Chapter 2 Synthesis of Siloxane-Imide-Containing Benzoxazines
2.1.2 Synthesis of the siloxane-containing dihydroxyl (A1-OH)
The siloxane-imide–containing dianhydride A1 and the siloxane-containing dihydroxyl compound A1-OH were synthesized according to the method reported by Li et al. [3] (Scheme 2-1). The siloxane— imide-containing dianhydride (A1) (30g, 0.065 mol) was dissolved in 90ml of dimethyl-formamide (DMF), and 4-aminophenol (14.9g, 0.136mol) in 40ml of DMF was gradually added. The solution was stirred for 6 hrs at ice-bath conditions, followed by imidization using a Dean-Stark instrument at 130℃ reflux for 4 hrs. A 1-OH in solid powder was obtained after vacuum drying, and
it was recrystallized from isopropanol (38g, yield = 91%, mp = 123℃).
Scheme 2-1. Preparation of the siloxane-imide–containing dianhydride (A1) and dihydroxyl compound (A1-OH)
The chemical structure of the light brown powder, A1-OH, was confirmed from
1H-NMR and FT-IR. 1H-NMR (CDCl3, ppm) δ: 0.01-0.02 (m, 12H), 0.61 (m, 2H), 1.54-1.62 (m, 8H), 2.74 (m, 2H), 2.78 (m, 2H), 3.12-3.17 (m, 4H), 6.70-6.73 (d, 4H), 6.90-6.94 (d, 4H), 7.42 (s, 2H). FT-IR: imide 1789 and 1720 cm-1, OH 3100-3500 cm-1 (yield: 88%)
2.1.3 Synthesis of the siloxane-imide–containing benzoxazine
N,N´-bis(N-phenyl-3,4-dihydro-2H-benzo[1,3]oxazine)-5,5´-bis(1,1´,3,3´-tetrameth
yldisiloxane-1,3-diyl)-bis(norborane-2,3-dicarboximide) (BZ-A1)
Aniline (1.88 g, 0.02 mol) was added dropwise into a mixture of A1-OH (6.44 g, 0.01 mol), paraformaldehyde (1.26 g, 0.04 mol), and 1,4-dioxane (100 mL) in a 250-mL round-bottom flask equipped with a magnetic stirrer bar. (Scheme 2-2) The mixture was then heated under reflux at 115 °C for 20 hrs, and gradually became homogeneous and turning dark brown. The resulting mixture was filtered and the solvent was evaporated under vacuum. The residue was dissolved in ethyl acetate and
washed five times sequentially with 1 N aqueous NaOH and distilled water.
Evaporation of the solvent and vacuum drying in an oven provided BZ-A1 as a brown powder (73.0%).
Scheme 2-2. Preparation of the siloxane-imide–containing benzoxazine monomer BZ-A1 from A1 and A1-OH
BZ-A1 was prepared according to Scheme 2 and its chemical structure was confirmed using FT-IR and 1H NMR spectroscopies and liquid chromatography/mass spectrometry (LC-MS). The IR spectrum of BZ-A1 (Figure 2-2) displays characteristic absorptions of a benzoxazine structure at 1498 cm–1and 1030 cm–1 (vibrations of the trisubstituted benzene ring), 1328 cm–1 (CH2 wagging of the oxazine unit) and 1230 cm–1 (asymmetric C–O–C stretching). The 1H NMR spectrum of BZ-A1 (Figure 2-3) displays aromatic protons at 6.60–7.40 ppm and characteristic peaks attributed to methylene units (oxazine Ar-CH2-N) at 5.30 and 4.60 ppm, respectively. LC-MS (Figure 2-4) provided a molecular weight of 881.1 g/mol, consistent with the calculated formula weight.
4000 3500 3000 2500 2000 1500 1000 500
Figure 2-2. IR spectrum of the siloxane-imide–containing benzoxazine BZ-A1.
