Bimodal Molar Mass Distribution Research Articles

Nitroxide-mediated polymerization of 1,3-butadiene in the presence of diphenylamine with hydrogen peroxide as initiator

Living behavior of a nitroxide-mediated polymerization of 1,3-butadiene in the presence of diphenyl nitroxide (DPN) as counter radical and hydrogen peroxide (H2O2) as initiator has been studied to elucidate the bimodal molar mass distribution of the product by four approaches such as hydrolyzation of the oligomeric product, peak separation technique, which defines the oligobutadiene molar mass dependency in relation to α as a ratio of [nitroxide] to [H2O2], variation of hydroxyl value with respect to α, and para-substituted DPN. It is demonstrated that the synthesized DPN shows efficient control of the polymerization of 1,3-butadiene, but because of the formation of difunctional nitroxide during the oxidation process of aromatic diphenylamine, a bimodal distribution in the final product is observed. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Isothermal crystallization of high density polyethylene with monomodal and bimodal molar mass distribution

AbstractThe isothermal crystallization behavior of two different high density polyethylene grades with monomodal and bimodal molar mass distribution was investigated by means of differential scanning calorimetry. The results indicate that extensive cocrystallization between linear short chains and long chains with short branches in bimodal polyethylene grade occurred. In contrast, polymer chains of different lengths in monomodal polyethylene exhibit different tendency to crystallize. This finding was explained qualitatively based on a general discussion of the effect of molar mass and branch content/length on crystallization tendency of polymeric chains.

Influence of molar mass distribution and long-chain branching on strain hardening of low density polyethylene

Low-density polyethylenes (LDPE) were synthesized in a laboratory-scale autoclave under high pressure. These samples were found to possess a high molar mass tail, resulting in a distinctly bimodal molar mass distribution and a lower concentration of long-chain branching than typical of commercial LDPEs. Rheological experiments in elongation showed that these samples exhibit a very pronounced strain hardening, which could be favorable for distinct processing operations. Although the samples have a rather high molar mass (\(M_{\rm w} = 2{\ldots}4 \times 10^{6}\) g/mol), their zero shear-rate viscosities η0 and their shear thinning behavior are still in a range, where thermoplastic processing is possible. A qualitative understanding of the experimental results is tried by the model of the Cayley tree.

Taylor Dispersion Analysis of Mixtures

Taylor dispersion analysis (TDA) is a fast and simple method for determining hydrodynamic radii. In the case of sample mixtures, TDA, as the other nonseparative methods, leads to an average diffusion coefficient on the different molecules constituting the mixture. We set in this work the equations giving, on a consistent basis, the average values obtained by TDA with detectors with linear response functions. These equations confronted TDA experiments of sample mixtures containing different proportions of a small molecule and a polymer standard. Very good agreement between theory and experiment was obtained. In a second part of this work, on the basis of monomodal or bimodal molar mass distributions of polymers, the different average diffusion coefficients corresponding to TDA were compared to the z-average diffusion coefficient (D(z)) obtained from dynamic light scattering (DLS) experiments and to the weight average diffusion coefficient (D(w)). This latter value is sometimes considered as the most representative of the sample mixture. From these results, it appears that, for monomodal distribution and relatively low polydispersity (I = 1.15), the average diffusion coefficient generally derived from TDA is very close to Dw. However, for highly polydisperse samples (e.g., bimodal polydisperse distributions), important differences could be obtained (up to 35% between TDA and D(w)). In all the cases, the average diffusion coefficient obtained by TDA for a mass concentration detector was closer to the Dw value than the z-average obtained by DLS.

