G. A. Shandryuk

Relation of glass transition temperature to the hydrogen-bonding degree and energy in poly(N-vinyl pyrrolidone) blends with hydroxyl-containing plasticizers. Part 1. Effects of hydroxyl group number in plasticizer molecule

Abstract The well-known Fox equation has been modified to express the glass transition temperature, Tg, as an explicit function of the number of polymer–plasticizer hydrogen bonds in miscible poly(N-vinyl pyrrolidone) (PVP) blends with ethyl alcohol, water, short-chain poly(ethylene glycol) (PEG), and glycerol. The plasticization effect has been found to be dependent on the fraction of hydroxyl groups in the blend rather than on the plasticizer weight fractions. Negative deviations of the blend Tg from the relationship predicted with the original form of the Fox equation were shown to be in direct proportion to the number of hydroxyl groups in the plasticizer molecule. The following quantities can be evaluated based on the Tg–composition profiles: binding degree, fraction of plasticizer hydroxyl groups forming hydrogen bonds to PVP repeat units, the fraction of plasticizer molecules crosslinking the polymer units by hydrogen bonding through two or more hydroxyl groups in their molecule. The dynamics of PVP–PEG hydrogen bonding over the entire compositional range has been evaluated in terms of hydrogen-bonded network density. The stoichiometric composition of the PVP–PEG hydrogen-bonded complex, determined from the Tg–composition relationship, corresponds to the data obtained with independent methods.

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.

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.

Development and stabilization of liquid crystalline phases in hydrogen-bonded systems

This review addresses the description of fundamental principles underlying the development of liquid crystalline phases in hydrogen-bonded complexes. Low-molecular-mass models and polymers systems are considered, where hydrogen bonding is used as an efficient means for modifying polymer structure and designing new materials.

Effect of Temperature on Probe Tack Adhesion: Extension of the Dahlquist Criterion of Tack

At a molecular level adhesive joint strength of pressure-sensitive adhesives (PSAs) is governed by the ratio between two generally conflicting factors: high energy of cohesive molecular interactions and large free volume. Increase in temperature leads to domination of the free volume contribution over the cohesive strength, affecting mechanisms of the debonding process, examined with a probe tack test. Linear viscoelastic properties and probe tack adhesion of five types of PSAs have been studied: polyisobutylene (PIB); acrylic, styrene-isoprene-styrene (SIS) triblock copolymer; hydrogen-bonded complex of high molecular weight poly(N-vinyl pyrrolidone), PVP; with oligomeric poly(ethylene glycol), PEG; and plasticized polybase—polyacid polyelectrolyte complex (PEC). The transition from solid-like mechanism of debonding to ductile type of adhesive bond failure with fibrillation of adhesive layer has been established to occur for all examined PSAs under temperature increase within the range from −20 to 80°C. The Dahlquist criterion of tack, which defines the value of the storage modulus, G′, below 0.1 MPa, featured for all the PSAs demonstrating maximum work of debonding, has been found to have a universal character and holds at corresponding temperatures for all the PSAs examined, including both typical and innovative adhesives. In addition to this adhesion predictor we have also established that for all the PSAs the transition from a solid–like debonding mechanism to a ductile type of debonding is observed in the range of G′ = 0.09–0.34 MPa. The value of the dissipation factor, tan δ, is also included in the analysis of correlation between linear viscoelasticity and probe tack behavior.

A new class of pressure-sensitive adhesives based on interpolymer and polymer-oligomer complexes

On the basis of previous concepts concerning the molecular nature of pressure-sensitive adhesion, a simple method of preparing new adhesives with the desired mechanical and adhesive behavior and water-absorbability via mixing of nonadhesive polymers has been developed. Pressure-sensitive adhesion is related to the combination of a high energy of cohesion and a large free volume, which leads to a high molecular mobility. This method is based on the formation of interpolymer or polymer-oligomer complexes during mixing of macromolecules capable of hydrogen, electrostatic, or ionic bonding. In interpolymer complexes, a high cohesion results from the formation of bonds between macromolecules carrying complementary groups in main chains, whereas free volume is related to defectiveness of the resulting network and formation of loops. In complexes formed by a high-molecular-mass polymer and an oligomer carrying complementary reactive groups at ends of short chains, a high energy of cohesion is related to their interaction with mainchain functional groups of the polymer, whereas a relatively large free volume is associated with the length and flexibility of intermacromolecular crosslinks via oligomer chains. The adhesive and viscoelastic properties of adhesives and their water absorbability are regulated by changes in the composition of mixtures of a film-forming polymer with a polymer or oligomer crosslinker and a plasticizer. In this case, an increase in cohesive strength is achieved owing to an increase in the crosslinker concentration, while the enhancement of free volume is ensured by the increasing plasticizer content in the blend. Adhesive materials capable of adherence to wet substrates, hydroactivated adhesives, and adhesion moisture sorbents have been prepared for the first time.

Synthesis of norbornene–cyclooctene copolymers by the cross-metathesis of polynorbornene with polyoctenamer

Copolymers of norbornene and cyclooctene were synthesized for the first time by the cross-metathesis of polynorbornene with polyoctenamer. This strategy made it possible to use the 1st generation Grubbs catalyst, which exhibits low activity toward copolymerization of those monomers. Statistical multiblock copolymers with average block lengths varying from 200 to 2 units were obtained.

Alignment of nanoparticles in polymer matrices

This paper briefly summarizes the state of the art in the field of designing composites containing semiconductor nanoparticles distributed in a polymer matrix. Special attention is focused on (i) nanocomposites based on block copolymers and (ii) LC polymer matrices capable of controlling the localization and alignment of nanoparticles.

Thermal fractionation of vinyl acetate-vinyl alcohol copolymers

Thermal fractionation via the method of successive self-nucleation and annealing was used for the first time to study the crystallinity of vinyl acetate-vinyl alcohol copolymers with different random distributions of chain units. The lamella-thickness distribution was calculated through the Gibbs-Thomson equation. It was shown that, for all samples, the minimum lamella thickness is the same and corresponds to a block of no less than 15 vinyl alcohol units. On the basis of these data and with the use of the computer simulation of the polymer-analogous reaction via the Monte Carlo method, the block-length distribution in the crystalline phase was found. It was shown through a comparison of the lamella-thickness and block-length distributions that the maximum lamella thickness increases with the block length and vinyl alcohol content in the copolymer. In crystallites, blocks with lengths exceeding the maximum lamella thickness comprise a significant fraction. Thus, it is probable that these blocks form folds. The dependences of melting temperatures of crystalline lamellas on their thicknesses, as well as the dependences of the melting temperatures of copolymers not subjected to thermal fractionation on the chain-structure parameters, are adequately described by the Flory crystallization theory.