PET Nanocomposites: Preparation and Characterization

6  PET Nanocomposites: Preparation and Characterization K. Priya Dasan Material Chemistry Division, SAS, VIT University, Vellore, Tamil Nadu, India

Polyethylene terephthalate (PET) is a plastic resin and is the most common type of polyester used commercially. It is also one of the most recycled plastics in the market. Soon after its commercial production in 1953 by DuPont, PET became the most widely produced synthetic fiber in the world. Later in the 1970s, the development of stretch-molding procedures facilitated the bulk production of PET into durable crystal-clear beverage bottles – an application that soon became second in importance only to fiber production. PET technology has advanced to such a level today that its presence is felt in almost every application. PET is generally produced by the polymerization of ethylene glycol and terephthalic acid. Ethylene glycol is a colorless liquid obtained from ethylene, and terephthalic acid is a crystalline solid obtained from xylene. When subjected to heat in the presence of chemical catalysts, ethylene glycol, and terephthalic acid produce PET in the form of a molten, viscous mass that can be spun directly to fibers or solidified for later processing as a plastic. Ethylene glycol is a diol, an alcohol with a molecular structure that contains two hydroxyl (OH) groups, and terephthalic acid is a dicarboxylic aromatic acid, an acid with a molecular structure that contains a large, six-sided carbon (or aromatic) ring and two carboxyl (CO2H) groups. Under the influence of heat and catalysts, the hydroxyl and carboxyl groups react to form ester (CO–O) groups, which serve as the chemical links joining multiple PET units together into long-chain polymers. This chemical structure provides PET many advantages, for example it is resistant to both heat and cold; it also has transparency, electrical qualities, and is chemical proof and abrasion proof. The presence of a large aromatic ring in the PET repeating units gives the polymer notable stiffness and strength, especially when the polymer chains are aligned with one another in an orderly arrangement by drawing or stretching. They are often used in blends with other fibers such as rayon, wool, and cotton, as fiber filling for insulated clothing and for furniture and pillows, as artificial silk when made in very fine filaments, and as carpet when

in large-diameter filaments. It is also used in automobile tire yarns, conveyor and drive belts, reinforcement for fire and garden hoses, seat belts, and nonwoven fabrics for stabilizing drainage ditches, culverts, and railroad beds, and for use as diaper topsheets and disposable medical garments. At a slightly higher molecular weight, PET is made into a high-strength plastic that can be shaped by all the common methods employed with other thermoplastics. Magnetic recording tape and photographic film are produced by extrusion of PET film. PET bottles are one of the most commercially preferred food containers due to their nontoxicity, strength, transparency, light weight, resealability, shatter resistance, and recyclability. When it comes to the application of thermoplastics such as PET, the most important criteria are their thermal stability, stiffness, and strength. At a commercial level these criteria are achieved through the addition of suitable reinforcing agents such as fibers or fillers. A recent trend is the utilization of these fillers at the nanoscale to achieve optimum properties even at very low loading. The formation of a hybrid nanocomposite results in a synergetic effect of the two respective components in the nanometer scale leading to considerable improvements in the mechanical, thermal, and gas barrier properties of polymer [1]. The small size of the fillers leads to a dramatic increase in interfacial area and this creates a significant volume fraction of interfacial polymer with properties different from the bulk polymer even at low loadings. Filler amounts of even less than 5 wt% result in effective enhancement of the nanocomposite properties [2]. There is a large variety of inorganic nanofillers under different stages of study as fillers in polymer. The most widely studied fillers include nanotubes, layered silicates (e.g., montmorillonite (MMT), saponite), metal nanoparticles, metal oxides, semiconductors (e.g., PbS, CdS), etc. [3]. In the past few years, a large number of works have been reported on the modification of PET with different types of fillers [4–10]. This chapter, however, mainly discusses the reinforcement of PET with

Poly(Ethylene Terephthalate) Based Blends, Composites and Nanocomposites. http://dx.doi.org/10.1016/B978-0-323-31306-3.00006-3 Copyright © 2015 Elsevier Inc. All rights reserved.

