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.
Variations in the crystalline structures of polyimides (PIs) were analyzed under high pressures up to 8 GPa using wide-angle X-ray diffraction. The compressibilities along the polymer chain axis (c-axis) of rigid-rod PIs increased with an increase in the number of phenyl rings in the diamine moiety (PMDA/PPD < PMDA/BZ < PMDA/DATP). This could be due to an increased shrinkage of the C−C bond lengths between the phenyl rings and/or a pressure-induced deformation of the periodic structure associated with changes in bond angles and dihedral angles. In contrast, PMDA/ODA, having an ether linkage, showed an increase in the lattice parameter along the c-axis up to 0.8 GPa, which could be due to a widening of the ether bond angle. Moreover, PMDA/PPD showed isotropic compression along interchain directions, whereas PMDA/DATP and PMDA/ODA showed anisotropic compression along the cofacial stacking direction, which resulted in the larger volumetric shrinkages of the latter PIs.
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