Semicrystalline polymers, effect

Creton, C., Kramer, E.J., Hui, C.-Y. and Brown, H.R., Failure mechanisms of polymer interfaces reinforced with block copolymers. Macromolecules, 25, 3075-3088 (1992). Boucher et al., E., Effects of the formation of copolymer on the interfacial adhesion between semicrystalline polymers. Macromolecules, 29, 774-782 (1996). 

When the polymeric material is compressed the local deformation beneath the indenter will consist of a complex combination of effects. The specific mechanism prevailing will depend on the strain field depth round the indenter and on the morphology of the polymer. According to the various mechanisms of the plastic deformation for semicrystalline polymers 40 the following effects may be anticipated ... 

Figure 8.4 shows generic load versus elongation curves for rubbery amorphous, glassy amorphous, and semicrystalline polymers. In each case, the effect of extension on a dogbone specimen is shown at various points along the curve. 

The effects of processing will be illustrated by considering injection moulding of a semicrystalline polymer. The molten plastic is injected into the mould under high pressure and temperature. The edges of the mould retard flow and cool more rapidly, leading to a boundary layer of high shear, which in semicrystalline polymers leads to orientation of the polymer chains and of short fibre reinforcements parallel to the direction of flow. At the centre the structure is less oriented. Where two separate flow streams meet, there is a... 

It is of course important to note that the overall rate of crystallization is not only determined by the growth rate of the spherulites, but also by the amount of nuclei being present in the system. This possibility is used as an effective method to influence the total crystallization rate of commercial polymeric materials in a controlled manner and to influence the size of spherulites and thus the physical properties of finished articles made from semicrystalline polymers. 

Fig. 3.14. The data is for a very broad range of times and temperatures. The superposition principle is based on the observation that time (rate of change of strain, or strain rate) is inversely proportional to the temperature effect in most polymers. That is, an equivalent viscoelastic response occurs at a high temperature and normal measurement times and at a lower temperature and longer times. The individual responses can be shifted using the WLF equation to produce a modulus-time master curve at a specified temperature, as shown in Fig. 3.15. The WLF equation is as shown by Eq. 3.31 for shifting the viscosity. The method works for semicrystalline polymers. It works for amorphous polymers at temperatures (T) greater than Tg + 100 °C. Shifting the stress relaxation modulus using the shift factor a, works in a similar manner.

Boston, Ma., 7th-llth May 1995, p.2183-8. 012 EFFECT OF MORPHOLOGY ON MICROCELLULAR FOAMING OF SEMICRYSTALLINE POLYMERS Doroudiani S Park C B Kortschot M T Cheung L K Toronto,University (SPE)... 

Composite-based PTC thermistors are potentially more economical. These devices are based on a combination of a conductor in a semicrystalline polymer—for example, carbon black in polyethylene. Other fillers include copper, iron, and silver. Important filler parameters in addition to conductivity include particle size, distribution, morphology, surface energy, oxidation state, and thermal expansion coefficient. Important polymer matrix characteristics in addition to conductivity include the glass transition temperature, Tg, and thermal expansion coefficient. Interfacial effects are extremely important in these materials and can influence the ultimate electrical properties of the composite. 

Experimental studies of the temperature changes and heat effects resulting from reversible deformations of glassy and semicrystalline polymers have been carried out since the 1950s. The early results were summarized and analysed by Muller 1). Since that time, a number of experimental results on thermomechanical behaviour of glassy and semicrystalline polymers have been obtained. This part of our review is dealing with critical consideration of these results. 

The thermomechanical behaviour of undrawn semicrystalline polymers above Tg is shown in Fig. 14. The values of the coefficients of thermal expansion calculated from the heat effects agree well with dilatometric results. For PE, the influence of degree of crystallinity on the value of thermal effects and thermal expansion coefficients was also studied 64). 

In Fig. 27 experimental 1/e values are represented as a function of the temperature for samples with different compositions. As shown, the experimental values qualitatively follow Eq. 7 although 1/e achieves a non-zero value at the critical temperature. The intrinsic composite structure of semicrystalline polymers has been invoked to understand this effect [8, 6]. The order of magnitude of the constant A has been reported to be around 103 °C [11] which is consistent with the relatively high polarizability of these materials. At this point it is important to emphasize that the knowledge of morphological aspects of these copolymers might help, in future, to develop a theoretical framework capable of accounting for the experimental observations. 

This relation holds well for semicrystalline polymers for amorphous polymers, it holds for T > Tg + 100°C. Below this region free volume effects predominate necessitating the use of the Arrhenius-WLF equation... 

Higher values can be reached for semi-crystalline polymers below Tg the crystalline phase is stiffer than the glassy amorphous phase (e.g. PEEK, E 4 GPa). Semicrystalline polymers above Tg have, however, a much lower E-value, such as PE (0.15 to 1.4 GPa) and PP 1.3 GPa) E is, in these cases, strongly dependent on the degree of crystallinity and on the distance to Tg. Sometimes a low modulus is also found for semi-crystalline polymers below Tg, due to the effect of one or more secondary transitions a strong example is PTFE (E = 0.6 GPa ). 

Because diffusion is limited to the amorphous phase of semicrystalline polymers, and the crystalline phase can additionally restrict chain motion in the amorphous phase, the value of D is dependent on the degree of crystallinity of the polymer. To a first approximation, this effect may be expressed by equation 7, where x is the crystalline volume fraction and D is the diffusion coefficient of the totally amorphous polymer. For example, diffusion coefficients for high density polyethylene are lower than for low density polyethylene (3). 

Like polyester, polyamides are synthetic fibers made from semicrystalline polymers which find use in a variety of applications in textiles almost similar to those of polyester. A recent work of Horrocks et al.55 has investigated the effect of adding selected intumescent FRs based on APP, MP,... 

Historically most of the microscopic diffusion models were formulated for amorphous polymer structures and are based on concepts derived from diffusion in simple liquids. The amorphous polymers can often be regarded with good approximation as homogeneous and isotropic structures. The crystalline regions of the polymers are considered as impenetrable obstacles in the path of the diffusion process and sources of heterogeneous properties for the penetrant polymer system. The effect of crystallites on the mechanism of substance transport and diffusion in a semicrystalline polymer has often been analysed from the point of view of barrier property enhancement in polymer films (35,36). 

The effects of morphology (i.e., crystallization rate) (6,7, 8) on the mechanical properties of semicrystalline polymers has been studied without observation of a transition from ductile to brittle failure behavior in unoriented samples of similar crystallinity. Often variations in ductlity are observed as spherulite size is varied, but this is normally confounded with sizable changes in percent crystallinity. This report demonstrates that a semicrystalline polymer, poly(hexamethylene sebacate) (HMS) may exhibit either ductile or brittle behavior dependent upon thermal history in a manner not directly related to volume relaxation or percent crystallinity. 

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