Oligosaccharide-based thermotropic liquid crystals, 4. Synthesis of cellobiose-based twin and triplet derivatives and their mesophase properties

Previous work from this laboratory has shown that cellobiose octaalkanoate (COA) functions as a thermotropic discotic mesogen, forming a hexagonal ordered columnar phase (D_ho). In this work, we prepared the COA-based discotic twin and triplet derivatives and examined the mesomorphic properties of the derivatives by DSC, polarization microscopy and X-ray diffraction. In the former derivative, two cellobiose heptadecanoate (CHD) molecules (monomers) are combined through an alkyl spacer of varying length by ether and ester linkages at the C-1 position of the reducing end units of the monomers. Three CHD molecules having an alkyl spacer are connected with a coupling agent for the latter derivative. The results revealed that (i) all the twin derivatives form a distinct discotic columnar phase, (ii) the thermal stability of the mesophases of the twins is enhanced, compared with that of the monomer, when the length of the flexible spacer is appropriate, (iii) the twins with a relatively short spacer form a discotic rectangular ordered (D_ro) phase, while those with a longer spacer form a pseudo-D_ho phase, (iv) the triplets also form a discotic columnar mesophase, but not a hexagonal ordered columnar (D_ho) phase, (v) the thermal stability of the mesophases of the triplets strongly depends on the chemical nature of coupling cores, and (vi) the packing structures of the cellobiose cores within the columns of both derivatives are markedly different from that of the monomer.     

Liquid‐Crystalline Complexes of Polyurethane Containing an Isonicotinamide Moiety with 4‐Dodecyloxybenzoic Acid

A series of supramolecular side‐chain liquid crystalline polyurethanes (SCLCPUs) was prepared by utilizing the selective hydrogen bonding interaction between polyurethane containing pyridine moieties ( 1 ) and 4‐dodecyloxybenzoic acid ( 2 ). Hydrogen bonding was confirmed by FTIR spectroscopy and supported by medium high level ab‐initio MO calculations. The components are miscible up to 0.7 mole of 2 versus the polyurethane repeat unit and the resulting complexes behave as uniform liquid‐crystalline polymers, which exhibit stable, reversible mesophases. The liquid crystalline behavior of these supramolecular polymeric complexes was established by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X‐ray diffractometry (XRD). The complexes show a highly ordered smectic phase, which does not appear for either of components 1 and 2 separately , and a nematic phase. Above the concentration limit of 0.7 mole of the acid 2 , the excess low molar mass molecules emerge to form a two phase system consisting of a polyurethane supramolecular complex and 4‐dodecyloxybenzoic acid ( 2 ). For complexes derived from polyurethane 1 and 4‐octyloxybenzoic acid ( 3 ) or 4‐butyloxybenzoic acid ( 4 ) with alkyl tails shorter than 2 , the phase separation occurs at lower molar ratios of 3 and 4 versus 1 . This means that the length of the alkyl tail in 4‐(alkoxy)benzoic acids plays an important role in the stabilization of supramolecular polyurethane complexes.     

Phase behaviour and structure of polymer surfactants in aqueous solution. The occurrence of lyotropic nematic phases

The binary phase diagrams are analyzed in aqueous solutions of monomeric, non-ionic surfactants and the corresponding polysiloxanes having attached the surfactants as side chains via the hydrophobic parts of the amphiphiles. In contrast to common surfactants these hydrophobic parts additionally contain a rigid, mesogenic biphenyl moiety. While the polymeric surfactants exhibit hexagonal and lamellar lyotropic liquid crystalline phases, similarly to the low-molar-mass surfactants, two important results have to be emphasized: (i) The liquid crystalline phase regions are markedly extended with respect to concentration and temperature compared with the corresponding monomers, as shown in a similar way for thermotropic polymer liquid crystals. (ii) The phase sequence of the polymorphic liquid crystalline phases can be changed by polymerization. While for the systems under investigation the monomeric surfactant exhibits a cubic phase, the polymeric surfactant shows no cubic but a nematic phase in front of the hexagonal phase at low surfactant concentration. The nematic phase of the polymeric surfactant is the first example for a nematic phase, which is formed for binary mixtures of non-ionic surfactants and water.     

