Polyesters, network structure

Poly(vinyl acetate) (PVAc) is very often used for low-profile applications. At low PVAc contents, the continuous matrix is a polyester network with PVAc inclusions. Increasing the PVAc amount leads first to a bicon-tinuous structure, and then to a phase-inverted system (Chapter 8). The low-profile action is observed in the concentration range where bicontinuous structures are formed (Pascault and Williams, 2000). However, the fracture energy attains a maximum value for lower PVAc concentrations (Bucknall et al., 1991). 

Experimental results on reactions forming tri- and tetrafunctional polyurethane and trifunctional polyester networks are discussed with particular consideration of intramolecular reaction and its effect on shear modulus of the networks formed at complete reaction. The amount of pre-gel intramolecular reaction is shown to be significant for non-linear polymerisations, even for reactions in bulk. Gel-points are delayed by an amount which depends on the dilution of a reaction system and the functionalities and chain structures of the reactants. Shear moduli are generally markedly lower than those expected for the perfect networks corresponding to the various reaction systems, and are shown empirically to be closely related to amounts of pre-gel intramolecular reaction. Deviations from Gaussian stress-strain behaviour are reported which relate to the low molar-mass of chains between junction points. 

The principal feature that distinguishes thermosets and conventional elastomers from thermoplastics is the presence of a cross-linked network structure. As we have seen from the above discussion, in the case of elastomers the network structure may be formed by a limited number of covalent bonds (cross-linked rubbers) or may be due to physical links resulting in a domain structure (thermoplastic elastomers). For elastomers, the presence of these cross-links prevents gross mobility of molecules, but local molecular mobility is still possible. Thermosets, on the other hand, have a network structure formed exclusively by covalent bonds. Thermosets have a high density of cross-links and are consequently infusible, insoluble, thermally stable, and dimensionally stable under load. The major commercial thermosets include epoxies, polyesters, and polymers based on formaldehyde. Formaldehyde-based resins, which are the most widely used thermosets, consist essentially of two classes of thermosets. These are the condensation products of formaldehyde with phenol (or resorcinol) (phenoplasts or phenolic resins) or with urea or melamine (aminoplastics or amino resins). 

Depending on whether there is a linear chain or network structure in the final form involved, either saturated (PET-S/PBT-S), or unsaturated (PET-U/(PBT-U) types are considered. PET-S and PBT-S are thermoplastics (thermoplastic polyesters), while PET-U is a thermoset. 

For example, an anhydride hardener (with tertiary amine accelerator) will produce a polyester -type structure with low polarity, whereas an amine hardener will form a P-hydroxyl amine network with high polarity. Hydrogen bonding of water molecules at these sites enhances the concentration of moisture which can be absorbed. However, with catalytic curing agents, polyether stmctures of relatively low polarity will form. Since it is quite common to use mixed curing agents for composite matrices, the cured... 

Both paints and adhesives are commonly formulated as polymer blends or grafts. In fact, some compositions resemble semi-IPN s or AB crosslinked copolymers (Section 8.7). For example, epoxy adhesive resins are often cured with polyamides (Bikerman, 1968). The product is tougher than materials cured with low-molecular-weight amines, possibly because of a separate amide phase in this AB crosslinked copolymer. A more complex molecular architecture is exhibited by the alkyd resins common in oil-based paints (Martens, 1968, Chapters 3 and 4). The major component is a polyester, which often forms a network structure on drying. The polyester component is reacted with various drying oils, such as linseed oil or tung oil (Martens, 1968, Chapters 3 and 4). These oils form an ester link to the polyester structures and also polymerize through their multiple double bonds. Latex paints always contain thickeners, such as cellulosics, poly(acrylic acid), casein. 

When the number of repeating units in a polymer chain is low, that is when the molecular weight of the polymer is low (2000-10000 g mol ), the polymer is defined as a resin, provided it possesses sufficient numbers of active sites in its structures for chemical cross-linking to occur. The resins can form three-dimensional network structures if sufficient external energy (heat/light/radiation) is applied, with or without the use of any other chemical(s) in their finished state. They are free flowing materials of low viscosity. Polyester resins, epoxy resins, and polyurethane resins are examples of this type of polymer. This book contains descriptions of the different types of resins derived from various vegetable oils. 

Fisher, J.E, Tirnmer, M.D., Holland, T.A., Dean, D., Engel, P.S., and Mikos, A.G. (2003) Photoinitiated cross-linking of the biodegradable polyester poly(propylene fumarate). Part I. Determination of network structure. Biomacromolecules, 4 (5), 1327—1334. 

The neat polyester resin showed very low water uptake due to the three dimensionally cross-linked network structure after curing. The -OH group in the chain end of polyester and oxygen of the ester linkage influences the formation of hydrogen bonds. However it absorbs 0.05 mol% of water due to the presence of micro cracks and also due to the hydrophihc nature of polyester. The fiber-reinforced composites absorb water very rapidly at the initial stage and later a saturation level is attained, and there is no further increase in water absorption. As the flber content increased the water absorption also increased due to the hydrophilic nature of the fiber [101]. 

The free radicals initiate the chain reaction which propagates through the unsaturated sites of the polyester and monomer. This leads to the formation of a network structure which is insoluble and infusible (Figure 2.17). The cobalt compound, when present in excess, reacts with the free radicals and converts them into ions. 

The crosslinking density of the flexibilized epoxy resin systems is variable over a wide range, and the chemical incorporation of the flexibilizer precludes the migration frequently observed with plasticizers. Figure 19 shows part of an idealized network structure for epoxy-polyester copolymers. 

Mortaigne, B., Vivien, B. (1996). Influence of the styrene-maleate ratio on the structure-property relationships of unsaturated polyester networks. Polymers for Advanced Technologies, 7(10), 813-821. 

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