SAXS Study of Oriented Crystallization of Polypropylene from a Quiescent Melt

Abstract

Uniaxially oriented polypropylene (PP) is molten in the synchrotron beam and crystallized from the quiescent melt, keeping its orientation in order to study the mechanisms of its crystallization. We document the different nanostructures observed as a function of melt-annealing temperature, undercooling, and time. In order to obtain a melt that crystallizes with high preferential orientation again, a melt-annealing temperature between 170 and 176 °C is chosen. Isothermal crystallization at 155 °C results in slow formation of (primary) lamellae placed at random. As the crystallization temperature is decreased (150, 145, and 140 °C), more and more secondary crystallites are observed which develop from a block mesostructure according to Strobl's mechanism. During the isothermal phase the blocks are fusing more or less to form imperfect lamellae. The structure evolution observed in the time-resolved small-angle X-ray scattering (SAXS) data during crystallization and remelting facilitates discrimination between the block structure and another frequently discussed morphology, Keller's cross-hatched structure. While after all our quiescent crystallization experiments most of the crystallites are blocks or incompletely fused lamellae, the hard-elastic precursor material which has been made under extreme gradients of temperature and pressure exhibits the melting of homogeneous and extended lamellae. As we apply a steep temperature gradient (−100 °C/min) to our melt in a nonisothermal crystallization experiment, we initially observe the formation of homogeneous and extended lamellae as well.

Tensile tests of polypropylene monitored by SAXS. Comparing the stretch‐hold technique to the dynamic technique

“…This is readily explained by the correlation among the crystalline bridges in equatorial direction. 43 These observations support a finding from a previous study 43 which indicated that the cross-hatched lamellae of PP are a secondary feature of its crystallization. We admit that this observation should be verified by a corresponding WAXS study.…”

Extruded blend films of poly(vinyl alcohol) and polyolefins: common and hard-elastic nanostructure evolution in the polyolefin during straining as monitored by SAXS

“…The scattering of the undrawn material is presented in the upper left corner. It shows the typical scattering of semicrystalline polypropylene with uniaxial orientation [22, 37, 38]. When unblended thermoplasts are sheet-extruded but not drawn, such a morphology is well known and called a precursor of a hard-elastic material [35].…”

Melting and Crystallization of Differently Oriented Sets of Crystallites in Hard-Elastic Polypropylene

“…In the interval 3 < t < 4.5 min, the WAXS exhibits the equilibrium-dominated regime of the crystallization. Here the SAXS, 15 ( Figure 9) detects a stable block structure of midrange correlation in both fiber and lateral directions. For t > 4.5 min, the WAXS peak-integral curves, S(t) and S*(t), indicate the growth-dominated regime of the crystallization.…”

“…Therefore, the period before the start of the nucleation (equilibrium-dominated crystallization) has not been resolved by the SAXS. The SAXS recorded during the regime of equilibriumdominated crystallization has shown a 3D arrangement 15 ( Figure 7) of blocks (i.e., nuclei) that turn from an entwined arrangement into a checkered arrangement, while at the same time, the gaps between the blocks shrink in both lateral and longitudinal directions. Because the WAXS shows that there is not much increase in crystallinity, the SAXS results could be explained by the notion that during the equilibrium-dominated crystallization, the amplitude of the size fluctuation of the nuclei appears to increase.…”

“…15 We reported that after shallow quench (155 and 150°C), the first discrete SAXS develops into meridional streaks. After medium and deep quench, the first discrete SAXS patterns conform to both the model of a cross-hatched 4,5,7,26 lamellar structure and Strobl's block structure.…”