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Morphology and properties of poly(benzoxazine-co-urethane)s based on bisphenol-s/Aniline benzoxazine.

INTRODUCTION

Benzoxazines is a newly developed class of thermosetting phenolic resins, and they can be used as matrix resins for high-performance composite because their corresponding polybenzoxazines possess many unique advantages, such as high stiffness, good thermal stability, high mechanical properties, and low absorption of moisture (1-3). Polybenzoxazines combine the thermal properties and inherent flame retardance of phenolic resins and hence found application where higher temperature resistance and processability are important.

In practical applications, benzoxazines can be combined with thermosetting and thermoplastic resins or nanomaterials to form polymer alloys or nanocomposites with super properties. The research on this aspect is active in many ways, and much effort has been devoted into the study. The most common approach to introduce high-performance polymers or nanomaterials into polybenzoxazine networks is by preparing copolymers, blends, and nano-composites, including rubber (4), polycarbonate (5), poly(s-caprolactone) (6), polyurethane (PU) (7-19), epoxy (20), (21), and organic--inorganic hybrid nanomaterials (22-24). Depending on the nature of components, melt-or solution-blending method can be used in the practical preparation and processing.

Alloying with PU is an efficient way to improve the toughness of polybenzoxazines and enhance the thermal stability of PU, and a series studies have been reported on the combinations of benzoxazines and traditional P[U.sub.s] (7-19) A commonly used approach to prepare poly(ben-zoxazine-co-urethane)s is by thermally polymerizing the mixture of benzoxazine/PU prepolymer. In the polymerization, a number of phenolic hydroxyl groups are formed from benzoxazine, and they react with the isocyanate (NCO) group in PU prepolymer, resulting in a cross-linked copolymer. Most studies are based on monofunc-tional or bisphenol-A type benzoxazine, and copolymer prepared from other type benzoxazines and PU are rarely reported.

Recently, bisphenol-S/aniline-type benzoxazine (6,6'-bis(3-phenyl-3,4-dihydro-2H-benzo[e] (1), (3)oxazinyl) sulfone, abbreviated as BS-a) was synthesized (25), and the properties of its polymer (PBS-a) are worth concerning. Owing to the rigidity of the sulfone group in BS-a molecular structure, the storage modulus and the glass transition temperature of PBS-a will be higher than those of poly-benzoxazine obtained from bisphenol-A/aniline-type ben-zoxazine (BA-a), and the effect of the improvement on the properties of PBS-a by introducing PU moiety will be evident. Although most findings had already been reported for BA-a--urethane systems (11), (12), (19), the morphology and water absorption of poly(benzoxazine-co-urethane) were rarely reported, and the changes on the properties of BS-a--urethane copolymer with urethane content will be different from those of BA-a--urethane copolymer. In this work, poly(benzoxazine-co-urethane)s were prepared from BS-a with PU prepolymer based on 2,4-toluene diisocyanate (TDI) and poly(ethylene glycol) (PEG), and the morphology and properties of poly(ben-zoxazine-co-urethane)s were studied.

EXPERIMENTAL

Materials

Bisphenol-S (4,4'-dihydroxydiphenyl sulfone) (99%) was purchased from Shanghai Chemical Reagent Co. (China). Formaldehyde (37 wt% aqueous), aniline, dioxane, chloroform, and TDI were obtained from Tianjin Chemical Reagent Co. (China). PEG with the number--average molecular weight of 2000 g/mol was obtained from Chengdu Kelong Chemical Reagent Factory (China). All chemicals were used as received.

Preparation of BS-a and Polybenzoxazine (PBS-a)

BS-a was synthesized via a solution method from bisphenol-S, aniline, and formaldehyde (25), and the corresponding PBS-a was prepared by thermally activated polymerizing of BS-a in an air-circulating oven at 200[degrees]C for 6 h.

Preparation of NCO-Terminated PU Prepolymer

In a 500-mL four-necked flask equipped with mechanical stirrer, reflux condenser, dropping funnel, and gas inlet adapter, 100 g PEG was introduced and heated to melt in nitrogen. Then, 17.4 g TDI was added dropwise into the flask, while maintaining the temperature at 60C. The two reactants were thoroughly mixed in the flask, and the mixture was continuously stirred under a nitrogen stream at 80-85'C for 2 h. Thereafter, the obtained clear and viscous PU prepolymer was cooled to room temperature and kept in a refrigerator to prolong its shelf life.

