Walther Burchard

Solution Properties of Branched Macromolecules

Dilute and semi-dilute solution properties of several classes of branched macromolecules are outlined and discussed. The dilute solution properties are needed for a control of the chemical synthesis. The molecular parameters also determine the overlap concentration which is an essential quantity for description of the semi-dilute state. This state is represented by a multi-particle, highly entangled ensemble that exhibits certain similarities to the corresponding bulk systems. Because of the rich versatility in branching the present contribution made a selection and deals specifically with the two extremes of regularly branched polymers, on the one hand, and the randomly branched macromolecules on the other. Some properties of hyperbranched chains are included, whereas the many examples of slight deviations from regularity are mentioned only in passing. The treatment of the two extremes demonstrates the complexity to be expected in the general case of less organized but non-randomly branched systems. However, it also discloses certain common features.

Static and dynamic light scattering from branched polymers and biopolymers

The striking properties of synthetic polymers and biological macromolecules are largely determined by their shape and the internal mobility. Both quantities are closely related to the architecture of the molecules. This article deals with branched macromolecules in dilute solution, where the individual molecules are observed. The common technique for determining the shape of macromolecules is static light scattering. Information on the internal mobility and the translational motion of the mass centre can be obtained from the more recent technique of quasi-elastic or dynamic light scattering.

About the structure of cellulose: debating the Lindman hypothesis

The hypothesis advanced in this issue of CELLULOSE [Springer] by Bjorn Lindman, which asserts that the solubility or insolubility characteristics of cellulose are significantly based upon amphiphilic and hydrophobic molecular interactions, is debated by cellulose scientists with a wide range of experiences representing a variety of scientific disciplines. The hypothesis is based on the consideration of some fundamental polymer physicochemical principles and some widely recognized inconsistencies in behavior. The assertion that little-recognized (or under-estimated) hydrophobic interactions have been the reason for a tardy development of cellulose solvents provides the platform for a debate in the hope that new scientific endeavors are stimulated on this important topic.

Star polymers from styrene and divinylbenzene

Abstract The principle of anionic block copolymerization of styrene and divinylbenzene, first employed by Rempp and his coworkers, is further developed. Rempps method (1) of divinylbenzene polymerization with long ‘living’ polystyrene chains, and our method (2) of the polymerization of styrene with polydivinylbenzene, bearing a number of ‘living’ carbanionic groups, are shown to be the two limiting cases of the styrene—divinylbenzene block copolymerization. By preparing ‘living’ polystyrene of shorter chain lengths than was used by Rempp et al . all stages of polymerization within the two limits are realized. In particular, star molecules with a large number of short side chains, according to Rempps method, are prepared in a first step, and these stars with ‘living’ nuclei are used to prepare stars (3) according to our method. The number of branches of the original star is doubled by this technique. Also mixed stars with both polystyrene and polyisoprene branches are prepared. The latter are characterized by light scattering in benzophenone, which is isorefractive with the polystyrene moiety. The g ′ factors of the stars are compared with existing theories, but in no case is satisfactory agreement obtained.

Solution properties of β-D-(1, 3)(1, 4)-glucan isolated from beer

Abstract β- d -(1, 3)(1, 4)-glucan isolated from beer, and in comparison to this, a purchased one isolated from barley, were investigated by combined static and dynamic light scattering and rheological methods. The weight average molecular weight of β- d -(1, 3)(1, 4)-glucan of beer and barley was determined as 175,000±5000 Da. The particle weight of β- d -(1, 3)(1, 4)-glucan strongly depends on concentration, solvent, temperature and external forces. The aggregates are assumed to be caused by association via hydrogen bonds. A specific effect resulted when maltose was included at different concentrations. Near 5% (w/v), a minimum of aggregation was found. The findings from static and dynamic light scattering and from viscometry indicate fringed micelle formation, a model which is described in detail. Variation of pH between 2 and 11 did not influence the viscosity.

Structures of cellulose in solution

Cellulose and their derivatives only rarely form molecularly dispersed solutions. Colloidal aggregates of yet not fully explosed solutions. Colloidal aggregates of yet not fully explored supramolecular structure remain preserved even at the highest dilution. The present contribution is concerned with the determination of the number of aggregated chains per colloid particle. In some cases the aggregation number could be directly determined but for the non-ionic, partial substituted cellulose-ethers the degree of polymerization of the non-aggregated cellulose was not sufficiently well known. A detailed analysis of the angular dependencies of scattered light revealed for the large aggregates a star-shaped structure. The smaller ones were better described by a worm-like structure. These observations led to the suggestion of a fringed micelle model where several chains are laterally assembled in a rigid stem leaving at both ends coronas of f/2 dangling chains. Recent SANS experiments suport this model. The estimated chain length of a single strand was found with DP W = (4-20) × 10 3 much too high and led to the conclusion that 4-20 chains must be co-linearly (staggered) associated to form one strand. The obtained overall aggregation numbers are close to those found for cellulose in NMMNO and vary from 10 to 800. The similarity of the supramolecular structures in aggregation numbers and dimensions is interpreted as reminiscent of the semi-crystalline structure of the parental native cellulose fibers. Much lower aggregation numbers were found for cellulose xanthogenates (viscose) in NaOH solution and for cellulose 2.5-acetate in acetone than for cellulose in NMMNO.

