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The effects of quenching from the melt on the isothermal crystallization of star-shaped four-armed stereo diblock poly(lactide) (4-LD) and four-armed poly(l-lactide) (4-L) polymers with different l-lactyl unit contents as low crystallizability models for the stereoblock copolymers or blends of poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) were investigated. Quenching induced both stereocomplex (SC)- and homo-crystallization of non-equimolar 4-LD polymers with l-lactyl unit contents around 30 and 70%, wherein both SC- and homo-crystallization were highly prohibited in other procedures, and enhanced nucleation of 4-LD and 4-L polymers compared to the samples without quenching, resulting in formation of a large number of small-sized crystalline assemblies. The results obtained in the present study strongly suggest that the procedure of isothermal crystallization after quenching from the melt is expected to be utilized for screening the crystallizability of SC- and homo-crystallites in various types of block copolymers and blends of PLLA and PDLA and for enhancing nucleation or formation of dense crystalline structure.

The crystalline species and crystallization behavior of four-armed stereo diblock poly(lactide) (4-LD) polymers and poly(L-lactide) polymers varied depending on crystallization method (precipitation, heating or slow cooling) and L-lactyl unit content. With the deviation of L-lactyl unit content from 50%, the crystalline species of 4-LD polymers changed from only SC-crystallites to amorphous and then to only homo-crystallites (heating and cooling) or from only SC-crystallites to only homo-crystallites (precipitation). Branching architecture rather than stereo diblock architecture interrupted the concurrent crystallization of SC-crystallites and homo-crystallites.

The isothermal crystallization of star-shaped four-armed equimolar stereo diblock poly(lactide) (4-LD) polymers with different molecular weights is investigated. Solely stereocomplex (SC) crystallites are formed in all equimolar 4-LD polymers, irrespective of molecular weight and crystallization temperature (Tc). The wide-angle X-ray diffractometry, differential scanning calorimetry, and polarized optical microscopy results for crystalline species, crystallinity, and maximum radial growth rate of spherulites values indicate that both branching and diblock architectures disturb the SC crystallization and spherulites growth of equimolar 4-LD polymers, and the disturbance effect is larger for branching architecture than for diblock architecture. The equilibrium melting temperature (Tm0) values are 181.9–266.0 °C, which are comparable with or lower than the value reported earlier (279 °C). The crystallite growth geometries of equimolar of 4-LD polymers are independent of molecular weight and Tc.

The incorporated star-shaped four-armed stereo diblock poly(lactide) (4-LD) with poly(l-lactide) (PLLA) core and poly(d-lactide) (PDLA) shell and equimolar l- and d-lactide units formed stereocomplex (SC) crystallites and facilitated and accelerated the homo-crystallization of linear one-armed PLLA during heating and slow cooling and of linear one-armed PDLA during heating. The accelerating effect was higher for PLLA/4-LD with the opposite configurations of one-armed PLLA and the PDLA shell of 4-LD than for PDLA/4-LD with the identical configurations of one-armed PDLA and the PDLA shell of 4-LD.

The effects of l-lactyl unit content on star-shaped four-armed stereo diblock poly(lactide) (4-LD) polymers and star-shaped four-armed poly(l-lactide) (4-L) on the isothermal crystallization from the melt were investigated. Solely stereocomplex (SC) crystallites were formed in equimolar 4-LD polymer with l-lactyl unit content of about 50%, irrespective of crystallization temperature (Tc) values. 4-L and 4-LD polymers with l-lactyl unit contents of 100 and 93% formed only homocrystallites, regardless of Tc, whereas only SC crystallites with traceable amounts were formed in 4-LD polymers with l-lactyl unit contents of 72 and 31% at a limited narrow Tc range of 110-120 °C, when crystallization was continued for as long as 24 h. About 20% deviation of l-lactyl unit content from 50% dramatically decreased the spherulite growth rate (G) values of SC crystallites, whereas a 7% decrease of l-lactyl unit content from 100% significantly decreased the G values of homocrystallites and largely decreased the overall homocrystallization rates. Branching architecture rather than diblock architecture hindered the simultaneous formation of SC and homocrystallites of non-equimolar 4-LD polymers.