聚乙二醇基多重响应的高强度智能水凝胶的制备与性能研究
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摘要
目前,智能水凝胶主要存在两点问题：(1)涉及双重和多重响应性的智能水凝胶十分有限；(2)智能水凝胶的机械性能普遍比较差。因此,无论在理论方面还是在实验研究方面,研发同时具有多重响应性和高强度的智能水凝胶将展现出重大的科学意义和应用价值。
自2006年Lutz研究小组首次报道利用两种乙二醇衍生物单体,甲基丙烯酸2-(2-甲氧基乙氧基)乙酯[2-(2-nethoxyethoxy) ethyl methacrylate,(MEO2MA)]与甲基丙烯酸寡(乙二醇)酯[oligo (ethylene glycol) methacrylate,(OEGMA)]共聚合成了一种新型温度敏感型水凝胶(P (MEO2MA-co-OEGMA))后,其优异的温敏性、可调的LCST值、无毒以及良好的生物相容性几乎可以用来取代传统的温敏性高分子聚(N-异丙基丙烯酰胺)(PNIPAAm),更适合生物医用材料的应用。
本论文从水凝胶的功能性、高强度性以及应用性角度出发,以MEO2MA和OEGMA作为温敏性单体,丙烯酸(AAc)作为pH敏感性单体,首先采用自由基聚合的方法制备了温度/pH双重敏感的P(MEO2MA-co-OEGMA-co-AAc)(简称：PMOA)水凝胶,系统地研究了PMOA水凝胶对外界环境变化的刺激响应行为。其次,分别以凹凸棒土(AT)和磁性凹凸棒土(AT-Fe3O4)作为增强体和磁功能性增强体,通过原位自由基聚合的方法制备了同时具有多重响应性和较好机械强度的AT/PMOA和PMOA/AT-Fe3O4纳米复合水凝胶,并着重地分析了AT和AT-Fe3O4的含量对复合水凝胶力学性能,功能性以及溶胀和消溶胀性能的影响。此外,还制备了PMOA/AT-Fe3O4纳米复合微凝胶,利用傅里叶红外光谱(FTIR),.场发射扫描电子显微镜(FESEM),原子力显微镜(AFM),纳米粒度仪以及磁性能测定(VSM)等表征手段对复合微凝胶的结构和响应行为进行了表征和机理分析。主要取得了以下研究成果：
(1)PMOA水凝胶不仅具有可逆的低临界相转变温度(LCST)行为,还具有pH响应行为。通过溶胀和消溶胀测试,结果发现PMOA水凝胶的温敏性和pH响应性与三种共聚单体MEO2MA、OEGMA和AAc的比例有关。在去离子水中时,PMOA的溶胀率随着OEGMA含量的增加而增大；随着AAc含量的增加而减小。然而,在碱性缓冲溶液中时,PMOA水凝胶的溶胀率随着AAc含量的增加而增大。另外,采用动态热机械分析(DMA)对水凝胶的温敏性进行了表征,研究发现PM9O1A-1和PM8O2A-1的LCST值分别为31℃和37℃,这与称重法所得到的结果是一致的。此外,通过改变外界环境的温度或(和)pH值研究了PMOA水凝胶的消溶胀动力学,其消溶胀过程均符合第一动力学方程。当同时改变温度(从18℃到55℃)和pH值(从8到2)两种刺激因素时,PMOA水凝胶的消溶胀过程最快,响应速度最大。特别是PM8O2A-1和PM8O2A-2,在80min里,它们的失水率分别达到了90%和100%。
(2)通过对纳米复合水凝胶拉伸性能的测试和断裂形貌的分析,结果发现AT/PMOA纳米复合水凝胶的力学性能较纯PMOA样品有大幅度的提高,并且其拉伸强度,拉伸模量和有效交联密度均随AT含量的增加而增大。分析认为拉伸过程中棒状的纳米粒子在聚合物基体中的取向作用,纳米粒子与聚合物分子链协同运动时“能量耗散”效应以及PMOA基体与纳米粒子之间的氢键吸附作用是样品拉伸强度和断裂伸长率同时提高的根本原因。除此之外,根据动态力学分析有效的表征了聚合物网络与纳米粒子之间的相容性以及纳米粒子对聚合物分子链运动的限制。通过分析储能模量,利用Arrenius方程进一步计算了聚合物网络的物理交联密度(Xc)和氢键解离活化能(Ea),发现xc随AT含量的增加而增大,这与静态拉伸的结果是一致的。另外,还分析了AT含量对纳米复合水凝胶溶胀和消溶胀性能的影响。发现添加lwt%AT时,复合水凝胶的溶胀率较纯的PMOA样品低。当AT含量较高时,它们的平衡溶胀率均比纯的PMOA样品高,并随着AT含量的增加而增大。然而,纳米复合水凝胶的消溶胀行为较纯的PMOA水凝胶出现了明显的滞后现象,并且其消溶胀速率随着AT含量的增加而降低。
