The dynamics of long chain polymers from intensity fluctuation spectroscopy

Linear, atactic polystyrene in the molecular weight range of 5 × 10⁶ to 20 × 10⁶ has been investigated in theta and non-theta solution conditions using the technique of Rayleigh Light scattering spectroscopy. An analysis of the scattered light line profile has allowed an approximate treatment based on the complete spectral analysis to determine the first Rouse mode as well as the centre-of-mass diffusion coefficient.     

Effects of polymer chain disentanglement on NMR properties. II. 13C magnetic relaxation in polystyrene

The transverse magnetic relaxation of ¹³C_α nuclei has been studied in concentrated solutions of polystyrene. The magnetic relaxation rate was measured as a function of molecular weight at several temperatures (313,318, and 323 K) and at several concentrations (0.53, 0.43, and 0.34 g/cm³). The spin-system response of these nuclei in natural abundance exhibits a characteristic evolution from pseudosolid properties to liquidlike one, induced by decreasing the molecular weight of polymer molecules. This evolution is analogous to that already observed in protons attached to polyisobutylene or polydimethylsiloxane chains; it is assumed to be induced by an increase of the disentanglement rate of polymer chains. The spin-system response may be considered as reflecting single-chain magnetic properties, because of the low concentration of ¹³CC_α nuclei, although all chains are in dynamic interaction with one another. The NMR disentanglement transition is interpreted in terms of a two-step motional averaging effect involving submolecules. A numerical analysis of NMR properties is given using a model of polymer chain relaxation based on a multiple-mode relaxation process, characterized by (i)a terminal relaxation time τ^v₁ depending upon M³, the molecular weight, and approximately proportional to the polymer concentration C (like the reptation time); (ii)a relaxation-time spectrum analogous to a Rouse spectrum; (iii)a terminal relaxation time τ^v₁ = 2.5 × 10^?2s for M = 2.5 × 10⁵, C = 0.53 g/cm³ in carbon tetrachloride at 313 K.     

Concentration dependence of the lifetime of the first excited singlet level of stilbene dissolved in chloroform

The S₁ lifetime τ₁ of dissolved stilbene has been measured in the concentration range from 5 × 10^-2 mol/? to 2 × 10^-1 mol/? using a probe beam method. The time dependence of the population of the S₁ level excited by two-photon absorption has been observed by single-photon absorption S₁ → S_x. A linear dependence of the reciprocal lifetime on stilbene concentration c_A has been found: 1/τ₁ = 6 × 10⁷ s^-1 + 6.7 × 10⁹ s^-1 ? mol^-1c_A.     

Intramolecular relaxation of polystyrene in a theta solvent by photon-correlation spectroscopy

Photon-correlation spectroscopy has been used to measure the diffusion coefficient D and first-mode intramolecular relaxation time τ₁ of polystyrene in a theta solvent, cyclohexane at 34.5°C. Measurements were made on five narrow fractions ($\text{M}$_w = 2.88 × 10⁶ to 9.35 × 10⁶) as a function of concentration c, in the dilute regime. D varied linearly with c, D = D_o (I + k_Dc), with D_o = (1.4 ± 0.2) × 10^?4$\text{M}{}_{\mathrm{w}}{}^{\mathrm{?(0.508\pm 0.007)}}$ cm² s^?1. Although the values obtained for τ₁ were reproducible to within 5%, no systematic variation with c was detected. The results are fitted by the relation τ₁ = (7.7 ? 0.3) × 10^?8$\text{M}{}_{\mathrm{w}}{}^{\mathrm{(1.42\pm 0.05)}}$ μs, which agrees well with the theoretical expression of Zimm for the non-draining bead-and-spring model, modified for the light-scattering case.     

