- Email: [email protected]
Study of ion beam induced depolymerization using positron annihilation techniques
Nuclear Instruments and Methods in Physics Research B 175±177 (2001) 605±609
www.elsevier.nl/locate/nimb
Study of ion beam induced depolymerization using positron annihilation techniques O. Puglisi
a,*
, M.E. Fragal a a, K.G. Lynn b, M. Petkov b, M. Weber b, A. Somoza c, A. Dupasquier d, F. Quasso d
a
Dipartimento di Scienze Chimiche, Universit a di Catania, viale A. Doria 6, I-95125 Catania, Italy b Department of Physics, Washington State University, Pullman, WA, USA c UNCentro, IFIMAT, Pinto 366, Tandil, Argentina d Dipartimento di Fisica del Politecnico, Istituto Nazionale di Fisica della Materia, Milano, Italy
Abstract Ion beam induced depolymerization of polymers is a special class of ion beam induced chemical reaction which gives rise to catastrophic ``unzipping'' of macromolecules with production of large amounts of the monomer, of the order of many hundreds monomer molecules per each macromolecule. The possible modi®cation of the density at microscopic level prompted us to undertake a study of this eect utilizing positron annihilation techniques in Poly(methylmethacrylate) (PMMA) before and after bombardment with He 300 keV ions at 200°C. Preliminary results shown here indicate that before bombardment there is a reproducible dependence of nano-hole distribution on the sample history. Moreover at 200°C we do not detect formation of new cavities as a consequence of the strong depolymerization that occurs under the ion beam. The possible correlation of these ®ndings with transport properties of PMMA at temperature higher than the glass transition temperature will be discussed. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 61.80; 82.30.L; 34.80 Keywords: PMMA; Ion beam; Unzipping; Depolymerization; Nano-cavity; Positronium
1. Introduction Ion beam induced depolymerization of macromolecular targets is a special class of ion beam induced chemical reaction which gives rise to catastrophic ``unzipping'' of the macromolecules with
*
Corresponding author. Tel.: +39-095-33-0533; fax: +39095-337-841. E-mail address: [email protected] (O. Puglisi).
production of large amounts of the monomer [1]. Unzipping under the ion beam has only been observed in Poly(methylmethacrylate) (PMMA), but there is no any good reason why this phenomenon should be limited to this particular polymer, so that it is plausible that many other macromolecular solids may give rise to the same eect. In PMMA the yield of this scission is strongly dependent on the temperature of the bombarded macromolecular target. In particular, under the ion beam, it can only be observed at a temperature
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 0 3 4 3 - 3
606
O. Puglisi et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 605±609
higher than the glass transition temperature Tg (105°C), while at lower temperature no trace of the methyl methacrylate monomer could be detected [2]. For comparison, while under ion beam (He 300 keV) a temperature of 170°C is sucient for observing intense monomer evolution from the bombarded PMMA ®lm, without the ion beam the same ¯ux can only be obtained at 300°C [1]. According to what is known about the chemistry of this polymer [3], the intense monomer (methyl methacrylate) evolution under ion beam bombardment can be explained in terms of disappearance of whole macromolecules or of signi®cant portion of the macromolecular chain [4]. For the PMMA targets here reported this depolymerization involves the production of many hundreds of monomer molecules per macromolecule. This local mass loss could give rise to the formation of new nano-cavities in the macromolecular ®lm and some gas ``bubbling''. Prompted by this hypothesis we have undertaken a study of ion beam induced depolymerization in PMMA using bulk positronium lifetime techniques and variable energy positrons to detect the production and the size of the nano-holes forming the free volume in the macromolecules before and after bombardment with He 300 keV ions at 200°C. Preliminary results shown here indicate that (i) the nano-hole distribution in thick ®lms before bombardment has a reproducible dependence on the sample history and (ii) at temperature of 200°C we do not detect formation of new cavities as a consequence of the strong depolymerization that occurs under the ion beam. The possible explanation of these ®ndings in terms of re-organization of PMMA thin ®lm at temperature higher than the Tg will be discussed. 2. Experimental PMMA thin ®lms have been prepared by spin coating on silicon substrates from chloroform so was measured lutions. The ®lm thickness (8000 A) by means of a stylus pro®lometer (Tencor a-step). The PMMA samples were commercially available. They were nearly monodisperse with (weight av-
