复杂结构微凝胶及纳米复合水凝胶的制备与功能化
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摘要
水凝胶材料与大多数生物组织一样含有大量的水,具有潜在的良好生物相容性,因而在生物医学方面具有巨大的应用价值而得到广泛的研究。本工作的主要目的是开创响应性复杂结构微凝胶和纳米复合水凝胶(nanocomposite hydrogels,NC凝胶)的制备方法。研究工作的基本思路是首先选择合适的体系制备无自交联网络可化学分解的微凝胶,然后采用多步沉淀聚合法制备多重响应性核壳分离式微凝胶以及包含纳米粒子的响应性微胶囊。再者,通过控制微凝胶制备过程中的静电相互作用获得伯胺化微凝胶并研究其与阴离子型微凝胶组装成微凝胶膜的过程与修复性能。最后,通过控制工艺条件以及选择合适的离子单体与N-异丙基丙烯酰胺(NIPAm)在Laponite分散液中原位共聚,制备具有超拉伸性的环境响应性透明NC凝胶。本工作的主要内容和结果如下:
1.利用过硫酸铵/四甲基乙二胺(APS/TEMED)氧化还原引发剂或紫外光照射2,2’-偶氮二异丁基脒二盐酸盐(V50)在37℃和45℃下引发沉淀聚合合成了聚N-异丙基丙烯酰胺(pNIPAm)微凝胶。光子相关光谱(PCS)及原子力显微镜(AFM)研究表明所得pNIPAm微凝胶的粒径大小分布比较窄,且所有微凝胶的体积相转变温度(VPTT)都在32℃左右。另外,APS/TEMED引发体系可以阻止自交联网络的形成,因而利用可分解交联剂N,N’-(1,2-二羟基乙烯基)双丙烯酰胺(DHEA)制备的微凝胶可被NaIO4完全分解。我们还利用一锅法在45℃下成功将蛋白质avidin共聚入pNIPAm微凝胶。
2.采用沉淀聚合法首先在聚(N-异丙基甲基丙烯酰胺)(pNIPMAm)微凝胶核上引入DHEA交联的pNIPMAm壳。随后以所得核壳微凝胶为种子,引入由N,N’-亚甲基双丙烯酰胺(BIS)交联的聚(N-异丙基丙烯酰胺-co-丙烯酸)(pNIPAm-AAc)壳,形成“核双壳”(CDS)微凝胶。CDS微凝胶中DHEA交联的pNIPMAm壳被NaIO4分解后,形成CDS-D微凝胶。CDS-D微凝胶在玻璃表面干燥后的AFM高度图及相图直观的显示了pNIPAm-AAc壳在失去pNIPMAm壳层的支撑后变得非常扁平。CDS和CDS-D微凝胶溶液荧光吸收强度的差异也证实了pNIPMAm壳被完全降解。微凝胶在不同pH缓冲溶液中的粒径及光散射强度随温度的变化情况进一步说明了多重响应性核壳分离式微凝胶的成功制备。
3.利用一锅沉淀聚合制备了pNIPAm/pNIPMAm核壳微凝胶。粒径和光散射强度随温度的变化情况表明所得核壳微凝胶具有双温度响应性。随后利用APS/TEMED引发剂在没有交联剂的情况下引发NIPAm聚合形成粒径窄分布的pNIPAm粒子分散液,并以所得pNIPAm粒子为种子原位引入BIS交联的pNIPMAm形成核壳微凝胶。温度降至室温后,核内pNIPAm粒子溶解成高分子链并透过pNIPMAm壳扩散至溶液。微凝胶溶液的光散射强度在35℃后逐渐增强表明凝胶内仍留有少量pNIPAm链,形成的是半中空pNIPMAm微胶囊。AFM高度图说明温度升至40℃时核内pNIPAm在溶胀态下的pNIPMAm内聚集成大量纳米粒子,形成包含有大量pNIPAm纳米粒子的pNIPMAm微胶囊。
4.以NIPMAm和N-(3-氨基丙基)甲基丙烯酰胺盐酸盐(APMH)为单体共聚沉淀聚合制备了伯胺化pNIPMAm微凝胶。利用AFM及PCS表征了微凝胶的形貌及流体力学直径。研究了反应体系中NaCl含量以及引发剂类型对所得微凝胶大小和产率的影响。结果表明,采用V50在纯水中引发聚合所得微凝胶产率很低,粒径在160 nm左右。若聚合在NaCl溶液中进行或由APS引发,微凝胶产率和粒径大小都得以提高。通过控制反应体系中NaCl含量(0 ~ 150 mM),可以生成直径在160到950 nm之间的伯胺化pNIPMAm微凝胶。微凝胶的溶胀大小或者?电位值随溶液pH、离子强度以及温度的变化情况表明所得伯胺化pNIPMAm微凝胶具有多重响应性。微凝胶与5(6)-羧基荧光素琥珀酰亚胺酯的反应表明微凝胶上的伯胺具有较强的化学反应活性,可用于微凝胶的后期改性。
5.以伯胺化pNIPMAm微凝胶和阴离子型pNIPAm-AAc微凝胶为组装单元,通过交替离心沉积逐层(LBL)组装成微凝胶膜。随着微凝胶层数的增加,AFM高度图显示玻璃基体表面微凝胶堆积密度逐渐提高;膜的荧光强度随膜中pNIPAm-AAc微凝胶层数的增加而直线增加;荧光显微镜测试也显示LBL过程中pNIPAm-AAc微凝胶被逐渐引入。随后,我们还采用AFM测量了不同微凝胶膜的厚度,结果表明微凝胶膜的厚度随着微凝胶层数的增加而增大。最后,对比研究了该微凝胶膜以及pNIPAm-AAc与聚烯丙基胺盐酸盐(PAH)组装的微凝胶膜的抗拉伸及修复能力。结果表明由pNIPAm-AAc及伯胺化pNIPMAm微凝胶组成的膜更脆,可修复能力更差。
6.首次以溶胶型锂藻土Laponite XLS为交联剂,采用甲基丙烯酸钠(SMA)与NIPAm在Laponite分散液中原位共聚,制备了透明、超拉伸性、具有温度和pH双响应的离子型pNIPAm-SMA/Laponite XLS NC凝胶。研究了离子型NC凝胶的温度和pH响应性、透光率以及力学性能。研究结果表明加入2 mol%的SMA就赋予NC凝胶于pH响应性,并能在真个pH范围内保持温敏性。所有NC凝胶的透光率都在75%以上。NC凝胶中SMA含量达到8 mol%时,拉伸强度由60 kPa降至45 kPa;断裂伸长率则随着SMA含量的升高而不断增大,含10 mol% SMA的NC凝胶的断裂伸长率高达2900%。根据小应变下等温频率扫描所得的平衡剪切模量Ge计算得出所有离子型NC凝胶的有效网链密度在0.28 mol/m~3左右。离子型NC凝胶的超拉伸性源于其较低的网链密度。
7.在吸附有聚乙二醇(PEG)的Laponite分散液中原位聚合NIPAm制备了pNIPAm/Laponite NC凝胶。Laponite分散液的(?)电位值随PEG含量的提高而降低表明PEG链可有效吸附在Laponite表面。力学性能测试结果表明凝胶的拉伸强度随PEG含量的升高而降低,断裂伸长率则升高。利用小应变下的等温频率扫描所得Ge得出NC凝胶的有效网链密度随PEG的加入而降低。表明PEG可优先吸附在Laponite表面,有效阻止pNIPAm高分子链从Laponite表面的生长接枝,从而降低Laponite对pNIPAm的交联作用。少量PEG的加入并没有明显影响NC凝胶的温度响应性,但由于交联密度的下降,含PEG的NC凝胶在室温下的平衡溶胀度更大。由此可见,通过调节PEG的加入量,可有效调节NC凝胶的力学及溶胀性能。
