Chunguang Shao

Unusual structural evolution of poly(lactic acid) upon annealing in the presence of an initially oriented mesophase

Uniaxial deformations of amorphous poly(lactic acid) (PLA) films were performed at two different temperatures, 70 and 80 °C, at various draw strains. The samples deformed at 70 °C showed a strain-induced mesophase, and the structural ordering and thermal stability increased as the draw strain increased. Further annealing was performed in situ at constant length at the drawing temperature of 70 °C for the films drawn to strains of 100% and 230%. Unusually, we found that after annealing, the crystal structure of the film at lower strain was more ordered than the one at higher strain. Further investigations revealed that upon annealing the structural evolution followed a distinct molecular mechanism for the samples stretched to the two draw strains. For the sample drawn to 100%, the mesophase melted very quickly upon annealing, resulting in chain randomization and the release of the constraints on the thermodynamic relaxation of the oriented amorphous chains. The chain relaxation motions had a beneficial effect on the occurrence of the conformational rearrangements that are necessary for crystalline ordering. By contrast, for the 230% sample, the melting of the mesophase was slow and most of the chain orientations were preserved upon annealing. As a result, a less ordered crystal structure was formed since the local relaxation motions that are necessary for promoting crystalline order via conformational rearrangements were hindered.

Nonisothermal crystallization kinetics of biodegradable poly(lactic acid)/zinc phenylphosphonate composites

The effect of zinc phenylphosphonate on the crystallization of poly(lactic acid) using differential scanning calorimetry operating in dynamical mode at various cooling rates is reported. Experimental data were analyzed using the Avrami, Tobin, and Ozawa models. It is concluded that the addition of zinc phenylphosphonate modifies the crystallization process of poly(lactic acid) (changing the value of the Avrami exponent). Various parameters such as the crystallization half-time and crystallization rate constant reflect that zinc phenylphosphonate significantly accelerates the crystallization process. The activation energy value of the crystallization of poly(lactic acid), determined by the Kissinger method, increases with the addition of zinc phenylphosphonate.

Transcrystallization in nanofiber bundle/isotactic polypropylene composites: effect of matrix molecular weight

Polyamide 66 (PA 66) nanofiber bundles were first electrospun and then introduced into isotactic polypropylene (iPP) melts to prepare nanofiber bundle/iPP composites. To reveal the influences of matrix molecular weight (Mn) on the transcrystalline layer, three kinds of iPP with different Mn were adopted. Polarized optical microscope was employed to investigate the transcrystallinity. In the presence of PA 66 nanofiber bundle, the heterogeneous nucleation distinctly happened in iPP melts. Moreover, the higher the iPP Mn, the denser the nuclei. Both a decrease in matrix Mn and an increase in isothermal crystallization temperature led to an increase in the induction time. The maximum temperature at which the transcrystalline layer can be optically observed increased with the increase of Mn. The growth rate of transcrystallinity decreased with the increasing Mn and crystallization temperature. Moreover, selective melting of the transcrystalline layers confirmed that it was merely composed of α form crystal for all composites.

Simulation of Jetting in Injection Molding Using a Finite Volume Method

In order to predict the jetting and the subsequent buckling flow more accurately, a three dimensional melt flow model was established on a viscous, incompressible, and non-isothermal fluid, and a control volume-based finite volume method was employed to discretize the governing equations. A two-fold iterative method was proposed to decouple the dependence among pressure, velocity, and temperature so as to reduce the computation and improve the numerical stability. Based on the proposed theoretical model and numerical method, a program code was developed to simulate melt front progress and flow fields. The numerical simulations for different injection speeds, melt temperatures, and gate locations were carried out to explore the jetting mechanism. The results indicate the filling pattern depends on the competition between inertial and viscous forces. When inertial force exceeds the viscous force jetting occurs, then it changes to a buckling flow as the viscous force competes over the inertial force. Once the melt contacts with the mold wall, the melt filling switches to conventional sequential filling mode. Numerical results also indicate jetting length increases with injection speed but changes little with melt temperature. The reasonable agreements between simulated and experimental jetting length and buckling frequency imply the proposed method is valid for jetting simulation.

Effect of Rapid Compression on the Crystallization Behaviour of Polyethylene

The crystallization behaviour of PE samples with different molecular weight was investigated through two different processes, concerning rapid compression (RC) and quench cooling (QC). The scanning electron micrographs, wide angle X-ray diffraction and Raman results showed that all the recovered samples were composed of spherulites. For the QC process, the effect of molecular weight on the spherulites morphologies was slight, but for the RC process, with increased molecular weight (MW), the growth rate as well as the final average diameter of the spherulites decreased remarkably. Additionally, for samples with the same MW, the mass fraction of the rigid amorphous phase of RC samples was lower. Furthermore, the crystallinity distributions in the longitudinal and latitude directions of the RC samples were both more uniform than those of the QC samples, implying that in the RC process, the pressure distribution was quite uniform and there was no pressure decay.