Dynamics of macromolecular chains. III. Time-dependent fluorescence polarization studies of local motions of polystyrene in solution

Fluorescence anisotropy decay experiments are described for polystyrene in various ethylacetate-tripropionin mixtures. Decay curve trends agree with the proposed theoretical autocorrelation function. Study of the effects of viscosity shows that the mean relaxation time varies according to a nonlinear law for low viscosities and that the relaxation time θ, reflecting the effects of the possible departures from the motions permitted by an ideal tetrahedral lattice, obeys a law of the type: θ = α + bη. Furthermore, the effects of the direction of the fluorophore transition moment are examined.     

Dynamics of macromolecular chains. VI. Carbon 13 and proton nuclear magnetic relaxation of polystyrene in solution

Spin-lattice ¹H and ¹³C nuclear magnetic relaxation (NMR) times T₁ have been measured for solutions of polystyrene in hexachlorobutadiene at two different frequencies. Some nuclear Overhauser enhancements and linewidths have also been determined. At 15 and 25 MHz the relaxation times T₁ of the ortho and meta carbons show two different dependences on temperature. These measurements indicate internal motion of phenyl groups around the C_α—C_para axis. A single isotropic correlation time is inadequate to explain the relaxation data for the para carbon. Use of a diamond-lattice motional model reveals that segmental reorientation of the chain backbone of polystyrene can be described in terms of two correlation times, ρ characterizing the three-bond motion process, and θ reflecting either isotropic motions of subchains or departure from an ideal lattice. Data on low-molecular-weight polystyrene indicate the participation of overall rotatory diffusion in the relaxation process. This motion is no longer efficient in high-molecular-weight polymers, where relaxation is due to segmental reorientation.     

Dynamics of macromolecular chains. VII. Nuclear magnetic relaxation of monosubstituted polystyrenes in solution

The ¹H spin-echo and ¹³C spin–lattice relaxation times have been measured for solutions of polystyrene derivatives: ortho, meta, and para-halo (F, Cl, Br) and ortho, meta, para, and α-methyl. Results obtained from these two techniques permit comparison of the intramolecular mobility of these polymers with that of polystyrene. Poly(α-methylstyrene) does not differ from polystyrene except for a slight slowing of both segmental reorientation and internal phenyl-group motions and apparent hindrance of the methyl-group rotation. Segmental reorientation of poly(m-methylstyrene) is similar to that of polystyrene; rotation of the methyl group is free, while the internal phenyl-ring process is slower. Poly(p-methylstyrene) and poly(o-methylstyrene) also contain freely rotating methyl groups; the intramolecular mobility decreases from the para to the ortho position of the substituent. Finally, in poly(o-bromostyrene) and poly(o-chlorostyrene), the internal motion of the phenyl ring is completely overshadowed by the segemental reorientation, which is itself quite reduced.     

Dynamics of macromolecular chains. V. Interpretation of the dielectric relaxation data

The autocorrelation function of orientations, derived for the model of conformational jumps in a chain described in a tetrahedral lattice, has been tentatively applied to dielectric relaxation data. The complex dielectric relaxation of polymers can be interpreted by the aid of only two relaxation processes. In the case of poly(p-chlorostyrene) in solution, the characteristic times that we have calculated agree favorably with the fluorescence-depolarization results. The model has been shown to be consistent with the empirical decay function exp[? (t/τ)^β] proposed by Williams.     

Luminescence studies in polymers—IV. Effect of orientation,tacticity and crystallinity on polystyrene and polyvinylcarbazole fluorescence

The fluorescence spectra of amorphous atactic, amorphous isotactic and crystallized isotactic polystyrene films have been compared. The effect of chain orientation has also been analysed on amorphous atactic samples. The results show that the fluorescence yield increases with crystallinity at room temperature and 77°K. The contribution of excimer fluorescence at 77°K increases according to the sequence: atactic < atactic oriented < isotactic amorphous < isotactic crystallized. An increase of the fluorescence yield with crystallinity was also observed for polyvinylcarbazole samples although the contribution of excimer fluorescence at 77°K is independent of crystallinity for this polymer. The results are interpreted in terms of energy migration.     

