Abstract
The morphology and crystallization behavior of two triblock terpolymers of polymethylene, equivalent to polyethylene (PE), poly (ethylene oxide) (PEO), and poly (ε-caprolactone) (PCL) are studied: PE227.1-b-PEO4615.1-b-PCL3210.4 (T1) and PE379.5-b-PEO348.8-b-PCL297.6 (T2) (superscripts give number average molecular weights in kg/mol and subscripts composition in wt %). The three blocks are potentially crystallizable, and the triple crystalline nature of the samples is investigated. Polyhomologation (C1 polymerization), ring-opening polymerization, and catalyst-switch strategies were combined to synthesize the triblock terpolymers. In addition, the corresponding PE-b-PEO diblock copolymers and PE homopolymers were also analyzed. The crystallization sequence of the blocks was determined via three independent but complementary techniques: differential scanning calorimetry (DSC), in situ SAXS/WAXS (small angle X-ray scattering/wide angle X-ray scattering), and polarized light optical microscopy (PLOM). The two terpolymers (T1 and T2) are weakly phase segregated in the melt according to SAXS. DSC and WAXS results demonstrate that in both triblock terpolymers the crystallization process starts with the PE block, continues with the PCL block, and ends with the PEO block. Hence triple crystalline materials are obtained. The crystallization of the PCL and the PEO block is coincident (i.e., it overlaps); however, WAXS and PLOM experiments can identify both transitions. In addition, PLOM shows a spherulitic morphology for the PE homopolymer and the T1 precursor diblock copolymer, while the other systems appear as non-spherulitic or microspherulitic at the last stage of the crystallization process. The complicated crystallization of tricrystalline triblock terpolymers can only be fully grasped when DSC, WAXS, and PLOM experiments are combined. This knowledge is fundamental to tailor the properties of these complex but fascinating materials.
Highlights
Crystallization in block copolymers is a subject widely studied in the past decades [1–11].It is vital to understand the morphology upon crystallization since it is directly related to the final properties of a material
differential scanning calorimetry (DSC) and WAXS results demonstrate that in both triblock terpolymers the crystallization process starts with the PE block, continues with the PCL block, and ends with the poly(ethylene oxide) (PEO) block
polarized light optical microscopy (PLOM) shows a spherulitic morphology for the PE homopolymer and the the PE227.1-b-PEO4615.1-b-PCL3210.4 (T1) precursor diblock copolymer, while the other systems appear as non-spherulitic or microspherulitic at the last stage of the crystallization process
Summary
Crystallization in block copolymers is a subject widely studied in the past decades [1–11].It is vital to understand the morphology upon crystallization since it is directly related to the final properties of a material. Crystallization in block copolymers is a subject widely studied in the past decades [1–11]. Many other factors such as composition, molecular weight, crystallization protocol, segregation strength, and block miscibility affect the crystallization behavior. AB-type diblock copolymers with one or two crystallizable blocks have been studied in the past few decades. Müller et al [19–21] reported strong segregation strength for these diblock copolymers and a lamellar morphology for compositions close to 50/50. They did not see any spherulitic-type morphology as expected. The overall crystallization rate of both PLLA block and PE block in the diblock copolymers [19–21] was slower than that of the corresponding PLLA and PE homopolymers. Coincident crystallization occurs, since the crystallization transitions of the PE block and the PLLA block overlap employing cooling rates higher than 2 ◦C/min
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