Interaction of Liquids at Solid Substrates

12 Polymer Solvent Interactions by Ultrasonic Impedometry J. K E N N E T H C R A V E R and D A V I D L . T A Y L O R

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Monsanto Company, St. Louis, Mo.

Some unexpectedly complex liquid solid interactions

have

been detected and studied by ultrasonic impedance measure ments (ultrasonic

impedometry).

Small amounts of water

and alcohols have pronounced effects on the physical state of hydrophilic

polymers; specifically,

the high

frequency

shear modulus and crystallinity index of a poly(vinyl alcohol) film increases with water content to a maximum before normal solution phenomena occur. These effects are attrib uted to the increased molecular order owing to water hydro gen bonded between polymer chains. The unusual effects of moisture on a novel poly(vinyl chloride)/plasticizer and on hydrophilic

system

polymers other than poly(vinyl alcohol)

are also described.

Τ η o u r studies o n the cellulose p u l p / w a t e r system a n d the v a r i o u s v i s c o *·*

elastic changes w h i c h take p l a c e i n this system d u r i n g d r y i n g

w e f o u n d M a s o n ' s u l t r a s o n i c shear i m p e d a n c e d e v i c e (12)

(5),

to b e a most

u s e f u l a n d versatile t o o l , c a p a b l e of m o n i t o r i n g r a p i d changes i n the shear stiffness of systems u n d e r g o i n g p o l y m e r i z a t i o n , c o a g u l a t i o n , or other l i q u i d - s o l i d transitions. T h e w o r k w e are r e p o r t i n g here relates to a v a r i e t y of transient changes i n the h i g h f r e q u e n c y shear m o d u l u s (stiff ness ) of p o l y m e r films i n d u c e d b y w e t t i n g of the films w i t h p o l a r solvents or near solvents. T h e s e transient effects are p a r t i c u l a r l y m a r k e d i n the case of p o l y ( v i n y l a l c o h o l ) films w h i c h are i n contact w i t h l i q u i d w a t e r . Methods and

"Procedures

T h e t e c h n i q u e of u l t r a s o n i c i m p e d o m e t r y w a s d e v e l o p e d b y M a s o n a n d M c S k i m m i n of B e l l T e l e p h o n e laboratories for m e a s u r i n g the c o m p l e x m e c h a n i c a l shear m o d u l u s of p o l y m e r l i q u i d s (11, 12). T h e y d e s c r i b e d three r e l a x a t i o n m o d e s for the s y s t e m s — c o n f i g u r a t i o n a l e l a s t i c i t y , l o n g 154 Alexander; Interaction of Liquids at Solid Substrates Advances in Chemistry; American Chemical Society: Washington, DC, 1968.


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155

Polymer Solvent Interactions

c h a i n entanglement, a n d c h a i n segment t w i s t i n g . L i t o v i t z a n d co-workers u s e d this h i g h f r e q u e n c y t e c h n i q u e i n t h e i r i n v e s t i g a t i o n of the v i s c o elastic properties of associated l i q u i d s (13, 1 7 ) . M y e r s a n d co-workers h a v e m o n i t o r e d the l i q u i d - s o l i d transitions associated w i t h the coalescence of latexes a n d the d r y i n g of l i n s e e d o i l ( J5,16). B a r l o w a n d L a m b s t u d i e d the h i g h shear rate b e h a v i o r of l u b r i c a t i n g oils at pressures u p to 1,000 atmospheres a n d at frequencies u p to 85 m e g a h e r t z ( 1 ). H u n t e r , M o n t rose, a n d S h i v e l y h a v e s t u d i e d the viscoelastic constants of s h o r t - c h a i n p o l y m e r s u s i n g this t e c h n i q u e (10). I n u l t r a s o n i c i m p e d o m e t r y , the test s a m p l e is s u b j e c t e d to a h i g h f r e q u e n c y c y c l i c shear strain b y a p l a n e - p o l a r i z e d , p u l s e d u l t r a s o n i c shear w a v e w h i c h is generated w i t h i n a q u a r t z echo b a r as s h o w n i n F i g u r e 1. T h e test m a t e r i a l is p l a c e d o n t o p of the b a r a n d the u l t r a s o n i c w a v e is generated i n the b a r b y means of a p i e z o e l e c t r i c c r y s t a l b o n d e d to one e n d of the b a r a n d d r i v e n at resonant frequencies b y a n e l e c t r i c a l i m p u l s e f r o m a p u l s e d oscillator. T h e p u l s e generator emits a r a d i o f r e q u e n c y e l e c t r i c a l p u l s e of 2 to 80 M h z . l a s t i n g a b o u t 5 microseconds a n d r e p e a t i n g a b o u t 400 times p e r second. T h e p a c k e t of u l t r a - s o u n d is reflected f r o m the test surface a n d is r e c e i v e d at the other e n d of the b a r b y a second p i e z o e l e c t r i c c r y s t a l w h e r e it is r e c o n v e r t e d to a n e l e c t r i c a l s i g n a l a n d d i s p l a y e d o n a n oscilloscope as a series of peaks of d e c r e a s i n g a m p l i t u d e . T h e geometry of the echo b a r is s u c h that the o r i g i n a l p u l s e is reflected b a c k a n d f o r t h c r e a t i n g the p u l s e echo p a t t e r n s h o w n i n F i g u r e 1. I n f a v o r a b l e cases, as m a n y as 50 echoes c a n b e observed. W i t h a test s a m p l e o n the o p t i c a l l y flat t o p surface of the b a r , t h e p u l s e echo t r a i n is r e d u c e d i n a m p l i t u d e . T h i s a t t e n u a t i o n is o w i n g to the r e f r a c t i o n of p a r t of the u l t r a s o n i c w a v e i n t o the test s a m p l e at the f r e q u e n c y used. T h e r a t i o of successive p e a k a m p l i t u d e s m a y b e m e a s u r e d o n the oscilloscope a n d expressed i n decibels loss p e r echo. F r o m this, t h e loss p e r echo w i t h n o s a m p l e o n the b a r c a n b e substracted t o give a v a l u e àdb w h i c h is r e l a t e d to the m e c h a n i c a l shear i m p e d a n c e of the sample. R a p i d changes c a n be c o n v e n i e n t l y m o n i t o r e d b y a r e c o r d e r w h i c h f o l l o w s the p e a k s i g n a l of a selected echo. T h e p o l y m e r i c films u s e d i n o u r studies w e r e p r e p a r e d b y a l l o w i n g a d i l u t e s o l u t i o n of the p o l y m e r i n a s u i t a b l e solvent to evaporate o n the test surface of the i m p e d o m e t e r b a r . F i l m thicknesses w e r e v a r i e d o v e r the range 10-100 m i c r o n s b y a p p r o p r i a t e c h o i c e of s o l u t i o n v o l u m e a n d c o n c e n t r a t i o n . T h e polyvinyl alcohols) ( P V A ) u s e d i n o u r w o r k w e r e M o n s a n t o ' s G e l v a t o l 20-30 a n d 20-90. T h e s e are 8 8 % h y d r o l y z e d p r o d u c t s a n d h a v e m o l e c u l a r w e i g h t s of 10,000 a n d 125,000 r e s p e c t i v e l y . T h e y b e h a v e d s i m i l a r l y i n t h e i r responses to w e t t i n g , h u m i d i f i c a t i o n , a n d redrying. T h e test f r e q u e n c y u s e d w a s n o r m a l l y 2.5 or 5.0 M h z . A spot c h e c k s h o w e d that the transient effects f o u n d w e r e m o r e p r o n o u n c e d at h i g h e r frequencies ( u p to 60 M h z . ) . Theory of Ultrasonic

