Polymer nanocomposites: Why their mechanical performance does not justify the expectation and a possible solution to the problem?

Abstract

The expected order-of-magnitude enhancement in modulus (at loadings less than 1%) of polymer nanocomposites has proven elusive-the observed improvements are 10-35% only. The failure of the concept has been attributed to poor dispersion, poor interfacial load transfer, process-related deficiencies, and others. Due to the inherent property of the nano-size materials, their extremely high specific surface, they tend to agglomerate, and their further dispersion in the matrix component is practically impossible. For this reason, the composites prepared via blending a polymer with a nano-size material are microcomposites instead of nanocomposites, as demonstrated by light scattering studies. So long as reliable tools and/or techniques for proper dispersion of nanomaterials are missing, we must use methods free of the dispersion step in the manufacturing process. The relatively new 'concept of converting instead of adding' offers two such techniques-instead to take the matrix and the reinforcement in their final form and blend them, one takes one component only and during the processing creates the missing second component. Both approaches result in true nanocomposites with superior mechanical performance-the improvements of the tensile strength for nanofibrillar polymer-polymer and single polymer composites are up to 200 and 440%, respectively, (or even up to 650% if trans-reaction catalyst is used).

Relationship between the Processing, Structure, and Properties of Microfibrillar Composites

“…The main differences between these two drawings lie in the temperature. The cold drawing happens slightly above the glass-transition temperature of the reinforcement and a highly oriented molecular fibrillar state may be achieved as the drawing goes via necking, [74,75] while the hot stretching goes directly from the melt. Hot drawing can also be called "melt stretching"; the molecular orientation, however, is typically much lower compared to the cold drawing techniq ue.…”

“…Hot drawing can also be called "melt stretching"; the molecular orientation, however, is typically much lower compared to the cold drawing techniq ue. [57,64,72,74,76,77] Both ratios may affect the aspect ratio of the fibrils, final morphology, and mechanical properties of the composite. The DR can be defined as the ratio of the areas (before and after the drawing), but it may also be calculated as the ratio of the velocity of the rollers.…”

“…Unlike other fiber‐reinforced composites, the fibrils in MFCs are formed in situ and there is no need for a dispersion step during the manufacturing. [ 74 ] Therefore, the agglomeration of the reinforcing component may be drastically reduced and perfect homogeneous distribution of the reinforcement in the matrix may be achieved, [ 74 ] which is the biggest issue when commercial fiber‐reinforced composites are processed. [ 18 ] Researchers have reported that applying the concept of MFCs to immiscible polymer blends is a good way to achieve high‐level mechanical properties.…”

Gelatin micro‐ and nanocapsules obtained via sonochemical method

“…Three different temperatures (24,34, and 42 C) were used in order to follow the influence of this parameter on particles size. The first two temperatures were chosen to be close to, but below the sol-gel transition temperature of gelatin which is 39-40 C. The last one was chosen to be above the sol-gel transition temperature.…”

“…The observed assembly in ribbons could be explained by the natural tendency of the nanosized particles (in this case capsules) to reduce their extremely large surface and the respective energy needed to maintain them stable in time. [24] The nanosize of GCs, obtained by sonochemical method, is quite unusual observation. So far, nanosize (400 ± 100 nm) was reported only for sonochemically prepared poly(glutamates) capsules, filled by vegetable oil.…”