Thermal Behaviour of Newly Synthesized Copolymer Resin Derived from 2, 4-Dihydroxyacetophenone and Biuret

S. S. Rahangdale

Department of Chemistry, Jagat Arts, Commerce and Indiraben Hariharbhai Patel , Science College, Goregaon - 441 801, Maharashtra, India

ABSTRACT:

The copolymer (2, 4-HABF) synthesized by the condensation of 2, 4-dihydroxyacetophenone (2, 4-HA) and biuret (B) with formaldehyde (F) in the presence of acid catalyst and using 2:1:3 molar proportions of the reacting monomers. The thermogravimetric analysis of copolymer resin prepared in present study has been carried out by non-isothermal thermogravimetric analysis technique in which sample is subjected to condition of continuous increase in temperature at linear rate. The advantage of Freeman-Carroll method is to calculate both the order of reaction (n) and energy of activation in one single stage by keeping heating rate constant. By using data of thermogravimetric analysis, various thermogravimetric parameters like frequency factor (Z), entropy change (ΔS), free energy change (ΔF) and apparent entropy (S*) have been determined using Freeman-Carroll method.

KEY WORDS: Resin, Synthesis, Thermal degradation.

INTRODUCTION:

Copolymer resins having good thermal stability have enhanced the development of polymeric materials. The thermal degradation study of copolymers has become a subject of interest. Study of thermal behaviour of copolymers in air at different temperature provides information about the nature of species produced at various temperatures due to degradation. In this laboratory, extensive work on the thermal degradation of copolymers has been undertaken ^1-2. Jadhao M. et al³ synthesized the terpolymer resin by the condensation of 2, 2’-dihydroxybiphenyl with urea and formaldehyde in the presence of 2M HCl as a catalyst. They reported thermal studies of the resins have been carried out to determine their mode of decomposition, activation energy, order of reaction, frequency factor, entropy change, free energy, and apparent entropy change. Patel and Patel⁴ were studied preparation and thermal degradation of 2-hydroxy-4-methoxypropiophenone-urea-formaldehyde copolymers. They observed that copolymers prepared using equimolar proportions of reactants are thermally more stable. Tang and coworkers studied thermal decomposition of copolyesters made from p-hydroxy benzoic acid^{5, 6}.

Thermal data have been analyzed by Freeman-Carroll and Sharp-Wentworth methods and various kinetic and thermodynamic parameters have also been evaluated. Comparable values of parameters indicates common decomposition reaction mode in all chelate polymers

EXPERIMENTAL:

MATERIALS:

2, 4-Dihydroxyacetophenone (SRL, Mumbai) and biuret (Merck, India) were purified by rectified spirit, formaldehyde (37 %). Double distilled water was used in all the experiments.

Synthesis of 2, 4-HABF copolymer resin:

Copolymer resin (2, 4-HABF-II) was prepared by condensing 2, 4-dihydroxyacetophenone (3.042 gm, 0.2 mole), biuret (1.3 gm, 0.1 mole) and formaldehyde (11.25 ml of 37 %, 0.3 mole) in the presence of 2M HCl (200 ml) as a catalyst at 124 +2 ⁰C in an oil bath for 5 h ^7-12. The solid product obtained was immediately removed from the flask as soon as the reaction period was over. It was washed with cold water, dried and powdered. The powder was repeatedly washed with hot water to remove unreacted monomers. The air-dried copolymer resin was extracted with petroleum ether and diethyl ether to remove copolymer. It was further purified by dissolving in 8 % NaOH and then was filtered. The copolymer was then precipitated by drop wise addition of 1:1 (v/v) conc. HCl/water with constant stirring and filtered. The process was repeated twice. The resulting polymer sample was washed with hot water, methanol and dried in a vacuum at room temperature. The purified copolymer resin was finely ground to pass through a 300 mesh size sieve and kept in a vacuum over silica gel. The yield of the copolymer resin was found to be 82%.

Characterization:

Instrumentation:

The non-isothermal thermogravimetric analysis was performed in air atmosphere with heating rate of 100 ⁰Cmin^-1 using 5 - 6 mg of samples in platinum crucible from temperature of 400 ⁰C to 800 ⁰C and thermograms

are recorded for 2, 4-HABF sample at SICART, Vallabhvidyanagar, Gujrat. With the help of thermogravimetric data the thermal activation energies (Ea) and order of reaction (n) calculated. Also other thermodynamic parameters such as entropy change (ΔS), apparent entropy change (S*) and frequency factor (z) are determined.

