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Polycarbonate (PC) foam is a versatile material with excellent properties, but its low thermal stability limits its application in high-temperature environments. The aim of this study was to improve the thermal stability of PC foam by adding glass fibers (GF) and to investigate the effect of GF on PC crystallization behavior and PC foam cell morphology. This study was motivated by the need to improve the performance of PC foams in various industries, such as construction, automotive, and medical. To achieve this goal, PC/GF composites were prepared by extrusion, and PC/GF composite foams were produced using a batch foaming process with supercritical carbon dioxide (SC-CO2) as the blowing agent. The results showed that the addition of GF accelerated the SC-CO2-induced crystallization stability of PC and significantly increased the cell density to 4.6 cells/cm3. In addition, the thermal stability of PC/GF foam was improved, with a significant increase in the residual carbon rate at 700 °C and a lower weight loss rate than PC matrix. Overall, this study highlights the potential of GF as a PC foam reinforcement and its effect on thermal and structural properties, providing guidance for industrial production and applications. [ABSTRACT FROM AUTHOR]

A combined melt-stretching and quenching setup is designed and developed to allow experimental investigations of polymer crystallization under the complex flow-temperature environments comparable to those encountered in the actual industrial processing. The melt-stretching proceeds by two drums rotating in the opposite directions with simultaneous recording of a stress–strain curve, where the Hencky strain and strain rate (≤233 s−1) are adjustable over a large range. After stretching, liquid N2 is used as a cooling medium to quench the free-standing melt, which is sprayed directly to the deformed melt driven by an electric pump. To ensure a high cooling efficiency, a three-way solenoid valve is employed to execute a sequential control of the liquid N2 flow direction to reduce the boil-off of liquid N2 before entering the sample chamber. The melt cooling rate depends on the liquid N2 flow rate controlled by a flow valve, which is up to 221 °C/s when quenching the isotactic polypropylene (iPP) melt with a thickness of 0.28 mm at 150 °C. Two independent temperature control modules are designed to meet the requirements of different stages of melt-stretching and quenching. To verify the capability of the setup, we have performed the melt-stretching and quenching experiments on iPP samples. The setup is demonstrated to be a valuable new tool to study polymer crystallization under coupled flow-cooling fields. [ABSTRACT FROM AUTHOR]

In this work, poly(ethylene glycol) (PEG)/boron nitride nanosheets (BNNS) masterbatch was prepared by freeze-drying their ultrasonically homogenized aqueous suspension, followed by melt-mixing with poly(lactic acid) (PLA) to fabricate the nanocomposites. The effect of PEG and BNNS on the crystallization behavior of PLA was investigated. The dynamic mechanical properties, thermal conductivity and thermal stability of the nanocomposites were also evaluated. It was found that, under quiescent conditions, PEG and BNNS had a synergistic effect on accelerating the nonisothermal crystallization of PLA. Unexpectedly, under injection molding conditions, the combination of PEG and BNNS had a negative effect on PLA crystallization. This can be ascribed to the plasticizing effect of PEG, which relaxes the orientation of PLA chains and retards flow-induced crystallization. Dynamic mechanical analysis (DMA) showed that the presence of BNNS enhanced the heat resistance of PLA only when PLA was crystallized, while further incorporation of PEG had a negative effect. Nevertheless, simultaneous use of PEG and BNNS had a synergistic effect on enhancing the thermal conductivity of PLA. [ABSTRACT FROM AUTHOR]

In this work, biodegradable poly(lactic acid) (PLA)/Ti3C2Tx MXene nanocomposites were prepared through melt compounding. The roles of MXene in nonisothermal melt and cold crystallization of PLA were explored. The morphology, structure and crystallization behavior were studied using polarized optical microscopy, wide-angle X-ray diffraction and differential scanning calorimetry. For nonisothermal melt crystallization, the presence of MXene increases the crystallinity and shortens the half-crystallization time but does not change the crystallization peak temperature of PLA. For nonisothermal crystallization from the glassy state, the incorporation of MXene decreases the cold crystallization peak temperature and reduces the crystallinity marginally but does not affect the half-crystallization time of PLA. The crystallization kinetics analysis based on the Avrami equation shows that, for both the melt and cold crystallization, the crystallization mechanism is unchanged irrespective of the addition of MXene. The investigation would be useful for understanding the correlation between processing and properties of PLA/MXene nanocomposites. [ABSTRACT FROM AUTHOR]

