Effects of Poly(ethylene-co-glycidyl methacrylate) on the Microstructure, Thermal, Rheological, and Mechanical Properties of Thermotropic Liquid Crystalline Polyester Blends

In this study, a series of thermotropic liquid crystalline polyester (TLCP)-based blends containing 1–30 wt% poly(ethylene-co-glycidyl methacrylate) (PEGMA) were fabricated by masterbatch-assisted melt-compounding. The scanning electron microscopy (SEM) images showed a uniformly dispersed microfibrillar structure for the TLCP component in cryogenically-fractured blends, without any phase-separated domains. The FT-IR spectra showed that the carbonyl stretching bands of TLCP/PEGMA blends shifted to higher wavenumbers, suggesting the presence of specific interactions and/or grafting reactions between carboxyl/hydroxyl groups of TLCP and glycidyl methacrylate groups of PEGMA. Accordingly, the melting and crystallization temperatures of the PEGMA component in the blends were greatly lowered compared to the TLCP component. The thermal decomposition peak temperatures of the PEGMA and TLCP components in the blends were characterized as higher than those of neat PEGMA and neat TLCP, respectively. From the rheological data collected at 300 °C, the shear moduli and complex viscosities for the blend with 30 wt% PEGMA were found to be much higher than those of neat PEGMA, which supports the existence of PEGMA-g-TLCP formed during the melt-compounding. The dynamic mechanical thermal analysis (DMA) analyses demonstrated that the storage moduli of the blends decreased slightly with the PEGMA content up to 3 wt%, increased at the PEGMA content of 5 wt%, and decreased again at PEGMA contents above 7 wt%. The maximum storage moduli for the blend with 5 wt% PEGMA are interpreted to be due to the reinforcing effect of PEGMA-g-TLCP copolymers.


Introduction
Thermotropic liquid crystalline polyesters (TLCPs)-aromatic polyesters exhibiting liquid crystallinity in a molten state-are used in a variety of industrial sectors because of their excellent thermal

Characterization
The morphological features of TLCP/PEGMA blends were characterized by using a scanning electron microscope (SEM, ZEISS, Merlin compact, Oberkochen, Germany) operating at the accelerating voltage of 8 kV. For obtaining cross-sectional SEM images, neat TLCP and its blends were fractured in a liquid nitrogen bath and then coated with platinum in a vacuum sputtering

Characterization
The morphological features of TLCP/PEGMA blends were characterized by using a scanning electron microscope (SEM, ZEISS, Merlin compact, Oberkochen, Germany) operating at the accelerating voltage of 8 kV. For obtaining cross-sectional SEM images, neat TLCP and its blends were fractured in a liquid nitrogen bath and then coated with platinum in a vacuum sputtering chamber for 60 s.
The chemical structures and molecular interactions of TLCP/PEGMA blends and their components were analyzed by using a FT-IR spectrometer (Nicolet iS10, Thermo Fisher Scientific, Waltham, MA, USA).
The thermal transition properties of neat TLCP, neat PEGMA, and TLCP/PEGMA blends were investigated with a differential scanning calorimeter (DSC6000, PerkinElmer Inc., Waltham, MA, USA) over the temperature range of 30-300 • C at a heating and cooling rate of 10 • C/min under nitrogen atmosphere.
The thermal stability and thermal degradation behavior of TLCP-based blends with different PEGMA contents were analyzed by using a thermogravimetric analyzer (TGA4000, PerkinElmer Inc., Waltham, MA, USA) in the temperature range of 30-800 • C at a heating rate of 10 • C/min under nitrogen atmosphere.
The rheological properties of neat TLCP, neat PEGMA, and TLCP/PEGMA blends under an oscillatory dynamic shear were measured by using a rheometer (MCR102, Anton Paar, Graz, Austria) with a parallel plate geometry with a 25 mm diameter and 1 mm gap. Dynamic frequency sweep measurements were performed at 300 • C in the frequency range of 0.1-100 rad/s. The temperature-dependent dynamic mechanical properties of TLCP/PEGMA blends were characterized by using a dynamic mechanical analyzer (DMA8000, PerkinElmer Inc., Waltham, MA, USA) with a single cantilever geometry at a frequency of 0.1 Hz. The DMA experiments were carried out in the temperature range from −50 to 240 • C at a heating rate of 5 • C/min.

