Miloslav Bohdanecký

The Huggins viscosity coefficient of aqueous solution of poly(vinyl alcohol)

Abstract Although water at room temperature is not a poor solvent for poly(vinyl alcohol) (PVA), the values of the Huggins viscosity coefficient k H are typically higher than 1/2. This is attributed to association. Based on the model put forward by Peterlin, the k H values reported in the literature for aqueous solutions of PVA were examined and the equilibrium constant K * was estimated. It turned out that it increases with increasing molecular weight and depends on the content of residual acetate units. The effect of temperature seems to be weak. Based on the theory of phase equilibria in solutions of crystalline polymers, the values of the Flory–Huggins interaction parameter χ , necessary for molecular dissolution in water of semicrystalline samples, were estimated and compared with the experimental data for samples of PVA with different content of vinyl acetate units. Differences in stability (aging) of PVA solutions can be explained on this basis. Existing experimental data on the viscosity of fresh dilute solutions of PVA in water are interpreted in terms of a model, where the association is due to intermolecular hydrogen bonds between the OH groups of polymer chains. On the contrary, aging phenomena may be attributed to an increase in size and amount of semicrystalline domains acting as multifunctional cross-linking agents.

Comparative studies on molecular chain parameters of chitosans and poly(diallyldimethylammonium chloride): the stiffness B-parameter and the temperature coefficient of intrinsic viscosity

Abstract The empirical coefficient B , a measure of polyelectrolyte chain stiffness was estimated for two types of polycations: chitosans with F A (mole fraction of N -acetyl- d -glucosamine units) ranging from 0.09 to 0.21 (determined by 1 H NMR method) and poly(diallyldimethylammonium chloride) (PDADMAC), in the presence of different counterions (Cl − , Br − , NO 3 − , ClO 4 − ).The method relies on the measurements of the intrinsic viscosity, [ η ], of polyelectrolyte chains at different solution ionic strength (I). Thus, the B -parameter shows the response of the hydrodynamic volume of polyelectrolyte molecules to the salt concentration in solution (the so-called salt tolerance ) referred to the [ η ] at I =0.1xa0M. The temperature coefficient of intrinsic viscosity of PDADMAC and chitosan was also determined. The comparison of the B -parameters indicates lower flexibility of chitosan than PDADMAC. The reasons for this difference are discussed.

Phase transition parameters of potential thermosensitive drug release systems based on polymers of N-alkylmethacrylamides

The phase separation and its thermohysteresis in dilute aqueous solutions of polymeric components of potential drug release systems (homopolymers and copolymers of N-isopropylacrylamide, N-isopropylmethacrylamide, N-propylmethacrylamide, N-sec-butylmethacrylamide, and N-(2-hydroxypropyl)methacrylamide) was studied, both on heating and cooling. Plots of light transmittance vs temperature were constructed and the parameters characterizing them were correlated with polymer structures. Qualitative information was obtained on the rate of formation of the concentrated phase on heating and its disappearance on cooling. Attention has been drawn to the improper identification of the cloud-point temperature, measured at an arbitrary concentration, with the lower critical solution temperature (LCST) as is frequently found in papers dealing with biomedical applications of thermosensitive polymers.

Note on the Relation between the Parameters of the Mark-Houwink-Kuhn-Sakurada Equation

The relation between the parameters K and a of the equation [η] = KM a , which has been empirically established by several authors, is discussed. Equations describing this relation are derived based on the two-parameter theory of the intrinsic viscosity for flexible chain polymers (where a<0.8) and on the worm-like cylinder model for stiff chains (a>0.8). The correspondence of calculated and empirical results is good.

Specific refractive index increments of segmented polyurethanes

Specific refractive index increments v of polyester-based segmented polyurethanes in N,N-dimethyl formamide have been determined, and the quotient dv/df d has been evaluated (where f d is the weight fraction of hard-segment units). The results are in good agreement with the values calculated from group contributions to the molar refraction, using the Vogel or the Gladstone-Dale equations. The values calculated with the Lorenz-Lorentz equation are too low. A potential explanation of this fact is proposed. The same methods have been applied to reported v values for polyether-based polyurethanes. An explanation is proposed for differences in dv/df d for polyesterand polyether-based polyurethanes.

Estimation of conformational characteristics of bisphenol-A based poly(hydroxyethers)

Two samples of bisphenol-A based epoxy resins (poly(hydroxy ethers)) were fractionated by precipitation. The fractions were characterized by the weight-average molecular weight (1.3<Mw×10−3<40) and the intrinsic viscosity in tetrahydrofuran and chloroform. Theories based on the worm-like touched-bead model were used to treat the molecular-weight dependence of the intrinsic viscosity and to estimate the conformational characteristics (Kuhn statistical segment length lK, Flory characteristic ratio C∞, and steric factor σ). Low values of these characteristics indicating rather high flexibility of the chain are discussed.

Determination of molecular dimensions of linear polyethylene by light scattering

Abstract The molecular dimensions of linear polyethylene published in the literature were reevaluated in order to discuss the radii of gyration RG and their dependence on molar mass, M. The relationships RG=bMa and the exponents a of this equation in comparison with the unperturbed dimensions of linear polyethylene obtained from viscometric measurements allowed the discussion of the reliability of the data and the reasons for deviations. The exponent a determined by light scattering for linear polyethylene in good solvent, tetraline, showed molecular dissolution, whereas in 1-chloronaphthalene and trichlorobenzene, the exponent a is lower than expected for good solvents. Microgels and aggregates of molecules affect the exponents. Radii of gyration measured directly by light scattering at fractions of linear polyethylene in the theta solvent, diphenylmethane, indicate incomplete molecular dissolution and the presence of branched structures.

The Mark-Houwink-Kuhn-Sakurada exponent of polymers with long side groups : is a0 = 1/2 a reliable criterion of the theta state ?

The intrinsic viscosity data for poly(decyl acrylate), poly(cetyl acrylate), poly(vinyl laurate), poly(vinyl stearate) and two polymethacrylates with long mesogenic side groups are reviewed and analysed by means of three sensitive plots. Unusual values of the Mark-Houwink-Kuhn-Sakurada exponent a0 under theta conditions or in good solvents (A2 ≠ 0 but a0 = 12) are assigned to a combination of the effects of long-chain branching, chain thickness and, in the last case, excluded volume.

Ubbelohde viscometer modified for foaming solutions of water soluble polymers

Abstract A modification of the standard Ubbelohde viscometer for measuring viscosity of foaming liquids such as solutions of water-soluble polymers is presented.

Free-rotation dimensions of some polyurethane chains

Abstract The ratio of the unperturbed mean-square end-to-end distance to the molecular weight in the random-coil limit, (〈 R 2 〉 0, f / M ) ∝ , in the freerotation approximation was calculated for chains of linear polyurethanes PUR-D k prepared from 4,4′-methylenediphenyl diisocyanates and low-molecular-weight diols HOue5f8(CH 2 ) k ue5f8OH ( k = 2, 3, 4 and 6). It was found that the experimental values of (〈 R 2 〉 0 / M ) ∝ estimated in the previous paper are very close to the values of (〈 R 2 〉 0. f / M ) ∝ calculated under the assumption that the groups of bonds CH 2 ue5f8C 6 H 5 ue5f8NH and NHue5f8COue5f8Oue5f8CH 2 can be replaced by rigid virtual bonds.