Structural Changes in PEO–PPO–PEO Gels Induced by Methylparaben and Dexamethasone Observed Using Time-Resolved SAXS

Abstract

Aqueous solutions of polyoxyethylene–polyoxypropylene–polyoxyethylene (PEO–PPO–PEO) triblock copolymers (commercially available as Pluronic surfactants) micellize and structurally arrange into cubic quasicrystalline lattices as their temperature is raised. This structural evolution is seen macroscopically as a gelation, and the presence of these ordered phases can be controlled through both polymer concentration and temperature. The presence of added solutes within the dispersions can also affect the onset and kinetics of structure formation. Here we investigate the structures formed in Pluronic F127 solutions ranging from 20 to 30% with two pharmaceutical additives [methylparaben (MP) and dexamethasone (DX)] using small-angle X-ray scattering (SAXS). We observe both the progressive evolution and breakdown of these structures as the temperature is increased from 0 to 80 °C. Additionally, we conducted time-resolved SAXS measurements to elucidate the kinetics of the structural evolution. On the basis of the evolution of scattering peaks as the samples were being heated, we suggest that added MP changes the nucleation behavior of fcc phases within the sample from a heterogeneous process to a more homogeneous distribution of nucleated species. MP and DX also stabilize the micelle lattices, allowing them to persevere at higher temperatures. We observed the unusual result that the presence of DX caused the primary peaks of the structure factor to be suppressed, while preserving the higher order peaks. The primary peaks reappeared at the highest temperatures tested.

Micellization and gelation of PEO-PPO-PEO binary mixture with non-identical PPO block lengths in aqueous solution

“…S8 in ESI. The relative positions of the peaks show a face centered cubic structure (FCC) for F127 gel in agreement with the literature [40]. Pure 25% L64 exhibits only a broad peak characteristic of a disordered system because it never forms a gel at any temperature.…”

“…We note that the fundamental peak (q 0 ) could not be seen for pure F127 at certain temperatures (not shown). This problem has also been reported previously for similar systems [40,41], but the exact cause is not fully understood. Fig.…”

“…As reported in the literature, the Bragg reflections always tend to become very weak in lyotropic liquid crystalline phases such as those found in Pluronics [39] due to short range disorder in the structure. However, these reflections are sufficient to reveal the packing of micelles [33,39,40]. The SAXS spectra of pure 25% F127 and pure 25% L64 are shown in Fig.…”

Near‐Infrared Optogenetic Genome Engineering Based on Photon‐Upconversion Hydrogels

“…-TTBP-Pluronic hydrogels before and after the annealing.I th as been reported that 30 wt %P luronic F127based hydrogels show SAXS peaks at 0.39 and 0.45 nm À1 , which become stronger after heat treatment and have been assigned to the micelle assemblies with fcc order. [12] Both of the Os(peptpy) 2 2+ -TTBP-Pluronic hydrogels without/with…”

Thermo-responsive polymers

“…It has also been shown that the CMC or CMT for a polymer depends on its molecular weight (PEO/PPO block length) and PEO (hydrophilic)-PPO (hydrophobic) content ratio (Deng, Yu, Price, & Booth, 1992;Price, 1983). This structural organization macroscopically leads to gelation (Meznarich, Juggernauth, Batzli, & Love, 2011;. By altering these parameters, the polymer properties can be finely tuned.…”