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接枝在纳米粒子表面的聚异丙基丙烯酰胺刷构象转变中的硫氰酸根离子效应

赵新军 李九智 石铭芸 马超

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接枝在纳米粒子表面的聚异丙基丙烯酰胺刷构象转变中的硫氰酸根离子效应

赵新军, 李九智, 石铭芸, 马超

Effect of thiocyanate anions on switching of poly (N-isopropylacrylamide) tethered to nanoparticle surface

Zhao Xin-Jun, Li Jiu-Zhi, Shi Ming-Yun, Ma Chao
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  • 应用分子场理论研究接枝在纳米粒子表面的聚异丙基丙烯酰胺(PNIPAM)球面刷构象转变中的硫氰酸根离子(SCN)效应, 理论模型考虑PNIPAM-SCN的结合(P—S键)和体系的静电特性. 研究发现, PNIPAM球面刷构象转变的低临界溶液温度(LCST)在较低SCN浓度条件下, 随着SCN浓度的增加会增大. 在高浓度条件下, 随着SCN浓度增加, LCST降低. 在低SCN浓度条件下, P—S键分数随着SCN浓度增加而变大, 在刷内产生静电排斥作用; 在高SCN浓度条件下, P—S键的形成趋于饱和, 较多的SCN结合到PNIPAM链中. 增加SCN浓度, 会增加抗衡离子浓度, 导致了以抗衡离子为中介的静电吸引和静电屏蔽, 以及PNIPAM链疏水性的增强. 理论结果符合实验观测, 并且可以预言, 在较强的P—S键作用下, 随着温度的降低, PNIPAM球面刷中出现垂直相分离结构, 出现两个转变温度, 这是由于PNIPAM-SCN结合导致PNIPAM的亲水作用与疏水作用、静电作用竞争平衡的结果.
    A recent experiment carried by Humphreys et al. (Humphreys B A, Wanless E J, Webber Grant B 2018 J. Colloid Interface Sci. 516 153) shows that when poly (N-isopropylacrylamide) (PNIPAM) tethered to nanoparticle surface is immersed in potassium thiocyanate solution, the thiocyanate anions (SCN) can increase the low critical solution temperature (LCST) of the PNIPAM below 500 mmol, though the LCST is reduced when at 1000 mmol. It is unclear why the SCN increases the LCST at low concentration and reduces the LCST at high concentration. In this paper, using a molecular theory, we investigate the effect of SCN on the switching and the structure of PNIPAM tethered to nanoparticle surface. In our model the PNIPAM-SCN bonding (P—S bonds), electrostatic effects and their explicit coupling to the PNIPAM conformations are taken into consideration. We find that under the low SCN concentration, as the SCN concentration increases, the SCN is associated with the PNIPAM chains through the PNIPAM—S bonds, and the PNIPAM segments become negatively charged, which makes electrostatic repulsion stronger and results in an increase in the LCST.According to our model, the reduction of LCST at high SCN concentration can be explained as follows: with the increase of SCN concentration, more and more PNIPAM-SCN bindings occur between SCN and PNIPAM segments, which will lead the hydrophobicity of PNIPAM chains to increase. On the other hand, the P—S bonds have been filled at the high SCN concentration, and the PNIPAM chains become more negatively charged. The increase of the SCN is accompanied with an increase in the concentration of counterions (K+). The increase of counterion concentration will give rise to the counterion-mediated attractive interactions along the chains and electrostatic screening within the negatively charged PNIPAM, thus the LCST can be reduced when further increasing the SCN concentration. The reduction of LCST can be attributed to the increased hydrophobicity of PNIPAM chains, or to the counterion-mediated attractive interaction along the chains and the screening of the electrostatic interactions. By analyzing the distribution of PNIPAM segments near the critical temperature, we find that the distribution of volume fractions of the PNIPAM tethered to nanoparticle surface shows a maximum when the hydration of PNIPAM and PNIPAM-SCN binding are stronger, which implies that a vertical phase separation may occur. Based on our theoretical model, a vertical phase separation and a two-step phase transition behaviors in the PNIPAM tethered to nanoparticle surface are predicted. We also analyze the height of the PNIPAM, which is a function of temperature at different SCN concentrations, and then obtain the critical temperature of the two-step phase transition. The results show that the vertical phase separation and the two-step phase transition are promoted by competition between hydrophobicity, hydrophilicity and electrostatic effects due to the P—S bonds. Our theoretical results are consistent with the experimental observations, and provide a fundamental understanding of the effects of SCN on the LCST of PNIPAM tethered to nanoparticle surface.
      通信作者: 赵新军, zhaoxinjunzxj@163.com
    • 基金项目: 新疆自然科学基金联合基金(批准号: 2019D01C333)和国家自然科学基金(批准号: 21764015)资助的课题
      Corresponding author: Zhao Xin-Jun, zhaoxinjunzxj@163.com
    • Funds: Project supported by the Joint Funds of Xinjiang Natural Science Foundation, China (Grant No. 2019D01C333) and the National Natural Science Foundation of China (Grant No. 21764015)
    [1]

