Comparison of the molecular properties and morphology of polypropylenes irradiated under different atmospheres and after annealing

Electron‐beam irradiation, a well‐known way of generating long‐chain branching, was used to modify polypropylene. Samples were investigated with differential scanning calorimetry, polarized light microscopy, and size exclusion chromatography. Independently of the atmosphere, postannealing led to the deactivation of residual radicals and to the reduction of the nucleus density. In comparison with the initial polypropylene, the crystallization temperatures increased for nonannealed samples but decreased for annealed samples. Stable products were obtained only by irradiation in nitrogen followed by annealing. A reaction including free radicals with oxygen in the ambient atmosphere led to increasing molar mass degradation and the formation of long‐chain branching after storage. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 634–639, 2006     

Long‐chain branching of polypropylene by electron‐beam irradiation in the molten state

The electron‐beam irradiation of polymers generates modification effects in the macromolecular structure and material properties. Therefore, irradiation processing is mostly realized in the polymer solid state. In this way, the modification of linear polypropylene may result in long‐chain branching of polypropylene macromolecules. The objective of this article is to investigate the effect of a polymer in the molten state during electron‐beam irradiation on the macromolecular structure and material properties of polypropylene. For this procedure, a special irradiation vessel (BG3) has been developed in which a rapid transfer of polymer films from the solid state to the molten state and a defined temperature during electron‐beam irradiation are realizable. The irradiated samples have been analyzed by high‐temperature size exclusion chromatography coupled with a multi‐angle laser light scattering detector and differential scanning calorimetry (DSC) measurements. With an increasing irradiation dose, a high reduction of the molar mass and an increasing amount of long‐chain branching are found. Compared with irradiation in the solid state, the modification in the molten state leads to a higher degree of branching. The rheological experiments in elongation flow clearly exhibit the existence of long‐chain branching. Furthermore, DSC measurements show that the glass‐transition temperature and peak temperatures of melting and crystallization decrease. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 260–265, 2006     

Crystallization behaviors of polypropylene and functional polypropylene

The crystallization behaviors of polypropylene (PP) homopolymer and its blends with 0–15% functional polypropylene (FPP), the backbones of which were grafted with guanidine and diamide polymer chains, were investigated with differential scanning calorimetry and wide‐angle X‐ray scattering. The crystallization kinetics were studied with spectral depolarization. The results revealed that the presence of FPP reduced the crystallinity and crystallite size of PP. Meanwhile, FPP increased the crystallization rate. Compared with that of the PP homopolymer, the crystallization temperature of PP/FPP blends was increased by more than 10°C. During isothermal crystallization, the relative crystallinity, developed as a function of time, was described by the Avrami equation. The half‐time of crystallization for PP/FPP blends was much shorter than that for the PP homopolymer. The half‐time of crystallization of PP/FPP blends depended much less on the crystallization temperature than that of the PP homopolymer. Therefore, FPP accelerated the crystallization rate of PP in a manner similar to that of a nucleator. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 872–877, 2003     

Isothermal crystallization behavior of polypropylene catalloys

The isothermal crystallization behavior of polypropylene (PP) catalloys and neat PP were studied with differential scanning calorimetry and polarized optical microscopy (POM). The crystallization kinetics of the samples were described with the well‐known Avrami equation. The crystallization rate depended remarkably on the content of the ethylene component in the PP catalloys. The crystallization half‐time increased obviously with the increase of the ethylene component in the PP catalloys. We also observed by POM that in isothermal crystallization, there were many more nuclei in the PP catalloys than that in neat PP and with an increase of the ethylene component, the average size of the spherulites decreased obviously. Even when ethylene content was as high as 27%, the crystallization rate still increased apparently, and this was quite different from common PP melting blends, in which the crystallization rate decreased when the ethylene content was relatively high because of the obstruction effect of dispersed droplets to the spherulite growth of the PP matrix. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 877–882, 2004     

