Chapter 9
Photophysics of Polymers Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SANTA BARBARA on 08/06/18. For personal use only.
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Fluorescence Probes the Study of Solvation and Diffusion of Reagents in Network Polymers K. J . Shea, G. J . Stoddard, and D. Y . Sasaki
Department of Chemistry, University of California, Irvine, CA 92717 A dansyl monomer, 1, prepared by the condensation of p-vinyl benzyl amine with dansyl chloride has been used as a fluorescent marker to probe the microenvironment of styrene-divinylbenzene networks. The probe readily reveals the degree of solvation of polymer chains which is found to correlate with the degree of crosslinking. Subtle yet important differences in polymer morphology are also uncovered by this method. The fluorescence emission intensity of polymer bound probe 1 is found to be quenched upon treatment with strong electrophiles (Ph C BF ). Monitoring the dimunition in fluorescence emission intensity permits study of the rate of diffusion of electrophilic reagents through styrene-divinylbenzene networks. +
3
4
Macroporous styrene-divinyl benzene (S-DVB) copolymers are widely used as supports for chemical reactions ( 1 ) . The surface area, pore volume, and pore size of these materials can be manipulated by a judicious choice of reaction conditions ( 2 ) . It i s recognized that reaction cosolvent and the r a t i o of monomer to cosolvent are important variables and considerable speculation has been offered regarding the relationship between polymerization conditions and polymer morphology ( 3 ) . On the basis of these studies a model has emerged to account for macroporosity i n these materials ( 4 ) . The continuous or gel phase i s found to consist of aggregated micro spheres. The macropores are defined by voids created by these aggregated microspheres. The gel or continuous phase of these materials i s produced by phase separation that occurs during polymerization. Properties of the gel phase are influenced by (a) the degree of c r o s s l i n k i n g , (b) cosolvent and (c) the monomer-cosolvent r a t i o . The l a s t two f a c tors w i l l affect the phase separation and thus the dimensions of the continuous phase as well as the degree of solvation of the polymer chains during phase separation. The objective of the present study i s to develop a diagnostic that w i l l enable us to evaluate solvation and chemical transport i n 0097-6156/87/0358-0097$06.00/0 © 1987 American Chemical Society
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PHOTOPHYSICS OF POLYMERS the continuous or g e l phase of h i g h l y c r o s s l i n k e d macroporous materials. In previous studies (5) we had noted that polymeriza tion conditions, cosolvent, and crosslinking monomer can exercise a dramatic e f f e c t on the chemical reactions of these supports. We undertook, therefore, a systematic study of these variables using a s e r i e s of macromolecules t h a t c o n t a i n an i n c r e a s i n g degree of crosslinking. Fluorescence spectroscopy was u t i l i z e d as the diagnostic to evaluate these phenomena. It w i l l be shown that a single probe molecule, a derivative of dimethylaminonaphthalenesulfonamide, (dansyl, V) can function both as an environmental probe to evaluate the degree of solvation of polymer chains i n the gel phase and also serve as a sensitive indicator for the d i f f u s i o n of ionic reagents through the crosslinked gel network. Results and Discussion Preparation of Materials. Dansyl probe J_ was prepared by conden sation of j>-vinyl benzyl amine with dansyl chloride (Equation J_)
Network polymers were prepared both by bulk and suspension free r a d i c a l polymerization techniques employing AIBN as i n i t i a t o r . The composition and method of preparation of these materials i s sum marized i n Table I. Non-porous "glass bead" polymers were prepared by suspension polymerization from neat mixtures of styrene and technical grade DVB with nominal crosslinking r a t i o s of 5, 20, and 50 mole %. Macroporous materials were also prepared by suspension polymerization using toluene as diluent ( f = 0.5) (2a) with crossl i n k i n g r a t i o s of 5, 20, and 50 mole %. Macroporous polymers prepared by bulk polymerization were also synthesized; the degree of crosslinking for these materials was 50 mole %. One type of bulk polymerization was formulated i d e n t i c a l l y with material pre pared by suspension techniques (toluene diluent and DVB as the crosslinking agent). The second type was prepared using acetonit r i l e as diluent. The polymers provide a spectrum of materials for analysis with varying degrees of crosslinking and a range of poly mer morphologies. The p a r t i c l e s i z e (125-150 μ) of the s o l i d materials were kept uniform by s i z i n g . The r a t i o of projje 2 monomer in a l l of the above materials was kept uniform (10~ ). t
o
9.
