Kunpeng Cui

Extensional rheometer for in situ x-ray scattering study on flow-induced crystallization of polymer

We designed and constructed an extensional rheometer for in situ small and wide angle x-ray study on flow-induced crystallization of polymer. Two rotating drums with an axis distance of 20 mm are employed to impose extensional deformation on the samples. With a constant angular velocity, the two drums generate a constant Henkcy strain rate as sample length for testing keeps constant during deformation. An ionic liquid is used as heating medium to prevent polymer melt from bending downward due to gravity, which is excellent in terms of high thermal stability, low viscosity, and relative low adsorption on x-ray. Flow-induced crystallization experiments are conducted with this apparatus on x-ray scattering station in Shanghai Synchrotron Radiation Facility (SSRF), which allows us to collect rheological and structural data simultaneously and may lead to a better understanding on flow-induced crystallization of polymer.

A simple constrained uniaxial tensile apparatus for in situ investigation of film stretching processing

A simple constrained uniaxial tensile apparatus was designed and constructed to obtain stress-strain curve during stretching and subsequent structural evolution of polymeric films. Stretch is carried out through two motor driven clamps in the machine direction and scissor-like clamps in the transverse direction keeping the sample width constant. The force information during film stretching process is recorded by a tension sensor and structural evolution can be obtained by in situ X-ray scattering technique. All parameters related to film stretching manufacturing, such as temperature, draw ratio, and stretching speed can be set independently, making the apparatus an effective method to explore the relationship between processing parameters and structure.

Multiscale and Multistep Ordering of Flow-Induced Nucleation of Polymers

Flow-induced crystallization (FIC) is a typical nonequilibrium phase transition and a core industry subject for the largest group of commercially useful polymeric materials: semicrystalline polymers. A fundamental understanding of FIC can benefit the research of nonequilibrium ordering in matter systems and help to tailor the ultimate properties of polymeric materials. Concerning the crystallization process, flow can accelerate the kinetics by orders of magnitude and induce the formation of oriented crystallites like shish-kebab, which are associated with the major influences of flow on nucleation, that is, raised nucleation density and oriented nuclei. The topic of FIC has been studied for more than half a century. Recently, there have been many developments in experimental approaches, such as synchrotron radiation X-ray scattering, ultrafast X-ray detectors with a time resolution down to the order of milliseconds, and novel laboratory devices to mimic the severe flow field close to real processing conditions. By a combination of these advanced methods, the evolution process of FIC can be revealed more precisely (with higher time resolution and on more length scales) and quantitatively. The new findings are challenging the classical interpretations and theories that were mostly derived from quiescent or mild-flow conditions, and they are triggering the reconsideration of FIC foundations. This review mainly summarizes experimental results, advances in physical understanding, and discussions on the multiscale and multistep nature of oriented nuclei induced by strong flow. The multiscale structures include segmental conformation, packing of conformational ordering, deformation on the whole-chain scale, and macroscopic aggregation of crystallites. The multistep process involves conformation transition, isotropic-nematic transition, density fluctuation (or phase separation), formation of precursors, and shish-kebab crystallites, which are possible ordering processes during nucleation. Furthermore, some theoretical progress and modeling efforts are also included.

The non-equilibrium phase diagrams of flow-induced crystallization and melting of polyethylene

Combining extensional rheology with in-situ synchrotron ultrafast x-ray scattering, we studied flow-induced phase behaviors of polyethylene (PE) in a wide temperature range up to 240 °C. Non-equilibrium phase diagrams of crystallization and melting under flow conditions are constructed in stress-temperature space, composing of melt, non-crystalline δ, hexagonal and orthorhombic phases. The non-crystalline δ phase is demonstrated to be either a metastable transient pre-order for crystallization or a thermodynamically stable phase. Based on the non-equilibrium phase diagrams, nearly all observations in flow-induced crystallization (FIC) of PE can be well understood. The interplay of thermodynamic stabilities and kinetic competitions of the four phases creates rich kinetic pathways for FIC and diverse final structures. The non-equilibrium flow phase diagrams provide a detailed roadmap for precisely processing of PE with designed structures and properties.

