All-trans Sequences Research Articles

Raman scattering in protonated and deuterated 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC): Indicators of conformational and lateral orders

The use of deuterocarbons is an effective method in the Raman spectroscopy of multicomponent lipid materials and biological samples. Here, Raman spectra of hydrated multilamellar vesicles of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), its deuterated analog 1,2-dipalmitoyl-d62-sn-glycero-3-phosphocholine (DPPCd62), and DPPC-DPPCd62 mixtures were studied in a wide temperature range to specify the Raman indicators of conformational and lateral orders. The temperature dependence of the 985 cm−1 line in the deuterated phospholipid unequivocally indicates that this line corresponds to the CC stretching vibrations of deuterated hydrocarbon chains in the all-trans conformation. It was also concluded that the ratio of Raman intensities at the maximum of the peak of the symmetric CD2 stretching and at a maximum near 2168 cm−1 reflects the conformational order of the hydrocarbon chain and can be used for an evaluation of the fraction of the all-trans sequences. The frequency of the symmetric CD2 stretching peak is sensitive to the phase state (gel or fluid) but has a low sensitivity to the partial conformational disordering within the gel phase. The Raman study of DPPC-DPPCd62 mixtures reveals that the lateral order contributes to the ratio of intensities of the antisymmetric and symmetric CH2 stretching peaks as a prefactor enhancing the effect of conformational ordering.

Application of temperature modulation to FTIR spectroscopy: an analysis of equilibrium and non-equilibrium conformational transitions of poly(ethylene terephthalate) in glassy and liquid states

In this paper, the application of a temperature modulation to the temperature-resolved FTIR analysis is reported. The advantage offered by the spectroscopic investigation, able to follow the micro-structural and conformational sample modification involved in sample thermal transformation, was merged to that of temperature modulation, related to the possibility to separate the reversing (in-equilibrium within the experimental condition) to the non-reversing (non-equilibrium) processes. The potentiality of the technique (modulated temperature FTIR, MTFTIR) is highlighted through the study of the thermal transitions of amorphous poly(ethylene terephthalate) from 50 °C to the cold-crystallization. After the presentation of the theoretical framework and the experimental conditions, a step-by-step description of acquired data elaboration is given. The total variation of a selected band intensity as function of mean temperature as well as its reversing and non-reversing components are obtained. The evolution of the bands at 1340 and 971 cm−1, assigned to the trans conformation of the ethylenic unit and to the all-trans conformation of the repeating unit, respectively, are investigated. As expected, the glass transition is observed in the reversing components meanwhile the recovery of the glass relaxation and cold crystallization in non-reversing ones. Particularly interesting resulted the behaviour of the sample in the supercooled liquid state, between the glass transition and the cold-crystallization onset, in which the results show that the ethylenic conformers are in-equilibrium while the all-trans sequences are not. MTFTIR is confirmed to be a technique particularly suitable for the characterization of non-equilibrium conformational states of polymers.

Defect-mediated polarization switching in ferroelectric films for low-power-consuming and ultra-high-density memories

Ferroelectric poly(vinylidene fluoride) (PVDF) based polymers are attracting tremendous interest because of their potential applications in flexible non-volatile memories. Current research suggests that the existence of a large hysteresis loop results in high-voltage operation and low writing/erasing speed, which is originated from uniform chain packing and coherent ferroelectric sequences in the crystalline regions. Here, we demonstrate the novel approach to understand the defect-mediated switching mechanisms in ferroelectric polymers by PFM-probe based technology. The single-point and linear polarization reversal is well controlled by defect-mediated ferroelectric phases that determine activation energy, switching rate, and the thermal stability. By the regulation of VDF/TrFE ratio and varying processing conditions, the coherent ferroelectric phase with all-trans sequence in the P(VDF-TrFE) has been disrupted by defects. Specially, the crystallites formed at high temperature in the copolymer P(VDF-TrFE) 50:50 mol% exhibit less ordered ferroelectric crystalline sequences, thus attaining the excellent features of the low operation voltage of about 4 V with a switching time of 100 ms, ultra-high memory density of 43.9 Gbit∙in−2 (by a 7 V, 1 ms pulse) and a high usage temperature of 60 °C. Compared with P(VDF-TrFE) 68:32 mol%, normal ferroelectric, it saves approximate 50% ratio of energy cost, and realizes four times higher resolution. Understanding and controlling defect functionality in ferroelectric materials is as critical for realizing their reliable applications in ferroelectric memories.

Fast Lithium-Ion Transportation in Crystalline Polymer Electrolytes.

