Polymer Characterization - American Chemical Society

17

Probing Polymer Structures Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 17, 2016 | http://pubs.acs.org Publication Date: May 5, 1990 | doi: 10.1021/ba-1990-0227.ch017

by Photoacoustic Fourier Transform Infrared Spectroscopy Marek W. Urban*, Scott R. Gaboury, William F. McDonald, and Ann M . Tiefenthaler North Dakota State University, Department of Polymers and Coatings, Fargo, N D 58105

This chapter presents recent developments as well as the theory of photoacoustic Fourier transform infrared (PA FTIR) spectroscopy. New techniques such as temperature photoacoustic and rheophotoacoustic (RPA) FTIR measurements and their applications to the surface analysis offibers and cross-linking reactions of amorphous networks are discussed. In situ photoacoustic FTIR detection of cross-linkingreactions permits monitoring such transitions as gelation and vitrification of the network as a function of temperature, and rheophotoacoustic FTIR spectroscopy allows one to relate the molecular deformations with external forces applied to a polymer.

T H E

T E R M " P H O T O A C O U S T I C " (PA) refers to the generation

of acoustic

waves by modulated optical radiation. This effect was discovered in 1880 by Alexander Graham Bell, who observed that audible sound is produced when sunlight modulated by a chopper is incident on optically absorbing materials (1,2). For almost 100 years, this 19th century concept has been overwhelmed by other spectroscopic techniques. It was rediscovered in the early 1970s with the advent of new sources of radiation. More sensitive detectors were followed by the theory of photoacoustic effect.

*Corresponding author

0065-2393/90/0227-0287$07.75/0 © 1990 American Chemical Society

Craver and Provder; Polymer Characterization Advances in Chemistry; American Chemical Society: Washington, DC, 1990.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 17, 2016 | http://pubs.acs.org Publication Date: May 5, 1990 | doi: 10.1021/ba-1990-0227.ch017

288

POLYMER

CHARACTERIZATION

Photoacoustic spectroscopy (PAS) detects the acoustic signal emitted from a sample that has absorbed a modulated electromagnetic radiation. A sample is placed in a small chamber to which a sensitive microphone is attached. Upon absorption of modulated light, the sample generates heat. Its release leads to temperature fluctuations at the surface. The frequency of the temperature fluctuations is in phase with the modulation frequency. The temperature fluctuations of the sample surface cause pressure changes in a surrounding gas, which, in turn, generate acoustic waves in the sample chamber. These pressure changes of the gas are detected by a sensitive microphone. Several processes may occur after light absorption. Depending upon the nature of detection, there are essentially four classes of PA signal: (1) PA spectroscopy that measures the amplitude of PA signal for a range of optical excitation wavelength; (2) PA spectroscopy that monitors deexcitation processes after optical excitation; various decays are possible including luminescence, photochemistry, photoelectricity, and heat that may be generated directly or through energy-transfer processes; (3) PA probing of thermoelastic or other physical properties of materials such as sound velocity, elasticity, flow viscosity, specific heat, substrate defects; and (4) PA generation of mechanical motion. Although these methods have generated various applications and have been the subject of numerous studies, the most common photothermal effect is caused by the heating of a sample after the absorption of optical energy. Other deexcitation processes besides heating may also occur. Figure 1 illustrates various paths producing the photoacoustic signal. In its broader sense, photoacoustics is the generation of acoustic waves or other thermoelastic effects by any type of energetic radiation, including electromagnetic radiation from radio frequency to X-rays, electrons, protons,

OPTICAL

•

•

• •

ABSORPTION

•

THERMAL DEEXCITATION # + + + + + + + + + + + +

•

• *

• * •

+ ++

# • # #

+ + + +

+ + + + +

HEAT

RADIATION TRAPPING * * + + + + + + LUMINESCENCE

• +

CHAIN REACTIONS PHOTOCHEMISTRY

PHOTOELECTRICITY

ENERGY

TRANSFER

+ + + + + + + +++ +

1

CARRIER RECOMBINATIONS * + + + + + + + + + + + + +

COLLISIONS + + + + + + + + + + + + + + + +

Figure 1. Various processes leading to the production of heat and generation of acoustic waves.


17.

URBAN E T AL.

Probing Structures by Photoacoustic FTIR Spectroscopy 289

ions, and other particles. As a consequence, quite a substantial amount of experimental and theoretical work has been presented in the literature on applications not only in spectroscopy, but also in many other disciplines such as physics, chemistry, biology, and medicine. Depending upon the method of PA signal generation, several criteria of classification have been established. Here, we will focus on the generation of an indirect PA signal. In direct PA generation, the acoustic waves are


created within the sample where the excitation energy is absorbed, but in indirect generation, the acoustic waves are generated in a coupling medium adjacent to the sample, usually because of heat produced at the sample surface and subsequent emission of acoustic waves in the coupling medium. This form of detection is essential in photoacoustic infrared spectroscopy. The infrared (IR) region of radiation offers considerable advantages in polymer science because the energy of vibrating atoms forming chemical bonds falls in this range. As a result, an IR spectrum can be obtained. This process is schematically depicted in Figure 2. Because of the energy-conversion processes (absorption of light-emission of acoustic waves), such detection

PHOTOACOUSTIC DETECTION

MICROPHONE

PAS

SIGNAL

Figure 2. Schematic representation of indirect photoacoustic signal detection for condensed samples.


