氧化鐵/聚異丙基丙烯醯胺核殼型奈米複合粒子之合成與性質研究

Abstract

本研究主要利用表面起始原子轉移自由基聚合法，在氧化鐵奈米粒子表面以'grafting from'的方式成長高分子－聚異丙基丙烯醯胺，形成核殼型之氧化鐵奈米複合粒子，其不僅保有氧化鐵原有之陶鐵磁性，表面所包覆之高分子－聚異丙基丙烯醯胺更進一步增加其親水性與生物相容性。 在合成上，我們利用熱裂解法合氧化鐵奈米顆粒，並藉由掌握錯合物的結構、熱裂解反應之反應溫度與反應系統中的油酸濃度，有效地控制氧化鐵奈米粒子之結構形態與粒徑分佈。而經由控制油酸在反應系統中之濃度，我們可以合成出不同粒徑大小之氧化鐵奈米顆粒，當油酸濃度愈高，氧化鐵奈米顆粒之粒徑愈大。接著為了進行氧化鐵之表面改質，我們利用配位基交換的方式，將2-bromo-2-methylpropionic acid (BrMPA)與citric acid sodium salt (CA-Na)吸附在氧化鐵之表面，並藉由UV-vis測得之檢量線做定量分析，得到BrMPA與CA-Na分別以1:1的比例吸附在氧化鐵之表面。 改質氧化鐵表面所使用之CA-Na可幫助氧化鐵良好分散於水中且不會破壞其表面結構，而BrMPA結構上之溴基則可起始ATRP聚合反應，與CuBr、tris[2-(dimethylamino)ethyl]amine (Me6TREN)所形成之催化劑進行氧化鐵表面之NIPAM單體的聚合成長。經由控制單體濃度在[M]/[I]＝1000、2000、3000之比例下，可聚合出分子量分別為55700 (PDI=1.43)、85900 (PDI=1.45)及108000 (PDI=1.41)之氧化鐵/聚異丙基丙烯醯胺奈米複合粒子。 我們利用穿透式電子顯微鏡來觀察氧化鐵於聚合前後之形態，發現當高分子由氧化鐵表面聚合成長後，會在氧化鐵表面形成一有機殼層，當高分子之分子量增加，殼層厚度也會隨之增加。接著我們利用動態光散射分析儀與紫外光可見光光譜儀分析氧化鐵/聚異丙基丙烯醯胺奈米複合粒子之性質，發現當溫度產生變化時，氧化鐵外圍所包覆之高分子會產生相變化。在超過低臨界溶液溫度時，氧化鐵/聚異丙基丙烯醯胺奈米複合粒子之粒徑會縮小，同時在水溶液中會發生兩相分離的現象，造成其穿透度下降。而隨著氧化鐵/聚異丙基丙烯醯胺奈米複合粒子表面之高分子分子量增加，其低臨界溶液溫度會由32 ℃略微上升至34 ℃。最後我們利用超導量子干涉磁量儀探討氧化鐵奈米粒子於聚合前後，其磁場與磁化強度之關係變化，發現在扣除PNIPAM之重量後，氧化鐵奈米粒子本身之飽和磁化量維持仍維持一定，並且在室溫下仍呈現陶鐵磁性，顯示出氧化鐵之磁性質不會因為表面成長高分子而有所影響。

Magnetic core/shell nanoparticles with well-defined thickness of poly(N-isopropylacrylamide) (PNIPAM) were synthesized by 'grafting from' route using surface-initiated atom transfer radical polymerization technique. The resulting core-shell magnetite poly(N-isopropylacrylamide) nanocomposites not only retain the magnetic properties of magnetite but also improve their hydrophile and biocompatibility by the polymer shell. Stable dispersions of monodispersed magnetite nanocrystal were prepared by the pyrolysis of iron carboxylate salts in the presence of oleic acid and then ligand-exchanged with a mixture of 2-bromo-2-methylpropionic acid (BrMPA)and citric acid sodium salt (CA-Na). CA-Na can help Fe3O4 to disperse in water without acid etching. The bromide groups in the surface-adsorbed BrMPA were used as initiator for the atom transfer radical polymerization (ATRP) of N-isopropylacrylamide (NIPAM) using CuBr/tris[2-(dimethylamino)ethyl]amine (Me6TREN) as catalyst. By varying the ratio of monomer to initiator concentration, Fe3O4@poly(N-isopropylacrylamide) with different molecular weights were successfully synthesized. The magnetite Fe3O4 nanoparticles and the resulting Fe3O4@PNIPAM core-shell nanocomposites were characterized by TEM. When the molecular weights of the grafted PNIPAM chains increase, the thickness of shell increases. DLS and UV-vis were then employed to study the thermal phase transitions of PNIPAM at the surface of Fe3O4 nanoparticles. The diameter of Fe3O4@PNIPAM nanocomposites decreases abruptly as the system temperature exceeded the lower critical solution temperature (LCST) of PNIPAM. Moreover, the transmittance decreases abruptly in the temperature range 26-40 ℃ due to the phase separation of PNIPAM in water. As the molecular weights of the grafted PNIPAM chains increase, the LCST value increases slightly from 32 ℃ to 34 ℃. Finally, the effect of PNIPAM thickness on the magnetic behavior of these Fe3O4@PNIPAM core-shell nanocomposites was examined by SQUID. The results indicate the presence PNIPAM had no effect on yhe magnetization of Fe3O4 core.