Polymer Degradation and Stability (v.85, #1)

Ultrasonic degradation of polybutadiene and isotactic polypropylene by Jayanta Chakraborty; Jayashri Sarkar; Ravi Kumar; Giridhar Madras (555-558).
The ultrasonic degradation of polybutadiene and isotactic polypropylene in solution was studied at different temperatures and in different solvents. The time evolution of molecular weight distribution was determined experimentally through gel permeation chromatography. Degradation rate coefficients were determined from a model based on continuous distribution kinetics and assuming mid-point chain scission. The variation of rate coefficients with vapour pressure and kinematic viscosity was also investigated.
Keywords: Ultrasound; Degradation; Polybutadiene; Isotactic polypropylene; Solvent effect; Continuous distribution kinetics;

A series of poly(butylene terephthalate-co-cyclohexyanedimethylene terephthalate)-b-poly(ethylene glycol) segmented random copolymer (PBCG) with poly(cyclohexyanedimethylene terephthalate) moieties (PCT) molar fraction (M PCT) varying from 10% to 60% were synthesized and characterized. The copolymers exhibit good symmetric peak in GPC chromatographs with the number average molecular weight more than 3.5×104 and polydispersity less than 1.70. It reveals that the two sorts of hard segment, PCT and poly(butylenes terephthalate) (PBT), take a random distribution along the copolymer chain and the number average block length of PBT decreases from 4.02 to 1.41, while that of PCT increases from 1.17 to 2.50 with the increasing M PCT by sequence distribution analysis. Micro-phase separation structure is validated for the appearance of two glass transition temperatures and one melting point in DSC thermograms. The minima of melting point, crystallinity as well as the discontinuous jump of glass transition temperature of hard segments at M PCT about 20–30% are derived from the crystal lattice transition from α-PBT to trans-PCT and discussed from the viewpoint of cohensive energy. The improved thermal stability is observed with ascending M PCT because of the high thermal stability of cyclohexylene group in the backbone. The mechanical testing results show that the lower crystallinity of hard segments imparts the copolymer higher elongation, while the higher one improves the elastic modulus, tensile strength and yield strength.
Keywords: Copolymerization; Crystallization; Polycondensation; Segmented random copolymer; Sequence distribution analysis;

Hydrolysis of poly(ethylene terephthalate) in a fluidised bed reactor by Guido Grause; Walter Kaminsky; Gerhard Fahrbach (571-575).
The hydrolysis of poly(ethylene terephthalate, PET) is the reverse of the direct esterification of terephthalic acid (TA) and ethylene glycol (EG). By this means the recovery of monomers is possible. These experiments have shown that 60%–72% of TA were recovered from virgin PET in a temperature range between 400 and 500 °C. Another 22%–27% TA remained in oligomers. The highest yield of TA (72%) was found at 450 °C. When real materials were used, still 60%–69% of TA were found at 450 °C. There was also a catalytic effect of transition metal oxides which reduced the content of TA in oligomers to 8%. The yield of EG was less than 10% in all experiments. Most EG reacted with water to form carbon oxides and hydrogen. Other decomposition products were acetaldehyde and ethene.
Keywords: Poly(ethylene terephthalate); Terepthalic acid; Hydrolysis; Fluidised bed; Ethylene glycol;

The flammability properties of polypropylene (PP) containing magnesium hydroxide can be significantly improved by using varying amounts of a newly prepared cobalt chelate. The chelate (resin) is based on an N-(4-methyl phenyl) acrylamide monomer crosslinked by 4,4′-bis-(acrylamido)diphenylsulfone. Different formulations of magnesium hydroxide and the cobalt chelate were used as flame retardant for PP. Thermogravimetric analysis, cone calorimetry and UL 94 techniques were used to assess the flammability properties of the formulated PP samples. It was found that 5 and 10 g of the prepared cobalt chelate mixed with 45 and 40 g, respectively, of Mg(OH)2 were the best among the investigated formulations.
Keywords: Monomer; Crosslinking agent; Resin; Complexation; PP formulation; TGA; Cone calorimetry;

