tailieunhanh - Mechanical properties of polymers and composites-Nielsen Episode 4

Tham khảo tài liệu 'mechanical properties of polymers and composites-nielsen episode 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Mechanical Tests and Polymer Transitions 23 of rubbers 116 . These Tg values were determined by varying experimental methods so hey are not always comparable. In any event the. effect is small. IV. CRYSTALLINITY Many polymers are not completely amorphous but are more or less crystalline. The degree of Crystallinity and the morphology of the crystalline material have profound effects on the mechanical behavior of polymers and since these factors can be varied over a wide range the mechanical properties of crystalline polymers take on a bewildering array of possibilities. The nature of the mechanical property changes is discussed in subsequent chapters. The degree of crystallinity is generally measured by x-ray diffraction techniques- or by measuring density 117 120 121 but some mechanical tests are- the most sensitive indicators of Crystallinity 4 . Morphological structure. including length of chains between folds in crystals and spherulitic structure. may be studied by light scattering small-angle way scattering . and electron microscopy 125 . Highly crystalline polymers such as polypropylene have a complex morphological structure. The polymer chains .generally appear to fold into laminar structures on the order of 100 A thick 125- 129 . with most chains turning and reentering the lamina from which they emerged. Figure 1. These lamellae stack together in layers to form ribbon-like structure . Between the layers are amorphous-like chain folds and some chains that go from one layer to the ne t to tie the entire structure together. Between he ribbons is more amorphous material. The lamellae often are part of a more complex spherulitic structure in which twisted lamellae ribbons radiate from a nuclcalion center 125 1 . Slow growth of the crystallites and annealing emphasize spherulitic structure whereas quenching minimizes it. Figures 8 and 9 illustrate .schematically some of the possible chain arrangements in crystalline polymers