Polymer Degradation and Stability (v.131, #C)

Synergistic environmental degradation of glass reinforced polymer composites by Tianyi Lu; Euripides Solis-Ramos; Yun-Bo Yi; Maciej Kumosa (1-8).
Synergistic effects involved in the environmental degradation of Glass Reinforced Polymer (GRP) composites were examined in this research. Six GRPs based on E-glass and ECR-glass fibers with four different polymer resins were exposed either individually or in combination to ultraviolet (UV) radiation, water condensation and elevated temperature for approximately 1000 h. The composites were monitored for weight changes as a function of time and their surfaces were examined after the tests using optical and scanning electron microscopes. A new model of synergistic aging of polymers under UV and water condensation was also proposed. It has been shown that the selected aging conditions created noticeable synergistic effects causing extensive erosion of the polymer matrices of the tested composites which appeared to be much stronger under the combined actions than under individual exposures. The differences in the aging rates under the individual and combined situations were adequately explained using the newly proposed model. It is suggested that the surface erosion of polymer matrices of the GRPs by combined UV and water condensation creates the most effective condition for small polymer particles formed by UV to be subsequently removed by water condensation exposing fresh still undamaged surfaces to further UV degradation. It has also been shown that depending on the size of the particles different particle removal mechanisms occur on polymer surfaces eroded by a cyclic exposure to UV and water condensation.
Keywords: Glass reinforced polymers; Environmental degradation; Synergistic effects; Ultraviolet degradation; Condensation degradation;

Polyhydroxyalkanoate (PHA) serves as an alternative to some petroleum-based materials due to its biodegradability nature. Nevertheless, no study has been conducted before to evaluate the impact of PHA degradation to the environment. The purpose of this research was to investigate the degradation of PHA and its effects towards the soil microbial community. Four types of PHAs with three different thickness were used in this study; poly(3-hydroxybutyrate) [P(3HB)], poly(3HB-co-8 mol% 3-hydroxyhexanoate) [P(3HB-co-8 mol% 3HHx)], P(3HB-co-12 mol% 3HHx) and P(3HB-co-21 mol% 3HHx). These films were buried at a secondary forest for a duration of 8 weeks. P(3HB-co-21 mol% 3HHx) showed the highest degradation than the rest of the films. 16S rDNA metagenomic analysis revealed that some of the major phyla that were found at the sampling sites included Actinobacteria, Firmicutes and Proteobacteria which had the ability to degrade PHA. 16S rDNA-Denaturing Gradient Gel Electrophoresis (DGGE) profiling successfully showed that the diversity and population of the soil microbial community were correlated with the degradation of PHA. There was a significant change in the soil microbial abundancy before and after the PHA was degraded.
Keywords: Polyhydroxyalkanoate (PHA); Biodegradation; Forest soil; 16S rDNA metagenomic sequencing; Denaturing gradient gel electrophoresis (DGGE);

A novel charring foaming agent N-methyl triazine-ethylenediamine copolymer defined as MTEC was synthesized from cyanuric chloride, methylamine and ethylenediamine through nucleophilic reaction. Its chemical structure was well characterized by Fourier transform infrared spectroscopy, elemental analysis and 13C solid-state nuclear magnetic resonance. The wettability of flame retardant was evaluated by water contact angle (CA) tests, and the synthesized MTEC present excellent hydrophobic property with the water CA of 117°. Meanwhile, the water CA of the prepared intumescent flame retardant (IFR) system containing MTEC, ammonium polyphosphate and silica reached 104° and also presented hydrophobic property. The obtained IFR was incorporated into polypropylene (PP) resin to prepare flame retardant PP composites, and the flame retardancy, thermal degradation behavior, water resistance and flammability behavior for IFR-PP were investigated by limiting oxygen index (LOI), vertical burning test (UL-94), cone calorimeter and thermogravimetric analysis (TGA) tests. Water resistant properties of IFR-PP composites were evaluated by soaking the samples into distilled water at 70 °C for 168 h. The results demonstrated that IFR-PP samples both passed UL-94 V-0 flammability rating before water treated and after hot water soaking and drying when the loading amount of IFR was 22 wt %, the LOI value of IFR-PP only decreased from 29.6 to 29.3% and the mass loss percent was only 0.17 and 0.69% for 3.2 and 1.6 mm samples after water soaking and drying. The TGA results indicated that the incorporation of IFR promoted PP matrix decomposition and charring at a relative low temperature, and then enhanced the char yield and thermal stability for IFR-PP composites at high temperature. The cone calorimeter tests revealed that the introduction of IFR greatly decreased the combustion parameters, such as heat release rate (HRR), smoke production rate (SPR) and so on. After water resistance test, the combustion parameter for IFR-PP was slightly increased. The scanning electron microscopy (SEM) tests indicated the introduction of IFR benefited to the formation of a sufficient, intumescent and homogeneous char layer on the materials surface during burning, which effectively prevented the underlying materials from further degradation and combustion. The structure and morphology of char layer for IFR-PP remained very well after water treated, consequently the water-treated IFR-PP presented excellent flame retardancy.
Keywords: Triazine charring agent; Hydrophobic property; Intumescent flame retardant; Polypropylene; Water resistance;

