Sergei A. Kuptsov

Coherence of thermal transitions in poly(N-vinyl pyrrolidone)-poly(ethylene glycol) compatible blends. 1. Interrelations among the temperatures of melting, maximum cold crystallization rate and glass transition

Abstract The phase behaviour of blends of high-molecular weight poly( N -vinyl pyrrolidone) (PVP) with short-chain poly(ethylene glycol) (PEG) of M w =400, prepared by drying their solutions in a common solvent (ethyl alcohol), was studied using DSC. Upon heating of cool-quenched samples a single glass transition was observed, followed by an exotherm corresponding to cold crystallization of excess PEG, a melting endotherm, and an endotherm corresponding to vaporization of absorbed water. The temperatures of glass transition ( T g ), PEG cold crystallization ( T c ), and melting ( T m ), along with the change in heat capacity (Δ C p ) between the polymers glassy and rubbery states at T g , vary with blend composition and hydration. As a result the T g / T m , T c / T m and T c / T g ratios for PVP–PEG blends are functions of composition. PVP–PEG compatibility is due to H-bonding of PEG terminal hydroxyls to the carbonyls in the PVP repeating units. Large negative deviations of T g values from the calculated weight averages, found mainly for PVP-overloaded blends, signify strong PVP–PEG interaction and free volume formation.

Monodomain liquid crystalline networks: reorientation mechanism from uniform to stripe domains

Monodomain acrylate-based networks have been synthesized by a two-step cross-linking procedure using gamma-irradiation. Strain-induced reorientation of the nematic director is studied by X-ray diffraction measurements. The geometrical shape (aspect ratio) of the monodomain films is found to affect drastically the dynamics and mechanics of the reorientation transition. Uniform continuous rotation of the director occurs in narrow samples with an aspect ratio AR=12 when the external mechanical field is applied perpendicular to the initial orientation. Under the same conditions, films having a lower aspect ratio (AR=2.5) demonstrate the formation of stripe domains with an alternating sense of director rotation (clockwise and counter-clockwise towards the stress axis). Deformation of a square film (AR=1) generates stripe domains in the geometrical centre of the sample, whereas a uniform continuous rotation is observed in other regions of the film. Finally, a comparison of experimental data and theoretical pred...

Coherence of thermal transitions in poly(N-vinyl pyrrolidone)–poly(ethylene glycol) compatible blends2. The temperature of maximum cold crystallization rate versus glass transition

Abstract Differential scanning calorimeteric (DSC) heating thermograms of amorphous poly( N -vinyl pyrollidone) (PVP) blends with short-chain poly(ethylene glycol) (PEG) feature exotherms of cold crystallization coupled with symmetric melting endotherms, which relate to the state of the crystalline component, PEG, while PVP–PEG hydrogen-bonded complex reveals itself as an amorphous phase. As PEG content in blends exceeds a characteristic level, PEG cold crystallization occurs upon heating of the cool-quenched samples through their glass transition temperatures ( T g ). The contributions of both thermodynamic and kinetic factors to the occurrence of non-crystallizable PEG have been analysed by considering the dependence of the PEG cold crystallization temperature, T c , on blend T g and composition along with the compositional dependence of the heat of melting of PEG. The stoichiometry of the PVP–PEG H-complex was evaluated from DSC thermograms as the amount of non-crystallizable PEG in PVP-underloaded blends.

Coherence of thermal transitions in poly(N-vinyl pyrrolidone)-poly(ethylene glycol) compatible blends. 3. Impact of sorbed water upon phase behaviour

Abstract The state of sorbed water in hydrogels based on hydrogen-bonded complexes between poly(N-vinyl pyrrolidone) (PVP) and short-chain poly(ethylene glycol) (PEG) has been examined by considering water vaporisation endotherms in d.s.c. heating traces and relating them to characteristics of the amorphous and crystalline phases, i.e. the thermodynamic parameters of glass transition and excess PEG melting. In compatible PVP–PEG blends water behaves as a third component in the PVP–PEG complex and is sorbed mainly by PVP, whereas PEG increases the mobility of sorbed water molecules evaluated in the terms of the entropy of water thermodesorption. Water sorption affects dramatically the state of crystalline (excess) PEG in blends, while the state of the amorphous phase, constituted by the PVP–PEG hydrogen-bonded complex, is practically unaffected by hydration.

Liquid crystal acrylate-based networks : polymer backbone-LC order interaction

Abstract New synthetic pathways to poly- and monodomain acrylate-based networks via gamma-irradiation and chemical crosslinking are described. Strain-induced orientation transitions and phase behavior are studied by X-ray scattering, differential scanning calorimetry and optical microscopy. Analysis of the observed phenomena is given in terms of crosslinking conditions, network topology, structure and spatial distribution of crosslinks. The pretransitional behavior and the observed shifts in clearing point Tn–i are in good agreement with theoretical predictions. The reorientation process in monodomain networks is regulated via strain gradients generated by external mechanical field. The variation in geometrical shape of the samples gives an opportunity to control and realize two different reorientation mechanisms: uniform director rotation and reorientation via stripe domains. Gamma-irradiation is found to be a powerful tool for fixation of any current distribution of poly- and monodomain regions within the LC film. ‘Macroscopic memory’ of the information recorded on the films is observed and discussed.

Antiferroelectric alignment and mechanical director rotation in a hydrogen-bonded chiral SmC* A elastomer

A new type of chiral smectic elastomer based on poly[4-(6-acryloyloxyhexyloxy)benzoic acid] is discussed. The layer structure and the molecular tilt stabilized by hydrogen bonding between side groups are identified by X-ray measurements. Well aligned and optically clear monodomain samples with smectic layers in the film plane are obtained by uniaxial stretching and then frozen-in by additional gamma-radiation crosslinking. In this monodomain state, two opposite orientations of director tilt are distributed through the sample thickness and alternate between neighbouring layers in a zigzag fashion. This structure of the stress-aligned chiral smectic C elastomer is similar to that of antiferroelectric liquid crystals of the smectic C* A type. Further mechanical stretching in the layer plane induces a gradual c-director reorientation along the new stress axis, when a threshold deformation ~ 20% is exceeded. The (reversible) transition proceeds as a director azimuthal rotation around the smectic C cone, with the layers essentially undistorted and the tilt angle of the side mesogenic groups preserved.

The products of water H-bonding to poly(N-vinyl pyrrolidone) in solid state

PVP is a highly hygroscopic amorphous polymer that is widely used in medicine as a drug-carrier, disintoxicant, blood-substituting agent and pharmaceutical excipient. In aqueous solution as many as 12 water molecules are reported to be associated with one PVP unit [1]. The PVP unit is capable of forming two H-bonds with water molecule through two unshared electron pairs at oxygen atom in carbonyl groups. Unshared p-electron pair at nitrogen atom has been shown to be never involved into H-bonding [2]. Consequently, the size of a first hydrate layer in solid polymer is limited by two water molecules associated with every PVP unit. This work focuses on eliciting the molecular mechanism of the first hydrate layer formation.