In this study, iPP was injection molded at 180°C, 200°C, and 220°C. According to polarization optical microscopy (POM) results, for a given part, the skin thickness steadily decreases along the flow direction. However, at the same distance from the gate, the skin thickness of the parts molded at lower melt temperature is larger than that molded at higher melt temperature. It is found that flow time (here, the time taken for melt to pass the specific position along the flow direction) and melt temperature are two significant factors leading to this phenomenon, while the gate size is another one. The DSC and WAXD results show that the relative fraction of β-form crystals, for a specific part, decreases along the flow direction, which is mainly determined by flow time. However, for the parts molded at different molding temperatures, the fraction of the β-form crystals is mainly determined by the molding temperature, though this influence is very complex. [ABSTRACT FROM AUTHOR]

A PC/PE(20/80) blend was molded by gas-assisted injection molding and, after being etched, its crystalline morphology was investigated. Microscopy results indicate that transcrystallinity is favorably formed on the surface of the thinner PC fibrils. However, transcrystallinity cannot be observed on the PC droplets' surface. In addition, in the non-isothermal crystallization process, there was no transcrystallinity formed around the PC fibrils under the condition without shear. Combining the characteristics of gas-assisted injection molding and the PC droplet's deformation, the transcrystallization origin is proposed to be the local shear stress, which is brought by the relative motion of the two components during PC droplet's deformation and solidification. [ABSTRACT FROM AUTHOR]

This work aims to perform a systematic investigation on the crystallization behavior and morphologies of carbon and glass fiber reinforced PEEK. The nonisothermal and isothermal crystallization behavior was investigated by differential scanning calorimetry (DSC). The resultant morphologies were assessed by wide angle X-ray diffraction (WAXD), small angle X-ray scattering (SAXS), and polarized optical microscopy (POM) to provide details on spherulitic level, crystalline structure at unit cell, and lamellar levels. It was found that the crystallization ability of carbon fiber filled PEEK was better than that of neat PEEK, while the behavior of glass fiber filled PEEK was in an opposite trend. The incorporation of carbon fiber (or glass fiber) led to a looser packing of the unit cell or a less crystal perfection of PEEK but did not change its crystal form as well as its long period of lamellae. The isothermal crystallization kinetics was analyzed by the Avrami model, suggesting that the crystallization mechanism of carbon fiber filled PEEK was different from that of neat PEEK and its glass fiber filled composites. Nevertheless, the POM results showed that fiber-induced transcrystallization in PEEK matrix was not evidenced for either carbon or glass fiber filled PEEK. Finally, the effect of carbon and glass fiber on the crystallization of PEEK matrix was discussed to some extent. [ABSTRACT FROM AUTHOR]

Rare‐earth nucleating agent (WBG‐II) is a kind of additive for isotactic polypropylene (iPP), which will dissolve into iPP melt during heating and re‐aggregate into diverse morphologies during cooling through self‐assembly. However, up to now, little work about the influence of graphene oxide (GO) on the self‐assembly of WBG‐II can be found. Results in this study suggest that via the hydrogen‐bond interaction, GO can provide additional nuclei for WBG‐II during self‐assembly. Consequently, during non‐isothermal crystallization, WBG‐II and GO present a synergistic effect rather than a superposition effect on the crystallization onset temperature (Tc‐on), the crystallization peak temperature (Tc‐peak), the crystallization half time (t1/2), and the relative content of β‐iPP (Kβ), especially when the final heating temperature (Tf) is higher than 240 °C. Specifically, as Tf is lower than 220 °C, Tc‐on, Tc‐peak, t1/2, and Kβ are insensitive to the weight concentration of GO‐ODA (ϕG), since that little WBG‐II is dissolved into iPP melt and the morphology of WBG‐II is changed only very slightly; as Tf is higher than 240 °C, Tc‐on, Tc‐peak, and Kβ first decrease to a minimum value with increasing of ϕG, and then increase when ϕG exceeded 0.2%. [ABSTRACT FROM AUTHOR]

