High-Temperature Properties and Applications of Polymeric Materials

Chapter 5

Positron Annihilation Lifetime Spectroscopy To Probe Free-Volume Effects in High-Temperature Polymers and Composites Downloaded by NORTH CAROLINA STATE UNIV on August 19, 2012 | http://pubs.acs.org Publication Date: October 13, 1995 | doi: 10.1021/bk-1995-0603.ch005

A. J. Hill Faculty of Engineering, Monash University, Clayton, Victoria 3168, Australia Positron annihilation lifetime spectroscopy (PALS) as a probe of free volume in polymers and composites is reviewed. The effect of temperature on the free volume distribution and studies of the effects of thermal history on room temperature free volume and related mechanical, physical and diffusional properties are presented. The free volume related phenomena discussed include the glass transition, physical aging, gas permeability, and crosslink density. Positron annihilation lifetime spectroscopy (PALS) provides an atomic scale probe of the free volume in polymers and polymeric composites. The free volume model applied to PALS [1] interprets the lifetime of orthoPositronium (oPs) localized in inter- and intra-molecular spaces as a measure of the size of those spaces. Typical oPs lifetimes in polymers range from 1 nanosecond (ns) to 3 ns corresponding to free volume cavity diameters ranging from 0.3 nm to 0.7 nm. These distances compare well with nonbonded interatomic distances in polymers and the root mean square end-to-end distance of an average polymer repeat unit. Numerous properties of importance to the performance of polymers and composites at high temperatures are related to the polymeric free volume. Perhaps the most commonly quoted polymer and composite properties of interest at high temperatures are the mechanical and transport properties which are affected by thermal history [2,3]. High temperature effects on polymer and composite properties can occur during processing, in service, or due to combined effects of processing and in-service thermal history. These high temperature effects can be beneficial (e.g. post cure) or deleterious (physical aging) to the particular property of interest. For example, physical aging, which is the structural relaxation that occurs in glassy polymers, causes changes in mechanical properties (decrease in ductility, increase in yield strength) and diffusional properties which have been linked to a decrease in free volume [4,5]. Free volume concepts were developed in order to model the relationship between viscosity and volume in polymer melts. Polymer behavior in the melt can be extended to the glass transition region [6,7] and below T [8-10] by free volume theories that can g

0097-6156/95/0603-0063$12.00/0 © 1995 American Chemical Society

In High-Temperature Properties and Applications of Polymeric Materials; Tant, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

Downloaded by NORTH CAROLINA STATE UNIV on August 19, 2012 | http://pubs.acs.org Publication Date: October 13, 1995 | doi: 10.1021/bk-1995-0603.ch005

64

HIGH-TEMPERATURE PROPERTIES AND APPLICATIONS OF POLYMERS

accurately model the effects of temperature, pressure and stress on the physical state of the polymer. As most engineering polymers have glass or melt transitions in the range 90°C to 180°C, operating temperatures above 50°C can lead to either stress relief, physical aging, or crystallization, each of which can alter the physical, mechanical, and diffusional properties of the polymer as well as cause dimensional changes. Models of physical aging [4,5], yield behavior [11], and permeability [12,13] are based on free volume, hence the PALS technique aids the development of an understanding, on the molecular level, of these and other free volume related phenomena. Two aspects of PALS make the technique particularly useful for polymer and composite characterization. Firstly, PALS provides a direct probe of the free volume sites which are used to model the phenomena mentioned previously as well as the glass transition [14], blend miscibility [15], plasticization [16], stress relaxation [14], and diluent diffusion [17]. Secondly, PALS is a nondestructive characterization technique with the potential to monitor the state of the polymer or composite during processing or while in service. This chapter will discuss the PALS technique as a free volume probe and review some of the literature of PALS applied to polymers and composites. The PALS Technique Positrons are positively charged electrons that are emitted by some unstable isotopes during decay. Positron emitting isotopes are available commercially and the isotope most commonly used for PALS is N a . This isotope emits a positron and a 1.28 MeV photon almost simultaneously through its nuclear decay to N e . In order to use positrons to study materials, the material of interest is placed next to the N a positron source. The positrons are emitted with a range of energies (maximum energy of 540 keV) and penetrate a few hundred microns in polymeric materials. Each positron eventually annihilates with an electron in the material. PALS measures the distribution of positron lifetimes which is related to the electron density of the material at the positron-electron annihilation site. The positron is repelled by nuclei and hence prefers sites of lower than average electron density such as vacancy-type defects in metals or free volume cavities in polymers. In order to determine the positron lifetime, a start and stop signal are used to mark the positron birth (emission from Na) and death (annihilation with an electron) respectively. The 1.28 MeV photon that is emitted almost simultaneously with the positron is used to mark the positron birth. During annihilation, the masses of the positron and electron are converted into energy, typically through the emission of two 0.511 MeV photons. One of the 0.511 MeV photons is used to mark the positron death. The lifetime of the positron is dependent on its state and the local electron density at the annihilation site. In polymers, the positron can remain in its free state, become trapped at a site of lower than average electron density, combine with molecular species in the solid, or extract an electron from the surrounding material to form a semi-stable bound state known as positronium (Ps). Positronium can exist in two ground states: paraPositronium (pPs) in which the positron and electron have antiparallel spins, and orthoPositronium (oPs) in which the positron and electron have parallel spins. 22