erage) molecular weight Mw 73 kDa and polydispersivity of the order of 1.07. Before spin coating purity and molecular weight parameters were checked by standard liquid chromatography techniques. After coating the Si wafers were cut in 4 cm2 squares and each piece was irradiated by 300 keV He . During irradiation the ion beam was swept along the sample surface and the current was kept below 0.1 lA/cm2 in order to avoid thermal eects. In these conditions the sample temperature does not increase more than 5°C. The basic pressure in the irradiation chamber was better than 1 10 5 Pa. For positron life time measurements, a stack of self-supporting PMMA foils was sandwiched with the positron 22 Na source between two Pb re¯ectors. The thickness of each foil was of the order of 10 lm. The lifetime spectra were collected by means of a conventional fast±fast coincidence setup, equipped with two NE111 25 mm 25 mm plastic scintillators, coupled to Philips XP2020 photomultiplier tubes. The resolution of the apparatus was about 230 ps. All the measurements were performed at room temperature. Each spectrum contained about 4 106 counts. For positrons annihilation spectroscopy (PAS) measurements, the positrons were implanted in the samples at dierent depths using a variable energy (0±70 keV) positron beam with a rate of 2 105 s 1 . The energy spectrum of the annihilation radiation was measured with a high-eciency high-purity Ge (HPGe) detector, with an energy resolution of 1.45 keV at 514 keV (85 Sr). Spectra with 2 106 events were acquired at each positron energy. Possible changes of the positronium (Ps) fraction, due to formation of cavities, were monitored by observing modi®cation of the spectrum in the 511 keV peak region and in the region of the maximum of 3c distribution. 3. Results and discussion Positrons, the antiparticles of the electron, can be used as a probe of the structure of the solids at a nanometric and sub-nanometric scale [5]. In the ®eld of polymer science, PAS is used as a nondestructive technique to observe the free volume
O. Puglisi et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 605±609
distribution [6]. The sensitivity of this technique to the presence and to the size of nano-cavities in a polymer is related to the formation in the cavities of Ps, i.e. the light isotope of hydrogen with a positron e taking the place of the proton. One fourth of the Ps atoms is formed in the para-state (p-Ps), which essentially annihilates in 2 c's by internal interaction of the two particles bound in the atom. The remaining fraction (3/4) is formed in the ortho-state (o-PS), which annihilates either in 3 c's by internal interaction or in 2 c's by pick-o, i.e. by interaction with the electrons in the walls of the cavity. Annihilation in 2 c's gives rise to a line in the energy spectrum of the annihilation radiation centered at 511 keV (annihilation energy 2m0 c2 ). This line is very narrow for p-Ps annihilation; in the case of annihilation of free positrons or of o-PS with the electrons of the medium, it is broadened by the Doppler eect related to the motion of the electrons. Annihilation in 3 c's gives a characteristic spectrum extended from 0 to 511 keV. However, in a nano-cavity, this annihilation channel is strongly suppressed by competition with the picko process. Under these conditions, the man life s of o-PS is related to the size of the hole where the atom is trapped. A spherical hole model [7] gives the following relationship between s and the radius R of the cavity: s
1
1
2ns 1
R 1 R sin 2p ; R0 2p R0
1
is an empirwhere R0 R dR, and dR 1:66 A ical parameter, related to the penetration of the Ps wavefunction into the bulk. When hole dimensions are distributed with a width DV around the average volume value hV i, a corresponding distribution of o-PS lifetimes is expected [8]. In the present work we want to investigate the behavior of the nano-cavities of PMMA after ion beam bombardment at a temperature higher than Tg (105°C). To do this it was important to have a good characterization of PMMA before bombardment and after simple thermal treatments. To this end we have studied two sets of samples: (1) thin (0.8 lm) ®lms deposited on Si substrates and (2) thick (10 lm) self supporting ®lms. The interest for these as received sample was due to
