Melting behavior of the oriented β-phase polypropylene texture crystallized by the temperature slope method

Isotactic polypropylene consisting of uniaxially oriented P-phase lamellae was crystallized in a temperature gradient. The β → α transition was investigated by simultaneous measurements with differential scanning calorimetry (DSC) and X-ray diffraction using synchrotron radiation (SR). To compare the transition mechanism, the β-phase sample was deformed by rolling it along the direction of the crystallization. During rolling, the β-crystal is deformed by interlamellar and interchain slip, which induces c-axis-oriented molecules along the rolling direction. The melting behavior is changed by the rolling deformation. For the as-grown β-crystal, the DSC thermogram has three peaks: the β-melting endotherm at 150°C, an exotherm by recrystallization into the °-form, and the endotherm at 167°C caused by melting of the recrystallized α-form. After the rolling deformation, the β-endotherm is extinguished by the successive exotherm. Simultaneous X-ray measurements reveal that the β → α transition is shifted to a lower temperature and that the recrystallized α-form has a c-axis-orientation caused by the rolling deformation. In the process of the β→ α transition, higher-order lamellar structure is developed earlier than formation of the crystalline structure. In this study, the heating phenomena, such as the β α transition and thickening of the β- and α-lamellae, are consistently explained by a mechanism involving melting and subsequent recrystallization.     

CO2-induced Crystallization of Isotactic Polypropylene by Annealing Near Its Melting Temperature

CO₂-induced crystallization of isotactic polypropylene (iPP) by annealing had been studied using differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). The iPP before annealed was in α-form and amorphous states. At lower temperatures by CO₂ isothermal treatments, iPP chains crystallized from the amorphous phase and only one crystal form, i.e., α-form, was observed. At higher temperatures by CO₂ isothermal treatments, both crystallization from the amorphous phase and thickening of existing crystal lamellae were observed. Moreover, light γ-form crystal appeared in the treated iPP. The crystalline lamellar thickness of iPP annealed at different CO₂ pressures had been determined. Using the Gibbs–Thomson plot method, the equilibrium melting temperature was found to be 187.6°C.     

Formation of β-iPP in Isotactic Polypropylene/Acrylonitrile–Butadiene–Styrene Blends: Effect of Resin Type,Phase Composition,and Thermal Condition

The formation of β-iPP (β-modification of isotactic polypropylene) in the iPP/ABS (acrylonitrile–butadiene–styrene), iPP/styrene–butadiene (K resin), and iPP/styrene–acrylonitrile (SAN) blends were studied using differential scanning calorimery (DSC), wide angle X-ray diffraction (WAXD), and scanning electron microscopy (SEM). It was found that α-iPP (α-modification of isotactic polypropylene) and β-iPP can simultaneously form in the iPP/ABS blend, whereas only α-iPP exists in the iPP/K resin and iPP/SAN blend samples. The effects of phase composition and thermal conditions on the β-iPP formation in the iPP/ABS blends were also investigated. The results showed that when the ABS content was low, the ABS dispersed phase distributed in the iPP continuous phase, facilitating the growth of β-iPP, and the maximum amount of β-iPP occurred when the composition of iPP/ABS blend approached 80:20 by weight. Furthermore, it was found that the iPP/ABS blend showed an upper critical temperature T _c * at 130°C for the formation of β-iPP. When the crystallization temperature was higher than the T _c *, the β-iPP did not form. Interestingly, the iPP/ABS blend did not demonstrate the lower critical temperature T _c ** previously reported for pure iPP and its blends. Even if the crystallization temperature decreased to 90°C, there was still β-iPP generation, indicating that ABS has a strong ability to induce the β-iPP. However, the annealing experiments results revealed that annealing in the melt state could eliminate the susceptibility to β-crystallization of iPP.     

Effects of molecular weight on the perforation impact behavior of injection-molded plaques of α-and β-phase isotactic polypropylenes

This study was devoted to the instrumented falling weight impact (IFWI) behavior of injection-molded a-and β-phase isotactic polypropylene (iPP) homopolymers. The perforation impact response of iPP with various melt flow indices (MFIs), and thus molecular weight (MW) characteristics, was studied at two different temperatures (T = 23°C and T =?40°C) and incident impact speeds (v_inc = 5 and 10 m/s). The impact resistance of β-iPP was superior to the α-modification. The absolute resistance to perforation increased with increasing MW or decreasing MFI, whereas the relative toughness improvement between the β-and α-iPPs followed an opposite tendency. The molding-induced skin-core morphology did not affect practically the out-of-plane response of the impacted plaques. Changes in the fractograms (viz. force-time curves) under various experimental conditions were traced to variations in the failure mode, showing a competition between radial and circumferential cracking with respect to the clamping ring. In the case of the more ductile β-iPP. circumferential cracking was favored.     