22

2 2

22



5. HILL

65

Positron Annihilation Lifetime Spectroscopy

The distribution of positron lifetimes is collected as a timing histogram of approximately one million sequential annihilation events in a material of interest. The positron decay curve (which contains the positron lifetimes and their relative intensities) is convoluted with the instrumental resolution function and contributions from source and background components. Computer programs are available to extract the positron lifetimes and relative intensities for the distinguishable positron states [18-20]. In PALS studies of polymers the decay curve is usually fitted with three lifetimes. The three lifetimes and their related annihilation mechanisms are commonly quoted as: Τι 0.1 - 0.3 ns pPs self-annihilation and pPs-molecular species, X2 0.3 - 0.5 ns free positrons, positrons trapped in free volume, and positron-molecular species, X3 0.5 - 4 ns oPs pickoff annihilation in free volume. OrthoPositronium pickoff anriihilation is a quenching process during which the positron in oPs (localized in free volume cavities) annihilates with an electron, of opposite spin, from the surrounding cavity wall. The PALS equipment is often a fast-fast coincidence system comprised of hardware supplied by E G & G Ortec that is either thermally or digitally stabilized [21]. The N a source is typically available as aqueous N a C l which is evaporated, drop by drop, on a thin (< 3 mg c m ) metal foil creating a spot source of 2 mm diameter and activity of 0.5 to 2 MBq. For sorption studies an impermeable thin film material is used in place of the metal foil. A second foil is placed on the source foil, and the foils are sealed (by crimping or adhesive). The foil is sandwiched between two identical pieces of the sample material such that all positrons emitting from the source enter the sample material. Depending on sample thickness (at least 0.5 mm) and source strength, it takes approximately 15 minutes to 2 hours to collect one spectrum for analysis. Typically several spectra are collected such that population standard deviations can be reported on the lifetime and intensity parameters. The population standard deviations usually quoted for the oPs pickoff components in polymer studies are +/- 0.03 ns for the lifetime, T3, and +/- 0.3 % for the intensity, I3. The fitting programs for data analysis can model the lifetime spectrum as a finite sum of decaying exponentials [18] or can perform a Laplace inversion of the decay curve and extract the continuous lifetime distribution [22]. The number of counts needed for statistical accuracy of the Laplace inversion is much greater than that needed for acceptable results from the exponential fitting routines resulting in measurement times an order of magnitude longer in order to extract continuous lifetime distributions [23]. Radiation safety is a primary concern when dealing with radionuclides and ionizing radiation. Although N a is a high level isotope due to the 1.28 MeV photon emission, the source strengths used in PALS are relatively low, and hence laboratories should meet a medium level standard by installing a barrier and an ionizing radiation monitor. The barrier should restrict laboratory access to radiation workers only, and the monitor should be available to measure radiation levels. Some samples may leach N a from the source and become contaminated. These samples must be bagged, labeled and properly disposed. Samples and sources should be handled using disposable gloves. Sources not in use should be stored in lead containers; lead shielding can be used around the source-sample sandwich avoiding obstruction of the detector heads. Individual 2 2

22

2

2 2

2 2


66


radiation monitors (film badges or thermoluminescent devices) should be worn by personnel working in a PALS laboratory.


