Influence of silanized low‐dimensional carbon nanofillers on mechanical,thermomechanical, and crystallization behaviors of poly(L‐lactic acid) composites—A comparative study

Composites were investigated regarding the comparison of multi‐walled carbon nanotubes (MWCNTs) with exfoliated graphene(EG) in poly(L‐lactic acid) (PLLA) and the effect of silane‐treated carbon nanofillers on properties of PLLA composites. Solution blending method was used to prepare PLLA composites at a filler content of 0.5 wt %. Fourier transform infrared spectroscopy and X‐ray photoelectron spectra results indicated the attachment of silane molecules on the surface of these nanofillers. It was found that the addition of these nanofillers greatly enhanced the mechanical, thermomechanical, and crystallization behaviors of PLLA due to the heterogeneous nucleation effect. Moreover, the silane‐treated fillers further enhanced the breaking elongation moderately (although the materials are still brittle), modulus and thermal property of the nanocomposites, without sacrificing the tensile strength, compared with the pristine nanocomposites. On the other hand, composites reinforced with MWCNTs and EG perform almost the same mechanical property. And EG outperformed MWCNTs in thermomechanical properties of composites when being used as the reinforcement of PLLA. Conversely, composites reinforced with MWCNTs showed better crystallization properties than those reinforced with EG. Interestingly, no significant changes were observed for the crystallization properties of PLLA composites when MWCNTs and EG had been treated by silane coupling agent. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1194‐1202, 2013     

Effectiveness of the preparation of maleic anhydride grafted poly (lactic acid) by reactive processing for poly (lactic acid)/carbon nanotubes nanocomposites

An effective strategy to increase the properties of poly (lactic acid) (PLA) is the addition of carbon nanotubes (CNT). In this work, aiming to improve the surface adhesion of PLA and CNT a new compatibilizer agent was prepared by reactive processing, PLA grafted maleic anhydride (PLA-g-MA) using benzoyl peroxide and maleic anhydride. The effectiveness of the PLA-g-MA as a compatibilizer agent was verified for PLA/PLA-g-MA/CNT nanocomposites. PLA and PLA-g-MA samples were characterized by Fourier transform infrared spectroscopy (FT-IR) to confirm the grafting reaction of maleic anhydride on PLA chains and by rheological analysis to prove the changes in the matrix PLA after the graphitization reaction. Thermal (differential scanning calorimetry and thermogravimetric analysis), mechanical tests (Izod impact strength and tensile test), and morphological characterization were used to verify the effect of the compatibilizer agent. The preparation of PLA-g-MA by reactive extrusion processing proved satisfactory and the nanocomposites presented good thermal and mechanical properties. The addition of the PLA-g-MA also contributed to the greater distribution of CNT and can be used as an alternative for the production of PLA/CNT nanocomposites.     

Comparison of covalent and noncovalent interactions of carbon nanotubes on the crystallization behavior and thermal properties of poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate)

In this study, multiwalled carbon nanotubes (MWCNTs) were dispersed into a poly(3‐hydroxybutyrate‐co?3‐hydroxyvalerate) (PHBV) matrix, in which PHBV was either covalently attached to the nanotubes through an esterification reaction between the carboxylic groups of functionalized MWCNTs and the hydroxyl groups of PHBV with toluene diisocyanate as a coupling agent or physically mixed to result in only noncovalent interactions. The structure, crystallization behavior, and thermal properties of the resulting nanocomposites were studied. We found that the crystallization of PHBV grafted onto the MWCNTs (PHBV‐g‐MWCNTs) was markedly hindered and exhibited an exothermic peak caused by cold crystallization, whereas the nonisothermal crystallization of PHBV was enhanced because a heterogeneous nucleation effect appeared in the PHBV/MWCNTs. Moreover, the maximum decomposition temperature of the PHBV‐g‐MWCNTs was improved by about 14.4°C compared with that of the PHBV/MWCNTs and by about 23.7°C compared with that of the original PHBV. Furthermore, the PHBV‐g‐MWCNTs exhibited the wider melt‐processing window than the PHBV/MWCNTs and original PHBV. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 4299–4307, 2013     

Polypropylene/poly(lactic acid)/carbon nanotube semi-biodegradable nanocomposites: The effect of sequential mixing approach and compatibilization on morphology,rheology and electrical conductivity

