Polymer Degradation and Stability (v.84, #3)

Thermal stabilities and the thermal degradation kinetics of polyimides by Liqing Li; Chunxiu Guan; Aiqing Zhang; Donghua Chen; Zhibao Qing (369-373).
The degradation kinetics of a series of polyimides containing maleic anhydride (MA), 4,4′-oxydianiline (ODA) and hexafluoroisopropylidene 2,2-bis (phthalic anhydride) (6FDA) were measured by thermogravimetric analysis (TGA) at different heating rates in flowing nitrogen. TGA experiments were performed to elucidate the thermal behaviour and supplied the data that characterise the degradation. The apparent activation energies E a were calculated by iterative isoconversional procedure, the mechanism and pre-exoponential factor A were determined by Coats–Redfern and Achar methods. This methodology compared four kinds of (MA + 6FDA + ODA) polyimides' excellent thermal stability: PI > PMI-1 > PMI-2 > PMI-3, it showed that E a for thermal degradation of polyimides reduced with decrease of 6FDA content and increase of MA content. The thermal decomposition kinetics of the four polymers obeyed the Avrami–Erofeev model function.
Keywords: Degradation kinetics; Polyimide; 6FDA; TGA; Thermal stability;

Poly(dimer acid–dodecanedioic acid) P(DA–DDDA) copolymers and poly(dimer acid–tetradecanedioic acid) P(DA–TA) copolymers have been prepared by melt polycondensation of the corresponding mixed anhydride prepolymers. The copolymers were characterized by FT-IR, gel permeation chromatography (GPC), differential scanning calorimetry (DSC), wide-angle X-ray powder-diffraction, and thermogravimetric analysis (TGA). In vitro studies showed that all the copolymers are degradable in phosphate buffer at 37 °C, and leave an oily dimer acid residue after hydrolysis for the copolymers with high content of dimer acid. The release profiles of a hydrophilic model drug, ciprofloxacin hydrochloride, from the copolymers, follow first-order kinetics. All the preliminary results suggested that the copolymers might be potentially used as drug delivery devices.
Keywords: Poly(dimer acid–dodecanedioic acid) copolymer; Poly(dimer acid–tetradecanedioic acid) copolymer; Degradable polymer; Drug slow-release;

UV degradability of aryl-substituted polysilylenes by F Schauer; I Kuřitka; S Nešpůrek (383-391).
The susceptibility of aryl-substituted polysilylenes to photodegradation by UV radiation was examined for poly[methyl(phenyl)silylene] (PMPSi), poly[(biphenyl-4-yl)methylsilylene] (PBMSi) and copolymer poly[methyl(phenyl)silylene{3(4)-[(2,4-dinitrophenylhydrazono)methyl]phenyl}methylsilylene] (PMPSi-DNPH). It is found that with increasing aromaticity of the aryl substituents the character of the first σ–σ optical transition (Si–Si excitation in PMPSi forming a photoluminescence band at about 355 nm) changes to the π–π type of transition. The photodegradation of PMPSi by UV radiation decreases the intensity of the exciton photoluminescence (PL) band. The time dependence is in accord with the idea of the scission of longer Si–Si segments. Visible PL is not changed during the photodegradation. In PBMSi and PMPSi-DNPH, the visible photoluminescence first decreased during the degradation by UV radiation, similarly to that in PMPSi caused by the absorption decrease, but then the rise and saturation of PL were observed. This is ascribed to the increased population of states by the progressive Si–Si chain scission and to the formation of states suppressing the non-radiative recombination. For demonstration purposes, a plasma-prepared PMPSi was examined, which exhibits all the PL bands mentioned.
Keywords: UV degradability; Polysilylenes; Weakened bond; Photoluminescence;

