Morphological changes in isotactic polypropylene as a function of cooling rate

A new method for monitoring rather than controling thermal history has been applied recently to analyze the crystallization kinetics at high cooling rate of a highly pure isotactic polypropylene, proving that the cooling history relevant to the structure of quenched samples is in the neighborhood of 90°C. A continuous variation of morphology and crystal structure was obtained with cooling rate. At cooling rates above 20°C/s, mesomorphic and α-monoclinic phases coexist with a morphology characterized by negatively birefringent spherulites surrounded by a weakly birefringent medium; below this value of cooling rate only α-monoclinic phase is formed, composed of mixed spherulites.     

Structure of trans-1,4-polybutadiene synthesized with a new catalyst system

Abstract

The structure, phase composition, and temperature behavior of two trans-1,4-polybutadienes (TPBs) were studied by means of x-ray scattering and differential scanning calorimetry (DSC) techniques. The two samples examined were (1) PB synthesized using an immobilized titanium-magnesium catalyst and (2) a random copolymer based on PB prepared with a homogeneous vanadium-containing catalyst used as a reference material. It was found that the nascent structure of the first PB involves three phases: crystalline, mesomorphic (low-temperature form), and amorphous. In the vicinity of 65°C, a first-order phase transition occurs. The system becomes biphasic and contains the high-temperature form of the mesophase, as well as the amorphous phase, component. Above 165°C, the polymer melts to form a single-phase isotropic melt with a structure typical of liquids. The lateral dimension of crystallites reversibly changes at the crystal-mesophase transition. It is suggested that during annealing of the mesophase formed by cooling of the isotropic melt, the chains acquire an extended conformation. Loss of regularity of the structure of macromolecules of TPB causes a reduction of phase transition temperatures, an increase of the imperfection of the crystalline phase, and a contraction of the temperature range of existence of the mesophase.     

Crystalline Morphology and Nonisothermal Crystallization Behaviors of iPP/PcBR Blends

Isotactic polypropylene/poly(cis‐butadiene) rubber (iPP/PcBR) blends were prepared by melt mixing. The influence of PcBR content on crystalline morphology and nonisothermal crystallization behaviors of iPP was investigated by polarized optical microscopy (POM), small angle light scattering (SALS), and differential scanning calorimetry (DSC). The POM showed that an increase of PcBR ranging from 10 vol% to 40 vol% led to less perfection of spherulites, vaguer boundaries between spherulites, and smaller spherulite size, which was quantitatively validated by SALS. The presence of PcBR also remarkably affected the nonisothermal crystallization behaviors of iPP. An addition of PcBR caused higher crystallization peak temperature and a faster crystallization rate, meaning a heterogeneous nucleation effect of PcBR upon crystallization of iPP. For the same sample, the crystallization peak temperature moved to lower temperature and the crystallization rate increased as the cooling rate increased. The Ozawa and combined Avrami and Ozawa equations were used to describe the nonisothermal crystallization process of iPP and blends. The combined Avrami and Ozawa equation was more appropriate for the crystallization of the blends. Crystallization activation energy of iPP and blends was calculated by the Kissinger equation; the result showed that crystallization activation energy decreased as the content of PcBR increased from 30 vol% to 40 vol%.     

Observation of transcrystalline growth of PLA crystals on sisal fibre bundles and the effect of crystal structure on interfacial shear strength

Hot-stage microscopy was used to characterise crystal growth at the interface between sisal fibre bundles and a polylactic acid (PLA) matrix in order to better understand the mechanical properties of sisal fibre–PLA composites. Cooling rates and crystallisation temperatures and times were varied to influence crystalline morphology at the interface. Single sisal fibre bundles were evaluated in their as received state or treated with 6 wt.% caustic soda solution for 48?h at room temperature. A microbond shear test was used to characterise the shear strength of the interface as a function of fibre surface treatment. These tests were performed on sisal fibre bundles carefully embedded in flat films of PLA supported on card mounts. Fibre bundles in a PLA matrix were cooled from 180?°C at rates from 2 to 9?°C/min and then crystallised isothermally. For as received fibre bundles uneven growth of PLA spherulites occurred at all cooling rates and crystallisation temperatures. For caustic soda treated fibres, uneven spherulitic growth was observed at crystallisation temperatures at and above 125?°C. In contrast, transcrystalline growth was observed for samples cooled to 120?°C at cooling rates from 2 to 6?°C/min and then allowed to crystallise. The microbond shear strengths of untreated and caustic soda treated fibre bundles were evaluated using Weibull statistics and the caustic soda treated fibres exhibited higher interfacial shear strengths in comparison to untreated fibres, reflecting the development of a transcrystalline layer at the fibre to matrix interface.     

