Effects Of Surface Groups Of SiO₂ Nanoparticles On The Kinetics Of In-situ Solution Polymerization And Its Influence Mechanism

In-situ free-radical polymerization in the presence of inorganic nanoparticles is still a research hotspot for the preparation of organic/inorganic nanocomposites.Previous studies have shown that the inorganic nanoparticles had significant impact on the polymerization process,including the polymerization rate,final conversion and the molecular weight of polymer matrix.Therefore,it is of great importance to investigate the intrinsic reason of above-mentioned problem,which will finally benefit to the improvement of the nanocomposite performance.In this study,monodisperse silica particles（SiO₂）and benzoyl peroxide（BPO）were used as the typical inorganic nanoparticles and oil-soluble peroxide initiator,respectively.3-（trimethoxysilyl）propyl methacrylate（MPS）and octyltrimethoxysilane（OTMS）were used as the active and inert silane coupling agents for the surface modification of SiO2.Then in-situ solution polymerization of methyl methacrylate（MMA）was performed in the presence of pristine or modified SiO₂ particles.The effects of SiO₂ particles on the kinetics of initiator decomposition and polymerization,as well as the evolution of grafting ratio of SiO2 particle,were examined systematically.Moreover,the influences of silicon hydroxyl,double bond and grafted polymer layer on particle surface on the initiator decomposition and in-situ polymerization process were discussed.On this basis,the influence mechanism of the surface group of inorganic particles toward in-situ solution polymerization was successfully illustrated.Firstly,the influence of SiO₂ particles on the initiator decomposition was studied.In detail,the effects of SiO₂ content and size,and surface-modification on the initiator decomposition kinetics and decomposition products were investigated.It was found that,with increasing amount of silicon hydroxyl,the activation energy of BPO decomposition was declined,and the BPO decomposition rate increased gradually.Moreover,the primary radicals preferred to transfer to the surface silicon hydroxyl,resulting in an increase of benzoic acid proportion in the main products of BPO decomposition.The polymerization rate at early stage indicated that the addition of SiO2 particles could shorten the induction period due to the acceleration of BPO decomposition,thereby accelerating its polymerization rate at early stage.Secondly,the influence of silicon hydroxyl on the kinetics of in-situ solution polymerization was studied.The effect of silicon hydroxyl on the polymerization rate,final conversion and molecular weight was examined.It was found that the polymerization rate at middle and later stage,final conversion and the molecular weight of resulting polymer were obviously reduced with increasing amount of silicon hydroxyl.It can be attributed to the acceleration of BPO decomposition and chain transfer reaction of free-radical to the silicon hydroxyl.Meanwhile,the influence of surface double bond and grafted polymer layer on the kinetics of in-situ solution polymerization was also studied.With increasing of MPS-anchoring density on SiO2 surface,the polymerization rate at middle and later stage and the final conversion increased gradually.On the one hand,the amount of residual silicon hydroxyl decreased as MPS-anchoring density increased;on the other hand,the in-situ generated grafting polymer layer could effectively shield the free-radicals from diffusing to surface of Si O2 and restrain the chain transfer to the silicon hydroxyl.In addition,the structure and composition of interfacial grafting layer was investigated by examining the elution process of free polymer chains in the interface layer and the size evolution of grafted SiO₂ particles during polymerization.Herein,the interfacial polymer layer contained three kinds of polymers,including the physical adsorbed free polymer,grafted polymer,and a large number of free polymers entangled in the graft layer.Due to the shielding effect of interfacial polymer layer,the grafted SiO₂ particles are separated and did not crosslink or aggregate with each other.In conclusion,this research work is focused on studying the influence of surface groups of SiO₂ particles on kinetics of initiator decomposition,in-situ polymerization rate,final conversion and the molecular weight of produced polymers.The influence mechanism of surface group on SiO₂ particles on the in-situ solution polymerization was explained via three aspects,including the acceleration of initiator decomposition,the chain transfer of free-radical to silicon hydroxyl and the shielding effect of grafted polymer layer.This research result can take an important guiding role in the design and fabrication of composite materials with high performance.