NASA-TM-I I0469

MICROSTRUCTURAL CHARACTERIZATION OF POLYMERS BY

POSITRON LIFETIME SPECTROSCOPY

By

JAG J. SINGH

NASA Langley Research Center

Hampton, VA 23681 - 0001

INVITED PAPER

Presented at

Fourteenth International Conference on the Applications of Accelerators

in Research and Industry

held at

University of North Texas at Denton, Texas

Nov. 6 - 9, 1996

https://ntrs.nasa.gov/search.jsp?R=19960054347 2018-05-11T16:25:26+00:00Z

MICROSTRUCTURAL CHARACTERIZATION OF

POLYMERS WITH POSITRONS

Jag J. Singh

NASA Langley Research Center, Hampton, VA 23681

Positrons provide a versatile probe for monitoring microstructural features of molecular solids. In this paper, we report on

positron lifetime measurements in two different types of polymers. The first group comprises polyacrylates processed on

earth and in space. The second group includes fully-compatible and totally-incompatible Semi-Interpenetrating polymer

networks of thermosetting and thermoplastic polyimides. On the basis of lifetime measurements, it is concluded that free

volumes are a direct reflection of physical/electromagnetic properties of the host polymers.

INTRODUCTION

We have been using positron annihilation

spectroscopies to infer host polymer properties for a

number of years. Our earlier studies of epoxies (1),

polyamides (2) and polyimides (3) have indicated that

positronium (Ps) atoms form readily in epoxies and

polyamides, but do not seem to form in polyimides

studied. These conclusions were drawn as a result of the

observations that lifetime spectra did not have a

significant longlife component in the case of polyimides.

Of course, one could argue that Ps atoms could have

formed and been quenched so fast that they could not be

distinguished from localized positron annihilations. This,

however, can not be the case since the availability of

quenching electrons is directly related to the formation of

positronium atoms. If the quenching electrons were that

readily available, they would rather annihilate with free

positrons than let them enter the microvoids to form Ps

atoms for subsequent fast quenching. Also, the

experimentally observed relative intensifies of the short

and intermediate lifetime components in polyimides

argue against the formation of Ps atoms in them (4). We

will review here the positron behavior in two different

categories of polymers: (i). Contact Lens Polymers

(Epoxies), and: (ii). Semi-Interpenetrating Polymer

Networks (S-IPN) of polyimides. The contact lens

samples studied included earth- and space- processed

rods. The S-IPN samples included fully- compatible and

totally-incompatible thermoset and thermoplastic

constituents.

EXPERIMENTAL PROCEDURES

Material Preparation

Contact lens samples were synthesized from methyl

methacrylate and varying amounts of cross-linking

ethylene glycol di-methacrylate. The polymerization

process was initiated at 40 °C by using thermal catalysts.

The S-IPN samples were syn_thesized from three different

types of polyimides. La_RCTM - RP46 / LaRC TM - IA

polymers where the thermosetting and the thermoplastic

components were fully compatible and LaRC TM - RP46 /

LaRC TM - SI polymers where the two components were

totally-incompatible. The relative concentration of the

two components varied from 100 : 0 to 0 : 100 percent by

weight.

Positron Lifetime Measurements

Positron lifetime measurements were made using a

standard fast - fast coincidence measurement system. A

25 ttC Na 22 positron source, deposited on a 2.54 ttm

thick kepton foil folded on itself, was sandwiched

between 2.54 cm x 2.54 cm x 0.25 cm test coupons and

the spectra were accumulated for 24 hours. This counting

period produced a total of more than 105 counts in the

peak and over 106 in each spectrum. The time resolution

of the lifetime system was 265 picoseconds. All

measurements were made at room temperature and in dry

samples. The lifetime spectra were analyzed with

PATFIT program (5).
EXPERIMENTAL RESULTS

Contact Lens Samples

The positron lifetime spectra in these materials

exhibited 3-clear components. The longest lifetime

components (_3) are associated with the quenching of

orthopositronium (O-Ps) atoms. These lifetimes are

quantitatively related (6) to the dimensions of the

microvoids where O-Ps atoms form and annihilate. The

free volume fractions (f) have been calculated using the

relation: f-- CI3Vf3, where C has been determined by

equating f in N4a samples with its saturation moisture

content. This is acceptable since H20 molecules can not

enter these materials chemically. The results are

summarized in Tables I and H. Also listed in the table

are the densities of various samples. The densities of

various samples are consistent with their flee volume

fractions. The designations Top/Bottom refer to the top

and bottom of the glass tube in which the sample rods

were polymerized.

TABLE I. Summary of Orthopesitronium Lifetimes and Associated Parameters of the Earth - Processed Contact Lens

samples.

