Low Molecular Weight Material Research Articles

Characterisation of humic substances by acid catalysed transesterification

The chemical composition of low molecular weight moieties linked to the core structures of humic substances (HS) are of substantial importance for the understanding of the chemical structures and mode of interactions of HS with other substances in the environment. In this study a novel approach to characterise certain low molecular weight compounds bound to HS is suggested. The method includes transesterification (TE) of ester and amide bound structures, and esterification (E) of free carboxylic groups using acid catalysed methanolysis followed by gas chromatography (GC)–mass spectrometry (MS) and GC-FID analysis. Methanolysis of five HS of different origin, demonstrated the presence of multifunctional hydroxy-substituted benzoic acids, hexoses, and long chain fatty acids. Based on GC-FID and addition of the internal standard before methanolysis, the total amounts of low molecular weight material could be estimated. In case of HS from the aqueous sources the yields were below 1%, whereas in case of the HS derived from lignite the yield was significantly higher. The hydrophobic long chain fatty acids constituted about one-third of this material. Principal component analysis (PCA), which was used for extended data evaluation, differentiated between the samples in terms of chemical composition.

Molecular weight distribution from viscoelastic data: The importance of tube renewal and Rouse modes

Rheological models based on molecular dynamics (as opposite to empirical relationships) are now preferred to link the molecular weight distribution (MWD) of linear polymers to their rheological properties. These models incorporate the double reptation concept, which represents the relaxation modulus as an integral over the molecular weight distribution. We propose a method that incorporates a detailed modeling of all the relevant relaxation processes, including Rouse fast and longitudinal modes and glassy relaxation. In addition, we take into account the effect of polydispersity on the relaxation times for reptation, i.e., “tube renewal.” In order to demonstrate the importance of these features of our technique, we compare it with one involving the direct inversion of the double reptation integral without accounting for tube renewal and additional relaxation processes. To invert the relaxation modulus in terms of the molecular weight distribution, one must either solve the ill-posed problem using an efficient numerical algorithm or postulate a function to describe the MWD. The second approach is more robust and less sensitive to noisy data, but one must assume the form of the MWD a priori. We present a procedure for selecting this function and use it to compare the two approaches to inverting a G(t) model. Data for several binary blends and commercial polymers are analyzed using both approaches. We conclude that the more detailed technique is necessary when the MWD is broad or when there is a significant amount of low-molecular weight material present.

Layer-by-Layer Self-Assembling of a Low Molecular Weight Organic Material by Different Electrostatic Adsorption Processes

A novel method has been reported to deposit low molecular weight organic materials by a layer-by-layer self-assembled film deposition method. A different electrostatic adsorption process has been followed to show its advantage over the conventional method. When an organic material, namely, alizarin violet, with fewer ionic groups was used as the anion for electrostatic self-assembly, the dye was found to come off the film during the subsequent polycation deposition. As an alternative approach, the dye has been anchored to the polycation via a physiabsorption process prior to deposition. By controlling the concentration ratio of the two, the combination of low molecular weight material and the polycation as a whole has been used as the polycation. The absorption spectra after each deposition showed that there was no material loss during layer depositions. The results showed that the method of anchoring low molecular weight molecules to polyions could effectively be used to deposit homogeneous layer-by-laye...

The influence of deep traps on transient current–voltage characteristics of organic light-emitting diodes

Abstract Transient current–voltage (I–V) characteristics of organic light-emitting diodes made from both conjugated polymers and low molecular-weight materials show hysteresis effects in the reverse bias regime depending on the direction and speed of the bias sweep. This behaviour is quantitatively investigated here for the example of devices based on N,N′-diphenyl-N,N′-bis(1-naphtyl)-1,1′-biphenyl-4,4′-diamine with Ca and indium–tin oxide as electrodes. To clarify the origin of this peculiarity numerical simulations have been carried out supposing the existence of deep acceptor-like trap states. Typical trends are shown by systematically varying parameters such as measuring conditions, trap characteristics, basic doping level, mobility and injection conditions. Based on the simulated potential and concentration profiles it is shown that the hysteresis of the I–V characteristics is caused by recharging of deep traps for holes. It occurs only if the reverse steady-state current is lower than the trap recharging current and if both currents have different bias dependencies. The origin for the large time needed for the traps to relax into the equilibrium state is clarified. In accordance with the high barrier for the holes at the cathode the calculated reverse current is much smaller than the measured one. Using a new analytical expression for the Schottky diode I–V characteristics for a low-doped thin film device, it is shown qualitatively that in real devices a leakage current should dominate for reverse bias.

