Size Analysis of a Block Copolymer-Coated Polystyrene Latex - ACS

Chapter 16

Size Analysis of a Block Copolymer-Coated Polystyrene Latex 1

1,3

2

Jenqthun Li , Karin D. Caldwell , and Julia S. Tan 1

Center for Biopolymers at Interfaces, Department of Bioengineering, University of Utah, Salt Lake City, UT 84112 Life Sciences Research Laboratories, Eastman Kodak Company, Rochester, NY 14650

Downloaded by CORNELL UNIV on August 2, 2012 | http://pubs.acs.org Publication Date: September 24, 1991 | doi: 10.1021/bk-1991-0472.ch016

2

A PEO-PPO-PEO triblock surfactant of type F108 M (W 14,600, the ratio of PEO/PPO/PEO is 129/56/129) has been adsorbed to two polystyrene standard particles with diameters of 272 nm and 69 nm respectively; the complex is found to be stable in several different media, including physiological saline containing 0.8% human serum albumin. The thickness of the ad-layer is determined by PCS after isolation of the monomer fraction of the coated particles by sedimentation FFF, or by flow FFF. Differences in layer thickness are found for the two particles, with the smaller particles showing a thinner coating. A direct PCS evaluation of the composite size without prior fractionation gives values which are larger than those measured on the monomer fraction, a fact explained by the presence of aggregates identified through the fractionation process and subsequently by SEM. SedFFF is found to permit quantification of the amount of surfactant adsorbed to each particle; these amounts are in good agreement with adsorption and quantification studies using radio-isotope labelled surfactant. In recent years, the use of c o l l o i d a l systems as vehicles f o r drug d e l i v e r y has received much attention. I t i s p a r t i c u l a r l y the slow release of drug from such c a r r i e r s , and the p o s s i b i l i t y to attach s p e c i f i c targeting moieties such as antibodies with a desired t i s s u e a f f i n i t y to these p a r t i c l e s that i s responsible for the attention. The a b i l i t y of a c o l l o i d a l c a r r i e r to reach i t s target before being cleared from the system depends primarily on i t s size; while p a r t i c l e s i n the 100-200 nm range are small enough to escape from the vascular system, larger p a r t i c l e s are confined within t h i s system and eventually end up i n the l i v e r or spleen. From recent studies of the systemic d i s t r i b u t i o n of c o l l o i d s , however, i t i s becoming c l e a r that not only size, but also surface composition has a strong e f f e c t on t h i s d i s t r i b u t i o n . For example, Ilium et a l . (l 2) have shown that surfactant Pluronic F108 are only to a minor degree trapped i n the f

3

Corresponding author © 1991 A m f r j t y f g f ^ e m i c a l Society

1155 16th St, N.W.

In Particle Size Distribution II; Provder, T.; Wishington, 0.CSociety: 20038Washington, DC, 1991. ACS Symposium Series; American Chemical

