Benzoxazine Monomer and Its Blends with
Poly(ethylene oxide)
JIEH-MING HUANG,1SHIAO-WEI KUO,2YUAN-JYH LEE,2FENG-CHIH CHANG2 1
Department of Chemical and Materials Engineering, Vanung University, 1 Van Nung Road, Chung-Li, 32054, Taiwan, Republic of China
2
Institute of Applied Chemistry, National Chiao-Tung University, Hsin-Chu, Taiwan, Republic of China
Received 12 May 2006; revised 10 November 2006; accepted 29 November 2006 DOI: 10.1002/polb.21090
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: A vinyl-terminated benzoxazine (VB-a), which could be polymerized through ring-opening polymerization, was synthesized through the Mannich condensa-tion of bisphenol A, formaldehyde, and allylamine. This VB-a monomer was then sub-jected to blending with poly(ethylene oxide) (PEO), followed by thermal curing, to form poly(VB-a)/PEO blends. The specific interactions, miscibility, morphology, and thermal properties of these blends were investigated with Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry, dynamic mechanical analysis (DMA), and scanning electron microscopy (SEM). Before curing, we found that PEO was misci-ble with VB-a, as evidenced by the existence of a single composition-dependent glass transition temperature (Tg) for each composition. The FTIR spectra revealed the pres-ence of hydrogen-bonding interactions between the hydroxyl groups of poly(VB-a) and the ether groups of PEO. Indeed, the ring-opening reaction and subsequent polymer-ization of the benzoxazine were facilitated significantly by the presence of PEO. After curing, DMA results indicated that the 50/50 poly(VB-a)/PEO blend exhibited two val-ues of Tg: one broad peak appeared in the lower temperature region, whereas the other (at ca. 3278C, in the higher temperature region) was higher than that of pristine poly (VB-a) (3018C). The presence of two glass transitions in the blend suggested that this blend system was only partially miscible. Moreover, SEM micrographs indicated that the poly(VB-a)/PEO blends were heterogeneous. The volume fraction of PEO in the blends had a strong effect on the morphology.VVC2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 644–653, 2007
Keywords: hydrogen bonding; polybenzoxazine; poly(ethylene oxide); polymer blends
INTRODUCTION
Polymeric materials play vital roles in the elec-tronics industry because of such factors as their ease of processing, low cost, low dielectric con-stants, and strong adhesive properties. Moreover,
many of the properties of polymers can be altered quite readily when they are processed into poly-mer blends and composites. Polybenzoxazines are novel thermosetting polymers that possess many physical properties that are superior to those of traditional polymers, such as epoxy and phenolic resins. Polybenzoxazines can be prepared by the Mannich condensation of phenol, formaldehyde, and primary amines.1 In addition, polybenzoxa-zines can be cured without the need of a strong Correspondence to: J.-M. Huang (E-mail: jiehming@msa.
vnu.edu.tw)
Journal of Polymer Science: Part B: Polymer Physics, Vol. 45, 644–653 (2007)
V
acid or base as a catalyst under conditions that do not produce toxic gases and other byproducts. Poly-benzoxazines have many outstanding performance features, such as low flammability, high thermal stability, low surface free energy, and low dielectric properties.2,3The glass transition temperature (Tg) of a typical polybenzoxazine—one prepared from a monomer having a difunctional oxazine ring (B-a), is 1808C, with a degradation temperature of about 310 8C.4In an effort to further improve the ther-mal stability of polybenzoxazines, polymerizable acetylene side groups have been introduced into the benzoxazine monomer.5,6 The acetylene-func-tionalized benzoxazines can be polymerized into three-dimensional network products having high thermooxidative stability and resistance to both solvents and moisture. The acetylene-functional-ized benzoxazines can be polymeracetylene-functional-ized at tempera-tures within the range of 190–220 8C, and the products have a char yield above 50% at 800 8C under nitrogen. Another approach toward improv-ing the properties of polybenzoxazines is to blend them with other polymers, such as