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 Gx-Hy 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.

RAFT‐Mediated Polymerization‐Induced Self‐Assembly

“…and they are usually explained by a change in the surface‐plasticization leading to a change in the packing parameter. The transition temperature is generally sensitive to the molar mass of the polymers, and can thus be tuned by the DP n or by copolymerization. For example, Warren, Plamper, and co‐workers copolymerized NIPAM with tert ‐butyl acrylamide (tBuAAm) to afford PDMAAm‐ b ‐P(NIPAM‐ co ‐tBuAAm) structures, morphological transitions of which could be induced by both temperature and hydrostatic pressure …”

In situ SAXS studies of a prototypical RAFT aqueous dispersion polymerization formulation: monitoring the evolution in copolymer morphology during polymerization-induced self-assembly

“…This is consistent with our earlier studies, which found that longer PHPMA chains become increasingly hydrophobic. 60,84 It is perhaps noteworthy that the substantially hydrated nature of the initial nascent micelles is likely to reduce the effective local concentration of HPMA monomer within their cores. This might explain why no rate enhancement was observed by 1 H NMR spectroscopy aer 9-10 min in Fig.…”

“…These observations are fully consistent with a worm growth mechanism based on the stochastic 1D fusion of multiple spheres. 59,84,88 To the best of our knowledge, this is the rst detailed study of the growth of spheres and worms as intermediate morphologies during PISA. This is important, because it provides an opportunity to compare the relative dimensions of such nano-objects during the sphere-to-worm (and worm-to-vesicle) transition.…”

Probing the mechanism for hydrogel-based stasis induction in human pluripotent stem cells: is the chemical functionality of the hydrogel important?

“…This new synthetic hydrogel comprises highly anisotropic worm-like particles formed by the self-assembly of an amphiphilic poly(glycerol monomethacrylate)-poly(2-hydroxypropyl methacrylate) [PGMA-PHPMA] diblock copolymer. 18,19 The worms are formed directly in aqueous solution during the synthesis of the diblock copolymer chains via a highly efficient and convenient process known as RAFT aqueous dispersion polymerization, [20][21][22][23][24][25][26][27][28][29][30][31][32] which is an example of polymerization-induced self-assembly (PISA). [33][34][35][36][37][38] Gelation is believed to be the result of multiple inter-worm contacts, which leads to local arrest of translational diffusion.…”