Thermal analyses of poly(N-isopropylacrylamide) in aqueous solutions and in nanocomposite gels

The coil-to-globule transition of poly(N-isopropylacrylamide) (PNIPA) prepared by free-radical redox polymerization in aqueous solutions and its nanocomposite (NC) gels were investigated by differential scanning calorimetery. The lower critical solution temperatures (LCST) of aqueous solutions of PNIPA of different molecular weights were not significantly affected by molecular weight (M _w: 0.19?×?10⁶?4.29?×?10⁶?g?×?mol^?1) or polymer concentration (1?10?wt%), although the enthalpy of transition increased with molecular weight, at M _w (<1.2?×?10⁶ g?×?mol^?1). The glass-transition temperature of PNIPA in the dried state also remained constant (138?°C), regardless of molecular weight. On the other hand, the enthalpy of the coil-to-globule transition of PNIPA in NC gels consisting of a PNIPA/clay network decreased with increasing clay concentration (C _clay), while the onset temperature (≡LCST) was almost constant, regardless of C _clay. The PNIPA chains in NC gels could be classified into the following three types: P-1, which exhibits a normal LCST transition, similar to that of linear PNIPA; P-2, exhibiting restricted transition at higher temperatures as a result of interactions with the clay; and P-3, which does not undergo that transition because of stronger restrictions. It was found that the proportion of P-3 increases with increasing C _clay. However, some P-1 and P-2 was still observed, even in NC gels with high C _clay. That the transition to the hydrophobic globular state was restricted by interactions with the clay was confirmed by measurements on PNIPA after removal of the clay from NC gels.     

Nuclear relaxation and molecular motion of 1,3,5-trifluorobenzene-d3 in liquid solutions

The ²D and ¹⁹F spin-lattice relaxation times of 1,3,5-trifluorobenzene-d₃ in 0.15 mol fraction solutions in various solvents have been measured over the temperature range 270–400 K. The relationship between the reorientational correlation times, τ_θ, and the angular momentum correlation times, τ_J, obtained from the nuclear relaxation data has been compared with the theoretical relationships predicted by the J-diffusion limit of the extended diffusion (EDJ) model and the Fokker-Planck-Langevin (FPL) model for symmetric top molecules. It was found that the rotational motion of 1,3,5-trifluorobenzene-d₃ in most solutions was better described by the FPL model with frictional anisotropy τ₆/τ_· in the range 1–2. (τ₆ and τ_· are the correlation times for the angular velocity components along the axes parallel and perpendicular to the molecular symmetry axis.) The viscosity and temperature dependence of τ_θ has been analyzed in terms of a modified Debye equation and values for the anisotropic interaction parameter, κ, in each solvent are reported. The variation in κ with solvent is attributed mainly to the variation in the dipole moment of the solvent molecules. The frictional anisotropy (τ₆/τ_·) is found to decrease as κ increases. This is interpreted in terms of the effects of molecular shape and electrostatic interactions between CF bond dipole moment and solvent dipole moments.     

Resonant electron capture by uridine

Processes of the resonant electron attachment to uridine molecules have been studied in the energy range 0–14 eV. The main fragmentation channels of negative molecular ions have been determined. Long-lived negative molecular ions have been detected in the thermal energy region. It was found that these ions form in a dipole-bound state. The lifetime of molecular ions was estimated at τ_a ～ 29 × 10^?6 s.     

