Characterizations of Nanocomposites of Liquid Crystalline Polymers
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Nanocomposites of three thermotropic liquid crystalline polymers (TLCPs) with organoclay were prepared. The first TLCP, poly(2-ethoxyhydroquinone-2-bromoterephthaloyl), EHBT, consists of wholly aromatic ester type mesogenic units containing an ethoxy side group, and the second poly(oxybiphenyleneoxy-2,5-dihexyloxyterephthaloyl) (OBDT) is an aromatic polyester TLCP having alkoxy side groups on the terephthaloyl moiety. The last TLCP polyazomethine (PAM) consists of diad aromatic azomethine type mesogenic units. An EHBT with an alkoxy side-group was synthesized from 2-ethoxyhydroquinone and 2-bromoterephthalic acid. Nanocomposites of EHBT with Cloisite 25A (C25A) as an organoclay were prepared by the melting intercalation method above the melt temperature (Tm) of the TLCP. Liquid crystallinity, morphology, and thermo-mechanical behaviors were examined with increasing organoclay content from 0 to 6 wt%. Liquid crystallinity of the C25A/EHBT hybrids was observed when organoclay content was up to 6 wt%. Regardless of the clay content in the hybrids, the C25A in EHBT was highly dispersed in a nanometer scale. The hybrids (0–6 wt% C25A/EHBT) were processed for fiber spinning to examine their tensile properties. Ultimate strength and initial modulus of the EHBT hybrids increased with increasing clay content and the maximum values of the mechanical properties were obtained from the hybrid containing 6 wt% of the organoclay. A TLCP (OBDT)/organoclay nanocomposite was synthesized via in-situ intercalation polycondensation of diethyl-2,5-dihexyloxyterephthalic acid and 4,4′-biphenol in the presence of organically modified montmorillonite (MMT). The organoclay, C18–MMT, was prepared by the ion exchange of Na+–MMT with octadecylamine chloride (C18–Cl−). OBDT/C18–MMT nanocomposites were prepared to examine the variations of the thermal properties, morphology, and liquid crystalline phases of the nanocomposites with clay content in the range 0–7 wt%. It was found that the addition of only a small amount of organoclay was sufficient to improve the thermal behavior of the OBDT hybrids, with maximum enhancement being observed at 1 wt% C18–MMT. Nanocomposites of PAM with the organoclay C12-MMT were also synthesized by using the in-situ interlayer polymerization method. The variations with organoclay content of the thermal properties, morphology, and liquid crystalline mesophases of the hybrids were determined for concentrations from 0 to 9 wt% C12-MMT. The wide-angle X-ray diffraction (XRD) analysis and transmission electron microscope (TEM) micrographs show that the levels of nanosize dispersion can be controlled by varying the C12-MMT content. The clay particles are better dispersed in the matrix polymer at low clay contents than at high clay contents. With the exception of the glass transition temperature (Tg), the maximum enhancement in the thermal properties was found to arise at an organoclay content of 1 wt%. Further, the PAM hybrids were shown to exhibit a nematic liquid crystalline phase for organoclay contents in the range 0–9 wt%.
KeywordsThermotropic liquid crystalline polymers Nanocomposite Organoclay Montmorillonite Intercalation method
TLCPs having an alkyl substituent with a base on a nematic liquid crystalline phase were reviewed. We also examined the correlation between the thermal properties and clay contents of TLCP nanocomposites and the dispersed morphology of the clay particles. Morphological studies showed that the levels of nanosize dispersion can be controlled by varying the organoclay content. The addition of only a small amount of organoclay was found to be sufficient to improve the thermal behavior of the TLCP hybrids.
Thermotropic liquid crystalline polymers (TLCPs) have already been established as high-performance commercial engineering polymers owing to their specific chemical structures, high strengths, high moduli, low viscosities, and other good mechanical properties. The structure–property relationships of TLCPs have been the subject of much research. Despite their inferior physical strength when compared with lyotropic liquid crystalline polyamides, TLCPs are attracting a great deal of interest based on their melt processability (Baird and Sun 1990; Lusignea 2001).
