Lyda Matisová-Rychlá

Morphological and Thermal Properties of Cellulose−Montmorillonite Nanocomposites

Cellulose-layered montmorillonite (MMT) nanocomposites were prepared by precipitation from N-methylmorpholine- N-oxide (NMMO)/water solutions. Two hybrid samples were obtained to investigate the influence of the reaction time on the extent of clay dispersion within the matrix. It was observed that longer contact times are needed to yield nanocomposites with a partially exfoliated morphology. The thermal and thermal oxidative properties of the hybrids, which might be of interest for fire-resistant final products, were investigated by thermogravimetry and chemiluminescence (CL). The nanocomposites exhibited increased degradation temperatures compared to plain cellulose, and the partially exfoliated sample showed the maximum stability. This result was explained in terms of hindered transfer of heat, oxygen, and degraded volatiles due to the homogeneously dispersed clay filler. Kinetic analysis of the decomposition process showed that the degradation of regenerated cellulose and cellulose-based hybrids occurred through a multistep mechanism. Moreover, the presence of nanoclay led to drastic changes in the dependence of the activation energy on the degree of degradation. CL analysis showed that longer permanence in NMMO/water solutions brought about the formation of carbonyl compounds on the polymer backbone. Moreover, MMT increased the rate of dehydration and oxidation of cellulose functional moieties. As a consequence, cellulose was found to be less stable at temperatures lower than 100 degrees C. Conversely, at higher temperatures, the hindering of oxygen transfer prevailed, determining an increase in thermo-oxidative stability.

Chemiluminescence from paper I. Kinetic analysis of thermal oxidation of cellulose

Abstract Kinetic analysis of chemiluminescence intensity—time and chemiluminescence intensity—temperature runs for oxidation of cellulose at elevated temperatures is presented showing a good coincidence of the first-order rate constants from non-isothermal chemiluminescence experiments with those obtained from polymerisation degree determinations and from the literature. Rate constants from isothermal runs are lower than those from non-isothermal runs, however, when repeating non-isothermal runs several times in so-called temperature cycling experiments, the non-isothermal rate constants approach isothermal ones. A method enabling the differentiation of faster and slower processes in oxidation of cellulose based on non-isothermal chemiluminescence runs is demonstrated.

Chemiluminescence during thermal and thermo-oxidative degradation of cellulose

The chemiluminescent phenomena during thermal and thermo-oxidative degradation of cellulosic material and model substances were found to originate in at least three relatively independent processes. The low-temperature chemiluminescent phenomenon typical of samples previously irradiated with an incandescent light source and dependent on both temperature and composition of the surrounding atmosphere during irradiation is attributed to the decay of charge-transfer complexes between oxygen and hydroxyl and/or ether groups in cellulose. A peak in chemiluminescence during dynamic experiments at 135°C typical of pre-oxidised samples can be associated with decomposition of peroxide groups present in the sample. An intense chemiluminescence at temperatures approaching 200°C was found to correlate with polymer chain scissions, resulting in the decrease in the degree of polymerisation determined by viscometry. The results show that the chemiluminescence approach may shed new light on the radical degradation mechanisms of cellulose and related polyhydroxylated polymers.

Assessment of the interrelation between photooxidation and biodegradation of selected polyesters after artificial weathering.

Three commercially available biodegradable polymers, two different aromatic-aliphatic copolyesters and polylactic acid, intended for the fabrication of agricultural mulching films, in addition to other applications, were subjected to a series of tests with the aim of studying the relationship between their photooxidation and biodegradation. Photooxidation resulted in the rearrangement of polymeric chains, in the case of both copolyesters the events led to polymeric chain crosslinking and the formation of insoluble polymeric gel. The tendency was significantly more pronounced for the copolyester with the higher content of the aromatic constituent. As regards polylactic acid photochemical reactions were not accompanied by crosslinking but instead provoked chain scissions. A biodegradation experiment showed that, despite marked structural changes, the extent of photooxidation was not the decisive factor, which significantly modified the rate of biodegradation in all three materials investigated. The specific surface area of the sample specimens was shown to be more important.

