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Role of Salt and Water in the Plasticization of PDAC/PSS Polyelectrolyte Assemblies Ran Zhang, Yanpu Zhang, Hanne S. Antila, Jodie L. Lutkenhaus, and Maria Sammalkorpi J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b12315 • Publication Date (Web): 13 Dec 2016 Downloaded from http://pubs.acs.org on December 19, 2016

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The Journal of Physical Chemistry

Role of Salt and Water in the Plasticization of PDAC/PSS Polyelectrolyte Assemblies Ran Zhang,† Yanpu Zhang,‡ Hanne S. Antila,¶ Jodie L. Lutkenhaus,∗,‡,§ and Maria Sammalkorpi∗,¶ †State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, P. R. China ‡Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843, United States ¶Department of Chemistry, School of Chemical Technology, Aalto University, P.O. Box 16100, FI-00076 Aalto, Finland §Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77843, United States E-mail: [email protected]; [email protected]

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Abstract In this work, we investigate the effect of salt and water on plasticization and thermal properties of hydrated poly(diallyldimethylammonium chloride) (PDAC) and poly(sodium 4-styrenesulfonate) (PSS) assemblies via molecular dynamics simulations and modulated differential scanning calorimetry (MDSC). Commonly, both water and salt are considered to be plasticizers of hydrated polyelectrolyte assemblies. However, the simulation results presented here show that while water has a plasticizing effect, salt can also have an opposite effect on the PE assemblies. On one hand, the presence of salt ions provides additional free volume for chain motion and weakens PDAC-PSS ion pairing due to electrostatic screening, which contribute toward plasticization of the complex. On the other hand, salt ions bind water in their hydration shells, which decreases water mobility and reduces the plasticization by hydration. Our MDSC results connect the findings to macroscopic PE plasticization and the glass-transition-like thermal transition Ttr under controlled PE hydration and salt content. This work identifies and characterizes the dual nature of salt both as plasticizer and hardener of PE assemblies and maps the interconnection of the influence of salt with the degree of hydration in the system. Our findings provide insight to the existing literature data, bear fundamental significance in understanding of hydrated polyelectrolyte assemblies, and suggest a direct means to tailor the mechanical characteristics of PE assemblies via interplay of water and salt.

Introduction Polyelectrolyte (PE) multilayers and complexes are versatile materials with a wide variety of applications in many fields ranging from biological and energy systems to responsive, smart materials and drug delivery. 1–7 The properties of PE assemblies depend on e.g. temperature, 8–10 solvent, 11 pH, 12,13 ambient water content, 14,15 and ionic strength 16–18 along with salt species. 19–22 Notably, both the hydration level and the ionic strength of the assembly solution or subsequent doping process influence the thermal and mechanical response of poly2 ACS Paragon Plus Environment

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electrolyte aggregates, see e.g. Refs. 14,15,22–31. For hydrated assemblies, the plasticization response with increasing temperature can bear also a thermal transition, which includes a decrease in modulus and an increase in chain mobility. 23,32–34 The presence of this thermal plasticization transition has been established under different conditions by extensive thermal characterization efforts. 23,28,29,32–34 Besides increasing temperature, the overall plasticization response of PE aggregates has been connected with the presence of water 14,35–37 and excess salt ions. 38,39 The glass transition-like thermal transition and the transition temperature Ttr appear to depend on factors such as the composition, the ionic strength in the sample processing, hydration level, and even the detection frequency, see e.g. Refs. 23,26,28,29,32,33,40. At a molecular level, these dependencies correspond to the density of ion pairs between oppositely charged PE chains (intrinsic vs extrinsic charge compensation), the amount of water molecules, the influence of salt ions on the molecular interactions, as well as, the relaxation of the PE chains (long and intermediate range diffusion). The effect of water content, i.e., the degree of hydration on PE assemblies has been connected with the mobility of the PEs in the thermal transitions of poly(diallyldimethylammonium chloride)/poly(sodium 4-styrenesulfonate) (PDAC/PSS) multilayers and complexes. 26,29,36,40 This translates to plasticization of the PE assembly and a decrease in the thermal transition temperature with increasing hydration. For example, dehydrated salt-free PDAC/PSS complexes have been reported to be brittle and show a Ttr at around 100◦ C, 41 whereas PDAC/PSS complexes with salt doping and a considerable hydration level (around 40 wt%) exhibit a Ttr at around room temperature. 26 For assemblies at higher hydration, the thermal transition occurs at lower temperatures and the transition temperature depends on the degree of hydration: a Ttr of 51◦ C for a PE multilayer with a water content of 12 wt% has been reported by Vidayasagar et al. 29 whereas Shamoun et al. found lower Ttr values for assemblies of same PEs at a more elevated water content of 37 − 41 wt%. 26 Furthermore, our recent work connects a decrease of hydrogen bonding life time between water and PEs,



