Page 284 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Copolymerization 247

The term “generation” describes the number of times “arms” have been extended. The nature of
each generation can be varied so that mixtures of steric requirements and hydrophobic/hydrophilic
character can be introduced, offering materials with varying structures and properties. By varying
the hydrophobic and hydrophilic interactions and steric nature of the arms, secondary and tertiary
structural preferences can be imposed on the dendrimer.
The dendrite structure can be used as a synthetic tool to craft a particular property into an overall
structure. For instance, structures have been formed that are a conducting rigid rod backbone with
dendritic structures radiating from the rigid rod. The backbone collects photons of visible light in
the range of 300–450 nm. The dendritic envelope collects light in the ultraviolet region from 220

to 300 nm and transfers it to the backbone that then fluoresces blue light at 454 nm. Unprotected
poly(phenyleneethynylene), the nondendritic backbone alone, does emit light but suffers from both

being brittle and from collisional quenching. The dendrimer product allows both flexibility and dis-
courages collisional quenching by forcing a separation between the backbone rods. This shielding
increases with each successive generation of the dendritic wedge.

7.16 IONOMERS

Ionomers are ion-containing copolymers typically containing more than 90% (by number) ethylene
units with the remaining being ion-containing units such as acrylic acid. These “ionic” sites are
connected through metal atoms. Ionomers are often referred to as processable thermosets. They are
thermosets because of the cross-linking introduced through the interaction of the ionic sites with
metal ions. They are processable or exhibit thermoplastic behavior because they can be reformed
through application of heat and pressure.
As with all polymers, the ultimate properties are dependant upon the various processing and
synthetic procedures that the material is exposed to. This is especially true for ionomers where
the location, amount, nature, and distribution of the metal sites strongly determines the properties.
Many of the industrial ionomers are made where a signifi cant fraction of the ionomer is unionized
and where the metal-containing reactants are simply added to the preionomer followed by heating
and agitation of the mixture. These products often offer superior properties to ionomers produced
from fully dissolved preionomers (Figure 7.4).
For commercial ionomers, bonding sites are believed to be of two different grouping densities.
One of these groupings involves only a few or individual bonding between the acid groups and the
metal atoms as shown in Figure 7.4. The second bonding type consists of large concentrations of acid
groups with multiple metal atoms (clusters) as shown in Figure 7.5. This metal-acid group bonding
(salt formation) constitutes sites of cross-linking. It is believed that the “processability” is a result of
the combination of the movement of the ethylene units and the metal atoms acting as “ball bearings.”
The “sliding” and “rolling” is believed to be a result of the metallic nature of the acid-metal atom
bonding. (Remember that most metallic salts are believed to have a high degree of ionic, nondirec-
tional bonding as compared with typical organic bonds where there exists a high amount of covalent,
directional bonding.) Recently, Carraher and coworkers have shown that the ethylene portions alone
are sufficient to allow ionomers to be processed through application of heat and pressure.

Ionomers are generally tough and offer good stiffness and abrasion resistance. They offer good
visual clarity, high-melt viscosities, superior tensile properties, oil resistance, and are fl ame retard-
ers. They are used in the automotive industry in the formation of exterior trim and bumper pads, in
the sporting goods industry as bowling pin coatings, golf ball covers, in the manufacture of roller
skate wheels, and ski boots. Surlyn (DuPont; poly(ethylene-comethacrylic acid)) is used in vacuum
packaging for meats, in skin packaging for hardware and electronic items (such as seal layers and as
foil coatings of multiwall bags), and in shoe soles.
Sulfonated ethylene–propylene–diene terpolymers (EPDM) are formulated to form a number of rub-
bery products, including adhesives for footwear, garden hoses, and in the formation of calendered sheets.

Perfluorinated ionomers marketed as Nafion (DuPont) are used for membrane applications, including

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