Stereocomplex temperature

Fig. 34 a, b. Effect of the mixing ratio of isotactic po y(methyi methacrylate) (iso-PMMA) and syndiotactic PMMA cm the melting temperature (Tm) and specific area of the melting endotherms299, (a) Relationship between Tm and crystallization temperature (Tc) for different mixtures of stereoregular PMMA [iso-PMMA]/[synd-PMMA] 1/2, O 1/1, C 2/1, 4/1, (b) Specific areas of melting endotherms of stereocomplexes with different [iso-PMMA]/[synd-PMMA] ratio in bulk mixtures. Ciystallization time O 4 h, O 16 h, 4 days, 2 weeks... 

When matrix polymerization of methyl methacrylate monomer was performed in the presence of iso-PMMA or of the stereocomplex ([iso-PMMA]/ [synd-PMMA] = 1/1), the tan 6 peaks of the a-process (caused by the micro-Brownian motions of main chain) and the / -process (caused by the motions of the ester side chains of PMMA) shifted to temperature ranges which were higher by 5-10°C and 30-40 °C, respectively, as compared with synd-PMMA312. From these results, Tanzawa et al. concluded that the stereospecific association drastically restricted the motion of the ester side chains of the two stereoregular PMMAs. In addition, the mutual interlocking of the side chains of PMMA also hindered, to a certain extent, the motion of the main chains of PMMA. Furthermore, they studied the matrix polymerization of methyl acrylate (MA) and ethyl acrylate (EA) in the presence of stereoregular PMMA and found ... 

It is reported that the maximum achieved tensile modulus and strength, for stereocomplex PLA films with an extrusion draw ratio of 16 and prepared by solid-state coextrusion, were 9500 and 410 MPa, respectively. In addition, these films exhibited excellent thermomechanical stability showing 9500, 7000, and 3000 MPa modulus [ ] values at room temperature, 100 and 200°C, respectively [209]. 

PDLA or PLLA when solely stereocomplexation takes place [107], However, spherulites with complicated morphologies are evident when stereocomplexation and homo-crystallization occur simultaneously [104,107,108]. Maillard and Prud homme observed mirror-image stereocomplex crystalline assemblies for PLLA- and PDLA-rich PLLA/PDLA mixtures [109]. As observed for crystalline polymers, PLLA spherulites become smaller with decreasing temperature and time [110,111]. 

The latter form can be prepared at a high draw ratio and a high drawing temperature [28]. The 7-form is formed by epitaxial crystallization [29]. It has been observed that a blend with equivalent poly(L-lactide) PLLA and poly(D-lactide) PDLA contents gives stereo-complexation (racemic crystallite) of both polymers. This stereocomplex has higher mechanical properties than those of both PLAs, and a higher melting temperature of 230°C. The literature reports different density data [4] for PLA, with most values for the crystalline polymer around 1.29 compared with 1.25 for the amorphous material. 

Stereocomplexes were prepared by systematically mixing i- and s-PMMA solutions of equal concentration at room temperature. They were kept in a thermostatically controlled bath at 25°C before the measurements were done. 

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