Low Molecular Weight Chains Research Articles

New insights on the structure of algaenan from Botryoccocus braunii race A and its hexane insoluble botryals based on multidimensional NMR spectroscopy and electrospray–mass spectrometry techniques

Through solution state NMR spectroscopy and quadrupole-time-of-flight mass spectrometry (Q-TOF MS) studies of the hexane insoluble botryal extract of the algae Botryococcus braunii race A, coupled with high-resolution magic-angle-spinning (HR-MAS) NMR spectroscopy of the algaenan from this alga, it has been possible to advance the structural understanding of this geochemically important biopolymer. It was found that the hexane insoluble botryals in this study constitute a mixture of low molecular weight unsaturated aliphatic aldehydes and unsaturated hydrocarbons with an average chain length of about 40 carbons. Exact assignments were provided for many of the structural units present and describe the average constitution of the mixture as a whole. Reticulation of the low molecular weight chains via acetal and ester links explains the primary make up of algaenan. In this study, it is concluded that the algaenan results from the reticulation of low molecular weight hexane insoluble botryal species rather than the polyaldehyde as previously observed in studies of algae at alternate stages of their growth cycle.

Adhesion of Thermoplastic Matrices to Carbon Fibers: Effect of Polymer Molecular Weight and Fiber Surface Chemistry

A structure property study between the interfacial adhesion and the polymer molecular weight, fiber chemistry and processing temperature in thermoplastic composites was undertaken. In the first phase of the study, four different molecular weight grades of pure bisphenol A polycarbonate (BPA-PC) and three different consolidation temperatures were used. Molecular weight and processing temperatures were found to have significant effect on interfacial adhesion, with a major increase in adhesion between the low molecular weight grade, which is below the critical molecular weight and the next higher molecular weight grade and a monotonous increase, but to a lesser degree above the critical limit. Further, the study shows that the interface might affect the segregation of polymer chains by their molecular weight resulting in a higher concentration of low molecular weight chains at the interface and consequent loss in mechanical properties [1]. In the second phase of the study, Hercules IM7 carbon fiber with four different levels of proprietary oxidative surface treatment and subsequent hydrogen passivation were employed to determine the effects of fiber chemistry on the interfacial adhesion. It was observed that, differing amount of surface oxygen functionalities have no effect on the level of adhesion, indicating insignificant polar or hydrogen bonding interactions between the fiber and the matrix. The level of adhesion was also found to be very weakly dependent on mechanical interlocking between the fiber and the matrix. In the third phase of the study, BPA-PC and polymethyl methacrylate (PMMA) were grafted on the surface of carbon fiber to study the effect of grafted chain length and entanglement on improvement in adhesion over ungrafted fiber composites. Grafting polymer chains to the fiber surface produced a penetrable interface, which improved adhesion levels by 25-100% over the ungrafted composites. The improvement in adhesion was independent of molecular weight of the grafted chains.

Poly(propylene carbonate). 1. More about Poly(propylene carbonate) Formed from the Copolymerization of Propylene Oxide and Carbon Dioxide Employing a Zinc Glutarate Catalyst

Propylene oxide, PO, reacts when heated to 60 °C over a zinc glutarate catalyst to form poly(propylene oxide), PPO, which is regioregular, HTHTHT, and favors isotactic triads ii. Under 50 bar of CO2, poly(propylene carbonate), PPC, is formed with less than 5% polyether linkages and with an even smaller component of propylene carbonate, PC. These reactions have been studied as a function of time, and the products have been analyzed by GPC, MALDI−TOF/MS, and 13C {1H} NMR spectroscopy. Polymerization of PO yields PPO with −OH and −H end groups, and in the copolymerization of PO and CO2 the low molecular weight chains are readily identified as an alternating copolymer represented as (PO)n-alt-(CO2)m, where m = n − 1, n − 2, n − 3, n − 4, n − 5, with terminal −OH and −H groups. These results, combined with NMR data, implicate Zn−OH groups as the active initiating species, and furthermore from the molecular weight of the polymer produced at short reaction times, we can infer that some Zn−OH sites are highly act...

