Session W25: Large-Strain Mechanical Properties of Polymer Networks

Bottlebrush elastomers as pressure sensitive adhesives

PSA materials possess unique rheological properties, exhibiting bonding-debonding dualism. During slow bonding, they display a low modulus that facilitates penetration of substrate roughness, whereas at faster debonding, the same material demonstrates a higher modulus, which is vital for generating high tack and preventing cohesive rupture at large deformations. To optimize the bonding-debonding interplay, PSAs are made by mixing polymer networks with large quantities of plasticizers and tackifiers that are prone to migration, resulting in ill-defined property variation over time and surface contamination. We have outlined a general framework for the design of additive-free adhesives, where properties are controlled by polymer brush architecture. Bottlebrushes suppress chain entanglements, enabling preparation of super-soft elastomers with a modulus as low as 100 Pa, which satisfies the Dahlquist criterion for spontaneous substrate wetting. Furthermore, by varying architectural parameters, we demonstrated unprecedented control over the work of adhesion spanning almost 5 orders of magnitude without changing chemical composition or using additives. This design platform was extended to the design of hot-melt pressure sensitive adhesives (HMPSAs), based on self-assembly of bottlebrush graft-copolymers, where side chains behave as softness, strength, and viscoelasticity mediators. These systems maintain moldability of conventional thermoplastic elastomers, while the brush architecture provides elastic softness at small deformations, intense strain-stiffening at large deformations, low melt viscosity, and a wide-ranged control of viscoelastic response.   

Interfacial Fatigue Fracture of Pressure Sensitive Adhesives

Pressure sensitive adhesives (PSAs) have been rapidly developed in recent decades and have found their broad application in various fields including construction, electronics, biomedical engineering, and daily life. It is foreseeable that PSAs will be required to sustain prolonged static and cyclic mechanical loads in emerging applications such as soft robotics and stretchable/flexible electronic devices. However, the interfacial fatigue fracture of PSAs has not been studied. In this presentation, we report interfacial fatigue fracture of PSAs, i.e., the growth of an interfacial crack between a PSA and another material under prolonged cyclic loads. We synthesize a PSA with a highly sticky surface and a well-controlled bulk mechanical hysteresis. We conduct cyclic peeling tests and measure the interfacial crack growth with cycles. We identify a steady state after thousands of peeling cycles, where the crack growth per cycle is nearly constant. We plot this steady-state crack growth per cycle as a function of the applied energy release rate and find out the threshold for cyclic peeling. In parallel, we conduct monotonic slow peeling experiments to plot the crack speed as a function of the applied energy release rate. We compare the two thresholds for monotonic peeling and cyclic peeling and discuss the possible fracture mechanisms at the molecular and microscopic levels based on the experimental data. It is hoped that this study will provide new fundamental knowledge for fracture mechanics of PSAs, as well as guidance for future tough and durable PSAs.   

Applying Time Temperature Superposition to Large Strain Shear data in Foldable Applications

Optically Clear Adhesives (OCA’s) are commonly used in electronics applications and serve important mechanical functions in addition to meeting optical needs. As technology progresses, electronics have moved from large stationary devices to smaller more mobile devices with curved or even foldable displays. This move has significantly increased the mechanical performance requirements from OCA’s. One such need is the ability to decouple stresses seen from large strains in a folded device. Current rheology test methods are available to measure the stress response of OCA’s at large strains. However, as OCA’s are viscoelastic materials, their performance is rate and temperature dependent, necessitating multiple rates and temperatures be run to assess the OCA’s performance in conditions relevant to the application. In this work we explore the use of the Time Temperature Superposition principle, commonly used in linear small strain experiments, on foldable OCA’s at large non-linear strain conditions in order to predict shear decoupling performance at multiple rates and temperatures.   

