Effet de la concentration sur la transition conformationnelle des copolymères séquencés et greffés

In this paper we have been studying the effect of concentration c on reduced viscosity η_sp/c for a triblock copolymer poly(methyl methacrylate)/polystyrene/poly(methyl methacrylate) and for a graft copolymer of the same nature. The temperature of the experiments has been chosen in the range where the conformation changes from a segregated form to a pseudogaussian one. For this temperature range η_sp/c is not a linear function of concentration but the points η_sp/c vs. c are situated on two straight line segments with a well defined intercept. We discuss the phenomenon in comparison to results obtained on mixtures of homopolymers. These results suggest a strong and anomalous dependance of molecular conformation on concentration.     

Étude viscosimétrique de solutions de polyélectrolytes par dilution isoionique. Effet de la densité de charge sur la conformation du polyion

The reduced viscosity η_sp/c of dilute solutions of carboxymethylcelluloses (CMC) was studied, keeping the total ionic strength of the solution constant; the variation of η_sp/c with concentration is often linear and the Huggins constant k′ increases with decreasing ionic strength. So it is possible to obtain the intrinsic viscosity [η]_{I = 0} at zero ionic strength I and to calculate the corresponding molecular dimensions. The variation of [η]_{I = 0} with the degree of substitution of CMC is explained by a progressive change from a zig-zag conformation to a rod when the linear charge of the polyion increases.     

Unexpected Rheology of Polydimethylsiloxane Liquid Blends

The rheological behaviour of polydimethylsiloxane (PDMS) liquid blends has been investigated both in rotational and oscillating mode. Liquid silicone rubbers based on PDMS at low, medium, and high viscosity have been used for low/high or medium/high liquid blends. The flow, viscosity, creep, and stress‐relaxation curves, strain‐spectra of temperature‐ or frequency‐sweep have been determined and the behaviour at slow shear rate investigated in detail. The low/high homologous blend shows an unexpected behaviour: the zero‐shear viscosity η₀ is higher, the compliance J ₀ lower and viscoelastic moduli G ₀, G(t), G′(ω) and G″(ω) higher, with respect to the components and medium/high blend. Moreover, the blend viscosity η(T), G′(T), and G″(T) exhibits an anomalous decreasing trend. The Entanglement Swelling Tube (EST) model is proposed. The nanosized chains swelled the entanglement of the long ones by means of dipolar interactions ? Si ^{δ +}–O ^{δ ?} in the sites between the blobs of the tube chains, where the free‐volume is highest. The linked entanglements are stabilized and the lifetime lengthened; the entanglement density increases at long deformation times. As the temperature rises, the anomalous behaviour is reduced due to the Entanglement Deswelling‐Solvation mechanism of short chains up to the transition to ordinary behaviour for T = 65 °C.     

Pressure and solvent influences on the shear thinning behavior of moderately concentrated solutions of poly(butyl methacrylate)

The viscosities of moderately concentrated solutions of poly(butyl methacrylate)—M?_w = 1820 kg/mol–in the thermodynamically good solvent toluene and in the theta solvent 2-propanol were measured up to pressures of 2000 bar as a function of shear rate $ \dot \gamma $ in the temperature interval from 40 to 70°C. Both systems obey Graessley's equation so that it is possible to obtain the following rheological parameters in addition to the zero-shear viscosity η₀ from the dependences $ \eta (\dot \gamma ) $. η can be split into an entanglement part η_ent plus a frictional contribution η_fric, and the characteristic viscometric relaxation time τ₀ becomes accessible. The present results demonstrate that the application of pressure raises the above parameters much less for good than for bad solvents. In contrast to η_fric (which can react differently to the application of pressure due to special circumstances) η₀ and τ₀ are always influenced to the same degree as T or p are changed. This result implies that the steady state shear compliance (τ₀/η₀) is independent of the variables of state irrespective of the thermodynamic quality of the solvent. For the good solvent the effects of temperature can be described by the Arrhenius equation, whereas the WLF equation has to be used for the bad one.     

Pressure dependence of viscometric relaxation times measured with a new apparatus. Williams-Landel-Ferry behaviour of moderately concentrated solutions of poly(butyl methacrylate)s in 2-propanol

A new rotational viscometer is presented which can be operated up to 2 000 bar and a maximum shear stress of 420 Pa. It allows, for the first time, to investigate the non-Newtonian flow behaviour of moderately concentrated polymer solutions. Results of measurements with two representatives of the system 2-propanol/poly(butyl methacrylate) with weight-average molecular weights M?_w = 520 000 and M?_w = 2 050 000, and ratios of weight- to number-average molecular weights M?_w/M?_n = 1,08 and M?_w/M?_w = 1,23, resp. in the region of moderate polymer concentrations are reported. For a ca. 7 wt.-% solution of the higher-molecular-weight polymer one obtains viscometric relaxation times τ₀ varying from 1 to 100 ms in the region from 1 to 2 000 bar and from 40 to 75°C; at the lower temperature the application of p can raise τ₀ by one order of magnitude. The steady-state shear compliance (proportional to τ₀/η₀, where η₀ is the zero-shear viscosity), is independent of pressure of varies only slightly. For the present system, which gels thermoreversibly upon cooling, η₀ and τ₀ as a function of temperature and pressure can be well represented by the Williams-Landel-Ferry equation.     

