Nano-scale interphase

In this chapter, the interphase phenomena in the polymer matrix composites at the micro- and nano-scales are briefly reviewed and thdr main differences are discussed. An approach for modeling mechanical properties of continuous macroscopic bodies considering peculiarities brought about by the discrete nature of the matter at the nano-scale is proposed based on the combination of gradient strain elasticity and chain reptation dynamics. 

Figure 7.4. (a) Visualizing the interphase considering only the micro-scale. Interphase is a continuum layer with a gradient of properties reflecting variations in its structure. The main role of the micro-scale interphase is to provide stable and effective ineans for stress transfer between inclusions and polymer matrix even under adverse conditions, (b) Visualizing the structure of a micro-composite considering also the nano-scale structural features when the discrete structure of the matrix and inclusions becomes evident [169]... 

The role of the nano-scale interphase differs when considering the two limiting cases of (i) low ty nanocomposites (ty< 0.05) and (ii) high ty nanocomposites Vf> 0.85). The first case represents the majority of the published data and can be assumed as a direction to preparing new nano-structmed advanced matrices, while the second case is more common in bio-nanocomposites [103,147] and its exploitation can result iu designing new nano-stnictured advanced reinforcements. With a few exceptions, most of the published literature on synthetic nanocomposites deals with low tynanocomposites, while, on the other hand, most of the literature published on high ty naiiocomposites is related to the mechanics of bio-composites, such as bones, teeth and shells [148,149]. 

Most of the experimental evidence related to the interphase in the low ty nanocomposites was obtained at temperatures below the polymer reusing meso-scale test specimens. Assuming the chain immobilization to be the primary reinforcing mechanism on the nano-scale, spatial distribution of the conformation entropy within the polymer phase is of primary importance. Hence, experimental data for nano-composites above the matrix Tg has to be considered. Sternstein et aL [24,26,27] published an interpretation of the viscoelastic response of rubbery nanocomposite above the matrix Tg, i.e., the Payne effect. Kalfus and Jancar [144] analyzed the viscoelastic response of poly vinylacetate filled with uaiio-sized hydi oxyapatite over the temperature range from -40 to - -120°C and observed strain softening similar to the Payne effect [133,150]. 

The modulus recovery experiments allowed measuring the terminal relaxation time of reptation motion of bulk and surface immobilized chains, supporting the hypothesis that theie is no interphase per se when nano-scale is considered. In order to bridge the gap between the continuum interphase on the microscale and the discrete molecular structure of the matrix consisting of freely reptating chains in the bulk and retarded reptatiug chains in contact with the inclusions, higher order elasticity combined with a suitable molecular dynamics model could be utilized [151-155]. 

Jancar J (2008), Review of the role of the interphase in the control of composite performance on micro- and nano-length scales, J Mater Sci 43 6747-6757. 

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