Time-Resolved Luminescence Anisotropy Studies of the Relaxation Behavior of Polymers. 1. Intramolecular Segmental Relaxation of Poly(methyl methacrylate) and Poly(methyl acrylate) in Dilute Solutions in Dichloromethane (original) (raw)
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The Journal of Physical Chemistry B, 2005
Steady-state and time-resolved emission spectroscopy (TRES) of the medium-sensitive probes 4-aminophthalimide (4-AP) and 6-propionyl-2-(dimethylamino)naphthalene (Prodan) were performed at 77 and 298 K in vacuum-sealed thin films of poly(vinyl alcohol) (PVA) and poly(vinyl acetate) (PVAc). The two probes show similar red-edge effect in steady state emission and a red shift with time in TRES in PVA. In PVAc the red shifts are much smaller and the spectral shift for 4-AP is slower. 4-AP locates in highly polar environments in PVA, where H-bond interaction with the polymer is important. Prodan locates in less polar environments, as evidenced by the position of the emission maximum with respect to reference solvents. Consequently, the observed monoexponential spectral red shift with time of 4-AP in PVA and in PVAc is attributed to relaxation of the interaction of the probe with the hydroxy and acetate moieties, respectively. The more intense interaction of the lighter -OH moiety with the probes explains the greater and faster spectral shift observed in PVA compared to PVAc. The lifetime of this monoexponential spectral shift is independent of temperature in PVA and takes place with a highly negative activation entropy. This fact is attributed to a collective rearrangement of -OH groups to better interact with the excited state. This relaxation nevertheless does not account for the complete accommodation of the excited state. Prodan shows a linear variation of the spectral shift with time that can be explained by microheterogeneity. In PVA, the width at half-maximum of the emission spectra does not change with time for Prodan and it decays with a lifetime similar to the lifetime of the spectral shift in the case of 4-AP. The differences in the behavior of the probes are attributed to their different average location in the polymer matrix.
Study of secondary relaxations in poly(vinyl chloride) by phosphorescence decay
Journal of Photochemistry and Photobiology A-chemistry, 2007
The phosphorescence emission of both naphthalene and pyridine can be used to detect the secondary () relaxation of PVC, as this relaxation manifests by a decrease in the emission from the grafted probe at the temperatures at which the local motion at the backbone begins. In this work, an extensive study of the kinetic and spectral features of the phosphorescence of 4-mercaptopyridine, 4-methoxybenzenethiol and 4-mercaptophenol groups as a function of temperature is presented. These three luminescent probes have been grafted onto PVC, with modification levels ranging from 3% up to 46%. The phosphorescence decay from −130 • C up to 30 • C has been followed and both the intensity of the emission and the spectral features have been studied as a function of temperature. The interaction between probes as the concentration increases leads in all cases to the emission from aggregates or excimers, which have features different to those of the isolated probe. Side reactions occur when grafting the hydroxyl containing probe, what also leads to surprising phosphorescence spectral and decay rate features. (P. Tiemblo).
Influence of structural chemical characteristics on polymer chain dynamics
The Journal of chemical …, 2008
A comparative study of the dielectric relaxation behavior of two structurally close polymers containing aliphatic-aromatic side groups was carried out in order to get a better understanding on how slight differences in chemical structure affect the molecular responses to perturbation fields. Specifically, chain dynamics of poly͑2-acryloyloxyethyl-2-naphthalene-2-ylacetate͒ and poly͑2-methacryloyloxyethyl-2-naphthalene-2-ylacetate͒ were studied by broadband dielectric spectroscopy in the frequency range of 10 −2-10 8 Hz and temperature window of 298-403 K. Also, the relaxation behavior of ͑2-acetyloxyethyl-2-naphthalene-2-ylacetate͒, model compound of the polymer side groups, was analyzed. The isotherms representing the dielectric loss in the frequency domain show important conductive contributions, especially at high temperature, which hide the low frequency side of the ␣ relaxation. Conductivity also increases the real component of the complex permittivity in the low frequencies region. Retardation spectra were obtained by minimizing the sum of the squares of the difference between the experimental values of the complex permittivity for each frequency and the analytical ones, predicted by the linear phenomenological theory, using a Tikhonov regularization technique. The spectra present an apparent ␣ peak with an excess wing at short time side resulting from the overlapping of the true ␣ relaxation and a  process. Three absorptions, named in increasing order of time ␥, , and ␣ relaxations, are separated by deconvolution methods. The activation energies associated with the ␥ process are 70.0Ϯ 1.8, 68.0Ϯ 1.4, and 74.8Ϯ 0.8 kJ mol −1 for ͑2-acetyloxyethyl-2-naphthalene-2-yl acetate͒, poly͑2-acryloyloxyethyl-2-naphthalene-2-yl acetate͒ and poly͑2-methacryloyloxyethyl-2-naphthalene-2-yl acetate͒, respectively. The respective activation energies associated with the  relaxation are 121.7Ϯ 2.4, 135.3Ϯ 1.4, and 141.6Ϯ 1.3 kJ mol −1. Values of the shape parameters and the strengths of the relaxation processes were obtained as a function of temperature. The dynamic fragility of the polymers and the model compound was studied and compared with that reported for macromolecular and monomeric systems. Also, the evolution of the size of the correlated domains associated with the ␣ relaxation was estimated. Finally, the ␥ relaxation rather than the  absorption obeys the criteria apparently held by the Johari-Goldstein  processes.
