Linear low‐density polyethylene nanocomposites by in situ polymerization using a zirconium‐nickel tandem catalyst system

A series of linear low‐density polyethylene (LLDPE) nanocomposites containing different types of nanofiller (TiO₂, MWCNT, expanded graphite, and boehmite) were prepared by in situ polymerization using a tandem catalyst system composed of {Tp^Ms}NiCl ( 1 ) and Cp₂ZrCl₂ ( 2 ), and analyzed by differential scanning calorimetry, dynamic mechanical analysis (DMA), and transmission electron microscopy (TEM). Based on these analyses, the filler content varied from 1.30 to 1.80 wt %. The melting temperatures and degree of crystallinity of the LLDPE nanocomposites were comparable to those of neat LLDPE. The presence of MWCNT as well as boehmite nucleated the LLDPE crystallization, as indicated by the increased crystallization temperature. The DMA results showed that the presence of TiO₂, EG, and CAM 9080 in the LLDPE matrix yielded nanocomposites with relatively inferior mechanical properties compared to neat LLDPE, suggesting heterogeneous distribution of these nanofillers into the polymer matrix and/or the formation of nanoparticle aggregates, which was confirmed by TEM. However, substantial improvement in the storage modulus was achieved by increasing the sonication time. The highest storage modulus was obtained using MWCNT (1.30 wt %). © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 3506–3512     

Structure and gas transport characteristics of triethylene oxide-grafted polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene

Polymeric membrane-based gas separation technology has significant advantages compared with traditional amine-based CO₂ separation method. In this work, SEBS block copolymer is used as a polymer matrix to incorporate triethylene oxide (TEO) functionality. The short ethylene oxide segment is chosen to avoid crystallization, which is confirmed by differential scanning calorimetry and wide-angle X-ray scattering characterizations. The gas permeability results reveal that CO₂/N₂ selectivity increased with increasing content of TEO functional group. The highest CO₂ permeability (281 Barrer) and CO₂/N₂ selectivity (31) were obtained for the membrane with the highest TEO incorporation (57 mol%). Increasing the TEO content in these copolymers results in an increase in CO₂ solubility and a decrease in C₂H₆ solubility. For example, as the grafted TEO content increased from 0 to 57 mol%, the CO₂ solubility and CO₂/C₂H₆ solubility selectivity increased from 0.72 to 1.3 cm³(STP)/cm³ atm and 0.47 to 1.3 at 35°C, respectively. The polar ether linkage in TEO-grafted SEBS copolymers exhibits favorable interaction with CO₂ and unfavorable interaction with nonpolar C₂H₆, thus enhancing CO₂/C₂H₆ solubility selectivity.     

Properties of biodegradable poly(butylene carbonate) (PBC) composites with fumed silica nanoparticles

Biodegradable poly(butylene carbonate)/fumed silica (PBC/SiO₂) nanocomposites were prepared by melt compounding. The PBC/SiO₂ nanocomposites exhibited a good dispersion of aggregates of SiO₂ in the PBC matrix, and an improvement in mechanical properties. Nanoparticles affect, also, the thermal properties of PBC and especially the crystallization rate, which in all nanocomposites is faster than that of pure PBC. Due to ongoing crystallization and the crystal perfection during heating process, the melting peak of PBC shifted to higher temperature when heating from amorphous state with decreasing heating rate. With increasing cooling rate, the non-isothermal crystallization exotherms became wider and shifted to lower temperature. At a given cooling rate, the crystallization peak temperature of neat PBC was lower than that of its nanocomposite. Non-isothermal crystallization kinetic procedure, the method of Ozawa, was applied to the first deconvoluted DSC peak only by processing the data related to DSC peak. The average value of Ozawa exponent m of pure PBC is 3.04, while the one of its nanocomposite is about 2.98. Moreover, the thermal stability of the nanocomposites was increased. The T _d enhancement of the nanocomposite was remarkable.     