Structural properties of polymerised lyotropic liquid crystals phases of 3,4,5‐tris(ω‐acryloxyalkoxy)benzoate salts

Nanostructured free‐standing films were obtained by polymerisation of lyotropic liquid crystal phases based on the monomer 3,4,5‐tris(11′‐acryloyloxyundecyloxy)benzoate. The structure of these films was investigated by X‐ray diffraction. No change of the lattice due to the polymerisation procedure was observed. The low water content (below 10 wt.‐%) of the liquid crystals in combination with the observed hexagonal lattice reveal that a reversed hexagonal structure (H₂ phase) is present. Different types of films were investigated, obtained by varying the preparation conditions of the used lyotropic liquid crystals, such as water content, type of metal‐counterion and length of the monomer. The lattice constant was found to be independent of the water content. It seems that at low water content of 8 wt.‐% the lyotropic structure is still fully hydrated. This unusual behaviour is related to the tapered structure of the monomer which is composed of by a small headgroup and three extended tails for each headgroup. The lattice constant can be changed by using differently charged metal‐counterions which are associated with the headgroup, as well as by varying the length of the tails on the monomer. Low resolution electron density maps of the hexagonal lattice give deeper insight in the arrangement of the monomers within the polymerised structure.     

Crystalline Polymorphism of Alkyl Chains in Supramolecular Complexes of Polyethyleneimine with Octadecanoic Acid

The supramolecular complexes of branched polyethyleneimine and octadecanoic acid (PEI(OA)_x) were prepared via interfacial reaction and homogeneous solution complexation. The binding mode and the crystalline polymorphism of alkyl chains in PEI(OA)_x were investigated by FTIR spectroscopy and WAXS. There are three kinds of OA exist in the complexes, the first one is bound by primary amines through proton transferring, the second one associated with PEI through hydrogen‐bonding, and the third one free OA. The crystal structure of alkyl chains in PEI(OA)_x is different from that of complexes of linear poly(4‐vinylpyridine) and pentadecylphenol reported by G. ten Brinke group.^[1] Hexagonal and orthorhombic and triclinic packing of alkyl chains may occur in PEI(OA)_x with 0.32 ⪇ x ⪇ 4.16 and the hexagonal packing still occurs at high content of OA. The ordered packing of alkyl chains is determined by the binding mode of OA in PEI(OA)_x as well as the specimen‐preparing process.     

Microstructure Induced Rigidity of Polysiloxane Yielding Hierarchical Self‐Assembly of Alkyl Side Chains

The influence of a backbone microstructure on the side chain crystallization of a comb‐like polymer is analyzed systematically using a tailor‐made random versus block siloxane copolymer system. While the side alkyl chains of the random siloxane undergo a stepwise order–disorder (OD) transition to form well‐ordered orthorhombic structure at low temperature, the packing structure of the alkyl chains pertaining to the block siloxane maintains their original hexagonal lattice up to a temperature of as low as 173 K. The unit lattice ordering of side alkyl chains in the random siloxane polymer is also accompanied by a major restructuring of the backbone conformation ultimately losing out long range ordered structure in the solid state. The OD transitions of side alkyl chains and their dynamic relationship with the backbone conformation are established unambiguously by a combination of temperature dependent small‐angle X‐ray and wide‐angle X‐ray scattering techniques. The observed conformational variations in random versus block polymers are explicitly discussed in terms of molecular chain mobility and theory of macromolecular chain conformation.     

Novel Poly(L-lactide) PLLA/SWNTs Nanocomposites for Biomedical Applications: Material Characterization and Biocompatibility Evaluation

Poly(L-lactide) (PLLA)/single-walled carbon nanotubes (SWNTs) nanocomposite films were produced using the solvent casting method, and morphological, thermal and mechanical properties were investigated. Biocompatibility was evaluated by using human bone cells, performing adhesion and proliferation studies. The role of single-walled nanotube incorporation and functionalization on PLLA bio-polymers was investigated. Pristine (SWNTs) and carboxylated (SWNTs–COOH) carbon nanotubes were considered in order to control the interaction between PLLA and nanotubes. SWNTs and SWNTs–COOH showed a good dispersion in the polymer matrix and improved the PLLA crystallinity. Thermal, morphological and dynamic-mechanical analyses revealed that carboxylic groups on the tube sidewalls increased compatibility between PLLA and nanostructures. Mechanical properties demonstrated an enhancement related to introduction and functionalization of carbon nanotubes. Biological investigations showed osteoblasts cultured on PLLA/SWNTs–COOH nanocomposites has higher cell adhesion and proliferation than osteoblasts cultured on PLLA and PLLA/SWNTs nanocomposites. These studies suggest that combination of biodegradable polymers and SWNTs opens a new perspective in the self-assembly of nanomaterials and nanodevices for biomedical applications with tunable properties.     

