Polymer Degradation and Stability (v.124, #C)

Graphene oxide (GO) modified with furfuryl alcohol and (3-aminopropyl) triethoxysilane (GOFASi) was used in two separate ways for preparation of novolac resin nanohybrids. In the first procedure, graphene-containing silica aerogel (GA) was obtained by incorporation of GOFASi into silica aerogel network using tetraethyl orthosilicate (TEOS). Then, GA was used as an additive in novolac resin matrix. In the second procedure, the synthesis of a hybrid novolac resin was accomplished by network formation via the reaction of GOFASi with (3-glycidyloxypropyl)trimethoxysilane-modified novolac resin and TEOS. The thermal stability and the carbon yield of the hybrid resins obtained by the two procedures were compared. Fourier transforms infrared spectroscopy, X-ray photoelectron spectroscopy, and thermogravimetric analysis showed that both GO and the novolac resin were successfully modified. Aerogel formation was proved by Raman, X-ray diffraction, N2 adsorption and desorption isotherms, and by scanning and transmission electron microscopies.
Keywords: Novolac phenolic resin; Graphene aerogel; Carbon yield;

Effect of swelling on fatigue life of elastomers by M.S. Loo; J.B. Le Cam; A. Andriyana; E. Robin; J.F. Coulon (15-25).
In a number of engineering applications, elastomeric components are exposed to aggressive solvent such as biodiesel. Since biodiesel is considered as a potential substitute for conventional fossil fuel, the study on the durability in service of elastomers exposed to biodiesel becomes essential. The present paper investigates the mechanical response of swollen elastomers, due to exposure to palm biodiesel, under fatigue loading conditions. To this end, fatigue tests are conducted on dry and swollen rubber specimens at various maximum strains and a zero strain ratio. The physical fatigue damage mechanism induced in swollen material is studied through FESEM analysis coupled with EDS. During the test, two definitions of specimen end-of-life are adopted: (i) the number of cycles required for a nucleated crack to reach 1 mm in length and (ii) the number of cycles required for a complete rupture to occur in the specimens. The fatigue lifetime curves are plotted where the maximum principal stretch is used as the predictor. It is shown that swollen rubbers have shorter lifetime compared to the dry ones. Moreover, FESEM results reveal that the swelling level has no effect on the morphology of crack nucleation and propagation, regardless of the imposed loading level.
Keywords: Elastomer; Fatigue; Swelling; Crack nucleation; Crack propagation;

Chain extension of virgin and recycled poly(ethylene terephthalate): Effect of processing conditions and reprocessing by Izarelle S. Duarte; Albaniza A. Tavares; Poliana Silva Lima; Daniela L.A.C.S. Andrade; Laura H. Carvalho; Eduardo L. Canedo; Suédina M.L. Silva (26-34).
Poly (ethylene terephthalate) (PET) is widely used in several segments of the plastic industry and PET-based products are extensively recycled. However, PET degrades during processing and use, which results in a decrease of molar mass and reduction of the material's performance. Degradation particularly affects recycled materials, subjected to repeated processing and use cycles, this limiting the applications of recycled PET. Chain extender additives rejoin polymer chain segments, compensate molar mass reduction due to degradation, and may be used to further increase molar mass. In the present work, an epoxidic multifunctional oligomer (Joncryl), recommended for use with condensation polymers was tested for chain extension of virgin and recycled PET. Molar mass decrease (degradation) or increase (chain extension) during processing in a laboratory internal mixer may be related to torque and temperature variations reported by the instrument. Here molar changes during processing were considered in terms of the chain extension additive concentration (0–1.5%) and the nominal rotor speed (30 rpm–120 rpm) in samples of bottle-grade virgin PET and postconsumer recycled PET. It was found that the molar mass changes depend strongly on additive content; 0.5% and 1.0% chain extender levels were sufficient to compensate the mild degradation observed in unadditivated samples. Reprocessing tests, with and without further additivation, showed that extra additivated PET (1.5% additive concentration) preserves chain extension capabilities to sustain reprocessing without molar mass decrease. The procedure adopted allows inline testing of additive effectiveness.
Keywords: PET; Joncryl; Chain extension; Reprocessing; Internal mixer;