Styrene–butyl acrylate copolymer (St–BA)/graphite oxide (GO) nanocomposites were prepared via exfoliation–adsorption with monomer followed by in situ emulsion polymerization. The structure of the nanocomposites was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and high resolution electron microscopy (HREM). Both differential scanning calorimetry (DSC) measurements and cone calorimetry experiments were carried out to evaluate their thermal stability and flammability properties. The cone calorimetry data showed that the peak heat release rate (HRR) value is reduced remarkably by 45% in an St–BA/GO nanocomposite with a GO content of only 1 wt%. The result suggests that the addition of GO even in 1 wt% content can greatly decrease the heat and smoke release rate of the St–BA/GO nanocomposites, and can increase their thermal decomposition temperature (T d) slightly. However, no effect of GO content on the reduction of HRR and THR in nanocomposites is apparent.
Keywords: Graphite oxide; St–BA copolymer; Nanocomposites; Flammability property;

A molecular scale model describing the molecular breakdown of low moisture amylopectin melts by shear forces was derived and compared to engineering scale experiments. The model is based on the assumption that the maximum shear stress on the polymer equals the effective surface stress on that polymer, which is related to the strength of the glycosidic bonds. The molecules are approximated as spheres, with a radius that exhibits fractal scaling with molecular weight. The molecular weight reduction as a function of applied shear stress as deduced from the model agrees satisfactorily with the results from experimentally obtained engineering scale data. The results show a successful example of a coupling between engineering scale experiments and micro-scale models and contribute to the understanding of fundamental changes in the properties of starch during industrial scale heating–shearing processes.
Keywords: Starch; Shear; Molecular weight reduction; Degradation;

Poly(ester amide)s derived from 1,4-butanediol, adipic acid and 6-aminohexanoic acid by Marcos Lozano; Lourdes Franco; Alfonso Rodrı́guez-Galán; Jordi Puiggalı́ (595-604).
BAK poly(ester amide)s with different adipic acid/terephthalic acid ratios have been synthesized and characterized considering spectroscopic data and both thermal and mechanical properties. Degradability under different media (water at 70 °C, acid or enzymatic catalysis) has also been studied by evaluating the changes in intrinsic viscosity, in the NMR spectra and in the surface texture of samples. The results indicate a decrease in the degradation ratio and an increase in chain stiffness as the content on the terephthalic units increases. Thermoenzymes such as Bacillus stearothermophilus are highly effective for the degradation of aromatic derivatives.
Keywords: Degradability; Poly(ester amide)s; Thermal properties; Mechanical properties;

Oxidative polymerization reaction of the protonated form of aniline (ANI-HCl) was investigated experimentally in acidic solutions with in situ spectrophotometric technique and computationally using molecular mechanics (MM+) calculations. Well-defined absorbance peaks of an intermediate and of an end product were observed after first scan of the spectrogram, using zero- and first-derivative spectroscopic treatment. Increases in the absorbance at λ∼420  nm with increasing the oxidant and/or anionic surfactant (sodium dodecyl sulfate, SDS) content in the polymerization medium were observed. This indicates that the presence of nitrenium cations and suggests that the polyaniline evolution goes towards a two-dimensional (D2) polymer with phenazine rings, which can be formed by a cross-linking reaction. It was confirmed by kinetic studies, as well as verified by MM+ calculations. Absorbance peak at λ∼520  nm (dark brown), in the first scan of the spectrogram was observed. This peak increases with increasing the acid surfactant [poly(acrylic acid)] (PAA) matrix content in the polymerization medium. Kinetic parameters of the oxidative polymerization reaction were deduced employing a computer-aided kinetic analysis of the absorbance (A) at ∼720 nm against oxidation time (t) data. The results obtained indicate that the rate controlling process are governed by the two-dimensional (D2) diffusion. Good linearity (r∼0.985) between ΔH # and ΔS # for the series of acids doped PANI matrix was obtained, which suggests the existence of a compensation (or isokinetic) effect. The proposed mechanism for the oxidation of ANI is also supported by MM+ calculation. Activation parameters for the rate of oxidation process of ANI have been computed and discussed.
Keywords: Aniline hydrochloride; Oxidative polymerization; UV–visible absorption spectra; Kinetic parameters; Molecular mechanics (MM+) calculations;

