Structural orientation and tensile behavior in the extrusion-stretched sheets of polypropylene/multi-walled carbon nanotubes' composite

In this paper, the structural orientation and tensile properties were investigated on the extrusion-elongated sheets of polypropylene (PP)/multi-walled carbon nanotubes' (MWCNTs) composites. The MWCNTs were modified via reactive grafting with octadecylamine to enhance the capability of dispersion in PP matrix. The orientation level of composite sheets was adjusted through adopting various drawn speeds after extrusion, and the structure-property relation was inspected systematically through the combination of two-dimensional wide-angle X-ray scattering, polarized light microscopy and tensile mechanical testing. The tensile behavior of the extrusion-drawn sheet was significantly impacted by both adding MWCNTs and increase of drawn speed. An optimization for simultaneously strengthening and toughening was achieved in the sample with relatively high content MWCNTs prepared upon fast drawn speed. Incorporation of nanotubes didn't alter the orientation degree of crystalline lamellae obviously, but the oriented nanotubes could act as the nucleating templates for the growth of oriented crystalline superstructure. Combined with the present morphological characterizations and the results in other published papers, it was postulated that the oriented superstructure of cylindrulites composed of rigid CNTs as nucleus and PP lamellae as kebabs might be generated in the extrusion-elongated sheets of PP/MWCNTs, which was regarded as the morphological reason for a reinforcing effect on tensile performances.     

Reactive extrusion of polypropylene with supercritical carbon dioxide: Free radical grafting of maleic anhydride

A reactive extrusion process for the functionalization of polypropylene with maleic anhydride in the presence of supercritical carbon dioxide was studied. Supercritical carbon dioxide was used in this reactive extrusion system to reduce the viscosity of the polypropylene melt phase by forming a polymer–gas solution in order to promote better mixing of the reactants. Subsequently, the effect of supercritical carbon dioxide on the level of grafting, product homogeneity, and molecular weight was evaluated. Analysis of the products revealed that the use of supercritical carbon dioxide led to improved grafting when high levels of maleic anhydride were used. The experimental results showed no evidence of an improvement in the homogeneity of the product, while melt flow rate measurements showed a reduction in the degradation of polypropylene during the grafting reaction when low levels of maleic anhydride were employed. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1116–1122, 2003     

Influence of chemical functionalization of carbon nanotubes on their dispersibility in alkyl methacrylate polymer matrix

Nanocomposites based on poly(methyl methacrylate) (PMMA) and poly(methyl methacrylate‐co‐octadecyl methacrylate) (M/O) matrices and four different types of multiwall carbon nanotubes: pristine, oxidized (MWCNT–COOH), methyl ester (MWCNT–COOCH₃), and dodecyl ester (MWCNT–COOC₁₂H₂₅) functionalized, were prepared in situ by radical (co)polymerization. The effectiveness of preparation of nanocomposites regarding dispersion and distribution of various MWCNT in polymer matrices was sized by Scanning electron microscopy. In case of PMMA matrix, the best dispersion and distribution were accomplished for MWCNT–COOCH₃ due to their chemical resemblance with polymer matrix. After the introduction of 10 mol % of octadecyl methacrylate in polymer matrix a fairly good dispersion and distribution of MWCNT–COOCH₃ were retained. The addition of 1 wt % of MWCNTs caused a significant reduction in the degree of polymerization of the PMMA matrix. But at the same time, the present MWCNTs increased storage modulus of PMMA nanocomposites except for dodecyl ester functionalized MWCNT. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46113.     