(3)利用化学共沉淀技术对AT表面磁功能化,成功地制备了AT-Fe3O4纳米粒子,经FTIR、X-射线衍射(XRD)、FESEM以及VSM分析可得,Fe3O4磁性纳米粒子能较好地附着在AT的表面；AT-Fe3O4纳米粒子在磁场作用下具有磁响应,是一种典型的超顺磁性材料,具有较高的比饱和磁化强度(σs)。
(4)系统地研究了磁性AT-Fe3O4的含量对复合水凝胶(mf-NC)的结构形态,响应行为以及拉伸性能的影响。结果表明,当添加少量AT-Fe3O4时,其在PMOA凝胶基体中分散性较好；而当AT-Fe3O4的含量增加到5et%时,其在网络结构中占据较多的位置,导致局部出现轻微的团聚现象。在对复合水凝胶的响应性行为研究中,首先,分别从施加静态和动态磁场的角度表征了水凝胶的磁响应行为,发现在外加静态磁场作用下,nf-NC水凝胶具有超顺磁行为,并且随AT-Fe3O4含量的增加其超顺磁性越显著；在交变磁场的作用下,已在去离子水中达到溶胀平衡的复合水凝胶可发生再溶胀现象。分析认为不停改变磁场的方向,使磁性纳米粒子在凝胶网络内部连续震动,从而导致磁性纳米粒子排列松散,分子链与分子链之间被粒子支撑开,增大了凝胶网络的空间,使更多的水分渗入到凝胶内部,继而产生了再溶胀现象。其次,分析了mf-NC水凝胶的温敏性和pH响应性,发现磁性纳米粒子的加入对水凝胶基体原有的温度/pH双重响应性不产生任何影响,mf-NC水凝胶依然具有温度/pH双重响应,并且其溶胀行为对温度和pH的依赖性是可逆的。此外,磁性凹土的加入明显改善了水凝胶基体的力学性能,其拉伸强度随AT-Fe3O4含量的增加而增大。
(5)以乙二醇甲基丙烯酸酯(EGDMA)为交联剂,采用原位自由基聚合制备了具有温度,pH和磁场三重响应性的PMOA/AT-Fe3O4纳米复合微凝胶。通过FTIR表征了磁性凹土与PMOA共聚物之间的氢键作用,对形成氢键的羰基进行分峰处理得到体系的氢键化程度(XH),并分析了氢键形成的机理。根据FESEM, AFM和VSM结果分析,认为PMOA/AT-Fe3O4微凝胶不仅具有典型的核壳式微观结构,还具有良好的超顺磁性行为。另外,利用PMOA/AT-Fe3O4微凝胶在不同pH介质和温度下的粒径变化表征了微凝胶的pH响应性、温敏性以及温度和pH响应性之间的协调作用机理。结果表明,PMOA/AT-Fe3O4微凝胶具有良好的pH响应性和温敏性。当pH<4.5时,微凝胶在分散介质中形成沉淀；而当pH>4.5时,微凝胶可以良好的分散在缓冲溶液中,并且其粒径随着pH值的增大而增大。PMOA/AT-Fe3O4微凝胶的粒径随温度的升高逐渐减小,体积相转变温度(VPTT)在36.5℃左右。此外,还研究了PMOA/AT-Fe3O4微凝胶在不同pH缓冲溶液中其粒径随温度的变化,分析认为当微凝胶在弱酸环境中时,其粒径随着温度的升高而下降,依然保持温敏性；然而,当微凝胶在强碱性缓冲溶液中时,由于凝胶网络中羧基的完全解离,其粒径随温度的升高不易发生收缩,从而使温敏性下降甚至消失。
At present, smart hydrogels mainly have two drawbacks:(1) Involving in double or multiple responses of smart hydrogels were very limited;(2) the mechanical properties of the smart hydrogels were generally poor. Therefore, it is of great scientific significance to research and develop the smart hydrogels with multiple responsive and excellent mechanical properties simultaneously from both theoretical and experimental research.
