Spherulite deformation mechanisms

Such information offers an opportunity to study details of the fibrillation mechanism. The fibers formed by stretching the spherulitic polymer representing nothing other than ribbon formations plastically deformed and oriented towards the mechanical stress that is released by comparatively weak mutual interconditions existing in an earlier formation (Figure 3). This behavior points to the existence of some weak surfaces in the crystalline polymers. Elements of the super-molecular structure detached by action of the external mechanical forces can slide on the weak surfaces. Evidence for the strain-destruction relationship must come from studies of the modification of the contact surfaces of two neighboring spherulites under mechanical stress. 

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. 

The results of Figure 11 indicate that the polymers studied were subject to different microstructural deformation mechanisms. In this connection it must be borne in mind that the maximum nominal deformation of POM and PA66 was only 1% whereas PP and PTFE were deformed up to 1.8% and 3.3% respectively. Therefore it may be assumed that for POM and PA66 only an instantaneously reversible deformation of the amorphous matrix of the spherulitic microstructure occurred (18) whereas for PP and PTFE some irreversible effects, like interlamellar shearing or reorientation of the lamellae may have taken place. 

Fig. 4.20a shows a variety of deformation mechanisms at the equator of the spherulite, due to differing orientations of the lamellar stacks relative to the tensile stress axis. Computer models are needed to consider the variety of lamellar stack orientations, and calculate the macroscopic stresses. Using an axisymmetric model of a spherulite (in a regular array), the tensile yield stress was predicted to be a nearly linear function of the crystallinity (Fig. 4.20b), and in the same range as experimental data. 

Matsuo, M. 1980. Deformation mechanism of polymer spherulite by linear isothermal viscoelastic theory. J. Chem. Phys. 72 899-910. 

In this chapter, firstly a mathematical representation of orientation distribution functions of structural units will be discussed in terms of an expansion of the distribution functions in a series of generalized spherical harmonics and generalized orientation factors. Secondly, the deformation mechanism of polymer spherulites will be shown to be one of the areas where the above theory can be applied. Thirdly, the relationship between the optical anisotropy in oriented polymeric materials and the orientation of the structural units will be described in general by several types of average degrees of molecular orientation. Finally, the mechanical anisotropy in oriented polymeric materials will be discussed in terms of orientation of the structural units. 

One of the most interesting applications of the theory of Krigbaum and Roe described above, is a computer simulation of the deformation mechanism of spherulitic crystalline texture. " The deformation mechanism of crystalline polymers has been extensively studied by a number of authors using different optical techniques in order to investigate the mechanism in terms of the change in crystalline texture during deformation, especially foj polyethylene specimens. "... 

Figures 14 and 15 show the orientation distribution functions, w(, 0, rj) and qjiCp 0) calculated as above for the extension ratios of the spherulite from 1.1 up to 1.4 by choosing the parameters so that the calculated results give the best fit to the observed orientation distribution functions in Figures 9 and 11. As can be seen by a comparison of Figure 14 with Figure 11, the development of the two populations in the orientation distribution function is well reproduced by theory. The orientation distribution functions, qjiCp 0), calculated for the principal crystallographic axes shown in Figure 15 also agree fairly well with the observed functions shown in Figure 9. The uniaxial deformation mechanism of spherulites can now be discussed in terms of the changes of the model parameters. ...

Another technique used for obtaining macroscopically polar films involves mechanical extension of the material. Uniaxial plastic deformation induces a destruction of the original spherulitic structure into an array of crystallites in which the molecules are oriented in the deformation direction. In case of PVF2 when such deformation takes place below 90 °C the original tg+ tg chains are forced into their most extended possible conformation which is all-trans [32]. 

Significant variation of the ultimate mechanical properties of poly(hexamethylene sehacate), HMS, is possible by con-trol of thermal history without significant variation of percent crystallinity. Both banded and unbanded spherulite morphology samples obtained by crystallization at 52°C and 60°C respectively fracture in a brittle fashion at a strain of r O.Ol in./in. An ice-water-quenched specimen does not fracture after a strain of 1.40 in./in. The difference in deformation behavior is interpreted as variation of the population of tie molecules or tie fibrils and variation of crystalline morphological dimensions. The deformation process transforms the appearance of the quenched sample from a creamy white opaque color to a translucent material. Additional experiments are suggested which should define the morphological characteristics that result in variation of the mechanical properties from ductile to brittle behavior. 

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