- Email: [email protected]
Chain atoms between entanglements and cross-sectional area per chain
Letters
R--NH--C
O II
I I (CH2)4 f Y I (CH2)4 I 0 I
O
0
RL--NH--C
II
R--NH --C A . -CO 2
R'NH"
II
/o
°
+
I
+H"
Y
R'NH 2
CH2--CH 2
0
R--NH--C
I
O il
-
C5
+
CH 2j
i H2
CH2--CH 2
1
-~+RNCO + H"
(I)
(rn) O
R--N
--~
.
,, RNCO+~H2(CH2)3__OH+~, -
+H',
CH3CH2CH2(]H2OH
(2)
(rT) ' (CH2) 4
J
Y
RNCO+CH2=CHCH 2CH 2O H + H Y - -
IV)
I atom (sulphur in the case of thioether and oxygen in the case of ethers used). If the bond ~ C - O - in polyurethane undergoes cleavage with subse-
quent hydrogen migration, instead of decarboxylation, a chain with OH groups at the ends can be formed. This prevents the cyclization reactions and
I
can lead to formation of alcohols with four carbon atoms (IV or V, scheme 2). This procedure can be accompanied by the Y - C bond cleavage with or without hydrogen atom migration as showri in scheme (2). The method presented here has been found useful for the identification of the diol constituent degradation products of polyurethanes of the type described This makes possible a partial qualitative characterization of polyurethane composition.
REFERENCES 1 Lesiak, T. and Prewysz-Kwinto, A. Chem. Stosow. 1972, 16, 259 2 Lesiak, T. and Prewysz-Kwinto, A. Chem. Stosow. 1974, 18,471 3 Lesiak, T. and Prewysz-Kwinto, A. Chem. Stosow. 1975, 19,477 4 Lesiak, T., Pielichowski, J. and PrewyszKwinto, A. Chem. Stosow. 1975, 19, 221 5 Hetper, J. and Zag6rski, W. Polimery 1973, 18, 327 6 Newell, J. E. 'Theory and Application of Gas Chromatography in Industry and Medicine', (Eds H. S. Kroman and S. R. Bender), Grune and Stratton, New York, 1968 7 Groten, B. AnaL Chem. 1964, 36, 1206 8 Corish, P. J. AnaL Chem. 1959, 31, 1298 9 Takeuchi, T., Tsuge, S. and Okumoto, T. J. Gas Chromatogr. 1968, 6,542
Letters
Chain atoms between entanglements and cross-sectional area per chain
We recently demonstrated a linear l o g - l o g correlation of slope - 3 between the ratio C2/C1 of the M o o n e y Rivlin constants (at 2C1 = 0.2 N/mm 2) and cross-sectional area per polymer chain 1"~. It was suggested 2 that either strain induced local order or chain entanglements might be involved in what appeared to be an intermolecular effect. Kraus and Moczvgemba 3 had proposed that the molecular origin of the C2 term was a result, at least in part, o f chain entanglements. Preliminary examination of an extensive tabulation by Porter and Johnson 4 of entanglement distances indicated a positive correlation with cross-sectional area per polymer chain. However, the scatter in data points was so great as to discourage any firm conclusions. The entanglement data in the literature
have now been examined in detail and a preliminary strong correlation has been made. A specific tabulation by Fox and Alien s was used as a source of Nc, the number of chain atoms between entanglement points determined from concentrated or bulk viscosity measurements as a function of molecular weight. Cross-sectional areas per polymer chain were calculated by a method described previously 2, which differs only in minor details from that used by Vincent 6 for a different purpose. Figure 1 shows a l o g - l o g correlation between Nc and chain areas in nm 2. The line as drawn has a slope o f approximately 0.5. The marked deviation of atactic PMMA from the trend line is considered to arise because of molecular association between isoand syndio-tactic species present in a
1112 P O L Y M E R , 1976, Vol 17, December
PMMA 7. Porter and Johnson note that any form of association tends to promote low values o f Nc. A detailed study of this subject will be presented elsewhere s. As one in2000 I000 500 300 lO0
o.,o c;.,s o'2o
'O10' ' o oL-"5.8o Chain cross-section ( n m 2)
Figure I
Number of chain atoms, Nc, between entangelements from data of Fox and Alien s vs. chain cross-section in nm 2. A, PE; B, poly(decamethylene adipate); C, poly(decamethylene sebacate); D, poly(ecaproamide); E, poly(propylene oxide); F, PIB; G, PDMS; H, PVAc; I, atactic PMMA; J, atactic PS.
