Crosslinking molar mass distribution

Polyurethane Networks. Andrady and Sefcik (1983) have applied the same relationship as Rietsch et al. (1976), to the glass transition temperature of networks based on poly(propylene oxide) diols with a controlled molar mass distribution, crosslinked by aromatic triisocyanates. They obtained a Kr value of 25 K kg mol-1, about twice that for PS networks. They showed that the length distribution of elastically active chain lengths, directly related to the molar mass distribution of the starting poly(propylene oxide), has practically no effect on Tg. 

The tests were carried out on 1 1 mixtures of two components (A component = polymer + Pt catalyst B component = polymer + crosslinker) which were prepared shortly before the test. The catalyst was a vinyl siioxane Pt complex with tetramethyltetravinylcyclotetrasiloxane as a ligand. All the polymers and crosslinkers used were equilibrated and devolatized products with a molar mass distribution typical for silicone polymers. 

Intermolecular reactions between carbon-centred radicals on adjacent chains will produce crosslinking, which will compete with the chain-scission and intramolecular reactions. The effect on the molar-mass distribution will depend on the competition between these reactions. For example the poly(acrylates) will preferentially crosslink rather than degrade and the recovery of monomer from these polymers is less than 1%, compared with close to 100% for poly(methacrylates). Poly(vinyl chloride) (PVC) is also a polymer that undergoes crosslinking in preference to scission, but the major observation of importance is the rapid elimination of hydrogen chloride by an auto-catalysed reaction as shown in Scheme 1.53. This is discussed in more detail later. 

When crosslinking predominates over disproportionation or scission, the molar-mass distribution will broaden and will shift to higher values. The process of crosslinking may be followed by extracting the polymer to determine the residual soluble (sol) fraction, S, i.e., the amount of material not yet incorporated into the infinite network. The equation describing this is the modified Charlesby-Pinner equation (Boyd and Phillips 1993) ... 

A model network should, at least, satisfy the following conditions [5] (i) the linear chain element of a model network should exhibit known length and, if possible, a narrow molar mass distribution ( >m)- ch elastic chain should be connected by its two ends to two different crosslink points (ii) a model network should be homogeneous crosslinking density should be constant throughout the gel and (iii) a model network should exhibit a known and constant functionality of crosslink points. 

Burchard [65] was the pioneer for the preparation of functional starshaped pol)miers in nonpolar solvents by an anionic core-first method. To build the cores pure / -DVB was reacted with /i-butyllithium in dilute cyclohexane solution. Suspensions of small crosslinked poly(DVB) noduli were obtained that contained numerous lithium organic sites. In a second step, styrene (or isoprene) was added to the living cores and polymerized. The polymeric species obtained exhibit huge molar mass distribution and rather large polydispersity indices. Even if these star-shaped polymers could exhibit active sites at the outer end of the branches, the efficiency of initiation of a second generation of monomers or of hmctionalization was never given by the authors. 

Consider the case of polymer chains with an arbitrary distribution of chain lengths and with a number of potentially reactive sites equal to the degree of polymerization (every monomeric unit of the polymer chain constitutes one potentially reactive site). The random crosslinking (vulcanization) of these linear primary chains may be considered a stepwise homopolymerization of Afl species, where the functionality of every species is directly proportional to its molar mass,... 

The influence of parameters such as crosslinking agent aluminium acetylacetonate content, introduction time of monomers, amount of reactor charge, distribution of N-vinyl caprolactam (VC), diverse crosslinkers, viscosity and molar mass of the synthesised pressure-sensitive adhesives (PSA) and solvent balance and transfer agent kind and content on such important parameters of PS A as shrinkage, plasticity, adhesion to steel and deformation are examined. Synthesised pressure-sensitive adhesives based on acrylic polymers and containing 2-ethylhexyl acrylate (2-EHA), methyl acrylate (MA), acrylic acid (AA) and VC are used for production of self-adhesives containing PVC carrier. 4 refs. 

Tang et al. (2004) presented a cure model that captures transient, thermal and chemical effects that are ignored in typical threshold-based models. This new model incorporates as inputs photoinitiation rates, reaction rates, diffusion and temperature distributions, and is able to determine the spatial and temporal distributions of monomer and polymer concentrations, molar masses, crosslink densities and degree of cure. 

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