STRUCTURE OF POLYMER-CHAINS CONFINED IN VYCOR

We observe by Small Angle Neutron Scattering the structure of polystyr ene chains in semi-dilute solutions confined in a model porous medium, Vycor. The size of the free chains in solution is always larger than the pore diameter, 70 Angstrom. The use of a suitable mixture of hydro genated and deuterated solvents and polymers enables us to measure dir ectly the form factor of one single chain among the others. The penetr ation of the chains in the porous medium is almost complete for the co ncentration (Phi = 20%) and the range of molecular weight (35 000 < M- w < 800 000) used. The radius of gyration of the confined chains is al ways smaller than the radius of gyration of the free chains in the equ ivalent bulk solution. Our measurements are in agreement with the theo retical predictions established by Daoud and de Gennes for chains conf ined in a cylindrical pore when the chains are entangled and laterally squeezed but remain ideal at large scale along the cylinder axis beca use of the screening of the excluded volume interactions (so-called re gime of ''semi-dilute cigars''). The values of the partition coefficie nt of the chains between the porous medium and the free solution and t he asymptotic behavior of the structure factors indicate that polystyr ene adsorbs onto the bare surface of Vycor. We show that silanizing Vy cor suppresses this adsorption.