High amylose hydroxypropylated starch films have been formed using traditional extrusion technology with a range of plasticisers and nanosilicate additives such as montmorillonite. After thermal processing, these materials are predominantly amorphous, however under controlled humidity over time the evolution of B-type (found in high amylose starches) and Vh type (generally induced via processing) crystallinity was observed. These structural changes have been analysed and correlated to final mechanical properties (modulus, tensile strength and break elongation). A “nanostabilization” effect was also observed over time in starch samples containing montmorillonite. It is proposed that in these systems the montmorillonite disrupts the retrogradation process reducing the rate of embrittlement over time. This is of significant importance for improving the shelf life of thermoplastic starches for industrial applications.