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Growth modes of multilayer films
PEM films grow in thickness as a function of the number of deposited layers following two possible regimes, the so-called linear or exponential growth regime. In the linear mode, the film thickness increases linearly with the number of deposited layers, unlike the exponential mode. Depending on the combination of polyelectrolytes and on the preparation conditions, some films may show a transition between the two types of growth.
Films with linear growth
In films with linear mode of growth, the increase in the thickness of the constructed film is always constant per number of deposited layers. In the linear growth, a polyelectrolyte interacts only with the last deposited polyelectrolyte layer [13] and cannot diffuse into the film, and at each deposition step the same amount of polyelectrolytes is adsorbed. In general, linearly growing films are thinner than exponentially growing ones.
It has been shown that there is a correlation between the complexation heat and the mode of growth of a PEM film. For linearly growing films, the polycation/polyanion complex has a negative enthalpy of complexation [28].
The poly(styrene sulfonate)/poly(allylamine hydrochloride) system is a reference example for linear growth mode (Figure 1.7). X-ray reflectometry and neutron reflectivity results showed that in films, polyelectrolyte layers only penetrate the adjacent layers constituting a pseudo-layer structure called “fuzzy layers” [13]. This gives the film certain homogeneity in structure and charge distribution [29, 30, 31].
Ladam et al. proposed a mechanism for the linear growth of multilayer films [20]. This model divides the film into three zones. Zone I, called the “precursor zone”, at the substrate/film interface and its vicinity, corresponds to the film precursor and is strongly influenced by the substrate. Zone III, at the interface film/solution, is called « external area”. Zone II is the intermediate zone and separates the two other zones and is not influenced by the interfaces. The boundaries between zones I and II and zones II and III are diffuse. The precise number of component layers of the zones I and III is not known but depends largely on the substrate, chemical structure of the used polyelectrolytes, and the physico-chemical parameters adopted for the film buildup. The model of the three zones is valid until a sufficient number of layers are deposited on the substrate.
Films with exponential growth
Such growth regime was first evidenced in 1990 with polypeptides- and polysaccharides-based films [32, 33]. Actually, most of biologically relevant films built from polypeptides and polysaccharides show an exponential growth. The mechanism leading to exponential growth is totally different from that which led to films with a linear growth. In exponentially growing films, at least one of the two polyelectrolytes has the ability to diffuse “in” and “out” of the core of the polyelectrolyte multilayer during its buildup [32, 33]. Such films possess physico-chemical properties similar to those of a hydrogel due to their high water content [34, 35]. The thicknesses of such films may reach several micrometers after the deposit of twenty layer pairs. Comparatively, a film with a linear growth has a typical thickness in the order of ten nanometers for 15 constructed bilayers under the same conditions of pH and ionic strength.
Isothermal titration microcalorimetry experiments revealed that the enthalpy of complexation of the polyelectrolytes in exponentially growing films is positive. Therefore, exponential growth is mainly driven by the entropy and is temperature sensitive [28]. The mechanism of construction of the exponentially growing hyaluronic acid/poly(L-lysine) ( films were described by Lavalle et al. (Figure 1.9) [36]. The ( multilayer film, terminated by a layer of polyanions HA (panel A in Figure 1.9), is brought into contact with the polycation PLL solution (panel B). The polycations form a layer at the surface creating a positive charge excess and diffuse at the same time into the film forming a reservoir of free polycations (panel C). During the rinsing phase, only a part of the free polycations leaves the film because of the barrier of positive charges at the surface (panel D). The ability of the film to store part of PLL chains is explained by excess loads of positive charges to the film surface, which creates an electrostatic potential barrier preventing the diffusion of all of the polycations in the film. When the film is then brought into contact with the polyanion solution, polyanions complex with the excess positive charges on the surface changing the sign of the potential barrier which becomes negative (E). The free polycations previously confined in the film then diffuse outwardly and interact with polyanions forming an additional deposit to the surface. During this single application process continues until the depletion of the polycations reservoir is reached (F). The mass of the layers formed is proportional to the amount of free polycations remaining in the film after the rinsing phase.