Morphological and Structural Characterization
In order to characterize the morphological features of TLC/PEGMA blends, SEM images of the fractured surfaces of TLCP and TLCP/PEGMA blends were obtained, as shown in Figure 2. In the case of neat TLCP, microfibrils with a highly orientated structure can be observed over the fractured cross-section due to the rigid backbone chains of TLCP. For TLCP/PEGMA blends, the cross-sectional morphological features are almost identical to those of neat TLCP, without showing apparent phase-separated domains, which suggests good compatibility between TLCP and PEGMA.
To identify the chemical structures and molecular interactions of TLCP and PEGMA, the FT-IR spectra of the neat TLCP, neat PEGMA, and TLCP/PEGMA blends were obtained, as can be seen in Figure 3. For neat TLCP, the characteristic bands associated with C=C stretching vibrations of the naphthalene and benzene rings in the backbone can be detected in the range of 1400-1650 cm −1 . In addition, characteristic C=O stretching (ν C=O, carboxyl ) and C-O stretching (ν C-O, carboxyl ) vibrations of the carboxyl group can be observed at~1724 and~1249/1178 cm −1 , respectively [26][27][28]. In the case of neat PEGMA, an asymmetric stretching vibrational band of the epoxy ring (ν epoxy, asym ) can be observed at 908 cm −1 . In addition, characteristic vibration bands related to the C-O stretching of O-CH 2 (ν C-O, O-CH2 ), C-O stretching of the carboxyl group of GMA (ν C−O, carboxyl ), symmetric deformation of the methyl group (δ CH3 ), bending deformation of the methylene group (δ CH2 ), and C=O stretching of carboxyl groups of GMA (ν C=O, carboxyl ) were detected at 1141, 1234, 1370, 1463, and 1733 cm −1 , respectively [29,30]. For the blends with 1-30 wt% PEGMA loadings, all of the FT-IR spectra were quite similar to those of neat TLCP. For a quantitative comparison, the characteristic vibrational bands Polymers 2020, 12, 2124 5 of 13 associated with carboxyl groups are summarized in Table 1. It can be seen that the C-O stretching bands (ν C-O, carboxyl ) at 1176 and 1249 cm −1 for neat TLCP were shifted to 1179 and 1253 cm −1 for the TLCP-E30 blend, as the PEGMA content increased in the blends. In addition, the C=O stretching band (ν C=O, carboxyl ) at 1724 cm −1 for neat TLCP was shifted to 1727 cm −1 for the TLCP-E30 blend. These results demonstrate that intermolecular interactions and/or a chemical reaction might exist between carboxyl/hydroxy groups of TLCP and epoxy/methacrylate groups of PEGMA [31], as represented schematically in Figure 4.

