Bioinspired Design and Fabrication of Polymer Composite Films

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Bioinspired Design and Fabrication of Polymer Composite Films Consisting of a Strong and Stiff Organic Matrix and Microsized Inorganic Platelets Donghwan Ji, and Jaeyun Kim ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b06767 • Publication Date (Web): 24 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019

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Bioinspired Design and Fabrication of Polymer Composite Films Consisting of a Strong and Stiff Organic Matrix and Microsized Inorganic Platelets Donghwan Ji1, Jaeyun Kim1,2,3*

1School

of Chemical Engineering, Sungkyunkwan University (SKKU), Suwon 16419, Republic

of Korea 2Department

of Health Sciences and Technology, Samsung Advanced Institute for Health

Science & Technology (SAIHST), Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea 3Biomedical

Institute for Convergence at SKKU (BICS), Sungkyunkwan University (SKKU),

Suwon 16419, Republic of Korea * Correspondence should be addressed to Prof. J. Kim (e-mail: [email protected]).

KEYWORDS: nacre-inspired design, layer-by-layer structure, hydrogel-film casting, alginate, ionic crosslinking, mechanical properties

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ABSTRACT Intensive studies on nacre-inspired composites with exceptional mechanical properties based on an organic/inorganic hierarchical layered structure have been conducted, however, integrating high strength, stiffness, and toughness for engineering materials still remains a challenge. We herein report the design and fabrication of polymer composites through a hydrogel-film casting method that allow for building uniformly layered organic/inorganic microstructure. Alginate (Alg) was used for an organic matrix, whose mechanical properties were controlled by Ca2+ crosslinking toward the simultaneously strong, stiff and tough resultant composite. Alumina (Alu) microplatelets were used for horizontally aligned inorganic phase, and their alignment and interactions with the organic matrix were improved by polyvinylpyrrolidone (PVP) coating on the platelet. The composite film exhibits well-balanced elastic and plastic deformation under tensile stress, leading to high stiffness and toughness, which have not been generally achieved in microplatelet-based composite films developed in previous studies. The synergistic effect of Ca2+ crosslinking and PVP-coated Alu platelets on the mechanical properties, improved polymerplatelet interfacial interactions, and platelet alignment are clearly demonstrated through mechanical tests, and FTIR and XRD analyses. We further demonstrate that the reinforcing effect of the Alu platelet and PVP-coated platelet on the mechanical properties is dependent on humidity. Such effects are maximized at highly dry condition, which is consistent to the model-estimation. Furthermore, a thick bulk composite was produced by laminating of thin films and showed high mechanical properties under flexural stress. Our design and fabrication strategies combined with the understanding of their mechanism yield an alternative approach to produce engineered composite materials.

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With the progress in industrial development, mechanically outstanding materials, such as lightweight damage-tolerant materials, have been intensively developed during the past decades. To develop these materials, many researchers have been focusing on the structure of natural materials that produce exceptional mechanical properties.1-4 Among the natural materials, nacre is a promising material owing to its exceptional strength and toughness that is derived from a layerby-layer structure composed of inorganic platelets and organic matrices.5-8 By mimicking the inorganic/organic layered structure, a number of artificial composite films with improved mechanical properties have been reported.9-20 The previously reported films typically consisted of nanosized inorganic platelets including graphene oxide9-15 and clays,16-20 and ductile organic polymers including cellulose11,14,17,19 and poly(vinyl alcohol);10,18,20 they were fabricated by evaporation-induced assembly,9,10,17 gel-film transformation,12,13 vacuum-assisted filtration,14-16 layer-by-layer assembly,18 or doctor-blading.19,20 However, these methods are not appropriate for fabricating microsized platelet-based films with uniform microstructure; thus, nacre-inspired films composed of microsized inorganic platelets with organic polymers have been relatively less reported.21-29 For microplatelet-based composite films, layered double hydroxide (LDH),21 alumina (Al2O3),22-27 and calcium carbonate (CaCO3)28,29 have been typically used. They were assembled into layered structures through several methods such as layer-by-layer assembly,21,22,29 magnetically assisted slip casting,23,25 and ice-templating.26-28 However, these approaches include complicated procedures and are not suitable for the large-scale engineering of the composite film. Although the resultant composite films exhibit improved mechanical behaviors compared to their single component film and even natural nacre, the integration of high strength, stiffness, and toughness still remains a challenge. A sharp change in the linear elastic region and a large plastic strain under high strength must be combined to guarantee high stiffness and toughness.  ACS Paragon Plus Environment

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We herein present inorganic microplatelet-reinforced, strong, stiff, and tough polymer composite films based on the rational design of each component, and the control of interfacial interactions between the organic and inorganic phases. The composite film consists of strong and stiff organic matrices and horizontally aligned two-dimensional (2D) alumina (Alu) microplatelets in a uniformly layered microstructure that was achieved by an alternative method i.e., hydrogel film casting suggested in the recent work by our group.30 An ionic crosslinked alginate (Alg) hydrogel maintains a three-dimensional structure, and allows for a uniform distribution of inorganic microplatelets without precipitation to produce composite films with desirable shapes, sizes, and thicknesses. The interfacial interactions between the microplatelets and polymer matrix was improved by polyvinylpyrrolidone (PVP) coating on the platelets, allowing for a wellbalanced elastic and plastic deformation of the resultant film to achieve the integration of high strength, stiffness and toughness. We demonstrate the effects of each constituent (Ca2+ crosslinking, Alu, and Alu-PVP platelets) on the improvement of the mechanical properties, interfacial interactions, and platelet alignment through mechanical tests, and Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD; 2theta- and omega-scans) analyses. We also found that the interfacial interaction between the PVP-coated platelets with the organic matrix is dependent on humidity; thus, humidity significantly affects the mechanical properties. The reinforcing effects of the inorganic Alu platelets and the PVP-coating for improving interfacial interactions were maximized at highly dry condition. Furthermore, a thick bulk composite was produced by laminating of the films and showed high mechanical properties under flexural stress. Our design and fabrication strategies combined with the understanding of their mechanism yield an alternative approach to produce engineered composite materials with extraordinary mechanical properties.  ACS Paragon Plus Environment

