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41576-AC7
Release of Macromolecules from Ultrathin Polyelectrolyte Films
The overall goal of our research is to provide new fundamental insights into stability and dynamics of macromolecules that are self-assembled at a solid-water interface. Our focus is on the effect of the number and the nature of intermolecular pairing on multilayer structure and on the chain dynamics in the polyelectrolyte films that are responsive to variations of the external pH. We have studied two related classes of responsive films, where interactions between adjacent layers are controlled either by electrostatic or by hydrogen-bonding forces. In particular, we (1) elaborated self-assembly conditions for polyelectrolyte multilayers of different thickness made of pH-responsive polyelectrolytes; (2) identified different regimes of film response to variations in pH and (3) studied a new regime in which one of the multilayer components is selectively released from electrostatically bound films. Specific emphasis was on multilayer internal structure at assembly and post-assembly steps. Importantly, we also performed kinetic measurements of this selective chain release for polymers of different chemistry as a function of pH and molecular weight of self-assembled polymers.
Study of self-assembly of the polymers has been done using in situ Fourier transform infrared spectroscopy in attenuated total reflection mode (ATR-FTIR) and ellipsometry. In addition, we have employed neutron reflectivity to probe the degree of layer interpenetration and the depth profiles for the selective pH-induced chain release from the film. Our studies have been performed with the polyelectrolyte multilayers composed of polymethacrylic acids (PMAA) with narrow molecular-weight distributions assembled either with neutral polymers or with polycations by means of hydrogen-bonding or ionic interactions, respectively. Polycations have been obtained by quaternization of poly-4-vinylpyridine (Mw 200 kDa) with ethyl-, propyl- or butylbromide to produce QPVP polymers with varying hydrophobicity and charge density.
Our main findings resulting from our studies for the 2005-2007 period (including a one-year no-cost extension) are listed below.
(1). We found that different regimes of film response can be achieved by choosing polycations with different charge densities. Using in situ ATR-FTIR spectroscopy, we have shown that when PMAA was assembled with ethyl-QPVP with 20% of pyridinium units (EtQ20) at pH 5 and then exposed to pH 7.5, the EtQ20/PMAA film selectively released PMAA to the solution. In situ ATR-FTIR confirms selectivity of polyacid release and shows that film response is caused by pH-induced charge imbalance. In contrast, EtQ98/PMAA films were capable of accumulating a significant amount of extra negative charge and remained stable at pH 7.5. This finding suggests that lower number of stitching points facilitates selective release of a weak polyelectrolyte.
(2).We applied neutron reflectivity (NR) at both NIST and ORNL to study the degree of layer interpenetration at assembly and post-assembly steps in hydrogen-bonded and electrostatically bound films. Modeling of the NR data of the measured systems was done in collaboration with John Ankner (SNS, ORNL). This collaboration, developed under ACS-PRF funding, turned out to be very fruitful. With John Ankner, we are now applying direct inversion program allowing visualization of the internal structure of responsive polymer films. We have shown that the Patterson function can be adapted to assist in determining the number of layers and their thicknesses. For conventionally collected reflectivity data, this represents a significant step toward real-space imaging of the internal film structure. Our NR results show that hydrogen-bonded layers are highly interdiffused, and that the degree of interpenetration is controlled by the strength of interlayer adhesion and by polymer nature. Specifically, high degree of interdiffusion and weak layering was observed for weakly bound systems. A Ph.D. student Eugenia Kharlampieva supported by this project got experienced in neutron reflectivity technique, and has written two proposals to study structural changes within the film upon pH variation which were approved. Eugenia is now advancing her academic career as a postdoc at GeorgiaTech.
(3). To study a newly discovered regime of pH-induced selective desorption of weak polyacid chains from polyacid/polybase layer-by-layer films, we have lead a team of an expert in theoretical polymer physics (Michael Rubinstein, UNC), an expert in NR (John Ankner, ORNL), and experts in experimental polymer science (our group, Stevens). This collaboration is very fruitful. Specifically, studies of kinetics of PMAA release using narrow fractions of PMAA reveal that the characteristic time of chain release, τ, scales with molar mass, Mw, as Mw1.1±0.1. In collaboration with Michael Rubinstein, we developed a theoretical model of the “sticky gel electrophoresis” of polyacids with excess charge driven out of the film by the entropy of counterions. This model of entangled polyacids agrees with experiments and predicts τ ~ Mw H, where H is film thickness. Finally, NR studies performed in collaboration with John Ankner showed that pH-triggered chain diffusion and release results in disordering of the multilayer film structure, as expected from the theoretical model. The obtained results might be used to rationally design multilayer films with desired characteristics for release of macromolecular components.