CRYSTALLIZATION OF A LOW MOLECULAR WEIGHT POLYETHYLENE AND PARAFFINS UNDER A TEMPERATURE GRADIENT

A low molecular weight polyethylene (PE) and even-number paraffins were crystallized under a temperature gradient. Highly oriented crystalline textures were developed by the temperature slope crystallization. The in situ crystallizing surface was observed by an optical microscope and X-ray diffraction. Polyethylene has a b-axis orientation in which the lamellar normal and crystalline c-axis are perpendicular to the temperature gradient. On the contrary, in the even-number paraffins, both axes are parallel to the temperature gradient. The results of the in situ measurements and the crystalline orientation are compared and discussed for both cases.     

A Comparison of the Effects of Calcium Glutarate and Pimelate on the Formation of β Crystalline Form in Isotactic Poly(propylene)

The effect of calcium glutarate (Cagt) and calcium pimelate (Capt) on the formation of β crystalline form in isotactic poly(propylene) in the crystallization temperature range of 110–130°C has been investigated. The content of β phase crystals increase with the addition of calcium glutarate. K (relative content of β crystalline form in the iPP sample) attains its maximum value for iPP doped with 0.3 wt.% Cagt isothermally crystallized at 110°C (26.71%) or 120°C (30.27%), and for iPP doped with 0.2 wt.% Cagt isothermally crystallized at 130°C (31.97%), respectively. Compared with the K values of iPP doped with 0.1 wt.% Capt (78.33–94.76%), the β nucleation ability of Cagt is inferior to that of Capt. The spherulite size of iPP doped with Capt is smaller than that of iPP doped with Cagt. The difference in the β nucleation ability between Cagt and Capt is explained by the difference between their crystal structure parameters and those of β‐iPP.     

Large Strain Tensile Deformation and Failure in Oriented Isotactic Polypropylene/Clay Nanocomposite: Role of Voiding and Molecular Orientation

Plane strain compression of isotactic polypropylene iPP)/clay nanocomposite in a channel die at 140 and 160°C, respectively, has been adopted to prepare oriented samples with well-controlled structure for comparative studies. Molecular orientation in the amorphous phase, independent of clay loadings, decreases with increasing preparation temperature, whereas crystallographic orientation is nearly the same for all oriented samples. Severer voiding and void coalescence during stretching, mostly induced by the crystals and inter-chain sliding in the amorphous phase, respectively, is suggested to be responsible for higher volume dilatation and lower failure strain in the oriented samples prepared at higher temperature (e.g., 160°C). Fracture toughness is well correlated with the molecular orientation and crystal-dependent voiding in the oriented samples with respect to preparation temperatures. Furthermore, debonding of clay in the iPP matrix, especially in the oriented samples prepared at 140°C, is another contributor to the enhanced toughness.     

Polypropylene Nanocomposites Based on Synthetic Organic-Soluble Ag Nanocrystals with Prominent β-nucleating Effect: Quiescent Crystallization and Melting Behavior

Differential scanning calorimetry, x-ray diffraction, and polarized optical microscopy were used to investigate the quiescent crystallization and melting behavior of isotactic polypropylene (iPP) nanocomposites based on synthetic organic-soluble Ag nanocrystals (NCs). The effects of Ag loading and crystallization temperature on the crystallization behavior and crystalline structure were studied. The results showed that the synthetic Ag NCs as a novel effective β-crystal nucleating agent for iPP could promote the overall crystallinity, decrease the size of spherulites, and induce the formation of large amounts of β-crystals in the nanocomposites under quiescent crystallization. The relative content of β-crystals significantly increased with increasing Ag loading, and slightly increased with decreasing crystallization temperature. The quiescent crystallization kinetics was analyzed using the Avrami model. The results showed that the iPP nanocomposites with added Ag NCs had higher crystallization rate constant (k) and lower crystallization half-times (t_1/2) as well as the Avrami exponent (n) than pure iPP, indicating that the presence of Ag NCs acted as heterogeneous nucleating sites and promoted the crystallization rate of iPP.     

