快速响应的温敏性PNIPAAm/粘土纳米复合水凝胶的制备与性能研究
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摘要
聚(N-异丙基丙烯酰胺)(PNIPAAm)凝胶是一种非离子型温度敏感性水凝胶，其体积相转变温度(VPTT)在32℃左右。在外界温度较低于其VPTT时，PNIPAAm水凝胶具有良好的亲水性；当温度上升至其VPTT以上时，PNIPAAm水凝胶体积收缩。基于这种特性，它在药物控制释放、酶的固定化及循环吸附材料等方面有着诱人的应用的前景。但是传统的PNIPAAm水凝胶存在机械性能差、易碎、响应速率慢及透明性差等缺点，大大限制了其应用。
本文通过添加制孔剂和冷冻干燥的方法制备了具有较快响应速率和良好力学性能的温敏性PNIPAAm／粘土纳米复合水凝胶(NC gels)，对这类凝胶的结构和形态、溶胀和消溶胀行为、力学性能等进行了系统的研究。主要研究结果如下：
1．通过制备快速响应PNIPAAm／Clay纳米复合水凝胶的探测性试验发现，相对于碳酸钠和碳酸钙，KPS与粘土片层具有更强的离子相互作用，从而能形成稳定的三维网络结构，而聚乙二醇通过与粘土片层表面的吸附作用把粘土包附起来，使粘土片层无法起到交联剂的作用，从而无法形成三维网络结构。
2．以CaCO_3为制孔剂制备得到具有快速响应速率的PNIPAAm／Clay纳米复合水凝胶(NCx／y(z)gels)。研究CaCO_3的粒径大小和不同含量对凝胶响应速率和力学性能的影响。实验结果表明，与没有加CaCO_3的PNIPAAm／粘土纳米复合水凝胶相比NCx／y(z)水凝胶的消溶胀速率得到了明显的提高，这是由于凝胶中孔洞结构的形成。凝胶中的孔洞结构由CaCO_3颗粒的大小和含量来决定的，3000目的CaCO_3比800目的CaCO_3制得的NCx／y(z)水凝胶孔洞更小，密度更高，消溶胀速度更快；并且随着CaCO_3含量的增加消溶胀速率也相应提高。由于凝胶中形成孔洞结构，NCx／y(z)水凝胶的机械性能随着CaCO_3含量的增加有所下降；但与传统的化学交联的PNIPAAm水凝胶相比，它的强度和韧性还是有明显的提高。
3．以Na_2CO_3为制孔剂制备得到具有快速响应速率的PNIPAAm／Clay纳米复合水凝胶(NCx／y gels)。研究了不同粘土含量和不同Na_2CO_3含量的NCx／y水凝胶的响应速率。研究结果表明，相对于NC水凝胶，NCx／y水凝胶具有快速的响应速率，但是粘土含量和Na_2CO_3含量的变化对消溶胀速率的影响不大。
4．通过冷冻干燥对NC水凝胶进行处理得到具有快速响应速率的PNIPAAm／粘土纳米复合水凝胶(FD gels)。研究了冷冻干燥过程中的水含量(Q)对于孔洞尺寸、水从凝胶中渗出的速度以及力学性能的影响。研究结果表明，在冷冻干燥过程中水含量是决定孔洞尺寸的一个很重要的因素，水含量(Q)越多，孔洞尺寸越大，水从凝胶中渗出的速度越快。FD水凝胶中的孔洞结构也导致了其抗张应力和断裂伸长的降低。
Poly(N-isopropylacrylamide)(abbreviated as PNIPAAm) has been one of the intensively studied thermosensitive polymers and exhibits a volume phase transition temperature (VPTT) around 32℃. Below this temperature, the PNIPAAm polymer is well soluble in aqueous media but precipitates from the solution as the temperature is increased above the VPTT. The properties of PNIPAAm hydrogel have been of great interest for a wide variety of applications, for instance, devices for controlled drug release or immobilization of enzymes and recyclable absorbents. Conventional PNIPAAm hydrogels, prepared using organic cross-linkers, had some serious disadvantages such as the lack of mechanical toughness, low swelling/deswelling rates and transparency which limited their applications greatly.