Morphological and Structural Characterization
In order to characterize the morphological features of TLC/PEGMA blends, SEM images of the fractured surfaces of TLCP and TLCP/PEGMA blends were obtained, as shown in Figure 2. In the case of neat TLCP, microfibrils with a highly orientated structure can be observed over the fractured cross-section due to the rigid backbone chains of TLCP. For TLCP/PEGMA blends, the cross-sectional morphological features are almost identical to those of neat TLCP, without showing apparent phaseseparated domains, which suggests good compatibility between TLCP and PEGMA. To identify the chemical structures and molecular interactions of TLCP and PEGMA, the FT-IR spectra of the neat TLCP, neat PEGMA, and TLCP/PEGMA blends were obtained, as can be seen in Figure 3. For neat TLCP, the characteristic bands associated with C=C stretching vibrations of the naphthalene and benzene rings in the backbone can be detected in the range of 1400-1650 cm −1 . In addition, characteristic C=O stretching (νC=O, carboxyl) and C-O stretching (νC-O, carboxyl) vibrations of the carboxyl group can be observed at ~1724 and ~1249/1178 cm −1 , respectively [26][27][28]. In the case of neat PEGMA, an asymmetric stretching vibrational band of the epoxy ring (νepoxy, asym) can be observed at 908 cm      To characterize the crystalline features of TLCP/PEGMA blends, X-ray diffraction patterns of neat TLCP, neat PEGMA, and their blends were obtained, as can be seen in Figure 5. For neat TLCP, a typical diffraction peak at ~19.8°, which corresponds to d-spacing of 4.5 nm based on Bragg's law, was observed and this is associated with the (110) plane of the TLCP crystalline structure [4,32]. In the case of PEGMA, strong diffraction peaks can be observed at 21.3 and 23.3°, which correspond to the (110) and (200) planes of PE crystals, respectively [33]. For all TLCP/PEGMA blends, the strong diffraction peak for TLCP crystals can be observed at 19.8°, whereas the characteristic diffraction peaks at 21.3 and 23.3° of PE crystals can only be detected for the blends with more than 10 wt% PEGMA. This result demonstrates that the PEGMA and TLCP in the blends do not noticeably affect each other's crystalline structure, although their relative diffraction intensities decrease when increasing the counterpart content in the blends. In particular, the crystalline diffraction peaks of the PEGMA component decrease significantly in the blends, which might be caused by the restricted crystallization of PEGMA in the blends owing to the formation of graft copolymers (PEGMA-g-TLCP) and the intermolecular interactions between TLCP and PEGMA components ( Figure 4). To characterize the crystalline features of TLCP/PEGMA blends, X-ray diffraction patterns of neat TLCP, neat PEGMA, and their blends were obtained, as can be seen in Figure 5. For neat TLCP, a typical diffraction peak at~19.8 • , which corresponds to d-spacing of 4.5 nm based on Bragg's law, was observed and this is associated with the (110) plane of the TLCP crystalline structure [4,32]. In the case of PEGMA, strong diffraction peaks can be observed at 21.3 and 23.3 • , which correspond to the (110) and (200) planes of PE crystals, respectively [33]. For all TLCP/PEGMA blends, the strong diffraction peak for TLCP crystals can be observed at 19.8 • , whereas the characteristic diffraction peaks at 21.3 and 23.3 • of PE crystals can only be detected for the blends with more than 10 wt% PEGMA. This result demonstrates that the PEGMA and TLCP in the blends do not noticeably affect each other's crystalline Polymers 2020, 12, 2124 7 of 13 structure, although their relative diffraction intensities decrease when increasing the counterpart content in the blends. In particular, the crystalline diffraction peaks of the PEGMA component decrease significantly in the blends, which might be caused by the restricted crystallization of PEGMA in the blends owing to the formation of graft copolymers (PEGMA-g-TLCP) and the intermolecular interactions between TLCP and PEGMA components ( Figure 4). a typical diffraction peak at ~19.8°, which corresponds to d-spacing of 4.5 nm based on Bragg's law, was observed and this is associated with the (110) plane of the TLCP crystalline structure [4,32]. In the case of PEGMA, strong diffraction peaks can be observed at 21.3 and 23.3°, which correspond to the (110) and (200) planes of PE crystals, respectively [33]. For all TLCP/PEGMA blends, the strong diffraction peak for TLCP crystals can be observed at 19.8°, whereas the characteristic diffraction peaks at 21.3 and 23.3° of PE crystals can only be detected for the blends with more than 10 wt% PEGMA. This result demonstrates that the PEGMA and TLCP in the blends do not noticeably affect each other's crystalline structure, although their relative diffraction intensities decrease when increasing the counterpart content in the blends. In particular, the crystalline diffraction peaks of the PEGMA component decrease significantly in the blends, which might be caused by the restricted crystallization of PEGMA in the blends owing to the formation of graft copolymers (PEGMA-g-TLCP) and the intermolecular interactions between TLCP and PEGMA components (Figure 4).

