Molecular Weight Distribution Characteristics Research Articles

Microstructural design characteristics of high-density polyethylene for thick-walled pressure pipe applications

Advancements in process technology and structure-property relationship-based design of high-density polyethylene has led to development of high-performance bimodal polyethylene grades suitable for large diameter thick-walled pressure pipe applications. These high-density bimodal products have enhanced design life, high pressure rating and sag resistance for fabrication of large diameter thick-walled pipe. Achieving optimal balance of these performance properties require controlled microstructural characteristics. The design characteristics are intricately influenced by catalyst type and process technology employed in resin manufacturing. In this study, we systematically establish correlations between microstructural characteristics of seven commercially available bimodal PE resins, produced using dual slurry or gas phase reactors, and their most relevant performance properties. We conducted comprehensive microstructural characterization of all samples using GPC-IR (high-temperature gel permeation chromatography coupled with an infrared composition detector), 13C NMR (nuclear magnetic resonance), TREF (temperature rising elution fractionation) and DSC (differential scanning calorimetry) techniques. Additionally, we measured mechanical, thermal, and rheological properties, focusing on crystallization kinetics, density, slow crack growth resistance (PENT and strain hardening modulus SHM) and low-frequency shear rheology of these products. Comparisons were made between key properties of interest, including slow crack growth resistance and low-frequency rheology (relevant for sag resistance), and microstructural characteristics. Theoretical and empirical correlations based on phenomenological models were employed for this analysis. The detailed study contributes to determining the critical levels of short and long chain branching, as well as molecular weight distribution characteristics essential to produce bimodal PE-4710 certifiable high-density polyethylene resins with low sag resistance. These design principles may serve as a foundation for the efficient design of reactor recipes and post-reactor modification processes, contributing to the evolution of pressure pipe polyethylene resin development and paving the way for design of next-generation pressure pipe grades.

The characterization of molecular weight distribution and aggregation by asymmetrical flow field-flow fractionation of unmodified and oxidized alginate

After oxidation, the presence of aldehydes on alginate chains allows reaction with, making oxidized alginate (OA) attractive for selective functionalization and gelation (i.e. covalent-crosslinks). The molecular weight (MW) of OA is significantly reduced after oxidation. As MW determines mechanical properties and degradation of OA-based hydrogels, it is essential to precisely measure the MW of OAs as function of its degree of oxidation (DO). Size exclusion chromatography is used for the analysis of polymers, but it has limited ability to measure the distribution of polymers with high MW, without recognizing the presence of aggregates. This work describes the use of asymmetrical flow field-flow fractionation (AF4) coupled with refractive index and multi-angle light scattering detectors as an analytical method for the quantification of the MW of OAs. OAs with low DO (5–10%) had a weight average molecular weight (Mw) in the range of 35–67 kDa, with aldehydes concentration of 1–1.8 mmol/g. OAs with higher DO (35–50%) had lower Mw (20 kDa), with aldehyde concentration of 4.3–6.5 mmol/g. This is the first study reporting the comprehensive characterization of OAs using the precision of AF4 method to quantify their MW, and will contribute substantially to the formulation of OA-based hydrogels towards more advanced biomedical applications.

Synthesis of Novel Type Terpolymer Poly(Eugenol-co-methylmethacrylate-co-hydroxyethylmethacrylate) Using Photopolymerization Method: Characterization and Investigation of Thermal Properties

In this study, it has been demonstrated that Poly(Eugenol-co-methylmethacrylate-co-hydroxyethylmethacrylate) terpolymers were successfully synthesized by using photopolymerization method. This synthesis was carried out in one step at ambient temperature, benzophenone and triethyl amine was used as the photoinitiator and hydrogen donor, respectively. Methyl methacrylate (MMA), 2-Hydroxyethyl methacrylate (HEMA) and Eugenol (Eg) were used as monomer in photopolymerization. Terpolymers are named as F-BAP1, F-BAP2, F-BAP3 and F-BAP4 according to the ratio of monomers used. Structural characterization and thermal properties of the synthesized terpolymers were carried out by FTIR (Fourier-transform infrared spectroscopy), 1H-NMR (Nuclear magnetic resonance spectroscopy), TG-DTG (Thermogravimetry-Differential Thermogravimetry) and DSC (Differential scanning calorimetry) methods. Characterization of molecular weight distributions were determined by viscometric method. SEM (Scanning electron microscope) was used for surface analysis. According to the data obtained from thermal analysis, two degradation steps are seen in thermograms of the terpolymers except FBAP-2. The maximum degradation temperature of F-BAP 2 is 422 oC, the first degradation temperature of the other terpolymers between 250-280 oC and the second one between 421-427 oC.

