Positron annihilation spectroscopy for surface and interface studies in nanoscale polymeric films

Spectrochimica Acta Part A 61 (2005) 1683–1691

Positron annihilation spectroscopy for surface and interface studies in nanoscale polymeric films Y.C. Jean∗ , Junjie Zhang, Hongmin Chen, Ying Li, Guang Liu Department of Chemistry, University of Missouri–Kansas City, 5009 Rockhill Road, Kansas City, MO 64110, USA Received 12 November 2004; received in revised form 13 December 2004; accepted 14 December 2004

Abstract Positron annihilation spectroscopy (PAS), coupled with a variable mono-energetic positron beam, has been used to investigate surface and interfacial properties in thin polymeric films. Free-volume properties have been measured from ortho-Positronium (o-Ps) lifetime and the S parameter of Doppler broadening of energy spectra from annihilation radiation as a function of the depth and of the temperature in thin polymeric films. Depth profiles of glass transition temperature and nanoscale layered structures in polystyrene (PS) thin films on the Si substrate are presented. © 2004 Elsevier B.V. All rights reserved. Keywords: Positron annihilation; Glass transition; Free-volume distribution; Polymer films

1. Introduction The positron is the anti-electron, which was theoretically predicted by Dirac in 1930 [1] and experimentally discovered by Anderson in 1932 [2]. When the positron encounters electrons, it annihilates into ␥-rays by Einstein’s equation E = mc2 . The characteristics of annihilation photons contain electronic properties, i.e. wave function, density, and energy levels of matter at the location where the positron annihilates [3]. Positron annihilation spectroscopy (PAS) is a branch of ␥-ray spectroscopy which monitors the lifetime, energy spectrum, and angular correlation of annihilation photons [4]. PAS has been used to study defects in solids for many decades [5]. Recently, it has been successfully applied to measure the free-volume [6] properties in polymers [7] and in thin films [8–19]. With its unique sensitivity to the atomic-level free volume in polymers, PAS is emerging as a promising tool to measure glass transition temperature, Tg , as a function of depth [10–18]. Positron annihilation lifetime (PAL) spectroscopy is capable of determining size, structure, distribution, and relative fraction of free volume in polymers due to the fact that the ∗

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ortho-Positronium (o-Ps, the triplet Positronium) is preferentially trapped in the subnanoscale free volume. In this paper, we first report the nanoscale structures from the measured free-volume data in thin polystyrene (PS) films supported on the Si substrate. Glass transition temperature, Tg , in thin films is one of the most fundamental physical properties for their applications to chemical and electronics industries [20]. A variety of techniques, such as Brillouin light scattering [21,22], ellipsometry [23], neutron scattering [24,25], differential scanning calorimetry (DSC) [26,27], scanning force microscopy (SFM) [28], atomic force microscopy (AFM) [29], Sumfrequency vibrational spectroscopy [30], and fluorescence spectroscopy [31,32] have been used to measure the Tg of thin polymeric films. However, despite much effort put into this area of research in recent years, existing reported results are still inconsistent and interpretations are not settled. In this paper, we also report the depth variation of Tg at the surface and interfaces in a thin polystyrene film on the Si substrate.

2. Experiments The polystyrene used in this study was purchased from Aldrich Chemicals (Mw = 212,400, Mw /Mn = 1.06). Silicon

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Fig. 1. A schematic diagram of the slow-positron beam at the University of Missouri–Kansas City [19]. (A) 50 mCi 22 Na positron source, (B) W-mesh moderator, (C) magnetic field (75 G) coils, (D) ExB filter, (E) positron accelerator, (F) correcting magnets, (G) gas inlet, (H) positron lifetime system for PAL, (I) turbo molecular pump, (J) samples, (K) sample manipulator, (L) cryo pump, (M) Ge solid state detector, and (N) lifetime detector.