Thermal Transition and the Decomposition Property
The thermal transition behaviors of neat TLCP, neat PEGMA, and TLCP/PEGMA blends were investigated by obtaining DSC heating and cooling thermograms, as shown in Figure 6. As a result, the peak temperatures (T m and T c ) and enthalpies (∆H m and ∆H c ) of the melting and crystallization transitions of TLCP and PEGMA components in the blends were summarized and are presented in Table 2. In the first heating thermograms of Figure 6A, the T m, TLCP and T m, PEGMA values of neat TLCP and PEGMA are characterized as~281 and~110 • C, respectively. In the cases of TLCP/PEGMA blends, the T m, TLCP values of 276-279 • C for the TLCP component are slightly lower than that of neat TLCP, whereas the T m, PEGMA values of the PEGMA component decrease noticeably from~101 • C of TLCP-E30 to~85 • C of TLCP-E1 when decreasing the PEGMA content in the blends. In the first cooling thermograms of Figure 6B, the crystallization temperatures (T c, TLCP and T c, PEGMA ) of neat TLCP and PEGMA are characterized as~237 and~90 • C, respectively. For TLCP/PEGMA blends, the T c, TLCP values of 235-237 • C for the TLCP component are quite comparable to that of neat TLCP, irrespective of the PEGMA content, while the T c, PE values of 76-79 • C for the PEGMA component are far lower than that of neat PEGMA. In the second heating thermograms of Figure 6C, the melting transition behavior of TLCP/PEGMA blends is quite similar to that in the first heating thermograms. In comparison to the TLCP component with a rigid backbone chain, the significantly lowered T m, PEGMA and T c, PEGMA values of the PEGMA component in the blends are conjectured to be caused by the restricted crystallization rate, as well as the thin crystal formation of PEGMA with a flexible backbone chain owing to the chemical reactions and specific interactions between PEGMA and TLCP, as supported by the above SEM, FT-IR, and X-ray diffraction analyses. Consistently, the experimental melting and crystallization enthalpies (∆H m and ∆H c ) of TLCP and PEGMA components in the blends (Table 2) were found to be much lower than the values calculated by the rule of mixtures. For instance, the crystallization enthalpies (∆H c, TLCP and ∆H c, PEGMA ) of TLCP and PEGMA components in the blends were plotted as a function of the PEGMA content and compared with the calculated values, as can be seen in Figure 7.       The influence of the PEGMA content on the thermal decomposition behavior of TLCP-based blends was characterized by using TGA and DTG thermograms of neat TLCP, neat PEGMA, and TLCP/PEGMA blends, as shown in Figure 8A,B. It could be found that neat TLCP and PEGMA exhibit single-step thermal decomposition behavior, whereas the blends with 7-30 wt% PEGMA contents show two-step thermal decomposition behavior. The thermal decomposition peak temperatures (T d, TLCP and T d, PEGMA ) of TLCP and PEGMA components in the blends, which were obtained from the DTG thermograms, were plotted as a function of the PEGMA content, as can be seen in Figure 8C The influence of the PEGMA content on the thermal decomposition behavior of TLCP-based blends was characterized by using TGA and DTG thermograms of neat TLCP, neat PEGMA, and TLCP/PEGMA blends, as shown in Figure 8A,B. It could be found that neat TLCP and PEGMA exhibit single-step thermal decomposition behavior, whereas the blends with 7-30 wt% PEGMA contents show two-step thermal decomposition behavior. The thermal decomposition peak temperatures (Td, TLCP and Td, PEGMA) of TLCP and PEGMA components in the blends, which were obtained from the DTG thermograms, were plotted as a function of the PEGMA content, as can be seen in Figure 8C. The Td, TLCP value of neat TLCP was measured to be ~513 °C and the Td, PEGMA

Rheological Property
The shear storage modulus (G'), shear loss modulus (G"), loss tangent (tan ™), and complex viscosity (η*) of neat TLCP, neat PEGMA, and TLCP/PEGMA blends at 300 • C were measured as a function of the angular frequency, as can be seen in Figure 9. The shear storage moduli, shear loss moduli, and complex viscosities of neat TLCP at 300 • C were much lower than those of neat PEGMA ( Figure 9A-C), which is due to the facile alignment of TLCP with a stiff chain backbone with the applied shear [5], compared to PEGMA with a high chain flexibility and entanglement in a melt state. The overall shear moduli and complex viscosities of the blends with 1-10 wt% PEGMA loadings are quite consistent with those of neat TLCP, although they increase slightly when increasing the PEGMA content in the blends. This demonstrates that the frequency-dependent shear moduli and complex viscosities of the blends are dominated by the TLCP component. On the other hand, the overall shear moduli and complex viscosities of the blends with 20 and 30 wt% PEGMA are highly increased compared with the values expected from the rule of mixtures. In particular, for the blend with 30 wt% PEGMA (TLCP-E30), the shear moduli and complex viscosities are far higher than those of neat PEGMA. In addition, for the blends with high PEGMA contents of 20-30 wt%, tan ™ peaks are observed at a lower frequency range, as can be seen in Figure 9D. These results are believed to be due to the fact that PEGMA-g-TLCP copolymers formed during the masterbatch-based melt-compounding contribute to enhancing the compatibility of TLCP and PEGMA components in the blend and restricting the alignment of TLCP chains with the applied shear.