ESR and NMR Studies of a Novel Conjugated Donor−Acceptor Polymer Containing Magnetic Spins: Poly[bicarbazolylene-alt-phenylene-bis(cyanovinylene)

A conjugated alternating copolymer, poly[bicarbazolylene-alt-phenylenebis(cyanovinylene)] (named PCPV), was prepared via the Knoevenagel condensation in the presence of tetrabutylammonium hydroxide. Tetrahydrofuran-soluble low molecular weight materials and insoluble polymer were obtained. The two fractions were characterized in detail by ESR and solid and solution NMR spectroscopies. Strong ESR signals were observed from the pristine PCPV polymer in both fractions. This was explained by the rotational cleavage of the cis vinylic bond to produce two π radicals, called biradicals, which diffuse along the main chains to form radical cations stabilized by conjugation in the alternating repeating moieties. In particular, the insoluble fraction shows an extremely strong and sharp line width signal with a spin concentration equal to 5.8 × 1017 spins/g. The alternating copolymer synthesized can be considered as a novel conjugated material analogous to substituted polyacetylenes with respect to the mobile unpaire...

Identification of Crystalline and Amorphous Regions in Low Molecular Weight Materials Using Microthermal Analysis

The use of microthermal analysis as a means of differentiating between amorphous and crystalline forms of a model low molecular weight solid has been investigated, with a view to establishing the strengths and limitations of the technique as a means of identifying different physical forms of the same substance within a single sample. Samples of the drug indometacin were prepared in amorphous, crystalline, and partially crystalline forms and the samples studied using microthermal analysis (MTA) and modulated temperature differential scanning calorimetry (MTDSC). MTDSC studies revealed a simple melting process for the crystalline material corresponding to the ? form of the drug, while a glass transition, recrystallization and subsequent melting was seen for the amorphous indometacin; in addition, the sample demonstrated a shift in the phase angle at approximately 65 °C which has been previously associated with flow of the sample in the DSC pan. MTA studies indicated that the thermal conductivity is dominated by the surface topology and no clear differentiation between amorphous and crystalline regions was obtained in this mode. A discontinuity was seen in the localized thermal analysis (LTA) profile for the crystalline sample at 152 °C corresponding to the melting of the material. Similarly, the amorphous material showed a shift in probe position at 64 °C, these responses being ascribed to sample softening. Topographical examination of the sample following LTA studies indicated the presence of a “crater” approximately 10-20 µm in diameter, giving an indication of the scale of scrutiny of the experiment. Studies using different scanning rates (2 to 20 °C/s) did not demonstrate significant changes to the temperatures of transition and crater size. Studies on partially crystalline samples clearly showed that the technique was able to differentiate between the two physical regions in LTA mode. The technique may therefore have considerable potential as a means of identifying distinct physical regions in a single sample. However, consideration needs to be given to the experimental and interpretative issues raised in this investigation.

Review Melt crystallisation of polymer materials: The role of the thermal conductivity and its influence on the microstructure

Even though many characteristics of the quiescent crystallisation process of polymers have been deeply investigated in the long run, some subjects are still open. Indeed, the theory of nucleation and growth, originally developed for metals and low molecular weight materials, have been extended to polymers without taking into account all the characteristics of macromolecules that can induce a crystallisation behaviour different from that of atoms or small molecules. This paper, within the framework of nucleation and growth, reviews the main aspects of the crystallisation process of polymers and discusses its influence on the microstructure, also in comparison with metallic and ceramic materials. Additional factors, often neglected, affecting the crystallisation process have been also identified. In particular, consideration is given to heat flow from solid to melt during polymer crystallisation that can provide an explanation for modified spherulitic morphologies found in fibre based composite materials and blends. Moreover, the circumstances under which thermal gradients within polymer samples may play a dominant role in crystallisation are underlined. Finally, several examples of polymer textures, that could have been originated by heat transfer anisotropy inside samples, are shown.

The Combined Influence of Molecular Weight and Temperature on the Physical Aging and Creep Compliance of a Glassy Thermoplastic Polyimide

The effect of molecular weight on the viscoelastic performance of anadvanced polymer (LaRC™-SI) was investigated through the use of creepcompliance tests. Testing consisted of short-term isothermal creep andrecovery with the creep segments performed under constant load. Thetests were conducted at three temperatures below the glass transitiontemperature of five materials of different molecular weight. Through theuse of time-aging-time superposition procedures, the material constants,material master curves and aging-related parameters were evaluated ateach temperature for a given molecular weight. The time-temperaturesuperposition technique helped to describe the effect of temperature onthe timescale of the viscoelastic response of each molecular weight. Itwas shown that the low molecular weight materials have higher creepcompliance and creep rate, and are more sensitive to temperature thanthe high molecular weight materials. Furthermore, a critical molecularweight transition was observed to occur at a weight-average molecularweight of $${\bar M}$$ w ∼ 25,000 g/mol below which, the temperature sensitivity of thetime-temperature superposition shift factor increases significantly. Theshort-term creep compliance data were used in association with Struik'seffective time theory to predict the long-term creep compliance behaviorfor the different molecular weights. At long timescales, physical agingserves to significantly decrease the creep compliance and creep rate ofall the materials tested. Long-term test data verified the predictivecreep behavior. Materials with higher temperature and lower molecularweights had greater creep compliance and higher creep rates.

