Chapter 2 The syntheses of octaphenol-POSS
3.3 Results and Discussion
In our previous study [11], we have discussed in details the hydrogen bonding interaction between the phenolic hydroxyl groups and the POSS siloxane groups using 1D and 2D FTIR spectra. Figure 3-1(a) shows the infrared spectra in the range from 2700 cm-1 to 4000 cm-1 of the pure phenolic and various phenolic/AS-POSS nanocomposites measured at room temperature. The pure phenolic polymer exhibits two bands in the hydroxyl stretching region of the infrared spectrum. A very broad band centered at 3350 cm-1 is attributed to the wide distribution of the hydrogen bonded hydroxyl group while a narrower shoulder band at 3525cm-1 is caused by the free hydroxyl group. Figure 3-1(a) clearly shows that the intensity of the free hydroxyl absorption (3525cm-1) decreases gradually as the AS-POSS content of the blend is increased from 5 to 90 wt%. The hydrogen-bonded hydroxyl band in the
phenolic tends to shift into a higher frequency with increasing AS-POSS content at the vicinity of 3465 cm-1. This change is due to the switch from the hydroxyl-hydroxyl bond to the hydroxyl-carbonyl or hydroxyl-siloxane bond.
Figure 3-1(b) displays the infrared spectra in the region from 1680 cm-1 to 1820 cm-1 for various phenolic/AS-POSS blend compositions measured at room temperature. The carbonyl stretching frequency is split into two bands at 1760 cm-1 and 1735 cm-1, corresponding to the free and the hydrogen-bonded carbonyl groups, respectively. The band can be easily decomposed into two Gaussian peaks, with areas corresponding to the hydrogen-bonded carbonyl (1735cm-1) and free carbonyl (1760cm-1) as shown in Figure 3-2. In order to obtain the fraction of the hydrogen-bonded carbonyl, the known absorptivity ratio for hydrogen bonded and free carbonyl contributions is required. We have employed a value of αHB/αF=1.5,
which was previously calculated by Moskala et al. [10]. Fractions of hydrogen bonded carbonyl through curve fitting are summarized in Table 3-1, indicating that the hydrogen bonded fraction of the carbonyl group increases with the increase of the phenolic content.
Table 3-1: Curve fitting of the area fractions of the carbonyl stretching bands in the FTIR spectra of phenolic/AS-POSS blends recorded at room temperature.
ν: wavenumber, W1/2: half width, *fb: fraction of hydrogen bonding interaction
Free C=O H-Bonding C=O
Phenolic/AS-POSS
Wt Ratio ν, cmP–1P
WB1/2B, cmP–1P
ABfB % ν, cmP–1P
WB1/2B, cmP–1P
ABb B% fBbPB*P
0/100 1763.2 23.5 100 - - - -
10/90 1765.2 18.2 52.6 1739.3 29.3 47.4 37.5 20/80 1764.8 16.5 41.5 1737.3 28.0 58.5 48.4 30/70 1764.5 17.0 36.1 1735.5 27.8 63.9 54.1 40/60 1763.4 16.3 32.6 1733.5 26.3 67.4 58.0 50/50 1763.6 16.9 29.9 1733.6 26.3 70.1 61.0 70/30 1762.8 17.0 25.6 1732.1 25.5 74.4 66.0 80/20 1762.1 18.0 23.3 1731.3 24.7 76.7 68.8 90/10 1762.5 18.6 22.1 1731.0 25.3 77.9 70.1
4 0 0 0 3 8 0 0 3 6 0 0 3 4 0 0 3 2 0 0 3 0 0 0 2 8 0 0
Fig. 3-1 Infrared spectra of phenolic/AS-POSS blend at room temperature in the hydroxyl stretching region (a) and carbonyl stretching region (b).
1 8 0 0 1 7 7 0 1 7 4 0 1 7 1 0 1 6 8 0
Fig. 3-2 Deconstructed models of the carbonyl stretching bands (in Figure 2) of the weight percent of phenolic/AS-POSS blends at various compositions.
