Conformations of single polymer chains on surfaces [Elektronische Ressource] : non-equilibrium, equilibrium and manipulation / von Christof Ecker

Christof Ecker

Sujets

• Physik

Informations

• Publié par: humboldt-universitat_zu_berlin
• Publié le: 01 janvier 2004
• Nombre de lectures: 12
• Langue: English
• Poids de l'ouvrage: 3 Mo

Extrait

Conformations of single polymer chains on
surfaces
non-equilibrium, equilibrium and manipulation
DISSERTATION
zur Erlangung des akademischen Grades
doctor rerum naturalium
(Dr. rer. nat.)
im Fach Physik
eingereicht an der
Mathematisch-Naturwissenschaftlichen Fakultat I
Humboldt-Universitat zu Berlin
von
Herr Dipl.-Phys. Christof Ecker
geboren am 10.7.1972 in Rheinberg
Prasident der Humboldt-Universitat zu Berlin:
Prof. Dr. Jurgen Mlynek
Dekan der Mathematisch-Naturwissenschaftlichen Fakultat I:
Prof. Thomas Buckhout, PhD
Gutachter:
1. Prof. Dr. J. P. Rabe
2. Prof. Dr. I. M. Sokolov
3. Prof. Dr. R. v. Klitzing
eingereicht am: 20. 9. 2004
Tag der mundlichen Prufung: 20. 12. 2004Abstract
Frontali et al. [137] showed in 1979 that it is possible to characterize the
structure of polymers by analyzing high resolution microscopy images. The
authors adsorbed DNA molecules onto mica and imaged them using electron
microscopy. From the vectorized contours of single chains, they were able to
determine both contour and persistence lengths so that the polymers were
fully described in the framework of the wormlike-chain (WLC) model. In
contrast to the conventional characterization methods for polymer solutions
thisapproachhastwoimportantadvantages: 1)singlemoleculesinsteadofan
ensembleareinvestigated,and2)theresultscanbecomparedtothemodelin
a much more direct way. As a disadvantage, high resolution microscopy such
as transmission electron microscopy (TEM) or scanning force microscopy
(SFM) require the polymers to be adsorbed onto a solid substrate surface
which inuences the chain conformations.
Until now, the method is not well established. In particular, basic knowledge
about single molecule conformations on surfaces is missing. Investigations in
literaturearemainlyconcernedwithadsorptionunderequilibriumconditions.
Often however, the interaction between molecule and substrate is strong so
that the equilibration is inhibited and chains remain trapped in the initial
conformation determined by the kinetics of the adsorption process.
The goal of this thesis was to develop and establish methods for the char-
acterization of single chains of polymers using scanning force microscopy.
Therefore, methods for the analysis and procession of SFM-images were de-
veloped. This includes a method for the determination of the persistence
length which is very e cient and features an analytic expression for the sta-
tistical error. Furthermore, a new algorithm for background removal was
found which replaces the commonly used attening . It avoids the mislead-
ing“shadows”characteristic for SFM-images and therefore enables elaborate
quantitative analysis of height values, e.g. along the contour of an adsorbed
polymer molecule. In addition an automated vectorization method has been
developed which avoids errors due to the discreteness of the image pixels.
Models were developed which allow for the rst time a detailed prediction of
conformational characteristics in the non-equilibrium. It is shown that twokinds of very regular conformations can appear on strong interacting sub-
strates: sine-like undulations and spiral shaped“tron”-conformations. Undu-
lations have been observed by other groups before but were probably misin-
terpreted, e.g. as helices.
Stronglyphysisorbedpolymerswereinvestigatedexperimentallyfortwomodel
systems: charged dendronized polymers on mica and DNA on poly-ornithine
layers. For the dendronized polymers conformations were found to be un-
dulated with a period of 133 nm. The adsorbed DNA molecules showed
“tron”features with a dependence on the ion concentration in agreement to
the proposed model.
The persistence length of poly(isocyanodipeptides) (PIC) has been deter-
mined for the rst time. The obtained value of 766 nm shows that these
polymer molecules are extraordinarily rigid, more rigid than double stranded
DNA and probably the most rigid synthetic polymers yet. This behavior
is attributed to the helical backbone which is additionally reinforced by a
networkof sidegroupsconnectedbyhydrogenbonds. Thepersistencelength
has also been determined for PICs where the polymerization was catalyzed
using acid instead of Ni, leading to a contour length of up to 5.3 m. Those
long chains were not able to equilibrate on the surface, nevertheless the same
value for the persistence length was obtained conrming the idea that chains
can equilibrate on the local scale of up to 100 nm.
