Solid-State Polymerization as a Vitrimerization Tool Starting from Available Thermoplastics: The Effect of Reaction Temperature

In the current work, solid-state polymerization (SSP) was studied for the synthesis of poly(butylene terephthalate), PBT-based vitrimers. A two-step process was followed; the first step involved alcoholysis reactions and the incorporation of glycerol in the polymer chains. The second step comprised transesterification reactions in the solid state (SSP) in the presence of zinc(II) catalyst resulting in the formation of a dynamic crosslinked network with glycerol moieties serving as the crosslinkers. The optimum SSP conditions were found to be 3 h at 180 °C under N2 flow (0.5 L/min) to reach high vitrimer insolubility (up to 75%) and melt strength (2.1 times reduction in the melt flow rate) while increasing the crosslinker concentration (from 3.5 to 7 wt.%) improved further the properties. Glass transition temperature (Tg) was almost tripled in vitrimers compared to initial thermoplastic, reaching a maximum of 97 °C, whereas the melting point (Tm) was slightly decreased, due to loss of symmetry perfection under the influence of the crosslinks. Moreover, the effect of the dynamic crosslinked structure on PBT crystallization behavior was investigated in detail by studying the kinetics of non-isothermal crystallization. The calculated effective activation energy using the Kissinger model and the nucleating activity revealed that the higher crosslinker content impeded and slowed down vitrimers melt crystallization, also inducing an alteration in the crystallization mechanism towards sporadic heterogeneous growth.