A water-free route to porous materials via cryoextraction and supercritical drying

Abstract

In this study a novel process combining freezing, cryoextraction and supercritical drying for producing porous materials is developed. The process consists of three main steps: first of all, freezing a polymer/DMSO solution at −28 °C to obtain a monolith. Second step is solvent exchange with EtOH at −28 °C to extract DMSO crystals, obtaining an alcogel at the end of this process. The last part of the process is drying the alcogel with scCO2 at 120 bar and 50 °C for 3 h. The method has been used for the production of polyacrylonitrile (PAN) aerogels, it is demonstrated that for this material a high quality aerogel is obtained at 5 wt % volumetric shrinkage 36.8 6.1 %, porosity is 93.5 0.6 % and 119 18m2 g−1 of specific surface area (SSA). In addition to PAN, other polymers such as poly(vinyl alcohol) (PVA) and agar-agar and their combination with PAN are tested. Pure PVA aerogels show the higgest value of SSA at 7.5 wt % of 107 15m2 g−1. Both combinations result in a aerogel with worse quality than pure PAN. In particular, porosity and linear shrinkage experimented by PAN/PVA aerogels have a exponential relation with PVA concentration. In addition to standard conditions, liquid nitrogen at −196 °C is used as a freezing agent obtaining aerogels with lower SSA that lack physical integrity. However, whereas in conventional freezing at −28 °C directional freezing is observed, more pronounced with the decrease in concentration, for aerogels frozen at −196 °C non-hierarchical pore structures are found. Generally speaking, it has been observed that lower concentrations leave more space for DMSO crystals to grow due to looser arrangement of polymer chains. At the same time, pore walls become thinner making the network weaker leading to higher shrinkage.

En el trabajo se ha desarrollado una nueva ruta, exenta de agua, para la preparación de materiales porosos a partir de polímeros que no pueden ser utilizados en la ruta convencional de producción de aerogeles. La nueva ruta está compuesta por cuatro etapas: disolución del polímero en DMSO, posteriormente congelación a -28º C, extracción del disolvente con etanol a baja temperatura- 28º c y secado supercrítico mediante CO2 a 120 bar y 50º C