the melt. Almost smooth, fully constrained fibrillar crystals grown at high temperature absorb more than 15.5 cal/g during this process, indicating that the polymer chains in such fibers must be highly extended.