SHEA ET AL.
Fluorescence Probes
99
Table I Summary of Polymerization Conditions
Designation
Monomers
% Crosslinking Monomer
Diluent m
Polymerization Conditions
DVB-5-S-N DVB-20-S-N DVB-50-S-N
ST-DVB ST-DVB ST-DVB
5 20 50
DVB-5-S-T
ST-DVB
5
DVB-20-S-T
ST-DVB
20
DVB-50-S-T
ST-DVB
50
DVB-50-B-T
ST-DVB
50
toluene (0.5)
bulk
DVB-50-B-A
ST-DVB
50
acetonitrile (0.5)
bulk
None None None
suspension^ suspension suspension
toluene (0.5) toluene (0.5) toluene (0.5)
suspension suspension suspension
0
a.
f i s the volume f r a c t i o n of diluent to monomer + diluent used during polymerization
b.
Typical polymerizations were carried out i n a morton flask containing a mixture of water (200 mL) diluent (20 mL), mono mers (20 g), methocel (90 mg) as dispersant, and AIBN (200 mg) as i n i t i a t o r . The mixture was s t i r r e d rapidly f o r 8 h a t 70°C. The polymer beads obtained were washed (refluxing acetone), dried (high vacuum), and sized with sieves (100-120 mesh).
m
t
c.
Polymerizations were carried out i n 35 mL capacity, mediumwalled glass tubes containing monomers (8 g), diluent (8 mL), and AIBN (80 mg) as i n i t i a t o r . The mixture was freeze-thaw degassed three times, sealed, and heated t o 80°C f o r 14 h. The temperature was increased t o 125°C for an additional 12 h. The polymer obtained was crushed, washed (refluxing acetone, 12h), and sized with sieves (100-120 mesh).
Solvation Studies. The fluoresence emission maximum of polymer bound probe provides a measure of the a b i l i t y of solvent molecules to solvate polymer chains i n the network. The solvatochromic s h i f t of probe ± has been l i n e a r l y related to a wide variety of organic solvents using the empirical relationship shown i n Equation 2 (6). (nm) = 53.45 π* + 20.48 α + 9.932 3 + 457. 1
(2)
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PHOTOPHYSICS OF POLYMERS
The terms Τ Ν at and 3 are empirical solvent parameters developed by Taft and Kamlet (7). The fluoresence emission maximum of probe ± in pure organic solvent defines the pure solvent reference l i n e in Figure 1. When probe i s covalently attached to a polymer back bone which in turn i s immersed i n solvent, the deviation i n f l u o r escence emission wavelength from the pure solvent correlation l i n e reveals how the polymer perturbs the raicroenvironment of the probe. This microenvironment can vary from pure solvent-like to one domi nated by the polymer backbone. The fluorescence emission of the probe in dry polymer i s also indicated in the f i g u r e . A summary of the solvatochromatic data i s discussed below. Solvation of "Glass Beads". The fluoresence emission of solventequilibrated "glass-beads" displays a predictable trend along the l i n e s of crosslink density (Figure 1). The 50% glass beads (DVB50-S-N) p a r a l l e l the "dry" polymer c o r r e l a t i o n l i n e i n d i c a t i n g l i t t l e probe solvation. However, as the crosslinking i s decreased solvation increases. Thus the probe ^ readily reveals that these g l a s s y , g e l - l i k e , m a t e r i a l s are s o l v a t e d to a degree which i s controlled in part by the number of c r o s s l i n k s . A departure from t h i s trend i s noted in poor swelling solvents (EtOH) where a l l polymers exhibit solvatochromic s h i f t s that approximate the dry s t a t e (no s o l v a t i o n ) , a s i t u a t i o n t h a t i n d i c a t e s the networks remain c o l l a p s e d and the polymer backbone dominates the probe microenvironment. It should be noted, however, that a l l polymers e x h i b i t a blue s h i f t upon changing from C H C 1 to EtOH. This finding indictes that a l l polymer chains, even DYB-50-S-N, are solvated to some degree in good swelling solvents ( i . e . , CH C1 ). 2