Dynamic mechanical properties of moderately concentrated polystyrene solutions

The storage (G′) and loss (G″) shear moduli have been measured in the frequency range from 0.04 to 630 Hz for solutions of narrow distribution polystyrenes with molecular weights (M) 19,800 to 860,000, and a few of poly(vinyl acetate), M = 240,000. The concentration (c) range was 0.014–0.40 g/ml and the viscosities of the solvents (diethyl phthalate and chlorinated diphenyls) ranged from 0.12 to 70 poise. Data at different temperatures (0–40°C) were combined by the method of reduced variables. Two types of behavior departing from the usual frequency dependence describable by the Rouse-Zimm-Tschoegl theories were observed. First, for M ? 20,000, the ratio (G″ ? ωη_s)/G′ in the neighborhood of ωτ₁ = 1 was abnormally large and the steady-state compliance J was abnormally small, especially at the lowest concentrations studied. Here ω is circular frequency, η_s solvent viscosity, and τ₁ terminal relaxation time. Related anomalies have been observed by others in undiluted polymers at still lower molecular weights. Second, at the highest concentrations and molecular weights, a “crossover” region of the logarithmic frequency scale appeared in which G″ ? ωη_s < G′. The width of this region is a linear function of log c; the frequency dependence under these conditions can be represented by a sequence of Rouse relaxation times grafted on to a sequence of Zimm relaxation times. For each molecular weight, the terminal relaxation time τ₁ was approximately a single function of c for different solvents of widely different η_s. At lower concentrations, τ₁ was close to the Rouse prediction of 6ηM/π²cRT, where η is the steady-flow viscosity; but at higher concentrations, τ₁ was proportional to η/c² and corresponded, according to a recent theory of Graessley, to an average molecular weight of 20,000 between entanglement coupling points in the undiluted polymer.     

Effect of molecular weight on radiation chemical degradation yield of chain scission of γ-irradiated chitosan in solid state and in aqueous solution

Chitosan A₁, A₂ and A₃ with molecular weight of 471, 207 and 100 kDa respectively, produced from squid pen chitin was degraded by gamma rays in the solid state and in aqueous solution with various doses in air at ambient temperature. Effect of molecular weight on radiation chemical degradation yield of chain scission and degradation rate constants of γ-irradiated chitosan in solid state and in aqueous solution was investigated. The radiation chemical degradation yield G_(s) and degradation rate values were calculated. The molecular weight changes were monitored by capillary viscometry method and the chemical structure changes were followed by UV analysis. The results showed that, the degradation of chitosan was faster in solution, than in solid state. The values of G_(s) in solid state and in aqueous solution were respectively 1.1×10^?8 mol/J and 0.074×10^?7 mol/J for A₁, 4.42×10^?8 mol/J and 0.28×10^?7 mol/J for A₂ and 6.08×10^?8 mol/J and 0.38×10^?7 mol/J for A₃. Degradation rate constants values ranged from 0.41×10^?5 to 2.1×10^?5 kGy^?1 in solid state, whereas in solution they ranged from 13×10^?5 to 68×10^?5 kGy^?1. The chitosan A₃ was more sensitive to radiolysis than A₁ and A₂. The chain scission yield, G_(s) and degradation rate constants seems to be greatly influenced by the initial molecular weight of the chitosan. Structural changes in irradiated chitosan are revealed by the apparition of absorption peaks at 261 and 295 nm, which could be attributed to the formation of carbonyl groups. In both conditions the peak intensity was higher in chitosan A₃ than in A₁ and A₂, the oxidative products decreased with increasing molecular weight of chitosan.     

Lifetime and collisional depopulation of the metastable 5D 3/2-state of Yb+

The lifetime and collisional depopulation rates of the metastable 5D _3/2 state of Yb⁺ have been determined in a radiofrequency ion trap by observation of the fluorescence count rate after ion excitation by a short laser pulse. From measurements using He, N₂ and H₂ as buffer gases between 10^?8 and 10^?6 mbar pressure and linear extrapolation to zero pressure we obtain a lifetime of τ=52.15±1.00 ms and rate constants ofR(H₂)=(1.02±0.10)×10^?9 cm³/s andR(N₂)=(1.78±0.19)×10^?10 cm³/s. The lifetime is in fair agreement with a calculated value of 74 ms.     

