Microrheological study of PVA/borax physical gels: Effect of chain length and elastic reinforcement by sodium hydroxide addition

In the present work, we have applied DWS-based microrheology to study poly (vinyl alcohol) (PVA) + borax physically cross-linked gels which are typically “living” networks. At observation times longer than the lifetime of the network connection (non-covalent reversible links based on hydrogen bonds) the system flows, but exhibits elastic character at shorter time scales. We determined gel points Cb* in borax concentration at ~27 °C using viscoelastic loss tangent uniqueness method for two molecular masses of PVA (89,000 and 205,000 g mol−1) at polymer concentration CP = 4.4% below Ce. We verified that loss tangent independency of frequency is equivalent to the Winter-Chambon (W-C) law in the critical regime for elastic and viscous moduli: G′ ∝ G″~ωn. The continuity between the terminal flow regime ω <100 rd/s and the high frequency W-C one is established allowing to cover [~1–105 rad/s]. The proportionality found between Cb* and the molecular mass needs to be verified on other chain lengths. We then studied the effect of sodium hydroxide addition to PVA/borax systems since borax dissociation in water gives borate ions (responsible for establishing “elastic active” reversible reticulations between PVA chains) and boric acid which doesn't contribute to elasticity building. The results show that NaOH addition (i.e. pH increase) transforms almost all boric acid, already attached to PVA chains by monodiol complexation, into borate ions leading to didiol complexation. This, therefore, induces gelation for systems initially in sol state or elastically reinforces physical gels. Lifetime measurements of physical reticulations with and without NaOH reveal a change in borax dependency, possibly due to the increase of the association rate compared to the dissociation one i.e. an enhancement in the reforming-breaking ratio rates.