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Bioinspired Design and Assembly of Platelet Reinforced Polymer Films

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Science  22 Feb 2008:
Vol. 319, Issue 5866, pp. 1069-1073
DOI: 10.1126/science.1148726

Abstract

Although strong and stiff human-made composites have long been developed, the microstructure of today's most advanced composites has yet to achieve the order and sophisticated hierarchy of hybrid materials built up by living organisms in nature. Clay-based nanocomposites with layered structure can reach notable stiffness and strength, but these properties are usually not accompanied by the ductility and flaw tolerance found in the structures generated by natural hybrid materials. By using principles found in natural composites, we showed that layered hybrid films combining high tensile strength and ductile behavior can be obtained through the bottom-up colloidal assembly of strong submicrometer-thick ceramic platelets within a ductile polymer matrix.

Steel and other metal alloys have long been used for the fabrication of strong and flaw-tolerant materials for structural applications. As opposed to metals, ceramic and polymeric materials do not exhibit the unique combination of high strength and flaw tolerance. Ceramics are typically strong but not tolerant to surface flaws and cracks, whereas most polymers are flaw-tolerant but deform extensively at rather low applied stresses.

Nature has found its way around this dilemma by combining plateletlike ceramic building blocks with polymeric matrices to render hybrid materials that are both strong and flaw-tolerant. Examples include mineralized tissues of vertebrates, such as bone, teeth, and calcified tendons, as well as the outer skeleton of invertebrates, such as the nacreous layer of mollusk shells (1).

The exquisite structure of these biological materials and the underlying concepts leading to their mechanical behavior have been extensively studied (25). Although substantial progress has been made on understanding the mechanical response of such structures (611), the manufacture of artificial composites that copy nature's designs remains a challenging goal.

A number of different materials have been used as inorganic reinforcing platelets for the fabrication of polymer-matrix artificial composites, including glass, graphite, SiC, AlB2, mica, talc, and clays (1215). Considerable increase in stiffness and strength has been achieved at rather low platelet concentrations, particularly in the case of polymers reinforced with nanosized clay particles (16). However, improvements in the composite strength at higher platelet concentrations [>10 weight % (wt %)] are often notably lower than that expected from theoretical models for reinforced polymers (12, 17, 18). This has been attributed to difficulties in achieving complete exfoliation and dispersion of platelets within the organic matrix and to poor bonding at the inorganic-organic interface (12, 18, 19). The strengthening seen with nanosized clay particles also seems to involve the cross-linking of polymer chains between neighboring platelets rather than solely depending on load transfer mechanisms operative in composites reinforced with micrometer-sized inorganic fibers (17, 18, 20). This cross-linking effect was used in making clay-based nanocomposites exhibiting tensile strength up to 400 MPa (18). However, such strong nanocomposites exhibit brittle fracture under tension (flaw intolerance), presumably because of the very high platelet concentration [50 volume % (vol %)] needed to reach this high strength level.

We applied some of the structural concepts found in biological materials to design and fabricate platelet-reinforced hybrid films that are both strong and ductile. The size and aspect ratio of the reinforcing platelets, as well as the mechanical properties of the individual inorganic and organic phases, were deliberately chosen to mimic the designing principles of strong and tough biological structures.

Biological materials such as bones, teeth, and mollusk shells are characterized by a layered structure consisting of strong inorganic platelets embedded in a soft, ductile organic matrix (1). In spite of the inherently weak inorganic constituents (e.g., silica, calcium carbonates, and phosphates), the high strength of the inorganic building blocks is ensured by limiting at least one of their dimensions to the nanoscale (3). These tiny building blocks are usually organized into a hierarchical structure spanning over various length scales. Changes in the fraction of inorganic phase (i.e., degree of mineralization) lead to hybrid materials ranging from soft tissues such as calcified tendons to strong, hard structures such as bone and nacre. Among the different models that have been proposed to describe the mechanical response of biological mineralized structures (811), a simple approach based on the mechanics of composite structures has been successfully applied to explain the strength and ductility of nacre (7).

The strength of nacre has been described on the basis of a simple shear lag model, according to which the applied load is transferred to the inorganic platelets through shear stresses developed in the organic matrix (7). For ductile matrices exhibiting a yield shear strength τy and a perfect bonding to the platelet surface (interfacial strength τi ≥ τy), the ultimate tensile strength of the hybrid material (σc) can be estimated from the volume fraction of platelets (Vp), the platelet aspect ratio (s), and the tensile strength of the platelets (σp) and of the organic matrix (σm), as follows (21): Math(1) with the factor α being a function of s, τy, and σp.

