Thermodynamics of Temperature-Sensitive Polyether-Modified Poly(acrylic acid) Microgels

Abstract

The temperature-induced structural changes and thermodynamics of ionic microgels based on poly(acrylic acid) (PAA) networks bonded with poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO−PPO−PEO) (Pluronic) copolymers have been studied by small-angle neutron scattering (SANS), ultra-small-angle neutron scattering (USANS), differential scanning calorimetry (DSC), and equilibrium swelling techniques. Aggregation within microgels based on PAA and either the hydrophobic Pluronic L92 (average composition, EO8PO52EO8; PPO content, 80%) or the hydrophilic Pluronic F127 (average composition, EO99PO67EO99; PPO content, 30%) was studied and compared to that in the solutions of the parent Pluronic. The neutron scattering results indicate the formation of micelle-like aggregates within the F127-based microgel particles, while the L92-based microgels formed fractal structures of dense nanoparticles. The microgels exhibit thermodynamically favorable volume phase transitions within certain temperature ranges due to reversible aggregation of the PPO chains, which occurs because of hydrophobic associations. The values of the apparent standard enthalpy of aggregation in the microgel suspensions indicate aggregation of hydrophobic clusters that are more hydrophobic than the un-cross-linked PPO chains in the Pluronic. Differences in the PPO content in Pluronics L92 and F127 result in a higher hydrophobicity of the resulting L92−PAA−EGDMA microgels and a larger presence of hydrophobic, densely cross-linked clusters that aggregate into supramolecular structures rather than micelle-like aggregates such as those formed in the F127−PAA−EGDMA microgels.

Preparation and characterization of thermal-sensitive ferrofluids for drug delivery application

“…Such aggregation is caused by the temperature sensitivity of the PPO segments of the Pluronic that being anchored to the gel network. Much like the uncrosslinked Pluronic, polymers can form intra-and inter-molecular micelle-like aggregates due to the hydrophobic interaction [3]. The gelation started at 21.8 1C and maximized at 32.5 1C for pure F127 fluids, and it commenced at 22.8 1C and maximized at 35.6 1C for F127 ferrofluids.…”

“…A delicate balance of hydrophilicity/hydrophobicity in the polymer structure is responsible for manipulating a lower critical solution temperature (LCST) phenomenon. A series of tri-block copolymers composed of poly (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide) (Pluronics) are a kind of temperaturesensitive polymers that demonstrate reversible solution transition behaviors in aqueous solutions [3,4]. Consequently, the novel thermo-sensitive ferrofluids that combine Pluronics-based polymers (Pluronics F127) with magnetite nanoparticles can lead to a temperature-responsive drug carrier system, because the magnetite itself provides a source of heat in the alternating magnetic field [5].…”

Gels with sense: supramolecular materials that respond to heat, light and sound

“…9), which exhibit lower polarity at higher temperatures due to increased conformational flexibility. 139,140 Heating therefore causes the fibres to become more hydrophobic, resulting in increased aggregation and expulsion of solvent beyond 70 o C. The swollen gel can be regenerated slowly by cooling or, more rapidly, through redox-induced sol-gel and gel-sol transitions.…”

Characterizing charge and crosslinker distributions in polyelectrolyte microgels

“…SANS has also been used to probe the morphology of the Pluronictemplated acrylic acid microgels produced by Bromberg et al [67]. When a more hydrophobic Pluronic surfactant (80% PPO) is used as the template, the microgel structure consisted of fractal-like structures of dense nanoparticles; alternately, when a more hydrophilic Pluronic (30% PPO) is used, micelle-like aggregates dominated the microstructure.…”