Recently there has been a lot of interest in mimicking lightweight natural bio-materials such as Nacre, Spider silk, Crustacean’s and various other materials for their unique high stiffness, strength properties. All these natural materials are formed by a combination of stepwise mineralization and a slow self-regulated growth of an organic component which allows the material to have high stiffness which can be attributed to the precise structuring of the individual layers within the material; This orderly arrangement of alternating hard/soft composite matrix has a very unique crack propagation mechanism and the soft has a unique energy dissipation mechanism. The mimicking of these bio-composites by using wholly synthetic materials has proved to be a challenge for many years, although recently artificial Nacre has garnered a lot of interest due to its high stiffness, mechanical properties which is comparable to natural Nacre. The remarkable mechanical properties of these bio-composites can be attributed to precise hierarchal arrangements in the microscopic level leading to amplification at the macro level. The challenge for producing these bio-inspired materials was the availability of a technique that can produce multifunctional materials by manipulations at the nano-scale.
Amongst the many techniques developed for the preparation of nanoscale multi-functional materials, the Layer by Layer (LbL) technique has proven to be by far the simplest and most effective method of producing multimaterial films by just tuning the spatial arrangement of functionality in one dimension. The major advantage of this technique is the application of the LbL assembly on different substrates yield identical properties, meaning films can be prepared on analytical surfaces such as Si-wafers and the LbL films can be put to the test by depositing it on surfaces such as textiles which prevent physio-chemical characterizations. Another key advantage of LbL is the relative ease with which functionalized free standing films can be prepared in comparison to other methods such as Langmuir-Blodgett technique which had painstakingly long processing times along with a large number of parameters that had to be accounted for.
The goal of this internship was to develop a route by which a bio-mimic of a crustacean based system could be prepared by the Layer by Layer (LbL) approach. During this internship period, I was given a training in LbL wherein I had to prepare multilayers of PSS/PSH by using both dipping and spray assisted LbL, the eventual goal of this internship was to prepare freely standing films of Cellulose Nanofibres (CNF) with Polyvinyl Amine (PVAm), PolyVinyl alcohol (PVA) by spin coating assisted LbL and dipping based LbL. By comparing these two methods I would be able to choose a suitable route to prepare micrometer size films for mechanical characterization and by utilizing two different systems namely CNF/PVAm, CNF/PVA I can compare the different layer pair growth mechanisms; Finally, by using QCM-D I would be able to analyze the kinetics of adsorption of PVAm VS PVA on CNF.
Bioinspired Layer by Layer approaches
In this manuscript I will detail a few of the bio-inspired systems considered for LbL assembly chosen because of its unique hierarchical arrangement and mechanical properties. The considered biological systems were: (1) Nacre shells (2) Dactyl club of Mantis shrimp (3) Silk Fibroin.
The structures of Haliotis tuberculata (commonly referred to as Nacreous shells) are formed by alternating hierarchical arrangements of porous hydrophobic a- Chitin sheets and a chemically stabilized amorphous CaCO3 precursor phase; Subsequent arrangement of these layers into a stack, followed by CaCO3 crystallization leads to lamellar stacks of ACC tablets with organic porous layers in between. Generally, the Nacre structure consists of 250-500 nm thick aragonite tablets separated by a 30-90 nm thick organic layer; In this structure the crystallinity is preserved throughout the structure and this stack of periodicity gives rise to its impressive mechanical properties along with its characteristic iridescence.
Xx et.al were able to successfully mimic Nacre artificially by using the LbL approach; the process involved the formation on Organic films by LbL followed by forming a porous film by surface functionalization, depositing the thin films onto mineral layers to form modified calcite and finally crystallizing the mineralized/organic films (In this study, the organic films were prepared by LbL deposition of PAA, PVP for 20 layer pairs by the use of a dip coating robot). One key advantage of this mimic process was the formation of porous organic intercrystalline layers interconnecting the mineral films by bridges thereby eliminating delamination effects and preserving the mechanical stability of the films.
In this study Xx et.al had compared the mechanical properties of natural, artificial Nacre as 69 Gpa, 38 Gpa respectively; For determining the mechanical properties they used cube – corner indentation. Finally, they were able to mimic natural Nacre in terms of its mechanical, optical properties by utilizing the unique route of synthesis employed to mimic Nacre.