poly(imide silox-ane), polyurethane, and poly(caprolactone) (PCL), or to incorporate clay into the polybenzoxazine ma-trix.7–9 Recently, Agag and Takeichi10 reported that the incorporation of allyl groups into benzoxa-zine monomers resulted in polybenzoxabenzoxa-zines hav-ing high crosslinkhav-ing densities and high Tgvalues. Encouraged by this result, for this study, we used allylamine as a raw material for the preparation of vinyl-terminated benzoxazines. We expected that the vinyl unit would serve as another polymeriz-able group and result in high-performance polyben-zoxazines. Furthermore, we blended the vinyl-based benzoxazine with poly(ethylene oxide) (PEO) to prepare polybenzoxazine/PEO blends. It is inter-esting that the resulting blends exhibited higher Tgvalues than the virgin polybenzoxazine. PEO is miscible with many polymers, such as poly(4-vinyl-phenol),11 phenoxy,12 and epoxy,13–15 because the ether groups of PEO form intermolecular hydrogen bonds with their hydroxyl groups. Because poly-benzoxazine contains many hydroxyl groups on its main chain after thermal curing, we anticipated that polybenzoxazine might be miscible with PEO as a result of such intermolecular hydrogen bond-ing. Because the physical properties of polymer blends are influenced strongly by the blending con-ditions and processes that, in turn, affect the level of mixing of the blends, there is growing interest in studying the miscibility and phase behavior of polymer blends. Various techniques have been em-ployed to investigate the miscibility of polymer
blends, including microscopy, thermal analysis, dy-namic mechanical analysis (DMA), dielectric mea-surements, and spectroscopy.16–19 Although there is growing interest in synthesizing and studying the properties of polybenzoxazines, little attention has been paid so far to the preparation of polyben-zoxazine/thermoplastic polymer blends. Ishida and
Lee9,20,21 studied polymer blends of
polybenzoxa-zine (B-a type) and poly(e-caprolactone) and found evidence from Fourier transform infrared (FTIR) spectra for hydrogen-bond formation between the hydroxyl groups of polybenzoxazine and the car-bonyl groups of poly(e-caprolactone). In addition, Lu and Zheng22studied polymer blends of polyben-zoxazine (B-a type) and PEO. Therefore, in this in-vestigation, we synthesized a benzoxazine mono-mer presenting a vinyl group, blended it with PEO, and then thermally cured the benzoxazine monomer to produce polybenzoxazine/PEO blends. Because of its incorporated polymerizable vinyl groups, the vinyl-based polybenzoxazine possessed a Tg(3008C) higher than those of traditional polybenzoxazines. We used differential scanning calorimetry (DSC), DMA, and FTIR spectroscopy to explore the misci-bility, specific interactions, and thermal behaviors of the blends, and scanning electron microscopy (SEM) to determine the morphology of the blends.
EXPERIMENTAL
Materials
An aqueous formaldehyde solution (37%), allyla-mine, bisphenol A, and PEO (Mn ¼ 10,000) were purchased from Aldrich Chemical Co. The benzox-azine monomer, B-a, was purchased from Shikoku Chemicals Co. (Japan). To prepare the polybenzox-azine/PEO blends, we synthesized a vinyl-termi-nated version of the benzoxazine monomer (VB-a). The presence of the vinyl group allowed ring-open-ing polymerization to be conducted under moder-ate conditions. The benzoxazine monomer VB-a was prepared according to the procedure outlined in Scheme 1. An aqueous formaldehyde solution (16.5 g) and bisphenol A (11.4 g) were mixed with methyl ethyl ketone (50 mL) in a 250-mL, three-necked flask. With a dropping funnel, allylamine (11.4 g) was added dropwise to the mixture, which was cooled in an ice bath. After an additional 30 min of stirring, the temperature of the mixture was raised gradually to 80 8C, and then it was heated under reflux for 3 h. The solvent and water were evaporated in vacuo, and the residue was
dissolved in ethyl ether (100 mL). The solution was washed several times with water and 2 N aqueous NaOH to remove any impurities and unreacted monomers. The ether solution was then dried (sodium sulfate), and the solvent was evapo-rated at room temperature. The product was ob-tained as a light yellow solid (23.5 g).
