Mei-Rong Huang

Thermal degradation of cellulose and cellulose esters

Cellulose, cellulose diacetate (CDA), cellulose triacetate (CTA), cellulose nitrate (CN), and cellulose phosphate (CP) were subjected to dynamic thermogravimetry in nitrogen and air. The thermostability of the cellulose and its esters was estimated, taking into account the values of initial thermal degradation temperature Td, the temperature at the maximum degradation rate Tdm, and char yield at 400°C. The results show that these polymers may be arranged in the following order of increasing thermostability: CN < CP < regenerated cellulose < filter cotton < CDA < CTA. The activation energy (E), order (n), and frequency factor (Z) of their degradation reactions were obtained following the Friedman, Chang, Coats–Redfern, Freeman–Carroll, and Kissinger methods. The dependence of Td, Tdm, E, n, Ln Z, and char yield at 400°C on molecular weight and test atmosphere is also discussed.

Thermal decomposition kinetics of thermotropic poly(oxybenzoate-co-oxynaphthoate) Vectra copolyester

An advanced heat-resistant thermotropic liquid crystalline Vectra aromatic copolyester prepared from p-hydroxybenzoic acid and 2,6-hydroxynaphthoic acid has been subjected to dynamic and isothermal thermogravimetries in nitrogen and air. The thermostability of the Vectra has been investigated in detail. The thermal decomposition kinetics have been analyzed using seven calculating techniques. These techniques gave an average activation energy (E) of 374 kJ/mol for the nonisothermal decomposition of Vectra in nitrogen, which is much higher than the E value of 255 kJ/mol determined from the isothermal thermogravimetric experiments. The average E value for the thermal decomposition in air was 288 kJ/mol from dynamic heating data. The E and Ln Z (frequency factor) are higher in nitrogen than in air from any of the calculating techniques used, but the nonisothermal decomposition order of 2.3 in nitrogen shows good agreement with the order of 2.1 of nonisothermal degradation in air and of isothermal degradation in nitrogen. The thermostability of Vectra is substantially higher in nitrogen than in air, and the decomposition mechanism in air is more complex because oxidation process is occurring in air.

Kinetics of thermal degradation of thermotropic poly(p-oxybenzoate-co-ethylene terephthalate) by single heating rate methods

The kinetics of decomposition of thermotropic liquid crystalline poly(p-oxybenzoate-co-ethylene terephthalate) (poly(B-co-E), BE polymer) with different monomer ratios in both nitrogen and air were studied by dynamic thermogravimetry (TG) from ambient temperature to 800°C. The kinetic parameters, including the activation energy E′, the reaction order n, and the pre-exponential factor Z, of the degradation of the BE polymers were evaluated by the single heating rate methods of Friedman, Freeman–Carroll and Chang. The BE polymers which degraded in two distinct stages in nitrogen and air, were stable under nitrogen, while almost completely burned in air. The weight losses in the first stage in nitrogen and air were dominated by the thermal degradation of both B and E segments, but the weight losses in the second stage were governed by the thermal degradation of B segment in nitrogen and by the oxidative degradation of both B and E segments in air. The maximum rate of weight loss increased linearly with the increase of E content and heating rate, but as E content increased, the char yield at 800°C in nitrogen decreased linearly. The E′, n and ln Z values of the BE polymers in the first stage of thermal pyrolysis are higher in nitrogen than in air, indicating that the degradation rate is slower in an inert atmosphere. The E′ value increased with increasing heating rate but varied irregularly with the variations of B/E ratios and molecular weight. The n and ln Z values for the BE polymers in nitrogen were found to be in the wide ranges 1·5–5·6 and 12–48min-1, respectively, suggesting a complex degradation process. The estimated lifetimes of the BE polymers at 250°C were calculated to be at least 33 days in nitrogen and 3h in air.

Thermal degradation of Kevlar fiber by high-resolution thermogravimetry

A novel high-resolution thermogravimetry (TG) technique in a variable heating rate mode that maximizes resolution and minimizes the time required for TG experiments has been performed for evaluating the thermal degradation and its kinet- ics of Kevlar fiber in the temperature range ; 25-900°C. The degradation of Kevlar in nitrogen or air occurs in one step. The decomposition rate and char yield at 900°C are higher in air than in nitrogen, but the degradation temperature is higher in nitrogen than in air. The initial degradation temperature and maximal degradation rate for Kevlar are 520°C and 8.2%/min in air and 530°C and 3.5%/min in nitrogen. The different techniques for calculating the kinetic parameters are compared. The respec- tive activation energy, order, and natural logarithm of preexponential factor of the degradation of Kevlar are achieved at average values of 133 kJ/mol (or 154 kJ/mol), 0.7 (or 1.1), and 16 min 21 (or 20 min 21 ) in air (or nitrogen). The technique based on the principle that the maximum weight loss rate is observed at the minimum heating rate gives thermal degradation results that were in excellent agreement with values deter- mined by traditional TG experiments.

