Synthesis And Properties Of Fluorine-containing Polyimides For Optical Waveguide Devices

Polymer optical waveguides have attracted considerable attentions for their possible application as light transmission components in optical communication systems. For advanced integrated optical waveguide applications such as optical attenuators, optical interconnects, splitters and arrayed waveguide gratings (AWG), the polymer optical materials should satisfy several requirements. First, it should have a low optical transmission loss especially at the telecommunication wavelength region of 1.3 and 1550 nm. Second, it should have the low birefringence that is the difference of the refractive index for the transverse electric (TE) and transverse magnetic (TM) modes in the waveguide. Last, it should have tunable refractive index (RI). For example, in the arrayed waveguide gratings, the polymer used as the core material must have higher RI than that of the cladding material, at the same time, the core and the cladding materials should be compatible to have similar thermal expansion coefficients.In the first part, fluorinated polyimides containing flexible ether linkages and pendent trifluoromethyl (CF3) and chloro (Cl) groups were synthesized from1,4-bis(3,4-dicarboxyphenoxy)benzene dianhydride (OPDA) (I), 4,4-oxydiphthalic anhydride (HQDPA) (II) and 4,4-hexafluoroisopropylidene diphthalic anhydride (6FDA) (III) with three types aromatic bis(ether amine)s [1,4-(4-amino-2-trifluoromethyl-phenoxy)-2-(3'-trifluoromethylphenyl)benzene(a )], [(1,4-(4-amino-2-trifluoromethylphenoxy)-2-(3'5'-ditrifluoromethylphenyl) benzene(b)) and [(1,4-(4-amino-2-trifluoromethylphenoxy)-2- (3'-trifluoromethyl -4'-chlorophenyl)benzene) (c)] via ring-opening polycondensation. The molecular weights (Mn's) and polydispersities (Mw/Mn's) of the polyimides were determined by GPC and were in the range 25 000-50 000 and 1.93-2.13, respectively. The glass transition temperatures (T_g) of the polymers varied from 236℃to 246℃. The temperatures at 5% weight loss were between 530℃and 590℃under nitrogen. The homopolymer and copolymer with both the bis(ether amine)s exhibited controllable refractive index. The refractive indices of the films at 1550 nm were in the range 1.5382-1.5991 and the birefringences at 650 nm were around 0.0122-0.0157. Near-IR results indicated that the polyimides had desirable properties for low loss optical waveguide applications.In the second part, a novel triamine monomer, 1,3,5-tris(2-trifluoromethyl- 4-aminophenoxy) benzene (TFAPOB), was synthesized from phloroglucinol and 3- trifluoromethyl-chloronitrobenzene, and it was successfully polymerized into soluble hyperbranched polyimides (HBPIs) with commercially available dianhydrides: 4,4-(hexafluoroisopropylidene) diphthalic anhydride (6FDA), 4,4-oxydiphthalic anhydride (ODPA), and 3,3,4,4-benzophenonetetracarboxylic dianhydride (BTDA). Different monomer addition methods and different monomer molar ratios resulted in HBPIs with amino or anhydride end groups. From ¹H NMR spectra, the degrees of branching of the amino-terminated polymers were estimated to be around 0.80, for anhydride-terminated HBPIs, and 0.60 for amino-terminated HBPIs, respectively. The HBPIs had moderate molecular weights with broad distributions. All polymers showed good thermal properties with 5% weight-loss temperatures (T5's) above 500℃and glass-transition temperatures (T_g's) of above 200℃for various dianhydrides. The anhydride-terminated HBPIs showed lower T₁₀ and T_g values than their amino-terminated counterparts.Furthermore, the fluorinated triamine monomer and commercially available dianhydride monomers were condensed to afford a series of anhydride-terminated poly(amic acid) precursors, then the precursors were end-capped by 3,5-ditrifuoro methylaniline (2CF₃) and 3,5-dichloroaniline (2Cl) and chemically imidized to yield a series of trifluoromethyl and chloro-terminated hyperbranched polyimides (HBPIs). The HBPIs also had high glass transition temperatures in the range of 200-240℃and good thermal stabilities in the range of 500-530℃. The refractive indices (RIs) of the polymers could be readily tuned by simply changing the ratio of the 2CF₃ and 2Cl terminated monomers used in the end-group modifications. Furthermore, the polymers showed low attenuation loss at optical communication wavelength due to the presence of the trifluoromethyl and chloro groups. The birefringences (Δn=n_TE-n_TM) of HBPIs are 0.006 and 0.002, respectively. It is known that the birefringence of polymers may be affected by several factors, for example, chain flexibility and geometry of the repeat units. The reduced birefringence is likely to be contributed to by the geometry of the repeat units but not by the few flexible ether groups contained in the polymers. In the hyperbranched polyimide, the triamine, i.e., the'core'molecule, is expected to define the configuration of the repeat units primarily. The single-crystal X-ray analysis reveals that the triamine in the repeat units is non-planar and asymmetric in geometry. Therefore, the triamine monomer in the polymers can reduce the orientation of the bonds involved in the polymer backbone and thus greatly reduces the birefringence. Based on this point, HBPI from the new triamine monomer could challenge the anisotropy and larger birefringence of the linear polyimide (PI).In the third par, the waveguide devices were fabricated by reactive ion etching (RIE). These fluorinated polyimides exhibited good optical propagation at wavelength of 1550 nm.