A modified method for isolating poly(vinyl alcohol)-degrading bacteria and study of their degradation patterns

An addition of catalase or peroxidase into an agar plate containing poly(vinyl alcohol) (PVA), was effective for the isolation of PVA-degrading microorganisms. A Gram-negative bacterium, strain TK-2 (γ-group of proteobacteria), rapidly degraded a high molecular weight PVA to low molecular weight material after 1 day thereby producing oligomers of PVA as shown by gel permeation chromatography. Conversely, Sphingomonas strain TJ-7 did not produce any PVA oligomers, suggesting that the strain TJ-7 degraded PVA from the terminal ends of the molecules, whereas the strain TK-2 cleaved PVA at random.

Generation of Hydroxyl Radicals from Polymer Containing Reducing Sugar Branches in the Presence of Hydrogen Peroxide

The generation of hydroxyl radicals from poly (6-O-vinyladipoyl-D-glucose) [poly(VAG) in the presence of hydrogen peroxide was detected by oxidation of p-nitrosodimethylaniline (p-NDA). Several hydroxyl radical scavengers inhibited the reaction, but inhibition by superoxide dismutase (SOD) was low. This suggests that hydroxyl radicals from Harber-Weiss reaction made relatively small contributions to this reaction, and that the enediol moiety of reducing sugar branches in poly(VAG) reacted with hydrogen peroxide to produce hydroxyl radicals. The polymer degraded to a low molecular weight material after reaction with hydrogen peroxide.

Observations on the α‐Amylolysis Pattern of Some Waxy Maize Starches from Inbred Line Ia453

ABSTRACTMaize starches of the endosperm mutants waxy (wx), dull:waxy (duwx), and amylose‐extender:dull:waxy (aeduwx) from inbred line Ia453 lack amylose. However, in addition to high molecular weight (HMW) amylopectin, the duwx and aeduwx starches contained 40 and 80%, respectively, intermediate branched material of low molecular weight (LMW). As gelatinized, the amylopectin of the wx starch was easily hydrolyzed into small dextrins by the α‐amylase of B. amyloliquefaciens, but components of duwx and aeduwx possessed partial resistance to amylolytic attack. Residual material of intermediate size obtained by a 4‐hr α‐amylolysis could not be separated from LMW dextrins by fractional precipitation in methanol. It is suggested that this material possessed a more regularly branched structure, in which the d‐glucosyl chain segments were too short to allow α‐amylase action. The granular starches of duwx and aeduwx genotypes were initially considerably more resistant than the wx sample to α‐amylase attack. This was possibly due to an altered structure in the amylopectin component or the high content of intermediate material in the former granules.

Novel cardiac peptide hormone in several teleosts.

To find out the significance of the newest member of the natriuretic peptide family, salmon cardiac peptide (sCP), we have determined the distribution of the peptide and its mRNA as well as the tissue and plasma molecular forms in several teleosts. Using probes based on the salmon sCP cDNA in Northern blot analysis we found mRNA homologous to that of sCP to be present in the heart of 15 fish species representing nine different genera. We developed a specific RIA for the salmon 29 amino acid peptide to be able to study the distribution of the peptide in the heart and plasma of different fish species. Despite the probable interspecies differences in the peptide sequence, large quantities of immunoreactive sCP were found in the atrium, ventricle and plasma of most of the fish species studied, suggesting that a cardiac hormone homologous to sCP has an endocrine function in a large variety of teleost species. The molecular form of the hormone secreted and stored in the tissue was determined by gel filtration high pressure liquid chromatography. In salmon, as in most of the other fish species studied, the predominant immunoreactive sCP in plasma corresponded to the low molecular weight form, with a size similar to that of the biologically active 29 amino acid sCP (sCP-29), whereas the form stored in the heart corresponded to the high molecular weight pro-sCP-sized material. The form secreted by isolated perfused salmon ventricle, in the basal state as well as when mechanically loaded, was the sCP-29-sized peptide, thus ruling out the possibility that the conversion from high to low molecular weight material is caused by plasma proteases. In conclusion, sCP-like peptides are produced and secreted from the heart of a large number of different fish species. Their post-translational processing appears to be remarkably similar to that of mammalian atrial natriuretic peptide.

