Fiber-Supported Hydrogels with Controlled surface Properties
Hydrogels are emerging materials that have the ability to hold a substantial amount of water. However, the low mechanical performance of hydrogels is often a limitation for extending the field of applications. This thesis describes the preparation and characterization of a new generation of materials that combines the benefits of hydrogels with the mechanical performance of fibers. In the present work, cellulose and PP-supported hydrogels were prepared by means of an ozone-induced graft polymerization process. The work focused on engineering and controlling surface properties such as the wettability and morphology as well as the swelling and environmental response of the hybrid materials. Materials were engineered by changing the ozonation and grafting conditions as well as the chemical composition of the monomer mixture. The techniques used for characterization were chiefly ESCA, DCA, SEM, ESEM and AFM.
It was demonstrated that it was possible to prepare fiber-supported hydrogels with controlled wettability by graft polymerization of HEMA, DEGMA and TEGMA onto PP and cellulose. Angle-dependent ESCA measurements showed that a gradient in chemical composition existed in the near-surface region. DEGMA and TEGMA-grafted PP fibers exhibited a less pronounced contact angle hysteresis in water as compared with HEMA-grafted fibers. Both these observations can be explained by the mobility of functional groups in the outermost surface layer.
It was possible to control the amount of grafted polymer on cellulose fibers by adding a small amount of bifunctional crosslinking agent to the monomer mixture used for grafting. A mechanism for the grafting enhancement is proposed. An increased yield of grafting resulted in a coverage of the cellulose fibers and, when certain monomers were used, surface microstructures were formed. It was possible to influence the appearance of the microstructure by controlling the grafting yield. pH-sensitive solid-supported hydrogels were prepared by graft polymerization of acrylic acid onto cellulose fibers. Crosslinking of the solid-supported hydrogels affected the grafting amount and resulted in a reversible swelling behavior of the fibers. DCA measurements showed substantial changes in the swelled perimeter in test liquids with various pH values. This work shows that controlled graft polymerization results in the ability to prepare a new generation of engineered materials.
electron spectroscopy for chemical analysis
dynamic contact angle
(environmental) scanning electron microscopy
atomic force microscopy