Hydrogels were widely studied because of their applications in biomedical areas for their high water content and potential biocompatibility. The purpose of this thesis is to develop methods to synthesis complex microgels and nanocomposite hydrogels (NC gels) with multiresponse. First, we used precipitation polymerization to prepare total degradable microgels free of self-cross-linking by selecting special monomers and initiator, and then prepared multicompartment core/shell microgels and nanostrucutred hydrogel microcapsules with multiresponse. Later, amine-laden microgels were prepared by controlling the repulsive interactions between polymers and then used to fabricate all microgel films with anionic microgels. At last, we prepared multiresponsive and transparent NC gels with ultrahigh tensibility by in-situ copolymerization of ionic monomers and N-isopropylacrylamide (NIPAm) in the Laponite suspension. The main works and results are as following:
1. Poly(N-isopropylacrylamide)(pNIPAm) microgels were synthesized by precipitation polymerization at temperatures ranging from 37 oC to 45 oC using the redox initiator system ammonium persulfate (APS)/N,N,N’,N’-tetramethylethylenediamine (TEMED), or the photoinitiator 2,2’-azobis(amidinopropane) dihydrochloride (V50). Photon correlation spectroscopy (PCS) and atomic force microscopy (AFM) studies revealed that spherical microgels with narrow size dispersities can be obtained with these methods, and that the resultant microgels have similar volume phase transition temperature around 32 oC . Additionally, the low temperature, redox initiator strategy produces microgels devoid of self-cross-linking, thereby permitting the synthesis of completely degradable microgels when using N,N’-(1,2-dihydroxyethylene)bisacrylamide (DHEA) as a cleavable cross-linker. We also demonstrate the potential utility of the approach in bioconjugate syntheses; in this case avidin immobilization is demonstrated by one-pot copolymerization at 45 oC.
2. Multiresponsive poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-AAc) microgels containing mechanically and thermodynamically decoupled poly(N-isopropylmethacrylamide) (pNIPMAm) cores have been prepared. To achieve this structure, pNIPMAm microgels were used as templates in the synthesis of a DHEA cross-linked pNIPMAm inner shell. A pNIPAm-AAc outer shell was then added, resulting in“core/double-shell”(CDS) microgels. Erosion of the inner shell via periodate-mediated cleavage of the 1,2-diol bond in DHEA produced multiresponsive core/shell microgels with independent cores. AFM height and phase images of CDS-D microgels clearly showed that the pNIPAm-AAc shell became really flat after drying on the glass without the support of pNIPMAm shell. The temperature dependence of size and light scattering intensity of microgels in different pH buffers indicated the successful preparation of core-shell separated multiresponsive core/shell microgels.
