Adsorption and Adhesion on Semi-Crystalline Polymers

Rarely can the perfect combination of bulk properties (strength, transparancy etc) and surface properties (water repellency, stickyness, etc) be found in any single material. Methods to achieve the desired surface properties of an object are commonly referred to as surface modification. Although the surface modification concept is not new - the ancient Egyptians achieved this with glue and varnish using natural products - the need to optimise these methods for new materials and to make efficient use of limited existing material resources is ever more important.Many everyday polymers such as polyethylene (PE), polypropylene (PP) and PET require surface modification to fulfil new functions. Certain applications, notably in printing, electronics and packaging, require modification of the native surface properties of these polymers. Many coating techniques have been developed to overcome this limitation, but most require additional processing steps. Furthermore, there is always some concern that the coating is either wasteful of material (too thick), or may be too thin or fragile to perform its function properly.We propose an alternative route to achieve surface modification, whereby the host polymer is blended with a small amount of a additive polymer, that differs only in that one end of each additive polymer molecule has been chemically modified. Provided the chemically modified groups can be attracted to the blend surface, the adsorption of the additive polymer to the surface of the blend will modify its external surface properties. In some cases this attraction is spontaneous and very easily achieved, in others, it may be induced by careful control of the processing conditions. Previously we have shown that this technique can work extremely well for several non-crystalline polymers, however it has never been attempted for a semi-crystalline polymer such as polyethylene. The semi-crystalline nature of PE, PP, and PET is vital to their excellent bulk properties, but also makes these materials more difficult to analyse. However, since the surface properties of these materials are also notoriously difficult to modify by other methods, there is a real need to explore this possibility further. Recently we have shown that the ion beam analysis techniques used at Durham are perfectly well suited to examine semi-crystalline polymer blends, therefore we are well placed to carry out this project.We are especially interested in the relationship between the adsorption of surface modifying polymers within the blend to its surface, and mineral deposition or adsorption to the external surface of the blend. Clearly these processes should be related since adsorption within the polymer blend will provide a template for adsorption from external media. This is potentially extremely very important for biophysical properties of the polymer, such as bone biocompatibility, and we plan to carry out the first sytematic study of this relationship.