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New diamines for high-performance poly(ester imide)s (PEsIs) containing dimethyl groups at the ortho-position of amino groups and ester derivatives have been developed to improve optical transparency and glass transition temperature (Tg) while maintaining a low coefficient of thermal expansion (CTE). Four kinds of PEsIs derived from bis(4-amino-3,5-dimethylphenyl) terephthalate (BADMT) and aromatic dianhydrides such as 1,2,4,5-benzenetetracarboxylic dianhydride (PMDA), 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), 4,4’-oxydiphthalic anhydride (ODPA), and 4,4'-(hexafluoroisopropylidene)diphthalic (6FDA) were synthesized via a two-step polycondensation. All PEsIs exhibited outstanding properties, such as light color, a good transmittance of >90% at 550 nm, high Tg above 289 °C and 5% weight loss temperature (483–511 °C), and low CTE (11–68 ppm/°C). The effect of the substituent on molecular packing and properties, including optical and thermomechanical properties for the resulting PEsIs were examined in detail. The temperature-dependent CTE of the PEsIs was determined using molecular dynamics (MD) simulation. The molecular orbital (MO) calculation models support the discussions on the electronic substituent effect of the PEsI main chains. Our molecular architecture and systematic property studies with MD and MO promote a better understanding of the polyimides.

A novel method for synthesizing a microsponge polyimide (PI) film with excellent thermal stability, chemical resistance, and heat insulation performance was developed. The synthesized microsponge PI film has open cells with sizes between 1 and 10 μm and a porosity of 76%. Furthermore, the film contains several layers overlapping in multiple grid structures, which complicates the heat transfer paths. Thus, the heat transfer coefficient of the microsponge PI film is 67% less than that of existing polyimide film (0.054 vs. 0.16 W/m·K). This reduced heat transfer coefficient results in excellent heat insulation performance of the microsponge PI film. The thermal decomposition (pyrolysis) of the microsponge PI starts at 498 °C and its glass transition temperature is 317 °C, which indicates excellent thermal stability. However, its Young's modulus, an indicator of mechanical strength, is nearly 74% less than that of conventional PI film (26 vs. 100.2 MPa).

To investigate the difference of the trifluoromethyl (CF 3 ) group and ether group affecting the optical property of fluorinated polyimides (PIs), we prepared 4,4′-bis(4-amino-2-trifluoromethylphenoxy)diphenyl ether ( 4 ) with three ether groups and 2,2-bis[4-(4-amino-2-trifluoromethylphenoxy)phenyl]hexafluoropropane ( 5 ) with four CF 3 groups with 2-chloro-5-nitrobenzotrifluoride and 4,4′-dihydroxydiphenyl ether or 2,2-bis(4-hydroxyphenol)hexafluoropropane. Two series of organosoluble and light-colored PIs ( 4a – 4c , 5a – 5c ) were synthesized from 4 and 5 with various aromatic dianhydrides: 3,3,4,4-benzophenonetetracarboxylic dianhydride (BTDA) ( a ), 4,4-oxydiphthalic anhydride (ODPA) ( b ), and 4,4-hexafluoroisopropylidenediphthalic anhydride (6FDA) ( c ), prepared through a typical two-step polymerization method. These PIs were soluble in amide polar solvents and even in less polar solvents. The glass-transition temperatures ( T g ) of 4a – 5c were 221–249 °C and the 10% weight-loss temperatures were above 530 °C. Their films had cutoff wavelengths between 339 and 399 nm and yellowness index ranges from 1.95 to 42.60. The dielectric constants estimated from the average refractive indices are 2.59–2.93 (1 MHz). In a comparison of the PI series based on 4 , 5 , and 4,4′-bis(4-amino-2-trifluoromethylphenoxy)biphenyl ( 6 ), we found that the CF 3 group and ether group on the diamine had almost same effect in lowering the color, but the ether group had better thermal stability. The color intensity of the three PI series was lowered in the following order: 6 > 4 > 5 . The PI 5c , synthesized from diamine 5 and dianhydride c , had six CF 3 groups in a repeated segment and ether group at the same time, so it exhibited the lightest color among the three series.

