Elastic Properties of Thin Polymer Films Investigated with Surface

Chapter 15

Elastic Properties of Thin Polymer Films Investigated with Surface Acoustic Wave Devices 1,3

2

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on March 16, 2018 | https://pubs.acs.org Publication Date: August 24, 1989 | doi: 10.1021/bk-1989-0403.ch015

David S. Ballantine, Jr. , and Hank Wohltjen

1Geo-Centers, Inc., 10903 Indian Head Highway, Fort Washington, MD 20744 Microsensor Systems, Inc., P.O. Box 8, Springfield, VA 22150

2

The observed glass transition temperatures (T ) of several thin polymer films on surface acoustic wave (SAW) devices are 50-60 °C higher than the T results reported using other methods such as DSC. The increase in the onset of Τ is the result of interaction of the high frequency SAW with the polymer film, consistent with the time-temperature superposition principle. The T were identified as localized minima in the frequency curves, or by changes in the slope of the curves, as the coated sensors were heated between 35-110 °C. Potential applications of SAWs for the characterization of polymer materials and the implications of these findings for the interpretation of SAW data are discussed. g

g

g

g

Polymeric materials are being employed in an increasing number of novel applications. As stronger, more flexible and more durable materials are discovered the demand for these materials will continue to grow. The chemical and physical properties of these materials will determine the applications for which they may be employed. Thus, the rapid and reliable characterization of these properties will be crucial. In the area of chemical sensors, thin polymer films are routinely used as coatings for the semi-selective sorption of chemical vapors. One sensor technology, the surface acoustic wave (SAW) device, has demonstrated excellent sensitivity as a vapor sensor when coated with films having appropriate solubility properties (1). To date, most sensor applications have utilized the extreme mass sensitivity of the devices. In this paper, we will examine the response mechanisms of the SAW sensor and demonstrate its sensitivity to changes in the elastic properties of the coating materials. Finally, we will discuss the significance of these results in terms of current sensor applications, and the advantages of the SAW for polymeric materials characterization. 3

Current address: Department of Chemistry, Northern Illinois University, DeKalb, IL 60115 0097-6156V89/O4O3-O222$06.00/0 © 1989 American Chemical Society

Murray et al.; Chemical Sensors and Microinstrumentation ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

15.

BALLANTINE AND WOHLTJEN

223

Properties ofThin Polymer Films


Background S i n c e i t s i n t r o d u c t i o n i n t h e e a r l y 1970's, t h e SAW d e v i c e has seen i n c r e a s i n g u t i l i z a t i o n as a c h e m i c a l sensor due t o i t s s e n s i t i v i t y , p o t e n t i a l l y r a p i d r e s p o n s e , and low c o s t . A b r i e f d e s c r i p t i o n o f t h e o p e r a t i n g p r i n c i p l e s o f t h e SAW w i l l a i d i n u n d e r s t a n d i n g t h e b a s i s o f t h i s s t u d y and i n t h e i n t e r p r e t a t i o n o f t h e r e s u l t s . Typically, a SAW d e v i c e c o n s i s t s o f a s e t o f two i n t e r d i g i t a l t r a n s d u c e r s w h i c h have been m i c r o l i t h o g r a p h i c a l l y f a b r i c a t e d on t h e s u r f a c e o f a p i e z o e l e c t r i c s u b s t r a t e . The a p p l i c a t i o n o f a t i m e - v a r y i n g e l e c t r i c p o t e n t i a l t o one t r a n s d u c e r causes a m e c h a n i c a l d e f o r m a t i o n o f t h e s u b s t r a t e , r e s u l t i n g i n t h e g e n e r a t i o n o f a s u r f a c e a c o u s t i c wave. The second t r a n s d u c e r c o n v e r t s t h e m e c h a n i c a l wave back i n t o an e l e c t r i c a l s i g n a l . The p r o p e r t i e s o f t h i s wave ( a m p l i t u d e , f r e q u e n c y , phase) a r e s e n s i t i v e t o p e r t u r b a t i o n s o c c u r r i n g on o r near t h e s u r f a c e o f t h e s u b s t r a t e . More d e t a i l e d d i s c u s s i o n o f t h e SAW o p e r a t i o n c a n be found i n r e f e r e n c e ( 2 ) . The g e n e r a t i o n o f d i f f e r e n t types o f s u r f a c e waves i s p o s s i b l e ( 3 ) ; f o r t h e purposes o f t h i s work, when we r e f e r t o t h e s u r f a c e a c o u s t i c wave we s p e c i f i c a l l y mean R a y l e i g h - t y p e s u r f a c e waves. The response o f t h e SAW d e v i c e i s t h e combined r e s u l t o f changes i n the mass l o a d i n g , c o n d u c t i v i t y , o r e l a s t i c p r o p e r t i e s o f t h e s u r f a c e f i l m . E q u a t i o n s d e s c r i b i n g t h e e f f e c t s o f changes i n these p r o p e r t i e s on t h e f r e q u e n c y o f t h e d e v i c e have been d e r i v e d p r e v i o u s l y ( 2 , 4 ) . F o r many sensor s t u d i e s , n o n - c o n d u c t i n g polymer f i l m s a r e employed. One e q u a t i o n , g i v e n below, d e s c r i b e s t h e response b e h a v i o r f o r a SAW d e v i c e coated w i t h a t h i n , l o s s l e s s , i s o t r o p i c , non-conducting f i l m , af - (k +k )phf 1

