Solid-state NMR studies on the orthorhombic-to-hexagonal phase transition for α,ω-alkanediol crystals

Article abstract of DOI:10.1016/S0022-2860(00)00965-0

Studies on the orthorhombic and hexagonal phases of α,ω-alkanediols have been performed by solid-state NMR. In the crosspolarization/magic angle spinning ¹³C NMR spectrum for the orthorhombic phase of 1,23-tricosanediol (HO-(CH₂)

Full text of DOI:10.1016/S0022-2860(00)00965-0

Journal of Molecular Structure 569 ꢀ2001) 55±64
www.elsevier.nl/locate/molstruc
Solid-state NMR studies on the orthorhombic-to-hexagonal phase
transition for a,v-alkanediol crystals
a
K. Kuwabara , F. Horii , Y. Ogawa
a,
b
*
a
Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan
b
Department of Chemistry, Faculty of Science, Kumamoto University, Kurokami 2-39-1, Kumamoto 860-8555, Japan
Received 13 October 2000; revised 28 November 2000; accepted 28 November 2000
Abstract
Studies on the orthorhombic and hexagonal phases of a,v-alkanediols have been performed by solid-state NMR. In the cross-
1
3
polarization/magic angle spinning C NMR spectrum for the orthorhombic phase of 1,23-tricosanediol ꢀHO±ꢀCH
resonance line assigned to the inner CH is observed at 32.9 ppm and two resonance lines appear for the a-CH and g-CH
2
2
)23±OH), the
2
2
carbons at chain ends. These doublets are due to the different chain ends with the all-trans and the gauche-included conforma-
1
3
tions. C spin-lattice relaxation time measurements reveal that the chain end including the gauche conformation is less mobile
compared to another chain end consisting of the all-trans conformation. It is also found that the monoclinic±hexagonal phase
transition behavior for the 1,20-eicosanediol ꢀHO±ꢀCH ) ±OH) crystallized from toluene solution by quenching is almost the
2
20
same as the case of the 1,20-eicosanediol sample crystallized by slowly cooling from the toluene solution which was previously
reported. q 2001 Elsevier Science B.V. All rights reserved.
Keywords: Solid-state NMR; Alkanediol; Orthorhombic phase; Gauche effect; n-Alkane; Polyethylene
1
. Introduction
molecular motions are depicted for the hexagonal
phase based on our previous work for 1,20-eicosane-
diol [9].
a,v-alkanediols have hydroxyl groups at both CH2
chain ends [1]. The crystal structure of long-chain
a,v-alkanediol is monoclinic [2±4] or orthorhombic
[5,6] at room temperature, depending on the crystal-
lization condition and the number of CH units [7].
With the increase of temperature, these a,v-alkane-
diols undergo monoclinic-to-hexagonal or orthor-
hombic-to-hexagonal phase transition prior to
melting [7,8]. Fig. 1 shows the schematic models for
the molecular arrangements of orthorhombic and
hexagonal phases of a,v-alkanediols [5,6,9]. The
Analogous premelting transitions are also found in
n-alkanes [10±13] and polyethylene [14±16] which
consist of CH2 sequences as major constituent.
These crystalline phase transitions and the molecular
motion in the hexagonal ꢀrotator) phase have drawn
particular interest, and many investigations have been
performed by DTA/DSC [17±19], X-ray [20±23],
infrared/Raman spectroscopies [24±27], and micro-
scopy [28,29]. Recently, the solid-state NMR tech-
niques have been developed remarkably to
characterize different phases of organic materials
2
[30±33] and more detail information has been
obtained for n-alkanes [34±38] and polyethylenes
[39±43].
*
Corresponding author. Tel.: 181-774-38-3150; fax: 181-774-
8-3148.
E-mail address: horii@molmat.kuicr.kyoto-u.ac.jp ꢀF. Horii).
3
0022-2860/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-2860ꢀ00)00965-0

