Pressure-Induced Variations of Aggregation Structures in Colorless and Transparent Polyimide Films Analyzed by Optical Microscopy, UV–Vis Absorption, and Fluorescence Spectroscopy

Abstract

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.

Synthesis and Characterization of White-Light Luminescent End-Capped Polyimides Based on FRET and Excited State Intramolecular Proton Transfer

“…However, since it has been reported that PI films with acid dianhydrides, such as PMDA and BPDA, emit fluorescence in the visible region, the improvement of the fluorescence property of PI is desired [16][17][18][19]. To suppress the CT and LE(n−π*) transitions and enhance the LE(π−π*) transitions between the S 0 and S 1 states, we have developed a series of highly fluorescent PIs with high Φ values by combining dianhydrides with high electron-accepting ability, whose HOMO-LUMO transitions are LE(π−π*), and alicyclic diamines with low electrondonating ability [15,[20][21][22][23][24][25][26][27][28][29]. With other methods, highly fluorescent fully aromatic PIs have been developed by introducing aggregation-induced emission (AIE)-active bulky moieties, such as modified triarylamine [30,31] or tetraphenylethylene structures [32], in their main or side chains.…”

Soluble Polyimides Bearing (cis, trans)-Hydrogenated Bisphenol A and (trans, trans)-Hydrogenated Bisphenol A Moieties: Synthesis, Properties and the Conformational Effect

“…Over the years, there have been numerous reports on the incorporation of cyclohexane into the backbone of polyimides to obtain highly transparent, readily soluble polyimides. S. Ando et al [18] utilized a-BPDA/CHDA to developed colorless and processable semialiphatic polyimide films; Hasegawa et al [19] synthesized 1S,2S,4R,5R-cyclohexanetetracarboxylic dianhydride (H′PMDA), and H′PMDA based polyimides were approximately colorless with a transmittance of up to 90.3% at 400 nm; Fang et al [20] developed cis -1,2,3,4-cyclohexanetetracarboxylic dianhydride ( cis -1,2,3,4-CHDA), and polyimides bearing cis -1,2,3,4-CHDA units presented good thermal, mechanical, and soluble properties, and the cut-off wavelength was around 270 nm. These results suggested that the cyclohexane moiety in the backbone of polyimides promoted the ideal properties of the polymer.…”

Measuring structural inhomogeneity of a helical conjugated polymer at high pressure and temperature

“…These studies were augmented by structural high pressure studies of polyphenylenes by Heimel et al and Gitsas et al, and by optical high pressure studies including studies of poly( p‐ phenylene vinylene) by Morandi et al and investigations of fluorene copolymers by Schmidtke et al Common to all these studies are planarization of otherwise twisted polymers followed by a redshift in photoluminescence (PL). Also, there are structural high pressure studies of oligoacenes and related molecules including fluorene as well as of related hairy rod type polymers like polyimides, which show diverse structural transitions and dependencies between the effect of pressure and side chain structure.…”