The Mechanism of Surface Cavitation in ... - ACS Publications

19 T h e M e c h a n i s m o f Surface C a v i t a t i o n i n Polyethylene Terephthalate F i b e r s 1

C. J. DURNING, H. D. WEIGMANN, L. REBENFELD, and W. B. RUSSEL

Downloaded by FUDAN UNIV on January 14, 2017 | http://pubs.acs.org Publication Date: August 29, 1984 | doi: 10.1021/bk-1984-0260.ch019

Textile Research Institute and Department of Chemical Engineering, Princeton University, Princeton, NJ 08544 Treating as-spun polyethylene terephthalate (PET) filaments with interactive solvents can cause surface cavitation, depending on the fiber/solvent contacting conditions. In this report, we discuss the preliminary results of an experimental investigation of cavitation and outline a theory for the process. The mathematical representations of four fiber/solvent treatment conditions are discussed: strongly interacting liquids and vapors, weakly interacting liquids, and mixtures of strongly and weakly interacting liquids. The preliminary experimental and theoretical results suggest that surface modification can be controlled by using liquid mixtures. Predominantly chemical methods are used to modify polyethylene terephthalate (PET) surfaces. Finishing, coating and grafting processes (1) impart antistatic properties, soiling resistance and flame resistance to polyester textiles. Plasma discharge treatments (2,3) can improve the adhesion properties of polyester films and fibers. Recently, Weigmann et al. (4,5) proposed a simple process for physically modifying polyester fiber surfaces via solvent induced crystallization (SINC). The treatment can increase the fiber specific surface area several hundredfold (6) without compromising the mechanical properties. The resulting fibers may find useful applications in composites or in filtration. The current version (6) of the fiber treatment process employs a drawing rig equipped with a solvent treatment tube where an asspun PET monofilament briefly contacts a swelling agent, causing rapid crystallization of surface layers. Solvent exchange and drying steps quench the crystallization before the fiber is drawn between feed and take-up rolls. A skin/core morphology results 'Current address: Department of Chemical Engineering and Applied Chemistry, Columbia University, New York, NY 10027. 0097-6156/ 84/0260-0309$06.00/0 © 1984 American Chemical Society

Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.


310

POLYMERS FOR

FIBERS A N D

ELASTOMERS

(Figure la); the solvent c r y s t a l l i z e d sheath, B, may possess extreme surface porosity, A, while the unpenetrated core, C, apparently develops a f i b r i l l a r morphology during the drawing step, imparting high tenacity to the modified filaments. Fiber-solvent contact times as short as 0.2 seconds are possible, allowing surface modif i e d filaments i n the 20-30 denier range to be produced. E a r l i e r workers (7-9) established phenomenologically the mechanisms of mass transport and c r y s t a l l i z a t i o n during SINC of amorphous PET i n " i n t e r a c t i v e " solvents. (Interactiveness indicates the a b i l i t y of a solvent to induce s t r u c t u r a l changes i n PET, which correlates with the l i q u i d ' s s o l u b i l i t y parameter (10) and with the glass t r a n s i t i o n temperature of the f u l l y swollen polymer (5).) During the sorption of i n t e r a c t i v e solvents, discontinuous swelling (11) occurs (Figure l b ) , with solvent d i f f u s i o n through the swollen surface layers (region B i n Figure la) c o n t r o l l i n g the transport process. A semicrystalline, s p h e r u l i t i c morphology develops r a p i d l y in the swollen portion of the f i b e r since the solvent enhances greatly the polymer c r y s t a l l i z a t i o n rate. One must c a r e f u l l y control the solvent contacting conditions to ensure c a v i t a t i o n of the surface layer ( 6 ) . This a r t i c l e discusses the e f f e c t s of several treatment conditions on the surface morphology. Experimental Investigations Because of the intimate coupling among sorption, c r y s t a l l i z a t i o n and c a v i t a t i o n during SINC, one must examine a l l three to f u l l y charact e r i z e the f i b e r treatment process. The preliminary r e s u l t s of our experimental investigations of PET exposed to methylene chloride at unit a c t i v i t y reveal several features of SINC not previously discussed i n the l i t e r a t u r e . Sorption. We recorded sigmoidal ( i . e . , "S"-shaped) p l o t s of r e l a t i v e weight gain versus the square root of the exposure time (/t) for MeCl i n PET films at 0°C. These indicate non-Fickian d i f f u s i o n (12). Although generally sigmoidal, the curve shapes depend somewhat on the f i l m thickness; thick films (= 0.09 cm) show a s i g n i f i cant portion increasing l i n e a r l y with Jt while thinner films (= 0.003 cm) do not. P l o t t i n g the r e l a t i v e weight gain against the parameter, /t/2Ap, where 2Ap i s the f i l m thickness, indicates that the d i f f u s i o n i s "history dependent" (13,14.) since the curves for thin films f a l l below those for thick films (Figure 2). The e f f e c t i s consistent with several theories for non-Fickian transport based on d i f f u s i o n with a simultaneous time-dependent transformation from glass to rubber-like states (15-17). 2

