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Sebastiao V Canevarolo - One of the best experts on this subject based on the ideXlab platform.
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high energy radiation forming Chain Scission and branching in polypropylene
Radiation Physics and Chemistry, 2010Co-Authors: Harumi Otaguro, M. A. Chinelatto, L F C P De Lima, D F Parra, Ademar B Lugao, Sebastiao V CanevaroloAbstract:Abstract The degradation of high molecular weight isotactic polypropylene (iPP) subjected to gamma rays irradiation up to 100 kGy in inert atmosphere was analyzed. The investigation relied upon complex viscosity, elastic modulus, gel fraction, morphology of the insoluble fraction and deconvoluted molecular weight distribution (MWD) curves. At low irradiation doses, already at 5 kGy, the MWD curve is strongly shifted to the low molecular weight side showing Chain Scission, which is confirmed using the calculated Chain Scission distribution function (CSDF). At high dose levels, the appearance of a shoulder in the high molecular weight side of the MWD curve indicates the formation of Chain branching. The presence of a considerable insoluble fraction at these high dose levels indicates also the formation of cross-linking, which has different morphology then the insoluble fraction present in the original iPP. The rheological results show changes in the molecular structure of irradiated samples in agreement with the gel content data. The chromatographic and rheological data has shown that gamma irradiation of iPP produces Chain Scission, branching and cross-linking.
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evaluation of philips and ziegler natta high density polyethylene degradation during processing in an internal mixer using the Chain Scission and branching distribution function analysis
Polymer Degradation and Stability, 2006Co-Authors: L. A. Pinheiro, M. A. Chinelatto, Sebastiao V CanevaroloAbstract:Abstract The oxidative and thermo-mechanical degradation of HDPE was studied during processing in an internal mixer under two conditions: totally and partially filled chambers, which provides lower and higher concentrations of oxygen, respectively. Two types of HDPEs, Phillips and Ziegler–Natta, having different levels of terminal vinyl unsaturations were analyzed. Materials were processed at 160, 200, and 240 °C. Standard rheograms using a partially filled chamber showed that the torque is much more unstable in comparison to a totally filled chamber which provides an environment depleted of oxygen. Carbonyl and transvinylene group concentrations increased, whereas vinyl group concentration decreased with temperature and oxygen availability. Average number of Chain Scission and branching (ns) was calculated from MWD curves and its plotting versus functional groups' concentration showed that Chain Scission or branching takes place depending upon oxygen content and vinyl groups' consumption. Chain Scission and branching distribution function (CSBDF) values showed that longer Chains undergo Chain Scission easier than shorter ones due to their higher probability of entanglements. This yields macroradicals that react with the vinyl terminal unsaturations of other Chains producing Chain branching. Shorter Chains are more mobile, not suffering Scission but instead are used for grafting the macroradicals, increasing the molecular weight. Increase in the oxygen concentration, temperature, and vinyl end groups' content facilitates the thermo-mechanical degradation reducing the amount of both, longer Chains via Chain Scission and shorter Chains via Chain branching, narrowing the polydispersity. Phillips HDPE produces a higher level of Chain branching than the Ziegler–Natta's type at the same processing condition.
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The role of Chain Scission and Chain branching in high density polyethylene during thermo-mechanical degradation
Polymer Degradation and Stability, 2004Co-Authors: L. A. Pinheiro, M. A. Chinelatto, Sebastiao V CanevaroloAbstract:The mechanical and thermo-oxidative degradation of high density polyethylene (HDPE) was measured in a twin-screw extruder using various processing conditions. Two types of HDPE, Phillips and Ziegler-Natta, having different levels of terminal vinyl unsaturation were analysed. Mild screw profiles, having mainly conveying elements, have short mean residence times then profiles with kneading discs and left hand elements. Carbonyl and trans-vinylene group concentrations increased, whereas vinyl group concentration decreased with number of extrusions. Higher temperature profiles intensified these effects. The thermo-mechanical degradation mechanism begins with Chain Scission in the longer Chains due to their higher probability of entanglements. These macroradicals then react with the vinyl terminal unsaturations of other Chains producing Chain branching. Shorter Chains are more mobile, not suffering Scission but instead are used for grafting the macroradicals, increasing the molecular weight. Increase in the levels of extrusion temperature, shear and vinyl end groups content facilitates the thermo-mechanical degradation reducing the amount of both, longer Chains via Chain Scission and shorter Chains via Chain branching, narrowing the polydispersity. Phillips HDPE produces a higher level of Chain branching than does the Ziegler-Natta type.