Residual dipolar coupling for the assessment of cross-link density changes in γ-irradiated silica-PDMS composite materials

We have measured changes in transverse relaxation times (T2e), residual dipolar couplings (〈Ωd2〉), and the mean-squared fluctuations in the residual dipolar couplings (〈δΩd2〉), associated with cross-link density changes in a complex silica-filled polydiphenylpolysiloxane/polydimethylsiloxane (PDPS/PDMS) block copolymer composite material. The crosslinks were induced by both chemical modification of the base polymer and by radiolytic aging. We have detected H1 NMR responses from polymer chains directly interacting with the silica filler (〈δΩd2〉≫2×106 rad2 sec−2), network polymer chains not directly interacting with the silica filler (〈Ωd2〉∼2×106 rad2 sec−2 and 〈δΩd2〉∼2×106 rad2 sec−2) and non-network, low molecular weight chains and chain ends (〈δΩd2〉∼1×105 rad2 sec−2). The network domain and the non-network domain were observed to exchange spin magnetization with a correlation time of 1 sec. No evidence of spin-exchange effects on the stimulated echo were observed between the PDMS and PDPS blocks, although the blocks were observed to be in spatial proximity by double quantum NMR methods. The residual dipolar couplings change in a straightforward manner with radiation and chemically induced cross-linking of the polymer network. The strength of the filler-polymer interaction was seen to affect only the residual dipolar couplings and the transverse relaxation times and not directly the mean-squared fluctuations of the residual dipolar couplings. Dipolar correlation effect NMR shows direct evidence for surface adsorbed species, however, and has measured changes in the amount of surface adsorption due to irradiation. The results suggest that siloxane polymer cross-linking was preceded by an initial disruption of the hydrogen bond interaction between the polymer backbone and the silica silanol groups at the polymer/silica interface and that noticeable radiation induced cross-linking then occurs at dosages above 100 kGray. The work reported here shows that detailed characterization of the relaxation processes of the various nuclei in the siloxane polymers under static conditions has the potential to provide detailed insight into changes in the mechanisms and energetics of motional processes brought about by polymer aging processes.

Molecular weight effect on swelling of polymer gels in homopolymer solutions: a fluorescence study

A novel technique based on in situ steady state fluorescence measurements is introduced for studying swelling processes of gels formed by free radical crosslinking copolymerization of methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDM) in homopolymer solutions. Gels were prepared at 55±2 °C for various EGDM contents. After drying these gels, swelling experiments were performed in chloroform solution of anthracene labeled poly(methyl methacrylate) (An-PMMA) in various molecular weights at room temperature by real time monitoring of anthracene fluorescence intensity. Anthracene labeled PMMA chains having various molecular weights were prepared by atom transfer radical polymerization at 90 °C. During the swelling experiments, it was observed that anthracene emission intensities increased due to trapping of An-PMMA chains into the gel as the swelling time is increased. The trapping of An-PMMA chains in swollen gel, increase by obeying parabolic law in time. Penetration time constant, τ of PMMA chains were measured and found to be increased as the crosslinker density of gel is increased. It is observed that τ values are much higher for high molecular weight An-PMMA chains than low molecular weight chains in all gel samples.

Reactive Blending of Functional PS and PMMA: Interfacial Behavior of in situ Formed Graft Copolymers

ω-Isocyanate PMMA, α-anhydride PMMA, and PS-co-PSNH2 have been prepared by atom transfer radical polymerization (ATRP) with controlled molecular weights (104 and 3.5 × 104) and low polydispersity (1.2). They have been used as precursors of PS-g-PMMA copolymers and let to react in the melt (170 °C, for 10 min) under moderate shear rate. The well-controlled molecular characteristics of these precursors are a substantial advantage to study the effect of the kinetics of the interfacial reaction on the phase morphology. When the grafting reaction is fast (NH2/anhydride pair) and low molecular weight chains are used, the interfacial reaction is quasi-complete and a nanophase morphology is observed, whereas limited reaction and formation of microphases are observed in all the other cases. A high reaction yield requires not only that the functional groups are highly mutually reactive but also that the interface is anytime made available to the functional polymers for the reaction to progress. Then, a nanophase mo...

Study of the hydrolytic degradation of polydioxanone PPDX

The hydrolytic degradation of bioabsorbable polydioxanone (PPDX) was studied in a phosphate buffer solution, pH=7.4, 37°C. The degradation was evaluated by analyzing the changes in the elastic modulus and morphological changes. After 10 weeks, the weight loss, pH and molecular weight changes suggested diffusion of low molecular weight chain segments into the reaction medium as a consequence of the breaking of ester bonds in the material. The crystallinity ( X c) increased with hydrolysis time, indicating the first step of the degradation process. Modulus was found to decrease, indicating that chain scission proceeded in two steps: the first occurring in the amorphous regions of microfibrils and intermicrofibrillar space; the second in the crystalline regions, resulting in a lower stress value for the chain segments when submitted to external forces. Surface morphological changes suggest a heterogeneous degradation mechanism by layers.

