Abstract
During the last few decades, wet adhesives have been developed for applications in various fields. Nonetheless, key questions such as the most suitable polymer architecture as well as the most suitable chemical composition remain open. In this article, we investigate the underwater adhesion properties of novel responsive polymer brushes with side graft chain architecture prepared using “grafting through” approach on flat surfaces. The incorporation in the backbone of thermo-responsive poly(N-isopropylacrylamide) (PNIPAm) allowed us to obtain LCST behavior in the final layers. PNIPAm is co-polymerized with poly(methyl ethylene phosphate) (PMEP), a poloyphosphoester. The final materials are characterized studying the surface-grafted polymer as well as the polymer from the bulk solution, and pure PNIPAm brush is used as reference. PNIPAm-g-PMEP copolymers retain the responsive behavior of PNIPAm: when T > LCST, a clear switching of properties is observed. More specifically, all layers above the critical temperature show collapse of the chains, increased hydrophobicity and variation of the surface charge even if no ionizable groups are present. Secondly, effect of adhesion parameters such as debonding rate and contact time is studied. Thirdly, the reversibility of the adhesive properties is confirmed by performing adhesion cycles. Finally, the adhesive properties of the layers are studied below and above the LCST against hydrophilic and hydrophobic substrates.
Highlights
Adhesive materials have a huge impact on our everyday routine
The responsive behavior of the final layers was characterized studying the surface-grafted polymer as well as the polymer from the bulk solution, and pure PNIPAm brush was used as reference
PNIPAm-g-poly(methyl ethylene phosphate) (PMEP) copolymers retain the responsive behavior of PNIPAm: when T > LCST, a clear switching of properties is observed
Summary
Adhesive materials have a huge impact on our everyday routine. Simple actions, such as holding objects in our hands, could not take place without adhesion phenomena. The exact mechanism involved in mussel adhesion is still unclear, but it has been demonstrated that these proteins contain a variety of chemical functionalities, being rich in 3,4-dihydroxyphenylalanine (l-DOPA), amino groups, and phosphates [2] This variety of functionalities allows the mussel to provide different kind of interactions, promoting adhesion against several organic and inorganic surfaces [3]. Non-specific and uncontrolled adhesion in wet environment is not always desirable, especially in biomedical applications [15] For these reasons, there is a demand for materials which are able to provide strong and reliable adhesion underwater and in wet conditions, and to control and tune the adhesion against different substrates. This is the so-called ‘salting out effect’ and it has been reported in previous studies [39]
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