The dual effect of α-pinene as a plasticizer on the reaction characteristics and mechanical properties of polydicyclopentadiene crosslinked polymers

In the development of solid propellants, the inherent brittleness and high glass transition temperature (T_g) of poly (dicyclopentadiene) (PDCPD) pose significant challenges for achieving the desired high strength, ductility, and low T_g properties in binders. Poly (dicyclopentadiene) (PDCPD)-based composites using α-pinene as a polymeric plasticizer, is synthesized as a novel binder for solid propellants requiring high strength, ductility, and low T_g. Through frontal ring-opening metathesis polymerization (FROMP), PDCPD/α-pinene (PDA) blends were fabricated with α-pinene (5–50 mol%) acting as a physical plasticizer rather than a comonomer, overcoming its kinetic limitations in copolymerization. The incorporation of α-pinene significantly altered material properties, demonstrating an inverse correlation between α-pinene content and tensile strength (38.3–0.57 MPa) alongside a positive concentration dependence on elongation at break (91–556 %), while systematically reducing T_g from 160 °C to 27–94 °C. Dynamic mechanical analysis and tensile testing revealed maintained crosslinking density (12–21 repeat units between crosslinks) despite enhanced ductility, mainly attributed to α-pinene's free volume modulation within the polymer network rather than chain scission mechanisms. Rheological and thermal analyses showed α-pinene's dual role as an effective FROMP inhibitor, extending resin pot life (6–45 min) through reduced reaction heat release (299–381 W g⁻¹) while enabling complete polymerization (>95 % conversion) via controlled frontal propagation. Thermal gravimetric analysis confirmed the composite's stability with three-stage degradation profiles, correlating α-pinene volatilization (100–305 °C) with theoretical content (2–40 % mass loss) and preserving PDCPD's crosslinked network integrity until structural carbonization (415–600 °C). The counterintuitive synergy between retained crosslinking and plasticization effects establishes α-pinene-modified PDCPD as a promising candidate for advanced propellant systems requiring tunable thermomechanical properties.