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clay, talc, silica, carbon nanotube, graphite, and a few other metals and their salts in their nanoscale. The effect of these reinforcements on the thermal and mechanical behavior of PET is also highlighted. The homogeneous dispersion of nanoparticles in polymer matrix and the interaction between the nanoparticles and PET matrix has been a huge challenge. The most common methods adopted for the preparation of PET nanocomposites are in situ polymerization or melt mixing. Each of these methods requires systematic investigation to obtain an optimum level of dispersion or intercalation. The various methods adopted to overcome these issues and the properties of composites thus obtained are given in the following discussions. One important aspect of PET is its crystallization behavior of PET nanocomposites. Previous reports suggest that microstructure and mechanical properties of PET can be significantly controlled by tailoring crystallization rate and degree of crystallinity with use of additives [11–14]. The role that nanoparticles play in the crystallization behavior of PET nanocomposites is widely reported in the literature [15,16]. Ou et al. [17,18] in their report have suggested that the half-life of crystallization can be lowered on clay intercalation in PET. Investigations by Barber et al. [19] have shown that the incorporation of organically modified MMT clays, such as Cloisite® 30A, via solution blending can significantly increase the crystallization temperature while lowering the crystallization half-life. They found exfoliated nanoparticles with bound ionomer to be less effective nucleants relative to the large clay aggregates. Investigations by some other research groups have pointed out that ionomeric content alone can also affect the overall crystallinity of PET [20,21]. Entezam et al. [22] suggested that the localization of nanoclay in the blend systems can be detected by analyzing changes in crystallization temperature and crystallinity of polymer components of the blend systems. The tensile test studies revealed that tensile strength of the blend systems was affected by localization of nanoclay in the matrix phase only. PET/clay nanocomposites has found a prominent place as a reinforcement in polymer for enhanced performance and low cost. The commercial application of clay/polymer nanocomposites started with the Toyota company, which began to produce clay intercalated thermoplastics for automobile applications. In the case of PET/clay nanocomposites, obtaining exfoliated or intercalated structures has been a huge challenge mainly due to the entropic and enthalpic energy factors and high PET processing temperatures of around 250°C. In general two different methods are reported

in the literature to prepare PET/clay nanocomposites. One is the direct melt compounding of an organically modified clay into PET and the other involves adding an organically modified clay into monomers or oligomers, followed by in situ polymerization. Hegde et al. [23] studied the crystallization kinetics and morphology of natural nanoclay (Closite Na+)-incorporated melt spun PET fibers. The clay tactoids were delaminated and lacked ordered structure in PET. Clay platelets showed a difference in distribution due to the “wall effect” and extent of intercalation at the near-fiber surface and at the fiber center. Even though nanoclay additives acted as nucleating agents, with accelerated crystallization kinetics, mechanical property benefits were not seen in fibers due to agglomeration. The strength and elongation of fibers decreased, and brittle failure mode was observed. Davis et al. [24] reported the preparation of PET-based nanocomposite via melt blending using a corotating mini twin-screw extruder operated at 285°C. They used 1,2-dimethyl-3-N-alkyl imidazolium salt modified MMT (hexadecyl-MMT) as the filler. Wide-angle X-ray diffraction analysis and transmission electron microscopy (TEM) observations established the formation of a mixed delaminated/ intercalated structure in the nanocomposites. Many researchers have attempted to facilitate better dispersion of nanofillers in PET nanocomposites by utilizing novel compatibilizers [25–27]. SanchezSolis et al. [28] used pentaerythritol and maleic anhydride as additives in the extrusion process in an attempt to compatibilize the PET with an organically modified MMT (Cloisite 15A). The morphology of the system was investigated by X-ray diffraction (XRD), and it was concluded that the system investigated did not display an exfoliated morphology. With large incorporations of the compatibilizers and increasing clay concentrations, an increase in the percent crystallinity was observed for the extruded pellets. This behavior was attributed to the large clay aggregates acting as a nucleating agent. Lai et al. [29] investigated the effect of epoxy monomer as a compatibilizer for PET copolymer/clay nanocomposite prepared via melt extrusion. The melt reactions between epoxide groups, hydroxyl groups in Cloisite 30B modifier, and the carboxyl end groups of polyethylene naphthalate (PETN) took place, thereby introducing extensive intercalation and exfoliation of Cloisite 30B. Besides improving the d spacing, the epoxy monomer acted as compatibilizer as well as the chain extender, improving the chemical interactions between PETN and organoclay, while discouraging the macromolecular mobility of polymer

6:  PET Nanocomposites: Preparation and Characterization

chains in the vicinity of clay particles. The improved polar interactions between the organoclay and the matrix were in turn found to enhance the thermal stability of the composites. The reason for using epoxides was that epoxide has the most suitable functionality to form covalent bonds with the nucleophilic end groups present in polyesters within the time constraint of extrusion operations. The random incorporation of ionic functionalities along the PET backbone was found to enhance interactions between the matrix polymer and MMT clay resulting in enhanced mechanical and thermal properties. Organically modified MMT clays and sodium MMT were introduced into pure PET and PET ionomers through a melt extrusion process and compared to determine the influence of the ionic functionality on clay platelet dispersion and crystallization behavior [30]. The ionic interactions created by the incorporation of random sulfonate groups into the backbone of PET aided in the dispersion of the organically modified MMT clay. The mechanism behind this was explained as the electrostatic interactions between the sulfonate groups along the polymer backbone and the edges of the clay platelets, facilitating movement of chain segments attached to the ionic group into the clay gallery. TEM and XRD data showed that increasing amounts of ionic comonomer in the PET backbone increase the apparent degree dispersion of the clay platelets. Goitisolo et al. [31] used polyamide 6 (PA6) to blend with PET to obtain better dispersion of nanoclay in PET nanocomposites. The researchers proceeded with PA6 since PET blends with PA6 have proven to be compatible due to the copolymers produced by reactions between the two components in the melt state [32–34]. Also, PA6 is prone to fibrillating in the melt state, which assured compatibility and allowed the development of these high surface–volume ratio structures. The organoclay used was Nanomer I30, which is known [35,36] to be able to effectively exfoliate in PA6. Morphological analysis showed organoclay located in the dispersed PA6 phase. The authors have claimed an increased mechanical behavior for the composite compared to any previously observed in the PET matrix. This was credited to the mixing procedure used and property of organoclay, which led to the location of the organoclay and to its consequent clear stiffening. Organoclays with various contents of hydroxyl groups and absorbed ammonium were prepared and compounded with PET, forming PET/clay nanocomposites via melt extrusion [37]. The main factors of polymer degradation were the amount of hydroxyl groups on the edge of clay