56
K. Kuwabara et al. / Journal of Molecular Structure 569 ꢀ2001) 55±64

K. Kuwabara et al. / Journal of Molecular Structure 569 ꢀ2001) 55±64
57
Such a situation for n-alkanes and polyethylene is
quite a contrast to the case of long-chain a,v-alkane-
diols, for which a limited number of studies has been
performed by X-ray [2±7], DSC [7±9], and FT-IR [9]
measurements. As for solid-state NMR, only the work
on the monoclinic-to-hexagonal phase transition for
the 1,20-eicosanediol sample, which was slowly crys-
tallized from the 1% toluene solution, has been
reported [9].
In this paper, to obtain more information on the
long-chain a,v-alkanediols, the sample which
includes the orthorhombic phase at room temperature
is characterized by high-resolution solid-state NMR
measurements. The structure and molecular motion
previously used [9], which was crystallized from the
1% toluene solution by slow cooling, includes only
monoclinic crystals at room temperature.
2.2. DSC
DSC measurements were performed on a TA
Instruments Model 2910 thermal analyzer at a heating
rate of 0.258C/h. The sample mass of 4±5 mg was
used. The calorimeter was calibrated with high-purity
indium standards.
2.3. Solid-state NMR
1
3
C
Solid-state
NMR measurements were
for inner CH sequences and chain ends are investi-
2
performed on a JEOL JNM-GSX 200 spectrometer
operating under a static magnetic ®eld of 4.7 T.
Each sample was packed into a 7-mm diameter
gated in detail in orthorhombic, monoclinic, and hexa-
gonal phases. The effects of the crystallization
condition and the length of the CH sequences are
2
1
13
zirconia rotor. Hand
gB /2p were 62.5 kHz throughout this work. The
C radio frequency ®elds
also discussed.
1
contact time for the cross-polarization ꢀCP) process
was 2.0 ms. To avoid the overlapping of spinning side
bands on other resonance lines, the magic angle spin-
ning ꢀMAS) rate was set to 3 kHz. C chemical shifts
were expressed as values relative to tetramethylsilane
2
. Experimental
1
3
2
.1. Samples
ꢀMe Si) by using the CH line at 17.36 ppm of hexam-
ethylbenzene crystals as an external reference.
4
3
In this work, two a,v-alkanediol samples, 1,23-
tricosanediol ꢀOH±ꢀCH ) ±OH) and 1,20-eicosane-
13
C
spin-lattice relaxation times ꢀT ) were measured by
2
23
1
C
diol ꢀOH±ꢀCH) ±OH), were used. These samples
2
0
the CPT1 pulse sequence [45]. Temperature calibra-
tion was done by the ethylene glycol method modi®ed
for NMR measurements under MAS [46±48].
were synthesized as follows [7]: 1,21-heneicosanoic
acid ꢀHOOC±ꢀCH ) ±COOH) was obtained from
2
21
short-chain diacids by ketene dimerization synthesis
44]. 1,18-Octadecanoic acid ꢀHOOC±ꢀCH ) ±
1
HCRAMPS measurements were performed on a
[
2
18
CMX-400 spectrometer operating at 399 MHz, using
the BR-24 pulse sequence with a cycle time of 103 ms,
corresponding to a basic pulse spacing of 3 ms and a
908 pulse width of 1.3 ms. The sample was packed into
a 5-mm diameter glass tube. The MAS speed was set
1
to 1.5 kHz. Hchemical shifts were expressed as
COOH) was purchased from Tokyo Kasei Co. Ltd.
These diacids were converted to methyl esters by
conventional procedures. The pure compounds were
obtained through fractional distillation and recrystal-
lization, and purity was estimated to be 99.8% by
capillary gas chromatography. The dimethyl esters
were further reduced to a,v-alkanediols by LiAlH4.
values relative to tetramethylsilane ꢀMe Si) by using
4
the CH line at 0.12 ppm of silicone rubber as an
3
1
,23-tricosanediol thus synthesized was crystal-
internal reference.
lized by slowly cooling the ethanol solution to room
temperature. This sample contains only orthorhombic
crystals at room temperature [7].
3. Results and discussion
1
,20-eicosanediol synthesized as described above
was crystallized from the 1% toluene solution by
quenching to 48C. This sample contains orthorhombic
crystals as well as monoclinic crystals at room
temperature. In contrast, the 1,20-eicosnaediol sample
3.1. DSC curves
Fig. 2 shows the DSC curves for the a,v-alkanediol
samples. For 1,23-tricosanediol crystallized from the

