High Molar Mass Fraction Research Articles

Changes in chemical structures and molar mass parameters of birch wood powder by ethylene diamine treatment

It is necessary to delignify wood samples and treat them with ethylene diamine (EDA) before they are dissolved in 8% (w/v) lithium chloride/N,N-dimethylacetamide (LiCl/DMAc) prior to size-exclusion chromatography (SEC) analysis. In the present study, the effects of delignifying birch wood powder 0–3 times with NaClO2 and subsequently treating it with EDA on its solubility in 8% (w/v) LiCl/DMAc and its SEC data. The neutral sugar composition of the birch powder was almost unaffected by either delignification or treatment with EDA. Treatment of the birch powder with EDA resulted in 28% solubilities in 8% (w/v) LiCl/DMAc. Approximately 11% of cellulose molecules in the birch wood powder was dissolved, and detected as a high-molar-mass (HMM) fraction in the SEC elution pattern. Each single delignification treatment increased the solubility in 8% (w/v) LiCl/DMAc to 68–74% after EDA treatment. Based on the glucose contents of the delignified samples, almost all cellulose molecules in the delignified samples were dissolved in 8% (w/v) LiCl/DMAc after EDA treatment, and detected as the HMM fractions in the SEC elution patterns. The HMM cellulose molecules in the EDA-treated birch powder had linear random-coil conformations in 1% (w/v) LiCl/DMAc. However, the SEC data suggest that there probably were some chemical linkages between the HMM cellulose molecules and lignin or NaClO2-treated lignin fragments in the HMM fractions.

Reverse Engineering of Chemically Similar Bimodal High Density Polyethylenes: A Comprehensive Study Using Advanced Chromatographic Techniques

AbstractBimodal high‐density polyethylene (bHDPE) is a complex, multicomponent polyethylene (PE) material whose synthesis in a multistage process can be challenging. Three bHDPEs with good and bad end‐use properties are reverse engineered using advanced analytical techniques. Average chemical composition is determined using 13C NMR and 1‐butene is identified as the comonomer for the good resin (bHDPE 1) while 1‐hexene is the comonomer in bHDPE 2 and 3. The presence of comonomer in the high molar mass fractions of the samples is shown using high‐temperature triple‐detection size exclusion chromatography (HT‐SEC‐d3). Chemical composition separation using high‐temperature interaction chromatography (HT‐IC) is achieved using porous graphitic carbon (PGC) and silica stationary phases. Some problems in temperature gradient interaction chromatography (TGIC) on PGC are overcome by using a non‐adsorptive stationary phase, enabling better separation and visualization of homopolymer and copolymer components. Coupling HT‐SEC in 2D liquid chromatography (2D‐LC) analyses at high temperatures reveals the presence of a larger copolymer component in bHDPE 1 at high elution volume. In contrast, bHDPE 2 and bHDPE 3 have copolymer components at low elution volumes, indicating poor comonomer distribution in the copolymer component which ultimately explains the poor mechanical properties at similar comonomer contents.

Acacia gums new fractions and sparkling base wines: How their biochemical and structural properties impact foamability?

Foam is the first attribute observed when sparkling wine is served. Bentonite is essentially used to flocculate particles in sparkling base wines but can impair their foamability. Gums from Acacia senegal and Acacia seyal improved the foamability of different bentonite-treated base wines. Our main goal was to see how the supplementation with new fractions separated from Acacia gums by Ion Exchange Chromatography affected foamability of sparkling base wines, deepening the relation between foam behavior and characteristics of wine and gums. High molar mass fractions increased the maximal foam height and the foam height during the stability period in, respectively, 11 out and 8 out of 16 cases (69% and 50%, respectively). The properties of the supplementing gums fractions obtained by IEC and, although to a minor extent, the wine characteristics, affected positively and/or negatively the foam behavior. Wine foamability also depended on the relationship between wine and gums fractions properties.