Enhancing the Colloidal Stability and Electrical Conductivity of Single‐Walled Carbon Nanotubes Dispersed in Water

In this article, the results of a focused systematic investigation of the nature, type, and concentration of cationic and anionic surfactants and proteins required to simultaneously disperse single‐walled carbon nanotubes (SWNTs) and enhance their electrical conductivities in water are presented. The dispersibility of SWNTs suspended in aqueous solutions is evaluated via light scattering to characterize the average particle size, particle size dispersion, electrophoretic mobility, and ζ‐potential of the dispersions. It is found that the colloidal stability of SWNT dispersions is influenced by the surfactant charge and concentration – i.e., above or below its critical micelle concentration and for proteins its charge. The amphiphilicity, concentration, and charge of the surfactant or protein determine their surface coverage on the SWNT and simultaneously increase electrostatic and steric repulsion and decrease surface chemical heterogeneity. It is found that the electrical conductivities of SWNT films stabilized with surfactant are as high as that without surfactants, with the added advantage of being homogeneously dispersed in water with significant enhancement in the long‐term stability of the nanotubes in water. These findings suggest that enhancement of the electrical properties of SWNTs requires selection of a surfactant that has strong adsorption, and thus strong interactions with the nanotube – i.e., π–π stacking – and using concentrations above the CMC. Overall, these results demonstrate the importance of understanding the structure/property relationships between SWNTs and their dispersants in order to achieve high colloidal stability and electrical conductivities.

     

Solid‐state NMR and ellipsometric investigations of C30 chains bonded to SiO2 surfaces

The morphology of C₃₀ alkyl chains bonded to different inorganic substrates such as fine ground glass particles, silica gel as well as silica wafers has been investigated by the combined use of solid‐state NMR spectroscopy and ellipsometry. It could be shown that the average phase thickness of the bonded C₃₀ layer of 29 Å is primarily dependent upon the employed polymeric surface reaction procedure and does not reflect the type of the employed inorganic substrate. A refined model of the C₃₀ alkyl chain organization for solution polymerization on silica surfaces could be developed.     

Dispersion of Single‐Walled Carbon Nanotubes with Poly(Pyridinium Salt)s Containing Various Rigid Aromatic Moieties

Dispersions of single‐walled carbon nanotubes (SWNTs) with several poly(pyridinium salt)s containing various aromatic diamine moieties are prepared via a non‐covalent coagulation in DMSO. The interactions between SWNTs and polymers are characterized with various spectroscopic techniques. The ¹H NMR spectra suggest that strong interactions exist. The UV?Vis absorption spectra are independent of the incorporation of SWNTs. The resulting composites display high quenching efficiency in the photoluminescent properties; and transmission electron microscopy studies reveal the well‐dispersed SWNTs in the ionic polymer matrices. The enhanced thermal stability of the composites is determined by thermogravimetric analysis. The fully grown lyotropic LC properties of the poly(pyridinium salt)s are disrupted by the introduction of SWNTs.     

Composites of Single‐Walled Carbon Nanotubes and Styrene‐Isoprene Copolymer Latices

Composites of a styrene‐isoprene copolymer and SWNTs were prepared by emulsion and miniemulsion polymerization in the presence of SWNTs, as well as by mixing dispersed SWNTs with a styrene‐isoprene copolymer latex after polymerization. For the former, the surfactant was displaced from SWNTs to monomer droplets leading to SWNT aggregation. Mixing dispersed SWNTs with latex after reaction was able to preserve SWNT dispersion and gave a polymer composite with an electrical percolation threshold of 0.2%. Dynamic mechanical measurements of film samples in tension showed that the composites had a measurable modulus above T_g, indicating entanglement formation. The modulus above T_g was strongly temperature dependent, confirming shear measurements that have shown entanglements in SWNT/polymer composites both polymer and nanotubes.

     

Packing and Uniaxial Alignment of Liquid Crystalline Oligofluorenes

Summary: Liquid crystalline oligomers of 9,9‐bis(2‐ethylhexyl)fluorene of defined degree of polymerization 4, 5, 6, and 7 were investigated by X‐ray diffraction in the non‐oriented and in the aligned state. The diffraction data give evidence for a smectic B type phase for all of the oligomers. Quenching below the glass transition does not change the structure of the liquid crystalline phase. This allows to align spin‐coated films of these oligomers on rubbed polyimide substrates to give monodomain films. These are stable against thermal disordering below T_g, e.g. at room temperature. The degree of alignment is characterized by the dichroic spectra and polarized fluorescence spectra. Dichroic ratios and polarization ratios increase substantially with the chain length and values as high as D = 23 and P = 41 are obtained for the heptamer. The type of packing of the oligomers in the LC phase is discussed based on the X‐ray single crystal structure of models. In one such model the packing of the 2‐ethylhexyl side chains could be fully resolved, while the other model reveals the torsional angle between adjacent fluorene units in the same molecule as 144.2° which corroborates earlier work based on fiber diffraction of corresponding polyfluorenes.