58
K. Kuwabara et al. / Journal of Molecular Structure 569 ꢀ2001) 55±64
Fig. 2. DSC curves for a,v-alkanediols: ꢀa) the 1,23-tricosanediol
sample with the orthorhombic phase; and ꢀb) the 1,20-eicosanediol
sample with the monoclinic and orthorhombic phases.
1
3
Fig. 3. CP/MAS C NMR spectra of the 1,23-tricosanediol sample
with the orthorhombic phase. The characters `o' and `h' denote the
ethanol solution by slowly cooling, two endothermic
peaks are found to appear at 91 and 1058C. The
endothermic peak at 918C is assigned to the crystalline
phase transition from the orthorhombic phase to the
hexagonal phase, whereas the endothermic peak at
2
CH resonance lines in the orthorhombic and hexagonal phases,
respectively. The characters `t' and `g' reveal the trans and gauche
ends in the orthorhombic phase, respectively.
1
[
058C is ascribed to melting of the hexagonal phase
7].
For the 1,20-eicosanediol sample crystallized from
contrast, the hexagonal phase of the 1,23-tricosane-
diol sample appears at 918C, which is somewhat lower
than the case ꢀ948C) for the 1,20-eicosaediol sample.
This result is interpreted in terms of the odd±even
the 1% toluene solution by quenching at 48C, three
endothermic peaks are observed at 86, 94, and 1018C.
These endothermic peaks are assigned to the crystal-
line phase transition from the orthorhombic to the
monoclinic phase, the crystalline phase transition
from the monoclinic to the hexagonal phase, and
melting of the hexagonal phase, respectively [9].
The shapes of endothermic peaks at 94 and 1018C
are similar to the respective peaks for the 1,20-eico-
sanediol sample crystallized from the 1% toluene
solution by slow cooling as previously reported [9].
The melting temperature of the hexagonal phase for
effect for the CH sequences [1,7].
2
13
.2. CP/MAS C NMR spectra of the 1,23-
3
tricosanediol sample with the orthorhombic phase
1
3
Fig. 3 shows the CP/MAS C NMR spectra of the
1,23-tricosanediol crystallized from the ethanol solu-
tion by slowly cooling and chemical shift values
obtained for the respective lines are plotted against
the temperature in Fig. 4. In accord with the appear-
ance of the orthorhombic±hexagonal transition peak
at 918C in the DSC curve, the resonance lines assigned
to the orthorhombic components are observed below
918C and the hexagonal lines appear above 918C. The
1
,20-tricosandiol is lower than that for 1,23-tricosane-
diol. Such a difference can be explained simply by the
difference in length of the methylene sequence. In