607
the fact that the ®rst set was representative of the nano-voids distribution in thin ®lms before bombardment with He ions at some hundreds keV energy (®lm thickness lower than the projected range), the second set was representative of the as received situation before bombardment with ions at some tens MeV. Lifetime spectroscopy (LS) was used for the thicker ®lms while variable energy positrons was used for the thin ®lms. For the LS investigations we used self supporting PMMA foils obtained from monodisperse polymer (molecular weight 73 kDa). Two batches (A and B) of samples were used. The eect of thermal depolymerization was studied with a series of dierent heat treatments and are reported in Table 1. Here s is the mean of the o-PS lifetime distribution, r is the width of the distribution, hV i and DV are the corresponding parameters of the 3 units. hole size distribution expressed in A The results of Table 1 show good reproducibility for the two series of samples. The thermal treatment decreases the average o-PS lifetime of about 15%, independent of temperature and duration of the treatment. The width of the lifetime distribution is zero, within the errors, in the asprepared samples while the heat treatment does introduce a dispersion, with very small dierences among the various treatments. In terms of the free volume model, this means that annealing produces a non-uniform shrinkage of the holes giving rise to a size distribution that is not observed in the asdeposited sample. A sample of the B series, after thermal treatments and LS measurement was dissolved in chloroform and reformed; in this case the o-PS parameters revert to the pristine values. The eect of ion bombardment was investigated on thin PMMA foils deposited by spin coating on Si(1 0 0) single crystal and bombarded at 200°C with 300 keV He ions (¯uence 5 1012 ions/cm2 ). As-prepared and thermally treated samples were also examined, as terms of comparison. For PAS measurements, the positrons were implanted in the samples at dierent depths using a variable energy (0±70 keV) positron beam with a rate of 2 105 s 1 (see Section 2). No 3c annihilation was detected. This is consistent with the short lifetimes reported in Table 1,
608
O. Puglisi et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 605±609
Table 1 Life-time spectroscopy data for self-supported PMMA samples subjected to heat treatments at various temperatures (without ion beam)a 3
3
Sample series
Thermal treatment
s (ns)
r (ns)
) hV i (A
) DV (A
A A A A A A A B B B B
As prepared 150°C ± 10 min 150°C ± 30 min 270°C ± 10 min 270°C ± 20 min 300°C ± 10 min 300°C ± 30 min As prepared 300°C ± 30 min 300°C ± 2 h Dissolved
2.14(6) 1.84(5) 1.80(5) 1.81(5) 1.80(5) 1.78(5) 1.78(5) 2.15(6) 1.86(5) 1.85(5) 2.10(6)
0 0.35(4) 0.27(4) 0.34(4) 0.32(4) 0.38(5) 0.36(4) 0 0.32(4) 0.37(5) 0.08(2)
114 77 76 74 74 70 71 115 79 76 107
± 30 24 29 28 32 30 ± 28 32 8
a
s is the mean of the o-PS lifetime distribution, r is the width of the lifetime distribution, hV i and DV are the corresponding parameters 3 units. of the hole size distribution expressed in A
which indicate that the pick-o process is strongly dominant over internal annihilation of o-PS. On the other hand, this fact also shows that ion bombardment does not produce large cavities where o-PS could survive with weak overlap to the bulk material. Absence of 3c annihilation does not exclude the formation of short-lived positronium in small cavities. Ps formation aects the conventional shape parameter S (fractional area of the annihilation spectrum in an energy window centered on the 511 keV peak) as given by the equation S S0 f
1
S0 4
;
with Eq. (2), it may be concluded that the possible change of the positronium fraction f is 1%. This means, in turn, that heat treatment and ion bombardment have a very small eect on the density of nano-voids. On the other hand, the intense depolymerization due to the thermal treatment and to the ion bombardment is clearly demonstrated by the thickness reduction of the PMMA foil, visible in the log. scale of Fig. 1 as change in the depth of the transition from the PMMA foil to Si substrate (plateau at S 0:461).
