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Original article
10 (
2_suppl
); S3732-S3739
doi:
10.1016/j.arabjc.2014.05.008

Thermal stability and degradation of poly (N-phenylpropionamide) homopolymer and copolymer of N-phenylpropionamide with methyl methacrylate

Department of Chemistry, Faculty of Science, Damietta University, Damietta, Egypt

⁎Corresponding author. Tel.: +20 1060081581; fax: +20 572403867. m.adiab@yahoo.com (M.A. Diab)

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Abstracted from her Ph.D.

Abstract

Different concentrations of copolymer of (N-phenylpropionamide) (PA) with methyl methacrylate (MMA) were prepared and the reactivity ratio values of copolymerization were calculated using the 1H-NMR technique. Thermal analysis of the copolymers showed that the thermal stability is an intermediate between poly(N-phenylpropionamide) and poly(methyl methacrylate) homopolymers. Thermal degradation products of the PPA were identified by GC–MS techniques. It seems that the mechanism of degradation of PPA homopolymer is characterized by free radical formation followed by recombination along the backbone chain. The activation energies of the thermal degradation of the copolymers were calculated using the Arrhenius relationship.

Keywords

N-phenylpropionamide
Thermal stability and degradation
Reactivity ratios
1

1 Introduction

Thermal analysis of polymers is very important in determining their utility under various environmental conditions, high temperature applications, in understanding molecular architecture, decomposition and mechanisms. Thermogravimetric analysis not only furnishes data on weight loss as a function of temperature but also provides a means to estimate kinetic parameters or thermal decomposition reactions. It is also possible to establish a pyrolysis mechanism and a rapid comparison of thermal stabilities and decomposition temperatures of different polymers. Polymers are usable over a certain range of temperatures. The working range can be increased by copolymerization (Diab et al., 1989; Zeliazkow, 2006; Cervantes et al., 2008; Cao et al., 2011), using additives (Diab et al., 2012; El-Sonbati et al., 2011; El-Bindary et al., 2012), by modification of molecular structure (Li et al., 2009; Shakya et al., 2010; Kumara et al., 2009) and by cross-linking (Shoa et al., 2009; Frohlich et al., 2012; Hearon et al., 2013). Thermally stable and heat resistant polymers are in good demand as insulators and enamels (Kagathara and Parsania, 2002).

Copolymerization of various monomers is one of the most important means to improve the performance of polymers. Copolymers are extensively used in industrial processes, because their physical properties, such as elasticity, permeability, glass transition temperature (Tg), and solvent diffusion kinetics can be varied within wide limits (Zhang et al., 2009; Dubreuil et al., 2001). Controlling the polymer property parameters, such as copolymer composition, copolymer sequence distribution and molecular weight overages, is of particular importance in copolymerization processes (Hou et al., 2007). Reactivity ratios are among the most important parameters for the composition equation of copolymers, which can offer information such as the relative reactivity of monomer pairs and help estimate the copolymer composition (Soykan et al., 2000).

In this paper, homopolymers of N-phenylpropionamide (PPA) and methyl methacrylate (MMA) and four different compositions of copolymers N-phenylpropionamide and methyl methacrylate (PA–MMA) were prepared, so that the reactivity ratios might be determined using the 1H-NMR method. The thermal stability of the homopolymer and copolymers was examined. Thermal degradation of the PPA homopolymer was studied using GC–MS apparatus and the activation energies of the thermal degradation of the homopolymers and copolymers were calculated using the Arrhenius relationship.

2

2 Experimental

2.1

2.1 Materials

Acryloyl chloride (AC) (Aldrich Chemical Co., Inc.) was used without further purification. It was stored below −18 °C in a tightly glass-stoppered flask. 2,2′-Azobisisobutyronitrile (AIBN) (Aldrich Chemical Co., Inc.) was used as an initiator for all polymerizations. It was purified by dissolving it in hot ethanol and filtering (Khairou and Diab, 1994). The solution was left to cool. The pure material was being collected by filtration and then dried. Methyl methacrylate (MMA) (BDH Chemical Ltd.), stabilized with 0.1% hydroquinone was washed with a small amount of sodium hydroxide solution, separated with a separating funnel, distilled on a vacuum line, dried over anhydrous sodium sulphate and stored below −18 °C. All other chemicals and solvents were purified by standard procedures.

