Results and Discussion

This study focused on comparing nanocomposites prepared using two different surfactant-modified clays. The CPC surfactant is an ammonium salt that contains a long aliphatic chain. The POSS surfactant is an ammonium salt that features a cube-like cage structure containing seven isobutyl groups located at the corner positions. Scheme 2-1 displays the structures of these two ammonium salts, CPC and POSS, used to prepare the modified clays. Conventional surfactants, such as CPC, are unstable at high temperatures. Thus, we expected that nanocomposites prepared from the POSS surfactant would possess better thermal stabilities relative to those formed using the CPC surfactant.

2.3.1 X-Ray Diffractions.

We used X-ray diffraction (XRD) to characterize the layered structures of the modified clays and polymer/clay nanocomposites, since changes in 2θ indicate changes in the gallery distance of the clay. Figure 2-1 displays X-ray diffraction patterns of organic-modified clays containing different CPC/clay ratios. The basal space indicates the interlayer spacing of the silicate layers which is calculated from the peak position using the Bragg equation. The pristine clay has this peak at 6.94°, which corresponds to a basal space of 1.26 nm. The insertion of the CPC surfactant between the galleries of the clay increases the d spacing as the CPC/clay ratio increases. When the CPC/clay ratio is greater than 0.4, the d spacing remains constant, which implies that oversaturation of the surfactant has occurred. This result indicates that the CPC surfactant becomes successfully intercalated into the galleries of the clay nanoparticles.

Figure 2-2 displays the XRD results of the POSS-intercalated clay. The pure POSS surfactant has characteristic diffraction peaks at 7.9° and 8.8° that arise from aggregation of the POSS. When the POSS surfactant is inserted between the galleries of the clay, the d spacing is increased from 1.26 nm for the pristine clay to 1.61 nm for the POSS surfactant-intercalated clay, which indicates that the POSS surfactant has indeed been intercalated successfully into the galleries of the clay. This d spacing is relatively smaller than that of the CPC-treated clay, but it does not affect the clay intercalation or exfoliation within the PS matrix. The POSS is known to interact with PS to promote intercalation and exfoliation [20]. This result agrees well with the TEM result that indicated that the degree of clay exfoliation of the POSS-treated nanocomposite is even greater than that of the CPC-treated material. The POSS surfactant-intercalated clay (POSS/Clay) exhibits only one diffraction peak; no peak was detected to arise from the aggregation of POSS. The POSS surfactant-intercalated clay is presented schematically in Scheme 2-2. The interlamellar distance indicated in the figure is calculated by the expression: △ d = d spacing – thickness of one platelet (~1.0 nm). In the case of the clay intercalated with POSS surfactant, the distance between two adjacent clay plates is 0.71 nm which is close to the particle size of the POSS. The R group (isobutyl group) of the POSS experiences van der Waals interactions with the styrene monomer, and thus allows the styrene monomer can polymerize within the galleries of the clay.

The formation of a true polymer/clay nanocomposite requires the insertion of the polymer between the layers of the clay. There are two terms used to describe the general classes of nanocomposites: intercalation and exfoliation (also called delamination). In the case of intercalation, polymer chains are inserted between galleries of the clay, and the spacing between the galleries is increased. On the other hand, in the case of exfoliation, these individual silicate layers are distributed

randomly such that they no longer interact with the cations. Characterizing the formation of a nanocomposite requires measurement of the d spacing by XRD and the use of TEM to determine the actual distribution of platelets within the polymer matrix.

The formation of an exfoliated structure usually results in the complete loss of registry between the clay layers so that no peak can be observed by XRD. The most likely occurrence is the formation of a mixture of exfoliated and intercalated structures; this phenomenon requires detection by TEM.

Figure 2-3 presents the XRD results for CPC/Clay/PS and POSS/Clay/PS nanocomposites. No peak is detected for the nanocomposites from both the CPC- and POSS-treated clays, which suggests that they have exfoliated structure. TEM is still required, however, to observe the true structure and distribution of the silica platelets.

There is an angle detecting limit for the XRD detector which is unable to detect the diffraction angle less than 3.5°.

2.3.2 TEM Measurements on the Nanocomposites.

Figure 2-4 displays TEM images of two nanocomposites prepared from the CPC- and POSS-modified clays. The layers of platelets observed for these CPC-treated nanocomposites are, in fact, an intercalated structure. For the POSS-treated nanocomposite, each clay layers is isolated and evenly distributed within the PS matrix, which implies that a full exfoliation is formed that is consistent with the XRD data.

