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The mechanical degradation of polystyrene and polymethylmethacrylate in the presence of various acceptors
Mechanical degradation of polymers in presence of aceeptors
41
6. S. S. M E D V E D E V , Zh. prikl, khim. 8: 472, 1937; S. S. M E D V E D E V and S. K A M E N SKAYA, Zh. fiz. k h i m . 14: 922, 1940; S. S. M E D V E D E V , O. K O R I T S K A Y A and Ye. A L E K SEYEVA,Zh. fiz. k h i m . 17, 391, 1943; A. I. Y U R Z H E N K O , G. N. GROMOVA a n d V. B. K H A I T S E R , Zh. obshch, khim 1 6 : 1 5 0 5 1946; B. A. DOLGOPLOSK, Issledovanie v oblast i
polimerizatsii. (Research in the Field of Polymerization.) Trudy VNIISK, 1948
T H E MECHANICAL D E G R A D A T I O N OF P O L Y S T Y R E N E A N D P O L Y M E T H Y L M E T H A C R Y L A T E IN T H E P R E S E N C E OF V A R I O U S ACCEPTORS * N.
K.
BARAMBOIM
Moscow Light Industry Technological Institute (Received 7 Febr~ar.q 1961)
THE study of the mechanical degradation of polymers ill the presence of various low-molecular compounds t h a t can act as aceeptors of the free radicals formed (luring the process is one of the main trends being followed in this field [1]. Investigation of the effect of the nature of the aeceptors on the process of mechanical degradation of elastic polymers is not only of theoretical interest but could also enable a number of practical recommendations to be made (for "chemical ptasticizers", anti-fatigue agents, modifiers, etc.). At the same time although elastic and rigid polymers are of equivalent practical value the mechanical degradation of the latter has been studied to a considerably lesser extent. This suggests, t h a t a study of the mechanical degradation of rigid polymers, and in particular of the effect of the chemical nature of acoeptoi's on this process will produce results of both theoretical and practical value just as fruitfully. In this communication results are presented of an investigation of the mechanical degradation of two typical rigid polymers, polystyrene and polymethylmethacrylate, in the presence of various acceptors. The t)olystyrene used was a standard, technical product, obtained by emulsion polymerization without further purification. The polymethylmethacrylate was obtained by autopolymerization of the monomer in bulk over a period of several months. The aeceptors investigated were aromatic amino and hydroxy eompoun
42
N.K.
BARAMBOI~[
The acceptors were stirred into 5% solutions of polystyrene in benzene or of polymethylmethacrylate in acetone to give a concentration of 5~o on the polymer. From these solutions films were poured on glass and the solvent was completely evaporated at room temperature in a dark chamber with a forced draught. The resulting films were cut into pieces approximately 2 × 2 ram, and were subjected to mechanical degradation in this form. Degradation was carried out in standard, four-compartment, laboratory vibro-mills of VNIITISM design. The inner surface of the body of the mills and the surface of t~he mill balls were chromium plated. The charge was 2 g of film in each cell. Four specimens, each with a different aceeptor, were milled at the same time, thus providing identical milling conditions. Before an experiment the compartments were chilled with liquid nitrogen. As a result of the high energy of vibro-milling the temperature of the ceils rose to -~ 10 ° in 30 seconds. Consequently during the milling process the temperature varied from --170 ° to ~-10 °. When a longer milling time was necessary the chilling was repeated every 30 seconds. For deciding the length of the milling period the following considerations were taken into account. In the overwhelming majority of cases known at present long processing times, up to some tens or even hundreds of hours, have been used. Long processes are advantageous in comparison with shorter periods with respect to the extent of the changes occurring in the polymer as a result of mechanical degradation. At the same time a number of negative factors are involved in long processing times. In addition to the accumulation of impurities caused by wearing of the grinding surfaces there is a continued increase in the changes brought about b y side reactions of the free-radical type. These changes complicate the assessment of the mechanism of the basic process from the properties of the end products. Consequently to facilitate the comparison of the effect of the chemical nature of the acceptors it seemed advantageous to use short periods and we chose a milling time of 120 seconds, chilling four times during the process. Since a comparative study was intended the properties of the degradation products were assessed b y only a limited number of characteristics, namely by the intrinsic viscosity, the concentration dependence of the viscosity and by turbidimetric titration. In all cases parallel tests were carried out on solutions of the original films and of the degradation products under identical conditions. The results are shown in Figures 1 and 2 and in the Table. Polystyrene. When the properties of the initial mixtures and of the products of mechanical degradation of polystyrene in the presence of the different acceptors are compared (Table, Fig. 1) it is evident t h a t all the compounds chosen function as acceptors. I t is important to note t h a t all these acceptors, of different chemical nature, have a substantial effect on the rheological properties of benzene solutions of polystyrene. This is not at all the same as an additive summation of the viscosities of the acceptors and the polymer.
