Formation of Multilayers by Star Polyelectrolytes: Effect of Number of Arms on Chain Interpenetration

Effect of polyacid architecture and polycation molecular weight on lateral diffusion within multilayer films

Despite the potential use of polyelectrolyte multilayers for biomedical, separation, and energy applications, their dynamic properties are not sufficiently understood. In this work, center-of-mass diffusion of a weak polyacid-poly(methacrylic acid) (PMAA) of linear and 8-arm architecture (L-PMAA and 8-PMAA, respectively) and matched molecular weight-was studied in layer-by-layer (LbL) assemblies with poly(diallyldimethylammonium) chloride (PDADMAC) of varied molecular weight. The film deposition at low-salt, acidic conditions when PMAA was only partially ionized yielded thicker, more diffused layers with shorter PDADMAC chains, and bilayer thickness decreased for multilayers constructed with longer PDADMAC. The molecular architecture of PMAA had a weak effect on film growth, with bilayer thickness being ∼20% larger for L-PMAA for the films constructed with the shortest PDADMAC (35 kDa) and identical film growth for L-PMAA and 8-PMAA with the longest PDADMAC (300 kDa). The exposure of the multilayer films to 0.2M NaCl triggered a reduction in PMAA ionization and significant lateral diffusivity of fluorescently labeled PMAA molecules (PMAA*), with diffusion coefficients D ranging from 10-13 to 10-12 cm2/s, as determined by the fluorescence recovery after photobleaching technique. For all the films, polymer mobility was higher for star polyacids as compared to their linear counterparts, and the dependence of PMAA diffusion coefficient D on PDADMAC molecular weight (D ∼ M-n) was relatively weak (n < 0.6). However, 8-PMAA demonstrated an approximately doubled power exponent compared to the L-PMAA chains, suggesting a stronger effect of the molecular connectivity of the partner polycation molecules on the diffusion of star polyelectrolytes.

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

This study explores the effect of salt on the diffusivity of polyelectrolytes of varied molecular architecture in layer-by-layer (LbL) films in directions parallel and perpendicular to the substrate using fluorescence recovery after photobleaching (FRAP) and neutron reflectivity (NR) techniques, respectively. A family of linear, 4-arm, 6-arm, and 8-arm poly(methacrylic acids) (LPMAA, 4PMAA, 6PMAA, and 8PMAA, respectively) of matched molecular weights were synthesized using atom transfer radical polymerization and assembled with a linear polycation, poly[2-(trimethylammonium)ethyl methacrylate chloride] (QPC). NR studies involving deuterated QPC revealed ∼10-fold higher polycation mobility for the 8PMAA/QPC system compared to all-linear LbL films upon exposure to 0.25 M NaCl solutions at pH 6. FRAP experiments showed, however, that lateral diffusion of star PMAAs was lower than LPMAA at NaCl concentrations below ∼0.22 M NaCl, with a crossover to higher mobility of star polymers in more concentrated salt solutions. The stronger response of diffusion of star PMAA to salt is discussed in the context of several theories previously suggested for diffusivity of polyelectrolyte chains in multilayer films and coacervates.

Synthesis and properties of well-defined carbazole-containing fluorescent star polymers of different arms

A series of fluorescent carbazole-containing star polymers with different arms were successfully synthesized using 9-(4-vinylbenzyl)9H-carbazole (VBCz) as monomer and multifunctional bromide as initiators via atom transfer radical polymerization (ATRP). The effect of the poly(9-(4-vinylbenzyl)-9H-carbazole) (PVBCz) star polymer architecture on their optical and electrochemical properties was investigated. All of the PVBCz star polymers absorbed light in the range of 280–360 nm both in solution and as polymer films. Meanwhile, the star polymers exhibited maximum fluorescent emission at 350 nm in solution, while at 406 nm as films. Moreover, the star polymers with different arm numbers showed different photoluminescence quantum efficiency and highest and lowest occupied molecular orbital (HOMO, LOMO, respectively) energy levels. It is proved that the PVBCz star polymers exhibited different photoelectronic properties by varying the molecular architectures.

Investigation on the fluorescence–(stimulus-response) properties of linear and star PVBCz-b-PDMAEMA block copolymers synthesized via ATRP

Linear and star PVBCz-b-PDMAEMA block copolymers were synthesized via ATRP, the properties of which were investigated including ultraviolet-visible absorption, fluorescence emission, stimulus-response and fluorescence-stimulus-response.