High-toughness biodegradable poly(lactic acid) (PLA) has always been intensively pursued on the way of replacing traditional petroleum-based plastics. Regulating microstructures to achieve self-toughening holds great promise due to avoidance of incorporating other heterogeneous components. Herein, we propose a straightforward and effective way to tailor microstructures and properties of PLA through melt-stretching and quenching of slightly crosslinked samples. The melt stretching drives chains orientation and crystallization at high temperature, while the quenching followed can freeze the crystallization process to any stage. For the first time, we prepare a type of transparent and low-crystalline PLA filled with rod-like β-form shish, which displays an outstanding tensile toughness, almost 17 times that of the conventional technique-processed one. This mechanical superiority is enabled by an integration of high ductility due to oriented chain network, and high tensile stress endowed by nanofibrous filler's role of β-form shish. Furthermore, the mechanically toughened PLA is demonstrated to generate the richest micro-cracks and shear bands under loading, which can effectively dissipate the deformational energy and underlie the high toughness. This work opens a new prospect for the bottom-up design of high-performance bio-based PLA materials that are tough, ductile and transparent by precise microstructural regulation through scalable melt processing route. • Melt stretching and quenching were combined to regulate microstructures of PLA. • High-toughness PLA composed of oriented chain network and β-form shish was fabricated. • Oriented chain network provided ductility and β-form shish imparted high tensile stress. • Numerous micro-cracks and shear bands were formed in toughened PLA under tensile deformation. [ABSTRACT FROM AUTHOR]

Fabrication of polymer films with superb mechanical properties is far from mature compared to that of fibers, hampering the further engineering applications of lightweight polymer materials. Herein, high density polyethylene (HDPE) films reinforced with re-dispersed or refined reduced graphene oxide (rGO) are prepared via a facile melt-stretching strategy. The incorporation of only 0.2 wt% rGO is demonstrated to endow the melt-stretched film (80× stretch ratio) with ultrahigh Young's modulus of 2.9 GPa and tensile strength of 162.2 MPa, which are comparable to many engineering plastics and also significantly superior to the existing HDPE/graphene composites. More importantly, these modulus and strength values are increased by 71 and 65% compared to the neat PE (80×), respectively, far more than the theoretical prediction with the mixing rule. Experimental characterizations indicate that the high-efficient mechanical enhancement is attributed to the strong synergy of melt stretching-driven crystallization and rGO re-dispersion or refinement in constructing the compact and robust shish-kebabs crystal network. Such microstructural characteristics greatly promote the load transfer along the covalently bonded molecular chains, and also from polymer to rigid fillers under loading. Furthermore, the excellent puncture resistance and thermo-mechanical properties of PE/rGO films have been demonstrated. This work provides a feasible way towards scalable manufacturing of high-performance polymer nanocomposite films from general plastics and carbonaceous nanomaterials. [Display omitted] • HDPE/rGO nanocomposite films are fabricated by a facile melt stretching process. • Superb mechanical performance of films is achieved at very low filler content. • Melt stretching-driven rGO re-dispersion or refinement is demonstrated. • rGO re-dispersion enhances the formation of robust shish-kebabs crystal network. • Good puncture resistance and thermo-mechanical properties of films are revealed. [ABSTRACT FROM AUTHOR]

Abstract The self-assembly behavior of N,N′,N ″-tricyclohexyl-1,3,5-benzenetricarboxylamide (BTCA) and its nucleating effect on poly(lactic acid) (PLA) crystallization were studied by differential scanning calorimetry and polarized optical microscopy. The effect of melting temperatures (T f) from 190 to 240 °C, BTCA concentrations from 0 to 0.5 wt%, and cooling rates from 2.5 to 20 °C/min was investigated. It was found that, when T f was 240 °C, BTCA could dissolve completely in PLA matrix, and then self-assemble into supermolecular frameworks upon decreasing temperature. The core findings are as follows: (1) for the two-step crystallization, the half-crystallization time of PLA showed a bell shaped dependence on the pre-crystallization temperatures of BTCA, which was higher than that without pre-treatment; (2) for nonisothermal crystallization, the crystallization peak temperature of PLA presented a bell shaped dependence on the concentration of BTCA at each rate. Meanwhile, the BTCA concentration, which corresponded to the maximum crystallization rate of PLA, shifted to higher value with the increase of cooling rate. The observations were discussed in term of the effective nucleating surface area of BTCA. Graphical abstract Image 1 Highlights • We examined the nucleating effect of BTCA on PLA crystallization. • PLA crystallization presents a bell-shaped dependence on BTCA concentration. • Optimized BTCA concentration for PLA crystallization depends on cooling rate. [ABSTRACT FROM AUTHOR]

