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Mechanism of the formation of rough surfaces on poly(ethylene terephthalate) films prepared by chemical etching in aqueous alkaline solutions
F o r m a t i o n of rough surfaces on P E T P films
2069
REI~#ENCE$
1. V. N. TSVETKOV, V. Ye. ESKIN a n d S. Ya. FRANKEL, S t r u k t u r a makromolekul v rastv o r a k h (Structure of Macromolecules in Solutions). Izd. " N a u k a " , 1964 2. M. WALES and K. E. van HOLDE, J. Polymer Sci. 14: 81, 1964 3. V. S. SKAZKA, G. V. TARASOVA and V. M. YAMSH~HKOV, Vestnik Leningrad State Univ. (LGU) 16: 69, 1973 4. H. Lt]TJE a n d G. M~TERHOFF, Makromolek. Chem. 68: 180, 1963 5. V. S. S K A Z K A and V. M. YAMSH~KI:KOV, Vysokomol. soyed. A15: 213, 1973 {Translated in Polymer Sci. U.S.S.R. 15: 1, 243, 1973) 6. B. ZIMM and D. LrROTHERS, Prec. Nat. Acad. Sei. USA 48: 905, 1962 7. W. H. STOCKMAYER a n d M. FIXMAN, J. Polymer Sci. C1: 137, 1964 8. {~. W. PYIN and M. FIXMAN, J. Chem. Phys. 41: 937, 1964 9. S. JMAI, J. Chem. Phys. 52: 4212, 1970 10. T. K O T A K A and N. DONKAI, J. Polymer Sei. 6; A-2: 1457, 1968 11. M. LECHNER a n d G. V. SCHULZ, Europ. Polymer J. 6: 945, 1970 12. V. PETRUS, J. DANIHEL a n d M. BOHDANECKY, Europ. Polymer J. 7: 143, 1971 13. 8. JMAI, J. Chem. Phys. 50: 2116, 1969 14. Ye. L. MOLODTSOVA a n d G. I. TIMOFEYEVA, Vysokomol. soyed. A16: 1163, 1974 (Translated in P o l y m e r Sci. U.S.S.R. 16: 5, 1347, 1974) 15. A. N. CHERKASOV, T. N. OSIPOVA and S. L KLENIN, Vysokomol. soyed. A I 0 : 1347, 1968 (Translated in Polymer Sci. U.S.S.R. 1O: 6, 1563, 1968)
MECHANISM OF THE FORMATION OF ROUGH SURFACES ON POLY(ETHYLENE TEREPHTH~LATE) FILMS PREPARED BY CHEMICAL ETCHING IN AQUEOUS ALKALINE SOLUTIONS* T. ¥E. RUDAKOVA, YU. V. MOISEYEV, V. I. ASTRII~A, L. L. RAZUMOVA, S. V. VLASOVAand G. ¥~.. ZAIKOV Chemical Physics Institute, U.S.S.R. A c a d e m y of Sciences
(Ree,eived 2 August 1974) A kinetic s t u d y was made of the etching of poly (ethylene terephthalate) (PETP) films in aqueous solutions of potassium hydroxide. An equation describing the accumulation of degradation products during the etching process was derived. A mechanism relating to the formation of rough surfaces of P E T P films is proposed.
ARTICL~.Smade from polymers, particularly poly (ethylene terephthalate) (PETP) articles, have smooth surfaces. However, P E T P articles with rough surfaces are required for a large number of practical purposes such as painting, bonding, and metallization, etc. One of the methods used for the creation of roughness on the surface in that * Vysokomol. soyed. A17: No. 8, 1797-1801, 1975.