The thermal degradation of various aliphatic polyesters such as poly(ε-caprolactone) (PCL), poly(glycolide) (PGA), and poly(d, l-lactide) (PLA), their physical blends and copolymers were investigated in a thermogravimetry analyzer under dynamic heating in an inert nitrogen atmosphere. The peak decomposition temperatures of the PCL, PGA, and PLA are 402, 360 and 295 °C, respectively. The Friedman plots showed a 2.5- to 3-fold increase in the activation energy from lower temperatures to higher temperatures during degradation. This suggested that the governing mechanism changes during degradation from random chain scission to specific chain end scission with increase in temperature. The activation energies for the random chain scission of PCL, PGA and PLA are 18.5, 16.5, and 10.2 kcal/mol, respectively and the corresponding activation energies for the specific chain scission are 55.5, 32.0, and 25.2 kcal/mol. Experiments were also conducted under isothermal holding to evaluate the activation energies for specific chain scission and are 47.5, 28.0, and 22.2 kcal/mol for PCL, PGA, and PLA, respectively. The degradation of the blends and copolymers was also investigated. The presence of PLA in the copolymer destabilized the other polymers (PCL and PGA). However, the presence of PLA in the mixture did not influence the degradation of PGA.
Keywords: Poly(ε-caprolactone); Poly(d, l-lactide); Poly(glycolide); Activation energy; Copolymers; Physical blends;

One-stage degradation process was observed for a poly(methyl methacrylate) (PMMA) sample. The fractional conversion α max at the maximum reaction rate is thus applied to determine reaction order of the thermal degradation. It is found that the PMMA degraded in the first order kinetics with weight loss. This is consistent with the fact that PMMA thermally degrades into its monomer. Activation energy was calculated. Compensation effect of the Arrhenius parameters is also found.
Keywords: PMMA; Thermal degradation; Kinetics; Reaction order; Compensation effect;

On the synthesis and photostability of some new naphthalimide dyes by T Konstantinova; R Lazarova; Al Venkova; V Vassileva (405-409).
Seven new 1,8-naphthalimide dyes, containing a hindered amine stabilizer in their molecule, have been synthesized. The properties of the dyes and their photostability in solution, in polymer and on polyamide fabrics were studied. The influence of the dyes on the photostability of PMMA coloured “in-mass” was examined.
Keywords: 1,8-Naphthalimide dyes containing 2,2,6,6-tetramethylpiperidine stabilizer; PMMA coloured “in mass”; Photostability of dyes and coloured PMMA; 1,8-naphthalimide dyes with tetramethylpiperidine stabilizer; PMMA;

Biodegradation of plastics was tested in the compost stored at −20 °C, 4 °C and 20 °C for different periods. Viable cells in the compost stored at −20 °C were expected to be fewer than those in the compost stored at 4 °C and 20 °C, because microbes may be under stress or even be killed due to the formation of ice crystals at the subzero temperature. Mesophilic bacteria and mesophilic actinomycetes were fewer in number in the compost stored at 20 °C than in the compost stored at the other two lower temperatures contrary to expectation. In contrast, both thermophilic bacteria and thermophilic actinomycetes were fewest in the compost stored at −20 °C as was expected, indicating that thermophilic microbes were more susceptible to stress in the freezing conditions than the mesophilic ones. Activity of the exo-enzymes plausibly excreted by the microbes in the compost decreased as a result of the storage. Nevertheless, biodegradation of cellulose in the compost was almost independent of the storage time and temperature. In contrast, biodegradability of both polycaprolactone (PCL) and poly(butylene succinate) (PBS) depended strongly on the storage conditions. From the point of view that the existing standards for biodegradation tests of plastics in compost accept reproducibility of cellulose biodegradability as a criterion for the validity of the biodegradation tests, a new standard of the compost preparation should be provided to guarantee more reliable results on the biodegradability of plastics.
Keywords: Compost; Storage; Biodegradation; Bacteria; Actinomycetes;

