posted on 2021-10-13, 09:03authored byRamkrishna Sarkar, Kamini Mishra, Harshita, Puspendu Kumar Das, S. Ramakrishnan
Periodically grafted amphiphilic
copolymers (PGACs) were earlier
shown by us to adopt a zigzag folded conformation in the solid state,
which enabled the backbone and pendant segments to segregate and occupy
alternate layers in a lamellar structure. The conformational transition
from a random coil to a zigzag folded chain in solution is an interesting
problem, which is largely unexplored. To examine this, an orthogonally
clickable parent polyester was sequentially clicked with two types
of poly(ethylene glycol) (PEG) segments: one is a simple PEG and the
other is a PEG that carries a dipolar chromophore. These two hydrophilic
PEG segments, installed in a periodic and alternating fashion along
the hydrocarbon-rich (HC) polyester backbone, ensure that the Janus folded chains are formed upon folding and carry chromophoric
dipoles oriented along the same direction, thereby generating a large
net dipole. The folding-induced alignment of chromophores in solution
was followed using second harmonic light scattering (SHLS), wherein
the intensity of the frequency-doubled scattered light (I2ω) is measured. Folding was induced by adding a
polar solvent, like methanol, to a chloroform solution of the polymer;
methanol desolvates the HC backbone but solubilizes the pendant PEG
segments, thus inducing folding. The second harmonic intensity (I2ω) increased initially with methanol
concentration and then saturated; in contrast, I2ω remained invariant with the solvent composition in
the case of an analogous model chromophore. Furthermore, in a model
PGAC carrying chromophore-bearing PEG segments on every repeat unit, I2ω decreased with increasing methanol
composition, revealing the formation of a centrosymmetric folded chain,
wherein the chromophoric dipoles on either side cancel each other.
Thus, this study clearly reveals that the zigzag chain folding of
PGACs can be induced by a segment-selective solvent, resulting in
the rather elusive directional ordering of chromophoric dipoles in
solution.