The buildup of polyelectrolyte multilayers is investigated in solution with multiple angle null-ellipsometry. Polyanion poly(styrenesulfonate) (PSS) and polycation poly(diallyldimethylammonium) (PDADMAC) are adsorbed sequentially from 0.1 M NaCl solution. First the films grow parabolically. After N trans deposited PDADMAC/PSS layer pairs a transition from a parabolic to a linear growth occurs. For molecular weights above a threshold (M w(PSS) > 25 kDa and M w(PDADMAC) > 80 kDa), N trans is 15, the thickness per layer pair in the linear growth regime is 12.3 nm. If either the PDADMAC or the PSS molecular weight is decreased below the threshold value, N trans either falls (for PDADMAC, lowest value observed is 8) or rises (for PSS, highest value observed is 33), respectively. Simultaneously, in the linear growth regime, the thickness per layer pair decreases (down to 4.3 nm) or rises (up to 25 nm). Furthermore, for low molecular weight PSS, three growth regimes are observed: exponential, parabolic, and linear. The opposite effect of PDADMAC and PSS molecular weight reduction is discussed in terms of persistence lengths and linear charge density. The data suggest that molecular weight provides a way to control growth and internal structure of polyelectrolyte multilayers.
Using neutron reflectivity, the internal structure of polyelectrolyte multilayers is described on the nanoscale. Each film consists of a protonated and a deuterated block, built from x protonated and y deuterated polycation/polyanion bilayers, respectively. The number of bilayers N = x + y is kept constant; the position of the interface between the blocks is varied systematically. The polyanion is poly(styrenesulfonate) (PSS), and the polycation is poly(allylamine hydrochloride) (PAH) or poly(diallyldimethylammonium chloride) (PDADMAC). Always, the first four to five bilayers are thinner than the average bilayer thickness, but the three terminating bilayers are sometimes thicker. In the core zone, the bilayer thickness is constant. The internal roughness is smallest next to the film/air interface and increases with the number of bilayers away from the film/air interface. This suggests that each deposition step promotes the interdiffusion of the supporting layers. At the selected preparation conditions, the internal roughness increases more for PDADMAC/PSS than for PAH/PSS; the diffusion constants differ by 2 orders of magnitude.
The internal interfaces of polyelectrolyte multilayers are investigated with neutron reflectivity. The films are made from poly(diallyldimethylammonium) (PDADMA), poly(styrenesulfonate) (PSS), and deuterated PSS-d. Each film consists of a protonated and a deuterated block. The internal roughness is smallest next to the film/air interface and increases with the number of layer pairs away from the film/air interface until a metastable state is reached. Both the final internal roughness and the interdiffusion constant increase with the salt concentration in the deposition solution and with PDADMA polymer weight. The increased mobility found with high molecular weight PDADMA is attributed to residual stresses occurring during film formation. The experiments suggest that PSS and PDADMA move partly as a complex. Postpreparation immersion in 1 M NaCl salt solutions has little effect if the multilayer is prepared from low salt solution and with high molecular weight PDADMA. However, almost complete intermixing is observed for multilayers prepared from 0.1 M NaCl and with low molecular weight PDADMA (diffusion length exceeds 30 nm).
Topographical and chemical features of biomaterial surfaces affect the cell physiology at the interface and are promising tools for the improvement of implants. The dominance of the surface topography on cell behavior is often accentuated. Striated surfaces induce an alignment of cells and their intracellular adhesion-mediated components. Recently, it could be demonstrated that a chemical modification via plasma polymerized allylamine was not only able to boost osteoblast cell adhesion and spreading but also override the cell alignment on stochastically machined titanium. In order to discern what kind of chemical surface modifications let the cell forget the underlying surface structure, we used an approach on geometric microgrooves produced by deep reactive ion etching (DRIE). In this study, we systematically investigated the surface modification by (i) methyl-, carboxyl-, and amino functionalization created via plasma polymerization processes, (ii) coating with the extracellular matrix protein collagen-I or immobilization of the integrin adhesion peptide sequence Arg-Gly-Asp (RGD), and (iii) treatment with an atmospheric pressure plasma jet operating with argon/oxygen gas (Ar/O). Interestingly, only the amino functionalization, which presented positive charges at the surface, was able to chemically disguise the microgrooves and therefore to interrupt the microtopography induced contact guidance of the osteoblastic cells MG-63. However, the RGD peptide coating revealed enhanced cell spreading as well, with fine, actin-containing protrusions. The Ar/O-functionalization demonstrated the best topography handling, e.g. cells closely attached even to features such as the sidewalls of the groove steps. In the end, the amino functionalization is unique in abrogating the cell contact guidance.