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0
2 # V o l u m e of "precipitant
0.375 0.75
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'2
FIG. 1. Mechanical d e g r a d a t i o n o f p o l y s t y r e n e in the presence of v a r i o u s acceptors: l initial p o l y m e r - a e e e p t o r m i x t u r e ; 2 - - d e g r a d a t i o n poduets. S o l v e n t - - b e n z e n e ; precip i t a n t - - 5 0 ° / o ethanol. A - - t u r b i d i m e t r i e t i t r a t i o n curves; iT--q~>/e f(c): 1 - - a t m o s p h e r i c oxygen; 2 diphenylamine; 3--p-aminophenol; 4--p-nitrophenol; 5--pentachlorophenol: 6 - - h y d r o q u i n o n e ; 7-- a - n a p h t h o l ; 8-- e - n a p h t h y l a m i n e .
N. K. BARAMBOIM
44
C t I A N G E I N VISCOSITY OF P O L Y M E R SOLUTIONS A F T E R D E G R A D A T I O N
Acceptor Polystyrene Atmospheric oxygen Diphenylamine p-Aminophenol p-nitrophenol Pentachlorophenol Hydroquinone -Naphthol a-Naphthylamine
Polymethylmethacrylate
Polystyrene
Polymethylmethacrylate
5"15 : 1"52--3"39 5.31 : 1"38=3"84 6"25 : 1"36=4.6 3"98 : 1"92=2"08 4"25 : 1"68=2"53 9"0 : I'76=5"1 3-12:1"3:2"4 5"35 : 1"26= 4"25
2.12:0.8 =2.5 2.28 : 0-82~2.78 2.15 : 0.85~2.94 1.94 : 0.92=2.11 1.94 : 0.77:2.52 2.86 : 0"75--3.8 1.42 : 0 " 5 : 2 . 8 4 2.03 : 0-73--2.78
0.87 : 0.35:2.5 0.82 : 0.32=2.56 0.96 : 0.32=3-0 0.73 : 0.43:1.7 0.77 : 0.38=2.08 1.24:0.4:3.1 0.62 : 0.59:2.14 0.88 : 0.30-2-94
I t is seen f r o m Fig. 1 a n d the T a b l e t h a t D P h A , p - A P h a n d H Q increase t h e v i s c o s i t y a n d c o n v e r s e l y t h e others lower it. This effect m a y be due to changes in t h e c o n f o r m a t i o n of t h e p o l y m e r m a c r o m o l e c u l e s in solution b r o u g h t a b o u t b y t h e acceptor, to t h e f o r m a t i o n b y t h e a c c e p t o r of b o n d s b e t w e e n t h e chains, to a change in t h e degree of association a n d c o n s e q u e n t l y in t h e s o l v a t i n g properties o f t h e s o l v e n t u n d e r t h e influence of t h e acceptor, or finally to s o l v a t i o n o f t h e p o l y m e r chains b y t h e a c c e p t o r . Since in this case we are dealing w i t h a solution of a n o n p o l a r p o l y m e r in a n o n p o l a r solvent, where s o l v a t i o n does n o t p l a y a decisive role in t h e m e c h a n i s m of solution a n d in t h e d e v e l o p m e n t of t h e rheological properties, t h e t h i r d a n d f o u r t h possibilities are less p r o b a b l e . T h e first possibility is closely associated w i t h t h e t h i r d a n d f o u r t h a n d is c o n s e q u e n t l y also i m p r o b a b l e . I t would be t h o u g h t t h a t t h e f o r m a t i o n o f a d d i t i o n a l b o n d s could e x p l a i n a n increase in viscosity b u t a n e x t e n s i o n of t h e e x p l a n a t i o n to all a c c e p t o r s is c o n t r a d i c t e d b y t h e decrease in v i s c o s i t y caused b y some of t h e a c c e p t o r s ( p - N P h , P C P h a n d N P ) since this w o u l d m e a n t h a t t h e f o r m a t i o n o f such b o n d s is o p p o s e d b y d e g r a d a t i o n in solution a t a c o m p a r a t i v e l y low t e m p e r a t u r e in t h e absence o f a c t i v e deg r a d a t i v e influences, w h i c h is i m p r o b a b l e . C o n s e q u e n t l y it is n o t possible to s t a t e w i t h o u t f u r t h e r i n v e s t i g a t i o n a general cause of t h e change in viscosity of p o l y s t y r e n e solutions w h e n t h e a c c e p t o r s are added. H o w e v e r , it is possible to suggest some p a r t i a l reasons for t h e viscosity variation. T h e solutions o f t h e original p o l y m e r , w i t h a n d w i t h o u t acceptors, were o b t a i n e d b y dissolving films f o r m e d b y e v a p o r a t i o n of a s o l v e n t f r o m p o l y m e r solutions a t r o o m t e m p e r a t u r e in t h e p r e s e n c e of air. O x i d a t i o n could occur during t h e film-forming process, giving rise to t h e f o r m a t i o n of linkages b e t w e e n t h e chains. O n l y a few of these b o n d s m a y be f o r m e d a n d t h e y could b e fairly
Mechanical degradation of polymers in presence of acceptors
45
labile b u t they could bring about an appreciable increase in the viscosity of the system when the film is subsequently dissolved. This is supported b y the fact that an increase in viscosity is brought about by those acceptors with the greatest tendency to oxidation, namely DPhA, p-APh and HQ, and those that are stable to oxidation, p-NPh, PCPh and NP, conversely lower the viscosity. The possibility of bond formation is also suggested b y a cbmparison of the rheological curves of solutions of the original polystyrene and of those containing acceptors (Fig. 1, B1, B2, B3 and B4) where the marked increase in slope of the curves of rls,/C against c reflects the degree of a s y m m e t r y of the particles. The comparative weakness of the bonds formed can be judged b y the rheological curves relating to the