Mechanism Of Poly Research Articles

Effects of Intermolecular Hydrogen Bondings on Isothermal Crystallization Behavior of Polymer Blends of Cellulose Acetate Butyrate and Poly(3-hydroxybutyrate)

Effects of hydrogen bondings (HBs) on the isothermal crystallization kinetics and mechanisms of poly(3-hydroxybutyrate) (PHB) in neat PHB and its blends with cellulose acetate butyrate (CAB), amorp...

Morphology, Crystallization and Formation Mechanism of Poly(Vinylidene Fluoride) Membranes Prepared with Immersion Precipitation: Effect of Maturation Time

Poly(vinylidene fluoride) (PVDF) membranes were prepared by the immersion precipitation method. Effects of the maturation time of dopes on the morphology and crystallization of the prepared membranes were investigated. The analysis showed that the maturation time played an important role in determining the morphology of the prepared membranes. For the dope prepared in the initial day, liquid–liquid demixing preceded solid–liquid demixing in the process of the membrane formation. The morphology of the cross section of the prepared membrane (M1) was finger-like structures with a sponge substrate beneath the porous skin. During the maturation, the dopes underwent a microscopic phase separation and the PVDF crystallized, which resulted in the existence of micro-liquid phases and micro-solid phase crystalline areas in the dopes. In the process of the membrane formation, liquid–liquid demixing took place by nucleation and growth of droplets of the polymer rich phase in the micro-liquid phase. The micro-solid phase crystallites were connected together by the polymer chains, and formed a three-dimensional network gelation morphology. The crystal structure of M1 was mainly β crystals. With increasing maturation time of the dopes, the proportion of β decreased crystals, but that of α crystals increased for the prepared membranes.

Getting in the Loop: New Insights into the Mechanism of Poly(A) Site Recognition

In this issue, Yang et al. (2011) show that the 3' end processing factor CFI(m) interacts with RNA in manner that facilitates RNA looping, suggesting mechanistic roles for this factor in the regulation of poly(A) site selection.

Mechanism for the Alteration of the Substrate Specificities of Template-Independent RNA Polymerases

PolyA polymerase (PAP) adds a polyA tail onto the 3'-end of RNAs without a nucleic acid template, using adenosine-5'-triphosphate (ATP) as a substrate. The mechanism for the substrate selection by eubacterial PAP remains obscure. Structural and biochemical studies of Escherichia coli PAP (EcPAP) revealed that the shape and size of the nucleobase-interacting pocket of EcPAP are maintained by an intra-molecular hydrogen-network, making it suitable for the accommodation of only ATP, using a single amino acid, Arg(197). The pocket structure is sustained by interactions between the catalytic domain and the RNA-binding domain. EcPAP has a flexible basic C-terminal region that contributes to optimal RNA translocation for processive adenosine 5'-monophosphate (AMP) incorporations onto the 3'-end of RNAs. A comparison of the EcPAP structure with those of other template-independent RNA polymerases suggests that structural changes of domain(s) outside the conserved catalytic core domain altered the substrate specificities of the template-independent RNA polymerases.

Gas sensitivity of poly (3, 4-ethylene dioxythiophene) prepared by a modified LB film method

An arachidic acid/poly (3, 4-ethylene dioxythiophene) (AA/PEDOT) multilayer Langmuir-Blodgett (LB) film was prepared by a modified LB film method. The theories were utilized to explain the effects between HCl molecule and LB film. The gas sensitivity mechanism of poly (3, 4-ethylene dioxythiophene) (PEDOT) multilayer film can be explained by the charge transfer between p system of PEDOT and oxidization HCl system. The gas sensitivity of PEDOT LB film deposited interdigital electrode to HCl was tested. The results showed that film thickness, treating temperature, deposition speed had different influence on film gas sensitivity. The AA/PEDOT film deposited device exhibited nonlinear behavior to HCl gas at lower concentration (20–60 ppm) and linear response behavior at higher gas concentration was observed. The time of the compound LB film of the AA/PEDOT responding to the 30 ppm HCl gas is about 20 seconds, which is far quicker than the time of the film to the PEDOT-PRESS film(about 80 seconds). It is not higher film press to better film. When the film press attains 45 mNs/m, the sensitivity of the AA/PEDOT film on the contrary descends.