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RESULTS AND DISCUSSION Designing strategy. To design polymer composite films exhibiting superior mechanical properties (integrated strength, stiffness, and toughness), we hypothesized that the constituents in the films must satisfy two criteria. First, a polymer matrix as an organic matrix must allow for a large plastic deformation after plastic yielding, leading to the toughening of the composite films via platelet pull-out and matrix plastic flow. Simultaneously, the polymer matrix must be strong and flaw-intolerant at elastic region (before plastic yielding). Second, inorganic platelets must efficiently reinforce the composites. The strong platelets with a high aspect ratio could carry more loads and strengthen the composites efficiently. In addition, the interactions between inorganic platelets and organic matrices must be sufficient. The sufficient interfacial interactions between them require a high amount of energy to break the interactions and contribute to improving the platelet alignment in the matrix, which can eventually enhance the mechanical properties of the composite films. A combination of such polymer matrix and inorganic microplatelets can create a synergistic effect on the strength, toughness, and stiffness of the composites. To fulfill the first criterion, we used an ionically crosslinked alginate (Alg) polymer in our study. The alginate is a linear polysaccharide copolymer consisting of β-D-mannuronate (M) and α-L-guluronate (G) residues.31 Owing to their functional groups including carboxyl and hydroxyl groups, the alginate chains easily interact with the surface of the inorganic materials. In addition, the G residues (G blocks) of the alginate can be coordinated with the metal cations, forming a hydrogel with three-dimensional structure. Employing Ca2+ crosslinking into the alginate matrix increases the strength and stiffness of the matrix after drying of hydrogel to form a film, and further enhanced the mechanical properties of the composites synergistically in the presence of inorganic

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platelets. Eventually, this approach could overcome the weakness of microplatelet-based polymer composites derived from a limited stiffness. To satisfy the second criterion, we used 2D Alu platelets as an inorganic component. The Alu platelets show a higher reinforcing efficiency than the montimorilonite platelets, which is typically used to fabricate nacre-inspired films. They exhibit a higher aspect ratio than aragonite platelets of natural nacre; therefore, the Alu platelets can carry more load and strengthen the composites compared to other inorganic platelets.22 In addition, the Alu microplatelet does not embrittle the polymer because the microplatelet does not create a confined space to induce polymer stiffening, and does not act as a crosslinker as opposed with nanoplatelets; thus, the microplatelet allows the composite to endure a large plastic deformation under the platelet pull-out mode.32 Furthermore, by modifying the alumina platelet surface with polyvinylpyrrolidone (PVP), interfacial interactions between inorganic platelets and organic matrices can be improved via hydrogen bonding rather than strong chemical bonds. Unlike strong covalent bonding between platelet/platelet or platelet/matrix that generally makes very stiffened composites, hydrogen bonding is weak enough to debond and to enable inelastic shear deformation between the platelet and the matrix as transferring load from the matrix to the platelets.18,32-34 In addition, as PVP is widely used stabilizing polymer to disperse the inorganic particles due to its amphiphilic characteristics, we supposed that the PVP-coating can improve the dispersion of micron-sized platelets in the organic matrix.35,36 Consequentially, the PVP coated on the platelet surface could contribute to effective aspect ratio of the platelet by enhancing the platelet alignment in the matrix compared to non-coated bare platelets. Fabrication of polymer composite films. Based on the aforementioned design principles, we fabricated a composite film composed of Ca2+-crosslinked alginate (Ca2+-Alg) incorporated  ACS Paragon Plus Environment

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with aligned PVP-coated alumina (Alu-PVP) platelets, designated as Ca2+-Alg/Alu-PVP film, through the hydrogel-film casting method (Figure 1a). First, Alu-PVP platelets were prepared by the adsorption of PVP on the surface of Alu platelets. The zeta potentials of bare Alu, PVP, and Alu-PVP revealed that PVP was indeed adsorbed on the Alu platelets owing to the electrostatic interactions (Figure S1a). The PVP-coating resulted in an enhanced water dispersion of the platelets (Figure S1b). A mixture solution of Alg, Alu-PVP, and a calcium sulfate (CaSO4) slurry was placed between two glass plates with a desired spacing and gelated for over a day. The resultant Ca2+-Alg/Alu-PVP hydrogel was cut into the desired shapes (e.g., a dog-bone shape for the subsequent studies), placed on a Petri dish, and dried at ambient temperature (20–25 °C) and humidity (RH 30–40%), resulting in a thin and flat film after around 12 h (Figure 1b). The drying of the composite hydrogel at higher temperature (60 ºC) and lower humidity (RH 10 %) led to faster drying (around 2h) but resulted in non-flat and distorted composite film (Figure S2); therefore, the optimized slow drying process is critical to achieve uniform composite film. No CaSO4 particles remained in the resultant film as observed in the X-ray diffraction (XRD) pattern of the Ca2+-Alg film compared to that of CaSO4 particles (Figure S3a, b). Energy dispersive X-ray spectroscopy (EDX) mapping on SEM also revealed that Ca2+ ions were homogeneously distributed throughout the film matrix (Figure S3c). These data represent that Ca2+ ions were released from CaSO4 microparticles during gelation and were fully participated in the ionic crosslinking of alginate. The hydrogel-film casting by drying the attached the hydrogels on the flat substrate (e.g., Petri dish in this study) enabled the unidirectional (z-axis) drying of hydrogels into thin films, such that the 2D Alu microplatelets were horizontally self-assembled in the resultant composite films after drying. The uniformly layered macroscopic structure of the microplatelet-based composite  ACS Paragon Plus Environment

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film was clearly identified in the cross-section SEM images (Figure 1c-e); the composite shows an overall uniform microstructure. From the top view of the composite, we also observed a wide top surface of the platelets (Figure 1f). Meanwhile, when a composite hydrogel, non-attached on the flat substrate, was dried in the air, the hydrogel was highly shrunken along all directions from the drying process, resulting in randomly embedded Alu platelets in the composites (Figure 1g, h). Furthermore, we could manipulate the macroscopic shape of the composite film by attaching the hydrogel onto the differently shaped (e.g., angled or curved) substrate. (Figure 1i). The freestanding dry films were obtained while the shapes of the substrates were maintained. The microplatelets were assembled in layer-by-layer along the drying direction (normal to substrate) regardless of the direction of gravity (Figure 1j). The macroscopically shape-adjusted-assembled platelets without agglomeration in the matrix enabled the film to free-stand. In addition, the very complex surface structure of the substrate could be successfully transferred into the resulting composite film (Figure 1k and Figure S4). These results suggest that this simple method starting from the hydrogels is applicable to the preparation of uniform composite films using diverse substrate regardless of the macroscopic shape or surface microstructure of the substrates. The significant enhancement of the degree of Alu platelet alignment in the film was investigated by XRD analysis (Figure 2). Most peaks corresponding to the primary crystallographic planes of 2D Alu platelet powder (α-Al2O3, JCPDS 46-1212, Figure S5), such as (012), (004), (113), (116), and (1010), were observed in the XRD pattern of the composite after the drying of hydrogels in the air, representing the random orientation of Alu platelets in the composite after an all directional drying process (Figure 2a). In contrast, highly intense peaks of (006) and (0012) planes that appeared weakly appeared on the XRD pattern of Alu powder, were clearly shown in the unidirectionally dried film, thus indicating that the [001]–type  ACS Paragon Plus Environment

