The influence of annealing on thermal transitions in a nematic copolyester

Thermal transitions of a glassy, main chain, liquid crystalline, random copolyester, HIQ‐40, have been characterized. HIQ‐40 is made from 40 mol percent p‐hydroxybenzoic acid (HBA) and 30 mol % each of p‐hydroquinone (HQ) and isophthalic acid (IA). This polymer is soluble in organic solvents, permitting the preparation of thin, solution‐cast films that are in a glassy, metastable, optically isotropic state. On first heating of an isotropic HIQ‐40 film in a calorimeter, one glass transition is observed at low temperature (approximately 42°C), and is ascribed to the glass/rubber transition of the isotropic polymer. A cold crystallization exotherm centered near 150°C is observed. This is associated with the development of low levels of crystalline order. A broad melting endotherm is centered at about 310°C; this endotherm marks the melting of crystallites and the transformation to a nematic fluid. A nematic to isotropic transition was not observed by calorimetry. After quenching from the nematic melt, a T_g is observed in the range of 110–115°C and is associated with the glass/rubber transition of the nematically ordered polymer. Annealing optically isotropic films at temperatures above the isotropic glass transition results in the systematic development of axial order. In these annealed samples, T_g increases rapidly until it is near the annealing temperature, then T_g increases more slowly at longer annealing times. In as‐cast films annealed at 120–135°C, the light intensity transmitted through a sample held between crossed polarizers in an optical microscope (a qualitative measure of birefringence and, in turn, axial order) initially increases rapidly and uniformly throughout the sample and, at longer annealing times, approaches asymptotic values that are higher at higher annealing temperatures. The increase in transmitted intensity is ascribed to the development of axial order. The uniform increase in transmitted intensity suggests that ordering occurs by a rather global process and not via a nucleation and growth mechanism. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 505–522, 1999     

Chain-backbone motion in glassy polycarbonate studied by polarized infrared spectroscopy

Chain-backbone motion in glassy polycarbonate has been investigated both under isothermal stress, and also under zero stress during isothermal annealing of freely contracting film specimens. In both types of experiment, backbone motion was detected by measuring the change in infrared dichroism. The dichroism of absorption bands at 1364 and 2971 cm^?1, which have transition moment vectors directly related to the chain-backbone orientation, was studied. Under tensile stress in the homogeneous region of deformation, changes of up to 2.2° in the mean chain-backbone orientation angle were measured at 23°C. With the onset of cold drawing a total orientation change of some 8° was observed. For the isothermal annealing experiments, a film specimen holder employing conductive heating with radiative losses was employed. It enables infrared measurements to be made while the temperature of the contracting specimen is maintained constant to ± 0.5°C. Oriented specimens were prepared by isothermal stretching of polycarbonate films to strains of the order of 100%. Changes in the mean chain-backbone orientation angle were observed during annealing of these oriented films at temperatures between 80°C and the glass transition (149°C). Chain motion proceeded during annealing, and chain segments were observed to move cooperatively. The temperature at which the polymer is prestretched has a pronounced effect on its subsequent relaxation during annealing: when the sample was stretched at 23°C. motions were detected during annealing at temperatures as low as 81°C, while, if it was stretched at 154°C, no motion was detected at annealing temperatures below 127°C. The data are discussed in comparison with theories of the glassy state that predict the absence of chain-backbone motion at temperatures significantly below the glass transition. A shift in frequency of the ν_a (CH₃) absorption peak in stretched polycarbonate was measured by using polarized radiation. The effect was interpreted in terms of changes in the intermolecular bonding structure of the oriented polymer.     

Fuming nitric acid treatment of polyethylene. IV. Morphology of drawn polyethylene

Drawn PE of different draw ratios (ranging from 1 to 25) and thermal treatment (annealing temperature 80, 100, 110, 120, 127°C.) was treated with fuming nitric acid at 80°C. Weight loss, molecular weight, elastic modulus, and thermograms were measured for annealed and unannealed samples as a function of the treatment time and draw ratio. As a consequence of the preferential oxidation of the noncrystalline portions, there occurs initially a high rate of weight loss and a steep drop in molecular weight, followed by a lower rate of weight loss at nearly constant molecular weight. The elastic modulus stays practically constant up to the moment where the brittleness of the sample prevents further measurement. During the later period the thermograms exhibit one melting peak during the first melting. The remelt of the same sample, however, has two melting peaks with a relative intensity independent of the treatment time. That the two melting peaks are caused by two components of different molecular weights present in the sample is substantiated by fractionation. At very high annealing temperature (127°C.), two peaks appear, not only in the first melting curve of the etched sample, but also in the melting curve of the unetched material. Such an effect is the consequence of partial melting during annealing followed by new crystallization during cooling the sample to room temperature. The findings are related to the morphology of the drawn material under the assumption of preferential scission of chain loops in the amorphous-crystalline sandwich layer model.     