2
where S0 is the value measured in the absence of Ps for a given material and f is the fraction of injected positrons forming Ps. A comparison of S depth pro®les for three samples is shown in Fig. 1 (®lled circles: as-deposited; open circles: heat treatment 200°C/10 min; ®lled squares: ion bombardment). Three plateaus, at increasing S-levels, are visible when going from the surface to the bulk. The middle plateau (at S 0:453) is associated with positrons annihilating in the PMMA foil. The level of this plateau is almost identical for the three samples (the average level dierence between the bombarded and the virgin sample is 1:3 0:3 10 3 ). In accordance
Fig. 1. Shape parameter S as a function of depth density in PMMA foils deposited on Si(1 0 0).
O. Puglisi et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 605±609
The lower plateau (at S 0:443), extended from the surface to a depth of about 20 nm, shows the existence of a surface layer with chemical or structural properties dierent than the bulk of the PMMA foil. The nature of this layer, whose existence has never been reported before, deserves further investigation and is currently under investigation.
4. Conclusions The above results lead us to the following conclusions: 1. The use of Positronium techniques on these systems has enormous potentiality, especially in the ®eld of very thin ®lms (see Table 1 and Fig. 1). 2. The study of the thick ®lms shows that the nano-void distribution does depend in reproducible way on the sample history. At our knowledge this is the ®rst time that this result is reported in literature. It could be important for studies in thick ®lms even outside the ion beam community. On the contrary the as received thin ®lms seems not very sensitive to this problem and this fact is not surprising due to dierent structure of a thin ®lm, with a high surface to bulk ratio, with respect to the thick one. 3. Unzipping, both pure thermal unzipping and ion beam induced unzipping, induces moderate shrinking of the nano-cavities in the macromolecular ®lm. Apparently, the intense gas bubbling that occurs during thermal depolymerization or during beam induced depolymerization gives rise to the same ®nal shrinking eect. At ®rst glance this result is surprising because in the studied samples depolymerization was so eective that the ®lm thickness was aected in not negligible amount (see Fig. 1), so that one can expect enlarged cavities after depolymerization. However, we must remind that these phenomena take place at temperature higher than the glass transition temperature which is
609
located ± for the macromolecular solids here reported ± at about 100°C. At this relatively high temperature chain mobility is large enough to allow ecient plasticity of the system. In other words, it is probable that the ``room'' left behind by the many macromolecules that undergo the depolymerization reaction: Macromolecule ! n monomers be soon occupied by the nearest macromolecular chains owing to their ¯exibility. However, it is not clear whether or not the time scale of this shrinking is comparable with that of the chemical reactions that lead to depolymerization: in any case the experimental set up employed here cannot give convincing answers to this last question because analysis of the bombarded ®lm is made ``ex situ''. Acknowledgements The authors wish to thank Giuseppe Compagnini and Luisa D'Urso for assistance in sample preparation and irradiation. Financial support from MURST and CNR (Rome) is gratefully acknowledged. References [1] M.E. Fragala, G. Compagnini, O. Puglisi, J. Mater. Res. 14 (1999) 228. [2] M.E. Fragala, G. Compagnini, A. Licciardello, O. Puglisi, J. Polym. Sci. B 36 (1998) 655. [3] L.E. Manring, Macromolecules 22 (1989) 2677 and references therein. [4] A. Raudino, M.E. Fragala, G. Compagnini, O. Puglisi, J. Chem. Phys. 111 (1999) 1721. [5] A. Dupasquier, A.P. Mills Jr. (Eds.), Positron Spectroscopy of Solids, IOS Press, Amsterdam, Ohmsha, Tokyo, 1995. [6] Y.C. Jean, in: A. Dupasquier, A.P. Mills Jr. (Eds.), Positron Spectroscopy of Solids, IOS Press, Amsterdam, Ohmsha, Tokyo, 1995, p. 563. [7] M. Eldrup, D. Lightbody, N.J. Sherwood, Chem. Phys. 63 (1981) 51. [8] R.B. Gregory, Y. Zhu, Nucl. Instr. and Meth. A 290 (1990) 172.