Kinetic study on thermal degradation of low-density polyethylene stabilized with chalcone derivatives by V. Dinoiu; L.M. Gorghiu; S. Jipa; T. Zaharescu; R. Setnescu; C. Dumitrescu (615-622).
The thermal oxidation of low-density polyethylene in the presence of some chalcone derivatives was studied by chemiluminescence. Several kinetic parameters were calculated to characterize the protection activity of the additives. Activation energies of thermal degradation were evaluated from oxidation induction time and maximum oxidation time in order to differentiate the effects of stabilizers on the early stage of degradation and the low activity of additives at the end of ageing.
Keywords: Low density polyethylene; Chalcone derivatives; Degradation; Chemiluminescence;

In our previous research, we developed flame retarding systems for cotton based on a hydroxy-functional organophosphorus oligomer (FR) and a binder, such as 1,2,3,4-butanetetracarboxylic acid (BTCA), dimethyloldihydroxyethyleneurea (DMDHEU) and melamine–formaldehyde (M–F), which are able to form covalent binding between FR and cotton. In this research, we used a factorial experimental design method to study the performance of the flame retarding system based on FR and the mixture of DMDHEU and M–F. We found that an increase in the DMDHEU/(DMDHEU + M–F) ratio in the mixture increases the amount of FR bound to cotton, increases the laundering durability of FR on cotton, and reduces the fabric tensile strength retention. DMDHEU and M–F also function as nitrogen providers and enhance the flame retarding performance of the treated fabric due to phosphorus–nitrogen synergism. M–F is a more efficient nitrogen provider than DMDHEU. The effect of reduced phosphorus–nitrogen synergism as a result of increasing the relative amount of DMDHEU in the mixture (DMDHEU + M–F) outweighs that of the improved bonding of FR to cotton. An increase in FR concentration in a formula increases the amount of FR bound to cotton, whereas it reduces the percent retention of FR on the fabric and also reduces the fabric strength loss. Desirable performance of treated cotton fabrics can be achieved by adjusting the concentrations of FR and the ratio of the two binders in a formula.
Keywords: Cotton; Flame retardant finishes; DMDHEU; Melamine–formaldehyde; Organophosphorus compounds; Statistical analysis;

The flammability characterization and synergistic flame retardant mechanism of fumed silica in the ethylene-vinyl acetate/magnesium hydroxide (EVA/MH) blends were studied by cone calorimeter test (CCT), thermogravimetric analysis (TGA), limiting oxygen index (LOI), and UL-94 test. The results showed that the addition of a given amount of fumed silica apparently increased the LOI value and decreased the loading of MH in EVA blends while keeping the V-0 rating in the UL-94 test. The data obtained from the CCT indicated that the addition of fumed silica not only greatly reduced the heat release and mass loss rates, but also depressed the smoke release during the combustion of EVA/MH blends. The morphological and TGA data showed further evidence that this synergistic flame retardant mechanism of fumed silica in the EVA/MH materials is mainly due to the physical process of fumed silica acting as enhanced char/silica layers in the condensed phase which prevents the heat and mass transfer in the fire. By adding fumed silica into EVA/MH blends and reducing the total amount of fillers, the elongation at break of samples is doubled, while the fire resistance of the blends is maintained.
Keywords: Fumed silica; Magnesium hydroxide; EVA; Synergistic effect; Flame retardant mechanism;

