Densities crosslink

Cross-link density and parameters relating to the network structure can be measured by NMR by analysis of the transverse relaxation decay (cf. Section 1.3) and the longitudinal relaxation in the rotating frame [67]. Combined with spatial resolution, the model-based analysis of relaxation yields maps of cross-link density and related parameters [68]. Often the statistical distribution of relaxation parameters over all pixels provides a reduced data set with sufficient information for sample characterization and discrimination [68]. 

Relaxation curves can be measured by simple NMR experiments. On the other hand, information about cross-link density and chain stiffness can be retrieved by exploiting 

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 some cases, the crosslink density of a polymer can be affected by the filler. These include  

Modification of the silica surface with mercaptosilane makes it reactive with rubber, resulting in an improvement in mechanical properties. Modified, precipi- 

One of the reactions which occurs on the surface of filler particles is that involving silanes. Vinyl silanes and mercapto silanes being typical examples. Kaolin modified with an isocyanate can react with polyols. Magnetic resonance spectroscopy was used to identify various crosslinks involving the filler and this shows that crosslinked rubber chains were attached to the surface of the carbon black. 

Zinc oxide is a reactive filler commonly used in rubber vulcanization. The crosslink density of rubber can be doubled by reaction of ZnO with HCl. Only a few specific fillers have the catalytic activity to promote crosslinking but fillers can take part directly in crosslinking processes initiated by an external source such as y-radialion. Generally, fillers reduce the effect of radiation. But y-rays are not screened by the filler so the protection given by fillers comes from reduction in chain mobility which lessens the probability of photoconversion. 

In summary, fillers have a very limited effect on matrix crosslinking except when they are used as crosslinkers or when the effect is caused by the physical properties of the filler (e.g., Al(0H)3 in UV crosslinked systems). 

According to classical rubber elasticity theory, intermolecular forces have no effect on the equilibrium chain configurations, and thus no effect on the stress. The relaxation of rubber toward mechanical equilibrium, however, is governed by the interactions among neighboring segments. This relationship has led to various attempts to interpret the elastic properties of rubber in terms of the network dynamics [47-52], 

Recent experimental advances reinforce the idea that the microscopic motions and the elastic properties can be usefully interrelated. For example, quasielastic neutron scattering measurements probe the motions of the network junctions [53,54]. Molecular dynamics simulations have also added new insight, for example, by demonstrating the existence of local constraints on the network chains at strand lengths less than the molecular weight necessary for chain entanglements [55]. Recently P NMR spin-lattice relaxation experiments on crosslinked rubber have been used to monitor specifically the dynamics of the network junctions [52,56]. 

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For thermosets with molecular weight between crosslinks M, the crosslink density Px, is described by Px N /Mx- As Px increases, the nets become tighter and stiffer, and thus require more stress to break via... 

There is considerable evidence in the thermoset literature that the fracture energy decreases with increasing crosslink density, consistent with the intuitive result that crosslinking inhibits flow. In the limit of very high crosslink density, where for example we approach the structure of diamond, fracture can occur on a single crystal plane such that... 

Acrylic polymers have the advantage that they can be formulated to be inherently tacky. However, for certain applications it may be desirable to adjust the rheological properties of the PSA beyond what can be obtained by selecting the right polymer composition and crosslink density. 

Fluorosilicones consist of PDMS backbones with some degree of fluoro-aliphatic side chains. The fluorinated group can be trifluoropropyl, nonafluorohexylmethyl, or fluorinated ether side group [78,28,79]. These polymers differ not only in substituent group, but also in the amount of fluoro-substitution relative to PDMS, the overall molecular weight and crosslink density, and the amount of branching. In most commercially available cases, these polymers are addition cure systems and the reactions are those discussed previously for silicone networks. 

Polmanteer, K.E. and Lentz, C.W., Reinforcement studies. Effect of silica structure on properties and crosslink density. Rubber Chem. Technol., 48(5), 795-809 (1975). 

An EB-curable struetural adhesive formulation usually eonsists of one or more crosslinkable oligomeric resins or prepolymers, along with such additives as reactive diluents, plasticizers, and wetting agents. The oligomer is an important component in terms of the development of mechanical properties. The adhesive and cohesive properties depend on the crosslink density, chemical group substitution, and molecular organization within the polymer matrix. Adhesion is achieved... 

E. Case V Predicting Crosslink Density Changes in Silicone Elastomers Due to Aging... 

Three different commercial formulations of silicone sealants from Dow Corning was used in the NSF sponsored studies. They were DC-790, DC-995, and DC-983, in the order of increasing modulus. Dumbbell test coupons (samples) were prepared as per the ASTM standards. Some test coupons were maintained at ambient conditions as control and the rest were subjected to simulated weathering. The weathered coupons were removed from the test layout at regular intervals of time and were tested for any changes in crosslink density due to exposure. 

In our study, the effect of moisture over the nonneutral pH range of 3-11, direct sunlight, ozone at a concentration level of 6000 ppm, and the effects of loading stresses, were investigated for the three commercial sealants. A characteristic variation of crosslink density for the typical silicone sealants is shown in Fig. 29. This figure depicts the results for the coupons exposed to moisture and sunlight. Initially upon exposure, the crosslink density of the sealants exhibit an increase due to the availability of residual uncurred crosslink sites... 

This transitory behavior was observed to arise from all the weathering agents considered in this study except ozone. Instead, test coupons exposed to ozone exhibited an initial decline in the crosslink density of the silicone with the formation of surface cracks, which were difficult to distinguish with the naked eye. With continued exposure to ozone, however, the material would begin to crosslink. We proposed that ozone s greatest affinity... 

Figure 29 Characteristic variation of crosslink density of silicone elastomers subjected to aging.

Experimental crosslink density changes due to weathering (moles/ cu.m)... 

Figure 30 ANN model training and testing results for crosslink density changes due to aging.

In general, the water uptake of D films tended to be higher than that of / films, but a more significant difference was shown by microhardness measurements. The results obtained with all three vehicles showed that the D areas were significantly softer than the / areas and that the distribution of the hardness values corresponded to that of the resistances. It was concluded that these films have a very heterogeneous structure and that / and D areas are brought about by differences in crosslinking density within the film. 

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