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Molecular orientation in PET studied by polarized Raman scattering
Letter Molecular orientation in PET studied by polarized Raman scattering We describe here the first quantitative experimental determination, as far as is known, of molecular orientation in a polymer by polarized Raman spectroscopy. For simplicity we have restricted ourselves to a transversely isotropic system by choosing uniaxially oriented specimens of poly(ethylene terephthalate) (PET), although the theory has been established for more general systems in a previous publicationL
Theory In the Raman experiment the intensity of the scattered radiation, Is, is given by: i
2
In this equation, I0 is a constant depending on the incident light intensity and instrumental factors, the direction cosines (11,/2, 13), (l~, l~, l~) define, respectively, the polarization directions of the incident light and the analyser (with respect to a set of axes O--xlx2x3 fixed in the sample) and alj is the (jth component of the Raman tensor for the vibration under examination. The scattered light intensity can therefore be regarded as depending on the experimentally chosen direction cosines l~ and lj, together with quantities Zaljavq, which depend on the distribution of molecular orientations and the principal components al, a2, aa of the Raman tensor. For the case of uniaxial symmetry, the Raman scattering depends on the distribution of orientations only through cos20 and cos40. The angle 0 is the angle between the molecular chain axis and the draw direction and the averages are taken over the distribution of orientations in the polymer. We shall assume that, in addition to the overall uniaxial statistical symmetry of the samples, the individual molecular chains have no preferred orientation around the chain axis. We shall further assume that for the particular Raman line on which measurements have been made the Raman tensor has cylindrical symmetry (~1 = a2 # ~3) and that its unique axis makes an angle ~, with the chain axis. If 0x3 is chosen parallel to the unique axis of the sample then we can write the quantity ~o~i]O~pqas:
Zoqjo~vq= 16zr4No ~
[2/(2• + 1)]l/2MzooDmoA~JoVoq (2)
/=0,2,4
[see equations (15) and 09) of ref. 1], where the summation on the left is over all No chain segments and ijpq is of the form iijj or ijij. In this expansion, the terms Mr00 are coefficients in the expansion of the molecular orientation distribution
398 POLYMER, 1973, Vol 14, August
in a series of Legendre polynomials Pl (cos0), the Dl00 are equal to Pl (cosy) (and take fixed values for a particular line) and the ~*100 zi~pq are linear sums of quadratic terms in the principal components of the Raman tensor, i.e. terms such as ~ , a1~2 etc. (see Table I of ref. 1). For an isotropic sample it follows from equation (2) that: (E~)/(Ea~i) = (1 --2r+r2)/(8r2+4r+3)
(3)
where r=al/a3. Thus by determining the scattered intensity from an oriented sample for at least three suitably chosen combinations of polarization vectors and combining this with the result for isotropic samples, we can find M200 and M400, and hence cos20 and cos40, together with the value of al/aa.
Experimental The PET samples were produced, by melt spinning, in the form of thin tapes with cross-section approximately 1.5 x 10-am by 1.0 x 10-4m. The number-average molecular weight was estimated from intrinsic viscosity measurements to be 2.3 × 104. The samples were subsequently oriented by drawing around a smooth stationary 'pin', heated to 80°C, located between rollers rotating at different rates. An additional specimen was produced by cutting a sample from an injection moulded plaque so that its cross-section was 3mm square and cold drawing it to a draw ratio of 3.25. X-ray photographs showed the crystallinity of the samples to be low, and measurements both of dimensions and refractive indices {at 551 nm) showed that, to a very good approximation, they were uniaxially oriented. Data from two randomly oriented samples were used. One was produced by annealing an as-spun tape at 90°C in an air oven until no birefringence was observable and one was produced from an injection moulded plaque. The Raman intensity measurements were made using a Coderg PHO spectrometer and a CRL 52A argon ion laser tuned to 488 nm. The tape samples were mounted parallel to the spectrometer slit. The partially focused laser beam was incident normally on them and the scattered light was collected in directions making approximately 180 ° with the incident light direction. The incident and scattered light polarization vectors could be chosen parallel or perpendicular to the length of the tapes so that intensities proportional to E~g3, Z ~ l , E~12z and Ea~l could be determined. The polarization of the incident beam was checked after it had passed through the specimen and the depolarization was found to be less than 1 ~. It was therefore assumed that the depolarization of both the incident and Raman-scattered light by its passage through the surface and the thickness of the specimen could be neglected. The Raman line chosen for this work was that at 1616cm -1, which is due 2, 3, to the benzene ring mode 8a (in Wilson's notation) the form of which is shown in Figure 1. The Raman tensor has one of its principal
Letter
5
mechanisms will be deferred until more extensive studies have been completed. Figures 2 and 3 also show points for the oriented bulk sample calculated from intensities proportional to Za~, E~122 and Z ~ s by assuming r = - 0 . 1 8 4 . The predicted value for the intensity proportional to Y'~2 was 21.1 and the experimental value was 20+3. This suggests that the assumption that r is independent of the distribution of orientations is reasonable. (The largest intensity, that proportional to Y~3, was 89 _+3 units.)
