High-performance liquid chromatography (HPLC)-size exclusion chromatography (SEC) for qualitative detection of humic substances and dissolved organic matter in mineral soils and peats in Lithuania

High-performance liquid chromatography (HPLC) with size exclusion (SEC) separation function was used to isolate and examine the molecular mass (MM) distributions and polydispersity of humic substances (HSs) and dissolved organic matter (DOM) from mineral soils and peats. The aim was to improve their detailed characterisation and to inform of their soil carbon (C) sequestration and environmental quality. This is the first study conducted in Lithuania in which HSs and DOM, separated from two soil types, have been used to characterise soil at the molecular level. The HPLC-SEC, as a separation method, was coupled with diode-array detection (DAD), thus enabling the separation of molecular fractions. Results showed that HPLC-SEC can be used to determine the MM of HSs in soil, provided that the relation between retention time and MM is known and a suitable method for fitting the HS peak is available. The UV-spectra analysis showed that DOM has a larger MM (M_w = 2439–3436 Da), which contains more aliphatic C. The HS fraction has a smaller MM (M_w = 2776 Da), with aromatic structures that reflect a higher aromaticity. Separated fractions had characteristic MMs of humic acid (HA) and fulvic acid (FA) and DOM. The HSs separated from peat samples were characterised by higher aromaticity, humification and stability. The HSs extracted from mineral soil samples showed a higher degradability level. The results also show the MM distribution and polydispersity of HS and DOM fractions (M_w/M_n = 1.009–1.252) are relatively homogenous in both soil types. Findings confirm that chromatographic and spectrometric parameters can be used for characterisation of both HSs and DOM, and for detecting changes in organic matter quality. Moreover, they can also be used for a further understanding the C-cycle and could be applied for enhancing soil C-sequestration and informing environmental quality management.     

Molecular weight dependence of crystal pattern transitions of poly(ethylene oxide)

Crystal patterns in ultrathin films of six poly(ethylene oxide) fractions with molecular weights from 25000 to 932000 g/mol were characterized within crystallization temperature range from 20 ℃ to 60 ℃.Labyrinthine,dendritic and faceted crystal patterns were observed in different temperature ranges,and then labyrinthine-to-dendritic and dendritic-tofaceted transition temperatures T L-D and T D-F were quantitatively identified.Their molecular weight dependences are T L-D(M w) = T L-D(∞) K L-D /M w,where T L-D(∞) = 38.2 ℃ and K L-D = 253000 ℃.g/mol and T D-F(M w) = T D-F(∞) K D-F /M w,where T D-F(∞) = 54.7 ℃ and K D-F = 27000 ℃.g/mol.Quasi two-dimensional blob models were proposed to provide empirical explanations of the molecular weight dependences.The labyrinthine-to-dendritic transition is attributed to a molecular diffusion process change from a local-diffusion to diffusion-limited-aggregation(DLA) and a polymer chain with M w ≈ 253000 g/mol within a blob can join crystals independently.The dendritic-to-faceted transition is attributed to a turnover of the pattern formation mechanism from DLA to crystallization control,and a polymer chain with a M w ≈ 27000 g/mol as an independent blob crosses to a depletion zone to join crystals.These molecular weight dependences reveal a macromolecular effect on the crystal pattern formation and selection of crystalline polymers.     

Enhancement of plant growth stimulation activity of irradiated alginate by fractionation

Alginate with the weight-average molecular weight (M_w) approximately 900 kDa and ratio of M (mannuronate)/G (guluronate) about 1.3 was irradiated by gamma Co-60 in aqueous solution at doses up to 200 kGy. The irradiation dose was shown to be a function for reducing M_w and molecular weight distribution of irradiated alginates. The distribution of oligomer fractions in irradiated products was also investigated by separation using ultrafiltration membranes. The irradiated alginate with M_w approximately 14.2 kDa was found to have a positive influence for growing of barley and soybean. The irradiated oligoalginate fraction with M_w ranging from 1 to 3 kDa displayed the strongest effect on the growth and development of the mentioned plants at low concentration (20 ppm). It is suggested that oligoalginate with M_w in the range 1–3 kDa is a trigger for the growth and development of plants.     

