VCL#53 b-grain Glassification v. Crystallization

"Rheo-Fluidification processing" is a new processing method which has been shown to create new entanglement states for polymeric melts, modifying the fluidity of the melt, its elasticity and its stability [1,2,3]. The classical concept of entanglement in polymer melt is seriously challenged when melt deformation takes place in the non-linear visco-elastic range, which is the working range for Rheo-Fluidification processing. In particular, we have discovered that “sustained-orientation” can occur, for which the melt retains its deformed state at temperatures well above its Tg, in apparent contradiction with the concept of reptation time which no longer represents the longest relaxation time. The stability of sustained-deformed melts can extend to 100,000 times the reptation time, thus questioning the current understanding of entanglements for such systems. It is anticipated that crystallization occurring under processing conditions which force the melt to be out of equilibrium, such as in Rheo-fluidification, may bring useful information not only with respect to the state of non-equilibrium of the melt (re its entanglements), but also with respect to the mechanisms of crystallization themselves, which are defined by nucleation and crystal growth, both strongly dependent on viscosity and local diffusion.

Polyethylene Terephtalate (PET) is chosen as a potential good candidate for modeling the complexity of the situation because it crystallizes from the melt when cooled slowly, yet remains amorphous when quenched. Additionally, its viscosity and crystallization is very sensitive to thermal history, especially under melt oscillation.

We analyze a series of Rheo-Fluidified PET samples by DSC, TMA, MFI (melt Flow Index) and dynamic rheometry. We show that for these samples, obtained from non-equilibrium melts, many discrepancies appear when characterizing and following the total crystallization in the sample, or comparing the heat capacity in the glassy and in the melt state with their theoretical values. The samples show a lack of free volume in the liquid and solid states which cannot be attributed to the crystalline phase.

This suggests that the entanglement state, varied by Rheo-Fluidification, correlates with the crystallization behavior, and, we postulate, with liquid-glassification (“Grain Glassification”), which occurs simultaneously and competes with crystallization. All measured parameters return to normal when we extensively anneal the samples, presumably resulting in the return to a stable network of entanglement.

We suggest a new understanding of the interactions between conformers in polymers and propose to use our model of Dual-Phase interactive coupling [4-6], which is our model to understand the rheology of entanglement networks in polymer melts, to make light on the various discrepancies observed experimentally for the heat capacity and the amount of crystallinity.

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