Letters
chides more bodies of data, and especially other measures of chain entanglement to be found in ref4, the scatter increases significantly. However, the trend shown in Figure I is not lost. Since both C2/C1 and Nc correlate with a common variable, area in nm 2, it follows that one should expect a linear correlation between C2/C1 and Nc. This indeed is the case with five data points for PE, cis-trans PBD, NR, PIB, and PDMS 8. This correlation does not necessarily mean that chain entanglements are solely responsible for the
References 1 Boyer, R. F. and Miller, R. L. Polymer 1976, 17, 925 2 Boyer,R. F. and Miller, R. L. Rubber Chem. TechnoL in press 3 Kraus,G. and Moczvgemba,G. A. ]. Polym. ScL {A-2) 1964, 277 4 Porter, R. S. and Johnson, J. F. Chem. Rev. 1966, 66, 1 5 Fox, T. G. and Allen, V. R. J. Chem. Phys. 1964, 41,344 6 Vincent, P. I. Polymer 1972, 13,558 7 Feitsma, E. L., de Boer, A. and Challa, G. Polymer 1975, 16, 515. This paper refers to earlier literature. 8 Boyer,R. F. and Miller, R. L. Rubber Chem. TechnoL in press
C2 term. The correlation line of Figure ] on a homogeneous set of data provides a criterion for judging and/or selecting from discordant literature values for these and other polymers.
Raymond F. Boyer and Robert L. Miller Midland Macromolecular Institute, 1910 W. St. Andrews Drive, Midland, Michigan 48640, USA (Received 19 July 1976)
The involvement of singlet oxygen in the sensitized photodegradation of cellulose The dye sensitized photo-oxidative degradation of cellulose has long been an area of practical and theoretical interest, but the mechanism of this process is still a controversial subject 1. Of the two principal mechanisms that have been proposed, namely the singlet oxygen theory 2 and the hydrogen (or electron) abstraction theory 3, the former has received relatively little credence. This is probably because it is relatively easy to demonstrate that a dye will photochemically abstract hydrogen or an electron from cellulose, whereas it is much more difficult to show unambiguously that singlet oxygen will react with this substrate. Almost all efficient sensitizers of singlet oxygen are good hydrogen abstractors (e.g. the quinonoid vat dyes, heterocyclic cationic dyes, and porphins), thus photodegradation studies with these compounds are ambiguous. The few known sensitizers that do not photoreduce (e.g. the acenes) are of little value because of their reactivity towards singlet oxygen. Non-photochemical sources of singlet oxygen are also suspect, as these systems are contaminated with other oxidizing species that could play an uncertain role in cellulose degradation. We have looked for new stable singlet oxygen sensitizers that have a minimal tendency to undergo photochemical reduction. A particularly suitable compound is (I), which is obtained by the reductive methylation of Vat Yellow 2 (C167 300), using zinc and methyl-p-toluenesulphonate in alkaline solution. This compound, which was characterized by elemental analysis, is soluble in benzene (Xmax 408,422 nm; loge 4.36, 4.29), and gives weakly fluorescent solutions. It
