Effect of oscillatory shear field on the morphology and mechanical properties of β‐nucleated isotactic polypropylene

The aim of this study is to investigate the effects of extra oscillatory shear field on morphology and mechanical properties of β‐nucleated isotactic polypropylene by using a homemade vibration‐assisted extrusion apparatus, in which oscillatory shear field was introduced into the extrusion head driven by crank and link mechanism. The results show that the extra oscillatory shear filed induces a significantly enhanced orientation of β‐crystal besides causing a better homogeneous dispersion of β‐nucleant, and the oriented β‐crystal has a high ratio of daughter lamellae to parent lamellae orientation, which was rarely reported in other literatures. The vibration increased the overall crystallinity and changed the relative content of β‐crystal. For the higher level nucleation agent content iPP(0.4 wt%), the relative contents of β‐crystal just changed a little or basically unchanged after vibration. However, for iPP contenting 0.2 wt% nucleation agent, the content of β‐crystal decreased. All the changes in the morphology, crystallinity, dispersion of β‐nucleant and β‐crystal content had a great influence on the mechanical properties, and that result was the improvement of yield stress and decrement of the elongation at break. POLYM. ENG. SCI., 57:838–845, 2017. © 2016 Society of Plastics Engineers     

Combined effect of β‐nucleating agent and processing melt temperature on the toughness of impact polypropylene copolymer

The phase morphology and toughening behavior of impact polypropylene copolymer (IPC) with and without nucleating agent (NA), prepared at different processing melt temperatures (T_p), were investigated. Interestingly, three different structures can be formed in the IPC samples by adding NA or tuning T_p. A well‐defined core–shell structure is obtained in samples with α‐NA or without NA prepared at all T_p. A developing multilayered structure is mainly formed at high T_p with added β‐NA, while an incomplete phase separation structure with interpenetrating chains is the dominant structure for IPC samples prepared at low T_p with added β‐NA. In this case, because of the synergistic effect between phase morphology and relatively high β‐form crystal content, the chain interaction among the components and chain mobility of the amorphous portion of IPC are distinctly improved, resulting in a largely improved toughness under 0 °C. This improvement in toughness is very important for applications. © 2012 Society of Chemical Industry     

Correlation between the fracture toughness and β‐crystal fraction in a β‐nucleated propylene‐based propylene–ethylene random copolymer

Propylene‐based propylene–ethylene random copolymer (PPR) has been widely used in the production of hot‐water pipes. To further improve its toughness and thermal resistance, β‐nucleating agents (β‐NAs) are frequently incorporated. In this study, PPR containing 5.6 mol % ethylene units was modified by two kinds of β‐NAs, that is, calcium pimelate and N,N′‐dicyclohexylterephthalamide. The notched Izod impact strength of PPR increased with the addition of the β‐NAs. Drastically different toughening effects were found between the two β‐NAs. The structure of PPR with and without a β‐NA was investigated by calorimetry, X‐ray diffraction, and thermomechanical analysis. The results indicated that the relative fraction of β crystals (k_β) in the injection‐molded specimens was determined by the type and content of β‐NA. The relationship between k_β and the impact toughness was summarized. A critical value for k_β (0.68) was identified for the brittle–ductile transition of PPR. PPR with β‐NA having a k_β greater than 0.68 displayed a higher impact strength than the other mixtures. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42930.     

Effect of α‐ and β‐nucleating agents on the fracture behavior of polypropylene‐co‐ethylene

The effect of α‐ and β‐nucleating agents (NA) of various amounts on the fracture behavior of polypropylene‐co‐ethylene (CPP) was evaluated using the essential work of fracture (EWF) method. The specific EWF values of CPPs incorporated with α‐NA of different amount were all lower than that of pure CPP, while the specific nonessential work of fracture was the highest at relative low α‐NA loading (0.1 wt %), and then decreased with further increasing amount of α‐NA. Similar trend of variation was observed with increasing amount of β‐NA in CPP, and it was found that the variation of K_β for β‐NA nucleated CPP versus NA content accorded well with the EWF versus NA content, which indicated that the addition of β‐NA could lead to effectively increased β‐crystal content and consequently improved fracture resistance of CPP. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Effects of rubber‐rich domains and the rubber‐plasticized matrix on the fracture behavior of liquid rubber‐modified araldite‐F epoxies