degradation products (curves 2 in the same figure). The decrease in slope of the curves after degradation probably results not only from degradation of the polymer itself (B1) but also from destruction of all the bonds formed with the acceptor in the original film. If the degree of degradation, expressed conventionally b y the fall in specific viscosity of 0.5% solutions of the polymer and acceptor in benzene (table) is taken as an indication of the efficiency of the acceptor, the acceptors studied can be arranged in order of efficiency as follows: hydroquinone >p-aminophenol > > a-naphthol > diphenylamine > a-naphthylamine > pentachlorophenol > oxygen > >p-nitrophenol. Thus the best acceptors are hydroquinone and p-aminophenol and the poorest are p-nitrophenol and pentachlorophenol. We shall discuss the special case of the position of oxygen in this series later, after consideration of the mechanical degradation of polymethylmethacrylate. From the point of view of chemical structure the general arrangement ()f the acceptors in this series is quite in order if one bears in mind the energy of bonds with easily substituting atoms when a substituent is present on the benzene ring. The most active acceptors are those with substituents of the first type. that promote further substitution, i.e. that lower the energy of the C - - H bonds in the nucleus of the acceptor. The poorest acceptors are those with substituents of the second type (NO 2 or halogen derivaties) that hinder further substitution. i.e. that increase the C - - H bond energy in the nucleus. It is naturally impossible to suppose that this relationship is general to all cases, for any conditions of mechanical degradation and for all polymers, because this does not take into account the specific activity of different macroradicals toward correspondingly different atoms or groups of atoms split off in the accepting process. For example pentachlorophenol is an efficient acceptor in the mechanical degradation (plasticization) of rubbers, where it is used under the name of "Renatsit V". Comparison of the turbidimetric
titration curves (Fig. I, A I-AS)
shows, as
would be expected, that these acce])tors promote the formation of products of lower molecular weight, and consequently products that are less easily affected b y the precipitant. The highly dispersed degradation products not only begin t<) precipitate at higher precipitant concentrations but also form more stable dis-
46
N.K.
BAI~A)IBOIM
persions of higher optical density, and consequently in most cases the upper sections of the turbidimetric curves of the degradation products pass above those of the original mixture. Further discussion of these results can be continued to the best advantages in the form of a comparison after the corresponding results for polymethyl methacrylate have been examined. Polymethylmethacrylate. The action of the above acceptors on the mechanical degradation of polymethylmethacrylate (Fig. 2) is in m a n y ways analogous to that discussed above for polystyrene. In spite of the difference in the nature of the polymers (polystyrene and polymethylmethacrylate) and the solvents (benzene and acetone), and consequently in spite of the very large effect of solvation on the properties of a solution of polymethylmethacrylate in acetone, the effect of the acceptors on the rheological properties is almost completely the same in this case. The fact that the effect of acceptors on the rheological properties of the solutions is common to polymers differing essentially in chemical nature makes it difficult to choose a single, probable explanation, and it requires independent investigation. The change in slope of the rheological curves (Fig. 2, B1-BS) can also be explained b y the formation of labile bonds as a result of oxidation during the formation of the film. If the same criterion is used as for polystyrene (Table) the acceptors can be arranged in the following descending order of efficiency in mechanical degradation: hydroquinone > p-aminophenol > a-naphthylamine > diphenylamine > oxygen > pentachlorophenol > ~-naphthol > p-nitrophenol. Although in general the order of the acceptors in this series is the same as that obtained with polystyrene, and can be explained b y the same reasoning, attention is drawn to the change in position of ~-naphthol and oxygen in the series. In the ease of polystyrene oxygen was less effective and ~-naphthol was more effective. This can be explained b y the specific reactivity of polystyrene macro-radicals toward oxygen. Further it is known that polystyrene readily reacts with oxygen to form relatively stable, polymeric peroxide radicals CH 2-CH-~0 2-+~CH
2CH--O--()
©
©
This comparatively stable and consequently "long-lived" peroxide radical can combine with another polymer radical and thus "knit together" again breaks in the degraded chains b y the formation of a fairly stable bond, which in this case is a peroxide linkage C H 2( ~ H
(') - - 0 - - C H
CH~ ~
~
C H ~ -- C H -- 0 -- 0 -- C H -- C H ~
