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A study of the mechanical degradation of polymethylmethacrylate and polystyrene by following molecular weight changes
A STUDY OF THE MECHANICAL DEGRADATION OF POLYMETHYLMETHACRYLATE AND POLYSTYRENE BY FOLLOWING MOLECULAR WEIGHT CHANGES* M. P. VERSHININA Institute of Macromolecular
and E. V. KUVSHINSKII Compounds,
U.S.S.R. Academy
of Sciences
(Received 21 April 1960) A REDUCTION in the molecular weight of the polymer occurs when polymeric materials are subjected to mecha~fical action such as b y rolling through calendars [1], grinding in ball mills [2], forcirg polymer solutions through a capillary [3] or allowing them to flow into a capillary [4], or b y cutting off shavings. This phenomenon can be attributed to a number of causes. In some cases high temperatures can be developed locally and this can promote the development of oxidative degradation. In other cases the considerable local deformations arising can cause the rupture of individual, strongly over-strained macromoleeules with the formation of free, polymeric radicals [5]. Some of the free radicals formed b y rupture can be stabilized b y recombination; the remainder must either disproportionate or react with neighbouring molecules (particularly oxygen) and initiate a chain of degradative decomposition. The last two processes result in a reduction in molecular weight of the polymeric molecules. Finally a reduction in molecular weight can result from the splitting off of monomer from the ends of the free, polymeric radical before it becomes stabilized. An indication of this is the strong odour of monomer that always accompanies the cutting of polymers. I t is clear that a study of characteristics reflecting changes in molecular weight and molecular-weight distribution can provide information on the course of the processes taking place during the comminution of a polymer. For this purpose we have carried out a systematic investigation of the intrinsic viscosities of solid polymers before and after cutting into shavings. EXPERIMENTAL METHOD
The polymers investigated were atactic polymethylmethacrylate (PMMA), prepared b y radical polymerization, with viscosity-average molecular weight, My, from 0.58×106 to 8.4×106, and atactic polystyrene (PS), prepared b y radical polymerization, with M r from 0.14×106 to 1.4×10% The polymers * Vysokomol. soedin. 2: No. 10, 1486-1493, 1960.
382
Degradation of polymethylmethacrylate and polystyrene
383
were formed into cylindrical rods of diameter 9 ram. Small pieces were cut off from these for the determination cf the intrinsic viscosity of the original material in benzene at 20 °. The viscosity-averagv molecular weights were calculated from the formulae: for polystyrene in benzene [6] log M v-- 1.344 log [r/] +5-405 for PMMA in benzene [7] log M , - - 1 . 3 log [~/]+5.62 where [~/] is the intrinsic viscosity in g/dl. We chose the cutting of shavings on a lathe at room temperature as a suitable method of mechanical degradation of the polymers. I t was only b y this method that it was possible to fix the cross-section of the shavings and the cutting conditions and thus to ensure the reproducibility of the experiments. The shavings were taken off in several passes over the cylindrical specimen. The cutting tool was set to give shavings with a rectangular cross-section. The cutting speed was varied between 1.6 and 70 cm/sec, the width of the shavings was kept constant at 0.5 mm and the thickness was varied between 3 and 75p by altering the settivg of the tool. In preliminary experiments in which the polymers were cut with a milling machine and in a medical microtome, it was found that the absolute value of the reduction in molecular weight was the greater the higher the molecular weight of the polymers and the thinner the shavings taken. It was also found that the molecular weight of the shavings did not vary on storing in air for a month. RESULTS AND DISCUSSION
Degradation at constant cutting rate. Figure 1 shows the dependence of the intrinsic viscosity of the shavings [~]~ for PMMA and PS of various molecular weights, on the intrinsic viscosity of the materials before cutting [~]]o. The curves correspond to various thicknesses of shavings. The rate of cutting was 2.4 cm/sec. As the diagram shows, PMMA and PS follow the same relationship found qualitatively in the preliminary experiments--the thinner the shavings and the higher the molecular weight of the original polymer the greater the reduction in molecular weight. With polymers of low molecular weight (PS with [~t] ~ 0-5 and PMMA with [~l] ~ 1) the removal of shavings of thickness 3/1 does not produce any perceptible effect. Obviously the observed reduction in molecular weight is connected primarily with mechanical degradation. The development of oxidative degradation was improbable because this requires high temperatures and at a cutting speed of 2.4 cm/sec the heating of the material was small. In view of the fa,ct that, 26 Polymer
l~. P. VERsnININAan:l E. V. KuVS~INSgH
384
the odour of monomer arises soon after cutting begins it m a y be supposed t h a t degradation begins at the new surfaces as t h e y are exposed. Let us establish how the degree and depth of degradation should be established if the process Pl].$
(a~
/
z
t
3
It' S
o
I
I
z
4
ffz
3
I
I
6 [~]o
o
z
4
6[~],.
