Polymer Degradation and Stability (v.72, #3)

The controlled thermal degradation of some samples of isotactic and syndiotactic polypropylenes with molecular weights ranging from 3.8×104 to 1.1×105 in Mn was examined to develop an effective method for selective formation of telechelic oligomers having two isopropenyl end groups, TVD. The principal conditions are at 370°C for 30–90 min. The average number of TVD per molecule f t determined from the 13C NMR spectrum of the nonvolatile oligomers (M n 3000–7000) isolated from the sample residues in the degradation ranged from 1.5 to 1.9 with an increase in the initial molecular weight of the original polymer, independent of the stereoregularity. The maximum f t value observed is 1.87, which indicates that about 88 mol% of the oligomer molecules are structurally symmetric telechelic oligomers having two TVDs. With an increase in the initial molecular weight M n0, the yield of residue decreases and the relative molecular weight (M n/M n0) of the nonvolatile oligomers decreases. These results strongly suggest that the intermolecular hydrogen abstraction of volatile radicals occurs more frequently than that of the terminal macroradicals, because almost all of the end double bonds of these oligomers are formed via the intermolecular hydrogen abstraction of various radicals followed by the β scission of on-chain macroradical and, thereby, the marked increase of f t value is caused by a greater contribution of volatile radicals to the scission of main chains of polymer molecules. Thus, it becomes clear from these results that the rates of elementary reactions in the degradation process increases with an increase in initial molecular weight owing to the diffusion-controlled mechanism of termination.
Keywords: Telechelic oligomer; Polypropylene; Controlled thermal degradation;

A new analytical method referred to as derivatization-electron probe X-ray microanalysis (XMA) has been developed to determine the distribution of 0.1% order functional groups in polymers with μm level lateral resolution. The fundamental performance of XMA in analyzing derivatized polymers has been investigated. The results showed that the detection limit for the concentration of Br in a brominated epoxy resin was 0.05% and the lateral resolution was 3 μm. Also, the optimal reaction conditions that make it possible to derivatize selectively the vinyl groups, the carboxyl groups and the carbonyl groups have been found. These conditions are the key to this method. In addition, the depth profile of the vinyl groups in amine-degraded poly (vinyl chloride) has been analyzed using this method. Since the depth profiles agreed well with those obtained by micro-IR spectroscopy, the reliability of the distribution measurement of functional groups by this method was confirmed. By this method, the sensitivity was 30 times higher, and the lateral resolution was 4 times higher than those obtained by the micro-IR spectroscopy. The actual applications to the depth analysis of degraded Nylon-6 and ethylene–carbon monoxide copolymer have proved that this method is useful for the characterization of polymers and the study of polymer degradation.
Keywords: Distribution analysis; Functional group; Polymer; Derivatization; Electron probe X-ray microanalysis; Chemical modification;

Cross-linking of polystyrene by Friedel–Crafts chemistry: reaction of p-hydroxymethylbenzyl chloride with polystyrene by Hongyang Yao; Michael A McKinney; Caroline Dick; John J Liggat; C.E Snape; Charles A Wilkie (399-405).
p-Hydroxymethylbenzyl chloride was found to be an effective cross-linking agent for polystyrene. The reaction was found to occur by Friedel–Crafts alkylation between the benzyl alcohol/chloride functional groups in the additive and phenyl ring in polystyrene. The reaction was studied by TGA–IR to monitor the evolution of hydrogen chloride and water, and the structure of the resultant gel was analyzed by solid state NMR and elemental analysis. The potential application in flame-retardancy was evaluated using Cone calorimetry.
Keywords: Friedel-Crafts; Polystyrene; Fire retardancy; Cross-linking; Cone calorimetry;

Light stabilization of metallocene polyolefins by F Gugumus; N Lelli (407-421).
Ultraviolet (UV) stabilization of metallocene polyolefins is examined from a practical and theoretical point of view. First of all, the possibility of using existing stabilizers and stabilizer systems is investigated. To this avail, the compatibility of the stabilizers needs to be checked at the concentrations most commonly used. It is found that, in this respect, there is no fundamental difference between conventional high-pressure polyethylene (PE-LD), linear low-density polyethylene (PE-LLD) and metallocene polyethylene (m-PE). With polypropylene (PP), stabilizer compatibility is generally not so much a problem as with polyethylene (PE). The assessment of the performance of light stabilizers is performed with compression molded films. The results so far available show that the UV absorbers and hindered amine light stabilizers (HALS) examined confer comparable UV stability to PE-LD, PE-LLD and m-PE. Similar results are observed with PP. It is found that the lifetimes obtained on artificial weathering of m-PP and third generation PP are comparable. Furthermore, the synergism between two polymeric HALS that has been observed previously with second and third generation PP is also found with metallocene polyolefins. The experimental laws relating the performance of the HALS to their concentration are deduced from the preliminary data available. It is found that they are the same as those found previously for PP and PE. With PP, UV stability increases linearly with HALS concentration as long as the last is not too high. With m-PE as with PE-LLD, the performance is directly proportional to the square root of the HALS concentration. From these and additional results it is concluded that the degradation and stabilization mechanisms with m-polyolefins are essentially the same as those developed for the previous types of polyethylene and polypropylene.
Keywords: Polyethylene; Polypropylene; Metallocene catalysts; Compatibility; Light stabilization; HALS; UV absorbers; Synergism; Mechanisms;

