Dear Shuaishuai,

Please let me ask first some questions.

1. Did you make any DSC test of the non-crosslinked sample as well?

2. How did you perform the DSC tests? In semicrystalline polymers one usually makes two subsequent runs. First heating above the highest melting temperature, followed by controlled cooling (to detect crystallization), and finally a second run. In the case of complicated melting (as is the case here) usually melting runs after various annealing times and temperatures are performed to assess the effect of crystallization conditions.

3. Do you know anything about the composition of the non-crosslinked resin? Is it perhaps a mixture of components of different moecular weights or densities? It would explain the multiple melting points. From the melting points it seems that there are at least three components, an ULDPE, and LDPE and an MDPE.

4. The reduction of OIT and of the decomposition temperatures the partial oxidation ad breakdown of the sturcture is evident. (If you have FTIR data, you would see the carbonyl index).

5. The irradiation enhances the crystallization of the lowest meting point component (fragmentation?) The effect is particlularly strong if the irradition is done at 90 oC, i.e. below the lowest melting point. At 115 the medium densitiy melting practically disappears. It would be useful to plot the partial crystalinities belonging to the various melting peaks. It would be also advisable to take WAXS curves to see the chnages in crystallinities. Perhaps optical micrographs with polarization and hot stage might help to folllow the spherulite distribution differences.

Thank you much, Gyorgy.

1. I do not have the exact the same sample but is non-crosslinked. The XLPE material I am studying here is purchased from RSCC company, it is used as cable insulation in nuclear power plants. But I do have done some DSC on pristine LDPE, and the LDPE after being gamma radiated for 20 days. These DSC are in the 1st page of the attachment here.

2. The the 2nd and 3rd pages of the attachment, multiple runs on the pristine XLPE is displayed, please have a look.

3. Yes, I do have some composition study results on the XLPE material. They are included in page 4 of the attachment. It is not pure polymer, but polymer-matrix composite with some inorganic crystals as fillers. However, I do not have the information about how many polymer components are there. Seeing from the TGA results, there should be at least two components, one decomposes between ~ 300-350 oC, the other decompose between about 350 -450 oC. From your suggestion they might be PE at different densities. My question is: Can PE at different densities have about 100 oC difference in their decomposition temperature? 

4. I do have FTIR data. They are in Page 5 of the attachment. I do not get much information from FTIR data, please have a look and I would appreciate your idea on them.

Thank you again.

Please see this attachment instead of the one uploaded above. A little more info is added to this attachment.

I am a little bit confused by the design of these experiments. Let us start with the beginning:

1. What experimental techniques are available to you?

2. What are the changes noticed immediately after the combined temperature and irradiation stops-if any?

3. Was the thermal and radiative processes occurring in air or in vacuum? Were the samples ever been exposed to air?

4. What are the dimensions of the samples (mainly if exposure to air is also a competing factor)?

Technically I do not believe the two cycle tests and I suggest the use of the three cycle test. In the first the samples is heated above the melting point to relax all stresses. In the second cycle (heating/cooling) the polymer behavior is recorded and in the third cycle the confirmation is provided by concluding that second and third cycle are identical.

How much time passed after irradiation stops? Some degradation processes may continue for a long time, mainly in crosslinked system.

Additional spectroscopic techniques (FTIR, RAMAN, RES, WAXS) can provide relevant information.

Thank you, Mircea.

1. I have conducted TGA, DSC, OIT, FTIR, dielectric constant&loss tangent, tests on the samples;

2. Yes, the color gets yellow from white after combined temperature and irradiation aging;

3. My samples are aged in air - in an circulating air, mechanical convection oven.

4. The sample is of straw geometry, with its outer diameter of about 2.8 mm and wall thickness of 0.7 mm, roughly.

The three-cycles DSC test results are displayed in the attachment in my previous answer, please have a look. 

It has been about a few months after irradiation stops.

Thank you.

I guess that samples are now at equilibrium and we can assume that all physical and chemical processes that are still occurring are very weak and slow.

Please contact your supervisor and ask if she is willing to collaborate with us. We are eager to help you but this may be a more complex project.

Please send us some samples:irradiated, not irradiated...probably position is also important as may be connected to the oxygen access. 

