Difference between revisions of "Polyelectrolyte Multilayers"

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__NOTOC__
 
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'''Polyelectrolyte Multilayers (under construction)'''
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== What is a Polyelectolyte Multilayer (PEM)? ==
  
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''UNDERSTANDING THE STRUCTURE, STABILITY AND DYNAMICS OF POLYELECTROLYTE MULTILAYERS''
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<onlyinclude>PEMs are composed of '''alternating layers of oppositely charged polyelectrolytes''' (PEs) (synthetic PEs or biomolecules), which are generally built up based on the Layer-by-Layer technique. [1,2] Due to their potential applications as membrane, encapsulation and matrix materials, and for enzymes and proteins in sensor applications, PEMs have stimulated great interests from both academic researchers and industries.[3] See also a [http://www.chem.fsu.edu/multilayers/ PEM website].  Despite the large number of experimental works, '''theoretical and computational studies''' toward understanding the microscopic structure of PEMs '''are scarce'''.[4]
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</onlyinclude>
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== Our Research ==
  
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* Self-assembly processes of charged polymers (polyelectrolytes) involving electrostatic interactions can be used to build-up multilayered materials with unique properties. In the early 90's Decher et al [1] demonstrated the feasibility of the self-assembly of polyelectrolyte multilayers (PEMs) using the so-called Layer-by-Layer (LbL) technique. The versatility of the LbL process has allowed the fabrication of thin multilayer films made of synthetic polyelectrolytes (PE), DNA, lipids and proteins, which has resulted in a boost of novel applications in recent years. For instance, PEMs are used as matrix materials for enzymes and proteins in sensor applications [2], and also as a matrix for active components in solar cells. PEMs are used as a coating for protecting and control the healing process of damaged arteries [3]. In addition, PEM's can be used as permeable membranes for nanofiltration [4], gas separation, and fuel cells. Furthermore, PEM's are also used in the fabrication of non-linear optical materials [5], coloured electrochromic electrodes (future display devices), and to tailor the properties of photonic crystalls [6]. Other uses of PEM's include analyte separation processes (chromatography) [7], and the fabrication of thin-walled hollow micro- and nanocapsules (see [8], and ref. therein). These capsules have a great potential for drug carrier and nanoreactors.
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In close collaboration with experimental investigations from groups of Prof. [http://www.chemie.tu-berlin.de/klitzing/menue/home/ von Klitzing] and Prof. [http://www.bio.ph.tum.de/index.php?id=69 Hugel], we are currently working to investigate the inner structure and dynamics of a small layer number of PEMs via '''all-atom''' (AA) and '''coarse-grained''' (CG) simulations. The project is funded by the [http://www.dfg-spp1369.de/front/joomla/ '''DFG SPP 1369''': Polymer-Solid Contacts: Interfaces and Interphases]
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[[Image:spp1369_logo_small.jpg|100px|]]
  