Role of Neutral Fraction of Dissolved Organic Matter in the Migration of Metals in Surface Waters: II. Neutral Metal Complexes in Water Bodies of Different Types

The results of long-term studies on the contents and seasonal dynamics of neutral carbohydrate metal complexes in the surface water bodies of different types (lakes, rivers, reservoirs) are summarized. It is established that the weight fraction of the mentioned complexes characteristically varies significantly depending on the type of the water body and the component composition of dissolved organic substances. In water bodies with a highly colored water, the relative contents of dissolved metals in neutral complexes are low due to their preferential binding to anionic complexes with humic substances. In small eutrophic water bodies with a relatively low content of humic substances, the carbohydrate concentrations are the highest and, accordingly, the weight fractions of metals in their neutral complexes is also very noticeable. The molecular weight distributions of neutral metal complexes are discussed. The neutral complexes with the molecular weight ≤5.0 kDa form a significant part of metals associated with carbohydrates. The seasonal dynamics of these metal complexes are discussed.     

Crystallizable Polyphenylacetylene-Preparation and Solution Properties

The molecular weight distribution (MWD) of crystallizable polyphenylacetylene prepared near room temperature in the presence of ferric acetylacetonate and triethylaluminum was determined through use of fractions characterized by vapor pressure osmometry and gel permeation chromatography (GPC). The number- and weight-average molecular weights (M _n and M _w) are both less than the molecular weight corresponding to the maximum of the weight distribution function, which lacks a high molecular weight tail. M _wandM _n is less than is consistent with models allowing for chain termination characteristic of vinyl polymers. GPC elution volumes are much less than those characteristic of polystyrene of similar molecular weight, and the Mark-Houwink exponent is high (2.4 for M _v 4800 to 6800). These data indicate more rodlike behavior than for polystyrene of equivalent molecular weight. The MWD and other data suggest intramolecular chain termination, possibly associated with the molecule's tendency to form paramagnetic defect states.     

Influences of molecular weight and crystalline structure on fracture behavior of controlled-rheology-polypropylene prepared by reactive extrusion

The molecular weight of ethylene-block-co-polypropylene (co-PP) was adjusted by reactive extrusion with the incorporation of dicumyl peroxide (DCP), and the effect of molecular weight on the crystallization behavior, crystal morphology, and fracture behavior was investigated. It was found that, with increasing DCP content, the molecular weight (MW) decreased and the polydispersity (M_w/M_n) slightly decreased. After modification, the number of spherulites with obscure boundaries increased, and the size of the spherulites was more even due to increasing amount of grafting and micro-cross-linking structures, generated in co-PP degradation, which were acting as nucleating agents. Evaluated by essential work of fracture method, the specific essential work of fracture, w_e, was found to be strongly dependent on the molecular weight, especially, on the number average molecular weight (M_n) linearly, while the specific non-essential work of fracture, βw_p, was enhanced with decreasing z-average molecular weight (M_z), probably owing to the reduction of ultra-high molecular weight component in degraded co-PP.     

ESI-MS analyses of lake dissolved organic matter in light of supramolecular assembly

