OrthoPositronium Annihilation in Nitric Acid Treated

OrthoPositronium Annihilation in Nitric Acid Treated Polypropylene A. Ogata and S. J. Tao Citation: Journal of Applied Physics 41, 4261 (1970); doi: 10.1063/1.1658454 View online: http://dx.doi.org/10.1063/1.1658454 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/41/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Mesoporous silica films with varying porous volume fraction: Direct correlation between ortho-positronium annihilation decay and escape yield into vacuum Appl. Phys. Lett. 95, 124103 (2009); 10.1063/1.3234381 Density and temperature dependencies of orthopositronium annihilation rates in ethane gas J. Chem. Phys. 75, 1226 (1981); 10.1063/1.442171 Positronium Annihilation in Molecular Substances J. Chem. Phys. 56, 5499 (1972); 10.1063/1.1677067 OrthoPositronium Annihilation and the Glass Transition of Nylon 6 J. Appl. Phys. 43, 737 (1972); 10.1063/1.1661191 Glass Transition of Atactic Polystyrene by OrthoPositronium Decay J. Appl. Phys. 41, 4273 (1970); 10.1063/1.1658456

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JOURNAL OF APPLIED PHYSICS

OCTOBER 1970

VOLUME 41, NUMBER II

Ortho-Positronium Annihilation in Nitric Acid Treated Polypropylene A. OGATA* AND S. J. TAO The N e'I1} England Institute, Ridgejield, Connecticut 06877

The mean lives of positrons annihilating in nitric acid treated polypropylene were determined. A longlifetime component with a mean life of about 50 nsec appeared when the polypropylene was treated in hot nitric acid for more than three hours. This was obviously due to the annihilation of ortho-positronium in the microcavities in the polypropylene etched by the action of nitric acid. It is slggested that nitric acid attacks the amorphous part of polypropylene first and leaves a skeleton of crystalline form filled with microcavities. The change of the mean life and the intensity of the long lifetime component with the change of the nitric acid treatment time is discussed. The value of the o-Ps diffusion coefficient in polypropylene is estimated to be D>4.5XlO-4 cm2 sec-1 and the potential of the barrier surrounding the o-Ps in the cavity is estimated to be less than 1 eV.

INTRODUCTION

Selective oxidation with nitric acid has been used as an effective means of studying the morphology of polypropylene.' The acid reacts with the amorphous region and removes it rapidly, leaving the more inert crystalline component in the solid phase. The reaction is said to occur uniformly throughout a fairly thick piece of polymer. If this is true, there should appear numerous small pores, formed by the removal of the amorphous region, embedded in the skeleton of the crystalline component after the reaction. The size of the pores is expected to become larger and larger as the reaction proceeds further and further. Positron annihilation lifetime spectra in solid powders of inorganic oxides depend strongly on the particle size. A very long lifetime component appears in the spectra if the particle size is about 100 A or less. 2 This is due to the effect of diffusion of positronium CPs) in solids. 3 A similar phenomenon has also been found for positron annihilation in silica gels. 4 The long lifetime component is attributed to the annihilation of ortho-positronium Co-Ps). Ortho-Ps is formed inside the solid phase. If it also annihilates inside the solid phase, its mean life will be quite short, in the order of a few nanoseconds. However, if it lives long enough to escape the solid phase into the voids by diffusion, its mean life will become much longer, approaching the theoretical o-Ps lifetime in free space, 140 nsec. This report concerns some recent investigations in the positron annihilation in nitric acid treated polypropylene. The effect of the duration of the treatment on the formation and annihilation of o-Ps is discussed. It is hoped this brief report will shed more light on the use of positron annihilation for the investigation of the properties of polymers and stimulate more work along this line.

are in the form of rectangular pellets with sides about 0.12S-in. long. No additional treatment was attempted before the nitric acid treatment. The procedure for the nitric acid treatment was basically the same as the one used by Hock.' Five grams of the polymer sample were immersed in 100 ml of 70% HN0 3, and the mixture was kept at the boiling point, about 120°C, for a certain time. After the acid treatment, the polymer was poured into an excess of distilled water, filtered, washed once with water and then several more times with methanol, and dried to a constant weight at room temperature. The sample and the 22Na source were enclosed in a glass tube, and then the contents were evacuated and kept under vacuum at a pressure of about 10-4 Torr. TABLE 1. Some properties of the polypropylene samples.