Thermal properties of single walled carbon nanotubes composites of polyamide 6/poly(methyl methacrylate) blend system

The nanocomposites of polyamide 6 (PA6)/poly(methyl methacrylate) (PMMA)/non-functionalized and functionalized [carboxylic acid (COOH) and hydroxyl (OH)] single wall carbon nanotubes (SWCNTs) were prepared in mass ratios of 79.5/19.5/1, 49.5/49.5/1, and 19.5/79.5/1 by melt–mixing method at 230 °C. The PA6/PMMA blends with mass ratios of 80/20, 50/50, and 20/80 served as references. The Fourier transform infrared analyses of nanocomposites showed the formation of hydrogen bond interactions among PA6, PMMA, and OH and COOH functional groups of SWCNTs. The nanocomposites and blends had higher thermal stability with respect to the PMMA. The differential scanning calorimeter (DSC) curves showed that the nanocomposites and blends exhibited two T _g values at around 51 and 126 °C for PA6 and PMMA, respectively. About 20 °C early crystallization was observed in nanocomposites compared to the blends. The dynamic mechanical analysis (DMA) results suggested that among all the compositions of blends and nanocomposites, storage modulus (E′) was higher for PMMA-rich blends and nanocomposites. At 25 °C, the E′ values were higher for blends and nanocomposites compared to the neat PA6. The tan δ curves indicated that the more heterogeneity of the hybrid nature resulted in PA6/PMMA/SWCNTs-OH or SWCNTs-COOH with 79.5/19.5/1 mass ratio nanocomposites compared to the PA6/PMMA with 80/20 mass ratio blend. The higher T _g values of PA6 and PMMA were observed in DMA studies compared to the DSC studies for PA6 and PMMA as neat and in blends and nanocomposites. The significant improvements in crystallization of nanocomposites were considered resulting from achieving better compatibility among the polymer components and carbon nanotubes.     

Influences of poly(lactic acid)‐grafted carbon nanotube on thermal,mechanical, and electrical properties of poly(lactic acid)

Poly(lactic acid)‐grafted multiwalled carbon nanotubes (MWNT‐g‐PLA) were prepared by the direct melt‐polycondensation of L ‐lactic acid with carboxylic acid‐functionalized MWNT (MWNT‐COOH) and then mixed with a commercially available neat PLA to prepare PLA/MWNT‐g‐PLA nanocomposites. Morphological, thermal, mechanical, and electrical characteristics of PLA/MWNT‐g‐PLA nanocomposites were investigated as a function of the MWNT content and compared with those of the neat PLA, PLA/MWNT, and PLA/MWNT‐COOH nanocomposites. It was identified from FE‐SEM images that PLA/MWNT‐g‐PLA nanocomposites exhibit good dispersion of MWNT‐g‐PLA in the PLA matrix, while PLA/MWNT and PLA/MWNT‐COOH nanocomposites display MWNT aggregates. As a result, initial moduli and tensile strengths of PLA/MWNT‐g‐PLA composites are much higher than those of neat PLA, PLA/MWNT, and PLA/MWNT‐COOH, which stems from the efficient reinforcing effect of MWNT‐g‐PLA in the PLA matrix. In addition, the crystallization rate of PLA/MWNT‐g‐PLA nanocomposites is faster than those of neat PLA, PLA/MWNT, and PLA/MWNT‐COOH, since MWNT‐g‐PLA dispersed in the PLA matrix serves efficiently as a nucleating agent. It is interesting that, unlike PLA/MWNT nanocomposites, surface resistivities of PLA/MWNT‐g‐PLA nanocomposites did not change noticeably depending on the MWNT content, demonstrating that MWNTs in PLA/MWNT‐g‐PLA are wrapped with the PLA chains of MWNT‐g‐PLA. Copyright © 2008 John Wiley & Sons, Ltd.     

Non-isothermal crystallization kinetics of exfoliated and intercalated polyethylene/montmorillonite nanocomposites prepared by in situ polymerization

Polyethylene/montmorillonite (PE/MMT) nanocomposites, one intercalated sample with higher MMT content and one exfoliated sample with lower MMT content, were prepared by in situ polymerization using MMT-supported metallocene as catalyst. Non-isothermal crystallization behaviors of these two nanocomposites were investigated and compared. The exfoliated sample exhibits higher crystallization temperature (T_c) than the neat PE, showing nucleation effect of MMT. The intercalated sample has lower T_c than the neat PE due to the confinement of MMT. It is observed that the intercalated sample has longer induction period and faster overall crystallization rate, indicating co-existence of suppression and nucleation effects in this sample. The Avrami plots show that the crystal growth of PE in the intercalated sample is two-dimensional, while it is three-dimensional in the exfoliated sample. The crystallization activation energy of the intercalated sample is slightly smaller than that of the exfoliated sample.     

