Self-Degrading Soft Materials: From Molecular Gels to Gel-Foams

Abstract

This dissertation introduces the concept of self-degrading soft materials and demonstrates three distinct platforms for such materials. Self-degrading materials autonomously transform from solids to thin liquids after a preset ‘degradation time’ without the need for external triggers or stimuli. The degradation time is programmed into the initial material through composition, allowing for precise control over the instant when the solid-liquid transformation occurs. In the first study, we develop self-degrading molecular organogels based on dibenzylidene sorbitol (DBS) in polar organic solvents. DBS molecules self-assemble into a nanofibrous network in these gels. In the presence of an aqueous acid, the gels slowly transform from robust solids (with shear moduli > 10,000 Pa) to thin sols. The degradation time can be tuned from hours to days through the acid concentration and temperature. Using NMR spectroscopy and mass spectrometry, we show that degradation occurs through the acid-catalyzed hydrolysis of DBS to form benzaldehyde and sorbitol. We demonstrate applications for these self-degrading gels in time-activated valves and flow diversion devices. The second study extends the above concept to non-aqueous systems (oils) suitable for oilfield applications, specifically addressing the critical problem of lost circulation during oil drilling. We show that DBS organogels containing organic acids like hexanoic acid degrade in a controlled manner through esterification rather than hydrolysis. These gels exhibit unique “shrinking core” behavior where their rheological properties remain constant even as the gel mass decreases linearly with time. The degradation kinetics can be tuned through multiple parameters: acid chain length (butanoic to octanoic), acid concentration (20–80 wt%), temperature (30–75°C), and DBS concentration (0.5–2 wt%). In the third study, we create self-degrading “gel-foams” based on polyethylene glycol diacrylate (PEGDA) and polyethyleneimine (PEI). We start with solutions of PEGDA and PEI and foam their mixture, generating bubbles of oxygen gas. Within seconds, PEGDA and PEI react to form a gel around the bubbles. Thus, the liquid foam transforms into a solid gel foam that can be lifted up by hand (bubbles remain trapped in the gel). Thereafter, the gel degrades into a thin liquid and this is because the crosslinks are cleaved by hydrolysis. The degradation time can be tuned from minutes to days based on the polymer composition. We show that such gel foams can slowly release solutes (such as dyes or antibiotics) into water over long time periods, thereby, such materials could be useful for environmental remediation of contaminated water bodies. Collectively, this work establishes self-degradation as a versatile concept that can be implemented across multiple chemical platforms and length scales. The ability to program materials to autonomously transform after a set time opens new possibilities across various fields, from oilfield operations to environmental remediation and controlled release applications.