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.     

Influence of Temperature and Time on Isothermal Crystallization of Poly-L-lactide

The thermal behavior of poly-L-lactide (PLLA) isothermal crystallization upon cooling from the melt was investigated using differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and polarizing microscope (POM) by changing the crystallization temperature and time. It was indicated that 110°C should be a critical temperature for PLLA melting crystallization. The melting point of crystallized PLLA discontinuously changed with crystallization temperature, increased with temperature, but decreased at about 110°C, and thereafter again increased with higher crystallization temperatures. At 110°C a multiple endothermic peak was observed. PLLA crystals of higher perfection form when crystallized under higher temperature, which reflects the effects of high chain mobility in higher temperatures. During isothermal crystallization, PLLA crystallites become increasingly perfect, and thicken with prolonged time, leading to an increasing melting point.     

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.     

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.     

Crystallization of polyamides under elevated pressure: 2. Pressure-induced crystallization of nylon-6 (polycapramide) from the melt

A study has been made on the crystallization of nylon-6 from the melt under elevated pressures. Crystallization induced by pressures of up to 8 kbar at temperatures between 270° and 310°C did not lead to a significant increase of the melting temperature for nylon-6 containing 8% caprolactam. However, the melting peak temperature, as determined by differential scanning calorimetry was found to increase from 220° to 250°C for nylon-6 without caprolactam and crystallized under pressures exceeding 5 kbar for 50 h. The heat of melting of the nylon specimen crystallized under these conditions increased from 14 to 37 cal/g. Thermal decomposition of the polymer could be diminished by heating under pressure and extruding the nylon under vacuum prior to the high pressure crystallization experiments. The specific volume diminished gradually during isothermal crystallization and the melting temperature was found to increase with crystallization time. These observations point to a one stage process for the development of extended-chain crystals of nylon-6. The highest melting peak temperature of 256°C was recorded on nylon-6 which was crystallized at 315°C and 8 kbar for a period of 320 h.     

Polymorphic behavior of some fully hydrogenated oils and their mixtures with liquid oil

Fully hydrogenated soybean oil, beef fat, rapeseed oil, a rapeseed, palm and soybean oil blend, cottonseed oil and palm oil were characterized by fatty acid composition, glyceride carbon number and partial glyceride content, as well as melting and crystallization properties. The latter were established by differential scanning calorimetry. Polymorphic behavior was analyzed by X-ray diffraction of the products in the flake or granulated form and when freshly crystallized from a melt. The hard fats were dissolved in canola oil at levels of 20, 50 and 80% and crystallized from the melt. Palm oil had the lowest crystallization temperature and the lowest melting temperature; rapessed had the highest crystallization temperature and soybean the highest melting temperature. All of the hard fats crystallized initially in the =00 form. When diluted with canola oil, only palm oil was able to maintain β′ stability.     

A study of the crystalline transitions of polyamides X 18

The crystalline transition behavior of polyamides X 18, where X = 2, 4, 6, 8, 10 and 12, has been studied by variable‐temperature wide‐angle X‐ray diffraction (WAXD) and real‐time Fourier‐transform infrared (FTIR) spectroscopy. It was found that polyamides 2 18 and 4 18 undergo the Brill transition on heating for both the melt‐crystallized samples and the solution‐crystallized samples. For polyamide 6 18, the melt‐crystallized sample undergoes Brill transition before melting but the solution‐crystallized sample melts prior to the complete crystalline transition. Polyamides 8 18, 10 18 and 12 18 do not undergo the Brill transition perfectly in advance of melting for either melt‐crystallized or solution‐crystallized samples. Real‐time FTIR analysis of polyamides X 18 provided helpful information for understanding the mechanism of the crystalline transition. Copyright © 2004 Society of Chemical Industry     

Crystallization of polyamides under elevated pressure: 4. Annealing of nylon-6 (polycapramide) under pressure

