外加小分子物质对聚（N-异丙基丙烯酰胺）在水溶液中线团—球体转变的影响
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摘要
聚(N-异丙基丙烯酰胺)是一种典型的水溶性温敏聚合物,在水溶液中305K附近具有下临界共溶温度。当温度达到下临界共溶温度时,很小的温度改变(1-2K)就可使得高分子发生从线团到球体的构象变化。由于这一转变行为,对研究许多与生命现象相关的问题具有很大的启示作用,如：蛋白质的折叠,DNA链的聚集与离解等,因此一直以来是高分子学界的重要科学前沿问题之一而外加小分子物质对这一构象转变具有很大影响,因此系统研究不同外加小分子,如表面活性剂以及无机盐等对聚(N-异丙基丙烯酰胺)在水溶液中线团—球体转变影响具有重要意义。
本课题具体以实验室自制的聚(N-异丙基丙烯酰胺)为研究对象,分别采用粘度法、激光光散射法和核磁共振法,系统研究了一种阴离子表面活性剂——十二烷基硫酸钠对聚(N-异丙基丙烯酰胺)在水溶液中线团—球体转变的影响,并分别在不同温度及表面活性剂浓度条件下考察了十二烷基硫酸钠与聚(N-异丙基丙烯酰胺)的相互作用及形成的复合结构特征；此外,还初步研究了八种无机盐对聚(N-异丙基丙烯酰胺)在水溶液中线团—球体转变的影响以及作用机制。结果表明：
(1)表面活性剂十二烷基硫酸钠的引入,可有效地延缓聚(N-异丙基丙烯酰胺)在水溶液中发生相转变,使得LCST升高。二维分子相关核磁谱以及扩散核磁谱结果表明,这是由于在低于相转变温度时,大约16%的十二烷基硫酸钠通过疏水作用与聚(N-异丙基丙烯酰胺)侧链作用,结合形成具有聚电解质特征的复合物。一方面由于高分子链内的静电排斥作用使得高分子链不易收缩,相转变温度升高；另一方面,高分子链间的静电排斥作用有效地阻止了高分子链间缠结作用,一定条件下,可观察到单链高分子乳液粒子。而在相转变温度以上,由于链内的静电和空间位阻作用,与聚(N-异丙基丙烯酰胺)结合的十二烷基硫酸钠会逐步解离下来形成自由的表面活性剂分子。当温度远高于相转变温度时,高分子与表面活性剂间基本已无相互作用。其不同温度下聚(N-异丙基丙烯酰胺)与十二烷基硫酸钠作用过程描述在图1上
(2)在低于相转变温度时,十二烷基硫酸钠与聚(N-异丙基丙烯酰胺)能有效结合形成复合物。而它们间的相互作用与表面活性剂的浓度有很大关联。粘度以及核磁实验结果均表明,当表面活性剂浓度小于0.86mM(接近十二烷基硫酸钠临界聚集浓度)时,高分子与表面活性剂间几乎没有相互作用；当表面活性剂浓度在0.86mM-7.0mM时,十二烷基硫酸钠通过疏水作用与聚(N-异丙基丙烯酰胺)结合,不断聚集在聚(N-异丙基丙烯酰胺)侧链上,使得高分子流体力学体积变大；当浓度超过7.0mM时,由于链内的静电和空间位阻作用,聚集在聚(N-异丙基丙烯酰胺)上的十二烷基硫酸钠数量已达到饱和,不再增加。十烷基硫酸钠浓度分别为1.0mM与12mM时与聚(N-异丙基丙烯酰胺)形成的复合物结构如图2所示。
(3)除了表面活性剂,无机盐对聚(N-异丙基丙烯酰胺)在水溶液中构象转变也有很大影响。粘度及动态光散射实验结果表明,在常温下绝大多数无机盐均能促进高分子构象发生转变,使得相转变温度下降；且盐浓度越高,影响程度越大。此外,无机盐中阴离子基团相比于阳离子基团,起到更为决定性作用。并且不同盐对高分子相转变影响程度的大小关系符合经典的Hofmeister序列。其中对于卤素系钠盐,NaI在较低浓度时表现出与其它盐相反的作用,使得聚(N-异丙基丙烯酰胺)链在水溶液构象伸展,高分子特性粘数及平均流体力学半径增大。进一步的核磁实验结果初步证实了无机盐对PNIPAM在水溶液中构象转变影响,主要是通过阴离子基团与溶剂分子间作用而进行的。
Poly(N-isopropylacrylamide)(PNIPAM) is a well-known thermo-sensitive polymer that exhibits a low critical solution temperature (LCST) at around305K in aqueous solutions. The coil-to-globule transition of PNIPAM can be induced by a small temperature variation (1～2K) accompanied by abrupt conformational changes. The LCST behavior of PNIPAM has been attracting research interests for several decades because of its implication in a number of living phenomena, especially on protein folding and DN A packing. However, the coil-to-globule transition of PNIPAM has been greatly affected by some additives, such as surfactants and inorganic salts. Therefore, the researches on the effects of the additives on the coil-to-globule transition of PNIPAM have gained great realistic significance.
