Controllable Synthesis Of Hybrid Silica Nanoparticles Densely Grafted With Poly(ethylene Glycol) And The Crystallization Behavior

Silica nanoparticles(SiO2-NPs)is a widely used platform to prepare nanoparticles owing to its robustness,biocompatibility and versatility in functionalization.Polymers tethering to silica surface through covalent bonds has attracted much attentions in past few decades.In recent years,polymer-inorganic nanocomposites have been applied in diverse fields due to excellent thermomechanical,electrical and mechanical properties.Grafting polymers generally play crucial role in their performance,which leads to industrial and scientific interest.In particular,interfacial effect of the polymer nanocomposites is highly crucial in terms of the applications,which a0ffects the conformation and crystallization of grafted polymer chains on the vicinity of the interface.In this thesis,we select poly(ethylene glycol)monomethyl ether(MPEG)as the semicrystalline polymer precursor(surface tethered MPEG confer desirable properties),by means of "grafting onto,strategy,a simple method that can control over the degree of polymerization under ambient conditions and has been widely used for modification of NPs.Epoxy-terminated MPEG(MPEG-EO)with various molar masses were grafted onto NH2-modified silica nanoparticles(SiO2-NH2)using "grafting onto" method.Effects of MPEG concentration and n-decane-to-toluene volume ratio on the grafting density were studied.The results demonstrate that the density increase with MPEG concentration.The grafting density can be tuned by altering n-decane-to-toluene ratio.In contrast to grafting density obtained in toluene,the maximum values of 200%.273%,and 321%higher than the former can be achieved in n-decane/toluene mixture,using MPEO-EO at the mean molecular weights of 750,2000 and 4000 g/mol,respectively.This strategy provides a controllable efficient method for tethering polymer onto SiO2-NPs.A two-step method was used for grafting bimodal polymer brushes on NPs,i.e.low-molecular-weight MPEG-EO(Mw = 750 g/mol)was first attached to SiO2-NH2 in toluene.Thereafter,as-obtained sample reacted with high-molecular-weight MPEG-EO(Mw = 4000 g/mol)in n-decane/toluene mixture.Grafting density reseachs 0.85,0.4 chain/nm2 using MPEG-EO(Mw = 750,4000 g/mol,respectively),as judged by the thermogramitric analysis(TGA).Effect of the grafting density on conformation and crystallization of MPEG were studied using Raman spectroscopy,differential scanning calorimetry(DSC)and thermogramitric analysis(TGA).The results demosntrate that the particle surface constrains chain mobility of interficial MPEG and increase the population of gauche conformation.In contrast to high grafting density,hydrogen-bonding interactions between interficial silanol groups and PEG units leads to chain confinement and impeded crystallization at low grafting density.In short,the grafting density of MPEG on SiO2-NPs was contralled by varying MPEO concentration and n-decane/toluene volume ratio.Grafting density increases with MPEG concentration and n-decane/toluene ratio.In contrast to high grafting density,hydrogen-bonding interactions between interficial silanol groups and MPEG units leads to chain confinement and impeded crystallization at low grafting density.