Polymer Degradation and Stability (v.98, #4)

Molecular simulation is used to probe the structure property relationships displayed by polysulphone (PS) and polyethersulphone (PES) and reproduces closely the temperatures at which thermal degradation occurs (and the glass transition temperatures). Both data sets agree well with those obtained empirically using TGA. The examination of the thermal and thermo-oxidative stability of thermoplastic oligomers (M w = 5454–33,866 g mol−1, PDI 1.33–1.82) based on PS, PES, polyetherimide (PEI) and poly(amide-imide) (PAI), is reported. TGA reveals the least thermally stable polymer is PES (T d = ∼250 °C), while PAI (T d = ∼350 °C) is the highest: the materials usually display two-step decomposition patterns: scission of bridging group and degradation of backbone structure. A possible mechanism for the degradation of a PAI is proposed on the basis of the empirical and simulation data. This work provides a general method for the prediction of the thermal stability of oligomeric modifiers (and high molecular weight polymers).
Keywords: Engineering thermoplastics; Low molecular weight; Thermo-oxidative/thermal stability; Modelling;

Synthesis and characterization of degradable polyimides from p-phenylenedioxybis(5-amino-2-pyridine) by Lin Chen; Yang Cao; Tiantian Fang; Wei Yue; Jing Wang; Xin Zhang; Fengchun Yang (839-843).
Polyimides are widely used in electronic, semiconductor industry, however, with the wide application, the degradation of the polymer has become a serious problem people have to face. In this work, a series of new fast degradable polyimides were synthesized from diamine p-phenylenedioxybis(5-amino-2-pyridine) with various commercially available aromatic dianhydrides via a conventional two-stage process. The resulting polyimides could degrade at room temperature under a certain condition, and we proposed a degradation mechanism by analyzing the degradation products. Compared with the traditional method, here, we provided a facile and effective method for polyimide degradation. The result suggested that introducing a phenyl-2-pyridyl ether structure into polymer chains may become a class of new method for polymer degradation.
Keywords: Degradable polyimide; Diamine; Degradation mechanism; Monomer synthesis;

Melt stereocomplexation from poly(l-lactic acid) and poly(d-lactic acid) with different optical purity by Yanlong Liu; Jingru Sun; Xinchao Bian; Lidong Feng; Sheng Xiang; Bin Sun; Zhiming Chen; Gao Li; Xuesi Chen (844-852).
Poly(l-lactic acid) (PLLA) and Poly(d-lactic acid) (PDLA) with different optical purity were blended in an internal mixer and their crystallization behavior were investigated. Nearly complete stereocomplex crystallites could be obtained at specific condition between PLLA and PDLA. The melting temperature of stereocomplex was related with the optical purity of PLLA and PDLA, and also with the mixing temperature and time. When the mixing temperature was set dozens of degrees above the melting temperature of pure PLLA and PDLA at a weight ratio 1:1, the stereocomplex could be formed within a short period of time, and no homocrystallization or trace homocrystallization in some samples could be found. The possible mechanism of stereocomplex formation was achieved in this article and the shear force play an important role during the melt mixing process.
Keywords: Poly(lactic acid); Melt stereocomplexation; Optical purity; Heat stability;

Effects of alkaline and acid solutions on glass/epoxy composites by A.M. Amaro; P.N.B. Reis; M.A. Neto; C. Louro (853-862).
Composite structures can be exposed to a range of corrosive environments during their in-service life, which causes degradation in terms of material properties. The effect of alkaline and acid solutions on the GRP mechanical properties can be found in open literature, but the studies presented are not sufficient to establish a full knowledge of this subject. In this paper the flexural properties and the impact strength of a glass fibre/epoxy composite after immersion in hydrochloric acid (HCl) and sodium hydroxide (NaOH) were analysed. Independently of the solution, the flexural strength and the flexural modulus decrease with the exposure time. However, alkaline solution promotes higher decrease of the flexural properties than the acid solution. The same tendency was observed for impact strength.
Keywords: Composites; Environmental degradation; Failure; Hardness measurement; Mechanical characterization;