The thermal stability behaviour and the flammability properties of poly(methyl methacrylate) (PMMA) filled with antimony oxide (Sb2O3), at different Sb2O3 amounts: 5, 10 and 20 wt%, were studied by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), cone calorimetry and limiting oxygen index (LOI) measurements. Samples were prepared using a twin-screw machine and subsequently injection molded. The experimental results obtained show that Sb2O3 particles retard the degradation and flammability of PMMA. The LOI increases to 24 for PMMA/Sb2O3 with just 20 wt% Sb2O3. An increase of the thermal stability between about 25 and 40 °C for 5–20 wt% of Sb2O3 was demonstrated by TGA in air. Cone calorimeter measurements showed that the heat release rate (HRR) significantly decreased in the presence of Sb2O3. Epiradiateur tests showed an increase of the ignition time between 10 and 20%. In a tentative way to establish how Sb2O3 acts as a flame retardant for PMMA, the activation energy of the thermal degradation was determined by the model-free isoconversional method of Vyazovkin.
Keywords: Thermal degradation; Poly(methyl methacrylate) (PMMA); Flame retardant; Antimony trioxide; LOI;

The effects of pH and an enzyme on the hydrolysis of poly(l-lactide) [i.e. poly(l-lactic acid) (PLLA)] crystalline residues or extended-chain crystallites containing a negligibly small amount of chains in an amorphous state were investigated in aqueous solutions at pH range from −0.9 to 12.8 and in Tris–HCl buffered solution with or without proteinase K by the use of gel permeation chromatography (GPC) and differential scanning calorimetry (DSC). GPC results showed that the hydrolysis of the crystalline residues proceeded from their surface composed of very short chains with a free end along the chain direction, irrespective of pH, but that from their lateral surface could not be traced. GPC and DSC results indicated that the hydrolysis of the crystalline residues was accelerated with the pH deviation from 7, reflecting that the hydronium and hydroxide ions have catalytic effects on the hydrolysis of the crystalline residues. The dependence of hydrolysis rates estimated by GPC on the concentration of hydronium or hydroxide ions revealed that the catalytic effect of hydroxide ions on the hydrolysis is much higher than that of hydronium ions. The significant T m decrease at high and low pH means that there is a decrease in thickness of the crystalline residues by the hydrolysis. On the other hand, GPC and DSC results also showed that proteinase K has no catalytic effect on the hydrolysis of the crystalline residues, confirming the reported finding that the hydrolysis of very short chains with a free end is not accelerated by the presence of proteinase K.
Keywords: Polylactide; Poly(lactic acid); Crystalline residues; Hydrolysis; pH effects; Proteinase K;

Thermal degradation behaviour of PE/clay nanocomposites by Marco Zanetti; Pierangiola Bracco; Luigi Costa (657-665).
Polymer nanocomposites based on organically modified clay (organoclay) and polyethylene (PE) were prepared by melt processing using poly(ethylene-co-vinyl acetate) (EVA) as compatibilizer. The degree of intercalation spacing was determined by X-ray diffraction (XRD). The thermal degradation behaviour was studied in thermogravimetry (TGA) and the chemical evolution in the solid residue was studied with an infrared microscope. During the thermal degradation of the nanocomposite in oxidant atmosphere the formation of a protective layer on the polymer surface was observed caused by a charring process of PE, which is normally a non-char-forming polymer. The protective effect of the char/clay layer against thermal oxidation was also observed by monitoring the retention of the long chain structure of PE.
Keywords: Clay; Compatibilization; Thermal degradation; Nanocomposites; Polyethylene (PE);

Logistic approach to polymer degradation in dynamic TGA by Ricardo Cao; Salvador Naya; Ramón Artiaga; Ana Garcı́a; Angel Varela (667-674).
The method presented in this work allows for fitting an overall TGA curve by generalized logistic regression. It is assumed that overlapping processes may be involved in a single step of a TGA trace. Since the logistic functions reproduce very well the typical asymptoticity at the beginning and end of degradation processes, the authors consider that each single degradation process can be fitted by a single logistic function or by the linear combination of a small number of them. Since the fitting is very good, the generalized function can replace the original TGA curve. This way, it is possible to differentiate directly the overall TGA curve fitted, overcoming the problems of noise and over–under smoothing, typical in DTG calculation. On the other hand, the separation of overlapping processes in several functions suggests that each single overlapping degradative process could be explained by one or the linear combination of few logistic functions. Particularly, one of the parameters obtained with this kind of logistic mixture represents the point in the time (or temperature) axis where each process is “centered”, while another parameter is related to the degradation rate.
Keywords: Polymer degradation; Logistic mixture; TGA; Fitting; Smoothing;