Figure 2-3. 1H NMR spectrum of the siloxane-imide–containing benzoxazine BZ-A1.
8 6 4 2 0 ppm
100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 m/z
%
0
100 x15 2.01e6
108.12
266.08 299.31
401.97 406.37
714.67
568.27 881.09
Figure 2-4. LC/Mass spectrum of the siloxane-imide–containing benzoxazine BZ-A1.
2.2 Synthesis of Siloxane-Imide-Containing Benzoxazine (BZ-A6)
2.2.1 Materials
5-Norbornene-2, 3-dicarboxylic anhydride (nadic anhydride) was purchased from Alfa Aesar (USA). Hydride-terminated polydimethylsiloxane, DMS-H03, was purchased from Gelest (USA) with molecular weight of ca. 400–500. Platinum divinyltetramethyldisiloxane complex was purchased from Gelest (USA). All chemicals were purified prior to use. 1,4-Dioxane and paraformaldehyde (95%) were purchased from TEDIA (USA) and Showa Chemicals (Japan), respectively. Ethyl acetate (EtOAc, 99.9%) was used as received from Mallinckrodt (USA). Aniline (99%), ethylene glycol (EG, ≥99%), and diiodomethane (DIM, 99%) were obtained from Aldrich (USA).
2.2.2 Synthesis of dinoborane anhydride terminated polydimethylsiloxane (A6) The synthesis of the siloxane-imide–containing dianhydride A1 has been reported in the literature. [3, 4] The higher-molecular-weight siloxane-imide– containing dianhydride, which is called A6, was prepared with reference to these previous methods. Pt catalyst (0.8 mL) was added dropwisely into a solution of nadic anhydride (82.1 g) in toluene (400 mL) in a three-neck round-bottom flask while stirring with a magnetic stirrer bar. DMS-H03 (112.5 g) was gradually added into the solution and then heated react to 70 °C for 48 hrs. The resulting mixture was filtered and the solvent was evaporated under vacuum. After the removal of residue nadic anhydride, A6 was obtained as a transparent liquid (yield: 75.0%). The chemical structure of the transparent liquid product, A6, was confirmed with 1H-NMR (Varian UNITY Inova-400NMR spectrometer) and FT-IR (Perkin Elmer, Spectrum one). 1H-NMR
(CDCl3, 400 MHz) δ: 0.03~0.05 ppm (12H, CH3–Si–CH3), 0.65 ppm (2H, –CH–Si–), 1.55~1.66 ppm (8H, cyclopentane CH2), 3.39~3.43 ppm (4H, –C(=O)). FT-IR (KBr):
1859 cm-1, 1778 cm-1 (anhydride, C=O stretching), 1222 cm-1 (C–Si stretching), 1078 cm-1 (Si–O–Si stretching); no 1680 cm-1 (norborane, C=C stretching) or 2150 cm-1 (Si–H, stretching).
2.2.3 Imidization of siloxane-imide–containing dianhydride (A6-OH)
4-Aminophenol (5.3 g, 0.0484 mol) in DMF (30 mL) was added gradually to a stirred solution of the siloxane-imide–containing dianhydride A6 (16.5 g, 0.022 mol) in dimethylformamide (DMF, 30 mL) in a 250-mL round-bottom flask (Scheme 2-3). The solution was stirred for 6 hrs in an ice-bath and the imidization was performed using a Dean–Stark apparatus. A6-OH was obtained as a viscous dark brown liquid after vacuum drying (yield: 86.8%). 1H-NMR (CDCl3) δ: 0.03~0.08 ppm (12H, CH3–Si–CH3), 0.65 ppm (2H, –CH–Si–), 1.55~1.66 ppm (8H, cyclopentane CH2), 6.23 ppm (2H, aromatic C–OH), 6.73~6.96 ppm (8H, benzene). FT-IR (KBr): 1720 cm-1 (imide), 3100~3500 cm-1 (OH, broad band).