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* The present knowledge about PEMs has been summarized in a few reviews [9-13]. Experiments have established several important features for PEM's: the thickness of each adsorbed layer shows almost a linear dependence on the salt concentration. Flexible polyelectrolytes in two component multilayers are known to intermix over several adjacent layers. This layer intermixing can be suppressed by using more rigid blocks for the assembly [13]. Intrinsic charge compensation by polyions accompanied by overcharging and the kinetically irreversible nature of deposition has also been reported. In the case of weak polyelectrolytes, very interesting variations of the stability, the thickness, the stiffness, the permeability and the porosity of PEMs have been observed by adjusting the pH of the dipping solutions; this change not only modifies the new layers, but also has an effect on previously adsorbed layers [14]. The measurements of the Young's modulus of PEMs show a strong correlation with ionomers, and therefore it could be possible that the structure of PEMs is close to that of ionomers. The role that the charge density of the PE plays on PEMs growth is somewhat unclear. Despite the existence of considerable data in the literature concerning the influence of the charge density on PEM formation, there is little consensus, and conflicting results have often been reported [15]. As an example of such controversy, some works have reported the existence of a critical minimum charge density below which is not possible the formation of PEMs whereas other works have observed multilayer buildup with PE of very low charge density [15]. From the extensive research over the past few years on PEMs, it is clear that unlike single layer films or polymer gels there is no general description of the physicochemical properties of different PEMs systems, especially for those assembled from weak polyelectrolytes. Another clear example of the previous claim is found in the buildup mechanism of PEMS [16]: whereas strong polyelectrolytes usually observe a linear growth for the PEM's thickness, weak polyelectrolytes do not always follow this pattern and can exhibit an exponential growth. The assembly conditions, even in the case of using the same PE, can modify the buildup regime. Thus, for instance, it has been shown very recently that an increase in temperature has a profound influence on the rate of the layer-by-layer buildup [17]. Furthermore, an elevated temperature is shown to swap the buildup from a linear to an exponential regime. The exact mechanism behind the phenomenon of the nonlinear growth is unknown at present. There are studies that suggest the surface roughening as responsible of the exponential growth (see ref. in [18]). Instead, other models are concerned with a certain active volume of the PEM, or the diffusion of polyions in and out of the film (see ref. in [17]). Another example of the complexity of the behaviour of weak PE is the shift in the charge density of weak PE depending on if they are adsorbed to the PEM or remain in solution. Furthermore, recent experimental reports ([18] and ref. therein) suggest that non-electrostatic short-range interactions, like for instance the hydrophobic interaction, also play an important role in the multilayering process. Moreover, it has been reported that the structures formed usually are metastable at some film buildup stages and little knowledge exist about the film relaxation during these stages. Thus, the complex nature of PEMs possesses a challenge when one tries to choose a PEM system for a particular application. Therefore, one must first try to learn more about the fundamental properties of PEMs before it is possible to understand how to use these films for specific applications without a large and exhausting process of trial and error.
 
  
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*There have been very few attempts to describe theoretically the electrostatic self-assembly, and all of them are build-up on severe assumptions hard to test experimentally; specifically they assume that PEM are equilibrium structures. Netz and Joanny [19] considered the formation of multilayers in a system of semiflexible polyelectrolytes (PE), assuming that the deposited layer structure was fixed, providing a solid charged substrate for the next layer. Mayes et al [20] applied a similar idea to flexible PE. Both these models neglect interpenetration (interdigitation) and chain complexation between the layers, which are usual features found in experimental works about PEMs. The oppositely limit of strong intermixing of PE between neighbouring layers was considered by Castlenovo and Joanny [21] by incorporating the complex formation between oppositely PEs into self-consistent field equations. Unfortunately this equations are limited to solutions of high ionic strength, were electrostatic interactions can effectively be treated as short-range interactions. Despite these huge efforts, the strong correlations existing between oppositely charged polyions, provides a formidable challenge for their theoretical description. In this respect, numerical simulations could be of great help.
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[[Image:scheme_PSS-PDADMA.png|200px|right|thumb|Schematic representation of PSS and PDADMA.]]
  
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* There exist few computational studies about electrostatic self-assembly of PEM. In a seminal paper on layer formation on a spherical substrate [22] it has been demonstrated that additional non-electrostatic forces are needed to produce nice PEMs. This suggests that multilayering is possibly not an equilibrium phenomenon evidence of which has been also observed experimentally [23]. Similar conclusions have been drawn for other geometries using either Molecular Dynamics (MD) or Monte Carlo (MC) techniques [24, 25]. Another outcome is, that these studies have confirmed the fuzzy nature of PEMs (molecules in one layer interpenetrate other layers) , although almost perfect periodic oscillations of the density differences between monomers belonging to positively and negatively charged PE have been found. MC studies (see refs. in [26]) have reported stability as well as the microstructure of the PE layers to be especially sensitive to the strength of the non-electrostatic short range attraction between the PE and the charged substrate. In addition, the thickness of the adsorbed layer is observed to decrease when increasing PE concentration or surface charge density, although the total adsorbed amount displays a non-monotonic dependence on polymer concentration. It was also demonstrated that the formation of multilayers as well as the extent of layer intermixing depends on the molecular weight of the PE chains and the fraction of charge on its backbone. The presence of ionic pairs between oppositely charged PE that form the layers have been claimed as a possible important factor in stabilizing the multilayer films. In the last year, MD studies for the assembly of flexible PE onto flat charged surfaces [26] have suggested the formation of thermodynamically stable structures from bulk mixture solutions. This is a controversial, very basic issue that opposes to some experimental reports, and we will try to resolve this issue.
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The AA level simulations proved to be '''consistent''' with existing experimental data on '''chain conformation''' of adsorbed poly(styrene sulfonate) (PSS) in PSS monolayer systems, '''dielectric permittivity''' and '''diffusion constant''' of water in PSS/PDADMA polyelectrolyte complexes (PDADMA stand for poly(diallyldimethylammonium)).
  
