Studies on Thermal Properties of PS Nanocomposites for the Effect of Intercalated Agent with Side Groups
2.3. Results and discussion
Structures of intercalated agents used to modify the clays are shown in Scheme 2-1.
2.3.1. Characterizations of C20-, C20-4VB, and C20-POSS modified clays.
Figure 2-1 shows the XRD curves for the starting and modified clays. The d spacing indicates the interlayer spacing of the silicate layers calculated from the peak position using the Bragg equation. The original clay with an intergallery spacing of 1.28 nm (2θ= 6.92°), increases to 3.95 nm for C20 (2θ = 2.23°), 3.33 nm for C20-4VB (2θ= 2.66°), and 3.80 nm for C20-POSS (2θ= 2.33°), respectively. The results indicate that these intercalated agents are successfully intercalated into the gallery and results are summarized in Table 2-1. 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 C20-POSS is inserted inside the gallery of the silicate.
2.3.2. Characterizations of polystyrene/clay Nanocomposites.
Figure 2-2 shows X-ray diffraction patterns of C20/clay/PS, C20-4VB/clay/PS, and C20-POSS/clay/PS nanocomposites. Essentially no peak can be detected for all nanocomposites from C20, C20-4VB, and C20-POSS modified clays, implying that they all have exfoliated structure. XRD alone may lead to false result in terms of the extent of exfoliation. As a result, TEM observations are necessary to further verify the extent of delamination and exfoliation. Either intercalated or exfoliated nanocompoaite can be distinguished on the bases of transmission electron microscopy (TEM). TEM images of C20/clay/PS, C20-4VB/clay/PS and C20-POSS/clay/PS are shown in Figure 2-3. In Figure 2-3(b), C20-4VB/clay/PS shows full dispersion and exfoliation of the
clay platelets within the PS matrix because the intercalated agent of C20-4VB containing a vinylbenzyl group allows for styrene polymerization taking place within the clay gallery. In Figure 2-3(a), these clay platelets of the C20/clay modified nanocomposite are mainly exfoliated, but in some regions they are aggregated and intercalated. In Figure 2-3(c), the image shows mainly the exfoliated structures for the C20-POSS modified nanocomposite.
2.3.3. Glass transition temperatures.
Figure 2-4 and Table 2-2 show that the pure PS exhibits a heat flow change at approximately 100 °C, corresponding to the Tg of PS. All DSC thermograms display single glass transition temperature in the experimental temperature range. C20/clay-, C20-4VB/clay-, and C20-POSS/clay- modified nanocomposites all show slightly higher Tg compared to the pure PS.
2.3.4. Molecular weights of the nanocomposites.
Table 2-3 shows the molecular weight and polydispersities of the PS and PS/silicate nanocomposites through emulsion polymerization. The sample for the molecular weights determination was filtered to remove all clay content before measurement and the results are shown in Table 2-3 From the table, Mn (or Mw) of the PS in PS/clay nanocomposites is higher, suggesting that clay may either act as an catalytic agent, resulting in a higher molecular weight of the PS with proceeding polymerization.
2.3.5 TGA analyses.
Figure 2-5 and Figure 2-6 present the thermal stabilities of the C20-, C20-4VB-,
and C20-POSS- modified clays and corresponding nanocomposites measured under nitrogen gas. Generally, the incorporation of clay into the polymer matrix is able to enhance thermal stability by acting as a superior insulator and mass transport barrier to the volatile products generated during decomposition. Figure 2-5 shows TGA traces for these intercalated clays. The C20-POSS/Clay decomposes at a higher temperature of 262 °C while C20/Clay and C20-4VB/Clay decompose at 189 and 243 °C, indicating the C20-POSS is thermally more stable than the C20 and C20-4VB. Thereby, the clay intercalated with C20-POSS has better thermal stability relative to those intercalated with C20 and C20-4VB.
All surfactant-modified nanocomposites show better thermal stability than the virgin PS as shown in Figure 2-6. The decomposition temperatures for 5% weight loss of C20-, C20-4VB, and C20-POSS- modified nanocomposites are 15, 12, and 25 °C higher than the virgin PS. This observed enhancement in the thermal properties is due to the presence of the clay to act as barriers to minimize the permeability of volatile degradation products out from the material. The values of 5 and 50% weight loss temperatures and the corresponding char yields are summarized in Table 2-1.
2.3.6 Coefficient of thermal expansion.
Dimensional stability is critical in many applications, poor dimensional stability can cause warping or other changes in sharp. Polymer/clay nanocomposites provide improvements both on thermal and dimensional stabilities. As shown in Figure 2-7 and Table 2-1, the CTE of the virgin PS is 164 μm/m °C while the CTEs for C20/clay/PS, C20-4VB/clay/PS, and C20-POSS/clay/PS are 120, 134, and 100 μm/m °C, respectively. The maximum reduction in coefficient of thermal expansion is from the C20-POSS/clay/PS, 39 % lower than the virgin PS.