3. One-pot precipitation polymerization was used to prepare pNIPAm/pNIPMAm core/shell microgels. The temperature dependence of size and light scattering intensity showed that the core/shell microgels had double temperature response. In addition, temperature responsive pNIPMAm microgel capsules containing multiple pNIPAm nanoscopic inclusions were prepared. This structure was achieved through the addition of a BIS cross-linked pNIPMAm shell to stable, low polydispersity aggregates of pNIPAm chains that resulted from APS/TEMED initiated free-radical precipitation polymerization of NIPAm in the absence of any cross-linker. Thus, upon decreasing the temperature following synthesis, the majority of the encapsulated pNIPAm chains escaped from the thin, porous pNIPMAm shell, resulting in nearly hollow pNIPMAm microcapsules. However, we have observed that there are remnant pNIPAm segments unable to escape from the microcapsule, which form nanoparticulate inclusions upon raising the temperature to 40 oC. AFM height images clearly showed that multiple pNIPAm nanoparticles can be formed in a swollen pNIPMAm shell.
4. Surfactant-free, radical precipitation co-polymerization of NIPMAm and N-(3-aminopropyl) methacrylamide hydrochloride (APMH) was carried out to prepare microgels functionalized with primary amines. The morphology and hydrodynamic diameter of the microgels were characterized by AFM and PCS, with the effect of NaCl concentration and initiator type on the microgel size and yield being investigated. When a V50-initiated reaction was carried out in pure water, relatively small microgels (~160 nm diameter) were obtained in low yield (~20%). However, both the yield and size increased if the reaction was carried out in saline or by using APS as initiator. Stable amine-laden microgels in the range from 160 nm to 950 nm in diameter with narrow size distributions were thus produced using reaction media with controlled salinity. Microgel swelling and electrophoretic mobility values as a function of pH, ionic strength and temperature were also studied, illustrating the presence of cationic sidechains and their influence on microgel properties. Finally, the availability of the primary amine groups for post-polymerization modification was confirmed via modification with fluorescein-NHS.
