Equilibrium and surface rheology of monolayers of insoluble polycations with side chains.

We have studied monolayers of poly(n-tetradecyl 4-vinylpyridinium-co-4-vinylpyridine) bromide with different degrees of quaternization at the air-water interface. The isotherms (surface pressure vs area) present several phase transitions: at low monolayer coverage, there is a phase transition over a characteristic area that increases on increasing the quaternization degree. This behavior can be rationalized in terms of a mean-field theory of 2D semiflexible polymeric chains and could be an indication of a disorder-order transition from a 2D isotropic liquid (IL) at low surface concentration to a 2D nematic phase (N) at higher concentrations. Low-frequency oscillatory strain experiments show that at low surface coverage the monolayers exhibit highly nonlinear behavior, even for low strain amplitude, whereas at higher surface coverage the response is linear for strains higher than 20%. In addition, stress relaxation experiments show a minimum in the characteristic times that coincide with the transition area. These unexpected results at low surface coverage might be characteristic of the system or related to the fact that the oscillatory experiments do not strictly correspond to constant surface-coverage conditions. However, they are in agreement with high-frequency viscoelasticity, obtained by surface quasielastic light scattering, that shows that the dilational viscosity is higher at low surface concentration than for concentrations beyond the surface phase transition. At higher coverage, there is a second phase transition, after which the isotherms present hysteresis, which is not observed below. Ellipsometry indicates that, after this transition, the monolayer thicken, which may be related to 3D growth into a multilayer.

Bio-foam enhances larval retention in a free-spawning marine tunicate.

Here we report a mechanism that reduces dispersal of early developing stages and larvae in a free-spawning intertidal and shallow subtidal tunicate, Pyura praeputialis (Heller 1878), in the Bay of Antofagasta, Chile. The spawning of gametes by the tunicate into the naturally turbulent aerated seawater decreases their surface tension and induces the formation of a bio-foam. Water collected from foamy intertidal pools and tide channels showed a high concentration of P. praeputialis early developing stages and tadpole larvae in the foam. Because gametes are synchronically spawned for external fertilization and larvae settle near adults, our results suggest that this bio-foam increases fertilization success and effective settlement of their short-lived larvae in the vicinity of the adults spawning the gametes. This mechanism reinforces published evidence suggesting that local retention of intertidal and inshore marine invertebrate larvae may be more common than previously thought, offering, for instance, new perspectives for the design and networking of marine protected and management areas.

New three-arm amphiphilic and biodegradable block copolymers composed of poly(epsilon-caprolactone) and poly(N-vinyl-2-pyrrolidone). Synthesis, characterization and self-assembly in aqueous solution.

The synthesis, characterization and the self-assembly process of a novel biodegradable block copolymer containing a poly(epsilon-caprolactone), PCL, central block and three poly(N-vinyl-2-pyrrolidone), PVP, arms are reported. Three samples with different amounts of PVP were investigated. The copolymers were characterized by FTIR spectroscopy, (1)H NMR and viscosity measurements. The composition and the molecular weights of the block copolymers were established using size exclusion chromatography SEC and (1)H NMR. Micelle formation by these copolymers was monitored by using the vibrational fine structure of pyrene monomer fluorescence and the critical aggregation concentrations, cac, of the copolymers in aqueous solution were determined using sigmoid Boltzmann-type fitting of the fluorescence data. Dynamic light scattering measurements showed a bimodal size distribution for the copolymers in solution, indicating that the micellization is an intermolecular process. Partitioning coefficients of pyrene between copolymer micelles and water were also determined and increase in magnitude with increasing epsilon-caprolactone content of the copolymer.

Surface properties of inclusion complexes between alpha-cyclodextrin and poly(ethylene oxide).

The surface properties of the supramolecular inclusion complex (IC) obtained from the threading of alpha-cyclodextrin (alpha-CD) onto poly(ethylene oxide) (PEO) free in solution are studied. The complexes were characterized by IR, (1)H NMR spectroscopy, and thermal analysis. The variation of the interfacial tension, gamma(int), with inclusion complex (IC) concentration and temperature were determined. The results were compared with those found for PEO under the same conditions. alpha-CD does not present surface activity. To quantify the adsorption process of IC and PEO in aqueous medium, the Gibbs equation was used. The driving force for adsorption of IC at the air/aqueous interface seems to arise from an enthalpic contribution. The wettability of the alpha-CD, PEO, and IC films with two liquids was determined by static contact angle measurements. The hydrophobicity degree was estimated. IC is more hydrophobic than PEO and alpha-CD.