Investigating the role of the different molar mass fractions of a pectin rich extract from onion towards its emulsifying and emulsion stabilizing potential

A pectin rich nitric acid extract produced from onion flesh (NA) showed promising emulsion stabilizing potential. At both pH 2.5 and pH 6.0, emulsions produced with the NA sample as sole emulsifying and emulsion stabilizing compound displayed stable oil droplet size distributions. Moreover, the emulsions showed good stability against creaming. Interestingly, the pectin rich acid extract was characterized by a bimodal molar mass distribution which led to the research objective to investigate the possible role of both polymer fractions in the promising emulsion stabilizing potential. To investigate this, both fractions were separated using centrifugal ultrafiltration.Structural characterization revealed the presence of mainly short chain galactans in the low molar mass fraction (NA-LMM) while the high molar mass fraction (NA-HMM) consisted mainly of pectin. Rheological analysis clearly demonstrated the viscosity increasing effect of NA-HMM in solution while the viscosity of a NA-LMM solution at equal concentration was comparable to pure water. Emulsions produced with NA-LMM destabilized quickly during refrigerated storage by extensive coalescence and creaming at pH 2.5. Despite presenting a somewhat better stability at pH 6, also in this case fast destabilization was observed. Contrarily, NA-HMM showed promising emulsion stabilizing potential. Oil droplet size distributions remained stable during storage at both pH 2.5 and pH 6.0. Creaming was observed for NA-HMM at pH 6.0 although to a lesser extent as for NA-LMM stabilized emulsions. Based on these results, it can be concluded that NA-LMM does not contribute to the emulsion stabilizing functionality of NA. Promising functionality is mainly attributed to the high molar mass polymers.

Tailored Thermosetting Wood Adhesive Based on Well-Defined Hardwood Lignin Fractions

By aiming at tailoring the bonding strength of a thermosetting lignin-containing phenol-formaldehyde (LPF) wood adhesive, different fractions of an industrial hardwood alkaline lignin have been prepared through sequential solvent fractionation (i-PrOH, EtOH, and MeOH). Those fractions were comprehensively characterized by GPC, GC, Py/GC–MS, and NMR techniques. Lignin fractions with low molar mass and narrow dispersity, including the i-PrOH-soluble and EtOH-soluble ones, were of high purity and had more reactive sites for LPF adhesive synthesis and better accessibility due to lower degree of condensation than the high molar mass ones. Some recalcitrance of integrating high molar mass fractions covalently into the PF adhesive was observed, which was also true in the case of lignin phenolation. The tailored bonding strength of the LPF adhesive, tested by gluing wood pieces, provided strong evidence for molecular structure–performance correlation; the i-PrOH-PF had the lowest activation energy, the highest curing enthalpy, and the strongest bonding strength of 2.16 MPa. This study demonstrates a clear structure–property-application relationship of technical hardwood lignin in the LPF adhesive field, which might pave the way for a more effective bulk valorization.

Selective adsorption of soil humic acid on binary systems containing kaolinite and goethite: Assessment of sorbent interactions

Inorganic colloids are ubiquitous in soils and coexist as simple mixtures or as strong associations of particles. The coexisting inorganic colloids with a variety of active sites can selectively adsorb humic substances and thus affect the availability and mobility of organic matter. To investigate the selective adsorption of organic matter by coexisting inorganic colloids, humic acid (JGHA) extracted from soil, a kaolinite and goethite synthetic association (KGA) and a kaolinite and goethite mixture (KGM) were prepared and studied. With the adsorption expressed in μmol m−2, the adsorption affinity of JGHA on KGA (initial slope of isotherms) was slightly higher than that on KGM, although KGM could adsorb more JGHA molecules at larger JGHA concentrations. At pH 4, JGHA was adsorbed on the kaolinite edges and goethite through electrostatic attraction and ligand exchange, and on the kaolinite siloxane plates through hydrophobic interaction. At pH 6 and 8, the adsorption occurred only on the kaolinite siloxane plates and goethite. The overall average adsorption Gibbs energies were moderately negative, and comprised a strongly negative average overall molar entropy and a negative average overall molar entropy contribution. Thus, overall JGHA adsorption on KGA and KGM was enthalpy controlled. The large exothermic enthalpy gains and entropy losses were a result of electrostatic attraction and ligand exchange. The entropy gains because of hydrophobic attraction between JGHA and the kaolinite siloxane plates are masked by the larger entropy loss of adsorption on the hydrophilic surfaces. The fractions of JGHA with molar mass >~12.9 kDa (1 Da = 1 g mol−1) were adsorbed selectively by KGM, whereas the fractions <~12.9 kDa were selectively adsorbed by KGA. High molar mass and hydrophobic fractions adsorb selectively on the kaolinite siloxane plates, whereas low molar mass fractions with hydrophilic groups adsorb selectively on goethite and the kaolinite edges.Highlights Selective adsorption behaviour of HA on binary systems containing kaolinite and goethite. High molar mass and hydrophobic HA fractions selectively adsorb on kaolinite siloxane plates. Low molar mass and hydrophilic HA fractions selectively adsorb on kaolinite edges and goethite. Binding affinity of HA to KGA is stronger than to KGM; mineral associations are more beneficial for fixing OM in soil.