Free‐radical grafting of acrylic acid onto isotactic polypropylene using styrene as a comonomer in supercritical carbon dioxide

Free‐radical grafting of acrylic acid (AAc) onto isotactic polypropylene (iPP) using styrene (St) as a comonomer in supercritical carbon dioxide (SCCO₂) medium was studied. The effects of temperature and pressure of reaction on functionalization degree (grafting degree of AAc) of the products were analyzed. The increase of reaction temperature increases the diffusion of monomers and radicals in the disperse reaction system of SCCO₂. In addition, the increase of temperature accelerates the decomposition rate of 2,2′‐azobisisobutyronitrile (AIBN), thus promoting grafting reaction. It was also observed that functionalization degree of the products decreases with the increase of pressure of SCCO₂ in the range of experiment. The effects of comonomer St on the functionalization degree of the products were investigated. The AAc graft degree of the resulting polymer was drastically higher in the present of St. It reached a maximum when the mass ratio of St and AAc was about 0.7 : 1. Because AAc is not sufficiently reactive toward iPP macroradicals, it would be helpful to use a second monomer that can react with them much faster than AAc. St preferentially reacts with the iPP macroradicals to form more stable styrene macroradicals, which then copolymerize with AAc to form branches. The highest functionalization degree was obtained when the AIBN was 0.75 wt %. When the initiator was used excessively, the functionalization degree decreased because of severe chain degradation of the iPP backbone. The morphologies of pure iPP and grafted iPP are different under the polarizing optical microscope. The diameter of the pure iPP spherulites is 20–38 μ and that of the grafted iPP spherulites is reduced with the increase of the functionalization degree of the products. This is proposed to be because the polar grafts formed during the reaction would have a tendency to associate in the hydrophobic PP environment. This might preserve some of the local crystalline order that existed during the reaction in the swollen iPP phase. It can be proven by a DSC cooling investigation that the crystallization temperature increased as the functionalization degree increased. This is proposed to be because the side‐chain of grafting polymer helps to bring about the heterogeneous nucleation in grafting polymer. Therefore, a large number of nuclei can emerge to a lesser supercooling degree. It can be also proven that the percent crystallization decreased as the functionalization degree increased, probably due to the grafted branches, which disrupted the regularity of the chain structure and increased the spacing between the chains. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2203–2210, 2004     

Effect of solvents and chain transfer agents on grafting on polyethylene films

Photosensitized and with UV-light irradiated low-density polyethylene was crosslinked and grafted with methacrylic acid at 95 and 96°C in the presence of toluene and chain transfer agents like tert-dodecyl mercaptan and carbon tetrachloride. The effect of the concentration of toluene and of chain transfer agents on the graft level has been studied. The graft level can be regulated varying the concentration of toluene and chain transfer agents. Low electrical resistance was obtained at a low concentration of the chain transfer agent and 30% toluene in methacrylic acid. Pore sizes and tensile strengths change in wider range by grafting in the presence of toluene than using chain transfer agents.     

Coagent assisted polypropylene radical functionalization: monomer grafting modulation and molecular weight conservation

The occurrence of significant degradation by β-scission reaction is a severe drawback affecting the classical procedure of polypropylene (PP) functionalization in the melt with maleic anhydride (MAH) or derivatives and free radical initiators. The present work deals with the control of the PP blocks degradation by using a specially designed furan derivative, butyl 3-(2-furyl) propenoate (BFA), as coagent able to control the PP blocks scission during PP functionalization process. More specifically MAH or BFA were used as functionalizing reagents for a propylene/ethylene 77/23 mol% semi-block copolymer (PPC) either separately or as mixture (MAH/BFA) with different molar ratios. The PPC functionalized samples were then characterized by determining the number of grafted groups (functionalization degree FD) and the molecular weight (MW). Finally, the effect of the feed conditions on the process and on functionalized PPC properties are discussed by hypothesizing a reaction mechanism, which takes into account all the occurring parallel reactions.     