Impedometry

T h e a m o u n t of a t t e n u a t i o n suffered b y the p u l s e echoes

depends

d i r e c t l y o n the shear m e c h a n i c a l i m p e d a n c e , Z , of the p o l y m e r film. Alexander; Interaction of Liquids at Solid Substrates Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

For

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INTERACTION

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LIQUIDS

AT

SOLID

SUBSTRATES

viscoelastic m a t e r i a l s Ζ has resistive a n d r e a c t i v e components, R a n d X ( u n i t s of m e c h a n i c a l o h m s ) .

T h e r e l a t i o n of R a n d X to shear stiffness

G a n d shear viscosity η is g i v e n b y : G=(R?-X*)/

(1)

η = 2RX/ωρ

(2)

p

and

w h e r e ρ is the d e n s i t y of the m a t e r i a l a n d ω is the c i r c u l a r f r e q u e n c y of the measurement.

A s discussed i n greater d e t a i l elsewhere ( 5 ) , the c o m


p o n e n t R is d e p e n d e n t almost solely o n the m e a s u r e d a t t e n u a t i o n of the p u l s e echo a m p l i t u d e . T h e c o m p o n e n t X is m u c h m o r e difficult to measure a c c u r a t e l y ( i n v o l v i n g s m a l l phase shifts at h i g h f r e q u e n c y )

and, i n our

w o r k w a s not m e a s u r e d . Sample Piezoelectric Transducer

ΖΖΣΖΣΖΖΖΖΖΖ

y Piezoelectric Transducer

Quartz impedometer bar (4x1 =1 inch) Calibrated Attenuator

Pulsed Oscillator

Oscilloscope

2-80 Mc

Amplifier Amplifier

Recorder Figure 1.

Ultrasonic Impedometer

F o r N e w t o n i a n l i q u i d s , X = R a n d therefore shear stiffness G , b y E q u a t i o n 1, is zero. V i s c o s i t y is t h e n g i v e n b y the f u n c t i o n 2 R / ω ρ . T h i s w i l l h o l d a p p r o x i m a t e l y t r u e i n t o the m e g a c y c l e f r e q u e n c y r a n g e for 2

Alexander; Interaction of Liquids at Solid Substrates Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

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157


l i q u i d s w i t h a n o r m a l viscosity b e l o w a b o u t 100 c p .

T h u s , w a t e r for

instance, w i l l s t i l l h a v e a v i s c o s i t y of 1 cp. at 10 M h z . a n d above. F o r elastic solids, s u c h as those w e are d e a l i n g w i t h e x p e r i m e n t a l l y i n this w o r k , X approaches zero a n d thus t h e shear stiffness is s i m p l y W/p

w h i c h is a d i r e c t f u n c t i o n of the m e a s u r e d echo a t t e n u a t i o n

Adb.

A n i n c r e a s e d a t t e n u a t i o n means i n c r e a s e d R a n d therefore i n c r e a s e d shear stiffness.

S u p e r f i c i a l l y , this w o u l d a p p e a r to b e opposite to the u s u a l

i n t e r p r e t a t i o n of " a t t e n u a t i o n " or " d a m p e n i n g " i n other d y n a m i c c h a n i c a l test methods.

me-

F o r e x a m p l e , i n t o r s i o n p e n d u l u m t y p e tests, a


r u b b e r y p o l y m e r gives a r a p i d l y d a m p e d o s c i l l a t i o n . A c t u a l l y , there is n o d i s c r e p a n c y at a l l because i n the t e c h n i q u e w e are u s i n g t h e w a v e a m p l i tudes m e a s u r e d o n the oscilloscope sample.

screen are those not e n t e r i n g the

M o r e energy escapes f r o m the q u a r t z b a r as the p o l y m e r

film

approaches the m e c h a n i c a l i m p e d a n c e of q u a r t z . T h e reader s h o u l d b e c a u t i o n e d to k e e p i n m i n d the different m e a n i n g w h i c h " a t t e n u a t i o n " has i n u l t r a s o n i c i m p e d o m e t r y as o p p o s e d to its use i n t r a d i t i o n a l d y n a m i c a l testing t e c h n i q u e s .