Nanoparticle-Induced Controlled Biodegradation and Its Mechanism in Poly(ε-caprolactone)

Poly(epsilon-caprolactone) (PCL)/layered silicate nanocomposites have been prepared via solution route. Two different organically modified nanoclays were used to compare the variation in properties based on organic modifications. The nanostructures, as observed from wide-angle X-ray diffraction and transmission electron microscopy, indicate intercalated and partially exfoliated hybrids depending on the nature of organic modification in nanoclay. The nanohybrids exhibit significant improvement in thermal and mechanical properties of the matrix as compared to neat polymer. The nanoclays act as nucleating agent for the crystallization of PCL. The biodegradability of pure PCL and its nanocomposites have been studied under controlled conditions in enzyme, pure microorganism (fungi), compost, Ganges water, and alkaline buffer solution. The rate of biodegradation of PCL has enhanced dramatically in nanohybrids and depends strongly on the media used. Scanning confocal, electron, and atomic force microscopes have used to demarcate the nature of biodegradation of pristine PCL and its nanocomposites. The change in biodegradation is rationalized in terms of the crystallization behavior and organic modification in nanoclays of the nanohybrids vis-a-vis the neat polymer. The extent of compatibility was measured quantitatively through the interaction parameter for two different nanoclays to compare and establish the reason for variation in their properties in nanohybrids. A biodegradation mechanism has been revealed for PCL and its nanocomposites through enzyme activity in varying pH environment.

The Debye-like relaxation mechanism in poly(vinylidene fluoride-trifluoroethylene) ferroelectric polymers

In this work we studied 65-nm-thick poly(vinylidene fluoride-trifluoroethylene) ferroelectric polymers films grown by Langmuir–Blodgett onto silicon substrates. Three dielectric anomalies have been unambiguously evidenced. The high temperature one near 380 K corresponds to the ferroelectric-paraelectric transition. The low temperature one near 270 K which is characterized by a β relaxation is attributed to structural defects in the crystalline phase leading to inhomogeneous strains detected by x-ray diffraction. At approximately 320 K, a Debye-like relaxation is clearly evidenced. Possible origins for this later relaxation are discussed.

Giant magnetoresistance due to electron-hole pair mechanism in poly( N-vinylcarbazole)

An amorphous molecular semiconductor, poly( N-vinylcarbazole) (PVCz), exhibits negative giant magnetoresistance (MR) under ambient conditions. The application of a weak magnetic field of 10 mT to PVCz films doped with lumichrome, in which light excitation immediately creates triplet electron-hole (e-h) pairs that are precursors of photocarriers, causes a steep decrease in resistivity by more than 20% at ambient temperature. Further, the resistivity of the doped film gradually reduces by approximately one-half under a field of 1 T, equivalent to an MR ratio of −55%. In addition, anomalous spikes are also detected at 0.07, 0.30, and 9.0 T, indicative of an exponential dependence with a decay distance of 0.1 nm in the exchange interaction of the e-h pair. A quantum mechanical calculation based on the density operator formalism clarifies that the observed MR effect can be comprehensively understood by the spin-selective charge dynamics and the coherent and incoherent spin dynamics of the e-h pair in a quasi-one-dimensional lattice for photocarrier generation. Model calculations also indicate the importance of the spin-lattice relaxation for the giant MR effect in organic molecular semiconductors.

The Wall Teichoic Acid Polymerase TagF Is Non-processive in Vitro and Amenable to Study Using Steady State Kinetic Analysis

Wall teichoic acids are a chemically diverse group of anionic polymers that constitute up to 50% of the Gram-positive cell wall. These polymers play a pivotal role in virulence and have been implicated in a diverse range of physiological functions. The TagF-like family of enzymes has been shown to be responsible for wall teichoic acid priming and polymerization events. Although many such enzymes are well validated therapeutic targets, a mechanistic understanding of this enzyme family has remained elusive. TagF is the prototypical teichoic acid polymerase and uses CDP-glycerol to catalyze synthesis of the linear (1,3)-linked poly(glycerol phosphate) teichoic acid in Bacillus subtilis 168. Here we used a synthetic soluble analog of the natural substrate of the enzyme, Lipid , to conduct the first detailed mechanistic investigation of teichoic acid polymerization. Through the use of a new high pressure liquid chromatography-based assay to monitor single glycerol phosphate incorporations into the Lipid analog, we conducted a detailed analysis of reaction product formation patterns and unequivocally showed TagF to be non-processive in vitro. Furthermore by monitoring the kinetics of polymerization, we showed that Lipid analog species varying in size have the same K(m) value of 2.6 microm and validated use of Bi Bi velocity expressions to model the TagF enzyme system. Initial rate analysis showed that TagF catalyzes a sequential Bi Bi mechanism where both substrates are added to the enzyme prior to product release consistent with a single displacement chemical mechanism.