2
2
2
Macroporous Polymers Prepared by Suspension Polymerization. Unlike the non-porous "glass beads", a l l macroporous polymers prepared by suspension polymerization with toluene as cosolvent exhibit f l u o r esence emission that p a r a l l e l s the solvent correlation l i n e (Figure 2). This finding indicates a substantial degree of probe solva tion. Indeed, even the 50% crosslinked material (DVB-50-S-T), exhibits solvation behavior similar to the l i g h t l y crosslinked 5% glass beads. An important difference in t h i s series occurs i n the poor s w e l l i n g s o l v e n t s (EtOH). In t h i s s o l v e n t the 5 and 20% crosslinked materials (DVB-5-S-T, DVB-20-S-T) exhibit a fluoresence emission that p a r a l l e l s the "dry" polymer, however, the emission of 50% crosslinked material (DVB-50-S-T) remains close to the pure solvent c o r r e l a t i o n l i n e . This observation reveals a t r u l y perman ent micropore structure that remains i n t a c t , regardless of solvent. Thus even the highly polar solvent EtOH can penetrate and solvate the probe i n 50% macroporous materials (DVB-50-S-T). Macroporous Polymers Prepared by Bulk Polymerization. With toluene as diluent (DVB-50-B-T) the highly crosslinked networks c l o s e l y p a r a l l e l the pure solvent correlation l i n e i n d i c a t i n g the probe i s highly solvated even in poor polymer solvents (Figure 3 ) . This result indicates a gel phase with a high degree of permanent micro pore structure. There i s e s s e n t i a l l y no difference between t h i s material and that prepared by suspension polymerization (DVB-50S-T).
SHEA ET AL.
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Fluorescence Probes
MeOH
440
460
480 λ
500
520
(nm) cal
F i g u r e 1. F l u o r e s c e n c e e m i s s i o n o f s o l v e n t e q u i l i b r a t e d " g l a s s beads".
F i g u r e 2. F l u o r e s c e n c e e m i s s i o n o f s o l v e n t e q u i l i b r a t e d macro porous s t y r e n e - d i v i n y l b e n z e n e copolymers prepared by s u s p e n s i o n polymerization.
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PHOTOPHYSICS OF POLYMERS
Quite i n t e r e s t i n g l y , when the bulk polymerization i s run with CH CN as diluent, the resulting polymer does not p a r a l l e l the s o l vent correlation l i n e , rather i t p a r a l l e l s very c l o s e l y the "dry" polymer region, indicating very l i t t l e solvation i n a l l solvents. This implies these materials are comprised of a gel phase that i s substantially less pervious to a l l solvents. Diffusion Studies We have observed that the fluoresence emission intensity of the dansyl probe i s diminished i n a c i d i c solvents. Indeed the f l u o r esence of can be completely suppressed i n the presence of strong acids (CF^C0 H) and by treatment with a variety of e l e c t r o p h i l e s . On the basis of NMR investigations, the mechanism of the f l u o r esence quenching i s found to involve reaction of the proton (electrophile) with the dimethylamino group of the dansyl probe (Equa t i o n 3 ) . The protonated or a l k a l a t e d ammonium i o n £ does not fluoresce when excited at 350-360 mm, the absorption maximum of J_. 2
N(CH ) 3
2
(3)
S0 NHCH R
S0 NHCH R 2
2
2
2
Non-Fluorescent
Fluorescent E = H*,Ph C B F " +
+
3
4
E t 0 BF +
3
4
The response of the fluoresence probe to added electrophile o f f e r s the p o t e n t i a l f o r u t i l i z i n g t h i s r e a c t i o n to study the migration of electrophiles i n macromolecules containing the dansyl probe by evaluating the rate at which the electrophile diminishes the net fluoresence i n t e n s i t y (8), Equation 4.
