M Hyla, J Filipecki, J Swiqtek - Thermally treated effects on polymers based on acrylate oligomers by positron annihilation lifetime spectroscopy - страница 1
ВІСНИК ЛЬВІВ. УН-ТУ
Серія фіз. 2009. Bun. 43. С. 110-117
VISNYKLVIV UNIV. Ser. Physics. 2009. Is. 43. P. 110-117
PACS number(s): 78.70.Bj
THERMALLY TREATED EFFECTS ON POLYMERS BASED ON ACRYLATE OLIGOMERS BY POSITRON ANNIHILATION
M. Hyla1, J. Filipecki1, J. Swiqtek1, J. Swiqtek-Prokop2
institute of Physics, Jan Dlugosz University al. Armii Krajowej 13/15, 42 201 Czqstochowa, POLAND
e-mail: firstname.lastname@example.org 2Institute of Technical Education, Jan Dlugosz University al. Armii Krajowej 13/15, 42 201 Czqstochowa, POLAND
Positron annihilation lifetime spectroscopy has been applied to the study of free volume properties in polymers based on the acrylate oligomers. The measurements have been made on samples heated to a temperature of 483 K. The longest lifetime, in three-component analyses of the spectra was associated with the pick-off annihilation of ortho-positronium trapped in free volumes. After the thermal treatment changes in the ortho-positronium lifetimes and the relative intensity of the longest component were observed. These results are discussed on the basis of free volume model.
Key words: positron annihilation; polymers, organics.
There has been interest in acrylate oligomers-based polymers due to the possibility of applications in different branches, such as optoelectronics, holography and polygraphical material engineering [1-3]. Since the materials properties of amorphous polymers are frequently interpreted in terms of the free volume concept, it is advantageous to have a spectroscopic technique by means of which quantitative information on free volume can be generated. During recent years, it has been established that positron annihilation lifetime spectroscopy (PALS) is a useful method for such analysis. It gives a measure of positron and positronium annihilation times and is the technique most commonly applied to polymers.
Positrons injected in substances lose their energy through elastic collisions and finally annihilate with electrons through several processes. In the case of non-conductive molecular materials in addition to the annihilation of the positron, formation and annihilation of positronium (Ps) take place. Ps is the bound state of positron and electron having an atomic radius comparable to that of the hydrogen atom. It exists in two spin states. One is called para-positronium (p-Ps) in which the positron and electron spins are antiparallel. The other state is ortho-positronium (o-Ps), in which the particle spins are parallel. Positronium appears in the para or ortho spin state with a relative formation rate of 1:3. Annihilation of p-Ps occurs with a lifetime in the order of 125 ps, whereas o-Ps decays after approximately 140 ns. However, in the condensed matter, the positron in oPs predominantly annihilates, during a collision with atoms or molecules, with an electron other than its bound partner and possessing an opposite spin. This process,
© Hyla M., Filipecki J., Swiatek J. et al., 2009
called pick-off annihilation, reduces the o-Ps lifetime in polymers to a few nanoseconds. Ps cannot form in materials with high electron densities. The positronium formation probability and lifetime are extremely sensitive to the electron density surrounding Ps. The o-Ps localises in the space between and along polymer chains and at chain ends (free volume holes), and the lifetime gives indication on the mean radii of these holes [4, 5]. The original free volume theory for the positron annihilation in molecular substances was proposed by Brandt, Berko and Walker . The free volume was defined as the cell volume minus the excluded volume, which was based on the Wigner-Seitz approximations. The free volume model expresses that Ps can only form in those free spaces of the lattice, having a size superior to some critical values. The electron pick-up depends on the overlap of the positron component of the Ps wave function with the lattice wave function. As the size of the free volume cavity increases, the local electron density, surrounding the o-Ps, decreases. Thus the o-Ps has a slower annihilation rate and longer lifetime. Tao and Eldrup et. al. [7, 8] derived the equation to correlate experimentally observed o-Ps lifetimes and free volume hole dimensions in polymers. They proposed a simple model in which the o-Ps particle resides in a spherical potential well, having an infinite potential barrier of radius R0. It is assumed that an electron layer forming a thickness AR is present on the wall of the hole, which effective radius is consequently R=R0-AR and that the lifetime of the o-Ps in the electron layer is the spin averaged Ps lifetime of 0,5 ns. Furthermore, a very successful semi-empirical equation has been established relating on the o-Ps lifetime to the size of the free volume hole in which it annihilates, thus т3 corresponds to a spherical space with a radius R, according to the following equation:
