吉林大学《材料科学与工程基础》chapter4.pptVIP

number-average degree of polymerization (nn) weight-average degree of polymerization (nw) Degree of polymerization (n:聚合度) the average number of mer units per polymer chain molecule. the mer molecular weight. For a copolymer: fj : the chain fraction of mer j mj: molecular weight of mer j Example problem 4.1 assume that the molecular weight distributions shown in Fig. 4.3 are for polyvinyl chloride. For this material, compute (a) the number-average molecular weight; (b) the number-average degree of polymerization; (c) the weight-average molecular weight. Solution (a) Table 4.4a: Data used for number-average molecular weight computations, as taken from Fig. 4.3a the atomic weights of C, H, and Cl are, respectively, 12.01, 1.01, and 35.45 g/mol. = 2(12.01 g/mol) + 3(1.01 g/mol) + 35.45 g/mol = 62.50 g/mol (b) for PVC: (c) Table 4.4b: Data used for weight-average molecular weight computations, as taken from Fig. 4.3b the magnitude molecular weight affect Various polymer characteristics molecular weight ? ( for M up to about 100,000 g/mol) melting temperature polymers with very short chains (molecular weight on the order of 100 g/mol) at room temperature as liquids or gases. exist with molecular weights of approximately 1000 g/mol waxy solids (柔软固体) and soft resins (软树脂). High polymers (molecular weights between 10,000 and several million g/mol) Solid polymers Fig. 4.5 Schematic representations of how polymer chain shape is influenced by the positioning of backbone carbon atoms (solid circles). For (a), the rightmost atom may lie anywhere on the dashed circle and still subtend a 109o angle with the bond between the other two atoms. Straight and twisted chain segments (链段) are generated when the backbone atoms are situated as in (b) and (c), respectively. 4.6 Molecular shape 109? (a) (b) (c) Single chain bonds are capable of rotation A single chain molecule might assume a shape having a multitude of