A Fourier transform infrared spectroscopy‐based method for tracking diffusion in organogels

Abstract

Organogels possess characteristics that make them promising materials for enhancing our understanding of nanostructure‐diffusion relationships in gels and for use in diffusion‐centered applications including drug delivery and nanoreactor media. Unlike hydrogels, however, there are no well‐recognized techniques for measuring the fundamental diffusion parameter of diffusivity, D, in organogels. The present work establishes a technique for measuring D based upon Fourier‐transform infrared spectroscopy. Physically crosslinked gels composed of poly[styrene‐b‐(ethylene‐butylene)‐b‐styrene] and aliphatic mineral oil are used to showcase the new technique's capability. Diffusivity of unimers—oleic acid—and reverse micelles—sodium dioctyl sulfosuccinate (AOT)—within as‐prepared and preswollen gels is quantified and resultant values are commensurate with studies of unimer and micelle diffusion in hydrogels. The case of AOT diffusion is further validated through small‐angle X‐ray scattering analysis, which is in close agreement (<20% difference).

Effect of network connectivity on the mechanical and transport properties of block copolymer gels

“…Nanostructure dimensions are quantified and compared by fitting each data set with an appropriate model. By considering both reverse micelles and crosslinks as spherical domains, 34–37 I ( q ) can be represented by where ϕ i , Δ ρ i , and P i ( q ) are the volume fraction, scattering length density contrast (see Table S1), and q ‐dependent form factor of i domains, respectively, V i is the volume of a single i domain, S cr ( q ) is the q‐ dependent crosslink domain structure factor, and bkg is the incoherent scattering background. A reverse micelle structure factor does not appear in Equation due to gels' low AOT concentration, which causes S RM ( q ) → 1.…”

“…Nanostructure dimensions are quantified and compared by fitting each data set with an appropriate model. By considering both reverse micelles and crosslinks as spherical domains, [34][35][36][37] I(q) can be represented by…”

Decoupling of Mechanical and Transport Properties in Organogels via Solvent Variation

“…Gels were regularly extracted from these solutions for transmission FTIR and gravimetric analysis. The latter validated that no significant swelling of gels occurred during diffusion experiments, whereas the former enabled retained mass profiles to be determined based upon peak absorbance values [18].…”

“…1.7 nm. Gels were initially formulated with 10 wt%, 20 wt%, or 30 wt% copolymer and 0.5 wt% OA or 1.0 wt% AOT (values based on previous work [17,18]), resulting in six series (in each series, the MO identity was varied). Note, 1.0 wt% AOT loading resulted in cloudy gels for HB 550 and HB 1000 MOs, suggesting that precipitation occurred.…”

“…Penetrant (i.e., OA or AOT) release experiments were conducted by submersing gels into pure MO (the identity of the oil matched that of the gel under observation) and monitoring the relative amount of OA or AOT retained within each gel over time. OA and AOT concentrations were measured using a previously described FTIR-based method that takes advantage of the isolated peak position of each penetrant compound's ester/acid carbonyl group(s) (OA = 1712 cm −1 , AOT = 1739 cm −1 ) [18]. Examples of the time evolution of FTIR spectra during release experiments are shown in Figure S2 (OA) and Figure S3 (AOT) and relative retained mass (i.e., m/m 0 where m and m 0 are penetrant mass in a gel at time t and time 0, respectively) corresponds directly to relative peak absorbance (A/A 0 where A and A 0 are carbonyl peak absorbance at time t and time 0, respectively).…”