Function Of Molecular Weight Research Articles

Flow-induced fractionation effects on slip of polydisperse polymer melts

The slip behavior of several high-density polyethylenes with a broad range of molecular weights (MWs) including bimodal is studied as a function of MW and its distribution (MWD). A formulation inspired by the reptation theory is used to predict the slip velocity of the studied polymers as a function of MWD coupled with a model of surface MW fractionation that includes (i) the entropy driven migration of short chains toward the die wall due to the concentration gradient and (ii) the flow (stress)-induced migration effects. While surface fractionation has a minor effect on slip of narrow to moderate MWD polymers (particularly unimodal), its role is significant for broad bimodal MWD polymers. The inclusion of both effects (concentration and flow gradients) accurately captures the slip velocity of broad MWD polymers.

N-Reacetylated Oligochitosan: pH Dependence of Self-Assembly Properties and Antibacterial Activity.

Oligochitosan (short chain chitosan) is more soluble in acidic aqueous media than a high molecular weight (MW) chitosan, but its antimicrobial activity decreases with increase in degree of acetylation (DA) and increase in pH above a critical pH threshold point. In the present study, oligochitosans varying in MW were additionally N-acetylated and their self-assembly properties and antibacterial activity toward Staphylococcus aureus and Escherichia coli were investigated in a wide pH range as a function of MW and DA. Light scattering studies reveals that reacetyleted oligochitosan with Mw ≤ 11 kDa is completely soluble in alkaline media (up to pH 12.5), if its DA is not less than 16%. Reacetylated chitosans with DA ∼ 30% are solubile in the entire pH range up to 12.5, if their Mw is not higher than 25 kDa, but they aggregate and precipitate from the solution at pH ≥ 8 when their Mw is above 25 kDa. Considering the influence of DA and MW, the antibacterial activity of reacetylated oligochitosans is maximal in the short interval of DA 16-28% at pH 7.4. These results are promising for expanding practical application of oligochitosan in pharmaceutical, cosmetic, and food compositions.

Impact of molecular weight and comonomer content on catalytic hydrogen-deuterium exchange in polyolefins

Abstract The effect of molecular weight (MW) and comonomer content (CC) on the catalytic hydrogen-deuterium exchange process (CHDEP) in a series of polyethylenes has been studied using a band filter based SEC-IR method. CHDEP permits deuterium labeling of polyolefins prepared by any synthetic route, a major advantage over the traditional method of polymerizing deuterated monomers. However, incompleteness in deuterium-exchange as well as heterogeneity in deuterium distribution are known challenges to deuterium labeling of heterogeneous polymers by this method. These factors complicate small angle neutron scattering (SANS) studies, in which the average deuterium-exchange level (DL) and deuterium distribution are critical. A band-filter based size exclusion chromatography (SEC)-infrared absorption (IR) technique is presented as a simple but accurate method to quantify the DL as a function of MW. The DL of a series of ethylene-alpha olefin copolymers has been found to increase with MW but does not depend strongly on CC for all the polyethylene samples. This behavior has been further confirmed with independent measurements of average DL as a function of weight-averaged MW of a series of fractions from a deuterated linear low density polyethylene. The SEC results for the fractions are in agreement with deuterium measurements on the fractions using NMR spectroscopy, density and FTIR.

High-Molecular-Weight β-Glucan Decreases Serum Cholesterol Differentially Based on the CYP7A1 rs3808607 Polymorphism in Mildly Hypercholesterolemic Adults