The fabrication of affinity improved nanocomposites with plasma treated polypropylene (PP) and alkaline cellulose nanofiber (CNF) suspension

Polypropylene (PP) nanocomposites containing cellulose nanomaterials have been studied for the last decade, but are still challenging due to the lack of affinity between PP and cellulose nanofiber (CNF), and the uneven dispersion of CNF in the PP matrix. In order to achieve the uniform dispersion of CNF in the PP matrix and improve affinity between PP and CNF, plasma-treated PP/aCNF nanocomposites were prepared by adding an alkaline CNF (aCNF) suspension to the nitrogen (N₂) and oxygen (O₂) plasma-treated PP. Based on the results of various analyses, the N₂ plasma-treated PP (npPP)/aCNF nanocomposite showed the best performance in tensile and oxygen barrier properties. The npPP/aCNF nanocomposite increased 32.8 and 26.3% in yield stress and Young's modulus, respectively, compared with neat PP, also in O₂ permeability, it showed a value of 94.31 cc-mm/m²-day-atm and showed a statistically significant decrease in contrast to neat PP. The npPP/aCNF nanocomposite has a great feasibility to be applied in various barrier packaging applications.     

Influence of the PVOH molar mass on the morphology and functional properties of EVOH/PVOH films prepared by melt blending

The influences of the molar mass (low, medium, and high) and content of poly(vinyl alcohol) (PVOH) dispersed by melt-blending in an ethylene vinyl alcohol (EVOH) copolymer on the morphology, microstructure, thermal, mechanical, and oxygen barrier properties were investigated. Multilayer films with external low-density polyethylene layers and inner EVOH/PVOH blend layer and respective monolayer films were elaborated and characterized. EVOH/PVOH blends exhibited a good compatibility because of the initial presence of PVOH segments in EVOH. The detailed quantitative analysis of the morphology performed for all blends showed that the finest dispersion was obtained with the PVOH with the lowest molar mass. The properties of the films as a function of the PVOH content and its molar mass were determined herein. Significant improvement of barrier properties was obtained at moderated water activities (up to a_w = 0.6) by using the PVOH with the lowest molar mass. Compared to the neat EVOH material, the oxygen permeability coefficients decreased by a factor 2 by adding 15 vol% PVOH while the thermal and mechanical properties remained similar.     

Microcellular foaming of polylactide and poly(butylene adipate‐co‐terphathalate) blends and their CaCO3 reinforced nanocomposites using supercritical carbon dioxide

Foamed polylactide (PLA), PLA–PBAT (poly (butylene adipate‐co‐terphathalate)) blend and their composites with CaCO₃ were prepared in a batch process using supercritical carbon dioxide (CO₂) at 12 MPa and 45°C. The solubility of CO₂ and its diffusion patterns in different PLA samples was investigated. PLA systems had a relatively high CO₂ solubility related to the carboxyl groups. CO₂ desorption behaviors in PLA systems first followed the Fickian diffusion mechanism in short time and then decreased slowly to a plateau. The addition of both PBAT and CaCO₃ into PLA impeded the desorption of CO₂. In the presence of second phase PBAT, nanoparticles CaCO₃ and dissolved CO₂, the PLA crystallization behavior investigated by DSC technique was greatly changed. As the desorption time increased, the gas induced crystallinity slightly decreased in consequence of less CO₂ content in each system and thus less plasticization effect. The cell morphology of foamed PLA and PLA composites showed interesting microstructure patterns. The prepared pure PLA foam exhibits a typical bimodal structure because of the foaming in both the amorphous and crystalline zones. With PBAT and CaCO₃ into PLA, the composite foam presented significant increase in cell uniformity and cell density. With less CO₂ content in each PLA sample, the cell structure showed interesting variation. Pure PLA foam presented transition from bimodal structure to more uniform cell structure with decreased cell density. In contract, PLA–PBAT foam show unfoamed regions because of none CO₂ left in the separated PBAT phase. Copyright © 2016 John Wiley & Sons, Ltd.     

