Hydrophobic Collapse Initiates the Poly(N-isopropylacrylamide) Volume Phase Transition Reaction Coordinate

Abstract

The best-known examples of smart, responsive hydrogels derive from poly( N-isopropylacrylamide) (PNIPAM) cross-linked polymer networks. These hydrogels undergo volume phase transitions (VPTs) triggered by temperature, chemical, and/or environmental changes. PNIPAM hydrogels can undergo more than 50-fold volume changes within ∼1 μs intervals. Studies have tried to elucidate the molecular mechanism of these extraordinarily large responses. Nevertheless, the molecular reaction coordinates that drive the VPT remain unclear. Using visible nonresonance Raman temperature-jump spectroscopy, we determined the molecular ordering of this VPT. The PNIPAM hydrophobic isopropyl and methylene groups dehydrate with time constants of 109 ± 64 and 104 ± 44 ns, initiating the volume collapse of PNIPAM. The subsequent dehydration of the PNIPAM amide groups is significantly slower, as our group previously discovered (360 ± 85 ns). This determination of the ordering of the molecular reaction coordinate of the PNIPAM VPT enables the development of the next generation of super-responsive materials.

Molecular mechanisms driving the microgels behaviour: A Raman spectroscopy and dynamic light scattering study

“…To investigate the molecular changes induced by temperature across the VPT, we focus our attention on the spectral range between 2850 cm −1 and 3000 cm −1 , where bands ascribed to vibrations of CH 2 (methylene group, Fig.1(a)(b)) and CH 3 (in isopropyl group, Fig.1(a)) are present. As recently observed in PNIPAM [60], changes of these bands, sensitive to hydrogen bond variations, are related to different interactions both among polymers and between polymer side groups and water molecules surrounding them. Raman spectra of PNIPAM and IPN microgels at two concentrations C w =0.3 % (critical concentration) and C w =5.0 % are reported in Fig.4 at temperatures T=297 K and T=316 K, below and above the VPTT (Fig.4(a,b,c,d)).…”

“…However, despite the numerous investigations on the VPT of PNIPAM microgels, a clear picture of the molecular mechanism behind swelling is still missing. To this aim Raman spectroscopy represents a powerful tool to highlight molecular changes related to the swelling behaviour as previously reported for linear PNIPAM [57,58] and PNIPAM microgels [59,60]. The main outcomes of these investigations underline that the principal groups involved in the swelling are the CH 2 stretching bands of the methylene group and the CH 3 stretching bands of the isopropyl group ( Fig.…”

A Light‐Driven Microgel Rotor

“…We note here that the bending rigidity of the ribbon and its stiffness are greatly affected by the water content in it. The light‐induced temperature‐jumps give rise to mechanical response over a few hundred milliseconds, where the rate‐limiting factor is the mass transport within the gel . We hypothesize that photothermal heating creates a transient state, where an imbalance exists between the stresses defining the local and the mean curvature of the ribbon .…”

Investigation of the Effects of Different Hydrophilic and Hydrophobic Comonomers on the Volume Phase Transition Temperatures and Thermal Properties of N-Isopropylacrylamide-Based Hydrogels

“…Below the volume phase transition temperature, the polymer chain is hydrated and adopts an extended coil conformation, while above it the polymer is dehydrated and adopts a globular conformation. Correspondingly, the cross-linked hydrogels obtained from these polymers swell under the volume phase transition temperature and shrink above it [7][8][9][10][11][12]. It is known that all factors affecting hydrogel swelling can be controlled by monomer selection and polymerization conditions.…”

“…If the amount of hydrophobic groups is reduced by further hydrophilic comonomers, the polymer hydrophilicity is increased due to the strong interaction between water and hydrophilic groups with the polymer [8,[17][18][19]. Incorporation of hydrophilic comonomer leads to an increase in the transition temperature, whereas incorporation of hydrophobic comonomer leads to a decrease [8,15,[17][18][19][20][21][22][23][24]. The swelling ratios of hydrogels depend on the ratio of hydrophilic and hydrophobic structural units in the polymer, as well as on the network structures of the hydrogels [24,25].…”