Initial Molar Mass Research Articles

Tailoring the mechanical and rheological properties of poly(lactic acid) by sterilizing UV-C irradiation

In this study, we irradiated amorphous (A) and semi-crystalline (SC) poly(lactic acid) (PLA) with different UV-C doses up to 2214 kJ/m2. We achieved an average crystallinity of 43 % by heat treatment, which was unaffected by UV-C irradiation. Modulated differential scanning calorimetry showed that crystal polymorphs and the ratio of rigid amorphous and mobile amorphous phases were also unaffected. Using gel permeation chromatography analysis, we showed that the degradation mechanism was noncatalytic random scission, and the initial molar mass was reduced by >90 % at a dose of 2214 kJ/m2 for both A- and SC-PLA samples. Our Raman spectroscopy results indicated that the probability of the formation of oxygen-containing groups increases with increasing UV-C doses. Since we found that the mechanical properties of PLA films can be tailored with UV-C light, we proposed a method to predict the overall tensile curve as a function of the UV-C dose. We also proposed a modified Cross-WLF model to describe the effect of UV-C irradiation on viscosity up to 55 % molar mass reduction. The models allow us to estimate the limits of recyclability and reusability of sterilised PLA products.

A Versatile Comonomer Additive for Radically Recyclable Vinyl-derived Polymers.

Radically-formed, vinyl-derived polymers account for over 30 % of polymer production. Connected through stable carbon-carbon bonds, these materials are notoriously challenging to chemically recycle. Herein, we report universal copolymerization of a cyclic allyl sulfide (CAS) additive with multiple monomers under free-radical conditions, to introduce main-chain dynamic motifs. Backbone allyl sulfides undergo post-polymerization radical rearrangement via addition-fragmentation-transfer (AFT) that fosters both chain scission and extension. Scission is selectively induced through allyl sulfide exchange with small molecule thiyl radicals, resulting in oligomers as low as 14 % of the initial molar mass. Crucially, oligomers retain allyl sulfide end groups, enabling their extension with monomer under radical conditions. Extended, i.e., recycled, product molar mass is tunable through the ratio of monomer to oligomer, and can surpass that of the initial copolymer. Two scission-extension cycles are demonstrated in copolymers with methyl methacrylate and styrene without escalation in dispersity. In illustration of forming higher-value products, i.e., upcycling, we synthesized block copolymers through the extension of oligomers with a different vinyl monomer. Collectively, our approach to chemical recycling is unparalleled in its ability to 1) function in a variety of vinyl-derived polymers, 2) complete radical closed-loop cycling, and 3) upcycle waste material.

A Versatile Comonomer Additive for Radically Recyclable Vinyl‐derived Polymers

AbstractRadically‐formed, vinyl‐derived polymers account for over 30 % of polymer production. Connected through stable carbon‐carbon bonds, these materials are notoriously challenging to chemically recycle. Herein, we report universal copolymerization of a cyclic allyl sulfide (CAS) additive with multiple monomers under free‐radical conditions, to introduce main‐chain dynamic motifs. Backbone allyl sulfides undergo post‐polymerization radical rearrangement via addition‐fragmentation‐transfer (AFT) that fosters both chain scission and extension. Scission is selectively induced through allyl sulfide exchange with small molecule thiyl radicals, resulting in oligomers as low as 14 % of the initial molar mass. Crucially, oligomers retain allyl sulfide end groups, enabling their extension with monomer under radical conditions. Extended, i.e., recycled, product molar mass is tunable through the ratio of monomer to oligomer, and can surpass that of the initial copolymer. Two scission‐extension cycles are demonstrated in copolymers with methyl methacrylate and styrene without escalation in dispersity. In illustration of forming higher‐value products, i.e., upcycling, we synthesized block copolymers through the extension of oligomers with a different vinyl monomer. Collectively, our approach to chemical recycling is unparalleled in its ability to 1) function in a variety of vinyl‐derived polymers, 2) complete radical closed‐loop cycling, and 3) upcycle waste material.

Influence of Rheological and Morphological Characteristics of Polyhydroxybutyrate on Its Meltblown Process Behavior.