5. All microgel films were fabricated by amine-laden pNIPMAm microgels and anionic pNIPAm-AAc microgels through centrifuge deposit layer-by layer (LBL) way. With the increase number of microgel layers in the film, AFM height images showed that the microgel packing density increased on the glass substrate and fluorescence microscopy images showed that the number of pNIPAm-AAc microgels increased. The fluorescence intensity of microgel films linearly increased with the number of pNIPAm-AAc microgel layer. Besides, AFM showed that the film thickness was also linearly increased with the number of microgel layer. At last, we comparatively studied the tensile and healing properties of microgel films from pNIPAm-AAc/PAH and pNIPAm-AAc/amine-laden pNIPMAm microgels. The results showed that film from pNIPAm-AAc and pNIPMAm microgels was easier to be broken and had worse healing ability.
6. Ionic NC gels cross-linked by Laponite XLS with ultrahigh tensibility were successfully synthesized for the first time via in-situ copolymerization of NIPAm and sodium methacrylate (SMA). The pH and temperature response, transparency, and mechanical properties of the ionic hydrogels were investigated. The results showed that the addition of only 2 mol% of SMA endowed the NC gels with pH response, while the temperature response remained in the whole pH range. All the as-prepared hydrogels demonstrated transparency higher than 75%. The tensile strength evidently decreased from 60 kPa to 45 kPa when the SMA content reached 8 mol%. The elongation at break increased with increasing SMA content and 2900% was achieved for the sample containing 10 mol% of SMA. The effective network chain density estimated from the equilibrium storage modulus was about 0.28 mol/m3 for all the samples. The low chain density was the intrinsic origin of the ultrahigh tensibility for these ionic NC gels.
7. pNIPAm/Laponite NC gels were synthesized via in-situ polymerization of NIPAm in the Laponite suspension containing polyethylene glycol (PEG). The adsorption of PEG on Laponite platelets was characterized by (?) potential, which decreased with the PEG adsorption. The tensile strength decreased and elongation at break increased with increasing PEG concentration. The effective network chain density of pNIPAm/Laponite NC gels determined from the equilibrium modulus Ge decreased upon adsorption of PEG on the Laponite. All of these results revealed the preferential adsorption of PEG on the Laponite platelets occupying the active sites for the pNIPAm chain anchoring, which hindered their cross-linking effect in the NC gels. However, the temperature sensitive swelling behavior still remained in the pNIPAm/Laponite NC gels containing PEG with higher swelling volume below the LCST due to the lower cross-linker density. By adjusting the amount of added PEG, we can easily control the properties of the pNIPAm/Laponite NC gels.
