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Abstract

Thermogravimetric analysis is always used to study the pyrolysis process of polymers and composites and can reflect their thermal degradation properties. In the article, an exfoliated nanocomposite was synthesized by vinyl acetate and organic montmorillonite through different processes. Linear macromolecular chains of polyvinyl acetate were formed in the layers of organic montmorillonite, and no chemical bond existed between them, but only physical effect. Organic montmorillonite was exfoliated into layers or sheets of nanoparticles, randomly dispersing in the matrix of polyvinyl acetate, so the different synthesis processes and the addition of organic montmorillonite had little effect on the structure of the exfoliated nanocomposite, but some certain effect on the pyrolysis properties. The pyrolysis was found to consist of eight phases, and those of polyvinyl acetate and the exfoliated nanocomposite were similar. The presence of organic montmorillonite had no obvious effect on the pyrolysis temperature, but mainly delayed the thermal degradation process. In addition, their pyrolysis kinetics was firstly analyzed by the Agrawal integral equation. The kinetic parameters, including the kinetics mechanism function, the activation energy, the frequency factor, the reaction order, the reaction rate constant, the form factor of heat flow curve, and the kinetic compensation effect equation, were obtained in detail.

Keywords

PVAc-OMMT structure pyrolysis kinetics kinetic compensation effect

Introduction

Generally, materials and their products are used at a certain environmental temperature, and in their use, they will reflect to different temperatures and show their different thermal physical properties, these properties are usually called asthermal properties or pyrolysis properties. The pyrolysis properties of polymers or composites are very important in their forming and application processes, so theyshould be investigated in detail. Thermogravimetric analysis (TGA) is always used to do this. Recently, montmorillonite (MMT) or organic montmorillonite (OMMT) has been applied in many polymers, such as polyacrylate ester,^1,2 poly (methyl methacrylate),^3,4 polyurethane,^5,6 epoxy,^7,8 polycarbonate,^9,10 polyethylene,^11,12 polyvinyl acetate (PVAc)^13–15 etc., to improve their chemical, physical, and mechanical properties, but few of them are specifically related to the pyrolysis properties, and till now, no systematic study on this subject has appeared.

Therefore, in the article, based on our previous work about PVAc,^16–19 OMMT,^20–24 and some exfoliated nanocomposites of PVAc and OMMT,^25–27 a new OMMT was prepared by MMT and octadecyl trimethyl ammonium bromide (STAB) according to refs 20 and 21, the intercalated nanocomposite of OMMT was obtained. STAB is a long alkyl chain quaternary ammonium salts, its cations arrange with the half paraffin-type molecular in the layers of MMT. Then to simplify the synthesis process, under the conventional conditions in the lab, with no ultrasonic dispersion and no radiation, PVAc-OMMT was polymerized by VAc and OMMT through different synthesis processes. In the synthesis, OMMT was 2% of VAc^25–27; it was a reasonable amount and would bring good effect on the pyrolysis properties. PVAc and PVAc-OMMT were homogeneous amorphous linear polymers; their structure was mainly investigated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and dispersion in water, then their TGA tests were also carried out, their pyrolysis processes were studied, their kinetics was also firstly analyzed by the Agrawal integral equation in detail.

Experimental

Samples

A total of 1.00 g of OMMT was immersed into 25.00 g of VAc for 24 h. It was then mixed with 70.00 g of 10% polyvinyl alcohol solution, 0.50 g of alkylphenol polyoxyethylene (10) ether, 3.75 g of 10% ammonium persulfate solution, 6.25 g of sodium lauryl sulfate, and 250 g (200 g, 150 g, 100 g, 50 g) of water while stirring vigorously for 8 h. When the mixture became a homogeneous emulsion, the temperature rose to 70°C. While stirring vigorously, 3.75 g of 10% ammonium persulfate solution and 25.00 g of VAc were gradually added into the homogeneous emulsion in 6 h (5 h, 4 h, 3 h, 2 h) for polymerization. Subsequently, the temperature rose to 85–90°C, the emulsion further polymerized for 0.5–1 h. After polymerization, when the temperature dropped to below 50°C, the emulsion was washed bywarm deionized water for several times to clean the residue of emulsifier, stabilization agent, initiator, and others, so the pure polymer was obtained with a high yield of 80%–90%. Then 4.00 g of ethanol, 3.00 g of water, 0.30 g of sodium benzoate, 0.18 g of sodium bicarbonate, and 6.00 g of Di-n-butyl phthalate were added, and finally PVAc-OMMT-A (PVAc-OMMT-B, PVAc-OMMT-C, PVAc-OMMT-D, PVAc-OMMT-E) was obtained. Here, we used A, B, C, D, and E to respectively correspond to the different synthesis processes with different polymerization time and different proportion of water. PVAc-A, PVAc-B, PVAc-C, PVAc-D, and PVAc-E were also synthesized by the same processes as PVAc-OMMT, butwithout OMMT. From our previous work,^26,27 their molecular weights were 1.0 × 10² (PVAc-A), 5.0 × 10² (PVAc-B), 4.0 × 10³ (PVAc-C), 8.1 × 10³ (PVAc-D), 1.4 × 10⁴ (PVAc-E), 3.8 × 10² (PVAc-OMMT-A), 1.2 × 10³ (PVAc-OMMT-B) 8.1 × 10³ (PVAc-OMMT-C), 2.0 × 10⁴ (PVAc-OMMT-D), and 4.0 × 10⁴ (PVAc-OMMT-E).

