(690b) Thermo-Responsive Bottlebrush Polymers

Bottlebrush
polymers are highly branched macromolecules with polymeric
side-chains on each repeat unit. The dense grafting of polymeric side-chains
results in both backbone and side-chain extension, giving rise to large, highly
extended macromolecules with individual molecules exceeding 100 nm in backbone
length in some cases. The novel structure of bottlebrush polymers may make them
particularly useful in specialty applications, and here we explore the surface
and solution properties of model thermoresponsive poly(N-isopropylacrilamide) (PNIPAAM) bottlebrush polymers.
PNIPAAM macromonomers are prepared through reversible
addition-fragmentation chain transfer using a norbornenyl-chain
transfer agent, and PNIPAAM bottlebrush polymers are subsequently prepared
using a "grafting-through" ring opening metathesis polymerization scheme
(Figure 1(a)). This synthetic method gives fully grafted bottlebrush polymers
with controlled backbone and side-chain lengths. The resulting PNIPAAM
bottlebrush polymers are fully soluble in water after removal of the terminal
chain transfer agent, reflecting a disproportionately strong effect of the
side-chain end groups on bottlebrush polymer solubility. Small-angle neutron
scattering (SANS) measurements of PNIPAAM bottlebrush polymers in D₂O
demonstrate that the side-chains collapse as the temperature increases towards
the lower critical solution temperature (LCST) (Figure (b)); both the LCST and
the collapse of the side-chains is dependent on the side-chain length.  Above the LCST, PNIPAAM bottlebrush
polymers aggregate, and we find that PNIPAAM bottlebrush polymers with 9 kg/mol side-chains form a lyotropic liquid crystal phase (Figure 1 (e) and (f)),
evidenced by low-angle scattering peaks and birefringence under polarized
optical microscopy at temperatures above the LCST. Bottlebrush polymers may
also be applicable to non-fouling and non-toxic surfaces due to the high
density of polymer chains. Bottlebrush thin films with poly(N-isopropylacrylamide) (PNIPAM) and 4-acryloylbenzophenone crosslinkable side-chains were prepared by spin casting
followed by UV irradiation. Cell attachment studies were conducted for
bottlebrush polymers at varying side-chain lengths and solution temperatures.

Figure1 Schemetic for the synthesis of P(NB-PNIPAM-SH) (a); SANS data for bottlebrush PNIPAM in D₂O
with temperature (b); Photographs of the aqueous solution of P(NB-PNIPAM-SH)
taken below LCST (c) and above LCST (d); POM images of P(NB-PNIPAM-SH) aqueous
solution taken below LCST (e) and above LCST (f).