Toughness mechanism in polypropylene composites: Polypropylene toughened with elastomer and calcium carbonate

Polypropylene (PP) was modified with elastomer or CaCO₃ particles of two different sizes (1 μm and 50 nm) in various volume fractions. The dispersion morphology and mechanical properties of the two systems were investigated as functions of the particle size and volume fraction of the modifier. The brittle‐to‐tough transition occurred when the matrix ligament thickness was less than the critical ligament thickness, which was about 0.1 μm for the PP used here, being independent of the type of modifier. At the same matrix ligament thickness, the improvement of the toughness was obviously higher with the elastomer rather than with CaCO₃, but adding CaCO₃ increased the modulus of PP. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1656–1662, 2004     

Impact‐specific essential work of fracture of maleic anhydride‐compatibilized polypropylene/elastomer blends and their composites

Charpy drop‐weight‐impact and essential work of fracture (EWF) characteristics of maleic anhydride (MA)‐compatibilized styrene–ethylene butylene–styrene (SEBS)/polypropylene (PP) blends and their composites reinforced with short glass fibers (SGFs) were investigated. MA was grafted to either SEBS copolymer (SEBS‐g‐MA) or PP (PP‐g‐MA). The mPP blend was prepared by the compounding of 95% PP and 5% PP‐g‐MA. Drop‐weight‐impact results revealed that the mPP specimen had an extremely low impact strength. The incorporation of SEBS or SEBS‐g‐MA elastomers into mPP improved its impact strength dramatically. Similarly, the addition of SEBS was beneficial for enhancing the impact strength of the SGF/SEBS/mPP and SGF/SEBS‐g‐MA/mPP hybrids. A scanning electron microscopy examination of the fractured surfaces of impact specimens revealed that the glass‐fiber surfaces of the SGF/SEBS/mPP and SGF/SEBS‐g‐MA/mPP hybrids were sheathed completely with deformed matrix material. This was due to strong interfacial bonding between the phase components of the hybrids associated with the MA addition. Impact EWF tests were carried out on single‐edge‐notched‐bending specimens at 3 m s^?1. The results showed that pure PP, mPP, and the composites only exhibited specific essential work. The nonessential work was absent in these specimens under a high‐impact‐rate loading condition. The addition of SEBS or SEBS‐g‐MA elastomer to mPP increased both the specific essential and nonessential work of fracture. This implied that elastomer particles contributed to the dissipation of energy at the fracture surface and in the outer plastic zone at a high impact speed of 3 m s^?1. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1881–1892, 2002     

Impact‐modified polypropylene/vermiculite nanocomposites

Impact‐modified polypropylene (PP)/vermiculite (VMT) nanocomposites toughened with maleated styrene–ethylene butylene–styrene (SEBS‐g‐MA) were compounded in a twin‐screw extruder and injection‐molded. VMT was treated with maleic anhydride, which acted both as a compatibilizer for the polymeric matrices and as a swelling agent for VMT in the nanocomposites. The effects of the impact modifier on the morphology and the impact, static, and dynamic mechanical properties of the PP/VMT nanocomposites were investigated. Transmission electron microscopy revealed that an exfoliated VMT silicate layer structure was formed in ternary (PP–SEBS‐g‐MA)/VMT nanocomposites. Tensile tests showed that the styrene–ethylene butylene–styrene additions improved the tensile ductility of the (PP–SEBS‐g‐MA)/VMT ternary nanocomposites at the expense of their tensile stiffness and strength. Moreover, Izod impact measurements indicated that the SEBS‐g‐MA addition led to a significant improvement in the impact strength of the nanocomposites. The SEBS‐g‐MA elastomer was found to be very effective at converting brittle PP/VMT organoclay composites into tough nanocomposites. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2332–2341, 2003     

Nonisothermal melting and crystallization of polypropylene in model composites: Kinetic analysis

The kinetics of nonisothermal melting and the crystallization of polypropylene (PP) in polypropylene/carbon‐fiber (C/PP) composites were studied by differential scanning calorimetry with the Nedkov and Atanasov method. Characteristic parameters such as the lamellar thickness, the transport energy through the phase boundary, and the surface free energy were determined and analyzed. In nonisothermal melting, the nucleation effect of carbon fibers was confirmed by decreasing transport energy (79 and 41 kJ/mol for PP and C/PP, respectively) and surface free energy (8 × 10^?4 and 7.9 × 10^?5 J/m² for PP and C/PP, respectively). Depending on the carbon‐fiber content, the lamellar thickness changed from 6.7 × 10^?9 m to 9.05 × 10^?9 m. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 66–73, 2005     