PALS as a Free Volume Probe Usually the third lifetime component (X3, I3) is associated with free volume related phenomena in polymers where X3 is related to the mean radius of the free volume sites and I3 is related to the concentration of free volume sites. There are, however, several free volume models for positron annihilation in polymers, some of which combine the second and third lifetime components in order to calculate a free volume size or fraction [1,24-29]. This section provides a generally accepted literature view of the free volume model applied to PALS based on the original work by Brandt et al. [1]. The Brandt free volume model was proposed in order to explain the observed increase in oPs lifetime with increasing temperature in molecular solids [1]. From rate kinetics considerations one might expect a decrease in lifetime (increase in annihilation rate) with increasing temperature; however, Brandt et aL [1] proposed a quantum mechanical explanation for the observed increase in oPs lifetime. According to the model, oPs localizes in free volume cavities in the polymer "lattice." The oPs lifetime is related to the probability of overlap of the oPs wave function with the wave function of an electron associated with the surrounding cavity wall. As the size of the free volume site increases, the local electron density decreases and the oPs lives longer resulting in an increase in X . The intensity of oPs annihilations, I3, increases with an increase in population of free volume sites due to a higher oPs formation and trapping rate. Thus PALS can yield information on the relative size and relative concentration of free volume sites; however, care must be taken in the interpretation of PALS results for molecular solids. For example, contact time with the N a source can create competitive processes (electron or positron scavengers, etc.) in some polymers which inhibit Ps formation thus reducing I3. The chemical properties of some functional groups also can affect Ps formation due to their ability to scavenge electrons or their electron or positron affinity [30]. In most cases Ps formation is not completely inhibited, and X3 remains unaffected. Hence X3 reflections of free volume cavity radius are less subject to the non-free volume related effects of radiation chemistry than I3 reflections of free volume concentration. 3

2 2

Models of lifetime related to a mean free volume cavity radius, R, have been developed [25,31], and the free volume size is calculated as V = 4ÏÏR /3. A Laplace inversion of the positron lifetime decay curve allows the continuous lifetime distribution to be obtained in place of the average lifetimes [22,23]. In the context of the free volume model, the free volume cavity size distribution is obtained in place of the mean free volume cavity size. The distribution of free volume sizes is important to free volume models of polymer behavior because only a part of the total free volume is available for molecular or segmental motion. Several recent PALS studies have been performed to investigate the effect of temperature [32,33], hydration [34], and pressure 3

f


5. HILL

67


[35,36] on the free volume size distribution. Other free volume probes such as fluorescent, electrochromic, and photochromic probes are, in general, larger than the intrinsic free volume cavities of polymers and measure different free volume distributions than the PALS probe [33,37]. This chapter will illustrate the applicability of the PALS results to free volume related properties of interest for high temperature polymeric materials.


PALS Applied to Polymers and Composites Molecular Relaxations and Free Volume Distributions. The classical free volume theories for liquids by Doolittle [6] and Fox and Flory [7] were developed for thermodynamic equilibrium and have not been successfully extended to temperatures far below T . An equation of state for glassy polymers, developed by Simha and Somcynsky [8] and subsequently modified [9], has successfully modeled the effects of temperature, pressure and stress on the free volume in the glassy state [10,38,39]. Comparison of the Simha and Somcynsky free volume parameter to the free volume parameters measured by PALS shows reasonable agreement [33,40]. PALS free volume measurements as a function of temperature have been used to study T and subT (Τβ, Τ ) mobility - free volume relationships in numerous polymers such as epoxy [41], polycarbonate [42], polyethylene terephthalate [43], polytetrafluoroethylene [44] polypropylene [45], polyethylene [46], and polystyrene [46,47,48]. The work of Kasbekar [48], shown in Figures 1 and 2, illustrates the dependence of the oPs pickoff annihilation parameters, T and I3, on temperature. The glass transition is indicated as a change in slope of X3 versus temperature similar to specific volume - temperature behavior. The I3 parameter, shown in Figure 2, also is sensitive to molecular motion and indicates a change in the relative number of free volume cavities rather than a change in their mean size. The glass transition indicated by PALS is usually evident at a lower temperature than that found by differential scanning calorimetry or dynamic mechanical analysis. It is postulated that the oPs free volume probe detects the changes in electron density due to molecular motion that occur at temperatures lower than the cooperative motion of chains measured by other techniques. The annihilation rate of oPs by pickoff in free volume cavities is approximately 10 Hz, hence molecular motions occurring at a lower frequency occupy dynamic free volume that is accessible to oPs. As the frequency of motion changes with temperature, these sites can become inaccessible to oPs, and this change in dynamic free volume allows the detection of subT transitions by PALS. g

g

g

γ

3

10

g

As polymers are cooled through T , the inability of molecular relaxations to occur on the time scale of cooling creates a nonequilibrium state, the glassy state. In the glassy state, thermodynamic properties change at constant temperature and pressure as a function of time. The gradual approach of polymer properties toward an equilibrium below T is termed physical aging. Physical aging can be accelerated by annealing glassy polymers at temperatures close to but below T . The extent of physical aging is g

g

g


68


Ό 08


> et U ο u

fa

10

30

50

70

130

90

150

Temperature (°C) Figure 1. The oPs pickoff lifetime, X3, as a function of temperature for atactic polystyrene. Adapted from ref. 48. 40

