Lateral Chain Packing Research Articles

Correlating morphology to thermal and electrical properties in imidazolium-poly(ethylene glycol) copolyesters

Ionic liquid-based copolyesters containing an imidazolium cation backbone and PF6− or Tf2N− counter-anion are synthesized from N,N′-bis(ω-hydroxyalkyl)imidazolium salts (PF6− or Tf2N−) (0.5 equiv.), poly(ethylene glycol) (PEG; number average molecular weight: 1000 or 2000) (0.5 equiv.), and sebacoyl chloride (1.0 equiv.). The imidazolium monomers are room-temperature ionic liquids. In the ionic copolymers, the 1:1 ratio between imidazolium and PEG, and number average molecular weights were confirmed using 1H NMR. Crystallization of the PEG segments was affected by the alkyl spacer length of the ionic units and by the anion type. Softer Tf2N− can prevent copolyester crystallization during cooling when the anions are close to the PEG segments; moreover, some Tf2N− copolymers show crystallization during a heating scan. The crystalline phase of the PEG-2000 segments gives distinct X-ray diffraction peaks at 2θ ≈ 19.0 and 23.1°, and the lateral chain packing of the C11-(imidazolium PF6)-C11 structures is shown at 2θ ≈ 21.5°. These crystalline domains are shown as Tc peaks during a cooling scan. The room-temperature ionic conductivities of the Tf2N− copolyesters are above 10−5 S cm−1; however, the ionic conductivity drops significantly below the melting temperatures of the PEG copolyesters. In contrast, the predominantly amorphous polymers that contain PF6− and Tf2N− counter-anions do not show a sudden conductivity drop over the entire temperature range. The imidazolium-PEG copolyesters, which have a low ion density, show a higher ionic conductivity than a reported poly(imidazolium-sebacate) without PEG segments because of the lower activation energy of the isolated conducting anions.

Non-symmetrical bis-silylated precursor can (also) self-direct the assembly of Silsesquioxane films

Symmetric α,ω-bis-silylated precursors are the standard building blocks of self-assembled bridged silsesquioxanes, a unique class of robust ordered nanomaterials prepared by sol-gel process without external surfactant. We report an unprecedented approach based on the utilization of a dissymmetric bis-silylated precursor, 1,2-bis(trimethoxysilyl)decane (1), in which the two alkoxy groups are carried by adjacent methylene groups. Extensive characterization—based on X-ray diffraction, real-time fourier transform infrared, electron and optical microscopy, 29Si solid-state nuclear magnetic resonance, thermogravimetry, and molecular modeling—shows surprisingly that such non-symmetrical precursor is highly conducive to achieve highly ordered lamellar mesostructure, able to sustain temperature up to 120 °C. To emphasize the effect of the alkoxy group functionality and position, comparison is made systematically with analogous silsesquioxanes derived from bis-(2) and mono-silylated (3) precursors. The sol-gel polymerization of 1 is unique by its ability to produce a homogeneous film possessing structural characteristic on multiple scales: uniform microcrystallites consisting of nanolamellar organosilica hybrid material. The most likely mesostructure consists of bilayers of slightly interpenetrated trans C8H9 chains, with a single central siloxy-hydrocarbon chain (Si2O(OH)4-C2H3) n . To permit a good lateral chain packing, the hydrocarbon chain of two adjacent silicon atoms point in opposite directions.

Self-segregated nanostructure in room temperature ionic liquids.

The nanosegregated bulk structure, and its evolution with the cation's alkyl length n, are studied by X-ray scattering for an unprecedentedly broad homologous series of a model room-temperature ionic liquid, [CnMIM][NTf2] (n = 4-22). A tri-periodic local structure is found, with the lateral periodicities, dII and dIII independent of n, and a longitudinal one, dI, linearly increasing with n. The results are consistent with a local structure comprising alternating layers of polar headgroups and apolar, interdigitated, partly overlapping, cations' alkyl tails, of an average macroscopic mass density close to that of liquid alkanes. A slope decrease in the linear dI(n) suggests a change from a lower to a higher rate of increase with n of chain overlap for n ≥ 12. The order decay lengths of the layering, and of the lateral chain packing, increase with n, as expected from the increasing van der Waals interaction's domination of the structure. The headgroups' lateral packing decay length decreases with n, due to increasing frustration between the longer lateral periodicity preferred by the headgroups, and the shorter lateral periodicity preferred by the chains. A comparison of the bulk and surface structures highlights the surface's ordering effect, which, however, does not induce here a surface phase different from the bulk, as it does in liquid crystals and liquid alkanes.