Thermophysical properties of polypropylene/aluminum composites

This article is dedicated to the study of the thermal parameters of composite materials. A nonlinear least‐squares criterion is used on experimental transfer functions to identify the thermal conductivity and the diffusivity of aluminum‐polymer composite materials. The density measurements were achieved to deduce the specific heat and thereafter they were compared to values given by differential scanning calorimetry measurement. The thermal parameters of the composite material polypropylene/aluminum were investigated for the two different types of aluminum filler sizes. The experimental data were compared with several theoretical thermal conductivity prediction models. It was found that both the Agari and Bruggeman models provide a good estimation for thermal conductivity. The experimental values of both thermal conductivity and diffusivity have shown a better heat transport for the composite filled with large particles. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 722–732, 2004     

Phase morphology development in PP/PA6 blends induced by a maleated thermoplastic elastomer

The effects of maleated thermoplastic elastomer (TPEg) on morphological development of polypropylene (PP)/polyamide 6 (PA6) blends with a fixed PA6 content (30 wt %) were investigated. For purpose of comparison, nonmaleated thermoplastic elastomer (TPE) was also added to the above binary blends. A comparative study of FTIR spectroscopy in above both ternary blends confirmed the formation of in situ graft copolymer in the PP/PA6/TPEg blend. Dynamic mechanical analysis (DMA) indicated that un‐like TPE, the incorporation of TPEg remarkably affected both intensity and position of loss peaks of blend components. Scanning electron microscopy (SEM) demonstrated that PP/PA6/TPE blends still exhibited poor interfacial adhesion between the dispersed phase and matrix. However, the use of TPEg induced a finer dispersion and promoted interfacial adhesion. Transmission electron microscopy (TEM) for PP/PA6/TPEg blends showed that a core‐shell structure consisting of PA6 particles encapsulated by an interlayer was formed in PP matrix. With the concentration of TPEg increasing, the dispersed core‐shell particles morphology was found to transform from discrete acorn‐type particles to agglomerate with increasing degree of encapsulation. The modified Harkin's equation was applied to illustrate the evolution of morphology with TPEg concentration. “Droplet‐sandwiched experiments” further confirmed the encapsulation morphology in PP/PA6/TPEg blends. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1050–1061, 2006     

Structure and mechanical properties of talc‐filled blends of polypropylene and styrenic block copolymers

The structure–property relationships of isotactic polypropylene (iPP)/styrenic block copolymer blends filled with talc were examined by optical and scanning electron microscopy, wide‐angle X‐ray diffraction, and tensile‐ and impact strength measurements. The composites were analyzed as a function of the poly(styrene‐b‐ethylene‐co‐propylene) diblock copolymer (SEP) and the poly(styrene‐b‐butadiene‐b‐styrene) triblock copolymer (SBS) content in the range from 0 to 20 vol % as elastomeric components and with 12 vol % of aminosilane surface‐treated talc as a filler. Talc crystals incorporated in the iPP matrix accommodated mostly plane‐parallel to the surface of the samples and strongly affected the crystallization process of the iPP matrix. The SBS block copolymer disoriented plane‐parallel talc crystals more significantly than the SEP block copolymer. The mechanical properties depended on the final phase morphology of the investigated iPP blends and composites and supermolecular structure of the iPP matrix because of the interactivity between their components. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1255–1264, 2004     

Nucleation ability of multiwall carbon nanotubes in polypropylene composites

The nonisothermal crystallization of multiwall carbon nanotube (MWNT)/isotactic polypropylene (iPP) nanocomposites was investigated. The results derived from the differential scanning calorimetry curves (onset temperature, melting point, supercooling, peak temperature, half‐time of crystallization, and enthalpy of crystallization) were compared with those of neat iPP. The data were also processed according to Ozawa's theory and Dobreva's approach. These results and X‐ray diffraction data showed that the MWNTs acted as α‐nucleating agents in iPP. Accordingly, MWNT/iPP was significantly different from neat iPP: A fibrillar morphology was observed instead of the usual spherulites. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 520–527, 2003     