where AR=0,l66 nm is the fitted empirical electron layer thickness. By fitting the above equation with the measured т3 values, R and free volume size as Vf = (4n/3)R3 can be evaluated. The relative intensity of the longest component, I3, is generally correlated to the density of the holes, which can be considered as a kind of trapping centres for Ps. A semi-empirical relation may be used to determine the fraction of free volume fV) in polymers as [4, 5]:
where Vf is the free volume calculated from т3, using eq. (l) with a spherical approximation, I3 (in %) is the intensity of long-lived component; C is an empirical parameter, which can be determined by calibrating with other physical parameters.
The studies of the changes occurring in the structure of materials after the thermal treatment are important for the material science. The aim of this paper is to investigate the changes induced by thermal treatment in the microscopic structure of a sample of acrylate oligomers-based polymers, using positron annihilation lifetime spectroscopy (PALS).
Positron lifetime studies were performed in the UV-cured composition based on the acrylate oligomers. The photocompositional mixtures identified as D-l, D-2, D-4, D-5-l, D-5-2 and D-5-2 were chosen for the further investigations. Mixture compositions are presented in table l. Their molecular structures are given in fig. l; isobutylbenzoin ether (signed as i-BEB) was used as a photoinitiator. The photodissociation of photoinitiator reaction is presented in fig. 2. R' and R" denote fee chemical radicals which are created
fv = CVfL
M. Hyla, J. Filipecki, J. Swiatek et al.
after a breaking of the photoinitiatior; Rl, R2, R3 and R4 (fig. l) denote the oligomer fragments which remain unchanged during photopolymerization. The number n of the identical oligomer fragments may usually vary from one to four.
Contents of the photocompositional mixtures in the investigated samples
о 0 II II
І і I
Fig. l. Chemical formulae of mixture content
The photochemical polymerization process takes place in the system due to the presence of the double C=C bonds at the ends of the chain. Under the influence of UV-irradiation the chemical bonds on the sides of oligomer constituents are broken. This leads to a "sewing" of single carbon bonds and occurrence of solid photopolymerized phase. All these components form a complex of a three-dimensional network.
,_. II 1
Fig. 2. Photodissociation reaction of photoinitiator
The photopolymerization was performed by an UV-hydrogen lamp (Xmax~350 nm). The UV fluence used to cure the polymers was 2,7 kJ/m2 for 1 min. UV-curing process was carried out for 20 min. The details concerning sample preparation have already been reported [2, 3].
The DSC analysis shows absence of any heat effects at the temperature of about 423 K, it indicates the high thermal stability of investigated polymers. The heating to temperatures higher than 483 K caused destruction of the samples. It is interesting to compare free volume hole sizes before and after heating the samples up to about 483 K to investigate the effect of their previous thermal history on the free volume hole sizes.
As-prepared polymer samples were heated to a temperature of 483 K before the measurements in an air oven. The heated samples were cooling down (10,0 K/min) to the room temperature before the positron annihilation measurements. All the samples proved amorphous using X-ray analysis.
The measurements of positron lifetimes were curried out after cooling down the samples, with an ORTEC spectrometer of about 270 ps FWHM (Full Width at Half Maximum) resolution. A Na22 isotope with 7,4-105 Bq activity was used as the positron source. It was placed between two identical samples, forming a „sandwich" system.