3
Conclusion
Figure 1 Form of vibration p-disubstituted benzenes
8a
in
axes parallel to CI-C4 (corresponding to ~3) and one perpendicular to the plane of the ring, corresponding to ~1 or ~2. The assumption that al = a~ is not easy to justify except by the results obtained, but inspection of the forms of the ,~i~p~ " l m n shows that provided these two components are of the same sign and not very different in magnitude equation (2) with al = ~2 is a very good approximation, particularly if 7 is small. The angle 7 is the angle between the C1-C4 direction in the benzene ring and the chain axis direction and has been assumed to be the same as in the crystal phase. It is calculated from the data of Daubeny et al. 4 to be 19o12 '. The chosen line does not appear to overlap any other Raman line and the peak height above the baseline was taken as a measure of the intensity. The directly measured intensities were corrected for the differential polarization sensitivity of the spectrometer and the inequality of the intensities of the exciting line for the two polarization directions. The intensities proportional to Z~8 and Z ~ l were then found to be equal within experimental error. The bulk samples were illuminated using the more conventional right-angle viewing technique, care being taken that the incident and scattered beams passed either through a large thickness of material, so that polarization scrambling was complete, or through such a small thickness that essentially no scrambling took place. From these samples it was possible to obtain intensities proportional to Zc~2, Zc~s, Ea~2, Ea~ and E%22. Results
The values of r determined from the data on the random samples were -0.192, -0"176. The mean value was used, together with the intensity data for the tapes, to calculate values of ~ and ~ from equation (2). A good straight line is obtained when co--ffff~is plotted against birefringence (Figure 2), as predicted theoreticallyS, 6, and the indicated maximum birefringence is 0.24, in good agreement with the value 0.23 given by Kashiwagi et aLL Figure 3 shows the relationship between cos--0-~0 and co--ffffa-0compared with theoretical curves for the affine rubber elasticity model 8 and for the pseudo-affine deformation schemeL This comparison is only given here to show that the values of c ~ are also reasonable; detailed discussion of deformation
The results reported here confirm that polarized Raman spectroscopy can give quantitative information about molecular orientation in polymers. More detailed studies are to be undertaken on PET and results will shortly be published of a detailed study of oriented poly(methyl methacrylate). IO
08 o
%
o
8o.e 04 000
004
008
i
L O.ii6
O 2 B irefringence
I
I
020
I
I
0.24
Figure 2 Variation of cos28 obtained from polarized Raman scattering with birefringence. O, Tape specimens; O, bulk specimen
0.8
/
0.6
/
.,4, 04
0.2 0.3
'
OI.5
'
J
O "7
,
O .9 cos 2 0 Figure 3 Comparison of c ~ and ~ obtained from polarized Raman scattering. 0 , Tape specimens; O, bulk specimen. Lower curve shows relationship between c---o-sT8and c-o-sT8according to affine rubber elasticity model and upper curve shows relationship according to the pseudo-affine deformation scheme
POLYMER, 1973, Vol 14, A u g u s t
399
Letter Acknowledgements
References
W e h a v e h a d useful d i s c u s s i o n s w i t h several colleagues. I n a d d i t i o n , we w i s h to t h a n k M r J. N o b b s a n d M r A . C u n n i n g h a m f o r p r e p a r i n g the o r i e n t e d s p e c i m e n s a n d ICI Fibres Division, Harrogate for providing melts p i n n i n g facilities. O n e o f us (J.P.) is i n d e b t e d to t h e Science R e s e a r c h C o u n c i l f o r t h e a w a r d o f a m a i n t e n a n c e grant.
