Thermal degradation and isothermal crystalline behavior of poly（trimethylene terephthalate）

Poly(trimethylene terephthalate)(PTT) is an excellent fiber material.Its thermal degradation and isothermal crystalline behaviors were in this study investigated using thermogravimetric analysis(TGA),thermogravimetric analysis-Fourier transform infrared spectroscopy(TGA-FTIR) analysis,differential scanning calorimetry(DSC) and X-ray diffraction(XRD).The thermal degradation mechanism of PTT follows Mclafferty rearrangement principle.The PTTwithintrinsicviscosity(Ⅳ) of 0.74 dL/g has a maximum crystallinity...     

Polymorphic crystalline forms of 8OCB

For most alkoxycyanobiphenyls (3OCB, 5OCB, 6OCB, 7OCB and 9OCB) it has been observed that slow or delayed cooling gave rise to a lower melting crystalline polymorphic form which then slowly converted into the higher melting stable crystalline structure. Although under slow evaporation from a solvent, polymorphic crystalline structures were observed also for 8OCB, a polymorphism on cooling like that in other nOCBs has not previously been reported. We have now observed that 8OCB also shows polymorphism, which brings it into line with the other homologues. On keeping the material between 35 and 38 °C after it was cooled from above the melting point of the stable crystal form (55 °C), regular platelets grow usually from one point and fill the whole sample. The texture is stable for weeks at room temperature. Upon heating it shows a melting point at 50 °C, i.e. 5 degrees below the stable crystalline melting point. It is interesting that all platelets are non-symmetric and have the same handedness.     

Polymorphic crystalline forms of 8OCB

For most alkoxycyanobiphenyls (3OCB, 5OCB, 6OCB, 7OCB and 9OCB) it has been observed that slow or delayed cooling gave rise to a lower melting crystalline polymorphic form which then slowly converted into the higher melting stable crystalline structure. Although under slow evaporation from a solvent, polymorphic crystalline structures were observed also for 8OCB, a polymorphism on cooling like that in other nOCBs has not previously been reported. We have now observed that 8OCB also shows polymorphism, which brings it into line with the other homologues. On keeping the material between 35 and 38 °C after it was cooled from above the melting point of the stable crystal form (55 °C), regular platelets grow usually from one point and fill the whole sample. The texture is stable for weeks at room temperature. Upon heating it shows a melting point at 50 °C, i.e. 5 degrees below the stable crystalline melting point. It is interesting that all platelets are non-symmetric and have the same handedness.     

New crystalline forms of permethylated beta-cyclodextrin

Two new crystalline forms of permethylated [small beta]-cyclodextrin are reported that contain methylglucose residues exclusively in the (4)C(1) conformation, in contrast to the known monohydrate, in which a single methylglucose residue adopts the (1)C(4) conformation.     

Thermal dissociation of crystalline substances

Processing of the reference data shows that the temperature dependences of the equilibrium dissociation pressure of crystalline substances of different classes under the studied and unstudied conditions can be described by equations of similar form. Similar semiempirical temperature dependences of the heat of formation of 1 mol of gaseous products were obtained after bringing the thermal dissociation constants of crystalline substances to consistency.     

Evaluation by Fourier Transform Infrared Spectroscopy of the different crystalline forms in syndiotactic polystyrene samples

Fourier transform infrared (FTIR) spectra of syndiotactic polystyrene (s-PS) semicrystalline samples have been examined by using the spectral subtraction approach. For the crystalline forms including trans-planar chains (trigonal α and orthorhombic β) a number of conformational and structural order effects, not previously described in the literature, have been identified. A method based on the results of the spectral subtraction analysis has been developed for the determination of the crystallinity degree and compared with the standard method based on the wide-angle X-ray diffraction patterns. The spectral subtraction analysis on FTIR spectra allows also an easy evaluation of the amount of α and β crystalline phases (often simultaneously present in melt-crystallized samples) although both contain chains in a same conformation. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1055–1066, 1997     

Thermal behaviour and degradation of a liquid crystalline alkylene-aromatic polyester: Poly(decamethylene-fumaroyl-bis-4-oxybenzoate)

The thermal behaviour and degradation of an alkylene-aromatic liquid crystalline polyester, poly(decamethylene-fumaroyl-bis-4-oxybenzoate), were studied by thermogravimetric analysis under dynamic conditions and by pyrolysis-gas chromatography and pyrolysis-gas chromatography/mass spectrometry in the temperature range 450-650 °C. Among the degradation products detected, maleic anhydride, phenol, 4-hydroxybenzoic acid and the corresponding decenyl ester were the most abundant. The type and the composition of the pyrolysis products gave useful information about the mechanism of thermal degradation. The polyester decomposition starts with a free-radical scission in the mesogenic fragment and continues with electrocyclic reactions in the spacer. The influence of fullerene C₆₀ addition and of the deuteration of the decamethylene spacer on thermal behaviour and degradation were investigated.     