Condition monitoring approaches applied to a polychloroprene cable jacketing material by Kenneth T Gillen; Roger A Assink; Robert Bernstein (419-431).
In this paper we examine the utility of several promising material condition monitoring (CM) techniques applied to a commercial polychloroprene cable jacketing material used in nuclear power plant applications. These include two relatively unknown approaches, cross-sectional modulus profiling and NMR T 2 relaxation time measurements of solvent-swelled samples, as well as three more commonly used approaches, density, gel fraction and solvent uptake. The results from each approach were compared to tensile elongation measurements, the usual standard approach for monitoring degradation of elastomers. Degradation was carried out at three temperatures and at four combined radiation plus thermal environments, all of which were selected (by theoretical modeling and later confirmed by cross-sectional degradation mapping) such that oxidation proceeded uniformly throughout the cross-section of the material. This allowed macroscopic condition monitoring measurements to be made in the absence of anomalous diffusion-limited oxidation effects. Of the techniques examined, modulus profiling, solvent uptake and NMR T 2 measurements correlated extremely well with the elongation measurements and therefore showed substantial potential as CM approaches for this material. This is not unexpected since all of these techniques are sensitive to crosslinking of the material and the deterioration of the elongation is itself dominated by material hardening and thus by crosslinking.
Keywords: Aging; Polychloroprene; Condition monitoring; Modulus; NMR relaxation; Solvent uptake; Swelling;

Degradation of poly(l-lactic acid) by a newly isolated thermophile by Kosuke Tomita; Tomohiro Nakajima; Yumi Kikuchi; Naoko Miwa (433-438).
A thermophile which degrades poly(l-lactic acid) (PLA) film was isolated from 153 soil samples with an enrichment culture technique at 60 °C. Under this temperature condition, the strain grew on PLA and the dissolved total organic carbon (TOC) concentration in the medium changed according to the growth stage, i.e., after the TOC rapidly reached the minimum, it increased gradually and then decreased again thereafter. As for residual PLA, viscosity number decreased rapidly but weight decreased initially slowly. Gel permeation chromatograms of the residual PLA showed a shift of the whole peak to low molecular weight region, which suggested the occurrence of degradation to the inside of the solid sample. The strain has high specificity for PLA, and the culture broth did not show esterase activity. It was identified as a neighboring species to Geobacillus thermocatenulatus, which has an optimum growth temperature of around 60 °C.
Keywords: Poly(l-lactic acid); Biodegradation; Thermophile; Geobacillus thermocatenulatus;

Physio-chemical degradation of thermally aged hypalon glove samples by Kennard V. Wilson; Bettina L. Smith; John M. Macdonald; Jon R. Schoonover; Julio M. Castro; Mark E. Smith; Michael E. Cournoyer; Rob Marx; Warren P. Steckle (439-449).
Attenuated total reflection (ATR) infrared spectra have been analyzed using multivariate curve resolution (MCR) to capture the chemistry of the thermal degradation in the aging of chlorosulfonated polyethylene (Hypalon®) glove samples. The analysis demonstrates the primary degradation pathways to be oxidation (formation of ketones and carboxylic acids), dehydrochlorination with formation of –CC– groups, and polymer crosslinking with changes in the C–H functional groups. From the multivariate analysis, the dominant degradation pathway involves carbon–carbon double bond formation and oxidation to form ketones. The tensile properties (modulus and elongation at break) demonstrate stiffening of the material with aging time. The dynamic mechanical data show that the storage modulus and mechanical loss tangent are also strongly affected with aging due to a hardening of the material. Taken together the mechanical and ATR data indicate that in thermal aging hypalon degrades by dehydrochlorination and loss of –SO2Cl functionality with –CC– formation, oxidation, and crosslinking causing the material to harden and become brittle.
Keywords: Chlorosulfonated polyethylene; Hypalon; Thermal aging; Thermal degradation; FTIR-ATR;

Melamine (MA) improves the fire retardant performance of poly(butylene terephthalate) (PBT) as measured by the limiting oxygen index test and the UL94 standard. Thermogravimetric and FT-IR studies revealed that MA promotes the formation of solid decomposition products that mostly consist of the condensation products of MA. As shown by GC/MS, MA affects the development of low and high boiling degradation products. As a result, aromatic nitriles, amides and alkenyl- and cycloalkylenemelamines were identified among the volatiles indicating the occurrence of an interaction between PBT and MA on heating. The fire retardant condensed-phase effect of MA is discussed.
Keywords: PBT; Melamine; Flame retardance; Thermogravimetry; FT-IR; GC/MS;

