How stress and temperature impact the movement of poly(lactic acid) (PLA) chains in the process of tensile film stretching was studied. The motion mode of chains was investigated through the study of the strain-induced crystallization and orientation through changes in the draw temperature (T_d), draw ratio, and draw rate. The crystallinity and orientation degrees of the PLA films were measured by differential scanning calorimetry, Fourier transform infrared spectroscopy, and polarized optical microscopy. According to the competition between the orientation caused by the stretching and relaxation of chains under the temperature field, the motion modes of PLA chains during strain were divided into four types, modes I–IV. When T_d was 100°C, the PLA chains acted in mode I, in which the relaxation rate of chains was so fast that no crystallinity or orientation could be obtained. Beyond a draw rate of 20 mm/min at a T_d of 90°C, the type of chain movement changed from mode I to II. In mode II, only crystallites could be reserved after unloading. Chains in the PLA film moved in mode III at a T_d of 80°C; then, both the crystallization and orientation were enhanced monophonically with increasing draw rate. Beyond the draw rate of 10 mm/min at a T_d of 70°C, the orientation rate of chains was much faster than the relaxation one, and the motion mode transformed from mode III to IV. Then, obvious decreases in the crystallinity and orientation were observed with further increases in the draw rate; this resulted from the destruction of the crystallites. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 42969.

In this study, an increase in the cooling rate of high-density polyethylene parts was carried out via a change in the fluid flow pattern to introduce gas cooling under a gas-assisted injection-molding process; this was conducive to the retention of orientation chains shaped during the injection stage and further developed into much more oriented crystals. Morphological observation showed that the parts without gas cooling (WOGC) were composed of oriented crystals except the gas channel zone, whereas the parts with gas cooling (WGC) were full of oriented crystals, especially much more interlocking shish-kebab structures in the subskin zone. The WGC parts had a higher degree of orientation than the corresponding zone of the WOGC parts. Although the lower crystallinity, the wider orientation regions, and much more interlocking shish-kebab structures led to considerable increases from 32 and 990 MPa in the WOGC parts to 36 and 1150 MPa in the WGC parts for the yield strength and elastic modulus, respectively. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40349.

The effect of annealing on the microstructural evolution and mechanical properties of high-density polyethylene parts molded via gas-assisted injection molding was investigated using scanning electron microscopy, differential scanning calorimetry, two-dimensional wide-angle X-ray diffraction and tensile testing. The results indicated that a variety of annealing temperatures could induce considerable variations in the hierarchical structures, crystallinity, lamellar thickness and yield stress of the molded bars. According to these results, the annealing temperatures could be divided into three regions. In the low-temperature region of annealing at 80 °C, the spatial variation of the superstructure developed along the thickness direction and mechanical properties of the annealed sample were mainly unchanged and similar to those of the original specimen. At 100 and 120 °C, the intermediate temperature region of annealing, the thickness of the crystals, degree of orientation and yield stress of annealed samples were greatly improved. Finally, at 127 °C, the degree of orientation decreased and yield stress slightly improved, an indication of the high-temperature annealing region being characterized by increasing melting/recrystallization and causing relaxation of oriented molecular chains. A model is proposed to interpret the mechanism of the annealing treatment of the samples at various temperatures. © 2013 Society of Chemical Industry

Wider zones with close-knit orientation crystals in high density polyethylene (HDPE) parts prepared via the gas-assisted injection molding (GAIM) process were obtained under high cooling gas pressure. In this study, compressed nitrogen, as a cooling medium, was introduced to retain a high cooling rate of the polymer melt. The high gas pressure leads to fast cooling of the polymer melt, which contributes to the stability of more oriented and stretched chains during the cooling stage. Then many more oriented structures are formed. SEM shows that many more oriented structures and interlocking shish-kebab structures are achieved in parts under highest cooling gas pressure (P3). The P3 parts possess a higher degree of orientation than the corresponding regions of parts under lowest cooling gas pressure (P1). Moreover, tensile testing indicates that, compared with P1 parts, although P3 parts have lower crystallinity, the mechanical properties are improved because of the wider orientation zone and many more interlocking shish-kebab structures. Combining the HDPE molecular parameters with the characteristics of the GAIM flow field and temperature field, the stability of oriented or stretched chains and the formation of orientation structures in various zones of the parts were analyzed. © 2014 Society of Chemical Industry

