Structure and mechanical properties of ultrafiltration membranes modified with Langmuir–Blodgett films

The structure and local mechanical properties of the surface of polysulfone(PS-100)- and polyacrylonitrile( PAN-100)-based source ultrafiltration membranes and ones modified with thin films of polyvinylpyridine (PVPyr) or latex particles applied by the Langmuir-Blodgett (LB) method and by the formation of a PVPyr/sodium polystyrene sulfonate polyelectrolyte complex have been studied using atomic force microscopy. It has been found that the application of a PVPyr layer on the PS-100 membrane results in a twofold decrease in the membrane pure water flux from 250–360 L/(m2 h), while a substantial increase in the rejection coefficient is observed: the nominal molecular weight cut-off limit is reduced from 100000 to 10000 Da. For the PAN membrane a 10% increase in water permeability was found, and the rejection coefficient for PVP K-30 increases from 55 to 65%. Modification with latex particles results in a sharp decrease in the membrane performance. Comparative analysis of the topography and the local elasticity modulus of the samples indicates almost complete blocking of the porous structure of the source membranes in the case of their modification with latex particles, as well as the destruction of the PVPyr layer with the formation of globular structures upon contact with water. The surface energy of the samples after the modification increases. Layer-by-layer applying of PVPyr and sodium polystyrene sulfonate results in a twofold decrease in the pure water flux of the PAN-100 membrane and in an increase in the rejection coefficient due to the formation of a polyelectrolyte complex. In addition, for the PAN/PVPyr/PSS membrane a substantial decrease in the degree of fouling after filtration of a model calibrant solution was detected. It was shown that depending on the type of the source matrix and the polymer used for the surface modification by the horizontal precipitation (LB) method, it is possible to produce membranes with a predetermined pore size, mechanical and transport properties; at the same time, the density and permeability of the modifying layer are determined by the physicochemical properties and the surface structure of the source membrane.