Fast lithium-ion transportation is found in the crystalline polymer electrolytes, α-CD-PEOn /Li+ (n=12, 40), prepared by self-assembly of α-cyclodextrin (CD), polyethylene oxide (PEO) and Li+ salts. A detailed solid-state NMR study combined with the X-ray diffraction technique reveals the unique structural features of the samples, that is, a) the tunnel structure formed by the assembled CDs, providing the ordered long-range pathway for Li+ ion transportation; b) the all-trans conformational sequence of the PEO chains in the tunnels, attenuating significantly the coordination between Li+ and the EO segments. The origin of the fast lithium-ion transportation has been attributed to these unique structural features. This work demonstrates the first example in solid polymer electrolytes (SPEs) for "creating" fast ion transportation through material design and will find potential applications in the design of new ionconducting SPE materials.

Does cyclic stress and accelerated ageing influence the wear behavior of highly crosslinked polyethylene?

First-generation (irradiated and remelted or annealed) and second-generation (irradiated and vitamin E blended or doped) highly crosslinked polyethylenes were introduced in the last decade to solve the problems of wear and osteolysis.In this study, the influence of the Vitamin-E addition on crosslinked polyethylene (XLPE_VE) was evaluated by comparing the in vitro wear behavior of crosslinked polyethylene (XLPE) versus Vitamin-E blended polyethylene XLPE and conventional ultra-high molecular weight polyethylene (STD_PE) acetabular cups, after accelerated ageing according to ASTM F2003-02 (70.0±0.1°C, pure oxygen at 5bar for 14 days). The test was performed using a hip joint simulator run for two millions cycles, under bovine calf serum as lubricant.Mass loss was found to decrease along the series XLPE_VE>STD_PE>XLPE, although no statistically significant differences were found between the mass losses of the three sets of cups. Micro-Raman spectroscopy was used to investigate at a molecular level the morphology changes induced by wear. The spectroscopic analyses showed that the accelerated ageing determined different wear mechanisms and molecular rearrangements during testing with regards to the changes in both the chain orientation and the distribution of the all-trans sequences within the orthorhombic, amorphous and third phases. The results of the present study showed that the addition of vitamin E was not effective to improve the gravimetric wear of PE after accelerated ageing. However, from a molecular point of view, the XLPE_VE acetabular cups tested after accelerated ageing appeared definitely less damaged than the STD_PE ones and comparable to XLPE samples.

Semiflexible Macromolecules with Discrete Bond Angles Confined in Nanoslits: A Monte Carlo Test of Scaling Concepts

Single semiflexible polymer chains confined in a planar slit geometry between parallel nonadsorbing repulsive walls a distance D apart are studied by Monte Carlo simulations of a lattice model, for the case of good solvent conditions. The polymers are modeled as self-avoiding walks on the simple cubic lattice, where every 90° kink requires a bending energy eb. For small qb = exp(−eb/kBT) the model has a large persistence length lp (given by lp ≈ 1/(4qb) in the bulk three-dimensional dilute solution, in units of the lattice spacing). Unlike the popular Kratky–Porod model of worm-like chains, this model takes both excluded volume into account and approximates the fact that bond angles between subsequent carbon–carbon bonds of real chains are (almost) restricted to large nonzero values, and the persistence length is controlled by torsional potentials. So the typical local conformation in the model is a straight sequence of (on average) lp bonds (roughly corresponding e.g. to an all-trans sequence of an alkan...

Giant Electrocaloric Effect in High-Energy Electron Irradiated P(VDF-TrFE) Copolymers

ABSTRACTA direct calorimetry method was developed and used to measure the electrocaloric effect (ECE). A temperature change ΔT of over 20 °C and an entropy change ΔS of over 95 J/(kgK) were procured at 33 °C and 160 MV/m in the high-energy electron irradiated poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) 68/32 mol% copolymers, which were larger than those of terpolymer blends (ΔT = 9 °C, ΔS=46 J/(kgK) at 180 MV/m and room temperature) and our earlier report on P(VDF-TrFE) 55/45 mol% normal ferroelectric copolymer (12 °C and 55 J/(kgK) at 80 °C). We observed that the β value ((8.7±0.6)×107 JmC-2K-1) in the equation of ΔS=1/2βΔD2 derived from ΔS - ΔD2 relation for irradiated copolymers was larger than that of the terpolymer blends ((5.4±0.5)×107 JmC-2K-1). It was also found that the irradiated copolymer showed a sharp depolarization peak at Td < Tm (maximum permittivity temperature), which is frequency independent, in the dielectric constant - temperature characteristics, a larger depolarization value at Td in the thermally stimulated depolarization current (TSDC) - temperature relationship, and a larger volume strain/longitudinal strain ratio over terpolymer blends. The giant ECE in irradiated copolymer is regarded as due to the greater randomness present in the relaxor state. In irradiated copolymers, the long all-trans chains are broken by the high-energy electrons, which make the small sized all-trans sequences more easily reorient along the electric field, more remarkably affecting the permittivity, TSDC, and volume strain.

