Self-Assembling Oligo(2-oxazoline) Organogelators for the Encapsulation and Slow Release of Bioactive Volatiles

Abstract

Herein, we report a class of distinctive supramolecular nanostructures in situ-generated from the cationic ring-opening polymerization of a particular 2-oxazoline monomer, i.e., 2-(N-tert-butyloxycarbonylaminomethyl)-2-oxazoline (Ox1). Driven by side-chain hydrogen bonding between neighboring molecules and van der Waals interactions, the growing oligomers of Ox1 precipitate in the form of macroscopic platelets when the degree of polymerization reaches 5-7. A similar self-assembly occurred in the block copolymerization of 2-ethyl-2-oxazoline (EtOx) or 2-pentyl-2-oxazoline (PeOx) and Ox1 as the second monomer. These polymeric aggregates were found to disassemble into rod-like nanoparticles under appropriate conditions, and to form stable organogels in some polar solvents like dimethylformamide as well as in natural liquid fragrances such as (R)-carvone, citronellal, and (R)-limonene. Scanning electron microscopy revealed that the morphology of their xerogels was solvent-dependent, mainly with a lamellar or fibrous structure. The rheology measurements confirmed the as-obtained organogels feature an obvious thixotropic character. The storage modulus was about 7-10 times higher than the loss modulus, indicating the physical crosslinking in the gel. The fragrance release profiles showed that the presented supramolecular gel system exhibits good sustained-release effect for the loaded bioactive volatiles.

Figure 1

Synthetic route for poly(2-oxazoline)s (A), schematic representation showing (B) the in situ generated supramolecular structures, (C) hydrogen-bonding network linking Ox1 segments, and (D) organogel formation.

Figure 2

(A) GPC traces (RI detection, TFIP, 0.8 mL min^–1, PMMA calibration) for the first block PEtOx (red) and PEtOx₁₀-b-POx1₆ as an example of block copolymers (black) prepared by a sequential polymerization route (CH₃CN, 80 °C). (B) Chemical structure and ¹H NMR spectrum (CDCl₃) of PEtOx₁₀-b-POx1₆ (see: Entry 3 in Table 1).

Figure 3

Nanoparticles formed by PEtOx_m-b-POx1_n copolymers of 0.5 mg mL^–1 in ethanol after sonication for 30 min in the presence of ice bath. TEM images of (A) PEtOx₄-b-POx1₅, (B) PEtOx₁₀-b-POx1₆, and (C) PEtOx₄₂-b-POx1₇. Insets display the length/size distribution histograms (based on measurements of ∼100 individual particles). (D) DLS measurements for the copolymers in ethanol (0.5 mg mL^–1; inset: cartoon representing the size difference in the micelle-like assemblies). Typical SEM images of the xerogels obtained by drying the organogels formed with 15 wt% of (E) PEtOx₁₀-b-POx1₆ and (F) PPeOx₁₀-b-POx1₆ in (R)-carvone in an oven (60 °C for 3 days). (G) Photographs of macroscopic gels formed by PPeOx₁₀-b-POx1₆ in (R)-carvone (a), citronellal (b), and (R)-limonene (c), and by PEtOx₁₀-b-POx1₆ in (R)-carvone (d) and citronellal (e).

Figure 4

Frequency-dependent storage (G′) and loss moduli (G″) of supramolecular organogels with different polymer contents performed at 0.1% strain, 25 °C. Gels formed by PPeOx₁₀-b-POx1₆ (the upper panel) in (R)-carvone (A) and citronellal (B), by PEtOx₁₀-b-POx1₆ (the lower panel) in (R)-carvone (C) and citronellal (D).

Figure 5

Step-strain measurements of organogels formed with 27.5 wt% of either PPeOx₁₀-b-POx1₆ or PEtOx₁₀-b-POx1₆ in (R)-carvone over three cycles (ω = 10 rad s^–1, 25 °C).

Figure 6

(A) Histogram of cumulative release percentages of fragrance organogels over a week, as measured by weighting samples under ambient conditions (25 °C, 75% humidity). Note: the release time was 2 days for the pure fragrances used as a control (3 mL). (B) Fragrance release curves (symbol) from different gel matrices. Dashed lines represent the corresponding fits with the Weibull equation. Gels were formed with 17.5 wt% of polymers in the corresponding liquid fragrances, and the initial loading contents were determined by TGA (see Figure S15).