Morphology and Formation Mechanism of Poly(Vinylidene Fluoride) Membranes Prepared with Immerse Precipitation: Effect of Dissolving Temperature

Poly(vinylidene fluoride) (PVDF) membranes were prepared by the immersion precipitation method. The effects of polymer dissolving temperature for the dopes on the morphology, crystallization and performance of prepared membranes were examined. Polymer dissolving temperature was varied from 50 to 120°C. N,N-dimethylacetamide (DMAc) and de-ionized water were used as solvent and non-solvent, respectively. Based on the membrane morphology and the schematic phase diagram of the ternary system, the membrane formation mechanism was analyzed theoretically. The binodal liquid-liquid demixing took place first for the nucleation and growth of droplets in the polymer poor phase; then consequently the spinodal liquid-liquid demixing occurred in the polymer rich phase. The demixings together resulted in the prepared membranes having a cross-section composed of interconnected globule-like particulates with bi-continuous structured surfaces. The dissolving temperature of the dopes had a remarkable effect on the morphology of the cross-section, even when the solution underwent a long time cooling before the demixing. The increase of the diameter of the particulates with the dissolving temperature was theoretically analyzed according to the conditions of the polymeric solution.

Apoptosis induced in chicken embryo fibroblasts in vitro by a polyinosinic:polycytidylic acid copolymer

This study investigated apoptosis induced in chicken embryo fibroblast (CEF) cells by the toxic effects of polyinosinic:polycytidylic acid copolymer (Poly I:C). Along with cell viability and morphological changes, other indicators were measured in vitro after incubation of the cells with Poly I:C, including phosphatidylserine translocation, the formation of DNA fragments, activity of caspase-3, caspase-8 and caspase-9, and the expression levels of RIPK1 mRNA and TNRSF8 mRNA. An inhibition of proliferation was found with increasing levels of Poly I:C, showing that Poly I:C was toxic to the cells. The findings of phosphatidylserine translocation and formation of DNA fragmentation suggested that Poly I:C induced apoptosis. With increased incubation time, the activities of caspase-3 and caspase-8 increased, while there was no significant change in caspase-9 activity. Accordingly, it is concluded that the apoptosis induced by Poly I:C involves a cell death receptor-mediated pathway. The transcription level of RIPK1 mRNA decreased, while that of TNFRSF8 mRNA increased, indicating that Poly I:C-induced apoptosis was related to upregulation of TNFRSF8. These observations provide insight into the potential mechanism of Poly I:C-induced toxicity.

The Tribological Mechanism of Poly(Vinylidene Fluoride) Composites Reinforced with Carbon Fibers

The current study examines the tribological performance of poly(vinylidene fluoride) (PVDF) and carbon fiber-reinforced poly(vinylidene fluoride) (CF/PVDF) under dry sliding condition. Different contents of carbon fibers were employed as reinforcement. All filled and unfilled polyimide composites were tested against CGr15 ball and representative testing was performed. The effects of carbon fiber content on tribological properties of the composites were investigated. The worn surface morphologies of neat PVDF and its composites were examined by scanning electron microscopy (SEM) and the wear mechanisms were discussed. Moreover, all filled PVDFs have superior tribological characteristics to unfilled PVDFs. The optimum wear reduction was obtained when the content of carbon fiber is 20 vol%.

