Before casting a film from polymer solution, the chemist has to be sure that all the polymer has dissolved in the solvent so 10 minutes are not enough. Complete polymer dissolution requires hours "until no traces of a solid are visible" & one has to be careful in trying to get a dilute solution since this will end up in forming random coil conformations which will not help in good crystallization of the ordered domain.

I think you need to approach this film casting with little patience & to accept that producing a "good" film may require several attempts.

Film cracking induced by drying can perhaps result from bubbles, among other possible reasons, such as increased polymer crystallinity induced by heat or mismatch between the expansion coefficients of the film and substrate.

As the film dries, its actual temperature is possibly somewhat lower than that selected for the evaporation chamber, from where it is often measured, because the film is cooled by the evaporating solvent. The initial drying temperature for the film made of the more concentrated polymer solution is possibly slightly higher than for the less concentrated solution. The actual (vacuum) pressure above the film also tends to be demoted by the vapour pressure of the solvent above the solution, and as the non-volatile polymer solute concentration increases, the vapour pressure decreases (cf. Raoult's law) and the vacuum level should less demoted close to the evaporating film surface. Hence, the more concentrated film may be initially drying at a slightly higher temperature and at a somewhat higher vacuum level, compared with the film of lower concentrated polymer solution. Eventually, bubbles may be more prone to nucleate and grow inside the former mentioned film. Such bubbles (if indeed formed) may possibly induce cracking at a later stage of drying, when the vapour pressure of the solvent nearly ceases to demote the vacuum level above the film while the film temperature also tends to increase, hence promoting bubbles expansion.

Try to use different substrate materials like PTFE.

as suggested, try maybe evaporation at lower temperature, for longer time, say 60 C for 4-5 hours, then compare the results ...

Also, what's the rationale of cooling the substrate to 0C while the solution is at over 100 C ?

Ion Laurentiu Tobos cooling is done to quench the solvent evaporation. I have noticed that the film porosity is much higher (macropores) if I skip the cooling step.I tried PTFE substrate but not much difference was observed. I think my main problem is balancing concentration, solution volume and viscosity.

Carlos Araújo Queiroz bubbling is not an issue I don't think. p-xylene does not boil under my oven's conditions. I purposely created bubbles by using 100 degrees under vacuum and I can clearly see bubble formation in the fabricated film, something I havent seen in the 80 degree condition. I tried heating the substrate to the solution's temperature to minimize the thermal expansion effect and it gave somewhat better results. I am not very confident of my understanding of LDPE crystallinity, so if you can explain that I would appreciate it.

(LDPE) generally has crystallinity in the range 35 - 55% which means that 45 to 65% of the regions, in a given LDPE sample, are amorphous or disordered . The reason for the low crystallinity is that LDPE is branched so there is an increase in volume (& decrease in density).

Many polymers of low crystallinity have shown what you observed (i.e. when you try to cast a film from a polymeric solution, the film crumbles and does not come out as one continuous piece). Therefore, I do not think that you have a problem. You only observed what is expected.

Thank you Nizar Matar . Do you think I can lower the effects of crystallinity if my casting solution is above the crystallization temperature (~128 degrees)? Also, do you think stirring the solution for longer than 10 minutes will help? I am using a very low LDPE concentration which makes me think that it can get well mixed in a short amount of time.

Before casting a film from polymer solution, the chemist has to be sure that all the polymer has dissolved in the solvent so 10 minutes are not enough. Complete polymer dissolution requires hours "until no traces of a solid are visible" & one has to be careful in trying to get a dilute solution since this will end up in forming random coil conformations which will not help in good crystallization of the ordered domain.

I think you need to approach this film casting with little patience & to accept that producing a "good" film may require several attempts.

Ok, so as I expected it was a mismatch between the solution volume and the petri dish inner diameter. I actually managed to make a decent film with 8 wt.% solution (~90um thick). However, the cracks are still seen, albeit on the micro-scale (1-10um) as well as macrovoids. Nizar Matar since my starting material is LDPE film, it immediately disappears into the solution upon contact with 125 degree C p-xylene, even after 1 minute of stirring the solution is clear; I tried stirring for hours but did not see a difference in casted film.

Dear Abd Rayes,

try to repeat[1] your process for a very thin, "critical" thickness, film (1-2 wt.% LDPE casting solution). The underneath (sacrificial, multi-)layers[2] will offer better ("critical" stress barrier, rearranging strain to an horizontal strain distribution, as stress barrier - relievers) protection for the vertical strain distribution, the cracks, beyond elastic limits, recognized in the thick(er) film cases.

Alternatively, you might be interested to replace your LDPE with XLPE (Crosslinked Polyethylene). XLPE has covalent bonds between its polymer chains. These crosslinked bonds tie the polymers together, lengthening the polymer chains, providing superior stress cracking[3] (and chemical) resistance.

1. Repeat n-times (n=2,3,4,...)

2. EVALUATION OF 9-LAYER FILM BLOWING PROCESS BY USING VARIATIONAL PRINCIPLES https://www.4spe.org/files/Chapters/D22/TechnicalArticles/2014-02-POTM.pdf

3. What are the differences between HDPE, LDPE, XLPE, LLDPE, and UHMWPE? http://www.usplastic.com/knowledgebase/article.aspx?contentkey=508

Ion Laurentiu Tobos I can get a decent film with 1-2 wt.% but unfortunately that is too thin for my application.

I tried a 4 wt.% solution, but coated the glass petri dish with silicon grease, and found that it formed a very nice film, where without the grease the film would be cracked at that concentration. Therefore I can safely say that my problem is the thermal expansion mismatch as well as the strong adhesion between LDPE and glass. I know I can use something like a silicon wafer but then I would have to use a casting knife. Do you have any recommendations as to what vessel I can use for casting other than the petri dish?

Some of the available PTFE[1] (Teflon) substrates might be good (to be tested) for your task.

1. Laboratory PTFE Petri Dishes https://www.alibaba.com/product-detail/AKMLAB-Manufacturer-Laboratory-PTFE-Teflon-Petri_60730155190.html?spm=a2700.7724857.normalList.2.199517f2TIh0b9&s=p

2. Smooth Surface Molding ptfe teflon sheet Virgin ptfe Board https://www.alibaba.com/product-detail/Smooth-Surface-Molding-ptfe-teflon-sheet_60454567963.html?spm=a2700.7724857.normalList.14.4fa357d6o13EZ7&s=p

I do not know but if thermal expansion mismatch is the problem you may want to try looking into different types of glass. I would think Pyrex is used largely in cooking wares for that reason, that it can withstand large temperature fluctuations, from which I surmise it may have a low thermal expansion coefficient. Also (not sure though, so do not quote me on that), if I remember correctly, heavy glass like flints (glass with certain lead compounds added) may have a lower expansion coeff. But that is from my old optics work days, a long long time ago.