The surface structure of Dow Cyclotene 3022, as determined by photoacoustic FTIR, confocal Raman and photoelectron spectroscopies

The cure of B-staged Cyclotene 3022 has been studied by photoacoustic FTIR and X-ray photoelectron spectroscopy (XPS). The data indicate that the cure reaction is second-order, consistent with a Diels–Alder reaction. The lack of any loss of aromaticity on cure, as observed by both photoacoustic FTIR and XPS, and confirmed by confocal Raman spectroscopy, indicates a reaction mechanism involving the production of tetrahydronaphthalene. The lack of structural differences between surface and bulk, determined by a comparison of photoacoustic and published transmission FTIR data, indicates that the cyclobutane homopolymerization pathway lies at an energy too high to be attained even with an added surface energy. The polymer loses a significant amount of low molecular weight material on curing.

Cellular Uptake of Clostridium botulinum C2 Toxin Requires Oligomerization and Acidification

The actin-ADP-ribosylating binary Clostridium botulinum C2 toxin consists of two individual proteins, the binding/translocation component C2II and the enzyme component C2I. To elicit its cytotoxic action, C2II binds to a receptor on the cell surface and mediates cell entry of C2I via receptor-mediated endocytosis. Here we report that binding of C2II to the surface of target cells requires cleavage of C2II by trypsin. Trypsin cleavage causes oligomerization of the activated C2II (C2IIa) to give SDS-stable heptameric structures, which exhibit a characteristic annular or horseshoe shape and form channels in lipid bilayer membranes. Cytosolic delivery of the enzyme component C2I is blocked by bafilomycin but not by brefeldin A or nocodazole, indicating uptake from an endosomal compartment and requirement of endosomal acidification for cell entry. In the presence of C2IIa and C2I, short term acidification of the extracellular medium (pH 5.4) allows C2I to enter the cytosol directly. Our data indicate that entry of C2 toxin into cells involves (i) activation of C2II by trypsin-cleavage, (ii) oligomerization of cleaved C2IIa to heptamers, (iii) binding of the C2IIa oligomers to the carbohydrate receptor on the cell surface and assembly with C2I, (iv) receptor-mediated endocytosis of both C2 components into endosomes, and finally (v) translocation and release of C2I into the cytosol after acidification of the endosomal compartment.

Quantification of Muconic Acid-Type Structures in High Molecular Weight Material from Bleach Plant Effluents

Summary Effluents from the chlorine dioxide bleaching of chemical pulps were characterized with respect to high and low molecular weight material, COD, TOC, elemental composition, and carboxylic acid and methyl ester content. A technique was developed for the quantitative estimation of muconic acid-type structures as moities of the high molecular weight material. The method is based on the use of quantitative 13C NMR and the pH-dependent cyclization of muconic acids in aqueous solution. 20–40 % of the carboxylic acids in the high molecular weight material isolated from bleach plant effluents from three softwood kraft mills were found to exhibit muconic acid-type behaviour.

Role of cyclization in the synthesis of hyperbranched aliphatic polyesters

The intramolecular cyclization in hyperbranching polycondensation reactions of the type AB2/Bf was investigated both theoretically and experimentally. Hyperbranched aliphatic polyesters prepared from 2,2-bis(hydroxymethyl)propionic acid (BisMPA) and 1,1,1-tris(hydroxymethyl)propane (TMP) as B3 core molecule as well as commercially available samples with ethoxylated pentaerythrol as B4 core (Perstorp, Boltorn®) have been investigated with a combination of methods (NMR, vapor pressure osmometry (VPO), size exclusion chromatography (SEC), MALDI-TOF-MS) with respect to the formation of intramolecular cycles during polymerization. Absolute molecular weights were determined for the first time and found to be in the range of 632 (G2) to 1046 g/mol (G5) and 827 (PG2) to 1974 g/mol (PG5) for the commercial samples, respectively, i. e. considerably lower than reported previously. Theoretical considerations as well as experimental data clearly demonstrate cyclization to be an important reaction regardless of the presence of a core. The fraction if cyclized molecules was found to increase with molecular weights from 32% to 48%, limiting molecular weights drastically. In summary, previously reported results could not be confirmed. Incremental addition of the BisMPA monomer to the TMP-core did not permit to control molecular weights and the term “pseudogeneration” is inappropriate for the low molecular weight materials formed.

