Short Flexible Spacer Research Articles

Wetting effect of branched anionic Gemini surfactant aqueous solution on PMMA surface.

In this paper, the adsorption behaviour and wetting modification ability of the sodium salts of bis-octadecenoyl succinate (GeminiC3, GeminiC6) and monomers on polymethyl methacrylate (PMMA) surfaces were investigated. The difference in spacer length led to slightly different behaviour of surfactant molecules in solution. The large molecular structure and short flexible spacer of GeminiC3 led to a complex self-aggregation behaviour in solution, forming micelles at low concentrations, leading to a rapid decrease in surface tension and subsequent transition to monolayer or multilayer vesicles. In GeminiC6, the longer flexible spacer groups act as spatial structure modifiers that hinder the formation of vesicles. The adsorption behaviour of the gas-liquid interface was analysed in three stages for the peculiar inflection points where surface tension appears. Combining contact angle measurements, adhesion tension and interfacial tension data showed that GeminiC3 and C6 formed a saturated monolayer on the adsorbed PMMA surface at low concentrations and a bilayer structure at high concentrations. Due to the low resistance of molecular space sites, the monomers adsorbed heavily on the PMMA surface, forming semi-colloidal aggregates with the lowest contact angle of monomeric surfactant solutions reaching 38° on the PMMA surface. Also, the monomer and GeminiC3 and C6 surfactants in this paper have a very high hydrophilic modification ability on the PMMA surface compared to other literature.

Synthesis of rod copolyaramids with short flexible spacers

AbstractRegular rigid‐flexible copolymers composed of aramid segments and aliphatic chains formed of 5 to 10 CH2 groups were synthesized using a two‐stage procedure. A first stage consisted of synthesizing, under Yamasaki–Higashi phosphorylation conditions, poly(paraphenylene terephthalamide) blocks or macromonomers of different lengths by using an excess of phenylene diamine. These macromonomers, with functionalized amine extremities, were reacted in a second stage, using the Higashi phosphorylation conditions, with a terephthalic‐acid end‐capped aliphatic amine. Macromonomer molecular weights and polydispersities were estimated from viscosity and NMR measurements, whereas for block copolymers viscosity measurements and size exclusion chromatography of allylated derivatives were performed. Block polydispersity increased with increasing length of block, but remained below 1.6. Copolymers were found to have Mw values from 2500 to 8600 g mol−1, depending on block length and aliphatic chain used. Polydispersity indices were systematically higher for block copolymers than for macromonomers, with values ranging from 1.6 to 4.7. Mv and Mw were found to be comparable for all copolyaramids, which indicates that addition of short aliphatic chains had a negligible effect on Mark–Houwink parameters. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5098–5112, 2004

Novel RGD lipopeptides for the targeting of liposomes to integrin-expressing endothelial and melanoma cells.

RGD peptides targeting alphav-integrins are promising ligands for the generation of vascular targeting agents. We isolated from phage display RGD motif libraries novel high-affinity cyclic RGD peptides by selection on either endothelial or melanoma cells. Although the starting sequences contained only two cysteine residues flanking the RGD motif, several of the isolated peptides possessed four cysteine residues. A high-affinity peptide (RGD10) constrained by only one disulfide bond was used to generate novel lipopeptides composed of a lipid anchor, a short flexible spacer and the peptide ligand conjugated to the spacer end. Incorporation of RGD10 lipopeptides into liposomes resulted in specific and efficient binding of the liposomes to integrin-expressing cells. In vivo experiments applying doxorubicin-loaded RGD10 liposomes in a C26 colon carcinoma mouse model demonstrated improved efficacy compared with free doxorubicin and untargeted liposomes.

Polyacetylenes Bearing Mesogenic Side Groups: Synthesis and Properties, 1. Mesogenic Substituents with a Short Flexible Spacer

We report on the synthesis and polymerization of novel acetylenic monomers derived from 3-butyn-1-ol and bearing different potentially mesogenic substitutuents. Polymerization is carried out in solution with typical metathesis catalysts based on Mo and W and yields polyacetylenes with fairly high molecular weight and soluble in common organic solvents. Polymers are fully characterized by the point of view of the molecular structure, by GPC, FT-IR, NMR and UV/VIS techniques, and of the thermal and morphological behavior, by TGA, DSC, POM and X-ray diffraction experiments. Polymers with different amounts of cis and trans units are obtained, depending upon polymerization conditions. Although the monomers do not show any liquid crystalline behavior, polymers P2 and P3, containing in the side-chain respectively biphenyl and benzoyloxy benzoate as mesogenic cores, exhibit an enantiotropic smectic phase.

