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. 3 Dip. di Ingegneria Chimica, Università di Napoli Federico II

J. BRAUN, H. WIPPEL, G. EDER, and H. JANESCHITZ-KRIEGL, Polym. Eng. Sci., 43, (2003) “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…” CRYSTALLIZATION UNDER ROCESSING CONDITIONS

Flow induces changes to crystallizationFlow induces changes to crystallization Crystallization induces changes to rheologyCrystallization induces changes to rheology Polymerprocessing Thermalhistory Flowhistory FinalPropertiesCRYSTALLINITY

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

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

THE STEP-STRAIN EXPERIMENT

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

MICRO-RHEOLOGICAL MODELING No flowFlow Isothermal nucleation rate*: * Lauritzen and Hoffman, 1960 and Ziabicki, 1996

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

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

CRYSTALLIZATION + MICRO- RHEOLOGY K n,  H 0, T m, M e,  d (in De) ARE NOT ADJUSTABLE PARAMETERS! (only  at one single temperature is fitted)

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%

Quiescent crystallization K n = 2.6  K J/m 3 and n = 1

System A: Linear viscoelasticity

Rheology during crystallization  10 min annealing at 160°C to erase any crystalline memory  Rapid cooling 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)

System A: crystallization under flow Sample A0Shear rate 0.01 s -1

System A: crystallization under flow

System B: Linear viscoelasticity

System B: crystallization under flow Sample B91Shear rate 0.01 s -1

System B: crystallization under flow

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 the double 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.