Molecular Dynamics Simulations of Short-Chain Branched Bimodal Polyethylene: Topological Characteristics and Mechanical Behavior

It has previously been shown that polyethylene (PE) with a bimodal molar mass distribution has a high fracture toughness. Our approach has been to use coarse-grained (CG) molecular dynamics (MD) simulations to investigate the effects of including short-chain branches in the high molar mass fraction of bimodal PE on topological features and mechanical behavior of the material. The CG potentials were derived, validated, and utilized to simulate melt equilibration, cooling, crystallization, and mechanical deformation. Crystallinity, tie chain, and entanglement concentrations were continuously monitored. During crystallization, the branched bimodal systems disentangled to a lesser degree and ended up with a higher entanglement density than the linear bimodal systems simulated in our previous study. The increase in entanglement concentration was proportional to the content of the branched high molar mass fraction. A significantly higher tie chain concentration was obtained in the short-chain branched bimodal s...

Molecular dynamics simulation of linear polyethylene blends: Effect of molar mass bimodality on topological characteristics and mechanical behavior

Blending different molar mass fractions of polyethylene (PE) in order to obtain materials with higher fracture toughness has previously proven beneficial. Our approach has been to use coarse-grained (CG) molecular dynamics (MD) simulations to obtain semicrystalline polyethylene systems on a nanoscale, and then draw them in order to mimic tensile testing. The CG potentials were derived, validated and utilized to simulate melt equilibration, cooling, crystallization and mechanical deformation. Crystallinity, tie chain and entanglement concentrations were continuously monitored. During crystallization, the low molar mass fraction disentangled to a greater degree and ended up with a lower entanglement density than the high molar mass fraction, although the tie chain concentration was higher for the low molar mass fraction. The deformation behavior of semicrystalline PE above its glass transition temperature was then assessed in a uniaxial tensile deformation simulation. The low-strain mechanical properties (i.e. elastic modulus, yield stress and strain) were in accordance with the literature. The high-strain mechanical features and toughness were improved in bimodal systems. The presence of a high molar mass fraction in bimodal systems was shown to affect the crystallinity and tie chain concentration during the strain hardening, leading to tougher model systems. Finally, the bimodal system with equal shares of the molar mass fractions showed the highest toughness and the best ultimate mechanical properties while having a concentration of tie chains and entanglements intermediate between the values for the other systems. This was a clear sign of the non-exclusive role of tie chains and entanglements in the mechanical behavior of bimodal PE and more generally of semicrystalline polymers at high strains.

Ball Milling's Effect on Pine Milled Wood Lignin's Structure and Molar Mass.

