A Molecular-Level Study of Metamorphosis and Strengthening of Gels by Spontaneous Polymorphic Transitions.

Many weak gels often undergo spontaneous transformation to form a stronger gel upon aging. Herein, the molecular-level changes that occur during the transformation of a weak gel into a strong gel are shown by using various time-dependent techniques. Diol 1 forms a metastable transparent gel (TG) in a mixture of CH2 Cl2 /hexane and undergoes a fast transition to an opaque gel (OG) accompanied by gradual strengthening of the gel, as evidenced from time-dependent Tgel and rheology studies. Differential scanning calorimetry and thermogravimetric analyses suggest that these two gels correspond to two different polymorphs. By using FTIR spectroscopy and powder XRD experiments, it is shown that the TG-containing kinetic polymorph, with weakly hydrogen-bonded self-assembly, spontaneously changes into the OG containing a strongly hydrogen-bonded stable polymorph and this leads to strengthening of the gel and metamorphosis. Time-dependent IR studies prove the gradual change in hydrogen-bonding pattern. This is the first molecular-level study of polymorphic transitions in gels.