Conformational Changes Of Poly Research Articles

Conformational Studies of Poly(9,9-dialkylfluorene)s in Solution Using NMR Spectroscopy and Density Functional Theory Calculations

Relationships have been obtained between intermonomer torsional angle and NMR chemical shifts ((1)H and (13)C) for isolated chains of two of the most important poly(9,9-dialkylfluorenes), poly[9,9-bis(2-ethylhexyl)fluorene-2,7-diyl] (PF2/6) and the copolymer poly(9,9-dioctylfluorene-co-[2,1,3]benzothiadiazole-4,7-diyl) (F8BT), using DFT calculations. The correlations provide a model for NMR spectral data interpretation and the basis for analysis of conformational changes in poly(9,9-dialkylfluorene-2,7-diyl)s. The correlations obtained for PF2/6 indicate that the (13)C chemical shifts of the aromatic carbons close to the intermonomer connection (C1, C2, and C3) have minimum values at planar conformations (0 degrees and 180 degrees ) and maximum values at 90 degrees conformations. In contrast, the (1)H chemical shifts of the corresponding aromatic ortho protons (Ha and Hb) are greatest for planar conformations, and the minimum values are seen for 90 degrees conformations. For the F8BT copolymer, similar relationships are observed for the (1)H (Ha, Hb, and Hc) aromatic shifts. Considering the aromatic carbons of F8BT, the behavior of C2, C4, C5, and C6 is similar to that found for the PF2/6 carbons. However, C1 and C3 of the fluorene moiety behave differently with varying torsion angle. These are in close proximity to the fluorene-benzothiadiazole linkage and are markedly affected by interactions with the thiadiazole unit such that delta(C1) is a maximum for 180 degrees and a minimum for 0 degrees , whereas delta(C3) is a maximum for 0 degrees and minimum for 180 degrees. We have studied the (1)H and (13)C spectra of the two polymers at temperatures between -50 degrees C and +65 degrees C. The observed changes to higher or lower frequency in the aromatic resonances were analyzed using these theoretical relationships. Fluorescence studies on PF2/6 in chloroform solution suggest there are no significant interchain interactions under these conditions. This is supported by variable-temperature NMR results. Polymer-solvent and polymer intramolecular interactions were found to be present and influence all of the alkylic and one of the aromatic (1)H resonances (Hb). The detailed attribution of the (1)H and (13)C NMR spectra of the two polymers was made prior to the establishment of the relationships between torsion angle and NMR chemical shifts. This was carried out through DFT calculation of the (1)H and (13)C shielding constants of the monomers, coupled with distortionless enhancement by polarization transfer and heteronuclear correlation NMR spectra. Several DFT levels of calculation were tested for both optimization of structures and shielding constants calculation. The B3LYP/6-31G(d,p) method was found to perform well in both cases.

Self assembly of poly(o-methoxy aniline) with RNA and RNA/DNA hybrids: Physical properties and conformational change of poly(o-methoxy aniline)

Biomolecular hybrids of a conducting polymer [poly( o-methoxy aniline) (POMA)] and RNA are prepared at the three different compositions by mixing aqueous solutions of diethyl, 2-hydroxy ethyl, ammonium salt of RNA (type IX from Torula Yeast) and POMA (ES, emeraldine salt; doping level [Cl]/[N] = 0.52). A slow increase of pH up to 30 h of aging occurs in the mixture till it levels up. The TEM micrographs indicate a fibrillar network structure in all the hybrid compositions (POMA: RNA = 1:3, 1:1, 3:1, by weight). In the complexes three types of supramolecular interactions, viz. (i) electrostatic, (ii) H-bonding and (iii) π– π interactions, are evident from the FTIR spectroscopy. The CD spectra indicate a small distortion of A-RNA conformation towards its B form during the hybrid formation. Time and temperature dependent UV–vis spectral studies indicate a slow red shift of the π-band to polaron band transition peak ( λ max) for the uncoiling of the POMA (P) chain on the RNA (R) surface. The repulsive interaction between the radical cations of POMA (ES) absorbed on the RNA surface is attributed to the conformational change causing the uncoiling of POMA chain. UV–vis spectral study indicates that the uncoiling and attachment of POMA on RNA surface is much faster than that on DNA (D). In POMA–RNA–DNA (PRD) hybrid solutions slower red shift of λ max indicates more disordered array of the phosphate groups than that in PR and PD systems. The conductivity values of the PR hybrids (10 − 6 S/cm − 1 ) are three orders higher than that of RNA, rendering the PR hybrids to be useful for fabricating good biosensors. In the PRD hybrids conductivity decreases by two orders than those of PR and PD hybrids suggesting a disorder arrangement of POMA chains in the PRD hybrids. The I– V characteristic curves of the PR and PRD hybrids indicate a semiconducting nature of the hybrids.

