Molecular Origin of the Glass Transition in Polyelectrolyte Assemblies.

Water plays a central role in the assembly and the dynamics of charged systems such as proteins, enzymes, DNA, and surfactants. Yet it remains a challenge to resolve how water affects relaxation at a molecular level, particularly for assemblies of oppositely charged macromolecules. Here, the molecular origin of water's influence on the glass transition is quantified for several charged macromolecular systems. It is revealed that the glass transition temperature (Tg) is controlled by the number of water molecules surrounding an oppositely charged polyelectrolyte-polyelectrolyte intrinsic ion pair as 1/Tg ∼ ln(nH2O/nintrinsic ion pair). This relationship is found to be "general", as it holds for two completely different types of charged systems (pH- and salt-sensitive) and for both polyelectrolyte complexes and polyelectrolyte multilayers, which are made by different paths. This suggests that water facilitates the relaxation of charged assemblies by reducing attractions between oppositely charged intrinsic ion pairs. This finding impacts current interpretations of relaxation dynamics in charged assemblies and points to water's important contribution at the molecular level.

Introduction

Charged assemblies bearing opposite or complementary charges span natural (proteins, enzymes, DNA)[1−4] to synthetic materials (surfactants, synthetic polyelectrolytes).[5,6] Assembly is facilitated by electrostatic attraction and entropic release of counterions, and most often occurs in aqueous media. These systems can range from solid-like to liquid-like, characteristic of the system relaxation dynamics. Similar to neutral macromolecules, charged assemblies can possess a Tg, which demarcates the glassy and rubbery states of the assembly. Notably decades ago, Michaels described synthetic polyelectrolyte complexes as brittle when dry but “leathery or rubberlike” when wet, which points to the strong effect of water on the mobility of a charged assembly.[7] Yet still there is not a clear view of how water facilitates the relaxation or, more specifically, controls the Tg of charged assemblies. To investigate this, we focus specifically upon synthetic polyelectrolyte complexes (PECs).[8−10]

The formation, structure, and properties of PECs are influenced by both the chemistry of the starting materials and the external conditions.[11−19] The resultant structure of the PEC, or for any charged assembly in general, is described in Figure . Polycations and polyanions assemble by forming intrinsic ion pairs, and the remaining unbound charged groups are compensated by small counterions to form extrinsic ion pairs. As this usually occurs in aqueous media, water molecules surround various ion pairs in hydration layers. The extent of intrinsic ion pairing influences various physicochemical aspects such as conductivity, self-healing, and mechanical modulus.[16,20,21] Ion pairing also influences the Tg, which in turn defines their processability and applications as thermally responsive materials.[22,23]

Figure 1

Schematic of a charged polyelectrolyte assembly. The charged polyelectrolyte assembly contains two different types of ion pairs: intrinsic and extrinsic. Intrinsic ion pairs are polycation–polyanion pairs, and extrinsic ion pairs are polyelectrolyte–counterion pairs.

Glass-transition-like thermal events have been identified in hydrated PECs and their counterparts, polyelectrolyte multilayers (PEMs). For PEMs, which are characterized more widely for their thermal behavior, the glass transition has manifested as a change in microcapsule or microtube size, heat capacity, mass, modulus, or impedance.[24−27] In all of these cases, water was present so as to achieve enough mobility to observe a Tg. The strong correlation between PEC/PEM physical properties and water has been noticed elsewhere.[28−32] These effects are generally attributed to water plasticization and a lowering of the Tg, but the underlying mechanism needs further explanation.[21,33] This is because PECs possess mixed ion pairing character (intrinsic and extrinsic), and the location of water molecules and their states have not been fully quantified.[34]

Recently, we experimentally examined the thermal behavior of weak polyelectrolyte complexes consisting of poly(allylamine hydrochloride) and poly(acrylic acid) (PAH and PAA) as a function of pH (i.e., intrinsic ion pairing density) and water content. Glass transition-like behavior was observed, and the Tg was labeled as a generic thermal transition (Ttr) since its mechanism was unclear. A correlation was speculated that connected Tg (or Ttr) to the molar ratio of water to intrinsic ion pairs.[35] Our complementary simulation work for a different PEC system suggested that the observed thermally induced events were initiated by the changing dynamics of water molecules instead of a traditional glass transition event.[36] However, as this was only one study, it was not clear if this could be applied to other charged assemblies and even those made by different means.

