Session: 13-11-01: Friction, Fracture, and Damage I

Paper Number: 173724

Environmental Stress Cracking of Biodegradable Polymers 

Plastics have revolutionized modern life. Disposable plastic products, serving essential roles in food, medical and hygiene industries, have significantly improved the quality and longevity of human life. However, the production and waste of fossil-based plastics have created serious environmental problems. The impacts of plastic pollution to ecosystems are poorly reversible, while the accumulation is continuously growing. Alternatively, bio-derived and biodegradable polymers, such as polyhydroxyalkanoates (PHAs), offer a more promising solution. They not only lower the carbon emissions from petroleum extraction and refinement, but more importantly, they can be decomposed by the action of living organisms, forming a close-loop carbon cycle.

This work studies the degradation kinetics and environmental stress cracking (ESC) of PHAs. High-throughput notched creep testing was conducted for (PHA) films in enzyme-rich aqueous environments to monitor their heterogeneous crack propagation under a combined loading of environment and stress. Samples were submerged in an environmental chamber with marine water, subjected to deadweight loads at capacities much lower than the critical energy release rate, and examined periodically. At the same time, the mechanical and surface property evolution of un-notched PHAs were probed under the same loading condition. We find that while mechanical properties – including modulus, strength, and toughness – remained relatively stable over four days of aging, the contact angle measurements revealed a marked increase in surface wettability after just two days. Notably, ESC in creep-loaded samples also initiated after two days of exposure in marine water, indicating that surface chemical evolution plays a more critical role in ESC onset than bulk mechanical degradation in PHA. The enzymatic degradation of PHAs undergoes a heterogeneous kinetic process involving sequential steps of enzymatic adsorption and hydrolysis reaction. They are reflected in the distinct regimes of the nonlinear crack growth of the PHA films during ESC: (1) an initial period of crack stagnation associated with enzymatic adsorption which transforms the PHA surface from a hydrophobic state to a hydrophilic state, followed by (2) a quasi-steady crack growth beginning after ~40 hours of aging, driven by stress-assisted hydrolysis that initiates ESC propagation. Tests were repeated in soil water and deionized (DI) water environments. Soil water produced similar ESC behavior to marine water but exhibited greater variability in the duration of enzymatic adsorption. In contrast, DI water did not induce ESC in PHA over seven days, during which no significant changes in surface wettability or mechanical properties were observed. The heterogeneous kinetics of PHA degradation therefore provides an additional buffer to stress-cracking vulnerability in non-enzymatic environments. Our study offers key insights into the degradation and ESC behavior of biodegradable polymers, paving the way for their practical applications.

Presenting Author: Lihua Jin University of California, Los Angeles

Presenting Author Biography: Lihua Jin is an associate professor in the Department of Mechanical and Aerospace Engineering at the University of California, Los Angeles (UCLA). Before joining UCLA in 2016, she was a postdoctoral scholar at Stanford University. In 2014, she obtained her PhD degree in Engineering Sciences from Harvard University. Prior to that, she earned her Bachelor’s and Master’s degrees from Fudan University. Lihua conducts research on mechanics of soft materials, stimuli-responsive materials, instability and fracture, soft robotics, and biomechanics. She was the winner of the Haythornthwaite Research Initiative Grant, Extreme Mechanics Letters Young Investigator Award, Hellman Fellowship, NSF CAREER Award, ACS PMSE Early Investigator Award, Sia Nemat-Nasser Early Career Award, and SES Huajian Gao Young Investigator Medal.

Authors:

Jonathan Gray University of California, Los Angeles
Jasmine Chen University of California, Los Angeles
Terrence Han University of California, Los Angeles
Lihua Jin University of California, Los Angeles