Preparation of VB-a-Type Polybenzoxazine and PEO Blends
VB-a/PEO blends of several different composi-tions were prepared via solution blending in di-chloromethane (10 mL). The mixture was stirred for 8 h at room temperature before being poured onto an aluminum plate, dried for 6 h in the open air, placed in an oven, and then heated in vacuo at 508C for 2 h. The cast film was polymerized in a stepwise manner: heating at 120, 140, 160, and 200 8C, each for 2 h. The product was postcured at 220 and 2408C for 30 min each.
Characterization
Proton Nuclear Magnetic Resonance (1H NMR) Spectroscopy
1
H NMR spectra were recorded on a Bruker DPX-300 spectrometer operating at 300 MHz with CDCl3 as the solvent. The relaxation time used in this
study was 2 s. Chemical shifts are reported in parts per million.
FTIR Spectroscopy
Infrared spectroscopy measurements were recorded on a PerkinElmer Spectra One infrared spectrom-eter; 32 scans were collected with a spectral reso-lution of 1 cm1. Infrared spectra of polymer blend films were recorded from samples prepared with conventional NaCl disk methods. Thus, the di-chloromethane solution containing the blend was cast onto a NaCl disk, which was dried under con-ditions similar to those used for the bulk prepara-tion. The films obtained in this way were suffi-ciently thin to obey the Beer–Lambert law.
DSC
The thermal properties of VB-a, PEO, and their blends were determined with a PerkinElmer DSC-7 differential scanning calorimeter under a nitrogen atmosphere. The samples (ca. 5 mg) were placed in a DSC pan, first heated from 25 to 1008C at a rate of 10 8C/min (first heating scan), and then maintained at that temperature for 5 min. The samples were then quenched by being placed in a liquid nitrogen bath. The second scan was re-corded upon reheating from 90 8C at the same heating rate (108C/min). The midpoint of the slope change of the heat capacity of the second heating scan was taken to be Tg. The melting temperature (Tm) was taken to be the maximum of the endo-thermic peak.
DMA
DMA measurements were performed with a TA Instruments DMA Q800 (DuPont) instrument op-erated in a single cantilever bending mode over a temperature range of100 to 350 8C. Data acqui-sition and analysis of the storage modulus (E0), loss modulus (E00), and loss tangent (tan d) were recorded automatically by the system. The heat-ing rate and frequency were fixed at 28C/min and 1 Hz, respectively. Samples for DMA experiments were prepared via molding; the sample dimen-sions were 3 0.8 0.2 cm.
Morphological Observations
The samples of the poly(VB-a)/PEO blends were fractured cryogenically with liquid nitrogen. The fractured surfaces were immersed in chloroform
Scheme 1. Preparation of benzoxazine monomers B-a and VB-a.
at room temperature for 20 min. The morpholo-gies of the cryogenically fractured surfaces of the specimens were examined with a Hitachi (Japan) S-570 scanning electron microscope. The frac-tured surfaces of the samples were coated with thin layers of gold (ca. 100 A˚ ).
RESULTS AND DISCUSSION
Characterization of VB-a
Figure 1 displays the1H and13C NMR spectrum of VB-a. In the 1H NMR spectrum, the vinyl group appears as two resonances, at 5.21 and 5.88 ppm, whose intensities have a 2:1 ratio. We assign the peaks at 3.9 and 4.8 ppm to protons in the methylene bridge of the oxazine. The signal of the protons located between the vinyl group and the nitrogen atom appears at 3.37 ppm. The peaks at 1.57 and 6.66–6.96 ppm are attributed to the C(CH3)2and aromatic protons, respectively. The 13C NMR spectrum of VB-a is presented in Figure 1(b). The signals of the carbon atoms of the terminal olefin unit appear at 116 and 138 ppm. We assign the characteristic signals at 52 and 82 ppm to the carbon atoms of the oxa-zine ring. These NMR spectra confirm that we successfully synthesized VB-a. The FTIR spec-trum of the difunctional benzoxazine compound VB-a has been reported previously.10,23Figure 2 displays infrared spectra of the B-a and VB-a benzoxazine monomers. We can observe the char-acteristic absorptions of the benzoxazine at 1230
(asymmetric stretching of COC units) and 1498 cm1(attributable to the 1,2,4-trisubstituted benzene ring). The characteristic absorption bands of the allyl group appear at 3075 (stretching of ¼¼CH bonds) and 1644 cm1(stretching of C¼¼C bonds). All results are indicative of the presence of vinyl-terminated benzoxazine groups.