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platelets and ammonium linkage on the clay for clayreinforced PET composites. The authors claimed that with the great increase in molecular weight, the clay dispersion state remains in the PET/clay nanocomposites. When the hydroxyl groups on clay platelets, which acted as Brønsted acidic sites to accelerate the polymer degradation, were grafted by silane, PET/H-MMT showed greater decrease on polymer viscosity due to the larger amount of Brønsted acidic sites via acid treatment. The absorbed ammonium of clay encountered Hofmann elimination reaction and produced Brønsted acidic sites during the melt extrusion, which accelerated further degradation of PET. The effect of ammonium on the polymer degradation weakened due to the shrinking amount of ammonium linkage on clay by ethanol washing and silane grafting. In addition, a better clay dispersion state resulted in greater polymer degradation because of the larger amount of clay surface and ammonium exposed to the PET matrix. Huo et al. [38] investigated the possibility of a silicon-based coating to introduce synergism with layered silicates in the flame retardancy of PET. Rather than being deposited on a polymer surface, a phenyl containing highly cross-linked polyborosiloxane (PBSiO) was synthesized to coat a commercial clay (Nanomer I.34 TCN, Nanocor, USA), which was subsequently blended with PET in the molten state. The decomposition of PBSiO coatings led to the formation of borosilicate glassy structures, which prevented the substrate from degradation. In combination with the organoclay, PBSiO was found to develop a protective borosilicate-carbonaceous char on the surface of PET, improving significantly its performance toward fire. The addition of 5 wt% PBSiO and 2.5 wt% organo-montmorillonite (OMMT) was found sufficient to obtain a reduction of 59% in pHRR (heat release rate) and of 51% in smoke yield. Ammala et al. [39] achieved improved dispersion of clay in PET by use of an aqueous ionomer. The researchers used PET ionomer to give an improved dispersion of both an organo-modified MMT clay (Cloisite 10A) and a modified synthetic fluoromica clay (Somasif MEE) in PET. The nanocomposites were prepared by first dispersing the clays in water with the PET ionomer, coating the suspension onto solid PET followed by the removal of water and then direct melt extrusion resulting in a greater degree of exfoliation of clay particles. This method takes advantage of the initial good separation of clay platelets that occurs when water causes the platelet layers to swell and disperse in solution. Ionic interactions of the negatively charged sulfonate groups on the PET

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Poly(Ethylene Terephthalate) Based Blends, Composites and Nanocomposites

ionomer with the positive charges on the edges of the clay platelets ensured that the platelets are kept apart after the removal of the water. Also, the high level of compatibility of the PET ionomer with the PET polymer led to greater dispersion of the clay in the final nanocomposite. Nanocomposites of PET with different types of organoclays were prepared using the melt mixing technique by Papageorgiou et al. [40]. Two types of commercial inorganic clays (Laponite-synthetic hectorite and Kunipia-MMT) were studied after cation exchange with hexadecyl trimethyl ammonium bromide while two commercial organo-modified MMT clays (Nanomer I.30E modified with primary octadecylammonium ions and Cloisite 10A modified with quaternary dimethyl benzyl hydrogenatedtallow ammonium ions) were also investigated. The organoclay I.30E was found to induce higher crystallization rates and lower activation energy. It was found to be more effective regarding the PET crystallization compared to the other types of organoclays. The I.30E organoclay nanocomposite exhibited higher immobilized amorphous fraction and the higher nucleation parameter Kg in the Lauritzen–Hoffman analysis. This is explained as being due to the better dispersion and exfoliation of the clay nanolayers into the PET matrix, compared to the other organoclays. The influence of OMMT nanomaterials on the thermal behavior of PET and its nonisothermal crystallization was reported by Antoniadis et al. [41]. They found that OMMT nanoparticles can act as nucleating agents enhancing the crystallization rate of PET. The easy recyclability of PET led many researchers to focus on recycled PET nanocomposite. Most of the works on recycled PET (rPET) focused on their mechanical and thermal properties. Pegoretti et al. [42] studied the dispersion of two different types of clay in recycled PET. Various amounts (1, 3, and 5 wt%) of a nonmodified natural MMT clay (Cloisite Na+) or of an ion-exchanged clay modified with quaternary ammonium salt (Cloisite 25A) were dispersed in recycled rPET by a melt intercalation process. Microphotographs of composite fracture surfaces showed that particles of Cloisite 25A are much better dispersed in the rPET matrix than those of Cloisite Na+. Moreover, wide-angle X-ray scattering measurements indicate that the lamellar periodicity of Cloisite 25A is increased in the composites, which evidences intercalation of rPET between silicate layers (lamellae) of the clay. Uniaxial tensile tests showed that both clays increase the modulus of the rPET composites; more effective Cloisite 25A accounts for a 30% increase at loading of 5 wt%. Yield