Dynamics of macromolecular chains. I. Theory of motions on a tetrahedral lattice

The bond correlation function for a macromolecular model chain on a tetrahedral lattice is derived by considering elementary three-bond and four-bond motions. The method of calculation allows the conformational structure of the chain to be involved in the final equations. Moreover, when the three-bond motions are considered alone, no linearization assumption is required; hence, the theory is valid at short times.     

Quenching of fluorescence of solutions of polystyrene and copolymers with methyl methacrylate

Quenching of excimer fluorescence from polystyrene and monomer emission from 5 and 10% methyl methacrylate copolymer with styrene has been studied in solution using dibromoethane and iodine as quenching species. From the measured rate constants along with polarization measurement of the emissions, it is concluded that energy migration does not occur in dilute solution of polystyrene. It is proposed that excimer is formed by rotation of a pendant, excited phenyl group with a face-to-face conformation with its nearest neighbour.     

Fluorescence quenching and polarization studies of segmental motion in polystyrene

Fluorescence quenching has been used in conjunction with measurements of fluorescence polarization to study the segmental relaxation of polystyrene. There is confirmation of the apparent variation in the intrinsic polarization of the bound fluorescent probe with the nature of the macromolecule. The results agree with previous observations, using a different fluorescence approach, that the rotational times for groups at the chain ends are less than one fifth of those for the same groups in the chain interiors. The variation of intrinsic polarization with polymer structure is discussed.     

Time-resolved polarization measurements of porphyrin fluorescence in solution and in single cells

Porphyrin monomers, dimers and aggregates, which can be differentiated on the basis of their fluorescence lifetimes, are shown to possess different degrees of fluorescence polarization. This opens up new possibilities for microscopic imaging of these individual components in photosensitization and tumour detection. A rough estimate of the size of the porphyrin aggregates is obtained from the data of time-resolved fluorescence anisotropy.     

Electron spin resonance studies of spin-labeled polymers. IV. Slow-motional effects in solid polystyrene

Shape changes in the electron spin resonance spectrum of spin-labeled polystyrene have been studied as a function of temperature in the range 77–340°K. Rotational correlation times have been calculated by using recent theories of slow-motional effects on ESR spectra. Three models were used, namely, Brownian, moderate jump diffusion, and large jump diffusion. The moderate jump model gave correlation times in good accord with those found by other techniques for the δ relaxation in polystyrene. The relaxation was found to be independent of molecular weight.     

Thermodynamic and conformational properties of polystyrene. III. Dilute solution studies on branched polymers

Light-scattering and viscometric measurements on dilute solutions of five branched polystyrene polymers are reported. The data include studies in decalin as a function of temperature, including the theta temperature, and in toluene. The results for the radius of gyration and the second virial coefficient are not in accord with the two parameter random-flight model. Possible causes of this descrepancy are considered. It is shown that the intrinsic viscosity of branched chains is not uniquely determined by the radius of gyration.     

Dynamics of macromolecular chains. II. Orientation relaxation generated by elementary three-bond motions and notion of an independent kinetic segment

In the preceding paper, general equations were established for the motions of chains confined to a tetrahedral lattice. In the present paper, bond orientation correlation and autocorrelation functions are explicitly calculated for the case where only three-bond elementary motions are considered. Effects due to the chain end are analyzed and the relaxation time distribution function is established. The expressions obtained reflect the influence of the chain structure. Finally, to characterize the dynamic behavior of chains in orientation relaxation experiments, the notion of an independent kinetic segment is proposed.     

Polychromatic femtosecond fluorescence studies of metal-polypyridine complexes in solution

Femtosecond-resolved broadband fluorescence studies are reported for[M(bpy)₃]²⁺ (M = Fe, Ru), RuN3 and RuN719 complexes in solution. We investigated the pump wavelength dependence of the fluorescence of aqueous [Fe(bpy)₃]²⁺ and the solvent and ligand dependence of the fluorescence of Ru-complexes excited at 400 nm. For all complexes, the ¹MLCT fluorescence appears at zero time delay with a mirror-like image with respect to the absorption. It decays in ?30-45 fs due to intersystem crossing to the ³MLCT states, but a longer lived component of ∼190 fs additionally shows up in RuN719 and RuN3. No solvent effects are detected. The very early dynamics are characterized by internal conversion (IC) and intramolecular vibrational redistribution (IVR) processes on a time scale which we estimate to ?10 fs using the ¹MLCT lifetime as an internal clock.     