Real-time investigation of crystallization in poly(vinylidene fluoride)-based nano-composites probed by infrared spectroscopy

Via time-resolved Fourier transform infrared spectroscopy (FTIR), we examined the real-time investigation of the conformational changes of poly(vinylidene fluoride) (PVDF) chain segment during crystallization of neat PVDF and the corresponding nano-composites having intercalated structure. It was shown that in the following crystallization processes the crystal growth was virtually the same in both nano-composites and neat PVDF. We have examined an annealing at an infinitely long time at 200°C (∼20min) to erase the thermal history in the nano-composites. The dispersed titanate nano-filler particles exhibited strong contribution to enhance the heterogeneous nucleation for the formation of both γ- and β-phase crystals.

Direct Visualization of Reversible Switching of Micropatterned Polyelectrolyte Brushes on Gold Surfaces Using Laser Scanning Confocal Microscopy

We apply confocal fluorescence microscopy for real time studies of reversible conformational changes of poly(methacryloyloxyethyl phosphate) (PMEP) brushes chemically grafted onto gold substrates. Oregon green 488 fluorophores chemically attached onto the PMEP polymers were used as reporters for probing the conformational changes. Use of a specially designed liquid flow microchamber allowed dynamic imaging of the brushes under varying environmental conditions. The fluorescence intensities exhibited fully reversible brightness changes on alternation of the solution in the chamber between water and KCl. This reversible quenching behavior is consistent with a conformational change between an extended and a collapsed brush configuration. The fluorescence quenching behavior of the brushes was found to be dependent on ion concentration as well as polymer grafting density and was caused by nonradiative energy transfer to the polymer scaffold and the gold substrate.

FTIR-ATR investigations of an α-helix to β-sheet conformational transition in poly( l-lysine)

A promising application of the multivariate curve resolution based on alternating least squares (MCR-ALS) method is reported for the analysis of the temperature-dependent conformational changes in poly( l-lysine) (PLL). The MCR-ALS approach has enabled detection of three structural components developed during the melting experiment. Most important is an infrared signature of the intermediate component described as a mixture of native α-helical conformation and its extended forms.

Water-induced (nano) organization in poly(ethyl acrylate- co-hydroxyethyl acrylate) networks

The conformational changes in poly(ethyl acrylate- co-hydroxyethyl acrylate), P(EA- co-HEA) chains, which constitute a copolymer network hydrogel, induced by the presence of water are investigated by different experimental techniques and compared with the behaviour of the corresponding xerogel. The mechanical relaxation spectrum shows the presence of a new water-induced relaxation, the water content dependence of the glass transition is measured by DSC, and the dielectric relaxation assesses the effect of water for the lower concentrations. Hydrophilic and hydrophobic monomeric units in the P(EA- co-HEA) network are able to aggregate to form two separated (nano)phases in the presence of water due to hydrophobic interaction. Phase separation takes place when the water content of the sample is higher than a critical value estimated as two water molecules per –OH group in the copolymer chain. The existence of the hydrophobic domains is detected by their glass transition being nearly independent on the water content of the sample. Phase separation is also clearly revealed by phase angle measurements in AFM experiments.