The fracture behavior of a bisphenol A diglycidylether (DGEBA) epoxy, Araldite F, modified using carboxyl‐terminated copolymer of butadiene and acrylonitrile (CTBN) rubber up to 30 wt%, is studied at various crosshead rates. Fracture toughness, K_IC, measured using compact tension (CT) specimens, is significantly improved by adding rubber to the pure epoxy. Dynamic mechanical analysis (DMA) was applied to analyze dissolution behavior of the epoxy resin and rubber, and their effects on the fracture toughness and toughening mechanisms of the modified epoxies were investigated. Scanning electron microscopy (SEM) observation and DMA results show that epoxy resides in rubber‐rich domains and the structure of the rubber‐rich domains changes with variation of the rubber content. Existence of an optimum rubber content for toughening the epoxy resin is ascribed to coherent contributions from the epoxy‐residing dispersed rubber phase and the rubber‐dissolved epoxy continuous phase. No rubber cavitation in the fracture process is found, the absence of which is explained as a result of dissolution of the epoxy resin into the rubber phase domains, which has a negative effect on the improvement of fracture toughness of the materials. Plastic deformation banding at the front of precrack tip, formed as a result of stable crack propagation, is identified as the major toughening process.     

Synergistic enhancement of crystallization and mechanical performance of polypropylene random copolymer by strong shear and β‐nucleating agent

Investigation of microstructure and properties is critical for the development and application of polymer materials. Polypropylene random copolymer (PPR) and β‐nucleated PPR are widely used in water pipe production. The effect of melt shear flow on the crystalline structure and mechanical properties of PPR containing β‐nucleating agent needs in‐depth understanding. In this paper, we demonstrated the preparation of PPR and PPR containing 0.1 wt% calcium pimelate (Ca‐Pim) samples by conventional injection molding (CIM) and oscillation shear injection molding (OSIM). The multilayer structures and morphologies of the samples were characterized by SEM, two‐dimensional X‐ray scattering and DSC. The mechanical properties and the microstructures of samples prepared by these two injection molding methods were compared. Compared with samples prepared by CIM, the stronger shear provided by OSIM induced the formation of a thicker layer of a shish‐kebab structure and a higher content of γ crystals, and dramatically suppressed the β‐nucleating effect of Ca‐Pim. The OSIM samples have more shish‐kebab structures and higher crystallinities than CIM samples and therefore the former exhibit better rigidity than the latter. The β crystals in the core layer and the thicker layer of shish‐kebab structure endow OSIM‐PPR/0.1 wt% Ca‐Pim with excellent impact toughness. © 2017 Society of Chemical Industry     

A novel β‐nucleating agent for isotactic polypropylene

The nucleating ability of p‐cyclohexylamide carboxybenzene (β‐NA) towards isotactic polypropylene (iPP) was investigated by differential scanning calorimetry, X‐ray diffraction, polarized optical microscopy and scanning electron microscopy. β‐NA is identified to have dual nucleating ability for α‐iPP and β‐iPP under appropriate kinetic conditions. The formation of β‐iPP is dependent on the content of β‐NA. The content of β‐phase can reach as high as 96.96% with the addition of only 0.05 wt% β‐NA. Under non‐isothermal crystallization the content of β‐iPP increases with increasing cooling rate. The maximum β‐crystal content is obtained at a cooling rate of 40 °C min^–1. The supermolecular structure of the β‐iPP is identified as a leaf‐like transcrystalline structure with an ordered lamellae arrangement perpendicular to the special surface of β‐NA. Under isothermal crystallization β‐crystals can be formed in the temperature range 80–140 °C. The content of β‐crystals reaches its maximum value at a crystallization temperature of 130 °C. © 2012 Society of Chemical Industry     

Characterization of the fracture behavior of glass fiber reinforced thermoplastics based on PP,PE‐HD,and PB‐1