Silver (II) was initially generated electrochemically in nitric acid. It was then used to oxidize the surface of vulcanized polybutadiene/organoclay nanocomposite (BRN) and the results were compared to those of polybutadiene rubber (BR). X-ray diffraction (XRD) pattern along with transmission electron microscopy (TEM) pictures proved intercalated organoclay/Cloisite 20A morphology in the BRN matrix. According to elemental analysis, surface oxygen content was increased markedly which is more pronounced for BR and promoted by oxidation time noticeably. The type of polymer and its bond strength can distinctly influence the extend of oxidation by silver(II). Intercalated organoclay obstructed oxidizing effect of silver (II) mainly by protecting polybutadiene chain bonds and partly by growing silver adsorption. Scanning electron microscopy (SEM) micrographs revealed that weak boundary layer (WBL) on the rubbers surface was clearly removed after oxidation. Atomic force microscopy (AFM) disclosed that surface roughness of both BR and BRN samples were drastically enhanced with a more prominent effect on BR. Although, surface roughness distribution was remained normal for BR it was changed to bimodal for the BRN after oxidation. Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy showed different surface functional groups such as hydroxyl and carbonyl formed after oxidation with a higher value for BR. The rubber content was found to have a negligible impact on the rate of surface oxidation. Kinetic study revealed that the concentration of silver (II) decreased quickly (<300 s) upon contacting rubbers and the kinetic data were fitted well to the second order rate equation, Morris-Weber and Longmuir isotherm models with a superior oxidation rate constant for BR. Finally it can be concluded that organoclay escalated oxidation resistivity and diminished oxidation rate constant of BRN.
Keywords: Polybutadiene/organoclay nanocomposite; Silver(II); Mediated electrochemical oxidation; Kinetic study; Characterization;

Poor thermal stability and thermoplastic processability are the main obstacles for wide application of polyvinyl alcohol, which is a typically water-soluble polymer with excellent mechanical properties, biocompatibility, barrier properties and biodegradability. In this work, a novel method was demonstrated for preparing PVA based material with improved thermal stability, low additive loading, and without impairing the inherent good properties of PVA. Poly (1-vinyl-3-ethyl-imidazolium bromide) (PEtVIm-Br) was synthesized and used as both a thermal stabilizer for PVA and a compatibilizer for PVA/graphene nanocomposites. A 49% improvement in tensile strength and 65% improvement in the elongation at break were achieved by incorporating 2.5 wt% of PEtVIm-Br and 0.5 wt% of graphene into PVA matrix. The thermal stability of the PVA/PEtVIm-Br-graphene composite has been investigated by thermogravimetric analysis (TGA), TGA-DSC simultaneous thermal analysis, and TGA coupled with FTIR. With the addition of PEtVIm-Br-graphene, the elimination reaction of PVA at the initial stage of pyrolysis was suppressed remarkably, while the activation energy for pyrolysis of the nanocomposite increased obviously, resulted in a much improved thermal stability, comparing to pure PVA. The volatile products of pure PVA and PVA composites were detected by TGA/FTIR and Py-GC/MS. The improvement in thermal stability of PVA/PEtVIm-Br and PVA/PEtVIm-Br-graphene composite comparing to pure PVA may attribute to the ability of capturing free radicals of PEtVIm-Br and the barrier effect of well-dispersed graphene.
Keywords: Thermal stability; Poly (vinyl alcohol); Poly (1-vinyl-3-ethyl-imidazolium bromide); Graphene; Nanocomposite;

