Prepolymer functionality, network

Table I. Correlation of Network Quality with Prepolymer Functionality...

Various factors such as prepolymer molecular weighty prepolymer functionality chemical conversion of crosslinking reaction and stoichiometry are related to the number of effective network chains (vg). One may speculate that the ultimate elongation and tensile strength are entirely determined by the number of effective network chains as is the case for networks in the rubbery state. However when all the data (Tables I to V) are plotted against calculated Ve no correlation is found. Similarly the variations in ultimate elongation or tensile strength cannot be related in any systematic fashion to the number of branch points or the number of chain ends per unit volume. 

The properties of a polymer network depend not only on the molar masses, functionalities, chain structures, and proportions of reactants used to prepare the network but also on the conditions (concentration and temperature) of preparation. In the Gaussian sense, the perfect network can never be obtained in practice, but, through random or condensation polymerisations(T) of polyfunctional monomers and prepolymers, networks with imperfections which are to some extent quantifiable can be prepared, and the importance of such imperfections on network properties can be ascertained. In this context, the use of well-characterised random polymerisations for network preparation may be contrasted with the more traditional method of cross-linking polymer chains. With the latter, uncertainties can exist with regard to the... 

It has been established by a variety of techniques that aromatic cyanate esters cyclotrimerize to form cross-linked cyanurate networks.1 Analogously, the fluoromethylene cyanate monomers cure to cyanurate networks. In addition to the 19F-NMR spectra shown in Figure 2.3, evidence includes an up-field shift of the methylene triplet (1H-NMR, 0.21 ppm 13C-NMR, 9.4 ppm), the disappearance of the cyanate functional group (IR, 2165 cm4 13C-NMR, 111.9ppm) and the appearance of the cyanurate functional group (IR, 1580 and 1370 cm4 13C-NMR, 173.6 ppm).9 Typically, monomers are advanced to prepolymers by thermal treatment at 120°C or just above the melting point. The prepolymers are then cured at 175°C and are postcured at 225°C. 

Polymeric silicones are extensively used in applications which require thermal stability and long-lasting retention of critical properties. They can be produced in various degrees of hardness and resiliency by combining prepolymer fluids, which contain reactive functional groups, in such ways as to form giant polymer networks with those desired properties. 

An analysis of network formation shows that terminal bifunctionality must be very close to the ideal of two functional groups per molecule to produce a high quality vulcanizate. The quality of a vulcanizate can be related to the required number average molecular weight, MR, of a polymer in an equivalent random network (6). MR values can be determined for an idealized network formed by coupling bi- or monofunctional prepolymers—i.e., no nonfunctional molecules—using the expression ... 

Mn = number average molecular weight of prepolymers XN = Nt/Mt = number average degree of polymerization Mr = (Xn)(Mn) = number average molecular weight of polymer in an equivalent random network A = mole fraction of difunctional prepolymer 1 — A = mole fraction of monofunctional prepolymer f = average functionality... 

The unreacted terminal epoxide groups can react with other diamine molecules to form a rigid network polymer. In this reaction the functionality of the bisphenol A-epichlorohydrin prepolymer 1-20 will be 2 since hydroxyl groups are not involved and the functionality of each epoxide group is one. 

This sudden gel formation is usually interpreted as the formation of a small per cent of infinite space network. The formation and control of the gel phase are vital in the processing of random thermosets and structosets. For example, too rapid gelation can cause poor bond strength in laminated and bonded structures. Too slow gelation could cause the collapse of a foam. The amount of reaction required for gelation can be controlled by the functionality of prepolymer units, /pp. 

In contrast to the linear thermoplastic polymers, which are soluble and fusible, the cross-linked network polymers are insoluble and infusible. They are formed from polymerizing systems containing monomers or prepolymers with a functionality of three or more. A good example is the phenol-formaldehyde resin systems. The cross-linking reaction takes place in the bond under applied pressure and heat, and the whole adhesive bond might consist of only one super giant molecule. Such resins are, therefore, called thermosetting resins. 

Degree of cure. The titration results given in Table III reveal that from 98% to 100% of the functional groups had reacted. Similarly no evidence for incomplete cure was observed by DSC or by dynamic mechanical spectroscopy (DMS). However, it may be pointed out that 2% unreacted functional groups could result in detectable incoherence (see reference 29) in the networks prepared from high-MW epoxy prepolymers. 

In some epoxy systems ( 1, ), it has been shown that, as expected, creep and stress relaxation depend on the stoichiometry and degree of cure. The time-temperature superposition principle ( 3) has been applied successfully to creep and relaxation behavior in some epoxies (4-6)as well as to other mechanical properties (5-7). More recently, Kitoh and Suzuki ( ) showed that the Williams-Landel-Ferry (WLF) equation (3 ) was applicable to networks (with equivalence of functional groups) based on nineteen-carbon aliphatic segments between crosslinks but not to tighter networks such as those based on bisphenol-A-type prepolymers cured with m-phenylene diamine. Relaxation in the latter resin followed an Arrhenius-type equation. 

In this study, the imperfection of the networks was varied by varying the stoichiometry of an Epon 828-methylene dianiline system. Related studies of morphology, other properties, and creep as a function of molecular weight and distribution of the prepolymer are described elsewhere (9-13). 

The silicone semi-lPNs consist of mixing a hydride-containing silicone prepolymer and a vinyl functionalized silicone polymer into a thermoplastic matrix such as PA, PBT, thermoplastic polyurethane (TPU) or styrene-ethylene/butylene-styrene (S-EB-S) block copolymer elastomer. The two silicone prepolymers co-react in the thermoplastic matrix during melt extrusion and injection molding to form a partially crosslinked network within the thermoplastic matrix. 

Hence a low molecular weight, reactive elastomer is normally used for impact modification of thermosets. The low molecular weight of the mbbery prepolymer aids its easy dissolution or dispersability in the thermosetting resin. The reactive functionality couples the rubber covalentiy to the growing polymer network during the curing reaction. Hence the rubber toughened thermosets may also be considered as co-reacted thermosets and not true blends. 

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