Crystallization kinetics of polypropylene under high pressure and steady shear flow

Comparison of the Foamability of Linear and Long-Chain Branched Polypropylene-The Legend of Strain-Hardening as a Requirement for Good Foamability

Polypropylene (PP) is an outstanding material for polymeric foams due to its favorable mechanical and chemical properties. However, its low melt strength and fast crystallization result in unfavorable foaming properties. Long-chain branching of PP is regarded as a game changer in foaming due to the introduction of strain hardening, which stabilizes the foam morphology. In this work, a thorough characterization with respect to rheology and crystallization characteristics of a linear PP, a PP/PE-block co-polymer, and a long-chain branched PP are conducted. Using these results, the processing window in foam-extrusion trials with CO₂ and finally the foam properties are explained. Although only LCB-PP exhibits strain hardening, it neither provide the broadest foaming window nor the best foam quality. Therefore, multiwave experiments were conducted to study the gelation due to crystallization and its influence on foaming. Here, linear PP exhibited a gel-like behavior over a broad time frame, whereas the other two froze quickly. Thus, apart from strain hardening, the crystallization behavior/crystallization kinetics is of utmost importance for foaming in terms of a broad processing window, low-density, and good morphology. Therefore, the question arises, whether strain hardening is really essential for low density foams with a good cellular morphology.

Hierarchical crystalline morphologies induced by a distinctly different melt penetrating process

It is the first time that hierarchically crystalline morphologies induced by melt penetration of the melt with different interactions.

CRISTAPRESS: an optical cell for structure development in high-pressure crystallization

An original optical high-pressure cell, named CRISTAPRESS, has been especially designed to investigate phase transitions of complex liquids, i.e., polymers, polymer blends, nano-composites, etc. The design of the cell is based on the optical properties of morphological entities through in situ light depolarizing microscopic observations. Pressure up to 200 MPa with a fine temperature control up to 300 °C can be applied. A striking advantage of this cell is the possibility to select the pressure transmitting medium that can be water, silicone oil, a fluid in the supercritical state, etc. The potential of the novel technique was demonstrated by carrying out time-resolved measurements during polymer crystallization induced by water pressure. These preliminary experimental investigations permit to discriminate the role of the barometric and thermal histories on the kinetics of polymer growth, as well as on the subsequent morphologies. It should lead to new reliable crystallization kinetics models.

Evolution of Morphology of iPP in Processing Conditions

A model for crystallization kinetics that accounts for the formation of different crystalline phases and is able to describe the morphological characteristics of samples solidified under quiescent conditions, has been enriched to account for the effect of solidification pressure. The effect of pressure was considered by assuming a linear increase of melting and glass transition temperatures (which are involved in the description of the growth rate and nucleation density of the alpha phase). Moreover, pressure was incorporated in the kinetic constant adopted to describe the evolution of the mesomorphic phase. The parameters of the model were identified on the basis of literature data on the distribution of crystalline phases in samples solidified under different pressures. The modified model also satisfactorily described PVT curves up to 100 MPa, and is now able to describe the evolution of morphology during solidification at cooling rates as fast as several hundreds of Kelvin degrees per second and under pressures of as high as 100 MPa.