Synthesis of novel polyimides from dianhydrides with flexible side chains - Polyimides and Other High Temperature Polymers - page 70

incorporation of thermally stable flexible or non-symmetrical linkages in the
backbone [2, 3], introduction of polar or non-polar bulky substituents [4-6], or
disruption of symmetry and recurring regularity through copolymerization. An-
other approach to enhance solubility is the insertion of alicyclic units into mono-
meric dianhydrides [7-9].
Among these structural modification methods, the appendage of flexible side
chains to rigid backbone polymers is a particularly interesting approach because
the entropy change on going from the crystalline to the nematic state is relatively
low, and these polymers exhibit high melting and decomposition temperatures. As
demonstrated by Majnusz et al. [10] and by Krigbaum et al. [11], the attachment
of substituents of varying structure and length lowered the thermal transition tem-
perature. A quantitative treatment of the phase transition behavior of rigid-rod
polymers with flexible side chains has been elaborated recently [12-18]. This the-
ory based on Flory's lattice model shows that the side chains act like a low mo-
lecular weight solvent.
Another very interesting property of the rigid-rod polymers with flexible side
chains is their ability to form layered structures in crystals and liquid crystals
when the flexible side chains reach a critical length, as shown in Figure 1. The
layered phases are characterized by a segregated structure in which the rigid main
chains form layers, while the flexible side chains occupy the space between the
layers. Depending on the degree of molecular interaction between rigid main
chain and flexible side chain, different layered structures are formed [19-21].
Recently, Matsumoto synthesized soluble polyimides using dianhydrides with
polyalicyclic (cycloaliphatic) structures and aromatic diamines, the so-called
Figure 1. Schematic representation of main and side chains in a rigid-rod polymer having flexible
side chains in the layered structure. (a) crystalline state (b) after side chain melting.