    Wu C, Zhou S 1995 Macromolecules 28 5388Google Scholar

    [2]

    Howard G, Schild H G, Tirrell D A 1990 J. Phys. Chem. 94 4352Google Scholar

    [3]

    Lahann J, Mitragotri S, Tran T N, Kaido H, Sundaram J, Choi I S, Hoffer S, Somorjai A, Langer R 2003 Science 299 371Google Scholar

    [4]

    Kanazawa H, Okano T 2011 J. Chromatography A 1218 8738

    [5]

    Tang Z, Akiyama Y, Okano T 2012 Polymers 3 1478

    [6]

    Zhang Y J, Furyk S, Bergbreiter D E, Cremer P S 2005 J. Am. Chem. Soc. 127 14505Google Scholar

    [7]

    Du H, Wickramasinghe R, Qian X H 2010 J. Phys. Chem. B 114 16594

    [8]

    Okur H I, Kherb J, Cremer S 2013 J. Am. Chem. Soc. 135 5062Google Scholar

    [9]

    Naini C A, Thomas M, Franzka S, Frost S, Ulbricht M, Hartmann N 2013 Macromol. Rapid Commun. 34 417Google Scholar

    [10]

    Riehemann K, Schneider S W, Luger T A, Godin B, Ferrari M, Fuchs H 2009 Angew. Chem. Int. Ed. 48 872Google Scholar

    [11]

    Xia T, Kovochich M, Liong M, Meng H, Kabehie S, George S, Zink J I, Nel A E 2009 ACS Nano 3 3273Google Scholar

    [12]

    Croissant J, Zink J I 2012 J. Am. Chem. Soc. 134 7628Google Scholar

    [13]

    Thornton P D, Heise A 2010 J. Am. Chem. Soc. 132 2024Google Scholar

    [14]

    Yu Z Z, Li N, Zheng P P, Pan W, Tang B 2014 Chem. Commun. 50 3494Google Scholar

    [15]

    Humphreys B A, Wanless E J, Webber Grant B 2018 J. Colloid Interface Sci. 516 153Google Scholar

    [16]

    Zhao X J, Gao Z F 2016 Chin. Phys. B 25 074703

    [17]

    Liu L, Shi Y, Liu C, Wang T, Liu G M, Zhang G Z 2014 Soft Matter 10 2856Google Scholar

    [18]

    Murdoch T J, Humphreys B A, Willott J D, Gregory K P, Prescott S W, Nelson A, Wanless E J, Webber G B 2016 Macromolecules 49 6050Google Scholar

    [19]

    Humphreys B A, Willott J D, Murdoch T J, Webber G B, Wanless E J 2016 Phys. Chem. Chem. Phys. 18 6037Google Scholar

    [20]

    Szleifer I, Carignano M A 2000 Macromol. Rapid Commun. 21 423Google Scholar

    [21]

    Ren C L, Nap R J, Szleifer I 2008 J. Phys. Chem. B 112 16238Google Scholar

    [22]

    Kundagrami A, Muthukumar M 2008 J. Chem. Phys. 128 244901

    [23]

    Fujishige S, Kubota K, Ando I 1989 J. Phys. Chem. 93 3313

    [24]

    Furyk S, Zhang Y, Ortiz-Acosta D, Cremer P S, Bergbreiter D E 2006 J. Polymer Sci.: Part A: Polymer Chemistry 44 1492Google Scholar

    [25]

    Lund M, Vacha R, Jungwirth P 2008 Langmuir 24 3387Google Scholar

    [26]

    Dormidontova E E 2002 Macromolecules 35 987Google Scholar

    [27]