Theoretical considerations:

To provide further evidence regarding the degradation system of analyzed compounds, we derived the TG curves by applying an analytical method proposed by Sharp-Wentworth and Freeman- Carroll.

Freeman-Carroll method:

The straight-line equation derived by Freeman and Carroll¹⁴, which is in the form of n

[∆log (dw / dt)] / ∆ log Wr = (-E / 2.303R) . ∆ (1/ T) / ∆ log Wr + n -----------------(1)

Where,

dw/dt = rate of change of weight with time.

Wr = Wc-W

Wc = weight loss at completion of reaction.

W = fraction of weight loss at time t.

Ea = energy of activation.

n = order of reaction.

The plot between the terms [Δ log (dw/dt)] / Δ log Wr Vs Δ (1/T) / Δ log Wr gives a straight line from which slope we obtained energy of activation (Ea) and intercept on Y-axis as order of reaction (n). The change in entropy (ΔS), frequency factor (z), apparent entropy (S*) can also be calculated by further calculations.

Sharp -Wentworth method:

Using the equation derived by Sharp and Wentworth¹⁵,

log [(dc/dT)/ (1-c)] = log (A/β) – [Ea/2.303R]. 1/T …………..(2)

Where,

dc/dT = rate of change of fraction of weight with change in temperature

β = linear heating rate dT/dt.

By plotting the graph between log dc/dt/ (1-c) vs 1/T we obtained the straight line which give energy of activation (Ea) from its slope. Where β is the conversion at time t, R is the gas constant (8.314 Jmol^-1K^-1) and T is the absolute temperature. The plots (Figure 1) give the activation energies at different stages of degradation reaction take place. This isoconversional (model-free) kinetic methods use to check the variation of the apparent activation energy values with degree of degradation. This kinetic analysis should be a starting point for obtain the useful information on the behaviour of the sample (Table 1).

RESULTS AND DISCUSSION:

Thermogravimetric analysis:

Decomposition pattern of representative system and TGA curve of 2, 4-HABF copolymer resins are shown in Fig 1. The kinetic parameters for the thermal degradation have been evaluated from the thermograms by using Freeman-Carroll Method and Sharp-Wentworth Method which are presented in Table 1.

Examination of thermogram and TG data of 2, 4-HABF copolymer resins showed that these polymers have resistance to elevated temperatures. Thermogram of all 2, 4-HABF copolymer exhibited two stages of decomposition after loss of water molecules. It is observed from TG data of these copolymer resins that the sample lost 3.81 to 6.58 % of its weight when the temperature was raised to 180 ⁰C. This initial weight loss may be attributed to solvent or moisture entrapped in the copolymer samples. First decomposition step in 2, 4-HABF copolymer resin represents degradation of both phenolic hydroxyl groups. Second decomposition step in case of all 2, 4-HABF copolymer resin shows the complete degradation of aromatic biphenyl nucleus and side chain attached to aromatic biphenyl ring.

Fig. 2: Thermal activation energy plot of 2, 4-HABF copolymer.

Fig. 3: Freeman-Carroll plot of 2, 4-HABF copolymer.

Table 1: Kinetic Parameters of 2, 4 - HABF copolymers

Sr. No.

Copolymer

Entropy

change ΔS (J)

Free energy

change ΔF (KJ)

Frequency factor Z (s^-1)

Apparent entropy
change S* (KJ)

Order of reaction
N

2, 4-HABF-I

2, 4-HABF-II

2, 4-HABF-III

2, 4-HABF-IV

-8.3866

-8.2705

-8.1751

-8.2286

22.58

16.70

19.11

18.96

814

457

794

630

-22.98

-23.56

-23.00

-23.24

1.1

1.2

0.98

0.99

FC = Freeman-Carroll Method.

SW = Sharp-Wentworth Method.

