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Morphology, crystallization and melting of single crystals and thin films of star–branched polyesters with poly(epsilon-caprolactone) arms as revealed by atomic force microscopy.
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
2008 (English)In: Journal of macromolecular science. Physics, ISSN 0022-2348, E-ISSN 1525-609X, Vol. 47, no 3, 589-607 p.Article in journal (Refereed) Published
Abstract [en]

The morphology and thermal stability of different sectors in solution- and melt-grown crystals of star-branched polyesters with poly(epsilon-caprolactone) (PCL) arms, and of a reference linear PCL, have been studied by tapping-mode atomic-force microscopy (AFM). Real-time monitoring of melt-crystallization in thin films of star-branched and linear PCL has been performed using hot-stage AFM. A striated fold surface was observed in both solution- and melt-grown crystals of both star-branched and linear PCL. The presence of striations in the melt-grown crystals proved that this structure was genuine and not due to the collapse of tent-shaped crystals. The crystals of the star-branched polymers had smoother fold surfaces, which can be explained by the presence of dendritic cores close to the fold surfaces. The single crystals of linear PCL grown from solution showed earlier melting in the {100} sectors than in the {110} sectors, whereas no such sectorial dependence of the melting was found in the solution-grown crystals of the star-branched polymers. The proximity of the dendritic cores to the fold surface yields at least one amorphous PCL repeating unit next to the dendritic core and more nonadjacent and less sharp chain folding than in linear PCL single crystals; this evidently erased the difference in thermal stability between the {110} and {100} sectors. Melt-crystallization in thin polymer films at 53-55 degrees C showed 4 times faster crystal growth along b than along a, and more irregular crystals with niches on the lateral faces in star-branched PCL than in linear PCL. Crystal growth rate was strictly constant with time. Multilayer crystals with central screw dislocation (growing with or without reorientation of the b-axis) and twisting were observed in both classes of polymers.

Place, publisher, year, edition, pages
2008. Vol. 47, no 3, 589-607 p.
Keyword [en]
star-branched polyesters, atomic-force microscopy, solution-grown crystals, real-time monitoring, melt-crystallization, melting
National Category
Polymer Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-5974DOI: 10.1080/00222340801955636ISI: 000254631800015Scopus ID: 2-s2.0-41849134394OAI: oai:DiVA.org:kth-5974DiVA: diva2:10526
Note

Uppdaterad från submitted till published: 20100914 QC 20100914

Available from: 2006-06-07 Created: 2006-06-07 Last updated: 2017-12-14Bibliographically approved
In thesis
1. Crystallization in Constrained Polymer Structures: Approaching the Unsolved Problems in Polymer Crystallization
Open this publication in new window or tab >>Crystallization in Constrained Polymer Structures: Approaching the Unsolved Problems in Polymer Crystallization
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The knowledge regarding certain issues in polymer crystallization e.g. the possible existence of short–lived mesophases remains inconclusive due to experimental limitations. Polymers undergo chain folding upon crystallization, which introduces some complications that are not found in crystallization of low molar mass materials. Chain–folded crystals are far from their equilibrium shape and they rearrange rapidly at the crystallization temperature. This, together with the slow experimental techniques traditionally used, impedes the observation of the originally formed structures. To approach this problem, molecularly constrained polymer structures (in which the crystallizing chains are fixed at one end whereas the other end is free to move) have been studied by X–ray diffraction, differential scanning calorimetry, polarized optical microscopy, transmission electron microscopy and atomic force microscopy.

The crystallization studies performed in star–branched polyesters showed that the dendritic cores have a pronounced effect on the crystallization of the linear poly(ε–caprolactone) (PCL) arms attached to them. The star–branched polymers showed slower crystal rearrangement, higher equilibrium melting point, higher fold surface free energy, moderately lower crystallinity, and a greater tendency to form spherulites in comparison with linear PCL. The crystal unit cell was the same in both linear and star–branched PCL. Single crystals of the star–branched polymers were more irregular and showed smoother fold surfaces than linear PCL crystals. No sectorial preference was observed in the crystals of the star–branched polymers upon melting while the single crystals of linear PCL showed earlier melting in the {100} sectors than in the {110} sectors. Some of the differences observed can be attributed to the dendritic cores, which must be placed in the vicinity of the fold surface and thus influence the fold surface structure, the possibility of major crystal rearrangement and the presence of a significant cilia phase during crystal growth causing diverging crystal lamellae and consequent spherulite formation. The attachment of the many crystallizable chains to a single core reduces the melt entropy, which explains the higher equilibrium melting point of star–branched PCL.

The crystallization behavior of a series of poly(ethylene oxybenzoate)s was also studied. The polymers showed a profound tendency for crystal rearrangement during melting even at high heating rates. The Hoffman–Weeks extrapolation method was found to be unsuitable to calculate the equilibrium melting point of the samples studied because the melting point vs. crystallization temperature data were sensitive to the variations in crystallisation time, which led to significant variations in the equilibrium melting points obtained.

Place, publisher, year, edition, pages
Stockholm: KTH, 2006. 72 p.
Series
Trita-FPT-Report, ISSN 1652-2443 ; 2006:17
Keyword
Crystallization, Constrained structures, poly(ε–caprolactone), poly(ethylene oxybenzoate).
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-4041 (URN)91-7178-375-X (ISBN)
Public defence
2006-06-16, Sal K2, Teknikringen 28, Stockholm, 14:00
Opponent
Supervisors
Note
QC 20100914Available from: 2006-06-07 Created: 2006-06-07 Last updated: 2010-09-14Bibliographically approved

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