2.2

2.2 Preparation of monomer and polymers

(N-phenylpropionamide) (PA) monomer was performed by the reaction of equimolar amounts of AC and aniline in dry benzene until the evolution of hydrogen chloride ceased forming a white powder of PA monomer. Poly(N-phenylpropionamide) (PPA) homopolymer was prepared by free radical initiation of PA using 0.1 w/v% AIBN as an initiator and DMF as solvent and reflux for 6 h. The polymer product was precipitated by pouring in distilled water and dried in a vacuum oven for several days at 40 °C.

Copolymers of PA with MMA were prepared using 0.2 w/v.% AIBN as a free radical initiator and 50/50 (v/v) DMF as solvent. Four different copolymer compositions of PA–MMA were prepared, so that the reactivity ratios might be determined. Polymerization was carried out to about 10% conversion. It can be controlled by weighing the resultant copolymers. The polymers were precipitated by pouring into a large excess of distilled water, filtered and dried in a vacuum oven at 40 °C for several days.

2.3

2.3 Analytical techniques

2.3.1

2.3.1 Infrared spectroscopy (IR)

Spectra were recorded on a Pye Unicam SP 2000 spectrometer, for the homopolymers and copolymers in the form of KBr discs.

2.3.2

2.3.2 Nuclear magnetic resonance spectroscopy (NMR)

1H-NMR spectra were obtained using a Varian EM 390 90 MHz spectrometer with integration and 20 mg samples. The integral values obtained for each value sample were used for determination of the polymer compositions.

2.3.3

2.3.3 Thermogravimetry (TG)

TG measurements were made with a Mettler TG 3000 apparatus. Finely powdered (∼10 mg) samples were heated at 10o/min in a dynamic nitrogen atmosphere (30 ml/min); the sample holder was boat-shaped, 10 mm × 5 mm × 2.5 mm deep and the temperature measuring thermocouple was placed 1 mm from the sample holder. TG was also used for the determination of rates of degradation of the homopolymers and copolymers in the initial stages of decomposition. The activation energies were obtained by the application of the Arrhenius equation.

2.3.4

2.3.4 Thermal degradation of the PPA homopolymer

Samples of ∼50 mg were heated under vacuum from ambient temperature to 500 °C. The volatile degradation product was collected for qualitative analysis by the GC–MS technique. A Saturn GC 3400 with fused quartz capillary column of 30 m × 0.25 mm coated methyl silicon under programmed heating condition from 60 to 200 °C was used for the identification of the condensable degradation products. The GC is interfaced with a Varian mass spectroscopy equipped with the standard electron impact (E1) or chemical ionization (CI) sources and a DS 55 data system scans from m/e 300 to 20 at a scan rate of 10 s/decade. Perfluorokerosene (PFK) was used for computer calibration and the ion source was maintained at 200 °C. Accurate mass measurements of the CI mass spectra were performed at 1000 resolving power using PFK as internal reference and by a computer interpolation data system or the mass spectra could be presented in NIST library.

3

3 Results and discussion

3.1

3.1 Characterization of PPA homopolymer and PA-MMA copolymers

The IR spectrum of PPA homopolymer shows a band at 1680 cm−1 is assigned to the antisymmetric stretching vibration of the amidic carbonyl group. The bands at 1600, 1545 and 1440 cm−1 are assigned to ν(C—H), ν(C⚌C) and ν(C—C) bands, respectively (Diab, 1994). The C—H in plane deformation in the region 1225–1045 cm−1, the ring breathing at 995 and 1005 cm−1, the out-of-plan C—H deformation vibration between 775 and 750 cm−1 and the C—C out-of-plan deformation at 500 cm−1 are assigned. The IR spectrum of PA–MMA copolymer shows bands at 1680 and 1730 cm−1 assigned to antisymmetric stretching vibration of the amidic carbonyl group of PP and the carbonyl group of MMA in the copolymers, respectively (El-Sonbati et al., 1990). The bands at 1600, 1545 and 1440 cm−1 are due to ν(C—H), ν(C⚌C) and ν(C—C) bonds (El-Sonbati et al., 1991), respectively.