FIG. 1. The dependence of [t/]s, for shavings of different thickness, on [t/]o; a--for polymethylmethaerylate, b--for polystyrene. Cutting speed 2.38 cm/sec; thickness of shaving (/~); 1--3; 2--4.5; 3--6.7; 4--10; 5--15; 6--22; 7--35; 8--50 and 75. takes place in the surface layers. Assuming t h a t all the polymer molecules in the layers next to the cut surfaces are degraded to form mainly low-molecular products, then the depth of degradation of the polymer in the shaving c a n obviously be measured by the total thickness of the surface layers in which the intrinsic viscosity is changed significantly on cutting in comparison with the viscosity of the original polymer. I f it is assumed t h a t degradation extends through the depth a (Fig. 2) the total thickness of the shaving, h, can be divided
Q
FIG. 2. Diagram of cutting of a shaving (for explanation see text). into two layers: a layer of thickness g = 2 a , where all the degraded molecules are concentrated, and a layer, b, where the molecules are unchanged. Then
Degradation of polymethyhnethacrylate and polystyrene
38.5
the intrinsic viscosity of the shaving, [~/]~, the latter being a two component system, will be determined b y [8]:
2a[~/]~ ÷ b[~Jb 2a Wb
g[v/]a ÷b[v/]b h
where [~/]a and [~]~ are the original intrinsic viscosities of layers a and b respectively. I f it is considered that after degradation the viscosity of the polymer: in the surface layers becomes negligibly small the intrinsic viscosity will be determined b y the relatior ship b
[~]s=~E~]o-
Then the relative fall in intrinsic viscosity will determine the lower limit of the relative total thickness of the degraded layer i.e. it will be a measure of the degree of degradation A[e]
[~]o--[~]s
[~]o
[~]o
(h--b) h
g h
The thickness of the degraded layer g=(A[vl]/[vl]).h will characterize the "depth" of degradation, being some absolute measure o f it. The magnitude of g can be estimated if it is considered that degradation, beginning at the surface, extends to a depth comparable with the length of the macromolecule. This layer for PMMA of different molecular weights should not exceed the values given below, where the values of twice the root mean square of the distance between the ends of themacromolecules, 2 ~/h,~ ----g, for PMMA in acetone [9], are given: [q] in b e n z e n e , g/all
. . . .
Molecular weight My
.......
.
.
D e p t h of' d e g r a d e d l a y e r , l~, g : 2 x / / ~ 2z
. 1.37,
6-3 × 10'~ 0.159
1.96,
l06 0.200
4.74
3:16 × 106 0.398
I f degradation extends through a layer of constant thickness, determined by the size of the macromolecules, the value of g should not vary with variation in thickness of the shaving. Experimentally we obtain values of g 10-100 times greater than that assumed. Figures 3a and b show the dependence of the depth cf degradation, g, on the thickness of the shaving, h, for PMMA and PS of different molecular weights at constant cutting speed. It is seen from the graphs that contrary to expectation the thickness of the degraded layer g is strongly dependent on the thickness of the shaving, h. The value o f g for a shaving with h----75/~ is 12 It and for h----3, 9-----1.5/~. '26*
M. P. VERSHININAand E. V. KUVSHINSKII
386
The curves shown in Figures 3a and b indicate t h a t for this series of specimens of PMMA and PS the depth of degradation increases somewhat with increasing molecular weight. However this increase is less than would be assumed on the (a [~11171)h: g cm
1
~ ld<
(a)
o/~
I0
o 2
o
J
8
q
crn $
M[~]/rrl])h=g
,I0 8
(b)
6
x2
#
2
o
I
I
zO
ao
1
I
60
r_
8o,,o ~ h. (cm)
0
20
/-t0
60
8 x/O #
h(cm)
Fla. 3. The dependence of the depth of degradation on the thiclmess of the shaving; a--for PMMA, b--for PS. a: [~]o: •--6.55, 2--4.9; 3--3.45; 4--1.35; b: [~/]o: 1--3.6; 2--2-16; 3--0-6: 4--1.9; cutting speed 2.38 cm/sec. basis of calculation. According to calculation a five-fold change in the molecular weight of PMMA should have altered the depth of the layer by a factor of at least 2-5. Figures 3a and b show t h a t with a five-fold increase in molecular weight the depth of degradation increases b y a factor of only 1.5. The results shown in Figure 4 indicate t h a t the dependence of the degree of degradation on molecular weight for other batches of the polymers is the reverse; g is somewhat lower in the case of the polymers of higher molecular weight. Evidently various kinds of impurities such as initiator and monomer residues, the content of which can vary from sample to sample, can affect the change in molecular weight of polymers on mechanical degradation. As is seen from Figures 3a and b the relationship g=g(h) is parabolic, whereas ff degradation took place uniformly through the whole thickness of the material a linear increase in g with increase in h would be expected. The cause of the deviation from linearity could also be t h a t the time factor in the cutting conditions is not identical, i.e. t h a t shavings of different thickness are subjected to mechanical action for a different time. I t is difficult to forecast how a change
Degradation of polymethylmethaerylate and polystyrene
387
in the time regime of euttivg can affect the degree of degradation of polymers. For example on increasing the cutting speed the shavings can increase in temperature in the cutting zone. This could result in a large number of macromolecules ,Cv]/[~]
•
0.5~
a
o- b
0 1 ~
0
~ll
I
I
go
40
~l
7' 11 810
60 v(cmlsec~
FIG. 4. The dependence of the degree of degradation on the linear cutting speed for PMMA. [0]o; a--6-1, b--4.85, c--3.4. Thickness of shaving (~): 1--3; /I--10; d - - r e s u l t s for PS of [0]o--3"25, thickness of shaving 75~.