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which is translated to partial dehydration, with the thermal transition. 42 In total, these observations present water as a plasticizer of PE systems: water molecules influence polymer motions via decreasing friction between polymer chains and increasing the effective volume of the PE assemblies, see e.g. Refs. 43–45. Decrease of the friction reduces the effective barriers against polymer motions and an increase of the volume enables enhanced mobility for the PE chains. Similar to water, salt can also act as plasticizer to PE assemblies. 24,26,46 Indeed, the role of salt ions in the plasticization of hydrated PE assemblies has received considerable attention. 26,29,47,48 Especially, Schlenoff and coworkers have studied the role of salt-doping level and the shearing frequencies on the plasticization of PE assemblies, and brought forward the concept of saloplasticity. 38,39 Additionally, a superposition of time, temperature, and salt has been proposed for the plasticization response of hydrated polyelectrolyte assemblies. 26 In hydrated PE assemblies, excess salt in the assembly bath or subsequent addition of salt has a key role: excess salt in the material allows compensation of the PE charges by the extrinsic salt ions. This decreases the crosslinking density of intrinsic PE-PE ion pairs. It weakens the bonding strength between the oppositely charged PE chains and results in looser PE assembly structure. 26 Furthermore, a "glass transition ionic strength" at which hydrated PE films experience a transition from glassy to rubbery akin to the thermal transition of hydrated PE assemblies has been suggested. 49 The dynamic mechanical analysis based characterizations of Shamoun et al. 26 and Jaber et al. 24 suggest that an increased salt doping of PE assemblies causes a decreased equilibrium modulus, as well as, a decrease in the thermal transition temperature Ttr . In these works, a significant decrease in the modulus was interpreted as a signal of the thermal transition Ttr . The method captures the relaxation of the PE chains in the interpenetrating structure and thus connects the mechanical response to the presence of the excess salt ions. Although salt can plasticize PE assemblies, the effect of salt appears to be more convo-


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luted than that of water and dependent on the water content of the sample: for example, dried PE complexes with salt have higher thermal transition temperatures than equivalent dried salt-free samples. 41 Additionally, calorimetry characterization measurements have failed to present a clear decrease of thermal transition temperature Ttr with increased salt in the system while maintaining the same water content in the PE assembly. 29 In experiments, increasing the salt content of PE assemblies usually results also in an increase of the overall water content as the salt ions carry water into the assembly, see e.g. Refs. 22,26. Thus, the observed plasticization response results both from the influence of salt and the water that the added ion content brings in. Furthermore, salt ions influence the water hydrogen bonding network (water structure) and water diffusion properties. 50,51 Highlighting the complexity of the actual nature of the proposed salt–water–time superposition of hydrated PE assemblies, 26 an interplay of the two contributions is to be expected: salt could both 1) contribute toward stiffening the material via slowing down the water dynamics and 2) act as a plasticizer via breaking PE-PE bonds and increasing the chain mobility. In this work, we focus on deciphering the role and interplay of salt ions and water in the plasticization of PE assemblies. The examination focuses on a model PE system of PDAC/PSS. Molecular dynamics simulations are employed to characterize the microscopic effects of increasing hydration and salt content in PDAC/PSS assemblies. The simulationsbased findings are connected to macroscopic scale with modulated differential scanning calorimetry of the corresponding PE multilayers.

Simulational and experimental methods Computational methods Hydrated poly(diallyldimethylammonium chloride)/poly(styrenesulfonate) (PDAC/PSS) assemblies were modeled in systems of 10PDAC40 /10PSS40 (Figure 1b) by classical molecular dynamics simulations using GROMACS 4.6.6 simulation engine, 52–54 OPLS-aa force field 55 5 ACS Paragon Plus Environment


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(a)

(b)

Figure 1: (a) Simulation snapshots of PSS40 and PDAC40 chains and their chemical structures. (b) Initial configuration of 10PDAC40 /10PSS40 system at 19 wt% hydration and salt molar ratio x = 1. PSS chains are in light brown, PDAC purple, N a+ ions white, and Cl− ions black. Water molecules are omitted in the visualization for clarity.

and explicit TIP4P water model. 56 The prefactor 10 signifies the number of PE chains of the given type in the system while the subscript 40 refers to the number of repeat units in each PSS and PDAC polyelectrolyte chain. The partial charges on PSS and sulfonate groups followed the work of Qiao et al. 57 For the simple salt NaCl, standard OPLS-aa ion parameters were used. Throughout the simulations, all covalent bonds were controlled by LINCS 58 while the SETTLE algorithm was used for water molecules. 59 For non-bonded short-range interactions, a cut-off distance of 1.0 nm was used both for the van der Waals interactions and electrostatic contributions in the real space. The long-range electrostatics were treated by the PME method. 60 3D periodic boundary conditions were employed throughout the study. The chemical structures of PSS and PDAC are presented in Figure 1a. PSS was set as syndiotactic and PDAC was in a trans conformation. Varying the amount of salt at constant hydration and varying the hydration while keeping salt molar ratio constant enable systematic study of the influence of salt and hydration separately on the plasticization of PE assemblies. The simulated systems are listed in Table 6 ACS Paragon Plus Environment