Synthèse et propriétés d'oligomères dièniques partiellement et totalement hydrogénés

Hydrogenations of several hydroxy-terminated low molecular weight polybutadienes (PB) or non functional PB and one polyisoprene by homogeneous catalysis were studied. Hydrogenation of 1,4-polybutadienes gave solid crystalline polymers. In the case of hydrogenated 1,2-polymers a decrease of the glass transition temperature was noted, the hydrogenation leading to amorphous polymers. The rheological behaviour of oligomers was found to be Newtonian at low shear rates (≤ 100 s^?1). The viscosity at T_g+100°C increased in every case with the hydrogenation rate. It was found that hydrogen bonding induced by hydroxyl groups has a great effect on the glass transition temperature but does not change the viscosity at T_g + 100°C.     

Incréments d'indice de réfraction de polymères en solution: Étude des lois de mélange. Influence de la masse moléculaire et de la structure

The variation of the refractive index increment (dn/dc) with the molecular weight and the structure of series of linear and branched well-defined polystyrenes dissolved in benzene was studied. Testing the Lorenz-Lorentz and Onsager-Böttcher mixture rules, we were able to show that:

• 1 Below a critical molecular weight of about 2·10⁴, the linear correlation dn/dc=f(1/M_n) is well explained by the influence of chemical heterogeneities included in the polymer chain (end-groups, branching points etc.).
• 2 These rules do not account for specific polymer-solvent interactions and are not quite rigorous.
• 3 The variations of (dn/dc) and the partial specific volume of the polymer, v?_p, observed already for molecular weights higher than 2·10⁴ are accompagnied by a change in the partial specific refractivity. These effects are related to the influence of intramolecular segment density in the interior of the linear or branched coil.

A homologous series of polyoxyethylene-glycols (α-hydro-ω-hydroxypoly(oxyethylene)s) in benzene presents the same behaviour.     

Copylymères séquences polyoxyéthylène-polyacrylonitrile. Essais de préparation par voie anionique

The anionic Polymerization of acrylonitrile has been intiated by sodium of lithium alcoholate of Polyxyethylene glycol. Experiments have been carried out at low temperature in dimethylformamide solution. Slightly coloured block copolymers have thus been obtained. The following conclusions have been drawn from the experimental results:

• the acrylonitrile consumption is quantitative,
• the efficiency of the macroinitiator is low,
• polyoxyethylene and polyacrylonitrile homopolymers are likely to be present in the crude reaction product,
• the anionic polymerization mechanism implies for the polyacrylonitrile blocks a highly branched structure.

     

Non-newtonian flow behaviour of poly(decyl methacrylate) solutions

The viscosity of solutions of poly(decyl methacrylate)s, PDMA, in 1-pentanol (thermodynamically poor solvent) and in toluence or isooctane (good solvents) was measured up to shear rates of 3 · 10⁴s^?1 by means of rotational viscosimeters. The observed pronounced shear thinning is for all solutions well described by the theory of Graessley, with the extension, introduced by Ito. Two parameters, resulting from the evaluation of the flow curves on the basis of the above theory, are discussed: τ₀, a characteristic relaxation time of the polymer chain, is found to be in the range of ms. η_fric, a frictional parameter, independent of shear rate, amounts upto 50% of the zero-shear viscosity for the measured solutions. Both, τ₀ and η_fric, increase with decreasing solvent quality, increasing molecular weight, and increasing concentration, in accord with theory. A comparatively sharp downward bend, separating two power-law regimes, shows up with moderately concentrated solutions, as the shear rate exceeds about 10³ to 10⁴ s^?1. This transition is tentatively interpreted as an orientation of the long side chains of PDMA.     

Melt rheology of compatible binary blends: Poly(ethylene oxide)-poly(methyl methacrylate) and poly(ethylene oxide)-poly(vinyl acetate)

A study of the rheological behaviour of poly(ethylene oxide)-poly(methyl methacrylate) and poly(ethylene oxide)-poly(vinyl acetate) compatible blends in the molten state is reported. Zero shear viscosity η₀ and the activation energy for the viscous flow ΔE* were obtained as function of both composition and molar mass of the components. Positive and negative deviations from the additivity rule were observed both in the case of log η₀ and ΔE*.     

