"...Tg can be significantly decreased by addition of plasticizers into the polymer matrix. Smaller molecules of plasticizer embed themselves between the polymer chains, increasing the spacing and free volume, and allowing them to move past one another even at lower temperatures..." - https://en.wikipedia.org/wiki/Glass_transition

If it available, new synthesis method  or chemical modification could help. All methods decrease ordering of polymer chains - by introduction of rotating groups ( -O-CF2-O- , -O-SiMe2-O-, -O- for instance) ,  bulky low-polar substituents, non-regularity of monomers bonding order, etc

Dear Stéphane, if you are above Tg the molecules exhibit considerable mobility. If you have a significant difference between the Tg and Tm (the melting point) then you may obtain relatively stable "frozen-in" amorphous structure by quenching (if you do not go the "pre-melting" region). Typical example is PLA, but there both Tg and Tm are far above room temperature. One other solution may be to crosslink your quenched sample e.g. by irradiation to fix the amorphous structure.

Hello Stephane,

I agree with Gyorgy. Irradiation may be a good solution but only if polymer crosslinks (if it undergoes chain scissions, small segments with high mobility can crytallize maybe very easily).

Irradiation under vacuum are thus recommended

E.

Thank you Emmanuel and Gyorgy.

Hint to cross-link polymer by irradiation (+vacuum!) is controversial. It is well known that cross-link points are good nucleation sites and promote rapid crystallization.

Because I am not sure to exactly understand what you say. Sorry.

Dear Andrei, In polyethylene XPE is usually less crystalline than the linear counterpart. Crosslinks are defect sites in crystals. I did not investigate the nucleation effect in other polymers. It is also true that in some polymers (e.g. PP) irradiation causes degradation and lower MW fragments may crystallize faster.

Ok.

Dear Gyorgy, at first thank you for upvote of my first answer  - indeed, plasisizer is exact "additive that prevent crystallization in polymer". Unfortunately, Stéphane have not accept this simple reality.

I agree with you that cross-linking (radiation or chemical) could lead to decrease of crystalline portion ( in very certain conditions). But it can not prevent it en masse.