This article deals with the influence of the polymeric matrix, such as isotactic polypropylene (iPP), polyethylene (PE‐HD), and isotactic polybutene‐1 (iPB‐1), and the glass fiber content on the material behavior of short glass fiber reinforced thermoplastics. The glass fiber content of all materials ranged between 0 and 50 wt %, which corresponds to a volume content between 0 and approx. 0.264. To describe the mechanical properties of all materials, the stiffness, strength, hardness, and toughness behavior were determined. The crack toughness behavior regarding unstable crack propagation was also assessed by applying fracture mechanics concepts. It was found that the energy‐determined J‐values for the PP material system reach their maximum at a glass fiber content of 0.135. In contrast, the crack toughness of the PE‐HD materials increases continuously with increasing glass fiber content due to the unchanged deformation ability at simultaneously increasing strength. The toughness level of the PB‐1 materials is nearly the same independent of the glass fiber content due to the opposite trend of the load and the deformation ability. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Essential work of fracture of poly(ϵ‐caprolactone)/boehmite alumina nanocomposites: Effect of surface coating

The essential work of fracture (EWF) approach was adopted to reveal the effect of nanofillers on the toughness of poly (?‐caprolactone) (PCL)/boehmite alumina (BA) nanocomposites. Synthetic BA particles with different surface treatments were dispersed into the PCL matrix by extrusion melt compounding. The morphology of the composites was studied by scanning electron microscopy. Differential scanning calorimetry and wide‐angle X‐ray scattering were used to detect changes in the crystalline structure of PCL. Also, mode I type EWF tests, dynamic mechanical analysis, and quasi‐static tensile tests were applied to study the effect of the BA nanofillers on the mechanical properties of the nanocomposites. BA was homogeneously dispersed and acted as heterogeneous crystallization nucleant and a nonreinforcing filler in PCL. The tensile modulus and yield strength slightly increased and the yield strain decreased with increasing BA content (up to 10 wt %). The effect of the BA surface treatment with octylsilane was negligible by contrast to that with alkylbenzene sulfonic acid (OS2). Like the tensile mechanical data, the essential and nonessential work of fracture parameters did not change significantly either. The improved PCL/BA adhesion in case of OS2 treatment excluded the usual EWF treatise. This was circumvented by energy partitioning between yielding and necking. The yielding‐related EWF decreased, whereas the nonessential EWF increased with BA content and with better interfacial adhesion. This was attributed to the effect of matrix/filler debonding. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Mechanical properties and morphology of poly(ethylene glycol)‐side‐chain‐modified bismaleimide polymer

Two maleimido‐end‐capped poly(ethylene glycol) (m‐PEG)‐modified bismaleimide (BMI) resins [4,4′‐bismaleimido diphenylmethane (BDM)] were synthesized from poly(ethylene glycol) (PEG) of two different molecular weights. A series of m‐PEGs and unmodified BDM were blended and thermally cured. The effect of incorporating m‐PEG side chains on the morphology and mechanical behaviors of BMI polymer were evaluated. The mechanical properties of these m‐PEG‐modified BMIs that were evaluated included flexural modulus, flexural strength, strain at break, fracture toughness, and fracture energy. The morphology of these blends was studied with scanning electron microscopy. All the m‐PEG‐modified BMI polymers showed various degrees of phase separation depending on the molecular weights and concentrations of the m‐PEG used. The effects of these morphological changes in the m‐PEG‐modified BMI polymers were reflected by the improved fracture toughness and strain at break. However, there was a reduction in the flexural moduli in all m‐PEG‐modified BMI polymers. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 715–724, 2002     

Investigation on the morphology and tensile behavior of β‐nucleated isotactic polypropylene with different stereo‐defect distribution