The tensile strength of the hybrid material depends on the operative failure mode, which in turn is determined by the aspect ratio of the inorganic platelets, s. For aspect ratios higher than a critical value (s > sc), the composite fails because of the fracture of platelets (Fig. 1A), leading to a brittle catastrophic rupture of the material. In this case (s > sc), the factor α in Eq. 1 is given by the following relation: Math(2)

Fig. 1.

Fracture mechanisms (A) and estimated tensile strength (B) of platelet-reinforced composites. The aspect ratio of the platelets (s) determines whether the composite fails under platelet fracture mode (s > sc) or platelet pull-out mode [s < sc (A)]. The tensile strength of polymer matrix composites reinforced with CaCO3 and Al2O3 platelets is estimated in (B) (black and blue surfaces, respectively) on the basis of a shear-lag mechanical model (21). The limit between the platelet fracture and pull-out modes is indicated by the black and blue full lines for CaCO3 and Al2O3 platelets, respectively. τy and σm of, respectively, 40 and 50 MPa were assumed here for the organic matrix in nacre (protein interlamella layer) (7) and in the artificial composites (chitosan, Fig. 3B). Platelet strength (σp) of 0.4 and 2.0 GPa were assumed for the CaCO3 (7) and Al2O3 platelets, respectively.

On the other hand, for aspect ratios lower than the critical condition (s < sc) the continuous matrix yields before the platelets break, thus leading to toughening mechanisms such as platelet pull-out and matrix plastic flow before the complete rupture of the composite (Fig. 1A). For s < sc, the factor α reads Math(3)

Composites that fail under the platelet fracture mode are strong but brittle and thus flaw-intolerant, whereas materials that rupture under the platelet pull-out mode are generally weak but ductile and thus flaw-tolerant (Fig. 1A). Note that for weak inorganic-organic interfaces fracture occurs at the interface before the matrix yields, so that τy has to be replaced by τi in the composite strength Eqs. 1 and 3.

The structure of biological materials like nacre has evolved in such a way to form platelets with s (s ∼8)(7) slightly below sc, so that a maximum strength is achieved without imparting brittleness to the composite (Fig. 1B) (7). sc is equal to the ratio σpy. Assuming σp between 360 and 500 MPa for the aragonite platelets in nacre [supporting online material (SOM) text], we expect a low sc in the range from 9 to 12.5 for such natural composite (τi = 40 MPa) (7). In contrast to the limited number of inorganic materials available for the synthesis of biological structures, artificial composites can benefit from the outstanding mechanical strength of synthetic inorganic fibers and platelets (σp ∼2 to 5 GPa). This leads to a higher scpy), allowing for a considerable increase of s before sc is reached (22).

Guided by these design principles, we selected inorganic platelets and an organic polymer with features that should lead to artificial hybrid materials exhibiting high strength and ductility. Alumina platelets with estimated σp of 2 GPa and a chitosan polymer with τy around 40 MPa were chosen. With a high sc of 50, the combination of these materials should lead to strength values higher than that of nacre while ensuring that fracture occurs by platelet pull-out, as indicated in Fig. 1B (blue surface). In order to maximize strength without impairing the polymer's ductility, 200-nm-thick platelets with an average s of about 40 were used (s slightly below sc, see dashed blue line in Fig. 1B). In spite of their higher s, the size of the artificial alumina platelets is in the same length scale as that of the aragonite platelets encountered in nacre (3, 7). With the exception of a recent study on composites with low platelet concentrations (Vp ≤ 0.03) (15), most investigations have been carried out with use of either silicate-based platelets with thickness around 1 nm (13, 14, 1618, 20, 23, 24) or artificial platelets with thickness larger than a few tens of a micrometer (19, 22). Platelets at the length scale we used are expected to be thin enough to possess the high tensile strength of small inorganic building blocks (3) and sufficiently thick to be fully dispersed in the polymer matrix so as to strengthen the composite through conventional load transfer mechanisms (22).

