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Internal Twisting Dynamics of Dicyanovinyljulolidine in Polymers Ah-Young Jee, Eunhye Bae, and Minyung Lee* Department of Chemistry and Nanoscience, Ewha Womans UniVersity, Seoul 120-750, Republic of Korea ReceiVed: September 1, 2009; ReVised Manuscript ReceiVed: NoVember 18, 2009
The fluorescence quantum yield of 9-dicyanovinyljulolidine (DCVJ) is very low in fluid solutions but increases markedly in solids because the medium rigidity slows down the internal motion, which acts as a major nonradiative decay channel. In this work, the excited-state twisting motion of DCVJ in polymers was investigated by time-resolved fluorescence spectroscopy, and it was observed that the fluorescence lifetime of DCVJ in polymers depends on the mechanical properties of the medium. Therefore, our results indicate that the elastic modulus is a determining factor for molecular rotor dynamics in soft matter, and its description requires a comprehensive visco-elasto-plastic theory. 1. Introduction The photoisomerization processes of electronically excited molecules in fluid media have drawn considerable attention. In comparison, studies in solid media have drawn less attention because of a preconception that the isomerism reaction would not be much affected by “rigid” environments. However, some fluorescence measurements obtained by steady-state and timeresolved spectroscopy indicate that photoisomerization reactions can occur effectively in the solid state under certain circumstances. Such rotamerizing molecules include stilbene and its analogs,1,2 azobenzene and its derivatives,3,4 3,3′-diethyloxacarbocyanine,5-7 rigidochromic coordination complexes,8 and rhodopsin.9 Thus far, free volume theory has been used to describe how small molecules can rotate in mechanically stiff media.10,11 9-Dicyanovinyljulolidine (DCVJ) is one of the molecules that undergo photoisomerism. Loutfy has first explored the interesting properties of electronically excited DCVJ in rigid media.12 Since then, the molecule has been widely used as a medium rigidity probe. For example, Iwaki et al. applied DCVJ as a molecular rotor to probe antibodies.13 Drickamer and co-workers have studied the pressure dependence on the dynamics of the molecule embedded in polymers.14 Allen et al. investigated the ground state and excited state motions of DCVJ and presented its excited potential surface.15 Lindgren et al. used the molecule as an amyloid aggregation probe.16 Macroncelli and co-workers used DCVJ to investigate medium dynamic heterogeneity.17 The photophysical properties of DCVJ were investigated in ionic liquids.18-20 Haidekker et al. synthesized DCVJ analogs and used them as molecular rotors in various applications.21 Figure 1 depicts the molecular structure of DCVJ in which julolidine and dicyano moieties are attached to 1 and 3 positions of propylene. A crystallography study showed that the ground state is nearly planar with a 3.5° twist angle between the aromatic and dicyanovinyl groups.22 Although the neutral form is shown in the figure, the bond length data imply a significant zwitterionic character in the ground state. Upon photoexcitation, it is weakly fluorescent with a very low quantum yield (∼10-4) in normal organic liquids.12 Internal twisting is the major nonradiative relaxation channel in the excited state. The quantum yield of DCVJ follows the fractional dependence of viscosity QF ) R/ηβ * To whom correspondence should be addressed. E-mail: mylee@ ewha.ac.kr.
Figure 1. The molecular structure of 9-dicyanovinyl julolidine.