Chain Length and Thermal Sensitivity of the Infrared Spectra of a Homologous Series of Anhydrous Silver(I)n-Alkanoates

The thermal and chain length sensitivity of the infrared spectra of some solid state anhydrous silver(I) salts (n-octanoate ton-eicosanoate, inclusive) are discussed. At ambient temperature, the tilted alkyl chains, anchored to the metal planes via chelating bidentate coordination to the silver ions, are crystallized in the fully extendedall-transconformation. Interestingly, though all compounds are crystallized in a monoclinic crystal system, their lateral chain packing,van der Waalseffects, and hence vibrational features are chain length-dependent. This is a direct result of electrostatic effects of the COO group in addition to vibrational coupling between CH2, CH3, and COO modes, an effect which varies significantly with chain length. Variable temperature infrared measurements indicate significant irreversible changes in the metal-carboxyl coordination sphere, most likely due to bond fission. For long chain adducts (nc>12), thermally induced crystal system switching, monoclinic to triclinic, indicates greater thermal sensitivity of their alkyl chains. During heating, the regions of the hydrocarbon chains, furthest from the COO, become increasingly molten and mobile as the stepwise melt advances towards the solid COO moieties. This solid-liquid melting behaviour is responsible for mesophase formation in metal carboxylate systems.

Studying the lateral chain packing in a ceramide bilayer with molecular dynamics simulations

In this work, we present a novel technique, based on molecular dynamics simulations, that allows the study of the lateral chain packing in a lipid bilayer. It utilizes the radial distribution function of the alkyl chains to determine the arrangement of the chains along the bilayer plane. The positions of the mass centres of the chains are projected onto the bilayer plane and a 2D radial distribution function is calculated for these projections. The proposed technique can be particularly useful for lipid bilayers in the gel (solid) phase where the chains present a limited degree of mobility. As a case study, we have examined a bilayer that consists of ceramide NS 24:0. Ceramide bilayers can be found in the lipid domain of the skin where they have a significant role in its barrier function. The specific bilayer was found (at 300 K) to adopt a strictly hexagonal chain packing with a separation distance between the chains of 0.466 nm, in good agreement with the available experimental data.

A comparative study of structure–property relationships in highly oriented thermoplastic and thermotropic polyesters with different chemical structures

AbstractStructure and properties of commercially available fully oriented thermoplastic and thermotropic polyester fibers have been investigated using optical birefringence, infrared spectroscopy, wide‐angle X‐ray diffraction and tensile testing methods. The effect of the replacement of p‐phenylene ring in poly(ethylene terephthalate) (PET) with stiffer and bulkier naphthalene ring in Poly(ethylene 2,6‐naphthalate) (PEN) structure to result in an enhanced birefringence and tensile modulus values is shown. There exists a similar case with the replacement of linear flexible ethylene units in PET and PEN fibers with fully aromatic rigid rings in thermotropic polyesters. Infrared spectroscopy is used in the determination of crystallinity values through the estimation of trans conformer contents in the crystalline phase. The analysis of results obtained from infrared spectroscopy data of highly oriented PET and PEN fibers suggests that trans conformers in the crystalline phase are more highly oriented than gauche conformers in the amorphous phase. Analysis of X‐ray diffraction traces and infrared spectra shows the presence of polymorphic structure consisting of α‐ and β‐phase structures in the fully oriented PEN fiber. The results suggest that the trans conformers in the β‐phase is more highly oriented than the α‐phase. X‐ray analysis of Vectran® MK fiber suggests a lateral organization arising from high temperature modification of poly(p‐oxybenzoate) structure, whereas the structure of Vectran® HS fiber contains regions adopting lateral chain packing similar to the room temperature modification of poly(p‐oxybenzoate). Both fibers are shown by X‐ray diffraction and infrared analyses to consist of predominantly oriented noncrystalline (63–64%) structure together with smaller proportion of oriented crystalline (22–24%) and unoriented noncrystalline (12–15%) structures. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 142–160, 2006

