Effect of pressure on the melting and crystallization behaviour of isotactic polybutene-1

The formation, melting and phase transition of isotactic polybutene-1 under high hydrostatic pressures were studied by high-pressure d.t.a. and X-ray diffraction up to 5 kbar. The d.t.a. thermogram of melting of form I shows a single endothermic peak up to 5 kbar. Form II crystallized directly from the melt at atmospheric pressure is metastable and it transforms to form I by the application of pressure. Above 900 bar, it transforms to form I completely and the endothermic peak of melting of form II is not observed. On crystallization from the melt under high pressure, the percentage content of form I' increases with crystallization pressure and at 1.6 kbar only form I' is crystallized. Above 2 kbar form II', which shows the same X-ray diffraction pattern as form II, is crystallized from the melt. The percentage content of form II' increases with pressure above 2 kbar, and that of form I' decreases up to 5 kbar. Upon heating under high pressure above 2 kbar, a solid-solid transition from form II' to form I' is observed in d.t.a. traces and the transition is confirmed by high-pressure X-ray diffraction. The melting temperature is expressed in the form of a quadratic equation as a function of pressure for four different forms in IPB-1.     

CO2-induced polymorphous phase transition of isotactic poly-1-butene with form III upon annealing

In-situ high-pressure FTIR was used to investigate the polymorphous phase transition of isotactic poly-1-butene (iPB-1) with form III upon annealing at temperatures ranging from 75 to 100 °C and CO₂ pressures ranging from 2 to 12 MPa. It was shown that the phase transition of form III changed from form III to II not through form III to I′ with increasing temperature and application of CO₂ increased the content of generated form I′. Wide-angle X-ray diffraction (WAXD) measurement on the annealed iPB-1 with form III verified the phase transition of form III. The crystalline morphology of the annealed iPB-1 films was investigated using polarized optical microscopy (POM). The results implied that the phase transition of form III to I′ might process via a solid-solid transition, which did not affect the orientation of the lamellar stacks. The orientation of form II lamellar stacks depended strongly on the formation process. To obtain strong orientation, the formation process displayed the following order: melt crystallization at ambient condition > melt recrystallization under CO₂ > phase transition upon annealing at ambient condition. Avrami equation could be well established to describe the phase transition of form III to I′ through a solid-solid phase transition.     

Kinetics of the melt – Form II phase transition in isotactic random butene-1/ethylene copolymers

The kinetics of formation of the Form II mesophase from the melt has been investigated as a function of the concentration of ethylene chain defects in isotactic random butene-1/ethylene copolymers, using standard and fast scanning chip calorimetry. Presence of ethylene co-units in the butene-1 chain leads to a distinct reduction of the melt – Form II phase transformation rate which has been quantified by evaluation of the critical cooling rate to suppress ordering, and by isothermal analysis of half-times of Form II mesophase formation. For the first time, the temperature-dependence of the rate of Form II mesophase formation has been evaluated for butene-1/ethylene random copolymers and the butene-1 homopolymer. This study needs to be considered as a complementary addendum to former work about the Form II to Form I polymorphic transformation in isotactic random butene-1/ethylene copolymers.     

Rheology,polymorphic crystal transformation,thermal, and mechanical properties of long-chain branched isotactic poly(1-butene)

Effects of long-chain branches (LCBs) on the rheology, crystal polymorphism, polymorphic transformation, and corresponding thermal and mechanical properties at different crystallization conditions, of isotactic poly(1-butene) (iPB-1) are systematically studied. The complex viscosity decreases and tangent increases with the increase of LCB concentration, and they inversely correlate with gels. The low branched samples crystallize into pure Form II by compression molding and cooling the melt to room temperature at a low crystallization cooling rate, whereas the moderate-to-highly branched samples crystallize into mixtures of Forms II and III, with a 1–30% fraction of crystals of Form III. The transformation of Form II into Form I in low branched iPB-1 was not significantly decelerated at different crystallization cooling rates, which is important in thermoforming, foaming, and extrusion blowing processes. Upon heating, Form III in highly branched iPB-1 with gels does not cold-crystallize into Form II even at a low heating rate. The low-to-highly branched samples mainly in Form I exhibit high yield strength, high melting temperature, and lower ductility, while the highly branched iPB-1 containing gels and mixtures of Forms I, III, and I′ possess brittleness. Under stretching, Form III predominantly transforms into Form I via a solid–solid crystal transition. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48411.     