with a β value of about 0.6.15 The absorption spectra of DCVJ shift to longer wavelengths in polar solvents, but its fluorescence quantum yield is almost insensitive to polarity, making DCVJ ideal as a rigidity probe. In this work, we measured the fluorescence lifetimes of DCVJ in various polymers including low density polyethylene (LDPE), polyvinyl alcohol (PVA), high density polyethylene (HDPE), ultrahigh molecular weight polyethylene (UHMWPE), polyvinyl chloride (PVC), polycarbonate (PC), polycaprolactam (Nylon 6), poly(methyl methacrylate) (PMMA), polystyrene (PS), and poly(acrylic acid) (PAA). We observed that the fluorescence lifetime of DCVJ is strongly correlated with the Young’s modulus of the polymer. On the basis of the observed fluorescence lifetimes, we attempted to delineate the twisting rate process of DCVJ in terms of polymer rigidity. 2. Experimental Section LDPE (MW 35 000), HDPE, (MW 125 000), UHMWPE (MW 4 500 000), PS (MW 192 000), PVA (MW 120 000), PC (MW 64 000), PMMA (MW 115 000) and PAA (MW 240 000) were purchased from Sigma-Aldrich. Nylon 6 (MW 18 000) was bought from Polyscience. DCVJ was obtained from Invitrogen. Different solvents were used to prepare the following polymer solutions: tetrahydrofuran (THF) for LDPE, PVC, PMMA and PC; decalin for HDPE and UHMWPE; 6:1 mixture of dimethyl sulfoxide (DMSO) and water for PVA; cyclohexane for PS; water for PAA; and 2, 2, 2-trifluoroethanol for Nylon 6. All films for the lifetime measurements were prepared by
10.1021/jp908430w 2009 American Chemical Society Published on Web 12/07/2009
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Figure 2. The resonance structure of DCVJ.
Figure 3. The graphical illustration of DCVJ emphasizing the isomerizing motion.
spin coating and drying in an oven at 70 °C for 24 h. Each of the polymer films contained ca. 5 µM of DCVJ. The fluorescence lifetime of DCVJ in polymer was measured by time-correlated single photon counting (TCSPC). The laser source for sample excitation was a picosecond diode laser operating at a wavelength of 467 nm at 20 MHz (Picoquant). A dichroic mirror and a 488 nm long pass filter (Semrock) were mounted to collect the emission and to eliminate the excitation wavelength. The detector used was a microchannel plate PMT (Hamamatsu 3809-07). Time-correlated single photon counting was carried out with a fast TCSPC board (Becker-Hickl SPC830). The instrument response function (IRF) of the system is about 150 ps. To avoid local bleaching in the polymer, the sample was continuously scanned at a rate of 1 Hz in the area of 100 µm x100 µm, during the measurement. The fluorescence lifetimes were extracted from the measured decay curves by a nonlinear least-squares fit with deconvolution of the instrument response function. 3. Results and Discussion DCVJ is a highly polar molecule with a large electric dipole moment. Its approximate values in the ground and first excited state are 9 and 24 D, respectively.12 Resonance Raman data and quantum mechanical calculations have shown that the ground and excited states of DCVJ consist of a mixture of neutral (N) and zwitterionic (Z) forms.23-26 Figure 2 shows the resonance structure of DCVJ in which N and Z forms are in equilibrium. In the two-state model, the average value of the dipole moment, 〈µ〉, can be defined by applying a rule of mixtures 〈µ〉n
) fnµN + (1 - fn)µZ
(n ) 0,1,...)
(1)
where µN and µZ are the dipole moments of the N and Z forms. The ground electronic state is n ) 0, the first excited electronic state is n ) 1, and so on. The fraction (fn) depends on the electronic state and solvent polarity. The dipole moments of N and Z forms of DCVJ are 2.76 and 37.9 D, respectively.24 The fraction of zwitterionic form is approximately 20% in the ground state but increases to 60% in the excited state. Figure 3 shows the graphical illustration of the molecular structure of DCVJ. The molecular volume of DCVJ is about 223 Å3.27 The two covalent C-C bonds responsible for isomerization are also shown. The julolidine (J) moiety is the electron donor and the dicyano (DC) moiety is the electron acceptor, and the intramolecular charge transfer occurs thorough the conjugate bridge, C1-C2-C3. The molecular volumes of J
Figure 4. The potential energy curve of DCVJ with respect to the twisting angle. The kiso is the twisting rate constant in the excited state.
