Controlling Morphology of Self Assembling Nanocrystalline Reinforcing Domains by Grafting Density Design*

Thermoplastic elastomers (TPEs) attain good mechanical performance by virtue of hard, reinforcing domains resulting 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 into β-sheet secondary structures via cooperative multiple hydrogen bonds has received attention as a strategy for forming reinforcing domains. The β-alanine trimer grafted polyisobutylenes (βA₃-g-PIB) studied here are TPEs. The β-sheet crystals provide physical crosslinks and reinforcement, and the PIB chains provide elasticity. In molecular designs f-PIB-g-βA₃ and p-PIB-g-βA₃ the chain tethering density on the surfaces of the β-sheet crystals was varied to change the long dimension of the reinforcing domains. Small Angle X-ray Scattering (SAXS) and Small Angle Neutron Scattering (SANS) measurements reveal that indeed when the chain tethering at the crystal surface is more crowded the domain dimension in the direction of β sheet stacking is reduced from over 200 nm to less than 10 nm, so all three domain dimensions are truly “nano”. SANS of samples swollen with deuterated cyclohexane show that portions of the backbone near the grafting points are stretched