Packing and Thermal Stability of Polyoctadecylsiloxane Compared with Octadecylsilane Monolayers

FTIR, Raman spectroscopy, and thermogravimetric analysis (TGA) were used to compare the structure and thermal properties of polyoctadecylsiloxane (POS) and octadecyl trichlorosilane (OTS) monolayers. The octadecyl chains in POS had higher conformational and intermolecular order than the same chains of OTS adsorbed as self-assembled monolayers (SAMs) on 106 nm hydrated silica beads. The latter were identical to the structures observed for OTS SAMs on superhydrated fumed silica that had primary particle sizes an order of magnitude smaller than the 106 nm beads, indicating that curvature did not affect the chain packing. The chains on POS were also more thermally stable to conformational and intermolecular disorder than the OTS chains, and this was attributed to increased free volume in the latter case. The differences in structure were partially determined by steric restrictions that arise because the Si−O−Si bond distance is less than the van der Waals radii of the alkyl chains. In POS, the most likely structure is therefore one in which the octadecyl chains are pointing in the same direction from every other Si atom, permitting good lateral chain packing. In the OTS SAMs, the octadecyl chains must all point in the same direction, away from the silica surface. Therefore, linear chains would be excluded but small clusters of dimers or trimers could be accommodated. These restraints increase the nearest neighbor distance between alkyl chains on SAMs and do not permit them to be as closely packed as for POS. This in turn contributes to the increased irreversible disordering of the chains with temperature.

Synthesis and characterization of fully rodlike poly(4,4?-biphenylene pyromellitimide)s with various short side groups

Fully rodlike poly(4,4-biphenylene pyromellitimide) (PMDA–BZ) is so brittle in spite of its extremely high modulus. In this study, the brittleness was attempted to be improved without a significant sacrifice of the high modulus by incorporating short side groups. For this, benzidine monomers, which contain methyl, methoxy, fluoro, and trifluoromethyl at the 2,2′-positions, were synthesized and then used for polycondensation reactions with pyromellitic dianhydride in N-methyl-2-pyrrolidone, producing soluble poly(amic acids)s. The synthesized poly(amic acid)s were converted to the fully rodlike polyimides in films by a conventional spin-coating on substrates, soft bake, and thermal imidization. The brittleness of PMDA–BZ was successfully healed with a small portion of sacrifice in the modulus by incorporating methyl, methoxy, and trifluoromethyl groups but could not be healed by the fluoro side group. The improvement in the brittleness might be contributed from the chain mobility and lateral chain packing order enhanced by the incorporation of the side groups, which are evident on the measured structures and properties. The structure and other properties were detected to be influenced by the incorporated side groups. The detailed structures and properties were interpreted by considering roles of side groups and the correlation between structure and properties, respectively. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 937–957, 1999

Phosphatidylcholine–fatty acid membranes: effects of headgroup hydration on the phase behaviour and structural parameters of the gel and inverse hexagonal (H II) phases

The phase behaviour and structural parameters of a homologous series of saturated diacyl phosphatidylcholine/fatty acid 1:2 (mol/mol) mixtures having chain lengths from C 12 to C 20 were studied by X-ray diffraction and calorimetry, as a function of water content. The chain–melting transition temperatures of the 1:2 PC/FA mixtures are found to be largely independent of the degree of hydration. For all chain lengths, the tilted L β′ and rippled P β′ gel phases of the pure PC component are replaced by an untilted L β gel phase in the 1:2 PC/FA mixtures. This gel phase swells considerably upon hydration, with a limiting water layer thickness in the range 18–24 Å, depending on the chain length. However, unlike pure phospholipid systems, the lateral chain packing within the gel phase bilayers is essentially identical in both the dry and the fully hydrated states. The fluid bilayer L α phase is suppressed in the 1:2 mixtures, being replaced by inverse non-lamellar phases for all chain lengths greater than C 12, and at all levels of hydration. For chain lengths of C 16 and greater, the inverse hexagonal H II phase is formed directly upon chain melting, at all water contents. For the shorter chain length mixtures, the behaviour is more complex, with the H II phase forming at low hydration, but with bicontinuous cubic phases appearing at higher levels of hydration. The implications of these surprising results are explored, in terms of the effective hydrophilicity of the associated PC and FA headgroups and the packing within the interfacial region. We suggest that the presence of the fatty acids significantly alters the lateral stress profile across the lipid monolayer in the fluid state, compared to that of the corresponding pure PC system, such that inverse phases, where the interface bends towards the water, become strongly favoured. Furthermore, for short chain lengths, packing constraints favour the formation of phases with negative interfacial Gaussian curvature, such as the bicontinuous cubic phases, rather than the H II phase, which has more severe chain packing frustration.