Multilayer ultrathin-film composite membranes for oxygen enrichment

Multilayer composite membranes were made of poly(4-methylpentene-1) (PMP), an ethyl cellulose (EC) + heptyl cellulose (HC) blend, polycarbonate (PC), polysulfone, poly(2,6-dimethylphenylene oxide), cellulose triacetate ultrathin films as selective layers, and polysulfone, poly(ether sulfone), and poly(sulfone amide) ultrafiltration membranes with a 10–45 nm pore size and 100–120 μm thickness as porous support layers. The effects of the ultrathin-film type and its casting solution concentration, operating pressure, temperature, as well as time on the oxygen-enriched air (OEA) flux and oxygen concentration in the OEA permeated in a single step through the composite membranes were investigated using a constant pressure—variable volume method. The OEA flux increases significantly with an increasing transmembrane pressure difference and operating temperature. The oxygen concentration in the OEA also increases with an increasing pressure difference but decreases slightly with an increasing operating temperature. In long-term tests, the oxygen-enrichment properties were maintained almost constant for as long as 170 h. The composite membranes consisting of the bilayer ultrathin film cast from a more dilute solution (0.11–0.26 wt %) on the porous support with a smaller pore size combine a higher oxygen-enriching ability and a higher stability than do those of monolayer and tetralayer ultrathin films. The maximum OEA flux and oxygen concentration produced at 20–75°C and a 500 kPa transmembrane pressure difference in a single pass across the PMP/98EC + 2HC bilayer and PC bilayer ultrathin-film composite membranes are 3.1 × 10−3 cm3(STP)/s cm2 and 50%, respectively.

Synthesis and characterization of liquid crystalline polymers from p-hydroxybenzoic acid, poly (ethylene terephthalate), and third monomers

Eight new p-hydroxybenzoic acid (PHB) and poly(ethylene terephthalate) (PET) copolymers containing vanillic acid (VA), p-aminobenzoic acid, m-hydroxybenzoic acid, hydroquinone/terephthalic acid (TPA), bisphenol A/TPA, 1,5-naphthalenediol/TPA, 2,7-naphthalenediol/TPA, and 1,4-dihydroxyanthraquinone/TPA as eight third monomers with a variety of structural features were synthesized by melted-state copolycondensation and were characterized through a thermal analyzer, proton nuclear magnetic resonance, wide-angle X-ray diffraction (WAXD), and a scanning electron microscope (SEM). The experimental results show that PHB/PET/VA copolymers exhibit a faster polycondensation rate, lower melting temperature, and higher thermostability than do the other seven copolymers and third monomer-free PHB/PET polymers. The as-spun fibers derived from the PHB/PET/VA copolymers with different VA contents show tensile strengths, Youngs moduli, and break elongations of 0.6–1.5 GPa, 28–67 GPa, and 7–9%, respectively. A highly oriented fibrillar structure in the PHB/PET/VA copolymer fibers was observed using WAXD and SEM. The most effective third monomer of the eight third monomers for an enhancing polycondensation rate and molecular weight of the PHB/PET polymers and for improving their thermal and mechanical properties is found to be vanillic acid (VA).

Thermal degradation of bisphenol A polycarbonate by high‐resolution thermogravimetry

Thermal degradation of bisphenol A polycarbonate (PC) has been studied in nitrogen and air from room temperature to 900 °C by high-resolution thermogravimetry (TG) with a variable heating rate in response to changes in the samples degradation rate. A three-step (in nitrogen) or four-step (in air) degradation process of the PC, which was hardly ever revealed by traditional TG, has been found. The initial thermal degradation temperature of the PC is higher in nitrogen than in air, but the three kinetic parameters (activation energy E, decomposition order n, frequency factor Z) of the major degradation process are slightly lower in nitrogen. The average E, n and lnZ values determined by three methods in nitrogen are 154 KJ mol−1, 0.8 and 21 min−1, respectively, which are almost the same as those calculated by traditional TG measurements. © 1999 Society of Chemical Industry

Glass transition of thermotropic polymers based upon vanillic acid, p‐hydroxybenzoic acid, and poly(ethylene terephthalate)

