Polymer Degradation and Stability (v.141, #C)

Research on thermal degradation process of p-nitrophenol-based polybenzoxazine by Yongfei Zhu; Yuhua Jiang; Runsheng Lin; Shujuan Yu (1-10).
Effect of nitro group on thermal degradation process of polybenzoxazine was explored in this work through investigating p-nitrophenol-aniline-based benzoxazine (np-a) and its polymer (Pnp-a). The DSC and FTIR results show that the polymerization of np-a could complete even after a process at 160 °C for 1 h. TGA results in N2 and air atmosphere indicate that the char yield, thermal and thermo-oxidation stability of Pnp-a were better than those of phenol-aniline-based polybenzoxazine. And the similar DTG curves of Pnp-a in N2 and air were observed, implying that the degradation mechanism of Pnp-a in N2 and air was alike. These suggest that the essence of thermal degradation of Pnp-a in N2 was oxidation degradation. Furthermore, TGA-FTIR results show that the obvious volatilization of CO2 and H2O was detected during the almost whole thermal degradation process of Pnp-a. Py-GC-MS measurements and the structure evolution of Pnp-a during the thermal degradation process demonstrate that CO2 mainly resulted from methylene on Mannich bridge being oxidized by hydroxyl radical. Besides, LOI test shows that the flame retardance of polybenzoxazine copolymer increased with the addition of Pnp-a.
Keywords: Benzoxazine; Nitro group; Thermal degradation process; CO2;

This research evaluates the effect that thermal and thermo-oxidative degradation conditions exert on the rheological, chemical and thermal properties of high-density polyethylene (HDPE). To that end, dynamic oscillatory rheology, solubility tests in xylene, TGA, DSC and FTIR were conducted on HDPE samples subjected to different degradation conditions: atmosphere (air or nitrogen), temperature (150, 175, 200, 225 or 250 °C) and time (10, 30 or 60 min). Thus, under nitrogen atmosphere, chain scission mechanism prevails over the formation of long chain branching/crosslinking phenomena, which is reflected in a decrease in complex viscosity (|η*|) and an increase in crystallinity (χc). Interestingly, under air atmosphere, two rheological responses were observed: a) a well-developed rubbery region, at degradation temperatures of 225 and 250 °C and, b) a so-called second “plateau” in G′ and G″, for lower degradation temperatures. Solubility tests in xylene point out that the rubbery region is consequence of both branching phenomena and the presence of crosslinking; however, the second “plateau” should be solely ascribed to the increase in the branching mechanism. In this case, the decrease in χc is consequence of both increase in branching and formation of degradation products (accordingly supported by FTIR results). Finally, TGA results reveal that thermal and thermo-oxidative degradation shift the characteristic temperatures (T5% and Tmax) to lower values, compared to virgin HDPE.
Keywords: High-density polyethylene; Thermo-oxidative degradation; Dynamic oscillatory rheology; Chain scission; Chain branching;

Preparation of halloysite nanotubes loaded antioxidant and its antioxidative behaviour in natural rubber by Bangchao Zhong; Jing Lin; Maolin Liu; Zhixin Jia; Yuanfang Luo; Demin Jia; Fang Liu (19-25).
Halloysite nanotubes (HNTs) loaded poly(1,2-dihydro-2,2,4-trimethyl-quinoline) (RD), HNTs-RD, was successfully prepared and added as filler and antioxidant to form natural rubber (NR) composites. The thermo-oxidative aging behaviour of NR composites and the migration of the loaded RD were investigated by mechanical testing, X-ray photoelectron spectroscopy, differential scanning calorimetry and energy dispersive spectroscopy. For the composites with only HNTs-RD as antioxidant, due to the changed conformation and mobility of the immobilized rubber chains approaching HNTs-RD surface, it took a long time for the released RD from the HNTs lumen to migrate into the free rubber phase, during which the free rubber chains were highly oxidized, leading to the poor thermo-oxidative stability of the rubber composites. For the NR composites with HNTs-RD and free RD, it was found that the free rubber chains were protected against oxygen and heat by the free RD. Besides, the released RD steadily migrated and dispersed into the free rubber phase, ensuring the relatively stable and high concentration of total antioxidant in the free rubber phase for a long time, leading to the much better thermo-oxidative stability compared to the rubber composites with only HNTs-RD. In general, this work is believed to provide better guidance for the preparation of rubber composites with long-term aging resistance.
Keywords: Loaded antioxidant; Halloysite nanotubes; Natural rubber; Antioxidative behaviour;