G Ο C

s e ο U 7

Ο >

fa

T3

32 H T

4>

!τ 30

1 -50

1

1 1 1—ι -30 -10 10

g

β

1

1—ι 30

1 50

1

i

1 • 1 • 1 • 1• 70

90

110

130

150

Temperature (°C) Figure 2. The oPs pickoff intensity, I3, as a function of temperature for atactic polystyrene. Adapted from ref. 48.


5. HILL

69


a function of aging time and temperature and is reflected in physical property changes, such as an increase in density and T , and in mechanical property changes, such as an increase in tensile yield strength and a decrease in fracture toughness. As mentioned previously the extension of free volume theories below T has been used to link molecular mobility with physical and mechanical property changes due to physical aging [4,10]. PALS measurements of the free volume change due to physical aging and the relaxation kinetics of free volume in the glassy state have been linked to molecular mobility, the structural state, and hence physical and mechanical properties [49-52]. Sensitivity of the oPs pickoff component, (13, I3) to structural relaxation (physical aging) in the glassy state has been reported in epoxy [50], polycarbonate [53], polyvinylacetate [54], and polystyrene [49]. The distribution of hole sizes probed by PALS ranges from the macroscopic pore size range in porous epoxy resins (and other high surface area materials) [55] to the atomic scale free volume cavities discussed thus far. Surface area studies of porous resins will not be discussed in this review; however, these measurements are useful for polymer and composite characterization in relation to moisture uptake [20]. The continuous distribution of positron lifetimes (free volume radii) can be calculated from PALS data using the program CONTIN(PALS2) [56]. Studies of the free volume distribution for polymers as a function of temperature [32,33], pressure [35,36], and hydration [34] have been reported. For example the temperature dependence of free volume in polyoxymethylene has been reported by Kristiak et al [32]. Figure 3 illustrates the change in the free volume distribution probed by PALS as temperature is varied. The lifetime X3 has been converted to a free volume cavity radius R using the model of Nakanishi et al. [25]. Figure 3 shows that the free volume cavity radii decrease as temperature is lowered and their distribution narrows. Deng and Jean [36] have measured the free volume distribution in an epoxy glass as a function of pressure. The second and third lifetime components are shown (Figures 4 and 5) as both can be related to free volume [26,31,57]. Increasing the pressure decreases the mean free volume cavity radii and narrows the distribution of free volume sizes. Polymers and composites usually are subject to thermal cycling and/or moist environments while under load (tensile or compressive), and knowledge of the free volume distribution as a function of temperature and stress can help predict service properties such as permeability, sorption, and stress corrosion cracking resistance. g


g

Gas Permeability. The distribution of free volume sizes is important to transport and sorptive properties, and the PALS free volume probe can be used to measure a particular fraction of the total static and dynamic free volume [33,58]. Water absorption in polymers and composites has been studied by PALS to determine the sensitivity of the oPs pickoff component (X3,13) to equilibrium moisture uptake [5964], moisture uptake rate [65], the states of water, whether bound or free [59,66], and the effect of thermal history on moisture uptake [62]. These studies have been performed on polyimides [62], epoxies [59-65], nylons [62], and polyvinylalcohol [66].


70

HIGH-TEMPERATURE PROPERTIES AND APPLICATIONS OF POLYMERS 0.4

•198°C 0.3


Λ

V Ε


Ο

%>° £ S e

10000

1000 4)

2 1 (μπι^ )

Diffusivity

Figure 7. The inverse of the cube of the oPs pickoff lifetime, I/X3 , (related to 1/V ) as a function of the natural logarithm of oxygen diffusivity showing reasonable agreement with Equation 1. Polymer abbreviations are defined in the text. Adapted from ref. 67. f

30

I 0

•

•

ι 2000

'

•

1

•

4000

M

c

1 6000

•

•

1 8000

(amu)

Figure 8. Molecular weight between crosslinks in epoxies as a function of the effective free volume cavity size calculated from the free volume theory of Singh and Eftekhari. Adapted from ref. 59.