The method of single chain nanomanipulation has been developed in our
group during the last years. It allows to move molecules in a precise way
on the substrate surface. The resulting lateral forces have been used in this
work to investigate the mechanical properties of adsorbed molecules. Of
special interest was the investigation of dendronized polymers because spe-
cial mechanical properties can be expected for this new class of molecules.
Dendronized polymers carry a regularly branched side group (dendron) at
each of the repeat units. The mass of a dendron grows exponentially with
the number of branching generations. For heigh generation numbers it can
be expected, that the mechanical bending behavior of the chain is strongly
inuenced by the dendrons and therefore deviates from the usual chain ex-
ibility. Our experiments indicate that a glassy state exists for molecules of
generation 3 and 4, in which the molecule no longer behaves as a exible
chain but instead plastically keeps the shape in which it is frozen-in, similar
to a macroscopic body. Further experiments show that also a liquid state
exists for elevated temperatures and good solvents in which molecules are
exible. The glassy state of a single molecule is a new and unusual property
iiifor polymers. It might be useful in nanotechnology as it enables e.g. the
creation of single molecular components with a selectable rigid shape. For
future research, it would be interesting to investigate the inuence of the
glassy state on macroscopic material properties. This question has to remain
unanswered for now as the amount of available dendronized polymers is not
sucient yet.
Keywords:
single molecules, polymer conformation, manipulation, scanning force
microscopy
ivZusammenfassung
In ihrer Arbeit von 1979 haben Frontali et al. [137] gezeigt, da es prinzipiell
moglich ist, die Struktur von Polymeren durch die Analyse hochaufgeloster
Mikroskopiebilderzucharakterisieren.DieAutorenhabenDNS-Molekuleauf
Glimmer adsorbiert und mit dem Elektronenmikroskop abgebildet. Aus den
vektorisierten Konturen einzelner Molekule konnten sie sowohl die Kontur-
als auch die Persistenzlange bestimmen und damit die Polymere im Rahmen
des Wormlike-Chain-Modells vollstandig beschreiben. Gegenuber gebrauch-
lichen Methoden zur Polymeruntersuchung hat der mikroskopische Ansatz
einigeVorteile:eswerdeneinzelneMolekulestatteinesEnsemblesuntersucht
und die Me ergebnisse k onnen viel direkter mit dem Modell verglichen wer-
den. Allerdings mussen Molekule, um sie mit mit dem Rasterkraftmikroskop
(RKM) oder dem Elektronenmikroskop zu untersuchen, auf einer Substra-
toberache adsorbiert werden, was die Interpretation der Konformationen
erschwert.
DieMethodeistbisherwenigetabliertundinsbesonderefehltgrundsatzliches
WissenuberdieKonformationenvonvereinzeltenMolekulenanOberachen.
So befassen sich die bisher in der Literatur beschriebenen Untersuchungen
hauptsachlichmitKonformationenunterGleichgewichtsbedingungen.Haug
besteht aber eine starke Wechselwirkung zwischen Molekul und Unterlage,
soda eine Aquilibrierung nicht moglich ist und die durch die Kinetik des
Adsorptionsprozesses bestimmte Anfangskonformation eingefroren wird.
Das Ziel der vorliegenden Arbeit war es, Methoden der Charakterisierung
einzelner Polymere mit dem Rasterkraftmikroskop zu erforschen und zu eta-
blieren. Es wurden zunachst Methoden der Auswertung von RKM-Bildern
undzurBildbearbeitungentwickelt.SowurdeeineMethodezurBestimmung
der Persistenzlange hergeleitet, die besonders e zient ist und es erlaubt, den
statistischen Fehler analytisch zu berechnen. Auerdem wurde ein einfacher
Algorithmus zur Korrektur des Untergrunds gefunden, der das ubliche Flat-
ten ersetzt. Es werden damit die fur RKM-Bilder typischen irrefuhrenden
“Schatten”vermieden und so eine quantitative Analyse der Hohenwerte, z.B.
entlang der Kontur einer Polymerkette, ermoglicht. Weiter wurde eine Me-
thode der automatisierten Vektorisierung entwickelt, die das Problem der
Fehler durch diskrete Pixel vermeidet.Es wurden Modelle entwickelt, die erstmals detaillierte Vorhersagen uber
Konformationen im Nichtgleichgewicht zulassen. Sie basieren auf der Annah-
me,dasichdieKonformationenbeidemAdsorptionsproze bildenundnicht
mehr verandern. Es wurde gezeigt, da zwei Arten von besonders regelma-
igen Konformationen auftreten konnen: sinusartige Undulationen und spi-
ralartige“Tron”-Konformationen. Erstere wurden in der Literatur verschie-
dentlich beschrieben, aber nach dem Resultat dieser Arbeit fragwurdig (z.B.
als Helizes) interpretiert.
KonformationenstarkphysisorbierterPolymerewurdenfurzweiModellsyste-
me experimentell untersucht: geladene dendronisierte Polymere auf Glimmer
und DNS auf Polyornithin-Schichten. Fur dendronisierte Polymere wurden
Undulationen mit einer Periode von 133 nm