Background: β-Glucan, a soluble fiber with viscous property, has a documented cholesterol-lowering effect. The molecular weight (MW) of β-glucan, which contributes to viscosity, and an individual's genotype might influence the cholesterol-lowering efficacy of β-glucan.Objectives: This study was designed to determine whether the cholesterol-lowering efficacy of barley β-glucan varied as a function of MW and the daily dose consumed. Our second aim was to determine whether any gene-diet interactions are associated with the cholesterol-lowering efficacy of β-glucan.Methods: In a randomized controlled crossover trial, 30 mildly hypercholesterolemic adults [12 men and 18 women, aged 27–78 y; body mass index (in kg/m2): 20–40; total cholesterol (TC): 5.0–8.0 mmol/L; LDL cholesterol: 2.7–5.0 mmol/L] were randomly assigned to receive a breakfast that contained either barley β-glucan at 3 g high MW (HMW)/d, 5 g low MW (LMW)/d, or 3 g LMW/d or a control diet, each for 5 wk. The washout period between the phases was 4 wk. Fasting blood samples were collected at the start and end of each phase for blood lipid analysis and genotyping.Results: Consumption of 3 g HMW β-glucan/d lowered TC by −0.12 mmol/L (95% CI: −0.24, −0.006 mmol/L) compared with the control diet (P = 0.0046), but the LMW β-glucan, at either 3 g/d or 5 g/d, did not change serum cholesterol concentrations. This effect of HMW β-glucan was associated with gene-diet interaction, whereby individuals with the single nucleotide polymorphism (SNP) rs3808607-G allele (GG or GT) of the cytochrome P450 family 7 subfamily A member 1 gene (CYP7A1) had greater responses to 3 g HMW β-glucan/d in lowering TC than TT carriers (P = 0.0006).Conclusions: The HMW β-glucan rather than LMW β-glucan reduced circulating TC effectively in mildly hypercholesterolemic adults. The cholesterol-lowering effect of β-glucan may also be determined by the genetic characteristics of an individual. These data show that individuals carrying the CYP7A1 SNP rs3808607-G allele are more responsive to the cholesterol-lowering effect of β-glucan with HMW than TT carriers. This trial was registered at clinicaltrials.gov as NCT01408719.

Thermal Analysis Goes With Mass Spectrometry to Evaluate the Molecular Weight of a Fluorinated Plasma Polymer

A combination of thermal analysis (differential scanning calorimetry DSC) and mass spectrometry (MS) has been proposed to estimate the molecular weight (MW) of fluorinated plasma polymers, not reachable by MS only owing to their high dispersity. A noticeable crystalline index has been previously reported for these plasma polymers leading to a detectable melting peak in DSC, itself not sensitive to dispersity. Relying on the variation of the melting temperature as a function of MW, a set of standards of various MW have been synthesized by atom transfer radical polymerization (ATRP). MW (from MS) plotted as a function of melting temperatures (from DSC) provided a calibration curve further used to estimate the MW of plasma polymers based on their own melting temperature. The comparison of the DSC thermograms of plasma polymer and another reference made by free radical polymerization (FRP) finally offered an original insight into the contributions from insoluble and soluble parts, mimicked by a FRP (branched, low MW) and a ATRP (linear, high MW) polymer respectively.

Defect-free Naphthalene Diimide Bithiophene Copolymers with Controlled Molar Mass and High Performance via Direct Arylation Polycondensation.

A highly efficient, simple, and environmentally friendly protocol for the synthesis of an alternating naphthalene diimide bithiophene copolymer (PNDIT2) via direct arylation polycondensation (DAP) is presented. High molecular weight (MW) PNDIT2 can be obtained in quantitative yield using aromatic solvents. Most critical is the suppression of two major termination reactions of NDIBr end groups: nucleophilic substitution and solvent end-capping by aromatic solvents via C-H activation. In situ solvent end-capping can be used to control MW by varying monomer concentration, whereby end-capping is efficient and MW is low for low concentration and vice versa. Reducing C-H reactivity of the solvent at optimized conditions further increases MW. Chain perfection of PNDIT2 is demonstrated in detail by NMR spectroscopy, which reveals PNDIT2 chains to be fully linear and alternating. This is further confirmed by investigating the optical and thermal properties as a function of MW, which saturate at Mn ≈ 20 kDa, in agreement with controls made by Stille coupling. Field-effect transistor (FET) electron mobilities μsat up to 3 cm(2)/(V·s) are measured using off-center spin-coating, with FET devices made from DAP PNDIT2 exhibiting better reproducibility compared to Stille controls.