Ill--7"5;
being "pulled out" without rupturipg and degradation would be reduced. B u t with a short cutting time the material m a y not have time to "relax" and the effect of rupture of the macromolecules could exceed that of their pulling out without damage. On the other hand an increase in temperature can increase thermal degradation and increase the degree of degradation etc. In our case where the thickness cf the shavings, h, is comparable with the "length" of the deformed region, 1 (Fig. 2), the time cf action on the shaving will be determined b y the ratio of the thickness of the shaving to the linear cutting speed, v=h/v. Consequently in order to keep the time factor constant in cutting shavings of different thickness the cutting speed should be varied so that shavings of greater thickness are cut at a proportionately higher speed, i,e. so that the cutting time remains constant. Such conditions were reproduced b u t in this case also the relationship g=g(h) remained curvilinear. This suggests that the
M. P. VERSHININAand E. V. KUVSHINSKII
388
curvilinearity of this function is an expression of essential.features of the course of the degradation process. Contrary to expectation the effect of Cutting speed on the degree of degra. dation was considerable and was given special study. The effect of cutting speed on the degree of mechanical degradation. Figure 4 shows the dependence of the degree of degradation of the cutting speed for PMMA of different molecular weights and at different thickneses of shavings. It is seen that degradation falls with increasing cutting speed; the shorter the time of action of the cutting tool on the shaving the lower the degree of degradation. These experimental results can be interpreted as an indication of the fact that the degradation of a polymer on cutting is limited b y a time-dependent process. At high speeds this does not have sufficient time for completion and hence the depth of degradation falls with cuttirg speed. Let us consider the validity of an explanation based on the assumption that the process limiting degradation is a diffusion process. cm
/
o
ttl
12 e
10
0":
'
~:"~ ~
i
'111
i
I11
"
8
tH
.
, / 6
~ ! :
0
l
ll
g
3
#
5
6 ' 7 ,
Lf~'TFT,-. ,
8~ Id z ,
~ , , / . (secl
,
FIG. 5. The dependence of the depth of degradation on the time taken in -cutting the shaving, for P~IMA. [7]o: a--6-1; b--4-85; c--3.4. Thickness of shaving (p): 1 - - 3 ; II--10; 111--75; d--results for PS of [g]o-~ 3"25, thickness of shaving 75 p.
Let us analyse our data from this point of view. It is known that the rate of any diffusion process is initially proportional to the square root of the time. Let us plot the thickness of the degraded layer g against the square root of the cutting time. It is seen from Figure 5 that this relationship gives a series of similar curves having coincident linear initial sections. The trend of the curves
Degradation of polymethyhnethacrylate
and polyst.yrene
389
toward zero thickness, g = 0 , at low cutting times, and the linearity of the initial l-
sections, i.e. the proportionality g ~ x/T, supports the suggestion of the existence of diffusion processes that limit degradation. This diagram shows that degradation proceeds differently in shavings of different thickness. The following are values of the depth of degradation and the time at which the gently sloping part of the curve is reached, for shavings of different thicknesses. T h i c k n e s s o f s h a v i n g , h, # . . . . . . . . . . 3 l0 75 T h i c k n e s s o f d e g r a d e d l a y e r , g, /~. . . . . . . 1.3 2.2 l l T i m e a t w h i c h g e n t l e s l o p e is r e a c h e d r=]~/~:, scc 0.0635 0.1 0 . ] 5 8
The thicker the shaving the greater the thickness of the degraded layer and the greater the time at which the g~ntle slope is reached. If a diffusion process, extendirg from the uncovered surface into the depth of the material. had limited degradation then a graph of degree of degradation, g/h, against
x/1/vh should give a universal curve for these materials. In fact, g/h=A[JI]/[rl] is a dimensionless meguitudc characteriziv~g the depth of degradation and ~,/iD/vha dimensionless argument, where D is the diffusion coefficient of the process. iimitirg degradation. Hence the function g/h=f(x/1/vh) should be universal and independent of the thickness of the shaving. Fig~lrc 6 shows that a uui-
O,z~
4~a~...e,..tl I
0.2
zz
o.I,~ I/'~
-~--- m
l
o.c
I
FIG. 6. T h e d e p e n d e n c e o f t h e d e g r e e o f d e g r a d a t i o n o n 4 1 / v h f o r P M M A . [ ~/]o: a -- 6.1 ; b - - 4.85; c - - 3.4. T h i c k n e s s o f s h a v i n g (/a): I - - 3; I I -- 10; I I I - - 75.