Mechanical degradation of polymers in presence of aeeeptors
47
This naturally reduced the degree of degradation and oxygen is the last trot one in the above decreasing series of efficieneies of the aeeeptors with respect to polystyrene. Polymethylmethacrylate shows no tendency to form such comparatively stable radicals and oxygen is a more efficient aeceptor in this case. Secondly, if it is assumed t h a t the polystyrene peroxide radical is just as specifically reactive toward e-naphthol as are the polymeric peroxide radicals <>frubbers [2], then the strong effect of e-naphthol in the ease of polystyrene is also understandable. I t should be borne in mind that the hydrogen atom of the hydroxyl group of e-naphthol can be abstracted particularly easily by peroxide radicals [2]. Consequently in the ease of polystyrene "ts peroxide radical becomes stabilized by reaction with a-naphthol in the following way: ,~CH2--CH--O--(')
©
HO Z-i
~CH 2 (!H ()--OH --)*
(') i
The naphthoxyl radical stabilizes another rupture of the polystyrene chain. For polymethylmethacrylate, which does not form peroxide radicals, ~.-naphthol is a less effective aeceptor. Finally, the most important difference in the behaviour of polymethylmethaerylate and polystyrene on mechanical degradation in the presence of these aeeeptors is disclosed by comparing the turbidimetric titration curves (see Figures 1 and 2). The polystyrene degradation products in all eases, in addition to a fall in viscosity, show a corresponding increase in resistance to the action of the precipitant as a result of linear degradation. In the case of polymethylmethaerylate the fall in viscosity as a result of degradation is accompanied by a relationship to the precipitant t h a t is opposite to t h a t tbr polystyrene. The resistance of the degradation products to precipitation falls considerably in all eases except t h a t of ~-naphthol. It is improbable t h a t this effect can be explained by linear degradation. The assumption t h a t branched and erosslinked fragments are formed t h a t are fairly symmetrical (reduction in viscosity) and less resistant to the precipitant is more probable. Consequently in contrast to polystyrene in this ease during mechanical degradation chain transfer occurs, in which the added aromatic compounds also take part. I f the milling were continued for a longer period further breakdown of the initially comparatively large branched and erosslinked fragments would lead not only to a further reduction in viscosity but also to an increase in resistance to precipitation, i.e. to the smoothing out and finally to the disappearance of the differences in properties of the products of mechanical degradation of polystyrene and polymethylmethacrylate, which could be brought to light by theological and turbidimetric methods. Thus our original idea t h a t short milling times would be highly advantageous in an examination of the details of the mechanism of the process is confirmed.
48
v.G. ALDOSHINand S. YA. FRENKEL' CONCLUSIONS
(l) A study has been made of the mechanical degradation of polystyrene and polymethylmethacrylate containing the following acceptors: diphenylamine, ~-naphthylamine, a-naphthol, hydroquinone, p-aminophenol, p-nitrophenol and pentachlorophenol. Degradation was carried out for 120 seconds in vibro-mills cooled with liquid nitrogen. (2) I t was found t h a t in the presence of all these acceptors polystyrene forms linear degradation products and t h a t the aeceptors can be arranged in a definite order of efficiency. In contrast to polystyrene, polymethylmethacrylate tends to form non-linear products and the order of the acceptors changes to some extent in conformity with their reactivity toward peroxide radicals. Translated by E. O. PHILLIPS REFERENCES
1. N. K. BARAMBOIM, Uspekhi khimii 28: 877, 1959 2. M. PICKE and W. F. WATSON, J. Polymer Sci. 9: 229, 1952
SELECTIVE INTERACTIONS IN POLYMER CHAINS---I. THE HYDRODYNAMIC PROPERTIES AND SOLUBILITY OF A 9 : 1 COPOLYMER OF METHYL METHACRYLATE AND METHACRYLIC ACID * V. G. ALDOSHIN and S. YA. FRENKEL' Institute of Macromolecular Compounds, U.S.S.R. Academy of Sciences (Received 10 February 1961)
IN THE preparation of glass-like polymers containing amide or carboxyl side groups, possible complications arising during the polymerization process and subsequent treatment as a result of intra- and intermolecular interactions must be taken into account. These interactions affect the conformation and solubility of the macromolecules over wider limits than in the case of ordinary polymers which vary in properties uniformly with progressive changes in temperature and solvent composition. In particular, with selective variation of the solvent macromolecules with - - C 0 0 H and --CONH 2 groups, as a result of cleavage and re-formation of hydrogen bonds or the formation of ionized groups, can be converted from an ordinary polymer to a polyelectrolyte or from a "vulcanized" * Vysokomol. soyed. 4: No. 1, 116-123, 1962.
