Polymers, stereocomplexes properties

Since there was no pathway towards syndiotactic PHB or unnatural isotactic (5)-PHB available for a long time, a more detailed investigation on material properties with regards to tacticity and stereocomplex formation is stiU missing. To date, it is not known whether syndiotactic PHBs crystallize in a similar manner to isotactic stereoisomers and therefore possesses similar properties nor how they are influenced by blending of polymers with different stereochemistry. 

The tacticity of PLA influences the physical properties of the polymer, including the degree of crystallinity which impacts both thermo-mechanical performance and degradation properties. Heterotactic PLA is amorphous, whereas isotactic PLA (poly(AA-lactide) or poly (55-lac tide)) is crystalline with a melting point of 170-180°C [26]. The co-crystallization of poly (RR-lactide) and poly(55-lactide) results in the formation of a stereocomplex of PLA, which actually shows an elevated, and highly desirable, melting point at 220-230°C. Another interesting possibility is the formation of stereoblock PLA, by polymerization of rac-lactide, which can show enhanced properties compared to isotactic PLA and is more easily prepared than stereocomplex PLA [21]. 

One of the cases of such complementarity may be the atactic polymer which can be considered as a copolymer of isotactic and syndiotactic links. Indeed, the composition and properties of the polyccanplex depend on the chain stereoregularity, for example in the case of poly(methyl methacrylate) stereocomplexes and poly(ethylene glycol) with isotactic or atactic PAA complexes ... 

Eukushima, K. and Kimura, Y. (2006) Stereocomplexed polylactides (Neo-PLA) as high-performance bio-based polymers their formation, properties, and application. Polymer International, 55, 626-642. 

Spectra obtained for atactic PMEPL and the stereocomplex formed by equal molar mixtures of the two isotactic polymers of opposite absolute configuration are presented in Fig. 3. In contrast to the isotactic case, no dependence on thermal history is found. These spectra are unexpectedly similar, and resemble the solution cast isotactic spectrum of Fig. 1. NMR spectra were also recorded for polymers of intermediate tacticity. PMEPL of optical purity 75% shows the same dependence on sample preparation as is found for the isotactic polymer whereas samples of optical purity 25% behave as the atactic case. Similar observations were made by Grenier and Prud homme by the comparison of other properties, including x-ray patterns, solubility and morphology. 

The above-mentioned chemical catalytic routes lead to racemic AHA mixtures. For the direct use of LA (or its esters) as a solvent or platform molecule for achiral molecules like acrylic acid and pyruvic acid, stereochemistry does not matter. The properties of the polyester PLA, the major application of LA, however, suffer tremendously if d and l isomers are built in irregularly [28]. This is exemplified by atactic PLA, made from racemic LA, which is an amorphous polymer with low performance and limited application. However, when l- and D-lactic acid are processed separately into their respective isotactic L- and d-PLA, as discovered by Tsuji et al., a stereocomplex is formed upon blending these polymers. This polymer exhibits enhanced mechanical and thermal properties [28, 164]. A productive route to D-Iactic acid is, however, missing today. If the chemocatalytic routes to LA are to become viable, enantiomer resolution of the racemate needs to be performed. Given separation success, a cheap source of o-lactic acid will be unlocked immediately, providing an additional advantage over the fermentation route (cfr. Table 2). 

PLA is a fascinating material and there are big differences in the thermomechanical properties of the polymer depending on the tacticity formed. For example, atactic and heterotactic PLA are amorphous and have no defined melting point. Polymerisation of either the pure l- or d- stereoforms produce isotactic PLA with a Tg of ca. 50 °C and a T , of 170-180 °C. However, a 50 50 mixture of PLLA and PDLA has a melting point of ca. 220-230 °C, this enhancement is due to the formation of a stereocomplex between opposite chiral chains. It is also possible, and indeed preferred, to produce the stereocomplexed polymer from the isoselective polymerisation of rac-LA forming stereoblock PLA. There are many examples of zirconium, and indeed group-4 metals as a whole, in the literature for the polymerisation of rac-LA, the next section will focus on the stereoselective examples of ROP. 

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. 

If stereocomplex formation were to occur only between I-PMMA and S-PMMA, then the mixing of isotactic-atactic-isotactic (lAtl)- and syndiotactic-atactic-syndiotactic (SAtS)-PMMA stereoblock polymers in complexing solvents should result in stereocomplexes imbedded in an atactic matrix (Figure 2). Here the stereocomplexes would act as physical cross-links providing a network structure from a single chemical composition. From an application viewpoint, such a material should possess interesting properties compared to conventional PMMA. 

Stereocomplexation results from stereoselective interactions, mainly van der Waals forces, between two opposite stereoregular polymers which interlock to form a new material with altered physical properties as compared to the parent polymers (S lager and Domb, 2003a Tsuji, 2016). Stereocomplexation between PLAioo and PLAq was first reported by Ikada et al. in the 1980s (Dcada et al., 1987), and is now a weU-known phenomenon for optically active PLA stereocopolymers (Tsuji and Ikada, 1993 Kakuta et al., 2009 Bao et al., 2013). The properties and potential applications... 

Tsuji, H., Ikada, Y., 1999. Stereocomplex formation between enantiomeric poly(lactic acid)s. XI. Mechanical properties and morphology of solution-cast films. Polymer 40, 6699. In vitro hydrolysis of blends from enantiomeric poly(lactide)s. Part 16708. 

It has been observed that a 1 1 mixture of pure PLLA with pure PDLA yields a stereocomplex of the two polymers during crystallization or polymerization. The PLA stereocomplex consists of racemic crystalline strucmres in which l-PLA and d-PLA chains are packed side by side, with a 1 1 ratio of l d monomer units [4, 32, 33]. While the melting temperature of a- and P-crystalline forms of PLA falls in the range 170-180°C, the 7) of PLA stereocomplex is between 220 and 230°C [33]. The high of PLLA/PDLA stereocomplex makes it a difficult material for processing however, it is interesting to note that the comparison between PLLA/PDLA equimolar blends and the starting materials shows mechanical properties that are markedly improved. 

The most common strategy to decrease the price or improve the properties of polylactide to fulfill the requirements of different applications is blending. Polylactide has been blended with degradable and inert polymers, natural and synthetic polymers, plasticizers, natural fibers and inorganic fillers. The most common blends include blends with other polyesters such as polycaprolactone or PLA/starch blends. Usually the compatibility between the two components has to be improved by addition of compatibilizers such as polylactide grafted with starch or acrylic acid (114,115). Recently a lot of focus was concentrated on the development of polylactide biocomposites, nanocomposites and stereocomplex materials. In addition various approaches have been evaluated for toughening of polylactide. 

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