Mn

M.. Crystallinity Recrystallization temperature

No. 6056

No. 6057

160 000 870000 61.1% 127.1°C

61 000 300 000 69.5% 129.0°C

Two time-to-amplitude converters were used for the lifetime measurements. One, with a fwhm of 300 psec for C0 60 'Y rays under the same energy selections as for 22N a and a linear range of 2S nsec,5 was used for measuring positron annihilation spectra without a very long lifetime component. Another, with a fwhm of 600 psec and a range of 300 nsec, was used for measuring positron annihilation spectra with a very long lifetime component. Two series of the positron annihilation lifetime spectra measured are shown in Figs. 1 and 2. A very EXPERIMENTAL long lifetime component appears in the spectra when the The polypropylene polymers were obtained from the treatment time is over one hour. These data have been Polymer Sample Bank, Distillation Products In- analyzed by a standard procedure. 6 After the analysis dustries, Eastman Kodak Company, Rochester, New the lifetime spectra which consists of a not very long York. Some of the properties of the two polymers lifetime component can be fitted to two-exponential investigated are shown in the Table 1. The samples component curves. The lifetime spectra which consists 4261


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their weight loss mcchanisms. 7 The first is the addition of heavy chemical groups, largely N02 , during the reaction. This is characterized by a very rapid small negative weight loss or weight gain. Then the fractional weight gain approaches a nearly steady value. The second is the rapid attack on the amorphous region. This is characterized by an initially rapid loss of weight, then the fractional weight loss remains practically unchanged. The third is the attack on the crystalline region. This is characterized by a slow loss of weight nearly linearly dependent on treatment time. By infrared spectroscopy, Hackl detected about 5% of N0 2 groups in the samples after one hour of treatment. This is roughly equivalent to one N02 group per 20 propylene groups. Thus, the surface of the pores produced

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FIG. 1. The positron annihilation lifetime spectra in polypropylene No. 6056 treated with nitric acid for various times. (1 channel = 1.50 nsec.)

of a very long lifetime component can hardly be fitted to simple multiexponential component curves. Apparently, as observed in Figs. 1 and 2, only the tail part of the very long lifetime component behaved like an exponential decay, but the initial part appeared curved in a semilog plot. Therefore, another method of analysis should be used. For such spectra we shall use the mean life of the tail part CTt) and the intensity of the whole very long lifetime component (It), i.e., the fractional area under the very long lifetime component for the discussion. These values are shown in Tables II and III. In Figs. 3 and 4 these values as well as the values of the weight loss are plotted against the duration of the nitric acid treatment. The effect of nitric acid treatment on polypropylene can be separated into three processes characterized by TABLE II. The values of the mean life (Tt) of the tail part of the long lifetime component and the intensity of the long lifetime component (It) for polypropylene. No. 6056 treated with nitric acid. Treatment time (hours) 1.0 1.5 2.0 2.5 3.0 6.0 12.0

Tt (nsee)

1.05±0.08 1O.9±0.6 11.2±0.5 34.0±0.7 42.1±0.6 38.0±0.6 46.0±0.7

It (%)

4.0±0.9 2.0±0.2 3.1±0.3 5.0±0.2 5.5±0.2 6.6±0.2 6.1±0.2

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FIG. 6. The relationship between the function F and the cubic root of weight loss %; circle, for sample No. 6056; square, for sample No. 6057; solid line, theoretical curve for kpro= 2.16X (wt loss %)1/3; dashed curve, theoretical curve for kpro= 2.46X (wt loss %)1/3.