An AFM study on the structure and melting behavior of melt-crystallized isotactic poly(1-butene)

Spherulites with closely packed edge-on lamellae and lathlike flat-on crystals of melt crystallized isotactic poly(1-butene) in ultra-thin films at different temperatures were studied by AFM in taping mode. Starting from these different crystals, the melting processes of them after aging at room temperature for different periods of time were monitored. Through selective melting of the thermally less stable Form II crystals, the solid phase transformation of iPB-1 from Form II to I was discussed. Based on the obtained results, it is concluded that nucleation of the iPB-1 stable Form I crystals is the rate-determining step of the Form II to I conversion. Moreover, from the facts that (i) nucleation of the stable Form I crystals starts most likely at crystalline side surfaces or corners, and (ii) the phase conversion rate of the melt grown flat-on crystals is much faster than that of the solution grown single crystals, we suggest that residual local thermal stresses exist at the edges of the microcrystallites and stacking regularity of the crystalline lamellae play a very important role in generating the nuclei of the iPB-1 Form I crystals.     

Structural changes in the melt spinning,cold drawing,and annealing of poly(4‐methylpentene‐1) fibers

Isotactic poly(4‐methylpentene‐1) melt‐spun fibers were investigated. Prior investigators of melt‐spun fibers found that these fibers have a tetragonal unit cell (Form I). We obtained the same unit cell structure in melt‐spun fibers. We found that higher draw‐down‐ratio fibers had d‐spacings closer to the previously cited values of Form I. We also found that cold‐drawn fibers had similar values to those of melt‐spun fibers. However, after these were annealed at 200°C, the unit cell was changed. It is possible that this new unit cell was the orthorhombic form of He and Porter. We also observed the birefringence of these fibers. The values changed after the melt‐spun fibers were cold drawn and annealed. The melt‐spun fiber values reached 0.006. The values for the drawn fibers were as high as 0.007. We suggest that the intrinsic birefringence is about 0.0075. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 130–137, 2005     

Zur Umwandlung der polymorphen Struktur von Polybuten-1 unter der Wirkung mechanischer Spannungen

Polybutene-1 crystallizes in the tetragonal form II during cooling of the melt. This form II is unstable and slowly transforms into the stable hexagonal form I. The rate of this crystal-crystal-transition can be considerably increased by applying mechanical stress to the sample. The effect of uniaxial tensile stresses on the II to I phase transition has been studied in a broad temperature range from ?196°C up to just below the melting point of the form II (ca. 115°C). The results can be summarized as follows:

• 1 Below the freezing temperature of the γ-process (?150°C) no Crystalline transformation can be observed. The polymer shows an “ideal” brittle fracture.
• 2 There is, furthermore a (macroscopic) brittle-like fracture at higher tempera-tures between ?150°C and ?70°C. On the fracture surface itself, however, a partial transformation from modification II to modification I has taken place during fracture. This can be considered as an evidence for a microscopic ductile fracture process.
• 3 Above ?70°C up to ?20°C necking occurs during stretching. Within the neck an almost quantitative transformation from I1 to I has taken place.
• 4 A t temperatures above the glass transition up to about 70°C a macroscopic homogeneous deformation of the samples is found. The amount of transformed material shows a complicated temperature dependence which can be explained by cooperative effects between the crystal-crystal transformation and the molecular mobility within the amorpheous regions on the basis of the relaxation behavior of this material.

     

The II-I crystal transformation of poly(vinylidene fluoride) under tensile and compressional stresses

The mechanism for a crystal transformation in poly(vinylidene fluoride) by a tensile deformation at atmospheric pressure was investigated in the temperature range 25–150 °C. Simultaneous X-ray and stress-strain relationship measurements during the drawing process revealed that the crystal transformation from Form II to Form I occurred at the temperatures below 130°C where the sample deformed by cold-drawing, and always initiated at the deformation stage where necking was completed at the centre of the tensile sample. Above 140°C, on the other hand, the sample deformed uniformly without necking and did not perform the crystal transformation. Thus, it was suggested that a heterogeneous stress distribution in the sample played a critically important role in the crystal transformation phenomenon. A uniaxial compressional deformation also caused the crystal transformation from Form II to Form I in this sample. The crystal conversion ratio varied with the conditions of compressional pressure and temperature, and the molecular orientation in resultant samples depended on the shapes of the anvils employed in the compression experiment. By applying a high d.c. voltage during the compressional deformation, a highly uniaxially oriented Form I film with prominent piezoelectric properties was obtained.     

