Session: ASME Undergraduate Student Design Expo

Paper Number: 173478

Mechanical Characterization of Self-Healing Polyurethanes Under Varying Thermal and Temporal Conditions 

Self-healing polymers are a class of materials designed to repair themselves after damage. They are being explored for applications that require durability, reusability, and long-term functionality. Among these, polyurethanes (PUs) are a class of elastomeric polymers known for their flexibility and tunable mechanical properties, making them promising candidates for such systems. When formulated with dynamic covalent bonds, such as disulfide (S–S) linkages, PUs can reestablish their original mechanical performance under moderate thermal conditions. This project investigates how healing time and temperature influence the mechanical recovery of PU-based elastomers following complete fracture.

Several PU formulations were synthesized using different isocyanates, including HMDI, IPDI, and MEDS, to evaluate how molecular structure and composition may interact with healing conditions. These were combined in varying molar ratios, such as HMDI:MEDS 2:1, 1:1, and 1:2, to compare how stiffness and flexibility changed depending on the relative amount of dynamic content. For example, the 2:1 ratio tended to produce stiffer materials with higher crosslink density, while the 1:2 ratio resulted in more flexible elastomers that were softer to the touch and easier to deform. The 1:1 ratio often exhibited an intermediate balance, and these variations directly impacted the healing behavior observed in mechanical testing.

In this study, samples were cut in half and realigned to simulate damage. They were then allowed to heal at controlled temperatures of 25 °C and 70 °C for eight time intervals: 10 minutes, 30 minutes, 1 hour, 2 hours, 6 hours, 12 hours, 24 hours, and 48 hours. For each condition, three samples were tested. After healing, a texture analyzer applied tension to pull the samples apart while recording stress-strain curves. These curves provided data on how well the materials recovered in terms of stiffness, toughness, and flexibility across multiple formulations.

Preliminary results show that longer healing times and higher temperatures generally lead to better recovery of mechanical strength. Shorter healing periods tend to result in lower recovery, likely because the dynamic bonds have less time to fully reform. These trends were mostly consistent across the different PU formulations and mixing ratios, although some variations were noted.

This research contributes to understanding how thermal and healing time factors influence the mechanical performance of self-healing polyurethanes. These findings are especially relevant for the development of adaptive, damage-tolerant materials used in soft robotics, flexible electronics, and prosthetic devices. Ongoing work will explore repeated damage cycles and more precise quantification of healing efficiency using modulus recovery and fracture energy comparisons.

Presenting Author: Amina Allab Pennsylvania State University

Presenting Author Biography: Amina Allab is a first-generation undergraduate student studying Electrical Engineering at Pennsylvania State University, University Park. She is a member of the Millennium Scholars Program, which supports high-achieving students committed to increasing diversity in STEM. Through a National Science Foundation Research Experience for Undergraduates (REU) at the University of Michigan, she is conducting research under Dr. Abdon Pena-Francesch on the mechanical performance and self-healing behavior of polyurethane-based elastomers under varying thermal and temporal conditions. This work contributes to the development of adaptive, damage-tolerant materials for use in soft robotics and other technologies. Outside of the lab, she serves as the Conference Planning Chair for the National Society of Black Engineers (NSBE) chapter at Penn State and is actively involved in efforts that promote student success and representation in STEM fields.

Authors:

Amina Allab Pennsylvania State University
Yiqun Li University of Michigan
Abdon Pena-Francesch University of Michigan