General properties of polymer networks - Physical Gels from Biological and Synthetic Polymers - page 110

called the entanglement plateau, and appears to correspond to a molecular mass at which
the zero-shear viscosity begins to rise as M 3.4 . The plateau value is related to the density
of entanglements and it de

nes the molecular mass M e between entanglements. M e tends

20 × 10 3 g mol − 1 .
Entanglements are present in melts and solutions when the product of normalized
concentration (assumed to be 1 in the melt) and molecular mass is >M e .
When dealing with physical gels, the local structures created by cross-linking
mechanisms are complex and depend on the type of local association of the chains
(crystallization, secondary structure formation, ionic interactions). The junctions are
multi-functional, although the functionality is not a

to be speci c to each polymer type but is typically in the range 5 -

fixed number and the junctions

either have finite lifetimes or can be disrupted by changes in temperature, solvent

quality etc. As well as those association mechanisms which occur in nature, some
polymers have been specially designed to create labile network structures under con-
trolled conditions. These are the so-called associative polymers, of which the most
common are the telechelics, polymers that have been end-functionalized. Telechelic
polymers have generated much scienti c activity and many practical applications. Yet

another type of system forming transient networks is made up of

flexible, entangled

chains bearing a fixed number of ' stickers ' at fixed positions along the chains, which

can associate reversibly.
In the next sections we present three theoretical models which are generally used to
describe the rheological properties of the above transient networks.

4.4.1 Model polymers with sticky end groups

One class of system that has attracted widespread interest is ,and

they have found applications in many surface coating formulations. These are solutions of

' associative thickeners '

flexible, water-soluble chains which bear associating groups at both ends. A well-known
example is poly(ethylene glycol) (PEG) chains extended by diisocyanates and end-capped
with long-chain alkanols, also called hydrophobic ethoxylated urethane (HEUR) thickeners
(Lundberg et al., 1991 ). The composition of a HEUR thickener is shown in Figure 4.5 .
With increasing chain length (molecular mass), two distinct regimes have been
considered. In the un-entangled network, the molecular mass M between the temporal
junctions is smaller than the entanglement molecular mass M e of the polymer. Here the
major part of the stress is sustained by the elastically active chain strands connecting the
junctions. In the second case, where when M >M e , localized entanglements play a role
similar to cross-linked junctions.

Structure of a HEUR thickener (Lundberg, 1991). Reproduced with permission of the American
Institute of Physics for The Society of Rheology.

Figure 4.5