Polymeric microstructures induced by freeze-drying process: Comparative study of PS41–PAA271 morphologies after impregnations in water and in an alkaline solution

Surface of polymeric coating can be modified after being submitted to treatment based on freeze-drying. Morphology of polystyrene-b-poly(acrylic acid) (PS41–PAA271) diblock copolymers was investigated by using scanning electron microscopy (SEM). After dip-coating on stainless steel substrate, influences of the nature and the pH of the immersing liquid, used before freeze-drying, were established. If water has impregnated the polymer in order to form a hydrogel, the surface could acquire a cellular monolayer structure after freeze-drying while stretched polymeric nodules, affected by the conditions of sublimation, cover some other parts of the surface. The porous monolayer happens when freeze-drying occurs in a homogenous and well-crystallised icy material. Observations of nucleation sites linked by polymeric filaments could be considered as the first step of growth of this regular network. In the other hand, if a KOH alkaline solution has impregnated the polymer before freeze-drying, ionic species affect the morphology by inducing specific lamellar structures that close the porous structure. XPS analyses highlight that the carboxylic functions of PS–PAA coating could be deprotonated during the immersion in the alkaline solution. This deprotonation induces polyacrylate-based compounds that could disturb the growth of the cellular polymeric network. This phenomenon should be avoided by using a neutral immersing solution in order to obtain a homogenous and porous monolayer of PS–PAA after freeze-drying.