Article pubs.acs.org/Macromolecules
Frequency Response of Polymer Films Made from a Precursor Colloidal Monolayer on a Nanomechanical Cantilever Ting Liu,† Sascha Pihan,† Marcel Roth,† Markus Retsch,†,‡ Ulrich Jonas,†,⊥ Jochen Stefan Gutmann,†,§,∥ Kaloian Koynov,† Hans-Jürgen Butt,† and Rüdiger Berger*,† †
Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States § Department of Chemistry and Center for Nanointegration Duisburg-Essen (CeNIDE), University Duisburg-Essen, Universitätsstr. 5, D-45117 Essen, Germany ∥ Deutsches Textilforschungszentrum Nord-West e.V., Adlerstr. 1, D-47798 Krefeld, Germany ⊥ Macromolecular Chemistry, Department Chemistry−Biology, University of Siegen, Adolf-Reichwein-Strasse 2, AR-G 213, D-57076 Siegen, Germany ‡
S Supporting Information *
ABSTRACT: Nanomechanical cantilevers (NMC) were used for the characterization of the film formation process and the mechanical properties of colloidal monolayers made from polystyrene (PS). Closely packed hexagonal monolayers of colloids with diameters ranging from 400 to 800 nm were prepared at the air−water interface and then transferred in a controlled way on the surface of NMC. The film formation process upon annealing of the monolayer was investigated by measuring the resonance frequency of the NMC (≈12 kHz). Upon heating of non-cross-linked PS colloids, we could identify two transition temperatures. The first transition resulted from the merging of polymer colloids into a film. This transition temperature at 147 ± 3 °C as measured at ≈12 kHz remained constant for subsequent heating cycles. We attributed this transition temperature to the glass transition temperature Tg of PS which was confirmed by dynamic mechanical thermal analysis (DMTA) and using the time temperature superposition principle. The second transition temperature (175 ± 3 °C) was associated with the end of the film formation process and was measured only for the first heating cycle. Furthermore, the transition of the colloidal monolayer into a homogeneous film preserved the mass loading on the NMC which allowed determination of the Young’s modulus of PS (≈3 GPa) elegantly.
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INTRODUCTION During the past years nanomechanical cantilevers (NMC) have been used more and more for the analysis of polymer properties. The interaction of polymers with solvents,1−3 the response of polymers to pH changes,4,5 the wetting properties between different polymers,6 swelling and deswelling,7,8 interfacial tension of polymers,9 and thermal transition temperatures of hydrogels10 have been investigated by NMC. Typically, picograms to nanograms of materials are enough for the analysis,11 and the method allows even screening of material properties.12 Recently, Jung and co-workers probed the glass transition temperature (Tg) of polystyrene (PS) and poly(vinyl acetate) (PVAc) and block copolymers using NMCs.13−15 In their experiments, the NMCs were coated with a polymer solution by inkjet printing. After solvent evaporation, only polymer remained on the NMC. Then the deflection and the resonance frequency of the NMC were probed at different temperatures. Changes in volume and in elastic properties of the polymer during heating induced a deflection of the NMC, which allowed determining the Tg of the polymer. Furthermore, vibrating the polymer coated NMC at its resonance frequency enabled a qualitative investigation of the change of the Young’s modulus of PVAc during heating.13,14 A better analysis was hampered by a nonuniform polymer coating. A quantitative © 2011 American Chemical Society
estimation of the Young’s modulus requires a separation of mass and rigidity induced changes in the resonance frequency of NMC.16 Grüter and co-workers reported on a method to disentangle mechanical and mass effects of thin metal film on NMC by including measurements of the quality factor Q of the vibrating NMC.17,18 On the basis of this method, they found that ≈5 nm thick isolated islands of Cu did not contribute to the rigidity but only to the mass loading of NMC. In order to characterize metal coatings, a mass loading
