Ru Xia

Study on the solidification kinetics of high-density polyethylene during thin-walled injection molding process

Abstract In this work, the effect of the initial and secondary temperature differences on the solidification behaviors of high-density polyethylene (HDPE) during the thin-walled injection molding (TWIM) was intensively investigated. Simulated temperature profiles using the enthalpy transformation methodology were compared with an in situ temperature measurement, and reasonable agreement was achieved between calculations and measurements. Two-dimensional wide-angle X-ray diffraction characterization shows that the formation of oriented crystal structures was considerably affected by the thermal gradient within the injection-molded article. The present study can be practically significant to the optimization of the cooling parameters during the TWIM of crystalline polymers as well as to the further study on the relationship among “processing-structure-property” of polymeric materials.

Correlation Between Solidification Behavior and Melt Crystallization Kinetics of Isotactic Polypropylene (iPP) during Injection Molding

The relationship between solidification behavior and crystallization kinetics, during the injection molding process of isotactic polypropylene (iPP) was investigated. It was found that the crystallization rate was in proportion to the cooling rate. The existence of a turning point (TP) in the dimensionless temperature (θ) vs. ln t curve, which is potentially applicable for the estimation of cooling time, was experimentally verified for iPP. The present study is expected to be useful for the optimization of the processing conditions during injection molding of crystalline polymers, and also supplies a good insight into further study of the “processing-structure-property” relationship of polymeric materials.

Thermorheological Properties and Thermal Stability of Polyethylene/Wood Composites

The thermal stability, flame retardancy, thermorheological, and mechanical properties of polyethylene/wood flour (PE/WF) composites were characterized. By time–temperature superposition treatment, addition of WF was found to lead to a complexity in the thermorheological behaviors in low-density PE/wood composites. However, high-density PE/wood counterparts showed no obvious thermorheological complexity. The effects of WF and ammonium polyphosphate contents on the thermorheological behavior and thermal stability were also studied. The current work should be of practical significance for the optimization of wood/plastic composite) formulae, as well as for further investigations on correlations between processing and performance of polymer composites.

Preparation and study on thermal conductive composites of chlorinated polyethylene rubber reinforced by boron nitride particles

The composites of chlorinated polyethylene rubber (CM) filled with boron nitride (BN) were prepared and examined systematically. Scanning electron microscopy (SEM) was used to observe microscopic morphology of composites. Mechanical properties of composites were analyzed by a rubber process analyzer (RPA) and dynamic mechanical thermal analyzer (DMTA). Thermal conductivity as well as thermo stability of composites was improved by adding BN particles into the CM rubber. It was found that BN particles can reinforce the rubber matrix while they also break down the network of polymer chains and ruin the physical properties of the rubber matrix. During the dynamic compressing process, BN particles can transfer heat from the matrix and alleviate the heat build-up phenomenon. The BN/CM composites (volume content CBN=18%) with thermal conductivity 1.179 W/(m·K) and good flexibility (Elongation at break=320%) were prepared, which may be used as thermal interface materials in a dynamic compressing process.

Effect of different nanofillers on non-isothermal crystallization kinetics and electric conductivity of dynamically-vulcanized PP-EPDM Blends

In this study, nanofillers composed of hydroxylated carbon nanotubes (h-CNT), carbon nanotubes (CNT) and graphene (GR) were separately added into the dynamically-vulcanized polypropylene (PP)/ethylene-propylenediene monomer (EPDM) blend. Differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and electrical resistivity measurements were employed to study the effect of nanofillers on the melt behavior, non-isothermal crystallization behavior and electrical conductivity of the prepared composites. WAXD results showed that h-CNT had a better induction effect of β-PP in the nanocomposites. The sequence of the activity in inducing the formation of β-PP was h-CNT>GR>CNT. However, the total crystallinity of the nanocomposites nearly remained constant. Non-isothermal crystallization kinetic analysis indicated that the presence of nanofillers improved the crystallization rate of the nanocomposites. The consequence of nucleation activity was as follows: CNT>GR>h-CNT. Although EPDM hindered the macromolecular motion of PP, interestingly it could increase the crystallization rate to an extent. Besides, the influence of nanofillers on enhancing the conductive property of the nanocomposites can be ranked as follow: CNT>GR>h-CNT.