Sample Top/Bottom z3(PS)/I3(%) f(%) O (gm/cc)

Designation (+ 0.0020)

N4a (Two percent cross- Top 2487 ± 13/20.8 ± 0.1 1.49 ± 0.02 1.1686

N4a linking agent) Bottom 2498 ± 11/21.3 ± 0.1 1.45 ± 0.03 1.1689

N4b {Five percent cross- Top 2122 d=20/23.2 ± 0.3 1.17 ± 0.04 1.1836

N4b linking agent} Bottom 2283 ± 14/22.4 ± 0.2 1.30 ± 0.03 1.1796

TABLE H. Summary ofOrthopositroninmLifetimesand Associated Parametersof Space-ProcessedContactLens

Samples.

Sample Top/Bottom x3(ps)/I3(%) f(%) p (gngcc)

Designation (± 0.0020)

N4a {Two percent cross- Top 2498 ± 14/21.9 ± 0.2 1.49 ± 0.03 1.1685

N4a linking agent} Bottom 2498 ± 14/22.2 ± 0.2 1.45 ± 0.03 1.1686

N4b (Five percent cross- Top 2046 ± 14/23.1 =L0.3 1.09 ± 0.03 1.1836

N4b linking agent} Bottom 2183 ± 13/22.7 ± 0.3 1.21 ± 0.03 1.1820

S-IPN Samples

The lifetime spectra in these samples were also

analyzed into 3-components. However, the intensities of

thethirdcomponents ('c3)inallS-IPN samples were very

low (< 1%) and thesewere,therefore,attributedto the

background. The intermediatelifetimecomponents ('c2)

were the strongestin all S-IPN samples. We attribute

thcsccomponents to the trapped positronannihilations

ratherthan fast-quenchedO-Ps atoms.JustliketheO-Ps

annihilationlifetimcs,thc trappedpositronannihilation

lifetimescan bc quantitativelyrelated(7) to the trap

dimensions.The lifetimeresults,freevolume fractions,

dcnsiticsand diclcctricconstantsof S-IPN samplesare

summarized in Tables HI and IV. The free volume

fractions (f) have been calculatedby equating f in

LaRC rM- IA with itssaturationmoisturecontent.This is

justifiablesincehydrationofthisthermoplasticpolyimide

isminiscule.Itis instructiveto note distinctdifferences

between the properties of the compatible and

incompatibleS-IPN samples.

DISCUSSION

Polymericrods produced in microgravityenvironment

should be homogeneous in composition and density,

compared with 1-g processed materials,where the

bottom samples may be more compacted than the
samples from the top section of the rods. However, the

data summarized in Tables I and ]I show no differences

in the free volume and densities of the two types of

contact lens samples. A closer investigation of the exact

geometrical orientations of the processing chambers

revealed that the polymerization cylinders in the ground

laboratory and the space shuttle bay were in horizontal

planes as opposed to the vertical orientations that the

experimental plans called for. Under these

circumstances, we should have expected what we actually

observedl We do, however, find some interesting

differences in the case of S-IPN materials.

TABLE I11. Summary of Positron Lifetime and Associated Parameters in Fully-Compatible (8) S-IPN Samples (8).

Sample Composition x 2 (ps) / 12 (%)

LaRC TM- RP46 : LaRC TM" IA

f(%) p(gm/cO e (10 Ghz)

( + 2%)

0 : 100 488 + 3/80.0 4- 2.0 2.81 4- 0.08 1.3402 + 0.0029 2.82

25 : 75 422 4- 5/79.3 4. 3.0 2.58 4. 0.12 1.3815 4- 0.0028 3.01

35:65 3994-4/78.94-2.9 2.484-0.10 1.36684-0.0020 3.11

50 : 50 3944- 1/79.44-0.1 2.484-0.09 1.3713±0.0041 3.14

65 : 35 4004-2/76.34- 1.4 2.424-0.07 1.36294-0.0032 3.14

75 : 25 399 4- 2/79.6 4- 1.8 2.50 4- 0.07 1.3546 4- 0.0036 3.13

100 : 0 400 4- 4/81.4 ± 2.2 2.56 4- 0.08 1.3572 + 0.0020 3.13

TABLE IV. Summary of Positron Lifetimes and Associated Parameters in Totally-Incompatible S-IPN Samples.

Sample Composition

LaRC TM- RP46 : LaRC TM - SI

x 2 (Ps) / I2 (%) f (%) p(gm/cc)

(4. 0.0020)

e (10 GHz)

(4-2%)

