The bulk phases were characterized with laboratory SAXS equipment (SAXSpoint 5.0 from Anton-Paar, Graz, Austria) and the Austrian SAXS beamline at ELETTRA (Trieste, Italy). The SAXSpoint 5.0 system used a Gallium enriched Metaljet X-ray source with a wavelength of 1.3 Å (9.2 keV) from Excillum (Kista, Sweden). The sample-todetector distance of 1608 mm and 489 mm provided a merged q-range from 0.02 < q < 6.5 nm -1 . q is the length of the scattering vector, defined by q = 4π/λ sin(θ/2), λ being the wavelength and θ the scattering angle. X-ray capillaries of 1.5 mm diameter (Hilgenberg GmbH, Malsfeld, Germany) were filled with the bulk samples on the top part of the capillaries and centrifuged at 3600 rpm for 3 min to transfer the viscous gel to the bottom of the capillary. A heated/cooled sample holder (Anton-Paar, Graz, Austria) adjusted the sample temperature. The temperature was increased from 25 °C to 95 °C with a rate of 5°C/min. The samples were then thermally equilibrated for 5 min and measured for 15 minutes (3 frames with 5 min each, averaged afterward). The 2D SAXS patterns were acquired using a EIGER2 R 1M Horizontal (Dectris Ltd., Baden, Switzerland) and integrated into one-dimensional scattering function I(q) using SAXSanalysis software (Anton-Paar, Graz, Austria) for further analysis. Time-resolved SAXS measurements were further conducted at the Austrian SAXS beamline at ELETTRA (Trieste, Italy). An X-ray source with a wavelength of 1.54 Å (8 keV) having a spot size of 0.2 × 0.5 (vertical × horizontal in mm) was used, at a sample-to-detector distance of 1235.2 mm, providing a q-range from 0.07 < q < 5.7 nm -1 . The bulk sample was loaded in 1.5 mm diameter X-ray quartz capillary (capillary Tube Supplies Ltd, Withielgoose, U.K.) and mounted vertically on a motorized sample stage. To find the bulk phase/air interface, the sample was measured at different points by changing the position of the capillary along the vertical axis with 0.5 mm increments (z_scan). 2D SAXS patterns were acquired for 10 s at each position at a temperature of 25°C. Then, a solution containing 4 wt% of LAP initiator in milli-Q water was placed on top of the bulk phase and exposed to UV light (Superfire S11-H 365nm, Shenzhen, China). The 2D SAXS patterns were acquired during HEA polymerization in the bulk sample (60 sec measurement with 1 sec delay between the frames).