2070
T. YE. ~:~UDAKOVAet al.
of chemical etching, whereby roughness can be brought about without detriment to the article itseff or to its properties. The mechanism of the formation of surface roughness can be better understood if one starts with the macrokinetics of the etching process and the extent t o w h i c h t h e s u r f a c e r o u g h n e s s is i n f l u e n c e d b y t i m e a n d t e m p e r a t u r e , a s w e l l as by the alkali concentration and the structure of the polymer. The P E T P films used for the investigation were of thickness 180-500/~ and varied as t o degrees of crystallinity and orientation. Crystallization of the films was carried out at 130 ° for varying time intervals in a nitrogen atmosphere. Orientation of the films was brought about b y drawing a t the rate of 5 mm/min at 85 °. Degrees of crystallinity were determined from density values, using r a p i d titration [1] and X - r a y difffraction [2]; the I-Iormans-Wcidinger method [3] was used to subdivide reflections into amorphous and crystalline fractions. The mean surface roughness was determined in accordance with State S t a n d a r d (GOST) 27892-59 on the basis of the p a r a m e t e r R a (the mean arithmetic deviation of microirregularities from an average profile line) on a " K a l i b r V E I - 2 I " profilograph-profilometer. The measurement error for R a amounts to _+0.05/~. The disrupted t o t a l internal reflection (DTIR) spectra were obtained in cells prepared at the Chemical Physics Institute, ~.S.S.R. Academy of Sciences; the cells have a germanium prism and a reflection factor of 8. The kinetic investigations were calTied out in a glass cell placed in a thermostat (water or off). The temperature of the reaction mixture was maintained accurate to ±0-1% A 10 ml alkaline solution and P E T P films measuring 4 × 1 cm • were used in all the experiments. Prior to the start of an experiment the reaction mixture was heated to a predetermined temperature, after which the P E T P film was lowered into t h e alkaline solution. The results of calculation show t h a t the film was heated to the solution temperature in less t h a n 1 see. A t the stipulated time intervals the films were rapidly takell out of t h e cell and placed in a twofold volume of distilled water to completely dissolve t h e reaction product (terephthalic acid (TPA)) t h a t is found on the film surface. Solutions o f t h e T P A in alkali and in water were mixed and cooled to room temperature. The mass of the T P A was determined by spectrophotometry [4]. Aqueous alkaline solutions of predetermined concentrations were prepared b y dilution of a saturated solution prepared from K O H of chemically pure grade.
Macrokinetics of the etching process. F i g u r e 1 s h o w s t h e k i n e t i c c u r v e s o f TPA accumulation during the etching of a PETP film of thickness 500±20/z w i t h 4 0 % c r y s t a l l i n i t y . I n 2 0 % K O H s o l u t i o n t h e r e a c t i o n is z e r o - o r d e r f o r TPA all the way to 0-7-0.8 conversion; in 49~ KOH solution deviation from l i n e a r i t y o c c u r s a t a c o n v e r s i o n d e g r e e o f ~ 0.3. A b o v e t h e l a t t e r d e g r e e o f c o n version an appreciable layer of TPA forms on the surface (owing to the poor solubility of TPA in conc.KOH), and the rate of the etching process begins to be limited by hydroxyl ions penetrating the negatively charged layer of TPA. A l l t h e e x p e r i m e n t s w e r e c o n d u c t e d u p t o ~ 0 . 1 5 d e g r e e s o f c o n v e r s i o n , i.e. u n d e r conditions that did not involve any appreciable amount of TPA forming on the film surface. The absence of variation in the molecular weight of the PETP films o f t h i c k n e s s 5/~ [4], a n d t h e i n v a r i a b i l i t y o f t h e D T I R s p e c t r a i n t h e I R r e g i o n * • No b a n d at 3290 em -1 corresponding to O H - valency vibrations in a carboxyl group was detected b y us. During the etching process the quality of the spectra deteriorated as a result of surface roughness appearing on the films.
F o r m a t io n of rough surfaces on P E T P films
2071
during the etching process, as well as the zero order for TPA mean t h a t the etching process takes place in a thin surface layer, which evidently measures no more than several monolayers. rn. I0 ~, mole
2
0
0
8
12
T / r o e . 17~/f,
:Fro. 1. Kinetic curves of TPA accumulation during the etching of a PETP film at 108° in 22 (1) and 49% KOH solutions (2). The roughness of the P E T P films changes during the etching process (Figs. 2, 3). As a first approximation one could say t h a t the number of projections remains constant during the etching process, the only change being in the size of the projections. To determine change in the surface in contact with the alkali solution one m a y assume t h a t the projections are conical ones; the mean surface of the film in a time t will then be
8,= (o 4(RoI)2+ ,
(1)
where So is the initial film surface, ~ is the mean base radius of projections in the profilogram. On analysing the experimental results one finds t h a t the maximally changed value of St amounts to not more than ~ 1-1 S0. The effective rate of the etching process was calculated by the equation [5]
dm ke~= dt St'
(2)
where m is the amount of TPA released at a moment of time t. I t was shown in ref. [4] t h a t the hydrolysis (etching) of ester bonds in a P E T P surface layer takes place through a mechanism whereby the hydroxyl ion is added in equilibrium at the carbonyl group of the ester bond, and the reactivity of the ionized form of the ester bond exceeds t h a t of the unionized form. The limiting step is t h e interaction of the ionized form of the ester bond with a molecule of water. According to this mechanism ktruoarqo
kerr-- 1 + KeJb o'
(a)'
