Polymer Degradation and Stability (v.139, #C)

Progress in biodegradable zwitterionic materials by Liuchun Zheng; Zhijuan Sun; Chuncheng Li; Zhiyong Wei; Priyesh Jain; Kan Wu (1-19).
Zwitterionic polymers, consisting of polymer backbone of ester or amide (meth) acrylic acid or pyrrolidinium, exhibit excellent non-fouling property, and have received great attention since last decade. However, these polymers lack important biodegradability for their application in biomedicine or marine field. Developing degradable zwitterionic polymers will solve problem associated with traditional zwitterionic polymers and increase their potential to be used in biomedicine for achieving both non-fouling and degradability. Herein, we present a comprehensive review describing various strategies developed till now for achieving degradable zwitterionic materials. Synthesis and applications of zwitterionic polymers based on various biodegradable polyesters, polypeptides, and natural polysaccharides (chitosan, starch and cellulose) have been critically reviewed and summarized. The basic structural characteristics and properties of different types of zwitterionic materials and biodegradable zwitterionic materials have also been introduced briefly. To the best our knowledge, this is the first review focusing on biodegradable zwitterionic materials.Display Omitted
Keywords: Zwitterions; Biodegradable; Non-fouling; Biocompatible;

Mild chemical recycling of aerospace fiber/epoxy composite wastes and utilization of the decomposed resin by Tuan Liu; Meng Zhang; Xiaolong Guo; Chengyun Liu; Tian Liu; Junna Xin; Jinwen Zhang (20-27).
Carbon fiber reinforced polymers (CFRPs) with high T g (>200 °C) are indispensable for aerospace industry where high service temperature is required. Chemical recycling of the matrix polymers for these CFRP composites is more difficult than that of their low T g analogues. In this work, an efficient approach for mild chemical recycling of CFRP with a T g of ∼210 °C was developed using a ZnCl2/ethanol catalyst system. The high efficiency of ZnCl2/ethanol was attributed to the strong coordination effect of ZnCl2 with the C-N bonds and the strong swelling ability of ethanol, which worked together to break down the chemical bonds of the cross-linked polymer. Also, mild degradation temperature (<200 °C) imparted little damage to the recovered fibers. The decomposed matrix polymer (DMP) was in the oligomer form and contained multifunctional reactive groups. When DMP was used as a reactive ingredient and added up to 15 wt% to the preparation of new epoxy materials, the resulting cross-linked polymers could still retain the high strength and modulus compared to the neat polymer without addition of DMP.
Keywords: Carbon fiber; Composite; Waste recycling; Thermosetting resin; Epoxy; Degradation; Mechanical property; Chemical recycling; Glass transition; Zinc chloride;

Polychloroprene behaviour in a marine environment: Role of silica fillers by S.T. Tchalla; P.Y. Le Gac; R. Maurin; R. Créac'hcadec (28-37).
Polychloroprene rubbers are widely used in marine structures and often filled with silica in order to increase mechanical properties. The presence of silica fillers leads to a complex degradation of the material. This study aims to understand the deeper degradation mechanisms involved when a silica filled polychloroprene is used in sea water. To do so, 4 polychloroprene rubbers filled with different amounts of silica (from 0 to 45 phr) were aged in natural sea water for 6 months at temperatures ranging from 25 to 60 °C. Moreover, a natural rubber with similar formulation was also considered in order to evaluate the role of the chlorine atom in the degradation. The chemistry and mechanics of the rubber degradation were also studied. In the presence of water and silica fillers, a large decrease in rubber stiffness was observed. This was attributed to the breakage of hydrogen bonds involved in the interaction between the silica and chloroprene matrix and the process is reversible. In the meantime, silica undergoes hydrolysis that leads to silanol formation and so an increase in rubber stiffness when water is removed; this process is irreversible.
Keywords: Polychloroprene; Seawater; Silica filler; Hydrolysis;