Here, the Tg of synthetic PECs consisting of strong polyelectrolytes is correlated to the ratio of water molecules to intrinsic ion pairs. Strong polyelectrolytes are not sensitive to pH; rather, their intrinsic ion pairing is controlled by salt concentration. This is important as the prior correlation was only sensitive to pH, and here connections to salt concentration are made. Synthetic PECs consisting of poly(diallyldimethylammonium) and poly(styrenesulfonate) (PDADMA and PSS) are investigated using neutron activation analysis (NAA), modulated differential scanning calorimetry (DSC), and molecular dynamics (MD) simulation (see Supporting Information, Figures S1 and S2, Tables S1–S3). Together, these give a complete charge balance on the system, specifying not only the composition, but also the doping level and the intrinsic/extrinsic ion pairing. With this information, the Tg is then compared against the water/intrinsic ion pair ratio. Interestingly, it is shown that the same correlation holds true for three separate systems: PDAMDA-PSS PECs, PDADMA-PSS PEMs, and PAH–PAA PECs. This spans weak and strong polyelectrolytes, as well as multilayers and complexes, which indicates the general role of water in controlling the glass transition in polyelectrolyte assemblies.

Results and Discussion

To fully correlate the Tg with the physical structure of the PEC, it is necessary to do a charge balance on the system, from which the doping level can be deduced. The doping level is an expression of the extrinsic ion pairing, whereby the process of doping with salt ions destroys intrinsic ion pairs and creates extrinsic ion pairs. Figure shows the compositions of PECs prepared and isolated from stoichiometric (1:1 by repeat unit) mixtures of PDADMA and PSS in the presence of various salt concentrations (see also Supporting Information, Figure S3). From NAA, the PDADMA and PSS compositions were calculated, Figure a. For PECs at all salt concentrations, PDADMA was in excess at 52–55 mol %, which is attributed to differences in linear charge density and hydrophilicity of PDADMA and PSS.[15] It is likely that uncomplexed polyelectrolyte was removed during the centrifugation step. This nonstoichiometric composition has also been observed elsewhere using scintillation counting.[37]

Figure 2

(a) PEC composition based on PDADMA and PSS repeat units for PECs prepared at varying NaCl concentrations measured. (b) Doping levels, for which y– and y+ are the molar ratios of Na+ to PSS and Cl– to PDADMA, respectively. The error bars represent the uncertainties (95% confidence level).

The salt doping process of PECs is represented in the Supporting Information, eq S1. Due to the observed nonstoichiometry of the PECs, the salt doping level y, indicating the fraction of extrinsic sites, in Supporting Information, eq S2, was divided into cationic and anionic doping level contributions:

NAA allowed for the estimation of these doping levels y and y (Figure b) by measuring the elemental Na+ and Cl– concentration in the PEC. If all Na+ and Cl– ions are assumed to participate in extrinsic ion pairing, then y may be considered equivalent to the fraction of extrinsic polyelectrolyte repeat units. The doping level, y and y, generally increased with increasing complexation salt concentration up to 1 M. A complexation concentration of 1.5 M NaCl resulted in a decreased doping level, but the meaning of this particular result is circumspect on account of the extreme softness of the material and the tendency of the PEC to lose salt during the isolation procedure. In all PECs, y+ was greater than y, which can be explained by the excess PDADMA in the PECs. The trend observed here using NAA most closely mimics that observed using inductively coupled plasma mass spectrometry (ICP-MS) and conductivity, Figure S4.[21,38]

The effect of hydration on the PEC’s Tg was examined using modulated DSC, Figure a. The inflection point of the sigmoidal reversing heat flow response was assigned as Tg. For a fixed complexation salt concentration of 0.5 M, Tg decreased from 387 to 300 K as water content increased from 16 to 32 wt %. An enthalpic relaxation peak in the nonreversing heat flow curve was also present, Figure S5a. For comparison, a dried PEC exhibited no remarkable thermal features, Figure S5b.

Figure 3

Reversing heat flow curves for PDADMA–PSS PECs of (a) varying water content and fixed 0.5 M NaCl complexation concentration, and (b) varying NaCl complexation concentration and fixed water content of 24 wt %. (c) Tg as a function of the molar ratio of water molecules to intrinsic ion pair in hydrated PDADMA–PSS complexes prepared from solutions of different NaCl concentrations. (d) Linear fitting of ln(nH/nintrinsic ion pair) vs 1000/Tg (dotted lines). For (a) and (b), second heating scans are shown with “exotherm down”, heating at 2 K·min–1, amplitude of 1.272 K for a period of 60 s. The legend in (c) also applies to (d). The left y-axis applies to PDADMA–PSS, and the right y-axis applies to both PDADMA–PSS and PAH–PAA. PAH–PAA data are from the authors’ previous work.[35] (e) ln(nH/nintrinsic ion pair) vs 1000/Tg for PDADMA-PSS polyelectrolyte complexes (PECs, black circles) and polyelectrolyte multilayers (PEMs, pink circles).

