Results and Discussion

The ¹H NMR spectrum of B-ala as shown in Figure 3-1 established that the structure of B-ala was recorded in deuterated chloroform (CDCl3) solution at 25° C by using a Varian UNITY INOVA-400 NMR spectrometer. The two multiples at 5.25 and 5.95 ppm were typical for the protons of ＝CH2 and ＝CH- in the allyl group, respectively. The protons of -CH2- of the allyl group showed a doublet at 3.36 ppm.

The characteristic protons of oxazine ring appeared at 3.92 and 4.82 ppm as assigned to -Ar-CH2-N- and -OCH2-N-, respectively, while the aromatic protons appeared as a multiplet at 6.77-7.0 ppm. Besides, ¹³C NMR spectrum and Mass spectrum of B-ala are shown in Figure 3-2 and 3-3 to provide evidences for the synthesis of B-ala monomer is successful.

The B-ala monomer contained the N-allyl group that can polymerize through free radical polymerization with the aid of a free radical initiator. The AIBN is known as an effective free radial initiator for addition polymerization of the N-allyl group [4, 6].

Figure 3-4 shows the DSC thermograms of the bifunctional allyl-containing benzoxazine monomer B-ala, with and without AIBN as an initiator. Two exothermic peaks were observed for B-ala, which correspond to the crosslinking of the N-allyl group (260 °C) and the ring-opening polymerization (210 °C) of the benzoxazine (Figure 3-5). [4] Another exothermic peak appeared at 120 °C when AIBN was added, which can be attributed to the breaking of its azo group. As shown in Figures 3-6 (a) and 3-6 (b), the characteristic absorption band assigned to the allyl group appeared at 1644 cm^-1 (stretching of C=C). The intensity of this allyl peak (1644 cm^-1) decreased in the presence of AIBN and with the increase of curing time (Figure 3-6 (b)), implying that the polymerization of N-allyl groups took place by the free radical mechanism. However, the tetrasubstituted benzene mode at 1484 cm^-1, corresponding

to benzoxazine polymerization via ring opening, was somewhat unexpected to have increased when the AIBN was added (Figure 3-6 (c)). Ishida et al. [7] studied the curing behavior of benzoxazine monomer and found that the benzoxazine precursor undergoes an autocatalytic type of curing mechanism as catalyzed by the phenol group formed by the ring opening of the oxazine ring. Based on a previous report, [8]

the decomposition heat from the breaking of the azo group as observed for AIBN at 363° K was around 123 J/g. Furthermore, certain phenolic-containing oligomers were formed due to the heat release from the breaking of the azo group of the AIBN. The phenol group of these oligomers provided catalytic effect for the ring opening of the oxazine ring in the subsequent curing. More oligomers formed during the earlier stages of the curing process resulting in the higher rate of ring-opening crosslinking of the oxazine ring. More AIBN was added to form more oligomers, thereby releasing more heat from the breaking of more azo groups and resulting in a higher rate of ring-opening crosslinking of the oxazine ring (Figure 3-6 (d)). As a result, the radical initiator AIBN, not only initiated the free radical polymerization for the allyl group but also catalyzed the ring-opening reaction of benzoxazine.

Table 3-1 lists the surface roughness and the advancing contact angles of the three test liquids and the respective surface free energies (γs) of B-ala and B-ala/AIBN PBZs with various curing times. The lowest surface free energy obtained was 15.3 mJ/m² from the B-ala/AIBN=5:1 PBZ system after 24 h of curing at 120 °C. In both B-ala and B-ala/AIBN PBZ systems at the curing temperature of 120 °C, the advancing contact angles of all the three test liquids (water, ethyleneglycol and diiodomethane) increased as curing time was increased. At the same curing time, all the advancing contact angles from B-ala/AIBN PBZ films were relatively higher than those from B-ala PBZ films, especially in the diiodomethane liquid system. The

surface free energy, γs, was caculated by using van Oss and Good’s three-liquid method [9]and two-liquid geomertric method. [10] The extremely low surface free energy (γ_S = 15.3 mJ/m²) from the B-ala/AIBN=5:1 after 24 h of curing was even lower than that of pure Teflon (γ_S = 16.43 mJ/m²). [3] This phenomenon can be explained in terms of the more intramolecular hydrogen bonding formed. To determine the extent of hydrogen bonding within B-ala and B-ala/AIBN PBZs, FTIR curve-resolving on all hydrogen bondings were performed. Figure 3-7 displays the FTIR spectra of the B-ala/AIBN PBZ thin films as a function of curing time (2, 4, 8, and 24 h) at 120 °C. The FTIR spectrum of the pure B-ala system is similar to the B-ala/AIBN system. The fraction of the peak at 3207 cm^-1, corresponding to the OH···N intramolecular hydrogen bond, increased when the curing time increased, while the OH···O intermolecular hydrogen bond at 3417 cm^-1 slight decreased when the curing time increased. When AIBN was added, the rate of the ring-opening reaction and the fraction of the intramolecular hydrogen bonding both increased (Table 3-2). The surface free energies versus the curing times at 120 °C of various molar ratios for B-ala monomer to AIBN are shown in Figure 3-8. The lowest surface free energy for the B-ala/AIBN=5:1 PBZ system was 15.3 mJ/m² which was even lower than the BA-m PBZ system cured in 1h at 210° C (16.4 mJ/m²). [3]

The thermal curing process of this B-ala/AIBN PBZ system is at 120 °C, therefore, it can be used to modify many polymer substrates. Figure 3-9 shows the advancing contact angles of water, ethylene glycol, and diiodomethane on the poly(4-vinyl pyridine) thin film before and after modification with B-ala/AIBN = 5/1 PBZ, those advancing contact angles of three test liquids all increase substantially.

Poly(4-vinyl phenol) film and polycarbonate substrates give the same trend as shown in Table 3-3.