Gelation Kinetics of Gelatin: A Master Curve and Network Modeling

The gelation kinetics of gelatin has been exhaustively studied using oscillatory rheometry for six molecular weight distributions, three concentrations, and four temperatures. Measurements lasted up to 31/2 months, much longer than previous studies. Remarkably, all of the data can be superimposed on a single master curve using suitable shift factors. The existence of a master curve shows that, over the broad range of variables studied, the gelation processes are identical; altering the variables just changes the scaling factors for elasticity and time. Empirically, it is found that neither high nor low molecular weight chains contribute to the elasticity. A statistical network model has been developed, based on Flory's phantom network formalism. It gives reasonable fits to the experimental gel times and is compatible with the observed dependence of elasticity on molecular weight distribution. A second order reaction kinetics model has also been developed which satisfactorily models the early stages of gelation.

Electrical and environmental aging of silicone rubber used in outdoor insulation

This is a laboratory study on the aging of SIR and hydrophobicity recovery. UV radiation, corona discharges, acid rain, dry-band arcing are employed as sources of the aging. Chemical and morphological analysis is used to detect the surface chemical and structural changes derived from these stresses. The nature of highly mobile low molecular weight (LMW) chains achieving a quick recovery of the hydrophobicity is carefully examined. Their generation and extinction caused by these stresses are investigated. From this study, it is shown that oxidation that induces crosslinking, branching, interchanging and a formation of silanol groups are the most dominant chemical reactions during the aging of SIR. Furthermore, silanol groups that are byprodncts of oxidation restrict the diffusion of mobile LMw chains, which decreases the recovery speed of hydrophobicity and accelerates the aging. A typical aging scenario of SIR from the installation to the end of their life is drawn.

Atom Transfer Radical Polymerization of (Meth)acrylates from Poly(dimethylsiloxane) Macroinitiators

The atom transfer radical polymerization (ATRP) of (meth)acrylates from poly(dimethylsiloxane) (PDMS) macroinitiators is described. Hydrosilylation of commercially available difunctional hydrosilyl terminal PDMS (Mw/Mn > 1.3) with allyl- or 3-butenyl 2-bromoisobutyrate resulted in the macroinitiators. Linear semilogarithmic kinetic plots were obtained for the ATRP of acrylates and methacrylates. A nonlinear dependence of molecular weight on conversion was ascribed to a high concentration of low molecular weight chains not readily observed in the size exclusion chromatograph using THF as eluent. Well-defined monofunctional, low-polydispersity PDMS macroinitiators were synthesized by the anionic ring-opening polymerization of hexamethylcyclotrisiloxane. ATRP of acrylates and methacrylates from those compounds showed linear increases in Mn with conversion, demonstrating the effectiveness of ATRP to synthesize a variety of inorganic/organic polymer hybrids. A monofunctional poly(styrene-b-dimethylsiloxane) macroinitiator was used to initiate ATRP of n-butyl acrylate and methyl methacrylate, resulting in ABC organic/inorganic/organic hybrid triblock copolymers.

Polymer solubility in mixed solvents

Modeling results indicate that polymer chains in mixtures of a good with a bad solvent exhibit preferential adsorption of the good solvent. That phenomenon is found to be strongly dependent on molecular weight and it increases with a decrease in chain length. These results have important consequences on polymer solubility. Thus, a low molecular weight chain in a solvent mixture behaves as if it were dissolved in the pure good solvent component, whereas the solubility of a longer chain is controlled by the average mixture composition. As a result, quenching a polydisperse system below the cloud point may induce molecular weight segregation between the two phases: the longer chains, which precipitate out first, tend to populate the polymer rich phase whereas the shorter chains, having greater solubility, remain in the solvent phase. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2782–2787, 1999

Degradation of DL-PLA-methadone microspheres during in vitro release

The in vitro degradation of polylactic acid of different weight average molecular weight ( M w) in microspheres containing variable amounts of methadone base (between 17 and 47%) was studied using the degradation index (DI), polidispersivity (pd) and molecular weights distribution. Both pd and DI are independent of initial methadone content and increase with M w polymer, showing values of DI around 6.6 for the high M w and 2.9 for the lower M w. The distribution functions in the number of the molecular weights during the degradation assays indicate the presence of different break mechanisms for high and low molecular weight chains. A semiempiric model is proposed to explain the evolution of M n during degradation in function of the number of bonds available for their breaking and hydrolysis rate constant. The first parameter ranges between 2.35 and 6.65 in function of M w; the hydrolysis rate constant is independent of M w and equal to 0.020 h −1 (SD = 0.011 h −1).

