Effects of Thermal Treatments and Dendrimers Chemical Structures on the Properties of Highly Surface Cross-Linked Polyimide Films Youchang Xiao, † Lu Shao, † Tai-Shung Chung,* ,† and David A. Schiraldi ‡ Department of Chemical & Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, and Department of Macromolecular Science & Engineering, Case Western Reserve University, Cleveland, Ohio 44106 We have conducted an extensive study to investigate the effects of thermal treatments and dendrimers’ structures on the chemical and physical properties of the surface-modified polyimide films. The amidation and cross-linking reaction between G0 PAMAM dendrimers and polyimide were examined by XPS (X-ray photoelectron spectrometer) and FTIR-ATR (attenuated total reflection) measurements after thermal treatments under different temperatures. Moderate thermal treatment (120 °C) is proven to be able to induce the highly amidation reaction and to increase the degree of cross-linking on the polyimide surface. The gas separation performance of modified polyimide films is significantly improved due to the enhanced “molecular sieving” ability by dendrimer modification and the stronger interactions between the polyimide chains, such as covalent cross-linking bonding and hydrogen bonding. When the temperature of treatment reaches 250 °C, 1 H NMR and GPC test implied that the cross-linking structure between polyimide chains is broken and that the degradation of polyimide backbone chains also occurs. Gas permeation tests also indicated that high-temperature treatment of dendrimer-modified polyimide films is not beneficial to separation. In addition, the performance comparison between different dendrimers, PAMAM- and DAB-modified films, is carried out. The chemical structural differences in PAMAM and DAB dendrimers have also been verified as one of the important factors in determining the properties of modified polyimide films. At room temperature, PAMAM dendrimers show stronger cross-linking ability. Introduction Polyimide films are of widespread interest in elec- tronics and gas separation applications because of their superior thermal, electrical, and membrane properties. 1-4 To enhance the selectivity and gas permeability of polyimides for gas separation, researchers have sug- gested that their polymer backbone chains must be stiffened by inhibiting the intra-segmental mobility and that the inter-segmental packing of polymer chains should be simultaneously prevented. 5 The syntheses of various polyimides containing different monomers have been carried out 6,7 as a means of modifying gas trans- port properties through changes in polymer solid-state structure. 8,9 These attempts appear to have reached the limit of the tradeoff curve for gas permeability and selectivity. 10,11 Therefore, an alternative method, cross- linking, has been developed as a means of altering membrane structure/performance and has utilized dif- ferent techniques including ultraviolet (UV) light ir- radiation, high-temperature thermal treatment, and chemical modification. 12-28 With an increasing degree of cross-linking, higher gas selectivity can be achieved due to reduced intra-segmental polymer chain mobility. It has also been shown that a cross-linked structure will limit material swelling in the presence of plasticizing agents as well as promoting chemical and thermal stability. A negative attribute of polymer cross-linking is a decrease in gas permeabilities due to higher chain packing. Novel cross-linking reagents, star-like dendrimers such as polyamidoamine (PAMAM) and diaminobutane (DAB), 29,30 have been recently reported. 31-33 The high- density functional groups at the surface or in the cavities of dendrimers offer numerous potential applica- tions based on their chemical, physical, optical, multi- redox, and catalytic properties. 34,35 The high density of terminal amine groups in dendrimers provides a large number of reactive sites for cross-linking when used as a modifier for polymides and offers the potential to improve the gas selectivity and physical properties of those polyimide films. Alternatively, the large molecular size of dendrimers may prevent polyimide chains from packing when these two materials are combined. More- over, the steric hindrance of dendrimers may restrict cross-linking modification to the surface of films, favor- ing high gas permeability. Dendrimers can play three important roles in this approach: (1) they are structural building blocks that provide multiple covalent branching sites for amidation to construct the cross-linked net- work; (2) they act as spacers in the polymer matrix that prevents the chain from packing; and (3) they offer free amine groups with strong affinity for CO 2 and facilitate the transport of that permeant gas. 36-38 Thus, the surface modification of polyimide films by dendrimers is a potential approach to obtain materials with better gas separation performance. The objective of this paper is to describe the effects of thermal treatment on the chemical and physical properties of dendrimer-modified polyimide films. The surface modification of the polyimide during thermal treatment was characterized by FTIR-ATR (attenuated total reflection), XPS (X-ray photoelectron spectroscopy), * To whom correspondence should be addressed. Fax: (65)- 67791936. E-mail: chencts@nus.edu.sg. † National University of Singapore. ‡ Case Western Reserve University. 3059 Ind. Eng. Chem. Res. 2005, 44, 3059-3067 10.1021/ie048837g CCC: $30.25 © 2005 American Chemical Society Published on Web 03/19/2005