XRD

The samples were tested by DX-2000 of X-ray diffraction under Cu Kα of radiating, 40 kV of tube voltage, 30 mA of tube current, scanning from 0.5° to 15° at a rate of 0.02°ċs⁻¹, and λ = 1.54184 Å of wavelength. The large value of d₍₀₀₁₎ forMMT or OMMT is calculated by Bragg law of λ = 2dsinθ, where λ is the wavelength representing the intensity of X-ray, d is the distance between layers of MMT, and θ is the diffraction angle.

FTIR

The samples mixed with kalium bromatum powder were pressed into plates, and then tested by the NICOLET 380 of FTIR.

Dispersion

About 3 to 5 drops of PVAc or PVAc-OMMT were added to 20 mL of water in a glass dish (diameter 90 mm). When they became homogeneous, the dispersion could be observed with water as reference.

TGA

PVAc and PVAc-OMMT were tested by NETZSCH TG209 F1 under 7–10 mg ofsample, 25–800°C at a heating rate of 10°Cċmin⁻¹, and a nitrogen flow rate of 60–80 mLċmin⁻¹. In the pyrolysis, the phase thermal weight loss (c, %) is calculated by:

c = \frac{m_{T_{1}} - m_{T_{2}}}{m_{i}} \times 100

Where m_i is the initial weight,

m_{T_{1}}

and

m_{T_{2}}

is the weight of T₁ and T₂, respectively.

Results and discussion

Structure

In the article, different nanocomposites of PVAc-OMMT were prepared throughdifferent synthesis processes, all of them were exfoliated nanocomposites shown in Figure 1. From the figure, we can see that the diffraction peaks of MMTand OMMT appeared, their 2θ were 7.00° and 4.18°, their d₍₀₀₁₎ were 1.263 nm and 2.114 nm, respectively.^20,21 After organic modification, OMMT was an intercalated nanocomposite; its XRD pattern still had the diffraction peak. Butfor PVAc-OMMT, as shown in the figure, including PVAc-OMMT-A, PVAc-OMMT-B, PVAc-OMMT-C, PVAc-OMMT-D, and PVAc-OMMT-E, no diffraction peaks appeared within 0.5–5° of θ on their XRD patterns.^28,29 In the emulsion polymerization, on the basis of the first intercalation of STAB into MMT, the linear macromolecular chains of PVAc were formed in the layers of OMMT, the formed chains made the second intercalation. The distance between MMT layers was greatly stretched, the original ordered crystal layers were completely destroyed, MMT layers were fully exfoliated, and they independently dispersed in the emulsion with nanometer-thick layers. In other words, OMMT wasgreatly exfoliated into layers or sheets of nanoparticles, and they randomly dispersed in the matrix of PVAc. The exfoliated nanocomposite of PVAc-OMMT was obtained.

Figure 1.

X-ray diffraction (XRD) patterns.

Then, to further study the interaction between PVAc and OMMT, they werealso tested by FTIR. The results are shown in Figure 2. From the figure, we can see that no chemical bond was found between OMMT and PVAc, but only physical adsorptions or the secondary bonds. The absorption bands of PVAc-OMMT were stacked by that of OMMT and PVAc. Neither were new absorption bands formed nor did existing absorption bands disappear. The different synthesis processes had no effect on the interaction. FTIR spectra of PVAc-A to PVAc-E were the same, and those of PVAc-OMMT were the same, too. In PVAc-OMMT, PVAc and OMMT were connected or absorbed together by physical effect. It was precisely because of these physical adsorptions or the secondary bonds; OMMT could still keep its good stability in the relatively harsh emulsion intercalation process and guarantee its structural continuity and stability.

