Browsing by Author "Shen, Shouqi"

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    Influence of monomer structure and catalyst concentration on topological transition and dynamic properties of dicarboxylic acid‐epoxy vitrimers
    (Wiley, 2024-12-31) Shen, Shouqi; Thakur, Vijay K.; Skordos, Alexandros A.
    This study delineates the dependence of thermophysical behavior of acid‐epoxy vitrimers on their formulation. The stress relaxation due to the bond exchange reaction and the glass transition temperature of acid epoxy vitrimers are investigated, with respect to the influence of catalyst content and acid chain length. This is carried out for a range of dicarboxylic acids and catalyst concentrations formulated and characterized using calorimetry and dynamic mechanical analysis. The influence of acid chain length on the bond exchange rate, topological transition, and glass transition temperatures of the vitrimers is found to be significant. The activation energy of the exchange reaction varies over a wide range from 73 to 104 kJ/mol and the topology freezing temperature from 66 to 136°C with the behavior governed by the interplay between crosslinking density, network flexibility and density and distance of functional groups, with an increase of catalyst concentration leading to lower topological transition temperature and the dependence on chain length showing non‐monotonic behavior. The glass transition decreases by about 30°C as the carbon chain length increases from 6 to 14 carbons due to enhanced monomer flexibility and is not affected by the concentration of catalyst.
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    Role of acid hydrocarbon chain length on the cure kinetics and thermal degradation of epoxy- dicarboxylic acid vitrimers
    (Elsevier, 2025-03-19) Shen, Shouqi; Skordos, Alexandros A.
    This study investigates the cure kinetics and thermal degradation of epoxy-dicarboxylic acid vitrimers, focusing on the effect of methylene chain length. A diffusion-controlled, modified autocatalytic kinetics model was applied, based on Differential Scanning Calorimetry (DSC) data, whilst Thermogravimetric Analysis (TGA) was used to assess degradation. Increasing the methylene chain length enhanced thermal stability, with decomposition temperatures ranging from 430 °C for the hexanedioic acid formulation to 500 °C for the tetradecanedioic acid formulation. The curing process transitioned through three distinct kinetics regimes: an initial non-catalysed phase, followed by an autocatalytic stage, and finally, a diffusion-limited phase at high crosslink density. This shift leads to a 30 %-70 % reduction in apparent activation energy during the early stages. The activation energy displays a complex behaviour, initially decreasing with longer methylene sequences before rising due to competing effects of chain flexibility and reduced reactivity. Kissinger and isoconversional analyses confirmed reliable activation energy values. Despite some discrepancies in the dodecanedioic acid formulation due to secondary reactions, the model exhibits a good approximation, with an average goodness-of-fit of 84.4 %. This analysis improves understanding of vitrimer cure kinetics and thermal behaviour, providing insights for optimising industrial applications.