1,4;3,6-Dianhydrohexitols (isohexides) consisting of
isosorbide,
isomannide, and isoidide are produced from cellulose and can be incorporated
into superengineered plastics while retaining characteristics
that stem from their bulky and rigid alicyclic structures. In this
study, seven types of polyimides (PIs) incorporating a chiral isosorbide
(ISS) skeleton in the main chain (ISS-PIs) were prepared using ISS
and a tetracarboxylic dianhydride derived from trimellitic anhydride.
ISS-PIs exhibited excellent optical transparency, low refractive indices,
low dielectric constants, and small birefringence compared to wholly
aromatic PIs. This is because the in-plane orientation of the main
chain and dense molecular packing between the PI chains were suppressed
by the bulky alicyclic ISS structure and the bent and flexible PI
main chains with reduced intermolecular interactions. ISS-PIs with
excellent optical properties also demonstrated lower dielectric constants
(D
k) with moderate dissipation factors
(D
f) at 10 and 20 GHz. Although the structure
dependence of D
k is similar to that of D
f, the values of D
k decrease but D
f increases at a higher
frequency of 20 GHz, which could be partly imparted by adsorbed moisture.
In particular, ISS-PIs and their model compounds demonstrated clear
circular dichroism in the UV region both in solution and in the solid
state because of the inherent optical activity of the biobased ISS,
which indicates that chiral structures were maintained even in the
solid PI films. Moreover, the ISS-PIs had sufficient thermal stability
with a glass transition temperature (T
g) of approximately 260 °C and the 5 wt % weight-loss temperatures
(T
d
5) of approximately 400
°C. This study clarifies the relationship between the excellent
optical, dielectric, and thermal properties; stereochemical structures;
and aggregation states of ISS-PIs.