Engineering the Cellulose Fiber Interface in a Polymer Composite by Mussel-Inspired Adhesive Nanoparticles with Intrinsic Stress-Sensitive Responsivity.

The interface between the fiber and matrix plays a key role in polymer composite performance and is adapted by chemical modification of the fiber surface. In this study, biomimetic adhesive nanoparticles formed by the self-assembly of polymer-peptide amphiphiles with a polydiacetelyene tail and local presentation of 3-hydroxyphenylalanine or DOPA adhesive groups at the outer surface are adsorbed on cellulose fiber surfaces for (i) probing the nanoscale adhesion in combination with a functionalized atomic force microscopy tip and (ii) evaluating the macroscale adhesion by single-fiber pull out tests from a solvent cast cellulose/poly(methyl methacrylate) composite. The interface properties are altered by changing the structure of the nanoparticles into either vesicular or planar shapes depending on the number of incorporated amphiphiles with adhesive groups and the nanoparticle concentration at the cellulose fiber surface. Based on nanoscale adhesive measurements, the adhesion force on modified cellulose fibers increases as a function of the nanoparticle concentration and is higher for the vesicular than for the planar nanoparticle structures. However, the local presentation and number of adhesive groups seems to rule over the surface roughness effects. From macrosale tests, an optimum concentration of adhesive vesicles provides maximum interface strength, while the formation of nanoparticle multilayers at higher concentrations results in lower interface adhesion. In addition, the intrinsic fluorescent properties of the adhesive vesicles under mechanical stress provide a unique tool to evaluate local failure and stress concentrations in the fiber/matrix interface. The incorporation of both adhesive and sensitive properties and versatility of the adhesive functional group may be an attractive strategy for the surface modification of fiber-reinforced composites in general.