Quantitative analysis of stereoscopic molecular orientations in thermally reactive and heterogeneous noncrystalline thin films via variable-temperature infrared pMAIRS and GI-XRD

Abstract

The quantitative estimation of stereoscopic molecular orientations in semicrystalline polymer thin films with characteristic heterogeneous structures is essential for the development of advanced film materials. Grazing-incidence X-ray diffraction (GI-XRD) is a powerful technique used to evaluate the orientational order of crystalline lattices; however, it provides the orientational order of only ordered regions (crystal and glassy liquid crystal regions). In contrast, infrared (IR) p-polarized multiple-angle incidence resolution spectrometry (pMAIRS) can provide the averaged orientational order of overall regions in a film regardless of sample crystallinity. This paper combines variable-temperature (VT) pMAIRS, static GI-XRD and spectroscopic ellipsometry and reports general insight into the molecular orientation mechanism occurring in polymer films revealed through the precise analysis of the structural evolution of linear poly(amic n-alkyl ester), PAE, with different lengths of n-alkyl chains during thermal curing, which involves solvent evaporation and dehydration imidization reactions to form the corresponding polyimide (PI). This combined method clarified that the PAE and PI thin films had a heterogeneous structure consisting of a glassy liquid-crystalline (ordered) region and a nonoriented amorphous region. Furthermore, imidization proceeded faster in the amorphous region than in the ordered (oriented) region. In addition, anomalies of refractive index and birefringence were found by comparing pMAIRS and spectroscopic ellipsometry, although the effect of the side chain length on the molecular orientation of PI was insignificant. The anomalies indicate that an isotropic void structures were generated by the elimination reaction of the side chains and that the volume fraction of voids increased with increasing side chain length. These results demonstrated that the combination of VT-pMAIRS and GI-XRD is a very powerful tool for the in situ structural analyses of noncrystalline (disordered crystal, glassy liquid crystal, and oriented amorphous) thin-film materials ongoing chemical reactions in their components and is inherently applicable to all kinds of interesting polymeric thin-film materials. Thin film aromatic polyimides were quantitatively analyzed by grazing-incidence X-ray diffraction (GI-XRD), variable-temperature infrared p-polarized multiple-angle resolution spectroscopy (VT-pMAIRS), and spectroscopic ellipsometry (SE) methods. Combining VT-pMAIRS and GI-XRD revealed characteristic heterogeneous structures comprising non-oriented amorphous and oriented liquid-crystalline glass regions. Additionally, SE indicated that the film involved anisotropic-shaped void inducing form-birefringence. Such an excess of birefringence can make it difficult to estimate orientational order using birefringence measurement. This methodology presented here will remarkably contribute to general structural analyses of all intriguing thin-film polymers with characteristic heterogeneity.