Influence of carbon nanofiber structure on properties of linear low density polyethylene composites

Three types of carbon nanofibers (MJ, Pyrograf®III PR‐19 and PR‐24) were incorporated into linear low density polyethylene (LLDPE) using intensive mixing. The electrical volume resistivity of composites decreased with the addition of CNFs from over 10¹² Ω cm for pure LLDPE to less than 10⁴ Ω cm for carbon nanofibers (CNF) contents of 15 wt% or more. Tensile modulus increased from 110 MPa for pure LLDPE to 200 MPa and 300 MPa for 15 wt% MJ and 15 wt% PR composites, respectively. However, the tensile strength remained fairly unchanged at about 20 MPa. Strain‐to‐failure decreased from 690% for pure LLDPE to 460% and 120% for 15 wt% MJ and 15 wt% PR composites, respectively. It was inferred that the interfacial interactions of LLDPE matrix with MJ fibers is better than that with PR fibers, resulting from the rougher surface of MJ fibers. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Effects of crystallization on dispersion of carbon nanofibers and electrical properties of polymer nanocomposites

Polymer nanocomposites filled with low volume fractions of carbon nanofibers (CNFs) were prepared by melt‐compounding. Three types of polymers with different crystallization behavior, i.e., weakly‐crystallized low density polyethylene (LDPE), strongly crystallized high density polyethylene (HDPE) and amorphous polystyrene (PS), were selected as matrices for the nanocomposites. The effects of polymer crystallization on the dispersion of CNFs were examined. Optical and electron microscopic examinations revealed that the dispersion of CNFs in the nanocomposite matrices was strongly depended on the crystallization behavior of polymer matrices. The CNFs were found to disperse uniformly in weakly crystallized LDPE and amorphous PS matrices, but agglomerated in HDPE due to its strong crystallization tendency. Such a distinct dispersion behavior of CNFs in polymers had a profound effect on the electrical properties of the nanocomposites investigated. The PS/CNF nanocomposites exhibited the lowest percolation threshold. The HDPE/CNF nanocomposites showed the largest percolation threshold due to the CNF agglomeration within the amorphous phase of HDPE. POLYM. ENG. SCI., 48:177–183, 2008. © 2007 Society of Plastics Engineers     

Effect of heat treatment of carbon nanofibers on the electromagnetic shielding effectiveness of linear low density polyethylene nanocomposites

The electrical properties and electromagnetic shielding effectiveness (EM SE) of nanocomposites consisting of heat‐treated carbon nanofibers (Pyrograf® III PR‐19, CNF) in a linear low density polyethylene (LLDPE) matrix were assessed. Heat treatment (HT) of carbon nanofibers at 2500°C significantly improved their graphitic crystallinity and intrinsic transport properties, thereby increasing the EM SE of the nanocomposites. Nanocomposites containing 11 vol% (20 wt%) PR‐19 HT displayed a DC electrical conductivity of about 1.0 ± 0.1 × 10¹ S/m (n = 4), about 10 orders of magnitude better than that of as‐received PR‐19 CNF nanocomposites. Over a frequency range of 30 MHz to 1.5 GHz, nanocomposites (2.5 mm thick) containing PR‐19 HT displayed EM SE average values of about 14 ± 2 dB (n = 4). Absorption was determined to be the main EM SE mechanism for the heat‐treated CNF nanocomposites. The nanocomposites possessed a modulus of 632 ± 36 MPa (n = 6) (nominally twice that of pure LLDPE) and a strain‐to‐failure of 180 ± 98% (n = 6), indicating that a significant ductility is retained in the nanocomposites. Such nanocomposites display potential as absorptive electromagnetic interference shielding materials for thin films and micromolding. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