41
6. S. S. M E D V E D E V , Zh. prikl, khim. 8: 472, 1937; S. S. M E D V E D E V and S. K A M E N SKAYA, Zh. fiz. k h i m . 14: 922, 1940; S. S. M E D V E D E V , O. K O R I T S K A Y A and Ye. A L E K SEYEVA,Zh. fiz. k h i m . 17, 391, 1943; A. I. Y U R Z H E N K O , G. N. GROMOVA a n d V. B. K H A I T S E R , Zh. obshch, khim 1 6 : 1 5 0 5 1946; B. A. DOLGOPLOSK, Issledovanie v oblast i
polimerizatsii. (Research in the Field of Polymerization.) Trudy VNIISK, 1948
T H E MECHANICAL D E G R A D A T I O N OF P O L Y S T Y R E N E A N D P O L Y M E T H Y L M E T H A C R Y L A T E IN T H E P R E S E N C E OF V A R I O U S ACCEPTORS * N.
K.
BARAMBOIM
Moscow Light Industry Technological Institute (Received 7 Febr~ar.q 1961)
THE study of the mechanical degradation of polymers ill the presence of various low-molecular compounds t h a t can act as aceeptors of the free radicals formed (luring the process is one of the main trends being followed in this field [1]. Investigation of the effect of the nature of the aeceptors on the process of mechanical degradation of elastic polymers is not only of theoretical interest but could also enable a number of practical recommendations to be made (for "chemical ptasticizers", anti-fatigue agents, modifiers, etc.). At the same time although elastic and rigid polymers are of equivalent practical value the mechanical degradation of the latter has been studied to a considerably lesser extent. This suggests, t h a t a study of the mechanical degradation of rigid polymers, and in particular of the effect of the chemical nature of acoeptoi's on this process will produce results of both theoretical and practical value just as fruitfully. In this communication results are presented of an investigation of the mechanical degradation of two typical rigid polymers, polystyrene and polymethylmethacrylate, in the presence of various acceptors. The t)olystyrene used was a standard, technical product, obtained by emulsion polymerization without further purification. The polymethylmethacrylate was obtained by autopolymerization of the monomer in bulk over a period of several months. The aeceptors investigated were aromatic amino and hydroxy eompoun
42
N.K.
BARAMBOI~[
The acceptors were stirred into 5% solutions of polystyrene in benzene or of polymethylmethacrylate in acetone to give a concentration of 5~o on the polymer. From these solutions films were poured on glass and the solvent was completely evaporated at room temperature in a dark chamber with a forced draught. The resulting films were cut into pieces approximately 2 × 2 ram, and were subjected to mechanical degradation in this form. Degradation was carried out in standard, four-compartment, laboratory vibro-mills of VNIITISM design. The inner surface of the body of the mills and the surface of t~he mill balls were chromium plated. The charge was 2 g of film in each cell. Four specimens, each with a different aceeptor, were milled at the same time, thus providing identical milling conditions. Before an experiment the compartments were chilled with liquid nitrogen. As a result of the high energy of vibro-milling the temperature of the ceils rose to -~ 10 ° in 30 seconds. Consequently during the milling process the temperature varied from --170 ° to ~-10 °. When a longer milling time was necessary the chilling was repeated every 30 seconds. For deciding the length of the milling period the following considerations were taken into account. In the overwhelming majority of cases known at present long processing times, up to some tens or even hundreds of hours, have been used. Long processes are advantageous in comparison with shorter periods with respect to the extent of the changes occurring in the polymer as a result of mechanical degradation. At the same time a number of negative factors are involved in long processing times. In addition to the accumulation of impurities caused by wearing of the grinding surfaces there is a continued increase in the changes brought about b y side reactions of the free-radical type. These changes complicate the assessment of the mechanism of the basic process from the properties of the end products. Consequently to facilitate the comparison of the effect of the chemical nature of the acceptors it seemed advantageous to use short periods and we chose a milling time of 120 seconds, chilling four times during the process. Since a comparative study was intended the properties of the degradation products were assessed b y only a limited number of characteristics, namely by the intrinsic viscosity, the concentration dependence of the viscosity and by turbidimetric titration. In all cases parallel tests were carried out on solutions of the original films and of the degradation products under identical conditions. The results are shown in Figures 1 and 2 and in the Table. Polystyrene. When the properties of the initial mixtures and of the products of mechanical degradation of polystyrene in the presence of the different acceptors are compared (Table, Fig. 1) it is evident t h a t all the compounds chosen function as acceptors. I t is important to note t h a t all these acceptors, of different chemical nature, have a substantial effect on the rheological properties of benzene solutions of polystyrene. This is not at all the same as an additive summation of the viscosities of the acceptors and the polymer.
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g.Z5
0
6L Bs aO I
0.5
c (g/lO0 g)
0
2 # V o l u m e of "precipitant
0.375 0.75
1.5
c (g/100 g)
(m11 Vm.