A small amount of high molecular weight molecules can have a dramatic influence on the flow-induced crystallization kinetics and orientation of polymers. To elucidate the effects of the high molecular weight component under a real processing process, we prepared model blends in which high density polyethylene with a high molecular weight and wide molecular weight distribution was blended with a metallocene polyethylene with a low molecular weight and very narrow molecular weight distribution. To enhance the shear strength, gas-assisted injection molding was utilized in producing the molded bars. The hierarchical structures and orientation behavior of the molded bars were intensively explored by using scanning electron microscopy and two-dimensional wide-angle X-ray diffraction, focusing on effects of the high molecular weight component on the formation of the shish kebab structure. It was found that there exists a critical concentration of high molecular weight component for the formation of a shish kebab structure. The threshold was about 5.5–7.0 times larger than the chain overlap concentration, suggesting an important role of entanglements of the high molecular weight component. Moreover, the rheological properties of molten polyethylene melts were studied by dynamic rheological measurements and a critical characteristic relaxation time for shish kebab formation was obtained under the processing conditions adopted in this research. © 2013 Society of Chemical Industry

The morphology distribution of a model polymer blend, polystyrene (PS)/polyethylene (PE), molded by multimelt multi-injection molding (MMMIM) process was studied by scanning electronic microscopy and polarizing light microscopy. An unusual double skin/core morphology was observed. The minor phase, PS, showed highly deformed morphology in both the skin layer near the mold wall and the core layer near the skin/core layer's interface. Meanwhile, in the regions that highly deformed PS phase showed, highly ordered cylindritic crystal structures of PE are also formed. As we all know the driving force and the basic prerequisite to deform the dispersed droplet and form the oriented crystal structure is the shear field. So an attempt was made to correlate the dispersed phase morphology, crystalline morphologies, and shear rate. The shear rate, estimated via the capillary number, across the thickness of the parts molded by MMMIM was bimodal. Even if the coalescence and relaxation of the dispersed phase during and after mold filling cannot be ignored, both the highly dispersed PS domains and the highly ordered crystal structure of PE showed in the regions with the maximum calculated shear rate, which is consistent with the generally accepted theories that strong shear flow is favorable to the formation of the oriented structures. POLYM. ENG. SCI., 54:2345–2353, 2014. © 2013 Society of Plastics Engineers

The relationship among the processing parameters, crystalline morphologies and mechanical properties of injected-molded bar becomes much complicated primarily due to the existence of temperature gradient coupled with the shear gradient along the sample thickness. The effect of thermal gradient field on the microstructural evolution, hierarchical structures and dynamic mechanical properties of high-density polyethylene parts molded via gas-assisted injection molding (GAIM) were investigated using scanning electron microscope, differential scanning calorimetry, dynamic mechanical analysis and two-dimensional wide-angle X-ray diffraction. The three-dimensional temperature profiles during the cooling stage under different melt temperatures of GAIM process were obtained by using a transient heat transfer model of the enthalpy transformation approach, and the phase-change plateaus were clearly observed in the cooling curves. It was found that a variety of melt temperatures could induce considerable variations of the hierarchical structures, orientation behavior and dynamic mechanical properties of the injection-molded bars. With reduced melt temperature, GAIM samples with higher molecular orientation and improved dynamic mechanical properties were obtained. Copyright © 2013 John Wiley & Sons, Ltd.

