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J. Phys. Chem. B 2004, 108, 18520-18529
Effects of Added CO2 on the Dynamics of Poly(dimethylsiloxane) Oligomers Dissolved in a Θ Solvent and a Poor Solvent Wendy E. Gardinier, Maureen A. Kane, and Frank V. Bright* Department of Chemistry, Natural Sciences Complex, UniVersity at Buffalo, The State UniVersity of New York, Buffalo, New York 14260-3000 ReceiVed: May 5, 2004; In Final Form: September 14, 2004
We report on the tail-tail cyclization and unfolding kinetics of a poly(dimethylsiloxane) oligomer that is end-labeled with pyrene (Py-PDMS-Py) when it is dissolved at low concentration in liquid ethyl acetate (a Θ solvent for PDMS) or methanol (a poor PDMS solvent) at 308 K as a function of added CO2 (0-1 mole fraction). In all liquids, the tail-to-tail cyclization rate increases as the CO2 mole fraction increases. The tail-to-tail unfolding rate is CO2 independent, and generally slower in comparison to the cyclization rates. As CO2 is added, toluene (a good solvent for PDMS) and ethyl acetate move toward becoming poorer PDMS solvents. At a given CO2 mole fraction in the liquid phase, the Θ solvent (ethyl acetate) quality for PDMS decreases ahead of and in greater magnitude than the good solvent (toluene). In methanol, the Py-PDMSPy molecules appear to aggregate below ∼0.2 CO2 mole fraction. Above ∼0.2 CO2 mole fraction, the PyPDMS-Py dynamics appear to obey a Birks-like model. As we increase the CO2 mole fraction in the PyPDMS-Py/methanol system above ∼0.2, there is not a significant change in the Py-PDMS-Py tail-to-tail distance; the internal polymer dynamics increase. The overall effects of solvent and CO2 mole fraction on the Py-PDMS-Py dynamics in CO2-expanded liquids are captured by a simple model.
Introduction The behavior of tail segments within flexible molecules is important because tail residues represent the sites at which chain growth occurs during polymerization, tails can wrap around a tethered reactive site to block access from other reagents/solvent, and tails/heads can react with one another within a molecule to form novel cyclic structures.1-9 In dilute solutions, experiments have shown that polymer tail-tail cyclization and unfolding kinetics depend on the polymer chemistry, polymer type and chain length,10-15 solvent quality (good, Θ, and poor),10,13,16,17 temperature,15-17 and hydrostatic pressure.18,19 Polymer tailtail cyclization dynamics have also been the subject of theoretical studies.20-25 One convenient way to investigate the behavior of polymer “tails” is to site selectively tag them with environmentally responsive fluorescent probe molecules like pyrene.2,10-19,26,27 In this way one can use the pyrene monomer and excimer emission28,29 to address questions about tail-tail cyclization/ unfolding dynamics and other inter- and intramolecular interactions. In this paper, we investigate the behavior of linear poly(dimethylsiloxane) oligomer tails that have be end-labeled with pyrene (Py-PDMS-Py)30,31 in ethyl acetate (Θ solvent for PDMS) and methanol (poor solvent for PDMS) at 308 K as a function of added CO2. Py-PDMS-Py has been studied previously in toluene/CO230 and pure supercritical CO2.31 CO2 was investigated in the current work because it is an environmentally responsible solvent,32-38 it has been used as an antisolvent for the controlled precipitation of polymers from solution,39-41 and CO2-dilated liquids have been a topic of recent * To whom all correspondence should be directed. Telephone: 716645-6800 ext. 2162 (voice); 716-645-6963 (Fax). E-mail: chefvb@ acsu.buffalo.edu.
Figure 1. Simplified schematic describing the photophysics of a flexible molecule like Py-PDMS-Py within a Birks framework. Rate coefficients (all unimolecular) are as follows: kM, excited-state monomer to ground-state monomer relaxation; kE, excited-state excimer to ground-state monomer relaxation; 〈kcyclize〉, excited-state intramolecular tail-tail cyclization; and 〈kunfold〉, excited-state tail-tail unfolding.
