Comparison of the Coil-to-Globule and the Globule-to-Coil Transitions of a Single Poly(N-isopropylacrylamide) Homopolymer Chain in Water

Abstract

Using a newly prepared nearly monodisperse (M w/Mn < 1.05) high molar mass (Mw ) 1.3 × 10 7 g/mol) poly(N-isopropylacrylamide) (PNIPAM) sample, we successfully, for the first time, made the conformation of individual PNIPAM chains change from a coil to a fully collapsed thermodynamically stable single chain globule and then back to a coil in an extremely dilute aqueous solution (∼6.7 × 10 -7 g/mL). The average chain density in the globule state is ∼0.34 g/mL, close to 0.40 g/cm 3 predicted on the basis of a space-filling model, indicating that the globule still contains ∼66% water even in its fully collapsed state. At a given temperature around the lower critical solution temperature, the chains are smaller in the globule-to-coil transition than in the coil-to-globule transition, revealing that the coil-toglobule transition is an irreversible process. The hysteresis can be attributed to the formation of intrachain structures, presumably the intrachain hydrogen bonding, in the globule state. We confirmed the existence of the crumpled coil and the molten globule states between the random coil and the collapsed globule states. The coil-to-crumpled coil transition can be reasonably described by the Birshtein and Pryamitsyn theory.

Stimuli‐responsive polypeptide materials prepared by ring‐opening polymerization of α‐amino acid N‐carboxyanhydrides

“…Usually, turbidity measurements are applied to determine the CP, at which the polymer solution changes from clear to cloudy. Two extensively studied thermo-responsive polymers are poly(N-isopropyl acryl amide) [35][36][37][38] and poly(oligo(ethylene glycol) methacrylate) (POEGMA). 39,40 From the thermodynamic consideration, the thermo-responsive property is due to a delicate amphiphilic balance between different subunits within polymer chains, which contains hydrophobic and hydrophilic moieties.…”

Thermo-responsive, UV-active poly(phenyl acrylate)-b-poly(diethyl acrylamide) block copolymers

“…Also, the block copolymer exhibited a slight hysteresis in cloud point temperature between the heating and the cooling cycles (T cp = 30°C for heating and T cp = 26°C for cooling), which has also been observed for thermo-responsive polymers bearing tertiary amines [40][41][42]. It is generally attributed to the inter-polymer hydrogen bonding of the tertiary amine groups [42,43]. In DLS measurements, the hydrodynamic diameter D h was below 100 nm at low temperature for all cases and sharply increased to above 130 nm when the critical temperature was reached, illustrating the transformation from small micelles to large aggregated particles upon heating.…”

How much weighs the swelling pressure

“…After the maximum in the m app curve at a temperature of 25.4 °C the deswelling of the PNIPAM governs the scene. This happens about 8 degrees below the LCST which is quite an astonishing result because so far, only moderate changes in the PNIPAM solution state have been described in this temperature range [24][25][26][27][28][29][30]. Moreover, 25 °C is frequently used as starting or reference temperature for deswelling studies [26,[29][30][31][32][33].…”