ac conductivity of anhydrous pellicular zirconium phosphate in hydrogen form

Aqueous suspensions of delaminated zirconium phosphate have been recently obtained in our laboratory by a suitable intercalation-deintercalation of alkylamines. By filtering these suspensions thin and flexible sheets or pellicles of lamellar α-zirconium phosphate, with the most part of the layers oriented parallel to the pellicle surface, can be easily prepared. ac conductivity measurements have been carried out, in the temperature range 150–300°C, on pellets made of α-Zr(HPO₄)₂ pressed pellicles, oriented both parallel and perpendicular to the electric field. Independently of the pellet orientation, a slope variation was observed in the Arrhenius plot at about 220°C. As already found for the α-Zr(HPO₄)₂ powder, this is associated with a phase transition at this temperature, which causes a discontinuous change of the interlayer distance from 7.4 Å to 6.8 Å. The conductivity of samples oriented parallel to the electric field ($\text{～10}{}^{\mathrm{?4}}\text{Ω}{}^{\mathrm{?1}}\text{cm}{}^{\mathrm{?1}}\text{at}\text{300°}\text{C}$) is about two orders higher than the conductivity of the anhydrous microcrystalline powder, which, in turn, is higher than that of perpendicularly oriented samples ($\text{～3 × 10}{}^{\mathrm{?7}}\text{Ω}{}^{\mathrm{?1}}\text{cm}{}^{\mathrm{?1}}\text{at}\text{300°}\text{C}$).     

Phase transformations in equiatomic alloy TiZr at pressure up to 70 kbar

The effect of pressure on the α ? β and ω ? β transformations in the equiatomic alloy TiZr is studied by the differential thermal analysis (DTA) and calorimetric technique. The α-β equilibrium at atmospheric pressure occurs at a temperature of 579°C, and the heat of transition ΔH is 40.9±2.0 J/g. As the pressure increases up to 28 kbar, the temperature of the α-β equilibrium linearly decreases, dT/dP=?2.2±0.3 K/kbar. In the pressure range 28–48 kbar, the β-phase undergoes a transition to the two-phase (α + ω) state upon cooling to room temperature. At pressures above the triple point with the coordinates P=49±3 kbar and T=460±30°C, the cooling of the β-phase gives rise to only the hexagonal ω-phase with the unit cell parameters a=4.843 Å, c=2.988 Å, and c/a=0.617 under normal conditions. The slope of the ω-β equilibrium boundary is positive at pressures up to 70 kbar, dT/dP≈0.46 K/kbar. The ω → α transformation at atmospheric pressure proceeds in the temperature range T=425–470°C with the enthalpy of transition ΔH=2.8 J/g.     

A Rule Governing Crystallization of Configurationally Directional Polymers: The Crystal Structure of the α and β Forms of Polypivalolactone

A “Rule” is proposed for incorporation of polymer chains having directional configuration, e.g. A‐B‐C‐A‐B‐C, into a crystal. Crystallization into a lamella morphology, as in slow crystallization from the melt, will incorporate antiparallel sequences (↑↓↑↓↑↓). Formation of a fiber by drawing the lamellar morphology must produce a different crystal structure containing parallel directional sequences. The drawn fiber must be polymorphic with a disordered aggregation of antiparallel and parallel crystal polymorphs. An example of this rule is found in the crystal structure of polypivalolactone. The melt crystallized α form is monoclinic, P2₁/c with a=9.05Å, b (fiber axis)=5.97Å, c=11.69Å, β=121.4° and consists of planar antiparallel sequences. The molecular conformation is a folded zig‐zag arrangement. On drawing a fiber, a disordered second phase of parallel plus antiparallel sequences is created. The chain conformation is a slightly distorted extended zigzag. The crystal structure of the directionally disordered β form is metrically monoclinic, with a=5.95Å, b=10.32Å, c (fiber axis)=4.94Å, β=101.3°. Examples of several classes of crystalline polymers demonstrating this Rule are presented.     