www.elsevier.nl/locate/nimb
Study of ion beam induced depolymerization using positron annihilation techniques O. Puglisi
a,*
, M.E. Fragal a a, K.G. Lynn b, M. Petkov b, M. Weber b, A. Somoza c, A. Dupasquier d, F. Quasso d
a
Dipartimento di Scienze Chimiche, Universit a di Catania, viale A. Doria 6, I-95125 Catania, Italy b Department of Physics, Washington State University, Pullman, WA, USA c UNCentro, IFIMAT, Pinto 366, Tandil, Argentina d Dipartimento di Fisica del Politecnico, Istituto Nazionale di Fisica della Materia, Milano, Italy
Abstract Ion beam induced depolymerization of polymers is a special class of ion beam induced chemical reaction which gives rise to catastrophic ``unzipping'' of macromolecules with production of large amounts of the monomer, of the order of many hundreds monomer molecules per each macromolecule. The possible modi®cation of the density at microscopic level prompted us to undertake a study of this eect utilizing positron annihilation techniques in Poly(methylmethacrylate) (PMMA) before and after bombardment with He 300 keV ions at 200°C. Preliminary results shown here indicate that before bombardment there is a reproducible dependence of nano-hole distribution on the sample history. Moreover at 200°C we do not detect formation of new cavities as a consequence of the strong depolymerization that occurs under the ion beam. The possible correlation of these ®ndings with transport properties of PMMA at temperature higher than the glass transition temperature will be discussed. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 61.80; 82.30.L; 34.80 Keywords: PMMA; Ion beam; Unzipping; Depolymerization; Nano-cavity; Positronium
1. Introduction Ion beam induced depolymerization of macromolecular targets is a special class of ion beam induced chemical reaction which gives rise to catastrophic ``unzipping'' of the macromolecules with
*
Corresponding author. Tel.: +39-095-33-0533; fax: +39095-337-841. E-mail address: [email protected] (O. Puglisi).
production of large amounts of the monomer [1]. Unzipping under the ion beam has only been observed in Poly(methylmethacrylate) (PMMA), but there is no any good reason why this phenomenon should be limited to this particular polymer, so that it is plausible that many other macromolecular solids may give rise to the same eect. In PMMA the yield of this scission is strongly dependent on the temperature of the bombarded macromolecular target. In particular, under the ion beam, it can only be observed at a temperature
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 0 3 4 3 - 3
606
O. Puglisi et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 605±609
higher than the glass transition temperature Tg (105°C), while at lower temperature no trace of the methyl methacrylate monomer could be detected [2]. For comparison, while under ion beam (He 300 keV) a temperature of 170°C is sucient for observing intense monomer evolution from the bombarded PMMA ®lm, without the ion beam the same ¯ux can only be obtained at 300°C [1]. According to what is known about the chemistry of this polymer [3], the intense monomer (methyl methacrylate) evolution under ion beam bombardment can be explained in terms of disappearance of whole macromolecules or of signi®cant portion of the macromolecular chain [4]. For the PMMA targets here reported this depolymerization involves the production of many hundreds of monomer molecules per macromolecule. This local mass loss could give rise to the formation of new nano-cavities in the macromolecular ®lm and some gas ``bubbling''. Prompted by this hypothesis we have undertaken a study of ion beam induced depolymerization in PMMA using bulk positronium lifetime techniques and variable energy positrons to detect the production and the size of the nano-holes forming the free volume in the macromolecules before and after bombardment with He 300 keV ions at 200°C. Preliminary results shown here indicate that (i) the nano-hole distribution in thick ®lms before bombardment has a reproducible dependence on the sample history and (ii) at temperature of 200°C we do not detect formation of new cavities as a consequence of the strong depolymerization that occurs under the ion beam. The possible explanation of these ®ndings in terms of re-organization of PMMA thin ®lm at temperature higher than the Tg will be discussed. 2. Experimental PMMA thin ®lms have been prepared by spin coating on silicon substrates from chloroform so was measured lutions. The ®lm thickness (8000 A) by means of a stylus pro®lometer (Tencor a-step). The PMMA samples were commercially available. They were nearly monodisperse with (weight av-