Growth Orientation of the Mechanosynthesized Fluorapatite-Based Composite Nanopowders: Influence of Subsequent Thermal Treatment

Growth orientation of fluorapatite–zirconia nanopowders was investigated after mechanical activation and thermal annealing process in the range of 600–1,300 °C for 1 h. Results revealed that during heating of the composite nanopowders the transition of the monoclinic zirconia to tetragonal form and its stabilization by calcium fluoride originating from the decomposition of fluorapatite as well as the formation of a solid solution of calcium fluoride in zirconia occurred. The influence of annealing on the growth orientation of fluorapatite–zirconia composite nanopowders indicated that the crystal growth of fluorapatite was preferentially accentuated on the (002) face in the direction of the crystallographic c-axis after heat treatment. Based on FE–SEM observations, the experimental outcome was composed of both agglomerates and fine particles (~33 nm) after 600 °C, while annealing of the sample at 1,300 °C demonstrated the occurrence of abnormal grain growth.     

Thermal transition of a wholly aromatic thermotropic liquid crystalline copolyester

A copolyester was prepared from p-hydroxybenzoic acid (HBA), 2,6-naphthalene dicaboxylic acid (NDA), and hydroquinone (HQ). Thermal transition behavior and the crystal structure of this copolyester were investigated by using polarized light microscopy (PLM), differential scanning calorimetry (DSC), and wide-angle X-ray diffraction (WAXD) after annealing at solid-phase polymerization conditions. A glass transition or newly ordered structure in the 270–290°C range was observed on annealing at 260°C, which increased with annealing time, attributed to mobility and reactive rearrangement in amorphous regions. Broad and unclear WAXD profiles and multimelting behaviors were found on annealing at 280°C, and explained by hexagonal and orthorhombic lattice formation and transformation. A large increase in melting temperature was observed only on annealing at a temperature (320°C) near the crystal–nematic transition, suggesting annealing temperatures near the melting point are required for sufficient mobility to afford crystalline rearrangement via transesterification. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3763–3769, 1999     

The effect of thermal annealing on the thermal properties and molecular weight of a segmented polyurethane copolymer

The influence of thermal annealing on molecular weight, microphase mixing, and multiple melting behavior of a segmented block copolyurethane is reported. The material studied contained 55% of hard segment consisting of 4,4'-diphenylmethane diisocyanate and butanediol, and a poly (propylene oxide) diol of molecular weight 2000 as the soft segment. The thermal stability of the materials was influenced greatly by the order-disorder transition, estimated to occur at ca. 191°C. Upon annealing above this temperature, molecular weight increased rapidly as a result of chain branching reactions. Microphase separation increased under these conditions, while the degree of hard segment crystallinity decreased. Annealing below the order-disorder transition temperature resulted in relatively small molecular weight increases for short annealing times, but large increases for annealing times greater than one hour. Glass transition temperature data for these thermal treatments was consistent with upper critical solution temperature behavior and selective solubility by hard segment sequence length according to the Koberstein-Stein hard microdomain model. The critical hard segment sequence for segregation was estimated (for 30 min annealing) to contain ca. 5 diisocyanate residues at 80°C, ca. 8 residues at 185°C, and increased slowly with annealing time. © 1994 John Wiley & Sons, Inc.     

Effect of heating and aging on microstructure of polyester-polyamide copolymer

Polyester-polyamide (PET/PA) copolymers offer a wide range of possible applications where both dimensional stability and good impact resistance can be achieved from their synergism. In order to ensure these properties for long-term use, however, problems of phase separation, volume change, and chemical reequilibration on heating or aging must be identified and overcome. Wide-angle x-ray diffraction (WAXD), small-angle x-ray scattering (SAXS), and thermal analysis were employed to follow the microstructure of a 70:30 PET/PA copolymer with several different processing histories as a function of heat history and both short-term and long-term aging. Heating the copolymer, as received, above the melting temperature of PET caused phase mixing. The effect of heating on crystallinity depended on starting crystallinity. Holding the copolymer at 50°C, just below the glass transition temperature of PET, caused phase separation in samples which had been annealed for 30 min at 150°C. On long-term aging samples gave evidence of phase mixing. The results indicate highly nonequilibrium structure in the material, regardless of the original processing.     

