Crystalline polymers stress

ABS, polycarbonate and polysulphone) but large effects on crystalline polymers. It is particularly interesting, as well as being technically important, that for many crystalline polymers the unfilled polymer has a heat deflection temperature (at 1.82MPa stress) similar to the Tg, whereas the filled polymers have values close to the T (Table 9.2). 

Distortion in mouldings can be worse in crystalline polymers than in amorphous plastics. This is because additional stresses may be set up as a result of varying crystallinity from point to point in the moulding so that the shrinkage on cooling from the melt also varies from point to point. This uneven shrinkage sets up stresses which may lead to distortion. 

Materials of these types have T s of some 290-300°C and some grades are claimed to be stable to about 400°C. Whilst resistant to hydrocarbons, halogenated hydrocarbons, ethers and acids the polymers are soluble in such materials as dimethylformamide, N-methylpyrrolidone and pyridine. Bases can cause stress cracking. These non-crystalline polymers are tough at temperatures as low as -46°C whilst at 260°C they have the strength shown by PTFE at room temperature. The polymers also exhibit excellent electrical insulation properties. 

The aim of this chapter is to describe the micro-mechanical processes that occur close to an interface during adhesive or cohesive failure of polymers. Emphasis will be placed on both the nature of the processes that occur and the micromechanical models that have been proposed to describe these processes. The main concern will be processes that occur at size scales ranging from nanometres (molecular dimensions) to a few micrometres. Failure is most commonly controlled by mechanical process that occur within this size range as it is these small scale processes that apply stress on the chain and cause the chain scission or pull-out that is often the basic process of fracture. The situation for elastomeric adhesives on substrates such as skin, glassy polymers or steel is different and will not be considered here but is described in a chapter on tack . Multiphase materials, such as rubber-toughened or semi-crystalline polymers, will not be considered much here as they show a whole range of different micro-mechanical processes initiated by the modulus mismatch between the phases. 

In the case of crystalline polymers it may be that solvents can cause cracking by activity in the amorphous zone. Examples of this are benzene and toluene with polyethylene. In polyethylene, however, the greater problem is that known as environmental stress cracking , which occurs with materials such as soap, alcohols, surfactants and silicone oils. Many of these are highly polar materials which cause no swelling but are simply absorbed either into or on to the polymer. This appears to weaken the surface and allows cracks to propagate from minute flaws. 

Copolymerization of ethylene and styrene by the INSITE technology from Dow generates a new family of ethylene-styrene interpolymers. Polymers with up to 50-wt% styrene are semicrystalline. The stress-strain behavior of the low-crystallinity polymers at ambient temperature exhibits elastomeric characteristics with low initial modulus, a gradual increase in the slope of the stress-strain curve at the higher strain and the fast instantaneous recovery [67], Similarly, ethylene-butylene copolymers may also be prepared. 

Polypropylene (PP) is a crystalline polymer suitable for low-stress applications up to 225°F (105°C). For piping applications this polymer is not recommended above 212°F (100°C). Polypropylene is shielded, pigmented, or stabilized to protect it from uv light. Polypropylene is often a combination of polyethylene and polypropylene which enhances the ductility of the polymer. 

For glassy and crystalline polymers there are few data on the variation of stress relaxation with amplitude of deformation. However, the data do verily what one would expect on the basis of the response of elastomers. Although the stress-relaxation modulus at a given time may be independent of strain at small strains, at higher initial fixed strains the stress or the stress-relaxation modulus decreases faster than expected, and the lloltz-nuinn superposition principle no longer holds. 

For elastomers, factorizability holds out to large strains (57,58). For glassy and crystalline polymers the data confirm what would be expected from stress relaxation—beyond the linear range the creep depends on the stress level. In some cases, factorizability holds over only limited ranges of stress or time scale. One way of describing this nonlinear behavior in uniaxial tensile creep, especially for high modulus/low creep polymers, is by a power... 

The temperature dependence of the compliance and the stress relaxation modulus of crystalline polymers well above Tf is greater than that of cross-linked polymers, but in the glass-to-rubber transition region the temperature dependence is less than for an amorphous polymer. A factor in this large temperature dependence at T >> TK is the decrease in the degree of Crystallinity with temperature. Other factors arc the reciystallization of strained crystallites ipto unstrained ones and the rotation of crystallites to relieve the applied stress (38). All of these effects occur more rapidly as the temperature is raised. 

