Crystalline and supermolecular structure evolution of poly(ethylene terephthalate) during isothermal crystallization and annealing treatment by means of wide and small angle X-ray investigations

Small-angle X-ray scattering, wide-angle X-ray diffraction and differential scanning calorimetry analysis were carried out to evaluate the evolution of the supermolecular structure of poly(ethylene terephthalate) (PET) during isothermal crystallization and annealing process. PET was crystallized from the melt by isothermal treatments at 226 °C. Partially crystallized samples were prepared interrupting the crystallization by quenching, while prolonged treatments were performed to prepare annealed samples. The adopted crystallization procedures allowed to form crystals which developed during primary and secondary crystallization, and the annealing process. On the basis of X-ray data, the lamellar and amorphous phases were unambiguously attributed. The lamellar thickness and the crystallinity progressively enhance with increasing the time of thermal treatment; on the contrary, the long period decreases and this effect is mainly due to the contraction of the amorphous phase. The melting behaviour of the annealed samples indicates that the heating-induced crystal reorganization phenomena are inconsistent. The interdependency between the melting temperature and the crystal thickness allowed to extrapolate the equilibrium melting temperature.     

Evaluation of the physical stability and local crystallization of amorphous terfenadine using XRD-DSC and micro-TA

It is very difficult to follow rapid changes in polymorphic transformation and crystallization and to estimate the species recrystallized from the amorphous form. The aim of this study was to clarify the structural changes of amorphous terfenadine and to evaluate the polymorphs crystallized from amorphous samples using XRD-DSC and an atomic force microscope with a thermal probe (micro-TA). Amorphous samples were prepared by grinding or rapid cooling of the melt. The rapid structural transitions of samples were followed by the XRD-DSC system. On the DSC trace of the quenched terfenadine, two exotherms were observed, while only one exothermic peak was observed in the DSC scan of a ground sample. From the in situ data obtained by the XRD-DSC system, the stable form of terfenadine was recrystallized during heating of the ground amorphous sample, whereas the metastable form was recrystallized from the quenched amorphous sample and the crystallized polymorph changed to the stable form. Obtained data suggested that recrystallized species could be related to the homogeneity of samples. When the stored sample surface was scanned by atomic force microscopy (AFM), heterogeneous crystallization was observed. By using micro-TA, melting temperatures at various points were measured, and polymorph forms I and II were crystallized in each region. The percentages of the crystallized form I stored at 120 and 135 °C were 47 and 79%, respectively. This result suggested that increasing the storage temperature increased the crystallization of form I, the stable form, confirming the temperature dependency of the crystallized form. The crystallization behavior of amorphous drug was affected by the annealing temperature. Micro-TA would be useful for detecting the inhomogeneities in polymorphs crystallized from amorphous drug.     

Thermal analysis of poly(2-methylpentamethylene terephthalamide)

Poly(2-methylpentamethylene terephthalamide) (Nylon M5T) is a new high temperature aromatic polyamide developed by Hoechst Celanese. In this paper thermal properties of Nylon M5T chips, as well as as-spun and drawn fibers were studied by DSC, DMA, hot stage microscopy and WAXS.T _g of the fully amorphous Nylon M5T is 143°C when measured by DSC;T _g increases with crystallinity to 151°C. The temperature dependence of the solid and melt specific heat capacities has also been determined. The heat capacity increase at the glass transition of the amorphous polymer is 103.9 J °C^–1 mol^–1.T _g by DMA for the as-spun fiber is 155°C, for a drawn fiber is 180°C. Three secondary transitions were observed by DMA in addition to the glass transition. These correspond to a local mode relaxation of the methylene groups at –120°C, onset of rotation of the amide-groups at –65°C and the onset of the rotation of the phenylenegroups (at 63°C). The crystallinity of Nylon M5T strongly depends on the rate of cooling from the melt. The isothermal crystallization data are melt temperature dependent: two-dimensional crystallization takes place when the samples are crystallized from higher melt temperatures, and this phase changes into a spherulitic structure during cooling to room temperature. Spherulitic crystallization occurs when lower melt temperatures are used. This polymer has three crystal forms as indicated by DSC, DMA and WAXS data. The crystal to crystal transitions are clearly visible when amorphous samples are heated in the DSC, or the DMA curves of as-spun fibers are recorded. It is experimentally shown that a considerable melting of the lower temperature crystal forms takes place during the crystal to crystal transitions. The equilibrium melting point as measured by the Hoffman-Weeks method, has been determined to be 339°C.Dedicated to Professor Bernhard Wunderlich on the occasion of his 65th birthday     