Phase Transition in β-nucleated Isotactic Polypropylene Induced by Combination of Annealing and High Pressure

The effects of annealing and high pressure on the microstructure of β-nucleated isotactic polypropylene (iPP) were investigated. Annealing treatment was carried out at different temperatures from 90–170°C under different pressure conditions, that is, atmospheric pressure and high pressures of 150, 250, and 350 MPa, respectively. The microstructure of the specimens was comparatively investigated through using wide angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM). Different from the common annealing treatment under atmospheric pressure, which induces the single-phase transition from metastable β-iPP to the more stable α-iPP (β→α) depending on the annealing temperatures, the application of high pressure induces not only the phase transition of β→α but also the phase transition of β→γ. Furthermore, the thermostability of β-iPP was improved greatly under high pressure. The higher the pressure applied, the smaller was the degree of the transition from β-iPP to α-iPP and/or γ-iPP. This work reports, for the first time, a novel crystalline morphology composed of β-iPP and γ-iPP.     

Microstructure of contact layers formed by the contact melting of copper and aluminum

The contact melting of copper and aluminum is performed in the nonsteady-state diffusion mode at a temperature of 570°C. Fragments of a spherical phase, presumably CuAl₂, 10–15 μm in size and a lamellar phase 70–200 μm in length and 10 μm in width are found on the cleaved facets of the contact layer formed.     

On the decomposition of the transition rhombohedral phase in the Al-30 wt.% Zn alloy

The decomposition of the transition rhombohedral α′_R-phase at the temperatures between 85 and 250 °C was investigated by transmission electron microscopy and by X-ray diffraction on single crystals. Below 161 °C the direct decomposition of α′_R-phase into the equilibrium hexagonal β-phase without the formation of the transition cubic α′-phase was found. Both transformation sequences α′_R → β and α/′_R→α′ → β were observed in the temperature range from 161 to 180 °C whereas only the previously known sequence α′_R → α′ → β was detected on ageing the alloy between 180 and 250 °C. The precipitation process at the temperatures from 161 to 180 °C is characterized by the decomposition of α′_R-phase into the equilibrium β-phase prior to the formation of the transition α′-phase and by the increased rate of decomposition of α′-precipitate. The observed transformation processes are related to the variation of strains at the partially coherent interface between α′_R-phase and α-matrix with the temperature in correlation with the metastable α′_R-phase boundary. These considerations allowed to estimate the relative stabilities of precipitated phases and their activation energies of formation and thus to discuss the decomposition mechanism of α′_R-phase at various temperatures.     

Structure and piezoelectric properties of microporous polyvinylidene fluoride films

A method to produce microporous polyvinylidene fluoride films characterized by a high content of a piezoactive crystalline phase and a developed relief surface is elaborated. The porous films are obtained in the process based on melt extrusion with subsequent isometric annealing and uniaxial extension. It is found that the samples have an oriented lamellar structure containing both α- and β-modifications of the crystallites. The effect of temperature and degree of uniaxial extention of the annealed films on the characteristics of the porous samples, that is, the content of the piezoactive crystalline β-phase, overall porosity, breakdown voltage and the piezoelectric modulus is studied.

     

Investigation of the Melt‐Crystallization of Polypropylene by a Temperature Slope Method

Abstract

To investigate the in‐situ ordering process of isotactic polypropylene (iPP) from a melt state, a stationary growth front was prepared by the temperature slope crystallization (TSC) method. During the melt‐crystallization, iPP was crystallized into the α‐phase or β‐phase depending on the crystallizing conditions. The mechanism of the melt‐crystallization at the growth front was precisely observed by wide‐angle and small‐angle x‐ray scattering (WAXS and SAXS) using a strong synchrotron beam. In the TSC apparatus, the sample was crystallized in between a heater, controlled to 220°C, and a cooler, cooled by water to 25°C. We define the z‐axis parallel to the temperature gradient. A‐lamellae and B‐lamellae are also defined as those whose lamellar normal are perpendicular and parallel to the z‐axis, respectively. In a sample‐stop (SS) stage before the TSC, the original α‐phase lamellae became thicker, approaching to the melt‐solid boundary by annealing. The annealing process showed that the α‐phase B‐lamellae remained and the SAXS reflection was stronger on the meridian near the melt‐solid boundary in the SS stage. In the beginning of the TSC, the α‐phase B‐lamellae developed as a primary crystallization. During secondary crystallization under high supercooling, the SAXS cross pattern appeared showing that the α‐phase developed both A‐ and B‐lamellae. As the growth direction of A‐lamellae is parallel to the z‐axis, A‐lamellae grow faster than B‐lamellae. By the self‐epitaxial mechanism on the side surface of the A‐lamellae, the B‐lamellae grow on the base of the A‐lamellae. Following appearance of a spontaneous β‐nucleus, the β‐phase lamellae grew preferentially, excluding the α‐phase, and occupied the whole area of the sample. In this case also, A‐lamellae are advantageous to grow because of the growth direction parallel to the z‐axis. As a result, the SAXS β‐phase reflection appeared on the equator.     

