Expanded Coil Conformation Research Articles

Ultrafast Laser‐Induced Excellent Thermoelectric Performance of PEDOT:PSS Films

Because poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate) (PEDOT:PSS) is water processable, thermally stable, and highly conductive, PEDOT:PSS and its composites have been considered to be one of the most promising flexible thermoelectric materials. However, the PEDOT:PSS film prepared from its commercial aqueous dispersion usually has very low conductivity, thus cannot be directly utilized for TE applications. Here, a simple environmental friendly strategy via femtosecond laser irradiation without any chemical dopants and treatments was demonstrated. Under optimal conditions, the electrical conductivity of the treated film is increased to 803.1 S cm−1 from 1.2 S cm−1 around three order of magnitude higher, and the power factor is improved to 19.0 μW m−1 K−2, which is enhanced more than 200 times. The mechanism for such remarkable enhancement was attributed to the transition of the PEDOT chains from a coil to a linear or expanded coil conformation, reduction of the interplanar stacking distance, and the removal of insulating PSS with increasing the oxidation level of PEDOT, facilitating the charge transportation. This work presents an effective route for fabricating high‐performance flexible conductive polymer films and wearable thermoelectric devices.

Conformational Behavior, Topographical Features, and Antioxidant Activity of Partly De-Esterified Arabinoxylans.

This study aimed to investigate the effect of arabinoxylans (AX) partial de-esterification with feruloyl esterase on the polysaccharide conformational behavior, topographical features, and antioxidant activity. After enzyme treatment, the ferulic acid (FA) content in AX was reduced from 7.30 to 5.48 µg FA/mg polysaccharide, and the molecule registered a small reduction in radius of gyration (RG), hydrodynamic radius (Rh), characteristic ratio (C∞), and persistence length (q). A slight decrease in α and a small increase in K constants in the Mark–Houwink–Sakurada equation for partially de-esterified AX (FAX) suggested a reduction in molecule structural rigidity and a more expanded coil conformation, respectively, in relation to AX. Fourier transform infrared spectroscopy spectra of AX and FAX presented a pattern characteristic for this polysaccharide. Atomic force microscopy topographic analysis of FAX showed a more regular surface without larger hollows in relation to AX. The antioxidant activity of FAX, compared to AX, was reduced by 30 and 41% using both 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulphonic acid) (ABTS+) and 1,1-diphenyl-2-picryl-hydrazyl (DPPH) methods, respectively. These results suggest that feruloyl esterase treatment of AX could offer a strategy to tailor AX chains conformation, morphological features, and antioxidant activity, impacting the development of advanced biomaterials for biomedical and pharmaceutical applications.

Effects of macromolecular crowding on the folding of a polymer chain: A Wang–Landau simulation study

A flexible polymer chain in the presence of inert macromolecular crowders will experience a loss of configurational entropy due to the crowder excluded volume. This entropy reduction will be most pronounced in good solvent conditions where the chain assumes an expanded coil conformation. For polymers that undergo a folding transition from a coil to a compact ordered state, as is the case for many globular proteins, macromolecular crowding is expected to stabilize the folded state and thereby shift the transition location. Here, we study such entropic stabilization effects for a tangent square-well sphere chain (monomer diameter σ) in the presence of hard-sphere (HS) crowders (diameter D ≥ σ). We use the Wang-Landau simulation algorithm to construct the density of states for this chain in a crowded environment and are thus able to directly compute the reduction in configurational entropy due to crowding. We study both a chain that undergoes all-or-none folding directly from the coil state and a chain that folds via a collapsed-globule intermediate state. In each case, we find an increase in entropic stabilization for the compact states with an increase in crowder density and, for fixed crowder density, with a decrease in crowder size (concentrated, small crowders have the largest effect). The crowder significantly reduces the average size for the unfolded states while having a minimal effect on the size of the folded states. In the athermal limit, our results directly provide the confinement free energy due to crowding for a HS chain in a HS solvent.

