Nanoporous Polymer Films from Immiscible Polymer Blends: Pore Size and Composition Dependence
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Abstract Nanoporous polymer films have been prepared using immiscible blends of polyetherimide (PEI) and poly(caprolactone diol) (PCLD). The films were prepared by spin-coating from a common solvent – dichloromethane (DCM). To create the nanoporous films, PCLD was removed by immersing the films in acetone, which dissolves PCLD only. Field emission scanning electron microscopy was used to study the porous structure. The pore structure of the films was affected by many factors such as composition of the blend, molecular weight, and various processing parameters. The formation of nanometer size pores was mainly due to the fast phase separation process during spin-coating. The pore size and pore size distribution was found to depend on the blend composition. Introduction In recent years, nanoporous materials have been receiving considerable interest from industry because of their several specific characters such as high surface area, low dielectric constant, and low refractive index. Some of the most interesting applications for nanoporous materials are as nanoporous films, which can be used as a catalyst, absorbent, filler system, gas separation layer, or biological sensor. There are many ways to make nanoporous films such as by lithography or the sol-gel method.1 One other possible approach is to create a 2-phase mixture and remove the minority phase through physical or chemical means. This is the approach taken in this study through the use of immiscible polymer blends. Although the preparation of the nanoporous structure is important, a more important issue is the control of the size, distribution and connectivity of the pores within the material. There are many factors that would influence these factors such as polymer blend composition, molecular weight of the blend components, thermal history, and processing conditions. In order to study the effects of these on the final pore structure, a careful characterization of the pores is necessary. Various characterization methods can be used for this purpose, such as mercury injection, gas sorption, permeation test, and optical methods (direct observation).2-5 Most of these methods characterize the pore structure by determining various macroscopic properties, which are unique to the pore structure.2 Pore structure models are then used to estimate average parameters. However, in the optical methods, pore structure can be directly observed and pore structure parameters can be obtained by image analysis. The disadvantage of these methods is that the statistics is limited due to the small amount of samples (observable pores). To obtain
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global information from the randomly selected regions (which are at the length scales of micrometers or less), statistical methods can be used.6 In our study, PEI and PCLD were used to prepare a nanoporous polymer film. The two polymers were dissolved in their common solvent (DCM) and phase separated during spin-coating. Then, the PCLD phase was extracted from the blend by a selective solvent (acetone), thereby creating the pores in the
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