Figure 2.

Fourier transform infrared spectroscopy (FTIR) spectra.

Besides these, their dispersion in water was also investigated. From the visual, all PVAc emulsions, including PVAc-A, PVAc-B, PVAc-C, PVAc-D, and PVAc-E, and some PVAc-OMMT emulsions, including PVAc-OMMT-A, PVAc-OMMT-B, and PVAc-OMMT-C, were the same. They were viscous, milk-white, homogeneous, fine emulsions. All had no coarse particles, no foreign bodies, and no delamination. Butfor PVAc-OMMT-D and PVAc-OMMT-E, they were greatly different. There appeared delamination, especially for PVAc-OMMT-E, its delamination was very severe. Actually, PVAc-OMMT-D was also viscous, milk-white, homogeneous, fine emulsions, but it had many large or small coarse particles. These particles deposited at the bottom, formed the white precipitate. Oppositely, for PVAc-OMMT-E it was not the case. Its delamination was very obvious. The top part was colorless, transparent liquid, like water, not viscous. The bottom part was viscous, milk-white floccus. Moreover, when some were dropped in water, their dispersion was also different to that shown in Figure 3. Comparing with water, the figure shows no coarse particles were found in the dispersion of all PVAc, PVAc-OMMT-A, PVAc-OMMT-B, and PVAc-OMMT-C, their dispersion was much better. But for PVAc-OMMT-D and PVAc-OMMT-E, their dispersion was another case, which was apparently different from others. As shown in the figure, the top sample of PVAc-OMMT-D dispersed in water appeared to be more viscous, its color was much deeper than others, like milk. The bottom sample dispersing in water showed some coarse particles, some were small and some were large; other part liquid was also like milk. From PVAc-A to PVAc-E and from PVAc-OMMT-A to PVAc-OMMT-D, we can know that the dispersion color became much deeper one than one;this phenomenon may be caused by the different synthesis processes that resulted in different emulsions with different viscosity and different molecular weight. Of course, PVAc-OMMT-E was an exception; it was completely different from all the others. Its dispersion with top sample was colorless, transparent liquid, like water, while there were white floccus and coarse particles in the dispersion withbottom sample. This may be also caused by its special synthesis process.

Figure 3.

Dispersion.

The dispersion was also further studied in our previous work^25–27 by transmission electron microscope. We found that the particle diameter of PVAc was from 250 nm to 500 nm while OMMT’s was from 50 nm to 100 nm, they randomly dispersed together, and the smaller OMMT particles were adsorbed around thelarger PVAc particles to form the ‘Strawberry’ structure, ‘Swallow’ structure, ‘Tactoids’ structure, ‘Half Core-Shell’ structure, or ‘Similar Core-Shell’ structure. And a kind of emulsion often contained one or more than one structures, this was due to most particles individually, randomly dispersed, and many of them were absorbed, gathered, or aggregated together to form different structures, and they all showed a good dispersion in the matrix of PVAc.

