The Preparation Of The Temperature-sensitive Microgels With Different Ionic Groups On Their Surfaces And The Study Of Their Colloidal Stability

The temperature-sensitive poly(N-isopropylacrylamide) (PNIPAM) microgels prepared by surfactant-free emulsion polymerization have good monodispersity and no surfactant pollution on their surfaces. The initiator for the preparation of the PNIPAM microgels is used to initiate the polymerization reaction. In addition, its decomposed fragments linked on the microgel surfaces by chemical bond can provide colloidal stability for the microgel particles dispersed in aqueous medium, and endow the microgels with some purposes, such as temperature-sensitive colloidal crystals, separation of bioactive substances, carrier of the reagent for medical test, etc.. Ammonium persulfate (APS), 2,2'-azobis(amidinopropane) dihydrochloride (V50) and4,4'-azobis(4-cyanovaleric acid) (ACVA) were respectively utilized to synthesize temperature-sensitive PNIPAM microgels by surfactant-free emulsion polymerization. The effect of the pH reaction medium on the particle sizes, the swelling ratios and the zeta potentials of the PNIPAM microgels, and the pH-dependent colloidal stability of the PNIPAMmicrogels with different ionizable groups on their surfaces, were investigated in this paper. The obtained results are as follows:1. The particle sizes and swelling ratios for the PNIPAM microgels synthesized using V50 or ACVA as initiator in the surfactant-free emulsion polymerization were more remarkably influenced by the pH value of the reaction medium than those for the PNIPAM microgels based on APS as initiator. This is due to the fact that APS is an initiator of strong acid salt, whereas V50 or ACVA is respectively the initiator of weakly basic or weakly acidic azo compound, which ionization degree is dependent on the pH value of the reaction medium. The lower the ionization degree of the initiator, the smaller the swelling ratios of the PNIPAM microgels, which meant that the crosslinking density inside the microgels increased with the decrease of the ionization degree of the initiator. These phenomena may be because the unionized initiators entered into the PNIPAM microgels and initiated the crosslinking reaction between the PNIPAM chains, leading to higher crosslinking density of the microgels.2. The charge property of the decomposed fragments of the initiator decided the zeta potential of the PNIPAM microgel surface was negative or positive. Using weakly acidic or basic azo compound as initiator, not only the particle sizes but also the zeta potentials were influenced by the pH value of the reaction medium. The lower the ionization degree of the initiator, the smaller the zeta potentials of the PNIPAM microgels. This further confirmed that the unionized initiators entered into the PNIPAM microgels and initiated the crosslinking reaction between the PNIPAM chains.3. The sulfate groups, amidine groups and carboxylic acid groups were linked to the surface of the PNIPAM microgels based on APS, V50 and ACVA as initiator, respectively. The effect of the pH value of thedispersion medium on the colloidal stability of three kinds of the PNIPAM microgels was different, and the colloidal stability of the microgels with amidine groups or carboxylic acid groups on their surfaces was dependent on the pH value. In the pH range from 4 to 10, no dependence of the colloidal stability of the PNIPAM microgels with sulfate groups on their surface on the pH value of the dispersion medium was observed, and the colloidal stability of the microgels with carboxylic acid groups was lowered at pH 4, whereas the colloidal stability of the microgels with amidine groups was decreased at pH 10. The results were explained by the relationship between the zeta potentials of the PNIPAM microgels and the pH value of the dispersion medium.4. The hydrophilic-to-hydrophobic transition of PNIPAM microgels was studied using contact angle measuring method. It was found that the contact angle values of PNIPAM microgel film were changed from 73.1° at 31 °C to 83.2° at 33 °C , indicating that hydrophilic-to-hydrophobic transition of the microgels occurred around 32°C. On the other hand, surface free energy of PNIPAM microgel film decreased from 38.5mN/m at 3 1°C to 32.08mN/m at 33 °C.Baolaiyan(Material Science) Supervised by Professor Zhaliusheng...