Results and

Discussion

P o l y ( v i n y l alcohol) / W a t e r .

W h e n d i s t i l l e d w a t e r is p o u r e d

onto

t h e surface of p o l y ( v i n y l a l c o h o l ) films d r i e d onto the i m p e d o m e t e r b a r , the i n i t i a l response is not the e x p e c t e d decrease i n stiffness c a u s e d

by

the s o l v a t i n g effect of the w a t e r , b u t r a t h e r a m a r k e d increase i n stiffness f o l l o w e d b y a s l o w r e l a x a t i o n as s h o w n i n F i g u r e 2.

( T h e Adb

scale o n

this a n d most f o l l o w i n g graphs has b e e n a d j u s t e d to r e a d as c h a n g e i n Adb film.

f r o m the i n i t i a l v a l u e . T h u s , the zero p o i n t is for the d r i e d p o l y m e r T h e absolute v a l u e Adb

16 Adb

f o r d r i e d p o l y m e r films r a n g e d f r o m 1 t o

d e p e n d i n g o n film thickness a n d p o l y m e r t y p e . )

T h i s u n u s u a l effect of i n c r e a s e d h i g h f r e q u e n c y

shear stiffness o n

i n i t i a l contact w i t h w a t e r w a s e v i d e n t e v e n w h e n the w a t e r was a p p l i e d as v a p o r . W i t h t h i n films of P V A , the Adb

increases r a p i d l y w i t h e v e n a

single m o i s t b r e a t h b l o w n o n the s a m p l e a n d t h e n returns to its i n i t i a l v a l u e s l o w l y as the s o r b e d w a t e r diffuses out of the moist a i r c o n t i n u o u s l y over a d r i e d P V A film, the Adb

film.

By blowing

c a n b e m a d e to

pass t h r o u g h a m a x i m u m just as i t does w h e n l i q u i d w a t e r is a p p l i e d . T h e s e responses are s h o w n s c h e m a t i c a l l y i n F i g u r e 2a. T h e same c u r i o u s effects o c c u r w h e n the m o i s t u r e sequence is r e versed. A n e x p e r i m e n t of this sort is s h o w n i n F i g u r e 3, w h e r e w e e q u i l i b r a t e d a P V A film w i t h a 1 0 0 % r e l a t i v e h u m i d i t y atmosphere a n d t h e n r e c o r d e d the Adb c h a n g e as this film d r i e d out i n a 5 0 % r e l a t i v e h u m i d i t y atmosphere.


158

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+0.5h

I

ι

ι

I

3

6

I

ι

10

30

ι

60

L

100

TIME AFTER H 0 APPLICATION, SEC. 2

Figure 2a. . Poly(vinyl alcohol): water interaction

Adb

TIME Figure 2b.

Response of PVA film to breath moisture (schematic)

T o c h e c k t h e p o s s i b i l i t y that this a n o m a l o u s m e c h a n i c a l b e h a v i o r m i g h t b e c a u s e d b y a solvent g r a d i e n t w i t h i n t h e p o l y m e r film o r b y


12.

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TAYLOR

some b u l k effect of w a t e r at the interface, another P V A film ( G e l v a t o l 20-30, 200 ft t h i c k ) w a s p r e p a r e d o n a n i m p e d o m e t e r b a r a n d d r i e d over p h o s p h o r u s p e n t o x i d e for one w e e k . It w a s t h e n c o n d i t i o n e d successively at r e l a t i v e h u m i d i t i e s of 50, 65, 75, 85, a n d 9 3 % . T h e film w a s e q u i l i b r a t e d at e a c h h u m i d i t y for 48 hours. M o i s t u r e content ( w e i g h t g a i n ) a n d

Adb

w e r e m e a s u r e d at i n t e r v a l s d u r i n g e a c h t w o d a y c o n d i t i o n i n g p e r i o d . T h e d a t a are g i v e n i n T a b l e I a n d F i g u r e 4.

N o t e that the stiffness of

the P V A film increases w i t h i n c r e a s i n g m o i s t u r e content u n t i l the n o r m a l solvent effect of w a t e r comes i n t o p l a y i n the r e g i o n b e t w e e n 85 a n d 9 5 % r e l a t i v e h u m i d i t y . D u r i n g e a c h a p p r o a c h to e q u i l i b r i u m , as s h o w n b y Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0087.ch012

the d o t t e d l i n e , the shear stiffness passes t h r o u g h a m a x i m u m i n d i c a t i n g that the transient m o i s t u r e g r a d i e n t i n the film also causes a n increase in

Adb.

Exposure Time at 50% RH,Sec. Figure 3. Mason (II)

Dehumidification of PVA film

has s h o w n t h a t the h i g h f r e q u e n c y shear stiffness of a

p o l y m e r is l a r g e l y a measure of the m o b i l i t y of segments of the p o l y m e r c h a i n . W e p r o p o s e that w a t e r m o l e c u l e s a c t i n g i n d i v i d u a l l y or i n s m a l l clusters serve as m o l e c u l a r crosslinks t h r o u g h h y d r o g e n b o n d i n g b e t w e e n adjacent p o l y m e r m o l e c u l e s or n e i g h b o r i n g c h a i n segments.