     

Synthesis and Properties of Side‐Chain Liquid Crystalline Polypeptides Bearing Various Alkyl Spacers and Oligo‐Ethylene‐Glycol Tails

A series of polypeptides bearing various alkyl (i.e., propyl, hexyl, or octyl) spacers, biphenyl mesogens, and oligo‐ethylene‐glycol (OEG) tails (i.e., PPLG₂₆‐BPOEG _m, PHLG₃₄‐BPOEG_m, and POLG₃₂‐BPOEG_m [m = 3 or 7]) has been synthesized via 1,3‐dipolar cycloaddition reactions with quantitative grafting densities. Fourier transform infrared results reveal that the polypeptides adopted an α‐helical conformation in the solid‐state. Differential scanning calorimetry (DSC) analysis reveals that the glass transition temperatures (T_gs) of the polypeptides with OEG₃ tails decrease with the increasing length of the alkyl spacers. For the samples containing alkyl spacers of the same length, T_g significantly decreases with the increasing lengths of the OEG tails. Polarized optical microscope (POM) results reveal that all polypeptides exhibit strong birefringence at room temperature. Polypeptides with long alkyl spacers (i.e., hexyl or octyl) or OEG₇ tails show isotropic transitions, while PPLG₂₆‐BPOEG₃ show no clear point temperature before thermal decomposition. DSC, POM, and wide‐angle X‐ray scattering results collectively reveal that polypeptides with long alkyl spacers (i.e., hexyl or octyl) and OEG₃ tails undergo a reversible smectic E phase to isotropic phase transition, while polypeptides with OEG₇ tails undergo a reversible liquid crystalline phase to isotropic phase transition. The transition temperatures increase with the increasing length of alkyl spacers.

     

Using single-walled carbon nanotubes nonwoven films as scaffolds to enhance long-term cell proliferation in vitro

Carbon nanotubes have attracted intensive interests in biomedical research in recent years. In this study, a novel type of carbon nanotubes material so called nonwoven single-walled carbon nanotubes (SWNTs) with nanotopographic structure and macroscopic volume was used as cell growing scaffold. The morphology and surface chemistry of nonwoven SWNTs were observed and characterized through scanning electron microscopy and X-ray photoelectron spectroscopy, respectively. The cells were cultivated in nonwoven SWNTs and in other types of substrate as control. The cells growth behaviors including adhesion, proliferation, and cytoskeletal development was investigated by using cell viability assay and confocal observation. The experimental results indicated that nonwoven SWNTs exhibited significant enhancement to the cells adhesion and proliferation in at least 3 weeks. Numerous and highly organized cytoskeletal structures were observed when the cells were cultured in nonwoven SWNTs. Furthermore, an obvious promotional influence of the cells cultivated in nonwoven SWNTs scaffold upon the proliferation of those growing in the other kind of substrate through cell-cell communication had been found. The results obtained in this work are of significance to in vitro cell amplification in large scale, tissue regeneration, or guided repair, as well as biomedical device application.     

PEG‐Based Microgels Formed by Visible‐Light‐Mediated Thiol‐Ene Photo‐Click Reactions

Step‐growth PEG‐based microgels are produced via three liquid–liquid two‐phase suspension polymerization systems: i) hexane with surfactants Span80/Tween80; ii) mineral oil with surfactant Pluronic F‐68; and iii) surfactant‐free dextran‐rich aqueous solution. Following short vortexing to create monomer droplets, microgels are polymerized by a visible‐light‐initiated thiol‐ene photo‐click reaction using eosin‐Y as the only photoinitiator. The use of hexane as the organic phase and Span‐80/Tween‐80 as the surfactants leads to PEG microgels with entrapped solvent droplets that dissolve rapidly with time. Microgels polymerized in mineral oil with surfactant Pluronic F‐68 contain no entrapped droplets and are more uniform with smaller sizes. Visible‐light‐cured step‐growth thiol‐ene microgels can also be photo­polymerized in a surfactant‐free aqueous two‐phase system. The sizes of the microgels formed in aqueous phase are one order of magnitude smaller than those formed in organic solvent. Dual‐layer microgels are also prepared using two‐step thiol‐ene reactions.

