Effect of Buffer Anions on Pearl-Necklace Morphology of Tertiary Amine-Containing Binary Heterografted Linear Molecular Bottlebrushes in Acidic Aqueous Buffers

Abstract

Molecular bottlebrushes can exhibit a multitude of distinct conformations under different conditions, and precise control of their morphology can facilitate better use of such materials in potential applications. Herein, we report a study on the effect of buffer anions on the pearl-necklace morphology of linear binary heterografted molecular brushes consisting of pHresponsive poly(2-N,N-diethylamino)ethyl methacrylate) (PDEAEMA) with a pK a of 7.40 and thermoresponsive poly-(ethoxydi(ethylene glycol) acrylate) (PDEGEA) with a lower critical solution temperature of 9 °C as side chains in various acidic aqueous buffers at 0 °C. The molecular brushes, denotated as BMB, were prepared by a grafting-to approach using copper(I)catalyzed azide−alkyne cycloaddition reaction. Dynamic light scattering studies showed that the apparent hydrodynamic size of BMB in aqueous buffers with a pH of 6.50 at 1 °C decreased with increasing valency of buffer anions, from acetate anions with a charge of 1−, to phosphate anions carrying charges of 2− and 1− and citrate anions bearing charges of 3− and 2− at pH = 6.50. Atomic force microscopy revealed that BMB exhibited a pearlnecklace morphology from all three aqueous buffers with a pH of 6.50 when spin-cast at 0 °C. Analysis of AFM images showed that the average length of BMB and the number of beads per brush molecule decreased with increasing valency of buffer anions while the size and height of the beads increased. The pearl-necklace morphology of BMB was believed to be the result of microphase separation of the neutral PDEGEA and the charged PDEAEMA side chains along the brush backbone. Multivalent kosmotropic buffer anions formed bridging linkages between protonated tertiary amine moieties and thus "crosslinked" the charged PDEAEMA side chains, resulting in the shrinkage of BMB and enhanced microphase separation of two side chain polymers.

Shape-Changing Bottlebrush Polymers

“…Historically, “grafting to” was not a very viable method for the synthesis of high grafting density BBPs. However, with the advent of highly efficient click reactions such as copper­(I)-catalyzed azide–alkyne cycloaddition (CuAAC), high density BBPs can be routinely synthesized. ,,,,, Scheme e shows the synthesis of poly­(ethylene oxide) (PEO) BBPs via CuAAC. A drawback of “grafting to” is the need to remove unreacted side chains from the formed BBP, which can be difficult.…”

“…A drawback of “grafting to” is the need to remove unreacted side chains from the formed BBP, which can be difficult. However, repeated fractionation or centrifugal filtration can completely remove the remaining unreacted side chains from the brushes. , …”

“…Copyright 2006 Springer Nature. (e–j) Reprinted and adapted with permission from refs , , and − , respectively. Copyright 2004, 2004, 2020, 2001, 2018, and 2004 American Chemical Society, respectively.…”

“…Through judicious control of environmental conditions, many different shapes have been experimentally observed for linear single- and multicomponent BBPs. These include wormlike (Figure c), rodlike (Figure d), C- and S-shaped (Figure e), pearl-necklace (Figure f,g), , spherical (Figure h and i), , tadpole (Figure j), and helical conformations (Figure k) . BBPs can be induced to convert from one shape to another by external stimuli. − For example, when P n BA BBP molecules on the water surface are laterally compressed in a Langmuir–Blodgett (LB) trough, pronounced rod-to-sphere shape transitions are observed .…”

“…These include wormlike (Figure c), rodlike (Figure d), C- and S-shaped (Figure e), pearl-necklace (Figure f,g), , spherical (Figure h and i), , tadpole (Figure j), and helical conformations (Figure k) . BBPs can be induced to convert from one shape to another by external stimuli. − For example, when P n BA BBP molecules on the water surface are laterally compressed in a Langmuir–Blodgett (LB) trough, pronounced rod-to-sphere shape transitions are observed . Moreover, for multicomponent BBPs, the worm-to-globule shape transitions are accompanied by reversible formation of unimolecular core–shell micellar nano-objects, , which could be used, for example, as drug carriers …”