Degradation of polycaproamide in hydrochloric acid vapour

~ol:ymer Science U.S.S.R. Vol. 21, pp. 1908-1013, Pergamon Press Ltd. 1980. Printed in Poland

0052--3950/79/0801-1908507.50J0

DEGRADATION OF POLYCAPROAMIDE IN HYDROCHLORIC ACID VAPOUR* L . P. •AZUMOVSKII, V. KH. I:~ODRIGEZ, V. S. MARKIN a n d G. YE. ZAIKOV Institute of Chemical Physics, U.S.S.R. Academy of Sciences

(Received 10 August 1978) The kinetics of hydrolysis of polyamide films at various temperatures and water and acid concentrations have been studied. By carrying out the reaction in hydrochloric acid vapour it has been possible to avoid a number of causes of difficulty in discussion of the process and it has been possible to demonstrate graphically the necessity of taking into account the structural and sorption properties of the polymer in investigating systems consisting of a polymer and an aggressive medium. On the basis of the results obtained, a scheme for hydrolysis has been proposed which described the experimental data well. The activation energy for the process is 19.5~:2.5 kcal/ /mole. The effect of the water vapour activity and temperature on the accessibility of the polymer was also studied. io_ s t u d y i n g the d e g r a d a t i o n of solid p o l y m e r s arises f r o m t h e necessity ~ f solving a p r o b l e m t h a t is i m p o r t a n t in practice, n a m e l y , predicting t h e d u r a bility of polymeric c o m p o n e n t s in aggressive media. This t y p e of d e g r a d a t i o n has, as yet, been t r e a t e d i n a d e q u a t e l y on t h e theoretical plane. I n the first instance, this is connected w i t h t h e c o m p l e x i t y of the s y s t e m s considered a n d t h e necessity of t a k i n g a c c o u n t of s t r u c t u r a l a n d sorption properties. The aim of the present work h a s to elucidate the m e c h a n i s m o f h y d r o l y s i s of p o l y a m i d e s in hydrochloric a c i d v a p o u r . T h e selection of a p o l y a m i d e as t h e material for t h e investigation arose because the diffusion a n d solubility of w a t e r h a v e been studied for the solid polymers a n d a m e t h o d of d e t e r m i n i n g the accessibility has been developed [1].

INTEREST

An industrial polycaproamide film PC-4, 60 ~m thick, and thin polycaproamide films 2-5 ~m thick, cast from a solution in 98~o formic acid 7 were used in the work. The PC-4 films were boiled beforehand for 5 hr in distilled water in an argon atmosphere to remove low molecular fractions. The kinetics of hydrolysis of the polymeric specimens were followed from the change in molecular mass and from the accumulation of end amide groups, as described in [2]. The accessibility was determined by the isotopic exchange H -* D using the absorption band of the ND group at 2480 cm -1 [11. In specially set experiments it was found that the coefficient of extinction ~ ) 0 was equal to 5"3 × 10-4 em2/mole. Changes in the concentration of the crystalline phase during degradation were followed by X-ray diffraction. A special vacuum cell with CaF2 windows was used for the deuteration of the thin * Vysokomol. soyed. A21: No. 8, 1732-1736, 1979. 1908

Degradation of polycaproamide

]g0D

~RImaand for the recording of their IR spectra. The construction of the ooll enabled the I R spectra to be recorded without changing the position of the film in the beam of the "Spo.cord-71 IR" spectrometer. In the first dat~ on the hydrolysis of solid polyamides, p r e s e n t e t in [3], t h e authors were limited to measurements of the number of end 5~H= groups formed during the reaction. Since hydrolysis could not be described b y a first orcler reaction, a number of papers have used the Frost and Dintsess equation in sub~equent papers: dx k(1--x) dt 1--fl(1--x) (1) where x is the concentration of the reagent that has reacted up to time t, given as a fraction; k is a certain constant in the reaction time and fl is a deceleration coefficient caused b y the diffusion of the reagent through the surface layer of t h e material.

fm=x I.OL

h I

a

¢

3

8.6

/

O'2 I

I

I

0.2

0.6

1.0

I

I

0"2

~6

l

/.O P/Po

FIe. 1. Dependence of f/fmax: a - - o n the water-vapour activity at the following temperatures; •--23; 2--35.5; 3--45 and 4--57°C; a n d b, as (a) b u t for films containing 0-4 mole/1. HC1 at •--57 and 2--25°C.

