Handbook of Polymer Synthesis Second Edition Episode 15 pdf

15 Crosslinking and Polymer Networks Manfred L. Hallensleben Institut fu ¨ r Makromolekulare Chemie, Universita ¨ t Hannover, Hannover, Germany I. INTRODUCTION In this article chemical crosslinking reactions are dealt with respect to network formation starting from individual monomeric, oligomeric or polymeric molecules. Any type of chemical reaction of functional groups may be used for the purpose of polymer network formation that allows for almost quantitative conversion of these functionalities; if not, considerable difficulties with respect to network stability, long term stability of physical properties of the network, and chemical transformation of unreacted functionalities may arise. Since the chemical and physical properties of a respective polymer network strongly depend on the chemical nature of the monomeric units in the chains and also depend on the crosslink density, a wide variety of physical properties of crosslinked polymeric materials is available and considerable technological input is made to design the network in order to match the demands. This contribution to chemical crosslinking does not include the use of electron beam or g-irradiation. These methods have some advantages over the use of chemical crosslinking agents as they do not leave behind toxic, elutable agents. Also it does not include peroxide initiated radical crosslinking of saturated polymers which proceeds randomly by hydrogen abstraction from chain segments and coupling reactions of these radical sites. This contribution does also not include ‘physical’ almost reversible crosslinking due to microphase separation of block copolymers, to strong hydrogen bonding or to ionic interactions or to crystallite formation. II. DEFINITION OF POLYMER NETWORKS Any formation of a polymer network starts from monomeric, oligomeric or polymeric individual molecules which react in solution, in melt or in the solid state. It is necessary that at least a small fraction of these molecules has a functionality f  3 to undergo bond formation with another individual. From each individual molecule may emanat e zero to f bonds to neighboring molecules and thus this molecule may participate in the formation of a large cluster of molecules which is called a macromolecule. In the so-called sol–gel transition, an infinitely large macromolecule is formed. This infinitely large macromolecule Copyright 2005 by Marcel Dekker. All Rights Reserved. is called a gel whereas a collection of finite clusters is called a sol independently from the fact that the gel may be formed by crosslinking the molecules in the solid state. A gel usually coexists with a sol: the finite clusters are then trapped in the interior of the gel. Gelation is the phase transition from a state without a gel to a state with a gel, i.e., gelation involves the formation of an infinite network [1,2,5,6–9]. The conversion factor p (see W. H. Carothers, p ¼ extent of reaction) is the fraction of bonds which have been formed between the monomer s of the system, i.e., the ratio of the actual number of bonds at the given moment to the maximally possible number of such bonds. Thus, for p ¼ 0, no bonds have been formed and all monomers remain isolated 1-clusters. In the other extre me, p ¼ 1, all possible bonds between monomers have been formed and thus all monomers in the system have clustered into one infinite network, with no sol phase left. Thus for small p no gel is present wher eas for p close to unity one such network exists. The gel is, in fact, considered as one molecule. Therefore, there is in general a sharp phase transition at some intermediate critical point p ¼ p c , where an infinite cluster starts to appear: a gel for p above p c , a sol for p below p c . This point p ¼ p c is the gel point and may be the analog of a liquid –gas critical point: For p below p c , only a sol is present just as for T above T c only a supercritical gas exists. But for p above p c , sol and gel coexist with each other; similarly for T below T c vapor and liquid coexist at equilibrium on the vapor pressure curve. However, we do not assert that these thermal phase transitions and gelation have the same critical behavior. Also, in gelation there is no phase separation: Whereas the vapor is above the liquid, the sol is within the gel. The liquid–gas transition is a thermodynamic phase transition whereas gelation deals with geometrical connections (i.e., with bonds). At least in simple gelation models the temperature plays only a minor role co mpared with its dominating influence on the thermodynamic phase transitions. Such simple gelation theories often make the assumption that the conversion p alone determines the behavior of the gelation process, though p may depend on temperature T, concentration c of monomers, and time t. Early theoretical approaches to the gel-formation [1–4] as the Flory–Stockmayer theory do not take into account several aspects which naturally occur as the individual molecules grow to form the gel, such as cyclic bond formation, excluded volume effects and steric hinderance. The Flory–Stockmayer