Chemical structure based prediction of PAN and oxidized PAN fiber density through a non-linear mathematical model by Khashayar Badii; Jeffrey S. Church; Gelayol Golkarnarenji; Minoo Naebe; Hamid Khayyam (53-61).
The production of carbon fiber, particularly the oxidation/stabilization step, is a complex process. In the present study, a non-linear mathematical model has been developed for the prediction of density of polyacrylonitrile (PAN) and oxidized PAN fiber (OPF), as a key physical property for various applications, such as energy and material optimization, modeling, and design of the stabilization process. The model is based on the available functional groups in PAN and OPF. Expected functional groups, including –C N, –CN–, –CH 2 , –CC–, and –CO, were identified and quantified through the full deconvolution analysis of Fourier transform infrared attenuated total reflectance (FT-IR ATR) spectra obtained from fibers. These functional groups form the basis of three stabilization rendering parameters, representing the cyclization, dehydrogenation and oxidation reactions that occur during PAN stabilization, and are used as the independent variables of the non-linear predictive model. The k-fold cross validation approach, with k = 10, has been employed to find the coefficients of the model. This model estimates the density of PAN and OPF independent of operational parameters and can be expanded to all operational parameters. Statistical analysis revealed good agreement between the governing model and experiments. The maximum relative error was less than 1% for the present model.
Keywords: FT-IR ATR; Deconvolution; Oxidized PAN fiber; Density; Non-linear mathematical model; Cross-validation method;

To expand the application of ammonium polyphosphate (APP) in epoxy resin (EP), hyperbranched polyethyleneimines (PEI) were selected to modify it via cation exchange reaction. Then, a highly-efficient flame-retardant hardener with poly-functionalities for EP was successfully prepared and named as PEI-APP. After curing, PEI-APP endowed the EP samples with good flame retardance and smoke suppression performance. Results suggested the total heat release (THR) and total smoke production (TSP) decreased 76.1% and 70.5% respectively. Thermogravimetric analyses (TGA) of the PEI-APP cured EPs displayed a slight improvement in the high temperature region compared with the reference sample (PEI/EP). Dynamic mechanical analysis (DMA) demonstrated that the glass transition temperature (Tg) of PEI-APP/EP also slightly increased compared with PEI/EP. Fourier transform infrared spectra (FTIR) was used to analyse the condensed products of PEI-APP/EP samples at different temperatures to investigate the flame-retardant mechanism. All the aforementioned results distinctly confirmed that PEI-APP did not only act as an effective flame-retardant hardener for EP, but also brought a good thermal stability and improved the smoke suppression to the system. This polyamine hardener provided a new platform for intumescent flame-retardant application in EP.
Keywords: Epoxy resin; Curing; Flame retardance; Thermal stability; Branched polyethyleneimines; Ammonium polyphosphate;

Surface properties of poly(lactic acid)/polyacrylate semi-interpenetrating networks – Effect of UVC radiation by Jolanta Kowalonek; Irena Vuković-Kwiatkowska; Dariusz Moszyński; Halina Kaczmarek (71-81).
Poly(lactic acid), PLA, was blended with multifunctional acrylate monomer (dipentaerythritol pentaacrylate) in various weight ratios (1:1 and 9:1). The reaction of photopolymerization of the acrylate monomer in poly(lactic acid) matrix was conducted to obtain semi-interpenetrating networks in the form of films dedicated for use in the packaging industry. Next, the PLA/polyacrylate films were exposed to UVC radiation in order to examine their photostability under the influence of high energy radiation, which can be used as sterilizing factor. In these studies, the following methods have been applied: contact angle measurements, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). These methods made it possible to determine the influence of sample composition on the surface properties and photochemical resistance. It was found that PLA was the most susceptible to UVC-irradiation among the studied samples. In PLA/polyacrylate films UV light induced a slight increase in surface roughness and only negligible changes in surface polarity and chemical composition. The mechanism of photodegradation in the tested specimens was also discussed.
Keywords: Poly (lactic acid); Polyacrylate; Semi-interpenetrating networks; UVC-irradiation; Contact angle measurements; XPS; AFM;

