Properties Of Polymer Molecules Research Articles

A simulation approach to study of entropic elasticity properties of polymer chain molecules using atomic scale Monte-Carlo

This paper describes atomic scale Monte–Carlo studies of entropic elasticity properties of individual polymer chain molecules. An efficient numerical Monte –Carlo sampling approach is outlined and used to evaluate the entropic contribution to the total elastic force.Theoretic predictions of mechanical properties of polymer molecules, particularly complex bio-molecules (proteins, lipids, etc.), are difficult due to effects of entropic elasticity. Entropic elastic force can be a significant contributor to the free energy F of the polymer chain and can even exceed interatomic potential energy U under external mechanical load. Monte-Carlo based approach allows to achieve atomic resolution for molecular structure in contrast to analytical methods. Specific load-extension curves are obtained numerically for a group of molecules with degenerate potential energy profiles. Results of the atomistic modeling are compared with the limiting continuum model of the same type of polymers. The extent of the linear and nonlinear elastic regimes and dependence on the molecular weight and geometric parameters of the molecules are discussed. A significant divergence with the continuummodel behavior is observed at smaller bond angles for all elongationsof the molecule. Linearity of the entropic force exists in a wide range ofthe elongations, however, molecules with low gyration radii(densely packedpolymers) are linear mostly in extensionor unfolding, while very sparsely packed molecules are linear mostly in thecontraction mode. The achieved result cannot be reproduced withinthe settings of the continuum model and required an application of atomic scale Monte-Carlo approach developed by our group.

Synthesis of molecular objects: Two‐dimensional polymers

AbstractThroughout this century polymer science has studied the linear chain and its architectural derivatives which include familiar forms such as the branched chain and the three dimensional network. Other derivatives with unique properties have been investigated more recently and include macromolecular rings, dendrimers, stars, combs and ladders. The objective of this work is to depart from the focus on linear chains and explore the “bulk” synthesis and properties of polymer molecules that can be considered molecular objects. Ideal molecular objects should be macromolecules with well defined shapes that persist as systems transform reversibly from solids to melts or solutions. The limited access to conformational space which is required in order to define and maintain shape in liquid and solid states of the system is an unusual molecular characteristic in common polymers. Our ability to create such objects through bulk reactions of reasonable scale will undoubtedly extend the current boundaries of polymer science and technology. Shapes that are particularly interesting are those not common in the conformational space of linear chains, for example, two‐dimensional polymers shaped as plates and macromolecular bundles shaped as cylinders or parallelepipeds. The molecular object to be discussed in this lecture is a rigid and anisotropic two‐dimensional polymer with planar dimensions greater than its thickness and a shape‐granting skeleton built by covalent bonds. We have so far developed three different strategies for their synthesis, all involving systems in which reactive oligomers organize spontaneously into the necessary planar assemblies to form the object. In one strategy molecular recognition driven by homochiral interactions plays a key role in the formation of two‐dimensional polymers.1 A different methodology relies on entropy‐driven nanophase separation in rodcoil block molecules in which a rigid segment is covalently bonded to a flexible one sharing the same backbone. The third strategy involves the folding of oligomers into hairpin structures which self assemble into two‐dimensional liquids. The lecture will also describe examples of unique properties that could be achieved in materials containing these rigid two‐dimensional objects. These examples will include bulk materials with self‐organized surfaces and also remarkably stable nonlinear optical properties.

Synthesis and ordering of two‐dimensional polymers

AbstractThe work to be discussed in this lecture departs from the usual focus of polymer science on linear chains and explores the “bulk” synthesis and properties of polymer molecules that can be considered shape‐persistent molecular objects. Shapes that are particularly interesting are those not common in the conformational space of linear chains, for example, two‐dimensional polymers shaped as plates or discs and macromolecules shaped as cylinders or parallelepipeds. The lecture focuses on a molecular object which is a rigid and internally anisotropic two‐dimensional polymer with planar dimensions greater than its thickness. The shape‐granting skeleton of this two‐dimensional polymer is built by covalent bonds. We have so far developed three different strategies for their bulk synthesis, all involving systems in which reactive oligomers organize spontaneously into the necessary planar assemblies to form the object. In one strategy molecular recognition events such as homochiral interactions play a key role in the formation of the two‐dimensional flat polymers /1/. A different methodology relies on nanophase separation in rodcoil block molecules in which a rigid segment is covalently bonded to a flexible one sharing the same backbone. The third strategy involves the folding of oligomers into hairpin structures which self assemble into two‐dimensional liquids. In these two last strategies the layered rodcoils or hairpins react to form the covalent backbones necessary to grant shape to the object. A computer simulation relevant to the experimental system suggests that large two‐dimensional polymers can be formed by extremely short backbones. The lecture will also describe examples of unique properties in advanced materials that could emerge from these rigid two‐dimensional objects. These examples include. materials with self‐organized surfaces of high chemical definition and temporal stability, self assembling membranes, molecular reinforcement, and films with remarkably stable electrical or optical properties.

Contribution of the Torsional Modes to the Equilibrium Distribution of Vibrational States and to the Specific Heat of a Chain Molecule

Abstract The torsional modes of a chain molecule were studied recently under the assumption that it has a well defined spatial configuration at any time. Here we show how the averages over the possible configurations, for a distribution in thermal equilibrium, should be performed in the calculation of the thermodynamical properties. Our results show that the mean density of torsional states increases in the low frequency region with increasing temperature. The specific heat behaviour shows a considerable difference from the result obtained in the previous paper, where only the lowest energetic configuration was considered. The consequences of this result with respect to the configurational properties of polymer molecules near the θ temperature are discussed.

First produce a crystal

The physical properties of polymer molecules are intrinsically anisotropic since the repeat units are chemically bonded along the chain while interactions between chains are much weaker. This anisotropy is obscured by the complex morphology of most synthetic and natural polymers.

Conformational properties of polymer molecules with side chains. Polycetylmethacrylate (PCMA)

TH~ size and structure of the lateral groups of macromolecular chains is known to affect the conformation and optical properties of the latter [1]. A sufficient increase of the lateral monomer radical at the same degree of polymerization makes it possible to observe some enlargement of the molecular coil dimensions in solution; this can be attr ibuted to a larger equilibrium rigidity of its main chain. The optical anisotropy of the polymer molecule is particularly sensitive to structural changes of the group in the side chain. The simultaneous study of the hydrodynamic and optical properties of molecular chains having branches of different length can therefore give some information about the effect of the latter on the rigidity of the main as well as the lateral chains of macromolecules [2-4, 20-26]. Systematic investigations in this direction were made only in the series of polymethacrylic acid esters. Promising dimensional data on molecules were obtained for up to polyarylmethacrylate [5-7], while the optical anisotropy was determined only for the lower homologues, including the oetyl ester [8, 9]. One of the articles [10] on asymmetrical light scattering gave the dimensions of PCMA maeromolecules in heptane. The polymers used in this s tudy had relatively low mol.wt., which makes the reliability of the results rather doubtful. Also, of greatest interest are the conformation properties of the polymer representing the alkyl methacrylate series, which has the longest groups in the branch. Sedimentation and diffusion analysis, coupled with flow birefringence, were used in this s tudy of the conformation and optical properties of PCMA fractions.

成型加工における高分子の物性論

成型加工における高分子の物性論