0 20 40 60 80 100 0.0
0.2 0.4 0.6 0.8 1.0
KA=38.6
Kc=26.0 KC=64.6
f HB
C=O
Phenolic Content (wt%)
Fig. 3-3 Fraction of the hydrogen-bonded carbonyl group versus composition:
(■)FT-IR data, (--) theoretical values from phenolic/PAS blend (KA=64.6) and (－) theoretical values from phenolic/AS-POSS (KA=26.0)
Figure 3-3 shows plots of the experimental data and theoretical predicted curve as a function of composition at room temperature that demonstrates the ability of PCAM to predict the degree of hydrogen bonding on the carbonyl group. Figure 3-3 indicates that the experimental values are generally lower than the predicted values based on using KA = 64.6 from phenolic/PAS blends. This result also indicates that the hydroxyl groups of phenolic not only interact with the carbonyl group of the acetoxystyrene but also with the siloxane group of the POSS, which is consistent with results from our previous study [11]. In other words, the AS carbonyl competes with
the siloxane of POSS to form a hydrogen bond with hydroxyl groups of the phenolic resin. The numerical method is employed to determine KA of the phenolic/AS-POSS blend according to the PCAM based on the fraction of the hydrogen bonded carbonyl group. The approximate equations are [11]:
⎥⎦
Where ΦA and ΦB denote volume fractions of non-self associated species A (AS-POSS) and self associating species B (phenolic), respectively. ΦA1 andΦB1 are
the corresponding volume fractions of the isolated AS-POSS and phenolic segments, respectively. r is the ratio of molar volume, VA/VB. Self-association equilibrium constants, KB and K2, describe the formation of multimers and dimers, respectively.
Finally, the KA is the equilibrium constant describing the association of A with B. In addition, KB and K2 are 23.3 and 52.3 at 25℃ of the pure phenolic [11]. In order to
calculate the inter-association constants (KA), the methodology of a least square method has been described in our previous study [12]. Table 3-2 lists all the
parameters required by the Painter-Coleman association model to estimate thermodynamic properties for this phenolic/AS-POSS blend. The inter-association equilibrium constant of 26.0 is obtained for the phenolic/AS-POSS blend. However, the KA=64.6 for the phenolic/PAS blend, implying that the KA between the hydroxyl group of phenolic and the siloxane group of POSS is equal to 38.6 (64.6-26.0 = 38.6), which is exactly consistent with the previously reported value based on the classical Coggeshall and Saier (C&S) methodology. Therefore, the inter-association equilibrium constant of hydroxyl-siloxane has good correlation between these two methods.
Table 3-2: Summary of the self-association and inter-association equilibrium constants and thermodynamic parameter of phenolic/AS-POSS blends at 25℃
Polymer V Mw Equilibrium Constant
K2 KB KA
Phenolic1 84 105 23.3 52.3
PAS2 128.6 162.2 64.6
8-isobutyl POSS3 778.6 872.2 38.6
AS-POSS 2058.6 2305.6 26.0
V: Molar Volume (ml/mol), Mw: Molecular Weight (g/mol), K2: Dimmer self-association equilibrium constant, KB: Multimer self-association equilibrium constant, KA: Inter-association equilibrium constant, 1: reference 11, 2: reference 12, 3: reference 11.
3.4 Conclusion
A new nanomaterial based on AS-POSS has been synthesized and the hydrogen bonding interaction of phenolic/AS has been investigated by using FTIR analyses.
The inter-association equilibrium constant between the hydroxyl group of the phenolic and the siloxane group of the POSS is determined indirectly from a least square fitting procedure based on the experimental fraction of hydrogen bonded carbonyl group in this blend system. The observed KA (38.6) of hydroxyl-siloxane is found equal to the value from classical classical Coggeshall and Saier (C&S) methodology.
References
[1] Schwab JJ, Lichtenhan JD. Appl Organomet Chem 1998;12:707.
[2] Lichtenhan JD, Otonari Y, Carri MG. Macromolecules 1995;28:8435.
