Evolution of Micropores in Polymer-Polymer Composite Thin Films
Discussion and Conclusions
The film which is made by LBL assembly technique can generate microporous structures as mentioned in the introduction. The formation of micro-pores starts from dissolving one component in extreme pH condition such as pH 2.5 or pH 12.5. Because the LBL film has complex weaving structures of two components (figure 1. (A)), the initial shape of pores which are generated by removing one component shows fairly serpentine nano-structures (figure 1. (B)). As the dissolution of a component proceeds, the initial serpentine nanopores become larger and rounder micro-pores (figure 2.). Given the composition of two components in the LBL film, the maximum volume ratio of the pores varies from 10% to 60%.
In this project, we simulated the evolution from nano-porous to micro-porous structures in the LBL thin film. The initial shape of the nanopores in the simulation is well matched to the serpentine nanopores. As time proceeds, the shape of the nanopore changes to smoother and larger micropore which is depicted by employing two simulation principles; surface diffusion and migration. Furthermore, the size of the resulting micropore is restricted to 60% of the area of whole matrix although the shape changing (annealing) step still proceeds. The result movies clearly show how the polymeric nanopore evolves to the micropore. As real phenomena involves the creation of serpentine nanopores, the growth of nanopores to micropores with continuous removing of one LBL polymer component, and the shape stabilizing of micropores with conserving the porous ratio in the film, the resulting simulation includes the last two evolution steps. The creation of serpentine nanopores is set to the initial shape of the nanopore.
(A) (B)
Figure 1. (A) The morphological structure of a LBL film which shows well mixed and fuzzy matching of two polymeric components. (B) The initial micro pores have serpentine structures because one component of the LBL film start to dissolve.
Figure 2. The micro-porous evolution displaying that the size of pores is increasing with merging many nanopores.