Thermal degradation of PE and PS mixed with ABS-Br and debromination of pyrolysis oil by Fe- and Ca-based catalysts by Mihai Brebu; Thallada Bhaskar; Kazuya Murai; Akinori Muto; Yusaku Sakata; Md.Azhar Uddin (459-467).
Polymer mixtures containing 8 g of high density polyethylene (PE) or polystyrene (PS) and 2 g of acrylonitrile–butadiene–styrene copolymer containing a polybrominated epoxy type flame retardant (ABS-Br) were thermally degraded at 450 °C. FeOOH and two carbon composites based on iron (Fe-C) and calcium (Ca-C) were used for catalytic decomposition and their effect on the bromine and nitrogen amount and distribution in pyrolysis oil was determined by GC-AED analysis. It was found that iron-based catalysts removed bromine from pyrolysis oil and decreased the nitrogen amount converting nitrile compounds into ammonia. Calcium carbon composite had lower effect especially on the pyrolysis of PS/ABS-Br mixture. These liquid products can be used as fuel oil or feedstock in petroleum refinery.
Keywords: PE/ABS-Br; PS/ABS-Br; Catalytic decomposition; Carbon composite; Debromination;

Crosslinked polydimethylsiloxane (PDMS) containing Irganox® 1076, Tinuvin® 770 or Irganox® 565, prepared by swelling PDMS in a solution of one of these stabilizers in n-hexane, was exposed to a corona discharge and the corona exposure time (τ crit) to form a brittle, silica-like layer was determined by optical microscopy. The critical corona exposure time showed a linear increase with increasing stabilizer concentration; Tinuvin 770 showed the highest efficiency and Irganox 1076 the lowest. The increase in τ crit on corona exposure of the stabilized samples with reference to the value for unstabilized PDMS was similar to that reported earlier for air plasma exposed samples. The efficiency of the stabilizers towards corona-induced surface oxidation of PDMS was also confirmed by X-ray photoelectron spectroscopy.
Keywords: Crosslinked polydimethylsiloxane; Surface oxidation; Cracking; Corona; Stabilizers;

Chemiluminescence study on irradiated low-density polyethylene containing various photo-stabilisers by Radu Setnescu; Mustapha Kaci; Silviu Jipa; Tanta Setnescu; Traian Zaharescu; Ghania Hebal; Aida Benhamida; Hocine Djedjelli (475-481).
The effect of hindered amine stabilisers (HAS) on the thermal oxidation of low-density polyethylene (LDPE) films subjected to various gamma radiation doses in the range of 68–812 kGy, was studied by chemiluminescence (CL) at 190 °C in the presence of air. Different types of HAS were used, commercially known as Tinuvin 123, Sanduvor PR31 and Uvasil 299. It was found that under CL conditions, an antioxidative effect occurs for all unirradiated samples, being more relevant in the case of Sanduvor PR31. Our results indicate a specific decrease in the stability (depending on HAS nature) of irradiated materials with increasing irradiation dose. The initial intensity of CL emission (I 0), which is related to the concentration of radiation-induced peroxy species was plotted as a function of dose and led to straight line dependence with the slope values placing the HAS effectiveness in the sequence: Tinuvin 123 > Sanduvor PR31 > Uvasil 299 ≫ none.
Keywords: Chemiluminescence; Polyethylene; Radiation induced oxidation; Hindered amine stabilisers;