Poly(ethylene oxide)s hydrophobically modified. Adsorption and spreading at the air-water interface.

A comparative study of spread and adsorbed monolayer of poly(ethylene oxide)s of different molecular weight hydrophobically modified with alkyl isocyanates of different length chain is reported. The modification of the polymer was carried out according to reported procedures. The polymers obtained were studied at the air-water interface by Langmuir isotherms for spread monolayers and by Gibbs isotherms for the adsorption process. Isotherms obtained are interpreted in terms of the hydrophobic and hydrophilic balance of the polymers. Limiting area per repeating unit (A(0)) and collapse pressure (pi(c)) from spread monolayers were obtained. Spread monolayers of the hydrophobically modified polymers show larger collapse pressure values than unmodified polymer monolayers. In the adsorption process the excess surface concentration Gamma(infinity), area per repeat unit sigma, and efficiency of the adsorption were determined. The values of the area occupied per repeat unit in adsorbed monolayer (sigma) were larger than those of the spread monolayer. The efficiency of the adsorption of poly(ethylene oxide)s increases with the hydrophobic modification and with the alkyl chain length.

Deconvolution of the relaxations associated with local and segmental motions in poly(methacrylate)s containing dichlorinated benzyl moieties in the ester residue.

The relaxation behavior of poly(2,3-dichlorobenzyl methacrylate) is studied by broadband dielectric spectroscopy in the frequency range of 10(-1)-10(9) Hz and temperature interval of 303-423 K. The isotherms representing the dielectric loss of the glassy polymer in the frequency domain present a single absorption, called beta process. At temperatures close to Tg, the dynamical alpha relaxation already overlaps with the beta process, the degree of overlapping increasing with temperature. The deconvolution of the alpha and beta relaxations is facilitated using the retardation spectra calculated from the isotherms utilizing linear programming regularization parameter techniques. The temperature dependence of the beta relaxation presents a crossover associated with a change in activation energy of the local processes. The distance between the alpha and beta peaks, expressed as log(fmax;beta/fmax;alpha) where fmax is the frequency at the peak maximum, follows Arrhenius behavior in the temperature range of 310-384 K. Above 384 K, the distance between the peaks remains nearly constant and, as a result, the a onset temperature exhibited for many polymers is not reached in this system. The fraction of relaxation carried out through the alpha process, without beta assistance, is larger than 60% in the temperature range of 310-384 K where the so-called Williams ansatz holds.

Surface characterization of poly(4-vinylpyridine) quaternized with tetradecyl bromide: effect of the degree of quaternization.

The surface behavior of poly(4-vinylpyridine) quaternized with tetradecyl bromide (P4VPC(14)) as function of the quaternization degree was studied. The percentage of vinylpyridine moieties quaternized was found to be 35 to 75%. Surface pressure-area isotherms (pi-A) at the air-water interface were determined. The polymer monolayers show particular shapes at different quaternization degrees. In order to get information about the hydrophobicity degree of the polymeric systems, the surface energy (SE), and their dispersion and polar contributions, gamma(D) and gamma(P) respectively, measurements of the contact angle (CA) with water, bromobenzene, and cis-decalin were performed. The results obtained are dependent on the quaternization percentage of the functionalized polymers. At high quaternization degree, hysteresis in the pi-A diagrams was observed. The change in the P4VPC(14) molecular organization in the monolayer was also investigated during the compression and expansion processes. During these processes the monolayer was monitored by Brewster angle microscopy (BAM). For several cycles, all compression curves and all expansion curves showed a common intersection point. We have tried to describe the P4VPC(14) molecular organization at the air-water interface by molecular dynamic simulation (MDS). The results are also discussed in terms of the effect of the counterions and water on the stability of the system over the more packed region.