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calculating the Chain Scission distribution function csdf using the concentration method
Polymer Degradation and Stability, 2004Co-Authors: Carlos Alberto Caceres, Sebastiao V CanevaroloAbstract:Abstract One of the possible ways to follow polymer degradation has been to calculate the average number of Chain Scissions ( n s ) that occurred during its thermal history. It is a single and average value, a ratio between the initial and final number average molecular weights. To have a complete coverage the calculation should be extended for each molecular weight fraction of the original MWD curve, which can be done by the Chain Scission distribution function, the CSDF curve. For this calculus a correlation rule must be set in order to allow finding the corresponding values in the initial and final MWD curves. In this paper we extend the previously presented method to calculate the CSDF curve proposing a new correlation rule, called the Concentration Method. Both methods were compared theoretically using an approximation of the MWD curves to a Gaussian shape and experimentally using polypropylene that had been subjected to thermo-mechanical degradation during multiple extrusions. We consider the Concentration Method the proper way to represent the correlation rule because it shows a true agreement with the conventional average n s value. It also presents lower data scattering and so can be used as another tool in understanding polymer degradation.
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Chain Scission distribution function for polypropylene degradation during multiple extrusions
Polymer Degradation and Stability, 2000Co-Authors: Sebastiao V CanevaroloAbstract:The degradation of polypropylene subjected to multiple extrusions was evaluated by following the changes in the molecular weight distribution curves (MWD) obtained by size exclusion chromatography (SEC). The curves shift to the low molecular weight side and we calculate the Chain Scission distribution function (CSDF) as a function of the initial molecular weight. Assuming that the SEC curves of both, virgin and degraded PP follow Gaussian behaviour the calculated CSDF data would be a straight-line function of the original molecular weight (MW). A random Chain Scission process gives a constant and positive CSDF value independent of MW. If the process is no longer random the CSDF value increases as a result of the increase in the Chain Scission probability. Polypropylene subjected to multiple extrusion shows that the Chain Scission processes during thermo-mechanical degradation display a CSDF curve with a constant positive value in the low molecular weight region which rises in the higher MW region. This indicates that the probability of Chain breaking is dependent on the degradation aggressiveness and the molecular weight of the Chain. At low MW, it is independent of it, i.e. the Chain Scission is random, but at higher MW it becomes dependent, increasing with MW. The behaviour has been measured during multiple extrusions and using different screw configurations. The CSDF values are higher the greater the aggressiveness of the degradation, i.e. number of extrusions and screw configuration: kneading elements with 45° (KB45) degrade more than conveying elements (CON), increasing the overall level of Chain Scission.
Bruno Fayolle - One of the best experts on this subject based on the ideXlab platform.
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Review: degradation-induced embrittlement in semi-crystalline polymers having their amorphous phase in rubbery state
Journal of Materials Science, 2008Co-Authors: Bruno Fayolle, Xavier Colin, Emmanuel Richaud, Jacques VerduAbstract:The literature dealing with degradation-induced embrittlement mechanisms in semi-crystalline polymers having their amorphous phase in rubbery state is reviewed. It is first demonstrated that the decrease of molar mass resulting from a quasi-homogeneous Chain Scission process is responsible for embrittlement. The main specificity of the polymer family under study is that embrittlement occurs at a very low conversion of the degradation process, while the entanglement network in the amorphous phase is slightly damaged. In these polymers, Chain Scission induces chemicrystallization. The analyses of available data on this process show that it is characterized by a relatively high yield: about one half entanglement strands integrate the crystalline phase after one Chain Scission. A simple relationship expressing the chemicrystallization yield for a given polymer structure is proposed. Chain Scission and chemicrystallization can lead to embrittlement through two possible causal Chains: (1) Chain Scission → molar mass decrease → chemicrystallization → decrease of the interlamellar spacing → embrittlement. (2) Chain Scission → molar mass decrease → chemicrystallization → decrease of the tie-macromolecule concentration → embrittlement. At this state of our knowledge, both causal Chains are almost undistinguishable.