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Table 1: Simulated systems details for 10PDAC40 /10PSS40 . Salt molar ratio x 0 0.5 1.0 1.5 1.0

Water Salt content ions 19 wt% 0 19 wt% 200 19 wt% 400 18 wt% 600 12 wt% 400 19 wt% 400 26 wt% 400

Water molecules 2000 2000 2000 2000 1120 2000 3000

1. Salt molar ratio x means the ratio of number of N a+ (or Cl− ) ions to polycation (or polyanion) charge. To explore the effect of salt on the PDAC/PSS, various salt molar ratios between 0 and 1.5 were examined at 18 − 19 wt% hydration and to investigate the role of hydration in the PDAC/PSS, water contents of 12, 19, and 26 wt% were examined at salt molar ratio 1.0. Note that the water content in wt% differs slightly between the different salt molar ratios because the added salt contributes to the total weight (number of water molecules and PE monomers is kept constant). For each system, the results presented are calculated from an average over simulations of 8 different initial configurations, unless otherwise specified. The initial configurations were generated using Packmol 61 with chain conformations extracted from dilute solution. Water molecules initially formed a solvation layer close to the polymers. Salt ions were added by replacing excess water molecules. An initial relaxation of the system was done by a 20 ns molecular dynamics simulation using a stochastic dynamics integrator 62 with an inverse friction constant of 2 ps and a time step of 2 fs. The metastable conformational states were annealed by increasing the temperature from 300 K to 380 K and back during 15 ns. After this, the temperature was maintained at 300 K for the remaining relaxation. System pressure was maintained at 1 bar with time coupling constant at 1 ps using Berendsen barostat. 63 This way of generating initial configurations creates intertwined, mixed PE assemblies with relatively uniform structure and water distribution. In the production runs, the standard leapfrog integrator was used with a 2 fs time step. 7 ACS Paragon Plus Environment


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The Nosé-Hoover thermostat 64,65 and Parrinello-Rahman barostat 66 were used with coupling constants at 0.1 ps and 1 ps, respectively. The system temperature was increased from 300 K to 375 K in 5 K steps of 5 ns each for a total duration of 75 ns. The last 4 ns of each temperature step were used for data analysis. VMD 67 was used for all molecular system visualizations. Different temperature ramping rates were tested; the employed rate is slow enough to capture water and ion dynamics whereas for capturing polymer relaxation, a significantly slower rate would be needed. In all analysis, binding between two atoms of functional groups are determined by a cut-off distance corresponding to the minimum after the first peak of the respective radial distribution function. Contact number to a certain functional group or atom is the count of these groups or atoms within the binding cut-off distance. For hydrogen bonds, a cut-off of 0.35 nm and an additional angle cut-off at 30◦ for an acceptor-donor-hydrogen are applied. Hydrogen bond lifetime is estimated as the integral of the correlation function of the hydrogen bonds. Notably, the simulational time scale here does not enable describing PE motion beyond local rearrangement and vibrations. Water and ions diffuse and they can induce structural and binding changes. This means that in preparing the initial configurations we have made the assumption that the dilute solution configurations provide a sufficiently representative set of conformations to describe the amorphous, disorganized polyelectrolyte assembly structure. This assumption was checked by preparing initial configurations in very different ways and mapping their characteristics: while absolute values can be biased by the assembly structure, trends appear to persist. The data here corresponds to PDAC/PSS assemblies with relatively uniform structure and water distribution. To check the validity of the PSS and PDAC models, we performed also MD simulations of the molecular radius of gyration (Rg ) with varying PSS and PDAC chain lengths. These simulation conditions followed Refs. 57,68. Our NPT simulation of PSS10 chains solvated by 750 water molecules resulted in an Rg of 0.71 ± 0.04 nm and 0.80 ± 0.03 nm for isotactic and syndiotactic configurations, respectively. The result is consistent with previously reported


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values of 0.72 ± 0.04 nm for the same OPLS-aa model 57 and 0.74 nm obtained using a united-atom model. 68 Notably, neither of the comparison references 57,68 specify the chain tacticity. On the other hand, our simulations show for a 20-mer PSS (PSS20 chain) solvated by 2000 water molecules an Rg of 1.08±0.06 nm (isotactic) and 1.32±0.05 nm (syndiotactic). Previously, a value of 1.11 ± 0.03 nm (united atom model, unspecified tacticity) has been reported. 68 For 10-mer PDAC chains, we obtain an Rg value of 1.00 ± 0.09 nm under the same conditions as for the PSS chains.

Experimental methods and materials Materials PDAC (Mw = 200000 − 350000 g mol−1 , 20 wt% in water, Sigma Aldrich) and PSS (Mw = 500000 g mol−1 , powder, Scientific Polymer Products) were used as received. Sodium chloride was used to adjust the salt molar ratio. The water used in all experiments was 18.2 MΩcm (Milli-Q water).