Poly(lactic acid) (PLA) chains are directly grafted onto a silicon surface by in situ amidation and PLA/PLA-grafted SiO₂ nanocomposites are compounded using a Haake torque rheometer. To have a better understanding of the interaction between grafted polymer chains and PLA matrix, thermal, and rheological properties of PLA/PLA-grafted SiO₂ nanocomposites are explored. DSC analysis shows that PLA-grafted-SiO₂ can accelerate the cold crystallization rate and increase the degree of crystallinity of PLA. Shear rheology testing indicates that PLA/PLA-grafted-SiO₂ nanocomposites still exhibits the typical homopolymer-like terminal behavior at low frequency range even at a content of PLA-grafted-SiO₂ of 5 wt %, compared to PLA/SiO₂, it is also found that the nanocomposites show stronger shear-thinning behaviors in the high frequency region after grafting. In addition, elongation viscosity testing shows the entanglement between grafted chains and matrix that is needed to improve the melt strength of PLA. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Graphene with amine group was covalently grafted on the polyurethane foam with nitrile group to form superhydrophobic foam. Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), thermogravimetric analysis (TGA), optical contact angle measuring device, and scanning electron microscopy (SEM) were used to characterize the foam, showing the dodecane diamine was successfully grafted onto graphene oxide and the graphene oxide modified by dodecane diamine was successfully grafted onto polyurethane foam. Moreover, the modified foam exhibited a high contact angle (159.1 ± 2.3°) compared to unmodified foam (121.4 ± 3.2°). And that is due to the foam modified by amidation of graphene oxide can enhance the surface roughness and reduce the surface energy of the foam. Owing to modified foam was extremely hydrophobic and preferentially adsorbed oil other than water, the sorption capacity of the modified foam for toluene, gasoline, and diesel is 41, 27, and 26 g/g, respectively. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3530–3536, 2013

The dependence of hierarchy in crystalline structures and molecular orientations of high density polyethylene parts with different molecular weights molded by gas-assisted injection molding (GAIM) was intensively examined by scanning electron microscopy, two-dimensional wide-angle X-ray scattering as well as dynamic rheological measurements. The non-isothermal crystallization kinetics of the samples were also analyzed with a differential scanning calorimeter at various scanning rates. It was found that oriented lamellar structure, shish-kebab and common spherulites were formed in different regions of the GAIM samples. The scanning electron microscope observations were consistent with the two-dimensional wide-angle X-ray scattering results and showed that the molecular chains near the mold wall had strong orientation behavior, revealing the distribution of the shear rate of the GAIM process. The differences in crystal morphologies can be attributed to molecular weight differences as well as their responses to the external fields during the GAIM process. The formation mechanism of the shish-kebab structure under the flow field of GAIM was also explored. Copyright © 2012 Society of Chemical Industry

In this work, gradient materials with low electrical resistivity were prepared by compounding isotactic polypropylene (iPP)/high density polyethylene (HDPE) blends with carbon black (CB) through extruding and injection molding. Contact angle measurements and morphology measurements showed that the CB particles were selectively located in HDPE phase and the final composites had a gradient structure that the HDPE/CB phase exhibited different morphologies in the skin layer and core layer of the composites under different processing procedures. The main factors influencing the formation of the functional gradient materials (FGM), including screw speed during extruding, iPP types and CB contents were discussed. They affect the phase morphology by shear stress, the restoration of HDPE phase, and the viscosity ratio of polymer blends, respectively. In conclusion, iPP/HDPE/CB FGM could be formed easily in the composites blending with the iPP type with narrow molecular weight distribution (MWD) and higher CB content extruded at higher screw speed. The electrical properties of iPP/HDPE/CB composites were studied and the results showed that screw speed in extrusion significantly influenced the percolation curve and electrical property of the final composites. Copyright © 2011 John Wiley & Sons, Ltd.

The effect of compounding procedure on morphology and crystallization behavior of isotactic polypropylene/high-density polyethylene/carbon black (iPP/HDPE/CB) composite was investigated. iPP/HDPE/CB composites were prepared by four compounding procedures (A: iPP + HDPE + CB; B: iPP/HDPE + CB; C: HDPE/CB + iPP; D: iPP/CB + HDPE). Scanning electron microscopy observation showed that CB particles are mainly distributed in HDPE in all composites, and the phase morphology of composites was obviously affected by a compounding procedure. The size of the HDPE/CB domains in the composites prepared by procedures A and D decreased with the increase in CB content, whereas that of HDPE/CB in the composites prepared by procedures B and C rarely changed with the increase in CB content. The crystallization behaviors of the composites were significantly affected by their phase morphology, which resulted from the variation of compounding procedure. The isothermal crystallization rate of iPP in the composites prepared by procedures A and D was obviously increased, which may originate from the small HDPE/CB droplets dispersed in the iPP phase. The non-isothermal crystallization curves of composites prepared by procedure D represented two peaks because the iPP component in these composites had the fastest crystallization rate, whereas the curves of composites prepared by other compounding sequences only exhibited one peak. Moreover, the crystallinity of HDPE almost increased by one time with the incorporation of only 1 phr CB because the CB particles selectively located in the HDPE phase, and the crystallinity of HDPE decreased with the further increase of CB content because of the strong restriction of CB on the HDPE chains. Copyright © 2011 John Wiley & Sons, Ltd.

