Transition Of Poly Research Articles

Synthesis and Structure Characterization of Liquid Crystalline Polyacrylates with Unconventional Fluoroalkylphenyl Mesogens

The synthesis, thermal behavior, and bulk microstructure of fluorinated new liquid-crystalline polyacrylates containing spaced fluoroalkylphenyl units in the side chains were studied. While in polyacrylates poly(1) and poly(2) there was a 4-(perfluorooctylethoxy)phenyl benzoate unit, i.e., a two-phenyl mesogenic core, poly(3) contained a 4-perfluorooctylethyl benzoate, i.e., a one-phenyl aromatic core that per se would not be able to promote a thermotropic behavior. X-ray diffraction studies allowed identification of the mesophases (hexatic smectic, disordered smectic, nematic) and study of the evolution of their structures and lattice parameters with temperature. It was found that each polymer presented a rich mesophase polymorphism, including the rather unusual smectic F2- (or I2-) to-nematic transition in poly(3). The electron density profiles along the smectic layer normal were drawn and provided a deeper insight into the packing of the side chains in tilted double layer structures. The strong mesogenic character of the polymers was attributed to the presence of the fluorinated tail on the aromatic core owing to its capability of adopting a rigid-rodlike conformation. A phase separation occurring at the molecular level of the different incompatible aliphatic−aromatic−fluoroaliphatic sections of the polymer repeat unit should efficiently cooperate to the onset of thermotropic mesomorphism.

A kinetic model for the B-Z transition of poly[d(G-C)].poly[d(G-C)] and poly[d(G-m5C)].poly[d(G-m5C)

The analysis of the kinetic data of the B-Z conformational changes induced by salt in sized double-stranded poly[d(G-C)].poly[d(G-C)] and poly[d(G-m5C)].poly[d(G-m5C)] polymers indicated that there exists a salt threshold which reveals some largely, as yet, unrecognised characteristics of the transition. It was observed that there is a direct correlation between the length of the polymer and the rate of the B-Z transition when the salt concentration in the polymer solution is lower than the salt threshold. The correlation is inverse when the salt concentration is higher than the salt threshold. Thus, the molecular mechanism of the B- to Z-DNA transition varies depending on whether the salt concentration is higher or lower than the threshold. In this context, we have found that the contrasting results reported in the literature describing the rate of the B-Z transition are not contradictory but complementary. The finding of a salt threshold leads to the establishment of a relationship between the cooperativity index of the B-Z transition and the polymer chain length. That relationship is dependent on the chemical structure of the polymer but is temperature independent.

1H NMR study of thermotropic phase transition of linear and crosslinked poly(vinyl methyl ether) in D2O

The changes in the dynamic structure during temperature-induced phase transition in poly(vinyl methyl ether) (PVME)/D2O solutions and gels in a broad range of concentrations (c=0.1–30wt%) and crosslinking densities, respectively, were studied by 1H NMR methods. Similar behaviour was found both for linear and crosslinked systems, indicating the formation of compact globular-like structures during the phase transition. The fraction of PVME segments in globular-like structures is ∼0.85 for solutions and ∼1.0 for swollen networks, independent of the polymer concentration or the crosslinking density. While for dilute PVME solutions the thermotropic transition as detected by NMR is virtually discontinuous, for semidilute and concentrated solutions, and for swollen networks, the transition sets in at lower temperatures and its width is several Kelvin broad.

Kinetics of the volume phase transition in poly(n‐isopropylacrylamide) gels prepared under high pressure

New poly(N-isopropylacrylamide) gels were prepared under high pressure (ca. 200 MPa) during gelation. The preparation-pressure dependence of the deswelling speed of the gels was measured with a conventional T-jump method. The deswelling time of a gel rod 2.2 mm in diameter prepared at 193 MPa was about 200 s, 1000 times faster than that of a homogeneous poly(N-isopropylacrylamide) gel. Moreover, the collective diffusion coefficient, the thermal fluctuation, and the ensemble-average intensity of the swollen gel networks were obtained with dynamic light scattering measurements. Both the enthalpy and entropy of the gels were estimated from equilibrium swelling curves with the Flory–Huggins interaction parameter evaluated with mean field theory based on the Flory-type of the Gibbs free-energy formula. It was found that the networks of the gels had an inhomogeneous structure newly introduced by the preparation pressure. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2315–2325, 2001

Kinetics of the volume phase transition in poly(n‐isopropylacrylamide) gels prepared under high pressure