Phase Transitions and Ferroelectric Relaxor Behavior in P(VDF−TrFE−CFE) Terpolymers

We show that the microstructures and phase transition behavior of vinylidene fluoride−trifluoroethylene−chlorofluoroethylene terpolymer, P(VDF−TrFE−CFE), with composition of 61.5/30.3/8.2 mol %, can be varied dramatically via different processing conditions. In the well-annealed sample, the polymer exhibits typical relaxor ferroelectric behavior with the nonpolar crystalline phase having a chain structure of random sequence of TT/TG/TG‘ conformations. In contrast, by lowering the crystallization temperature in the solution-cast films, the polar crystal phase with all-trans (Tm>4) planar zigzag conformation (similar to the ferroelectric phase in the corresponding copolymer) can appear or even become a dominating phase. Four types of transitions can be detected in this terpolymer, besides the crystal melting, the glass transition in amorphous region, and the transition between the ferroelectric-like and paraelectric crystals (30−90 °C), an additional transition appears near 10 °C which can be attributed to the crystalline domains with all-trans sequence randomly disrupted by defects, and the coherence length of these domains is estimated to be less than 4.7 nm (polar nanodomains). The large dielectric response of the terpolymer comes from the two crystalline phases: the ferroelectric-like crystals and polar nanodomains. The former is responsible for the frequency-independent dielectric maxima while the latter generates the strong frequency dependence in the dielectric spectra, a typical feature of relaxor ferroelectrics, which we suggest to come from the short-range molecular motion in the polar nanodomains.

LAM Study of Chain Straightening During Stress Relaxation in Gel-Spun Polyethylene Fibers

The low-frequency Raman spectra of gel-spun polyethylene fibers were recorded in the frequency range of the longitudinal acoustic modes (LAM) localized on all-trans sequences, i.e., on straight-chain segments (SCS). During the experiment, the samples were stressed and held with their ends fixed. As soon as the stress was applied, there appeared a substantial amount of short (as compared to the size of the crystal) all-trans sequences randomly distributed in the amorphous phase. These primary SCS are not stable and decay (coil) within approximately half an hour. Further chain straightening continues in the interfacial regions between the crystalline and amorphous phases. In the course of this process, the individual all-trans sequences in the amorphous bulk are virtually replaced by the SCS adjacent to crystal cores. The final SCS length distribution in the relaxed sample is characterized by an increased fraction of the crystalline SCS penetrating deeply into the amorphous regions with formation of some quantity of taut-tie molecules.

Acyl Chain Conformation and Packing in Dipalmitoylphosphatidylcholine Bilayers from MD Simulation and IR Spectroscopy

The lipid chain conformation predicted by a recent molecular dynamics simulation (MD) of the fluid and gel phases of a dipalmitoylphosphatidylcholine (DPPC) phospholipid bilayer is described in detail and, where possible, is compared to the results of IR measurements. Conformational statistics for the MD simulated fluid-phase chains are compared with those for an unconstrained liquidlike n-C16H34 system estimated using a rotational isomeric state model. The constraints imposed on the bilayer chains are found to reduce the concentration of gauche bonds, suppress sterically unfavorable conformational sequences, and increase trans/gauche bond segregation. The concentrations of short (1−3 bond) conformational sequences for the MD-simulated bilayer are found to be nearly the same as those for the unconstrained RISM n-C16 system with the same gauche bond concentration as the constrained chains. For long sequences, however, especially long all-trans sequences, there are large differences between the systems. The bilayer-normal distribution of CC bonds across the gel phase bilayer is found to be largely determined by longitudinal chain displacement, whereas for the fluid phase, the distribution is determined by conformational disorder. Order−disorder heterogeneity is observed in the MD-simulated fluid-phase bilayer in the form of a low concentration of more or less well-defined, highly transient microdomains consisting of the order of 20 all-trans or nearly all-trans chains. The packing of these chains is similar to that found for the simulated gel phase. Both chain-tilt and chain order/disorder correlations between opposing monolayers were found. The two most reliable IR methods for determining chain conformation are critically reviewed. Previously reported IR measurements on the gel- and fluid-phase DPPC bilayer using these methods are reinterpreted. The distribution of gauche bonds in the gel is found to be determined primarily by longitudinal displacements of the chain. By combination of the measured concentrations of bond-pair conformations tt, gt, and gg in the fluid phase, the overall concentration of gauche bonds is estimated to be 0.14 ± 0.04. This value is significantly lower than most previously reported values and much lower than the 0.28 derived from the MD simulation.