The effect of ball milling expressed as the yield of milled wood lignin (MWL) on the structure and molar mass of crude milled wood lignin (MWLc) preparation is studied to better understand the process’ fundamentals and find optimal conditions for MWL isolation (i.e., to obtain the most representative sample with minimal degradation). Softwood (loblolly pine) MWLc preparations with yields of 20–75% have been isolated and characterized based on their molar mass distribution (by Size Exclusion Chromatography (SEC)), hydroxyl groups of different types (31P NMR), methoxyl groups (HS-ID GC-MS), and sugar composition (based on methanolysis). Classical MWL purification is not used to access the whole extracted lignin. The results indicate that lignin degradation during ball milling occurs predominantly in the high molar mass fraction and is less pronounced in the low molar mass fraction. This results in a significant decrease in the Mz and Mw of the extracted MWLc with an increase in the yield of MWLc, but has only a very subtle effect on the lignin structure if the yield of MWLc is kept below about 55%. Therefore, no tedious optimization of process variables is necessary to achieve the required MWLc yield in this range for structural studies of softwood MWL. The sugar composition shows higher amounts of pectin components in MWLs of low yields and higher amounts of glucan and mannan in high-yield MWLs, confirming that lignin extraction starts from the middle lamella in the earlier stages of MWL isolation, followed by lignin extraction from the secondary wall region.

Deconstruction of hybrid poplar to monomeric sugars and aromatics using ethanol organosolv fractionation

Acidic ethanol organosolv fractionation of hybrid poplar was investigated to determine the impact of pretreatment conditions on the resulting biomass and lignin properties and to assess the subsequent deconstruction of the cell wall biopolymers to monomeric sugars and aromatics. It was found that increasing reaction severity (i.e., time and temperature) during the organosolv fractionation increased the rate of delignification and xylan solubilization while the lignins recovered from the liquors were found to exhibit lower degrees of polymerization. Glucose hydrolysis yields > 75% at moderate enzyme loadings (30 mg/g glucan) could be obtained for the more severe pretreatment conditions. The lignins recovered from the pretreatment liquors were subjected to fractionation using a sequential extraction with solvents of increasing polarity. It was found that the low molar mass, low polydispersity lignins increased in pretreatment liquors with increasing time and temperature and were concentrated in the methanol fraction while a high molar mass fraction was extracted with the diethyl ether. We hypothesize that the extraction of the high molar mass fraction with diethyl ether is due to partial ethyl O-alkylation of lignin hydroxyl groups during pretreatment, rendering lignins more soluble in the non-polar solvent. Finally, depolymerization of unfractionated lignins by thioacidolysis resulted in mass yields of aromatic monomers ranging from 80 to 157 mg monomer per gram of lignin and that these yields exhibited strong positive correlations to the lignin β-O-4 content, molar mass, and strong negative correlations to the pretreatment temperature.

Characterization of high-molar-mass fractions in a Scots pine (Pinus sylvestris L.) knotwood ethanol extract

AbstractThe identification of compounds in Scots pine (Pinus sylvestrisL.) knotwood (KnW), obtained by extracting with hydrophilic organic solvents, has been performed previously almost exclusively by gas chromatography-mass spectrometry (GS-MS) equipped with long GC columns (≥25 m). That means that the molar mass (MM) of the majority of the identified compounds was below 500–600 Da, and the analytical data accounted for only about half of the dry extract weight. In the present work, high-molar-mass (HMM) fractions in a Scots pine KnW-EtOH extract were isolated and chemically characterized by means of several advanced analytical techniques, such as high performance size-exclusion chromatography-evaporative light scattering detector (HPSEC-ELSD), high performance liquid chromatography (HPLC)-electrospray ionization-ion trap-mass spectrometry [(HPLC)ESI-IT-MS], ESI-quadrupole time of flight-mass spectrometry (QTOF-MS), pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS), thermally assisted hydrolysis and methylation-gas chromatography-mass spectrometry (THM-GC-MS), nuclear magnetic resonance (NMR) and GC-MS. The results indicate that the MM maxima of the HMM fractions ranged from approximately 500 to 2200 Da, and that the compounds consist mainly of oligomers of hydroxylated resin acids (RAs), especially dehydroabietic acid, but also of fatty acids (FAs), stilbenes and sterols. A large number of RA dimers were tentatively identified in the HMM fractions. However, it remains unknown how the monomer units are linked together, as it was not possible to isolate a RA dimer fraction pure enough for NMR characterization. RA dimers in native KnW have not been identified previously.