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So far, we are building the PSS/PDADMA bilayers based on the previously obtained PSS monolayers and we are expecting to extract exciting information from these studies. The simulation of a bilayer represents our final goal for atomistic simulations of PMEs so far, due to the high requirement of computer resources (ca. 1000 000 cpu hours in total).
  
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* In short, despite the amount of work done during the last 15 years, the understanding of the multilayer formation process and the knowledge about how slight differences during the growth process are able to strongly modify the properties of the multilayer materials is still in its infancy. Doubtless, the understanding of such issues is of paramount importance to improve current building-up methods and devices, tune finely the properties of such materials for specific purposes, and in turn devise new potential applications for such materials. Such knowledge will not be only of benefit for the  Scientific Community but also for industry as well as society due to the huge potentiality of such materials for new devices and applications.  
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Due to the limitations of atomistic simulations, further insight into structure and dynamics of PEMs can be achieved only with simulations at CG level. Qualitative understanding and agreement with experiments has been obtained by us using the already existing generic bead-spring PE model. However, a refined CG PE model is needed in order to be quantitatively predictive. This is part of our next working program.  
  
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== Useful References ==
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Some selected results obtained during the last 2.5 years are:
  
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[1] Decher G, Hong JD, and Schmitt J, Thin Solid Films, 210, 831, (1992).
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'''a) Bilayer thermodynamical instability'''
  
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[2] Tran D, and Renneberg R, Biosensors and Bioelectronics, 18, 1491, (2003).
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[[Image:thermodynamic_stability_bilayer_vs_trilayer.png|200px|right|thumb|Thermodynamic instability of the bilayer and the stability of the fast deposited tri-layer.]]
  
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[3] Thierry B, Winnik FM, Merhi Y, and Tabrizian M, J. Am. Chem. Soc., 125, 7494, (2003).  
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Our CG level simulations have shown that depending on the relative strength of the monomer-monomer and monomer-surface interaction energies, a '''progressive redissolution''' of the first bilayer or a '''partial dewetting''' resulting in a disordered melt can happen.
  
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[4] Malaismy R, and Bruening M, Langmuir, 21, 10587, (2005)
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We have shown that a '''fast enough deposition of the third layer''' -- before the aging process  --  can '''prevent such redissolution or partial dewetting''' and provide the stability
 +
needed to form a PEM. We have checked that the deposition of further layers is a stable process. This suggests that the first PE bilayer is not thermodynamically stable, while tri-layers and higher layers are stable, at least within the long run time of our simulations.
  
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[5] Jiang L, Lu F, Chang Q, Liu Y, Liu H, Li Y, et al., Chem. Phys. Chem., 6, 481,(2005).
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'''b) Charge compensation mechanism'''
  
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[6] Arsenault AC, Halfyard J, Wang Z, Kitaev V, Ozin GA, et al., Langmuir, 21, 499, (2005).
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[[Image:intrinsic_vs_extrinsic_result_in_PEC.png|200px|right|thumb|Possibilities of sulfurs from PSS and nitrogens from PDADMA which are intrinsically and extrinsically charge compensated.]]
  
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[7] Kamande MW, Fletcher KA, Lowry M, and Warner IM, J. Sep. Sci., 28, 710, (2005).
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In our AA level simulations on PSS/PDADMA complexes, intrinsic (polyanions pair with polycations) and extrinsic (polyions pair with salt ions) charge compensation mechanisms have been found to co-exist, although '''the intrinsic one is predominant''' in the investigated salt (NaCl) concentration range from 0.17 to 1.00 mol/L.
  
