Session: ASME Undergraduate Student Design Expo

Paper Number: 173416

Thermal and Photothermal Healing of Thermoplastic Polyurethane Nanocomposites 

This study explores the thermal and photothermal healing capabilities of thermoplastic polyurethane (TPU) nanocomposites, with a focus on their ability to recover mechanical integrity after damage and large deformations. TPUs are segmented block co-polymers composed of sequences of flexible polyols, separated by hard diisocyanate segments. Since the mutual solubility of flexible and hard segments is limited, TPUs exhibit microphase separation, resulting in a complex heterogeneous morphology. The hard segments self-assemble into discrete glassy or crystalline domains dispersed within a continuous amorphous matrix formed by the soft segments. The hard segments act as the physical crosslinks and account for TPU’s rubbery behavior, high elasticity, and thermal responsiveness. TPU films were synthesized by dissolving TPU resins in a polar solvent. Nanoparticles such as carbon nanofibers, nanotubes, or graphene platelets were added to the solutions at a low weight fraction. The solutions were subsequently stirred, heated, and degassed to remove residual solvent. This process yielded uniform and flexible films with rubber-like mechanical properties at room temperature. Standard tensile test specimens were punched from the films and tested under uniaxial deformation to evaluate their mechanical response. Strong hysteresis, followed by a large residual strain was observed after cyclic finite deformations, indicating energy dissipation and microstructural rearrangement at large deformations. All samples showed a shape-memory behavior and recovered their original shape and geometry upon mild thermal treatment. This behavior is rooted in a unique mechanical feature of TPUs, often referred to as strain-induced crystallization. The amorphous chains of TPU possess a conforming atomistic structure during large deformations and rearrange themselves into ordered crystals. Melting the crystals at a temperature slightly higher than room temperature facilitates the shape-memory of TPU films. This recovery process suggested a thermally reversible mechanism that can be exploited for self-healing. To show this, the test specimens were cut, and the cut pieces were put against each other while being heated by a heat-gun.  Following thermal treatment, samples regained much of their original mechanical strength and integrity. In addition, the incorporation of plasmonic silver nanoparticles to TPUs reinforced with graphene nanoplatelets induced an ultrafast photothermal healing response upon laser exposure. The rapid healing and recovery of mechanical properties were attributed to the localized heating triggered by plasmonic effects of silver nanoparticles and graphene which can be facilitated in a matter of seconds. These findings highlight the significant potential of thermal and photothermal healing strategies to restore the mechanical performance of damaged TPU nanocomposites, extending their service life and contributing to sustainable material use.

Presenting Author: Aylar Gayypova Mercer University

Presenting Author Biography: Aylar is a junior undergraduate student in the Department of Mechanical Engineering at Mercer University. She is also a participant in the Honors Program.

Authors:

Aylar Gayypova Mercer University
Alireza Sarvestani Mercer