Ellipsometric measurements give information on two film properties with high precision, thickness and refractive index. In the simplest case, the substrate is covered with a single homogenous, transparent film. Yet, with ellipsometry, it is only possible to determine the two film properties thickness and refractive simultaneously if the layer thickness exceeds 15 nm - a restriction well known for a century. Here we present a technique to cross this limitation: A series expansion of the ellipsometric ratio ρ to the second order of the layer thickness relative to the wavelength reveals the first and second ellipsometric moment. These moments are properties of the thin film and independent of incident angle. Using both moments and one additional reference measurement enables to determine simultaneously both thickness and refractive index of ultra-thin films down to 5 nm thickness.
Electrostatic forces at the cell interface affect the nature of cell adhesion and function; but there is still limited knowledge about the impact of positive or negative surface charges on cell-material interactions in regenerative medicine. Titanium surfaces with a variety of zeta potentials between −90 mV and +50 mV were generated by functionalizing them with amino polymers, extracellular matrix proteins/peptide motifs and polyelectrolyte multilayers. A significant enhancement of intracellular calcium mobilization was achieved on surfaces with a moderately positive (+1 to +10 mV) compared with a negative zeta potential (−90 to −3 mV). Dramatic losses of cell activity (membrane integrity, viability, proliferation, calcium mobilization) were observed on surfaces with a highly positive zeta potential (+50 mV). This systematic study indicates that cells do not prefer positive charges in general, merely moderately positive ones. The cell behavior of MG-63s could be correlated with the materials' zeta potential; but not with water contact angle or surface free energy. Our findings present new insights and provide an essential knowledge for future applications in dental and orthopedic surgery.
Assembly of oppositely charged macromolecules (proteins, DNA, polyelectrolytes) is often used for surface modification and functionalization. Yet, it remains a challenge to control the position and mobility of the molecules within the assembly. Using polyelectrolyte multilayers as model systems, we study the diffusion constant of the polyanion PSS. D PSS could be varied by 5 orders of magnitude. Two parameters were found to be important: (i) the conformation of the polyelectrolytes and (ii) the molecular weight of the polycation (M w(PDADMA)); the latter was the dominant parameter. Independent of conformation, by increasing M w(PDADMA), D PSS decreased by at least 3 orders of magnitude when M w(PDADMA) increased by a factor of seven. The decrease was stronger than predicted by any scaling law; it was either exponential or abrupt after D PSS was almost constant for low M w(PDADMA). The polymer conformation was adjusted with the salt concentration in the preparation solution. Flatter and less entangled chains led to an increase in D PSS. These findings on the time dependence of the internal structure of assemblies are discussed in the context of network theory.
Linearly assembled polyelectrolyte multilayers (PEMs) are prepared by sequential adsorption of polyanions and polycations from 0.1 mol/L NaCl. The internal structure of PEMs is investigated with neutron reflectivity. The films are made from poly(ethylenimine) (PEI), poly(diallyldimethylammonium) (PDADMA) and poly(styrenesulfonate) (PSS or deuterated PSS-d). Each film consists of a protonated and a deuterated block, built from m protonated and n deuterated polycation/polyanion layer pairs, respectively. Annealing in salt solution (1 mol/L NaCl) allows the polyelectrolytes to gain entropy by adopting a more coiled conformation and by intermixing. During annealing the internal interface between the two blocks broadens due to interdiffusion; thus, the PSS diffusion coefficient is measured. Eventually the annealing leads to a uniform distribution of protonated and deuterated PSS throughout the film. Yet, if one polycation layer in the film center is branched PEI, then this PEI layer serves as a diffusion barrier, which is impenetrable for up to 33% of PSS macromolecules. The equilibration time of the remaining mobile PSS fraction increases which is attributed to the low permeation rate through the barrier layer. Possibly, some PSS molecules have a conformation that hinders them to cross the barrier layer, or the barrier layer gets clogged with time.
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