Chain Entanglemen链缠结

1、1chm 585 / 490chapter 7properties2chapter 7 molecular weight tg melting point3chain entanglement4the 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 p

2、olymerization 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. c chhhh()n5averaging because polymeric materials do not consist of strictly identical molecules, but instead are a mixture of many spec

3、ies having different values of dp, the dp of the material only is an average value, . hence, measured polymer molecular weights are only average values.6weight average / number average the number average is the simple arithmetic mean, representing the total weight of the molecules present divided by

4、 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. 7the ratio of the weight-average and number-average molecular weights, , is a measure of the polydispersity of a polymer mixture - how wide

5、ly distributed the range of molecular weights are in the mixture. a ratio that is around 1.0 indicates that the range of molecular weights in the mixture is narrow; a high ratio indicates that the range is wide. 8startdestinationmiles1: atlanta, ga2: columbia, sc2102: columbia, sc3. charlotte, nc903

6、: charlotte, nc4. new york, ny6204. new york, ny5. los angeles, ca2790suppose that you want to travel from atlanta to los angeles, visiting your friends along the way. unfortunately your friends live only on the eastern and western coasts, leading to a somewhat unconventional journey.9startdestinati

7、onmiles1: atlanta, ga2: columbia, sc2102: columbia, sc3. charlotte, nc903: charlotte, nc4. new york, ny6204. new york, ny5. los angeles, ca2790the number-average of the miles traveled in the four legs of the journey is 928 miles. this is the average distance traveled in each leg of the journey. it p

8、laces equal emphasis on each leg.the mass-average of the miles traveled in the four legs of the journey is 2216 miles. this average places a greater emphasis on the leg of the journey with the largest mass - that is the fourth leg in which you travel 2790 miles. it is more representative of the majo

9、r part of your journey.101112if you look at the numbers you can see that the average person doesnt live in a town of a population of 180,000. take a look there. most of the people in the combined populations of the four towns live in memphis, a town with a lot more than 180,000 people. so how do we

10、calculate the size of town that the average person lives in, if the simple average doesnt work? what we need is a weighted average.13take memphis. it has a population of 700,000. the total population of our four cities is 723,500. so the fraction of people who live in memphis is. 14152 chainsone at

11、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 = 1.71617181920glass transition tgthe polymer samples may be thought of as a crowd of people on a d

12、ance floor. while each whole body tends to stay in the same spot, various arms, legs, and whatnot are changing position a lot. when the temperature drops below the tg, for polymers the partys over, and the long-range segmental motion grinds to a halt. when this long-range motion ceases, the glass tr

13、ansition occurs, and the polymer changes from being soft and pliable to being hard and brittle. 21how easily the chains move.a polymer chain that can move around fairly easily will have a very low tg, while one that doesnt move so well will have a high one. this makes sense. the more easily a polyme

14、r can move, the less heat it takes for the chains to commence wiggling and break out of the rigid glassy state and into the soft rubbery state. 22for polysulfone tg 500c; flexible ether linkages increase mobility and bring the tg to 190c232425plasticizers sometimes, a polymer has a tg that is higher

15、 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. 26dopoooo27tg

16、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 arranged in ordered crystals, but are just strewn around in any old fashion, even thoug