The prediction of mechanical performance of isotactic polypropylene on the basis of processing conditions

A filament stretching rheometer for in situ X-ray experiments: Combining rheology and crystalline morphology characterization

We present a rheometer that combines the possibility to perform in situ X-ray experiments with a precise and locally controlled uniaxial extensional flow. It thus allows us to study the crystallization kinetics and morphology evolution combined with the rheological response to the applied flow field. A constant uniaxial deformation rate is ensured, thanks to a fast control scheme that drives the simultaneous movement of the top and bottom plates during a pulling experiment. A laser micrometer measures the time evolution of the smallest diameter, where the highest stress is concentrated. The rheometer has a copper temperature-controlled oven with the ability to reach 250 °C and a N₂ connection to create an inert atmosphere during the experiments. The innovation of our rheometer is the fixed location of the midfilament position, which is possible because of the simultaneous controlled movement of the two end plates. The copper oven has been constructed with four ad hoc windows: two glass windows for laser access and two Kapton windows for X-ray access. The key feature is the ability to perfectly align the midfilament of the sample to the laser micrometer and to the incoming X-ray beam in a synchrotron radiation facility, making it possible to investigate the structure and morphologies developed during extensional flow. The rheological response measured with our rheometer for low-density polyethylene (LDPE) is in agreement with the linear viscoelastic envelope and with the results obtained from the existing extensional rheometers. To demonstrate the capability of the instrument, we have performed in situ-resolved X-ray experiments on LDPE samples exhibiting extensional flow-induced crystallization.

Effect of Rapid Mold Heating on the Structure and Performance of Injection-Molded Polypropylene

The tailoring by the process of the properties developed in the plastic objects is the more effective way to improve the sustainability of the plastic objects. The possibility to tailor to the final use the properties developed within the molded object requires further understanding of the relationship between the properties of the plastic objects and the process conduction. One of the main process parameters that allow adjusting the properties of molded objects is the mold temperature. In this work, a thin electrical heater was located below the cavity surface in order to obtain rapid and localized surface heating/cooling cycles during the injection molding process. An isotactic polypropylene was adopted for the molding tests, during which surface temperature was modulated in terms of values and heating times. The modulation of the cavity temperature was found able to control the distribution of relevant morphological characteristics, thus, properties along the sample thickness. In particular, lamellar thickness, crystallinity distribution, and orientation were analyzed by synchrotron X-ray experiments, and the morphology and elastic modulus were characterized by atomic force microscopy acquisitions carried out with a tool for the simultaneous nanomechanical characterization. The crystalline degree slightly increased with the cavity temperature, and this induced an increase in the elastic modulus when high temperatures were adopted for the cavity surface. The cavity temperature strongly influenced the orientation distribution that, on its turn, determined the highest values of the elastic modulus found in the shear layer. Furthermore, although the sample core, not experiencing a strong flow field, was not characterized by high levels of orientation, it might show high values of the elastic modulus if temperature and time during crystallization were sufficient. In particular, if the macromolecules spent adequate time at temperatures close to the crystallization temperature, they could achieve high levels of structuring and, thus, high values of elastic modulus.

Effect of Thermal History and Shear on the Viscoelastic Response of iPP Containing an Oxalamide-Based Organic Compound

We report on the role of temperature and shear on the melt behavior of iPP in the presence of the organic compound N1,N1′-(propane-1,3-diyl)bis(N2-hexyloxalamide) (OXA3,6). It is demonstrated that OXA3,6 facilitates a viscosity suppression when it resides in the molten state. The viscosity suppression is attributed to the interaction of iPP chains/subchains with molten OXA3,6 nanoclusters. The exact molecular mechanism has not been identified; nevertheless, a tentative explanation is proposed. The observed viscosity suppression appears similar to that encountered in polymer melts filled with solid nanoparticles, with the difference that the OXA3,6 compound reported in this study facilitates the viscosity suppression in the molten state. Upon cooling, as crystal growth of OXA3,6 progresses, the decrease in viscosity is suppressed. Retrospectively, segmental absorption of iPP chains on the surface of micrometer-sized OXA3,6 crystallites favors the formation of dangling arms, yielding OXA3,6 crystallites decorated with partially absorbed iPP chains. In other words, the resulting OXA3,6 particle morphology resembles that of a hairy particle or a starlike polymer chain. Such hairy particles effectively facilitate a viscosity enhancement, similar to branched polymer chains. This hypothesis and its implications for the shear behavior of iPP are discussed and supported using plate–plate rheometry and slit-flow experiments combined with small-angle X-ray scattering analysis.