[3] Shochey EG, Bolf AG, Jones PF, Schwab JJ, Chaffee KP, Haddad TS, Lichtenhan JD. Appl Organomet Chem 1999;13:311.
[4] Haddad TS, Lichtenhan JD. Macromolecules 1996;29:7302.
[5] Abad MJ, Barral L, Fasce DP, Williams RJJ. Macromolecules 2003;36:3128.
[6] Fu BX, Zhang W, Hsiao BS, Johansson G, Sauer BB, Phillips S, Balnski R, Rafailovich M, Sokolov J. Polym Prepr 2000;41:587.
[7] Leu CM, Chang YT, Wei KH. Macromolecules2003;36:9122.
[8] Mather PT, Jeon HG., Romo-Uribe A, Haddad TS, Lichtenhan JD. Macromolecules
1999;32:1194.
[9] Xu H, Kuo SW, Lee JS. Chang FC. Polymer 2002;43:5117.
[10] Coleman MM, Graf JF, Painter PC. Specific Interactions and the Miscibility of Polymer Blends; Technomic: Lancaster, PA, 1991.
[11] Lee YJ, Kuo SW, Huang WJ, Lee HY, Chang FC. J Polym Sci., Polym Sci Ed 2004;42:1127.
[12] Kuo SW, Chang FC. Macromol Chem Phys 2002;203:868.
[13] Kuo SW, Chang FC. Macromolecules 2001;34:4089.
[14] Coggesthall ND, Saier EL. J Am Chem Soc 1951;71:5414.
[15] Coleman MM, Painter PC. Prog Polym Sci 1995;20:1
Chapter 4. A Novolac type of phenolic nanocomposite from octaphenol-POSS
Abstract
By adding the octaphenol-POSS before the polymerization process (which we
called the reaction system) of phenolic resin to join the reaction with phenol and formaldehyde, we can synthesized a series novel novolac type of phenolic/POSS nanocomposite. Each octaphenol-POSS has eight phenol groups, and they can be viewed as eight polymerizable groups in the condensation polymerization process of phenolic resin. In order to demonstrate the occurrence of the reaction, we synthesized another series novolac type of phenolic/POSS nanocomposite by adding the octaphenol-POSS after the polymerization process (which we called the blend system) of phenolic resin for comparison. The thermal properties of these two materials are investigated by DSC, TGA, and the molecular weight distribution of the nanocomposite and dispersion condition of POSS are investigated by MOLDI-TOF and WAXD, respectively.
4.1 Introduction
Despite the emergence of several new classes of thermosets, high performance polymers and several other new generation materials that are superior in some respects, phenolic resins retain industrial and commercial interest, a century after its introduction. Phenolic resins are preferred in a wide range of applications, from commodity and construction materials to high technology aerospace industry. This recognition emerges from the fact that these resins have several desirable characteristics, such as superior mechanical strength, heat resistance and dimensional stability, as well as, high resistance against various solvents, acids and water. They are inherently flame resistant, and evolve low smoke upon incineration. Although phenolics cannot be substituents for epoxies and polyimides in many engineering areas, their composites still find a major market in thermo-structural application in the aerospace industry due to good heat and flame resistance, excellent ablative properties and low cost. These key properties add to their market growth, and as a result of innovative research, new products and applications continue to emerge, demonstrating the versatility and the potential of phenol resins to cope with the ever-changing
requirements and challenges of advanced technology. [1-2]
Undisputedly, classical phenolic resins based on resole and novolac dominate the resin market. However, their acceptance as a universal material in many engineering areas is hampered by some of the inherent qualities derived from their special
chemical structures. These resins cure at moderately high temperature by a condensation mechanism with the evolution of volatiles, which necessitates application of pressure during molding to form void-free components. The need for the use of catalyst for curing and the limited shelf life of resin at ambient conditions are also major shortcomings of these systems. When compared to many known thermally stable polymers, their thermo-oxidative stability is low. The rigid aromatic units tightly held by the short methylene linkages make the matric brittle. In view of this, a new chemistry is needed to modify the cure of phenolic resins, in particular, a new method is needed to chain extend and/or to cross-link phenolic resins without production of volatiles and allow for extended shelf stability at ambient conditions for the formulated thermosets. In doing so, it is imperative that the modifications do not impair the thermo-mechanical characteristics of the resultant system.
