UV Resonance Raman Determination of Molecular Mechanism of Poly(N-isopropylacrylamide) Volume Phase Transition

Abstract

Poly (N-isopropylacrylamide) (PNIPAM) is the premier example of a macromolecule that undergoes a hydrophobic collapse when heated above its lower critical solution temperature (LCST). Here we utilize, dynamic light scattering, H-NMR, steady-state and time-resolved UVRR measurements to determine the molecular mechanism of PNIPAM's hydrophobic collapse. Our steady-state results indicate that in the collapsed state the amide bonds of PNIPAM do not engage in inter-amide hydrogen bonding, but are hydrogen bonded to water molecules. At low temperatures, the amide bonds of PNIPAM are predominantly fully water hydrogen bonded, whereas, in the collapsed state one of the two normal C=O hydrogen bonds is lost. The NH-water hydrogen bonding, however, remains unperturbed by the PNIPAM collapse. Our kinetic results indicate a mono-exponential collapse with τ~360 (±85) ns. The collapse rate indicates a persistence length of n~10. At lengths shorter than the persistence length the polymer acts as an elastic rod, whereas, at lengths longer than the persistence length the polymer backbone conformation forms a random coil. Based on these results we propose that at low temperatures PNIPAM adopts an extended, water-exposed conformation that is stabilized by favorable NIPAM-water solvation shell interactions which stabilize large clusters of water molecules. At elevated temperatures, thermal agitation disrupts these interactions. The PNIPAM+water polymer undergoes a volume phase transition, expels water and shrinks to a compact conformation that reduces its hydrophobic solvent accessible surface area. In this compact state, PNIPAM forms small hydrophobic nano-pockets where the (i, i +3) isopropyl groups make hydrophobic contacts. A persistent length of n~10 suggests a cooperative collapse where hydrophobic interactions between adjacent hydrophobic pockets stabilize the collapsed PNIPAM.

Ionic liquid modulation of swelling and LCST behavior of N-isopropylacrylamide polymer gels

“…Experimentally, below the LCST, pNIPAAM is swelled with water through hydration of the polymerised aliphatic chain and hydrogen bonding interactions with its amide moiety, generating a gel network 8 . Above the LCST, the gel collapses along the polymer backbone before water 35 molecules are expelled 9, 10 . This process is driven by a changeover from polymer-solvent interactions dominating at low temperatures to polymer-polymer and solvent-solvent interactions and structures dominating at the higher temperatures 6 .…”

“…-anion 10 was apparent. ionogels and the hydrogel sample, as measured using differential scanning calorimetry.…”

“…At t = 5 min, the sample plate is heated from 20 to 45 o C, ensuring that any thermally sensitive processes related to the LCST are completed. As the temperature is increased, the storage modulus increases as the polymer contracts and its 10 mechanical energy is stored internally in response to the applied stress. The sharp peaks produced by the hydrogel are due to the instrument readjusting the normal force on the sample.…”

Search for the Origin of Discrepancies in Osmotic Measurements of the PNIPAM - Water System

“…Such local interactions have already been detected in homopolymer PNIPAM microgels by FTIR and Raman spectroscopy [60], while with foreign probe molecules, such as phenol, they are visualised directly by 1 H CRAMPS solid-state NMR [61].…”

Rupturing cancer cells by the expansion of functionalized stimuli-responsive hydrogels

“…37 At high temperature, the interaction between the polymer and the water molecules reduced. 38,39 In this case, entropy dominates: water is expelled out of the hydrogel, and the polymeric chains adopt a collapsed globule-like conformation.…”