Polyhydroxybutyrate (PHB) is a promising biopolymer. However, processing PHB in pure form in thermoplastic processes is limited due to its rapid degradation, very low initial crystallization rate, strong post-crystallization, and its low final stretchability. In this article, we screened commercial PHBs for morphological characteristics, rheological properties, and "performance" in the meltblown process in order to reveal process-relevant properties and overcome the shortcoming of PHB in thermoplastic processes for fiber formation. An evaluation of degradation (extruded (meltblown) material vs. granules) was performed via rheological and SEC analysis. The study revealed large differences in the minimum melt temperature (175 up to 200 °C) and the grade-dependent limitation of accessible throughput on a 500 mm plant. The average fiber diameter could be lowered from around 10 μm to 2.4 μm in median, which are the finest reported values in the literature so far. It was found that the determination of the necessary process temperature can be predicted well from the complex shear viscosity. Different to expectations, it became apparent that a broader initial molar mass distribution (>8) is suitable to overcome the state-of-the art limitations of PHAs in order to stabilize fiber formation, increase the productivity, and obtain better resistance towards thermal degradation in process. Accordingly, longer polymer chain fractions could be more affected by degradation than medium and short polymer chains in the distribution. Further, a low initial narrow distributed molar mass resulted in too brittle fabrics.

Comparison of the performances of different drying enhancers for waterborne polyvinyl alcohol films

AbstractWaterborne polyvinyl alcohol (PVA) films show lower environmental impact but render high residual solvent on account of hydrophilicity of PVA and low volatility of water. Plasticizers have shown high potential to act as drying enhancers in some recent studies. Plasticizers triphenyl phosphate (TPP) and polyethylene glycol (PEG400) with their favorable chemical structures and relatively low molar masses are promising options. Their performances in drying waterborne PVA films are compared with the plasticizers used in earlier works, at different initial wet film thicknesses, polymer contents, enhancer contents, and enhancer molar masses. Films were solvent casted by drying PVA‐water solutions in a substrate. TPP causes maximum lowering of the residual solvent and does this at high drying rate corresponding to low initial wet film thickness and PVA content. The least value of residual solvent is 0.20% at optimum TPP loading. The next best residual solvent results of 0.21% and 0.59% are, respectively, exhibited by PEG400 and PEG6000, but at low drying rate for high wet film thickness, for both the enhancers; however, at high drying rate for low wet film thickness anomalous skinning increased the residual solvent. At 0.51%, Capstone FS‐63 also gives good results and at high drying rate. Their relative performances are correlated to the viscosity of cast solutions, glass transition temperature of films (DSC), and chemical structures of the polymer and enhancers. TPP thus proved to be most effective. SEM showed dense films and disappearance of defects with optimum loading of TPP, and TGA/DTG showed thermally stable coatings. In spite of the higher cost of TPP it can have useful specialized applications that exploit its excellent flame retardance features.

Estimations of the upper and lower depth limits for kerogen to generate oil/gasworldwide: A hypothesis

This study focuses on the upper and lower depth limits for kerogen to generate oil/gas worldwide. For the first time, the upper and lower depth limits of the conversion process from kerogen to oil/gas are given by the equations with clear physical significance. The method for obtaining the parameters in the equations is also proposed. The results show that the upper limit of hydrocarbon generation of kerogen mainly depends on the ratio of the minimum dissociation energy and initial molar mass of hydrocarbon generation. The smaller the ratio is, the shallower the upper limit of hydrocarbon generation of kerogen is. The upper depth limit of hydrocarbon generation ranges from 688 m to 4925 m, with an average of 1944 m. The lower limit of hydrocarbon generation mainly depends on the ratio of the dissociation energy and molar mass at the end of hydrocarbon generation. The lower depth limit of hydrocarbon generation is proportional to the above ratio. The lower limit of hydrocarbon generation of kerogen ranges from 2539 m to 16 337 m, with an average of 6926 m. This study not only solves a major controversial issue which is the minimum and maximum depth of drilling required to capture oil/gas but also helps select the location of carbon storage. It will be conducive to the effective utilization of fossil energy and the early realization of global carbon neutralization.

