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Séminaires à venir

Flow-Induced Crystallization of Engineering Thermoplasticse

Ralph H. Colby, Materials Science and Engineering, Penn State University, University Park, PA 16802 USA

le jeudi 12 septembre à 15h30 à l’amphi Boreau

 

Brief intervals of shear flow at rates exceeding the reciprocal of the Rouse time of the longest chains create precursors that nucleate orders of magnitude more crystals and change the morphology from 30 μm spherulites to far smaller 1 μm crystallites. This flow-induced crystallization (FIC) at low shear rates builds with shearing time and eventually saturates. In contrast, at much higher stress levels that might occur in processing flows, a second morphology transition to shish-kebabs is observed when a critical shear stress ( 0.14 MPa for iPP) is exceeded. The shish-kebab transition is evident in subsequent oscillatory shear as a weak gel and as a sudden jump in the pressure needed to push the material through the die in capillary rheometry. Flow-induced crystallization is studied in detail for isotactic polypropylenes [1-3] and poly(ether ether ketone)s [4] representing flexible and semi-rigid polymers, and for Polyamide 6,6 representing a flexible polymer with strong hydrogen bonding, [5,6] to see which aspects of FIC are universal to all polymers and which aspects are polymerspecific. The fact that the precursors are quite stable allows the sheared samples to be removed from the rheometer and studied extensively with DSC and optical microscopy, while annealing at elevated temperatures allows the study of precursor stability.

Figure 1. Polyamide 66 shish morphology after shearing at 10 s-1 at 270 oC for 1 min.


[1] F. G. Hamad, R. H. Colby and S. T. Milner, Macromolecules 48, 3725 and 7286 (2015).
[2] F. G. Hamad, R. H. Colby and S. T. Milner, Macromolecules 49, 5561 (2016).
[3] B. Nazari, H. Tran, B. Beauregard, M. Flynn-Hepford, D. Harrell, S. T. Milner and R. H. Colby, Macromolecules 51, 4750 (2018).
[4] B. Nazari, A. M. Rhoades, R. P. Schaake and R. H. Colby, ACS Macro. Lett. 5, 849 (2016).
[5] A. M. Rhoades, A. M. Gohn, J. Seo, R. Androsch and R. H. Colby, Macromolecules 51, 2785(2018).
[6] J. Seo, H. Takahashi, B. Nazari, A. M. Rhoades, R. P. Schaake and R. H. Colby, Macromolecules 51, 4269 (2018).


Solution Rheology of Dry Native Cellulose in Ionic Liquids : Weakly Associating Polymers ?

Ralph H. Colby, Materials Science and Engineering, Penn State University, University Park, PA 16802 USA

le jeudi 19 septembre à 14h à l’amphi Holweck

 

We report solution rheology of six dry native cellulose samples of different molecular weight in three different ionic liquids ; 1-ethyl-3-methylimidazolium acetate [EMIm]Ac, 1-butyl-3-methyl imidazolium chloride [BMIm]Cl1 and 1-ethyl-3-methylimidazolium methylphosphonate [EMIm][P(OCH3)(H)O2].2 These solutions do not crystallize, making it possible to measure the glass transition using oscillatory shear rheology. Trace amounts of water impart a yield stress to these solutions so the experience 20 min at 80 C in the rheometer after loading to remove water.3 Concentration dependences of specific viscosity and relaxation time make each solution appear to be a ‘normal’ polymer solution, with dilute, semidilute unentangled and entangled regimes of concentration. However, the same cellulose sample in different solvents at the same concentration has a different width of the rubbery plateau and there is a strange failure of the Cox-Merz rule (shear viscosity larger than linear complex viscosity in the shear-thinning region) for cellulose solutions in ionic liquids. Both of those observations suggest that native cellulose is not a simple flexible polymer in solution and that it instead has some inter-chain hydrogen bonds between cellulose chains. Urea is known to compete for hydrogen bonds and adding urea to these solutions lowers the viscosity, most dramatically for the solutions with the widest rubbery plateau. An additional complication with these solutions is that cellulose seems to adsorb to the air interface to create a viscoelastic film and the consequences of that adsorption on literature for the intrinsic viscosity of cellulose in [EMIm]Ac will be discussed.


1. X. Chen, Y. Zhang, H. Wang, S.-W. Wang, S. Liang and R. H. Colby, Solution Rheology of Cellulose in 1-butyl-3-methyl Imidazolium Chloride, J. Rheol. 55, 485 (2011).
2. X. Chen, S. Liang, S.-W. Wang, and R. H. Colby, Linear Viscoelastic Response and Steady Shear Viscosity of Native Cellulose in 1-ethyl-3- methylimidazolium methylphosphonate, J. Rheol. 62, 81 (2018).
3. B. Nazari, N. W. Utomo, and R. H. Colby, The Effect of Water on Rheology of Native Cellulose/Ionic Liquids Solutions, Biomacromolecules 18, 2849 (2017).