Copolyimide thin films, which has the low level stress and stress relaxation induced by water sorption, were characterized for potential application as encapsulant, stress-relief buffer, and interlayer dielectrics. The polyimides studied are poly(p-phenylene pyromellitimide) (PMDA-PDA), poly(p-phenylene 4,4′-hexafluoroisopropylidene diphthalimide) (6FDA-PDA) and their random copolyimides with various composition. These copolyimide films exhibited good combinations of physical and mechanical properties with low thermal expansion coefficient (TEC) and residual stress behavior by appropriately selecting the ratios of the acid dianhydride component. For these copolyimides, residual stress increased in the range of −5 to 50 MPa, whereas stress relaxation induced by water uptake decreased in the range of 20.5–4.5 MPa at 30°C, 100% relative humidity with increasing 2,2′-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) composition, respectively. The degree of in-plane chain orientation was directly correlated to the residual stress and stress relaxation coefficient (S) in the film, which is an in-plane characteristic. However, the stress relaxation caused by the water uptake was significantly influenced by packing order induced by chain mobility, which might occur during thermal imidization. Overall, the candidate for the low level stress buffer application from the PMDA/6FDA-PDA copolyimide was both the 90/10 (=PMDA/6FDA in molar ratio) and 70/30 copolyimides.

Four different polyimides: rodlike poly( p -phenylene pyromellitimide) (PMDA-PDA), pseudo-rodlike poly( p -phenylene biphenyltetracarboximide) (BPDA-PDA), semiflexible poly(4,4-oxydiphenylene pyromellitimide) (PMDA-ODA) and poly(4,4-oxydiphenylene biphenyltetracarboximide) (BPDA-ODA) were prepared from two different precursors, poly(amic diethyl ester) (PAE) and poly(amic acid) (PAA). For polyimides prepared from the PAE precursors, stress relaxation coefficient at 25°C, 100% RH was 14.76×10 −10 cm 2 /s for PMDA-PDA, 3.29×10 −10 cm 2 /s for BPDA-PDA, 24.91×10 −10 cm 2 /s for PMDA-ODA and 9.34×10 −10 cm 2 /s for BPDA-ODA. For polyimides prepared from the PAA precursors, stress relaxation coefficient at 25°C, 100% RH was 10.50×10 −10 cm 2 /s for PMDA-PDA, 1.91×10 −10 cm 2 /s for BPDA-PDA, 28.84×10 −10 cm 2 /s for PMDA-ODA and 9.10×10 −10 cm 2 /s for BPDA-ODA. The effect of bulky ethyl ester group on the stress relaxation behavior is significantly high in rodlike PMDA-PDA or pseudo-rodlike BPDA-PDA having high chain rigidity and intermolecular packing order. The decreased birefringence, crystallinity and the looser packing in amorphous region of rigid polyimide derived from PAE may induce fast stress relaxation compared to flexible polyimide. Specially, the different chain mobility of polyimide also induced significant effect on the stress relaxation.