Preparation of Poly(benzoxazine-co-urethane)

BS-a was mixed with the NCO-terminated PU prepolymer to provide BS-a/PU prepolymer mixtures at different weight ratios, i.e., 90/10, 80/20, and 70/30. The BS-a/PU prepolymer mixture was heated to about 80[degrees]C and stirred until a homogeneous transparent solution was obtained, then the molten clear homogenous mixture was poured into a steel mold, thereafter, the mold was kept in a vacuum oven conditioned at 120[degrees]C for 2 h to remove the entrapped air from the molten specimen and polymerized in an air-circulated oven at 200[degrees]C for 6 h. Finally, the sample was cooled freely to room temperature.

Measurements

Fourier transform infrared spectroscopy (FTIR) spectra were obtained with a Nicolet 380 FTIR spectrometer at a resolution of 4 [cm.sup.-1]. BS-a/PU prepolyiner mixture was finely ground with KBr powder and pressed into a disk, and then the disk was placed in an air-circulating oven with a fixed temperature. During the polymerization reaction, the disk was removed periodically for measurement.

The morphologies of the fracture surfaces of PBS-a and poly(benzoxazine-co-urethane)s were observed by a KYKY-2800B scanning electron microscope operating at an accelerating voltage of 25 kV. The specimens were freeze fractured in liquid nitrogen, and the fracture surfaces were then coated with a thin layer of gold prior to examination.

A Perkin-Elmer DMA-8000 dynamic mechanical analyzer was used to determine the storage modulus (E'), loss modulus (E"), and loss factor (tan [sigma]) of PBS-a and poly(benzoxazine-co-urethane) by using the single cantilever bending method. Measurement was performed from 25 to about 300[degrees]C at a heating rate of 5[degrees]C/min in static air atmosphere, and the testing frequency was set at 1 Hz. The dimension of the specimen was approximately 10.0 mm X 5.8 mm X 2.4 mm.

The amounts of water absorption of PBS-a and poly(ben-zoxazine-co-urethane)s were measured according to ASTM D570 using a disk-shaped specimen 50.8 mm in diameter and 3.2 mm in thickness. The specimens were weighed with a Mettler Toledo AG245 balance. Three samples of each composition were conditioned at 50[degrees]C for 24 h prior to the test, then cooled in a dessicator and immediately weighed, and submersed in deionized water at 25[degrees]C. The specimens were periodically removed, wiped dry, weighed, and then immediately returned to the water bath. The amount of the absorbed water was calculated as the difference between the weight at each of these measurement times and the initial conditioned weight. The water absorption is calculated by the following formula:

Water absorption (%) = [W.sub.wet] - [W.sub.dry]/[W.sub.dry] (1)

where [W.sub.wet and ][W.sub.dry] are the weight of wet and dry specimens, respectively.

A Shitnadzu TGA-40 thermogravimeter was used to .determine the weight loss behavior of PBS-a and poly(ben-zoxazine-co-urethane)s in the thermal degradation process. The samples of about 5 mg were heated from 25 to 800[degrees]C at a heating rate of 10[degrees]C/min in nitrogen atmosphere.

RESULTS AND DISCUSSION

Copolymerization Reaction

The NCO-tertninated PU prepolymer was prepared by reacting TDI with PEG at a molar ratio of 2/1, and the reaction can be expressed as Scheme 1.

The poly(benzoxazine-co-urethane)s were prepared by reacting BS-a with the NCO-terminated PU prepolymer, and the preparation involves the polymerization of BS-a and the chemical reaction between isocyanate groups in PU prepolymer and phenolic hydroxyl groups in PBS-a. The copolymerization reaction can be described by Scheme 2.

Figure 1 shows the FTIR spectra of the BS-a/PU prepolymer mixture before and after polymerization. The characteristic absorption band of the stretching vibration of NCO group is at 2273 [cm.sup.-1] (Fig. 1 a), and the urethane groups are confirmed by the absorptions of N--H stretching at 3432 [cm.sup.-1], C=0 stretching from both urethane and carboxyl groups at 1725 and 1665 [cm.sup.-1]. Meanwhile, the characteristic absorption peaks of BS-a can be seen at 1030, 1237, and 1119 [cm.sup.-1], which are assigned to the symmetric and asymmetric stretching of C--N--C asymmetric stretching of C--N--C of the oxazine ring, respectively.