Semi-biodegradable polypropylene (PP)/poly(lactic acid) (PLA) (50:50 vol%) blend loaded with 0.6 vol% of pristine carbon nanotube (CNT) were prepared by melt compounding the components using different sequential mixing strategies: (i) all components together (PP/PLA/CNT); (ii) PP first mixed with CNT (PP@CNT/EVA) and (iii) EVA first mixed with CNT (EVA@CNT/PP). The composites presented co-continuous structure and the CNT selectively localized inside the PP phase or at the interface, regardless the order of the CNT addition into the mixing. These features were confirmed by selective extraction experiments and morphological studies: optical, scanning electron, and transmission electron microscopy. However, the preferential localization at the interface was predicted from wetting coefficient, determined from interfacial energy. Higher electrical conductivity values were achieved by using the one-step mixing approach, were all components were mixed together, whose value of around 10⁻⁴ S/m was achieved by adding 0.6 vol% of CNT to the blend. The compatibilization with polypropylene-g-maleic anhydride increased the melt viscosity of the blend and composite but did not affect the conductivity or the tensile properties of the CNT-based composite.     

Toughness improvement of poly(lactic acid) with poly(vinyl propionate)-grafted natural rubber

Natural rubber (NR) grafted with poly(vinyl propionate) (NR-g-PVP) was prepared by emulsion polymerization. The monomer content was set at 5, 10, 20, and 30 wt%. The chemical structure of NR-g-PVP was confirmed by ¹H-NMR and FTIR techniques. The grafting parameters of purified NR-g-PVP were evaluated. Binary (PLA/NR and PLA/NR-g-PVP) and ternary (PLA/NR/NR-g-PVP) blends were prepared by melt blending using a twin-screw extruder. The percentage of grafted PVP on NR affected morphology, thermal and mechanical properties of the blends. In binary blends, 5% grafting showed the greatest improvement of toughness and ductility with PLA, whereas there was no improvement in the mechanical properties of PLA/NR blend from using NR-g-PVP as a compatibilizer. The mechanical properties of the blends are related to mutual compatibility of the components. Good interfacial adhesion and proper particle size of NR were the key factors contributing to mechanical properties.     

Influence of processing condition and carbon nanotube on mechanical properties of injection molded multi‐walled carbon nanotube/poly(methyl methacrylate) nanocomposites

In this work, multi‐walled carbon nanotubes (MWCNT) and poly(methyl methacrylate) (PMMA) pellets were compounded via corotating twin‐screw extruder. The produced MWCNT/PMMA nanocomposite pellets were injection molded. The effect of MWCNT concentration, injection melt temperature and holding pressure on mechanical properties of the nanocomposites were investigated. To examine the mechanical properties of the MWCNT/PMMA nanocomposites, tensile test, charpy impact test, and Rockwell hardness are considered as the outputs. Design of experiments (DoE) is done by full factorial method. The morphology of the nanocomposites was performed using scanning electron microscopy (SEM). The results revealed when MWCNT concentration are increased from 0 to 1.5 wt %, tensile strength and elongation at break were reduced about 30 and 40%, respectively, but a slight increase in hardness was observed. In addition, highest impact strength belongs to the nanocomposite with 1 wt % MWCNT. This study also shows that processing condition significantly influence on mechanical behavior of the injection molded nanocomposite. In maximum holding pressure (100 bar), the nanocomposites show highest tensile strength, elongation, impact strength and hardness. According to findings, melt temperature has a trifle effect on elongation, but it has a remarkable influence on tensile strength. In the case of impact strength, higher melt temperature is favorable. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43738.     

On the use of ball milling to develop PHBV–graphene nanocomposites (I)—Morphology,thermal properties,and thermal stability

In the first part of this work, novel nanocomposites based on poly (3‐hydroxybutyrate co‐3‐hydroxyvalerate) (PHBV) and functionalized graphene nanosheets (FGS) were prepared through ball milling. As revealed by morphological characterization, this blending methodology was able to allow proper nanofiller dispersion and distribution into the matrix. Thermal properties were studied under non‐isothermal and isothermal conditions and the addition of FGS into PHBV matrix, although no changes in crystallization mechanism were observed, it modified the crystallization kinetics leading to increased crystallinity. Thermal stability analysis revealed that FGS affected the mechanism of oxidative thermal degradation and had no effect on thermal degradation by pyrolysis. Furthermore, an analysis of isothermal degradation kinetics showed that FGS speeded up the degradation rate. The Sestak‐Berggren model was used as a model to explain the isothermal degradation behavior of the obtained materials in good agreement with the experimental data. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42101.     