248

P A R T I C L E S I Z E D I S T R I B U T I O N II

l i v e r the major portion being found elsewhere i n the carcass. Accumulation i n the l i v e r i s the f i n a l result of a process that begins with the sequential adsorption of an array of plasma proteins to the p a r t i c l e . The suppression of protein adsorption by d i f f e r e n t surface treatments i s therefore a major goal i n current research on targeted drug delivery. Numerous research groups have shown that surfaces coated with polyethyleneoxide (PEO), i n e i t h e r grafted or adsorbed form, are s i g n i f i c a n t l y less prone to protein adsorption than t h e i r untreated counterparts (3-5). The e f f e c t i s thought to be the result of s t e r i c exclusion and i s strongly correlated with the chain length of the polymer. Just as a repulsion of protein i s accomplished by the dynamics of the PEO chains i n the surface coating, the s t e r i c exclusion e f f e c t i s also responsible f o r reducing i n t e r - p a r t i c l e a t t r a c t i o n and suppressing aggregation. Because of i t s importance i n c o l l o i d s t a b i l i z a t i o n , the s t e r i c exclusion e f f e c t created by adsorption of polymeric surfactants onto p a r t i c l e s of d i f f e r e n t surface composition has been studied extensively. Aside from surface densities of adsorbed polymer, and s t a b i l i t i e s of the adsorption complex upon changes i n composition of the suspension medium, the parameter most frequently studied i s the thickness of the adsorbed layer. Since, f o r a given molecular weight, t h i s thickness bears an inverse r e l a t i o n s h i p to the mobility of the polymer chains (£), i t may constitute a measure of the s t a b i l i z i n g e f f e c t of the coating. I f so, one may question whether the layer thickness i s the same regardless of substrate geometry or whether the d i f f e r e n t curvatures presented by core p a r t i c l e s of d i f f e r e n t diameters i n any way a f f e c t the thickness of the coating. On p a r t i c u l a t e surfaces, coating thicknesses are frequently analyzed by photon c o r r e l a t i o n spectroscopy (PCS). As long as dust contamination can be minimized, t h i s technique o f f e r s a rapid and convenient way of determining the Z-average of p a r t i c l e diameters present i n a suspension, and the thickness of an adsorbed layer can, i n p r i n c i p l e , be determined from the size difference between coated and uncoated p a r t i c l e s . For monodisperse populations, the PCS technique reproducibly y i e l d s diameters i n good agreement with values from other well accepted methods, such as a n a l y t i c a l u l t r a c e n t r i f u g a t i o n (2), and electron microscopy (&). However, f o r polydisperse samples of unknown d i s t r i b u t i o n the technique i s less well suited as a characterization t o o l , as i t only provides values for the average diameter together with a p o l y d i s p e r s i t y index. This index i s derived by assuming the recorded autocorrelation function to be the sum of the several s p e c i f i c exponential functions, generated by each s i z e present i n the population and weighted by the concentration of p a r t i c l e s of t h i s s i z e . The second moment of t h i s f i t t e d d i s t r i b u t i o n i s then reported as the p o l y d i s p e r s i t y index f o r the d i s t r i b u t i o n . Since larger p a r t i c l e s scatter l i g h t more strongly than fines, even minor contaminations of p a r t i c l e s larger than the average tend to s h i f t the average size toward larger values without noticeably a f f e c t i n g the measured polydispersity. Several comparative studies of polymer coatings on p a r t i c l e s and on f l a t surfaces indicate a lack of agreement between coating thicknesses determined by PCS, on the one hand, and by u l t r a c e n t r i f u g a t i o n , ellipsometry, and neutron scattering, on the other (£.). Various geometrical correction factors are applied to the non-PCS measurements to bring them into conformity with the PCS data, often with less than s a t i s f a c t o r y result (10.) . In order to develop a better understanding of the question of polymer conformation on the surface of c o l l o i d a l p a r t i c l e s , we have


f

In Particle Size Distribution II; Provder, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


16. L I E T A L .

Analysis of Copolymer- Coated Polystyrene Latex

249

u s e d two forms o f t h e f i e l d - f l o w f r a c t i o n a t i o n (FFF) t e c h n i q u e , namely t h e s u b t e c h n i q u e s o f s e d i m e n t a t i o n and f l o w FFF, t o f r a c t i o n a t e adsorbates i n t o cuts of uniform p a r t i c l e s i z e . Due t o t h e u n i f o r m i t y o f t h e s e c u t s , PCS c o u l d be e x p e c t e d t o g i v e r e l i a b l e i n f o r m a t i o n about t h e s i z e o f p a r t i c l e s i n e a c h c u t . For p a r t i c l e s f r a c t i o n a t e d by f l o w FFF t h e r e t e n t i o n volume i s a d i r e c t measure o f p a r t i c l e s i z e , and t h e c o m b i n a t i o n o f FFF and PCS t h e r e f o r e y i e l d s two i n d e p e n d e n t measurements o f p a r t i c l e s i z e . A l t h o u g h t h e s i z e s e l e c t i v i t y o f t h e s e d i m e n t a t i o n FFF t e c h n i q u e i s h i g h e r t h a n t h a t o f t h e f l o w a n a l o g u e and s e p a r a t i o n s by t h i s method p r o d u c e s h i g h l y u n i f o r m f r a c t i o n s , t h e i n t e r p r e t a t i o n o f r e t e n t i o n d a t a i n terms o f p a r t i c l e d i a m e t e r i s c o m p l i c a t e d by t h e r e q u i r e d i n p u t o f an e x a c t v a l u e f o r t h e p a r t i c l e d e n s i t y . While t h i s p o s e s no problems f o r u n c o a t e d p a r t i c l e s whose c o m p o s i t i o n i s u n i f o r m t h r o u g h o u t , i t i s i m p o s s i b l e t o implement f o r c o a t e d p a r t i c l e s where t h e c o a t i n g d e n s i t y d i f f e r s from t h a t o f t h e c o r e . S i n c e t h e composite d e n s i t y depends on t h e unknown d e g r e e o f s o l v a t i o n o f t h e a d s o r b e d l a y e r , t h e s i z e o f a c o m p o s i t e p a r t i c l e can not be d e t e r m i n e d from sedFFF r e t e n t i o n d a t a , a l t h o u g h i t i s r e a d i l y measured on c o l l e c t e d sedFFF f r a c t i o n s u s i n g PCS. While the t h i c k n e s s o f an a d s o r b e d l a y e r e l u d e s measurement by t h i s t e c h n i q u e , t h e p r e s e n t s t u d y e x p l o r e s t h e p o s s i b i l i t y o f d e t e r m i n i n g t h e amount o f polymer a d s o r b e d t o t h e s u r f a c e . As w i l l be shown below, t h i s can be done p r o v i d e d t h e d e n s i t y o f t h e u n s o l v a t e d polymer i s known. The f r a c t i o n a t i o n o f c o a t e d p a r t i c l e s i d e a l l y t a k e s p l a c e i n a c a r r i e r f r e e of s o l u b l e s u r f a c t a n t . In o r d e r f o r the r e c o r d e d d i a m e t e r v a l u e s t o be m e a n i n g f u l , one must make s u r e t h a t t h e c o a t i n g i s s t a b l y a d s o r b e d so t h a t no d e s o r p t i o n o f polymer t a k e s p l a c e during the s i z i n g process. S i n c e an a d d i t i o n a l i n t e r e s t o f o u r s i s t o b e t t e r understand the p r o t e i n r e s i s t a n c e g i v e n a polymeric s u r f a c e upon t r e a t m e n t w i t h P E O - c o n t a i n i n g s u r f a c t a n t s , we have a l s o examined the s t a b i l i t y of the c o a t i n g i n a p r o t e i n environment.