Synthesis and thermal decomposition of antimony(III) oxide hydroxide nitrate

The thermal decomposition of the only known antimony nitrate antimony(III) oxide hydroxide nitrate Sb₄O₄(OH)₂(NO₃)₂, whose synthesis routes were reviewed and optimized was followed by TG-DTA under an argon flow, from room temperature up to 750°C. Chemical analysis (for hydrogen and nitrogen) performed on samples treated at different temperatures showed that an amorphous oxide hydroxide nitrate appeared first at 175°C, and decomposed into an amorphous oxide nitrate above 500°C. Above 700°C, Sb₆O₁₃ and traces of -Sb₂O₄ crystallized.Author to whom all correspondence should be addressed     

Structural investigations of sol-gel-derived LiYF4 and LiGdF4 powders

A soft synthesis route based on the sol-gel process was used for preparing rare-earth tetrafluoride powders from alkoxide precursors. In-situ fluorination was performed by decomposition of a fluorine containing organic compound named 1,1,1-trifluoro-5-methyl-2,4-hexanedione when sintering the as-prepared xerogel to produce crystallized samples. Both to insure complete departure of organic residues as well as to avoid any oxidation into oxyfluoride, annealing treatment was carried out under fluorine atmosphere. Free-oxygen content of resulting samples was evidenced by infrared and Raman spectroscopies. X-ray absorption spectroscopies (XAS) and ¹⁹F nuclear magnetic resonance (NMR) studies showed that samples heat treated at 300 °C are already crystallized but for a full crystallization in LiGdF₄ and LiYF₄ a thermal treatment at 550 °C is needed. Temperature dependence of powder morphology was analyzed by scanning electron microscopy (SEM).     

DSC analysis of isothermal melt-crystallization, glass transition and melting behavior of poly(l-lactide) with different molecular weights

Isothermal melt-crystallization, glass transition and melting behavior of poly(l-lactide) (PLLA) with different molecular weights were investigated by using differential scanning calorimetry. Analysis by Avrami equation showed that crystallization was initiated by heterogeneous nucleation, followed by 3-dimensional growth. The maximum reciprocal half-time of crystallization (1/t_1/2) was detected at 105 °C. Double endothermic peaks were observed around the glass transition for PLLA with intermediate crystallinities, indicating the coexistence of bulk-like and confined amorphous regions. Double-melting behavior was analyzed and combined with the equilibrium melting temperature evaluation by non-linear Hoffman-Weeks extrapolation, from which a value of 207.6 °C was deduced for PLLA of infinite molecular weight. Lauritzen-Hoffman theory was employed to analyze the crystallization kinetics. Regime II-III transition was found to occur at 120 °C for PLLA of lower molecular weight. The crystal morphology was also examined by scanning electron microscopy through chemical etching method.     

Studies of structures of poly-ε-caprolactam/montmorillonite nanocomposite: Part 1. Tests of compression moulded specimens

A poly-ε-caprolactam (PA6) taken as a reference and its nanocomposite (PNC) containing 1.6 wt.% of montmorillonite were examined. The specimens as discs were prepared by compression moulding at 235 °C and 70 MPa. Using the novel version of TMA, in the PA6 and PNC specimens, within the temperature range from −100 to 250 °C, a semi-crystalline structure with anisotropy of distribution of the more ordered (crystalline?) portion was found in the surface layer up to 0.5 mm thick. The amorphous regions have differed in a state of order (different transition temperatures) and related compactness. The free volume fraction in amorphous regions determined in machine direction (normal to the surface) is increased when in transverse direction is reduced; simultaneously molecular weights of polymer chains between junctions were increased by incorporation of the nanofiller.     