The reaction between BS-a and the NCO-terminated PU prepolymer was examined by FTIR. Figure lb-e shows the structure changes in the copolymerization reaction at 160[degrees]C. Owing to the opening of the oxazine ring in BS-a, the intensity of the absorption peaks at 1030 and 1237 [cm.sup.-1] decreased gradually, whereas the intensity at 1119 [cm.sup.-1] increased. Correspondingly, the intensity of some absorptions decreased, such as the stretching vibrations of the benzene ring with an oxazine ring attached to it at 1500 [cm.sup.-1], the out-of-plane bending mode at 920 [cm.sup.-1], and the in-plane bending mode at 577 [cm.sup.-1]. Similar to the polymerization of the neat BS-a, the absorption intensity at 1143, 970, 920, 721, and 662 [cm.sup.-1] decreased with the reaction, and some absorption peaks disappeared (25).

Oh the other hand, the absorption intensity of NCO group at 2273 [cm.sup.-1] decreased rapidly and the absorption peak completely disappeared in 5 min. Meanwhile, the intensity of the characteristic absorption of the C=0 stretching of nonhydrogen bonded carbonyl group of urethane at 1725 [cm.sup.-1] decreased gradually and the absorption peak disappeared eventually, whereas new absorption peaks appeared and increased at bands of 1698 and 1631 [cm.sup.-1] that are assigned to the C=0 stretching of hydrogen-bonded urethane carbonyl. The absorption at 1514 [cm.sup.-1] is due to the N--H out-of-plane bending combined with C--N asymmetric stretching of urethane group, and the absorption at 1352 [cm.sup.-1] is assigned to the C--N stretching.

In addition, the intensity of the strong broad absorption centered at 3432 [cm.sup.-1] decreased with the copolymerization reaction proceeding, and the band consists of the N--H stretching vibration of urethane and the O--H stretching vibration of unreacted phenolic hydroxyl groups. As more phenolic hydroxyl groups generated in the BS-a polymerization, more phenolic hydroxyl groups were consumed by reaction with NCO groups, forming urethane groups, and the absorption intensity of phenolic hydroxyl group may become weaker. After all the NCO groups had reacted, no obvious changes can be seen in the absorption intensity of phenolic hydroxyl groups.

The above absorption changes demonstrate the reaction between BS-a and NCO functional groups, and confirm the formation of poly(benzoxazine-co-urethane).

Morphology

Figure 2 shows the scanning electron micrographs of the fracture surfaces of PBS-a and poly(benzoxazine-co-urethane)s with different weight percent of urethane. As shown in Fig. 2a, the fracture surface of PBS-a is smooth with uninterrupted crack propagation, which indicating the typical morphological features of brittle fracture and poor fracture toughness. The relatively coarse structure with rough and ridge like patterns and river marks can be seen on the fracture surfaces of the poly(benzoxazine-co-urethane)s in Fig. 2b--d, indicating the bulk plastic deformation. The roughness of the fracture surfaces is associated with the ductile nature of the crack, which is due to the formation of the ductile cross-linked network between PBS-a and PU. The phenolic hydroxyl groups formed in the polymerization of BS-a reacts with the NCO groups in PU prepolymer and chemically bonded PU with PBS-a, which enhances the ability of plastic deformation of poly(benzoxazine-co-urethane)s. Moreover, the fracture surfaces of poly(benzoxazine-co-urethane)s with different weight percent of urethane did not display any phase separation, indicating that a homogeneous structure was formed in poly(benzoxazine-co-urethane)s. With the increase in the urethane content. the fracture surfaces became rougher and more deformed, and the roughness indicates an increase in the required energy for the propagation of the cracks. Thus, the toughness may be enhanced by increasing the fracture energy through the mechanism of the plastic deformation in poly(benzoxa-zine-co-urethane)s.