Effect of D-lactide content of annealed poly(lactic acid) on its thermal,mechanical, heat deflection temperature,and creep properties

In our research, the effect of D-lactide content in injection-molded and annealed poly(lactic acid) (PLA) was investigated for crystallinity as well as crystalline forms (less-ordered α′ and more-ordered α crystalline forms) and thus on the mechanical, heat deflection temperature (HDT), and creep properties. Three different PLA grades 3052D, 3001D, and 3100HP were investigated with D-lactide contents of 4%, 1.4%, and 0.5%, respectively. The injection-molded PLA specimens were then postproduction annealed in a heat chamber at 80–140 °C for 1 h to develop various crystallinities and various ratios of α′ and α crystal forms. It was demonstrated that not only annealing but also the D-lactide content significantly influences the crystallization, crystal structure, and accordingly properties of PLA like notched Charpy impact strength, HDT, and creep characteristics. Higher HDT, impact properties, and creep resistance could be reached by using PLA with lower D-lactide content with a certain annealing temperature range to develop definite crystal structures. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47103.     

On the use of ball milling to develop poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate)‐graphene nanocomposites (II)—Mechanical,barrier, and electrical properties

In this work, poly (3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) (PHBV) nanocomposites containing functionalized graphene sheets (FGS) were prepared by means of high‐energy ball milling. The crystalline structure, oxygen barrier, mechanical and electrical properties, and biodegradability of the developed nanocomposites were analyzed and correlated with the amount of FGS incorporated and with their morphology, which was reported in a previous study. Addition of FGS into the PHBV matrix did not affect the crystal morphology of the material but led to somewhat enhanced crystallinity. The good dispersion and distribution of the nanofiller within the polymeric matrix, revealed in the first part of this study, was thought to be crucial for the mechanical reinforcing effect of FGS and also resulted in enhanced gas barrier properties at high relative humidity. Additionally, the conducting behavior of the nanocomposites, as interpreted by the percolation theory, displayed a very low percolation threshold set at ～0.3 vol % of FGS, while the materials exhibited an overall significantly enhanced conductivity. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42217.     

Quantification of carbon nanotube dispersion and its correlation with mechanical and thermal properties of epoxy nanocomposites

An experimental study is carried out to quantitatively assess the dispersion quality of carbon nanotubes (CNTs) in epoxy matrix as a function of CNT variant and weight fraction. To this end, two weight fractions (0.05% and 0.25%) of as-grown, oxidized, and functionalized CNTs are used to process CNT/epoxy nanocomposites. Scanning electron microscopy, X-ray diffraction, and Fourier transform infrared analysis of different variants of CNTs are used to establish the efficiency of purification route. While the relative change in mechanical properties is investigated through tensile and micro-hardness testing, thermal conductivity of different nanocomposites is measured to characterize the effect of CNT addition on the average thermal properties of epoxy. Later on, a quantitative analysis is carried out to establish the relationship between the observed improvements in average composite properties with the dispersion quality of CNTs in epoxy. It is shown that carboxylic (-COOH) functionalization reduces the average CNT agglomerate size and thus ensures better dispersion of CNTs in epoxy even at higher CNT weight fraction. The improved dispersion leads to enhanced interfacial interaction at the CNT/epoxy interface and hence provides higher relative improvement in nanocomposite properties compared to the samples prepared using as-grown and oxidized CNTs. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48879.     

Thermal and mechanical behavior of poly(vinyl butyral)‐modified novolac epoxy/multiwalled carbon nanotube nanocomposites

The effects of poly(vinyl butyral) (PVB) and acid‐functionalized multiwalled carbon nanotube modification on the thermal and mechanical properties of novolac epoxy nanocomposites were investigated. The nanocomposite containing 1.5 wt % PVB and 0.1 wt % functionalized carbon nanotubes showed an increment of about 15°C in the peak degradation temperature compared to the neat novolac epoxy. The glass‐transition temperature of the novolac epoxy decreased with increasing PVB content but increased with an increase in the functionalized carbon nanotube concentration. The nanocomposites showed a lower tensile strength compared to the neat novolac epoxy; however, the elongation at break improved gradually with increasing PVB content. Maximum elongation and impact strength values of 7.4% and 17.0 kJ/m² were achieved in the nanocomposite containing 1.5 wt % PVB and 0.25 wt % functionalized carbon nanotubes. The fractured surface morphology was examined with field emission scanning electron microscopy, and correlated with the mechanical properties. The functionalized carbon nanotubes showed preferential accumulation in the PVB phase beyond 0.25 wt % loading. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43333.     