'2
FIG. 1. Mechanical d e g r a d a t i o n o f p o l y s t y r e n e in the presence of v a r i o u s acceptors: l initial p o l y m e r - a e e e p t o r m i x t u r e ; 2 - - d e g r a d a t i o n poduets. S o l v e n t - - b e n z e n e ; precip i t a n t - - 5 0 ° / o ethanol. A - - t u r b i d i m e t r i e t i t r a t i o n curves; iT--q~>/e f(c): 1 - - a t m o s p h e r i c oxygen; 2 diphenylamine; 3--p-aminophenol; 4--p-nitrophenol; 5--pentachlorophenol: 6 - - h y d r o q u i n o n e ; 7-- a - n a p h t h o l ; 8-- e - n a p h t h y l a m i n e .
N. K. BARAMBOIM
44
C t I A N G E I N VISCOSITY OF P O L Y M E R SOLUTIONS A F T E R D E G R A D A T I O N
Acceptor Polystyrene Atmospheric oxygen Diphenylamine p-Aminophenol p-nitrophenol Pentachlorophenol Hydroquinone -Naphthol a-Naphthylamine
Polymethylmethacrylate
Polystyrene
Polymethylmethacrylate
5"15 : 1"52--3"39 5.31 : 1"38=3"84 6"25 : 1"36=4.6 3"98 : 1"92=2"08 4"25 : 1"68=2"53 9"0 : I'76=5"1 3-12:1"3:2"4 5"35 : 1"26= 4"25
2.12:0.8 =2.5 2.28 : 0-82~2.78 2.15 : 0.85~2.94 1.94 : 0.92=2.11 1.94 : 0.77:2.52 2.86 : 0"75--3.8 1.42 : 0 " 5 : 2 . 8 4 2.03 : 0-73--2.78
0.87 : 0.35:2.5 0.82 : 0.32=2.56 0.96 : 0.32=3-0 0.73 : 0.43:1.7 0.77 : 0.38=2.08 1.24:0.4:3.1 0.62 : 0.59:2.14 0.88 : 0.30-2-94
I t is seen f r o m Fig. 1 a n d the T a b l e t h a t D P h A , p - A P h a n d H Q increase t h e v i s c o s i t y a n d c o n v e r s e l y t h e others lower it. This effect m a y be due to changes in t h e c o n f o r m a t i o n of t h e p o l y m e r m a c r o m o l e c u l e s in solution b r o u g h t a b o u t b y t h e acceptor, to t h e f o r m a t i o n b y t h e a c c e p t o r of b o n d s b e t w e e n t h e chains, to a change in t h e degree of association a n d c o n s e q u e n t l y in t h e s o l v a t i n g properties o f t h e s o l v e n t u n d e r t h e influence of t h e acceptor, or finally to s o l v a t i o n o f t h e p o l y m e r chains b y t h e a c c e p t o r . Since in this case we are dealing w i t h a solution of a n o n p o l a r p o l y m e r in a n o n p o l a r solvent, where s o l v a t i o n does n o t p l a y a decisive role in t h e m e c h a n i s m of solution a n d in t h e d e v e l o p m e n t of t h e rheological properties, t h e t h i r d a n d f o u r t h possibilities are less p r o b a b l e . T h e first possibility is closely associated w i t h t h e t h i r d a n d f o u r t h a n d is c o n s e q u e n t l y also i m p r o b a b l e . I t would be t h o u g h t t h a t t h e f o r m a t i o n o f a d d i t i o n a l b o n d s could e x p l a i n a n increase in viscosity b u t a n e x t e n s i o n of t h e e x p l a n a t i o n to all a c c e p t o r s is c o n t r a d i c t e d b y t h e decrease in v i s c o s i t y caused b y some of t h e a c c e p t o r s ( p - N P h , P C P h a n d N P ) since this w o u l d m e a n t h a t t h e f o r m a t i o n o f such b o n d s is o p p o s e d b y d e g r a d a t i o n in solution a t a c o m p a r a t i v e l y low t e m p e r a t u r e in t h e absence o f a c t i v e deg r a d a t i v e influences, w h i c h is i m p r o b a b l e . C o n s e q u e n t l y it is n o t possible to s t a t e w i t h o u t f u r t h e r i n v e s t i g a t i o n a general cause of t h e change in viscosity of p o l y s t y r e n e solutions w h e n t h e a c c e p t o r s are added. H o w e v e r , it is possible to suggest some p a r t i a l reasons for t h e viscosity variation. T h e solutions o f t h e original p o l y m e r , w i t h a n d w i t h o u t acceptors, were o b t a i n e d b y dissolving films f o r m e d b y e v a p o r a t i o n of a s o l v e n t f r o m p o l y m e r solutions a t r o o m t e m p e r a t u r e in t h e p r e s e n c e of air. O x i d a t i o n could occur during t h e film-forming process, giving rise to t h e f o r m a t i o n of linkages b e t w e e n t h e chains. O n l y a few of these b o n d s m a y be f o r m e d a n d t h e y could b e fairly
Mechanical degradation of polymers in presence of acceptors
45
labile b u t they could bring about an appreciable increase in the viscosity of the system when the film is subsequently dissolved. This is supported b y the fact that an increase in viscosity is brought about by those acceptors with the greatest tendency to oxidation, namely DPhA, p-APh and HQ, and those that are stable to oxidation, p-NPh, PCPh and NP, conversely lower the viscosity. The possibility of bond formation is also suggested b y a cbmparison of the rheological curves of solutions of the original polystyrene and of those containing acceptors (Fig. 1, B1, B2, B3 and B4) where the marked increase in slope of the curves of rls,/C against c reflects the degree of a s y m m e t r y of the particles. The comparative weakness of the bonds formed can be judged b y the rheological curves relating to the degradation products (curves 2 in the same figure). The decrease in slope of the curves after degradation