The thermal degradation of natural rubber/polystyrene (NR/PS) interpenetrating polymer networks (IPNs) was studied and the effects of blend ratio, crosslink level and initiating system were analysed. The thermogravimetric analysis (TGA) shows that the IPNs are more stable than the pure components. The full-IPNs have better stability than semi-IPNs which is due to higher entanglement density of full-IPNs. The initial decomposition temperature and temperature for 50% decomposition (T 50) increase with increase in concentration and crosslinker level of the plastic phase. The kinetics of degradation were analysed using nine mechanistic equations. The kinetic parameters such as activation energy, Arrhenius parameter and entropy of activation were determined. Thermal ageing studies of the samples were also conducted. The IPNs aged for 72 h at 100°C showed enhanced mechanical strength due to crosslinking on post curing.
Keywords: Thermal degradation; Interpenetrating polymer networks; Natural rubber; Polystyrene;

Thermal behaviour and molecular modelling of some aromatic polyethers containing a hexamethylenic spacer by Nicolae Hurduc; Mihaela Prajinaru; Bogdan Donose; Dumitru Pavel; Natalia Hurduc (441-445).
We report a study of the thermal stability of some aromatic copolyethers containing a hexamethylenic spacer. The polymers were synthesized by phase transfer catalysis (in a liquid/liquid system) starting from 1,6-dichlorohexane and different bisphenols: 4,4′-dihydroxyazobenzene, 4,4′-dihydroxydiphenyl, bisphenol A and 2,7-dihydroxynaphthalene. Thermal stability was investigated by thermogravimetric analysis, in a static air atmosphere the heating rate being 10°C/min. Molecular modeling was used as a complementary analysis method for the best understanding of the relationship between the chain conformation and polarity and the thermal behavior. CERIUS2 and HYPERCHEM programs were used to perform the molecular modelling. All the synthesized polymers present similar values of the starting point of the weight loss. This behavior can be explained by supra-molecular ordering, which is probably more important than the chemical structure. The presence of the hexamethylenic spacer leads to a micro-phase separation, with a favourable influence on the ordering in the solid state. All polymers showed low values of the polar surface, with interchain interactions playing a secondary role in the thermal stability.
Keywords: Thermal stability; Molecular modelling; Liquid crystals; Polyethers;

Three types of ageing (thermal oxidation, radiochemical ageing and hydrolytic ageing) have been applied to a series of epoxy networks which can be used for the embedding of low activity radioactive wastes. The effects on the chemical composition and structure were studied and related to the evolution of the gas diffusion properties of these networks. In the case of thermal oxidation, the detailed kinetic study of the degradation and the measurement of O2 solubility and diffusivity allowed precise modelling of the oxidation profile.
Keywords: Thermal oxidation; Radiochemical ageing; Hydrolytic ageing; Epoxy-amine networks; Water and gas transport;

Plasticiser migration and structural changes in an aged poly(vinyl chloride) coating by L. Monney; M. Jamois-Tasserie; C. Dubois; P. Lallet; F. Villa; C. Renaud (459-468).
A filled PVC coating used for the head and armrests of trains was aged naturally in a railway environment or artificially. Ageing caused the coating to harden and in most cases to crack. The loss in suppleness was linked to the PVC matrix's loss of plasticisers. Two parameters were studied by artificial thermal ageing carried out at about 100°C: the role of the temperature inside the train, and the influence of the polyurethane foam inside the arm and headrests on the migration of the plasticisers. The loss in plasticisers, which led to hardening, was accompanied by a structural heterogeneity in the filled PVC matrix. The mineral fillers, initially spread out evenly, were concentrated in one part of the matrix due to the formation of non-filled PVC nodules. Under these ageing states, the material cracked in response to the mechanical constraints caused by the users.
Keywords: Poly(vinylchloride); Railway environment; Plasticiser; Structure;