Packing characteristics of two-component bilayers composed of ester- and ether-linked phospholipids

The miscibility properties of ether- and ester-linked phospholipids in two-component, fully hydrated bilayers have been studied by differential scanning calorimetry (DSC) and Raman spectroscopy. Mixtures of 1,2-di-O-hexadecyl-rac-glycero-3-phosphocholine (DHPC) with 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DHPE) and of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) with 1,2-di-O-hexadecyl-sn-glycero-3-phosphoethanolamine (DHPE) have been investigated. The phase diagram for the DPPC/DHPE mixtures indicates that these two phospholipids are miscible in all proportions in the nonrippled bilayer gel phase. In contrast, the DHPC/DPPE mixtures display two regions of gel phase immiscibility between 10 and 30 mol% DPPE. Raman spectroscopic measurements of DHPC/DPPE mixtures in the C-H stretching mode region suggest that this immiscibility arises from the formation of DHPC-rich interdigitated gel phase domains with strong lateral chain packing interactions at temperatures below 27 degrees C. However, in the absence of interdigitation, our findings, and those of others, lead to the conclusion that the miscibility properties of mixtures of ether- and ester-linked phospholipids are determined by the nature of the phospholipid headgroups and are independent of the character of the hydrocarbon chain linkages. Thus it seems unlikely that the ether linkage has any significant effect on the miscibility properties of phospholipids in biological membranes.

Polymorphic behavior of 1,2-dipalmitoyl-3-lauroyl(PP12)- and 3-myristoyl(PP14)-sn-glycerols

The polymorphic behavior and molecular packing in different polymorphic forms of stereospecific triacylglycerols, 1,2-dipalmitoyl-3-lauroyl-sn-glycerol (PP12) and 1,2-dipalmitoyl-3-myristoyl-sn-glycerol (PP14) were examined by X-ray diffraction, differential scanning calorimetry, infrared and Raman spectroscopy techniques. The molecular packing and the polymorphic behavior of the metastable forms of these two compounds are very similar. In both compounds the isotropic liquid, on quenching, crystallizes into a hexagonally packed alpha-phase. The long spacing periodicity of the alpha-phase indicates a tilted bilayer structure to compensate the voids created by the short acyl chains. Upon heating, the alpha-phase converts into an orthorhombic perpendicular (O perpendicular) beta'2-phase. The beta' 2-phase, on further heating, exothermally converts to beta' 1-phase with slightly different O perpendicular subcell. On incubation of PP12 near the melting temperature of beta' 1-phase, there is a slow (hours) conversion to a beta-phase with triclinic parallel (T//) packing. However the beta' 1-phase of PP14 is the most stable structure and the beta-phase is absent. The thermodynamic parameters of the O perpendicular packings of these compounds compared to those of the higher homologue, tripalmitoylglycerol, suggest that the O perpendicular subcell is more stable in PP12 and PP14. The X-ray diffraction long spacings indicate that all the polymorphic forms of these compounds pack in a bilayer structure. The vibrational spectra confirm the lateral chain packing and inter- and intra-molecular order/disorder in the various polymorphic forms. The Raman and infrared spectra further indicate perturbation in the carbonyl and the end methyl plane regions of the beta'-phases.

