Polymer Degradation and Stability (v.98, #3)

Ultraviolet resistance of azo-containing poly(1,3,4-oxadiazole) fibres by Wanli Zhou; Xiao Yang; Erpeng Jia; Xu Wang; Jianjun Xu; Guangdou Ye (691-696).
To improve the ultraviolet (UV) resistance of poly(1,3,4-oxadiazole) (POD) fibres, 4,4′-azodibenzoic acid (ADA) was introduced into the POD fibres through copolymerization and blending respectively. The influence of UV light on the intrinsic viscosity and mechanical properties of POD fibres with and without azo groups was investigated in detail. The results revealed that the tensile strength retention of the POD fibres containing ADA was much higher than that of the POD fibres without the azo groups (p-POD) after accelerated irradiation using an iodine-gallium lamp. It was also found that POD with ADA in the molecular chain (c-POD) by copolymerization showed better UV stability than that of the POD blended with the same amount of ADA (b-POD). The UV spectrum was used to explore the UV resistance mechanisms of the series of the POD containing azo groups and it was demonstrated that the mechanisms of b-POD and c-POD were different.
Keywords: Poly(1,3,4-oxadiazole); 4,4′-azodibenzoic acid; Copolymerization; Blending; UV resistance;

Bis(2-aminoethyl) terephthalamide (BAET, or trimer) and α,ω-aminoligo(ethylene terephthalamide) (AOET, or oligomers) were obtained from the reaction of poly(ethylene terephthalate) (PET) waste with excess amount of ethylenediamine (EDA). The structures of trimer BAET and oligomers AOET were confirmed by FTIR, NMR, HPLC-MS analysis. The influence of input molar ratio of reactants on oligomer yield was also investigated. The formation of oligomers competed with the formation of trimer even though EDA was used in excess amount. The melting points, thermal properties of BAET and AOET were determined by DSC, TGA techniques. The physical melting process and chemical transamidation of amino end capped BAET/AOET occurred concurrently before thermal decomposition.
Keywords: Poly(ethylene terephthalate); Ethylenediamine; Bis(2-aminoethyl) terephthalamide; α,ω-aminoligo(ethylene terephthalamide); Aminolysis; Thermal decomposition;

Stereo multiblock and diblock poly(lactic acid)s (PLAs), poly(DL-lactide) (PDLLA), and poly(DL-lactic acid), with block lengths comprising 3.8–61.9 lactyl units were synthesized. The effects of the block length sequence on the hydrolytic degradation behavior were investigated at 37 °C and pH 7.4 using amorphous (quenched) samples of these polymers, in comparison with quenched neat poly(l-lactide) (PLLA), poly(d-lactide) (PDLA), and PLLA/PDLA blend. 13C NMR spectra revealed that chain cleavage during the hydrolytic degradation occurred rapidly on the atactic segments connecting relatively long isotactic l-lactyl and d-lactyl segments. The rates of weight loss and hydrolytic degradation of stereoblock PLAs, PDLLA, and poly(DL-lactic acid) decreased with an increase in the average stereoblock length. The hydrolytic degradation of the stereo multiblock PLAs with average block lengths higher than 27 and 15 lactyl units was suppressed as compared to that of the PLLA/PDLA blend and the neat PLLA or PDLA, respectively. The stereocomplex crystallization occurred during the hydrolytic degradation of stereoblock PLAs and PDLLA with average stereoblock lengths higher than 7 lactyl units, in agreement with the results reported for the stereocomplex crystallization from the melt. The crystallization rate of stereoblock PLAs during the hydrolytic degradation increased on increasing the block length; however the rate of the hydrolytic degradation of the diblock PLA with the highest average stereoblock length was lower than that of the PLLA/PDLA blend.
Keywords: Poly(lactic acid); Polylactide; Hydrolysis; Stereocomplex; Stereoblock copolymer;

Degradation mechanism of silicone glues under UV irradiation and options for designing materials with increased stability by Hartmut R. Fischer; Christopher Semprimoschnig; Cathal Mooney; Thomas Rohr; Ernst R.H. van Eck; Margriet H.W. Verkuijlen (720-726).
The degradation of silicone glues used, for example, in the assembly of solar modules for use in space, has been investigated and possible mechanisms which lead to colouration and possible embrittlement are analysed. Both effects are connected to the generation of radicals upon exposure to UV irradiation. As a follow up-reaction in vacuum, cross-linking and subsequently embrittlement and cracking due to shrinkage can occur. The degradation of the polymers via a radical mechanism can also lead to the formation of small metal clusters containing residues of the Karstedt catalyst and also silicon. Both types of nanoparticles contribute to colouration of the silicone glues. Possible ways to avoid this is the use of ultra-pure starting substances, a minimum amount of catalyst and a higher cross-link density of the silicone, minimizing the mobility of dangling chains formed upon UV-exposure and their tendency to form metal clusters.
Keywords: Silicone rubber; UV irradiation; Degradation mechanism; ESR; Space environment;