C r y s t a l l i z a t i o n . We measured the density of i n i t i a l l y amorphous PET films following exposures to methylene chloride at unit a c t i v i t y , thereby obtaining c r y s t a l l i z a t i o n k i n e t i c s . The density gradient technique was used (18) with carbon tetrachloride/n-heptane mixtures as the immersion media. To avoid spurious readings, e f f o r t s were made to remove r e s i d u a l solvent from the c r y s t a l l i z e d samples and to prevent the entrapment of a i r i n surface c a v i t i e s during the density


19.

Surface Cavitation in PET Fibers

DURNING ET AL.

311


Fiber Cross Section

D

Figure l a . Schematic representation of skin/core morphology r e s u l t i n g from solvent contact. (A) surface layer, possibly cavitated; (B) penetrated region, solvent c r y s t a l l i z e d ; (C) dry core, subsequently drawn. Figure l b . Schematic representation of discontinuous swelling accompanying sorption of an i n t e r a c t i v e solvent during the f i b e r treatment process. (D) moving boundary separating swollen from unswollen polymer; (E) threshold concentration f o r polymer swelling.

Weight Gain (%dry polymer)

60 f

0

80

160

VT/2A

240 (VS7M

P

320 3

*10" )

Figure 2. /

Relative weight gain versus v t/2A

p

for i n i t i a l l y amorphous P E T

films exposed to methylene chloride at 0°C and unit a c t i v i t y . 2A

- f i l m thicknesses:

0.03 cm

( 0 ) ; 0.09 cm

(Q)

.


312

POLYMERS FOR FIBERS AND ELASTOMERS

measurements. After c r y s t a l l i z a t i o n , samples were dried under high vacuum and then prewetted with carbon tetrachloride before placement i n the density column. Using the constants developed by K i l i a n (19), the densities measured by our procedure provide quite reasonable values (20) for the ultimate degree of c r y s t a l l i n i t y induced (25-35 percent). The shape of the k i n e t i c carves for c r y s t a l l i z a t i o n depend on the f i l m thickness (Figure 3); thicker films show a l i n e a r increase i n density with / t , while thin films give sigmoidal, S -shaped curves resembling those predicted for bulk c r y s t a l l i z a t i o n by Avrami's theory (21). The former result corroborates e a r l i e r work by Desai and Wilkes (7). Sigmoidal k i n e t i c curves have been observed during SINC i n thin polycarbonate sheets (22) ; these suggest decoupling between solvent transport and l o c a l c r y s t a l l i z a tion. Downloaded by FUDAN UNIV on January 14, 2017 | http://pubs.acs.org Publication Date: August 29, 1984 | doi: 10.1021/bk-1984-0260.ch019

