Energy transfer in illuminated PMMA

Letters Energy transfer in illuminated P M M A It has been known for a long time that y-irradiation of poly(methyl methacrylate) (PMMA) produces degradation of the polymer. For every 61 eV of ionizing energy absorbed, there is a main chain break 1, 3. However, when the polymer contains an aromatic as a solute, the amount of chain break is lowered7, owing to the protection by the solute on the polymer. For instance, PMMA containing 5 ~o by wt of pyrene is completely protected from chain break by ionizing radiation s. Others such as p-terphenyl, xylene and benzene are also effective in this protective action. Wundrich 4 attributed the inhibiting effect of pyrene and naphthalene on the degradation of 7-irradiated PMMA to energy transfer from the polymer to the aromatic solute. Aromatic amines added to PMMA are also instrumental in quenching degradation by 7-irradiation 5, 6; the protection provided by the guest molecules on the host is explained by the transfer of electrons from the amines to the irradiated polymer, since the cation radicals of amines have been detected in the latter 7. Thin film samples of PMMA have been illuminated, using an Osram HBO 200W super high pressure mercury arc and a stable radical in the polymer was found at room temperature. The electron spin resonance (e.s.r.) spectrum of this 9-line radical is shown in Figure 1, similar to the e.s.r, spectrum observed in 7-irradiated PMMAS. This radical has been identified as the --CH2C(CHs)COOCH3 species9. Since the effective shortest wavelength of the mercury lamp is 253.7nm, corresponding to a 4.9eV photon, it is clear that the radical is produced by polymer segments in the highly excited state following the adsorption of the u.v. light.

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Figure 1 E.s.r. spectrum of u.v. irradiated poly(methyl methacrylate) film at room temperature. The g-line signal is due to the radical --CH2C(CHs)COOCH3. The small sharp line denoted by * is due to the quartz signal. The field strength increases from left to right

60 POLYMER,1974, Vol 15, January

It appears, therefore, that the polymer is capable of transmitting the excitation along the polymer chain until it is localized at an appropriate site, at which a carbon--carbon bond dissociation results. Evidence of excitation energy transfer in PMMA comes from the observation that polymer films containing various solutes exposed to u.v. light for the same length of time as the solute-free polymer contain the above radicals at concentrations much lower than those of the latter. For example, triphenylamine, triphenylphosphine or naphthalene, present separately in polymer samples as a guest molecule to the extent of 2-3 mol ~o (the monomer of the plastic is used as the molecular weight) will lower the radical concentration by 50yo. Polymer films containing 8 mol ~o of any of the above three solutes have no radical signal within the sensitivity of the e.s.r, spectrometer. Apparently the solute molecules act as energy sinks in the polymer. The excitation energy in the polymer (which may exist as excitons) is transmitted to the solute molecules, in preference to being localized in the polymer chains, thus resulting in decreased chain scission. The protective effect by the solute on the polymer is not due to a filter effect on the incident photons from the light source; this possibility is precluded by the work of Gardner et aL z. Neither is it likely that the observed lowerings of radicals in the solute-containing polymer are due to radical scavenging, since the mobility of the solute molecules is very limited, in view of the fact that the quenching effect persists even at liquid nitrogen temperature 5. Other molecules used in the same manner are found to be ineffective as energy interceptors, viz. tetracyanoethylene (a very efficient electron scavenger) and brilliant green (a strongly absorbing dye) have no effect on the radical formation in the illuminated polymer. The PMMA films, after exposure to the intense radiation from the mercury arc, acquire a light brown colour. When a piece of the irradiated film is observed in a Cary 14 spectrophotometer, a rather sharp band appears at 342.5nm, as shown in Figure 2. It has not been ascertained whether this band is related to the radical spectrum shown in Figure 1. The following is a brief description of the experimental procedure. The polymer, obtained from Borden Chemical Co., was dissolved in ethyl acetate and precipitated in cyclohexane. The dried, gummy polymer was redissolved in acetone. Thin films of the material were obtained by evaporating the acetone solution on a mercury surface very slowly. The films were then lifted from the mercury surface, cut to appropriate dimensions and rolled into cylinders and inserted into quartz tubings (about 9 mm o.d.) and evacuated overnight to a residual pressure of about 5 x 10-5 mmHg on a grease-free vacuum manifold. The quartz tubes were then flame-sealed, placed in an unsilvered quartz Dewar containing ice-water and photolysed by the Osram lamp located 6in ( ~ 15cm) away from the polymer sample. After irradiation, the

Letters I

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Quantitative morphological characterization of semi-crystalline PET Introduction

In this report it is shown that semi-crystalline poly(ethylene terephthalate) (PET) samples with the same degree of crystallinity but different morphological structures are obtained by suitable crystallization and annealing treatments. The morphological structure of the samples is related to their mechanical properties.