2.2.4 Synthesis of siloxane-imide-containing benzoxazine (BZ-A6)
Aniline (3.8 g, 0.04 mol) was added dropwisely into a mixture of A6-OH (18.95 g, 0.02 mole), paraformaldehyde (2.4 g, 0.08 mole), and 1,4-dioxane (120 ml) in a 250 ml round-bottom flask equipped with a magnetic stirrer bar (Scheme 2-4). The mixture was then heated under reflux at 115 °C for 20 hrs, gradually becoming homogeneous and turning dark brown. The resulting mixture was filtered and the solvent was evaporated under vacuum. The residue was dissolved in ethyl acetate and washed five times sequentially with 0.5 N aqueous NaOH and distilled water. Evaporation of the
solvent and vacuum drying in an oven provided BZ-A6 as a viscous dark brown liquid product (yield: 87.7%). 1H-NMR (CDCl3) (Figure 2-5) δ: 6.70~7.30 ppm (aromatic protons), 5.35 ppm (OCH2N), 4.65 ppm (Ar–CH2–N). FT-IR (KBr) (Figure 2-6): 1256 cm-1 (C–O–C, stretching), 1178 cm-1 (C–N–C, stretching), 1307 cm-1 (CH2, wagging of oxazine), 1502 cm-1 (trisubstituted benzene ring).
8 7 6 5 4 3 2 1 0
Figure 2-5.1H-NMR spectrum of the siloxane-imide–containing benzoxazine BZ-A6.
4000 3500 3000 2500 2000 1500 1000 500
Si CH3
CH3 O Si
CH3
CH3 O Si CH3
CH3 N N
O
O
O
O O O
N N
n
1178cm-1 1502cm-1
v (cm-1)
T (%)
1256cm-1 1307cm-1
Figure 2-6. FT-IR spectrum of the siloxane-imide–containing benzoxazine BZ-A6.
O
Scheme 2-3. Syntheses of compounds A6 and A6-OH
Si
Scheme 2-4. Preparation of compound BZ-A6
References
[1] Holly, F. W.; Cope, A. C. J. Am. Chem. Soc. 1944, 66, 1875.
[2] Ghosh, N. N.; Kiskan, B.; Yagci, Y. Porg. Polym. Sci. 2007, 32, 1344.
[3] Li, H.T; Chang H.R.; Wang, M. W; and Lin, M. S. Polym Int 2005, 54, 1416.
[4] Eddy, V. J; Hallgren, J. E. and Robert, E. J Polym Sci Part A: Polym Chem 1990, 28, 2417.
Chapter 3
Curing Behavior of Siloxane-Imide-Containing Benzoxazines
To understand the polymerization reaction of benzoxazines, an understanding of the chemical structure of its oxazine ring is very important. The ring opening of the benzoxazine was first discussed by Burke et al. [1] In the reaction of 1,3-dihydrobenzoxazine with a phenol, having both ortho and para position free, it was found that aminoalkylation occurred preferentially at the free ortho position to form a Mannich base bridge structure, along with small amount reaction at para position. A cross-linked network structured polybenzoxazines, with higher Tg and degradation temperature, can be obtained when benzoxazines undergo polymerization.
It has been observed that during synthesis of a difunctional benzoxazine (from bisphenol A, formaldehyde and aniline) form by the subsequent reactions between the rings and ortho position of bisphenol A hydroxyl groups. These free phenolic hydroxyl structure containing dimmers and oligomers trigger the monomer to be self-initiated towards polymerization and crosslinking reactions. [2] The curing behavior of siloxane-imide-containing benzoxazines, BZ-A1 and BZ-A6, are discussed in this section.
3.1 Curing behavior of the siloxane-imide–containing benzoxazine BZ-A1
Typically, benzoxazines undergo exothermic ring opening reactions at ca.
