Layer-by-Layer Assembly and Electrochemical Study of Alizarin Red S-Based Thin Films.

Electroactive organic dyes incorporated in layer-by-layer (LbL) assemblies are of great interest for a variety of applications. In this paper, Alizarin Red S (ARS), an electroactive anthraquinone dye, is employed to construct LbL (BPEI/ARS)n films with branched poly(ethylene imine) (BPEI) as the complementary polymer. Unconventional LbL methods, including co-adsorption of ARS and poly(4-styrene sulfonate) (PSS) with BPEI to assemble (BPEI/(ARS+PSS))n, as well as pre-complexation of ARS with BPEI and further assembly with PSS to fabricate ((BPEI+ARS)/PSS)n, are designed for investigation and comparison. Film growth patterns, UV⁻Vis spectra and surface morphology of the three types of LbL assemblies are measured and compared to reveal the formation mechanism of the LbL films. Electrochemical properties including cyclic voltammetry and spectroelectrochemistry of (BPEI/ARS)120, (BPEI/(ARS+PSS))120 and ((BPEI+ARS)/PSS)120 films are studied, and the results show a slight color change due to the redox reaction of ARS. ((BPEI+ARS)/PSS)120 shows the best stability among the three samples. It is concluded that the manner of dye- incorporation has a great effect on the electrochemical properties of the resultant films.

Comparison of KBr and NaCl effects on the glass transition temperature of hydrated layer-by-layer assemblies.

The influence of assembly and post-assembly conditions on the glass transition temperature of free-standing poly(diallyldimethyl ammonium) (PDADMA)/poly(4-styrene sulfonate) (PSS) layer-by-layer (LbL) films assembled in 0.5M NaCl and 0.5M KBr are explored using modulated differential scanning calorimetry. Upon completion, PDADMA/PSS LbL assemblies are hydrated using solutions containing various concentrations of KBr. The data indicate that water provides the primary driving force for changes in the glass transition temperature of completed films rather than the post-assembly salt type. However, upon changing the assembly salt conditions from NaCl to KBr, the glass transition temperature shows a decrease of nearly 20 °C. Additionally, the composition of the films upon analysis with 1H NMR spectroscopy and neutron activation analysis indicates an elevated number of extrinsic binding sites within the film structure when KBr is the assembly salt. This shows a clear link between the assembly conditions and the internal structure and, therefore, the thermal properties of PDADMA/PSS LbL assemblies.

Hydration and Temperature Response of Water Mobility in Poly(diallyldimethylammonium)-Poly(sodium 4-styrenesulfonate) Complexes.

The combination of all-atom molecular dynamics simulations with differential scanning calorimetry (DSC) has been exploited to investigate the influence of temperature and hydration on the water distribution and mobility in poly(diallyldimethylammonium) (PDADMA) and poly(sodium 4-styrenesulfonate) (PSS) complexes. The findings show that the vast majority of the water molecules hydrating the polyelectrolyte complexes (PECs) with 18-30 wt % hydration are effectively immobilized due to the strong interactions between the PE charge groups and water. Temperature and hydration were found to decrease similarly the fraction of strongly bound water. Additionally, at low hydration or at low temperatures, water motions become dominantly local vibrations and rotations instead of translational motion; translation dominance is recovered in a similar fashion by increase of both temperature and hydration. DSC experiments corroborate the simulation findings by showing that nonfreezing, bound water dominates in hydrated PECs at comparable hydrations. Our results raise attention to water as an equal variable to temperature in the design and engineering of stimuli-responsive polyelectrolyte materials and provide mechanistic explanation for the similarity.

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.

QCM-D Investigation of Swelling Behavior of Layer-by-Layer Thin Films upon Exposure to Monovalent Ions.

