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| Sketch of the Dual-Split Statistics for an entangled polymer that can crystallize |
"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with the concept of reptation time which no longer represents the longest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 (see the Figure at the top) 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.
3 lectures are available in the Video Clip Format (VCL # 53) to explain (5 hours total):
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Introduction to Rheo-Fluidification processing. Introduction to Entanglement Instability
and to "Sustained-Orientation". The Crossed-dual-Phase model of Entanglements.
Analysis of DSC Results.
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| Analysis of TMA results. |
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Dynamic Temperature Sweeps at constant Frequency. Grain-Field Statistics for
un-entangled, entangled, amorphous and semi-crystalline polymers
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You can also listen to a video introduction to VCL #53 by clicking the following link:
REFERENCES:
1.
J.P.Ibar,
“Processing polymer melts under
Rheo-Fluidification flow conditions: Part 1. Boosting shear-thinning by adding
low frequency non-linear vibration to induce strain softening.”. J.
Macromol. Sci. Part B, Phys,, 52:411-445, 2013 (publication on line November 1st 2012. DOI: 10.1080/00222348.2012.711999).
2.
J.P. Ibar,
“Processing polymer melts under
Rheo-Fluidification flow conditions: Part 2. Simple flow Simulation”. J.
Macromol. Sci. Part B, Phys., 52:446-465, 2013 (publication on line : November
1st 2012) DOI: 10.1080/00222348.2012.712004).
3.
J.P. Ibar, “Mixing
Polymers under Rheo-Fluidification Conditions”, Macromolecular Symposia,
Special Issue, 11th International European Symposium on Polymer
Blends, 2012. Volume 321-322, Issue 1, p. 30-39.
4. J.P Ibar, “ The
Great Myths of Polymer Rheology. Part I.: Comparison of Experiment and Current Theory', Journal of
Macromolecular Science, Part B, 48: 6, 1143 — 1189 (2009).
5. J.P. Ibar “ The
Great Myths of Polymer Rheology” Part II. Transient and Steady State. The
question of the entanglement stability. Journal of Macromolecular Science,
Part B, 49, 1148 -1258 (2010).
6. J.P. Ibar, “The
Great Myths in Polymer Rheology, Part III: Elasticity of the Network of
Entanglements”, J. Macrom. Sci. Part B, Phys. 52:222-308, 2013.
