Crosslinked Chains

Statistically defined structures may also arise from the formation of crosslinks in a melt the resulting gels are described within a percolation framework which predicts the existence of definite meshes [7, 8]. Contact-lenses, jellies or even jellyfish are common examples of gels. Latex beads with specific functionalities attached, such as antigens, are used in biodiagnostics. 

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P Stjarnkvist, I Sjoholm, T Laasko. Biodegradable microspheres. XII. Properties of the crosslinking chains in polyacryl starch microparticles. J Pharm Sci 78 52-56, 1989. 

Secondary processes are normally employed to crosslink chain growth polymers. In one example a linear thermoplastic, such as polyethylene, is compounded with an organic peroxide that is thermally stable at standard processing temperatures but decomposes to chemically react with the polymer chain at higher temperatures creating crosslinks. 

A host of scientists in universities and industrial laboratories are finding that by adding side chains to the crosslinked chains or otherwise altering their structure they can control when and how the materials respond. It is therefore to be expected that they will design new smart gels and ideas for gel-using systems in the future. 

Some TICT-forming fluorescent probes containing the / j - /V. /V - cl i a I k I a m i n o benzylidene malononitrile motif (usually related to 30 in Fig. 11) have been applied to monitor and quantify polymerization reaction, crosslinking, chain relaxation,... 

Select examples of organosilicon resist chemistries that undergo crosslinking, chain scission or solution inhibition. 

For low extents of crosslinking the curves lie between the limits of the curve near the gel point and that for linear chains. This behavior is understood when recalling that at low crosslinking the system mainly consists of linear chains. The highly crosslinked chain on the other hand approaches a molar mass independent master curve that, as expected, lies in the region of hard sphere behavior. 

The division by two denotes the topological requirement (6) of two inter-crosslink chains per DVB, in the present model of a closed X-type network structure. (Consider two X units with their chain ends joined together. There are four chain lines and two vertices.)... 

Figure 11. Scheme for crosslinking chain copolymerization x free radical site, a inter-and a intramolecular reaction. 

Figure 2. Effect of crosslinking, chain termination, and plasticization on effective strain. Numbers on points refer to percent plasticizer, moles of chains X 10A/cc. or moles of chains X 10A /cc. dangling, respectively...

For indefinite continuation of a structure of crosslinked chains it is necessary that e > 1. At the gel point e equals unity, which leads to... 

In the following, we will briefly outline the use of the link p.g.f. (l.p.g.f.) for the calculation of the gel point in /-functional polycondensation without and with cyclization including the f.s.s.e. In Chapter II, section 2.2 we will consider an application in connection with the number of elastically active network chains in random polycondensates or in a collection of randomly crosslinked chains. 

Fig. 22. Schematic diagram of coordinate system related to one inter-crosslink chain (adapted from Ref.3I)...

Similar effects are observed in y-irradiated n-C H (530 kGy) in the molten state. Three new structures are identified as a) one-bond crosslinks (H-structure), b) trans-vinylene groups and c) long branches (T- or Y-structure)144). However, highly crystalline polyethylene y-irradiated in the solid state at low doses (up to 40 kGy) yields predominantly the branched Y-structure. A failure to detect the cross-linked H-structure could arise from a) insufficient abundance of crosslinks to give a detectable signal and b) insufficient mobility of crosslinked chains in the polyethylene gel which results in very broad resonance lines, not observable during normal data acquisition in the solution 13C NMR experiment145). 

Thermal analysis (DSC, DMTA, DETA, etc.) gives valuable information on eventual changes of glass transition temperature, which can be interpreted in terms of crosslinking/chain-scission processes. 

An average expectance of additional chains v, which is greater than unity allows for a non-zero probability of the randomly selected crosslinked chain belonging to an infinitely large system [79,83]. This gives the critical value of crosslinking density ac corresponding to the gel point. 

Permanent networks formed from crosslinked chains are of particular interest for illustrating the NMR approach. In this Section, it is shown how the proton transverse relaxation observed on networks is characterised in a standard way and provides values of relevant parameters. 

One important feature of randomly crosslinked chains is that they can be highly strained and/or swollen they recover their initial shape when the stress is interrupted and/or after deswelling. Neither the dependence of the modulus of elasticity, E, upon the variable e nor that of the swelling ratio, Qm, have been exactly predicted, until now. Nevertheless, these two physical quantities as well as the rate %c are functions of the variable, e. 

Considering calibrated gels or randomly crosslinked chains, the effect of linkage of monomeric units on NMR is well detected in spite of the purely statistical definition of the segmental spacing between crosslinks in vulcanised chains. 

Figure 10.11 The structure of an EPDM/N550 (phr=100) vulcanisate according to the results of NMR and extraction studies - (A) and mechanical data for the case of pure hydrodynamic reinforcement - (B) [62], The volume fraction of microphases/ components is given in vol.%. According to the NMR data, the total network density in the rubber phase, l/2Mc+e+ad, equals 425 mmol/kg, where subscripts c, e and ad stand for chemical crosslinks, chain entanglements and adsorption rubber-filler junctions. The density of the adsorption junctions in the loosely bound rubber, 1/...

Figure 15.1 a 2H NMR spectrum of perdeuterated crosslinked chains in a PDMS network (average molecular weight between junctions Mn= 10500 g.mol 1, i.e.,... 

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