1.利用过硫酸铵/四甲基乙二胺(APS/TEMED)氧化还原引发剂或紫外光照射2,2’-偶氮二异丁基脒二盐酸盐(V50)在37℃和45℃下引发沉淀聚合合成了聚N-异丙基丙烯酰胺(pNIPAm)微凝胶。光子相关光谱(PCS)及原子力显微镜(AFM)研究表明所得pNIPAm微凝胶的粒径大小分布比较窄,且所有微凝胶的体积相转变温度(VPTT)都在32℃左右。另外,APS/TEMED引发体系可以阻止自交联网络的形成,因而利用可分解交联剂N,N’-(1,2-二羟基乙烯基)双丙烯酰胺(DHEA)制备的微凝胶可被NaIO4完全分解。我们还利用一锅法在45℃下成功将蛋白质avidin共聚入pNIPAm微凝胶。
2.采用沉淀聚合法首先在聚(N-异丙基甲基丙烯酰胺)(pNIPMAm)微凝胶核上引入DHEA交联的pNIPMAm壳。随后以所得核壳微凝胶为种子,引入由N,N’-亚甲基双丙烯酰胺(BIS)交联的聚(N-异丙基丙烯酰胺-co-丙烯酸)(pNIPAm-AAc)壳,形成“核双壳”(CDS)微凝胶。CDS微凝胶中DHEA交联的pNIPMAm壳被NaIO4分解后,形成CDS-D微凝胶。CDS-D微凝胶在玻璃表面干燥后的AFM高度图及相图直观的显示了pNIPAm-AAc壳在失去pNIPMAm壳层的支撑后变得非常扁平。CDS和CDS-D微凝胶溶液荧光吸收强度的差异也证实了pNIPMAm壳被完全降解。微凝胶在不同pH缓冲溶液中的粒径及光散射强度随温度的变化情况进一步说明了多重响应性核壳分离式微凝胶的成功制备。
3.利用一锅沉淀聚合制备了pNIPAm/pNIPMAm核壳微凝胶。粒径和光散射强度随温度的变化情况表明所得核壳微凝胶具有双温度响应性。随后利用APS/TEMED引发剂在没有交联剂的情况下引发NIPAm聚合形成粒径窄分布的pNIPAm粒子分散液,并以所得pNIPAm粒子为种子原位引入BIS交联的pNIPMAm形成核壳微凝胶。温度降至室温后,核内pNIPAm粒子溶解成高分子链并透过pNIPMAm壳扩散至溶液。微凝胶溶液的光散射强度在35℃后逐渐增强表明凝胶内仍留有少量pNIPAm链,形成的是半中空pNIPMAm微胶囊。AFM高度图说明温度升至40℃时核内pNIPAm在溶胀态下的pNIPMAm内聚集成大量纳米粒子,形成包含有大量pNIPAm纳米粒子的pNIPMAm微胶囊。
4.以NIPMAm和N-(3-氨基丙基)甲基丙烯酰胺盐酸盐(APMH)为单体共聚沉淀聚合制备了伯胺化pNIPMAm微凝胶。利用AFM及PCS表征了微凝胶的形貌及流体力学直径。研究了反应体系中NaCl含量以及引发剂类型对所得微凝胶大小和产率的影响。结果表明,采用V50在纯水中引发聚合所得微凝胶产率很低,粒径在160 nm左右。若聚合在NaCl溶液中进行或由APS引发,微凝胶产率和粒径大小都得以提高。通过控制反应体系中NaCl含量(0 ~ 150 mM),可以生成直径在160到950 nm之间的伯胺化pNIPMAm微凝胶。微凝胶的溶胀大小或者?电位值随溶液pH、离子强度以及温度的变化情况表明所得伯胺化pNIPMAm微凝胶具有多重响应性。微凝胶与5(6)-羧基荧光素琥珀酰亚胺酯的反应表明微凝胶上的伯胺具有较强的化学反应活性,可用于微凝胶的后期改性。
5.以伯胺化pNIPMAm微凝胶和阴离子型pNIPAm-AAc微凝胶为组装单元,通过交替离心沉积逐层(LBL)组装成微凝胶膜。随着微凝胶层数的增加,AFM高度图显示玻璃基体表面微凝胶堆积密度逐渐提高;膜的荧光强度随膜中pNIPAm-AAc微凝胶层数的增加而直线增加;荧光显微镜测试也显示LBL过程中pNIPAm-AAc微凝胶被逐渐引入。随后,我们还采用AFM测量了不同微凝胶膜的厚度,结果表明微凝胶膜的厚度随着微凝胶层数的增加而增大。最后,对比研究了该微凝胶膜以及pNIPAm-AAc与聚烯丙基胺盐酸盐(PAH)组装的微凝胶膜的抗拉伸及修复能力。结果表明由pNIPAm-AAc及伯胺化pNIPMAm微凝胶组成的膜更脆,可修复能力更差。
6.首次以溶胶型锂藻土Laponite XLS为交联剂,采用甲基丙烯酸钠(SMA)与NIPAm在Laponite分散液中原位共聚,制备了透明、超拉伸性、具有温度和pH双响应的离子型pNIPAm-SMA/Laponite XLS NC凝胶。研究了离子型NC凝胶的温度和pH响应性、透光率以及力学性能。研究结果表明加入2 mol%的SMA就赋予NC凝胶于pH响应性,并能在真个pH范围内保持温敏性。所有NC凝胶的透光率都在75%以上。NC凝胶中SMA含量达到8 mol%时,拉伸强度由60 kPa降至45 kPa;断裂伸长率则随着SMA含量的升高而不断增大,含10 mol% SMA的NC凝胶的断裂伸长率高达2900%。根据小应变下等温频率扫描所得的平衡剪切模量Ge计算得出所有离子型NC凝胶的有效网链密度在0.28 mol/m~3左右。离子型NC凝胶的超拉伸性源于其较低的网链密度。
7.在吸附有聚乙二醇(PEG)的Laponite分散液中原位聚合NIPAm制备了pNIPAm/Laponite NC凝胶。Laponite分散液的(?)电位值随PEG含量的提高而降低表明PEG链可有效吸附在Laponite表面。力学性能测试结果表明凝胶的拉伸强度随PEG含量的升高而降低,断裂伸长率则升高。利用小应变下的等温频率扫描所得Ge得出NC凝胶的有效网链密度随PEG的加入而降低。表明PEG可优先吸附在Laponite表面,有效阻止pNIPAm高分子链从Laponite表面的生长接枝,从而降低Laponite对pNIPAm的交联作用。少量PEG的加入并没有明显影响NC凝胶的温度响应性,但由于交联密度的下降,含PEG的NC凝胶在室温下的平衡溶胀度更大。由此可见,通过调节PEG的加入量,可有效调节NC凝胶的力学及溶胀性能。
