Cross-linking and chain-branching

In conventional organic compounds, any branching is characteristic of the molecular structure and is reflected in its lUPAC name. For instance, the alkane molecule 3-ethyl hexane has a branch in the structure, an ethyl group on the third carbon. With a polymer such as PE, short, medium length, long, or even complex branches can be 

FIGURE 3.6 Left An illustration of crystallites embedded in an amorphous polymer matrix. Right plastic crystals imaged by AFM. Source Reproduced with permission from Mouras et al. (2011). 

FIGURE 3.7 The change in elastic modulus of a semicrystaUine and amorphous polymers with the temperature. 

Individual polymer chains in an ensemble can also be covalently joined to other chains around it at discrete points along it. This yields a 3D network of chains (or open-tree structures of chains or a mix of both). Cross-linking is desirable where insolubility and high mechanical strength are demanded of aplastic. Ideally, each and every chain will be linked to each other so that the entire ensemble of chains is a single giant molecule (this actually does occur in natural rubber when vulcanized or cross-linked.) An automobile or aircraft tire is an example of a fully cross-linked polymer. On heating, cross-linked polymers do not convert into a viscous liquid melt as the molecules are chemically linked to one another and cannot flow independently. 

FIGURE 3.8 Illustration of different types of copolymers. Sections of polymer chains are shown and each circle represents a repeat unit, (a) Alternating copolymer, (b) random copolymer, (c) block copolymer, and (d) branched block copolymer. 

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In systems where the interactions are dominated by the van der Waals forces, solubility is favored by chemical similarity of solvent and polymer. Molecular weight, chain branching and cross-linking of individual polymers slightly influence the solubility parameter, 5 [Hildebrand, 1916]. In a strict sense, the molecular interactions should be non-specific, without forming associations or orientation, hence not of the hydrogen or polar type. The intermolecular interactions 1-2, are assumed to follow the geometric mean of the intramolecular interactions, 1-1 and 2-2 ... 

It becomes evident that with a reasonably simple monomer (chloroprene being a substituted butadiene), one must consider four structures for the incorporation of monomer units, two types of interunit connections, two potential comonomers, long-chain branching, and cross-linking, as well as molecular weight distribution. The sum of all of these structural variables determines, for the most part, the end-use properties of the polymer and aU are strong functions of polymerization conditions. 

When -xylene is used as the monomer feed in a plasma polymer process, PX may play an important role in the formation of the plasma polymer. The plasma polymer from -xylene closely resembles the Gorham process polymer in the infrared, although its spectmm contains evidence for minor amounts of nonlinear, branched, and cross-linked chains as well. Furthermore, its solubiUty and low softening temperature suggest a material of very low molecular weight (15). 

Glass-Transition Temperature. The T of PVP is sensitive to residual moisture (75) and unreacted monomer. It is even sensitive to how the polymer was prepared, suggesting that MWD, branching, and cross-linking may play a part (76). Polymers presumably with the same molecular weight prepared by bulk polymerization exhibit lower T s compared to samples prepared by aqueous solution polymerization, lending credence to an example, in this case, of branching caused by chain-transfer to monomer. 

The presence of pendant reactive vinyl groups through 1,2 and 3,4 addition provides a site for branching and cross-linking since these may be involved in other chain reactions. Because of this a 1,4 polymer is generally to be desired. 

Nonlinear structures may arise in vinyl polymerizations through chain transfer with monomer or with previously formed polymer molecules, but such processes usually occur to an extent which is scarcely significant. A more common source of nonlinearity in the polymerization of a 1,3-diene is the incorporation in a growing chain of one of the units of a previously formed polymer molecule. The importance of both branching by chain transfer and cross-linking by addition of a polymer unit increases with the degree of conversion of monomer to polymer. 

In the absence of adsorption, inclusion, or exclusion, a polymer is fractionated on a GPC column according to the hydrodynamic volume.40138 The hydrodynamic volume is a function of monomer identity, as well as polymer molecular weight, branching, and cross-linking. The polymer chains in any given chromatographic fraction have roughly the same hydrodynamic volume. 

Note Excess can lead to undesirable molecular branching and cross-linking. Chain branching can severely hamper the crystallization capability of PET and this may lead to a loss in performance properties. 

PET chain-extended with a diepoxide as chain extender can exhibit varying degrees of branching and cross-linking depending on the level of chain extender used [13, 14], The branched and cross-linked PETs exhibit significant improvement in tensile properties. 

The degradation and combustion behavior of polycarbonate/POSS hybrid system has been reported recently.48 Different loading contents of trisilanolphenyl-POSS (TPOSS) were melt blended with polycarbonate matrix (PC). The data shown in Table 8.4 indicate that no improvement in thermal stability parameters (i.e., onset decomposition temperature and peak decomposition temperature) was observed compared to the neat polycarbonate. The thermo-oxidative degradation process of the hybrid system proved to be a complicated process, which includes hydrolysis/alcoholysis of the carbonate linkage, free radical oxidative chain degradation, reformation, and branching and cross-linking reactions. 

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