Abstract
Stimuli-responsive polymer materials are used in smart nanocarriers to provide the stimuli-actuated mechanical and chemical changes that modulate cargo delivery. To take full advantage of the potential of stimuli-responsive polymers for controlled delivery applications, these have been grafted to the surface of mesoporous silica particles (MSNs), which are mechanically robust, have very large surface areas and available pore volumes, uniform and tunable pore sizes and a large diversity of surface functionalization options. Here, we explore the impact of different RAFT-based grafting strategies on the amount of a pH-responsive polymer incorporated in the shell of MSNs. Using a “grafting to” (gRAFT-to) approach we studied the effect of polymer chain size on the amount of polymer in the shell. This was compared with the results obtained with a “grafting from” (gRAFT-from) approach, which yield slightly better polymer incorporation values. These two traditional grafting methods yield relatively limited amounts of polymer incorporation, due to steric hindrance between free chains in “grafting to” and to termination reactions between growing chains in “grafting from.” To increase the amount of polymer in the nanocarrier shell, we developed two strategies to improve the “grafting from” process. In the first, we added a cross-linking agent (gRAFT-cross) to limit the mobility of the growing polymer and thus decrease termination reactions at the MSN surface. On the second, we tested a hybrid grafting process (gRAFT-hybrid) where we added MSNs functionalized with chain transfer agent to the reaction media containing monomer and growing free polymer chains. Our results show that both modifications yield a significative increase in the amount of grafted polymer.
Highlights
The development of smart polymer materials has been inspired by the response to stimuli present in all living organisms
Another strategy is to use a “hybrid grafting” approach where both monomer and free-growing Reversible addition fragmentation chain transfer (RAFT) polymer chains are present when chain transfer agent (CTA)-functionalized Mesoporous Silica Nanoparticles (MSNs) are introduced in the reaction mixture
We prepared a nanocarrier for pH-actuated controlled release, with a mesoporous silica core and a pH-responsive polymer shell prepared by RAFT polymerization
Summary
The development of smart polymer materials has been inspired by the response to stimuli present in all living organisms. Stimuli-response in polymers materials is generally related to changes in the polymer chain conformation [6] that can be induced by temperature [7,8], pH [9], light [10], proteins [11], ionic strength [12], and so forth or combinations thereof [13,14]. Among these stimuli, pH has many potential applications because it can be used as an intrinsic trigger in processes where variations in pH. The conformation change of the polymer can be tuned to the desired application by selecting the acid or basic monomer building blocks [20,21]
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