Showing papers by "Nicholas A. Peppas published in 1977"

Crosslinked poly(vinyl alcohol) hydrogels as swollen elastic networks

Abstract: Transparent crosslinked PVA hydrogels were prepared by electron beam irradiation of aqueous solutions under nitrogen. These weak hydrogels, upon swelling at 30°C in water, showed low elastic moduli (up to 50 psi), low ultimate tensile strength (up to 4 psi), and low extensibility to break (not higher than 85%). Values of the molecular weight between crosslinks Mc were calculated from swelling and from tensile experiments. In fact, two values of Mc were calculated for each swelling experiment, (a) allowing for observed variation in the polymer–solvent interaction parameter χ1 with concentration, and (b) fixing χ1 = 0.494 according to literature data. The correlation of the Mc obtained from tensile data with the Mc obtained from swelling data, by (a) or (b), was approximately linear and gave the same per cent agreement.

Development of semicrystalline poly(vinyl alcohol) hydrogels for biomedical applications.

Abstract: Swollen crosslinked poly(vinyl alcohol) (PVA) networks were developed and tested as potential biomaterials. They were prepared by electron-beam irradiation of aqueous PVA solutions at various temperatures and doses of irradiation. These materials were characterized by low mechanical properties and especially by low elongation at break and ultimate tensile strength. Reinforcement was achieved by a two-stage dehydration-annealing process, introducing crystallites in the polymeric network. Improved mechanical properties were achieved due to the presence of this crystalline PVA phase. Due to their structure, these hydrogels are proposed to be used as biomembranes for selective transport of macromolecules and as biomaterials for synthetic articular cartilage applications. Surface heparinization was accomplished by reacting heparin to the hydroxyl groups of the PVA surface, through covalent acetal bridges.

Crosslinking of polystyrene by mono‐ and difunctional agents

Abstract: Slightly crosslinked polystyrene networks are preferable to linear polystyrene in commercial uses where increased thermal properties are favoured. A novel method of production of macrocrosslinked networks has been developed by reaction of polystyrene with mono- and difunctional derivatives of p-xylene, durene and oligomeric chains (n<10) thereof. The reaction system consists of a common organic solvent such as acetic acid or butyl acetate and a catalyst such as H2SO4 or HClO4; the reaction temperature varies between 60°C and 100°C. The degrees of crosslinking and thermal properties of the produced networks depend on the reaction system, the molar ratio of polymer to crosslinking agent and the reactivity of the functional groups; the gelation time varies between 3–12 hours for a fully crosslinked network. Promotion of the formation of regular structure networks without branches in the chains between crosslinks is achieved by the use of difunctional monomers, which favour the formation of linear polybenzylene chains between the polystyrene chains. Use of monofunctional monomers leads usually to branched and slightly crosslinked or grafted polystyrene. In both cases the regions of Tg and Tm increase up to 115°C and 350°C respectively as judged by DSC analysis. This novel crosslinking method has been also applied to crosslinking of copolymers of polystyrene and polymeric chains with aromatic structure in their backbone chain. Die Gebrauchseigenschaften von leicht vernetztem Polystyrol sind denen von linearem Polystyrol vorzuziehen, wenn hohere thermische Stabilitat erforderlich ist. Eine neue Methode zur Darstellung makrovernetzter Polymerer wurde entwickelt, indem Polystyrol mit mono- und difunktionellen Derivaten von p-Xylol, Durol und deren Oligomeren umgesetzt wurde. Das Reaktionssystem bestand aus einem normalen organischen Losungsmittel wie Essigsiiure oder Butylacetat und einem Katalysator wie H2SO4 oder HClO4: die Reaktionstemperatur wurde zwischen 60 und 100°C variiert. Vernetzungsgrad und thermische Eigenschaften der erhaltenen Netzwerke hangen vom Reaktionssystem. dem Molverhaltnis von Polymerem zu Vernetzungsmittel und der Reaktivitat der funktionellen Gruppen ab. Die Zeit fur die vollstandige Vernetzung variiert zwischen 3 und 12 Stunden. Die Bildung von Netzwerken eineitlicher Struktur ohne Kettenverzweigungenzwischen den Netzstellen wird durch die Verwendung von difunktionellen Monomeren erreicht, die die Bildung linearer Polybenzylenketten zwischen den Polystyrolketten begunstigen. Der Einsatz von monofunktionellen Monomeren fuhrt normalerweise zu verzweigtem und schwach vernetztem oder gepfropftem Polystyrol. In beiden Fallen steigen die Bereiche fur Tg und Tm bis auf 115 bzw. 350°C. wie sich aus DSC-Messungen ergibt. Diese neue Vernetzungsmethode wurde auch auf Styrol-Copolymere und auf Polymere mit aromatischer Struktur in der Hauptkette angewandt.