Direct Comparison of Three Buckling-Based Methods to Measure the Elastic Modulus of Nanobiocomposite Thin Films

Abstract

To engineer tunable thin film materials, accurate measurement of their mechanical properties is crucial.However, characterizing the elastic modulus with current methods is particularly challenging for sub-micrometer thick films and hygroscopic materials because they are highly sensitive to environmental conditions and most methods require free-standing films which are difficult to prepare. In this work, we directly compared three buckling-based methods to determine the elastic moduli of supported thin films: 1) biaxial thermal shrinking, 2) uniaxial thermal shrinking, and 3) the mechanically compressed, strain-induced elastic buckling instability for mechanical measurements (SIEBIMM) method. Nanobiocomposite model films composed of cellulose nanocrystals (CNCs) and polyethyleneimine (PEI) were assembled using layer-by-layer deposition to control composition and thickness. The three buckling-based methods yielded the same trends and comparable values for the elastic moduli of each CNC-PEI film composition (ranging from 15 -44 GPa, depending on film composition). This suggests that the methods are similarly effective for the quantification of thin film mechanical properties. Increasing the CNC content in the films statistically increased the modulus, however, increasing the PEI content did not lead to significant changes. The standard deviation of elastic moduli determined from SIEBIMM was 2-4 times larger than for thermal shrinking, likely due to extensive cracking and partial film delamination. In light of these results, biaxial thermal shrinking is recommended as the method of choice because it affords the simplest implementation and analysis and is the least sensitive to small deviations in the input parameter values, such as film thickness or substrate modulus. File list (3) download file view on ChemRxiv TStimpson_CNC-PEI-Buckling-Methods.pdf (1.52 MiB) download file view on ChemRxiv TStimpson_CNC-PEI-Buckling-Methods_SI.pdf (588.65 KiB) download file view on ChemRxiv wrinkleAnalysis_portable.m (109.02 KiB)

Thick Polyvinyl Alcohol Films Reinforced with Cellulose Nanocrystals for Coating Applications

“…It is worth noting that no previous work has reported such high Young’s moduli for CNC-reinforced PVOH films or coatingsvalues are usually in the range of several GPa. ,, However, the literature does not report mechanical properties of CNC-reinforced PVOH coatings measured by SIEBIMM, and as mentioned earlier, a comparison of absolute elastic moduli determined by different techniques on materials prepared in different ways is challenging. We have shown that a variety of film buckling/structuring methods all give similar moduli and that, for example, the values are similar to those obtained from AFM modulus mapping . Furthermore, for PVOH-based materials, characterization is often performed on thicker (freestanding) samples using conventional methods.…”

“…These studies have demonstrated that the Young’s modulus of various polymer thin films can be improved by the addition of nanofillers, , inducing chemical cross-linking within the polymer matrix, , controlling the polymer composition, or even altering the architecture of the films . Additionally, SIEBIMM has been implemented to investigate the effect of nanofiller loading − and film deposition technique on the mechanical properties of polymer thin films. However, all of these studies were carried out on polymeric thin films with thicknesses under 400 nm, as opposed to the micrometer thick coatings demonstrated here …”

Graft-Then-Shrink: Simultaneous Generation of Antifouling Polymeric Interfaces and Localized Surface Plasmon Resonance Biosensors

“…Conversely, SEM characterization of Graft-then-Shrink dry surfaces with 25 and 60 kDa PCB showed increased heterogeneity in Au layer wrinkled pattern sizes with islands of large micrometer-scale wrinkles surrounded by nanoscale wrinkling (Figure e,f). These unique wrinkled Au structures may indicate increased film stiffness, as the modulus of the rigid skin material determines the wrinkle size, , which may be due to interchain polymer interactions during the shrinking process. No change in Au wrinkling was observed on the 10 kDa PCB Graft-then-Shrink discs, indicating a reduced impact from polymer interchain interactions on stiffness due to the polymer’s smaller MW.…”

Efficient Multi-Material Structured Thin Film Transfer to Elastomers for Stretchable Electronic Devices

“…The addition of the photoresist layer increased the wrinkle size compared to wrinkles formed on 20 nm-thick gold films deposited directly onto the PS substrate. This is because the wavelength of the wrinkles is proportional to the combined thickness of the stiff film and the sacrificial layer, and because the sacrificial layer can soften and flow during the shrinking process [11,16,17]. Shrunken electrode with no sacrificial layer on PS substrate.…”