Molecular weight-dependent heterogeneities in photochemical formation of hydroxyl radical from dissolved organic matters with different sources

Dissolved organic matter (DOM) is ubiquitous in aquatic ecosystem and characterized by a wide range of molecular weight (MW) distribution. In this study, a total of nine bulk DOM samples, including five International Humic Substances Society (IHSS) standards and four naturally collected samples, were fractionated into low MW (LMW-, <1 kDa) and high MW (HMW-, 1 kDa~0.45 μm) fractions, with MW-dependent heterogeneities in photochemical formation of hydroxyl radical (HO) was investigated. The formation rate of HO (RHO) for the bulk samples were 4.60–7.27 × 10−12 M/s/mg-C/L for IHSS standards and 4.63–7.66 × 10−12 M/s/mg-C/L for naturally collected samples. Regardless of sample types, the LMW fraction was found to exhibit generally higher RHO values than the HMW counterparts. For IHSS standards, the RHO decreased from 4.68–8.46 × 10−12 M/s/mg-C/L for LMW fraction to 3.67–6.66 × 10−12 M/s/mg-C/L for HMW fraction, and for naturally collected samples, the value of RHO decreased from 5.21–12.04 × 10−12 M/s/mg-C/L for LMW fraction to 3.25–6.49 × 10−12 M/s/mg-C/L for HMW counterpart. A positive correlation between the net RHO and the normalized intensities of fluorescent peak [Em/Ex: (400–500)/(230–250) nm] was found, showing that the HO formation was strongly coupled to the abundance of humic-like substances. The results indicate that aquatic DOM is an important pool for HO formation, and characterization of MW distribution rather than average MW is thus required to explain the DOM-induced formation potential of HO.

Effects of molecular weight fractions and chemical properties of time-series cyanobacterial extracellular polymeric substances on the aggregation of lake colloidal particles.

Colloidal particles in lake waters interact inevitably with cyanobacterial extracellular polymeric substance (EPS), which will change their behavior and fate. Quantitative prediction of the effects of cyanobacterial EPS on colloidal behavior is difficult due to its variability and heterogeneity. To explore the effects of molecular weight (MW) fractions and chemical properties of cyanobacterial EPS on aggregation kinetics of colloidal particles, time-series cyanobacterial samples were collected in Lake Taihu, China, from April to November (during blooming and maintenance period), with the bulk EPS matrix fractionating into low MW (LMW-, <1 nm) and high MW (HMW-, 1 nm-0.45 μm) fractions. HMW-EPS was generally characterized with higher absorbance and predominant distribution of protein-like substances, while LMW-EPS contained mainly the humic- and fulvic-like substances. The absorbance, molecular size, and humification degree for each MW fraction consistently increased from April to November, showing obvious temporal variations from blooming period to maintenance period. As for the MW-dependent aggregation behaviors, the HMW-EPS provided better stability against aggregation than the LMW-EPS, and the bulk EPS matrix that consisted of HMW- and LMW-fractions exhibited the effects intermediate between that of each fraction alone. Regardless of MW fractions, the effects of EPS-induced stability enhancement were more evident in maintenance period than in blooming period. Further analysis showed that the colloidal stability was correlated positively with SUVA254 (R2 = 0.82-0.93) but negatively with Slope275-295 (R2 = 0.53-0.91) of UV-Vis absorption spectra, indicating that aromaticity and MWs were two critical parameters controlling colloidal aggregation. Therefore, cyanobacterial EPS can exhibit variable effects on colloidal stability, and characterization of MW distribution is strongly required in predicating the behavior and fate of colloidal particles in water environments.