1 Bower, D. I. d. Polym. Sci. (Polym. Phys.) 1972, 10, 2135 2 Varsanyi, G. 'Vibrational Spectra of Benzene Derivatives', Academic Press, New York, 1969, p 152 3 Julien-Laferriere, S. and Lebas, J-M. Spectrochim. Acta 1971, 27A, 1337 4 Daubeny, R. de P., Bunn, C. W. and Brown, C. J. Prec. R. Soc. 1954, 226A, 531 5 Hermans, P. H. 'Physics and Chemistry of Cellulose Fibres', Elsevier, New York, 1949, Part 2, Ch 4 6 Pinnock, P. R. and Ward, I. M. Br. d. AppL Phys. 1964, 15, 1559 7 Kashiwagi, M., Cunningham, A., Manuel, A. J. and Ward, 1. M. Polymer 1973, 14, 111 8 Roe, R. J. and Krigbaum, W. R. J. AppL Phys. 1964, 35, 2215 9 Ward, I. M. 'Mechanical Properties of Solid Polymers', Wiley, London, 1971, p 258
d. P u r v i s , D. I. B o w e r a n d I. M. W a r d
Department of Physics, University of Leeds, Leeds L$2 9JT, UK (Received 9 May 1973; revised 24 May 1973)
Book Review I U P A C International Conference on Chemical Transformations of Polymers Edited by R. Redo
Butterworths, London, 1972,300 pp. £.8.50 It is a sobering reflection on progress in chemical transformations in polymers that if Charles Goodyear were miraculously resurrected for the Conference whose Plenary and 18 Main Lectures make up this book, he would have found the vulcanization of rubber by sulphur still a subject for discussion, and could probably have contributed a few sensible remarks on the practical aspects of a process which he discovered in 1839. Wohler, who transmuted ammonium cyanate to urea only 11 years earlier, would hardly fare so well at a modern conference on the metamorphosis of small molecules. Insofar as 'chemical transformation' implies purposeful improvement, polymer chemists are scarcely off the starting-line in comparison with their small-molecule confreres, partly of course because of the late recognition of large molecules, but also because until relatively recently most of the effort expended on polymeric transformations was designed to avoid them; the more appropriate term 'degradation' was not coined by a man who was pleased with what he saw. This preventive aspect of polymer transformation underlies 7 of the main lectures, with predominant emphasis on photochemical processes which, combined with ketone carbonyl groups produced by atmospheric oxidation, are the main cause of deleterious 'weathering'. The general background of energy transfer from excited states in small molecules is discussed by Fox, who goes on to examine emission spectra (fluorescent/phosphorescent) in polymers, where the photophysical process may be influenced by the orbital overlap of a succession of chromophores allowing energy migration along a chain to a weak link 'energy-trap'. Guillet examines practical aspects of u.v. stabilization with screening and absorbing agents, and mentions briefly the formation of chromophores in polyolefins by oxidative processes. (The testing and performance of antioxidants are reviewed by Scott, and that of specific u.v. absorbers by Heller and Blattman.) Photochemical aspects of degradation are rounded off by Golub on polydienes, a specialized but important area which is starting to make progress again after twenty years' neglect following Bateman's pioneering work in 1945. There is a satisfying chapter by Grassie on the thermal degradation of acrylates (which fragment) and methacrylates (which unzip)--satisfying because the careful and detailed experiments described lead to an integrated mechanism which resolves the question of why such closely allied polymers break down in different ways. Finally, a relative newcomer to the degradation scheme is the break down of polymers caused by Ziegler-Natta catalyst residues, on which Vesely and coworkers make a start by analysing similar reactions of small molecules to isolate individual steps in the overall process.