Thermal behavior and morphological and rheological properties of polypropylene and novel elastomeric ethylene copolymer blends

The thermal behavior including melting and crystallization behavior and morphological and rheological properties of the blends based on an isotactic polypropylene and a novel maleated elastomeric ethylene copolymer were investigated in this work. The addition of an elastomer to polypropylene (PP) was found not to change the PP crystalline structure significantly when cooled quickly from the melt. On recrystallization at a lower cooling rate, the elastomer promotes the formation of β?pseudohexagonal PP in PP‐rich blends. In elastomer‐rich compositions, heterogeneous nucleation is hindered and homogeneous nucleation takes place. These phenomena are revealed by morphology observation: that, with increasing of the elastomer content, the system undergoes PP continuous, dual‐phase continuity and PP‐dispersed morphologies. The blend viscosity at a low shear rate range increases continuously with increasing elastomer content and shows positive deviations from the additivity rule. In the terminal zone, the dynamic storage modulus of the blends shows positive deviation from the simple mixing rule and the maximum deviation lies in the composition range of dual‐phase continuity which could be caused by a large increase in the interfacial tension. The Cox–Merz rule does not hold for the blends because of the two‐phase heterogeneous structure and its variation in steady and oscillatory shear flow. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 3430–3439, 2002     

Effects of single‐walled carbon nanotubes on the crystallization behavior of polypropylene

Polypropylene matrix composites reinforced with single‐walled carbon nanotubes (SWNTs) were produced with different nanotube concentrations. The characterization of these new materials was performed by differential scanning calorimetry and Raman and Fourier transform infrared spectroscopy to obtain information on the matrix–nanotube interaction, on the crystallization kinetics of polypropylene, and especially on the macrostructure and organization of the nanotubes in the composite. On the one hand, the results confirmed the expected nucleant effect of nanotubes on the crystallization of polypropylene, but on the other hand, this effect was not linearly dependent on the SWNT content: there was a saturation of the nucleant effect at low nanotube concentrations. Raman spectroscopy was successfully applied to demonstrating that in the composite films, the crystallization kinetics were strongly affected by the distance between the nanotube bundles as a result of a different intercalation of the polymer. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 708–713, 2003     

Thermal properties and morphology of isotactic polypropylene/acrylonitrile–butadiene rubber blends in the presence and absence of a nanoclay

A thermoplastic elastomer (TPE) nanocomposite based on polypropylene (PP), acrylonitrile–butadiene rubber (NBR), and a nanoclay (NC) was prepared in a laboratory mixer with a 54/40/6 weight ratio. The effects of NC on the thermal properties, crystalline structure, and phase morphology of the TPE nanocomposite were studied in this work. The results obtained from the nonisothermal crystallization of PP, PP/NBR, and PP/NBR/NC, which was carried out with differential scanning calorimetry, revealed that the overall rate of crystallization of PP decreased with the addition of NBR to PP and increased when NC was incorporated into the nanocomposite. In addition, the crystallite size distribution was more uniform for the PP phase crystallized in the nanocomposite versus the PP itself. Also, although the PP in the reference blend (PP/NBR) crystallized only in the α form, the crystalline structure of the PP incorporated into the nanocomposite was a mixture of α‐ and γ‐crystalline forms. The effects of NC on the phase morphology of PP/NBR blends prepared with three different cooling methods (quenching in liquid nitrogen, cooling between two metal plates at room temperature, and molding at a high temperature in a hot press) were studied. For the samples quenched in liquid nitrogen or cooled between metal plates, a particulate–cocontinuous morphology formed. However, for the samples prepared under a hot press, a laminar‐like morphology was observed. In all three cases, a similar particulate–cocontinuous morphology formed for the reference blend, but the rubber inclusions were always smaller than those of the TPE nanocomposite. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Thermal stability and crystallization kinetics of isotactic polypropylene/organomontmorillonite nanocomposites

