Effect of Heating Rate on the Pathway for Vesicle Formation in Salt-Free Aqueous Solutions of Thermosensitive Cationic Diblock Copolymers

Abstract

Amphiphilic diblock copolymers composed of a thermo-responsive poly(2-isopropyl-2-oxazoline) (PIPOZ) block (M n = 7000 g/mol) linked to a poly(3acrylamidopropyl)trimethylammonium chloride) (PAMPT-MA) block (M n = 3500 or 8200 g/mol) were prepared by cationic ring-opening polymerization (PIPOZ block) and subsequent atom transfer radical polymerization (PAMPTMA block). Salt-free dilute aqueous solutions of the diblock copolymers underwent a reversible phase transition upon heating past 40 °C that led to the formation of vesicles, as established by light scattering (LS) and transmission electron microscopy (TEM) studies of solutions heated to 60 °C. The pathway toward vesicles varied depending on the heating rate. A slow heating rate (0.2 °C/min) led to the formation of small spheres as the temperature reached 40 °C; the spheres grew in size as the temperature increased and eventually converted into vesicles by diffusion of some hydrophilic charged blocks toward the center. Upon application of a fast heating rate (0.7 °C/min or faster), the copolymers formed rod-like and/or worm-like micelles at 40 °C and, at higher temperature, oblate membranes that closed up into vesicles. The kinetic control of the pathway toward vesicles may be attributed to the existence of several metastable states in the system, by analogy with studies of the isothermal self-assembly of diblock copolymers in selective solvents.

Redox-active organic–inorganic hybrid polyoxometalate micelles

“…Note that addition of further D2O to the D2O NMR sample led to no change in the spectra. Similar phenomena have been observed for thermosensitive block-copolymers, 28 where proton signals disappeared upon thermal stimulation and subsequent coiling of the polymer. Interestingly however, such behaviour has not been reported for other POM-surfactant systems to date.…”

Temperature-Dependent Adsorption and Adsorption Hysteresis of a Thermoresponsive Diblock Copolymer

“…This water content is due to both water trapped within the layer and water directly hydrating the polymer chain, and we note that Korchagina et al reported that the PIPOZ chains of the diblock copolymer were not fully dehydrated even above the phase-transition temperature. 22 …”

“…Poly(2-isopropyl-2-oxazoline) 60 -b-poly((3-acrylamidopropyl)trimethylammonium) 17 , abbreviated as (PIPOZ 60 -b-PAMPTMA 17 ), was prepared as described previously with a molecular weight M n of 10.3 kDa (Figure 1). 22 The nonionic block, PIPOZ, has a M n of 7.0 kDa with M w /M n = 1.06 as determined by GPC, 31 corresponding to ∼60 repeat units. This polymer was used here in control experiments aimed at evaluating the adsorption of PIPOZ on silica surfaces.…”

“…The number of charged units per copolymer (∼17) was determined by 1 H NMR spectroscopy. 22 The size and size distribution of the diblock copolymer could not be determined by GPC due to irreversible adsorption of the copolymer on the GPC packing material. 31 Sodium chloride (NaCl, BioXtra, ≥99.5%) was purchased from Sigma-Aldrich and used as received.…”

“…The experimental procedure can be found in the Supporting Information, but we note already here that the refractive index increment is insensitive to temperature. 22 2.3. Quartz Crystal Microbalance with Dissipation, QCM-D. QCM-D measurements enable determination of the mass of an adsorbed layer, including the associated solvent, by continuously measuring the change in frequency and dissipation at the fundamental frequency as well as at six overtone frequencies (15,25,35,45,55,65 MHz).…”