Although it satisfactorily describes the kinetics of the process, eqn. (1) does n o t enable the reaction rate to be pre~icte~ as a function of the experimental conditions (k and fl were calculated e~ch time directly from the experiment itself). In addition to this it is not clear w h y the coefficient fl depends slightly on temperature, acid concentration and the nature of the polyamide. Loknando etal., [7] ascribed the slowing down of the reaction (the deviation from a first order equation) to the existence of two rate constants. Initially t h e accessible part of the polymer was rapidly hydrolyse2 arid then th~ inaccessible part was slowly hydrolysod. B u t there is some contra=lictioa i n the experimental d a t a presented in the paper. On the one hand, after approximately one rupture

1910

L. P. RAZUMOVSX_TIet a/.

"has occurred p e r macromolecule, the inaccessible part begins to be hydrolysed (that is, t h e accessibility is approximate]y one per cent). On the other hand: the value of accessibility, obtained from acetylation of the end NH~ groups, v a r i e d in t]~.e range 75-84~/o. The slowing down of the reaction m a y be caused b y a number of factors: 1) ~low molecular fractions formed during the reaction become washed out into the external solution; 2) some of*the bonds in t h e film are in a stressed condition and their hydrolysis occurs more rapidly [8]; 3) the concentration of "free" ~acid (not combined with end groups) decreases during the reaction and 4) the accessibility of the polymer sl0ecimens is not taken into account.

-lo /

,

f

I'0

0.8 ~

Z

×3

..

m

0.6 -~ 20

;I 6O Fzo. 2

" I

0.6

IOOT,°C

5 '

15 Time,hr Fzo. 3

FIG. 2. Change in the accessibility of PC-4 films with temperature; / ' i n the absence of HC1; 2 and 3--with HC1 concentrations of 0.2 and 0.8 mole/h resl~ctively. FIG. 3. Dependence of log I[B]0-[P]} On time in the acidic hydrolysis of PC-4 at 96°C; [H=O]0~ 3.2 mole/1., B0--0"34 mole/1. In order to take account of possible reasons for the slowing down of the rate of t h e process, hydrolysis was carried out in hydrochloric acid vapour in the present work. This enabled direct contact between the specimens and the liquid phase to be avoided but it required that the dependence i f the accessibility f on the water vapour activity should be determined. The fact is that accessibility is a function not only of temperature [9, 10], as is true of solutions, b u t also of vappur pressure [9, 1 l]. For a number of reasons, the conditions of the experiment required the reaction to be carried out in unsaturated vapour. It m a y be seen from the data shown in Fig. la, that the accessibility increases as both p/p~ and temperature increase. As the temperature is increased, the limiting values o f accessibility, /max, are tb.ns reached at lower relative pressures. A similar relationship is found if the initial film contains a certain amount of hydrochloric acid, Fig. lb; even at p/~0~0.5, accessibility is found to be the same as in the saturated vapour. Figure 2 shows the temperature dependence of the acces,sibility of ,PC-4 films deuterated in saturated vapour or in aqueous solutions of

Degradation

of polycaproamide

1911

H ~ 0 + D 2 0 . If the film had adsorbed acid beforehand, with the method described in [2], values of accessibility were obtained that were slightly reduced. Probably this is connected with the fact that the acid, by blocking the amide groups, prevents the isotopic H - * D exchange reaction. The data obtained thus enabled it to be suggested that all the experiments on the hydrolysis of PC-4 were carried out under conditions when the accessibility did not depend on the water vapour activity (see Table). The concentration of the crystalline phase remained unchanged during the reaction. KII~ETIC

PARAMETERS

FOR

THE

HYDROLYSIS

OF

PC-4

POLYAMIDE

FILMS

UNDER

VARIOUS

CONDITIONS

T , °C

p/p.

[I-I,O]o*

Bo* mole/1.

105 96 96 96 96 96 96 76 65 65

saturated 0.80 0.84 0.61 0.70 saturated

9~

5.2 5.7 5-3 3.9 3.5 3.2 6-0 6.3 6.9 7-3

t X 104 kett

k~: e~f X 10'

1.]mole. m i n 0.74 0.77 1.08 0.45 0-69 0.34 0.79 0.79 1.13 0.79

4.1 2.7 2.1 1.6 1.3 1.3 1.5 0.70 0.21 0.27

8"7 4.8 4"3 4"0 4"5 4"2 2"7 1'1 0.35 0.41

* Per litre of the amorphous phase. t Values of kat calculated for mechanism (2). Values of k,tt calculated for mechanisms (3, 4).

