Interrogating the relationship between the microstructure of amphiphilic poly(ethylene glycol-b-caprolactone) copolymers and their colloidal assemblies using non-interfering techniques

Abstract

Understanding the microstructural parameters of amphiphilic copolymers that control the formation and structure of aggregated colloids (e.g., micelles) is essential for the rational design of hierarchically structured systems for applications in nanomedicine, personal care and food formulations. Although many analytical techniques have been employed to study such systems, in this investigation we adopted an integrated approach using non-interfering techniques - diffusion nuclear magnetic resonance (NMR) spectroscopy, dynamic light scattering (DLS) and synchrotron small-angle X-ray scattering (SAXS) - to probe the relationship between the microstructure of poly(ethylene glycol-b-caprolactone) (PEG-b-PCL) copolymers [e.g., block molecular weight (MW) and the mass fraction of PCL (f_PCL)] and the structure of their aggregates. Systematic trends in the self-assembly behaviour were determined using a large family of well-defined block copolymers with variable PEG and PCL block lengths (number-average molecular weights (M_n) between 2 and 10 and 0.5-15 kDa, respectively) and narrow dispersity (Ð < 1.12). For all of the copolymers, a clear transition in the aggregate structure was observed when the hydrophobic f_PCL was increased at a constant PEG block M_n, although the nature of this transition is also dependent on the PEG block M_n. Copolymers with low M_n PEG blocks (2 kDa) were observed to transition from unimers and loosely associated unimers to metastable aggregates and finally, to cylindrical micelles as the f_PCL was increased. In comparison, copolymers with PEG block M_n of between 5 and 10 kDa transitioned from heterogenous metastable aggregates to cylindrical micelles and finally, well-defined ellipsoidal micelles (of decreasing aspect ratios) as the f_PCL was increased. In all cases, the diffusion NMR spectroscopy, DLS and synchrotron SAXS results provided complementary information and the grounds for a phase diagram relating copolymer microstructure to aggregation behaviour and structure. Importantly, the absence of commonly depicted spherical micelles has implications for applications where properties may be governed by shape, such as, cellular uptake of nanomedicine formulations.

Keywords: Aggregate; Aggregated colloid; Diffusion NMR spectroscopy; Micelle; Microstructure; Poly(ethylene glycol-b-caprolactone) copolymers; Self-assembly; Small angle X-ray scattering.

Figure 1:

¹H NMR spectra (600 MHz, 25 °C) of (a) PEG₅ homopolymer in D₂O, and PEG₅PCL_2.4 copolymer in (b) CDCl₃ and (c) D₂O. Aggregation of the block copolymer in D₂O results in broadening of the characteristic PCL signals and a reduction in their intensity relative to the PEG signal indicative of the changing physical environment and reduced rotational mobility of protons on the hydrophobic block.

Figure 2:

(a) PGStE NMR spectroscopy attenuation of signal intensity and (b) D distributions for the methylene protons of the PEG backbone (δ_H 3.65 ppm) of the PEG₂ (blue), PEG₅ (red) and PEG₁₀ (green) homopolymers in D₂O. The scale of the x axis was chosen for ease of comparison with subsequent figures which exhibit self-diffusion coefficients across this range.

Figure 3:

D distributions for the PEG (solid red) and PCL (dashed blue) blocks of (a) PEG₂ homopolymer and the PEG₂PCL_y copolymer series, (b) PEG₅ homopolymer and PEG₅PCL_y copolymer series, and (c) PEG₁₀ homopolymer and PEG₁₀PCL_y copolymer series in D₂O. Differences between PEG and PCL D distributions arise due to relaxation weighting and indicate aggregated structure heterogeneity.

Figure 4:

Mean (marker) and standard deviation (error bars) of $\langle D\rangle$ measured on three separate self-assembled samples (independent experiments) for each PEG_xPCL_y copolymer, plotted against f_PCL. In most cases, error bars are smaller than symbols. The solid and open symbols represent the $\langle D\rangle$ of the PEG and PCL blocks, respectively, for the PEG₂PCL_y (blue), PEG₅PCL_y (red) and PEG₁₀PCL_y (green) copolymer series. Dashed lines are included as a guide to the eye and join the PEG $\langle D\rangle$ for each series.

Figure 5:

Mean (marker) and standard deviation (error bars) of PDI measured on three independent experiments of each PEG_xPCL_y copolymer solution, plotted against f_PCL.

Figure 6:

Synchrotron small-angle X-ray scattering profiles and model fitting for PEG-b-PCL block copolymers in water. Illustrations indicate the proposed structure of the aggregates, wherein the PCL cores are depicted as red, and the PEG coronas are shown in blue. The models which have been applied for fitting are denoted as follows: MF = mass fractal (random-walk polymers loosely clustered), PL = power-law scattering from secondary polymer aggregates, CY = cylindrical micelles, GP = Guinier-Porod (collapsed polymer chain), EL = ellipsoidal micelles.

Figure 7:

Phase diagram for aggregates formed from the PEG₂PCL_y (diamonds), PEG₅PCL_y (squares) and PEG₁₀PCL_y (circles) block copolymer series in water, indicating the aggregate structure with respect to f_PCL. The diagram combines aggregation states measured by SAXS measurements (shown by colors defined in the figure legend) and by NMR and DLS (with the dashed and solid black lines dividing the samples based on aggregation stability).