Organic-Inorganic Polymer Associations

Organic-inorganic association systems are those in which there is no covalent bonding between the organic polymer and the inorganic network but the systems exhibit characteristics similar to systems which are covalently bonded. This synergistic relationship can occur through two types of associative behavior: hydrogen bonding, and association/coupling equilibrium.

A wide variety of polymers contain moieties which would ideally have the capability to hydrogen bond. Various polyesters, polycarbonates, polyacrylates, and polyamides possess moieties such as carbonyls, ethers, and amine groups which have free electrons available for hydrogen bonding. Yet most polymers, when mixed non-covalently with inorganic sol-gel systems, are completely phase separated.

Some polymers do exist (See Figure 1) which possess moieties which are capable of hydrogen bonding with an inorganic sol-gel network. 23 Hydrogen bonding of

Figure 1: Several Polymers that Interact with an Inorganic Phase

the sol-gel with these polymers retards severe phase separation during network densification. Researchers in this area of organic-inorganic associations have utilized the specific polymers that have hydrogen bonding capabilities in copolymers. The other polymer in the copolymer typically does not have the capability of interacting with an inorganic phase. In these copolymers, the hydrogen bonding polymer is used as a compatibilizer with the other polymer system which is of interest for inorganic modification. Ideally, the compatibilizing polymer, which is only a small quantity of the copolymer, will diffuse to the interface of the two immiscible polymer systems and reduce the interfacial tension. This reduction of the interfacial tension creates a better organic-inorganic composite interface by reducing the dispersed-phase particle size and increasing the adhesion between the two phases.

An association/coupling equilibrium occurs between polymer systems which hydrogen bond. Here there is a dynamic equilibrium between hydrogen bonded groups and a covalently bound inorganic network, producing -C-O-Si- bonds (See Figure 2).

Figure 2: Inorganic Association/Coupling Equilibrium with Polymers

The reason for the equilibrium is the hydrolytic instability of -C-O-Si- bonds. Generally, the equilibrium is shifted towards the hydrogen bonded moieties. Polymers containing carboxylic acid endgroups can also undergo this type of association/coupling equilibrium (See Figure 3). The carboxylic acid endgroups are

Figure 3: Endgroup "In-Situ" Association Materials

acidic enough to catalyze the reaction. The endgroups are then involved in the dynamic equilibrium between hydrogen bonded chain ends and covalently bound chain ends. This in situ behavior also occurs in template systems; where an ionomeric membrane is utilized as a morphological template for in situ sol-gel reactions.

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Written by: Sandra Young (Partially From Her Research Prospectus)

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