Hydrogels were widely studied because of their applications in biomedical areas for their high water content and potential biocompatibility. The purpose of this thesis is to develop methods to synthesis complex microgels and nanocomposite hydrogels (NC gels) with multiresponse. First, we used precipitation polymerization to prepare total degradable microgels free of self-cross-linking by selecting special monomers and initiator, and then prepared multicompartment core/shell microgels and nanostrucutred hydrogel microcapsules with multiresponse. Later, amine-laden microgels were prepared by controlling the repulsive interactions between polymers and then used to fabricate all microgel films with anionic microgels. At last, we prepared multiresponsive and transparent NC gels with ultrahigh tensibility by in-situ copolymerization of ionic monomers and N-isopropylacrylamide (NIPAm) in the Laponite suspension. The main works and results are as following:
1. Poly(N-isopropylacrylamide)(pNIPAm) microgels were synthesized by precipitation polymerization at temperatures ranging from 37 oC to 45 oC using the redox initiator system ammonium persulfate (APS)/N,N,N’,N’-tetramethylethylenediamine (TEMED), or the photoinitiator 2,2’-azobis(amidinopropane) dihydrochloride (V50). Photon correlation spectroscopy (PCS) and atomic force microscopy (AFM) studies revealed that spherical microgels with narrow size dispersities can be obtained with these methods, and that the resultant microgels have similar volume phase transition temperature around 32 oC . Additionally, the low temperature, redox initiator strategy produces microgels devoid of self-cross-linking, thereby permitting the synthesis of completely degradable microgels when using N,N’-(1,2-dihydroxyethylene)bisacrylamide (DHEA) as a cleavable cross-linker. We also demonstrate the potential utility of the approach in bioconjugate syntheses; in this case avidin immobilization is demonstrated by one-pot copolymerization at 45 oC.
2. Multiresponsive poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-AAc) microgels containing mechanically and thermodynamically decoupled poly(N-isopropylmethacrylamide) (pNIPMAm) cores have been prepared. To achieve this structure, pNIPMAm microgels were used as templates in the synthesis of a DHEA cross-linked pNIPMAm inner shell. A pNIPAm-AAc outer shell was then added, resulting in“core/double-shell”(CDS) microgels. Erosion of the inner shell via periodate-mediated cleavage of the 1,2-diol bond in DHEA produced multiresponsive core/shell microgels with independent cores. AFM height and phase images of CDS-D microgels clearly showed that the pNIPAm-AAc shell became really flat after drying on the glass without the support of pNIPMAm shell. The temperature dependence of size and light scattering intensity of microgels in different pH buffers indicated the successful preparation of core-shell separated multiresponsive core/shell microgels.
