Nanomechanical viscoelastic characterization of rubber with atomic force microscopy (AFM)

Poss, Matthew Michael

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https://hdl.handle.net/2142/115818 Copy

Description

Title

Nanomechanical viscoelastic characterization of rubber with atomic force microscopy (AFM)

Author(s)

Poss, Matthew Michael

Issue Date

2022-04-21

Director of Research (if dissertation) or Advisor (if thesis)

Tawfick, Sameh

Doctoral Committee Chair(s)

Tawfick, Sameh

Committee Member(s)

• Jasiuk, Iwona
• Chasiotis, Ioannis
• Hutchens, Shelby

Department of Study

Mechanical Sci & Engineering

Discipline

Mechanical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Atomic Force Microscopy
• AFM
• Nanomechanics
• Natural Rubber
• Fast Force Mapping
• FFM
• Amplitude-Modulation Frequency-Modulation
• AM-FM
• Force Curve
• Force-Indentation Curve
• Force-Distance Curve
• Adhesive Viscoelastic Contact
• Adhesive Viscoelastic Nanomechanics
• Experimental Mechanics
• Experimental Nanomechanics
• Ting
• JKR
• DMT
• Quasi-static AFM
• Sub-resonant AFM
• PeakForce
• Force Map

Abstract

In elastomeric materials, such as rubbers and other soft-hard nanocomposites, far field stresses generate highly heterogeneous deformations at the nanoscale. Interfacial strains around particles, strain distributions within interphase regions, and other molecular-scale mechanisms such as strain-induced crystallization are critically important to a material’s fracture behavior. Due it its ability to provide highly sensitive mechanical measurements of nonconductive substances at nanometer resolutions, atomic force microscopy (AFM) is particularly suited for the nanoscopic analysis of soft materials. The longstanding goal of rubber research is the a priori prediction of the mechanical behavior of complex rubber formulations. Existing analysis techniques, such as dynamic mechanical analysis (DMA), fatigue-crack growth, and tearing energy tests, are limited in their ability to relate macroscale properties, including fracture behavior, to molecular-scale interactions. By providing highly sensitive mechanical measurements at nanometer resolutions, nanomechanical AFM promises to revolutionize existing material models and catalyze new predictive capabilities. The first contribution of this work was to develop a methodology to mechanically probe local changes in rubber surface stiffness at the highest possible resolutions. Toward the goal of quantitative nanomechanical measurements, a viscoelastic contact model with ad hoc adhesion was proposed. A workflow for the identification of appropriate model parameters was developed. The model was verified by varying imaging parameters and observing changes in the resulting moduli. Sources of error were analyzed to understand the sensitivity and robustness of measurements. The developed methodology permits viscoelastic characterization of rubbers at nanometer resolutions. Future applications of this technique will provide critical mechanical information that will advance the scientific understanding of soft material fracture. In the first thrust of this project, a method was developed to automatically fit an ad hoc adhesive viscoelastic contact model to force-indentation curves collected with an atomic force microscope. The automated method featured a reliable snap-on location routine, an adaptation of Ting’s viscoelastic contact model with ad hoc adhesive corrections, numerous options for the specification of a stress-relaxation function, and a method for weight-averaging the obtained viscoelastic stress-relaxation functions for comparison with elastic modulus measurements. This method enabled the successful acquisition of mechanical measurements from noisy, adhesive, viscoelastic force-indentation curves. To evaluate both the quality of the fitting procedure and the validity of various model specifications, obtained viscoelastic modulus functions were compared to macroscopic dynamic mechanical measurements extended to the appropriate timescale via time-temperature superposition. Nanomechanical and macroscopic measurements of a natural-rubber sample differed by 10% to 70%, evidencing order-of-magnitude agreement. Possible causes of this disagreement were examined. To simplify comparisons and ensure accurate moduli, the most appropriate set of model parameters was identified: a spherical tip, a power-function modulus, and JKR adhesion. Moduli obtained in air and aqueous environments showed good agreement, evidencing the functionality of the adhesive component of the ad hoc contact model. Moduli obtained at different cantilever z-rates mirrored master-curve trends, evidencing the functionality of the viscoelastic component of the ad hoc model. A rate-dependent work of tip-sample separation indicated the necessity of viable models of adhesive viscoelastic contact. A theoretical analysis of under-tip stresses established the near impossibility of linear viscoelastic indentations with commercially available tips of nanometer dimension. This analysis also highlighted the dominance of adhesive deformations and demonstrated the sensitivity of strains, but not stresses, to small variations in Poisson's ratio. Numerous sources of error were identified and quantified throughout the entirety of the investigation. Experimental corrections and future investigations were proposed. A tensile stretching apparatus was designed and constructed to enable nanomechanical characterization of soft materials at the large strains necessary for fracture mechanics investigations. The tensile apparatus was specifically designed to fit in the tight confines of Asylum Research’s Cypher atomic force microscope with alternative mounts for a commercial focused ion beam (FIB) system and other scanning electron microscopes (SEM). Numerous fixtures and cases accompany the stretcher to facilitate sample mounting, ensure safe and secure transport, and assist with apparatus assembly. This thesis details the design of the tensile device and its associated suite of tools and fixtures. A full set of technical drawings is provided in the appendix. Several examples of the high-strain and high-resolution imaging capabilities are provided. Toward the long-term fracture mechanics objectives of this work, the stretcher was used to stretch filled rubber samples in an AFM to strains of up to 1000%. Several nanomechanical AFM techniques were used to qualitatively probe a variety of filled and blended rubber samples with and without applied strain. Primarily, the bimodal amplitude-modulated frequency-modulated (AM-FM) and quasi-static fast force-mapping (FFM) techniques were used to obtain images where the apparent contrast correlated with the rubber’s local mechanical behavior. Differentiation of mechanically similar rubber phases was achieved. Optimized bimodal imaging differentiated hard silica and soft carbon black particles embedded in a rubber matrix. Ripple features, believed to be strain-induced crystallization, were observed at very high strains. The demonstrated sensitivity of these advanced AFM techniques coupled with the large strains enabled by the tensile device permit mechanical characterization of important fracture phenomena at relevant strains. While the images obtained are qualitative in nature, in future work they can be used for quantitative spatial nanomechanical analysis. In summary, accurate mechanical maps of filled and blended rubbers, obtained at numerous strains, will greatly advance the science of soft material fracture. This thesis documents a method for the nanomechanical viscoelastic characterization of soft, adhesive, dissipative materials: the method's development, validation, optimization, uncertainty, and suggestions for future improvements. Spanning engineering polymers, composites, and cellular biology, the potential scientific impact of such capabilities cannot be understated. Fields such as mechanobiology would benefit from the elucidation of vital nanoscopic structure-property relationships. By revealing untold mysteries of rubber behavior, such measurements would catalyze new predictive capabilities and accelerate the virtual design and optimization of polymer formulations.

Graduation Semester

2022-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Matthew Michael Poss

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