Rongni Du

Molecular weight dependence of hybrid shish kebab structure in injection molded bar of polyethylene/inorganic whisker composites.

In our previous work, a hybrid shish kebab structure, with polyethylene (PE) crystal lamellae periodically decorated on the surface of an inorganic whisker (SMCW) and aligned approximately perpendicular to the long axis of the whisker, has been observed in the injection molded bar of PE/SMCW composites. To investigate the effect of the molecular weight of the PE matrix on the formation of the hybrid shish kebab structure and the corresponding physical properties of HDPE/SMCW composites, in this work, three types of PE with different molecular weights were used to prepare the composites. They were first melt blended and then subjected to dynamic packing injection molding (DPIM), in which the prolonged shear was exerted on the melt during the solidification stage. An obvious hybrid shish kebab (HSK) structure, with PE crystal lamellae closely packed on the surface of the SMCW, was found in the samples with a low molecular weight PE (LMW-PE) matrix and a medium molecular weight PE (MMW-PE) matrix. However, in samples with a high molecular weight PE (HMW-PE) matrix, an incomplete HSK structure with PE crystal lamellae loosely decorated on the surface of the SMCW was observed. Furthermore, DSC results indicated that SMCW served as a good nucleating agent only for the composite with a LMW-PE matrix and the nucleation efficiency decreased with increasing PE molecular weight. Correspondingly, the tensile strength of the PE/SMCW composites was significantly improved by adding SMCW for the samples with a LMW-PE or MMW-PE matrix. Especially for samples with a LMW-PE matrix, the tensile strength was remarkably enhanced by the presence of only 1 wt % SMCW. For the composites with a HMW-PE matrix, the addition of SMCW had almost no reinforcing effect on the composites. The molecular weight dependence of the formation of HSK and property enhancement was discussed on the basis of the chain mobility and crystallization capability of the PE matrix.

Unexpected molecular weight dependence of shish-kebab structure in the oriented linear low density polyethylene/high density polyethylene blends

In this study, highly oriented shish-kebab structure was achieved via imposing oscillatory shear on the melts of linear low density polyethylene (LLDPE)/high density polyethylene (HDPE) blends during the packing stage of injection molding. To investigate the effect of molecular weight of HDPE on the formation of shish-kebab structure, two kinds HDPE with large melt flow index (low molecular weight) and small melt flow index (high molecular weight) were added into LLDPE matrix. The structural characteristics of LLDPE/HDPE blends were systematically elucidated through two-dimensional wide-angle x-ray scattering, scanning electron microscopy, and differential scanning calorimetry. Interestingly, an unexpected molecular weight dependence of shish-kebab structure of the prepared samples was found that the addition of HDPE with low molecular weight resulted in an higher degree of orientation, better regularity of lamellar arrangement, thicker lamellar size, and higher crystal melting temperature than that adding HDPE with high molecular weight. Correspondingly, the blend containing low molecular weight HDPE had better tensile strength. A possible mechanism was suggested to elucidate the role of HDPE molecular weight on the formation of shish-kebab structure in the oriented blends, considering the change of chain mobility and entanglement density with change of molecular weight.

EFFECT OF MOLECULAR WEIGHT OF PDMS ON MORPHOLOGY AND MECHANICAL PROPERTIES OF PP/PDMS BLENDS

A series of polydimethylsiloxane (PDMS) with varied molecular weights (Mw = 3 × 106, 1 × 106 and 0.5 × 106) were melt blended with PP to investigate the effect of PDMS molecular weight (MW) on the morphology and mechanical properties of PP/PDMS blends. Scanning electron microscopic (SEM) examination showed that the size of PDMS domains was dependent on the MW of PDMS. It was found that the lower the value of PDMS MW, the better dispersion of the PDMS domains in the PP matrix. Tensile and Izod impact tests revealed that the addition of PDMS with lower MW would lead to a more significant increase in impact strength of the blends compared with the blends with higher MW ones, while the influence of the molecular weight on tensile strengths of the blends was relatively small in the MW range studied. Differential scanning calorimetry (DSC) results also showed that the crystallization temperature of PP was increased with decreasing PDMS MW, indicating a better nucleation capability of lower MW of PDMS. Melting flow rate (MFR) measurements indicated that the processibility of PP could be enhanced by adding PDMS, and again the lower MW PDMS resulted in better data. Our work demonstrates that not only the processibility but also the mechanical properties of PP could be enhanced to a more significant degree by using low MW PDMS than the higher ones.

Preparation of a thermally conductive biodegradable cellulose nanofiber/hydroxylated boron nitride nanosheet film: the critical role of edge-hydroxylation

Chemical modification of boron nitride nanosheets (BNNSs) is a simple but effective method to improve their dispersion and compatibility within polymer nanocomposites. The key point is how to import compatible functional groups but simultaneously retain the intact crystal lattice. In this study, two different hydroxylation methods, namely nitric acid oxidation and aqueous ball milling, were adopted for surface modification of BNNSs. And a comparative study was carried out in terms of hydroxyl (OH) content, position of modification and the thermal conductivity (TC) after mixing them with a biodegradable cellulose nanofiber (CNF). TEM mapping, XRD and HRTEM indicated that nitric acid oxidation mainly resulted in an in-plane hydroxylation of BNNSs (POH-BNNSs) whose crystal lattice was seriously damaged, while aqueous ball milling led to most likely an edge hydroxylation of BNNSs (EOH-BNNSs) whose crystal lattice could be well preserved. As a result, strikingly, the TC of POH-BNNS filled nanocomposites only showed a little growth (10.2%) at a low OH content or even decreased at a higher OH loading, while an impressive thermal enhancement of almost 100% was achieved for CNF/EOH-BNNS nanocomposites. Such a substantial improvement signifies the critical role of edge-hydroxylation, which can simultaneously reduce interfacial thermal resistance and retain the intact basal crystal lattice to facilitate acoustic phonon transfer. This research provides deep insight into modification of two-dimensional nanomaterials from the point of view of the defect position, and an edge-selective modification strategy is expected to be of great advantage for the fabrication of highly thermally conductive nanocomposites.

MORPHOLOGY AND MECHANICAL PROPERTIES OF POLYPROPYLENE WITH in situ FORMED POLYAMIDE-6 FIBRILS

in situ Fibril formation of polyamide-6 (PA6) in isotropic polypropylene (iPP) was first fabricated using a slit die extrusion and hot stretching process. Then the prepared materials were subjected to injection molding in the temperature range higher than the melting temperature of iPP but lower than that of PA6. The obtained injection-molded samples were characterized via scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and two-dimensional wide-angle X-ray scattering (2D-WAXS). Mechanical properties were also investigated. The SEM result shows that the optimum fibril formation could be only achieved in the range of 20 wt% to 30 wt% of PA6 content for the studied system. The fibril morphology changes along the sample thickness in the injection-molded bars. The fibril morphology in the skin layer was better than that in the core layer. 2D-WAXS results showed that the orientation of PP decreased with the increase of PA6 content, which indicated that the orientation of PP was confined by PA6 fibrils. Combined consideration of mechanical properties and morphology indicates that only PP/PA6 composites with 20 wt% of PA6 content show better properties because of the better fibril morphology and PP chain orientation.