Isothermal crystallization kinetics of poly(butylene terephthalate)/attapulgite nanocomposites

Poly(butylene terephthalate) (PBT)/organo‐attapulgite (ATT) nanocomposites containing 2.5 and 5 wt % nanoparticles loadings were fabricated via a simple melt‐compounding approach. The crystal structure and isothermal crystallization behaviors of PBT composites were studied by wide‐angle X‐ray diffraction and differential scanning calorimetry, respectively. The X‐ray diffraction results indicated that the addition of ATT did not alter the crystal structure of PBT and the crystallites in all the samples were triclinic α‐crystals. During the isothermal crystallization, the PBT nanocomposites exhibited higher crystallization rates than the neat PBT and the varied Avrami exponents when compared with the neat PBT. At the same time, the regime II/III transition was also observed in all the samples on the basis of Hoffman‐Laurizten theory, but the transition temperature increased with increasing ATT loadings. The fold surface free energy (σ_e) of polymer chains in the nanocomposites was lower than that in the neat PBT. It should be reasonable to treat ATT as a good nucleating agent for the crystallization of PBT, which plays a determinant effect on the reduction in σ_e during the isothermal crystallization of the nanocomposites, even if the existence of ATT could restrict the segmental motion of PBT. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2112–2121, 2006     

Poly(ε-caprolactone) (PCL)/cellulose nano-crystal (CNC) nanocomposites and foams

Poly(ε-caprolactone) (PCL)/cellulose nanocrystal (CNC) nanocomposites were produced via twin-screw extrusion. Microcellular nanocomposite samples were produced with microcellular injection molding using carbon dioxide (CO₂) as physical blowing agent. The foaming behavior, physical properties, thermal properties, crystallization behavior, and biocompatibility were investigated. It was found that the CNCs interacted with the PCL matrix which led to a strong interface. The CNCs effectively acted as nucleation agents in microcellular injection molding. Both solid and foamed samples with higher levels of CNC content showed higher tensile moduli, complex viscosities, and storage moduli due to the reinforcement effects of CNCs. Furthermore, improvement in the foamed samples was more significant due to their fine cell structure. The addition of CNCs caused a reduction of the decomposition temperature and an increase in the glass transition temperature, crystallization temperature, and crystallinity of PCL. Moreover, the biocompatibility of the foamed nanocomposites with low CNC content was verified by 3T3 fibroblast cell culture.     

Nonisothermal crystallization behavior of polypropylene-clay nanocomposites

The influence of two concentrations of clay nanoparticles on the nonisothermal crystallization behavior of the intercalated polypropylene-clay nanocomposites is investigated here. It is observed that the crystallization peak temperature (T_p) of PP-clay nanocomposites is marginally higher than neat PP at various cooling rates. Furthermore, the half-time for crystallization (t_0.5) decreased with increase in clay content, implying the nucleating role of clay nanoparticles. The nonisothermal crystallization data is analyzed using Avrami, Ozawa and Mo and coworkers methods. The validity of kinetic models on the nonisothermal crystallization process of PP-clay nanocomposites is discussed. The approach developed by Mo and coworkers successfully describes the nonisothermal crystallization behavior of PP and PP-clay nanocomposites. The activation energy for nonisothermal crystallization of pure PP and PP-clay nanocomposites based on Kissinger method is evaluated.     

The effect of halloysite nanotubes and N,N'- ethylenebis (stearamide) on the properties of polylactide nanocomposites with amorphous matrix

Amorphous polylactide/halloysite nanotube (PLA/HNT) nanocomposites were prepared and examined. Neat HNT and HNT treated with N,N'- ethylenebis(stearamide) (EBS) were used as nanofillers. The role of HNT and/or EBS content on the cold crystallization of amorphous PLA matrix, HNT dispersion, as well as on the dynamic mechanical and optical properties of the materials was determined.The PLA/HNT-based nanocomposites contained well-distributed nanotubes and occasionally micron-sized aggregates, especially at high loading. HNT, EBS treated HNT and EBS influenced the cold crystallization of PLA, therefore the formation of the disorder α′ and the order α crystallographic forms of PLA.The nanocomposites exhibited increased stiffness and decreased transparency compared to the neat PLA. Due to the reinforcing effect and additional specific features of HNT, the addition of the nanofiller allows tuning of the properties of the nanocomposites with amorphous PLA matrix.     