Free radical grafting of glycidyl methacrylate onto polypropylene in a co-rotating twin screw extruder

Free radical grafting of glycidyl methacrylate (GMA) onto molten polypropylene (PP) was studied in a co-rotating twin screw extruder. Grafting yields of GMA obtained under various experimental conditions along the screw length allowed for a good appreciation of the effects of chemical parameters (the presence of styrene and the concentrations of peroxide and monomers) and those of processing parameters (feed rate, screw speed, and specific throughput). Similar to the results obtained in a batch mixer,¹ free radical grafting of GMA carried out in the extruder in the presence or absence of styrene proceeded rapidly, as it was virtually completed half-way down stream of the extruder. Additionally, the presence of styrene as a second monomer increased the GMA grafting yield reatly with reduced PP chain degradation. The ultimate GMA grafting yield increased with increasing concentration of peroxide, 1,3-bis(tert-butylperoxyisopropyl)benzene. This similarity between the batch mixer and the extruder is related to the fact that in both cases it is the concentration of the peroxide and its half lifetime that determine the grafting rate and the ultimate grafting yield. On the othe hand, the GMA grafting yield decreased with increasing screw speed or feed rate. For a particular specific throughput (the ratio of throughput to screw speed), an increase in throughput with a concomitant increase in screw speed brought about a decrease in GMA grafting yield. It was concluded that the GMA grafting yield is affected primarily by the residence time in the zone in which free radicals are not depleted. The effects of screw speed, feed rate, and specific throughput manifest mainly through this local residence time distribution. Specific energy is not a good measure of the performance of the extruder with respect to the free radical rafting of GMA onto PP. © 1995 John Wiley & Sons, Inc.     

Effect of stabilizer on the radiation grafting of methylmethacrylate onto polypropylene

The effects of the molecular weight of polypropylene, nucleating agent, and types of stabilizers on the grafting of methylmethacrylate onto polypropylene sheets were examined. The oxidation of polypropylene sheet surfaces exposed to air after irradiation was studied by measuring survey scan spectra with an X-ray photoelectron spectrometer (XPS). The 1,1-diphenyl-2-picrylhydrazyl (DPPH) technique was utilized to evaluate the concentration of peroxide formed on the irradiated polypropylene sheet by counting the quantity of DPPH consumed by the reaction of peroxide radicals with DPPH. Plastics stabilizers caused a decrease in the concentration of peroxide in the polymer as well as trapped radicals. It was shown that the stabilizer in polypropylene markedly decreased the grafting of methylmethacrylate onto polypropylene. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 2323–2330, 1998     

Effect of matrix features on polypropylene layered silicate nanocomposites

The aim of this study was to examine the effect of the polypropylene based resin on the properties of organoclay-PP nanocomposites prepared by melt compounding using a twin screw extruder.The characterization of the obtained materials was performed by X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), melt flow rate (MFR) and mechanical tests.The study has shown the effect of the polymer matrix molecular weight on the possibility of producing by melt compounding nanocomposites based on PP homopolymers or heterophasic PP-PE copolymers and an organically modified montmorillonite, in presence of maleic anhydride-modified polypropylene (PP-g-MAH).An increase of mechanical properties was achieved both for homopolymers and heterophasic copolymers. However the reinforcing effect of clay dispersed in heterophasic copolymers was not as high as found for the homopolymers.     

Transcrystallization of maleated polypropylene in the presence of various carbon fibers

Summary In this paper, the influence of the fiber surface sizing on development of transcrystallization, TCR, as well as the fiber-matrix interactions were analysed. Sized (CT) and unsized (CU) high-strength carbon fibers and maleated polypropylene, mPP, were used in model systems. Transcrystallization was followed by polarizing optical microscopy, in isothermal regime (Tc=124–130 °C). Besides the radius, distribution of the thickness and growth rate of the transcrystalline layer and the surface energetic parameters have been also determined. Lower value for was obtained for the system with surface treated C fibers ( ^CT =1,145x10 ^-7 J/cm ² ) which also contribute for their higher nucleation activity. Using FTIR microscopy and model reaction, an attempt was made to analyse the chemical interactions at fibedmatrix interface.     

