Polymorphism in polyamide 6 and polyamide 6/clay nanocomposites molded via water-assisted injection molding

Differing from the conventional injection molding, being featured by a monotonous decrease of cooling rate from the outer layer to the core layer, the water-assisted injection molding (WAIM) is characterized by a unique distribution of cooling rate across the part wall, which would be expected to result in the formation of unique hierarchical structures in WAIM parts. So the crystal structures of the WAIM parts of two grades of polyamide 6 (PA6s) with different molecular weights (MWs) and PA6 (with higher MW)/clay nanocomposite were investigated. It is found that the α-form is predominant at low cooling rates, whereas the γ-form is predominant when the cooling rate is high or in the presence of the clay combining with an appropriate cooling rate, which may be confined by both upper and lower critical values. The MW of the PA6 exhibits more effect on the formation of the γ-form as the cooling rate decreases. Moreover, the shear is found to hinder the formation of the γ-form for the PA6 with higher MW. This work help further understand the crystal formation mechanism in the PA6 and its nanocomposites upon processing.