erage) molecular weight Mw 73 kDa and polydispersivity of the order of 1.07. Before spin coating purity and molecular weight parameters were checked by standard liquid chromatography techniques. After coating the Si wafers were cut in 4 cm2 squares and each piece was irradiated by 300 keV He . During irradiation the ion beam was swept along the sample surface and the current was kept below 0.1 lA/cm2 in order to avoid thermal eects. In these conditions the sample temperature does not increase more than 5°C. The basic pressure in the irradiation chamber was better than 1 10 5 Pa. For positron life time measurements, a stack of self-supporting PMMA foils was sandwiched with the positron 22 Na source between two Pb re¯ectors. The thickness of each foil was of the order of 10 lm. The lifetime spectra were collected by means of a conventional fast±fast coincidence setup, equipped with two NE111 25 mm 25 mm plastic scintillators, coupled to Philips XP2020 photomultiplier tubes. The resolution of the apparatus was about 230 ps. All the measurements were performed at room temperature. Each spectrum contained about 4 106 counts. For positrons annihilation spectroscopy (PAS) measurements, the positrons were implanted in the samples at dierent depths using a variable energy (0±70 keV) positron beam with a rate of 2 105 s 1 . The energy spectrum of the annihilation radiation was measured with a high-eciency high-purity Ge (HPGe) detector, with an energy resolution of 1.45 keV at 514 keV (85 Sr). Spectra with 2 106 events were acquired at each positron energy. Possible changes of the positronium (Ps) fraction, due to formation of cavities, were monitored by observing modi®cation of the spectrum in the 511 keV peak region and in the region of the maximum of 3c distribution. 3. Results and discussion Positrons, the antiparticles of the electron, can be used as a probe of the structure of the solids at a nanometric and sub-nanometric scale [5]. In the ®eld of polymer science, PAS is used as a nondestructive technique to observe the free volume
O. Puglisi et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 605±609
distribution [6]. The sensitivity of this technique to the presence and to the size of nano-cavities in a polymer is related to the formation in the cavities of Ps, i.e. the light isotope of hydrogen with a positron e taking the place of the proton. One fourth of the Ps atoms is formed in the para-state (p-Ps), which essentially annihilates in 2 c's by internal interaction of the two particles bound in the atom. The remaining fraction (3/4) is formed in the ortho-state (o-PS), which annihilates either in 3 c's by internal interaction or in 2 c's by pick-o, i.e. by interaction with the electrons in the walls of the cavity. Annihilation in 2 c's gives rise to a line in the energy spectrum of the annihilation radiation centered at 511 keV (annihilation energy 2m0 c2 ). This line is very narrow for p-Ps annihilation; in the case of annihilation of free positrons or of o-PS with the electrons of the medium, it is broadened by the Doppler eect related to the motion of the electrons. Annihilation in 3 c's gives a characteristic spectrum extended from 0 to 511 keV. However, in a nano-cavity, this annihilation channel is strongly suppressed by competition with the picko process. Under these conditions, the man life s of o-PS is related to the size of the hole where the atom is trapped. A spherical hole model [7] gives the following relationship between s and the radius R of the cavity: s
1
1
2ns 1
R 1 R sin 2p ; R0 2p R0
1
is an empirwhere R0 R dR, and dR 1:66 A ical parameter, related to the penetration of the Ps wavefunction into the bulk. When hole dimensions are distributed with a width DV around the average volume value hV i, a corresponding distribution of o-PS lifetimes is expected [8]. In the present work we want to investigate the behavior of the nano-cavities of PMMA after ion beam bombardment at a temperature higher than Tg (105°C). To do this it was important to have a good characterization of PMMA before bombardment and after simple thermal treatments. To this end we have studied two sets of samples: (1) thin (0.8 lm) ®lms deposited on Si substrates and (2) thick (10 lm) self supporting ®lms. The interest for these as received sample was due to
