Generally, the a-form is the most stable crystal with excellent
tensile strength but inferior toughness [17–20] while the b-form
has greater toughness but lower thermal stability. Several advantages
of the b-NA has been reported including lower crystalline density,
lower melting point [15,21,22] and higher toughness [23–25].
It was believed that the b-crystal can be achieved under stipulated
conditions such as a temperature gradient, under shearing or by
using a specific nucleating agent [4,15,20].