theory assumes that in the gelation process each bond between two individual monomeric, oligomeric or polymeric molecules is formed randomly. Thus this theory assumes point-like monomers. This apparently is not the case when already existing macromolecules are crosslinked, i.e., in vulcanization reactions as well as in copolymerization reactions of macromolecules with the functionality f  3 with bifunctional monomers. Besides the polymer networks which are generated from homogeneous solution or in bulk either by crosslinking processes of already existing pre-polymers or by crosslinking copolymerization reactions and which are completely insoluble in any solvent, there are also existing network particles in much smaller dimensions which are called microgels and which form in very dilute solution in copolymerizing a monofunctional and a difunctional vinyl monomer, e.g., such as styrene and divinylbenzene, or which are formed in emulsion copolymerization of such comonomers. A microgel is an intramolecularly crosslinked macromolecule which is dispersed in normal or colloidal solutions, in which, depending on the degree of crosslinking and on the nature of the solvent, it is more or less swollen [10]. The IUPAC Commission on Macromolecular Nomenclature recommended mic ronetwork as a term for microgel [11] and defi ned it as a highly ramified macromolecule of colloidal dimensions. However, ‘micro’ refers to dimensions of more than one micrometer whereas the dimensions of the so-called microgels are in the range of nanometers. Copyright 2005 by Marcel Dekker. All Rights Reserved. Probably most network structures obtained by copolymerization reactions of bifunctional monomers and larger fractions of monomers with a higher functionality are inhomogeneous, consisting of more densely crosslinked domains embedded in a less densely crosslinked matrix, often with fluent transitions. Besides the inhomogeneity due to a non-uniform distribution of crosslinks, other inhomogeneities due to pre-existing orders, network defects (unreacted groups, intramolecular loops and chain entanglements) or inhomogeneities due to phase separation during the crosslinking process may contribute to network structures [7]. It may be concluded therefore that network inhomogeneity is a widespread structural phenomenon of crosslinked polymers. For any existing polymer network the most important parameters are the crosslink density, the functionality of the crosslinks, that is the number of elastic network chains tied to one given crosslink, the number of dangling chains (with only one end attached to the network), molecular weight and molecular weight distribution of the elastic chains in the network, the number of loops and the number of trapped entanglements. III. THEORETICAL CONSIDERATIONS Polymerization reactions comprising monomers of the A–B plus A f type (with f > 2) in the presence of B–B monomers will lead not only to branching but also to a crosslinked polymer structure. Branches from one polymer molecule will be capable of reacting with those of another polymer molecule because of the presence of the B–B reactant. Crosslinking can be pictured as leading to the structure I in which two polymer chains have been joined together (crosslinked) by a branch. The branch joining the two chains is referred to as a crosslink. Copyright 2005 by Marcel Dekker. All Rights Reserved. A crosslink can be formed whenever there are two branch es that have different functional groups at their ends, that is, one has an A group and the other a B group. Crosslinking will also occur in other polymerization reactions involving reactants with functionalities f greater than two. These include the polymerizations A A þ B f ! A A þ B B þ B f ! A f þ B f ! In order to control the crosslinking reaction so that it can be used properly it is important to understand the relationship be tween gelation and conversion, that is consumption of monomers and/or functional groups, that is also called extent of reaction. Two general approaches have been used to relate the extent of reaction at the gel point to the composition of the polymerization system based on calculating when X n and X w , respectively, reach the limit of infinite size. X n !1 The first one considering the gel point when the number average degree of polymerization X n becomes infinite X n !1in a polycondensation reaction was given by the pioneer W. H. Carothers himself [12]. This approach is based on the simple assumption that the reactive groups in the system only are consumed by chemical reaction; no branching or cyclization events are taken into account. If the average functionality of all functional groups present in the system of two monomers A and B in equimolar amounts is named f avg , the average functionality of a mixture of monomers is the average number of functional groups per monomer molecule and is given by f avg ¼ X N i f i . X N i which of course is the general formula to calculate the average specifics of a great number of individuals. Thus for a system consisting of 2 moles of lycerol (a triol, f ¼ 3) and 3 moles of adipic acid (a diacid, f ¼ 2), the