versal curve is not altogether obtained. The curves are closer tcgether than those in Figure 5 and possibly the process initially follows the diffusion law the initial sections coincide), b u t then saturation occurs and the effect of thick-
390
M.P. VERSHININAand E. V. KUVSHINSKII
ness ~gain appears, inversely to a n d less sharply t han t h a t in Figure 5. The diffusion coefficient D, calculated fcom the slope of the common initial sections accordirg to the equation 4 x/D =(if~h) x/-~ (the case of diffusion from a surface into a semi-infinite layer), was 10-4 cm2/sec. For comparison, the diffusion coefficient for oxygen into rubber a t room t em perat ure is 10-6 cm2/sec [10]. The unusually high calculated value of the diffusion coefficient indicates t h a t it is doubtful whether we are concerned with any single diffusion process ext e n d i r g from the surface into the depth of the material. The calculated value of D = 1 0 -4 cm2/sec is an order of magnitude lower t han the thermal diffusivity X----x/cpp=lO -3 cm2/sec. This shows t h a t degradation is also not limited b y t he distribution of t e m per a t ur e in the shavirg duripg the cutting process. I t is evident t h a t the degradation process brought about b y cutting bears a far from simple relationship to the rupture of chemical bonds at the surface of the shaving. U n d o u b t e d l y degradation begins and develops in the deeper layers of the material. Moreover the results obtained indicate t h a t the degradation process is time dependent and reaches completion during the process of obtaining the shavings.
CONCLUSIONS (1) The degree of degradation of PMMA and PS on cutting is dependent on the thickness of the shaving cut, on the rate o f cutting and on the molecular weight of the polymer. (2) E v e n if the degradation process begins at the uncovered surfaces it is completed in layers of considerable thickness and is a time-dependent process. (3) There are certain factors t h a t limit the depth in which degradation occurs. The thinner the shaving the earlier the process of degradation is terminated and the level of degradation is correspondingly lower.
Translated by E.O. PHILLIPS REFERENCES 1. B. KA_1%MINand B. BETTS, Issledovaniya v oblasti vysokomolekulyarnykh soedinenii. (Researches in the Field of Macromolecular Compounds.) pp. 129-137; Izd. Akad. Nauk SSSR, 1949; W. F. WATSON and M. PIKE, J. Polymer Sei. 13: 229, 1952; J. Sei. Instr. 3 1 : 98, 1954 2. S. A. PAVLOV and N. K. BARAMBOIM, Kolloid. zh. 11: 420, 1949; N. K. BARAMBOIM, Zh. fiz. khim. 32: 433, 1958; N. K. BARAMBOIM, Legkaya prom. 4: 22, 1950; N. K. BARAMBOIM and V. N. (~ORODILOV, Vysokomol. soedin. 2: 197, 1960; N. K. BARAMBOIM, Kolloid. zh. 13: 84, 1951; N. K. BARAMBOIM, Nauchnye trudy Mosk. in-ta legkoi prom. 4: 104, 1954; N. K. BARAMBOIM, ibid. 7: 53, 1957 3. G. SIITAUDINGER, sb. Vysokomolek. soed. (Collected Papers, Maeromoleeular Compounds.) p. 398, Moscow, 1935; A. B. BESTUL, J. Applied Phys. 29: 1069, 1954; J. Chem. Phys. 24: 1197, 1956; J. Phys. Chem. 61, 418, 1957
Electron spin resonance spectra of linear aromatic polymers
391
4. E. V. KUVSHINSKII, Doktorskaya dissertatsiya. (Doctorate Thesis.) 1948 5. S. E. BRESLER, S. N. ZHURKOV, E. N. KAZBEKOV, E. M. SAMINSKII and E. E. TOMASHEVSKII, Zh. tekh. fiz. 29: 458, 1959; G. L. SLONIMSKII, Khim. nauka i prom. 4: 73, 1959; A. A. BERLIN, Dokl. Akad. Nauk SSSR 110: 401, 1956; A. A. BERLIN. Uspekhi khimii 27: 94, 1958; N. K. BARAMBOIM, Zh. fiz. khim. 32: 806, 1958 6. P. FLORY, J. Polymer Sci. 11: 37, 1953 7. V. N. TSVETKOV, Zh. eksp. i. teor. fiz. 26: 357, 1954 8. E. V. M ELEK H I N A and E. V. KUVSHINSKII, Zh. fiz. khim. 24: 199, 1950 9. V. N. TSVETKOV, K. V. FATTAKHOV and O. V. KALLISTOV, Zh. eksp. i teor. fiz. 351, 1954 10. S. A. REITLINGER, Uspekhi khimii 20: 213, 1951
26:
POLYMERS WITH CONJUGATED BONDS AND HETERO-ATOMS IN THE CONJUGATED CHAIN--XI. ELECTRON SPIN RESONANCE SPECTRA OF LINEAR AROMATIC POLYMERS* B. I. LIOGON'KII,
L. S. LYUBCHENKO, A.A. BERLIN, and V.P. PARINI
Institute of Chemical
Physics, U.S.S.R. Academy
L. A. BLYUMENFEL'D of Sciences
(Received 25 April 1960) IT was reported previously that b y reacting bis-diazotized benzidine, benzidine-3,3-dicarboxylic acid and o-tolidine with monovalent copper salts the aromatic polymers I, II and I I I are obtained, with a partial content of azo groups and, evidently, a linear structure [1, 2]. From a consideration of the mechanism of the Sandmeyer reaction [3] and the properties of these polymers it m a y be assumed that radical reaction centre.~ arise as a result of the reduction of the bis-diazonium salts
Cu +
; R
R
.......