Experimental S i z i n g Methods. Sedimentation Field-Flow Fractionation (sedFFF). Under t h e i n f l u e n c e o f an a p p l i e d c e n t r i f u g a l f i e l d , a p a r t i c u l a t e sample i s t r a n s p o r t e d by a f l o w i n g c a r r i e r t h r o u g h t h e t h i n FFF c h a n n e l w i t h a v e l o c i t y which d i r e c t l y r e f l e c t s i t s buoyant mass. The r e l a t i o n s h i p between t h e sample c h a r a c t e r i s t i c s mass and d e n s i t y , and t h e r e s u l t a n t m i g r a t i o n v e l o c i t y has been d e s c r i b e d i n d e t a i l by G i d d i n g s and o t h e r s (11-13). F o r a l l FFF t e c h n i q u e s o p e r a t i n g i n t h e " n o r m a l " mode (2A), a sample's r e t e n t i o n r a t i o R i s e x a c t l y r e l a t e d t o t h e t h i c k n e s s 1 o f t h e l a y e r t h a t forms as t h e f i e l d c o n c e n t r a t e s t h e sample p a r t i c l e s n e a r t h e a c c u m u l a t i o n w a l l .

(1) Here, V i s t h e average m i g r a t i o n r a t e o f a g i v e n p a r t i c l e t y p e , i s t h e a v e r a g e c a r r i e r v e l o c i t y , and w i s t h e t h i c k n e s s o f t h e channel. For convenience, the dimensionless r e l a t i v e l a y e r t h i c k n e s s 1/w, which i s c h a r a c t e r i s t i c o f e a c h p a r t i c l e t y p e , i s g i v e n t h e symbol X . E a c h s u b t e c h n i q u e o f FFF i s a s s o c i a t e d w i t h a s p e c i f i c r e l a t i o n s h i p between p a r a m e t e r X , on t h e one hand, and t h e p r o d u c t o f f i e l d s t r e n g t h and sample c h a r a c t e r i s t i c s on t h e o t h e r ; i n t h e c a s e o f sedFFF, t h e s e c h a r a c t e r i s t i c s a r e mass m and d e n s i t y pp, as n o t e d above z

o

n

e


250


X =

kT m(Ap/ )Gw Pp

(

2

)

I n t h i s e q u a t i o n , kT has t h e u s u a l meaning o f B o l t z m a n n c o n s t a n t and t e m p e r a t u r e , Ap s y m b o l i z e s t h e d i f f e r e n c e i n d e n s i t y between p a r t i c l e and c a r r i e r , a n d G i s t h e c e n t r i f u g a l a c c e l e r a t i o n . For spherical p a r t i c l e s o f u n i f o r m d e n s i t y , t h e mass m i n E q u a t i o n 2 may be e x p r e s s e d as t h e p r o d u c t o f volume a n d d e n s i t y . Under such circumstances, X i s a simple f u n c t i o n o f p a r t i c l e s i z e d !