total number of functional groups is 12 per 5 monomer molecules, and f avg therefore simply is 12/5 or 2.4. For a system consisting of equimolar amounts of glycerol, adipic acid, and acetic acid (a monoacid), the total number of functional groups is 6 per 3 monomer molecules and f avg simply is 6/3 or 2. In a system containing stiochiometric numbers of A and B groups, the number of monomer molecules present initially is N 0 and the corresponding total number of functional groups is N 0 f avg .IfN is the number of molecules after reaction has occurred, then 2(N 0 N) is the number of functional groups that have reacted. The extent of reaction p is the fraction of functional groups lost p¼ 2ðN 0  NÞ=N 0 f avg while the degree of polymerization is X n ¼N 0 =N Copyright 2005 by Marcel Dekker. All Rights Reserved. This is the so-called Carothers equation which relates the degree of polymerization to the number of mo lecules present in the polymerizing system. From combination of both these equations it follows that X n ¼ 2=2  pf avg or by rearrangement p¼ 2=f avg  2=X n f avg This equation is equivalent to the Carothers equation, and in this expression it relates to the extent of reaction and degree of polymerization to the average functionality f avg of the system. At the gel point the number average degree of polymerization X n becomes infinite and therefore the secon d term in the previous equation is zero. Thus, the critical extent of reaction p c at the gel point is given by p c ¼ 2=f avg This equation allows us to calculate the extent of reaction to which the reaction has to be pushed to reach the onset of gelation in the reaction mixture of reacting monomers from its average functionality. In the example given above of reacting a dibasic acid, adipic acid, with a trifunctional alcohol, glycerol, which is of the type A 2 B 3 , we have to take 2 moles of glycerol and 3 of adipic acid, or 5 altogether, containing 12 equivalents and f avg ¼ 12/5 ¼ 2.4. Then at X n ¼1, p ¼ 2/2.4 and the limit of reaction will be 5/6 ¼ 0.833. This, in fact, represents the maximum amount of reaction that can occur before gelation under any distribution of combinations, provided only, that the reaction is all intermolecular. X w !1 Flory [1,2] and also Stockmayer [3,4] used a statistical approach to derive an expres- sion for predicting the extent of reaction at the time where gelation will occur by calculating when X w approaches infinite size. This statistical approach in its simplest form assumes that the reactivity of all functional groups of the same type is the same and independent of molecular size and shape. It is further assumed that there are no intramolecular reactions between functional groups on the same molecule such a s cyclizatio n reactions. For the ease of demonstration how the branching reaction in a step-growth polymerization reaction of A–A þ B–B þ A f molecules proceeds, Flory has used a simple picture to sketch the branching procedure which at some critical point finally leads to gelation [13] A A þ B B þ A f ! A ð f1 Þ AðB BA AÞ n B BA A ð f1 Þ The center unit in Figure 1 is given by the segme nt to the right of the arrow with the two A f at the end as branching sites. Infinite networks are formed when n number of chains or chain segments give rise to more n chains through branching of some of them. The criterion for gelation in a system containing a reactant of functionality f is that at least Copyright 2005 by Marcel Dekker. All Rights Reserved. one of the ( f  1) chain segments radiating from a branch unit will in turn be connected to another branch unit (note: f is not identical to f avg used by Carothers [12]). The probability for this occurring is simply 1/( f  1) and the critical branching coefficient a c for gel formation is a c ¼ 1ð f  1Þ When a( f  1) equals 1, a ch ain segment will, on average, be succeeded by a( f  1) chains. Of these a( f  1) chains a portion a will each end in a branch point so that a 2 ( f  1) 2 more chains are creat ed. The branching process continues with the number of succeeding chains becoming progessively greater through each succeeding branching reaction. If all groups (of the same kind) are equally reactive, regardless of the status of other groups belonging to the same unit, the probability P A that any particular A group has reacted equals the fraction of the As which have reacted; similarly, P B is defined. If r is the ratio of all A to all B groups, then P B ¼rP A since the number of reacted A groups equals the number of reacted B groups. The probability that a given functional group (A) of a branch unit is connected to a sequence of 2n þ 1 bifunctional units followed by a branch unit is ½P A P B ð1  rÞ n P A P B r where r is the ratio of As belonging to branch units to the total number of As. Then a ¼ X 1 n¼0 P A P B ð1  rÞ½ n P A P B r ¼ P A P B r=½1  P A P B ð1  rÞ ¼ rP 2 A r=½1  rP 2 A ð1  rÞ ¼P 2 B r=½r  P 2 B ð1  rÞ Figure 1 Schematic representation of a trifunctionally branched three-dimensional polymer molecule [13]. Copyright 2005 by Marcel Dekker. All Rights Reserved. It will depend on the analytical circumstances which