Critical Dependence of Molecular Weight on Thermoresponsive Behavior of Diblock Copolymer Worm Gels in Aqueous Solution

Abstract

Reversible addition-fragmentation chain transfer (RAFT) aqueous dispersion polymerization of 2-hydroxypropyl methacrylate was used to prepare three poly(glycerol monomethacrylate) _x -poly(2-hydroxypropyl methacrylate) _y (denoted G _x -H _y or PGMA-PHPMA) diblock copolymers, namely G₃₇-H₈₀, G₅₄-H₁₄₀, and G₇₁-H₂₀₀. A master phase diagram was used to select each copolymer composition to ensure that a pure worm phase was obtained in each case, as confirmed by transmission electron microscopy (TEM) and small-angle x-ray scattering (SAXS) studies. The latter technique indicated a mean worm cross-sectional diameter (or worm width) ranging from 11 to 20 nm as the mean degree of polymerization (DP) of the hydrophobic PHPMA block was increased from 80 to 200. These copolymer worms form soft hydrogels at 20 °C that undergo degelation on cooling. This thermoresponsive behavior was examined using variable temperature DLS, oscillatory rheology, and SAXS. A 10% w/w G₃₇-H₈₀ worm dispersion dissociated to afford an aqueous solution of molecularly dissolved copolymer chains at 2 °C; on returning to ambient temperature, these chains aggregated to form first spheres and then worms, with the original gel strength being recovered. In contrast, the G₅₄-H₁₄₀ and G₇₁-H₂₀₀ worms each only formed spheres on cooling to 2 °C, with thermoreversible (de)gelation being observed in the former case. The sphere-to-worm transition for G₅₄-H₁₄₀ was monitored by variable temperature SAXS: these experiments indicated the gradual formation of longer worms at higher temperature, with a concomitant reduction in the number of spheres, suggesting worm growth via multiple 1D sphere-sphere fusion events. DLS studies indicated that a 0.1% w/w aqueous dispersion of G₇₁-H₂₀₀ worms underwent an irreversible worm-to-sphere transition on cooling to 2 °C. Furthermore, irreversible degelation over the time scale of the experiment was also observed during rheological studies of a 10% w/w G₇₁-H₂₀₀ worm dispersion. Shear-induced polarized light imaging (SIPLI) studies revealed qualitatively different thermoreversible behavior for these three copolymer worm dispersions, although worm alignment was observed at a shear rate of 10 s^-1 in each case. Subsequently conducting this technique at a lower shear rate of 1 s^-1 combined with ultra small-angle x-ray scattering (USAXS) also indicated that worm branching occurred at a certain critical temperature since an upturn in viscosity, distortion in the birefringence, and a characteristic feature in the USAXS pattern were observed. Finally, SIPLI studies indicated that the characteristic relaxation times required for loss of worm alignment after cessation of shear depended markedly on the copolymer molecular weight.

Scheme 1. (a) Chemical Structure of Poly(glycerol monomethacrylate)–Poly(2-hydroxypropyl methacrylate) Diblock Copolymer (G_x-H_y; Where x = 37, 54, and 71 and y = 80, 140, and 200); (b) Schematic Representation of the Increase in Cross-Sectional Worm Radius When Increasing the Mean Degree of Polymerization of the Corona PGMA and the Core-Forming PHPMA Block

Figure 1

Phase diagram constructed for G_x-H_y diblock copolymer nano-objects to determine the precise copolymer composition for the pure worm phase. Each point represents the copolymer morphology assigned on the basis of post-mortem TEM studies. Green squares indicate spheres, red circles indicate worms, and blue squares indicate vesicles. Shaded boundaries represent regions of uncertainty. The three stars indicate the specific copolymer compositions used in this study. All copolymer syntheses were conducted at 20% w/w solids except for those involving PGMA DPs below 47, which were conducted at 10% w/w solids. These can be included in this phase diagram because the copolymer morphologies produced using such short stabilizer blocks exhibit no concentration dependence.

Figure 2

DMF gel permeation chromatograms recorded for the three diblock copolymers used in this work: G₃₇-H₈₀, G₅₄-H₁₄₀, and G₇₁-H₂₀₀. M_n and M_w/M_n values were calculated relative to a series of 16 near-monodisperse poly(methyl methacrylate) calibration standards.

Figure 3

TEM images obtained for (a) G₃₇-H₈₀, where no discernible copolymer morphologies were observed owing to molecular dissolution on dilution, (b) G₅₄-H₁₄₀ copolymer worm, and (c) G₇₁-H₂₀₀ copolymer worms. Each copolymer dispersion was diluted from 20% to 0.20% w/w using water (pH 6) at 20 °C. Inset digital images were recorded for 20% w/w dispersions.

Figure 4

Small-angle X-ray scattering (SAXS) patterns for aqueous dispersions of G₃₇-H₈₀ (1.0% w/w), G₅₄-H₁₄₀ (1.0% w/w), and G₇₁-H₂₀₀ (1.0% w/w) diblock copolymer worms. G₅₄-H₁₄₀ and G₇₁-H₂₀₀ were recorded at 25 °C, whereas G₃₇-H₈₀ was recorded at 35 °C to ensure that its original worm morphology was retained on dilution. The red solid curves are calculated fits to the data using a wormlike micelle model (see the Supporting Information). A gradient of −1 is indicated as guidance for the eye. Each SAXS pattern is offset by an arbitrary multiplication factor to avoid overlap of the data. The fitting results are presented in Table 1.