5. HILL


75

Much effort has been devoted to the development of real time cure sensors based on dielectrics, ultrasonics, and fiber optics. The collection times of 15 minutes to 2 hours for each PALS spectrum precludes real time monitoring for industrial applications; however, laboratory research at lower temperatures (to slow reaction kinetics) can contribute to the understanding of the role of free volume in network development. The work of Suzuki et al. [80] on epoxies illustrates the sensitivity of the oPs pickoff components (13 and I3) to the network development during a cure cycle. The results are shown in Figures 9 and 10. Curing takes place at 70°C over a 30 hr period followed by postcures at 100°C and 150°C. With the initial temperature increase, T3 and I3 increase due to thermal expansion. As polymerization begins, there is a decrease in X3 and a slope change in I3 indicative of the consumption of monomer and the change in packing. Gelation is marked by a leveling off of both the T3 and I3 parameters as the 3-D network is formed. In this study, further cure and postcure are evident only from the PALS parameter response to temperature change. On final cooling of the glass, the 13 value returns to that characteristic of gelation; however, I3 indicates retained free volume concentration (excess free volume) which is time dependent. The time dependent I3 response to temperature or pressure change has been characterized in other glassy polymers and related to structural relaxation [31,53]. The relaxation kinetics are indicative of the state of the glass and have been used to characterize physical aging as discussed in a previous section of this review. The lifetime X3 and intensity I3 results show that the PALS parameters related to free volume can detect the commencement of polymerization, propagation of the 3-D network, termination of polymerization, and are sensitive to the cure temperature and thermal history. Conclusions and Future Work This review has presented and discussed a number of examples of PALS studies of polymers and composites. PALS gives a measure of the positron lifetime which is related to the electron density at the annihilation site. Changes in chemistry, temperature, pressure or some other variable which affect the electron density at the oPs localization (annihilation) site are reflected in the oPs pickoff lifetime. The oPs localizes in regions of reduced electron density such as free volume and interfacial sites in polymers and composites. Hence the variations in the lifetime, X3, and intensity, I3, of the oPs pickoff component have been related to free volume behavior for the examples discussed in this review. The sensitivity of the PALS probe to the free volume that affects physical, diffusional, and mechanical properties indicates that the PALS technique provides a useful characterization tool. The measurement of free volume distributions as functions of temperature, pressure, hydration, or stress can provide information to help understand the mechanisms of polymer and composite degradation under load in hot-wet environments, leading to the design of better polymers, fillers, and interfaces.


76



Response to Temperature


00

Time (hrs) Figure 9. The oPs pickoff lifetime, 13, as a fonction of time during the curing cycle for an epoxy. Adapted from ref. 80. Response to Temperature 1

0


Development of Network

20

40

/

60

80

100

Time (hrs) Figure 10. The oPs pickoff intensity, I3, as a function of time during the curing cycle for an epoxy. Adapted from ref. 80.



5. HILL


77

PALS studies of moisture uptake in thermosets have indicated that larger free volume cavities lead to higher absorption rates [65]. The distribution of free volume sizes is obviously important to diffusional properties, and as noted by Jeffrey and Pethrick [64], the presence of submicron and micron size pores contributes significantly to the moisture uptake. The oPs pickoff component can be used to measure pores as well as atomic-scale free volume cavities; hence the technique is useful for the prediction of hot-wet behavior. Also important to composites is the interfacial free volume. Relatively few PALS studies of composites have been reported [63,81,85]; however, PALS sensitivity to interfacial effects in phase separated homopolymers and polymer blends has been reported [74-77]. Systematic studies of composites with varying interfacial chemistry and thermal history could help in the identification of annihilations at interfaces and correlation of free volume or chemistry related interfacial effects with composite properties. The ability to follow cure reactions as functions of time and temperature is important to process control In epoxy cure, it is the final etherification reactions which are inaccessible to DSC characterization of cure but followed rather well by near infrared and dielectric loss spectroscopies. PALS studies in conjunction with near infrared and dielectric loss could shed light on the effect of these reactions on the network structure and free volume related properties. The depth of penetration of positrons emitted from N a is approximately a few hundred microns in polymers. The development of slow and/or monoenergetic positron beams [86] allows depth profiling of materials and should provide useful results for polymers and composites including network inhomogeneities due to cure history, fiber placement, damage sites, weak interface, etc. The recent and forthcoming advances in the use of PALS to characterize polymers and composites should help scientists and engineers progress in their understanding of free volume and its effect on properties. 2 2

Acknowledgments The author would like to thank J. Gonis, M . D. Zipper, R. M . Hodge, G . P. Simon, K. J. Heater, A. Siegle, H. Schneider and T. J. M . Bastow for reference help and useful discussions. Literature Cited

1. 2. 3. 4. 5. 6. 7. 8.

