Chapter 19
Photophysical and Rheological Studies of the Aqueous-Solution Properties of Naphthalene-Pendent Acrylic Copolymers Mark D. Clark, Charles L. McCormick, and Charles E. Hoyle Department of Polymer Science, The University of Southern Mississippi, Hattiesburg, MS 39406-0076 The effects of pH and the addition of various additives on the photophysical solution properties of the amphiphilic polyelectrolyte poly(2-(1-naphtylyacetyl)ethylacrylate-co-methacrylic acid) (NAEAMAA) have been investigated. Unlike previously reported systems, an almost four-fold increase in excimer emission relative to monomer emission (I /I ) is observed upon increasing pH and, consequently, the degree of ionization. An increase in the dilute solution viscosity of NAEA-MAA solutions with increasing degree of ionization indicates a somewhat expected transition from a compact coil to a more expanded coil at pH 7.5. The hydrophobic character of both the naphthyl groups of NAEA and the pendent methyl groups of MAA and/or the effective decoupling of the naphthyl chromophores from the polymer backbone via a spacer linkage are believed to be responsible, at least in part, for this behavior. Experiments utilizing urea, a water-structure breaker, indicate that at high pH hydrophobic interactions between naphthyl chromophores stabilize the copolymer in an intramolecular "hypercoil". Finally, synthesis of a hydrolytically stable analog of NAEA, necessary for further investigations, is outlined, and initial photophysical studies of its copolymers with methacrylic and acrylic acid are presented. E
M
Amphiphilic polyelectrolytes containing aromatic chromophores have been the focus of a number of studies. These polymers can form heterogeneous, often hydrophobic, microenvironments in dilute aqueous solution. These "pseudomicellar microenvironments or "hypercoils* are formed as the hydrophobic aromatic groups are clustered toward the center of the coil, while the solubilizing hydrophilic ionic groups are located on the exterior (1.2). Not only does this pseudo-micellar conformation enhance the migration of energy within the polymer coil (3), it is also able to solubilize large hydrophobic compounds (4.5) which may in turn act as energy traps. Since the initial work of Guillet and coworkers with copolymers of acrylic acid and naphthylmethyl methacrylate (1), recognition of the potential importance of these types of synthetic polymers in "photon-harvesting* or artificial photosynthetic applications has fueled efforts to create a number of modified water-soluble antennae systems (6-10). ,,
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However, most investigations to date have been conducted on solutions of polymers i n the electrolyte form. Few comprehensive studies directed toward understanding the effect of p H on the conformation, and consequently the photophysical and rheological properties of the pseudo-micellar polymers have been conducted. Similarly the effect of "decoupling" the chromphore from the polymer backbone via a spacer group has only recently received attention. In this chapter, we report the results of both fluorescence emission and viscosity studies of a polymer of methacrylic acid containing 20 mol% of 2-(lnaphtylacetyl)ethylacrylate (NAEA). Data suggest that the observed behavior relates, at least i n part, to hydrophobic naphthyl group associations within the polymer coil. However, further investigations of this system dictate that a more hydrolytically stable label than N A E A (11) be used. Therefore, we also outline the preparation of the amide analog to N A E A and present initial photophysical data from those respective copolymers with methacrylic acid. Experimental Materials. The synthesis of the 2-(l-naphtylyacetyl)ethylacrylate (NAEA) monomer and copolymers with methacrylic acid have been outlined elsewhere (12). Scheme 1 illustrates the synthesis of the hydrolytically stable amide analog of N A E A , 2-(l-naphtylyacetamido)ethylacrylamide (ΝΑΕAm). The activated succinimido ester of naphtylacetic acid (NAA-NHS) was prepared by reacting the acid with ο I CH 0-OH
°v V
ο I
HO-Ν
/J
2
ο °
ΟΗ Ο-Ο-ΝΠ 2
CO'
CO ° O-N-C-N-O ^
HjjN-R-NHg
Sponge
Scheme I. N A E A m Monomer Synthesis. N-hydroxysuccinimide in CHC1 utilizing dicyclohexylcarbodiimide (DCC) as a coupling agent. 2-(l-Naphthylacetamido)ethylamine (NAA-EDA) was prepared via modification of a high-dilution procedure for the reaction of acid derivatives with symmetrical diamines (13). The N A E A m monomer was prepared via addition of acryloyl chloride to N A A - E D A at 0 C using "Proton Sponge" (1,8dimethylamino)naphthylene (Aldrich) in CHC1 . Purification of the material was accomplished via multiple recrystallizations from a mixture of CHC1 and CH OH. 3
e
3
3
3
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Urea (Aldrich, 99+%) was recrystallized three times from methanol. Copper nitrate (Aldrich 99.99%) was used as received. A l l polymer solutions were prepared i n deionized water. Due to the inherent hydrolytic instability of N A E A (11), solutions of N A E A - M A A were discarded after 48 hrs., and fresh solutions prepared for subsequent studies. Methods. Polymer Characterization. U V absorbance measurements (PerkinElmer Lambda 6B) and elemental analysis (MHW Laboratories, Phoenix, AZ) were used to determine copolymer compositions. The number average molecular weight, M , of the N A E A - M A A copolymer was estimated to be 140,000, based on osmotic pressure measurements (Knauer Osmometer with 600W membrane, Arro Laboratories, Inc.) i n Ν,Ν-dimethylacetamide. Residual N A E A monomer in the purified polymers was found to be