Until2006, Lutz et al., who used two kinds of PEG derivatives of2-(2-methoxyethoxy)ethyl methacrylate (MEO2MA) and oligo(ethylene glycol) methacrylate (OEGMA) have synthesized adjustable temperature-sensitive copolymer (P(MEO2MA-co-OEGMA)), ending that only the amide type polymer has temperature-sensitive of the times. Therefore, PNIPAM as an example of a thermo-sensitive polymer was challenged by the discovery that the random copolymer of P (MEO2MA-co-OEGMA) exhibits a LCST in water, which can be tuned between26℃to90℃depending on the OEGMA content. Moreover, in contrast with PNIPAM, P (MEO2MA-co-OEGMA) hydrogels are expected to be more suitable for the application of biomedical materials because of their non-toxic and non-immunogenic.
In this thesis, from the views of the functionality, mechanical properties and application of hydrogels, first, dual temperature-and pH-sensitive P (MEO2MA-co-OEGMA-co-AAc)(PMOA) hydrogels were prepared by free-radical polymerization of MEO2MA and OEGMA as thermo-sensitive monomers, and acrylic acid (AAc) as a pH-sensitive monomer. The stimuli-responsive properties of PMOA hydrogles were studied in detail by means of swelling and de-swelling experiments. Secondly, dual-and multiple-responsive organic/inorganic nanocomposite (NC) hydrogels (AT/PMOA and PMOA/AT-Fe3O4) with excellent mechanical properties were synthesized by in situ polymerization of MEO2MA, OEGMA and AAc, as the polymeric matrix (PMOA), and fibrillar attpulgite (AT) and magnetic attapulgite (AT-Fe3O4), as the reinforcer and magnetic functional enhancers, respectively. The mechanical properties, stimuli-responsive properties as well as the swelling/de-swelling behaviors were investigated in detail. Furthermore, we successfully fabricated PMOA/AT-Fe3O4nanocomposite microgel. The morphology, structure and responsive behaviors of the prepared microgels were systematically characterized by fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FESEM), atomic force microscope (AFM), nano-particle size analyzer and vibrating sample magnetometer (VSM). The main contributions are shown as following:
(1) The PMOA hydrogels exhibit reversible LCST behaviors in the deionized water with changing temperatures and excellent pH-responsive behaviors in the alkaline buffer solutions. The results of the swelling and de-swelling tests showed that the thermo-and pH-responsive behaviors of the hydrogels can be tuned by three comonomers of MEO2MA, OEGMA and AAc. The equilibrium swelling ratio of hydrogels in the deionized water increased with increasing OEGMA content and decreasing AAc content. However, the equilibrium swelling ratio of hydrogels in the alkaline buffer solutions increased with increasing AAc content. In addition, the phase transition temperature determination by tracking the change of equilibrium swelling ratios with temperature was consistent with the DMA testing results. The LCST values corresponding to PM9O1A-1and PM8O2A-1are around31℃and37℃, respectively. Moreover, the de-swelling kinetics was studied by changing temperature and/or pH, and they could be well described with a first-order kinetics equation, In comparison of three given conditions, the PMOA hydrogels exhibited faster de-swelling rates by changing the temperature and pH values simultaneously from pH8/18℃to pH2/55℃. Especially PM8O2A-1and PMgO2A-2hydrogels, the amounts of release of water almost reach90%and100%at80min, respectively.
(2) The results of the tensile tests and the fracture morphology analysis demonstrated that the incorporation of AT nanoparticles significantly enhanced the mechanical properties of AT/PMOA NC hydrogels. As the content of AT increased, the tensile strength, tensile modulus and effective cross-linked chain density increased. The fracture morphology analysis suggested the orientation effect of the nanorods during tension, the energy dissipation mechanism as well as the hydrogen bonding interactions between PMOA matrix and nanoparticles may be the main reasons for such mechanical properties improvement. In addition, the compatibility and interaction between the nanoparticles and polymer matrix were effectively characterized by dynamic mechanical analysis (DMA). The results showed that the storage modulus of AT/PMOA NC hydrogels were increased and the glass transition temperatures shifted to higher temperature compared to the pure PMOA hydrogel, which further indicated that the enhancement of mechanical property depended upon the presence and content of AT. According to the Arrenius equation, the physical crosslink density (Xc) and the activation energy for hydrogen bonding dissociation (Ea) have been evaluated from the elastic modulus-temperature relationship. The results showed that Xc increased with increasing the AT content, which was consistent with the results of static stretching. Furthermore, the content of AT has also an obviously effect on the swelling/de-swelling behaviors of the nanocomposite hydrogels. It was found that the faster swelling rates of the NC hydrogels were observed in comparison to the corresponding physically cross-linked PMOA hydrogel, except for1%AT/PMOA sample. However, the de-swelling kinetics of NC hydrogels was obviously retarded, and the de-swelling rates decreased with increasing the AT content.