Convection enhanced delivery of different molecular weight tracers of gadolinium-tagged polylysine

Convection enhanced delivery (CED) is a powerful method of circumventing the blood-brain barrier (BBB) to deliver therapeutic compounds directly to the CNS. While inferring the CED distribution of a therapeutic compound by imaging a magnetic resonance (MR)-sensitive tracer has many advantages, however how the compound distribution is affected by the features of the delivery system, its target tissue, and its molecular properties, such as its binding characteristics, charge, and molecular weight (MW) are not fully understood. We used MR imaging of gadolinium diethylenetriaminepentaacetic acid (Gd-DTPA)-tagged polylysine compounds of various MW, in vitro and in vivo, to measure the dependence of compounds MW on CED distribution. For the in vitro studies, the correlation between volume of distribution (Vd) as a function of MW was determined by measuring the T1 of the infused tracers, into 0.6% agarose gels through a multiport catheter. The compounds distributed in the gels inversely proportional to their MW, consistent with convection and unobstructed diffusion through a porous media. For the in vivo studies, Gd-DTPA tagged compounds were infused into the non-human primate putamen, via an implanted multiport catheter connected to a MedStream™ pump, programmed to deliver a predetermined volume with alternating on–off periods to take advantage of the convective and diffusive contributions to Vd. Unlike the gel studies, the higher MW polylysine-tracer infusions did not freely distribute from the multiport catheter in the putamen, suggesting that distribution was impeded by other properties that should also be considered in future tracer design and CED infusion protocols.

Effect of polydispersity on natural organic matter transport

The mobility of humic-substance dominated natural organic matter (NOM) concentrated from a freshwater wetland by reverse osmosis was examined in sand columns at pH 5–8, in 0.001 M and 0.01 M NaClO4. Greater mobility was observed at higher pH and lower ionic strength, although breakthrough curves (BTCs) for bulk NOM exhibited extensive tailing under all conditions examined. Based on observations from previous batch experiments indicating preferential adsorption of intermediate to high molecular weight (MW) NOM, we postulate that ‘adsorptive fractionation’ of the NOM pool leads to the observed tailing behavior, and develop a novel approach to assess the effects of polydispersity on transport of NOM and associated contaminants. BTCs for different NOM fractions were constructed by separating column effluent MW distributions determined by high-pressure size exclusion chromatography into five discrete intervals or ‘bins’ and calculating the mass of NOM within each bin at four sampling times. Observed retardation factors (Ro), reflecting median arrival time relative to that of a nonreactive tracer, ranged from 1.4 to 7.9 for the various bins and generally increased with MW. NOM retarded transport of the contaminant metal Cd (2.5 ppm, in 0.01 M NaClO4) slightly at pH 5 and more substantially at pH 8. Although Cd had little or no effect on bulk NOM transport, retention of the more aromatic, IMW-HMW NOM appeared to be slightly enhanced by Cd. Study results demonstrate that heterogeneity in retardation as a function of MW is likely a major factor contributing to bulk NOM BTC tailing and may have important implications for contaminant transport.

Comparative Pharmacodynamic Assessment of the Antiangiogenesis Activity of Heparin and Low-Molecular-Weight Heparin Fractions

Effects of unfractionated heparin and low-molecular-weight heparins (LMWHs) on human microvascular endothelial cell sprouting (tube formation assay) in vitro were determined. Antiangiogenesis efficacy of commercially available LMWHs tinzaparin and enoxaparin in the chick chorioallantoic membrane (CAM) model of growth factor-induced angiogenesis was compared. The LMWH tinzaparin was fractionated into different molecular weight (MW) pools by size exclusion chromatography; they inhibited CAM angiogenesis depending on their MW distribution, with optimal inhibition at 8 to 12 kDa and no inhibition at <2 kDa. Tinzaparin demonstrated greater antiangiogenesis efficacy than enoxaparin (P < .001); these CAM results correlated with the endothelial tube formation assay results (P < .001, tinzaparin vs enoxaparin). These data point to the variable antiangiogenesis efficacy of different LMWHs as a function of MW and perhaps other structural differences. Our hypothesis confirmed a relationship between lower release of tissue factor pathway inhibitor by lower MW fractions of tinzaparin or enoxaparin leading to reduced antiangiogenesis efficacy.

