Hydroxypropyl lignins (HPLs) from several sources were reacted with propylene oxide to produce chain-extended ) hydroxypropyl lignin (CEHPL) copolymers with molar substitutions (MS) between 1 and 7 propylene oxide units. Isolated copolymers were characterized with respect to their chemical composition, molecular weight and thermal properties. These techniques confirmed the presence of a copolymer with between 20 and 67% lignin. Glass transition temperatures (Tgs) of the CEPHLs followed the behavior predicted by the Gordon-Taylor equation. Properties of the CEHPLs were independent of the original lignin source.

The CEHPL copolymers were incorporated into lignin polyurethane networks (LPUs). The LPUs contained 17 to 43% lignin and showed a single Tg. Both the Tg and the Youngs modulus (MOE) of the LPUs were strongly correlated to the lignin content and type of diisocyanate used to prepare the network. Swelling studies indicated that the LPUs prepared from CEHPLs with a high MS (5-7) were not highly crosslinked networks. The LPU properties also appeared to be independent of the lignin source.

In another set of experiments a HPL was separated into five fractions (F-HPLs) with molecular weights (MWs) between 1.5 and 10x10³ daltons. The Tg of the F-HPLs was correlated to molecular weight by the Fox-Flory equation. The fractionated HPLs were incorporated into polyurethane networks. The Tgs of these networks were related to the MW of the F-HPL. Swelling studies indicated that low molecular weight monofunctional fragments limited network formation.

The LPUs were also used as one component in LPU/polymethyl methacrylate (PMA) interpenetrating polymer networks (IPNs). These IPNs varied in their LPU/PMMA composition and the presence of crosslinking. Dynamic mechanical and thermal analysis showed two phases in all of the IPNs. Mechanical properties were dependent on the IPN composition and phase crosslinking. For IPNs with a crosslinked LPU phase, the MOE values indicated the presence of dual phase continuity.

A second series of IPNs was prepared to investigate the effects of lignin content on IPN properties. Phase separation appeared to be related to the lignin content. Mechanical properties were related to lignin content and not the phase behavior.