Polyimide Nanofoams

Last but not least, some of us have recently synthesized polyimide-aliphatic polyester triblock and graft copolymers in collaboration with Hedrick and his coworkers [ 97,98]. Well-defined aminophenyl or diaminophenyl end-functional polyester oligomers have been synthesized on purpose and used as end-cappers or macromonomers leading to the aforementioned triblock or graft copolymers, respectively. The polyimide-polyester copolymers so obtained proved to be highly efficient promoters of polyimide nanofoams (for more details see Sect. 4.2). 

Fig. 7. Approach to the preparation of polyimide nanofoam using microphase separated block copolymers...

The successful implementation of the block copolymer approach to polyimide nanofoams requires the judicious combination of polyimide with the thermally labile coblock. The material requirements for the polyimide block are stringent and require thermally stable, high Tg polyimides which can be readily... 

J.L. Hedrick, K.R. Carter, R. Richter, R.D. Miller, T.P. Russell, Polyimide nanofoams from aliphatic polyester-based copolymers, Chem. Mater. 10 (1) (1998) 39 9. 

The glass transition temperatures ofthe polyimides are 195-250 °C their 10% weight loss temperamres (dynamic thermogravimetric analysis, air, AT = 4.5 °C/min) are 390-422 °C. Of particular interest are the dielectric constants of these polyimides. At a relative humidity of 50% these constants are 2.70-2.90 and are comparable with constants of the best fluorinated polyimides [21, 50-55]. The lowest dielectric constant (2.70) was observed for polyimide based on 6F dianhydride, containing the highest amount of fluorine. Thermal treatment of this polymer film at 280-290 °C for 1 hour led to a decrease (2.45) of dielectric constant due to the possible formation of nanofoams [56]. 

Although nanofoam formation was observed in the amorphous high T polyimides containing PPO blocks, the volume fraction of voids does not directly correspond to the volume fraction of propylene oxide in the initial copolymer. A decrease in the volume fraction of voids present in the matrix in comparison to the initial volume fraction of the propylene oxide in the copolymer can be rationalized by considering the distribution of sizes in the propylene oxide phase. Since the copolymer blocks are not monodisperse, a distribution of propylene oxide microdomain sizes is expected. The pressure exerted on a pore will vary as y/r, where y is the surface tension and r is the pore radius. Consequently, the higher pressure on the smaller pores will tend to cause them to collapse. In addition, there will always be a small amount of propylene oxide co-mixed within the imide phase, which will also be removed upon decomposition. However, this... 

Table 6. Nanofoam properties from polyimide/PPO, ABA triblock copolymers...

The generation of nanofoam materials by the self-assembled block copolymer approach has been demonstrated. The nanofoam approach has been shown to work for a variety of different polyimide matrices, in combination with a variety of different thermally labile coblocks. Numerous synthetic approaches were surveyed as a means to appropriately functionalized thermally labile coblocks, including group transfer, anionic, ROP living free radical and others. (The block copolymers were prepared either by the poly(amic acid) or poly(amic alkyl ester) precursor to the polyimide.) In each case, microphase separated morpholo-... 

Y. Charlier, et al.. High temperature polymer nanofoams based on amorphous, high Tg polyimides, Polymer 36 (5) (1995) 987-1002. 

Charlier, Y, J. L. Hedrick, T. P. RusseU, A. Jonas, and W. VoUcsen. 1995. High temperature polymer nanofoams based on amorphous, high Tg polyimides. Polymer 36 (5) (March) 987-1002. doi 10.1016/0032-3861(95)93599-H. http //linkinghub.elsevier.com/retrieve/ pii/003238619593599H. 

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