Chemiluminescence from oxidation of polyamide 6,6. I. The oxidation of pure polyamide

The chemiluminescence (CL) accompanying the oxidation of both polyamide 6,6 and a model diamide with no reactive endgroups is reported. CL emission for neat diamide starts after melting of crystallites, and the shapes of the intensity-time curve show a sigmoidal behaviour, typical of the chain oxidation of organic compounds. Adipic acid shortens induction period of chemiluminescence increase. A bimolecular hydroperoxide decomposition model can successfully describe the kinetic runs of CL. Polyamide 6,6 CL runs are apparently composed of 3 different kinetic stages: a decay from an initial CL value, due to the termination of peroxyl radicals being trapped in the polymer after processing and storage (stage I), a sudden increase of emission, related to depletion of terminal amino groups (stage II), and a major maximum of emission, decaying again to a lower level of chemiluminescence, indicating chain oxidation of –CONH–CH2– structural units (stage III). Terminal carboxyl groups show the general tendency to shift the induction time of the third stage to a shorter time. # 2003 Elsevier Ltd. All rights reserved.

Initial degradation processes of cellulose at elevated temperatures revisited - chemiluminescence evidence

The initial processes governing degradation of cellulose at elevated temperatures are re-examined. The influences of moisture, alkali-earth metal carbonates, and atmosphere at 180 degreesC, were studied. The various effects were studied both conventionally, by determining the changes in the degree of polymerisation by viscometry, and by chemiluminescence measurements. The moisture initially contained in the sample was shown to have no effect on degradation in dry nitrogen although the chemiluminescence of moist samples is initially considerably higher. It was associated with production of singlet oxygen. On the other hand, presence of moisture in oxygen atmosphere enhances oxidation. Some initial degradation occurs during rapid heating and was attributed to mechanical degradation, though no chemiluminescent phenomenon could be associated with it. The degradation rates during the more advanced phases were shown to be related to the presence of earth-alkali metal cations. It is suggested that complex formation between a metal cation and glycosidic oxygen might play an important role in the degradation mechanism

Kinetics of mass changes in oxidation of polypropylene

Abstract Isothermal ageing of isotactic polypropylene films was studied in air, in the temperature range 50–150°C, by using gravimetry. The curves display a nonmonotonic shape owing to the occurrence of mass gain by oxidation and mass loss by evaporation of small molecular fragments. We have tried to construct a kinetic model of these mass changes using a ‘closed-loop’ model in which initiation is only due to bimolecular decomposition of hydroperoxides and volatile formation results directly from the initiation or/and termination reactions. This model gives better results than the corresponding one with unimolecular POOH decomposition but it needs a temperature variation of the initial POOH concentration, which seems physically unrealistic. Certain slightly more complex schemes could be the solution in mathematical terms, but they carry some ambiguity and are presumably more difficult to check.

Chemiluminescence and thermogravimetric study of thermal oxidation of polypropylene

Abstract Thermooxidation of unstabilized polypropylene powders was investigated by chemiluminescence and thermogravimetry methods in the temperature interval 100–190 °C and a model of formal termination of reaction sites has been used for the description of the kinetics of chemiluminescence.

Thermooxidation of lactam-based polyamides with amino end-groups. thermooxidation of hexano-6-lactam and decomposition of 6-hydroperoxyhexano-6-lactam in the presence of primary amines*

Oxygen consumption, reaction products and chemiluminescence in the course of the thermooxidation of hexano-6-lactam melt in the presence of primary amines as well as thermooxidation of lactam-based polyamides with amino end-groups were studied. Amines induce the homolysis of amide hydroperoxides, primary products of the chain oxidation of lactams, and react with aldehydes originating from hydroperoxide homolysis. By condensation of amines with aldehydes, aldimines are produced together with singlet oxygen, the initiator of N-alkylamide oxidation. In polylactams prepared by hydrolytic polymerization, the carboxylic and amino end-groups are associated and their effect on polymer thermooxidation is thus suppressed.

Rate constants of oxidation of unsaturated fatty esters studied by chemiluminescence

This study deals with oxidation kinetics of three unsaturated fatty esters: methyl oleate, methyl linoleate and methyl linolenate at temperatures ranging from 90 to 150 °C. The reaction was monitored by chemiluminescence. A kinetic model was derived from a simple mechanistic scheme, in which initiation is due to hydroperoxides decomposition, whereas propagation results from the abstraction of the most labile hydrogen and termination results only from the bimolecular combination of peroxyl radicals. Analysis of induction period duration indicated that hydroperoxides mainly decompose by a bimolecular process. The model well predicts the main features of the experimental chemiluminescence curves. Kinetic parameters of the three unsaturated fatty esters were assessed from inverse method and discussed.