Kinetic analysis of the thermal degradation of polystyrene–montmorillonite nanocomposite by Serge Bourbigot; Jeffrey W. Gilman; Charles A. Wilkie (483-492).
Nanocomposites exhibit a combination of unique properties, such as increased heat distortion temperature, reduced permeability, reduced flammability and improved mechanical properties. In this work, a polystyrene (PS) clay nanocomposite was prepared via bulk polymerization using a novel organically modified montmorillonite (MMT). The organic-modifier is the N,N-dimethyl-n-hexadecyl-(4-vinylbenzyl) ammonium chloride (VB16). The thermal stability of PS–VB16 compared to pure PS is examined in pyrolytic and thermo-oxidative conditions. It is then studied using a kinetic analysis. It is shown that the stability of PS is significantly increased in the presence of clay. The thermal behavior of PS and PS nanocomposite is modeled and simulated. A very good agreement between experimental and simulated curves both in dynamic and isothermal conditions is observed. Using kinetic analysis associated to the reaction to fire of PS nanocomposite simulated in a cone calorimeter, the peak of heat release rate is half that of virgin PS, it is suggested that the clay acts as a char promoter slowing down the degradation and providing a protective barrier to the nanocomposite. The combination of these two effects is an important factor lowering the HRR.
Keywords: Kinetic; Thermal degradation; Nanocomposite; Organo-clay; Polystyrene;

Catalytic degradation of low-density polyethylene and polypropylene using modified ZSM-5 zeolites by Qian Zhou; Li Zheng; Yu-Zhong Wang; Guo-Ming Zhao; Bo Wang (493-497).
The catalytic degradation of low-density polyethylene (LDPE) and polypropylene (PP) using a modified ZSM-5 zeolite, DeLaZSM-5, has been investigated. The results obtained have been compared with those of USY, ZSM-5, and also thermal degradation. The degradation behaviours of LDPE and PP using DeLaZSM-5 were quite different from those observed in thermal degradation and in studies using USY as catalysts. LDPE exhibited much higher degradation activity using the DeLaZSM-5 than PP did, especially in relatively lower temperature stage, indicating that DeLaZSM-5 shows a shape-selective effect in catalytic degradation of LDPE and PP. The factors affecting the shape-selective effect of DeLaZSM-5 are discussed. It was concluded that the pore structure and unique acid properties of the DeLaZSM-5 as well as proper reaction temperatures were important factors to fully exert this shape-selective effect.
Keywords: Waste plastics; DeLaZSM-5 zeolite; Catalytic degradation activity; LDPE; PP;

The monomeric antioxidants 13 were grafted onto low molecular weight polyethylene (LMWPE) by melt processing with free radical initiators. From the infrared spectra of the grafted LMWPE, it was found that the monomeric antioxidants were introduced onto the polymeric chain. IR spectroscopic methods were used for the quantitative determination of the extent of grafting of monomeric antioxidant. To optimize the reaction conditions, the influences of the concentration of initiator, monomeric antioxidant, reaction time and temperature on the extent of grafting were speculated.
Keywords: Hindered phenol antioxidant; Melt free radical grafting; Low molecular weight polyethylene; Polymer-bound antioxidant;

Thermal degradation kinetics of poly(n-butyl acrylate) initiated by lactams and thiols by Yu-Hsiang Hu; Chuh-Yung Chen; Cheng-Chien Wang (505-514).
The thermal degradation of poly(n-butyl acrylate) (PBA) has been studied by using thermogravimetry under nitrogen atmosphere. It is found that the thermal degradation of PBA initiated by thiols and ε-caprolactam has only a one-step degradation mechanism, and the degradation temperatures (360–370 °C) are higher than AIBN system (304 and 361 °C). FT-IR, 1H NMR, and 13C NMR are applied to characterize the polymer structures. The terminal group of the PBA chain has a great influence on the degradation, and the initial degradation does not appear to be ester group scission. In order to determine the effect of terminal groups, several approaches (such as Flynn, Flynn–Wall–Ozawa, and Kissinger methods) have been applied to evaluate the kinetic parameters, including activation energy, frequency factor, and rate constants. These values are compared with those obtained for PBA initiated by AIBN. These results indicate that the end groups do have an influence on thermal stability, and the strength of the effect follows the order –R12>–COOH>–OH>–AIBN.
Keywords: Poly(n-butyl acrylate); Degradation kinetics; Activation energy; Thiols;