2.3.3 Infrared Spectroscopy.

IR analysis further confirms the existence of the POSS in the intercalated clay sample. Figure 2-5 presents the IR spectra of pure POSS, pure clay, and the POSS-treated clay. The pure clay exhibits a strong absorbance at 1040 cm^-1 for the Si

－O － Si bond in montmorillonite silicate. The pure POSS surfactant exhibits characteristic absorption peaks for its C－H bonds at 2950－2800 cm^-1, the Si－C bonds at 1230 cm^-1, and the Si－O－Si bonds of the cage structure at 1109 cm^-1. The POSS-treated clay possesses all of the characteristic peaks of pure POSS and pure clay, which is an indication that the POSS surfactant is intercalated into the host galleries of the clay. This result is consistent with XRD data.

2.3.4 Analyzing Glass Transition Temperatures.

Figure 2-6 displays the DSC thermograms for the virgin PS and surfactant-modified clay nanocomposites. The Tg of the virgin PS is 100 °C while the values of Tg for the CPC/clay and POSS/clay nanocomposites are 102 and 108 °C, respectively. The presence of the clay layer tends to retard PS chain movement near its value of Tg. The better-dispersed clay nanocomposite (POSS/clay) retards chain movement more effectively than does the CPC/clay-modified nanocomposite.

2.3.5 Molecular Weights of the Nanocomposites.

Table 2-2 lists the molecular weights of PS in nanocomposites under similar emulsion polymerization conditions. The virgin PS has higher average molecular weights (Mn and Mw) and a lower polydispersity index (PDI) than the two nanocomposites. It has been reported [21] that clay may act as additional micelles that

are responsible for the observed lower molecular weight.

2.3.6 Characterization by TGA.

Figure 2-7 displays the TGA traces of pure clay, clay treated with the POSS, and clay treated with the CPC. As expected, the inorganic montmorillonite silicate possesses exceptionally high thermal stability. The total weight loss of the pristine clay is only 5.7% up to 800 °C, which corresponds to three different types of water present in the montmorillonite clay [22]. In contrast, the surfactant-intercalated clays are more easily decomposed. Both CPC- and POSS-modified clays begin to decompose at much lower temperatures than the pure clay. The modified clays possessing identical component weight ratio (surfactant/clay = 0.4) give different temperatures of the 5% weight loss. The POSS-modified clay decomposes at a higher temperature of 386 ℃ while the CPC-modified clay decomposes at 278 ℃.

The POSS-intercalated clay is relativity more thermally stable than the CPC-intercalated clay. Thermal decompositions and removal of surfactants are responsible for the observed weight losses of these intercalated clays. This result also agrees well with the IR result that suggested that the clay indeed contains the organic surfactant.

Figure 2-8 presents TGA thermograms of nanocomposites and polystyrene. Both of the surfactant-modified PS nanocomposites display higher decomposition temperatures than the virgin PS. The onset of thermal decomposition for the nanocomposites is shifted to higher temperatures. The POSS-intercalated clay nanocomposite is the most thermally stable one among these three samples. The nanocomposite prepared from clay/POSS displays a 21 ° C increase in the

nanocomposite is 18 °C higher. Table 2-1 summarizes the TGA results for the nanocomposites. The highest temperature of 50% weight loss is observed for the POSS-intercalated clay nanocomposite. In the case of the CPC-intercalated clay nanocomposite, the temperature of 50% weight loss is actually the same as that for the virgin polystyrene. Therefore, the nanocomposite prepared from CPC-modified clay exhibits no improvement over the virgin PS, but, in the contrast, the nanocomposite prepared with POSS-modified clay has an improved thermal stability.

2.4 Conclusions.

We have prepared polystyrene/clay nanocomposites that have (a) an exfoliated structure when derived from POSS treatment and (b) an intercalated forms when treated with CPC. The intercalation of surfactants into montmorillonite clay nanoparticles was confirmed by XRD and FTIR spectroscopy. Results of XRD indicated that the d spacing increased from 1.26 nm for pristine clay to 1.61 nm for the POSS-intercalated clay. TGA of the nanocomposites suggests that the onset of thermal degradation occurs at a higher temperature for the nanocomposite formed from POSS than for either the virgin PS or the nanocomposite derived from CPC treatment. It appears that the presence of the POSS in the clay enhances the thermal stability of polystyrene. Surfactant-modified clays give PS having lower molecular weights (Mn and Mw) and a higher MW distribution (polydispersity index, PDI) relative to the virgin PS formed under similar emulsion polymerization conditions.

The glass transition temperatures of the nanocomposites incorporating CPC or POSS are higher than that of the virgin PS.

