Theta solvent

Abstract: Carbosilane dendrimers with 64 and 128 surface Si-Cl bonds were used as coupling reagents for monodisperse poly(butadienyl)lithium. Two series of regular star polybutadienes with 64 and 128 arms were prepared. The arm molecular weight was varied between 6400 and 72 000. The dilute-solution properties of the stars were determined in a good solvent (cyclohexane) and in a θ-solvent (dioxane) for polybutadiene. Measurements of R G . A 2 , D 0 , and [η] indicate that the isolated stars behave as hard spheres. The ratio of the hydrodynamic radius over the radius of gyration is slightly larger than (5/3) 1/2

Abstract: We revive an old conjecture that a fixed number of binary contacts between chains collec- tively make up the topological constraint commonly referred to as an entanglement. This leads us to a general scaling theory for semidilute polymer solutions which involves two length scales, the screening length ( and the tube diameter a. In good solvents these two lengths have the same concentration depen- dence and we recover the de Gennes results. In 9 solvents the two length scales depend on concentration differently. Combining the concentration dependences of these two length scales with concepts from the- ories of rubber elasticity and reptation leads to new predictions for the plateau modulus G - kT/(a2{) - c7/3 and viscosity 9 - M2/3(c/c*)14/3 in 9 solvents, where M is the polymer molecular weight and c* is the overlap concentration. These predictions are found to compare favorably with available experimental data.

Abstract: We present some conjectures concerning the equilibrium statistics and dynamics of ring polymers. We argue that unconcatenated ring polymers in the melt may have statistics intermediate between those of collapsed and Gaussian chains. An extremely crude (Flory-like) treatment suggests that the radius R of such a ring scales with its polymerization index N as R ∼ N 2/5. In contrast, a ring in a melt of long linear chains (of the same chemical species) should be swollen. Moreover, rings of one chemical species (A) can be compatible with linear chains of another (B), even when linear chains of A and B are not compatible. Rings in a network of fixed obstacles are also discussed. A simple analysis of their dynamics shows that the diffusion constant D of such a ring (in three dimensions) scales with its polymerization index N as D ∼ N-2. This prediction is confirmed by computer simulations. Finally, we consider a knotted ring formed irreversibly in a theta solvent, and argue that, under appropriate formation conditions, such a ring may remain Gaussian when placed in a good solvent.

Abstract: The aim of this study was to define the parameters determining an optimized yield of monodisperse, nanosized particles after nanoprecipitation of a biodegradable polymer, with a view to industrial scale-up the process. Poly(d,l)-lactides (PLAs) from a homologous series of different molar masses were nanoprecipitated at different initial polymer concentrations from two organic solvents, acetone and tetrahydrofuran (THF), into water without surfactant according to a standardized procedure. Quasi-elastic light scattering and gel permeation chromatography with universal detection were used respectively to size the particles and to determine the molar mass distribution of the polymeric chains forming both nanoparticles and bulk aggregates. The intrinsic viscosity of the polymers as a function of molar mass and solvent were determined by kinematic viscosity measurements in organic solutions. High yields of small nanoparticles were obtained with polymers of lower molar mass (22600 and 32100 g/mol). For a given polymer concentration in organic solution, the particle diameter was always lower from acetone than from THF. For initial molar masses higher than 32100 g/mol, only dilute organic solutions gave significant yields of nanoparticles. Furthermore, polymer mass fractionation occurred with increasing initial molar mass and/or concentration: the nanoparticles were formed by polymeric chains of molar masses significantly lower than the average initial one. In general, nanoparticle production was satisfactory when the initial organic solution of polymer was in the dilute rather than the semi-dilute regime. Moreover, acetone, which acted as a theta solvent for PLA, always led to smaller particles and better yields than THF.

Abstract: Self-avoiding walks, lattice trees, and related geometrical models provide a link between the physics of polymers and the study of critical phenomena. In particular, these models in the presence of a surface provide insight into surface adsorption in dilute polymer systems in a good solvent. The theme of this review is the influence of polymer structure (topology) on the critical properties of these models. Emphasis is placed on recent results by rigorous methods, scaling theory, and conformal covariance theory. Numerical results that may be used to test the predictions of scaling of scaling and conformal covariance theories are also summarized. Related topics such as the adsorption of directed polymers, the semidilute regime, the theta point and theta solvents, and percolation (polymer gels) are briefly discussed in the final section.