Poly(lactic acid) (PLA)/poly(ethylene oxide) (PEO) blends nucleated by a self-assembly nucleating agent, N,N′,N″-tricyclohexyl-1,3,5-benzenetricarboxylamide (BTCA), were prepared by melt blending. The crystallization behavior and mechanical properties of the materials were investigated by differential scanning calorimetry, polarized optical microscopy, wide-angle X-ray diffraction, dynamic mechanical analyzer and tensile testing. It was found that PEO had a synergistic effect together with BTCA on promoting PLA crystallization, besides its toughening effect on the material. Moreover, BTCA revealed prominent reinforcement effect on both neat PLA and PLA/PEO blends in the glass transition region and above, indicating the improvement on the heat resistance of the materials. [ABSTRACT FROM AUTHOR]

In many industrial processes, polymer melt undergoes complex flow-cooling environments before being solidified into the final products. Investigating polymer crystallization under processing conditions is a necessary step to predict and customize the structures and properties of semicrystalline polymers. Herein, the crystallization behavior of high-density polyethylene (HDPE) under combined melt stretching and fast cooling fields was studied through a homemade extensional rheometer. A series of HDPE samples were prepared by cooling the stretched melt at different cooling rates (0.02–220 °C/s) immediately after flow, or after waiting for different times of isothermal crystallization at 128 °C. Microstructural characterizations confirm four different structural states related to the evolution of shish-kebabs after melt stretching, namely generation of shish precursors, shish crystal growth, kebab lamellar growth, and perfection of shish-kebabs, which diversify greatly the structures and morphologies of the final solidified samples. By prolonging the isothermal crystallization time at high temperature, the fast cooling-induced lamellae keep decreasing in thickness. This supports the occurrence of molecular segregation caused by shish-kebabs growth at high temperature, which reduces the molecular weight of the remaining un-crystallized melt. In addition, quantitative analysis of crystalline orientation manifests that the lamellar growth during isothermal process takes strictly the shish as a template and adopts a high orientation, while the fast cooling-induced lamellae are only related to the chains conformation of un-crystallized melt prior to cooling. This work would be essential to advance the in-depth understanding on the actual industrial processing of semicrystalline polymeric materials. [Display omitted] • Melt crystallization of HDPE under combined stretching and fast cooling fields was studied. • Four different structural states related to the evolution of shish-kebabs after flow were identified. • Molecular segregation caused by shish-kebabs growth at high temperature was suggested. • Isothermal and non-isothermal crystallization under flow presented different lamellar growth modes. [ABSTRACT FROM AUTHOR]

Abstract: The study of crystallization behavior and crystalline morphology of polymer melt under shear flow is of great interest due to the strong effect of flow field on the final properties of polymer products in the practical processing. In this respect, the shearing hot stage provides a unique tool which monitors sensitively the changes in crystalline structure induced by precise experimental conditions. Herein, the impacts of both melting temperature and shear rate on the crystallization behavior of isotactic polypropylene (iPP) melt are investigated. Under static conditions, there are only random spherulite structures. Once shear is involved, the cylindrite‐layers appear near both surfaces of the sample, which is consistent with the skin‐core structure in the injection molded parts. Meanwhile, the β‐crystals can be developed and are related to the molecular orientation, depending on the applied melting temperatures and shear rates. More interestingly, the crystallinity of β‐crystal in the pure iPP can reach 15%. The above results indicate that the melting temperature and shear rate are important factors in determining the β‐form crystal development of iPP matrix. [ABSTRACT FROM AUTHOR]

Abstract: Shish‐kebab, which is endowed with superior strength and modulus, provides the potential to fabricate self‐reinforced polymer products. However, the injection‐molded product usually exhibits a typical skin–core structure, and the shish‐kebab is only located in an extremely thin shear layer. Therefore, the controlling and tailoring of crystal structures in complex flow field to improve the mechanical properties of the injection‐molded sample are still a great challenge. Herein, for the first time, high‐density polyethylene sample with a novel macroscopic alternating skin–core structure is achieved using a melt multi‐injection molding technique. Results show that, with increasing the amount of melt injection, the layers of skin–core structure increase in the form of arithmetic progression, and therefore the tensile strength of the samples progressively increases due to an increase of shish‐kebab content. This study demonstrates a new approach to achieve multilayer homogeneous materials with excellent tensile strength via macroscopic structural design during the practical molding process. [ABSTRACT FROM AUTHOR]