Modification of polypropylene via the free‐radical grafting ternary monomer in water suspension systems

Maleic anhydride, styrene, and butyl acrylate were grafted onto polypropylene (PP) via free‐radical polymerization. The grafted product, polypropylene‐g‐(maleic anhydride–styrene–butyl acrylate) (PP‐g‐PMSB), was prepared in a water suspension system, and a nongrafted polymer, poly(maleic anhydride–styrene–butyl acrylate) (PMSB′), was produced at the same time. The optimal synthesis conditions were determined by orthogonal experiments. The crystallinity, thermal stability, melt flow rate, and hydrophilicity of the grafting samples were investigated in the presence or absence of PMSB′. The results indicate that the grafting percentage (G_p) of PP‐g‐PMSB and the content of PMSB′ (C_m) increased as the monomer content increased under the optimum reaction conditions. All of these ternary monomers were grafted onto the PP backbone as long‐chain branches. With increasing G_p, PP‐g‐PMSB's polarity and thermal stability increased, the crystallinity decreased, and the molecular distribution became narrower. The contact angle decreased to 72.12° when G_p was 6.87%. With increasing C_m, the crystallinity and thermal stability of the grafting products decreased compared to PP‐g‐PMSB and the molecular distribution grew wider. The contact angle decreased to 63.51° when C_m was 3.64%; this indicated that the presence of PMSB′ further improved the hydrophilicity of the grafted products. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

A new grafting monomer for synthesizing long chain branched polypropylene through melt radical reaction

A grafting monomer (p-(3-butenyl)styrene, BS)) containing two carbon–carbon double-bonds with different reactivity was used in melt radical reaction to prepare long chain branched polypropylene (LCB-PP). High-temperature size-exclusion (HT-SEC) coupled with triple detectors, rheometer and NMR were used to characterize the microstructure of the resultant LCB-PP. BS showed double functions in mediating radical reaction, i.e. stabilizing macroradicals and promoting branching reaction. Compared to styrene and divinylbenzene, using BS as a grafting monomer led to the formation of more uniformly distributed LCB structure on the PP backbone. Thus the resultant sample showed strain hardening in extensional flow and had high melt strength.     

pH Sensitive polypropylene porous membrane prepared by grafting acrylic acid in supercritical carbon dioxide

pH sensitive membrane was prepared by grafting acrylic acid (AA) on the porous polypropylene (PP) membrane using supercritical (SC) CO₂ as a solvent. The monomer (AA) and the initiator (benzyl peroxide, BPO) were impregnated into the PP substrate with the aid of SC CO₂, and were grafted onto the microporous PP substrate. The grafted membranes were characterized by Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FE-SEM), and the water permeability of the virgin and grafted membranes were determined at different pH values. It was demonstrated that the grafting degree (Dg) could be easily controlled by varying operating conditions, such as the monomer concentration, pressure, and temperature during the soaking process. The water permeation of the virgin membrane is nearly independent of pH. However, the water permeation of grafted membranes decreases dramatically with pH as the pH varies from 3 to 6 because the conformation of the PAA changes significantly with the pH of the contacting solution.     

Effect of solvents on gamma radiation induced graft copolymerization of methyl methacrylate onto polypropylene

Methyl methacrylate was grafted onto polypropylene films using gamma radiation. Effects of different parameters, such as radiation dose, inhibitor concentration, monomer concentration, and type of solvents on the graft yield were investigated. It was established that the graft yield depends on the proper choice of solvent mixtures, and its maximum value can be obtained by using decalin/methanol mixture in the ratio 2 : 3 wt %. Thermal characteristics using differential scanning calorimetry shows that grafting has practically no effect on the crystallinity of PP. © 1997 John Wiley & Sons, Inc.     

Effect of chemical solvents on the technological characteristics of hemp fibre/polypropylene composites

The effects of different chemical solvents on the technological properties of hemp fibre-reinforced polypropylene composites were evaluated in this experimental work. Composite profiles consisting of hemp fibre and polypropylene at 50% weight ratios, with 2% of coupling agent were fabricated using melt compounding followed by injection moulding. The composite specimens were then immersed in CH₃COOH, HNO₃, H₂SO₄, NaOH, NH₃, and C₆H₅NH₂ for different time intervals. Then, the weight loss and mechanical strength of samples were measured. Results indicated that the chemical reagents had significant effect on the weight loss of the composites. The weight loss ratio of the control samples was lower than that of those samples exposure to the chemical solvents. The tensile strength and modulus, and impact strength of composite specimens decreased after exposure to the chemical solvents. The highest mechanical reduction was observed in the case of NaOH. SEM micrographs showed that the extent of degradation increased in composites when they are exposed to chemical solvents.     