CNFs的掺杂与取向对CNFs/LDPE纳米复合材料结晶性能的影响

通过微波法化学镀镍对纳米碳纤维(CNFs)进行表面改性,采用熔融共混法制备CNFs/低密度聚乙烯(LDPE)纳米复合材料,在模压硫化过程中施加磁场,实现表面镀镍CNFs在LDPE基体中的取向,研究了CNFs的掺杂对CNFs/LDPE纳米复合材料结晶性能的影响,采用场发射扫描电子显微镜(FE-SEM)观察刻蚀后样品中CNFs分散、取向情况和球晶形貌,采用X射线衍射仪(XRD)分析纳米复合材料结晶性能。研究发现,在LDPE基体中CNFs的掺杂对纳米复合材料结晶性能有着较大的影响;掺杂CNFs使LDPE的结晶度下降,且掺杂样品需较长时间的刻蚀才能看到清晰的球晶结构,LDPE的球晶结构较明显,其直径约6μm,CNTs的取向使纳米复合材料的结晶度增大。     

Poly(ϵ‐caprolactone)‐functionalized carbon nanofibers by surface‐initiated ring‐opening polymerization

Carbon nanofibers (CNFs) were covalently functionalized with biodegradable poly(?‐caprolactone) (PCL) by in situ ring‐opening polymerization (ROP) of ?‐caprolactone in the presence of stannous octoate. Surface oxidation treatment of the pristine CNFs afforded carboxylic CNFs (CNF‐COOH). Reaction of CNF‐COOH with excess thionyl chloride (SOCl₂) and glycol produced hydroxyl‐functionalized CNFs (CNF‐OH). Using CNF‐OH as macroinitiator, PCL was covalently grafted from the surfaces of CNFs by ROP, in either the presence or absence of sacrificial initiator, butanol. The grafted PCL content was achieved as high as 64.2 wt %, and can be controlled to some extent by adjusting the feed ratio of monomer to CNF‐OH. The resulting products were characterized by FTIR, NMR, Raman spectroscopy, TGA, DSC, SEM, TEM, HRTEM, and XRD. Core–shell nanostructures were observed under HRTEM for the PCL‐functionalized CNFs because of the thorough grafting. The PCL‐grafted CNFs showed different melting and crystallization behaviors from the mechanical mixture of PCL and CNF‐OH. This approach to PCL‐functionalized CNFs opens an avenue for the synthesis, modification, and application of CNF‐based nanomaterials and biomaterials. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

On the melting behavior of isothermally crystallized 1‐octene linear low‐density polyethylene copolymers

The melting behavior of two 1‐octene linear low‐density polyethylene (LLDPE) copolymers is investigated. One made using Dow′s INSITE constrained geometry catalyst technology (LLDPE‐A) and the other using titanium‐based Ziegler–Natta catalysts (LLDPE‐B). Both have similar comonomer content as well as melt flow index. Differential scanning calorimetry (DSC) was used throughout the work. Isothermal crystallizations in the DSC for several times were carried out at various temperatures between 90 and 100°C for LLDPE‐A and between 105 and 112.5°C for LLDPE‐B. As a result of the isothermal crystallizations for both copolymers, multiple melting peaks are found in the DSC traces on subsequent heating. The melting behavior was also examined as a function of heating rate (1, 2.5, 5, 10, and 20°C/min). The multiple melting behavior indicates that they are inhomogeneous. In addition, a melting–recrystallization process was shown to be responsible for the appearance of one of the melting peaks in LLDPE‐B. A lowering in heating rate from the crystallization temperature favors the occurrence of melting–recrystallization during the dynamic experiment. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 2022–2028, 2001     

Crystallization behaviors and mechanical properties of poly(ethylene 2,6‐naphthalate)/multiwall carbon nanotube nanocomposites