We have prepared a polyether-type polyurethane (PU) containing 0.043% gold nanoparticles (∼5 nm) by mixing the waterborne PU with gold suspension, casting and drying at 60 °C. The gold nanoparticles were found to be well dispersed in PU. A significant increase in the pyrolysis temperature and the glass transition temperature was demonstrated in the nanocomposite PU films. The biostability was tested in a rat subcutaneous model. After 19 days of implantation, the PU containing gold showed less surface degradation as well as much reduced tissue reaction.
Keywords: Biostability; Microphase separation; Biodegradation; Tissue reaction;

The commonly observed effect of weathered polymer samples continuing to change color in the dark was studied in some detail for polycarbonate, styrene–acrylonitrile copolymer (SAN), and various combinations with SAN–polybutadiene rubber graft (SRG) copolymer. All of the formulations became more yellow upon storage in the dark after accelerated weathering, and all behaved very similarly. There was a fairly rapid shift of 1–2b units (as defined in CIELAB color space) over the first two weeks followed by a slower, linear shift that continued for many months. This amount of color shift is enough to be visible to the eye in side-by-side comparisons. Most of the additional color generated in the dark could be bleached by exposing the samples to sunlight through a south-facing window. The samples usually reached the original color after 2–4 days of window exposure. Putting the samples back in the dark caused the yellowing to start up again, although the rapid yellowing often was not as much as the first time. Samples aged under natural conditions outdoors also underwent a small color shift upon storage for 5 months and substantial color shifts after two years. The results are consistent with the photo–thermal equilibrium between ZE isomers as suggested in the literature, although some aspects remain unexplained.
Keywords: Polycarbonate; SAN; ABS; Accelerated testing; Color measurement; Post-actinic yellowing;

Crosslinked PCL was achieved by electron beam irradiation in the presence of polyfunctional monomers (PFM). Using the PFM to crosslink PCL reduces the irradiation dosage required for producing gel. The amount of gel formed was determined as a function of both the irradiation dose and PFM concentration. The higher the PFM concentration, the higher is the gel content. However, the amount of gel increases with irradiation dose to reach its maximum at 40 kGy and thereafter, it tends to level off. As the gel content of PCL increases, the tensile strength increases, but the elongation at break decreases. The crosslinked PCL has higher heat resistance compared with the uncrosslinked one. The enzymatic and soil burial degradation of uncrosslinked and crosslinked PCL was investigated. It was found that the biodegradation rate decreased and strongly depended not only on the degree of crosslinking of PCL but also on the degradation reaction conditions. The results obtained indicated that, by introducing crosslinking into PCL, its physical characters such as heat stability and mechanical properties were greatly improved, whilst, its ability to biodegrade was diminished.
Keywords: Radiation; Polycaprolactone; Heat resistance; Physical properties; Biodegradation;

Discoloration of celluloses treated with polyethylenimines by M.U. de la Orden; M.C. Matı́as; J. Martı́nez Urreaga (697-703).
Celluloses treated with polyethylenimines (PEIs) suffer severe degradation in air at moderate temperatures, yielding strongly coloured materials. The discoloration was found to depend on the PEI, the pH and the solvent used in the treatment of the cellulose. To study these effects, cotton cellulose samples were treated with two PEIs, using methanol and water (at pH=6 and pH=11) as solvents, and the PEI-treated celluloses were subjected to thermal treatment in air. The discoloration was studied using diffuse reflectance UV–Vis spectroscopy, diffuse reflectance FTIR spectroscopy and standard colorimetry. The results were explained considering that the main source of chromophores in the degradation at moderate temperatures is the reaction between PEI amino groups and cellulose carbonyl groups to form coloured imines.
Keywords: Cellulose; Polyethylenimine; Diffuse reflectance; Colour; Discoloration;