Since polycaproamide has selective sorption with respect to acid molecules, the experimental conditions selected were such that all the acid was sorbed from solution by the polymer specimens in a time small compared with the reaction time (the film was placed in an ampoule above the HC1 solution). During the reaction, the concentration of "free" HC1 in the specimen must decrease continuously as a result of its combining with newly formed end NH2 groups. In view of this, the process of change in the concentration of amide groups should not be described by a first order equation. We have previously [8] propose~l the following possible mechanism for the reaction: A~-B ~*~ P ,

(2)

where A represents the hydrolysed amide groups (A0= 10-13 mole/1.), B the "free" acid and P the reaction product. A model of this type described the experimental data well: namely, the consumption of amide bonds according to a first order 0quation and also the consumption of acid according to a pseudo-first order equation ([A]>>[B]) (Fig. 3),

1912

L. P, RaZlIMOVSKII et al.

Suck a scheme does not, however, reflect the process entirely' correctly. It is a fact that the values of ke~fcalculated with this scheme depend on the experimental conditions, there being a clear connection between the water concentration in the polymer and the value of keff (see Table). L e t us consider a kinetic scheme for the hydrolysis of polyamides under t h e action of acids, which takes into account the part played by water: Kp

:

A + B ~--~ AB kz

A B + H 2 0 --~ P ,

(3) (4)

where AB is the reaction species, Kp the equilibrium constant and kl is the rate constant for the limiting stage. According to this scheme, HC1 molecules are linked rapidly and in an equilibrium fashion to the amide bond and the complex formed then reacts with water giving the reaction products. The rate of accumulation of the products will be: d[P] -- kl [AB] [H,O] dt

(5)

I f we take into account the balance equation, [B0]----[B]+[AB]+[P],

(6)

where Be is the initial concentration of "free" HC1 in the polymer, and t h e existence of the equilibrium Kp----[AB]/[A] [B],

(7)

as well as the fact that the change in the concentration [A] may be neglected in comparison with [B], we obtain the solution of eqn. (5): In [[H,0]o-- [P]] [B]o = keff[H20]o_ [B]0]t ' [H,O]o [Be--P]

(8)

whore

ke. =

l + l / K p [A]o

(9)

I f the concentration of H20 changes only slightly during the reaction, the solution of eqn. (5) takes on the form:

[Bo] __--~]geff[HuO]ot iSle--IF]

In -

(10)

Equations (8) and (10) enable values of kett to be calculated from the data for the accumulation of new end NH~ groups in the polymer or from the consumption of amide groups (see Table). In fact, if w'e differentiate the balance

Degradation of polycaproamide

1913.

[A]0= [A]-{-[AB] ~- [P],

(11)

equation then we obtain

d([A]+[AB])/dt=

d[P]/dt

(12)

I t is impossible to determine the value of kl within the framework of t h i s model since it is necessary to carry out experiments with different initial concentrations of amide groups [A]o in order to do this. The temperature dependence of keff becomes linear with the coordinates of the Arrhenius' equation and the activation energy is found to be 19-5±2.5 kcal/mole. Carrying out the hydrolysis in hydrochloric acid vapour under special conditions thus enabled a number of causes of difficulty in a kinetic description of the process to be avoided. In order to discuss the degradation process correctly in the general case, it is necessary, however, to take account of the change in structural and sorption properties in the system, consisting of the polymer and the aggressive medium, during the reaction. A scheme for the hydrolysis o f polycaproamide, put forward on the basis of the experimental data, describes the kinetic data well. Translated by G. F. M O D L E N REFERENCES 1. V. S. MABKIN, Yu. G. TKACH, Yu. V. MOISEYEV and G. Ye. ZAIKOV, VysokomoL soyed. A15: 2744, 1973 (Translated in Polymer Sci. U.S.S.R. 15: 12, 3116, 1973) 2. L. P. RAZUMOVSKII, V. Kh. RODRIGES, V. S. MARKIN and G. Ye. ZAIKOV, Vysokomol, soyed. AI9: 1357, 1977 (Translated in Polymer Sci. U.S.S.R. 19: 6, 1562, 1977) 3. E. EL~D and H. G. FR~JHLICH, Mell. Text 30: 103, 1949 4. Ye. K. MANKASH and A. B. PAKSHVER, Zh. fiz. khimii 25: 468, 1951 5. B. DOLE~EL, Chem. prfimysl. 6: 463, 1956 6. $. RUSZHAK a n d G. LEPENYE, Kolorisztikai Err 1O: 226, 1968 7. N. BHATTACHARYYA a n d H. T. LIKNANDE, J. Appl. Polymer Sci. 20: 873, 1976 8. V. A. BERSHTEIN, L. M. YEGOROVA and V. V. SOLOV'YEV, Mekhanika polimerov, 854, 1977 9. V. S. MARKIN, L. P. RAZUMOVSKII, Yu. V. MOISEYEV and G. Ye. Z~EKOV, Vysokomol, soyed. A18: 1187, 1976 (Translated in Polymer Sci. U.S.S.R. 18: 6, 1361, 1976) 10. P. SCHMIDT and B. SCHNEIDER, Collect. Czechosl. Chem. Commun. 31, 1896, 1966. 11. R. PUFFR and J. ~EBENDA, Collect. Czechosl. Chem. Commun. 29: 75, 1964