Investigation of stability of branched structures in softwood cellulose using SEC/MALLS/RI/UV and sugar composition analyses

Size-exclusion chromatography using multi-angle laser-light scattering, refractive index, and ultraviolet absorption (SEC/MALLS/RI/UV) detection was applied to Japanese cedar (JC) and eucalyptus (E) powders after delignification (D), extraction with 4% NaOH (H), and acid hydrolysis (A), with different sequences of the D, H, and A treatments. All the delignified wood samples were dissolved in 8% (w/w) LiCl/N,N-dimethylacetamide (DMAc) after ethylenediamine pretreatment. The wood sample/LiCl/DMAc solutions were diluted to 1% (w/v) LiCl/DMAc and subjected to SEC/MALLS/RI/UV analysis. The cellulose molecules in the high-molar-mass (HMM) fractions of the eucalyptus samples, i.e., E-D, E-DHA, and E-ADH, were linear polymers with random coil conformations. In contrast, the cellulose molecules in the HMM fractions of the Japanese cedar samples, i.e., JC-D, JC-AD, JC-DHA, and JC-D-α-cellulose (prepared from JC-D by soaking in 17.5% NaOH), had branched structures. However, the JC-ADH HMM fraction contained no branched structures. The results of SEC/MALLS/RI/UV and neutral sugar composition analyses of the Japanese cedar samples showed the presence of chemical linkages between cellulose and glucomannan molecules through lignin or lignin fragments. The stability of the cellulose/glucomannan linkages in the Japanese cedar holocellulose was investigated based on the results for samples prepared using different sequences. The obtained results were consistent with those of SEC/MALLS/RI analysis of softwood acid-sulfite and kraft pulps; softwood kraft pulp has branched structures in the HMM fraction, whereas softwood acid-sulfite pulp has no such branched structures.

Characterization of polymeric substance classes in cereal-based beverages using asymmetrical flow field-flow fractionation with a multi-detection system.

Cereal-based beverages contain a complex mixture of various polymeric macromolecules including polysaccharides, peptides, and polyphenols. The molar mass of polymers and their degradation products affect different technological and especially sensory parameters of beverages. Asymmetrical flow field-flow fractionation (AF4) coupled with multi-angle light scattering (MALS) and refractive index detection (dRI) or UV detection (UV) is a technique for structure and molar mass distribution analysis of macromolecules commonly used for pure compound solutions. The objective of this study was to develop a systematic approach for identifying the polymer classes in an AF4//MALS/dRI/UV fractogram of the complex matrix in beer, a yeast-fermented cereal-based beverage. Assignment of fractogram fractions to polymer substance classes was achieved by targeted precipitations, enzymatic hydrolysis, and alignments with purified polymer standards. Corresponding effects on dRI and UV signals were evaluated according to the detector's sensitivities. Using these techniques, the AF4 fractogram of beer was classified into different fractions: (1) the low molar mass fraction was assigned to proteinaceous molecules with different degrees of glycosylation, (2) the middle molar mass fraction was attributed to protein-polyphenol complexes with a coelution of non-starch polysaccharides, and (3) the high molar mass fraction was identified as a mixture of the cell wall polysaccharides (i.e., β-glucan and arabinoxylan) with a low content of polysaccharide-protein association. In addition, dextrins derived from incomplete starch hydrolysis were identified in all fractions and over the complete molar mass range. The ability to assess the components of an AF4 fractogram is beneficial for the targeted design and evaluation of polymers in fermented cereal-based beverages and for controlling and monitoring quality parameters.

