The SEM images of mica without and with sonication during fabrication are shown in Fig. 1(b), (c) and (d), respectively. Without sonication, mica exists as blocks of irregular shapes. The length of a block unit can be as long as 10 ~ 40 µm. This is due to the high surface energy, causing the thin flakes to merge with each other. With sonication, thinner and smaller flakes of ~ 500 nm are kept separate from each other and more evenly distributed in the suspension. Besides, there are also some thinner flakes of ~ 10µm in the suspension.
Hardness and elastic modulus of the P-M-P film
Effect of the ratio of P-P to P-M-P film. Nanoindentation was used to test the hardness and indentation modulus of the films. Mica belongs to monoclinic crystal, whose crystal is scaly, with silk luster (Muscovite is glass luster), pure block is gray, purple rose color, white, etc., diameter thickness ratio > 80, specific gravity 2.6 ~ 2.7, molar hardness 2 ~ 3, elastic, bendable, good wear resistance and wear resistance; heat-resistant insulation, difficult to dissolve in acid-based solution, stable chemical properties.
The hardness and elastic modulus of films with different ratios of P-P to P-M-P film are shown in Table 1. Films were prepared with 1 and 5 cycles. For examples, a P-P film with 1 cycle was prepared by depositing PVA one cycle followed by PMMA in one cycle. a P-M10-P film with 1 cycle was prepared by depositing PVA one cycle followed by PMMA in one cycle and sandwich mica in ten cycles. The hardness and indentation modulus of a P-P film with 1 cycle were 0.323 GPa and 19.43 GPa, respectively. With the increase of the P-P cycles to 5, the hardness decreased from 0.323 to 0.277 GPa, and the indentation modulus decreased from 19.43 to 6.56 GPa. This is because PVA and PMMA are flexible material. The more PVA/PMMA “mortar” layers, the tougher the film. When the mica with 10 cycles, the hardness increased from 0.323 to 3.71 GPa, and the indentation modulus increased from 19.43 to 72.99 GPa. This can be explained by the fact that mica “bricks” is quite rigid itself. While P-M10-P with 5 cycles, the indentation modulus significantly decreased from 72.99 to 4.95Gpa, the hardness decreased from 3.71 to 0.187 GPa, indicating the “mortar” layers contact between PVA and PMMA significantly reduces the hardness and indentation modulus of the P-M-P film.
Table 1
Mechanical properties of films with different ratios of PVA/PMMA to P-M-P film.
sample | Cycle | Hardness (GPa) | Indentation modulus (GPa) |
P-P | 1 | 0.323 | 19.43 |
P-P | 5 | 0.277 | 6.56 |
P-M10-P | 1 | 3.71 | 72.99 |
P-M10-P | 5 | 0.187 | 4.95 |
Effect of the ratio of mica to P-M-P film. Figure 3a shows the load-displacement curves for P-M-P film with mica prepared under different mica pretreatment methods. All films were prepared with an equal number of cycles of PVA and PMMA. two pretreatment methods were compared, i.e. sonication, mechanical stirring high concentration for 24 hours. Under the load of 6000µN, the depths of P-P film, P-M1-P film with the sonicated suspension mica and P-MH-P with high concentration are 863.5, 350 and 248.4 nm, respectively. The displacement is between P-P film and P-MH-P film for P-M1-P film, indicating that the sonicated suspension mica was evenly distributed in the PVA and PMMA. This suggests that non-dispersed mica in large pieces had limited effect on the improvement of the film stiffness.
We further studied the hardness, indentation modulus, and load-displacement curves of P-M-P film with the different mica cycles 1, 10, 20, 30, 40 and 50 of assembly. All films were prepared with an equal number of PVA and PMMA. It is obvious that the P-M30-P film had the best ability to recover deformation (Fig. 3b), indicating the mica 30 cycles was the best radio of P-M-P to form homogeneous film. The hardness and indentation modulus of the P-M30-P film were 6.14 GPa and 68.41 GPa, respectively (Fig. 3c). The hardness of the film increased with the number of mica self-assembly cycles, and the indentation modulus was significant decline in P-M30-P film. It shows that the more mica content of the film, the more rigid and flexible the film. This also can be verified from the Fig. 3d. The calculated values of H/E to P-M-P film was increased with the mica cycles from 1 to 30 and decreased at 50, indicating the P-M30-P was the best plasticty.
Formation mechanism of the "brick-and-mortar" structure. The physical properties of PVA and PMMA are affected by chemical structure and polymerization degree. The common in their chemical structure is "head tail". Mica is a kind of aluminosilicate mineral with continuous layered silica tetrahedral structure. Mica is mostly monoclinic, in the form of laminated or book like crystals. The fully developed ones are rhombus or hexagon with six crystal faces, sometimes forming pseudo hexagonal columnar crystals.
As shown in Fig. 4, PVA is adsorbed onto the substrate via the interaction between the hydroxyl groups of PVA and the glass surface in first step of the self-assembly process, i.e. the immersion of the substrate in the PVA solution. When the mica layer is deposited, the silanol groups of mica form hydrogen bonds with the hydroxyl groups of PVA, resulting in self-assembly. The multi-mica-layer assembly is to repeat the above process. After the substrate is pulled up, PVA sheet will be laid on the substrate slowly because of gravity on the wood, the result of directional arrangement is produced. The last layer was PMMA, the PMMA solution would permeate into the mica and act as hydrogen bonds with ester group, yielding alternating layers of PVA, mica and PMMA.
Because of the PMMA permeate into mica and yielding alternating PVA and mica, the P-M-P film exhibit outstanding plasticty. the mechanical properties of P-M-P film to nacre are list in Table 2. When the Mica cycles is 30, the P-M-P film exhibited a significate increase in hardness and modulus compared to P-P film. Meanwhile, calculated values of H/E for P-M30-P film is 0.090, which is closed to nacre.
Table 2
Mechanical properties of nanocomposites.
sample | Hardness (GPa) | modulus (GPa) | H/E | Macroscale |
PVA/PMMA | 0.323 | 19.43 | 0.017 | film |
PVA/Mica30/PMMA | 6.14 | 68.41 | 0.090 | film |
Nacre [23] | 10.8 ± 1.5 | 114 ± 8.8 | 0.095 ± 0.011 | bulk |