Isothermal Crystallization Behavior of the Poly(L-lactide) Block in Poly(L-lactide)-Poly(ethylene glycol) Diblock Copolymers: Influence of the PEG Block as a Diluted Solvent

Abstract

ABSTRACT:Isothermal crystallization kinetics and morphology of the poly(L-lactide) block in poly(L-lactide)-poly(ethylene glycol) diblock copolymers were studied by differential scanning calorimetry (DSC) and polarized optical microscopy (POM), respectively. The results were compared with that of the PLLA homopolymer. The introduction of the PEG block accelerated the crystallization rate of the PLLA block and promoted to form ring-banded spherulites. The analysis of isothermal crystallization kinetics has shown that the PLLA homopolymer accorded with the Avrami equation. But the PLLA block of the diblock copolymers deviated from the Avrami equation, which resulted from increasing of the crystallization rate and occurring of the second crystallization process. The equilibrium melting temperature (T o m ) of the PLLA block fell with its molecular weight decreasing. The conditions to obtain more regular ring-banded spherulites were below: the sample was the PLLA block of LA 5 EG 5 ; the crystallization temperature was about from 95 C to 100 C, which almost corresponded to regime II. Poly(L-lactide)(PLLA) is possessed of many predominant properties, such as biodegradability, biocompatibility, innocuity, tissue absorbability and so on. Therefore, it is widely used in medical area 1-4 as an important biomaterial. But it also has some flaws. For example, bad watersolubility, friability etc. Moreover, the properties are not able to meet the demand for expandedness of its application area. In order to improve its property, the synthesis of PLLA-based block copolymer is one of the most important ways. Poly(ethylene glycol) (PEG) is the simplest polyether. It has good biocompatibility, blood-compatibility, hydrophilicity and flexibility. 5,6 Hydrophilic PEG is introduced to form block copolymer which hopes to improve hydrophilicity, biocompatibility and other properties.7-10 Therefore, PLLA-PEG block copolymers have attracted many researchers' attention and have been studied on their properties and application. 11-15Although crystallization process of block copolymer that have double-crystallizable blocks is more complex than other block copolymer systems, the studies on crystallization process and morphology also attracted many people's attention in recent years owing to their important meaning in practical application. [16][17][18][19] As a double-crystallizable block copolymer, it is out of question that the properties and application of PLLA-PEG block copolymers have intimate relations with their crystallization process, crystallinity and crystallization morphology. Therefore, studying them is the indispensable contents. Fujiwara conducted detailed investigation on the crystallizationinduced morphological change, macromolecular selforganization process and crystalline structure. [20][21][22] Lee reported that PLLA and PEG blocks were phase separated, and the crystallization behavior of one block was markedly affected by the presence of the other block. 23 Luo thought that the crystallization morphology was deeply depen...

Effect of biobased and biodegradable nucleating agent on the isothermal crystallization of poly(lactic acid)

“…Moreover, the value of the heat crystallization (∆H c ) of talc1.0 is very similar to that of PLA/CMPS blends and starch1.0. In earlier studies [19,[29][30][31], polymers with talc did not show a heat of crystallization on reheating process after quenching, indicating that most of the crystallization process was completed during the quenching procedure. But, in this study, PLA containing talc showed the heat of fusion, which might be due to the intrinsically slow crystallization rate of PLA.…”

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.…”

CHAPTER 3. Crystallization of PLA-based Materials

“…PLLA-b-PEG diblock copolymers exhibit banded spherulites when isothermally crystallized at T c Z110 1C. 47,48 Yang et al 48 proposed that the existence of the PEG block as a diluent played an important role in the formation of ring-banded spherulites. Huang et al 49 studied crystalline morphologies of PLLA-b-PEO diblock copolymers.…”