Semicrystalline polymers chain conformations

We have, in conclusion, presented a systematic approach for investigating polymer chain conformations in the amorphous and semicrystalline materials. Fourier transform infrared spectroscopy has been utilized and combined with the techniques of factor analysis, absorbance ratioing, and least squares curve-fitting. 

The crystallization process of flexible long-chain molecules is rarely if ever complete. The transition from the entangled liquid-like state where individual chains adopt the random coil conformation, to the crystalline or ordered state, is mainly driven by kinetic rather than thermodynamic factors. During the course of this transition the molecules are unable to fully disentangle, and in the final state liquid-like regions coexist with well-ordered crystalline ones. The fact that solid- (crystalline) and liquid-like (amorphous) regions coexist at temperatures below equilibrium is a violation of Gibb s phase rule. Consequently, a metastable polycrystalline, partially ordered system is the one that actually develops. Semicrystalline polymers are crystalline systems well removed from equilibrium. 

Many semicrystalline polymers are polymorphic and exist in different crystal forms. When PBT fiber is uniaxially stretched [75], the contracted gauche-trans-gauche a-crystal chain is extended to a fully trans conformation of a y-crystal. Above 20% strain, the crystal form is 100% y-crystal with a longer c-axis triclinic cell dimension. Thus, it is reasonable to ask whether the... 

J. A. Kulkarni and A. N. Bens, Lattice-based Simulation of Chain Conformation in Semicrystalline Polymers with Application to Flow Induced Crystallization, J. Non-Newt. Fluid Meek, 82, 331-336 (1999). 

Figure 4.5(a) shows the three principal regions of a semicrystalline polymer namely the crystalline region with a three-dimensional ordered structure, the interfacial region and the interzonal or amorphous regions, which consist of chain units in non-ordered conformations that connect crystallites. 

Accordingly, the influence of MW on the crystallization behaviors of semicrystalline polymers has been studied in various articles. For example, linear crystal growth rates of poly(ethylene oxide) and poly(ethylene succinate) (PES) reach a minimum value at a critical MW. This value is related to the crystallization transition from an extended chain to a folded chain conformation [96,97], suggesting that high MW polymers require sufficient reconformation time to achieve an ordered structure. As evidence of this MW dependence of the semicrystalline polymer on... 

The unordered (amorphous) state of aggregation in which the polymer chains also assume random conformations represents one extreme in the physical state of the polymer. This is the state that exists in such amorphous states as solution, melts, or some solids, the randomness being induced by thermal fluctuations. The other extreme is the case where the molecules are able to pack closely in perfect parallel alignment as is found in those polymers that exhibit fibrous behavior— that is, in those possessing a high degree of crystallinity and crystal orientation. In between these two extremes of amorphous and crystalline polymers there is a wide spectrum of polymeric materials with different degrees of crystallinity and amorphous character. These are called semicrystalline. 

For the purpose of this work, processing includes three steps PVTFA sample preparation, solvolysis, and hydration. PVTFA is a semicrystalline polymer that has not been studied extensively because of its poorly defined crystallinity. Wide-angle X-ray diffraction of unoriented polymer tends to give very diffuse scattering maxima. The polymer exhibited a distinct melting endotherm with a small AH of around 13 cal/g. In the most detailed structural study published, Bohn et al. (14) suggested that the X-ray data indicated a large-pitch helical structure for the chain. PVA, on the other hand, is believed to crystallize in a planar zig-zag conformation (15) with a much more typical AH of fusion of 30 cal/g (16). 

---