(3) The magnetic nanocomposite particle was successfully prepared via co-precipitation technique in aqueous suspension of purified attapulgite. The obtained AT-Fe3O4nanoparticle was characterized by FT-IR, XRD, FESEM and VSM. The results showed that Fe3O4nanoparticles were well deposited on the surface of AT and the AT-Fe3O4nanocomposite particle exhibited better the superparamagnetic behavior.(4) The effect of the magnetic AT-Fe3O4content on the morphology, various responsive behaviors, as well as tensile properties of the multi-functional AT-Fe3O4/PMOA nanocomposite hydrogels (mf-NC hydrogels) was systematically investigated. The results showed that the addition of the small amount of AT-Fe3O4nanoparticles which were well dispersed in the hydrogel matrix. With further addition of AT-Fe3O4nanoparticles (5wt%), more space of the polymer network was occupied which lead to a slight agglomeration. The magnetic properties of the mf-NC hydrogels were characterized by applying the static and dynamic magnetic fields, respectively. The magnetic hysteresis loops indicated that the mf-NC hydrogel exhibited superparamagnetic behavior, and the saturation magnetization of that increased with increasing the AT-Fe3O4content. The mf-NC hydrogel can continue to swell in an alternating magnetic field (AMF) after equilibrium swelling in deionized water. Due to the effect of the AMF, the magnetic nanoparticles have uninterrupted vibration in the hydrogel network, and thus the arrangement of the magnetic nanoparticles become more loose. It is reasonable to suggest that an increase of the distance between the molecular chains lead to a larger space in the hydrogel networks, which allows water to enter further. This phenomenon can affect the swelling behavior of the whole system. In addition, the mf-NC hydrogel still possesses temperature-and pH-sensitive simultaneously. The temperature and pH dependent swelling of the hydrogel is reversible, although the magnetic AT-Fe3O4nanoparticles were introduced into the PMOA system. Moreover, the mechanical properties of the mf-NC hydrogels were significantly improved by addition of the magnetic AT-Fe3O4nanoparticles into the PMOA matrix, and the tensile strength of that increases with increasing of AT-Fe3O4content.
(5) The temperature-, pH-and magnetic-field-sensitive PMOA/AT-Fe3O4nanocomposite microgel was prepared by in situ free-radical polymerization with ethylene glycol dimethacrylate (EGDMA) as a chemical cross-linker. FT-IR spectrum has been used to characterize the hydrogen bonding interactions between AT-Fe3O4and PMOA copolymer. The degree of hydrogen bonding (XH) was obtained from the 'peak resolve processing' on the corresponding H-bonded carbonyl peaks. The results illustrated that the XH of PMOA/AT-Fe3O4nanocomposite microgel was higher than that of the pure PMOA microgel. Simultaneously, the mechanism of the formation of the hydrogen bond between AT-Fe3O4and PMOA matrix was analyzed. According to the results of FESEM, AFM and VSM, PMOA/AT-Fe3O4nanocomposite microgel had not only the typical core-shell microstructure, but also had good superparamagnetic behavior. In addition, the pH-and (or) temperature-responsive properties of microgels were analyzed by the particle size variation in different pH media and temperatures. The results showed that PMOA/AT-Fe3O4nanocomposite microgel has a good pH-and temperature-sensitive. When the pH value of the buffer solution was below4.5, the micrlge was easily to form precipitate in the dispersing medium. However, when the pH value was above4.5, the microgel could be well dispersed in the buffer solution, and the particle sizes increased with increasing the pH value of the dispersing medium. The particle sizes of PMOA/AT-Fe3O4microgel was gradually decrease with increasing temperature, and the volume phase transition temperature (VPTT) was around36.5℃.Simultaneously, the temperature dependence of the particle size of the PMOA/AT-Fe3Q4microgel in different pH buffer solution was also investigated. The results showed that when the microgel was dispersed in weak acidic mediums, which remained temperature-sensitive, and the particle size of the microgel was decreased with increasing the temperature; however, when the microgel was dispersed in strong alkaline buffer solution, the particle size of that was less prone to shrinkage with increasing temperature. It may be attributed that the completely dissociated carboxy groups in the polymer network, leading to the temperature-sensitive of the microgel decreased or even disappeared.
