Perfect polymer interlocking by spherical particles: Capillary force shapes hierarchical composite undercuts

Polymers often do not provide all necessary properties for certain applications. In this case polymer composites can be used to combine beneficial qualities of each single component. In many cases, especially for surface-bulk-composites, adhesion is an issue because polymers with complementary properties are necessary. If the chemical binding of the pristine polymers does not allow for good adhesion, advanced strategies have to be employed which most of the time lead to chemical alteration of the surface, its destruction or else low adhesion. While roughening of the surface leads to an increase in the interface area, it will not provide a stable bond, if the adhesion between the two polymers is low in the first place. By selforganized introduction of special undercuts onto one of the polymer surfaces and by applying the second polymer in a liquid form, a mechanical interlocking composite can be achieved. In these composites adhesion can be so strong that only cohesive failure in one of the polymers will occur. In this work we evaluated simple fabrication techniques for the design of simple and complex undercuts and the adhesion between the exemplary composite PEEK and PDMS. We find that by utilizing the capillary effect, spherical standard particles can be used to create a surface structure for mechanical interlocking. Additionally, we obtain a 5.6 times higher adhesion between PEEK and PDMS. We come to the conclusion that a multi-scale undercut is necessary to obtain a strong adhesion between soft polymers like PDMS and stiff polymers like PEEK by looking at the detachment mechanism for these different undercut systems. Lastly, the composite is evaluated by blood contact tests to verify the intactness of the blood repellant effects of the PDMS layer.