Brandt, W.; Berko, S.; Walker, W. Phys. Rev. 1960, 120, 1289. Tant, M . R.; Henderson, J. in International Encylcopaedia of Composites,Vol.6, Lee, S. M.; ed. VCH Publishers: New York, 1990. Hollaway, E., Ed. Handbook of Polymer Composites for Engineers, Woodhead Publ. Ltd.: England, 1994. Struik, L. C. E. Ann. Ν. Y. Acad. Sci. 1976, 279, 78. Struik, L. C. E. Physical Aging in Amorphous Polymers and Other Materials, Elsevier: Amsterdam, 1978. Doolittle, A. K. J. Appl. Phys. 1951, 22, 471. Fox, T. G.; Flory, P. J. J Amer.Chem.Soc. 1948, 70, 2384. Simha, R.; Somcynsky, T. Macromolecules 1969, 2, 342.



78


9. Neis, E.; Stroeks, A. Macromolecules 1990, 23, 4088. 10. Curro. J. G.; Lagasse, R. R., Simha, R. J. Appl. Phys. 1981, 52, 5892. 11. Glasstone, S.; Laidler, K. J.; Eyring, H. The Theory of Rate Processes, McGrawHill: New York, 1941. 12. Maeda, Y.; Paul, D. R. J. Polym. Sci. Β 1987, 25, 1005. 13. Cohen, M. H.; Turnbull, D. J. Chem. Phys. 1959, 31,1164. 14. Ferry, J. D. Viscoelastic Properties of Polymers, John Wiley and Sons: New York, 1980. 15. Patterson, D.; Robard, A. Macromolecules 1978, 77, 690. 16. Doolittle, A. K. The Technology of Solvents and Plasticisers, Wiley: New York, 1954. 17. Vrentas, J. S.; Duda, J. L.; Ling, H.-C. J. Polym. Sci. Β 1985, 23, 275. 18. Puff, W. Comput. Phys. Commun. 1983, 30, 359. 19. Seigel, R. W. Ann. Rev. Mater. Sci. 1980, 10, 393. 20. Schrader, D. M; Jean, Y. C, Eds. Positron and Positronium Chemistry: Studies in Physical and Theoretical Chemistry, Elsevier: New York, 1988. 21. Paulus, T. J. Optimization of a State-of-the-Art Positron Measurement Apparatus, EG&G Ortec, Oakridge, TN, 1988. 22. Gregory, R. B. J. Appl. Phys. 1991, 70, 4665. 23. Jean, Y. C.; Dai, G. H. Nucl. Instrum, Methods Phys. Res. Β 1993, B79, 356. 24. Tao, S. J. J. Chem. Phys. 1972, 56, 5499. 25. Nakanishi, H.; Wang, S. J.; Jean, Y. C. International Symposium on Positron Annihilation Studies of Fluids, Sharma, S. C. Ed., World Scientific: Singapore, 1987, 292. 26. Singh, J. J.; Eftekhari, A. Nucl. Instrum. Methods Phys. Res. Β 1992, B63, 477. 27. Eldrup, M.; Lightbody, D.; Sherwood, J. N. Chem. Phys. 1981, 63, 51. 28. Thosar, Β. V.; Kulkarni, V. G.; Lagu, R. G.; Chandra, G. Phys. Lett. 1969, 28A, 760. 29. Gol'danskii, V. I.; Onishchuk, V. A; Shantorovich, V. P.; Volkov, V. V; Yampol'skii, Y. P. Khim Fiz. 1988, 7, 616. 30. Mogensen, Ο. E. J. de Physique IV, 1993, 3, 1. 31. Jean, Y. C. Microchem J. 1990, 42, 72. 32. Kristiak, J.; Kristiakova, K.; Sausa, O.; Bandzuch, P.; Bartos, J. J. de Physique IV, 1993, 3, 265. 33. Lui, J.; Deng, Q.; Jean, Y. C. Macromolecules 1993, 26, 7149. 34. Gregory, R. B; Chai, K.-J. J. de Physique IV, 1993, 3, 305. 35. Jean, Y. C.; Deng, Q. J. Polym. Sci. Β 1992, 30, 1359. 36. Deng, Q.; Jean, Y. C. Macromolecules 1993, 26, 30. 37. Yampol'skii, Y. P.; Shantorovich, V. P.; Chernyakovskii, F. P.; Kornilov, A. I.; Plate, N . A. J. Appl.Polym.Sci. 1993, 47, 85. 38. Robertson, R. E.; Simha, R.; Curro, J. G. Macromolecules 1984,17,911. 39. McKinney, J. E.; Simha, R. Macromolecules 1976, 9, 430. 40. Vleeshouwers, S.; Kluin, J.-E.; McGervey, J. D.; Jamieson, A. M.; Simha, R. J. Polym. Sci. Β 1992, 30, 1429. 41. Jean, Y. C.; Sandreczki, T. C.; Ames, D. P. J. Polym. Sci. Β 1986, 24, 1247. 42. Kluin, J.-E.; Yu, Z.; Vleeshouwers, S.; McGervey, J. D; Jamieson, A. M.; Simha, R. Macromolecules 1992, 25, 5089. 43. Tseng, P.; Chang, S.; Chuang, S. Positron Annihilation (Proc. of 6th International Conference on Positron Annihilation) Coleman, P. G.; Sharma, S. C.; Diana, L. M. Eds., North Holland: New York, 1982, 730. 44. Kindl, P.; Sormann, H.; Puff, W. Positron Annihilation (Proc. of 6th International Conference on Positron Annihilation) Coleman, P. G.; Sharma, S. C.; Diana, L. M. Eds., North Holland: New York, 1982, 685. 45. Lind, J. H.; Jones, P. L.; Pearsall, G. W. J. Polym Sci. A 1986, 24, 3033.