AbstractNew poly(N‐isopropylacrylamide) gels were prepared under high pressure (ca. 200 MPa) during gelation. The preparation‐pressure dependence of the deswelling speed of the gels was measured with a conventional T‐jump method. The deswelling time of a gel rod 2.2 mm in diameter prepared at 193 MPa was about 200 s, 1000 times faster than that of a homogeneous poly(N‐isopropylacrylamide) gel. Moreover, the collective diffusion coefficient, the thermal fluctuation, and the ensemble‐average intensity of the swollen gel networks were obtained with dynamic light scattering measurements. Both the enthalpy and entropy of the gels were estimated from equilibrium swelling curves with the Flory–Huggins interaction parameter evaluated with mean field theory based on the Flory‐type of the Gibbs free‐energy formula. It was found that the networks of the gels had an inhomogeneous structure newly introduced by the preparation pressure. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2315–2325, 2001

Polymer-solvent interactions during phase transition in poly(vinyl methyl eiher)/D2O solutions as studied by NMR methods

The structural-dynamic changes and polymer-solvent interactions during temperature-induced phase transition in poly(vinyl methyl ether) (PVME)/D 2 O solutions in a broad range of concentrations (0.1-30 wt.-%) were studied by 'H NMR methods. In the whole concentration range the phase transition is manifested by line broadening (linewidth 350-500 Hz) of a major part of PVME units, evidently due to the formation of globular-like structures. Above the LCST transition, the fraction of phase-separated PVME segments is equal to 0.8 ± 0.1, independent of polymer concentration. While at low concentrations the transition is virtually discontinuous, at high concentrations the transition region is ∼ 3 K broad. Measurements of nonselective and selective 1 H spin-lattice relaxation times T 1 of solvent (HDO) molecules evidenced that at elevated temperatures, where most PVME forms globular structures, a part of solvent molecules is bound to PVME forming a complex; the lifetime of the bound water (HDO) molecules is ≤ 2 s.

Structural, Electronic, and Morphological Changes in Poly(phenylenesulfide phenyleneamine) upon Electrochemical Doping

Structural and electronic transitions in poly(phenylenesulfide phenyleneamine) (PPSA) upon electrochemical doping have been investigated. The results indicate that polarons are the predominant charge defects at low doping levels, yielding an electronically and mechanically stable material. The electrochemical doping at high potentials induces a transition of the polarons to a bipolaron state. Due to the heterogeneous nature of the polymer chain and different oxidation potentials associated with aniline and phenylene sulfide units, the formation of polaron and bipolaron is distinguished as two separated steps. The dopant perchlorate ion exists in the polymer matrix, not only as charge neutralizer but also as a ligand that is simultaneously Coulombically bound to the positively charged S or N sites on one polymer chain and hydrogen bonded to the N−H group on the neighboring chain. The formation of such perchlorate anion centered Coulombic/hydrogen-bonded complexes has a major impact on the electrochemical activity and the morphology of the doped polymer. Also due to stabilization of the dopant ions, the doped polymer can be retained in a stable and desirable oxidation state.

Tuning of Electronic Properties in Conducting Polymers

Structural and electronic transitions in poly(thiophenyleneiminophenylene), usually referred to as poly(phenylenesulfidephenyleneamine) (PPSA) upon electrochemical doping with LiClO4 have been investigated. The unusual electrochemical behavior of PPSA indicates that the dopant anions are bound in two energetically different sites. In the so-called "binding site", the ClO4- anion is Coulombically attracted to the positively charged S or N sites on one chain and simultaneously hydrogen-bonded with the N-H group on a neighboring polymer chain. This strong interaction causes a re-organization of the polymer chains, resulting in the formation of a networked structure linked together by these ClO4- Coulombic/hydrogen bonding "bridges". However, in the "non-binding site", the ClO4- anion is very weakly bound, involves only the electrostatic interaction and can be reversibly exchanged when the doped polymer is reduced. In the repeated cycling, the continuous and alternating influx and expulsion of ClO4- ions serves as a self-organizing process for such networked structures, giving rise to a diminishing number of available "non-binding" sites. The occurrence of these ordered structures has a major impact on the electrochemical activity and the morphology of the doped polymer. Also due to stabilization of the dopant ions, the doped polymer can be kept in a stable and desirable oxidation state, thus both work function and conductivity of the polymer can be electrochemically controlled.