In this work, with the applications of laser light scattering (LLS), viscosmetry as well as high-resolution nuclear magnetic resonance (NMR), the effects of the anion surfactant sodium n-dodecyl sulfate (SDS) on the coil-to-globule transition of PNIPAM under various temperatures and surfactant concentrations have been systematically studied. Besides, the effects of eight inorganic salts on the coil-to-globule transition of this polymer have been also explored. Several interesting results have been drawn:
(1) SDS can increase LCST of PNIPAM solutions obviously for peculiar interactions with PNIPAM. Results from2D NOESY and pulsed-field gradient diffusion NMR show that a small proportion of SDS (around16%) binds to the PNIPAM chain and forms a polyelectrolyte-like complex via hydrophobic interactions below LCST. The polymer-bound SDS plays an important role in the retardation of the PNIPAM chain collapse and the inhibition of inter-chain aggregation. Interestingly, the polymer-bound SDS gradually dissociates from PNIPAM because of electrostatic repulsion and steric hindrance effects resulting from the coil-to-globule transition. Almost no cross-peak in the2D NOESY NMR spectra above the LCST is observed because of the reduced interactions between PNIPAM and SDS after the polymer chain transforms into a compact globule. The role of SDS during the coil-to-globule transition in the PNIPAM/SDS aqueous solution at various temperatures was illustrated in Figure1.
(2) The complex structure of and interaction between PNIPAM and SDS in aqueous solutions below the LCST were investigated further. Results from viscometry and NMR have both indicated that the interactions of PNIPAM and SDS were found to exhibit strong dependences on the SDS concentration. Two critical SDS concentrations for this system were found, i.e.,0.86and7.0mM, within the current measured SDS concentration range. The former was the onset concentration of formation of the PNIPAM and SDS aggregates. The latter was the saturation concentration at which the surfactant molecules could not bound to the polymer chain. At SDS concentrations below0.86mM, the interaction between PNIPAM and SDS was very minimal, resulting in almost no change in the intrinsic viscosity and hydrodynamic size of the polymer. At SDS concentrations below7.0mM but above0.86mM, the polymer-free SDS concentration remained unchanged at around0.86mM near the critical aggregation concentration (CAC). Hence, excess SDS molecules preferred to attach onto the PNIPAM chain until saturation and formed a polyelectrolyte-like complex. Consequently, the intrinsic viscosity and hydrodynamic size of the polymer in solution increased. The amount of polymer-bound SDS and intensities of the cross-peaks in the2D NOESY NMR spectra also increased. At SDS concentrations above7.0mM, the number of polymer-bound SDS reached saturation and no more SDS molecule bound to the polymer chain due to steric hindrance. Subsequently, the amount of polymer-free SDS began to increase. The morphological structures of PNIPAM/SDS at SDS concentrations1.0mM and12mM have shown in Figure2, respectively.
(3) Besides SDS, the additives of inorganic salts have also shown great effects on the conformational change of PNIPAM in aqueous solutions. Results from viscometry and LLS indicated that most of tested salts would accelerate the coil-to-globule transition of PNIPAM and reduce the LCST. These effects exhibited strong dependence on the salts concentrations, and the higher salts concentration, the lower of LCST. Furthermore, it was found that the anionic groups of salts played the key roles to affect the coil-to-globule transition behavior of polymer, and followed the order of classic Hofmeister sequence. However, NaI at low concentration region showed opposite effects, which could increased the hydrodynamic size of PNIPAM slightly. The NMR experiments primarily indicated that the effects of the inorganic salts on the conformational change of PNIPAM in aqueous solutions were carried on mainly through interaction between the anionic groups and solvent molecules and destroying the solvation effects of polymer.
