F. Zamora

Thermodegradation and thermal transitions of an exopolysaccharide produced by Pediococcus damnosus 2.6

The thermodegradative behavior of an exopolysaccharide (EPS), produced by Pediococcus damnosus 2.6 from a strain isolated from a ropy Basque Country cider, was studied. Two endothermic processes at 118 and 163°C were detected by differential scanning calorimetry and considered as sub and main glass transitions, respectively, for the EPS. Dynamic thermogravimetry experiments showed: a first weight loss, below 100°C, due to alcohol molecules trapped in the structure of the EPS, a main degradation process involving both dehydration reactions, which yield levoglucosan structures, and scissions of C–O bonds, either in the inner glucan rings or between rings, going on with depolymerization reactions up to 350°C, and char-forming reactions at higher temperatures. From isothermal degradation processes, at short reaction time, an activation energy of 33 kJ/mol was determined. This value changed to 50.4 kJ/mol for conversion ranges of 20–35%, rising to 138 kJ/mol at 50% (determined by dynamic methods). Analyzing the whole process by McCallums dynamic method, the energy value reaches 293 kJ/mol. Differential thermal analysis indicated at least two simultaneous reactions that are heating rate dependent and influence the whole degradation process.

The influence of molecular weight on the degradation of poly(N-vinylcarbazole)

Abstract The methods of isothermal and dynamic thermogravimetry have been used to study the degradation of poly(N-vinylcarbazole) (PNVC). The multiple heating rate method has been used, as a dynamic method, to obtain kinetic parameters. A linear relationship between the activation energy. E, and M w −1 ( M w being weight average molecular weight) was found. From isothermal experiments, a temperature was found for which E was independent of molecular weight. We could then refer to a degradation characteristic- temperature of the polymer. On the other hand, altering the heating rate leads to changes in the values of E for each molecular weight indicating two kinds of scission: one occurs in the backbone, producing mainly monomer; in the other, both side-group and backbone scissions occur producing different products.

Inclusion of the cation iodonium (I3+) into the structure of poly(N-vinylcarbazole)

Abstract Iodine is an usual doping agent used in order to improve the conductivity of insulator polymers with either aromatic and conjugated double bonds in their structure. This paper evidences the action of the cation iodonium on the conductivity of poly( N -vinylcarbazole) (PVK) and nitrosilated-PVK (PVK-NO). The cation is introduced via bis-pyridine-iodonium tetrafluoroborate salt (IPYBF 4 ) in both polymers yielding PVK–IPY-P and PVK-NO–IPY-P complexes, respectively, with a 1:1 ratio (carbazolyl/iodonium) in each. The complex extends the absorption band in the UV–Visible region and its IR spectrum shows characteristic bands. Both inter- and intra-molecular distances were modified in the polymer by the presence of iodonium as it is manifested by WAXS experiments. The thermal behaviour of the complex was studied by simultaneous thermogravimetry and differential thermal analysis being quite different from that of PVK. The polymer–IPY complex is stable till 200°C at which point the pyridine ligands loss takes place and results in a new structure of PVK-I + which is stable till 380°C and then is suddenly degraded. Elecrical conductivity of the complex improves with respect to that of PVK, but it does not overcome the semiconducting level achieved with other dopants.

Degradation of partially mononitrated-poly(N-vinylcarbazole) by thermogravimetry

Fractions of cationic poly(N-vinylcarbazole) (PNVC-A) and unfractionated radical poly(N-vinylcarbazole) (PNVC-B) have been nitrated in dimethylsulphoxide using a sulphuric-nitric acid mixture. Proportions of 3-nitrocarbazoyl units, ranging from 70 to 90% were found in PNVC-A samples depending on the molecular weight. An i.r.-band at 650 cm−1, present in dinitrated carbozoyl compounds but absent from mononitrated, could be used for analysis of polymer mixtures or copolymers, containing those units. By differential scanning calorimetry, exothermal and endothermal processes were observed. An average value of ΔH = 272 ± 8 J/g in samples A and ΔH = 218 ± 8 J/g in samples B were found for the exothermal process. This process corresponds with the first stage observed by thermogravimetry in degradation. Samples with nitration degree <76% did not show that behaviour. The activation energy during degradation depended on both molecular weight and nitration degree.