6kT

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d

ftApGw

(3)

Equation 3 applies only t o p a r t i c l e s o f uniform density o r t o c o m p o s i t e p a r t i c l e s whose o v e r a l l d e n s i t y i s e x a c t l y known. This i s n o t t h e c a s e f o r a d s o r p t i o n complexes, where a c o r e p a r t i c l e o f known s i z e a n d d e n s i t y i s c o v e r e d by a s o l v a t e d a d - l a y e r o f unknown t h i c k n e s s and d e n s i t y . However, i f t h e d e n s i t y o f t h e u n s o l v a t e d c o a t i n g i s known, t h e measured X - v a l u e c a n be u s e d t o c a l c u l a t e t h e d r y mass o f t h e c o a t i n g , as seen from a r e a r r a n g e m e n t o f E q u a t i o n 2 i n w h i c h t o t a l mass m i s r e p l a c e d by t h e sum o f masses f o r t h e c o r e (mi), t h e u n s o l v a t e d c o a t ( m 2 ) , and t h e mass o f bound s o l v e n t (1113), and t h e p a r t i c l e d e n s i t y i s e x p r e s s e d as t o t a l mass d i v i d e d by t o t a l volume V, i n t u r n c a l c u l a t e d from masses m\ m 2 , a n d 1113, and d e n s i t i e s p i , P 2 and p, r e s p e c t i v e l y . By w r i t i n g t h e buoyant mass as (m - Vp), i . e . t h e d i f f e r e n c e between t h e t o t a l mass a n d t h e mass o f d i s p l a c e d s u s p e n s i o n medium, a n d e x p r e s s i n g V as [ ( m i / p i ) + ( m 2 / p 2 ) + ( m 3 / p ) ] , one a r r i v e s a t t h e f o l l o w i n g r e l a t i o n s h i p : f

kT —

= m (

i

+

m

2 )

-

P {

(m /p ) i

i

+

(m /p )} 2

2

(4)

where p s y m b o l i z e s c a r r i e r d e n s i t y . The s e d i m e n t a t i o n F F F u n i t u s e d i n t h i s work was b u i l t i n house, e s s e n t i a l l y a c c o r d i n g t o (15). The n o m i n a l d i m e n s i o n s o f t h e f l o w c h a n n e l a r e 96.0 X 2.0 X 0.0254 cm^, a n d t h e measured v o i d volume V ° o f t h i s u n i t i s 4.76 mL. The emergence o f p a r t i c l e s f r o m t h e s e p a r a t o r was m o n i t o r e d by a L i n e a r UV d e t e c t o r w i t h a 254 nm l i g h t source. E x p e r i m e n t a l r e t e n t i o n r a t i o s R were computed i n t h e u s u a l way a s r a t i o s o f V ° and e l u t i o n volumes V . A l l a n a l y s e s were p e r f o r m e d under a f i e l d o f 173 g r a v i t i e s (1000 rpm) and a c h a n n e l f l o w o f 2.7 mL/min. Samples o f 5 JIL volume were i n j e c t e d d i r e c t l y o n t o t h e c h a n n e l a n d r e l a x e d a t z e r o f l o w f o r 15 m i n . e

Flow Field-Flow Fractionation (flow FFF). T h i s s u b t e c h n i q u e a c c o m p l i s h e s r e t e n t i o n and s e l e c t i v i t y b y a l l o w i n g a f l o w o f c a r r i e r a c r o s s t h e semi-permeable w a l l s o f t h e c h a n n e l t o c o n c e n t r a t e t h e sample p a r t i c l e s i n t h i n l a y e r s n e a r one o f t h e w a l l s . The l o n g i t u d i n a l c a r r i e r v e l o c i t y , w h i c h i s t y p i c a l l y a r o u n d two o r d e r s o f magnitude h i g h e r t h a n t h a t o f t h e c r o s s - f l o w , p e r f o r m s t h e t r a n s p o r t o f p a r t i c l e s from i n l e t t o o u t l e t a t r e l a t i v e r a t e s w h i c h a r e d e t e r m i n e d by t h e t h i c k n e s s o f e a c h p a r t i c l e l a y e r . As i n t h e c a s e o f sedFFF, E q u a t i o n 1 above r e l a t e s an o b s e r v e d r e t e n t i o n r a t i o R t o i t s corresponding X-value, i . e . t o the dimensionless thickness o f t h e sample l a y e r . However, i n c o n t r a s t t o sedFFF, r e t e n t i o n i n f l o w F F F i s a f u n c t i o n o f a s i n g l e p a r t i c l e c h a r a c t e r i s t i c , namely t h e d i f f u s i v i t y (lfL) .


16. L I E T A L .

Analysis of Copolymer-Coated Polystyrene Latex kTV

251

o

37mvw

(5)

The " f i e l d strength" i n t h i s case i s the rate of cross-flow, V; the column void volume i s V° and D i s the sample's d i f f u s i o n c o e f f i c i e n t . In the right hand formulation, the Stokes-Einstein equation i s used to express D i n terms of the c a r r i e r v i s c o s i t y x\ and the hydrodynamic diameter d of the sample p a r t i c l e


D =

37rnd

(6)