During the construction of multilayer films, a transition from exponential to linear growth may occur. Hübsh et al. were the first to observe such a transition [37]. They studied the growth of a multilayer film based on a polycation, poly(allylamine hydrochloride) (PAH), and a binary mixture of polyanions, poly(glutamic acid) (PGA) and poly(sterene sulfonate) (PSS). These polyanions were chosen because PGA induces exponential growth with PAH while PSS causes a linear growth is solely interacting with PAH. Other studies have also
described this transition in the case of system. This film reaches a critical thickness at which free polyelectrolytes stop diffusing into the film. After constructing a large number of layers, a structure comprising the three zones of the linear model would be obtained. This transition phenomenon would be established when the film thickness is between 150 and 200 nm.
Physico-chemical parameters affecting the growth of PEM films
The physico-chemical parameters defining the conditions of the deposition cycle such as the pH, the ionic strength, and the temperature of the polyelectrolyte solutions mainly determine the structural, morphological and mechanical properties of the deposited PEMs.
Solution pH
The pH of the polyelectrolyte solutions plays a key role especially in cases where at least one of the two polymers used for constructing the PEM is a weak polyelectrolyte. The degree of charge dissociation or the polyelectrolyte ionization is pH-sensitive in the case of weak polyelectrolytes, thus the charge density can be regulated by a simple change of the pH of the polyelectrolyte solutions [2]. The total charge density determines the degree of interaction between the two polyelectrolyte solutions used in the film deposition as well as the ions permeability within the films. This degree of interaction in turn greatly affects the structural and mechanical properties of the deposited films.
Studies showed that the young modulus of PEM films is tunable upon simple adjustment of pH. Lulevich and Vinogradova showed that the swelling and the stiffness of polyelectrolyte multilayer capsules are pH-controlled processes. For multilayer capsules, the stiffness (young modulus) can be adjusted by changing the pH. The Young modulus varies dramatically from 70-100 MPa at a neutral pH to the order of 10-20 MPa at pH 10 [38]. These results were confirmed by Kim and Vinogradova [39]. This shift in Young modulus is explained by the fact that PAH is a weak polycation, so its linear charge density is pH sensitive. The charge density of PAH decreases at high pH where the ammonium groups are deprotonated. This affects the PAH conformation and reduces the density of ionic cross-links resulting in a smaller Young modulus.
Rubner also showed that the pH-controlled LBL assembly of weak polyelectrolytes yields a huge flexibility for controlling the molecular organization and the properties of PEM films [40]. For multilayer film of weak polyelectrolytes, poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH), it is possible to shift the thickness of a deposited polyelectrolyte layer from 0.5 nm to 8 nm just by changing the pH of the polyelectrolytes solutions used for film construction (Figure 1.10). At pH=7 the polymeric chains are significantly ionized and exhibit an elongated conformation due to strong intra-chains charges repulsion and thus the process ends up with thinner films [41]. Assembling PAA and PAH with strong polyelectrolytes led to similar results. In both cases, the dramatic change in thickness and the transition from thin to much thicker layers occurred when the charge density of the weak polyelectrolyte decreased from its fully charged state to 70-90% charged units. Moreover, FTIR analyses of the pH-dependent degree of ionization of the weak polyelectrolyte showed that the ionization behavior of a weak polyelectrolyte can differ substantially from the solution state when it is incorporated into a multilayer film [42].
Ionic strength of the buffer and polyelectrolyte solutions
Solution ionic strength plays a major role in the PEM films construction mechanism. In general, the increase in ionic strength is accompanied by an increase in the multilayer thickness (at least up to a certain ionic strength, which is specific for each multilayer system) and by an increase in film roughness [12, 18, 20].
