Crosslinking, thermal properties and relaxation behaviour of polyisoprene under high-pressure

The transient hot-wire method has been used to measure the thermal conductivity κ and heat capacity per unit volume ρc_p of untreated (virgin) and crosslinked cis-1,4-poly(isoprene) (PI) in the temperature range 160-513 K for pressures p up to 0.75 GPa. The results show that the crosslinking rate of the polymer chains becomes significant at ∼513 K on isobaric heating at 0.5 GPa changing PI into an elastomeric state within 4 h without the use of crosslinking agents. The crosslinked PI and untreated PI have about the same κ = 0.145 Wm⁻¹ K⁻¹ and c_p = 1.81 kJ kg⁻¹ K⁻¹ at 295 K and 20 MPa, but different relaxation behaviours. Two relaxation processes, corresponding to the segmental and normal modes, could be observed in both PI and crosslinked PI but these have a larger distribution of relaxation times and become arrested at higher temperatures (∼10 K) in the latter case. The arrest temperature for the segmental relaxation of untreated and crosslinked PI, for a relaxation time of ∼1 s, are described well by the empirical relations: T(p) = 209.4 (1 + 4.02 p)^0.31 and T(p) = 221.3 (1 + 2.33 p)^0.40 (p in GPa and T in K), respectively, which thus also reflects the pressure variations of the glass transition temperatures.