The thermal stability and crystallization kinetics of isotactic polypropylene (iPP) and iPP/organomontmorillonite (organo‐MMT) nanocomposites were investigated with differential scanning calorimetry and thermogravimetry. The incorporation of organo‐MMT up to a concentration of 4 wt % did not affect the melting temperature of iPP but did increase the peak thermal degradation temperature by 60°C. The isothermal crystallization kinetics showed that the addition of organo‐MMT increased the crystallization rate of iPP but reduced the isothermal Avrami exponent. The crystallization temperature of the nanocomposites measured with nonisothermal crystallization was higher than that of plain iPP, and this indicated an enhanced crystallization rate. The nonisothermal Avrami exponent, like the isothermal exponent, decreased with the addition of organo‐MMT, and this suggested changes in the crystallite growth geometry. Subsequently, the tensile yield strength and the tensile modulus both increased, but the elongation at break and the notched Izod impact strength did not change significantly. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3404–3415, 2003     

Effect of long‐chain branches of polypropylene on rheological properties and foam‐extrusion performances

The effect of modifying polypropylene by the addition of long‐chain branches on the rheological properties and performance of foam extrusion was studied. Three polypropylenes, two long‐chain‐branched polypropylenes and a linear polypropylene, were compared in this study. The modification was performed with a reactive‐extrusion process with the addition of a multifunctional monomer and peroxide. The rheological properties were measured with a parallel‐plate and elongational rheometer to characterize the branching degree. The change from a linear structure to a long‐chain‐branched nonlinear structure increased the melt strength and elasticity of polypropylene. Also, there was a significant improvement in the melt tension and sag resistance for branched polypropylenes. Foaming extrusion was performed, and the effect of the process variables on the foam density was analyzed with Taguchi's experimental design method. For this study, an L₁₈(2¹3⁵) orthogonal array was used on six parameters at two or three levels of variation. The considered parameters were the polypropylene type, the blowing agent type, the blowing agent content, the die temperature, the screw speed (rpm), and the capillary die length/diameter ratio. As a result, the most significant factor that influenced the foam density was the degree of long‐chain branching of polypropylene. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1793–1800, 2005     

Effect of wood species on the mechanical and thermal properties of wood–plastic composites

The effect of wood species on the mechanical and thermal properties of wood–plastic composites (WPCs) was explored. Various wood species, including cherry, sweet gum, hickory, yellow poplar, Osage orange, walnut, eastern red cedar, pine, maple, and red oak, were compounded with virgin isotactic polypropylene in a 50 : 50 weight ratio and injection‐molded. The tensile strength of WPCs made with cedar and hickory was higher than that of WPCs made with maple, oak, and Osage orange. The tensile modulus of WPCs made with gum and walnut was higher than that of oak WPCs. The tan δ peak temperatures and peak values from dynamic mechanical analysis indicated that pine and hickory WPCs had higher amorphous or void contents than walnut and cherry WPCs. The induction time during isothermal crystallization suggested that red cedar, cherry, and gum WPCs had higher nucleation density than walnut, pine, and oak WPCs. Dynamic mechanical properties of the WPCs appeared to be related to the crystallization behavior of the wood flour, which depends on the surface roughness. Although there were statistically significant differences in mechanical properties among the species, the differences were small, implying that wood flours from many species can be used successfully as raw materials for WPCs. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Influence of γ irradiation on the properties of polyacrylonitrile films

