Transparent fluorinated poly(imide siloxane) copolymers with good adhesivity

A series of fluorinated poly(imide siloxane) (PIS) copolymers were prepared with fluorine dianhydride 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and two kinds of functional diamine, 2,2'-bis(trifluoromethyl)-[ 1,1'-biphenyl]-4,4'-diamine (TFMB) and 1,3-bis(3-aminopropyl) tetramethyldisiloxane (GAPD) via a conventional two-stage process with thermal imidization. The soft and hard segments in the copolymer chain were composed of 6FDA-GAPD and 6FDA-TFMB, respectively. The structure of the resulting copolymers was determined by Fourier transform infrared (FT-IR) and H-1 nuclear magnetic resonance (H-1 NMR) spectroscopy. Optical, adhesion, mechanical, and thermal properties of these PIS copolymers have been evaluated with respect to real siloxane loading kept to 0, 1.4, 2.6, 4.1, and 5.5 wt% in the copolymers, which is lower than the theoretical content of siloxane loading due to a disproportionation reaction of the siloxane chain. The resulting PIS copolymers showed excellent optical transparency in the UV-visible range and good adhesion property. The cutoff absorption wavelength (lambda (0)) of the resulting PIS copolymers films was less than 345 nm and the maximum transmittance (lambda (max)) was up to 91%. The introduction of siloxane segment dramatically improved the adhesion property of PIS copolymers from 1.29 to 2.26 MPa (measured by means of shear strength) compared to the fluorinated homopolyimide (6FDA-TFMB) synthesized without siloxane moiety. The thermal and mechanical properties of the resulting PIS copolymers decreased slightly with the increase in siloxane loading but still remain a good level. Due to the addition of trifluoromethyl (-CF3) group and siloxane moiety, the solubility of PIS films was greatly enhanced and they were able to be dissolved in polar aprotic solvents, such as DMAC, DMF, and NMP. The contact angle, surface energy and water absorption were also studied in order to discuss their hydrophobic property.