Chain Entangleent链缠结.pptVIP

CHM 585 / 490 Chapter 7 Properties Chapter 7 Molecular weight Tg Melting Point Chain Entanglement The degree of polymerization (DP) DP in a polymer molecule is the number, n, of repeating units in the polymer chain. ? The molecular weight of a particular polymer molecule is a product of the degree of polymerization and the molecular weight of the repeating unit. For instance a particular polyethylene molecule with DP = 1000 will have a molecular weight of 28,000. Averaging Because polymeric materials do not consist of strictly identical molecules, but instead are a mixture of many species having different values of DP, the DP of the material only is an average value, . Hence, measured polymer molecular weights are only average values. Weight Average / Number Average The number average is the simple arithmetic mean, representing the total weight of the molecules present divided by the total number of molecules The probability factor in a weight-average emphasizes the mass of the molecules so that the heavier molecules are more important. 2 ChainsOne at 10,000;One at 100,000 Mn = (100,000 + 10,000) /2 = 55,000 Mw 100,000 / 110,000 = .91 10,000 / 110,000 = .09 (100,000 x .91) + (10,000 x .09) = 91,900 Mw = 91,900 Polydispersity = Mw/Mn = 91,900/55,000 = 1.7 Glass Transition Tg How easily the chains move. Plasticizers Sometimes, a polymer has a Tg that is higher than wed like. Thats ok, we just put something in it called a plasticizer. This is a small molecule which will get in between the polymer chains, and space them out from each other. We call this increasing the free volume. When this happens they can slide past each other more easily. DOP Tg versus Melting Point. Melting is a transition which occurs in crystalline polymers. Melting happens when the polymer chains fall out of their crystal structures, and become a disordered liquid. The glass transition is a transition which happens to amorphous polymers; that is, polymers whose chains are not arra