of the unreacted groups, A or B, is the one to determine which of the equations will be used. Combination of a c ¼ 1( f  1) and a ¼ rP 2 A r /[1rP 2 A (1  r)] ¼ P 2 B r /[r  P 2 B (1  r)] yields a useful expression for the extent of reaction (of the A functional groups) at the gel point p c ¼1=fr½1 þ rð f  2Þg 1=2 When the two functional groups are present in equivalent numbers, r ¼ 1 and P A ¼ P B ¼ P, then a ¼ P 2 r=½1  P 2 ð1  rÞ and p c ¼ 1=½1 þ rð f  2Þ 1=2 In the reaction of glycerol, f ¼ 3, with equivalent amounts of several diacids, the gel point was observed [14,15] at an extent of reaction of 0.765. The predicted values of p c are 0.709 and 0.833 calculated from [13] (Flory, statistical) and [12] (Carothers), respectively. Flory [13] studi ed several syst ems composed of diethylene glycol ( f ¼ 2), 1,2, 3-propane- tricarboxylic acid ( f ¼ 3), and either succinic or adipic acid ( f ¼ 2) with both stoichiometric and nonstoichiometric amounts of hydroxyl and carboxyl groups, see Table 1. The observed p c values as in many other similar systems fall approximately midway between the two calculated values. The Carothers equation [12] gives a high v alue for p c . The experimental p c values are close to but always higher than those calculated from the Flory equation [13]. Two reasons can be given for this difference: first the occurence of intramolecular cyclization and second unequal functi onal group reactivity. Both factors were ignored in the theoretical derivations for p. Although both the Carothers and statistical approaches are used for the practical prediction of gel points, the statistical approach is the more frequently employed. The statistical method is preferred, since it theoretically gives the gel point for the largest sized molecules in a size distribution. Some theoretical evaluations of the effect of intramolecular cyclization on gelation have been carried out [6,16,17]. The main conclusion is that, although high reactant concentrations decrease the tendency toward cyclization, there is at least some cyclizati on occurring even in bulk polymerizations. Thus, even after correcting for unequal reactivity of functional groups, one can expect the actual p c in a crosslinking system to be larger than a calculated p c value. Table 1 Gel point for polymers containing tricarboxylic acid [13]. Extent of reaction at gel point ( p c ) r ¼ [CO 2 H]/[OH] r Calculated from [12] Calculated from [13] Observed 1.000 0.293 0.951 0.879 0.911 1.000 0.194 0.968 0.916 0.939 1.002 0.404 0.933 0.843 0.894 0.800 0.375 1.063 0.955 0.991 Copyright 2005 by Marcel Dekker. All Rights Reserved. IV. CROSSLINKING — CONCEPT Among all crosslinking strategies which are used to synthesize polymer networks, three different classes are in common application: 1. One-shot crosslinking of multifunctional monomers or copolymerization with difunctional monomers, 2. two-stage crosslinking via prepolymers, 3. crosslinking of high molecular weight polymers. Into the first category of crosslinking strategies fall the formation of poly(styrene-co- divinylbenzene) resins, the methacrylic resins and some others, and among those also a small fraction of the so-called microgels. In general, these resins are formed of monomers which in linear polymerization lead to thermoplastic polymers such as poly(styrene), polyacrylics or methacrylics a.s.o. High glass trans ition temperature of the linear polymers and high melt viscosity makes it unattractive to process premade linear thermoplastics prior to a second step of crosslinking reaction. Incorporation of pendant C–C– double bonds into the linear chains by copolymerization with small quantities of a difunctional monomer and thereby avoiding early stage crosslinking is difficult to handle and such polymers would be very sensitive to undergo uncontrolled network formation. One-shot crosslinking of multifunctional monomers and copolyme rization therefore is limited to the radical induced copolymerization of styrene and some derivatives with divinylbenzene or of methacrylates with ethyleneglycol dimethacrylate as crosslinker in suspension polymerization to form densely crosslinked polymer beads for applications such as ion exchange resins, Merrifield resins, polymer supports for chemical reagents especially with the aspect of combinatorial syntheses. Into the second category of crosslinking strategies fall the processes of preparing polymer networks which make use of prepolymers. These are two-stage processes in which in the first stage, overhelmingly in step-growth polymerization reactions, prepolymers are prepared with molecular weight mostly ranging from 1 to 6  10 3 which are soluble in organic solvents, fusible and have low melt viscosity. The second stage curing is achieved either by heat — thermosetting — or, when necessary, by the addition of appropriate curing agents. Most prominent examples are epoxy resins, phenol-formaldehyde resins, unsaturated polyesters, and the polyurethane networks. Into the third catagory fall the vulcani zation reactions of elastomers. These polymers expose C–C double bonds incorporated in the main chain segments which are