Interaction of polymyxin B 1 and polymyxin B 1 nonapeptide with phosphatidic acid monolayer and bilayer membranes

The interactions of the antibiotic polymyxin B 1 and its enzymatic cleavage product polymyxin B 1 nonapeptide with phosphatidic acid monolayers and with bilayer membranes were investigated. Temperature-dependent pressure-area analysis of the monolayer reveals a linear increase of the lipid mean molecular area in the liquid condensed state for polymyxin concentrations between 10 −6 and 4 × 10 −2 M. Depending on the surface pressure, the area increase amounts to 30–70 Å 2. A linear dependence was also observed in the liquid expanded state but saturation is reached already at 10 −7 M polymyxin. The adsorption of polymyxin to phosphatidic acid bilayers is also linear and of a Langmuir type. Saturation is reached at a 1:4 polymyxin/lipid molar ratio. Polymyxin induces a phase separation in phosphatidic acid monolayers which was concluded from the thermotropic phase transition curves. In agreement with earlier bilayer experiments a second lowered phase transition appears in the presence of polymyxin. These fluidized domains again exhibit a linear polymyxin uptake comparable to the one of the liquid expanded monolayer at a temperature, where the undisturbed lipid is still in the condensed state. Polymyxin nonapeptide also causes an expansion of phosphatidic acid monolayers but only by maximally 10 Å 2. The thermotropic phase transition of the monolayer is reduced and considerably broadened by the nonapeptide. In phosphatidic acid bilayers we observed a decrease of the lipid phase transition temperature by 24°C. The lateral chain packing is considerably disturbed by the peptide part of polymyxin. The branched fatty acid tail may only serve to anchor the peptide within the matrix but does not contribute considerably to the formation of less ordered subcells within the membranes. For polymyxin B 1 nonapeptide, the binding curve of peptide to bilayers is linear but saturation was not reached.

The high temperature transition of polyethylene into a Condis phase, described on the basis of the cluster-entropy hypothesis (CEH)

On the basis of atomistic calculations a rotator model of the high temperature Condis phase of polyethylene is proposed. Its cooperative statistical treatment (using CEH for the lateral packing of chains) explains the first order transition from the all-trans crystal as well as the transition dataT t ,ΔS t ,ΔV t . We found that the normal melting (i. e. at pressures below the triple point) is also governed by the onset of the Condis phase.

Lateral chain packing in lipids and membranes

The aliphatic chains of many biologically important lipids are heterogeneous and often related to the functions of the molecules. Certain phospholipids containing arachidonic acid may serve as precursors for prostaglandins, certain diglycerides may serve as second messengers for certain membrane-triggered reactions (43), and other phospholipids containing a very short chain in the two position may serve as vasoactive hormones (44). The packing of such molecules is of interest. The evidence is quite clear from both the conformation of saturated and unsaturated molecules and from mixing experiments in the solid state that long and short chains don't mix well, nor do unsaturated and saturated chains, even if they are of the same chain length. There is even some evidence to indicate that some degree of chain segregation occurs even in the liquid state. However, different chains are often associated through covalent bonds, e.g., in wax esters, diacylglycerols, triacylglycerols, and phospholipids. A variety of possibilities for chain segregation are present in the neat phases of wax esters, ceramides, diacylglycerols, and triacylglycerols. However, in the unique case of membrane lipids like phospholipids or sphingolipids, the two chains are forced to lie side by side by virtue of the interaction of the polar group with water, and thus interactions between different chains must occur. Most of the evidence suggests that, when a solid phase results in these systems, the nonspecific chain packing mode (hexagonal chain packing) is preferred. In fact, for all of the phospholipids studied thus far, clearcut evidence of specific chain-chain interaction in molecules having both unsaturated and saturated chains has never been observed. However, for mixed chain triacylglycerols, evidence of specific chain-chain interactions (beta' and even beta) has been found and some suggestions have been given as to how this might occur through chain segregation mechanisms in the neat state. The literature suggests that further work needs to be done on the interaction of different chains that are covalently linked to the same molecule. Such studies will lead to a better understanding of the structure of lipid bilayers, membranes, lipoproteins, and lipid deposits.

