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Crystallization in Constrained Polymer Structures: Approaching the Unsolved Problems in Polymer Crystallization
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
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 [en]
Crystallization, Constrained structures, poly(ε–caprolactone), poly(ethylene oxybenzoate).
National Category
Polymer Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-4041ISBN: 91-7178-375-X (print)OAI: oai:DiVA.org:kth-4041DiVA: diva2:10527
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
List of papers
1. Crystal structure, melting behaviour and equilibrium melting point of star polyesters with crystallisable poly(ε–caprolactone) arms
Open this publication in new window or tab >>Crystal structure, melting behaviour and equilibrium melting point of star polyesters with crystallisable poly(ε–caprolactone) arms
Show others...
2004 (English)In: Polymer, ISSN 0032-3861, E-ISSN 1873-2291, Vol. 45, no 15, 5251-5263 p.Article in journal (Refereed) Published
Abstract [en]

Star polymers consisting of poly(ε-caprolactone), PCL, attached to third generation dendrimer, hyperbranched and dendron cores have been studied by differential scanning calorimetry and wide-angle X-ray scattering. The degree of polymerisation of the PCL arms of the star polymers ranged from 14 to 81. The crystal unit cell was the same for the star polymers as for their linear PCL analogues. The star polymers showed a lower degree of crystallinity than the linear PCL, suggesting that the dendritic cores imposed restriction on PCL crystallisation. Slow heating of rapidly cooled samples led to crystal rearrangement - a gradual increase in melting point with decreasing heating rate and recrystallisation followed by additional high temperature melting. The tendency for crystal rearrangement was less pronounced in star polymers based on dendrimer or hyperbranched cores, suggesting that the dendritic cores constitute an obstacle to crystal rearrangement. The star polymers showed higher equilibrium melting points than the linear PCL analogues. It is suggested that covalent attachment of the PCL arms to the dendritic core reduced the positional freedom and the entropy of the melt with respect to that of linear PCL.

Keyword
Equilibrium melting point, Poly(ε-caprolactone), Star polymer
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-5970 (URN)10.1016/j.polymer.2004.05.047 (DOI)000222844500028 ()2-s2.0-3042584200 (Scopus ID)
Note
QC 20100907Available from: 2006-06-07 Created: 2006-06-07 Last updated: 2011-09-26Bibliographically approved
2. Crystallization behaviour and morphology of star polyesters with poly(ε–caprolactone) arms
Open this publication in new window or tab >>Crystallization behaviour and morphology of star polyesters with poly(ε–caprolactone) arms
Show others...
2004 (English)In: Journal of macromolecular science Part B, ISSN 0022-2348, Vol. 43, no 6, 1143-1160 p.Article in journal (Refereed) Published
Abstract [en]

Star polymers consisting of poly(ε-caprolactone), (PCL), grafted onto third generation dendrimer, which had hyperbranched and dendron cores, were studied by polarized light microscopy together with reference linear PCL. The degree of polymerization of the PCL arms in the star polymers ranged between 14 and 81. The star polymers exhibited a greater tendency than the linear polymers to form spherulites. It is suggested that the preference of the star polymers for forming spherulites is due to the presence of amorphous material—dendritic cores and PCL cilia—between crystal lamellae that generates the necessary pressure to force the lamellae to diverge at lamellar branch points. The linear growth rate data followed a single crystallization regime. The fold surface free energy was higher for the star polymers than for their linear analogs. It is proposed that the presence of the large and rigid dendritic cores on the fold surfaces of the star polymer crystals increases the fold surface energy.