A comparison of Equations 3 and 5 reveals that sedFFF, where X varies with d~^, i s more s e l e c t i v e to samples of d i f f e r e n t sizes than i t s flow counterpart for which X i s proportional to d " . The absence of any influence on retention by a second sample c h a r a c t e r i s t i c , e . g . density, and the s i g n i f i c a n t l y larger separation range ( t y p i c a l l y 11000 nm for flow FFF as compared to 100-1000 nm for sedFFF) makes the flow analogue an a t t r a c t i v e complement to the sedimentation subtechnique. The flow FFF instrument used i n t h i s study was purchased from FFFractionation; the dimensions of i t s flow channel are 27.6 X 2.0 X 0.0254 cm^, and i t s measured void volume i s 1.17 mL. Samples of 5 |1L were i n j e c t e d and relaxed at zero a x i a l flow for a time period s u f f i c i e n t to l e t 1.5 void volumes of cross flow pass through the channel. As i n the sedFFF experiment, the effluent from the flow FFF channel was monitored by UV detection (Linear) at 254 nm. 1

Photon

Correlation

Spectroscopy

(PCS).

Light

from a

monochromatic source which impinges on a p a r t i c u l a t e suspension i s scattered to varying degrees i n a l l d i f f e r e n t d i r e c t i o n s . The scattered l i g h t from neighboring p a r t i c l e s w i l l i n t e r f e r e c o n s t r u c t i v e l y or d e s t r u c t i v e l y depending upon the wavelength of t h i s l i g h t , the observation angle and the distance between p a r t i c l e s . This distance i s not constant, but varies i n time as a result of Brownian motion; by mapping the time course of registered i n t e n s i t y fluctuations through the use of an autocorrelator one can e s t a b l i s h an average d i f f u s i v i t y for the sample. As i n the case of flow FFF above, the Stokes-Einstein equation can then be used to convert values for the d i f f u s i v i t y D into corresponding values for the diameter d (12) . The autocorrelator assembles a c o r r e l a t i o n function G(T) from p a i r s of measurements of scattered photons, recorded at times t and t + t. For samples of uniform size and composition, the autocorrelation function i s given by Q

Q

G(T) = 1 + P e x p 2 ( - Pi)

(7)

where T i s the shorthand notation for the product of the sample's d i f f u s i o n c o e f f i c i e n t D and the factor (4?cn s i n (8/2) /X ) 2, whose value depends on the r e f r a c t i v e index n of the suspension medium, the observation angle 0, and the wavelength X' of the incident l i g h t . Factor P i n Equation 7 i s a system s p e c i f i c constant. From the d i f f u s i o n c o e f f i c i e n t D one calculates a hydrodynamic diameter d for the sample using the Stokes-Einstein r e l a t i o n s h i p (Equation 6). In t h i s work, PCS s i z i n g was performed using e i t h e r the f i x e d angle (90-degree) BI-90, or the multiangle BI-2030AT p a r t i c l e s i z e r s 1