3.2

3.2 Determination of reactivity ratios of PA-MMA copolymers

Four different copolymers of PA–MMA with 1:30, 1:40, 1:50 and 1:60 mol of PA–MMA covering the entire composition range between PPA and PMMA homopolymers were prepared, so the reactivity ratios might be determined using the 1H-NMR method. This method has already been used for the determination of reactivity ratios for styrene–MMA (Kato et al., 1964), methacrylate–acrylate copolymers (Grassie et al., 1965) and recently for copolymers of 4-nitro-3-methylphenylmethacrylate and N-(4-bromophenyl)-2-methacrylamide with glycidyl methacrylate, respectively (Vijyanand et al., 2007; Soykan et al., 2008). Fig. 1 shows the 1H-NMR spectrum of PA–MMA copolymers. The bands at 2.490 and 2.696–2.856 ppm are due to CH2 and CH protons of PA and MMA in the copolymers (Williams and Fleming, 1966). The band at δ 7.880 ppm is due to –NH proton of PA in the copolymer (Mochel, 1976).

1H-NMR spectrum of PA–MMA copolymers.
Figure 1 1H-NMR spectrum of PA–MMA copolymers.

The peak (A) at δ 7.096–7.267 ppm is due to phenyl protons of PA in the copolymer and peak (B) at 3.594 ppm is due to –OCH3 protons of MMA units in the copolymers. Dividing peak A by five and peak B by three, the monomer composition of the copolymer can be calculated. By knowing the number of moles of the monomer mixture and the molar ratio of the copolymer, reactivity ratios can be calculated by applying the following equation (Billmeyer, 1971): f 1 ( 1 - 2 F 1 ) ( 1 - f 1 ) F 1 = f 1 2 ( F 1 - 1 ) ( 1 - f 1 ) 2 F 1 r 1 + r 2 where, F 1 = M 1 / M 2 M 1 / M 2 + 1 is the mole fraction of MMA (M1) in copolymers f 1 = n 1 n 1 + n 2 is the mole fraction of M1 in feed and r1 and r2 are the reactivity ratios of MMA and PA, respectively. Fig. 2 is a plot of f 1 2 ( F 1 - 1 ) ( 1 - f 1 ) 2 F 1 versus f 1 ( 1 - 2 F 1 ) ( 1 - f 1 ) F 1 , and Fig. 3 is a plot of f 2 2 ( F 2 - 1 ) ( 1 - f 2 ) 2 F 2 versus f 2 ( 1 - 2 F 2 ) ( 1 - f 2 ) F 2 where F 2 = M 2 / M 1 M 2 / M 1 + 1 is the mole fraction of PA (M2) in copolymer and f 2 = n 2 n 1 + n 2 is the mole fraction of M2 in feed. From the slope and intercept in Figs. 2 and 3 reactivity ratio values for PA–MMA copolymer are: r 1 ( MMA ) = 26 and r 2 ( PA ) = 0.4 .

Graph of f 1 2 ( F 1 - 1 ) ( 1 - f 1 ) 2 F 1 versus f 1 ( 1 - 2 F 1 ) ( 1 - f 1 ) F 1 for PA–MMA copolymers.
Figure 2 Graph of f 1 2 ( F 1 - 1 ) ( 1 - f 1 ) 2 F 1 versus f 1 ( 1 - 2 F 1 ) ( 1 - f 1 ) F 1 for PA–MMA copolymers.
Graph of f 2 2 ( F 2 - 1 ) ( 1 - f 2 ) 2 F 2 versus f 2 ( 1 - 2 F 2 ) ( 1 - f 2 ) F 2 for PA–MMA copolymers.
Figure 3 Graph of f 2 2 ( F 2 - 1 ) ( 1 - f 2 ) 2 F 2 versus f 2 ( 1 - 2 F 2 ) ( 1 - f 2 ) F 2 for PA–MMA copolymers.

3.3

3.3 Thermogravimetry (TG)

TG curves of PPA and PMMA homopolymers and PA-MMA copolymers are shown in Fig. 4. PPA homopolymer degrades in three stages. The first starts at ∼180 °C with a weight loss of ∼13%. The second stage starts at ∼280 °C with a weight loss of ∼39%. The third stage starts at ∼390 °C with a weight loss of ∼43%.

TG curves for PPA and PMMA homopolymers and PA–MMA copolymers.
Figure 4 TG curves for PPA and PMMA homopolymers and PA–MMA copolymers.

PMMA homopolymer shows two TG decomposition stages. The first starts at ∼210 °C with a weight loss of ∼16%. The second stage starts at ∼295 °C with a weight loss of ∼80%. There are three TG degradation stages for all the PA–MMA copolymers. The degradation temperature started at ∼190, 195, 200 and 205 °C for the copolymers 1:30, 1:40, 1:50 and 1:60 mol of PA–MMA. Table 1 represents the weight loss percentage and the maximum rate of weight loss shown by derivative TG apparatus. TG curves of the copolymers reveal that the stability of copolymers is intermediate between PPA and PMMA homopolymers.