Synthesis of Poly(ether‐ester‐urethane) with a Controlled Molar Mass Based on Poly(ε‐caprolactone) and Poly(ethylene glycol) Segments

AbstractPoly(ether‐ester‐urethanes) (PEEUs) derived from poly(ε‐caprolactone) and poly(ethylene glycol) (PCL‐b‐PEG) have a high potential for use in biomedical applications or as compatibilizers of immiscible poly(acid lactic) and poly(ε‐caprolactone) (PLA/PCL) blends. This study focused on the synthesis of PEEUs with same molar mass by one‐step bulk polycondensation of PCL and PEG macrodiols and hexamethylene diisocyanate (HDI). The copolymers, distinguished by the size of hydrophilic PEG and hydrophobic PCL segments and symmetry of the blocks, are characterized by SEC, FTIR, H1‐NMR, TGA, DSC, and water absorption testing. FTIR and H1‐NMR results demonstrated the formation of a hydroxy‐terminated multiblock copolymer with urethane bonds. Additionally, the PEG:PCL compositions measured by H1‐NMR are close to the theoretical compositions. Molar mass distribution curves revealed that copolymers with molar mass around 45 000 Da are synthesized by varying extents of ratio, given by NCO/OH molar ratio, between 0.9 and 1.0 depending on the initial macrodiols molar masses. The block size ratio of hydrophilic and hydrophobic blocks is an important parameter in water absorption behavior, which can generate a water‐uptake of 50% or even 350% depending on the symmetry of the segments.

Analysis of high melt‐strength poly(ethylene terephthalate) produced by reactive processing by shear and elongational rheology

Three grades of poly(ethylene terephthalate) (PET) produced by different synthesis routes and with different molar masses were reactively extruded with two tetra‐functional chain extenders, i.e., pyromellitic dianhydride (PMDA) and tetraglycidyl diamino diphenyl methane (TGDDM). The preferred reactivity of the coupling agents led to different long‐chain branched (LCB) structures, which can be related to different hydroxyl and carboxyl end group concentrations of the PET grades investigated. The complex viscosity and the transient elongational viscosity increased by up to two decades. Both the activation energy of flow and the loss angle indicate long‐chain branching. A more quantitative assessment of the extent of strain hardening was achieved by application of the molecular stress function (MSF) model. The two material parameters of the model revealed different behaviors depending on the chain extender used. The initial molar mass of PET and the concentration of end groups, i.e., hydroxyl and carboxyl, determine the structure of the polymer molecules. PMDA proved to be an excellent coupling agent for industrial processing which induces reproducibly either a star‐like, comb‐like, or randomly branched structure depending on the concentration of coupling agent added and the hydroxyl concentration of the PET employed. TGDDM led to a hyperbranched structure. POLYM. ENG. SCI., 59:396–410, 2019. © 2018 Society of Plastics Engineers

Molar mass dependence of structure of xanthan thermally denatured and renatured in dilute solution

Double helical polysaccharide, xanthan samples with varying molar mass were thermally denatured and renatured in dilute solutions. Both the weight average molar mass, which was determined by size exclusion chromatography (SEC), and viscosity average molar mass decreased after denaturation and renaturation for samples with an initial molar mass of 106 g mol−1, but those for the samples with an initial molar mass of 105 or 107 g mol−1 decreased only slightly. Although the double helices of xanthan only partially dissociated into single chains, the molar mass distribution estimated by SEC did not broaden drastically by the denaturation and renaturation. These results can be explained by the formation of hairpin structures by the single chains, which are restricted to xanthan with a molar mass of 105 g mol−1 by the strain of the hairpin loop, and for xanthan molar masses of 107 g mol−1 by the incomplete dissociation of its long polymer chains. Double helical polysaccharide, xanthan samples with varying molar mass were thermally denatured and renatured in dilute solutions. The molar mass decreased after denaturation and renaturation for samples with an initial molar mass of 106 g mol−1, but those for the samples with an initial molar mass of 105 or 107 g mol−1 decreased only slightly. A model explaining this molar mass dependence of the denaturation and renaturation behaviors was proposed based on the experimental results.

