Influence of hydrogen bonding on the melt rheology of polypropylene

The rheological behavior of hydroxyl-functionalized polypropylene (PPOH) copolymers modified with up to 3.9 mol% hydroxyl groups was compared with unmodified polypropylene (PP) in the melt state. The hydroxyl groups interacted via hydrogen (H-) bonding, the strength of which increased with increasing hydroxyl concentration. The hydrogen bonds persisted at temperatures as high as 250 °C, even though the H-bonding strength decreased with increasing temperature. A direct consequence of H-bonding was an increase in the elasticity, viscosity and relaxation time of the PPOH copolymers compared to PP, and also an increase in these properties with increasing hydroxyl concentration. The zero-shear viscosity exhibited a power-law relationship with the hydroxyl concentration, η₀ ∝ [M_v]^3.6 [OH]², which was found to be consistent with the predictions from Leibler-Rubinstein-Colby (LRC) theory for associating polymers. The PPOH copolymers exhibited apparent molecular weights 2–8 times higher than the actual molecular weights. The PPOH copolymer with 3.9 mol% hydroxyl groups displayed a gel-like behavior that suggested the formation of an elastic network in the melt presumably due to the phase separation of H-bonded OH groups into nanoclusters. The network, however, was weak since non-Newtonian viscous flow occurred under non-linear deformations.