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<p>Understanding how structural modifiers influence glass-transition dynamics in thermosets remains an important problem with broader implications for glass-forming systems. In an earlier work we demonstrated that epoxy networks modified with glycidyl-terminated polyurethane (GPU) exhibit enhanced toughness, tunable reductions in <i>T</i><sub>g</sub>, and functionalities such as self-healing and shape-memory behavior. This study investigates how GPU incorporation alters free-volume characteristics and their effect on both macroscopic properties and glass-transition dynamics. Free volume (hole size and distribution) was quantified at room temperature using positron annihilation lifetime spectroscopy (PALS), while modulated differential scanning calorimetry (MDSC) and dynamic mechanical analysis (DMA) were used to determine cooperativity length scales and fragility indices at <i>T</i><sub>g</sub>. The pristine epoxy exhibited a single o-Ps lifetime (τ₃) of 1.59 ± 0.01 ns and a fractional free volume of 16.3 ± 0.1%. GPU incorporation led to a bimodal free-volume distribution, indicating heterogeneous network formation. With increasing GPU content, the cooperativity length scale (ξ) decreased from 2.0&#xa0;nm to 1.54&#xa0;nm (20 wt% GPU). The fragility index (<i>m</i>) showed non-monotonic behavior, decreasing from 116 to 90 at 5 wt% GPU, followed by an increase at higher loadings. Correlations between free-volume microstructure, macroscopic response, and segmental dynamics are established.</p>

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Sub-nanoscopic free volume in glycidyl-terminated polyurethane-toughened epoxy thermoset: correlations with fragility and cooperative length scale

  • 期刊论文

  • 发表日期：

    2026年6月19日

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  • Sangram K. Rath,
  • Srikanth Billa,
  • Kathi Sudarshan,
  • Debdatta Ratna

摘要

Understanding how structural modifiers influence glass-transition dynamics in thermosets remains an important problem with broader implications for glass-forming systems. In an earlier work we demonstrated that epoxy networks modified with glycidyl-terminated polyurethane (GPU) exhibit enhanced toughness, tunable reductions in Tg, and functionalities such as self-healing and shape-memory behavior. This study investigates how GPU incorporation alters free-volume characteristics and their effect on both macroscopic properties and glass-transition dynamics. Free volume (hole size and distribution) was quantified at room temperature using positron annihilation lifetime spectroscopy (PALS), while modulated differential scanning calorimetry (MDSC) and dynamic mechanical analysis (DMA) were used to determine cooperativity length scales and fragility indices at Tg. The pristine epoxy exhibited a single o-Ps lifetime (τ₃) of 1.59 ± 0.01 ns and a fractional free volume of 16.3 ± 0.1%. GPU incorporation led to a bimodal free-volume distribution, indicating heterogeneous network formation. With increasing GPU content, the cooperativity length scale (ξ) decreased from 2.0 nm to 1.54 nm (20 wt% GPU). The fragility index (m) showed non-monotonic behavior, decreasing from 116 to 90 at 5 wt% GPU, followed by an increase at higher loadings. Correlations between free-volume microstructure, macroscopic response, and segmental dynamics are established.