Crystallinity—about.i to 15% (213,232). The creep of plasticized poly(vinyl chloride) polymers as a function of temperature, concentration, and kind of plasticizer has been studied by many workers, including Aiken et ai. (232), Neilscn ct ai. (234), and Sabia and Eirich (243). These last workers also studied stress relaxation (244). In the case of crystalline polymers, plasticizers and Copolymerization reduce the melting point and the degree of Crystallinity. These factors tend to increase the creep and stress relaxation, especially at temperatures approaching the melting point. 

Host irradiated polymers show a continuing change in properties for a long period after irradiation. These post-irradiation effects may be attributed to (1) trapped radicals which react slowly with the polymer molecules and with oxygen which diffuses into the polymer (2) peroxides formed by irradiation in the presence of air or trapped within polymers irradiated in vacuum or an inert atmosphere) and slowly decompose with formation of reactive radicals, usually leading to scission, (3) trapped gases in glassy and crystalline polymers which cause localized stress concentrations. 

The critical state of stress-induced crystallization at high spinning speeds is governed by the viscoelasticity of the polymer in combination with its crystallization behavior. Any kind of coarse particle obviously disturbs the structure and affects the resistance against deformation. The development of stress is controlled by the rheological properties of the polymer. Shimizu et al. [4] found that increasing the molecular weight drastically promotes the crystallinity under stress conditions. 

D. Cai, M. Song, A simple route to enhance the interface between graphite oxide nanoplatelets and a semi-crystalline polymer for stress transfer, Nanotechnology, 20 (2009) 315708. 

In terms of the mechanical behavior that has already been described in Sections 5.1 and Section 5.2, stress-strain diagrams for polymers can exhibit many of the same characteristics as brittle materials (Figure 5.58, curve A) and ductile materials (Figure 5.58, curve B). In general, highly crystalline polymers (curve A) behave in a brittle manner, whereas amorphous polymers can exhibit plastic deformation, as in... 

The thermal properties of fillers differ significantly from those of thermoplastics. This has a beneficial effect on productivity and processing. Decreased heat capacity and increased heat conductivity reduce cooling time [16]. Changing thermal properties of the composites result in a modification of the skin-core morphology of crystalline polymers and thus in the properties of injection molded parts as well. Large differences in the thermal properties of the components, on the other hand, lead to the development of thermal stresses, which also influence the performance of the composite under external load. 

The degree of crystallinity may be calculated from the density of the polymer if the density is known for the amorphous and crystalline states. Some crystallizable polymers are polymorphic, i.e., they may exist in more than one crystalline form. An unstable crystalline form may change to a more stable form, and crystalline forms may change under stress. For example, hdpe changes from an orthorhombic crystalline polymer to a monoclinic form when subjected to compressive forces. 

Table 5.2 lists polymers and their tendency toward crystallinity. Yield stress and strength, and hardness increase with an increase in crystallinity as does elastic modulus and stiffness. Physical factors that increase crystallinity, such as slower cooling and annealing, also tend to increase the stiffness, hardness, and modulus of a polymeric material. Thus polymers with at least some degree of crystallinity are denser, stiffer, and stronger than amorphous polymers. However, the amorphous region contributes to the toughness and flexibility of polymers. 

The dynamic behavior of liquid-crystalline polymers in concentrated solution is strongly affected by the collision of polymer chains. We treat the interchain collision effect by modelling the stiff polymer chain by what we refer to as the fuzzy cylinder [19]. This model allows the translational and rotational (self-)diffusion coefficients as well as the stress of the solution to be formulated without resort to the hypothetical tube model (Sect. 6). The results of formulation are compared with experimental data in Sects. 7-9. 

In recent years, the behaviour of liquid crystalline polymers including elastomers has been a subject of considerable interest 104,105). It is known that small molecule liquid crystals turn into a macroscopic ordered state by external electric or magnetic fields. A similar behaviour seems to occur for liquid-crystalline polymer networks under mechanical stress or strain. 

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