A new approach, mild blending method, to prepare carbon black (CB) filled polypropylene (PP) nanocomposite using CB aqueous suspension was reported in this study. In this compounding process, the CB particles were first dispersed in aqueous suspension by using an ultrasonic irradiation. Subsequently, the CB suspension was blended with melting PP using an extruder with low shear strength screw configuration, followed by removing the vapor from the vent by vacuum. The morphological observation showed that the CB particles were dispersed at a nanometer level in the nanocomposites as they were in aqueous suspension and distributed homogeneously in PP matrix. The CB/PP nanocomposite prepared by this method exhibited a very low percolation threshold, i.e., 2.49 vol %, and a high-critical resistance exponent t (t = 5.82). These phenomena, which deviated from the classical percolation theory, were likely to come down to the homogeneous distribution of CB particles and the tunneling conduction. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

The dynamic rheological and mechanical properties of the binary blends of two conventional high-density polyethylenes [HDPEs; low molecular weight (LMW) and high molecular weight (HMW)] with distinct different weight-average molecular weights were studied. The rheological results show that the rheological behavior of the blends departed from classical linear viscoelastic theory because of the polydispersity of the HDPEs that we used. Plots of the logarithm of the zero shear viscosity fitted by the Cross model versus the blend composition, Cole–Cole plots, Han curves, and master curves of the storage and loss moduli indicated the LMW/HMW blends of different compositions were miscible in the melt state. The tensile yield strength of the blends generally followed the linear additivity rule, whereas the elongation at break and impact strength were lower than those predicted by linear additivity; this suggested the incompatibility of the blends in solid state. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Four kinds of ethylene–proplene–diene/maleic anhydride compatibilized polyamide 6/polypropylene samples were prepared with different mixing sequences and showed significant differences in the Izod impact strength. The morphological features of these samples were characterized with scanning electron microscopy, and a heterogeneous dispersion of the compatibilizer in the injection-molded samples was observed; this was related to the shear field in the skin and subskin layers during injection. A parameter, the transfer energy, is put forward to interpret the dispersion of the compatibilizer in the injection-molded blends, and the results show that the transfer energy is a key factor in determining the transfer of the compatibilizer. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

The thermal degradation of polycarbonate (PC) and polycarbonate/hydroxyapatite (PC/HAP) composite was investigated by the thermogravimetric analysis (TGA) and the decomposition activation energy was calculated by Kissinger method. It was found that the first step decomposition activation energy (E_a1) was higher than that of pure PC while the second step decomposition (E_a2) decreased a little. The limiting oxygen index (LOI) of PC/HAP composite with 0.5 wt % HAP reached 34. The carbon char of PC and PC/HAP composite after combustion was analyzed by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and flourier transform infrared spectroscopy (FTIR) and similar morphology, components, and the same functional groups were found in the char residues. Based on these results, it can be concluded that a low content of HAP in PC matrix can significantly improve the thermal properties and flame retardancy of the composite. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

In this article, gas penetration-induced skin-core structure of isotactic polypropylene(iPP), which is molded by gas-assisted injection molding at different gas pressures, was investigated. For comparison, the counterpart was also molded by conventional injection molding (CIM) using the same processing parameters but without gas penetration. They were characterized via PLM, DSC, and SEM. And the crystal morphology at different gas pressures was principally concerned. For the GAIM parts, highly oriented structure is formed in the skin zone, and much less oriented structure in the inner zone (near the gas channel surface). Furthermore, it is suggested that the naked shish structure can be developed in the skin zone of GAIM part, which is molded at higher gas pressures, and shish-kebab structure is mainly formed in the skin zone of that, which is molded at lower gas pressure. However, for the CIM part, from the skin to the core zone, the dominant morphological feature is spherulite. In a word, the presence of gas penetration notably enhances the oriented structure formation and gives rise to the skin-core structure. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers