Semicrystalline polymers homopolymers

SEM micrographs of two members of these polymers (HB and HBIB-50) are shown in Figure 7 to provide further evidence for superstructure on the micron level within the solution cast films. One can directly observe the surface of the spherulitic structure of the HB homopolymer as well as in that of the copolymer HBIB-50. Clearly, the level of structure (-5 pm) is well above that of the individual domains of either HB or HI and reflects the possible primary nucleation and subsequent growth behavior common to spherulitic semicrystalline polymers. The Hv patterns shown in... 

In this final section, emphasis will be placed on the relationship between the behaviour of the homopolymer and that of heterogeneous systems containing interfaces. Thus, in Sect. 4.2, rather than dwell on the (albeit very important) technological aspects of welding, the discussion centres on the extent to which studies of interfaces might help in understanding the fundamentals of fracture in semicrystalline polymers, as they have in the case of interfaces between amorphous polymers [137]. 

In this chapter we will not discuss cavitational response of many semicrystalline polymers which may exhibit forms quite similar to the conventional crazing behavior of both glassy homopolymers and rubber-modulated heterogeneous polymers. In view of its great complexity, the behavior of such materials has not been widely studied. For a general discussion of such phenomena, however, the reader is referred to Chapter VIII and to the earlier work of Friedrich... 

This review has illustrated various properties of multiphase polymer systems obtained from computer simulation. Three modeling techniques - atomistic, coarse-grained, and atomistic-continuum modeling - are applied to miscibility of homopolymer/copolymer and homopolymer/homopolymer blends, compat-ibilizing effect of block copolymers, and mechanical properties of semicrystalline polymers, respectively. 

Benkoski et al. have utilized diblock copolymers, composed of a glassy block and a semicrystalline block, to reinforce an interface [9]. Their studies indicate that the penetration of the chains from the diblock into the homopolymer allow a transfer of stress across the interface that is dependent on parameters related to the crystalline chains in the diblock. By increasing the length of the crystalline portion of the diblock, values of the fracture energy increased from 1 to 700 J mT [9]. As with the experiments of Bidaux et al, the largest values of the fracture energy were attributed to plastic deformation and cohesive failure within the semicrystalline polymer. 

The crystallization of blends tends to depend on the level of mutual miscibility of the components. In miscible blends, the general result is that suppression or otherwise of crystallization with miscibility is dependent on the relative glass transition temperatures of both phases [33, 34]. For example, in a blend of an amorphous and semicrystalline polymer, if the amorphous material has the higher Tg, the miscible blend will also have a higher Tg than that of the semicrystalline homopolymer and, at a given temperature, the mobility and thus the efficacy of the semicrystalline phase molecules to crystallize is reduced. The converse is often true if the amorphous phase has a lower glass transition. Effects such as chemical interactions and other thermodynamic considerations also play a role and the depression of the melting point in a miscible blend can be used to determine the Flory interaction parameter x [40]. 

The dimensions of the crystal and amorphous subsystems vary from nanometers to micrometers, i.e., polymeric materials are nanophase- or microphase-separated and have only a partial crystallinity. If we treat homopolymers as one-component systems, the phase rule of Sect. 2.5.7 does not permit equilibrium between two phases, except at the transition temperature. Partially crystalline homopolymers are, thus, not in equilibrium. The properties of semicrystalline polymers are critically influenced by the interactions between the amorphous and crystalline domains, as is seen in the formation of rigid amorphous fractions, discussed in Sect. 6.1.3 and 6.3.4. 

As demonstrated before, the shifting involves three shift factors, one horizontal, usually expressed as a-j- = bi-rio(T)/rio(To), where b-j- = PoTo/pT is the first vertical shift factor that originates in the thermal expansion of the system (p is density). The subscript o indicates the reference conditions, defined by the selected reference temperature To, usually taken in the middle of the explored T-range. For homopolymer melts as well as for amorphous resins, the two shift factors, a and bj, are sufficient. However, for semicrystalline polymers the second vertical factor Vj has been found necessary - it accounts for variation of... 

Reorganization processes, however, are in principle more developed in semicrystalline polymers, where the application temperature lies above Tg, which for polypropylene (PP) homopolymers is at about 0-+4°C (in random copol3miers with ethylene, Tg can be lowered to -10°C). Struik [1] considers crystalline polymers as inhomogeneous systems with reduced mobility. Below the glass transition Tg, the same behavior as for amorphous ones is expected (in the amorphous regions) above Tg, the main effect should be due to a widened glass transition Struik does not accept any contribution of the crystalline regions. 

In the case of semicrystalline polymers, glass temperature is not easily determined with the usual measurements. For such polymers, an indirect technique can be used. Basically, this involves taking two semicrystalline homopolymers and preparing a group of amorphous copolymers that have varying mass fractions of each of the homopolymer materials. Then, according to the equation... 

The chemical iadustry manufactures a large variety of semicrystalline ethylene copolymers containing small amounts of a-olefins. These copolymers are produced ia catalytic polymerisation reactions and have densities lower than those of ethylene homopolymers known as high density polyethylene (HDPE). Ethylene copolymers produced ia catalytic polymerisation reactions are usually described as linear ethylene polymers, to distiaguish them from ethylene polymers containing long branches which are produced ia radical polymerisation reactions at high pressures (see Olefin POLYMERS, LOWDENSITY polyethylene). 

Vinyhdene chloride copolymers are available as resins for extmsion, latices for coating, and resins for solvent coating. Comonomer levels range from 5 to 20 wt %. Common comonomers are vinyl chloride, acrylonitrile, and alkyl acrylates. The permeability of the polymer is a function of type and amount of comonomer. As the comonomer fraction of these semicrystalline copolymers is increased, the melting temperature decreases and the permeability increases. The permeability of vinylidene chloride homopolymer has not been measured. 

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