A study has been made on the annealing of nylon-6 under elevated pressure. Heat treatment of meltcrystallized nylon-6 at 6.5 kbar and 20°C below the beginning of melting for a period of 120 h yielded an increase in the heat of fusion from 14.2 to 41.2 cal/g and an increase in atmospheric melting temperature from 222° to 256°C (1 kbar = 100 MN/m²; 1 cal/g = 4.187 kJ/kg). Stepwise annealing by exposing nylon-6 to progressively higher temperatures at 6.5 kbar led to a heat fusion of 40.8 cal/g and a melting temperature of 269°C. Annealing was found to be particularly effective in improving the crystalline structure at pressures exceeding 4 kbar. The rate of annealing at 6.5 kbar increased with temperature in the range between 260° and 280°C. Electron microscopy of fracture surfaces disclosed that annealing could give rise to a marked increase in lamellar thickness. Wide-angle X-ray diffraction showed that crystal growth also occurred in the lateral direction and that the alpha-crystalline modification was preserved during annealing. From a comparison between the melting characteristics of nylon-6 obtained by pressure-induced crystallization from the melt and by annealing under pressure of folded-chain material, it is inferred that the folded-chain lamellar state may be an essential intermediate stage of the chain extension in polyamides under pressure.     

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.     

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.

     

Structure of syndiotactic propylene-ethylene copolymers: Effect of the presence of ethylene units on the structural transitions during plastic deformation and annealing of syndiotactic polypropylene

Syndiotactic propylene-ethylene copolymers have been synthesized with a single-center C_s-symmetric syndiospecific metallocene catalyst. A study of the effect of the presence of ethylene comonomeric units on the polymorphic behavior of syndiotactic polypropylene (sPP) and on the structural transitions occurring during stretching is reported. For copolymer samples with low ethylene contents, in the range 2-7 mol%, crystals of the helical form I, present in the melt-crystallized samples, transform into the trans-planar form III by stretching at high deformation. Form III transforms in part into the helical form II by releasing the tension, as it occurs for sPP. Samples with ethylene contents in the range 8-10 mol% are crystallized from the melt as a mixture of crystals of helical form I and form II. Both helical forms transform by stretching at low values of deformation (lower than 300%) into the trans-planar mesomorphic form, which transforms into the trans-planar form III by further stretching at higher deformations (higher than 500%). For these samples form III transforms back into the mesomorphic form, rather than into the helical forms, by releasing the tension. Unoriented samples of copolymers with ethylene content in the range 13-18 mol% are mainly crystallized in the helical form II, which transforms into the trans-planar mesomorphic form by stretching. Upon releasing the tension the mesomorphic form remains stable and no polymorphic transition is observed. The presence of ethylene comonomeric units stabilizes the trans-planar forms in fibers of the copolymer samples. This has been confirmed by the result that for high ethylene contents the trans-planar form III and the mesomorphic form do not transform in helical forms by annealing of fibers stretched at high deformations.     

Crystallization of polyamides under elevated pressure: 5.Pressure-induced crystallization from the melt and annealing of folded-chain crystals of nylon-11, poly(aminoundecaneamide) under pressure

High pressure dilatometry, differential scanning calorimetry, electron microscopy, X-ray diffraction, and infra-red spectroscopy to study how the crystallization of nylon-11 from the melt, as well as annealing of the folded-chain crystals, are affected by pressure in the range from 1 to 10 kbar (1 kbar = 100 MN/m²) and temperature in the range from 200° to 320°C. Pressures exceeding 3 kbar and temperatures higher than 230°C are sufficient for growth of the chain-extended crystals of nylon-11 either by pressure-induced crystallization from the melt or by annealing of the folded-chain crystals. Crystallization from the melt or annealing at 320°C or higher, and 10 kbar, resulted in crosslinking of the polymer. The highest melting temperature and heat of melting found for the chain-extended crystals of nylon-11 were 226°C and 35 cal/g respectively, as compared to 190°C and 13.6 cal/g for the folded-chain material. The texture of the chain-extended crystals of nylon-11 was found to be spherulitic with well developed striations forming circle patterns, and polymer chains passing through several lamellae. No sharp boundaries were found between the chain-extended lamellae. The alpha-crystalline modification, found for the folded-chain crystals of nylon-11, was preserved in the high pressure crystallization and annealing experiments. Infra-red absorption bands at 1420 and 1225 cm^?1 seem to be associated with the presence of folds in the nylon-11 crystals. It is suggested that, during the initial stage of crystallization under pressure, folded-chain crystals are formed, with a crystalline order and long spacing larger than that of the starting nylon-11.     