The Journal of Physical Chemistry B, 2012
Dielectric relaxation spectroscopy has been utilized for studying the molecular dynamics of polymer solutions. 1À6 In the case of polymer solutions composed of polar solvents in a solvent-rich region, relaxation processes due to the reorientation of dipoles of solvents and polymer chains are observed separately at higher and lower frequencies, respectively. 3À5,7,8 Typically, the relaxation process observed at frequencies on the order of 10 GHz is associated with the molecular motion of solvent molecules, while the relaxation process observed at kHz-MHz frequencies is attributed to the relaxation modes of polymer chains. The relaxation process owing to solvent molecules is affected by the addition of polymers. 3,4,9À11 This implies that the dynamical structures of solvent molecules are related to the polymers through interactions at the molecular level. The relaxation process that arises from the polymer chains should also be affected by the solvent molecules. This interdependence of polymer chains and solvent molecules can be analyzed by investigating the dielectric relaxation spectrum as a function of concentration and/or temperature. Dielectric relaxation spectra can be described by, for example, the relaxation time, the relaxation strength, and the shape parameter characterizing the distribution of the relaxation process. Therefore, the relaxation parameters obtained by the variation of polymer concentration or temperature, as well as the solvent species, can provide important information leading to greater understanding of molecular interactions. Recently, the relaxation processes of polymer chains and solvent molecules have been studied systematically for the poly(vinylpyrrolidone) [PVP] system in various polar and nonpolar solvents in broad temperature and frequency ranges. 3À5 It has been revealed that the cooperation between segmental motion and the reorientation of solvent molecules provides intrinsic information about the molecular dynamics of polymer solutions. In this study, we report the experimental results of dielectric relaxation behavior for the systems of poly(N-isopropylacrylamide) (PNiPAM) in protic and aprotic solvents as a function of PNiPAM concentration studied by dielectric relaxation spectroscopy. An aqueous solution of PNiPAM has a Θ-temperature of 30.6°C and undergoes a coilÀglobule transition upon heating. 12À15 The transition of PNiPAM chains in water is also observed upon the addition of a second water-miscible solvent, such as methanol,
Physical Chemistry Chemical Physics, 2007
New low and high molecular weight poly(acrylic acid), PAA, 2000 g mol À1 and 450 000 g mol À1 , respectively, were tagged with pyrene (low and high contents of probe) and its behaviour in solution was investigated using absorption and fluorescence (steady-state and time-resolved) techniques. Fluorescence data shows that the degree and level of intramolecular association strongly depends on the molecular weight. With the short pyrene-labeled PAA chains in aqueous solution, the excimer-to-monomer fluorescence ratio I E /I M decreases with the increase of pH, oppositely to the increase in the I E /I M ratio with the increase in pH previously observed with the long chain PAA. Time-resolved data suggest that excimer formation with the short pyrene-labeled PAA polymers (ca. 28 acrylic acid monomers per chain) in water is largely due to excitation of Ground State Dimers, GSD. The increment of pH, and the consequent gradual ionization of the carboxylic groups in the chain, initially increases the fraction of GSD, possibly due to the occurrence of special micelle-like chain conformations, inside which the pyrene units are accommodated. A further increase of the pH above the pK a values, resulting in the full ionization of carboxylic groups, apparently destabilizes such chain conformations, which leads to a pH effect on the photophysical properties identical to that of the long chain polymers. In water, the dynamic data shows the existence of two excimers coexisting with two monomer classes. In methanol and dioxane (good solvents for the pyrene probe) at room temperature, where one excimer and two monomers are present, all rate constants could be obtained, as well as the fractions of ground-state species. It is thus shown that different types of interactions are produced with small-and long-sized PAA polymers, i.e., the size of the polymer matters.
