Computer Study of Chromatographic Separation Process: A Monte Carlo Study of H‑Shaped and Linear Homopolymers in Good Solvent

The partitioning of linear (L) and H-shaped polymers between bulk solvent and narrow pores with inert and attractive walls and the conformational behavior of chains in pores was studied by Monte Carlo simulations. The polymer chains were modeled as self-avoiding walks in a good solvent. The concentration profiles in the pores, partition coefficients <i>K</i>_H and <i>K</i>_L, and various structural characteristics were calculated as functions of pore size and interaction parameter ε, ranging from 0 to −0.26. <i>K</i>_H is higher than <i>K</i>_L in pores with nonattractive walls, but the difference decreases with increasing |ε|. Both partition coefficients equal for ε* ca. −0.2, and later their sequence inverts. ε* depends only slightly on chain architecture and chain length. The results are important from the experimental point of view because they show that the improperly chosen experimental conditions can deteriorate SEC analysis of branched samples

Dissipative Particle Dynamics Study of Electrostatic Self-Assembly in Aqueous Mixtures of Copolymers Containing One Neutral Water-Soluble Block and One Either Positively or Negatively Charged Polyelectrolyte Block

The paper describes the general features and trends of the electrostatic assembly (EA) of block polyelectrolytes. We performed computer simulations of the associative behavior of aqueous mixtures of diblock copolymers containing one neutral water-soluble block and one either positively or negatively charged polyelectolyte (PE) block. While the neutral block is readily soluble in water, the hydrophilic vs hydrophobic nature of the neutral backbone of the PE block and the compatibility of the blocks vary in a broad range. We investigated the role of (i) electrostatics, (ii) solvophobicity of the PE block, (iii) compatibility of the polymer blocks, and also (iv) compatibility of small ions with the polymer blocks. We employed the dissipative particle dynamics (DPD) method and used the generally recognized formula (J. Chem. Phys. 1997, 107, 4423) for recalculating the Flory–Huggins interaction parameters in DPD parameters of soft coarse-grained repulsion forces. The Coulomb interactions are described by the rigorous expression derived for the exponentially smeared charge with a fairly low charge decay length λ = 0.2. A low λ value has been used to reproduce the behavior of small counterions as realistically as possible at the DPD level. We compared the self-assembling behavior of charged and neutral copolymers for all the systems. The conclusions of the study can be briefly outlined as follows: Even though long-range electrostatic interactions are a prerequisite for electrostatic self-assembly and the increase in entropy due to liberation of mobile counterions represents an important driving force in all cases, the solvent quality for the PE backbone and incompatibility of blocks play an important role and substantially modify the association process. Only dimers containing one positively and one negatively charged chain are formed in systems with readily soluble PE blocks. The formation of large core–shell associates assumes (in addition to the effect of electrostatics) significantly unfavorable interactions of PE segments with water and with segments of the water-soluble block. The presence of opposite charges on different chains promotes the formation of associates, i.e., both the fraction of associates and association number increase, but the latter increase is fairly small (taking into account the value that is attained in corresponding neutral system)

Influence of the Chain Architecture and the Presence of End-Groups or Branching Units Chemically Different from Repeating Structural Units on the Critical Adsorption Point in Liquid Chromatography

The critical adsorption point (CAP) of linear and star-shaped polymers was investigated by normal phase and reversed phase liquid chromatography (NPLC and RPLC) and computer simulation. Three sets of polystyrenes (PS) differing in chain architecture and chemically distinct groups were prepared: linear PS (<i>sec</i>-butyl and hydrogen end group), 2-arm PS (linear, two <i>sec</i>-butyl end groups and one silyl group in the middle of the chain) and 4-arm star-shaped PS (four <i>sec</i>-butyl end groups and one silyl group in the center of the star). It was found that the column temperature at CAP, <i>T</i>_CAP (linear PS) = <i>T</i>_CAP (2-arm PS) > <i>T</i>_CAP (4-arm PS) in both RPLC and NPLC which can be attributed to the variation in chain architecture. However, the elution times at CAP of three polymers are all different: In NPLC, <i>t</i>_E,CAP (linear) > <i>t</i>_E,CAP (2-arm PS) > <i>t</i>_E,CAP (4-arm PS) while in RPLC, <i>t</i>_E,CAP (4-arm PS) > <i>t</i>_E,CAP (2-arm PS) > <i>t</i>_E,CAP (linear). The variation of <i>t</i>_E,CAP can be explained by the contribution of the chemically distinct groups. The computer simulation results are in good agreement with the chromatography experiments results and support the interpretation of experimental data

Modeling of Ionization and Conformations of Starlike Weak Polyelectrolytes

The target of this work is to study conformational properties of starlike polyelectrolytes with pH-sensitive (annealed) dissociation in salt-free solutions. We confront hybrid Monte Carlo (HMC) simulations with computationally less expensive approximate numerical self-consistent field (SCF) calculations and with analytical theories. We demonstrate when the mean-field results are reliable and their advantage over MC in terms of efficiency can be exploited and when not. In the interior of the star, where inter-arm interactions dominate over intra-arm ones, the mean-field approximation works well and SCF agrees with the MC results. Intra-arm interactions dominate at star periphery, and their role is underestimated by the mean field. Here, conformations and dissociation resemble those of linear polyelectrolytes. Consequently, the dissociation profile along the chain contour is qualitatively different between MC and SCF. Comparison of the two methods and a distinction between intra-arm and inter-arm contributions to interactions enables us to understand the transition in behavior from linear to starlike chain topology