自2006年Lutz研究小组首次报道利用两种乙二醇衍生物单体,甲基丙烯酸2-(2-甲氧基乙氧基)乙酯[2-(2-nethoxyethoxy) ethyl methacrylate,(MEO2MA)]与甲基丙烯酸寡(乙二醇)酯[oligo (ethylene glycol) methacrylate,(OEGMA)]共聚合成了一种新型温度敏感型水凝胶(P (MEO2MA-co-OEGMA))后,其优异的温敏性、可调的LCST值、无毒以及良好的生物相容性几乎可以用来取代传统的温敏性高分子聚(N-异丙基丙烯酰胺)(PNIPAAm),更适合生物医用材料的应用。
本论文从水凝胶的功能性、高强度性以及应用性角度出发,以MEO2MA和OEGMA作为温敏性单体,丙烯酸(AAc)作为pH敏感性单体,首先采用自由基聚合的方法制备了温度/pH双重敏感的P(MEO2MA-co-OEGMA-co-AAc)(简称：PMOA)水凝胶,系统地研究了PMOA水凝胶对外界环境变化的刺激响应行为。其次,分别以凹凸棒土(AT)和磁性凹凸棒土(AT-Fe3O4)作为增强体和磁功能性增强体,通过原位自由基聚合的方法制备了同时具有多重响应性和较好机械强度的AT/PMOA和PMOA/AT-Fe3O4纳米复合水凝胶,并着重地分析了AT和AT-Fe3O4的含量对复合水凝胶力学性能,功能性以及溶胀和消溶胀性能的影响。此外,还制备了PMOA/AT-Fe3O4纳米复合微凝胶,利用傅里叶红外光谱(FTIR),.场发射扫描电子显微镜(FESEM),原子力显微镜(AFM),纳米粒度仪以及磁性能测定(VSM)等表征手段对复合微凝胶的结构和响应行为进行了表征和机理分析。主要取得了以下研究成果：
(1)PMOA水凝胶不仅具有可逆的低临界相转变温度(LCST)行为,还具有pH响应行为。通过溶胀和消溶胀测试,结果发现PMOA水凝胶的温敏性和pH响应性与三种共聚单体MEO2MA、OEGMA和AAc的比例有关。在去离子水中时,PMOA的溶胀率随着OEGMA含量的增加而增大；随着AAc含量的增加而减小。然而,在碱性缓冲溶液中时,PMOA水凝胶的溶胀率随着AAc含量的增加而增大。另外,采用动态热机械分析(DMA)对水凝胶的温敏性进行了表征,研究发现PM9O1A-1和PM8O2A-1的LCST值分别为31℃和37℃,这与称重法所得到的结果是一致的。此外,通过改变外界环境的温度或(和)pH值研究了PMOA水凝胶的消溶胀动力学,其消溶胀过程均符合第一动力学方程。当同时改变温度(从18℃到55℃)和pH值(从8到2)两种刺激因素时,PMOA水凝胶的消溶胀过程最快,响应速度最大。特别是PM8O2A-1和PM8O2A-2,在80min里,它们的失水率分别达到了90%和100%。
(2)通过对纳米复合水凝胶拉伸性能的测试和断裂形貌的分析,结果发现AT/PMOA纳米复合水凝胶的力学性能较纯PMOA样品有大幅度的提高,并且其拉伸强度,拉伸模量和有效交联密度均随AT含量的增加而增大。分析认为拉伸过程中棒状的纳米粒子在聚合物基体中的取向作用,纳米粒子与聚合物分子链协同运动时“能量耗散”效应以及PMOA基体与纳米粒子之间的氢键吸附作用是样品拉伸强度和断裂伸长率同时提高的根本原因。除此之外,根据动态力学分析有效的表征了聚合物网络与纳米粒子之间的相容性以及纳米粒子对聚合物分子链运动的限制。通过分析储能模量,利用Arrenius方程进一步计算了聚合物网络的物理交联密度(Xc)和氢键解离活化能(Ea),发现xc随AT含量的增加而增大,这与静态拉伸的结果是一致的。另外,还分析了AT含量对纳米复合水凝胶溶胀和消溶胀性能的影响。发现添加lwt%AT时,复合水凝胶的溶胀率较纯的PMOA样品低。当AT含量较高时,它们的平衡溶胀率均比纯的PMOA样品高,并随着AT含量的增加而增大。然而,纳米复合水凝胶的消溶胀行为较纯的PMOA水凝胶出现了明显的滞后现象,并且其消溶胀速率随着AT含量的增加而降低。