Water immersion aging of polydicyclopentadiene resin and glass fiber composites by Yinghui Hu; Xiaochen Li; Augustus W. Lang; Yunpeng Zhang; Steven R. Nutt (35-42).
We present an investigation of the effect of long-term water immersion aging on the thermal/mechanical properties of a ruthenium-catalyzed polydicyclopentadiene (pDCPD) polymer and associated composites, using an epoxy resin system as a benchmark. The pDCPD neat polymer showed low-level water absorption after one year of aging due to inherent hydrophobicity. No plasticization was observed for pDCPD, while significant plasticization occurred for the epoxy. Salt water aging and deionized water aging had identical effects on pDCPD and composites, while epoxy materials aged in salt water showed less water absorption compared to aging in deionized water. We also measured the fiber-interface strength before and after aging of composites using single-fiber push-out tests. Aging caused decreases in interface strength, and these decreases led to a decline in composite fatigue strength.
Keywords: Polymer-matrix composites (PMCs); Environmental degradation; Fiber–matrix interface;

A novel ricinoleic acid (RA) based phosphorus and nitrogen-containing flame retardant polyols (FRPE) was successfully synthesized and characterized by FTIR, 1H NMR and 13C NMR. The flame retardant polyurethane sealants (FR-PUS) were prepared by curing FRPE with methylene diphenyl diisocyanate (MDI-50). The flame retardant properties and thermal decomposition of FR-PUS were investigated by the limiting oxygen index (LOI), cone calorimeter testing (CCT) and thermogravimetric analysis (TGA). The results showed that FRPE could enhance the thermal stability and improve the flame retardancy of polyurethane sealants (PUS) without any other flame retardant. Moreover, the gaseous degradeation products of FR-PUS were analyzed by thermogravimetric analysis/infrared spectrometry (TG-IR), providing insight into the thermal degradation mechanism. FTIR and dynamic mechanical analysis (DMA) were used to explore the chemical components of the char after CCT and mechanical properties of PUS.
Keywords: Polyurethane sealant (PUS); Flame retardant polyols (FRPE); Ricinoleic acid (RA); Flame retardancy; Thermal degradation;

Mechanism and kinetics in catalytic hydrocracking of polystyrene in solution by Edwin G. Fuentes-Ordóñez; Joseba A. Salbidegoitia; María P. González-Marcos; Juan R. González-Velasco (51-59).
A mechanism for polystyrene hydrocracking on a bifunctional Pt/H-Beta catalyst has been proposed. The mechanism consists of three steps for polymer degradation: initiation, propagation and termination, and an additional step for transformation of aromatics, direct product of polystyrene degradation, into other hydrocarbons. The most important reactions in the last step are hydrogenation–dehydrogenation and ring opening reactions, to produce naphthenics, paraffins and isoparaffins, with higher added value as fuels. Finally, degradation of polystyrene by catalytic hydrocracking has been described by means of continuous distribution kinetics, which allows for modelling of the evolution of polymer molecular size and conversion with reaction time, and estimation of kinetic parameters associated to catalyst characteristics when working in the kinetic regime.
Keywords: Polystyrene; Hydrocracking; Catalyst; Mechanism; Kinetics;

The thermal degradation and flame retardancy of poly(methyl methacrylate) containing three mineral fillers, with different geometry, MMT: plate-like, sepiolite: fibrilar and zirconium oxide ZrO2: spherical, in combination with ammonium polyphosphate (APP) were studied. The flame behavior of composites was investigated using pyrolysis-combustion flow calorimeter, cone calorimeter and LOI tests. The results revealed that the composites containing APP and sepiolite present the best fire behavior. A rheological study was performed on composites to establish a relationship between viscosity and flammability. The residues of samples after cone calorimeter test were studied using X-ray diffraction and micro-tomography in order to investigate their chemical composition and also repartition of fillers. A better repartition of sepiolite during combustion could be the origin of the best results obtained for sample containing this filler.
Keywords: PMMA; Flammability; Sepiolite; Montmorillonite; ZrO2;

The effect of structure on thermal stability and anti-oxidation mechanism of silicone modified phenolic resin by Shan Li; Yue Han; Fenghua Chen; Zhenhua Luo; Hao Li; Tong Zhao (68-76).
In order to improve the thermal properties of the phenolic resin, silane with different degree of polymerization was introduced into phenolic resin by controllable reactions. Silicone phenolic resin was first synthesized via esterification reaction between methyltrimethoxysilane and novolac phenolic resin; then the degree of self-polymerization of silane was controlled by the hydrolyzation reaction. The designed structure of the hybrids was confirmed by Fourier Transform Infrared Spectroscopy (FTIR), Nuclear Magnetic Resonance (1H-NMR and 29Si-NMR), and curing of the hybrids was performed to obtain samples for microstructure observation and anti-oxidation evaluation. In situ self-polymerization of the silane during both the hydrolyzation and the curing processes led to phase separation in the cured hybrids, and the size of the phase structure increased with the increase of self-polymerization degree of the silane. Thermogravimetric analysis and high-temperature oxidation test were performed, and silane introduction was found to be beneficial to the enhancement of both thermal stability and oxidation resistance. Anti-oxidation mechanisms for the phenolic resin modified with silanes at different polymerization degree were compared and discussed.
Keywords: Silane; Phenolic; Structure; Thermal stability; Anti-oxidation mechanism;

