Thermal and mechanical analysis characterization

Thermal stability of nanocomposite was investigated by a TA Instruments Q50 apparatus. The sample ~5 mg was placed in a Pt cell with scan rate of 20 ˚C /min from 30 to 800 ˚C under a 40 mL/min flow of nitrogen gas. Thermal analysis through differential scanning calorimetry (DSC) was performed using a Du-Pont (DSC-2010) to measure the glass transition temperature (Tg) of the nanocomposite. The sample was preheated at a scan rate of 20 ˚C /min from 30 to 150 ˚C under a nitrogen atmosphere. A small sample (ca. 5-10 mg) was weighted and sealed in an aluminum pan. The sample was quickly cooled to 10 ˚C from the first scan and then scanned

between 30 and 150 ˚C at the scan rate of 20 ˚C /min. The glass transition temperatures are taken as the midpoint of the heat capacity transition between the upper and lower points of deviation from the extrapolated glass and liguid lines. The coefficient of thermal expansion (CTE) was measured using a thermomechanical analyzer (TMA TA 2940) by recording the change in dimension of the specimen with temperature. The specimen was heated from 25 to 150 ˚C at a heating rate of 5 ˚ C/min.

3.3 Results and discussion

3.3.1 Morphologies of modified clays and nanocomposites.

Microstructures of polymer-layered silicate nanocomposites were characterized by XRD and TEM. Figure 3-1 shows the X-ray diffraction curves of the pristine clay and modified clays in the 2θ region of 2-10°. For the pristine clay, the Bragg diffraction peak at 2θ=6.92° corresponds to d-spacing of 1.28 nm. For the POSS-NH2/clay and C20-POSS/clay, the 2θ value shifts from 6.92° (1.28 nm) to 5.51° (1.61 nm) and 2.33°

(3.80 nm) after ion exchange, indicating that the basal spacing is expanded as the sodium cations in the interlayer galleries are replaced by intercalated agents of POSS-NH2 and C20-POSS. The increase of the basal spacing indicates that the clay can be efficiently intercalated by POSS-NH2 and C20-POSS. The d-spacing of the C20-POSS-modified clay is substantially greater than the POSS-NH2-modified clay.

Larger interlayer spacing favors the penetration of styrene monomer and the formation of exfoliated nanocomposite by providing more hydrophobic environment.

The pure POSS-NH2 has characteristic diffraction peaks arising from the aggregation of the POSS [23]. Figure 3-2 shows XRD patterns of the unmodified clay and the C20-POSS modified clay. The pure C20-POSS has a broad peak in the region of 5-12°

arising from the C20 long aliphatic chain. When the C20-POSS is inserted between the galleries of the clay, the d spacing is increased from 1.28 nm for original clay to 3.80 nm, implying that the organic modifier is incorporated between and pushing the clay layers. Both nanocomposites do not show XRD diffraction peak as shown in Figure 3-3, indicating the silicate layers are exfoliated in the polymer matrix. TEM images for POSS-NH2/clay and C20-POSS/clay nanocomposites at 3% inorganic clay loading are shown in Figures 3-4(a) and 3-4(b), indicating that the exfoliated clay platelets are distributed in the matrix homogeneously and randomly.

3.3.2 Fourier transfer infrared analyses.

The representative FT-IR spectra of the organophilic clay, POSS-NH2-modified and C20-POSS modified clays are given in Figure 3-5. After ion exchange, FT-IR spectroscopy can provide important information regarding the difference between intercalated agents and modified clays. In Figure 3-5(a), characteristic vibration bands of the pure clay are 1030 cm^-1 (Si-O), 520 cm^-1 (Al-O), and 470 cm^-1 (Mg-O) [24-26].

In Figure 3-5(b), the absorption peaks in the region of 2950-2800 is assigned to the stretching vibration of aliphatic C-H. The symmetrical Si-O-Si band in the silsequioxane cage is characterized by the stretching band at 1109 cm^-1. In Figure 5(d), C20-POSS contains both alkyl chain and POSS moiety where the POSS moiety exhibits characteristic absorption peaks at 2950－2800 cm^-1 (C－H bonds ), 1230 cm^-1 (Si－C bonds), 1109 cm^-1 (Si－O－Si bonds of the cage structure). The characteristics of the vibration band of alkyl chain appear at 2920, 2850, and 1475 cm^-1 (–CH2– vibration bands). Figures 3-5(c) and 3-5(e) show the features of combination of characteristic bands of pure clay, POSS-NH2, and C20-POSS. IR analysis further confirms the existence of these intercalated agents in these intercalated clay samples, implying that these intercalation of the intercalated agents are indeed present within the gallery gap. These observations support the explanation in the earlier observation from XRD.

3.3.3 Thermal properties.

Figure 3-6 presents DSC traces of these nanocomposites with different intercalated agents. All DSC thermograms display single glass transition temperatures in the experimental temperature range. The glass transition temperature of PS occurs

at 100 °C. With the addition of the POSS and C20-POSS modified MMT to the polymer matrix, the glass transition temperatures (Tgs) of the POSS-NH2/clay/PS and C20-POSS/clay/PS are 108, and 105 °C, respectively. From the DSC results that the incorporation of the organoclay resulted in an increase in the Tg relative to virgin PS, as summerized in Table 3-1. The addition of clay results in Tg increase which can be attributed to the retardation of PS chain movement.

Figure 3-7 presents the thermal stabilities of POSS-NH2- and C20-POSS-modified clays and nanocomposites investigated by TGA. Both nanocomposites show improved thermal stabilities than the virgin PS. The improvement in the degradation temperature is mainly due to the homogeneous dispersion of silicate nanoplatelets in the PS matrix [27-30]. In Figure 3-7, the C20-POSS/clay decomposes at 262 ˚C while the POSS-NH2/clay decomposes at higher temperature of 386˚C. The POSS-NH2-modified clay is relatively more stable than the C20-POSS-modified clay. Essentially all nanocomposites give higher decomposition temperatures than the pristine PS and the improved thermal stability can be attributed to the diffusion hindrance of the decomposited volatiles. The values of 5% and 50% weight loss temperatures and the char yields are summarized in Table 1.