DSC and dynamic mechanical and thermomechanical analyses are used to study the T g of the thermotropic liquid crystalline polyesters prepared by molten polycondensation of 4-acetoxyvanillic acid (V), 4-acetoxybenzoic acid (B), and poly(ethylene terephthalate) (E). The polyesters exhibit two glass transition ranges at 66-83 as well as 136-140°C, which are attributed to E segmental motion in the E-rich/B-rich phases of the polyesters, as well as to the local mobility of the B component in the B-rich phase. The lower T g of the polyesters increases with an increase in the B and V unit contents or with increasing heating rate in the DSC measurements, but the higher T g decreases slightly. The experimental T g values are compared with the T g values predicted from three equations on the basis of the sequence distribution and the monomer reactivity ratio of the polyesters. Uematsus, Johnstons, and Couchmans equations all give a better fit with the experimental results.

Thermal decomposition kinetics of liquid crystalline p-oxybenzoate/ethylene terephthalate/third monomer terpolymers

The thermal stability and decomposition kinetics of two series of thermotropic liquid crystalline terpolymers consisting of p-oxybenzoate, ethylene terephthalate and third monomer unit including vanillate or phenylene terephthalate with several monomer ratios have been studied using dynamic thermogravimetry (TG) based on three single heating rate methods of kinetic analysis. The results indicate that the thermal stability and degradation kinetics of the terpolymers are dependent of monomer ratio, third monomer content, heating rate, test atmosphere, measuring method, and calculating method. The rigidity of p-oxybenzoate appears to be responsible for the improved the rmostability and resistance of main-chain scission in the terpolymers. Less p-oxybenzoate- or third monomer-containing terpolymers seem to exhibit larger activation energy, frequency factor, maximum decomposition rate and lifetime, but smaller decomposition order of thermal decomposition. The maximum degradation rate of the terpolymers increases linearly with increasing heating rate. The activation energy, preexponential factor, and decomposition order for the first stage thermal decomposition of the p-oxybenzoatelethylene terephthalate/vanillate terpolymers under nitrogen range between 217 and 260 kJ/mol, 35 and 43 min−1, and 3.1 and 4.7, respectively, which are larger than the three kinetic parameters of the thermal degradation for the p-oxybenzoate/ethylene terephthalate/phenylene terephthalate terpolymers under air. The longest estimated lifetimes of the terpolymers are 59.1 min at 350°C and 1.1 min at 410°C.

High‐resolution thermogravimetry of liquid crystalline copoly(p‐oxybenzoate–ethylene terephthalate–m‐oxybenzoate)

Thermotropic liquid crystalline terpolymers consisting of three units of p-oxybenzoate (B), ethylene terephthalate (E), and m-oxybenzoate (M), were investigated through high-resolution thermogravimetry to evaluate their stability and kinetic parameters of thermal degradation in nitrogen and air. Overall activation energy data of the first major decomposition was calculated through three calculating methods. Thermal degradation occurs in three major steps in both nitrogen and air. Three kinds of degradation temperatures (T d , T m1 , T m2 ) are slightly higher and the first maximum weight-loss rates are slightly lower in nitrogen than in air, suggesting a higher thermostability in nitrogen. The thermal degradation temperatures range from 450 to 457°C in nitrogen and 441 to 447°C in air and increase with increasing B-unit content at a fixed M-unit content of 5 mol %. The temperatures at the first maximum weight loss rate range from 452 to 466°C in nitrogen and 444 to 449°C in air and increase slightly with an increase in B-unit content. The first and second maximum weight-loss rates are maintained at almost 9.2-10.8 and 4.0-6.1%/min in nitrogen (11.2-12.0 and 3.9-4.2%/min in air) and vary slightly with copolymer composition. The residues after the first major step of degradation are predicted on the basis of the complete exclusion of ester and ethylene groups and hydrogen atoms and compared with those observed experimentally. The char yields at 500°C in both nitrogen and air are larger than 42.6 wt % and increase with increasing B-unit content. However, the char yields at 800°C in nitrogen and air are different. The activation energy and In(pre-exponential factor) for the first major decomposition are slightly higher in nitrogen than in air and increase with an increase in B-unit content at a given M-unit content of 5 mol %. There is no regular variation in the decomposition order with the variation of copolymer composition and testing atmosphere. The activation energy, decomposition order, and In(preexponential factor) of the thermal degradation for the terpolymers are located in the ranges of 212-263 kJ mol -1 , 2.4-3.5, 33-41 min -1 , respectively.