290

POLYMER CHARACTERIZATION

can be a valuable tool w h e n the optical absorption is so strong that it p r e v e n t s light passage t h r o u g h the sample.

Theory of Photoacoustic Effect F i g u r e 3 schematically depicts the generation of photoacoustic signals. T h e m o d u l a t e d I R radiation w i t h intensity I

enters the sample w i t h refractive

0


index n a n d absorption coefficient ($. T h e i n t e n s i t y of the I R radiation d i m i n i s h e s e x p o n e n t i a l l y as it penetrates the s a m p l e , g i v i n g rise to the i n t e n s i t y at d e p t h x:

I(x) = I ( l - n ) e x p ( - 0x)

(1)

0

T h e a m o u n t of l i g h t absorbed w i t h i n the thickness x is e q u a l to: E(x)

=

0J(x) = p/o(l -

n) exp ( - p x )

(2)

T h e d e p t h o f o p t i c a l p e n e t r a t i o n is d e f i n e d as o p t i c a l absorption l e n g t h , L , p

a n d is i n v e r s e l y p r o p o r t i o n a l to 0 : 1

h

= X

(3)

0

I n other w o r d s , L is the distance from the surface at w h i c h the i n i t i a l I R intensity, I , attenuates to ( l / e ) I . T h e absorbed energy is released i n a f o r m of heat that is transferred to the sample surface. T h e efficiency o f the heat transfer is d e t e r m i n e d b y the t h e r m a l diffusion coefficient of the sample, a , a n d the m o d u l a t i o n frequency of the i n c i d e n t r a d i a t i o n , co: p

0

0

s

(4)

w h e r e a is the t h e r m a l diffusivity [ a = fc/pC, that is, t h e r m a l c o n d u c t i v i t y d i v i d e d b y (density X specific heat)]. T h e t h e r m a l diffusion l e n g t h , |x , is th

related to the t h e r m a l diffusion coefficient, a , as s

f2aT/2

1

(5)

L J

a*

T h e a m o u n t of heat p e r i o d i c a l l y transferred to the surface t h r o u g h the sample is t h e n e q u a l to

H

M

^

^

U

x

)

^

^

^

;

^


(6)


URBAN ET AL.

Probing Structures by Photoacoustic FTIR Spectroscopy

3

3 o

S2


292

POLYMER

CHARACTERIZATION

Applications of PA FTIR Spectroscopy


Although the theory of indirect PA signal generation has been described previously (3-5), we will briefly focus on its applications to the analysis of polymers. As suggested by Gersho and Rosencwaig (6-8), materials are classified according to their thermal and optical properties. Table I summerizes these results and demonstrates the relationship between photoacoustic intensity and modulation frequency. For example, most polymeric materials are optically thin and thermally thick. For such materials, the Rosencwaig-Gersho (RG) theory predicts that PAoca)- ^

(7)

:

where PA is the intensity of the photoacoustic signal and co is the modulation frequency of the incident light. The modulation frequency of the F T I R instrument is related to the velocity of the moving mirror of the interferometer: for Michelson interferometer: o> = 2Vv

(8)

for Ganzel interferometer: a) = 4Vv

(9)

where V is the mirror velocity (cm/s), and v is the vibrational frequency. The thermal diffusion length, that is, a distance below the surface from which the generated heat can communicate with the surface, is related to the modulation frequency by equation 5. Thus, by changing the mirror velocity of the interferometer, it is possible to vary the thermal diffusion length which, in turn, is the effective penetration depth. However, the thermal diffusion length is also a function of IR wavenumber. For a typical polymer, k = 0.0003 cal/(m s °C); p = 1.2 g / c m ; C = 0.35 cal/g °C; and the thermal diffusion length is of the order of 6.5, 6.9, 8.4, and 11.4 |xm at 3

Table I. Dependence of Modulated Frequency on Magnitude of Photoacoustic Signal Optical Property Thermally Thin Thermally Thick Optically 1. C (Xth » 0 ; |x > Lp 1. |Xth > 0 ; fXth < L transparent P A oc o)- 2 PA OC O) p

th

1

3/

2. fJlth < 0 ; u, « PA oc a , - * th

Optically opaque

L

p

1. u,th < 0 ; flu, > Lp PA

2. jxth > 0 ; |xh < Lp PA oc o r t

1

1. u,th » PA OC

oc a ) - 2 3 /

0 ; [ith O)

»

1

2. jx « 0 ; |i h < L$ PA oc to th

t

3/2

N O T E :

Polymer Characterization - American Chemical Society

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