Strain‐Tunable Thermal Conductivity in Largely Amorphous Polyolefin Fibers via Alignment‐Induced Vibrational Delocalization

Developing fast, reversible, and recyclable thermal switches is essential to advance adaptive thermal management. Here, we present a strain-tunable thermal switch based on largely amorphous olefin block copolymer (OBC) fibers, achieving a continuous switching ratio above 2 over 1000 cycles, as well as very short response times below 0.22 s. Using Raman spectroscopy, we quantify vibrational delocalization with increasing strain and demonstrate its direct connection to the observed thermal conductivity changes. We show that unlike prior assumptions linking propagating heat carriers primarily to crystalline domains, alignment in amorphous systems can enable phonon-like modes that dominate transport. To our best knowledge, this work is the first to experimentally probe vibrational delocalization using Raman spectroscopy and to demonstrate that alignment alone can govern the dominant carrier in disordered polymers. These findings establish design strategies for fatigue-resistant, high-performance, and recyclable polymer thermal switches for advanced thermal energy transport applications.