Large amount of work has been published on the tacticity‐properties relationship of isotactic polypropylene (iPP). However, the stereo‐defect distribution dependence of morphology and mechanical properties of β‐nucleated iPP (β‐iPP) is still not clear. In this study, two different iPP resins (PP‐A and PP‐B) with similar average isotacticity but different uniformities of stereo‐defect distribution were selected, their β‐iPP injection molding specimens were prepared, and the morphology evolution and tensile behaviors were studied by means of differential scanning calorimetry (DSC), 2D wide‐angle X‐ray diffraction (2D‐WAXD) and scanning electron microscope (SEM). DSC results showed that with the same concentration of β‐nucleating agent (0.3 wt % WBG‐II), PP‐B with more uniform stereo‐defect distribution exhibited more amount of β‐phase than that of PP‐A with less uniform stereo‐defect distribution, indicating that PP‐B is more favorable for the formation of β‐phase. SEM results showed that PP‐B formed more amount of β‐crystals with relatively high structural perfection, while in PP‐A a mixed morphology of α‐ and β‐phase with obviously higher amount of structural imperfection emerges. The results of room‐temperature tensile test indicated that the yield peak width of PP‐B was obviously wider, and the elongation at break of PP‐B was higher than that of PP‐A, showing a better ductile of PP‐B. The morphology evolution results of SEM, 2D‐WAXD and DSC suggest that, a combination of lamellar deformation and amorphous deformation occurred in PP‐A, while only amorphous deformation mainly took place in PP‐B, which was thought to be the reason for the different tensile behaviors of the samples. In the production of β‐PP products via injection molding, the uniformity of stereo‐defect distribution was found to be an important factor. PP with more uniform distribution of stereo‐defect favors the formation of large amount of β‐phase with high perfection, which exhibit superior ductile property. The related mechanism was discussed. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014 , 131, 40027.     

Enhanced β‐crystal formation of isotactic polypropylene under the combined effects of acid‐corroded glass fiber and preshear

The acid‐corroded glass fiber (GF)/isotactic polypropylene (iPP) composite was injection molded by mixing–injection molding (MIM). Through this method, preshear can be imposed on melt during mix–plasticization process. The crystalline structure across the thickness direction of the injection‐molded bars was investigated by wide‐angle X‐ray diffraction and differential scanning calorimetry (DSC). It was unexpectedly found that, in core region, the acid‐corroded GF/iPP sample has the highest content of β‐form crystals, followed by uncorroded GF/iPP and neat iPP. Additionally, the crystalline morphology was investigated by polarized optical microscopy (POM) and scanning electron microscopy, and the results showed that β‐transcrystallization is preferably present in the acid‐corroded GF/iPP system. Confirmed by POM and DSC, the acid‐corroded GF shows strong β‐nucleation ability to iPP under static condition. Combined with the main features of MIM, three β‐nucleation origins in the acid‐corroded GF/iPP system under injection molding condition are proposed: (1) precursors induced by preshear in the barrel, (2) row‐nuclei induced by local shear, and (3) the acid‐corroded GF nuclei. POLYM. COMPOS. 34:1250–1260, 2013. © 2013 Society of Plastics Engineers     

Uni‐ and biaxial impact behavior of double‐gated nanoclay‐reinforced polypropylene injection moldings

Polypopylene/nanoclay three‐dimensional parts were produced without intermediate steps by direct injection molding to explore the influence of flow features and nanoclay incorporation in their impact performance. The nanocomposite was obtained by direct compounding of commercial PP with nanoclay masterbatch. The as‐molded morphology was analyzed by X‐ray and TEM analyses in terms of skin‐core structure and nanoclay particle dispersion. The nanoclay particles induced the reduction of β‐form spherulites, a known toughener. The impact behavior was assessed in tensile and biaxial modes. The PP nanocomposite molding toughness was practically unaffected by the processing melt temperature and flow rate. Conversely the nanoclay presence is influent in the impact performance. Under biaxial stress impact, the regions close to weld lines are tougher than the bulk and the fracture develops with main crack paths along the flow direction and the weld line. Cracking along the weld line results from less macromolecular interpenetration and chain entanglement, and unfavorable nanoparticle orientation. It seems that a failure mechanism which involves nanoclay delamination and multiple matrix crazing explains the toughening of PP in the directions where the nanoparticle orientation with respect to loading is adequate. POLYM. ENG. SCI. 2013. © 2012 Society of Plastics Engineers     