Stability of form II of syndiotactic polypropylene confirmed by cold and melt crystallization in supercritical carbon dioxide

The form II of syndiotactic polypropylene (sPP) has been found more thermodynamically stable than form I when melt crystallized at pressures above 150 MPa, while the reverse occurs below 150 MPa. In the present study, through the cold and melt crystallization in supercritical CO₂ the stability of various polymorphic forms of sPP, especially form II, was confirmed by using Fourier-transform infrared spectroscopy and wide-angle X-ray diffraction. Compared with the formation of pure form I at high temperatures under ambient condition, a mixture of forms I and II was formed by both the cold and melt crystallization in supercritical CO₂. This atmosphere changed the relative stability of forms I and II, and made the form II more thermodynamically stable than form I. The increased solubility parameters of the surroundings, at which the form II was formed, also confirmed the stability of form II over form I in supercritical CO₂. The incubation pressure was the key factor affecting the formation and amount of form II. Supercritical CO₂ provides a combining severe condition to obtain the form II crystal, although its pressure was much lower than the elevated pressures (>150 MPa) reported before.     

Antioxidants and stabilizers

Summary The efficiency of polyolefine melt stabilizer 2, 2-thiobis-(4,6-di-tert.butylphenol) (I) as a hydroperoxide decomposing antioxidant was studied under model conditions.tert.Butylhydroperoxide was used to simulate the reaction with polypropylene hydroperoxide. The reaction was performed at 85°C in chlorobenzene solution. Decomposition of tert.butylhydroperoxide by I and formation of the sulphoxide II and the sulphone III from sulphide I were followed quantitatively. Formation of small amounts of effective peroxidolytic species from II or III, responsible for acceleration of hydroperoxide decomposition was considered.     

Conformation transition and molecular mobility of isolated poly(ethylene oxide) chains confined in urea nanochannels

Inclusion compounds formed from host small molecules and guest polymers have provided a novel platform to study the behavior of isolated polymer chains confined in nanochannels. In this article, the PEO chain conformation in the metastable poly(ethylene oxide) (PEO)-urea inclusion compound (IC) and its transition was characterized via a combination of different analytical methods. Based on the FTIR and Raman spectroscopy results, PEO chains in the metastable tetragonal IC are tentatively assigned to the tgg′ conformation. The structural changes of the metastable tetragonal IC to the stable trigonal form were observed via in situ FTIR and ex situ WAXD. The transformation is a kinetic solid-solid process and can even occur at room temperature. The activation energy of about 222 kJ/mol indicates that the transition occurred via cooperative disruption of several hydrogen bonds. Measurement of the laboratory frame spin-lattice relaxation time T₁ (¹³C) shows that molecular motions of the nanoconfined PEO chains are more intensive than the neat crystalline PEO but weaker than those of the neat amorphous PEO. Second harmonic generation microscopy demonstrates that the trigonal IC exhibits stronger nonlinear optical activity than the tetragonal IC. The intermolecular hydrogen bonding is attributed to the driving force for the transformation of the metastable tetragonal IC into the stable trigonal form.     

Ceria-doped zirconia-graphite as possible refractory for tundish nozzles in steelmaking

In zirconia-graphite refractories, calcia-stabilized zirconia is sensitive to silica coming from oxidized additives (Si, SiC). Accordingly, ceria is considered here as an alternative cheap stabilizer.

Ceria-doped zirconia has been prepared and the monoclinic/tetragonal distribution has been analysed. At 1500°C in oxygen, the solution of ceria in zirconia is sluggish; to obtain a pure tetragonal phase, a 16 mol% ceria concentration (equal to the solubility limit) is necessary. Such samples are stable below 1050°C, in spite of previous claims to the contrary.

At 1300°C, silica does not destabilize these samples, even in reducing conditions. However, at low oxygen pressure, the pyrochlore Ce₂Zr₂O₇ is formed, and the remaining zirconia transforms into the monoclinic modification. Nevertheless, below 1175°C, a metastable, purely tetragonal modification is obtained, because the activation energy needed for the formation of Ce₂Zr₂O₇ is too high.

In any case, the reduction does not destroy the polycrystalline texture. This fact, combined with the resistance to silica, favours the use of these materials in steelmaking.     