Crystallization of polyamides under elevated pressure: 6.Pressure-induced crystallization from the melt and annealing of folded-chain crystals of nylon-12, polylaurolactam under pressure

The influence of pressure on the crystallization and annealing of polylaurolactam, nylon-12, has been investigated. The increase of the final melting temperature of this polyamide with pressure amounted to 20°C per kbar as determined by high pressure dilatometry. Crystallization as well as annealing under pressure led to a partial transformation of the pseudo-hexagonal or monoclinic crystal structure to an alpha modification. Samples crystallized at a pressure of 4.9 kbar (1 kbar = 100 MN/m²) displayed multiple melting behaviour, whereas annealing under pressure gave rise to one melting peak in the d.s.c. thermograms. The heat of fusion could be enhanced from 16 to 32 cal/g and the melting peak temperature could be increased from 179° to 209°C by annealing under 4.9 kbar and 260°C for 336 h. Small-angle X-ray scattering curves reveal that annealing brings about considerable broadening of the distribution of the crystal dimensions. The pressure treated nylon-12, consisting of well developed spherulites, could be fractured very easily along inter-spherulitic and trans-spherulitic planes. The striations in the fracture surfaces as observed in the electron microscope were arranged perpendicular to the radial arms of the spherulites. Annealing at 320°C and 10 kbar for 48 h caused efficient crosslinking of the polylaurolactam.     

Cyclic diamides with (CH2) x segments: Phase transitions in the solid state

Summary Cyclic diamides with various CH₂ segment lengths were synthesized. Cyclo-(octamethylene sebacamide) I, x=8, was investigated as a model system by means of calorimetry, X-ray diffraction and Raman spectroscopy with special emphasis on its phase behaviour. In the low temperature phase the molecules exist as collapsed rings where the amide groups are located in the folds and the straight stems consist of nondisordered CH₂ segments. During the solid phase transition at about 84°C a strong increase of conformational disorder (4 additional gauche defects per molecule) occurs which does not change significantly during subsequent melting. It is shown that the conformation of the molecules in the high temperature phase is similar to that in the melt (liquid-like). In spite of conformational disorder the linewidth of the X-ray reflection peaks is smaller in the high temperature phase compared to that in the solution crystallized phase.     

Multiple melting behavior of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) investigated by differential scanning calorimetry and infrared spectroscopy

The multiple melting behavior of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (P(HB-co-HHx)) (HHx = 12 mol%) isothermally crystallized from the melt state has been characterized by differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy. The influence of different experimental variables (such as crystallization temperature, time, and heating rate) on the multiple melting behavior of P(HB-co-HHx) was investigated by using DSC. Moreover, it has been further examined by monitoring intensity changes of the characteristic IR bands during the subsequent heating process. For the isothermally crystallized P(HB-co-HHx) samples, triple melting peaks were observed upon heating. The weak lowest-temperature DSC endotherm I always appears at the position just above the crystallization temperature, and shifts to a higher temperature linearly with the logarithm of the crystallization time. The combination of DSC and IR results suggested that the occurrence of peak I was a result of the melting of crystals formed upon long-time annealing. As for the other two main melting endothermic peaks, endotherm II corresponds to the melting of crystals formed during the primary crystallization, and endotherm III is ascribed to the melting peak of the crystals formed by recrystallization during the heating process.     

Crystallization from the melt of α and β forms of syndiotactic polystyrene

The crystallization of trans-planar α and β forms of syndiotactic polystyrene is studied through X-ray diffraction and DSC analyses of melt-crystallized samples. The factors controlling the crystallization of the two forms are analyzed. Pure α and β forms of syndiotactic polystyrene can be easily obtained setting the maximum temperature at which the melt is heated and the permanence time of the melt at this temperature. The crystallization of the α and β forms does not depend on the crystallization temperature, at least in the range of accessible crystallization temperatures, between 240 and 270 °C, but only depends on the presence of the ‘memory’ of the α form in the melt. The most important factors are, indeed, the crystalline form of the starting material used in the melt crystallization experiments and the maximum temperature of the melt. Relevant recrystallization phenomena, occurring during the melting of the samples crystallized from the melt at low crystallization temperatures, are responsible for the complex melting behavior of the α and β forms. The recrystallization involves only lamellar thickening of the crystals of the same form (α or β) and not structural transformation.     