is particularly resistant to photo-oxidation, and no decomposition was observed after irradiating an oxygen saturated solution for 48 h (600 W tungsten lamp). The compound was an efficient sensitizer of singlet oxygen, as shown by the rapid photo-oxidation of tetraphenylcyclopentadienone when solutions of the two were exposed to visible light 4. The reaction could be supressed by the known singlet oxygen quencher, diazabicyclo [2,2,2 loctane s. As (I) is an aromatic system that is, in effect, a prereduced form of a known photodegrading dye, it is unlikely to take part in further photoreduction processes with any degree of efficiency. To verify this, comparison experiments were carried out with (I) and 1-methoxyanthraquinone (II), an efficient hydrogen abstractor. The latter compound undergoes photoreduction in degassed isopropanol with a quantum yield of 0.536. In isopropanol-benzene mixtures (3:2 by vol) under nitrogen the dimethoxy compound (I) underwent only a slow irreversible fading, in contrast to the rapid reversible photoreduction of (II). Under conditions of equal light absorption, photoreduction of(II) was at least 200 times more rapid than the fading of(I). Thus (I) can be regarded as a useful sensitizer of singlet oxygen that is free from the usual complications of competing hydrogen abstraction. Photodegradation of regenerated cellulose film was most conveniently followed by measurement of the intrinsic viscosity [r/] of solutions in Cadoxen. Measurements were carried out at 30°C with an Ubbelohde suspended-level dilution viscometer, and the viscosity of solutions measured
~S Ph
OMe
$___~:h
OM¢ 0
(I)
o
{~1
over a range of concentrations. The intrinsic viscosity of the undergraded samples had an average value of 1.30 dl/g. Cellulose films freed from plasticizer were immersed in benzene solutions of(I) (4 x 10-4M) and were irradiated with a fluorescent lamp (aw) for 530 h. Control samples containing no sensitizer were irradiated at the same time. The unsensitized samples showed a 13% decrease in intrinsic viscosity, whereas the sensitized samples showed a corresponding decrease of 30%. Thus it can be concluded that singlet oxygen causes degradation of cellulose. It was of interest to examine the relative efficiencies of the singlet oxygen and hydrogen abstraction mechanisms, and thus a comparison experiment was carried out with (I) and (II). These two compounds absorb over roughly the same region of the spectrum above 350 nm, and so rate differences due to different light absorption characteristics could be minimized by using appropriate concentrations of the two compounds. Cellulose fdms were immersed in benzene solutions of (I)
POLYMER, 1976, Vol 17, December 1113
R--NH--C
O II
I I (CH2)4 f Y I (CH2)4 I 0 I
O
0
RL--NH--C
II
R--NH --C A . -CO 2
R'NH"
II
/o
°
+
I
+H"
Y
R'NH 2
CH2--CH 2
0
R--NH--C
I
O il
-
C5
+
CH 2j
i H2
CH2--CH 2
1
-~+RNCO + H"
(I)
(rn) O
R--N
--~
.
,, RNCO+~H2(CH2)3__OH+~, -
+H',
CH3CH2CH2(]H2OH
(2)
(rT) ' (CH2) 4
J
Y
RNCO+CH2=CHCH 2CH 2O H + H Y - -
IV)
I atom (sulphur in the case of thioether and oxygen in the case of ethers used). If the bond ~ C - O - in polyurethane undergoes cleavage with subse-
quent hydrogen migration, instead of decarboxylation, a chain with OH groups at the ends can be formed. This prevents the cyclization reactions and
I
can lead to formation of alcohols with four carbon atoms (IV or V, scheme 2). This procedure can be accompanied by the Y - C bond cleavage with or without hydrogen atom migration as showri in scheme (2). The method presented here has been found useful for the identification of the diol constituent degradation products of polyurethanes of the type described This makes possible a partial qualitative characterization of polyurethane composition.
