[aisha]

For DSC experiements, it says that for polymers that can form crystals completely a crystallisation and melting temperature is observed, and that an amorphous polymer will not show any crystallisation or any melting either. My question is that I have a glass transition temperature on my DSC graph for PVC but no melting or crystallisation, so I am thinking that PVC is amorphous. Does this mean that PVC will not show a melting on DSC experiments or it will not ahve a melting point altogether? (but it does)

Can someone please clarify??

Thanksssss

[Alpha-Omega]

Polyethylene is a crystalline plastic.  PVC is an amorphous plastic.  Amorphous means nebulous, fluid, unstructured… that is, it does NOT have a crystalline molecular structure like polyethylene.

DSC:  Differential Scanning Calorimetry


Differential scanning calorimetry measures the heat flow between a sample and its surroundings when the sample is subjected to a temperature program (heating or cooling), i.e.the heat absorbed (endotherm) or liberated (exotherm) by the sample.

The physical and chemical processes that occur on heating are coupled with a loss or gain in energy or with a
change of the specific heat capacity of the sample. Such processes are therefore shown as peaks or steps in the measurement curve. The integration of a peak (peak area) yields the transition energy or the heat of reaction.

The glass transition with the relaxation peak. This depends on the thermal history of the sample. Plasticizer-free PVC typically has a glass transition temperature (Tg) of about 80°C.

I am attaching a word document that outlines an experiment done determining the glass transition for PVC.

[aisha]

Thanks a lot...very helpful.

[Polleke]

silly question: how can you have a crystalization peak when working with amorphous substances?
(as seen in the graph attached at your answer)

And then I am speaking about a 100% amorphous solution.

Normally an amorphous substance does not show a crystalization peak, right?


so the peak can only be seen when working with a substance thats part amorphous and part crystaline.

Or am I missing something here?

[Alpha-Omega]

silly question: how can you have a crystalization peak when working with amorphous substances?
(as seen in the graph attached at your answer)
And then I am speaking about a 100% amorphous solution.
Normally an amorphous substance does not show a crystalization peak, right?
so the peak can only be seen when working with a substance thats part amorphous and part crystaline.
Or am I missing something here?

Yes...you are there is a glass transition point determined for PVC using DSC..

The question was good.  Terminology can be confusing.

It was NOT a SILLY QUESTION...no question is SILLY....especially if you do not understand....DSC is often usde to determine the degree of gelation or PVC.

A good place to learn about things like this is a text on the behavior of non-metallic materials. Try doing an internet search on materials that are crystalline and amorphous.

[Polleke]

I allready did some research about cyrstaline and amorphous substances as this is part of my "big" research.

I just got a little bit confused thats all.

anyway, amorphous= no crystalization (fluid, rubbery state, glassy state)
crystaline = crystalization (fluid, solid cristal)

THis is hower pure theory, most substances are not 100% pure, so you get a sort of mix.

And then you have susbstances that can be both , depending on the rate of cooling you use

its really confusing and hard.
And then try to write a theoretical paper about it , there are more exceptions then those that follow the rules looooool

[Alpha-Omega]

Take a look at this post in the Materials Board:

http://www.chemicalforums.com/index.php?topic=22118.0

The question was about XRD and amorphous materials.  The way te question was answered was GOOD!!!!  Accurate and Correct!!!!!

Most people assume that all "amorphous" materials will give no XRD pattern.  That is not true.  XRD is often used to determine the "degree of crystallinity."

Then check this link on the PANalytical site that gives you an idea of the power XRD has with regard to monitoring crytallinity changes in "amorphous" materials. 

And, YES, the rate of crystallinity in a specific material can be induced by adjusting the rate of cooling....

For a specfic sample: By determining the amount of crystallinity you can determine the amount of amorphous material present in a sample.

LINK:http://www.panalytical.com/index.cfm?pid=776&words=amorphous

[Polleke]

I see.

Anyway, thats just stating what I allready knew are had written down.

Its not always either this on or the other one..

Anyway, it makes it all harder for me
too bad most articles I used for my research are a little bit blind for this difficulties. They are to theoretical and always refer to either amorph or cristalyne, while there isnt an abrupt line between those two.

[aisha]

I'm trying to work out the degree of crystallinity for the polymer PET. I have been given the following equation to solve this
C=deltaHf/deltaHc X100

where deltaHf is the measured heat of fusion of the sample deltaHc is the heat of fusion of the pure crystallite (literature value) can anyone tell me where I can obtain the literature value??

Furthermore I have a melting temperature of 260 degrees celsius for PET but have a
-47.66J/g for the heat of fusion...is this negative number correct because I have seen literature values for 66J/g

Please help