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crystallographic planes were the strongly preferred orientation in the composite film (Figure 2a). The 2D Alu platelets were horizontally aligned in the matrix parallel to the surface of the substrate; thus, the (00l) reflections derive a distinctly enhanced diffraction intensity of the [001] type. The sharp intense peak generated by the strong preferred orientation is generally observed by aragonite crystals in natural nacre,37 synthetic nacre,28 and crystalline-textured alumina ceramics fabricated in a strong magnetic field.38,39 The preferred orientation of the platelets in the films was maintained even at a high platelet content (Figure 2b). The formation of hydrogel as a matrix to maintain the dispersed status of micro-sized platelets and to prevent their precipitation during the drying process is important to obtain a uniformly layered organic/inorganic microstructure. The spontaneous layering of 2D inorganic platelets is generally observed in composite films prepared from the drying of a mixture solution of polymer and nanosized platelets, caused by the synergistic result of the Brownian motion of well-dispersed nanoplatelets and gravitational field.40,41 However, the microsized Alu platelets in Alg polymer solution were readily precipitated during drying to from a film owing to its heavy weight, as revealed in the SEM image (Figure S6). In contrast, those heavy Alu microplatelets could be spatially fixed without precipitation in Ca2+-crosslinked Alg hydrogel matrix, resulting in a horizontally aligned, layer-by-layer structure in the resultant film after unidirectional drying process on the substrate. Therefore, the hydrogel-film casting is an appropriate and effective method for the microplatelets to form composite films with uniform microstructures. Mechanical properties and behaviors of the films. The effects of each constituent on the mechanical properties were investigated: the effects of Ca2+ crosslinking (from CaSO4), and inorganic microplatelets (Alu and Alu-PVP). The dog-bone-shaped films prepared at room temperature and at a relative humidity of around 30% were analyzed in a tensile test. We first  ACS Paragon Plus Environment

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investigated the effect of Ca2+ crosslinking on the mechanical properties of the Ca2+-Alg film to obtain an optimized amount of calcium sulfate to fabricate the composite films. The comparison of the strength, elastic modulus, and toughness of Alg and Ca2+-Alg films fabricated with different Ca2+ concentrations show that Ca2+-crosslinking enhanced the mechanical properties of the film. Further, we determined the optimized concentration of CaSO4 as 20 wt% of the Alg polymer (Figure S7). Next, to optimize the amount of platelets, Ca2+-Alg/Alu and Ca2+-Alg/Alu-PVP composite films were prepared with different platelet contents from 5 to 40 wt% to the total mass of alginate and platelets, using 20 wt% CaSO4. (Figure S8). The platelet content was confirmed from the thermogravimetric analysis (TGA) of the composite films (Figure S9). The Ca2+-Alg/Alu-PVP composite film with 20–30 wt% of inorganic platelets exhibited the maximum enhancement on the tensile strength, elastic modulus, and toughness compared to the Ca2+-Alg/Alu film. When excessive amounts of microplatelets were incorporated in the composites, the dry film was slightly twisted (not flat) because of platelet misalignment and voids between the platelets. The misaligned platelets were not completely bound to the matrix and the polymer chains could not stretch enough for plastic yielding, and thus the composite films with high microplatelet content (40 wt% or more in this study) exhibited weaker mechanical properties (Figure S10). We also found that 20 wt% CaSO4 resulted in the highest mechanical properties in Ca2+-Alg/Alu-PVP composite films (Figure S11). Based on these results, we used 20 wt% CaSO4 and 20 wt% Alu platelets as the optimized condition to prepare the composites for the subsequent experiments. Figure 3a represents the representative stress-strain curves of the Alg, Ca2+-Alg, Ca2+Alg/Alu, and Ca2+-Alg/Alu-PVP films. Compared to the Alg film, although the strain at rupture was decreased, the Ca2+-Alg film showed a significant increase in tensile strength and elastic  ACS Paragon Plus Environment

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modulus. In addition, the incorporation of Alu-PVP microplatelets resulted in a slight improvement on strength, and a significant improvement on both elastic modulus and extension of plastic deformation. Consequently, the Ca2+-Alg/Alu-PVP film exhibits integrated mechanical properties: tensile strength of 204 MPa, elastic modulus of 16.7 GPa, and toughness of 8.79 MJ m−3, which are 2.0-, 2.6-, and 1.5-fold higher than that of the Alg film (Figure 3b-d), respectively. The PVP on the Alu surface certainly created improved interfacial interactions with the organic matrix; thus, a substantial fraction of the load was effectively transferred to the aligned Alu-PVP platelets, leading to an enhanced stiffness with increased yield strength of the resultant film compared to the bare Alu platelets. In addition, yielding of the strengthened polymer matrix confined between the PVP-coated platelets allowed for a large plastic deformation with the platelet pull-out under tensile load. Accordingly, the combination of Ca2+ crosslinking of Alg matrix and PVP coating on Alu microplatelets synergistically led to a pronounced reinforcing effect on the mechanical properties, particularly resulting in a sharp change in the linear elastic region and a large plastic deformation. The integration of properly balanced stiffness and toughness is in significant contrast to the previously reported microplatelet-based polymer composite films that usually showed a low stiffness owing to the composite rupture under the platelet pull-out in the ductile matrix.21-23,33,42 Meanwhile, nanoplatelet-based polymer composite films generally show high stiffness and low toughness because nanoplatelets induce ordering/crystallization of organic matrices and stiffening of composite with limited plastic deformation.18,32 For example, clay-nanoplatelet-based polymer composite film exhibited high tensile strength and stiffness, while toughness was relatively low with a limited strain.17-19,43 Graphene oxide-nanoplatelet-based polymer composite film also exhibited high stiffness with low toughness;44,45 occasionally, there were some other cases that  ACS Paragon Plus Environment