Keyword
Star polymers, Poly(epsilon-caprolactone), Crystallization; Morphology
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-5971 (URN)10.1081/MB-200026519 (DOI)000225403700003 ()2-s2.0-9944252431 (Scopus ID)
Note
QC 20100914 QC 20110916Available from: 2006-06-07 Created: 2006-06-07 Last updated: 2011-09-16Bibliographically approved
3. Single crystal morphology of star polyesters with crystallisable poly(ε–caprolactone) arms
Open this publication in new window or tab >>Single crystal morphology of star polyesters with crystallisable poly(ε–caprolactone) arms
2005 (English)In: Polymer, ISSN 0032-3861, E-ISSN 1873-2291, Vol. 46, no 16, 5992-6000 p.Article in journal (Refereed) Published
Abstract [en]

Star-branched polymers consisting of poly(epsilon-caprolactone) (PCL) attached to third generation dendrimer, hyperbranched and dendron cores have been studied together with linear PCL analogues. The degree of polymerisation of the PCL arms of the star-branched polymers ranged from 14 to 81. Single crystals were grown from dilute solution and studied by transmission electron microscopy. Single crystals of linear PCL were multilayer hexagons with flat or slightly curved {110} and {100} faces. These single crystals were larger along [010] than along [100]. Single crystals of star-branched PCL showed the same basic shape, but with many crystallographic and irregular steps on the lateral crystal faces. The width of the micro-faces was typically 100-300 nm. These single crystals were more extended along [100] than along [010]. It is proposed that the high fold surface free energy and the constrained character of the star-branched polymers favour the formation of steps on the growth faces. Globular polycrystalline aggregates were also observed. They originated from a more concentrated polymer phase following phase separation of the solution. In the case of the star-branched polymers, lamellar branching was observed with a 30 angle between the crystals arms.

Keyword
star-branched poly(epsilon-caprolactone), single crystal, transmission electron microscopy
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-5972 (URN)10.1016/j.polymer.2005.05.083 (DOI)000230714200019 ()2-s2.0-21744456720 (Scopus ID)
Note
QC 20100914Available from: 2006-06-07 Created: 2006-06-07 Last updated: 2010-09-14Bibliographically approved
4. Crystallisation behavior and crystal rearrangement of poly(ethylene oxybenzoate)
Open this publication in new window or tab >>Crystallisation behavior and crystal rearrangement of poly(ethylene oxybenzoate)
2005 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 413/414, no Sp. Iss. SI, 435-441 p.Article in journal (Refereed) Published
Abstract [en]

One complex fact of polymer crystallisation is that polymer crystals have a tendency to rearrange with time. In this paper, poly(ethylene oxybenzoate)s (PEOB) with different degrees of polymerisation ranging from 5 to 30 have been studied by differential scanning calorimetry and polarised microscopy. The samples showed a great tendency for crystal rearrangement during heating to the melting point, even at high heating rates. The relationship between melting point and crystallisation temperature was analyzed and the Hoffman-Weeks method was found to be unsuitable for determining the equilibrium melting point of these polymers. It is proposed that fast crystal rearrangement, which is a characteristic feature of poly(ethylene oxybenzoate), is the reason for the inadequacy of the Hoffman-Weeks method to obtain reliable estimates of the equilibrium melting point. Polarised microscopy showed, remarkably in view of the low molar mass of the polymers, the formation of perfect banded spherulites. Linear growth rate data suggested that the branched polymers crystallised more slowly than their linear analogues, presumably due to differences in the equilibrium melting point.

Keyword
Crystal rearrangement, Equilibrium melting point, Melting, Polymer crystallisation
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-5973 (URN)10.1016/j.msea.2005.08.168 (DOI)000234202900071 ()2-s2.0-29444447766 (Scopus ID)
Note

QC 20100907

Available from: 2006-06-07 Created: 2006-06-07 Last updated: 2016-12-16Bibliographically approved
5. 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.
Open this publication in new window or tab >>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.
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.

Keyword
star-branched polyesters, atomic-force microscopy, solution-grown crystals, real-time monitoring, melt-crystallization, melting
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-5974 (URN)10.1080/00222340801955636 (DOI)000254631800015 ()2-s2.0-41849134394 (Scopus ID)
Note

Uppdaterad från submitted till published: 20100914 QC 20100914

Available from: 2006-06-07 Created: 2006-06-07 Last updated: 2016-12-20Bibliographically approved

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