252


from Brookhaven I n s t r u m e n t s . A l l l i q u i d s u s e d f o r sample d i l u t i o n were p r e v i o u s l y p a s s e d t h r o u g h a M i l l i p o r e f i l t e r w i t h a 200 nm p o r e size. O t h e r Methods. Scanning Electron Microscopy. F r a c t i o n s from sedFFF and f l o w FFF were c o l l e c t e d on N u c l e o p o r e f i l t e r s w i t h p o r e s i z e s o f e i t h e r 200 nm o r 100 nm. A f t e r a i r d r y i n g , t h e s e f i l t e r s were c u t t o s i z e , mounted on s t u b s , and s p u t t e r c o a t e d w i t h g o l d . Specimens t r e a t e d i n t h i s way were t h e n s u b j e c t e d t o e l e c t r o n m i c r o s c o p y u s i n g a JEOL model JFM 35 s c a n n i n g e l e c t r o n m i c r o s c o p e . Measurements of Surfactant Adsorption and Leakage. The PEO s i d e - a r m s o f t h e P l u r o n i c F108 s u r f a c t a n t were r a d i o i s o t o p e l a b e l l e d w i t h 1 2 5 i u s i n g a method t o be d e s c r i b e d i n d e t a i l e l s e w h e r e ( L i , J . ; L i n , J . ; C a l d w e l l , K.D. M a n u s c r i p t i s i n p r e p a r a t i o n ) . F o l l o w i n g a d s o r p t i o n o f l a b e l l e d s u r f a c t a n t from a 4 % F108 s o l u t i o n , which i s w e l l i n t o t h e p l a t e a u r e g i o n o f t h e a d s o r p t i o n i s o t h e r m (3), t h e c o a t e d p o l y s t y r e n e s p h e r e s were p e l l e t e d i n a F i s h e r M i c r o c e n t r i f u g e , model 235A, and t h e s u p e r n a t a n t was removed. After r e s u s p e n s i o n i n phosphate b u f f e r e d s a l i n e (PBS, 0.15M, pH 7 . 4 ) t h e p a r t i c l e s were a g a i n c e n t r i f u g e d and t h e s u p e r n a t a n t c o l l e c t e d f o r measurement o f i t s r a d i o a c t i v i t y u s i n g a Beckman 170M Radiocounter. These washing c y c l e s were r e p e a t e d u n t i l no r a d i o a c t i v i t y was measured i n t h e s u p e r n a t a n t . Samples were run i n t r i p l i c a t e . By suspending the coated p a r t i c l e s f o r d i f f e r e n t l e n g t h s of time i n media o f d i f f e r e n t c o m p o s i t i o n , c e n t r i f u g i n g , and m e a s u r i n g t h e r a d i o a c t i v i t y i n the supernatant, the r e l a t i v e r a t e o f s u r f a c t a n t d e s o r p t i o n / d i s p l a c e m e n t was a s s e s s e d f o r e a c h medium. The s m a l l e r PS p a r t i c l e (69 nm) c o a t e d w i t h I - l a b e l l e d F108 can not be f o r c e d t o s e t t l e u s i n g t h e above method. I n s t e a d , a c e n t r i f u g e tube c o n t a i n i n g a f i l t e r w i t h MW 30,000 c u t o f f (Amicon, C e n t r i c o n - 3 0 ) was u s e d t o s e p a r a t e s o l i d s from s u p e r n a t a n t . D u r i n g c a r e f u l washing o f the s o l i d s c o l l e c t e d on t h e f i l t e r , t h e f i l t r a t e was c o n t i n u o u s l y c o u n t e d f o r r a d i o a c t i v i t y ; t h e washing c o n t i n u e d u n t i l no r a d i o a c t i v i t y c o u l d be d e t e c t e d i n t h e f i l t r a t e . At t h i s p o i n t , the r a d i o a c t i v i t y of the f i l t e r was c o u n t e d and r e l a t e d t o t h e known amount o f p a r t i c l e s present. From t h e r a d i o a c t i v i t y measured on a g i v e n amount o f p a r t i c l e s , t h e i r s u r f a c e d e n s i t y o f F108 was determined. Measurements of Latex Concentration and Surfactant Density. The d e n s i t y o f t h e p o l y s t y r e n e l a t e x i s known t o be 1.053 g/mL (value p 2 i n the e q u a t i o n below). By m e a s u r i n g t h e d e n s i t y p o f a s u s p e n s i o n o f PS l a t e x i n a medium o f known d e n s i t y p i , one d e t e r m i n e s t h e mass c o n c e n t r a t i o n C 2 (g/mL) o f p o l y m e r i c p a r t i c l e s from t h e f o l l o w i n g relationship 1 2 5

(8) The d e n s i t y ( 2 ) o f t h e F108 s u r f a c t a n t was s i m i l a r l y measured f r o m s o l u t i o n s o f known c o n c e n t r a t i o n C 2 r o b t a i n e d by c a r e f u l w e i g h i n g o f t h e d r y polymer and d i s s o l v i n g i n a c c u r a t e l y d e t e r m i n e d volumes o f d i s t i l l e d water. A l l d e n s i t i e s were d e t e r m i n e d t o 6-th p l a c e a c c u r a c y u s i n g a M e t t l e r PAAR model DMS 60 h i g h p r e c i s i o n densitometer. P

Materials. Two t y p e s o f p o l y s t y r e n e l a t e x s p h e r e s w i t h n o m i n a l d i a m e t e r s o f and 272 nm (Seradyn) were u s e d as s u b s t r a t e s i n t h e a d s o r p t i o n


69


16.

LI ET AL.

Analysis of Copolymer-Coated Polystyrene Latex

253

experiments; i n a d d i t i o n , other PS l a t e x standards from the same manufacturer were used t o v e r i f y the performance of the s i z i n g equipment. The polymeric surfactant focused on i n t h i s study was Pluronic F108, generously donated by the BASF corporation. This material i s a t r i b l o c k with a molecular weight of 14,600 dalton, c o n s i s t i n g of PEO/PPO/PEO i n the monomeric r a t i o of 129/56/129. The density of t h i s material was determined t o be 1.186 g/mL, as described above. A l l standard p a r t i c l e s were s i z e d p r i o r t o any exposure t o the polymeric surfactant, using a 0.1% aqueous s o l u t i o n of the FL-70 (low molecular weight) surfactant from Fisher S c i e n t i f i c . This suspension medium has been used extensively i n FFF p a r t i c l e c h a r a c t e r i z a t i o n experiments, and has been found t o y i e l d diameters i n good agreement with other techniques. During the adsorption experiments, the l a t e x p a r t i c l e s (2.5% w/w) were incubated with F108 (4.0% w/w) i n PBS f o r periods of 24 hours under constant end-over-end shaking. The coated l a t e x p a r t i c l e s were then s i z e d using c a r r i e r s containing e i t h e r 0.1% FL-70, or 0.1% F108 i n deionized water alone or with the a d d i t i o n of 12 mM NaCl. The desorption of F108 surfactant was studied i n media c o n s i s t i n g of deionized water, F108 i n Dl water, 0.1% aqueous FL-70, and 0.1% human serum albumin (HSA, ICN 82-301-1) i n PBS. R e s u l t s and D i s c u s s i o n