References

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[3] Kojima Y, Usuki A, Kawasumi M, Okada A, Kurauchi T, Kamigaito O. J Polym Sci Part A: Polym Chem 1993; 31: 983.

[4] Usuki A, Kojima Y, Kawasumi M, Okada A, Fukushima T, Kurauchi T, Kamigaito O. J Mater Res 1993; 8: 1179.

[5] Yano K, Usuki A, Okada A, Kurauchi T, Kamigaito O. J Polym Sci Part A:

Polym Chem 1993; 31: 2493.

[6] Okada A, Usuki A. Mater Sci Eng 1995; 3: 109.

[7] Lan T, Pinnavaia T. J Chem Mater 1994; 6: 2216.

[8] Lan T, Pinnavaia T. J Chem Mater 1994; 6: 573.

[9] Lan T, Kaviratna PD, Pinnavaia T. J Chem Mater 1995; 7: 2144.

[10] Messersmith P, Giannelis EP. J Polym Sci Part A: Polym Chem 1995; 33: 1047.

[11] Messersmith P, Giannelis EP. Chem Mater 1994; 6: 1719.

[12] Gilman JW, Jackson CL, Morgan A, Harris R, Manias E, Giannelis EP, Wuthenow M, Hilton D, Phillips SA. Chem Mater 2000; 12: 1866.

[13] Burnside SD, Giannelis EP. Chem Mater 1995; 7: 1597.

[14] Tseng CR, Wu JY, Lee HY, Chang FC. Polymer 2001; 42: 10063.

[15] Tseng CR, Lee HY, Chang FC. J. Polym Sci Part B: Polym Phys 2001; 39: 2097.

[16] Wu HD, Tseng CR, Chang FC. Macromolecules 2001; 34: 2992.

[17] Tseng CR, Wu JY, Lee HY, Chang FC. J Appl Polym Sci 2002; 85: 1370.

[18] Tseng CR, Wu HD, Wu JY, Chang FC. J Appl Polym Sci 2002; 86: 2492.

[19] Jin Z, Alexander BM, Frank JL, Charles AW. Chem Mater 2001; 13: 3774.

[20] Wenhua Z, Bruce XF, Seo Y, Eric S, Hsiao B, Patrick TM, Yang NL, Dayi X, Harald A, Miriam R, Jonathan S. Macromolecules 2002; 35: 8029.

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Table 2-1. Results of TGA and DSC for Polystyrene Nanocomposites

Sample T_g, °C ^a T_0.05, °C ^b T_0.5, °C ^c Char at 600 °C, %

PS 100 390 424 0

CPC/Clay/PS 102 408 424 2.9

POSS/Clay/PS 108 411 446 2.8

aGlass transition temperature (Tg). ^b5% degradation temperature (T0.05). ^c50%

degradation temperature (T0.5).

Table 2-2. Molecular Weights of Polystyrene Nanocomposites

Sample M_n (×10³)^a M_w (×10³)^b PDI (M_w/M_n)^c

PS 26.1 31.8 1.22

CPC/Clay/PS 22.5 30.8 1.37

POSS/Clay/PS 21.9 31.1 1.42

aNumber-average molecular weights (Mn) and ^bweight-average molecular weights (Mw) were determined by GPC. ^cPolydispersity index, Mw/Mn.

Si O

-Scheme 2-1. Chemical structures of the surfactants used to prepare the modified clays.

Scheme 2-2. Schematic drawing of the clay intercalated by the POSS and polystyrene.

4 6 8 10 12 CPC/Clay=0.5 CPC/Clay=0.4 CPC/Clay=0.3 CPC/Clay=0.2 Pure

Intensity

2θ

Figure 2-1. XRD spectra of materials featuring the various ratios of the organic ammonium salt.

4 5 6 7 8 9 10 11 12

POSS/Clay

Clay

POSS

Intensity

2θ

Figure 2-2. X-Ray powder diffraction patterns of pure POSS, pure clay, and intercalated clay.

4 5 6 7 8 9 10 POSS/Clay/PS CPC/Clay/PS

Intensity

2θ

Figure 2-3. XRD spectra of the two surfactant-containing nanocomposites indicating the degree of exfoliation.

Figure 2-4. TEM micrographs of (a, top) the CPC-treated nanocomposite and (b, bottom) the POSS-treated nanocomposite.

4000 3750 3500 3250 3000 2750 2500 2250 2000 1750 1500 1250 1000 750 500 Clay intercalated with POSS

Pure POSS

Pure clay

Wave number (cm^-1)

Figure 2-5. IR spectra of pure clay, pure POSS, and intercalated clay.