Since last decade, momentous development has been made in the biomedical field. However, the contribution of polymer hydrogels as drug delivery carrier cannot be ignored. Crosslinking of natural and synthetic polymer has established promising characteristics in stimuli sensitive hydrogels for targeted drug delivery application. In this work, we synthesized N-succinyl chitosan and characterized via Fourier transform infrared (FTIR) spectroscopy, Nuclear magnetic resonance (NMR) spectroscopy, X-ray diffraction (XRD), and differential scanning calorimetry (DSC). Then, we synthesized N-succinyl chitosan-g-Poly (acrylamide-co-acrylic acid) hydrogel via chemical crosslinking of N-succinyl chitosan, acrylamide and acrylic acid. Crosslinking was performed using free radical mechanism. Here, N, N’-methylenebisacrylamide (MBA) and ammonium persulfate (APS) were used as crosslinking agent and initiator, respectively. The hydrogels were synthesized by varying the concentration of monomers, initiator, and crosslinking agent. The chemical interactions in the synthesized hydrogel network were evaluated by FTIR, XRD, and DSC. Field emission scanning electron microscope (FESEM) showed porous morphology of hydrogels. The yield %, gel contents, and gel time were thoroughly examined and excellent results were found. The rheology study was performed to measure the mechanical strength. Moreover, swelling study revealed that hydrogels were highly swellable and depended on the various concentrations of initiator, monomers, and crosslinking agent. The results also showed that pH, type and concentration of salt solutions have strong influence on the swelling properties. Hydrogels expressed reversible swelling behavior in buffer solutions and buffer and salt solutions. Furthermore, theophylline was encapsulated during the synthesis of hydrogel and encapsulation efficiency was found in the range of 59–83%. In addition, theophylline release was observed in buffer solutions of pH 1.2 and pH 7.4. The results revealed the significant effect of pH and hydrogel composition on drug release. The theophylline release was found in the range of 14–24% in buffer solution pH 1.2 and 67–93% in buffer solution pH 7.4. The drug release data showed good fitting to Ritger- Peppas model and release kinetics followed non-Fickian anomalous transport mechanism and relaxation of polymers (case II transport). It was also observed that theophylline maintained its chemical activity after in vitro release. According to the observed facts, synthesized hydrogels can be adapted for their prospective exploitation in targeted drug delivery applications.
Keywords: N-succinyl chitosan; Free radical mechanism; Mechanical and swelling properties; Controlled release; Drug activity;

Effects of bridged DOPO derivatives on the thermal stability and flame retardant properties of poly(lactic acid) by Lijuan Long; Qifeng Chang; Wentao He; Yushu Xiang; Shuhao Qin; Jingbo Yin; Jie Yu (55-66).
Three bis P-C bridged 9,10-dihydro-9-oxa-10-phosphaphe nanthrene-10-oxide (DOPO) derivatives including ethyl-(DiDOPO1), phenethyl-(DiDOPO2) and naphthalene-(DiDOPO3), were synthesized and incorporated in poly(lactic acid) (PLA). Then, the flame retardant and thermal properties of the obtained PLA composites were investigated in detail. At 10 wt% addition of the derivatives, all PLA composites achieved a V-0 rating (UL-94, 3.2 mm), and the LOI values of PLA/DiDOPO1, PLA/DiDOPO2 and PLA/DiDOPO3 increased from 20.0% (pure PLA) to 36.4%, 27.1% and 24.3%, respectively. The introduction of DiDOPO1, DiDOPO2, and DiDOPO3 decreased the peak heat release rate value of PLA composites from 561 kW/m2 (pure PLA) to 490, 488 and 434 kW/m2, respectively. In addition, the derivatives significantly enhanced the thermal stability of PLA especially under air condition. It was found that the bridged DOPO derivatives shows a dominant flame inhibition role in PLA. Furthermore, the investigation of the residue micromorphology of PLA composites revealed that a cross-linked structure likely formed with the increased of aromatic groups in the molecule.
Keywords: Flame retardant; Bridged DOPO derivatives; Poly(lactic acid);