Conformational change of poly(methylphenylsilane) induced by the photoisomerization of pendant azobenzene moiety in the film state

Conformational change of the main chain of poly(methylphenylsilane) functionalized with pendant azobenzene has been studied in the solution and in the film state. The conformation of the main chain was found to depend on the molar ratio of the Si units with and without the pendant azobenzene moiety. The inflection region for the conformational change was 0.3–0.5 of the molar ratio. The conformation of the main chain changed by the photoisomerization of the pendant azobenzene moiety in the film state. The efficiency of the conformational change of the main chain increased with the decrease of the molar ratio, accompanied by the highest final value of degree of the conformational change at the molar ratio of 0.5. The reverse conformational change was induced by heating.

Insight into the Origin of the Thermosensitivity of Poly[2‐(dimethylamino)ethyl methacrylate

We investigate the effects of pH and temperature on the conformational changes of poly[2-(dimethylamino)ethyl methacrylate] (PDEM) chains at the air/water interface by using Langmuir balance and sum frequency generation vibrational spectroscopy. At pH 4, the tertiary amine groups are fully charged and the PDEM chains are so hydrophilic that they completely enter into the water phase and do not exhibit thermosensitivity. At pH 7, these groups are only partially charged, and the accompanying hydration/dehydration--followed by repartitioning into the water and air phases--gives rise to a marked thermosensitivty. Finally, at pH 10, the tertiary amine groups become uncharged and thus preferentially stay in the hydrophobic air phase, devoid of associated water molecules, which results in the surface-pressure change (DeltaPi) being nearly independent of the temperature. Our Langmuir-balance experiments, coupled with surface-sensitive spectroscopy, demonstrate that: 1) the thermosensitivity of the PDEM chains relates to the hydration/dehydration of the tertiary amine groups, 2) the phase transition of thermosensitive polymers is most likely initiated by the dehydration of the chains, and 3) the phase transition of thermosensitive polymers at the air/water interface is markedly different from that in aqueous solution because of the redistribution of the macromolecular segments induced by the asymmetric forces at the air/water interface.

Spectroscopic and calorimetric studies on the binding of the phototoxic and cytotoxic plant alkaloid sanguinarine with double helical poly(A)

The interaction of the phototoxic and cytotoxic DNA binding plant alkaloid sanguinarine with double stranded polyadenylic acid has been investigated from multifaceted spectroscopic and calorimetric techniques. The hypochromic and bathochromic shifts in the absorption spectrum, and large enhancement of the fluorescence intensity of sanguinarine indicated strong binding of the alkaloid to the duplex poly(A). The corresponding intrinsic binding constant for the complexation, obtained from visible absorbance and fluorescence spectroscopic methods, respectively, was found to be in the order of 10 4 M −1. The binding was further characterized by strong energy transfer from the adenine base pairs to the alkaloid, significant polarization of the fluorescence of the alkaloid, stabilization against thermal strand separation of the duplex, and also resulting in remarkable conformational changes in poly(A) with concomitant generation of optical activity in the bound alkaloid molecules that are otherwise achiral. Isothermal titration calorimetric and differential scanning calorimetric studies revealed the binding of the alkaloid to be exothermic and enthalpy driven. All these results together with fluorescence quenching experiments advance good evidence for the intercalation of the alkaloid sanguinarine into the duplex poly(A).