study.42 We use steady-state and time-resolved fluorescence spectroscopy to investigate the pyrene monomer and excimer emission and we determine the Py-PDMS-Py tail-tail cyclization and unfolding kinetics as a function of added CO2. Theory Figure 1 presents a simplified energy-level diagram describing the behavior of a flexible molecule like Py-PDMS-Py undergoing intramolecular excimer formation (i.e., tail-tail cyclization) within the idealized Birks framework.2,10,28 Here, a monomeric form of the Py-PDMS-Py molecule (1) is photoexcited (hνex) to produce an excited-state (*) monomer (2). In this simple scheme, the excited-state monomer has two possible fates. It can deexcite back to the ground state with overall rate kM. In a second pathway, the tails on the excited-
10.1021/jp0480706 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/05/2004
Dynamics of Poly(dimethylsiloxane) Oligomers
J. Phys. Chem. B, Vol. 108, No. 48, 2004 18521
state monomer can cyclize intramolecularly with rate 〈kcyclize〉, forming an intramolecular excited-state (/) excimer (3). The so formed excimer also has two possible fates. It can unfold with rate 〈kunfold〉 to re-form the excited-state monomer or it can deexcite back to the ground-state monomer with overall rate kE. (Note: 〈 〉 is used with some of the rates to remind the reader that all oligomers and polymers exhibit some degree of polydispersity.) Figure 1 predicts2,10,28 that the monomer time-resolved fluorescence intensity (IM(t)) will decay as the sum of two exponentials and the excimer time-resolved emission intensity (IE(t)) will decay as the difference of two exponentials
IM(t) ) a1 exp(-λ1t) + a2 exp(-λ2t)
(1)
IE(t) ) a3 exp(-λ3t) + a4 exp(-λ4t)
(2)
with the following constraints: λ1 ) λ3, λ2 ) λ4, and a4/a3 ) -1. If Figure 1 describes the system photophysics, one can recover all the kinetic terms from the following relationships:
2λ1 or 3 ) (Ax + Ay) - [(Ax - Ay)2 + 4〈kcyclize〉〈kunfold〉]1/2 (3) 2λ2 or 4 ) (Ax + Ay) + [(Ax - Ay)2 + 4〈kcyclize〉〈kunfold〉]1/2 (4) where
Ax ) kM + 〈kcyclize〉
(5)
Ay ) kE + 〈kunfold〉
(6)
a2/a1 ) (Ax - λ1)/(λ2 - Ax)
(7)
and
In the current work, we use a global analysis strategy43 to determine the λx and ax terms by acquiring time correlated single photon counting data at two or more emission wavelengths across the Py-PDMS-Py emission spectrum and analyzing these data sets simultaneously to recover the desired λx and ax terms. We determine kM by using a model monomer, 1-ethylpyrene (i.e., kM ) τ1-EP-1). Experimental Section Chemicals and Reagents. NH2-PDMS-NH2 (Mn ) 2500 g/mol) was obtained from United Chemical Technologies. 1-Pyrenebutanoic acid succinimidyl ester and 1-ethylpyrene (1EP) were purchased from Molecular Probes, Inc. HPLC grade ethyl acetate (99.8%) and methanol (99.8%) were purchased from Sigma-Aldrich. CO2 (SCF grade) was from Scott Specialty Gases. All chemicals and reagents were used as received. Py)PDMS)Py Synthesis.30 NH2-PDMS-NH2 was reacted in anhydrous toluene with a 10-fold molar excess of 1-pyrenebutanoic acid succinimidyl ester for 24 h with stirring in the dark under an N2 atmosphere. The desired product was isolated by gel permeation chromatography and its structure was confirmed by matrix-assisted laser desorption/ionization timeof-flight mass spectrometry (MALDI-TOF MS). Only the doubly labeled Py-PDMS-Py is present in our samples following purification. Instrumentation. The high-pressure optical system consists of an Isco model 260-D syringe pump that is coupled to a pressure monitoring system ((0.2 bar) and a temperaturecontrolled ((0.1 K) optical cell (internal volume ) 3.5 mL).44
All electronic absorbance measurements were performed by using Milton-Roy model 1201 or HP model 8452A UV-vis spectrophotometers. Steady-state excitation and emission spectra were recorded with an SLM-AMINCO model 48000 MHF spectrofluorometer. The excitation source was a 450 W xenon arc lamp. Wavelength selectors were single grating monochromators. The spectral band-pass for a given scan was fixed at 1 nm. All spectra were corrected by using appropriate blanks. The blank contribution to the total emission was always
