Polymorphic behavior of SPS

The effects of molecular orientation on the crystallization and polymorphic behavior of SPS and SPS/poly(2,6-dimethyl-l,4-phenylene oxide) (PPO) blends were studied with wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry [37]. The oriented amorphous films of SPS and SPS/ PPO blends were crystallized under constraint at crystallization temperatures ranging from 140 to 240 °C. The degree of crystallinity was lower in the cold-crystallized oriented film than in the cold-crystallized isotropic film. It was inferred that the oriented mesophase was obtained in drawn films of SPS and that the crystallization of SPS was suppressed in that phase. The WAXD measurements showed that the crystal phase was more ordered in SPS/PPO blend than in pure SPS under the same annealing conditions. It was principally due to the decrease in the mesophase content. The crystal forms were found to be dependent on the crystallization temperature, blend composition, and... 

The great number of papers and patents that appeared on this polymer in the last years are the main results of these studies. The principal objective of these studies was the crystallization behavior of SPS, the structure of the ordered forms, and the properties of SPS with the respect to the processing conditions. The possibility of controlling the conditions for obtaining controlled structures indeed is particularly interesting in view of the different physical properties that the various polymorphic structures show. 

A cold compaction of SPS powders at a temperature well below its melting temperature (273 °C) was found possible [108]. In addition, the mechanical properties of the resulting material were comparable to those of the SPS submitted to compression molding after melting. Parallel experiments on poly(ethylene terephthalate) (PET) and linear low-density polyethylene (LLDPE) suggest that the behavior of SPS is a peculiarity of such polymers, which is likely connected to their polymorphic nature. 

In the section on structure and fundamental properties of SPS, Chapter 9 summarizes the polymorphic behavior of this polymer, the structure of the different forms, and the crystallization and melting behavior. Chapter 10 describes co-crystals and nanoporous crystalline phases of SPS regarding preparation, structure, properties, and new interesting applications, for example, molecular sensors. The section concludes with Chapter 11 on selected topics of crystallization thermodynamics and kinetics of SPS. 

Whereas atactic PS is an amorphous polymer with a Tg of 100 CC, syndio-tactic PS is semicrystalline with a Tg similar to aPS and a Tm in the range 255-275 °C. The crystallization rate of sPS is comparable to that of polyethylene terephthalate). sPS exhibits a polymorphic crystalline behavior which is relevant for blend properties. In fact, it can crystallize in four main forms, a, (3, -y and 8. Several studies [8] based on FTIR, Raman and solid-state NMR spectroscopy and WAXD, led the a and (3 forms to be assigned to a trans-planar zig-zag molecular chain having a (TTTT) conformation, whereas the y and 8 forms contain a helical chain with (TTG G )2 or (G+G+TT)2 conformations. In turn, on the basis of WAXD results, the a form is said to comply with a unitary hexagonal cell [9] or with a rhombohedral cell [10]. Furthermore, two distinct modifications called a and a" were devised, and assigned to two limiting disordered and ordered forms, respectively [10]. 

As fully described below, sPS has been found to be miscible with aPS, PPE, PYME, TMPC and styrene-l,l-diphenylethylene copolymer. Generally the reported investigations deal with the effect of the second component on crystalline features of sPS, such as polymorphic behavior, crystallization kinetics, morphology and growth rate of crystallites. Just one study reports on toughening sPS by adding suitable components. 

Figure 20.2 sPS/PPE blends (75 25 wt%) at different isothermal crystallization temperatures. (a) DSC thermograms (b) WAXD patterns. Reprinted from Polymer, vol. 39, Hong B. K., Jo W. H., Lee S. C., Kim J., Correlation between melting behavior and polymorphism of sPS and its blends with PPE , p. 1793, Copyright 1998, with permission from Elsevier Science... 

Su et al. [30] also performed direct and nonintrusive observations of crystallization and melting behavior of a and p polymorphs in bulk SPS by means of temperature-programmed X-ray diffraction. Results indicated that the perfection of the less ordered a form into the better ordered a" form within the a family occur in the vicinity of 270 °C. 

As discussed in detail in the previous chapter of this book, SPS is a stereoregular polymer presenting a very complex polymorphic behavior including five crystalline phases (two of which being nanoporous) and is capable to form co-crystals with several low-molecular-mass guest molecules [17,19,21-24],... 

---