Water contributes to PEC plasticization via (1) enhancing free volume which facilitates polymer chain motion,[33,39−42] (2) lubricating the polymer chains by decreasing the internal resistance of polymer sliding motions,[33,43] and (3) decreasing electrostatic attractions between polyelectrolyte intrinsic ion pairs. Whereas the first two effects are associated with traditional plasticization,[39,42] the last effect in polyelectrolyte complexes and assemblies makes water a nontraditional plasticizer, analogous to salt. Zhang et al. observed that the direct contact number of intrinsic ion pairs decreased as water content increased.[41] Elsewhere, organic alcohol solvents (n-butanol, ethylene glycol, 1-propanol, and propanediol) bore no plasticization effect on PECs or PEMs.[35,36]

Figure b shows the response for PECs of varying complexation NaCl concentration at a fixed hydration level of 24 wt %. Tg decreased from 345 to 325 K as the NaCl concentration increased from 0 to 1.5 M, indicative of a salt-plasticization effect. Doping has an intense local effect on screening between intrinsic ion pairs, which decreases physical cross-linking and increases polymer mobility.[44−47] This is consistent with the trend in doping levels displayed in Figure b.

The effects of hydration and salt on the Tg are summarized in Figure S6. As shown, the effect of salt doping on the Tg is much less pronounced than the effect of hydration. This emphasizes the general trend that Tg decreases with increasing water content and complexation NaCl concentration, confirming the plasticizing effect of water,[32,33,35] and also the effects of salt doping, which breaks intrinsic ion pairs and lowers the Tg.[21,41] For example, salt-time superpositioning of PECs has demonstrated strong effects on dynamic behavior.[48] The question is, then, how water and doping are together quantifiably interrelated to the glass transition.

Recently, the molar ratio of water molecules to intrinsic ion pairs has been suggested to control Tg (or Ttr)[35] in PAH–PAA complexes. Also, water and salt influence PAH–PAA complexation behavior.[49] To explore water–salt–temperature relations for the PDADMA–PSS system, Tg values for PDADMA–PSS complexes were plotted against the ratio of water molecules to intrinsic ion pairs , Figure c. The water content nH was taken as the controlled amount of water added to the system, and nintrinsic ion pair was taken from the charge balance and the doping level. This ratio can also be expressed in terms of the doping level y or y, depending on which species is a minority. Here, the molar ratio of water to intrinsic ion pairs becomes . Remarkably, the Tg values all collapsed into a single master curve for all hydration levels and salt concentrations examined.

Figure d shows the linearization of this master curve in the form of ln() = 1.30 × (1000/Tg) – 1.98 (R2 = 0.984). This relationship provides an energy of −10.8 kJ·mol–1, which is close in value to the van’t Hoff enthalpy ΔH associated with the disruption of one O–H···O unit (10.5 ± 2.5 kJ·mol–1).[50] This possibly suggests a link between the glass transition and hydrogen bond distribution in PECs. The enthalpy is also similar to values obtained from the proton conductivity of polymer membranes and Nafion nanofibers.[51,52] The successful collapse and linearization of Tg values for PDADMA–PSS complexes is notably similar to that observed in PAH–PAA complexes, shown in Figure d for comparison, with an energy of −10.5 kJ·mol–1.[35] The slopes and energies for both types of complexes are similar, which further suggests the generalized role of water in the relaxation. It is curious that the y-intercepts for the linear fits in Figure d are different for PDADMA-PSS and PAH–PAA PECs. The difference may be attributed to the differences in charged group size or ion pair size (e.g., PDADMA repeat unit is larger than PAH repeat unit), linear charge density, and/or the water distribution around the charged groups between these two PEC systems. The significance of the y-intercept is a focus of ongoing work.

These results show generalized behavior between two very different types of PECs (strong vs weak), yet there still remains the question of if this behavior is similar for charged assemblies made by a different path. To examine this, we performed the same analysis for PDADMA-PSS PEMs made using the layer-by-layer assembly technique. Whereas PECs are made here by the rapid mixing of polycation and polyanion solutions, PEMs are made by the alternate and sequential adsorption of polycations and polyanions from solution to a surface. PEMs can possess stratified or mixed structures,[53,54] and the assembly conditions here were selected so as to give a more mixed structure within the layers. PDADMA-PSS PEMs were exposed to various salt concentrations and water contents, and their Tg’s were measured. Elemental analysis was performed as before to obtain the doping level and intrinsic/extrinsic ion pair content. The results, plotted in Figure e, show the nearly perfect overlay of PEM and PEC behavior. The first heating scans from PECs and PEMs also show the same results (Figure S7). Despite being formed by very different methods, the manner in which water controls the Tg appears to be essentially the same, pointing to similar local environments.