Despite the increasing interest and numerous papers on composites with TLCPs, most studies have treated only wholly aromatic rigid-rod-type polymers; less attention has been paid to the processing and properties of TLCPs containing substituents, side group structures, or a flexible alkyl group in the main chains. Although wholly aromatic rigid-rod-type TLCPs exhibit very attractive mechanical properties, they generally have high melting points, which can give rise to difficulties in processing. The presence of substituents or side groups in main chain structures in otherwise aromatic polyesters lowers their melting points, thus widening the processing window. Thus, despite the expected loss of thermomechanical properties that results from their use (compared to the properties of wholly aromatic TLCPs), they may have considerable advantages in particular applications (McArdle 1989).
The recent development of nanocomposites was stimulated by work with nylon 6/clay by Toyota (Kojima et al. 1994; Usuki et al. 1995). Progress, especially with respect to nanocomposite formation, has been reviewed by many researchers (Giannelis 1996; Yang et al. 1998). Nanocomposites are one of the most important classes of synthetic engineering materials. Their makeup is such that they can be transformed into new materials possessing the advantages of both organic materials, such as light weight, flexibility, and good moldability, and inorganic materials, such as high strength, heat stability, and chemical resistance. The incorporation of organic/inorganic hybrids can yield materials possessing excellent stiffness, strength, and gas barrier properties with far less inorganic content than is used in conventionally filled polymer composites: the higher the degree of delamination in polymer/clay nanocomposites, the greater the enhancement of these properties. Nanocomposites such as these can also be employed as scratch- and abrasive-resistant hard coatings, nonlinear optical materials, and reinforcements for elastomers and plastics (LeBaron et al. 1999; Vaia 2000).
Nanofillers can be categorized on the basis of their dimensions, such as one-dimensional fillers (nanotubes and nanowires), two-dimensional fillers (clays and graphene), and three-dimensional fillers (spherical and cubic nanoparticles). Among them, clays have sandwich structures with one octahedral Al sheet and two tetrahedral Si sheets, the so-called phyllosilicates. There are many types of phyllosilicates, including kaolinite, montmorillonite (MMT), hectorite, saponite, and synthetic mica. These clays consist of stacked silicate sheets with lengths of about 46 nm for hectorite, 170 nm for saponite, 218 nm for MMT, 1230 nm for synthetic mica, and so on. They have the same sheet thickness of 1 nm. Therefore, the only difference among them is the length of the silicate sheets (Yano et al. 1997; Garcia et al. 2000).
To overcome problems of macro- and micro-phase separation between organic polymers and inorganic clays, organic/inorganic polymer hybrids have mostly been synthesized using three methods: solution intercalation, melt intercalation, and in-situ intercalation polymerization. Additionally, other approaches, such as the sol–gel process and monomer/polymer grafting to clay layers, have resulted in organic/inorganic polymer hybrids (Ishida et al. 2000; Shen et al. 2002).
Solution intercalation is based on a solvent system in which the polymer is soluble and the clay layers swell. The layered clay is first swollen in a solvent, such as N,N′-dimethylacetamide (DMAc). When the polymer and clay solutions are mixed, the polymer chains intercalate and displace the solvent from between the layers of clay. Upon solvent removal, the intercalated structure remains, resulting in hybrids with nanoscale morphology. In the process of melt intercalation, the layered silicate is mixed with a molten polymer matrix. If the silicate surfaces are sufficiently compatible with the chosen polymer, the polymer can enter the interlayer space and form an intercalated or an exfoliated nanocomposite. Finally, in-situ intercalation polymerization is based on the use of one or more monomers that may be in-situ linearly polymerized or crosslinked and was the first method used to synthesize polymer-layered silicate nanocomposites based on nylon 6. In situ intercalation relies on swelling of the organoclay due to the monomer, followed by in-situ polymerization initiated thermally or by the addition of a suitable compound. Chain growth in the clay galleries triggers clay exfoliation and nanocomposite formation. Thus, an advantage of the in-situ method is the preparation of polymer hybrids without physical or chemical interactions between the organic polymer and the inorganic material (Min and Chang 2012).