High‐temperature mastication of raw natural rubber: Changes in macrostructure and mesostructure

AbstractRaw natural rubber samples of different Mooney viscosity values and inherent molar mass distributions (MMD) were used to characterize changes occurring during high‐temperature mastication and to identify the structural components conditioning mastication‐induced degradation. The macrogel degraded rapidly to essentially microgel, the latter of which was also continuously degraded, and at times completely. Mastication provoked decreases in the average molar mass of polyisoprene chains and changes in their MMD. The MMDs evolved differently as mastication progressed, differences that could be linked to the inherent MMD of each rubber. The non‐viscosity stabilized grade rubber (TSR 10) of monomodal MMD was progressively degraded by mastication, whereas rubber of the same grade but with a bimodal MMD was degraded much slowly and even stagnated on prolonged mastication. The stage at which stagnation occurred was characterized by an offset of the balance between long and short polyisoprene chains due to increases in the quantity of short chains, which facilitate the sliding of the longer ones. POLYM. ENG. SCI., 46:222–227, 2006. © 2005 Society of Plastics Engineers

Pulsed Laser Polymerization of Alkyl Acrylates: Potential Effects of the Oxygen Presence and High Laser Power

AbstractSummary: Unexpected difficulties are encountered in the determination of propagation rate coefficients (kp) in free radical polymerization of alkyl acrylates by pulsed laser polymerization (PLP), mainly due to intramolecular transfer to polymer.1 This article is focused on the role played by the high laser power in these difficulties and the possible reactions of mid‐chain radical with residual oxygen. Removing the oxygen by simple bubbling of nitrogen is sufficient to avoid alteration of the polymerization kinetics of acrylates by residual oxygen under PLP conditions. Moreover, no degradation of polymer (or solvent) has been detected after irradiation with the high laser power typically used in PLP experiments. However, it has been shown that this high laser power completely prevents from having a temporally and spatially homogeneous radical concentration in the PLP cell. A model is proposed here to simulate the pulsed laser polymerization taking initiator consumption and laser energy absorption into account. According to our simulation results, this non‐negligible initiator consumption and laser power absorption can indeed have a positive influence, i.e., it favors the obtainment of a bimodal molar mass distribution fulfilling the IUPAC consistency criteria. This observation may contradict the idea that PLP‐SEC is not suitable to determine accurate kp values for acrylates above 20–30 °C.Instantaneous MMDs formed after N pulses. Simulation taking initiator consumption and laser absorption into account.imageInstantaneous MMDs formed after N pulses. Simulation taking initiator consumption and laser absorption into account.

Evolution in the natural rubber native structure and plasticity retention index from the first tapping of clonal trees

AbstractThis study was carried out with five rubber clones planted in Côte d'Ivoire from the first tapping up to the 18th month of tree tapping (1 tapping/4 days). Changes in the natural rubber native mesostructure (macromolecular structure, macrogel, and microgel) of films prepared from fresh field latex were monitored. At the same time, the evolution of the thermooxidation sensitivity of raw rubber samples [grade 10 technically specified rubber (TSR10)] was also monitored with the plasticity retention index (PRI). The substantial initial macrogel rate (70–86%, depending on the clone) fell during the first 18 months of tree tapping to reach a few percent. However, during the same period, the initially low microgel rate (5–15%) increased and then remain stabilized around 55% rubber. The macromolecular structure [weight‐average molecular weight (Mw) and molar mass distribution (MMD)] also changed after tree opening. Mw increased and stabilized after 7.5 months of tapping. The bimodal MMD primarily involved short chains (molar mass < 400 kg/mol) at the opening of the trees. The TSR10 samples, prepared with latex from virgin trees, showed high PRIs and, therefore, low sensitivity to thermooxidation. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 903–909, 2005

Crystallisation behaviour of high density polyethylene blends with bimodal molar mass distribution 1. Basic characteristics and isothermal crystallisation

