5
The
W a t e r - P o l y m e r Interface
ARTHUR W. ADAMSON
Downloaded by UCSF LIB CKM RSCS MGMT on December 2, 2014 | http://pubs.acs.org Publication Date: August 19, 1980 | doi: 10.1021/bk-1980-0127.ch005
Department of Chemistry, University of Southern California, Los Angeles, CA 90007
We present here some aspects of the surface chemistry and some explanatory models for water-polymer and related interfaces. The term "polymer" will be taken to mean an essentially organic material, of sufficiently high molecular weight and (or) sufficiently cross-linked that a stiff (as opposed to fluid) phase is involved. The material is insoluble in water, so that the term "water-polymer" interface refers to what is macroscopically an ordinary phase boundary. Typical polymers in the present context will be polytetrafluorethylene (PTFE), and polyethylene (PE). Solutions of macromolecules are thus not considered, nor is the related topic of so-called hydrophobic bonding, although some of what is discussed here is relèvent to that subject. The phenomenological surface chemistry of the water-polymer interface owes its modern development to the studies of Zisman and co-workers, (1) beginning in the 1950's. The useful and widely used critical surface tension quantity, Yc, allowed practicing surface chemists to estimate contact angle on a polymer surface, given the liquid surface tension. The empirical observation was that cos Θ, where θ is the solid-liquid-vapor contact angle, is linear in YL, the surface tension of the non-wetting liquid; the intercept at cos θ = 1 being defined as Y . Each type of polymer can be characterized by a Y value; further, Y is an approximate ly constitutive quantity in the sense of being additive in func tional group contributions. Although Zisman has appeared never to attribute a specific fundamental meaning to his Y , others have considered y to be the surface tension that the solid would have were its cohesive forces the same as those acting across the solid-liquid interface. Thus for a hydrocarbon solid, which has been supposed to interact only through dispersion forces both cohesively and across the solid-liquid interface, Y should be the actual surface tension of the solid-^vapor interface. This is essentially what is observed; Y for PE is approximately that measured for a high molecular weight liquid hydrocarbon. On the other hand, i f water is con sidered to interact with a hydrocarbon only through dispersion c
C
C
C
c
C
C
0-8412-0559-0/ 80/ 47-127-087S05.75 / 0 © 1980 American Chemical Society
In Water in Polymers; Rowland, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
WATER IN POLYMERS
88
forces (again, a common s u p p o s i t i o n ) , i t s γ r e l a t i v e to such i n t e r a c t i o n s should be much lower than the a c t u a l surface t e n s i o n of water. Figure 1 shows a Zisman p l o t for v a r i o u s l i q u i d s on i c e (2); the l i n e drawn i n d i c a t e s a Y value around 28 erg c m " at - S ^ C , or much l e s s that the about 110 erg c m " estimated for the i c e - v a p o r i n t e r f a c e . (3) T h i s Y c a n , however, be thought of as the c o n t r i b u t i o n of d i s p e r s i o n forces to the surface t e n s i o n of i c e , o r , a l t e r n a t i v e l y and very q u a l i t a t i v e l y as the surface t e n s i o n of a h y p o t h e t i c a l s o l i d having the s t r u c t u r e of i c e but whose cohesive f o r c e s were d i s p e r s i o n only i n nature. Q u a l i t a t i v e ideas such as the above were made manageable by Good and co-workers, (4) w i t h a key assumption, namely that for substances 1 and 2, 1-2 i n t e r a c t i o n s could be i n t e r p o l a t e d from 1-1 and 2-2 i n t e r a c t i o n s by a geometric mean law. On then equat ing energy and f r e e energy of i n t e r a c t i o n , that i s , on n e g l e c t i n g the entropy of b r i n g i n g two phases i n t o c o n t a c t , due to s t r u c t u r a l changes, the equation 0
2
c
2
Downloaded by UCSF LIB CKM RSCS MGMT on December 2, 2014 | http://pubs.acs.org Publication Date: August 19, 1980 | doi: 10.1021/bk-1980-0127.ch005
c
(1) was obtained. Here, φ i s a term which a r i s e s from the d i f f e r e n t molar volumes of the two substances; e m p i r i c a l φ values were u s u a l l y w i t h i n 10% to 20% of u n i t y . The model i s i l l u s t r a t e d i n Figure 2. Fowkes (5) introduced the important q u a l i f i c a t i o n that for the c r o s s , 1-2, term, only that c o n t r i b u t i o n to y χ and Y2 corresponding to the nature of the 1-2 i n t e r a c t i o n should be used. The square r o o t term becomes, for the general case, / Y i Υ2"· Thus f o r a water (W)-Hydrocarbon (H) i n t e r f a c e s , across which o n l y d i s p e r s i o n i n t e r a c t i o n s were though to be important, one would w r i t e Y » = Y^d, where Yyd i s the d i s p e r s i o n component of the water surface t e n s i o n . However, s i n c e hydrocarbons i n t e r a c t by d i s p e r s i o n forces o n l y , Υ Η ' = γ ^ . For the reasons of t r a c t a b i l i t y , the molar volume c o r r e c t i o n term, φ, was equated to u n i t y . Thus 1
w
^w-H -
+
ΎΗ -
2
^w
(2)
V
d
Although the o p e r a t i o n a l o r i g i n s are d i f f e r e n t , the e x p l a n a t o r y concepts are s i m i l a r enough that Y and γ
