John Eckelt

Pullulan and Dextran : Uncommon Composition Dependent Flory-Huggins Interaction Parameters of their Aqueous Solutions

Vapor pressure measurements were performed for aqueous solutions of pullulan ( M w 280 kg/mol) and dextran ( M w 60 and 2100 kg/mol, respectively) at 25, 37.5, and 50 degrees C. The Flory-Huggins interaction parameters obtained from these measurements, plus information on dilute solutions taken from the literature, show that water is a better solvent for pullulan than for dextran. Furthermore, they evince uncommon composition dependencies, including the concurrent appearance of two extrema, a minimum at moderate polymer concentration and a maximum at high polymer concentration. To model these findings, a previously established approach, subdividing the mixing process into two clearly separable steps, was generalized to account for specific interactions between water and polysaccharide segments. Three adjustable parameters suffice to describe the results quantitatively; according to their numerical values, the reasons for the solubility of polysaccharides in water, as compared with that of synthetic polymers in organic solvents, differ in a principal manner. In the former case, the main driving force comes from the first step (contact formation between the components), whereas it is the second step (conformational relaxation) that is advantageous in the latter case.

Interpolymer Complexes and Polymer Compatibility

A reliable method to decide whether two polymers A and B are miscible or incompatible would be very helpful in many ways. In this contribution we demonstrate why traditional procedures cannot work. We propose to use the intrinsic viscosities [η] of the polymer blends instead of the composition dependence of the viscosities as a criterion for polymer miscibility. Two macromolecules A and B are miscible because of sufficiently favorable interactions between the two types of polymer segments. For solutions of these polymers in a joint solvent, this Gibbs energetic preference of dissimilar intersegmental contacts should prevail upon dilution and lead to the formation of interpolymer complexes, manifesting themselves in deviations from the additivity of intrinsic viscosities.

Thermodynamic Interaction Parameters for the System Water/NMMO Hydrate

Vapor pressures of water were measured for aqueous solutions of N-methyl-morpholine N-oxide (NMMO) at 80, 90 and 100 degrees C. The Flory-Huggins interaction parameters, chi, calculated from these data as a function of phi, the volume fraction of NMMO, are negative at all concentrations; at low phi, they decrease by more than a factor of 2 as T is raised, whereas they remain almost unchanged as phi approaches unity. Accordingly, the heat of mixing is pronouncedly endothermal at low NMMO concentrations but close to athermal at low water content. The composition dependence of chi can be equally well described by the Redlich-Kister equation and by an approach subdividing the mixing process into two separate steps. The opportunities of the latter modeling for a better molecular understanding of the mixing processes are discussed.

Polydispersity effects on the phase diagram of the system chloroform/poly-l-(lactic acid)/poly(methyl methacrylate) and morphology of PLA/PMMA films

Abstract Cloud point curve, critical composition and several critical coexistence curves were measured at 25°C for the ternary system poly- l -(lactic acid) (PLA), poly(methyl methacrylate) (PMMA) — where both polymers exhibit broad molecular weight distributions — and the common solvent chloroform. In contrast to the situation encountered in the absence of the second polymer both branches of the critical coexistence curves are located without any doubt inside the miscibility gap as defined by the cloud point curve. This unexpected experimental finding is corroborated by model calculations on the basis of continuous thermodynamics. The removal of solvent from the ternary mixtures yields films of different morphology, depending on the particular path through the metastable or unstable regions of the phase diagram. The structures observed by means of optical microscopy confirm the theoretically postulated phase separation mechanisms.

Liquid/gas and liquid/liquid phase equilibria of the system water/bovine serum albumin.

The thermodynamic behavior of the system H2O/BSA was studied at 25 °C within the entire composition range: vapor pressure measurements via head space sampling gas chromatography demonstrate that the attainment of equilibria takes more than one week. A miscibility gap was detected via turbidity and the coexisting phases were analyzed. At 6 °C the two phase region extends from ca. 34 to 40 wt % BSA; it shrinks upon heating. The polymer rich phase is locally ordered, as can be seen under the optical microscope using crossed polarizers. The Flory-Huggins theory turns out to be inappropriate for the modeling of experimental results. A phenomenological expression is employed which uses three adjustable parameters and describes the vapor pressures quantitatively; it also forecasts the existence of a miscibility gap.

Fractionation of Cellulose

Cellulose samples with molecular weight distributions that are considerably narrower than those of the natural products can be obtained by at least three fundamentally different routes. (i) Synthesis of easily soluble derivatives, fractionation by means of well-established methods and subsequent regeneration, (ii) selective extraction of short chains from activated cellulose, using solvents of suitable marginal quality, and (iii) partition of the homologs between two coexisting phases formed by the demixing of homogeneous solutions. All three methods can be applied successfully. However, the efforts in terms of labor and required solvent differ considerably. Most of the experiments were performed with the following three cellulose samples: Avicel (Mw = 30 kg mol−1, U = (Mw/Mn)−1 = 2.0), Solucell (Mw = 230 kg mol−1, U = 1.8), and Stockstadt (Mw = 320 kg mol−1, U = 5.7). Options (ii) and (iii) emerged most promising for large scale fractionation. The mixed solvent consisting of DMAc and LiCl turned out to be particularly versatile in both cases. In the pure state it can be used for incremental extraction (yielding quick access to orienting information on the width of the molecular weight distribution) as well as for one-step extraction. In combination with suitable precipitants (like acetone) it enables the realization of the coexistence of two liquid phases required with route (iii). One obstacle for fractionation that all methods share is the high viscosity of cellulose solutions. With the last method it is possible to mitigate this limitation considerably by the use of spinning nozzles for the mixing of feed and extracting agent.

Forces Between Solid Surfaces Across Polymer Melts as Revealed by Atomic Force Microscopy

Forces between solid surfaces across polymer melts are poorly understood despite their fundamental importance and their relevance for making composite materials. Such force measurements reveal information on the structure of polymers at surfaces and of confined polymers. Experiments with the atomic force microscope and polyisoprene (PI) confirmed theoretical predictions that no long‐range force should be present in thermodynamic equilibrium. In poly(dimethyl siloxane) (PDMS) repulsive forces are observed at high molar mass. We attribute this to the formation of an immobilized layer caused by a slow release of adsorbed segments enhanced by entanglement. In low molar mass PDMS attractive forces were observed which we can not yet explain. Attaching a hydroxyl end group to PI or PDMS chains lead to repulsive forces caused by the formation of a brush‐like structure.

Fractionation of Poly(vinyl methyl ether): Comparison of Two Large-Scale Fractionation Techniques

The efficiencies of two large-scale fractionation techniques namely continuous polymer fractionation (CPF) and continuous spin fractionation (CSF) are compared. To this end, we used a commercially available sample of poly(vinyl methyl ether). Both methods are suitable to reduce the nonuniformity of the sample below 0.6. The results demonstrate that CSF has a better fractionation performance than the progenitor method, CPF. A further advantage is the three times higher throughput of CSF.