Preparation and properties of semi-alicyclic thermoplastic polyimide sheets with improved optical properties and good mechanical properties from stereoisomeric hydrogenated pyromellitic dianhydrides and fluorinated diamine

Abstract Two semi-alicyclic thermoplastic polyimides (PI) have been prepared from stereoisomeric hydrogenated pyromellitic dianhydrides, including 1S,2R,4S,5R-hydrogenated pyromellitic dianhydride (ccHPMDA or H-PMDA) and 1R,2S,4S,5R-hydrogenated pyromellitic dianhydride (ctHPMDA or Hʹʹ-PMDA) and a fluorine-containing diamine, 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (BDAF), respectively. The derived PI-1 (ccHPMDA-BDAF) and PI-2 (ctHPMDA-BDAF) resins were easily soluble in polar aprotic solvents, such as N -methyl-2-pyrrolidinone (NMP) and N,N -dimethylacetamide (DMAc) and meanwhile they also exhibited good thermoplasticities. Thus, the PI films (thickness: ~25 µm) and sheets (thickness: ~3 mm) were separately prepared from the PI solutions in DMAc and the PI powders, respectively. The stereoisomeric effects in the ccHPMDA-derived PI-1 and the ct-HPMDA-derived PI-2 apparently affected the melt-flowability of the polymers. PI-2 exhibited lower melting viscosities than that of the PI-1 counterpart. The PI sheets exhibited obviously reduced optical transparency compared with those of the PI films; however better optical transparency than those of the common thermoplastic PI sheets. For example, PI-2 sheet showed the optical transmittance value at the wavelength of 760 nm (T 760 ) of 69.6%, which was 19.5% lower than that of the analogous PI-2 film (T 760 = 89.1%), however 50.3% and 65.7% higher than those of the commercially available PI-ref1 sheet (T 760 = 19.3%) derived from 3,3ʹ,4,4ʹ-oxydiphthalic dianhydride (ODPA) and 4,4ʹ-oxydianiline (ODA) and the PI-ref2 sheet (T 760 = 3.9%, Aurum® PL450C, Mitsui Chem. Co, Ltd., Japan), respectively. Meanwhile, the PI-1 and PI-2 sheets showed the heat deflection temperatures (HDT) of 261.0 o C and 265.0 o C, respectively, which were obviously higher than those of the referenced PIs. At last, the PI sheets exhibited good mechanical properties, including high flexural strength (130.0 ~ 132.0 MPa), flexural modulus (2.87 ~ 2.96 GPa), compression strength (124.4 ~ 149.3 MPa), compression modulus (1.49 ~ 1.59 GPa), and impact strength (129.0 ~ 139.6 KJ/m 2 ).