Structure–activity relationship of new NLO organic materials based on push–pull azodyes: 4. Side chain polymers

Several new polymeric materials containing push–pull azobenzene derivatives in the side chains were synthesized by polymer-analogous reactions involving the coupling of the diazonium salts of 4-substituted anilines with a preformed polymer, poly(N-methacryloyl-N′-phenylpiperazine). The second harmonic generation properties of these glassy macromolecular materials were investigated in order to understand the influence of the chromophore chemical structure on dipole orientability and stability of orientation. The results thus obtained were compared with those of analogous guest–host systems and a lack of chromophore mobility was highlighted for the side chain polymers. The use of a short flexible spacer should probably improve the NLO performance.

In VitroCharacterization of Late Steps of RNA Recombination in Turnip Crinkle Virus: II. The Role of the Priming Stem and Flanking Sequences

Turnip crinkle carmovirus (TCV) has a uniquely high recombination frequency and nonrandom cross-over site distribution among the recombining TCV-associated satellite RNAs. Anin vitrosystem has been developed that includes a partially purified TCV replicase preparation (RdRp) and chimeric RNAs that resemble the putativein vivorecombination intermediates (Nagy, P. D., Zhang, C., and Simon, A. E.,EMBO J.17, 2392–2403, 1998). This system mimics the strand transfer and primer extension steps of recombination events. We characterize in detail three RNA factors that, in addition to the previously characterized motif1-hairpin, can influence the efficient generation of 3′-terminal extension products: (i) a primer binding region, termed the priming stem; (ii) a spacer region; and (iii) a U-rich sequence located 5′ of the motif1-hairpin. The priming stem is formed between the acceptor RNA and the nascent RNA synthesized from the donor RNA template in the recombinants. The stability and location of the priming stem relative to the motif1-hairpin can influence both the efficiency and initiation site of 3′-terminal extension. A short flexible spacer region between the motif1-hairpin and the priming stem can increase the efficiency of 3′-terminal extensions. A U-rich sequence 5′ of the motif1-hairpin facilitates 3′-terminal extensions and its function partly overlaps with that of the spacer region. These RNA factors may also affect the late steps of RNA recombination in TCV.

Morphology and Dynamic Interaction of Defects in Polymer Liquid Crystals

The liquid crystal phase is an anisotropic mesophase, intermediate in order between the liquid and crystal phases. Liquid crystals have less translational order than crystals and more rotational order than isotropic liquids. The liquid crystal phase does not support finite shear stresses and thus behaves like a fluid. Molecules that display a liquid crystal phase are referred to as mesogenic. Mesogenic molecules exhibit shape anisotropy: either large length to diameter ratio (needlelike) or large diameter to thickness ratio (disklike). Because of their shape anisotropy, all liquid crystals display orientational order of their molecular axes.Until 1956, all known examples of liquid crystals were low molecular weight compounds. Robinson was the first to identify liquid crystallinity in a liquid crystalline polymer (LCP) as the explanation for “a birefringent solution” of a polymeric material, poly-y-benzyl-L-glutamate, in chloroform, previously observed by Elliott and Ambrose. Chemists soon discovered that LCPs may be readily synthesized by covalently stitching small mesogenic units (e.g., rigid monomers) together into a chain using short flexible spacers. Mainchain or sidechain liquid crystal polymers may be formed (Figure 1). An example of a polymer molecule possessing a liquid crystal phase is shown in Figure 2. Liquid crystals may be thermotropic, where liquid crystallinity is exhibited over a range of temperatures, or lyotropic, where nonmesogenic solvent molecules are present in addition to the mesogens, and liquid crystallinity is observed over a range of concentrations as well.