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Figure 2 Optical spectrum of u.v. irradiated poly(methyl methacrylate) film. A, before irradiation; B, after 80rain of irradiation. The band maximum is at 342.5nm

sample tube was inserted into the cavity of the Varian Associates 4500 e.s.r, spectrometer for radical detection. For optical work, a piece of the polymer film was cut to size and inserted into a special cell of 2 mm optical path (from Pyrocell Co.). After evacuation and illumination as described above, the optical spectrum was obtained in a Cary 14 spectrophotometer. More detailed work on the kinetics of radical growth and decay and on the effects of various solutes on the radical species is being carried out. P. K. W o n g Department of Chemistry, Queensborough Community College, City University of New York, Bayside, NY 11364, USA, and Brookhaven National Laboratory, Upton, NY 11973, USA (Received ~ October 1973)

References

1 Alexander, P. and Charlesby, A. Nature 1954, 173, 578 2 Alexander, P., Charlesby, A. and Ross, M. Proc. R. Soc. 1954, A223, 392 3 Gardner, D. G. and Epstein, L. M. J. Phys. Chem. 1961, 34, 1653 4 Wundrich, K. '3rd Tihany Symposium on Radiation Chemistry' (Eds J. Dob6 and P. Hedvig), Akadrmia6 Kiadr, Budapest, 1972, Vol. 1, p 747 5 Bagdasar'yan, Kh. S., Krongauz, V. A. and Kardash, N. S. Pror. Akad. ScL USSR (Chem. Ser.) 1962, 144, 37a 6 Milyutinskaya, R. I. and Bagdasar'yan, Kh. S. Rum. J. Phys. Chem. 1964, 38, 419 7 Borovkova, L. Ya. and Bagdasar'yan, Kh. S. High Energy Chem. 1967, 1,295 8 Kaul, W. and Kevan, L. '3rd Tihany Symposium on Radiation Chemistry' (Eds J. Dob6 and P. Hedvig), Akad~miafi Kiadr, Budapest, 1972, Vol 1, p 919 9 Iwasaki, M. and Sakai, Y. J. Polym. ScL (,4) 1969, 7, 1537

PET sheets supplied by Agfa-Gevaert, Antwerp, were thermally crystallized and annealed in an oven under vacuum. The volume crystallinity, X~, was obtained from density measurements. Small-angle X-ray scattering (SAXS) measurements were made with a Kratky camera (slit collimation) and desmeared using a computer program kindly provided by Vonk t. Mechanical measurements were performed with an Instron testing machine. PET samples with similar degree of crystallinity were prepared in two different ways. Sample I was crystallized from the glassy state at 200°C; the other samples were crystallized at 100°C (samples II and III) and at room temperature in the presence of acetone (sample IV). They were heated in different ways to 200°C and annealed at this temperature for a certain time (see Table 1). The crystallinity of samples II, III and IV was measured after the annealing. Results and Discussion Morphological structure from S A X S . Desmeared SACKS curves of samples I to IV exhibit one maximum which shifts to greater angles from I to IV. The Tsvankin analysis 2 as modified by Buchanan 3 was applied to the desmeared scattering curves; the morphological parameters are listed in Table 1. The experimental observed long period, d, decreases considerably from sample I to IV and the calculated values obtained for the mean long period, L, the average lamellar thickness, /, and the average thickness of the amorphous regions, a, follow the same trend. The linear crystallinities of these samples, k [=l/(a+l)], which corresponds to the one-dimensional alternation of crystalline and amorphous domains, are higher than the corresponding volume crystallinities, X~. This suggests that part of the amorphous phase is outside the lamellar bundles. Indeed, k is not related to amorphous regions which do not form part o f the regular alternating two phase structure 2. It should be noted that in the temperature range between 100°C and 200°C, there is a gradual increase in Xv and k. The structural changes induced by thermal treatments of sample V depend on the heating rate (Table 1, samples II and III). For sample III (slow heating) a decrease of the mean long period is observed, suggesting interlamellar crystallization (see evolution of L, k and a). In sample II (quickly heated) the mean periodicity is almost constant. However, the values of k and I increase while a decreases. Tentatively this is ascribed to intralamellar crystallization, i.e., incorporation of interlamellar amorphous chain segments into the existing lamellae. It is clear that during the heating process lamellar thickening

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