Cyclic Topologies in Linear α,ω-Dihydroxy Polyisoprenes by Dielectric Spectroscopy

A series of mono- and di-functionalized polyisoprenes (PIs) bearing hydroxyl (OH−) end-group(s) with different molar masses ranging from 2 to 30 kg mol–1 were synthesized and studied by a combination of temperature- and pressure-dependent dielectric spectroscopy and rheology. In the di-functionalized PIs, the −OH end-group interactions result in a mixture of linear and cyclic configurations (up to 45% cyclic configurations for the lower molar masses). The formation of cyclic topologies due to increased H-bonding interactions restricted the backbone mobility and increased the glass temperature, Tg, especially for the lower molar masses. Moreover, an additional process (termed α*) was evidenced in the dielectric spectroscopy in the range between the segmental process and the global chain relaxation. It followed a Vogel–Fulcher–Tammann temperature dependence, freezing at a temperature in the vicinity of the liquid-to-glass temperature, being independent of molar mass. Its molecular origin was identified by employing the pressure sensitivity of the characteristic relaxation times and the pressure dependence of Tg. It reflects the relaxation of segments in the vicinity of the H-bonded groups. Overall, this study provided information on the impact of weakly associating polar end-groups (hydroxyl) on the molecular dynamics of type-A polymers. Furthermore, it suggested promising routes for designing polymers with a higher concentration (>50%) of cyclic topologies, for example, by employing (i) short chains with (ii) strongly associating end groups (stronger than the hydroxyl end-groups).