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showed high toughness with low stiffness.12-14 Although their mechanical properties could be further improved by additional ionic crosslinking 14,17,44 or reduction of graphene oxide,10,12,44 the integration of the stiffness and toughness was still restricted. In this study, the employing Ca2+crosslinking into the alginate matrix generated a stronger organic matrix, which could remedy the shortcoming of the weak and ductile matrix. Further, the use of PVP-coated alumina microplatelets also led to a large plastic deformation. Therefore, the incorporation of PVP-coated alumina microplatelets in ionically crosslinked alginate matrix could lead to the integrated high stiffness, toughness and strength of composite films, which is an advantage over the previously reported composite films from the viewpoint of mechanical properties. Investigation of the role of each constituent. Through FTIR spectroscopy, we investigated the effect of the Ca2+ crosslinking and alumina platelets on the interactions between the polymer chains, and between the polymer and the platelets (Figure 4a). The Ca2+ crosslinking induced three major changes in the IR spectra of the films. First, the coordination of Ca2+ with COO− on the alginate chains was observed. Compared to the Alg film, asymmetric and symmetric stretching peaks of COO−, vas(COO−) and vs(COO−), shifted to higher wavenumbers from 1597 to 1599 cm−1 and from 1410 to 1416 cm−1, respectively. These results demonstrate that the extent of shift in vas(COO−) was smaller than that in vs(COO−). As both Ca2+ ions (from calcium sulfate) and Na+ ions (from sodium alginate) coexist in the composite film, we assume that both Ca2+ and Na+ cations could participate in various coordinations with COO− groups, including chelating, bridging bidentate, and pseudo-bridging, thereby resulting in the peak shifts in vas(COO−) and vs(COO−).46-50 Next, the appearance of a new peak near 1262 cm−1 was observed as a result of Ca2+ crosslinking. This peak is probably assigned to C–O–C stretching and correlated with the ionic bonding of alginate chain.51,52 Finally, a peak shift was found toward the lower wavenumber from 1084 cm−1  ACS Paragon Plus Environment

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to 1078 cm−1, and a significant increase in intensity of the 1120 and 1078 cm−1 peaks were observed. As these peaks are associated with the C–O and C–C stretching of the G blocks of the alginate chain, those changes were derived from the weakening in the C–O and C–C bonds by calcium cations.50,53 The incorporation of platelets also induced three major changes. First, the formation of bridging bidentate and unidentate coordination was observed. In the Ca2+-Alg/Alu and Ca2+Alg/Alu-PVP composite films, the vas(COO−) and vs(COO−) shift to higher wavenumbers from 1597 to 1601 cm−1 and 1410 to 1413 cm−1, respectively. Compared to the Ca2+-Alg film, the extent of shift in vas(COO−) increased, whereas the extent of shift in vs(COO−) decreased, thus indicating that the Alu microplatelets interact with the COO− of alginate through the unidentate and bridging bidentate in the composite film.49,54,55 Next, the peak intensity at 1262 cm−1 was increased. Ca2+ crosslinking created a peak at 1262 cm−1 and Alu-PVP microplatelets amplified the peak intensity, indicating that the PVP coated on the platelet induced some interactions. Finally, the peak intensity at approximately 780–818 cm−1 was increased upon the addition of Alu platelets. The M blocks and G blocks of alginate exhibit the peaks at 818 cm−1 and 781 cm−1, respectively,56-58 and the Al-O stretching also exhibits broad peaks near 750–850 cm−1.59 Owing to the interfacial interactions between alginate and alumina, the peak intensity at approximately 780–820 cm−1 show a substantial increase compared to the Ca2+-Alg film and alumina powder. The PVP coated on the platelets further amplified the peak intensity, and likely contributed to the efficient interfacial interactions. Based on all FTIR results, we conclude that the Alu platelets sufficiently strongly interact with the alginate matrix, and the PVP coating on the platelets allows for an additional improvement in the interfacial interactions between organic and inorganic constituents.

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We further verified the effect of the PVP on the platelet orientation through numerical comparison based on distinct (006) or (0012) peaks on the XRD patterns of Ca2+-Alg/Alu and Ca2+-Alg/Alu-PVP films, as shown in Figure 4b. As the degree of preferred orientation (η) is inversely related to March parameter (r),37 we calculated the ratio of March parameters of the composite films consisting of bare Alu platelet or PVP-coated Alu platelets, 𝑟Alu 𝑟Alu ― PVP (the detailed calculation is described in the supporting information). The calculated ratio was greater than 1, representing that the degree of preferred orientation of the platelets in the composite film was improved by the PVP coated on the platelets. Additionally, the degree of horizontal platelet alignment in the composite film was experimentally estimated through an omega-scan (rocking curve) from the XRD analysis.22,60,61 For the omega-scan, the most intense peak at approximately 2θ = 90.6° that was (0012) plane was first selected by the 2theta-scan. The plane (2θ = 90.6°) was subsequently set, and the film was tilted by an angle, ω. The all directionally dried composite (shown in Figure 1g) showed no defined diffraction peak in the omega-scan (Figure 4c, black line) owing to the randomly embedded platelets (Figure 1h, 2a), whereas both the Ca2+-Alg/Alu and Ca2+-Alg/Alu-PVP films exhibited well-defined peaks. However, the angular distribution (peak sharpness) was clearly different. The peak width of the Ca2+-Alg/Alu-PVP film was smaller than that of the Ca2+-Alg/Alu film. Therefore, the degree of horizontal alignment of the Alu-PVP platelets were higher than that of the bare Alu platelets. We further confirmed that the PVP coated on the platelets led to a better even distribution in the polymer matrix by comparing the composite films with different platelet contents using rocking curves (Figure S12). Based on all FTIR measurements, the calculated value of March parameter, and the omegascan in XRD, we concluded that PVP coating on Alu platelets enhances the interfacial interaction  ACS Paragon Plus Environment

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between the platelets and polymer matrix, and the alignment of the platelets within the film. Therefore, Alu-PVP platelets contributed to a higher effective aspect ratio in the composites compared to bare platelets. Eventually, the load effectively transfers to the ordered Alu-PVP platelets, thus resulting in the improved mechanical properties. The side-view SEM images of the fractured surfaces of the Ca2+-Alg/Alu-PVP film revealed that the platelets were pulled-out by tensile stress (Figure 4d). The voids between matrices and platelets, and the holes in which the platelets were completely pulled out from the matrix were also observed after rupture (Figure 4e-g). A similar phenomenon was reported in chitosan/alumina platelet composite; these voids were formed owing to the strong plastic deformation derived from the strong chitosan/alumina interfacial bonding in the relatively weak and ductile chitosan matrix.32 In our results, the combination of the sufficient interfacial interactions between organic and inorganic phases achieved by PVP coating on the platelets, and the strong, stiff Ca2+crosslinked alginate matrix resulted in some voids forming between the matrix and the platelet, and holes formed by the pulled-out platelets, thus leading to the integration of stiffness and toughness. Interfacial bonding was clearly observed as shown in Figure 4f, g. The external load was transferred to the platelets, and then gradually pulled out from the polymer matrix under the interruption of interfacial bonding between alginate and alumina (Figure 4f). After the complete pulling-out of the platelet, some residues that originated from the matrix were discernible on the platelet (Figure 4g). Humidity-dependence of mechanical properties. The mechanical properties of natural nacre, biomaterials, and even synthetic composites are generally dependent on wet (hydration) or dry (dehydration) conditions.62-64 We stored the composite films in a specific humidity-adjusted container (humidities of 15, 30, 50, and 70%) for at least a day, and investigated the mechacanical  ACS Paragon Plus Environment