本课题具体以实验室自制的聚(N-异丙基丙烯酰胺)为研究对象,分别采用粘度法、激光光散射法和核磁共振法,系统研究了一种阴离子表面活性剂——十二烷基硫酸钠对聚(N-异丙基丙烯酰胺)在水溶液中线团—球体转变的影响,并分别在不同温度及表面活性剂浓度条件下考察了十二烷基硫酸钠与聚(N-异丙基丙烯酰胺)的相互作用及形成的复合结构特征；此外,还初步研究了八种无机盐对聚(N-异丙基丙烯酰胺)在水溶液中线团—球体转变的影响以及作用机制。结果表明：
(1)表面活性剂十二烷基硫酸钠的引入,可有效地延缓聚(N-异丙基丙烯酰胺)在水溶液中发生相转变,使得LCST升高。二维分子相关核磁谱以及扩散核磁谱结果表明,这是由于在低于相转变温度时,大约16%的十二烷基硫酸钠通过疏水作用与聚(N-异丙基丙烯酰胺)侧链作用,结合形成具有聚电解质特征的复合物。一方面由于高分子链内的静电排斥作用使得高分子链不易收缩,相转变温度升高；另一方面,高分子链间的静电排斥作用有效地阻止了高分子链间缠结作用,一定条件下,可观察到单链高分子乳液粒子。而在相转变温度以上,由于链内的静电和空间位阻作用,与聚(N-异丙基丙烯酰胺)结合的十二烷基硫酸钠会逐步解离下来形成自由的表面活性剂分子。当温度远高于相转变温度时,高分子与表面活性剂间基本已无相互作用。其不同温度下聚(N-异丙基丙烯酰胺)与十二烷基硫酸钠作用过程描述在图1上
(2)在低于相转变温度时,十二烷基硫酸钠与聚(N-异丙基丙烯酰胺)能有效结合形成复合物。而它们间的相互作用与表面活性剂的浓度有很大关联。粘度以及核磁实验结果均表明,当表面活性剂浓度小于0.86mM(接近十二烷基硫酸钠临界聚集浓度)时,高分子与表面活性剂间几乎没有相互作用；当表面活性剂浓度在0.86mM-7.0mM时,十二烷基硫酸钠通过疏水作用与聚(N-异丙基丙烯酰胺)结合,不断聚集在聚(N-异丙基丙烯酰胺)侧链上,使得高分子流体力学体积变大；当浓度超过7.0mM时,由于链内的静电和空间位阻作用,聚集在聚(N-异丙基丙烯酰胺)上的十二烷基硫酸钠数量已达到饱和,不再增加。十烷基硫酸钠浓度分别为1.0mM与12mM时与聚(N-异丙基丙烯酰胺)形成的复合物结构如图2所示。
(3)除了表面活性剂,无机盐对聚(N-异丙基丙烯酰胺)在水溶液中构象转变也有很大影响。粘度及动态光散射实验结果表明,在常温下绝大多数无机盐均能促进高分子构象发生转变,使得相转变温度下降；且盐浓度越高,影响程度越大。此外,无机盐中阴离子基团相比于阳离子基团,起到更为决定性作用。并且不同盐对高分子相转变影响程度的大小关系符合经典的Hofmeister序列。其中对于卤素系钠盐,NaI在较低浓度时表现出与其它盐相反的作用,使得聚(N-异丙基丙烯酰胺)链在水溶液构象伸展,高分子特性粘数及平均流体力学半径增大。进一步的核磁实验结果初步证实了无机盐对PNIPAM在水溶液中构象转变影响,主要是通过阴离子基团与溶剂分子间作用而进行的。
Poly(N-isopropylacrylamide)(PNIPAM) is a well-known thermo-sensitive polymer that exhibits a low critical solution temperature (LCST) at around305K in aqueous solutions. The coil-to-globule transition of PNIPAM can be induced by a small temperature variation (1～2K) accompanied by abrupt conformational changes. The LCST behavior of PNIPAM has been attracting research interests for several decades because of its implication in a number of living phenomena, especially on protein folding and DN A packing. However, the coil-to-globule transition of PNIPAM has been greatly affected by some additives, such as surfactants and inorganic salts. Therefore, the researches on the effects of the additives on the coil-to-globule transition of PNIPAM have gained great realistic significance.
