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Unusual crystals of poly(epsilon-caprolactone) by unusual crystallisation: The effects of rapid cooling and fast solvent loss on the morphology, crystal structure and melting
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Fibre Technology.
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2013 (English)In: Polymer, ISSN 0032-3861, E-ISSN 1873-2291, Vol. 54, no 5, 1497-1503 p.Article in journal (Refereed) Published
Abstract [en]

The lateral habit, unit cell structure and melting behaviour of single crystals of poly(epsilon-caprolactone) (PCL) prepared by the rapid expansion of a supercritical solution technique was studied by AFM at ambient and higher temperatures and by grazing-incident X-ray scattering using a synchrotron source. After dissolving PCL in a solution of supercritical CO2 and 0.1 vol.% chloroform, an extremely fast phase transfer from a supercritical to a gas-like state occurred during expansion into atmospheric conditions, leading to a temporary temperature drop to below -;50 degrees C at the silica surface where the crystals were deposited. Single crystals of a hitherto unreported rectangular lateral habit were observed. Six-sided crystals were also observed, but they were fewer than the rectangular crystals and in addition the angles between the lateral faces were different from the theoretical angles between adjacent {110} faces and {110} and {100} faces. X-ray scattering indicated a polymorphic structure also including the orthorhombic (110) and (200) diffraction peaks. Distinct low angle peaks essentially along the c-axis indicated a stacking on a very fine scale (3.7-4.7 nm) within the crystals. The equatorial diffraction peaks indicated a less dense packing of the PCL stems. Rectangular single crystals with a height according to AFM of 11-27 nm melted between 40 and 45 degrees C, which is lower than the melting points (55 degrees C) recorded for the distorted six-sided crystals. The unusual conditions for crystallisation used gave the polymer molecules a severe limitation to rearrange from the initial random coil state. The facetted crystals consisted of a stack of 4 nm thick blocks; these blocks most probably constituted a regular variation in molecular packing, i.e. molecular order. The pronounced changes in the angles between adjacent faces from those observed in mature PCL crystals and the wide-angle X-ray scattering data indicated the presence of conformational disorder in the crystals.

Place, publisher, year, edition, pages
2013. Vol. 54, no 5, 1497-1503 p.
Keyword [en]
Poly(ε-caprolactone), Polymer single crystal, Rapid expansion of a supercritical (CO2) solution
National Category
Polymer Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-109808DOI: 10.1016/j.polymer.2013.01.014ISI: 000315746300005Scopus ID: 2-s2.0-84873715767OAI: oai:DiVA.org:kth-109808DiVA: diva2:584499
Funder
Swedish Research Council, 2006-3559 2009-3188Knut and Alice Wallenberg Foundation
Note

QC 20130404. Updated from submitted to published.

Available from: 2013-01-09 Created: 2013-01-09 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Different paths to explore confined crystallisation of PCL
Open this publication in new window or tab >>Different paths to explore confined crystallisation of PCL
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this work, different paths to confined crystallisation of poly(ε-caprolactone) (PCL) havebeen explored. Innate confinement effects due to the presence of bulky end groups wereshown to affect crystalline characteristics for strictly monodisperse ε-caprolactone oligomers. The interaction between end groups and end groups, as well as that between end groups andε-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-ε-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.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. 72 p.
Keyword
crystallisation, poly-ε-caprolactone, end groups, morphology, nanoconfinement, melting, monodisperse, RESS, injet printing
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-109792 (URN)
Public defence
2013-01-25, D3, Lindstedtsvägen 5, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Foundation for Strategic Research , 2006-3559, 2009-3188
Note

QC 20130109

Available from: 2013-01-09 Created: 2013-01-08 Last updated: 2013-01-09Bibliographically approved

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