Reduced titanium dioxide as support for Ziegler–Natta catalysts

Titanium tetrachloride heterogenized on reduced TiO₂ has been studied as a catalyst for ethylene polymerization. The catalyst has good storage stability and exhibits good activity for ethylene polymerization. The polymer chains grow linearly during ca. 1 h, giving an average molecular weight of up to 2.5 × 10⁶ which indicates that practically no β-elimination occurs. The activity of the catalyst at 50°C, based on Ti(III), is 7.6 × 10⁶ PE/mol Ti h bar and based on the quantity of polyethylene formed it is 1.25 × 10⁶ g PE/mol Ti h bar. The molecular weight of the polymer can be controlled with the addition of hydrogen, under 0.5 bar hydrogen, polyethylene with a molecular weight of 411,000 and a relatively low polydispersity index of 2.2 is obtained. The catalyst shows good thermal stability; the Arrhenius activation energy is 31.8 kJ/mol for the polymerization. The catalyst is also active for propylene polymerization, giving 3 × 10⁶ g PP/mol Ti h bar with the high isotacticity of 93%. © 1994 John Wiley & Sons, Inc.     

Two-step picosecond UV excitation of polynucleotides and energy transfer

We studied the dependence of the efficiency of two-step photoreactions in several polynucleotides: poly-U, C_spd-phage DNA and TMV RNA, on the chain length and spatial structure of the excited biopolymer under high-intensity picosecond UV irradiation (λ = 266nm, I = 10¹¹ ?10¹⁴ W/m², τ_p = 23 ps). It has been found that the effective energy transfer distance Λ in single-stranded polynucleotides is ≈ 1 or 2 nucleotides. In double-stranded nucleic acids Λ < 170. The lifetime τ₁ of the excited electronic state S₁ and the absorption cross section σ₂ from the level S₁ have been estimated for nucleotide pU (τ > 6.6 ps, σ₂ <1.7 × 10^?16 cm²).     

Spatially intermittent polymerization

A novel reactor has been designed which permits the precise determination of absolute rate constants in photoinitiated free-radical vinyl polymerization. A solution of monomer and initiator flows through a dark tubular reactor past regularly spaced slots through which light shines. The alternating dark and light regions produce spatially intermittent polymerization (SIP) and make the system analogous to the well-known rotating-sector technique. However, the SIP reactor has the advantage of producing large volumes of reaction product, at low conversion, suitable for analysis of both conversion and molecular weight. This supplies the necessary data, from a single set of experiments, for the simultaneous determination of the rate constants for propagation and termination. Experimental data are reported at 25°C for methyl methacrylate which indicate that k_p = 315 I./mole-sec, independent of polymer molecular weight, and k_t is dependent on molecular weight especially at low molecular weight, approaching a lower value of k_t = 30 × 10⁶ I./mole-sec at a molecular weight of 10⁶. For styrene, measurements being made only at high molecular weight, k_p = 74 ± 5 and k_t = 37 ± 0.3 × 10⁶ l./mole-sec at 25°C.     

Flow properties of poly(methyl methacrylate) solutions

Viscosity and normal stress behavior were measured for poly(methyl methacrylate) samples of various average molecular weights in diethyl phthalate solution at 30 and 60°C. All samples conformed approximately to the most probable distriution (M?_w/M?_n = 2). Concentrations ranged from 0.113 to 0.38 g/ml, and M?_w from 53,800 to 1,620,000. Despite considerable evidence in the literature of unusual linear viscoelastic behavior for this polymer, its nonlinear properties appear to be rather conventional. The viscosity–shear rate master curve was similar to that found earlier for concentrated solutions of polystyrene and poly(vinyl acetate) of comparable molecular-weight distribution. The viscosity time constant τ_o parallels τ_R, the characteristic time of the Rouse model, although the residual dependence of τ_o/τ_R on concentration and molecular weight appears to be slightly different from that for polystyrene and poly(vinyl acetate). Similar conclusions apply to the recoverable compliance J_e,^o estimated from the normal stress behavior of each solution, and its relationship to the Rouse model compliance J_R.     

Anionic polymerization initiated by lithium amides for preparing high molecular weight polyacrylonitrile

Lithium amides have been proved to be effective anionic initiators for the anionic polymerization of acrylonitrile to get high molecular weight polyacrylonitrile in this study. Polyacrylonitrile with weightaverage molecular weight ranging from 1.02 × 10~6 g/mol to 1.23 ×10~6 g/mol (M_w/M_n= 1.9-2.2) could be prepared utilizing lithium amides derived from diisopropylamine, diethylamine, hexamethyldisilazane,dicyclohexylamine, and 2,2,6,6-tetramethylpiperidine as initiators. The polymerization of acrylonitrile proceeded in a homogeneous manner in N,N-di methyl for mamide and insignificant contribution of side reactions was confirmed.     