Influence of Thermal Treatment on the Thermal Behavior of Poly‐L‐lactide

The influences of thermal treatment on cold crystallization and the thermal behavior of poly‐L‐lactide (PLLA) were investigated by DSC and polarizing microscopy. Both the cooling and heating rates had effects on cold crystallization. Double peaks were observed for the samples on subsequently heating at 10°C min^?1 after cooling between 5 and 20°C min^?1. The degrees of crystallinity dramatically increased with decreasing cooling rate, and the size of PLLA spherulites increased with a decrease in the cooling rate. Double cold crystallization peaks were also observed during heating traces at higher rates for this material after fast cooling (20°C min^?1) from the melt. The competition between the crystallization from the nuclei formed during cooling, and that from spontaneous nucleation might be responsible for the appearance of double peaks.     

Chemical states of GeTe thin-film during structural phase-change by annealing in ultra-high vacuum

The chemical states of GeTe thin film are investigated using high-resolution X-ray photoelectron spectroscopy (HRXPS) with synchrotron radiation, during amorphous to crystalline structural phase transition. As the temperature increases from 250 to 400 °C, we observe the rock-salt crystalline structure and phase with X-ray diffraction (XRD) and transmission electron microscopy (TEM). Spin-orbit splitting of the Ge 3d core-level spectrum clearly appears after annealing at 400 °C for 5 min. However, the binding energy of the Ge 3d_5/2 core-level peak of 29.8 eV does not change in the amorphous to crystalline structural phase transition. In the case of the Te 4d core-level, change in binding energy and peak shapes is also negligible. We assume that the Te atom is fixed at a site between the amorphous and crystalline phases. Although the structural environment of the Ge atoms changes during the structural phase transition, the chemical environment does not.     

Solidification of selenium melt under pressure up to 6.5 GPa

Abstract

High purity selenium samples were melted under high pressure (≤6.4 GPa) and quenched at various rates ranging from 2 K s^?1 to 500 K s^?1 and the recovered material was examined by X-ray diffraction and electron microscopy. In the entire range of pressure and cooling rate, the melt was found to solidify into a polycrystalline aggregate of the trigonal phase of selenium. The samples obtained by slow cooling of the melt at 6.4 GPa contain, in addition to crystalline phase, regions which appear to be amorphous.     

Ringed spherulitic and undulated textures in the nematic phase of a mixture of trans-4-hexylcyclohexanecarboxilic and benzoic acids

We present a polarizing optical microscopy study of a liquid-crystal mixture of trans-4-hexylcyclohexanecarboxilic (C6) and benzoic acids. Both materials have carboxylic groups that can form dimers through hydrogen bonding. The mixture is nematic at room temperature and has the clearing point at 88°C. The nematic phase shows ringed spherulites, sometimes looking like spiral structures, and undulated textures, which remain visible when heating the sample till 71°C. This is the temperature of a texture transition inside the nematic phase.     

Influence of Shearing on Crystallization Behavior of Polyoxymethylene/Poly(Ethylene Oxide) Crystalline/Crystalline Blends

The effect of shearing on crystallization behavior of a crystalline/crystalline blend, polyoxymethylene [POM]/poly(ethylene oxide) [PEO], was investigated using polarized light microscopy connected with a CSS450 shearing hot-stage, scanning electron microscopy, differential scanning calorimetry [DSC], and x-ray diffraction [XRD]. The experimental results indicated that the shearing made POM and PEO disperse more evenly and increased the inclusion and entanglement effects between the molecular chains of POM and PEO and therefore enhanced the influence of PEO on the crystallization of POM. As a result, the blend sheared at a shear rate of 20 s^?1 for 10 min at 160°C formed shish–kebab crystals and produced more interlamellar structures compared with the formation of perforated spherulites in the unsheared blend. Moreover, a more obvious shoulder melting peak of POM appeared in the DSC heating trace and a new diffraction peak occurred at 2θ = 31.7° in the XRD pattern for the sheared POM/PEO [50/50] blend.     

Deformation behavior of tetramethyl-p-silphenylenesiloxane (TMPS)-dimethylsiloxane (DMS) block copolymer spherulitic films

A study of the morphology of uniaxially drawn spherulites of TMPS-DMS block copolymers with TMPS contents ranging from 90 to 30 wt % was made especially by electron and optical microscopy. For the entire range of polymer compositions, the copolymers, like the TMPS homopolymer, form negatively birefringent spherulites when crystallized from solution, or from the supercooled melt. A banded spherulitic morphology is observed. The deformability of copolymer spherulites increases as the DMS amorphous component increases. The DMS component coexists with, but is physically excluded from the ordered TMPS “crystalline” phase. In samples having the higher TMPS compositions, deformation occurs through crack formation, mechanical slip, tilting and/or twisting of the lamellae that help comprise the spherulites. At high DMS contents this “soft” component is undoubtedly responsible for the flexible character of these copolymers. Sample clarity also increases with DMS content as the crystallinity decreases.     

