Nanoscale rheology: dynamic mechanical analysis over a broad and continuous frequency range using photothermal actuation atomic force microscopy

Polymeric materials are widely used in industries ranging from automotive to biomedical. Their mechanical properties play a crucial role in their application and function and arise from the nanoscale structures and interactions of their constitutive polymer molecules. Polymeric materials behave viscoelastically, i.e., their mechanical responses depend on the time scale of the measurements; quantifying these time-dependent rheological properties at the nanoscale is relevant to develop, for example, accurate models and simulations of those materials, which are needed for advanced industrial applications. In this paper, an atomic force microscopy (AFM) method based on the photothermal actuation of an AFM cantilever is developed to quantify the nanoscale loss tangent, storage modulus, and loss modulus of polymeric materials. The method is then validated on styrene–butadiene rubber (SBR), demonstrating the method’s ability to quantify nanoscale viscoelasticity over a continuous frequency range up to 5 orders of magnitude (0.2–20,200 Hz). Furthermore, this method is combined with AFM viscoelastic mapping obtained with amplitude modulation–frequency modulation (AM–FM) AFM, enabling the extension of viscoelastic quantification over an even broader frequency range and demonstrating that the novel technique synergizes with preexisting AFM techniques for quantitative measurement of viscoelastic properties. The method presented here introduces a way to characterize the viscoelasticity of polymeric materials and soft and biological matter in general at the nanoscale for any application

Atomic force microscope-based methods for the nano-mechanical characterisation of hydrogels and other viscoelastic polymeric materials for biomedical applications

Hydrogels, porous hydrated polymeric materials, are being increasingly used in biomedical applications. The mechanical properties of hydrogels play an important role in the success of many of these applications (e.g. tissue engineering and drug delivery), and can be modulated by external stimuli and/or by changing hydrogel structure and composition. Therefore, it is important to measure and understand these mechanical properties at cellular length scales (nano/micro-scale) to improve the efficacy of hydrogels in biomedical applications. Hydrogels are often studied in terms of their elasticity. In this thesis, elastic properties of hydrogels were studied in terms of their elasticity via Atomic Force Microscopy (AFM) quasi-static micro-indentations. It is important to consider that hydrogels are not just elastic, but exhibit viscoelasticity, a time-dependent mechanical behaviour typical of hydrogels and polymeric materials in general. In this thesis, a novel AFM-based technique was developed to measure nano/micro-scale viscoelasticity. First, changes in the local elastic properties of chitosan (a biopolymer) hydrogels in response to an external magnetic field were measured, demonstrating magneto-mechanical coupling. Second, this coupling was further explored by incorporating magnetic nanowires into the hydrogels. Both experiments suggest this coupling could be exploited to control hydrogel mechanics. Third, elasticity of hydrogels based on fullerenes was measured, and depended on the size of the fullerene used. Hydrogel elasticity in all three experiments matched that of biological tissues, suggesting biomechanical compatibility. The possibility of controlling hydrogel properties either by external stimuli or by composition, opens the door for all three of these materials to be used for therapeutic applications. Lastly, a novel technique based on photothermal excitation of the AFM cantilever was developed and proved to quantify viscoelasticity of polymeric materials over a frequency range up to five orders of magnitude (0.2 Hz to 20200 Hz), in good agreement with macroscopic data, revealing a promising tool for the viscoelastic characterisation of hydrogels