Comparative low-frequency Raman study of straight-chain-segment length distributions in gel-derived and melt-crystallized polyethylenes

The Raman longitudinal acoustic mode (LAM) has been examined for two ultra-high-molecular-weight reactor powders and melt-crystallized (MC) and gel-derived (GD) films and hot-drawn fibers prepared from them. The analysis of the straight-chain-segment (SCS) length distributions obtained from the LAM spectra demonstrates complete decomposition of the initial crystalline structure of the powders in the GD samples, while in MC films one could distinguish some features reminiscent of the powder structure. In addition, a certain amount of all-trans bridges linking adjacent crystallites in unoriented MC films was detected. In drawn MC samples, the average SCS length grows gradually with the draw-ratio increase, thus indicating the molecular ordering in the interface zones between the crystallite cores and the amorphous phase. A similar effect in GD fibers is accompanied by the formation of all-trans sequences whose length is much more than the crystallite core thickness, i.e., taut-tie molecules. The calculated SCS length distributions manifest clearly the presence of population of the taut-tie molecules entering a series of a few crystallites consecutively.

Studying the length of trans conformational sequences in polyethylene using Raman spectroscopy: a computational study

The long standing question of how long a trans sequence in a polyethylene needs to be in order to contribute to the all-trans Raman bands at 1060 and 1130cm−1 is addressed. Various hexadecane, C16H34, conformers were studied using ab initio type calculations revealing vibrational frequencies and intensities. We found that only trans sequences longer than 10 trans bonds reveal Raman intensity primarily selectively at the same frequencies as the C–C stretching modes arising from crystalline polyethylene. Since it is statistically highly unlikely that such long trans sequences occur in the amorphous phase, intensity arising from all-trans sequences not belonging to crystalline structure may only be expected as a consequence of strong non-equilibrium conditions (e.g. shear), or the presence of tie-molecules.

Correlation of Structure and Molecular Weight Distributions during the Formation of Poly(methylphenylsilylene) by the Wurtz Reductive-Coupling Reaction

Variations of yields and molecular weight parameters of poly(methylphenylsilylene) formed through sodium-mediated Wurtz-type reductive coupling of dichloromethylphenylsilane in refluxing toluene are described. The intermediate molecular weight fractions within the polymodal distributions that result from such syntheses are shown to correlate well with the length of all-trans sequences within the polymer as revealed through the near-ultraviolet spectra of the fractionated polymers. The origins of the polymodal distributions are explained as being the consequence of a competition between termination through a backbiting reaction and a continued growth reaction whenever additions in gauche conformation arise. The likelihoods of such additions are reasoned to increase with the lengths of the all-trans sequences that arise from the preceding additions, and the termination reaction is rationalized as having a maximum probability when the polymer consists of only one such sequence. The reasoning is shown to corr...

Structure of the tie chains in oriented polyethylene: dependence of all-trans stem lengths on draw ratio

By the Raman LAM experiments the length distributions of straight all-trans sequences were obtained for oriented polymers films of varied draw ratios λ. A shape of distribution function depended on λ and draw temperature as well: the higher λ the more long trans-stems could be observed in the oriented material. The experimental facts were matched to the results of the computer structure simulation based on ‘stencil crystallization’ model which was described previously[1]. Large deviation of experimental data from theoretical ones is discussed briefly. In essence, this deviation may be considered as a measure of superstructure imperfection of a real oriented polymer. © 1997 Elsevier Science Ltd.

Low-Temperature Phase of Tetraethylammonium Bromide

The title compound, C8H20N+.Br−, exhibits a trigonal structure. Pairs of alkyl chains form all-trans sequences through the N atom.