Plasticization and Crystallization of Poly(ethylene Terephthalate) Induced by Water

Commercial poly(ethylene terephthalate) (PET) was treated at R. H.>80% and room temperature for a set time. The glass transition temperature (Tg) decreases with the time of exposure to high humidity. The decrease in Tg is a result of plasticization. Our data indicate that the Tg of dry PET of 76-78°C may decrease to as low a temperature as 65-67°C when it is wet. Induced crystallization of PET in the presence of water reduces the cold crystallization temperature (Tc). The structure of water-induced crystals is imperfect and can be improved in perfection by annealing. This revised version was published online in July 2006 with corrections to the Cover Date.     

The dependence of slow crack growth in a linear polyethylene on test temperature and morphology

The slow crack growth behavior of a linear polyethylene with different morphologies was studied by using three point bending with a single edge notched specimen at testing tem-peratures from 30 to 80°C. The morphology was varied by annealing the quenched material at temperatures from 86°C to 135°C. It was found that at test temperatures of 60°C or less, the changes in failure time with annealing temperature are very similar to the change in density with a maximum at 130°C. At testing temperatures above 60°C, the relationship of between failure time and annealing temperature is altered when the test is in the range of the α transition temperature. These results indicate that with respect to slow crack growth in the case of a homopolymer the strength of the crystals is relatively more important than the number of tie molecules. © 1993 John Wiley & Sons, Inc.     

Electronic Structure and Size of TiO2 Nanoparticles of Controlled Size Prepared by Aerosol Methods

Summary.  A complete characterization of nanostructures has to deal both with electronic structure and dimensions. Here we present the characterization of TiO₂ nanoparticles of controlled size prepared by aerosol methods. The electronic structure of these nanoparticles was probed by X-ray absorption spectroscopy (XAS), the particle size by atomic force microscopy (AFM). XAS spectra show that the particles crystallize in the anatase phase upon heating at 500°C, whereas further annealing at 700°C give crystallites of 70% anatase and 30% rutile phases. Raising the temperature to 900°C results in a complete transformation of the particles to rutile. AFM images reveal that the mean size of the anatase particles formed upon heating at 500°C is 30 nm, whereas for the rutile particles formed upon annealing at 900°C 90 nm were found. The results obtained by these techniques agree with XRD data. Received October 5, 2001. Accepted (revised) December 6, 2001     

Influence of bond interchange reactions on mechanical behavior of solid poly(ethylene terephthalate)

Reported are a pair of studies exploring the possible role of transesterification during the deformation of poly(ethylene terephthalate) (PET) near its glass transition temperature. SANS experiments on PET-D/PET-H blends were carried out on films drawn according to the method of Petermann and Gohil at 70 and 90°C. In order to avoid effects of orientation on the SANS pattern, the measurements were made on liquid solutions made from the drawn material. The results show a decrease in the D-sequence length upon deformation, while the molecular weight of the chain is unchanged. This result indicates that bond exchange has taken place. Based upon the SANS results, an approximate activation energy in the 8–24 kcal/mol range is reported for bond interchange. To test the hypothesis that transesterification may play a role in the mechanical behavior of PET near T_g, creep measurements were carried out over a temperature range on either side of T_g. Activation energies obtained from time–temperature superposition shift factors showed a dual behavior: above T_g the activation energy matches those for diffusive processes in PET; below T_g the activation energy coincides with activation energies reported for exchange reactions in this material. The role of annealing prior to deformation is also reported, and it is shown that the activation energy increases rapidly with increasing annealing temperature. © 1996 John Wiley & Sons, Inc.     

Thermally stimulated depolarization currents of crosslinked polyethylene relaxations in the fusion range of temperatures

With thermally stimulated depolarization currents, we researched the relaxations of crosslinked polyethylene as it is used in medium‐voltage cable insulation. Through conventional polarization two heteropolar peaks stand up in the spectra, at 80 and 105 °C. As the sample is annealed, a homopolar peak is developed at about 99 °C. With window polarization, our results indicated that the 80 °C peak is a structured peak related to polar crosslinking subproducts and impurities. The 105 and 99 °C peaks are fitted to the general kinetic‐order model because the 105 °C peak is related to free‐charge detrapping at the crystalline phase, in the bulk and maybe at the amorphous‐crystal interphases, and the peak that is observed at 99 °C is due to injected charge. Annealing at high temperatures promotes the creation of traps in the material. Charge trapping at T < 70 °C seems to be related to the increased insulator resistivity with annealing time. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1412–1421, 2003     

