Which would be the choice of solvent for better quaternization of pDMAEMA on nanoparticle?

As per available literature I did used THF to disperse my pDMAEMA grafted nanoparticle system (which is in water) and added a quaternizing agent of aryl halides for 48h at RT. After the reaction, it was purified by rota-vap and dialysed against water.

While DLS measurements with pH ranges of 3-12 the system's surface charge went to negative after pH 9 and the particle diameter became much bigger >500 nm.

So I feel like the system was not quaternized well. Since im having poor knowledge in this area, a better solution would be a great help.