Effect of crystallinity on microhardness

As pointed out above, the semicrystalline polymer can be considered as a two-phase composite of amorphous regions sandwiched between hard crystalline lamellae (Fig. 4.2(a)). Crystal lamellae ( c) are normally 10-25 nm thick and have transverse dimensions of 0.1-1 pm while the amorphous layer thickness, a, is 5-10 nm. As mentioned in the previous section, melt-crystallized polymers generally exhibit a spherulitic morphology in which ribbon-like lamellae are arranged radially in the polycrystalline aggregate (Bassett, 1981). Since the indentation process involves plastic yielding under the stress field of the indenter, microhardness is correlated to the modes of deformation of the semicrystalline polymers (see Chapter 2). These 

In dealing with the hardness of semicrystalline polymers it has long been recognized that the following general empirical relationship (parallel model) holds (Balta Calleja etal., 1981 Balta Calleja, 1985)  

This equation assumes that the semicrystalline polymer is a two-phase system and that the microhardness (yield) is due to plastic deformation taking place only in the crystalline regions. However, for polymers like PET or PEEK, when Tg T the microhardness of the amorphous phase Ha 0 (Deslandes etal, 1991). 

A typical good linear relationship between the microhardness and degree of crystallinity has been observed for melt-crystallized PP (Martinez-Salazar et al. 1988). For a series of i-PP samples crystallized within a wide range of crystallinity values the microhardness varies with crystallinity We according to eq. (4.3)  

A similar linear relationship between H and We has also been obtained for other polymers, such as PET (Vanderdonckt et al., 1998) and nylon 6 (Krumova et al, 1998) annealed at various temperatures. The extrapolation of the linear plot of H Vi We allows one to estimate again the H values for the completely amorphous (H 120 MPa 50 MPa) and for the fully crystalline 

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Let us next illustrate the effect of crystallinity on microhardness for a polymer with Tg above room temperature. For this purpose the correlation of H and the microstructure of PET, a polyester of typically low crystallinity having a Tg value well above room temperature, was examined. PET can be easily prepared in the form of a glassy amorphous material by quenching from the melt. 

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