Hard domains

Block copolymers with stmctures such as A—B or B—A—B ate not thermoplastic elastomers, because for a continuous network to exist both ends of the elastomer segment must be immobilized in the hard domains. Instead, they are much weaker materials resembling conventional unvulcanized synthetic mbbers (4). 

In TPE, the hard domains can act both as filler and intermolecular tie points thus, the toughness results from the inhibition of catastrophic failure from slow crack growth. Hard domains are effective fillers above a volume fraction of 0.2 and a size <100 nm [200]. The fracture energy of TPE is characteristic of the materials and independent of the test methods as observed for rubbers. It is, however, not a single-valued property and depends on the rate of tearing and test temperature [201]. The stress-strain properties of most TPEs have been described by the empirical Mooney-Rivlin equation... 

Komfield J.A., Spiess H.W., Nefzger H., Hayen H., and Eisenbach C.D. Deuteron NMR measurement of order and mobility in the hard segments of a model polyurethane, Macromolecules, 24, 4787, 1991. Meltzer A.D., Spiess H.W., Eisenbach C.D., and Hayen H. Motional behaviour within the hard domain of segmented polyurethane A NMR study of a triblock model system. Macromolecules, 25, 993, 1992. 

PU elastomers contain alternative soft and hard segments, which separate into different phases. Hard domains play a role of cross-links, whereas soft blocks provide extensibility. Therefore, morphology and properties of PU are defined by relative amount of soft and hard segments. For example, at 70% concentration of soft segments, the material is described as a mbbery matrix with... 

For instance, inaccurate positions of spherical hard-domains in their lattice of colloidal dimensions 2SIn real space there is a convolution of the ideal atom s position (a delta-function) with the real probability distribution to find it. 

For instance, crystalline lamellae in an amorphous matrix (semicrystalline polymer materials), hard domains in a soft matrix (thermoplastic elastomers)... 

Let us consider the other example. In an anisotropic material we select the fiber axis, r3, project the intensity on this direction and compute an IDF. Then the meaning of the thickness distributions is quite similar as in the aforementioned example. Let us identify the first thickness distribution, /i/,(r3), by a distribution of hard-domain thicknesses. Then the next thickness distribution, hs (r3), is the thickness distribution... 

The Material of the Example. Poly(ether ester) (PEE) materials are thermoplastic elastomers. Fibers made from this class of multiblock copolymers are commercially available as Sympatex . Axle sleeves for automotive applications or gaskets are traded as Arnitel or Hytrel . Polyether blocks form the soft phase (matrix). The polyester forms the hard domains which provide physical cross-linking of the chains. This nanostructure is the reason for the rubbery nature of the material. 

How should such rigid domain coupling work In principle domains can only be rigidly coupled by a bridge of hard-phase material which has a different density. We know that the polyester hard-phase is semicrystalline. So the observation is indicative for a structure in which the hard domains are subdivided into crystalline and amorphous zones. 

The size of the hard domains of the multiblock and triblock copolymers is in the range 30-100X and 100-300, respectively. 

The nature of the hard domains differs for the various block copolymers. The amorphous polystyrene blocks in the ABA block copolymers are hard because the glass transition temperature (100°C) is considerably above ambient temperature, i.e., the polystyrene blocks are in the glassy state. However, there is some controversy about the nature of the hard domains in the various multiblock copolymers. The polyurethane blocks in the polyester-polyurethane and polyether-polyurethane copolymers have a glass transition temperature above ambient temperature but also derive their hard behavior from hydrogen-bonding and low levels of crystallinity. The aromatic polyester (usually terephthalate) blocks in the polyether-polyester multiblock copolymer appear to derive their hardness entirely from crystallinity. 

The block copolymer made from connecting blocks of PS with blocks of polybutadiene illustrates another use of soft and rigid or hard domains in TPEs. The PS blocks give rigidity to the polymer, while the polybutadiene blocks act as the soft or flexible portion. The PS... 