Contribution à l'étude de la métallation de structures polyisoprène, 1. Mèthode de dosage

Squalene, as a model compound, and polyisoprene were metalated by the complex of sec-butyllithium (sec-BuLi) with N,N,N′,N′-tetramethylenediamine. The rate of metalation was determined indirectly by further reaction with a deactivating reagent. Amongst the various agents tested, a solution of chlorotrimethylsilane (Me₃SiCl) in diethyl ether was preferred, as deactivation by this reagent does not lead to secondary reactions. The rate of silylation of sec-BuLi by Me₃SiCl was found to be influenced by the solvent and the temperature. The presence of diethyl ether, as solvent of silylation, and a reaction temperature of 0°C are suitable conditions for the reaction between lithiated polymers and Me₃SiCl.     

Phénomènes cinétiques associés à la transition vitreuse du polyisoprène de structure 1,4 prépondérante

The thermal behavior of polyisoprene with a high content of 1,4 structures prepared by anionic polymerization has been investigated in the glass transition interval by means of differential scanning calorimetry (DSC). This polymer is of particular interest since it has a low glass transition temperature T_g and shows a substantial increase in heat capacity in the glass transition interval. When the thermal history of the polymer is fixed by linear cooling, the dependence of T_g (measured at the inflexion point of the experimental curve) on heating rate q, can be expressed by q=constant × exp{?E/(RT_g)} where E varies from 127 to 144 kJ/mol when the cooling rate decreases from 80 to 1.25 K/min. It is shown that this behavior is consistent with a kinetic model for glass transition involving a simple relaxation process characterized by an energy of activation close to E.     

Polymérisation anionique de l'acroléine, 2. Influence de la nature du contre-ion et de la température sur la cinétique de la réaction

In tetrahydrofuran, with Na⁺ and Li⁺ as counter-ions, the kinetic order of the anionic polymerization of acrolein is unity for monomer and for initiator. These results indicate that the living ends are not associated at the studied concentrations of initiator. The polymerization rate depends on the nature of the counter-ion. Transfer reactions to monomer do not affect the polymerization rate but greatly change the molecular weights M _n of polyacroleins. The experimental M are much lower than the theoretical M . The propagation constants k_pr and the transfer constant h_m are determined using these experimental values of M . From these results, we can conclude that with Li⁺ as counter ion, transfer reactions are much more numerous than with Na⁺. Furthermore, the polymerization rate increases with temperature. The activation energies of the propagation (E_a,pr) and transfer reactions (E_a,t) can be determined separately. When the temperature increases the propagation reaction is promoted in comparison to the transfer reaction to monomer, and simultaneously transfer reactions to polymer take place. This last phenomenon restricts the upper value of the polymerization temperature.     

Measurement of human blood viscosity by an electromagnetic spinning sphere viscometer

We herein applied an electromagnetic spinning sphere (EMS) viscometer to the measurement of human blood viscosity for the first time. We collected blood samples from 100 healthy outpatient volunteers in order to analyse viscosity dependence on blood cell parameters and on the shear rate with a simple approximation formula [η_i (γ)\, = A_i γ^{- pi} + η₀]. Viscosity dependence on blood cell parameters was relatively high at a high shear rate, but became lower as the shear rate decreased. The approximation formula with appropriate parameters of A_i and p_i nearly faithfully reproduced actual blood rheological behaviour with a standard deviation of 1.5%. The distributions of A_i and p_i values were broad, suggesting that the pattern of viscosity dependence on the shear rate varied with individual differences. The results obtained using the EMS viscometer suggest that blood viscosity values are individual-specific and actual individual measurements are important for understanding rheological conditions.     

Etude de la structure ramifiée du polyéthylène basse densité par viscosité et diffusion de la lumière de fractions issues de la chromatographie d'exclusion gpc

Fractions of high density polyethylene (PEhd) and low density polyethylene (PEld) obtained from gel permeation chromatography were studied by viscometry and light scattering. The variations of the intrinsic viscosity [η], the molecular weight M?_w, and the radius of gyration R_g for PEhd and PEld were compared with one another and the relationships [η] vs M and R_g vs. M for PEhd were established. The dependence of structural parameters g = (R_g²)_branch/(R_g²)_linear and g′ = [η]_branch/[η]_linear on the molecular weight was determined for three samples of PEld.     