strength was found to remain practically unaffected by the used fractions of the clays while tensile strength slightly decreased with the clay content. Recycled PET nanocomposites with various amounts of MMT have been manufactured by using twin-screw extrusion by Parvinzadeh et al. [43]. By rising MMT weight content up to 1 wt% it was possible to increase yield strength and ultimate strength of the composite by 17% and 27%, respectively, in comparison to neat rPET. Introduction of MMT in the rPET also led to considerable increase of the modulus of elasticity and reduced creep of the composites. Yuan et al. [44] prepared water-soluble polyvinylpyrrolidone-treated fibrous silicate (palygorskite, PT)/PET/silica nanocomposites with good dispersion of the PT nanoparticles via in situ polycondensation. They found that during thermal decomposition in nitrogen, the clay as a mass-transport protective barrier can slow down degradation of polymer, but the catalytic effect of metal derivatives in clays can accelerate the decomposition behavior of PET. The combination of these two effects determined the final thermal stability of nanocomposite. However, in air atmosphere, the oxidative thermal stability of PET/PT nanocomposite was found to be superior to that of pure PET. PET/silica nanocomposites were synthesized by using the in situ polymerization approach by He et al. [45]. Sol-gel transformation based on the hydrolysis and condensation of tetraethoxysilane was used to prepare the inorganic phase, concurrent with condensation polymerization of terephthalic acid and ethylene glycol to produce the PET matrix. Due to the simultaneous formation of the polymer matrix and the inorganic networks, a macrophase separation was avoided, and the resulting materials were found to have high degree of homogeneity. Differential scanning calorimetry (DSC) measurements indicated that the silica nanoparticle can act as an efficient nucleating agent for PET crystallization, resulting in significant increase in PET crystallization rate. Ke et al. used two kinds of inorganic particles of SiO2 and LS (layered silicate) to disperse in PET [46]. Nanoscale silica was first treated with organic molecules, covered by PET oligomer, and then mixed or polymerized with PET to form a composite. Similarly, LS micron particles were treated by a quaternary salt, and then mixed with PET oligomers to form a precursor of submicron particles, which were exfoliated into nanoparticles through subsequent polymerization. The dispersion results showed that both LS and silica are homogeneously dispersed in PET, while they have different dispersion

6:  PET Nanocomposites: Preparation and Characterization

mechanisms. Silica was dispersed by shear force, and LS by in situ exfoliation. The resulting PET-based nanocomposites showed excellent barrier properties to oxygen compared to pure PET films. The light transmittance by PET-SiO2/PS (porous silicon) nanocomposite films were found to be much higher than PET alone due to strongly heterogeneous nucleation of PS-encapsulated SiO2 nanoparticles in PET [47]. The nanocomposite films were prepared by melting PET with the core–shell SiO2/PS nanoparticles. Crystallization temperature of nanocomposite films with 2 wt% PS-encapsulated SiO2 nanoparticles was 11.6°C higher than that of PET. For crystallized PET-SiO2/PS nanocomposite films, double-melting peaks appeared in DSC curves similar to PET. With the increase of annealing temperature from 110°C to 150°C, the transmittance of PET-SiO2/PS nanocomposite films decreased slowly from 69.9% to 46.9%, while their haziness increased slightly from 45.8% to 48.2%. The roughness of nanocomposites was found to be higher than the pure PET. The migration of hydrophilic nanosilica particles to the surface of the PET matrix was observed in these composites leading to the belief that this causes increased roughness. Two different types of PET nanocomposites were prepared using hydrophilic (i.e., Aerosil 200 or Aerosil TT 600) or hydrophobic (i.e., Aerosil R 972) nanosilica by melt compounding [48]. Contact angle measurements of the resultant PET composites demonstrated that the wettability of such composites depends on surface treatment of the particular nanosilica particles used. Zheng and Wu [49] in their study have shown that nanosilica does not behave as a nucleating agent but rather retards the appearance of the microcrystalline phase that enhances spinnability. Liu and coworkers [50–52] have carried out extensive research on silica/ PET nanocomposites. The effect of the nanoparticles on the crystallization temperature, on the melting point of the polymer, and also on the mechanical behavior of PET is extensively reported by them. Tammaro et al. [53] reported the preparation of seven modified hydrotalcites via coprecipitation method and the characterization of these new hybrids. The organic molecules intercalated in the layered double hydroxide (LDH) were chosen for their possible oxygen barrier properties and their dispersion in the PET matrix was achieved by high-energy ball milling. The study of the structural, thermal, and oxygen barrier properties of PET/LDH composites was performed. The authors suggested the composites as promising active food packaging systems due to the presence of agents that can control undesirable oxidation on the