Preparation of free-standing layer-by-layer assemblies PDAC and PSS solutions were made from their respective homopolymers and water at a concentration of 1 g/L. The automated slide stainer (HMS series, Carl Zeiss, Inc.) was employed to fabricate the LbL assemblies. Teflon substrates were cleaned using sonication for 15 min in methanol and then 15 min in water. The substrates were dried using highvelocity nitrogen. Teflon substrates were dipped in PDAC solution for 15 min, followed by three separate rinses with water for 2, 1, and 1 min, respectively. 29 The substrates were then dipped in PSS solution for 15 min, followed by another series of water (with matching NaCl concentration) rinses as before. Two NaCl concentrations, 0.5 M or 1.0 M, were used for all baths. This was repeated for 140 cycles to generate a PDAC/PSS multilayer of 140 layer pairs. Freshly prepared LbL films were dried in ambient air overnight and then in vacuum oven at 30 ◦ C for 24 h. The films were stored in a beaker sealed with Parafilm. Before MDSC 9 ACS Paragon Plus Environment


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characterization, the films were dried again at 115◦ C for 30 min under nitrogen purge.

Modulated differential scanning calorimetry (MDSC) measurements MDSC was performed on hydrated PDAC/PSS multilayers using a heat-cool-heat-cool cycle model. The sample size was ca. 10 mg. Tzero pans and hermetic lids were used for hydrated samples. The hydration content varied from 16.0, 20.6, and 24.8 wt% water of matching NaCl concentration. The thermal program consisted of ramping from 5 to 115◦ C at a rate of 2◦ C min−1 , amplitude of 1.272◦ C for 60 s, followed by cooling and heating in the same manner.

Results and discussion Characterization of the thermal transition In our previous work, 42 we reported that the thermal transition of PDAC/PSS assemblies was characterized by a significant increase of water mobility, which originated from the decrease of hydrogen bond life-time in the water-PE hydrogen bonding network as temperature rises. In this work, our focus is on mapping the role of hydration and ions in the plasticization of PDAC/PSS assemblies. As a comparison, we first discuss the appearance of the thermal transition in our previous work and here. In Ref. 42, the thermal transition was visible in simulated PDAC/PSS complexes with 4PDAC25 − 4PSS25 composition and 18 wt% water as a sudden increase of water mobility which manifested as a kink in the water diffusion coefficient plotted as a function of temperature. Furthermore, water, PSS, and PDAC chain mean square displacement (MSD) underwent a sudden increase at the corresponding temperature with water initiating the MSD increase. The discontinuity of MSD at a temperature T was interpreted as the Ttr . Figure 2a shows the mean diffusion coefficient of water in the 10PDAC40 /10PSS40 system with 18 wt% hydration and salt molar ratio of 1.5. This system is both larger and more 10 ACS Paragon Plus Environment

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D

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(a)

(b)

Figure 2: (a) Averaged water diffusion coefficient D and (b) the averaged density of the 10PDAC40 /10PSS40 simulation system as a function of temperature at salt molar ratio 1.5. The inset in (a) shows the water diffusion coefficients of individual simulations. The corresponding pure water diffusion coefficient in the simulation model is provided in the Figure S1.

interpenetrated than the system in Ref. 42. The averaged diffusion coefficient data shows a smooth transition; fitting linear portions to the low-T and high-T regions provides an interception point of the extrapolated linear portions at T = 340 K. However, a transition in this data set is not as clearly visible as is in Ref. 42. Yet in individual samples, the data show more visible kinks also for the larger system, see the inset of Figure 2a. This results from the system constructs of this work being larger and more interpenetrated than those of Ref. 42 which also leads to more homogeneous water distribution than in Ref. 42; averaging over multiple widely different initial configurations smoothens the discontinuity in water diffusion. The water diffusion data here bears resemblance to experimentally measured ion diffusion in Ref. 34. At the molecular level, the more homogeneous water distribution in a system where the PEs and the salt are partially hydrated spreads the distribution of the hydrogen bond strengths and consequently their expected breakage in the system. On the other hand, an aqueous pore has more hydrogen bonds of similar strength. This means that, if the thermal



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transition Ttr is associated with a decrease of hydrogen bonding life time, the presence of aqueous pores could enhance the visibility of the transition. Indeed, these observations provide microscopic insight to the reasons why the transition is often hard to capture using MDSC and similar testing methods, see e.g. Refs. 24,26. Our findings lead us to speculate that the strength of the transition is influenced by heterogeneity in the water environment. It is beyond this work, but we speculate there is an optimal structure heterogeneity range for Ttr . This range is dependent on the PEs, degree of hydration, and added salt (extrinsic charge compensation). Figure 2b shows the density of the simulation cell. The density of the simulation cell averaged over all the simulation runs of the larger 10PDAC40 /10PSS40 system presents a clear change of slope within the same temperature range. This means the averaged cell volume experiences a sudden change. Indeed, a decrease of density is a typical feature of glass transitions in amorphous polymers. 69 Like amorphous polymers, plasticizer molecules such as water here are thought to be embedded between PE chains increasing the free volume. 69 Therefore, the free volume theory of glass transition in amorphous polymers sheds light on the role of water in plasticization of this PE assembly, and associates the thermal response at Ttr with a decrease of density. The systems examined in Ref. 42 showed no such change of density as visible in this work. This is because a comparable change of system volume with the temperature is masked by fluctuations in smaller systems. The larger, more interpenetrated simulation system enables capturing this feature. Besides the diffusion data in the simulations, this decrease of density also appears to be another robust means of capturing the Ttr , given that the system size is sufficient to show the change. The interception point of the linear fits to the simulations data both for water diffusion and density are higher than the Ttr of layer-by-layer assemblies at similar water hydration level in thermal characterization experiments. This could reflect the elevated salt content in the simulations. Furthermore, the sample size, examined configurations, and the employed model can bias the transition to higher temperatures in the simulations.


