Different paths to explore confined crystallisation of PCL
Sammanfattning: In this work, different paths to confined crystallisation of poly(?-caprolactone) (PCL) havebeen explored. Innate confinement effects due to the presence of bulky end groups wereshown to affect crystalline characteristics for strictly monodisperse ?-caprolactone oligomers. The interaction between end groups and end groups, as well as that between end groups and?-caprolactone repeating units, created an obstacle for unfolding the crystal structures that hadinitially formed even at the high-temperature limit of crystallisation where crystallisationoccurred over hundreds of hours. Very rapid X-ray imaging of the in situ crystallisationprocess showed that rapid shifts in the unit cell occurred during the first minute ofcrystallisation due to the difficulty of fitting the bulky end groups in a stable unit cell.Confinement effects also arose when polymer chains were crystallised in systems with smallpore sizes. For linear poly-?-caprolactone, chains confinement depended mainly on thedimensionalities of the pores. Linear polymers with Mn = 10 000 and 42 500 were stronglyinhibited from forming crystal structures in 10 nm pore systems, but not hindered in 23 nmpore systems. Linear polymers with Mn = 80 000 also experienced limited confinement in the23 nm pores. A star-shaped oligomer with four arms of approximately Mn = 1 000 each evenexperienced confinement in 290 nm pores, although having smaller molecular size and radiusof gyration compared to the linear chains. The innate confinement created by the challenge ofpacking four arms amplified the effect of physical confinement. Another limitation wascreated on the crystallisation process by solving PCL in supercritical CO2 and depositingduring extremely fast phase transfer to gas-like state. The formed structures were limited bythe very low temperature that resulted from the phase change and by the rapid evaporation ofthe solvent. These limitations resulted in entrapment of crystal structures in metastablephases. As a consequence, crystals of hitherto unreported rectangular form were observed aswell as the common six-sided form. The former crystals had considerably lower melting pointcompared to the latter. X-ray analysis showed that two sets of lattice constants existed,supporting the notion of entrapment in metastable phases. Another way of achievingconfinement was precise deposition of droplets in the pikolitre volume range of highly dilutesolutions. The microcrystals which formed were confined by both the low polymer content ineach droplet and by the time constraint on crystal formation that arose by the rapidevaporation of the small droplets. Confinement led to entrapment into metastable phases,evident by the presence of unusual eight-sided and rectangular crystals.
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