High-Stress Shear-Induced Crystallization in Isotactic Polypropylene and Propylene/Ethylene Random Copolymers

Abstract

IntroductionSemicrystalline polymers are usually processed from their molten state and subjected to intense shear and/or elongation flows. Such flow fields not only accelerate crystallization kinetics, which shortens the processing time, but can also change the morphology from isotropic spherulites to highly oriented shish-kebab structures 1-3 and, as a consequence, determine the ultimate product properties. Therefore, understanding the interplay between strong flow fields and the resulting structures is of importance for designing processing procedures to tailor these end product properties.Considerable work [4][5][6][7] has been devoted to this topic in the past half century. Many researchers have focused on the relation between shear flow and polymer crystallization, because shear fields are easily created with rotational 3 or sliding 8,9 plate-plate devices, on rotational rheometers, 10,11 and in pressure driven slit flows. 12,13 These test geometries are typically combined with time-resolved characterization techniques like mechanical spectrometry, 10, 11, 14 light scattering, 15,16 birefringence, 13, 17 X-ray scattering 7,18,19 and Fourier transform infrared (FTIR) spectroscopy, 9, 20, 21 etc. Significant progress has been made in understanding shear-induced crystallization, 4-7 while some of the fundamental issues remain unsolved. In particular, knowledge of basic mechanism of crystallization under high shear rates and stress-close to realistic processing conditions-is still limited. The need for such information is becoming urgent in order to improve the latest simulation models, since the results of numerical predictions of, for example, injection molding, 22 have to be validated and further refined from experimental evidence.Imposing a strong shear flow at chosen high shear rates or stresses under well-defined conditions requires a specially designed flow device. Both the pressure-driven slit flow apparatus constructed by Janeschitz-Kriegl et al. 12 and improved by Kornfield et al.,13 and the piston-driven slit rheometer developed by Mackley et al. 23 and modified by Peters and coworkers, 24,25 can operate in the high stress region (of the order of 0.1 MPa) and are easily combined with time-resolved birefringence 24,26 and/or X-ray scattering [27][28][29][30] AbstractCrystallization of an isotactic polypropylene (iPP) homopolymer and two propylene/ethylene random copolymers (RACO), induced by high-stress shear, was studied using in situ synchrotron wide-angle X-ray diffraction (WAXD) at 137 °C. The "depth sectioning" method (Fernandez-Ballester, Journal of Rheology 53:5 (2009), pp. 1229−1254) was applied in order to isolate the contributions of different layers in the stress gradient direction and to relate specific structural evolution to the corresponding local stress. This approach gives quantitative results in terms of the specific length of fibrillar nuclei as a function of the applied stress. As expected, crystallization becomes faster with increasing stress-from the inner to the outer layer-for...

Chemical Cross‐linking of Polypropylenes Towards New Shape Memory Polymers

“…This can be explained by the literature known effect of iPP homoepitaxy, where “daughter” crystals grow onto the (010)‐planes under 80° and 100° apart to the “mother” crystals, that are parallely oriented to the stretching direction during programming . Typically, the amount of mother crystals is larger than that of daughter crystals . As seen in Figure f, we found a higher fraction of daughter crystals compared to the mother crystals as suggested by a mother to daughter crystal ratio of 1:1.37, calculated by integration of the peak areas of the (110) d ‐ and (110) m ‐reflections …”

“…Typically, the amount of mother crystals is larger than that of daughter crystals . As seen in Figure f, we found a higher fraction of daughter crystals compared to the mother crystals as suggested by a mother to daughter crystal ratio of 1:1.37, calculated by integration of the peak areas of the (110) d ‐ and (110) m ‐reflections …”

“…In contrast to x‐sPP, not all crystals are oriented parallel but a significant amount of crystals is oriented at angles of 80° and 100° to the stretching direction as indicated by the (110)‐reflections at 2 θ = 14.1° (see Figure d) . This can be explained by the literature known effect of iPP homoepitaxy, where “daughter” crystals grow onto the (010)‐planes under 80° and 100° apart to the “mother” crystals, that are parallely oriented to the stretching direction during programming . Typically, the amount of mother crystals is larger than that of daughter crystals .…”

Flow Induced Crystallization of Poly(ether‐block‐amide) from the Microinjection Molding Process and its Effect on Mechanical Properties

“…Relying on the fact that shear stress in pressure driven flow varies linearly along the channel thickness direction from zero at the center line to the maximum at the wall (see Figure ), we used in‐line process monitoring and morphology observation to study flow induced crystallization in injection molding process. The linear profile of shear stress is derived directly from a force balance, which is independent of the material's viscoelastic properties in the channel and velocity boundary conditions in the wall .…”

“…The shear pulse is then applied by an actuator, after the melt is cooled down to the target crystallization temperature, which is above the melting temperature of spherulite crystals ( T mc ) to eliminate temperature‐driven nucleation . The shear time can be tens of seconds and shear stress is usually <100 kPa, despite so‐called “high stress” conditions . The corresponding shear rate is <100 s −1 in order to sustain the measurement resolution at the boundary conditions .…”