Network heterogeneity

Mechanical properties of crosslinked elastomers are influenced not only by the volume-average crosslink density but also by network heterogeneity. The influence of structural defects (such as residual sol, dangling chains, chain loops and the heterogeneity of the junction distribution) on the viscoelastic properties and the equilibrium swelling data is still under discussion. Local methods which probe molecular properties are very suitable for the determination of the degree of network heterogeneity [11]. 

A significant difference between the large spatial-scale mobility of network chains and that of network defects allows us to determine the degree of network heterogeneity. The most reliable data are obtained for swollen samples, because an increasing solvent content results in the disentanglement of network defects from network chains [52, 61, 62], The molecular mobility of network chains is consequently decoupled from that of network defects, resulting in a major distinction in the relaxation behaviour. 

The CCA-model considers the filler network as a result of kinetically cluster-cluster-aggregation, where the size of the fractal network heterogeneity is given by a space-filling condition for the filler clusters [60,63,64,92]. We will summarize the basic assumptions of this approach and extend it by adding additional considerations as well as experimental results. Thereby, we will apply the CCA-model to rubber composites filled with carbon black as well as polymeric filler particles (microgels) of spherical shape and almost mono-disperse size distribution that allow for a better understanding of the mechanisms of rubber reinforcement. 

Overall it is considered that the spatially independent network models, whilst simpler, should strictly be used for lightly crosslinked, homogeneous networks, whereas the spahally dependent models, although computationally intensive and limited by pre-defined lattice structure, provide a better understanding of network heterogeneities in highly crosslinked systems. 

Gel-type networks with enlarged swelling can also be obtained by the copolymerization of styrene with a relatively small amount of DVB in the presence of thermodynamically good solvents, such as toluene [93—95]. As mentioned above, the solvent would have to assist in increasing network heterogeneity. However, a more powerfijl factor consists of the fact that a... 

Polymeric systems displaying tunable and dynamic surface patterns are emerging as important platforms for many applications. Polymer networks swell and de-swell when exposed to a solvent. Controlling network heterogeneity enables to tune the... 

There has been some NMR work on trimodal PDMS elastomers, with regard to thermal degradation and the presence of network heterogeneities. The idea of employing trimodality has also been employed in some triblock copolymers. - ... 

E. Bowen, D. Stephens, T., Investigation of Network Heterogeneities in Filled, Trimodal, Highly Functional PDMS Networks by H Multiple Quantum NMR. Macromolecules 2007,40,4953-4962. 

Szczepanski CR, Pfeifer CS et al (2012) A new approach to network heterogeneity polymerization induced phase separation in photo-initiated, free-radical methacrylic systems. Polymer 53 4694-4701... 

Cooperative diffusion of transient polymer networks, heterogeneity mode related to polymer self-diffusion in diblock copolymers, entanglement mode, chain reptation, viscoelastic relaxation, diffusion of clusters Viscoelastic relaxation, a- and 3-relaxation... 

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