The PAL spectra were measured at room temperature and analysed through the common "Microcomputer program for analysis of positron annihilation lifetime spectra LT" designed by Kansy  with a three-component model. The four-component analysis generally does not give better fitting for the variance value. Therefore, only three-component results are presented here. In polymers, the shortest lived component is usually attributed to p-Ps annihilation. In our case, because of the relatively poor time resolution and no constraints on lifetimes during computer analyses, it might contain not only p-Ps annihilation contribution but also contributions from the positron compounds. Therefore, during the fitting for the shortest lifetime, zb it was fixed at 125 ps (the p-Ps lifetime). The intermediate lifetime (т2 ~ 0,36 ns) is due to the free positrons annihilation with electrons in the bulk material. It shows some quite small variations after the heating. The results of the calculation of the mean values of positron lifetimes for the investigated samples showed the existence of a long-lived component in the positron annihilation lifetime spectra. According to the common interpretation we attribute the longest component т3, to the pick-off annihilation of o-Ps trapped by free volumes. In any given, sample all the free volume holes are not the same size. The LT results are the averaged values, but the real long-lived annihilation events have some
114_M. Hyla, J. Filipecki, J. Swiatek et al.
time-distribution around the averaged value. So, the concept of the average free volume size is used in practice.
As the o-Ps component is relevant to the free-volume properties, and it is markedly sensitive to the microstructure changes, in this paper, our main attention is paid to the т3 and I3 . The lifetime parameters for the samples in the initial state, before thermal treatment, have already been reported [10-12]. The variations of o-Ps pick-off lifetime and its intensity for the investigated samples before and after thermal treatment are presented in table 2. The errors are the results of the mathematical analysis.
Mean values of the lifetime, т3, and relative intensity, I3, of the o-Ps
The average size of the free-volume holes Vf was calculated according to eq. 1. The values of the Vf and Vf -I3 = fV/C for the investigated polymers before and after the thermal treatment are shown in fig. 3 and in fig. 4, respectively. The errors bars are smaller than symbol plots.
As it can be seen in table 2, all the samples exhibite a change in o-Ps lifetimes, x3i as well as the relative intensity of o-Ps, I3, which indicates that the thermal treatment changes the microstructure of the investigated polymers.
In the case of the samples D-1, D-2 and D-4, a decrease of т3 can be observed. It does not change significantly in the samples D-1, D-2 and D-4, a decrease of about 1%, taking into account the initial samples and the heated ones, can be detected. It follows that the free volume size values change nearly more than 2-10-30 m3. As it can be seen in table 2, the thermal treatment in these samples leads to decrease of the relative intensity of o-Ps, I3. It suggests that the o-Ps formation probability slightly decreases after heating.
In the case of the sample D-5-3 a decrease of т3 can be observed too, but difference is much larger and additionally the relative intensity of o-Ps, I3 slightly decreases after heating of this sample.
However, in the case of D-5-2 sample the difference of free volume size values before and after heating is in the range of the errors of the mathematical analysis. It seems that in the sample D-5-2, the thermal treatment does not change the microstructure. But simultaneously, the relative intensity of o-Ps, I3 increases significantly, from 5,38 % for the initial state sample, to 8,64 % for the heated one (see table 2). Because the concentration of the free volume sites is proportional to the
intensity of o-Ps, we assumed that the concentration of free volumes in the sample D-5-2 is much larger after the thermal treatment.
Fig. 3. Average free volume size, Vf for the investigated samples before and after thermally treatment. Lines are drawn as a guides for the eye
Fig. 4. The Vf I3 = fV /C values for the investigated samples before and after thermally treatment. Lines are drawn as a guides for the eye
Absolutely different is behaviour of the sample D-5-1. As can be seen in table 2 only in D-5-1, thermally treatment leads to increase the o-Ps lifetime. It means that the mean free volume holes size increases after the heating only in D-5-1 (see fig. 3).
