A Robust Cross-Linking Strategy for Block Copolymer Worms Prepared via Polymerization-Induced Self-Assembly

Abstract

A poly(glycerol monomethacrylate) (PGMA) chain transfer agent is chain-extended by reversible addition-fragmentation chain transfer (RAFT) statistical copolymerization of 2-hydroxypropyl methacrylate (HPMA) with glycidyl methacrylate (GlyMA) in concentrated aqueous solution via polymerization-induced self-assembly (PISA). A series of five free-standing worm gels is prepared by fixing the overall degree of polymerization of the core-forming block at 144 while varying its GlyMA content from 0 to 20 mol %. ¹H NMR kinetics indicated that GlyMA is consumed much faster than HPMA, producing a GlyMA-rich sequence close to the PGMA stabilizer block. Temperature-dependent oscillatory rheological studies indicate that increasing the GlyMA content leads to progressively less thermoresponsive worm gels, with no degelation on cooling being observed for worms containing 20 mol % GlyMA. The epoxy groups in the GlyMA residues can be ring-opened using 3-aminopropyltriethoxysilane (APTES) in order to prepare core cross-linked worms via hydrolysis-condensation with the siloxane groups and/or hydroxyl groups on the HPMA residues. Perhaps surprisingly, ¹H NMR analysis indicates that the epoxy-amine reaction and the intermolecular cross-linking occur on similar time scales. Cross-linking leads to stiffer worm gels that do not undergo degelation upon cooling. Dynamic light scattering studies and TEM analyses conducted on linear worms exposed to either methanol (a good solvent for both blocks) or anionic surfactant result in immediate worm dissociation. In contrast, cross-linked worms remain intact under such conditions, provided that the worm cores comprise at least 10 mol % GlyMA.

Figure 1

Synthesis of a PGMA₅₆ macro-CTA via RAFT solution polymerization of GMA in ethanol using a CPDB RAFT agent and its subsequent chain extension via statistical copolymerization of varying molar ratios of HPMA and GlyMA to form diblock copolymer worms in aqueous solution via polymerization-induced self-assembly (PISA). Such worms are then cross-linked using APTES in a two-step post-polymerization process involving (i) an epoxy–amine reaction with the GlyMA residues and (ii) hydrolysis–condensation reaction with the hydroxyl groups on the HPMA residues.

Figure 2

Conversion vs time curves obtained by ¹H NMR for the (co)polymerization of HPMA (red circles), GlyMA (black circles), and the overall comonomer mixture (blue circles) at 50 °C using a PGMA₅₆ macro-CTA when targeting diblock copolymer compositions of (a) PGMA₅₆–PHPMA₁₄₄, (b) PGMA₅₆–P(HPMA₁₃₀-stat-GlyMA₁₄), and (c) PGMA₅₆–P(HPMA₁₁₅-stat-GlyMA₂₉). All syntheses were conducted at 15% w/w solids.

Figure 3

DMF GPC curves obtained for PGMA₅₆ macro-CTA (black curve) and the corresponding traces for four PGMA₅₆–P(HPMA_y-stat-GlyMA_z) (where y + z = 144; these copolymers are denoted as G₅₆-(H_y-stat-E_z) for brevity) diblock copolymers prepared at 50 °C. Molecular weights are expressed relative to a series of near-monodisperse poly(methyl methacrylate) calibration standards.

Figure 4

Representative TEM images obtained for dried 0.1% w/w aqueous dispersions of PGMA₅₆–P(HPMA_y-stat-GlyMA_z) linear diblock copolymers prior to cross-linking (where y + z = 144; these copolymers are denoted as G₅₆-(H_y-stat-E_z) for brevity). Digital photographic images of the corresponding free-standing gels recorded at 7.5% w/w solids are shown as insets.

Figure 5

Variation of the storage modulus (G′; denoted by red data set) and the loss modulus (G″; denoted by blue data set) as a function of temperature (closed circles denote a 25 to 5 °C temperature sweep, and open circles denote a 5 to 25 °C temperature sweep) for a 7.5% w/w aqueous dispersion of (a) PGMA₅₆–PHPMA₁₄₄, (b) PGMA₅₆–P(HPMA₁₃₇-stat-GlyMA₁₄), and (c) PGMA₅₆–P(HPMA₁₁₅-stat-GlyMA₂₉) worms before cross-linking. Conditions: angular frequency = 1.0 rad s^–1, applied strain = 1.0%, and rate of cooling/heating = 0.50 °C min^–1.