Pressure-Temperature Phase Diagram of Zirconia

The pressure-temperature phase diagram of zirconia was determined by optical microscopy and X-ray diffraction techniques using a diamond anvil pressure cell. At room temperature, monoclinic ZrO₂ transforms to a tetragonal phase ( t II) which is related to the high-temperature tetragonal structure ( t I). The transformation pressure exhibits hysteresis and is cycle dependent. At room temperature, the initial transformation pressure for the monoclinic- t II transition on a virgin monoclinic crystal can be as high as 4.4 GPa; on subsequent cycling the transition pressure ultimately lowers to 3.29 ± 0.06 GPa. The pressure for the reverse transition is essentially constant at 2.75 ± 0.06 GPa. At pressures > 16.6 GPa, the t II form transforms to the orthorhombic cotunnite (PbCl₂) structure. With increasing temperature, the t II form transforms to the high-temperature tetragonal phase. For increasing P and T , the monoclinic- t I- t II triple point is located at T = 596°± 18°C and P = 2.26 ± 0.28 GPa, whereas for decreasing P and T , the triple point is found at T = 535°± 25°C and P = 1.7 ± 0.28 GPa.     

Direct melt-crystallization of isotactic poly-1-butene with form I′ using high-pressure CO2

In this work, we found a new method to obtain isotactic poly-1-butene (iPB-1) with form I′ through direct melt-crystallization using high-pressure CO₂. The non-isothermal melt-crystallization behaviors of iPB-1 under atmospheric N₂ and 0.5-10 MPa CO₂ at cooling rates ranging from 0.25 to 5 °C/min were carefully studied using high-pressure differential scanning calorimeter (DSC) and analyzed using the modified Avrami method. Wide-angle X-ray diffraction (WAXD) measurements showed that the crystal structure of non-isothermally melt-crystallized iPB-1 changed from form II under atmospheric N₂ and 0.5-8 MPa CO₂ to form I′ under 10 MPa CO₂. In-situ high-pressure Fourier transform infrared (FTIR) was also used to investigate the non-isothermal melt-crystallization at CO₂ pressure up to 18 MPa at the cooling rate of 1 °C/min. Likewise, it was found that form II crystallized under atmospheric N₂ and 0.5-8 MPa CO₂, and form I′ melt-crystallized directly at CO₂ pressures higher than 10 MPa, which was confirmed by the followed DSC and WAXD characterizations on the iPB-1 films after FTIR measurements. The crystal morphology of the melt-crystallized iPB-1 films, characterized by using polarized optical microscopy (POM), showed that the Maltese cross pattern of iPB-1 spherulite became more diffuse with increasing CO₂ pressure, and the spherulite size decreased abruptly at the CO₂ pressure of 10 MPa.     

A hot melt adhesive from polyester waste

Hot melt adhesives were prepared from polyester waste in three steps: Step I: Polyester waste was degraded to a monomer stage, i.e. bis-(2-hydroxyethyl)terephthalate (BHET) (1) by means of different metal acetates in the presence of ethylene glycol. The reaction in the presence of zinc acetate gave the highest yield of BHET (95%). Step II: Esterification of (I) with isophthalic acid in situ led to the formation of an esterified product (II). Step III: Further esterification of (II) with sebacic acid in situ, followed by polycondensation in the presence of antimony oxide and triphenylphosphate, led to the formation of the hot melt adhesive (III) based on BHET via the intermediate esterified product (IV). The hot melt adhesive, possessing a highest adhesive strength of 620 (psi) was obtained in a total reaction period of five hours. The present paper also details the effect of isophthalic acid and sebacic acid on the adhesive strength of (III).     

Electrochemical extraction of samarium from molten chlorides in pyrochemical processes

This work concerns the electrochemical extraction of samarium from molten chlorides. In this way, the electrochemical behaviour of samarium ions has been investigated in the eutectic LiCl–KCl at the surface of tungsten, aluminium and aluminium coated tungsten electrodes. On a W inert electrode the electro-reduction of Sm(III) takes place in only one soluble–soluble electrochemical step Sm(III)/Sm(II). The electrochemical system Sm(II)/Sm(0) has not been observed within the electrochemical window, because of the prior reduction of Li(I) ions from the solvent, which inhibits the electro-extraction of Sm species from the salt on such a substrate. Sm metal in contact with the melt react to give Li(0) according to the reaction: Sm(0) + 2Li(I) ↔ Sm(II) + 2Li(0).On the contrary, on reactive Al electrodes the electrochemical system Sm(II)/Sm(0) was observed within the electroactive range. The potential shift of the redox couple is caused by the decrease of Sm activity in the metal phase due to the formation of Sm–Al alloys at the interface. The formation mechanism of the intermetallic compounds was studied in a melt containing: (i) both Sm(III) and Al(III) ions, using W and Al coated tungsten electrodes, and (ii) Sm(III) ions using an Al electrode. Analysis of the samples after potentiostatic electrolysis by X-ray diffraction and scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS), allowed the identification of Al₃Sm and Al₂Sm.     