20/2
T. YE. RUDAKOVA et al.
where/¢true is the true rate of hydrolysis, Keq is the equilibrium constant for the process of hydroxyl ion addition to the ester bond, b0 is alkalinity and aH,o is the reactivity of water in the alkaline solutions. Provided that the degree of ionization is low, equation (3)may be reduced to ktrue Keq b°aH'°
heft •
(4)
Table 1 gives the effective rate constants obtained from the experiments results. The temperature dependences of the tabulated constants are satisfacto
FIG. 2
FIG. 3
I~IG. 2. Profilogram of P E T P film (degree of crystallinity 40%) etched in 49% K O H at 119 ° for 2 mira Vertical magnification of the profilogram 400, horizontal 200. ~ G . 3. Photomicrograph of P E T P film surface. For the conditions see Fig. 2 ( × 450).
rily described by the Arrhenius equation. The effective activation energies and the thermodynamic parameters for the films of varying crystallinity are given in Table 2. As can be seen from Fig. 4a log ke~ 2~ is a linear function of Be on COaH,O
ordinates of equation (4). In concentrated KOH solutions (>46%) the position of experimental values of log-/ce~25: is lower than it ought to be according to aH~O
equation (4). The departure from linearity could be due to marked ionization of ester bonds. In that case the experimental results should be described by equation (3) (Fig. 4b). Combining equations (2) and (3) we obtain a general expression describing TPA accumulation during etching relative to time and temperature and thermodynamic parameters b0 and a ~ o
2A exp (-- ~-~)aH,oS,t m-~
1 -~-]¢eq/b0
where A is the pre-exponential factor.
'
(5)
F o r m a ti o n of rough surfaces on P E T P films
2073
As can be seen from Table 2, the degree of crystallinity has practically no effect on kinetic parameters of the etching process. This is apparently because for processes occurring in the monomolecular surface layer, the accessibility and reactivity of the ester bonds are identical in amorphous and crystalline regions. Orientation likewise has no effect on kinetic parameters of the etching process.
log (kea/a.zo)25.
ka:~O, lO-r,rnin.cmZ/rzo/e
a
/
/
/
20 -y
/0
-70
i
l
I
Z
I
-Ba
I
/Q
o
I
1
ze
FIo. 4. Plots of log /¢~f t& ° vs. --Be (a) and-a~'--£° vs. 1/bo (b) for the etching of P E T P
kerr
aHto
films in K O H . L e t us n o w c o n s i d e r h o w v a r i o u s f a c t o r s i n f l u e n c e t h e f o r m a t i o n o f r o u g h s u r f a c e s . A s c a n b e s e e n f r o m F i g . 5, t h e r e is f i r st o f all a m a r k e d r i s e i n R , d u r ing the etching of a PETP a t 108 ° i n 49~o K O H ,
f i l m o f t h i c k n e s s 5 0 0 4 - 2 0 g w i t h 40~/o c r y s t a l l i n i t y
after which the value of Ra goes through a maximum,
a n d t h e n falls a n d r e m a i n s n e a r l y c o n s t a n t u p t o c o m p l e t e c o n v e r s i o n o f t h e f i l m . TABI~ 1. EFFECTIVE R&TE CONSTANTSFOR THE ETOHII~G OF P E T P FILMS I1~ K O H OF VA.RYI'lqG- COlqCENTRATIOlqAT DIFFEREI~VJ:TEM'PERATU'RES
T, °C
ken × 10L mole/rain, •cm s
[KOH], wt.%
8.20
66 78 88 93
0"45 1"0 1"7 3"1
33.6
48 58 66 78
0.85 1.8 2.8 7.6
14.3
66 78 88 93
0"95 2"3 4"2 6"2
39"0
27 37 48
0-39 1.4 2.2
58 66 78
0"95 2-0 3"9 7"8
100 108 119
4.0 × 102 6.0 × 10 s 9.1 X l0 s
[KOH], wt.%
22"0
88
46"3
bet t X 10L T, °C mole/rain, •cm s
bat X 103, T, °C mole/rain. . em a
[KOH], wt.%
49.0
I 53.0
50 80 I00 108 119
0'1 × 10 a 1"2 X 10 I 4"0 × l0 s 6-5 × l0 s 1"2× 10'
50 80 100 108 119
0"6 × 6-0 × 3.0× 5.6 × 1-0 x
10 I 10 s l0 s l0 s 10a
2074
T. YE. R17DAXOVA e¢ al.
I t was calculated that maximum roughness corresponds to an etched layer thickness of 25-30 p. Two effects come to light on analysing the experimental results: 1) for all the films under varying conditions /~a goes through a maxim u m relative to time, the maximum corresponding to an "etched" layer thickness of 25-30/1; 2) the maximum value of R a decreases with rising degree of crystaUinity, and is practically independent of orientation. Ra ,~m
1"0 R a ,/lrn
l'O
0.5
/ 0"5
T .c
t
0
q
I
I
I
[
8 lZ Time, rain
FIG. 5
T J.
