The oxidative degradation of PP/OMMT nanocomposites under γ‐irradiation was studied. Changes in structure and properties resulting from γ‐exposure in the range 0–100 kGy were investigated. The results were analyzed by comparing the influence of PP‐g‐MA and pristine OMMT on the oxidation kinetics of neat PP. γ‐Irradiation in the presence of air strongly degraded the properties of PP materials, particularly for radiation doses above 20 kGy. The rate of oxidative degradation of PP/OMMT/PP‐g‐MA nanocomposites was much faster than that of neat PP. This suggests that PP‐g‐MA and pristine OMMT components behave as oxidation catalysts, leading to the formation of free radicals in the polymer matrix.
Full Paper: The objective of this work is aimed at determining the kinetics of HALS photografting in PP films as a function of the additive concentration under both natural weathering and accelerated UV conditions. The stabilizer studied had a single structure that combined HALS and a UV absorbing unit detectable at 308 nm in the UV spectrum of PP, capable of inducing a photoreaction with the polymer matrix. The kinetics of HALS photografting on PP films at various additive concentrations was determined by direct spectroscopic measurements on film samples through the absorption band of the stabilizer positioned at 308 nm in the UV spectra, which was attributed to the highly conjugated methylenic double bond. The content of free HALS was determined as a function of exposure time by UV spectroscopy for all the additive concentration ratios used. It is shown that the kinetics of HALS photografting increase with a decrease in the additive concentration ratio. The whole grafting process under conditions of natural weathering occurred in the PP film after almost 350 h of exposure, while approximately 20 h were necessary under accelerated UV conditions. Moreover, these kinetics are well described by a third order polynomial model, validated by a maximum value of the coefficient of correlation close to unity. This was also confirmed by a comparison of the time values measured at 50% of the total grafting calculated from the model with those observed experimentally. This finding was consistent with the data obtained on the free HALS content determined by UV spectroscopy.
The percentage of HALS photografting in PP films as a function of time under natural weathering, determined by UV spectroscopy. 相似文献
Summary: Poly(propylene) (PP)‐clay nanocomposites were prepared from unmodified montmorillonite clays (NaMMT), with poly(ethylene oxide)‐based nonionic surfactants as dispersants/intercalants/exfoliants. The primary objective of this research was to find dispersants that (a) allow PP nanocomposites to be formed by direct melt mixing; (b) are effective with unmodified clays and (c) comprise of only a minor component with respect to both the clay and the overall composition. Linear, branched, gemini and sugar‐based surfactants and structures containing poly(dimethyl siloxane) and poly(methyl methacrylate) blocks were examined. These additives were found to be effective in breaking down the clay agglomerates to tactoids, giving some expansion of the clay structure and partial exfoliation and providing substantially improved clay dispersion. The properties of the derived nanocomposites depend on the level of additive and its structure. Tensile and impact properties show significant improvement over the precursor PP. Also notable are the significantly better thermal and thermo‐oxidative stabilities, as compared to both PP and “clay alone” composites. For optimal properties, it is both necessary and desirable that the surfactant should only be a minor constituent (20–50%) of the composition, with respect to the clay. A preferred surfactant is linear PE‐block‐PEO, with a short PEO block and an alkyl chain with approximately 30 carbon atoms (C30).
Summary: N‐Isopropylacrylamide (NIPAAm) was graft‐polymerized from its acetone solution onto poly(propylene) (PP) films, after electron‐beam irradiation in the presence of air oxygen. The effects of pre‐irradiation dose as well as monomer concentration, reaction temperature and reaction time on the grafting efficiency were investigated. Typical conditions for achieving maximum grafting yield were observed for 1 M monomer concentration, after PP pre‐irradiation with a 300 kGy dose and a reaction temperature of 50 °C. The location of the graft polymerization was examined by different methods including measurements of dimensional variations, calorimetry, SEM and AFM. The temperature‐responsive behavior of grafted copolymer was studied by swelling and contact angle measurements at different temperatures.
