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Preparation and characterization of liquid crystalline polyurethane-imide modified epoxy resin composites.

INTRODUCTION

The use of thermotropic liquid crystalline polymers as the minor phase of polymer blends has attracted considerable attention in the past three decades. This is because thermotropic liquid crystalline polymers can improve the mechanical properties of polymers matrix. Thermotropic liquid crystalline polymers tend to deform into elongated fine fibrils under appropriate processing conditions. The thermotropic liquid crystalline polymers fibrils can reinforce polymers matrix effectively [1].

Epoxy resin is an important polymeric material with outstanding properties of high modulus, high electrical resistance, and excellent adhesive property. However, its brittleness limits it application to projects, which require high-impact strength [2-4]. In this respect, many efforts have been made to improve the toughness of cured epoxy resins by introducing rubbers [5-7], thermoplastics [8-10], rigid particles [11, 12], hyperbranched polymers [13, 14] and liquid crystal polymers [15-17] within the matrix. Rubbers and thermoplastic as the modifier may initially be immiscible or may phase-separate during cure [18, 19]. Williams and coworkers [20, 21] studied reaction-induced phase separation in poly(butylene terephthalate)-epoxy systems and generated conversion-temperature-transformation diagrams (CTT) for poly(butylene terephthalate)-epoxy systems with different curing agents and studied the different morphologies, developed by different cure cycles. The incorporations of rubber, thermoplastics into epoxies can effectively increase their fracture toughness. However, this improvement is compromised by reduction in some basic properties, such as strength, modulus, and glass transition temperature. So, Deng et al. [22] toughening epoxies with halloysite nanotubes, and concluded that halloysite nano-particles were effective additives in increasing the fracture toughness of epoxy resins without sacrificing other properties. Rubber, thermoplastic, or rigid particles as tougheners are always limit the processability of the resin systems, so Manson and coworkers [23] use dendritic hyperbranched polymers as tougheners for epoxy resins. The chemical and physical structure of hyperbranched polymers, induce unique properties that can solve the problems related to processability, property compromises, and compatibility, which are found with commercial additives and modifiers. The main drawback with dendrimers and hyperbranched polymers is that they are normally synthesized using a stepwise process, which make them far too expensive to be used as polymer additives [24], Liquid crystal polymer used as modifier of epoxy resin is an effective method to improve its toughness. The mechanical reinforcing of the composites by mesogenic group is due to the alignment of liquid crystal polymer arrangement along outside force direction when there exists outside force to the composite, and the mutual traction of liquid crystal polymer with matrix polymer also make the matrix orientation to some extent, so the tensile strength of materials modified by liquid crystal polymer are enhanced. Meanwhile, the liquid crystal polymer modified composite has flexible and rigid space structures, and those structures are useful for improving impact resistance of the composite [25, 26].

Polyimide is a kind of materials with high performances for their excellent thermal stability, dimensional stability, radiation resistance, mechanical properties, and electric properties, and is mainly used in the aerospace and electronic industries in the form of films and moldings [27-33], However, polyimide is generally insoluble and infusible, and this makes it difficult to process. Modified polyimide with flexible linear macromolecule can reduce the polymer melting point effectively [34]. Huang et al. [35] synthesized thermotropic polyurethanes containing biphenylnate and imide units, and all of the polyurethane polymers exhibited thermotropic nematic liquid crystalline textures in the range of 160-190[degrees]C. Jiang et al. [36] synthesized a kind of poly (urethane-imide) (PUI) by alternating copolymerization. The inherent viscosity of PUI was 0.83-0.99 dl/g and exhibited improved solubility in organic solvents, but the thermal stability of materials reduced. It is an effective method to reduce the melting temperature of composite materials by using flexible macromolecule polyimide as modifier.

In this research, liquid crystalline polyurethane-imide (PUI) was synthesized and used as modifier of epoxy resin. The mechanical properties, thermal property, and morphology of the PUI modified epoxy resin were characterized.