Effect of compatibilizer on the dispersion of untreated silica in a polypropylene matrix

BACKGROUND: A new processing method for polypropylene–untreated precipitated silica (PP/SiO₂) composites based on the incorporation of a second polymer phase of polyamide 6 (PA6) is presented and compared with a more classic one making use of compatibilizers: glycerol monostearate (GMS), ethylene acrylic acid ionomer (IAAZE) and maleic anhydride grafted polypropylene (MA‐graft‐PP). The effects of processing methods and conditions on the microstructure and properties of PP/SiO₂ composites prepared by melt compounding are investigated with a view to reduce the size of aggregates of silica from the micrometre to the nanometre scale and to improve the link between filler and matrix. RESULTS: On the one hand, the presence of GMS and IAAZE compatibilizers significantly improves the dispersion of the silica particles. On the other hand, when using a PA6 second phase, the SiO₂ particles are dispersed in PA6 nodules. Within these nodules, SiO₂ appears dispersed at the nanoscale but with larger particles (‘aggregates’) of about 200 nm. Significant improvements in tensile strength and modulus are obtained using MA‐graft‐PP compatibilizer. An increase in impact strength is observed in the case of GMS compatibilizer. Thermal parameters indicate also that silica plays the role of nucleation agent for PP matrix. All improvements (tensile strength, modulus and impact strength) increase with the addition of compatibilized PA6 second phase. CONCLUSION: By the incorporation of masterbatch of silica in PA6 as a second polymer polar phase, a successful new production method for PP/SiO₂ nanocomposites has been developed. Interestingly, this method does not require any (expensive) pre‐treatment of the silica. Copyright © 2007 Society of Chemical Industry     

Synthesis of highly concentrated suspension of chemically converted graphene in organic solvents: Effect of temperature on the extent of reduction and dispersibility

We report the effect of temperature on the extent of graphene oxide reduction by hydrazine and the dispersibility of the resulting chemically converted graphene (CCG) in polar organic solvents. The extent of graphene oxide reduction at high temperatures was only slightly higher than at low temperatures (30–50 °C), while the dispersibility of the resulting CCG in organic solvents decreased markedly with increasing temperature. The low dispersibility of CCGs prepared at high temperatures was greatly affected by reduction and influenced by the formation of an irreversible agglomerate of CCG at high temperatures. The reduction of graphene oxide at low temperatures is necessary to prepare highly dispersible CCG in organic solvents. CCG prepared at 30 °C is dispersible in N-methyl-2-pyrrolidone concentrations as high as 0.71 mg/mL. The free-standing paper made of this CCG possessed an electrical conductivity of more than 22,000 S/m, one of the highest values ever reported.     

Silane grafting and moisture crosslinking of polypropylene

Different approaches for free‐radical initiated grafting of functional monomers to polypropylene (PP) have been investigated with a view to enabling crosslinking of the polymer upon subsequent exposure to moisture, resulting in gel contents as high as 50–60 wt%. The first approach involved grafting a co‐agent such as triallyl trimellitate (TATM) to the polymer using peroxide initiator to add carbon–carbon double bond functionality to the polymer, followed by peroxide‐initiated grafting of mercaptopropyltrimethoxysilane (MPTMS) to the PP‐TATM adduct. The second approach was the more traditional peroxide‐mediated grafting of vinyltrimethoxysilane (VTMS) to polypropylene, and was most effective at increasing melt state dynamic storage modulus. In the third approach, peroxide‐mediated grafting of VTMS was followed by peroxide‐initiated grafting of MPTMS. This process took advantage of the unsaturation created via chain scission in the first step to provide graft sites for the MPTMS in the second step. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers