Glycine phases formed from frozen aqueous solutions: Revisited

Glycine phases formed when aqueous solutions were frozen and subsequently heated under different conditions were studied by Raman scattering, x-ray diffraction, and differential scanning calorimetry (DSC) techniques. Crystallization of ice I(h) was observed in all the cases. On cooling at the rates of 0.5 K∕min and 5 K∕min, glassy glycine was formed as an intermediate phase which lived about 1 min or less only, and then transformed into β-polymorph of glycine. Quench cooling of glycine solutions (15% w∕w) in liquid nitrogen resulted in the formation of a mixture of crystalline water ice I(h) and a glassy glycine, which could be preserved at cryogenic temperatures (80 K) for an indefinitely long time. This mixture remained also quite stable for some time after heating above the cryogenic temperature. Subsequent heating under various conditions resulted in the transformation of the glycine glass into an unknown crystalline phase (glycine "X-phase") at 209-216 K, which at 218-226 K transformed into β-polymorph of glycine. The "X-phase" was characterized by Raman spectroscopy; it could be obtained in noticeable amounts using a special preparation technique and tentatively characterized by x-ray powder diffraction (P2, a = 6.648 A?, b = 25.867 A?, c = 5.610 A?, β = 113.12[ordinal indicator, masculine]); the formation of "X-phase" from the glycine glassy phase and its transformation into β-polymorph were followed by DSC. Raman scattering technique with its power for unambiguous identification of the crystalline and glassy polymorphs without limitation on the crystallite size helped us to follow the phase transformations during quenching, heating, and annealing. The experimental findings are considered in relation to the problem of control of glycine polymorphism on crystallization.     

等规聚丙烯β晶型向α晶型的相转变行为研究进展

等规聚丙烯(iPP)是典型的多晶型半结晶性聚合物,其常见晶型有单斜(α),三方(β),三斜(γ)以及四方或双四方(e),其中稳定性最好的α晶型和处于亚稳态的β晶型工业和经济价值较大,因此二者之间的相转变行为得到了人们的广泛关注.本文综述了近年来β→α-iPP生长相转变的研究进展.在高临界温度(141°C)和低临界温度(100°C)区间内,β-iPP生长速率高于α-iPP,而温度高于141°C,或低于100°C,由于α-iPP在动力学上占优势,β-iPP会发生向α-iPP的生长转变.但由于α-iPP是热力学上最稳定的晶型,β-iPP熔融重结晶过程也会发生β→α-iPP相转变.此外,拉伸形变过程中也会发生β→α-iPP相转变,广泛用于制备聚丙烯气体交换膜、过滤膜或锂电池隔膜等.目前对变形过程中的β→α-iPP相转变机理还存在争议,本文也对2种主要的机理进行了介绍,并对聚丙烯晶型转变行为的研究方向进行了展望.     

Formation of a metastable phase induced by a liquid crystalline phase in a novel chloropoly(aryl ether ketone)

The phase transition behavior of a thermotropic liquid crystalline poly(aryl ether ketone) synthesized by nucleophilic substitution reactions of 4,4′‐biphenol (BP), and chlorohydroquinone (CH) with 1,4‐bis(4‐fluorobenzoyl)benzene (BF) has been investigated by differential scanning calorimetry (DSC) and wide angle X‐ray diffraction (WAXD). The copolymer exhibits multiple first order phase transitions, which are associated with crystal‐to‐smectic liquid crystal transition and smectic liquid crystal‐to‐isotropic transition. When the cooling rate is low (< 10°C/min), only stable crystal form I is formed. With the cooling rate being high (>20°C/min), the metastable crystal form II is formed, which always coexists with form I. The liquid crystalline phase plays an important role in the formation of metastable phase form II.     

Direct imaging of smectic B phase of poly(p-xylylene)

Single crystals of poly(p-xylylene) were grown in dilute α-methylnaphthalene solution and studied by bright-field and high-resolution electron microscopy (HREM). The crystallization process was discussed in terms of the dependence of crystal form on crystallization conditions. A 0.15 nm resolution was achieved from high-resolution images of a frozen liquid crystalline phase. High-temperature electron diffraction patterns confirmed the existence of the liquid-crystalline phase in agreement with previous work of Lieser. The HREM images show the molecular packing in the smectic B phase. © 1992 John Wiley & Sons, Inc.     