Pyrolysis properties

From the results described above, we can see that the different synthesis processes had little effect on the structure of PVAc and PVAc-OMMT; PVAc-OMMT wasan exfoliated nanocomposite, from PVAc-A to PVAc-E and from PVAc-OMMT-A to PVAc-OMMT-E, their FTIR spectra were almost the same. Butfor the dispersion in water, all PVAc, PVAc-OMMT-A, PVAc-OMMT-B, and PVAc-OMMT-C, their dispersion was much better, while those of PVAc-OMMT-D and PVAc-OMMT-E were great different. Therefore, in this part, on behalf of all PVAc, PVAc-A was chosen and tested by TGA, and all PVAc-OMMT, including PVAc-OMM-A, PVAc-OMMT-B, PVAc-OMMT-C, PVAc-OMMT-D, and PVAc-OMMT-E were tested by TGA. Their TGA spectra are shown in Figure 4. In the figure, each one contains three curves; the curve of weight loss is the thermogravimetric (TG) curve, the curve of weight loss rate is the derivative thermogravimetric (DTG) curve, and the curve of heat flow is the differential thermal analysis (DTA) curve. They will describe the pyrolysis process in detail. As shown in Figure 4, the pyrolysis was found to consist of eight phases, phase 1 from 25°C to 150°C, phase 2 from 150°C to 230°C, phase 3 from 230°C to 290°C, phase 4 from 290°C to 350°C, phase 5 from 350°C to 410°C, phase 6 from 410°C to 510°C, phase 7 from 510°C to 710°C, and phase 8 from 710°C to 800°C. TG curves of PVAc and PVAc-OMMT were similar. From Table 1, we can see that all the total weight loss was more than 90%, only less than 10% was left, these burnt residue may be some fire-retardant materials, such as MMT, which is a mineral, it cannot be burnt, as well as some other inorganic additives added in the polymerization. We can also find out that the pyrolysis mainly occurred in phase 2, phase 3, phase 4, and phase 6. In these phases, small molecules and macromolecules all were completely burnt, that is to say that with the temperature rising, PVAc as the polymer matrix was gradually pyrolysized, and finally was completely burnt. For PVAc-OMMT, besides the pyrolysis of PVAc, it also included the pyrolysis of OMMT, the organic content of OMMT here we used was 40%–60%, and OMMT was mainly pyrolysized during 300–400°C.^22–24 Even so, from our above description about structure, PVAc and PVAc-OMMT were synthesized through five different processes, but their structures were similar, so their TG curves and weight loss were bound to be similar.

Figure 4.

Thermogravimetric analysis (TGA) curves.

Table 1.

Phase weight loss.

c(%)	PVAc-A	PVAc-OMMT
c(%)	PVAc-A	A	B	C	D	E
Phase 1	5.23	6.51	6.03	6.01	6.68	5.58
Phase 2	29.31	25.89	23.01	25.10	26.02	12.18
Phase 3	11.96	11.95	10.32	12.34	9.60	8.15
Phase 4	20.06	25.56	27.45	25.67	23.62	41.04
Phase 5	4.61	5.34	5.82	5.34	6.46	6.19
Phase 6	15.94	16.87	17.01	17.77	15.08	22.60
Phase 7	0.02	0	0	0	0.19	0
Phase 8	3.04	1.93	1.69	1.99	2.76	0.90
Total loss	90.17	94.05	91.33	94.22	90.41	96.64

However, as an exfoliated nanocomposite, the pyrolysis process of PVAc-OMMT, after all, was somewhat different from PVAc, this could be reflected by DTG curves, their weight loss rate was different. As shown in Table 2, coinciding with the results shown in Table 1 and from the pyrolysis valley temperature (T_v) and the maximum pyrolysis rate (v_v) obtained from DTG curves, we can find out that the pyrolysis mainly occurred in phase 2, phase 4, and phase 6, especially in phase 4, its v_v was the maximum among these three phase, and then were phase 2 and phase 6. In phase 2 and phase 6, the pyrolysis was relatively complex; they appeared two or three, even five T_v and v_v. Maybe many small molecules were mainly pyrolysized in phase 2, many macromolecules were mainly pyrolysized in phase 4 and phase 6, and OMMT was also burnt during these phases. These DTG curves showed that the presence of OMMT had no obvious effect on the pyrolysis temperature, but mainly delayed the thermal degradation process. For our exfoliated nanocomposites, the layered structural silicate would reduce their absorption to solvents and other small molecules, then extend the transmission channels of small molecules, and finally effectively improve their barrier properties.^30–33

Table 2.

Pyrolysis valley characteristics of differential thermal analysis (DTG) curve.

