# Thermal Stability

## Thermal Stability

7 Thermal Stability 7.1 Introduction Thermogravimetry (TG) is a technique used to measure the change in mass of a sample as a function of temperature...

7

Thermal Stability 7.1 Introduction Thermogravimetry (TG) is a technique used to measure the change in mass of a sample as a function of temperature, time, or both. TG is used to measure the thermal stability of polymeric materials. The instrument used is called a thermogravimetric analyzer (TGA). Changes of mass occur during sublimation, evaporation, decomposition, chemical reaction, and magnetic or electrical transformations [1]. The choice of purge gas and the conditions present in the specimen chamber are crucial factors in TG analysis. When heating occurs under the flow of an inert gas, such as nitrogen, helium, or argon, nonoxidative degradation of the specimen being tested occurs, whereas the use of air or oxygen allows oxidative degradation of the specimen. The extent of heat transfer to the specimen also depends on the gas flow rate. In recent years, researchers combined TGA with FTIR or mass spectrometry to analyze polymeric materials. Such combinations are always advantageous when substances are identified by methods involving a certain loss of mass. The underlying principle is that the gaseous components generated during heating in the TGA are transferred by a constant gas stream into another test chamber [1]. In the case of polymeric materials, changes of mass may involve more than one step. For a single-step change of mass, the percentage of mass ML is calculated from the masses ms (initial mass) and mf (mass after the test) using the following equation: ML ¼

m s  mf  100% ms

ð7:1Þ

In multistep losses of mass, in addition to ms and mf, the mass mi, an intermediate between the other two losses of mass, is determined. For example, the first loss of mass ML1, the second loss of mass ML2, . . ., are calculated from the following equations: ML1 ¼

m s  mi  100% ms

ML2 . . . ¼

m i  mf  100% ms

ð7:2Þ ð7:3Þ

In general, the incorporation of clay into a polymer matrix enhances the thermal stability of the neat polymer. This improvement in thermal stability could be due to the different effects of dispersed silicate layers [2], such as a high surface to volume ratio, very low permeability, a decrease in the rate of evolution of the volatile products formed, and the formation of high-performance carbonaceous silicate chars on the clay surface that insulate the bulk material and slow the escape of volatile products generated during decomposition and absorption of formed gas into clay platelets. Clay-Containing Polymer Nanocomposites: From Fundamentals to Real Applications © 2013 Elsevier B.V. All rights reserved.

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Dispersed silicate layers, except at decomposition temperatures, can lead to a drastic change in the degradation mechanism of a polymer matrix, leading to products that differ completely in quantity and quality from neat polymers. The results also show that, with higher clay loading, the promoter effect rapidly increases and becomes imprintable. In such cases, the overall thermal stability of nanocomposites decreases.

7.2 Overview of Thermal Stability Blumstein [3] first reported the improved thermal stability of polymer–clay nanocomposites that combined poly(methyl methacrylate) (PMMA) and MMT. These PMMA nanocomposites were prepared by free-radical polymerization of MMA intercalated in the clay. The author showed the PMMA that was intercalated (d(001) spacing increase of 0.76 nm) between the galleries of MMT resisted thermal degradation under conditions that would otherwise completely degrade neat PMMA. TGA data revealed that both linear and cross-linked PMMA intercalated into MMT layers had a 4050 C higher decomposition temperature. Blumstein argues that the stability of the PMMA nanocomposite is due not only to its different structure but also to the restricted thermal motion of the PMMA in the gallery. Like Blumstein, Liaw et al. [4] found that intercalation of PMMA chains into the silicate galleries of clay plays a significant role in the higher thermal stability of PMMA–clay nanocomposites. However, a decrease in thermal stability was observed in the case of PMMA– kaolinite composites when compared with neat PMMA [5, 6]. A similar decrease in the thermal stability of a PMMA matrix is reported by Laachachi et al. [7] after the formation of nanocomposites with organically modified MMT. Such an occurrence may be due to the degradation of functional groups at higher temperatures, which eventually leads to a decrease in the overall thermal stability of PMMA–OMMT nanocomposites. Over the last decade, many reports have been concerned with the improved thermal stability of nanocomposites prepared with various types of pristine and organically modified clays and polymer matrices [8–43]. For example, Zanetti et al. [44] conducted detailed TG analyses of nanocomposites based on EVA. The inorganic phase was fluorohectorite (FH) or MMT; both exchanged with a octadecylammonium cation. They found that the deacylation of ethyl (vinyl acetate) (EVA) in nanocomposites is accelerated and may occur at temperatures lower than those for the neat polymer or the corresponding microcomposite due to catalysis by the strongly acidic sites created by thermal decomposition of the silicate modifier. These sites are active when there is intimate contact between the polymer and the silicate. Slowing down the volatilization of the deacylated polymer in nitrogen may occur because of the labyrinth effect of the silicate layers in the polymer matrix [45]. In air, a nanocomposite exhibits a significant delay in weight loss that may derive from the barrier effect caused by the diffusion of both volatile thermo-oxidation products to the gas phase and oxygen from the gas phase to the polymer. According to Gilman et al. [46], this barrier effect increases during volatilization because of the reassembly of the reticular of the silicate on the surface. In the case of EVA–clay nanocomposites, Costache, Jiang, and Wilkie [47] found that dispersed silicate layers in an EVA matrix do affect the degradation mechanism (refer

