The major molecular features generating high-performance polymers are structural rigidity and high-bonding energies, both mainly obtained by using aromatic an/or heteroaromatic moieties. In addition to the rigidity on the level of a single macromolecule, long-range order between neighboring polymer chains, i.e. crystallinity, further enhances the outstanding stability of HPPs. The stability enhancement is a direct consequence of the additional lattice energy: if one was to e.g. thermally degrade a crystalline HPP material, one would not only have to supply the thermal energy to break the covalent bonds in the polymer molecules, but in addition destroy the crystal and thus supply the lattice energy. With respect to mechanical properties, crystallinity is also desirable: with increasing crystallinity, the bulk modulus of a polymer is approaching its crystalline modulus, which is often considerably higher.
The crystallinity of chain-flexible polymers can be increased by recrystallizing them from solution or the melt, or e.g. by shearing. Unfortunately, these approaches are not applicable for most HPPs, as they are so-called rigid-rod polymers with little flexibility of the polymer chain. Fully aromatic polyimides for instance have such high molecular rigidity that they are often thermosets without even being cross-linked. Hence, crystallinity of polyimides can often only be achieved during the polymerization reaction, but not afterwards. Conventional polyimide syntheses yield semicrystalline materials at best. On a molecular level, this means that the chains are irregularly kinked, which does not allow for their packing into a crystal. In fact, if the chain conformation is highly irregular, there is no long-range order between the rigid-rods, and amorphous phases are obtained (see below, left). Slight kinking leads to liquid-crystalline nematic-type ordering (see below, middle). If one can access a uniform chain-conformation, however, these chains can nicely packed into a crystalline array (see below, right).
We have recently developed a non-classical polymerization technique, hydrothermal polymerization (HTP), which directly yields highly crystalline polyimides. The techniques is geomimetic, i.e. inspired by natural mineral formation.[2-3] The polyimides obtained by HTP were so crystalline, that we could refine their crystal structures from powder X-Ray diffraction data.[2,4] Below, the crystal structures of three polyimides synthesized via HTP are depicted, from left to right: poly(p-phenylene pyromellitimide), poly(p-phenylene benzophenone imide) and poly(4,4′-biphenyl benzophenone imide).
Figure: Crystalline polyimides obtained by hydrothermal polymerization A-B) crystal structure of poly(p-phenylene pyromellitimide) ; C) crystal structure of poly(p-phenylene benzophenone imide) ; D) crystal structure of poly(4,4′-biphenyl benzophenone imide) .
Read more about hydrothermal polymerization, the technique with which polyimides of outstanding crystallinity can be obtained.
 K. Tashiro, Prog. Polym. Sci. 1993, 18, 377-435. “Molecular theory of mechanical properties of crystalline polymers“
 B. Baumgartner, M. J. Bojdys and M. M. Unterlass*, Polym. Chem. 2014, 5, 3771-3776. “Geomimetics for Green Polymer Synthesis: Highly Ordered Polyimides via Hydrothermal Techniques”
 M. M. Unterlass*, Mater. Today 2015, 18, 242-243. “Creating geomimetic polymers“
 B. Baumgartner, M. J. Bojdys, P. Skrinjar and M. M. Unterlass*, Macromol. Chem. Phys. 2016, in press. DOI: 10.1002/macp.201500287. “Design Strategies in Hydrothermal Polymerization of Polyimides”