Cationic Temperature-Responsive Poly(N-isopropyl acrylamide) Graft Copolymers: from Triggered Association to Gelation

Abstract

In this work temperature-triggered association and gel formation within aqueous solutions of a new family of cationic poly( N-isopropyl acrylamide) (PNIPAm) graft copolymers have been investigated. Five copolymers were synthesized using aqueous atom transfer radical polymerization (ATRP) involving a macroinitiator based on quaternarized N, N-dimethylaminoethyl methacrylate units (DMA+). The PDMA+) x - g-(PNIPAmn)y copolymers have x and y values that originate from the macroinitiator; values for n correspond to the PNIPAm arm length. The copolymer solutions exhibited temperature-triggered formation of nanometer-sized aggregates at the cloud point temperature, which was 33-34 degrees C. The aggregates were investigated using variable-temperature turbidity, hydrodynamic diameter, and electrophoretic mobility measurements. The aggregates were clearly evident using SEM and flowerlike or spherical morphologies were observed. Variable-temperature electrophoretic mobility measurements revealed that the zeta potentials of the aggregates increased with DMA+ content. A study of the effect of added NaNO3 showed that electrostatic interactions controlled the size of the aggregates. The concentrated graft copolymer solutions showed temperature-triggered gelation when the copolymer concentrations exceeded 5 wt %, Fluid-to-gel phase diagrams were constructed. It was found that electrostatic interactions also controlled the gelation temperature. A correlation was found between aggregate size and the minimum copolymer concentration needed to form a gel. A mechanism for the temperature-triggered structural changes leading to the formation of aggregates (in dilute solution) or gels (in concentrated solutions) is proposed.

Thermally reversible colloidal gels for three-dimensional chondrocyte culture

“…However, within the range of concentrations assessed (up to 2.5 wt%), the strength of the structure was observed to decrease substantially with decreasing concentration. Examples of this type of study are rarely reported in the literature, but Liu et al [42] reported that the minimum gel concentration (resistant to inversion with 'vigorous shaking by the hand') of quaternized poly(DMAEMA-graft-NIPAM)s was 6.4 wt%. The concentration reported for cell culture studies is typically 5 -10 wt% [19,20,26].…”

Thermoresponsive surfaces prepared using adsorption of a cationic graft copolymer: A versatile method for triggered particle capture

“…x -g-ðPNIPAm n Þ y copolymers has been described in detail elsewhere [1]. A brief description is given below.…”

“…The copolymers were prepared by aqueous ATRP using a macroinitiator, MI. The MIs were prepared from [1] poly(DMA-stat-HEMA) poly(N,N-dimethylaminoethyl methacrylate-stat-hydroxyethyl methacrylate) followed by its conversion to a quaternized, isobutyrate functionalized version using the method established by Chen and Armes [34]. To prepare PDMA þ…”

“…The values for n correspond to the PNIPAm side-chain lengths. The copolymers have [1] lower critical solution temperatures (LCST) of 33-34°C. We have shown previously that addition of PDMA þ…”

“…Cationic PNIPAm copolymers been prepared as random [4], block [5,6], star [7,8] and graft copolymers [1,2,[9][10][11]. They have been actively studied in the context of gene delivery [9,10,12] and temperature-sensitive flocculants [4].…”