Polystyrene (PS) is a thermoplastic material having so many using areas like home appliances, construction, packaging, and other industrial applications. It is cheap and easily processable. However, it has restricted mechanical properties and is very flammable. Due to its advantages, different kinds of derivatives have been produced. To enhance PS’s competition capacity in engineering resin applications, to protect its properties like thermal stability is crucial. In this work, poly(para-substituted styrene)s have been investigated using nonisothermal degradation kinetics.

Usually, polystyrene, exposed to thermal degradation, softens and melts at 160˚C. Up to 275˚C, volatility of fused polymer with high molecular weight is lowered and this molten version of the polymer keeps its stability. However, polymer fragments with lower molecular weight have higher volatility form and the decomposition is completed after the temperature reaches to 470˚C [1] . In addition, molecular weight rapidly decreases in the beginning of the degradation and the rate of this decline slows down at later stages [2] . This case applies to not only polystyrene but also some of its substituted derivatives. Literature suggests that styrene is the major decomposition product in the complete depolymerization taking place below 500˚C. After this temperature, further fragmentation carries out and the oligomers turn into gaseous species having lower molecular weight [3] .

The decomposition of polystyrene is a radical chain process composed of sequential 1) initiation, 2) propagation, and 3) termination steps [4] . C-C bond strength determines the thermal stability of polystyrene. Random scission or breakage of “weak links” can cause initiation reactions followed by C-C bond chain-end cleavage producing radicals. In the second step of the degradation, hydrogen-abstraction and unzipping reactions occur. The reaction of free radicals produced and polystyrene results in short chain radicals. Termination generally happens as a result of the recombination of two radical chains. There are many examples for thermal degradation of different PS materials [5] [6] .

For kinetic interpretations in this study, poly(p-substituted styrene)s’ thermal degradation data were collected in nitrogen atmosphere. With intent to understand the origin of the equations to determine the kinetic parameters, a brief derivation is included below. For a simple degradation step, the rate of conversion is described as follows:

In this equation, a is the degree of chemical conversion, n is the reaction order, k is the reaction rate, and d/dt is the time derivative. A relation between the reaction rate and temperature is usually assumed:

Here, Z is the preexponential constant, R is the ideal gas constant, E_a is the activation energy, and T is the temperature in Kelvin. This is known as the Arrhenius relationship. Equation (3) is attained by combining Equations (1) and (2):

In order to obtain a precise solution, one of three variables in Equation (3) either may be held as a constant or may be related to another variable when the reaction process is carried out. In this way, the reaction process can be defined. For years, a differential or an integral version of Equation (3) has been applied by researchers to calculate the kinetic parameters consisting of E_a, Z, and n. While some of the techniques rely on the functional version of conversion, f(a), in the literature [7] - [12] , activation energy values have also been indicated to be estimated without any preceding knowledge of the functional form.

There is a linear relationship between temperature and time as following:

and

where T is the current temperature, T(0) is the initial temperature, t is time, and b is the scanning rate. Combining Equation (3) with Equation (5) yields:

This equation shows the theoretical shape of the TGA curve that is determined in the Kinetics Software. Logarithmic form of the equation is

A multilinear regression is performed by using ln(bda/dT), −l/RT, and ln(1−a) which have been appraised from TGA curve, and solving for Z, E_a, and n.

Kinetic parameters of degradation are determined upon the observations of Ozawa [13] and Flynn and Wall [14] . Their statements involve an approximation that has been discussed by Doyle [15] and by Flynn and Wall [14] , among others. This approximation comprises the so-called “exponential integral” and methods lowering error to a negligible level and by this way improving the results. Zsako and Zsako have suggested a closed form approximation for the exponential integral [16] . Although the determination of the activation energy is straightforward, one cannot carry out further kinetic computations without further data or assumptions as to the form of the kinetic equation. It has often been observed that the initial portion of a TG curve can usually be well-fitted by a first-order reaction equation. Since the interest in lifetime or stability studies is most often concerned with the initial portion (low conversion level), it has become a procedural practice to assume first-order kinetics and compute the rate constants, pre-exponential factors, etc. on this assumption. When the value of n = 1 is used (first order kinetics), the method is essentially the same as that used by ASTM Standart Test Method E 1641.

In this work, the mathematical treatment followed in calculating the parameters are called as Standard method. A head start of this method is to obtain the activation energy exactly independently of any assumptions regarding the relation between the reaction rate and the percent reacted. The quality of the statistical fit can be judged from the 95% confidence limits and from observing the lnβ vs 1/T plot. In the ideal case, the data points fall on parallel straight lines, the slope of which is used to calculate the activation energy.

Low heating rate degradation experiments were conducted in a Perkin Elmer PYRIS Diamond TG/DTA (DSC) apparatus in a dynamic nitrogen atmosphere which was calibrated with calcium oxalate. All TGA experiments were carried out with a linear heating rate of 5˚C - 25˚C/min up to 600˚C under nitrogen atmosphere. Kinetic parameters of degradation were determined by the Scanning Kinetics Software.

Polystyrene with Mw = 230,000 g/mol (430,102), poly(p-methylstyrene) with Mw = 72000 (182,273), poly(p-chlorostyrene) with Mw = 75,000 (434,124), and poly(p-bromostyrene) with Mw = 65,000 (181,366) beads were all supplied from Aldrich company and used without any further process. TG analysis was carried out on approximately 10 mg samples.

Thermal decomposition kinetics of poly(p-substituted styrene)s was examined in nitrogen atmosphere. Five different heating rates were applied to the samples and as heating rate increased, the degradation shifted to higher temperatures accompanied with an increase in the rate of weight loss. From the nonisothermal degradation data, kinetic parameters were calculated using Flynn-Wall-Ozawa method and the values of the reaction order of poly(p-substituted styrene)s were deduced as 1 with the correlation factor higher than 0.98. Activation energies of thermal degradations are increasing individually in PS, PMS, PClS with weight loss as it is decreasing in PbrS with weight loss. In general, the decrease in the activation energies of poly(p-substituted styrene)s with PS/PMS/ PClS trend is consistent with the increase in repeating unit molecular weight except for PBrS. Therefore PBrS is suggested to decompose with somehow different mechanism.

AyşegülŞenocak,CemilAlkan,AhmetKaradağ, (2016) Thermal Decomposition and a Kinetic Study of Poly(Para-Substituted Styrene)s. American Journal of Analytical Chemistry,07,246-253. doi: 10.4236/ajac.2016.73021