A polarized infra-red spectroscopic study of mechanically induced orientation in side-chain liquid crystalline polymers

We exploit the utility of infra-red dichroism for studying the orientation in thin films of side-chain liquid crystalline polymers (LCPs). Mechanical stretching of these polymers is difficult to perform because of difficulties in preparing the actual film samples necessary for the experiments. The method we used in this study consists of casting a thin LCP film onto the surface of a supporting conventional polymer film which can easily be prepared and stretched over a wide temperature range. Three nematic polyacrylates were investigated in this way, using poly(vinyl alcohol) as the supporting polymer. It is shown that stretching of the LCP film is sustained by extension of the supporting film and, similar to cross-linked nematic elastomers, an order parameter of approximately 0.4 for the mesogenic groups, determined by infra-red dichroism measurements, is obtained at a draw ratio as small as 1.5, i.e. only 50% deformation, indicating a rapid macroscopic alignment of the nematic domains along the mechanical field direction. Comparisons show that the mechanical stretching is particularly efficient for obtaining high orientation in side-chain LCPs having a short flexible spacer, the orientation of which is difficult to develop in a magnetic field, revealing an opposite influence of the length of the flexible spacer on the alignment of the nematic domains in both magnetic and mechanical fields. The mechanism of the orientation process in this stretching method is also examined. It is proposed that the stress field is essentially transferred to the nematic polymer through the interface between the nematic and supporting films, and that the alignment of the nematic domains, which starts in the interfacial region, could propagate throughout the sample by virtue of a strong cooperative movement among the nematic domains. The efficiency of the alignment propagation is higher for LCPs containing a short flexible spacer because of the stronger couplings between the chain backbone and the mesogenic groups.

Molecular Composites Based on Rigid Rod Polymers for Electrooptical Applications

ABSTRACTNovel rigid rod polymers substituted with NLO-active chromophores have been developed towards application in electrooptical signal Modulation. The Materials are based on rigid polyesters and polyesteramids, in which the chromophores are either covalently linked to the backbone by short flexible spacers or directly incorporated into the main chain. In the bulk these systems form macroscopically ordered structures with layers of rigid rod backbones separated by the side chain segments. The properties and stability of the induced polar order can thus be adjusted by morphological parameters like the layer distance or the orientation of the main chains relative to the substrate. The relaxation of the NLO activity with time is described by a Multi-exponential decay and shows enhanced time-temperature stability even above 100°C. The temperature dependence of the relaxation times exhibits unusual features that distinguishes these systems from conventional NLO side chain polymers.

Can the rigidity of a side-chain liquid-crystalline polymer backbone influence the mechanism of distortion of its random-coil conformation?

The relationship between the isotropization transition temperatures and the associated entropy changes of two series of side-chain liquid-crystalline polymers based on three different polymer backbones (polymethacrylate, polyacrylate and polymethylsiloxane) and 4-methoxy-4′-hydroxy-α-methylstilbene and 4-hydroxy-4′-methoxy-α-methylstilbene mesogenic side-groups attached to the polymer backbone through different flexible spacers is discussed. In the case of polymers containing long flexible spacers, isotropization transition temperatures, which are mostly dictated by the side-groups, are higher for polymers based on flexible backbones, suggesting that they provide the highest degree of order in the mesophase. Therefore, their mesophase should exhibit the lowest entropy and the highest entropy change of isotropization. However, experimentally determined entropy changes of isotropization, which refer to the overall degree of order of the polymer, present the highest values for polymers based on the most rigid backbone. Two different mechanisms of distortion of the random-coil conformation of flexible and rigid polymer backbones are suggested to account for this contradictory result. A squeezed random-coil conformation, which in the case of smectic polymers is confined between the smectic layers, is considered for flexible backbones. An extended-chain conformation is considered for rigid backbones. The entropy associated with the squeezed random-coil conformation is higher than that associated with the extended conformation, and therefore the overall order that results from the combination of backbone and the organization of the mesogenic side-groups may explain the experimentally observed isotropization entropy changes. Polymers based on short flexible spacers exhibit similar entropy changes of isotropization irrespective of the nature of their backbone. This may suggest an extended backbone conformation for polymers based on short spacers and rigid or flexible backbones.

Terminal–Terminal Types of Liquid Crystals. I. Synthesis and Electrooptical Properties of a Ferroelectric Liquid Crystal

Abstract A novel ferroelectric liquid crystal, (S)-bis(2-methylbutyl) 3,3′-[1,2-ethanediylbis[oxy-4,1-phenylenemethylidyne nitrilo-4,1-phenylene]]bis[2-propenoate] (EBOPBP) was prepared, which are composed of two chiral mesogenic groups interconnected by a very short flexible spacer chain. EBOPBP exhibts a chiral smectic C phase (Sc*). From an electrooptical effect due to a deformation of helical structure in the Sc* state, the rise time (τr) was estimated to be 240 μs at 180 °C.