Blowing-out effect in flame retarding epoxy resins: Insight by temperature measurements during forced combustion by Wenchao Zhang; Alberto Fina; Fabio Cuttica; Giovanni Camino; Rongjie Yang (82-90).
The temperature measurements within burning epoxy resins (EPs) were used to study the thermo physical evolution up and inside of samples during different applied heat fluxes cone calorimeter tests. A series of flame retarded epoxy resins (EP) were prepared with polyhedral oligomeric silsesquioxane containing 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO-POSS). The flame retardancy of these EPs was tested by both flammability tests (LOI, UL-94) and forced combustion in cone calorimeter, to assess DOPO-POSS effects on the flame retardancy of EP composites. 2.5 wt% DOPO-POSS incorporation into epoxy resin perform interesting blowing-out effect, which results in a LOI value 27.1 and UL-94 V-1 (t 1 = 8 s and t 2 = 5 s) rating. The details of fire behavior, such as the values of TTI, HRR, p-HRR, and THR have been tested using a cone calorimeter. The temperature measurements during cone calorimeter tests indicate that the char layer of EP/2.5 DOPO-POSS has the best performance on heat insulation and play an effective role (heat insulation) quicker than that of EP/5 DOPO-POSS or EP/10 DOPO-POSS. This heat insulation performance of char layer produced by EP/2.5 DOPO-POSS explains its good performance on LOI and UL-94 test.
Keywords: Epoxy resin; DOPO-POSS; Temperature measurement; Blowing-out effect; Cone calorimeter;

The role of quinoid derivatives in the UV-initiated synergistic interaction mechanism of HALS and phenolic antioxidants by Leila Maringer; Lukas Roiser; Gernot Wallner; David Nitsche; Wolfgang Buchberger (91-97).
Within this work, UV-initiated interaction mechanisms between the phenolic antioxidant Irganox 1330 and nine commercially used hindered amine light stabilizers (HALS) were successfully studied using high-performance liquid chromatography (HPLC) coupled to UV and high-resolution mass spectrometric detection (MS). An analytical evaluation of the stabilizer performances in the polypropylene-mimicking solvent squalane revealed that all investigated HALS exhibited a strong synergism when combined with the phenolic antioxidant. Up to now, the synergistic interaction was described as a result of the hydrogen transfer from a hydroxylamine derived from HALS to the oxidized form of a primary antioxidant, whereby the phenol is regenerated. Investigations on degradation products, however, indicated that the proposed interaction mechanism cannot be applied to a sterically hindered phenol such as Irganox 1330. Instead, a completely new stabilization mechanism of phenolic antioxidants in the presence of HALS, involving the formation of quinoid derivatives, was discovered and confirmed by using a model compound.
Keywords: HALS; Phenolic antioxidants; Synergism; UV aging; Quinoid; UV-absorber; High-performance liquid chromatography; Mass spectrometry; NMR;

Thermal curing and degradation behaviour of silicon-containing arylacetylene resins by Kangkang Guo; Ping Li; Yaping Zhu; Fan Wang; Huimin Qi (98-105).
Silicon-containing aryacetylene resins (PSAs) have already shown potential application as heat-resistant materials due to their excellent thermal stability. The formation and degradation of structures in cured PSAs are very important to evaluate the thermal properties during their applications. Herein, the thermal curing and degradation behavior of PSAs with different substituents were investigated by DSC, FT-IR, Py-GC-MS, TGA and TG-GC-MS. DSC and FT-IR analysis reveals that the curing reaction is dominated by the crosslinking reaction of CC―H and Si―CC―, meanwhile, the reactive substituents (such as Si―H and Si―CH=CH2) could promote the CC―H and Si―CC― reaction with higher conversion. Py-GC-MS was used to detect the pyrolysis products at 650 °C and 750 °C, respectively, and the results could provide important information about the curing reaction and structures of cured PSAs. The curing reaction of PSAs mainly contains cyclotrimerization and Diels-Alder reaction of CC―H and Si―CC―, and some addition reaction to form polyene structure. The main structures in cured PSAs contain a lot of phenyl rings, some aromatic fused rings and a little of polyene structure, moreover, their contents in cured PSAs are dependent on the substituents of the precursors. The TGA results show that the substituents of Si―H and Si―CH=CH2 can effectively improve the crosslinking index of the cured PSA-H and PSA-V, and endow them with better thermal stability. Additionally, the degradation behavior of cured PSAs have been well studied by TG-GC-MS. When the temperature is 400 °C, the Si―CH3 and some aliphatic structure begin to degrade and form CH4, and with the temperature increasing to 500 °C, the unsaturated and aliphatic structures decompose into C2H4 and C2H6, and after 600 °C, the aromatic rings and fused rings in cured PSAs are dehydrogenated and release H2. In addition, the content of the degradation products and their forming temperature are also related with the substituents.
Keywords: Silicon; Arylacetylene; Cure; Degradation;