Table 1 Weight loss % of PPA and PMMA homopolymers and PA–MMA copolymers.
Polymer mole PA–MMA Volatilization temperature (°C) First stage Second stage Third stage Remaining wt % after 500 °C
Tmax (°C) Wt loss (%) Tmax (°C) Wt loss (%) Tmax (°C) Wt loss (%)
PPA 180 220 13 280 39 390 43 4
1:30 190 225 22 285 38 393 36 4
1:40 195 225 23 290 38 395 35 4
1:50 200 270 26 312 34 433 32 8
1:60 205 264 27 318 34 421 33 6
PMMA 210 230 16 295 80 4

The most clear result is the increase of the thermal stability of PPA homopolymer and PA–MMA copolymers towards PMMA homopolymer. The effective activation energies for the thermal degradation of PPA and PMMA homopolymers and PA–MMA copolymers were determined from the temperature dependence of the chain rupture rate. The rate constant of the thermal degradation was plotted according to the Arrhenius relationship (Fig. 5). Table 2 lists the activation energies of the homopolymers and copolymers, from which the values of activation energy of the copolymers increasing from 47.29 to 249.42 kJ/mol were obtained as the MMA concentration in the copolymer increases. It is clear that the rate of activation energies is in the same order as the stabilities.

Arrhenius plots of the rate constants of degradation of PPA homopolymer and PA–MMA copolymers.
Figure 5 Arrhenius plots of the rate constants of degradation of PPA homopolymer and PA–MMA copolymers.
Table 2 Activation energies of the thermal degradation of PPA and PMMA homopolymers and PA–MMA copolymers.
Polymer mole PA–MMA Activation energy (Ea) kJ/mol
PPA 47.29
1:30 71.71
1:40 83.14
1:50
PMMA 249.42

3.4

3.4 Thermal degradation of PPA homopolymer

50 mg of PPA homopolymer was heated under vacuum from ambient temperature to 500 °C. The volatile products of degradation were collected in a small gas cell for identification by IR spectroscopy. Benzene, aniline and ammonia were among the degradation products of PPA homopolymer. The liquid fractions from the degradation of the homopolymer were injected into the GC–MS apparatus. Fig. 6 shows the GC trace for the liquid products of degradation of PPA homopolymer at 500 °C. Table 3 gives the results of the degradation which were identified by mass spectroscopy. The various degradation products of PPA homopolymer indicate that the mechanism of degradation is characterized by the elimination of low molecular weight radicals rather than monomer formation in the early stage of degradation, followed by random scission mechanism along the backbone chain. It seems that the breakdown of PPA homopolymer occurs mainly in the C—N bond producing the radicals.

GC curve of the liquid fraction of degradation of PPA homopolymer.
Figure 6 GC curve of the liquid fraction of degradation of PPA homopolymer.
Table 3 GC–MS data of the liquid fraction of the degradation of PPA homopolymer.
Compound No. Retention time (min) Major MS fragment Suggested structure
1 8.72 137, 120, 92, 77
2 9.15 182, 169, 109, 77
3 10.08 197, 105, 95, 82, 77
4 10.90 212, 137, 119, 93, 77
5 11.97 240, 165, 150, 122, 107, 77

The radicals III and V abstract H and produce benzene and aniline as major products.

Compound 1 in the GC curve listed in Table 3 could be formed by abstraction of OH by the radical IV.

Compound 2 could be formed by a dimerization of V.

The mass spectrum of N-phenylbenzamide (compound 3) is a termination reaction of the radicals III and IV.

The suggested structure of compound 4 is formed by the reaction between the radicals IV and V.

The assignment of structure 5 is a dimerization reaction of IV.

4

4 Conclusion

Four different compositions of copolymers of N-phenylpropionamide and methyl methacrylate (PA–MMA) were prepared and the reactivity ratios were determined using the 1H-NMR method. Thermal degradation of poly(N-phenylpropionamide) (PPA) was studied and the products of degradation were identified by GC–MS techniques. Benzene, aniline, ammonia, phenylcarbamic acid, 5,10-dihydrophenazine, N-phenylbenzamide, 1,3-diphenylurea and N1,N2-diphenyloxalamide were the main degradation products. Accordingly, it seems that the mechanism of degradation of PPA is characterized by breaking down in the C—N bond producing low-molecular radicals. Combination of these radicals and random scission mechanism along the backbone chain are the main source of the degradation products.

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