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Crystallization and Melting Behavior of Monodisperse Oligomers of epsilon-Caprolactone
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
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2012 (English)In: Journal of macromolecular science. Physics, ISSN 0022-2348, E-ISSN 1525-609X, Vol. 51, no 10, 2075-2092 p.Article in journal (Refereed) Published
Abstract [en]

Monodisperse epsilon-caprolactone (CL) oligomers with different end groups (t-butyldimethylsilyl, benzyl, hydroxyl, and carboxylic acid) and different numbers of repeating units (4-64) have been studied by differential scanning calorimetry and small-angle X-ray scattering (SAXS) in order to gather information regarding the melting temperature, long period, and melting enthalpy. Oligomers crystallized at their maximum temperatures (different for the different oligomers) to full crystallinity yielded extended-chain crystals for oligomers with 4, 8, and 16 repeating units with the important exception of the oligomers with four and eight repeating units and hydroxyl and benzyl end groups that showed double-layer crystals. Oligomers with 32 and 64 repeating units exhibited remarkably stable once-folded (32-mer) and thrice-folded (64-mer) crystals. Only the oligomer with 16 repeating units showed two crystallization temperature regimes resulting in once-folded crystals (low temperatures) and extended-chain crystals (high temperatures). The end groups had a profound effect on the structures. Hydrogen-bonding groups promoted the formation of crystal bilayers and led to a very high melting enthalpy (150 J g(-1)) exceeding the melting enthalpy of 100% crystalline poly (epsilon-caprolactone). The bulky end groups, in particular t-butyldimethylsilyl, reduced the crystallinity and favored chain tilting and probably preventing the unfolding of crystal stems in the oligomers with 32 and 64 repeating units. Melting temperatures of mature crystals obeyed a linear relationship with inverse CL stem length. The intercept (equilibrium melting temperature) was in the range of 350 to 357 K.

Place, publisher, year, edition, pages
2012. Vol. 51, no 10, 2075-2092 p.
Keyword [en]
end groups, long period, melting, monodisperse, poly-epsilon-caprolactone
National Category
Polymer Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-103105DOI: 10.1080/00222348.2012.661678ISI: 000308022800014OAI: oai:DiVA.org:kth-103105DiVA: diva2:558701
Funder
Swedish Research Council, 2006-3559 2009-3188
Note

QC 20121004

Available from: 2012-10-04 Created: 2012-10-04 Last updated: 2017-12-07Bibliographically 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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