4.1.1 Strategies for designing addition-cure phenolics
Several approaches have been reported for modification of phenolic resins and their cure chemistry. Structural modification to confer addition-cure character has been one thrust area of research [3,4]. Additional-curable phenolic resins with improved thermal and pyrolysis characteristics will be the desirable resins in composites for thermo-structural applications. [5] Higher char-yield leads also to a
better heat shielding. Such high char phenolics could be potential candidates as matrices in carbon/carbon composites with obvious advantages [6]. The major strategies in designing addition-cure phenolics are:
(1) Incorporation of thermally stable addition-curable groups on to novolac backbone (2) Structural modification (transform) involving phenolic hydroxyl groups
(3) Curing of novolac by suitable curatives through addition reactions of OH groups (4) Reactive blending of structurally modified phenolic resin with a functional
reactant
4.1.2 Hybrid inorganic/organic crosslinked resins containing polyhedral oligomeric silsesquioxanes
As POSS macromers are incorporated, a kinetic race occurs. The solubility of the POSS in the organic monomer mixture of the resin (phenolics, epoxies, vinyl esters, methacrylics, styrene-divinylbenzene, cyanate esters or dicyclopentadiene) decreases during the cure as the number of monomer molecules (hence the entropy of mixing) decreases. Therefore, phase separation of the POSS macromer may occur sometime during the cure. This would occur in competition with the POSS macromer’s chemical incorporation into the resin. After such phase separation, the POSS macromer may homopolymerize (or copolymerize with smaller amounts of
resin monomers) to form POSS-rich phases within the composite as the resin curing continues. This process can compete against random POSS incorporation into the developing resin network. Random incorporation leads to molecularly dispersed POSS monomer units within a homogeneous resin phase. Phase separation will be increasingly favored if the POSS monomer has a low relative reactivity. This will cause monomer drift during the cure which raises POSS concentration relative to the other monomers, favoring phase separation. Anything that increasing the early incorporation of POSS will lower this phase separation. Another type of phase separation process can also operate, even if all the POSS is polymerized randomly into the resin network. POSS macromers bonded into chain segments of the resin may preferentially self-associate with each other, forming POSS aggregates whose structures and sizes are limited by the freedom of motion permitted by the developing resin’s chain segmental mobility and the POSS stoichiometry. This process is favored at low crosslink densities where segmental mobility is larger. POSS aggregation has been previously observed in uncrosslinked thermoplastics. Coughlin et al have demonstrated that linear copolymers of ethylene with a mono-alpha-olefin-substituted POSS can form POSS nanocrystalline domains due to self-aggregation of pendant POSS moieties [7]. The extent of such aggregation depends on the mole fraction of POSS present and the method of solidifying the polymer (cooling a melt, precipitation
from solution etc.). Such aggregation in crosslinked resin matrices will certainly be dependent on (1) the buildup of crosslinked density as a function of the degree of cure, (2) the relative reactivity ratios of the monomers, (3) the solvent used (if any) and (4) other factors (processing condition etc.).
In this thesis, we used octaphenol-POSS to enhance the thermal properties of phenolic resin, the suitable time of adding POSS and the changes in thermal properties will be discussed.
4.2 Experimental section
Scheme 4-1. Preparation of phenolic/octaphenol-POSS nanocomposite
Synthesis process of phenolic/octaphenol-POSS nanocomposite:
reaction system
formaldehyde(aq) (137 g, 37wt%) was placed in a 1000 mL Schlenk flask equipped with a reflux condenser and a mechanical stirrer. Octaphenol-POSS at desired composition was added to phenol solution (188 g), stirred 5 minute, and then added the solution to the flask. Sulfuric acid (10%, 10ml) was added via a syringe.