wafers were from Wafer World Inc. (West Palm Beach, FL). The polystyrene films were prepared by dissolving different wt.% polystyrene into toluene, then spin-coated onto Si wafers at a spin rate of 2000 rpm for 5 min. The films were annealed in vacuum at 140 ◦ C for 12 h before mounting on a Kapton film heater in a vacuum system for PAS measurement. The film thickness was found to be 82 ± 5 nm on Si using profilometry (Tencor alfa-step 200, Adv. Surf. Tech., Cleveland, OH) as described elsewhere [18]. The surface roughness of Si wafers was measured to be 3 ± 1 nm. Doppler broadening energy spectroscopy (DBES) and PAL were measured at the University of Missouri–Kansas City (UMKC) and at the National Institute of Advanced Industrial Science and Technology in Japan (AIST). Conventional positron lifetime measurement was performed with 22 Na sandwiched between two thick polystyrene films (2 mm) as a function of temperature at UMKC. DBES was measured with positron energy from 0.1 to 30 keV at a counting rate of 2000 cps at UMKC [19]. A schematic diagram of the UMKC positron beam is shown in Fig. 1. The obtained DBES spectrum is expressed as the S parameter, which is defined as a ratio of integrated counts near 511 keV at the central part to the total counts with a window of ±0.53 keV, as schematically shown in Fig. 2. The S parameter represents a measure of the free-volume quantity in polymeric materials from two main contributions: the paraPositronium (p-Ps, singlet Ps) annihilation and the energy broadening due to the uncertainty principle for Ps and the positron localized in a free-volume hole. The energy resolution of the solid detector was 1.5 keV at 511 keV. Similarly, the W parameter contains the high momentum contribution of electrons in polymers. Since W is a reciprocal quantity of the S parameter, we only present the S data to discuss the free-volume properties in polymers in this paper. One advantage of measuring the S parameter over the positron lifetime

is the short time of data acquisition (of the order of minutes) as opposed to hours for a PAL spectrum total of one million counts. A detailed description of DBES and the S parameter can be found elsewhere [12,17,19]. PAL experiments were performed using a 0–15 keV positron beam from AIST in Japan [33]. The PAL data thus contain free-volume properties for polymers from the surface, any interfaces, and to the bulk. The lifetime resolution was 250–350 ps at a counting rate of 1000–2000 cps. Each PAL spectrum contains two million counts. The obtained PAL data were fit into three lifetime components using the PATFIT program [34] after the longest lifetime component (>10 ns) was subtracted from the raw spectra, and the subtracted spectra was then fit in the range to 25 ns, as we have reported in the

Fig. 2. A schematic presentation for a typical DBES spectrum, N(E) counts vs. energy, and S parameter collected near center of the positron annihilation radiation 511 keV while W parameter near the wings of annihilation radiation.

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Fig. 3. Raw PAL spectra in different energies of incident positron (depths) in an 80-nm PS film.

past [35,36]. PAL spectra for a polystyrene film at different depths are shown in Fig. 3. It is seen that both o-Ps lifetime and intensity vary as a function of implanted positron energy due to different Ps–molecular interactions and polymeric properties at different depths from the surface. Each PAL spectrum contains p-Ps, the positron, and o-Ps annihilation radiations in polymers. Fortunately, in complicated polymeric systems, it has been shown that Ps preferentially localizes in defect sites [37], particularly in the free volume before annihilation takes place. Therefore, all Ps signals contain the electron properties of free volume. Three resolved lifetimes in polymers are τ 1 ∼ 0.125 ns which corresponds to p-Ps annihilation, τ 2 ∼ 0.45 ns from the positron annihilation, and τ 3 due to o-Ps annihilation. The o-Ps lifetime τ 3 is relatively longer than the others, of the order of 1–5 ns in polymeric materials, the so-called pickoff annihilation with electrons in molecules [7]. A correlation between the measured o-Ps lifetime τ 3 and the free-volume radius R, based on a spherical-cavity model [38], has been established as: 
 R 1 2πR −1 −1 τ3 (ns ) = 2 1 − (1) + sin R0 2π R0 where R0 = R + R, and R is an empirical parameter deter˚ [39] by fitting the observed lifetimes with mined to be 1.66 A the known hole and cavity sizes in molecular substrates. Furthermore, the intensity or the probability of o-Ps lifetime I3 may be used as information about the relative numbers of free volume [35,36], then free volume Vf based on R from Eq. (1) and I3 are the foundation for the determination of fractional free volume (ffv) [7]. A good correlation has been calibrated for hole sizes up to about a radius of 1 nm. Recently, this equation has been extended for hole sizes larger than 1 nm or o-Ps lifetimes longer than 20 ns [40,41]. It is assumed that in the larger pores, the o-Ps behaves more like a quantum particle, bouncing back and forth between the energy barriers as the potential well becomes large. A PAL spectrum in a polymeric material could be resolved into a continuous lifetime distribution since the free volume