Table of contents :
GENERAL INTRODUCTION
CHAPTER I. LITERATURE REVIEW
I. Polyelectrolyte multilayer (PEM) films
I.1. Polyelectrolyte multilayer (PEM) films obtained by electrostatic interactions
I.1.1. Properties of polyelectrolytes
I.1.2. Strong polyelectrolytes versus weak polyelectrolytes
I.1.3. History of polyelectrolyte multilayer films
I.1.4. Principles of the Layer by Layer (LBL) deposition of PEM films
I.1.5. Interactions in multilayer LBL films
I.1.6. Charge balance in polyelectrolyte multilayers
I.1.7. Growth modes of multilayer films
I.1.7.1. Films with linear growth
I.1.7.2. Films with exponential growth
I.1.8. Physico-chemical parameters affecting the growth of PEM films
I.1.8.1. Solution pH
I.1.8.2. Ionic strength of the buffer and polyelectrolyte solutions
I.1.8.3. Buildup temperature
I.1.8.4. Effect of salt anion on the film construction
I.1.9. Mechanical properties of PEM films
I.1.10. Water content in PEM films
I.2. Polyelectrolyte multilayer films obtained by hydrogen bonding
I.3. Electrochemical properties of polyelectrolyte multilayers
I.3.1. Ion Permeability of polyelectrolyte multilayers
I.3.2. Donnan Potential in PEM films
I.4. Applications of PEM films
I.5. Incorporation of Nanoparticles and Biomolecules into PEM Films
I.5.1. Incorporation of bioactive molecules and ions
I.5.2. Incorporation of Nanocolloids
I.5.3. Incorporation of proteins and peptides
I.5.4. Loading with Multivalent ions
I.5.5. Loading of nanoparticles in PEM films
II. Electrokinetic Investigations of Nanodendrimers
II.1. Concept of soft particles
II.2. Electrokinetics of soft particles
II.3. Polyamidoamine (PAMAM) carboxylated nanodendrimers
II.3.1. Synthesis and structure
II.3.2. Advances in nanodendrimers research and applications
II.3.3. Investigations of dendrimer toxicity
III. References
CHAPTER II. MATERIALS AND CHARACTERIZATION METHODS
I. Materials and samples preparation
I.1. The polyelectrolyte solutions
I.2. Construction of the films
I.3. Heating and aging of the films
I.4. Electrokinetics of the nanodendrimers
I.5. Loading the films with nanodendrimers
II. Characterization Methods
II.1. Atomic force microscopy (AFM)
II.1.1. AFM technique and principles
II.1.2. AFM cantilevers and probes
II.1.3. Different AFM modes
II.1.4. Force measurements
II.1.5. Determining film elastic modulus using AFM
II.2. Quartz Crystal microbalance-dissipation (QCM-D)
II.3. Confocal Raman Spectroscopy (CRS)
II.4. Dynamic light scattering (DLS) and Phase Analysis light Scattering (PALS)
II.4.1. Size measurements by DLS
II.4.2. Electrophoretic mobility measurements by PALS
III. References
CHAPTER III. REMARKABLE ELECTROKINETIC FEATURES OF CHARGE-STRATIFIED SOFT NANOPARTICLES: MOBILITY REVERSAL IN MONOVALENT AQUEOUS ELECTROLYTE
CHAPTER IV. REMARKABLE STRUCTURE AND ELASTICITY RELAXATION DYNAMICS OF POLY(DIALLYLDIMETHYLAMMONIUM CHLORIDE)-POLY(ACRYLIC ACID) MULTILAYER FILMS
CHAPTER V. LOADING OF PAMAM G6.5 DENDRIMERS IN (PDADMAC-PAA) MULTILAYER FILMS
I. Characterization of (PDADMAC-PAA) films
II. Loading of the films with G6.5 PAMAM nanodendrimers
II.1. Effect of G6.5 dendrimers concentration on the morphology of P A MAC PAA films
II.2. Effect of G6.5 concentration on Young modulus of P A MAC PAA films …. 209
III. Conclusions
III. References
GENERAL CONCLUSION AND PERSPECTIVES