where Aw is the annihilation rate of o-Ps in the wall material, AO is the annihilation rate of o-Ps in free space, and F is the ratio of the probability of o-Ps staying inside the pore to the probability of o-Ps penetrating the wall material. For large F

of the wall of the cavity remain unchanged. These plots are shown in Fig. 6. The solid lines are the best fit theoretical curves to high values of F. It is very interesting to note that the data points for short treatment time (low weight loss) fall far below the theoretical curves. This implies that the average cavity size at this time is smaller than the one assumed so that each cavity occupies an amorphous site and is enlarged gradually from a small initial cavity. At first there may be more than one small cavity in an amorphous site. During the treatment, these cavities may merge into a larger cavity. The sizes of the cavities are not expected to be highly uniform, particularly in the initial stage of the reaction. This explains the curved nature of the long lifetime component in the lifetime spectra and also why the curved part gradually disappears after a prolonged treatment. The potential of the barrier of the cavity can be estimated using the following procedure. After more than six hours of nitric acid treatment T('-'60 nsec. As the value of Aw should be approximately 0.9 nsee!, the reciprocal of the mean life of o-Ps in nitrocompounds, we obtain F",70 by using Eq. (5). Then from Fig. 5 we obtain kpro"'8.0. If the radius of the cavity is assumed to be 50 A the potential is about 0.05 eV. If the radius of the cavity is assumed to be 18 A the potential is about 0.5 eV. Whether it is 0.05 eV or 0.5 eV, at least we are sure the potential barrier surrounding the o-Ps atom in the cavity which traps the o-Ps atom is not large, i.e., less than 1 eV.

A= Aw/(l+F)+Ao.

ACKNOWLEDGMENTS

as a condition such that most of the o-Ps atoms will react with N02 groups outside the nonreacted region. If the values of Rand Tp are known we have (3) After we have inserted R= 50 A and Tp= 2.2 nsec into (3) we obtain D>4.5XlO-4 cm 2 sec!. This value is greater than the value of D in Si02 , 6X 10-5 cm2 sec! determined by Brandt and Paulin. 3 Since the structure of Si02 is more rigid than that of polypropylene, a higher value of D for o-Ps in polypropylene is a reasonable result. The Properties of the Cavities

The annihilation rate (A= l/T) of o-Ps in a pore or a cavity can be represented by a formulas AAw - (1+F)

AO

+ (1+1/F)

,

(4)

(5)

Since the values of AO and Aw are known we can calculate the values of F using the above formula. If the cavities are considered to be spherical in shape and the walls of the cavities rigid we can treat the cavities as simple square well barriers to positronium atoms inside and obtain the values of F. The solution is shown in Fig. 5 where the value of F is plotted against kpToi To is the radius of the cavity and kp= (4 mV)!/2 with V the potential of the barrier. If each cavity is enlarged gradually from a small initial cavity during the treatment with nitric acid, a plot of F vs the cubic root of the weight loss should give us a curve similar to the theoretical F vs kpTo plot, provided the properties

This work is partially supported by USAEC contract AT(30-1)3661. The authors wish to thank Dr. S. Y. Chuang, Dr. John Lee, and Dr. J. H. Green for their assistance. 1 C. W. Hock, Polymer Lett. 3, 573 (1965); J. Polymer Sci. Part A 4, 227 (1966). 2 R. Paulin and G. Ambrosino, J. Phys. 29,263 (1968). 'W. Brandt and R. Paulin, Phys. Rev. Lett. 21,193 (1968). • S. Y. Chuang and S. J. Tao, J. Chern. Phys. 52, 749 (1970). 5 A. Ogata and S. J. Tao, Nuc!. Instrum. Methods 69, 344 (1969). • S. J. Tao, IEEE Trans. Nue!. Sci. 15, (1) 175 (1968). 7 D.]. Blundell, A. Keller, and T. M. Connor, ]. Polymer Sci. Part A 5,991 (1967). 8 W. Brandt, S. Berko, and W. W. Walker, Phys. Rev. 120, 1289 (1960).