If the l i f e -

t i m e of these crosslinks is l o n g e r t h a n the f r e q u e n c y u s e d to m e a s u r e m e c h a n i c a l stiffness, i n our case 10"

T

second, t h e n one w o u l d expect to

see a n i n c r e a s e d stiffness o w i n g to a d s o r b e d w a t e r w h i c h c o u l d not b e m e a s u r e d b y t r a d i t i o n a l m e c h a n i c a l t e s t i n g methods o p e r a t i n g at m u c h l o w e r frequencies. W h a t w o u l d l o o k v e r y r i g i d at 1 0 c.p.s. m i g h t a p p e a r 7

q u i t e flexible i n a c o n v e n t i o n a l t o r s i o n p e n d u l u m or tensile tester. T h u s , it is c o n c e i v a b l e

t h a t the " s t a t i c " m o d u l u s w o u l d b e r e d u c e d b y

the

a d s o r p t i o n of m o i s t u r e u n d e r the e x p e r i m e n t a l c o n d i t i o n s w e h a v e u s e d


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here e v e n t h o u g h the " h i g h f r e q u e n c y " m o d u l u s is i n c r e a s e d w i t h i n t h i s range. Several

findings

d e n s i t y measurements

i n t h e l i t e r a t u r e s u p p o r t this v i e w .

F o r example,

i n other h y d r o p h i l i c p o l y m e r systems

(collagen

a n d g e l a t i n ) h a v e r e v e a l e d a m a x i m u m u p o n t h e a d d i t i o n of w a t e r ( 7 ) . T h e d e n s i t y m a x i m u m w a s a t t r i b u t e d t o t h e f o r m a t i o n of w a t e r b r i d g e s , doubly hydrogen-bonded alignment.

structures, j o i n i n g accessible c h a i n s i n closer

Also, thermodynamic

i n t e r p r e t a t i o n of s o r p t i o n

isotherms

i n d i c a t e that p o l y m e r / w a t e r m o l e c u l a r contacts are f a v o r e d o v e r w a t e r / w a t e r contacts i n some h y d r o p h i l i c p o l y m e r s u p t o r e l a t i v e h u m i d i t i e s of Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0087.ch012

8 0 % a n d o v e r ( 1 9 ) . I n e a r l i e r w o r k , Z i m m suggested that t h e effect of w a t e r m o l e c u l e s o n p r o t e i n s t r u c t u r e c o u l d give rise to a n e n t r o p y l o s s — i.e., i n c r e a s e d o r d e r i n g — of c o n s i d e r a b l e m a g n i t u d e Table I.

(21).

Humidification of P V A F i l m

% Relative Humidity

Conditioning Time, Hrs.

Moisture Content %

Adb at 5 MHz.

0 50 50 50 50 65 65 65 75 75 80 80 93

(1 week) 1 4 24 46 5 22 28 2 66 6 24 22

0 1.0 2.1 3.5 3.8 7.7 10.9 11.2 13.7 20.4 23.2 27.2 47.9

15.0 15.0

— 15.6 15.6 16.7 16.1 16.2 17.5 17.9 19.6 20.0 16.8

I n a s t u d y of P V A / w a t e r relations, i t w a s f o u n d that a m a r k e d r e d u c t i o n i n P V A s w e l l i n g results f r o m p r e t r e a t m e n t of t h e p o l y m e r b y exposure to a h i g h h u m i d i t y atmosphere (18).

films

T h e explanation was

that t h e c h a i n segments, a l l o w e d greater f r e e d o m of m o t i o n b y t h e a b s o r b e d w a t e r , c o u l d assume configurations f a v o r a b l e f o r g r o w t h of n e w o r d e r e d regions. O t h e r w o r k e r s h a v e f o u n d large interactions of P V A w i t h w a t e r , as m e a s u r e d b y changes i n specific v o l u m e (20).

Russian workers

h a v e s h o w n b y heat c a p a c i t y a n d t o r s i o n t e c h n i q u e s that t h e p l a s t i c i z i n g a c t i o n of w a t e r o n a h y d r o p h i l i c p o l y m e r ( r a y o n ) consists " n o t o n l y i n increasing the

flexibility

of t h e m o l e c u l a r c h a i n s b u t also i n i n c r e a s i n g

t h e i r orderedness" (14).

C r o s s l i n k i n g of p o l y e t h y l e n e g l y c o l d e r i v a t i v e s

w i t h w a t e r m o l e c u l e s has b e e n i n d i c a t e d b y y i e l d v a l u e (9).

measurements

T h u s , f r e q u e n t reference has b e e n m a d e to t h e o r g a n i z i n g influence

of w a t e r o n h y d r o p h i l i c p o l y m e r s .


12.

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Polymer Solvent Interactions H 0 CONTENT, %


2

40

60

80

100

RELATIVE HUMIDITY, % Figure 4.

PVA humidification

— Equilibrium — Approach to equilibrium (schematic) If w a t e r acts as a c r o s s l i n k i n g agent, it s h o u l d increase the m o l e c u l a r o r d e r i n g i n the p o l y m e r film a n d this effect s h o u l d b e detectable b y x - r a y d i f f r a c t i o n t e c h n i q u e s . P V A films w e r e p r e p a r e d o n glass slides that w e r e c o n d i t i o n e d at 0, 30, a n d 8 5 % r e l a t i v e h u m i d i t y . X - r a y diffractograms of these films are p r e s e n t e d i n F i g u r e 5, w i t h a n a l y t i c a l d a t a i n T a b l e I I . At 0%

r e l a t i v e h u m i d i t y , o n l y a b r o a d a m o r p h o u s b a n d is present.

3 0 % r e l a t i v e h u m i d i t y , a p e a k at 20 = h u m i d i t y , a s e c o n d p e a k at 20 =

At

19.3° appears; at 8 5 % r e l a t i v e

22.2° is b a r e l y e v i d e n t .

are c h a r a c t e r i s t i c of the P V A c r y s t a l l i n e phase ( 3 ) .