“Green” molecular weight degradation of chitosan using microwave irradiation by Jaroslaw M. Wasikiewicz; Stephen G. Yeates (863-867).
Chitosan degradation in 0.1 M acetic acid solution by microwave radiation alone (without any additives and initiators) is reported. At the condition of reduced thermal effects of microwave heating degradation manifested most of typical features associated with mechanical (and or mechanochemical) degradations induced by shear forces, in this particular case oscillating molecules. At the absorbed energy dose of 9 MJ kg−1 chitosan in 1% w/v solution degrades within 20 min to the weight average molecular weight of 30 kDa.Comparative evaluation of degradation yield of chain scission (G s) with other high energy radiations such as ultraviolet- and gamma radiation showed microwave to be the least efficient, but considering the environmental impact and low running costs it presents a viable alternative to obtain targeted molecular weights of charged polysaccharides.
Keywords: Chitosan; Microwave; Degradation; Molecular weight; Energy efficiency;

The synergistic effects of boroxo siloxanes with magnesium hydroxide in halogen-free flame retardant EVA/MH blends by Lei Ye; Yueying Miao; Hong Yan; Ze Li; Yulong Zhou; Jianxun Liu; Hong Liu (868-874).
The synergistic effects of boroxo siloxanes (BSil) with magnesium hydroxide in halogen-free flame retardant EVA/MH blends have been studied by cone calorimeter test (CCT), Fourier transfer infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), transmission electron microscopy (TEM), scanning electron microscopy (SEM), mechanical properties measurements, limiting oxygen index (LOI), UL-94 and torque tests. The LOI and UL-94 data show that when 3 wt% boroxo siloxane substituted for the magnesium hydroxide (MH) can increase 8% LOI values of the EVA/MH blends and keep the V-0 rating. The data obtained from the CCT tests indicate the heat release rates (HRR) reduced by about 23% for the EVA/MH sample with 3 wt% BSil when compared with the EVA/MH sample without BSil. The experimental observations from the torque, FTIR, TEM and SEM give positive evidence that the synergistic effects mechanism of BSil with MH can be described to: (i) BSil react with MH and increases the melt viscosity; (ii) BSil promote the homogeneous dispersion of MH; (iii) BSil promote the formation of compact charred layers and prevent the charred layers from cracking, which effectively protects the underlying polymer from burning. The TGA and dynamic FTIR spectral reveal that EVA/MH/BSil samples show slower thermal degradation rate and higher thermal degradation temperature than the EVA/MH sample. The mechanical tests show that the mechanical properties of EVA/MH/BSil blends are markedly improved by suitable amount of BSil.
Keywords: Boroxo siloxane; Magnesium hydroxide; Synergistic effect; Flame retardant; Ethylene-vinyl acetate copolymer;

Comparison of biodegradability of various polypropylene films containing pro-oxidant additives based on Mn, Mn/Fe or Co by Stéphane Fontanella; Sylvie Bonhomme; Jean-Michel Brusson; Silvio Pitteri; Guy Samuel; Gérard Pichon; Jacques Lacoste; Dominique Fromageot; Jacques Lemaire; Anne-Marie Delort (875-884).
The biodegradability of two polypropylene films with low content of ethylene (a statistical copolymer (PPs) and a block copolymer (PPb) with balanced additions of phenolic antioxidant and pro-oxidants based on Mn, Mn/Fe or Co was studied. Abiotic pre-treatments by accelerated artificial photooxidation and thermooxidation representing about 3–4 years of outdoor weathering, including 3–4 months of exposure to daylight and 3 years in soil were followed by FTIR and SEC measurements. When a controlled oxidation was reached in the films, they were inoculated, in a second step, with the strain Rhodococcus rhodochrous in mineral medium and incubated up to 180 days. The metabolic activity of bacteria was evaluated by measuring ATP content, ADP/ATP ratio and cell viability. Complementary 1H NMR experiments were conducted on the incubation media, with and without cells, in order to monitor the consumption of soluble compounds excreted from the oxidized polymers by R. rhodochrous cells. The main conclusions are that the Co derivatives (with Co content ≥ 150 ppm) must be considered toxic for R. rhodochrous. PP films containing pro-oxidants based on Mn and Mn + Fe give positive results for the biotest (low ADP/ATP ratio, post-development in Petri dishes). However the biodegradability of oxidized PP films is less efficient in comparison to oxidized PE films (see paper published in this journal). This observation may be correlated with the accumulation in the incubation media of oxidized oligomers that cannot be metabolized rapidly by the bacterial cells and/or by the residual crystallinity of PP derivatives.
Keywords: Polypropylene; Biodegradation; Pro-oxidants; Thermooxidation; Photooxidation;