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properties depending on the humidity (Figure 5a). A high relative humidity (RH 70%) resulted in a relatively ductile behavior with a strain of 10% compared to that at non-humid condition, however, the film was weak and soft; the Ca2+-Alg/Alu-PVP film showed tensile strength of 87.9 MPa, modulus of 3.72 GPa, and toughness of 7.04 MJ m−3. Meanwhile, at a highly dry condition (RH 15%), the film was very strong and stiff, but exhibited a limited toughness with a slight plastic deformation; the Ca2+-Alg/Alu-PVP film showed tensile strength of 304 MPa, modulus of 20.1 GPa, and toughness of 4.50 MJ m−3. Accordingly, a moderate condition (RH 30%) was found to exhibit a well-balanced elastic and plastic deformation, resulting in a sufficiently high strength of 204 MPa, elastic moulus of 16.7 GPa, and toughness of 8.79 MJ m−3. This humidity-dependent trend in the mechanical behaviors is caused by the plasticizing effect of water included in the materials; the water molecules caused the softening and large plastic deformation.64 We also found that the PVP coated on the platelets significantly improved the mechanical properties at both the highly and moderately dry conditions, as compared with the Ca2+-Alg/Alu film (Figure 5b). When we compared the Ca2+-Alg, Ca2+Alg/Alu, and Ca2+-Alg/Alu-PVP films under RH 15 and 30%, the incorporation of bare Alu platelets in the film contributed to a less improved on mechanical properties at RH 30% than that at RH 15%. At the highly dry condition (RH 15%), the Ca2+-Alg film was basically brittle and exhibited almost only elastic deformation, whereas the Ca2+-Alg/Alu composite film was ruptured with a slight plastic deformation under the platelet pull-out mode, of which the phenomenon is the same with our previous report.30 At the moderately dry condition (RH 30%), the water molelcules existing in the film softened the polymer matrix and allowed for a higher plastic deformation even in the Ca2+-Alg film. In this environment, employing bare Alu platelets that have relatively weak interfacial interactions with the softened matrix did not significantly enhanced the mechanical properties. We inferred that the incorporation  ACS Paragon Plus Environment

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of bare Alu platelets can exhibit large reinforcing effects at the highly dry condition rather than at the moderately dry condition. Meanwhile, improved interfacial interactions with the matrix achieved by the PVP coating on the platelets effectively obstructed the pulling-out of the platelet and resulted in enhanced mechanical properties even at the modertely dry condition; the reinforcing effect of the PVP was maximized at the highly dry condition. Despite the improved interactions, relatively many water molecules at the normal and humid conditions (RH 50% or above) dominantly weakened the materials (Figure S13); thus, the PVP could not effectively enhanced the mechanical properties at the humid condition. To demonstrate the effect of hydration on the polymer dynamics, we tried to compare the glass transition temperature (Tg) or melting temperature (Tm) of the polymer composite films at the designated humidity.64 However, Tg of the alginate polymer could not be determined precisely both in differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). Although the Tm of the fully dried film could be observed near 150‒200 ºC, it was not technically possible to measure the Tm of the film under the specific humidity. Instead, we estimated platelet– matrix interfacial strength at different humidity based on a simple shear lag model, and compared them with experimental values.8,65 Thus, we could compare the reinforcing effect of the bare platelet and the PVP-coated platelet on mechanical properties of the resultant films through the model-estimation, and further demonstrated their reinforcing effects depending on the humidity in the following section. Simple shear lag model for analysis of mechanical behaviors. A shear yield strength (τy) of the polymer matrix can be estimated from a tensile yield strength (σy) of the polymer film with the von Mises criterion,66

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𝜏𝑦 =

𝜎𝑦 3

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(1)

The measured σy is about 220 MPa at RH 15% or 150 MPa at RH 30%, respectively, and thus τy is approximately 86–127 MPa at RH 15–30%. As the interfacial-shear strength (τi) between alumina platelets and polymer matrix without strong chemical covalent bonds is 15–50 MPa in previous reports,22,32 the τi in our composite film without chemical bonds can be expected to be smaller than the τy (86–127 MPa). For τi < τy, a critical aspect ratio (sc) of platelet is defined as follows 𝑠𝑐 =

𝜎𝑝 𝜏𝑖

(2)

where σp is the platelet tensile strength that is 2–5 GPa.22,32 Therefore, the sc can be approximately calculated to be above 40. Meanwhile, an aspect ratio (s) of the alumina microplatelet measured using SEM images is around 25 (Figure S14). The s is lower than sc, which represents yielding of the polymer matrix occurs before platelet fracture. Taken together, the interfacial strength is lower than the shear strength (τi < τy) and the aspect ratio of the platelets is smaller than the estimated critical aspect ratio (s < sc). This indicates that fracture of the platelet–polymer interface occurs before yielding of the polymer matrix under tensile stress, and then the composite ruptures under platelet pull-out mode. Therefore, the enhanced stiffness with increased yield strength and the improved toughness with plastic deformation could be resulted in the Ca2+-Alg/Alu-PVP composite film consisting of the strong, stiff organic matrix and inorganic microplatelets that have sufficient interfacial strength. Furthermore, the tensile strength of the composite (σc) can be estimated by a simple shear lag model as follows8,22,65 𝜎𝑐 = 𝛼𝜎𝑝𝑉𝑝 + (1 ― 𝑉𝑝)𝜎𝑚

(3) 

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where Vp is the volume fraction of the platelet, σp is the tensile strength of the Alu platelet, σm is the tensile strength of the Ca2+-Alg matrix, and α is given by 𝜏𝑖𝑠

𝛼 = 2𝜎𝑝 .