607
the fact that the ®rst set was representative of the nano-voids distribution in thin ®lms before bombardment with He ions at some hundreds keV energy (®lm thickness lower than the projected range), the second set was representative of the as received situation before bombardment with ions at some tens MeV. Lifetime spectroscopy (LS) was used for the thicker ®lms while variable energy positrons was used for the thin ®lms. For the LS investigations we used self supporting PMMA foils obtained from monodisperse polymer (molecular weight 73 kDa). Two batches (A and B) of samples were used. The eect of thermal depolymerization was studied with a series of dierent heat treatments and are reported in Table 1. Here s is the mean of the o-PS lifetime distribution, r is the width of the distribution, hV i and DV are the corresponding parameters of the 3 units. hole size distribution expressed in A The results of Table 1 show good reproducibility for the two series of samples. The thermal treatment decreases the average o-PS lifetime of about 15%, independent of temperature and duration of the treatment. The width of the lifetime distribution is zero, within the errors, in the asprepared samples while the heat treatment does introduce a dispersion, with very small dierences among the various treatments. In terms of the free volume model, this means that annealing produces a non-uniform shrinkage of the holes giving rise to a size distribution that is not observed in the asdeposited sample. A sample of the B series, after thermal treatments and LS measurement was dissolved in chloroform and reformed; in this case the o-PS parameters revert to the pristine values. The eect of ion bombardment was investigated on thin PMMA foils deposited by spin coating on Si(1 0 0) single crystal and bombarded at 200°C with 300 keV He ions (¯uence 5 1012 ions/cm2 ). As-prepared and thermally treated samples were also examined, as terms of comparison. For PAS measurements, the positrons were implanted in the samples at dierent depths using a variable energy (0±70 keV) positron beam with a rate of 2 105 s 1 (see Section 2). No 3c annihilation was detected. This is consistent with the short lifetimes reported in Table 1,
608
O. Puglisi et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 605±609
Table 1 Life-time spectroscopy data for self-supported PMMA samples subjected to heat treatments at various temperatures (without ion beam)a 3
3
Sample series
Thermal treatment
s (ns)
r (ns)
) hV i (A
) DV (A
A A A A A A A B B B B
As prepared 150°C ± 10 min 150°C ± 30 min 270°C ± 10 min 270°C ± 20 min 300°C ± 10 min 300°C ± 30 min As prepared 300°C ± 30 min 300°C ± 2 h Dissolved
2.14(6) 1.84(5) 1.80(5) 1.81(5) 1.80(5) 1.78(5) 1.78(5) 2.15(6) 1.86(5) 1.85(5) 2.10(6)
0 0.35(4) 0.27(4) 0.34(4) 0.32(4) 0.38(5) 0.36(4) 0 0.32(4) 0.37(5) 0.08(2)
114 77 76 74 74 70 71 115 79 76 107
± 30 24 29 28 32 30 ± 28 32 8
a
s is the mean of the o-PS lifetime distribution, r is the width of the lifetime distribution, hV i and DV are the corresponding parameters 3 units. of the hole size distribution expressed in A
which indicate that the pick-o process is strongly dominant over internal annihilation of o-PS. On the other hand, this fact also shows that ion bombardment does not produce large cavities where o-PS could survive with weak overlap to the bulk material. Absence of 3c annihilation does not exclude the formation of short-lived positronium in small cavities. Ps formation aects the conventional shape parameter S (fractional area of the annihilation spectrum in an energy window centered on the 511 keV peak) as given by the equation S S0 f
1
S0 4
;
with Eq. (2), it may be concluded that the possible change of the positronium fraction f is 1%. This means, in turn, that heat treatment and ion bombardment have a very small eect on the density of nano-voids. On the other hand, the intense depolymerization due to the thermal treatment and to the ion bombardment is clearly demonstrated by the thickness reduction of the PMMA foil, visible in the log. scale of Fig. 1 as change in the depth of the transition from the PMMA foil to Si substrate (plateau at S 0:461).