Crystallization kinetics and morphology of poly(trimethylene terephthalate)

In this work, the isothermal crystallization kinetics of polytrimethylene terephthalate (PTT) was first investigated from two temperature limits of melt and glass states. For the isothermal melt crystallization, the values of Avrami exponent varied between 2 and 3 with changing crystallization temperature, indicating the mixed growth and nucleation mechanisms. Meanwhile, the cold crystallization with an Avrami exponent of 5 indicated a character of three-dimensional solid sheaf growth with athermal nucleation. Through the analysis of secondary nucleation theory, the classical regime I→II and regime II→III transitions occurred at the temperatures of 488 and 468 K, respectively. The average work of chain folding for nucleation was ca. 6.5 kcal mol⁻¹, and the maximum crystallization rate was found to be located at ca. 415 K. The crystallite morphologies of PTT from melt and cold crystallization exhibited typical negative spherulite and sheaf-like crystallite, respectively. Moreover, the regime I→II→III transition was accompanied by a morphological transition from axialite-like or elliptical-shaped structure to banded spherulite and then non-banded spherulite, indicating that the formation of banded spherulite is very sensitive to regime behavior of nucleation.     

Crystallization of nanocomposites of an isotactic random butene-1/ethylene copolymer and layered double hydroxide

The effect of the presence of a layered double hydroxide (LDH) nanofiller on the crystallization behavior of a random isotactic butene-1/ethylene copolymer was investigated. Addition of LDH enhanced heterogeneous nucleation of the ordering process of the polymer matrix leading to an increase of the temperature of formation of the Form II mesophase on cooling the melt. Consequently, the size of spherulites of the polymer matrix was markedly reduced in the nanocomposites. In contrast, the Form II mesophase–Form I crystal phase transition kinetics and the final crystallinity were not affected by the presence of LDH. Addition of the LDH nanofiller led to a beneficial increase of the stiffness which suggests a route for compensating of the lower stiffness of the random copolymer compared to the homopolymer. Random copolymerization accelerates the disadvantageous room-temperature mesophase–crystal transition, but results in a reduction of the crystallinity. The addition of LDH counterbalances the lowering of the crystal fraction.     

Crystalline transformation of isotactic polybutene-1 in supercritical CO2 studied by in-situ fourier transform infrared spectroscopy

The solid-solid crystalline transformation of isotactic polybutene-1 (iPB-1) from tetragonal form II to hexagonal form I could be accelerated by supercritical carbon dioxide (scCO₂). In this study, in-situ Fourier transform infrared spectroscopy (FTIR) and two dimensional correlation spectroscopy (2DIR) is used to observe and investigate the crystallization behaviour of iPB in scCO₂ and compressed CO₂. Based on the transform sequence given by 2DIR analysis, this transformation of helical chain structures is found to be initiated with the motion of side chains and followed by the movement of main chains. It is speculated that the motion of polymer chains was enhanced with the diffusion of CO₂. Also this crystalline transition is observed even in compressed CO₂, suggesting that CO₂ could also diffused into polymer under high pressure near the critical pressure. This diffusion of CO₂ is indicated by the growth of IR bands being assigned to the stretching vibration of C–O. A further investigation on the mechanically heating and freely cooling of iPB provides more evidences on the process of structure transition. The result implies that the nucleus of tetragonal form II formed in the melt is not affected by the existence of scCO₂, but the crystallization temperature become obviously lower.     

Estimation of thermal transitions in poly(ethylene naphthalate): Experiments and modeling using isoconversional methods

Thermal transitions of PEN, such as the glass transition temperature and those occurring during isothermal or nonisothermal crystallization were investigated based on careful experiments and modeling with isoconversional methods. The latter was applied to DSC data to determine the effective activation energy for the glass transition in PEN. Using the same data and different thermal methods the dynamic fragility of PEN was evaluated. The Lauritzen-Hoffman (LH) parameters K_g and U^∗ were estimated using the secondary nucleation theory from both PLM and isothermal DSC after self-nucleation measurements. Regime II to I and III to II transition at about 253 °C and 243 °C were concluded. Elliptical-shaped hedrite-like morphology was observed above 253 °C. Finally, isoconversional analysis was applied to both melt and glass non-isothermal crystallization data and the combined set of activation energies was found to be described by the theoretical Vyazovkin-Sbirrazzuoli equation using a single set of LH parameters coming from PLM measurements.