(3)利用化学共沉淀技术对AT表面磁功能化,成功地制备了AT-Fe3O4纳米粒子,经FTIR、X-射线衍射(XRD)、FESEM以及VSM分析可得,Fe3O4磁性纳米粒子能较好地附着在AT的表面；AT-Fe3O4纳米粒子在磁场作用下具有磁响应,是一种典型的超顺磁性材料,具有较高的比饱和磁化强度(σs)。
(4)系统地研究了磁性AT-Fe3O4的含量对复合水凝胶(mf-NC)的结构形态,响应行为以及拉伸性能的影响。结果表明,当添加少量AT-Fe3O4时,其在PMOA凝胶基体中分散性较好；而当AT-Fe3O4的含量增加到5et%时,其在网络结构中占据较多的位置,导致局部出现轻微的团聚现象。在对复合水凝胶的响应性行为研究中,首先,分别从施加静态和动态磁场的角度表征了水凝胶的磁响应行为,发现在外加静态磁场作用下,nf-NC水凝胶具有超顺磁行为,并且随AT-Fe3O4含量的增加其超顺磁性越显著；在交变磁场的作用下,已在去离子水中达到溶胀平衡的复合水凝胶可发生再溶胀现象。分析认为不停改变磁场的方向,使磁性纳米粒子在凝胶网络内部连续震动,从而导致磁性纳米粒子排列松散,分子链与分子链之间被粒子支撑开,增大了凝胶网络的空间,使更多的水分渗入到凝胶内部,继而产生了再溶胀现象。其次,分析了mf-NC水凝胶的温敏性和pH响应性,发现磁性纳米粒子的加入对水凝胶基体原有的温度/pH双重响应性不产生任何影响,mf-NC水凝胶依然具有温度/pH双重响应,并且其溶胀行为对温度和pH的依赖性是可逆的。此外,磁性凹土的加入明显改善了水凝胶基体的力学性能,其拉伸强度随AT-Fe3O4含量的增加而增大。
(5)以乙二醇甲基丙烯酸酯(EGDMA)为交联剂,采用原位自由基聚合制备了具有温度,pH和磁场三重响应性的PMOA/AT-Fe3O4纳米复合微凝胶。通过FTIR表征了磁性凹土与PMOA共聚物之间的氢键作用,对形成氢键的羰基进行分峰处理得到体系的氢键化程度(XH),并分析了氢键形成的机理。根据FESEM, AFM和VSM结果分析,认为PMOA/AT-Fe3O4微凝胶不仅具有典型的核壳式微观结构,还具有良好的超顺磁性行为。另外,利用PMOA/AT-Fe3O4微凝胶在不同pH介质和温度下的粒径变化表征了微凝胶的pH响应性、温敏性以及温度和pH响应性之间的协调作用机理。结果表明,PMOA/AT-Fe3O4微凝胶具有良好的pH响应性和温敏性。当pH<4.5时,微凝胶在分散介质中形成沉淀；而当pH>4.5时,微凝胶可以良好的分散在缓冲溶液中,并且其粒径随着pH值的增大而增大。PMOA/AT-Fe3O4微凝胶的粒径随温度的升高逐渐减小,体积相转变温度(VPTT)在36.5℃左右。此外,还研究了PMOA/AT-Fe3O4微凝胶在不同pH缓冲溶液中其粒径随温度的变化,分析认为当微凝胶在弱酸环境中时,其粒径随着温度的升高而下降,依然保持温敏性；然而,当微凝胶在强碱性缓冲溶液中时,由于凝胶网络中羧基的完全解离,其粒径随温度的升高不易发生收缩,从而使温敏性下降甚至消失。
At present, smart hydrogels mainly have two drawbacks:(1) Involving in double or multiple responses of smart hydrogels were very limited;(2) the mechanical properties of the smart hydrogels were generally poor. Therefore, it is of great scientific significance to research and develop the smart hydrogels with multiple responsive and excellent mechanical properties simultaneously from both theoretical and experimental research.