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The data of Figure 2 and comparison to Ref. 42 reveals that the change involved in the thermal transition of hydrated PE assemblies depends on the assembly structure: Our earlier work on PDAC/PSS assemblies 42 established that local dehydration drives the thermally induced change in plasticization of PDAC/PSS assemblies. Here, we continued and extended that work. In line with Ref. 42, a sudden increase in the water diffusion of the PE assemblies upon increasing temperature was observed also in this work in individual simulations. On the other hand, an increase of the system density at a corresponding temperature is observed in this work. In Ref. 42, we noted that the mechanism of the transition bears resemblance to lower critical solution temperature (LCST)-type transitions in which entropic considerations drive dehydration; a density increase is associated with LCST transitions. 70

Salt influence on PDAC/PSS charge compensation and water diffusion Figure 3a presents the average number of PDAC-PSS contacts measured as PDAC N – PSS S atoms within direct interaction range, as well as, PSS sulfonate group oxygen O hydration measured by direct contacts with water molecules as a function of salt molar ratio x at 300 K. The plotted contact number count is the average number of PSS S atoms and PDAC N atoms or PSS sulfonate group O atoms and water hydrogens Hw within the binding distance cut-off of each other. The figure shows that when no additional salt is present, the S − N contact number count is 2.7. The PDAC/PSS monomer ratio in the system is 1 : 1 so having a contact number count greater than 1 indicates that the charged groups coordinate with multiple oppositely charged groups; the PE-PE ion pairs have a 3D packing order instead of coordinating one-to-one. Typically, a 1 : 1 ion pair formation between the oppositely charged functional groups in the PEs is assumed in experimental research. 24,26,36 Increasing the salt molar ratio x in the PE assemblies decreases the direct S − N contacts significantly. As shown in Figure 3a, the S − N contact number count decreases from 2.7 to 2.4 when salt molar ratio increases from 0 to 0.25. With further addition of salt, the 13 ACS Paragon Plus Environment


average number of S − N contacts decreases linearly. In contrast, the no-added salt system deviates from the linear relation, which may indicate a threshold for which salt breaks ionic

D/D0

crosslinks, as has been argued in e.g. Ref. 49.

S-N O-Hw Contact number count

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a)

x

b)

Figure 3: (a) Salt influence on PDAC/PSS S − N contact number count (PE-PE ion pairs) and PSS sulfonate group O atom – water hydrogen Hw contact number count (PSS sulfonate group hydration) at T = 300 K. (b) Relative water diffusion coefficient D/D0 as a function of temperature T for systems with different salt molar ratio. The pure water diffusion coefficient D0 is presented in the SI.

In salt-induced plasticization of PE complexes discussed, e.g., in Ref. 25, the amount of salt blended in the PE complexes measured by thermal gravimetric analysis corresponded to a salt molar ratio much lower than x = 1. In the current work, we examined salt molar ratios up to x = 1.5 with relatively lower water content than we assume the PE complexes to have (water content is around 40 wt% in Ref. 25). In our work, the examined water content combined with the highest salt molar ratio results in excessive and, in some sense, artificial salt compensation of the polyions in the polyelectrolyte assemblies. As a consequence of this and a possible enhanced propensity for salt crystal formation in classical NaCl force-fields, 71 small salt crystals were observed in the simulated systems at the higher salt molar ratios. For the salt molar ratio 1.5, the presence of these crystals reduces the number of free salt ions which affects the absolute values presented.


