Session N17: Structure, Dynamics, and Mechanics of Polymer Networks II

Characterizing failure behavior of ductile glassy polymers

To improve the mechanical characteristics of existing and upcoming polymers, particularly sustainable polymers, we need to understand how polymers resist crack propagation.  In contrast to brittle polymers where fracture mechanics has been routinely applied to gather phenomenology, a subject addressed elsewhere by our coworker (C. Gupta), ductile polymers are more challenging to rank.   While work fracture (the area under the stress-strain curve) is effective to quantify toughness, it does not indicate how a ductile polymer deforms in presence of a crack, e.g., single-edge notch.  In this work, we apply concepts familiar fracture mechanics, e.g., stress intensification ahead of crack, and show how ductile glassy polymers shows brittle response when the combination of nominal load and crack size defines a threshold toughness.   This condition is different from one under which they respond in a ductile manner in presence of smaller cracks.  We explore this interesting phenomenon to provide new insight into what a fracture mechanical and chain-network picture of polymer deformation can provide in terms of prediction and explanation.   We will explore a potential correlation between this newly defined toughness and work of fracture.   

Fracture behavior of elastomers: case of EPDM

Brittle fracture of elastomers has been treated with the paradigm of fracture mechanics.  For example, Rivlin and Thomas proposed a formula to evaluate the critical energy release rate G_c for pure shear to understand fracture of crosslinked rubber polymers.  Unlike the case of brittle plastics, the fracture mechanics paradigm appears more successful because Lake-Thomas model provides an estimate of G_c comparable to experimental observation.  Nevertheless, inspired by our parallel investigation of plastics, we have pursued an alternative approach to fracture of elastomers. In this presentation, we show, based on ethylene-propylene diene monomer (EPDM) rubbers (acquired from Lion Elastomers), how the new approach compares with the Rivlin-Thomas method and indicate that a different framework may offer more effective instructions on how to improve mechanical characteristics of polymers.   

Dynamic Properties of Filled Elastomers

Crosslinked rubber materials reinforced with nanofillers are important to many technologies and have a broad impact on the economy, climate and environmental sustainability, the most obvious being energy efficiency of tire technologies. Much of the work in this area is empirical and hence it has been difficult to a priori produce materials with tailored properties. Previous work in the group on polymer nanocomposite melts have shown that tuning parameters such as grafting density and the ratio of matrix to graft chain length results in various dispersion states, which in turn have a direct correlation with reinforcement. We extended the study to elastomers where we studied different morphologies. Linear tensile tests for the crosslinked state reveal that sparsely grafted nanoparticles with more particle-particle contact gives a significantly higher reinforcement whereas non-linear properties show very little dependence on the NP dispersion state. Proton MQ NMR spectroscopy method is used to measure the crosslink density of the composites. With an ultimate goal in mind to understand the reinforcement and hysteresis in rubber tires, we further aim to understand the conditions responsible behind it through the study of Payne effect.   

Spatially-resolved Dynamics of Poly Vinyl Alcohol Gels.

Hydrogels are cross-linked polymers held together in a network by covalent bonds and swollen in water. The large volume fraction of solvent gives these materials unique physical properties, which makes them ideal for applications like absorbents, drug delivery, and tissue engineering. Studying the dynamics of the hydrogels is necessary to understand and engineer hydrogels with respect to each application. While much research effort has focused on understanding gel dynamics in the bulk, very little is understood about the effect of directional stresses, and how the networks respond locally. Here, we stretch poly(vinyl alcohol) gels and study the resulting dynamics parallel and perpendicular to the direction of deformation using dynamic small angle light scattering. We find that the rate of elastic motion increases in the direction of stretching and decreases in the perpendicular direction. This study will help understand how gels perform locally under stress.   

Dilute surfactant induced swelling and dissolution of hydrophobically crosslinked hydrogels

Non-covalently crosslinked hydrogels can exhibit toughness and mechanical adaptability associated with biological tissues, but interactions with molecules in the environment can significantly alter their properties. Here,the swelling and rheological properties of hydrogel films crosslinked by hydrophobic associations based on a random copolymer of poly(N,N-dimethylacryalmide-co-2-(N-ethylperfluorooctanesulfonamido)ethyl acrylate (DMA-FOSA) from dilute sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) solution are quantified with quartz crystal microbalance with dissipation (QCM-D). At concentrations much less than the CMC, the DMA-FOSA hydrogel swells signficiantly with a decrease in the elastic modulus and increased tan δ. At 0.1x CMC for SDS and 0.5x CMC for CTAB partial dissolution occurs, but the kinetics are slow with limited swelling occuring over the first 30 min and no signatures of dissolution until after 1000 min of exposure in some cases. The long incubation times, order of hours, associated with dissolution of thin hydrogel films from introduction of two simple ionic surfactants may challenge identification of long-term instabilities with physically crosslinked hydrogels from exposure to complex aqueous environments.    

