Interphase thickness, effect composites

Other methods for estimating the volume percentage of the interphases in a composition have been proposed, too, for example, measurements of density variations [76, 77], volume of compressed sample [78], the dielectric constant [77], etc. The important thing is that the interphase thickness determined in one way or another is an effective value dependent upon the conditions and type of the experiment by which it was determined [51]. 

Lipatov et al. [116,124-127] who simulated the polymeric composite behavior with a view to estimate the effect of the interphase characteristics on composite properties preferred to break the problem up into two parts. First they considered a polymer-polymer composition. The viscoelastic properties of different polymers are different. One of the polymers was represented by a cube with side a, the second polymer (the binder) coated the cube as a homogeneous film of thickness d. The concentration of d-thick layers is proportional to the specific surface area of cubes with side a, that is, the thickness d remains constant while the length of the side may vary. The calculation is based on the Takayanagi model [128]. From geometric considerations the parameters of the Takayanagi model are related with the cube side and film thickness by the formulas ... 

Unlike in the case of spherical inclusions where the rigid inclusions cause stiffening of the composite by excluding volume of a deformable mattix, the presence of an interphase layer affects the tme reinforcing efficiency of the inclusions. Hence, the effective filler modulus of the inclusions have to be calculated as a function of interphase thickness and elastic modulus. This can be done effectively using simple rule of mixture ... 

Lipatov (22) investigated the effects of interphase thickness on the calorimetric response of particulate-filled polymer composites. Based on experimental evidence, his analysis led to the conclusion that the interphase region surrounding filler particles had sufficient thickness to give rise to measurable calorimetric response. The proposed existence of a thick interphase region correlates with limitations of molecular mobihty for supermolecular structures extending beyond the two-dimensional filler boundary surface. 

In this conceptual framework it is naturally impossible to simulate the effect of the interphases of complex structure on the composite properties. A different approach was proposed in [119-123], For fiber-filled systems the authors suggest a model including as its element a fiber coated with an infinite number of cylinders of radius r and thickness dr, each having a modulus Er of its own, defined by the following equation ... 

It is now well established that a fiber coating must be deposited on the fiber prior to infiltration of the matrix, in order to control the fiber-matrix bonding and the mechanical behavior of the composite. Pyrocarbon (PyC), boron nitride or (PyC-SiC)- and (BN-PyC)-multilayers, with an overall thickness ranging from about 0.1 pm to about 1 pm, and displaying a layered crystal structure (PyC, BN) or a layered microstructure (multilayers), are the most common interphase materials in nonoxide CMCs. The main role of the interphase is to deflect the microcracks which form in the matrix under loading, and hence to protect the fiber from notch effect. 

This effect is fully corroborated by a dart drop test (DIN EN ISO 6603-2) with a 100 J dart and injection molded plates of 2 mm thickness, as shown in Fig. 18.24. In particular, the absorbed energy during the puncture event for the weak PP-g-MAH interphase is more than doubled compared to the normal composite without decoupling (moderate interphase). 

FIG. 11 Effect of interfacial interaction on the thickness of the interphase determined in various polymer/CaC03 composites (PP, LDPE, pPVC, PVC in increasing order). 

The coverage of the surface of a filler with a polymer layer which is capable of interdiffusion with the matrix proved to be very effective both in stress transfer and in forming a thick, diffuse interphase with acceptable deformability. In this treatment the filler is usually covered by a functionalized polymer, preferably by the same polymer as the matrix, which is attached to the surface by secondary, hydrogen, ionic or sometimes even by covalent bonds. The polymer layer inter-diffuses with the matrix, entanglements are formed and, thus strong adhesion is created. Because of its increased polarity, in some cases reactivity, usually maleic anhydride or acrylic acid modified polymers are used, which adsorb to the surface of most polar fillers even from the melt. This treatment is frequently used in polyolefin composites, since other treatments often fail in them, on the one hand, and functionalization of these polymers is relatively easy, on the other. Often a very small amount of modified polymer is sufficient to achieve significant improvement in stress transfer [126, 127]. 

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