Conformation of oligo(ethylene glycol) grafted polystyrene in dilute aqueous solutions

Temperature induced conformational changes of poly( p-oligo(ethylene glycol) styrene) (POEGS) in aqueous solutions were investigated by small angle neutron scattering (SANS), neutron transmission and dynamic light scattering (DLS). The molecular weight of the polymer studied was 9400 g/mol with a polydispersity index of 1.18 and each repeat unit of the polymer had four ethylene glycol monomer segments. The polymer was water soluble due to the hydrophilicity of the OEG side chains and these solutions showed lower critical solution temperature (LCST) depending on the concentration of the polymer. Measurements of solution behavior were made as a function of temperature in the range of 25–55 °C for three polymer concentrations (0.1 wt%, 0.3 wt%, and 1.8 wt%). Neutron transmission measurements were used to monitor the amount of polymer which precipitated or remained in solution above the cloud point temperature ( T CP). DLS revealed the presence of large clusters in all solutions both below and above T CP while SANS provided information on the structure and interactions between individual chains. It was found that in the homogeneous region below T CP the shape of individual polymers in solution was close to ellipsoidal with the dimensions R a = 37 Å and R b = 14 Å and was virtually independent of temperature. The SANS data taken for the most concentrated solution studied (1.8 wt%) were fit to the ellipsoidal model with attractive interactions which were approximated by the Ornstein–Zernike function with a temperature-dependent correlation length in the range of 24–49 Å. The collapse of individual polymers to spherical globules with the radius of 15 Å above T CP was observed.

Conformational Changes of Poly(N-isopropylacrylamide) Chains at Air/Water Interface: Effects of Temperature, Compression Rate, and Packing Density

This study concerns the effects of temperature, compression rate, and packing density on conformational properties of polystyrene-block-poly(N-isopropylacryamide) (PS-b-PNIPAM) diblock copolymer monolayers at the air/water interface using the Langmuir balance technique. The dependency of surface pressure changes on temperature and compression rate is strongly influenced by the conformations of PNIPAM chains at the interface, which can be adjusted by varying the packing density. Specifically, when loops or tails are formed at the interface, PNIPAM chains display thermosensitive properties due to hydration/dehydration and obvious dependence on compression rate. When PNIPAM chains take train conformation at the air/water interface, however, the surface pressure changes are nearly independent of temperature and compression rate because almost all segments of the PNIPAM chains are adsorbed at the interface and the nonpolar isopropyl groups are preferentially oriented toward the air. Our work reveals that one could manipulate stimuli-responsive properties of PNIPAM chains at the interface simply by adjusting the conformations of PNIPAM chains.

Solid‐state 19F MAS NMR study on the conformation and molecular mobility of poly(chlorotrifluoroethylene)

The temperature dependence of molecular mobility and conformational changes of poly(chlorotrifluoro- ethylene) (PCTFE) have been investigated by solid-state (19)F magic angle spinning (MAS) NMR spectroscopy. The pulse techniques of dipolar-filter and T(1rho)-filter allow selective observation of the amorphous and crystalline domains, respectively. The temperature dependence of T(1rho) (F) revealed that the segmental motion in the amorphous domain becomes vigorous above ca 80 degrees C, which is well above the glass transition (T(g)) temperature (52 degrees C) and more close to the beta-relaxation temperature (95 degrees C). On the other hand, vigorous molecular motions in the crystalline domain occur above 120 degrees C, which is much below the melting temperature (212 degrees C). This indicates that the polymer chains in the PCTFE crystallites are more mobile than those of typical semicrystalline fluoropolymers like poly(vinylidene fluoride) (PVDF), which can be associated with structural imperfections in the crystallites. In addition, the density functional theory (DFT) calculations of (19)F magnetic shielding suggest that the high-frequency shifts observed for the crystalline signals above 80 degrees C can be ascribed to the conformational change around meso diads toward more twisted and/or helical conformations in the main chain.

