The formation of polyvinylidene fluoride (PVDF) membranes by vapor-induced phase separation (VIPS) is known to depend strongly on the dissolution temperature of the casting solution, yet the mechanism linking dissolution conditions to membrane morphology has remained unclear. Here we investigate this relationship by correlating dissolution temperature, solution rheology, polymer crystallization behavior, and membrane microstructure within the theoretical framework of viscoelastic phase separation (VPS). Rheological measurements, FTIR-ATR spectroscopy of solutions and membranes, and scanning electron microscopy of the resulting membranes reveal that PVDF/DMSO solutions exhibit a distinct temperature window bounded by the minimum and critical dissolution temperatures \(T_{d,min}\) and \(T_{d,crit}\) . Below \(T_{d,crit}\) , incomplete dissolution leaves residual \(\alpha\) -polymorph microcrystallites in solution that act as thermoreversible multichain junctions. These junctions form a transient stress-bearing network that increases solution viscoelasticity and promotes elastic phase separation (EPS). Above \(T_{d,crit}\) these junctions dissolve, reducing the viscoelastic constraint and allowing phase separation to proceed predominantly in the fluid phase separation (FPS) regime. Within the VPS framework, the Weissenberg number ($Wi$), defined as the ratio between the stress-relaxation time of the polymer-rich phase and the deformation time scale generated during phase separation, provides an experimentally accessible descriptor of this transition. A morphology regime diagram relating the initial Weissenberg number ( \(Wi_i\) ) to the observed membrane structures shows that bicontinuous morphologies occur for \(Wi_i \ge 1\) , whereas nodular structures form when \(Wi_i < 1\) . Remarkably, the condition \(Wi \approx 1\) lies within the dissolution-temperature window previously associated with \(\alpha\) -polymorph prevalence, while higher dissolution temperatures favor \(\beta\) -phase formation due to increased chain mobility during demixing. These findings establish a mechanistic connection between dissolution temperature, transient microcrystalline junction networks, solution viscoelasticity, and membrane microstructure. More broadly, they demonstrate how viscoelastic phase-separation theory can provide a unified framework for understanding and rationally controlling membrane formation from semicrystalline polymer solutions.
