A Morphological Study of a Semicrystalline Poly(l-lactic acid-b-ethylene oxide-b-l-lactic acid) Triblock Copolymer

Abstract

Poly(l-lactic acid-b-ethylene oxide-b-l-lactic acid) (PLLA-b-PEO-b-PLLA) triblock copolymers were synthesized by a ring-opening polymerization. PLLA and PEO sequentially crystallized by slowly cooling (−2 °C/min) from the melt. In the resultant spherulitic morphology, the retardation of polarized light was additive, and the sign of the spherulite (negative) was preserved when the PEO crystallized within the framework established by the PLLA crystals. Homopolymer blends of PLLA and PEO having the same composition as the block copolymer showed similar optical behavior. However, the change in the optical retardation upon crystallization of the PEO was much greater for the triblock copolymer than for the blend. Upon heating, the small-angle X-ray scattering from both the triblock copolymer and homopolymer mixture showed a stepwise increase in the long period at ∼60 °C, i.e., when the PEO crystals melted. For comparable volume fractions (φPLLA = 0.57), the increase in long period was greater for the triblock copolymer than for the blend. Wide-angle X-ray diffraction studies on shear aligned triblock copolymers indicate that the PLLA and PEO crystals adopt the same average orientation; though, in the case of the copolymer, the orientation is more strongly coupled.

Poly(ethylene glycol)-poly(tetrahydrofuran)-poly(ethylene glycol) triblock copolymer : Synthesis, crystallization behavior and novel morphology

“…When phase separation occurred, due to the dynamic competition between phase separation and crystallization process, PEG lamellas tended to array along different directions and formed different spherulitic morphology forms [32,44]. As Block7800 was proved to be crystalline-amorphous, the resulting crystal morphologies can be significantly influenced by microphase separation [5,6]. That may help to explain the formation of double concentric spherulites discovered in our study.…”

“…Block copolymers, consisting of both crystalline and amorphous segments were widely recognized that their crystallization processes and the resulting crystal morphologies can be significantly influenced by microphase separation in melt [6,7]. However, block copolymers, consisting of different crystalline segments such as the double crystalline block copolymers, usually exhibit much more complicated crystallization behaviors [8,9], including confined crystallization [10][11][12][13], competitive or interactive crystallization and so on [14][15][16].…”

“…Unfortunately, reports on this field are still inadequate till now. Recently, with the comprehensive investigations on crystallization process for block copolymers, the high order structures containing crystal lamellae were frequently discovered [6,[21][22][23][24][25], where several types of spherulites with unusual morphologies such as ring banded spherulite, spherical granular aggregates and double concentric spherulites have been reported [14,15,[26][27][28][29][30][31]. Especially, the double concentric spherulites were only found in poly(ethylene oxide)/poly(!-caprolactone) PEG/PCL block copolymer up to now [14,[26][27][28][29].…”

Effect of block asymmetry on the crystallization of double crystalline diblock copolymers

“…27 Similar diluent effects on the PLLA blocks have been observed in PLLA-b-PEO diblock copolymer with increasing the composition of poly(ethylene oxide) (PEO) block. 40,41 In a recent work we have demonstrated the effect of segregation strength on the development of crystallinity in symmetric diblock copolymers. 42 We have observed that with increasing value of segregation strength, the degree of crystallinity gradually decreases producing smaller and thinner crystals.…”

Micromorphology Memory in Amphiphilic Polypeptides

“…Generally the spherulite sign is derived from the chain orientation along or across the spherulite circle. 10 In X-ray diffraction (XRD) diagrams of as-cast films (Figure 2a), some intense diffraction peaks assigned to PEG crystals 11 appeared in the 2 (: diffraction angle) range more than 10 while a very weak peak of PBLA blocks 12 appeared at 2 ¼ 5 , indicating that spherulites formed mainly as a result of PEG block crystallization. In addition, a sharp diffraction peak appeared at 2 ¼ 2 corresponding to a d-spacing of 4.4 nm, which demonstrates the formation of a long-range structure.…”