0
[
I
2
1
q 6 m~lOa,mole
Fro. 6
FIG. 5. Plot of the roughness R a vs. etching time of PETP film in 49 % KOH. Fzo. 6. Plot of R a vs. amount of TPA received for PF~TP films with degree of crystallinity 0 (1), 30 (2), 40 (3) and 58% (4), etched in 49% KOH at 108°. No convincing explanation has yet been found to account for the appearance and mechanism of surface roughness. One of the explanations is that preferential etching of amorphous phase takes place, and that unevenness appears as a result of "exposed" crystaUites. However, this is at variance with the fact that the roughness of amorphous films exceeds that of crystalline ones. Moreover, X-ray structural analysis of the etched films showed that no development of crystalline structure is found in the polymer. I t could well be that the surface roughness appears as a result of the laying bare of microdefects during the etching process, i.e. defects appearing during the preparation of polymers from melts have been reported b y a number of authors [5, 6]. According to views currently held b y authors, the number of defects in the surface layers of films exceeds the amount that are present in the interior layers. As the degree of crystallinity rises, the degree of defectiveness of samples is reduced. This results in reduced surface roughness in the etching of films with a
Formation of rough surfaces on PETP films
2075
TABLE 2. EFFECTIVW.RATES OF ETCHINGOF PETP FILMSIN" K O H OF VARYING COI~CENTRATIOI~AT DIFFERENT TEMPERATURES [KOtt], wt.%
[--logke,t,5o J
8.2 14.3 22.0 33.6 39.0 46.3 49.0 53-0
9.94 9.59 9.29 8.74 8-54 8-41 8.52 8-61
--B*
--loga~,o
0.12 0"48 0.94 1"64 2.02 2.53 2.75 2.98
0.02 0.05 0-10 0.25 0-36 0-61 0.78 1"00
--log -ke't a m ' ° × 1 0 - ' 1 Ee,,4-0"5, a~2o ke~t bo × 103 kcal/molo 9"92 9"54 9"19 8"49 8"18 7"80 7"74 7"61
831 347 155 31 15 6"30 5"30 4"00
58 31 15 23.0 10 31 2.0 1.0
18.0 18.0 18.0 18.0 18"0 16'5 16.5 16"5
*Bo ffilog bo.
high degree of crystallinity. J u d g i n g f r o m our results, it is clear t h a t the n u m b e r o f microdefects is largest a t a d e p t h o f 25-30/~, w h i c h gives rise to m a x i m u m r o u g h n e s s in the etching o f a l a y e r o f this v e r y thickness. The degree of r o u g h n e s s is d e t e r m i n e d solely b y e t c h e d l a y e r thickness, the alkali c o n c e n t r a t i o n a n d t e m p e r a t u r e p l a y i n g o n l y a s e c o n d a r y role, a n d influencing solely the r a t e o f r o u g h surface f o r m a t i o n . Translated by R. J. A. ~_EI~DRY REFERENCES