,f

n

Cavitation. We also obtained scanning electron micrographs of PET films or f i b e r s c r y s t a l l i z e d i n four d i f f e r e n t environments: strongly interacting l i q u i d s and vapors, weakly interacting l i q u i d s , and mixtures of strongly and weakly interacting l i q u i d s . The strongly i n t e r a c t i v e and weakly i n t e r a c t i v e solvents were methylene chloride and methanol, respectively. As found by Wilkes et a l . 07*8)* b r i e f contact with l i q u i d methylene chloride produced severe surface c a v i t a t i o n (Figure 4). Fracture cross-sections showed that the c a v i t a t i o n i s confined to the immediate v i c i n i t y of the surface (Figure 5). Despite inducing discontinuous swelling and s i g n i f i c a n t c r y s t a l l i n i t y , saturated methylene chloride vapor did not induce appreciable surface c a v i t a t i o n , confirming our e a r l i e r findings (5). (Very early work (7) suggested that swelling vapors could induce surface porosity comparable to that r e s u l t i n g from l i q u i d contact; however, our investigations do not support t h i s conclusion.) S i m i l a r l y , contact with weakly i n t e r a c t i v e , l i q u i d methanol also causes swelling and c r y s t a l l i z a t i o n (23), but only minor surface roughening r e s u l t s . Liquid mixtures of methylene chloride and methanol apparently cause intermediate l e v e l s of surface modification, depending on the composition of the mixture (Figure 6 ) . The micrographs indicate a continuous decline i n surface roughness as the concentration of the strong swelling agent decreases. The Mechanism of Cavitation A theory for SINC must predict simultaneously the sorption, c r y s t a l l i z a t i o n and c a v i t a t i o n behavior mentioned above. To accomplish t h i s , one must combine adequate descriptions of non-Fickian d i f f u sion, polymer/diluent c r y s t a l l i z a t i o n , and l o c a l c a v i t a t i o n . Although well developed approaches exist for the former two (9,11-17) none are available for the l a t t e r . We postulate the c a v i t a t i o n r e s u l t s from the accumulation of the pure l i q u i d solvent during the development of polymer c r y s t a l l i t e s . Since the solvent cannot remain i n c r y s t a l l i n e regions, c r y s t a l l i t e development expels the diluent to neighboring amorphous polymer, increasing the l o c a l concentration of the penetrant.



DURNING ET AL.

Density (gm/cm ) 3


1.395

•

1.335

* 0

10

20

VT

(VT )

30

Figure 3. Apparent density versus /t f o r i n i t i a l l y amorphous P E T films exposed to methylene chloride at 38°C.

Film thicknesses:

0.0025 cm ( < § ) , 0.03 cm ( O ) .




Figure 4. Surface c a v i t a t i o n of 0.09 cm PET f i l m r e s u l t i n g from b r i e f contact (3 min) with methylene chloride l i q u i d at room temperature.



19. DURNING ET AL.


315

Figure 5. Fracture cross-section showing surface c a v i t a t i o n of 0.09 cm PET f i l m r e s u l t i n g from b r i e f contact (3 min) with methylene chloride at room temperature.




Figure 6. Surface modification of 360 denier as-spun PET monofilament r e s u l t i n g from b r i e f contact (4.8s) with methylene chloride/ methanol mixtures at 13°C. Filaments were drawn to a draw r a t i o of four a f t e r contacting the l i q u i d mixture. tions (volume percent methanol):

Mixture composi-

(A) 100; (B) 30; (C) 20; (D) 0.


19.

DURNING ET

Surface Cavitation in PET

AL.