T h e s e peaks

T h e i n t e n s i t y of the

a m o r p h o u s phase d e f r a c t i o n h a l o decreases w i t h i n c r e a s i n g r e l a t i v e h u midity.

It appears, therefore, that w a t e r i n d u c e s the a g g r e g a t i o n

of

s m a l l crystallites ( w h i c h c o n t r i b u t e to the a m o r p h o u s p a t t e r n ( 2 ) ) i n t o groups large e n o u g h to be d e t e c t e d b y x - r a y as b e i n g t r u l y c r y s t a l l i n e . I n a s t u d y of the s w e l l i n g b e h a v i o r of P V A i n w a t e r , P r i e s t m a d e reference to e a r l i e r u n p u b l i s h e d w o r k w h e r e the x - r a y d i f f r a c t i o n p a t t e r n for P V A b e c a m e sharper o n h u m i d i f i c a t i o n ( 1 8 ) . i n c r e a s i n g the Adb

T h u s , the effect of m o i s t u r e i n

of P V A films correlates w i t h s t r u c t u r a l changes i n

the p o l y m e r as v i e w e d b y x - r a y t e c h n i q u e s . Poly (vinyl alcohol)/Methanol.

W h e n m e t h a n o l is u s e d i n s t e a d of

w a t e r , the increase i n a t t e n u a t i o n is not as r a p i d as that w i t h w a t e r b u t


INTERACTION


162

OF

LIQUIDS

A T SOLID

SUBSTRATES

Ο

| = VERTICALLY SHIFTED FOR CLARITY J

35

I

30

I

I

25

L

20

15

10

DIFFRACTION ANGLE,2Θ Figure 5.

X-ray diagram polyvinyl alcohol)

0% Rehtive Humidity 30% Revive Humidity 85% Revive Humidity Table II.

-

Effect of Water Vapor oit Poly (vinyl alcohol) Crystallinity Peak Intensity

Sample Condition

Rroad

Structure

20: 19.5

19.3

22.2

0-5

46.5

—

Lab Air

30

41.0

11.5

— —

28

Humid

87

34.0

12.5

5

64

Redried

0-5

46.5

12.5

—

27

Dry

% RH

Crystallinity Index 0


12.

CRAVER

AND

TAYLOR


the p e a k i n t e n s i t y is m o r e persistent as is s h o w n i n F i g u r e 6.

163 Ethyl


a l c o h o l has a s i m i l a r response, b u t the i n t e n s i t y is o n l y 1 / 1 0 as great.

Figure 6.

Poly(vinyl alcohol): methanol interaction

P r o p y l a l c o h o l , b u t y l a l c o h o l , b e n z e n e , a n d octane s h o w n o effect.

These

results correlate w i t h the r e l a t i v e h y d r o g e n b o n d i n g c a p a b i l i t i e s of the a l c o h o l series. A p p a r e n t l y m e t h a n o l is also c a p a b l e of f o r m i n g h y d r o g e n b o n d i n g crosslinks b e t w e e n p o l y ( v i n y l a l c o h o l ) c h a i n segments b u t w i t h out d i s s o l v i n g the p o l y m e r . E t h y l a l c o h o l w i t h its i n c r e a s i n g m o l e c u l a r v o l u m e is s t i l l c a p a b l e of f o r m i n g some h y d r o g e n b o n d i n g crosslinks, p e r h a p s o n l y i n the m o r e r e a d i l y accessible regions of the p o l y m e r n e t w o r k w h i l e t h e h i g h e r a l c o h o l s — p r o p y l a l c o h o l , b u t y l a l c o h o l , etc., i f t h e y are h y d r o g e n b o n d e d , d o not f o r m crosslinks w h i c h are detectable at the frequencies w e used. B e c a u s e m e t h a n o l is not a c t u a l l y a solvent for P V A , some i n t e r e s t i n g s o r p t i o n / d e s o r p t i o n c y c l i n g experiments c a n b e c o n v e n i e n t l y r u n o n P V A films

w i t h o u t d i s r u p t i n g the film i n t e g r i t y . A s n o t e d a b o v e , m e t h a n o l

p r o d u c e s a s l o w increase of shear stiffness ( Adb ) w h e n l a y e r e d o n P V A . T h e t i m e scale a n d c u r v e shape are suggestive of a d i f f u s i o n process ( l i q u i d into polymer).

B y r e m o v i n g the l i q u i d l a y e r , w e s h o u l d p e r m i t

m e t h a n o l to evaporate f r o m the film a n d thus reverse the effect.


The

164

INTERACTION

OF

LIQUIDS

AT

SOLID

SUBSTRATES

results s h o w n i n F i g u r e 7 are s u r p r i s i n g . T h e first a p p l i c a t i o n of m e t h a n o l p r o d u c e d a s l o w increase i n Adb

as before.

T h e excess m e t h a n o l w a s

d r a w n off t h e surface b y syringe at p o i n t B . W e t h e n expected to see Adb

r e t u r n s l o w l y to t h e i n i t i a l v a l u e as t h e film d r i e d out. R e m o v a l of

t h e l i q u i d l a y e r ( a t B ) c a u s e d a decrease i n a t t e n u a t i o n u n t i l t h e film surface a p p e a r e d d r y ( at C ). T h e n , as m e t h a n o l e v a p o r a t e d f r o m w i t h i n the film, t h e Adb passed t h r o u g h a v e r y h i g h m a x i m u m . A second

flooding

of t h e surface after a i r d r y i n g r e s u l t e d i n a n i m m e d i a t e increase i n Adb i n contrast to t h e s l o w b u i l d u p o n i n i t i a l w e t t i n g . A f t e r t h e first w e t t i n g , the sequence of responses to subsequent w e t t i n g / d r y i n g cycles w a s r e Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0087.ch012

markably reproducible.

