Ultrastrong and stiff layered polymer nanocomposites, Podsiadlo, Kaushik, et al., Kotov, Science, 2007
Novelty/impact/significance:
Based on a traditional layer-by-layer (LBL) assembly, multilayered clay/poly(vinyl alcohol)(PVA) nanocomposites with highly ordered and uniformly distributed planar nanosheets are fabricated. The nanocomposites show exceptionally high stiffness and tensile strength (one order of magnitude greater than those of analogous nanocomposites), comparable to the modulus of Kevlar.
Scientific question:
How to scale/retain the ideal strength/robustness of the nanoscale building blocks to/in the mechanical properties of macroscale composites / to produce composites that show properties approaching the theoretical maxima?
Key of how:
Using LBL via spatial and orientational control of the nanoscale building blocks, a high level of orderness of the clay platelets (70wt%) and the extensive effective interactions between the organic and inorganic components (hydrogen bonding, Al-PVA covalent linkages, GA crosslinking) stiffening the matrix (constrained motion) lead to highly effective load transfer between the nanosheets and the polymer.
Major points:
1. Despite using nanoscale reinforcements with exceptional/ideal strength and stiffness, the bulk nanocomposites usually show mechanical properties that fall far below the expected theoretical values, except at low volume fractions.
This is due to the difficulty to obtain well-dispersed and/or well-ordered large volume fractions of the reinforcing nanofillers (structural control), to realize an effective load transfer to the nanoreinforcements, and to fully understand the mechanical interactions of the constituents at the nanoscale.
2. Montmorillonite (MTM) clay nanoplatelets reinforced polymer composites have been extensively studied, while the stiffness values are still one order of magnitude lower than theoretical ones, which is due to, similarly, the above issue (less than 10wt% clay can be well-dispersed or well-intercalated, more addition causes aggregation and property deterioration).
3. The bottom-up LBL assembly includes sequential adsorption of nanometer-thick monolayers of oppositely charged compounds (charged nanoparticles, polyelectrolytes) to form a multilayered structure, thus having control over the nanometer-level architecture.
Prior work on LBL fabricated nanocomposites show analogous structure to the natural nacre and fairly high strength and modulus, but are still below theoretical limits.
4. Using a traditional LBL, the poly(vinyl alcohol) (PVA)/MTM multilayered nanocomposites are fabricated; characterization and measurements reveal the dense coverage of the nanoplatelets and their strictly planar orientation, and an average of ~5 nm per bilayer (total 1.0-1.5 µm thick composites).
5. Despite of uncharged PVA, the PVA/MTM nanocomposites are stronger than other charged polymers with the clay sheets. This is due to the high efficiency of hydrogen bonding (corroborated by atomic modeling) and the extensive Al-PVA covalent crosslinking (supported by FTIR, NMR, XPS).
Next treatment of glutaraldehyde (GA) can further increase the crosslinking/bonding between the clay sheets and the PVA. The GA treated PVA/MTM show high uniformity, strength, modulus, flexibility, and transparency, with 80 to 90% transparency across the visible light (PS: so the transparency with 70wt% addition demonstrates the uniform dispersion and perfect orientation?).
6. The GA treated PVA/MTM show outstandingly high strength and modulus, (~480 MPa and ~125 GPa), being ~4 times and by one order of magnitude than PVA/MTM without GA, and much higher than the PVA. The modulus is comparable to that of Kevlar (E ~80 to 220 GPa) and exceeds that of the CNT-based fibers.
7. Without available correlated theories (predicting properties of nanocomposite with nanometer-scale spaced nanoreinforcements at a large volume fraction) at that time, it was believed that the impressive stiffening lies in the effective stiffening of the PVA matrix (constrained polymer chain motion), resulted from the close proximity to and extensive interactions (hydrogen bonding, Al-PVA covalent linkages, GA crosslinking) with the MTM platelets. This resulted in a highly efficient load transfer from the polymer matrix to the stiff MTM platelets.
It is clear and concise, an impressive work.
Here is the link of the fulltext: https://science.sciencemag.org/content/318/5847/80