Carbon nanotube (CNT)‐reinforced poly(ethylene 2,6‐naphthalate) (PEN) nanocomposites were prepared by direct melt blending process in a twin‐screw extruder. There is significant dependence of the crystallization and melting behavior of PEN/CNT nanocomposites on CNT content and crystallization temperature. The incorporation of CNT may favor the formation of the β‐form crystals in PEN/CNT nanocomposites, and more CNT content amplified this effect. In this PEN/CNT nanocomposite system, the CNT promoted the nucleation and the growth with higher crystallization rate of PEN/CNT nanocomposites, and simultaneously reduced the fold surface free energy and the work required in folding polymer chains in the polymer nanocomposites. In addition, the incorporation of a very small quantity of CNT significantly improved the mechanical properties of PEN/CNT nanocomposites. POLYM. ENG. SCI., 47:1715–1723, 2007. © 2007 Society of Plastics Engineers     

Linear low-density polyethylene/poly(ethylene-ran-butene) elastomer blends: Miscibility and crystallization behavior

The phase and crystallization behavior of the blends consisting of LLDPE (0.7 mol% hexene copolymer) and PEB (26 mol% butene copolymer) have been investigated using optical microscopy (OM), differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). The blends exhibited an upper critical solution temperature of 162°C. The solubility parameter analysis showed that the solubility parameter of LLDPE decreased more rapidly than that of PEB with temperature. However, due to the slow kinetics of phase separation, at lower crystallization temperatures, the crystallization and melting behavior of LLDPE mainly reflected the miscibility between LLDPE and PEB. Crystallization from the two-phase state could present two crystallization peaks. PEB didnt change the crystal cell unit and crystallinity of LLDPE, but changed its distribution of lamellar thickness or crystal perfection. The dilute effect of PEB also changed the overall nature of the nucleation and growth process of LLDPE. The equilibrium melting temperature in this blend could be obtained by the Hoffman-Weeks method, and comparing with that of the pure LLDPE, it was reduced and kept relatively constant in the bi-phase state. The phase diagram made up of the LLPS boundary, equilibrium melting temperatures and melting temperatures observed may be better to indicate the phase and crystallization behavior of LLDPE/PEB blends.     

Significantly improving electro‐activated shape recovery performance of shape memory nanocomposite by self‐assembled carbon nanofiber and hexagonal boron nitride

This study presents two effective approaches to significantly improve the electro‐thermal properties and electro‐activated shape recovery performance of shape memory polymer (SMP) nanocomposites that are incorporated with carbon nanofibers (CNFs) and hexagonal boron nitrides (h‐BNs), and show Joule heating triggered shape recovery. CNFs were self‐assembled and deposited into buckypaper form to significantly improve the electrical properties of SMP and achieve the shape memory effect induced by electricity. The h‐BNs were either blended into or self‐assembled onto CNF buckypaper to significantly improve the thermally conductive properties and electro‐thermal performance of SMPs. Furthermore, the shape recovery behavior and temperature profile during the electrical actuation of the SMP nanocomposites were monitored and characterized. It was found that a unique synergistic effect of CNFs and h‐BNs was presented to facilitate the heat transfer and accelerate the electro‐activated shape recovery behavior of the SMP nanocomposites. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40506.     

Preparation and characterization of low‐density polyethylene/thermoplastic starch composites reinforced by cellulose nanofibers

In the current study, the effect of extracted cellulose nanofibers (CNFs) on rheological and mechanical properties and biodegradability of polyethylene/starch blend was investigated. The CNFs were extracted from wheat straws using a chemo‐mechanical method. Polyethylene/starch blend was reinforced by different amounts of CNF (6–14 wt%) using an internal mixer followed by a single screw extruder. The flow properties of nanocomposites were investigated by determining Melt Flow Index (MFI) and viscosity. Due to the weak interaction of cellulosic nanofibers and polymers, the flow behavior of nanocomposites was undesirable. Tensile tests were performed to evaluate the mechanical performance of nanocomposites. By increasing the CNF content, the tensile strength and elongation at break declined; whereas, the Young's modulus was improved. The biodegradation of cellulose nanocomposites was investigated by water absorption and degradability tests. Both experiments confirmed the progressive effect of cellulose nanofibers on the degradation of the composites. POLYM. COMPOS., 36:2309–2316, 2015. © 2014 Society of Plastics Engineers     