Characteristics of molecular weight distribution of dissolved organic matter in bromide-containing water and disinfection by-product formation properties during treatment processes

The characteristics of dissolved organic matter (DOM) and bromide ion concentration have a significant influence on the formation of disinfection by-products (DBPs). In order to identify the main DBP precursors, DOM was divided into five fractions based on molecular weight (MW), trihalomethane formation potential and haloacetic acid formation potential were determined for fractions, and the change in contents of different fractions and total DBPs during treatment processes (pre-chlorination, coagulation, sand filtration, disinfection) were studied. Moreover, the relationship between bromide concentration and DBP generation characteristics in processes was also analyzed. The results showed that the main DBP precursors were the fraction with MW <1kDa and fraction with MW 3−10kDa, and the DBP's generation ability of lower molecular weight DOM (<10kDa) was higher than that of higher molecular weight DOM. During different processes, pre-chlorination and disinfection had limited effect on removing organics but could alter the MW distribution, and coagulation and filtration could effectively remove organics with higher MW. For DBPs, trihalomethanes (THMs) were mainly generated in pre-chlorination and disinfection, while haloacetic acids (HAAs) were mostly generated during pre-chlorination; coagulation and sand filtration had little effect on THMs but resulted in a slight removal of HAAs. In addition, the results of ANOVA tests suggested that molecular sizes and treatment processes have significant influence on DBP formation. With increasing bromide concentration, the brominated DBPs significantly increased, but the bromine incorporation factor in the processes was basically consistent at each concentration.

The Scaling Law between Molecular Mass and Diffusivity and its Influence on the Molecular Weight Distribution as Derived from a Stretched Exponential PGSTE NMR Response Curve

AbstractThe molecular weight distribution characteristics derived from an inverse Laplace transformation (ILT) of a stretched exponential PGSTE NMR response function (SEF) is discussed with reference to the scaling law between diffusivity (D) and molecular mass (M) (D = KM−α). Two general equations are presented, enabling the polydispersity index (PDI) and the average molecular weights (Mw, Mn, and Mmed) to be determined from α and the two characteristic parameters β and D0 of the (normalized) SEF (= exp [(−D0t)β]). A detailed discussion regarding the derivation of the two equations is presented.

Molecular Weight Distribution Characteristics (of a Polymer) Derived from a Stretched‐Exponential PGSTE NMR Response Function—Simulation

AbstractAssuming the signal response in a pulsed gradient stimulated echo (PGSTE) experiment (on a polymer) to be described by a simple stretched‐exponential function (SEF) and knowing the scaling law between diffusivity and molecular weight, the molecular weight distribution (MWD) characteristics (kurtosis, skewness, moment, and width) are derived and compared to the corresponding distribution characteristics obtained by a log‐normal function fit (to the same MWD). Also, the challenge involved in obtaining a reliable weight average molecular weight from an SEF response function is discussed.

Rheological behaviors and molecular weight distribution characteristics of bimodal high‐density polyethylene

AbstractA series of bimodal high density polyethylene (PE) with different molecular weight distributions (MWDs) were prepared by melt blending, and the fitting multipeaks on Gaussian were used to analyze the MWD curves, and the ratio of the areas under unimodal curves was as a tool to characterize the MWD; the phase behaviors and rheological behaviors were studied by dynamic rheological. The results showed that homogeneous bimodal high density PEs could be successfully prepared via melt blending, and the bimodal characteristic could be adjusted as expected. For samples with the MWD peak positions unvaried, the storage modulus, complex viscosity, and zero‐shear viscosity decreased rapidly with the value of AL/U increasing. Especially in the low frequency region, the loss modulus surpassed the storage modulus (G″ &gt; G′) when AL/U &gt; 10.17 and the dynamic cross‐point Gx appeared and increased with increasing AL/U, with an increasing extent much larger than that due to the width of MWD. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Fracture behavior of bimodal polyethylene: Effect of molecular weight distribution characteristics

A series of bimodal polyethylenes with different molecular weight distribution characteristics were prepared by melt blending, and the fracture behavior of these bimodal polyethylenes was studied by the method of essential work of fracture. The results show that specific essential work of fracture, we, increases obviously with the molecular weight distribution characteristic, AL/U, indicating the improvement of the resistance to crack propagation. By means of successive self-nucleation and annealing analysis, obvious variations in the crystal structures of bimodal polyethylenes with increasing AL/U have been found. That is, the crystal size and the amount of relatively thick lamellas increase with AL/U, but no large variation of crystallinities has been observed. So, the influence of AL/U is mainly on the crystal perfection, the improvement of which produces an enhancement of fracture toughness since more energy would be dissipated in the superior network structure constructed from crystalline zones and amorphous zones.