Design and characterization of a programmable self regulation hybrid material system: wrinkled hydrogels and plasmonic nanoparticl lattices

In this research, a self-regulating system of wrinkle-patterned hydrogel with tunable feedback cycle timescales will be presented where wrinkled pattern senses humidity variation and plasmonic nanoparticle (PNP) lattice regulates through the plasmonic heating.  In the feedback loop, moisture absorption makes the hydrogel’s wrinkled surface to flatten thus, less light scattered and more transmitted through the flattened surface of the system to the plasmonic PNP lattice at the bottom layer. Due to light absorption, PNP lattice generates heating that makes the hydrogel dries out and ultimately leading to the formation of wrinkles at the surface. Thus, less light transmitted and more scattered from the wrinkled surface of the system. Therefore, cooling and moisture absorption flatten wrinkle patterns thus initial configuration is restored. Time-dependent finite element analysis reveals thermal and mechanical mechanisms of wrinkle formation. Simulations were consistent with the experiments in terms of wrinkle characteristics, timescale and temperature range. Thermomechanical modeling could aid engineering the time-scale of regulatory systems by tuning the system parameters for the design of future responsive materials and smart systems.   

Contraction response of a polyelectrolyte hydrogel structure to a spatially non-uniform electric field

We study the shape fluctuations of a semi-infinite polyelectrolyte hydrogel slab to transient and spatially nonuniform sinusoidal electric fields by using extensive coarse-grained molecular dynamics simulations. Our simulations show that if the electric field is applied on a small volumetric section of the hydrogel slab, the slab contracts reversibly and uniaxially in the direction perpendicular to the electric field. The hydrogel contracts almost half of its field-free initial length before retracting its original shape, and eventually its length fluctuations decay similar to an under-damped oscillator. The contraction speed and efficiency can be controlled by tuning the frequency and amplitude of the external electric field. No contraction is observed if the field is applied uniformly on the whole hydrogel structure. The contraction response of the hydrogel is robust against various backbone charge fractions and dielectric constants but weakens at high salt concentrations. Our results demonstrate the effectiveness of spatially nonuniform electric fields to manipulate hydrogels mechanically.   

Reversing fatigue damage in self- healing vitrimer and its composites

Thermosetting polymeric composites which are ubiquitous in structural applications due to their superior strength and stiffness compared to their thermoplastic counterparts. These superior properties are principally imparted by dense and permanent crosslinking present at the molecular network level. However, as thermosets are subjected to fatigue loading, they accumulate damage in terms of ruptured crosslinks which eventually result in catastrophic failure. Nanocomposites fabricated by the addition of nanofillers create interactions with incipient cracks and prolong the fatigue life¹. But since the crosslinking rupturing is irreversible process, the cracks eventually propagate to a critical size which leads to failure. Thus, the same crosslinking which imparts strength and stiffness to thermosets is responsible for the accumulation of irreversible fatigue damage. If crosslinks in the thermosets are made reversible under application of external stimulation, thermosets can retain their superior properties while potentially possessing the ability of reversing the fatigue damage. Liebler et al proposed an epoxy-based network called vitrimer which can reform crosslinks at elevated temperature². Vitrimers have attracted wide attention in the past decade, however their fatigue performance remains widely unexplored.

We report a vitrimeric system which is prepared using epoxy resin and adipic acid with a suitable catalyst. The system has mechanical properties comparable to conventional epoxies and is defined by two characteristic temperatures namely, the glass transition temperature (T_g) and topology freezing temperature (T_v). We demonstrate that the vitrimer can be periodically healed by heating at elevated temperature. Further, we also report a carbon fiber vitrimer composite which can be similarly healed by periodic healing. Such composites can pave way for next generation of structural components which are recyclable and have much longer service lifespans.   

Using Thiol-Thioester Bond Exchange to Elucidate the Interplay Between Crystallinity and Dynamic Bond Exchange in Semi-Crystalline Polymer Networks