A universal strategy for decoupling stiffness and extensibility of polymer networks

Stiffness and extensibility are two fundamental mechanical properties of polymer networks. However, they are intrinsically coupled: stiffer networks are less extensible. Here, we design and synthesize networks by crosslinking foldable bottlebrush polymers, which feature a collapsed backbone grafted with many linear side chains. During stretching, the collapsed backbone unfolds, releasing stored length and enabling remarkable extensibility. Simultaneously, the network stiffness is determined by the side chains. We create polymer networks with nearly constant stiffness while increasing the tensile breaking strain by ~100 times from 0.23±0.04 to 23.62±3.64. Furthermore, we demonstrate that this concept applies to polymers of different chemical species. Our discovery provides a universal strategy for decoupling the stiffness and extensibility of polymer networks, opening an avenue for developing polymers with extraordinary mechanical properties.   

Effect of Loop and Dangling End Defects on Strain Stiffening in Double Network Elastomers

We investigate the impact of loop and dangling end defects on the strain stiffening behavior of regularly-crosslinked polymer networks using a double network strategy. Regularly-crosslinked networks are synthesized by crosslinking tetrafunctional poly(n-butyl acrylate) star polymers using a thiol-bromine click reaction. In a first series of samples, the polymer concentration is varied during synthesis to introduce loop defects. In a second series of samples, the polymer concentration is held constant, but some of the difunctional crosslinker is replaced with a monofunctional thiol to introduce dangling ends. We find that both types of defects result in softer networks with delayed strain stiffening. As in previous work, the moduli of the double networks are higher than expected from the moduli of the precursor networks from which they are prepared. Comparison of the loop and dangling end samples suggests that loops in the initial samples may become elastically-effective once entangled with the second network. This work provides new insights into how the interactions of the first and second networks affect the mechanical properties of double network elastomers, and the extent to which double network can (and cannot) be used to understand the behavior of the underlying first networks.   

Elastomer Mechanics of Cross-linked Ring-Linear Polymer Blends

Recent studies have demonstrated the distinctive physical properties of non-concatenated ring polymers, such as the softness and stretchability of cross-linked ring polymers. Nevertheless, synthesizing ring polymers with high purity has remained demanding in polymer chemistry. A blend of ring and linear polymers occurs more readily in experiments and brings blend composition as a new parameter for the designing of ring-polymer-based materials. Large-scale molecular simulations are performed to investigate the elastomer mechanics of cross-linked polymer blends. The tensile stress σ as a function of the stretch ratio λ in a uniaxial tensile test is calculated for varying volume fractions Φ_R of ring polymers. As Φ_R increases, both the network shear modulus G and the maximum stretch ratio λ_p before the network failure do not change much until a large Φ_R^c, beyond which G decreases while λ_p increases towards their respective values for the super-soft and super-stretchable pure ring elastomer. Meanwhile, the network strength as characterized by the peak stress σ_p depends on Φ_R non-monotonically, exhibiting a maximum around Φ_R^c before decreasing towards the value of the pure ring elastomer. Further analysis of the topology reveals that the threading of rings by linear chains plays a crucial role in the structure-property relationship for the dependencies of G, λ_p, and σ_p on Φ_R.   

Hyperelastic swelling of tough hydrogels

Hydrogels are biphasic, swollen polymer networks where elastic deformation is coupled to nanoscale fluid flow. As a consequence, hydrogels can withstand large strains and exhibit nonlinear, hyperelastic properties. For low-modulus hydrogel and semiflexible biopolymer networks, previous studies have shown that these materials universally contract when sheared on timescales much longer than the poroelastic relaxation timescale. Using rheological and tribological measurements, we find that tough polyacrylamide and polyacrylic acid hydrogels, with moduli of order ~ 10-100 kPa, exclusively exhibit dilatancy when sheared. The poroelastic relaxation process was examined using strain-controlled compression, indicating a diffusion constant of order 10^-9 m^2/s. At both short (minutes) and long (hours) timescales, an applied shear stress induced an increase in normal stress. Creep experiments revealed that tough hydrogels can ``remember'' the initial direction of applied shear, suggesting an evolution of the polymer network. Moreover, we show that this dilatant behavior manifests as swelling during tribological sliding, imbibing the hydrogel with fluid. We suggest that this inherent, hyperelastic dilantancy is an important feature in all tough hydrogels, and may explain rehydration and mechanical rejuvenation in biological tissues such as cartilage.   