Structure of casein micelles I. Small angle neutron scattering and light scattering fromβ- andχ-casein

Casein is the main protein component of milk and is of remarkable colloidal stability. Under the influence of milk clotting enzymes casein shows the striking behaviour of coagulation. This clotting process has already been studied by other groups, neglecting the fact that casein is not a homogeneous protein. The purpose of the present study is focused, in this first stage, on the determination of the structure of the various casein components. In cooperation with other laboratories we have been able to obtain the well separated individual proteins. Studies have been performed so far withβ- andχ-casein. For detailed structural information we carried out small angle neutron scattering and combined static and dynamic light scattering measurements and determined the molecular weight,Mw, the radius of gyration, 〈S2〉 the hydrodynamic radius,RH, theϱ-value and the particle scattering factor, Pz(q). The two caseins show a strikingly different behaviour. For theβ-casein we found a star-like structure, i. e. an aggregation pattern that is expected for a common micelle. The micelle consists of about 38 monomer chains. The aggregates ofχ-casein appear to be composed of star-like submicelles, where each submicelle contains nineχ-casein chains and the total degree of aggregation is about 140.

Quasi-elastic light scattering from branched polymers: 1. Polyvinylacetate and polyvinylacetate—microgels prepared by emulsion polymerization

Emulsion polymerization of vinylacetate leads to branched polymers which at high monomer conversions form microgels of the shape and size of the latex particles. Quasielastic light scattering measurements from samples in the pre-gel state give at small q2 a linear angular dependence of Dapp = Гq2 which resembles that of randomly branched chain molecules, where Г is the decay constant of the time correlation function. Extrapolation of Dapp towards zero scattering angle yields the translational diffusion constant Dz. The diffusion constant follows the molecular weight dependence Dz = 9.78 10−5Mw−0.478. The diffusion constant of the microgels, i.e. at molecular weights Mw > 14 106, remains constant because of the finite and constant size of the latex particles. The coefficients kf and kD in the concentration dependence of the frictional and diffusion coefficients are related according to the equation kD = kf − 2A2Mw − v where A2 is the second virial coefficient and v the partial specific volume of the particle. The coefficient kf is calculated from the experimentally determined quantities kD, A2 and Mw, and the result is compared with the theory by Pyun and Fixman. Accordingly the branched coils in the pre-gel state resemble soft spheres, but the microgels behave more like spheres of some rigidity.

Solubility and Solution Structure of Cellulose Derivatives

Strongly interacting solvents are needed to dissolve cellulose; therefore, in the past the interpretation of the uncommon solution behavior of cellulose and its derivatives was based mainly on energetic (enthalpic) considerations, for example, hydrogen bonding. These attempts have not been very successful. The present paper demonstrates that entropic effects influence the solution behavior much stronger than hitherto supposed. In the well-known Flory–Huggins theory the driving force for dissolution of flexible chains is the configurational entropy of mixing. This large entropy is strongly reduced by the chain stiffness of the cellulose backbone and by the strictly regular primary structure of this polysaccharide. It strongly reduces the driving force for dissolution. The entropy of mixing becomes largely increased again by the attachment of long side chains and causes solubility with surprising efficiency (hairy rod principle). This effect is demonstrated with several examples. Among others, the surprising insolubility of short, regular-selectively substituted cellulose chains is explained, although long chains of the same substitution pattern are soluble. The striking behavior of cellulose ethers in water is based on the hydrophobic effect, which causes an increased order of the polymer surrounding water molecules. The induced order results in a very pronounced decrease of entropy of mixing that overcompensates the positive configurational entropy of mixing. Common rules of basic thermodynamics now predict phase separation on heating, contrary to the Flory–Huggins theory, which can only predict phase separation on cooling.

Gel point in physical gels: rheology and light scattering from thermoreversibly gelling schizophyllan

Abstract The β-1.3/1.6 glucan schizophyllan is a fungal polysaccharide that is known to suppress tumor growth. Thermoreversible, optically transparent gels of schizophyllan were obtained on addition of sorbitol to aqueous solutions of schizophyllan. Gel temperatures were independent of the history of sample preparation. The gelation process was followed by low-amplitude oscillatory shear and by static and dynamic light scattering. Power law behavior of storage modulus G′(ω) and loss modulus G″(ω) was found at the gel temperature with the exponent n=0.5. The static LS measurements showed a strong irregularity at the gel temperature. The intensity time correlation functions (TCFs) as measured by dynamic LS during the gelation process were transformed into the electric field TCFs, taking into account the heterodyne contributions. At the gel temperature which was estimated by rheology, DSC and static LS, no power law was found for the TCF. The angular dependence of the intensity TCF was measured and converted to the field TCF by use of a non-ergodicity treatment and by the heterodyne approximation, respectively. Both methods result in mean relaxation times that agree quite well. Two relaxation modes were found in the whole temperature range (above and below the gel temperature) indicating the existence of inhomogeneities in the gel.