Research on the friction and wear mechanism of Poly(vinyl alcohol)/hydroxylapatite composite hydrogel

Poly(vinyl alcohol)/Hydroxylapatite (PVA/HA) composite hydrogel was prepared with poly(vinyl alcohol) and hydroxylapatite as raw materials, using the method of repeated freezing and thawing. The morphologies of PVA/HA composite hydrogel were observed by means of high-accuracy 3D profiler and scanning electron microscope (SEM). The compressive elastic modulus and the stress relaxation characteristics of PVA/HA composite hydrogel were measured using the flat-head cylinder indenter. The friction and wear tests between PVA/HA composite hydrogel and bovine knee articular cartilage were performed on the micro-tribometer. The worn morphologies of PVA/HA composite hydrogel were observed with environmental scanning electron microscope (ESEM). The results showed that PVA/HA composite hydrogel has the cross-link network microstructure which is similar to that of the natural bovine knee articular cartilages. With the increase of freezing-thawing cycles and the HA content, the degree of cross-link and the crystallization of PVA/HA composite hydrogel both increase, the elastic modulus increases evidently, the rate of stress relaxation is improved and the value of balance stress decreases. The friction coefficient decreases with the increase of the freezing-thawing cycles and the HA content. The more the freezing-thawing cycles are, the earlier the friction coefficient reaches the stable balance value. The friction deformation depth between PVA/HA composite hydrogel and bovine knee articular cartilage is inversely proportional to freezing-thawing cycles and the HA content. The main wear mechanisms of PVA/HA composite hydrogel are plastic flowing and adhesive flaking. The wear severity degree decreases with the increase of freezing-thawing cycles and the HA content.

Adsorption of poly(ethylene oxide) on smectite: Effect of layer charge

The adsorption of polymers on clay is important in many applications. However the mechanisms of poly(ethylene oxide) (PEO) adsorption on smectite is not well elucidated at present. The aim of this study was to investigate the effect of layer charge density on the adsorption of PEO by smectite. The results indicated that both the hydrophobic interaction (between CH 2 CH 2 groups and siloxane surface) and the hydrogen bonding (between ether oxygen of PEO and structure OH of smectite) lead to PEO preferential adsorption on the surface of low-charge smectite. In addition, the delamination of low-charge smectite in water is enhanced upon PEO adsorption presumably due to the hydrophilic ether oxygen of adsorbed PEO.

Interaction of α‐gliadin with poly(HEMA‐co‐SS): Structural characterization and biological implication

The wheat gluten protein alpha-gliadin, a well known trigger of celiac disease, can be complexed by random copolymers of hydroxyethyl methacrylate (HEMA) and sodium 4-styrene sulfonate (SS). In this work, influence of alpha-gliadin and poly(HEMA-co-SS) concentrations on alpha-gliadin structure was studied using spectroscopic techniques and dynamic light scattering. In 70% ethanol or 0.06M HCl (pH 1.2), alpha-gliadin was found to self-associate upon increasing its concentrations and displayed decreased alpha-helical content and increased beta-turn and beta-sheet contents. At pH 1.2, alpha-gliadin interacted with poly(HEMA-co-SS) to form supra-molecular complex particles. Poly(HEMA-co-SS) induced alpha-gliadin structural changes that mimicked those obtained by varying the protein concentration in pure solution. At pH 6.8, alpha-gliadin was poorly soluble and formed large particles but alpha-helix is still main secondary structure. The influence of the polymer on protein structure was weaker at neutral than acidic pH. Interaction with poly(HEMA-co-SS) disrupted alpha-gliadin conformation and self-association to form new complex particles at neutral pH. This study provides insight into the mechanism of poly(HEMA-co-SS)/alpha-gliadin interaction and the polymer as alpha-gliadin sequestering agents in the supportive treatment of celiac disease.

Effect of electrical operation on the defect states in organic semiconductors

We have investigated the role of the trapping process in degradation mechanisms of poly(9,9-dihexylfluorene-co-N,N-di(9,9-dihexyl-2-fluorenyl)-N-phenylamine) (PF) based diodes, after aging (at half lifetime) by electrical stress. By using the Charge based Deep Level Transient Spectroscopy, we have determined the trap parameters in PF light emitting devices. The mean activation energies of the traps are in the range 0.13–0.60 eV from the band edges with capture cross sections of the order of 10–18 to 10–20 cm2. The trap densities are in the range of 10–16 to 10–17 cm−3. Upon aging, no new trap levels have been found indicating that the electrical stress did not create additional defect level in the polymer in contrast to previous investigations on other organic materials, which reported that the degradation of devices in humid atmosphere lead to the onset of new traps acting as recombination centers. Furthermore, aging would not affect uniformly the defect levels in the polymer. Shallow trap states (below 0.3 eV) remain stable, whereas the enhancement in trap density of deeper trap levels (above 0.3 eV) have been observed, suggesting that degradation by electrical stress leads to an increase in density of deep levels.

