Temperature-Dependent Capsule Shell Bonding and Destruction Based on Hindered Poly(urea-urethane)Chemistry

摘要

Capsules with responsive shells that,for example,can be destroyed to release a payload or fused together to create a monolith are needed to improve performance in,for example,controlled release,material storage/transport,molecular separation,and so on.Most commonly,these shells contain pH-responsive functional groups or temperature-responsive polymers and undergo changes in shell permeability,for example,upon decreased pH or increased temperature.Herein,we report a new approach to fabricating responsive capsules for controlled fusion or destruction by the incorporation of hindered poly(urea-urethane)chemistry into capsule shells.Using a non-aqueous Pickering emulsion as a template,we demonstrate that interfacial polymerization between three different secondary diamines and four different diisocyanates can be used to prepare capsules with a core of polar oil(N,N-dimethylformamide,DMF)or a mixture of DMF and ionic liquid(IL)with shells containing hindered urea bonds that undergo dynamic bond exchange in response to slightly elevated temperatures.For the capsules in which the core is oil,the temperature of responsivity is based on the hindrance of the diamine(35,55,or 80 ℃),consistent with the bulk polymer,and supported by variable-temperature Fourier transform infrared spectroscopy.In contrast,for capsules in which the core contains IL,the temperature required is significantly decreased,suggesting that the shell is plasticized with the core liquid.Heating isolated capsules to the relevant temperature leads to capsule shell fusion into a monolith,whereas addition of a primary amine to dispersed capsules at elevated temperature leads to shell destruction.Scanning electron microscopy(SEM)and optical microscopy were used to confirm the morphology of capsules,monoliths,or emulsion droplets,and focused ion beam-SEM was utilized to demonstrate the core-shell structure of the capsules.Furthermore,comparison of mechanical tests of the capsules and fused monoliths highlight the importance of bond exchange on bulk properties.This work demonstrates that capsule shells containing dynamic covalent bonds are a new exciting class of materials that can be used to tailor morphology beyond the traditional use of responsive shells to change permeability.