0 : 100 400 4. 2/93.2 4. 0.4

25 : 75 394 4. 1/93.4 4- 0.5

35 : 65 397 4- 2/95.0 4- 1.0

50 : 50 395 + 1192.3 ± 0.4

65 : 35 396 ± 2/93.14. 0.8

75 : 25 398 4- 2/93.2 + 0.3

100 : 0 396 4. 1/94.7 4. 0.6

2.48 4- 0.12 1.3727 3.13

2.47 4- 0.12 1.3696 3.13

2.524-0.14 1.3630 3.12

2.44 ± 0.11 1.3667 3.13

2.47 4- 0.13 1.3620 3.13

2.48 4- 0.11 1.3693 3.13

2.51 4- 0.08 1.3789 3.12

For the compatible LaRC TM - RP46/LaRC TM - IA

samples, it is found that the free volume appears to go

through a minimum at 50:50 composition. The densities

and dielectric constants also follow a consistent trend as

seen in data summarized in Table V and illustrated in

Figures 1 and 2. For the incompatible LaRC TM - RP46/

LaRC TM - SI samples, on the other hand,no such trends

were observed. The stiffness of the LaRC _lv,. SI
TABLEV. Comparison Between the Experimental and Computed Values of the Densities and Dielectric Constants of

Fully-Compatible S - IPN Samples (8).

Sample Composition Density (p), gm/cc Dielectric Constant (_) at 10 GHz

LaRC TNL RP46 • LaRC TM- IA Experimental Computed Expofinmntal Computed

0 : 100 1.3402 ± 0.0029 1.3402 ± 0.0029 2.82 ± 0.06 3.10 ± 0.02

25 : 75 1.3815 + 0.0028 1.3558 + 0.0023 3.01 4- 0.06 3.11 ± 0.02

35 : 65 1.3668 ± 0.0010 1.3620 ± 0.0025 3.11 4- 0.06 3.12 4- 0.02

50:50 1.37134-0.0041 1.37144-0.0020 3.144.0.06 3.124.0.02

65 : 35 1.36294-0.0016 1.36724-0.0017 3.144-0.06 3.134-0.02

75 : 25 1.3546 4- 0.0018 1.3643 4- 0.0015 3.13 4- 0.06 3.12 4- 0.02

100:0 1.35724-0.0010 1.35724-0.0010 3.134-0.06 3.114-0.02

p (computed) = Wlpi + w2P2 + (1/2)aJ3(pro2), where (x = 0.0168 4- 0.0031 and p = Molecular chain overlap parameter.

c (computed) = {(1-f)/(cD + (f)/(_)}-t, where _R= 3.30.

chain precluded any electrostatic effects observed for the

more compliant LaRC TM - IA member (8).

g

3

g

e_

1.394

1.374

1.3341._ 1.g._ 1.:_r4 1.3_

Density (comput_:l), gin/co

Figure 1. Comparlslon Between the Experimental

and the Computed Vsluee of Densltlee

of Fully-Compatible S-IPN Samples.

4.5.

i 3,5

0

2.5

+

2

z3 zs _'7 io a3 a.3

Free VolamD Fraction (f}, %

Figure 2. Dielectric Constant vs Free Volume Frmctlon

in Fully-Compatible S-IPN SampleL

CONCLUDING REMARKS

Positrons are ideal probes for monitoring

microstructural properties of polymers. We have observed

direct correlations between the atomic scale free volume

holes and macroscopic mechanical and electromagnetic

properties of the polymeric materials not only

qualitatively but also quantitatively.

REFERENCES

1. Sigh, .LJ.; Stoffdty, D.M.; Holt, W.H.; Mock, W.M.; tad Teter,

J.P.: Effect of Ttamition Metnl Ions on Positron Annihilation

Chanz'terislies in Epoxies, Nucl. Instr. and Methods, 26(13), 598(1987).

2. Sigh, J.J.; St. Clair, T.L; Holt, W.H.; and Mock, W.M.: Moisture

Dependence of Positron Annihilation Spectra in Nylon-6, Nucl Instr. and

Methods, 221, 427(1984).

3. Singh, J.J.; _ A.; and St. Clair, T.L: A Low Energy Positron

Flux Camm'ator for Miermtroctural ChanO.erb_on of Thin Polymer

Films, in AIP Conf, rtr._ Proceedings #303, ¢dit_ by E. Ottewitt¢ and

A.H. Weiss (AlP pre_ New York, 1994) pp. 516 - 525.

4. West, R.N.: Pt_itron Studies of Condensed Matter, Advances in

Physics, 22,263(1973).

5. Kirkegaard, P.; Pederk-_ N.J.; and Eldrup, E.: PATFIT-g8

(RISO-M- 2740), 1988.

6. NakanJahi, H.; Wang, S.J.; and Jean, Y.C.: in Positron Annihilation

Studies of Fluids, edited by S.C. Sharma (World Scientific, Singapore,

1988) p.292.

7. Den& Q.; Sundar, C.S. and Jean, Y.C.: Prcuur¢ Dependence of Free

Volume Hole Properties in an Epoxy Polymer, .7. Phys. Chem, 96(1),

492(1992).

8. ,'Ringh, J.J.; I)aSef, R.H.; and Eflekhari,, A.: Mierosltuetural

Characterization of Semi- Interpenetrating Polymer NetworkJ by Positron

Lifetime Spectroscopy, NASA Technical Paper# 3617. 1996.