necessary for the crosslinking process referred to as vulcani zation. Natural rubber and the synthetic elastomers have glass transition temperatures far below the temperature range in which the crosslinked rubbers are used. The molecular weight of the applied polymers is in the range of 2–5  10 5 , and natural rubber with an upper molecular weight fraction of 2–4  10 6 has to be degraded to this molecular weight level by mechanical treatment referred to as mastication. The basis of all processes that come after mastication and before vulcanization are the operations of blending rubber mixtures, mixing with all the vulcanization ingredients, calendering, frictioning, extrusion, moulding and combining with textile fabrics or cords is the flow or viscous deformation of the rubber, more precisely the rheological behavior. Extrusion, calendering and frictioning all involve vigorous mechanical working in large machines and hence enormous energy consumption and heat generation. Copyright 2005 by Marcel Dekker. All Rights Reserved. A. General Class ification of Prepolymers [18] Curing reactions applied to epoxy prepolymers, unsaturated polyesters, resoles, and novolacs make use of three general classes of prepolymers which are distinguished by the number and location of sites of functional groups available for subsequent crosslinking reac- tions. These three general classes have been defined as discussed in the following sections. 1. Random prepolymers. Random prepolymers are those built up from polyfunc- tional step-growth monomers which have been reacted randomly and which are capable of forming crosslinked polymers directly. Monomer conversion in the first-stage polymerization reaction for the formation of these prepolymers is stopped short and kept below the critical conversion at which network formation woul d occur. Crosslinking in the second-stage, step-growth poly- merization reaction is achieved simply by heating to carry the original reaction past the critical conversion. For this reason, the term thermoset is applied to these prepolymers, and these are exemplified by the phenol-formaledehyde resole resins and the glycerol polyesters. The term structoset has been applied to the other two classes of prepolymers to distinguish them from the thermoset type because in the other two classes the second-stage crosslinking reaction requires the addition of a catalyst or monomer, and generally proceeds by a reaction different from the first-stage reaction. 2. Structoterminal prepolymers. Structoterminal prepolymers are those in which the reactive sites are located at the ends of the polymer chains. These first-stage polymers give maximum control of the length and type of chain in the final network polymer. The epoxy prepolymers may be considered examples of this class if the second-stage reaction occurs overwhelmingly through reaction of the terminal epoxide functional groups. If the aliphatic hydroxyl groups along the chain in epoxy prepolymers become significantly involved in the crosslinking reaction, then these polymers are more properly included in the third class of prepolymers. 3. Structopendant prepolymers . Structopendant prepolymers are those in which the crosslink sites are distributed in either a regular or random order along the chain. Examples of this class are the unsaturated polyesters and the novolac resins. V. PHENOL-FORMALDEHYDE RESINS Phenol-formaldehyde condensates were among the first synthetic polymeric materials on the market. It was Baekeland at the beginning of the 20th century who in 1907 defined the differences between basic or acidic react ion conditions and the different molar ratios on the reaction procedure and the resulting molecular structure. He was able to manufacture a thermosetting resin and made applications for a patent [19] (Bakelite). Most phenolic resins are heat hardenable or thermosetting. The resin may be delivered to the user ready to be cured or it may be in the temporarily thermoplastic novolac form to which a hardener, commonly hexamethylenetetramine–urotropin, will be added. The major categories of uses for phenolics are  Molding compounds  Coatings  Industrial bonding resins. Copyright 2005 by Marcel Dekker. All Rights Reserved. The latter includes resins for grinding wheels and coated abrasives, laminating, plywood adhesives, glass wool thermal insulation and bonded organic fiber patting, foundry sand bonding, wood waste bonding, and other miscellaneous applications. A. Reaction of Phenol and Formaldehyde Under Basic Conditions The base-catalyzed first-step reaction of phenol ( f ¼ 3, because reaction can take place in two ortho and one para position) and formaldehyde ( f ¼ 2) with an excess of formaldehyde of about 15 mol% closely resembles an aldol addition and yields mixtures of monomolecular methylolphenols and also dimers, trimers and the corresponding polynuclear compounds according to a generalized reaction scheme given in (1b). In commercial processes formaldehyde is added in aqueous solution. Sodium hydroxide, ammonia and hexamethylenetetramine–urotropin, sodium carbonate, calcium-, magnesium-, and barium-hydroxide and tertiary amines are used as catalysts. After the hydroxybenzyl alcohol has been formed in the first step, the condensation steps to form oligomers are likely to be a Michael type of addition to a base-induced dehydration product of the hydroxybenzyl alcohol. Detailed studies have been presented by Martin [20] and Megson [21]. ð1Þ Copyright 2005 by Marcel Dekker. All Rights Reserved. [...]