This work was conducted on Pinctada maxima nacre (mother of pearl) in order to understand its multiscale ordering and the role of the organic matrix in its structure. Intermittent-contact atomic force microscopy with phase detection imaging reveals a nanostructure within the tablet. A continuous organic framework divides each tablet into nanograins. Their shape is supposed to be flat with a mean extension of 45nm. TEM performed in the darkfield mode evidences that at least part of the intracrystalline matrix is crystallized and responds like a 'single crystal'. The tablet is a 'hybrid composite'. The organic matrix is continuous. The mineral phase is thus finely divided still behaving as a single crystal. It is proposed that each tablet results from the coherent aggregation of nanograins keeping strictly the same crystallographic orientation thanks to a hetero-epitaxy mechanism. Finally, high-resolution TEM performed on bridges from one tablet to the next, in the overlying row, did not permit to evidence a mineral lattice but crystallized organic bridges. The same organic bridges were evidenced by SEM in the interlaminar sequence.

We present experimental results on the friction force exerted by a network of poly(dimethylsiloxane) (PDMS) moving on a solid substrate coated with the same polymer. Two different coatings are compared: a surface, densely grafted with short chains (which can be seen as a model impenetrable surface) onto which a well-defined number of long chains (connectors) can be gradually added and adsorbed PDMS layers. Increasing the sliding velocity between 10 -5 and 10 -1 m/s suggests a transition between "liquidlike" and "solidlike" frictional behavior. Increasing the molecular weight of adsorbed or grafted chains shifts this transition to higher sliding velocities. Increasing systematically the areal density (Σ) of connectors yields two opposite trends: (i) At our lowest velocities, an increase of Σ results in higher friction. This increase of friction is due to the pull-out process of the grafted chains from the network. (ii) At higher velocities, the same connectors lead to a reduction of friction (lubrication effect!) with respect to a surface without connectors.

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