SKIN-CORE MORPHOLOGY

The blends of thermotropic LCPs and thermoplastics are generally two-phase systems where the dispersed LCP phase exists as small spheres or fibers within the thermoplastic matrix. Often a skin/core morphology is created with well-fibrillated and oriented LCP phases in the skin region and less-oriented or spherical LCP domains in the core. 

Hobbs, S. Y. and Pratt, C. F., The effect of skin-core morphology on the impact fracture of polybutylene terephthalate, J. Appl. Polym. Sci., 19, 1701-1722 (1975). 

Figure 5.74 Skin-core morphology of an injection-molded polypropylene structural foam. Reprinted, by permission, from P. R. Hornsby, in Two-Phase Polymer Systems, L. A. Utracki, ed., p. 102. Copyright 1991 by Carl Hanser Verlag.

The thermal properties of fillers differ significantly from those of thermoplastics. This has a beneficial effect on productivity and processing. Decreased heat capacity and increased heat conductivity reduce cooling time [16]. Changing thermal properties of the composites result in a modification of the skin-core morphology of crystalline polymers and thus in the properties of injection molded parts as well. Large differences in the thermal properties of the components, on the other hand, lead to the development of thermal stresses, which also influence the performance of the composite under external load. 

Figures 11 to 13 are dark field micrographs of 66 polyamide monofilaments. Figure 11 show an Ag-S stained filament. Silver sulfide precipitates, which appear as black areas (as they did in bright field images) as well as polyamide crystallites (bright spots) are visible. Figure 12 corresponds to a type 4 fiber (with skin-core morphology) where there is a lower density of crystallites in the skin region. Figure 13 corresponds to the case of type 5 fiber which has smaller crystallites.

Whatever their origin, the bands reflect the susceptibility of the fibrils to transverse kinking or buckling. Development of a skin-core morphology in the coagulation process may explain the... 

Figure 8. Skin/core morphology of injection-molded PP—polarizing optics, (a, top) Skin and shear zone (b, bottom) spherulitic core.

SEM micrographs and visual appearances of the fracture surfaces revealed the presences of a hierarchical organization and microstructures in the macrolayers. Starting torn the macroscopic level, five macrolayers were observed two outer skins, two mid layers with a core in between. Due to differences in color, the macrolayers were readily visible to the naked eye. This skin-core morphology is a characteristic of many injection-molded LCPs (7-81. 

C02-induced crystallization of polymers presents a unique, tunable degree of morphological control of the polymeric matrix, as revealed by CRM. Control over the operating conditions enables control of the diffusivity of CO2 combined with control over the kinetics of crystalhzation. This degree of control enables the distribution of morphological changes in the polymer to be tailored, which may facilitate the formation of novel materials with a skin-core morphology with possible unique mechanical properties. 

Skin/core morphologies are common in blends of LCP s and thermoplastic polymers and they play a significant role in defining the properties of both extruded and injection molded samples. Usually, LCP s in the skin have a higher degree of orientation than in the core when the blends are extruded or injection molded (Husman et al. 1980 Hedmark et al. 1989 Lee 1988). Baird et al. (Baird and Mehta 1989 Baird and Sukhadia 1993) observed a skin/core morphology in blends of PA 66 with HBA/HNA and 40 PET/60 PHB and 20 PET/80 HBA copolyesters. More LCP fibers were present in the skin than in the core for both systems. Isayev and Swaninathan (1994) also reported shell-core structure in the fracture surfaces of injection molded blends of HNA/HBA liquid crystalline copolyesters and poly (etherimide). 

Karger-Kocsis, J. and Friedrich, K. (1989) Effect of skin-core morphology on fatigue crack-propagation in injection molded polypropylene homopolymer, Int. J. Fatigue, 11, 161-168. 

The balance between these characteristics and the molecular weight can explain some important differences in flow-induced crystallization during transformation processes for these samples (skin-core morphology, biorientability, etc). 

Fig. 5.38 SEM of a fractured, molded POM test bar, containing a high void level, shows a skin-core morphology. Elongation of the voids at the skin surface is due to high orientation whereas the more roimded voids and the semicircular flow front in the core results from less orientation in that region of the mold.

Thermotropic LCP molded bars exhibit a layered structure as shown by reflected light (Fig. 5.91A) of a cut and polished bar. Thin sections of a molded bar show fine, nematic domains with superimposed flow lines (Fig. 5.91B, color section), especially near the center of the bar (Fig. 5.91C, color section). Skin-core morphologies are obvious in injected molded bars and extrudates with domains aligned in the flow direction. Complementary SEM assessment of fractured injection molded bars provides an overall view of the layered structure (Fig. 5.92A), the surface skin (Fig. 5.92B) and the internal fibrillar structures of the inner skin (Fig. 5.92C) and core (Fig. 5.92D). 

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