Effect of Self-Assembly of Oxalamide Based Organic Compounds on Melt Behavior, Nucleation, and Crystallization of Isotactic Polypropylene

We report on the effect of an aliphatic oxalamide based nucleating agent (OXA3,6) on the melt and crystallization behavior of isotactic polypropylene (iPP) under defined shear conditions. Through polarized optical microscopy, we demonstrate that OXA3,6 self-assembles from the iPP melt into rhombic crystals whereas their size and distribution proved highly dependent on the employed cooling rates. The presence of 0.5 wt % of OXA3,6 in iPP results in a significant suppression in iPP melt viscosity, which could not be explained via molecular modeling. A possible cause for the drop in viscosity in the presence of OXA3,6 is attributed to the interaction (absorption) of high molecular weight iPP chains with the nucleating agent, thereby suppressing their contribution to the viscoelastic response of the melt. This proposed mechanism for the suppression in melt viscosity appears similar to that encountered by the homogeneous distribution of nanoparticles such as CNTs, graphene, and silica. Shear experiments, performed using a slit flow device combined with small-angle X-ray diffraction measurements, indicate that crystallization is significantly enhanced in the presence of OXA3,6 at relatively low shear rates despite its lowered sensitivity to shear. This enhancement in crystallization is attributed to the shear alignment of the rhombic OXA3,6 crystals that provide surface for iPP kebab growth upon cooling. Overall, the suppression in melt viscosity in combination with enhanced nucleation efficiency at low as well as high shear rates makes this self-assembling oxalamide based nucleating agent a promising candidate for fast processing.

Structure⁻Properties Relations for Polyamide 6, Part 1: Influence of the Thermal History during Compression Moulding on Deformation and Failure Kinetics

The deformation and failure kinetics of polyamide 6 samples prepared by several thermal histories were investigated by tests at different temperatures and relative humidities. PA6 samples were produced in quiescent condition and multiple cooling procedure. A characterization was performed to investigate the effect of the different thermal histories and the effect of hydration on both structures and glass transition temperature. The mechanical properties were investigated by tensile and creep tests at different temperatures and relative humidity. In order to describe the experimental results, the Ree–Eyring equation, modified with the “apparent temperature”, was employed. In addition, the results of time-to-failure (creep tests) were described by the use of the “critical strain” concept. Eventually, a link between the Eyring theory and the structure evolution was made, i.e., a relation between the rate factors and the average lamellar thickness.

Time-Resolving Study of Stress-Induced Transformations of Isotactic Polypropylene through Wide Angle X-ray Scattering Measurements

The development of a highly oriented fiber morphology by effect of tensile deformation of stereodefective isotactic polypropylene (iPP) samples, starting from the unoriented γ form, is studied by following the transformation in real time during stretching through wide angle X-ray scattering (WAXS) measurements. In the stretching process, after yielding, the initial γ form transforms into the mesomorphic form of iPP through mechanical melting and re-crystallization. The analysis of the scattering invariant measured in the WAXS region highlights that the size of the mesomorphic domains included in the well oriented fiber morphology obtained at high deformations increases through a process which involves the coalescence of the small fragments formed by effect of tensile stress during lamellar destruction with the domain of higher dimensions.

Deformation-Induced Phase Transitions in iPP Polymorphs

This detailed study reveals the relation between structural evolution and the mechanical response of α -, β - and γ -iPP. Uni-axial compression experiments, combined with in situ WAXD measurements, allowed for the identification of the evolution phenomena in terms of phase composition. Tensile experiments in combination with SAXS revealed orientation and voiding phenomena, as well as structural evolution in the thickness of the lamellae and amorphous layers. On the level of the crystallographic unit cell, the WAXD experiments provided insight into the early stages of deformation. Moreover, transitions in the crystal phases taking place in the larger deformation range and the orientation of crystal planes were monitored. At all stretching temperatures, the crystallinity decreases upon deformation, and depending on the temperature, different new structures are formed. Stretching at low temperatures leads to crystal destruction and the formation of the oriented mesophase, independent of the initial polymorph. At high temperatures, above T α c , all polymorphs transform into oriented α -iPP. Small quantities of the initial structures remain present in the material. The compression experiments, where localization phenomena are excluded, show that these transformations take place at similar strains for all polymorphs. For the post yield response, the strain hardening modulus is decisive for the mechanical behavior, as well as for the orientation of lamellae and the evolution of void fraction and dimensions. β -iPP shows by far the most intense voiding in the entire experimental temperature range. The macroscopic localization behavior and strain at which the transition from disk-like void shapes, oriented with the normal in tensile direction, into fibrillar structures takes place is directly correlated with the strain hardening modulus.