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surface of food. 10-[3,5-bis(methoxycarbonyl) phenoxy]decyltriphenylphosphonium bromide (IP10TP) was used as a compatibilizer in PET/mica nanocomposite prepared via a two-step polymerization procedure. The presence of compatibilizers led to better dispersion of the mica [26]. The resulting nanocomposite exhibited higher tensile properties when compared to a PET/mica nanocomposite prepared without compatibilizer. Effect of surface modification on exfoliation and dispersion of micrometer-sized LDH in PET and the thermal stability of the resultant PET/LDH nanocomposites or composite was studied by Xu et al. [54]. First, large micrometer-sized LDH with carbonate anions were prepared by the urea hydrolysis method. Then, various anions including nitrate (NO3−), dodecyl sulfate (DS−), or dodecyl benzene sulfonate (DBS−) were intercalated into the interlayer space via anion exchange reaction. By a solution intercalation process, PET/LDH_DS and PET/ LDH_DBS nanocomposites were successfully obtained, while the LDH_NO3 could not be exfoliated enough in the PET matrix. After appropriate surface modification by DS− or DBS−, large micrometersized LDH synthesized by the urea hydrolysis method could be dramatically exfoliated and homogeneously dispersed in the PET matrix to afford PET/LDH_DS or PET/LDH_DBS nanocomposites and the degree of exfoliation for LDH_DBS was observed to be better than that for LDH_DS. The PET/LDH_DBS nanocomposite possesses the highest thermal stability and the largest charred residue. The effect of processing techniques on the dispersion of nanofillers in PET was investigated by Martínez-Gallegos et al. [55]. An organic–inorganic hybrid compound constituted by an LDH with the hydrotalcite-type structure intercalated with dodecyl sulfate (LDH-DS) was dispersed in PET through chemical reaction or by mechanical treatment in an agate mortar or a ball mill. Exfoliation and dispersion of the inorganic–organic hybrid in the polymer phase was achieved by chemical processes. Mechanical grinding of the LDH-DS and PET mixtures only led to partial dispersion after long milling times, both for the PET-LDH composites and for the LDH-DS hybrid. In situ preparation of the polymer by chemical reactions yielded optimum exfoliation at low LDH-DS loadings. Carbon in various forms has been studied as fillers for PET/graphene nanocomposites. Structurally graphene is a monolayer of carbon atoms arranged in a honeycomb network. The unique properties of graphene make it a choice as inorganic fillers to improve electrical, thermal, and mechanical properties of composite materials [56–58]. Several

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Poly(Ethylene Terephthalate) Based Blends, Composites and Nanocomposites

effective techniques have been developed for preparing graphene nanosheets, including chemical [56] and mechanical exfoliation [59], alkali metal intercalation and expansion [60], microwave chemical vapor deposition [61], substrate-based thermal decomposition [62], and thermal exfoliation of graphite oxide [63]. Among them, the thermal exfoliation and in situ reduction method can conveniently produce graphene nanosheets for mass production. The presence of oxygen-containing groups in graphene nanosheets as a result of thermal exfoliation can facilitate the dispersion of the nanosheets in polar polymers [64]. PET/graphene nanocomposites with uniform dispersion were prepared by melt compounding by Zhang et al. [65]. Graphene nanosheets were prepared by complete oxidation of pristine graphite followed by thermal exfoliation and reduction. The incorporation of graphene greatly improved the electrical conductivity of PET, resulting in a sharp transition from electrical insulator to semiconductor with a low percolation threshold of 0.47 vol%. A high electrical conductivity of 2.11 S/m was achieved with only 3.0 vol% of graphene. A series of nanocomposites based on PET and exfoliated graphite (EG) were prepared by the melt-compounding method and their morphology, structures, thermal stability, and mechanical and electrical properties were then investigated by Li and Jeong [66]. For manufacturing the nanocomposites, EG, which is composed of disordered graphene sheets, was prepared by acid treatment followed by rapid thermal expansion of micronsized crystalline natural graphites. The disordered graphene sheets of EG were well dispersed in the PET matrix without forming crystalline aggregates even at a high EG content of 7.0 wt%. Accordingly, thermal and thermo-oxidative degradation temperatures of PET/EG nanocomposites were improved substantially with the increment of EG content. In addition, dynamic storage moduli of the nanocomposites increase noticeably by increasing the EG content. The extraordinary properties that can be achieved with PET/carbon nanotubes (CNTs) composites prompted many researchers to look into these materials as a reinforcement for polymeric systems. The unique and extraordinary mechanical, electrical, and thermal conductivity properties of multiwalled carbon nanotubes (MWCNTs) make them ideal candidates as functional fillers for polymeric materials. Alongi et al. [67] reported on fibers spun from polyester/carbon nanofiber (CNF) composites prepared by melt blending. A homogeneous and fine dispersion of CNF was observed in the PET matrix. Meanwhile, the presence of CNF increased the elongation at break and