Twist Phase Transition in Poly(p-phenylene) Oligomers: Heat Capacity of p-Quinquephenyl

Abstract The heat capacity of crystalline p-quinquephenyl was measured by adiabatic calorimetry below room temperature. A broad hump was detected around 264 K and attributed to a phase transition, through which an average molecular conformation changes from the planar to the twisted one on cooling. A similar anomaly was also detected for crystalline p-sexiphenyl around 295 K by chopped-light ac calorimetry. The broad shape of the thermal anomalies arising from the twist transitions are discussed in relation to the number of possible intramolecular twisting modes. Properties of the twist transition in polymer are discussed on the basis of those of oligomers. Standard thermodynamic functions are tabulated for p-quinquephenyl.

Конформационные переходы поли (С) и поли(сІС): исследование методом протонной буферной емкости

Конформационные переходы поли (С) и поли(сІС): исследование методом протонной буферной емкости

Optical constants of highly stretch-oriented poly(p-phenylene-vinylene): A joint experimental and theoretical study

We have determined the frequency dispersion of the anisotropic complex optical constants of highly stretch-oriented poly(p-phenylene-vinylene) chains from Kramers-Kronig transformation of polarized near-normal-incidence absolute reflectivity data, inversion of polarized reflectance and transmittance measurements, and spectroscopic ellipsometry. The spectroscopic data are discussed in comparison to the results provided by correlated quantum-chemical calculations. On this basis, we assign the nature and polarization of the optical transitions in poly(p-phenylene-vinylene) chains.

Electron-Irradiation Induced Phase Transition in Poly (Vinylidene Fluoride-Trifluoroethylene) Copolymer

The electron-irradiation induced ferroelectric-to-relaxor phase transition in poly (vinylidene fluoride-trifluoroethylene) (P(VDF/TrFE)) copolymer was investigated. One polymer with a composition of 53 mol% VF2 (PVDF53) and a Curie temperature of 67°C, and another with 76 mol% VF2 (PVDF76) and a Curie temperature of 136°C, were used in the research. Changes in hysteresis loop, permittivity and electrostriction during the phase transition were measured. X-ray diffraction showed that PVDF53 had a second-order phase transition during the irradiation in which the structure was changing continuously, while PVDF76 had a first-order phase transition in which the structure changed discontinuously. Field-induced piezoelectric response was measured using a loudspeaker which provided a vibrational force, and d31 = 135 pC/N at bias E3 = 40 MV/m was obtained. The conditions for electron irradiation and composition of the copolymers were discussed.

Alpha−Beta Transition in Poly(butylene terephthalate) As Revealed by Small-Angle X-ray Scattering

The stretching-induced alpha−beta polymorphic transition in the crystalline regions of poly(butylene terephthalate) (PBT) is studied by small-angle X-ray scattering (SAXS), instead of by the wide-angle X-ray scattering (WAXS) usually used for this purpose. An expression for the increase of the long spacing upon the transition is derived from a simple model of oriented PBT as a set of microfibrils, each consisting of alternating crystalline and amorphous regions. This increase is calculated on the basis of the c unit cell parameter of both alpha and beta phases and the degree of crystallinity as obtained from differential scanning calorimetry. There is a fairly good agreement between this value and the experimentally measured one. The affine deformation model holds for both the alpha and beta phases in the whole interval of the strain, from zero up to the strain at break. Without external stress the microfibrils are in the fully relaxed state, as concluded from the values of the corresponding long spacing ...

00/01561 Temperature-modulated calorimetry of the frequency dependence of the glass transition of poly(ethylene terephthalate) and polystyrene

00/01561 Temperature-modulated calorimetry of the frequency dependence of the glass transition of poly(ethylene terephthalate) and polystyrene

Amine induced Z-DNA in poly(dG-dC)·poly(dG-dC): Circular dichroism and gel electrophoresis study

Spermine and [Co(NH3)6]3+ are known to induce the B → Z transition in poly(dG-dC)·poly(dG-dC) DNA. In this paper gel electrophoresis shift assays using the long DNA poly(dG-dC)·poly(dG-dC) are developed to complement circular dichroism (CD) measurements of the conformation of the DNA in the presence of amines including spermine, spermidine, [Co(NH3)6]3+, (+)-[Co(en)3]3+ and (−)-[Co(en)3]3+ (en: ethylenediamine). CD is used to monitor the average conformation of the DNA and the gels show when there is a population of a given type of DNA in the solution. Spermidine is found to induce the transition at very low ionic strength, in contrast to literature wisdom. The cobalt amine-induced Z-DNA melts, with a broad transition, at a lower temperature than B-DNA. The electrophoresis data shows the presence of a DNA more flexible than B-DNA at low amine concentrations. This is ascribed to the existence of B/Z junctions. At higher amine concentrations, a DNA (Z-DNA) that is stiffer than B-DNA is observed. The Z-DNA induced by spermine is stiffer (runs more slowly on the gels) than that induced by the other amines. The spermine–DNA interaction is found to be very cooperative, whereas that of the cobalt amines is uncooperative. The kinetics of the B → Z transition with spermine at room temperature is very slow, whereas the cobalt amines induce it within seconds.