i R
R
I
where R =
I--H I I - - COOH, / III--CHa
* Vysokomol. soedin. 2" No. 10, 149t---1499, 1960.
l
R
R
and E. V. KUVSHINSKII Compounds,
U.S.S.R. Academy
of Sciences
(Received 21 April 1960) A REDUCTION in the molecular weight of the polymer occurs when polymeric materials are subjected to mecha~fical action such as b y rolling through calendars [1], grinding in ball mills [2], forcirg polymer solutions through a capillary [3] or allowing them to flow into a capillary [4], or b y cutting off shavings. This phenomenon can be attributed to a number of causes. In some cases high temperatures can be developed locally and this can promote the development of oxidative degradation. In other cases the considerable local deformations arising can cause the rupture of individual, strongly over-strained macromoleeules with the formation of free, polymeric radicals [5]. Some of the free radicals formed b y rupture can be stabilized b y recombination; the remainder must either disproportionate or react with neighbouring molecules (particularly oxygen) and initiate a chain of degradative decomposition. The last two processes result in a reduction in molecular weight of the polymeric molecules. Finally a reduction in molecular weight can result from the splitting off of monomer from the ends of the free, polymeric radical before it becomes stabilized. An indication of this is the strong odour of monomer that always accompanies the cutting of polymers. I t is clear that a study of characteristics reflecting changes in molecular weight and molecular-weight distribution can provide information on the course of the processes taking place during the comminution of a polymer. For this purpose we have carried out a systematic investigation of the intrinsic viscosities of solid polymers before and after cutting into shavings. EXPERIMENTAL METHOD
The polymers investigated were atactic polymethylmethacrylate (PMMA), prepared b y radical polymerization, with viscosity-average molecular weight, My, from 0.58×106 to 8.4×106, and atactic polystyrene (PS), prepared b y radical polymerization, with M r from 0.14×106 to 1.4×10% The polymers * Vysokomol. soedin. 2: No. 10, 1486-1493, 1960.
382
Degradation of polymethylmethacrylate and polystyrene
383
were formed into cylindrical rods of diameter 9 ram. Small pieces were cut off from these for the determination cf the intrinsic viscosity of the original material in benzene at 20 °. The viscosity-averagv molecular weights were calculated from the formulae: for polystyrene in benzene [6] log M v-- 1.344 log [r/] +5-405 for PMMA in benzene [7] log M , - - 1 . 3 log [~/]+5.62 where [~/] is the intrinsic viscosity in g/dl. We chose the cutting of shavings on a lathe at room temperature as a suitable method of mechanical degradation of the polymers. I t was only b y this method that it was possible to fix the cross-section of the shavings and the cutting conditions and thus to ensure the reproducibility of the experiments. The shavings were taken off in several passes over the cylindrical specimen. The cutting tool was set to give shavings with a rectangular cross-section. The cutting speed was varied between 1.6 and 70 cm/sec, the width of the shavings was kept constant at 0.5 mm and the thickness was varied between 3 and 75p by altering the settivg of the tool. In preliminary experiments in which the polymers were cut with a milling machine and in a medical microtome, it was found that the absolute value of the reduction in molecular weight was the greater the higher the molecular weight of the polymers and the thinner the shavings taken. It was also found that the molecular weight of the shavings did not vary on storing in air for a month. RESULTS AND DISCUSSION
Degradation at constant cutting rate. Figure 1 shows the dependence of the intrinsic viscosity of the shavings [~]~ for PMMA and PS of various molecular weights, on the intrinsic viscosity of the materials before cutting [~]]o. The curves correspond to various thicknesses of shavings. The rate of cutting was 2.4 cm/sec. As the diagram shows, PMMA and PS follow the same relationship found qualitatively in the preliminary experiments--the thinner the shavings and the higher the molecular weight of the original polymer the greater the reduction in molecular weight. With polymers of low molecular weight (PS with [~t] ~ 0-5 and PMMA with [~l] ~ 1) the removal of shavings of thickness 3/1 does not produce any perceptible effect. Obviously the observed reduction in molecular weight is connected primarily with mechanical degradation. The development of oxidative degradation was improbable because this requires high temperatures and at a cutting speed of 2.4 cm/sec the heating of the material was small. In view of the fa,ct that, 26 Polymer
l~. P. VERsnININAan:l E. V. KuVS~INSgH
384
the odour of monomer arises soon after cutting begins it m a y be supposed t h a t degradation begins at the new surfaces as t h e y are exposed. Let us establish how the degree and depth of degradation should be established if the process Pl].$
(a~
/
z
t
3
It' S
o
I
I
z
4
ffz
3
I
I
6 [~]o
o
z
4
6[~],.
