NON-ISOTHERMAL KINETIC AND THERMODYNAMIC STUDIES ON POLYESTERS AT DIFFERENT VISCOSITY AVERAGE MOLECULAR MASSES

Non-Isothermal kinetics of solid-state decomposition of Polybisphenol-A-Carbonate [PC; (Mt\: 1.5 3.68 x 10 4 ) and polymethyl metacrylate [PMMA Miv1.55-3.52 x 10 4 ] has been investigated under nitrogen atmosphere in view to study the effect of molecular mass on their thermal decompositions in solid state. Thermal data deduced through simultaneous thermogravimetric-differential thermal analysis-differential thermo gravimetry [TG-DTA-DTG] was used for calculations of thermodynamic parameters through Horowitz-Metzger and coats-Redfern methods, kinetics and mechanism of degradations of each polymer at different molecular masses. [PC] showed two step decomposition with slower rate than [PMMA], which showed single step decomposition at each of molecular mass ranges. It was observed that ester linkages in [PC] backbone provided satisfactory over [PMMA] which contain ester linkages in side chain, to Horowitz-Metzger, coats-Redfern methods kinetic equations of solid-sate decompositions In general, Horowitz-Metzger methods have shown greater values of energy of activation than Coats-Redfern methods for [PC]. Both of methods have provided a wide difference in frequency factor but almost same entropy change in most of the cases* Thermodynamic and kinetic data were mostly satisfied for first order and zero order kinetics in case of [PC] whereas were not satisfactory for [PMMA].


INTRODUCTION
The quality control and assessment in the industrial polymers have been found to be tedious and a series of experimental techniques were developed since past few decades [1][2] .Among such techniques, thermo analytical methods have emerged as a rapid and cost effective substitute over other analytical methods with reproducible results 3 -4 .Most of the modern chemical and metalurgical industries employ process involving solid-state gas-solid interface reactions.A detailed knowledge of the thermodynamics, kinetic and reaction mechanisms of these thermally induced processes is of immense help to optimize and control the operating parameters leading to design of proper process equipment.Thermal analysis techniques like thermogravimetry (TG), differential thermal analysis (DTA), differential scanning calorimetry (DSC) etc, where the property of a substance measured, as a function of temperature is extremely useful in this context 5 .In addition, if real time analysis of the product gases evolved during a thermally induced reaction will greatly enhance the possibility of correctily interpreting the kinetic data and arriving at most appropriate mechanism governing the process.Thus, knowledge of single particle kinetics becomes a pre-requisite for studying interface transport phenomena between moving gas stream and porous of solid materials 6 .Such single particle kinetics can be easily studied through simultaneous TG-DTA-DTG, evolved gas analysis EGA and thermo mechanical analysis TMA.Among such the technique of TG-DTA-DTG is frequently used as a. means of assessing the thermal stability and conveniently provide valuable information about kinetics and thermodynamics of the solid state decompositions of polymers and related heat resistant materials x 10 4 ) was synthesized through benzoyl peroxide catalyzed free radial polymerization of freshly distilled monomer.The monomer and polymers were thermally characterized through simultaneous TG-DTA-DTG in nitrogen.Thermal data were used for calculations of thermodynamic parameters through Horowitz-Metzger 7 and coats-Redfern methods 8 , kinetics and mechanism of degradations of each polymer at different molecular masses (Table 1) 6 .

Starting materials (a) Poly bisphenol-A-Carbonate [PC]:
[PC] was purchased with melt index 7 was purchased from Ms Sigma Aldrich Chemicals USA It was decomposed with aqueous HCI solution (10-30% v/v) at 100 ±1°C in the range of 0.5 to 1.0 h.This result a series of [PC] samples with different [PC] samples.Their molecular masses were deduced in tetrahydrofuran (K = 38.9x 10 3 mL/g,α = 0.70) through Mark Houwink equation using Oswald's capillary viscometer 9 and was found in the range of Mn=1.51-3.68 x 10 4 .

(b) Polymethyl metharcrylate [PMMA]
PMMA solutions in benzene were prepared at a concentration of 10, 20 and 30% (VW).50 ml of each solutions were separately polymerized in presence of benzoyl perioxide (0.1 M: 10 ml) for one hour at 100 ± 1 °C.PMMA of varying molecular masses were separatly isolated from each of the solutions, by evaporating the solvent.Viscosity average molecular masses of each of PMMA specimens was separated investigated in benzene (K = 5.5 x 10 -3 mL, g , α = 0.76) 9 and were found in the range of 1.55 -3.52 x 10 4 .

(c) Thermal Analysis
A simultaneous thermo gravimetric (TG) differential thermo-gravimetric (DTG) Differential thermal analysis (DTA) of each of the polymeric samples has been performed over NETZSCH-Geratebau GmbH Thermal Analyzer moder STA 409C.The sample size was ranged from 13 to 19.8 mg.The samples were allowed to decomposed in nitrogen at heating rate of 25°C/10.0 (K/min) 1400°C using alumina as a reference.All such thermo analytical procedures were executed at RSIC, Indian Institute of Technology (IIT), Madras.
Non-isothermal kinetics of solid-state decompositions of samples has been calculated from TG data to ascertain the rate controlling process according to the procedure reported by Sestak and Berggen and Satava 6 .In this procedure evaluations were made from different integral forms of kinetic expressions and the declaring rate equation based on diffusion was studied in terms of parabolic law satisfied to one dimensional transport (D1), two dimensional diffusion (D2), three dimensional diffusion according to Jander equation (D3) and three dimensional diffusion according to Ginstling-Brounshtein equation (D4) and the rate laws at one dimensional, zero order (R1), two dimensional; cylindrical symmetry (R2) and three dimensional spherical symmetry (R3) [Table I ].

(d) Kinetics and Thermodynamics
Thermogravimetric (TG) data of each of the polymers were evaluated for their weight loss (%) with reference to their temperature, decomposition stages involved and kinetic parameters with reference to their order (n) and energy of activation of thermal degradation (E), Entropy change (As) through a series of calculation procedures such as Horowitz-Metzger method 6 and Coate-Redfern method 7 .Thermal properties [    2).Kinetic and thermodynamic parameters of [PC] during thermal decomposition process were claculated for zero and first order kinetics in terms of energy of activation, frequency factors, and entropy changes according to Horwitz metzer (HM) 7 and coats redfern (CR) 8 methods.At molecular mass range 1.51 x 2.65 x 10 4 the values calculated for energy of activation were observed at lower values for CR method than HM which was calculated in the reversed order at molecular mass 3.68 x 10 4 .At molecular mass 2.65x10 4 HM method showed negative values for entropy change than other methods (Table 3).

Thermal properties of [PC] and
Mechanism of thermal decomposition was studied under non-isothermal conditions at definite intervals of time (Table 1 ) 6 .This has been done by solving the thermo gram for a series of diffusion and phase boundary / and other related equations.A comparative account has beed made between [PMMA] and [PC] samples towards their thermal decomposition reaction, and related non-isothermal relative parameters studies conducted and indicated that the calculation involved, was found to be more suitable for [PC] than [PMMA].These observations concluded that PBA like material can be more suitable studied in terms of their non-isothermal and other kinetic parameters than other thermoplastics like PMMA.

Figure- 1 .
Figure-1.Mechanism of thermal decomposition of PC at MT| 1.51 x 10