Phase transitions in liquid crystal 6O.4 (p-<Emphasis Type="Italic">n</Emphasis>-hexyloxybenzylidine-p′-<Emphasis Type="Italic">n</Emphasis>-butylaniline)

DSC measurements on p-n-hexyloxybenzylidine-p′-n-butylaniline (6O.4) showed that the crystalline to liquid crystalline (K-S _H) transition at 33.7°C observed in the heating cycle does not revert even when the sample is cooled down to −100°C. Hence it is inferred that a physically stable supercooled liquid crystalline phase is formed on cooling 6O.4. To investigate the K-S _H transition further the techniques of polarized microscopy and X-ray diffraction were used which concurred with the DSC results. Quasielastic neutron scattering measurements carried out to study the re-orientational motions in the ordered phases of 6O.4 (K and S _H) show that while in the crystalline phase (at RT) the re-orientational motion is found to involve only the core of the molecule, in the S _H phase (at 45°C) the dynamics involves the whole molecule and this motion is found to persist even when the sample cools back to room temperature corroborating the results of the DSC, microscopy and X-ray diffraction.      

Processes of the solidification of Ni-B alloys

The temperature and concentration conditions of metastable phase formation in a Ni-B system at melt cooling rates of 10² to 10⁴ °C/s were investigated by X-ray structural analysis, metallography, and transmission electron microscopy. An increase in the cooling rate was shown to yield a decline in the concentration range of the metastable phase’s existence.     

Temperature dependence of morphology of supermolecular structures growing in the thin layer melt of fractions of poly(ethylene oxide)

Poly(ethylene oxide) fractions with molecular weights in the range from 1.55 × 10³ to 1.5 × 10⁵ were studied. The optical microscopic observations of the morphology of supermolecular structures grown isothermally from the melt were used to determine the dependance of structural-morphological changes on temperature. Two morphological transitions were established: (1) a high temperature transition at 49 [Ptilde] 3° C from hedritic structures to positive spherulitoids and (2) a low temperature transition at 38 [Ptilde] 2° C from positive spherulitoids to negative spherulites. It is supposed that the transition from hedrites to positive spherulitoids proceeds through a transition from b-axis-oriented lamellae with tilted chains to [401] lamellae with chains perpendicular to the lamellar surface.     

Molecular order and dynamic mechanical behavior of polyurethanes based on liquid crystalline diol

Synthesis of the liquid crystalline (LC) diol 6,6′-[ethylenebis(l,4-phenylene-oxy)]-dihexanol (I) is described. The structure of polyurethanes prepared from diol I and 4,4′-methylenedi(phenyl isocyanate) (MDI), 4,4′-methylenedi(cyclohexyl isocyanate) (HMDI), or 2(4)-methyl-l,3-phenylene diisocyanate (TDI) at 1:1 molar ratios of isocyanate and hydroxy groups is studied by dynamic mechanical spectroscopy, differential scanning calorimetry (DSC), polarizing microscopy, and x-ray scattering. The polymer prepared from HMDI and the diol (I/HMDI) shows, on cooling, thermal behavior typical of amorphous polymers. A frequency-temperature superposition could be applied to the mechanical data, and the horizontal shift factor satisfied the Williams-Landel-Ferry (WLF) equation. A more-complex thermal behavior was found for I/HMDI polymer during subsequent heating; above 70°C, the formation of an ordered structure takes place, and this structure melts at about 120°C. Complex thermal behavior is exhibited by I/TDI polymer. On cooling its melt, the polymer forms a nematic phase at about 80°C, which freezes into the LC glassy state. On heating, the mesophase melts, and. subsequently, a better-ordered smectic phase is formed at 95°C, which melts at 120°C. This structure buildup is accompanied by a rapid increase in storage modulus G′, and the sample shows thermorheologically complex mechanical behavior. The polymer formed from the diol and MDI (I/MDI) exhibits a most-complex thermal behavior. On cooling and heating, four transitions can be detected in its thermal mechanical behavior, and the structure of the polymer is strongly dependent on its thermal history.     