A high-performance size-exclusion chromatography (SEC) system was coupled on-line to an electrospray ionization (ESI) interface to detect gas-phase ions by an API 365 LC/MS/MS triple quadrupole analyzer. The SEC fractions of a strongly coloured freshwater solution containing dissolved organic matter-humic substances (DOM-HS) were screened both by UV₂₅₄ and by ESI mass spectrometry (ESI-MS) in the full-scan mode within the m/z range of 100–2,900 amu in negative and positive polarities. The ESI-MS spectra were also collected by direct infusion of the DOM-HS solution in both polarities. ESI-MS spectra did not primarily favour low mass compounds, and negative and positive total ion chromatograms were parallel to the SEC elution profile obtained by UV₂₅₄ detection from DOM-HS solution. The UV₂₅₄ detection overestimated the SEC portion of higher size/mass solutes and underestimated that of solutes of smaller sizes/masses as compared with the total ion chromatogram intensities in negative or positive polarities. The change of mass-weighted and number-weighted average sizes/masses (M _w and M _n) of different SEC fractions was fairly small, in contrast to UV₂₅₄ detection, with increasing elution volume. A reasonable explanation for the great differences between M _w and M _n values, obtained by UV₂₅₄ and ESI-MS detections for eight different SEC fractions, seems to be a supramolecular-type association of relatively small components through weak dispersive forces. M _n values obtained by vapour-pressure osmometry for different SEC fractions were to some extent analogous with those of negative and positive ESI-MS. The shapes obtained by either negative or positive polarities and calculated M _w and M _n values indicated a close structural similarity between each SEC fraction. Positive ion and negative ion spectra of different humic fractions represented quite similar components, and there was no evidence for noteworthy occurrence of multiply charged ions being able to lower mass distributions of negative ion spectra. The effect of nitrogen on the mass spectra seemed to be unimportant, and the weak ions observed at even m/z values correspond most likely to the ¹³C counterparts of the more abundant ¹²C odd ions. No uncontrolled ESI fragmentation was observable and humic solutes seemed to be quite heat-resistant. Direct infusion of the untreated DOM-HS solution and statistical calculation verified that the SEC-separated different fractions really represent distinct entities of the original DOM-HS mixture. ESI-MS results support the opinion that the structural composition of humic solutes in their original combined mixture resembles supramolecular-type associations of smaller molecular size entities possessing similar structural functionalities.     

Manipulation of chain transfer agent and cross-linker concentration to modify latex micro-structure for pressure-sensitive adhesives

N-Dodecyl mercaptan (NDM) chain transfer agent and allyl methacrylate (AMA) cross-linker were used to manipulate latex properties in a starved seeded semi-batch emulsion polymerization of butyl acrylate (BA) and methyl methacrylate (MMA) or with a third monomer, acrylic acid (AA). Latexes with higher gel content and lower sol polymer molecular weight (M_w) were produced by adding only AMA. On the other hand, latexes with lower gel content and M_w were produced by adding only NDM. In addition, at a constant AMA concentration (0.2 phm), the addition of NDM (0.2 phm) decreased gel content, increased molecular weight between cross-linking points (M_c), and decreased M_w. Adding more NDM (to a total of 0.4 phm) further decreased the gel content, while decreasing the tested M_c and increasing M_w. It was also found that using higher concentrations of both AMA and NDM could produce latex with similar gel content, but smaller M_c and M_w, compared to the latex produced at lower concentrations of both NDM and AMA. Regarding the influence of AA, gel content was increased and M_w was significantly decreased with an increase in AA concentration and a decrease in MMA concentration. The performance of the latexes was evaluated for application as a pressure-sensitive adhesive (PSA).     

On the validity of the Mark-Houwink relationship at very high molecular weights

Adsorption-entanglement layers have been shown to interfere with the measurement of intrinsic viscosity of high-molecular weight (M _w10⁶) atactic polystyrene solutions. Values of the intrinsic viscosity, corrected for the pressure of any such layers are reported. The Mark-Houwink relationship is thus shown to fail at high molecular weights.     

Controlling the molecular weight of poly(2,5-bis[N-methyl-N-hexylamino]phenylene vinylene) using nitrobenzene as an inhibitor

Poly(2,5-bis[N-methyl-N-hexylamino] phenylene vinylene) (BAM-PPV) has been studied for several decades as an anti-corrosion coating. Although the polymer is readily synthesized via base-promoted radical chain polymerization, BAM-PPV exhibits poor solubility at high-molecular weights (M_w > 100 kDa), which limits its applications. In this work, the molecular weight of BAM-PPV was modulated with nitrobenzene. At nitrobenzene loading as low as 0.9 mol%, the polymerization reaction produced low-molecular weight BAM-PPV (M_w = 5–46 kDa). The polymerization of BAM-PPV was systematically studied via size-exclusion chromatography (SEC), liquid chromatography–mass spectrometry (LC-MS), and ¹H NMR spectroscopy, which revealed suppression of the dimer intermediate before chain polymerization. Collectively, this work could expand the practical applications of BAM-PPV for use in optoelectronic devices or corrosion inhibition coatings.     