Fibers

317

(Physical evidence supporting the idea of solvent accumulation i n amorphous regions due to c r y s t a l l i z a t i o n i s discussed by Zachmann (23).) If t h i s causes l o c a l saturation of the amorphous polymer, then continued c r y s t a l l i z a t i o n would produce l o c a l phase separation, creating pockets of pure l i q u i d solvent and a network of holes and c a v i t i e s i n the dried specimen. Accepting t h i s , an immediate conclusion i s that saturation of the amorphous component must precede complete c r y s t a l l i z a t i o n for c a v i t a t i o n to develop. We now relate the rate of l i q u i d accumulation i n c a v i t i e s to the c r y s t a l l i z a t i o n rate. Within a c r y s t a l l i z i n g volume element having a saturated amorphous component (Figure 7), the masses of the solvent, M , and polymer, M^, are g


M

M

s

p

=

p V + c V s s o a

(la)

=

P V + p V c c a a

(lb)

where V , V and V are the volumes of the pure l i q u i d , mixed and c r y s t a l l i n e phases, respectively; p , p and p are their o v e r a l l densities; and c i s the solvent's concentration i n the mixed phase at saturation ( i . e . , the solvent's s o l u b i l i t y i n the amorphous component of the polymer). The t o t a l volume of the element, V is g

a

c

Q

e>

V

-

V

+ V s

e

+ V a c

Next, we assume that c r y s t a l l i z a t i o n does not a l t e r the s o l u b i l i t y c , and does not cause substantial hydrodynamic flow of the solvent. Flory's theory (24) for semicrystalline polymer/solvent solutions shows that the former i s v a l i d for small changes i n the c r y s t a l l i n e volume f r a c t i o n . Hydrodynamic flow, r e s u l t i n g from a hydrostatic pressure developed during d e n s i f i c a t i o n , can also be neglected for small changes i n c r y s t a l l i n i t y . Since the t o t a l change i n the c r y s t a l l i n e volume f r a c t i o n for solvent induced c r y s t a l l i z a t i o n i s only 25-35 percent (20), both assumptions represent reasonable f i r s t approximations i n describing the c a v i t a t i o n process. In t h i s case, a change i n the phase volumes within the c r y s t a l l i z i n g element must s a t i s f y the r e s t r i c t i o n s Q

3M

3V

IF

=

-

0

3M

Tt~

P TT S

3V +

C

OTF

3V =

0

=

(2A

3V

P c i r ^ . i r


( 2 b )

>



\W

\\\\\

V/>A

dt

Figure 7. Schematic of a small, c r y s t a l l i z i n g volume element within a sample undergoing SINC, having a saturated amorphous component. (a) saturated, amorphous phase; (c) pure c r y s t a l l i n e (s) pure l i q u i d phase.


phase;

av

e

3V

_

at

Partial


°o

P

av/

p

p

e

so

that

can

c

o

p p c

a

s

a

changes

a, v

Substituting

at

volume and

element

has

a

rearranging

p

a

c

(3b) p

s aJ

(3c)

experimentally fraction,

in

volume

av

the

av

c

f

"

av

s

e

o

p

p p s

crystalline

defined

fractions

volume

fraction,

by

are

defined

by

(4a)

e

(4b)

e

at

3V /3t

c

the

e,

at

at

for

I

c

respectively,

_af at

(3a) C

c oJ

at

at

_1_ V

at

3b,

the

Combining

p p 1

volume

av

_3e

av

observe

void

at

p

L

c

P s

_f

and

r

Z

the

because

sample.

+

t

One

appear the

p

at

and

within

av/ r ± i ~a L

at

f,

(2c)

gives

2

at

av

c

at

differentials

e

av

3V

a

at

location

Equations

av

3V

s

at

specific

319


DURNING ET AL.

19.

c a

in

Equations

4a

then

employing

Equation

-

e(c

-

f(p

o s

p

c

p

a

-pp

s

- p p s

c c

and

4b

using 3c,

Equations

3a

gives

+pp) s

+ p

c

a

c

o

)


(5)

320


r e l a t i n g the rate of c a v i t a t i o n , 3e/8t, to the c r y s t a l l i z a t i o n r a t e , 3f/3t, when the amorphous component i s saturated. By assuming that the t o t a l volume of the element remains constant (an approximation) and that the polymer and solvent volumes i n the mixed phase are additive, one can simplify Equation 5 to

de

at


3t

(6)