200–250 °C, which can be monitored using DSC. DSC was performed using a TA Instrument DSC-Q10 apparatus operated at a heating rate of 10 °C/min under a N2
atmosphere. The gas flow rate was 40ml/ min. Benzoxazine samples of approximately 5 mg were scanned in hermetic aluminum sample pans. The reaction point of the bisphenol A–type benzoxazine Ba is 228.7 °C; the energy of the exothermic ring opening reaction is 296.0 J/g (Figure 3-1). The thermogram of BZ-A1 in Figure 3-1 reveals a ring opening exothermic reaction having an onset temperature at 194.9 °C and a peak point at 232.7 °C. The exothermic energy of BZ-A1 is 173.7 J/g; i.e., it is lower than that of Ba, presumably due to molecular weight effect, molecular weight of BZ-A1 (879 g/mol) is significantly higher than that of Ba (462 g/mol). The PBZs of Ba (PBa) and BZ-A1 (PBZ-A1) were then cured in an oven under the curing conditions listed in Table 3-1.
Exo Up Universal V4.4A TA Instruments
Figure 3-1. DSC thermograms of Ba and BZ-A1.
Table 3-1. Curing conditions for PBZs
Benzoxazine Ba BZ-A1
200 °C/2 hrs + 230 °C/2 hrs 200 °C /2 hrs + 230 °C/4 hrs Curing conditions
200 °C /2 hrs + 230 °C/6 hrs
PBZs usually exhibit good thermal properties after polymerization. [3] The glass transition temperature of PBZ-A1 after cross-linking was 186.1 °C (Figure 3-2), which is substantially higher than that of typical PBZs (PBa: Tg= 150.0 ℃). [4] In general, the longer and flexible of siloxane segments in the matrix structure results in lower of Tg(Tg from tan δ peak of CP-F-Bz/BATMS-Bz-100 is 116 ℃) as discussed by Liu et al. [5] Our PBZ-A1 structure features both siloxane and imide segments in the benzoxazine monomer where the imide segment tends to raise the glass transition temperature.
186.1 deg. C
-0.60 -0.55 -0.50 -0.45 -0.40 -0.35
Heat Flow (W/g)
100 120 140 160 180 200 220
Temperature (°C)
Exo Up Universal V4.4A TA Instruments
Figure 3-2. Glass transition temperature (Tg) of PBZ-A1, determined from the DSC trace.
3.2 Curing behavior of the siloxane-imide–containing benzoxazine BZ-A6
In general, benzoxazines undergo exothermic ring opening at temperatures of ca.
200–250 °C [6-9] which can be monitored using DSC. The thermogram of BZ-A6 in Figure 3-3 reveals a ring opening exothermic reaction having an onset temperature at 153.7 °C and a peak maximum at 214.2 °C with exothermic energy of 57.9 J/g. After curing at 200℃ for 2 hrs, the reaction heat is decreased to be 37.9 J/g from 57.9 J/g.
We performed the polymerization of BZ-A6 using a two-step process; the first step
involved benzoxazine ring opening at 200 °C and the second involved post curing at a 230 °C. PBZs were cured in an oven under the curing conditions listed in Table 3-2.
BZ-A6 monomer
200℃/ 2h
Tp = 214.2 ℃ Delta H = 57.9 J/g
Tp=221.1 ℃ Delta H = 37.9 J/g
200℃/ 2h + 230℃/ 2h
200℃/ 2h + 230℃/ 4h
200℃/ 2h + 230℃/ 6h
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4
Heat Flow (W/g)
75 125 175 225 275
Temperature (°C)
Exo Up Universal V4.4A TA Instruments
Figure 3-3. DSC thermograms of BZ-A6 monomer and polymerized BZ-A6 (after curing).