FTIR spectroscopic characterization of structural changes in polyamide‐6 fibers during annealing and drawing

First, we report the development of Fourier transform infrared (FTIR) spectroscopic methods to determine the α/γ‐crystalline phase ratio of polyamide‐6 fibers and, in combination with density measurements, the total crystallinity. Using density determinations of the crystallinity of pure α and pure γ samples, we found the absorption coefficient ratio for the 930 (α) and 973 cm⁻¹ (γ) bands to be 4.4, from which we could obtain the α/γ ratio for any polyamide‐6 sample. The application of this FTIR method to the quantitative analysis of phase changes during thermal treatment and the drawing of polyamide‐6 was then made. We confirmed that crystallization during thermal treatments involved increases in both phases and did not involve crystal‐to‐crystal transformation, whereas drawing involved both crystallization of the amorphous phase in the α form and γ→α transformation. Finally, we revisited the band assignments for the amorphous phase of polyamide‐6 and found that the band at 1170 cm⁻¹ was not an amorphous band but, because its absorbance was independent of crystallinity, could be used as an internal reference band. The band at 1124 cm⁻¹ was reliably attributed to the amorphous phase. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 536–547, 2001     

Formation and phase transition of cobalt molybdate in cobalt oxide-molybdenum trioxide-alumina system

In the cobalt oxide-molybdenum trioxide-alumina system with a molar ratio of 1:1:1, the amounts of the high-temperature modification (a) of CoMoO₄ formed during heating from 500 to 800°C and the low-temperature modification (b) formed by phase transition during the subsequent cooling to room temperature are influenced by the kinds of alumina used, such as α-, γ- and calcined γ-aluminas. Powder X-ray diffraction analysis revealed that in an α-alumina system formation of a-CoMoO₄ is most favorable at a calcination temperature of 500°C and phase transition from a- to b-CoMoO₄ during cooling is enhanced by higher calcination temperatures. In the γ-alumina system, formation of a-CoMoO₄ is slight at 500°C but increases with increase in the calcination temperature, as does slightly the degree of phase transition from a- to b-CoMoO₄ upon cooling. In a system containing calcined γ-alumina, formation of a-CoMoO₄ similar to α- and γ-alumina systems was observed to occur at 500°C and 800°C, respectively, together with phase transition to b-CoMoO₄ during cooling. The degree of dispersion in the CoOMoO₃ coexistent system is affected by the particle size of aluminas, such as coarse α-, fine amorphous γ- and calcined γ-alumina consisting of both sizes, as observed with electron microscopy. Presence of finer γ-alumina is considered to suppress or retard the solid state reaction and phase transition.     

Transformation of synthetic γ-FeOOH (lepidocrocite) in aqueous solutions of ferrous sulphate

The course of the transformation of synthetic γ-FeOOH (lepidocrocite) samples, prepared by different methods was investigated and the effect of a preliminary heat treatment of the initial material on the product phase composition and morphology studied. Both the goethite-tohematite ratio and the particle size of the products depend on the preparation method and on the preliminary heat treatment of the initial lepidocrocite. The results are discussed in terms of the dissolution of the less stable lepidocrocite followed by the crystallization of the more stable phases α-FeOOH and α-Fe₂O₃ from the solution. The final product phase composition and the particle size are affected considerably by a presence of goethite and/or hematite germs in the initial γ-FeOOH.     

Order-order and order-disorder transitions in thin films of an amphiphilic liquid crystalline diblock copolymer

In this study, we quantitatively investigated the temperature-dependent phase transition behaviors of thin films of an interesting amphiphilic diblock copolymer, poly(ethylene oxide)-b-poly(11-[4-(4-butylphenylazo)phenoxy]undecyl methacrylate) (p(EO)-b-p(MAAZ)) and the resulting morphological structures by using synchrotron grazing incidence X-ray scattering (GIXS) and differential scanning calorimetry. The quantitative GIXS analysis showed that the diblock copolymer in the homogeneous, isotropic melt state undergoes phase-separation near 190 degrees C and then forms a body-centered cubic (BCC) structure of spherical p(EO) domains in the p(MAAZ) matrix, at which point the p(EO) domains and the p(MAAZ) matrix are both in amorphous, liquid states. The BCC structure of spherical p(EO) domains is converted to a hexagonal cylinder structure near 120 degrees C, which is induced by the transformation of the isotropic phase of the p(MAAZ) matrix to the smectic A phase, which is composed of a laterally ordered structure of p(MAAZ) blocks with fully extended side groups. The resulting hexagonal cylinder structure is very stable below 120 degrees C. This microscopic hexagonal cylinder structure is retained as the smectic A phase of the p(MAAZ) matrix undergoes further transitions to smectic C near 104 degrees C and to a smectic X phase near 76 degrees C, while the amorphous, liquid phase of the p(EO) cylinders undergoes crystallization near -15 degrees C. These complicated temperature-dependent disorder-order and order-order phase transitions in the films were found to take place reversibly during the heating run. A face-centered orthorhombic structure of p(EO) domains was also found during the heating run and is an intermediate structure in the hexagonal cylinder structure to BCC structure transformation. We use these structural analysis results to propose molecular structure models at various temperatures for thin films of the diblock polymer.     