1. Ye. S. KHOROSHAYA, Plast. massy, No. 10, 60, 1961 2 . L. L. RAZUMOVA, T. Ye. RUDAKOVA, Yu. V. MOYISEYEV, L. A. MEL'NIKOV and G. Ye. ZAIKOV, Vysokomol. soyed. AI7: 861, 1975 (Translated in Polymer Sci. U.S.S.R. 17: 4, 990, 1975) 3. P. H. HERMANS and A. WEIDINGER, Makromolek. Chem. 98: 50, 1961 4. T. Ye, RUDAKOVA, Yu. V. MOISEYEV, A. Ye. CHALYKH and G. Ye. ZAIKOV, Vysokomol, soyed. A14: 449, 1972 (Translated in Polymer Sci. U.S.S.R. 14: 5, 505, 1972) 5, M. SHEN and M. B. BEVER, J. Mater. Sci. 71: 741, 1972 6. N. P. MEL'TSEVA, V. V. STRYUKOV and E. G. ROZANTSEV, Dokl. AN SSSR, 203: 1313, 1972
2069
REI~#ENCE$
1. V. N. TSVETKOV, V. Ye. ESKIN a n d S. Ya. FRANKEL, S t r u k t u r a makromolekul v rastv o r a k h (Structure of Macromolecules in Solutions). Izd. " N a u k a " , 1964 2. M. WALES and K. E. van HOLDE, J. Polymer Sci. 14: 81, 1964 3. V. S. SKAZKA, G. V. TARASOVA and V. M. YAMSH~HKOV, Vestnik Leningrad State Univ. (LGU) 16: 69, 1973 4. H. Lt]TJE a n d G. M~TERHOFF, Makromolek. Chem. 68: 180, 1963 5. V. S. S K A Z K A and V. M. YAMSH~KI:KOV, Vysokomol. soyed. A15: 213, 1973 {Translated in Polymer Sci. U.S.S.R. 15: 1, 243, 1973) 6. B. ZIMM and D. LrROTHERS, Prec. Nat. Acad. Sei. USA 48: 905, 1962 7. W. H. STOCKMAYER a n d M. FIXMAN, J. Polymer Sci. C1: 137, 1964 8. {~. W. PYIN and M. FIXMAN, J. Chem. Phys. 41: 937, 1964 9. S. JMAI, J. Chem. Phys. 52: 4212, 1970 10. T. K O T A K A and N. DONKAI, J. Polymer Sei. 6; A-2: 1457, 1968 11. M. LECHNER a n d G. V. SCHULZ, Europ. Polymer J. 6: 945, 1970 12. V. PETRUS, J. DANIHEL a n d M. BOHDANECKY, Europ. Polymer J. 7: 143, 1971 13. 8. JMAI, J. Chem. Phys. 50: 2116, 1969 14. Ye. L. MOLODTSOVA a n d G. I. TIMOFEYEVA, Vysokomol. soyed. A16: 1163, 1974 (Translated in P o l y m e r Sci. U.S.S.R. 16: 5, 1347, 1974) 15. A. N. CHERKASOV, T. N. OSIPOVA and S. L KLENIN, Vysokomol. soyed. A I 0 : 1347, 1968 (Translated in Polymer Sci. U.S.S.R. 1O: 6, 1563, 1968)
MECHANISM OF THE FORMATION OF ROUGH SURFACES ON POLY(ETHYLENE TEREPHTH~LATE) FILMS PREPARED BY CHEMICAL ETCHING IN AQUEOUS ALKALINE SOLUTIONS* T. ¥E. RUDAKOVA, YU. V. MOISEYEV, V. I. ASTRII~A, L. L. RAZUMOVA, S. V. VLASOVAand G. ¥~.. ZAIKOV Chemical Physics Institute, U.S.S.R. A c a d e m y of Sciences
(Ree,eived 2 August 1974) A kinetic s t u d y was made of the etching of poly (ethylene terephthalate) (PETP) films in aqueous solutions of potassium hydroxide. An equation describing the accumulation of degradation products during the etching process was derived. A mechanism relating to the formation of rough surfaces of P E T P films is proposed.
ARTICL~.Smade from polymers, particularly poly (ethylene terephthalate) (PETP) articles, have smooth surfaces. However, P E T P articles with rough surfaces are required for a large number of practical purposes such as painting, bonding, and metallization, etc. One of the methods used for the creation of roughness on the surface in that * Vysokomol. soyed. A17: No. 8, 1797-1801, 1975.