We fabricated layered hybrid materials by using an approach that relies on the sequential deposition of inorganic and organic layers at ambient conditions (Fig. 2). Because the artificial platelets are submicrometer-sized in thickness, colloidal-based techniques can be used for the assembly of the inorganic building blocks. The ability of colloidal platelets to adsorb at air-water interfaces was used as a means to direct their assembly into a highly oriented two-dimensional (2D) structure. The adsorption of platelets at the air-water interface was favored through the attachment of slightly hydrophobic amine-terminated silane species on the platelet surface (fig. S1) (25). Amine groups at the end of the silane hydrophobic tail are expected to form hydrogen bonds with the oxygen atoms of the chitosan backbone, increasing the adhesion between inorganic platelets and the organic matrix. An ethanol suspension containing 1 vol % of modified platelets was spread over a water surface. A smooth and perfectly oriented monolayer of platelets was formed at the surface of water upon ultrasonication (Fig. 2). The formation of a repulsive electrical double layer between like-charged platelets at neutral pH allowed for extensive rearrangement of particles during sonication. The 2D assembled platelets were transferred to a glass substrate by dip-coating and were then spin-coated with an organic layer of chitosan solution (25). The thickness of the polymer layer was controlled by changing the chitosan concentration in the spin-coating solution (fig. S2). Repetition of these steps in a sequential manner leads to multilayered inorganic-organic films with a total thickness typically less than a few tens of a micrometer. Free-standing films were obtained by peeling them off from the substrate with a razor blade (Fig. 2).

Fig. 2.

Bottom-up colloidal assembly of multilayered hybrid films. Surface modified platelets are assembled at the air-water interface to produce a highly oriented layer of platelets after ultrasonication. The 2D assembled platelets are transferred to a flat substrate and afterwards covered with a polymer layer by conventional spin-coating.

Flexible and thin hybrid films containing Vp up to 0.2 exhibited a brick-mortar structure with strongly aligned platelets surrounded by a ductile organic matrix (Fig. 3A). For inorganic volume fractions higher than 0.2, extensive swelling of the thin chitosan layer during dip coating led to platelet misalignment and the incorporation of voids within the film (Fig. 4A and fig. S3). As a result of strong platelet alignment for volume fractions lower than 0.2 (Fig. 4A), a substantial fraction of a load applied parallel to the ordered layers can be taken by the stronger inorganic phase, increasing the material's elastic modulus (Ec) from 2 up to 10 GPa (Figs. 3B and 4B).

Fig. 3.

Microstructure and mechanical behavior of artificial and natural hybrid materials with different concentrations of inorganic platelets. The highly oriented microstructure of an Al2O3-chitosan artificial film is shown in (A) for Vp = 0.11. The use of strong platelets with s slightly lower than sc leads to light and flexible composites combining high tensile strength and high strain at rupture (B). Chitosan-based films prepared with platelet concentrations as low as 10 and 15 vol % exhibit tensile strength comparable to or higher than that of natural layered materials (C). The stress versus strain curves for nacre (red abalone, Haliotis rufescens), bone, dentin, and calcified tendon were obtained from Wang et al.(2), Landis et al. (after Ashman) (29), Sano et al.(28), and Landis et al. (29), respectively. The scale bar in (A) indicates 5 μm. The stress versus strain curves shown in (B) and (C) are representative curves of the total of five evaluated samples.

Fig. 4.

Platelet orientation and mechanical properties of Al2O3-chitosan artificial films as a function of Vp. A high degree of orientation was achieved for values of Vp up to 0.2 [(A), solid symbols]. Load transfer from the continuous chitosan phase to the highly oriented inorganic platelets led to a substantial increase in Ec (B) and σc of the hybrid film (C), in accordance with mechanical models for bulk composite materials [continuous fitting curves in (B) and (C)] (21, 27). Although the strain at rupture of the hybrid films decreases by increasing the concentration of platelets, values as high as 4% are still achieved for Vp = 0.2 (D). The elastic moduli data (B) were obtained from the initial elastic region of the stress versus strain curves exemplified in Fig. 3B. Error bars correspond to the standard deviation obtained from a total of five samples.

The mechanical behavior of the hybrid film strongly deviated from the linear elastic regime, when the yield tensile strength of the organic matrix was reached. At this stress condition, yielding of the polymer phase between the inorganic platelets led to a pronounced plastic deformation of the composite film (Fig. 3B). Because of load transfer to the platelets, the tensile stresses required for plastic yielding increased from 50 MPa to values as high as 300 MPa when the Vp was increased from 0 to 0.15 (Figs. 3B and 4C). Most remarkably, films containing inorganic volume fractions up to 0.15 fractured at a total strain (ϵrupt) typically between 4 and 35% (Fig. 4D), as a result of extensive plastic yielding of the polymeric matrix before rupture. Flexible hybrid films that are simultaneously strong (tensile strength σc ∼ 300 MPa) and ductile (ϵrupt ∼ 20%) were successfully produced at rather low Vp values (Vp = 0.15) (Fig. 3B).