Study of the degradation mechanism of Chinese historic silk (Bombyx mori) for the purpose of conservation by Mei-Ying Li; Yang Zhao; Tong Tong; Xiao-Hui Hou; Bei-Song Fang; Shun-Qing Wu; Xin-Yu Shen; Hua Tong (727-735).
Chinese historic silk fabrics dated from the Warring States Period (B.C. 475–B.C. 221), the Han Dynasty (B.C. 202–A.D. 220) and the Ming Dynasty (A.D. 1368–A.D. 1644) as well as artificial aged silk specimens were examined by SEM, TEM, amino acid analysis, FTIR, WAXD, SAED and DSC to investigate the degradation mechanism for the purpose of conservation. The variations of the relative concentrations of Gly and Ala and the crystallinity revealed that the degradation mainly occurred at the amorphous region at the initial degradation stage; both the amorphous and the crystalline regions were degraded at the same speed at the late degradation stage. Combined with the fact that the mechanical strength decreased over the degradation process, it could be concluded that the degradation of both the amorphous and the crystalline regions contributed to the decrease of the mechanical strength. During the degradation process, the morphology of the cross-section of the silk fiber was found to change from stacking particles to lamellar sheets, with decreasing size of segments of silk fibril. The changes of morphology and degree of orientation during the degradation process exhibited correlations with the change of secondary structure, which was greatly affected by aging method. The change of thermal stability exhibited correlations with both the secondary structure and the degree of orientation.
Keywords: Historic silk; Firoin; Degradation mechanism; Hierarchical structure;

Metathetic degradation of synthetic cis-1,4-polyisoprene (PI) and styrene–butadiene copolymer (SBR) was performed with cis-1,4-diacetoxy-2-butene (DAB) as chain transfer agent (CTA) using Grubbs II catalyst. Well-defined acetoxy telechelic polyisoprene structures were obtained in a selective manner with a wide range of targeted average molecular weights from 350 g mol−1 to 98,000 g mol−1, with a polydispersity index of around 2. Starting from SBR, a similar structure control was obtained with a range of M n from 1400 g mol−1 to 65,000 g mol−1. It was found that precise selection of catalyst concentration and solvent is the major factor to obtain targeted products with high M n control. Moreover, preliminary tests using such a procedure have shown its efficiency starting from waste tyre. This methodology thus suggests the possibility to recycle such rubber waste leading to high value liquid rubber.
Keywords: Rubber; Recycling; Chemical degradation; Metathesis; Waste tyre;

Thermal degradation of crosslinked copolyimide membranes to obtain productive gas separation membranes by R.M. Huertas; A. Tena; A.E. Lozano; J. de Abajo; J.G. de la Campa; E.M. Maya (743-750).
By removing anhydride groups from crosslinked copolyimide membranes at 430 °C, partially pyrolysed membranes were obtained and gas separation properties were evaluated and related to the content of labile groups. The spaces occupied by the linking anhydride groups prevailed as micropores, which greatly improved the gas transport properties.The release of the labile groups was confirmed by TGA and FT-IR spectroscopy which permitted us to establish that polymer main chains did not suffer any substantial change during the pyrolytic treatment. During the heating step new cross-linking points are created, no Tg below 450 °C could be observed and the process prevents the collapse of micropores.The pyrolysed membrane obtained from the precursor with the lowest content of anhydride groups showed the highest permeability values: 101, 25, 17 and 446 barrers to O2, N2, CH4 and CO2 respectively, and a selectivity to CO2/CH4 of 26, good enough to place this membrane close to Robeson's new upper bound (2008), while the crosslinked precursor membrane fell below Robeson's old upper bound (1991).
Keywords: Copolyimides; Anhydride groups; Membranes; Pyrolysis; Gas permeation;

The thermal degradation behaviour of poly(ε-caprolactone) (PCL), poly(l-lactide) (PLLA) and poly(lactide/ε-caprolactone) (PLCL) and of composites of these polymers filled with bioglass particles was investigated by means of thermogravimetric analysis (TGA). According to the activation energies calculated by the Friedman approach, PCL showed the highest resistance to thermal degradation. Addition of bioglass induced a reaction between the ester groups of the polyesters and the SiO groups present in the surface of bioglass particles which caused a 1.3–1.9 fold decrease in activation energies of the composites with respect to their unfilled polymer counterparts. This reaction was proven by the increase in the absorbance of the carboxylate peak in the infrared spectra of the composite films maintained at 210 °C and confirmed the random chain scission of the polymer chains. This fact caused a significant decrease in the featured thermal transitions of the polymers as determined by the differential scanning calorimetry (DSC) measurements.
Keywords: Bioglass; Polyesters; Thermal degradation; Composites;