In this paper, thermosensitive PNIPAAm/Clay nanocomposite hydrogels with fast responsive rate and improved mechanical properties were synthesized by using different pore-foaming agents and freeze-drying. The structure and morphology, the swelling and deswelling behaviors and the mechanical properties of these novel hydrogels were investigated in detail. The main results obtained are as follows:
1. The exploring experiments show that the interaction between KPS and clay platelets is much stronger than those between Na_2CO_3. or CaCO_3 with clay platelets, sothe clay still acts as an-effective multi-functional cross-linker and the stable PNIPAAm/Clay nanocomposite hydrogels could be formed. Poly(ethylene glycol), (PEG) chains can absorbs onto clay platelets and form a polymer layer around the particles, which prevented the clay to be a cross-linker, and the hydrogels couldn't to be prepared.
2. Fast responsive thermosensitive PNIPAAm/Clay nanocomposite hydrogels (NCx/y(z) hydrogels) were synthesized by using CaCO_3 particles as a pore-foaming agent. The effects of meshes and content of CaCO_3 particles on responsive rates and tensile mechanical properties of these hydrogels were investigated. It was found that compared with the PNIPAAm/Clay hydrogels without introduction of CaCO_3, the deswelling rate of NCx/y(z) hydrogels in response to temperature was improved significantly, owing to the formation of a porous structure within these hydrogels. And the porous structure was determined by the meshes and content of CaCO_3 particles. The smaller CaCO_3 particles can get smaller and denser pores which can lead to faster deswelling rates of the hydrogel. With increasing of the CaCO_3 content, the deswelling rate of hydrogels was improved more significantly. The mechanical properties of NCx/y(z) hydrogels were weakened with the increasing of the CaCO_3 content, which is due to the formation of a porous structure within these hydrogels. However the tensile strength and toughness of NCx/y(z) hydrogels were improved significantly compared with the conventional PNIPAAm hydrogels crosslinked by chemical crosslinker.
3. Fast responsive thermosensitive PNIPAAm/Clay nanocomposite hydrogels (NCx/y hydrogels) were synthesized by using Na_2CO_3 as a pore-foaming agent. The effects of Na_2CO_3 content and clay content on responsive rates of hydrogels were syudited. It was found that compared with the PNIPAAm/Clay hydrogels the deswelling rate of NCx/y hydrogels in response to temperature was improved significantly compared with the PNIPAAm/Clay hydrogels, and the variety of contents of clay or Na_2CO_3 has tiny influence on the deswelling rates of NCx/y hydrogels.
4. PNIPAAm/Clay hydrogels treated by freeze-drying (FD hydrogels) exhibit fast deswelling rates. The size of pores, responsive rates and tensile mechanical properties of FD hydrogels were investigated, especially the effect of water content in PNIPAAm/Clay hydrogels during the freeze-drying process. It was found that water content in the process of freezing gel was an important factor to control the pore size, and the deswelling rate was accelerated with increasing water content on the FD treatment. The formation of a porous structure within FD hydrogels also leaded to the decrease of tensile strength and elongation at break.
本文通过添加制孔剂和冷冻干燥的方法制备了具有较快响应速率和良好力学性能的温敏性PNIPAAm／粘土纳米复合水凝胶(NC gels)，对这类凝胶的结构和形态、溶胀和消溶胀行为、力学性能等进行了系统的研究。主要研究结果如下：
1．通过制备快速响应PNIPAAm／Clay纳米复合水凝胶的探测性试验发现，相对于碳酸钠和碳酸钙，KPS与粘土片层具有更强的离子相互作用，从而能形成稳定的三维网络结构，而聚乙二醇通过与粘土片层表面的吸附作用把粘土包附起来，使粘土片层无法起到交联剂的作用，从而无法形成三维网络结构。
2．以CaCO_3为制孔剂制备得到具有快速响应速率的PNIPAAm／Clay纳米复合水凝胶(NCx／y(z)gels)。研究CaCO_3的粒径大小和不同含量对凝胶响应速率和力学性能的影响。实验结果表明，与没有加CaCO_3的PNIPAAm／粘土纳米复合水凝胶相比NCx／y(z)水凝胶的消溶胀速率得到了明显的提高，这是由于凝胶中孔洞结构的形成。凝胶中的孔洞结构由CaCO_3颗粒的大小和含量来决定的，3000目的CaCO_3比800目的CaCO_3制得的NCx／y(z)水凝胶孔洞更小，密度更高，消溶胀速度更快；并且随着CaCO_3含量的增加消溶胀速率也相应提高。由于凝胶中形成孔洞结构，NCx／y(z)水凝胶的机械性能随着CaCO_3含量的增加有所下降；但与传统的化学交联的PNIPAAm水凝胶相比，它的强度和韧性还是有明显的提高。