Conformational Change of Poly(ethylene Glycol) near the Critical Point of Isobutyric Acid + Water

In solutions of isobutyric acid + water, poly(ethylene glycol) (PEG) can assume a helical conformation. [Alessi, M. L.; Norman, A. I.; Knowlton, S. E.; Ho, D. L.; Greer, S. C. Macromolecules 2005, 35, 9333-9340.] Here we report new measurements of the kinematic viscosity, nu, as a function of temperature for a solution of isobutyric acid + water at the critical composition, to which PEG (number average molecular weight = 1.01 x10(3) g/mol) was added at a concentration of 6.01 mg/mL. The data show that nu decreases near the critical point, with a maximum in nu at about 0.05 degrees C above the critical temperature, Tc. We interpret this change in nu in terms of a change in conformation of the polymer from helix to coil. This interpretation is supported by polarimetry measurements on the same mixture doped with (S)-(+)-1,2-propanediol, which indicates the loss of helicity near Tc and also a second helix-to-coil transition at about 60 degrees C.

Reentrant conformation transition in poly( N, N-dimethylacrylamide) hydrogels in water–organic solvent mixtures

Conformational changes in poly( N, N-dimethylacrylamide) (PDMA) networks swollen in aqueous solutions of organic solvents are studied both experimentally and theoretically. PDMA hydrogels of various charge densities were prepared by free-radical crosslinking copolymerization. Swelling behavior of the hydrogels was investigated in aqueous organic solvent mixtures as functions of solvent species and the concentration. With increasing volume fraction ϕ of acetone, tetrahydrofuran, or 1,4-dioxane in the aqueous solution, PDMA hydrogels exhibit reentrant conformation transition. During this transition, the gel first deswells in the range of ϕ between 0.4 and 0.9, and then rapidly reswells if ϕ is monotonically increased. The reswelling of the collapsed PDMA gel occurs in a narrow of ϕ above ϕ=0.97. It was shown that the reentrant transition in PDMA gels requires moderate hydrogen bonding organic solvents, so that the hydrophobic interactions between PDMA and the organic solvent dominate the swelling process. The results were interpreted using the theory of equilibrium swelling. The interaction parameters in the gel system as well as the partition parameter of the organic solvent between the gel and the solution phases were calculated.

The role of solvation on the conformational change during repeated freezing–thawing treatment to an extremely dilute aqueous solution of poly(vinyl alcohol)

The effect of solvation–desolvation process on the conformational change of poly(vinyl alcohol) in an extremely dilute aqueous solution during the repeated freezing–thawing treatment was investigated by a differential refractometer. It was found that the hydrodynamic volume of PVA decreased regularly and smoothly with the number of freezing–thawing cycles treatment increasing, as the intra-molecular entanglement points made the polymer chain contract, and even in a full compacted globule state, there were still some solvent molecules in the chain. The extraordinary phenomenon was successfully explained and treated by a quantitative theory in terms of accompanying solvation–desolvation process during the macromolecular chain having contracted.

Surface-Induced Conformational Changes in Poly(3-hexylthiophene) Monolayer Films

With the aim of elucidating the surface-induced molecular ordering in regioregular poly(3-hexylthiophene) (P3HT) monolayer films, we have controlled the intermolecular interactions at the interface between P3HT and the insulator substrate by using self-assembled monolayers (SAMs) functionalized with two kinds of groups (-NH2 and -CH3). We have found that, depending on the surface properties of such modified insulator substrates, the P3HT chains in the monolayer films can adopt two different conformations (edge-on and face-on). This surprising variation in chain conformation arises because of the specific interactions of the P3HT chains with the modified insulator substrates, which can be explained in terms of the following factors: the unshared electron pairs of the SAM end groups (in the -NH2 system), the pi-H interactions between the thienyl backbone bearing pi systems and the H (hydrogen) atoms of the SAM end groups, and interdigitation between the alkyl chains of P3HT and the alkyl chains of the SAMs (in the -NH2 system).