Chapter 7 • Thermal Stability

Pm

( )p

Pn O

245

O

O

O

350 deg C -CH3COOH Pm

( )p

Pn

(P-H)

450 deg C

Pn

Pn

+

( )p

(Pn•)

+

Pm

(Pm•)

Pm

FIGURE 7.1 Thermal degradation of EVA by allylic scission of the main chain. Source: Reproduced from Costache, Jiang, and Wilkie [47] by permission of Elsevier Science Ltd.

to Figure 7.1) of the matrix. According to the authors, the difference in the degradation mechanism of an EVA matrix in the presence of clay may be related to the secondary reactions by which secondary allylic radicals can form [47]. In the case of a nanocomposite, dispersed clay platelets can confine these radicals so that a recombination reaction is much more probable than in the case of neat EVA, as shown in Figure 7.2. Peeterbroeck et al. [48] observed quite different results when they studied the effect of clay and similar materials on the thermal degradation of EVA nanocomposites. Different

Pm

Nanocomposite + Po

Virgin EVA

P-H

R Pm

Pm

+

P•

Po R volatilization/random scission FIGURE 7.2 Possible radical recombination reactions for EVA nanocomposites. Source: Reproduced from Costache, Jiang, and Wilkie [47] by permission of Elsevier Science Ltd.

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CLAY-CONTAINING POLYMER NANOCOMPOSITES

degrees of interfacial interaction between clay surfaces and an EVA matrix lead to the formation of different types of composites—from microcomposites to nanocomposites. The TGA data showed that a higher degree of delamination of the silicate layers in EVA leads to higher thermal stability of EVA–clay composites. This finding supports the hypothesis that thermal degradation delay is due to a decrease in the rate of evolution of volatile products, because gas permeability is significantly improved in the case of highly delaminated nanocomposites. The thermal stability of PS is significantly improved after nanocomposite formation with clay in both pyrolytic and thermo-oxidative environments [49]. The thermal stability of PS and its clay-containing nanocomposites has been modeled and simulated, and the results show very good agreement between experimental and simulated curves in both dynamic and isothermal conditions [50]. As with most nanocomposite systems, the thermal stability of PS–clay nanocomposites is directly related to the degree of dispersion of silicate layers in the nanocomposites. In the case of PS–Na-MMT, PS–C-MMT (MMT modified with cetyltrimethyl ammonium bromide), and PS–A-MMT (MMT modified with ammonium persulfate) composite systems, the degree of dispersion of silicate layers, and hence the onset degradation temperature, follows the order PS–Na-MMT < PS–CMMT < PS–A-MMT, as shown in Figure 7.3. In the last decade, researchers used different types of organically modified clays for the preparation of PS nanocomposites; and in all cases, the thermal stability of neat PS has been shown to be moderately improved after nanocomposite formation [51–55]. The most interesting behavior was observed when PS nanocomposites were prepared using phosphonium cation-modified MMT [49]. The TGA scans of neat PS and various

100

80

60

40

Weight (%)

Weight (%)

100

80

20

0 100

d

d

b 60 300

c

c

a

a 400 Temperature (∞C)

200

500

b

300 400 Temperature (∞C)

500

600

FIGURE 7.3 Thermal gravimetric diagrams for PS and PS–MMT composites: (a) PS, (b) PS–Na-MMT, (c) PS–C-MMT (MMT modified with cetyltrimethyl ammonium bromide), and (d) PS–A-MMT (MMT modified with ammonium persulfate). Source: Reproduced from Li, Yu, and Yang [49] by permission of Elsevier Science Ltd.

