Transformation of β to α phase of isotactic polypropylene nucleated with nano styrene butadiene rubber-based β-nucleating agent under microwave irradiation

This paper investigates the effect of microwave irradiation on theβtoαphase transformation of theβ-nucleated isotactic polypropylene(iPP).Ten microwave irradiation cycles was applied to the iPP and iPP modified with 0.3 wt%and 0.5 wt%β-NA,and the data at 2nd,4th,6th,8th and 10th irradiation were reported.As expected,the sample temperature was found to increase with the irradiation time,by more than 130°C,due to high frequency of microwave processing.This was the major factor that induced theβ-phase transformation and structural change.Both the differential scanning calorimetry(DSC)and X-ray diffraction(XRD)results indicated thatβ-phase was mainly transformed toα-phase and partially converted to the amorphous section.It was reflected as 1)the reduction of the enthalpy ofβ-crystal melting(ΔHmβ),2)the increased enthalpy ofα-crystal melting(ΔHmα),3)the decreasedβ-crystalline phase fraction(Kβ)and 4)the decrease of the overall degree of crystallinity(Xall).Under impact force,neat iPP showed a slight increase in the impact strength with the irradiation time,due to the increase of amorphous region.For theβ-iPP,it decreased due to the reduction of theβ-phase content.

Enhanced permeation performance of polyether-polyamide block copolymer membranes through incorporating ZIF-8 nanocrystals

Membrane-based CO_2 separation is a promising alternative in terms of energy and environmental issues to other conventional techniques. Polyether-polyamide block copolymer(Pebax) membranes are promising for CO_2 separation because of their excellent selectivity, but limited by their moderate gas permeability. In this study,fresh-prepared zeolitic imidazolate framework-8(ZIF-8) nanocrystals were integrated into the Pebax?1657matrices to form mixed matrix membranes. The resulting membrane exhibits significantly improved CO_2permeability(as high as 300% increase), without the sacrifice of the selectivity, to the pristine polymer membrane. Several physical characterization techniques were employed to confirm the good interfacial interaction between ZIF-8 fillers and Pebax matrices. The effect of added ZIF-8 fillers on the transport mechanism through MMMs is also explored. Mixed-gas permeation for both CO_2/N_2 and CO_2/CH_4 was also evaluated. The separation performance for CO_2/CH_4 mixtures on the ZIF-8/Pebax MMMs is very close to the Roberson upper bound, and thus is technologically attractive for purification of natural gas.

Impact of Zn(Ⅱ)Ions on Crystallization and Thermal Properties of Poly(lactic acid)

The issues of low crystallinity and slow crystallization rate of poly(lactic acid)(PLA)have been widely addressed.In this work,we find that doping PLA with Zn(Ⅱ)ions can speed up the process of crystallization of PLA.Three kinds of Zn(Ⅱ)salts(ZnCl2,ZnSt and ZnOAc)were tested in comparison with some other ions such as Mg(Ⅱ)and Ca(Ⅱ).The increased crystallinity and crystallization rate of PLA doping with Zn(Ⅱ)are reflected in FT-IR and variable temperature Raman spectroscopy.The crystallinity is further confirmed or measured with differential scanning calorimetry and X-ray diffraction.The crystallinity rate of the PLA/ZnSt-0.4 wt%material can reach 22.46% and the crystallinity rate of the PLA/ZnOAc-0.4 wt%material can reach 24.83%,as measured with differential scanning calorimetry.

Microstructure, morphology, crystallization and rheological behavior of impact polypropylene copolymer

Impact polypropylene copolymer (IPC), named polypropylene catalloy, not only possesses excellent impact property, but also presents good rigidity. Its superior performances result from the complicated composition and microstructure. In the present article, recent progress in the studies on microstructure, morphology, crystallization and rheological behavior of IPC is summarized, and findings of the authors and their collaborators are reported. In general, IPC is divided into three components, i.e., ethylene-propylene random copolymer (EPR), a series of different segment lengths ethylene-propylene copolymer (EbP) and propylene homopolymer. The reasonable macromolecular structures of EbP and a multilayered core-shell model of dispersed phase structure in IPC were proposed, in which the dispersed phase consists of an outer EbP shell, an inner EPR layer and an EbP core. It is found that the annealing at melt-state may lead to an abnormal phase inversion, and the phase inversion disappears when temperature cools down to room temperature. The cause of phase inversion is ascribed to the existence of EbP component, which results in the stronger activity of the dispersed phase. The crystalline structure and morphologic results confirm the formation of β-iPP in IPC. Furthermore, it is found that the ethylene content in IPC and cooling rate of the samples have an important influence on the formation of β-iPP. Based on the crystallization kinetics analyzed by Lauritzen-Hoffman theory, crystallization behavior of different IPC samples is discussed and it is proposed that the dilution effect of ethylene propylene copolymer has a more remarkable influence on surface nucleation than on crystal growth. In addition, annealing at high temperature can result in the changes of chain structure for IPC, and this instability is ascribed to the oxidative degradation and crosslink reaction mainly in iPP component.