Poly(styrene‐block‐butadiene‐block‐styrene‐block‐butadiene) and poly(styrene‐block‐butadiene‐block‐methyl methacrylate) copolymers as compatibilizers in PPO–SAN blends. I. Morphology and fracture behavior

The toughness behavior of PPO–SAN blends with the modifier poly(styrene‐block‐butadiene) (SBSB) and with poly(styrene‐block‐butadiene‐block‐methyl methacrylate) copolymers (SBM) under impact loading conditions has been investigated. The observed morphology of blends compatibilized with SBM, in which the rubber phase discontinuously accumulated at the PPO–SAN interface, correlated with about 20 times higher energy dissipation up to maximum force and about seven times higher deformation capacity compared to pure PPO–SAN blends. In contrast, the fracture behavior of the SBSB‐modified blends was not as strongly dependent on the rubber content. It is especially noteworthy that although the SBM modification resulted in a strong increase in toughness of the PPO–SAN blends, no decrease in stiffness could be found with up to 15% rubber additions. The values of Young's moduli remained at the same high level of the nonmodified material. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 2037–2045, 2000     

Poly(methyl methacrylate)‐modified vinyl ester thermosets: Morphology,volume shrinkage,and mechanical properties

Thermoset materials obtained from styrene/vinyl ester resins of different molecular weights modified with poly(methyl methacrylate) (PMMA) were prepared and studied. Scanning electron microscopy and transmission electron microscopy micrographs of the fracture surfaces allowed the determination of a two‐phase morphology of the modified networks. Depending on the molecular weight of the vinyl ester oligomer, the initial content of the PMMA additive, and the selected curing temperature, different morphologies were obtained, including the dispersion of thermoplastic‐rich particles in a thermoset‐rich matrix, cocontinuous structures, and the dispersion of thermoset‐rich particles in a thermoplastic‐rich matrix (phase‐inverted structure). Density measurements were performed to determine the effect of the PMMA‐modifier concentration and curing temperature on the volume shrinkage of the final materials. The development of cocontinuous or thermoplastic‐rich matrices was not too effective in controlling the volume shrinkage of the studied vinyl ester systems. The evaluation of the dynamic mechanical behavior, flexural modulus, compressive yield stress, and fracture toughness showed that the addition of PMMA increased the fracture resistance without significantly compromising the thermal or mechanical properties of the vinyl ester networks. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

The formation of β‐polypropylene crystals in a compatibilized blend of isotactic polypropylene and polyamide‐6

Compatibilized polypropylene (PP)/polyamide (PA6) blends with and without β nucleating agent (β‐NA) are prepared, and are designated as Blend‐0.3 and Blend‐0, respectively. The melting and crystallization characteristic of the blends crystallized under different cooling rates and different crystallization temperatures are studied. It is observed that high β‐PP content can be developed in Blend‐0.3 only at slow cooling rates (<5°C/min), whereas high α‐PP content is formed at fast cooling rates. Isothermal crystallization analysis of Blend‐0 indicates that PA6 is an effective NA for α‐PP in the lower temperature range, whereas the α‐nucleating effect disappears in the higher temperature range. Blend‐0.3 can, therefore, be viewed as a system containing both α‐ and β‐NAs, simultaneously. PA6 is competing with β‐NA in inducing PP crystallization. Under the normal injection of Blend‐0.3, the melt will be cooled through the higher temperature that favors the effectiveness of β‐NA rapidly because of the faster cooling rate. However, the α‐nucleation effect from PA6 predominate at the lower temperature. This explains the difficulty in obtaining high β‐PP content in Blend‐0.3 from injection molding. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers     

Nonisothermal crystallization and multiple melting behaviors of β‐nucleated impact‐resistant polypropylene copolymer