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[8] Khopade AJ, Arulsudar N, Khopade SA, Hartmann J, Biomacromolecules, 6, 229, (2005).
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Furthermore, the relative scale of the interaction energy of the ion-pairs in such PSS/PDADMA mixture is calculated to follow (in kJ/mol): Na-Cl (-520) > PSS-Na (-420) > PDADMA-Cl (-280) ~ PSS-PDADMA (-270). The relative scale of the interaction energy can be very useful to explain some experimental finding, such as PSS is found to be in a higher concentration than PDADMA in PSS/ PDADMA complexes [5]. This information is also valuable to properly model the interactions between ion-pairs in the upcoming refined CG model.
  
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[9] Messina R, Holm C, Kremer K, J. Poly. Sci. B, 42, 3557, (2004). [10] Klitzing RV, Wong JE, Jaeger W, and Steiz R, Current Op. Coll. Interf. Sci., 9, 158,(2004).
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'''c) PSS adsorption monolayer'''
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[[Image:PSS_momolayer.png|200px|right|thumb|PSS adsorption monolayer]]
  
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[11] Schönhoff M, Current Op. Coll. Interf. Sci., 8, 86, (2003). [12] Hammond PT, Current Op. Coll. Interf. Sci., 4, 430, (2000).
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The PSS monolayer is diposited from a PSS solution via atomistic simulations. Our results demonstrate that short-range interactions of van der Waals origin from the adsorbing substrate play a significant role in the layer structure of the adsorbed PSS, and they alone are already sufficient to induce a stable PSS adsorption layer. The PSS chains are found to behave as hydrophilic PEs, two kinds of conformations of which are observed: flat PSS adsorption layer dominates with some adsorbed PSS chains dangling into the above PSS solution.
  
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[13] Decher G, Science, 277, 1232, (1997).
 
  
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[14] Kharlampieva E, and Sukhishvili SA, Langmuir, 19, 1235, (2003).
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'''d) PE chain pulling experiment'''
  
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[15] Schoeler B, Kumaraswamy G, and Caruso F, Macromolecules, 35, 889, (2002).
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[[Image:PE_pulling_experiment.png|200px|thumb|Results from our CG simulations]]
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[[Image:PE_pulling_experiment_Hugel.png|200px|thumb|Exp. data from Hugel's group]]
  
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[16] Kujawa P, Moraille P, Sanchez J, Badia A, Winnik FM, J.Am.Chem.Soc, 127, 9224, (2005) .
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The present, non-refined CG model yields a qualitative agreement with the experiments by the Hugel group. This makes us confident that maybe even a quantitative comparison might be obtainable once the refined coarse-grained model will be ready.
  
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[17] Salomäki M, Vinokurov IA, Kankare J, Langmuir, 21, 11232, (2005). [18] Guyomard A, Muller G, Glinel K, Macromolecules, 38, 5737, (2005).
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In the computer simulations on PE pulling experiments, a PE chain, which is similar to the PE chains of the capping layer, is introduced with the corresponding counterions. The averaged force, that is needed to keep one of the chain ends fixed at a given point Z_{tip}, is measured by performing several independent runs. The position of the chain tip is slowly increased to a new value where a new measurement was performed.
  
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[19] Netz RR, Joanny JF, Macromolecules, 32, 9013, (1999).
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== Scientists ==
  
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[20] Park SY, Rubner MF, and Mayes AM, Langmuir, 18, 9600, (2002).
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* ''[[Christian Holm]]''
 +
* ''[[Baofu Qiao]]''
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* ''[[Joan Josep Cerdà]]''
 +
* ''[[Marcello Sega]]''
  
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[21] Castlenovo M, and Joanny JF, Langmuir, 16, 7524, (2000).  [22] Messina R, Holm C, Kremer K, Langmuir, 19, (10), 4473, (2003).
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== Publications ==
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<bibentry> messina03a, messina04b, cerda09b,cerda09c, qiao10a, qiao11a</bibentry>
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[23] D. Kovacevic, S. van~der Burgh, M.A. Cohen-Stuart, Langmuir 18, 5607 (2002).
 
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[24] Patel PA, Jeon J, Mather PT, and Dobrynin AV, Langmuir, 21, 6113, (2005).
 
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[25] Panchagnula V, Jean J, Rusling JF, and Dobrynin AV, Langmuir, 21, 1118, (2005).
 