3．以Na_2CO_3为制孔剂制备得到具有快速响应速率的PNIPAAm／Clay纳米复合水凝胶(NCx／y gels)。研究了不同粘土含量和不同Na_2CO_3含量的NCx／y水凝胶的响应速率。研究结果表明，相对于NC水凝胶，NCx／y水凝胶具有快速的响应速率，但是粘土含量和Na_2CO_3含量的变化对消溶胀速率的影响不大。
4．通过冷冻干燥对NC水凝胶进行处理得到具有快速响应速率的PNIPAAm／粘土纳米复合水凝胶(FD gels)。研究了冷冻干燥过程中的水含量(Q)对于孔洞尺寸、水从凝胶中渗出的速度以及力学性能的影响。研究结果表明，在冷冻干燥过程中水含量是决定孔洞尺寸的一个很重要的因素，水含量(Q)越多，孔洞尺寸越大，水从凝胶中渗出的速度越快。FD水凝胶中的孔洞结构也导致了其抗张应力和断裂伸长的降低。
Poly(N-isopropylacrylamide)(abbreviated as PNIPAAm) has been one of the intensively studied thermosensitive polymers and exhibits a volume phase transition temperature (VPTT) around 32℃. Below this temperature, the PNIPAAm polymer is well soluble in aqueous media but precipitates from the solution as the temperature is increased above the VPTT. The properties of PNIPAAm hydrogel have been of great interest for a wide variety of applications, for instance, devices for controlled drug release or immobilization of enzymes and recyclable absorbents. Conventional PNIPAAm hydrogels, prepared using organic cross-linkers, had some serious disadvantages such as the lack of mechanical toughness, low swelling/deswelling rates and transparency which limited their applications greatly.
In this paper, thermosensitive PNIPAAm/Clay nanocomposite hydrogels with fast responsive rate and improved mechanical properties were synthesized by using different pore-foaming agents and freeze-drying. The structure and morphology, the swelling and deswelling behaviors and the mechanical properties of these novel hydrogels were investigated in detail. The main results obtained are as follows:
1. The exploring experiments show that the interaction between KPS and clay platelets is much stronger than those between Na_2CO_3. or CaCO_3 with clay platelets, sothe clay still acts as an-effective multi-functional cross-linker and the stable PNIPAAm/Clay nanocomposite hydrogels could be formed. Poly(ethylene glycol), (PEG) chains can absorbs onto clay platelets and form a polymer layer around the particles, which prevented the clay to be a cross-linker, and the hydrogels couldn't to be prepared.
2. Fast responsive thermosensitive PNIPAAm/Clay nanocomposite hydrogels (NCx/y(z) hydrogels) were synthesized by using CaCO_3 particles as a pore-foaming agent. The effects of meshes and content of CaCO_3 particles on responsive rates and tensile mechanical properties of these hydrogels were investigated. It was found that compared with the PNIPAAm/Clay hydrogels without introduction of CaCO_3, the deswelling rate of NCx/y(z) hydrogels in response to temperature was improved significantly, owing to the formation of a porous structure within these hydrogels. And the porous structure was determined by the meshes and content of CaCO_3 particles. The smaller CaCO_3 particles can get smaller and denser pores which can lead to faster deswelling rates of the hydrogel. With increasing of the CaCO_3 content, the deswelling rate of hydrogels was improved more significantly. The mechanical properties of NCx/y(z) hydrogels were weakened with the increasing of the CaCO_3 content, which is due to the formation of a porous structure within these hydrogels. However the tensile strength and toughness of NCx/y(z) hydrogels were improved significantly compared with the conventional PNIPAAm hydrogels crosslinked by chemical crosslinker.
3. Fast responsive thermosensitive PNIPAAm/Clay nanocomposite hydrogels (NCx/y hydrogels) were synthesized by using Na_2CO_3 as a pore-foaming agent. The effects of Na_2CO_3 content and clay content on responsive rates of hydrogels were syudited. It was found that compared with the PNIPAAm/Clay hydrogels the deswelling rate of NCx/y hydrogels in response to temperature was improved significantly compared with the PNIPAAm/Clay hydrogels, and the variety of contents of clay or Na_2CO_3 has tiny influence on the deswelling rates of NCx/y hydrogels.
