Jian-Ming Feng

Poly(ethylene terephthalate)/polyethylene composite based on in-situ microfiber formation

Poly(ethylene terephthalate) (PET) microfiber was in-situ formed by compounding PET with polyethylene (PE) through a single screw extruder of a Haake rheometer system, where a rod die with comparatively smaller diameter (2.1 mm) was used, and the extrudate was drawn in a certain drawing ratio (3.1:1) and quickly cooled in cold water. Subsequently, the in-situ PET/PE composite was injection molded into specimens at temperatures that were lower than the melting temperature of the PET to keep the original shape of the PET fibers. The result from morphology observations of the composite showed that when the die diameter of the extruder is 2.1 mm and the drawing ratio of the extrudate is 3.1:1, PET was more or less changed into microfibers. The PET almost changed into fibers when the concentration was 15 wt%; concentrations below and above which decreased the fiber content. For the PET/PE blend prepared by conventional mixing technology, the dispersed PET formed spheres and no microfibrilar structure were found. The reinforcing effect of the PET fibers on the corresponding composite was significant at 15 wt% PET concentration observed from the correlation between the PET content and the tensile properties of the PET/PE in-situ composite. Besides, in general, the tensile strength and modulus of the composite increased with the PET concentration, and was higher than the conventional PET/PE blend without microfibers.

Role of gas delay time on the hierarchical crystalline structure and mechanical property of HDPE molded by gas-assisted injection molding

The relationship among the processing parameters, crystalline morphology, and macroscopic properties in injected molded bar becomes very complicated due to existence of temperature gradient and shear gradient along the sample thickness. To enhance the shear strength, gas-assisted injection molding (GAIM) was utilized in producing the molded bars. The aim of our research was to explore the relationship between processing conditions and the spatial variation of the hierarchy structure as well as the mechanical properties of high-density polyethylene (HDPE) obtained via GAIM. In our previous work [Wang L, Yang B, Yang W et al (2011) Colloid Polym Sci 289:1661–1671], we found that the enhancement of the gas pressure can remarkably increase the degree of molecular orientation in the HDPE samples, which turns out to improve the mechanical performances of GAIM parts. In this work, the hierarchy structure, orientation behavior, and mechanical properties of molder bars under different gas delay time were investigated using a variety of characterization techniques including rheological experiments, scanning electron microscope, tensile testing, differential scanning calorimetry, and two-dimensional wide-angle X-ray scattering. Moreover, the temperature field during the short shot stage of GAIM process was simulated using an enthalpy transformation approach. Our results indicate that these properties were intimately related to each other, and with prolonged gas delay time, GAIM samples with higher degree of orientation and improved mechanical properties were obtained.

Suppressing phase coarsening in immiscible polymer blends using nano-silica particles located at the interface

The morphologies of polymer blends generated during processing are usually unstable and phase coarsening often occurs in the melt state, so suppressing the morphology coarsening is crucial to obtain polymer blends with tailored and stable structure and properties. The effect of nano-silica particles located at the interface on the phase coarsening of a polypropylene (PP)/polystyrene (PS) blend was studied in this work. In co-continuous 50/50 PP/PS blend, the particles at the interface can effectively suppress the coarsening process even at a very low particle loading. Real-time observation conducted by using an optical microscope equipped with a camera and a hot stage showed that a small loading of particles has little effect on the retraction process but can suppress the coalescence and at high loading of particles, both the retraction and coalescence process can be significantly suppressed. The suppressing effect towards the coalescence was confirmed in 70/30 blend with PS phase as the dispersed phase. The stabilization mechanism used in particle stabilized emulsions was adopted to explain the suppressing effect of nano-silica particles located at the interface towards the phase coarsening of PP/PS blends.

Hierarchically oriented crystalline structures of HDPE induced by strong second melt penetration

Recently, a melt-penetrating process in which the first melt suffered from only one direction penetrating action was achieved by our home-made multi-melt multi-injection molding (MMMIM). In this work, a high-density polyethylene (HDPE) melt was penetrated by a high-speed second HDPE melt via an MMMIM instrument. It was found that hierarchically oriented crystalline structures were generated in the melt-penetrating sample along the thickness, investigated by SEM, synchrotron 2D-WAXD and 2D-SAXS; however, only isotropic spherulites were formed in non-melt-penetrating samples. 2D-WAXD/2D-SAXS results demonstrated that in the melt-penetrating sample, the degree of orientation in the subskin layer was larger than that in other layers, and confirmed the existence of the shish–kebab structures.

