Effect of hydroxyl-functionalization on the structure and properties of polypropylene

Hydroxyl-modified polypropylenes (PPOH) with side chains containing OH groups were synthesized by copolymerization of the propylene and undecenyloxytrimethylsilane monomers. Copolymers with hydroxyl concentration up to ∼4 mol % were produced by hydrolysis of the propylene- undecenyloxytrimethylsilane copolymers and their structural, thermal and rheological properties were compared with that of unmodified polypropylene (PP). The presence of H-bonding between the hydroxyl groups led to a densification of the amorphous domains and an increase in the glass-transition temperature with increasing hydroxyl concentration. The incorporation of comonomer in the PP backbone led to a reduction in the melting point, as predicted by Flory's theory of melting point depression in copolymers. During crystallization, the OH containing polymer segments are excluded from the crystalline domains and their presence disrupt the growth behavior leading to reduced lamellar thickness, leaving the α-monoclinic crystal structure unaffected. Because of the steric hindrance by the comonomer side-chains to the ordered packing of PP backbone chains into the lamellar structure, a reduction in crystallinity by as much as 15% and a significant reduction in the overall rate of crystallization was observed for the PPOH copolymers. Both the storage and the loss moduli decreased for the PPOH copolymers below the melting point, owing to a lower crystallinity than PP, but increased above the melting point due to the dominating effect of H-bonding interactions in the melt state.