SEC-MALS method for the determination of long-chain branching and long-chain branching distribution in polyethylene

This paper systematically describes a LCB determination method that can quantify both LCB content and LCB distribution across the molecular weight distribution in polyethylene homopolymers as well as copolymers. Coupling size-exclusion chromatography with multi-angle light scattering (SEC-MALS), this method quantifies molecular weights (MW) and radii of gyration ( R g) simultaneously. The number of LCB per molecule and LCB frequency as a function of MW can be calculated by comparing R g of a branched polymer with that of a linear control at the same MW using the Zimm–Stockmayer approach. Because the presence of short-chain branching in copolymers results in changes in R g of the copolymers, their LCB contents cannot be obtained before the short-chain branching (SCB) effect is corrected. Using well-characterized linear PE copolymers as standards, an empirical method is successfully established in this paper to correct the SCB effect. Consequently, this method can be applied to determine LCB in PE copolymers as well. Some practical aspects, such as the selection of formalism for data processing, the LCB detection sensitivity and precision, and long-term reproducibility of this method are also discussed. Finally, examples are given to demonstrate how this method is applied to determine LCB and LCB distribution in practical PE homopolymers and copolymers.

Distribution of molecular weight and composition in diblock copolymers

Abstract The distribution of molecular weight (MW) and composition of diblock copolymers is considered theoretically. Assuming that the chain end of each block is coupled randomly, the weight-average MW of the block copolymers is given by Mw = w1 (Mw,1 + Mn,2) + w2 (Mw,2 + Mn,1), irrespective of the shape of the distribution of each block, where wi is the weight fraction, and Mn,i and Mw,i are the number- and weight-average MW of each block. In copolymer chains, the chemical compositions as well as the MWs cannot be identical for all polymers, and there exists a bivariate distribution of MW and composition. When the MW distribution (MWD) of both blocks follows the Schulz-Zimm distribution, the bivariate distribution can be obtained analytically. In addition to the bivariate distribution, the full MWD, the average composition as a function of MW, the composition distribution of copolymers having a specified MW, and the overall composition distribution are obtained. The composition distribution, as well as the average composition, becomes independent of MW under the condition σ1/Mn,1 = σ2/Mn,2, where σ i is a parameter indicating the narrowness of the Schulz-Zimm distribution. The present theoretical analysis provides new insight into the design of diblock copolymers.

Traube-Rule Interpretation of Protein Adsorption at the Liquid−Vapor Interface

Pendant-drop tensiometry of aqueous-buffer solutions of purified human proteins spanning nearly 3 orders of magnitude in molecular weight (MW) reveals that reduction in liquid−vapor (LV) interfacial tension γlv followed a systematic progression in MW with the molar concentration required to reach a specified γlv value decreasing with increasing MW in a manner reminiscent of the Traube rule for linear hydrocarbon surfactants. Furthermore, the concentration dependence of interfacial tension (dγlv/d ln CB, where CB is bulk-solution concentration) is observed to be surprisingly invariant among this disparate group of proteins (i.e., approximately constant apparent Gibbs' surface excess Γ = −1/RT dγlv/d ln CB). These findings are interpreted through a model of protein adsorption predicated on the interfacial packing of spherical molecules with dimensions scaling as a function of MW. The Traube-rule-like ordering is rationalized as a natural outcome of an invariant partition coefficient that entrains a fixed fr...

Characterization of DOM as a function of MW by fluorescence EEM and HPLC-SEC using UVA, DOC, and fluorescence detection