Silk fibrion has garnered a lot of interest as a key biomaterial component in hybrid nanocomposites due to its good biocompatibility and biodegradability in biomedical applications. Silk fibrion is extracted primarily from silk worm cocoons, can be applied to a range of materials from thin films to electrospun fibers; It also has very good mechanical properties such as high young’s modulus, elongation at break and toughness but lacks the robustness required in biotechnology inspired applications.
In this study Xc. Et al, the authors reinforce silk fibrion with two inorganic components in separate namely Montmorillonite (MMT) and Polyhedral oligomeric silesquioxane (POSS – M) to enhance the mechanical robustness of the silk composites; These composites were made by conventional LbL and modified LbL wherein the inorganic components (MMT, POSS-M) were combined with silk matrices which were crosslinked with Gluteraldehyde (GA) prior to LbL to enhance their interfacial strength.
In applying this modified LbL synthesis route, the authors observed that the nanocomposites of (Silk+GA+MMT) had a thickness increment of 5.35 nm/bilayer, whereas (Silk+MMT) had a thickness increment of 4.7 nm/bilayer. Mechanical tests performed on the composites had showed that modified silk – MMT composites had a Young’s modulus of 25±2 GPA in contrast with Silk-MMT composites with a Young’s modulus of 12±2 GPA. The authors had showed that the inclusion of nanofillers, crosslinkers combined with LbL assembly had a considerable effect in thickness of the composites and showed a 15% increase in Young’s modulus in comparison with composites that were prepared by traditional LbL assembly.
Recently the raptorial appendage of the stamatopods (a species of marine crustaceans) have been studied in depth due to its remarkable mechanical properties; the stamatopods use this dactyl clubs to smash its prey at amazing speeds of 23 m/s and are able to endure repetitive forces of about 1500N, which makes it a very unique damage tolerant natural material and has been investigated in depth by Xv.et al.
The dactyl club is mostly made up of two regions: Impact region, Periodic region; Impact region is made up of an Impact surface which is a thin layer on the surface of the dactyl club of about 70 µm followed by a well-defined herringbone pattern (elastic modulus oscillates between 30-45 Gpa) and the periodic region consists of a characteristic helicoidal arrangement of mineralized chitin fibers which have an elastic modulus of 25 Gpa which could be attributed to the gradient in mineralization. The helicoidal arrangement is compacted laterally in the impact region leading to the formation of a herringbone pattern, these fibers rotate about the normal axis (surface of the club) which yields a sinusoidal pattern. The out of plane mineralized fibers which are normal to the fracture plane are scattered in between the vertically aligned fibrous pore canal tubes; the propagation of fibrous pore canal tubes from the periodic to impact region not only serves as a route for material transfer but also strengthens the material especially when experiencing shear, tensile stress during impact.
In X.c1 et al the authors had devised a route to synthesize a crustacean mimic by concentration induced self-assembly of Cellulose Nanocrystals (CNC) in Polyvinyl Alcohol (PVOH); By alternating the CNC/PVOH ratio, the authors were able to demonstrate the effect of tuning the pitch height on the nematic cholesteric phase. Sequential deposition of PVOH/CNC on top of each other produces materials containing different periodicities and properties. The authors also observed that by varying the PVOH content more than 40 % the cholesteric nematic phase had become unstable and had made these observations by adjusting the half pitch height of the chiral nematic phase from ca. 355 nm for pure CNC to ca. 486 nm for 60/40 of CNC/PVOH. Producing materials with stacked periodicity would not only allow structural control but encoding the material with different mechanical properties as a function of the helical pith is possible. The authors performed tensile tests on individual nanocomposite films at a relative humidity of 55% by using a wet/humidity based drawing device in line with a linear actuator to perform strain controlled drawing; the highest mechanical properties were observed for 90/10 (CNC/PVOH) films with Young’s modulus of 10±2.8 Gpa and the lowest Young’s modulus was observed for 40/60 (CNC/PVOH) films. Although the films with higher content of CNC has a better elastic modulus, they have a reduced elongation at break which could be attributed to the stiffness of CNC’s and by observing the crack propagation mechanisms the authors had concluded that by incorporating higher amount of polymer, the composites have a higher number of toughening mechanisms which is similar to that observed in crustaceans.
