Semicrystalline polymers yielding behavior

Practical problems associated with infrared dichroism measurements include the requirement of a band absorbance lower than 0.7 in the general case, in order to use the Beer-Lambert law in addition infrared bands should be sufficently well assigned and free of overlap with other bands. The specificity of infrared absorption bands to particular chemical functional groups makes infrared dichroism especially attractive for a detailed study of submolecular orientations of materials such as polymers. For instance, information on the orientation of both crystalline and amorphous phases in semicrystalline polymers may be obtained if absorption bands specific of each phase can be found. Polarized infrared spectroscopy can also yield detailed information on the orientational behavior of each component of a pol3mier blend or of the different chemical sequences of a copoljnner. Infrar dichroism studies do not require any chain labelling but owing to the mass dependence of the vibrational frequency, pronounced shifts result upon isotopic substitution. It is therefore possible to study binary mixtures of deuterated and normal polymers as well as isotopically-labelled block copolymers and thus obtain information simultaneously on the two t3q>es of units. 

The restraining influence of the crystallite alters the mechanical behavior by raising the relaxation time T and changing the distribution of relaxation and retardation times in the sample. Consequently, there is an effective loss of short T, causing both the modulus and yield point to increase. The creep behavior is also curtailed and stress relaxation takes place over much longer periods. Semicrystalline polymers are also observed to maintain a relatively higher modulus over a wider temperamre range than an amorphous sample. 

For polyamide 6 crystals the slip systems are (001) [010] chain slip at 16.24 MPa, (100)[010] chain slip at 23.23 MPa and (001)[100] transverse slip [71]. Relatively little attention was paid to the plastic deformation of other semicrystalline polymers [98,110]. In particular, there are only a few papers [111,112] describing the investigations of the yield behavior and plastic resistance of oriented iPP. 

The stress-strain curves of semicrystalline polymers show, at low strain rate, a sharp drop in stress after the yield point. After the neck formation, and during a certain time period, the stress does not change appreciably with further strain. Finally, there is a slight increase in stress and then the specimen breaks. Such behavior is typical for HDPE but not for LDPE. The drop in stress after the neck... 

The deformation behavior of amorphous polymers has been studied extensively, partly because the structure is rather simple as compared with semicrystalline polymers thus, the relationship between structure and properties can be established with relative ease. It is well known that two major micromechanisms are involved in the deformation and subsequent fracture of glassy polymers [1,2,13] (see Figs. 18.1 and 18.2). These are crazing and shear yielding, and both involve localized plastic deformation and some energy is dissipated during the deformation. In a craze, polymer chains are stretched along the stress direction and... 

It is well known that the mechanical behavior of glassy amorphous polymers is strongly influenced by hydrostatic pressure. A pronounced change is that polymers, which fracture in a brittle manner, can be made to yield by the application of hydrostatic pressure Additional experimental evidence for the role of a dilatational stress component in crazing in semicrystalline thermoplastics is obtainai by the tests in which hydrostatic pressure suppresses craze nucleation as a result, above a certain critical hydrostatic pressure the material can be plastically deformed. 

The same type of methodology was also used to prepare ferrocene-containing arylidene polyesters 122 in good yields from dicarboxyl ferrocenes and organic diols. These materials were characterized by elemental analysis, IR spectroscopy, viscometry, and WAXS. The polymers were found to be semicrystalline but were soluble in polar organic solvents. Conductivity studies showed an n-type semiconductor behavior (cr = 3 x 10 Scm at room temperature) that followed a one-term Arrhenius-type equation with increasing conductivity over the range 25-220 °G. 

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