Table 3-2. Curing conditions for PBZs
Benzoxazine Ba BZ-A1 BZ-A6
200 °C/2 hrs + 230 °C/2 hrs 200 °C/2 hrs+ 230 °C/4 hrs Curing conditions
200 °C/2 hrs+ 230 °C/6 hrs
References
[1] Burke, W. J.; Bishop, J. L.; Glennie, E. L. M.; Bauer, W. N. J. Org. Chem.1965, 30, 3423.
[2] Ning, X.; Ishida, H. J. Polym. Sci., Part A: Polym. Chem. 1994, 32, 1121.
[3] Ghosh, N. N.; Kiskan, B. and Yagci, Y. Prog Polym Sci 2007, 32, 1344.
[4] Ishida, H. and Allen, D. J. J Polym Sci Part B: Polym Phys 1996, 34, 1019.
[5] Liu, Y. L.; Hsu, C. W. and Chou, C. I. J Polym Sci Part A: Polym Chem 2007, 45, 1007.
[6] Chen, K. C.; Li, H. T.; Chen, W. B.; Liao, C. H.; Sun, K. W. and Chang, F. C.
Polym Int in press
[7] Takeichi, T.; Kano, T and Agag, T. Polymer 2005, 46, 12172.
[8] Agag, T. and Takeichi, T. Macromolecules 2003, 36, 6010.
[9] Takeichi, T.; Agag, T. and Zeidam, R. J Polym Sci Part A: Polym Chem 2001, 39, 2633.
Chapter 4
Thermal/ Mechanical Properties of Siloxane-Imide-Containing Polybenzoxazines
The physical, mechanical and thermal properties of polybenzoxazines are primarily decided by the nature of the diphenol and diamine. The properties of polybenzoxazines are shown to compare very favorably with those of conventional phenolic and epoxy resins. DMA reveals that these candidate resins for composite applications possess high modulus and glass transition temperatures. Long-term immersion studies indicate that they have low water absorption and loe saturation compact. Impact, tensile and flexural properties are also good. [1] BZs are cured usually in the temperature window of 160-220℃. The polymer exhibit Tg in the range 160-340℃ depending on the structure, and have higher stability. The high TGA decomposition onset temperature and char yield are attributed to the very strong intramolecular H-bonding between phenolic OH and the Mannich bridge. [2]
In this section, we discussed the thermal and mechanical properties of polymerized siloxane-imide-containing PBZ-A1 and PBZ-A6.
4.1 Thermal stability of the poly-siloxane-imide–containing benzoxazine PBZ-A1
4.1.1 Materials and Characterization
The bifunctional bisphenol A–type benzoxazine (Ba, Figure 4-1) was purchased from Shikoku Chemicals (Japan). The siloxane-imide-containing benzoxazine, BZ-A1, was synthesized from the according method in chapter 2, the structure is shown in
Figure 4-2. DSC was performed using a TA Instrument DSC-Q10 apparatus operated at a heating rate of 10 °C/min under a N2 atmosphere. The gas flow rate was 40ml/ min.
Benzoxazine samples of approximately 5 mg were scanned in hermetic aluminum sample pans. TGA was performed using a TA Instrument TGA-Q500 apparatus operated at a heating rate of 20 °C/min under an atmosphere of N2 or air, respectively.
An energy dispersive system (EDS) was used for element test, which was recorded
Figure 4-1. Structure of the bifunctional bisphenol A–type benzoxazine Ba.
O N
Figure 4-2. Structure of the BZ-A1.
4.1.2 TGA of the poly-siloxane-imide–containing benzoxazine PBZ-A1
PBZs usually exhibit good thermal properties after polymerization. [3] The glass transition temperature of PBZ-A1 after cross-linking was 186.1 °C (Figure 4-3), which is substantially higher than that of typical PBZs (PBa: Tg= 150.0 ℃). [4] In general, the longer and flexible of siloxane segments in the matrix structure results in lower of Tg(Tg from tan δ peak of CP-F-Bz/BATMS-Bz-100 is 116 ℃) as discussed by Liu et al. [5] Our PBZ-A1 structure features both siloxane and imide segments in the
benzoxazine monomer where the imide segment tends to raise the glass transition temperature.