decreased tenacity and tensile strength. The combustion tests by cone calorimetry revealed a relevant decrease of heat-release rate, total heat evolved, and total smokes released by the nanocomposites as compared to neat PET. The extent of MWCNT dispersion and its effect on the mechanical behavior of PET was evaluated by Yoo et al. [68]. The investigations were carried out by correlating microscopic observations with electrical and rheological percolation measurements. Prior to incorporating, the MWNTs were acid treated, and functionalized with 2-phenylethyl alcohol and 4-phenyl-1-butanol. The incorporation of MWNTs into the melt-spun fibers resulted in increased crystallization of PET but lower breaking stress than that of pure PET fibers, even in those containing well-dispersed functionalized MWNTs. The breaking stress of drawn composite fibers was also lower than that of pure PET fibers prepared at the same draw ratio. Time annealing effect was more dominant for composite fibers with functionalized MWNTs. These findings indicated that the presence of well-dispersed MWNTs disturbs the crystallization and orientation of PET molecules in highly stressed fibers, which differs from MWNT-induced crystallization of PET molecules in relaxed fibers. The fracture process of CNT/PET composites was investigated using time-resolved small-angle X-ray scattering measurements during tensile deformation [69]. The influences of CNT addition on the mechanical properties of PET were found to vary depending on the specimen geometries used for the mechanical tests and marked influences were obtained with surface-notched specimens. The CNT addition increased the energy needed to widen the crazes and retarded the growth and fracture of the crazes during deformation. Gomez-del Rio et al. [70] studied the mechanical behavior of PET/single-walled carbon nanotube (SWCNT) composites prepared by in situ polymerization. Young’s modulus was found to increase continuously with the volume fraction of SWCNT added, while Poisson ratio decreased. Regarding the plastic properties of the PET and its nanocomposites, yield stress exhibited a minor increase with SWCNT content, but the failure strain was dramatically reduced with the presence of SWCNT. The effect of blend component on the CNT dispersion and other properties of composites was evaluated for blends PET prepared using a twin-screw microcompounder [71]. r-PET and polyethylene naphthalate (PEN) were mixed with functional elastomers – terpolymer of ethylene-ethyl acrylate-maleic anhydride (E-EA-MAH) and terpolymer of ethylene-methyl acrylate-glycidyl methacrylate (EMA-GMA) – to

6:  PET Nanocomposites: Preparation and Characterization

ensure the miscibility between PET and PEN during the preparation of the blends and composites. Samples prepared with E-EA-MAH had better mechanical properties than the ones containing E-MAGMA due to the better elastomer phase dispersion. Moreover, addition of CNT also improved the mechanical properties of the samples for both elastomer types. In contrast to mechanical test results, samples prepared with E-MA-GMA had higher electrical conductivity values when compared with those containing E-EA-MAH due to the differences in the selective distribution of CNT particles between the polymer phases in the samples. A series of PET/MWCNT nanocomposites were prepared by in situ polymerization using different amounts of MWCNTs to study the crsytallization process in the filled composites [72]. The values of the activation energy of the nanocomposites were found to be larger than the ones of pristine PET. The kinetics of crystallization are described in detail by the authors. PET control fibers (nominal diameter 24 ± 3 mm) and PET fibers with embedded vapor-grown CNFs (PET-VGCNF) (nominal diameter 25 ± 2 mm) were exposed to cyclic loading and monotonic tensile tests to study the effects of fatigue and residual strain on mechanical behavior [73]. The control fibers were processed through a typical meltblending technique and the PET-VGCNF samples were processed with approximately 5 wt% CNFs present in the sample. A strengthening mechanism (strain hardening effect) in the low residual strain limit was observed for fatigued PET samples. In comparison with the unreinforced PET sample, the PET-VGCNF fibers showed greater degradation of mechanical properties as a function of residual strain due to fatigue when cycled at 60% of the fracture stress. The results from this study indicated that pure PET samples can withstand a larger accumulation of strain from the fatigue process conducted at the same maximum stress level as compared with the PET-VGCNF nanocomposite samples. The scattered intensity at small angles by samples consisting of different concentrations of SWCNTs dispersed in molten PET was studied using synchrotron radiation [74]. The scattered intensity was found to follow power law, characteristic of the scattering by fractal objects, which can be interpreted as being due to the presence of branched rope-like structures of bundled nanotubes precluding the existence of isolated CNTs. Logakis et al. [75] investigated the influence of processing conditions on PET/MWCNT. The composites were prepared by three different methods: in situ polymerization technique (I-S), direct mixing in the