P e r h a p s the first w e t t i n g is u n i q u e i n that i t

"opens u p " t h e surface s t r u c t u r e of t h e film, p e r m i t t i n g r a p i d p e n e t r a t i o n of m e t h a n o l i n t o t h e film o n s u c c e e d i n g r e w e t t i n g s .

Figure 7.

Poly(vinyl alcohol): methanol interaction

Polymer: Gelvatol 20-30 Film thickness = 50 μ Frequency = 5 MHz. Film Adb = 6.0

A: MeOH added Β: MeOH drawn off C: Polymer surface dry

A n o t h e r e x a m p l e of a n i n t r i g u i n g c y c l i n g effect r e s u l t e d f r o m a n e x p e r i m e n t i n w h i c h w e c r o s s l i n k e d the surface of P V A film b y p a i n t i n g the film w i t h b o r a x s o l u t i o n . T h e surface w a s n o w v i s i b l y less r e c e p t i v e to w a t e r ( d r o p l e t s d i d n o t s p r e a d ) , b u t t h e film Adb

was unchanged.

M e t h a n o l w e t t i n g p r o d u c e d t h e c u r i o u s d o u b l e d p e a k e d increase i n Adb s h o w n i n F i g u r e 8. B y r e d r y i n g t h e film i n a v a c u u m o v e n ( 5 0 ° C ) , t h e p o s i t i o n of t h e first p e a k ( u p o n r e w e t t i n g w i t h m e t h a n o l ) w a s g r a d u a l l y s h i f t e d to longer times. T h e s e c o n d p e a k p o s i t i o n w a s u n c h a n g e d . A p p a r -


12.

CRAVER

AND

165


TAYLOR

ently, whatever was causing the early peak was being

modified

or

r e m o v e d b y the a l c o h o l , r e s u l t i n g i n longer times r e q u i r e d f o r subsequent


m e t h a n o l a d d i t i o n s to r e a c h the u n m o d i f i e d r e g i o n of t h e film.

I I

I

I

3

I

I

I

6

I

I

I

I

I

10

I

30

I

I

I

I

I

60

I

I

100

Time after MeOH application, sec. Figure 8.

Poly(vinyl alcohol) crosslinked: methanol interaction Polymer: Gelvatol 20-30 Film thickness = 65 μ Frequency = 5 MHz.

O t h e r P o l y m e r s . O t h e r h y d r o p h i l i c p o l y m e r s also e x h i b i t t h e shear stiffness a n o m a l y w h e n c o n t a c t e d w i t h h y d r o g e n b o n d i n g solvents.

Films

o f a m y l o s e r e s p o n d to w a t e r a n d d i m e t h y l s u l f o x i d e as s h o w n i n T a b l e I I I . W h e n w e t w i t h w a t e r , there is a c o m p a r a t i v e l y l a r g e increase i n a t t e n u a t i o n f o l l o w e d b y a s l o w d e c l i n e to a l e v e l p l a t e a u . W e a t t r i b u t e t h i s l e v e l i n g o u t to t h e c o m p a r a t i v e w a t e r i n s o l u b i l i t y of r e t r o g r a d e d amylose. D i m e t h y l sulfoxide is a m u c h stronger solvent f o r amylose t h a n is w a t e r a n d t h e increase i n stiffness w h i c h is f o l l o w e d b y a r a p i d d e c a y to z e r o i n d i c a t e s c o m p l e t e film s o l u t i o n as w a s t h e case w i t h w a t e r o n P V A . T h e


166

INTERACTION

OF

LIQUIDS

AT

SOLID

SUBSTRATES

p e a k h e i g h t is l o w e r w i t h D M S O because r a p i d s o l u t i o n prevents a t t a i n m e n t of m a x i m u m stiffening effect. O t h e r h y d r o p h i l i c p o l y m e r s i n c l u d i n g g e l a t i n , casein, a n d polyvinyl p y r r o l i d o n e ) a l l r e s p o n d l i k e P V A w h e n c o n t a c t e d b y w a t e r ; there is a r a p i d increase i n m o d u l u s f o l l o w e d b y film s o l u t i o n . E v e n t h o u g h cellulose fiber mats are d i s c o n t i n u o u s r a t h e r t h a n h o m o geneous films, a sheet p r e p a r e d f r o m w e l l m a s c e r a t e d cellulose fibers w i l l e x h i b i t this same response to w a t e r as does P V A . A m o r e d e t a i l e d t r e a t m e n t of cellulose fiber w a t e r interactions has b e e n g i v e n elsewhere

(5).


H y d r o p h o b i c p o l y m e r s a n d films, s u c h as paraffin w a x a n d p o l y ( v i n y l c h l o r i d e ) , s h o w n o c h a n g e i n stiffness m o d u l u s w h e n flooded w i t h w a t e r . O n the other h a n d , b u t y l a l c o h o l o n a paraffin w a x film causes a s l o w , slight increase of stiffness m o d u l u s over a p e r i o d of several m i n u t e s . T h e effect of b e n z e n e o n paraffin films is s i m i l a r to that of m e t h a n o l o n P V A . A p o l y ( v i n y l c h l o r i d e ) film is unaffected b y p l a s t i c i z e r s s u c h as d i m e t h y l a n d d i e t h y l p h t h a l a t e o v e r short t i m e p e r i o d s . H o w e v e r , P V C gives rise to the u l t r a s o n i c m o d u l u s a n o m a l y w h e n c o n t a c t e d w i t h t e t r a h y d r o f u r a n , a strong solvent for P V C . T h e p l a s t i c i z i n g effect of d i b u t y l p h t h a l a t e o n p o l y s t y r e n e has b e e n

demonstrated b y ultrasonic impedometry

(16).

N o anomalies w e r e f o u n d . Table III.