Vacuum pyrolysis of commingled plastics containing PVC I. Kinetic study by Rosa Miranda; Jin Yang; Christian Roy; Cornelia Vasile (469-491).
Thermal decomposition of commingled plastics comprising high and low density polyethylene, polystyrene, polypropylene and poly(vinyl chloride) was investigated under vacuum conditions by dynamic thermogravimetric analysis in the temperature range of 25–600°C. The mixtures were representative of the major polymeric materials found in a municipal plastic waste (MPW) stream. The study focused on the role of each polymer in the stabilization or destabilization of other interacting plastics, as well as the fate of chlorine produced from PVC. First, the decomposition of each single plastic was investigated. Then the binary combination of all above polymers and finally the multicomponent mixture similar to a MPW sample were studied. Two approaches have been applied to study the interaction between components in the commingled plastics. The first approach involved a comparison of the experimental curve of the decomposition of the mixture with a calculated non-interacting component decomposition curve based on the behavior of the individual polymers. The second approach involved a comparison of the kinetic parameters of each plastic in the mixture as determined by two different methods: (1) fitting the single polymer decomposition curve and (2) fitting the peak corresponding to the pyrolysis of one particular polymer on the mixture pyrolysis curve. The results obtained indicate that some interactions occurred during MPW pyrolysis, mainly at high temperatures (>375°C). PS and PVC appear to be the plastics responsible for the interactions through their intermediate pyrolysis products. The chlorine from PVC in the mixture is released almost completely before 375°C, when the conversion of the MPW to pyrolysis products has reached ca. 13%.
Keywords: Commingled plastics; PVC; Thermal degradation; Kinetics; Vacuum; Pyrolysis;

Dioxane lignin was isolated from manchurian ash by means of Pepper's method. The thermal degradation and charring of the manchurian ash treated with ammonium polyphosphate, ammonium dihydrogen or boric acid in the condensed phase were investigated by the combination of XPS, FTIR and TGA. TGA results under pure nitrogen showed that the flame retardants reduced the initial temperature of thermal degradation of the lignin and promoted char formation compared with the control sample. XPS data obtained in high vacuum showed that an increase of C1s and C1s (C–C) intensity, a decrease of C1s (C–O) intensity and a decrease of C1s binding energy were promoted by the flame retardants. FTIR data of char residue showed that flame retardants reduced the relative absorbance of the C–O group and enhanced the relative absorbance of C–H vibration in aromatic ring and of aromatic ring vibration. No increase of the CO group vibration was observed. The results from XPS, FTIR and TGA demonstrate that the flame retardants catalyzed the cleavage of C–O bonds in the lignin to form a highly conjugated system and promoted crosslinking and char formation. Ammonium polyphosphate and ammonium dihydrogen phosphate were more effective in char forming than was boric acid.
Keywords: Manchurian ash lignin; Thermal degradation; Charring; Flame retardants;

Blends of a polyurethane with two different functionally reactive elastomers have been prepared by two different blending techniques. The thermal stability of the blends was studied to correlate the effect of blending technique on the degradation temperatures of the blends. The blending technique plays an important role in determining the thermal stability of the blends. For the same elastomer ratio, the preblended sample degraded at a lower temperature and the degradation temperatures for the preheated preblended samples were delayed. This may be due to the interchain crosslinking reaction between the two elastomers on heat treatment.
Keywords: Polyurethane; Preblending; Preheating preblending; Interchain crosslinking; Polyblend; Dicumyl peroxide;

We discuss some thermal properties, as well as the thermal stability, of irradiated isotactic polypropylene. Sol–gel analysis showed that, despite the fact that irradiation doses were relatively high, no gel was formed. Values for DSC melting temperature, specific enthalpy of melting, crystallization temperature and crystallization enthalpy are different for the first and second scan, but the trend was similar. The melting temperature and specific melting enthalpy decreased with an increase in irradiation dose. TGA results showed that irradiation lowered the thermal stability of PP.
Keywords: Polypropylene; Gamma irradiation; Degradation; DSC; TGA;

This paper examines the microstructural aspects of vernonia oil-added starch–polyethylene and vernonia oil added starch–polyethylene–polylactic acid composite. Polymer composite films containing different percentage of additives were melt processed and acid hydrolyzed. Scanning electron microscopy of the fracture surfaces of OsO4 stained vernonia oil-added composite showed that the vernonia oil is present at the interface of the starch–polyethylene. Tapping mode atomic force microscopy (TMAFM) was used to obtain pore size of the hydrolyzed polymer composite. The progressive hydrolysis of the degradable component in the composite was studied by mass loss measurement and FTIR analyses. The quantity of water passing through the porous acid hydrolyzed composite was found to depend on the thickness of film.
Keywords: AFM; Composite; Polylactic acid; Acid hydrolysis; Porous material;