4. PNIPAAm/Clay hydrogels treated by freeze-drying (FD hydrogels) exhibit fast deswelling rates. The size of pores, responsive rates and tensile mechanical properties of FD hydrogels were investigated, especially the effect of water content in PNIPAAm/Clay hydrogels during the freeze-drying process. It was found that water content in the process of freezing gel was an important factor to control the pore size, and the deswelling rate was accelerated with increasing water content on the FD treatment. The formation of a porous structure within FD hydrogels also leaded to the decrease of tensile strength and elongation at break.
引文
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8． Haraguchi K,Taniguchi S,Takehisa T.Nanocomposite Hydrogel Consisting of Poly(N-isopropylacrylamide)and Clay.Chem.Phys,2005,6:238-241．
9． Haraguchi K,Takada T.Macromol.Characteristic Sliding Frictional Behavior on the Surface of Nanocomposite Hydrogels Consisting of Organic-Inorganic Network Structure,Macromol.Chem.Phys.,2005,206:1530-1540．
10． Haraguchi K,Huan-Jun Li.Mechanism of Forming Organic/Inorganic Network Structures during In-situ Free-Radical Polymerization in PNIPA-Clay Nanocomposite Hydrogels.Macromolecules,2005,38,3482-3490．
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8．陈兆伟，陈明清，刘晓亚，等．温敏性聚(N-异丙基丙烯酰胺)水凝胶的合成与表征．功能高分子学报，2004，17(1)：46-50．
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58．刘晓华，王晓工，刘德山．快速响应的温敏性聚(N-异丙基丙烯酰胺)水凝胶-Ⅰ．以CaCO_3为成孔剂制备方法、表征及动力学研究．高分子学报，2002．3：354-357．
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63．陈兆伟，陈明清，刘晓亚，等．温敏性聚(N-异丙基丙烯酰胺)水凝胶的合成与表征．功能高分子学报，2004，17(1)：46-50．
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1. Park T G. Temperature modulated protein release from pH/temperaturesensitive hydrogels. Biomaterials, 1999, 20:517-521.
2. Sassi A P, Shaw A J, Han S M. Partitioning of proteins and small biomolecules in temperature-and pH-sensitive hydrogels. Polymer, 1996, 37(11): 2151-2164.
3. Liu F, Zhuo R X. Synthesis of thermal phase separating reactive polymers and their application in immobilized enzymes. Polymer, 1993, 25:561-563.
4. Takigawa T, Araki H, Takahashi K, Masuda T. J. Chem. Phys, 2000, 113:7640-7646.
5. Yoshida R, Uchida K, Kaneko Y, Sakai K, Kikuchi A, Sakurai Y, Okano T. Comb-type grafted hydrogels with rapid deswelling response to temperature changes.Nature,1995,374:240-242．
6． Haraguchi K,Takehisa T.Fan S.Effects of Clay Content on the Properties of Nanocomposite Hydrogels Composed of Poly(NIsopropylacrylamide)and Clay.Macromolecules,2002,35:10162-10171．
7． Shibayama M,Suda J,Karino T,Okabe S,Takehisa T.Haraguchi K.Structure and Dynamics of Poly(N-isopropylcarylamide)-Clay Nanocomposite Gels.Macromolecules,2004,37:9606-9612．
8． Haraguchi K,Taniguchi S,Takehisa T.Nanocomposite Hydrogel Consisting of Poly(N-isopropylacrylamide)and Clay.Chem.Phys,2005,6:238-241．
9． Haraguchi K,Takada T.Macromol.Characteristic Sliding Frictional Behavior on the Surface of Nanocomposite Hydrogels Consisting of Organic-Inorganic Network Structure,Macromol.Chem.Phys.,2005,206:1530-1540．
10． Haraguchi K,Huan-Jun Li.Mechanism of Forming Organic/Inorganic Network Structures during In-situ Free-Radical Polymerization in PNIPA-Clay Nanocomposite Hydrogels.Macromolecules,2005,38,3482-3490．
11． Elena Loizou,Paul Butler,Large Scale Structures in Nanocomposite Hydrogels.Macromolecules,2005,38,2047-2049．
12． Dudley W.Thompson,James T.Butterworth.The Nature of Laponite and Its Aqueous Dispersions.Journal of Colloid and Interface Science.Vol.151． No.1． June 1992．
1. Zhou R X, Li W. Preparation and characterization of macroporous poly (N-isopropylacrylamide) hydrogels for the controlled release of proteins. J.Polym. Sci, Part A: Polym. Chem., 2003, 41: 152-159.