... entering of different epoxy groups into the reaction, irrespective of the acidic or basic character of the curing agent, is a very important feature of the crosslinking process of ACECs because it conditions the formation of a regular polymer network [51] B Curing The epoxy prepolymers are considered as structopendant prepolymers because of the pendant aliphatic hydroxyl groups or as structoterminal prepolymers... of a small amount of a polyfunctional alcohol In the presence of small quantities of water, carbon dioxide is liberated from hydrolysis of some isocyanate groups and acts as a foaming agent in polyurethane foam production Two-stage crosslinking, in which in the first stage is the synthesis of a prepolymer containing two isocyanato endgroups in the classical way of reaction (a diol either of low or of. .. involvement of zinc in increasing the efficiency of crosslinking is regarded to occur by some chelation of zinc with electron donating sites as ligands L such as amines, carboxylic groups of the activator, or the double bonds of the polymer chain XIV CONCLUDING REMARKS Crosslinking reactions to form polymer networks has already created new polymeric materials in prehistoric time measured on the time-scale of polymer. .. the development of any kind of absorption chromatography The first chemical synthesis with the support of a crosslinked polymer carrier was reported in 1963 [147] by Robert Bruce Merrifield and the Nobel Prize awarded to him in 1984 underlines the importance of this first example of a solid phase synthesis on polymer support which was followed by the development of numerous polymeric reagents [148] Presently,... oligomers with a degree of polymerization up to 15 or 20, but it is also possible to prepare high molecular weight linear polymers from this reaction by careful control of monomer ratio and reaction conditions [45] The two ring-opening reactions occur almost exclusively by attack of the nucleophile on the primary carbon atom of the oxirane group [46] Depending on the conditions of the polymerization reaction,... class of crosslinked polymers are prepared by a two-step polymerization sequence The first step which provides prepolymers, or more exactly: preoligomers, is based on the step-growth polymerization reaction of an alkylene epoxide which contains a functional group to react with a bi- or multifunctional nucleophile by which prepolymers are formed containing two epoxy endgroups In the second step of the... agents, and because of the great number of hydroxyl groups in the prepolymer, curing with dianhydrides can form very densely crosslinked, second- stage polymers if used in relatively high concentrations The prepolymer is a structoterminal prepolymer when amines are used as crosslinkers Crosslinking in this case involves the base-catalyzed ring-opening of the oxirane groups Both primary and secondary amines... formation of the methylene bridges B Curing of Resol Prepolymers Heat curing of resols usually is carried out at temperatures in the range 130–200  C Below 150  C the formation of dibenzyl ether bridges is predominant whereas at higher temperatures methylene bridge formation is favored This was nicely shown by the investigations of Kammerer et al who carried out polycondensation reaction of 2,6¨ bis(hydroxymethyl-4-methylphenol... Methods for Preparing Microgels 1 Emulsion (co)polymerization of Monomers Emulsion polymerization — macroemulsion or microemulsion — is the most efficient synthetic route to prepare microgels In emulsion polymerization, the dimensions of the micelles as the micro-continuous reactors in which conversion of monomers to polymers is performed, determine the size of the netted particles Hence, although these... reaction conditions formation and reaction of alkoxide ions become competitive and polymer chain branching may occur Polymers of this type with molecular weight exceeding 8000 are undesirable because of their high viscosity and limited solubility, which make processing in the secondstage, crosslinking-reaction difficult to perform The oligomers of the diglycidylether of bisphenol-A (DGEBA) are the most commonly . oligomeric or polymeric molecules. Any type of chemical reaction of functional groups may be used for the purpose of polymer network formation that allows for almost quantitative conversion of these. microphase separation of block copolymers, to strong hydrogen bonding or to ionic interactions or to crystallite formation. II. DEFINITION OF POLYMER NETWORKS Any formation of a polymer network starts. crosslinked polymer structure. Branches from one polymer molecule will be capable of reacting with those of another polymer molecule because of the presence of the B–B reactant. Crosslinking can be pictured