400 POLYMER, 1973, Vol 14, August
Turning to purposeful transformations, the formation of block, graft and network copolymers (a distant descendant of Goodyear's vulcanization) is most commonly done with carbanionic systems, here reviewed by Rempp and Franta, but there is an interesting account by Ashworth, Bamford and Smith of network formation in a free-radical system using molybdenum carbonyl catalysts, where polymer chains containing C1 atoms yield free radicals (which subsequently act as growth sites) in a redox process. The vulcanization of rubbers using peroxides instead of sulphur, giving a simpler system which is also chemically related to electron irradiation curing has a chapter by Loan, and the preparation of semi-permeable membranes is discussed by Pegoraro. The general behaviour of free radicals in polymeric systems covers three chapters: Braun on stable polyradicals (in German), Butiagin on the decay of radicals, and a brief account by Jenkins of the application of the Patterns system to polyradicals. The starting-point for most polymer modification is the introduction of functional groups along the polymer chain, and since most of our knowledge of the reactivity of such groups is derived from their behaviour in small molecules, it is important to know if and how this reactivity is affected in polymeric surroundings. These 'polymeric effects' are discussed in detail by R. C. Schulz for three specific instances (optical racemization, electron donoraccepter complex formation, and the reactivity of chlorine in N-chlorinated nylon-6,6). Chemically attached groups scattered along the polymer chain may also have a profound direct effect on the physical properties of the polymer, acting as built-in plasticizers or modifying crystallization behaviour. This complex field is reviewed by Plat6, with a final hint of things to come in the effects of such 'micromodification' on the catalytic activity of synthetic proteolytic enzymes. The remaining two lectures scarcely fit in the general context of the book--Chapiro on controlled propagation in associated monomer aggregates and Okamura on solid-state polymerization--and though both competent reviews on their own ground add little to the main theme. Finally, the Plenary Lecture by Smets on the photochromic behaviour of polymeric systems is perhaps over-detailed on too narrow a front, considering the scope of the book as a whole. Reversible colour formation on exposure to light is a feature of certain small-molecule organic structures, and the author is chiefly concerned with the 'polymeric effect' on the photochromism of spirobenzopyrans, which is most marked in the solid state and depends strongly on the glass transition temperature of the polymer. One of the more interesting practical applications of such systems is the possibility of using photochromes to detect local motions in a polymer chain and to pinpoint secondary glass transition temperatures. Overall, the lectures are a rather diverse collection gathered somewhat uneasily under one roof. No single reader is likely to find more than two or three chapters of direct interest, and at £8.50 it is a book for the library rather than the private shelf.
R. O. Colclough
Theory In the Raman experiment the intensity of the scattered radiation, Is, is given by: i
2
In this equation, I0 is a constant depending on the incident light intensity and instrumental factors, the direction cosines (11,/2, 13), (l~, l~, l~) define, respectively, the polarization directions of the incident light and the analyser (with respect to a set of axes O--xlx2x3 fixed in the sample) and alj is the (jth component of the Raman tensor for the vibration under examination. The scattered light intensity can therefore be regarded as depending on the experimentally chosen direction cosines l~ and lj, together with quantities Zaljavq, which depend on the distribution of molecular orientations and the principal components al, a2, aa of the Raman tensor. For the case of uniaxial symmetry, the Raman scattering depends on the distribution of orientations only through cos20 and cos40. The angle 0 is the angle between the molecular chain axis and the draw direction and the averages are taken over the distribution of orientations in the polymer. We shall assume that, in addition to the overall uniaxial statistical symmetry of the samples, the individual molecular chains have no preferred orientation around the chain axis. We shall further assume that for the particular Raman line on which measurements have been made the Raman tensor has cylindrical symmetry (~1 = a2 # ~3) and that its unique axis makes an angle ~, with the chain axis. If 0x3 is chosen parallel to the unique axis of the sample then we can write the quantity ~o~i]O~pqas:
Zoqjo~vq= 16zr4No ~
[2/(2• + 1)]l/2MzooDmoA~JoVoq (2)
/=0,2,4
[see equations (15) and 09) of ref. 1], where the summation on the left is over all No chain segments and ijpq is of the form iijj or ijij. In this expansion, the terms Mr00 are coefficients in the expansion of the molecular orientation distribution
398 POLYMER, 1973, Vol 14, August
in a series of Legendre polynomials Pl (cos0), the Dl00 are equal to Pl (cosy) (and take fixed values for a particular line) and the ~*100 zi~pq are linear sums of quadratic terms in the principal components of the Raman tensor, i.e. terms such as ~ , a1~2 etc. (see Table I of ref. 1). For an isotropic sample it follows from equation (2) that: (E~)/(Ea~i) = (1 --2r+r2)/(8r2+4r+3)
(3)
where r=al/a3. Thus by determining the scattered intensity from an oriented sample for at least three suitably chosen combinations of polarization vectors and combining this with the result for isotropic samples, we can find M200 and M400, and hence cos20 and cos40, together with the value of al/aa.
