tailieunhanh - Coatings of Polymers and Plastics Part 2

Tham khảo tài liệu 'coatings of polymers and plastics part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 14 Babinec and Cornell 2 CHEMICAL AND PHYSICAL PROPERTIES OF POLYMERS The behavior of a polymer is based on both its chemical composition and its physical state. The chemical characteristics of homopolymer and copolymers are fundamentally derived from the choice of monomers. Table 2 lists common engineering polymers. In addition to the chemical composition there are several distinct physical arrangements of polymer chains which contribute to performance as illustrated in Figure 16. In the melt state the arrangement uniquely affects the rheological behavior as a function of shear and temperature. In the solid state the arrangement affects the viscoelastic properties and the mechanical properties such as strength and ductility. In the transition from liquid to solid the crystallization kinetics are also a strong function of the precise arrangement. For copolymers the full chemical description also includes the sequencing of monomers with common patterns listed in Table 3. Block copolymers are interesting materials in many applications because the high level of control of their sequencing results in the formation of precise morphologies on a very small scale less than 100 nm . Figure 17 shows the morphology map for certain well-controlled diblock copolymer microstructures. Although the precise details of the diblock phase diagram depend on the composition and on both the absolute and relative lengths of the A and B blocks all diblocks will form these same phases. Selecting an appropriate material frequently begins with strength requirements. Figure 18 shows the stress strain response patterns that characterize different types of polymers. Every material follows one of these basic behaviors but with quantitatively unique values. The slope of a stress strain curve is the modulus. High-modulus materials typically require a large force for a small deformation but brittle fracture at low elongation. Low-modulus materials elastomers require comparatively little force for a