Crosslinked fractions

The crosslink fraction a is related to the number of crosslinked sites and the amount of polymer as... 

Rather than measuring the amount of material in the gel fraction, it might be easier to determine the time at which gelation of a network takes place. (For example, one could focus a laser beam on a sample and note when the speckle interference pattern freezes. ) Gelation occurs in a developing network if the crosslink fraction a exceeds a critical value a, given as(23)... 

Beyond the gel point, the shear modulus is related to the number of primary network chains n and the crosslink fraction a as(23) ... 

Below 200°C, the heat-induced changes in dry wool structure are confined to the amorphous parts of the protein. Above 200°C two melting endotherms are present, a small one at 215 °C from the melting of a low crosslinked fraction of the helix and a major one at 235°C as a result of the melting of the higher crosslinked fraction of the helix (49). In wool, the most direct evidence for this interpretation of helix melting is provided by x-ray diflFraction patterns at the various temperatures. 

The crosslinking of polyethylene resulting from its reaction with peroxides is more profound if maleic anhydride is present in the system [113]. The crosslinked fraction contains a higher proportion of carbonyl groups from grafted maleic anhydride than... 

BMl) - sequential interpenetrating polymer networks (30 wt.-% BMl, different crosslinker fraction) 85.4 - 130 99Nai... 

Irradiation of PP/EPDM blends causes pronounced differences in rheological behaviour compared with irradiated PP, which cannot be explained only by crosslinking of the EPDM phase. A possible explanation is the formation of grafted copolymers from PP onto the EPDM rubber particles. In blends a higher crosslinked fraction (based on 100% EPDM) is found than in irradiated EPDM (Table 1). The authors, however, are well aware that this is not an absolute prove of the presence of a graft copolymer and certainly is not a quantitative method to measure the amount of grafting. In order to get more evidence IR and NMR experiments will be performed in the future. 

Table 1. Crosslinked fractions in EPDM and PP/EPDM blends based on 100% EPDM. Soxhlet extraction in xylene.

The crosslinking poly(ethylene vinylacetate) (EVA) is not prevented in the presence of fire retardants. Even for 75 phr (parts per hundred of rubber) of hydrotalcit and 5 phr red phosphorus in polyethylene vinylacetate (14 % vinylacetate), the crosslinked fraction increase rapidly from 70 % for 50 kGy to about 85 % for 100 kGy, followed by slow enhance to 87 % for 300 kGy [06J1]. 

Table 48 Increase in the crosslinked fraction in y ( °Co)-irradiated SBS prepared in two formulations. The data were taken from [08Z1]. ...

The MAH contents of the soluble and insoluble fractions obtained in the presence of 2.0 wt-% BPO were greater than those of the corresponding fractions obtained in the presence of 0.25 wt-% BPO. The crosslinked fraction had a higher MAH content than the uncrosslinked fraction, irrespective of the BPO concentration in the charge (Table I),... 

Standard characterization methods are applicable to most polymers. However, as polymerization conditions are pushed toward high temperature and pressure, the side reactions may lead to contaminants on both sides - the low molecular weight fragments and partially crosslinked fraction. Thus, these three parts may require separate characterization. 

Suppose the un-cross-linked polymer chain has a molecular weight M which, upon crosslinking, is divided into subchains of molecular weight M, . This means that each subchain is a fraction of the original chain. Since the crosslink... 

An important application of Eq. (3.39) is the evaluation of M, . Flory et al.t measured the tensile force required for 100% elongation of synthetic rubber with variable crosslinking at 25°C. The molecular weight of the un-cross-linked polymer was 225,000, its density was 0.92 g cm , and the average molecular weight of a repeat unit was 68. Use Eq. (3.39) to estimate M. for each of the following samples and compare the calculated value with that obtained from the known fraction of repeat units cross-linked ... 

Between T j, and Tg, depending on the regularity of the polymer and on the experimental conditions, this domain may be anything from almost 100% crystalline to 100% amorphous. The amorphous fraction, whatever its abundance, behaves like a supercooled liquid in this region. The presence of a certain degree of crystallinity mimics the effect of crosslinking with respect to the mechanical behavior of a sample. 

Tirrell et al. [42,43] studied the role of interfacial chains in a more detailed fashion. Tirrell et al. [42,43] used a crosslinked PDMS cap in contact with a silicon wafer on to which a,o)-hydroxyl terminated PDMS chains are tethered by adsorption from a solution. The molecular weight of the narrow disperse PDMS samples was in the range of 20,000-700,000. The surface chain density was given by27 yj g e 0 is the volume fraction of PDMS in solution. 

It is basically a fractionation process that depends not only on molecular size, but also on chemical composition, stereo-configuration, branching, and crosslinking. For multicomponent systems, fractionation with different ion polymolecularity, chemical heterogeneity and sequence length distribution, solubility or elution fractionation is of primary importance. Therefore, gel permeation chromatography or size exclusion chromatography is used as an important tool for the characterization of PBAs. 

PF.O)l, crosslinked network, t (Polyin), various polymers, t Current fraction. 

Fig. 19. Selective bonding of novobiocin by crosslinked polyelectrolytes ARA-3p-T-40 (1 and 4) and ARA-lp-T-40 (2 and 3) with different grain sizes I) and 2) 10 pm 3) and 4) 200 pm. Nrmolar fraction of novobiocine in the ion exchanger...

Serious deviations of the polymer network structure from the ideal one can have several causes. One of them is the crosslinking agent involvement in intramolecular cycle formation. The contribution of this reaction grows with the system dilution as well as when the crosslinker units in the chain are close one to the other, i.e. its fraction in the copolymer increases. All this is in good agreement with the observed trend. 

The second reason is of kinetic nature and due to a substantially lower reactivity of the pendant acrylic groups. Some experimental facts may be considered a direct evidence of this possibility. Thus, the absorption in the region of 280 nm in the UV spectrum points to a growth of the residual concentration of acrylamide groups with an increasing fraction of the crosslinker [25]. The same groups can... 

The free volume model seems to be more adequate to describe the plasticization behaviour of the systems of lower amine content. According to Eq. (5), the higher is the change of the expansion coefficient the lower is the influence of the diluent volume fraction. The three TGDDM-DDS mixtures cured with 20, 30 and 50 PHR of hardener were characterized l2) by changes of the expansion coefficient at the glass transition, respectively, of 0.63, 1.08 and 2.94x 10 3 °C l. The more dense and stiffer resin crosslinked with 50 PHR of DDS should be, in principle, the less... 

The pre-gel model calculates the weight average molecular weight of the reaction mixture, while the post-gel model calculates the weight of the sol fraction and the effective crosslink density. A simple computer program using the derived expressions has been written in BASIC and runs on IBM-PC compatible computers. The importance of secondary reactions on cure in typical coatings is discussed. 

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