Chemical cross-links, modulus

Molecular movement above the Tg is restricted by crystallinity and, as with chemical cross-linking, the more the crystallinity, the more rigid the polymer. Some polymers tend to melt over a wide temperature range, in which case the modulus may fall over a range of temperatures leading up to the melting point T . The above effects are summarised in Figure 9.1. 

Networks with tri- and tetra-functional cross-links produced by end-linking of short strands give moduli which are more in accord with the new theory if quantitative reaction can be assumed (3...13) However, the data on polydimethylsiloxane networks, may equally well be analyzed in terms of modulus contributions from chemical cross-links and chain entangling, both, if imperfect reaction is taken into account (J 4). Absence of a modulus contribution from chain entangling has therefore not been demonstrated by end-linked networks. 

Figure 3. Modulus contributions from chemical cross-links (Cx, filled triangles) and from chain entangling (Gx, unfilled symbols) plotted against the extension ratio during cross-linking, A0, for 1,2-polybutadiene. Key O, GN, equibiaxial extension , G.v, pure shear A, Gx, simple extension Gx°, pseudo-equilibrium rubber plateau modulus for a polybutadiene with a similar microstructure. See Ref. 10.

The two-network method has been carefully examined. All the previous two-network results were obtained in simple extension for which the Gaussian composite network theory was found to be inadequate. Results obtained on composite networks of 1,2-polybutadiene for three different types of strain, namely equibiaxial extension, pure shear, and simple extension, are discussed in the present paper. The Gaussian composite network elastic free energy relation is found to be adequate in equibiaxial extension and possibly pure shear. Extrapolation to zero strain gives the same result for all three types of strain The contribution from chain entangling at elastic equilibrium is found to be approximately equal to the pseudo-equilibrium rubber plateau modulus and about three times larger than the contribution from chemical cross-links. 

Identical to chemically cross-linked (vulcanized) elastomers, the modulus of radiation cured gum elastomers depends on the concentration of elastically effective network strands and temperature. ... 

Conventionally, cross-link density is determined by measurements of the modulus, the glass transition temperature T, and by solvent uptake in swelling experiments. In these procedures, the chemical cross-link density cannot be discriminated from network-... 

In both cases of copolymerization, there is a noticeable decrease in the slope of the modulus curve in the r on of the inflection point. This, in essence, means a decrease in the modulus in the rubbery region. This contrasts with the chemically cross-linked systems where the modulus in the rubbery region shows some increase with increasing temperatures. In the copolymer system, the molecules are inter-coimected by physical cross-hnks due to secondary forces. These cross-hnks can be disrupted reversibly by heating, and this forms the basis of the new class of copolymers referred to as thermoplastic elastomers. 

By proper choice of either the isocyanate or the polyol, actual chemical cross-links can be introduced in either the hard or soft segments that may be beneficial to some properties. The effectiveness of these cross-links is offset by a disruption of the hydrogen bonding between polymer chains. Highly cross-linked polyurethanes are essentially amorphous in character exhibiting high modulus, hardness, and few elastomeric properties. Many adhesives fall into this category. 

Li et al. synthesized a PMMA-PEG semi-IPN by radical polymerization and cross-linking of PMMA in the presence of linear PEG, which exhibits two independent shape memory effects at two transition temperatures, the of the PEG crystal and the Tg of the semi-IPN [39]. In the IPN, a single Tg appeared due to the miscibility of the amorphous phase of the two polymers. Based on a reversible order-disorder transition of the crystals below and above the of PEG, and the large difference in storage modulus below and above the Tg of the semi-IPN, the polymer has a recovery ratio of 91 and 99%, respectively For the shape-memory behavior at the of PEG crystals, the fixing phase was the PMMA network and the reversible phase was PEG crystals. For the shape memory behavior at the Tg of the semi-IPNs, the fixing phase was the chemical cross-linked point, while the reversible phase was the PMMA-PEG complex phase. 

Figure 9.20. Elastic protein-based fibers a few hundred micrometers in diameter, prepared by extrusion and chemical cross-linking. Impressive mechanical properties of elastic modulus at 20%...

Post-Curing. Whenever production techniques or economics permit, it is recommended that compoimds based on terpolymer grades be post-cured for optimum properties. Relatively short press cures can be continued with an oven cure in order to develop full physical properties and maximum resistance to compression set. Various combinations of time and temperature may be used, but a cycle of 4 h at 175°C is the most common. The post-cure step increases modulus, greatly improves compression set performance, and stabilizes the initial stress-strain properties. During the post-cure step, the chemical cross-link is converted from an amide linkage to a more stable imide linkage. Peroxide-cured dipolymer compounds need not be post-cured. 

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