This is an editorial article. It has no abstract.
Poly(L-lactide)s (PLLA) tethered with octaglycidylether polyhedral oligomeric silsesquioxane (OPOSS) with contents of 0.02–1.00 mol% were successfully prepared by solution ring-opening polymerization of L-lactide in the presence of Sn(Oct)2 catalyst. Fourier transform infrared (FTIR) and proton nuclear magnetic resonance 1H-NMR spectroscopic techniques confirm the formation of secondary hydroxyls due to the reaction between PLLA main chains and OPOSS cages. X-ray analyses prove that the presence of OPOSS does not alter the packing structure of PLLA chain in the crystals but enhances the crystallinity of PLLA hybrids. PLLA/PLLA-OPOSS nanocomposites with PLLA-OPOSS contents of 1–30 wt% were prepared by solution-mixing of the neat PLLA polymer with various contents of the PLLA-OPOSS hybrid with 0.50 mol% OPOSS. It is found that the glass transition temperature of the PLLA/PLLA-OPOSS nanocomposites remains the same as that of pristine PLLA and the thermooxidative stability of the nanocomposites is improved with the PLLA-OPOSS content of 1–20 wt%, as compared to that of the neat PLLA polymer. PLLA/PLLA-OPOSS nanocomposites, except for PLLA/PLLA-OPOSS1, has higher crystallization rate at 120°C. The nucleation density increases with increase in the content of PLLA-OPOSS.
Ultrahigh molecular weight polyethylene (UHMWPE) was ram extruded using a temperature window effect. The extrusion pressure abruptly drops at a very narrow extrusion temperature window which is about 10°C higher than the theoretical melting point of orthorhombic polyethylene crystals under quiescent and equilibrium states. The correlation between extrusion pressure and parameters such as extrusion temperature, annealing condition, thermal history, piston velocity, L/D ratio of the die, and molecular weight of UHMWPE, was studied. The temperature window increases with molecular weight and is unaffected by thermal history and annealing. The stable extrusion pressure and the critical piston velocity decrease with the rise in the extrusion temperature. The flow resistance reversely depends on the L/D ratio of the die. This phenomenon is attributed to an extensional flow-induced chain alignment along the streamline, which results in the formation of a metastable mesophase with higher chain mobility.
The viscoelastic nature of the Poisson’s ratio of a semicrystalline poly (butylene terephthalate) is highlighted by investigating its dependence on time, temperature and strain rate, under two types of loading conditions: i) constant deformation rate tests, in which the transverse strain is measured in tensile ramps at various temperatures and at two strain rates; and ii) constant deformation tests, in which, under a constant axial deformation, the transverse strain is measured as a function of time in isothermal experiments performed at various temperatures. In both testing configurations, axial and transverse deformations are measured by means of a biaxial contact extensometer, and a correction procedure is adopted in order to compensate the lateral penetration of the extensometer knives. Poisson’s ratio displays the typical features of a retardation function, increasing with time and temperature, and decreasing with strain rate. This behaviour has been compared to that of simultaneously measured relaxation modulus.
Intrinsic fluorescence method was applied to study the miscibility and interactions of thermoplastic phenol formaldehyde resin (TPF) / poly(ε-caprolactone) (PCL) blends. The characteristic intrinsic fluorescence emission of TPF at 313 nm showed the very good sensitivity to monitor the macromolecular chain motion in the TPF/PCL blends. The glass transition (Tg), crystallization (Tc), and melting transition point (Tm) of TPF/PCL blends were measured by the temperature dependence of intrinsic fluorescence intensities upon heating or cooling process. Interestingly, when TPF/PCL ≥ 5/5, besides a Tg for the amorphous phase of blend, another transition at temperature a little higher than Tg of PCL can be observed by intrinsic fluorescence method. This microheterogeneity can be explained by the so-called ‘rigid amorphous phase’ (RAP) due to the good flexibility and the strong self-association of PCL chains in amorphous phase. Besides, the analysis of the dependence of Tg on the content of PCL suggests that this microheterogeneity can attenuate the interactions between TPF and PCL chains and result in a lowering of Tgs of blends. In view of the simplicity and sensitivity of measurement as well as affordability of instrument, intrinsic fluorescence proved to be an effective means for characterization of microstructural variation in polymer blends.