Entanglement of semiflexible polyelectrolytes: Crossover concentrations and entanglement density of sodium carboxymethyl cellulose

Viscosity data for aqueous solutions of sodium carboxymethyl cellulose (NaCMC) as a function of polymer concentration ( c), added salt concentration ( c S), and degree of polymerization ( N) are presented. In dilute solution, NaCMC adopts an expanded coil conformation in excess salt ( R ∝ N 0.59, [ η ] ∼ N 0.786) and a rodlike conformation ( R ∝ N, [ η ] ∼ N 2) in salt-free water. The total persistence length in 0.1M NaCl is calculated from the N dependence of the intrinsic viscosity to be l p ≃ 5.5 nm. The entanglement crossover, evaluated from the c and N dependence of the specific viscosity, is found to be independent of c S. Our results suggest that polymer conformation (e.g., l p or solvent quality) within the range of parameters studied does not affect the formation of entanglements. The scaling model of Dobrynin et al. correctly describes the nonentangled rheology of NaCMC in salt-free and excess salt solutions but does not explain the c S independence of the entanglement crossover. Modifications to this model are proposed, which result in better but still limited agreement with experiments.

Preparation and Physicochemical Characterization of Dual Responsive and Chemically Modified Cellulose Based Copolymer Hydrogels

Abstract This research work is based on the preparation and physicochemical characterization of Poly(N-isopropylacrylamide)–Cellulose–Poly(Acrylic acid) [PNIPAAm–Cellulose–PAAc] based terpolymer hydrogels. The free radical polymerization reaction was initiated by the presence of ammonium persulphate (APS) and crosslinking between different monomers was occurring through N,Nl- Methylene bis-acrylamide (MBA). Confirmation of polymerization process was done by FT-IR and UV-visible spectroscopy. The prepared hydrogels were further characterized by different physicochemical techniques like rheology, Ostwald viscometry and dynamic light scattering (DLS). The effect of external stimuli like temperature, pH and composition of the samples on the physicochemical behavior was also carried out by dynamic rheology, swelling measurement and DLS. Various other properties like elasticity, shear stress, shear strain, loss modulus, storage modulus and complex viscosity was investigated by rheology. DLS was used to trace the size and swelling behavior of the samples. From the results obtained it was found that all the microgel samples are stimuli responsive and most of their physicochemical properties were prominently varying while changing the internal as well as the external experimental variable. These changes in physicochemical behavior of the gel can be attributed to two possibilities; the change in the hydrophobic character of gel (PNIPAAm) with temperature and also to the weakening of intermolecular hydrogen bonds with increase in temperature. As a result of this the PAA chains may undergo a transition from a compact conformation to an expanded coil conformation, resulting in the swelling of the hydrogels.

Modified Electrodes and Electrochemical Systems Switchable by Temperature Changes

AbstractThis article is an overview of extensive research efforts in the area of temperature‐controlled electrochemical systems. Electrochemical reactions, including electrocatalytic and bioelectrocatalytic processes, have been reversibly activated and inhibited by temperature changes. This was achieved by modification of electrode surfaces with thermo‐sensitive polymers (e.g., poly(N‐isopropylacrylamide), PNIPAM) which are reversibly switched by temperature changes between two different structures: swollen expanded coil conformation and shrunken collapsed globule state. While the swollen hydrophilic state allows penetration of redox species to the electrode conducting support and activates electrochemical reactions, the collapsed hydrophobic state isolates the electrode surface and inhibits electrochemical processes. Electrodes modified with the thermo‐switchable polymers have been additionally functionalized with photo‐switchable molecules (e.g., spiropyran derivatives) to achieve double‐controlled electrochemical reactions switchable by temperature changes and light signals. Incorporation of metallic nanoparticles or graphene species in the temperature‐sensitive polymer films resulted in sophisticated features and multi‐signal controlled behavior of the nano‐composite systems.

Design of Environmentally Responsive Fluorescent Polymer Probes for Cellular Imaging.