Flow field‐flow fractionation and multiangle light scattering for ultrahigh molecular weight sodium hyaluronate characterization

This review describes the utility of flow field-flow fractionation coupled with multiangle light scattering and differential refractive index (FlFFF-MALS-DRI) detection methods for the separation of ultrahigh molecular weight sodium hyaluronate (NaHA) materials and for the characterization of molecular weight distribution as well as structural determination. The sodium salt of hyaluronic acid (HA), NaHA, is a water-soluble polysaccharide with a broad range of molecular weights (10(5) -10(8) ) found in various naturally occurring fluids and tissues. Basic principles of FlFFF-MALS using field programming for the separation of the degraded products of NaHA prepared by treating raw materials with depolymerization or degradation processes such as membrane filtration, enzymatic degradation, ultrasonic degradation, alkaline reaction, irradiation by γ-rays, and thermal treatment for the development of pharmaceutical applications are introduced. Changes in molecular weight distribution and conformation of NaHA materials due to external stimuli are also discussed.

Size Characterization of Sodium Hyaluronate by Field Programming Frit Inlet Asymmetrical Flow Field-Flow Fractionation/Multiangle Light Scattering

Sodium hyaluronate (NaHA), water soluble polymer having ultra-high molecular weight, is characterized by using on-line frit inlet asymmetrical flow field-flow fractionation (FI-AFlFFF) and multiangle light scattering (MALS). This study demonstrates the capability of power programming FI-AFlFFF for the separation of NaHA and the applicability of FI-AFlFFF with MALS for the characterization of molecular weight distribution and their structural information. Since sample injection and relaxation in FI-AFlFFF are achieved by using hydrodynamic relaxation, separation of high molecular weight polymers can be achieved smoothly without halting the separation flow. Experiments are carried out with the two different NaHA products (a raw NaHA sample and a thermally degraded NaHA product) and molecular weight distribution and conformations in solution are determined. Influence of sample filtration on the change of molecular weight distribution is also discussed.碏�⨀؀က碏�⨀邏�⨀਀邏�⨀႐�⨀က႐�⨀ꂏ�⨀Ԁꂏ�⨀࢐�⨀؀က࢐�⨀ₐ�⨀਀ₐ�⨀‘�⨀က‘�⨀ゐ�⨀਀ゐ�⨀�⨀؀က�⨀႑�⨀਀႑�⨀邑�⨀က邑�⨀ₑ�⨀Ԁₑ�⨀袑�⨀؀က袑�⨀ꂑ�⨀਀ꂑ�⨀䂓�⨀Ԁက䂓�⨀낑�⨀낑�⨀碒�⨀☀碒�⨀ᄐ돀㡧�⨀堘�⨀悕�⨀돐肕�⨀��⨀袕�⨀섏덐颕�⨀쀏ꀏ
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Characterization of molecular weight distribution of oligomers from autocatalyzed batch hydrolysis of xylan.

Oat spelt xylan was treated with water in a batch reactor at temperaturesof 180 and 200 degrees C. Ion-moderated partition (IMP) chromatography was thenapplied to separate oligomers in solution according to their molecular size. Calibration of the IMP measurements based on peak height was found toquantify dissolved monomer and oligomer yields well. Oligomer concentrationsin the liquid hydrolysate were also determined from the difference inxylose monomer concentrations measured by high-performance liquid chromatographybefore and after posthydrolysis of dissolved xylooligosaccharidesto xylose. Delayed formation and then rapid disappearance of oligomersfrom DP10 to DP2 was observed by IMP, and total oligomer yields measuredby IMP and posthydrolysis were very similar at these times. However, whileIMP detected virtually no oligomers initially, posthydrolysis measurementsgave significant amounts of soluble oligomers at these times, indicating thatoligomers with chain lengths >10 were in solution but not detectable by theIMP system used.

Rheological properties of polypropylenes with different molecular weight distribution characteristics

AbstractRelationships between the rheological properties and the molecular weight distribution of two polypropylene series with different molecular weight distribution characteristics were studied. The end correction coefficient in capillary flow is determined by the molecular weight Mw and the molecular weight distribution Mw/Mn, and is higher as both characteristic values are larger. The die swell ratio at a constant shear rate depends on Mw, Mw/Mn, and Mz/Mw, and is higher as the three characteristic values are larger. The critical shear rate at which a melt fracture begins to occurs depends on the molecular weight Mw and the molecular weight distribution Mz/Mw, and is proportional to Mz/Mw2 in a log–log plot. The critical shear stress does not depend on the molecular weight, and is higher as Mz/Mw is higher. The zero‐shear viscosity is determined by a molecular weight of slightly higher order than Mw, and the characteristic relaxation time is determined by Mz. The storage modulus at a constant loss modulus scarcely depends on the molecular weight, and is higher as the molecular weight distribution Mw/Mn is higher. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2128–2141, 2002