Bisphenol-A is one of the phenolic compounds often used as the starting material for the synthesized of polybenzoxazines. PBa shows high decomposed temperature (T5% c.a. 300-330 ℃) and high char yield (c.a. 30-42 %) from TGA. [3, 6-9] Liu et al.
[5] investigated that siloxane-containg polybenzoxaizne, CP-F-Bz/BATMS-Bz-100, has Td at 369℃ in air. Figure 4-4 displays TGA thermograms recorded in air and results are summarized in Table 4-1. The 5% and 10% weight loss temperatures (T5%
loss and T10% loss, respectively) for PBZ-A1 cured at 200 °C/ 2hrs and 230 °C/ 2hrs were 380.1 °C and 441.1 °C, respectively, which are both higher than those of PBa or siloxane-containing polybenzoxazine. The PBZ-A1 shows higher thermal stability than PBa because of the presence of the siloxane-imide–containing segment. In Liu et al.
siloxane-containing polybenzoxazine TGA study, they found high thermal stability silica layers formation during the thermal oxidation process and the layer structured protect the polybenzoxazine. [10] PBa-PDMS hybrids was investigated that introduced PDMS into PBa results in the improvement of thermal stability of the hybrid. [11] The better thermal stability of PBZ-A1 with higher decomposed temperature and high char amount is come from siloxane and imide group.
In contrast, the presence of siloxane-imide groups improved the thermo-oxidative stability of the benzoxazine by increasing the char yield to 10–12 wt% in air. This char yield is close to the content of inorganic content (Si–O–Si, 8.2%) in the BZ-A1 structure. EDS analysis was employed to analyze the elemental composition of the PBZ-A1 residue after TGA testing in air. Figure 4-5 displays an image of the residue from PBZ-A1 and its EDS data. The silicone content in the residue was significantly higher than those of C and O atom, the residue from PBZ-A1 after TGA testing in air
was primarily inorganic in nature. Thus, the siloxane units of BZ-A1 provide an inorganic content in its structure, therefore, improve its thermo-stability properties after cross-linking.
The same phenomena occurred in the TGA thermograms recorded under a N2
atmosphere (Figure 4-6, Table 4-2). The 5% weight loss temperature of PBa was ca.
328–337 °C under the N2 atmosphere, whereas that of PBZ-A1 was significantly higher (ca. 355–362 °C). The temperatures for 5 and 10 wt% losses of PBZ-A1 were both higher than those for PBa. PBZ-A1 also featured a high weight residue after high temperature decomposition. The char yield of PBZ-A1 after curing at 200 °C for 2 hrs and then 230 °C for 2 hrs was high (48.0 %), i.e., it was improved by the presence of the siloxane-imide groups. It appears that the PBZ-A1 has the potential use as flame-retardant material.
186.05°C
-0.60 -0.55 -0.50 -0.45 -0.40 -0.35
Heat Flow (W/g)
100 120 140 160 180 200 220
Temperature (°C)
Exo Up Universal V4.4A TA Instruments
Figure 4-3. Glass transition temperature (Tg) of PBZ-A1, determined from the DSC trace.
Figure 4-4. TGA thermograms of PBa and PBZ-A1 (in air).
Table 4-1. Thermostabilities of the cured PBZs PBa and PBZ-A1 (in air) Polymer Curing Conditions T5% loss
(°C)
T5% loss: Temperature at which the weight loss was 5%.
T10% loss: Temperature at which the weight loss was 10%.
Td: Decomposition temperature, onset point temperature.
-20
Figure 4-5. Residue and EDS analysis of PBZ-A1 after TGA testing.
Figure 4-6. TGA thermograms of Ba and BZ-A1 (under N2).