Miscibility of the Uncured VB-a/PEO Blends
DSC is used extensively to investigate the misci-bility of polymer blends. A single composition-dependent glass transition is an indication of full miscibility at a dimensional scale between 20 and
Figure 1. (a)1H and (b)13C NMR spectra of VB-a.
Figure 2. FTIR spectra, recorded at room tempera-ture, of B-a and VB-a.
40 nm.24,25 Figure 3 displays the DSC thermo-grams for quenched VB-a, PEO, and VB-a/PEO blends recorded at a heating rate of 10 8C/min; Table 1 summarizes the data. We observed essen-tially only one value of Tg for each composition, with the value of Tgfor PEO being58.5 8C. The value of Tg rose monotonically with increasing VB-a content in the blends, suggesting full
misci-bility of these blends. Furthermore, Tm of the blend decreased with increasing VB-a content in the blends, as indicated in Figure 4. The 80/20 VB-a/PEO blend exhibited a cold crystallization temperature before Tm (as indicated by the arrow). This result implies that the presence of a higher VB-a content retards PEO crystallization from the glassy state; this phenomenon is nor-mally expected for a miscible polymer pair. We observed no trace of a melting endotherm for the blend containing 90% VB-a. This result is similar to the situation observed in our previous study of poly(butylene-2,6-naphthalate) (PBN) and poly(ether imide) (PEI) blends.17
Curing and Polymerization of the VB-a/PEO Blends Figure 5 displays FTIR spectra of 80/20 VB-a/ PEO blends cured at 180 8C for different curing times. The significant decreases in the intensity of the bands at 926 and 1232 cm1, which is the band indicating the presence of the benzoxazine ring,23imply that ring-opening reactions occurred. This result was confirmed by the appearance of new bands within the range of 3100–3600 cm1,
Figure 3. DSC thermograms, recorded within the temperature range of80 to 20 8C, of PEO, VB-a, and their blends.
Table 1. Thermal Properties of the VB-a/PEO Blends VB-a/PEO Tg(8C) Tm(8C) DHf(J/g)a 0/100 58.5 65.3 199.4 10/90 48.2 64.0 185.3 20/80 39.6 62.3 176.5 30/70 27.4 61.1 168.4 40/60 21.2 59.7 162.7 50/50 12.4 58.3 156.2 60/40 3.1 57.8 123.8 70/30 0.9 54.7 115.4 80/20 — 52.3 90.5 90/10 — — 18.2 a
The heat of fusion is based on the weight fraction of PEO present in the blend.
Figure 4. DSC thermograms, recorded during the second heating scan, of PEO, VB-a, and their blends.
which we assign to the hydrogen-bonded hydroxyl units of the opened oxazine ring species. The degree of polymerization of the vinyl groups in the VB-a monomer can be followed by the moni-toring of the changes in the intensity of the band at 3075 cm1, which is associated with CH stretching of the vinyl group. In addition, a new band for the tetrasubstituted aromatic ring of the polymerized VB-a appears at 1480 cm1, with a corresponding decrease in the intensity of the band representing the trisubstituted aromatic ring of VB-a (1498 cm1).
Hydrogen Bonding between Poly(VB-a) and PEO FTIR spectroscopy is a powerful tool for investi-gating specific intermolecular interactions. It is known that intermolecular hydrogen bonding plays a dominant role in determining the misci-bility of polymers containing ether and hydroxyl groups.26 Figure 6 displays infrared spectra of VB-a and VB-a/PEO blends (containing 20, 40, or 60 wt % PEO) that were cured isothermally at 1808C for 120 min. In the region of 1600–1400 cm1, the intensity of the band at 1480 cm1, with re-spect to that at 1498 cm1, was higher when a higher PEO content was present in the blend. This phenomenon implies that the ring-opening
and subsequent polymerization reactions were facilitated by the presence of PEO as a modifier. In the region associated with hydroxyl group stretching (4000–2350 cm1), four different kinds of hydrogen-bonding interaction were involved: the OHþN intramolecular hydrogen bonding at about 2750 cm1, the OHN intramolecular hydrogen bonding at about 3200 cm1, the OHO intermolecular hydrogen bonding at about 3400 cm1, and the OHp intramolecular hydrogen bonding at about 3550 cm1, which have been discussed in
Figure 5. FTIR spectra of 80/20 VB-a/PEO blends that had been cured at 1808C for different lengths of time: (A) 0, (B) 20, and (C) 120 min.