Degradable tri-component copolymers were synthesized by the bulk copolymerisation of 2,2-dimethyl trimethylene carbonate (DTC), ϵ-caprolactone (CL) and glycolide (GA) using stannous octanoate as catalyst at 140 °C for 36 h. The mole fraction ratio in the feed of DTC and CL was fixed at 20/80 and the mole percent of GA was varied to obtain copolymers with different properties. The polymerisation products were characterized by 1H NMR, 13C NMR, FT-IR, GPC and DSC. It was found that the hydrophilicity of these materials increased with increasing GA content, according to measurements of static contact angles of distilled water on the surface of polymer films. Mechanical tests and hydrolytic degradation assays showed that copolymers of different degradability and mechanical properties could be tailored by adjusting the compositions. For the polymer P-3 (GA content about 10 mol%), the tensile strength and the elastic modulus could reach 4.3 MPa and 22 MPa, respectively. It took 4 weeks for the copolymer P-3 to degrade to about 82% (M n,t /M n,0) and 10 weeks to 40%. These low crystallinity copolymers could be processed into porous films with stable structures by means of combination of solvent volatilisation with salt leaching. These materials are expected to be useful for nerve reconstruction and other biomedical applications.
Keywords: Biodegradable polymers; Poly(ϵ-caprolactone); Polyglycolide; Random copolymers; Poly(2,2-dimethyl trimethylene carbonate-co-ϵ-caprolactone-co-glycolide);

The effect of a curing agent on the thermal degradation of fire retardant brominated epoxy resins by A.I. Balabanovich; A. Hornung; D. Merz; H. Seifert (713-723).
The diglycidyl ether of tetrabromobisphenol A, the diglycidyl ether of bisphenol A and their mixture was cured by 4,4′-diaminodiphenyl methane. The pyrolysis of the obtained epoxy resins was studied by TG, DSC, TG/FTIR as well as FTIR characterization of pyrolysis residues. The gaseous and high boiling pyrolysis products were collected, characterized by GC/MS and their formation is discussed. The brominated epoxy resins are thermally less stable than the non-brominated ones. This effect is caused by the amine-containing hardener. The degradation initiation reaction is associated with the formation of hydrogen bromide which further destabilizes the epoxy network. The effect of the curing agent can be used in recycling of epoxy resins to separate brominated pyrolysis products from non-brominated ones.
Keywords: Brominated epoxy resin; Pyrolysis; GC/MS; FTIR; TG/FTIR; WEEE;

Two kinds of curing agents, triethanolamine and polysilazane (KH-CL), were used to make polymethylphenylsiloxne (PMPS) cured at room temperature. Their effects on the short-term and long-term thermal stability of PMPS are reported and discussed. The influences of the curing agents on the thermal degradation mechanisms of PMPS were studied by thermogravimetric analysis (TG), differential thermogravimetry (DTG) and Fourier-transform infrared spectroscopy (FTIR). The results indicate that both kinds of PMPS had higher short-term thermal stability than traditional heat-cured PMPS. For long-term thermal stability, the PMPS cured with KH-CL was much higher than the one cured with triethanolamine. The “unzip degradation” of PMPS was eliminated through the reaction between KH-CL and silanol groups, and then the thermal stability of PMPS was improved. However, it was initiated at a low temperature (about 300 °C) with catalysis by triethanolamine, and the thermal stability was decreased.
Keywords: Polymethylphenylsiloxane; Polysilazane; Triethanolamine; Thermal stability; Thermogravimetric analysis;

A copolymer of acrylonitrile with N-(4-chlorophenyl)acrylamide was prepared and was reacted with ferric chloride, cobalt acetate and zinc acetate to obtain the corresponding metal chelates. The metal chelates were used with 16% kaolin as flame retardants for polypropylene. The flammability properties of the various polypropylene compositions were evaluated by thermogravimetric analysis (TGA) and cone calorimetry. It was found that the iron chelate showed the best synergistic effect with kaolin. This synergistic mixture reduced the average heat release rate of polypropylene by about 60% and the smoke emitted by 50%.
Keywords: Copolymer; Metal chelation; Polypropylene formulation; TGA and cone calorimetry;