Nanocomposites of poly(ether ether ketone) with carbon nanofibers: Effects of dispersion and thermo-oxidative degradation on development of linear viscoelasticity and crystallinity

Poly(ether ether ketone), PEEK, is a widely used engineering plastic that is especially suitable for high temperature applications. Compounding of PEEK with carbon nanofibers, CNF, has the potential of enhancing its mechanical and thermal properties further, even at relatively low CNF concentrations. However, such enhancements can be compromised by myriad factors, some of which are elucidated in this study. Considering that the dispersion of the CNF into any high molecular weight polymer is a challenge, two different processing methods, i.e., melt and solution processing were used to prepare PEEK nanocomposites with low aspect ratio carbon nanofibers. The linear viscoelastic material functions of PEEK nanocomposites in the solid and molten states were characterized as indirect indicators of the dispersion state of the nanofibers and suggested that the dispersion of nanofibers into PEEK becomes difficult at increasing CNF concentrations for both solution and melt processing methods. Furthermore, the time-dependence of the linear viscoelastic material functions of the PEEK/CNF nanocomposites at 360-400 °C indicated that PEEK undergoes thermo-oxidative cross-linking under typical melt processing conditions, thus preventing better dispersion by progressive increases of the mixing time and specific energy input during melt processing. The crystallization behavior of PEEK is also affected by the presence of CNF and degree of cross-linking, with the rate of crystallization decreasing with increasing degree of cross-linking and upon the incorporation of CNFs both for the solution and melt processed PEEK nanocomposites.     

Curing behavior and physical properties of epoxy nanocomposites comprising amine‐functionalized carbon nanofillers

Carbon nanofillers like nanotubes and nanofibers have been used to reinforce various epoxy systems. The incorporation of carbon nanofillers into a thermosetting epoxy system enhanced the thermal and mechanical properties of the epoxy system. The best performance of an epoxy nanocomposite system with carbon nanofillers would be resulted from the homogeneous dispersion of the nanofillers and strong interfacial adhesion between the epoxy matrix and the nanofillers. Therefore, amine‐functionalization of carbon nanofibers (CNFs) and multiwalled carbon nanotubes (MWNTs) was carried out via treating them with 4‐aminobenzoic acid in polyphosphoric acid. FTIR spectroscopy, XPS, TGA, and FE‐SEM analyses confirmed that the functionalization was successful. Curing behavior and thermo‐physical properties of the nanocomposites comprising the pristine or functionalized carbon nanofillers were investigated and compared with each other. Fractured surfaces of the nanocomposites were investigated by FE‐SEM. The functionalized MWNTs induced stronger interfacial adhesion than the functionalized CNFs and resulted in considerable improvement in the physical properties of the epoxy nanocomposites. POLYM. COMPOS., 31:1449–1456, 2010. © 2009 Society of Plastics Engineers     

Effects of carbon nanofibers on crystalline structures and properties of ultrahigh molecular weight polyethylene blend fabricated using twin‐screw extrusion

Melt mixing in an extruder with polymers is an effective approach for forming nanocomposites, allowing mass production applications. The intent of this study is to investigate carbon nanofiber composites with ultrahigh molecular weight polyethylene (UHMWPE) matrix using the twin‐screw extruder. To decrease the high viscosity of UHMWPE, a low density polyethylene (LDPE) was added into the UHMWPE. The effects of carbon nanofibers (CNFs) on the crystalline structures and properties of the nanocomposites were analyzed. The differential scanning calorimetry (DSC) and X‐ray diffraction (XRD) measurements showed the addition of CNFs decreases the degree of crystallinity, but does not impart significant effects on the crystalline structure of the UHMWPE/LDPE blend. Tensile test results showed that the nanocomposite with loading of 3 wt % CNFs had an increase of 38% in tensile strength and 15% in modulus. The thermal stability and thermal conductivity of UHMWPE/LDPE blends were also enhanced by the addition of CNFs. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Synthesis and characterization of linear low‐density polyethylene/sepiolite nanocomposites