Chain-folding measurements in annealed poly(ethylene terephthalate)

Chain folding in unoriented poly(ethylene terephthalate) (PET) films has been investigated as a function of annealing time and temperature. To meet this objective dynamic mechanical, infrared, and molecular weight measurements were used, together with selective chemical degradation to remove chain folds and amorphous regions. The β dispersion in the dynamic mechanical spectrum of PET is here tentatively associated with motions of methylene and/or carboxyl groups in irregular chain folds; the β dispersion is not found in quenched amorphous polymer, in polymer where amorphous regions and chain folds have been removed, or in highly annealed PET where the irregular folds have regularized. Upon mild crystallization and annealing (30 min at 110°C) of initially amorphous film a large β dispersion appears and then diminishes upon further annealing at 220°C. As the β dispersion diminishes, the infrared regular fold band increases more than the crystallinity band, indicating regularization of folds. The molecular weight of the degraded residue corresponds approximately to typical fold-period dimensions (～130 Å), and increases on annealing as expected from lamellar thickening. The degradation process has, by fold removal, reduced the chains in the crystals to a very short, uniform length.     

Emanation thermal analysis study of synthetic gibbsite

Emanation thermal analysis (ETA) was used for thermal characterization of microstructure changes taking place during heating of synthetic gibbsite sample in argon in the range of 25–1200°C. Microstructure development and the increase of the surface area under in-situ conditions of the sample heating were characterized. The increase of the radon release rate from 130–330°C monitored the increase of the surface area due to the dehydration of Al(OH)₃. During heating of the sample in the range 450–1080°C the ETA results characterized the annealing of surface and near surface structure irregularities of intermediate products of gibbsite heat treatment. The mathematical model for the evaluation of the ETA experimental results was proposed. From the comparison of the experimental ETA results with the model curves it followed that the model is suitable for the quantitative characterization of microstructure changes taking place on heating of gibbsite sample. This revised version was published online in July 2006 with corrections to the Cover Date.     

Fluorescence study on a thermotropic liquid-crystalline polyester at high temperatures

A thermotropic liquid-crystalline (LC) polyester, poly[(ethylene terephthalate)-co-(p-oxybenzoate)] (PET40/OBA60) (OBA content: 60 mol %), is investigated by fluorescence technique using two model compounds: dimethyl terephthalate (DMT) and methyl methoxybenzoate (MMB) and is demonstrated to form an intermolecular ground-state complex between the terephthalate and OBA moieties. The change in fluorescence of PET40/OBA60 film is studied from 25°C to 450°C. The peak wavelength change for fluorescence of the intermolecular ground-state complex from 394 to 430 nm was observed in the temperature range between T_g and the LC transition temperature (115～ 250°C). This is attributed to the electronic distribution change between terephthalate and OBA moieties in the excited state, which play roles of acceptor and donor, respectively. The increase in the fluorescence intensity from the temperature near the annealing temperature to the temperature near the isotropic temperature (287～370°C) is suggested to be the increase in LC configuration and the formation of a more stable excited state due to the electronic distribution change between terephthalate and OBA moieties. The lifetime of PET40/OBA60 film quenched from LC temperature (300°C) to room temperature is in agreement with that of the nonannealed one, which is due to the fact that the deactivation process of the sample quenched from LC temperature is in accord with that of the nonannealed one. © 1995 John Wiley & Sons, Inc.     

Ordering processes in the 2,6-hydroxynaphthoic acid (HNA) rich copolyesters of p-hydroxybenzoic acid and HNA

The nature of the microstructure of several copolyesters of p-hydroxybenzoic acid (HBA) and 2,6-hydroxynaphthoic acid (HNA) with compositions of 20/80, 24/76, and 30/70 was studied. Annealing the compositions at 50–80 °C below the crystal nematic transition resulted in a physical ordering process, whereas heating the polymers at the crystal nematic transition resulted in a more highly ordered phase. The ordered phases could be randomized by heating to a temperature at or above the transition of the more highly ordered phase.     

