Lingpu Meng

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.

A Universal Equipment for Biaxial Stretching of Polymer Films

A biaxial stretching equipment was designed and constructed to enable fundamental studies of the relationship between film processing conditions and structures of oriented film products. With programmable drive motors and scissor-like mechanism, all stretching modes, including uniaxial stretching with constant and free width, simultaneous and sequential biaxial stretching, can be applied to a square-shaped sheet. Parameters related to film stretching manufacturing, such as temperature, draw ratio and stretching speed can be set independently to meet the requirement of different polymers. The force information during stretching is recorded by two miniature tension sensors in two directions independently, which can monitor the mechanical stimulus and stress response. Using this equipment, experiments are conducted to investigate the influence of stretching parameters on the structure of polypropylene films, which provides an effective method to tailor the processing conditions to obtain the films with desired properties.

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.

How flow affects crystallization in a heterogeneous polyethylene oxide melt

Extension-induced crystallization in a heterogeneous polyethylene oxide (PEO) melt is investigated by small angle X-ray scattering (SAXS) and extensional rheology. The crystalline complex of PEO and sodium bromide is the heterogeneous component, which simulates the roles of crystal or strongly interacted additives during flow. It is aimed to demonstrate how the altered chain relaxation by the heterogeneous particle affects the crystallization. The main findings are listed as follows: (i) strain hardening occurs in heterogeneous melts but not in pure PEO. This indicates that the crystalline complex strengthens the entanglement network of free chains, leading to slower chain relaxation. (ii) In morphology, the orientation of lamellae becomes easier with the existence of the crystalline complex. A jump in the long period is also observed with large strain, accompanied by changes in the evolutional trend at the early stage. (iii) The crystallization kinetics converge with increasing strain; even the stress response significantly differs in different samples. Based on these findings, a quasi-network consisting of the crystalline complex and the entangled free chain is anticipated. Nucleation induced by the stretch of the quasi-network is supposed to lead to the change in crystallization observed.

Transition from chain- to crystal-network in extension induced crystallization of isotactic polypropylene

With a combination of extensional rheology and in-situ small-angle X-ray scattering measurements, the protocol of two-step extension is proposed to investigate the early stage of flow-induced crystallization (FIC) in supercooled isotactic polypropylene melt at 138 °C. After both step strains, the crystallization half-time presents a nonmonotonic dependence on the interval time between two extensional operations, based on which three different stages of structural evolution are defined. In stage I, both nucleation and chain relaxation occur, which enhances the crystallization rate but reduces the final lamellar crystal orientation. In this stage, each part of the melt is considered to have approximately the same dynamics to respond homogeneously to the second extension and thus the system is still dominated by a chain-network. When entering into stage II, the sparse large-scaled crystal is formed to construct a heterogeneous crystal-network superimposed on the chain-network, which decelerates the second ex...

Improving the Softness of BOPP Films:From Laboratory Investigation to Industrial Processing

Young’s modulus of biaxially oriented polypropylene (BOPP) films prepared with homemade film stretcher was investigated, which can be used to indicate the softness of films. It was found that the modulus of films was decreased by about 69% as the content of polyethylene (PE) added into polypropylene (PP) reached 30%. Also, increasing draw temperature can induce lower stress level during stretching, which may lead to the formation of crystals with low orientation level and thus decreased modulus of films. Based on laboratory study, BOPP films produced on commercial line were studied by differential scanning calorimetry (DSC), wide and small-angle X-ray scattering (WAXS, SAXS) with varying contents of PE. SAXS results show that the crystals are oriented in both machine direction (MD) and transverse direction (TD), and the crystals are more oriented in TD than MD according to the WAXS results for all films. Also, the orientation parameter of crystal along TD increases from 0.68 to 0.83 as the contents of PE increase from 0% to 25%. Meanwhile, the modulus of films in MD declines with increase of PE contents generally, improving the film softness. Orientation of crystals is thus an effective structure parameter to adjust the film softness. The relationship of processing-structure-property is also established.

Time-resolved orientation detection system with quantum cascade lasers

A system with the combination of quantum cascade lasers (QCLs) and a photoelastic modulator (PEM) has been designed and constructed, which can achieve orientation detection with a time resolution of nearly 20 µs based on the principle of infrared dichroism, several orders of magnitude higher than that of the general mid-infrared spectrum. PEM with an inherent frequency of 50 kHz is employed to modulate the polarization direction of infrared light rapidly, the controller of which is used to provide the external trigger signal. A double frequency and delay circuit is fabricated to match the frequency of QCLs and PEM as well as overcome the delay during transfer of the trigger signal to a QCL controller, which can realize a minimum delay resolution of 5 ns. Also, a data acquisition program is compiled to reduce the data size, making continuous collection possible and lowering difficulty in data processing. The system is combined with the home-made biaxial stretching equipment to conduct the sequential biaxial stretching of β-polypropylene (PP) films. It shows that the orientation factor of polymer chains increases from 0.04 to 0.36 during machine stretching, which decreases to 0 during transverse stretching, fitting well with the orientation factors estimated with FTIR. The result robustly proves the feasibility of the system for rapid orientation detection.

Counterion-Induced Nanosheet-to-Nanofilament Transition of Lyotropic Bent-Core Liquid Crystals

The smart flexibility of phase transitions in liquid crystals (LCs) makes them suitable for various applications and is an important research field in contemporary science, engineering, and technology. Unlike most reports focused on bent-core LCs in the thermotropic situation, in our present study, we designed and synthesized a fully rigid bent-core molecule with the sulfonic acid group replacing conventional flexible chains. A rich variety of counterion-induced supramolecular LC phase behaviors have been systematically investigated. It was found that the smectic phase with nanosheets tends to transform to the hexagonal phase with nanofilaments when the protons of the sulfonic acid group are partially replaced by alkali metal ions. The experimental results show that the nanoaggregate and phase transition are controlled by the displacing ratio of alkali metal ions rather than the molecular concentration. Another interesting feature is that the achiral bent-core molecules self-assemble into columns by helical stacking and present macroscopic chirality, indicating that spontaneous chiral symmetry breaking occurs in the columnar phase. The fully rigid bent-core molecules reveal surprisingly hierarchical molecular self-assemblies with the smectic-to-hexagonal phase transition, which was not previously observed in supramolecular complexes. The findings will provide new possibilities for applications in LC-based photonic devices, biosystem switches, and supramolecular actuators.