Hybrid Organic-Inorganic Composites - American Chemical Society

Chapter 23

Preparation and Mechanical Properties of Polybenzoxazole—Silica Hybrid Materials

Downloaded by UNIV OF SYDNEY on May 3, 2015 | http://pubs.acs.org Publication Date: March 21, 1995 | doi: 10.1021/bk-1995-0585.ch023

J . P. Chen1, Z.Ahmad2,4,Shuhong Wang2,5, J. E . M a r k 2 , and F. E . Arnold3 1Systran Corporation, Dayton, O H 45432 2Department of Chemistry and Polymer Research Center, University of Cincinnati, Cincinnati, O H 45221-0172 3Wright Laboratory, Materials Directorate, Wright-Patterson Air Force Base, Dayton, O H 45433 High molecular weight benzoxazole copolymers prepared from 4,4'-[2,2,2trifluoro-l-(trifluoro-methyl)ethylidene]bis[2-aminophenol], 4,4'-oxybis(benzoic acid) and 5-hydroxyisophthalic acid or 5-phosphonoisophthalic acid exhibited solubility in tetrahydrofuran. These thermo-oxidative stable copolymers with the Tg of ca. 330°C can be incorporated with alkoxysilane by the sol-gel method. Hydroxypolybenzoxazoles could be reacted with an isocynatosilane coupling agent and then hybrid consequently with silica. The resulting hybrid films containing one third of silica were transparent. The mechanical properties of the hybrid materials were highly dependent on the nature of the organic polymers. In general, the tensile modulus of the hybrid materials increased with addition of silica, but the elongation at break decreased at higher silica contents. Organic and inorganic hybrid materials prepared through sol-gel processing have the potential to possess the desired properties of both organic and inorganic components, such as high tensile modulus, scratch resistance, thermal and dimensional stability from inorganic network or toughness, flexibility, and lightweight from the organic portion. A particular interest in this area is to investigate a new hybrid material which would possess excellent mechanical properties for use in structural applications. Choosing a tough, processible, high temperature polymer and developing a well-controlled sol-gel process which can produce the hybrid material with a uniform structure are likely to be the two basic requirements in this investigation. Among the high temperature polymers (7,2), polybenzoxazoles show excellent thermal and chemical stabilities and good tensile strength. One of the benzoxazole polymers, poly(phenoxyphenyl-6F-benzoxazole) (PP6FAP) prepared from 4,4'-oxybis(benzoic acid) (PP) and 4,4-[2,2,2-trifluoro-l4Permanent address: Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan 5Current address: PPG Industries, Inc., 440 College Park Drive, Monroeville, P A 15146

0097-6156/95/0585-0297$12.00/0 © 1995 American Chemical Society

In Hybrid Organic-Inorganic Composites; Mark, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

HYBRID ORGANIC-INORGANIC COMPOSITES

298

PP-6FAP


Scheme I. High temperature benzoxazole polymer (PP-6FAP), Tg at 300°, is soluble in tetrahydrofuran. (trifluoromethyl)ethylidene]bis[2-aminophenol] (6FAP) (Scheme I), is soluble in tetrahydrofuran (3). The good solubility in a water miscible organic solvent of this polymer leads to the possibility of making hybrid materials from the benzoxazole polymers and metal alkoxides. Based on this polymer, we have successfully prepared a series of copolymers with hydroxy or phosphonic acid pendants (Scheme II). We anticipate that the polar pendants will increase the interaction and compatibility between organic polymers and oligomers of the metal oxides, and also to bridge the organic and inorganic phases through covalent bonding after the heat treatment of dried gel or by applying a silane coupling agent. The synthesis of the functionalized copolymers, the preparation of hybrid gels from the copolymers and tetramethoxysilane, and their mechanical properties are reviewed in this paper. Experimental Monomer Compounds. 5-Hydroxyisophthalic acid (HIPA) and 4,4-oxybis(benzoic acid) (PP) were received from Aldrich. 4,4-[2,2,2-Trifluoro-l-(trifluoromethyl)ethylidene]bis[2aminophenol] (6FAP) was obtained from Day Chem Inc. (Dayton, Ohio). 5-Phosphonoisophthalic acid (PA) was prepared from 5-bromo-1,3-xylene in three steps. According to Tavs' method (4), treatment of bromoxylene with triethyl phosphite in the presence of N i B r produced oil-like diethyl xylenylphosphonate in good yields. The phosphonate diester was converted to acid, mp 197-9°, with ethyl bromide as by-product by refluxing the diester with 48% hydrobromic acid overnight (5). Oxidation of xylenyl phosphonic acid in aqueous potassium permanganate gave phosphono-isophathalic acid as white needle crystals from HC1 aqueous solution, mp 360°, m/e 246 (M) . 2