K. Kuwabara et al. / Journal of Molecular Structure 569 ꢀ2001) 55±64
59
results indicate the occurrence of some disordering
such as twisting in the trans end with increasing the
temperature.
Secondary, the chemical shift value for the inner
CH carbon of 32.9 ppm agrees well with the values
2
for the inner CH carbons of the orthorhombic n-
2
alkane [36±38] and orthorhombic polyethylene [39].
In our previous work [9], to calculate the content of
the gauche conformation in the hexagonal phase of
1,20-eicosanediol, the chemical shift value for the
inner CH carbon of n-alkane was used as a standard
2
material for the crystals with the all-trans conforma-
tion. At that time additional NMR measurements
seemed to be necessary to prove the reliability of
the use of n-alkane as a standard material for the
methylene crystal because no solid-state NMR
datum had been reported which indicates that the
OHgroups at chain ends have no effect on the
1
3
chemical shift value of the inner CH carbons. The
2
Fig. 4. C chemical shift values of the 1,23-tricosanediol sample
with the orthorhombic phase: A, a-CH ꢀorthorhombic); W, a-CH
hexagonal); B, inner CH ꢀorthorhombic); X, inner CH ꢀhexa-
gonal); S, g-CH ꢀorthorhombic); K, g-CH ꢀhexagonal).
result obtained here for the orthorhombic 1,23-trico-
sanediol con®rms that the inner CH carbons for n-
2
2
ꢀ
2
2
2
2
2
alkane and a,v-alkanediol orthorhombic crystals have
1
3
the same C chemical shift value probably due to the
same states in conformation and packing.
assignment of each resonance line is performed by
reference to data on liquid n-alcohol [49,50] and
solid n-alkanes [34±38].
Above 948C, the resonance lines assigned to the
hexagonal phase are observed in Fig. 3. The line
width of each CH carbon is narrower compared to
2
Some interesting features are observed: the reso-
nance lines for the a-CH and g-CH carbons appear
the resonance line in the orthorhombic phase. Such
decrease in line width suggests the occurrence of the
molecular motion with the rate signi®cantly higher
than the amplitude of the dipolar decoupling ®eld
2
2
as doublet. Such resonance lines re¯ect the different
chain ends; one chain end is composed of the all-trans
conformation, whereas another chain end includes the
gauche conformation [5,6]. The difference in
chemical shift between the trans and gauche confor-
mation can be explained in terms of the g-gauche and
vicinal gauche effects [51]. These gauche effects have
5
ꢀ,10 Hz) [52±54]. The resonance line for the
inner CH carbon in the hexagonal phase appears
2
at 32.9 ppm, showing somewhat up®eld shift
compared to the corresponding line in the hexagonal
phase of n-alkane [36±38]. This value is the same as
already been revealed for the 'inner' CH sequences
2
observed for the inner CH carbon in the hexagonal
2
by the previous work for cycloalkane crystals [51].
However, as far as we know, no direct evidence at
chain ends has been reported so far. This work for
the orthorhombic 1,23-tricosanediol has con®rmed
the g-gauche and vicinal gauche effects for CH2
chain ends with the experimental data.
phase of the 1,20-eicosanediol crystallized from the
1% toluene solution by slow cooling [9]. According
to the previous calculation considering the packing,
g-gauche, and vicinal gauche effects [9], such a
lower chemical shift means the inclusion of 3%
gauche defects and the occurrence of the rapid tran-
sition between the trans and gauche conformations
in the hexagonal phase of the 1,23-tricosanediol. As
in the hexagonal phase of polyethylene [39], such
gauche defects are required to ful®ll the cylindrical
symmetry in the hexagonal phase because crystalline
As seen in Fig. 4, the chemical shift value for the
a-CH carbon in the trans end shows up®eld shift
2
with the increase of temperature, whereas the
chemical shift value for the a-CH carbon in the
gauche end is independent of temperature. These
2

60
K. Kuwabara et al. / Journal of Molecular Structure 569 ꢀ2001) 55±64
2
Fig. 5. T1C values for the respective CH carbons of the 1,23-trico-
sanediol sample with the orthorhombic phase.
CH chain ends are not mobile unlike the chain ends
2
of n-alkane [37].
1
Fig. 6. HCRAMPS spectra of a,v-alkanediols in different phases:
A, this work; B, previous work.
1
.3. C spin-lattice relaxation behavior of the 1,23-
3
3
tricosanediol sample with the orthorhombic phase
1
CH carbons. On the basis of the HCRAMPS spectra
2
To obtain further information on the molecular
motion for the orthorhombic and hexagonal phases
of 1,23-tricosanediol, T1C measurements have been
performed at different temperatures. Fig. 5 shows
the plots of T1C values against the respective CH2
shown below and the structure model previously
proposed [9], it should be pointed out that these results
indicate the occurrence of the two-site exchange
motion for the OHgroups in the hexagonal phase as
a result of the jump rotation around the C±O bond.
carbons. In the orthorhombic phase, the a-CH and
2
g-CH carbons at the gauche chain end have longer
2
1
3.4. H CRAMPSspectra of the 1,23-tricosanediol
sample with the orthorhombic phase
T1C values compared to the corresponding CH2
carbons at the trans end. This fact indicates that the
chain end including the gauche conformation is less
mobile compared to another chain end consisting of
the all-trans conformation. Such a difference in mobi-
lity between the gauche and trans ends cannot be
interpreted in terms of the crystal structure shown in
Fig. 1 which was obtained by the recent wide-angle X-
ray diffraction analysis [5,6]. In other words, present
solid-state NMR work has pointed out for the ®rst
time such a difference in chain dynamics in the ortho-
rhombic phase of a,v-alkanediol.
1
Fig. 6 shows the HCRAMPS spectra of the 1,23-
tricosanediol and the previous 1,20-eicosanediol
samples at different temperatures. The resonance
line at about 1.5 ppm is assigned to the CH protons,
2
whereas the line at 3.5±4.0 ppm is ascribed to the OH
protons. These assignments were performed by refer-
ence to data on liquid n-alcohols [50]. Because of the
1
13
lower resolution of HNMR compared to C NMR,
no chemical shift change for the CH resonance line is
observed at each spectrum.
2
1
As seen in Fig. 6, Hchemical shift values for the
T1C values for the hexagonal phase of the 1,23-
tricosanediol are similar to those for the previous
1,20-eicosanediol sample crystallized from the 1%
toluene solution by slow cooling [9]: more external
OHprotons of 1,23-tricosanediols are in accord with
0
those for the OHprotons of the monoclinic ꢀM ) and
hexagonal 1,20-eicosanediols [9]. These results mean
that the electronical environment of the OHprotons
CH carbons are more mobile compared to the inner
2