Until2006, Lutz et al., who used two kinds of PEG derivatives of2-(2-methoxyethoxy)ethyl methacrylate (MEO2MA) and oligo(ethylene glycol) methacrylate (OEGMA) have synthesized adjustable temperature-sensitive copolymer (P(MEO2MA-co-OEGMA)), ending that only the amide type polymer has temperature-sensitive of the times. Therefore, PNIPAM as an example of a thermo-sensitive polymer was challenged by the discovery that the random copolymer of P (MEO2MA-co-OEGMA) exhibits a LCST in water, which can be tuned between26℃to90℃depending on the OEGMA content. Moreover, in contrast with PNIPAM, P (MEO2MA-co-OEGMA) hydrogels are expected to be more suitable for the application of biomedical materials because of their non-toxic and non-immunogenic.
In this thesis, from the views of the functionality, mechanical properties and application of hydrogels, first, dual temperature-and pH-sensitive P (MEO2MA-co-OEGMA-co-AAc)(PMOA) hydrogels were prepared by free-radical polymerization of MEO2MA and OEGMA as thermo-sensitive monomers, and acrylic acid (AAc) as a pH-sensitive monomer. The stimuli-responsive properties of PMOA hydrogles were studied in detail by means of swelling and de-swelling experiments. Secondly, dual-and multiple-responsive organic/inorganic nanocomposite (NC) hydrogels (AT/PMOA and PMOA/AT-Fe3O4) with excellent mechanical properties were synthesized by in situ polymerization of MEO2MA, OEGMA and AAc, as the polymeric matrix (PMOA), and fibrillar attpulgite (AT) and magnetic attapulgite (AT-Fe3O4), as the reinforcer and magnetic functional enhancers, respectively. The mechanical properties, stimuli-responsive properties as well as the swelling/de-swelling behaviors were investigated in detail. Furthermore, we successfully fabricated PMOA/AT-Fe3O4nanocomposite microgel. The morphology, structure and responsive behaviors of the prepared microgels were systematically characterized by fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FESEM), atomic force microscope (AFM), nano-particle size analyzer and vibrating sample magnetometer (VSM). The main contributions are shown as following:
(1) The PMOA hydrogels exhibit reversible LCST behaviors in the deionized water with changing temperatures and excellent pH-responsive behaviors in the alkaline buffer solutions. The results of the swelling and de-swelling tests showed that the thermo-and pH-responsive behaviors of the hydrogels can be tuned by three comonomers of MEO2MA, OEGMA and AAc. The equilibrium swelling ratio of hydrogels in the deionized water increased with increasing OEGMA content and decreasing AAc content. However, the equilibrium swelling ratio of hydrogels in the alkaline buffer solutions increased with increasing AAc content. In addition, the phase transition temperature determination by tracking the change of equilibrium swelling ratios with temperature was consistent with the DMA testing results. The LCST values corresponding to PM9O1A-1and PM8O2A-1are around31℃and37℃, respectively. Moreover, the de-swelling kinetics was studied by changing temperature and/or pH, and they could be well described with a first-order kinetics equation, In comparison of three given conditions, the PMOA hydrogels exhibited faster de-swelling rates by changing the temperature and pH values simultaneously from pH8/18℃to pH2/55℃. Especially PM8O2A-1and PMgO2A-2hydrogels, the amounts of release of water almost reach90%and100%at80min, respectively.
(2) The results of the tensile tests and the fracture morphology analysis demonstrated that the incorporation of AT nanoparticles significantly enhanced the mechanical properties of AT/PMOA NC hydrogels. As the content of AT increased, the tensile strength, tensile modulus and effective cross-linked chain density increased. The fracture morphology analysis suggested the orientation effect of the nanorods during tension, the energy dissipation mechanism as well as the hydrogen bonding interactions between PMOA matrix and nanoparticles may be the main reasons for such mechanical properties improvement. In addition, the compatibility and interaction between the nanoparticles and polymer matrix were effectively characterized by dynamic mechanical analysis (DMA). The results showed that the storage modulus of AT/PMOA NC hydrogels were increased and the glass transition temperatures shifted to higher temperature compared to the pure PMOA hydrogel, which further indicated that the enhancement of mechanical property depended upon the presence and content of AT. According to the Arrenius equation, the physical crosslink density (Xc) and the activation energy for hydrogen bonding dissociation (Ea) have been evaluated from the elastic modulus-temperature relationship. The results showed that Xc increased with increasing the AT content, which was consistent with the results of static stretching. Furthermore, the content of AT has also an obviously effect on the swelling/de-swelling behaviors of the nanocomposite hydrogels. It was found that the faster swelling rates of the NC hydrogels were observed in comparison to the corresponding physically cross-linked PMOA hydrogel, except for1%AT/PMOA sample. However, the de-swelling kinetics of NC hydrogels was obviously retarded, and the de-swelling rates decreased with increasing the AT content.