The generally good agreement between p a r t i c l e s i z e s determined by FFF and other methods has been discussed previously (11-13). In Figure l a we have compiled data from a study by L i , J-M.; Caldwell, K.D., and Machtle, W. (12.) i n which 9 PS l a t e x standards were characterized using a n a l y t i c a l u l t r a c e n t r i f u g a t i o n (AUC), sedFFF, PCS, and SEM. The good c o r r e l a t i o n between the manufacturer's (BASF) AUC data, and data from the other techniques supports the notion that p a r t i c l e diameters are accurately determined using e i t h e r of the techniques, provided the samples are uniform i n s i z e and of known density. However, while polydisperse samples are r e a d i l y s i z e d by sedFFF and AUC, and somewhat l e s s e a s i l y s i z e d by SEM, t h e i r d e t a i l e d s i z e d i s t r i b u t i o n can not be established using the PCS technique (12). Instead, t h i s method i s generally l i m i t e d t o providing a Z-average of a l l s i z e s present i n the sample as w e l l as a p o l y d i s p e r s i t y index f o r the population. Only f o r bimodal samples with a l a r g e r than 100% s i z e d i f f e r e n c e between the modes i s i t possible t o s i z e the two populations separately (12) . Conversely, since the c e n t r i f u g a l force a f f e c t i n g the suspended p a r t i c l e s depends both on t h e i r s i z e and density (1£), the two sedimentation techniques AUC and sedFFF are by themselves unsuited f o r s i z i n g samples with nonuniform density. I t i s , however, possible to u t i l i z e the high f r a c t i o n a t i n g power of the sedFFF technique t o produce narrow f r a c t i o n s of such samples, and t o subsequently l e t the PCS provide a s i z e assignment f o r the now uniform p a r t i c l e s present i n each f r a c t i o n . Flow FFF presents an altogether d i f f e r e n t s i t u a t i o n , since from Equation 5 above i t i s c l e a r that s i z e i s the only p a r t i c l e c h a r a c t e r i s t i c which a f f e c t s retention i n t h i s technique. For monodisperse standard p a r t i c l e s , Figure l b i l l u s t r a t e s the good agreement found between diameters determined by flow FFF and those determined by PCS. Adsorption of the F108 t r i b l o c k t o two PS standard l a t e x samples with nominal diameters of 69 and 272 nm was performed at a surfactant concentration w e l l above the onset of the plateau region of the adsorption isotherm (2SD and resulted i n s i g n i f i c a n t s i z e increases,


254



d

(FFFF),

nm

F i g u r e 1. a) Comparison o f p a r t i c l e s i z e s d e t e r m i n e d by sedFFF and o t h e r t e c h n i q u e s ; b) Comparison o f s i z e d e t e r m i n a t i o n s by PCS and f l o w FFF.