    Nap R J, Park S H, Szleife I 2018 Soft Matter 14 2365Google Scholar

    [28]

    Mahalik J P, Sumpter B G, Kumar R 2016 Macromolecules 49 7096Google Scholar

    [29]

    Humphreys B A, Prescott S W, Murdoch T J, Nelson A, Gilbert E P, Webber G B, Wanless E J 2019 Soft Matter 15 55Google Scholar

    [30]

    Mason P E, Neilson G W, Dempsey C E, Barnes A C, Cruickshank J M 2003 Proc. Natl. Acad. Sci. U S A 100 4557Google Scholar

  • 图 1  接枝在纳米粒子表面的PNIPAM球面刷系统(其中SCN通过P—S键与PNIPAM结合)

    Fig. 1.  Schematic representation of the PNIPAM tethered to nanoparticle surface. Bonding between PNIPAM and SCN by formation of P—S bonds.

    图 2  接枝在纳米粒子表面的PNIPAM球面刷高度随温度的变化关系(其中结合能参数为${E_{\rm{p}}}/{k_{\rm{B}}}=1000\;{\rm{K}}$, 熵的损失为$\Delta {S_{\rm{p}}}= - 2.25$, ${\chi _{{\rm{p}}{\rm{w}}}}= - 0.45 + 135/T$, 接枝密度为$\sigma = 0.\, 05\;{\rm{n}}{{\rm{m}}^{ - 2}}$)

    Fig. 2.  Height of the grafted PNIPAM brushes as a function of temperature. The P—S bond energetic gain is chosen as ${E_{\rm{p}}}/{k_{\rm{B}}}=1000\;{\rm{K}}$, and the entropic loss is given by $\Delta {S_{\rm{p}}}{\rm{ = }} - 2.25$. The surface coverage is $\sigma = 0.05\, \, {\rm{n}}{{\rm{m}}^{ - 2}}$.

    图 3  P—S 键分数在垂直纳米粒子表面方向的分布(其中温度T = 31 ℃, 其余参数与图2相同)

    Fig. 3.  Local fraction of P—S bond as a function of SCN concentration at a given temperature of T = 31 ℃. All parameters are the same as those in Fig. 2.

    图 4  体系静电势在距离垂直纳米粒子表面方向的分布(温度T = 31 ℃, 其余参数与图2相同)

    Fig. 4.  Electrostatic potential as a function of the distance from the surface at different thiocyanate anion concentrations at a given temperature of T = 31 ℃. All parameters are the same as those in Fig. 2

    图 5  C = 750 mmol时PNIPAM分子链单体的平均体积分数在垂直纳米粒子表面方向的分布 (a) ${\chi _{{\rm{p}} {\rm{w}}}}= - 0.45 + 135/T, $${E_{\rm{p}}}/{k_{\rm{B}}}=1000{\kern 1 pt} \, {\kern 1 pt} {\rm{K;}}$ (b) ${\chi _{{\rm{p}} {\rm{w}}}}= - 2.25 + 95/T, {E_{\rm{p}}}/{k_{\rm{B}}}=2000\; {\rm{K}}$; 其余参数与图2相同

    Fig. 5.  Average volume fractions of the grafted PNIPAM chains as a function of the distance from the surface for C = 750 mmol: (a) ${E_{\rm{p}}}/{k_{\rm{B}}}=1000\; {\rm{K, }}\;{E_{\rm{p}}}/{k_{\rm{B}}}=1000\; {\rm{K}}$; (b) ${\chi _{{\rm{p}}{\kern 1 pt} {\rm{w}}}}= - 2.25 + 95/T,\; {E_{\rm{p}}}/{k_{\rm{B}}}=1800\; {\rm{K}}$. All parameters are the same as those in Fig. 2

    图 6  C = 750 mmol时PNIPAM球面刷高度随温度的变化(其中${\chi _{{\rm{p}}{\rm{w}}}}= - 2.25 + 95/T$, 其余参数与图2相同)

    Fig. 6.  Height of the grafted PNIPAM brushes as a function of temperature at C = 750 mmol. The ${\chi _{{\rm{p}}{\rm{w}}}}= $ – 2.25 + 95/T. All parameters are the same as those in Fig. 2.

