Impact of Star Polyacid Branching on Polymer Diffusion within Multilayer Films

Abstract

A series of linear and star poly­(acrylic acids) (LPAA and star PAAs) were synthesized to explore the effect of molecular architecture on the stratification and polymer dynamics of electrostatic layer-by-layer (LbL) films. Studies of LbL deposition of LPAA and star PAAs with poly­[2-(dimethylamino)­ethyl methacrylate] (PDMAEMA) at acidic pH showed an ∼30% increase in film dry thickness with increased polymer branching. Consistent with a greater mass of star polymer deposited within the films, in situ ellipsometric measurements of PAA uptake from solution revealed ∼3.5-fold greater diffusion coefficients for 8-arm PAA in comparison to linear PAA. For comparison, the dynamics of the linear PDMAEMA partner was explored via neutron reflectometry (NR) studies of stacked multilayers containing hydrogenated and deuterated polycations, hPDMAEMA and dPDMAEMA. The stacked multilayers deposited from low-ionic-strength solutions were stratified, exhibiting interfacial widths between hydrogenated and deuterated stacks of ∼15 and 10 nm for films constructed with star and linear PAAs, respectively, suggesting relatively low mobility of the polycation in both assemblies. Further exposure of these films to 0.5 M sodium chloride solutions enhanced the mobility of PDMAEMA, revealing an order magnitude faster diffusion of PDMAEMA in films of 8-arm PAA relative to linear PAA. The faster diffusion of polymers within films of star polyacids was correlated not only with the compactness of star polymers but also with an ∼2-fold lower ionization of assembled 8-arm PAA as determined by Fourier transform infrared spectroscopy and thus a lower number of polycation–polyacid ionic contacts in the case of star polymers as compared to their linear counterparts. The significant influence of molecular architecture on the number of polymer–polymer contacts was further confirmed by isothermal titration calorimetry studies of polyelectrolyte complexes in solution.

Salt-Induced Diffusion of Star and Linear Polyelectrolytes within Multilayer Films

“…However, this model could not be used for the 8PMAA/QPC system due to strong intermixing between hQPC and dQPC at early times of salt annealing. In this case, we used the limited-source model, 10,18,41 which correlates changes in concentration of dQPC in the stacks at different…”

“…However, this model could not be used for the 8PMAA/QPC system due to strong intermixing between h QPC and d QPC at early times of salt annealing. In this case, we used the limited-source model, ,, which correlates changes in concentration of d QPC in the stacks at different diffusion times as true( C ( 0 ) C ( t ) true) 2 = 2 D ⊥ σ 0 2 t + 1 , where C (0) is the relative concentration of d QPC in the D-stack at zero time, C ( t ) is relative concentration of d QPC in a D-stack at a given time of exposure, and σ 0 is the initial internal roughness between stacks. Figure B shows that true( C ( 0 ) C ( t ) true) 2 0.25em scaled linearly with time for the 8PMAA/QPC system, yielding 2 D ⊥ σ 0 2 as a slope.…”

“…The deprotection procedure of linear and star PTBMAs was adapted from previous publications. , In brief, 0.6 g (0.004 mol of polymer units) of linear or star PTBMA was dissolved in 15 mL of dichloromethane in a 20-mL glass vial under vigorous stirring. Then 1 mL (0.013 mol) of TFA was added to the vial under continuous stirring and stirred overnight.…”

“…11,24 Synthesis and characterization of tetrakis(2-bromoisobutyrate) (4f-BiB), dipentaerythritol hexakis(2-bromoisobutyrate) (6f-BiB), and tripentaerythritol octakis-(2-bromoisobutyrate) (8f-BiB) were described in our previous publications. 41,42 Synthesis of Linear and Star Poly(tert-butyl methacrylates). Linear and star poly(tert-butyl methacrylates) (PTBMA) were synthesized using activators regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP), as described previously for methacrylate monomers.…”

“…Recently, we explored the effect of polymer architecture on the diffusion of polymer chains using NR and in situ ellipsometry techniques. The systems of interest included assemblies of star poly­(acrylic acid)­s with a linear polycation at low pH, where ionization of the weak polyelectrolyte stars was largely suppressed, contributing to differences in mobility of star and linear polyacids perpendicular to the substrate . In this paper, we employ NR and FRAP to investigate the diffusion of polymers perpendicular and lateral to the surface, respectively.…”