In this paper, a novel phosphorus/nitrogen-containing compound (DMT) constructed by maleimide, phosphaphenanthrene and triazine-trione was successfully synthesized. The chemical structure of DMT was characterized by Fourier transform infrared spectroscopy (FTIR), 1H and 31P nuclear magnetic resonance (NMR). DMT was then blended with diglycidyl ether of bisphenol-A (DGEBA) to prepare a series of flame-retardant epoxy resins. The flame retardancy and combustion behavior were evaluated using limited oxygen index (LOI), vertical burning (UL94) and cone calorimeter test. The results indicated that DMT dramatically enhanced the flame retardancy of epoxy resin. When the phosphorus content was only 1.0%, the EP/DMT-1.0 sample had a LOI value of 35.8%, and achieved UL94 V-0 rating. The combustion parameters disclosed that DMT obviously weakened the heat release intensity. Compared with the neat EP sample, the peak of heat release rate (pk-HRR), average of heat release rate (av-HRR) and total heat release (THR) of EP/DMT-1.25 sample were decreased by 59.4%, 28.2% and 27.4%, respectively. The decreased av-EHC of EP/DMT thermosets indicated that DMT inhibited the gaseous phase combustion due to the gaseous phase quenching effect of phosphorus-containing free radicals derived from the decomposed DOPO group. The condensed phase studies indicated that DMT promoted the charring of EP matrix and formation of phosphorus-containing char layer with compact and intumescent structure. DMT exerted bi-phase flame retardant effect.
Keywords: Epoxy resin; Flame retardant; Phosphaphenanthrene; Triazine-trione; Maleimide;

The enzymatic degradation of oligo(ε-caprolactone) (OCL) based films at the air-water interface is investigated by Langmuir monolayer degradation (LMD) experiments to elucidate the influence of the molecular architecture and of the chemical structure on the chain scission process. For that purpose, the interactions of 2D monolayers of two star-shaped poly(ε-caprolactone)s (PCLs) and three linear OCL based copolyesterurethanes (P(OCL-U)) with the lipase from Pseudomonas cepacia are evaluated in comparison to linear OCL. While the architecture of star-shaped PCL Langmuir layers slightly influences their degradability compared to OCL films, significantly retarded degradations are observed for P(OCL-U) films containing urethane junction units derived from 2, 2 (4), 4-trimethyl hexamethylene diisocyanate (TMDI), hexamethylene diisocyanate (HDI) or lysine ethyl ester diisocyanate (LDI). The enzymatic degradation of the OCL based 2D structures is related to the presence of hydrophilic groups within the macromolecules rather than to the packing density of the film or to the molecular weight. The results reveal that the LMD technique allows the parallel analysis of both the film/enzyme interactions and the degradation process on the molecular level.
Keywords: Langmuir technique; Oligo(ε-caprolactone); Enzymatic degradation; Polymer architecture;

