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

Arabian Journal of Chemistry (2017) 10, S3732–S3739

King Saud University

Arabian Journal of Chemistry www.ksu.edu.sa www.sciencedirect.com

ORIGINAL ARTICLE

Thermal stability and degradation of poly (N-phenylpropionamide) homopolymer and copolymer of N-phenylpropionamide with methyl methacrylate M.A. Diab *, A.Z. El-Sonbati, A.A. El-Bindary, H.M. Abd El-Ghany

1

Department of Chemistry, Faculty of Science, Damietta University, Damietta, Egypt Received 26 December 2012; accepted 20 May 2014 Available online 2 June 2014

KEYWORDS N-phenylpropionamide; Thermal stability and degradation; Reactivity ratios

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. ª 2014 Production and hosting by Elsevier B.V. on behalf of King Saud University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

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 * Corresponding author. Tel.: +20 1060081581; fax: +20 572403867. E-mail address: [email protected] (M.A. Diab). 1 Abstracted from her Ph.D. Peer review under responsibility of King Saud University.

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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,

http://dx.doi.org/10.1016/j.arabjc.2014.05.008 1878-5352 ª 2014 Production and hosting by Elsevier B.V. on behalf of King Saud University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

Thermal stability and degradation of poly (N-phenylpropionamide) homopolymer 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.

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. 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.

CH

H2C

H2C

NH2

CH O

O

C NH

Dry Benzene

+

C

S3733

Cl

N-Phenylpropionamide (PA) H2 C

CH

H2C O

C

H C O

NH

n

C NH

AIBN DMF

Poly(N-Phenylpropionamide) (PPA)

2. Experimental 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. 2,20 -Azobisisobutyronitrile (AIBN) (Aldrich Chemical Co., Inc.) was used as an initiator for all polymerizations. It was purified by dissolving it in hot

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.

S3734

M.A. Diab et al. cation 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.

2.3. Analytical techniques 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. Nuclear magnetic resonance spectroscopy (NMR) 1 H-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.

3. Results and discussion

2.3.3. Thermogravimetry (TG)

3.1. Characterization of PPA homopolymer and PA-MMA copolymers

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.

The IR spectrum of PPA homopolymer shows a band at 1680 cm1 is assigned to the antisymmetric stretching vibration of the amidic carbonyl group. The bands at 1600, 1545 and 1440 cm1 are assigned to m(CAH), m(C‚C) and m(CAC) bands, respectively (Diab, 1994). The CAH in plane deformation in the region 1225–1045 cm1, the ring breathing at 995 and 1005 cm1, the out-of-plan CAH deformation vibration between 775 and 750 cm1 and the CAC out-of-plan deformation at 500 cm1 are assigned. The IR spectrum of PA–MMA copolymer shows bands at 1680 and 1730 cm1 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 cm1 are due to m(CAH), m(C‚C) and m(CAC) bonds (El-Sonbati et al., 1991), respectively.

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 identifi-

CH3 CH2

CH2

CH O

C NH

C O

C

n OCH3

(B) (A)

Figure 1

1

H-NMR spectrum of PA–MMA copolymers.

Thermal stability and degradation of poly (N-phenylpropionamide) homopolymer 3.2. Determination of reactivity ratios of PA-MMA copolymers

S3735

f2(1-2F2 )/(1-f2)F2

0.4

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 1 H-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 o 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 d 7.880 ppm is due to –NH proton of PA in the copolymer (Mochel, 1976). The peak (A) at d 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):

1 =M2 where, F1 ¼ MM1 =M is the mole fraction of MMA (M1) in 2 þ1 1 copolymers f1 ¼ n1nþn is the mole fraction of M1 in feed and 2 r1 and r2 are the reactivity ratios of MMA and PA, respec-

a plot of

f22 ðF2 1Þ ð1f2 Þ2 F2

f21 ðF1 1Þ ð1f1 Þ2 F1

versus

f1 ð12F1 Þ , ð1f1 ÞF1

and Fig. 3 is

f2 ð12F2 Þ ð1f2 ÞF2

versus

fraction of PA (M2) in

0.2

0.1

0.0

0.000

-0.005 -0.010 -0.015 -0.020 -0.025 -0.030 -0.035 -0.040 2

2

f2 (F2-1)/(1-f 2 ) F2

Figure 3 Graph of copolymers.