REFERENCES 1 Lesiak, T. and Prewysz-Kwinto, A. Chem. Stosow. 1972, 16, 259 2 Lesiak, T. and Prewysz-Kwinto, A. Chem. Stosow. 1974, 18,471 3 Lesiak, T. and Prewysz-Kwinto, A. Chem. Stosow. 1975, 19,477 4 Lesiak, T., Pielichowski, J. and PrewyszKwinto, A. Chem. Stosow. 1975, 19, 221 5 Hetper, J. and Zag6rski, W. Polimery 1973, 18, 327 6 Newell, J. E. 'Theory and Application of Gas Chromatography in Industry and Medicine', (Eds H. S. Kroman and S. R. Bender), Grune and Stratton, New York, 1968 7 Groten, B. AnaL Chem. 1964, 36, 1206 8 Corish, P. J. AnaL Chem. 1959, 31, 1298 9 Takeuchi, T., Tsuge, S. and Okumoto, T. J. Gas Chromatogr. 1968, 6,542
Letters
Chain atoms between entanglements and cross-sectional area per chain
We recently demonstrated a linear l o g - l o g correlation of slope - 3 between the ratio C2/C1 of the M o o n e y Rivlin constants (at 2C1 = 0.2 N/mm 2) and cross-sectional area per polymer chain 1"~. It was suggested 2 that either strain induced local order or chain entanglements might be involved in what appeared to be an intermolecular effect. Kraus and Moczvgemba 3 had proposed that the molecular origin of the C2 term was a result, at least in part, o f chain entanglements. Preliminary examination of an extensive tabulation by Porter and Johnson 4 of entanglement distances indicated a positive correlation with cross-sectional area per polymer chain. However, the scatter in data points was so great as to discourage any firm conclusions. The entanglement data in the literature
have now been examined in detail and a preliminary strong correlation has been made. A specific tabulation by Fox and Alien s was used as a source of Nc, the number of chain atoms between entanglement points determined from concentrated or bulk viscosity measurements as a function of molecular weight. Cross-sectional areas per polymer chain were calculated by a method described previously 2, which differs only in minor details from that used by Vincent 6 for a different purpose. Figure 1 shows a l o g - l o g correlation between Nc and chain areas in nm 2. The line as drawn has a slope o f approximately 0.5. The marked deviation of atactic PMMA from the trend line is considered to arise because of molecular association between isoand syndio-tactic species present in a
1112 P O L Y M E R , 1976, Vol 17, December
PMMA 7. Porter and Johnson note that any form of association tends to promote low values o f Nc. A detailed study of this subject will be presented elsewhere s. As one in2000 I000 500 300 lO0
o.,o c;.,s o'2o
'O10' ' o oL-"5.8o Chain cross-section ( n m 2)
Figure I
Number of chain atoms, Nc, between entangelements from data of Fox and Alien s vs. chain cross-section in nm 2. A, PE; B, poly(decamethylene adipate); C, poly(decamethylene sebacate); D, poly(ecaproamide); E, poly(propylene oxide); F, PIB; G, PDMS; H, PVAc; I, atactic PMMA; J, atactic PS.
Letters
chides more bodies of data, and especially other measures of chain entanglement to be found in ref4, the scatter increases significantly. However, the trend shown in Figure I is not lost. Since both C2/C1 and Nc correlate with a common variable, area in nm 2, it follows that one should expect a linear correlation between C2/C1 and Nc. This indeed is the case with five data points for PE, cis-trans PBD, NR, PIB, and PDMS 8. This correlation does not necessarily mean that chain entanglements are solely responsible for the
References 1 Boyer, R. F. and Miller, R. L. Polymer 1976, 17, 925 2 Boyer,R. F. and Miller, R. L. Rubber Chem. TechnoL in press 3 Kraus,G. and Moczvgemba,G. A. ]. Polym. ScL {A-2) 1964, 277 4 Porter, R. S. and Johnson, J. F. Chem. Rev. 1966, 66, 1 5 Fox, T. G. and Allen, V. R. J. Chem. Phys. 1964, 41,344 6 Vincent, P. I. Polymer 1972, 13,558 7 Feitsma, E. L., de Boer, A. and Challa, G. Polymer 1975, 16, 515. This paper refers to earlier literature. 8 Boyer,R. F. and Miller, R. L. Rubber Chem. TechnoL in press
C2 term. The correlation line of Figure ] on a homogeneous set of data provides a criterion for judging and/or selecting from discordant literature values for these and other polymers.