The primary focus of this study was to evaluate the effects of different montmorillonite nanoclays (MMT) on the thermal stability and degradation of epoxy composites exposed to UV radiation and elevated temperatures. Diglycidyl ether of bisphenol A (DGEBA) epoxy resin, SC15 was reinforced with three different montmorillonite nanoclays, Nanomer® I.28E, Cloisite® 10A and Cloisite® 30B. Thermal properties of modified DGEBA nanocomposites were characterized. Subsequently, neat and nanocomposites were subjected to 500 h of UV radiation and characterized to determine the effects of various nanoclays on the degradation. Addition of nanoclays increased the thermal properties compared to the unmodified composite and better retention of material properties after exposure to UV radiation. Viscoelastic properties increased with addition of nanoclays in both unexposed and UV radiation exposed samples.
Keywords: MMT – montmorillonite nanoclay; DGEBA – diglycidyl ether of bisphenol A; UV radiation; Degradation; Thermal stability;

A comparison is made between available literature and present results of the three major types of polylactide (PLA) degradation giving a compiled view on the details of degradation mechanisms with relation to explicit factors affecting degradation. The temporal decrease of molar mass has been analyzed under isothermal conditions at 220 °C, biological and photodegradation conditions using a polylactide (PLA) with ∼4 mol% D units. The decrease of molar mass with time during biodegradation follows a first order process (M = M o e kt ) while the molar mass of specimens tested during thermal and photodegradation follows a second order law (1/M = (1/M o ) + kt) Literature data obtained in similar degradation conditions were also adequately fitted with these equations. This allows us to conclude that the main step in the three types of degradation is a random chain excision, with some differences in the algebraic functionality. Under the degradation conditions tested, the degradation rate follows the progression thermal > photo > biological. For equivalent molar mass, the effect of degradation type on cold crystallization and melting is significant indicating that degradation cannot be explained by a solely outcome of chains breakage and molar mass reduction. This feature is especially prominent when the linear growth rates of specimens subjected to bio or photo degradation are compared. Anhydride groups that are formed during photodegradation decrease the crystallization rate compared to biodegraded specimens of equivalent molar mass. The molar mass dependence of the maximum growth rate follows a power law with exponents 1.3 for bio and 1.0 for photodegraded specimens, representative of semi-entangled systems. The temperature coefficient of the growth rate, analyzed according to secondary nucleation leads to a linear dependence for bio and photodegraded specimens, and to values of the surface free energy of crystallites that decrease from ∼85 to 55 erg/cm2 with decreasing molar mass. Combinations of molar mass characterization, FTIR, and thermal and crystallization rate analysis are proven useful strategies to assess and discriminate macroscopic changes of PLA structure induced by different types of degradation. This work also underlines the importance of analyzing the linear growth rates as a parameter that uncovers specific structural changes during degradation.
Keywords: Polylactide; Thermal degradation; Degradation in soil; Photodegradation; Degradation kinetics; Crystallization kinetics;

Recycling of glass fibre reinforced composites using subcritical hydrolysis: Reaction mechanisms and kinetics, influence of the chemical structure of the resin by Géraldine Oliveux; Jean-Luc Bailleul; Eric Le Gal La Salle; Nicolas Lefèvre; Gwenaël Biotteau (785-800).
Hydrolysis in batch conditions was used to recycle composite materials reinforced with long glass fibres and made of unsaturated polyester resin cross-linked with styrene. Subcritical conditions of water (200 °C < temperature < 374 °C and pressure < 221 bars) were chosen regarding the involved chemistry for the case of simple esters. Experiments performed in light and heavy water confirmed the specific acid catalysis of the hydrolysis of ester bonds ( k H 2 O / k D 2 O < 1 ), indicating that it can be described by the Aac2 mechanism identified for simple esters. The hydrolysis kinetics was thus defined for three widespread commercial resins and showed a good fit to the experimental data. Secondary reactions like decarboxylation of carboxylic acid and dehydration of glycol were also described in terms of mechanisms and kinetics. They also showed a specific acid catalysis.The obtained results enabled the definition of smooth conditions for the recovery of valuable products. They also showed that the chemical structure of the resin has an influence on the kinetics of hydrolysis but also on the kinetics of the main secondary reactions. However iso- and ortho-phthalic polyesters could be treated together.
Keywords: Glass fibres; Polymer-matrix composites; Unsaturated polyester; Chemical structure; Recycling; Kinetics;