FIG. 1. The dependence of [t/]s, for shavings of different thickness, on [t/]o; a--for polymethylmethaerylate, b--for polystyrene. Cutting speed 2.38 cm/sec; thickness of shaving (/~); 1--3; 2--4.5; 3--6.7; 4--10; 5--15; 6--22; 7--35; 8--50 and 75. takes place in the surface layers. Assuming t h a t all the polymer molecules in the layers next to the cut surfaces are degraded to form mainly low-molecular products, then the depth of degradation of the polymer in the shaving c a n obviously be measured by the total thickness of the surface layers in which the intrinsic viscosity is changed significantly on cutting in comparison with the viscosity of the original polymer. I f it is assumed t h a t degradation extends through the depth a (Fig. 2) the total thickness of the shaving, h, can be divided
Q
FIG. 2. Diagram of cutting of a shaving (for explanation see text). into two layers: a layer of thickness g = 2 a , where all the degraded molecules are concentrated, and a layer, b, where the molecules are unchanged. Then
Degradation of polymethyhnethacrylate and polystyrene
38.5
the intrinsic viscosity of the shaving, [~/]~, the latter being a two component system, will be determined b y [8]:
2a[~/]~ ÷ b[~Jb 2a Wb
g[v/]a ÷b[v/]b h
where [~/]a and [~]~ are the original intrinsic viscosities of layers a and b respectively. I f it is considered that after degradation the viscosity of the polymer: in the surface layers becomes negligibly small the intrinsic viscosity will be determined b y the relatior ship b
[~]s=~E~]o-
Then the relative fall in intrinsic viscosity will determine the lower limit of the relative total thickness of the degraded layer i.e. it will be a measure of the degree of degradation A[e]
[~]o--[~]s
[~]o
[~]o
(h--b) h
g h
The thickness of the degraded layer g=(A[vl]/[vl]).h will characterize the "depth" of degradation, being some absolute measure o f it. The magnitude of g can be estimated if it is considered that degradation, beginning at the surface, extends to a depth comparable with the length of the macromolecule. This layer for PMMA of different molecular weights should not exceed the values given below, where the values of twice the root mean square of the distance between the ends of themacromolecules, 2 ~/h,~ ----g, for PMMA in acetone [9], are given: [q] in b e n z e n e , g/all
. . . .
Molecular weight My
.......
.
.
D e p t h of' d e g r a d e d l a y e r , l~, g : 2 x / / ~ 2z
. 1.37,
6-3 × 10'~ 0.159
1.96,
l06 0.200
4.74
3:16 × 106 0.398
I f degradation extends through a layer of constant thickness, determined by the size of the macromolecules, the value of g should not vary with variation in thickness of the shaving. Experimentally we obtain values of g 10-100 times greater than that assumed. Figures 3a and b show the dependence of the depth cf degradation, g, on the thickness of the shaving, h, for PMMA and PS of different molecular weights at constant cutting speed. It is seen from the graphs that contrary to expectation the thickness of the degraded layer g is strongly dependent on the thickness of the shaving, h. The value o f g for a shaving with h----75/~ is 12 It and for h----3, 9-----1.5/~. '26*
M. P. VERSHININAand E. V. KUVSHINSKII
386
The curves shown in Figures 3a and b indicate t h a t for this series of specimens of PMMA and PS the depth of degradation increases somewhat with increasing molecular weight. However this increase is less than would be assumed on the (a [~11171)h: g cm
1
~ ld<
(a)
o/~
I0
o 2
o
J
8
q
crn $
M[~]/rrl])h=g
,I0 8
(b)
6
x2
#
2
o
I
I
zO
ao
1
I
60
r_
8o,,o ~ h. (cm)
0
20
/-t0
60
8 x/O #
h(cm)
Fla. 3. The dependence of the depth of degradation on the thiclmess of the shaving; a--for PMMA, b--for PS. a: [~]o: •--6.55, 2--4.9; 3--3.45; 4--1.35; b: [~/]o: 1--3.6; 2--2-16; 3--0-6: 4--1.9; cutting speed 2.38 cm/sec. basis of calculation. According to calculation a five-fold change in the molecular weight of PMMA should have altered the depth of the layer by a factor of at least 2-5. Figures 3a and b show t h a t with a five-fold increase in molecular weight the depth of degradation increases b y a factor of only 1.5. The results shown in Figure 4 indicate t h a t the dependence of the degree of degradation on molecular weight for other batches of the polymers is the reverse; g is somewhat lower in the case of the polymers of higher molecular weight. Evidently various kinds of impurities such as initiator and monomer residues, the content of which can vary from sample to sample, can affect the change in molecular weight of polymers on mechanical degradation. As is seen from Figures 3a and b the relationship g=g(h) is parabolic, whereas ff degradation took place uniformly through the whole thickness of the material a linear increase in g with increase in h would be expected. The cause of the deviation from linearity could also be t h a t the time factor in the cutting conditions is not identical, i.e. t h a t shavings of different thickness are subjected to mechanical action for a different time. I t is difficult to forecast how a change
Degradation of polymethylmethaerylate and polystyrene
387
in the time regime of euttivg can affect the degree of degradation of polymers. For example on increasing the cutting speed the shavings can increase in temperature in the cutting zone. This could result in a large number of macromolecules ,Cv]/[~]
•
0.5~
a
o- b
0 1 ~
0
~ll
I
I
go
40
~l
7' 11 810
60 v(cmlsec~
FIG. 4. The dependence of the degree of degradation on the linear cutting speed for PMMA. [0]o; a--6-1, b--4.85, c--3.4. Thickness of shaving (~): 1--3; /I--10; d - - r e s u l t s for PS of [0]o--3"25, thickness of shaving 75~.