(4)
Fluorescent
Probe
in Polymer
Matrix
Chemically Modified Probe Non-Fluorescent
Thus the dansyl probe can serve two f u n c t i o n s , f i r s t as a solvatochromic diagnostic to evaluate solvation of polymer chains and second as a probe to monitor the d i f f u s i o n of electrophiles i n polymer networks.
9. SHEA ET AL.
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Fluorescence Probes
The technique used to study the rate at which e l e c t r o p h i l i c reagents diffuse into the network domain i s straightforward. A s t i r r e d solution or suspension of the probe-labeled polymer i n CH Cl :hexane (13:4) i s continuously i r r a d i a t e d i n a fluorescence spectrometer c e l l . The fluorescence emission intensity i s adjusted to 100%. At t , a solution of the electrophile i s added via s y r inge and the fluoresence emission intensity i s continuously r e corded. A trace of the dimunition of fluorescence i n t e n s i t y w i l l r e f l e c t the o v e r a l l rate f o r the d i f f u s i o n of reagent into the polymer domain and reaction with the fluorescence probe. In con t r o l experiments, the fluorescence emission of was found to be quenched "instantaneously" when electrophile was added to homo geneous solutions of J _ , thus the p r i n c i p a l contribution to the intensity-time curve w i l l be to reveal the influence of the polymer on impeding transport of electrophile through the network. To f a c i l i t a t e a comparative study of materials, the r a t i o of polymer bound fluorescence probe to added electrophile was held constant at 10:1. This r a t i o was chosen to y i e l d convenient f l u o r e s c e n c e quenching times while revealing the differences i n penetrability of related polymeric materials. Triphenylmethyltetrafluoroborate (Ph^C B F J J " ) was used as the electrophile i n these reactions. The restai ts are summarized i n Figures 4-6. 2
2
Glass Beads (DVB-5, 20, 50-S-N). A rather straightforward trend i s noted f o r t h i s s e r i e s o f m a t e r i a l s - the r a t e o f f l u o r e s c e n c e quenching i s inversely proportional to the nominal c r o s s l i n k i n g density (Figure 4). The fluorescence emission of 5% crosslinked glass beads (DVB-5-S-N) i s quenched fastest ( t . = 15 sec) and completely while the 50% material (DVB-50-S-N) has l o s t less than 50% of i t s intensity after 9 min. / 2
Macroporous Suspension Polymers (DVB-5, 20, 50-S-T). Copolymerization i n the presence of diluents r e s u l t s i n formation of macro porous materials. As can be seen i n Figure 5, macroporosity exerts a dramatic e f f e c t upon the quenching rate. Not s u r p r i s i n g l y , LPS i s quenched " i n s t a n t l y " upon addition of e l e c t r o p h i l e , what i s most interesting however, i s that for a l l p r a c t i c a l purposes, there i s no difference i n quenching rate between DVB-5-S-T and DVB-20-S-T macroporous materials. Quite remarkably, a l l domains are r e a d i l y accessible since the fluorescence emission i s completely quenched in these materials. The l e v e l i n g o f f of fluorescence emission intensity f o r the 50% crosslinked material (DVB-50-S-T) reveals k i n e t i c a l l y inaccessible domains under the conditions of the ex periment . Bulk Macroporous Polymers (DVB-50-B-T, DVB-5Q-B-A). Comparison of DVB-50-B-T with DVB-50-S-T r e v e a l s there i s l i t t l e d i f f e r e n c e between polymers prepared i n bulk and by suspension techniques. Both materials, prepared with toluene as cosolvent, reveal f l u o r escence quenching traces that are almost superimposable. There i s , however, a very dramatic difference between material prepared by bulk polymerization using d i f f e r e n t cosolvents. Polymer DVB-50-B-T reveals a large f r a c t i o n of a l l s i t e s are quenched within 15 sec (Figure 6), a small t a i l on t h i s curve indicates a component (