Microstructure and thermal stability in metallocene iPP-materials: 1-pentene and 1-hexene copolymers by Alberto García-Peñas; María L. Cerrada; José M. Gómez-Elvira; Ernesto Pérez (77-86).
Two series of propylene based copolymers with 1-pentene and 1-hexene, respectively, have been synthesized via metallocene catalysis up to comonomer contents around 14 mol %. The pyrolysis behavior of these propylene-rich copolymers has been studied and compared with the corresponding thermal response of the referential isotactic polypropylene (iPP) homopolymer. Dynamic TGA analysis at different heating rates has been carried out in order to evaluate kinetic changes, all along the weight loss interval, through either the Friedman's method or a pseudo-first order model. A thorough characterization of chain microstructure has allowed screening the chain features which could determine the existence of an initial stage involving a low value of the apparent activation energy. Namely, oxidation degree, molecular weight and comonomer content have been considered. Changes encountered in the pyrolysis kinetics have been found to be consistent with the expected evolution of chain mobility, as the parameter that seems to rationalize the influence of distinct chain characteristics all together.
Keywords: Pyrolysis; Polypropylene; Propylene-alpha-Olefin-Copolymers; Chain microstructure;

The complex (LaL3) of lanthanum (III) with N-(2-amino ethyl) maleamic acid radical (L) was synthesized and characterized by Fourier infrared spectrum (FT-IR), elemental analysis and melting point measurements. The complex LaL3 was a chelate, in which lanthanum (III) ion was coordinated with the carboxylate oxygen and amide nitrogen atoms of N-(2-amino ethyl) maleamic acid radical (L). The stabilization effect of LaL3 as a thermal stabilizer on polyvinyl chloride (PVC) was evaluated by static thermal stability test, ultraviolet–visible spectrum (UV–vis), FT-IR and thermogravimetric analysis (TGA). The results showed that the addition of LaL3 as thermal stabilizer could significantly enhance static stability time of PVC. Compared with the TG of pure PVC and LaL3/PVC, within the range of 200–300 °C, the degradation rate of pure PVC was significantly accelerated, whereas the degradation rate of the PVC added LaL3 stabilizer decreased obviously. The stabilizing efficiency of PVC improved by the addition of LaL3 is partially attributed by the formation of coordinate bonds between lanthanum (III) and chloride atoms in PVC main chain.
Keywords: N-(2-amino ethyl) maleamic acid radical (L); Lanthanum (III); Polyvinyl chloride (PVC); Heat stabilizer; Thermogravimetric analysis (TGA);

The effects of tensile stress on degradation of biodegradable PLGA membranes: A quantitative study by Meng Guo; Zhaowei Chu; Jie Yao; Wentao Feng; Yuxing Wang; Lizhen Wang; Yubo Fan (95-100).
The inhomogeneous stress distribution of biodegradable stents after implantation affects the local degradation rate of the stents, leading to stress concentration and hence stent fracture. The quantitative relationship between the tensile stress and degradation rate of stent polymer is first investigated in this work. To implement the study, an in vitro degradation of poly(l-lactide-co-glycolide) (PLGA) membranes was incubated in deionized water under different applied tensile stress levels from 0.1 MPa to 0.5 MPa. By a special designed device, the tensile stress level can be maintained constant during degradation. The mass loss and mechanical properties of the membranes during the degradation were sampled each week until the membranes broke. The experimental results showed that over a range of tensile stress, higher tensile stress might lead to quicker loss of mechanical properties. Specifically, remarkable decreases of elastic modulus and tensile strength in 0.5 MPa group were observed. As the magnitude of tensile stress increased, more mass loss was observed in the loaded groups. In conclusion, the mass loss rate and mechanical properties of PLGA was sensitive to the tensile stress level during the in vitro degradation. The load dependency of our data demonstrates the importance of quantifying the effects of tensile stress on the degradation of biodegradable polymers. Moreover, this quantification model could be used as a prediction tool for the optimization of biodegradable polymer stents.
Keywords: Biodegradable stents; Non-uniform degradation; PLGA; In-vitro; Quantitative study;