Degradation profiles of biodegradable plastic films by biodegradable plastic-degrading enzymes from the yeast Pseudozyma antarctica and the fungus Paraphoma sp. B47-9 by Shun Sato; Azusa Saika; Yukiko Shinozaki; Takashi Watanabe; Ken Suzuki; Yuka Sameshima-Yamashita; Tokuma Fukuoka; Hiroshi Habe; Tomotake Morita; Hiroko Kitamoto (26-32).
Esterases from the yeast Pseudozyma antarctica (PaE) and the fungus Paraphoma sp. B47-9 (PCLE) can degrade biodegradable plastics (Shinozaki et al., 2013; Suzuki et al., 2014). The degradation profiles of plastic films composed of poly(butylene succinate), poly(butylene succinate-co-adipate), or poly(butylene adipate) by these enzymes were characterized by liquid chromatography-mass spectroscopy in terms of the molecular structures and molecular weights of the degradation products. Monomers and oligomers with molecular weights corresponding to dimers to octamers were identified as products of degradation by PaE in an aqueous reaction solution, irrespective of the type of biodegradable plastic film. Size-exclusion chromatography indicated that the number-average molecular weight of degraded films decreased with reaction time, suggesting that PaE degraded polyester films randomly into monomer units (endo-type degradation). PCLE also degraded polyester films randomly into monomer units, albeit more slowly than did PaE.
Keywords: Biodegradable; Biodegradable plastics; Pseudozyma antarctica; Polyester hydrolase;

Laser-induced blackening on surfaces of thermoplastic polyurethane/BiOCl composites by Zheng Cao; Yanchao Hu; Ying Lu; Yinglin Xiong; An Zhou; Cheng Zhang; Dun Wu; Chunlin Liu (33-40).
Using Bismuth Oxychloride (BiOCl) as a laser-sensitive component, and thermoplastic polyurethane (TPU) as the matrix, the TPU/BiOCl composite capable of laser marking can easily be produced by melt blending of TPU with various proportions of BiOCl. The TPU/BiOCl composite samples showed high contrast black markings on the surface after laser treatment, depending on the BiOCl loading and laser fluence in terms of visual and microscopic analysis. The laser-induced blackening on material surfaces was examined by XPS, XRD, and Raman spectroscopy. The results revealed that the laser-induced blackening on the composite surface entails laser absorption of the BiOCl particles and local heating of the surrounding TPU chains. This led to the generation of amorphous black carbonised materials and the reduction of BiOCl into black bismuth metal. The carbonisation of TPU chains and the photo-thermal conversion of BiOCl particles synergistically contributed to the formation of the black-coloured marking on composite surfaces.
Keywords: Laser marking; Thermoplastic polyurethane; Bismuth oxychloride; Carbonisation; Reduction;

Thermal degradation of MWCNT/polypropylene nanocomposites: A comparison of TGA and laser pulse heating by Karen E. Supan; Celeste Robert; Michael J. Miller; Jeffrey M. Warrender; Stephen F. Bartolucci (41-44).
The kinetics of thermal degradation of isotactic polypropylene (PP) and PP with Multi-Walled Carbon Nanotubes (MWCNT) was studied using thermogravimetric analysis (TGA). Nanocomposites with 0.5, 1.0, and 2.0 wt % MWCNT showed enhanced thermal stability compared to the pure polymer. Higher heating rates showed a higher temperature for the 5% decomposition point and throughout the decomposition of the samples. This trend was seen at all heating rates ranging from 30 to 180 °C/min. The activation energy, calculated using the Flynn-Wall method, increased with the addition of the MWCNT, demonstrating that MWCNT contribute to the improved thermal stability of the polymer. In addition to TGA, the materials were examined at heating rates approximately 5 orders of magnitude higher than TGA with laser pulse heating. A TGA-analogue curve was constructed for the laser pulse heating case and the results were compared to traditional TGA. Laser irradiance and TGA heating rates exhibited similar effects, where higher heating rates showed lower mass loss than lower heating rates, in both regimes. MWCNTs were demonstrated to improve thermal stability in both the TGA and laser pulse heating experiments. In this work, we have established that there are similarities in nanocomposite thermal degradation behavior between the two regimes.
Keywords: Nanocomposites; Carbon nanotubes; Thermal analysis; Laser pulse heating; Polymers;