Figure 5

Schematic representation of the various structural parameters obtained by fitting a wormlike micelle and/or a spherical micelle model to experimental SAXS data. R_c corresponds to R_cs when considering the sphere core radius and R_cw when considering the worm core cross-sectional radius. Similarly, d corresponds to d_s when considering the sphere core radius and d_w when considering the worm core cross-sectional diameter.

Figure 6

Variable-temperature dynamic light scattering studies showing the sphere-equivalent diameter determined during thermal cycles conducted on 0.1% w/w dilute aqueous dispersions of (a) G₃₇-H₈₀, (b) G₅₄-H₁₄₀, and (c) G₇₁-H₂₀₀ worms. Filled symbols indicate the (first) cooling cycle, whereas hollow symbols indicate the heating cycle.

Figure 7

Temperature-dependent oscillatory rheology studies obtained on cooling (red data) and heating (black data) aqueous dispersions of three types of G_x-H_y worms: (a) 10% w/w G₃₇-H₈₀, (b) 10% w/w G₅₄-H₁₄₀, (c) 10% w/w G₇₁-H₂₀₀, (d) 20% w/w G₃₇-H₈₀, (e) 20% w/w G₅₄-H₁₄₀, and (f) 20% w/w G₇₁-H₂₀₀. Closed symbols represent G′, and open symbols represent G″. Oscillatory shear conditions: angular frequency = 10 rad s^–1, applied strain amplitude = 1.0%.

Figure 8

Representative small-angle X-ray scattering (SAXS) patterns recorded for aqueous dispersions of a G₅₄-H₁₄₀ diblock copolymer (a) at 5.0% w/w and (b) 0.50% w/w. Black data were recorded at 25 °C before conducting the thermal cycle. Blue data were obtained after cooling to 2 °C and equilibrating for 30 min. Red data were recorded after reheating to 25 °C and equilibrating for 24 h. A gradient of −1 is provided as a guide for the eye. A full set of SAXS data for various sample concentrations is given in the Supporting Information (Figure S7).

Figure 9

Small-angle X-ray scattering (SAXS) patterns obtained for a 5.0% w/w aqueous dispersion of G₅₄-H₁₄₀ diblock copolymer nano-objects recorded on heating from 5 to 25 °C. The red solid lines are calculated fits to the data using a combination of spherical micelle and wormlike micelle models. Gradients of 0 and −1 are provided as guidance for the eye. Each SAXS pattern is offset by an arbitrary multiplication factor (shown at the right side of the patterns) for clarity. These data confirm a fully reversible sphere-to-worm transition for this copolymer under the stated conditions.

Figure 10

Temperature dependence of the mean worm contour length, L_c (black squares), and the relative volume fraction of the copolymer in the solution comprising worms (x_worm; solid red circles) and spheres (x_sphere; hollow red circles) determined by SAXS for a 5.0% w/w aqueous dispersion of a G₅₄-H₁₄₀ diblock copolymer. Lines are a guide for the eye.

Figure 11

(a) Viscosity vs temperature plots obtained from continuous shear rheology studies conducted on G₅₄-H₁₄₀ copolymer dispersions at concentrations of 5% w/w (green symbols), 10% w/w (blue symbols), 15% w/w (red symbols), and 20% w/w (black symbols). The shaded region indicates the temperature range in which the characteristic Maltese cross was observed during the heating cycle. For the 5% w/w worm dispersion, the Maltese cross was only visible at 26 °C. (b) SIPLI images obtained at various temperatures during a thermal cycle conducted for G₅₄-H₁₄₀ worms at copolymer concentrations of 5, 10, 15, or 20% w/w. The Maltese cross indicates birefringence, which is the result of worm alignment under a shear flow. Arrows on PLI indicate orientation of the polarizer (P) and the analyzer (A).

Figure 12

Viscosity vs temperature recorded for a 10% w/w G₅₄-H₁₄₀ worm gel while shearing at 1 s^–1. Inset: SIPLI images recorded at 35, 20, and 15 °C during the temperature ramp. Arrows on PLI indicate orientation of the polarizer (P) and the analyzer (A)

Figure 13

(a) Viscosity vs temperature plots recorded for 20% w/w aqueous dispersions of the three G_x-H_y diblock copolymers examined in this study. Maltese cross symbols indicate the temperature at which maximum worm alignment was observed. (b) SIPLI images obtained at 2 °C (the first column), at the characteristic temperature where the Maltese cross was judged to be most intense (i.e., 6, 8, or 22 °C) (the second column), and at 35 °C (the third column). Arrows on PLI indicate orientation of the polarizer (P) and the analyzer (A). (c) Schematic representation of the proposed mechanism for the transition from spheres to worms to branched worms that occurs on raising the temperature from 5 to 35 °C.

Figure 14

SIPLI relaxation studies conducted on 20% w/w aqueous dispersions of G₃₇-H₈₀, G₅₄-H₁₄₀, and G₇₁-H₂₀₀ worms at various temperatures (20–40, 2–10, and 2–6 °C, respectively): (a–c) show the decay of the image intensity vs time after cessation of shear (the solid line represents a fit to an exponential decay); (d–f) show the temperature dependence of the characteristic half-life time, τ_1/2, required for worm relaxation (here the solid lines are merely a guide for the eye).