probably results not only from degradation of the polymer itself (B1) but also from destruction of all the bonds formed with the acceptor in the original film. If the degree of degradation, expressed conventionally b y the fall in specific viscosity of 0.5% solutions of the polymer and acceptor in benzene (table) is taken as an indication of the efficiency of the acceptor, the acceptors studied can be arranged in order of efficiency as follows: hydroquinone >p-aminophenol > > a-naphthol > diphenylamine > a-naphthylamine > pentachlorophenol > oxygen > >p-nitrophenol. Thus the best acceptors are hydroquinone and p-aminophenol and the poorest are p-nitrophenol and pentachlorophenol. We shall discuss the special case of the position of oxygen in this series later, after consideration of the mechanical degradation of polymethylmethacrylate. From the point of view of chemical structure the general arrangement ()f the acceptors in this series is quite in order if one bears in mind the energy of bonds with easily substituting atoms when a substituent is present on the benzene ring. The most active acceptors are those with substituents of the first type. that promote further substitution, i.e. that lower the energy of the C - - H bonds in the nucleus of the acceptor. The poorest acceptors are those with substituents of the second type (NO 2 or halogen derivaties) that hinder further substitution. i.e. that increase the C - - H bond energy in the nucleus. It is naturally impossible to suppose that this relationship is general to all cases, for any conditions of mechanical degradation and for all polymers, because this does not take into account the specific activity of different macroradicals toward correspondingly different atoms or groups of atoms split off in the accepting process. For example pentachlorophenol is an efficient acceptor in the mechanical degradation (plasticization) of rubbers, where it is used under the name of "Renatsit V". Comparison of the turbidimetric
titration curves (Fig. I, A I-AS)
shows, as
would be expected, that these acce])tors promote the formation of products of lower molecular weight, and consequently products that are less easily affected b y the precipitant. The highly dispersed degradation products not only begin t<) precipitate at higher precipitant concentrations but also form more stable dis-
46
N.K.
BAI~A)IBOIM
persions of higher optical density, and consequently in most cases the upper sections of the turbidimetric curves of the degradation products pass above those of the original mixture. Further discussion of these results can be continued to the best advantages in the form of a comparison after the corresponding results for polymethyl methacrylate have been examined. Polymethylmethacrylate. The action of the above acceptors on the mechanical degradation of polymethylmethacrylate (Fig. 2) is in m a n y ways analogous to that discussed above for polystyrene. In spite of the difference in the nature of the polymers (polystyrene and polymethylmethacrylate) and the solvents (benzene and acetone), and consequently in spite of the very large effect of solvation on the properties of a solution of polymethylmethacrylate in acetone, the effect of the acceptors on the rheological properties is almost completely the same in this case. The fact that the effect of acceptors on the rheological properties of the solutions is common to polymers differing essentially in chemical nature makes it difficult to choose a single, probable explanation, and it requires independent investigation. The change in slope of the rheological curves (Fig. 2, B1-BS) can also be explained b y the formation of labile bonds as a result of oxidation during the formation of the film. If the same criterion is used as for polystyrene (Table) the acceptors can be arranged in the following descending order of efficiency in mechanical degradation: hydroquinone > p-aminophenol > a-naphthylamine > diphenylamine > oxygen > pentachlorophenol > ~-naphthol > p-nitrophenol. Although in general the order of the acceptors in this series is the same as that obtained with polystyrene, and can be explained b y the same reasoning, attention is drawn to the change in position of ~-naphthol and oxygen in the series. In the ease of polystyrene oxygen was less effective and ~-naphthol was more effective. This can be explained b y the specific reactivity of polystyrene macro-radicals toward oxygen. Further it is known that polystyrene readily reacts with oxygen to form relatively stable, polymeric peroxide radicals CH 2-CH-~0 2-+~CH
2CH--O--()
©
©
This comparatively stable and consequently "long-lived" peroxide radical can combine with another polymer radical and thus "knit together" again breaks in the degraded chains b y the formation of a fairly stable bond, which in this case is a peroxide linkage C H 2( ~ H
(') - - 0 - - C H
CH~ ~
~
C H ~ -- C H -- 0 -- 0 -- C H -- C H ~
Mechanical degradation of polymers in presence of aeeeptors