Role of cyclization in the synthesis of hyperbranched aliphatic polyesters

The intramolecular cyclization in hyperbranching polycondensation reactions of the type AB2/Bf was investigated both theoretically and experimentally. Hyperbranched aliphatic polyesters prepared from 2,2-bis(hydroxymethyl)propionic acid (BisMPA) and 1,1,1-tris(hydroxymethyl)propane (TMP) as B3 core molecule as well as commercially available samples with ethoxylated pentaerythrol as B4 core (Perstorp, Boltorn®) have been investigated with a combination of methods (NMR, vapor pressure osmometry (VPO), size exclusion chromatography (SEC), MALDI-TOF-MS) with respect to the formation of intramolecular cycles during polymerization. Absolute molecular weights were determined for the first time and found to be in the range of 632 (G2) to 1046 g/mol (G5) and 827 (PG2) to 1974 g/mol (PG5) for the commercial samples, respectively, i. e. considerably lower than reported previously. Theoretical considerations as well as experimental data clearly demonstrate cyclization to be an important reaction regardless of the presence of a core. The fraction if cyclized molecules was found to increase with molecular weights from 32% to 48%, limiting molecular weights drastically. In summary, previously reported results could not be confirmed. Incremental addition of the BisMPA monomer to the TMP-core did not permit to control molecular weights and the term “pseudogeneration” is inappropriate for the low molecular weight materials formed.

Influence of physico-chemical properties of chitin and chitosan on complement activation

To recognize the complement activation by chitin and chitosan, various physico-chemical aspects were studied. Complement activation was determined by change of plasma C3 concentration using single radial immunodiffusion method. Results were as follows: in samples with homogeneous acetylation (oDAC), C3 concentration was decreased with the increase in the degree of acetylation of oDAC. In oDACs 50 and 42 (water soluble materials), however, C3 was not decreased. In samples with heterogeneous acetylation or deacetylation samples (eDAC), all samples showed C3 activation even in eDAC 53 (solid material) and have almost same activity on complement activation. C3 activation was not seen in low molecular weight materials including d-glucosamine. The important factors inducing complement activation by chitosan-based mucopolysaccharide should be solids. The samples with heterogeneous acetylation (eDACs) had a stable ability on complement activation than with homogeneous acetylation (oDACs).

Effects of the chemical structure on the miscibility level and properties of phenoxy/polymethacrylate blends

The important effect that even a small change in the nature of the side chain of a component of a blend has in its miscibility level was observed in a series of blends of phenoxy (Ph) with poly(methacrylates). Thus, while the Ph/poly(methyl methacrylate) blends are miscible and the Ph/poly(ethyl methacrylate) blends partially miscible, Ph/poly(butyl methacrylate) blends were almost fully immiscible. The observed miscibility of Ph/poly(butylmethyl methacrylate) indicates that the change in a component of a miscible blend of some pendant units that give rise to miscibility, by those from a different second component, which give rise to immiscibility is less important. The observed decrease in the strength of the β secondary transition of Ph was clearly related to the miscibility level of the blends. The negative effects on properties of a very low molecular weight material can be overcame by blending with a miscible second component, rendering the overall molecular weight of the blend above the critical value. The change in the nature of the side chain, apart from the negative effect on fracture properties such as ductility, also had considerable effect on the short-term mechanical properties such as modulus of elasticity and yield stress. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2978–2986, 2000

Soluble fibrin species in arterial thrombi

The aim of this study was to characterize soluble fibrin(ogen) species in human, arterial, in-vivo-formed thrombi, using the immunoblotting technique. Specimens were collected via intra-arterial catheters in six patients scheduled for catheter-directed thrombolysis. Unreduced and reduced samples of the supernatants from the arterial thrombi-derived specimens were electrophoresed on polyacrylamide gels and immunoblotted, using specific mono- and polyclonal anti-fibrin(ogen) antibodies. The reduced samples disclosed substantial amounts of high molecular weight material, consistent with alpha-chain polymers and gammagamma-dimers, as well as lower molecular weight material, such as alpha-, beta- and gamma-chains. No fibrinogen with intact fibrinopeptide A was detectable, and des-AABB fibrin represented a major fibrin derivative in the soluble part of the arterial thrombi. The alpha-chains were C-terminally degraded, most of them distal to position 259. In conclusion, we have demonstrated the presence of cross-linked fibrin derivatives in soluble, arterial thrombus-related material, without signs of fibrinogen-fibrin hybrids. The fibrin derivatives were C-terminally degraded, thus representing X-oligomeric material, most probably originating from plasmin degradation of insoluble thrombus fibrin. The present study supports the hypothesis of a dynamic equilibrium between clotting and lysis in thrombi.