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mechanism of degradation induced embrittlement in polyethylene
Polymer Degradation and Stability, 2007Co-Authors: Bruno Fayolle, Lida Audouin, Xavier Colin, Jacques VerduAbstract:Abstract The thermal oxidation of polyethylene films in air at 80 °C and 90 °C has been studied by tensile testing, IR spectrophotometry and molar mass determination from rheometric measurements. In the conditions under study, the polymer predominantly undergoes Chain Scission and embrittles suddenly when the weight average molar mass reaches a critical value (90 kg mol −1 ), far before significant damage of the entanglement network ( M e = 1.9 kg mol −1 ) in the amorphous phase. The following embrittlement mechanism is proposed: Chain Scission in the amorphous phase induces chemicrystallization. The thickness of the interlamellar amorphous layer ( l a ) decreases until a critical value of the order of 6–7 nm, below which plasticity cannot be activated and the polymer behaves in a brittle manner, as previously shown for virgin polyethylene. Using ( l a , M W ) maps, it is possible to explain the differences observed in the embrittlement behaviour of semi-crystalline polymers predominantly undergoing Chain Scission.
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oxidation induced embrittlement in polypropylene a tensile testing study
Polymer Degradation and Stability, 2000Co-Authors: Bruno Fayolle, L Audouin, J VerduAbstract:Abstract The thermal oxidation of polypropylene films (100 μm) in air at 90°C was studied by IR spectroscopy, rheometry at 210°C and tensile testing. Tensile testing reveals a sudden embrittlement before the end of the induction period determined from carbonyl build-up curves. Embrittlement occurs at a very low conversion of the Chain Scission process (only 1 Chain Scission per 3 initial Chains) and it can be demonstrated that it results from a decrease in polymer toughness rather than from the build-up of defects linked to a presumed heterogeneity of the oxidation process. Tensile testing is not necessarily a good tool to characterize embrittlement given that phenomena such as necking and crack initiation at geometric defects are not taken into account with the usual measuring methods. The sudden drop of ultimate elongation could be, at least partly, an artifact.
Jacques Verdu - One of the best experts on this subject based on the ideXlab platform.
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Review: degradation-induced embrittlement in semi-crystalline polymers having their amorphous phase in rubbery state
Journal of Materials Science, 2008Co-Authors: Bruno Fayolle, Xavier Colin, Emmanuel Richaud, Jacques VerduAbstract:The literature dealing with degradation-induced embrittlement mechanisms in semi-crystalline polymers having their amorphous phase in rubbery state is reviewed. It is first demonstrated that the decrease of molar mass resulting from a quasi-homogeneous Chain Scission process is responsible for embrittlement. The main specificity of the polymer family under study is that embrittlement occurs at a very low conversion of the degradation process, while the entanglement network in the amorphous phase is slightly damaged. In these polymers, Chain Scission induces chemicrystallization. The analyses of available data on this process show that it is characterized by a relatively high yield: about one half entanglement strands integrate the crystalline phase after one Chain Scission. A simple relationship expressing the chemicrystallization yield for a given polymer structure is proposed. Chain Scission and chemicrystallization can lead to embrittlement through two possible causal Chains: (1) Chain Scission → molar mass decrease → chemicrystallization → decrease of the interlamellar spacing → embrittlement. (2) Chain Scission → molar mass decrease → chemicrystallization → decrease of the tie-macromolecule concentration → embrittlement. At this state of our knowledge, both causal Chains are almost undistinguishable.
