Polymeric microstructures induced by freeze-drying process: Comparative study of PS_(41) - PAA_(271) morphologies after impregnations in water and in an alkaline solution

摘要

Surface of polymeric coating can be modified after being submitted to treatment based on freeze-drying. Morphology of polystyrene-b-poly(acrylic acid) (PS_(41)-PAA_(271)) 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.