50 60 70 80 90 100 110 120 130 140 150

POSS / Clay / PS

CPC / Clay / PS

Heat Flow (Endo, ) PS

Temperature (^oC)

Figure 2-6. DSC curves for determining the glass transition temperature of the nanocomposites.

100 200 300 400 500 600 700 800

Figure 2-7. TGA traces of (a) pure clay, (b) clay intercalated with POSS, and (c) clay intercalated with CPC.

100 200 300 400 500 600

0

Figure 2-8. TGA curves for the nanocomposites under a nitrogen atmosphere: (a) pure PS, (b) the nanocomposite formed with CPC, and (c) the nanocomposite formed with

POSS.

Chapter 3

Thermal Properties of Exfoliated Polystyrene Nanocomposites formed from Rigid Intercalation Agents-Treated Montmorillonite

Abstract

We synthesized intercalation agent APB and prepared polystyrene/clay nanocomposites using an emulsion polymerization technique. We used two different intercalation agents to treat clay: the phosphonium salt (APP) and the ammonium salt (APB). Both intercalation agents can intercalate into the layers of the pristine clay dispersed in water. We expected that the intercalation agent APB containing rigid adamantine group also has high thermal stability besides phosphonium group. We used X-ray diffraction (XRD) and transmission electron microscopy (TEM) to characterize the structures of the nanocomposites. The nanocomposites prepared from the APB- and APP-treated clay have exfoliated structures. The molecular weights of polystyrene (PS) obtained from the nanocomposite is slightly lower than the virgin PS formed under similar polymerization conditions. The coefficient of thermal expansion (CTE) was obtained from thermomechanical analysis. A 44~55 % decrease of CTE is observed for APB- and APP-intercalated clay nanocomposites relative to the pure PS.

The value of Tg of the PS component in the nanocomposite is higher than the virgin PS and its thermal decomposition temperature is also higher significantly. It appears that the presence of the clay enhances the thermal stability of polystyrene.

3.1 Introduction

Polystyrene (PS) is one of the most mass-productive and commercialized polymers, and copolymers are being produced by various methods such as solution polymerization and emulsion polymerization. Hence, PS/clay nanocomposites may have huge the applicability, so many people have been focusing on the PS/clay nanocomposites with various methods. The clay in nanocomposites has been intercalated by in situ polymerization [1－6] and melts intercalation [7] of PS using organically modified silicate, and recently exfoliated by a few researchers. [8－11]

On the polymer-clay nanocomposites, number of papers that show higher thermal stability and a reduced rate of heat release in the cone calorimeter. [12－17] Barrier properties could include both the thermal barrier, which protects the polymer from fire, and the mass transport barrier, which makes it difficult for degradation products to leave the polymer.

The nanocomposite typically comprises the organically modified clay and the mother polymer. Montmorillonite (MMT), which is an aluminosilicate mineral with sodium counterions present between the layers, is the most commonly used clay. The space between these clay layers is referred to as the clay gallery. To make this inorganic clay compatible with organic polymers, the sodium counterions are usually ion-exchanged with an organic ammonium or phosphonium salt to convert the material into hydrophobic ammonium- or phosphonium-treated clays. The nanocomposites may be prepared either by a blending process (either melt blending or solution blending) or by an in situ polymerization process in the presence of the organically modified clay.

In this paper, we describe the preparation of two new organically modified clays and the preparation of nanocomposites of these clays by emulsion polymerization. An

organic ammonium salt (APB) and a phosphonium salt (APP) were shown in scheme 3-1. We synthesized intercalation agent of APB and compared thermal stability with APP. In general, intercalation agent usually is the linear aliphatic chain, but it is not good for thermal stability. [18 － 22] Some literatures report that they used phosphonium cation as intercalation agent to enhance thermal stability. [23, 24] The APB contains the rigid group of adamantane and be expected to enhance thermal stability as good as phosphonium cation. We used the APP not only contains phosphonium cation to keep from inflammation, but also has a double bond to participate in polymerization.

PS/clay nanocomposites were prepared through emulsion polymerization by suspending the organic-treated clay in styrene monomer. We used both X-ray diffraction (XRD) and transmission electron microscopy (TEM) to characterize the clay structure. The thermal properties of these PS/clay nanocomposites were characterized by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Coefficient of thermal expansion was obtained using thermomechanical analyzer (TMA). The molecular weight was obtained by gel permeation chromatography (GPC).