and DC moieties are 152 and 57 Å3, respectively. The figure depicts the relative size of J and DC (1:0.7 in radius), assuming the moving parts are spherical. Because of the single and double bond character, the twisting motion occurs around the C1-C2 bond in the N form, but it occurs around the C2-C3 bond in the Z form. The excited state potential surface is more zwitterionic in character, and thus the C2-C3 bond is responsible for the twisting. DCVJ is a highly polarizable molecule. A description of this push-pull type molecule is made by bond length alternation (BLA)28
χ ) r(C1 - C2) - r(C2 - C3)
(2)
where χ is the BLA index and r is the equilibrium bond distance. The BLA is an indication of the resonance character between the neutral and charge transfer states. The BLA value is positive for an N form and negative for a Z form (Figure 2). The value is zero for a delocalized form (Figure 3). The quantum mechanical calculation shows that the BLA value of DCVJ correlates strongly with solvent polarity. The BLA index of DCVJ is positive in the ground state but becomes negative in the excited state, further evidencing that the excited state is mostly zwitterionic. Figure 4 depicts the schematic illustration of the potential energy surface of DCVJ along the dihedral angle between the julolidine and dicyano groups. When DCVJ is electronically excited into the Franck-Condon region, A* returns to the ground state radiatively or undergo a twisting motion nonradiatively. The ground state isomerizing motion does not occur at ambient temperature because the potential barrier for the twisting motion of DCVJ, A f A′, is very high (210 kJ mol-1).15 Note that the isomerizing motion of DCVJ is unique because the molecular structure does not change after internal rotation. That is, A and A′ actually have the same molecular structure. On the excited state potential surface, only one direction, A* f B*, is shown in the figure, but the isomerization motion in the other direction is equally probable. The torsional motion is affected by the potential barrier, which largely determines the fluorescence lifetime. The twisting motion of the excited DCVJ is a barrierless process, which explains the ultrafast isomerization motion of DCVJ in solution. On the excited state potential surface, the well depth in the twisted geometry, corresponding to the nonfluorescing B* state, is about 60 kJ mol-1.15 The magnitude of the well depth indicates that the reverse reaction, B* f A*, is not possible at room temperature because the
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Figure 5. The fluorescence decay curves of DCVJ in glycerol and in PMMA. Also shown is the instrument response function (dotted line).
Figure 6. The fluorescence decay curves of DCVJ in several polymers. Also shown is the instrument response function (dotted line).
TABLE 1: The Fluorescence Lifetimes of DCVJ in Various Polymers at Room Temperature (24 °C)a
potential barrier for the process is 24 times higher than the thermal energy at ambient temperature. The nonradiative relaxation from the excited state to the ground state, B* f B, occurs in the perpendicular geometry where the excited state potential is a minimum and the ground state is a maximum. This one-dimensional picture of the isomerism process is an ideal case because the issue of multidimensionality in photoisomerization reactions has been subjected to intensive debates for a long time.29-34 Figure 5 shows the fluorescence decay curves of DCVJ in glycerol and in PMMA. None of these data show a single exponential. Therefore, the curves were fit by a double exponential form and the average lifetime, defined as R1τ1 + R2τ2 /(R1 + R2), was obtained. The average lifetime of DCVJ was 170 ps in glycerol and was 1.27 ns in PMMA. Since the radiative lifetime of DCVJ in solution was reported as 3.6 ns,15 the twisting motion is effective even if the viscosity of glycerol is very high. The fluorescence decay of DCVJ in PMMA is much longer than in glycerol, meaning that the polymer matrix effectively slows down the isomerizing motion. However, the lifetime is still shorter than the radiative lifetime even if it is rigid. This indicates that the twisting motion of DCVJ still occurs in the polymer. In addition to PMMA, the fluorescence lifetimes of DCVJ in other polymers were measured. As shown in Figure 6, the fluorescence decay curves of DCVJ in polymers exhibit multiple exponentials, reflecting the structural heterogeneities of the medium. We did not carry out any lifetime distribution analysis because all decay curves were well fit to a double exponential. The analyzed data were all shown in Table 1. The average lifetime was used as a characteristic time constant that represents the excited state dynamics of DCVJ in polymers. The experimental findings of molecular motion in soft matter have been described by free volume theory;35,36 that is, although polymers are more rigid than liquid, the internal rotation of a dye still occurs in the free volume holes contained in the matrix. The idea, however, has not been tested in a quantitative way. According to the Williams-Landel-Ferry (WLF) model,37 the free volume is related to the glass transition temperature (Tg) by
f(T) ) fg + ∆R(T - Tg)
(3)
where fg is the free volume at Tg, and ∆R is the difference in the thermal expansion coefficient of the rubbery and glass states (∆R ) Rr - Rg).
