Polymer-like networks

ANDRZEJ R. ALTENBERGER, JOHN S. DAHLER AND MATTHEW TIRRELL Tracer Diffusion in Polymer-like Networks 285... 

We have studied self-diffusion in polymer-like networks self-diffusion of transient chains (long entangled micelles) and tracer diffusion in the solvent (silica gels). Our results illustrate the role of reversible chain breaking and the scale-dependence of diffusion in the solvent. 

As has been discussed before, hydrolysis of silicon alkoxides is faster than condensation under acidic conditions. Since all species are hydrolyzed at an early stage of the reaction, they can condense to form small oligomeric species (clusters) with reactive Si—OH groups. Under these conditions, reactions at terminal silicon atoms are fevored (see above). This results in polymer-hke gels that is, small clusters undergo condensation reactions with each other to give a polymer-like network with small pores. 

Cables are available in a variety of constmctions and materials, in order to meet the requirements of industry specifications and the physical environment. For indoor usage, such as for Local Area Networks (LAN), the codes require that the cables should pass very strict fire and smoke release specifications. In these cases, highly dame retardant and low smoke materials are used, based on halogenated polymers such as duorinated ethylene—propylene polymers (like PTFE or FEP) or poly(vinyl chloride) (PVC). Eor outdoor usage, where fire retardancy is not an issue, polyethylene can be used at a lower cost. 

Block (Star) Arrangement. The known star polymers, like their linear counterparts, exhibit microphase separation. In general, they exhibit higher viscosities in the melt than their analogous linear materials. Their rheological behavior is reminiscent of network materials rather than linear block copolymers (58). Although they have been used as compatibiUzers in polymer blends, they are not as effective at property enhancements as linear diblocks... 

The less simple polymers (like the epoxies, the polyesters and the formaldehyde-based resins) are networks each chain is cross-linked in many places to other chains, so that, if stretched out, the array would look like a piece of Belgian lace, somehow woven in three dimensions. These are the thermosets if heated, the structure softens but it does not melt the cross-links prevent viscous flow. Thermosets are usually a bit stiffer than amorphous thermoplastics because of the cross-links, but they cannot easily be crystallised or oriented, so there is less scope for changing their properties by processing. 

The synthesis of polycatenanes requires, like the synthesis of catenanes, the preorientation of the macrocycle precursors into a favorable geometry before cycliza-tion (Scheme 4) [5], This pre-orientation is commonly achieved via a template, resulting from rc-donor-acceptor interactions, hydrogen-bonding, and coordination bonds [1-3, 5, 41], The use of a template in catenane synthesis is the subject of Chapters 4 and 6-8 and will not be treated further in this section. The aim of this section is to present the state of the art of the various synthetic approaches leading to the polycatenane polymers and networks. 

Bulky crosslinks or side-groups in the network chains, e.g., dendritic wedges [73], may also influence molecular mobility and viscoelastic properties of polymer networks. For example, UV curing of difunctional acrylates results in the formation of zip-like network junctions, which may be regarded as extreme cases of bimodal networks [52], Results obtained with the NMR T2 relaxation method agree well with those of mechanical tests... 

Network polymers are formed either if tri-functional or even tetra-functional monomers are present during the polymerisation reaction or by cross linking of high molecular weight polymers, like vulcanisation of rubber. 

Natural polymer-based networks have also been investigated. The proteins etc comprising antibodies represent the largest group [164, 166, 169, 189] but this is of course a specialised area. Poly(saccharides), in particular starch [60], dextran [161], dextrin [161] and maltohexose [161], and also natural polypeptides, mainly enzymes [162-165], embody the more accessible biopolymers. In some instances imprinting is achieved through formation of covalent bonds, with crosslinkers like cyanuric chloride or glutaraldehyde. Likewise chitin derivatives similarly crosslinked have been exploited [136]. 

In network polymers, of course, surfaces can only be created by breaking primary bonds, but these may be relatively widely spaced. Crystalline thermoplastics and indeed amorphous polymers with very long molecules may behave (in this respect) more like network polymers because crystals or entanglements act as effective aross-links. 

If the solution is cooled slowly (8 C/mln to 1350 C/min), Sol 2 micelles appear. If, however, cooling is too rapid ( b2,000 C/min), a continuous lace-like, noncellular polymer network is apparent in photomicrographs. This lace-like network is the frozen Sol 1 structure which, for kinetic reasons, is unable to assume the Sol 2 configuration before it becomes immobilized. 

Dimerization of carboxylic acids is also used for the formation of nonmesomorphic molecular assemblies in solid states such as one-dimensional polymeric structures [131, 132], two- or three-dimensional networks [133], and dendritic materials [134]. Dimer formation due to quadruple H-bonding has been designed to obtain highly stable molecular association [135]. The quadruply H-bonded polymer associates from bifunctional compounds show polymer-like behavior in the solution state due to large association constants [135]. 

Nevertheless, there is a phase transition from the gas- to the liquid-expanded and solid phase. A collapse of the isotherms could never be observed. This is a further indication for the formation of a polymer-like hydrogen bonding network. 

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