Effect of organoclays on the degradation of polyoxymethylene homopolymer during melt processing by Alexis Ditta; Hervé Laurandel; François Breynaert; Arnaud Travert; Loïc Le Pluart (122-131).
The improvement of Polyoxymethylene (POM) thermomechanical properties through the incorporation of nanofillers such as organoclays can be hindered by a detrimental effect of the latter on the thermal stability of the polymer. In order to be able to develop polyoxymethylene-based nanocomposites, it is necessary to assess the relation between nanofillers chemical properties and the thermal stability of POM. For this purpose, polyoxymethylene nanocomposites containing various organoclays have been processed and thoroughly characterized in this work. It is confirmed that organoclay containing nanocomposites are less thermally stable than neat polyoxymethylene. In this study, we bring evidence that the degradation of the alkylammonium ions of the organoclay is not responsible for polyoxymethylene decreased thermal stability. Moreover, it is demonstrated that the quantity of acidic sites on the organoclay surface accelerates the degradation and more surprisingly that their nature (Brönsted or Lewis) can induce different degradation mechanisms.
Keywords: Polyoxymethylene; Nanocomposites; Organoclay; Thermal stability; Degradation mechanism;

The thermo-mechanical recycling of poly(lactide) (PLA) by reprocessing was recently considered as a new end-of-life scenario for this biosourced and biodegradable polymer. With this regard, the degradation mechanisms of plasticized PLA (pPLA) engendered by multiple reprocessing were little investigated to date, and hence, the relevancy of its reprocessing was not assessed. They were identified in this paper by a multiscale approach in the case of a lab-developed grade of pPLA obtained by the reactive extrusion of PLA with acrylated poly(ethylene glycol) (acryl-PEG) as reactive plasticizer. pPLA structure consisted of a semi-crystalline PLA matrix in which grafted poly(acryl-PEG) micro-inclusions were dispersed. Up to 5 successive processing cycles including extrusion and compression-molding, the tensile and impact properties drastically dropped indicating an embrittlement of pPLA. Structural analyses revealed that reprocessing caused these mechanisms: chain scission of PLA, crystallization of PLA, damaging of the inclusions, decrease of the size of poly(acryl-PEG) phases within the inclusions, and cracking of PLA. At the same time, the amount of grafted poly(acryl-PEG) was not influenced by the reprocessing. Inclusion damaging and matrix cracking are believed to be responsible for the embrittlement of pPLA after multiple reprocessing, which makes it not suitable for being reused for its initial application.
Keywords: Poly(lactide); Plasticization; Thermo-mechanical degradation; Reprocessing; Mechanical properties;

A non-linear viscoelastic model to describe the mechanical behavior's evolution of biodegradable polymers during hydrolytic degradation by Q. Breche; G. Chagnon; G. Machado; B. Nottelet; X. Garric; E. Girard; D. Favier (145-156).
The biodegradable triblock copolymer PLA-b-PEG-b-PLA presents, in its initial state, a non-linear viscoelastic behavior. Its mechanical properties evolves during the in vitro degradation process. Tensile and relaxation tests are performed at 2%, 4% and 6% of load strain for different degradation steps. In order to describe the behavior of the polymer during degradation, an adaptive quasi-linear viscoelastic model is considered. In a first step, the model calibrated on the non-degraded state, perfectly fits the load and relaxation curves for every strain. Then, based on considerations about the preservation of the normalized relaxation curves over degradation time, the adaptive quasi-linear viscoelastic model is adapted to degradation. A degradation parameter that drives the mechanical degradation kinetics is deduced for every tested degradation states. A physically motivated model is finally used to describe the degradation parameter at every degradation step. The whole constitutive model is very accurate to fit the mechanical curves at every strain during degradation.
Keywords: PLA-b-PEG-b-PLA; Mechanical behavior's evolution; Biodegradable; Viscoelasticity; Mechanical model;

Experimental data prove a significant mutual interference between the chemical nature of the monomer and the particularities of the radical polymerization procedure which influences the thermal decomposition (depropagation) of the corresponding polymer. Results are presented for polystyrene, poly(methyl methacrylate), and poly(α-methylene-γ-valerolactone) which were made via different radical polymerization techniques (thermally self-initiated bulk polymerization at 80 °C, photoinitiated bulk and aqueous heterophase polymerization at 25 °C). Also, experimental data are presented after thermally annealing the polymers at 200 and 300 °C.Display Omitted
Keywords: Thermal decomposition; Polysytrene; Poly(methyl methacrylate); Poly(α-methylene-γ-valerolactone); Radical polymerization procedure;