polymer
R1
τ1 (ns)
R2
τ2 (ns)
〈τ〉 (ns)
χ2
LDPE PVA HDPE UHMWPE PVC PC Nylon 6 PS PMMA PAA
0.93 0.93 0.91 0.90 0.89 0.87 0.79 0.75 0.64 0.53
0.25 0.27 0.28 0.24 0.34 0.36 0.43 0.44 0.61 0.97
0.07 0.07 0.09 0.10 0.11 0.13 0.21 0.25 0.36 0.47
2.62 2.56 2.72 3.24 2.34 2.82 2.67 3.04 2.44 2.46
0.41 0.43 0.50 0.54 0.56 0.68 0.90 1.09 1.27 1.67
1.00 1.22 1.15 1.01 1.19 1.08 1.09 1.04 0.98 1.30
a
The average lifetimes were obtained from a double exponential fit to the data.
TABLE 2: The Thermophysical and Mechanical Parameters of Polymers and the Photophysical Parameters of DCVJa Tg
free volume hole
Young’s modulus
lifetime
kiso
polymer
(K)
(Å3)
(GPa)
(ns)
(109 s-1)
LDPE PVA HDPE UHMWPE PVC PC Nylon 6 PS PMMA PAA
158 333 153 148 346 398 326 375 392 379
145a 50b 156c 140d 72e 110f 93g 98e 95h 93i
0.14a 0.35b 0.65c 0.98d 1.12e 1.80f 2.82g 3.10h 3.37i 4.15j
0.41 0.43 0.50 0.54 0.56 0.68 0.90 1.09 1.27 1.67
2.19 2.06 1.73 1.58 1.52 1.20 0.85 0.65 0.52 0.32
a The values of the free volume hole size and Young’s modulus were taken from ref 38 and ref 40, respectively.
Positron annihilation lifetime spectroscopy (PALS) measures the free volume hole size (Vf) distribution of polymer. Table 2 shows the average values of Vf taken from PALS data for ten polymers.38 We have chosen the similar molecular weight polymers as used in this work. The large Vf values of the polyethylenes (LDPE, HDPE, and UHMWPE), compared to other polymers, are due to their low Tg. At room temperature, these three polymers are in the rubbery state, while the other seven polymers are in the glassy state. PVA and PC are interesting to compare. Among the ten polymers, the glass transition temperature of PC is the highest (125 °C), but its Vf value is about 110 Å3. PVA has the smallest Vf (50 Å3) in spite of the fact that its Tg is much lower than that of PC. The average
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Figure 7. The average lifetime of DCVJ versus the free volume hole size in ten different polymers.