Isothermal crystallisation of high density polyethylene (HDPE) blends and their parent polymers was investigated. The blends having broad bimodal molar mass distributions and various compositions were prepared by blending a high molar mass ( M w=330 kg/mol, M w/ M n=4.8) and a low molar mass HDPE ( M w=34 kg/mol, M w/ M n=10) in different ratios in xylene solution. The blends and their parent components were characterised by size-exclusion chromatography, dynamic rheological and density measurements. Crystallisation kinetics were studied using a polarised light microscope equipped with an in-house built hot stage and by differential scanning calorimetry. The Avrami theory was applied for crystallisation kinetics analysis. Such crystallisation kinetics parameters as nucleation rate, nucleation density, the Avrami index and cystallisation rate contant were determined for the blends and their parent polymers. According to the results obtained an increasing polydispersity of the sample had a slight increasing effect on the Avrami index, indicating gain in prevalence of the thermal nucleation over the athermal one. In all samples nucleation density increased continuously during crystallisation verifying that the presence of a certain thermal nucleation was typical for all the materials studied. Both the crystallisation rate constant and the nucleation rate decreased with increasing molar mass of the sample. The nucleation density increased proportionally to the increase in average molar mass and the values were larger at lower crystallisation temperatures. The formed supermolecular structure was found to be sensitive to the blend composition and crystallisation temperature. Irregular banded or non-banded spherulites were observed in the materials. Banding of spherulites was typical for the samples having higher average molar mass. The superstructures observed in this work were smaller and vaguer than the superstructures reported in the earlier studies of polyethylene materials having similar average molar mass but narrow molar mass distribution.

Crystallisation behaviour of high density polyethylene blends with bimodal molar mass distribution: 2. Non-isothermal crystallisation

Crystallisation of high density polyethylene (HDPE) blends with broad bimodal molar mass distribution was investigated by differential scanning calorimetry (DSC) under non-isothermal conditions. The blends were prepared by blending a high molar mass PE ( M w=330 kg/mol, M w/ M n=4.8) and a low molar mass linear PE ( M w=34 kg/mol, M w/ M n=10) in different ratios in xylene solution. The samples were analysed by the normal DSC at different crystallisation rates and by a thermal fractionation technique. The blends and their parent polymers behaved according to general expectations i.e., crystallinity and density decreased when the molar mass of the samples increased. Additionally, non-linear relationships between MM and different analysed parameters were found. Small addition of the high molar mass parent polymer to the low molar mass parent polymer increased crystallisation temperature, although the general trend was decreasing. Furthermore, a complicated relationship between the reciprocal of crystallisation half-time and sample composition was found. The value increased first with increasing molar mass, reached a maximum when the average molar mass of the blend was between 150 and 200 kg/mol and then decreased. The detected maximum correlated with the broadest molar mass distribution of the blends. The crystallinities and densities of the blends with the broadest molar mass distribution also deviated from the linear correlation between them and molar mass. The Avrami index under non-isothermal conditions was analysed with a method developed by Harnisch and Muschik. The results indicated that thermal nucleation and spherical growth regimes are present in all studied materials.

Determination of the Molar Mass and the Radius of Gyration, Together with their Distributions for Methylhydroxyethylcelluloses

Three molar mass series were produced by different methods of degradation (namely ultrasonic (seven samples), oxidation (seven samples) and autoclaving (eight samples)) from a methylhydroxyethylcellulose (MHEC) sample with an average degree of substitution (DS) of 1.3, a molar degree of substitution (MS) of 0.46, a radius of gyration of 67 nm and a weight-average molar mass, Mw, of 318,000 g/mol. The degraded samples were characterized in terms of their molar mass and particle size together with their respective distributions with a hyphenated apparatus consisting of size exclusion chromatography and multi-angle laser light scattering and concentration detector (SEC/MALLS/DRI) at 25 °C in 0.1 M NaNO3 solution (with 200 ppm NaN3 as antibactericide). The refractive index increment was determined as dn/dc = 0.135 cm3/g. It was possible to reduce the weight-average molar mass down to approximately 10% of the initial molar mass using all the methods. In a comparison of the three degradation methods it was shown that only ultrasonic degradation retains the monomodal distribution, whereas the other two degradation methods yield a bimodal molar mass distribution. Consequently, only ultrasonic degradation represents a suitable method for producing homologous molar mass series. An RG–M relationship of RG = 0.0511 × M0.56 was established for the sample used in this case, and from this it was possible to calculate an [η]–M relationship of [η] = 0.3587 × M0.68.