In this study, we examined the effects of γ irradiation on the mechanical, thermal, structural, and electrical properties of polyacrylonitrile (PAN). Irradiation doses of 20, 40, 60, 80, and 100 kGy were used, with nonirradiated PAN films serving as control samples. Microhardness measurements, mechanical tests, Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry analysis, color determination, X‐ray diffraction, and electrical properties were investigated to evaluate the effects of the irradiation treatments on the PAN films. A fair consistency was observed between the microhardness results. Irradiation caused a significant deterioration in the mechanical properties of the samples. The tensile strength and percentage elongation at break decreased with increasing irradiation dose. Similarly, Young's modulus and toughness also decreased with increasing irradiation dose. The melting and crystallization temperatures decreased, whereas the degree of yellowness increased with increasing irradiation dose. The percentage crystallinity of the PAN film increased with increasing irradiation dose. The FTIR spectra showed that there was a tendency toward a greater effect of γ irradiation on the structure of PAN at higher irradiation doses. The values of the electrical parameters, such as capacitance in parallel, dielectric constant, dielectric loss, resistance in series, resistance in parallel, reactance, and quality factor, increased, whereas the values of capacitance in series, impedance, conductance, susceptance, admittance, and phase angle decreased because of the γ irradiation. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Effects of additives on non-isothermal crystallization kinetics and morphology of isotactic polypropylene

In this study, effects of commercial additives such as antioxidant and stabilizer on the non-isothermal crystallization kinetics of isotactic polypropylene without nucleating agents were investigated by differential scanning calorimetry (DSC) method. Kinetic parameters by Osawa, Avrami and Liu-Mo models and apparent activation energy of the crystallization by Kissinger model were calculated. A polarized optical microscope was also used to observe crystalline morphology of the polypropylene samples crystallized at different cooling rates. On the contrary rate inducing effects of the nucleating agents on the crystallization kinetics of the polypropylene, interestingly, it was found that such types of commercial additives reduced the overall crystallization rate of the polypropylene. Based on the crystallization kinetics and morphology of the samples, it was observed that commercial additives inhibit the chain diffusion toward the growing crystal faces thus slow the crystal growth rate. Furthermore, calculated nucleation activity (ϕ) for the additives showed that they do not act as effective nucleating agents. It was found that the crystallization activation energy of additive-free sample was higher than that of the sample which has commercial additives. Activation energies were found to be 233.6 and 276.7 kJ mol⁻¹ for the PP-1 and PP-2, respectively. Kinetic results also show importance of using of nucleating agents to increase the crystallization rate of polypropylene by increasing the nucleation and thus overall crystallization rate during polypropylene processing operations (esp. for a fast processing cycle in injection molding).     

Differential scanning calorimetry studies on high‐ and low‐density annealed and irradiated polyethylenes: Influence of aging

The effect of γ‐radiation, followed by 10 years storage at ambient conditions, on the thermal behavior of different types of high‐ and low‐density commercial polyethylenes was studied. First, samples were annealed to improve the crystalline content. Next, they were irradiated, after which fusion endotherms, melting temperatures, crystallinity indices, and lamellar thicknesses were obtained by differential scanning calorimetry (DSC). The change in the thermal parameters for the first and second meltings were related to the absorbed doses. Afterward, the samples were stored at ambient conditions for 10 years and then scanned again by DSC to assess the influence of aging on previously irradiated samples. The results showed that the changes on the morphological structure undergone by the samples with the storage time were highly dependent on the polyethylene type and the absorbed radiation dose. The high‐density polyethylene was the most sensitive to radiation and storage, whereas the low‐density polyethylene with the lowest molecular weight and the highest degree of branching was the least affected. In general, the changes observed during irradiation can be explained in terms of an increase of imperfections and chain scissions. The storage can be understood as a slow crystallization process at low radiation doses, and as a decrease of the crystalline structure at high doses. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3260–3271, 2003     

Effect of heat‐sealing temperature on the properties of OPP/CPP heat seals. Part II. Crystallinity and thermomechanical properties

The crystalline structure of the heat‐sealed part of oriented polypropylene (OPP) and cast polypropylene (CPP) films was investigated by differential scanning calorimetry, Fourier transform infrared spectroscopy, and thermal mechanical analysis (TMA). The relationship between the crystalline structure and the mechanical properties was analyzed. It was found that the high total crystallinity of both OPP and CPP gave rise to good mechanical properties and that the orientation of the crystalline structure in the OPP film also was an important factor. The optimum condition for heat sealing was the temperature at which total crystallinity was highest while still retaining the crystalline orientation of OPP. The assessment of crystalline orientation by TMA is an innovation initiated by the authors. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 513–519, 2006     