Photochimie de la polyacrylophénone en solution, 1. Cinétique de transfert d'énergie triplet-triplet

The kinetics of triplet-triplet energy transfer is studied in fluid solution with polyacrylophenone, (poly(1-benzoylethylene)), (PAP) as the energy donor and 1-methylnaphthalene (MeN) as quencher. The rate constant of quenching K_q is obtained from the Stern-Volmer constant K_qτ for the inhibition of main chain scission in PAP, the lifetime τ of the excited state being measured by picosecond laser photolysis. K_q does not verify the scheme of a diffusion controlled reaction generally applied for small molecule as valerophenone. We proposed a kinetic scheme based on the Voltz theory of energy migration where a migration term Ω of the excitation energy in PAP adds to the material diffusion of MeN. The good agreement obtained for the value of Ω determined by extrapolation at infinite viscosity of solvent, where the material diffusion is negligible, supports our hypothesis of energy migration in PAP at room temperature.     

Micellisation de copolymères séquencés polystyrène-polyvinylpyridinium, 3. Influence de la concentration en sel et de la température

The effects of the salt concentration and temperature on the micellization phenomenon of polystyrene-poly(4-vinyl-N-ethylpyridinium bromide) block copolymers in dilute solution are investigated in solvents selective for the polyelectrolytic part (water-methanol-LiBr mixtures). The polystyrene moiety being in non-solvent media remains rather collapsed whatever temperature and salt concentration. On the contrary, the latter parameters modify strongly the conformation of the soluble polyelectrolytic chains and therefore determine the “quality” of the solvent essentially towards the polyvinylpyridinium blocks. Consequently an increase of the temperature or a decrease of the salt concentration have the same effect on the behaviour of the copolymers. In the case of a copolymer with a small molecular weight polystyrene block (M_w = 2 700), the unimer-multimer (single molecules/micelles) equilibrium is easily shifted in favour of the single molecules by increasing the temperature or decreasing the ionic strength. On the contrary, if the polystyrene moiety is longer (M_w = 6 000 – 13 000) the micellar weight of the system is little or even not at all influenced by the ionic strength and the temperature. These results emphasize that, for the investigated systems, it is the “insoluble” blocks which essentially govern the micellization phenomenon.     

Synthèse et copolymérisation avec l'acrylamide de macromonomères d'acrylate de dodécyle

Graft copolymers of well-defined structure and composition were prepared by radical copolymerization of acrylamide and poly(dodecyl acrylate) macromonomers. The macromonomers were prepared by telomerization of dodecyl acrylate in the presence of 2-mercaptoethanol as a transfer agent, followed by reaction with acryloyl chloride. Poly(dodecyl acrylate) macromonomers with M _n = 1100, 1100, 2200 and 4600 were synthesized. The kinetics of the radical telomerization of dodecyl acrylate with 2-mercaptoethanol and AIBN was studied at 66°C in THF. Under these conditions we obtained the transfer constant C_T = 0,9. The graft copolymerization of acrylamide with the poly(dodecyl acrylate) macromonomer was carried out in the presence of the transfer agent C₆F₁₃C₂H₄SH. If C₀ = [AIBN]/[Acrylamide] = 0,01 and R₀ = [C₆F₁₃C₂H₄SH]/[Acrylamide] = 0,01, for acrylamide a DP _n = 160 was obtained in all cases, for the macromonomer of M _n = 1100 a DP _n = 2,5, and for the macromonomer of M _n > 1100 a DP_n = 1.     

Étude viscosimétrique du système ternaire polystyrène/poly(méthacrylate de méthyle)/p-xylène

Some theoretical considerations and experimental results are presented on the limiting viscosity number of polystyrene, (PS), dissolved in a mixture of poly(methyl methacrylate), (PMMA), and p-xylene, and of PMMA in a mixture of PS and p-xylene. At relatively low concentration, this problem can be treated using the classical formalism of dilute solutions, introducing in addition a new parameter b_AB characterizing the interactions between the polymers A and B. The quantitative use of this apparently simple method involves some difficulties owing to the fact that terms of higher order have to be taken into account even at moderate concentrations. The results can be explained by a decrease of the polymer dimensions which is more pronounced when the “solvent polymer” is of low molecular weight. At higher concentration, there is a critical point which seems to appear when the total volume of the two polymers is equal to the volume of the solution.     

Relation entre la solvatation préférentielle et la densité en segments de polymères de structure différente

This paper presents results on preferential solvation and viscosity in three ternary systems, (polystyrene (PS)-benzene-methanol, poly(2-vinylpyridine)-ethanol-cyclohexane, and PS-cyclohexane-ethanol) using linear and branched polymers. It is shown that the representation ΔΛ′ (variation of the preferential solvation coefficient) as a function of Δ_Q (variation of the segment density), gives curves which are not sensitive to branching and, therefore, caracteristic of the nature of the polymer and not of its structure. When there is an inversion in preferential solvation one can define “isosbestic” points, i.e. compositions of the solvent mixture for which Λ′ or Δ_Q are independent of the molecular weight of the polymer.