Thermal Transitions and Structural Characteristics of Poly(3,4-ethylenedioxythiophene/cucurbit[7]uril) Polypseudorotaxane and Polyrotaxane Thin Films

Abstract

Herein, we report the thermal transitions and structural properties of poly(3,4-ethylenedioxythiophene/cucurbit[7]uril) pseudopolyrotaxane (PEDOT∙CB7-PS) and polyrotaxane (PEDOT∙CB7-PR) thin films compared with those of pristine PEDOT. The structural characteristics were investigated by using variable-temperature spectroscopic ellipsometry (VTSE), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and atomic force microscopy (AFM). VTSE and DSC results indicated the presence of an endothermic process and glass transition in the PEDOT∙CB7-PS and PEDOT∙CB7-PR thin films. X-ray diffraction of PEDOT∙CB7-PS and PEDOT∙CB7-PR powders displayed the presence of interchain π-π stacking revealing a characteristic arrangement of aromatic rings in the internal structure of the crystallites. AFM imaging of PEDOT∙CB7-PS and PEDOT∙CB7-PR thin films exhibited significant differences in the surface topographies compared with those of PEDOT. A high degree of crystallization was clearly visible on the surface of the PEDOT layer, whereas the PEDOT∙CB7-PS and PEDOT∙CB7-PR thin films exhibited more favorable surface parameters. Such significant differences identified in the surface morphology of the investigated layers can, therefore, be clearly associated with the presence of surrounding CB7 on PEDOT skeletons.

Keywords: PEDOT; cucurbit[7]uril; organic semiconductors; supramolecular encapsulation; surface morphology; thin polymer films; variable-temperature spectroscopic ellipsometry.

Figure 1

Chemical structures of pristine PEDOT (a), PEDOT∙CB7-PS (b) PEDOT∙CB7-PR (c), and CB7 (d).

Figure 2

Optical model for ellipsometric measurements.

Figure 3

Refractive indexes and extinction coefficients of PEDOT (a), PEDOT∙CB7-PS (b), and PEDOT∙CB7-PR (c) as functions of wavelength.

Figure 4

Ellipsometric angle Δ at λ = 930 nm as a function of temperature and DSC plots for PEDOT (a,b), PEDOT∙CB7-PS (c,d), and PEDOT∙CB7-PR (e,f).

Figure 5

Phase diagram constructed based on DSC and VTSE results.

Figure 6

XRD analysis: XRD patterns of the PEDOT powders (a); optimized geometry of two PEDOT oligomers calculated with MOPAC 2016 using the PM6-DH+ semi-empirical method (b,c)—atoms: O (red), S (yellow), H (white) and C (grey); schematic arrangement of the EDOT units in the unit cell of the PEDOT crystallite (d,e), where a, b, and c are the parameters of the lattice.

Figure 7

The AFM surface morphology over areas of 15 × 15 µm, 5 × 5 μm 2D, and 5 × 5 μm 3D topographic images of PEDOT (a–c), PEDOT∙CB7-PS (d–f), and PEDOT∙CB7-PR (g–i), respectively.