Hydrodynamic and conformational properties of cellulose myristate molecules in solution

Cellulose myristate samples with a degree of substitution of 230–250 have been studied by the methods of molecular hydrodynamics (viscometry, analytical ultracentrifugation (flotation), and isothermal translational diffusion) in chloroform in the range M = (56–652) × 10³. The experimental evidence has been interpreted within the framework of the generalized wormlike Yamakawa-Fuji model with the following parameters: the persistence length a = 115 × 10^?8 cm, the chain diameter d = 45 × 10^?8 cm, and the molecular mass per unit chain length M _L = 270 × 10⁸ cm^?1. It has been inferred that the polymer dissolves in chloroform in the form of dimers.     

Conductance of aqueous solutions of cobalticyanides,ferrocyanides, and molybdocyanides of potassium and of tetraalkylammonium lons

The conductances of aqueous solutions of the above-mentioned compounds have been measured in the range of ionic strength between 5×10^?4–10^?1 at 25°C. The decrease in equivalent conductance due to the increase in concentration always becomes smaller with increasing molecular weight of the tetraalkylammonium ions. K₄Fe(CN)₆ is comparable to (Me₄N)₄Fe(CN)₆, K₄Mo(CN)₆ is intermediate between the tetraethyl- and the tetra-n-propylammonium salts, and K₃Co(CN)₆ has a conductance larger than the tetra-n-butylammonium salt. These results are compared with the kinetic salt effects due to the tetraalkylammonium ions.     

The mechanism and kinetics of energy transfer processes in Xe–CCl4–M (M=CO,CO2) mixtures irradiated by xenon resonance light

The mechanism and kinetics of energy transfer from the Xe(6s[3/2]₁) resonance state to CO and CO₂ molecules have been investigated by XeCl(B–X) (λ_max=308 nm) fluorescence intensity measurements at stationary conditions in Xe–CCl₄–M systems. Steady-state analysis of the fluorescence intensity dependence on the xenon and M pressure at constant CCl₄ concentration shows that these processes occur in two- and three-body reactions: Xe(6s[3/2]₁⁰)+M→products; Xe(6s[3/2]₁⁰)+M+Xe→products. The two-body rate constants for above reactions have been found to be (0.7±0.2)×10⁻¹⁰ and (4.9±0.4)×10⁻¹⁰ cm³ s⁻¹ for CO and CO₂, respectively. The three-body rate constants have been found to be (3±1)×10⁻²⁹ and (2.4±0.3)×10⁻²⁸ cm⁶ s⁻¹ for CO and CO₂, respectively. It has been shown that the third order reaction is a very effective channel of xenon excited atoms decay at high xenon pressures (P(Xe)>50 Torr).     

Proton spin-lattice and spin-spin relaxation times in isotactic polypropylene—III. Effects of molecular weight

Proton spin-lattice (T₁) and spin-spin (T₂) relaxation times of isotactic polypropylene (molecular weights from 1.95 × 10⁵ to 1.78 × 10⁶) were measured at 40° using a wide line pulsed spectrometer operating at 19.8 MHz. Two series of samples with different thermal histories were prepared viz. a “melt-quenched sample” and an “annealed sample”. For all samples two T₁'s, the longer (T₁₁) and the shorter (T_1s), were observed. T₁₁ and T_1s decrease with increasing molecular weight. On the other hand, T_2a the longest T₂, associated with the most mobile amorphous region, increases with increasing molecular weight. The mass fraction of the rigid crystalline region decreases and those of the intermediate and mobile amorphous regions increase with increasing molecular weight. T₁₁ changes approximately linearly with the mass fraction of the crystalline region in the “melt-quenched sample”.