Network cross-links temperature dependency

Linear polymers are often thermoplastic, meaning they can flow at some temperature depending on the molecular weight. Polymers need not be simple chains but can be branched or networked. Cross-linked polymers can be gels that swell in a solvent or thermosets, which form three-dimensional networks for example, epoxy resins. This interconnectedness allows long-range coupling. 

The photomechanical effect of stretched rubbery networks cross-linked by isomerizable spirobenzopyran chromophores [see Table 4 Structure (14)] has been examined [49,54]. The effect was studied under constant stress at constant temperature by following the contraction as a function of time of irradiation. On irradiation, 2% contraction was observed in the dark, length recovery occurs. The photomechanical effect increased with decreasing temperatures and showed a maximum at optimum stress, depending on value of the network. The light-dark cycle could be repeated several times and was reversible. 

We noted above that the presence of monomer with a functionality greater than 2 results in branched polymer chains. This in turn produces a three-dimensional network of polymer under certain circumstances. The solubility and mechanical behavior of such materials depend critically on whether the extent of polymerization is above or below the threshold for the formation of this network. The threshold is described as the gel point, since the reaction mixture sets up or gels at this point. We have previously introduced the term thermosetting to describe these cross-linked polymeric materials. Because their mechanical properties are largely unaffected by temperature variations-in contrast to thermoplastic materials which become more fluid on heating-step-growth polymers that exceed the gel point are widely used as engineering materials. 

Such considerations appear to be very relevant to the deformation of polymethylmethacrylate (PMMA) in the glassy state. At first sight, the development of P200 with draw ratio appears to follow the pseudo-affine deformation scheme rather than the rubber network model. It is, however, not possible to reconcile this conclusion with the temperature dependence of the behaviour where the development of orientation reduces in absolute magnitude with increasing temperature of deformation. It was proposed by Raha and Bowden 25) that an alternative deformation scheme, which fits the data well, is to assume that the deformation is akin to a rubber network, where the number of cross-links systematically reduces as the draw ratio is increased. It is assumed that the reduction in the number of cross-links per unit volume N i.e. molecular entanglements is proportional to the degree of deformation. 

Whereas polymers of sufficiently high molecular weight may be soluble in the common solvents with some difficulty, network polymers do not dissolve, even at elevated temperature. They usually swell depending on the nature and cross-link density. Marcus [10] described the swelling of polystyrene cross-linked by divinylbenzene. 

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... 

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. ... 

At their melting temperature these powders can be dissolved in the epoxy monomers and they are able to react and participate in the cross-linking reaction through the amide groups (Lennon et al., 2000). For this reason, the cure cycle must be selected in order to keep the polyamide particles below their melting point (in the range 170°C or 220°C, depending on the type of polyamide used), and thus keep their initial shape and size. But in some cases a partial dissolution of the powder surface can improve the particle-polymer network interactions. 

The fracture energy cannot be related to the failure of chemical bonds which may contribute only with a few Jm-2. Furthermore, the possibility of crazing is not allowed in thermosets because fibrils cannot exist due to the high crosslink density. So, in the case of high-Tg cross-linked materials the main source of energy absorption before failure is the yielding of the network. This assumption is obviously valid only above the ductile-brittle transition temperature (Fig. 12.5), where yielding is temperature-dependent. ... 

In elastomer samples with macroscopic segmental orientation, the residual dipolar couplings are oriented as well, so that also the transverse relaxation decay depends on orientation. Therefore, the relaxation rate 1/T2 of a strained rubber band exhibits an orientation dependence, which is characteristic of the orientational distribution function of the residual dipolar interactions in the network. For perfect order the orientation dependence is determined by the square of the second Legendre polynomial [14]. Nearly perfect molecular order has been observed in porcine tendon by the orientation dependence of 1/T2 [77]. It can be concluded, that the NMR-MOUSE appears suitable to discriminate effects of macroscopic molecular order from effects of temperature and cross-link density by the orientation dependence of T2. 

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