Bio‐based diblock copolymers prepared from poly(lactic acid) and natural rubber

New bio‐based diblock copolymers were synthesized from poly(lactic acid) (PLA) and natural rubber (NR). NR polymer chains were modified to obtain hydroxyl telechelic natural rubber oligomers (HTNR). Condensation polymerization between PLA and HTNR was performed at 110°C during 24 or 48 h. The molecular weight of PLA and HTNR and the molar ratio PLA : HTNR were varied. The new ester linkage in the diblock copolymers was determined by ¹H‐NMR. The molecular weight of the diblock copolymers determined from SEC agreed with that expected from calculation. The thermal behavior and degradation temperature were determined by DSC and TGA, respectively. The diblock copolymers were used as a toughening agent of PLA and as a compatibilizer of the PLA/NR blend. PLA blended with the diblock copolymer showed higher impact strength, which was comparable to the one of a PLA/NR blend. The former blend showed smaller dispersed particles as showed by SEM images, indicating the increase in miscibility in the blend due to the PLA block. The compatibilization was effective in the blends containing ～10 wt % of rubber. At a higher rubber content (>10 wt %), coalescence of the NR and diblock copolymer was responsible of the larger rubber diameter in the blends, which causes a decrease of the impact strength. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41426.     

Preparation and thermomechanical properties of nanocrystalline cellulose reinforced poly(lactic acid) nanocomposites

A two‐step process was developed to prepare nanocrystalline cellulose (NCC) reinforced poly(lactic acid) (PLA) nanocomposites using polyethylene glycol (PEG) as a compatibilizer. It was composed of solvent mixing and melt blending. The NCC was well dispersed in the PLA matrix. A network was formed at high NCC‐to‐PEG ratio at which the amount of the PEG was not enough to cover all the surfaces of the NCC. The formation of the network was confirmed by the occurrence of a plateau for the storage modulus at low frequency. The incorporation of the PEG and NCC could improve the crystallinity of the PLA. The elongation at break increased from 11.0% for the neat PLA to 106.0% for the composites including 6 wt % NCC, impact strength was improved from 0.864 to 2.64 kJ m^?2 and tensile strength did not change significantly for the same 6 wt % NCC composites. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134 , 44683.     

In situ polymerization of bio‐based thermosetting polyurethane/graphene oxide nanocomposites

Novel bio‐based polyurethane/graphene oxide (GO) nanocomposites have been successfully synthesized from biorenewable epoxidized soybean‐castor oil fatty acid‐based polyols with considerable improvement in mechanical and thermal properties. The GO was synthesized via a modified pressurized oxidation method, and was investigated using Raman spectra, AFM and XPS, respectively. The toughening mechanism of GO in the bio‐based polyurethane matrix was explored. The elongation at break and toughness of polyurethane were increased by 1.3 and 0.8 times with incorporation of 0.4 wt % GO, respectively. However, insignificant changes in both mechanical strength and modulus were observed by adding GO. The results from thermal analysis indicated that the GO acts as new secondary soft segments in the polyurethane which lead to a considerable decrease in the glass transition temperature and crosslink density. The SEM morphology of the fracture surface after tensile testing showed a considerable aggregation of graphene oxide at concentrations above 0.4 wt %. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41751.     

Green composites of poly(3‐hydroxybutyrate) and curaua fibers: Morphology and physical,thermal, and mechanical properties

In this article, we report the morphology and thermal, mechanical and physical properties of poly(3‐hydroxybutyrate) (PHB)/curaua composites containing triethyl citrate (TEC) as the plasticizer. The composites were prepared by mechanical mixing using pristine and chemically treated fibers (10 wt %) and TEC (30 wt %) and characterized by differential scanning calorimetry, dynamic mechanical analysis, X‐ray diffraction, small angle X‐ray scattering, polarized optical microscopy, scanning electron microscopy, tensile tests, impact resistance test, thermodilatometry, and thermal conductivity measurements. The curaua fibers acted as nucleating agent and strongly influenced the morphology of the crystalline phase of PHB, increasing the lamella thickness, decreasing the crystal size and inducing spherulite–axialite transition. These characteristics of the PHB crystalline phase determined all the properties of the composites. The tensile properties of the composites were comparable with those of neat PHB, while the impact resistance of composites was comparable with that of plasticized PHB. The higher heat capacity and thermal expansion coefficient and the lower thermal conductivity of the composites compared with neat PHB reflect the morphological changes in the PHB crystalline phase. The strategy of developing a green polymeric material from ecofriendly components exhibiting a good balance of properties by combining curaua fibers, TEC, and PHB was successful. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44676.     