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melt (DM), and dilution of a 0.5 wt% masterbatch, synthesized via in situ polymerization, using melt mixing (MB). The I-S series of samples exhibited an extremely low electrical percolation threshold (pc: 0.06 wt%), as compared to values of similar systems previously mentioned in the literature. The MB series showed a comparable pc value (pc: 0.05–0.10 wt%), whereas the investigation revealed a higher pc in the DM series (pc: 0.10–0.20 wt%). Finally, selected concentrations of samples were prepared using OHfunctionalized MWCNT, following the I-S procedure. The conductivity of these samples was found to be lower than that of samples with nonfunctionalized MWCNT. Anoop Anand et al. [76] suggested that the ultrasound-assisted dissolution-evaporation method enables more effective dispersion of SWNTs in the PET matrix as compared to the melt-compounding method. DSC studies showed that SWNTs nucleate crystallization in PET at weight fractions as low as 0.3%, as the nanocomposite melt crystallized during cooling at temperatures 24°C higher than neat PET of identical molecular weight. Isothermal crystallization studies also revealed that SWNTs significantly accelerate the crystallization process. Mechanical properties of the PET/SWNT nanocomposites improved as compared to neat PET, and SWNTs at concentrations exceeding 1 wt% in the PET matrix resulted in electrical percolation. The role of metal oxides as fillers has been evaluated by many research groups. The investigation by Chae and Kim [77] revealed that the introduction of ferrite nanoparticles increased heat of crystallization and fusion with ferrite content and the crystallization temperature by about 3°C, while it was found to have no effect on the melting temperature (Tm) of PET. The thermal stability and dynamic viscosity of PET nanocomposites were also found to be increasing with ferrite content. TiO2 is the most widely investigated photocatalyst because of its potential application in environmental purification. PET fabrics embedded with TiO2 can have self-cleaning properties under solar light and thus have good potential for commercialization. However, the preparation of this material at the nanolevel and its composites possess lots of difficulties. Generally TiO2/PET nanocomposites are generally prepared by coating processes such as dip-pad-dry-cure in which PET fibers are first dipped into the as-prepared TiO2 sol, and then padded, dried, and cured at high temperatures (100– 160°C) [78,79]. However, the requirement of special equipment and high-temperature treatment hampers the large-scale production of TiO2/PET composites. With the development of synthetic technology

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to produce TiO2 at low temperatures, in situ growth of TiO2 nanoparticles on polymeric substrates has been achieved by some groups [78,80,81]. Wang et al. [82] achieved in situ growth of TiO2 in PET nanofibers via hydrothermal synthesis and electrospinning. Han and Yu [78] fabricated PET/nano-TiO2 composites via in situ polymerization followed by being spun into fibers with excellent UV-blocking properties. TiO2 nanoflowers were in situ grown on PET nonwoven fabric by hydrolysis of TiCl4 in aqueous solution in the presence of nanocrystal cellulose (NCC)-grafted PET fabric (NCC-g-PET) at a low temperature of 70°C by Peng et al. [82]. NCC pregrafted on PET fabric acted as a hydrophilic substrate and morphology-inducing agent to promote the nucleation and crystal growth of TiO2. Rutile TiO2 nanoflowers were found to be growing abundantly on PET nonwoven fabric, and the established hydrogen bonding strengthened the interfacial interaction between the inorganic particles and the polymeric substrates. Bhimaraj et al. [83] observed that the addition of 38 nm alumina nanoparticles to PET resulted in an optimum filler content of about 2 wt%, which provided a 50% reduction in wear rate and a 10% reduction in coefficient of friction. Crystallinity was found to be a function of the processing conditions as well as the particle size and loading, while tribological properties were affected by crystallinity, filler size, and loading. Wear rate and friction coefficient were lowest at optimal loadings that ranged from 0.1 to 10 depending on the crystallinity and particle size. Wear rate decreased monotonically with decreasing particle size and decreasing crystallinity at any loading in the range tested. The same group reported the friction and wear properties of PET filled with alumina nanoparticles (1–10 wt%) [84]. The incorporation of the alumina particles increased the wear resistance by nearly 2× and decreased the average coefficient of friction. Wear resistance of PET was found to increase on addition of nanoparticles at low loadings irrespective of the crystallinity of the polymer matrix. The synergistic flame retardant effects and mechanisms of nano-Sb2O3 in combination with aluminum diethylphosphinate (AlPi) in PET were investigated by means of limiting oxygen index (LOI), UL 94 vertical burning test, thermogravimetric analysis (TGA), and thermogravimetric analysis [85]. A good dispersion of nano-Sb2O3 and AlPi into polymer matrix was achieved as shown by high-resolution transmission electron microscope and scanning electron microscope. Obvious synergistic effects between nanoSb2O3 and AlPi were observed for flame retardant