  
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[26] Abu-Sharkh B, J. Chem Phys., 123, 114907, (2005).
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== References ==
 +
[1] J. Schmitt, G. Decher and G. Hong. ''Thin Solid Films'', '''1992''', ''210/211'', 831.[http://www.sciencedirect.com/science/article/B6TW0-47X13HN-47/2/e56caa8317193804cc1863ce3db4a32b URL]
  
 +
[2] G. Decher, ''Science'','''1997''', ''277'', 1232. [http://www.sciencemag.org/cgi/content/abstract/277/5330/1232 URL]
  
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== Coworkers ==
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[3] J. B. Schlenoff, ''Langmuir'', '''2009''', ''25'', 14007.[http://pubs.acs.org/doi/abs/10.1021/ja802054k URL]
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* [[Christian Holm]]: Project supervisor
 
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* [[Joan J. Cerda]]  
 
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== Collaborators ==
 
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*
 
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== Publications ==
 
  
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[4] A. V. Dobrynin. ''Curr. Opin. Colloid Interface Sci'', '''2008''', ''13'', 376.[http://www.sciencedirect.com/science/article/B6VRY-4S7JFXX-2/2/fd13c66c913bca456ed80d11acd1da39 URL]
  
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== Links ==
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[5] H. H. Hariri, and J. B. Schlenoff, ''Macromolecules'', '''2010''', DOI: 10.1021/ma1012978. [http://dx.doi.org/10.1021/ma1012978 URL]
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* {{Download|xxxx|Posters, talks etc presented ...}}
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[[Category:Research]]

Latest revision as of 20:21, 5 September 2011


What is a Polyelectolyte Multilayer (PEM)?

PEMs are composed of alternating layers of oppositely charged polyelectrolytes (PEs) (synthetic PEs or biomolecules), which are generally built up based on the Layer-by-Layer technique. [1,2] Due to their potential applications as membrane, encapsulation and matrix materials, and for enzymes and proteins in sensor applications, PEMs have stimulated great interests from both academic researchers and industries.[3] See also a PEM website. Despite the large number of experimental works, theoretical and computational studies toward understanding the microscopic structure of PEMs are scarce.[4]

Our Research

In close collaboration with experimental investigations from groups of Prof. von Klitzing and Prof. Hugel, we are currently working to investigate the inner structure and dynamics of a small layer number of PEMs via all-atom (AA) and coarse-grained (CG) simulations. The project is funded by the DFG SPP 1369: Polymer-Solid Contacts: Interfaces and Interphases Spp1369 logo small.jpg


Schematic representation of PSS and PDADMA.

The AA level simulations proved to be consistent with existing experimental data on chain conformation of adsorbed poly(styrene sulfonate) (PSS) in PSS monolayer systems, dielectric permittivity and diffusion constant of water in PSS/PDADMA polyelectrolyte complexes (PDADMA stand for poly(diallyldimethylammonium)).

So far, we are building the PSS/PDADMA bilayers based on the previously obtained PSS monolayers and we are expecting to extract exciting information from these studies. The simulation of a bilayer represents our final goal for atomistic simulations of PMEs so far, due to the high requirement of computer resources (ca. 1000 000 cpu hours in total).

Due to the limitations of atomistic simulations, further insight into structure and dynamics of PEMs can be achieved only with simulations at CG level. Qualitative understanding and agreement with experiments has been obtained by us using the already existing generic bead-spring PE model. However, a refined CG PE model is needed in order to be quantitatively predictive. This is part of our next working program.

Some selected results obtained during the last 2.5 years are:

a) Bilayer thermodynamical instability

Thermodynamic instability of the bilayer and the stability of the fast deposited tri-layer.

Our CG level simulations have shown that depending on the relative strength of the monomer-monomer and monomer-surface interaction energies, a progressive redissolution of the first bilayer or a partial dewetting resulting in a disordered melt can happen.

We have shown that a fast enough deposition of the third layer -- before the aging process -- can prevent such redissolution or partial dewetting and provide the stability needed to form a PEM. We have checked that the deposition of further layers is a stable process. This suggests that the first PE bilayer is not thermodynamically stable, while tri-layers and higher layers are stable, at least within the long run time of our simulations.

b) Charge compensation mechanism

Possibilities of sulfurs from PSS and nitrogens from PDADMA which are intrinsically and extrinsically charge compensated.