3.2 Experimental

3.2.1 Materials

Most of chemicals used in this study, including monomeric styrene, chemically pure acetone, methanol, tetrahydrofuran, potassium iodide (KI), and potassium hydroxide (KOH) were acquired from the Aldrich Chemical Co., Inc. The styrene monomer was purified by removing the inhibitor with the aid of an inhibitor-removal column, which was also acquired from Aldrich. Both Sodium dodecyl sulfate (SDS) and hydrochloride acid were obtained from Curtin Matheson Scientific, Inc.

Potassium persulfate (K2S2O8) and aluminum sulfate [Al2(SO4)3] were acquired from Fisher Scientific Co., USA. Allyl-triphenyl-phosphonium chloride (APP), chloroform, and hydrazine were obtained from Acros Organics, USA. Both 4-(1-adamantyl)phenol and n-(4-bromobutyl)phthalimide were obtained from TCI, Inc. Pristine Na-MMT was provided by Telekal Co., Taiwan.

3.2.2 Synthesis of Intercalation Agent of 4-(4-adamantyl phenoxy)-1-butanamine

(APB)

APB that contains a rigid group of adamantane was prepared according to the pathway shown in Scheme 3-2. First, 4-(4-adamantylphenoxy)butyl phthalimide (1) was prepared by the reaction between 4-(1-adamantyl)phenol (0.8 g, 3.5mmol) and N-(4-bromobutyl)phthalimide (1.19 g, 4.2 mmol) dissolved in 50 ml of acetonitrile.

Both potassium hydroxide (KOH, 0.5 g) and potassium iodide (KI, 0.006g) were added with magnetic stirring to obtain a homogeneously system and its contents were heated to 70 ^oC for 24 h. Removal of the solvent in a rotary evaporator at 50 ^oC gave a

Then the ether solution was dried with anhydrous sodium sulfate, followed by evaporation of ether under vacuum to afford yellow product (1).

Second, A mixture of 1 (0.438 g, 1.02 mmol), hydrazine monohydrate (0.3 g, 6 mmol), chloroform/methanol (7:3, 10 ml) was heated overnight at 55~60 ^oC. After cooling to RT, the solid by-product was filtered off. The filtrate was diluted with CHCl3 (100 ml), washed with water (2 x 100 ml), dried over MgSO4, and concentrated. The residue was dissolved in ether (10 ml) and CH2Cl2 (5 ml) to which was added a solution of methanesulfonic acid (0.1 g, 1.04 mmol) in ether (30 ml). The resulting mixture was concentrated to 10 ml and placed at -20 ^oC overnight. The solid was collected by filtration to afford pale yellow product (2).

3.2.3 Preparation of Modified Clays

The prewashed clay (1 g) and water (50 mL) were placed into a 100-mL two-neck round-bottom flask and stirred continuously for 4 h. The intercalation agents (APP and APB) in water (10 mL) was placed into another flask and then 10 % hydrochloric acid (1 mL) was added and then stirring for 1 h. This intercalation agent solution was then added into the suspended clay and the mixture was stirred overnight.

The mixture was filtered, washed several times with deionized water, and then dried overnight in a vacuum oven at room temperature.

3.2.4 Preparation of Polystyrene/Clay Nanocomposites

Emulsion polymerization was performed as follows. A suspension of clay (0.3 g) in deionized water (40 ml) was stirred for 4 h at room temperature. A aqueous solution of surfactant (APP or APB; 0.12 g) was added and the mixture was stirred for 4h.

KOH (0.02 g) and SDS (0.4 g) were added into the solution and the temperature was raised to 50 °C. Styrene monomer (10 g) and K2S2O8 (0.05g) were added slowly to the flask. Polymerization was performed at 50 °C for 8 h. After cooling, 2.5 % aqueous aluminum sulfate (10 mL) was added to the polymerized emulsion, followed by dilute hydrochloric acid (10 mL), with stirring. Finally, acetone was added to break down the emulsion completely. The polymer was washed several times with methanol and distilled water and then dried overnight in a vacuum oven at 80 °C. Similar procedures were employed to prepare virgin polystyrene.

3.2.5 Instrumentations

X-Ray diffraction spectra were collected on an M18XHF-SPA X-ray diffraction instrument (MacScience Co., Japan), using Co Kα radiation; Bragg`s law (λ = 2dsinθ) was used to compute the spacing. Transmission electron microscopy (TEM) images of the composites were obtained at 100 kV using a Hitachi H-7500 Electron Microscope.

The sample was ultramicrotomed at room temperature using a diamond knife on a Leica Ultracut UCT Microtome to give 70 nm-thick sections. The contrast between

The sample was ultramicrotomed at room temperature using a diamond knife on a Leica Ultracut UCT Microtome to give 70 nm-thick sections. The contrast between