Development of a new series of polyamine-type polymeric surfactants used for emulsion liquid membranes

By summarizing studies of surfactants used for emulsion liquid membrane, a new polyamine-type surfactant called LMA has been developed. This type of surfactant is composed of copolymer of isobutene and isoprene in hydrophobic site and polyethylene polyamine [NH₂(CH₂CH₂NH)_nCH₂CH₂NH₂, n≥1] in hydrophilic site. Experimental results show that the characteristics of this surfactant mainly depends on its mean molecular weight and its distribution of molecular weight, and the suitable surfactants are those with number-average molecular weight (M_n) of 5000–9000 and proper molecular weight distribution (usually M_w/M_n=3.0–6.0).     

Molecular weight and antioxidant effects on the structure of irradiated linear low density polyethylene

Linear copolymers of ethylene and butene-1 with uniform chemical microstructure and very narrow molecular weight distribution are used to study the effects of ionizing radiation. The well characterized copolymers are irradiated at room temperature with γ-rays from a ⁶⁰Co source. To follow the evolution of the molecular structure with the radiation doses, changes in molecular weight averages M_n and M_w are measured by membrane osmometry, light scattering and GPC.The influence of the original linear polymer molecular weight is examined in the range of 50,000–100,000. The effects of antioxidant are explored irradiating samples with and without additive.     

Molecular weight influence on viscosimetric parameters of poly(4-vinylpyridine) polymers

This work describes the effect of the molecular weight on the viscosimetric parameters of poly(4-vinylpyridine) (P4VP) polymers in ethanolic solution. Numerous studies concerning this question have been reported in very separate intervals of molecular weight. We have observed a discordance (discontinuity) in the variation of the intrinsic viscosity as a function of the molecular weight of these polymers ([η]=f(M_w)). In order to establish a general relationship between viscosimetric parameters and M_w, we have considered 10 P4VP samples in a wide interval of molecular weights: 0.75×10⁴ to 153×10⁴. These results have been compared and completed with that of the literature. We have observed that:

(i)

All viscosimetric parameters (intrinsic viscosity [η], Huggins constant k_H, second virial coefficient, viscosimetric expansion coefficient α_η, and critical concentration) change according to a continuous function without a break.

(ii)

The lower is the molecular weight of P4VP; the higher are the variations of the expansion coefficient and the interaction effects.

(iii)

The variation of the intrinsic viscosity versus the molecular weight follows a unique relation in the whole M_w range. In fact, the Berkowitz equation (1), described for a limited range of relatively high M_w (10⁵ to 18.5×10⁵) is extended for all M_w interval values.

(iv)

Empiric laws for [η], k_H, A₂ and C^* and variations as a function of molecular weight were proposed for the P4VP in ethanol.

     

Direct construction of silicon-silicon bond by using the low-valent titanium reagent

The reductive dimerization or polymerization of organochlorosilanes has been achieved by using the low-valent titanium reducing agent other than the alkali metals that are invariable used in the Wurtz-type coupling reaction. Applying this method, the corresponding disilanes or poly(methylvinylsilane) was obtained in good yields. The poly(methylvinylsilane) synthesized by this method is highly pure with a high molecular weight and a narrow molecular weight distribution (M_w/M_n = 1.6, M_n = 16,860).     

Chemical composition effects on the fracture of polystyrene-block-poly(methyl methacrylate)block copolymers

The crazing and fracture behaviors of glassy–glassy block copolymers were investigated for polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymers that had similar overall molecular weights but different poly(methyl methacrylate) (PMMA) molar fractions. A liquid chromatography technique was applied to separate as-synthesized PS-b-PMMA [(1) weight-average molecular weight (M_w) = 94,000 g/mol and PMMA molar fraction = 0.35 and (2) M_w = 65,000 g/mol and PMMA molar fraction = 0.28] into three fractions with different chemical compositions. With a copper-grid technique, the fracture behaviors of 0.5-μm-thick PS-b-PMMA films were studied as a function of the applied strain. For the higher M_w PS-b-PMMA samples, the median strains at crazing and fibril breakdown increased with an increase in the PMMA molar fraction from 0.24 to 0.46, corresponding to an increase in the chain entanglements in the PMMA domains. In contrast, for the lower M_w samples, the two values were not significantly changed even when the PMMA molar fraction was varied from 0.16 to 0.35. M_w of the minor component in PS-b-PMMA played a critical role in controlling the fracture behaviors of the block copolymers. Specifically, M_w/M_e of the minor component (where M_e is the molecular weight between entanglements) had to be roughly larger than 2 for the block copolymers to sustain sufficient strains before fracture. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3612–3620, 2006     