Effect of selective nanoparticles on phase separation of copolymer–nanoparticle composites confined between two neutral surfaces

Using Monte Carlo simulation, we investigated the phase separation of copolymer–nanoparticle composites confined between two neutral surfaces. The incorporated nanoparticles are selective to the copolymer blocks, being repulsive to one block and attractive to the other. We constructed a series of phase diagrams for copolymer nanocomposites phase separated under varied film thicknesses. With the increase of concentration of nanoparticles, a morphological transition from lamellae to perforated lamellae, cylinders, and spheres was observed in the films. The effect of system parameters on the formation and orientation of these structures was studied in detail. We also investigated the evolution of the configuration of the chains and their composed blocks, as a function of concentration and selectivity strength of the nanoparticles.

Three-Dimensional Simulation of the Shrinkage Behavior of Injection-Molded Poly Lactic Acid (PLA): Effects of Temperature, Shear Rate and Part Thickness

The effects of injection temperature, shear and part thickness on the linear shrinkage of injection-molded poly (lactic acid) (PLA) were intensively analyzed using the Autodesk Moldflow software. The obtained results showed that both melt temperature and shear rate had obvious effects on the linear shrinkage of PLA, i.e., the linear shrinkage of PLA increases significantly with the increase of melt temperature and shear rate. In addition, the shrinkage of high-crystallinity PLA was remarkably larger than that of low-crystallinity PLA, and thin-walled parts was larger than thick-walled ones in shrinkage.

Preparation of pH-sensitive amphiphilic block star polymers, their self-assembling characteristics and release behavior on encapsulated molecules

Poly(ethylene glycol) (PEG), a polymer with excellent biocompatibility, was widely used to form nanoparticles for drug delivery applications. In this paper, based on PEG, a series of pH-sensitive amphiphilic block star polymers of poly(ethylene glycol)-block-poly(ethoxy ethyl glycidyl ether) (PEG-b-PEEGE) with different hydrophobic length were synthesized by living anionic ring-opening polymerization method. The products were characterized using 1H NMR and gel permeation chromatography. These copolymers could self-assemble in aqueous solution to form micellar structure with controlled morphologies. Transmission electron microscopy showed that the nanoparticles are spherical or rodlike with different hydrophilic mass fractions. The pH response of polymeric aggregates from PEG-b-PEEGE was detected by fluorescence probe technique at different pH. A pH-dependent release behavior was observed and pH-responsiveness of PEG-b-PEEGE was affected by the hydrophobic block length. These results demonstrated that star-shaped polymers (PEG-b-PEEGE) are attractive candidates as anticancer drug delivery carriers.

High-efficiency synthesis of dendrimer-like poly(ethylene oxide) via “arm-first” approach

In this study, a dendrimer-like polymer based on poly(ethylene oxide) (PEO) was synthesized through a combination of anionic ring-opening polymerization (AROP) and click reaction via arm-first method. Firstly, the polymeric arm, a linear PEO with one alkynyl group and two bromo groups, was synthesized by AROP of ethylene oxide followed by functionalization with propargyl bromide and esterified with 2-bromopropionic bromide. Second, a star PEO carrying three azide groups was synthesized though AROP of ethylene oxide used 1,1,1-tris(hydrosymethyl) ethane as initiator followed esterificated with 2-bromopropionic acid and azidation. By azide–alkyne click reactions between the azide-terminated PEO star polymer and linear PEO with functionalization alkynyl group, a three generation dendrimer-like PEO, G3-PEO-24Br, was successfully synthesized. The resulting polymers were observed to have precisely controlled molecular weights and compositions with narrow molecular weight distributions.

Monte Carlo simulations of the phase separation of a copolymer blend in a thin film.

Monte Carlo simulations were carried out to study the phase separation of a copolymer blend comprising an alternating copolymer and/or block copolymer in a thin film, and a phase diagram was constructed with a series of composed recipes. The effects of composition and segregation strength on phase separation were discussed in detail. The chain conformation of the block copolymer and alternating copolymer were investigated with changes of the segregation strength. Our simulations revealed that the segment distribution along the copolymer chain and the segregation strength between coarse-grained beads are two important parameters controlling phase separation and chain conformation in thin films of a copolymer blend. A well-controlled phase separation in the copolymer blend can be used to fabricate novel nanostructures.