Melt Structure and its Transformation by Sequential Crystallization of the Two Blocks within Poly(L‐lactide)‐block‐Poly(ε‐caprolactone) Double Crystalline Diblock Copolymers

Abstract

Sequential crystallization of poly(L-lactide) (PLLA) followed by poly(e-caprolactone) (PCL) in double crystalline PLLA-b-PCL diblock copolymers is studied by differential scanning calorimetry (DSC), polarized optical microscopy (POM), wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS). Three samples with different compositions are studied. The sample with the shortest PLLA block (32 wt.-% PLLA) crystallizes from a homogeneous melt, the other two (with 44 and 60% PLLA) from microphase separated structures. The microphase structure of the melt is changed as PLLA crystallizes at 122 8C (a temperature at which the PCL block is molten) forming spherulites regardless of composition, even with 32% PLLA. SAXS indicates that a lamellar structure with a different periodicity than that obtained in the melt forms (for melt segregated samples). Where PCL is the majority block, PCL crystallization at 42 8C following PLLA crystallization leads to rearrangement of the lamellar structure, as observed by SAXS, possibly due to local melting at the interphases between domains. POM results showed that PCL crystallizes within previously formed PLLA spherulites. WAXS data indicate that the PLLA unit cell is modified by crystallization of PCL, at least for the two majority PCL samples. The PCL minority sample did not crystallize at 42 8C (well below the PCL homopolymer crystallization temperature), pointing to the influence of pre-crystallization of PLLA on PCL crystallization, although it did crystallize at lower temperature. Crystallization kinetics were examined by DSC and WAXS, with good agreement in general. The crystallization rate of PLLA decreased with increase in PCL content in the copolymers. The crystallization rate of PCL decreased with increasing PLLA content. The Avrami exponents were in general depressed for both components in the block copolymers compared to the parent homopolymers.Polarized optical micrographs during isothermal crystallization of (a) homo-PLLA, (b) homo-PCL, (c) and (d) block copolymer after 30 min at 122 8C and after 15 min at 42 8C.

Crystalline and Spherulitic Morphology of Polymers Crystallized in Confined Systems

“…This makes the crystallizable block copolymers representative systems having confined crystallization behavior, especially in the strong segregation regime [52]. Confined crystallization has been widely observed in the diblock copolymers having double crystalline blocks, such as PEO-b-PCL [112][113][114][115][116], PLLA-b-PEO [117][118][119][120][121][122], PLLA-b-PCL [123][124][125], poly(p-dioxanone) (PPDX)-b-PCL [126][127][128], polyolefin-based block copolymers [129][130][131][132][133][134][135], and so on. The confined crystallization kinetics and crystalline morphology of such block copolymers have been extensively studied.…”

Composition Dependence of Crystallization Behavior Observed in Crystalline-Crystalline Diblock Copolymers

“…Therefore the crystallized morphology formed by high-T m blocks is a kind of spatial confinement for the subsequent crystallization of low-T m blocks, which will be qualitatively different from the confinement imposed by amorphous or glassy microdomains observed in crystallineamorphous diblock copolymers. [9][10][11] There are some studies so far on the morphology formation of double crystalline block copolymers with different T m values, [12][13][14][15][16][17][18][19][20][21][22][23][24] where they are mainly focused on the characteristic morphology finally formed in the system.…”

Morphological Evolution during Isothermal Crystallization Observed in a Crystalline-Crystalline Diblock Copolymer

“…This crystallization-induced melting was first reported by Hamley et al 22 using a time-resolved WAXD method in a crystalline-crystalline diblock copolymer, poly(L-lactide)-block-poly("-caprolactone) (PLLA-b-PCL), where the primary crystallization of amorphous PCL blocks (lower-T m blocks) yielded the partial melting of pre-existing PLLA crystals (higher-T m blocks) just after quenching PLLA-b-PCL into low temperatures. They concluded that the melting of PLLA crystals occurred partially in order to compensate the space previously occupied by amorphous PCL blocks.…”

“…However, DSC and FT/IR results revealed that the crystallinity of PCL blocks significantly increased and simultaneously that of PE blocks decreased moderately with increasing crystallization time at the late stage. These results suggest the partial melting of PE crystals facilitated by further crystallization of PCL blocks in order to compensate the space previously occupied by amorphous PCL blocks.KEY WORDS: Crystalline-crystalline Diblock Copolymer / Isothermal Crystallization / Morphological Evolution / Crystalline-crystalline diblock copolymers show an interesting morphology formation by the interplay between two kinds of crystallization starting from the microdomain structure formed in the melt.1 There are many studies on the crystallization behavior and resulting morphology of double crystalline diblock copolymers, such as poly("-caprolactone)-blockpoly(ethylene oxide) (PCL-b-PEO), 2-11 poly(p-dioxanone)-15-17 poly(L-lactide)-block-PCL (PLLA-b-PCL), [18][19][20][21][22] and PCL-b-PE. [23][24][25][26][27] In PCL-b-PEO, both blocks have a close melting temperature T m and eventually two kinds of crystallization occur simultaneously to result in a characteristic morphology in the system.…”