Retraction Behavior of Stretchable Hydrogels

Elastic biopolymers such as resilin exhibit exceptional stretchability and resilience, which many species exploit in nature for mechanical energy storage to facilitate their movement. Inspired by that, we synthesized highly stretchable hydrogels with low dissipation and high resilience. When released from a highly stretched state, these gels retract rapidly. Capturing the retraction velocity and acceleration allows us to investigate the mechanical energy storage and release behavior of these hydrogels. Two different gel systems have been considered: UV-cured polyacrylamide (PAAm) hydrogels with different volume fractions and free radical polymerized hydrogels composed of acrylic acid (AAc), alkyl-acrylamide, such as methacrylamide (MAM), and poly(propylene glycol diacrylate) (PPGDA). The PAAm hydrogels mostly consist of hydrophilic components, whereas moderately hydrophobic PPGDA leads to hydrophobic domains in the second gel system. The effect of different microstructures for these two hydrogels on the tensile properties, retraction velocity, and acceleration will be presented. Our study broadens the understanding of the structure-property relationships for hydrogels at large strain, essential for their applications in numerous areas.   

Fracture of ductile plastics

Ductile plastics can be characterized in terms of their Young’s modulus, yield stress and specific work of fracture.  In presence of a notch, as often introduced in packaging, mechanical failure is localized due to the notch, and fracture necessarily involves yielding and plastic deformation.   This work examines whether ductile fracture of certain plastics (PC, PP, HIPS, HDPE) can still be characterized in terms of a framework like one that has been successfully applied to describe phenomenology of brittle fracture [1].  Specifically, we investigate whether some material specific quantity, e.g., toughness like, can be identified and how such a parameter compares with the traditional characterization based on evaluation of the essential work [2].   

Exploring the Effects of Nanoparticle Loading, Dispersion and Structure on the Stress Response of Elastomeric Nanocomposites

For over a century, nanoparticles have been utilized to mechanically reinforce elastomers. Evidence spanning several decades suggests that yielding of the percolating filler network, occurring at strains on the order of 10%, leads to softening, a phenomenon known as the Payne effect. However, the microscopic origins of the dissipative nature and mechanical reinforcement of the elastomer beyond this point in the nonlinear deformation regime remain the subject of debate. Understanding these underlying mechanisms will be instrumental in designing new elastomers with tunable properties, as well as optimizing those currently in use.

In this study, we employ molecular dynamics simulations to determine the contributions of nanoparticulate structure, dispersion state, and interactions to the mechanical properties of filled elastomers. Specifically, we present results that probe local and global mechanical properties, providing insights into the precise spatiotemporal origins of reinforcement in the nonlinear regime. These findings offer valuable insights for the rational design and optimization of elastomeric nanocomposites with targeted mechanical properties.   

Effect of Hydrogen Bonds on Thermomechanical Properties of Polyamide Ionene

In this study, we use a combination of molecular dynamics simulations and experimental characterization to understand the chemical composition-structure-thermomechanical properties of polyamide ionene, a self-healing elastomer. We specifically examined the role of hydrogen bonds in thermomechanical properties and analyzed their competition with ionic interactions.   

Beyond linear response: Time-resolved rheology of interpenetrating biopolymer composites

The design of functional biological materials for use as three dimensional cellular scaffolds and bioprinting requires exquisite control of both structure and mechanical properties of multicomponent systems. In 3-D printing applications, functionality and microscale structure must be maintained in conditions where large strain deformations occur at all scales. In this talk, we will discuss the nonlinear mechanical response of a biologically derived interpenetrating network composed of gelatin, fibrin and a non-specific enzymatic crosslinker through the use of Large Amplitude Oscillatory Strain (LAOS) rheology. Using a time-resolved approach, the sequence of physical processes (SPP) framework, we quantify the relationship between the microscopic interplay of the constituents to determine the non-additive rheology of the composite when subjected to large deformations. Specifically, our results show that under fixed protein concentrations and polymerization conditions, the enzymatic crosslinking moieties are not equivalently distributed throughout the composite network compared to the single component systems. Additionally, we will show that our statistical analysis of the SPP results provides a generic and robust method for straightforward physical interpretation of LAOS rheology.