Effect of reprocessing and clay concentration on the degradation of polypropylene/montmorillonite nanocomposites during twin screw extrusion by Josiane da R. Silvano; Sabrina A. Rodrigues; Juliano Marini; Rosario E.S. Bretas; Sebastião V. Canevarolo; Benjamim de M. Carvalho; Luís A. Pinheiro (801-808).
An evaluation was made of the degradation of polypropylene (PP) and montmorillonite (MMT) nanocomposites. The nanocomposites were compatibilized with polypropylene grafted with maleic anhydride (PP-g-MA) and with acrylic acid (PP-g-AA). The concentration of MMT varied from 1 to 5% (w/w). Nanocomposites were processed in a twin screw extruder up to five times. X-ray diffraction showed a decrease in the refraction angle with reprocessing steps and for PP-g-MA based nanocomposites, indicating an increase in the interplanar distance due to clay intercalation and exfoliation. Infrared analysis showed an increase in the concentration of carbonyl and unsaturated groups with the increase in the number of reprocessing cycles, and reduction as the MMT concentration increased. Colorimetry assays indicated a decrease in the luminosity and degree of redness due to the oxidation of the iron ions present in MMT to Fe2O3. The intensity of the yellow color of the b* coordinate was augmented with the number of reprocessing cycles but decreased with MMT concentration, confirming the FTIR analysis. Rheological measurements showed no crossing point between storage (G′) and loss (G″) moduli versus frequency, which is characteristic of a percolated nanocomposite networks. Within a range of frequencies, complex viscosity decreased with the number of reprocessing cycles, but the presence of montmorillonite was found to be responsible for lower differences between viscosity of nanocomposites reprocessed one and five times.
Keywords: Clay/polymer nanocomposite; Polypropylene; Montmorillonite; Reprocessing; Degradation;

Durability of polydicyclopentadiene under high temperature, high pressure and seawater (offshore oil production conditions) by P.Y. Le Gac; D. Choqueuse; M. Paris; G. Recher; C. Zimmer; D. Melot (809-817).
In the offshore industry polymer coatings are widely used to ensure thermal insulation of steel pipes, and to avoid over-cooling of the hot oil inside. Because of very severe service conditions (i.e. high temperature, high pressure and presence of seawater) and an expected life time of 20 years, durability of these coatings is a major issue for this industry. Polypropylene and polyurethane are often used for this application, nevertheless these polymers have some limitations in terms of processing time for polypropylene and maximum service temperature for polyurethane. Polycyclopentadiene (pDCPD) shows good processing characteristics and low thermal conductivity, so this polymer could be a good alternative coating in the offshore industry, but the durability of this polymer under offshore conditions is unknown. This paper present results from an accelerated ageing study of pDCPD in seawater at temperatures from 90 °C to 180 °C for 18 months. Polymer evolution during ageing is characterized using both mechanical (tensile test and DMA) and chemical (FTIR, NMR) analyses. For ageing at temperatures below T g (i.e. 155 °C) the only degradation mechanism is oxidation, whereas for ageing temperatures above T g secondary polymerization process of the material is observed.
Keywords: Polydicyclopentadiene; Seawater; Accelerated ageing; Oxidation; Offshore; Field joint;

Thermal decomposition of phenoxy/clay nanocomposites: Effect of organoclay microstructure by Maria Angeles Corres; Manuela Zubitur; Milagros Cortazar; Agurtzane Mugica (818-828).
The influence of organoclays on the thermal decomposition pathway of poly(hydroxyl ether of bisphenol A) (Ph) was analysed under nitrogen and air. In the two clays employed, montmorillonite and vermiculite, the organic clay modifier, a polar alkylammonium surfactant, led to different interlayer arrangements. Nanocomposite formation was established by X-ray diffraction and transmission electron microscopy. Thermal degradation of phenoxy and its nanocomposites was analysed by thermogravimetry (TGA/DTGA) and the evolved gases were investigated using thermogravimetric analysis coupled with Fourier transform infrared spectroscopy (TGA/FTIR). In both inert and oxidative atmosphere, it was observed that for the same organoclay proportion, nanocomposites containing organo-modified montmorillonite (Cloisite 30B) decompose at higher temperatures than those containing organo-modified vermiculite (VMT-ETO). This behaviour was attributed to the presence of the original metallic cations as a consequence of the incomplete exchange obtained in the organic modification of vermiculite. FTIR spectra of the evolved gases demonstrated that the incorporation of organoclays into phenoxy resin does not modify the degradation pathway of phenoxy, although the decomposition of the matrix is accelerated, possibly as a consequence of acidic sites created in organical decomposition.
Keywords: Phenoxy resin; Nanocomposites; Thermal and thermo-oxidative degradation; TGA/FTIR;