We are intending to do a chemical mapping across the thickness of your sample by Raman spectroscopy. Our resolution is theoretically 1 micron. I want to check and to see how inhomogeneous is the chemical degradation (oxidation). Unfortunately a map at a single temperature, a single sample can take 6-10 h

After that, if the vibrations connected to crystalline domains can be isolated, we may do Raman at various temperatures and to measure the distribution of the melting temperatures across the thickness of the sample. This may be a very complex and time consuming study. Nothing like that was ever done!

Oxidative radiation aging of polyethylene has been done back to the 1950s and 1960s. (Charlesby). My first boss, Dr. Peter Hedvig togeher with D. Janos Dobo also made several tsts at the former Research Institute for Plastics in Budapest, partly published e.g. in a series of conrefrence proceedings, the Tihany Symposia on radiation Chemistry an Physics, back to the 1970s and 1980s. It is well known that somewhat different processes undergo in vacuo and in air. (The differences could also be detected by ESR). In addition to the IR or Raman miscroscopy suggested by Mircea several others, such as thermally stimulated currrents and thermally stimulated lumiescence are also available.

Hereby I also attach a relatively new review on the radiation effects on PE and cellulose

https://helda.helsinki.fi/bitstream/handle/10138/42163/lowenerg.pdf?sequence=1

Here is a short review on crosslinked PEs

http://nopr.niscair.res.in/bitstream/123456789/9558/1/IJCT%2011(6)%20853-864.pdf

This is a presentation about condition monitoring of nuclear power plant cables:

http://www.nist.gov/el/building_materials/upload/Watson_SLPMarchMeeting_2.pdf

Here is an older PhD on fluorescence detection of defects in cables:

https://ttu-ir.tdl.org/ttu-ir/bitstream/handle/2346/18201/31295007118036.pdf?sequence=1

Here is a compilation of DSC and FTIR data:

https://cds.cern.ch/record/1248718/files/COMPILATION_DSC_FTIR_CERN_IRR_CABLES_SI.pdf

The paper below contains oxidative index mapping in cables:

http://www.jsme.or.jp/monograph/pes/1999/ICONE7/PAPERS/TRACK04/FP7166.PDF

Here is another paper on the IR analysis of oxidatively aged XPE

https://www.researchgate.net/publication/282657951_Oxidation_Evaluation_of_Cross-linked_Polyethylene_XLPE_under_Thermal_Degradation_FTIR_Study

I also attach some papers I found on the TGA of XPE

http://sklfs.ustc.edu.cn/yjdw/js/201108/W020121202624029385031.pdf

http://www.sciencedirect.com/science/article/pii/S1877705813003093

http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1516-14392015000200313

The following paper is an interesting combination of volatile analysis and other techniques in XPE (including IR mapping):

http://www.jicable.org/2007/Actes/Session_A10/JIC07_A101.pdf

You are right, most "pure" XPE-s produce a single TGA step. The presence of multiple steps in your systems may indicate blending of the modifying effect of fillers.

I will study your attached files and will come back with comments.

Conference Paper Oxidation Evaluation of Cross-linked Polyethylene (XLPE) und...

Some further notes on the more detailed data you attached yesterday. Obviously your cable is flame retarded (combination of Sb2O3 with bromine compounds). ZnO is presumably used as a white fine filler - the reason is not fully understood. From the IR it is clear, that the material is a PE although there is something broad carbonyl-tpye absorption already in the pristine sample. Interestingly the enhancement of the carbonyl signal is very sight after irradiation in air. Apparently the oxidation process was diffusion limited. The complex melting behavior indicates that probably the PE is a mixture of more components of various densities.

Indeed, there is a high probability that the degradation was mainly oxidative and diffusion controlled. The Raman chemical mapping may answer to this question.

Even if the initial compound was more or less homogeneous the diffusion controlled degradation may result in such a multicomponent behavior. The fact that the experiment was done on a commercial formulation is another source of pain(s) as the behavior may be very complicated.

Thank you, Dr. Banhegyi and Dr. Chipara.

I am spending time on the literatures that you suggested above, Dr. Banhegyi.

Dr. Chipara,  I have conveyed the message to my supervisor Dr. Nicola Bowler about your help. I expect that she will contact you later, maybe after she gets back from her trip. 

Best regards to you.

After reading the discussion above, i think, it will be more interesting if you chose the best  heating and or cooling rate of the DSC. The slower heating or cooling rate, 2oC/mnt, you will get the better curve.

With such an extensive and valuable comments of Prof Banhegyi, I believe you've got the best. Good luck