Effect of moisture on the dynamic mechanical relaxation of polyamide-6/clay nanocomposites

This article investigates the effect of moisture on the dynamic mechanical behavior of polyamide-6 (PA6)/clay nanocomposites with dynamic mechanical analysis from −130 to 110 °C. The storage moduli increase with the clay loading for dried and moisture-absorbed samples because of the enhancing effect from the high-aspect-ratio nanoclay. Storage moduli for moisture-exposed samples are lower than those for dried samples; the longer the moisture absorption period is, the lower the moduli are for neat PA6 and PA6/clay nanocomposites. At temperatures below about 10 °C, however, samples exposed to moisture for longer periods tend to be stiffer than dried samples, probably because of the stiffening effect of ice. The peak temperature of the β relaxation shifts from −53 to −65 °C as the moisture content increases. The glass-transition temperature (T_g) or α relaxation dramatically shifts; its position is significantly lowered from 62 to 17 °C as the moisture content increases (longer moisture absorption period) and from 62 to 50 °C as the clay loading increases. The observed depression of the storage modulus and T_g may be attributed to the plasticization effect of moisture absorption. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1823–1830, 2004     

Isothermal crystallization kinetics and melting behaviour of PET/ATO nanocomposites prepared by in situ polymerization

Neat poly(ethylene terephthalate) (PET) and PET/antimony doped tin oxide (ATO) nanocomposites were prepared by in situ polymerization. The study of the isothermal crystallization behaviors of neat PET and PET/ATO nanocomposites was carried out using differential scanning calorimetry (DSC). The crystallization kinetics under isothermal conditions could be described by the Avrami equation. For neat PET and PET/ATO nanocomposites, the Avrami exponent n both decreased with increasing crystallization temperature. In addition, for the same crystallization temperature, the value of n increased with increasing ATO content. These suggested that the crystallization types related to the values of n in the Avrami theory could not be suitable for the crystallization of PET and its nanocomposites. The change of the n values indicated that the addition of ATO resulted in the increase of the crystallizing growth points. That is a heterogeneous nucleating effect of ATO on crystallization of PET. In the DSC scan after isothermal crystallization process, multiple melting behavior was found. And the multiple endotherms could be attributed to melting of the recrystallized materials or the secondary lamellae produced during different crystallization processes. The equilibrium melting temperature of PET in the nanocomposites increased with increasing the ATO content. Surface free energy of PET chain folding for crystallization of PET/ATO nanocomposites was lower than that of neat PET, confirming the heterogeneous nucleation effect of ATO.     

Poly(lactic acid)/organoclay nanocomposites: Thermal,rheological properties and foam processing

In this study, polymer nanocomposites based on poly(lactic acid) (PLA) and organically modified layered silicates (organoclay) were prepared by melt mixing in an internal mixer. The exfoliation of organoclay could be attributed to the interaction between the organoclay and PLA molecules and shearing force during mixing. The exfoliated organoclay layers acted as nucleating agents at low content and as the organoclay content increased they became physical hindrance to the chain mobility of PLA. The thermal dynamic mechanical moduli of nanocomposites were also improved by the exfoliation of organoclay; however, the improvement was reduced at high organoclay content. The dynamic rheological studies show that the nanocomposites have higher viscosity and more pronounced elastic properties than pure PLA. Both storage and loss moduli increased with silicate loading at all frequencies and showed nonterminal behavior at low frequencies. The nanocomposites and PLA were then foamed by using the mixture of CO₂ and N₂ as blowing agent in a batch foaming process. Compared with PLA foam, the nanocomposite foams exhibited reduced cell size and increased cell density at very low organoclay content. With the increase of organoclay content, the cell size was decreased and both cell density and foam density were increased. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 689–698, 2005     

Fire reaction properties of polystyrene-based nanocomposites using nanosilica and nanoclay as additives in cone calorimeter test

Using nanofiller additives in the polymer matrix to form nanocomposites is a potential way of reducing the flame spread and enhancing flame retardancy of polymeric materials during fire. To understand the fire reaction properties and the relative performance of flame-retardant additives in polymer during well-developed fire, neat polystyrene, polystyrene–silica and polystyrene–nanoclay (MMT) have been tested in a cone calorimeter. The neat polystyrene and the polystyrene nanocomposites have been prepared via an in situ polymerization method. An external heat flux of 50 kW m^?2 was applied in the test, and parameters such as heat release rate, peak heat release rate, time to ignition, smoke toxicity, CO and CO₂ yield have been investigated. Both neat polystyrene and polystyrene nanocomposites have shown the trend of a thermally thick charring polymer in the heat release rate over time data. The nanocomposites had an overall better flame retardancy than the neat polystyrene in terms of lower peak heat release rate, lower average mass loss rate and enhanced char formation. The nanocomposites had also reduced smoke emission with lower CO and CO₂ yield compared to the neat polystyrene. The overall flame retardancy was enhanced as the nanofiller loading was increased for both the nanosilica and MMT nanocomposites.