Linear low‐density polyethylene (LLDPE)/sepiolite nanocomposites were prepared by melt blending using unmodified and silane‐modified sepiolite. Two methods were used to modify sepiolite: modification before heat mixing (ex situ) and modification during heat mixing (in situ). The X‐ray diffraction results showed that the position of the main peak of sepiolite remained unchanged during modification step. Infrared spectra showed new peaks confirming the development of new bonds in modified sepiolite and nanocomposites. SEM micrographs revealed the presence of sepiolite fibers embedded in polymer matrix. Thermogravimetric analysis showed that nanocomposites exhibited higher onset degradation temperature than LLDPE. In addition, in situ modified sepiolite nanocomposites exhibited higher thermal stability than ex situ modified sepiolite nanocomposites. The ultimate tensile strength and modulus of the nanocomposites were improved; whereas elongation at break was reduced. The higher crystallization temperature of some nanocomposite formulations revealed a heterogeneous nucleation effect of sepiolite. This can be exploited for the shortening of cycle time during processing. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

纤维素纳米纤维对聚乳酸结晶、流变和拉伸性能的影响

采用熔融共混⁃注射成型制备了聚乳酸（PLA）/纤维素纳米纤维（CNFs）可生物降解纳米复合材料,利用差示扫描量热仪、流变测试、拉伸性能测试等手段,考察了CNFs含量对PLA/CNFs复合材料结晶行为、流变特性和力学性能的影响规律。结果表明,少量的CNFs能均匀分散在PLA基体中,CNFs可作为PLA的异相成核剂,提高结晶速率常数,缩短半结晶时间,CNFs的含量为5 %（质量分数,下同）时,半结晶时间由纯PLA的10.4 min缩短至2.9 min;CNFs体现出润滑作用,使PLA/CNFs复合材料的储能模量和损耗模量均低于纯PLA;CNFs的含量为3 %时,复合材料的断裂伸长率较纯PLA提高了41.2 %。     

Preparation and characterization of carbon nanofiber reinforced thermoplastic polyurethane nanocomposites

The effect of CNFs on hard and soft segments of TPU matrix was evaluated using Fourier transform infrared (FTIR) spectroscope. The dispersion and distribution of the CNFs in the TPU matrix were investigated through wide angle X‐ray diffraction (WAXD), field emission scanning electron microscope (FESEM), high resolution transmission electron microscope (HRTEM), polarizing optical microscope (POM), and atomic force microscope (AFM). The thermogravimetric analysis (TGA) showed that the inclusion of CNF improved the thermal stability of virgin TPU. The glass transition temperature (T_g), crystallization, and melting behaviors of the TPU matrix in the presence of dispersed CNF were observed by differential scanning calorimetry (DSC). The dynamic viscoelastic behavior of the nanocomposites was studied by dynamical mechanical thermal analysis (DMTA) and substantial improvement in storage modulus (E') was achieved with the addition of CNF to TPU matrix. The rheological behavior of TPU nanocomposites were tested by rubber processing analyzer (RPA) in dynamic frequency sweep and the storage modulus (G') of the nanocomposites was enhanced with increase in CNF loading. The dielectric properties of the nanocomposites exhibited significant improvement with incorporation of CNF. The TPU matrix exhibits remarkable improvement of mechanical properties with addition of CNF. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Effect of carbon nanofiber (CNF) silanization on the properties of CNF/epoxy nanocomposites

Carbon nanofibers (CNFs) were functionalized by a multistage process including oxidation, reduction and silanization. The chemical modifications were examined by Fourier transform infrared spectroscopy, X‐ray photoelectron spectrometry, Raman spectroscopy and thermogravimetric analysis. The silanized CNFs were then added into an epoxy resin (EPON 828) to study the effect of the surface modification of CNFs on the properties of nanocomposites. For comparison, nanocomposites containing original unmodified CNFs were also investigated. Scanning electron microscopy indicates better dispersion of modified fibers in the epoxy polymer matrix; the mechanical and thermal properties of composites are also improved; the electrical conductivity of the composites is reduced. Copyright © 2011 Society of Chemical Industry     