The thermodynamic properties of flow-induced precursor of polyethylene

Flow-induced preordering or precursor (FIP) has been studied in a series of lightly cross-linked high-density polyethylene with a combination of extensional rheology and in situ synchrotron radiation small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) measurements. Based on the incipient strains of SAXS and WAXD signals during extension in a large temperature range, strain-temperature diagrams for flow-induced preordering and nucleation were constructed and revealed that flow-induced crystallization (FIC) undergoes two stages: melt-precursor transition (MPT) and precursor-nuclei transition (PNT). At different temperatures, FIP with different inner structures and morphologies can be induced by strain; these embryos have shape and structure that are related to those of the corresponding critical nuclei. With the strain-temperature diagrams, the thermodynamic properties of FIP are deduced, which shows that compared with the relative nuclei the FIP always has a lower bulk free energy (ΔH) and a much lower surface free energy (σe). In extreme cases (high temperature), the σe of FIP can be negligible. The quantitative estimation of the thermodynamic parameters suggests the existence of variant FIPs, which plays a vital role for the subsequent progress of PNT and the whole process of FIC.

A small-angle x-ray scattering system with a vertical layout.

A small-angle x-ray scattering (SAXS) system with a vertical layout (V-SAXS) has been designed and constructed for in situ detection on nanostructures, which is well suitable for in situ study on self-assembly of nanoparticles at liquid interface and polymer processing. A steel-tower frame on a reinforced basement is built as the supporting skeleton for scattering beam path and detector platform, ensuring the system a high working stability and a high operating accuracy. A micro-focus x-ray source combining parabolic three-dimensional multi-layer mirror and scatteringless collimation system provides a highly parallel beam, which allows us to detect the very small angle range. With a sample-to-detector distance of 7 m, the largest measurable length scale is 420 nm in real space. With a large sample zone, it is possible to install different experimental setups such as film stretching machine, which makes the system perfect to follow the microstructures evolution of materials during processing. The capability of the V-SAXS on in situ study is tested with a drying experiment of a free latex droplet, which confirms our initial design.

Relaxation propelled long period change in the extension induced crystallization of polyethylene oxide

The time evolution of the long period in the extension-induced crystallization of polyethylene oxide (PEO) is investigated with a combination of rheological measurement and in situ small angle X-ray scattering (SAXS), aiming to show the dynamic change in the spatial arrangement of nuclei in terms of both the chain stretching and orientation. The main findings are summarized as follows. (i) Strain hardening is observed when the strain is larger than 3.0, indicating a transition from chain orientation to chain stretching. (ii) The time evolution of the long period differs before and after strain hardening, reflecting changes in the evolution of nuclei arrangement. The full strain range is thus divided into two regions, namely the large strain region and the small strain region. In the large strain region the long period exhibits a two-step monotonic decrease. For the small strain region, an increase-plateau-decrease behavior is observed. (iii) A large gap exists between the initial values of the long period in each of the two strain regions. (iv) The crystallization kinetics shows no secondary acceleration after strain hardening. Based on these results, a qualitative model emphasizing the chain relaxation propelled rearrangement of nuclei is proposed to explain the structural evolution.

A New Three-Dimensional (3D) Multilayer Organic Material: Synthesis, Swelling, Exfoliation, and Application

A novel fully rigid, rod-shaped oligo(p-benzamide) (OPBA-6) molecule was designed and synthesized, which can be recrystallized into a three-dimensional (3D) multilayer material via an antiparallel molecular packing model. Intermolecular hydrogen bonding and π-π interaction are brought to ensure a strong intralayer interaction, while decoration of layer surface with sulfonic groups promotes water to enter interlayer space and facilitates the swelling and exfoliation of sample. With a simple dispersion in water, the obtained multilayer material can be easily swollen by water without destruction of in-plane morphology and subsequently delaminated into 2D nanosheets with thickness of about 5.38 nm. This achievement may be the first attempt to exfoliate layered organic materials and thus provide a new strategy to prepare 2D organic nanosheets without using any substrates or templates as required by conventional and widely used self-assembly routes. Based on exfoliated nanosheets, poly(vinyl alcohol) nanocomposites were prepared using a simple water solution processing method. A 64% increase in tensile stress and a 63% improvement in Youngs modulus were achieved by addition of 7 wt % OPBA-6 loading.