In general, block copolymers are heterogeneous (multiphase) polymer systems, because the different blocks from which they are built are incompatible with each other, as for example, in diene/styrene-block copolymers. This incompatibility, however, does not lead to a complete phase separation because the polystyrene segments can aggregate with each other to form hard domains that hold the polydiene segments together. As a result, block copolymers often combine the properties of the relevant homopolymers. This holds in particular for block copolymers of two monomers A and B. 

From the molecular mobilities of the soft and hard phases, it appears that the increase in phase mixing caused by crosslinking, is more important in determining phase mobility than a simple increase in crosslink density. The fraction of protons in the hard phase (f) is relatively independent of crosslink density at 28 °C. However, f in linear and crosslinked PEU have different temperature behaviors (Fig. 12). Incurred PEU exhibits a single FID below —20 °C and again above 80 °C, when both phases are below and above T, respectively. In the region between approximately 10 and 75 °C, at which temperatures the hard and soft phase undergoes a transition, respectively, the fraction of protons in hard domains remains relatively constant. However, f in the crosslinked PEU decreases in a continuous fashion over a relatively... 

In the case of Fig. 7.6a the cluster formation and the size distribution can be influenced not only by chemical reactions but also by partial miscibility of the substructures during reaction. Polyurethane networks prepared from polyolefin instead of polyester or polyether as macrodiol, can serve as an example. In this particular case an agglomeration of hard domains takes place in the pregel stage, produced by a thermodynamic driving force. 

Reaction-induced phase separation is certainly also the reason for which an inhomogeneous structure is observed for photocured polyurethane acrylate networks based on polypropylene oxide (Barbeau et al., 1999). TEM analysis demonstrates the presence of inhomogeneities on the length scale of 10-200 nm, mostly constituted by clusters of small hard units (the diacrylated diisocyanate) connected by polyacrylate chains. In addition, a suborganization of the reacted diisocyanate hard segments inside the polyurethane acrylate matrix is revealed by SAXS measurements. Post-reaction increases the crosslink density inside the hard domains. The bimodal shape of the dynamic mechanical relaxation spectra corroborates the presence of a two-phase structure. 

Thermoplastic elastomers (TPE s) are characterized by the exceptional property that, without vulcanization, they behave as cross-linked rubbers. They are block-copolymers, in which blocks of the same nature assemble in hard domains, acting as cross-links between the rubbery parts of the chain. These hard domains lose their function when they reach their softening temperature, so that the material can then be processed as a thermoplast. One of the oldest member of the family of TPE s is SBS (styrene-butadiene-styrene block copolymer), but several other TPE s have been developed, i.a. on the basis of polyesters, polyurethanes and polyolefins. In their properties these polymers cover a broad range between conventional rubbers and soft thermoplastics. 

The desirability of segregation in block copolymers can be demonstrated by considering the behaviour of SBS, which is one of the oldest types. It has about the same chain composition as SBR, but, rather than SBR, it shows two glass-rubber transitions, namely that of polybutadiene and that of polystyrene. Between these two temperatures it behaves as a rubber, in which the PS domains act as cross-links it is, therefore, a self-vulcanizing rubber (see also Figure 3.8 see Qu. 9.14). Moreover, the hard domains play the role of a reinforcing filler. 

Commercial silyl-epoxy hybrid adhesives have been developed by Collano AG. These are polymer alloys consisting of a matrix of silyl reactive polymers (elastic phase), which host domains of epoxy reactive polymers (hard domains). With a different choice of components, mixing ratios, and the size of respective domains, the overall properties of the bond line can be tailored for specific purposes. 

The presence of hard and soft domains in segmented polyurethanes also has been confirmed by experimental results using pulsed NMR and low-frequency dielectric measurements. Assink (55) recently has shown that the nuclear-magnetic, free-induction decay of these thermoplastic elastomers consists of a fast Gaussian component attributable to the glassy hard domains and a slow exponential component associated with the rubbery domains. Furthermore, the NMR technique also can be used to determine the relative amounts of material in each domain. 

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