Fig. 4. Mean stopping distance (a) as a function of positron incident energy and stopping profiles and (b) for positrons as a function of mean depth.

has a distribution. All PAL spectra were further analyzed into continuous lifetime distributions using three existing programs: CONTIN [42], LT [43], and MELT [44]. While they provide similar results, we only present the smoothed lifetime distributions from MELT analysis here. When a positron enters the polymeric surface, it loses its energy via inelastic collision processes, which could be expressed by a Makhovian implantation profile. The mean depth Z of the polymer where the positron annihilation occurs is calculated from E+ using Eq. (2) below [45]:   40 × 103 1.6 Z(E+ ) = E+ (2) ρ where Z is expressed in nm, ρ is the density in kg/m3 , and E+ is the positron incident energy in keV. The corresponding depth Z and its distribution with energy E+ are plotted versus the positron incident energy and the depth in Fig. 4. It is seen that the depth resolution is better near the surface than in the bulk. A typical depth resolution is estimated to be about 10% of the depth of interest. All positron beam experiments were performed under a high vacuum of 10−8 Torr. For DBES and PAL spectra, there were one and two million total counts, respectively. The polymeric samples were heated using a resistance Kapton device, which was controlled by an Omega temperature controller with accuracy of ±1 ◦ C. The heating process started from RT with 5 ◦ C intervals to 130 ◦ C, then cooled to RT.

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3. Results and discussion 3.1. Nanoscale layer structures of thin polymeric films Fig. 5 shows the variation of the S parameter measured by DBES as a function of positron energy or mean depth (top) as calculated from the positron energy according to the established Eq. (2) for different thicknesses of PS prepared with different wt.% of PS (0.3, 0.5, 0.8, 1.0, 1.5, and 2.0%) in toluene on Si and spun dry. A low value of S parameter at low positron energy for thin films is due to back-diffusion of the implanted positron and Ps from the polymer. In a polymer, the diffusion length of Ps and positron is small, of the order of 1–10 and 100 nm for Ps and the positron, respectively [37], due to the trapping in the free volume. When the positron reaches the interfacial region of polymer and substrate, the S parameter starts to decrease again as shown in Fig. 5. A thinner film shows that the peak is closer to the surface. We define the full-width-half-maximum (FWHM) of the peak as the thickness of polymer film. They are found to be 5 nm (0.3%), 12 nm (0.5%), 20 nm (0.8%), 30 nm (1.0%), 42 nm (1.5%), and 55 nm (2.0%) with wt.% of polystyrene in toluene as shown in parenthesis. Fig. 6 shows the variation of S parameter with respect to the positron incident energy and the mean depth (top x-axis) as calculated from an established Eq. (2) for an 80 ± 8 nm thin polystyrene film (prepared from 3.0 wt.% of PS in toluene) on Si wafers. From Fig. 6, four regions for a polystyrene film are identified: the near surface (region I), the polymeric film (region II), the interfacial layer (region III) between the polymer and the Si substrate, and the bulk of the substrate (region IV). Although it is known that the depth of the implanted positron spreads out as the positron enters further into the bulk, the division of three regions in a thin polymer film is still clearly identifiable from S parameter versus energy plot, as shown in Fig. 6. We fitted the S variation with the depth in a multi-layer model by using a computer program VEPFIT

Fig. 5. Plot of S parameter vs. implantation positron energy (depth) for blank silicon wafer, polystyrene films in different thicknesses, and a thick polymeric film.

