We discuss possible motions for one polymer molecule P (of mass M) performing wormlike displacements inside a strongly cross‐linked polymeric gel G. The topological requirement that P cannot intersect any of the chains of G is taken into account by a rigorous procedure: The only motions allowed for the chain are associated with the displacement of certain “defects” along the chain. The main conclusions derived from this model are the following:(a) There are two characteristic times for the chain motion: One of them (Td) is the equilibration time for the defect concentration, and is proportional to M2. The other time (Tr) is the time required for complete renewal of the chain conformation, and is proportional to M3.(b) The over‐all mobility and diffusion coefficients of the chain P are proportional to M−2.(c) At times t < Tr the mean square displacement of one monomer of P increases only like 〈(rt − r0)2〉 = const t1/4.These results may also turn out to be useful for the (more difficult) problem of entangle...

Reptation of a Polymer Chain in the Presence of Fixed Obstacles

We present an extensive molecular‐dynamics simulation for a bead spring model of a melt of linear polymers. The number of monomers N covers the range from N=5 to N=400. Since the entanglement length Ne is found to be approximately 35, our chains cover the crossover from the nonentangled to the entangled regime. The Rouse model provides an excellent description for short chains N<Ne, while the dynamics of the long chains can be described by the reptation model. By mapping the model chains onto chemical species we give estimates of the times and distances of onset of the slowing down in motion due to reptation. Comparison to neutron spin‐echo data confirm our mapping procedure, resolving a discrepancy between various experiments. By considering the primitive chain we are able to directly visualize the confinement to a tube. Analyzing the Rouse mode relaxation allows us to exclude the generalized Rouse models, while the original reptation prediction gives a good description of the data.

Dynamics of entangled linear polymer melts: A molecular‐dynamics simulation

Current status and future perspectives in atom transfer radical polymerization (ATRP) are presented. Special emphasis is placed on mechanistic understanding of ATRP, recent synthetic and process development, and new controlled polymer architectures enabled by ATRP. New hybrid materials based on organic/inorganic systems and natural/synthetic polymers are presented. Some current and forthcoming applications are described.

Atom Transfer Radical Polymerization (ATRP): Current Status and Future Perspectives

http://wwwcourses.sens.buffalo.edu/ce435/Pedersen97.pdf

Analysis of small-angle scattering data from colloids and polymer solutions: modeling and least-squares fitting

Recent attempts have been made to assess the relative merits of the free volume and entropy theories of viscous flow in glass‐forming liquids by accurate measurement of viscosity over wide temperature ranges, and subsequent comparison with the equations derived from these theories. In the author's view, this effort is misguided. The theories are crude and qualitative, and such tests are too stringent. It is better to make qualitative or semiquantitative comparison of a wide variety of physical phenomena; judged by this criterion, the entropy theory appears more successful. It is conjectured that further progress can be made by accepting the crude, naive character of any model we are likely to find tractable for the foreseeable future, and recognizing both the values and limitations of such models. A picture of the flow process in viscous liquids is proposed, in an attempt to answer certain questions about the molecular steps in flow either answered unsuccessfully or ignored by present theories, in the hop...

/pdf/viscous-liquids-and-the-glass-transition-a-potential-energy-47wz1nu6jb.pdf

Viscous Liquids and the Glass Transition: A Potential Energy Barrier Picture

The viscosity of polymers and their concentrated solutions

The results of an extensive study of some dilute solution properties of a series of linear, homogeneous (anionically prepared) polystyrenes (48<10−4M<440) over a wide temperature interval (10°<T<110°C) in decalin and toluene are described The data are used to obtain detailed comparisons among the second virial coefficient A2, the mean‐square molecular radius 〈s2〉, the molecular weight M, and the temperature The empirical correlations thus obtained are compared to theoretical predictions In particular, it is found that A2M½(〈s2〉/M)0−32 and α2=〈s2〉/〈s2〉0 are each single‐valued functions of the interaction parameter z=B(〈s2〉0/M)0−32M½, when B is assumed to have the temperature dependence B=B0[1—(Θ/T)] Certain theoretical predictions for the dependence of A2M½/α3 on z/α3 are found to adequately describe the form of the observed behavior and can be used as the basis for a satisfactory empirical relation The observed dependence of α2 on z is satisfactorily predicted by recent calculations of Flory and Fis

Thermodynamic and Conformational Properties of Polystyrene. I. Light‐Scattering Studies on Dilute Solutions of Linear Polystyrenes

Molecular brushes as super-soft elastomers

Rheo‐optical studies on shear deformation of well‐aligned nematic solutions of the rodlike poly (1,4‐phenylene‐2,6‐benzobisthiazole), PBT, are reported. Conoscopic microscopy was used to follow the director during shear, and during relaxation on cessation of flow. The results show that the molecules are not homogeneously aligned in shearing flow. Rather, various complex director field distortions may occur, depending on the relative orientation of the initial director to the flow direction. The results are compared quantitatively with predictions based on the Ericksen–Leslie constitutive equation for nematic fluids.

Rheo‐optical studies on aligned nematic solutions of a rodlike polymer

Static and dynamic polarized and depolarized light scattering characterization of dilute solutions of regioregular poly(3-dodecylthiophene) is described to study intermolecular association as a function of the thermal history of the solutions. It is shown that metastable aggregation obtains under all of the conditions studied, including temperatures as high as 65 °C. Under some conditions, the aggregated moiety appears to have a disklike shape at room temperature, with appreciable depolarized scattering, attributed to an extended chain structure, consistent with the formation of the lamellar suprastructure characteristic of the bulk, with the polythiophene chains adopting an extended conforma- tion and forming a nematic phase in a polythiophene lamella faced by lamellae rich in alkyl chains. Under other conditions, especially at low temperature, elongated supramolecular structures are formed, with the polythiophene chains in an extended conformation. The reversible thermochromic effect is associated with enhanced order of the alkyl side chains with decreasing temperature, facilitating coplanar conformers in the polythiophene backbones, with the attendant enhancement in the ﷿-﷿* transitions of the thiophene ring electronic absorption spectra. This behavior is analogous to the events in the thermochromic event in the bulk. Such supramolecular structures could intervene in normal film casting solution processing, with effects on the electronic or optical properties of the cast film. This interpretation of the light scattering data suggests strategies to enhance or suppress lamella formation in solvent-cast films.

/pdf/intermolecular-association-and-supramolecular-organization-2jtzmvf7yt.pdf

Guy C. Berry

Papers

The viscosity of polymers and their concentrated solutions

Thermodynamic and Conformational Properties of Polystyrene. I. Light‐Scattering Studies on Dilute Solutions of Linear Polystyrenes

Molecular brushes as super-soft elastomers

Rheo‐optical studies on aligned nematic solutions of a rodlike polymer

Intermolecular Association and Supramolecular Organization in Dilute Solution. 1. Regioregular Poly(3-dodecylthiophene)