A thermo-kinetic study on the pyrolysis of a series of starch-g-poly(phenyl acrylate) copolymers showing differences in their grafting percentages at temperatures ranging from 40 to 1000 °C under inert conditions by applying the TG/DSC/FTIR/QMS-coupled technique was carried out. As for the pyrolysis kinetics of the novel materials, two model-free-kinetics algorithms were evaluated and discussed: the differential Friedman (F) and integral Kissinger-Akahira-Sunose (KAS) methods. The degradation pathways of the copolymers occurred in two large stages connected with the emission of various volatile compounds resulting from the cleavage of the bonds existing in the structures of all the studied copolymers, which followed a similar pattern. The conducted analyses revealed that the thermal stability, decomposition course of the copolymers and the types of gaseous decomposition products were independent of the grafting percentage of the copolymers. However, this percentage exerted an effect on the heat of the pyrolysis processes (ΔH) and the mass losses under heating. Moreover, the activation energy (E) of the pyrolysis process was directly dependent upon the conversion degree (α) and the grafting percentage of the copolymers.
Keywords: Potato starch; Phenyl acrylate; Copolymers; TG/DSC/FTIR/QMS; Pyrolysis; Kinetics;

Accelerated hydrothermal aging of biocarbon reinforced nylon biocomposites by Emmanuel O. Ogunsona; Manjusri Misra; Amar K. Mohanty (76-88).
Nylon/biocarbon biocomposites show great potential for semi-structural auto-part applications due to their enhanced mechanical and physical properties. However, the long term durability of these biocomposites has never been studied before. The durability of the biocomposite was compared to that of talc reinforced nylon composite at 20 wt. % for both composites. They were subjected to accelerated and aggressive conditioning by complete immersion in water at 85 °C for prolonged periods of time up to 28 days. The water uptake of the nylon/biocarbon biocomposite was reduced in comparison to that of the nylon suggesting that biocarbon particles act as a barrier while also concentrating the absorbed moisture within its pores and around the interface with nylon. Thermal properties revealed little changes in the crystallinity of the nylon/biocarbon composites. However, the glass transition temperature was observed to decrease significantly after conditioning due to the presence of bound water molecules within the amorphous phase of nylon. The impact strength remained mostly unchanged even after conditioning, suggesting interaction between biocarbon and nylon to restrict the chain mobility and thereby annulling the effect of moisture on the nylon. The morphology of the impact fractured surface of the conditioned sample revealed two distinct phases. One was significantly affected by moisture, revealed swelling and debonding of biocarbon particles from the nylon matrix while the other remained unchanged when compared to that of the unconditioned sample.
Keywords: Hydrothermal; Nylon; Biocarbon; Biocomposites; Durability;

Initial hydroperoxide (ROOH), which exists even in freshly produced polymers, acts as an initiator for the oxidative degradation of the polymer. Electron spin resonance (ESR) with the spin trapping agent 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) was used to investigate the presence of ‘initial ROOH’ in a polyethylene oxide (PEO) sample. The degradation of PEO was investigated by heating or irradiating mixtures of PEO/DMPO/H2O with or without dissolved oxygen by ultraviolet-visible (UV-vis, λ > 250 nm) and visible (λ > 350 nm) light. The concentration of ‘initial ROOH’ and the photolysis and thermolysis rate constants were estimated from the concentrations of the DMPO–H, DMPO–C–PEO, and DMPO–OH spin adducts. Knowledge of the oxidative degradation mechanism of polymers allows the ‘initial ROOH’ concentration to be used as a quality control indicator for polymers.
Keywords: PEO; Hydroperoxide; Photolysis; Thermolysis; Rate constant; ESR spin trap;

