| Porous polymer monolith is a kind of shaped material with porous structure.Owing to the advantages of low density, large specific surface area, high porosity, fast mass transport rate and ease of chemical modification, polymer monolith has become a hotspot in research and been widely applied in the field of chromatographic separation, adsorption and catalysis support, etc. Up to now, there are various methods to fabricate porous polymer monolith, such as in situ polymerization, gas foaming, self-assembly, phase separation and so forth. Among these methods, phase separation has received extensive attention from scientific researchers in the field of polymer monolith fabrication due to the superiorities of simplicity, flexibility and low-costing. In the phase separation method,after the addition of nonsolvent into polymer solution or cooling it down, phase separation takes place. Subsequently,polymer-rich phase and solution-rich phase are formed. Followed by exchange and freeze-drying, which aiming to remove residual solvent, the skeleton and pore structure of polymer monolith are formed. By varying the fabrication parameters like polymer concentration, phase separation temperature and nonsolvent type, the porous structure of monolith can be controlled.In this paper, porous polycarbonate(PC) monolith with three-dimensional interconnected nanofibrous skeleton was prepared via thermally impacted nonsolvent induced phase separation(TINIPS) method. The impact and mechanism of fabrication parameters(nonsolvent type, phase separation temperature and polymer concentration) on the microstructure of PC monolith was investigated. The main results can be concluded as following:(1) Under the conditions of using water as nonsolvent, high phase separation temperature and low polymer concentration, phase separation was more likely to follow the spinodal decomposition mechanism and resulted in the formation of nanofibrous skeleton and three dimensional interconnected porous structure.(2) Optimized parameters(polymer concentration at70 mg/mL, water used as nonsolvent and phase separation at 4 oC) were proposed.The corresponding sample showed a nest-like structure with nanofibrous skeleton and interconnected mesopores, large specific surface area and narrow pore size distribution were obtained as well.(3) Based on fabrication process and phase separation mechanism, a morphology evolution mechanism was proposed: at the initial stage, a bicontinuous structure is formed and followed by the dispersion of solution-rich droplets in polymer-rich phase, then the droplets gradually grow up and coarsening, an interconnected fibrous network and mesoporous structure is obtained in the end.(4) The microcrystal structure that created by partially crystallized PC chains served as crosslinking points and facilitated the formation of nanofibrous structure.Subsequently, the hydrophicity, oil absorbency and thermal stability of porous PC monolith were investigated and the internal relation between structure and properties was established. The main results are listed as following:(1) Due to the improved surface roughness and ‘air pocket’ effect derived from porous structure, the porous PC monolith was highly hydrophobic(with water contact angle of 143.9o) and showed outstanding properties of self-cleaning and high oil absorbency.(2) The interconnected pore structure promoted diffusion of volatile products and heat transfer during thermal degradation, the onset degradation temperature was lower and the thermal stability was impaired subsequently.(3) The main pathways of thermal degradation for porous PC monolith were hydrolysis/alcoholysis of carbonate,scission of isopropylidene and rearrangement of carbonate, etc.(4) Porous structure was destroyed and a dense surface char layer was gradually formed, which resulted in the increasing of activation energy for thermal degradation. |
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