Application of biodegradable superabsorbent hydrogel composite based on Gum ghatti-co-poly(acrylic acid-aniline) for controlled drug delivery by Kashma Sharma; Vijay Kumar; Babulal Chaudhary; B.S. Kaith; Susheel Kalia; H.C. Swart (101-111).
In this work, biodegradable hydrogel composite gum ghatti-co-poly(acrylic acid-aniline) (Gg-co-poly(AA-ANI) was prepared through graft copolymerization of ANI onto Gg-co-poly(AA) chains, in the presence of N,N’-methylene-bis-acrylamide (MBA) and ammonium persulphate (APS) as a crosslinker-initiator system in an aqueous solution. The matrix Gg-co-poly(AA) was synthesized by polymerizing AA onto Gg backbone using MBA and APS as a crosslinker-initiator system. The characterization of the crosslinked hydrogels has been carried out by Time of flight secondary ion mass spectroscopy, Fourier transform infrared, X-ray diffraction and thermogravimetric analysis. These spectroscopic studies confirmed the successful graft polymerization. The biodegradation of the crosslinked hydrogels was analysed using the composting soil method for two months. The initial and final weight of the crosslinked hydrogels were compared as well as the percentage degradation was calculated. The capability of the synthesized hydrogels to be employed as a colon-specific drug delivery vehicle was executed at various pH media using amoxicillin trihydrate as a model drug. The crosslinked hydrogel with the maximum percentage swelling was observed to show best drug absorption. Preliminary kinetic studied were conducted to get an estimated view of the release mechanism.
Keywords: Cross-linked hydrogels; ToF-SIMS; Biodegradation; Drug delivery;

Chain extension and oxidation stabilization of Triphenyl Phosphite (TPP) in PLA by Xin Meng; Guotao Shi; Chushi Wu; Weijie Chen; Zhong Xin; Yaoqi Shi; Yan Sheng (112-118).
PLA, which is synthesized from natural resource, has good mechanical property and biodegradable ability, so it is well recognized as a reasonable substituent of petroleum base plastic. Triphenyl Phosphite can increase the molecular weight of PLA through the chain extension with PLA in the melt processing. In this paper, the factors that influence the chain extension of TPP in PLA were studied in detail. The results show that the adding amount of TPP, processing temperature and processing time affect the chain extension of TPP in PLA. When the adding amount of TPP is about 2wt% compared to PLA, the melt temperature is 180 °C and the melt time lasts 15 min, the chain extension is the best. On the conditions, the molecular weight of PLA increases about 56% compared to virgin PLA and can reach 2.52 × 105 g/mol. Furthermore, the chain extension makes the activation energy of oxidation degradation of PLA increases by 14.3%, so the chain extended PLA named PLA-TPP indicates the better oxidation stability.
Keywords: PLA; Chain extension; Phosphite; Oxidation degradation; Activation energy;

Currently available biodegradable scaffolds do not ensure mechanical stability combined with degradation profile needed for an optimal support of bone tissue regeneration. In the present study a step towards scaffolds with tuned degradation rates was made by correlating the lactide to glycolide ratio of the PLGA part of ternary composite scaffolds with the resulting hydrolysis kinetics. The scaffolds were composed of a poly(ε-caprolactone) (PCL) matrix containing 5 wt% of tricalcium phosphate and 25 wt% of PLGAs with three different lactide to glycolide ratios. The scaffolds were fabricated using Fused Deposition Modelling and subsequently incubated in phosphate buffered saline and simulated body fluid for up to 81 weeks. Degradation was analysed by taking change of mass of the samples, water absorption, pH, molecular weight, mechanical properties, surface morphology and crystallinity as indices. Our findings suggest that the degradation rate of the ternary composite scaffolds was inversely correlated to the degradation rate of the PLGA: slower degrading PLGAs were retained longer in the PCL matrix and caused its more advanced hydrolysis. The release of acidic degradation products of the PLGA hindered precipitation of calcium phosphates (CaPs). A late-reinforcement phenomenon was observed simultaneously with precipitation of the CaPs. Thus the ternary composite system represents a suitable tool to tune degradation rate of polyesters for applications as biodegradable implants or tissue engineering constructs.
Keywords: Degradation rate; Mechanical properties; Scaffolds; Composite; Polycaprolactone;