Polyethylene loss of ductility during oxidation: Effect of initial molar mass distribution

This paper reports a study of thermal oxidation induced embrittlement in several polyethylene grades differing mainly by the broadness of the molar mass distribution (ranging for lower than 3 to more than 30). Thermal oxidation was monitored at macromolecular scale (Gel Permeation Chromatography, Differential Scanning Calorimetry) and macroscopic scale (tensile tests). As expected, the samples undergo predominant chain scission and plastic deformation is suppressed below a critical molar mass value (M’C). Even though this latter was previously reported to be independent of the initial weight average molar mass, it is shown here that it depends on initial polydispersity index. Samples were also shown to undergo chemicrystallization, i.e. that segments released by chain scissions migrate into the crystalline phase with a yield increasing with initial polydispersity index. Finally, the main novelty of this work is to evidence that the previously proposed end-of-life criteria at macromolecular level linked to loss of ductility (critical molar mass, crystallization yield) depend on the initial polydispersity index.

Hydrolytically degradable poly(ethylene glycol) based polycarbonates by organocatalyzed condensation

Poly(ethylene glycol) is one of the most used stealth polymer in the field of polymer-based drug delivery. In spite of its excellent biocompatibility, high molecular poly(ethylene glycol) can be accumulated in the body, in contrast to low molar mass one. This is due to the non-degradability of poly(ethylene glycol) which can cause some negative side effects. In order to overcome this drawback, in this work we present new degradable high molecular poly(ethylene glycol) by simply incorporating carbonate groups between poly(ethylene glycol) units. Thus, poly(ethylene glycol) based polycarbonates have been synthesized by polycondensation of dimethyl carbonate and poly(ethylene glycol) of four different low molar masses; 600, 1000, 1500 and 2000gmol−1. Optimum bulk polycondensation reactions were run in two steps in the presence of 4-dimethylaminopyridine organocatalyst. By this method, poly(ethylene glycol) based polycarbonates were prepared and characterized showing molar masses between 10 and 35kgmol−1 and crystallinity was increased compared to the initial low molar mass poly(ethylene glycol)s. The hydrolytic degradation of the poly(ethylene glycol) based polycarbonates was investigated in vitro using a phosphate buffer solution at pH = 7.2. The results showed that the initial poly(ethylene glycol)-polycarbonates hydrolyzed by the carbonate units showing a decrease of molar mass as a function of time. It was found that all polycarbonates underwent almost complete hydrolysis after 50days leading to the initial low molar mass poly(ethylene glycol) units that are bellow renal clearance threshold.

Investigation of polyamide 11 embrittlement during oxidative degradation

Embrittlement processes occurring during thermal oxidation are investigated for stabilized and unstabilized polyamide 11 samples differing by their thicknesses and initial molar masses. Tensile tests were carried out in the temperature range between room temperature and 110 °C in order to investigate the influence of mechanical testing temperature on the embrittlement coordinates. In the same time, molar mass and crystalline morphology are monitored by size exclusion chromatography (SEC) and DSC/SAXS measurements respectively. The experimental results point out the existence of a critical molar mass for ductile-brittle transition M′c about 10 kg mol−1, independent of sample initial molar mass or stabilization, but depending on tensile testing temperature. However, even if oxidation chain scissions are shown to be clearly responsible for the loss of mechanical properties at failure, the structure-property relationships governing ductile-brittle transition require a mixed criterion involving molar mass and crystalline morphology, especially the interlamellar distance. For this purpose, specific molar mass – crystalline morphology relationships are investigated.

Thermal oxidation kinetics of additive free polyamide 6-6

Thermal aging of an additive free PA 6-6 has been elucidated at 90, 100, 120, 140, 150 and 160 °C in air-ventiled ovens by Fourier transform infrared spectrophotometry, viscosimetry in molten state and uniaxial tensile testing. Oxidation of methylene groups starts after a considerably shorter induction period but reaches a lower maximal rate than in additive free PE. Cleavage of C–N bonds constitutes the main source of chain scissions. It leads to the formation of aldehyde chain-ends and a catastrophic decrease in molar mass. Embrittlement occurs at a very low conversion ratio of the oxidation process, in particular when the concentration of aldehyde chain-ends reaches a critical value of [PH=O]F ≈ 5.6 10−3 mol l−1, corresponding to a critical value of the number average molar mass of MnF ≈ 17 kg mol−1. At this stage, the entanglement network in the amorphous phase is deeply damaged.A non-empirical kinetic model has been derived from the oxidation mechanistic scheme previously established for PE, but improved by adding elementary reactions specific to polyamides such as the rapid decomposition of unstable hydroxylated amide groups. This model describes satisfyingly the main features of the thermal oxidation kinetics of PA 6-6, but also of other types of aliphatic polyamides studied previously in the literature such as: PA 6, PA 12 and PA 4-6, as long as it is not controlled by oxygen diffusion. At the same time, it confirms the existence of an universal character for the thermal oxidation kinetics of aliphatic polyamides whatever their origin, i.e. their initial molar mass, crystallinity ratio, concentration of impurities, structural irregularities, etc.