Fig. 4: Sharp-Wentworth plot of 2, 4-HABF copolymer

The data on the thermogravimetric analysis of copolymer resins were analyzed using the methods described above. These methods were used to determine the kinetic parameters like activation energy (Ea) and order (n) of the decomposition reaction. By using Freeman-Carroll method different thermodynamic parameters (Table 1) have been calculated such as, entropy change, free energy change, frequency factor and apparent entropy change etc. However, due to economies the space the thermal data (Table 1) and kinetic plots (Fig. 2 to 4) for only one representative case has been given.

Thermograms of 2, 4-HABF terpolymer resins are shown in Fig. 2 to 4. The results of thermogravimetric analysis of copolymer are listed in Table 1. Examination of thermograms and TG data of these 2, 4-HABF terpolymer resins revealed that the sample lost 2-6 % of its weight when the temperature was raised to 443 K. This initial weight loss may be due to solvent or moisture entrapped in the copolymer sample¹³. Thermogram of all the 2, 4-HABF copolymers have exhibited three stages of decomposition, after loss of water molecules. The temperature range of first, second and third decomposition stage for different copolymer is found to be different. The first stage of decomposition represents decomposition of side chain attach to aromatic nucleus in polymer i. e. phenolic - OH and acetyl - COCH₃ groups linked to aromatic ring. The second stage of decomposition corresponds to loss of aromatic nucleus from the polymer. The third stage of decomposition may be assigned to removal of side chain of biuret polymer and consequently residue remained may be assigned as imide. From the results of kinetic parameters it is concluded that the terpolymer resins prepared from a higher molar ratio of 2, 4-hydroxyacetophenone exhibited a lower rate of decomposition suggesting the order of stability as: 2, 4-HABF –I < 2, 4-HABF –II < 2, 4-HABF –III < 2, 4-HABF

–IV. This fact is further supported by the increasing order of melting points (Viz. 2, 4-HABF-I < 2, 4-HABF –II < 2, 4-HABF –III < 2, 4-HABF –IV) of these copolymer resins. The above mentioned order of stability may be due to possibility of an almost linear structure of the copolymer having higher molar ratio of 2, 4-dihydroxyacetophenone which may give rise to a stable structure to the copolymer chain¹⁴.

Sharp - Wentworth method² is applied to calculate the activation energy, which is in agreement with the activation energy calculated by Freeman - Carroll method (Table 1). Thermodynamic parameters have been calculated³ on the basis of thermal activation energy and these values are cited in Table 1. The similarities of the values indicate a common are about the same. The similarities of the values indicate a common reaction mode². From the abnormally low values of frequency factor (Z), it may be concluded that the decomposition reaction of 2, 4-HABF terpolymer resins can be classed as a “slow” reaction. There seems to be no other obvious reason¹². Higher value of activation energy suggests the higher stability¹⁴. Thus the order of thermal activation energy is: 2, 4-HABF-I < 2, 4-HABF –IV < 2, 4-HABF –III < 2, 4-HABF –II. The negative values for entropy (ΔS) indicate that the activated polymer has a more ordered structure than the reactants and the reactions are slower than normal. This is further supported by low Z values⁴.

However, in the Freeman - Carroll method (Fig. 2) some abnormal points were ignored to get a clear picture about most of the points. Similarly, in the Sharp - Wentworth method, (Fig. 3) some points at the beginning or at the end did not fall on the straight line. This is expected since the decomposition of terpolymer is known not to obey first order kinetics perfectly². These observations were in harmony with the findings by earlier workers³

CONCLUSIONS:

The 2, 4-HABF copolymer based on the condensation polymerization of 2,4-dihydroxyacetophenone and biuret with formaldehyde in the presence of acid catalyst has been prepared. In TG the energy of activation evaluated from the Freeman-Carroll and Sharp-Wentworth methods are found to be nearly equal and the thermodynamic parameters obtained from Freeman-Carroll method are found to similar, indicating the common reaction mode. Low values of frequency factor (Z) may be concluded that the decomposition reaction of 2, 4-dihydroxyacetophenone-biuret-formaldehyde copolymer can be classified as ‘slow reaction’.

ACKNOWLEDGEMENT:

The author expresses his sincere thanks to The Director, Laxminarayan Institute of Technology, Nagpur for cooperation and for providing necessary laboratory facilities and also thankful to the UGC for financial support.

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Received on 28.10.2012

Modified on 11.11.2012

Accepted on 05.12.2012

Research J. Science and Tech. 4(6): November –December, 2012: 285-289