K. Sunitha

pH-responsive superomniphobic nanoparticles as versatile candidates for encapsulating adhesive liquid marbles

Conventional adhesives are rarely used in sophisticated applications such as micro-fluidic devices or ‘operations of bonding from a distance’ due to their permanent wetting characteristics. Liquid marbles offer exceptional switching between non-wetting and wetting on demand. In this contribution, we present a novel approach to encapsulate both hydrophilic (epoxy resin) and hydrophobic (siloxane polymer) liquids via wrapping them with superomniphobic nanoparticles. The free energy for marble formation is lower for a hydrophobic liquid (0.931 × 10−16 J), whereas a hydrophilic liquid registers a higher value of 1.86 × 10−16 J. The mechanical bursting energy for hydrophobic marbles (20 μJ) is lower than that for their hydrophilic counterpart (48.6 μJ). The static friction coefficients of epoxy-based liquid marble are between 0.015 and 0.020 on glass, aluminium and stainless steel substrates. As a highlight, the nanoparticle coating is responsive to pH, and the bursting time of the liquid marbles can be tuned from <1 minute to several hours. It is demonstrated that the adhesive strength of cross-linked epoxy obtained by a liquid marble route is higher than that obtained vis-a-vis a conventional wetting route. The liquid marbles presented in this work can be ruptured by changing the pH, have a lower friction coefficient compared to the bare liquids (more rolling distance, which is highly essential for bonding of an intricate space from a distance) and are useful as dry adhesives.

Novel superabsorbent copolymers of partially neutralized methacrylic acid and acrylonitrile: synthesis, characterization and swelling characteristics

Novel superabsorbent copolymers (SAP) based on random copolymers of sodium methacrylate, methacrylic acid, and acrylonitrile (ACN) of varying compositions and cross-link densities were synthesized by free radical polymerization and were characterized. ACN content in the copolymers varied from 0 to 15 mol%. The SAPs were capable of absorbing 350–990 times their weight of water and 60–130 times their weight of saline (0.3 wt. % solution of sodium chloride in deionized water) depending on the composition and cross-link density of the copolymer. Swelling of the copolymers followed first-order kinetics with non-Fickian swelling behavior in deionized water. Water absorbency (Sw) was found to decrease with pressure on the gel side and a pressure coefficient (dSw/dp) of −0.6%/Pa was estimated for a given composition of the copolymer. Strength of the hydrogels increased with increasing cross-link density. Shear modulus of the hydrogels was found to vary from 1000 to 6000 Pa depending on the composition and cross-link density. Higher cross-link density of the SAP and higher salt concentration of the swelling medium reduced the swelling ratio, while higher extent of neutralization and lower salt concentration of the medium enhanced it. Different copolymer hydrogels were able to retain 50–78% of absorbed water at 37 °C after a period of 20 h, with the systems having lower ACN content retaining higher amount of water. The copolymers exhibited absorbency in the range of 1500–2000% by weight under a pressure of 6200 Pa when swollen in 0.9% aqueous solution of sodium chloride. Increase in ionic strength of the medium, pressure on the hydrogel, and temperature (of the medium) were proven to decrease the swelling commensurate with the thermodynamics of the system.

One-component novel allyl–maleimide Alder ene resins: synthesis, curing, and thermal properties

Novolac resins, bearing both allyl and maleimide groups in varying proportions and capable of self-curing via Alder ene reaction, were synthesized by anchoring maleimidobenzoyl groups onto allyl novolac resins. These systems are capable of self-curing at moderate temperature by undergoing two-step cure reaction, i.e. ene reaction (~60 °C) followed by Diels–Alder reaction (~180 °C) to form thermal stable resins. Anchoring of the allyl and the maleimide groups in the same molecule decreases the cure activation energy tremendously due to increased proximity of reactive species and favorable activation entropy factors. The increase in activation energy with conversion implied that the reaction migrates from chemically controlled zone to diffusion-controlled zone beyond ~50% conversions. The resins showed increasing thermal stability with enhanced maleimide content.