Sensitivity of Polymer Crystallization to Shear at Low and High Supercooling of the Melt

Abstract

Flow-induced crystallization (FIC) is a dominant mechanism of polymer self-assembly, but the process is poorly understood at high supercooling and under fast cooling conditions because of structural rearrangements that occur during slow heating and cooling conditions typically used for investigation. Incorporating fast-scanning chip calorimetry techniques, the influence that specific amounts of shear flow have on the subsequent crystallization of polyamide 66 over a wide range of temperatures, 85–240 °C, is determined. At high temperatures, heterogeneous nucleation dominates and crystallization rate increases with increasing shear. Low-temperature crystallization, driven by homogeneous nucleation, is not influenced by previous shear flow, but sheared samples are able to crystallize via the heterogeneous nucleation route at temperatures 15 K lower than unsheared materials. The magnitude of previous shear flow also dictates α-/γ-crystalline phase development and crystallization during cooling at rates below 200 K/s. This approach provides a route to develop important thermodynamic and kinetic insights that describe the crystallization behavior of many important polymers to enable the advanced engineering of polymer processing and injection molding applications, where practical cooling rates typically range between 10 K/s and 1000 K/s.

Nucleation‐controlled semicrystalline morphology of bulk polymers

“…After their generation in the melt, these nuclei remain stable below a critical temperature of the order of magnitude of the equilibrium melting temperature, T m,0 . As such, on cooling the melt, these nuclei lead to an acceleration of the crystallization rate already at low supercooling, as demonstrated in Figure . The various curves were obtained on samples of PA 66 subjected to different amount of specific shear work in a rotational rheometer at 270°C, being close to T m,0 = 277°C, as indicated in the legend.…”

“…At the right‐hand side are shown POM micrographs of samples crystallized in an FSC at 70°C (top) and 200°C (bottom), after being subjected to a specific shear work of 22 MPa. Data and images were adapted from , with permission from ACS…”

Critical specific work of flow for shear‐induced formation of crystal nuclei in poly (l‐lactic acid)

“…For the sample volume subjected to a shear rate of close to 2.5 s −1 an almost space‐filled spherulitic morphology is observed, which agrees with the prediction of Figure A (see black diamonds, obtained after shearing the melt at 2 s −1 ). Noteworthy, though the thin section of Figure shows the structure perpendicular to flow, there are also observed row‐like aligned spherulites pointing to turbulent flow conditions (see top‐right image in Figure ) in the performed experiment, similar to those observed in polyamide 66 …”

“…These are, in particular, aligned spherulites grown from the thread‐like crystallization precursors or so‐called shish kebab structures being composed of long extended‐chain‐like crystals (shish) with lamellae grown perpendicular to the shish (kebab) . Flow‐induced crystallization may affect the crystal growth rate; however, it is mainly the largely increased number of rather stable nucleation sites, which leads to an increase of the overall crystallization rate.…”

Crystallization of poly(butylene 2,6‐naphthalate) containing diethylene 2,6‐naphthalate constitutional defects

“…It is assumed that a different nucleation mechanism becomes active at high supercooling. Indeed, double minima in the peak or half‐crystallization time curves are often observed in polymer crystallization, and commonly attributed to the dominance of homogeneous nucleation and heterogeneous nucleation at high and low melt supercooling, respectively. The copolymerization greatly slows down crystallization kinetics, with the peak crystallization time increasing several orders of magnitude with respect to the homopolymer, when the same crystallization temperature is considered.…”