Recent efforts to blend and compatibilize intrinsically immiscible polymers in the solid state by high-energy methods have shown that macromolecules may undergo scission, crosslinking or amorphization, depending on the chemical nature of the repeat unit, the processing temperature and the initial degree of polymer crystallinity. Identical process-induced molecular and structural modifications have been previously observed in polymers exposed to large doses of electron and γ radiation, suggesting that the responses of polymers to high-energy processes may be mechanistically similar. In this work, we explore a variety of similarities between mechanically-milled and irradiated polymers in terms of molecular characteristics, process temperature and polymer crystallinity, and we demonstrate that these similarities provide predictive guidance for the selection of polymers to be subjected to solid-state processing.
Keywords: Mechanical milling; Solid-state processing; Chemical structure; Radiation damage;

Thermal behaviour of palladium complexing polyacrylamidoxime polymer by Gabriela Moroi; Doina Bı̂lbă; Nicolae Bâlbă (525-535).
A fibrous material with metal complexing ability, suitable for retention, concentration and recovery of the palladium ions from acidic aqueous solutions, was prepared by partially converting the nitrile groups of a polyacrylonitrile fibre into amidoxime groups. The thermal behaviour of the resulting polyacrylamidoxime fibre in comparison with its polyacrylonitrile precursor, as well as that of the sorbent material after the retention of palladium ions were studied by thermogravimetric analysis and differential scanning calorimetry. By using the thermogravimetric data the activation energy of the thermo-oxidative decomposition was evaluated by Coats–Redfern and Levi–Reich methods. The introduction of amidoxime groups into the structure of the polyacrylonitrile fibre decreases its initial thermal stability, but has a rather little influence on the main step of its thermo-oxidative decomposition. The polyacrylamidoxime fibre containing palladium ions retained by complexation exhibits a better initial thermal stability than the original polyacrylamidoxime fibre due presumably to the crosslinking formed by intermolecular complexation of the ions; by contrast, the palladium ions have a strong catalytic effect on the reactions involved in the main step of polyacrylamidoxime fibre decomposition.
Keywords: Thermal behaviour; Polyacrylamidoxime fibre; Palladium ions; Thermogravimetry; IR spectroscopy;

The oxidative degradation of poly(methyl methacrylate) (PMMA) was investigated in chlorobenzene at 50–100°C for various peroxide and polymer concentrations. Unlike thermal degradation, the chain scission of PMMA occurs randomly across the polymer backbone. The samples were analyzed by gel permeation chromatography (GPC) to obtain the time evolution of molecular weight distributions (MWDs). A continuous distribution model for polymer degradation and simultaneous peroxide deactivation was developed assuming that the rate coefficients depend linearly on MW. This model was used to determine the rate coefficients for the oxidative degradation of PMMA. The activation energy, determined from the temperature dependence of the rate coefficients, was 48.2 kcal/mol.
Keywords: Oxidative degradation; Random chain scission; Poly(methyl methacrylate); Rate coefficients; Radical mechanisms;

Through grafting styrene onto UV irradiated polyolefins, the hydroperoxidised sites of the substrates were amplified. These grafted sites can be seen from scanning electron microscopy or detected by Raman micro-spectroscopy of fractured samples. The grafting spots are heterogeneous as observed in SEM images and in Raman maps where they appeared as 1–4 μm nodules attached to the substrates and distributed through the sample thickness in a profile consistent with diffusion control of photo-oxidation. Catalyst residues were also detected through Raman mapping and EDX. Their distribution is also heterogeneous but maxima in catalyst concentration are not coincident with the sites for grafted polystyrene.
Keywords: Polyolefins; Polypropylene; Degradation; Photo-oxidation; Hydroperoxides; Grafting; Raman mapping; SEM;

Thermal and mechanical behaviour of some new crosslinked poly(ester-siloxane)urethanes by Aurelian Stanciu; Victor Bulacovschi; Stefan Oprea; Stelian Vlad (551-558).
New cross-linked poly(ester-siloxane)urethanes were prepared by multistep polyaddition from aromatic diisocyanates and diglycerin maleate, as the hard segment, and poly(ethylene glycol adipate) or poly (diethylene glycol adipate) and poly(dimethyl siloxane), as the soft segment. In some cases the soft segment was extended with polybutadiene segments. The influence of structure and position across the chain of the hard and soft containing segments upon mechanical and thermal properties were studied using thermogravimetric and mechanical techniques.
Keywords: Poly(ester-siloxane)urethanes; Cross-linking; Network; Elastomers; Mechanical properties; Thermal stability;