5. HILL Positron Annihilation Lifetime Spectroscopy

46. 47. 48. 49.


50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77.

79

Varisov, A. Z.; Kuznetsov, Y. N.; Prokop'ev, E. P.; Filip'ev, A. I. Russian Chem. Rev. 1981, 750, 991. Stevens, J. R.; Mao, A. C. J. Appl. Phys. 1970, 41, 4273. Kasbekar, A. D. M. Sc. Thesis, Duke University, Durham, NC, 1987. McGervey, J.; Panigrahi, N.; Simha, R.; Jamieson, A. Positron Annihilation (Proc. of 7th International Conference on Positron Annihilation) Jain, P. C.; Singru, R. M ; Gopinathan, K. P.. Eds., World Scientific Publ. Co.: Singapore, 1985, 690. Sandreczki, T. C; Nakanishi, H.; Jean, Y. C. International Symposium on Positron Annihilation Studies of Fluids, Sharma, S. C. Ed., World Scientific: Singapore, 1987, 200. Hill, A. J.; Katz, I. M.; Jones, P. L. Polym. Sci. Engr. 1990, 30,762. Heater, K. J.; Jones, P. L. Mat. Res. Soc. Symp. Proc. 1991, 275, 207. Hill, A. J.; Heater, K. J.; Agrawal, C. M. J. Polym. Sci. Β 1990, 28, 387. Kobayashi, Y.; Zheng, W.; Meyer, E. F.; McGervey, J. D.; Jamieson, A. M.; Simha, R. Macromolecules 1989, 22, 2302. Venkateswaran, K.; Cheng, K. L; Jean, Y. C. J. Phys. Chem. 1983, 88, 2465. Gregory, R. B.; Shu, Y. Nucl. Instrum. Methods Phys. Res. A 1990, A290, 172. Deng, Q.; Sundar, C. S.; Jean, Y. J. Phys. Chem. 1992, 96, 492. Simon, G. P.; Zipper, M. D.; Hill, A. J. J. Appl. Polym. Sci. 1994, 52, 1191. Singh, J. J; Eftekhari, A.; Schultz, W. J.; St. Clair, T. L. NASA Technical Memorandum 4390, 1992. MacQueen, R. C.; Granata, R. D. Mater. Sci. Forum 1992, 705-770, 1649. MacQueen, R. C.; Granata, R. D. J. Polym, Sci. Β 1993, 31, 971. Singh, J. J. Proc. 33rd Internatl. SAMPE Symp., 7-10 March 1988, SAMPE: Covina, CA, 407. Singh, J. J.; Holt, W. H.; Mock, W. Jr. NASA Technical Paper 1681, 1980. Jeffrey, K.; Pethrick, R. A. Eur. Polym. J. 1993, 30, 153. Suzuki, T.; Oki, Y.; Numajiri, M.; Miura, T.; Kondo, K.; Shiomi, Y.; Ito, Y. J. Appl. Polym. Sci. 1993, 49, 1921. Hodge, R. M.; Bastow, T. J.; Edward, G. H.; Simon, G. P.; Hill, A. J. Proc. 3rd Pacific Polym. Conf. 13-17 Dec. 1993, Pacific Polym. Fed.: Gold Coast, Australia, 451. Kobayashi, Y.; Haraya, K.; Hattori, S.; Sasuga, T. Polymer 1994, 35, 925. Lui, L. B.; Lewis, J. C.; Yee, A. F.; Gidley, D. W. 8th Intl. Conf. on Deformation, Yield, and