Microstructure of Semicrystalline Polymers

Tlie term crystallite is used in polymer science to imply a component of an interconnected microcrystalline structure. Metals also belong to the class of microcrystalline solids, since they consist of tiny ordered grains connected by strong boundaries. 

Crystallization begins from a nucleus that may derive from surfaces of adventitious impurities (heterogeneous nudeation) or from the aggregation of polymer segments at temperatures below (homogeneous nudeation). The latter process is reversible up to the point where a critical size is reached, beyond which further growth results in a net decrease of free energy of the system. Another 

Once nucleated, crystallization proceeds with the growlh of folded chain ribbon-like crystallites called lamellae. The arrangement of polymer chains in 

There is uncertainty about the regularity and tightness of the folds in solution-grown single crystals. Three models of chain conformations in a single crystal are illustrated in Fig. 11-4. 

The morphology of a crystallizable polymer is a description of the forms that result from crystallization and the aggregation of crystallites. The various morphological features which occur in bulk crystallized polymers are reviewed in this section. 

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As PET has the ability to be obtained either in the amorphous state or not an additional scientific goal exists that is studying the interrelations between the microstructure of semicrystalline polymers and the efficiency of rubber toughening [ 1 ]. 

In spite of much research, some details of the microstructure of semicrystalline polymers are still unknown. Polymer development has proceeded empirically, with microstructural knowledge being acquired later, and then used to explain mechanical and physical properties. The order of presentation is that of increasing size scale Bonding in the crystal unit cell, the shape of lamellar crystals, the microstructure of spherulites, the overall crystallinity and the processes of crystallisation. Details of polymer crystal structures and microstructures can be found in literatures listed in Further Reading . 

Improved dispersion of nanoscaled fillers in polymers is expected to lead to better mechanical properties. The question is if by using a nano-modified matrix instead of a laminate with neat polymer as the matrix, in a fiber-reinforced composite can lead to improve delamination resistance. In the case of semicrystalline polymer matrices, the scenario is even more complicated as the microstructure is not only influenced by the processing but also by the presence of nanofillers. The addition of different types of nanoscaled reinforcements such as carbon nanotubes, nanofibers or... 

The microstructure of semicrystalline multicomponent polymers can often be determined by... 

Schrauwen BAG, Janssen RPM, Govaert LE, Meijer HEH (2004) Intrinsic deformation behavior of semicrystalline polymers. Macromolecules 37 6069-6078 Schultz JM (1974) Polymer materials science. Prentice Hall, Englewood Cliffs, NJ Schultz JM (1984) Microstructural aspects of failure in semicrystalline polymers. Polym Eng Sci 24 770-785... 

Applications and examples of microscopy imaging and analysis to multicomponent polymers follow. The microstructure of semicrystalline multicomponent polymers can often be determined by polarized light microscopy of thin sections. A blend of two polyacetals, a homopolymer and a copolymer, is shown in the micrograph of a thin section (Fig. 5.68). The structure is rather interesting... 

The volume inside the semicrystalline polymers can be divided between the crystallized and amorphous parts of the polymer. The crystalline part usually forms a complicated network in the matrix of the amorphous polymer. A visualization of a single-polymer crystallite done [111] by the Atomic Force Microscopy (AFM) is shown in Fig. 9. The most common morphology observable in the semicrystalline polymer is that of a spherulitic microstructure [112], where the crystalline lamellae grows more or less radially from the central nucleus in all directions. The different crystal lamellae can nucleate separately... 

The time needed for polymer chains of amorphous thermoplastics above Tg and semicrystalline thermoplastics above Tm to diffuse across the interface and randomize is relatively short compared with the time needed for resin flow. It is believed, therefore, that diffusion bonding is completed immediately after the two molten surfaces merge, and that the microstructure of the contact zone is also assumed to be identical to that of the intraply sections [12,13],... 

Through the use of multiple experimental techniques, we have shown how both the NXL and XL phases of PILE interact and respond to applied tensile deformation. Strains transmitted to PILE crystals lead to two distinct slip modes and, at higher strains, to the breakup and alignment of lamellar fragments. In our experiments, crystallites in PTFE orient fuUy with respect to the draw direction at strains between 70 to 200%. With increasing strain, some chains originally in the XL phase are transformed to NXL material. Noncrystalline chains continue to orient until macroscopic failure is reached. This could be a fairly general microstructural response for semicrystalline polymers. 

Crystalline lamellae are the basic units in the microstructures of solid semicrystalline polymers. The lamellae are observed to be organized into two types of larger structural features depending on the conditions of the bulk solidification process. 

The interest in multicomponent materials, in the past, has led to many attempts to relate their mechanical behaviour to that of the constituent phases (Hull, 1981). Several theoretical developments have concentrated on the study of the elastic moduli of two-component systems (Arridge, 1975 Peterlin, 1973). Specifically, the application of composite theories to relationships between elastic modulus and microstructure applies for semicrystalline polymers exhibiting distinct crystalline and amorphous phases (Andrews, 1974). Furthermore, as discussed in Chapter 4, the elastic modulus has been shown to be correlated to microhardness for lamellar PE. In addition, H has been shown to be a property that describes a semicrystalline polymer as a composite material consisting of stiff (crystals) and soft, compliant elements. Application of this concept to lamellar PE involves, however, certain difficulties. This material has a microstructure that requires specific methods of analysis involving the calculation of the volume fraction of crystallized material, crystal shape and dimensions, etc. (Balta Calleja et al, 1981). 

Fig. 3.4 Possible effects of processing (injection molding) on the microstructure of a semicrystalline polymer. In contact with the mold wall (which is assumed to be perpendicular to the plane of the scheme) a surface skin morphology is formed. An isotropic microstructure can be observed in the center core interior. Adopted with permission from [15]...

Poly(ethylene terephthalate) Poly(ethylene terephthalate) is a widely used semicrystalline polymer. The macroscopic properties of PET such as thermal, mechanical, optical, and permeation properties depend on its specific internal morphologies and microstructure arrangement. It can be quenched into the completely amorphous state, whereas thermal and thermomechanical treatments lead to partially crystallized samples with easily controlled degrees of crystallinity. The crystallization behavior of thermoplastic polymers is strongly affected by processing conditions [91-93]. 

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