Phase Separation Behavior in Tough and Self-Healing Polyampholyte Hydrogels

Abstract

Polyampholyte hydrogels (PA gels) are drawing great attention for their excellent mechanical properties including self-healing, high toughness and fatigue resistance.These mechanical performances are found to be attributed to the hierarchical structure of the PA gels, consisting of reversible ionic bonds at the 1 nm scale, permanent polymer network at the 10 nm scale, and bicontinuous phase network at the hundred nm scale. In this work, we systematically studied the phase network formation of these gels, aiming to answer the following three questions: (1) how the phase separation occurs? (2) what determines the phase structure? and (3) is this structure in thermal dynamically equilibrium or not? Our results show that, the phase separation occurs during dialysis of counterions from the gels and it is driven by the Coulombic and hydrophobic 2 interactions. The phase size d0 and the number of aggregated chains in a unit cell of phase structure n scale with the molecular weight of partial chain between permanent effective crosslinking Meff as d0 ~ Meff and n ~ Meff 2 , respectively. Chemical crosslinker and topological entanglement suppress phase separation while hydrophobic interaction favors phase separation. An intrinsic correlation between the polymer density difference () between two phases and d0 is observed,   d0 2 , as a result of the competition between the driving force to induce phase separation and the resistance to suppress the phase separation. The phase separated structure is metastable, which is locally trapped by strong intermolecular interactions.

Effect of mesoscale phase contrast on fatigue-delaying behavior of self-healing hydrogels

“…Our previous study showed that the level of phase contrast and characteristic phase length (d 0 ) of PA gels depend on the effective chain density of permanent polymer network  e and temperature (T dial ) used to dialyze the small counterions from the gels during the sample preparation (24,25).  e is due to the presence of chemical cross-links and trapped topological entanglements.…”

“…Accordingly, tuning the monomer concentration C m at the gel synthesis step can change the polymer concentration and thereby the concentration of the trapped topological entanglements. In this work, we vary the chemical cross-linker density (C MBAA ) to tune chemical cross-linking density and the total monomer concentration (C m ) to tune the topological entanglement density (24,28). Two series of samples were prepared by one-step random copolymerization of the anionic monomer sodium p-styrenesulfonate (NaSS) and the cationic monomer methyl chloride quarternized N,N-dimethylamino ethylacrylate (DMAEA-Q).…”

“…S1). The mesh size  of the permanent polymer network, estimated by the relation  e ≅  −3 (24), is in the range of 4 to 9 nm (table S1).…”

“…The intensity of the scattering ring decreases markedly with increasing C MBAA , and last, the ring disappears for PA-2.5-1.0, indicating that the phase contrast declines and the sample becomes homogeneous as C MBAA increases. A S2), due to an increase in the topologically trapped entanglements that serve as effective permanent cross-linking density (24). For comparison, results of sample series PA-2.0-0.1 (T dial ) dialyzed at different T dial [extracted from our previous work (25)] are also shown in table S2.…”

“…The hard/soft microphase separated network consists of bicontinuous dense/sparse polymer phases in water. The polymer strands are in their collapsed globule conformation (24). The PA gels exhibit an extremely slow crack growth mode when the applied energy release rate (G) is above the fatigue threshold G 0 and that slow propagation mode is maintained until a transition point G tran that has a value several times that of G 0 , at which point a fast crack propagation mode is observed.…”