0.0 0.5 1.0 1.5 2.0
Table 4-2. Thermostabilities of the cured PBZs PBa and PBZ-A1 (under N2) Polymer Curing Conditions T5% loss
(°C)
T5% loss: Temperature at which the weight loss was 5%.
T10% loss: Temperature at which the weight loss was 10%.
Td: Decomposition temperature, onset point temperature.
4.2 Thermal stability of the poly-siloxane-imide–containing benzoxazine PBZ-A6
4.2.1 Materials and Characterization
The bifunctional bisphenol A–type benzoxazine (Ba, Figure 4-1) was purchased from Shikoku Chemicals (Japan). The siloxane-imide-containing benzoxazine, BZ-A6, was synthesized from the according method in chapter 2, the structure is shown in Figure 4-7. DSC was performed using a TA Instrument DSC-Q10 apparatus operated at a heating rate of 10 °C/min under a N2 atmosphere. The gas flow rate was 40ml/ min.
Benzoxazine samples of approximately 5 mg were scanned in hermetic aluminum sample pans. TGA was performed using a TA Instrument TGA-Q500 apparatus operated at a heating rate of 20 °C/min under an atmosphere of N2 or air, respectively.
Si
4.2.2 TGA of the poly-siloxane-imide–containing benzoxazine PBZ-A6
Polybenzoxazines usually exhibit good thermal properties. [3] Figure 4-7 displays TGA thermograms recorded under air atmosphere. No residue remained after burning PBa at high temperature, the char was almost zero at 850 °C. The char yield of PBZ-A1 at 850 °C was 10–11 wt% and the elemental analysis confirmed that the residue was inorganic silicon oxide. [12] PBZ-A6 exhibited a higher char yield of 16–17 wt%, presumably due to the longer siloxane chain in the BZ-A6 backbone than that in BZ-A1. Thus, the char yield increased upon increasing the siloxane content in the polymer. The PBa started to decomposed, T5% loss, around 330-350℃ and it was obviously that PBZ-A1 has higher decomposed temperature to 380-395℃ from the result in Figure 4-8. Polybenzoxazine which contained the siloxane-imide segment in the main-chain, PBZ-A1 and PBZ-A6, could improve the thermal stability. The highest T5% loss was observed in the PBZ-A6 curve. The weight residue of PBZ-A6 is 16-18wt% at 850℃ under air atmosphere, which is list in Table 4-3. It was obviously that the char yield is higher than that of shorter siloxane containing PBZ-A1 (10-12 wt%) or the conventional bisphenol A type polybenzoxazine, PBa (almost 0%). It was indicated that longer siloxane chain could make further improvements in the thermal stability since the more siloxane contaning was incorporated into the main chain of PBZ-A6.
PBZ-A6
Figure 4-8. TGA thermograms of PBa, PBZ-A1 and PBZ-A6 (in air).
Table 4-3. Thermostability of cured polybenzoxazine PBa, PBZ-A1 and PBZ-A6 (in air)
2 hrs 337.3 365.5 349.3 0.2%
4 hrs 345.0 376.8 346.4 -0.3%
PBa 2 hrs
6 hrs 349.1 381.5 357.9 -0.1%
2 hrs 380.1 441.1 472.2 10.1%
4 hrs 389.4 444.2 480.7 11.3%
PBZ-A1 2 hrs
6 hrs 392.0 449.3 478.1 11.6%
2 hrs 435.4 497.9 498.3 17.8%
4 hrs 426.7 491.0 498.6 17.4%
PBZ-A6 2 hrs
6 hrs 433.3 482.4 491.5 16.9%
T5% loss: The temperature for which the weight loss is 5%.
T10% loss: The temperature for which the weight loss is 10%.
Td: The decomposed temperature
TGA analysis has revealed that PBa exhibits high decomposition temperature (T5%, ca. 300–330 °C) and high char yield (ca. 30–42%). [3, 6-8] In a previous study, we found that PBZ-A1 exhibited superior thermal properties relative to that of PBa.