To investigate the composition of dissolved organic matter (DOM) as a function of apparent molecular weight (MW) by rapid analytical methods, high performance liquid chromatography (HPLC)-size exclusion chromatography (SEC) was conducted with sequential on-line detectors consisting of UV, fluorescence, and quantitative DOC measurement. Fluorescence excitation-emission matrix (EEM) spectrophotometry was used to select wavelengths for the HPSEC on-line fluorescence system. The chosen peak maxima locations of excitation-emission wavelengths were 278–353 nm for protein-like substances and 337–423 nm for fulvic-like substances based on an analysis of EEM spectra for various samples and reference materials. This system provides quantitative and qualitative information on the specific MW components of DOM, including proportion of DOC (by DOC measurement), aromaticity (by comparison of UV and DOC measurements), and chemical properties (by fluorescence measurement). It further allows determination of organic matter characteristics (e.g., fulvic-like, protein-like, and polysaccharide-like substances) as a function of MW. Three types of samples (Irvine Ranch ground water (IRWD-GW), Barr Lake surface water (BL-SW), and Hawaii wastewater secondary effluent) were analyzed by the HPSEC-UVA-fluorescence-DOC system. These results were compared with fluorescence EEM for samples fractionated by HPLC-SEC. The DOM fraction in the high apparent MW range (over 10,000 g/mol) consisted of polysaccharide-like substances for IRWD-GW and a mixture of polysaccharide-like/protein-like substances for BL-SW and wastewater secondary effluent. Minimal amounts of fulvic-like substances were found in the wastewater secondary effluent sample. The DOM fractions in a medium apparent MW range (5000–1000 g/mol) showed higher aromaticity (fulvic in character) than any other fractions for all samples. For the DOM fraction in the low apparent MW range (below 680 g/mol), additional aliphatic organic matter was found in IRWD-GW, while BL-SW contained protein-like processes. DOM plays an important role in drinking water and wastewater treatment processes. An enhanced HPSEC technique with multiple on-line detectors enables a better understanding of quantitative and qualitative DOM properties and can help to design and optimize water/wastewater treatment facilities.

Molecular Weight-Retention Volume Relation for Narrow and Broad Polystyrenes by Size Exclusion Chromatography Equipped With a Light Scattering Detector

The difference of the relation of molecular weight (MW) and retention volume of narrow polystyrene (PS) samples from broad ones in the size exclusion chromatography - light scattering (LS)/refractive index (RI) detectors is discussed. Effects of interdetector delay volume on the relation was serious and the approach to adjust the interdetector delay volume so as to get a plateau relation of MW and retention volume for PS samples with a narrow MW distribution was proposed. The interdetector delay volume was found to be a function of MW. PS samples with a broad MW distribution had the similar relation of MW and retention volume to the calibration curve constructed with PS standards with narrow MW distributions; and the MW averages obtained by the LS/RI approach gave the comparable values to those calculated by a calibration curve method.

Rapid measurement of dialyzer clearance as a function of molecular weight.

The relationship between solute clearance and molecular weight (MW) is an important performance characteristic of hemodialyzers. Traditionally, this relationship has been determined using a range of solutes of diverse properties that require a variety of analytical procedures. We describe a simple method for obtaining clearance as a function of MW using a homologous series of glucose oligomers, continuous from the monomer to the octamer. Using HPLC, single analyses of the inflow and outflow streams of the dialyzer allow the determination of clearances over a MW range of up to 1314 daltons.

Fundamental aspects of adsorption chromatography of polymers and their experimental verification by thin-layer chromatography

The dependence of changes in the free energy (−Δ F/ kT) accompanying the adsorption of polystyrenes on the energy of interaction of a polymer segment with the surface of a porous adsorbent (−ε) has been investigated. It is shown that, for macromolecules of any molecular weight (MW) this dependence intersects the −ε axis at the point −ε = −ε cr and, hence, the existence of the critical energy, ε cr, has been verified experimentally. At −ε < −ε cr the probability of entry of macromolecules into the adsorbent pores is determined by the molecular-sieve effect, at −ε > −ε cr the adsorption effect begins to predominate and at −ε = −ε cr these two effects mutually compensate and the separation of macromolecules according to MW (or according to size) does not occur (the distribution coefficient, K d, is unity for macromolecules of any MW). Since the molecular-sieve and the adsorption effects are interrelated, in the range of the molecular-sieve effect, the entry into the pores of macromolecules of higher MW is favoured by adsorption, whereas in the adsorption range the adsorption of macromolecules within the pores is favoured by the molecular-sieve effect. Over a wide range of energies, a continuous transition occurs from gel-permeation chromatography, in which the components are separated mainly owing to the macromolecular-sieve effect, to adsorption chromatography, in which the separation is due mainly to differences in the adsorption of components of different molecular weight. A linear calibration of K d is possible as a function of MW, within the limits of a given homologous series, and itis desirable to use this calibration both for gel-permeation and for adsorption chromatography of polymers.