Polymers and polymer solutions

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Introduction

The Nobel Prize in Chemistry 1974 to Paul J. Flory:

"This year's Nobel prize in chemistry has been awarded to Professor Paul Flory for his fundamental contributions to the physical chemistry of macromolecules.

Macromolecules include biologically important materials such as cellulose, albumins and nucleic acids, and all of our plastics and synthetic fibers.

Macromolecules are often referred to as chain molecules and can be compared to a pearl necklace. They consist of long chains of atoms which, when magnified one hundred million times, appear as a pearl necklace. The pearls represent the atoms in the chain. One should realize that this chain is much longer than the necklaces being worn here this evening. To obtain a representative model of a macromolecule all of the necklaces here in this hall should be connected together in a single long chain.

One can readily appreciate that the development of a theory for these molecules presented considerable difficulties. The forms of the chain itself, whether extended or coiled, represents a property difficult to rationalize.

A statistical description is of necessity required, and Professor Flory has made major contributions to the development of such a theory. The problem is more difficult, however. How can one compare different molecules in different solvents?

When chain molecules are dissolved in different solvents they become coiled to different degrees, depending on the interaction between repulsive and attractive forces in the solution. In a good solvent the chain molecules are extended. In a poor solvent, in contrast, the chain molecules assume a highly coiled form.

Professor Flory showed that if one takes a solution of extended chain molecules in a good solvent, and slowly cools the solution, then the molecules become progressively more coiled until they are no longer soluble.

Thus, there must be an intermediate temperature where the attractive and repulsive forces are balanced. At this temperature the molecules assume a kind of standard condition that can be used, generally, to characterize their properties.

This temperature Professor Flory named the theta temperature. A corresponding temperature exists for real gases at which they follow the ideal gas law. This temperature is called the Boyle temperature after Robert Boyle who discovered the gas laws. By analogy, the theta temperature for macromolecules is often referred to as the Flory temperature.

Profssor Flory showed also that it was possible to define a constant for chain molecules, now called Flory's universal constant, which can be compared in significance to the gas constant."


Topics

Extended Reading

  • Cates
    • Chapter 3. Polymer physics: from basic concepts to modern
  • de Gennes (1990)
    • Chapter 1. General aspects of polymer chains
      • The physical states of flexible linear polymers:
        • It is a liquid of entangled chains at high temperature. p. 2
        • Upon cooling, most often the polymer forms a glass. p. 2
        • Under some favorable circumstances the polymer can crystallize upon cooling p. 2
        • Vulcanization of polymers involves joining some of the chains together and results in a random lattice of connected chains. Locally this network is still fluid, but macroscopically, the network resists compression with a non-zero elastic modulus. p. 3
      • "Polyelectrolytes are chains which carry a certain amount of charge. In the presence of salt (i.e., when the screening radius is small in comparison with the chain size) polyelectrolytes arrange themselves in almost classical coils." p. 9
      • "Molten polymers flow like (high viscous) ordinary liquids when they are acted upon by very slow perturbations. However, at slightly high frequencies, they behave like a rubber." "At small times, the knots open within the chain to not have time to break, resulting in a rubbery behavior. At large times, the knots open by Brownian motion: the chains slide past one another, resulting in a behavior similar to a liquid." p. 9-10
    • Chapter 2. Minimum number of aminoacids required to build up a specific receptor with a polypeptide chain.
  • de Gennes (1997)
    • Chapter 2. Mobile borders: the dynamics of wetting (or dewetting)
    • Chapter 3. Decorated borders: slippage between a solid and a polymer melt
      • When two polymer networks (or rubbers) are facing each other there are some mobile chains which wander around and may provide a transient bridge between the two sides. p. 17
      • Concerning a polymer melt facing a passive (negligible interaction) surface, "The polymer liquid has an enormous viscosity (because it is an entangled system), but it need not have a large friction coefficient: the wall friction acting on any unit in a polymer chain is still exactly the same as it would be in the corresponding simple liquid of monomers." This thinking seems to cast doubt on the no-slip condition normally invoked, but in the a majority of cases the assumption of a passive surface is not appropriate (the surfaces are too dirty). Though there are a number of cases where slippage can be observed. p 19-26
    • Chapter 5. Polymer/polymer welding.
      • For the joining of two identical polymers, the chain ends play a crucial role. p. 80
  • Jones (1999)
    • Chapter 2: The surface of a polymer melt
    • Chapter 5. Adsorption and surface aggregation from polymer solutions
    • Chapter 6. Tethered polymer chains in solutions and melts.
  • Norde
    • Chapter 12. Polymers
      • "Polymers in solution are lyophobic or reversible colloidal systems, which implies that the polymeric material dissolves spontaneously with a decrease in the Gibbs energy of the solution." p. 215
      • Dissolution of a polymer involves the following changes:
        • "Interactions between segments (monomers) of the polymer and between solvent molecules are disrupted and, on the other hand, interactions between polymer segments and solvent molecules are formed." p. 215
        • There is "an increased number of configurational possibilities and therefore higher configurational entropies of both components (solvent and polymer), that is, a positive mixing entropy." p. 216
      • The mixing entropy is due mainly to the solvent. p. 216
      • "The quality of the solvent, that is, the preference of polymer-solvent interactions over interactions between the pure components, and consequently, the solubility of the polymer may be altered by changing the composition of the solvent. For instance, for an aqueous solution the solvent quality usually decreases by adding (low molecular weight) electrolytes or alcohols. These additives compete for hydration with the polymer segments causing the polymers to become insoluble. This phenomenon is known as the salting-out effect. Salting-out is more effective when the additives are more strongly hydrated." p. 217
      • Considering dilute solutions where interactions between individual molecules are negligible, there are three extreme shapes the polymer can take depending on the strength of the interactions between the polymer chain with itself and/or the solvent:
        • When monomer-monomer interactions are favored, the polymer chain folds back on itself in a "compact sphere" to minimize monomer-solvent interactions. p. 218
        • In some cases, the polymer forms a stiff, rod-like shape (e.g. directive forces are involved which are maximally effective when aligned, such as hydrogen bonds in helical or pleated structures). p. 218
        • A highly solvated disordered coil-like structure is formed when polymer-solvent interactions are favored. p. 218
        • Block copolymers and random copolymers will fit somewhere in between these shapes depending on the details of the polymers chains. p. 218
      • "In aqueous solutions polyelectrolytes bear charged groups along their chains. If the charged groups are strong acids or strong bases the charge is essentially invariant with pH and the polymer is called a strong polyelectrolyte. Weak polyelectrolytes contain weak acid or base groups so that their charge depends on pH." "The presence of charge influences both inter- and intramolecular interactions." "Because of the electrostatic contribution, the persistence length varies with ionic strength." p.223-224
    • Chapter 13. Proteins
  • Rubenstein
    • Chapter 1.1-1.5 Introduction
    • Chapter 5. Polymer solutions
    • Chapter 7. Networks and gels.