Rheological Property
The shear storage modulus (G'), shear loss modulus (G"), loss tangent (tan ), and complex viscosity (*) of neat TLCP, neat PEGMA, and TLCP/PEGMA blends at 300 °C were measured as a function of the angular frequency, as can be seen in Figure 9. The shear storage moduli, shear loss moduli, and complex viscosities of neat TLCP at 300 °C were much lower than those of neat PEGMA ( Figure 9A-C), which is due to the facile alignment of TLCP with a stiff chain backbone with the applied shear [5], compared to PEGMA with a high chain flexibility and entanglement in a melt state. The overall shear moduli and complex viscosities of the blends with 1-10 wt% PEGMA loadings are quite consistent with those of neat TLCP, although they increase slightly when increasing the PEGMA content in the blends. This demonstrates that the frequency-dependent shear moduli and complex viscosities of the blends are dominated by the TLCP component. On the other hand, the overall shear moduli and complex viscosities of the blends with 20 and 30 wt% PEGMA are highly increased compared with the values expected from the rule of mixtures. In particular, for the blend with 30 wt% PEGMA (TLCP-E30), the shear moduli and complex viscosities are far higher than those of neat PEGMA. In addition, for the blends with high PEGMA contents of 20-30 wt%, tan  peaks are observed at a lower frequency range, as can be seen in Figure 9D. These results are believed to be due to the fact that PEGMA-g-TLCP copolymers formed during the masterbatch-based meltcompounding contribute to enhancing the compatibility of TLCP and PEGMA components in the blend and restricting the alignment of TLCP chains with the applied shear.

Dynamic Mechanical Property
To characterize the effects of the PEGMA content on the dynamic mechanical thermal properties of TLCP/PEGMA blends, the changes of the storage modulus (E') of neat TLCP, neat PEGMA, and their blends as a function of the temperature were obtained, as shown in Figure 10A. For neat TLCP, the storage modulus of~10 11 Pa at −50 • C decreased slightly to~10 10 Pa at~100 • C owing to the glass transition, and remained at a level of~10 10 Pa up to 230 • C. In the case of neat PEGMA, the storage modulus of~10 9 Pa at −50 • C decreased when increasing the temperature and decreased significantly above 50 • C owing to the beginning of the melting of PE crystals. The overall temperature-dependent storage modulus changes of the blends with 1-30 wt% PEGMA are almost identical to the neat TLCP, although the storage moduli are slightly lower over the temperature range.
To characterize the effects of the PEGMA content on the dynamic mechanical thermal properties of TLCP/PEGMA blends, the changes of the storage modulus (E') of neat TLCP, neat PEGMA, and their blends as a function of the temperature were obtained, as shown in Figure 10A. For neat TLCP, the storage modulus of ~10 11 Pa at −50 °C decreased slightly to ~10 10 Pa at ~100 °C owing to the glass transition, and remained at a level of ~10 10 Pa up to 230 °C. In the case of neat PEGMA, the storage modulus of ~10 9 Pa at −50 °C decreased when increasing the temperature and decreased significantly above 50 °C owing to the beginning of the melting of PE crystals. The overall temperature-dependent storage modulus changes of the blends with 1-30 wt% PEGMA are almost identical to the neat TLCP, although the storage moduli are slightly lower over the temperature range. For a quantitative comparison, the storage moduli of neat TLCP, neat PEGMA, and TLCP/PEGMA blends at constant temperatures of −50, 30, and 100 °C were plotted as a function of the PEGMA content, as shown in Figure 10B. It was found that the storage moduli of the blends at three different temperatures decreased slightly with the PEGMA contents up to 3 wt%, increased at the PEGMA content of 5 wt%, and decreased again at PEGMA contents above 7 wt%. It is believed that the increased storage moduli for the blend with 5 wt% PEGMA loading stem from the dominant reinforcing effects of PEGMA-g-TLCP copolymers on the compatibility and mechanical performance of TLCP/PEGMA blends.

