Sequential crystallization and multi-crystalline morphology in PE-b-PEO-b-PCL-b-PLLA tetrablock quarterpolymers
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Date
2021-08-10Author
Matxinandiarena Almandoz, Eider
Mugica Iztueta, Miren Agurtzane
Ladelta, Viko
Zapsas, George
Hadjichristidis, Nikos
Cavallo, Dario
Flores, Araceli
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Macromolecules 54(15) : 7244–7257 (2021)
Abstract
We investigate for the first time the morphology and crystallization of two novel tetrablock quarterpolymers of
polyethylene (PE), poly(ethylene oxide) (PEO), poly(ε-caprolactone) (PCL), and poly(L-lactide) (PLLA) with four potentially
crystallizable blocks: PE18
7.1-b-PEO37
15.1-b-PCL26
10.4-b-PLLA19
7.6 (Q1) and PE29
9.5-b-PEO26
8.8-b-PCL23
7.6-b-PLLA22
7.3 (Q2) (superscripts
give number average molecular weights in kg/mol, and subscripts give the composition in wt %). Their synthesis was
performed by a combination of polyhomologation (C1 polymerization) and ring-opening polymerization techniques using a
″catalyst-switch″ strategy, either ″organocatalyst/metal catalyst switch″ (Q1 sample, 96% isotactic tetrads) or ″organocatalyst/
organocatalyst switch″ (Q2 sample, 84% isotactic tetrads). Their corresponding precursorstriblock terpolymers PE-b-PEO-b-PCL,
diblock copolymers PE-b-PEO, and PE homopolymerswere also studied. Cooling and heating rates from the melt at 20 °C/min
were employed for most experiments: differential scanning calorimetry (DSC), polarized light optical microscopy (PLOM), in situ
small-angle X-ray scattering/wide-angle X-ray scattering (SAXS/WAXS), and atomic force microscopy (AFM). The direct
comparison of the results obtained with these different techniques allows the precise identification of the crystallization sequence of
the blocks upon cooling from the melt. SAXS indicated that Q1 is melt miscible, while Q2 is weakly segregated in the melt but
breaks out during crystallization. According to WAXS and DSC results, the blocks follow a sequence as they crystallize: PLLA first,
then PE, then PCL, and finally PEO in the case of the Q1 quarterpolymer; in Q2, the PLLA block is not able to crystallize due to its
low isotacticity. Although the temperatures at which the PEO and PCL blocks and the PE and PLLA blocks crystallize overlap, the
analysis of the intensity changes measured by WAXS and PLOM experiments allows identifying each of the crystallization processes.
The quarterpolymer Q1 remarkably self-assembles during crystallization into tetracrystalline banded spherulites, where four types of
different lamellae coexist. Nanostructural features arising upon sequential crystallization are found to have a relevant impact on the
mechanical properties. Nanoindentation measurements show that storage modulus and hardness of the Q1 quarterpolymer
significantly deviate from those of the stiff PE and PLLA blocks, approaching typical values of compliant PEO and PCL. Results are
mainly attributed to the low crystallinity of the PE and PLLA blocks. Moreover, the Q2 copolymer exhibits inferior mechanical
properties than Q1, and this can be related to the PE block within Q1 that has thinner crystal lamellae according to its much lower
melting point.