Ill--7"5;
being "pulled out" without rupturipg and degradation would be reduced. B u t with a short cutting time the material m a y not have time to "relax" and the effect of rupture of the macromolecules could exceed that of their pulling out without damage. On the other hand an increase in temperature can increase thermal degradation and increase the degree of degradation etc. In our case where the thickness cf the shavings, h, is comparable with the "length" of the deformed region, 1 (Fig. 2), the time cf action on the shaving will be determined b y the ratio of the thickness of the shaving to the linear cutting speed, v=h/v. Consequently in order to keep the time factor constant in cutting shavings of different thickness the cutting speed should be varied so that shavings of greater thickness are cut at a proportionately higher speed, i,e. so that the cutting time remains constant. Such conditions were reproduced b u t in this case also the relationship g=g(h) remained curvilinear. This suggests that the
M. P. VERSHININAand E. V. KUVSHINSKII
388
curvilinearity of this function is an expression of essential.features of the course of the degradation process. Contrary to expectation the effect of Cutting speed on the degree of degra. dation was considerable and was given special study. The effect of cutting speed on the degree of mechanical degradation. Figure 4 shows the dependence of the degree of degradation of the cutting speed for PMMA of different molecular weights and at different thickneses of shavings. It is seen that degradation falls with increasing cutting speed; the shorter the time of action of the cutting tool on the shaving the lower the degree of degradation. These experimental results can be interpreted as an indication of the fact that the degradation of a polymer on cutting is limited b y a time-dependent process. At high speeds this does not have sufficient time for completion and hence the depth of degradation falls with cuttirg speed. Let us consider the validity of an explanation based on the assumption that the process limiting degradation is a diffusion process. cm
/
o
ttl
12 e
10
0":
'
~:"~ ~
i
'111
i
I11
"
8
tH
.
, / 6
~ ! :
0
l
ll
g
3
#
5
6 ' 7 ,
Lf~'TFT,-. ,
8~ Id z ,
~ , , / . (secl
,
FIG. 5. The dependence of the depth of degradation on the time taken in -cutting the shaving, for P~IMA. [7]o: a--6-1; b--4-85; c--3.4. Thickness of shaving (p): 1 - - 3 ; II--10; 111--75; d--results for PS of [g]o-~ 3"25, thickness of shaving 75 p.
Let us analyse our data from this point of view. It is known that the rate of any diffusion process is initially proportional to the square root of the time. Let us plot the thickness of the degraded layer g against the square root of the cutting time. It is seen from Figure 5 that this relationship gives a series of similar curves having coincident linear initial sections. The trend of the curves
Degradation of polymethyhnethacrylate
and polyst.yrene
389
toward zero thickness, g = 0 , at low cutting times, and the linearity of the initial l-
sections, i.e. the proportionality g ~ x/T, supports the suggestion of the existence of diffusion processes that limit degradation. This diagram shows that degradation proceeds differently in shavings of different thickness. The following are values of the depth of degradation and the time at which the gently sloping part of the curve is reached, for shavings of different thicknesses. T h i c k n e s s o f s h a v i n g , h, # . . . . . . . . . . 3 l0 75 T h i c k n e s s o f d e g r a d e d l a y e r , g, /~. . . . . . . 1.3 2.2 l l T i m e a t w h i c h g e n t l e s l o p e is r e a c h e d r=]~/~:, scc 0.0635 0.1 0 . ] 5 8
The thicker the shaving the greater the thickness of the degraded layer and the greater the time at which the g~ntle slope is reached. If a diffusion process, extendirg from the uncovered surface into the depth of the material. had limited degradation then a graph of degree of degradation, g/h, against
x/1/vh should give a universal curve for these materials. In fact, g/h=A[JI]/[rl] is a dimensionless meguitudc characteriziv~g the depth of degradation and ~,/iD/vha dimensionless argument, where D is the diffusion coefficient of the process. iimitirg degradation. Hence the function g/h=f(x/1/vh) should be universal and independent of the thickness of the shaving. Fig~lrc 6 shows that a uui-
O,z~
4~a~...e,..tl I
0.2
zz
o.I,~ I/'~
-~--- m
l
o.c
I
FIG. 6. T h e d e p e n d e n c e o f t h e d e g r e e o f d e g r a d a t i o n o n 4 1 / v h f o r P M M A . [ ~/]o: a -- 6.1 ; b - - 4.85; c - - 3.4. T h i c k n e s s o f s h a v i n g (/a): I - - 3; I I -- 10; I I I - - 75.