The obtained lower CTE from these composites can be attributed to the particle rigidity and fine dispersion of the clay platelets in the PS matrix. The incorporation of the modified clay can reduce CTE and provides products with good dimensional stability.
2.4. Conclusions
In this paper, we have employed the novelty of the preparation of the C20-POSS and C20-4VB containing intercalated agents. Syntheses of exfoliated nanocomposites via emulsion polymerization of styrene in the presence of 3 wt % clay containing the C20, C20-4VB, and C20-POSS intercalated agents, were prepared respectively. XRD results show that d spacing increases from 1.28 nm to 3.95, 3.33, and 3.80 nm after intercalation. The C20-4VB containing vinyl benzyl group results in more effective in promoting fully exfoliated structure in polystyrene matrix. All modified clay nanocomposites result in higher Tg and higher thermal degradation temperature than the virgin PS, especially for the C20-POSS/clay nanocomposite resulting in 25 °C increase in the thermal degradation temperature. These well distributed clay platelets tend to retard the segmental movement of PS and result in reduced CTE. The incorporation of the 3 wt% clay leads to improvements in thermal stability, slight increase in glass transition temperature, and decrease in coefficient of thermal expansion.
Acknowledgments
The authors thank the National Science Council, Taiwan, for financially supporting this research under Contract NSC-95-2221-E-009-118 and NSC-96-2120-M-009-009
References
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Table 2-1. Basal spacing and organic fraction of the OMMT.
Sample (001) Basal spacing(nm) Organic fraction (%)a
Clay 1.28 -
C20/Clay 3.95 53.3
C20-4VB/Clay 3.33 47.3
C20-POSS/Clay 3.80 37.5
a organic fraction was based on TGA analyses.
Table 2-2. Results of thermal and mechanical properties of polystyrene and polystyrene nanocomposites.
Sample Tg(°C)a T0.05(°C)b T0.5(°C)c Char at 600 °C d (％)
CTE (μm/m °C)
PS 100 390 424 0 164
C20/Clay/PS 107 405 456 1.59/1.4 120
C20-4VB/Clay/PS 108 402 454 1.59/1.59 134
C20-POSS/Clay/PS 105 415 457 1.98/2.4 100
a Glass transition temperature(Tg).
b 5％ Degradation temperature(T0.05).
c 50％ Degradation temperature(T0.5).
d Chat at 600(°C)％ (expected %/experiment %)
Table 2-3. 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
C20/Clay/PS 47.5 58.6 1.23
C20-4VB/Clay/PS 45.2 57.4 1.27
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.
Scheme 2-1. Intercalation agents for organic modified clays preparation.
Scheme 2-2. Synthesis of the POSS -Cl compound.
Scheme 2-3. Synthesis of the C20-4VB and C20-POSS intercalated agents.
N
1 2 3 4 5 6 7 8 9 10
Intensity (a. u.)
2θ
Clay C20/Clay C20-4VB/Clay C20-POSS/Clay
Figure 2-1. X-Ray diffraction patterns of pure clay, and intercalated clays.
2 3 4 5 6 7 8 9 10
Intensity (a. u.)
2θ
C20/Clay/PS C20-4VB/Clay/PS
C20-POSS/Clay/PS
Figure 2-2. XRD spectra of the three surfactant-containing nanocomposites indicating the extent of delamination.
Figure 2-3. TEM images of (a) C20, (b) C20-4VB, (c) C20-POSS (low magnification), and (d) C20-POSS (high magnification)-treated nanocomposites.
60 80 100 120 140 108 oC
105 oC
107 oC
Heat Flow(Endo,←))
Temperature(oC)
a b c d
100 oC
Figure 2-4. DSC curves glass transition temperature of (a) PS, (b) the nanocomposites formed used C20, (c) the nanocomposites formed used C20-4VB, and (d) the
nanocomposites formed used C20-POSS.
100 200 300 400 500 600 700 800 0
20 40 60 80 100
(a) Pure clay (b) C20-POSS/clay (c)C20-4VB/clay (d)C20/clay
Weight(%)
Temperture(oC) (a) (c) (b)
(d)
Figure 2-5. TGA curves of (a) Pure Clay, (b) C20-POSS/Clay, (c) C20-4VB/Clay, and (d) C20/Clay.
200 300 400 500 600
Figure 2-6. (A)TGA and (B) DTG curves of the nanocomposites under a nitrogen atmosphere: (a) pure PS, (b) the nanocomposite formed with C20, (c) the nanocomposite formed with C20-4VB, and (d) the nanocomposite formed with
C20-POSS.
a b c d 0
20 40 60 80 100 120 140 160 180
CTE (μm/mo C)
Sample
Figure 2-7. Coefficient of thermal expansion of (a) pure PS, (b) the nanocomposite formed with C20, (c) the nanocomposite formed with C20-4VB, and (d) the
nanocomposite formed with C20-POSS.