A possible charge-transfer complex in nitrated poly(N-vinylcarbazole)

Abstract Radical and cationic poly(N-vinylcarbazole) were fractionated and the fractions nitrated to give mono- and di-nitrated copolymers. Depending on the composition, inter- and intramolecular interactions manifested as charge-transfer complexes were found, as suggested by absorption and emission spectra and by measurements of thermally stimulated depolarization currents.

Determination of the flexibility parameter of polymers

Abstract This paper deals with the determination, in theta conditions, of the flexibility parameter λ defined by Kratky-Porod for random-coiled polymers, using three different methods: (a) from hydrodynamic data in θ conditions, (b) by using a new form of extrapolation to zero of the corresponding values for the average molecular weight, and (c) by extrapolation of λ tending to zero. From method (b), values of the interaction parameter, B, were in agreement with those from Stockmayer-Fixman theory. A linear relationship was obtained from (c) by plotting (R2/L)½ versus λ. The slope, h∗, of this plot was found to be related with λθ in the following way: where A1 and A2 are constants, and h is the monomeric unit projection length in terms of the wormlike theory.

Alternating current conductivity of iron(III)-phthalocyanine grafted poly(N-vinylcarbazole)

Abstract Differences in alternating current conductivity measurements carried out on Fe(III)-phthalocyanine grafted poly(N-vinylcarbazole), compared to that of the fresh polymer, are attributed to both the linking chemical group involved and resonant structures developed. Polar and magnetic characteristics of the samples create capacitative and inductive behaviors. Frequency and temperature variations cause nonlinear electrical responses and enhanced conductivities up to 6 orders of magnitude, a.c. conductivity values, over 10−2 S m−1 obtained at discrete frequency and temperature values, are unusual responses of electrical measurements in vinylaromatic-based semiconducting polymer materials. Experiments using varying temperatures from room temperature to 200°C, gave conductivity peaks that are heating rate dependent. The bond flexibility of the link between the metal-phthalocyanine and polymer is of primary importance to the conduction behavior.

Determination of the monomeric unit projection length from gel permeation chromatography and viscometry

Abstract Several homodispersed samples of poly(N-vinylcarbazole), poly(acenaphthylene), poly(methyl methacrylate), and poly-(vinyl chloride) have been analyzed in order to obtain information about the monomeric unit projection length, h, from gel permeation chromatography and viscometry measurements. Although the studied polymers are vinyl type, the h values obtained depend on the bulk of their side groups.

Degradation of dinitrated-poly(n-vinylcarbazole) by thermogravimetry

Abstract The techniques of isothermal and dynamic thermogravimetry have been used to study the degradation of six nitrated homodisperse fractions of poly(N-vinylcarbazole). The effects of nitrogen content and molecular weight of the samples on kinetic parameters are reported. Increasing the heating rate or using higher isothermal temperatures causes a sudden mass-loss and an increasing sample temperature was observed. An induction period in which −NO2 scission took place may be the cause of this sudden mass-loss.

Thermal transitions and degradative processes in Fe(III)-tetracarboxyphthalocyanine-grafted poly(N-vinylcarbazole)

Abstract The introduction of 2,9,16,23-tetracarboxy-Fe(III)phthalocyanine [Fe(III)tcPc] units onto fractions of poly(N-vinylcarbazole) (PVK) by Friedel-Crafts reaction leads to amorphous PVK-based grafted compounds that show major interchain distances by the wide-angle x-ray scattering (WAXS) technique. Glass transition temperatures of the products are lower than the main glass transition temperature of PVK and are attributed to local movements of the voluminous added groups. Dynamic thermogravimetric experiments show decarboxylation processes within the 250°C-500°C range and little weight loss. The large remaining residue, up to 70% of the sample at 800°C, is suggested to be the consequence of successive cross-linking reactions. The Fourier transform infrared (FTIR) spectrum of the residue is similar to that of the grafted sample before degradation.