EXPERIMENTAL

Materials

N-Methyl pyrrolidone (NMP, AR) was purchased from Tianjin Fuchen Chemical Reagents, China and vacuum dried before use. Polyethyleneglycol-600 (PEG-600, AR) was purchased from Tianjin Ruijin Chemical Reagents, China and vacuum dried at 80[degrees]C for 2 h. Pyromellitic dianhydride (PMDA, AR) and 4, 4'-Methylenedianiline (DDM, AR) used as curing agent were supplied by Aladdin Reagent, China. Methanol (AR) was purchased from Dongguan Dongjiang Chemical Reagents, China. Toluene 2, 4-diisocyanate (TDI, AR) was purchased from the First Chemical Reagents of Shanghai, China. Epoxy resin (ER, E-51) was supplied by Shenzhen Jitian Chemical, China.

Synthesis of Polyurethane-Imides

21.8 g (0.1 mol) PMDA was dissolved in 150 ml toluene in a 500 ml, three-necked round-bottom flask equipped with a reflux condenser, a thermometer and a mechanical stirrer. Thirty-five gram (0.2 mol) TDI was added into 150 g NMP, then fully mixed and added it into the 500 ml three neck flask at 110[degrees]C. The reaction mixture was stirred at 110[degrees]C for 2 h, and then the mixture was cooled down to 100[degrees]C, and 60 g (0.1 mol) PEG-600 solution was added dropwise to the reaction solution in 30 min. The reaction was maintained for 3 h, and the solution turned to brownish black. At last the reaction liquid was poured into a big beaker from flask and added enough methanols, and then target polymer was separated out, purified and dried in vacuum oven at 50[degrees]C for 2 h. The reaction equation is shown in Fig. 1.

Fabrication of Polymer Composites with PUI

Epoxy resin, PUI and curing agent DDM were mixed according to a certain proportion of quality to make fabrication of the composites with various weight percents of PUI. The dosage of DDM was determined according to the Eq. 1.

M (DDM)= m(resin) G/N(H) (1)

where G is the epoxy value of epoxy resin, and A(H) is the number of active hydrogen atoms in curing agent. Solid PUI was mixed with epoxy resin by stirring at 95[degrees] C. DDM was then added subsequently and mixed thoroughly. The samples were degassed in vacuum oven, poured into a mold and then heated at 120[degrees]C for 2 h and 165[degrees]C for 2 h to obtain cured specimens. The formulas of the composites were listed in Table 1.

Characterization

The chemical structure of PUI was tested by Fourier transform infrared spectroscopy (FTIR, 8300 PCS, Shimadzu). Differential scanning calorimetry (DSC) was performed in a 202 Netzsch to insure the range of liquid crystal temperature of PUI. Polarization microscope (POM, Leica dmlp, Germany) was used to observe the formation of liquid crystal in the process of temperature rise. The decomposition temperatures of composites were characterized by Thermogravimetric analysis (TGA, STA409PC, Netzsch, Germany). Electronic universal test machine (CMT4304, Shenzhen xinsansi material detection, China) was used to measure the toughness of the composites. Dynamic mechanics analyzer (DMA, Q800, TA) was used to analyze phase separation condition of the materials. The morphologies of the composites were observed through scanning electron microscope (SEM, S-3400N (II), Japan).

RESULTS AND DISCUSSION

Structure Characterization of PUI

Figure 2 provides the FTIR spectrum of PUI, which exhibits the characteristic absorptions of the amide group at ~3250 [cm.sup.-1]. The adsorption band of 2800-2900 [cm.sup.-1] is attributed to the C-H stretch in the [CH.sub.2], [CH.sub.3] groups. The absorption band at 1778 [cm.sup.-1], 1725 [cm.sup.-1], and 1375 cm-1 indicate the presence of 0=C=NH group [36]. The peaks at ~1540 [cm.sup.-1] are assigned to --CO--N[H.sub.2] groups. In addition, the C--O--C stretching is positioned at 1109 [cm.sup.-1]. The structure characterization of the polymer is in good agreement with the prediction.

Thermal Behavior of PUI

The thermal properties of PUI investigated by means of DSC correlate highly with the microscopic observations. The DSC measurements were performed at heating rate of 20[degrees]C/min, 15[degrees]C/min, 10[degrees]C/min as well as 5[degrees]C/ min to 300[degrees]C. The results are shown in Fig. 3.