Ratios of average molecular weights and molecular weight distributions in polymers

If M_min and M_max are lower and upper bounds, respectively, to the molecular weights of different molecular weight species contained in a polymer, the weight-average to number-average molecular weight ratio M _w/M _n cannot exceed (1 + M_max/M_min)²/(4M_max/M_min). The ratio attains this maximum possible value if the masses of the two species with molecular weights M_min and M_max are equal and the masses of all the other species are negligibly small, corresponding to maximum spread in the molecular weight distribution within the specified bounds. Also for a given value of M _w/M _n = α, the M_max cannot be smaller than [2α ? 1 + 2α^1/2(α ? 1)^1/2]M_min. The minimum possible value of M_max/M_min consistent with α given is obtained in the case of maximum spread described above. If only one species is predominant, then both M _w/M _n and M_max/M_min approach unity, as is well known. Similar relations hold for the ratios of higher-order average molecular weights for which the role of the mass fractions is replaced by higher-order distribution functions.     

Atom transfer radical polymerization of n-octyl acrylate under microwave irradiation

The atom transfer radical polymerization (ATRP) of n-octyl acrylate (OA) was successfully carried out using ethyl-2-bromobutyrate as an initiator, and CuBr/2,2^′-bipyridine (bpy) as a catalyst under microwave irradiation (MI) at 76.8 °C. The polymerization of n-octyl acrylate under MI showed linear first-order rate plots, a linear increase of the number-average molecular weight M_n with conversion, and low polydispersities, 1.1<M_w/M_n<1.4, where M_w is weight-average molecular weight. The ATRP of n-octyl acrylate is well controlled. Under the same experimental conditions, the apparent rate constant, k_p^app, under MI is larger apparently than that under conventional heating. In addition, the effects of concentration of catalyst and other factors on polymerization are reported.     

New Interpretations: Molecular Weight Averages for a Polymer

The statement is often made in the polymer literature, without proof, that M _z ≤ M _w ≤ M _n, where M _z, M _w, and M _n are the z-, z weight-, and number-average molecular weights respectively. Four proofs of a generalization of these inequalities are given. It is shown that a higher-order molecular weight average is larger than a lower-order one, regardless of the form of the molecular weight distributions, except for the case when all the molecules have the same molecular weight. A brief discussion of the viscosity-average molecular weight is also included.     

Dendritic analogues of engineering plastics - a general one-step synthesis of dendritic polyaryl ethers

The first general single-step route to dendritic or cascade polyaryl ethers analogous to common linear polyaryl ethers is described. The sodium salts of four AB₂ monomers each containing a single phenolic hydroxyl group and two aryl fluorides activated toward nucleophilic substitution by carbonyl, sulphonyl, or tetrafluorophenyl moieties are shown to polymerize in hot N, N-dimethylacetamide. The products are high molecular weight polymers (7000 < M_n < 36000), have narrow polydispersities (1.50 < M_w/M_n < 4.50), and are highly soluble in organic solvents. The molecular weights of two of the polymers increase with monomer concentration. The polymers are thermally stable (500 °C under N₂) and have glass transition temperatures ranging from 135 to 231 °C.     

Chromatographic mass of a polymer and its use for determining the molecular mass characteristics

The method has been proposed for determining the molecular characteristics of flexible-chain polymers that obey the universal calibration principle and for which there are available experimental data on the intrinsic viscosity. This method is based on studying the dependence of the hydrodynamic volume M _n[η], M _w[η], M _z[η], and M _η[η] on parameter a in the Mark-Kuhn-Houwink equation. It has been found that, for parameter a varying in the range from 0.5 to 0.8, the weight-average hydrodynamic volume M _w[η] remains almost unchanged. This allows estimation of M _w based on a single intrinsic viscosity value. The notion of the chromatographic mass of a polymer is advanced and is employed for determining other molecular mass parameters.