2070
T. YE. ~:~UDAKOVAet al.
of chemical etching, whereby roughness can be brought about without detriment to the article itseff or to its properties. The mechanism of the formation of surface roughness can be better understood if one starts with the macrokinetics of the etching process and the extent t o w h i c h t h e s u r f a c e r o u g h n e s s is i n f l u e n c e d b y t i m e a n d t e m p e r a t u r e , a s w e l l as by the alkali concentration and the structure of the polymer. The P E T P films used for the investigation were of thickness 180-500/~ and varied as t o degrees of crystallinity and orientation. Crystallization of the films was carried out at 130 ° for varying time intervals in a nitrogen atmosphere. Orientation of the films was brought about b y drawing a t the rate of 5 mm/min at 85 °. Degrees of crystallinity were determined from density values, using r a p i d titration [1] and X - r a y difffraction [2]; the I-Iormans-Wcidinger method [3] was used to subdivide reflections into amorphous and crystalline fractions. The mean surface roughness was determined in accordance with State S t a n d a r d (GOST) 27892-59 on the basis of the p a r a m e t e r R a (the mean arithmetic deviation of microirregularities from an average profile line) on a " K a l i b r V E I - 2 I " profilograph-profilometer. The measurement error for R a amounts to _+0.05/~. The disrupted t o t a l internal reflection (DTIR) spectra were obtained in cells prepared at the Chemical Physics Institute, ~.S.S.R. Academy of Sciences; the cells have a germanium prism and a reflection factor of 8. The kinetic investigations were calTied out in a glass cell placed in a thermostat (water or off). The temperature of the reaction mixture was maintained accurate to ±0-1% A 10 ml alkaline solution and P E T P films measuring 4 × 1 cm • were used in all the experiments. Prior to the start of an experiment the reaction mixture was heated to a predetermined temperature, after which the P E T P film was lowered into t h e alkaline solution. The results of calculation show t h a t the film was heated to the solution temperature in less t h a n 1 see. A t the stipulated time intervals the films were rapidly takell out of t h e cell and placed in a twofold volume of distilled water to completely dissolve t h e reaction product (terephthalic acid (TPA)) t h a t is found on the film surface. Solutions o f t h e T P A in alkali and in water were mixed and cooled to room temperature. The mass of the T P A was determined by spectrophotometry [4]. Aqueous alkaline solutions of predetermined concentrations were prepared b y dilution of a saturated solution prepared from K O H of chemically pure grade.
Macrokinetics of the etching process. F i g u r e 1 s h o w s t h e k i n e t i c c u r v e s o f TPA accumulation during the etching of a PETP film of thickness 500±20/z w i t h 4 0 % c r y s t a l l i n i t y . I n 2 0 % K O H s o l u t i o n t h e r e a c t i o n is z e r o - o r d e r f o r TPA all the way to 0-7-0.8 conversion; in 49~ KOH solution deviation from l i n e a r i t y o c c u r s a t a c o n v e r s i o n d e g r e e o f ~ 0.3. A b o v e t h e l a t t e r d e g r e e o f c o n version an appreciable layer of TPA forms on the surface (owing to the poor solubility of TPA in conc.KOH), and the rate of the etching process begins to be limited by hydroxyl ions penetrating the negatively charged layer of TPA. A l l t h e e x p e r i m e n t s w e r e c o n d u c t e d u p t o ~ 0 . 1 5 d e g r e e s o f c o n v e r s i o n , i.e. u n d e r conditions that did not involve any appreciable amount of TPA forming on the film surface. The absence of variation in the molecular weight of the PETP films o f t h i c k n e s s 5/~ [4], a n d t h e i n v a r i a b i l i t y o f t h e D T I R s p e c t r a i n t h e I R r e g i o n * • No b a n d at 3290 em -1 corresponding to O H - valency vibrations in a carboxyl group was detected b y us. During the etching process the quality of the spectra deteriorated as a result of surface roughness appearing on the films.
F o r m a t io n of rough surfaces on P E T P films
2071
during the etching process, as well as the zero order for TPA mean t h a t the etching process takes place in a thin surface layer, which evidently measures no more than several monolayers. rn. I0 ~, mole
2
0
0
8
12
T / r o e . 17~/f,
:Fro. 1. Kinetic curves of TPA accumulation during the etching of a PETP film at 108° in 22 (1) and 49% KOH solutions (2). The roughness of the P E T P films changes during the etching process (Figs. 2, 3). As a first approximation one could say t h a t the number of projections remains constant during the etching process, the only change being in the size of the projections. To determine change in the surface in contact with the alkali solution one m a y assume t h a t the projections are conical ones; the mean surface of the film in a time t will then be
8,= (o 4(RoI)2+ ,
(1)
where So is the initial film surface, ~ is the mean base radius of projections in the profilogram. On analysing the experimental results one finds t h a t the maximally changed value of St amounts to not more than ~ 1-1 S0. The effective rate of the etching process was calculated by the equation [5]
dm ke~= dt St'
(2)
where m is the amount of TPA released at a moment of time t. I t was shown in ref. [4] t h a t the hydrolysis (etching) of ester bonds in a P E T P surface layer takes place through a mechanism whereby the hydroxyl ion is added in equilibrium at the carbonyl group of the ester bond, and the reactivity of the ionized form of the ester bond exceeds t h a t of the unionized form. The limiting step is t h e interaction of the ionized form of the ester bond with a molecule of water. According to this mechanism ktruoarqo
kerr-- 1 + KeJb o'
(a)'