Thermosetting polyurethane‐multiwalled carbon nanotube composites: Thermomechanical properties and nanoindentation

In this study, composites based on a thermoset polyurethane elastomer (PU) and multiwalled carbon nanotubes (MWCNT) in the case of a PU of high elastic modulus (>200 MPa) are analyzed for the first time. As‐grown and modified nanotubes with 4 wt % of oxygenated functions (MWCNT‐ox) were employed to compare their effect on composite properties and maxima mechanical properties (elastic modulus and tensile strength) were reached at 0.5 wt % of MWCNT‐ox. Furthermore, by examining the morphology using optical and electron microscopies better dispersion and interaction of the nanotube‐matrix was observed for this material. DMTA data supports the observation of an increase in the glass transition temperature of ～20°C in the nanocomposites compared with the thermoset PU, which is an important result because it shows extended reliability in extreme environments. Finally, nanoindentation tests allowed a comparison with the conventional mechanical tests by measuring the elastic modulus and hardness at the subsurface of PU and the nanocomposites. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41207.     

Study on flax fiber toughened poly (lactic acid) composites

Poly (lactic acid) (PLA) is a renewable and biodegradable polymer with high modulus, high strength but low toughness. Blending PLA with plant fiber has been believed an available strategy to improve the toughness of PLA. PLA/Flax composites were fabricated by extrusion and injection molding processes. The flax fiber surfaces were modified before blending to improve the compatibility, and the chemical structures of both untreated and treated fiber were characterized by Fourier transform infrared spectroscopy. Results of mechanical test showed that the impact strength and elongation at break of PLA/Flax composites were remarkably higher than PLA. The impact fractures of PLA/Flax composites were also observed by scanning electron microscope. The results showed uniform dispersion of fibers in PLA matrix and good compatibility between treated fibers and PLA matrix. Moreover, it can be observed that crazing propagation was hindered by fibers and transcrystalline developed along fibers by polarized optical microscope. Differential scanning calorimetry analysis was carried out to study the crystallinity of PLA and it was found that incorporation of fiber improved the crystallinity of PLA. The toughening mechanism of PLA/Flax composites was discussed according to the results. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42573.     

Combined effect of a plasticizer and carvacrol and thymol on the mechanical,thermal, morphological properties of poly(lactic acid)

In this study, the effects of the monotherpenic phenol concentration on the properties of biocomposites containing plasticized poly(lactic acid) (PLA) with acetyl tributyl citrate (ATBC) were investigated. The monotherpenic phenols carvacrol (C) and thymol (T) were added to PLA by a melt‐blending method. The prepared samples were characterized by means of tensile testing, Fourier transform infrared spectroscopy, thermogravimetric analysis, differential scanning calorimetry, scanning electron microscopy (SEM), and antibacterial activity tests. The addition of ATBC to PLA resulted in hydrogen bonding between ATBC and PLA. We observed that ATBC, C, and T reduced the glass‐transition temperature of PLA. The presence of C and T decreased the maximum degradation temperature slightly. Because of the plasticization effect of the additives, the tensile strength and Young's modulus of PLA decreased, whereas the extent of elongation they experienced before failure increased. This effect was also observed with SEM analysis in terms of plastic deformation at break. The antibacterial activity tests showed that samples containing high concentrations of C demonstrated an improved antibacterial activity against Staphylococcus aureus, Salmonella typhimurium, and Listeria monocytogenes bacteria. We observed that C exhibited a higher inhibition against bacterial strains than T. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45895.     

A liquid‐like multiwalled carbon nanotube derivative and its epoxy nanocomposites

Multiwall carbon nanotubes (MWCNTs) with liquid‐like behavior at room temperature were prepared with sulfonic acid terminated organosilanes as corona and tertiary amine as canopy. The liquid‐like MWCNT derivative had low viscosity at room temperature (3.89 Pa s at 20°C) and exhibited non‐Newtonian shear‐thinning behavior. The weight fraction of MWCNT in the derivative was 16.72%. The MWCNT derivative showed very good dispersion in organic solvents, such as ethanol and acetone. The liquid‐like MWCNT derivative was incorporated into epoxy matrix to investigate the mechanical performance of the nanocomposites and the distribution of MWCNTs in the matrix. When the liquid‐like MWCNT derivative content was up to 1 wt %, the flexural strength and impact toughness of composites were 12.1 and 124% higher than the pure epoxy matrix, respectively. Transmission electron microscope (TEM) confirmed the very good dispersion of the liquid‐like MWCNT derivative in epoxy matrix. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 2217–2224, 2013