Multiple melting behavior in isothermally crystallized poly(trimethylene terephthalate)

Melting behavior of poly(trimethylene terephthalate) (PTT) after isothermal crystallization from the melt state was studied using differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) techniques. The subsequent melting thermograms for PTT isothermally crystallized within the temperature range of 182-215 °C exhibited triple (for crystallization temperatures lower than ≈192 °C), double (for crystallization temperatures greater than ≈192 °C but lower than ≈210 °C), or single (for crystallization temperatures greater than ≈210 °C) endothermic melting phenomenon. These peaks were denoted peaks I, II, and III for low-, middle-, and high-temperature melting endotherms, respectively. For the triple melting phenomenon, it was postulated that the occurrence of peak I was a result of the melting of the primary crystallites, peak II was a result of the melting of recrystallized crystallites, and peak III was a result of the melting of the recrystallized crystallites of different stabilities. In addition, determination of the equilibrium melting temperature T_m⁰ for this PTT resin according to the linear and non-linear Hoffmann-Weeks extrapolation provided values of 243.6 and 277.6 °C, respectively.     

Analysis of thermally induced solid-solid interactions in vanadia-alumina system

V₂O₅/Al₂O₃ solids of varying compositions were prepared, dried at 100°C and calcined in air at 400–1000°C. The solid-solid interactions between the mixed oxides were investigated by means of DTA, TG and XRD techniques.The results revealed that ammonium metavanadate and aluminium hydroxide decomposed at 260 and 290°C, respectively, to yield an ammonium vanadium intermediate compound and Al₂O₃ as solids. The intermediate compound readily decomposed at 360°C to give V₂O₅.Solid V₂O₅ generally catalyses the crystallization of Al₂O₃ to an extent proportional to its amount present. The solid-solid interactions between Al₂O₃ and V₂O₃ to produce AlV₂O₄ and AlVO₄ took place at 750 and 900°C. These solids decomposed entirely at 1000°C, producing V₂O₅ and alpha-corundum. The pure Al₂O₃ samples employed existed as amorphous solids even when heated in air at 750°C, but in the presence of V₂O₅ (7–18 wt.%) they crystallized to thetaalumina at 600°C. The pure solid alumina crystallized at 1000°C to a mixture of theta and kappa-alumina. In the presence of V₂O₅, alpha-corundum together with kappa and theta phases was obtained on heating at 900°C.     

TMDSC and Atomic Force Microscopy Studies of Morphology and Recrystallization in Polyesters Including Oriented Films

The thermal and crystal morphological properties of poly[ethylene teraphthalate] (PET) and poly(ethylene-2,6-naphthalenedicarboxylate) (PEN) biaxially oriented films were compared to amorphous and other isotropic semi-crystalline samples. Crystal melting as a function of temperature was characterized by temperature modulated DSC (TMDSC) and found to begin just above the glass transition for both oriented films. About 75°C above the glass transitions, substantial exothermic recrystallization begins and continues through the final melting region in oriented films. The maximum in the non-reversing TMDSC signal for the oriented films signifies the maximum recrystallization exothermic activity with peaks at 248°C and 258°C for PET and PEN, respectively. The final melting endotherm detected was 260°C and 270°C for PET and PEN, and is shown by the TMDSC data and by independent rapid heating rate melting point determinations to be due to the melting of species recrystallized during the heating scan. The results are compared with TMDSC data for initially amorphous and melt crystallized samples. The volume fraction of rigid species (F_rigid=total crystal fraction plus rigid amorphous or non-crystalline species) were measured by TMDSC glass transition data, and contrasted with the area fraction of rigid species at the oriented film surface characterized with very high resolution atomic force microscopy (AFM) phase data. The data suggest that the 11 nm wide hard domains in PET, and 21 nm wide domains in PEN film detected by AFM consist of both crystal and high stiffness interphase species.This revised version was published online in November 2005 with corrections to the Cover Date.     

