I am trying to make "films" of polyethylene glycol by melting them, pouring them into a mold and then cooling the mold down. The plan is to do this with different molecular weights. The problem is that depending on the molecular weight, the crystallinity will be different. Is there a way of ensuring that the crystallinity of the polymer is the same for all of them? Thanks


1 Answer 1


Is there a way of ensuring that the crystallinity of the polymer is the same for all of them?

First you need a method to assess the degree of crystallinity. Samples of the same material that differ in degree of crystallinity should show different light transmission intensities due mainly to increased light scattering in the more crystalline samples. If you have access to a spectrophotometer you could insert the films and measure the light transmission. If you don't have access to a spectrophotometer, a "poor-man's" approach would be to use visual comparison of the transmitted light (use a black piece of paper and a pencil beam flashlight in a dark room). It is important that the film thicknesses all be the same. If there is variability in your film thickness, then cast thicker films so that the percentage variation will be smaller.

By cooling your sample more quickly, you will tend to preserve more of the amorphous nature found in the solution. Conversely, cooling more slowly or annealing your sample will tend to increase the degree of crystallinity.

Adjust your rate of cooling on each of the films until they all produce the same transmission intensity, then the degree of crystallinity should be similar in your films.

  • $\begingroup$ So once the crystallinity is found for each sample (via your described method or any other) the only way would be to change the rate of cooling via trial and error? $\endgroup$
    – Daniel AG
    Nov 20, 2014 at 18:43
  • $\begingroup$ That's how I see it, unless you're running these experiments in a research environment where other high dollar value equipment (x-ray equipment, etc.) is available. It wouldn't surprise me if you see a trend between cooling rate, crystallinity and molecular weight, that might make it possible to estimate or project cooling rates for later samples. $\endgroup$
    – ron
    Nov 20, 2014 at 18:50
  • $\begingroup$ This way you might get the same degree of crystallinity with one method, but very likely the morphology will be different, so other methods give different values (there is no method that gives a undoubtable absolute value). The whole thing sounds rather pointless to me. $\endgroup$
    – Karl
    Oct 18, 2015 at 16:25

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