Glassy domains

The outstanding morphological feature of these rubbers arises from the natural tendency of two polymer species to separate one from another, even when they have similar solubility parameters. In this case, however, this is restrained because the blocks are covalently linked to each other. In a typical commercial triblock the styrene content is about 30% of the total, giving relative block sizes of 14 72 14. At this level the styrene end blocks tend to congregate into spherical or rod-like glassy domains embedded in an amorphous rubbery matrix. These domains have diameters of about 30 nm. 

In Chapters 3 and 11 reference was made to thermoplastic elastomers of the triblock type. The most well known consist of a block of butadiene units joined at each end to a block of styrene units. At room temperature the styrene blocks congregate into glassy domains which act effectively to link the butadiene segments into a rubbery network. Above the Tg of the polystyrene these domains disappear and the polymer begins to flow like a thermoplastic. Because of the relatively low Tg of the short polystyrene blocks such rubbers have very limited heat resistance. Whilst in principle it may be possible to use end-blocks with a higher Tg an alternative approach is to use a block copolymer in which one of the blocks is capable of crystallisation and with a well above room temperature. Using what may be considered to be an extension of the chemical technology of poly(ethylene terephthalate) this approach has led to the availability of thermoplastic polyester elastomers (Hytrel—Du Pont Amitel—Akzo). 

Triblock copolymers, as shown in Fig. 5.8 d), comprise a central homopolymer block of one type, the ends of which are attached to homopolymer chains of another type. As with other block copolymers, the components of triblocks may be compatible or incompatible, which will strongly influence their properties. Of particular interest are triblocks with incompatible sequences, the middle block of which is rubbery, and the end blocks of which are glassy and form the minor phase. When such polymers phase-segregate, it is possible for the end blocks of a single molecule to be incorporated into separate domains. Thus, a number of rubbery mid-block chains connect the glassy phases to one another. These materials display rubber-like properties, with the glassy domains acting as physical crosslinks. Examples of such materials are polystyrene/isoprene/polystyrene and polystyrene/polybutadiene/polystyrene triblock copolymers. 

Note The junction points in a reversible network are usually small erystallites or glassy domains sueh as those formed within block copolymers. 

The synthesis and characterization of these ABA block copolymers of styrene and dienes have been described elsewhere (JO, 11). Since the polystyrene end blocks aggregate into glassy domains which act as network junctions, the elastic center blocks must virtually represent the network chains/ The polystyrene domains should also act as a finely divided filler. Hence it might be expected that the mechanical properties of these materials could depend on the two basic parameters polystyrene content and length of center block ( molecular weight between crosslinks ). 

It is also possible to obtain reinforcement of a PDMS elastomer by polymerizing a monomer such as styrene to yield hard glassy domains within the elastomer.5051 Low concentrations of styrene give low-molecular-weight polymer that acts more like... 

These new variables are necessary to take into account viscoelastic effects linked to molecular motions. These effects are non-negligible in the glassy domain between boundaries a and (3 in the map of Fig. 11.2, and they are very important in the glass transition region (around boundary a). Here, we need relationships that express the effects of s, d (the stress rate may be used instead of the strain rate), and T on the previously defined elastic properties. Also numerical boundary values of elastic properties are required, characterizing unrelaxed and relaxed states (see Chapter 10). 

The relative errors on both complex modulus components are expected to be almost equal. These errors can be of the order of 50% or more for certain commercial apparatus, and they vary presumably with temperature, so that E measurements have to be calibrated in both rubbery and glassy domains. 

The situation is quite different with block copolymers. As an example we again take a copolymer of styrene and butadiene, but now as a three-block copolymer, SBS. The incompatibility of polystyrene and polybutadiene now results in a phase separation, which is enabled by the circumstance that the blocks can live their own life . The polystyrene chain ends clog together into PS domains, which lie embedded in a polybutadiene matrix. These glassy domains act as physical cross-links, so that the polymer has the nature of a thermoplastic rubber. The glass-rubber transitions of PS and BR both remain present in between these two temperatures the polymer is in a, somewhat stiffened, rubbery condition (see Figure 3.8). This behaviour is dealt... 

In the present study, the effects of composition, molecular weight, and heat treatment on the relaxation behavior of styrene—butadiene-styrene (SBS) block polymers are investigated. There is evidence (e.g., 6,7,8) that these types of multicomponent multiphase systems exhibit unusual phenomena in their dynamic mechanical behavior and in other physical properties. These are apparently related to the presence of the so-called interphase mixing region between the elastomeric and glassy domains. Similar evidence has been obtained by gas diffusion and sorption studies on the copolymer samples used in this investigation (9). 

C),than for block copolymers with NBCN and MTD, which showed glass transition temperatures above the isotropization temperature of I-n (poly-MTD Tg=214 °C, poly-NBECN Tg=116 °C). The amorphous glassy domains... 

The thermoplastic elastomer/PA blend was also prepared in order to avoid extra crosslinking steps. Electron microscopic results indicated that polyacetylene moieties incorporated into the rubbery matrix rather than into the polystrene glassy domains. 

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