	PVAc-A			PVAc-OMMT-A			PVAc-OMMT-B
	T_v(°C)	v_v(mg•min⁻¹)	k_v(min⁻¹)	T_v(°C)	v_v(mg•min⁻¹)	k_v(min⁻¹)	T_v(°C)	v_v(mg•min⁻¹)	k_v(min⁻¹)
Phase 1	70	0.0524	0.04 × 10⁻⁴	65	0.0553	2.79 × 10⁻⁷	68	0.0340	1.46 × 10⁻⁷
Phase 2	180	0.3941	0.91 × 10⁻³	173	0.3178	3.26 × 10⁻⁴	175	0.2848	2.32 × 10⁻⁴
	195	0.3944	3.92 × 10⁻³	200	0.3520	3.38 × 10⁻³	197	0.3240	1.93 × 10⁻³
Phase 3	—	—	—	—	—	—	—	—	—
Phase 4	325	0.4186	0.49	318	0.4973	0.40	322	0.4854	0.40
Phase 5	—	—	—	—	—	—	—	—	—
Phase 6	425480490	0.25720.21620.0881	3.3317.6222.91	425445475485495	0.22410.20640.10940.10740.1044	0.020.020.030.030.03	445475492509	0.20480.14000.10310.0645	8.8824.5441.3367.23
Phase 7	—	—	—	—	—	—	—	—	—
Phase 8	754	0.0771	0.03	760	0.0761	280.07	737	0.0474	205.76
	PVAc-OMMT-C			PVAc-OMMT-D			PVAc-OMMT-E
	T_v(°C)	v_v(mg•min⁻¹)	k_v(min⁻¹)	T_v(°C)	v_v(mg•min⁻¹)	k_v(min⁻¹)	T_v(°C)	v_v(mg•min⁻¹)	k_v(min⁻¹)
Phase 1	77	0.0479	98.55	82	0.0469	1.04 × 10⁻⁵	65	0.0471	1.17 × 10⁻⁷
Phase 2	167	0.3046	1.74 × 10⁻⁴	165	0.2775	2.67 × 10⁻⁴	176	0.1911	4.88 × 10⁻⁵
	202	0.3381	3.44 × 10⁻³	200	0.2996	4.74 × 10⁻³	197	0.1860	1.93 × 10⁻⁴
Phase 3	—	—	—	—	—	—	—	—	—
Phase 4	320	0.5164	0.40	317	0.3690	0.32	326	0.9545	0.27
Phase 5	—	—	—	—	—	—	—	—	—
Phase 6	427440452477502	0.23750.23860.23050.14200.1428	4.126.6710.1322.6646.76	427445500	0.18770.18410.0675	0.160.230.62	441473481498	0.29110.19760.18760.1605	0.300.040.040.04
Phase 7	—	—	—	—	—	—	—	—	—
Phase 8	740	0.0716	171.91	745	0.0979	172.94	741	0.0342	294.12

As far as the pyrolysis of PVAc and PVAc-OMMT were concerned, their pyrolysis kinetics needed to be further studied. For the kinetics, the mechanism function is always expressed as:

G (a) = \int_{T_{0}} T e^{- \frac{E_{a}}{RT}} d t

where a is the relative weight loss calculated by:

a = \frac{m_{i} - m_{T}}{m_{i} - m_{f}}

where m_i is the initial weight, m_f is the final weight, m_T is the weight of T.

The Agrawal approximate equation is³⁴:

\int_{T_{0}} T e^{- \frac{E_{a}}{RT}} d T = \frac{{RT}^{2}}{E_{a}} [\frac{1 - 2 (\frac{RT}{E_{a}})}{1 - 5 (\frac{RT}{E_{a}})^{2}}] e - \frac{E_{a}}{RT}

Combining the above two equations, the Agrawal integral equation is writtenas:

ln [\frac{G (a)}{T^{2}}] = ln {\frac{AR}{β E_{a}} [\frac{1 - (\frac{RT}{E_{a}})}{1 - 5 (\frac{RT}{E_{a}})^{2}}]} - \frac{E_{a}}{RT}

For the general reaction temperature and most of the activation energy (E_a), the relationship among E_a, R, and T are:

\frac{E_{a}}{RT} >> 1; 1 - (\frac{RT}{E_{a}}) \sim 1; 1 - 5 (\frac{RT}{E_{a}}) \sim 1

So the Agrawal integral equation is simplified as:

In [\frac{G (a)}{T^{2}}] = In (\frac{AR}{β E_{a}}) - \frac{E_{a}}{RT}

The relationship between ln $[\frac{G (a)}{T^{2}}]$ and $\frac{1}{T}$ will be linear if with a suitable G (a), then E_a can be calculated from the linear slope, the frequency factor (A) can be calculated from the linear intercept.