Cell thickness dependence of electrically tunable infrared reflectors based on polymer stabilized cholesteric liquid crystals

We reported here the fabrication of the elec- trically tunable infrared （IR） reflectors based on the polymer stabilized cholesteric liquid crystal （PSCLC） with negative dielectric anisotropy. A systematic study of the influence of cell gap on the electrically tunable reflection bandwidth was performed. When a direct current （DC） electric field was ap- plied, the reflection bandwidth red shifted in the cells with small cell gap, whereas the bandwidth broadening was ob- served in the cells with large cell gap. It is therefore reasonable to deduct that the reflection is dictated by the pitch gradient steepness which strongly relies on the cell thickness. The re- sults reveal that for making PSCLC based IR reflector windows with electrically induced bandwidth broadening, a minimal cell gap thickness is required. The resulted IR reflectors pos- sess a short native switching time and long-term operation stability, and are potentially applicable as smart energy saving windows in buildings and automobiles.

Performance improvement of organic phototransistors by using polystyrene microspheres

Organic phototransistors （OPTs） have been intensively studied in recent years due to the combined ad- vantages of phototransistors and organic semiconductors （OSCs）. However, the electrical performance of OPTs is lar- gely limited by OSCs themselves, posing a challenge to further improve the performance of the devices. Preparing nano/mi- cro-structures of OSCs is considered as an effective way to improve the performance of OPTs. Polystyrene （PS） micro- sphere, as a kind of insulating and low-cost material, is ex- tensively used in fabricating nano/microporous structures, and the resulting devices exhibit high response to external stimuli. Therefore, we combined PS microspheres with OSCs to fabricate PS/OSC OPTs, and the Ilight/Idark ratio was en- hanced by two orders of magnitude compared with the pris- tine counterparts, which can be modulated from 46 to 1800 by controlling the diameters of PS microsphereso This strategy paves a way for developing high-performance OPTs with nano/microporous structures with potential applications in organic optoelectronics.

Mechanical and thermal properties of mesogen-jacketed liquid crystalline polymer/epoxy resin composites

The effects of mesogen-jacketed liquid crystalline polymer poly(dipropyl vinylterephthalate)(PDPVT) on the mechanical and thermal properties of diglycidyl ether of bisphenol-A(DGEBA) epoxy resin were investigated by impact test, tensile test and thermogravimetric analysis(TGA). The mechanism underlying the enhancement of mechanical properties of epoxy resin was studied using 1D wide-angle X-ray diffraction(WAXD) and scanning electron microscope(SEM). It was found that the mechanical properties of 1 wt%–5 wt% PDPVT/DGEBA composites were significantly improved compared to neat epoxy resin. Especially, the epoxy resin with 3 wt% PDPVT had the greatest increase in mechanical properties, with the impact strength, tensile strength and elongation while breaking increased by 87%, 59% and 174%, respectively. The increased mechanical strength was due to the fact that PDPVT maintained liquid crystalline phase in cured PDPVT/DGEBA composites, which would blunt the crack tip and prevent crack propagation. Moreover, PDPVT had slight effect on the thermal stability properties of epoxy resin.

Enhanced performance of field-effect transistors based on C_(60) single crystals with conjugated polyelectrolyte

Contact resistance at the interface between metal electrodes and semiconductors can significantly limit the performance of organic field-effect transistors, leading to a distinct voltage drop at the interface. Here, we demonstrate enhanced performance of n-channel field-effect transistors based on solution-grown C60 single-crystalline ribbons by introducing an interlayer of a conjugated polyelectrolyte （CPE） composed of poly[（9,9-bis（3＇-（（N,N-dimethyl）-N-ethylammonium）-propyl）-2,7-fluorene）- alt-2,7-（9,9-dioctylfluorene）] dibromide （PFN＋Br-）. The PFN＋Br- interlayer greatly improves the charge injection. Consequently, the electron mobility is promoted up to 5.60 cm2 V-1 s-1 and the threshold voltage decreased dramatically with the minimum of 4.90 V.