Femtosecond fluorescence studies of self-assembled helical aggregates in solution

For the diamino-bipyridine based C(3)-symmetrical disk molecule, TAB, (sub)picosecond fluorescence transients have been observed by means of femtosecond fluorescence upconversion and picosecond time-correlated photon counting techniques. The dodecyl peripheral side chains of the synthetic compound are large enough to allow, in apolar solvents, self-assembling of the discotic molecules to helical aggregates. In polar solvents, the hydrogen bonding and pi-pi interactions pertaining to the chiral aggregation are compensated by solvation and self-assembling of the disklike molecules is disrupted. For comparison, time-resolved fluorescence measurements have been performed for the subgroup molecule, DAC, which is the (asymmetric) building block for TAB. It is concluded that, after pulsed photoexcitation, TAB and DAC exhibit excited-state intramolecular double proton transfer (ESIDPT) with a typical time of approximately 200-300 fs, irrespective of the degree of aggregation. Picosecond components in the fluorescence of TAB and DAC, ranging from 3 to 25 ps, are representative of vibrational cooling effects in the excited product state. Only aggregated TAB shows a rapid ( approximately 1 ps) decay of its fluorescence anisotropy. This component is attributed to excited-state energy transfer within the aggregate. Finally, the excited-state lifetime of TAB in the aggregated form is found to be an order of magnitude longer than that for TAB in its nonaggregated form. It is inferred that aggregation diminishes the influence of low-frequency twisting motions in the radiationless decay of the excited state.     

Interpenetration of polystyrene chains in bulk polymer

Polystyrenes lightly substituted with either tertiary amine groups or nitrophenol groups have been allowed to interact in concentrated t-butylbenzene solution and in bulk polymer. Above the glass transition temperature a temperature dependent equilibrium is achieved in the bulk polymer between the two substituents and the amine salt of the phenol. The degree of association found is considerably greater in the bulk polymer than in solution. The results show that much interpenetration of the polymer occurs, and the results are discussed in terms of models of the bulk amorphous state of the polymer.     

Oxidation of polystyrene in solution

Polystyrene (PS) has been oxidized in carbon tetrachloride, chloroform, methylene chloride and cyclohexane under O₂ at atmospheric pressure using γ-initiation. Benzaldehyde, acetophenone and reductions in molecular weight were observed in all solvents. Yields of benzaldehyde and acetophenone were used to show that attack in chlorinated solvents is essentially random along the polymer chains and is predominantly by Cl^? radicals. Intramolecular propagation is much faster for attack on tertiary carbons than on secondary carbons. There are more neighboring hydroperoxide groups in PS oxidized in carbon tetrachloride and methylene chloride than in PS oxidized in chloroform, because, in chloroform, the solvent hydrogen is abstracted by polymer-based peroxyl radicals. For one set of conditions, about six intramolecular propagation steps took place in carbon tetrachloride and one in chloroform. At lower rates of initiation, the kinetic oxidation chains are longer and more intramolecular propagation occurs. HCl added in excess of that formed in these experiments decomposes hydroperoxide groups to give an autoaccelerating effect.Rates of scission were proportional to $\text{G}{}_{\mathrm{Cl?}}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, where G_Cl? is proportional to the rate of production of Cl? radicals. The alkoxyl radicals on polystyrene, produced in bimolecular interactions of peroxyl radicals, did not abstract hydrogen from the solvents, and thus appear to be very short-lived.The amount of oxidation of polystyrene in cyclohexane is much smaller than in chlorinated solvents. The analytical techniques used were too imprecise to allow meaningful comparison with the oxidations in chlorinated solvents.     

Block copolymerization. VI. Polymerization of pivalolactone with macromolecular initiators from polystyrene and polytetrahydrofuran

Polymerization of pivalolactone with polystyryl sodium or polystyryl ethoxysodium in tetrahydrofuran resulted in homopolymer mixtures. Block copolymers of pivalolactone and styrene were obtained by the polymerization of pivalolactone with sodium polystyrene carboxylate in tetrahydrofuran containing dimethyl sulfoxide. Block copolymers of pivalolactone and tetrahydrofuran were obtained by the polymerization of pivalolactone with polytetrahydrofuran containing carboxylate endgroups. The mechanism of the initiation reaction and various factors affecting block efficiency are discussed.