Polyelectrolyte multilayers and layer-by-layer assemblies are susceptible to structural changes in response to ionic environment. By altering the salt type and ionic strength, structural changes can be induced by disruption of intrinsically bound ion pairs within the multilayer network via electrostatic screening. Notably, high salt concentrations have been used for the purposes of salt-annealing and self-healing of LbL assemblies with KBr, in particular, yielding a remarkably rapid response. However, to date, the structural and swelling effects of various monovalent ion species on the behavior of LbL assemblies remain unclear, including a quantitative view of ion content in the LbL assembly and thickness changes over a wide concentration window. Here, we investigate the effects of various concentrations of KBr (0 to 1.6 M) on the swelling and de-swelling of LbL assemblies formed from poly(diallyldimethylammonium) polycation (PDADMA) and poly(styrene sulfonate) polyanion (PSS) in 0.5 M NaCl using quartz-crystal microbalance with dissipation (QCM-D) monitoring as compared to KCl, NaBr, and NaCl. The ion content after salt exchange is quantified using neutron activation analysis (NAA). Our results demonstrate that Br- ions have a much greater effect on the structure of as-prepared thin films than Cl- at ionic strengths above assembly conditions, which is possibly caused by the more chaotropic nature of Br-. It is also found that the anion in general dominates the swelling response as compared to the cation because of the excess PDADMA in the multilayer. Four response regimes are identified that delineate swelling due to electrostatic repulsion, slight contraction, swelling due to doping, and film destruction as ionic strength increases. This understanding is critical if such materials are to be used in applications requiring submersion in chemically dynamic environments such as sensors, coatings on biomedical implants, and filtration membranes.

Role of Salt and Water in the Plasticization of PDAC/PSS Polyelectrolyte Assemblies.

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 contributes 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 into 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.

Electropolymerized Polythiophenes Bearing Pendant Nitroxide Radicals.

We report a facile way to synthesize polythiophenes carrying pendant 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) radicals, here called PTATs, by electropolymerization in boron trifluoride diethyl etherate (BFEE). The spacing between the TEMPO radical and the polythiophene backbone is varied by an alkyl spacer (n = 2, 4, 6), and the electronic and electrochemical properties are examined using UV-vis spectroscopy, cyclic voltammetry, and electrochemical impedance spectroscopy. Film morphologies are also studied via scanning electron microscopy (SEM) and atomic force microscopy (AFM), which show that the longer octyl chain placed between thiophene and TEMPO effectively suppresses aggregation. The highest conductivity and electroactivity are observed for n = 4 and n = 6, respectively. Such morphology differences provide an opportunity to better understand the charge transport and energy storage properties in electronic materials.

The influence of ionic strength and mixing ratio on the colloidal stability of PDAC/PSS polyelectrolyte complexes.

Polyelectrolyte complexes (PECs) form by mixing polycation and polyanion solutions together, and have been explored for a variety of applications. One challenge for PEC processing and application is that under certain conditions the as-formed PECs aggregate and precipitate out of suspension over the course of minutes to days. This aggregation is governed by several factors such as electrostatic repulsion, van der Waals attractions, and hydrophobic interactions. In this work, we explore the boundary between colloidally stable and unstable complexes as it is influenced by polycation/polyanion mixing ratio and ionic strength. The polymers examined are poly(diallyldimethylammonium chloride) (PDAC) and poly(sodium 4-styrenesulfonate) (PSS). Physical properties such as turbidity, hydrodynamic size, and zeta potential are investigated upon complex formation. We also perform detailed molecular dynamics simulations to examine the structure and effective charge distribution of the PECs at varying mixing ratios and salt concentrations to support the experimental findings. The results suggest that the colloidally stable/unstable boundary possibly marks the screening effects from added salt, resulting in weakly charged complexes that aggregate. At higher salt concentrations, the complexes initially form and then gradually dissolve into solution.

Thermal Transitions in Polyelectrolyte Assemblies Occur via a Dehydration Mechanism.

Hydrated polyelectrolyte (PE) complexes and multilayers undergo a well-defined thermal transition that bears resemblance to a glass transition. By combining molecular simulations and differential scanning calorimetry (DSC) of poly(diallyldimethylammonium) (PDAC) and poly(styrenesulfonate) (PSS) multilayers, we establish for the first time that dehydration drives the thermally induced change in plasticization of the complex and in the diffusion behavior of its components. DSC experiments show that the thermal transition appears when the assemblies are hydrated in water but not in the presence of alcohols, which supports that water is required for this transition. These findings connect PE complexes more generally to thermoresponsive polymers and liquid crystal phases, which bear phase transitions driven by the (de)hydration of functional groups, thus forming a fundamental link toward an integrated understanding of the thermal response of molecular materials in aqueous environments.