Polymerisation-Induced Self-Assembly of Graft Copolymers

Abstract

We report the polymerisation-induced self-assembly of poly(lauryl methacrylate)-graft-poly(benzyl methacrylate) copolymers during reversible addition-fragmentation chain transfer (RAFT) grafting from polymerisation in a backbone-selective solvent. Electron microscopy images suggest the phase separation of grafts to result in a network of spherical particles, due to the ability of the branched architecture to freeze chain entanglements and to bridge core domains. Small-angle X-ray scattering data suggest the architecture promotes the formation of multicore micelles, the core morphology of which transitions from spheres to worms, vesicles, and inverted micelles with increasing volume fraction of the grafts. A time-resolved SAXS study is presented to illustrate the formation of the inverted phase during a polymerisation. The grafted architecture gives access to unusual morphologies and provides exciting new handles for controlling the polymer structure and material properties.

Keywords: Graft Copolymer; PISA; Polymerization; RAFT; Self-Assembly.

Scheme 1

Synthetic route used in this study: RAFT copolymerisation of LMA and HEMA, post‐modification, and dispersion polymerisation of BzMA.

Figure 1

Representative ¹H NMR spectra of polymers prepared in this study (400 MHz, CDCl₃). A) Linear p(LMA₈₁₆‐s‐HEMA₉₉) (1 a). B) Functionalised backbone pLMA₉₁₅‐CTA_10% (1 b). C) Reaction mixture after PISA with pLMA₉₁₅‐CTA_10% targeting graft DP 5 (6.5).

Figure 2

SEC profiles of A) p(LMA‐s‐HEMA) copolymers 1 a–5 a, B) pLMA‐CTAs 1 b–5 b, and C) pLMA‐g‐pBzMA graft copolymers 6.1–6.12 prepared with pLMA₉₁₅‐CTA_10%. Analysis in CHCl₃ with DRI detection and PMMA calibration.

Figure 3

A) Reaction mixtures after PISA using pLMA₉₁₅‐CTA_10% to target various graft lengths at 20 wt% and 10 wt% solids content, and using pLMA₉₃₉‐CTA_2% to study the effects of grafting density. TEM (▪), SEM (▴), and cryo‐SEM (•) images show nanostructures prepared using pLMA₉₁₅‐CTA_10% at 20 wt% targeting graft lengths of 1, 10, and 105 repeat units. B) Connected spheres obtained using pLMA₂₀₆‐CTA_10%, pLMA₉₁₅‐CTA_10%, and pLMA₄₇₄‐CTA_10% at 20 wt%. Illustration shows the proposed origin of sphere clusters: phase separation of grafts leads to physical crosslinks arising from backbone entanglements and bridging of core domains.

Figure 4

SAXS data (points) and associated structural fits (lines) of PISA reaction mixtures at 20 wt% prepared from pLMA₉₁₅‐CTA_10% (1b) to yield pLMA₉₁₅‐g‐(pBzMA _x ) _y graft copolymers (6.1–6.12). Graft length increases from bottom to top (data vertically offset for clarity). Scheme illustrates the suggested multicore micelle (DP 1–15), rigid cylinder (DP 18), flexible cylinder (DP 24–31), vesicle (DP 53), and inverse multicore micelle (DP 105) morphologies of the pBzMA cores (in white) against pLMA and n‐dodecane (in black; not to scale).

Figure 5

A) Time‐resolved SAXS data (black traces) collected in situ during PISA of pLMA‐g‐pBzMA. Fits to the data are shown as coloured lines. Error bars have been omitted for clarity and datasets have been vertically offset. B) Zero‐angle intensity (I₀) and radius of gyration (R _g) values obtained through fitting data between 5–50 min to Gaussian coil models. C) Equatorial and polar radii (r) obtained through fitting data between 55–85 min to ellipsoidal models. D) Radii, radial polydispersities (PDI) and volume fractions (χ) obtained through fitting data between 140–320 min to raspberry models describing inverse multicore micelles.