Effects of polypeptide-phospholipid interactions on bilayer reorganizations Raman spectroscopic study of the binding of polymyxin B to dimyristoylphosphatidic acid and dimyristoylphosphatidylcholine dispersions

The interactions of the antibiotic polymixin B, a polycationic cyclic polypeptide containing a branched acyl side chain, with dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidic acid (DMPA) bilayers were investigated by Raman spectroscopy for a wide range of lipid/polypeptide mole fractions. Temperature profiles, constructed from peak height intensity ratios derived from the lipid methylene C-H stretching and acyl chain C-C stretching mode regions, reflected changes originating from lateral chain packing effects and intrachain trans / gauche rotamer formation, respectively. For DMPC/polymyxin B bilayers the temperature dependent curves indicate a broadening of the gel-liquid crystalline phase transition accompanied by an approx. 3 C deg. increase in the phase transition temperature from 22.8°C for the pure bilayer to 26°C for the polypeptide complex. For a 10:1 lipid/polypeptide mole ratio the temperature profile derived from the C-C mode spectral parameters displays a second order/disorder transition, at approx. 35.5°C, associated with the melting behavior of approximately three bilayer lipids immobilized by the antibiotic's charged cyclic headgroup and hydrophobic side chain. For the 10:1 mole ratio DMPA/polypeptide liposomes, the temperature profiles indicate three order/disorder transitions at 46, 36 and 24°C. Pure DMPA bilayers display a sharp lamellar-micellar phase transition at 51°C.

Hydrocarbon chain disorder in lipid bilayers: Temperature dependent ram an spectra of 1,2-diacyl phosphatidylcholine-water gels

Vibrational Raman spectra of multilayers of 1,2-dimyristoyl, 1,2-dipalmitoyl and 1,2-distearoyl phosphatidylcholine were recorded as a function of temperature for the gel phase from −180° C to values slightly below the pretransition temperature. Temperature profiles for band intensity ratios involving the acyl chain C-C stretching modes define a characteristic temperature T g which denotes the onset of hydrocarbon chain trans-gauche isomerization in the gel. T g for the dimyristoyl (C 14 chain lengths), dipalmitoyl (C 16 chain lengths) and distearoyl (C 18 chain lengths) systems are approximately −40, −40 and 5°C, respectively. The higher T g value for distearoyl phosphatidylcholine is associated with increasing interactions between the terminal chain areas of the individual monolayers forming the bilayer unit. This increased interaction between bilayer halves is presumably a consequence of the larger angle through which the hydrocarbon chains are tilted in the C 18 phospholipid gel. Temperature-dependent peak height intensities for the 2800–3000 C-H stretching region provide convenient probes for quantitatively describing the increase in lateral chain packing disorder as the gel assemblies are warmed. The temperature profiles for various methylene mode peak height ratios indicate that both the inter- (lateral packing) and intramolecular ( trans-gauche) disordering processes are related and are initiated at nearly the same temperatures for each lipid system. Increases in the van 't Hoff enthalpies which result from the disordering of the lipid gel matrix by a lattice expansion were estimated for the C 14, C 16 and C 18 phospholipids to be approximately 1.5, 1.2 and 3.2 kcal/ mol, respectively.

Molecular motion in polyamides

AbstractIt is well known that aliphatic polyamides with even numbers of methylene groups between amide groups melt at higher temperatures than those with odd numbers of methylene groups. Crystallographic studies in the past have sought to relate the packing of the amide groups to the variations in melting point, but these studies have been made almost exclusively at room temperature. In the present study, series of polyamide fibers were examined by x‐ray diffraction over a wide range of temperatures. In the even‐numbered polyamides, the triclinic structure occurring at room temperature shifts to a more symmetrical structure in which the chains assume hexagonal packing. This latter packing occurs in the odd‐numbered polymers at room temperature. The polar groups in the latter occur in layers which are normal to the chain axes. The layers of polar groups in the even‐numbered polyamides are tilted with respect to the chain axes at room temperature, and remain tilted at high temperatures even though the lateral chain packing becomes the same as the lower‐melting (odd‐numbered) members of the series. The spacing between polar groups along the chain decreases with increasing temperature, but the decrease is relatively less in the even‐numbered than in the odd‐numbered polymers, and is less in the more polar than in the more paraffinic compounds. These changes are ascribed to torsional and rotational motions of chain segments in the crystallites. Proton magnetic resonance studies demonstrate the onset of motion of the hydrocarbon segments and compare qualitatively the constraints to chain motion among isomeric members of the series. Some implications regarding the melting process are discussed.