Conclusions
In summary, TLCP-based blends with 1-30 wt% PEGMA loadings were fabricated by facile and efficient masterbatch-based melt-compounding, and their morphology, microstructures, and physical properties were investigated systematically by considering the chemical reaction and intermolecular interaction between TLCP and PEGMA components. The SEM images of cryogenic fracture surfaces revealed that microfibrils of the TLCP component with stiff backbone chains were dispersed uniformly for all blends, without exhibiting any phase-separated domains, which indicates a good compatibility between TLCP and PEGMA components. From the FT-IR spectra, the characteristic vibrational bands associated with the carboxyl groups of TLCP components were found to shift to higher wavenumbers when increasing the PEGMA content in the blends. X-ray diffraction patterns showed that the crystalline diffraction intensity of the PEGMA component in the blends decreased more significantly compared with that of the TLCP component. The morphological and microstructural features obtained from SEM, FT-IR, and XRD data suggested the presence of specific intermolecular interactions between carboxyl/hydroxy groups of TLCP and glycidyl methacrylate groups of PEGMA, as well as the formation of PEGMA-g-TLCP copolymers during the meltcompounding. As the results show, the melting and crystallization temperatures of the PEGMA For a quantitative comparison, the storage moduli of neat TLCP, neat PEGMA, and TLCP/PEGMA blends at constant temperatures of −50, 30, and 100 • C were plotted as a function of the PEGMA content, as shown in Figure 10B. It was found that the storage moduli of the blends at three different temperatures decreased slightly with the PEGMA contents up to 3 wt%, increased at the PEGMA content of 5 wt%, and decreased again at PEGMA contents above 7 wt%. It is believed that the increased storage moduli for the blend with 5 wt% PEGMA loading stem from the dominant reinforcing effects of PEGMA-g-TLCP copolymers on the compatibility and mechanical performance of TLCP/PEGMA blends.

Conclusions
In summary, TLCP-based blends with 1-30 wt% PEGMA loadings were fabricated by facile and efficient masterbatch-based melt-compounding, and their morphology, microstructures, and physical properties were investigated systematically by considering the chemical reaction and intermolecular interaction between TLCP and PEGMA components. The SEM images of cryogenic fracture surfaces revealed that microfibrils of the TLCP component with stiff backbone chains were dispersed uniformly for all blends, without exhibiting any phase-separated domains, which indicates a good compatibility between TLCP and PEGMA components. From the FT-IR spectra, the characteristic vibrational bands associated with the carboxyl groups of TLCP components were found to shift to higher wavenumbers when increasing the PEGMA content in the blends. X-ray diffraction patterns showed that the crystalline diffraction intensity of the PEGMA component in the blends decreased more significantly compared with that of the TLCP component. The morphological and microstructural features obtained from SEM, FT-IR, and XRD data suggested the presence of specific intermolecular interactions between carboxyl/hydroxy groups of TLCP and glycidyl methacrylate groups of PEGMA, as well as the formation of PEGMA-g-TLCP copolymers during the melt-compounding. As the results show, the melting and crystallization temperatures of the PEGMA component in the blends were greatly lowered compared to the TLCP component with a rigid backbone chain. The thermal decomposition peak temperatures of PEGMA and TLCP components in the blends were higher than those of neat PEGMA and neat TLCP, respectively. The shear moduli and complex viscosities for the blend with 30 wt% PEGMA at a melt state of 300 • C were far higher than those of neat PEGMA, which supports the presence of PEGMA-g-TLCP formed during the melt-compounding. The DMA results revealed that the storage moduli of TLCP/PEGMA blends at a constant temperature decreased slightly with the PEGMA content up to 3 wt%, increased at the PEGMA content of 5 wt%, and decreased again at PEGMA contents above 7 wt%. The maximum storage moduli for the blend with 5 wt% PEGMA were also conjected to be due to the reinforcing and synergistic effects of PEGMA-g-TLCP copolymers on the compatibility and mechanical performance of TLCP/PEGMA blends. Overall, it is valid to contend that TLCP/PEGMA blends with enhanced thermal and mechanical properties, as well as an increased price-performance ratio, could be utilized as advanced engineering materials for high-end applications in automotive components, electronic devices, and super fibers.