Graduation Year

2023

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Physics

Major Professor

Robert S Hoy, Ph.D.

Committee Member

Dario Arena, Ph.D.

Committee Member

Inna Ponomareva, Ph.D.

Committee Member

David Simmons, Ph.D.

Keywords

Cavitation, Crazing, Granular, Polymer glasses, Voids

Abstract

Soft matter, as a subfield of condensed matter physics, has gained lots of interest from the academic and industrial communities due to its complexity and intellectual challenges. Soft materials, such as liquids, polymers, and granular materials, are widely used in real-world applications like foams, paints, and lubricants. Furthermore, their cost-effective and unique features like extreme ductility also make soft materials suitable for many advanced applications such as bioimplantable devices.

In Chapter 2, we examine thermodynamic cavitation, a transition from liquid to gas that occurs when the ambient pressure drops below the liquid’s vapor pressure. Varying the interatomic pair potential strongly affects the structure and the macroscopic properties of liquids, and can be understood in terms of the energy landscape. To learn more about how the pair potential affects cavitation, we examine the Sastry transition, a cavitation transition that occurs in the energy landscape of model liquids. We find that Lennard-Jones as well as shorter- and longer-ranged pair potentials yield the following behavior: Low-temperature thermodynamically stable liquids have ρ < ρS, where ρS is the density at which the pressure PIS(ρ) within liquids’ inherent structures (IS) is minimal, except when the attractive forces are long-ranged. For moderate- and short-ranged attractions, stable liquids with ρ > ρS exist at higher temperatures; the temperature T∗ at which stable ρ > ρS liquids emerge is 0.84ε/kB for Lennard-Jones liquids where ε is the energy scale; T∗ decreases (increases) rapidly with increasing (decreasing) pair-interaction range. In particular, for short-ranged potentials, T∗ is above the critical temperature. All liquids’ inherent structures are isostructural (isomorphic) for densities below (above) the Sastry density ρS. We conclude that the barriers to cavitation in most simple liquids under ambient conditions for which significant cavitation is likely to occur are primarily vibrational-energetic and entropic rather than configurational-energetic in character. The most likely exceptions to this rule are liquids with long-ranged pair interactions, such as alkali metals.

Next, we study the character of glassy-polymeric cavitation in Chapter 3. Cavitation in glassy polymers can lead to either void growth and coalescence (i.e., brittle fracture) or “ductile” void growth. However, determining which type of cavitation will occur for a given sample and deformation protocol is quite difficult in general. We employ generic polymer glass model, contrasting results for flexible loosely entangled chains and semiflexible tightly entangled chains to discover the relation between cavitation and entanglement density. Strain-controlled deformation with a wide range of Poisson ratios ν reveal features that are not apparent from the uniaxial-deformation or uniaxial-stress protocols employed by the vast majority of previous studies. In particular, for 0.125 . ν . 0.45, we find that the semiflexible tightly entangled chains have a significantly lower void volume fraction (but far more voids) at the same strain and volumetric expansion ratio than their flexible loosely entangled counterparts. Voids in loosely entangled glasses coalesce more often and grow faster, whereas voids in tightly entangled glasses continue to nucleate well into the strain-hardening regime. In other words, cavitation that does not lead to crazing (and hence to brittle fracture) can be an effective means of energy dissipation during ductile deformation.

In Chapter 4, we focus on investigating the mechanical properties, specifically ductility, of model semiflexible conjugated polymers (SCP) glasses. SCP glasses are expected to be brittle because classical formulas for their craze extension ratio λcraze and fracture stretch λfrac predict that systems with Ne = C∞, where Ne is their entanglement length and C∞ is their constituent chains characteristic ratio, have λcraze = λfrac = 1 and hence cannot be deformed to large strains. We show that in fact such glasses can form stable crazes with λcraze N 1/4e ≃ C 1/4∞ , and that they fracture at λfrac = (3N1/2e −2) 1/2 ' (3C 1/2∞ −2) 1/2 . We argue that the classical formulas for λcraze and λfrac fail to describe SPGs’ mechanical response because they did not account for Kuhn segments’ ability to stretch during deformation.

Continuing the work presented in Chapter 4, in Chapter 5, we examine uniaxial-strain tensile deformation of SPGs with a wide range of chain stiffnesses. We show that SPGs with a wide range of Ne/C∞ can exhibit a ductile mechanical response, including stable craze drawing. Our results show that the novel equation we come up with in Chapter 4 successfully estimates the craze extension ratio for the entire range of stiffnesses. Our simulation results for λfrac also indicate the model discussed in Chapter 5 overpredicts data less than 10%, which is acceptable due to the bead-spring model’s finite-strength covalent bonds. Our study presents a novel approach to predicting deformation in polymer glasses: it trends entangled strands as extensible rather than rigid (as assumed by Kramer’s model). This holds true across a broad spectrum of polymer glasses, as evidenced in both experimental findings and simulation results.

In Chapter 6, we examine how the structure of marginally jammed granular systems of grains with varying degrees of friction and cohesion depends on the protocol with which these systems are prepared. We compare results for systems with four types of intergrain interactions: (1) no friction or cohesion, (2) friction but no cohesion, (3) cohesion but no friction, and (4) both cohesion and friction. The final packing fractions (φsettled) of systems prepared by beginning with a dilute state and then ramping the pressure under the time τramp to a fixed, small Pfinal range from .64 for system (1) to .35 for system (4). We find that while these φsettled are almost independent of the pressure ramp rate for systems (1-3), φJ in system (4) is φsettled decreases substantially with decreasing ramp rate. We introduce an inverse-logarithmic rate law to represent the packing fractions of frictional cohesive grains accurately. Our findings indicate that the synergistic combination of cohesion and realistic friction promotes the stabilization of structural voids during pressure-ramp compression. This stabilization process involves an ultraslow dynamic phenomenon. By examining this stabilization of structural voids, we develop a kinetic free-void-volume theory that elucidates the ultraslow settling kinetics of cohesive frictional grains, even when the ramp time (τramp) is significantly large. Importantly, this theory should also be applicable to predicting the settling kinetics of real powders, particularly in scenarios where strong intergrain cohesion and friction are present.

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