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When mapping the effect of salt molar ratios x, the absolute water amount was maintained constant. This allows us to examine the effect of salt ions on the hydration of polyions in the PE complexes. In Figure 3a, with an increasing salt molar ratio x, the number of O − Hw connections (between the PSS oxygens O and water hydrogens Hw ) show a similar non-linear decline, suggesting a decrease in the PSS hydration level. In Figure 3b, the diffusion of water in the hydrated PE complex system is shown as a ratio of water diffusion coefficient D in the PE complex system and the diffusion coefficient of pure water D0 . Water diffusion in the PE assembly increases with increasing temperature. The range of D/D0 calculated for the system at different temperatures in Figure 3b suggests water in the PDAC/PSS system at all studied salt contents is significantly confined in comparison to bulk water. Moreover, increasing the salt molar ratio in the system causes a significant drop of water mobility with increasing the salt molar ratio from x = 0 to 0.5 resulting in the most prominent decrease of water mobility. An increase of the salt molar ratio from 0.5 to 1.0 has less effect but a further increase to 1.5 salt molar ratio decreases the mobility significantly. The observed decrease of water mobility with the addition of salt ions suggests that the presence of salt suppresses the water mobility and water becomes less efficient in decreasing the friction of PE chain motion in the system. In simple terms, if the surrounding water cannot move, PE chain motion is pinned. For prior considerations of this pinning, see e.g. Refs. 72–74. At the same time, salt can also enhance PE mobility (and the thermal transition) via breaking the polyion pairs. This breaking of polyion pairs and its chemistry specificity has been discussed further in Refs. 75,76. Therefore the above analysis of S − N contacts, PSS group hydration, and the drop of water mobility with increasing salt content reveals that salt ions have dual effect on plasticization of PDAC/PSS assemblies. In particular, (1) the additional salt ions decrease the number of intrinsic ion pairs in PDAC/PSS assemblies. This weakens the binding between the chains (plasticizes the assembly) and also lowers the barrier for thermal transition. At the same time, (2) the presence of salt ions redistributes the water molecules as salt ions attract water into their hydration shells. This



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reduces the plasticization by water and thus can be viewed as a negative contribution to the plastification. In order to elucidate this via experiments, we performed MDSC measurements on PDAC/PSS multilayers assembled at 0.5 M or 1.0 M NaCl salt and hydrated with water of matching salt concentration in the samples. This maintains a controlled hydration level while changing salt concentration. Figure 4 shows that an increasing water content decreases significantly Ttr at both assembly salt concentrations. This confirms the prominent role of water in the plasticization of the assemblies. However, Ttr appears relatively insensitive to the change in the salt concentration of the assembly solution: the mean Ttr values show a Ttr response that follows the simulations prediction of salt increasing the Ttr at low water content and decreasing at higher water content, but the data is inconclusive as the error estimate covers the range of difference in Ttr at all examined water contents. The data confirms salt does not have a major plastification effect and points toward salt having a dual effect of salt on the plasticization response. Additionally, the data of 24.8 wt% hydrated sample points to this system having sufficient water to counter the effect of water binding by the added salt ions. At high-enough water content, the water in the system becomes the dominating factor in dictating the plasticization response and the properties of the material. For this system, sufficient hydration to counter the effect of 1.0 M NaCl appears to be 24.8 wt%. Due to the sensitivity of PDAC/PSS assemblies to water content in the system, see for example Ref. 36, the effect of salt redistributing water can be easily countered by increasing water content jointly with the addition of salt. We emphasize that this is usually the case in experimental work: salt doping is typically accompanied by increased water content, see e.g. Refs. 22,25,26. The additional water brought into the PE assembly by the salt ions leads to countering the negative influence of salt on the plasticization; one can expect the water slowdown by salt to be significant only at incomplete hydration and to depend on the amount of salt. As a consequence, the properties of the material (such as thermal transition and elastic modulus) are likely mainly dominated by the change in water content upon changing


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Figure 4: Thermal transition temperatures Ttr for PDAC/PSS LbL films assembled at 0.5 M or 1.0 M NaCl.

the salt content. In this light, it is quite expected that with a much higher water content, such as 37 wt% to 41 wt% as in the work of Shamoun et al., 26 the negative contribution of salt to thermal plasticization is not visible. Interestingly, PDAC/PSS complexes have been found by DSC characterization to bear lower thermal transition temperatures Ttr at lower hydration (hydration between less than 10 wt% of water to extreme dehydration) when free salt in the PE complexes was separated by dialysis beforehand. 41 This is in agreement with the observations presented there. On the other hand, our earlier work 29 where MDSC is used to characterize PDAC/PSS multilayers with a 12 wt% water content does not show Ttr dependency with increasing assembly bath salt concentration. 29 To further study the effects of salt on water distribution in the system, we next examine the water binding with PSS and the ions through the number density distribution. Figure 5 shows the number density distribution ρg(r) of water hydrogen atoms Hw around PSS O atoms at different salt concentrations. Here ρ is the global number density and g(r) the radial distribution function (RDF). The first peak corresponds to the water layer that is in direct contact with the O atoms; it reflects the level of PSS hydration. The density