Structural investigation of water‐induced phase transitions of poly(ethylene imine). III. The thermal behavior of hydrates and the construction of a phase diagram

The thermal behavior of poly(ethylene imine) (PEI) hydrates in a water vapor atmosphere was investigated through temperature‐dependent measurements of infrared spectra and X‐ray diffraction. Almost perfectly dried anhydrate melted at about 60 °C during the heating process. Anhydrate containing a small amount of water showed a phase transition to a mixture of hemihydrate and sesquihydrate around 40 °C, at which point the ethylene imine (EI)/water ratio was 1/0.5 in the hemihydrate and 1/1.5 in the sesquihydrate. The hemihydrate transferred to the sesquihydrate around 60 °C, and the latter melted above 80 °C. When the starting PEI sample contained a greater amount of water and consisted of hemihydrate and sesquihydrate, the hemihydrate transferred to the sesquihydrate via heating, and the latter melted around 75 °C. For a sample of dihydrate (EI/water ratio = 1/2) containing an appreciably large amount of water, it transferred to the sesquihydrate around 65 °C, and the latter melted above 90 °C. A sample of dihydrate with a much higher water content existed up to 110 °C and then melted; during this period, no transition to the sesquihydrate was observed. In this way, the starting crystalline phases were found to change for anhydrate and various types of hydrates. Their transition behaviors varied according to the water content. From these data, a phase diagram was successfully derived as a function of the temperature and water content. This phase diagram allowed us to predict the transition behavior during the hydration process at various constant temperatures. For example, at 60 °C, a molten sample should crystallize into a mixture of hemihydrate and sesquihydrate at first, and the hemihydrate should transfer to the sesquihydrate with increasing water content. The latter should change to the dihydrate in the final stage. This prediction was checked with time‐resolved measurements of X‐ray diffraction and infrared spectra during the hydration process at the corresponding temperature; this led to the establishment of the phase diagram. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2937–2948, 2003     

Melting behavior of linear polyethylene crystallized by solution stirring

Calorimetric and dilatometric studies have been made of the fusion process of linear polyethylene crystals precipitated by high speed stirring from solution. It is shown that long-time annealing at elevated temperatures alleviates the superheating observed when rapid heating rates are employed. By the annealing procedures that have been adopted, a small but demonstrable fraction of high melting material can be produced whose melting temperature depends on the crystallization temperature. For crystallization at 105°C, followed by annealing at 142°C, a melting temperature of 146.0 ± 0.5°C is observed. The dissolution temperature in xylene, determined for the same sample, is consistent with the high melting temperature observed for the pure polymer. It is recognized that a state of high axial orientation need not necessarily be identified with extended chain crystals. Consequently, the increased melting temperature can result from either an increase in the crystallite size or a reduced interfacial free energy relative to crystallites produced by the more conventional mode of crystallization.     

Maximizing the grain growth rate during the disorder‐to‐order transition in block copolymer melts

The factors controlling grain growth during the disorder‐to‐order transition in a polystyrene‐block‐polyisoprene copolymer melt were studied with time‐resolved depolarized light scattering. The ordered phase consisted of hexagonally packed polyisoprene cylinders, and the order–disorder‐transition temperature of the block copolymer (T_ODT) was 132 ± 1 °C. Our objective was to identify the temperature at which the grain growth rate was maximized (T_max) and compare it with theoretical predictions. We conducted seeded grain growth experiments, which comprised two steps. In the first step, which lasted for 43 min, the sample was cooled from the disordered state to 124 °C. This resulted in the formation of a small number of ordered grains or seeds. This was followed by a second step in which the sample was heated to temperatures between 124 and 132 °C and the seeds grew with time. Our objective was to study grain growth at different temperatures starting from the same initial condition. The value of T_max obtained experimentally was 128 °C. The theoretically predicted value of T_max, based entirely on the rheological properties of the disordered sample and T_ODT, was also 128 °C. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2231–2242, 2001     

热处理条件对R-PET/LLDPE-g-MA共混物中PET玻璃化转变行为的影响

对回收聚对苯二甲酸乙二酯(R-PET)/LLDPE-g-MA马来酸酐改性的线性低密度聚乙烯共混物进行不同条件的热处理, 采用差示扫描量热仪(DSC)研究共混物基体PET的玻璃化转变行为. 结果表明, 当热处理温度低于PET的玻璃化转变温度(Tg)时, PET的玻璃化转变区域出现热焓松弛现象. 随着热处理温度的增加, PET的Tg逐渐升高; 在50~70 ℃下热处理48 h后, PET的Tg逐渐稳定. 当热处理温度高于PET的Tg而低于100 ℃时, PET的玻璃化转变区域出现2个热流转变, FTIR分析表明, PET分子构象开始发生变化. 当热处理温度为100 ℃时, DSC曲线上PET的玻璃化转变消失, PET的结晶度明显增加, 说明PET开始冷结晶的温度在90~100 ℃之间.