Crystallization behavior of SPS

The crystallization rate is dependent on the crystallization temperature, T, and molecular weight of the polymer. It is important to study the effect of T and molecular weight on the crystallization nature of SPS. 

JJ = activation energy for diffusion T0c = temperature where flow stops Tm 0 = equilibrium melting temperature Kg = nucleation constant. 

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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. 

La Carubba et al. [85] proposed a two-phase model to describe the crystallization behavior of SPS. The samples were analyzed by macroscopic methods, such as density, WAXD, and microhardness (MH) measurements. The density was strictly related to the phase content, as confirmed by WAXD deconvolution [85]. 

McKeiman RL, Heintz AM, Hsu SL, Atkins EDT, Penelle J, Gido SP. Influence of hydrogen bonding on the crystallization behavior of semicrystalline polyurethanes. Macromolecules 2002 35 6970-6974. 

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]. 

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... 

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... 

Gausepohl et ah [31] investigated the behavior of blends between sPS and random styrene-l,l-diphenylethylene copolymers obtained by anionic synthesis. The blends were miscible for copolymer contents of 1,1-diphenylethylene lower than 15 wt% as indicated by the occurrence of a single Tg (114°C). Tm and crystallization rate were not influenced. 

Most papers published on single-crystal-face behavior deal with this situation because most ions adsorb on solid electrodes of sp and sd metals. Adsorption depends on the nature of the ions and the metal, the interaction between ions and solvent in the dl,t the interactions between electrode metal and solvent, and the influences of these interactions on each other. All this exists already for electrodes which are not single-crystal faces, but the situation is complicated by the fact that the charge is distributed at the surface in an uncontrolled way. This is not the case for single-crystal faces for which the strength of adsorption, as well as its variation with charge density at the electrode surface, could depend on the atomic structure of the face which is the electrode. However, despite these complications, some progress has and can be made. 

Some works on sPS are present, but essentially they regard the incorporation of organophilic clays (Park et al., 2001) and the characterization of sPS/clay nanocomposites with respect to the crystallization behavior (Tseng, Lee, and Chang, 2001 Wu et al., 2004), mechanical properties (Ho Kim et al., 2004), and moldability by means of injection-molding process (Sorrentino, Pantani, and Brucato, 2006). A work on the reinforcing of sPS by means of carbon nanocapsules is also present (Wang et al., 2008). 

Poly(cyclohexyl acrylate) was shown to be miscible with PS with ucst behavior [720]. Random copolymers of cyclohexyl acrylate with n-butyl acrylate showed miscibility with PS above 50% cyclohexyl acrylate[721]. Poly(cyclohexyl methacrylate)/isotactic PS blends showed miscibility based on calorimetry and NMR studies [722]. The NMR results showed homogeneous behavior at a scale of 2.5-3.5 nm. Poly(4-trimethylsilyl styrene) miscibility with polyisoprene was observed with a lest behavior (critical temperature = 172 ° C at degree of polymerization of 370) [723]. The interaction parameter, showed the following relationship = 0.027—9.5/T. Isotactic and syndiotactic polystyrene both exhibit crystallinity, whereas atactic polystyrene is amorphous. Atactic PS/isotactic PS blends exhibited crystallization kinetics, which decreased linearly with atactic PS addition indicating miscibility [724]. The TgS of aPS and iPS are identical, thus Tg methods could not be employed to assess miscibility. Atactic PS/syndiotactic PS blends were also noted to be miscible with rejection of atactic PS in the interfibrillar region between the lamellar stacks of sPS [725]. 

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. 

By strain-induced crystallization, the pure a form is obtained even at temperatures for which thermal crystallization from the glassy state is not observed. De Candia et al. [36] analyzed the drawing behavior of amorphous films of SPS at different temperatures. They found that strain-induced or thermal-induced crystallization was obtained, depending on the drawing temperature. At 110 °C thermal crystallization was not observed, whereas strain-induced crystallization occurred at high draw ratios, resulting in a substantial increase in the elastic modulus of the obtained samples. 

Lawrence and Shinozaki [84], for an SPS with a molecular weight of Mw = 372,000 g/mol, determined the crystallization parameters by simultaneously fitting data of both melt and cold isothermal crystallization. The parameters identified were used to predict the crystallization behavior during non-isothermal experiments. The predictions were found to agree with the data of crystallization from the melt vice versa, the predicted rates underestimated the data of crystallization from the amorphous solid. 

However, as reported by Hohne [88,89], this behavior is not true for all the crystal phases. In particular, for the SPS, the a phase shows a decrease in the melting point with pressure, whereas that of the P phase shows an opposite trend. The opposite behavior of the melting temperature of the two crystal phases with pressure is a consequence of the fact that the density of the amorphous phase is smaller than the density of the P and larger than the density of the a phase. 

Multiple melting peaks (two to four peaks) have been observed on heating scans of SPS samples that present a combination of a and p crystals. However, multiple melting peaks can also be observed in samples that are known to contain only a single type of unit cell [73]. Apparently, the multiple peaks phenomenon cannot be entirely attributed to multiple types of unit cells. Variation in the lamellar morphology may also be responsible for the complex thermal behavior. 

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. 

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