Polymers Main Page > Research Highlights > Detail     Technical Highlights in Nanotechnology   High-throughput Measurements of Elastic Moduli of Polymer Thin Films   As technology continues to strive towards smaller, thinner, and lighter devices, more stringent demands are being placed on polymer films as diffusion barriers, dielectric coatings, electronic packaging, etc. The material properties of thin films can be drastically different from that of the bulk material. Therefore, there is a growing need for testing platforms that allow for rapid determination of the mechanical properties of thin polymer films/coatings. We demonstrate here a novel measurement technique that yields the elastic modulus of supported polymer films in a rapid and quantitative manner without the need for expensive equipment or material-specific modeling.   Christopher M. Stafford   There exist few techniques to measure the mechanical properties of polymer thin films (e.g., nanoindentation, atomic force microscopy (AFM), surface acoustical wave spectroscopy, and brillouin light scattering), each having their own limitations and none of which are positioned to be applied as high-throughput measurements. We present a reliable, high-throughput technique by which the elastic modulus of thin polymer films can be measured both rapidly and quantitatively. Indeed, this technique is very simple and practically any laboratory, academic or industrial, can perform such measurements with only modest investment in equipment.   This measurement platform exploits a classic mechanical instability that occurs in laminates and sandwich structures under compression. In this geometry, a highly periodic buckling instability arises from a mismatch of the elastic moduli of a relatively stiff polymer coating that has been applied to a soft polydimethylsiloxane (PDMS) substrate. The wavelength of the wrinkles can be measured rapidly by conventional light scattering. In addition, the sample can be rastered across the beam to map out the properties of the entire film if the sample is comprised of a gradient library, or if the sample is uniform, a multitude of images can be acquired to generate sufficient statistics. We have denoted this technique as Strain Induced Elastomer Buckling Instability for Mechanical Measurements (SIEBIMM). Since this is an intrinsically local measurement, this technique is well suited for measurements of combinatorial libraries with spatially varying properties that can be prepared by existing methodologies developed at NIST.   In general, thin polymer films are prepared on silicon substrates either by flow coating or spin coating. The thickness (h) of each film is measured extensively by conventional interferometry. Films are transferred to PDMS strips via aqueous immersion to produce a laminate structure. Upon application of strain, the disparity in the Poisson's ratios (n) between the PDMS substrate and the polymer film results in a net compressive stress on the glassy film perpendicular to the strain direction. Conversely, higher degrees of compression can be obtained by pre-stretching the PDMS prior to transfer of the polymer film and then releasing the stressed PDMS/film laminate. One mechanism by which the film can relieve this applied stress is to buckle, thus producing a sinusoidal phase grating whose periodicity or wavelength (d) can be rapidly quantified by conventional light scattering.   The modulus of the upper film is calculated by the following equation (as detailed by H.G. Allen, Analysis and Design of Structural Sandwich Panels):   where Ep is the modulus of the upper film, and Em and nm are the modulus and Poisson's ratio of the PDMS substrate, respectively. The Poisson's ratio for PDMS was measured to be 0.50 ± 0.05. In this formalism, q=2p/d where d is the periodicity of the wrinkles. We find that this equation is valid for Ep/Em > 20, which allows a wide variety of materials to be measured.   Figure 1: (a) Optical image of a thickness gradient of polystyrene, (b) optical images of the buckling pattern as a function of film thickness, (c) light diffraction patterns from the periodic wrinkles shown in (b).   Initial validation of this concept focused on a thickness gradient of polystyrene ranging from 140 nm to 280 nm in thickness. According to Equation 1, the buckling wavelength should be inversely proportional to the thickness of the upper film for a film of constant modulus, Ep. Figure 1 demonstrates the validity of Equation 1: as the film thickness increases by a factor of two, the buckling wavelength also increases by a factor of two as measured by both optical microscopy and light scattering. Through Equation 1, the modulus of the polystyrene film is calculated to be 3.17 GPa ± 0.11 GPa for the entire range of thickness. These values are in excellent agreement with the measured bulk value of 3.22 GPa ± 0.05 GPa for the same polymer.   This method was also applied to a model system that varied in mechanical properties. Here, a common plasticizer (dioctyl phthalate) was solution blended with polystyrene prior to spin coating. As shown in Figure 2, the measured modulus (blue) of the polystyrene film decreases in a sigmoidal manner with increasing concentration of added plasticizer. These results were compared against nanoindentation data (red) obtained on identical films supported on silicon wafers. The agreement between the buckling method and nanoindentation is markedly good.   Figure 2. Modulus of polystyrene thin films as a function of mass fraction of added plasticizer, dioctyl phthalate.   Measurements were also performed on blends of polystyrene-polyisoprene-polystyrene (P(S-I-S)) triblock copolymers to demostrate the applicability to soft, structured materials. Two commercially available copolymers of P(S-I-S) with different fractions of the glassy component (PS) were solution blended prior to spin-coating into micron thick films. Since the periodicity of block copolymer microdomains is on the order of 30 nm, thick films should exhibit bulk-like mechanical behavior. Also, the microphase-separated structure of the copolymer should not interfere with the light scattering experiment due to the small domain sizes. The results of this experiment can be seen in Figure 3, where thin film data are shown in blue and bulk tensile tests are shown in red. While the bulk tests suffer from a great deal of scatter, perhaps due to variations in sample preparation and handling, buckling measurements show a comparatively smooth increase in modulus with increasing PS fraction. As shown in Figures 2 and 3, the moduli range accessible by this method is as low as 5 MPa and as high as several GPa.   Figure 3. Modulus of P(S-I-S) triblock copolymer blends with an increasing glassy component, PS.   We have developed a novel, high-throughput technique that allows the elastic modulus of polymer thin films to be determined rapidly and quantitatively. The SIEBIMM technique exploits a strain-induced buckling instability in elastomer-supported films that exhibits a modulus-dependent wavelength. Conventional light scattering can quickly access the wavelength of the wrinkles, and the modulus can be calculated using a simple model. We measured films with thicknesses ranging from several micrometers to 20 nm, and films with a range of moduli from several GPa to 5 MPa.   Research is continuing in the areas of nanoporous low-k films, metal and ceramic thin films, and UV-curable materials. In addition, the method shows promise for the study of such issues as aging effects, creep, stress-relaxation, and viscoelasticity. There is also interest in exploiting these sinusoidal surfaces as tunable phase gratings and using these tunable textured surfaces to examine a number of scientific problems, such as patterned cell growth and anisotropic adhesion.   A detailed description of the experimental aspects of this technique can be found in C.M. Stafford, C. Harrison, 'SIEBIMM Instrumentation Documentation' available at www.nist.gov/combi.   For More Information on this Topic C.M. Stafford, A. Karim, E.J. Amis (Polymers Division, NIST), C. Harrison (Schlumberger-Doll Research)               NIST Material Science & Engineering Laboratory - Polymers Division