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

Abstract

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 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 plate 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 N 2 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 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, allowing to investigate structure and morphologies developed during extensional flow. The rheological response measured with our rheometer for LDPE is in agreement with the linear viscoelastic envelope and with the results obtained from existing extensional rheometers.To demonstrate the capability of the instrument we have performed in-situ time-resolved X-ray experiments on LDPE samples exhibiting extensional flow-induced crystallization.

Towards a universal shear correction factor in filament stretching rheometry

“…In case of uniaxial extension, a falling mass was initially used to generate the stretching motion (Matta and Tytus 1990). Later, several research groups have built variations of the so-called filament stretching rheometer (Sridhar et al 1991;Tirtaatmadja and Sridhar 1993;McKinley et al 1999;Anna et al 2001;Bach et al 2003;Chellamuthu et al 2011;Pepe et al 2020). In this device, a cylindrical fluid sample is placed between two parallel disks, that are then pulled apart by moving one plate while a load cell connected to the other plate measures the generated force.…”

“…Later, several control schemes were implemented that use feedback and feed forward control combined with in situ measurements of the filament diameter to ensure a constant uniaxial extension rate at the middle of the filament (Anna et al 1999;Marín et al 2013). Recently, within our group, a further improved filament extensional stretching rheometer (FiSER) design was realized that allows to measure the extensional rheological properties while in situ structure characterizations can be performed (Pepe et al 2020). Thereto, both pistons move at equal speed in opposite directions relying on the underlying control mechanism whereby the stagnation point is positioned at the center of the filament.…”

Multi-Scale Simulation of Injection Molding Process with Micro–Features Replication: Relevance of Rheological Behaviour and Crystallization

“…Furthermore, as the micro-corner is a convergent cavity (see sketch showed in Figure 4), the extensional flow should also be considered. The extensional flow is recognized to be more efficient than the shear flow in orienting molecules, thus enhancing the crystallization kinetics [26], leads to a significant increase in viscosity. This phenomenon, during The simulations consistently reproduce the experimental results for the cases with 27 • C and 150 • C cavity surface temperatures.…”

A numerical study of extensional flow‐induced crystallization in filament stretching rheometry

“…17 Therefore, recently, within our group, a FiSER design was realized wherein both pistons move at equal speed in opposite directions. 18 As a result, a stagnation point is formed at the center of the filament, while ensuring a constant uniaxial extension rate with a radius-based controller. This makes it possible to measure the extensional rheological properties while in situ structure characterizations can be performed by means of X-ray scattering.…”

“…This makes it possible to measure the extensional rheological properties while in situ structure characterizations can be performed by means of X-ray scattering. 18 Thereby, shishkebab growth in LDPE was followed during extensional flow.…”