(417f) Morphology of Nanocrystalline Domain Reinforced Rubber

Thermoplastic elastomers (TPEs) attain good mechanical performance by virtue of hard, reinforcing domains that result from microphase separation of two immiscible, covalently connected parts of the chains. The use of monodisperse, hard segments with a strong tendency to self-assemble to form well-defined β-sheet secondary structures via cooperative multiple hydrogen bonds has received considerable attention as a strategy for forming hard reinforcing domains more effective than the types previously known. The reinforcement in such a supramolecular elastomer containing crystalline domains formed by β-sheet stacking mimics the reinforcement present in silks. However, the crystalline domains in silks are smaller than ten nanometers in all three dimensions. It has proved difficult to obtain synthetically domains that are so small in three dimensions.

Recently, we succeeded in synthesizing TPEs with hard domains having all three dimensions well below 100 nm by entropically limiting the degree of association due to favorable hydrogen bonding. Polyisobutylene (PIB) chains were synthesized with graft side chains, each containing a few β-alanine units. Mechanical studies have shown that this nanocrystalline structure is extraordinarily effective in reinforcing elastic networks. The nanocrystalline domains must have PIB chains tethered to their surfaces and we imagine this leads to a structure around the nanocrystallite that is similar to that of commonly known “polymer brushes.” In our samples, the surfaces to which the chains are grafted are the surfaces of the nanocrystallites. The brush “chains” are portions of PIB backbones that go into and come out of a spot on the surface of the nanodomain where the oligo-β-alanine graft has been crystallized into the domain. Due to the kind of tethering that occurs here, the “brush” may look like a “loop” brush, which can have properties quite different from those of brushes of linear chains. Details of the brush character can be tailored by tailoring the synthesis of the grafts. Small angle neutron scattering measurements have been used to probe the structure of the “brush” around the nanodomains.