Characterization of oxidized cellulose with ultraviolet–visible spectroscopy

AbstractThis article describes some properties and behavior of oxidized cellulose evaluated with ultraviolet–visible spectroscopy. Newly defined analytical parameters are described. Mechanisms of poly(1,4‐β‐D‐anhydroglucuronic acid) degradation and destruction are formulated. The feasibility of using the same Staudinger–Mark–Houwink equation to calculate the degree of polymerization of cellulose and the degree of polymerization of oxidized cellulose is demonstrated. Utilization of ultraviolet–visible spectroscopy is shown for the optimization of oxidized cellulose production and aging. Advantages and limitations of this method are discussed. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Mechanism of Poly(A) Polymerase: Structure of the Enzyme-MgATP-RNA Ternary Complex and Kinetic Analysis

We report the 1.8 A structure of yeast poly(A) polymerase (PAP) trapped in complex with ATP and a five residue poly(A) by mutation of the catalytically required aspartic acid 154 to alanine. The enzyme has undergone significant domain movement and reveals a closed conformation with extensive interactions between the substrates and all three polymerase domains. Both substrates and 31 buried water molecules are enclosed within a central cavity that is open at both ends. Four PAP mutants were subjected to detailed kinetic analysis, and studies of the adenylyltransfer (forward), pyrophosphorolysis (reverse), and nucleotidyltransfer reaction utilizing CTP for the mutants are presented. The results support a model in which binding of both poly(A) and the correct nucleotide, MgATP, induces a conformational change, resulting in formation of a stable, closed enzyme state. Thermodynamic considerations of the data are discussed as they pertain to domain closure, substrate specificity, and catalytic strategies utilized by PAP.

Deciphering the Role of Hydrogen Bonding in Enhancing pDNA−Polycation Interactions

There is considerable interest in the binding and condensation of DNA with polycations to form polyplexes because of their possible application to cellular nucleic acid delivery. This work focuses on studying the binding of plasmid DNA (pDNA) with a series of poly(glycoamidoamine)s (PGAAs) that have previously been shown to deliver pDNA in vitro in an efficient and nontoxic manner. Herein, we examine the PGAA-pDNA binding energetics, binding-linked protonation, and electrostatic contribution to the free energy with isothermal titration calorimetry (ITC). The size and charge of the polyplexes at various ITC injection points were then investigated by light scattering and zeta-potential measurements to provide comprehensive insight into the formation of these polyplexes. An analysis of the calorimetric data revealed a three-step process consisting of two different endothermic contributions followed by the condensation/aggregation of polyplexes. The strength of binding and the point of charge neutralization were found to be dependent upon the hydroxyl stereochemistry of the carbohydrate moiety within each polymer repeat unit. Circular dichroism spectra reveal that the PGAAs induce pDNA secondary structure changes upon binding, which suggest a direct interaction between the polymers and the DNA base pairs. Infrared spectroscopy experiments confirmed both base pair and phosphate group interactions and, more specifically, showed that the stronger-binding PGAAs had more pronounced interactions at both sites. Thus, we conclude that the mechanism of poly(glycoamidoamine)-pDNA binding is most likely a combination of electrostatics and hydrogen bonding in which long-range Coulombic forces initiate the attraction and hydroxyl groups in the carbohydrate comonomer, depending on their stereochemistry, further enhance the association through hydrogen bonding to the DNA base pairs.

Effects of poly(ethylene oxide) adsorption on the dispersion of smectites

The adsorption of polymers on clay is important in many applications. However the mechanisms of poly(ethylene oxide) (PEO) adsorption on smectite and its effect on suspension rheology are not well elucidated at present. The aim of this study was to investigate the mechanisms responsible for PEO sorption on smectite and how this impacts on dispersive behavior of smectite suspension. The results indicated that the hydrophobic interaction between CH 2 CH 2 groups and siloxane surface is the major driven force for PEO adsorption on smectite. In addition, PEO adsorbed preferentially on the surface of low-charge smectite and the delamination of low-charge smectite in water was enhanced due to PEO adsorption presumably due to the hydrophilic ether oxygen of adsorbed PEO.