Viscoelastic relaxation mechanisms of conventional polypropylene toughened by a plastomer

Plastomers are new alternatives that provide a higher impact strength in polypropylene synthesized by conventional methods. This study presents an analysis of the structure and thermal characterization of two sets of polypropylene/plastomer blends. The difference between the two polypropylene samples is their molecular weight. With the structural knowledge obtained, a comprehensive analysis of the different relaxation processes has been performed. Neither component is miscible. The area under the loss tangent curves provides preliminary information about how the toughness is enhanced with this type of impact modifier. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1878–1888, 2003     

Isothermal crystallization kinetics of polypropylene/POSS composites

Polypropylene (PP)/octavinyl polyhedral oligomeric silsesquioxane (POSS) composites were prepared by two different processing methods: reactive blending and physical blending, and the crystallization behavior of PP and PP/POSS composites was studied by means of differential scanning calorimetry and polarized optical microscope. The results showed that the crystallization of PP in PP/POSS composites was strongly influenced by the different processing methods. POSS particles can act as effective nucleating agent, accelerating the crystallization of PP. The crystallization rate increased more dramatically for the reactive blending composite due to the stronger nucleating effect of PP grafted POSS. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1762–1772, 2008     

Effects of clay on polymorphism of polypropylene in polypropylene/clay nanocomposites

The effects of clay on polymorphism of polypropylene (PP) in PP/clay nanocomposites (PPCNs) under various thermomechanical conditions were studied. In extruded PP and PPCN pellet samples, only α-phase crystallites existed, as they were prepared by rapidly cooling the melt extrudates to room temperature. Under compression, β-phase crystallites can develop in neat PP under various thermal conditions, of which isothermal crystallizing at 120 °C gave the highest content of β-phase crystallites. In contrast, no β-phase crystallite was detected in the PPCN samples prepared under the same conditions. This indicated that clay significantly inhibits the formation of β-phase crystallites. The likely reason is that the presence of clay in PPCNs greatly sped up the crystallization process of the α phase, whereas it had an insignificant effect on the crystallization rates of the β phase. The results also showed that clay may slightly promote the formation of γ-phase PP crystallites in PPCNs. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1810–1816, 2004     

Annealing induced electrical conductivity jump of multi‐walled carbon nanotube/polypropylene composites and influence of molecular weight of polypropylene

Multi‐walled carbon nanotube (MWCNT)/polypropylene (PP) composites were prepared by a micro melt mixing process. As‐prepared composites had relatively low electrical conductivity due to the disruption of MWCNT network by strong shear. The electrical conductivity jumped to high values throughout an annealing process above the melting temperature of PP. The significant enhancement of electrical conductivity was influenced by annealing time, temperature, and content of MWCNTs. In particular, molecular weight of PP played an important role in affecting the conductivity enhancement. The molecular weight of PP was varied from 190,000 to 340,000 to examine its effect on the electrical conductivity. By comparing the conductivity enhancement behavior of composites with different molecular weight PPs and observing the morphology evolution during annealing, it was found that reaggregation of MWCNTs and the subsequent formation of MWCNT network during annealing are the main reasons for the jump of electrical conductivity. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Viscoelastic behavior of polypropylene/nitrile rubber thermoplastic elastomer blends: Application of Kerner's models for reactively compatibilized and dynamically vulcanized systems