As a substitute of isotactic polypropylene in applications requiring excellent fracture resistance, impact‐resistant polypropylene copolymer (IPC) has attracted much attention in recent years. In this study, a highly effective β‐form nucleating agent (β‐NA; an aryl amide compound) was introduced into IPC, and our attention was focused on the nonisothermal crystallization and subsequent melting behaviors of the nucleated samples. The nonisothermal crystallization behaviors were investigated on the basis of the different cooling rates and different concentrations of β‐NA with differential scanning calorimetry, wide‐angle X‐ray diffraction (WAXD), and polarized optical microscopy. The results show that both the cooling rate and concentration of β‐NA greatly determined the nonisothermal crystallization process and subsequent multiple melting behaviors. Further results show that the multiple melting behaviors were related to the transition in β crystallites and those between the β and α crystallites. The morphologies of the dispersed particles and the supermolecular structure of the matrix were characterized with scanning electron microscopy. Finally, the effect of the β‐NA concentration on the fracture resistance of IPC was evaluated by measurement of the notched Izod impact strength. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Mechanical characterization of woven carbon fiber composites with poly(methyl methacrylate)–modified epoxy matrices

The influence of the modification of epoxy matrices with poly(methyl methacrylate) (PMMA) on the fracture behavior of composite laminates based on woven carbon fibers has been investigated. Three‐point flexural, short beam shear (SBS) and end‐notched flexural tests (ENF) have been carried out. Microstructural features have been investigated by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Dynamic mechanical thermal analysis of the different epoxy matrices and their corresponding composites shows the power of this technique for microstructural studies. Fracture behavior is compared with that shown by similar bifunctional (DGEBA) epoxy matrix composites. In spite of the two‐phase structure obtained in tetrafunctional (TGDDM) epoxy matrix‐based systems for all PMMA contents, only a small improvement in fracture toughness and interlaminar shear strength properties was obtained. In contrast, for DGEBA bulk matrices and composites, a higher enhancement of fracture toughness was obtained, as a consequence of the lower crosslink densities of bifunctional matrices.     

Calcium dicarboxylates nucleation of β‐polypropylene

Seven dicarboxylates of calcium were synthesized. The effect of dicarboxylate on the formation of β‐form polypropylene was investigated by X‐ray diffraction. Calcium pimelate, calcium suberate, calcium phthalate, and calcium terephthalate have been found to be an effective β‐nucleator. The K_x values of the isotatic propylene samples with 0.5 wt % of the nucleators above are 0.95, 0.96, 0.93, and 0.62, respectively. Calcium succinate, calcium adipate, and calcium sebacate behave invalidly on the nucleating of the β‐phase. We conducted an investigation on the affect of particle shape, crystal form, and crystallinity upon the level of the β‐form. The X‐ray diffraction of the effective nucleators reveals a common character that their first reflection locate at the d‐spacing between 10 to 13 Å, indicating structural similarity of the nucleators with β‐polypropylene. The nucleation mechanism is explained by the cooperative effect of the nonpolar and polar part of nucleating agents in the crystallization of polypropylene. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 633–638, 2002     

Variation of non‐isothermal crystallization behavior of isotactic polypropylene with varying β‐nucleating agent content

The non‐isothermal crystallization behavior, the crystallization kinetics, the crystallization activation energy and the morphology of isotactic polypropylene (iPP) with varying content of β‐nucleating agent were investigated using differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). The DSC results showed that the Avrami equation modified by Jeziorny and a method developed by Mo and co‐workers could be successfully used to describe the non‐isothermal crystallization process of the nucleated iPPs. The values of n showed that the non‐isothermal crystallization of α‐ and β‐nucleated iPPs corresponded to a tridimensional growth with homogeneous and heterogeneous nucleation, respectively. The values of crystallization rate constant showed that the rate of crystallization decreased for iPPs with the addition of β‐nucleating agent. The crystallization activation energy increased with a small amount (less than 0.1 wt%) of β‐nucleating agent and decreased with higher concentration (more than 0.1 wt%). The changes of crystallization rate, crystallization time and crystallization activation energy of iPPs with varying contents of β‐nucleating agent were mainly determined by the ratio of the content of α‐ and β‐phase in iPP (α‐PP and β‐PP) from the DSC investigation, and the large size and many intercrossing lamellae between boundaries of β‐spherulites for iPPs with small amounts of β‐nucleating agent and the small size and few intercrossing bands among the boundaries of β‐spherulites for iPPs with large amounts of β‐nucleating agent from the SEM examination. Copyright © 2010 Society of Chemical Industry