In our AA level simulations on PSS/PDADMA complexes, intrinsic (polyanions pair with polycations) and extrinsic (polyions pair with salt ions) charge compensation mechanisms have been found to co-exist, although the intrinsic one is predominant in the investigated salt (NaCl) concentration range from 0.17 to 1.00 mol/L.

Furthermore, the relative scale of the interaction energy of the ion-pairs in such PSS/PDADMA mixture is calculated to follow (in kJ/mol): Na-Cl (-520) > PSS-Na (-420) > PDADMA-Cl (-280) ~ PSS-PDADMA (-270). The relative scale of the interaction energy can be very useful to explain some experimental finding, such as PSS is found to be in a higher concentration than PDADMA in PSS/ PDADMA complexes [5]. This information is also valuable to properly model the interactions between ion-pairs in the upcoming refined CG model.

c) PSS adsorption monolayer

PSS adsorption monolayer

The PSS monolayer is diposited from a PSS solution via atomistic simulations. Our results demonstrate that short-range interactions of van der Waals origin from the adsorbing substrate play a significant role in the layer structure of the adsorbed PSS, and they alone are already sufficient to induce a stable PSS adsorption layer. The PSS chains are found to behave as hydrophilic PEs, two kinds of conformations of which are observed: flat PSS adsorption layer dominates with some adsorbed PSS chains dangling into the above PSS solution.


d) PE chain pulling experiment

Results from our CG simulations
Exp. data from Hugel's group

The present, non-refined CG model yields a qualitative agreement with the experiments by the Hugel group. This makes us confident that maybe even a quantitative comparison might be obtainable once the refined coarse-grained model will be ready.

In the computer simulations on PE pulling experiments, a PE chain, which is similar to the PE chains of the capping layer, is introduced with the corresponding counterions. The averaged force, that is needed to keep one of the chain ends fixed at a given point Z_{tip}, is measured by performing several independent runs. The position of the chain tip is slowly increased to a new value where a new measurement was performed.

Scientists

Publications

  • René Messina, Christian Holm, Kurt Kremer.
    Polyelectrolyte Multilayering in Spherical Geometry.
    Langmuir 19:4473–4482, 2003.
    [PDF] (963 KB) [Preprint]
  • René Messina, Christian Holm, Kurt Kremer.
    Polyelectrolyte Adsorption and Multilayering on Charged Colloidal particles.
    Journal of Polymer Science, Part B: Polymer Physics 42:3557, 2004.
    [PDF] (620 KB) [DOI]
  • Juan J. Cerdà, Baofu Qiao, Christian Holm.
    Understanding polyelectrolyte multilayers: an open challenge for simulations.
    Soft Matter 5:4412–4425, 2009.
    [PDF] (1.7 MB) [DOI]
  • Juan J. Cerdà, Baofu Qiao, Christian Holm.
    Modeling strategies for polyelectrolyte multilayers.
    European Journal of Physics Special Topics 177(1):129–148, 2009.
    [PDF] (971 KB) [DOI]
  • Baofu Qiao, Juan J. Cerdà, Christian Holm.
    Poly(styrenesulfonate)-Poly(diallyldimethylammonium) Mixtures: Toward the Understanding of Polyelectrolyte Complexes and Multilayers via Atomistic Simulations.
    Macromolecules 43(18):7828–7838, 2010.
    [PDF] (2.3 MB) [DOI]
  • Baofu Qiao, Juan J. Cerdà, Christian Holm.
    Atomistic Study of Surface Effects on Polyelectrolyte Adsorption: Case Study of a Poly(styrenesulfonate) Monolayer.
    Macromolecules 44(6):1707–1718, 2011.
    [PDF] (3.6 MB) [DOI]


References

[1] J. Schmitt, G. Decher and G. Hong. Thin Solid Films, 1992, 210/211, 831.URL

[2] G. Decher, Science,1997, 277, 1232. URL

[3] J. B. Schlenoff, Langmuir, 2009, 25, 14007.URL

[4] A. V. Dobrynin. Curr. Opin. Colloid Interface Sci, 2008, 13, 376.URL

[5] H. H. Hariri, and J. B. Schlenoff, Macromolecules, 2010, DOI: 10.1021/ma1012978. URL

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