Comparative rheological study of ionic semi-IPN composite hydrogels based on polyacrylamide and dextran sulphate and of polyacrylamide hydrogels

Ionic semi-interpenetrating polymer networks composite hydrogels were synthesized by free-radical polymerization using dextran sulphate (DxS), acrylamide as monomer and N,N′-methylene(bis)acrylamide as cross-linking agent. The viscoelastic properties of these composite hydrogels were investigated by oscillatory shear measurements under small deformation conditions comparative with those of polyacrylamide gels. Changes of the rheological properties of composite hydrogels have been studied in terms of polymerization temperature, cross-linker ratio, initial monomer concentration and molar mass of DxS. The results showed that the stability of the composite hydrogels obtained at room temperature (22 °C) was relatively low because the storage modulus (G′) was only eight times higher than the loss modulus (G″), while for those obtained by cryopolymerization (−18 °C), the stability was improved, the G′ values being about 30 times higher than those of G″. This behaviour indicated that, by conducting the synthesis of hydrogels below the freezing point of the reaction solutions, an enhancement of the hydrogels elasticity was achieved. The network parameters, i.e. the average molecular weight between two cross-links and the cross-link density of the composite hydrogels prepared at −18 °C, were estimated from rheological data.

Thioketone‐Mediated Polymerization with Dithiobenzoates: Proof for the Existence of Stable Radical Intermediates in RAFT Polymerization

A novel dithioester control agent [dimethyltetrathioterephtalate (DMTTT)] is presented for the thioketone-mediated radical polymerization (TKMP) of n-butyl acrylate. The rate of polymerization is significantly decreased in the presence of DMTTT indicating formation of dormant radical species. During polymerization, molar masses increase linearly with monomer conversion with reasonably narrow initial molar mass distributions (PDI between 1.3 and 1.8), whereas the dispersity increases during the course of the polymerization due to irreversible termination of both propagating and dormant radicals. The present results thus highlight the possibility of a mixed mechanism operating in RAFT polymerization, which combines slow fragmentation (long-lived intermediates) and intermediate radical termination.

Random Scission of Polymers: Numerical Simulations, and Experiments on Hyaluronan Hydrolosis

We present a study of the random scission process of polymers, in which numerical simulations are com- bined with experimental investigations into the acid hydrolysis ofthepolysaccharidehyaluronan(HA),inparticularatpH1.1and 50 � C. As input for the simulations, an initial molecular weight distribution (MD(0)) and the hydrolysis rate constant (kh) are needed.The firstwasobtainedusingagarosegelelectrophoresis followed by densitometric analyses, by which we determined the evolution of the full molar-mass distribution during hydro- lysis. The rate constant was experimentally obtained by two independent techniques. Kinetic plots were obtained from both the number- and the weight-average molar mass (Mn and Mw, respectively), converted to degrees of polymerization (Nn and Nw, respectively),andfromthese,khvaluesforHAhydrolysiswerederived.Forconfirmation,khwasalsodeterminedfromtheevolution oftheconcentrationofreducingchainends.Experimentallydeterminedkhvalueswereusedasinputforthemodel,andtheevolution of MD during hydrolysis was simulated for various hypothetical MD(0) functions as well as for an experimentally determined MD(0) curve.Agreementbetweenexperimentalandsimulatedresultsdemonstratedconsistencyofthemodelandindicatesthatacidhydrolysisof hyaluronan can indeed be considered a random scission process. We show that for cases where the initial molar mass distribution is notoftheKuhn-type—whichisoftenthecase—thevalueforthekhdeterminedfromtheevolutionofMnismoreaccuratethanthat obtained from Mw.