Vf of polymers with Tg above 100 °C is ca. 100 Å3 on average. The direct comparison of PALS data with the free volume theory of WLF, however, is somewhat qualitative because PALS measures the free volume hole size distribution, but polymer contains the interstitial and hole free volume in the void space. It has been reported that the elastic modulus of polymer is proportional to the free volume with a negative slope.39 Of course, this is only valid when the free volume varies with changing pressure or temperature in one polymer. As shown in Figure 7, the average fluorescence lifetime does not correlate with the average free volume hole size of the ten polymers chosen in this work. The Vf values of the polymers except for PVA are all larger than the molecular volume of the dicyanovinyl moiety. This means that the photoisomerization of DCVJ can occur freely in the polymers, except in PVA. However, this is not the case for our experimental observations. The fluorescence decay time increases as a function of the elastic modulus of polymer. The bulk values of Young’s modulus were obtained from the literature and were shown in Table 2.40 The fluorescence lifetime of DCVJ strongly correlates with the Young’s modulus of the polymer. Recently, it was observed that the excited-state twisting motion of Nile Red in various polymers depends on the Young’s modulus of the polymer.41 The fluorescence lifetime of Nile Red was nonlinearly proportional to the Young’s modulus, showing a downward curvature. The fluorescence lifetime of DCVJ is also nonlinearly proportional to the Young’s modulus, but it behaves very differently. As seen in the figure, the 〈τ〉 of DCVJ increases gradually from LDPE (0.14 GPa) to PVC (1.12 GPa), and increases rapidly from Nylon 6 (2.82 GPa) to PAA (4.15 GPa). The fluorescence lifetime is associated with the rate constant of the twisting motion by
τF )
1 kr +
∑ knr
1 〈τ〉
- kr
2.8 × 108 s-1, does not accommodate the different polymer environments.15,42 Table 2 shows kiso with the Young’s modulus of various polymers. Figure 8 shows the average lifetime and the twisting rate constant as a function of the Young’s modulus. The kiso decreases monotonously with increasing the Young’s modulus of polymer. The internal motions of small molecules in rigid media have been described by the free volume theory,43 but an endeavor was made in this work to quantitatively describe the phenomena by invoking the elastic modulus of the medium. We found that the Young’s modulus of the medium is a determining factor for molecular rotor dynamics of DCVJ in polymers. The driving source for the motion is an absorbed photon that induces the molecule to rotate internally, causing the matrix to deform. The deformation of a material, which can be either time-dependent or independent, is influenced by its mechanical properties. The photomechanical effects in reversible (elastic) and irreversible (plastic) deformations will influence the twisting dynamics in a different manner. On the basis of viscoelastic theory, the solution data are described by viscosity and the polymer data by elasticity. However, plasticity should also be considered in polymers. That is, if the twisting motion is undergone in an elastic environment, the reaction volume returns to the original state (reversible). If the polymer response is plastic, then the reaction volume is created permanently after the reaction (irreversible). It is possible that the twisting motion is better described by the effective modulus which should be different from the bulk value because the Young’s modulus is a macroscopic quantity, but the twisting motions occur at the molecular level. The frequency dependence of the elastic modulus should also be applied whenever it is necessary. We think a more comprehensive visco-elasto-plastic theory is necessary to explain our experimental observations.
(4) 4. Conclusions
where kr is the radiative rate constant, and knr is the sum of the nonradiative rate constants responsible for internal conversion, intersystem crossing, and twisting. If the twisting motion is much faster than internal conversion or intersystem crossing, which is the case for DCVJ,15 then the rate constant for the twisting motion, kiso, can be calculated by
kiso )
Figure 8. The dependence of the average lifetime and isomerization rate constant of DCVJ on the Young’s modulus of polymer.