The Journal of Chemical Physics, 2005
The relaxation behavior of poly͑2,3-dichlorobenzyl methacrylate͒ is studied by broadband dielectric spectroscopy in the frequency range of 10 −1 -10 9 Hz and temperature interval of 303-423 K. The isotherms representing the dielectric loss of the glassy polymer in the frequency domain present a single absorption, called  process. At temperatures close to T g , the dynamical ␣ relaxation already overlaps with the  process, the degree of overlapping increasing with temperature. The deconvolution of the ␣ and  relaxations is facilitated using the retardation spectra calculated from the isotherms utilizing linear programming regularization parameter techniques. The temperature dependence of the  relaxation presents a crossover associated with a change in activation energy of the local processes. The distance between the ␣ and  peaks, expressed as log͑f max; / f max;␣ ͒ where f max is the frequency at the peak maximum, follows Arrhenius behavior in the temperature range of 310-384 K. Above 384 K, the distance between the peaks remains nearly constant and, as a result, the ␣ onset temperature exhibited for many polymers is not reached in this system. The fraction of relaxation carried out through the ␣ process, without  assistance, is larger than 60% in the temperature range of 310-384 K where the so-called Williams ansatz holds.
Influence of oxycycloaliphatic groups in the relaxation behavior of acrylic polymers
Journal of Polymer Science Part B: Polymer Physics, 1999
A comparative study on the mechanical and dielectric relaxation behavior of poly(5-acryloxymethyl-5-methyl-1,3-dioxacyclohexane) (PAMMD), poly(5-acryloxymethyl-5-ethyl-1,3-dioxacyclohexane) (PAMED), and poly(5-methacryloxymethyl-5-ethyl-1,3-dioxacyclohexane) (PMAMED) is reported. The isochrones representing the mechanical and dielectric losses present prominent mechanical and dielectric  relaxations located at nearly the same temperature, approximately Ϫ80°C at 1 Hz, followed by ostensible glassrubber or ␣ relaxations centered in the neighborhood of 27, 30, and 125°C for PAMMD, PAMED, and PMAMED, respectively, at the same frequency. The values of the activation energy of the  dielectric relaxations of these polymers lie in the vicinity of 10 kcal mol Ϫ1 , ϳ 2 kcal mol Ϫ1 lower than those corresponding to the mechanical relaxations. As usual, the temperature dependence of the mean-relaxation times associated with both the dielectric and mechanical ␣ relaxations is described by the Vogel-Fulcher-Tammann-Hesse (VFTH) equation. The dielectric relaxation spectra of PAMED and PAMMD present in the frequency domain, at temperatures slightly higher than T g , the ␣ and  relaxations at low and high frequencies, respectively. The high conductive contributions to the ␣ relaxation of PMAMED preclude the possibility of isolating the dipolar component of this relaxation in this polymer. Attempts are made to estimate the temperature at which the ␣ and  absorptions merge together to form the ␣ relaxation in PAMMD and PAMED. Molecular Dynamics (MD) results, together with a comparative analysis of the spectra of several polymers, lead to the conclusion that flipping motions of the 1,3-dioxacyclohexane ring may not be exclusively responsible for the -prominent relaxations that polymers containing dioxane and cyclohexane pendant groups in their structure present, as it is often assumed. The diffusion coefficient of ionic species, responsible for the high conductivity exhibited by these polymers in the ␣ relaxation, is semiquantitatively calculated using a theory that assumes that this process arises from MWS effects, taking place in the bulk, combined with Nernst-Planckian electrodynamic effects, due to interfacial polarization in the films.