(3) The magnetic nanocomposite particle was successfully prepared via co-precipitation technique in aqueous suspension of purified attapulgite. The obtained AT-Fe3O4nanoparticle was characterized by FT-IR, XRD, FESEM and VSM. The results showed that Fe3O4nanoparticles were well deposited on the surface of AT and the AT-Fe3O4nanocomposite particle exhibited better the superparamagnetic behavior.(4) The effect of the magnetic AT-Fe3O4content on the morphology, various responsive behaviors, as well as tensile properties of the multi-functional AT-Fe3O4/PMOA nanocomposite hydrogels (mf-NC hydrogels) was systematically investigated. The results showed that the addition of the small amount of AT-Fe3O4nanoparticles which were well dispersed in the hydrogel matrix. With further addition of AT-Fe3O4nanoparticles (5wt%), more space of the polymer network was occupied which lead to a slight agglomeration. The magnetic properties of the mf-NC hydrogels were characterized by applying the static and dynamic magnetic fields, respectively. The magnetic hysteresis loops indicated that the mf-NC hydrogel exhibited superparamagnetic behavior, and the saturation magnetization of that increased with increasing the AT-Fe3O4content. The mf-NC hydrogel can continue to swell in an alternating magnetic field (AMF) after equilibrium swelling in deionized water. Due to the effect of the AMF, the magnetic nanoparticles have uninterrupted vibration in the hydrogel network, and thus the arrangement of the magnetic nanoparticles become more loose. It is reasonable to suggest that an increase of the distance between the molecular chains lead to a larger space in the hydrogel networks, which allows water to enter further. This phenomenon can affect the swelling behavior of the whole system. In addition, the mf-NC hydrogel still possesses temperature-and pH-sensitive simultaneously. The temperature and pH dependent swelling of the hydrogel is reversible, although the magnetic AT-Fe3O4nanoparticles were introduced into the PMOA system. Moreover, the mechanical properties of the mf-NC hydrogels were significantly improved by addition of the magnetic AT-Fe3O4nanoparticles into the PMOA matrix, and the tensile strength of that increases with increasing of AT-Fe3O4content.
(5) The temperature-, pH-and magnetic-field-sensitive PMOA/AT-Fe3O4nanocomposite microgel was prepared by in situ free-radical polymerization with ethylene glycol dimethacrylate (EGDMA) as a chemical cross-linker. FT-IR spectrum has been used to characterize the hydrogen bonding interactions between AT-Fe3O4and PMOA copolymer. The degree of hydrogen bonding (XH) was obtained from the 'peak resolve processing' on the corresponding H-bonded carbonyl peaks. The results illustrated that the XH of PMOA/AT-Fe3O4nanocomposite microgel was higher than that of the pure PMOA microgel. Simultaneously, the mechanism of the formation of the hydrogen bond between AT-Fe3O4and PMOA matrix was analyzed. According to the results of FESEM, AFM and VSM, PMOA/AT-Fe3O4nanocomposite microgel had not only the typical core-shell microstructure, but also had good superparamagnetic behavior. In addition, the pH-and (or) temperature-responsive properties of microgels were analyzed by the particle size variation in different pH media and temperatures. The results showed that PMOA/AT-Fe3O4nanocomposite microgel has a good pH-and temperature-sensitive. When the pH value of the buffer solution was below4.5, the micrlge was easily to form precipitate in the dispersing medium. However, when the pH value was above4.5, the microgel could be well dispersed in the buffer solution, and the particle sizes increased with increasing the pH value of the dispersing medium. The particle sizes of PMOA/AT-Fe3O4microgel was gradually decrease with increasing temperature, and the volume phase transition temperature (VPTT) was around36.5℃.Simultaneously, the temperature dependence of the particle size of the PMOA/AT-Fe3Q4microgel in different pH buffer solution was also investigated. The results showed that when the microgel was dispersed in weak acidic mediums, which remained temperature-sensitive, and the particle size of the microgel was decreased with increasing the temperature; however, when the microgel was dispersed in strong alkaline buffer solution, the particle size of that was less prone to shrinkage with increasing temperature. It may be attributed that the completely dissociated carboxy groups in the polymer network, leading to the temperature-sensitive of the microgel decreased or even disappeared.
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