20/2
T. YE. RUDAKOVA et al.
where/¢true is the true rate of hydrolysis, Keq is the equilibrium constant for the process of hydroxyl ion addition to the ester bond, b0 is alkalinity and aH,o is the reactivity of water in the alkaline solutions. Provided that the degree of ionization is low, equation (3)may be reduced to ktrue Keq b°aH'°
heft •
(4)
Table 1 gives the effective rate constants obtained from the experiments results. The temperature dependences of the tabulated constants are satisfacto
FIG. 2
FIG. 3
I~IG. 2. Profilogram of P E T P film (degree of crystallinity 40%) etched in 49% K O H at 119 ° for 2 mira Vertical magnification of the profilogram 400, horizontal 200. ~ G . 3. Photomicrograph of P E T P film surface. For the conditions see Fig. 2 ( × 450).
rily described by the Arrhenius equation. The effective activation energies and the thermodynamic parameters for the films of varying crystallinity are given in Table 2. As can be seen from Fig. 4a log ke~ 2~ is a linear function of Be on COaH,O
ordinates of equation (4). In concentrated KOH solutions (>46%) the position of experimental values of log-/ce~25: is lower than it ought to be according to aH~O
equation (4). The departure from linearity could be due to marked ionization of ester bonds. In that case the experimental results should be described by equation (3) (Fig. 4b). Combining equations (2) and (3) we obtain a general expression describing TPA accumulation during etching relative to time and temperature and thermodynamic parameters b0 and a ~ o
2A exp (-- ~-~)aH,oS,t m-~
1 -~-]¢eq/b0
where A is the pre-exponential factor.
'
(5)
F o r m a ti o n of rough surfaces on P E T P films
2073
As can be seen from Table 2, the degree of crystallinity has practically no effect on kinetic parameters of the etching process. This is apparently because for processes occurring in the monomolecular surface layer, the accessibility and reactivity of the ester bonds are identical in amorphous and crystalline regions. Orientation likewise has no effect on kinetic parameters of the etching process.
log (kea/a.zo)25.
ka:~O, lO-r,rnin.cmZ/rzo/e
a
/
/
/
20 -y
/0
-70
i
l
I
Z
I
-Ba
I
/Q
o
I
1
ze
FIo. 4. Plots of log /¢~f t& ° vs. --Be (a) and-a~'--£° vs. 1/bo (b) for the etching of P E T P
kerr
aHto
films in K O H . L e t us n o w c o n s i d e r h o w v a r i o u s f a c t o r s i n f l u e n c e t h e f o r m a t i o n o f r o u g h s u r f a c e s . A s c a n b e s e e n f r o m F i g . 5, t h e r e is f i r st o f all a m a r k e d r i s e i n R , d u r ing the etching of a PETP a t 108 ° i n 49~o K O H ,
f i l m o f t h i c k n e s s 5 0 0 4 - 2 0 g w i t h 40~/o c r y s t a l l i n i t y
after which the value of Ra goes through a maximum,
a n d t h e n falls a n d r e m a i n s n e a r l y c o n s t a n t u p t o c o m p l e t e c o n v e r s i o n o f t h e f i l m . TABI~ 1. EFFECTIVE R&TE CONSTANTSFOR THE ETOHII~G OF P E T P FILMS I1~ K O H OF VA.RYI'lqG- COlqCENTRATIOlqAT DIFFEREI~VJ:TEM'PERATU'RES
T, °C
ken × 10L mole/rain, •cm s
[KOH], wt.%
8.20
66 78 88 93
0"45 1"0 1"7 3"1
33.6
48 58 66 78
0.85 1.8 2.8 7.6
14.3
66 78 88 93
0"95 2"3 4"2 6"2
39"0
27 37 48
0-39 1.4 2.2
58 66 78
0"95 2-0 3"9 7"8
100 108 119
4.0 × 102 6.0 × 10 s 9.1 X l0 s
[KOH], wt.%
22"0
88
46"3
bet t X 10L T, °C mole/rain, •cm s
bat X 103, T, °C mole/rain. . em a
[KOH], wt.%
49.0
I 53.0
50 80 I00 108 119
0'1 × 10 a 1"2 X 10 I 4"0 × l0 s 6-5 × l0 s 1"2× 10'
50 80 100 108 119
0"6 × 6-0 × 3.0× 5.6 × 1-0 x
10 I 10 s l0 s l0 s 10a
2074
T. YE. R17DAXOVA e¢ al.
I t was calculated that maximum roughness corresponds to an etched layer thickness of 25-30 p. Two effects come to light on analysing the experimental results: 1) for all the films under varying conditions /~a goes through a maxim u m relative to time, the maximum corresponding to an "etched" layer thickness of 25-30/1; 2) the maximum value of R a decreases with rising degree of crystaUinity, and is practically independent of orientation. Ra ,~m
1"0 R a ,/lrn
l'O
0.5
/ 0"5
T .c
t
0
q
I
I
I
[
8 lZ Time, rain
FIG. 5
T J.