PET when the mass ratio of AlPi and nano-Sb2O3 was from 1:1 (4 wt%) to 4:1 (10 wt%). Attapulgite or AT is a crystalloid, hydrous magnesium-aluminum silicate mineral, which consist of a randomly oriented network of densely packed rods. A fibrillar single crystal is the structural unit of AT, 20–30 nm in diameter and 0.5–2.0 mm in length [86–88]. Owing to the low price, relatively high surface area, good mechanical strength, and thermal stability, AT is attracting more and more attention in the preparation of polymer/ clay composites [89–92]. However, since AT is highly anisotropic and is typically agglomerated in the as-received condition, appropriate organic modification is a crucial point in the design and preparation of polymer/AT nanocomposites. Chen et al. [93] in their work used an aromatic diisocyanate (MDI) to capture hydroxyl groups on the AT surface. The MDI-grafted AT (MAT) was then introduced to the PET matrix via in situ polymerization. This organic modification and composites synthesis process allowed PET chains to be linked on the MAT nanorod surface forming a novel polymer grafting interface structure. Owing to the unique interfacial structure in PET/MAT composites, their thermal and mechanical properties were greatly improved. Compared with neat PET, the elastic modulus and the yield strength of PET/MAT were significantly improved by about 39.5 and 36.8%, respectively, by incorporating only 2 wt% of MAT. LDHs, known as a class of anionic clays, have attracted increasing interest due to their broad applications in areas such as catalysis, materials, medicine, and environmental protection. The polymer/LDH systems have been much less studied than the cationic clays due to the delamination difficulties of LDHs with the small gallery space and hydrophilic surface character. However, the highly tunable properties and the anion exchange capacity have converted these materials into a new emerging class of layered crystals, which seem to be better suited for the preparation of multifunctional polymer/layered crystal nanocomposites. To apply anionic LDHs to a nanocomposite system, preparation of organo-modified LDHs resulting in an increase in gallery spacing is very important. Different methodologies have been adopted by research groups to overcome these factors. Lee et al. [94] prepared PET/LDH nanocomposites using anionic surfactant-intercalated LDHs by a direct melt-compounding method. Rives and coworkers prepared PET/LDH nanocomposites through a microwave [95] and mechanical grinding method [96]. In another work, Rives and coworkers [97] used dodecylsulfate-intercalated LDHs to enhance the compatibility between the PET polymer

6:  PET Nanocomposites: Preparation and Characterization

and the LDHs. It was found that the microwave process improves the dispersion and the thermal stability of nanocomposites. The structure, morphology, and thermal property of PET/LDH nanocomposites prepared via in situ polymerization were investigated by Cui et al. [98]. TA-intercalated LDHs were previously dispersed in ethylene glycol (EG, 1 mol) using a sonicating homogenizer for 15 min. The EG slurry containing TA-intercalated LDHs was then mixed with dimethyl terephthalate (DMT) (0.5 mol) and 0.020 wt% (with respect to the DMT amount) of manganese acetate and magnesium acetate as catalyst. TA-intercalated LDH contents were 0, 0.5, 1.0, and 2.0 wt% with respect to the DMT amount. The ester interchange reaction was carried out at 190–230°C with a continuous removal of byproduct (methanol). Polycondensation reaction was carried out at 280°C with EG antimony catalyst at a pressure of 0.1 torr for 2 h. The as-synthesized nanocomposite samples were dried in a vacuum oven for 1 day at 70°C. The TA-intercalated LDHs were partially exfoliated and well dispersed in the PET matrix. The PET/LDH nanocomposites exhibit enhanced thermal stability relative to pure PET, as confirmed by the thermogravimetric analysis results. The results of differential scanning calorimetry suggest that LDH nanoparticles could effectively promote the nucleation and crystallization of PET. CaCO3 nanoparticles have been widely studied and utilized in academic and industry settings because of their commercial availability and the substantial improvement in properties of polymers. Chen et al. [99] prepared highly dispersible CaCO3 for PET. CaCO3 nanoparticles were successfully prepared via a carbonization route with polyethylene glycol phosphate 1000 (PGP) as the modifying agent to improve the dispersion and increase the compatibility between nanoparticles and the PET matrix. PET/CaCO3 nanocomposites were prepared by further in situ polymerization with 5 wt% CaCO3 nanoparticles. A good dispersion of nanofillers was obtained up to 5 wt% of the surface-treated CaCO3 loading for PET/CaCO3 nanocomposites. Compared to the nanocomposites filled with the blank CaCO3, the resulting nanocomposites filled with the modified CaCO3 exhibited a better dispersion of the nanoparticles, a higher polymerization degree, a better thermal stability, and superior crystallinity properties. It was confirmed that PGP induced the growth of calcite and coated the surface of calcite by the covalent bond. Hydrophobic BaSO4 nanoparticles, which were compatible with the polymer PET, were designed by Gao et al. [100]. Hydrophobic

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BaSO4 nanoparticles were successfully prepared by one-step precipitation reaction in an aqueous solution of Na2SO4 and BaCl2 with stearic acid (SA) as the modifying agent. BaSO4/PET nanocomposites were prepared by further in situ polymerization of purified terephthalic acid, EG, and BaSO4 nanoparticles. Two types of nanoparticles were used: one type was made of the blank BaSO4, the other kind was composed of hydrophobic BaSO4 modified by 2 wt% SA. The aim was to determine the efficiency of the SA coating in improving dispersion into the polyester matrix and promoting interfacial adhesion between the phases, and analyzing the effect of the dispersion of BaSO4 nanoparticles on the properties of PET. Compared to the nanocomposites filled with the blank BaSO4, the resulting nanocomposites filled with the modified BaSO4 exhibited better dispersion of the nanoparticles, superior crystallinity properties, and better thermal stability. The experimental results also suggested that the properties of nanocomposites were correlated with the dispersion of BaSO4 in PET and the interfacial interactions between BaSO4 and PET matrix.

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