(1 - v j )

where v j i s the volume f r a c t i o n of the solvent i n the mixed phase at saturation. To c a l c u l a t e c a v i t a t i o n i n a macroscopic specimen, one must determine the concentration and c r y s t a l l i z a t i o n h i s t o r y of each volume element i n the sample during the sorption process and apply Equation 6 to each from the time saturation occurs. This has been done f o r films by solving a non-Fickian transport equation, an equat i o n for l o c a l c r y s t a l l i z a t i o n and Equation 6 simultaneously. The transport and c r y s t a l l i z a t i o n equations (without t h e i r i n i t i a l and boundary conditions) are

-

A r

/

J

(f o - f)

(8)

respectively (25). The symbols are those usually associated with d i f f u s i o n problems: D i s the d i f f u s i v i t y , c the concentration, x the distance, etc. A i s the polymer c r y s t a l l i z a t i o n rate, while fo i s the ultimate c r y s t a l l i n e volume f r a c t i o n . Equations 6-8 apparently represent the sorption and c r y s t a l l i zation behavior i n films reasonably well (26), and should adequately represent f i b e r treatments for short contact times. In the remaining section, we discuss some preliminary c a l c u l a t i o n s with Equations 6-8, employing physical constants c h a r a c t e r i s t i c of SINC of PET at room temperature. Surface Conditions during Fiber Modification One can simulate f i b e r treatment conditions by specifying approp r i a t e surface boundary conditions. We consider the situations encountered experimentally. Strongly i n t e r a c t i v e l i q u i d s . Here we assume that the f i b e r surface layers achieve equilibrium instantaneously, so the surface boundary condition i s constant surface concentration (27), c = c . With t h i s , Equations 6-8 predict exactly the r e s u l t s depicted i n Figures 4 and 5: severe surface porosity confined to the surface l a y e r . s

Q


19.

DURNING ET AL

Surface Cavitation in PET

Fibers

321

Strongly i n t e r a c t i v e vapors. For saturated vapors, a large density difference exists across the surface i n t e r f a c e , and an external mass transfer resistance i s present since solvent molecules must migrate through the background a i r before reaching the f i b e r surface. The mathematical representation of these conditions i s (27)

k K(C

o

- c ) s

=

-D

. 3xI surface

(9)

where k i s the external mass transfer c o e f f i c i e n t , K i s the r a t i o of the external to i n t e r n a l solvent density at equilibrium, and c i s the surface concentration. Using a stagnant f i l m model, we e s t i mated k for a f i b e r i n quiescent solvent vapor 1 cm above the l i q u i d solvent, which corresponds to the conditions for the vapoi treatment procedure used i n our e a r l i e r work (5). We estimated K from the i d e a l gas law and t y p i c a l equilibrium weight gains for solvents i n PET. The surface concentration predicted by Equations 6-9 remains below the saturation value throughout the sorption process (Figure 8), thereby preventing s i g n i f i c a n t c a v i t a t i o n .


g

Weakly i n t e r a c t i v e l i q u i d s . Such solvents do not provide s u f f i c i e n t free volume for polymer chain mobility c h a r a c t e r i s t i c of the rubbery state (5). The PET/methanol system i l l u s t r a t e s t h i s ; amorphous PET imbibed with methanol at = 25°C remains noncrystalline i n d e f i n i t e l y (20), i n d i c a t i n g severely r e s t r i c t e d chain mobility even i n the swollen state. For such systems, the k i n e t i c r e s t r i c t i o n s to chain rearrangement prevent the rapid achievement of equilibrium i n surface layers C5). As a r e s u l t , the solvent surface concentration increases slowly during the sorption process. Several authors (12, 17,28-30) have suggested s p e c i f i c r e l a t i o n s h i p s governing the time dependence of the surface concentration i n such cases; we have employed a simple exponential increase, v i z . :

for t < 4$t*

c

= c o

s

for t > 43t* —

(10)

where c i s the surface concentration at time t and c i i s the value immediately a f t e r contacting the solvent. The c h a r a c t e r i s t i c time t * i s the time scale for the non-Fickian transport (defined i n references 16^ and 25), so that $ i s the time constant for a t t a i n i n g equilibrium at the surface, scaled by t * . The constant 43 therefore represents the number of c h a r a c t e r i s t i c times required for surface saturation. Figure 9 shows the surface c a v i t a t i o n predicted by Equations 6-8 and 10, s e t t i n g c i equal to the "threshold" concentration (5) for PET swelling. As the time scale for surface saturation g