We report the development of environmentally responsive fluorescent polymers. The reversible temperature-induced phase transition of copolymers composed of N-isopropylacrylamide and a fluorescent monomer based on the fluorescein (FL), coumarin (CO), rhodamine (RH), or dansyl (DA) skeleton was used as a molecular switch to control the fluorescence intensity. The poly(N-isopropylacrylamide) (PNIPAAm) chain showed an expanded coil conformation below the lower critical solution temperature (LCST) due to hydration, but it changed to a globular form above the LCST due to dehydration. Through the combination of a polarity-sensitive fluorophore with PNIPAAm, the synthetic fluorescent polymer displayed a response to external temperature, with the fluorescence strength dramatically changing close to the LCST. Additionally, the P(NIPAAm-co-FL) and P(NIPAAm-co-CO) polymers, containing fluorescein and coumarin groups, respectively, exhibited pH responsiveness. The environmental responsiveness of the reported polymers is derived directly from the PNIPAAm and fluorophore structures, thus allowing for the cellular uptake of the fluorescence copolymer by RAW264.7 cells to be temperature-controlled. Cellular uptake was suppressed below the LCST but enhanced above the LCST. Furthermore, the cellular uptake of both P(NIPAAm-co-CO) and P(NIPAAm-co-RH) conjugated with a fusogenic lipid, namely, l-α-phosphatidylethanolamine, dioleoyl (DOPE), was enhanced. Such lipid-conjugated fluorescence probes are expected to be useful as physiological indicators for intracellular imaging.

Molar mass characterization of sodium carboxymethyl cellulose by SEC-MALLS

Two series of sodium carboxymethyl celluloses (NaCMCs) derived from microcrystalline cellulose (Avicel samples) and cotton linters (BWL samples) with average degrees of substitution (DS) ranging from DS=0.45 to DS=1.55 were characterized by size exclusion chromatography with multi-angle laser light scattering detection (SEC-MALLS) in 100mmol/L aqueous ammonium acetate (NH4OAc) as vaporizable eluent system. The application of vaporizable NH4OAc allows future use of the eluent system in two-dimensional separations employing evaporative light scattering detection (ELSD). The losses of samples during filtration and during the chromatographic experiment were determined. The scaling exponent as of the relation 〈s2〉1/2~Mas was approx. 0.61, showing that NaCMCs exhibit an expanded coil conformation in solution. No systematic dependencies of as on DS were observed. The dependences of molar mass on SEC-elution volume for samples of different DS can be well described by a common calibration curve, which is of advantage, as it allows the determination of molar masses of unknown samples by using the same calibration curve, irrespective of the DS of the NaCMC sample. Since no commercial NaCMC standards are available, correction factors were determined allowing converting a pullulan based calibration curve into a NaCMC calibration using the broad calibration approach. The weight average molar masses derived using the so established calibration curve closely agree with the ones determined by light scattering, proving the accuracy of the correction factors determined.

AFM Imaging of Adsorbed Nafion Polymer on Mica and Graphite at Molecular Level

Perfluorosulfonic acid ionomer (PFSA, specifically Nafion at EW = 975 g/mol) was visualized at the single molecule level using atomic force microscopy (AFM) in liquid. The diluted commercial Nafion dispersion shows an apparent M(w) = 1430 kg/mol and M(w)/M(n) = 3.81, which is assigned to chain aggregation. PFSA aggregates, imaged on mica and HOPG during adsorption from EtOH-H(2)O solvent at pH(e) 3.0 (below isoelectric point), showed a stable, segmented rod-like conformation. This structure is consistent with earlier NMR, SAXS/SANS, and TEM results that support a stiff helical Nafion conformation with long persistence length, a sharp solvent-polymer interface, and an extension of the sulfonated side chain into solution. Adsorption of Nafion structures on HOPG was observed at even higher pH(e) from EtOH due to screening of the repulsive electrostatic interaction in lower dielectric constant solvent, while the chain adopted an expanded coil conformation. These measurements provided direct evidence of the chain aggregation in EtOH-H(2)O solution and revealed their equilibrium conformations for adsorption on two model surfaces, highly ordered pyrolitic graphite (HOPG) and mica. The commercial Nafion dispersion was autoclaved at 0.10% w/w in nPrOH/H(2)O = 4:1 v/v solvent at 230 °C for 6 h to give a single-chain dispersion with M(w) = 310 kg/mol and M(w)/M(n) = 1.60. The autoclaved chains adopt an electrostatically stabilized compact globule conformation as observed by AFM imaging of the single PFSA molecules after rapid deposition on mica and HOPG at a low surface coverage.

