Thermodynamic stability of polyacrylamide and poly(N,N‐dimethyl acrylamide)

Based on theoretical thermodynamic principles, the possibility of environmental degradation of polyacrylamide to its starting monomer was investigated. Theoretical electronic structure studies on the geometry and fragmentation energy of acrylamide and N,N‐dimethyl acrylamide tetramer models were carried out using a first principles gradient corrected density functional approach. Thermal degradation to form a radical would require the cleavage of carbon–carbon bonds in the polymer chain; the energy needed for this cleavage was found to depend on the structure of the repeat unit which ranged from low of 72.5 kcal for a rare head‐to‐head construct to 86.2 kcal for a normal head‐to‐tail polymer construct (therefore, for the cleavage of a normal head‐to‐tail repeat unit, temperatures of approximately 450°C would be required). The thermodynamics of the unzipping, disproportionation, and back‐biting reactions in the resulting radical fragments were also investigated; the back‐biting process was found to require the least energy and provided the most stable radical fragment with a low probability for disproportionation or releasing of monomer to occur. The effect of solvation on the hydrogen‐bonding network in the acrylamide tetramer was studied by adding explicit molecules of water to the tetramer models. The addition of water had a significant effect on the stability of the model polymer slightly stabilizing the head‐to‐head polymer, and slightly destabilizing the head to tail polymer. Copyright © 2007 John Wiley & Sons, Ltd.     

Development of a novel potentiometric titration method for determination of polypropylene degradation

In this study, a novel potentiometric titration of hydroperoxide in degraded polypropylene (PP) is proposed. This titration is quite sensitive compared with the conventional ones such as UV and manual titrations, and its detection limit was about 2 meq/kg. The sensitivity was equal to that of molecular weight measurement by GPC for the degraded PP and, in addition, the volatilization behavior of the hydroperoxide could be detected. This titration was found to be very effective for the determination of PP degradation.     

Effects of EG and MoSi2 on thermal degradation of intumescent coating

An intumescent flame retardant coating was prepared with resin, solvent and flame retardant system composed of ammonium polyphosphate-APP, pentaerythritol-PER and melamine-MEL. The modifiers such as molybdenum disilicide (MoSi₂) and expandable graphite (EG) were used to improve the performances of the APP-PER-MEL coating. The effects of EG, MoSi₂ and MoSi₂/EG on the fireproofing time and char formation of the coating were investigated by using heat insulation test, thermogravimetric analysis (TGA), differential thermal analysis (DTA), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electric microscope (SEM). The results showed that by adding modifiers, the fireproofing time was prolonged and char formation rate was evidently enhanced. The largest improvement was achieved with 9 wt.% MoSi₂/5 wt.% EG, XPS analysis indicated that the performance of anti-oxidation of the coating was improved by adding EG and MoSi₂, and SEM images showed that a good synergistic effect was obtained through a ceramic-like layer produced by MoSi₂ covered on the surface of “open-cellular” structural char.     

Comparison of EI and metastable atom bombardment ionization for the identification of polyurethane thermal degradation products

Polyurethanes are widely used in the manufacture of commercial products such as foams and paints. During combustion, these polymers can generate isocyanates, which induce adverse health effects. Polymer pyrolysis (Py) hyphenated with mass spectrometry (MS) allows the investigation of polymer thermal degradation over time/temperature. A diphenylmethanediisocyanate (MDI) polyurethane foam was analyzed with electron ionization (EI) and metastable atom bombardment (MAB) ionization at a pyrolysis temperature of 400 °C. The recently introduced MAB ionization source uses discrete energy stored in metastable atoms of gases to ionize the analytes. This characteristic allows modulation of the ionization energy by simply changing the ionization gas. The extensive fragmentation of molecular ions observed using EI 70 eV is not totally eliminated with EI 10 eV. However, only molecular ions are observed with MAB using N₂ as the ionization gas. Temperature gradients were used to separate the products generated during the thermal degradation of a 1,6-hexamethylenediisocyanate (HDI) polyurethane paint. The analysis of mass spectra was facilitated owing to a selective desorption of pyrolysis products. Furthermore, changing the MAB ionization gas allows elucidation of the structure of the pyrolysis products by controlling the extent of their fragmentation. During these experiments, isocyanic acid, methyleneisocyanate, ethyleneisocyanate, propylisocyanate and butylisocyanate were detected.     