Transreactions in poly trimethylene terephthalate/bisphenol-A polycarbonate (PC) blends analysed by pressure-volume-temperature measurements

The effect of annealing on the miscibility and thermal properties of poly trimethylene terephthalate (PTT)/bisphenol-A polycarbonate (PC) blends was examined using pressure-volume-temperature (PVT) measurements. The PTT/PC blends were thermally annealed at 260 °C for different times to induce various extents of transesterification reactions between the two polymers. The non-annealed blends are immiscible and exhibit the thermal properties of the blend components. Upon annealing, the original semi-crystalline morphology transforms to an increasingly amorphous nature. PVT and WAXS analysis confirmed that the PTT/PC blends completely lost their crystallinity when annealed at 260 °C for a period of 120 min or longer, indicating the formation of random co-polyesters due to chemical transreactions between the PTT and PC. The further increase in the specific volume with annealing time also indicates that after reaching a completely amorphous co-polymer the transesterification continuous until a fully random copolymer is formed.     

Short range ordering in rapidly quenched Fe83P17 amorphous alloys

Mössbauer spectroscopy, X-ray diffractometry and differential scanning calorimetry were used to study the effect of the preparation parameters on the short range ordering of Fe₈₃P₁₇ amorphous alloys. The samples were quenched with different quenching rates from the melt which was kept either at 1100°C or 1250°C for 20 min. The short range order of the amorphous samples was found to depend on the quenching rate due to the relaxation taking place during rapid quenching of the melt. In one case, in the sample quenched with the lowest quenching rate from the melt of 1250°C, crystalline phases identified as Fe₃P and -Fe were also found. This phase separation can be understood by taking into consideration the effect of both the melt temperature and the quenching rate. In this case, the cooling time can be long enough for separation of the phases which can be expected within the range of 1040–1250°C according to the phase diagram.     

Preparation and characterization of thin TiO2-films on gold/mica

Flat and highly (111) oriented gold and silver films were prepared by physical vapour deposition (PVD) using optimized deposition parameters. On these films, which were characterized with atomic force microscopy (AFM), scanning tunneling microscopy (STM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), titanium dioxide films were deposited by electron beam evaporation and dip coating. Dip coating from titanium tetraisopropoxide solutions resulted in films with different morphology and coverage depending on the alkoxide concentration (0.009 mol/L – 0.60 mol/L) and the post-treatment. Scanning electron microscopy (SEM) and AFM revealed that the deposited TiO₂ consists of amorphous, highly porous islands when the applied alkoxide concentration is high (0.05 mol/L – 0.6 mol/L). At higher temperatures these amorphous TiO₂ islands sintered significantly and crystallized to anatase. In contrast, transparent TiO₂ films were obtained from low concentrated alkoxide solutions (< 0.01 mol/L) which covered the whole substrate, similar to electron beam evaporated thin films. Sputter profiles with ion scattering spectroscopy (ISS) indicated that the film thickness is in the range of 2 nm when alkoxide solutions with a concentration of 9 mmol/L are used. The deposition of TiO₂ by electron beam evaporation normally resulted in significantly reduced TiO₂ films, completely oxidized ones were obtained when deposition was performed at elevated oxygen partial pressures (p(O₂) > 2 × 10^–5 mbar).     

