Model networks, preparation

As a conclusion, we may state that the very intensive studies of model networks prepared by anionic block polymerization provided much interesting experimental data. However, they failed to prove the validity of the current network elasticity theories and provided rather ambiguous explanations of the swelling and mechanical properties of model networks. Moreover, the results obtained raised more questions regarding the evaluation and interpretation of the neutron scattering technique data. 

Mark and co-workers have studied the effect of dangling chains on the ultimate properties of model networks prepared by end-linking vinyl-terminated PDMS chains with a tetra functional crosslinker [34]. In this case the crosslinker was used in varying amounts smaller than those corresponding to a stoichiometric balance. The... 

Figure 18(a) presents normalized proton DQ buildup functions of end-linked polydimethylsiloxane model networks prepared by mixing and subsequent cross-linking of very short and rather long chains. Such bimodal networks are known to consist of clusters of highly cross-linked regions embedded in a long-chain elastomer matrix. The buildup curves as well as the cross-link density distributions (i.e., the distributions of D s) derived from these show clear bimodal... 

Model networks were prepared using hydroxyl terminated polymer and isocyanates, (a) Bifunctional hydroxyl terminated polybutadiene (Butarez, from Phillips Petroleum) was crosslinkined with tris (p-isocyanatophenyl)-thiophosphate (Desmodur RF, from Mobay Chemical Co.). This crosslinked... 

Dendrimers represent a model for compact multifunctional precursor of polymer networks. Polymer networks prepared by crosslinking of dendrimers were suggested several years ago [64]. Since then, some experimental work has been performed, but there are still many points in structural interpretation of network formation and network properties that are not well understood. 

It is shown that model, end-linked networks cannot be perfect networks. Simply from the mechanism of formation, post-gel intramolecular reaction must occur and some of this leads to the formation of inelastic loops. Data on the small-strain, shear moduli of trifunctional and tetrafunctional polyurethane networks from polyols of various molar masses, and the extents of reaction at gelation occurring during their formation are considered in more detail than hitherto. The networks, prepared in bulk and at various dilutions in solvent, show extents of reaction at gelation which indicate pre-gel intramolecular reaction and small-strain moduli which are lower than those expected for perfect network structures. From the systematic variations of moduli and gel points with dilution of preparation, it is deduced that the networks follow affine behaviour at small strains and that even in the limit of no pre-gel intramolecular reaction, the occurrence of post-gel intramolecular reaction means that network defects still occur. In addition, from the variation of defects with polyol molar mass it is demonstrated that defects will still persist in the limit of infinite molar mass. In this limit, theoretical arguments are used to define the minimal significant structures which must be considered for the definition of the properties and structures of real networks. 

Note 1 A model network can be prepared using a non-linear polymerization or by crosslinking of existing polymer chains. 

Note 2 A model network is not necessarily a perfect network. If a non-linear polymerization is used to prepare the network, non-stoichiometric amounts of reactants or incomplete reaction can lead to network containing loose ends. If the crosslinking of existing polymer chains is used to prepare the network, then two loose ends per existing polymer chain result. In the absence of chain entanglements, loose ends can never be elastically active network chains. 

Functionally terminal polymers are valuable material intermediates. The di- and polyfunctional varieties (telechelic polymers) have found theoretical (e.g., model network) and commercial (e.g., liquid rubber) applications (1, ). On the other hand, macromolecules with a functional group at one chain end (semitelechelic polymers) have been used to prepare novel macromolecular monomers (Macromers ), as well as block and graft copolymers ( -8). 

Values of the mean-square dipole moment, , of PDEI are determined as a function of temperature. The value of the dipole moment ratio is 0.697 at 303 K. Trifunctional model networks are prepared. From thermoelastic experiments performed on the networks over a temperature range 293 - 353 K, it is found that the value of the temperature coefficient of the unperturbed dimensions amounts to 1.05 0.17 K-1. The dipole moments and the temperature coefficients of both the dipole moments and the unperturbed dimensions are critically interpreted in terms of the RIS model, and are found to be in a reasonable agreement. 

Model urethane networks prepared from polyoxypropylene tetrols and hexamethylene diisocyanate are studied with the aid of the computer, and good agreement of theory with experiment is found for gel paints when the crosslinkers are treated as sticks. 

Model networks of known structure can be prepared by end-linking functionally-terminated polymer chains [3, 4, 157]. Because of the nature of this network formation process, the molecular weight of the starting chains becomes the critically important molecular weight between cross-links, Mc. Control of Mc and its... 

New methods are being developed in various laboratories to synthesize well-defined networks exhibiting structures as close as possible to ideality. The principle of these so-called endlinking methods is to separate the polymerization process from the network-forming reaction. The first step aims at preparing a linear precursor polymer, fitted at both ends with reactive groups. In the second step bonds are established between several precursor chain ends to form the crosslinks. The methods which were used to synthesize so-called model networks have already been described, and we shall only summarize them here ... 

Figure 10.1 Temperature dependence of the H T2 relaxation time of well-defined end-linked (PPO) networks with narrow molecular mass distributions between chemical crosslinks [44], The molecular mass of network chains (in g/mol) is shown in this figure. The temperature dependence of a linear, high-molecular-mass polypropylene oxide) prepared from a polypropylene glycol precursor (with a molecular mass of 4000 g/mol) using a chain extender with a chemical structure similar to that of the crosslinker is shown for comparison. The synthesis of the model networks has been...

Figure 1.8 Dependence of volume swelling in benzene of model networks vs. molecular weight M of chain precursors for three sets of networks each set prepared at a constant concentration and constant portion of DVB per living end. (After [126]).

Undoubtedly, in the preparation of model networks from rather long polymeric precurson by the end-hnking reactions, the strands between the junction points wid maintain the conformation of coils. In accordance with classical ideas, the cods in the dry model network should acquire unperturbed dimensions, simdar to cods in a -solvent. However, on contacting a model polystyrene network with cyclohexane at room temperature, which is far below the -point for linear polystyrene (34.5°C), its volume increases by a factor of 3. Thus, the solvent breaks some polymer-polymer interactions, stimulates swelling of the network, and may result in an increase of chain dimensions. Hence, even at room temperature, the polystyrene chains prefer replacing the alien chain segments by cyclohexane molecides. 

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