Raymond F. Boyer and Robert L. Miller Midland Macromolecular Institute, 1910 W. St. Andrews Drive, Midland, Michigan 48640, USA (Received 19 July 1976)
The involvement of singlet oxygen in the sensitized photodegradation of cellulose The dye sensitized photo-oxidative degradation of cellulose has long been an area of practical and theoretical interest, but the mechanism of this process is still a controversial subject 1. Of the two principal mechanisms that have been proposed, namely the singlet oxygen theory 2 and the hydrogen (or electron) abstraction theory 3, the former has received relatively little credence. This is probably because it is relatively easy to demonstrate that a dye will photochemically abstract hydrogen or an electron from cellulose, whereas it is much more difficult to show unambiguously that singlet oxygen will react with this substrate. Almost all efficient sensitizers of singlet oxygen are good hydrogen abstractors (e.g. the quinonoid vat dyes, heterocyclic cationic dyes, and porphins), thus photodegradation studies with these compounds are ambiguous. The few known sensitizers that do not photoreduce (e.g. the acenes) are of little value because of their reactivity towards singlet oxygen. Non-photochemical sources of singlet oxygen are also suspect, as these systems are contaminated with other oxidizing species that could play an uncertain role in cellulose degradation. We have looked for new stable singlet oxygen sensitizers that have a minimal tendency to undergo photochemical reduction. A particularly suitable compound is (I), which is obtained by the reductive methylation of Vat Yellow 2 (C167 300), using zinc and methyl-p-toluenesulphonate in alkaline solution. This compound, which was characterized by elemental analysis, is soluble in benzene (Xmax 408,422 nm; loge 4.36, 4.29), and gives weakly fluorescent solutions. It
is particularly resistant to photo-oxidation, and no decomposition was observed after irradiating an oxygen saturated solution for 48 h (600 W tungsten lamp). The compound was an efficient sensitizer of singlet oxygen, as shown by the rapid photo-oxidation of tetraphenylcyclopentadienone when solutions of the two were exposed to visible light 4. The reaction could be supressed by the known singlet oxygen quencher, diazabicyclo [2,2,2 loctane s. As (I) is an aromatic system that is, in effect, a prereduced form of a known photodegrading dye, it is unlikely to take part in further photoreduction processes with any degree of efficiency. To verify this, comparison experiments were carried out with (I) and 1-methoxyanthraquinone (II), an efficient hydrogen abstractor. The latter compound undergoes photoreduction in degassed isopropanol with a quantum yield of 0.536. In isopropanol-benzene mixtures (3:2 by vol) under nitrogen the dimethoxy compound (I) underwent only a slow irreversible fading, in contrast to the rapid reversible photoreduction of (II). Under conditions of equal light absorption, photoreduction of(II) was at least 200 times more rapid than the fading of(I). Thus (I) can be regarded as a useful sensitizer of singlet oxygen that is free from the usual complications of competing hydrogen abstraction. Photodegradation of regenerated cellulose film was most conveniently followed by measurement of the intrinsic viscosity [r/] of solutions in Cadoxen. Measurements were carried out at 30°C with an Ubbelohde suspended-level dilution viscometer, and the viscosity of solutions measured
~S Ph
OMe
$___~:h
OM¢ 0
(I)
o
{~1
over a range of concentrations. The intrinsic viscosity of the undergraded samples had an average value of 1.30 dl/g. Cellulose films freed from plasticizer were immersed in benzene solutions of(I) (4 x 10-4M) and were irradiated with a fluorescent lamp (aw) for 530 h. Control samples containing no sensitizer were irradiated at the same time. The unsensitized samples showed a 13% decrease in intrinsic viscosity, whereas the sensitized samples showed a corresponding decrease of 30%. Thus it can be concluded that singlet oxygen causes degradation of cellulose. It was of interest to examine the relative efficiencies of the singlet oxygen and hydrogen abstraction mechanisms, and thus a comparison experiment was carried out with (I) and (II). These two compounds absorb over roughly the same region of the spectrum above 350 nm, and so rate differences due to different light absorption characteristics could be minimized by using appropriate concentrations of the two compounds. Cellulose fdms were immersed in benzene solutions of (I)
POLYMER, 1976, Vol 17, December 1113