31 research outputs found
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A morphological, mechanical and thermodynamic investigation of the isotactic polyvinylmethylether/polystyrene polymer blend/
A novel technique for the production of toughened polymers using LCST behavior as a mechanism for the production of rubbery domains is discussed. The polymer blend of isotactic polyvinylmethylether (PVME) with polystyrene (PS) is used. Synthesis, fractionation and characterization of isotactic PVME is reviewed. Thermodynamic effects of tacticity on miscibility are extensively investigated using light and neutron scattering. A simple critical point analysis is presented which indicates an entropic nature to the tacticity effect in this blend. Flory-Huggins-Staverman (F-H-S) theory is next applied to the tacticity effect. This more elaborate analysis also indicates an entropic nature to the tacticity effect. Accounting for polydispersity results in a predicted fractionation in the phase separated blends which is supported by mechanical data. F-H-S theory was used to generate a functional form for the interaction parameter in terms of the temperature and composition dependence of miscibility. From these functions a dramatic shift in the kinetics of phase separation with tacticity is predicted. Experimental data affirms this prediction. A novel, modified Cahn-Hillard analysis is presented for analysis of intermediate stages. Neutron scattering data yields the composition and temperature dependence of the statistical segment length, b, of tactic PVME. A functional form for b is derived which predicts the equilibrium melting point and melting point depression behavior for the blends. A relationship between b and thermodynamic miscibility from a geometric perspective is discussed. The tacticity effect can be described in terms of an interaction parameter whose change with tacticity in terms of entropy is functionally related to the volume of an interacting group and in terms of enthalpy is functionally related to the surface area of an interacting group. Two related studies are presented. The first pertains to a shift in glass transition of thin PS films with thickness as investigated using ellipsometry. The second study used neutron reflection data to disprove the supporting argument for a minus two thirds dependence of surface tension with molecular weight. An alternative theory for the molecular weight dependence of surface tension is presented
Quantification of interaction and topological parameters of polyisoprene star polymers under good solvent conditions
Dimensional Description of Cyclic Macromolecules
ABSTRACT: Cyclic structures are often used to model and simulate long chain molecules due to the simplification of no chain-end effects. Many technically important and biologically relevant molecules are cyclics. Further, ring polymers display dramatic viscosity enhancement when blended with linear polymers in the melt. It has been proposed that cyclic melts may display a topologically driven coil collapse at high molecular weights reminiscent of cyclic DNA. Despite the structural simplicity and importance of cyclics a quantitative analytic distinction between cyclics and linear chains in the melt or in solution has been elusive since both linear and cyclic macromolecules display similar disordered, fractal structures. A dimensional analysis of cyclic polymers and its use to describe scattering data from cyclic macromolecules is presented. The validity of the new approach to describe cyclic structures is demonstrated using experimental data, and the Casassa form factor, previously used for cyclic polymers, is critically revisited. The scaling model is also used to quantify cyclic coil collapse in simulations from the literature
Toward Resolution of Ambiguity for the Unfolded State
The unfolded states in proteins and nucleic acids remain weakly understood despite their importance in folding processes; misfolding diseases (Parkinson's and Alzheimer's); natively unfolded proteins (as many as 30% of eukaryotic proteins, according to Fink); and the study of ribozymes. Research has been hindered by the inability to quantify the residual (native) structure present in an unfolded protein or nucleic acid. Here, a scaling model is proposed to quantify the molar degree of folding and the unfolded state. The model takes a global view of protein structure and can be applied to a number of analytic methods and to simulations. Three examples are given of application to small-angle scattering from pressure-induced unfolding of SNase, from acid-unfolded cytochrome c, and from folding of Azoarcus ribozyme. These examples quantitatively show three characteristic unfolded states for proteins, the statistical nature of a protein folding pathway, and the relationship between extent of folding and chain size during folding for charge-driven folding in RNA
Hierarchical approach to aggregate equilibria
Hierarchical aggregation is generally viewed as a kinetic phenomenon governed by kinetic growth laws, such as in the Smoluchowski equation, and modeled using diffusion or reaction limited kinetic growth models. Some aggregates, especially those controlled by surface grafting or surfactants, display reversible stability. For these equilibrated aggregates a simple thermodynamic model is proposed to describe the size distribution and the enthalpy and entropy of aggregation. The model uses the average degree of aggregation, z_{i(i−1)}, as the central quantifying parameter. Here i is an index reflecting the hierarchical level of structure in an aggregate, for instance, composed of crystals (i=0), clustered primary particles (i=1), aggregates (i=2), and agglomerates of aggregates (i=3). A change in Gibbs free energy for aggregation is given by ΔG_{i(i−1)}=−RTln(1/z_{i(i−1)}) for each level (i>0). This expression is advantageous since the degree of aggregation is directly determined in small-angle neutron and x-ray scattering, by transmission electron microscopy, simulation, or through spectroscopy. The atomistic hierarchical model enables an understanding of the mechanism of equilibrium aggregation since it provides expressions for entropy and enthalpy of aggregation at each structural/thermodynamic level. The model can be extended to describe pseudoequilibrium for industrially relevant materials such as condensation polymers. Applications in organic pigments and wormlike micelles are also briefly demonstrated
PEO−PPO−PEO Block Copolymer Micelles in Aqueous Electrolyte Solutions: Effect of Carbonate Anions and Temperature on the Micellar Structure and Interaction Volume 34, Number 3, January 30, 2001, pp 552−558
Branch content of metallocene polyethylene
ABSTRACT: Small-angle neutron scattering (SANS) is employed to investigate the structure and longchain branch (LCB) content of metallocene-catalyzed polyethylene (PE). A novel scaling approach is applied to SANS data to determine the mole fraction branch content (φ br ) of LCBs in PE. The approach also provides the average number of branch sites per chain (n br ) and the average number of branch sites per minimum path (n br,p ). These results yield the average branch length (z br ) and number of inner segments n i , giving further insight into the chain architecture. The approach elucidates the relationship between the structure and rheological properties of branched polymers. This SANS method is the sole analytic measure of branch-onbranch structure and average branch length for topologically complex macromolecules
Free Energy of Scission for Sodium Laureth-1-Sulfate Wormlike Micelles
Wormlike
micelles (WLMs) are nanoscale, self-assembled components
of many products from shampoos to fracking fluids due to their viscoelasticity.
Their rheological behavior is largely governed by the contour length
of the micelles and the concomitant propensity of the micelles to
overlap and entangle. The large contour lengths, on the order of micrometers,
is the result of a delicate balance between the scission enthalpy
of the wormlike micelles on the one hand and entropic factors such
as the mixing entropy of dispersion, the ordering of water molecules
and counterions, and the mobility of branch points on the other hand.
The structure and contour length of wormlike micelles assembled from
sodium laureth-1-sulfate was determined at various temperatures using
small-angle neutron scattering. The results allow the calculation
of the enthalpy and entropy as well as the free energy of scission
and are employed to critically evaluate the common methods to determine
micellar scission energy from mean-field theory. Interesting behavior
is observed when comparing branched and unbranched WLMs that may reflect
on mechanistic differences in chain scission