0
[
I
2
1
q 6 m~lOa,mole
Fro. 6
FIG. 5. Plot of the roughness R a vs. etching time of PETP film in 49 % KOH. Fzo. 6. Plot of R a vs. amount of TPA received for PF~TP films with degree of crystallinity 0 (1), 30 (2), 40 (3) and 58% (4), etched in 49% KOH at 108°. No convincing explanation has yet been found to account for the appearance and mechanism of surface roughness. One of the explanations is that preferential etching of amorphous phase takes place, and that unevenness appears as a result of "exposed" crystaUites. However, this is at variance with the fact that the roughness of amorphous films exceeds that of crystalline ones. Moreover, X-ray structural analysis of the etched films showed that no development of crystalline structure is found in the polymer. I t could well be that the surface roughness appears as a result of the laying bare of microdefects during the etching process, i.e. defects appearing during the preparation of polymers from melts have been reported b y a number of authors [5, 6]. According to views currently held b y authors, the number of defects in the surface layers of films exceeds the amount that are present in the interior layers. As the degree of crystallinity rises, the degree of defectiveness of samples is reduced. This results in reduced surface roughness in the etching of films with a
Formation of rough surfaces on PETP films
2075
TABLE 2. EFFECTIVW.RATES OF ETCHINGOF PETP FILMSIN" K O H OF VARYING COI~CENTRATIOI~AT DIFFERENT TEMPERATURES [KOtt], wt.%
[--logke,t,5o J
8.2 14.3 22.0 33.6 39.0 46.3 49.0 53-0
9.94 9.59 9.29 8.74 8-54 8-41 8.52 8-61
--B*
--loga~,o
0.12 0"48 0.94 1"64 2.02 2.53 2.75 2.98
0.02 0.05 0-10 0.25 0-36 0-61 0.78 1"00
--log -ke't a m ' ° × 1 0 - ' 1 Ee,,4-0"5, a~2o ke~t bo × 103 kcal/molo 9"92 9"54 9"19 8"49 8"18 7"80 7"74 7"61
831 347 155 31 15 6"30 5"30 4"00
58 31 15 23.0 10 31 2.0 1.0
18.0 18.0 18.0 18.0 18"0 16'5 16.5 16"5
*Bo ffilog bo.
high degree of crystallinity. J u d g i n g f r o m our results, it is clear t h a t the n u m b e r o f microdefects is largest a t a d e p t h o f 25-30/~, w h i c h gives rise to m a x i m u m r o u g h n e s s in the etching o f a l a y e r o f this v e r y thickness. The degree of r o u g h n e s s is d e t e r m i n e d solely b y e t c h e d l a y e r thickness, the alkali c o n c e n t r a t i o n a n d t e m p e r a t u r e p l a y i n g o n l y a s e c o n d a r y role, a n d influencing solely the r a t e o f r o u g h surface f o r m a t i o n . Translated by R. J. A. ~_EI~DRY REFERENCES
1. Ye. S. KHOROSHAYA, Plast. massy, No. 10, 60, 1961 2 . L. L. RAZUMOVA, T. Ye. RUDAKOVA, Yu. V. MOYISEYEV, L. A. MEL'NIKOV and G. Ye. ZAIKOV, Vysokomol. soyed. AI7: 861, 1975 (Translated in Polymer Sci. U.S.S.R. 17: 4, 990, 1975) 3. P. H. HERMANS and A. WEIDINGER, Makromolek. Chem. 98: 50, 1961 4. T. Ye, RUDAKOVA, Yu. V. MOISEYEV, A. Ye. CHALYKH and G. Ye. ZAIKOV, Vysokomol, soyed. A14: 449, 1972 (Translated in Polymer Sci. U.S.S.R. 14: 5, 505, 1972) 5, M. SHEN and M. B. BEVER, J. Mater. Sci. 71: 741, 1972 6. N. P. MEL'TSEVA, V. V. STRYUKOV and E. G. ROZANTSEV, Dokl. AN SSSR, 203: 1313, 1972