Temperature dependence of the swelling ratio in water as a function of temperature. 相似文献
Extrusion flow experiments of linear and branched syndiotactic poly(propylene)s were carried out. The work was focused on flow instabilities. Ionized radiation was employed to induce long chain branching in linear samples. Sharkskin and melt fracture were postponed in the case of slightly long branched samples, which possess an enhanced melt elasticity compared to linear samples. For the most elastic samples the nature of the flow instability changed: sharkskin disappeared and melt fracture was observed instead. The correlation between sharkskin and melt strength results is discussed.
Well‐defined graft copolymers based on poly(ε‐caprolactone) (PCL) via poly(linoleic acid) (PLina), are derived from soybean oil. Poly(linoleic acid)‐g‐poly(ε‐caprolactone) (PLina‐g‐PCL) and poly(linoleic acid)‐g‐poly(styrene)‐g‐poly(ε‐caprolactone) (PLina‐g‐PSt‐g‐PCL) were synthesized by ring‐opening polymerization of ε‐caprolactone initiated by PLina and one‐pot synthesis of graft copolymers, and by ring‐opening polymerization and free radical polymerization by using PLina, respectively. PLina‐g‐PCL, PLina‐g‐PSt‐g‐PCL3, and PLina‐g‐PSt‐g‐PCL4 copolymers containing 96.97, 75.04 and 80.34 mol% CL, respectively, have been investigated regarding their enzymatic degradation properties in the presence of Pseudomonas lipase. In terms of weight loss, after 1 month, 51.5 % of PLina‐g‐PCL, 18.8 % of PLina‐g‐PSt‐g‐PCL3, and 38.4 % of PLina‐g‐PSt‐g‐PCL4 were degraded, leaving remaining copolymers with molecular weights of 16,140, 83,220 and 70,600 Da, respectively. Introducing the PLina unit into the copolymers greatly decreased the degradation rate. The molar ratio of [CL]/[Lina] dramatically decreased, from 21.3 to 8.4, after 30 days of incubation. Moreover, reduced PCL content in PLina‐g‐PSt‐g‐PCL copolymers decreased the degradation rate, probably due to the PSt enrichment within the structure, which blocks lipase contact with PCL units. Thus, copolymerization of PCL with PLina and PSt units leads to a controllable degradation profile, which encourages the use of these polymers as promising biomaterials for tissue engineering applications. 相似文献
Summary: Dielectric cyclosiloxane bearing polysilsesquioxanes (CS‐PSSQs) were prepared by acid catalyzed polymerization using 2,4,6,8‐tetramethyl‐2,4,6,8‐tetra(trimethoxysilylethyl)cyclotetrasiloxane. The molecular weight, and content of the functional end‐groups of the CS‐PSSQs were found to be dependent on the process parameters, such as the molar ratio of water and catalyst to the ? OCH3 group of the silane monomer, the amount of solvent, and the poly(ε‐caprolactone) (PCL) content, etc. Based on these CS‐PSSQ prepolymers, nanoporous organic/inorganic hybrid thin films were fabricated by spin‐coating mixtures of these prepolymers with star‐shaped PCL on the silicon substrate, and subsequently heating them at 150 and 250 °C for 1 minute each and then at 420 °C for 1 hour. A dielectric constant as low as 2.28 was achieved for the nanoporous PCL/CS‐PSSQ (3:7 v/v) film, along with low moisture absorption in ambient and water conditions, primarily due to the presence of hydrophobic moieties such as ethylene and methyl groups of the polymer and nano‐sized hydrophobic pores inside the matrix, resulting in the film having stable dielectric properties. Moreover, the PCL/CS‐PSSQ (3:7 v/v) nanohybrid film revealed a nanoporous structure containing ca. 1.52 nm of average‐sized mesopores, as measured by the N2 adsorption method. The CS‐PSSQ‐only film showed high mechanical strengths having an elastic modulus and hardness of 6.64 and 0.88 GPa, respectively, for the 7 500 Å thick film and 2.41 and 0.38 GPa, respectively, for the PCL/CS‐PSSQ (3:7 v/v) film. In addition, the crack velocity of the CS‐PSSQ‐only film was found to be ca. 10?11 m · s?1 in ambient conditions and an aqueous environment, probably due to the enhanced hydrophobicity and mechanical toughness of the incorporated methyl group, siloxane unit and ethylene moieties in the polymer matrix.