Organic modification of the clay surface introduces reactive moieties that disrupt the bundle structure and can potentially make it possible to obtain individual sheets. Organic modification involves attachment of functional moieties to the open ends and sidewalls of the clay, primarily to improve the solubility and dispersibility of the clay sheets. Accordingly, one of the best methods of achieving a homogeneous dispersion of clays in a polymer matrix is the use of organoclays. This involves organically modifying the clays with polymers that are structurally similar to the matrix polymer to ensure that the dispersed clays are compatible with the polymer matrix and to limit any microscopic phase separation in the nanocomposites. Many papers reported large improvements in the thermal stabilities of TLCP nanocomposites by using organoclay. This enhancement of the thermal stabilities explains reasonably well the dispersed structure of clay in the nanocomposites caused by the formation of a large aspect ratio of the clay particles (Chang 2014).
The aim in this chapter was to investigate the effectiveness and influence of two different processes (melt and solution intercalation) on the morphology and thermal properties of TLCP/organoclay nanocomposites in order to obtain a material that combines the excellent thermal behavior provided by addition of inorganic particles with the versatility and easy processing characteristics of TLCP composites. The general goal of this work was to use a minimum amount of clay in the hybrids and still obtain thermal properties significantly superior to those of the matrix polymer. The properties of these nanocomposites were studied as a function of the organoclay content of the TLCP matrix.
The source clay, Kunipia-F (Na+-MMT), was obtained from Kunimine Co. (Tokyo, Japan). By screening this Na+-MMT clay with a 325-mesh sieve to remove impurities, we obtained a clay with a cationic exchange capacity of 119 meq/100 g. Cloisite 25A (organically modified MMT; C25A) was obtained from Southern Clay Product Co. (Gonzales, LA, USA). All reagents were purchased from Aldrich Chemical Co. (Yongin, Korea). Commercially available solvents were purified by distillation.
Syntheses of Organoclays (C12- and C18-MMT)
A dispersion of Na+-MMT was added to solutions of the ammonium salts of dodecylamine (C12) and octadecylamine (C18). These organophilic MMTs were obtained through a multi-step route (Park and Chang 2000) and have been termed C12-MMT and C18-MMT, respectively.
Syntheses of TLCPs and Their Nanocomposites
Preparation of Poly(2-ethoxyhydroquinone-2-bromoterephthaloyl) (EHBT) Nanocomposites
General properties of EHBT hybrids with various organoclay contents
0 (pure EHBT)
For simplicity, the hybrids are referred to in this paper as 0 wt% organoclay/TLCP, 3 wt% organoclay/TLCP, and so on, in which 3 wt% organoclay and TLCP are the organoclay and polymer components used to prepare the hybrids, respectively, and the number denotes the organoclay weight percent in the hybrid.
Preparation of Poly(Oxybiphenyleneoxy-2,5-Dihexyloxyterephthaloyl) (OBDT) Nanocomposites
General properties of OBDT hybrids with various organoclay contents
0 (pure OBDT)
Preparation of Polyazomethine (PAM) Nanocomposites
General properties of PAM hybrids with various organoclay contents
0 (pure PAM)
The EHBT hybrids were processed for fiber spinning to examine their tensile properties. The dried blends were pressed at 160 °C and 2500 kg/cm2 for a few minutes on a hot press. The film-type blends were dried in a vacuum oven for 24 h before being extruded through the die of a capillary rheometer. From the capillary rheometer, the hot extrudates were immediately drawn at constant take-up speed to form extended extrudates having the same diameters. The cylinder temperature of the extruder was 190 °C, and the mean residence time in the capillary rheometer was about 2–3 min.
Thermal and thermogravimetric analyses of the hybrids were conducted under N2 atmosphere using DuPont 910 equipment (New Castle, DE, USA). The samples were heated and cooled at a rate of 20 °C/min. Wide-angle X-ray diffraction (XRD) measurements were performed at room temperature on a Rigaku (D/Max-IIIB) X-ray diffractometer (Tokyo, Japan) using Ni-filtered Cu-Kα radiation. The scanning rate was 2°/min over a range of 2θ = 2–14°.
The tensile properties of the extrudate were determined using an Instron Mechanical Tester(Model 5564) (Norfolk County, USA) at a crosshead speed of 2 mm/min. The specimens were prepared by cutting strips 5 × 70 mm2 in size. An average of at least eight individual determinations was obtained. The experimental uncertainties in the tensile strength and modulus were ±1 MPa and ±0.05 GPa, respectively.
