The continuous cooling transformation (CCT) as a flexible tool to investigate polymer crystallization under processing conditions

Abstract

An experimental route for investigating polymer crystallization over a wide range of cooling rates (from 0.01 to 1000◦C/s) and pressures (from 0.1 to 40 MPa) is illustrated, using a method that recalls the approach adopted in metallurgy for studying structure development in metals. Two types of experimental setup were used, namely an apparatus for fast cooling of thin films (100–200 μm thick) at various cooling rates under atmospheric pressure and a device (based on a on-purpose modified injection molding machine) for quenching massive samples (about 1–2 cm3) under hydrostatic pressure fields. In both cases, ex situ characterization experiments were carried out to probe the resulting structure, using techniques such as density measurements and wide-angle x-ray diffraction (WAXD) patterns. The cooling mechanism and temperature distribution across the sample thickness were analyzed. Results show that the final structure is determined only by the imposed thermal history and pressure. Experimental results for isotactic polypropylene (iPP), poly(ethylene terephthalate) (PET), polyamide 6 (PA6), and syndiotactic polystyrene (sPS) are reported, showing the reliability of this experimental approach to assess not only quantitative information but also a qualitative description of the crystallization behavior of different classes of semicrystalline polymers. The present study gives an opportunity to evaluate how the combined effect of the cooling rate and pressure influences the crystallization kinetics for various classes of polymer of commercial interest. An increase in the cooling rate translates into a decrease in crystallinity and density, which both experience a sudden drop around the specific “crystallizability” (or “critical cooling rate”) of the material examined. The exception is sPS where competition among the various crystalline modifications determines a minimum in the plot of density vs. cooling rate. As for the effect of pressure, iPP exhibits a “negative dependence” of crystallization kinetics upon pressure, with a decrease of density and degree of crystallinity with increasing pressure, owing to kinetic constraints. PA6 and PET, on the other hand, due to thermodynamic factors resulting in an increase in Tm with pressure, exhibits a “positive dependence” of crystallization kinetics upon pressure. Finally, recent original results concerning sPS have shown that the minimum in the density vs. cooling rate curve shifts toward larger cooling rates upon increasing pressure

Crystallization and Mechanical Properties of Polypropylene under Processing-Relevant Cooling Conditions with respect to Isothermal Holding Time

“…At temperatures less than or equal to 50 ∘ C the mesomorphic phase occurs. Brucato et al [18,19] asserts that, with an increasing cooling velocity, the amount of the mesomorphic phase increases and the density decreases. With a special focus on the crystalline structure and morphology with respect to the cooling rate Piccarolo et al [20,21] described the formation of the mesomorphic phase with isolated, negatively birefringent spherulites for cooling rates greater than 80 K/s.…”

Laser Ablation of Poly(lactic acid) Sheets for the Rapid Prototyping of Sustainable, Single-Use, Disposable Medical Microcomponents

“…To investigate this hypothesis, four types of PLA sheets were produced with differing degrees of crystallinity using a compression molding technique: ultrahigh cooling rate (UHCR), high cooling rate (HCR), low cooling rate (LCR), and ultralow cooling rate (ULCR) (Supporting Information, section 1). The purpose was to investigate the influence of the cooling protocol on the overall morphology on the nanoscale (size of crystal lamellae) and on the mesoscale (size and amount of crystalline aggregates such as spherulites) (according to Continuous Cooling Transformation proposed for polymeric materials 35 ) and, in turn, to study the behavior of these different PLA types when subjected to CO 2 laser cutting. During the phase transition from liquid to solid, a rapid cooling rate does not provide the time necessary for crystallites to form and, by freezing the polymer into a disordered structure, generates an amorphous status.…”

Measurement of cloud point temperature in polymer solutions

“…For instance, in the case of crystallization from the melt, the cooling rate influences the crystallinity value and the crystal morphology obtained. 4,5 It is thus reasonable to assume that also the phase behavior of polymer solutions could be influenced by the thermal history, and not only by its thermodynamic properties. Many features of polymer solutions are temperature dependent (e.g., driving force for phase separation, demixing mechanism, system viscosity, polymer mobility, and conformation in solution): therefore, an influence of thermal history on the phase separation processes could be reasonably envisaged.…”