+

Functionalized Copolymers Syntheses. Hydroxy functionalized copolymers were prepared from 5-hydroxyisophthalic acid (HIPA), 4,4-oxybis(benzoic acid) (PP), and 4,4-[2,2,2trifluoro-1 -(trifluoromethyl)ethylidene]bis[2-aminophenol] (6FAP). Polycondensations were carried out in polyphosphoric acid with P O content at 83-84% at 180° for 64 h. In the work-up step, the reaction mixture was cooled to 90° and diluted with 85% phosphoric acid and water to reduce the P 0 content to ca. 67%. The homogeneous solutions were stirred at 70° overnight to hydrolyze the phosphate ester, a side reaction product from polymer hydroxy pendant and polyphosphoric acid, then poured into water to coagulate the polymer. Phosphonic acid functionalized copolymers were prepared by using various amounts of 5-phosphonoisophthalic acid (PA), 4,4-oxybis(benzoic acid) (PP), and 4,4-[2,2,2trifluoro-l-(trifluoromethyl)ethylidene]bis[2-aminophenol] (6FAP). Polycondensations were carried out by the same method as the preparation of copolymers with the hydroxy pendants (Scheme Π). 2

2

s

5



^

"COOH

*

2

(PA)

2

5t

3

^

C

2

5

x+y

6FAP

P

F

3

70°, 16 h

2) Phosphoric Acid/water (P 0 , ca. 67%)

180°, 64 h

1) PPA (P 0 83-84%)

C

2

OH

yy "0C

HIPA(x)-PP(y)-6FAP or PA(x)-PP(y)-6FAP

PP

Hooc-Q-o-Q-cooH.

ΗΝ

Scheme II. High molecular weight hydroxy- and phosphono-benzoxazole copolymers with Tg at 320-330° were prepared in polyphosphoric acid.

~P(OH)