Study of gelatin renaturation in aqueous solution by AFlFFF–MALS: Influence of a thermal pre-treatment applied on gelatin

Asymmetrical Flow Field-Flow Fractionation coupled to Multi-Angle Light Scattering has been applied to study and understand the renaturation phenomenon of gelatin in dilute aqueous solutions. Renaturation behaviour has been studied on a native gelatin powder and compared to the same gelatin, pre-heated at 75 °C. An increase of molar mass, due to gelatin chains association at room temperature was observed for native gelatin. AFlFFF–MALS analysis allowed to follow α, β, γ chains association and the occurrence of other fractions with high molar mass. This process was absent in pre-heated gelatin which suggested that pre-heating process could lead to interactions between gelatin chains which limited renaturation process in aqueous solutions. For pre-heated gelatin samples, the average initial molar mass was higher and hardly changed during incubation of a dilute solution.

Abiotic and biotic degradation of oxo-biodegradable foamed polystyrene

An alternative for improving the degradability of polyolefins and polystyrene is the addition of pro-oxidant substances to their formulations. The materials obtained are then called oxo-biodegradable. This work aimed to assess the biotic and abiotic degradation of atactic polystyrene (PS), utilising as test material foamed PS plates used in the manufacture of trays, formulated with Co- and Mn-based pro-oxidant additives. The plates were exposed to artificial weathering (ultraviolet radiation and heat) and were periodically analysed for changes in structural properties. The oxidised surface residues detached from the samples were incubated in a stabilised compost of urban waste (58 °C) or in an aqueous mineral medium (25 °C), the latter being inoculated with urban waste compost and also with a microbial consortium. It was found that the molar masses of the eroded materials from the pro-oxidant activated samples were significantly lower than the initial sample molar masses, with simultaneous incorporation of oxygen into the chains during the accelerated weathering. These samples underwent biodegradation and gave mineralisation values of 2–5% over 2–3 months of incubation in compost and perlite or in mineral aqueous medium. Biodegradation of the residues from the samples not containing pro-oxidant additives was also observed, but at levels which were lower than those obtained for oxo-biodegradable samples.

Relations between high‐temperature mastication and Mooney viscosity relaxation in natural rubber

AbstractMastication of natural rubber (NR) is undertaken as a preliminary step towards the preparation of NR‐based vulcanizates, a process during which the elastomer is broken down to a homogeneous matrix of lower viscosity. Several tests and indicators are in use for characterizing the behavior of elastomers but these have mostly been adopted for the nonmasticated product. This study uses coefficients generated from modeled Mooney relaxation data, as indicators of elasticity, to examine the effect of high‐temperature mastication on the processability of the masticated rubber. Some derived coefficients such as the terminal relaxation time (τ) from Maxwell's triexponential model, the elastic component (a) from Wu‐Abbott model [Y = 1 + a* ln(t) − bt/(c + t)], and the constant (b) from the Power law model (Y = at−b), adequately characterized the effect of mastication on NR. Although the NR grades studied were quite different with respect to their initial molar mass distributions, they followed a similar response to the mechanical models before and after mastication, indicating therefore that mastication decomposes to a similar extent, the various components (long isoprene chains, densely crosslinked solvent‐insoluble gel, etc.) that account for the viscoelastic behavior of the raw elastomer. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Thermooxidative aging of polyoxymethylene, part 2: Embrittlement mechanisms

AbstractThe thermal oxidation at 130°C of polyoxymethylene homopolymer (H) and copolymer (C) samples of close initial molar masses has been studied by gravimetry, rheometry, X ray scattering, and tensile testing. Both samples undergo random chain scission and depolymerization. Their crystallinity ratio increases, whereas their long period decreases. All these changes are faster for H than for C. Tensile tests reveal that there is no significant change of behavior law except for the ultimate strain, which decreases abruptly when the weight average molar mass reaches a value of the order of 70 kg mol−1 for H and 90 kg mol−1 for C. At these molar masses values, the entanglement network of the amorphous phase has undergone only small damagement. In contrast, a chemi‐crystallization process has induced significant morphological changes, especially a decrease of the interlamellar thickness. It is suggested that this latter phenomenon could be responsible for embrittlement. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009