High-Strength Multifunctional Conducting Polymer Hydrogels

A simple and versatile method has been invented to fabricate conducting polymer hydrogels via supramolecular self-assembly between polymers and multivalent cations. As-prepared hydrogels composed of poly(3,4-ethylenedioxythiophene) and poly(styrenesulfonate) (PEDOT-PSS) exhibit expanded-coil conformation in polymer chains, phase-separate at nanometer scale, possess controllable microstructure, and is responsive to external stimulus. The conducting PEDOT-PSS hydrogels have then been introduced into multiple-network hydrogels to obtain composite hydrogels combining enhanced mechanical strength and excellent electrical activity. Triple-network (TN) and special double-network (sDN) hydrogels, containing poly(acrylic acid) (PAA) and poly(acrylamide) (PAAm) as the matrix respectively, are successfully prepared. Finally, PEDOT-PSS hydrogels with self-strengthening function are directly fabricated via a one-step process under optimized conditions. The strengthening mechanisms for each kind of hydrogels are proposed, and the applicability in electrosensors, supercapacitors and electromechanical actuators are briefly demonstrated.

Systematic evaluation of the preparation of conducting PANI/ABS blends

AbstractFilms of blended poly(acrylonitrile‐butadiene‐styrene) (ABS) and polyaniline (PANI) were produced by codissolving both components in a common organic solvent, which was then evaporated. The influence of the preparation conditions on the properties of the blends was analyzed by factorial design. The factors evaluated were the PANI content in the blend, the m‐cresol to chloroform solvent ratio, the dopant used (dodecylbenzenesulfonic acid (DBSA) or camphor sulfonic acid) and its concentration, and the acrylonitrile content in the ABS. The responses analyzed were the flexibility and electrical conductivity of the blends. The results showed that the PANI content in the blend and the acrylonitrile content in the ABS were the major factors influencing both of the assessed responses. The dopant affected only the conductivity, DBSA being preferred for the development of more conductive PANI/ABS blends. The solvent ratio did not have any influence, owing to the uniform expanded coil conformation expected for PANI molecules at the studios ratios. After the best conditions had been established, a percolation threshold study was performed that pointed to a low threshold of 3 wt % PANI necessary in the blend, giving a flexible blend with a conductivity of 3 S/cm. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Phase transitions of a single polymer chain: A Wang–Landau simulation study

A single flexible homopolymer chain can assume a variety of conformations which can be broadly classified as expanded coil, collapsed globule, and compact crystallite. Here we study transitions between these conformational states for an interaction-site polymer chain comprised of N=128 square-well-sphere monomers with hard-sphere diameter sigma and square-well diameter lambdasigma. Wang-Landau sampling with bond-rebridging Monte Carlo moves is used to compute the density of states for this chain and both canonical and microcanonical analyses are used to identify and characterize phase transitions in this finite size system. The temperature-interaction range (i.e., T-lambda) phase diagram is constructed for lambda<or=1.30. Chains assume an expanded coil conformation at high temperatures and a crystallite structure at low temperatures. For lambda>1.06 these two states are separated by an intervening collapsed globule phase and thus, with decreasing temperature a chain undergoes a continuous coil-globule (collapse) transition followed by a discontinuous globule-crystal (freezing) transition. For well diameters lambda<1.06 the collapse transition is pre-empted by the freezing transition and thus there is a direct first-order coil-crystal phase transition. These results confirm the recent prediction, based on a lattice polymer model, that a collapsed globule state is unstable with respect to a solid phase for flexible polymers with sufficiently short-range monomer-monomer interactions.