While crosslinks impart mechanical strength and solvent resistance to polymer networks, they preclude reprocessing and recycling. One way to make such networks reprocessable – and thus more sustainable – is through the incorporation of dynamic covalent bonds (DCBs) to form covalent adaptable networks (CANS). In CANS, bond exchange reactions in response to specific stimuli enable plasticity, allowing for materials to be reconfigured over their lifetime. In recent years, significant efforts have been made to incorporate various species of DCBs within polymeric materials of various backbones, architectures, and morphology to elucidate the effects of bond exchange on rheological properties. However, much less work has been undertaken to understand how bond exchange processes influence the self-assembly and morphology of polymers which are critical to realizing high performance reprocessable materials. In this work, we explore the effect of bond exchange on the crystallization of semi-crystalline polymer networks. By exploiting control over thiol-thioester exchange kinetics using different catalysts, we systematically probe how bond exchange kinetics in chemically-identical systems influences crystallization kinetics and crystalline assembly. We find that as bond exchange kinetics increase, the melting temperature of semi-crystalline networks is systematically depressed and crystallization kinetics are drastically slowed. Coupled with microscopy and x-ray scattering, we present a rationale for these observations and how dynamic bonds can be harnessed in polymer networks to control both rheology and structure. Finally, we show how semi-crystallinity and DCBs can be combined toward high-performance recyclable materials by applying these systems to additive manufacturing.   

Shear Banding in Entangled Polymers: Stress Plateau, Banding Location, and Lever Rule

Using molecular dynamics simulation, we study shear banding of entangled polymer melts under steady shear. The steady shear stress vs. shear rate curve exhibits a plateau spanning nearly two decades of shear rates in which shear banding is observed, and the steady shear stress remains unchanged after switching the shear rates halfway in the range of shear rates within the plateau region. In addition, we find strong correlation in the location of the shear bands between different shear rates starting from the same microstate configurations at equilibrium, which suggests the importance of the inherent structural heterogeneity in the entangled polymer network for shear banding. Furthermore, for the steady shear bands persisting to the longest simulated time of 9 τ_d0 (disengagement time), the shear rate in the slow band and the relative proportion of the bands do not change very much with the increase of imposed shear rate, but the shear rate in the fast band increases approximately in proportion to the imposed shear rates, in contradiction to the lever rule.   

Tuning mechanical properties of a stretchable hydrogel system

Hydrogels mimicking the mechanical responses of biomaterials can lead to new applications. Resilin is a bioelastomer found in the tissues of many species, allowing them to perform power amplified activities, including hopping of a froghopper and appendage strike of a mantis shrimp. Developing a synthetic material with resilin-like properties requires high stretchability to store elastic energy, low hysteresis for high energy conversion, and high retraction velocity when released from a stretched state for power amplification. Here, we present a hydrogel system capable of mimicking some of these properties. These gels are synthesized through a free-radical reaction of acrylic acid, methacrylamide, and poly(propylene glycol) diacrylate. The gel structures have similarities to resilin as the hydrophobic aggregates are connected through hydrophilic polymer chains. By varying the chemical composition, the low-strain modulus of these gels can be tuned from 15 to150 kPa, and the stretchability from 2 to 10 times the original length. These gels retract rapidly to their original length when released from a stretched state. The stability of these gels at high salinity aqueous solutions opens up their applications in developing undersea soft-robotics and engineered devices.   

Microstructure and Dynamics of Nanocellulose Network: Insights into the Deformational Behaviors

Cellulose nanocrystals (CNCs) thin films draw considerable interest in engineering and technological applications due to their excellent mechanical and physical properties associated with dynamic and microstructural features. Here, we employ coarse-grained molecular dynamics (CG-MD) simulations to investigate how the dynamics and microstructure change in the CNC films under tensile deformation. Our results show that the Young's modulus can be quantitatively predicted by power-law scaling relationship with initial packing density, where higher density leads to an increase in both modulus and strength. By evaluating the molecular local stiffness during the tensile process, our findings show that CNC film with higher density exhibits a higher degree of dynamic heterogeneity, which is greatly reduced under deformation. Our results further demonstrate that randomly oriented CNCs tend to be more aligned with the tensile direction associated with higher free volume and porosity during the deformation; however, the dynamics of CNC are more associated with the degree of local packing and density rather than the CNC orientation. Our study provides fundamental insights into deformational mechanisms of CNC films at a molecular level, aiding in the tailored design of cellulose-based materials for their mechanical performance.   

Theory and Simulation for elasticity of slide-ring gels

Polymer networks with crosslinks slidable along chains was first used to model entangled polymer networks, and later achieved chemically by connecting slide-rings on the chains. This kind of networks are called slide-ring gels. These gels are shown to exhibit non-Hookean elasticity and higher toughness and extensibility without hysteresis. The most striking difference between these networks from conventional polymer networks is that strand sections can exchange monomers and thus homogenize the tension. We develop single-chain analytical models designed to quantitatively describe the sliding effects on the elasticity of slide-ring gels and test these models by simulations. We find that slide-ring gels are getting softer under both uniaxial deformation and swelling. The fraction of chains with high tension is significantly reduced in comparison with crosslinked network. The tension homoginezation and much smaller fraction of strands with high tension delays the fracture to much larger deformation and leads to much higher toughness of slide-ring gels in comparison with conventional cross-linked gels.