Glassy organic networks

Preston s mechanism was adopted to explain the "feature-track" effect observed In the fractography of glassy organic networks... 

Thus, although some degree of local organization may indeed occur in amorphous systems, and may even have some effect on the mechanical properties of polymers in the glassy state, the influence on the mechanical properties of melts, concentrated solutions and networks appears to be negligible. 

However, experimental results do not support these expectations. Young moduli of all the polymers considered here are rather close to those of organic glasses (polymeric and non-polymeric)40,62) (see also Sect. 5) at 25 °C, E25 = 3.0-3.7 GPa. These values show that the U term, even in densely crosslinked epoxy network glasses, has an intermolecular origin and is determined by Van-der-Waals interactions. Also the deformation of H bonds and the deformation of chemical bonds and valence angles of network chains do not play any role in the elasticity of glassy networks. 

Microporous beads are weakly crosslinked resins obtained by suspension polymerisation of styrene and divinylbenzene in the absence of any porogen agent. This process leads to the formation of a homogeneous network evidenced by a glassy and transparent appearance. The most commonly used supports for solid-phase organic synthesis and catalysis are styrene-divinylbenzene copolymers crosslinked with only 1-2% DVB. Many of their derivatives are commercially available [20]. 

Biobased epoxy nanocomposites can be reinforced with organo montmorillonite clay and carbon fibers obtained from poly(acryl-onitrile) (45). To get the organically modified clay into the glassy biobased epoxy networks, a sonication technique was used. In this way, clay nanoplatelets were obtained that were homogeneously dispersed and completely exfoliated in the matrix. 

The dramatic change observed in the electrical behavior of SiC based fibers with increasing pyrolysis temperature is related to the formation or/and the organization of free carbon around the SiC crystals. In Si-C-0 fibers it occurs above 1200°C. Below 1200°C, the microstructure of the fibers is either amorphous or nanocrystalline, and carbon exists as isolated small BSUs. The material has semiconducting properties (E. = 0.4 eV). Above 1200 C, decomposition occurs with formation of jJ-SiC crystals and free carbon. The increase in electrical conductivity might be related to the removal of the glassy silicon oxycarbide and the formation of a continuous network of carbon around the SiC crystals [18]. 

Block copolymers with incompatible blocks which are able to microphase separate are good candidates for PSA properties. Indeed, blends of ABA triblocks and AB diblocks, where the rubbery midblock of the ABA is the majority phase and the glassy endblocks self organize in hard spherical domains and form physical crosslinks, are widely used as base polymers for PSA. The actual adhesives are always compounded with a low molecular weight tackifier resin able to swell the rubbery phase and dilute the entanglement network. Linear styrene-rubber-styrene copolymers, with rubber being isoprene, butadiene, ethylene/propylene or ethylene/butylene, are the most widely used block copolymers in this category. 

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