Investigation on the thermal degradation of acrylic polymers with fluorinated side-chains

The thermal degradation of poly-2,2′,3,3′,4,4′,5,5′,6,6′,7,7′,7″-tridecafluoroheptylacrylate and poly-2,2′,3,3′,4,4′,5,5′,6,6′,7,7′-dodecafluoroheptylmethacrylate has been studied in isothermal conditions at 450-750 °C using pyrolysis-gas chromatography. The type and composition of the pyrolysis products give useful information about mechanism of thermal degradation. It was shown that the main thermal degradation process for both polymers is random main-chain scission. The major degradation products for fluorinated polyacrylate are monomer, dimer, saturated diester, trimer, and corresponding methacrylate. The fluorinated polymethacrylate gives monomer as the main product of thermal destruction. As a result of side-chain reaction, the thermal degradation of the fluorinated polyacrylate also produces remarkable amounts of alcohol. On the other hand, the respective alcohol is only a minor component among the pyrolysis products of the fluorinated polymethacrylate. For both polymers, the main nontrivial degradation product coming from the alkyl ester decomposition is the corresponding fluorinated cyclohexane. The formation of the fluorinated cyclohexanes may be accounted for a nucleophilic bimolecular substitution pathway.     

Selective degradation of lignin and elimination of HO radicals in pulps by O<Subscript>3</Subscript> and UV laser flash irradiation

HO^. radical is an aggressive reagent to abstract hydrogen from diverse substitutes and lead them to degradation, however, in reaction of active oxygen species with lignins, complex phenolic polymers, in dispersed lignocellulose such as pulp for environment-benign delignification, HO^. radicals should be eliminated as more as possible to prevent cellulose from unfavorably concomitant degradation. A reaction system of O₃ is constructed under UV laser flash irradiation, and HO^. radicals are controlled efficiently by it. A new mechanism is proposed, for the first time, that O^. radicals generated from reaction of O₃ with UV laser flash irradiation might be the contributor to scavenge HO^. radicals.     

TG and DSC studies of filled porous copolymers

A complex approach including thermogravimetry, differential thermal analysis and differential scanning calorimetry was applied to study characteristics of non-filled and filled porous copolymers of divinylbenzene with styrene or some acrylic monomers: di(methacryloyloxymethyl)naphthalene, methacrylic ester of p,p’-dihydroxydiphenylpropane diglycidyl ether, and dimethacrylglycolethylene. High disperse silicas with the grafted methyl and silicon hydride groups in the surface layer were used as fillers. The kinetic parameters of thermal degradation for composites obtained were determined by different methods.     

Exchange reactions occurring through active chain ends: Melt mixing of nylon 6 and polycarbonate

The chemical reactions occurring in the melt mixing of nylon6/polycarbonate (Ny6/PC) at 240°C were investigated. The reaction of equimolar Ny6/PC blends can be reconciled within the overall scheme of an exchange reaction occurring with the attack of active amino terminals on the inner carbonate groups. We have performed the synthesis of low molecular weight amino-terminated nylon 6 and the effect of the active amino terminal groups on the exchange kinetics was investigated. The exchange reaction yields sizeable amounts of copolymer, in fact after 75 min of melt mixing the (initially equimolar) blend contains 30 mol of unreacted PC and 70 mol of Ny6/PC copolymer (all the Ny6 was therefore incorporated in the copolymer). Trifluoroacetylation of nylon 6 was used to produce CHCl₃-soluble Ny6/PC copolymers, that could be analyzed by NMR. The NMR analysis yielded, beside the copolymer composition, evidence of the presence of urethane units interconnecting the Ny6 and PC blocks. The amount of urethane units increased with the reaction time, indicating a reduction of the block size as a function of the extent of exchange. Our study established the structure of the products formed, provided the materials balance of the process, and investigated some salient kinetic aspects. A thermal degradation study was also performed by thermogravimetry and direct pyrolysis mass spectrometry, to identify the products formed in the thermal treatment of the blends and to investigate the possible role of the inner amide groups in the intermolecular exchange reactions occurring between Ny6 and PC. Our results prove that these reactions occur above 300°C, and that only the cleavage of carbonate groups, by means of Ny6 amino end groups, is actually occurring at 240°C. © 1994 John Wiley & Sons, Inc.     

Prediction of paper permanence by accelerated aging II. Comparison of the predictions with natural aging results

Accelerated aging tests are credible and useful to predict paper permanence only if such tests can be shown to correlate with natural aging. In the first part of this study, a kinetic model was developed based on the accelerated aging results. In this report, we have shown that this kinetic model can indeed predict the natural aging results of lignin-free sheets with a statistical confidence. This is the first quantitative comparison of accelerated aging with natural aging.     

Fe3+-TiO2/SiO2光催化降解罗丹明B的研究

以硅胶为载体,采用溶胶-凝胶法制备了不同掺杂量的Fe3+-TiO2/SiO2光催化剂,并采用SEM,Raman和DRS等手段对其进行了分析和表征.以氙灯为光源,通过对可溶性染料罗丹明B的降解反应,考察了Fe3+-TiO2/SiO2催化剂的光催化活性,探讨了光催化反应中溶液pH值和起始浓度对催化反应的影响.