Mechanically Enhanced Layer-by-Layer Assembled Polymer Films

The driving forces of the layer-by-layer membranes include the hydrogen bond,van der Waals bond and the electrostatic force which are mostly weak and sensitive tothe environment. On the one hand, the responsiveness of the membranes are mainlydependent on their sensitivity to the environment; on the other hand, thelayer-by-layer membranes based on these weak forces are inferior in mechanicalproperties than that based on covalent bond. Therefore, improving the stability and themechanical properties of the layer-by-layer membranes is quietly an important issuein this area. In this paper, we enhanced the mechanical properties of thepolyelectrolyte layer-by-layer membranes by two methods--adding inorganicnanomaterial and adjusting the pH of the solution as following:（1） MCNT was dispersed into water by the help of the polyelectrolyte PEI andassembled into the layer-by-layer membranes （PAA/PEI&MCNT_x）₃₀. The multilayerfilms can be constructed rapidly by an exponential assemble process. With the help ofthe MCNT, the mechanical properties of the self-supporting membranes weresignificantly enhanced: the ultimate tensile strength and young’s modulus is183.74±32.87Mpa and3.43±0.38GPa respectively with a5%MCNT contant, whichare nearly2.5or1.0times higher than the membranes without MCNT.（2） PEI and PAA were layer-by-layer assembled into membranes. The ultimatemechanical properties of the multilayer membranes altered according the pH of thePEI solution. Results show that the pH of the solution has influence both on themolecular structure of the PEI and the charge on it which may further impact on thethickness and the mechanical properties of the multilayer membranes. At the sametime, the iron ion would improve the mechanical properties of multilayer membranesunder the wet environment.