The reaction mixture was then heated to 100 under nitrogen and was complete℃ d in 22 hours. The solution was washed by hot water (90℃) three times to remove
unreacted monomer and extract to remove water layer. The product was dried in a vacuum oven at 180 for 24 hour℃ s. Phenolic/octaphenol-POSS nanocomposite:
reaction system can be obtained.
Synthesis process of phenolic/octaphenol-POSS nanocomposite:
blend system
formaldehyde(aq) (137 g, 37wt%) and phenol solution (188 g) were placed in a 1000 mL Schlenk flask equipped with a reflux condenser and a mechanical stirrer.
Sulfuric acid (10%, 10ml) was added via a syringe. The reaction mixture was then heated to 100 under nitrogen and was complete℃ d in 22 hours. The solution was washed by hot water (90℃) three times to remove unreacted monomer and extract to remove water layer. The product (pure phenolic resin) was dried in a vacuum oven at 180 for 24 hour℃ s. Dissolve the pure phenolic resin and octaphenol-POSS at desired composition in tetrahydrofuran (THF) and dried in a vacuum oven at 80 for 24 ℃ hours.Phenolic/octaphenol-POSS nanocomposite: blend system can be obtained.
4.3 Characterizations
Differential scanning calorimeter (DSC)
Thermal analysis was performed with a differential scanning calorimeter from DuPont (DSC-9000) with a scan rate of 20 ℃/min and a temperature range of -50~150 ℃. Temperature and energy calibrations were carried out with indium.
Approximately 5-10 mg of each blend was weighted and sealed in an aluminium pan.
This sample was quickly cooled to -50℃ from the melt for the first scan and then scanned between -50 to 150℃ at 20℃/min. The glass transition temperature is at the
midpoint of the specific heat increment.
Wide Angle X-ray Diffraction (WAXD)
XRD spectra was collected on a M18XHF-SPA X-ray diffraction instrument (MacScience Co, Japan), used Co Kα radiation; Bragg’s law(λ=2d sinθ) was used
to compute the spacing.
Thermogravimetric Analysis (TGA)
Thermal stability of the cured sample was investigated by a Du Pont 2050 TGA.
The cured sample of 5-10mg was placed in a Pt cell and heated at a heating rate of 10
℃/min from 30 to 800℃ at a nitrogen flow of 90 mL/min.
MALDI-TOF
All the mass spectra were obtained using a Biflex 3 (Bruker) time-of-flight mass spectrometer. The mass spectrometer was equipped with a 337-nm nitrogen laser, a 1.25-m flight tube, and a sample target having the capacity to load 384 samples simultaneously. The accelerating voltage was set to 19 Kv.
4.4 Results and Discussion
Figure 4-1 displays the DSC thermograms of octaphenol-POSS blended with phenolic (the blend system) as a function of composition, and Figure 4-2 displays the DSC thermograms of the reaction system. For comparison, Table 4-1 shows the summary of Tg of these two figures. The Tg of the blend system decreases with increasing POSS content, while the Tg of the reaction system increases with increasing POSS content (except the 10wt%), which verifies that in reaction system, the octaphenol-POSS joins the polymerization reaction and acts as a crosslinking point to make the thermal properties of the material improve. A Tg depression in the blend system is characteristic of a poor dispersion of POSS, as we mentioned before at Chapter 3, the phenol groups of phenolic resin will form hydrogen bonds with the siloxane groups of the octaphenol-POSS. Apparently, the force of the hydrogen bonds are not strong enough to make the octaphenol-POSS disperse well. For the reaction type, Tg increases with increasing POSS content and then decreases again (10wt%), indicates that octaphenol-POSS joins the polymerization of phenolic and acts as a crosslinking points to make the nanocomposite crosslinke, reduces the free volume and raises the Tg. Figure 4-3 displays the MALDI-TOF of the reaction system phenolic/POSS nanocomposites. The low molecular weight portions (<1500) increase
significantly with increasing POSS content, indicates that as the POSS content increases, the steric hindrance of the POSS will lower the reactivity of the phenol groups. In other words, the effect of POSS acts as a crosslinking point competes with the effect of steic hindrance POSS, once the POSS content exceeds a certain degree, the low reactivity will make the thermal property of the material decreases.
