Abstract
Pre-stretching raises the glass transition temperature (Tg) of polymers, yet the underlying microscopic mechanisms remain unclear. This work employs molecular dynamics simulations of polymer systems with different entanglement densities (Ne) to unravel this mechanism. The simulations reproduce the experimentally observed increases in Tg, structural relaxation time (τα), and density after stretching. During stretching, stress-activated flow accelerates segmental dynamics, lowers the density, and raises the enthalpy. Upon subsequent free relaxation, these quantities not only recover but also over-recover their unstretched values, indicating that stretching drives the system into a denser, dynamically slower state. The over-recovery strengthens with increasing Ne and is more pronounced under biaxial stretching. Two quantitative correlations pinpoint the underlying mechanism. First, the evolutions of free volume and τα collapse onto a single linear relation, consistent with the Doolittle equation of free-volume theory. Second, the density scales linearly with the local orientational order parameter. These correlations reveal that stretching induces local orientational ordering, which packs segments more densely, reduces the free volume, and thereby suppresses segmental dynamics. Consistently, upon heating, the internal-energy and density differences between stretched and unstretched systems reach their extrema at the temperature where the local orientation begins to decay, corroborating the coupling between local ordering and dense packing. Entanglements sustain the stretch-induced oriented state, so the effect is amplified at high Ne. These findings clarify how stretching raises Tg and highlight the role of entanglements in stabilizing the stretch-induced glassy state.