A polarizing microscope (Leitz, Ortholux) (Lahn-Dill-Kreis, Germany) equipped with a Mettler FP-5 hot stage was used to examine the liquid crystalline behavior. The morphology of the fractured surfaces of the extrusion samples was investigated using a Hitachi S-2400 scanning electron microscope (SEM) (San Jose, CA, USA). The fractured surfaces were sputter-coated with gold for enhanced conductivity using an SPI Sputter Coater. Transmission electron microscope (TEM) photographs of ultrathin-section polymer/organoclay hybrid samples were taken on an EM 912 OMEGA (Carl Zeiss) TEM (Tokyo, Japan) using an acceleration voltage of 120 kV.
Results and Discussion
Dispersibility of Organoclay in TLCP
Small peaks at d = 14.30 Å (2θ = 6.18°), 11.92 Å (2θ = 7.42°), and 9.74 Å (2θ = 9.08°) appeared in the XRD results for pure OBDT. Similar XRD peaks appeared for the OBDT hybrid with 1 wt% organoclay content. The intensity of the XRD peak at d = 14.63 Å (2θ = 6.04°), however, increased as the clay loading was increased from 3 to 7 wt%, suggesting that dispersion is more effective at lower clay loadings than at higher clay loadings. Higher clay loadings are expected to result in increased agglomeration of some portion of the clay within the TLCP matrix; however, the presence of the organoclay was found to have no effect on the location of the peak, which indicates that perfect exfoliation of the clay layer structure of the organoclay does not occur in the TLCP matrix (Chang et al. 2006).
XRD is the conventional method of determining the interlayer spacing of clay layers in the original clay and in intercalated polymer/clay nanocomposites. Unfortunately, XRD cannot detect regular stacking exceeding a layer spacing of 88 Å. Note that the commonly used definition of an exfoliated nanocomposite is based on layer spacing larger than this value. It was the electron-microscopic analyses that provided evidence for the formation of nanoscale hybrids.
The thermal properties of EHBT hybrids with different organoclay contents are listed in Table 1. The glass transition temperatures (Tg) of EHBT hybrids increased linearly from 92 to 98 °C with clay loading from 0 to 4 wt% and leveled off at more than 4 wt% of organoclay. The increase in the Tg value of these hybrids could be the result of two factors (Agag et al. 2001). First, the effect of small amounts of dispersed clay layers on the free volume of TLCP is significant and does influence the glass transition temperature of TLCP hybrids. The second factor is ascribed to confinement of the intercalated polymer chains within the clay galleries, which prevents segmental motions of the polymer chains.
These decreases seem to be the result of clay agglomeration, which occurs upon the addition of clay to the polymer matrix above a critical clay loading. Agglomeration also reduces the heat insulation effect of the clay layers in the polymer matrix. However, agglomerated structures form and become denser in the TLCP matrix above a critical clay content (1 wt%). The presence of organoclay agglomeration in TLCP was confirmed using the XRD and electron microscopy results (see Figs. 4, 8, 9, and 10).
The thermal properties of PAM hybrids with various C12-MMT contents are listed in Table 3. The Tg values of the PAM hybrids increase linearly from 113 to 126 °C as the clay loading increases from 0 to 9 wt%. These increases in the Tg values of these hybrids might be the result of two factors, as described above. In contrast to the Tg values, the Tm, Ti, and TDi values of the hybrids increase with increasing C12-MMT content up to 1 wt% and then decrease with further increases in the organoclay loading up to 9 wt%. For example, the Tm, Ti, and TDi values of the C12-MMT/PAM hybrid with 1 wt% clay loading are higher by 17, 7, and 8 °C, respectively, than those of pure PAM.
As mentioned above, the increase in Tm upon the addition of the organoclay might result from the heat-insulating effects of the clay layer structure, as well as from interactions between the organoclay and the PAM molecular chains. The presence of the clay also enhances the initial decomposition temperatures by acting as an insulator and a mass-transport barrier to the volatile products generated during decomposition. This increase in the thermal stability can also be attributed to the high thermal stability of the clay and to interactions between the clay particles and the polymer matrix (Gilman 1999).
