Abstract
Lamellar crystals of diblock, triblock and four-arm poly(ethylene glycol)-b-poly(ɛ-caprolactone) (PEG-b-PCL) crystalline-crystalline copolymers were successfully obtained from their solution. Morphology and structure of lamellar crystals of crystalline-crystalline copolymers were investigated using tapping-mode atomic force microscopy (AFM) and selected area electron diffraction (SAED). All of these samples showed the truncated-lozenge multilayer basal shapes with central screw dislocation or central stack, which were all obtained simultaneously from the oil bath. The diffraction pattern of PEG block lamellar crystal is attributed to the (120) diffracting planes and the pattern of PCL block lamellar crystal is attributed to the (110) diffracting planes and (200) diffracting planes according to the SAED results. Four (110) crystal growth planes and two (200) crystal growth planes are discovered for the PCL blocks, but the (120) crystal growth planes of PEG blocks are hided in the figure of AFM. The crystalline structure of the four-arm copolymers (FA) is more disorder and confused than that of the diblock (DI) copolymer and the striated fold surface structures of lamellar crystals of four-arm copolymers (FA) are smoother than these of linear analogues, owing to the confused crystallization of blocks caused by the mutual restriction of blocks and the hindrance of the dendritic cores. In addition, the aspect ratio of FA is greater than that of the others. It is hypothesized that there are two reasons for the change of aspect ratios. First, the (200) diffracting planes of PCL crystals grew slowly compared to their (110) diffracting planes because of difference in the energy barrier. Secondly, edge dislocations on the (200) diffracting planes are also responsible for the variation of the aspect ratio. Consequently, the crystalline defects are augmented by the competing blocks crystallized simultaneously and the hindrance of the dendritic cores.
Highlights
Over the past few decades block copolymers have received tremendous scientific and industrial interest [1]
(DI and TRI) (Fig. 2), whereas the rings of diffraction in selected area electron diffraction (SAED) for four-arm copolymers (FA) are more disorder and confused than those of DI. It reveals that positional restriction caused by the dendritic core for FA is more than that of linear analogues, so that the PEG blocks and PCL blocks are hindered to crystallize orderly or the defects of crystals are increased
It is similar to PEG-b-PCL copolymers that a few PEG or PCL repeating units nearest to the dendritic cores or the points of connection are unable to crystallize and that chain folds are least tight in FA
Summary
Over the past few decades block copolymers have received tremendous scientific and industrial interest [1]. The central cores located on the lamellar surface had great influence on the crystallization and biodegradation of each arm of star-shaped PCL[4]. Due to the central core hindered the chain to crystallize, the crystals of the star-shaped polymers had smoother fold surfaces than those of the linear counterpart did. Müller et al.[7] expected that both blocks of the poly(L-lactide)b-poly(ε-caprolactone) (PLLA-b-PCL) diblock copolymers should be able to form two separate lamellar crystals that should be located close to each other because of the covalent bond in the chains, which was concluded from the DSC results. The attention is focused on solution crystallization for a series of block PEG-PCL, including diblock, triblock and four-arm block, which represents the different topological structure of macromolecules. Solution Crystallization Behavior of Linear and Star-shaped PEG-b-PCL Block Copolymers.
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