The effect of a self-assembly nucleating agent, N , N ′, N ″-tricyclohexyl-1,3,5-benzenetricarboxylamide (BTCA), on the crystallization behavior of poly(lactic acid) (PLA) was probed by time-resolved Fourier transform infrared spectroscopy (FTIR) and wide angle X-ray diffraction (WAXD). The vibrational changes associated with inter- and intra-chain interactions during crystallization were monitored. In the initial period of crystallization, the order of intensity changes is as follows: 1458 cm −1 > 1210 cm −1 » 921 cm −1 , 1458 cm −1 ∼ 1210 cm −1 > 921 cm −1 , and 1458 cm −1 ∼ 1210 cm −1 ∼ 921 cm −1 for neat PLA, PLAs containing 0.1 wt% and 0.3 wt% BTCA, respectively. This indicates that BTCA can accelerate both the formation of skeletal conformational-ordered structure and, especially, the 10 3 helix one. The incorporation of BTCA changes the crystallization mechanism but has no impact on the crystal form of PLA. [ABSTRACT FROM AUTHOR]

The crystallization behavior of partially melted poly(ether ether ketone) (PEEK) was investigated by differential scanning calorimetry. The influence of pre-crystallization conditions was studied in details. After partial melting, the maximum peak shifts to higher temperature by 8.4-11.8 °C during subsequent cooling, indicating that self-nucleation remarkably enhanced the overall crystallization rate of PEEK. An interesting finding is that the self-nucleation temperature domain of PEEK is way outside the melting curve, and its magnitude is greatly large (28-35 °C). Moreover, samples that were pre-crystallized nonisothermally or from the glassy state exhibit a larger self-nucleation effect, and a larger self-nucleation temperature domain. The isothermal crystallization data were analyzed by the Avrami model. The Avrami exponent is found to be remarkably different for melts with and without self-nucleation effects, indicating that self-nucleation changes the crystallization mechanism of PEEK. [ABSTRACT FROM AUTHOR]

Benefiting from its superior thermal and mechanical properties, the stereocomplex crystal (SC) of PLLA/PDLA is catching increasing interest. In this work, a systematic investigation was performed to reveal the effect of solid-state stretching on the formation of SC during annealing. Results show that as the stretching temperature was close to the glass transition temperature (T s = 60 °C), the polymer chains orientation was increased gradually with draw ratio (λ). As a result, the crystallization induction time (t 0) was shortened, the growth rate (G r) of SC was enlarged, and the crystallinity of SC (X SC) was elevated as a result of orientation-induced crystallization. Whereas, the saturation of t 0 , G r , and X SC appeared quickly when λ reached 2.7, although the orientation of polymer chains continued to increase. The reason is that in addition to orientation, strong phase separation between PLLA and PDLA was also triggered therein. The phase separation prohibited greatly the crystallization of SC due to a long time would be needed for PLLA or PDLA chains to diffuse to a distance close enough to activate the hydrogen-bonding interaction, which is the prerequisite of SC crystallization. When the stretching temperature was elevated to 75 °C, the phase separation became much weaker. So X SC was higher compared to that of the blend stretched at 60 °C to the same λ. Moreover, in the annealing temperature from 180 °C–190 °C, t 0 was much smaller for the blend stretched at 75 °C since the crystallization of SC in this temperature range is a diffusion-controlled process. [Display omitted] • Strong orientation of polymer chains and phase separation are induced by solid-state stretching. • Orientation of polymer chains and phase separation exhibit a competition effect on the crystallization of SC. • The more intense phase separation at lower stretching temperature gives rise to a longer crystallization. induction time [ABSTRACT FROM AUTHOR]

The influence of crystalline memory effect on poly(lactic acid) (PLA) crystallization was probed by time-resolved Fourier transform infrared spectroscopy (FTIR). The melt with different degrees of order was prepared by melting at various temperatures. The vibrational changes associated with inter- and intra-chain interactions during crystallization were monitored. In the initial period of crystallization, the order of intensity changes is as follows: 1458 cm −1 > 1210 cm −1 ≫ 921 cm −1 , 1458 cm −1 > 1210 cm −1 > 921 cm −1 , and 1458 cm −1 > 1210 cm −1 ∼ 921 cm −1 for PLA melted at 190, 170, and 168 °C, respectively. With increasing the degree of order, both the formation of skeletal conformational-ordered structure and, especially, the 10 3 helix structure are accelerated. The influence of memory effect on the crystallization kinetics was also discussed based on the Avrami model. [ABSTRACT FROM AUTHOR]

