Non-covalent columnar cyclodextrin-based structures

Examples of the formation of ordered ensembles of α-, β-, and γ-cyclodextrins (CDs) molecules with a columnar packing of macrocycles are reported. These ensembles are formed by (1) the supramolecular dissociation of polymer inclusion complexes under the action of organic solvents that are selective with respect to a polymer guest and (2) the fixation of columnar CD aggregates self-organized in aqueous solutions at high temperatures upon the precipitation from water into organic solvents. Specific features of the organization of cyclodextrins in the thus-synthesized structures are studied by X-ray diffraction. Preliminarily oriented polymer inclusion complexes based on corresponding CDs are used as a model with the columnar arrangement of macrocycles.     

New mesomorphic organocyclosiloxanes II. Thermal behaviour and mesophase structure of organocyclohexasiloxanes

Investigations of thermotropic phase transitions performed on organocyclosiloxanes [PhSi(O)OSiR]₆, where R is Me₃, Me₂(CH₂Cl) or Me₂(CH≃CH₂), have revealed that all these hexamers are mesomorphic compounds. The hexamers exhibit uncommon polymesomorphic behaviour forming two quite different mesomorphic structures. The molecular arrangement in the low temperature (LT) modification is characterized by two-dimensional (2D) long-range order with hexagonal packing. The X-ray diffraction pattern and peculiarities of molecular packing in the crystal lead us to suggest that the LT-mesophase is columnar, presumably of the Col_hd type. The LT-mesophase is formed by dimeric moieties, which associate with each other in column-like substructures, the ring planes not orthogonal to the stack axis. The high temperature (HT) mesophase is a plastic crystal (3D-order), where molecules take up positions in a face-centred cubic lattice. This is a very uncommon example of thermal behaviour for plastic crystals that provides a unique opportunity to bridge the gap between plastic crystalline and liquid crystalline mesomorphic behaviour. The thermal and structural properties of the mesophases depend upon the type of side groups of the hexamers. The size of the ring also affects the phase behaviour and the mesomorphic structure. This conclusion is consistent with data obtained by us earlier for cyclotetrasiloxanes.     

Aggregation of inclusion complexes formed by noncovalent columnar structures based on α- and γ-cyclodextrins and poly(alkylene glycols)

Kinetic analysis of the aggregation of complexes formed by columnar types of α- and γ-cyclodextrins (α-CD_col and γ-CD_col) and poly(alkylene glycols) is performed by the dynamic light scattering method. For comparison, analogous studies were conducted for systems containing initial α- and γ-cyclodextrins (α-CD and γ-CD). Upon the aggregation of systems containing α-CD, the number of nuclei with critical sizes slowly increases at the initial part of kinetic curve throughout the solution bulk; when some limiting concentration and sizes of formed aggregates are achieved, the system is transformed into the gel-like state. The aggregation of γ-CD_col-poly(ethylene glycol) system proceeds into two stages. At the first fast stage, aggregates are formed by particles representing single-strand inclusion complexes composed of one γ-CD_col molecule and two units of ethylene oxide. At the second, much slower stage, aggregates are formed by two-strand complexes composed of one γ-CD_col molecule and four units of ethylene oxide. It follows from the comparison of aggregative properties of γ-CD_col-poly(ethylene glycol) and γ-CD_col-poly(propylene glycol) systems that the rate of aggregation is much higher in the second case.     

Modeling of structures of poly(oxyhexaorganocyclotetrasiloxy-2,6-diyl) mesophases

The idealized liquid-crystal structures of two cyclolinear organosilicon polymers, viz., poly(oxyhexaorganocyclotetrasiloxy-2,6-diyls), where the alkyl substituent is Et (1b) or Pr (1c), were modeled. The columns of monomer units in polymers have different conformations, viz., a distorted long boat in 1b and a distorted long chair in 1c. Both polymers have columnar structures with the most probable antiparallel arrangement of the columns.     

Effect of the method used for crystal structure formation on the rheological behavior of organocyclotetrasiloxane in the plastically crystalline state

The rheological properties of cis-cyclotetrasiloxane, cis-[PhSi(O)(OSiMe₃)]₄, in the plastically crystalline state were investigated. The yield stress and non-Newtonian character of the flow indicates that cis-[PhSi(O)(OSiMe₃)]₄ is a viscoplastic material with respect to its rheological behavior. The conditions of crystal structure formation determine the rheological properties of organocyclotetrasiloxane in the mesophase. The temperature and stress during capillary flow were shown to affect the size and orientation of crystallites formed upon cooling of extrudates.     

Chiral liquid-crystalline copolyacrylates with biphenyl and phenyl benzoate side groups

The phase behavior and structure of liquid-crystalline phases of three series of copolymers with mesogenic units belonging to the families of biphenyls and phenyl benzoates have been studied, with the former mesogenic units containing a chiral center in a flexible spacer. A variation in the isotropization temperature as a function of composition is well described by the additivity rule, whereas the tilt angle of mesogenic groups in smectic layers changes nonlinearly. This angle decreases beginning from homopolymers and attains the minimum value for the copolymers of the equimolar composition, which show the highest ferroelectric activity among the polymer systems under study.     

Noncovalent columnar structures based on β-cyclodextrin

A new method for the synthesis of associates of cyclodextrins (CDs) of the columnar type consisting of the precipitation of CDs from aqueous solutions into acetone at lowered temperatures is developed. It is shown that columnar structures exist in both a crystalline state and in aqueous solutions. Hydrodynamic radii and molecular masses of noncovalent columnar structures (NCSs) in aqueous solutions are determined by the dynamic and static light scattering methods. The degree of association of noncovalent columnar polymers is ～40. It is revealed the NCS associates based on β-CD are stable and their hydrodynamic radius R _h is equal to 100 ± 10 nm. The kinetics of interactions of initial β-CD and NCS with poly(propylene oxide) (PPO) is studied. The pattern of kinetic curves of R _h growth upon interaction between NCS and PPO indicates that the aggregation of the particles of polymer inclusion complex proceeds in the regime of reaction-limited cluster-cluster aggregation. Kinetic curves describing the interaction processes between β-cyclodextrin and PPO are characterized by the presence of induction period t ₀. At t > t ₀, R _h ∞ t ^0.56 which is typical for the diffusion-limited cluster-cluster aggregation. Schemes of the formation of polymer inclusion complexes between initial β-CD or NCS and poly(propylene oxide) are proposed. Comparison of kinetic data on the complexation of β-CD in solution in the form of associates of two types with PPO demonstrates that columnar forms of associates are reactive species acting as macroreceptors.