Response of Amylose Films to Wetting (Film Thickness: 10 μ ) Change inAdb from dry film value after wetting surface with:

Time after Wetting (sec.) 0 (dry) 2 5 10 30

Water

Dimethyl sulfoxide

0 2.1 1.4 1.2 1.0 0.8

oo

Other Transient Interactions.

0 0.8 -0.1 -0.1 -0.1 -0.1

T h e P V A / b o r a x example dealt w i t h

the effects of surface exposure of a t w o - c o m p o n e n t p o l y m e r system to a w e t t i n g l i q u i d . W e d i s c o v e r e d a p r a c t i c a l e x a m p l e of this class of p h e n o m e n a i n a p l a s t i c i z e d p o l y ( v i n y l c h l o r i d e ) ( P V C ) system. A p e c u l i a r plasticizer

(tetraethyl methylene bisphosphonate )

found

w h i c h has the u n u s u a l p r o p e r t y of

(6)

for P V C has

been

e x u d i n g out of

PVC

i m m e d i a t e l y u p o n exposure to h i g h h u m i d i t y . T h e p l a s t i c i z e r q u i c k l y re-enters the r e s i n w h e n the h u m i d i t y is r e d u c e d . and 4 0 %

plasticizer was

A film of 6 0 % r e s i n

solvent cast f r o m t e t r a h y d r o f u r a n onto a n

i m p e d o m e t e r b a r a n d w a s subjected to s u d d e n h i g h h u m i d i t y exposure.


12.

CRAVER

AND

167


TAYLOR


+ 2.0

Time after moisture vapor application,sec. Figure 9.

Resin: plasticizer: moisture interaction

Resin: polyvinyl chloride)—Opalon 650 Plasticizer: 40% tetraethyl methylene bisphosphonate A — D = See text Film thickness = 4 mg./cm. Frequency = 5 MHz. 2

T h e response o b t a i n e d is s h o w n i n F i g u r e 9. O u r i n t e r p r e t a t i o n i s : at A — s o r p t i o n of s m a l l a m o u n t

of moisture v a p o r

effect a n d reduces the shear r i g i d i t y .

enhances the p l a s t i c i z i n g

A t Β—water v a p o r

at the

film

surface d r a w s p l a s t i c i z e r out of the r e s i n , l e a v i n g the film m o r e r i g i d . P l a s t i c i z e r c a n be seen a n d felt o n the surface at this p o i n t . is l o s i n g adhesion to the q u a r t z surface.

At C—film

A t D — p l a s t i c i z e r is r e t u r n i n g

to t h e r e s i n ( o b s e r v e d ) a n d film re-adheres to the surface, r e t u r n i n g to its o r i g i n a l state. Supplementary e q u i l i b r a t e d at 5 0 %

experiments

support

this i n t e r p r e t a t i o n :

(a)

films

R . H . h a d l o w e r r i g i d i t y t h a n one e q u i l i b r a t e d at

0 % R . H . — s u g g e s t i n g b e h a v i o r at A , ( b ) film e q u i l i b r a t e d at 1 0 0 % R . H . separated

f r o m q u a r t z surface,

(c)

a b r u p t h u m i d i t y changes h a v e no

effect o n P V C or t h e p l a s t i c i z e r separately. A mundane

e x a m p l e w h i c h w e i n c l u d e for its n o v e l c o m p l e x i t y

the effect of w a t e r w e t t i n g o r d i n a r y c e l l o p h a n e adhesive tape.

is

T h e tape


168

INTERACTION

provides a highly complex a n d other c o m p o n e n t s ) .

OF

LIQUIDS A T

substate ( c e l l o p h a n e ,

SOLID

SUBSTRATES

p l a s t i c i z e r , adhesive,

T h e t a p e w a s pressed onto the i m p e d o m e t e r b a r

and wetted w i t h water.

A c o m p l e x response w o u l d b e e x p e c t e d

of a

c o m p l e x system i f the a t t e n u a t i o n changes p r e v i o u s l y o b s e r v e d o n s i m p l e r systems w e r e not of t r i v i a l origin—e.g., i n s t r u m e n t effects.

T h e response

s h o w n i n F i g u r e 10 is q u i t e c o m p l e x a n d r e p r o d u c i b l e ( p a r t i c u l a r l y a l o n g the t i m e a x i s ) . A d o u b l e l a y e r of t y p e ( C u r v e B ) gave a s i m i l a r response s h i f t e d to l o n g e r times a n d s m a l l e r a m p l i t u d e s . W e d o not p r e t e n d to b e a b l e to i n t e r p r e t the b e h a v i o r d e p i c t e d i n F i g u r e 10, b u t i t is c e r t a i n that some subtle l i q u i d - s o l i d interactions are r e s p o n s i b l e as w e l l as gross Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0087.ch012

effects s u c h as s e p a r a t i o n at the c e l l o p h a n e / a d h e s i v e interface. + 3.0

I

1

I

I

I

I

3

I M i l l

6

I

10

I

30

I

I—I

60

Time after Η£θ application, sec. Figure 10.

Cellophane tape/water interaction Frequency = 2.5 MHz.

Summary and Conclusions. " S i m p l e shear is b y f a r the most i m p o r tant t y p e of d e f o r m a t i o n i n studies of viscoelastic bodies, because [for e x a m p l e ] the absence of a v o l u m e c h a n g e facilitates i n t e r p r e t a t i o n of the b e h a v i o r i n m o l e c u l a r t e r m s ( 8 ) . ,,


12.

CRAVER

AND

TAYLOR

169


T h e sequence of events w h i c h w e h a v e d e t e c t e d at u l t r a s o n i c f r e quencies w h e n h y d r o p h i l i c p o l y m e r s are c o n t a c t e d b y strongly h y d r o g e n b o n d i n g solvents i n v o l v e s a s h a r p increase i n shear stiffness f o l l o w e d b y a m o r e or less r a p i d decrease to a t t e n u a t i o n levels close to that of the solvent. W e a t t r i b u t e the i n i t i a l increase i n stiffness to w a t e r h y d r o g e n b o n d i n g p o l y m e r c h a i n s w i t h consequent

increase i n order.