Fracture of Polymers, 8-11 Apr. 1991, Cambridge UK, 29/1. Lui, L. B.; Gidley, D. W.; Yee, A. F. J. Polym. Sci. Β 1992, 30, 231. Elwell, R. J.; Pethrick, R. A. Eur. Polym. J. 1990, 26 853. Okada, T.; Nishijima, S.; Honda, Y.; Kobayashi, Y. J. de Physique IV, 1993, 3, 291. Zipper, M . D.; Simon, G. P.; Cherry, P.; Hill, A J. J. Polym. Sci. Β 1994, 32, 1237. Zipper, M . D.; Simon, G. P.; Tant, M. R.; Small, J. D.; Stack, G. M.; Hill, A. J. Polym. Intl. in press. Zipper, M . D.; Simon, G. P.; Raris, V.; Campbell, J. A.; Hill, A. J. Mater. Sci. Forum, in press. Wang, C. L.; Wang, S. J; Zheng, W. G.; Qi, Z. N. J. de Physique IV, 1993, 3, 261. Ravindrachary, V.; Sreepad, H. R.; Chandrashekara, A.; Ranganathaiah, C.; Gopal, S. Phys. Rev. Β 1992, 46, 11471. Chanyuan, Y.; Weizheng, X.; Qingchao, G.; Dexun, S. Positron Annihilation (Proc. of 8th International Conference on Positron Annihilation) 29 Aug. - 3 Sept., 1988, Gent, Belgium, World Scientific Publ. Co.: Singapore, 1989, 573.


80

78. 79. 80. 81.


82. 83. 84. 85. 86.


Ratner, M . Polymer Electrolyte Reviews, MacCallum, J. R.; Vincent, C. Α., Eds., Elsevier: London, 1987, 185. Peterlin, A. Interrelations between Processing Structure and Properties of Polymeric Materials, Seferis, J. C.; Theocaris, P. S., Eds., Elsevier Science Publ. Β. V.: Amsterdam, 1984, 585. Suzuki, T.; Oki, Y.; Numajiri, M.; Miura, T.; Kondo, K.; Ito, Y. Polymer 1993, 34, 1361. Dale, J. M; Hulett, L. E.; Roseel, T. M.; Fellers, J. F. J. Appl. Polym. Sci. 1987, 33, 3055. Waber, J. T.; Wang, R.; Williams, J. Positron Annihilation (Proc. of 9th International Conference on Positron Annihilation) Sombathy, World Scientific Publ. Co.: Singapore, 1991, 1761. Gonis, J.; Simon, G. P.; Hill, A. J. Proc. 3rd Pacific Polym. Conf. 13-17 Dec. 1993, Pacific Polym. Fed.: Gold Coast, Australia, 533. Singh, J. J.; Eftekhari, Α.; Hinkley, J. Α.; St. Clair, T. L.; Jensen, B. J. NASA Technical Memorandum 4326, 1992. Tino, J.; Kristiak, J.; Hlouskova, Z.; Sausa, O. Eur. Polym. J. 1993, 29, 95. Positron Beams for Solids and Surfaces; Schultz, P.; Massoumi, G. R.; Simpson, P. J., Eds.; American Institute of Physics: London, Ontario, 1990.

RECEIVED November 17, 1994


High-Temperature Properties and Applications of Polymeric Materials

Recommend Documents