16. L I ET A L .

255

Analysis of Copolymer-Coated Polystyrene Latex


as judged from PCS a t a f i x e d o b s e r v a t i o n a n g l e . Indeed, t h i s t e c h n i q u e showed t h e d i a m e t e r s o f t h e ( f i l t e r e d ) a d s o r p t i o n complexes t o be 84 and 320 nm, r e s p e c t i v e l y . I n PCS, t h e a n g u l a r dependence o f t h e s c a t t e r i n g i n t e n s i t y i s known t o d i f f e r m a r k e d l y w i t h p a r t i c l e size. A good way t o m o n i t o r a sample f o r changes i n i t s p o l y d i s p e r s i t y , such as might o c c u r d u r i n g t h e a d s o r p t i o n o f a p o l y m e r i c s u r f a c t a n t , i s t h e r e f o r e t o scan f o r angular v a r i a t i o n s i n t h e d i f f u s i o n c o e f f i c i e n t D o f E q u a t i o n 6 b e f o r e and a f t e r t h e a d s o r p t i o n OSL) . I f t h e r e i s no change i n t h e p o l y d i s p e r s i t y as a r e s u l t o f t h e c o a t i n g , t h e r e l a t i o n s h i p s between D and sin^G/2 s h o u l d remain c o n s t a n t i n b o t h c a s e s t h r o u g h o u t a l l o b s e r v a t i o n a n g l e s . A l t h o u g h F i g u r e 2 shows a s l i g h t s c a t t e r i n t h e D v a l u e s r e c o r d e d f o r t h e F 1 0 8 - c o a t e d PS 272 nm a t d i f f e r e n t o b s e r v a t i o n a n g l e s , t h e d a t a g i v e no c l e a r i n d i c a t i o n o f any i n c r e a s e i n sample p o l y d i s p e r s i t y . An a l t o g e t h e r d i f f e r e n t c o n c l u s i o n i s r e a c h e d when t h e sample i s examined by sedFFF. The f r a c t o g r a m s i n F i g u r e 3 show t h e e l u t i o n p r o f i l e s o f PS 272 b e f o r e and a f t e r c o a t i n g . In t h e " b e f o r e " c a s e , t h e f r a c t o g r a m c o n t a i n s one s i n g l e peak i n d i c a t i v e o f a u n i m o d a l s i z e distribution. As t h e d e n s i t y o f t h e naked p o l y s t y r e n e p a r t i c l e s i s w e l l known, t h e i r average s i z e can be c a l c u l a t e d from t h e e l u t i o n p o s i t i o n u s i n g E q u a t i o n s 1 and 2. T h i s v a l u e , r e p o r t e d i n Table I, i s seen t o be i n c l o s e agreement w i t h t h e s i z e d e t e r m i n e d from PCS. By c o n t r a s t , t h e f r a c t o g r a m r e p r e s e n t i n g t h e F 1 0 8 - c o a t e d p a r t i c l e s d i s p l a y s q u i t e a b i t o f c o m p l e x i t y , w i t h t h e major peak b e i n g t r a i l e d by s e v e r a l minor components. Since the d e n s i t y of the coated p a r t i c l e s i s unknown, no s i z e can be a s s i g n e d t o t h e s e components b a s e d on t h e i r sedFFF e l u t i o n b e h a v i o r . However, e l e c t r o n m i c r o g r a p h s o f f r a c t i o n s c o l l e c t e d under each peak v e r y c l e a r l y r e v e a l e d t h e p r e s e n c e o f monomers, d i m e r s , t r i m e r s , and h i g h e r o r d e r aggregates. The monomer f r a c t i o n i s by f a r t h e most abundant, and an average d i a m e t e r f o r p a r t i c l e s i n t h i s f r a c t i o n i s e a s i l y d e t e r m i n e d by PCS (see T a b l e I ) . A d i r e c t PCS a n a l y s i s o f t h e F 1 0 8 - c o a t e d PS p a r t i c l e s had shown an average d i a m e t e r o f 365 nm; a f t e r p a s s a g e t h r o u g h a N u c l e o p o r e f i l t e r w i t h 0.8 pm p o r e s i z e , t h e average PCS s i z e was r e d u c e d t o 320 nm i n d i c a t i v e o f a c o a t i n g t h i c k n e s s o f 24 nm. However, by e x p o s i n g t h e sample t o t h e sedFFF p r o c e d u r e and t h e r e b y removing a l l a g g r e g a t e s from t h e c o a t e d p a r t i c l e s , one a r r i v e s a t t h e much s m a l l e r v a l u e o f 14 nm f o r t h i s t h i c k n e s s , as seen i n T a b l e I .

Table I.

d,

S i z e s o f Bare and F108-Coated PS P a r t i c l e s , Relative Coating Thickness, 8

Nominal (nm)

d, PS (nm)

d, F108 c o a t e d (nm)

and

PS

the

5 (nm)

FFF 272 69

275±2 69±2

-

82±2

-

7±2

PCS 272 272±3 69 68±3 * Measured on c o l l e c t e d f r a c t i o n ** Measured on f i l t e r e d s o l u t i o n

301±2* 84+2**


14±3 8±3


256


2.0e-8

Bare PS 272

1.8e-8 "

1.6e-8 "

1.4e-8 -

PS 272 c o a t e d w i t h F108 1.2e-8 "

1.0e-8 0.0

— i

1 0.1

1

1

i

0.2

|

i

0.3

iin

'

1

i

0.4

1 0.5

i

—

0.6

(B/2)

F i g u r e 2. A n g l e dependence o f d i f f u s i o n c o e f f i c i e n t s 272-F108 a d s o r p t i o n complex d e t e r m i n e d by PCS.

f o r t h e PS



F i g u r e 3. SedFFF f r a c t o g r a m o f PS 272 b e f o r e and a f t e r c o a t i n g w i t h t h e F108 s u r f a c t a n t . C a r r i e r : 0.1% F108, 12 mM N a C l ; f l o w r a t e 2.7 niL/min; f i e l d s t r e n g t h 173 g r a v i t i e s .


Size Analysis of a Block Copolymer-Coated Polystyrene Latex - ACS

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