Synthesis of polycaprolactone-polyimide-polycaprolactone triblock copolymers: Via a 2-step sequential copolymerization and their application as carbon nanotube dispersants

Abstract

Polyimides have attractive properties such as a strong interaction with 1D/2D carbon nanomaterials but their solubilities in common organic solvents are limited. We report a new synthesis route for triblock copolymers of polycaprolactone-polyimide-polycaprolactone (PCL-PI-PCL) via polycondensation followed by ring-opening polymerization. The prepared OH-PI-OH homopolymer precursors were reacted with two equivalents of stannous(ii) octoate (Sn(Oct)2) to afford α,ω-dihydroxyl-terminated polyimide macroinitiators which can polymerize with ϵ-caprolactone to obtain the final triblock copolymers (PCL-PI-PCL). Four different molecular weights of PCL-PI-PCL triblock copolymers with different lengths of PCL and PI blocks were synthesized to assess the best composition for carbon nanotube dispersion in a low boiling organic solvent (tetrahydrofuran, THF). The polyimide block interacts strongly with single walled carbon nanotubes (SWNTs) through charge transfer, as shown by Raman spectroscopy, while the polycaprolactone block has a good solubility in THF. An optimised triblock copolymer disperses the carbon nanotubes in THF well even after standing for 1 h, while the PI homopolymer-dispersed SWNTs settled completely under the same conditions. We applied the new PCL-PI-PCL in SWNT-reinforced epoxy composites with the use of THF as the casting solvent. The optimised triblock copolymer-dispersed SWNTs (2 wt%) increased the tensile strength, modulus, and elongation at maximum stress by 74%, 35%, and 62% respectively compared to the neat resin blend. The new synthesis route of the triblock copolymer is amenable to the synthesis of diverse PI-based triblock copolymers with various desired functionalities for myriad applications, such as for carbon nanotube-reinforced epoxy-based composites, water-based antibacterial dispersions, etc.