Fig. 6. Plot of S parameter vs. positron incident energy (depth) in an 80-nm PS film (top) and depth (y-axis) vs. scanning distance (x-axis) in PS film on Si from profilometry.

[46], which has taken the positron implantation profile into consideration. The resolved film thickness (80 ± 8 nm) from VEPFIT analysis agrees well with the result obtained from profilometry (82 ± 5 nm) [18] as shown in the lower plot of Fig. 6. Fig. 7 are two schematic diagrams which show the result of fitted nanoscale layer structure for a thin polymer film on substrate: surface layer (I), film layer (II), interface layer (III), and substrate layer (IV). The main conclusions from the fitted VEPFIT results of the S parameter versus depth for an 80-nm PS film are: the density of the interface layer (0.4 ± 0.3 g/cm3 ) is significantly lower than that of the film layer (1.1 g/cm3 ); the thickness of the film layer and the interface layer is 80 ± 8 and 21 ± 3 nm, respectively; the Ps diffusion length is very short, 3 ± 1 nm, and the positron diffusion length in the film and in the interface are 40 ± 5 and 9 ± 3 nm, respectively. The S parameter data are thus useful for the determination of layer structures for polymeric thin films. Fig. 8 shows the o-Ps lifetime τ 3 and intensity I3 versus energy or mean depth in an 80-nm PS film. The o-Ps lifetime is large at the surface and decreases to 2.1 ns at the film layer, then further decreases after the interface layer to 1.7 ns in the substrate. The observed o-Ps lifetime (1.7 ns) component in Si at a very small intensity (I3 < 0.5%) is attributed to some Ps

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Fig. 7. Sketches of the layer structure of a polymer thin film on a substrate from this work. Nanoscale layer structure of thin polymeric film supported on Si is composed of four layers: near surface of polymer film (I), the film (II), the interface between polymer film (III), and substrate (IV).

annihilation with different depths of the film as the positron energy is spread out when the positron enters the substrate. It is interesting to observe that the variation of o-Ps intensity (I3 of Fig. 8) is similar to that of the S parameter in Fig. 6. This is expected because S mainly determines p-Ps, while I3 is the intensity of o-Ps. At the surface, the low intensity is due to back-scattering of positron and Positronium. Similar to S data, it increases to its peak value at the film layer, then decreases at the interface layer, then to nearly zero due to no o-Ps formation in the Si substrate. All PAL spectra were further analyzed into continuous lifetime distributions (or radius distributions), as shown in the lower plot of Fig. 9. The distribution becomes wider as the mean depth is closer to the surface. This is consistent with that reported in different polyurethanes [47]. FWHMs of the distribution in the free-volume radius at different depths of the thin film are listed in Table 1 to compare with the thick film result. The broadening in free-volume distribution is related to the degree of incomplete entanglement of polymer chains and to the increased chain motion near the surface and in the interface. For the currently studied polystyrene, Mw = 212,400, the gyration radius is 13 nm calculated from ˚ = 0.28 Mw1/2 [11]. It has a significant effect on polymer Rg (A) chain ends, near the surface (5 nm), and the interface (70 nm) as seen in a broad free-volume distribution.

Fig. 8. o-Ps lifetime (τ 3 ) and intensity (I3 ) vs. positron energy or mean depth (top) and free-volume radius distribution (bottom) at different depths of an 80-nm PS film.

