Molecular bottlebrushes: new routes to self-assembled morphologies with small periodicities

Abstract

Linear A/B block polymer self-assembly offers exciting opportunities for bottom-up nanostructured materials design. For any chemically incompatible A/B monomer pair, there is a minimum degree of polymerization (N) required for melt self-assembly that sets the smallest microdomain (d) periodicity that such a material can form. This thermodynamic limitation impacts some block polymer applications, especially sub-10 nm templating for microelectronic device manufacture. Nonlinear polymer architectures offer opportunities to manipulate A/B block polymer self-assembly thermodynamics without resorting to new monomer chemistries. This thesis compares the morphologies and self-assembly thermodynamics of parent poly(lactide)-block-poly(𝜀-decalactone)-block-poly(lactide) (LDL) polymers bearing a midchain norbornene functionality to those of daughter core-shell bottlebrushes (csBBs) with varied backbone degrees of polymerization (Nbb), which derive from living “grafting through” polymerization of the norbornyl moieties. We specifically quantify how the d-spacings and order-disorder transition temperatures of the parent LDL triblocks change on enchainment into csBBs using small-angle X-ray scattering. Across lactide volume fractions fL = 0.27–0.73, we demonstrate that the csBB architecture thermodynamically stabilizes microphase separated melts and that it can drive ordering of disordered parent LDL triblocks. On this basis, we develop general phenomenological relationships for the composition-dependent critical degree of polymerization of the LDL triblock arms (Narm) for melt self-assembly in terms of Nbb, which enable new materials designs with ever smaller d-spacings. In compositionally asymmetric LDL triblocks and their daughter csBBs, we establish that the csBB architecture also alters the observed microphase separated morphologies. When the total volume fraction of the L block is fL = 0.27-0.34, the parent LDL polymers form hexagonally-packed cylinders and micellar Frank-Kasper A15, σ, and dodecagonal quasicrystal phases. However, enchainment into csBBs induces a spheres-to-cylinders transition with increasing Nbb. When fL = 0.73, the parent LDL triblocks form hexagonally-packed cylinders yet the csBBs form both double gyroid and metastable modulated lamellar phases. Thus, the csBB architecture induces an interfacial curvature reduction in the observed morphology relative to the parent triblock. Thus, this work establishes that the csBBs of A/B block polymers offer new opportunities to tune both the accessible morphologies and their thermodynamic stabilities.