Fractionation, semicrystalline polymers

Similarly to Crystaf, Tref is an analytical technique that fractionates semicrystalline polymers on the basis of chain crystallizabilities. However, Tref involves two consecutive steps, crystallization and elution, while Crystaf can perform a similar analysis in a single crystallization step. 

Stretched Polymers MF membranes may be made by stretching (Fig. 20-68). Semicrystalline polymers, if stretched perpendicular to the axis of crystallite orientation, may fracture in such a way as to make reproducible microchannels. Best known are Goretex produced from Teflon , and Celgard produced from polyolefin. Stretched polymers have unusually large fractions of open space, giving them very high fluxes in the microfiltration of gases, for example. Most such materials are very hydrophobic. 

Anantawaraskul, S., Soares,. B. P. and Wood-Adams, P. M. Fractionation of Semicrystalline Polymers by Crystallization Analysis Fractionation and Temperature Rising Elution Fractionation. Vol. 182, pp. 1-54. 

In principle, the calorimetric determination of the fraction of crystallinity of a semicrystalline polymer is simple. One needs only to know (1) the enthalpy change required to convert the sample to the completely... 

The study of the phase structure of semicrystalline polymers, particularly that of the interfacial region, is of foremost importance, since it has an intimate relationship with the macroscopic properties of polymers. Many techniques have been used to study this problem. To determine the mass fraction of each phase of the three-phase structure, such techniques as H broad-line NMR or Raman... 

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). 

Glass transition temperature—The temperature at which an amorphous polymer (or the amorphous fraction of a semicrystalline polymer) changes from a hard glassy state to a rubbery, flexible state abbreviated Tg. 

Considering the semicrystalline polymers as a two-phase system with a sharp delineation between the crystalline and the amorphous material, we can use the specific volumes to calculate the weight-fraction degree of crystallinity. 

The interest in multicomponent materials, in the past, has led to many attempts to relate their mechanical behaviour to that of the constituent phases (Hull, 1981). Several theoretical developments have concentrated on the study of the elastic moduli of two-component systems (Arridge, 1975 Peterlin, 1973). Specifically, the application of composite theories to relationships between elastic modulus and microstructure applies for semicrystalline polymers exhibiting distinct crystalline and amorphous phases (Andrews, 1974). Furthermore, as discussed in Chapter 4, the elastic modulus has been shown to be correlated to microhardness for lamellar PE. In addition, H has been shown to be a property that describes a semicrystalline polymer as a composite material consisting of stiff (crystals) and soft, compliant elements. Application of this concept to lamellar PE involves, however, certain difficulties. This material has a microstructure that requires specific methods of analysis involving the calculation of the volume fraction of crystallized material, crystal shape and dimensions, etc. (Balta Calleja et al, 1981). 

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