3.2. Glass transition in polymeric thin films Glass transition temperature Tg can be defined accurately by measuring the molecular level of free-volume variation with respect to temperature. The DSC measures the variation of heat capacity rate with temperature, and the intercept point was found to be Tg = 100 ± 1 ◦ C for a thick PS film [12]. We measured the free-volume size (from o-Ps lifetime or τ 3 ) in the same thick PS film and plotted the results versus temperature as shown in Fig. 9 (upper). There is a slope change at Tg on the free volume versus temperature plot, which is measured to be 97 ± 2 ◦ C. The difference from PAL is slightly lower than DSC due to the slower rate of PAL experiments (hours to days) as opposed to DSC (seconds to minutes). Furthermore, we obtained the free-volume distributions at different temperatures in a thick PS film and plotted them in Fig. 9 (bottom). It is interesting to observe that the free volume has a wider distribution as the temperature increases. We calculated FWHMs and found that the onset temperature (96 ± 5 ◦ C) of FWHMs versus temperature plots coincides with the Tg as determined from the o-Ps data (Fig. 9) as

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Table 1 Free-volume thermal expansion coefficients and FWHM of distribution in an 80-nm polystyrene film Locations

Tg (◦ C) (PAL)

βg (Vf /Vf T (below Tg ), K−1 )

βr (Vf /Vf T (above Tg ), K−1 )

˚ of free-volume FWHM (A) radius distribution

Surface (5 nm) of 80-nm film Center (36 nm) of 80-nm film Interface (70 nm) of 80-nm film Bulk of thick film

80 ± 5 98 ± 3 86 ± 2 97 ± 2

2.3 ± 0.3 × 10−3 1.6 ± 0.3 × 10−3 1.3 ± 0.2 × 10−4 2.0 ± 0.2 × 10−3

5.6 ± 0.4 × 10−3 7.0 ± 0.4 × 10−3 6.7 ± 0.4 × 10−3 6.2 ± 0.3 × 10−3

0.263 ± 0.010 0.159 ± 0.006 0.167 ± 0.008 0.156 ± 0.005

shown in Fig. 10. A wide distribution is due to the increase of free volume in both small and large sizes, which are generated from polymeric end chains and the formation of larger holes due to molecular motions at higher temperature. PAL is a powerful technique to determine Tg at the molecular level. The S parameter from DBES could be used to determine the variation of Tg at different depths as a function of temperature. We have measured S versus temperature for six thicknesses of PS films (5, 12, 20, 30, 42, and 55 nm). Fig. 11 shows two such plots for 30 and 42 nm thin films, respectively. Since the Ps formation shows a very weak response to

temperature (see later Section 3.3 and Fig. 13), the S variation as a function of temperature is attributed to an expansion of free volume with temperature. It is resulted from Ps localization in the free-volume hole and according to the uncertainty principle as stated in the Section 2 and described else [12]. The steepest increase occurs at Tg . In Fig. 11, the intercept temperatures are found to be 89 ± 5 and 97 ± 5 ◦ C, for 30 and 42 nm thickness of PS film, respectively. The result of Tg as obtained from S data versus different PS films thickness is shown in Fig. 12. This variation is compared with the reported equation of Tg (d) variation with respect to the film thickness d for a weak interacting substrate by the following equation [11]
 γ  ξ0 Tg (t) = Tg (∞) 1 − (3) d where ξ 0 is the length scale of the mobile region, γ a constant, Tg (∞) the asymptotic Tg in the limit of large length, and ξ 0 and γ are fitting parameters which depend on the properties of the polymer [11,12]. The fitted curve is shown in Fig. 12 with ξ 0 = 1.21 ± 0.54 nm and γ = 1.21 ± 0.21, which are comparable to the reported values ξ 0 = 1.46 ± 0.23 nm and γ = 1.37 ± 0.1 by spectroscopic ellipsometric measurement [23] as shown in the dashed line of Fig. 12. The consistent variation of Tg versus film thickness shows that the S parameter is a useful method to determine Tg for nanoscale polymeric films at the molecular level.