Processing characteristics and structure development in solid-state extrusion of a new semicrystalline polyimide (BTDA–DMDA)

In this article, we present detailed processing characteristics and structure development in a thermoplastic polyimide BTDA–DMDA in the solid-state extrusion process. This fully imidized polyimide polymer is known to crosslink at fast rates when it is brought to a molten phase even for short periods of time. This characteristic makes it difficult to process it in the molten phase and attempts at melt processing result in melt fracture and highly distorted extrudates. However, this polymer can be shaped into high-quality extrudates when it is processed below its melting temperature directly from its postpolymerization powdered state. The solid-state extrusion of precompacted BTDA–DMDA powder was studied in the temperature range from 250 to 320°C. At the temperatures from 290 to 320°C, high-quality extrudates were obtained. Below 290°C, solid-state extrusion was not possible due to the limitation of the load cell capacity of the capillary rheometer used in this research. Above 320°C, the extrudates were found to be of poor quality as a result of degradation and crosslinking in the molten phase. Structural characteristics of the samples produced by solid-state extrusion was investigated by the microbeam X-ray diffraction technique. The thermal behavior of the extrudates was also characterized by differential scanning calorimetry (DSC). The DSC results show that at low extrusion temperatures the samples exhibit dual endothermic peaks and are highly crystalline in an extruded state. The higher melting peak located at about 350°C is due to the melting of the new crystalline phase that has developed partially during the solid-state extrusion process and partially during the recrystallization process that takes place at temperatures at and slightly above the primary melting process during the DSC heating scan. This has been confirmed by DSC, depolarized light hot-stage video microscopy, and wide-angle X-ray diffraction studies. The long spacing of the higher melting crystals was found to be much larger than that of the lower melting crystals, as evidenced by the small angle X-ray scattering studies. © 1995 John Wiley & Sons, Inc.     

A unique crystallization and double melting behavior of a polyimide derived from 3,3′,4,4′-biphenyltetracarboxylic dianhydride and 1,4-bis(3-aminopropyl)piperazine

A unique crystallization and melting behavior of a novel semicrystalline polyimide derived from 3,3′,4,4′-biphenyltetracarboxylic dianhydride and 1,4-bis(3-aminopropyl)piperazine were studied by differential scanning calorimetry (DSC) with and without temperature modulation and wide-angle X-ray diffraction (WAXD). Polymer samples isolated from a chloroform solution showed melting transitions in the DSC. However, WAXD traces showed crystallinity only after annealing above the glass transition, for about 2 h. For samples crystallized from the melt, crystallization could be achieved only in a narrow crystallization range of 200-220 °C, after 10 h. A maximum crystallinity of this polyimide was found to be 30%. Two distinct melting transitions were observed by DSC, which could be explained using a partial disordering—reorganization—final melting model.     

Melting and crystallization behavior of poly(trimethylene 2,6-naphthalate)

The melting and crystallization behavior of poly(trimethylene 2,6-naphthalate) (PTN) are investigated by using the conventional DSC, the temperature-modulated DSC (TMDSC), wide angle X-ray diffraction (WAXD) and polarized light microscopy. It is observed that PTN has two polymorphs (α- and β-form) depending upon the crystallization temperature. The α-form crystals develop at the crystallization temperature below 140 °C while β-form crystals develop above 160 °C. Both α- and β-form crystals coexist in the samples crystallized isothermally at the temperature between 140 and 160 °C. When complex multiple melting peaks of PTN are analyzed using the conventional DSC, TMDSC and WAXD, it is found that those arise from the combined mechanism of the existence of different crystal structures, the dual lamellar population, and melting-recrystallization-remelting. The equilibrium melting temperatures of PTN α- and β-form crystals determined by the Hoffman-Weeks method are 197 and 223 °C, respectively. When the spherulitic growth kinetics is analyzed using the Lauritzen-Hoffmann theory of secondary crystallization, the transition temperature of melt crystallization between regime II and III for the β-form crystals is observed at 178 °C. Another transition is observed at 154 °C, where the crystal transformation from α- to β-form occurs.