Figure 6. FTIR spectra of VB-a and VB-a/PEO blends that had been cured at 180 8C for 120 min. The PEO contents were (A) 0, (B) 20, (C) 40, and (D) 60 wt %.
a previous study.27 Figure 7 shows their corre-sponding curve-fitting results with various poly (VB-a)/PEO blends, indicating that the fraction of intramolecular hydrogen bonding decreases sig-nificantly at about 2750 and 3200 cm1, whereas the OHO intermolecular hydrogen bonding at about 3400 cm1 increases with the increase in the PEO content. The increase in the intermolec-ular hydrogen bonding comes from the interac-tion between the hydroxyl group of poly(VB-a) and the ether group of PEO. In conclusion, the re-sult indicates that the hydrogen-bonding interac-tion is from the intramolecular hydrogen bonding (OHN) to the intermolecular hydrogen bonding (OHO) between poly(VB-a) and PEO with the increase in the PEO content, which enhances the miscibility behavior in the poly(VB-a)/PEO blend system.
DMA
We examined the viscoelastic properties of the vinyl polybenzoxazine poly(VB-a) along with those of the typical polybenzoxazine poly(B-a). Figure 8
indicates the temperature dependence exhibited by E0, E00, and tan d of the polybenzoxazines. In the case of the typical polybenzoxazine [poly(B-a)], the glass transitions appeared to occur at 160 8C, as determined from the maxima of E00; for poly-(VB-a), the corresponding values of Tg shifted to as high as 2948C as a result of the introduction of the vinyl groups. Thus, these analyses of the visco-elastic properties reveal that a significant increase in Tg (ca. 1348C) occurred for the vinyl polyben-zoxazine, indicating the crosslinking density that was afforded by the introduction of the vinyl groups as additional crosslinkable units. Agag and Takeichi10reported a similar finding. Figure 9 dis-plays E0and E00 of pure PEO and 20/80 and 40/60 poly(VB-a)/PEO blends recorded over the temper-ature range of80 to 80 8C. From a comparison of the E0values of PEO and its blends, it is clear that those of the latter were higher than that of pure PEO. As indicated in Figure 9, the maximum of E00 of PEO, corresponding to its Tg, was 56 8C. From the E00 curves, we can clearly observe a
Figure 7. Curve-fitting results in FTIR spectra at the hydroxyl stretching region of poly(VB-a)/PEO blends.
Figure 8. E0, E00, and tan d of poly(B-a) and poly(VB-a).
Figure 9. E0and E00of PEO and 20/80 and 40/60 poly (VB-a)/PEO blends.
decreased peak intensity accompanying an upshift of the temperature positions after the addition of poly(VB-a). A peak shift in the dynamic properties of a blend results primarily from strong interac-tions between its components. Because the hydroxyl groups of poly(VB-a) are involved in intermolecular hydrogen bonding, the a-relaxation process of the PEO blends will be hindered to
some extent, and thus it will require a higher tem-perature to become activated. Figure 10 displays E0 and tan d for the 90/10 and 50/50 poly(VB-a)/ PEO blends. In the tan d curve of the 50/50 pol-y(VB-a)/PEO blend, the peak centered at 60 8C is due to the melting of PEO (see also Fig. 4). Fur-thermore, the 50/50 poly(VB-a)/PEO blend exhib-its two values of tan d: one broad peak appears in the lower temperature region, which we attribute to the a relaxation of the PEO, and the other, in the higher temperature region (at ca. 3278C), rep-resents the a relaxation of the poly(VB-a); this lat-ter value is higher than that of pristine poly(VB-a) (301 8C). The result is different with polybenzoxa-zine blending with PCL: the a relaxation of poly(B-a) is reduced with the increase in the PCL con-tent.9The presence of two values of tan d for this blend suggests that it is immiscible. In addition, in Figure 10, we can observe a slight depression of the peak intensity with increasing PEO content. Because the damping property is provided by the ratio of the viscous and elastic components, we sur-mise that the reduced peak height is associated with lower segmental mobility and fewer relaxa-tion species, and thus it is indicative of stronger
Figure 10. E0and tan d of 50/50 and 90/10 poly(VB-a)/ PEO blends.