Chemical characterization of high-molar-mass fractions in a Norway spruce knotwood ethanol extract

The low-molar-mass (LMM) fraction, only, i.e., the GC-eluting compounds, which are mainly lignans, has been characterized in Norway spruce knotwood hydrophilic extracts previously. Of this fraction, many lignans and sesquilignans and all GC peaks supposedly representing dilignans remain unidentified. In this work, dilignans and the GC non-eluting compounds (the high-molar mass fractions, HMM) were characterized in a 7-hydroxymatairesinol-reduced knotwood ethanol extract of Norway spruce by using several fractionation and analytical techniques. A methyl tert-butyl ether (MTBE) insoluble fraction of the extract contained mainly HMM material, of which the main part was shown to consist of lignan oligomers. The oligolignans (with a molar mass up to approximately 3700 Da) seemed to be linked by 55′ bonds, some of them containing one or two guaiacylglycerol ether units linked to the lignan by βO4 or β5 bonds. Several oligolignans were identified or tentatively identified. The MTBE soluble fraction, which accounted for the major part (81%) of the extract, contained mainly LMM material (lignans, sesqui- and dilignans). The part of the HMM material in the MTBE soluble fraction that was easily isolable (2%) seemed to contain polymers of fatty acids and alcohols, resin acids, and sterols.

Multicyclic Polyesters of Trimesic Acid and Alkanediols and the Theory of Network Formation

Trimesoyl chloride is polycondensed with various α,Ώ-alkanediols in dichloromethane at different concentrations using equifunctional feed ratios. As evidenced by MALDI-TOF (matrix assisted laser desorption/ionization-time of flight) mass spectrometry the soluble reaction products mainly consist of perfect multicyclic oligomers and polymers. The solphase extracted from the gels also consists of perfect multicycles. SEC (size exclusion chromatography) measurements show that both soluble reaction products and extracted solphases also contain a high molar mass fraction of perfect and nonperfect multicycles extending up to masses beyond 105 g mol−1. When the polycondensation is stopped after a few minutes perfect multicycles are already detectable in the reaction mixture along with functional (multi)cyclic oligomers. These results prove that at initial monomer concentrations < 0.2 mol L−1 networks and large multicyclic polymers are synthesized from functional cyclic oligomers formed in early stages of the polycondensation and not from hyperbranched polymers. This interpretation is presented as “egg-first theory” and compared with the “hen-first theory” of Stockmayer and Flory.

Aggregates of Chemically Functionalized Multiwalled Carbon Nanotubes as Viscosity Reducers.

Confinement and surface effects provided by nanoparticles have been shown to produce changes in polymer molecules affecting their macroscopic viscosity. Nanoparticles may induce rearrangements in polymer conformation with an increase in free volume significantly lowering the viscosity. This phenomenon is generally attributed to the selective adsorption of the polymer high molar mass fraction onto nanoparticles surface when the polymer radius of gyration is comparable to the nanoparticles characteristic dimensions. Carbon nanotubes seem to be the ideal candidate to induce viscosity reduction of polymer due to both their high surface-to-volume ratio and their nanometric sizes, comparable to the gyration radius of polymer chains. However, the amount of nanotube in a polymer system is limited by the percolation threshold as, above this limit, the formation of a nanotubes network hinders the viscosity reduction effect. Based on these findings, we have used multiwalled carbon nanotubes MWCNT “aggregates” as viscosity reducers. Our results reveal both that the use of nanotube clusters reduce significantly the viscosity of the final system and strongly increase the nanotube limiting concentration for viscosity hindering. By using hydroxyl and carboxyl functionalized nanotubes, this effect has been rather maximized likely due to the hydrogen bridged stabilization of nanotube aggregates.

Characterization of Chemical Composition along the Molar Mass Distribution in Polyolefin Copolymers by GPC Using a Modern Filter‐Based IR Detector

SummaryGel permeation chromatography (GPC), also known as size exclusion chromatography (SEC), is the technique routinely used at high temperature to analyze the molar mass distribution in polyolefins. The distribution of comonomer along the molar mass distribution in a copolymer is a key microstructural feature that determines the macroscopic properties of the material, and thus, its range of possible applications and performance. The direct coupling of a modern filter‐based infrared (IR) detector to a high temperature GPC instrument, by means of a heated flow‐through cell, is here described. The analyses are carried out by recording the continuous IR absorbance chromatograms at selected bands, which show different sensitivities to the different monomer units. A slice‐by‐slice ratio of the IR bands is further calculated to determine the average chemical composition of each molar mass fraction, after GPC separation. The high sensitivity of this IR detection method allows the injection of low concentrations of sample and the use of standard GPC analysis conditions, so that chromatographic quality is not compromised even in cases where very high molar mass fractions are present. The analysis of comonomer variations along the molar mass distribution in polyolefin copolymers is discussed. Selected applications of the method to polyethylene and poly(propylene) copolymers are presented.