Weight loss (%)

Chapter 7 • Thermal Stability

247

PS-VB16

80 PS-P16

PS

40

PS-OH16

0 200

300 400 Temperature (8C)

500

FIGURE 7.4 Thermogravimetric analysis results of polystyrene and its nanocomposites prepared with alkylphosphonium cation modified MMT. Source: Reproduced from Zhu, Morgan, Lamelas, and Wilkie [56] by permission of the American Chemical Society.

nanocomposite samples are shown in Figure 7.4. These scans show that the thermal stability of the nanocomposite is enhanced relative to that of virgin PS [56] and that the typical onset temperature of the degradation is approximately 50 C higher for the nanocomposites. Figure 7.4 clearly shows that the degradation mechanism of phosphonium nanocomposites is somehow different from the others; the degradation has a second step. This second step accounts for approximately 30% of the degradation of the phosphonium–PS nanocomposite and must be attributed to some interaction between the clay and the PS that serves to stabilize the nanocomposite. The most likely explanation is that the higher decomposition temperature of the phosphonium clay provides for the formation of char at a time more opportune to retain the PS. In the case of ammonium clays, char formation occurs earlier and can be broken up by the time the polymer degrades. The variation ln the temperature at which 10% degradation occurs for all three nanocomposites is shown as a function of the amount of clay in Figure 7.5 [56]. Even with Temperature of 10% Weight Loss 430 420 Temperature (8C)

410 400 390 380 370

ps-vb16

360

ps-oh16

350

ps-p16

340 0

3

6 9 Clay Content (%)

12

FIGURE 7.5 Temperature of 10% mass loss for nanocomposites as a function of the fraction of clay. Source: Reproduced from Zhu, Morgan, Lamelas, andWilkie [56] by permission of the American Chemical Society.

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as little as 0.1 wt % of clay present in the nanocomposite, the onset temperature was significantly increased. The thermal stability of PE-based nanocomposites has also been studied extensively in both inert and oxidative environments [57–64]. Organically modified clays have a very low, thermodynamically favorable interaction with the PE matrix. For this reason, compatibilizers, such as maleic anhydride (MA), can be used. In comparing the thermal stability of neat PE and its clay-containing nanocomposites, the presence of a small amount of MA has been found to improve the thermal stability of nanocomposites tremendously, as shown in Figure 7.6. Furthermore, the higher thermal stabilization of PE-based nanocomposites is directly related to the clay loading. However, in most cases, the thermal stability starts to decrease after incorporation of more than 4 wt % clay. Similar behavior was observed in the case of PP–clay nanocomposites [65–68]. For example, Sharma, Nema, and Nayak [67] found that the incorporation of 20 wt % PP-gMA with C20A improved the thermal stability of neat PP at approximately 90 C. This drastic improvement in thermal stability could be due to the high level of confinement of PP chains in nano-dimensional silicate galleries of clay. The improvement could also be due to the formation of high-performance carbonaceous silicate char buildup on the surface, which insulates the bulk material and slows the escape of volatile products generated during decomposition [50].

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Weight (%)

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0 300

Neat LDPE LDPE-Clay 1.0 wt% LDPE-Clay 2.0 wt% LDPE-Clay 3.0 wt% LDPE-Clay 4.0 wt% LDPE-Clay 5.0 wt% 350

400 450 Temperature (∞C)

500

550

FIGURE 7.6 Thermogravimetric analysis scans of solid maleic-anhydride-grafted low-density polyethylene (LDPE-gMA)–clay nanocomposites. Source: Reproduced from Hwang, Hsu, Yeh, Yang, Chang, and Lai [63] by permission of Elsevier Science Ltd.