Figure 11. SEM micrographs of (a) 90/10, (b) 80/20, (c) 70/30, and (d) 60/40 poly(VB-a)/PEO blends.
hydrogen bonding for the 50/50 poly(VB-a)/PEO blend. Moreover, the peak width at half-height increases with increasing PEO content, and this is a result of decreasing network homogeneity.28 After PEO is added to poly(VB-a), the tempera-ture distribution at which the different mobile net-work segments become activated is increased.
Morphology of the Poly(VB-a)/PEO Blends
We investigated the morphology of the poly(VB-a)/ PEO blends with SEM. Figure 11 displays SEM micrographs of the chloroform-etched fracture sur-faces of the blends. We observed heterogeneous morphologies for each of the blends investigated, which correspond well to the results that we obtained through DMA; that is, the blends were phase-separated. For the 90/10 poly(VB-a)/PEO blend, after the rinsing of the PEO phase, we ob-served that PEO had been dispersed uniformly in the continuous matrix, with cavity diameters of about 0.5–1.5 lm [Fig. 11(a)]. With increasing PEO content, the blends displayed remarkably dif-ferent morphologies [Fig. 11(b–d)]. For the 80/20 poly(VB-a)/PEO blend [Fig. 11(b)], we found that the PEO domains began to interconnect and that they exhibited irregular shapes. Because the PEO phase had been dissolved in chloroform, the spher-ical particles, having a broad size distribution, were composed of the poly(VB-a) component. It is clear that phase inversion had begun to appear; indeed, we observed totally phase-inverted mor-phologies for blends having PEO contents above 20 wt %. The average size of the spherical par-ticles decreased with increasing PEO content in the blends, as indicated in Figure 11(c,d). These results are similar to those of the poly(B-a)/PCL blend system.29 The connected-globule structures visible in our SEM images imply the presence of a two-phase morphology of interconnected spherical domains of a poly(VB-a)-rich phase dispersed reg-ularly in a matrix of PEO. When the PEO volume fraction was further increased, the diameters of the poly(VB-a) particles became even smaller (av-erage: 0.4 lm), and their shapes became more reg-ularly spherical. The greater volume fractions of PEO probably cushioned the poly(VB-a) spheres from direct impingement. Similar observations have been described for some thermoplastic-modified epoxy systems,22,30 for which the sizes of the ep-oxy particles decreased with an increasing con-centration of the thermoplastic; this phenomenon was attributed to the deceleration of phase
sepa-ration and coarsening that resulted from the inclusion of a high-viscosity thermoplastic.31
CONCLUSIONS
We synthesized and characterized a vinyl-termi-nated benzoxazine monomer (VB-a) featuring terminal vinyl groups. We prepared VB-a/PEO blends by the solution blending of this benzoxa-zine monomer and PEO, followed by thermal cur-ing of the benzoxazine. DSC analysis indicated that a single Tg existed for the uncured blend and that its value increased with increasing con-tent of VB-a. In addition, both Tmand the degree of crystallinity of the PEO component in the blend decreased with increasing VB-a content. The addition of PEO to the poly(VB-a) network greatly increased the hydrogen bonding and strongly influenced its thermal properties. FTIR spectra indicated that, after curing, hydrogen bonds existed between the ether groups of PEO and the hydroxyl groups of poly(VB-a). At rela-tively low volume fractions of PEO in the blends, a full-scale phase inversion was observed in the cured poly(VB-a)/PEO networks. With an increas-ing PEO volume fraction in the blends, the spheres not only became smaller in size but also became more regularly spherical in their geome-try as a result of a lower degree of impingement. DMA indicated that Tgincreased from 3018C for the neat poly(VB-a) to 327 8C for the 50/50 poly (VB-a)/PEO blend as a result of the latter system’s increased hydrogen bonding.
The authors thank the National Science Council (Taiwan, Republic of China) for supporting this re-search financially under contract no. NSC-92-2216-E-238-002.
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