Thermal conductivity of oriented crystalline polymers

Thermal conductivities of six oriented semicrystalline polymers which range from 0.37 to 0.63 in crystallinity and 1 to 5 in draw ratio λ (up to about 15 for two polymers) have been measured between 100 and 340 K. It was found that for increasing λ the conductivity K_∥ (along the draw direction n?) increases rapidly while K_⊥ (normal to n?) decreases slightly; K_∥ also increases with temperature, but K_⊥ shows no simple pattern in temperature dependence. These general features can be reproduced reasonably well at low draw ratio (λ < 5) by the modified Maxwell model, and the discrepancy in details may be attributed to the fact that the model does not take into account the possible anisotropy of the amorphous phase of the oriented polymers. At high draw ratio the intercrystalline bridge effect becomes important, and one must resort to the Takayanagi model, but the lack of corroborating x-ray data has rendered a detailed comparison impossible.     

Thermal degradation of UHMWPE

The thermal behaviour of ultra-high molecular weight polyethylene (UHMWPE) of different molecular weights was examined by thermal analysis methods. The melting temperatureT _m and the heat of melting H were measured by the DSC method. The thermooxidative degradation process was investigated by using a MOM Q-1500 D derivatograph at various heating rates in air atmosphere. The initial decomposition temperatureT _i was determined from the TG curves, and other characteristic temperatures of decomposition were calculated. It was found thatT _m and H are higher for UHMWPE than those for HDPE, i.e. 146C and 195 J g^–1 for UHMWPE as compared with 133C and 166 J g^–1 for HDPE. The thermal behaviour of the investigated UHMWPE samples is not significantly influenced by molecular weight.     

Thermal degradation of polystyrene

Molecular weight change studies have shown that the thermal degradation of random copolymers of styrene — namely HIPS, SAN, and ABS-at low temperatures and in air involves random chain scission. The dominant process in the degradation of HIPS is random chain scission due to weak links, whereas in SAN it is intermolecular chain transfer. In ABS, the degradation is initially random scission due to weak links and then mainly intermolecular chain transfer. The infrared spectra show that during degradation the labile weak links are attacked by oxygen and peroxidic free radicals are produced. Via hydrogen abstraction or autoxidation of olefinic links, these free radicals are responsible for the formation of aliphatic ketonic or peroxyester structures, and for isomerization and cyclization. The activation energies of overall degradation of HIPS, SAN, and ABS are 134, 142, and 92 kJ.mol^–1 respectively.Part of the PhD dissertation of Mrs. Jaya Nambiar, University of Gorakhpur, Gorakhpur-273001, 1980.     

Thermal degradation of Levan

The thermal degradation of the polysaccharide Levan, of various molecular weights, was studied by means of isothermal and dynamic TG, X-ray diffraction, and IR measurements. From dynamic TG traces, two main degradation stages were observed for HL-Levans (hydrolyzed) whereas, three such stages were observed for UL-Levans (unhydrolyzed). These stages were correlated with branching effects, an interpenetrating network, and ring-fragmentation and other reactions in the late stages of the thermal degradation which result in the formation of infusible material that chars on further heating. Isothermal curves obtained for the first stage indicated the possibility of a random type of thermal degradation. Anticipated linear plots were obtained, using expressions from random degradation theory, which added further support for this type of degradation for Levan (stage 1). Various parameters were obtained for the thermal degradation of Levan, e.g., activation energies, and various experimental observations have been correlated in terms of postulated microstructures for Levan and in terms of parameters such as entropy of activation.     

Thermal degradation of polyethylene

Thermal degradation of low-density polyethylene (LDPE) in the temperature range from 450 to 525°C has been studied using a sieve-bottom reactor with inert gas as heat-transferring agent bubbled through the PE melt. Temperature dependence of the degradation rate was determined. Full degradation of LDPE into gaseous and wax-like hydrocarbons (alkanes and 1-alkenes) was achieved. Temperature rise and prolonging of the contact time increased the yield of the gaseous hydrocarbons and decreased the molecular weight of the wax-like product.     