T h i s stage is


f o l l o w e d b y progressive s o l v a t i o n of the p o l y m e r film, p l a s t i c i z a t i o n , a n d solution.

W a t e r vapor produces

alcohol)

either t h r o u g h c r o s s l i n k i n g or

mobility.

increased organization i n poly ( v i n y l through increased

molecular

T h i s is m a n i f e s t e d b y the i n c r e a s e d sharpness of the x - r a y

d i f f r a c t i o n p a t t e r n a n d b y the i n c r e a s e d h i g h f r e q u e n c y shear stiffness of the h u m i d i f i e d

films.

T h e m o l e c u l a r b r i d g i n g hypothesis has

been

u s e d b y others to e x p l a i n d e n s i t y m a x i m a i n h u m i d i f i e d c o l l a g e n a n d g e l a t i n , the " b o u n d " w a t e r p h e n o m e n a i n l i v i n g organisms, a n d t h e b o n d i n g of cellulose fibers i n p a p e r m a k i n g . T h e h y d r o g e n b o n d s i n l i q u i d w a t e r h a v e a n average l i f e t i m e of less than 10"

10

s e c o n d as m e a s u r e d b y d i e l e c t r i c r e l a x a t i o n times (4).

But

the h y d r o g e n b o n d s b e t w e e n w a t e r a n d a p o l y m e r c o u l d exist for longer t h a n 10"

7

seconds.

S u c h a s t r u c t u r e w o u l d a p p e a r " p e r m a n e n t " at the

frequencies u s e d i n the u l t r a s o n i c i m p e d o m e t e r ( 1 0

7

cycles/sec.

range),

a n d s h o u l d demonstrate a m e a s u r a b l e shear stiffness at these frequencies. T h e case for i n c r e a s e d o r g a n i z a t i o n i n h y d r o p h o b i c films c a u s e d b y n o n h y d r o g e n - b o n d i n g solvents is not as w e l l d o c u m e n t e d .

It is difficult

for us to see h o w b e n z e n e c a n increase the shear m o d u l u s of paraffin w a x or h o w t e t r a h y d r o f u r a n c a n d o the same for p o l y ( v i n y l c h l o r i d e ) . It is possible that w e are d e a l i n g w i t h a v a n d e r W a a l s b o n d i n g i n s w o l l e n p o l y m e r s i n these cases b u t m u c h m o r e e v i d e n c e w o u l d b e n e e d e d b e f o r e a definite theory c a n b e a d v a n c e d . T h e use of the shear-reflectance i m p e d o m e t e r has p e r m i t t e d us to observe p o l y m e r - s o l v e n t interactions i n t h i n films a n d to m o n i t o r transient effects w h i c h are not r e a d i l y accessible b y other t e c h n i q u e s .

This sug-

gests t h a t this t e c h n i q u e c o u l d b e a p p l i e d to the s t u d y of p o l y m e r - s o l v e n t interactions i n other r e l a t e d research areas s u c h as the diffusion

of

solvents i n t o films, p h y s i c a l effects at p i g m e n t - p o l y m e r surfaces, or t h e n a t u r e of the interface b e t w e e n l i v i n g c e l l surfaces a n d aqueous m e d i a .

Acknowledgment T h e x-ray crystallinity data were supplied b y E m i l e D . Pierron, Central Research Department, Monsanto C o .


170

INTERACTION

OF

LIQUIDS

AT

SOLID

SUBSTRATES

Literature Cited (1) (2) (3) (4) (5)


(6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21)

Barlow, A. J., Lamb, J., Proc. Roy. Soc. A253, 52 (1959). Bosley, D. E., J. Appl. Poly. Sci. 8, 1521 (1964). Bunn, C. W., Nature 161, 929 (1948). Collie, C. H . , et al., Proc. Phys. Soc. 60, 145 (1948). Craver, J . K., Taylor, D . L . , 'Consolidation of the Paper Web," p. 445, F. Bolam, Ed., Tech. Section, Β. P. & B.M.A., London, 1966. Darby, J. R., Monsanto (personal communication, 1964). Fels, I. G., J. Appl. Poly. Sci. 8, 1813 (1964). Ferry, J. D., "Viscoelastic Properties of Polymers," p. 6, Wiley, New York, 1961. Gluzman, M . Kh. et al., Polymer Sci. USSR 7, 2216 (1965). Hunter, J. L . , et al., J. Acoustical Soc. Am. 36, 953 (1964). Mason, W. P., McSkimin, H . J., Bell System Tech. J. 31, 122 (1952). Mason, W . P., et al., Phys. Rev. 75, 936 (1949). Meister, R., et al., J. Appl. Phys. 31, 854 (1960). Mikhailov, Ν. V., et al., Poly. Sci. USSR 6, 582 (1964). Myers, R. R., Schultz, R. K., J. Appl. Poly. Sci. 8, 755 (1964). Myers, R. R., et al., J. Paint Tech. 38, 479 (1966). Piccirelli, R., Litovitz, Τ. Α., J. Acoustical Soc. Am. 29, 1009 (1957). Priest, W. J., J. Poly. Sci. 6, 699 (1951). Starkweather, H . W., Jr., Poly. Letters 1, 133 (1963). Tadokoro, H . , et al.,Bull.Chem. Soc., Japan 27, 451 (1954). Zimm, Β. H . , Londberg, J. L . , J. Phys. Chem. 60, 425 (1956).

RECEIVED October 26, 1967.


Interaction of Liquids at Solid Substrates

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