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The response of a permanently charged polyelectrolyte brush to external ions: the aspects of structure and dynamics bintao zhao, Guangcui Yuan, Xiao Chu, Jingfa Yang, and Jiang Zhao Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01195 • Publication Date (Web): 21 May 2018 Downloaded from http://pubs.acs.org on May 21, 2018
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Langmuir
The response of a permanently charged polyelectrolyte brush to external ions: the aspects of structure and dynamics Bintao Zhao,a,b Guangcui Yuan,c Xiao Chu,a,b Jingfa Yang,a,b Jiang Zhaoa,b,* a
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
b
University of Chinese Academy of Sciences, Beijing 100049, China
c
Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA
Abstract Structure and dynamics inside permanently charged polyelectrolyte brushes, sodium polystyrene sulfonate brushes (NaPSS), during their response to the introduction of external ions (NaCl) are investigated by neutron reflectivity and dielectric spectroscopy. Neutron reflectivity measurements show that the segmental density of the inner part of the brushes decreases and that of the outer part increases when the salt level is tuned from the salt-free condition to a moderate level (1.0 M), exhibiting a similar trend as the frequency change in the imaginary part. The real part tells the strength of the overall polarization, which is proportional to number of ions participating in the polarization, i.e. the field-induced charge separation. The continuous increase of the polarization with the salt concentration tells the increasing number of the adsorbed counterions at the elevated salt concentrations. The increase of amplitude of the real part of permittivity also saturates at high salt concentration, in a good agreement of the saturated counterion adsorption at the very high salt level – an entropy-driven process. The amplitude of the real permittivity at the frequency of maximum of the imaginary part scales roughly with the grafting density (the amplitude for the grafting density of 14 ACS Paragon Plus Environment
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0.02 and 0.04 chainnm2 is 6.0104 and 1.8105, respectively). This result strongly supports the picture of saturated counterion adsorption at high enough salt concentrations. The experimental results described above agree well with the mechanism of enhanced counterion adsorption onto the PSS chain with the increase of salt concentration – the process of charge regularization.66, 69 The effect of salt concentration to dynamics of the adsorbed counterions is through the initial effective charge density of the chain. Under all salt concentrations, the ionic strength within the brushes becomes very low right after the electric field is applied, due to the depletion of free counterions because the dynamics of these free counterions is orders of magnitude faster. Regarding the initial charge density of the polyelectrolyte chain, the higher the initial salt concentration is, the bigger amount of counterions adsorb on the polyelectrolyte chain and the lower initial charge density does the chain have. The lower effective charge density results in the weaker attraction to the counterions and therefore the faster dynamics of the counterions – the faster dynamics of counterion desorption and migration. Further investigations on this issue is certainly very desirable. The lower frequency of the counterions’ relaxation in the sample with a higher grafting density indicates a longer residing time of the counterions on the charged chains. This is attributed to the higher probability for the counterions to be attached to the chains. When electric field is applied, the free ions, which are not bound to the PSS chains, are immediately depleted from the brush layer, creating a situation with a large value of Debye length within the brushes. When a previously adsorbed counterion detaches from the polyelectrolyte chains, it is attracted by multiple charged chain segments. It is envisioned that the probability of the counterion to be adsorbed on the PSS chains is higher for the case of higher grafting density, and therefore a longer overall residing time is resulted, leading to a slower dynamics of counterions. It is noted that because of the different methods of fabricating NaPSS brushes on silicon surface and gold surface in the current study, the grafting density of the brushes differs 5-10 times between the samples for neutron reflectivity and dielectric spectroscopy measurements, a direct comparison is not available here. It is envisioned that the relaxation frequency of the adsorbed counterions should be even lower for very high grafting density and also possible inhomogeneity can emerge due to the inhomogeneous distribution of segmental density. 15 ACS Paragon Plus Environment
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Figure 5 The frequency of the maximum of imaginary permittivity and the thickness of NaPSS brushes with two grafting density, as a function of salt concentration (NaCl). The red data points: 0.02 chainnm2; black data points: 0.04 chainnm2. The thickness of the brushes were measured by ellipsometry. The comparison of the variation of frequency of the imaginary permittivity with that of thickness as a function of salt concentration helps to understand the mechanism of the brushes’ swelling and collapsing (Figure 5). With the increase of salt concentration, the thickness of the brushes experiences an increase at moderate salt concentration (
