Network junction

Traditional rubbers are shaped in a manner akin to that of common thermoplastics. Subsequent to the shaping operations chemical reactions are brought about that lead to the formation of a polymeric network structure. Whilst the polymer molecular segments between the network junction points are mobile and can thus deform considerably, on application of a stress irreversible flow is prevented by the network structure and on release of the stress the molecules return to a random coiled configuration with no net change in the mean position of the Junction points. The polymer is thus rubbery. With all the major rubbers the... 

The 6-function makes sure that if two segments and 2 meet on the huge network chain they can form a permanent constraint R( i) = R( 2)- Hence, this process will produce a network junction of functionality/n = 4, usually realized as sulfur bridges in technical elastomers like, for example, tire treads. 

In this review, we have given our attention to Gaussian network theories by which chain deformation and elastic forces can be related to macroscopic deformation directly. The results depend on crosslink junction fluctuations. In these models, chain deformation is greatest when crosslinks do not move and least in the phantom network model where junction fluctuations are largest. Much of the experimental data is consistent with these theories, but in some cases, (19,20) chain deformation is less than any of the above predictions. The recognition that a rearrangement of network junctions can take place in which chain extension is less than calculated from an affine model provides an explanation for some of these experiments, but leaves many questions unanswered. 

The equilibrium shear modulus of two similar polyurethane elastomers is shown to depend on both the concentration of elastically active chains, vc, and topological interactions between such chains (trapped entanglements). The elastomers were carefully prepared in different ways from the same amounts of toluene-2,4-diisocyanate, a polypropylene oxide) (PPO) triol, a dihydroxy-terminated PPO, and a monohydroxy PPO in small amount. Provided the network junctions do not fluctuate significantly, the modulus of both elastomers can be expressed as c( 1 + ve/vc)RT, the average value of vth>c being 0.61. The quantity vc equals TeG ax/RT, where TeG ax is the contribution of the topological interactions to the modulus. Both vc and Te were calculated from the sol fraction and the initial formulation. Discussed briefly is the dependence of the ultimate tensile properties on extension rate. 

When the network junctions are entirely immobilized by the surrounding chains, h equals zero. Then the junctions in a deformed specimen are displaced in proportion to the macroscopic strain, i.e., the deformation is affine. Alternatively, h equals unity when junction fluctuations are not impeded, the defining characteristic of a phantom network (16, 17). The parameter h was introduced (13) to allow empirically for different degrees of fluctuations. For undiluted networks at small deformations, h should usually be small, though not necessarily zero. 

Interlocking of polymer chains in a polymer material forming a transient or permanent network junction over the time-scale of the measurement. 

Note 2 The polymer network can be a network formed by covalent bonds or by physical aggregation with region of local order acting as network junctions. 

Scanlan has suggested another criterion (282). An effective network junction point is a crosslink in which at least three of the four strands radiating from it lead independently to the network. A crosslink with only two strands anchored to the network simply continues an active strand a crosslink with only one anchored strand is part of a dangling end and can make no elastic contribution at equilibrium. An elastically effective strand is therefore one which joins two effective network junction points. Accordingly, the total number of active strands is simply one half the number of gel-anchored strands radiating from effective junction points ... 

CTBN oligomers leads to a higher molecular weight of chains between network junction points and consequently, a lower crosslink density. 

The synthesis and characterization of these ABA block copolymers of styrene and dienes have been described elsewhere (JO, 11). Since the polystyrene end blocks aggregate into glassy domains which act as network junctions, the elastic center blocks must virtually represent the network chains/ The polystyrene domains should also act as a finely divided filler. Hence it might be expected that the mechanical properties of these materials could depend on the two basic parameters polystyrene content and length of center block ( molecular weight between crosslinks ). 

Nikolic, B., MacNulty, E., Mir, B., and Wiche, G. (1996). Basic amino acid residue cluster within nuclear targeting sequence motif is essential for cytoplasmic plectin-vimentin network junctions./. Cell Biol. 134, 1455-1467. 

Bulky crosslinks or side-groups in the network chains, e.g., dendritic wedges [73], may also influence molecular mobility and viscoelastic properties of polymer networks. For example, UV curing of difunctional acrylates results in the formation of zip-like network junctions, which may be regarded as extreme cases of bimodal networks [52], Results obtained with the NMR T2 relaxation method agree well with those of mechanical tests... 

Figure 10.8 The density of network junctions as a function of the volume fraction of paraffinic oil in EPDM/oil vulcanisates [74], The solid line represents the result of a linear regression analysis of the dependence (intercept = 453 5 mmol/kg slope = -6.2 0.0.3 mmol/kg the correlation coefficient = 0.996). Maximum torque in the rheometer curve for the vulcanisates is shown on the right ordinate...

Figure 10.9 A simplified graphic representation of EPDM chains at the carbon black surface [62], Monomer units with low mobility in the interface and mobile chain units outside of interface are represented by solid and open points, respectively. The rotational and translational mobilities of a few chain units next to the adsorption layer along the chain (dashed points) are hindered somewhat more than those of the chain units in the matrix. The chain fragments with low mobility in the interface provide adsorption network junctions for the rubber matrix. At the bottom of the figure, the spatial profile of the correlation time Tc of the chain motion is schematically represented as a function of the distance, r, from the carbon black surface. The xc is the average time of a single reorientation of a chain unit...

It is suggested that the sites of interaction between carbon black and rubbery chains cause physical (adsorption) network junctions in the rubber matrix [20, 62] (Figure 10.10). 

The first is an EPDM fraction which is loosely bound to the carbon black due to adsorption interactions. This loosely bound rubber has numerous adsorption network junctions, similar to those in bound rubber. The second EPDM fraction, consisting of extractable rubber, contains a relatively small number of adsorption network junctions and can apparently be extracted from the compounds. The fraction of loosely bound EPDM chains determined with the aid of NMR increases with an increase in the maximum possible EPDM-carbon black contact area per unit volume of the elastomer, regardless the type of carbon black used, and is relatively close to the content of bound rubber [62]. 

A good understanding of the structure of the network in filled rubbers is of great importance, because the rubber s elastic properties are determined primarily by the density of chemical and physical network junctions and their ability to fluctuate. The following types of network junctions occur in filled rubbers ... 

The relative contribution of each of these types of network junctions to the overall crosslink density in silica-filled PDMS was estimated by means of T2 relaxation experiments [113, 118-121], To determine the relative contributions of the different types of network... 

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