[12] Figure 4-9 displays TGA thermograms recorded under N2 atmosphere and Table 4-4 summarizes the results. The 5 and 10% weight loss temperatures (T5% loss and T10%
loss, respectively) for PBZ-A6 (437.1 and 481.3 °C, respectively) were the highest among the polymers investigated in this study. The thermal decomposition temperature of PBZ-A6 was in the range 460–471 °C. PBZ-A1 and PBZ-A6 exhibited higher thermal stability than PBa because of the presence of the siloxane-imide–containing segment. Furthermore, the siloxane content of PBZ-A6 is higher than that of PBZ-A1 and the decomposition temperature of PBZ-A6 was higher accordingly. The siloxane-imide–containing PBZs also featured high weight residues after TGA. The highest char yield was 50.9% from PBZ-A6 due to the presence of longer siloxane-imide group. PBZ-A6 exhibited good thermal stability, the highest decomposition temperature, and the highest char yield. It appears that incorporating siloxane and imide moieties into the benzoxazine main chain can significantly enhance the thermal properties of PBZs, providing the potential to be used as flame-retardant materials.
PBa
Figure 4-9. TGA thermograms of PBa, PBZ-A1 and PBZ-A6 (in N2).
Table 4-4. Thermostability of cured PBZ PBa, PBZ-A1, and PBZ-A6 (in N2) Curing Conditions
2 hrs 334.6 356.8 344.2 34.3%
4 hrs 328.8 360.7 342.7 42.7%
PBa 2 hrs
6 hrs 336.5 369.8 341.6 46.3%
2 hrs 355.7 417.8 452.9 48.0%
4 hrs 361.5 427.2 448.4 48.4%
PBZ-A1 2 hrs
6 hrs 358.5 415.8 446.5 49.3%
2 hrs 437.1 474.2 471.0 45.1%
4 hrs 437.2 481.3 459.9 48.1%
PBZ-A6 2 hrs
6 hrs 430.6 477.4 463.7 50.9%
T5% loss: Temperature at which the weight loss reached 5%.
T10% loss: Temperature at which the weight loss reached 10%.
Td: Decomposition temperature
4.2.3 DMA of the poly-siloxane-imide–containing benzoxazine PBZ-A6
Figure 4-10 displays DMA thermograms of the PBZ-A6 under three curing conditions and results are summarized in Table 4-5. The curing profiles revealed that the storage modulus at room temperature was 600–800 MPa which is much lower than conventional PBa’s. In general, a higher shear storage modulus in the rubbery state indicates a polymer having a high crosslinking density [5]. The storage modulus reached the highest value after longer post curing time at rubbery state which was indicated at 200℃ and 220℃ in Table 4-5. Thus, a longer curing time improved the crosslinking density as would be expected. These results are consistent with the fact that PBZ-A6 exhibited the highest Tg (186.4 °C) of the studied polymers. From a previous study [11] the storage modulus of the brittle PBa was found to be ca. 3.2 GPa at room temperature and Tg(derived from tan δ) of 174 °C.
In generally, it is difficult to obtain free-standing PBZ films without adding plasticizers. Since the siloxane-imide–modified benzoxazine PBZ-A6 exhibits superior flexibility and toughness, PBZ-A6 readily formed a free-standing, bendable film after polymerization at a thickness of ca. 200 μm (Figure 4-11). Notably, the PBZ-A6 film exhibited not only excellent flexibility but also a high value of Tg due to the presence
In generally, it is difficult to obtain free-standing PBZ films without adding plasticizers. Since the siloxane-imide–modified benzoxazine PBZ-A6 exhibits superior flexibility and toughness, PBZ-A6 readily formed a free-standing, bendable film after polymerization at a thickness of ca. 200 μm (Figure 4-11). Notably, the PBZ-A6 film exhibited not only excellent flexibility but also a high value of Tg due to the presence