2
where S0 is the value measured in the absence of Ps for a given material and f is the fraction of injected positrons forming Ps. A comparison of S depth pro®les for three samples is shown in Fig. 1 (®lled circles: as-deposited; open circles: heat treatment 200°C/10 min; ®lled squares: ion bombardment). Three plateaus, at increasing S-levels, are visible when going from the surface to the bulk. The middle plateau (at S 0:453) is associated with positrons annihilating in the PMMA foil. The level of this plateau is almost identical for the three samples (the average level dierence between the bombarded and the virgin sample is 1:3 0:3 10 3 ). In accordance
Fig. 1. Shape parameter S as a function of depth density in PMMA foils deposited on Si(1 0 0).
O. Puglisi et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 605±609
The lower plateau (at S 0:443), extended from the surface to a depth of about 20 nm, shows the existence of a surface layer with chemical or structural properties dierent than the bulk of the PMMA foil. The nature of this layer, whose existence has never been reported before, deserves further investigation and is currently under investigation.
4. Conclusions The above results lead us to the following conclusions: 1. The use of Positronium techniques on these systems has enormous potentiality, especially in the ®eld of very thin ®lms (see Table 1 and Fig. 1). 2. The study of the thick ®lms shows that the nano-void distribution does depend in reproducible way on the sample history. At our knowledge this is the ®rst time that this result is reported in literature. It could be important for studies in thick ®lms even outside the ion beam community. On the contrary the as received thin ®lms seems not very sensitive to this problem and this fact is not surprising due to dierent structure of a thin ®lm, with a high surface to bulk ratio, with respect to the thick one. 3. Unzipping, both pure thermal unzipping and ion beam induced unzipping, induces moderate shrinking of the nano-cavities in the macromolecular ®lm. Apparently, the intense gas bubbling that occurs during thermal depolymerization or during beam induced depolymerization gives rise to the same ®nal shrinking eect. At ®rst glance this result is surprising because in the studied samples depolymerization was so eective that the ®lm thickness was aected in not negligible amount (see Fig. 1), so that one can expect enlarged cavities after depolymerization. However, we must remind that these phenomena take place at temperature higher than the glass transition temperature which is
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located ± for the macromolecular solids here reported ± at about 100°C. At this relatively high temperature chain mobility is large enough to allow ecient plasticity of the system. In other words, it is probable that the ``room'' left behind by the many macromolecules that undergo the depolymerization reaction: Macromolecule ! n monomers be soon occupied by the nearest macromolecular chains owing to their ¯exibility. However, it is not clear whether or not the time scale of this shrinking is comparable with that of the chemical reactions that lead to depolymerization: in any case the experimental set up employed here cannot give convincing answers to this last question because analysis of the bombarded ®lm is made ``ex situ''. Acknowledgements The authors wish to thank Giuseppe Compagnini and Luisa D'Urso for assistance in sample preparation and irradiation. Financial support from MURST and CNR (Rome) is gratefully acknowledged. References [1] M.E. Fragala, G. Compagnini, O. Puglisi, J. Mater. Res. 14 (1999) 228. [2] M.E. Fragala, G. Compagnini, A. Licciardello, O. Puglisi, J. Polym. Sci. B 36 (1998) 655. [3] L.E. Manring, Macromolecules 22 (1989) 2677 and references therein. [4] A. Raudino, M.E. Fragala, G. Compagnini, O. Puglisi, J. Chem. Phys. 111 (1999) 1721. [5] A. Dupasquier, A.P. Mills Jr. (Eds.), Positron Spectroscopy of Solids, IOS Press, Amsterdam, Ohmsha, Tokyo, 1995. [6] Y.C. Jean, in: A. Dupasquier, A.P. Mills Jr. (Eds.), Positron Spectroscopy of Solids, IOS Press, Amsterdam, Ohmsha, Tokyo, 1995, p. 563. [7] M. Eldrup, D. Lightbody, N.J. Sherwood, Chem. Phys. 63 (1981) 51. [8] R.B. Gregory, Y. Zhu, Nucl. Instr. and Meth. A 290 (1990) 172.