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distributions of Figure 5 indicate similar close-range hydration as in dilute PSS solutions. The position of the binding shells at different salt concentrations overlaps but the amount of water directly bound by the PSS O atom decreases with the addition of salt. This indicates that the absolute amount of hydration water for PSS decreases because salt is hydrated. Indeed, Figure 6a presents the average number of water molecules forming hydrogen bonds with the PSS O atom and shows PSS hydration decreases significantly with increasing salt molar ratio. An increase of salt molar ratio from 0 to 0.5 causes a 0.25 decrease in the mean number of water molecules bonded to PSS. With higher salt molar ratio, the decreasing trend persists and the same pattern is seen in the influence of salt on the hydrogen bonding life time of PSS-water bonds in Figure 6b. At a given salt molar ratio, the number of hydrogen bonds per PSS O atom shows a smooth decrease with increasing temperature. This sensitivity decreases for a higher salt molar ratio. The relative hydration of the salt ions is presented as supporting information in Figure S3 via the respective number density distribution functions calculated for the salt ions and water. The data of Figure 6b shows that the corresponding hydrogen bond relative life times decrease with increasing temperature. This indicates an increase of water mobility. However, addition of salt makes the hydrogen bond life time longer. This signifies the hydrogen bonds formed between water and the polyion charged group are stronger when N a+ ions are around: the N a+ ions take those water molecules that have weakest binding with the polyion to their own hydration shells which leaves the more strongly bound water in the PE hydration shell. Notably, the hydrogen bonding life time in these assemblies is much longer than in dilute solutions due to confinement and locally low hydration in the former. For comparison, Table 2 shows the water binding to the salt ions is more persistent than water binding to PEs or the water-water binding. The data also shows a consistent increase of water dynamics with increasing hydration. Interestingly, slow-down of water dynamics significantly influencing the characteristics of the system has been reported in dehydrated soft matter systems both experimentally, see e.g. Refs. 77,78 as well as computationally, see e.g. Refs. 79,80.


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Figure 5: Number density distribution functions ρg(r) calculated for PSS O - water hydrogen Hw . Here ρ is the global number density and g(r) the radial distribution function (RDF) of O − Hw at different salt molar ratios x at temperature T = 300 K. Table 2: Relative life time of water-salt (N a+ − Hw and Cl− − Ow ), water-PSS (O − Hw ), and water-water hydrogen bonds in ps between different atoms or groups at 300 K. The salt molar ratio x is 1.0. Water N a+ − Ow content 12 wt% 1483 19 wt% 1195 26 wt% 688

Cl− − Hw

O − Hw

1112 798 299

202 193 166

WaterWater 156 144 97

The influence of hydration and salt diffusion on PE plasticization So far, we have explored the significant influence of varying salt content on the PDAC/PSS ionic pairs and on the hydration of the system components, as well as, the difference in bonding characteristics. Next, we investigate the system response to changing water content while maintaining the constant salt molar ratio x = 1.0. Three different water contents are examined, 12, 19 and 26 wt%. The water content is expected to lead to significant changes in the mechanical and thermal properties of PDAC/PSS assemblies. The corresponding structural explanation of such changes at the microscale could be a decrease of PDAC/PSS ionic pair number. As shown in Figure 7a, the number of PDAC N atoms within the binding cut-off distance from a PSS 19 ACS Paragon Plus Environment

(a)

(b) Na+

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Hydrogen bonds per PSS O atom


(c) Figure 6: (a) Number of hydrogen bonds formed by water and PSS O atoms at 18 wt% hydration and (b) corresponding hydrogen bond life time as a function of temperature at varying salt molar ratios. (c) Fraction of water molecules in the first binding shell of N a+ ions (first peak of RDF) as a function of temperature at different salt molar ratios x.

S atom (S − N contacts) decreases as the water content increases. The constant salt molar ratio and the constant water conditions data in Figure 7a and 3a show that salt ions are in dominating role, rather than water, in controlling the number of polyion-polyion pairs in the system. On the other hand, Figure 7b shows that a significant increase of water mobility at salt molar ratio 1.0 is obtained at the 26 wt% hydration in comparison to the lower hydration: at any temperature throughout the examined range, the increase of D/D0 resulting from increasing the water concentration from 19 wt% to 26 wt% is almost double


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in comparison to the change resulting from increasing the water concentration from 12 wt% to 19 wt%. Furthermore, Figure 7c suggests that the number of water molecules in the vicinity of PSS and in direct contact with N a+ ions increases almost monotonically with increasing water content. This means that the components of the system are incompletely hydrated at the lower water contents. The observation is a direct sign of both the PE chains and the ions having increasing access to water with increasing water content in the system but it also signifies the build-up of hydration shells as well as a water hydrogen-bonding network in the system. In compliance with the prior deduction this data also shows that the effect of confinement of water by the higher salt molar ratios is countered by increasing the water content in the system. To further characterize the system, we calculated the average exchange time of water (water hydrogen bond life time) at different binding locations in the system. As Table 2 shows, the exchange time of water bound to salt ions is much longer than that of water bound to PSS O atoms or that of water-water hydrogen bonding. In general, the water binding dynamics (relative hydrogen bonding life times) reflect the partial charge distribution in the molecules and the chemistry of the components in the system. As the water content increases from 12 to 19 wt%, the relative life time of N a+ −Ow bonds decreases from 1483 ps to 1195 ps at salt molar ratio x = 1.0. The bond life time continues to decrease with increased hydration and is 688 ps at 26 wt% water content, indicating significantly faster dynamics. Cl− − Hw , PSS-water and water-water bonds behave similarly, that is, the relative life time of the bonds decreases with increasing water content. However, they exhibit much faster dynamics than the N a+ − Ow bond (the life time is shorter). Similar trend is observed for water bound to PEs. Regardless of the examined salt molar ratio, the salt ions are located close to the ionized groups of PSS and PDAC, see Figure 8. N a+ ions condense very close to PSS O atoms, while Cl− ions distribute around PDAC N atom with lower peak values, suggesting Cl−


