Banded spherulites in poly(L‐lactic acid): Effects of the crystallization temperature and molecular weight

Abstract

The spherulitic morphology of pure poly(Llactide) (PLLA) was investigated with polarized optical microscopy as a function of the crystallization temperature and molecular weight. After being melted at 2108C for 3 min, samples were cooled quickly to designated temperatures for isothermal crystallization. It was shown for the first time that a clear banding-to-nonbanding morphological transition took place at a critical temperature for PLLA with a number-average molecular weight of 86,000. With the increasing molecular weight of the material, the spherulite growth rates decreased notably, and the band spacing decreased significantly. On the basis of the main-chain chirality in PLLA and the observation of a nonbanded spherulitic morphology in a certain temperature region, it was suggested that the crystallization temperature might have an effect on the relationship between the sense of lamellar twisting and the main-chain chiral structure in PLLA.

CHAPTER 3. Crystallization of PLA-based Materials

“…Moreover, the spherulite growth rate decreases with an increase in molecular weight because of more restricted chain mobility. 3,9,14,29,39,42,60 This behaviour has been also reported for several synthetic polymers, such as isotactic polystyrene, nylon-6, poly(oxypropylene) and poly(chlorotrifluoroethylene), among others. 61 However, it must be pointed out that an unusual behaviour in the spherulite growth rate curve has been reported by several researchers for intermediate molecular weight PLLAs.…”

“…Yasuniwa et al 42 observed circular ring morphology on spherulites grown at T c ¼ 121 1C and 131 1C, via isothermal crystallization. Wang et al 39 showed a clear banding-to-non-banding morphological transition in PLLA at a critical crystallization temperature. The PLLA samples of different M w (151 and 301 kg mol À1 ) were isothermally crystallized at different T c , and exhibited a band spacing pattern depending on M w .…”

“…Nevertheless, the spherulitic morphology of PLLA is widely debated in the literature. 39 In most published scientific research, PLLA has shown non-banded spherulites. 8,9,14,36,37,40 Gazzano et al 41 investigated the spherulitic structure of PLLA by using microfocus X-ray diffraction, and they concluded that no lamellar twisting existed in neat PLLA.…”

Optimization of melting conditions for the analysis of crystallization kinetics of poly(3-hydroxybutyrate)

“…PHB spherulites are characterized by a maltese cross and circular bands [1]. For PHB [1,32], as well as for other polymers [30][31], the band spacing increases with molecular mass, which, in turn, decreases with the temperature of fusion that precedes the crystallization step. For PHB [1,32], as well as for other polymers [30][31], the band spacing increases with molecular mass, which, in turn, decreases with the temperature of fusion that precedes the crystallization step.…”

Synthesis, morphology, and nonisothermal crystallization behavior of poly(trimethylene terephthalate)/poly(propylene glycol) segmented random copolymers

“…[9,27] This phenomenon was because of the decreasing degree of supercooling, resulted from the decreasing average sequence length of PTT blocks from PTPG1 to PTPG3. [28] Figure 3a′-c′ shows the POM micrographs of PTPG3 cooling from the melt to room temperature at various cooling rates. Obviously, the nonisothermal crystallization morphology of PTPG3 copolymers samples strongly depended on the cooling rate.…”