Thermal contraction and extension in fibrous crystals of polyethylene

Summary Solution grown polyethylene shish-kebabs have a core, diameter 20–30 nm, which is highly superheatable. When a mat of shish-kebabs is heated to 132 °C the lamellar material melts, leaving birefringent fibres in an isotropic matrix as seen under the polarizing microscope. Microscopic observations reveal that these fibres, aggregates of many shish-kebabs, contract as they are melted and extend on subsequent recrystallization from the partially molten state as the temperature is lowered. This extension is completely reversible on temperature cycling. For fibres originally crystallized at below 96 °C this effect results in crimping on recrystallization and straightening from the crimped state on remelting. This is because these latter fibres are isolated from each other by the lamellar overgrowth and they buckle rather than extending the sample. Analogous reversible length changes and crimping effects were observed also in fibrous crystals produced from the melt.The above observations allow certain conclusions to be reached as regards the nature of the shish-kebab fibres. Thus an estimate of the forces involved in the bucking indicates that the basic unit which buckles is less than 50 nm in diameter in agreement with the single shish-kebab core when this is directly observable. Similar buckling in the melt crystallized fibrous entities indicates a core of similar size, which is not so easily seen. Further, the observed shrinkage behaviour, together with the associated birefringence and calorimetric information collected in this study, can only be accounted for if melting is visualised as the formation of disordered amorphous regions which alternate with perfect crystal regions along the shish-kebab core. This leads to a model equivalent to a linked row of fringed micelles within a given fibre. The chain straightening of the amorphous sections on recrystallization would then be responsible for the fibre extension. It follows further that for the amorphous sections to form in the first instance there must be preexisting centres of imperfections along the fibre. A model of randomly distributed imperfections (visualised as interlocking chain loops) can account for the present observations in a quantitative manner with realistic predictions as regards the lengths of the undisturbed crystal regions.Finally there is an obvious analogy with the crystallization of oriented polymer networks (e.g. stretched rubber) as regards the observed extension —contraction behaviour, providing a link between recent findings and long established experience.With 12 figures and 1 table     

Structural features of crystallized poly(ethylene terephthalate) polymers

Polyethylene terephthalate (PET) is a widely used polymeric material. In this work, the microstructural features before and after the solid‐state polymerization (SSP) of several DuPont PET products were investigated by low‐voltage scanning electron microscopy (LV‐SEM) and atomic force microscopy (AFM). The microstructural features on the cross section of various PET samples included crystallites, voids, boundaries, defects, and amorphous phases. The SEM images revealed layered and stepped structural features at the micron and 10‐micron scales that are highly crystallized at the near‐edge region of the cross section for both linear and branched PET samples after the SSP process. The AFM images demonstrate that the degree of crystallization for the linear and branched PET samples increases gradually from the central area to the edge on the cross section. The linear crystallized PET has a higher degree of orientation than the branched crystallized PET in the 10‐micron to micron scales, but their crystalline structures have no significant differences in the submicron to nanometer scales. The PET crystallization process occurs when the molecular chains in the amorphous phase are aligned and folded to form straight molecular chains at the nanometer scale, and small crystallites are formed. The crystallites aggregate and align together into a polygon rod‐like‐shaped crystallites at the submicron scale. Finally, large crystallites at the micron size are formed that appear on the edge area of the cross section. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 245–254, 2002     

Formation of micro-crystals in poly(ethylene terephthalate) fiber by a short heat treatment and their influence on the mechanical properties

The morphological change of poly(ethylene terephthalate) (PET) fibers by a short heat treatment under free-to-relax condition (i.e., without mechanical constraints imposed on the fibers during the treatment) was investigated. Heat treatment on polymeric fibers, in particular free-to-relax condition, has been known to lower the initial elastic modulus due to the relaxation of the amorphous molecules; however a short heat treatment at 190 °C for 1.2 s in the present study increased the initial modulus, whereas the yield strength was decreased significantly. During the short heat treatment, the PET molecules in the PET fibers were relaxed and became crystallized to some extent. The PET chains in the amorphous regions were also relaxed, promoting the formation of micro-crystals. These micro-crystals in the amorphous region can explain the increase in the initial modulus. The mechanism for such unusual behavior was investigated using mechanical tests, thermal stress analysis, wide and small angle X-ray diffraction, and FTIR spectrum analysis. Furthermore, a morphological model for the molecular arrangements in the PET fibers due to the short heat treatment is proposed, offering the possibility of developing PET fibers with shape retention function that can behave similarly to metal fibers.     