Distribution of molar mass and branching index of natural rubber from hevea brasiliensis trees of different age by size exclusion chromatography coupled with online viscometry

Natural rubber from hevea brasiliensis trees (Thailand, RRIM 600 clone) of different age (8, 20, and 35 years) were characterized by size exclusion chromatography (SEC) coupled with online viscometry with respect to their distribution of molar mass and branching index at a temperature of 70°C using cyclohexane as solvent. The natural rubber samples were purified by washing with an aqueous solution of sodium dodecylsulfate and subsequent saponification. With this procedure physical branching points caused by phospholipids, proteins and hydrophobic terminal units, mainly fatty acids, of the natural rubber (cis-1,4-polyisoprene) molecule, could be removed leading to completely soluble polymer samples. All samples investigated possess a very broad (10 to 50,000 kg/mol) and distinct bimodal molar mass distribution. With increasing age of the trees the peak area in the low molar mass region decreases favoring the peak area in the high molar mass region. By plotting the branching index as a function of the molar mass it could be shown that branching of the natural rubber increases with both, the molar mass and the age of the trees.

MORPHOLOGY OF STEREOBLOCK POLYPROPYLENE

The microstructure and morphology of stereoblock polypropylenes containing sequences of isotactic (i) and syndiotactic (s) polypropylene (PP) were studied by 13C NMR spectroscopy, wide angle x-ray scattering (WAXS), size exclusion chromatography (SEC) combined with FT-IR analysis of the fractions, and atomic force microscopy. The 13C NMR spectra show the presence of long stereoregular sequences in the respective stereoblock polypropylene which are able to form the crystal modification known from the respective stereoregular homopolymers as revealed by WAXS measurements. A bimodal molar-mass distribution is detected by SEC. All SEC fractions contain i-PP and s-PP sequences, proven by FT-IR spectroscopy. It is shown by atomic-force microscopy that the stereoblock polypropylene, synthesized by homopolymerization of propylene using iso- and syndiotactic catalysts, form a microphase-separated morphology. The microphase separation is preserved also after treatment with hot toluene. A parallel investigation was conducted on a blend of i-PP and s-PP; it exhibits macrophase separation. Treatment of the blend with hot toluene results in the dissolution of the s-PP phase.

Living-Like Polymerization of Propylene with Mixed Metallocene Catalyst Systems

Propylene polymerization was conducted with the Cp2ZrHCl/B(C6F5)3/[tBuNSiMe2Flu]TiMe2 catalyst system with AlOct3 as a scavenger at –50°C. The polymer obtained displayed a bimodal molar mass distribution. It could be confirmed that the polymer with higher M̄n was produced from Zr active sites and the polymer with lower M̄n resulted from Ti active sites. In both fractions, M̄n was increased linearly with increasing polymerization time. The MWD (M̄w/M̄n) values of each fraction were around 1.1. Thus, it could be said that propylene polymerization proceeded in a living manner even with zirconocene active species by using the mixed metallocene system. The living-like polymerization of propylene with Cp2ZrMe2/B(C6F5)3 /Cp*TiCl3 was also demonstrated at –50°C. Under the reaction between carbon monoxide (CO) and this living polypropylene (PP) at –78°C, it could be found that CO was quantitatively incorporated into living PP.

Probing Thermoplastic Matrix−Carbon Fiber Interphases. 1. Preferential Segregation of Low Molar Mass Chains to the Interface

The molecular mobility of polystyrene (PS) chains near the surface of carbon fibers was investigated. The glass transition temperature of very thin coatings deposited on carbon fibers was measured by supported DMTA in order to probe the properties of the thermoplastic interphase. Depending on the fiber impregnation method from the melt or from solution, T-g changed in an opposite way for the thinnest coatings, decreasing with coating thickness in the case of impregnation in the melt and increasing when a polymer solution was used. This observation is explained by degradation of PS in the melt and subsequent preferential segregation of low molar mass chains to the interface. This conclusion is supported by the study of the T-g of coatings made from a bimodal molar mass distribution.

Synthesis of sulphated acrylic polymers and their evaluation as inhibitors of basic fibroblast growth factor dependent cell growth

Sulphated acrylic polymers have been synthesised and evaluated as potential inhibitors of basic fibroblast growth factor-dependent cell growth. The polymers were synthesised with a range of molar masses using both a thermal radical method and a room temperature reduction activation route. The polymers had bimodal molar mass distributions, but much of the low molar mass component could be removed by dialysis. Some of these polymers were effective inhibitors, their effectiveness being dependent on the molar mass. IC50 values as low as 5 μg/ml could be obtained with up to 73% inhibition by 100 μg/ml. These polymers are of interest as potentially readily accessible materials for cancer therapy.