2

2 0

2

2

- k h f ( 4 y / V ) [ (λ + 10/(λ+2μ) ] 2

0

(1)

R

where k and k a r e m a t e r i a l c o n s t a n t s f o r t h e q u a r t z s u b s t r a t e , V i s the R a y l e i g h wave v e l o c i t y , h i s t h e f i l m t h i c k n e s s , ρ i s t h e d e n s i t y , μ i s t h e shear modulus, A i s t h e Lame c o n s t a n t , and f i s t h e fundamental frequency o f t h e d e v i c e . T y p i c a l v a l u e s f o r these parameters f o r ST-cut q u a r t z s u b s t r a t e s a r e g i v e n i n T a b l e I . The f i r s t h a l f o f t h e e q u a t i o n y i e l d s t h e frequency s h i f t r e s u l t i n g from mass l o a d i n g , w h i l e t h e second h a l f d e s c r i b e s t h e e f f e c t o f changes i n the e l a s t i c p r o p e r t i e s o f t h e f i l m on t h e r e s o n a n t f r e q u e n c y . To d a t e , t h e s e l e c t i o n o f c o a t i n g s f o r v a p o r sensor a p p l i c a t i o n s has been m o s t l y e m p i r i c a l , r e q u i r i n g t h e s c r e e n i n g o f a l a r g e number of candidate m a t e r i a l s to i d e n t i f y coatings w i t h s u f f i c i e n t s e n s i t i v i t y t o t h e vapor o f i n t e r e s t . To address t h i s problem, r e c e n t work has f o c u s e d on c h a r a c t e r i z i n g t h e o b s e r v e d sensor r e s p o n s e s i n terms o f s o l u b i l i t y i n t e r a c t i o n s ( 1 , 1 ) . The s o r p t i o n o f a s o l u t e v a p o r i n t o a s o l v e n t c o a t i n g c a n be q u a n t i t a t i v e l y d e f i n e d as K, t h e p a r t i t i o n c o e f f i c i e n t . A m o d i f i e d v e r s i o n o f E q u a t i o n 1 has been used t o p r e d i c t the frequency response o f c o a t e d SAWs t o s p e c i f i c v a p o r s u t i l i z i n g Κ v a l u e s c a l c u l a t e d from g a s - l i q u i d chromatography (GLC) d a t a ( 5 ) . The i n h e r e n t assumptions i n t h a t work a r e t h a t (1) t h e polymer c o a t i n g i s a l o s s l e s s f i l m ( t h a t i s , t h e r e i s no s i g n i f i c a n t a t t e n u a t i o n o f t h e s u r f a c e wave r e s u l t i n g from i n t e r a c t i o n s w i t h t h e s u r f a c e f i l m ) , and (2) t h e c o n t r i b u t i o n s t o t h e o b s e r v e d response from e l a s t i c p r o p e r t i e s o f t h e f i l m a r e n e g l i g i b l e . To j u s t i f y t h e second a s s u m p t i o n , polymer f i l m s were s e l e c t e d t h a t would be above t h e i r g l a s s t r a n s i t i o n temperatures (T ) a t t h e o p e r a t i n g temperature o f t h e d e v i c e . 1

2

R

Q


224

CHEMICAL SENSORS AND

TABLE I.

parameter

k


1 *2 V

MICROINSTRUMENTATION

TYPICAL PARAMETER VALUES*

description

value (units)

material constant

-8.7 χ 10" m s/kg -3.9 χ 10" "

8

2

8

R

Rayleigh wave v e l o c i t y

3158

density of polymer

1000 kg/m

h

film

f i l m thickness SAW

resonant frequency

shear modulus (glass) * (rubber)

μ

M

3

7

1 χ 10" m 158 MHz 10 10

10

*

Values f o r k , kg, and V 1

R

2

dyne/cm

7

0.85

( λ + u/λ + 2U)

m/s

( t y p i c a l value)

(for ST-quartz) are taken from reference (j>) .