Effects of sample preparation conditions and short chains on space charge formation in LDPE

The effects of chemical change during sample preparation and low molecular weight chains of low density polyethylene (LDPE) have been investigated in order to clarify the origins for heterocharge in LDPE which does not contain antioxidants. The carbonyls formed at the surface of polyethylene (PE) sheet prepared at 180/spl deg/C with polyethylene terephthalate backing films were found to contribute to the increase of heterocharge in LDPE. All aspects of the processes used for the preparation of samples must be studied carefully to ascertain possible effects on the properties of samples. The type of charge accumulated in LDPE was also found to depend on the concentration of short PE chains. Details of experimental results and their possible causes are described.

Measurement of Amylose/Amylopectin Ratio by High‐Performance Liquid Chromatography

AbstractAmylose content has been measured by iso‐amylase digestion of starch followed by quantitation of the amounts of the higher molecular weight chains (from amylose) and the lower molecular weight chains (from amylopectin). The separatrion and quantitation has been performed by size‐exclusion high performance liquid chromatography on hydrophilic columns. Three columns were tested and an Ultrahydrogel 250 was found to provide the optimum separation. Starches of up to 30‐35 per cent amylose may be analysed in this way without the need for first removing the lipids. With starch of higher amylose content, precipitation of the amylose in the sample may occur while it is awaiting HPLC analysis. This precipitation can be prevented in starches of up to 40 per cent amylose by the use of dimethyl sulfoxide in the reaction mixture. Samples with amylose content higher than this always show some precipitation and the amylose contents obtained are similar for all starches with amylose in the range from 40 to 80 per cent, measured by iodine determination. The method gives results which are more repeatable than those obtained by spectrophotometric determination of iodine binding, and which appear to be unaffected by the lipid content of the starch.

Phase Behavior and Morphology of Poly(ethylene oxide) Blends

Crystallization studies on blends of high and low molecular weight poly(ethylene oxide) (PEO) have been carried out to characterize phase behavior and morphology. At low crystallization temperatures, both components cocrystallize into a common crystal lattice. The thickness of the cocrystal is found to vary with the blend composition. At high fractions of the low molecular weight polymer, the cocrystal is thin, suggesting a folding of the long-chain polymer into the lattice of the low molecular weight chain. At lower fractions of the low molecular weight polymer, the cocrystal is thicker, resembling the higher molecular weight polymer. At higher crystallization temperatures, the components form a defective cocrystal which dynamically phase splits into its components. The crystal transformation in the blend occurs by thinning of low molecular weight chains and thickening of high molecular weight chains. The formation of crystal templates of both components eventually results in phase-segregated growth. Models for cocrystallization and dynamic phase separation of components are discussed. A time-resolved small-angle X-ray scattering study along with complementary calorimetry and optical microscopy results confirms the mechanism of cocrystallization and dynamic phase separation in PEO blends.

Influence of the functionality of the quaternizing agent and the polymer molecular weight on the viscoelastic behaviour of α,ω-(dimethylamino)polyisoprenes

Quaternization of the end-groups of α,ω-(dimethylamino)polyisoprenes with monofunctional, difunctional and trifunctional halides (iodides and bromides) has been undertaken on polymers of different molecular weights ( M n =6000 and 38000 g mol −1 ). The thermal stability of the ammonium end-groups is somewhat better when iodide counteranions are present. Quaternization increases the T g of the material. This effect is more pronounced for low molecular weight chains, especially when a short alkyl group quaternizing agent is used. However, the size of the alkyl or aryl radical has a minor effect on polymer T g when difunctional quaternizing agents are used. Nevertheless, the modulus of the rubber-like plateau of materials with difunctional agents is higher than with monofunctional quaternizing agents, owing to coupling reactions between the chain-ends. Rheological measurements indicate that the quaternized low molecular weight polyisoprenes exhibit an Arrhenius-type temperature dependence characterized by activation energies which vary between 121 and 146 kJ mol −1 and suggest that the relaxations attributed to dipole—dipole interactions dominate over long chain entanglements.

