Surface Modification of Cellulose Nanocrystals with CO2-Responsive Polymers via Reversible Deactivation Radical Polymerization
Cellulose is the most abundant natural polymer on Earth. From cellulose, cellulose nanocrystals (CNC) can be obtained through various hydrolysis processes. There is interest in grafting different types of polymers onto CNC as their applications are limited due to their poor dispersibility in low polarity and non-polar systems. The potential versatility and value of modified CNC can be enhanced even further if the polymers used in the grafting process are CO2-responsive, including for example poly(dimethylaminoethyl methacrylate) (PDMAEMA), poly(diethylaminoethyl methacrylate) (PDEAEMA) and poly(diisopropylaminoethyl methacrylate) (PDPAEMA), since stimuli responsive composites have shown promise in applications such as Pickering emulsifiers, water treatment and as polymer reinforcing agents. In this work, reaction conditions for the homopolymerization of dialkylaminoethyl methacrlyates (DMAEMA, DEAEMA and DPAEMA) by Cu(0) and RAFT-mediated polymerizations were determine. Cu(0)-ATRP polymerization showed a linear evolution of Mn vs. conversion and first order kinetics; however, poor livingness of the macro-initiators was observed. RAFT-mediated polymerization showed good livingness and the Mn of exclusively the living chains of the macro-RAFT agents was determined by GPC-UV. These macro-RAFT agents were suitable to be used for the grafting-to approach. The grafting of CNC surfaces via grafting-from and grafting-to using Cu(0)-mediated atom transfer radical polymerization (ATRP) and reversible addition fragmentation chain transfer (RAFT) radical polymerization is also reported. For the grafting-from approach, the CNC surfaces were first chemically modified with an ATRP initiating group or a RAFT agent molecule capable of mediating the polymerization of dialkylaminoethyl methacrylates and their corresponding macro-RAFT agents. A grafting-from polymerization was then performed using the CO2 switchable monomers DEAEMA, DMAEMA and DPAEMA via SI-Cu(0)-ATRP and SI-RAFT. For the grafting-to approach, the CNC surfaces were modified with a double bond containing molecule which was then reacted with fully characterized macro-RAFT agents. All of the grafted products were found to be CO2-responsive. Products made via grafting-from were colloidally stable under CO2; whereas the products made via grafting-to were colloidally unstable under CO2, but dispersible under mildly basic conditions. It was determined that both the Mn of the polymer grafts and graft densities play an important role on grafted CNC dispersibility.