The viscoelastic properties of binary blends of nitrile rubber (NBR) and isotactic polypropylene (PP) of different compositions have been calculated with mean‐field theories developed by Kerner. The phase morphology and geometry have been assumed, and experimental data for the component polymers over a wide temperature range have been used. Hashin's elastic–viscoelastic analogy principle is used in applying Kerner's theory of elastic systems for viscoelastic materials, namely, polymer blends. The two theoretical models used are the discrete particle model (which assumes one component as dispersed inclusions in the matrix of the other) and the polyaggregate model (in which no matrix phase but a cocontinuous structure of the two is postulated). A solution method for the coupled equations of the polyaggregate model, considering Poisson's ratio as a complex parameter, is deduced. The viscoelastic properties are determined in terms of the small‐strain dynamic storage modulus and loss tangent with a Rheovibron DDV viscoelastometer for the blends and the component polymers. Theoretical calculations are compared with the experimental small‐strain dynamic mechanical properties of the blends and their morphological characterizations. Predictions are also compared with the experimental mechanical properties of compatibilized and dynamically cured 70/30 PP/NBR blends. The results computed with the discrete particle model with PP as the matrix compare well with the experimental results for 30/70, 70/30, and 50/50 PP/NBR blends. For 70/30 and 50/50 blends, these predictions are supported by scanning electron microscopy (SEM) investigations. However, for 30/70 blends, the predictions are not in agreement with SEM results, which reveal a cocontinuous blend of the two. Predictions of the discrete particle model are poor with NBR as the matrix for all three volume fractions. A closer agreement of the predicted results for a 70/30 PP/NBR blend and the properties of a 1% maleic anhydride modified PP or 3% phenolic‐modified PP compatibilized 70/30 PP/NBR blend in the lower temperature zone has been observed. This may be explained by improved interfacial adhesion and stable phase morphology. A mixed‐cure dynamically vulcanized system gave a better agreement with the predictions with PP as the matrix than the peroxide, sulfur, and unvulcanized systems. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1417–1432, 2004     

Nonisothermal crystallization kinetics of polypropylene/montmorillonite nanocomposites

The nonisothermal crystallization kinetics of poly(propylene) (PP) and poly(propylene)/organic‐montmorillonite (PP/Mont) nanocomposite were investigated by differential scanning calorimetry (DSC) with various cooling rates. The Avrami analysis modified by previous research was used to describe the nonisothermal crystallization process of PP and PP/Mont nanocomposite very well. The values of half‐time and Z_c showed that the crystallization rate increased with increasing cooling rates for both PP and PP/Mont nanocomposite, but the crystallization rate of PP/Mont nanocomposite was faster than that of PP at a given cooling rate. The activation energies were estimated by the Kissinger method, and the values were 189.4 and 155.7 kJ/mol for PP and PP/Mont nanocomposite, respectively. PP/Mont nanocomposite could be easily fabricated as original PP, although the addition of organomontmorillonite might accelerate the overall nonisothermal crystallization process. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 408–414, 2002; DOI 10.1002/polb.10101     

Crystallization behavior of dynamically cured polypropylene/epoxy blends

Dynamically cured polypropylene (PP)/epoxy blends compatibilized with maleic anhydride grafted PP were prepared by the curing of an epoxy resin during melt mixing with molten PP. The morphology and crystallization behavior of dynamically cured PP/epoxy blends were studied with scanning electron microscopy, differential scanning calorimetry, and polarized optical microscopy. Dynamically cured PP/epoxy blends, with the structure of epoxy particles finely dispersed in the PP matrix, were obtained, and the average diameter of the particles slightly increased with increasing epoxy resin content. In a study of the nonisothermal crystallization of PP and PP/epoxy blends, crystallization parameter analysis showed that epoxy particles could act as effective nucleating agents, accelerating the crystallization of the PP component in the PP/epoxy blends. The isothermal crystallization kinetics of PP and dynamically cured PP/epoxy blends were described by the Avrami equation. The results showed that the Avrami exponent of PP in the blends was higher than that of PP, and the crystallization rate was faster than that of PP. However, the crystallization rate decreased when the epoxy resin content was greater than 20 wt %. The crystallization thermodynamics of PP and dynamically cured PP/epoxy blends were studied according to the Hoffman theory. The chain folding energy for PP crystallization in dynamically cured PP/epoxy blends decreased with increasing epoxy resin content, and the minimum of the chain folding energy was observed at a 20 wt % epoxy resin content. The size of the PP spherulites in the blends was obviously smaller than that of PP. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1181–1191, 2004     

Influence of microstructure on space charges of polypropylene

The correlation of chemical structure, crystalline morphology, and space charge distribution under a dc electrical field was investigated with three kinds of poly(propylene) (PP) with a different chemical structure, that is, homogeneous PP and block copolymer and random copolymer of PP. The space charge distribution of the samples was prominently affected by their chemical sequence structure and crystalline microstructure. Among samples of different PPs, all isothermally crystallized at 140 °C, the sample of random coPP represents the most well proportional space charge distribution and the smallest number of space charges. The effect of thermal history on the space charge distribution was also investigated by the samples of block coPP. The sample thermally treated at 50 °C clearly represents a better proportional distribution than that at higher temperature of 140 and 100 °C. Subsequent experiments indicate that the better proportional distribution attributes to imperfect and fine sperulites with the fine distribution of the “amorphous” region. The imperfect and fine sperulites originate from the random incorporation of ethylene segments or units into PP chains or from the low annealing temperature, and play an important role in the formation of shallow traps and transportation of space charges. © 2002 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 365–374, 2002; DOI 10.1002/polb.10100     