Degradation of polydispersed poly( l-lactic acid) to modulate lactic acid release

Polydispersed poly( l-lactic acid) (PLLA) membranes comprised of blends of monodispersed PLLA of weight average molecular weight of 82500 and 7600 were fabricated to investigate the effect of polydispersity on degradation characteristics. The PLLA blends exhibited large spherulites of high molecular weight chains embedded in a low molecular weight matrix. During degradation in phosphate buffer at pH 7.4 and 37 °C for 28 d, the release rate of lactic acid increased as the percentage of the low molecular weight component in the blend was increased. For low molecular weight compositions larger than 50%, voids were created in the degrading blends due to the degradation of low molecular weight chains and the concurrent dissolution of lactic acid, and also the release of undegraded particles of high molecular weight. These studies demonstrate the feasibility of modulating lactic acid release during in vivo degradation of PLLA implants by adjusting the polymer polydispersity.

Hypoallergenicity of guayule rubber particle proteins compared to Hevea latex proteins

Hevea brasiliensis Muell. Arg. is currently the sole commercial source of natural rubber. However, products made from Hevea latex are responsible for causing allergies affecting an increasing number of people world-wide. In this paper we test the hypothesis that latex allergy can be circumvented by using rubber of low hypoallergenicity from other species. We also report studies to determine the feasibility of ameliorating latex allergy in Hevea. We show that antibodies raised against proteins extracted from films of ammoniated Hevea latex did not recognize latex proteins from two other rubber producing plant species, guayule ( Parthenium argentatum Gray) and Ficus elastica Roxb., indicating that Hevea latex allergy can be circumvented using rubber from other species. The hypoallergenicity of guayule and F. elastica latex was confirmed in preliminary clinical trials (reported elsewhere). F. elastica rubber is mostly of low molecular weight and short chain, whereas guayule rubber is comparable in quality to that of Hevea. Guayule latex should be suitable for the manufacture of high-quality, hypoallergenic natural rubber products for the Hevea-hyperallergic individual. The Hevea latex protein antibodies recognized not only proteins from raw Hevea latex, but also rubber particle preparations from which the soluble latex proteins were removed. Hence extensive removal of rubber particle-bound proteins, in addition to the soluble latex proteins, would be required to produce a safe Hevea latex product. Latex samples from different commercial lines of H. brasiliensis reacted similarly to the antibodies, indicating that clonal selection is unlikely to be helpful in eliminating latex antigens.

Structures of sulfated oligosaccharides in human trachea mucin glycoproteins.

The structures of high molecular weight sulfated oligosaccharide chains in mucins purified from the sputum of a patient with cystic fibrosis and blood group H determinant were established. Reduced oligosaccharides released by treatment with alkaline borohydride were separated by ion exchange chromatography on DEAE-Agarose and a fraction containing multisulfated chains was further purified by lectin affinity chromatography to completely remove small amounts of sialylated chains. A major sulfated oligosaccharide fraction containing chains with an average of 160 to 200 sugar residues was isolated by gel filtration on BioGel P-10 columns and individual subfractions were characterized by methylation analysis, periodate oxidation and sequential glycosidase digestion before and after desulfation. Carbohydrate analysis yielded Fuc, Gal and GldNAc in a ratio of 1:2:2.1 and only one galactosaminitol residue for every 160- to 200 sugar residues. The average molecular weight of oligosaccharide chains in these fractions was between 27,000 and 40,000 daltons. Structural analysis showed that these high molecular weight chains contained varying amounts of the repeating unit shown in the following oligosaccharide. Only one in about every 10 repeating units contained sulfate esters. Several shorter chains which contain 2 to 3 sulfate esters were also isolated from this multisulfated oligosaccharide fraction. The structures proposed for these oligosaccharides indicate that they are lower molecular weight chains with the same general structure as those found in the high molecular weight sulfated oligosaccharides. Taken collectively, the results of these studies show that a major sulfated oligosaccharide fraction in respiratory mucin purified from the mucus of patients with cystic fibrosis contains high molecular weight branched chains that consist of a repeating oligosaccharide sequence with sulfate linked to the 6 positions of galactose and possibly GlcNAc residues in the side chains.

Swelling and deswelling of polyamide 11 with formic acid

The swelling and deswelling of isotropic and oriented polyamide 11 (PA11) with formic acid have been determined as functions of temperature and acid solution concentration. During swelling, dimension relaxation, partial dissolution of crystallites and extraction of low molecular weight chains occur. During deswelling, amorphous chains recrystallize and after complete drying, the original morphology with the same orientation is restored. This memory effect is due to strain-induced crystallization in the PA oriented gel.