Models of Adjacent Re-Entry Folds in the β Form of Syndiotactic Polystyrene. Conformational and Packing Analysis by Molecular Mechanics

Abstract

The adjacent re‐entry folds of chains of syndiotactic polystyrene crystallized in the β form have been investigated by molecular mechanics. Various models of fold of chains along bilayers have been found. The results are in agreement with the literature experimental data indicating that the fold surface is irregular. Both the conformational and the packing energy of folded chains have been minimized by various techniques using several set of potential functions. A theoretical prediction of the work of fold is given.

Thermodynamic Characterization of Polymorphs in Bulk-Crystallized Syndiotactic Polystyrene via Small/Wide-Angle X-ray Scattering and Differential Scanning Calorimetry

“…19 kJ mol -1 ) obtained by Chen et al 20 from growth rate measurements (without specifying the phase, but likely the β form in view of their experimental T mc range of 236-244 °C). Both g values are much lower than the broad and similar range of estimates (45-90 kJ mol -1 ) for the two phases by Napolitano and Pirozzi 44,45 via extensive molecular mechanics calculations based on various schemes of adjacently re-entered tight folds with different force fields. Nevertheless, the low g values observed are consistent with the TEM observations 23 that decorating polyethylene crystals are randomly oriented on both R and β basal planes, indicating that the folds are generally disordered.…”

On the Crystal Structure of cis‐1,4‐Poly(2,3‐dimethyl‐1,3‐butadiene)

“…(ii) A good capability in the prediction of crystal structures of polymers is displayed, e.g., for the crystal structures of a and b modifications of syndiotactic polystyrene. [26,27] Calculated spectra (b) and (c) of Figure 2 correspond, respectively, to a model optimized by varying all the parameters and to a model obtained by keeping the c axis at the value found in the minimization of the isolated chain (7.31 Å ). Both models exhibit a very high increase in the a axis with respect to that of cis-PBD, caused by the necessity to remove the strong nonbonded interactions arising from the substitution of the hydrogen atoms with the bulkier methyl groups.…”

Crystal Structure and Adjacent Reentry Folds of the α Form of Syndiotactic Polystyrene. Conformational and Packing Analysis by Molecular Mechanics

“…A very similar range (from 50 to 85 kJ/mol of monomeric unit) was obtained for the for the β form of sPS. 30 These results indicate that the energy required to fold a chain of sPS in due prevailingly to the constitution and configuration of the polymer chains rather than to the packing of the chains.…”

“…The analysis of the models of fold reported in this paper is particularly elaborate because it takes into account not only the interactions between the atoms of the folds and the atoms of the crystal as performed for the β form of sPS 30 but also gradual adjustments of the chains reentering the crystal, as calculated in the refined folds. These adjustments seem significant because the results obtained for the refined folds are generally more consistent with respect to the corresponding results obtained for the optimized folds.…”

“…The internal energy values of the folds have been used to estimate the work of fold. Energy calculations of adjacent reentry folds taking also into account the packing of the chains have been performed in the past for polyethylene − and recently by us for syndiotactic polypropylene, poly( p -phenylene sulfide), and the β form of sPS . This work presents a more complete analysis of the influence of the crystalline field on the folds for a polymer with crystal structure more complex than those studied in the past.…”