Abstract: Both poly(lactic acid) (PLA) and cellulose acetate butyrate (CAB) are bio-based and biodegradable. In this work, the effect of CAB on the cold and melt crystallization behavior of PLA was investigated by differential scanning calorimetry and polarized optical microscopy. Both the cold and melt crystallization of PLA was dramatically inhibited even at a very low loading of 0.5wt% CAB. The isothermal crystallization kinetics was analyzed by the Avrami model. It was found that the Avrami exponent of PLA crystallization was not significantly influenced by the addition of CAB, indicating that the crystallization mechanism of PLA almost did not change in the blends. The crystallization half-time of the blends increased significantly with increasing CAB loading. The observation from optical microscopy further revealed that the presence of CAB reduced remarkably the nuclei density but had nearly no influence on the spherulite growth rate of PLA. These observations indicate that CAB has dramatic “nucleation density reduction” effect on PLA. [Copyright &y& Elsevier]

The nonisothermal and isothermal crystallization behavior of pre-crystallized poly(lactic acid) (PLA) after partial or complete melting was investigated. After partial melting, the maximum crystallization peak shifts to higher temperature by 30 °C during subsequent cooling, indicating that self-nucleation greatly enhances the overall crystallization rate of PLA. Partial melting leads to predetermined athermal nucleation at higher temperature, while complete melting results in sporadic nucleation at lower temperature. Spherulitic and irregular morphologies are formed for the sample partially melted at 178 °C and 176 °C (or 175 °C), respectively. The isothermal crystallization data were analyzed by the Avrami model. The values of the Avrami exponent for partially melted PLA are comparable to that of completely melted material, indicating that self-nucleation does not change the crystallization mechanism of PLA. Irregular spherulites are formed for PLA partially melted at 176 °C during subsequent isothermal crystallization, and the nucleation density decreases with increasing crystallization temperature. [ABSTRACT FROM AUTHOR]

Abstract: The effect of one of the 1,3,5-benzenetrisamide derivatives, N,N',N''-tricyclohexyl-1,3,5- benzenetricarboxylamide (defined here as TMC), on the crystallization behavior of isotactic polypropylene (iPP) was investigated by differential scanning calorimetry. TMC shows excellent nucleating effect on iPP. The nucleation efficiency of the blends reaches 30% even at a very low loading of 0.05 wt% TMC. For 0.1 wt% TMC, the crystallization half-time of the blend decreases from 39.41 to 3.97 min at 136 °C, compared with pure iPP. The isothermal crystallization data were further analyzed by the Avrami model. The values of the Avrami exponent of the blends are comparable to that of pure iPP, indicating that the presence of TMC does not change the crystallization mechanism of the matrix. The presence of TMC increases significantly the nuclei density of iPP but has no discernible effect on its crystalline structure, evidenced by polarized optical microscopy and wide angle X-ray diffraction measurements. [Copyright &y& Elsevier]

In order to further investigate shear-induced interfacial crystallization of polymer-based composites, an improved fiber-pulling device was designed and built. Its peculiar characteristic is that a force transducer is assembled to in situ monitor the variation of interfacial shear stress between the polymer matrix and pulling fiber. Thus, the relationship between interfacial shear stress and the subsequent crystalline morphology can be quantitatively established. In the preliminary study via this device, isotactic polypropylene (iPP)/glass fiber composite was adopted as a model system. The results indicate that interfacial crystallization kinetics is promoted by the presence of interfacial shear stress. Furthermore, there are two thresholds of interfacial shear stress for interfacial crystalline morphology. To be specific, one (0.017 MPa) is for the induction of iPP nucleation, above which α-form iPP crystals are obviously encouraged during the subsequent isothermal crystallization; the other is for the generation of β-form iPP crystals (0.042 MPa), above which β-form crystals are favored to be triggered in the transcrystalline region. [ABSTRACT FROM AUTHOR]

In this study, the crystallization behavior of oriented isotactic polypropylene during solid-state stretching was investigated by in-situ wide-angle X-ray scattering. Results showed that a new arc appeared on the (040) reflection with the previous arcs remained when the stretching direction was inclined to the initial orientation direction, as a result of stretching induced crystallization. The crystallization process occurred later than the yielding but earlier than the strain hardening. The crystallinity of the newly formed crystal showed a linear dependence on the Hencky strain, and the slope of the plot was more sensitive to the stretching angle rather than the stretching temperature. Lastly, the stress relaxation test showed that the crystallinity was linear correlated to the amorphous phase orientation. The Pearson correlation coefficient was as high as −0.99, suggesting that the crystallization process is originated from the orientation of the chain network during stretching. [Display omitted] • The crystallization behavior of oriented iPP during solid state stretching is studied by in-situ wide angle X-ray scattering. • The crystallinity shows a linear dependence on the macroscopic Hencky strain. The slope of the plot is more sensitive to the stretching angle instead of the stretching temperature. • The crystallization during solid-state stretching correlates with the orientation of chain network in the amorphous phase. [ABSTRACT FROM AUTHOR]