Electron beam irradiation effect on the mechanical properties of nanosilica-filled polyurethane films by Fei Dong; Suraj Maganty; Stephan J. Meschter; Shinji Nozaki; Takeshi Ohshima; Takahiro Makino; Junghyun Cho (45-53).
Polyurethane (PU) is a good candidate to be used as conformal coatings for space electronic components and boards due to its manufacturability and desirable mechanical properties. It also keeps tin whiskers from growing on the tin-rich surfaces under a long-term usage that will ultimately result in electrical failure. PU coating can, however, be susceptible to the irradiation damage in space environment that ultimately alters its chemical structure and mechanical behavior. In this study, four variations of PU-based coatings (PU filled with and without nanosilica particles; polyurethane acrylate (PUA) filled with and without nanosilica particles) were investigated to understand the irradiation damage on chemical structure and the corresponding mechanical properties under three electron beam irradiation fluences (1 × 1014 cm−2, 1 × 1015 cm−2, 1 × 1016 cm−2). Infrared spectroscopy was used to examine the degradation of chemical bonding with irradiation. Microphase separation, the degree of curing, and molecule chain mobility were examined via differential scanning calorimetry (DSC) by monitoring the shift in glass transition temperatures after the irradiation. The electron beam irradiation on PU films resulted in the quinone structure formation whereas, on PUA film, microphase separation increased, thereby making both films stronger and stiffer, but less ductile and more brittle. As a result, PU and PUA coatings under a high fluence of irradiation will be more prone to tin whisker penetration. This study also shows the potential effect of nanosilica on retarding the irradiation damage in PU and PUA films.
Keywords: Conformal coating; Electron beam irradiation; Polyurethane; Polyurethane acrylate; Nanosilica; Mechanical properties;

Hysteresis in the relation between moisture uptake and electrical conductivity in neat epoxy by Gilles Lubineau; Anwar Sulaimani; Jalal El Yagoubi; Matthieu Mulle; Jacques Verdu (54-57).
Monitoring changes in electrical conductivity is a simple way to assess the water uptake from environmental moisture in polymers. However, the relation between water uptake and changes in conductivity is not fully understood. We monitored changes in the electrical volume conductivity of an anhydride-cured epoxy polymer during moisture sorption-desorption experiments. Gravimetric analysis showed that the polymer exhibits a two-stage sorption behavior resulting from the competition between diffusive and reactive mechanisms. As expected, the macroscopic electrical conductivity increases with the diffusion of water. However, our most surprising observation was severe hysteresis in the relation between water uptake and electrical conductivity during the sorption and desorption experiments. This indicates that change in the electrical conductivity depends on both the water uptake and the competition between the diffusive and reactive mechanisms. We studied samples with various thicknesses to determine the relative effects of the diffusive and reactive mechanisms. This is an important observation as it means that general electrical monitoring techniques should be used cautiously when it comes to measuring the moisture content of polymer or polymer-based composite samples.
Keywords: Epoxy; Moisture sorption; Hydrolysis; Electrical properties; Diffusion;

The effect of percent hydrogenation and vulcanization system on ozone stability of hydrogenated natural rubber vulcanizates using Raman spectroscopy by Korn Taksapattanakul; Tulyapong Tulyapitak; Pranee Phinyocheep; Polphat Ruamcharoen; Jareerat Ruamcharoen; Fabienne Lagarde; Philippe Daniel (58-68).
The ozone stability of partially hydrogenated natural rubbers (HNRs) was evaluated. HNRs with the hydrogenation levels of 14, 33 and 65 mol% including with vulcanization systems of peroxide and sulfur on ozone stability comparing with natural rubber (NR) and ethylene-propylene-diene-rubber (EPDM) vulcanizates were studied. The chemical structures of rubber vulcanizates were characterized by Raman spectroscopy. The surface cracks were observed by Raman optical microscopy. The results clearly exhibited that the ozone stability of HNRs vulcanizates was much greater than that of the NR vulcanizates. The difference between the integral intensities of C=C bonds of isoprene units in rubber chains by Raman spectroscopy before and after ozone exposed was minimized with the degree of hydrogenation. The depth of cracking observed by three-dimensional (3D) modes clearly decreased with an increase in the degree of hydrogenation, while no cracks on the surface of EPDM were found. These findings indicated that ozone stability increased with the degree of hydrogenation. Regarding the effect of vulcanizing systems, sulfur cure showed greater resistance to ozone degradation than peroxide cure.
Keywords: Ozone stability; Raman spectroscopy; Hydrogenated natural rubber; Vulcanization systems;

In the creation of controllable selective adhesive materials, we have utilized a stereocomplex (SC) on an adhesive interface. We selected poly(L,L-lactide) (PLLA) and poly(D,D-lactide) (PDLA) initiated benzyl alcohol (PLLAb and PDLAb, respectively) as a substrate and gold nanoparticle-coated PLLA and PDLA (PLLA-Au and PDLA-Au, respectively) as an injection material. Before performing measurements, SC formation was examined via Fourier Transform Infrared Spectroscopy (FT-IR), X-ray diffraction (XRD), and Differential Scanning Calorimetry (DSC). Subsequently, the force acting on the interface after drying additional chloroform between two substrates was measured using lap-shear testing by changing the molecular weight of the PLLA and PDLA adsorbed on gold nanoparticles or the gold nanoparticle concentration. As a result, it was found that optimal conditions exist to show differences. Additionally, until present SC formation has required an organic solvent such as chloroform or acetonitrile. We have discovered that SC formation can be induced using novel organic solvents approved by the Food and Drug Administration (FDA). We anticipate applications for this study in the creation of biodegradable medical adhesive materials where these materials can be released based on environmental considerations using the selective adhesion behavior and induce SC formation through the use of novel solvents.
Keywords: Polylactide; Stereocomplex; Interface; Adhesion; Polymer-polymer interaction;

Meso-lactide (MLA) was synthesized by thermal degradation of poly(l-lactide) [i.e., poly(l-lactic acid) (PLLA)] at relatively high temperatures of 250 and 300 °C for thermal degradation periods up to 750 and 360 min, respectively. Maximum MLA yields as high as 30 and 40 mol% were successfully attained at 250 and 300 °C, respectively, when thermal degradation was continued for 750 and 180 min, respectively. This indicates that the intentional thermal configurational inversion and depolymerization of PLLA and configurational inversion of lactides (LAs) at the higher temperature are favorable for the synthesis of MLA in a short period. The molecular weight and optical purity and tacticity of poly(lactide) [i.e., poly(lactic acid) (PLA) decreased and the d-lactyl unit content of PLA increased with an increase in reaction time. The higher MLA fraction for 250 °C compared to that for 300 °C at the reaction times when PLA samples had similar number-average molecular weights can be ascribed to the lower tacticity or higher racemization at chiral carbons in PLA chains.Display Omitted
Keywords: Poly(lactide); Poly(lactic acid); Poly(l-lactide); Poly(l-lactic acid); Lactides; Meso-lactide; Thermal degradation; Thermal depolymerization; Configurational inversion; Racemization;

Microstructure and ablation behavior of an affordable and reliable nanostructured Phenolic Impregnated Carbon Ablator (PICA) by Maurizio Natali; Ivan Puri; José M. Kenny; Luigi Torre; Marco Rallini (84-96).
Among polymeric Thermal Protection System (TPS) materials, Phenolic Impregnated Carbon Ablators (PICAs) offer substantial specific weight reduction. A traditional NASA developed PICA is made starting from a preform or using Milled Carbon Fibers (MCFs) to produce a porous skeleton impregnated with a liquid phenolic matrix; the production methods, allow to obtain a low density porous material showing high ablation resistance. The aim of this research was to produce a nanostructured PICA (n-PICA) based on the use of MCFs, a liquid phenolic matrix, and selected nanofillers. Two different nanofillers were used: Multi-Walled Carbon Nanotubes (MWNTs) and nanoclays. The synergistic effect of the two nanofillers was evaluated in terms of thermal and dimensional stability, mechanical properties and ablation resistance via oxy-acetylene torch. Scanning electron microscopy was used to investigate the exposed and in depth surface morphologies of the oxy-acetylene torch tested specimens in order to highlight the differences in the thermal erosion mechanisms.
Keywords: PICA; Thermal protection system; Nanocomposites;

Effect of end-group modification of poly(lactide)s by cinnamoyl chloride on their thermal stability by Yuushou Nakayama; Naoki Matsubara; Zhennguo Cai; Takeshi Shiono; Kei Inumaru; Hiroyuki Shirahama (97-103).
In order to improve thermal stability of poly(l-lactide) (PLLA), the terminal hydroxy groups of PLLAs were converted to cinnamate esters by the treatment with cinnamoyl chloride. The resulting end-group modified PLLAs exhibited much higher thermal degradation temperature at 10% weight loss (T d10) at around 320 °C than those of the starting hydroxy-terminated PLLAs (ca. 240 °C). The amounts of the residual Sn in the PLLAs were estimated by ICP measurements before and after modification, which indicated the modified PLLAs were still contaminated with ca. 600 ppm of Sn. This suggested that the end-group modification is effective for improving thermal stability of PLLA even in the presence of Sn. The end-group modification was also applicable to the stabilization of stereocomplex poly(lactide)s, whose thermal stability is important due to their high processing temperature.
Keywords: Poly(l-lactide); End-group modification; Cinnamoyl chloride; Thermal stability; Residual Sn; Stereocomplex polylactide;

The thermal degradation of polyisocyanurate foam samples were studied by TG/DTA, FTIR, and SEM. All samples with different isocyanate index (NCO/OH = 100, 200, 300) were pre-treated by H2SO4, K2CO3, and NaOH before heating. The measurements of DTG and DTA presented corresponding variability for different acidic and alkaline treatments. The activation energy of thermal decomposition was calculated based on kinetic reaction evaluation. The pronounced polyol and isocyanate regenerations were observed over degradation. Further FTIR measurements at elevated temperatures suggested the possibility of acidic hydrogen bonding catalyzation and alkaline reversible amide regeneration during degradation by chemical treatments. The morphology and growth mechanism of localized corrosion and microcrystallization were characterized based on SEM micrographs.
Keywords: Polyisocyanurate; Chemical activation; Thermal degradation; Decomposition; Kinetics; Morphology;