Hydrolytic degradation of PLA/PEO/PLA triblock copolymers prepared in the presence of Zn metal or CaH 2

Various PLA/PEO/PLA triblock copolymers were prepared by ring-opening polymerization of L-lactide in the presence of poly(ethylene glycol), using CaH 2 or zinc metal as co-initiator. The degradation behavior of these copolymers was investigated in a pH=7.4 phosphate buffer at 37°C. Various techniques such as weighing, size exclusion chromatography, differential scanning calorimetry, X-ray diffractometry, and infra-red and 1H nuclear magnetic resonance spectrometries were used to monitor the changes in water absorption, weight loss, molar mass distribution, thermal properties, degree of crystallinity and composition. The results showed that introduction of PEO sequences considerably increased the hydrophilicity of the copolymers as compared with PLA homopolymers. However, the degradability of PLA blocks was not enhanced because of the phase separation between PLA and PEO blocks. Bimodal molar mass distributions were detected at the later stages of degradation, which were assigned to the formation of crystalline degradation byproducts within the bulk material.

Synthesis of a telechelic polypropylene with a vanadium-based living polymerization catalyst

The V(acac)3 (acac = acetylacetonato)/AlEt2Cl catalyst is known to promote the living polymerization of propylene at low temperature. In the present study, the catalyst was modified by contacting with α,ω-unconjugated diene compounds, and the resulting catalyst was applied to propylene polymerization. The polymer produced shows a bimodal molar mass distribution, where the molecular weight of the higher molecular weight portion is twice that of the lower molecular weight portion. By using the CO quenching technique, it was found that the polymer with the higher molecular weight has a telechelic structure.

Synthesis of a telechelic polypropylene with a vanadium-based living polymerization catalyst

The V(acac)3 (acac = acetylacetonato)/AlEt2Cl catalyst is known to promote the living polymerization of propylene at low temperature. In the present study, the catalyst was modified by contacting with α,ω-unconjugated diene compounds, and the resulting catalyst was applied to propylene polymerization. The polymer produced shows a bimodal molar mass distribution, where the molecular weight of the higher molecular weight portion is twice that of the lower molecular weight portion. By using the CO quenching technique, it was found that the polymer with the higher molecular weight has a telechelic structure.

Synthesis of Functional Polyethylenes with Soluble Metallocene/Methylaluminoxane Catalyst

This paper describes an easy method for preparing functional polyolefin copolymers of ethylene and 10-undecen-1-ol with a soluble metallocene/methylaluminoxane catalyst. The GPC profiles of the samples showed a reduction in the molar mass and a marked broadening in the molar mass distributions of the polymers with the addition of the polar compound. The bimodal molar mass distribution curve obtained indicated the presence of two or more active sites in the catalyst. The same results were obtained in DSC analysis. Two transition temperatures can be taken as evidence of the presence of a macroscopic phase separation of the polymer and the nonuniformity of the catalyst. The melt flow indices of the polymers were dramatically increased with the addition of functionalities to the polyethylene backbone, providing marked improvement in the processability of the polymers.

An improved algorithm for calculating relaxation time spectra from material functions of polymers with monodisperse and bimodal molar mass distributions

Using the basic concept of Emri and Tschoegl, the algorithm for calculating relaxation time spectra has been improved such that excellent results are provided in the difficult case of polymers with narrow molar mass distributions. These spectra can be compared with those calculated by nonlinear regularization (Weese 1992), which we regard as a very exact method, and show equally good results with even less mathematical effort. Examples of dense relaxation time spectra (up to eight points per decade) are given for nearly monodisperse polystyrene melts and for mixtures of these up to four components. The relaxation time spectra describe the dynamic mechanical experimental data in each case with an overall error of less than 3%. The filtering method used to avoid physically senseless oscillations has been proven to resolve the characteristic peaks contributed by monodisperse polymers accurately.