A novel hierarchical crystalline structure of injection-molded bars of linear polymer: co-existence of bending and normal shish–kebab structure

The hierarchy structures and orientation behavior of high-density polyethylene (HDPE) molded by conventional injection molding (CIM) and gas-assisted injection molding (GAIM) were intensively examined by using scanning electronic microscopy (SEM) and 2D wide-angle X-ray diffraction (2D-WAXD). Results show that the spatial variation of crystals across the thickness of sample molded by CIM was characterized by a typical skin–core structure as a result of general shear-induced crystallization. Unusually, the crystalline morphologies of the parts prepared by GAIM, primarily due to the penetration of secondary high-compressed gas that was exerted on the polymer melt during gas injection, featured a richer and fascinating supermolecular structure. Besides, the oriented lamellar structure, general shish–kebab structure, and common spherulites existed in the skin, sub-skin, and gas channel region, respectively; a novel morphology of shish–kebab structure was seen in the sub-skin layer of the GAIM parts of HDPE. This special shish–kebab structure (recognized as “bending shish–kebab”) was neither parallel nor perpendicular to the flow direction but at an angle. Furthermore, there was a clear interface between the bending and the normal shish–kebab structures, which may be very significant for our understanding of the melt flow or polymer rheology under the coupling effect of multi-fluid flow and complex temperature profiles in the GAIM process. Based on experimental observations, a schematic illustration was proposed to interpret the formation mechanism of the bending shish–kebab structure during GAIM process.

An unusual transition from point-like to fibrillar crystals in injection-molded polyethylene articles induced by lightly cross-linking and melt penetration

Recently, a melt penetrating process in which a first polymer melt is rapidly penetrated by a second polymer melt has been realized on our home-made multi-melt multi-injection molding (MMMIM) machine. Although great deformation can be provided by the rapid melt penetration process, it has been found that hardly any oriented crystalline structures can be kept and formed due to the quick chain relaxation at high temperatures. In the present work, lightly cross-linked structures were introduced to prolong the relaxation time of linear high density polyethylene (HDPE) molecular chains. The hierarchical structures of MMMIM samples were characterized by scanning electron microscopy (SEM), polarized light microscopy (PLM) and two-dimensional small angle X-ray scattering (2D-SAXS). It was found that the melt penetrating process promoted the formation of cylindritic crystalline structures in the subskin layer, whereas only isotropic spherulites were formed in the subskin layer of the corresponding conventional injection molding (CIM) sample. From linear to lightly cross-linked macromolecular chain structures, a transition from cylindritic structures composed of banded-spherulites along the flow direction towards shish–kebab-like structures was observed in the subskin layer of the MMMIM samples, and also the distances between two nuclei decreased as well as the orientation degree increased gradually in the transition layer. These results indicate that lightly cross-linked HDPE structures with longer relaxation times are beneficial to keep the point-like nuclei along the flow direction and are helpful for the transition to shish–kebab-like structures with thread-like nuclei. Modified models are proposed to interpret the mechanism of the formation of shish–kebab-like structures under the melt penetrating of samples with lightly cross-linked structures.

Thermorheology and Crystallization Behaviors of Polyethylenes: Effect of Molecular Attributes

Correlations between polyethylenes of different compositions and branching architectures and the temperature dependence of their viscoelastic behavior as well as the dependence of the nonisothermal crystallization behaviors on the cooling rate were described. To analyze the thermorheological behavior of the various classical polyethylenes, a method proposed by van Gurp and Palmen was utilized and the classical high-pressure low-density polyethylene (LDPE) was found to be thermorheologically complex, while for high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE), thermorheological simplicity was observed. The Avrami and Mo methods were applied to describe the nonisothermal crystallization kinetics of the different PEs for various cooling rate. The values of the kinetic parameter F(T), kinetic crystallization rate constant (Zc), and half-time of crystallization (t1/2) indicated that long-chain branching (LCB) had the role of being a heterogeneous nucleating agent and accelerated the ...