The poly(l-lactide) [i.e. poly(l-lactic acid) (PLLA)] films prepared by crystallization or annealing at different temperatures (T as) were hydrolyzed at 97 °C in a phosphate-buffered solution (PBS) and changes in their long period (L), lamellae thickness (l c), physical properties, and morphology were investigated by small angle X-ray scattering (SAXS), gel permeation chromatography (GPC), light-scattering (LS) measurements, differential scanning calorimetry (DSC), and scanning electron microscopy (SEM). The L and l c values of the PLLA films before hydrolysis and the l c values of the crystalline residues after the hydrolysis for 40 h became higher with increasing T a. The l c values of the crystalline residues (extended-chain crystallites) after the hydrolysis, assuming that crystalline residues were composed solely of the crystalline regions, were found to be in complete agreement with the L values of the films before the hydrolysis and the l c values were increased by the hydrolysis in PBS at 97 °C and also at 37 °C. The reasons for the decreased melting temperature (T m) after the hydrolysis despite the increased l c are discussed.
Keywords: Polylactide; Poly(lactic acid); Hydrolysis; Degradation; Crystalline residues;

Tri(acryloyloxyethyl) phosphate (TAEP) and di(acryloyloxyethyl)ethyl phosphate (DAEEP) were synthesized from phosphorus oxychloride and hydroxylethyl acrylate and used as flame retardant multifunctional monomers for UV curable systems. The UV cured TAEP and DAEEP films have limiting oxygen indexes of 36 and 29, respectively. Their thermal behaviours were studied by thermogravimetric analysis, and they show three characteristic degradation temperature regions, attributed to the decomposition of phosphate, thermal pyrolysis of acrylate side chains, and decomposition of unstable structures in char, respectively. This was further demonstrated by in situ Fourier-transform infrared and direct pyrolysis/mass spectrometry measurements, and monitored by inductively coupled plasma-atomic emission spectrometry. As flame-retardant mechanism it is proposed that the degraded products of phosphate form poly(phosphoric acid), which further catalyses the breakage of carbonyl groups to form an intumescent char, preventing the samples from further burning.
Keywords: Flame-retardant; Phosphate; UV curing; Thermodegradation;

Evaluation of polar ethylene copolymers as fire retardant nanocomposite matrices by Christopher M.L Preston; Gandara Amarasinghe; Jefferson L Hopewell; Robert A Shanks; Zenka Mathys (533-544).
A range of nanocomposite materials was prepared under high shear based on polar polyethylene (PE) copolymer matrices. An organically modified bentonite was used as the smectite clay, with intercalation shown to occur in the presence of poly(ethylene-co-methyl acrylate) (EMA), poly(ethylene-co-vinyl acetate) (EVA), poly(ethylene-co-methyl acrylate-co-acrylic acid) (EMAAA) and poly(ethylene-g-maleic anhydride) as evidenced by small angle X-ray scattering measurements. The thermal stability and flame retardant properties of the polymer–clay composites were measured by thermogravimetry and cone calorimetry, with significant improvements in the heat-release rates and thermal stability of all systems. Enhanced char formation was shown to be responsible for reducing degradation under oxidative conditions.
Keywords: Halogen-free flame retardant; Nanocomposites; Polyethylene copolymer; Thermal degradation;

Viscoelastic properties and thermal degradation kinetics of silica/PMMA nanocomposites by Yu-Hsiang Hu; Chuh-Yung Chen; Cheng-Chien Wang (545-553).
PMMA-silica nanocomposites were prepared using a bulk polymerization technique. Three organic silica groups, two modified with methyl groups and the third an octane, made these inorganic silica particles more hydrophobic. These silica/PMMA nanocomposites exhibited higher storage and loss moduli than those of pristine PMMA. The T g of these composites increased with the silica content. The thermal characteristics of these composites were also enhanced by incorporating silica into the PMMA matrix. The degradation temperature at 10% weight loss was approximately 30 °C higher than that of pristine PMMA, depending upon the silica content. The rate of weight loss at 220 °C for 2 h was also markedly reduced in the presence of these modified silicas. The results might be attributed to “trapping effect”. The activation energies for these silica/PMMA nanocomposites were enhanced according to Flynn, Ozawa–Flynn, and Kissinger methods.
Keywords: PMMA; TGA; Activation energy; Silica;