Synthesis, properties, and morphology of lignin based epoxy resins
Star-like lignin-poly(propylene oxide) copolymers were prepared by chain-extending steam exploded lignins (tulipifera liriodendron) with propylene oxide and by subsequent endcapping with ethylene oxide. Epoxidation of these copolymers was carried out with epichlorohydrin at room temperature, using KOH as oxyanion forming reagent. The epoxidized compounds were fractionated by solvent precipitation to remove poly(alkylene oxide) homopolymers and to prepare fractions of narrow molecular weight distributions.
The epoxides were cross-linked with meta phenylene diamine yielding thermosets which were, depending on lignin content, either low modulus elastomers, or high modulus materials with considerable ductility. The modulus of elasticity was a strong and linear function of lignin content, whereby the highest value was 1100MPa (57% lignin).
The curing reaction was of nth-order type, whereby the reaction order changed from close to one at the beginning of the curing reaction to 2, once the reaction becomes diffusion controlled. Curing induced partial demixing of the lignin and poly(propylene oxide) phases which yielded a secondary structure where lignin rich domains in the order of 10 nm were interspersed in a matrix of lignin poor material. However, from TEM and ¹³C solid state cross-polarized NMR analyses it was evident that the domain structure was not that of a classical micro-phase separated copolymer with well defined phase boundaries, but rather had broad interphases. Additionally, the results of multifrequency dynamic mechanical thermal analysis showed that the lignin containing thermosets have very broad glass transition ranges which most likely were due to transitional phase inhomogeneities and provided these materials with good vibrational damping ability.