Importance of Multi‐Angle Light Scattering in Polyolefin Characterization

SummaryThe paper reviews basic principles and applications of multi‐angle light scattering (MALS) employed as a detector for high temperature size exclusion chromatography (SEC). The presented MALS data were acquired with state‐of‐the‐art MALS instrumentation. A special attention is paid to the specific elution behavior of branched polyolefins during their SEC separation, to the characterization of long chain branching, and to overcoming the limitations arising from abnormal SEC elution. The MALS detection is also presented as highly reproducible method of the determination of molar mass averages and extremely sensitive technique for the detection of high molar mass fractions.

Increase of long-chain branching by thermo-oxidative treatment of LDPE: Chromatographic, spectroscopic, and rheological evidence

Low-density polyethylene was thermo-oxidatively degraded at 170 °C, i.e., degraded in the presence of air, by a one thermal cycle (1C) treatment during times between 30 and 90 min, and by a two thermal cycles (2C) treatment, i.e., after storage at room temperature, an already previously degraded sample was further degraded during times between 15 and 45 min. Characterization methods include gel permeation chromatography (GPC), Fourier transform infrared (FTIR) spectroscopy, as well as linear and nonlinear rheology. A reduction of molar mass was detected for all degraded samples by GPC, as well as an increase of the high molar mass fraction of the 1C sample degraded for the longest time. Intrinsic viscosity measurements indicate also a reduction of molar mass with increasing degradation times for both 1C and 2C samples. Thermo-oxidation is confirmed for 1C and 2C samples by analyzing specific indices in FTIR. Linear viscoelasticity seems to be in general only marginally affected by thermo-oxidative exposure, while the enhanced strain-hardening effect observed in uniaxial extension experiments presents a clear evidence for an increased long-chain branching (LCB) content in both 1C and 2C samples. Elongational viscosity data were analyzed by the molecular stress function (MSF) model as well as the Wagner-I model, and for both models, quantitative description of the experimental data for all samples was achieved by fit of only one nonlinear model parameter. Time-deformation separability was confirmed for all samples degraded, 1C as well as 2C, for cumulative degradation times of up to 90 min. The characterization by GPC was confronted with the characterization obtained from nonlinear rheology. It can be stated that elongational rheology is a powerful method to detect structural changes due to thermo-oxidative degradation, especially the formation of enhanced LCB. It has the further advantage that experimental data can be quantified by a single nonlinear model parameter of constitutive equations like the MSF or the Wagner-I model.

Carbocationic polymerization of isoprene initiated by dimethylallyl derivatives associated with B(C6F5)3

The cationic polymerization of isoprene using dimethylallyl derivatives (3,3-DMAX)–B(C6F5)3 as initiating systems has been investigated in detail. In most cases, oligomers of low molar mass (Mn ≤ 1500 g mol−1) and narrow molar mass distribution (Mw/Mn ≤ 2) were formed, which contain isoprene units almost exclusively in a trans-1,4-configuration in combination with saturated ones, with sometimes a higher polymer fraction of high dispersity. Most of the oligomers are terminated by an olefinic terminal group due to intensive transfer via proton elimination whereas a small proportion of X-functionalized oligomers results from chain end recombination with the counterion. Besides, the high molar mass fraction is formed firstly via protonation of oligomer double bonds followed by propagation at low conversion and then via intermolecular branching yielding polyisoprenes with a high branching degree and/or gel formation at high monomer conversion. Most of the elementary reactions involved in these processes were shown to be dependent on the counteranion X of 3,3-DMAX.