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標題: Swelling and Elastic Modulus of Acrylic Acid/2-Hydroxyethyl Methacrylate Hydrogels
AAc/HEMA 水膠之平衡膨潤與彈性模數性質
作者: 林明宏
關鍵字: hydrogel;水膠;swelling ratio;elastic modulus;膨潤性質;彈性模數
出版社: 化學工程學系
In this study, pH-sensitive AAc/HEMA hydrogels were prepared by radical copolymerization of acrylic acid (AAc) with 2-hydroxyethyl methacrylate (HEMA), using N,N'-methylene bis acrylamide (MBAAm ) as crosslinking agent and ammonium peroxydisulfate (APS) as initiator at 60 ℃. Based on the Donnan equilibrium theory, the total ion concentration difference inside and outside the hydrogel was calculated. The dissociation degree of AAc in hydrogels and the chemical potential that resulted in ion accumulation in hydrogel at high pH were evaluated. It was clearly observed that the effects of the ionic strength on swelling change with changing pH. The swelling behavior of hydrogels was mainly governed by the ionic osmotic pressure.
In addition, the elastic modulus of swollen hydrogels was measured by mechanical uniaxial compression. The non-Gaussian behavior of effective chains was observed due to high swelling of polymer networks at high pH. For the non-Gaussian behavior, the treatment employed herein was based on the inverse Langevin function and non-Gaussian entropy function and the results interpret well the change in the elastic modulus of elastically effective chains at high extents of swelling. The effects of the swelling degree of hydrogels on the flexible units of elastically effective chains were determined.

本實驗所規劃之水膠是以2-hydroxyethyl methacrylate (HEMA) 和acrylic acid (AAc) 為單體,在水相中以ammonium peroxydisulfate (APS) 為熱起始劑和N,N’-methylene bis acrylamide (MBAAm) 為交聯劑來進行自由基共聚合反應,形成具有酸鹼應答且親水性佳的水膠。並測量不同AAc含量之水膠於不同pH值下,其Donnan ratio、解離程度、水膠內外離子的濃度差、swelling ratio及離子造成之化學位能差的變化。研究中我們發現引起水膠膨潤之離子化學位能差會隨著pH值的增加而增加,且AAc含量越多,其值越大。另外也發現,隨著AAc含量的增加,其解離程度會降低。
此外還測量不同AAc含量之水膠,在不同V2,s (於swelling state的高分子體積分率) 下其達到平衡膨潤時彈性模數“elastic modulus”之變化。且發現水膠的G值會隨著V2,s的下降而呈現先驟降而緩降再緩升的三階段趨勢。第一階段的驟降現象,我們推測為氫鍵所造成的物理交聯;第二階段的緩降現象為標準的高斯分佈之行為。在第三階段的部分,為高分子鏈被拉伸至極限而造成彈性鏈段 (n)逐漸減少的影響下,造成高分子鏈較堅硬 (stiffness),使得彈性模數增加。因此水膠彈性力的變化已不屬於高斯分佈之行為。
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