Transport properties of mesomorphic syndiotactic polystyrene

Amorphous syndiotactic polystyrene (sPS) films were annealed at 110° and 115°C for 24 h. The annealing at the higher temperature produced the mesomorphic form of sPS, characterized by conformational order, as shown both by the presence of the zigzag band in the infrared spectrum and by the presence of characteristic peaks in the x-ray diffractogram. The crystalline form was not formed at the investigated temperatures. Differential scanning calorimetry of the mesomorphic sample shows that the transformation of the mesophase into the crystalline phase occurs during the heating run. The transport properties of dichloromethane show that the mesophase is impermeable to the vapors at low activity and becomes permeable at higher activities. At low activity it is therefore possible to derive the fraction of impermeable phase, which corresponds to the fraction of mesophase. This fraction was about 50%.     

Multiordered Phase Structures Of Nematic Pbo/ppa Solution

Liquid crystalline solutions of poly (p-phenylene benzoxazole) (PBO) in polyphosphoric acid (PPA) were prepared and studied. The typical banded texture and disclination of sheared PBO/PPA solution were observed by polarizing optical microscopy (POM), and the microstructure of the periodic aligned polymer chain that induced the texture was investigated by scanning electron microscopy (SEM). Spherulite structure was found in the PBO/PPA system with little shearing. Differential scanning calorimeter (DSC) analysis on the system revealed the existence and melting behavior of two ordered phases. An exothermic peak at 85°C was interpreted as the enhancement of the interaction between PBO molecules and PPA solvent, and the endothermic peaks at 140 to 150°C and 180 to 200°C are explained as the melting of the ordered phases. The changes of small-angle light scattering (SALS) patterns of PBO/PPA solution at different temperature further supported these results.     

Freezing behavior of one-dimensional copper nanowires

The freezing behavior of copper nanowires at different cooling rates was studied by using molecular dynamics (MD) simulation via embedded atom methods (EAM) potential. The simulation results indicate that the structure of the nanowires changed from amorphous to crystalline via helical multi-shelled structure with decreasing cooling rates. The curves of the energy-temperature and the local clusters were used to study the phase transition. The variation of local clusters implies that the order of the system increases as the temperature drops. The fcc crystalline structure of the copper nanowires with the diameter of 1.63 nm was proved to be the most stable form.     

Phase organization of solvent-crystallized syndiotactic polystyrene films

Sorption and diffusion of dichloromethane vapor in the helical forms—that is, δ and γ—of syndiotactic polystyrene were investigated. The thermal transition from the δ to the γ form occurs through an intermediate mesomorphic form, also characterized by chains in helical conformation but lacking true crystalline order. The mesomorphic phase was found impermeable to the vapor at low activity, becoming permeable at higher activities. The presence of a mesomorphic phase, along with the crystalline and the amorphous forms, was suggested in the crystallized samples, too, and its fraction increases greatly going from the δ to the γ form.     

Ferroelectric properties of vinylidene fluoride copolymers

Ferroelectric properties of copolymers of vinylidene fluoride with trifluoroethylene and tetrafluoroethylene are described with special interest in their polarization reversal and phase transition behavior. The ferroelectric phase consists of all-trans molecules packed in a parallel fashion while molecules adopt irregular TT, TG, T[Gbar] conformations in the paraelectric phase. In the ferroelectric phase, polarization reversal occurs at very high fields (> 100 MV/m) as a result of eventual 180° rotations of individual chain molecules around their axes. The switching time ranges from sec to nsec depending upon the strength of the applied field according to an exponential law with a particularly large activation field (～ 1 GV/m). The value of the observed remnant polarization is consistent with prediction from a simple dipole sum implying a minor contribution from the Coulomb interaction. The ferroelectric-to-paraelectric transition appears most clearly for copolymers containing 50-80 mol% vinylidene fluoride at temperatures from 70 to 140°C. The transition accompanies cooperative dipole motions associated with elementary trans-gauche transformations. Changes in various properties as well as in structures in the vicinity of the Curie point are consistent with an order-disorder transition of first order. Detailed characteristics specifying their ferroelectricity are summarized and discussed in relation to the molecular, crystalline, and higher-order structures inherent to ferroelectric polymers.     

EXAFS investigation of amorphous-to-crystal transition in Ge

The structure of a-Ge as a function of the deposition temperature T_s has been studied by the EXAFS technique for the first time. The results demonstrate the sensitivity of EXAFS to structural differences in the medium range. The data provide strong evidence for a continuous transition from the amorphous to the crystalline phase over a temperature interval form 130°C to 300°C. The analysis of the data is in line with the microcrystal model of amorphous Ge films and allows an estimate of the average grain size for the lower range T_s used.