Effect of blending high‐pressure low‐density polyethylene on the crystallization kinetics of linear low‐density polyethylene

It is well known that the addition of a small amount of high‐pressure low‐density polyethylene (HP‐LDPE) to linear low‐density polyethylene (LLDPE) can improve the optical properties of LLDPE, and LLDPE/HP‐LDPE blend is widely applied to various uses in the field of film. The optical haziness of polyethylene blown films, as a result of surface irregularities, is thought to be as a consequence of the different crystallization mechanisms. However, not much effort has been directed toward understanding the effect of HP‐LDPE blending on the overall crystallization kinetics (k) of LLDPE including nucleation rate (n) and crystal lateral growth rate (v). In this study, we investigated the effect of blending 20% HP‐LDPE on the crystallization kinetics of LLDPE polymerized by Ziegler‐Natta catalyst with comonomer of 1‐butene. Furthermore, by combining depolarized light intensity measurement (DLIM) and small‐angle laser light scattering (SALLS), we have established a methodology to estimate the lateral growth rate at lower crystallization temperatures, in which direct measurement of lateral growth by polarized optical microscopy (POM) is impossible due to the formation of extremely small spherulites. This investigation revealed that HP‐LDPE blending leads to enhanced nucleation rate, reduced crystal lateral growth rate, and a slight increase in the overall crystallization kinetics of pure LLDPE. From the estimated crystal lateral growth rate, it was found that the suppression in v from HP‐LDPE blending is larger at lower temperatures than at higher temperatures. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Effect of polyaniline surface modification of carbon nanofibers on cure kinetics of epoxy resin

Polyaniline (PANI) “nanograss” was grown on carbon nanofibers (CNFs). The cure behavior of an epoxy resin with and without unmodified CNFs or PANI modified CNFs was studied by means of non‐isothermal and isothermal differential scanning calorimetry (DSC). CNFs accelerated the reaction of epoxy and diamine. PANI surface modification further increased the reaction rate and the extent of reaction. An autocatalytic cure kinetic model was used to fit the reaction curves. It was found that activation energies of the epoxy reaction decreased in the presence of CNFs and PANI modified CNFs. The observed catalytic effect of CNF and PANI surface coating can be very useful for low temperature cure of large epoxy composite products. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Combination of cellulose nanofibers and chain‐end‐functionalized polyethylene and their applications in nanocomposites

In this study, nanofiber cellulose (NFC) based on a 2,2,6,6‐tetramethylpiperidine‐1‐oxyl radical oxidization method was successfully combined with chain‐end‐functionalized polyethylene containing alkoxysilane via silanization. Fourier transform infrared spectroscopy, transmission electron microscopy, contact angle measurements, Molau tests, and X‐ray photoelectron spectroscopy analyses provided further evidence for the effectiveness of the surface modifications. The hydrophilic surface characteristics of NFC were changed to apparently hydrophobic for the modified nanofiber cellulose (M‐NFC). Then, the linear low‐density polyethylene (LLDPE)/M‐NFC nanocomposite was prepared, and the mechanical properties, thermal properties, and crystallization properties of the LLDPE–M‐NFC were investigated by tensile testing, thermogravimetric analysis, differential scanning calorimetry, and dynamic mechanical analysis. The results show that after modification, the thermal stability of NFC was enhanced. The interface between M‐NFC and the matrix was good. The tensile strength and Young's modulus values of the nanocomposites were enhanced compared with those of LLDPE; in particular, the tensile strength and Young's modulus of the blend with 5 wt % M‐NFC increased by 56 and 106%, respectively. The storage modulus of the nanocomposites was enhanced obviously over a wide temperature range. The addition of a small amount of M‐NFC had slight effects on the crystallinity and melting temperature of LLDPE. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45387.