Session Q26: Fracture, Yielding, and Failure of Soft Materials

Understanding the role of crosslink density and linear viscoelasticity on the shear failure of PSAs

Pressure-sensitive-adhesives (PSAs) are ubiquitous in electronic, automobile, packaging, and biomedical applications due to their ability to stick to numerous surfaces without undergoing chemical reactions. These materials date back to the 1850s but their resistance to static shear loads remains challenging to predict from molecular design. In this talk, we will discuss the role of crosslink density and linear viscoelasticity on the shear failure of PSAs. The key result is that crosslinking acrylic PSAs with a metal chelate like Al(acac)₃ leads to notable stress concentrations ahead of the peel front, as well as a transition from bulk to interfacial crack growth. The shear stress distributions, as evaluated by means of a linearly viscoelastic shear lag model, suggest that this transition is related to the evolution of ratio of the load transfer length to the bond length as dictated by the mechanical properties of the backing and adhesive layers, and the tape geometry.     

Impact of Topological Defects on Fracture and Fatigue of Polymer Networks

Polymer networks are pervasive in biological organisms and engineering materials. Topological defects such as cyclic loops and dangling chains are ubiquitous in polymer networks. While fracture is a dominant mechanism for mechanical failures of polymer networks, existing models for fracture of polymer networks neglect the presence of topological defects. Here, we report a defect-network fracture model that accounts for the impact of various types of topological defects on fracture of polymer networks. We show that the fracture energy of polymer networks should account for the energy from multiple layers of polymer chains adjacent to the crack. We further show that the presence of topological defects tends to toughen a polymer network by increasing the effective chain length, yet to weaken the polymer network by introducing inactive polymer chains. Such competing effects can either increase or decrease the overall intrinsic fracture energy of the polymer network, depending on the types and densities of topological defects. This model provides theoretical explanations for the experimental data on the intrinsic fracture energy of polymer networks with various types and densities of topological defects. We further synthesize end-linked star-shaped polymer networks with controlled dangling-chain defects, experimentally investigating their impacts on fracture and fatigue of polymer networks through strain-induced crystallization. This work not only sheds light on the critical role of topological defects in toughening mechanism adopted in load-bearing biological tissues but also facilitate healthcare and sustainability challenges associated with the robustness and longevity of soft materials.   

Essence of elastomeric fracture

Gum rubbers have been found to be stronger and tougher (i.e., more resistant to crack propagation) at lower temperature since nearly seventy years ago.  According to the standard and prevailing rationale this rises from a rise of energy dissipation with decreasing temperature (increasing internal viscosity).  Since our recent spatial-temporal resolved polarized optical microscopic observations reveal^1,2that (a) tensile strenght reflects the inherent strength and (b) toughness is proportional to inherent strength, it is clear to us that the temperature dependence of strenght and toughness has the same physical origin.  Bond dissociation kinetics are slower at lower temperature, permitting greater degree of rubber strength to high critical load and tensile stress³.  In this study we further examine the temperature and rate dependencies by clarifying the role of chain dynamics in various gum rubbers prepared by crosslinking of pre-polymer of high molecular weight.   

Large strain micromechanics of thermoplastic elastomers with random microstructures

Thermoplastic polyurethanes (TPU) are block copolymeric materials composed of plastomeric "hard" and elastomeric "soft" domains, by which they exhibit highly resilient yet dissipative large deformation features depending on volume fractions and microstructures of the two distinct domains. In this work, we present a new methodology to address the microscopic deformation mechanisms in TPU materials with highly disordered microstructures. We propose new micromechanical models for randomly dispersed (or occluded) as well as randomly continuous hard domains, each within a continuous soft structure as widely found in representative TPU materials over a wide range of volume fractions of hard and soft components. The micromechanical modeling results are compared to experimental data on the macroscopic large strain behaviors reported in our prior work (Cho et al., Polymer, 128 (16), 2017; https://doi.org/10.1016/j.polymer.2017.08.065). We explore the role of the dispersed vs. continuous nature of the geometric and topological features of the random microstructures on shape recovery and energy dissipation at the microstructural level in this important class of phase-separated copolymeric materials.   

Unveiling the fracture mechanism for entangled polymer melts under extensional flow with molecular dynamics simulations

Entangled polymer melts exhibit fracture behavior under strong extensional flows. The rapid deformation rate at high Weissenburg number surpasses the reciprocal relaxation time of the polymer chains leading to strongly deformed chain conformations. Consequently, stress accumulates within the melts, and ultimately results in cavitation. We conducted molecular dynamics simulations of polymer melts under extensional flow using the Kremer-Grest model incorporating a breakable bond potential. The simulation results demonstrate the possibility of cavitation without rupture of bonds and melt fracture by chain pullout, suggesting that the critical stress for fracture can be related to cavitation rather than bond ruptures. Bond scission after cavitation was observed at higher extension rates due to fracture of the entanglement network. The distribution of broken bond positions is peaked at the location of higher tension in the middle of the chains.   

Molecular Simulations of Polymer Thin Film Necking: Ductility from Entanglements and Plane Stress Condition

Recent advances in the nanotechnology of fabricating and characterizing polymer thin films have enriched conventional thermoplastic mechanics. One intriguing observation is a polymer that is brittle in the bulk state, such as polystyrene, exhibits ductility in the thin film state via the shear deformation resulting from necking. To reveal the microscopic picture of necking in a glassy polymer thin film, large-scale coarse-grained molecular simulations are performed. The simulations demonstrate that stable necking relies on an entanglement network that prevents the catastrophic chain pullout. The neck propagates under a constant tensile force, for which strain hardening in the necked region results in higher local stress that compensates for the reduction in the thickness. The necked film is perforated with voids, exhibiting a morphology different from fibrils in crazing, which is a brittle deformation mode unique to glassy polymers. The replacement of crazing with necking and thereby an enhanced ductility is facilitated by the free boundary that promotes plane stress. Despite the critical role of entanglements, the width of the neck is much larger than the entanglement spacing. The Considère construction predicts well the onset of necking but not the draw ratio of necked polymers, where voids break down the conservation of volume. A simple geometric argument based on the extension of entanglement strands is able to relate the draw ratio in necking to that in crazing.   

Predicting failure locations in model end-linked polymer networks

The fracture of end-linked polymer networks and gels has a significant impact on the performance of these versatile and widely used materials, and a molecular-level understanding of the fracture process is crucial for the design of new materials. Network analysis techniques, especially geodesic edge betweenness centrality (GEBC), have been proven effective in failure locations across various network materials. In this talk, we introduce an approach combining coarse-grained molecular dynamics simulations and network analysis techniques to investigate the effectiveness of GEBC and polymer strand orientation in predicting failure locations in model end-linked polymer networks. We demonstrate that polymer strands with fewer topological defects in their local surroundings, higher GEBC values compared to the system average, and greater alignment to the deformation axis are more prone to breaking under uniaxial tensile deformation. Our results can be used to further refine the description of the processes at play during the failure of polymer networks and provide valuable insights into the inverse design of network materials with desired fracture properties.   

Viscoelasticity and the Persson-Brener Model

Polymeric coatings are widely produced by industry and used to create barriers between structures and the elements. Recently, effort has focused on soft coatings that prevent ice and other unwanted foulants from adhering to surfaces. While low surface energies of soft materials, such as polydimethylsiloxanes (PDMS), promise easy removal of attached foulants and ice, the simple physical limit for adhesion strength based on surface energy has not been achieved in practice. It is largely believed that the discrepancy is due to viscoelastic losses in the soft coating materials. To better understand the viscoelastic losses in soft adherent PDMS materials, we perform JKR adhesion experiments on several elastomers, at different temperatures and over a set of speeds that spans several orders of magnitude. Each elastomer also undergoes DMA experiments to characterize the dynamic mechanical modulus from the glassy to rubbery regime. We demonstrate that the adhesion tests are qualitatively related to the dynamic moduli and use a more direct comparison (the Persson-Brener model of crack propagation [1]) to show a quantitative relationship between adhesion and dynamic moduli at low speeds.

 

[1] B. N. J. Persson and E. A. Brener, “Crack propagation in viscoelastic solids,” PHYSICAL REVIEW E 71, 036123 (2005).   

Stability of ductile fracture of plastics

Mechanical behavior of highly ductile plastics (PET, LLDPE) does not require fracture mechanics to describe unless a large notch is present.  We study characteristic responses of such plastics in terms of the nature of mechanical failure, e.g., whether test geometry affects the mode of macroscopic separation.  The question challenges the established notion that mechanical behavior is material specific, independent of specimen geometry except for the case of notch-brittle PC where a thick PC specimen with notch turns brittle.  We will explore the relationship between essential work [1] and specific work of fracture based on such super ductile plastics to expand beyond our knowledge [2] of brittle fracture in brittle glassy polymers.

1.  Ward, I.M. and J. Sweeney, Mechanical properties of solid polymers. 2012: John Wiley & Sons.

2.  Smith, T., et al., Toughness arising from inherent strength of polymers. Extreme Mechanics Letters, 2022. 56.   

Accounting for brittle yielding in soft materials

Many soft materials yield, changing their mechanical response from that of a soft solid to that of a viscous fluid, but how this transition occurs can vary significantly. Understanding the physics behind this transition is of great interest for the behavior of biological, environmental, and industrial materials. Some materials yield gradually, while others yield more abruptly and are referred to as being "brittle". The key rheo-physical signatures of brittle yielding include a stress overshoot in steady-shear-startup tests and a sharp increase in the loss modulus during oscillatory tests. We account for brittility with a small modification to our recently proposed continuum model for yield stress materials (Kamani et al., Phys. Rev. Lett. 126, (2021)). We account for brittility by modifying the contribution of the recoverable component to the total strain rate, which impacts the rate at which yielding occurs. The model results are successfully compared to results of different rheological protocols from a number of model yield stress fluids having different microstructures, indicating the generality of the approach. Our study shows that the brittility of soft materials can be described as a continuum, and plays a critical role in determining the rate of yielding transition.   

Characterization of Near-Wall Effects During the Puncture of Soft Solids

Performing conventional mechanical characterization techniques on soft materials can be challenging due to issues such as limited sample volumes and clamping difficulties. Puncture tests are a promising alternative technique for probing soft solids as they are an information-rich measurement with the potential to be performed in a high-throughput manner. Despite its promise, current experimental methods lack standardized protocols, and open questions remain about possible limitations. Addressing these shortcomings is vital to ensure consistent methodology, measurements, and interpretation across samples and labs. To fill this gap, we examine the role of finite sample dimensions (and by extension, volume) on measured forces to determine the sample geometry needed to perform and unambiguously interpret puncture tests. Through measurements of puncture on a well-characterized elastomer using systematically varied sample dimensions, we show that the apparent mechanical response of a material is in fact sensitive to near-wall effects.   

Polymers for Impact Mitigation: New Measurements Provide Insights into an Old Problem

Polymers are widely used in protective equipment or structures where the objective is to absorb or dissipate the energy from a mechanical impact.  It is generally understood there is a link between the relaxations in a glassy polymer and its corresponding toughness. Our understanding is that relaxations dissipate the energy imparted during impact and thereby enhance toughness.  Decades of research have focused on correlating the mechanical toughness of a polymer with the relaxation processes quantified by relatively slow techniques such as dynamic mechanical analysis, dielectric spectroscopy, or solid-state nuclear magnetic resonance.  However, there is a disconnect when it comes to understanding impact resistance at the strain rates of 10⁶ sec^-1 or higher that are relevant for ballistic impact events.  These time scales are typically several orders of magnitude faster than the time scales of the characterization techniques used by this community to quanitfy the relaxation processes. We revisit these correlations between toughness and polymer relaxations by using quasielastic neutron scattering (QENS) to quantify the collective excitations and molecular relaxations that occur on the time scale of pico- to nanoseconds, and then see how these polymer motions correlate with mechanical toughness. What emerges is a strong correlation between the ratio of the population of the fast relaxations (dissipative) to the fast collective (many atom) vibrations deep in the glassy state and the mechanical toughness.   

Regarding viscoelastic effects in elastomeric fracture

In our initial investigations of elastomeric fracture [1], we stayed in elastic limit where stress-strain curves overlap, independent of rate and temperature.  In this limit there is no correlation between temperature dependence of toughness and that of dynamic modulus, yet there is strong temperature and rate dependencies, which can only be explained by acknowledging bond dissociation as the controlling event in fracture [2].  In this study, we move into viscoelastic regime and investigate how our formulation of elastomeric fracture needs to be generalized.

[1] Fan, Z.; Wang, S.-Q. Resolving stress state at crack tip to elucidate nature of elastomeric fracture. Extreme Mechanics Letters 2023, 62, 101986.

[2] Wang, S.-Q.; Fan, Z. Investigating dependence of elastomeric fracture on temperature and rate. Rubber Chem. Technol. 2023, in press.