Dynamic Mechanical

Properties Dynamic mechanical properties such as E', E", and tan [sigma] of PBS-a were determined and compared with those of poly(benzoxazine-co-urethane)s with different weight percents of urethane. Figure 3 shows the temperature dependence of E', E", and tan [sigma] for the prepared PBS-a and poly( benzoxazine-co-urethane)s.

In Fig. 3, one-step decrease can be seen in E' of PBS-a and poly(benzoxazine-co-urethane)s, which corresponds to the single damping peak observed in the tan [sigma] curves, followed by an obvious rubbery plateau. The E' value of PBS-a is around 2.7 GPa at room temperature (25[degrees]C) and the glass transition temperature ([T.sub.g]) is 224[degrees]C determined from the peak temperature on tan [sigma] curve (Table 1). Incorporation of urethane results in a remarkable decrease in storage modulus and increase in tan [sigma] value of poly (benzoxazine-co-urethane)s. The E' values of poly(ben-zoxazine-co-urethane)s are lower than that of PBS-a in the glassy region, and an increase in the urethane content led to a progressive decrease in E' of poly(benzoxazine-co-urethane)s, due to the plasticity effect from the soft moiety of urethane. In the glassy state, the mobility of the molecule segments is weak and more energy can be stored and less energy dissipation takes place, so the E' value decreases slightly with the temperature increasing. However, with continuous increase in the temperature, a sharp decrease in E' can be observed near [T.sub.g], which is attributed to the rapidly increased mobility of polymer chains above [T.sub.g]. Consequently, opposite to the variation trend in the glassy state, the E' values of poly(benzoxa-zine-co-urethane)s (73/30 and 80/20) are higher than that of poly(benzoxazine-co-urethane) (90/10). With the temperature increasing, PBS-a and poly(benzoxazine-co-ure-thane)s step into elastic state, and the value of the elastic E' is very small. As a result, the difference between the moduli of the glassy state and rubbery state is smaller in poly(benzoxazine-co-urethane)s than in the neat PBS-a.

TABLE 1. Properties of PBS-a and poly(benzoxazine-co-urethane)s
with various urethane contents.

Urethane       E' (GPa)     [T.sub.g]    [T.sub.5%]   [T.sub.10%]
conical             (at  ([degrees]C)  ([degrees]C)  ([degrees]C)
(wt%)     25[degrees]C)

0                  2.70           224           366           386

10                 2.03           243           362           384

20                 1.80           255           356           382

30                 1.78           261           347           380


Concomitantly, as can be seen, with the temperature increasing, E" and tan (5 of PBS-a and poly(benzoxazine-co-urethane)s rapidly rise until they reach the maximum values and then decrease. The peaks of tan (5 appear 2530[degrees]C higher than those the corresponding E" peaks. With increasing the weight percent of urethane, the peak temperature of tan (5 shifted to a higher temperature value, whereas the peak height decreased. Similar behavior was observed for BA-a and urethane copolymers (18), (19). However, it can be noticed that the peak height of tan o of the poly(benzoxazine-co-urethane) (90/10) is higher than that of the neat PBS-a, whereas other poly(benzoxazine-co-ure-thane)s (80/20 and 70/30) are lower. The phenomenon can be attributed to the molecular structure of PBS-a, for the rigidity of sulfone in PBS-a restricts the mobility of the PBS-a molecules. As is well known, the tan (5 peak corresponds to the cc-relaxation, and the relaxation strength corresponds to the height of the tan peak, which represented the mobility of the segments of polymer chain. The significant increase in the peak temperature and decrease in the peak height in tan [sigma] curves reflect the reduced mobility of poly(benzoxazine-co-urethane) segment, which could be attributed to the constraint by the increasing number of chemical bonding between polybenzoxazine and PU prepolymer segments. When a small dose of urethane is introduced into BS-a, such as 10/90, the cross-linking density of the resultant poly(benzoxazine-co-urethane) may be enhanced in some extent, but the flexibility of the network is higher than that of the neat PBS-a due to the plasticity effect of the urethane segments, and the mobility of the poly(benzoxazine-co-urethane) is higher than that of the neat PBS-a. So the height of the tan [sigma] peak is higher than that of the PBS-a. As the urethane fraction increased in poly(benzoxazine-co-urethane), the mobility of the chain segments decreases with increasing cross-linking density, for the degree of cross-linking density is controlled by the number of isocyanate groups in the PU prepolymer and the phenolic hydroxyl groups generated in the PBS-a formation.