In order to assess the v a l i d i t y of the second assumption, a b r i e f discussion of the e l a s t i c properties of polymers i s needed. The viscoe l a s t i c behavior of a polymer i s depicted schematically i n Figure 1. The parameter of interest i n the case of SAWs i s the shear modulus, denoted as G i n Figure 1 and as μ i n Equation 1. Simply stated, the modulus i s a measure of the r i g i d i t y of the polymer. The regions of interest are the glassy region (where the polymer i s a hard, r i g i d material) and the elastomeric region (where the polymer i s a rubber). Rigid, glassy polymers t y p i c a l l y have high modulus values on the order of 10 - 10 dyne/cm . In this region, the polymer chains are locked into the lowest energy conformations and there i s i n s u f f i c i e n t energy i n the system to allow free rotation around the polymer backbone. As the temperature increases the polymer becomes an elastomer. In t h i s region, there i s s u f f i c i e n t energy i n the system f o r free r o t a t i o n to occur. This additional r o t a t i o n a l freedom i s manifested as a softening of the polymer, with a corresponding decrease i n the modulus to 10 10 dyne/cm . The temperature at which t h i s softening occurs i s the Τ . Other changes occur at this temperature that can be monitored to i d e n t i f y the T experimentally. These include changes i n s p e c i f i c volume of the polymer, index of r e f r a c t i o n , gas d i f f u s i o n c o e f f i c i e n t s , thermal expansion c o e f f i c i e n t s (measured by dilatometry) , and s p e c i f i c heat (measured by d i f f e r e n t i a l scanning calorimetry (DSC) or by d i f f e r e n t i a l thermal analysis (DTA)). General discussions of the e l a s t i c properties of polymers can be found i n references (6,2)· The T of a polymer increases as a function of the o s c i l l a t i n g frequency of an applied stress. This phenomenon was f i r s t described by Williams, Lande1 and Ferry i n 1955, and became the basis of the time- temperature superposition p r i n c i p l e (8). Previous work 1

2

2

g

fl


15.

BALLANTINE AND WOHLTJEN

Properties of Thin Polymer Films 225

demonstrated that the SAW could be used to characterize polymeric materials (9,10). Both t h i n films and bulk samples were characterized by monitoring changes i n the amplitude of the surface wave as the temperature of the polymer samples was increased. The results indicated that, f o r t h i n polymer films, s i g n i f i c a n t i n t e r a c t i o n of the polymer f i l m with the high frequency surface wave can occur, r e s u l t i n g i n an increase i n the T of the polymer. This observation i s consistent with the time-temperature superposition p r i n c i p l e . The work presented here was motivated by two factors. F i r s t , i t had been assumed that polymer films used i n previous sensor applications were above t h e i r Τ at the SAW operating temperature. I f the T of a given polymer f i l m increases s i g n i f i c a n t l y due to effects of the high frequency surface wave, then the e l a s t i c properties of the f i l m must be taken into consideration when interpreting sensor responses. Second, since frequency measurements with the SAW device are inherently more sensitive than amplitude measurements, such measurements may prove useful i n the area of materials characterization. The following experiments were performed to v e r i f y t h i s p o t e n t i a l and to investigate the possible e f f e c t of e l a s t i c properties on sensor responses.


g

g

Experimental The polymer coatings studied are given i n Table I I , along with pertinent physical parameters. These coatings were selected based on a v a i l a b i l i t y , since they are among coatings previously used f o r chemical sensor studies at the Naval Research Laboratory (NRL). In addition, the T and melting temperature (T ) are within the range that could be e a s i l y investigated using our experimental apparatus. Of these coatings, fluoropolyol (FPOL) and poly(ethylene maleate) (PEM) were provided by the Polymeric Materials Branch, Chemistry Division, of the NRL i n Washington D.C. They are both l i n e a r polymers with no observed c r y s t a l l i n i t y . PEM i s a polyester material with a repeating monomer unit of 35-50. FPOL i s a highly viscous epoxy pre-polymer with g

TABLE I I .

m

POLYMER COATINGS AND PHYSICAL PARAMETERS

T

polymer

fluoropolyol (FPOL) poly(ethylene maleate) (PEM) ethyl c e l l u l o s e (ECEL) poly(caprolactone)

g

(°C)

T

m

(°C)

10

-10

43

Elastic Properties of Thin Polymer Films Investigated with Surface

Recommend Documents