D/D0

(a)

(b) Na+-Ow O-Hw

Contact number count

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PDAC N - PSS S contact number count


(c) Figure 7: (a) Average number of PDAC N atoms around PSS S atoms and (b) the corresponding relative water diffusion coefficient D/D0 as a function of temperature for systems of different wt% hydration. The open symbols present data at salt molar ratio 1.0 and the tip-down triangles data at 19 wt% hydration with different salt molar ratios. (c) Average number of water molecules in direct contact with N a+ ions or hydrogen bonded to PSS O atoms at salt molar ratio 1.0 as the function of water content in the system.

is more spread out and mobile. The first peaks of density distribution, i.e., the contact layer of O − N a+ and N − Cl− , of different water contents overlap, but the decrease of the number contacts (insets Figure 8) indicates that the corresponding numbers of salt ions directly bound to the O or the N atom are decreasing with increased hydration. The stronger condensation of N a+ cations in comparison to Cl− anions results from the hydration characteristics of the ions and the ionic groups, as well as, their charge distribution. This


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Number of Cl- per N

r

(a)

(b)

r

Figure 8: Number density distribution ρ g(r), where ρ is the global number density and g(r) the radial distribution function (RDF), of (a) O − N a+ and (b) N − Cl− at 300 K for different water content. Salt molar ratio x = 1.0. The insets show the number of salt ions (N a+ and Cl− ) in the first binding shell of PSS O atom and PDAC N atom.

difference in the ion condensation could, on a molecular level, provide an explanation for the different physical behavior of PDAC/PSS multilayers depending on the termination layer, see e.g. Refs. 29,81,82. Dsalt/D0

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Number of Na+ per O

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12% Na 19% Na 26% Na 12% Cl 19% Cl 26% Cl

Temperature (K)

Figure 9: Salt ion diffusion coefficient ratio (Dsalt /D0 ) as a function of temperature at different water contents in the PE assembly at salt molar ratio of 1.0. Dsalt is the salt ion diffusion coefficient and D0 the pure bulk water diffusion coefficient.

As the salt ions bind rather strongly with the polyions, the simulations do not provide 23 ACS Paragon Plus Environment


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reasonable statistics for calculating a life time of the salt ion - polyion bonding. To characterize the mobility of the salt ions, we present the averaged diffusion coefficient of the salt ions, Figure 9. The figure shows that the diffusion coefficient of salt ions increases with increasing temperature and water content. At higher hydration, adequate water supply relieves the competition for water between the polyions and the salt ions and leads to an increase of mobility for both. The observed diffusion coefficients of salt ions in the simulations are less than 1/10 of water diffusion, see Figure 7b. This is actually quite expected as each ion drags water in its hydration shell. Cl− is more mobile than N a+ as the hydrated radius of Cl− is smaller 83 and also because PSS charge is more localized at the sulfonate group atoms than PDAC charge which leads to weaker ionic binding to PDAC.

Conclusion In this paper, we examined water and simple salt plasticization of hydrated polyelectrolyte PDAC/PSS assemblies by molecular dynamics simulations and MDSC characterization. The findings can be summarized by (1) the change involved in the thermal transition of hydrated PE assemblies depends on the assembly structure and (2) salt has a dual nature in plasticization of PE assemblies (salt acts both as a plasticizer via breaking PE-PE ion bonds and against the plasticization via influencing water structure and dynamics). On one hand, salt ions weaken PE-PE ion pairs and thus facilitate the motion of the chains. On the other hand, salt ions bind water decreasing the lubricating effect that mobile water has on PE chain motions. Depending on the salt ion concentration and degree of hydration of the system, this can suppress the plasticizing effects of water, especially at lower hydration levels. In summary, our work provides the first systematic mapping of role of hydration and simple ions on the PE plasticization response. The findings set the amount of salt, chemical species, and water as keys to guide PE assembly thermal plasticization. Additionally, the dual nature of salt in these systems could enable enhanced utilization of the saloplastic response.


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Acknowledgement This work is supported in part by Academy of Finland and the National Science Foundation (Grant No. 1312676). Computational resources from the CSC IT Centre for Science (Finland) are gratefully acknowledged. The authors thank Dr. Piotr Batys for useful discussions.

Supporting Information Available 1) Pure water diffusion coefficient D0 in the simulation model. 2) Mean square displacement (MSD) of PDAC and PSS molecules. 3) Ion hydration expressed by number density distribution functions.

This material is available free of charge via the Internet at

http://pubs.acs.org/.

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