Ν

X = -OH (HIPA), or

HIPAorPA

HOOC

Λ



300


Sol-Gel Synthesis of Polymer-Silica Hybrid Materials. A general procedure for preparing polymer-silica hybrid materials was to dissolve the polymer in anhydrous THF at the concentration of ca. 7% weight to volume. A measured amount of water and tetramethyl orthosilicate (TMOS, Aldrich) at the mole ratio 3.0-3.5 to 1 and a catalytic amount of triethylamine were added. A viscous, homogeneous solution was obtained after stirring for 2 h. The solution was transferred to a Petri dish and dried slowly over a period of 16 to 24 h at room temperature. The resulting film was further dried under vacuum for 48 h at 80-100°. In case of the use of a silane coupling agent, hydroxypolybezoxazole reacted with isocynatopropyltriethoxysilane (Huls America) (6) in anhydrous THF and triethylamine as catalyst at 35° overnight. The resulting polymer and coupling agent product solution was treated with water and TMOS as described above. The tensile properties of the unfilled polymers and the polymer-silica hybrid films were measured using an Instron Universal Testing Instrument (Model 1122) at the drawing rate of 0.2 in/min at room temperature. Results and Discussion Copolymers Synthesis and Properties. A series of high molecular weight functionalized benzoxazole copolymers were prepared from 4,4'-[2,2,2-trifluoro-l-(trifluoro-methyl)ethylidene]bis[2-aminophenol], 4,4'-oxybis(benzoic acid) and 5-hydroxyisophthalic acid or 5-phosphonoisophthalic acid. Due to the low nucleophilicity of hexafluoroisopropylbis(aminophenol) (6FAP) (7), the polycondensations were carried out in polyphosphoric acid with a P 0 content at 84% at 180° for 64 h. From previous work (8) and model compound studies, it was found that phosphate ester was formed during the polymeriza tion by the side reaction of the pendent hydroxy groups and polyphosphoric acid. The phosphate ester can be hydrolyzed in situ without cleaving the polymer chain by diluting the polyphosphoric acid with phosphoric acid and water to hydrolytic conditions and stirring at 70° overnight. After several trials, most copolymers exhibited intrinsic viscosities above 1.0 dl/g, as measured in methansulfonic acid (Table I.) The solubility of hydroxypolybenzoxazoles in THF remains the same as PP-6FAP, but the solubility of phosphonopolybenzoxzaoles decreases while increasing the amount of phosphonic acid moiety; presumably, the strong hydrogen bonding of the phosphonic acid between polymer chains retards the solubility. The hydrogen bonding can be interrupted by adding a small amount of water to the THF. It was shown in the case of the polymer (PA(17)-PP(83)-6FAP) which was only swollen in THF but was dissolved in THF including 3-5 % of water. 2

5

Table, Τ Intrinsic viscosities and solubilitv in THF of the resulting cooolvmers Soluble in THF copolvmers Γ i.v. fdl/g^l x/v Yes A: HIPA(25VPPf75V6FAP Γ0.9 - 1.191 25/75 Yes B: HIPA(40VPP(60V6FAP Γ0.95 - 1.341 40/60 Yes C: PAC10VPPÎ90V6FAP Γ 1.0-1.461 10/90 porom. Swollen* D: PAC 17 VPPf 83 V6FAP Π. 161 17/83 ΡΟΓΟΗΥ, NO E: PAC25VPPÎ75V6FAP Γ0.901 25/75 POiOHU * Clear solution can be obtained by adding 3 - 5 % of water. X OH OH


23.

CHEN ET AL.

Polybenzoxazole—Silica Hybrid Materials

301

The functionalized copolymers exhibited Tg's at 320-330° which are slightly higher than that of homopolymer PP-6FAP. Tough transparent films could be obtained by casting from the THF solutions of those copolymers. Infrared spectra of hydroxy or phophono polybenzoxazoles are similar to that of PP-6FAP except for the presence of a strong broad absorption at 3100-3200 cm" and a weak to medium broad absorption at 2300-3400 cm" which correspond to the hydroxy (-OH) and phosphonic acid (-POH), respectively. Thermal stability was investigated by using TGA-MS. The major degradation fragments were at 525-550° and 650-660° corresponding to the decomposition of 6F-isopropyl, phenyl ether, and benzoxazole. 1


