Hole Structure In Ethyl Cellulose Membrane Studied By Electrochemical Impedance Spectroscopy

Polymer functional membrane materials have the advantages of high efficiency,low energy consumption,low cost,cleaning and so on.They are widely used in medicine,food,environmental protection,electronics,chemical industry and other fields.Cellulose ester membrane material is one of the earliest and most widely used polymer functional membrane materials.Among them,ethyl cellulose(EC)has the properties of good film-forming,hydrophobic and stable chemical,which can be used to prepare polymer membranes with excellent permeability.With the application of EC membrane in erasable coating,drug sustained-release coating,metal protective film,liquid band aid and so on,it is particularly important to explore the hole structure and screening mechanism of EC membrane in acid environment.Electrochemical impedance spectroscopy(EIS)is a non-destructive in-situ detection method,which can monitor the behavior of the membrane in the actual working environment.It is of great significance to analyze the relationship between the hole structure of EC membrane and the transport behavior of ions in the membrane.In order to investigate the correlation between the hole structure of EC membrane and the acid treatment time,the microstructure of EC membrane soaked in HCl solution for different time was evaluated.SEM showed the holes around 100 nanometer enlarged in the samples immersed for longer time.XRD and DSC showed that EC remained amorphous and its crystallinity almost being unchanged before and after EC membrane soaking.ATR-FTIR indicated that the hydroxyl group on the EC molecular chain had undergone a substitution reaction and the ether bond had undergone a cleavage reaction.The contact angle results showed that EC membrane remained hydrophobic after soaking.EIS implied EC membrane resistance decreased,while positron annihilation lifetime measurement indicated the free-volume holes enlarged with increasing soaking time.All the results are in agreement with each other.Based on the pore model of membrane materials,a semi-empirical formula is proposed in this paper.It is found that the membrane resistance is inversely proportional to the cross-sectional area of the tunnels for ion permeation.The experimental data confirmed that free-volume holes acting as the tunnels for ions permeating through the EC membrane.This could be a potential method to evaluate the tortuosity of holes in membranes.In this paper,EC/MMT composite membranes were prepared.The hole structure of EC/MMT composite membrane during the acid treatment were characterized by XRD,SEM and EIS.SEM pictures showed the formation of micrometer-size holes after adding montmorillonite(MMT)in EC matrix.A new time constant appeared in the impedance spectrum of EC/2%MMT,EC/3%MMT and EC/5%MMT composite membrane,which meant the structure of composite membrane had changed.These rigid platelets would confine the relaxation of EC chain segments around MMT in the phase inversion process.During the acid treatment,EIS results indicated these micrometer-size holes would tend to shrink while the small holes in EC matrix enlarged.The shrinkage of the micrometer-size holes could be contributed by the chain disentanglement due to the acid treatment.The above results suggested that it was easier to distinguish the hole variation in EC/MMT composite membrane by EIS,which was beneficial for analyzing the mechanism of the drug releasing process.