     

Facile Fabrication of PBS/CNTs Nanocomposite Foam for Electromagnetic Interference Shielding

In order to reduce the pollutants of environment and electromagnetic waves, environment friendly polymer foams with outstanding electromagnetic interference shielding are imminently required. In this paper, a kind of electromagnetic shielding, biodegradable nanocomposite foam was fabricated by blending poly (butylene succinate) (PBS) with carbon nanotubes (CNTs) followed by foaming with supercritical CO₂. The crystallization temperature and melting temperature of PBS/CNTs nanocomposites with 4 wt % of CNTs increased remarkably by 6 °C and 3.1 °C compared with that of pure PBS and a double crystal melting peak of various PBS samples appeared in DSC curves. Increasing the CNT content from 0 to 4 wt % leads to an increase of approximately 3 orders of magnitude in storage modulus and nearly 9 orders of magnitude in enhancement of electrical properties. Furthermore, CNTs endowed PBS nanocomposite foam with adjustable electromagnetic interference (EMI) shielding property, giving a specific EMI shielding effectiveness of 28.5 dB cm³/g. This study provides a promising methodology for preparing biodegradable, lightweight PBS/CNTs foam with outstanding electromagnetic shielding properties.     

The morphology and non-isothermal crystallization characteristics of poly(trimethylene terephthalate)/BaSO4 nanocomposites prepared by in situ polycondensation

A simple method was developed for preparing nano-BaSO₄ suspension by reacting H₂SO₄ with Ba(OH)₂ in 1,3-propanediol (PDO). The zymotechnics 1,3-propanediol and newly prepared nano-BaSO₄ suspension were used for fabricating PTT/BaSO₄ nanocomposites by in situ polymerization. The size distribution curves revealed that most of the nano-BaSO₄ particles in PDO have a diameter of 15–23 nm. Transmission electron microscopy (TEM) showed that BaSO₄ disperses uniformly in PTT matrix when BaSO₄ content was no more than 12 wt%. The non-isothermal crystallization behavior was studied quantitatively by modified Avrami equation and Ozawa theory. Both theories can successfully describe the non-isothermal crystallization of pure PTT and PTT/nano-BaSO₄ composites. The crystallinity and crystallization rate of nanocomposites was greatly increased by addition of BaSO₄. The maximum enhancement of crystallization rate for the nanocomposites was observed in nanocomposites containing about 12 wt% BaSO₄ with a range of 2–16 wt%, which was confirmed by both Avrami crystallization rate parameter (Z_c) and Ozawa crystallization rate parameter logK(T). The Avrami and Ozawa mechanism exponents, n and m of the nanocomposites, were higher than those of neat PTT, suggesting more complicated interaction between molecular chains and the nanoparticles that caused the changes of the nucleation mode and the crystal growth dimension. Effective activation energy calculated from the Friedman formula was reduced as nano-BaSO₄ content increased, suggesting that the nano-BaSO₄ made the molecular chains of PTT easier to crystallize during the non-isothermal crystallization process. The polarizing micrographs showed that much smaller or less perfect crystals formed in composites due to the interaction between molecular chains and nano-BaSO₄ particles.     

Effect of MWCNTs and graphene on the crystallization of polyurethane based nanocomposites,analyzed via calorimetry,rheology and AFM microscopy

Isothermal crystallization of two nanocomposites composed of multiwalled carbon nanotubes (MWCNT) and graphene dispersed in a hot melt adhesive polyurethane (PUR) is investigated. Rheological percolation is reached for PUR/MWCNT, but not for PUR/Graphene nanocomposites. Analysis of the elastic modulus versus time indicates that the crystallization process is more hastened by MWCNTs than by graphene, favouring the welding process of the corresponding adhesives. Correlative AFM images show that the crystal size follows the order: Crystals in neat PUR > Crystals in PUR/Graphene > Crystals in PUR/MWCNT. The nucleation density is higher in the case of MWCNTs due to the poorer dispersion of graphene in the PUR matrix. Analysis of DSC results by the Avrami equation and the fitting of G′-time results to a rheological equation adapted from the Avrami equation, show that the parameter n decreases from n = 3 for neat PUR to n = 2 for both nanocomposites. The coherence between the value n = 2 and AFM images is demonstrated: The lamellae organization in two dimensional growth, that brings about nucleated axialites, as expected for n = 2, is confirmed by AFM microscopy.