Conformational changes of poly(ethylene oxide) in poly(ethylene oxide)/poly(methyl methacrylate) blends by supercritical carbon dioxide

AbstractThe effects of supercritical carbon dioxide (SC CO2) fluids on the morphology and/or conformation of poly(ethylene oxide) (PEO) in PEO/poly(methyl methacrylate) (PMMA) blends were investigated by means of differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction (WAXD), and Fourier transform infrared (FTIR). According to DSC data for a given blend, the melting enthalpy and, therefore, degree of crystallinity of PEO were increased, whereas the melting temperature of PEO was decreased, with SC CO2 treatment. The enhancement of PEO crystallization with SC CO2 treatment, as demonstrated by DSC data, was supported by WAXD data. According to FTIR quantitative analyses, before SC CO2 treatments, the conformation of PEO was transformed from helix to trans planar zigzag via blending with PMMA. This helix‐to‐trans transformation of PEO increased proportionally with increasing PMMA content, with around 0.7% helix‐to‐trans transformation per 1% PMMA incorporation into the blend. For a given blend upon SC CO2 treatments, the conformation of PEO was transformed from trans to helix. This trans‐to‐helix transformation of PEO decreased with increasing PMMA contents in the blends because of the presence of interactions between the two polymers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2479–2489, 2004

Imaging the Coil-to-Globule Conformational Transition of a Weak Polyelectrolyte by Tuning the Polyelectrolyte Charge Density

Conformational changes of poly(vinylamine) (PVA) upon adsorption onto mica were determined by AFM. The polyelectrolyte charge can be tuned through the solution pH. At pH 3, where the PVA is highly charged, an extended coil conformation is observed. When the pH is increased, the polyelectrolyte charge decreases and induces a segmental collapse through pearl-necklace structures. Above pH 9, where the PVA is only weakly charged, one can observe a globular conformation. By analyzing the volume of the imaged objects, it could be clearly demonstrated that single molecules have been observed. By examining PVA adsorbed in its most globular conformation, the molecular mass distribution has been determined. The PVA molecules could be also detected after being grafted to an epoxy terminated surface, which has been obtained through a silanization reaction. The observed polyelectrolyte collapse through a sequence of pearl-necklace structures is likely related to analogous conformations in solution, which are predicted on theoretical grounds.

Study of conformational changes in poly(3-dodecylthiophene) dependent on backbone stereoregularity using a spin-probe technique

Poly(3-dodecylthiophene) (P3DDT), with different amounts of head-to-head configuration defects, was characterized by ultraviolet–visible, 1H-NMR, photoluminescence, and Raman spectra. The heat-induced conformational sample changes were studied by electron paramagnetic resonance (EPR). For the study of these changes, a spin-probe technique was used, in which 5-doxyl-stearic acid methyl ester was applied as a spin probe. From the EPR spin-probe spectra, rotational correlation times and order parameters were calculated. The heat-induced conformational changes of P3DDT were accompanied by a monotonic decrease in the rotational correlation time up to approximately −33°C (240 K) and then by an increase in the range of the glass-transition temperatures, with the maximum being near room temperature and depending on the effective conjugation length. Afterward, the rotational correlation times had a decreasing tendency up to 90°C (363 K). © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2215–2223, 2003

Photothermally Induced Conformational Changes in Poly(Substituted Thiophene) Film Leading to Nanometer Surface Protrusion: A Near-Field Fluorescence Microspectroscopic Study

The relationship between changes of fluorescence spectrum, polymer conformation, and surface morphology of a poly{3-[2-(N-dodecylcarbamoyloxy)ethyl]thiophene- 2,5-diyl} (P3DDUT) film were examined by near-field fluorescence microspectroscopy. By a 488 nm excitation using the near-field probe, a nanometer scale protuberance was formed on the surface. The excitation also induced a blue shift in the fluorescence spectrum. By analyzing the spectral change, it was confirmed that photothermally induced conformational changes (rod-coil transformations) lead to the nanometer surface protrusion. Moreover, in exposure time dependence measurements, it was found a time lag in appearances of the blue shift and the protrusion, indicating that an accumulation of transformed polymers is necessary to evolve into the surface protrusion.