47
This naturally reduced the degree of degradation and oxygen is the last trot one in the above decreasing series of efficieneies of the aeeeptors with respect to polystyrene. Polymethylmethacrylate shows no tendency to form such comparatively stable radicals and oxygen is a more efficient aeceptor in this case. Secondly, if it is assumed t h a t the polystyrene peroxide radical is just as specifically reactive toward e-naphthol as are the polymeric peroxide radicals <>frubbers [2], then the strong effect of e-naphthol in the ease of polystyrene is also understandable. I t should be borne in mind that the hydrogen atom of the hydroxyl group of e-naphthol can be abstracted particularly easily by peroxide radicals [2]. Consequently in the ease of polystyrene "ts peroxide radical becomes stabilized by reaction with a-naphthol in the following way: ,~CH2--CH--O--(')
©
HO Z-i
~CH 2 (!H ()--OH --)*
(') i
The naphthoxyl radical stabilizes another rupture of the polystyrene chain. For polymethylmethacrylate, which does not form peroxide radicals, ~.-naphthol is a less effective aeceptor. Finally, the most important difference in the behaviour of polymethylmethaerylate and polystyrene on mechanical degradation in the presence of these aeeeptors is disclosed by comparing the turbidimetric titration curves (see Figures 1 and 2). The polystyrene degradation products in all eases, in addition to a fall in viscosity, show a corresponding increase in resistance to the action of the precipitant as a result of linear degradation. In the case of polymethylmethaerylate the fall in viscosity as a result of degradation is accompanied by a relationship to the precipitant t h a t is opposite to t h a t tbr polystyrene. The resistance of the degradation products to precipitation falls considerably in all eases except t h a t of ~-naphthol. It is improbable t h a t this effect can be explained by linear degradation. The assumption t h a t branched and erosslinked fragments are formed t h a t are fairly symmetrical (reduction in viscosity) and less resistant to the precipitant is more probable. Consequently in contrast to polystyrene in this ease during mechanical degradation chain transfer occurs, in which the added aromatic compounds also take part. I f the milling were continued for a longer period further breakdown of the initially comparatively large branched and erosslinked fragments would lead not only to a further reduction in viscosity but also to an increase in resistance to precipitation, i.e. to the smoothing out and finally to the disappearance of the differences in properties of the products of mechanical degradation of polystyrene and polymethylmethacrylate, which could be brought to light by theological and turbidimetric methods. Thus our original idea t h a t short milling times would be highly advantageous in an examination of the details of the mechanism of the process is confirmed.
48
v.G. ALDOSHINand S. YA. FRENKEL' CONCLUSIONS
(l) A study has been made of the mechanical degradation of polystyrene and polymethylmethacrylate containing the following acceptors: diphenylamine, ~-naphthylamine, a-naphthol, hydroquinone, p-aminophenol, p-nitrophenol and pentachlorophenol. Degradation was carried out for 120 seconds in vibro-mills cooled with liquid nitrogen. (2) I t was found t h a t in the presence of all these acceptors polystyrene forms linear degradation products and t h a t the aeceptors can be arranged in a definite order of efficiency. In contrast to polystyrene, polymethylmethacrylate tends to form non-linear products and the order of the acceptors changes to some extent in conformity with their reactivity toward peroxide radicals. Translated by E. O. PHILLIPS REFERENCES
1. N. K. BARAMBOIM, Uspekhi khimii 28: 877, 1959 2. M. PICKE and W. F. WATSON, J. Polymer Sci. 9: 229, 1952
SELECTIVE INTERACTIONS IN POLYMER CHAINS---I. THE HYDRODYNAMIC PROPERTIES AND SOLUBILITY OF A 9 : 1 COPOLYMER OF METHYL METHACRYLATE AND METHACRYLIC ACID * V. G. ALDOSHIN and S. YA. FRENKEL' Institute of Macromolecular Compounds, U.S.S.R. Academy of Sciences (Received 10 February 1961)
IN THE preparation of glass-like polymers containing amide or carboxyl side groups, possible complications arising during the polymerization process and subsequent treatment as a result of intra- and intermolecular interactions must be taken into account. These interactions affect the conformation and solubility of the macromolecules over wider limits than in the case of ordinary polymers which vary in properties uniformly with progressive changes in temperature and solvent composition. In particular, with selective variation of the solvent macromolecules with - - C 0 0 H and --CONH 2 groups, as a result of cleavage and re-formation of hydrogen bonds or the formation of ionized groups, can be converted from an ordinary polymer to a polyelectrolyte or from a "vulcanized" * Vysokomol. soyed. 4: No. 1, 116-123, 1962.