versal curve is not altogether obtained. The curves are closer tcgether than those in Figure 5 and possibly the process initially follows the diffusion law the initial sections coincide), b u t then saturation occurs and the effect of thick-
390
M.P. VERSHININAand E. V. KUVSHINSKII
ness ~gain appears, inversely to a n d less sharply t han t h a t in Figure 5. The diffusion coefficient D, calculated fcom the slope of the common initial sections accordirg to the equation 4 x/D =(if~h) x/-~ (the case of diffusion from a surface into a semi-infinite layer), was 10-4 cm2/sec. For comparison, the diffusion coefficient for oxygen into rubber a t room t em perat ure is 10-6 cm2/sec [10]. The unusually high calculated value of the diffusion coefficient indicates t h a t it is doubtful whether we are concerned with any single diffusion process ext e n d i r g from the surface into the depth of the material. The calculated value of D = 1 0 -4 cm2/sec is an order of magnitude lower t han the thermal diffusivity X----x/cpp=lO -3 cm2/sec. This shows t h a t degradation is also not limited b y t he distribution of t e m per a t ur e in the shavirg duripg the cutting process. I t is evident t h a t the degradation process brought about b y cutting bears a far from simple relationship to the rupture of chemical bonds at the surface of the shaving. U n d o u b t e d l y degradation begins and develops in the deeper layers of the material. Moreover the results obtained indicate t h a t the degradation process is time dependent and reaches completion during the process of obtaining the shavings.
CONCLUSIONS (1) The degree of degradation of PMMA and PS on cutting is dependent on the thickness of the shaving cut, on the rate o f cutting and on the molecular weight of the polymer. (2) E v e n if the degradation process begins at the uncovered surfaces it is completed in layers of considerable thickness and is a time-dependent process. (3) There are certain factors t h a t limit the depth in which degradation occurs. The thinner the shaving the earlier the process of degradation is terminated and the level of degradation is correspondingly lower.
Translated by E.O. PHILLIPS REFERENCES 1. B. KA_1%MINand B. BETTS, Issledovaniya v oblasti vysokomolekulyarnykh soedinenii. (Researches in the Field of Macromolecular Compounds.) pp. 129-137; Izd. Akad. Nauk SSSR, 1949; W. F. WATSON and M. PIKE, J. Polymer Sei. 13: 229, 1952; J. Sei. Instr. 3 1 : 98, 1954 2. S. A. PAVLOV and N. K. BARAMBOIM, Kolloid. zh. 11: 420, 1949; N. K. BARAMBOIM, Zh. fiz. khim. 32: 433, 1958; N. K. BARAMBOIM, Legkaya prom. 4: 22, 1950; N. K. BARAMBOIM and V. N. (~ORODILOV, Vysokomol. soedin. 2: 197, 1960; N. K. BARAMBOIM, Kolloid. zh. 13: 84, 1951; N. K. BARAMBOIM, Nauchnye trudy Mosk. in-ta legkoi prom. 4: 104, 1954; N. K. BARAMBOIM, ibid. 7: 53, 1957 3. G. SIITAUDINGER, sb. Vysokomolek. soed. (Collected Papers, Maeromoleeular Compounds.) p. 398, Moscow, 1935; A. B. BESTUL, J. Applied Phys. 29: 1069, 1954; J. Chem. Phys. 24: 1197, 1956; J. Phys. Chem. 61, 418, 1957
Electron spin resonance spectra of linear aromatic polymers
391
4. E. V. KUVSHINSKII, Doktorskaya dissertatsiya. (Doctorate Thesis.) 1948 5. S. E. BRESLER, S. N. ZHURKOV, E. N. KAZBEKOV, E. M. SAMINSKII and E. E. TOMASHEVSKII, Zh. tekh. fiz. 29: 458, 1959; G. L. SLONIMSKII, Khim. nauka i prom. 4: 73, 1959; A. A. BERLIN, Dokl. Akad. Nauk SSSR 110: 401, 1956; A. A. BERLIN. Uspekhi khimii 27: 94, 1958; N. K. BARAMBOIM, Zh. fiz. khim. 32: 806, 1958 6. P. FLORY, J. Polymer Sci. 11: 37, 1953 7. V. N. TSVETKOV, Zh. eksp. i. teor. fiz. 26: 357, 1954 8. E. V. M ELEK H I N A and E. V. KUVSHINSKII, Zh. fiz. khim. 24: 199, 1950 9. V. N. TSVETKOV, K. V. FATTAKHOV and O. V. KALLISTOV, Zh. eksp. i teor. fiz. 351, 1954 10. S. A. REITLINGER, Uspekhi khimii 20: 213, 1951
26:
POLYMERS WITH CONJUGATED BONDS AND HETERO-ATOMS IN THE CONJUGATED CHAIN--XI. ELECTRON SPIN RESONANCE SPECTRA OF LINEAR AROMATIC POLYMERS* B. I. LIOGON'KII,
L. S. LYUBCHENKO, A.A. BERLIN, and V.P. PARINI
Institute of Chemical
Physics, U.S.S.R. Academy
L. A. BLYUMENFEL'D of Sciences
(Received 25 April 1960) IT was reported previously that b y reacting bis-diazotized benzidine, benzidine-3,3-dicarboxylic acid and o-tolidine with monovalent copper salts the aromatic polymers I, II and I I I are obtained, with a partial content of azo groups and, evidently, a linear structure [1, 2]. From a consideration of the mechanism of the Sandmeyer reaction [3] and the properties of these polymers it m a y be assumed that radical reaction centre.~ arise as a result of the reduction of the bis-diazonium salts
Cu +
; R
R
.......
i R
R
I
where R =
I--H I I - - COOH, / III--CHa
* Vysokomol. soedin. 2" No. 10, 149t---1499, 1960.
l
R
R