1

Sol-Gel Process of Copolymers and Tetramethyl Orthosilicate. The extraordinary shrinkage accompanied with cracking is the most difficult problem for producing sol-gel glasses. A wet gel with weak interparticle interaction and uneven micropores is most likely to have cracking. The problem can be minimized to some extent by using very time consuming processes such as extensive aging to increase the strength of the interparticle networks, the enlarging of the pore size, and the very slow evaporation rate to reduce the capillary stresses. Alternatively, a sol-gel process consisting of a tough organic polymer and inorganic particles has the advantage of excluding the cracking problem because of the formation of tough organic polymer matrix interaction during the drying process. On the other hand, phase separation between organic polymer and inorganic particles might be the critical point of the sol-gel processing for hybrid materials. At the early stage of the sol-gel process, the incomplete hydrolysis and condensation gives the low molecular weight silica particle containing organic alkoxy and polar hydroxy groups which are compatible with organic polymer in sol-gel solvent; but the growth of silica particle leads to macro phase separation of the mixture. The growth and fractal structure of silica particle are highly influenced by the hydrolysis and condensation reactions. It is known that the rate of these reactions can be affected by the pH of the solution, the catalysts, the amount of water, and even the silicon-containing starting materials. After preliminary trials, we found that the use of tetramethyl orthosilicate as the starting compound and triethylamine as catalyst gave better results in homogeneity compared with the use of ethyl silicate - 40 and HC1 catalyst. A sol-gel solution was prepared as described above. The solvent of the clear sol-gel solution was removed slowly under atmosphere over a period of 16 to 24 h. The resulting film was further dried under vaccum at 80-100° for 48 h to complete the hydrolysis and to increase the condensation of silica. By using this approach, we have successfully prepared the transparent, uniform, and thick hydroxypolybenzoxazole-silica hybrid films with 33 wt % of silica (Figure. 1). In attempts to verify the formation of covalent bondings between functionalized copolymers and silica, it was found that the hybrid films redissolved in THF after soaking in the solvent overnight, which showed no significant bonding between polymer and silica. The crosslinking between polymer and silica can be achieved by utilizing a coupling agent. Hydroxy functional copolymer was dissolved in anhydrous THF; a stoichiometric amount of isocyanatopropyltriethoxysilane (6) and a catalytic amount of triethylamine were added (Scheme III). The solution was heated at 35° overnight. Infrared spectrum of the product showed the absorption of -C=0 of carbamate at 1760 cm" indicating the 1



302


Figure 1. The transparent thick hybrid films, 1.7 inch in diameter, including 33.3% of silica and 66.6% of HIPA(40)-PP(60)-6FAP, shows left: 0.45g, without silane coupling agent; right: 0.50g, with silane coupling agent.

HIPA(40)--PP(60)--6FAP + OCN(CH2) Si(OEt)3 3

THF Et N 3

0-C(0)-NH(CH ) Si(OEt) 2

3

POLYMER

3

0-C(0)-NH(CH ) Si(OEt) 2

3

3

«ΑΛΑΛΑΑΛΑΛΛ^^ΑΑΛΛΑΑΑΑΛ

TMOS

Moisture

H 0 2

Hybrid Film

Gel

Scheme III. The preparation of hybrid materials from hydroxybenzoxazolecopolymer, silane coupling agent, and TMOS.


23.

CHEN ET AL.