Layer by Layer assembly
Ideally a self-assembly experiment involves different chemical species attaining an equilibrium state but it is very likely that the experiment results in the formation of a material with less than optimal properties. Usually the adopted strategy would be testing the material in different assembly conditions by trial and error, usually followed by a rather arduous series of optimization cycles to follow. One popular strategy developed was to prepare hierarchically organized composite materials that use an assembly procedure that bypasses equilibrium by confining the compound kinetically in a predetermined spatial arrangement. In the early 20th century, the Langmuir- Blodgett (LB) technique was used to create multi-functional materials by forming monolayers on the surface of water and transferring them to a solid support by the use of a trough; Although this technique had many advantages it was limited to the number of components that were suitable for LB deposition and most importantly the molecules would rearrange frequently after deposition resulting in inhomogeneity of films.
During the 1980’s Decher’s group had introduced a technique called “Layer-by-Layer” deposition which involved sequential deposition of polycation over polyanion on solid surfaces with an intermediate rinsing step; this technique allowed the fabrication of nanometre scale multilayers with a plethora of different materials in a single device with relative ease. In this technique, it is very important to keep in mind certain parameters that effect the multilayer build-up such as solvent concentration of adsorbing species, deposition time, salt concentration, rinsing time, drying rate, dipping speed and surface roughness.
Photo of LbL
LbL deposition techniques
LbL deposition by dipping
The dipping assisted LbL is by far the simplest, earliest form of LbL which requires two solutions of oppositely charged polyelectrolytes and a positively charged substrate. This technique involves dipping the positively charged substrate into a polyanion solution resulting in a charge reversal on the surface, subsequently rinsing the substrate would remove weakly attached chains and finally dipping the substrate into a polycation solution would cause a new adsorption step that results in a charge reversal to a positive charge on the surface. These steps are repeated continuously in order to produce a multi-layered film. The charge reversal can be attributed to the unbalanced nature of excess charge present on the surface with the surface charge during adsorption and this allows the deposition of the next layer. In the dipping process, there is a formation of a zone with a very low polyelectrolyte concentration further referred to as “depletion zone” close to the surface of the substrate. This depletion layer is formed mainly due to hydrodynamic phenomena and gradual depletion of polyelectrolyte adsorption on the surface. The depletion zone is a function of the polyelectrolyte concentration gradient which varies from zero at the surface and increases rapidly in the free solution. Thus, the dipping process is diffusion controlled at the surface as the polyelectrolyte chains would have to diffuse through depletion zone before arriving to the surface and is controlled by both convection, diffusion when solution is flowed over the surface of the substrate.
LbL deposition by spraying
One of the major disadvantages of LbL assembly by dipping was the time required for film deposition which varies between few hours for one-layer pair to days for obtaining a thick film. Spray assisted LbL assembly was introduced by (51,52) , which considerably shortens the deposition time while keeping the quality of the film intact. In the spraying process, usually the polyanion solution is sprayed onto the positively charged substrate followed by spraying the substrate with water and finally the polycation is sprayed onto the substrate with identical deposition times. It has been suggested that since there is a continuous drainage of liquid, the rinsing step can be avoided and thus speed up the process.
In this process, the receiving surface is kept vertical and the spraying cone is oriented horizontally; there is a perpendicular orientation between spray axis and the receiving surface and this orientation will allow the entire liquid that comes into contact with the receiving surface to drain at the maximum speed. As the thickness of the film is thin, it is assumed that any droplet arriving at the surface would instantaneously fuse to the liquid film and replace the draining liquid. Since the concentration of solution in adsorbed liquid film and spray reservoir are identical, it can be assumed that the depletion zone would be formed close to the surface and the adsorbed species diffusing through this zone would have a negligible effect which is in contrast with that of the dipping process.
Spin assisted LbL
Spin assisted LbL takes advantage of small amount of liquids required for the process and short contact times with the substrate. In this process, polyelectrolyte solutions are spun onto the substrate at optimum speeds, followed by typically two rinsing steps maintained at the same deposition conditions and sequentially depositing multiple layers in a relatively shorter period of time.
In spin assisted LbL, the adsorption and rearrangement of the adsorbed chains on the surface and the elimination of weakly bound chains on the surface is simultaneously taking place while spinning. By eliminating water at high speeds, the concentration of polyelectrolytes increase during the short deposition time