(4)

Vp is calculated from a wt% of Alu platelet in the resultant composite film with an alginate polymer density of ≈1.6 g cm−3 and alumina platelet density of ≈3.98 g cm−3, and σm is 226.7 MPa at RH 15% or 180.6 MPa at RH 30%. Based on the experimental tensile strength and the estimated values, we fitted eq (3) along the Vp (Figure 5c, d). Note that the measured s is still smaller than sc calculated by estimated τi (20–90 MPa) from the theoretical predictions. Except high Vp, the modelestimated τi is well-fitted with the experimental values in both bare and PVP-coated platelets at RH 15%. In contrast, the estimated τi of the Ca2+-Alg/Alu-PVP film at RH 30% especially at high Vp was not well-matched with the experimental values, which is probably due to unexpected interactions by relatively many water molecules existing in the film. According to the modelestimation, the level of the interfacial interaction at the lower water content (RH 15%) was 2‒3 times higher than the moderate water content (RH 30%), and the reinforcing effect of the PVPcoating was maximized at the dry environment. In the theoretically ideal case of this type of composite films, inorganic–organic interfaces break completely before polymer matrix yields, which leads to the enhancement of the strength and stiffness and do not affect much to the plastic behavior of the composite. According to this behavior, at RH 15%, PVP-coating enhanced the tensile strength and stiffness of the composite film, but did not increase the tensile strain (Figure 5b). On the contrary, at RH 30%, there was an increase of plastic deformation (higher strain) by PVP coated on the platelet, but the tensile strength of the composite film was slightly improved compared to bare Alu platelets (Figure 5b).  ACS Paragon Plus Environment

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Through these different behaviors, it is supposed that there might be unexpected interactions between PVP-water-alginate and partial crystallization of polymers, contributing to the effective matrix yielding with the extended plastic deformation at the relatively high humidity (RH 30%). The reinforcing effect of the platelet in our composite was relatively low compared to previous reports22,32 because the reinforcing effect would be more predominant in the weak and ductile polymer matrix with low τy, not in the strong and stiff polymer matrix with high τy (like our composite). Therefore, the use of stronger platelets with higher aspect ratio close to sc (s ≈ sc) and enhancement of interfacial interactions (τi ≈ τy or τi > τy) would be able to generate mechanically far superior composite materials in the future study. Preparation of thick bulk composite and its mechanical properties. The preparation of the thicker composite mimicking nacre is a challenging issue in this field.28,67 The characteristics of the hydrogel-film casting method could be extended to prepare thicker bulk composite materials. Based on the possibility of reversible ionic crosslinking of alginate matrix, thicker bulk composites could be easily prepared by laminating of thin composite films. For example, several thin composite films prepared from composite hydrogels were soaked into Ca2+ solution, and then treated with ethylenediaminetetraacetic acid (EDTA) solution. The slightly softened films were stacked layer-by-layer and hot-pressed, creating very uniform and thick composite specimen with 2–3 mm thickness (Figure 6a). The softened and chelated surface of the films by Ca2+ and EDTA solutions probably could allow for additional ionic crosslinking between the films under pressure. This resulted in no interfacial separation between films while the well-aligned microplatelets were maintained in the final thick specimen, as shown in cross-sectional SEM images (Figure 6b). To investigate flexural mechanical properties under 3-point bending test, the laminated thick composite block was polished and cut into a beam shape with 45 mm length, 4 mm width, and 2  ACS Paragon Plus Environment

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mm depth, and the resultant specimens were tested on 16 mm supporting span according to previous reports (Figure 6c).67-70 The observed flexural strength of the composite was around 236 MPa with relatively large strain (Figure 6d), which is correlated with crack deflection and multiple cracking during crack propagation (Figure 6e). The flexural strength was comparable to the natural nacre and synthetic composites exhibiting flexural strength of 185–235 MPa.67-69 The flexural modulus of the composite specimen was around 6.7GPa which is comparable to the result of other previous work.70 Although the fabrication process needs to be further optimized to obtain improved mechanical properties, the hydrogel-film casting method show potential for producing composite materials with uniform microstructures and superior mechanical properties.

CONCLUSIONS Inspired from the hierarchical layer-by-layer structure of nacre, we demonstrated the design strategy for polymer composite films consisting of strong and stiff organic matrices and microsized inorganic platelets. Through the hydrogel-film casting method, the composite films exhibited a uniform layered microstructure with horizontally self-assembled alumina platelets, in addition to the integration of strength, stiffness, and toughness. The PVP coated on the platelets especially reinforced the platelet and polymer interfacial interactions, leading to improved mechanical properties and platelet alignment. The reinforcing effects of the PVP were clearly identified by mechanical tests, FTIR, and XRD analyses. Additionally, using SEM, we observed voids between matrix and platelet, holes formed by the pulled-out platelets, and interfacial bonding acting as a glue between matrix and platelet. The proposed method can allow fabrication of mechanically stable and strong composites with desirable shapes and scales, which would be potentially useful in several applications. One  ACS Paragon Plus Environment

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of the applications of the composite films based on hydrogel-film casting could be an alternative material to plaster casts. Unlike plaster casts that often cause the patient discomfort and pain especially at joints such as ulnar head bone or patch bone because of the complex curved surface structure, the composite film could be shaped according to the surface structure. In addition, the resultant composite is composed of biocompatible constituents such as alginate, alumina, and PVP, which would be adaptable for bioapplications, such as partial bone and tooth substitutes. Furthermore, based on humidity-dependent mechanical properties and interfacial interactions between the organic matrix and inorganic platelets, it would enable to fabricate humidity responsive or sensitive actuators.71,72 We believe that our strategy and results (from moderate inorganic contents and tunable organic matrix) are beneficial to develop engineering materials with the integrated strength, stiffness, and toughness for various applications.

MATERIALS AND METHODS Materials.

Alginic

acid

sodium

salt

from

brown

algae

(Sigma-Aldrich),

polyvinylpyrrolidone (Sigma-Aldrich, MW 40000), calcium sulfate dihydrate (Samchun), and alumina microplatelets (RonaFlair) were purchased and used without further purification. Preparation of non-crosslinked Alg films. Alginic acid sodium salt was dissolved in distilled water and 2 wt% alginate solution was obtained. The 2 wt% alginate solution was poured into a Petri dish and dried at the ambient temperature and humidity to prepare a non-crosslinked alginate film. For the mechanical test, the non-crosslinked alginate film was carefully cut into a dog-bone shape. Preparation of Ca2+-Alg, Ca2+-Alg/Alu and Ca2+-Alg/Alu-PVP composites. Ca2+crosslinked films were prepared following our previous work through the hydrogel-film casting  ACS Paragon Plus Environment

