Poly(N-isopropylacrylamide) - from single-chain dynamics inpolymer physics to intelligent gels in biomedical applications
Zhuojun Dai and Chi Wu
Department of Chemistry, The Chinese University of Hong Kong, Sha Tin, N.T. Hong Kong, China
As a thermally sensitive polymer, poly(N-isopropyl acrylamide) (PNIPAM), especially its so-called volume
phase transition, has been extensively studied because of its convenient and adjustable (by copolymerization)
lower critical solution temperature (LCST ~ 32 oC). Among these studies, our bottom-up approach is rather
different and unique. Namely, we started from the coil-to-globule transition of individual chains, which had
been predicted in theory, but not been observed, for a long time because interchain aggregation always spoils
intrachain contraction. After a lengthily preparation of narrowly
distributed linear PNIPAM chains, we have finally shown that
individual coiled chains can form single-chain globules bypassing a newly discovered molten globular state when solvent isswitched from good to poor [1-4]. The right figure shows staticand dynamic laser light-scattering results; namely, the solution
temperature dependence of “the Rayleigh ratio (Rvv(q)) vs thescattering vector (q)” and the hydrodynamic radius distribution(f(Rh)). We found that even at the collapsing limit, each globule
still contains ~70% of water in their hydrodynamic volume, notas “dry” as we thought. The time of transition, involving an
initial nucleation followed by a coarsening process, is in theorder of ms, too short to support a previously suggested highly chain-knotting inside each globule. Our study
has been extended to some PNIPAM copolymers. We haverevealed that after the introduction of second hydrophilic orhydrophobic monomer into the PNIPAM chain backbone [5,6],
heat
heat
the chain folding could lead to some unique nanostructures,such as a single-chain micelle and an ordered coil state, as
shown in the right figure. We have successfully explained theCollapsed
Random coil
Ordered coilformation of stable mesoglobules made of a limited number ofcore-shell
nanoparticle
collapsed chains on the basis of the viscoelasticity in dilutecopolymer solutions. Further, by cross-linking linear chains into a microgel with a size comparable to a longchain, we have demonstrated that the previously observed sharp shrinking of PNIPAM gels is not a volume
phase transition, but due to a sudden breakdown of the balance between the bulk shear module and theinhomogeneous shrinking-induced internal stress built up during
the coil-to-globule transition of sub-chains between two
neighboring cross- linking points. Armed with a fundamental
understanding of such a wonderful system, we have developed andpatented some fast-shrinking, biocompatible and biodegradable
thermally sensitive hydrogels with potential biomedical
applications, as shown in the right figure [7]. In this lecture, wewould like to illustrate and emphasize how important to combine
synthetic chemistry, polymer physics and biology together because
our future in education and research must be multi-disciplinary.
[1] C. Wu, S.Q. Zhou, Macromolecules, 28, 5388; and 8381 (1995); 29, 1574; and 4998 (1996)[2] C. Wu, S.Q. Zhou, Physics Review Letters, 77(14), 3053 (1996)[3] X.H. Wang, X.P. Qiu, C. Wu, Macromolecules, 31, 2972 (1998)[4] C. Wu, X.H. Wang, Physical Review Letters, 80(18), 4092 (1998); and 83, 4105 (1999)[5] C. Wu, X.P. Qiu, Physical Review Letters, 80(3), 620 (1998)[6] G.Z. Zhang, C. Wu, Physical Review Letters, 86(5) 822 (2001).
[7] C. Wu, J. Hong,, U.S. Patent 6,030,634 (2000); & 6,238,688 (2001); Chinese Patent 971 80823.6 (2003)

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