In this work, with the applications of laser light scattering (LLS), viscosmetry as well as high-resolution nuclear magnetic resonance (NMR), the effects of the anion surfactant sodium n-dodecyl sulfate (SDS) on the coil-to-globule transition of PNIPAM under various temperatures and surfactant concentrations have been systematically studied. Besides, the effects of eight inorganic salts on the coil-to-globule transition of this polymer have been also explored. Several interesting results have been drawn:
(1) SDS can increase LCST of PNIPAM solutions obviously for peculiar interactions with PNIPAM. Results from2D NOESY and pulsed-field gradient diffusion NMR show that a small proportion of SDS (around16%) binds to the PNIPAM chain and forms a polyelectrolyte-like complex via hydrophobic interactions below LCST. The polymer-bound SDS plays an important role in the retardation of the PNIPAM chain collapse and the inhibition of inter-chain aggregation. Interestingly, the polymer-bound SDS gradually dissociates from PNIPAM because of electrostatic repulsion and steric hindrance effects resulting from the coil-to-globule transition. Almost no cross-peak in the2D NOESY NMR spectra above the LCST is observed because of the reduced interactions between PNIPAM and SDS after the polymer chain transforms into a compact globule. The role of SDS during the coil-to-globule transition in the PNIPAM/SDS aqueous solution at various temperatures was illustrated in Figure1.
(2) The complex structure of and interaction between PNIPAM and SDS in aqueous solutions below the LCST were investigated further. Results from viscometry and NMR have both indicated that the interactions of PNIPAM and SDS were found to exhibit strong dependences on the SDS concentration. Two critical SDS concentrations for this system were found, i.e.,0.86and7.0mM, within the current measured SDS concentration range. The former was the onset concentration of formation of the PNIPAM and SDS aggregates. The latter was the saturation concentration at which the surfactant molecules could not bound to the polymer chain. At SDS concentrations below0.86mM, the interaction between PNIPAM and SDS was very minimal, resulting in almost no change in the intrinsic viscosity and hydrodynamic size of the polymer. At SDS concentrations below7.0mM but above0.86mM, the polymer-free SDS concentration remained unchanged at around0.86mM near the critical aggregation concentration (CAC). Hence, excess SDS molecules preferred to attach onto the PNIPAM chain until saturation and formed a polyelectrolyte-like complex. Consequently, the intrinsic viscosity and hydrodynamic size of the polymer in solution increased. The amount of polymer-bound SDS and intensities of the cross-peaks in the2D NOESY NMR spectra also increased. At SDS concentrations above7.0mM, the number of polymer-bound SDS reached saturation and no more SDS molecule bound to the polymer chain due to steric hindrance. Subsequently, the amount of polymer-free SDS began to increase. The morphological structures of PNIPAM/SDS at SDS concentrations1.0mM and12mM have shown in Figure2, respectively.
(3) Besides SDS, the additives of inorganic salts have also shown great effects on the conformational change of PNIPAM in aqueous solutions. Results from viscometry and LLS indicated that most of tested salts would accelerate the coil-to-globule transition of PNIPAM and reduce the LCST. These effects exhibited strong dependence on the salts concentrations, and the higher salts concentration, the lower of LCST. Furthermore, it was found that the anionic groups of salts played the key roles to affect the coil-to-globule transition behavior of polymer, and followed the order of classic Hofmeister sequence. However, NaI at low concentration region showed opposite effects, which could increased the hydrodynamic size of PNIPAM slightly. The NMR experiments primarily indicated that the effects of the inorganic salts on the conformational change of PNIPAM in aqueous solutions were carried on mainly through interaction between the anionic groups and solvent molecules and destroying the solvation effects of polymer.
引文
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