Transformation of Aluminosilicate Wet Gel to Solid State

Optically clear, homogeneous, monolithic aluminosilicate gels were prepared directly from solutions of inorganic aluminum salt, tetraethoxysilane, and alcohol. The dried aluminosilicate gels show new special properties, e.g. piezoelectricity and variable porosity (1-2000 m²/g) due to the fractal-like gel structure. Ceramic products, which keep the chemical bonds and structure of the gels, were produced by heat treatment at 300-350°C. The processes that occurred during heat treatment were investigated by thermal analysis and mass spectrometry. The temperature limits of the gel state were determined by differential scanning calorimetry and small angle X-ray scattering. The collapse of the gel structure began around 350°C; however, the amorphous character remained until the temperature (ca. 970°C) of the phase transmission.     

Sol-gel processed barium titanate waveguides

Monolayer monomode and multilayer multimode BaTiO₃ waveguides have been prepared on amorphous silica substrates by using the dip-coating technique and the sol-gel process. After heat treatment, these waveguides were hard and of good optical quality (losses as low as 2.6 dB/cm were measured) but, their structure was found to depend strongly on the number of layers. Monolayer waveguides were totally amorphous even when heated at 1000°C, while multilayer ones exhibited the tetragonal BaTiO₃ structure at an annealing temperature of 600°C.     

Molecular Precursor Role in the Synthesis of ZrMVI2O8, MVI = Mo,W: Isolation and Properties of Zr3Mo8O24(OiPr)12(iPrOH)4

The studies of zirconium isopropoxide solvate, Zr(OiPr)4·iPrOH(I), interaction with molybdenum oxoisopropoxide in different solvents (HOiPr, hexane) revealed I to be the only isolable crystalline product in alcohol. In hexane I crystallized from Zr-rich solutions (up to Zr : Mo = 1 : 2 ratio). From the solutions with Zr : Mo = 1 : 2 ratio repeatedly dried in vacuo and subsequently redissolved in hexane, the crystallization of Zr3Mo8O24(OiPr)12(iPrOH)4(II) occurred slowly with 12% yield. II can be recrystallized from hexane, but is destroyed by iPrOH which causes the formation of I. Thermal decomposition of II in air at 800°C (2 h) gives a single phase ZrMo2O8(III). Hydrolysis studies of hexane solutions, prepared in a manner analogous to those, from which II was obtained, showed that single-phase samples of III could be obtained when a thin layer of solution was left for hydrolysis and evaporation of solvent in a moist air and then annealed at 800°C (2 h). The same kind annealing of a xerogel, obtained by hydrolysis with water solutions in iPrOH and subsequent drying at 100°C gave a complex mixture of phases where III was not even the major component. No Zr-W complex was isolable under analogous conditions. From the xerogel, obtained by hydrolysis with H2O/iPrOH, ZrO2 and WO3 crystallized separately on heating. Only traces of ZrW2O8(IV) were obtained along with individual oxides when 1% water in ether was applied for hydrolysis. Molecular precursor is therefore crucial for obtaining III, IV.     

Glass transition and enthalpy relaxation of ethylene glycol and its aqueous solution

Differential scanning calorimetry (DSC) and cryomicroscopy were employed to investigate the glass transition and enthalpy relaxation behaviors of ethylene glycol (EG) and its aqueous solution (50% EG) with different crystallization percent. Isothermal crystallization method was used in devitrification region to get different crystallinity after samples quenched below glass transition temperature. The DSC thermograms upon warming showed that the pure EG has a single glass transition, while the 50% EG solution has two if the solution crystallized partially. It is believed that the lower temperature transition represents the glass transition of bulk amorphous phase of EG aqueous solution glass state, while the higher one is related to ice inclusions, whose mobility is restricted by ice crystal. Cryomicroscopic observation indicated that the EG crystal has regular shape while the ice crystal in 50% EG aqueous solution glass matrix has no regular surface. Isothermal annealing experiments at temperatures lower than T_g were also conducted on these amorphous samples in DSC, and the results showed that both the two amorphous phases presented in 50% EG experience enthalpy relaxation. The relaxation process of restricted amorphous phase is more sensitive to annealing temperature.