HEATS OF COMBUSTION OF HIGH TEMPERATURE POLYMERS
HEATS OF COMBUSTION OF HIGH TEMPERATURE POLYMERS
HEATS OF COMBUSTION OF HIGH TEMPERATURE POLYMERS
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oxygen. In equation 2 the quantity, ro = [nO2 MO2 /npMp], is the stoichiometric oxygen-to-fuel<br />
mass ratio.<br />
To illustrate this thermochemical calculation the net heat of combustion was determined for<br />
poly(methylmethacrylate) (PMMA) which has the chemical structure<br />
[ CH2 C ] n<br />
O<br />
C<br />
OCH3<br />
3<br />
CH 3<br />
The methylmethacrylate repeat unit shown has the atomic composition C5H8O2 so the balanced<br />
chemical equation for complete combustion is<br />
C5H8O2 + 6 O2 → 5 CO2 + 4 H2O (3)<br />
From equation 3 it is seen that 6 moles of O2 are required to completely convert 1 mole of PMMA<br />
repeat unit to carbon dioxide and water. From equation 1 E was calculated for PMMA using the<br />
measured net heat of combustion in the present study, ∆hc = – 25.0 kJ/g (see table 1)<br />
E = (± 25.0 kJ/g±PMMA)<br />
(1 mol PMMA)(100 g±PMMA/mol)<br />
(6 mol O 2 consumed)(32 g/mol±O 2 )<br />
= ± 13.02 kJ<br />
g±O 2<br />
This calculation was performed for each polymer to determine E from its known atomic<br />
composition and measured heat of combustion. Inverting equation 1<br />
∆h c = E n O 2 M O2<br />
n p M p<br />
= Er o<br />
shows that the net heat of complete combustion of a polymer is simply calculated if the atomic<br />
composition of the polymer is known a priori so that the balanced thermochemical reaction<br />
equation can be written.<br />
Molar Group Additivity of Heats of Formation: Calculation of the heat of the combustion<br />
reaction of polymers was carried out using the principle of molar additivity of the heats of<br />
formation of the combustion products and reactants [13, 14]. The concept derives from the fact that<br />
enthalpy (H) is a state function and therefore its change in any process is independent of the path<br />
from reactants to products. Thus, the overall enthalpy of a reaction is the simply the sum of the<br />
enthalpies of the component reactions. In practice, the heat of combustion of the reaction can be<br />
calculated by subtracting the heat of formation of the products from the heat of formation of the<br />
reactants<br />
∆h c = np∆h o Σi f, p<br />
± nr∆h o Σj f, r<br />
where p and r denote products and reactants, respectively, in the standard state at temperature, 298<br />
K.<br />
For polymeric reactants the molar heat of formation can be estimated from the tabulated molar<br />
contributions of the chemical groups which constitute the monomer or repeat unit. Using PMMA<br />
as an example again with the monomer/repeat unit chemical structure shown previously, the heats of<br />
formation of the methylmethacrylate constituent groups at T = 298K are [13]:<br />
(4)<br />
(5)