The effects of silicon and ferrocene on the char formation of modified novolac resin with high char yield by Jin Yun; Lixin Chen; Xiaofei Zhang; Hui Zhao; Ziyou Wen; Chi Zhang (97-106).
Novolac resin (NR) modified with 1, 1’-bis(dimethylsilyl) ferrocene (FeNR) was synthesized. The thermal properties and structural evolution of cured FeNR were studied to analyze the reason for high char yield of cured FeNR. Thermal stability of cured FeNR has been improved, and the char yield of cured FeNR at 800 °C can be increased by 8.82% in comparison to that of cured NR. The results show that silicon atom and ferrocene contribute to the high char yield of cured FeNR. On one hand, the Si-O structure can be kept in the system during the whole pyrolysis process, which could help to reduce the weight loss. On the other hand, introducing ferrocene into novolac resin promotes the graphitization degree and graphite crystallites of pyrolyzed resins. Moreover, the pyrolyzed products possess different magnetic properties owing to the formation of iron nanocrystal at 800 °C and Fe5Si3 nanocrystal at 1200 °C.
Keywords: Novolac resin; 1, 1’-bis (dimethylsilyl) ferrocene; Silicon; Char yield; Graphitization; Magnetic material;

Thermal stabilization of polyacrylonitrile fibers is an important step in the production of carbon fibers. A detailed map of stabilization reactions is necessary to adjust the processing parameters - such as temperature and tension - that lead to high performance carbon fibers with minimum structural imperfections. We have investigated the effect of comonomer type and heating conditions on the thermal behavior of polyacrylonitrile fibers during stabilization process. In doing so, three types of PAN fibers having different chemical composition and characteristics were studied using Differential Scanning Calorimetry, Thermogravimetry Analysis and Thermomechanical Analysis. It was observed that chemical composition and characteristics of PAN fibers affect the initiation mechanism and temperature of the stabilization reactions, i.e. the sequence and temperature range of the stabilization reactions change in presence of different types of comonomers. However, heating rate as an important processing parameter does not affect the sequence of these reactions. An increase in the heating rate can only shift the reaction temperature to higher values. These results help the design of the processing unit and adjusting the tension.
Keywords: Carbon fiber precursor; Thermal behavior; Processing parameters; Chemical composition; DSC; TGA; TMA;

An adduct of myrcene and maleic anhydride (AMMA) was synthesized using a Diels-Alder reaction. The adduct was transformed into the corresponding sodium, calcium salt and zinc salts (Na-AMMA, Ca-AMMA, and Zn-AMMA) using a metathesis reaction. The chemical structures of AMMA, Na-AMMA, Ca-AMMA, and Zn-AMMA were confirmed with Fourier transform infrared spectroscopy (FTIR), Inductively coupled plasma-atomic emission spectrometry (ICP-AES), 1H and 13C nuclear magnetic resonance (1H NMR and 13C NMR), and X-Ray Diffraction (XRD). Mixed Ca/Na/Zn salts from AMMA provided thermal stabilization for poly (vinyl chloride) (PVC). The thermal and mechanical properties of the PVC samples were investigated using discoloration and Congo red tests, scanning electron microscopy analysis (SEM), thermal gravity analysis (TGA), TGA–FTIR analysis, thermal gravity-mass spectrometer analysis (TGA-MS), and dynamic mechanical analysis (DMA). Optimal design of the Ca/Na/Zn-AMMA formulation was studied. The application of Ca/Na/Zn-AMMA in an optimized formulation for a biobased, primary thermal stabilizer for PVC could significantly improve PVC thermal stability. The optimized Ca/Na/Zn-AMMA formulation had higher thermal stability, higher storage modulus, and lower glass transition temperatures than PVC blends without any thermal stabilizers and using commercial CaSt2/ZnSt2. The weights of the Ca/Na/Zn-AMMA samples were 33.3–75.0% that of CaSt2/ZnSt2, but provided higher thermal stability. Myrcene-derived Ca/Na/Zn-AMMA was shown to have strong potential as a primary PVC thermal stabilizer.Display Omitted
Keywords: Poly(vinyl chloride); Myrcene; Thermal stabilizer; Ca/Na/Zn composite stabilizer;