Therefore, this master curve identifies a unifying parameter, namely, the molar ratio of water to intrinsic ion pairs, that controls the Tg in charged assemblies and reflects the roles of both water and ionization. The Tg for PECs and PEMs is concluded to follow the general relationship:

Molecular dynamics (MD) calculations in all-atom detail were performed to further investigate the water distribution and molecular interactions that give rise to the behavior at the Tg. Figure displays a simulated PEC structure and the visualizations of extrinsic and intrinsic ion pairs corresponding to 1.5 M NaCl concentration and 30 wt % water. The magnification of an extrinsic pair site shows a Na+ counterion surrounded by multiple charged PSS groups, demonstrating a three-dimensional packing configuration. Due to this three-dimensional packing character, a specific polyelectrolyte site can bear both intrinsic and extrinsic charge compensation character, see Supporting Information. Figure S8 compares the mole fraction of intrinsically compensated PSS groups calculated from simulations and experiments; the generally good agreement between the experimental and simulation approaches supports the validity of the quantitative calculation in Figure .

Figure 4

Representative snapshot of a hydrated PDADMA–PSS complex, prepared from 1.5 M NaCl solution first and then hydrated with 30 wt % water and its extrinsic (left) and intrinsic (right) ion pairs.

The distribution and location of water molecules in the PECs were further investigated. The analysis focused on the PSS group because our recent work on PDADMA/PSS assemblies has shown indications that the observed thermal transition behavior is linked with shortening of the hydrogen bond lifetime between PSS and water.[36] Upon heating, water experiences faster rotation and vibration movements, as indicated by the shortening of H-bond lifetime and increase of molecular diffusion coefficient.[36,41] The data in Figure a,b show that the total number of water–sulfonate hydrogen bonds decreases with increasing temperature and that the decrease results dominantly from water at the intrinsic ion pairs. For extrinsically compensated ion pairs, the number of hydrogen bonds with water was relatively insensitive to temperature, Figure c. Likely, this results from the Na+ ion in the extrinsic ion pairs pinning nearby water molecules. The number of intrinsically charge compensated ion pairs exceeded the number of extrinsically compensated ones, see Figure , so the decrease in the total water–sulfonate hydrogen bonds tracks closely with that of the intrinsic ion pairs. The PSS hydration shell size (in water molecules) depended on hydration, salt concentration, and whether the PSS sulfonate group was intrinsically or extrinsically compensated, Figure S9.

Figure 5

PSS–water hydrogen bond analysis for simulations of PECs of different water content prepared from 0.5 M NaCl solution (a) total, (b) intrinsically, and (c) extrinsically compensated ion pairing of PSS. Panels (a–c) correspond to the average of three simulations.

In prior simulation work, a clear transition in the lifetime, and consequently the number of hydrogen bonds between PSS sulfonate groups and water, was observed and interpreted to be associated with the thermal transition or Tg.[36] The data shown in Figure a,b correspond to a much larger simulation system, so the transition here was relatively weak. Individual simulation runs are presented in Figure S10. The plot in Figure c presents the insensitivity of hydrogen bonds between water and extrinsically compensated PSS. These molecular simulations show the partitioning of water to intrinsic and extrinsic sites, specifically the water-activated Tg at the intrinsic ion pair.

Conclusion

The origin of how water at the intrinsic ion pair facilitates the relaxation of charged polyelectrolyte assemblies has been demonstrated. The fact that the scaling of 1/Tg ∼ ln() experimentally applied to pH-sensitive and salt-sensitive systems, as well as systems made by different approaches, points to water’s generalized role. This scaling gave identical energies of ∼ −10.8 kJ/mol, similar in value to the van’t Hoff enthalpy of a hydrogen bond,[50] for all charged polyelectrolyte assemblies. MD simulations of PDADMA–PSS directly observed the intrinsic and extrinsic ion pairs separately, showing that the Tg was connected to a decrease in H-bonds between water–PSS at intrinsic ion pairs. These findings imply an underlying mechanism for the glass transition: water plasticizes the PECs by weakening intrinsic ion pairing and water surrounding the intrinsic ion pair facilitates the sliding motion and relaxation of polyelectrolytes within the assembly. These findings may potentially be generalized to any aqueous macromolecular assembly containing charged ion pairs, whether that be natural or synthetic.