Fig. 9. o-Ps lifetime (hole radius) vs. temperature in a thick PS film (top) and the o-Ps lifetime distributions at different temperatures in a thick PS film (bottom). The intercept of two lines defines Tg = 97 ± 2 ◦ C.

Fig. 10. The full-width-half-maximum (FWHM) of free-volume radius distribution vs. temperature in a thick PS film. The FWHMs were calculated from the data in Fig. 9(bottom).

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abruptly. Below Tg , the polymer is glassy and the structure of the polymer chain is more confined than the rubbery state above Tg . According to the free-volume theory [6], at temperature below Tg , the free volume in the polymer increases slowly as a function of temperature, while above Tg , it increases greatly. Thus when the temperature is above Tg , the S parameter or o-Ps lifetime as a function of temperature is expected to show a larger slope because of the expansion and increase of free-volume holes in the rubbery state. Therefore, we expect the onset of S parameter or o-Ps lifetime versus temperature to be at Tg . In a confined nanoscale environment, polymer chains are relaxed and reoriented at a partially entangled manner. Since Si has a weak surface interacting with PS, the chain movement of the polymer shows more elasticity and is softer at a confined dimension compared to the bulk state. Tg is thus suppressed as the thickness decreases in PS films when their thickness is of the order of the radius of gyration. Fig. 12 also indicates that when the thickness of polymeric film is greater than 80 nm, the polymer behaves as the bulk state for PS with Mw = 212,400 at Tg (∞) = 100.2 ◦ C. It could be interpreted that for a film thickness of 80 nm, that is almost five times the gyration radius, the confinement effect of the polymer chain becomes insignificant. This result also is consistent with that from the local thermal analysis method [27]. 3.3. Glass transition in the surface and interfaces Fig. 11. S parameter vs. temperature at the center of the 30-nm (top) and of the 42-nm (bottom) PS films. Lines were the best linear fits in two regions of temperature and the intercept is defined as Tg = 89 ± 5 and 97 ± 5 ◦ C, for 30 and 42 nm thickness of PS films, respectively.

The Tg suppresses as the thickness of thin PS films (of the order of radius of gyration 13 nm) decreases. It is an interesting and ongoing research subject for the thin film applications. The nature of polymeric motion changes greatly when crossing Tg because the chain flexibility and mobility change

Fig. 12. Plot of glass transition temperature vs. the thickness of polystyrene. The solid curve is fit to Eq. (3) from the current data and compares with the existing data [11] in the dashed line.

Fig. 13 shows the variation of o-Ps lifetime (top) and intensity (bottom) as a function of temperature for an 80-nm PS film at three different depths: near the surface (5 nm), the center of the film (36 nm) and near the interface (70 nm) with Si. In all plots, we observe a large expansion of τ 3 at Tg as due to the existence of free-volume expansion form the glassy state to the rubbery state. The free-volume expansion coefficients (β = Vf /Vf T) above and below Tg at different depths of the film, along with the bulk PS data are listed in Table 1. It is interesting to observe nearly the same βr in the rubbery state for different depths, but a decreasing βg in the glassy state as the depth increases from the surface, the center, and to the interface. The clear onset temperature of the fitted lines on τ 3 represents Tg . For a thick polystyrene film, as shown in Fig. 9, Tg is determined to be 97 ± 2 ◦ C, which is close to the DSC measurement (100 ± 1 ◦ C). For the thin film, we observe a significant suppression of Tg near the surface (80 ± 5 ◦ C) and in the interface (86 ± 2 ◦ C), but no suppression at the center of the film (98 ± 3 ◦ C). The current result of lower Tg near the surface and interface is consistent with the reported Tg suppression near the surface in a thick film [12]. In Fig. 13, we only observe a slight trend of increase in I3 across Tg . This is typical for polymeric materials where I3 responds to temperature across Tg less dramatic as o-Ps lifetime [7]. We further analyzed all PAL data as a function of temperature into lifetime distribution for three depths of 80 nm PS film. In Fig. 14 (top), we plotted the result of o-Ps lifetime

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Fig. 13. o-Ps lifetime (top) and intensity (bottom) vs. temperature in an 80-nm polystyrene film at different depths and o-Ps lifetime (free-volume radius). The intercept of two fitted lines defines the Tg .

