12 Degradation of Polymers ROBERT SIMHA Department of Chemistry, University of Southern California, Los Angeles, Calif.
In the main chain breakdown of a polymer by pyrolysis, two competing processes are decisive: a random scission caused by intermolecular radical Downloaded by FUDAN UNIV on December 22, 2016 | http://pubs.acs.org Publication Date: January 1, 1962 | doi: 10.1021/ba-1962-0034.ch012
attack and a depropagation of macroradicals.
In
terms of these, the observed correlations between monomer yields, rates of volatilization, and decrease of average molecular weight can be interpreted.
The relative importance of the
two
mechanisms for a specific polymer is a function of its structure and this accounts for the wide qualitative and quantitative differences between different systems. average
For a given structure, the initial
molecular weight, the distribution of
chain lengths, branching and impurities or structural inhomogeneities in the chain have important kinetic consequences.
The role of these factors
presents important experimental and theoretical problems which have been investigated conclusively only in part.
Another problem is a quan-
titative or qualitative analysis of the radicals present in a pyrolyzing polymer.
|^ inetic studies of free radical mechanisms or heat stability tests of n e w polymers have evolved from the large amount of experimental work, during the past 15 years, o n the rates of volatilization of polymers (9, 11, 22). Concurrently, general theories on the degradation process were developed (18, 22) and their quantitative conclusions were compared w i t h experimental results. T h e limited experimental data and the complexity of the equations involved have restricted the comparisons for the most part to special cases w h i c h could be treated analytically. In this article, w e summarize the basic kinetic observations and principles involved i n pyrolytic degradations and present some selected and partially solved problems on reactions based on chain breakdown of vinyl-type polymers. Basic Observations After a polymerization has been initiated, the important step is the propaga tion of active centers, the rate of w h i c h equals k MR, where M and R represent p
157 PLATZER; POLYMERIZATION AND POLYCONDENSATION PROCESSES Advances in Chemistry; American Chemical Society: Washington, DC, 1962.
ADVANCES IN CHEMISTRY SERIES
158
monomer and (in a free radical process) radical concentrations, respectively. This latter rate increases w i t h increasing temperature. A t the same time, however, k R the rate of the reverse process—namely, the depropagation or evolution of monomer from a radical—also increases. W e have the following relation between the corresponding activation energies 2
y
E + AH = E p
where AH is the thermodynamic heat of polymerization. Hence, the depropaga tion increases more rapidly w i t h temperature. F i n a l l y , at the so-called ceiling temperature, T , the two rates become equal ( 5 ) . T is a function of monomer concentration and is conventionally referred to one mole per liter or 1 at. for a gaseous monomer. T h e evaluation of ceiling temperatures has been discussed i n detail by Dainton and I vin (5) who have tabulated values for a series of v i n y l polymers. It is of interest to compare these temperatures with another set of tem peratures, characteristic of the degradation process. T h e y involve an arbitrary choice of a reference rate of decomposition. Table I shows decomposition tem peratures T (28), referred to a rate of volatilization of 1% per minute i n vacuum. T h e latter varies w i t h the degree of conversion to volatiles, either decreasing monotonically or reaching a maximum. T h e reference rates used i n the construction of Table I are the largest ones throughout. There are considerable uncertainties i n the magnitudes of the T s, augmented by the fact that the reference states vary for dif ferent polymer-monomer systems. Nevertheless, for most polymers shown, T is considerably larger than T —as expected—but i n polyethylene, both are about equal. F o r polytetrafluoroethylene, on the other hand, T is lower b y about 150° C . w h i c h implies a relatively high rate of decomposition. This cannot be associated w i t h a low value for the activation energy E i n Equation 1, since AH is about 40 kcal. per mole (15), considerably higher than for hydrogenated polymers. Hence, the radical concentration must be high. This, i n turn, w o u l d indicate a h i g h rate of initiation a n d / o r low probability of termination w h i c h may be a chemi cal or simply a physical effect, arising from a diffusion controlled and, therefore, reduced termination rate. c
c
Downloaded by FUDAN UNIV on December 22, 2016 | http://pubs.acs.org Publication Date: January 1, 1962 | doi: 10.1021/ba-1962-0034.ch012
(1)
2
d
f
c
d
c
d
2
Table I.
Decomposition Temperatures, l , for a Series of Polymers, Referred to 1%/Minute(28) d
Polymer Tetrafluoroethylene Ethylene Propylene Styrene Methyl methacrylate Methacrylonitrile Isobutylene a-Methylstyrene Formaldehyde
Td °C. 510 400 380 360 330 >200 340 290 >100
T h e TVs i n Table I i m p l y wide variations i n rates, but not necessarily i n mechanism, w h i c h could possibly be simple monomer depropagation. T h a t this is not so, i n a l l cases, is concluded at once from an inspection of the volatile de composition products. Table II (18) summarizes the mass spectrometric yields of monomer, obtained b y Madorsky and his colleagues at the National Bureau of Standards. Briefly, we note the h i g h yields of monomer i n the α-substituted and fluorinated vinyls. Elimination of the α-substituents or the fluorine atoms reduces considerably the monomer yield. This is illustrated b y a comparison of polyPLATZER; POLYMERIZATION AND POLYCONDENSATION PROCESSES Advances in Chemistry; American Chemical Society: Washington, DC, 1962.
SI ΜΗ A
159
Degradation of Polymers
(methyl methacrylate) w i t h poly (methyl acrylate), poly(-a-methylstyrene) w i t h polystyrene, and polytetrafluoroethylene w i t h polytrifluoroethylene. Thus, w e observe a wide spectrum of monomer yields ranging from practically zero i n polyethylene to almost 100% i n α-substituted polymers and Teflon. Table II. Polymer
Formula CH
Methyl meth acrylate
Monomer Yields
Weight %
Weight
Polymer
%
3
1
>95
-CH —C— 2
/3-Deuterostyrene
42
CH 0(X> Downloaded by FUDAN UNIV on December 22, 2016 | http://pubs.acs.org Publication Date: January 1, 1962 | doi: 10.1021/ba-1962-0034.ch012
3
Methyl acry late
Η
—CHz
Propylene
—CH*
^20
3
>95
-CH2—C—
(DeHs Ethylene
D a-Deuterostyrene
Isobutylene
-GHr-C—
hoho CH
a-Methylstyrene
2
I
-CH —Ο 2
Ι
0.025
70
Vinylcyclohexane
-0.1
52
Tetrafluoroethylene
>95
CeHg
Η m-Methylstyrene
I
-CH —C— 2
-CH Styrene
Η -CHz—G—
75
Trifluorostyrene
8
42
Trifluorochloroethylene Trifluoroethylene Vinylidene fluoride Vinyl fluoride
-CF2CFCI—
28
-CF CFH—
