Experimental study of shear-induced crystallization of an impact polypropylene copolymer

Abstract

International audienceThe crystallization kinetics of polypropylene was observed during shear and after shear experiments under isothermal condition. The crystallizations were performed in a plate-plate and a fiber pull-out device. The nucleation density, the crystalline growth and the overall kinetics were measured and compared with data obtained in a similar way but during static experiments. The morphologies are spherulitic and formed from nuclei which seem to be randomly distributed, α-phase spherulites are always observed but with a nucleation density and a growth rate which depend on shearrate. The nucleation density is strongly enhanced by shear and acts as the main factor on the overall kinetics. The overall kinetics can be analyzed with a two-step Avrami model, where an Avrami exponent n1 with a very high value is always observed first after shear and a more usual parameter n2 for the subsequent crystallization period. This high value of n1 seems to be related to the strong enhancement of nucleation density. The growth rate increases with the shear-rate, but the basic growth mechanisms do not seem to be modified. For crystallizations after shear the growth rate decreases with a long-time delay after shear but not down to the static value. The effect is characteristic of a partial relaxation of chain orientation after shear but with a very unusual time constant

Investigating the Morphological Variations Due to Processing and Thermomechanical Treatment of Poly(propylene) Using Raman Microscopy

“…The increase in X c seen across the weld seam, making it similar to the X c of the surrounding PP-R plate, can be attributed to the shear forces dominating the crystallization behavior along the weld seams. [ 41 ] The weld seams experience both the highest amount of shear and the highest rate of cooling, and in the case of PP-R mechanical abrasion→DSC and μ-FT-IR microscopy are limited to the low spatial resolution in measurements and intricate variations across the weld seams are averaged out. Using Raman microscopy it could be shown that these variations in X c were present in narrow channels along the weld seams and a gradient of ≈18% in the crystallinity could be observed.…”

Modification of polypropylene by melt vibration blending with ultra high molecular weight polyethylene

“…There have been lots of reports about shear-induced ␤ form of PP. 41,42 The spatially and temporally varying shear gradient field in the vibration internal mixer could give rise to the increment in possibility of the accidental discrepancy of molecular chain arrangement, which favors polypropylene chains to alter packing mode into the ␤ form unit cell in the subsequent crystallization. The long entangled molecular UHMWPE or its crystals formed also curled the growing polypropylene lamellae inducing more ␤ crystals.…”

The influence of chain defects on the crystallisation behaviour of isotactic polypropylene

“…The mechanical load can be introduced by pulling a fibre through an undercooled melt at a certain temperature T pull . The shear stress generated by fibre pulling produces row nuclei consisting of the !-phase that induce the growth of a cylindritic growth front [13][14][15][16][17][18][19]37]. These row nuclei surround the fibre in a cylindritic fashion thus the growth front is cylindrical and proceeds in the radial direction.…”

“…Increasing concentration of the chain defects in the polymer chain reduces both the spherulitic growthrate and the overall crystallization rate. Sanchez and Eby [23] derived a general expression for the spherulitic growth rate G (assuming a coherent surface nucleus and regime III growth) (see Equation (18)): (18) It is possible to express the nucleation rate of the copolymer in terms of the homo-polymer nucleation rate plus a term that contains the comonomer concentration if the following function is defined (Equation (19)): (19) The term /G 0 is the bulk free energy difference for the complete defect-free (homo) polymer with infinite lamellar thickness and molar mass compared to the defect containing 'real' lamella of finite thickness and is usually approximated by Equation (20): (20) For /G 0 it is assumed that at moderate undercooling the heat of fusion (/H 0 ) and entropy of fusion (/S 0 ) are temperature independent [45]. From the nucleation theory [45] follows Equation (21): (21) where l c,i is the initial crystal thickness and " e is the surface free energy.…”

“…Shear stress generated for example by pulling a fibre through an undercooled melt produces row nuclei consisting of the !-phase, inducing the growth of a cylindrical growth front consisting of the "-phase. Several studies [14][15][16][17][18][19] were dealing with parameters that influence the formation of such "-cylindrites, like the temperature of pulling (T pull ), crystallization temperature (T c ), time of pulling (t pull ), speed of fibre pull (v pull ), etc. The conclusion from these studies was that samples rich in "-phase are formed when T !"…”