Stability of poly( l -lactide)/TiO2 nanocomposite thin films under UV irradiation at 254 nm by Wei-Wei Wang; Chang-Zhen Man; Chun-Mei Zhang; Long Jiang; Yi Dan; Thien-Phap Nguyen (885-893).
Poly( l -lactide)/titania (PLLA/TiO2) nanocomposites were in situ synthesized through ring-opening polymerization of l -lactide. Two types of TiO2, anatase and rutile, were used in the synthesis of composites. The chemical structure, crystallinity, thermal stability and morphology of composite thin films were analyzed by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). The effects of TiO2 on the stability of the nanocomposites under UV irradiation as a function of time were investigated by FTIR, XRD, UV absorption spectroscopy and differential scanning calorimetry (DSC). The obtained results indicated that rutile TiO2 particles have a screening effect while anatase TiO2 acts as a photocatalyst. The DSC results revealed that anatase TiO2 embedded in bulk of the films produces the same effect as rutile TiO2.
Keywords: Poly( l -lactide); Rutile titania; Anatase titania; Nanocomposite; UV irradiation; Stability;

Studies of hair for use in lime plaster: Implications for conservation and new work by Craig J. Kennedy; William A. Revie; Lore Troalen; Matthew Wade; Tim J. Wess (894-898).
Historic buildings constructed with lime plasters often require repairs and re-plastering of areas as part of a maintenance and conservation regime. Hair is commercially available for use in lime plaster and mortar, as it is still used today to provide additional strength and crack resistance to fresh plaster. In this study we examine commercially available imported hair from a number of species as well as fresh, untreated horse hair. Small angle X-ray scattering (SAXS) demonstrates a loss of keratin structure in most of the imported, treated hair samples, including horse hair compared to untreated indigenous horse hair samples. Fourier transform infrared spectroscopy (FT-IR) shows that the imported horse hair has high levels of cysteic acid present which is not shown in the fresh, untreated hair. The results obtained suggest that imported hairs are treated with an oxidising agent such as bleach or acid prior to sale, and this weakens the hair making it more susceptible to failure in a building context.
Keywords: Hair; Keratin; Lime; Plaster; Heritage; Repair;

Photo-induced oxidation of polythiophene derivatives: Dependence on side chain structure by Yoshinori Aoyama; Toshihiro Yamanari; Nagatoshi Koumura; Hiroto Tachikawa; Masaru Nagai; Yuji Yoshida (899-903).
The photooxidation of poly(3-hexylthiophene) (P3HT) has previously been investigated and it is known that the sulfur atom of the thiophene ring is oxidized into sulfur oxides by a hydroxyl radical that arises through hydrogen abstraction at the hexyl side chain at the α-position. In this study, in order to further investigate the photooxidation mechanism of the polymer, the side chain at α-position was varied and three different polythiophene derivatives with phenyl (P3PhT), tert-pentyl (P3DMPT), and alkoxy (P3OOT) groups were synthesized with no α-hydrogens. We observed the differences in the photochemical behavior of the polythiophene derivatives by UV–vis–NIR and FT-IR (ATR) spectroscopy during simulated solar light irradiation. It was found that the photooxidation of P3PhT and P3DMPT progressed in the same way as P3HT, whereas the analogous decomposition of P3OOT was not observed.
Keywords: Photooxidation; Poly(3-hexylthiophene); Side chain; Polymer solar cells; Fourier transform infrared spectroscopy;