Sequence-dependent conformational changes in DNA induced by polynuclear platinum complexes

In this work, the B→Z transition of poly(dG-dC)·poly(dG-dC) and the B→A transition of poly(dG)·poly(dC) and of calf thymus (CT) DNA fragments modified by antitumor bifunctional polynuclear platinum complexes were investigated by circular dichroism (CD). The transition from the B- to Z-form of DNA was inducible with all three compounds studied, as indicated by an inversion of the B-form spectra. The B→A transition in poly(dG)·poly(dC) was induced easily by platinum complex binding alone, while the B→A transition in CT DNA was induced by ethanol but inhibited by coordination of all polynuclear platinum compounds used here. It was shown that the compound [{ cis-PtCl(NH 3) 2} 2 μ-{H 2N(CH 2) 6NH 2}](NO 3) 2 (1,1/c,c) was most effective at inhibiting the B→A transition in CT DNA, and [{ trans-PtCl(NH 3) 2} 2 μ-{ trans-Pt(NH 3) 2(H 2N(CH 2) 6NH 2) 2}](NO 3) 4 (1,0,1/t,t,t) was least effective, while the effectiveness of [{ trans-PtCl(NH 3) 2} 2 μ-{H 2N(CH 2) 6NH 2}](NO 3) 2 (1,1/t,t) fell between the two. This corresponded to the relative amounts of interstrand crosslinks in double-stranded DNA caused by each compound.

Poly-N-vinylcaprolactamdashhydrate complex as a model of detector of associate states and interactions in aqueous solutions

The “dissolving — precipitation” transition of poly- N -vinylcaprolactam (PVCL) macromolecules in water at heating was studied. The structure of the PVCL-water complex was determined by ab initio quantumdashchemical analysis and DSC method. It was found that the latent heat of transition is low (−1.7 kJ/mol). This permits one to use hydrated PVCL macromolecules as highly sensitive detectors of water associates and interactions occurring in a water medium. The influence of different water-soluble compounds on phase transition indirectly reflects a structure change in water caused by the latter. Various changes of the “dissolving-precipitation” transition temperature depending on the concentrations of various substances in water seem to indicate that each compound affects in its own way the water molecular structure in solution.

Direct Evidence for a Lipid Alkyl Chain Ordering Transition in Poly(ethylene oxide) Lipopolymer Monolayers at the Air−Water Interface Obtained from Infrared Reflection Absorption Spectroscopy

Lipopolymers are lipids with a polymer chain covalently attached to the lipid. At the air−water interface such lipopolymers may undergo up to two monolayer phase transitions. One is correlated with desorption of the polymer group from the surface (the so-called pancake−mushroom transition) and a second is “native” to lipopolymers in the sense that it can only be observed when lipid and polymer are present in the same molecule. In this study we present direct evidence from infrared spectroscopy showing that the “native” transition, although requiring the presence of the polymeric headgroup, is a transition solely within the lipid alkyl chains. This transition correlates to a strong reduction of the number of gauche isomers within the lipid alkyl chains. We used two poly(ethylene oxide) lipopolymers which differed only in the lipid alkyl chains. These were either fully deuterated or fully protonated. The polymer chains were protonated in both cases. Within the protonated lipopolymer, a strong ordering of th...

Effect of Molecular Weight on αβ Splitting Region of Dynamic Glass Transition in Poly(ethyl methacrylate)

Effect of Molecular Weight on αβ Splitting Region of Dynamic Glass Transition in Poly(ethyl methacrylate)

Conformational analysis of the 13/4 to 4/1 helical transition in poly( α-isobutyl β-L-aspartate)

The mechanism for the conversion of the 13/4 helix into the 4/1 helix of poly( α-isobutyl β-L-aspartate) was simulated at the atomic level. A computational procedure based on a potential of mean force capable of creating multiple conformation states compatible with a given helical symmetry was used. Intermediate helical arrangements differing in 0.1 residues/turn were modeled and energetically evaluated to delineate the reaction coordinate. The conversion was found to proceed through poorly defined hydrogen bonded structures having an energy surplus of about 6 kcal/mol·residue. Results were consistent with experimental data and helpful for understanding fundamental aspects of the transition as it is observed in the solid state.