Summary: Poly(ε‐caprolactone)‐polyglycolide‐poly(ethylene glycol) monomethyl ether random copolymers were synthesized from ε‐caprolactone (ε‐CL), glycolide (GA) and poly(ethylene glycol) monomethyl ether (MPEG) using stannous octoate as catalyst at 160 °C by bulk polymerization. The copolymers with different composition were synthesized by adjusting the weight ration of reaction mixture. The resultant copolymer with a weight ratio (10:15:75) of MPEG2000, GA, and CL was characterized by IR, 1H NMR, GPC and DSC. The new biodegradable copolymer has potential for medical applications since it is combined with properties of PCL, PGA and MPEG.
Efficient one‐step syntheses of α,β‐ and β,β‐dihaloenones were achieved by ruthenium(II)‐catalyzed reactions between cyclic or acyclic diazodicarbonyl compounds and oxalyl chloride or oxalyl bromide in moderate to good yields. This methodology offers several significant advantages, which include ease of handling, mild reaction conditions, one‐step reaction, and the use of an effective and non‐toxic catalyst. The synthesized compounds were further transformed into highly functionalized novel molecules bearing aromatic rings on the enone moiety using the Suzuki reaction.
In this study, isotactic poly(propylene) (PP)/poly(oxymethylene) (POM) blend films, including of POM as minor phase in the range of 10–30 wt%, are prepared in a twin screw extruder equipped with a slit‐die and cast film haul‐off unit. It is found that the blend films show characteristic immiscible matrix‐droplet morphology. Short‐term uniaxial tensile creep behaviors of films imply that the introducing of POM significantly improves the elastic modulus and decreases the total creep strain of PP/POM blends. Creep tests are also performed at various temperatures and long‐term deformations of samples are predicted by applying of time‐temperature superposition principle and the Findley model. It is found that the presence of POM domains into PP matrix enhances the creep resistance of PP especially at high temperatures. It is concluded that the PP‐rich PP/POM blend films show much lower short and long‐term creep strains compared to PP.
It is still not clear why the long‐term properties of plastic weld seams can only be differentiated by the very expensive medium tensile creep tests. One hypothesis for justifying this is based on the change in the structure of the weld seam surroundings, another cites the consumption of antioxidants and the following ageing in the weld seam area to be responsible for this. Butt‐welded weld seams made of poly(propylene) were systematically produced under different process parameters. Corresponding to the particular hypothesis, these weld seams were then analyzed in various ways to find correlations or to prove one of the hypotheses. Regarding their short‐term weld seam quality, the analyzed weld seams could not be differentiated through short‐term tensile or short‐term bend test. However, the medium tensile creep tests showed significant differences in both time until failure and long‐term weld seam quality. Under long‐term loading, the start of the brittle crack could be detected in most weld seams in the fine spherulite‐zone or between this zone and the area of the flow lines. This demonstrated again that only long‐term tests are suitable for examining different weld seam qualities. Depending on the welding parameters, times until failure decline with increasing heated‐tool temperature and heating time. Though these parameters lead to a higher consumption of antioxidants in the weld seam, a degradation was not detected in the breaking area. In fact, increasing heated‐tool temperatures and heating times, as well as higher joining pressures lead to a change in the internal structure of the material. This can be seen in morphological structure analyses in the larger bend of the entire weld seam area. A larger bend, however, correlates with higher residual stresses in the weld seam. In the medium tensile creep tests, these residual stresses as well as the tensile stress in the border region and the compressive stress in the middle are superimposed by the tensile stress resulting from the test stress. Thus a greater bend of the weld seam area and higher residual stresses in the weld seam itself lead to shorter times until failure in medium tensile creep tests.
Schematic representation of the formation of residual stresses in a weld seam and residual stresses in the different bended weld seam areas. 相似文献
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