Enhancement of Field-Effect Mobility and Stability of Poly(3-hexylthiophene) Field-Effect Transistors by Conformational Change

With the aim of enhancing the field-effect mobility and air stability of self-aligned regioregular poly(3-hexylthiophene), P3HT, by promoting two-dimensional molecular ordering, we transformed the chemical structure of the P3HT chain from a benzoid to a quinoid structure by doping the P3HT solution with HAuCl4 prior to film formation. We found that, for the appropriate HAuCl4 concentration, the P3HT nanocrystals adopt a highly ordered molecular structure with a field-effect mobility of 0.03 cm2 V-1 s-1, which is an improvement by a factor of more than 100. This increase in field-effect mobility is due to a significant enhancement of molecular ordering and the perpendicular orientation of the nanocrystals with respect to the insulator substrate. This resulted from the change in the P3HT chain conformation from a benzoid to a quinoid structure due to oxidation by HAuCl4. The quinoid structure favors a linear or expanded-coil conformation, so the thiophene inter-ring bonds have increased double-bond character, which improves the molecular ordering. Furthermore, the electrical properties of a doped P3HT device had highly improved stability to air without encapsulation. These results suggest that the effect of unintentional doping by oxygen on HAuCl4-doped P3HT film is insignificant because the p-type doping effect of HAuCl4 is the major contributor.

Increasing the work function of poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) by ultraviolet irradiation

The effects of ultraviolet (UV) irradiation on the work function of poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT:PSS) have been investigated in this study. Spectroscopic methods [x-ray photoelectron spectroscopy and Raman spectroscopy (532nm excitation)] and electrical conductivity measurements were used to characterize the electrical conducting polymer PEDOT:PSS prepared by UV irradiation. The authors found that UV irradiation could lead to an increase in the work function and the enhancement of electrical conductivity of PEDOT:PSS, resulting from a decrease in the number of the charge-trapping-related defects and the formation of linear or expanded-coil conformation.

Enhanced Conductivity of Thin Film Polyaniline by Self-Assembled Transition Metal Complexes

In a recent study, the transition metal complex, cis-dichlorobis(2-,2'-dipyridyl)ruthenium (II) (Ru(bpy)2Cl2), and the macrocycle Ru(TPP)CO (TPP:- tetraphenylporphine) were bound to pyridine terminated self-assembled monolayers on quartz. Following modification of the quartz surface with metal complexes, the conducting polymer polyaniline was deposited via in situ polymerization. The sheet conductivity (as measured by the four-probe method) of the resulting polyaniline films deposited onto Ru(bpy)2Cl2 and Ru(TPP)CO surfaces was significantly enhanced relative to films deposited onto unmodified quartz. It is postulated that either the macrocycle or the transition metal complex-modified surface interacts with the conducting polymer as it is forming, resulting in a more ordered expanded coil conformation for the polymer. The net result of such an interaction is a thin film possessing significantly greater electrical conductivity.

Modification of Conducting Polymer Thin Film Interfaces Using Self-Assembled Monolayers Containing Transition Metal Complexes

ABSTRACTThin films of polyaniline (PANi) have been in-situ deposited onto fused silica plates modified with ruthenium porphyrin and polypyridyl complexes coordinated to self-assembled monolayers of pyridyl-terminated alkylsilanes. Based on UV-vis and AFM studies, PANi exhibits a slower deposition rate and an earlier onset of secondary nucleation on the metal complex-modified interfaces. Electrical conductivity of the HCl-doped emeraldine salt form of PANi is increased to as high as 40 S/cm, relative to &lt;1 S/cm on bare silica, measured under the same conditions and in the absence of any external modification to the polymer films. These changes are attributed to favorable π interactions between the aromatic surface species and the conjugated polymer that lead to a more expanded coil conformation for PANi.

