Spherulite boundaries

Polymers below the glass transition temperature are usually rather brittle unless modified by fibre reinforcement or by addition of rubbery additives. In some polymers where there is a small degree of crystallisation it appears that the crystallines act as knots and toughen up the mass of material, as in the case of the polycarbonates. Where, however, there are large spherulite structures this effect is more or less offset by high strains set up at the spherulite boundaries and as in the case of P4MP1 the product is rather brittle. 

Increasing EPDM content results in irregular spherulitic texture, smaller spherulite size, and loss of sharpness in the spherulite boundaries. 

Open questions also exist in the case of macroscopically inhomogeneous deformation, as it occurs for instance in the presence of aggressive environments Even less well known are those inhomogeneous deformation mechanisms which are induced by certain morphological features crack-like defects in the spherulitic morphology, second phases, void or crack nucleating particles, or certain micro-structural elements behaving differently such as spherulites and spherulite boundaries. 

Fig. 22a—c. Microtome section of an intersperujitic crack path in bulk coarse spherulitic PP 1120 a. Figure b indicates as an SEM-micrograph taken from the surface of the specimen the interspherulitic craze formation prior to the cracking process, c shows a site of shear along a spherulite boundary oriented under an angle of about 60° to the horizontal crack direction... 

Fig. 30a--c. Interaction between shear bands of type A with bands of type B a, and with spherulite boundaries perpendicular b and parallel c to shear band package A in PP 1320... 

In addition to the described role of shear bands in crack initiation, the influence of spherulitic boundaries on crack propagation has to be considered, especially in coarse spherulitic, highly isotactic PP. Cracks which may have been formed at a particular point of a shear band according to the mechanism described above can leave their shear plane with further growth in order to move into an adjacent spherulite boundary (Fig. 36, Point I). This is especially probable when the boundaries are cut by the shear plane under a small angle. The selection of the particular crack path is determined by the partial fracture mechanical properties of the shear band and the spherulite boundary, respectively, as well as by the local geometrical conditions... 

Fig. 36a and b. Crack initiation as a result of interactions between shear bands (A, B) and coarse spherulite boundaries (SB) a before crack initiation (x = shear stress in the bands) b after crack initiation (C) ... 

Fig, 37a—d. Structure of shear fracture surface a SEM-micrograph of a shear fracture in PP 1120 (T = —80 °C) b secondary crack formation (white arrow) in one of the shear bands of type B in fine spherulitic PB-l (T = -196 °C) c traces of shear bands B containing fibrillated polymer substance on a shear fracture plane in fine spherulitic PP 1120 (T = —196 °C) d preferred shear fracture along spherulite boundaries (SB) of coarse spherulitic PP 1120 (T = —80 °C)... 

Further fracture surface features, especially distinct for the highly isotactic, coarse spherulitic PP, are polyhedron shaped regions at which the shear crack has left the shear fracture plane in order to stay in a spherulitic boundary region oriented at a flat angle to the sliding direction (Fig. 37d). These sites often constitute secondary crack nuclei in boundaries perpendicular to the shear fracture plane. 

The formation of shear bands under compression is found in crystalline polymers when loaded at temperatures lower than 0.75 T. Under such a condition the shear bands interact with certain morphological features such as spherulite boundaries or lamellar arrangements inside the spherulites. The band initiation stress, ct, increases and the strain at break, Cp, decreases with decreasing temperature and increasing stiffness of the tested polymer, i.e. increasing degree of crystallinity. 

Figure 1.10. (a) The spherulitic habit of a semi-crystalline polymer on cooling from the melt. (A micrograph of polyethylene glycol viewed under crossed polarizers), (b) A schematic diagram of lamellar fibrils that have nucleated from the points shown and grow to the spherulite boundaries. Tie molecules connecting lamellae are shown. 

In previous sections we have shown that the redistribution of additives at the spherulite boundaries during polymer crystallization leads to the additives uneven distribution, whose form is determined by the kinetics of the growth rejection process. In time, this initial dynamic distribution should relax to an equilibrium form in which the noncrystalline polymer is uniformly permeated by the additive, whose distribution reflects that of the noncrystalline polymer. The relevanoe of these observations to oxidative degradation processes in semi-crystalline polyolefins is discussed in this section. 

Figure 16.7 Temporal evolution of the crystalline microstructure in the 50/50 iPP/EPDM blend, following a T-quench from the isotropic melt to a supercooled temperature below both the UCST spinodal gap, showing the growth of spherulitic front in the concentration field, but the overgrowth of this spherulitic boundary on the bicontinuous SD domain structures can be seen clearly only in the enlarged version.

If these oxidized sections were flexed the cracks ran apparently at random. Similar samples which were crystallised and oxidised as thin films tended to crack along spherulite radii. If the sample was first oxidised to sibout the induction time and then crystallised there was again no evident relationship between oxidation and morphology. However the staining process had clearly extracted material from the spherulite boundaries leaving them as channels in the film. If the unstained sections were flexed they frequently cracked along the spherulite boundaries. 

We conclude from this that crystallisation of pre-oxidised polymer does lead to segregation of partly oxidised, isopropanol soluble material to the spherulite boundaries which weakens them. At low levels of oxidation where the isopropanol extractable material is only a small fraction of the total carbonyl content, this segregation does not lead to increased oxidation rates locally. At higher oxidation levels much of the oxidised material is extracted so that we cannot follow the process. 

As crystallization proceeds, the growth fronts of two different spherulites meet, and the lamellae extend across spherulite boundaries into uncrystallized material available, thus holding the material together. In addition, interlamellar fibrils tie two or more lamellae together and also bridge spherulite themselves. 

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