PolybenzoxazoleSilica Hybrid Materials

303


reaction of functionalized copolymer and silane coupling agent. Before gelation, TMOS and water were added, and the mixture was heated at 40° with stirring for 2 h, followed by the procedure mentioned above for hybrid film formation. The resulting hybrid films including functionalized copolymer, silane coupling agent, and silica (Figure 1) were completely insoluble in THF. The results agree well with the occurrence of crosslinking of polymer and silica. Homopolymer, PP-6FAP, and TMOS were not miscible entirely in THF. Compared to PP-6FAP, the phosphonobenzoxazolecopolymer, PA(10)-PP(90)-6FAP, showed limited compatibility with TMOS. The sol-gel solution including the copolymer and 10% silica was clear at the beginning but turned cloudy before the solvent was removed. The resulting films were opaque indicating the occurrance of macro phase separation. Mechanical Properties of Copolymers and the Hybrid Films. Four unfilled copolymers (see Table I), a (HIPA(25)-PP(75)-6FAP, i.v.: 0.9 dl/g), b (HIPA(25)-PP(75)-6FAP, i.v.: 1.19 dl/g), ç (HIPA(40)-PP(60)-6FAP, i.v.: 0.95 dl/g), and d (PA(10)-PP(90)-6FAP, i.v.: 1.11 dl/g), and the transparent hybrid films with various amount of silica from TMOS were prepared for stress-strain measurement. Figure 2 shows the stress-strain curves for the unfilled polymers. Polymers a and b have the same structure and composition, but polymer b film is much tougher than the film from polymer a. It indicates that the mechanical properties of polymers are highly influenced by the polymer molecular weight. Since both the composition and the structure of ç and d are different from a or b, it is not clear that the lack in strength of ç and d is caused by the lower molecular weights, or the amount of meta structure segments, or the structure of functional groups. Figures 3 and 4 show the mechanical strength of hybrid films derived from polymers a and b. The modulus and tensile strength increased with addition of silica in both cases, but the elongation at break decreased at the higher silica contents. This is a typical mechanical behavior of polymer-silica hybrid composites which possess well-dispersed micro size silica particles in the polymer matrix. It was also consistent with the interaction between polymer and silica through the secondary bonding and large surface area of the micro silica filler. In case of polymer ç (Figure 5), there is a considerable increase in mechanical strength with increasing silica contents. Although the strength of unfilled polymer ç is lower than that of polymers a and b, the polymer c and silica (20%) hybrid film has the strength as good as those from polymers a and b. It is interested to note that the elongation at break even increased with addition of silica in case of polymer ç. This may be due to the strong interaction including the slightly crosslinking between polymer and silica resulting from the number of hydroxy groups in polymer ç. In conclusion, this study presents the results of preparing high Tg, THF soluble, thermally stable functionalized copolymers to incorporate with TMOS. Hydroxypolybenzoxazoles can chemically hybrid with a silane coupling agent and TMOS through solgel processing. Phosphonopolybenzoxazole was miscible with TMOS in THF but failed to form homogeneous gels due to the occurrence of phase separation. The mechanical behavior of the hybrid materials showed a reinforcement in strength, indicating a strong interaction between the polymers and silica through the homogeneous dispersion of the micro silica particles in the polymer matrix.



304


Figure 2. Stress-strain curves for unfilled copolymers a, h, ç, and d.

0 I

0.00

»

•

•

t

0.10

ι

0.20

0.30

.

.

.

I

0.40

Strain Figure 3. Stress-strain curves of hybrid films prepared from polymer a and TMOS.



23. C H E N E T AL.

PolybenzoxazoleSilica

' 0

·

' 0.2

·

'

305

Hybrid Materials

·

I

0.4

0.6

.

I Ο.Β

.

1 1

Strain Figure 4. Stress-strain curves of hybrid films prepared from polymer b and TMOS.

Strain Figure 5. Stress-strain curves of hybrid films prepared from polymer c and TMOS.



306 Acknowledgment

The authors are pleased to acknowledge the financial support provided by the Air Force Office of Scientific Research.


References (1) Morikawa, Y.; Iyoku, M.; Kakimoto, M.; Imai, Y. Polym. J. 1992, 24, 107 (2) Ahmad, Z.; Wang, S.; Mark, J. E. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Sci.) 1993, 34(2), 745 (3) Unroe, M. R. et. al. 22nd Science of Advanced Materials and Process Engineering Conference Proceedings, 1990, 22, 186 (4) Tavs, P. Chem. Ber. 1970, 103, 2429 (5) Nagarajar, K. et. al. Can. J. Chem. 1987, 65, 1731 (6) Noell J. L.; Wilkes G. L.; Mohanty, D. K.; MacGrath, J. E. J. Appl. Polym. Sci. 1990, 40, 1177 (7) Maruyama, Y.; Oishi, Y.; Kakimoto, M.; Imai, Y.; Macromolecules, 1988, 21, 2307 (8) Arnold, F. E.; Chen, J. P. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Sci.) 1991, 32(2), 209 RECEIVED August 3, 1994


Hybrid Organic-Inorganic Composites - American Chemical Society

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