The initial guess on the phase identity for the simulation was narrowed to the Ia3d and Pn3m cubic phases based on the GMO/HEA/water phase diagram in bulk. The experimental 2D GISAXS data matched the simulated scattering pattern of a Pn3m cubic phase with the plane (111) parallel to the wafer surface (see Figure ). This agrees with previous GISAXS studies on GMO thin films showing the (110) and (111) planes set parallel to the substrate to be the most thermodynamically favorable orientation for the Ia3d and Pn3m cubic phases, respectively. The lattice constant of ~ 9 nm of the hydrated films, calculated from the radially integrated GISAXS curves in Figure S8, is in the range of Pn3m lattice constant measured by increasing the amount of water in the bulk phases: 8.9 -11.3 nm. The next step was investigating the effect of HEA polymerization on the structure using in situ GISAXS in the GMO60HEA5 film. The surrounding water was replaced with 0.1 wt% of LAP solution, and the film was UV irradiated at 365 nm to initiate the reaction. Figure shows representative 2D GISAXS patterns at selected time points. The initial GISAXS pattern is from an oriented Pn3m phase with a lattice constant of 9.1 nm. Additional diffraction spots are observed after 15 min of UV irradiation that could be related to a coexisting Im3m cubic phase, which aligns with the results for polymerization in bulk phases. The Pn3m scattering pattern intensity decreases over time, signifying a decrease in its content relative to that of the Im3m structure. During polymerization in the films, the peaks of the Im3m structure are gradually shifted to higher q values, indicating a shrinking of the structure (See movies SI3 and SI4 showing the 2D images during the reaction). The corresponding lattice constant decreases from 12.4 after 15 min to 12.2 nm at 30 min of polymerization. Furthermore, a reorientation of the Im3m phase with the (200) plane parallel to the surface is observed. The polymerization-triggered transformation of the Pn3m to the Im3m structure may be interpreted as the formation of new water channels directed by the growth of linear HEA polymers within the water channels of the Pn3m cubic phase. The water channels of the Pn3m cubic phase meet in a four-way junction at an angle of 109°, while the Im3m cubic phase presents six-way junctions meeting at an angle of 90°.