Surfactant assisted processing of carbon nanotube/polypropylene composites: Impact of surfactants on the matrix polymer

Multiwalled carbon nanotubes (MWCNT) were dispersed in presence of various surfactants and introduced into isotactic polypropylene (PP) via melt blending. The effect of the surfactants on the crystallization and mechanical properties was studied on the injection molded specimens by isothermal differential scanning calorimetry and tensile tests, respectively. The results reveal that the presence of surfactants affects the mechanical and thermal properties of the nanocomposite. This information is supported by thermogravimetric analysis of the surfactants to examine their behavior at temperatures relevant for the processing of PP. In addition, they have to be taken into account as a third phase between the MWCNT and polymeric matrix, affecting the crystallization and failure behavior of PP or causing the formation of pores at elevated temperatures. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

The crosslinking of poly (ether ether ketone): Thermally and by irradiation

Crosslinking of amorphous poly (ether ether ketone) films was carried out by means of thermal annealing at 400°C as well as by irradiation with 11.0 MeV proton beam at different dose rate ranging from 1.75 to 15.5 kW g^?1. The materials properties of the resulting films were investigated by mean of light microscopy, sol‐gel analysis, two‐dimensional infra‐red correlation spectroscopy, 2DCOS‐IR, and differential scanning calorimetry (DSC). It was found that both chain scission and crosslinking yields were decreased by an increase of dose rate and the ratio of crosslinks to chain scission was increased from 0.9 to 1.4. The 2DCOS‐IR analysis in the region 1400–1800 cm^?1 showed progressive development of new bands at 1470 and 1740 cm^?1, which have been used to support the presence of crosslinking and chain scission reactions. The glass transition temperature also increased in line with increasing crosslinking density, but the results showed a limiting plateau value for the glass transition , which depended only on the absorbed dose. This suggests that crosslinking was limited and did not increase after a specific value of the dose rate. Crosslinking by irradiation has many advantages over thermal annealing; in particular it is a one step rapid process producing a variety of homogeneously crosslinked, good quality films available for chemical modification. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41999.     

Comparative study of the rheological behavior of multiwalled carbon nanotubes and nanofiber composites prepared by the dilution of a masterbatch of polypropylene

In this investigation, the characteristics and the rheological properties of two different nanocomposite systems were investigated. These systems consisted of a dispersion of carbon nanotubes (CNTs) and carbon nanofibers (CNFs) in a polypropylene (PP) matrix. The mixing process was carried out by melt compounding with a twin‐screw corotating extruder with different reinforcement amounts (0.2–20 wt %) from concentrated masterbatches (20 wt %) of PP/CNT and PP/CNF. The results show a remarkable increase in the viscosity for both blends as the reinforcement amount was increased. It was important to evaluate the rheological behavior to understand the effect of the nanocarbon particles on the internal structures and their processing properties of the obtained composites. CNFs were a more viable reinforcement from a processability point of view because the obtained viscosities of the PP/CNF blends were more manageable. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Applications of rheological torque–time curves to the study of thermooxidative degradation of polypropylene powder

