Network chains motions

Figure 2 Schematic drawing of a slip link, with its possible motions along the network chains specified by the distances a, and its locking into position as a cross-link.

Solid-state NMR methods have been much used to study the characteristics of the network chains themselves, particularly with regard to orientations [265], molecular motions [266], and their effects on the diffusion of small molecules [267], Aspects related to the structures of the networks include the degree of cross-linking [268,269], the distributions of cross-links [270] and stresses [271], and topologies [272,273]. Another example is the use of NMR to clarify some issues in the areas of aging and phase separation [274],... 

These cyclics can change the properties of the network in which they are trapped. Because they restrict to some extent the motions of the network chains, they should increase the modulus of an elastomer. Some small but possibly significant increases in low-deformation moduli have, in fact, been observed [192], Also, when PDMS cyclics are trapped in a thermoplastic material, they can act as a plasticizer that is in a sense intermediate to the usual external (dissolved) and internal (copolymerized) varieties. Interesting changes in mechanical properties have been observed in materials of this type [197]. 

The dynamic mechanical properties of the siloxane-modified epoxy networks were also investigated. The DMTA curves for the control epoxy network exhibit the two major relaxations observed in most epoxy polymers 39 40,41>. A high temperature or a transition at 150 °C corresponds to the major glass transition temperature of the network above which large chain motion takes place. The low temperature or (5 transition is a broad peak extending from —90° to 0 °C with a center near —40 °C. It has been attributed predominantly to the motion of the CH2—CH(OH)—CH2—O (hydroxyether) group of the epoxy 39-40 2 ... 

Above Tg, the network chains have sufficient thermal energy to overtake the potential barriers linked to Van der Waals interactions. They undergo fast conformational changes through cooperative segmental motions, but cross-linking prevents any liquid flow. We are thus in the presence of a peculiar state of matter, which displays at the same time liquid and solid (elastic) properties the rubbery state. 

Solid-state NMR magnetisation relaxation experiments provide a good method for the analysis of network structures. In the past two decades considerable progress has been made in the field of elastomer characterisation using transverse or spin-spin (T2) relaxation data [36-42]. The principle of the use of such relaxation experiments is based on the high sensitivity of the relaxation process to chain dynamics involving large spatial-scale chain motion in elastomers at temperatures well above the Tg and in swollen networks. Since chain motion is closely coupled to elastomer structure, chemical information can also be obtained in this way. 

Figure 10.9 A simplified graphic representation of EPDM chains at the carbon black surface [62], Monomer units with low mobility in the interface and mobile chain units outside of interface are represented by solid and open points, respectively. The rotational and translational mobilities of a few chain units next to the adsorption layer along the chain (dashed points) are hindered somewhat more than those of the chain units in the matrix. The chain fragments with low mobility in the interface provide adsorption network junctions for the rubber matrix. At the bottom of the figure, the spatial profile of the correlation time Tc of the chain motion is schematically represented as a function of the distance, r, from the carbon black surface. The xc is the average time of a single reorientation of a chain unit...

A major focus of this chapter is the effect of network formation and network density on the relaxation times of the H and 13C nuclei in the NMR experiment. The changes in relaxation times can be exploited to obtain information about crosslink density and chain motion, and must be taken into account in the design of experiments to determine changes in chemical structure. In this section we examine how crosslinking changes the transverse (T2) relaxation times of nuclei, and how this information can be of use. Two different approaches have been taken in the literature, namely changes in T2 can be used to estimate crosslink density, or used to develop and verify models of chain motion. 

Annealing of the nascent powder sample passed through Process 1, and above 90 °C, dynamic molecular motion started, as defined by Process 2. This critical temperature is slightly higher than that of the solution-crystallised sample. This difference indicates the restricted crystalline chain motion for the domain network structure crystallised during polymerisation. In Process 2, the crystallinity remained at a constant level for the nascent powder sample. This shows that the lamellar thickening is limited for the nascent powder morphology. 

Thermosets normally are rigid materials and are network polymers in which chain motion is greatly restricted by a high degree of crosslinking. As for elastomers, they are intractable once formed and degrade rather than melt upon the application of heat. 

Summary Solid state NMR studies of molecular motions and network structure in poly(dimethylsiloxane) (PDMS) filled with hydrophilic and hydrophobic Aerosil are reviewed and compared with the results provided by other methods. It is shown that two microphases with significantly different local chain mobility are observed in filled PDMS above the glass transition, namely immobilized chain units adsorbed at the filler surface and mobile chain units outside this adsorption layer. The thickness of the adsorption layer is in the range of one to two diameters of the monomer unit ( 1 nm). Chain units in the adsorption layer are not rigidly linked to the surface of Aerosil. The chain motion in the adsorption layer depends significantly on temperature and on type of the filler surface. With increasing temperature, both the fiaction of less mobile adsorbed chain units and the lifetime of the chain units in the adsorbed state decrease. The lifetime of chain units in the adsorbed state approaches zero at approximately 200 K and 500 K for PDMS chains at the surface of hydrophobic and hydrophilic Aerosil, respectively. 

Finally, the NMR and the dynamic mechanical study show that two regions are present in filled silicone rubbers above the Tg, which differ significantly in local chain mobility immobilized chain units adsorbed at the filler surface and mobile chain units outside the adsorption layer. The local chain motions outside the adsorption layer are similar to those for unfilled rubbers. Chain motions in the adsorption layer however are strongly restricted. The frequency of chain motions in the adsorption layer at 300 K is comparable to the fi-equency of chain motions in a crosslinked PDMS containing 3-4 elementary chain units between network junctions [26]. 

Adsorption junctions at the surface of active fillers are of importance due to the large total elastomer-filler interfacial area. The adsorption of chain units at the Aerosil surface causes a significant restriction of local chain motions in the first layer adjacent to the filler surface. The low mobile adsorbed chain units represents another type of network junction in filled elastomers. However, the adsorbed chain units are not rigidly linked to the surface of Aerosil above T. The lifetime of chain imits in the adsorbed state is already very short at room temperature chain units adhere to the filler surface for only tens of microseconds [9], It was shown in Part I that the fraction of adsorbed chain units decreases on heating due to chain desorption. Therefore, the amount of adsorption junctions decreases with the increase of temperature as shown in Fig. 13. 

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