The crystallization in isotactic polypropylene (iPP) melt inclusions, encapsulated by transcrystallinity and spherulites, was investigated by polarized light microscope. Generally, owing to the proceeding of crystallization in melt inclusions, a negative pressure will be built up. Up to a critical value, the negative pressure is released by the formation of cavitation in melt. Unexpectedly, β-crystals with different morphologies emerge at different locations in the melt inclusions. For low molecular weight iPP, only smaller β-crystals around bubbles are discernable. Turn to higher molecular weight iPP, apart from the smaller β-crystals, fan-shaped ones can be developed under suitable conditions (e.g., lower than 131 °C) at the growth front of spherulites and transcrystallinity. Considering the characteristics of molecular weight and morphology, it is proposed that β-crystals are induced by different mechanisms. That is, smaller β-crystals are resulted from the stress caused by the appearance of bubbles while the fan-shaped ones are derived due to the temperature gradient during the release of negative pressure. [ABSTRACT FROM AUTHOR]

Pure isotactic polypropylene (iPP) and that containing 0.2 wt% of a β-nucleating agent (β-NA) were extruded through a slit die. Simultaneously, the extruded melt was stretched at the die exit with different stretching rates (SR). The change of β-phase content with different SR was investigated by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). The results indicate that, for pure iPP, the content of β-phase first increases with increasing SR till it reaches a maximum and then it gradually decreases. However, for the case of β-nucleated iPP, it decreases monotonously with increasing SR. The spatial confinement is considered as the best explanation for the suppression of β-phase in the nucleated iPP melt upon stretching. [ABSTRACT FROM PUBLISHER]

Abstract: Glass fiber (GF) was immersed in diluted acid solution and corroded for 4h. The interfacial morphology of isotactic polypropylene (iPP) reinforced by the corroded GF was studied via polarized optical microscope. It was found that the corroded GF exhibits anomalous nucleating ability to induce ringed nuclei, and β-transcrystallinity can be developed unexpectedly from these nuclei during isothermal crystallization. The present work provides a novel approach, via which β-transcrystallinity can arise in static condition from the surface of the acid-corroded GF. [Copyright &y& Elsevier]

Abstract: Polyamide 66 (PA 66) nanofibers, with the mean diameter of about 140nm, were prepared by electrospinning. Nano-hybrid shish-kebab (NHSK) structure was achieved in PA 66 nanofibers/isotactic polypropylene (iPP) composites via isothermal solution crystallization. The morphology of such NHSK was observed by scanning electron microscopy (SEM). It was found that PA 66 nanofibers act as “shish”, and iPP crystals serve as “kebabs”. Furthermore, the concentration of iPP solution remarkably affects the morphology of the NHSK, i.e., the size of iPP crystals becomes much bigger with the increasing iPP solutions'' concentration. The reason for this can be explained as that the high concentration of iPP solutions contain more free chains which can participate into the process of crystallization. [Copyright &y& Elsevier]

Using a homemade pressure device, we explored the synergistic effect of pressurization rate and β-form nucleating agent (β-NA) on the crystallization of an isotactic polypropylene (iPP) melt. The obtained samples were characterized by combining small angle X-ray scattering and synchrotron wide angle X-ray diffraction. It was found that the synergistic application of pressurization and β-NA enables the preparation of a unique multi-phase crystallization of iPP, including β-, γ- and/or mesomorphic phases. Pressurization rate plays a crucial role on the formation of different crystal phases. As the pressurization rate increases in a narrow range between 0.6–1.9 MPa/s, a significant competitive formation between β- and γ-iPP was detected, and their relative crystallinity are likely to be determined by the growth of the crystal. When the pressurization rate increases further, both β- and γ-iPP contents gradually decrease, and the mesophase begins to emerge once it exceeds 15.0 MPa/s, then mesomorphic, β- and γ- iPP coexist with each other. Moreover, with different β-NA contents, the best pressurization rate for β-iPP growth is the same as 1.9 MPa/s, while more β-NA just promotes the content of β-iPP under the rates lower than 1.9 MPa/s. In addition to inducing the formation of β-iPP, it shows that β-NA can also significantly promote the formation of γ-iPP in a wide pressurization rate range between 3.8 to 75 MPa/s. These results were elucidated by combining classical nucleation theory and the growth theory of different crystalline phases, and a theoretical model of the pressurization-induced crystallization is established, providing insight into understanding the multi-phase structure development of iPP. [ABSTRACT FROM AUTHOR]