The paper investigates the effect of voids and hygrothermal conditions on bending, compressive, and interlaminar shear strength (ILSS) of T300/914 composite laminates. By adopting different autoclave pressures during a cure cycle, specimens with three different void contents ranging from 0.33% to 1.50% were obtained. Experimental results reveal that compressive, bending strength, and ILSS decrease with increasing void contents and immersion time. The most significant decrease in strength of aged specimens is in the ILSS and compressive strength, followed by the bending strength. The effect of voids on the absorption/desorption behavior is also discussed. For a similar porosity, the compressive strength and ILSS of dried specimens are higher than that of the aged specimens, while they are lower than that of the unaged specimens. The bending strength of the dried specimens is higher than that of the unaged specimens. The experimental results are explained by the supportive micrographs that illustrate different types of voids and their morphology before and after moisture absorption/desorption.
Preparation of nanocomposites by in situ polymerization of sodium chloroacetylaminohexanoate in the presence of Cloisite 20A (C20A) or Cloisite 30B (C30B) organo-modified montmorillonites was studied. Both clays rendered an intercalated structure that contrasts with the exfoliated structure previously found with the use of the C25A montmorillonite. Polymerization under non-isothermal and isothermal conditions was evaluated by Wide Angle X-ray Diffraction (WAXD) synchrotron radiation and Fourier Transform Infrared Spectroscopy (FTIR) experiments. Results indicate that C20A and C30B had a similar influence on the polymerization kinetics. Thus, the activation energy and the Arrhenius preexponential factor decreased compared to those calculated for the neat monomer. Clear differences were also found when using the C25A clay since, in this case, polymerization had similar activation energy to that determined for the neat monomer. The crystallization kinetics of the intercalated C20A and C30B nanocomposites was studied by FTIR and optical microscopy. The incorporation of clay particles increased the overall rate kinetic constant due to the enhancement of the primary nucleation. On the contrary, the spherulitic growth rate was slightly disfavored by the clay.
A new process technology modified from conventional coaxial electrospinning process has been developed to prepare polymer fibers from a high concentration solution. This process involves a pure solvent concentrically surrounding polymer fluid in the spinneret. The concentric spinneret was constructed simply by inserting a metal needle through a high elastic silica gel tube. Two syringe pumps were used to drive the core polymer solution and the sheath solvent. Using polyvinylpyrrolidone (PVP) as the polymer model, which normally has an electrospinnable concentration of 10% w/v in ethanol, it was possible to electrospin 35% w/v of PVP in the same solvent, when pure N, N-dimethylacetamide (DMAc) was used as sheath fluid. The resultant fibers have a smooth surface morphology and good structural uniformity. The diameter of the fibers was 2.0±0.25 µm when the DMAc-to-polymer-solution flow rate ratio was set as 0.1. The process technology reported here opens a new window to tune the polymer fibers obtained by the electrospinning, and is useful for improving productivity of the electrospinning process.
Polycaprolactone (PCL)/calcium sulfate (CS) whisker composites have been fabricated by melt blending and coprecipitation methods respectively. Scanning electron microscope (SEM) was used to observe the microstructure of the composites. The crystallization and thermal properties were characterized by polarized optical microscope (POM), X-ray diffractometry (XRD), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). For composites prepared by melt blending method, experiment results show that average length of the whiskers is shortened. The crystallization perfection of PCL in composites is improved by adding whiskers. The flexural strength increases whereas the impact strength decreases. For composites prepared by coprecipitation method, whisker addition worsens the crystallization perfection of PCL. An improvement of 21% in flexural strength and 22% in impact strength has been achieved for the composite with 15 wt% of whiskers.