Fabrication of polymer nanodots with single electron patterning technology

Polymer nanodots 3–5 nm in diameter have been fabricated using conventional polymer resists. Negative-type resists of cresol novolak resins combined with crosslinkers are exposed with a uniform 50-kV electron beam. The uniform beam is considered to be a flux of ultrafine beams of individual electrons that causes crosslink reactions. By selecting electron beam doses and wet development conditions, dense patterns of polymer nanodots are successfully obtained. We call this process ‘‘single electron patterning technology’’ (SEPT). Sizes of the nanodots reflect the molecular-weight distribution characteristics of the resin polymers. A resist polymer with a small molecular distribution (polydispersity) leads to uniform dots. The dots are thought to be polymer microgels that consist of a few crosslinked resin molecules, or the molecules themselves. X-ray photoelectron spectroscopy confirms that the dots consist of resist polymers.

Polymer molecular weight characterization by temperature gradient high performance liquid chromatography

We report a novel method of temperature gradient interaction h.p.l.c. for the characterization of molecular weight distribution of macromolecules. A very fine and reproducible control of interaction between polymer chains and the packing material can be achieved by altering the column temperature. A mixture of 10 polystyrene standard samples (molecular weight range: 1700-2 890 000) of narrow molecular weight distribution are analysed by this method. Near complete separation down to the baseline is achieved with the use of a single C18 bonded silica column. This method is thus proven to provide a much superior resolution to the conventional size exclusion chromatography.

Light‐scattering and size‐exclusion chromatographic characterization of hydroxyethyl cellulose acetate

AbstractA recently developed analytical method of combining off‐line laser light scattering (LLS) and size exclusion chromatography (SEC) was used to investigate a set of moderately distributed hydroxyethyl cellulose acetate (HECA) samples in tetrahydrofuran (THF) at room temperature. Our results have shown that this new LLS + SEC method is suitable for the characterization of molecular weight distribution of HECA. By using this method, we have simultaneously determined two calibrations of V (cm3) = 45.3 − 1.89 log (M) and D (cm2/s) = 2.45 × 10−4 M−0.60, where M is the molecular weight of HECA; V, the elution volume in SEC; and D, the translational diffusion coefficient in dynamic LLS. In addition, our results have also indicated that the chain conformation of HECA in THF at room temperature is a slightly extended linear coil. © 1995 John Wiley &amp; Sons, Inc.

NMR characterization of erodible copolymers

The determination of copolymer composition and comonomer sequence distributions in copolymers of sebacic anhydride, 1,3-bis (p-carboxyphenoxy) propane anhydride and 1,6-bis (p-carboxyphenoxy) hexane anhydride was discussed. In addition, the characterization of molecular weight distributions by intrinsic viscosity measurements and by gel permeation chromatography using the universal calibration procedure was made

Sedimentation equilibrium in a density gradient and characterization of the molecular weight distribution of poly(methyl methacrylate)

AbstractThe determination of average molecular weights and the characterization of molecular weight distributions by the ratio of the weight‐ to number‐average molecular weights M̄w/M̄n from sedimentation equilibrium in the ultracentrifuge are described. The essential measurement of the density profile of the mixed solvent is conducted according to Munk (Macromolecules 15, 500 (1982)). Special evaluation of the polymer band in the equilibrium allowed to deduce from the observed concentration profile M̄w and M̄n of a series of poly(methyl methacrylate) samples and thus to characterize the molecular weight distribution of the samples.

Evaluation and Application of a Commercial Single Capillary Viscometer System for The Characterization of Molecular Weight Distribution and Polymer Chain Branching

Abstract The objective of this paper is to critically evaluate the performance of the newly introduced Millipore Waters GPC-viscometer system. In particular, the capabilities of the data analysis methodology are evaluated and limitations are identified. Potential modifications to improve the data analysis methodology and to make it more user friendly are suggested. The characterization of molecular weight, molecular weight distribution and polymer branching for polymer standards and polymers of commercial interest are reported.