POLYMERS FOR FIBERS AND ELASTOMERS 1000

k/f/U = 0

0996

C = threshold concentration for polymer swelling t = time required for penetration of sample by moving boundary p


0-992

0-988 0,0

0.25

0.5 1/

Figure 8 .

1

0.75

I.O

p

Prediction of the f i b e r surface concentration during b r i e f exposure to the saturated vapor of a strong swelling agent.

The

model parameters are estimated f o r the system PET/methylene chloride vapor at room temperature. the dimensionless

The combination k K/UO i s

external mass transfer c o e f f i c i e n t ; k K / U 0

corresponds to a constant surface concentration.

Void Fraction at Fiber Surface

0.2 Figure 9.

0.4

0.6

0.8

1.0

40

The effect of a time dependent surface concentration on f i b e r surface morphology. Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

00

19.

DURNING ET AL.


323


approaches that for transport ( i . e . , 43 1)» the theory predicts s i g n i f i c a n t l y l e s s c a v i t a t i o n . Assuming 43 = 1 represents the surface h i s t o r y for weakly i n t e r a c t i v e solvents, Equation 10 o f f e r s a tentative explanation of the experimental r e s u l t s . Liquid mixtures of strongly and weakly i n t e r a c t i v e solvents. Because of the external mass transfer resistance imposed by the inert component, the strongly i n t e r a c t i v e solvent cannot r e a d i l y access the f i b e r surface. The extent of t h i s e f f e c t depends on the Schmidt and Reynolds number (31) i n the treatment zone. Also, because of i t s reduced a c t i v i t y , the strongly i n t e r a c t i v e component may not induce the chain mobility required for rapid achievement of surface equilibrium. By analogy with the preceding cases, f i b e r treatment with such mixtures should induce less c a v i t a t i o n than the strongly swelling agent alone. By adjusting the composition and flow rate of the mixture i n the treatment tube (6), i t should be possible to control the surface conditions and thereby the surface c a v i t a t i o n . Our preliminary experimental r e s u l t s (Figure 6) support t h i s conclusion. Summary This report summarizes recent work on a process f o r the physical modification of polyester f i b e r surfaces v i a solvent induced c r y s t a l l i z a t i o n (SINC). New experimental r e s u l t s show: -

Sorption during SINC i s non-Fickian; Local c r y s t a l l i z a t i o n and transport decouple i n t h i n specimens; Liquid mixtures of strong and weak swelling agents induce intermediate l e v e l s of surface c a v i t a t i o n .

A theory for c a v i t a t i o n was outlined, and preliminary calculations for four f i b e r treatment conditions were discussed. The theory corroborates the experimental r e s u l t s and reinforces the idea that l i q u i d mixtures of strong and weak swelling agents can be used to control surface modification. Acknowledgments The authors thank Dr. A. Gozdz for h e l p f u l discussions, and Ms M.G. Scott and Ms S. Reutsch for experimental assistance. We g r a t e f u l l y acknowledge the National Science Foundation (Grant //DMR-7905980) f o r f i n a n c i a l support.

Literature Cited 1. 2. 3.

Szegö, L. Adv. Polym. Sci. 1979, 31, 89. Briggs, D.; Ranee, D.C.; Kendall, C.R.; Blythe, A.R. Polymer 1980, 21, 895. Amouroux, J.; Goldman, M.; Revoil, M.F. J . Polym. Sci. Chem. Ed. 1982, 19, 1373.


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