Polyesters, network preparation

Figure 11. Mooney-Rivlin plot of stress-strain data (32) for three triol-based polyester networks prepared from sebacoyl chloride and LHT240 at various initial dilutions in diglyme as solvent. Conditions P100 is 0% solvent P130 is 30% solvent PI 65 is 65% solvent.

Polyester-based networks are typically prepared from polyester prepolymers bearing unsaturations which can be crosslinked. The crosslinking process is either an autoxidation in the presence of air oxygen (alkyd resins) or a copolymerization with unsaturated comonomers in the presence of radical initiators (unsaturated polyester resins). It should also be mentioned that hydroxy-terminated saturated polyesters are one of the basis prepolymers used in polyurethane network preparation (see Chapter 5). 

In the case of Fig. 7.6a the cluster formation and the size distribution can be influenced not only by chemical reactions but also by partial miscibility of the substructures during reaction. Polyurethane networks prepared from polyolefin instead of polyester or polyether as macrodiol, can serve as an example. In this particular case an agglomeration of hard domains takes place in the pregel stage, produced by a thermodynamic driving force. 

Diaz-Calleja. R.. Ricard, E., and Guzman, J.. Influence of static strain on DM behaviour of amorphous networks prepared from aromatic polyesters. J. Polymer. Science. F olymer Physics Edn. 23 (1986). 

When a three-dimensional network is prepared by polycondensation, a monomer with two functional groups is taken for the construction of linear chains, whereas a monomer with three or more functional groups is used for cross-linking. For example, in the preparation of polyester networks, some amount of polyol or multifunctional acid is copolymerized with the principal monomers diol and dicarboxylic acid (Figure 3). Thus, to get a polymer gel by polycondensation, at least one monomer should have more than two functional groups. 

To the best of our knowledge, no fully aromatic polyester networks have been reported in the literature until now. Several reports appeared describing the preparation of liquid crystal thermosets from rodlike polyester segments and branchpoints or junctions created by the reaction of reactive end-caps with one another [417-419]. These end-capped polyester networks will be discussed, however, with all other fully or mostly aromatic networks made in two steps by reacting the end-caps of stiff segments with each other. 

Over the years many blends of polyurethanes with other polymers have been prepared. One recent example is the blending of polyurethane intermediates with methyl methacrylate monomer and some unsaturated polyester resin. With a suitable balance of catalysts and initiators, addition and rearrangement reactions occur simultaneously but independently to give interpenetrating polymer networks. The use of the acrylic monomer lowers cost and viscosity whilst blends with 20% (MMA + polyester) have a superior impact strength. 

Liquid organic rubbers with reactive functionality can be prepared by several methods. End-functional oligomers are preferred. Chains attached to the network at only one end do not contribute as much strength to the network as those attached at both ends [34], Urethane chemistry is a handy route to such molecules. A hydroxy-terminated oligomer (commonly a polyester or a polyether) can be reacted with excess diisocyanate, and then with a hydroxy methacrylate to form a reactive toughener [35]. The methacrylate ends undergo copolymerization with the rest of the acrylic monomers. The resulting adhesive is especially effective on poIy(vinyl chloride) shown in Scheme 2. 

Siloxane containing interpenetrating networks (IPN) have also been synthesized and some properties were reported 59,354 356>. However, they have not received much attention. Preparation and characterization of IPNs based on PDMS-polystyrene 354), PDMS-poly(methyl methacrylate) 354), polysiloxane-epoxy systems 355) and PDMS-polyurethane 356) were described. These materials all displayed two-phase morphologies, but only minor improvements were obtained over the physical and mechanical properties of the parent materials. This may be due to the difficulties encountered in controlling the structure and morphology of these IPN systems. Siloxane modified polyamide, polyester, polyolefin and various polyurethane based IPN materials are commercially available 59). Incorporation of siloxanes into these systems was reported to increase the hydrolytic stability, surface release, electrical properties of the base polymers and also to reduce the surface wear and friction due to the lubricating action of PDMS chains 59). 

IPNs are found in many applications though this is not always recognised. For example conventional crosslinked polyester resins, where the polyester is unsaturated and crosslinks are formed by copolymerisation with styrene, is a material which falls within the definition of an interpenetrating polymer network. Experimental polymers for use as surface coatings have also been prepared from IPNs, such as epoxy-urethane-acrylic networks, and have been found to have promising properties. 

Instead of a, co-dihydroxylated polysiloxanes, other a, co-dihydroxy-lated organic polymers such as polyether, polyester, or polybutadiene can be used to prepare hybrid organic-inorganic networks. 

Fixation of dyestuffs and pigments by incorporation in the crosslinked cellulose and in the finish network, also providing better wet fastness for conventional dyeings and printings Cellulosics mainly with direct and acid dyes, cotton/polyester blends with reactive dyes, preparation for dry heat transfer printing of cellulosics ... 

Two- and three-component interpenetrating polymer network (IPN) elastomers composed of polyurethanes (PU), epoxies (E), and unsaturated polyester (UPE) resins were prepared by the simultaneous technique. Fillers and plasticizers were... 

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