Poly(N-ethyl-4-vinylpyridinium bromide) (a polycation with a degree of polymerization of 1100) was adsorbed onto liposomes composed of egg lecithin with a 0.05-0.20 molar fraction (nu) of anionic headgroups provided by cardiolipin (a doubly anionic lipid). According to electrophoretic mobility data, this led to total charge neutralization of the liposomes, whereupon the liposomes adopted a positive charge as additional polymer continued to adsorb. Although the liposomes aggregated at the charge-neutralization point, they disassembled into individual liposomes after becoming positively charged. The degree of polymer adsorption was shown to reach a limit. Thus, by measuring the free polymer content in a liposome suspension, it was possible to determine the polymer concentration at which the liposome surface became saturated with polymer. Beyond this point, an electrostatic/steric barrier at the surface suppressed further adsorption. Dynamic light scattering studies of liposomes with and without adsorbed polymer allowed calculation of the polymer film thickness which ranged from 22 to 35 nm as the molar fraction of cardiolipin (nu) increased from 0.05 to 0.20. The greater the content on the anionic lipid in the bilayer, the thicker the polymer film. The maximum number of polymer molecules adsorbed onto the liposomes was estimated: 1-2 molecules for nu = 0.05; 3 molecules for nu = 0.1; 4- molecules for nu = 0.15; and 6 molecules for nu = 0.2. The polymer appears to lie on the liposome surface, rather than embedding into the bilayer, because addition of NaCl easily dislodges the polymer from the liposome into the bulk water.

The relationship between processes of thermal denaturation and heat-induced aggregation of tobacco mosaic virus (TMV) coat protein (CP) was studied. Judging from differential scanning calorimetry "melting" curves, TMV CP in the form of a trimer-pentamer mixture ("4S-protein") has very low thermal stability, with a transition temperature at about 40 degrees C. Thermally denatured TMV CP displayed high propensity for large (macroscopic) aggregate formation. TMV CP macroscopic aggregation was strongly dependent on the protein concentration and solution ionic strength. By varying phosphate buffer molarity, it was possible to merge or to separate the denaturation and aggregation processes. Using far-UV CD spectroscopy, it was found that on thermal denaturation TMV CP subunits are converted into an intermediate that retains about half of its initial alpha-helix content and possesses high heat stability. We suppose that this stable thermal denaturation intermediate is directly responsible for the formation of TMV CP macroscopic aggregates.

A new type of temperature-sensitive polymer systems based on mixtures of poly(acrylic acid) (PAA) and poly(diallyldimethylammonium chloride) (PDADMAC) in strongly acidic aqueous medium is described. The mixtures exhibit reversible separation into two liquid phases upon a decrease in temperature. The phase separation is induced by the intermolecular binding between PAA and PDADMAC due to iondipole interactions between protonated carboxylic groups of PAA and quaternary nitrogen atoms of PDADMAC. Molecular and hydrodynamic characteristics of the interpolymer complexes are probed by static and dynamic light scattering.

Dilute-semidilute regime crossover in aqueous solutions of partly neutralized poly(acrylic acid) and of its complex with tetradecyltrimethylammonium bromide was studied by light scattering and viscometry methods. The chain charge growth causes the decrease of overlap concentration (c*) and the increase of the entanglements formation concentration (ce), hence, the semidilute unentangled regime of solution expands. Complexation of the polyelectrolyte with an oppositely charged surfactant leads to c* increase and to ce decrease. It is shown that in semidilute entangled solutions the surfactant acts as an effective structuring agent because of the binding of polyelectrolyte chains via surfactant micelles.

No abstract

No abstract

Bulk and solution studies revealed a strongly pronounced effect of chain structure on the rheological and relaxation behavior of wellcharacterized vinyl acetate−vinyl alcohol copolymers of similar composition and polymerization degree. The frequency−temperature superposition principle is fully applicable to the random copolymers, which demonstrate all expected relaxation states, whereas a divergence of the reduced dynamic moduli−frequency dependences is observed for the multiblock copolymers. In the latter case, the terminal zone is sensitive to the self-assembling of vinyl alcohol blocks into (depending on the copolymer composition) crystalline or amorphous microstructures. The monomer unit distribution particularly affects properties of the copolymer solutions in N,Ndimethylformamide (DMF). 5% solutions behave as simple viscoelastic liquids at 20°C and show viscoplastic behavior at −20°C, where more blocky chains are characterized by up to 4 orders of magnitude higher yield stress values. The multiblock copolymer solutions demonstrate a pronounced viscosity hysteresis in the heating−cooling cycle, being absent in the random copolymers. 10% solutions of multiblock copolymers are practically gelatinous even at room temperature. The observed effects are explained by examining the peculiarities of hydrogen bonding in vinyl acetate−vinyl alcohol copolymers using FTIR spectroscopy. The multiblock copolymers are characterized by stronger hydroxyl−hydroxyl H-bonds and greater fraction of interchain hydroxyl−acetyloxy H-bonds providing aggregation of chains and high viscosity of the corresponding samples, whereas the random copolymers more strongly interact with the residual solvent. Dynamic light scattering studies prove that the relaxation of concentration fluctuations is completely diffusive, being bimodal in the random copolymers and trimodal in the multiblock ones. The fast mode in the latter case demonstrates anomalous concentration behavior. In the dilute regime, up to very low concentrations, multiblock copolymer chains form stable aggregates, and this fact correlates with an unusual growth of the reduced viscosity in the corresponding rheological experiments.

The formation and structure of water‐soluble complexes of partly neutralized poly(acrylic acid) (PA) and poly(diallyldimethylammonium chloride) (PDADMAC) with oppositely‐charged surfactants [tetradecyltrimethylammonium bromide (C14AB) and sodium dodecyl sulfate (SDS)] have been studied by elastic and quasi‐elastic laser light‐scattering, high‐speed sedimentation and viscometry techniques. It was shown that minimum aggregation number (nmin) of the surfactant ions in the complex micelle depends on the chemical structure of the polyelectrolyte. In particular, C14AB interacting with PA in dilute water solutions forms PA bound micelles with nmin of 102 similar to spherical micelles formed by free C14AB. This is in contrast to soluble PDADMAC–SDS complex species, which are most probably vesicles, each containing ca 105 surfactant ions instead of 102 characteristic for free SDS spherical micelles. A shift from a dilute to a semi‐dilute solution regime with respect to the free PDADMAC results in drastic rearrangement of the PDADMAC–SDS complex structure. Copyright © 2003 Society of Chemical Industry