(free-volume radius) distributions at different temperatures at the center (36 nm depth) of the PS film. As expected, the free-volume radius is distributed wider as the temperature increases and a large increase occurs at Tg similar to that in the thick PS film (Fig. 9). The measured FWHMs of the freevolume distributions at different depths are plotted versus temperature in Fig. 14 (bottom). It is interesting to observe that the FWHM of the free-volume distribution has an onset temperature similar to the free-volume radius plots (Fig. 13). The onset temperatures from FWHM-temperature plots are close to the Tg s as determined from o-Ps lifetime plot. A broad free-volume distribution near the surface and at the interface indicates the existence of more end chains and the increase of polymeric chain mobility as evidence from the observed lower Tg . The existing reported Tg suppression for a thin film has been interpreted mainly in terms of incomplete entanglement of polymeric chains and broadening of Tg [11–14,21–32]. The new information from the current PAL is the depth dependence of free-volume distributions, which are used to interpret the observed Tg suppression near the surface and in the interface. For the interfacial Tg , since PS–Si is a weak

Fig. 14. The o-Ps lifetime (radius) distribution function (top) at different temperatures at the center (36 nm) of an 80-nm polystyrene film and the FWHM of free-volume radius distributions (bottom) vs. temperature at different depths of an 80-nm polystyrene film. The bulk data from the thick PS film from Fig. 10 is included for comparison. Lines were drawn through data points below and above Tg .

interaction interface, we expect Tg suppression (11 K) to be less than on the surface (17 K) where there is no interaction. This is also consistent with the reported less Tg suppression reported in capped PS films [48]. The current result of free-volume distribution by PAL offers a new interpretation that Tg suppression in a thin film is due to different degrees of free-volume distribution at different depths. The free volume has the widest distribution near the surface which leads to large Tg suppression; in the interface, it is slightly broadened but to a lesser extent, which leads to less Tg suppression. In the center of the thin film, it has a distribution similar to that of thick film, which has no observable Tg suppression. The current interpretation is consistent with a theoretical calculation which indicates increased fluctuation of free-volume holes and molecular self-diffusion in nanoscale thin polymer films to account for the Tg suppression [49].

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4. Conclusion PAS is a novel spectroscopic method, which contains the most fundamental information about physical properties in molecules, i.e. wave function, electron density, and energy level. PAL and DBES techniques provide unique scientific information about the defect properties at the atomic and molecular levels. In this paper, we demonstrate PAS’s novelty and sensitivity in measuring the nanoscale layer structures in thin polymeric films. We have observed a significant variation of Tg suppression as a function of depth in an 80-nm polystyrene thin film on Si: 17 K lower near the surface and 11 K lower in the interface of the Si substrate than the center of the film or in the bulk. This depth dependence of Tg suppression is interpreted as a broadening of free-volume distribution in the surface and interfaces. Although the positron has been discovered over 70 years, the uses and applications of PAS to molecular and biological systems are still at the developing stage and more systematic works along this line of research are needed in the near future.

Acknowledgments This research has been supported by NSF and NIST. We appreciate Drs. R. Suzuki and T. Ohdaira of AIST, T.N, Nguyen of NIST, and Professor T.C. Sandreczki for their collaboration. We would like to thank Professor Durig for his excellent leadership as the Dean of the College of Arts and Sciences at UMKC when the current slow-positron beam (Fig. 1) was successfully built in 1999.

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