3.3.4 Molecular weights of the nanocomposites.

Molecular weight and molecular weight distribution (PDI) by GPC analyses of polymer samples recovered after excluding all clay content are listed in Table 3-2.

From the Table 3-2, molecular weight (Mw or Mn) of the PS in the PS/clay nanocomposites is higher than the pure PS, suggesting that clay may act as a catalytic agent responsible for the observed higher molecular weight of the PS with the

proceeding emulsion polymerization.

3.3.5 Coefficient of thermal coefficient.

Thermal mechanical analyzer (TMA) was used to determine the coefficient of thermal expansion of the POSS/clay nanocomposites. The thermal expansion coefficient is an important issue for polymers in engineering applications. The CTE was measured from the initial linear slope of the thermal strain-temperature plot. A low thermal expansion coefficient is often desirable to achieve dimensional stability and can be achieved by incorporation of a rigid and low CTE filler material. From the data in Table 3-1, the CTE of the virgin PS is 164 μm/m ˚C and the addition 3 wt % organically modified clays reduces the CTE values to 98 and 100 μm/m ˚C for POSS/clay/PS and C20-POSS/clay/PS approximately 40 % reduction relative to the virgin PS.

In general, the extent of CTE reduction depends on the particle rigidity and fine dispersion of the clay platelets in the PS matrix and also duo to efficient stress transfer to clay layers. The retardation of PS chain segmental movement through incorporation of organically modified clays also leads to decrease in the coefficient of thermal expansion (CTE). The incorporation of the organically modified clays results in significant improvement in dimensional stability of the PS matrix.

3.4 Conclusions.

The POSS-clay hybrids of polystyrene are prepared via emulsion polymerization using two organically modified clays, POSS-NH2 and C20-POSS, as intercalated agents. X-ray diffraction (XRD) results indicate that the clay is successfully intercalated by POSS-NH2 and C20-POSS. The random dispersion of these exfoliated silicate layers in these nanocomposites are identified by XRD and TEM. These well dispersed clay platelets in PS matrix result in improved thermal properties in terms of thermal decomposition temperature (Td) and glass transition temperature (Tg). In addition, the incorporation of these organoclay results in significant reduction in coefficient of thermal expansion of virgin PS.

Acknowledgments

The authors thank the National Science Council, Taiwan, for financially supporting this research under Contract NSC-95-2221-E-009-118.

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Table 3-1. Results of Thermal and Mechanical properties of Polystyrene and Polystyrene Nanocomposites.

Sample Tg,(℃)^a T0.05,(℃)^b T0.5,(℃)^c Char at 600 ℃ (％) CTE (μm/m ^。C)

PS 100±0.5 390±1.7 424±0.8 0 164±2

POSS/Clay/PS 108±0.6 411±1.3 446±1.2 2.9 98±1

C20-POSS/Clay/PS 105±0.3 415±1.1 457±0.9 2.4 100±3

a Glass transition temperature(Tg).

b 5％ Degradation temperature(T0.05).

c 50％ Degradation temperature(T0.5).

Table 3-2. Molecular Weights of Polystyrene and Polystyrene Nanocomposites Sample Mn(×10⁴)^a Mw(×10⁴)^b PDI(Mw/Mn)^c

PS 34.5 53.1 1.54

POSS-NH2/Clay/PS 40.9 54.0 1.32

C20-POSS/Clay/PS 47.7 58.7 1.23

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

O Si

Scheme 3-1. Chemical structures of the intercalated agents used to prepare the modified clays.

Si OH

Scheme 3-2. Synthesis of the POSS -Cl compound.

N

Scheme 3-3. Synthesis of the C20-POSS intercalated agent.

2 4 6 8 10

Intensity (a. u.)

2θ

Clay

POSS-NH₂/Clay C₂₀-POSS/Clay

Figure 3-1. X-Ray diffraction patterns of pure clay, and intercalated clay.

2 3 4 5 6 7 8 9 10

Intensity (a. u.)

2θ C20-POSS

Clay

C₂₀-POSS/Clay

Figure 3-2. XRD spectra of C20-POSS, pure clay, and C20-POSS/Clay.

2 4 6 8 10 12 C₂₀-POSS/Clay/PS

POSS-NH

2/Clay/PS

Intersity (a. u.)

2θ

Figure 3-3. XRD spectra of the two surfactant-containing nanocomposites indicating the extent of delamination.

Figure 3-4. TEM images of (a) POSS-NH2 and (b) C20-POSS-treated nanocomposites.

4000 3000 2000 1000

Wave number (cm^-1) (a)

(b) (c) (d) (e)

Clay POSS-NH

2

POSS-NH₂/Clay C20-POSS C₂₀-POSS/Clay

Figure 3-5. IR spectra of the two intercalated agent, intercalated clay, and pure clay.

60 80 100 120 140

a: PS

b: POSS-NH

2/Clay/PS c: C20-POSS/clay/PS

Heat Flow (Endo,←)

Temperature(^oC)

a b c

Figure 3-6. DSC curves glass transition temperature of (a) PS, (b) the nanocomposites formed used POSS-NH2, and (c) the nanocomposites formed used C20-POSS.

100 200 300 400 500 600 700 800 pure PS, (e) the nanocomposite formed with POSS-NH2, and (f) the nanocomposite

formed with C20-POSS.

.

Chapter 4

Effect of the organically modified Nanoclay on