f22 ðF2 1Þ ð1f2 Þ2 F2

versus

f2 ð12F2 Þ ð1f2 ÞF2

for PA–MMA

fraction of M2 in feed. From the slope and intercept in Figs. 2 and 3 reactivity ratio values for PA–MMA copolymer are:

f1 ð1  2F1 Þ f2 ðF1  1Þ ¼ 1 r1 þ r2 ð1  f1 ÞF1 ð1  f1 Þ2 F1

tively. Fig. 2 is a plot of

0.3

2 =M1 where F2 ¼ MM2 =M is 1 þ1 2 copolymer and f2 ¼ n1nþn is 2

the mole the mole

f1(1-2F1)/(1-f1)F1

27

18

r1 ðMMAÞ ¼ 26

and

r2 ðPAÞ ¼ 0:4:

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%. PMMA homopolymer shows two TG decomposition stages. The first starts at 210 C with a weight loss of

PMMA PA:MMA(1:50) PA:MMA(1:30) PPA

9

-60

-40

-20 2

0

2

f1 (F1-1)/(1-f1) F1

Figure 2 Graph of copolymers.

f21 ðF1 1Þ ð1f1 Þ2 F1

versus

f1 ð12F1 Þ ð1f1 ÞF1

for PA–MMA

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

S3736 Table 1

M.A. Diab et al. Weight loss % of PPA and PMMA homopolymers and PA–MMA copolymers.

Polymer mole PA–MMA

Volatilization temperature (C)

PPA 1:30 1:40 1:50 1:60 PMMA

180 190 195 200 205 210

First stage

Second stage

Wt loss (%)

Tmax (C)

Wt loss (%)

Tmax (C)

Wt loss (%)

220 225 225 270 264 230

13 22 23 26 27 16

280 285 290 312 318 295

39 38 38 34 34 80

390 393 395 433 421 –

43 36 35 32 33 –

PPA homopolymer 1:30 PA:MMA 1:40 PA:MMA 1:50 PA:MMA 1:60 PA:MMA

-1.2 -1.6

Ln(K)

-2.0 -2.4 -2.8 -3.2 -3.6

1.95

1.98

2.01

2.04 -3

2.07

Third stage

Tmax (C)

2.10

2.13

-1

1 / T X 10 K

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 1:30 1:40 1:50 PMMA

47.29 71.71 83.14 – 249.42

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,

Remaining wt % after 500 C

4 4 4 8 6 4

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. 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. 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 CAN bond producing the radicals.

C O

CH2 CH

I

NH

IV

V

CH2 CH

C O

NH

C O

NH

II

III

Thermal stability and degradation of poly (N-phenylpropionamide) homopolymer

Figure 6

Table 3

S3737

GC curve of the liquid fraction of degradation of PPA homopolymer.

GC–MS data of the liquid fraction of the degradation of PPA homopolymer.

Compound No.

Retention time (min)

Major MS fragment

1

8.72

137, 120, 92, 77

Suggested structure

OH C O

NH

Phenylcarbamic acid 2

9.15

182, 169, 109, 77

H N N H 5,10-Dihydrophenazine

3

10.08

197, 105, 95, 82, 77

O C

H N

N-Phenylbenzamide

S3738

M.A. Diab et al. (continued)

Table 3 4

10.90

212, 137, 119, 93, 77

O H H N C N

1,3-Diphenylurea 5

11.97

240, 165, 150, 122, 107, 77

O H N

C

O

H C N

N1,N2-Diphenyloxalamide

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.

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

NH

OH C O

O

NH

C

H C N

+

NH IV

OH

+

O

H N

NH

Compound 4 m/e 212 1,3-Diphenylurea

V

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

Compound 1 m/e 137 Phenylcarbamic acid

IV

Compound 2 could be formed by a dimerization of V. NH

O

C NH

IV

dimerization

2

C

O

H C N

2

H N

O H N

Compound 5 m/e 240 N1,N2-Diphenyloxalamide

N

H

4. Conclusion

Compound 2 m/e 182 5,10-Dihydrophenazine

V

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

NH

O C

H N

+

III

IV

Compound 3 m/e 197 N-Phenylbenzamide

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 CAN 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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