Biaxial stretching modulated montmorillonite filled polyvinyl alcohol composite film with mechanics enhancement

While incorporating nanofillers into polymer matrices significantly enhances macroscopic performance, excessive concentrations inevitably trigger severe filler agglomeration. To overcome this dispersion bottleneck, synchronous biaxial stretching emerges as a highly efficient, easily operable, and solvent-free post-processing strategy. In this study, poly(vinyl alcohol) (PVA) matrices with two distinct degrees of polymerization (17PVA and 24PVA) are integrated with organo-montmorillonite (OMMT) and subjected to a 1.3 × 1.3 biaxial stretch. Microstructural characterizations reveal two fundamental mechanisms driving the performance upgrade. First, the two-dimensional mechanical field physically regulates the spatial distribution of OMMT, breaking up initial tactoids while effectively reducing large OMMT-rich aggregates and promoting a more homogeneous microscale morphology to form a structurally balanced architecture. Second, stretching strongly induces PVA crystallization. Specifically, biaxial stretching increases the base crystallinity of pure 17PVA and 24PVA from 36.4% to 45.0% and 36.0% to 39.0%, respectively. For the 3 wt% OMMT-loaded 24PVA composite, the crystallinity increases from a baseline of 36.4% to 38.4% upon filler addition, then further increases to 42.0% post-stretching. Macroscopically, this strain-induced highly ordered network yields massive mechanical enhancements. Adding 3 wt% OMMT initially increases the tensile strength of both 17PVA and 24PVA films from approximately 10 MPa to 14 MPa; subsequent biaxial stretching then increases these values to 45 MPa and 60 MPa, respectively. Thermally, a 1 wt% OMMT loading optimally increases the melting points by 3.0 °C (17PVA) and 7.9 °C (24PVA), a stability that remains robustly uncompromised by the stretching process. Ultimately, this work demonstrates that biaxial stretching provides a scalable pathway for fabricating high-performance nanocomposites.