Experimental The PET samples were produced, by melt spinning, in the form of thin tapes with cross-section approximately 1.5 x 10-am by 1.0 x 10-4m. The number-average molecular weight was estimated from intrinsic viscosity measurements to be 2.3 × 104. The samples were subsequently oriented by drawing around a smooth stationary 'pin', heated to 80°C, located between rollers rotating at different rates. An additional specimen was produced by cutting a sample from an injection moulded plaque so that its cross-section was 3mm square and cold drawing it to a draw ratio of 3.25. X-ray photographs showed the crystallinity of the samples to be low, and measurements both of dimensions and refractive indices {at 551 nm) showed that, to a very good approximation, they were uniaxially oriented. Data from two randomly oriented samples were used. One was produced by annealing an as-spun tape at 90°C in an air oven until no birefringence was observable and one was produced from an injection moulded plaque. The Raman intensity measurements were made using a Coderg PHO spectrometer and a CRL 52A argon ion laser tuned to 488 nm. The tape samples were mounted parallel to the spectrometer slit. The partially focused laser beam was incident normally on them and the scattered light was collected in directions making approximately 180 ° with the incident light direction. The incident and scattered light polarization vectors could be chosen parallel or perpendicular to the length of the tapes so that intensities proportional to E~g3, Z ~ l , E~12z and Ea~l could be determined. The polarization of the incident beam was checked after it had passed through the specimen and the depolarization was found to be less than 1 ~. It was therefore assumed that the depolarization of both the incident and Raman-scattered light by its passage through the surface and the thickness of the specimen could be neglected. The Raman line chosen for this work was that at 1616cm -1, which is due 2, 3, to the benzene ring mode 8a (in Wilson's notation) the form of which is shown in Figure 1. The Raman tensor has one of its principal
Letter
5
mechanisms will be deferred until more extensive studies have been completed. Figures 2 and 3 also show points for the oriented bulk sample calculated from intensities proportional to Za~, E~122 and Z ~ s by assuming r = - 0 . 1 8 4 . The predicted value for the intensity proportional to Y'~2 was 21.1 and the experimental value was 20+3. This suggests that the assumption that r is independent of the distribution of orientations is reasonable. (The largest intensity, that proportional to Y~3, was 89 _+3 units.)
3
Conclusion
Figure 1 Form of vibration p-disubstituted benzenes
8a
in
axes parallel to CI-C4 (corresponding to ~3) and one perpendicular to the plane of the ring, corresponding to ~1 or ~2. The assumption that al = a~ is not easy to justify except by the results obtained, but inspection of the forms of the ,~i~p~ " l m n shows that provided these two components are of the same sign and not very different in magnitude equation (2) with al = ~2 is a very good approximation, particularly if 7 is small. The angle 7 is the angle between the C1-C4 direction in the benzene ring and the chain axis direction and has been assumed to be the same as in the crystal phase. It is calculated from the data of Daubeny et al. 4 to be 19o12 '. The chosen line does not appear to overlap any other Raman line and the peak height above the baseline was taken as a measure of the intensity. The directly measured intensities were corrected for the differential polarization sensitivity of the spectrometer and the inequality of the intensities of the exciting line for the two polarization directions. The intensities proportional to Z~8 and Z ~ l were then found to be equal within experimental error. The bulk samples were illuminated using the more conventional right-angle viewing technique, care being taken that the incident and scattered beams passed either through a large thickness of material, so that polarization scrambling was complete, or through such a small thickness that essentially no scrambling took place. From these samples it was possible to obtain intensities proportional to Zc~2, Zc~s, Ea~2, Ea~ and E%22. Results