Owing to the cross-linking density of poly(benzoxa-zine-co-urethane)s increases with the rising weight fraction of urethane, the mobility of the molecular segments with sulfone decreases, and more energy is dissipated for molecular segments to move in the glassy region. Consequently, the E" value of poly(benzoxazine-co-urethane)s increases with the rising weight fraction of urethane, and a broad peak is appeared in the E" curve of poly(benzoxa-zine-co-urethane) (70/30) in the temperature range of about 50-160C due to the higher cross-linking density. However, with temperature rising into the glass transition region, the opposite trend is observed in the E" curves. In the glass transition region, the mobility of the molecular segments increases significantly, but the energy dissipated decreases with the reduction in the molecular segment mobility due to the enhanced cross-linking density as the urethane fraction increasing. Therefore, the peak temperature of E" of poly(benzoxazine-co-urethane) (70/30) is found to decrease, whereas the opposite is observed in the tan curves.

Of course, it can be seen that poly(benzoxazine-co-ure-thane)s exhibit a single [T.sub.g] corresponding to the cooperative segmental motions of the molecular chains, which implies no phase separation in the poly(benzoxazine-co-urethane)s, and the compatibility between PBS-a and the PU is good, which is coincident with the results observed by scanning electron microscopy.

Water Absorption

Figure 4 shows the plots of the percentage of water absorption versus time for PBS-a and poly(benzoxazine-co-urethane)s. It can be seen that the amount of the water absorption by the neat PBS-a is minimum, i.e., about 0.73% for 20 days. For poly(benzoxazine-co-urethane), the increase in the weight of the samples due to water absorption gradually increased as the urethane contents increased, especially for poly(benzoxazine-co-urethane) with higher urethane content. The significant increase in water absorption for poly(benzoxazine-co-urethane) may be attributed to effect of the hydrogen bonding developed between urethane groups with water molecules.

Thermal Stability

The TG curves of PBS-a and .poly(benzoxazine-co-ure-thane)s are shown in Fig. 5. It can be seen that the initial onset of weight loss temperatures of the poly(benzoxa-zine-co-urethane)s are lower than that of PBS-a, and the 5, and 10% weight loss temperatures (T5% and T10%) are listed in Table 1. These data indicate that incorporation of PU moiety into polybenzoxazine network structure reduces the thermal stability. In the thermal degradation, the amount of the weight loss increase with the urethane content increasing at the same temperature. The degradation of PU is generally initiated at the urethane linkages, and the scission of the urethane bonds make PBS-a thermal instable and accelerate the thermal degradation of PBS-a moiety in the poly(benzoxazine-co-urethane)s. As a result, the char yield at 800C decreases with the urethane content increasing poly(benzoxazine-co-urethane)s.

CONCLUSIONS

Poly(benzoxazine-co-urethane) was prepared by melt-blending BS-a with NCO-terminated PU prepolymer based on TDI and PEG, followed by thermally activated polymerization of the blend. The chemical reaction between the OH group in PBS-a and the NCO in PU prepolymer results in the formation of poly(benzoxazine-co-urethane). The alterations in the ratio of BS-a/PU prepolymer cause morphological changes and lead to variation in dynamic mechanical properties, water absorption, and thermal stability of the poly(benzoxazine-co-ure-thane)s.

Correspondence to: Yanfang Liu: e-mail: liuyanfang@msn.com

DOL 10.1002/pen.23520

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

[c] 2013 Society of Plastics Engineers

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Zhihong Li, (1) Yanfang Liu, (2) Jie Zhang, (2) Shasha Zhao, (2) Shanshan Luo, (2) Zaiqin Yue, (2) Mingtao Run (2)

(1) Office of Educational Administration, Hebei University, Baoding 071002, People's Republic of China

(2) College of Chemistry and Environmental Science, Hebei University, Baoding 071002, People's Republic of China
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Author:Li, Zhihong; Liu, Yanfang; Zhang, Jie; Zhao, Shasha; Luo, Shanshan; Yue, Zaiqin; Run, Mingtao
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:9CHIN
Date:Dec 1, 2013
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