Manganese-, cobalt-, and zinc-based mixed-oxide spinels as novel catalysts for the chemical recycling of poly(ethylene terephthalate) via glycolysis by Muhammad Imran; Do Hyun Kim; Waheed A. Al-Masry; Asif Mahmood; Azman Hassan; Sajjad Haider; Shahid M. Ramay (904-915).
Chemical recycling of post-consumer poly(ethylene terephthalate) (PET) into useful feedstock was carried out in the presence of novel mesoporous metal oxide spinel catalysts. ZnO (hexagonal), metal oxide spinels (Co3O4 and Mn3O4), and mixed metal oxide spinel (ZnMn2O4, CoMn2O4, and ZnCo2O4) catalysts were synthesised via the precipitation or co-precipitation method. The structural, textural, and acidity properties of the materials were examined using various characterisation tools, such as XRD, SEM/EDX, TEM, FT-IR, NH3-TPD, and BET surface area analyser. The depolymerisation of waste PET (mostly soft-drink bottles) to the monomer bis(2-hydroxyethyl) terephthalate (BHET) via glycolysis was performed using excess ethylene glycol (EG) in the presence of fabricated metal oxides as transesterification catalysts. The effect of different parameters, such as temperature, catalyst type, reaction time, EG/PET molar ratio, and catalyst/PET weight ratio, on the monomer yield were investigated. The results revealed that the catalyst that yielded the highest amount of BHET (92.2 mol%) under mild reaction conditions (260 °C and 5.0 atm) was zinc manganite tetragonal spinel (ZnMn2O4), which has tetrahedral Zn+2 ion and octahedral Mn+3 ion coordination with the spinel crystal structure. The high catalytic activity of this spinel may be attributed to its greater surface area, the presence of mild and strong acid sites, and its overall higher concentration of acid sites. Furthermore, the ZnMn2O4 structural properties were examined, and it was determined that the Zn+2/Mn+3 metal cations pair, their positioning in the crystal structure, and that the spinel geometry has a pronounced effect on the catalytic efficiency. The monomer, dimer, and oligomers were separated and characterised by thermal (DSC and TGA) and structural (NMR) analyses, which confirmed the purity and structure of the monomer and dimer. Based on the experimental observations, a reaction mechanism was also proposed. In conclusion, the present approach was an attempt to demonstrate a process consisting of a set of new catalysts with optimised process conditions for the maximum production of highly pure BHET.
Keywords: PET; Glycolysis; Depolymerisation; Metal oxides; Spinel catalysts;

Synthesis and characterization of addition-type silicone rubbers (ASR) using a novel cross linking agent PH prepared by vinyl-POSS and PMHS by Yifu Zhang; Yongyun Mao; Dongzhi Chen; Weibing Wu; Shengping Yi; Shaobo Mo; Chi Huang (916-925).
A novel cross-linker (denoted as PH) prepared using vinyl-POSS and poly(methylhydrogensiloxane) (PMHS), was introduced to vinyl terminated polydimethylsiloxane (VPDMS) matrix to prepare a series of addition-type silicone rubbers (ASR) for the first time. Morphology, thermal properties and mechanical properties of these ASR were respectively studied by means of scanning electron microscope (SEM), thermal gravimetric analysis (TGA) and universal tensile testing machine. The results revealed that the thermal stabilities and mechanical properties of ASR were significantly enhanced by the addition of PH compared with the traditional cross-linker PMHS. It was found that the tensile strength of ASR prepared using PH as cross-linker was strikingly increased by 43.8% compared with the one prepared using PMHS as cross-linker. The striking enhancements in thermal properties and improvements on mechanical properties of ASR were likely attributed to the increase of dimensionality of novel cross-linked networks in novel ASR.
Keywords: POSS; Addition-type silicone rubber; Cross-linker; Thermal stability; Mechanical property;