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Effects of molecular weight distribution on the flow-enhanced crystallization of poly(1-butene)

Effects of molecular weight distribution on the flow-enhanced crystallization of poly(1-butene). Stefano Acierno 1 , Salvatore Coppola 2 , Nino Grizzuti 3 1 Dipartimento di Ingegneria, Università del Sannio di Benevento 2 Centro Ricerche Elastomeri, Polimeri Europa S.p.A.

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Effects of molecular weight distribution on the flow-enhanced crystallization of poly(1-butene)

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  1. Effects of molecular weight distribution on the flow-enhanced crystallization of poly(1-butene) Stefano Acierno1, Salvatore Coppola2, Nino Grizzuti3 1Dipartimento di Ingegneria, Università del Sannio di Benevento 2Centro Ricerche Elastomeri, Polimeri Europa S.p.A. 3Dip. di Ingegneria Chimica, Università di Napoli Federico II

  2. CRYSTALLIZATION UNDER ROCESSING CONDITIONS “Depending on the shear rates and shearing times, either spherulitic or shish-kebab crystallization takes place. In the mechanical work done on the sample, the number of spot-like nuclei increases tremendously…” “In duct flow, high shear rates lead to highly oriented surface layers, consisting of a kind of shish-kebab…” “Shear-induced crystallization is apparently caused by a change in the structure of the polymer melt…” J. BRAUN, H. WIPPEL, G. EDER, and H. JANESCHITZ-KRIEGL, Polym. Eng. Sci., 43, 188-203 (2003)

  3. Flow induces changes to crystallization Crystallization induces changes to rheology CRYSTALLIZATION UNDER ROCESSING CONDITIONS Thermal history Final Properties CRYSTALLINITY Polymer processing Flow history

  4. Outline • Motivation • Rheological behaviour of the molten phase • Materials: HMW – LMW iPB blends • Crystallization under shear flow • Model comparison • Concluding remarks

  5. RHEOLOGY OF THE MOLTEN PHASE • Crystallization implies a reorganization of the molten phase • A good micro-rheological model is highly desirable Doi-Edwards model

  6. THE STEP-STRAIN EXPERIMENT

  7. Chain neither oriented nor stretched Chain oriented but not stretched Chain oriented and stretched ORIENTATION VS. STRETCHING Characteristic time Shear rate

  8. MICRO-RHEOLOGICAL MODELING Isothermal nucleation rate*: No flow Flow * Lauritzen and Hoffman, 1960 and Ziabicki, 1996

  9. Reptation is considered as the only relaxation mechanism (no constraint release) Chain segments are considered as non-interacting rigid rods (Independent Alignment Approximation, IAA) FLOW-INDUCED FREE ENERGY For shear deformation*: * Marrucci & Grizzuti, 1983

  10. Memory function For simple reptation* the memory function is given by: Simple reptation does not account for any constraint release coming from reptation of the surrounding chains. For this reason we choose the double reptation** approach: *Doi & Edwards, 1986 **des Cloizeaux,1990

  11. CRYSTALLIZATION + MICRO-RHEOLOGY Kn, H0, Tm, Me, d (in De) ARE NOT ADJUSTABLE PARAMETERS! (only  at one single temperature is fitted)

  12. Materials & methods Blends of two isotactic iPB’s System A: “diluted”, i.e. H-Molecular weight component up to 10 wt% System B: “concentrated”, i.e. H-Molecular weight component form 30 to 90 wt%

  13. Quiescentcrystallization Kn = 2.6  1010 K J/m3 and n = 1

  14. System A: Linear viscoelasticity

  15. 10 min annealing at 160°C to erase any crystalline memory Rapidcooling to the crystallization temperature of 95°C A constant shear rate is applied and the polymer viscosity is monitored The crystallization time scale is characterize by an induction time (time needed for the viscosity jump) Rheology during crystallization

  16. System A: crystallization under flow Sample A0 Shear rate 0.01 s-1

  17. System A: crystallization under flow

  18. System B: Linear viscoelasticity

  19. System B: crystallization under flow Sample B91 Shear rate 0.01 s-1

  20. System B: crystallization under flow

  21. Conclusions • Shear flow accelerates crystallization kinetics and higher molecular weights are more sensitive to flow intensity (i.e., the shear rate). • The addition of a small amount of high MW-polymer (< 6 wt%) to a low MW sample does not produce any appreciable effect upon the crystallization kinetics under both quiescent and shear flow conditions. • Greater elevated amounts of high MW-polymer produce evident effects upon (both quiescent and flow-enhanced) crystallization. Nevertheless the effect is not dramatic. • This behavior can be attributed to constraint release of high MW chains due to the relaxation of the shorter chains. Such a physical phenomenon is successfully described by thedouble reptation theory, which can be used to predict the flow-induced enhancement in crystallization rate under steady flow conditions. In the case of steady shear flow the agreement between calculations and experimental results is good.

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