It can be seen from the DSC curves that there are no transition peaks of liquid crystalline state at 20 or 15[degrees]C/min. That is because the molecular chains have no time to move into the lowest energy state while the heating rate is too high. However, it has a larger endothermic peak at the temperature of about 124[degrees]C, showing that PUI changes from the elastomeric state to the viscous state. It appears a smaller endothermic peak at a lower heating rate of 10[degrees]C/min between 150 and 200[degrees]C, due to the change of the molecular chains from disorder arrangement to order arrangement, which needs to absorb heat energy that is less than the energy of transformation from the elastomeric state to the viscous state. Although the liquid crystal transition peak can be clearly seen from the DSC thermogram at the heating rate of 5[degrees]C/min, the phase transition peak is not obvious because not only the molecular chains have enough time to move at low heating rate, but also the molecular weight distribution of polymer is wide so that the temperature range of energy absorption is also wide.

Polarized Light Property of PUI

Polarized light microscopy is used to observe the crystal growth of PUI at the heating rate of 10[degrees]C/min, which is displayed in Fig. 4.

Figure 4 indicates that PUI begins to appear the Maltese cross phenomenon at the temperature of 145[degrees]C, which is the unique property of polymer spherulite, demonstrating that part of polymer has been transformed from the isotropic state into the anisotropic state. With increasing of the temperature, the spherulites continue to grow, which is on account of the uneven distribution of the molecular weight. As the temperature goes up, those shorter molecular chains are able to transform from the disordered arrangement into the ordered arrangement earlier, thus resulting in accelerating the change from the anisotropic state to the isotropic state. Yet, the long molecular chains need more time to achieve that because they require higher energy to move. Therefore, polarized light property of the liquid crystalline begins to decline when the temperature rises to 200[degrees]C. On the whole, the polymer has a wide range of liquid crystal temperature.

Analysis of the Crystallinity of PUI

A decalescence peak appears in the DSC curve of PUI while the temperature is above 110[degrees]C as shown in Fig. 3, and there is a smaller one after the temperature of 150[degrees]C. To make sure that the polymer shows liquid crystal between 150 and 240[degrees]C, we prepared two samples by employing the way of relatively rapid cooling to keep the molecular chains maintaining at the state of high temperature. One is rapidly cooled at 110[degrees]C while the other at 180[degrees]C. Then the crystallinity of those samples are tested, and the results are revealed in Fig. 5.

Figure 5 presents that the X-ray diffraction (XRD) curve of PUI cooled rapidly at 110[degrees]C has no crystallization peak while the curve of the sample cooled rapidly at 180[degrees]C appears some crystallization peaks. This is because the molecular chains are free to move to the lowest energy state and array orderly at the temperature of 180[degrees]C due to the existence of hydrogen bonds as well as the molecular force of hard- and soft-segments among the molecular chains, whereas the chain segments of polymer have not enough freedom and fail to arrange orderly at 110[degrees]C. Hence, we can draw the conclusion that PUI is a crystalline polymer at 180[degrees]C.

Mechanical Property of PUI/ER Composites

The results of maximum bending strain, fracture strain, and bending strength of PUI/ER composites are summarized in Figs. 6 and 7.

The maximum bending strain and fracture strain of PUI/ER composites increases with the growth of mass fraction of PUI when the mass fraction of PUI is under 15 wt%. The largest value of strain of the composites is 10.65% as the mass fraction of PUI is 15 wt%, which is higher than that of pure ER about 33%. However, the maximum bending strain and fracture strain will fall down if the mass fraction of PUI rises continuously. The PUI is a kind of thermotropic liquid crystal, and after it is cured at the high temperature, there are a lot of semirigid structures existing in PUI/ER composites, which can form interpenetrating polymer networks. The existence of semirigid structures reduces the crosslinking degree and internal tension of the composites. What's more, the excess of PUI will weaken the compatibility between PUI and ER [37], so the bending strength of the PUI/ER composites will decrease if the mass fraction of PUI is more than 15 wt%. When the mass fraction of PUI is 15 wt%, the bending strength of the composite is 178 MPa, about 32% higher than that of pure EP whose bending strength is 135 MPa. In a word, both the toughness and strength of PUI/ER composites are enhanced when the content of PUI in epoxy is appropriate.