Thermal degradation of polyquinoxalines

A systematic investigation of the thermal stability of nine structurally related polyquinoxalines has been conducted. The relative oxidation resistance of these polymers is controlled by two opposing structural effects. Phenyl sidegroup substitution in the heterocycle greatly improves oxidative stability, while the introduction of oxygen into the main polymer chain, in the form of ether groups, produces a negative effect of equal magnitude. These results are discussed from a mechanistic point of view. Simultaneous, dynamic thermal analysis in vacuum up to 1400°C and analysis of volatile and nonvolatile products indicates three major decomposition regions. Between 500 and 640°C, main polymer degradation takes place involving the heterocycle. This event is followed by dehydrogenation of a stable degradation product between 640 and 690°C. Above 1360°C an exothermic reaction takes place to yield highly condensed aromatic residues.     

Thermal degradation of polybenzyl

Polybenzyl has been synthesized from the benzyl compounds, chloride, methyl ether, and isopropyl ether by using stannic chloride as the Friedel-Crafts catalyst. The detailed structures of these polybenzyls have been determined by use of methods including elemental analysis, molecular weight determination, and infrared, ultraviolet/visible, and NMR spectra. The structure of these polybenzyls is globular with a high degree of branching. Thermal degradation was studied by using DTA and stepwise and isothermal degradation with analysis of both the volatile fraction and the residue. The activation energy for scissions was determined from changes in the molecular weight of the sample with time at a range of degradation temperatures of 340–440°C. The composition of the volatile fraction changed with both temperature and time of degradation. Thermal scission is directed to internal units and depends on the presence of weak links such as substituted anthracene units.     

Thermal degradation mechanisms of liquid crystalline aromatic polyesters studied by pyrolysis–gas chromatography/mass spectrometry

The thermal degradation mechanisms of liquid crystalline aromatic polyesters (LCPs) prepared from p-hydroxybenzoic acid (PHB), biphenol (BP), and terephthalic acid (TA) were studied by pyrolysis–gas chromatography/mass spectrometry (Py-GC/MS). The LCP containing deuterated terephthalate units and the LCPs that have different comonomer ratios were examined. On the basis of the pyrolysis products determined, the origin of the main pyrolysis products (benzene, phenol, biphenyl, phenyl benzoate, etc.) from the corresponding comonomer units were estimated and their thermal degradation mechanisms were eventually discussed in detail.     

Morphologies and crystalline forms of polyvinylidene fluoride membranes prepared in different diluents by thermally induced phase separation

We have studied the morphologies and crystalline forms of polyvinylidene fluoride (PVDF) membranes separately prepared in four different diluents bearing >C?O groups, namely 1,2‐propylene glycol carbonate (PGC), dimethyl phthalate (DMP), diphenyl ketone (DPK), and dibutyl phthalate (DBP), by the thermally induced phase separation (TIPS) method. The permittivities of the diluents and PVDF were measured to compare the different PVDF–diluent systems. The results showed the permittivity of PGC to be much greater than that of PVDF, and those of DMP and DBP to be lower than that of PVDF. The permittivity difference between DPK and PVDF was not apparent above 120 °C. On cooling mixtures with a PVDF concentration of 10 wt %, PVDF crystallization was observed in the PVDF–DMP, PVDF–DBP, and PVDF–PGC systems, while liquid–liquid phase separation occurred in the PVDF–DPK system. A cross‐section of the PVDF–PGC membrane presented smooth PVDF particles in the β‐phase crystalline form. Those of the PVDF–DMP and PVDF–DBP membranes presented PVDF particles consisting of a fibrillar network in the α‐phase. The PVDF–DPK membrane preferentially adopted an α‐phase bicontinuous channel structure. When the concentration of PVDF was 60 wt %, the cross‐sections of the above four membranes revealed PVDF polyhedra, among which the PVDF–DMP, PVDF–DBP, and PVDF–DPK membranes retained the α‐phase crystalline form, and the diffraction peak of the α‐phase became visible in the X‐ray diffraction (XRD) spectrum of the PVDF–PGC membrane. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Thermal analysis of binary liquid crystalline mixtures

Two series of binary mixtures composed of bent shaped and rod like molecules are reported. The first star shaped bent core molecules were synthesized and used as a component of binary mixtures. The chiral rod like compounds having commensurable length with the arms of the bent core compounds have been chosen for these mixtures. The resulted compositions show various thermotropic liquid crystalline phases that are characteristic to both types of liquid crystalline materials. In case of mixing the rod like molecules to the bent core compound the B2, B7 and induced B1 phases have been observed. While using the star-shaped bent core and chiral rod like compounds in mixture, the paraelectric smectic A, ferroelectric smectic C* and orthogonal hexatic smectic B phases were preferred. The appearing mesophases were investigated by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction methods.     

Thermal characterization of the plastic crystalline phase

Differential scanning calorimetry provides a simple and rapid method for the identification and characterization of plastic crystalline materials. The methods used are illustrated by applying them to 2,2-dinitropropane.