The values of r determined from the data on the random samples were -0.192, -0"176. The mean value was used, together with the intensity data for the tapes, to calculate values of ~ and ~ from equation (2). A good straight line is obtained when co--ffff~is plotted against birefringence (Figure 2), as predicted theoreticallyS, 6, and the indicated maximum birefringence is 0.24, in good agreement with the value 0.23 given by Kashiwagi et aLL Figure 3 shows the relationship between cos--0-~0 and co--ffffa-0compared with theoretical curves for the affine rubber elasticity model 8 and for the pseudo-affine deformation schemeL This comparison is only given here to show that the values of c ~ are also reasonable; detailed discussion of deformation
The results reported here confirm that polarized Raman spectroscopy can give quantitative information about molecular orientation in polymers. More detailed studies are to be undertaken on PET and results will shortly be published of a detailed study of oriented poly(methyl methacrylate). IO
08 o
%
o
8o.e 04 000
004
008
i
L O.ii6
O 2 B irefringence
I
I
020
I
I
0.24
Figure 2 Variation of cos28 obtained from polarized Raman scattering with birefringence. O, Tape specimens; O, bulk specimen
0.8
/
0.6
/
.,4, 04
0.2 0.3
'
OI.5
'
J
O "7
,
O .9 cos 2 0 Figure 3 Comparison of c ~ and ~ obtained from polarized Raman scattering. 0 , Tape specimens; O, bulk specimen. Lower curve shows relationship between c---o-sT8and c-o-sT8according to affine rubber elasticity model and upper curve shows relationship according to the pseudo-affine deformation scheme
POLYMER, 1973, Vol 14, A u g u s t
399
Letter Acknowledgements
References
W e h a v e h a d useful d i s c u s s i o n s w i t h several colleagues. I n a d d i t i o n , we w i s h to t h a n k M r J. N o b b s a n d M r A . C u n n i n g h a m f o r p r e p a r i n g the o r i e n t e d s p e c i m e n s a n d ICI Fibres Division, Harrogate for providing melts p i n n i n g facilities. O n e o f us (J.P.) is i n d e b t e d to t h e Science R e s e a r c h C o u n c i l f o r t h e a w a r d o f a m a i n t e n a n c e grant.
1 Bower, D. I. d. Polym. Sci. (Polym. Phys.) 1972, 10, 2135 2 Varsanyi, G. 'Vibrational Spectra of Benzene Derivatives', Academic Press, New York, 1969, p 152 3 Julien-Laferriere, S. and Lebas, J-M. Spectrochim. Acta 1971, 27A, 1337 4 Daubeny, R. de P., Bunn, C. W. and Brown, C. J. Prec. R. Soc. 1954, 226A, 531 5 Hermans, P. H. 'Physics and Chemistry of Cellulose Fibres', Elsevier, New York, 1949, Part 2, Ch 4 6 Pinnock, P. R. and Ward, I. M. Br. d. AppL Phys. 1964, 15, 1559 7 Kashiwagi, M., Cunningham, A., Manuel, A. J. and Ward, 1. M. Polymer 1973, 14, 111 8 Roe, R. J. and Krigbaum, W. R. J. AppL Phys. 1964, 35, 2215 9 Ward, I. M. 'Mechanical Properties of Solid Polymers', Wiley, London, 1971, p 258
d. P u r v i s , D. I. B o w e r a n d I. M. W a r d
Department of Physics, University of Leeds, Leeds L$2 9JT, UK (Received 9 May 1973; revised 24 May 1973)
Book Review I U P A C International Conference on Chemical Transformations of Polymers Edited by R. Redo
Butterworths, London, 1972,300 pp. £.8.50 It is a sobering reflection on progress in chemical transformations in polymers that if Charles Goodyear were miraculously resurrected for the Conference whose Plenary and 18 Main Lectures make up this book, he would have found the vulcanization of rubber by sulphur still a subject for discussion, and could probably have contributed a few sensible remarks on the practical aspects of a process which he discovered in 1839. Wohler, who transmuted ammonium cyanate to urea only 11 years earlier, would hardly fare so well at a modern conference on the metamorphosis of small molecules. Insofar as 'chemical transformation' implies purposeful improvement, polymer chemists are scarcely off the starting-line in comparison with their small-molecule confreres, partly of course because of the late recognition of large molecules, but also because until relatively recently most of the effort expended on polymeric transformations was designed to avoid them; the more appropriate term 'degradation' was not coined by a man who was pleased with what he saw. This preventive aspect of polymer transformation underlies 7 of the main lectures, with predominant emphasis on photochemical processes which, combined with ketone carbonyl groups produced by atmospheric oxidation, are the main cause of deleterious 'weathering'. The general background of energy transfer from excited states in small molecules is discussed by Fox, who goes on to examine emission spectra (fluorescent/phosphorescent) in polymers, where the photophysical process may be influenced