Figure 6

Reaction scheme illustrating worm core cross-linking chemistry by (i) epoxy ring-opening via nucleophilic attack with APTES and (ii) intermolecular cross-linking via hydrolysis-condensation. The latter step involves either reaction of the APTES with hydroxyl groups on HPMA residues on another copolymer chain (denoted as 1) and/or condensation with other APTES groups (denoted as 2). In reality, ¹H NMR studies indicate that these two steps occur more or less simultaneously, rather than consecutively as shown (see main text for details). Moreover, the chemistry is likely to be more complex than that shown as the secondary amine species may react further.

Figure 7

(a) ¹H NMR spectra obtained at various time points following the reaction of APTES with PGMA₅₆–P(HPMA₁₁₅-stat-GlyMA₂₉) after dilution into CD₃OD. The amine reacts with GlyMA as judged by the reduction in the epoxy signal peak at 3.0 ppm compared to the internal standard TMSP. (b) Kinetics of the ring-opening epoxy–amine reaction (blue data set) as judged by the attenuation in the relative integral of the epoxide signal at 3.0 ppm compared to an internal standard by ¹H NMR spectroscopy. Kinetics of worm core cross-linking as judged by the relative attenuation in the integrated pendent methyl group signal at 0.9 ppm assigned to the HPMA residues (red data set) and the relative attenuation in the integrated methyl signal at 1.2 ppm assigned to the methacrylate backbone (green data set) compared to the same internal standard at 0 ppm.

Figure 8

Representative TEM images obtained for dried 0.1% w/w aqueous dispersions of PGMA₅₆–P(HPMA_y-stat-GlyMA_z) diblock copolymers after APTES cross-linking of 7.5% w/w worm dispersions at 20 °C. Inset digital photographic images were recorded for the same aqueous copolymer dispersions at 7.5% w/w solids; free-standing gels are observed in each case.

Figure 9

Representative TEM images obtained for core cross-linked PGMA₅₆–P(HPMA_y-stat-GlyMA_z) diblock copolymers (abbreviated G₅₆-(H_y-stat-E_z) for the sake of brevity) after drying 0.1% w/w methanolic dispersions at 20 °C. (a) No well-defined nano-objects were observed at 5 mol % GlyMA, whereas the original worm morphology persists when core cross-linked worms contain higher proportions of GlyMA; see images (b), (c), and (d).

Figure 10

Zeta potential versus pH curves obtained at 25 °C for 0.1% w/w aqueous dispersions of linear PGMA₅₆–PHPMA₁₄₄ diblock copolymer worms and four examples of APTES cross-linked PGMA₅₆–P(HPMA_y-stat-GlyMA_z) diblock copolymer worms in the presence of 10^–3 M KCl.

Figure 11

Variation in storage modulus (G′; red circles) and loss modulus (G″; blue circles) as a function of temperature (closed circles denote the cooling temperature sweep and open circles denote the heating temperature sweep) for 7.5% w/w aqueous dispersions of (a) PGMA₅₆–P(HPMA₁₃₀-stat-GlyMA₁₄) and (b) PGMA₅₆–P(HPMA₁₁₅-stat-GlyMA₂₉) after worm core cross-linking using APTES (final solution pH 9–10). Conditions: angular frequency = 1.0 rad s^–1; applied strain = 1.0%; heating/cooling rate = 0.5 °C min^–1.

Figure 12

Values for tan δ at 25 °C for a series of PGMA₅₆–P(HPMA_Y-stat-GlyMA_Z) diblock copolymer worms before (blue data set) and after (red data set) after cross-linking. Covalent stabilization of the worms leads to a reduction in tan δ.

Figure 13

Representative TEM images obtained for dried dispersions of 0.1% w/w APTES cross-linked PGMA₅₆–P(HPMA_y-stat-GlyMA_z) diblock copolymer worms exposed to the presence of 1.0% w/w SDS at 20 °C.