Chapter 7 • Thermal Stability

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These two possible reasons for an increase in thermal stability at approximately 90 C explain the formation of exfoliated structures in the presence of 20 wt % PP-g-MA. The presence of clay platelets was confirmed by the residue content, which was approximately 3% in all the nanocomposites. Further investigation of the kinetics of degradation using the Coats–Redfern model shows that simultaneous incorporation of organically modified MMT nanocomposites significantly enhances the activation energy of degradation. Completely different thermal behavior was observed in the case of PVC nanocomposites, in which the incorporation of OMMT accelerates the degradation of the polyvinyl chloride (PVC) matrix [69]. The onset degradation temperature for PVC nanocomposites is lower than for neat PVC. The organic ammonium cations act as a Lewis acid and accelerate chlorine ion separation from the PVC matrix and then absorb it to form the hydrochloric salt of organic amine. This salt easily releases hydrochloric gas (HCl) at high temperatures and induces the PVC to self-catalyze degradation. Such an effect is more obvious in samples without compatibilizers. However, the thermal degradation mechanism of PVC is completely changed when a small amount of MA is added to PVC–OMMT composites. MA tends to hinder direct interaction between the amine group and PVC. MA also acts as a covering layer on the surface of OMMT through the formation of hydrogen bonds with the OMMT surface. Furthermore, the maximum decomposition temperatures, Tmax for PVC–OMMT, were not significantly different from Tmax for PVC. Meanwhile, Tmax for PVC–OMMT–MAH was higher than Tmax for PVC. OMMT possesses higher thermal stability, and its layer structure exhibits a strong barrier effect that hinders the evaporation of the small molecules generated in the thermal decomposition of the PVC matrix. Dispersion (intercalation and partial exfoliation) of OMMT is better in the presence of MA, and its barrier properties are consequently improved. Researchers also studied the effect of clay-surface functionality on overall thermal stability and color formation in PVC–clay nanocomposites [70–73]. PVC composites containing different types of pristine and organically modified MMT were heated in an oven, removed after a defined time period then compared [70]. The temperature at which samples are heated has an enormous influence on the nanocomposite’s color and is different from one nanofiller to another. To be more precise, samples can be separated into two groups. The first is a group of neat PVC composites and nanocomposites with pristine MMT as nanofiller; the second is a group of nanocomposites containing chemically modified types of MMT as nanofillers. The changes between the neat PVCs and nanocomposites in the first group are almost the same over time. The first change that occurs in samples in this first group seems to appear after 30 min of thermal treatment. In samples in the second group, on the other hand, the first change appears after 10 min. Less responsive thermal behavior was found for nanofiller C93A than for C30B, where the visible color change was shifted to an earlier time period and occurred in less than 20 min. This difference in the behavior between C93A and C30B arises from the fact that C93A contains a ternary ammonium salt, which is less thermally stable than the quaternary ammonium salt present in the interlayer space of C30B. Similar results are reported by Awad et al. [72].

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CLAY-CONTAINING POLYMER NANOCOMPOSITES

Chapter 7 • Thermal Stability

251

H+ H+

H+

HO

Increasing hydroxyl groups

H+

H+

H+

Grafting

H+

OCH2 OH

HO

HO

Si

O

OH

OCH2

H2CO

HO

O

OH HO

HO

HO

HO

O

O

HO S

H-MMT

O

O

S

O O

O

OCH2 S

Si

OCH2

Si

O

Acid treatment

S

S-MMT

Ammonium HO

Intercalating

Physisorption

OH

Coating

HO

N

OH

HO

OH

OH

OH

+ + N

N+

HO

Na-MMT

OH

+

+ N

HO

HO N HO

N

+

+ N

+

HO

OH

+ N

N

OH

N

HO

HO

HO

N+

HO

+ N

+

H2CO

OH HO

e

N + N

OH

HO

N+

Silan

HO

+ HO N HO

N+

A-MMT

ed

sh

Wa

+ N

OH

HO HO

+ HO

N

OCH2

+ N

+

O

O

S

O

O Si

O O

Si

H

OCH2

+ N

O

O Si

Si OCH2

O

OCH2 Si

O

W-A-MMT S-A-MMT FIGURE 7.7 Preparation of clay with different contents of hydroxyl groups on the edge of the clay platelets and ammonium linkage on clay. Source: Reproduced from Xu, Ding, Qian, Wang, Wen, and Zhou [95] by permission of Elsevier Science Ltd.

100

Weight (%)

80 60

a

40

b

20 0 300

400

500

600

Temperature (8C)

c

700

800

FIGURE 7.8 TGA curves (relative weight loss as a function of temperature) for (a) pure polysulfone, (b) nanocomposite with 1 wt % clay, and (c) nanocomposite with 5 wt % clay. Source: Reproduced from Sur, Sun, Lyu, Mark [96] by permission of Elsevier Science Ltd.

Chapter 7 • Thermal Stability

253

100

weight (%)

75

PCL

50

1 wt% 3 wt%

25

5 wt% 10 wt% 0 250

300

350 400 temperature ⬚C

450

500

FIGURE 7.9 Temperature dependence of the weight loss under an air flow for neat PCL and PCL nanocomposites containing 1, 3, 5, and 10 wt % (relative to inorganics) of MMT-Alk. Source: Reproduced from Lepoittevin, Devalckenaere, Pantoustier, Alexandre, Kubies, Calberg, Jerome, and Dubois [98] by permission of Elsevier Science Ltd.