Time-resolved mechanical spectroscopy (TRMS) was conducted to study the thermo-oxidative degradation of linear low density polyethylene (LLDPE) samples with different thermal histories and their blends with a polyamide (PA6) in the melt state. Neat LLDPE was first melt-processed at 180, 220, 250, and 260 °C in an extruder and then pre-processed LLDPE samples were further extruded with PA6 at 260 °C to form various LLDPE/PA6 blends. TRMS measurements were conducted under an air atmosphere at 0.5% strain and a constant frequency of 0.1 rad/s for LLDPE samples and at a range of frequencies between 0.1 and 60 rad/s for LLDPE/PA blend samples, over a 1 h period. In the case of LLDPE samples, time-sweep experiments were carried out at 190, 220, and 240 °C, whereas the temperature was fixed at 240 °C for the LLDPE/PA blend samples. The observed rheological behaviors revealed that the degradation resulted in an increase in the elastic moduli of the LLDPE and LLDPE/PA blends regardless of the thermal history. LLDPE processed at different processing temperatures produced different viscoelastic behaviors in cases where the LLDPE samples were processed at lower temperatures (180 and 220 °C) where a rapid increase in the modulus over a short period was seen. On the other hand, a change in the pre-processing temperature of the LLDPE had no effect on the rheological property of the corresponding LLDPE/PA6 blends. Cross-linking reactions during measurements under an air atmosphere could be the main reason for the growth in the modulus as a result of thermo-oxidative degradation. It was found that degradation was only a function of the temperature and exposure time, not the frequency. The most important result of this study was that collecting data on the isochronal moduli at different scanning frequencies was a more accurate way to understand the degree of cross-linking compared to running conventional frequency sweep tests, where the molecular structure of the material was unstable as a result of rapid degradation.
Keywords: Time-resolved mechanical spectroscopy; Thermo-oxidative degradation; Rheology; LLDPE; Cross-linking;

Three amphiphilic bioresorbable copolymers derived from lactide (LA), ε-caprolatcone (CL) and poly(ethylene glycol) (PEG) were investigated for their emulsifying properties and degradation characteristics. Polymers consisted of 80 wt% hydrophilic PEG block and 20 wt% lipophilic PLA, PCL, PLACL block were synthesized by ring-opening polymerization of LA and/or CL in the presence of monomethoxy PEG. By possessing similar hydrophilic-lipophilic balance (HLB) values, these polymeric emulsifiers have an equivalent ability to stabilize squalane/water interfaces during emulsification. Degradation of polymers in aqueous solution and within emulsion was carried out in water at 37 °C selected to mimic the human body conditions. Our results demonstrated that the degradability intrinsic to each polymer is the predominant cause of destroying the emulsion. Moreover, polymer matrices within the emulsion exhibited lower degradation rates than the corresponding polymers in aqueous solution. These features are of great interest in pharmaceutical applications, especially for the design of sustained delivery systems.Display Omitted
Keywords: Bioresorbable polymer; Hydrolytic degradation; Oil-in-water emulsion;

Recent advances on the ageing of flame retarded PLA: Effect of UV-light and/or relative humidity by Nicolas Lesaffre; Séverine Bellayer; Hervé Vezin; Gaëlle Fontaine; Maude Jimenez; Serge Bourbigot (143-164).
Three different accelerated ageing scenarios were applied on flame retarded polylactic acid (FR-PLA). The influence of (i) Temperature and UV-light exposure (T/UV), (ii) Temperature and relative humidity exposure (T/RH) and (iii) combined Temperature, UV-light and relative humidity (T/UV/RH) exposure on (i) the morphology, (ii) the physico-chemical properties and (iii) the fire performances of FR-PLA were evaluated. The influence of FR fillers (melamine, ammonium polyphosphate and organomodified montmorillonite) as well as the associated mechanisms of degradation (i.e. photo-degradation under T/UV, hydrolysis under T/RH and a combination of both phenomena under T/UV/RH) were explained. The flame retardant properties of FR-PLA before and after ageing were compared using Limiting Oxygen Index (LOI) and Mass Loss Calorimetry (MLC). It was reported that FR-PLA aged under T/UV/RH conditions is more degraded compared to FR-PLA exposed to T/RH or T/UV. Surprisingly, the FR properties are enhanced after ageing, even if the degradation of the samples is observed.
Keywords: Polylactic acid (PLA); Flame retardancy; Ageing; Temperature; Photo-oxidation; Hydrolysis; Decomposition;