Micro spherulitic structure

Optical micrographs of cross sections of the extruded HDPE/iPP (25/75) filaments show a fine morphological structure with micro-spherulites aligned along telescopic rings (Fig. 11). In the decentralized centre of the filament there is a zone of almost circular shape with different morphology this region contains mainly HDPE. 

On annealing the PC/PET micro- and nanolayered composites, the PET iayers were found to undergo coid crystallization and to form lamellar crystals independent of the thickness of individual layers see Fig. 9.6. Due to the physical confinement imposed by the adjacent PC layers, the PET layers (and in general semicrystalUne polymers) are unable to develop well-defined spherulitic structures, as in bulk melt-crystaUized samples [8,13]. 

Fig. 13.12. Mechanical deformation (drawing) of spherulitic structures breaks the constituent lamellae into blocks and tie molecules. The blocks tilt, with the chains aligning along the load axis, and make up highly oriented micro-...

Polymer micro-Znano-structure including crystallinity, orientation of amorphous chains, and spherulite size. 

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. 

Another aspect of the multiphase rheometry is related to the interrelations between the flow field and system morphology. In this text the term morphology will refer to the overall physical form or shape of the physical structure of a material, usually described as either a dispersed phase (particles or domains), co-continuous lamellae, fibrils or spherulites. Furthermore, morphology considers distribution and orientation of the phases, the interfacial area, the volume of the interphase, etc. However, the term must be distinguished from micro-morphology, which describes structures of the crystalline phase. Flow may induce two modifications of morphology that may complicate interpretation of data the concentra-... 

Polymers can exhibit a hierarchical organization of structure at four successive levels, the molecular, nano-, micro-, and macrolevel [33, 34], On the scale of tens of microns, semicrystalline polymers contain spherulites, the spherulites have a lamellar texture, and the molecules within the lamellae are organized in crystals and amorphous domains. Amorphous polymers are structured on the molecular and macroscopic scale only [34]. Thermoplastic SMPs are usually phase-segregated materials, i.e., they consist of at least two different domains, which are related to different thermal transition temperatures (Tinms)- Therein hard domains have a TJrans (glass transition temperature Tg or melting temperature 7]n) usually much higher than room temperature and determine the permanent shape, while switching domains show a lower thermal transition (Tg or 7]n). SMP networks contain chemical crosslinks instead of hard domains to fix the permanent shape. 

The crystallization temperature peak of PLLA has been estimated at about 123°C, and therefore a proper temperature for annealing is around 100°C. At this thermal condition, crystal growth is faster than at lower temperature. This is illustrated in Figure 11.2, where the micro-structures of PLLA specimens annealed at 70 and 100°C at different treatment times are shown with polarized optical microphotography. It is clear that the density and size of spherulites increase with annealing time and temperature [23]. 

Invariably the fractional n values are close to 2 and this has been attributed to the growth of rods whose numbers are increasing with time and inherent in the structure of spherulites. Calvert et al. using a u.v. optical microscope, have obtained some evidence from the variation in amorphous content within spherulites of the complexity of the micro-structure of spherulites. The central regions of the spherulites contained less amorphous-regions than the outer, and... 

Of special interest are the results of studying jump-like creep on the micro-scale level when the controlled structural heterogeneities of the same sizes are present in polymers, namely, for epoxy networks with a globular sUiicture [311], for epoxy composites containing diabase microparticles [310], for POM plastics with different spherulite sizes [320], and for Pl-graphite composites [320], These materials can be considered as models for checking up the micro-plasticity vs sUiicture correlations. It was possible to compare directly the sizes of heterogeneities (solid microparticles or densely packed micro-domains of polymers) with the creep micro-jumps, and to draw the conclusions about their interrelationship. 

In [320], jump-like creep was studied for two POM semi-crystaUine polymers. Analysis of their polarizing microscope images showed that homoPOM contained predominantly spherulites 1-5 pm in diameter, whereas poly(oxymethylene-co-oxyethylene) (95POM/5POE) sample contained both the same small spherulites and larger ones, up to 25 pm in diameter. It might be supposed that the size of these more dense structural units will be reflected in the character of variation of the creep rate and the values of deformation jumps L. Then the loosely packed inter-spherulite boundaries could be considered presumably as the most probable points for local shear displacements (micro-plasticity). 

The found coincidence of deformation micro-jumps in creep with the sizes of spherulites may be associated with the fact that creep occurs initially preferably through micro-shears along the boundaries of spherulites. Of course, at larger strains the transformation of the sPucture can result in the manifestation of new structural units and another deformation jumps on the meso-scale level. 

Micro-diffraction techniques have been developed mainly at the ID 13 beamline of the European Synchrotron Radiation Facility (ERSF) with a beam size of 3-10 pm for viscose rayon fibers, spider silk, spherulites of P(3HB), and a poly(lactic acid)/(atactic-P(3HB)) blend. Recently, we developed the micro-diffraction techniques with 0.5 pm beam size for analysis ultra-high-molecular-weight-P(3HB) mono-filament ° and P(3HB) copolymer spherulites. To reveal the detail fiber structure and the distribution of two types of molecular conformations in drawn P(3HB-co-8%-3HV) mono-filament, a micro-beam X-ray diffraction experiment was performed with synchrotron radiation at SPring-8, Japan. The beam size was focused to 0.5 pm with the Fresnel Zone Plate technique and the P(3HB-co-8%-3HV) mono-filament was scanned linearly perpendicular to the fiber axis with a step of 4 pm. 

Figure 7 shows that the micro-injection molded polyethylene parts exhibit typical skin-core morphology similar to that observed for conventional injection molding parts. While the interface between the skin layer and the transitional shear zone is apparent, the interface between the transitional shear zone and the spherulitic core is hard to locate. The skin layer probably has shish-kebab structural characteristics. The Kebabs , which are crystalline lamellae, fill the crystalhzed space. Fibrous crystals, or the Shishs , are ahgned parallel to the injection direction. They penetrate those lamellae. The fibrillar structure follows the direction of the flow, as shown in Figure 7. The transitional shear zone may be thought of as crystalline ribbons that branch and fill crystallized space with some loss of orientation. Crystallization occurring at the sites of both the skin layer and the transitional shear zone is significantly influenced by shear or elongational stress history. On the other hand, the influence of shear on the crystalhzation occurring in the spherulitic core is negligible. The crystalline structure...

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