Polyethylene oxide (PEO) was introduced to improve the dispersion of graphene nanoplatelets (GNPs) in poly (lactic acid) (PLA) matrix. GNPs were dispersed into PEO aqueous solution and freeze-dried to form PEO/GNPs masterbatch, followed by melt-mixed with PLA to prepare the nanocomposites. Three grades of PEO with different molecular weights were evaluated. It is found that adding very low loading of PEO (1 wt%) enhances the dispersion of GNPs in PLA matrix, resulting in remarkable improvement in elongation at break of PLA/GNPs without obviously sacrificing its modulus and tensile strength. Meanwhile, PEO reveals a synergistic effect with GNPs in promoting PLA crystallization. Moreover, GNPs enhances significantly the thermal stability of PLA, while further addition of PEO does not apparent compromise this effect. In addition, PEO with low molecular weight presents the best effect in improving the properties of PLA/GNPs nanocomposites. • Freeze-dried PEO/GNPs masterbatch was melt-mixed with PLA. • PEO effectively promotes the dispersion of GNPs in PLA matrix. • PEO enhances the toughness of PLA/GNPs. • PEO enhances the crystallization ability of PLA/GNPs. • Effect of the molecular weights of PEO was addressed. [ABSTRACT FROM AUTHOR]

Isotactic polypropylene filled with 1 wt.% multi-walled carbon nanotubes (iPP/MWCNTs) were prepared, and their crystallization behavior induced by pressurizing to 2.0 GPa with adjustable rates from 2.5 to 1.3 × 104 MPa/s was studied. The obtained samples were characterized by combining wide angle X-ray diffraction, small angle X-ray scattering, differential scanning calorimetry, transmission electron microscopy and atomic force microscopy techniques. It was found that pressurization is a simple way to prepare iPP/MWCNTs composites in mesophase, γ-phase, or their blends. Two threshold pressurization rates marked as R1 and R2 were identified, while R1 corresponds to the onset of mesomorphic iPP formation. When the pressurization rate is lower than R1 only γ-phase generates, with its increasing mesophase begins to generate and coexist with γ-phase, and if it exceeds R2 only mesophase can generate. When iPP/MWCNTs crystallized in γ-phase, compared with the neat iPP, the existence of MWCNTs can promote the nucleation of γ-phase, leading to the formation of γ-crystal with thicker lamellae. If iPP/MWCNTs solidified in mesophase, MWCNTs can decrease the growth rate of the nodular structure, leading to the formation of mesophase with smaller nodular domains (about 9.4 nm). Mechanical tests reveal that, γ-iPP/MWCNTs composites prepared by slow pressurization display high Young's modulus, high yield strength and high elongation at break, and meso-iPP/MWCNTs samples have excellent deformability because of the existence of nodular morphology. In this sense, the pressurization method is proved to be an efficient approach to regulate the crystalline structure and the properties of iPP/MWCNTs composites. [ABSTRACT FROM AUTHOR]

The agglomeration problem of nanofillers, for instance, carbon nanotubes (CNTs) in a poly(ether ether ketone) (PEEK) matrix, is still a challenging assignment due to the intrinsic inert nature of PEEK to organic solvents. In this work, organically modified montmorillonite (MMT) was introduced as a second filler for improving the dispersion of CNTs in the PEEK matrix and enhancing the mechanical properties, as well as reducing the cost of the materials. The nanocomposites were fabricated through melt-mixing PEEK with CNTs/MMT hybrids, which were prepared in advance by mixing CNTs with MMT in water. The introduction of MMT improved the dispersion stability of CNTs, as characterized by sedimentation and zeta potential. The CNTs/MMT hybrids were maintained in PEEK nanocomposites as demonstrated by the transmission electron microscope. The mechanical and thermomechanical measurements revealed that CNTs together with MMT had a strong reinforcement effect on the PEEK matrix, especially at high temperature, although it had a negative effect on the toughness. A maximum increase of 48.1% was achieved in storage modulus of PEEK nanocomposites with 0.5 wt% CNTs and 2 wt% MMT at 240 °C, compared to that of neat PEEK. The differential scanning calorimetry results revealed that CNTs accelerated the crystallization of the PEEK matrix while a further addition of MMT played an opposite role. The nucleation activity of the fillers was also evaluated by the Dobreva method. [ABSTRACT FROM AUTHOR]