Crystallization during polymerization of poly-p-xylylene

Crystallization during polymerization of p-xylylene from the gas phase has been studied between 200 and ?196°C. From room temperature to ?17°C the polymer crystal morphology changes in that the crystallinity decreases. In this range the process is thought to be of the successive polymerization and crystallization type. The morphology is in agreement with this mechanism, of the folded-chain β-polymorph type with proper epitactic orientation of the chains with respect to the support surface. At ?78°C an intermediate, poorly crystallized polymer results. At 196°C the reaction is most likely of the simultaneous polymerization and crystallization type. The morphology is, in agreement with the changed mechanism, of a metastable, irregularly folded β-polymorph type with no orientation of the chains relative to the support surface. No significant changes in molecular weight were observed in the polymers produced between 26 and ?196°C.     

Thermal Expansion of Liquid Crystalline Polyesters Having Phenanthrene Moieties. Simultaneous DSC-XRD method

Phase transition process of polyester from phenanthrene, poly(oxyheptamethyleneoxy-2,7-phenantrenedicarbonyl), a main chain type liquid crystalline polymer, was investigated by simultaneous DSC-XRD measurements using the synchrotron radiation facility (PF). Three exothermic DSC peaks were observed during cooling from the isotropic liquid state. These DSC peaks were assigned to the transition from the isotropic liquid to the smectic A, that from the smectic A to C, and that from the smectic C to the crystalline state, respectively, as determined by XRD profiles. The rate of transition from the smectic A to C was very slow compared with the liquid crystalline transition and the crystallization. From the DSC-XRD results, the thermal expansion along c-axis in the crystal and smectic phases are 4.1×10⁻⁴ and 1.4×10⁻³ nm K⁻¹ , respectively. This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of molecular weight and spinning conditions on the crystalline morphology of polypropylene fibres

In the spinning of polypropylene the “smectic” phase generally appears in the as spun fibres obtained from high fluidity polypropylene (for example M̄_w = 118000) at 200°C and more. The amount of the “smectic” form decreases as the draw down ratio increases. At 200°C and for a draw ratio of 40 the “smectic” form is about 70% of the crystalline phase. At 200°C and for a draw ratio of to 130 or higher, only the monoclinic form is observed. At 180°C, and whatever is the draw ratio, only the monoclinic form is observed. For high-molecular - weight polypropylene (for example M̄_w = 320000) the “smectic” form has never been observed whatever is the spinning temperature within the range 220–260°C. The fibres with high amount of smectic phase show low orientation factors, whereas the fibres having only monoclinic form show the highest orientation factors. These results, compared with elongational behaviour of the polypropylene, suggest that the smectic form appears in the fibres as the elongation rate on the spinning line is lower than the relaxation rate of polypropylene chains.     

HBTU Mediated Synthesis of α,β-Unsaturated γ-Lactams from E-α,β-Unsaturated γ-Amino Acids

The α,β-unsaturated γ-lactams have been found in many biologically active peptide natural products. Due to their biological activities, extensive efforts have been made in the literature to synthesize the α,β-unsaturated γ-lactams. Here, we are reporting the spontaneous transformation of E-α,β-unsaturated γ-amino acids into α,β-unsaturated γ-lactam through in-situ activation of free carboxylic acid using peptide coupling reagent HBTU and base DIPEA at room temperature. The transformation also involves the E→Z isomerization of α,β-unsaturated γ-amino acids. The reaction is also compatible with the peptides consisting of E-α,β-unsaturated amino acids at the C-terminus. The α,β-unsaturated γ-lactams were isolated in very good yields. Even though the reaction required very mild conditions, the products were isolated in the form of a racemic mixture. However, the products can be separated under a chiral environment. No α,β-unsaturated γ-lactams were observed if the reaction was performed in the presence of free amines. In addition, no racemization was observed during the peptide synthesis. The analysis of the reactions of various substrates revealed that amide NH and γ-CH are important for lactamization. No α,β-unsaturated γ-lactams or E→Z isomerization products were observed in the case of N-Me-(E)-α,β-unsaturated γ-amino acids, whereas in the case of E-α,β-unsaturated γ,γ-dimethyl amino acid α,β-unsaturated γ-lactam was isolated, however, with low yield.     

Formation and transformation of smectic polypropylene nanodroplets

A dispersion of isotactic polypropylene (PP) nanoparticles was produced by interfacially‐driven breakup of PP nanolayers. Layer‐multiplying coextrusion was used to fabricate an assembly of 257 alternating PP nanolayers about 12 nm thick sandwiched between thicker polystyrene (PS) layers. Characterization by thermal analysis and wide‐angle X‐ray diffraction (WAXD) confirmed that PP crystallized primarily in the smectic form when confined as nanolayers. When the layered assembly was heated into the melt, the PP nanolayers broke up to form a dispersion of PP droplets in a PS matrix. After solidification, particle size analysis revealed that 90% of the PP was present as 30 nm nanoparticles. The particles were small enough and numerous enough that most did not contain a primary nucleus. When cooled from melt at 10 °C min^?1, the droplets crystallized by homogeneous nucleation at 40 °C. The droplets were found to be in the smectic form by WAXD. Because crystallization occurred below the temperature of the smectic to α‐form transformation, the intermediate smectic form was stable and did not convert to the α‐form until heated above 70 °C. This result provided direct evidence for an intermediate smectic phase in the process whereby homogeneous nucleation leads to α‐form crystals in confined nanoparticles. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1795–1803, 2006     

Effect of morphology on the dynamic mechanical properties of a semicrystalline polysiloxane

The dynamic mechanical properties of semicrystalline poly(tetramethyl-p-silphenylene siloxane) in three morphological preparations were measured over the wide frequency range of about 0.002 Hz to 500 Hz and the temperature range of about ? 190°C to 100°C. The three samples were all isothermally crystallized at 125°C. Two samples had a spherulite size of 25 μ diameter but differed in the time allowed for secondary crystallization. The other sample had a smaller spherulite size. By assuming compliance additivity, the viscoelastic behavior could be separated into five relaxation processes with an indication that a sixth existed at low temperature. Two processes called γ₁ and γ₂ could be resolved at low temperatures. The γ₁ process was associated with the amorphous region since the peak strength was affected by the rate of cooling through the glass transition region; the γ₂ peak, unaffected by cooling rate, is attributed to the crystalline part. In the high-temperature region, the β peak is associated with the glass transition and has a shape and location that is essentially independent of the morphology. The highest temperature α₂ process, whose maximum was not observed in the experimental range covered, is attributed to the crystalline region and is sensitive to changes in crystallization history. The strength of the α₁ process unlike that of the other processes was found to be a function of temperature; it was associated with the noncrystalline region.     

Synthesis and thermal behaviour of salicylaldimine-based liquid crystalline symmetrical dimers

A series of symmetrical dimers consisting of salicylaldimine moieties connected by flexible alkylene central spacer via ether linkages has been synthesized. In order to validate the empirical rule suggested by Date et al. to account for the smectic behaviour of such dimers, the chain length of the terminal alkoxy chain has been kept constant (C₈) while the number of methylene units in the central spacer was varied from C₃ to C₁₁. Another aim of the present investigation was to understand structure-property relationship in these dimers in which the salicylaldimine mesogenic segment has been used for the first time in dimers. The mesomorphic behaviour of these dimers was evaluated using optical microscopy and differential scanning calorimetry and the structure of some of the mesophases has been further investigated with the help of X-ray diffraction. Our studies reveal that the dimers consisting of 3 to 8 methylene units in the flexible spacer show only smectic (smectic C and smectic A) phases. For the dimers containing 4, 6 and 8 methylene units in the central spacer, a unique filament growth pattern has been observed in the smectic A phase while cooling from the isotropic phase. The dimers containing of C₉ to C₁₁ methylene groups exhibit the nematic phase in addition to smectic modifications. This observation indicates that when the terminal chains are shorter than the spacer, the tendency to form smectic phases is not fully extinguished but is perhaps reduced.     

Crystallization kinetics of a thermotropic liquid crystalline polyimide derived from 1,3-bis[4-(4′-aminophenoxy) cumyl] benzene

We have studied the nonisothermal and isothermal crystallization kinetics of an aromatic thermotropic liquid crystalline polyimide synthesized from 1,2,4,5-benzenetetracarboxylic dianhydride (PMDA) and 1,3-bis[4-(4′-aminophenoxy) cumyl] benzene (BACB) by means of differential scanning calorimetry (DSC). Polarized light microscopy (PLM) and wide-angle X-ray diffraction (WAXD) results confirm that this polyimide exhibits a smectic texture. Nonisothermal crystallization showed two strong and one weak exothermic peaks during cooling. The phase transition from isotropic melt to liquid crystalline state is extremely fast which completes in several seconds. The mesophase transition has a small Avrami parameter, n, of approximate 1. The isothermal crystallization of 253–258°C has been examined. The average value n is about 2.6 and the temperature-dependent rate constant k changes about two orders of magnitude in the crystallization temperature range of 6°C. The slope of ln k versus 1/(T_cΔT) is calculated to be −2.4 × 10⁵, which suggests nucleation control, via primary and/or secondary nucleation for the crystallization process. During the annealing process, a new phase (slow transition) is induced, which grows gradually with annealing time. At lower annealing temperatures (220–230°C), the slow transition process seems not to be influenced by the crystals formed during cooling process and its Avrami parameter n is ca. 0.3–0.4. However, the slow transition was hindered by the crystals formed during cooling process when annealed at higher temperature (230–240°C). © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1679–1694, 1998     

Polymorph specific RMSD local order parameters for molecular crystals and nuclei: α-, β-, and γ-glycine

Crystal nucleation is important for many processes including pharmaceutical crystallization, biomineralization, and material synthesis. The progression of structural changes which occur during crystal nucleation are often described using order parameters. Polymorph specific order parameters have been developed for crystallization of spherically symmetric particles; however, polymorph specific order parameters for molecular crystals remain a challenge. We introduce template based polymorph specific order parameters for molecular crystals. For each molecule in a simulation, we compute the root mean squared deviation (RMSD) between the local environment around the molecule and a template of the perfect crystal structure for each polymorph. The RMSD order parameters can clearly distinguish the α-, β-, and γ-glycine polymorph crystal structures in the bulk crystal and also in solvated crystallites. Surface melting of glycine crystallites in supersaturated aqueous solution is explored using the newly developed order parameters. The solvated α-glycine crystallite has a thinner surface melted layer than the γ-glycine crystallite. α-glycine forms first out of aqueous solution, so surface melted layer thickness may provide insight into interfacial energy and polymorph selection.     

Textural transformations of a smectic phase with asymmetric banana-shaped molecules

Materials consisting of asymmetric banana-shaped molecules may form a smectic C_G phase having C₁ symmetry. We have studied textural transformations in a smectic phase of an asymmetric chlorine-substituted banana-shaped material under electric and mechanical fields. We observed two novel features that have not been observed so far on corresponding materials with symmetric banana-shaped molecules. These observations, however, could be explained by the same arguments as were used for the symmetric molecules. Although our studies do not exclude the possibility that the material has C₁ symmetry, we suggest that the chlorine molecules are positioned arbitrarily and the bulk has C₂ symmetry.     

Solution crystallization of nylon 12

Electron microscopy and x-ray diffraction data have been obtained on nylon 12 crystallized from 1-hexanol, 1,6-hexanediol, and hexylene glycol. Ribbonlike lamellar crystals of the γ form are obtained by crystallization from all the solutions and elongated flat crystals of the α form by crystallization from the 1-hexanol and hexylene glycol solutions. The direction of the hydrogen bond in these crystals is almost parallel to that of maximum crystal elongation. α- and γ-form crystals both grow from 1-hexanol and hexylene glycol at appropriate crystallization temperatures. γ-form crystals alone are obtained from 1,6-hexanediol solution at every crystallization temperature. The long periods measured by small-angle x-ray diffraction for the solution-grown crystals are in the range 7.6–10.6 nm. The melting behavior of the solution-grown crystals is examined and discussed. The melting temperatures of the γ form may be lower than that of the α form. An equilibrium melting temperature of 208.4°C for γ-form crystals is obtained by using a relation between thickness of lamellar crystals and their melting temperatures observed by differential scanning calorimeter measurements. Solvents affect the growth of the two crystalline forms in solution crystallization.