Dendritic-to-faceted crystal pattern transition of ultrathin poly„ethylene
and Gerhard Wegner
The Key Laboratory of Functional Polymer Materials of Ministry of Education and Institute of Polymer
Chemistry, College of Chemistry, Nankai University, Tianjin 300071, China
Max-Planck-Institute for Polymer Research, Ackermannweg 10, Postfach 3148, D-55128, Mainz, Germany
͑Received 8 May 2008; accepted 3 November 2008; published online 12 December 2008͒
The detailed T
-sensitive crystal pattern transition from dendrites through fourfold-symmetric
structures to faceted crystals of ultrathin poly͑ethylene oxide͒ ﬁlms has been experimentally
observed using atomic force microscopy. The transition has been quantitatively described by the
-dependences of the fractal dimension and of the velocity ratio caused by forward and transverse
growths in crystal tips. The essential aspect of the pattern selection and transition is mainly the
competition of two macroscopic mechanisms: Nucleation-limited and diffusion-limited growths
which create faceted and dendritic crystal patterns, respectively. Their combination is a facet growth
within a diffusion ﬁeld which will create a faceted dendrite. © 2008 American Institute of Physics.
The formation of diverse patterns, such as dendrite, sea-
weed, and faceted crystals, etc., has been one of the most
fascinating topics of condensed matter physics and statistical
physics over the past several decades.
Previous effort was
directed at understanding the physical mechanisms govern-
ing the formation of these patterns in different conditions far
from equilibrium to near equilibrium. Nowadays, it is well
known that the ramiﬁed patterns are created by solidiﬁcation
limited by diffusion because the growth of a ﬂat interface is
whereas faceted crystals can grow in nucleation-
limited condition. Recently, theoretical
investigations have been focused on the transition be-
tween unlike patterns to understand the turnover of driving
forces during pattern transition.
Because of their long chain nature, polymer crystalliza-
tion via chain folding is a complicated process in comparison
with low-mass compounds and crystal morphology is highly
dependent of crystallization temperature.
tion from thin layer of polymers
and other soft
on the surface of solid substrates and thin ﬁlm
samples of polymer blends
sandwiched in-between two
glass slides is a quasi two-dimensional ͑2D͒ solidiﬁcation
process. A theoretical simulation has been also performed to
understand the characteristic of crystal patterns of long chain
Therefore, the further study on the crystal
patterns can help explore the formation essence on the basis
of previous theoretical studies carried out in 2D condition.
In our previous study, we have demonstrated crystal pattern
transitions of ultrathin poly͑ethylene oxide͒͑PEO͒ ﬁlms
from dendrite through seaweed to square-shaped crystals
with increasing crystallization temperature, T
To have an
in-depth understanding of the pattern transition, a thorough
observation from dendrite through fourfold-symmetric pat-
tern to faceted crystal has been carefully performed. The
transition will be quantitatively described through the
-dependences of the fractal dimension and forward-to-
transverse velocity ratio of the growing tips. These quantita-
tive descriptions can reveal the inﬂuences of T
of nucleation-limited and diffusion-limited growths ͑NLG
and DLG͒ on the pattern transition.
The polymer used in this study is a poly͑ethylene oxide͒
with weigh-average molecular weight M
and polydispersity index PDI=1.23, purchased from Fluka. It
contains a methyl group at one end and a hydroxyl group at
the other end. Its equilibrium melting point is T
=67.8 ° C.
Toluene solutions of this PEO sample with a concentration of
0.01–0.02 wt. % were prepared in glassware. Silicon wafers
with a size of about 0.8ϫ0.8 cm
were treated in the solu-
tion of H
=3:1 at 120 °C for 30 min and
then cleaned in an ultrasonic water bath. Ultrathin PEO ﬁlms
were prepared simply by dropping the polymer solution
l͒ on the surface of the silicon wafers at room tem-
perature. The samples were dried at normal atmosphere over-
night and then treated in a vacuum oven at room temperature
for 12 h. Normally, ultrathin ﬁlms of lamellar crystals with
fractal-like patterns formed on the surface of the silicon
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Present address: Microscopy/Thermal Analytical Science, Asia Paciﬁc Re-
search, Dow Chemical ͑China͒ Investment Co. Ltd., 5/F, No. 512 Yutang
Road Building C, Songjiang Industrial Park, Shanghai 201613, China.
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THE JOURNAL OF CHEMICAL PHYSICS 129, 224708 ͑2008͒
0021-9606/2008/129͑22͒/224708/6/$23.00 © 2008 American Institute of Physics129, 224708-1