  • [1]

    Wu C, Zhou S 1995 Macromolecules 28 5388Google Scholar

    [2]

    Howard G, Schild H G, Tirrell D A 1990 J. Phys. Chem. 94 4352Google Scholar

    [3]

    Lahann J, Mitragotri S, Tran T N, Kaido H, Sundaram J, Choi I S, Hoffer S, Somorjai A, Langer R 2003 Science 299 371Google Scholar

    [4]

    Kanazawa H, Okano T 2011 J. Chromatography A 1218 8738

    [5]

    Tang Z, Akiyama Y, Okano T 2012 Polymers 3 1478

    [6]

    Zhang Y J, Furyk S, Bergbreiter D E, Cremer P S 2005 J. Am. Chem. Soc. 127 14505Google Scholar

    [7]

    Du H, Wickramasinghe R, Qian X H 2010 J. Phys. Chem. B 114 16594

    [8]

    Okur H I, Kherb J, Cremer S 2013 J. Am. Chem. Soc. 135 5062Google Scholar

    [9]

    Naini C A, Thomas M, Franzka S, Frost S, Ulbricht M, Hartmann N 2013 Macromol. Rapid Commun. 34 417Google Scholar

    [10]

    Riehemann K, Schneider S W, Luger T A, Godin B, Ferrari M, Fuchs H 2009 Angew. Chem. Int. Ed. 48 872Google Scholar

    [11]

    Xia T, Kovochich M, Liong M, Meng H, Kabehie S, George S, Zink J I, Nel A E 2009 ACS Nano 3 3273Google Scholar

    [12]

    Croissant J, Zink J I 2012 J. Am. Chem. Soc. 134 7628Google Scholar

    [13]

    Thornton P D, Heise A 2010 J. Am. Chem. Soc. 132 2024Google Scholar

    [14]

    Yu Z Z, Li N, Zheng P P, Pan W, Tang B 2014 Chem. Commun. 50 3494Google Scholar

    [15]

    Humphreys B A, Wanless E J, Webber Grant B 2018 J. Colloid Interface Sci. 516 153Google Scholar

    [16]

    Zhao X J, Gao Z F 2016 Chin. Phys. B 25 074703

    [17]

    Liu L, Shi Y, Liu C, Wang T, Liu G M, Zhang G Z 2014 Soft Matter 10 2856Google Scholar

    [18]

    Murdoch T J, Humphreys B A, Willott J D, Gregory K P, Prescott S W, Nelson A, Wanless E J, Webber G B 2016 Macromolecules 49 6050Google Scholar

    [19]

    Humphreys B A, Willott J D, Murdoch T J, Webber G B, Wanless E J 2016 Phys. Chem. Chem. Phys. 18 6037Google Scholar

    [20]

    Szleifer I, Carignano M A 2000 Macromol. Rapid Commun. 21 423Google Scholar

    [21]

    Ren C L, Nap R J, Szleifer I 2008 J. Phys. Chem. B 112 16238Google Scholar

    [22]

    Kundagrami A, Muthukumar M 2008 J. Chem. Phys. 128 244901

    [23]

    Fujishige S, Kubota K, Ando I 1989 J. Phys. Chem. 93 3313

    [24]

    Furyk S, Zhang Y, Ortiz-Acosta D, Cremer P S, Bergbreiter D E 2006 J. Polymer Sci.: Part A: Polymer Chemistry 44 1492Google Scholar

    [25]

    Lund M, Vacha R, Jungwirth P 2008 Langmuir 24 3387Google Scholar

    [26]

    Dormidontova E E 2002 Macromolecules 35 987Google Scholar

    [27]

    Nap R J, Park S H, Szleife I 2018 Soft Matter 14 2365Google Scholar

    [28]

    Mahalik J P, Sumpter B G, Kumar R 2016 Macromolecules 49 7096Google Scholar

    [29]

    Humphreys B A, Prescott S W, Murdoch T J, Nelson A, Gilbert E P, Webber G B, Wanless E J 2019 Soft Matter 15 55Google Scholar

    [30]

    Mason P E, Neilson G W, Dempsey C E, Barnes A C, Cruickshank J M 2003 Proc. Natl. Acad. Sci. U S A 100 4557Google Scholar

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出版历程
  • 收稿日期:  2019-05-06
  • 修回日期:  2019-07-02
  • 上网日期:  2019-11-01
  • 刊出日期:  2019-11-05

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