Elastomeric behavior

Stress-strain tests of these perfectly alternating PDMS-PSF copolymers show that the mechanical behavior is dictated by the volume fraction of PDMS present in the system. At high siloxane content (> 70 wt %), copolymers show elastomeric behavior Hue to the presence of continuous PDMS matrix. An increase in the PSF content resulted in an increase in the initial modulus and the ultimate tensile strength of these materials, while a decrease in the ultimate elongation was also observed, as expected. 

In the case of poly(alkoxyphosphazenes) (IV) or poly(aryloxyphos-phazenes) (V) a dramatic change in properties can arise by employing combinations of substituents. Polymers such as (NP CHjCF ) and (NP CgH,).) are semicrystalline thermoplastics (Table I). With the introduction of two or more substituents of sufficiently different size, elastomers are obtained (Figure 4). Another requirement for elastomeric behavior is that the substituents be randomly distributed along the P-N backbone. This principle was first demonstrated by Rose (9), and subsequent work in several industrial laboratories has led to the development of phosphazene elastomers of commercial interest. A phosphazene fluoroelastomer and a phosphazene elastomer with mixed aryloxy side chains are showing promise for military and commercial applications. These elastomers are the subject of another paper in this symposium (10). 

With regard to elastomers of controlled structure, those having unusual distributions of network chain lengths have been of particular interest [88,89]. The most novel elastomer of this type consists of a binary combination of unusually short network chains (molecular weights of a few hundred) and the much longer chains typically associated with elastomeric behavior (molecular weights of ten or twenty thousand). Such a network is sketched in Figure 6. 

Thermoplastic elastomeric behavior requires that the block copolymer develop a microheterogeneous two-phase network morphology. Theory predicts that microphase separation will occur at shorter block lengths as the polarity difference between the A and B blocks increases. This prediction is borne out as the block lengths required for the polyether-polyurethane, polyester-polyurethane, and polyether-polyester multiblock copolymers to exhibit thermoplastic elastomeric behavior are considerably shorter than for the styrene-diene-styrene triblock copolymers. 

PSA compositions must have a good balance of different properties, depending on its specific application. PSAs for adhesive tapes for instance, must have a good adhesion, cohesion, stretchiness and elasticity. Furthermore the base polymers must be elastomers at room temperature [124]. Due to its elastomeric behavior and biodegradability, poly(HAMCL) seems to be a promising base polymer for the development of a completely biodegradable PSA. 

These observations can be qualitatively explained in terms of the constrained-junction theory. If a network is cross-linked in solution and the solvent then removed, the chains collapse in such a way that there is reduced overlap in their configurational domains. It is primarily in this regard, namely reduced chain-junction entangling, that solution-cross-linked samples have simpler topologies, and these diminished constraints give correspondingly simpler elastomeric behavior. 

The character of the counterion and the solvent both affect the microstruclure of polymers made anionically from dienes. In general, the proportion of 1,4 chains is highest for Li and decreases with decreasing clecironegativity and increasing size of the alkali metals in the order Li > Na > K > Rb > Cs. A very high (>90%) 1,4 content is achieved only with lithium alkyl or lithium metal initiation in hydrocarbon solvents. The properties of polymers of conjugated diolefins tend to be like those of thermoplastics if the monomer enchainment is 1,2 or 3,4 [reactions (4-3) and (4-4)]. Elastomeric behavior is realized from 1,4 polymerization and particularly if the polymer structure is cis about ihe residual double bond. 

The properties of block copolymers that are most affected by molecular architecture are elastomeric behavior, melt processability, and toughness in the solid state. The effects of such copolymers in polymer blends can obviously also be strongly influenced by the same factors. 

Since 1980, siloxane-urea block copolymers prepared by the reaction of bisaminoalkyl-terminated silicones and diisocyanates [1] have been well known in the literature. Silicone is responsible for the elastomeric behavior and urea for the thermal behavior. Nevertheless, to obtain high-molecular-weights in the polymerization of such copolymers, the starting compounds have to be sufficiently pure [2]. Therefore, one key step in the synthesis of such block copolymers is the preparation of... 

Thermoplastic elastomers are a more recently developed class of polymer in which the neat polymer has inherently elastomeric properties, yet it behaves thermoplastically. With some limitations, these materials can be formed by essentially the same operations as thermoplastics, but the final object has elastomeric behavior. Objects molded with thermoplastic elastomers can be melted and re-shaped. Since these are not crosslinked, their creep resistance and extended high temperature use are limited. 

The use of monomers that produce amorphous hard segments produces highly compatible systems with poorly defined rubbery plateau and elastomeric behavior. Because no heterogenous conditions develop during the reaction, molecular weight in amorphous polyurethane systems tends to be less affected by reaction rates or polymerization conditions. 

The performance expected from a sealant and the polymer used in its production determine the type of fillers used. From the point of view of performance, sealants can be divided into these having plastic behavior and these with elastomeric behavior. Sealants which have plastic behavior are low-cost and low-performance products which are being gradually eliminated from the market. These sealants use inexpensive fillers to lower cost and regulate non-sag properties. Typical fillers used in these products are calcium carbonate and some fibrous materials which are used as replacement for asbestos which was very popular in the past. This combination of fibrous and spherical particles provided a useful tool to the sealant formula-tor to regulate non-sag properties which are very important in sealants. Fibers have also been used to reinforce these products because the properties of polymers were poor. 

The properties of polymers of conjugated diolefins tend to be like those of thermoplastics if the monomer enchainment is 1,2 or 3,4. Elastomeric behavior is shown by 1,4-polymer, particularly if the polymer structure is cis about the residual double bond. Natural rubber is head-to-tarl cis-polyisoprene. 

The simple treatment provided above captures the most essential aspects of the response of elastomers to mechanical deformation. Building a more profound understanding of the subtler aspects of elastomeric behavior requires detailed (and computationally intensive) numerical simulations, as illustrated for example in the recent work of Grest et al [146]. 

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