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method.30 The PVP-coated alumina platelets were prepared as follows; 400 mL of a 2 wt% of PVP solution and 8 g of alumina platelets were mixed, stirred at room temperature for half a day, vacuum-filtered and washed several times to obtain PVP-coated alumina platelets. For the Ca2+crosslinked Alg films, alginic acid sodium salt was dissolved in distilled water and the 3 wt% alginate solution was mixed with different amount of ionic crosslinker calcium sulfate slurries (CaSO4) to eventually obtain a 2 wt% alginate mixture. The amount of CaSO4 (added to 3 wt% alginate solution) was 10, 15, 20, and 23 wt% with respect to the amount of alginate, which was corresponding to 0.2, 0.3, 0.4, and 0.46 wt% to the total weight of mixture. The mole ratio between calcium cations and carboxylate groups of the alginate was 12, 17, 23, and 26 mol%. The mixture was subsequently gelled inside a closed glass mold (2-mm thickness) at room temperature over a day. For the Ca2+-crosslinked Alg/Alu or Ca2+-crosslinked Alg/Alu-PVP composite films, bare or PVP-coated alumina platelets were added to the alginate solution before adding CaSO4. The number of platelets was altered from 5 to 40 wt% of the total mass of the polymer and the platelet. For the tensile test of the films, the flat hydrogels were cut into a dog-bone shape of 10 mm narrow-width, 16 mm broad-width, 20 mm long of the narrow part, and 50 mm overall length. Subsequently, the hydrogels were dried at ambient temperature (20–25 °C) and humidity (RH 30– 40%). On average, the dimensions of the dry films were 7.5 mm narrow-width, 13 mm broadwidth, 18 mm long of the narrow part, and 45 mm overall length. On average, film thickness was between 30 and 48 µm, depending on the Ca2+ concentration and the platelet content measured by SEM. For the bulk composite, dry Ca2+-crosslinked Alg/Alu-PVP composite films were soaked into 100 mM CaCl2 solution for a day and rinsed with water. The films softened without significant swelling due to sufficient calcium ions in the solution. The slightly swollen and softened films  ACS Paragon Plus Environment

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with 135–150 µm thickness were stacked layer-by-layer with applying 1 M EDTA solution between the films for temporary Ca2+-chelating of the surface, and dried under 5 kg-weight at 50 °C. The resulting laminated composite was very slightly softened again in 1 M CaCl2 solution for a day, and subsequently hot-pressed with 10 MPa at 70 °C. The final thick bulk composite was polished and cut into a beam shape with 45 mm length, 4 mm width, and 2 mm depth for the 3point bending test. Characterization. Tensile and flexural tests were conducted by Comtech QC-508E using a 500 N load cell of 5 mm/min load speed. Before the tensile tests, the dry films were stored in a specific humidity-adjusted container for at least one day to compare the mechanical properties of the composite films according to humidity. The gauge length for the tensile test was 24–30 mm for all measurements, and at least five samples in each condition were measured. The support span for the flexural test was 16 mm. Scanning electron microscopy (SEM) images were obtained using JEOL JSM-6510. Energy dispersive X-ray spectroscopy (EDX) analysis in SEM was conducted using JEOL 7500F. X-ray diffraction (XRD) analyses were conducted using D8 ADVANCE, Bruker Corporation, and the analyses for the omega-scan (rocking curve) were performed using SmartLab, Rigaku. High-resolution 2theta-scans were first conducted and a high intense peak around 2θ = 90.6° (0012 crystallographic plane) was detected. The plane (2θ = 90.6°) was subsequently set, and the omega-scan was conducted. Three different points were scanned in the XRD, and almost same results were obtained. Fourier transform infrared spectroscopy (FTIR) spectra were obtained using Bruker IFS-66/S, TENSOR27. Thermogravimetric analysis (TGA) was performed using TG/DTA7300, SEIKO Instruments under air at a heating rate of 10 °C/min.

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Numerical comparison of the degree of preferred orientation of platelet; (Figure S1) Zeta potential values and sedimentation rates; (Figure S2) Composite films at different drying condition; (Figure S3), XRD patterns of CaSO4 particles, Alg film, and Ca2+-Alg film, SEM images of CaSO4 particles, and EDX mapping of films; (Figure S4) A composite film dried on the complex surface structure; (Figure S5) A SEM image of alumina microplatelets; (Figure S6) A cross-section SEM image of a non-crosslinked Alg/Alu film; (Figure S7) Mechanical properties of the Ca2+-Alg films; (Figure S8) Mechanical properties of Ca2+-Alg/Alu and Ca2+-Alg/Alu-PVP composite films; (Figure S9) TGA curves of composite films with different inorganic contents; (Figure S10) Crosssection SEM images of composite films composed of high microplatelet contents; (Figure S11) Stress-strain curves of Ca2+-Alg/Alu-PVP composite films; (Figure S12) Rocking curves of different kinds of composite films; (Figure S13) Effect of humidity on mechanical properties; (Figure S14) An aspect ratio of alumina platelets.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] ORCHID Donghwan Ji: 0000-0003-2266-1654 Jaeyun Kim: 0000-0002-4687-6732 Author Contributions D.J. and J.K. conceived the concepts and ideas. D.J. conducted the experiments and collected and analyzed the data. D.J. and J.K. discussed the data and wrote the manuscript.  ACS Paragon Plus Environment

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ACKNOWLEDGEMENTS This work was supported by the Basic Science Research Program through the National Research Foundation (NRF) funded by the Ministry of Science and ICT, Republic of Korea (20100027955, 2014M3A9B8023471)

