Insight into precipitation polymerization of N-isopropylacrylamide : reaction mechanism, particle formation and particle structure

Stimuli-sensitive microgels are soft, spherical nanoparticles composed of thermosensitive polymers, such as poly(N-isopropylacrylamide) (PNIPAM) and their copolymer derivatives. PNIPAM microgels are conveniently prepared by aqueous precipitation polymerization of N-isopropylacrylamide (NIPAM). The appeal of these particles is based on the fundamental physical aspects related to the stimuli-sensitive volume-phase transition (VPT) and interaction of the soft colloids with their surroundings, as well as specific applications enabled by the stimulus- controlled VPT behavior.The particle formation in precipitation polymerization of thermosensitive polymers is a question of fundamental interest to colloid and polymer chemistry, although insufficiently addressed in previous work. The aim of this thesis is to recognize the important polymerization parameters that affect the particle formation, establish the polymerization mechanism in precipitation polymerization and connect the mechanism to observable particle properties such as particle volume at full conversion. All the experiments are carried out with NIPAM system.A rapid small-scale synthesis method in conjunction with dynamic light scattering characterization is used to investigate effects of reaction conditions on PNIPAM particle volume. Other main characterization methods used are static light scattering and small-angle neutron scattering, which is used to follow the particle growth in situ during the polymerization. It is shown that the PNIPAM particle volume follows power laws in respect to batch monomer and initiator concentration. The power law behavior arises from charge density of PNIPAM chains generated during the particle nucleation phase early in the reaction, which determines the number density of particles in the batch. Evidence is laid out in support of the reaction likely proceeding by radical solution polymerization mechanism during the initial nucleation stage, which is the underlying factor explaining how the charge density of PNIPAM changes with reaction parameters.In addition it shown by static scattering analysis that commonly used peroxide initiators can have a drastic effect on the particle structure of ultra-low cross-linked microgels. These particles are shown to be hollow-like PNIPAM shells in contrast to fuzzy sphere structure commonly attributed to PNIPAM microgels. The unexpected build of the ultra-low cross-linked microgels is explained by peroxide radical induced cross-linking on the microgel surface during the reaction. Choice of initiator system and the amount of additional crosslinker therefore play a decisive role in determining the particle structure