     

Bulk Polymerization Photo‐Initiator ZnO: Increasing of the Benzoyl Formic Acid Concentration and LED Illumination

Polymerization in volume/bulk is one of the most important processes used in science and in industry to form solid polymers used in multiple different applications. The migration and transport of molecular polymerization initiators are problematic for commercially used photo‐polymerization procedures. ZnO not only is a well‐known photo‐semiconductor but has the potential to be used as an enlarged size photo‐initiation system. A high reactivity modification, firstly systematically investigated using novel light‐emitting diode illumination and two benchmarked resins is presented here. The curing potential of layers up to 1.4 mm is proven and first indications of differences in oxygen dependencies of such initiating systems are discovered. Additionally, the optimized synthesis procedure (0.45 mol L^?1 Zn²⁺ at room temperature) is presented which allows to perform systematic variation of the ZnO surface. Starting from one approach/batch, the content of the modification is systematically increased.     

Side-chain liquid-crystalline polysalts. Synthesis and thermal properties of poly{1-[ω-(4′-methoxy-4-biphenylyloxy)alkyl]-4-vinylpyridinium bromide} or poly[1-(2-{2-[2-(4′-methoxy-4-biphenylyloxy)ethoxy]ethoxy}ethyl)-4-vinylpyridinium bromide]

The synthesis and thermal properties of a new type of side-chain liquid-crystalline polymers, incorporating ionic groups, are described. The polymers are obtained upon spontaneous polymerization (simultaneous quaternization and polymerization) of 4-vinylpyridine in the presence of ω-(4′-methoxy-4-biphenylyloxy)alkyl or 2-{2-[2-(4′-methoxy-4-biphenylyloxy)ethoxy]ethoxy}-ethyl bromide. Fully quaternized polymers of high molecular weight are obtained. As the analogous low-molecular-weight molecules previously studied these polymers form smectic mesophases, although the mesogenic groups, which include the pyridinium rings, are directly connected to the polyvinylic backbone.     

Size Tunable Core Crosslinked Micelles from HPMA‐Based Amphiphilic Block Copolymers

A variety of core crosslinkable hydroxypropylmethacrylamide‐based block copolymers are synthesized by reversible addition‐fragmentation chain transfer (RAFT) polymerization, which are composed of hydroxypropyl‐methacrylate as hydrophilic block combined with a statistical hydrophobic block from laurylmethacrylate and the photo crosslinkable monomer. It is discovered that the self‐assembled micellar aggregates from these systems vary strongly in size depending not only on the velocity of the polarity switch (nanoprecipitation or slow dialysis) but also on the solvent from which they were dialyzed. In this way micellar aggregates with an R _h varying between 15 and 80 nm can be prepared from the same block copolymer. In addition, a new hydrophobic crosslinkable hymecromone moiety is introduced. In this way the differently sized nanoparticles can be effectively stabilized by photo‐crosslinking, leading to a high stability of the photo crosslinkable polymeric micelles against disintegration by detergents.     

Determination of Head‐Addition Incidence of Methyl Acrylate and Temperature Dependence in Radical Polymerization by Coupling Reversible Addition‐Fragmentation Chain Transfer Block Polymerization Derivatization and Gradient Polymer Elution Chromatography

Regioselective addition of nonsymmetrical monomers in radical polymerization is a basic issue for control of the reaction kinetics and structure of the resulting products. In reversible addition‐fragmentation chain transfer (RAFT) polymerization, head addition generates primary alkyl adducts when trapped by RAFT agents. Due to the low leaving ability of primary alkyl radicals, this kind of adduct in RAFT polymerization will become nonactive macro‐RAFT agents, that is, dead dormant species (DDS). Based on the previously‐developed equation for the correlation of DDS content (α) with head‐addition incidence (ρ₁) in RAFT polymerization and the quantitative determination method of DDS content by coupling a block‐polymerization derivatization with gradient polymer elution chromatography analysis, the ρ₁ values of methyl acrylate (MA) at different temperatures are determined with benzyl N‐carbazolecarbodithioate (BCBD) as RAFT agent and 2‐methyloxyethyl acrylate as the second monomer for block‐polymerization derivatization. The results show that the ρ₁ values of MA at 60, 70, 80, and 90 °C are 2.04 × 10^?3, 2.37 × 10^?3, 2.72 × 10^?3, and 3.28 × 10^?3, respectively, and the activation energy for ρ₁ is 15.8 kJ mol^?1. The DDS in BCBD‐mediated RAFT polymerization of MA is separated by column chromatography and characterized by ¹H‐NMR and MALDI‐TOF MS.