The fractional free volume is proportional to Vf I3, because C in eq. 2 is constant. Hence, these results suggest that after the thermal treatment, the fractional free volume, slightly decreases in the samples D-1 and D-2, but significantly in the sample D-4, as it is presented in fig 4. Increasing of the fractional free volume is observed in the samples
M. Hyla, J. Filipecki, J. Swiatek et al.
D-5-1 and D-5-2. It is interesting that in spite of the largest decrease in free volume size value after heating, the fractional free volume in D-5-3 does not change.
The effect of the heating is such that either free volume size value or the fractional free volume change in the investigated polymers after the thermal treatment. In the samples marked as D-5-1, D-5-2 and D-5-3 (i.e. polymers based on the oligomer DMOEM) quite large variations are observed. Among of these three polymers D-5-2 sample indicates the least change and it seems to be result of the lowest presence of DMOEM in the composition of this sample.
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3. Merwinskij R.I., Gudzowskaja L.A., Roter W.E. Chimija Wysokich Energij. 1994. Vol. 28. 182 p.
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ЕФЕКТИ ТЕРМІЧНОЇ ОБРОБКИ В ПОЛІМЕРАХ НА ОСНОВІ ОЛІГОМЕРІВ АКРИЛАТУ, ВИВЧЕНІ МЕТОДОМ ПОЗИТРОННОЇ АНІГІЛЯЦІЙНОЇ СПЕКТРОСКОПІЇ
М. Хиля1, Я. Філіпецький1, Й. Свіонтек1, Й. Свіонтек-Прокоп2
'інститут фізики, Університету Яна Длугоша вул. Армії Крайової '3/'5, 4220' Ченстохова, Республіка Польща
2Інститут технічної освіти, Ууніверситету Яна Длугоша вул. Армії Крайової '3/'5, 4220' Ченстохова, Республіка Польща
Вивчено властивості вільного об'єму в полімерах на основі олігомерів акрилату методом позитронної анігіляційної спектроскопії. Дослідження проводили на зразках, нагрітих до температури 483 K. Під час аналізу спектрів довготривалий час життя третьої компоненти був пов' язаний з процесором pick-off анігіляції орто-позитронію, який захоплювався у вільному об'ємі. Після термічної
обробки були виявлені зміни в часі життя орто-позитронію та відносній інтенсивності довготривалої компоненти. Отримані результати проаналізовані на основі моделі вільного об' єму.
Ключові слова: позитронна анігіляція, полімери, органіка.
ЭФФЕКТЫ ТЕРМИЧЕСКОЙ ОБРАБОТКИ В ПОЛИМЕРАХ НА ОСНОВЕ ОЛИГОМЕРОВ АКРИЛАТА, ИЗУЧЕННЫЕ МЕТОДОМ
ПОЗИТРОННОЙ АННИГИЛЯЦИОННОЙ СПЕКТРОСКОПИИ
М. Хиля1, Я. Филипецкий1, Й. Свионтек1, Й. Свионтек-Прокоп2
'Институт физики, Университет Яна Длугоша ул. Армии Краевой '3/'5, 4220' Ченстохова, Республика Польша 2Институт технического образования, Университет Яна Длугоша ул. Армии Краевой '3/'5, 4220' Ченстохова, Республика Польша
Изучены свойства свободного объема в полимерах на основе олигомеров акрилата методом позитронной аннигиляционной спектроскопии. Исследования проводили на образцах, нагретых до 483 K. Во время анализа спектров долговременное время жизни третьей компоненты было связано с процессором pick-off аннигиляции орто-позитрония, который захватывался в свободном объеме. После термической обработки были обнаружены изменения времени жизни орто-позитрония и относительной интенсивности долговременной компоненты. Полученные результаты проанализированы на основе модели свободного объема.