Our results indicate that the interfacial bonding between platelets and the organic matrix plays a crucial role on the load transfer efficiency from the ductile polymer to the strong inorganic phase. For τi values lower than the matrix τy, the ultimate strength of the composite is controlled by τi rather than τy (Eqs. 1 and 3). A comparison of our experimental results with those expected from the simplified shear-lag model (Fig. 4C) shows that most of the strength data agree well with the theoretical predictions when τi is varied from 25 to 50 MPa (21). Films exhibiting the highest σcc ∼ 300 MPa) are stronger than the model predictions, suggesting that in this case τi or τy might have been improved during the processing of the hybrid film. A possible reason for an increase of the τy would be the crystallization of chitosan molecules on the surface of the alumina platelets.

The Ec data for the layered composites are also in reasonable agreement with theoretical predictions (26, 27) (Fig. 4B), except for some data sets at Vp between 0.1 and 0.2. Scattering of the strength and elastic modulus at this range of platelet concentration (Fig. 4, B and C) was probably caused by the limited control over the organic-inorganic interface of the Al2O3-chitosan films or by the possible presence of processing defects (voids) within the composite microstructure. Improvements in the sequential assembly process to avoid processing defects and a better control over the interfacial bonding by chemically coupling the platelet surface groups to the organic matrix should considerably reduce scattering of the strength and Ec values.

The combination of high tensile strength and ductility (inelastic strain) is the most distinct feature of our composites when compared to other platelet-reinforced polymers (Table 1) (14, 18, 20, 23). Composites with 15 vol % of submicrometer alumina platelets showed tensile strength up to 315 MPa and an inelastic deformation of 17%, as opposed to the catastrophic brittle failure observed for nanocomposites containing up to 50 vol % of clay platelets (Table 1) (14, 18). Such inelastic deformation should lead to very high fracture energy and toughness. A rough estimation based on the area under the stress versus strain curve indicates that the energy required to rupture our composites is two orders of magnitude higher than that needed to fracture ultrastrong and stiff clay-based nanocomposites (14, 18). The lower platelet concentration of the alumina-reinforced polymer led to relative strength (σcm) and Ec values that are, respectively, 40% and 10-fold lower than that of ultrastrong nanocomposites (Table 1) (18). It is important to note, however, that the submicrometer-thick platelets used exhibit a higher reinforcing efficiency compared with the clay platelets investigated by Podsiadlo et al.(18). On the basis of the factor α of Eq. 1, we estimated that the submicrometer alumina platelets are at least four times more efficient than clay platelets as load-bearing units of the composite (SOM text).

Table 1.

Summary of the mechanical properties of artificial composites reinforced with submicrometer-thick alumina platelets in comparison to clay-based nanocomposites (14, 18). Data refer to mean values and corresponding standard deviation. N indicates the minimum number of samples evaluated. PDDA, poly(diallyldimethylammonium chloride); PVA, poly(vinyl alcohol).

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The mechanical behavior of the artificial composites was also compared with that of hybrid biological structures in Fig. 3C. In spite of their lower inorganic content, the artificial composites exhibit yield strength higher than or comparable to that of natural materials. As anticipated by the mechanical model shown in Fig. 1, the high strength of artificial composites prepared at rather low inorganic contents results from the enhanced tensile strength of the synthetic inorganic platelets (σp ∼ 2 GPa) as compared to that of platelets found in biological structures [360 < σp < 500 MPa in nacre (SOM text)]. The strain to rupture of the artificial composites is also considerably higher than that of natural materials because of the extensive plastic flow of the chitosan matrix (Fig. 3C). The area under the stress versus strain curves obtained for the artificial materials is more than one order of magnitude larger than that for nacre and other biological materials (Fig. 3C). The elastic moduli of up to 10 GPa achieved by the artificial composite is comparable to those of dentin and bone (9, 28, 29) and about 10-fold lower than that of nacre (7).

The use of high-strength artificial platelets as mechanical reinforcement imposes less-stringent microstructural requirements to the composites in comparison to natural hybrid structures. Artificial composites reinforced with strong platelets show remarkable mechanical properties in spite of their less-elaborate microstructure. In contrast, the relatively weak inorganic compounds available in nature for biomineralization require a far more sophisticated architecture to render materials with comparable mechanical behavior. Nature still remains supreme in its ability to build hybrid materials with unique structures and properties using a relatively limited variety of inorganic building blocks. Further advances in this area might allow us to replicate the microstructure of natural materials by using strong artificial building blocks in the future. If successful, this bio-inspired approach would eventually lead to man-made hybrid materials with unprecedented mechanical properties.

Supporting Online Material

www.sciencemag.org/cgi/content/full/319/5866/1069/DC1

Materials and Methods

SOM Text

Figs. S1 to S3

References and Notes

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