In the above equations, T is the temperature, R is the universal gas constant that is 8.314 J•(mol•K)⁻¹, β is the heating rate that is 10°C•min⁻¹, E_a is the activation energy, and A is the frequency factor. Then, according to the simplified Agrawalintegral equation, T and a in each pyrolysis phase were linear fit by thetrial-and-error method, and based on the linearly dependent coefficient, the pyrolysis kinetics of PVAc-A and PVAc-OMMT were obtained as shown in Table 3, and their reaction order (n) was also shown in Table 4. In these kinetic parameters, G (a) was the kinetics mechanism function; it illustrated the pyrolysis process of PVAc or PVAc-OMMT. If their G (a) were different, their linear fitequation, linearly dependent coefficient, E_a, A, and n were different, too. E_a was the minimum energy required from a reactant molecule to an activated molecule in achemical reaction. It was the different energy between the onset and the offset of apyrolysis process. A was a constant determined by the reaction essence, had nothing to dowith the reaction temperature and concentration in the system. Itwas one of the important reaction kinetic parameters. From these two tables, we can see thatthe pyrolysis kinetics of PVAc and PVAc-OMMT were similar, thisis because the pyrolysis mainly happened to PVAc, which was the polymer matrix.

Table 4.

Reaction order.

n	PVAc-A	PVAc-OMMT
n	PVAc-A	A	B	C	D	E
Phase 1	3	4	4	3	3	4
Phase 2	4	4	4	4	4	4
Phase 3	4	4	4	4	4	4
Phase 4	4	4	4	4	4	4
Phase 5	4	4	4	4	4	4
Phase 6	4	2	4	4	3	2
Phase 7	—	—	—	—	—	—
Phase 8	2	4	4	4	4	4

Table 3.

Pyrolysis kinetics.

	PVAc-A			PVAc-OMMT-A			PVAc-OMMT-B
	G (a)	E_a(kJ•mol⁻¹)	A(min⁻¹)	G (a)	E_a(kJ•mol⁻¹)	A(min⁻¹)	G (a)	E_a(kJ•mol⁻¹)	A(min⁻¹)
Phase 1	$[- ln (1 - a)]^{3}$	1.86	8.70 × 10⁻⁵	$[- ln (1 - a)]^{4}$	1.95	1.03 × 10⁻⁵	$[- ln (1 - a)]^{4}$	1.50	2.08 × 10⁻⁶
Phase 2	$[- ln (1 - a)]^{4}$	28.38	1.57 × 10⁵	$[- ln (1 - a)]^{4}$	24.92	1.09 × 10⁴	$[- ln (1 - a)]^{4}$	27.58	3.96 × 10⁴
Phase 3	$[- ln (1 - a)]^{4}$	9.84	2.26	$[- ln (1 - a)]^{4}$	10.00	1.37	$[- ln (1 - a)]^{4}$	8.90	0.47
Phase 4	$[- ln (1 - a)]^{4}$	33.04	1.01 × 10⁵	$[- ln (1 - a)]^{4}$	42.41	3.74 × 10⁶	$[- ln (1 - a)]^{4}$	47.70	2.21 × 10⁷
Phase 5	$[- ln (1 - a)]^{4}$	3.93	0.49	$[- ln (1 - a)]^{4}$	5.36	1.30	$[- ln (1 - a)]^{4}$	6.49	1.82
Phase 6	$[- ln (1 - a)]^{4}$	51.37	6.86 × 10⁶	$[- ln (1 - a)]^{4}$	13.22	0.78	$[- ln (1 - a)]^{4}$	59.58	8.75 × 10⁷
Phase 7	$[- ln (1 - a)]^{4}$	—	—	$[- ln (1 - a)]^{4}$	—	—	$[- ln (1 - a)]^{4}$	—
Phase 8	$[- ln (1 - a)]^{4}$	26.22	2.18	$[- ln (1 - a)]^{4}$	178.06	4.85 × 10¹⁴	$[- ln (1 - a)]^{4}$	164.50	9.39 × 10¹³
	PVAc-OMMT-C			PVAc-OMMT-D			PVAc-OMMT-E
	G (a)	E_a(kJ•mol⁻¹)	A(min⁻¹)	G (a)	E_a(kJ•mol⁻¹)	A(min⁻¹)	G (a)	E_a(kJ•mol⁻¹)	A(min⁻¹)
Phase 1	$[- ln (1 - a)]^{4}$	0.42	189.93	$[- ln (1 - a)]^{4}$	0.57	2.40 × 10⁻⁵	$[- ln (1 - a)]^{4}$	1.60	2.25 × 10⁻⁶
Phase 2	$[- ln (1 - a)]^{4}$	23.89	5.18 × 10³	$[- ln (1 - a)]^{4}$	22.55	3.68 × 10³	$[- ln (1 - a)]^{4}$	18.88	19.59
Phase 3	$[- ln (1 - a)]^{4}$	10.72	1.80	$[- ln (1 - a)]^{4}$	9.79	1.57	$[- ln (1 - a)]^{4}$	10.44	0.09
Phase 4	$[- ln (1 - a)]^{4}$	42.11	3.02 × 10⁶	$[- ln (1 - a)]^{4}$	32.55	7.46 × 10⁴	$[- ln (1 - a)]^{4}$	78.56	1.06 × 10¹²
Phase 5	$[- ln (1 - a)]^{4}$	5.10	0.92	$[- ln (1 - a)]^{4}$	9.15	6.03	$[- ln (1 - a)]^{4}$	7.85	2.56
Phase 6	$[- ln (1 - a)]^{4}$	57.70	4.72 × 10⁷	$[- ln (1 - a)]^{4}$	32.93	1.70 × 10³	$[- ln (1 - a)]^{4}$	11.59	0.70
Phase 7	$[- ln (1 - a)]^{4}$	—	—	$[- ln (1 - a)]^{4}$	—	—	$[- ln (1 - a)]^{4}$	—
Phase 8	$[- ln (1 - a)]^{4}$	160.36	3.59 × 10¹³	$[- ln (1 - a)]^{4}$	199.34	1.64 × 10¹⁶	$[- ln (1 - a)]^{4}$	144.61	4.60 × 10¹²