2．刘晓华，王晓山，刘德山．快速响应的温敏性聚(N-异丙基丙烯酰胺)水凝胶1．以CaCO_3为成孔剂制备方法、表征及动力学研究。高分子学报，2002，3：354-357．
3. X Z Zhang, Ren-Xi Zhou. Synthesis and properties of thermosensitive poly(N-isopropylacrylamide-co-methyl methacrylate) hydrogel with rapid response. Materials Letters, 2002,52:5-9.
4. Zhang X Z, Zhou R X. Preparation of fast responsive, thermally sensitive poly(N-isopropylacrylamide) gel. Eur. Polym. J., 2000, 36:2301.
5. J Chen, K Park. Synthesis and characterization of superporous hydrogel composites. Journal of Controlled Release, 2000, 65:73-82.
6. J Chen, H Park, K Park. Synthesis of superporous hydrogels: Hydrogels with fast swelling and superabsorbent properties. Biomed. Mater. Res., 1999, 44:53-62.
7. J Chen, K Park. Synthesis of fastswelling, superporous sucrose hydrogels. Carbohydrate Polymer, 2000, 41:259-268.
8．陈兆伟，陈明清，刘晓亚，等．温敏性聚(N-异丙基丙烯酰胺)水凝胶的合成与表征．功能高分子学报，2004，17(1)：46-50．
1. Hirokawa Y, Tanaka T. Volume phase-transition in a nonionic gel [J]. J Chem Phys, 1984, 81(12): 6379-6380.
2. Haraguchi K, Takehisa T, Fan S. Effects of Clay on the Properties of Nanocomposite Hydrogels Composed of Poly(N-isopropylacrylamide) and Clay. Macromolecules, 2002,35:10162-10171.
3. Zhang XZ, Yang Y, Chung TS. Effect of mixed solvent on characteristics of poly(N-isopropylacrylamide) hydrogels [J]. Langmuir 2002,18:2538-2542.
1．马晓梅，赵喜安，唐小真．智能型水凝胶，化学通报，2004，2：117-123．
2．陈兆伟，陈明清，刘晓亚，杨成，朱雪燕．温敏性聚(N-异丙基丙烯酰胺)水凝胶的合成与表征．功能高分子学报，2004，17：46-50．
3. Norihiro K, Yasuzo S, Shihoko S. Wide-range control ofdeswelling time for. thermosensitive poly(N-isopropylacrylamide) gels treated by freeze-drying. Macromolecules, 2003,36:961-963.
4. Haraguchi K, Takehisa T. Nanocomposite hydrogels: a unique organicinorganic network structure with extraordinary mechanical and swelling/deswelling properties. Adv. Mater., 2002, 14:1120-1124.
5. K, Takehisa T, Fan S. Eftiects of Clay on the Properties of Nanocomposite Hydrogels Composed of Poly(N-isopropylacrylamide) and Clay. Macromolecules, 2002,3 Haraguchi 5:10162-10171.
6. Haraguchi K, Farnworth R, Ohbayashi A, et al. Compositional Effects of Mechanical Properties of Nanocomposite Hydrogels Composed of Poly(N,N'-dimethylacrylamide) and Clay. Macromolecules, 2003, 36: 5732-5741.
7. Zhang XZ, Yang Y, Chung TS. Effect of mixed solvent on characteristics of poly(N-isopropylacrylamide) hydrogels [J]. Langmuir 2002,18:2538-2542.
8. Kato, N.; Takahashi, F. Deswelling Kinetics of a Porous Poly(N-isopropylacrylamide/methacrylic acid) Gel Prepared by Freeze-Drying. Bull. Chem. Soc. Jpn. 1997, 70, 1289.
9. Kabra, B. G.; Gehrke, S. H.; Spontak, R. J. Macromolecules 1998, 31, 2166.