Coefficients of thermal linear and volumetric expansion (CTE, CVE) of crystal lattice for 13 fully aromatic crystalline polyimides (PIs) were evaluated from lattice parameters measured from variable-temperature (VT) synchrotron X-ray diffraction patterns, and the effects of chemical structure on CTE and CVE are discussed. The smallest CVE (116 ppm K–1) was observed for PMDA-PPD with the simplest rigid-rod structure, and the largest CTE anisotropy was observed for PMDA-ODA containing an ether linkage with an extraordinarily negative CTE a (−44 ppm K–1). The values and anisotropy of the CTEs strongly depended on the crystalline structure, whereas the CVEs were negatively correlated with the weight density, regardless of the PI type. The correlation was explained using the Grüneisen equation, (∂V/∂T) P /V = γC v,interχ, assuming that isothermal compressibility χ dominates the equation. An increase in the weight density and/or molecular weight of repeating units effectively suppresses the CVEs of crystalline PIs.

This study proposes a molecular design and synthetic route to novel colorless, transparent polyimides that exhibit dual fluorescence/phosphorescence emission at long wavelengths applicable to solar energy conversion. Partially alicyclic polyimides (Ac-PI-TBs) based on Tröger’s base (TB) and denoted as Ac-PI-TB-1, Ac-PI-TB-2, and Ac-PI-TB-3 were synthesized by in situ TB formation. The resulting Ac-PI-TBs are readily soluble in common organic solvents and have good mechanical properties with tensile strengths of 72.5–102.3 MPa, elongations at breaks of 12.5–75.0%, low dielectric constants (∼2.66) and low thermal diffusivities (D ⊥ ≤ 7.7 × 10–8 m2/s), and good thermal stability. The films are totally colorless and transparent with transmittances above 77% at 400 nm. The films also show dual fluorescence and phosphorescence emissions with Stokes shifts as large as 11 421 cm–1 at low temperatures. The results highlight the possible application of these films in the spectral conversion of unused UV solar radiation to useful visible light.

Pressure-induced variations in the main chain and aggregation structures of colorless and transparent semialiphatic polyimide (PI) films were investigated by optical microscopy, UV-vis absorption, and fluorescence spectroscopy up to 8 GPa. Upon application of pressures up to 2 GPa, a gradual volumetric compression was clearly observed by microscopy, and definite bathochromic shifts of locally excited (LE) absorption bands were detected, which was attributed to the compression of interchain free volume and enhanced intermolecular interactions. In addition, a significant reduction in fluorescence intensity was observed for PIs with quasilinear structures below 2 GPa due to enhanced energy transfer in the excited states caused by the densification of PI chain packing. In contrast, the volumetric compression of the PI films and bathochromic shifts of the LE absorption bands were gradually reduced at pressures above 2 GPa. The former is closely correlated with the bulkiness and flexibility of the alicyclic diamine structure. The latter reflects the intense compression stress generated around the dianhydride moiety, associated with the deformability and in-plane orientation of the main PI chains. High-pressure experiments on PI films are beneficial to investigate variations in aggregation structures and local electronic structures of PI chains induced by dense molecular packing and enhanced intermolecular interactions.

A series of semi-aliphatic polyimide (PI) copolymers (CoPIs) were prepared through the copolymerization of two dianhydrides, 1-hydroxy pyromellitic dianhydride (PHDA) and 4,4′-oxydiphthalic anhydride (ODPA), with 4,4′-diaminodicyclohexylmethane, in which the PHDA molar ratio was controlled at less than 5% to suppress aggregate formation in a solid state. Upon increasing the molar ratio of the PHDA unit, the fluorescence color of the CoPI films continuously changed from pink to yellow through an orange color owing to the enhanced emission from the anionic form of the PHDA unit, and the efficiency of the energy transfer from the locally excited S1 state to the anionic state simultaneously increased. Moreover, the CoPI film with a PHDA content of 3 mol % that formed on a silica substrate was colorless and transparent and showed bright orange fluorescence. However, this CoPI film formed on a soda-lime-silica (soda) glass substrate showed a pale-yellow color as well as yellowish fluorescence originating from the anionic form owing to the high basicity of the soda glass. To suppress the absorption and emission from the anions, a small amount of sulfuric acid was doped into the CoPI film, and a colorless and transparent film exhibiting large Stokes-shifted orange fluorescence was successfully obtained on the soda glass substrate. The wavelength-converting spectrum of the CoPI demonstrated that UV irradiation in solar light was efficiently absorbed, and its energy was converted into visible light between 500 and 700 nm with a quantum efficiency of 20%. These CoPI films exhibiting large Stokes-shifted ESIPT fluorescence on both silica and the soda glass substrates are promising materials for solar spectral conversion applications.

The relationship between the photoexcitation dynamics and the structures of semi-aliphatic polyimides (3H-PIs) was investigated using ultrafast fluorescent emission spectroscopy at atmospheric and increased pressures of up to 4 GPa. The 3H-PI films exhibited prominent fluorescence with extremely large Stokes shifts (Δν > 10 000 cm −1 ) through an excited-state intramolecular proton transfer (ESIPT) induced by keto−enol tautomerism at the isolated dianhydride moiety. The incorporation of bulky −CH 3 and −CF 3 side groups at the diamine moiety of the PIs increased the quantum yields of the ESIPT fluorescence owing to an enhanced interchain free volume. In addition, 3H-PI films emitted another fluorescence at shorter wavelengths originating from closely packed polyimide (PI) chains (in aggregated forms), which was mediated through a Forster resonance energy transfer (FRET) from an isolated enol form into aggregated forms. The FRET process became more dominant than the ESIPT process at higher pressures owing to an enhancement of the FRET efficiency caused by the increased dipole−dipole interactions associated with a densification of the PI chain packing. The efficiency of the FRET rapidly increased by applying pressure up to 1 GPa owing to an effective compression of the interchain free volume and additionally gradually increased at higher pressures owing to structural and/or conformational changes in the main chains.