In the pyrolysis kinetics, the reaction rate constant (k) is always to quantitatively explain the reaction rates, it is usually related to A, E_a, and T, and calculated by Arrhenius equation:

k=A $e^{- \frac{E_{a}}{RT}}$

k in each pyrolysis phase of PVAc-A and PVAc-OMMT was obtained as shown in Table 2. The variation trend of k was similar to those of T_v and v_v. From phase 1 to phase 8, k was gradually getting greater and greater, from some minimum k to some maximum k. As we know, k is closely related to the reaction temperature, reaction medium (or solvent), catalyst, and so on, even to the shape and characteristics of reactors, so during the pyrolysis, with the pyrolysis temperature rising, k increased.

In addition, the linear relationship between lnA and E_a is named as the kinetic compensation effect. It is a main characteristic in kinetics, explaining that A partly compensates to the change of E_a. The equation³⁵ is:

lnA = KE_a +Q

where K and Q are the kinetic compensation effect parameters which can be calculated by linear fit between E_a and A. Then, the pyrolysis kinetic compensation effect of PVAc-A and PVAc-OMMT was obtained as shown in Table 5. From the table, we can see that the pyrolysis kinetic compensation effect in each phase was different, this indicated that although the pyrolysis mainly happened to PVAc, different molecules or different materials were burnt in each phase. And from phase 1 to phase 8, the whole pyrolysis kinetic compensation effect of PVAc-A and PVAc-OMMT was also obtained (Table 5). From the whole pyrolysis kinetic compensation effect equations, we can see that K was from 0.1927 (PVAc-OMMT-D) to 0.2258 (PVAc-A) and Q was from –3.0856 (PVAc-A) to –1.6194 (PVAc-OMMT-B), these K and Q were very close to each other, this also showed that the pyrolysis was mainly carried out on PVAc. Because the kinetic compensation effect parameters have nothing to do with the experimental factors, the kinetic compensation effect equations could clearly explain the pyrolysis properties of PVAc and PVAc-OMMT and also were their theoretical expression.

Table 5.

Kinetic compensation effect equations.