-50 0 50 100 150
heat flow(mW)
temperature(OC)
0%
2%
5%
10%
Fig. 4-1 DSC scans of phenolic/octaphenol-POSS (blend system)
-50 0 50 100 150
Fig. 4-2 DSC scans of phenolic/octaphenol-POSS (reaction system)
Table 4-1 Comparison of phenolic/octaphenol-POSS for reaction system and blend system(DSC)
1000 1500 2000 2500 3000 3500 4000
absolute intensity
m/z
0%
2%
5%
10%
Fig. 4-3 MALDI-TOF of phenolic/octaphenol-POSS (reaction system)
Figure 4-4 and figure 4-5 display the TGA thermograms of the blend system and the reaction system phenolic/POSS nanocomposites, respectively. The decomposition parameters obtained from TGA thermograms are compiled in Table 4-2, including the 5 wt% loss temperatures and char yields. The thermal stability and anaerobic char residue of the blend type decrease with increasing POSS content, while for the reaction type, they increase. The totally different trends between the two types are
resulted from the role of octaphenol-POSS play in phenolic: In the reaction system, the POSS joins the polymerization and acts as a crosslinking point, it will definitely increase the thermal stability of the nanocomposite due to the better dispersion of POSS induced by the chemical bond formation. However, in the blend system, the POSS just acts as a nano-filler. Moreover, even at very low POSS loading aggregation appears easily, the poor dispersion will lower the thermal stability.
0 100 200 300 400 500 600 700 800
30 40 50 60 70 80 90 100
weight percent(wt%)
temperature(OC) Pure phenolic 10%
Fig. 4-4 TGA of phenolic/octaphenol-POSS (blend system) (For clarity, data of 2% and 5% are not shown.)
0 100 200 300 400 500 600 700 800 0
10 20 30 40 50 60 70 80 90 100
weight percent age (%)
temperature (
oC)
10%
5%
2%
pure phenolic
Fig. 4-5 TGA of phenolic/octaphenol-POSS (reaction system)
Table 4-2 Comparison of phenolic/octaphenol-POSS for reaction system and blend
Operations: Import
Q8M8H - File: Q8M8H.raw - Type: 2Th/Th locked - Start: 1.000 ? - End: 40.000 ? - Step: 0.100 ? - Step
Operations: Y Scale Add 3500 | Import
Phenolic10% - File: Phenolic10%.raw - Type: 2Th/Th locked - Start: 1.000 ? - End: 40.000 ? - Step: 0.1 Operations: Y Scale Add 3000 | Import
phenolic5% - File: phenolic5%.raw - Type: 2Th/Th locked - Start: 1.000 ? - End: 40.000 ? - Step: 0.100 Operations: Y Scale Add 2500 | Import
phenolic1% - File: phenolic1%.raw - Type: 2Th/Th locked - Start: 1.000 ? - End: 40.000 ? - Step: 0.100 Operations: Y Scale Add 500 | Y Scale Add 500 | Y Scale Add 500 | Y Scale Add 500 | Import phenolic0% - File: phenolic0%.raw - Type: 2Th/Th locked - Start: 1.000 ? - End: 40.000 ? - Step: 0.100
Lin (Counts)
Figure 4-6 is the WAXD of phenolic/octaphenol-POSS nanocomposite (reaction system). From 0% to 10%, traces of WAXD show only amorphous halo, leading to the conclusion that at reaction system, POSS aggregation is small.
10%
5%
2%
0%
QB8BM
Fig. 4-6 WAXD of phenolic/octaphenol-POSS (reaction system) and Q8M8H
4.5 Conclusions
We have successfully synthesized a new novolac type phenolic resin based on the octaphenol-POSS, through carefully choosing the time of adding octaphenol-POSS, the thermal properties will improve dramatically. Compared to the blend system of
We have successfully synthesized a new novolac type phenolic resin based on the octaphenol-POSS, through carefully choosing the time of adding octaphenol-POSS, the thermal properties will improve dramatically. Compared to the blend system of