Dual aging behaviour in a clay-polymer dispersion.

Clay-polymer compounds have recently attracted increasing attention due to their intriguing physical properties in colloidal science and their rheological non-trivial behaviour in technological applications. Aqueous solutions of Laponite clay spontaneously age from a liquid up to an arrested state of different nature (gel or glass) depending on the colloidal volume fraction and ionic strength. We have investigated, through dynamic light scattering, how the aging dynamics of Laponite dispersions at fixed clay concentration (Cw = 2.0%) is modified by the addition of various amounts of poly(ethylene oxide) (PEO) (CPEO = (0.05 ÷ 0.50) %) at two different molecular weights (Mw = 100 kg mol(-1) and Mw = 200 kg mol(-1)). A surprising and intriguing phenomenon has been observed: the existence of a critical polymer concentration C that discriminates between two different aging dynamics. With respect to pure Laponite systems the aging will be assisted (faster) or hindered (slower) for PEO concentrations respectively lower (CPEO < C) or higher (CPEO > C) than the critical concentration. In this way a control on the aging dynamics of PEO-Laponite systems is obtained. A possible explanation based on the balance of competitive mechanisms related to the progressive saturation of the clay surface by polymers is proposed. This study shows how a real control on the aging speed of the PEO-Laponite system is at hand and renders possible a real control of the complex interparticle interaction potential.

Biomimetic poly(amidoamine) hydrogels as synthetic materials for cell culture.

BACKGROUND: Poly(amidoamine)s (PAAs) are synthetic polymers endowed with many biologically interesting properties, being highly biocompatible, non toxic and biodegradable. Hydrogels based on PAAs can be easily modified during the synthesis by the introduction of functional co-monomers. Aim of this work is the development and testing of novel amphoteric nanosized poly(amidoamine) hydrogel film incorporating 4-aminobutylguanidine (agmatine) moieties to create RGD-mimicking repeating units for promoting cell adhesion. RESULTS: A systematic comparative study of the response of an epithelial cell line was performed on hydrogels with agmatine and on non-functionalized amphoteric poly(amidoamine) hydrogels and tissue culture plastic substrates. The cell adhesion on the agmatine containing substrates was comparable to that on plastic substrates and significantly enhanced with respect to the non-functionalized controls. Interestingly, spreading and proliferation on the functionalized supports are slower than on plastic exhibiting the possibility of an easier control of the cell growth kinetics. In order to favor the handling of the samples, a procedure for the production of bi-layered constructs was also developed by means the deposition via spin coating of a thin layer of hydrogel on a pre-treated cover slip. CONCLUSION: The obtained results reveal that PAAs hydrogels can be profitably functionalized and, in general, undergo physical and chemical modifications to meet specific requirements. In particular the incorporation of agmatine warrants good potential in the field of cell culturing and the development of supported functionalized hydrogels on cover glass are very promising substrates for applications in cell screening devices.

Structural iridescent tuned colors from self-assembled polymer opal surfaces.

Structural colors are the object of a wide scientific interest, not only for the potential technical applications of their intriguing optical properties but also for the need of coloring agents to replace toxic and carcinogenic dyes. We present a simple methodology to obtain polymer opal surfaces of self-assembled core-shell nanoparticles with different degree of order for structural color applications. Polymer nanospheres prepared by surfactant-free emulsion radical copolymerization of an hydrophobic and an hydrophilic comonomer (styrene and methacrylic acid) spontaneously assemble into core-shell particles. Nanoparticles with identical composition and different diameters were prepared by modulating the degree of ionization of the weakly acidic comonomer. We report experimental results revealing how the synthesis parameters affect the properties of the core-shell particles and their influence on the optical properties of the final polymer opal surfaces, which depend on size, charge, and packing arrangement of the constituent nanoparticles.