by the orbital overlap of a succession of chromophores allowing energy migration along a chain to a weak link 'energy-trap'. Guillet examines practical aspects of u.v. stabilization with screening and absorbing agents, and mentions briefly the formation of chromophores in polyolefins by oxidative processes. (The testing and performance of antioxidants are reviewed by Scott, and that of specific u.v. absorbers by Heller and Blattman.) Photochemical aspects of degradation are rounded off by Golub on polydienes, a specialized but important area which is starting to make progress again after twenty years' neglect following Bateman's pioneering work in 1945. There is a satisfying chapter by Grassie on the thermal degradation of acrylates (which fragment) and methacrylates (which unzip)--satisfying because the careful and detailed experiments described lead to an integrated mechanism which resolves the question of why such closely allied polymers break down in different ways. Finally, a relative newcomer to the degradation scheme is the break down of polymers caused by Ziegler-Natta catalyst residues, on which Vesely and coworkers make a start by analysing similar reactions of small molecules to isolate individual steps in the overall process.
400 POLYMER, 1973, Vol 14, August
Turning to purposeful transformations, the formation of block, graft and network copolymers (a distant descendant of Goodyear's vulcanization) is most commonly done with carbanionic systems, here reviewed by Rempp and Franta, but there is an interesting account by Ashworth, Bamford and Smith of network formation in a free-radical system using molybdenum carbonyl catalysts, where polymer chains containing C1 atoms yield free radicals (which subsequently act as growth sites) in a redox process. The vulcanization of rubbers using peroxides instead of sulphur, giving a simpler system which is also chemically related to electron irradiation curing has a chapter by Loan, and the preparation of semi-permeable membranes is discussed by Pegoraro. The general behaviour of free radicals in polymeric systems covers three chapters: Braun on stable polyradicals (in German), Butiagin on the decay of radicals, and a brief account by Jenkins of the application of the Patterns system to polyradicals. The starting-point for most polymer modification is the introduction of functional groups along the polymer chain, and since most of our knowledge of the reactivity of such groups is derived from their behaviour in small molecules, it is important to know if and how this reactivity is affected in polymeric surroundings. These 'polymeric effects' are discussed in detail by R. C. Schulz for three specific instances (optical racemization, electron donoraccepter complex formation, and the reactivity of chlorine in N-chlorinated nylon-6,6). Chemically attached groups scattered along the polymer chain may also have a profound direct effect on the physical properties of the polymer, acting as built-in plasticizers or modifying crystallization behaviour. This complex field is reviewed by Plat6, with a final hint of things to come in the effects of such 'micromodification' on the catalytic activity of synthetic proteolytic enzymes. The remaining two lectures scarcely fit in the general context of the book--Chapiro on controlled propagation in associated monomer aggregates and Okamura on solid-state polymerization--and though both competent reviews on their own ground add little to the main theme. Finally, the Plenary Lecture by Smets on the photochromic behaviour of polymeric systems is perhaps over-detailed on too narrow a front, considering the scope of the book as a whole. Reversible colour formation on exposure to light is a feature of certain small-molecule organic structures, and the author is chiefly concerned with the 'polymeric effect' on the photochromism of spirobenzopyrans, which is most marked in the solid state and depends strongly on the glass transition temperature of the polymer. One of the more interesting practical applications of such systems is the possibility of using photochromes to detect local motions in a polymer chain and to pinpoint secondary glass transition temperatures. Overall, the lectures are a rather diverse collection gathered somewhat uneasily under one roof. No single reader is likely to find more than two or three chapters of direct interest, and at £8.50 it is a book for the library rather than the private shelf.
R. O. Colclough