	PVAc-A	PVAc-OMMT-A	PVAc-OMMT-B
Phase 1	lnA =–1.6352E_a–7.2196	lnA =–0.759E_a–9.6251	lnA =–2.677E_a–9.0564
Phase 2	lnA = 0.5329E_a–3.578	lnA = 0.4947E_a–3.6869	lnA = 0.4857E_a–3.5158
Phase 3	lnA = 0.7183E_a–6.0534	lnA = 0.7732E_a–6.8106	lnA = 0.702E_a–6.6893
Phase 4	lnA = 0.4941E_a–4.2426	lnA = 0.4454E_a–3.7731	lnA = 0.4265E_a–3.5852
Phase 5	—	lnA = 0.7544E_a–3.7848	lnA = 0.3407E_a–1.6914
Phase 6	lnA = 0.4015E_a–4.3962	lnA = 0.3782E_a–4.1213	lnA = 0.3811E_a–4.2075
Phase 7	—	—	—
Phase 8	lnA = 0.2017E_a–3.7266	lnA = 0.1863E_a–1.5761	lnA = 0.1912E_a–2.4155
Whole	lnA = 0.2258E_a–3.0856	lnA = 0.1934E_a–2.3456	lnA = 0.196E_a–1.6194
	PVAc-OMMT-C	PVAc-OMMT-D	PVAc-OMMT-E
Phase 1	lnA =–2.586E_a–7.3396	lnA =–1.1319E_a–8.7703	lnA =–1.163E_a–10.734
Phase 2	lnA = 0.4885E_a–3.7642	lnA = 0.5129E_a–3.911	lnA = 0.3685E_a–4.5454
Phase 3	lnA = 0.6858E_a–6.4127	lnA = 0.7689E_a–6.5851	lnA = 0.4923E_a–7.2039
Phase 4	lnA = 0.4353E_a–3.6889	lnA = 0.4623E_a–4.0515	lnA = 0.386E_a–3.1206
Phase 5	lnA = 0.7329E_a–3.8148	lnA = 0.6354E_a–3.6475	lnA = 0.667E_a–4.0501
Phase 6	lnA = 0.383E_a–4.1821	lnA = 0.4099E_a–4.395	lnA = 0.3664E_a–4.0641
Phase 7	—	—	—
Phase 8	lnA = 0.1924E_a–2.5476	lnA = 0.1877E_a–2.8882	lnA = 0.1922E_a–1.9202
Whole	lnA = 0.2011E_a–2.4714	lnA = 0.1927E_a–2.9051	lnA = 0.212E_a–2.7239

Finally, the form factor of DTA curve (ϕ) should also be taken into account, it indicates the symmetry of DTA curve, the symmetry will be worse when n decreases. ϕ is defined by Kissinger³⁶ as shown in Figure 5; it is only closely related to n as:

Figure 5.

ϕ of differential thermal analysis (DTA) curve.

ϕ = 0.63n²

So ϕ in each pyrolysis phase of PVAc-A and PVAc-OMMT were calculated as shown in Table 6. From the table, we can see that DTA curves all had good symmetry, but because a DTA curve was a relatively more complex and continuous curve as shown in Figure 4, and the pyrolysis was found to consist of eight phases, so the symmetry of the DTA curve in each pyrolysis phase of PVAc-A and PVAc-OMMT was difficult to directly find out from figure 4, but only could be judged from the calculated ϕ.

Table 6.

ϕ of differential thermal analysis (DTA) curve.

ϕ	PVAc-A	PVAc-OMMT
ϕ	PVAc-A	A	B	C	D	E
Phase 1	5.67	10.08	10.08	5.67	5.67	10.08
Phase 2	10.08	10.08	10.08	10.08	10.08	10.08
Phase 3	10.08	10.08	10.08	10.08	10.08	10.08
Phase 4	10.08	10.08	10.08	10.08	10.08	10.08
Phase 5	10.08	10.08	10.08	10.08	10.08	10.08
Phase 6	10.08	2.52	10.08	10.08	5.67	2.52
Phase 7	—	—	—	—	—	—
Phase 8	2.52	10.08	10.08	10.08	10.08	10.08

Conclusions

In the article, an exfoliated nanocomposite was prepared by VAc and OMMT through different processes. Linear macromolecular chains of PVAc were formedin the layers of OMMT, and no chemical bond existed between OMMT and PVAc, but only physical effect. OMMT was exfoliated into layers or sheets ofnanoparticles, randomly dispersing in the matrix of PVAc, so the different synthesis processes and the addition of OMMT had little effect on the structure of PVAc-OMMT, but some certain effect on the pyrolysis properties. The pyrolysis was found to consist of eight phases, and those of PVAc and PVAc-OMMT weresimilar. The presence of OMMT had no obvious effect on the pyrolysis temperature, but mainly delayed the thermal degradation process. In addition, their pyrolysis kinetics was firstly analyzed by the Agrawal integral equation. The kinetic parameters, including G (a), E_a, A, n, k, ϕ, and the kinetic compensation effect equation, were obtained in detail.

Footnotes

Acknowledgement

The authors are grateful for the financial support of the Program for New Century ExcellentTalents in University of the Ministry of Education of China (NCET-06-0825).

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Pyrolysis properties of exfoliated nanocomposite synthesized by vinyl acetate and organic montmorillonite through different processes

Abstract

Keywords

Introduction

Experimental

Samples

XRD

FTIR

Dispersion

TGA

Results and discussion

Structure

Pyrolysis properties

Conclusions

Footnotes

Acknowledgement

References