(5)
The radiative rate constant is usually obtained by applying the Strickler-Berg equation, and the use of the solution kr value,
We measured the fluorescence lifetimes of DCVJ in various polymers including LDPE, PVA, HDPE, UHMWPE, PVC, PC, Nylon 6, PMMA, PS, and PAA. The fluorescence quantum yield of 9-dicyanovinyljulolidine (DCVJ) is very low in fluid solution but increases markedly in polymer because the medium rigidity slows down the internal motion. We described the measured fluorescence lifetimes of DCVJ in terms of the free volume size and Young’s modulus of the polymers. Our results clearly indicate that the elastic modulus is a determining factor for molecular rotor dynamics in soft matter. Therefore, a more
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comprehensive visco-elasto-plastic theory may be necessary to explain our experimental observations. Acknowledgment. This work was supported by the Acceleration Research Program of the National Research Foundation of Korea. References and Notes (1) Waldeck, D. H. Chem. ReV. 1991, 91, 415. (2) Liu, R. S. H.; Hammond, G. S. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 11153. (3) Lee, G. J.; Kim, D.; Lee, M. Appl. Opt. 1995, 34, 138. (4) Serra, F.; Terentjev, E. M. J. Chem. Phys. 2008, 128, 224510. (5) Ponterini, G.; Caselli, M. Ber. Bunsenges. Phys. Chem. 1992, 96, 564. (6) Scaffradi, L.; Paolo, R. E.; Duchowicz, R. J. Photochem. Photobiol. A: Chem. 1997, 107, 185. (7) Pillai, Z. S.; Sudeep, P. K.; Thomas, K. G. Res. Chem. Intermed. 2003, 29, 293. (8) Polo, A. S.; Itokazu, M. K.; Frin, K. M.; Patrocinio, A. O.; Iha, N. Y. Coord. Chem. ReV. 2006, 250, 1669. (9) Chosrowjan, H.; Mataga, N.; Shibata, Y.; Tachibaanaki, S.; Kandori, H.; Schichida, Y.; Okada, T.; Kouyama, T. J. Am. Chem. Soc. 1998, 120, 9706. (10) Budd, P. M.; McKeown, N. B.; Fritsch, D. J. Mater. Chem. 2005, 15, 1977. (11) Moorthy, J. N.; Venkaatakrishnan, P.; Savitha, G.; Weiss, R. G. Photochem. Photobiol. Sci. 2006, 5, 903. (12) Loutfy, R. O. Pure Appl. Chem. 1986, 58, 1239. (13) Iwaki, T.; Torigoe, C.; Noji, M.; Nakanishi, M. Biochemistry 1993, 32, 7589. (14) Dreger, Z. A.; White, J. O.; Drickamer, H. G. Chem. Phys. Lett. 1998, 290, 399. (15) Allen, B. D.; Benniston, A. C.; Harriman, A.; Rostron, S. A.; Yu, C. Phys. Chem. Chem. Phys. 2005, 7, 3035. (16) Lindgren, M.; Sorgjerd, K.; Hammarstrom, P. Biophys. J. 2005, 88, 4200. (17) Jin, H.; Li, X.; Maroncelli, M. J. Phys. Chem. B 2007, 111, 13473. (18) Lu, J.; Liotta, C. L.; Eckert, C. A. J. Phys. Chem. A 2003, 107, 3995. (19) Gutkowski, K. I.; Japas, M. L.; Aramendia, P. F. Chem. Phys. Lett. 2006, 426, 329. (20) Paul, A.; Samanta, A. J. Phys. Chem. 2008, 112, 16626. (21) (a) Haidekker, M. A.; Ling, T.; Anglo, M.; Stevens, H. Y.; Frangos, J. A.; Theodorkis, E. A. Chem. Biol. 2001, 8, 123. 22. (b) Haidekker, M. A.; Brady, T. P.; Chalian, S. H.; Akers, W.; Lichlyter, D.; Theodorkis, E. A. Bioorg. Chem. 2004, 32, 274. (c) Haidekker, M. A.; Brady, T. P.; Lichlyter, D.; Theodorkis, E. A. J. Am. Chem. Soc. 2006, 128, 398. (d) Haidekker, M. A.; Theodorkis, E. A. Org. Biomol. Chem. 2007, 5, 1669. (e) Zhu, D.; Haidekker, M. A.; Lee, J.-S.; Won, Y.-Y.; Lee, J. C.-M. Macromolecules 2007, 40, 7730. (22) Liang, M.; Yennawar, H.; Maroncelli, M. Acta Crystallogr. 2009, E65, o1687. (23) Moran, A. M.; Egolf, D. S.; Blanchard-Desce, M.; Kelley, A. M. J. Chem. Phys. 2002, 116, 2542.
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