Preparation and characterization of polypropylene/mesoporous silica nanocomposites with confined polypropylene

Polypropylene (PP)/Ti-MCM-41 nanocomposites were prepared by isospecific propylene polymerization with Ti-MCM-41/Al(i-C₄H₉)₃ catalyst. The cross polarization/magic angle spinning (CP/MAS) ¹³C NMR spectrum of the composite was similar to that of the conventional isotactic PP, and the decrease in the pore volume of Ti-MCM-41 in the nanocomposites, as measured by N₂ adsorption, was consistent with the value calculated from the weight loss in the thermogravimetric analysis (TGA) curve; both these facts attest to propylene polymerization within the mesopores of Ti-MCM-41. Alkali treatment followed by extraction with o-dichlorobenzene allows us to extract the confined PP out of the Ti-MCM-41 mesopores. Although the PP/Ti-MCM-41 nanocomposites do not exhibit a crystalline melting point, the same PP when extracted from the mesopores showed a clear melting point at 154.7 °C; this indicates that the crystallization of PP confined in mesopores is strongly hindered. For the PP polymerized within the confinement, the molecular weight (M_w) and molecular weight distribution (M_w/M_n) were 84,000 and 4.3, respectively; these values were considerably smaller than those of the PP polymerized concurrently outside the Ti-MCM-41 mesopores (M_w = 200,000–450,000, M_w/M_n = 40–75). Therefore, the confinement also has a marked effect on the molecular weight of the PP. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 3324–3332, 2003     

Kinetics study on melt compounding of carbon nanotube/polypropylene nanocomposites

The multiwalled carbon nanotubes/polypropylene nanocomposites (PP/CNTs) were prepared by melt mixing using maleic anhydride grafted polypropylene (mPP) as the compatibilizer. The effect of mPP on dispersion of CNTs was then studied using the tool of rheology, aiming at relating the viscoelastic behaviors to the mesoscopic structure of CNTs. To further explore the kinetics of hybrid formation, a multilayered sample with alternatively superposed neat mPP and binary PP/CNTs microcomposites (without addition of mPP) sheets was prepared and experienced dynamic annealing in the small amplitude oscillatory shear flow. The results show that melt blending CNTs with PP can only yield the composites with microscale dispersion of CNTs, while adding mPP promotes nanoscale dispersion of CNTs as smaller bundles or even as individual nanotubes, reducing percolation threshold as a result. However, the values of apparent diffusivities of the composites are in same order with that of self‐diffusion coefficients of the neat PP, indicating that the presence of detached CNTs nearly does not inhibit PP chain motion. Hence, the activation energy of hybrid formation is close to the self‐diffusion of PP. This also indicates that although addition of mPP can improve the compatibility between CNTs and PP thermodynamically, those dynamic factors, such as shear flow, however, may be the dominant role on hybrid formation. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 608–618, 2009     

Melting behavior in blends of isotactic polypropylene and a liquid crystalline polymer

The influence of low contents of a liquid crystalline polymer on the crystallization and melting behavior of isotactic polypropylene (iPP) was investigated using electron and optical microscopy, differential scanning calorimetry, and X-ray diffraction. In pure iPP, the α modification was found, whereas for iPP/Vectra blends at Vectra concentration <5%, both α and β forms were observed. The amount of β phase varied from 0.23 to 0.16. Optical microscopy showed that Vectra was able to nucleate both α and β forms. Non-isothermal crystallization produces a material with a strong tendency for recrystallization of the α and β forms (αα′ and ββ′ recrystallization) leading to double endotherms for both crystalline forms in DSC thermograms. Melting thermograms after isothermal crystallization at low temperatures showed a similar behavior. At values of T_c > 119 °C for the α form and T_c > 125 °C for the β form, only one melting endotherm was observed because enough perfect crystals, not susceptible to recrystallization, were obtained. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1949–1959, 2004