Morphology of strain-induced crystallization of natural rubber. Part II. X-Ray studies on cross-linked vulcanizates

The results of wide- and small-angle X-ray studies on bulk vulcanizates of natural rubber are described. At room temperature no discrete X-ray peaks occur at small angles, even for highly strained (700%) specimens for which a significant degree of crystallinity is present at room temperature. Below the isotropic melting point of the network, discrete small-angle X-ray scattering develops for both unstrained and strained specimens. The Bragg periodicity corresponds closely to the longitudinal lamellar spacing from electron micrographs of thin films under corresponding conditions. The X-ray long period exhibits significant thermal reversibility. Upon heating, following initial crystallization at ?25°C, the long period continuously increases from about 140 Å at ?25°C to about 250 Å at 17°C. After heating, progressive cooling from room temperature results in continuous decreases in long period and at least 80% of the initial increase on heating is recovered. These results in conjunction with electron microscopy data (Part I) strongly suggest that thermally reversible changes in physical lamellar periodicity occur for natural rubber and are discussed in detail in light of current concepts concerning long period reversibility in bulk polymers.     

Structural and magnetic properties of thin epitaxial Fe films on (1 1 0) GaAs prepared by metalorganic chemical vapor deposition

Iron films have been grown on (1 1 0) GaAs substrates by atmospheric pressure metalorganic chemical vapor deposition at substrate temperatures (T_s) between 135°C and 400°C. X-ray diffraction (XRD) analysis showed that the Fe films grown at T_s between 200°C and 330°C were single crystals. Amorphous films were observed at T_s below 200°C and it was not possible to deposit films at T_s above 330°C. The full-width at half-maximum of the rocking curves showed that crystalline qualities were improved at T_s above 270°C. Single crystalline Fe films grown at different substrate temperature showed different structural behaviors in XRD measurements. Iron films grown at T_s between 200°C and 300°C showed bulk α-Fe like behavior regardless of film thickness (100–6400 Å). Meanwhile, Fe films grown at 330°C (144 and 300 Å) showed a biaxially compressed strain between substrate and epilayer, resulting in an expanded inter-planar spacing along the growth direction. Magnetization measurements showed that Fe films (>200 Å) grown at 280°C and 330°C were ferromagnetic with the in-plane easy axis along the [1 1 0] direction. For the thinner Fe films (⩽200 Å) regardless of growth temperature, square loops along the [1 0 0] easy axis were very weak and broad.     

Fabrication of Degradable Bone-Like Substitutes Based on Poly-L-lactide and β-tricalcium Phosphate

Composite bone-like substitutes composed of poly-L-lactide (PLLA) and β-tricalcium phosphate (β-TCP) (average particle size: 4.43 μm) were fabricated and the properties were investigated. β-TCP was prepared by wet chemical precipitation, followed by calcining at 800°C. Composite films were obtained by completely mixing dissolved PLLA with granules of β-TCP; the agglomerated β-TCP powder granules were distributed homogeneously in the PLLA matrix. PLLA/β-TCP composite materials were obtained by cold and hot pressing the composite film at a pressure of 130 MPa and temperature of 185°C–195°C. With increase of the amount of β-TCP powder, the bending strength of the composites decreased while the bending modulus increased. The fracture mechanism of the composites was significantly influenced by the content of β-TCP powder, from ductile fracture to brittle fracture as the β-TCP powder content increased.     

Skin Thickness and β-Crystals Development in Injection-Molded iPP Along the Flow Direction

In this study, iPP was injection molded at 180°C, 200°C, and 220°C. According to polarization optical microscopy (POM) results, for a given part, the skin thickness steadily decreases along the flow direction. However, at the same distance from the gate, the skin thickness of the parts molded at lower melt temperature is larger than that molded at higher melt temperature. It is found that flow time (here, the time taken for melt to pass the specific position along the flow direction) and melt temperature are two significant factors leading to this phenomenon, while the gate size is another one.

The DSC and WAXD results show that the relative fraction of β-form crystals, for a specific part, decreases along the flow direction, which is mainly determined by flow time. However, for the parts molded at different molding temperatures, the fraction of the β-form crystals is mainly determined by the molding temperature, though this influence is very complex.