3. One-pot precipitation polymerization was used to prepare pNIPAm/pNIPMAm core/shell microgels. The temperature dependence of size and light scattering intensity showed that the core/shell microgels had double temperature response. In addition, temperature responsive pNIPMAm microgel capsules containing multiple pNIPAm nanoscopic inclusions were prepared. This structure was achieved through the addition of a BIS cross-linked pNIPMAm shell to stable, low polydispersity aggregates of pNIPAm chains that resulted from APS/TEMED initiated free-radical precipitation polymerization of NIPAm in the absence of any cross-linker. Thus, upon decreasing the temperature following synthesis, the majority of the encapsulated pNIPAm chains escaped from the thin, porous pNIPMAm shell, resulting in nearly hollow pNIPMAm microcapsules. However, we have observed that there are remnant pNIPAm segments unable to escape from the microcapsule, which form nanoparticulate inclusions upon raising the temperature to 40 oC. AFM height images clearly showed that multiple pNIPAm nanoparticles can be formed in a swollen pNIPMAm shell.
4. Surfactant-free, radical precipitation co-polymerization of NIPMAm and N-(3-aminopropyl) methacrylamide hydrochloride (APMH) was carried out to prepare microgels functionalized with primary amines. The morphology and hydrodynamic diameter of the microgels were characterized by AFM and PCS, with the effect of NaCl concentration and initiator type on the microgel size and yield being investigated. When a V50-initiated reaction was carried out in pure water, relatively small microgels (~160 nm diameter) were obtained in low yield (~20%). However, both the yield and size increased if the reaction was carried out in saline or by using APS as initiator. Stable amine-laden microgels in the range from 160 nm to 950 nm in diameter with narrow size distributions were thus produced using reaction media with controlled salinity. Microgel swelling and electrophoretic mobility values as a function of pH, ionic strength and temperature were also studied, illustrating the presence of cationic sidechains and their influence on microgel properties. Finally, the availability of the primary amine groups for post-polymerization modification was confirmed via modification with fluorescein-NHS.
5. All microgel films were fabricated by amine-laden pNIPMAm microgels and anionic pNIPAm-AAc microgels through centrifuge deposit layer-by layer (LBL) way. With the increase number of microgel layers in the film, AFM height images showed that the microgel packing density increased on the glass substrate and fluorescence microscopy images showed that the number of pNIPAm-AAc microgels increased. The fluorescence intensity of microgel films linearly increased with the number of pNIPAm-AAc microgel layer. Besides, AFM showed that the film thickness was also linearly increased with the number of microgel layer. At last, we comparatively studied the tensile and healing properties of microgel films from pNIPAm-AAc/PAH and pNIPAm-AAc/amine-laden pNIPMAm microgels. The results showed that film from pNIPAm-AAc and pNIPMAm microgels was easier to be broken and had worse healing ability.
6. Ionic NC gels cross-linked by Laponite XLS with ultrahigh tensibility were successfully synthesized for the first time via in-situ copolymerization of NIPAm and sodium methacrylate (SMA). The pH and temperature response, transparency, and mechanical properties of the ionic hydrogels were investigated. The results showed that the addition of only 2 mol% of SMA endowed the NC gels with pH response, while the temperature response remained in the whole pH range. All the as-prepared hydrogels demonstrated transparency higher than 75%. The tensile strength evidently decreased from 60 kPa to 45 kPa when the SMA content reached 8 mol%. The elongation at break increased with increasing SMA content and 2900% was achieved for the sample containing 10 mol% of SMA. The effective network chain density estimated from the equilibrium storage modulus was about 0.28 mol/m3 for all the samples. The low chain density was the intrinsic origin of the ultrahigh tensibility for these ionic NC gels.
7. pNIPAm/Laponite NC gels were synthesized via in-situ polymerization of NIPAm in the Laponite suspension containing polyethylene glycol (PEG). The adsorption of PEG on Laponite platelets was characterized by (?) potential, which decreased with the PEG adsorption. The tensile strength decreased and elongation at break increased with increasing PEG concentration. The effective network chain density of pNIPAm/Laponite NC gels determined from the equilibrium modulus Ge decreased upon adsorption of PEG on the Laponite. All of these results revealed the preferential adsorption of PEG on the Laponite platelets occupying the active sites for the pNIPAm chain anchoring, which hindered their cross-linking effect in the NC gels. However, the temperature sensitive swelling behavior still remained in the pNIPAm/Laponite NC gels containing PEG with higher swelling volume below the LCST due to the lower cross-linker density. By adjusting the amount of added PEG, we can easily control the properties of the pNIPAm/Laponite NC gels.
引文
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