Ca 2+-induced gelation of low methoxy pectin in the presence of oxidised starch. Part 1. Collapse of network structure

The effect of oxidised starch on the networks formed by low-methoxy pectin (2.0 wt%), on cooling (from 90 to 5°C) in the presence of Ca 2+ (10–100% stoichiometric) has been explored by rheological measurements under low-amplitude oscillatory shear, and by light microscopy. At low concentrations of Ca 2+, incorporation of increasing concentrations of starch (across the range 0–30 wt%) causes a progressive increase in modulus ( G′ at 5°C), attributed to segregative interactions between the two polymers promoting conversion of pectin from the expanded coil conformation to a more compact associated form. Incorporation of starch at higher Ca 2+ concentrations, however, causes large reductions in gel strength, which arise from sharp drops in G′ during cooling. The Ca 2+ concentration at the transition from enhancement to depletion of network strength is independent of starch concentration, and increases with increasing degree of esterification of the pectin component (across the DE range 31.1–55.8%). Reduction in gel strength is attributed to incipient precipitation of pectin into large aggregates within a supporting calcium pectinate network. This interpretation is supported by microscopy, which shows conversion from a homogeneous distribution of the two polymers to a grossly heterogeneous structure (length-scale ∼10–50 μm) with increasing concentration of Ca 2+, and by quantitative analysis of gel moduli reported in the following paper.

Smart Polymers and Protein Purification

Smart polymers undergo fast and reversible changes in microstructure triggered by small changes of medium property (pH, temperature, ionic strength). These properties of smart polymers were exploited for the development of new protein purification techniques: affinity precipitation, affinity partitioning, and temperature-induced elution in dye-affinity chromatography. Different smart polymers were used for affinity precipitation: Eudragit S 100 (a copolymer of methacrylic acid and methyl methacrylate which precipitates on decreasing pH); stoichiomet-ric polyelectrolyte complexes of poly(methacrylate) and poly(N-ethyl-4-vinylpyridium bromide); poly(N-vinyl caprolactam), and poly(N-iso-propylacrylamide) (both precipitate on heating aqueous solutions to 35–40°C or increase in ionic strength). Recombinant thermostable lac-tate dehydrogenase from a thermophile Bacillus stear other mophilus was purified by affinity partitioning in an aqueous two-phase polymer system formed by dextran and a copolymer of N-vinyl caprolactam and 1-vinyl imidazole. The enzyme partitioned preferentially into the copolymer phase in the presence of Cu ions. The thermoprecipitation of the copolymer at 45°C resulted in an aqueous solution of the purified en- At elevated temperatures, poly-VCL molecules are in a compact globule conformation, capable of binding only to a few ligands on the matrix. The enzyme lactate dehydrogenase from porcine muscle has better access to the ligands and binds to the column. With a decrease in temperature, the polymer molecules undergo transition to a more expanded coil conformation. Polymer molecules interact now with more ligands and begin to compete with the bound enzyme for the ligands. Finally, the bound enzyme is replaced by the expanded polymer chains. This system was used for lactate dehydrogenase purification. Porcine muscle homogenate was applied on a column until breakthrough at 40°C. The foreign proteins were washed out with 0.1 M KC1; then the column was cooled to room temperature and virtually homogeneous lactate dehydrogenase was eluted with the same buffer. The purification factor was 17; recovery was 90% (13).

Effect of Polymeric Additives on Turbulent Flow in Opposed Jets

In this work we study the effect of polymeric additives on the transition to turbulence in opposed-jets flow. In this type of flow, the transition to turbulence for Newtonian fluids is characterized by a decrease in the rate of change of pressure drop with flow rate. We have used various polymers whose equilibrium molecular conformation in aqueous solution is different: poly (ethylene oxide), which exists in a conformation close to a random coil, hydroxypropyl guar, which adopts an expanded coil conformation, and hydrolyzed polyacrylamide, whose conformation is close to a random coil in the presence of an electrolyte (sodium chloride) but it changes to an expanded coil in distilled water. The results show that small amounts of either flexible or semi-rigid polymers induce a delay in the critical Reynolds number at which turbulence sets in. This delay seems to be a result of the suppression of flow instabilities in a region close to the stagnation point, which is linked to macromolecular orientation. Since, for Newtonian fluids, this flow has an increase of drag with flow rate that is slower in the turbulent flow regime than in the laminar regime, the addition of polymer causes a substantially higher pressure drop in turbulent flow with respect to that of the pure solvent. Therefore, polymer addition causes, in this particular case, a drag increase in turbulent flow, as opposed to the commonly observed drag reduction in turbulent flow through pipes.