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mechanism of degradation induced embrittlement in polyethylene
Polymer Degradation and Stability, 2007Co-Authors: Bruno Fayolle, Lida Audouin, Xavier Colin, Jacques VerduAbstract:Abstract The thermal oxidation of polyethylene films in air at 80 °C and 90 °C has been studied by tensile testing, IR spectrophotometry and molar mass determination from rheometric measurements. In the conditions under study, the polymer predominantly undergoes Chain Scission and embrittles suddenly when the weight average molar mass reaches a critical value (90 kg mol −1 ), far before significant damage of the entanglement network ( M e = 1.9 kg mol −1 ) in the amorphous phase. The following embrittlement mechanism is proposed: Chain Scission in the amorphous phase induces chemicrystallization. The thickness of the interlamellar amorphous layer ( l a ) decreases until a critical value of the order of 6–7 nm, below which plasticity cannot be activated and the polymer behaves in a brittle manner, as previously shown for virgin polyethylene. Using ( l a , M W ) maps, it is possible to explain the differences observed in the embrittlement behaviour of semi-crystalline polymers predominantly undergoing Chain Scission.
Michael J Solomon - One of the best experts on this subject based on the ideXlab platform.
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universal scaling for polymer Chain Scission in turbulence
Proceedings of the National Academy of Sciences of the United States of America, 2006Co-Authors: Siva A Vanapalli, Steven L Ceccio, Michael J SolomonAbstract:We report that previous polymer Chain Scission experiments in strong flows, long analyzed according to accepted laminar flow Scission theories, were in fact affected by turbulence. We reconcile existing anomalies between theory and experiment with the hypothesis that the local stress at the Kolmogorov scale generates the molecular tension leading to polymer covalent bond breakage. The hypothesis yields a universal scaling for polymer Scission in turbulent flows. This surprising reassessment of over 40 years of experimental data simplifies the theoretical picture of polymer dynamics leading to Scission and allows control of Scission in commercial polymers and genomic DNA.
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inertial effects on polymer Chain Scission in planar elongational cross slot flow
Macromolecules, 2004Co-Authors: Mohammad T Islam, Siva A Vanapalli, Michael J SolomonAbstract:The molar mass and molar mass distribution of polymers subjected to Chain Scission in planar elongational flow are profoundly affected by the inertial character of the flow, as quantified by the Reynolds number. The degradation of dilute poly(ethylene oxide) (PEO) Chains in aqueous-based solvents of varying viscosity was quantified in the planar elongational flow of a cross-slot flow device by gel permeation chromatography with multiangle laser light scattering detection. At low Reynolds number (Re ∼1000), the observed scaling was e ∝ Mw,f-1.04±0.07. Differences of this kind, first quantified by comparing results from stagnation point elongation flows and contraction flows, have previously been attributed to different molecular me...
Jan Vicha - One of the best experts on this subject based on the ideXlab platform.
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mechanism of sulfonation induced Chain Scission of selectively oxidized polysaccharides
Carbohydrate Polymers, 2020Co-Authors: Lukas Munster, Barbora Hanulikova, Michal Machovský, F Latecka, Ivo Kuřitka, Jan VichaAbstract:Abstract Oxidation of polysaccharides to 2,3-dicarboxypolysaccharides is a two-stage process, where selective oxidation by periodate is followed by secondary oxidation by chlorite. Addition of sulfamic acid before the secondary oxidation influences the molecular weight and degree of oxidation of the product. Here, mechanism of sulfamic acid-catalysed Chain Scission is elucidated for selectively oxidized cellulose and dextrin. Initially, sulfamic acid sulfonates the aldehyde groups of 2,3-dialdehydepolysaccharide. Introduced –SO3H groups are in ideal position to protonate the oxygen atom of 1-4’ glycosidic bond and to trigger acidic hydrolysis. This can be used to obtain a direct control over the molecular weight of the product. Observed slightly lower degree of oxidation was ascribed to the ability of sulfamic acid to scavenge the hypochlorite and thus protect the intramolecular hemiacetals from oxidation. Usually undesirable hypochlorite thus seems to be necessary for preparation of selectively oxidized polysaccharides with degree of oxidation above 90 %.