Raman Vibrational Studies of Syndiotactic Polystyrene. 2. Use of the Fundamental ν1 Vibrational Mode as a Quantitative Measure of Crystallinity within Isotropic Material

Syndiotactic polystyrene (sPS) has been found to exhibit two distinct vibrational peaks in the Raman spectrum corresponding to the ν1 vibrational mode. These features have been assigned to the symmetric phenyl ring breathing mode coupled to the polymer backbone. It has been shown that the presence of long all-trans sequences gives rise to a peak at ∼773 cm-1, whereas less well-defined trans/gauche conformations result in a separate peak at ∼798 cm-1. With increasing levels of crystallinity, the integrated intensity of the former feature grows at the expense of the high-frequency feature. The use of a series of annealed isotropic films of glassy sPS of increasing crystallinity and molten sPS has made it possible to examine the precise relationship between these two features and other neighboring vibrational peaks. The level of crystallinity within each sample has been determined independently using wide-angle X-ray scattering, differential scanning calorimetry, and infrared spectroscopic measurements. It h...

Raman Vibrational Studies of Syndiotactic Polystyrene. 1. Assignments in a Conformational/Crystallinity Sensitive Spectral Region

A detailed analysis of the Raman spectrum of syndiotactic polystyrene, sPS, in the region 600−850 cm-1 has been undertaken. As sPS exhibits considerable polymorphism, spectra of various preparations including melt-crystallized sPS (α/β form), solvent-crystallized sPS (γ/δ forms), and quenched glassy material were studied. The ν1 vibration of the phenyl ring (ring breathing mode) has been shown to manifest itself through the presence of two peaks resulting from local conformational changes in the alkyl backbone. The peak centred around 773 cm-1 has been assigned to an all-trans backbone sequence whereas the higher frequency feature at ∼798 cm-1 is attributed to mixed trans/gauche conformations. For comparison the spectrum of atactic polystyrene was also studied, and the presence of a weak shoulder is assigned to a syndiotactic all-trans component within the polymer. Further study of a cross section of molded material exhibiting a skin/core structure reveals the continuous way in which the relative size, position, width, and intensity of these features vary with changing crystallinity.

Reappraisal of the Origins of the Polymodal Molecular Mass Distributions in the Formation of Poly(methylphenylsilylene) by the Wurtz Reductive-Coupling Reaction

Variations of yields and molecular weight parameters of poly(methylphenylsilylene) formed through sodium-mediated Wurtz reductive coupling of dichloromethylphenylsilane in toluene, xylene, diethyl ether, tetrahydrofuran, and diphenyl ether both in the presence and in the absence of 15-crown-5 and at different temperatures are described. The effects of various terminating reagents are also considered. The crown ether when it is present in the reaction mixture is shown to act as a phase transfer agent for the sodium and this is interpreted as having two effects. The first is to activate the alkali metal for reductive coupling, which at lower temperatures is a necessary condition for polymer formation. The other is to transport the alkali metal to points on the high molecular weight polymer chains where, as a prelude to backbiting, it induces a highly selective chain scission; this is explained by it occurring only at gauche defects in the otherwise rodlike all-trans sequences within the polymer in solution....

Structure and interfacial properties of spontaneously adsorbed n-alkanethiolate monolayers on evaporated silver surfaces

Monolayer films of n-alkanethiols (CH 3 (CH 2 ) n SH, n=3-15, 17) have been spontaneously adsorbed from solution at evaporated Ag films. These monolayers have been characterized with optical ellipsometry, contact angle measurements, and external reflection infrared spectroscopy (IRS). The IRS data indicate that the monolayers with long chains (n≥10) are predominantly densely packed, crystalline-like structures with all-trans conformational sequences exhibiting average tilts of ∼13 o from the surface normal

Ferroelectric Transition in Vinylidene–Trifluoroethylene Copolymer Studied by Nuclear Magnetic Resonance Method

NMR absorption lines for a uniaxially drawn film of VDF–TrFE copolymer (72:28) were measured as a function of the alignment angle γ between the drawn axis and the magnetic field, over a temperature range from 23 to 130°C. The line shape consisted of inner, intermediate and outer peaks below the Curie temperature Tc. The line width of the intermediate peak narrowed at about 40°C with increasing temperature, and above 80°C it increased gradually by 1—3 Oe up to Tc. This line width as well as the line width of the outer peak strongly depended on γ, indicating that the nuclear dipole interactions in the crystalline region contribute to these peaks. Above Tc the width of the outer peak and the second moment decreased markedly owing to the chain motion accompanying conformational changes from the all-trans sequence to the combination of TG, TḠ, T3G, and T3Ḡ groups. The remarkable γ-dependence of the line shape as well as the line width observed in the paraelectric phase may be interpreted by a one-dimensional diffusion motion of conformational defects, and compared well with the ESR of a one-dimensional magnetic substance (CH3)4 NMnCl3 (TMMC).