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46. Deacon, G. B.; Huber, F.; Phillips, R. J. Diagnosis of the Nature of Carboxylate Coordination from the Direction of Shifts of Carbon–Oxygen Stretching Frequencies. Inorg. Chim. Acta 1985, 104, 41—45. 47. Nara, M.; Tasumi, M.; Tanokura, M.; Hiraoki, T.; Yazawa, M.; Tsutsumi, A. Infrared Studies of Interaction between Metal Ions and Ca2+-Binding Proteins Marker Bands for Identifying the Types of Coordination of the Side-Chain COO− Groups to Metal Ions in Pike Parvalbumin (pI = 4.10). FEBS Letters 1994, 349, 84—88. 48. Nara, M.; Torii, H.; Tasumi, M. Correlation between the Vibrational Frequencies of the Carboxylate Group and the Types of Its Coordination to a Metal Ion:  An ab initio Molecular Orbital Study. J. Phys. Chem. 1996, 100, 19812—19817. 49. Papageorgiou, S. K.; Kouvelos, E. P.; Favvas, E. P.; Sapalidis, A. A.; Romanos, G. E.; Katsaros, F. K. Metal-Carboxylate Interactions in Metal-Alginate Complexes Studied with FTIR Spectroscopy. Carbohydr. Res. 2010, 345, 469-473. 50. Sartori, C.; Finch, D. S.; Ralph, B.; Gilding, K. Determination of the Cation Content of Alginate Thin Films by FTI.R. Spectroscopy. Polymer 1997, 38, 43—51. 51. Furukawa, T.; Sato, H.; Murakami, R.; Zhang, J.; Duan, Y.-X.; Noda, I.; Ochiai, S.; Ozaki, Y. Structure, Dispersibility, and Crystallinity of Poly(hydroxybutyrate)/Poly(L-lactic acid) Blends Studied by FT-IR Microspectroscopy and Differential Scanning Calorimetry. Macromolecules 2005, 38, 6445—6454. 52. Cathell, M. D.; Schauer, C. L. Structurally Colored Thin Films of Ca2+-Cross-Linked Alginate. Biomacromolecules 2007, 8, 33—41. 53. Daemi, H.; Barikani, M. Synthesis and Characterization of Calcium Alginate Nanoparticles, Sodium Homopolymannuronate Salt and Its Calcium Nanoparticles. Sci. Iran. 2012, 19, 2023—2028. 54. Landry, C. C.; Pappe, N.; Mason, M. R.; Apblett, A. W.; Tyler, A. N.; MacInnes, A. N.; Barron, A. R. From Minerals to Materials: Synthesis of Alumoxanes from the Reaction of Boehmite with Carboxylic Acids. J. Mater. Chem. 1995, 5, 331—341. 55. Derakhshan, A. A.; Rajabi, L.; Karimnezhad, H. Morphology and Production Mechanism of the Functionalized Carboxylate Alumoxane Micro and Nanostructures. Powder Technol. 2012, 225, 156—166. 56. Mackie, W. Semi-Quantitative Estimation of the Composition of Alginates by Infra-Red Spectroscopy. Carbohydr. Res. 1971, 20, 413—415. 57. Chandía, N. P.; Matsuhiro, B.; Mejías, E.; Moenne, A. Alginic Acids in Lessonia Vadosa: Partial Hydrolysis and Elicitor Properties of the Polymannuronic Acid Fraction. J. Appl. Phycol. 2004, 16, 127—133. 58. Leal, D.; Matsuhiro, B.; Rossi, M.; Caruso, F. FT-IR Spectra of Alginic Acid Block Fractions in Three Species of Brown Seaweeds. Carbohydr. Res. 2008, 343, 308—316. 59. Kim, Y.-C.; Park, H.-H.; Chun, J. S.; Lee, W.-J. Compositional and Structural Analysis of Aluminum Oxide Films Prepared by Plasma-Enhanced Chemical Vapor Deposition. Thin Solid Films 1994, 237, 57—65.  ACS Paragon Plus Environment

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Figure 1. (a) Schematic illustration representing fabrication process of the hydrogel-film casting. Randomly embedded alumina microplatelets are assembled into a horizontally layered structure during unidirectional drying. (b) Photographs of the dog-bone-shaped composite hydrogels and dry films before/after unidirectional drying. The overall shape maintains well during the drying process. SEM images of (c-e) cross-section, and (f) top-view of the composite film fabricated by unidirectional drying. The macroscopically uniform layer-by-layer microstructure is clearly observed at both low and high magnifications, and the alumina platelets are horizontally well aligned. (g) Photographs of a composite hydrogel and a dry film before/after all directional drying, and (h) a top-view SEM image of the dry film. Compared to the unidirectionally dried film, the all directionally dried film shows a highly shrunken shape with randomly embedded Alu platelets. (i) Photographs of composite hydrogels and dry films after unidirectional drying on the angled or curved substrate, and (j) a cross-section SEM image of angled dry film. (k) A photograph of the composite film dried on the substrate with complex surface structure.  ACS Paragon Plus Environment

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Figure 2. (a) XRD patterns of Alu platelets powder, all directionally dried film in the air, and unidirectionally dried film on the dish. Most peaks of all directionally dried film correspond to Alu powder due to the randomly embedded platelets, whereas the unidirectionally dried film exhibit strong peaks corresponding to the preferred planes, (006) and (0012). (b) XRD patterns of the composite films incorporated with various platelet contents. The preferred orientation of the platelets is maintained even at high inorganic contents.

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Figure 3. (a) Representative stress-strain curves, (b) tensile strength, (c) elastic modulus, and (d) toughness of the films of the non-crosslinked alginate (Alg), Ca2+-crosslinked alginate (Ca2+-Alg), Ca2+-crosslinked alginate/alumina (Ca2+-Alg/Alu), and Ca2+-crosslinked alginate/PVP-coated alumina (Ca2+-Alg/Alu-PVP) films. The Ca2+ crosslinking and horizontally aligned Alu-PVP platelets synergistically generate an increase in mechanical properties. For the accurate mechanical test, dog-bone-shaped films were prepared as shown in Figure 1b.

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Figure 4. (a) FTIR spectra of the Alg, Ca2+-Alg, Ca2+-Alg/Alu, and Ca2+-Alg/Alu-PVP films. The Ca2+ crosslinking, Alu, and Alu-PVP platelets induce an appearance of new peak, peak shifts, and increase in peak intensity. (b) XRD patterns (2theta-scan), and (c) rocking curves (omega-scan) of the composite films incorporated with bare or PVP-coated platelets. The PVP-coating improves the preferred orientation of the platelets, (006) and (0012) planes. (d-g) SEM images of side-view or cross-section of the fractured composite film after the tensile test. Some voids between the the matrix and the platelet, and holes formed by pulled-out platelet are observed. Interfacial bonding as like an organic glue exists, and residues remain at the platelet surface.

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Figure 5. (a, b) Effect of humidity on mechanical properties and (c, d) estimated interfacial strengths based on a simple shear lag model. (a) Stress-strain curves of the Ca2+-Alg/Alu-PVP composite films depending on humidity from 15 to 70 %. (b) Stress-strain curves of the Ca2+-Alg film and Ca2+-Alg/Alu, Ca2+-Alg/Alu-PVP composite films at highly and moderately dry conditions. Humidiy-dependent tensile strength (σc) as a function of the platelet volume fraction and the estimated interfacial strength (τi) based on the shear-lag model (continuous fitting curves) in (c) Ca2+-Alg/Alu and (d) Ca2+-Alg/Alu-PVP composite films at RH 15% and RH 30%. The platelet-polymer interfaces interact more efficiently at drier condition and PVP-coating improves the ineteractions resulting in higher τi.

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Figure 6. (a) Photographs of layered thin films before drying (left), a laminated bulk composite block after hot-pressing and polishing (middle), and a bulk composite specimen prepared for flexural test (right). (b) Cross-section SEM images of the composite with well-aligned microplatelets. (c) A photograph of the thick composite specimen for 3-point bending test. (d) A representative stress-strain curve of the specimen. (e) Fracture surface SEM images of the thick composite showing crack deflection and multiple cracking during crack propagation.

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TOC figure

Polymer composites consisting of a strong and stiff organic matrix and inorganic microplatelets within a uniformly layered structure are fabricated through a hydrogel-film casting method, based on rational design of each component and control of interfacial interactions between organic and inorganic phases. The hydrogel-film casting with unidirectional drying enables to build the microstructure with horizontally aligned alumina platelets, thus resulting mechanically improved composite materials.

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