The torque–time curves of polypropylene (PP) powder treated under various thermooxidative degradation conditions were obtained through processing in the mixing chamber of a rheometer. Meanwhile, the Fourier transform infrared (FTIR) spectra of the corresponding samples were determined, and the quantitative analysis of the carbonyl indices of the FTIR spectra of the samples of the PP powder was carried out to provide evidence for the rheological characterization. PP granules, to which an antideteriorant was added before they were commercially supplied so good antidegradation could be achieved, was investigated for the sake of contrast. The analysis of the experimental results showed that the height of the torque–time curve of the PP powder and the corresponding value of the equilibrium torque could be used to characterize or evaluate the variations of the thermooxidative degradation of the PP powder. Under the same processing conditions, the heights of the torque–time curves of the PP powder and the corresponding values of the equilibrium torque decreased with the enhancement of the thermooxidative degradation treatment before mixing; on the contrary, the heights and areas of the characteristic bands of the carbonyl groups in the FTIR spectra of the PP powder and the corresponding values of the carbonyl index increased. The quantitative analysis of the FTIR spectra provided evidence for the conclusion that the heights of the torque–time curves of the PP powder and the corresponding values of the equilibrium torque could be used to characterize or evaluate the thermooxidative degradation of the PP powder. If the treatment under thermooxidative degradation conditions weakened or the degradation of the PP powder just began (i.e., in the viscosity range for processability), the evaluation method using the heights of the torque–time curves of the PP powder or the corresponding values of the equilibrium torque could provide more sensitivity than the method using the values of the carbonyl index. Consequently, the method using the heights of the torque–time curves to evaluate the thermooxidative degradation of the PP powder had its advantages. The application of the torque–time curves could be used to evaluate not only the variation of the thermooxidative degradation of the PP powder treated under aging conditions before mixing but also the variation of the degradation, including the mechanochemical degradation, of the PP powder during the period of mixing. The dependence of the variation of the degradation of the polymer on the processing time during mixing could be evaluated by the study of the variation of the torque–time curves. It can be concluded that the application of torque–time curves to the evaluation of degradation of PP powder has the advantages of being convenient, real‐time, in situ, online, and production‐oriented. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Influence of magnesium sulfate whiskers on the structure and properties of melt‐stretching polypropylene microporous membranes

The influence of magnesium sulfate (MgSO₄) whiskers on the structure and properties of polypropylene cast films and stretched microporous membranes was investigated. We found that for the cast films, MgSO₄ showed some nucleation effects, and the introduction of MgSO₄ led to the decrease of the orientation degree along the machine direction (MD), whereas that along the transverse direction (TD) was improved; this indicated that MgSO₄ whiskers were mainly arranged along the TD. The introduction of MgSO₄ up to 10 wt % did not induce apparent changes in the pore structure and air permeability properties of the stretched microporous membranes but improved the electrolyte absorption ability. The most pronounced change for the stretched microporous membranes was the strength along the TD. It was increased by 110% when the MgSO₄ content was 2 wt %. During the fabrication of microporous membranes, only stretching along the MD was carried out to initiate pore formation; this resulted in a lower strength along the TD. This study gave us a method for improving the mechanical properties of stretched microporous membranes along the TD. The obtained microporous membranes with better electrolyte absorption and higher mechanical strength along the TD could be used in lithium‐ion batteries as separators. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43884.     

Differential scanning calorimetry of copolymer of isotactic polypropylene backbone with grafted poly(ethylene‐co‐propylene) branches

A series of graft polymers having polypropylene (PP) backbone and poly(ethylene‐co‐propylene) (EPR) side chains was prepared. PP backbone molecular weight (M_n) was 28–98 kg/mol, EPR side chain M_n was 2.6–17 kg/mol, and EPR content was 0–16 wt %. In this work, thermal analysis of the copolymers was performed using differential scanning calorimetry (DSC). Nonisothermal crystallization was performed at different cooling rates. The DSC thermograms revealed multiple melting peaks for slowly cooled samples, most likely the result of the melting of thinner tangential lamellae followed by the melting of thicker radial lamellae. Equilibrium melting temperature (T_m⁰) was determined using the linear Hoffman–Weeks method. Another approach was also used for determining T_m⁰: melting temperature (T_m) and crystallization temperature (T_c) were plotted as functions of logarithmic cooling rate. Linear relationships were observed for all samples with the cross points as T_m⁰'s. As cooling rate decreased, T_c, T_m, and enthalpy of fusion (ΔH_f) increased. T_m and T_m⁰ increased with increasing PP M_n. T_c and T_m were unaffected by the grafting of EPR onto the PP backbone. T_m⁰ and ΔH_f decreased as EPR content increased. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 99: 3380–3388, 2006