Thermogravimetric Analysis of PUIIER Composites

The heat resistance of pure ER and PUI/ER materials was studied with thermogravimetry under nitrogen atmosphere and the results are given in Fig. 8 and Table 2. Table 2 lists the temperatures of cured PUI/ER materials at onset and weight residues of cured PUI/ER materials at 500[degrees]C. The result suggests that the thermal decomposition temperature of PUI/ER cured samples with different content of PUI have no obvious decline as compared with that of pure epoxy resin. It can be contributed to imide bond in PUI, which is difficult to decompose at high temperature [36]. Therefore, the thermal stability of epoxy resin after modification with PUI has not obvious decrease because of good heat resistance of PUI although the addition of PUI in epoxy leads to the decrease of crosslinking density of the composites.

Morphology Analysis of PUI I ER Composites

Figure 9 manifests SEM images of fracture surfaces of PUI/ER composites with different mass fraction of PUI.

The fracture surfaces of pure ER have a little bit of fracture strips with smoothness, which is a distinguished feature of brittle fracture. Fracture surfaces of PUI/ER composites with different mass fraction of PUI such as 5, 10, 15, and 20 wt% appear many strips as well as reticular ravines, explaining that PUI has taken shape in the anisotropic fibers inside ER. With the growing of PUI content, the ravines become more and more obvious and thick shown as 25% PUI in Fig. 9, which means that the more the mass fraction of PUI is, the more obvious the phase separation of the PUI/ER composites will become. Meanwhile, the ravines on the fracture surface of the composites become wider and larger as the content of PUI increases such as 30 and 35 wt% because the crosslinking density decreases with the content of PUI in epoxy resin and a lot

of debris are chipped with impact fracture. It also illustrates that too much PUI may decrease the toughness of the composites instead.

The reason why this kind of phenomenon comes out is that PUI is a type of thermotropic liquid crystalline. When cured after high temperature, it is like some rigid rods interspersed in the material. As a result, PUI can enhance the strength and toughness of composites when it blends with ER at a suitable content, but if too much PUI exists in ER, the composites will turn to crisping.

CONCLUSIONS

Liquid crystalline PUI was synthesized by means of polymer polymerization, characterized, and used to modify epoxy resin for improving thermal and mechanical properties of composite materials. The results show that PUI is a kind of liquid crystal material within a wide range of temperature from 150 to 240[degrees]C.

The PUI/ER materials have not only good thermal stability but also excellent mechanical performances. Compared with the unmodified epoxy resin, PUI/ER composite achieves the maximum mechanical properties with bending strength of 178 MPa and bending strain at break of 10.65%, increased by about 32% and 33%, respectively when the content of PUI is 15wt%. The mechanical behaviors of the PUI/ER are confirmed by morphology analysis of the fracture surfaces of PUI/ER composites from SEM.

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Dayong Gui, Xue Gao, Jingfeng Hao, Jianhong Liu

School of Chemistry and Chemical Engineering, Shenzhen University, Shenzhen 518060, People's Republic of China

Correspondence to: Dayong Gui; e-mail: dygui@szu.edu.cn

Contract grant sponsor: National Basic Research Program of China; contract grant number: Program 973, 2011CB605603, 613142; contract grant sponsor: R&D Funds for Basic Research Program of Shenzhen; contract grant number: JCYJ20120613110532389.

DOI 10.1002/pen.23712

Published online in Wiley Online Library (wileyonlinelibrary.com).

TABLE 1. The formula of PUI/ER composites.

PUI loading         ER            PUI            DDM
(wt%)          quality (g)    quality (g)    quality (g)

0                   9              0             2.28
5                   9             0.6            2.28
10                  9             1.25           2.28
15                  8             1.77           2.02
20                  8             2.5            2.02
25                  8             3.34           2.02
30                  8             4.3            2.02
35                  8             5.4            2.02

TABLE 2. The initial decomposition temperatures of the cured PUI/EP
materials and the weight residue at 500[degrees]C.

Content of     Temperature of          Weight residue
PUI(wt %)    onset decomposition   at 500[degrees]C (wt%)
                ([degrees]C)

0                   370.6                   24.1
10                  363.1                   26.8
15                  365.6                   28.4
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Author:Gui, Dayong; Gao, Xue; Hao, Jingfeng; Liu, Jianhong
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:9CHIN
Date:Jul 1, 2014
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