A two-plate buckling technique for thin film modulus measurements: Applications to polyelectrolyte multilayers

We present a modified version of a strain-induced buckling instability technique that relies on the analysis of a two-plate composite film deposited on an elastomeric poly(dimethylsiloxane) (PDMS) substrate. We have previously shown that the strain-induced elastic buckling instability for mechanical measurements (SIEBIMM) technique is suitable for testing polyelectrolyte multilayers (PEMs) that are amenable to deposition directly on the testing substrate. The method presented in this paper broadens the applicability of the SIEBIMM technique through the transfer of a thin layer of polystyrene (PS) to the PDMS surface prior to film deposition, which creates a deposition surface that can be treated to promote adhesion of films not amenable to transfer or assembly directly onto PDMS. Multilayers assembled onto the PS-coated PDMS substrates yield thin two-plate PS-PEM composite films on the surface of the PDMS substrates that buckle like their homogeneous counterparts. The mechanical contribution of the PS layer is mathematically deconvoluted from the behavior of the composite film to arrive at a Young's modulus value for the PEM part of the two-plate film. We test the new method by comparing results from two systems evaluated with both conventional SIEBIMM and the two- plate technique. Following this, we use the two- plate method to perform measurements on two PEM assemblies comprised of poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) that could not be measured by the conventional SIEBIMM approach. In addition to confirming the accuracy of the two- plate approach, our results yield new insights into the mechanical properties of PEM films. We find that the dry-state stiffness of PEM films is affected primarily by the choice of polyelectrolytes and the ambient humidity and secondarily by assembly conditions. In addition, films assembled from PAH and PAA have moduli on the order of 10 GPa at low (20%) ambient humidities, an unusually high value for a nonreinforced cross-linked polymer network.