The thermal stability of PBS–clay and PBSA–clay nanocomposites has been studied by various researchers [99–106] using TGA Weight loss due to the formation of volatile products after degradation at high temperatures, either in an inert gas atmosphere or in air, is usually monitored as a function of temperature. This illustrates, once again, the dependence of the thermal stability of nanocomposites on the organic modifier used in the preparation of organoclays. Different behavior is observed in synthetic biodegradable aliphatic polyester (BAP)– organoclay nanocomposite systems, in which the thermal degradation temperature and thermal degradation rate systematically increase with increasing amounts of organoclay, up to 15 wt % [100]. The TGA results for neat BAP and various nanocomposites are presented in Figure 7.10. As with PS-based nanocomposites, a small amount of clay increases the residual weight of BAP–OMMT because of the restricted thermal motion of the polymer in the silicate layers. The residual weight of various materials at 450 C increases in the following order: BAP < BAP03 < BAP06 < BAP09 < BAP15 (where the number indicates the wt % of clay). These improved thermal properties are also observed in other systems, such as SAN [107, 108], the intercalated nanocomposite prepared by emulsion polymerization. The thermal stability of clay-containing nanocomposites of epoxy is quite interesting and directly related to the curing process and clay loading used. For example, Ingram, Liggat, and Petbrick [109] report an increase in the thermal stability of an epoxy–clay nanocomposite when it was cured at 180 C. This was confirmed using TGA analysis. The onset of the degradation temperature was delayed, and the temperature window of degradation was strongly influenced by the addition of clay. In contrast, however, when the nanocomposite material was postcured at 220 C, the addition of nanoclay resulted in a decrease in the thermal stability of the epoxy resin. This may be attributed to dissociated alkyl chains,

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CLAY-CONTAINING POLYMER NANOCOMPOSITES

100 100

Residual Weight [%]

80

95

90

60 85 300

350

400

BAP00 40

BAP03 BAP06 BAP09

20

BAP15 0 100

150

200

250

300

350

400

450

500

Temperature [⬚C] FIGURE 7.10 TGA of BAP–organically modified MMT with different organoclay. Source: Reproduced from Lim, Hyun, Choi, and Jhon [100] by permission of the American Chemical Society.

after the resin is subjected to the high temperature and its thermal properties are destabilized [41]. This finding is critical for maximizing the enhancements possible with the use of nanoclay. All cure cycle temperatures must be carefully selected; otherwise, detrimental effects may result, even when similar levels of exfoliation and mechanical properties are achieved. However, Carrasco and Page`s [110] found that the thermal decomposition of cured materials was independent of cure temperature but dependent on the clay content. With the incorporation of crude clay, the Ti and Tmax shifted upward [111]. The enhancement of the resin’s thermal stability was more significant in the initial stage of decomposition. This behavior may be attributed to the protection of epoxy polymer chains present between hard MMT–clay nanolayers, which act as a barrier protecting against volatilization of the epoxy polymer matrix [38]. The polymer networks between the clay layers undergo restricted segmental motion, which is reflected in better thermal stability properties than in unmodified epoxies (UME) systems.

7.3 Conclusions In summary, the addition of pristine and organically modified clays generally improves the thermal stability of neat polymer matrices, and this improved thermal stability is directly related to the degree of thermodynamically favorable interactions that occur between the matrix and clay surfaces. These favorable interactions lead to the homogeneous dispersion of silicate layers in the polymer matrix. This is the most significant factor in the improved thermal stability of clay-containing polymer nanocomposites.

Chapter 7 • Thermal Stability

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On the other hand, highly dispersed clay particles can act as a heat barrier, which enhances the overall thermal stability of the system, and they can assist in the formation of char after thermal decomposition. In the early stages of thermal decomposition, the clay shifts the onset of decomposition to a higher temperature. Subsequently, the heat barrier effect results in reduced thermal stability. In other words, the stacked silicate layers can hold accumulated heat that can be used as a heat source to accelerate the decomposition process, in conjunction with the heat flow supplied by the outside heat source. A more thermally stable surfactant without the aforementioned issues can be used in the formation of polymer/clay nanocomposites with much higher thermal stability. Despite the strong interfacial interaction, clay loading plays a crucial role in the thermal stabilization enhancement of nanocomposites. The presence of loose surfactant molecules and water in organically modified clays can have a serious effect on the thermal behavior of a polymer matrix. For example, the presence of a small amount of water can accelerate the degradation of the polymer matrix, particularly for condensation polymers, due to the catalytic role of water molecules during processing at elevated temperatures.

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