We conclude that the introduction of an inorganic clay component into an organic polymer can improve the polymer’s thermal properties because of the good thermal stability of the clay. However, in this hybrid system, the maximum effect on the thermal properties, with the exception of Tg, was obtained at an organoclay loading of 1 wt%. The weight of the residue at 650 °C ranges from 68% to 75% and increases as the clay loading increases from 0 to 9 wt%.
Considering the above results, it is consistently believable that the introduction of inorganic components into organic polymers can improve their thermal stability on the basis of the fact that clays have good thermal stability.
The pure EHBT and EHBT hybrids were extruded through a capillary die with a draw ratio (DR) of 1 to examine the tensile strength and modulus of the extrudates. The DR was calculated from the ratio of the diameter of the drawn extrudate to that of the extruder die.
This large increase in the tensile properties of hybrids owing to the presence of organoclay can be explained as follows: the amount of the increase due to the clay layers depends on interactions between rigid, rod-shaped TLCP molecules and layered organoclays, as well as on the rigid nature of the clay layers (Fornes et al. 2002). Moreover, the clay was much more rigid than the TLCP molecules and did not deform or relax as the TLCP molecules did. This improvement was possible because organoclay layers could be highly dispersed and exfoliated in the TLCP matrix. This is consistent with the general observation that the introduction of organoclay into a matrix polymer increases its strength and modulus. The percent elongation at break of all samples, however, decreases from 2% to 1% and then remains constant with further clay addition.
Liquid Crystalline Mesophase
In this chapter, TLCP hybrids with different organoclay contents were prepared using different intercalation methods and monomers containing different substituents to investigate their thermomechanical properties, morphology, and liquid crystallinity. We found that the addition of a small amount of organoclay was sufficient to change the thermal properties of the TLCP matrix polymer. The TLCP hybrid used in this study showed considerably higher thermal properties than neat TLCP. In the OBDT and PAM hybrids, the thermal properties (Tm, Ti, and TDi) of the TLCP hybrids increase with the addition of the organoclay up to a critical content and then decrease with further organoclay loading. The XRD, SEM, and TEM results for the TLCP hybrids show that the clay particles are better dispersed in the matrix TLCP at low clay contents than at high clay contents. Higher clay loadings are expected to result in increased agglomeration of some portion of the clay within the TLCP matrix. EHBT hybrids of different C25A contents were extruded with a draw ratio of 1 from a capillary rheometer to investigate their mechanical properties. The ultimate strength and initial modulus of the hybrids increased with increasing C25A content. When the amount of organoclay in EHBT reached 6 wt%, a 1.6-fold increase in the ultimate strength and a 2.0-fold increase in the initial modulus were obtained, as compared to the strength and modulus of the pure polymer matrix.
Nanocomposites are a class of composites derived from ultrafine inorganic particles, such as clays (with sizes in the nanometer range), that are homogeneously dispersed in a polymer matrix. Because of the nanometer sizes of the particles, these materials possess properties that are superior to those of conventional composites. In particular, the high interfacial adhesion in the nanocomposites resulting from their nanometer particle dimensions improves the physical material properties. Many works demonstrated a potential fabrication technique for nanocomposites of a TLCP with an organoclay that yields high-performance materials. Nanostructured materials often possess a combination of physical and mechanical properties that are not present in conventional composites. Even at low clay concentrations (<10 wt%), the thermomechanical properties can be substantially improved.
The various structural factors influencing the thermal and thermotropic properties of main-chain, aromatic polyesters with and without substituents or side-groups based on nanocomposites have not been systematically studied. Thus, essential studies of the structure and length of mesogenic units and substituents or side-groups, the nature of lateral substituents in the mesogenic parts, inclusion of a kink unit in the mesogenic structure, the molecular weight and thermal history of the polymer, and the copolymers’ structures and nanocompositions are the most important factors investigated. Further investigation of the thermal properties should account for the chemical structures in side groups, given that the morphology and liquid crystallinity depend strongly on the side group of the TLCP.
This research was supported by the Ministry of Trade, Industry & Energy (MOTIE, Korea) under the Industrial Technology Innovation Program (No. 10063420, Development of high strengthen thermotropic liquid crystal polyester fiber).
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