Poly(amic diethyl ester) [PAE] precursors of four different polyimides with various chain rigidities were synthesized and thermally converted to the polyimides in thin films: rodlike poly( p -phenylene pyromellitimide) [PMDA-PDA] and poly(4,4′-biphenylene pyromellitimide) [PMDA-BZ], pseudo-rodlike poly( p -phenylene biphenylenetetracarboximide) [BPDA-PDA] and semiflexible poly(4,4-oxydiphenylene pyromellitimide) [PMDA-ODA PAE]. For the polyimide films, water sorption behaviors were investigated by gravimetry and then compared with those of the polyimides prepared from the corresponding poly(amic acid) [PAA] precursors. In addition, their morphological structures were examined by X-ray diffraction. For all the polyimides, both water sorption and morphological structure were dependent upon the history of precursor origin. All the polyimides except PMDA-ODA revealed relatively higher diffusion coefficient of water, higher water sorption, lower intermolecular packing order and lower population of a more ordered phase in the PAE-derived polyimide films than the corresponding PAA-derived polyimide films. For PMDA-ODA, the variations in the water diffusion and sorption due to the precursor origin were relatively small in spite of a relatively large change in the morphological structure. Overall, the effect of precursor origin on the water diffusion and sorption is significantly high in the rodlike or pseudo-rodlike polyimides having a high chain rigidity and high intermolecular packing order, but relatively low in the semiflexible polyimide with a relatively low chain rigidity. In comparison, regardless of the precursor origins, the diffusion coefficient of water was in the decreasing order PMDA-ODA>PMDA-PDA>PMDA-BZ>BPDA-PDA and, however, the water sorption was in the decreasing order PMDA-PDA>PMDA-BZ>PMDA-ODA>BPDA-PDA. These water diffusion and sorption behaviors were understood by considering their chemical affinities to water in addition to the morphological structures.

Several photosensitive polyimide (PSPI) precursors were synthesised by the acid/base complexations of conventional poly(amic acid) precursors with photochemically cross-linkable 2-(dimethylamino)ethyl methacrylate. PSPIs in films were prepared from the photosensitive precursors by thermal imidisation, whereas the corresponding PIs in films were prepared from the conventional poly(amic acid)s: rod-like poly(p-phenylene pyromellitimide) (PMDA-PDA), rigid (poly(p-phenylene biphenyltetracarboximide) (BPDA-PDA), semi-flexible poly(4,4′-oxydiphenylene biphenyltetracarboximide) (BPDA-ODA), semi-flexible poly(4,4′-oxydiphenylene pyromellit-imide) (PMDA-ODA), and flexible poly(4,4′-oxydiphenylene benzophenonetetracarboximide) (BTDA-ODA). Water sorption and diffusion behaviours in the PSPIs were gravimetrically measured at 25°C in 100% relative humidity and compared with those of the corresponding PIs. For the PSPIs as well as the PIs, the water sorption and diffusion behaviours were nearly Fickian, regardless of the backbone chemistry. However, those were strongly dependent upon the polyimide backbone chemistry and precursor origin. In addition, those in polyimides were affected by the bulky photosenistive groups, even though they were temporarily linked to the precursor polymers and then debonded from the backbones and ultimately outgassed during the thermal imidisation process. For PMDA-PDA, BPDA-PDA and BPDA-ODA, the PSPIs absorbed water more quickly than the corresponding PIs, whereas for PMDA-ODA and BTDA-ODA, the PSPIs absorbed water less quickly than the corresponding PIs. In contrast to the water diffusion, all the PSPIs absorbed slightly more or a great deal more water than the corresponding PIs, depending on the backbone chemistry. All the measured water sorption and diffusion behaviours in the PSPIs were understood by considering changes in the morphological structure (namely, chain order and orientation), voids, and residues possibly induced by the bulky photosensitive groups, in addition to the Tg as well as the chemical affinity to water.

Poly(4,4′-oxydiphenylene pyromellitimide) (PMDA—ODA) films of 10–109 μm thickness were prepared from its poly(amic acid) precursor by thermal imide-ring-closure formation at various temperatures. Water sorption in the films was measured at 25°C over 22–100% relative humidity using an electromicrobalance. Water diffusion in all the films was a nearly Fickian process despite the morphological heterogeneity due to the ordered and less ordered phases. Depending upon humidity, film thickness and imidization history, the diffusion coefficient and water uptake varied in the ranges of 1 × 10 −9 to 3 × 10 −9 cm 2 s −1 and 0.4 to 4.5 wt%, respectively. Overall, both the diffusion coefficient and the water uptake increased with increasing humidity and film thickness, but decreased as the imidization temperature and time increased. The water sorption results were interpreted by consideration of morphological variations (molecular order, chain orientation and microvoids) due to film thickness and imidization history.