K. Kuwabara et al. / Journal of Molecular Structure 569 ꢀ2001) 55±64
61
components. The resonance lines assigned to the
monoclinic phase of the 1,20-eicosanediol sample
are observed at 63.7, 34.7, 33.8, and 29.3 ppm,
which correspond to the a-CH , inner CH ꢀb-CH
2
2
2
is also included), d-CH , and g-CH carbons, respec-
2
2
tively. These results for the monoclinic phase are in
good accord with the previous results for the 1,20-
eicosanediol sample with the pure monoclinic phase,
which was crystallized from the 1% toluene solution
by slow cooling [9]. As for the orthorhombic compo-
1
3
nent, the C chemical shift values are similar to those
for the orthorhombic component of the 1,23-eico-
sanediol sample shown in Fig. 4. The vicinal gauche
effect [51] for the a-CH carbons and the g-gauche
2
effect [51] for the g-CH carbons are clearly observed
2
in Fig. 7.
At 90 and 928C, only the resonance lines assigned
to the monoclinic phase are found to appear. The
resonance lines found here are similar to the
previously reported resonance lines for the mono-
0
clinic ꢀM ) phase of 1,20-eicosanediol [9]: the reso-
nance lines are broader compared to those for the
monoclinic ꢀM) phase of 1,20-eicosanediol which
was produced below 808C for the sample slowly crys-
tallized from the 1% toluene solution. Moreover, the
1
3
Fig. 7. CP/MAS C NMR spectra of the 1,20-eicosanediol sample
with the monoclinic and orthorhombic phases. The characters `o',
resonance lines for the a-CH carbon seems doublet.
2
Therefore, this monoclinic phase corresponds to the
0
`
h', `t', and `g' correspond to those denoted in Fig. 3. The character
m' denotes the CH resonance lines in the monoclinic phase.
monoclinic ꢀM ) phase which appears at 90 and 928C
in a previous work [9].
`
2
are almost the same in these samples even if there are
differences in mobilities at the chain ends as revealed
At 96 and 988C, the spectra for the hexagonal phase
of the 1,20-eicosanediol sample is almost the same as
that for the hexagonal phase of the 1,23-tricosanediol
1
by T1C measurements. As described above, these H
1
3
chemical shift values imply the two-site exchange
motion for the OHgroups around the C±O bonds in
the hexagonal phase [9]. Under such a jump motion,
shown in Fig. 3. As far as the C chemical shift and
line width are concerned, the odd±even effect for the
number of CH carbons are negligible in the CP/MAS
2
1
3
the inner CH sequences in the hexagonal phase are
2
C NMR spectra.
allowed to jump at random around the chain axis with
keeping the almost planar zig±zag conformation as
shown in Fig. 1.
13
.6. C spin-lattice relaxation behavior of the 1,20-
3
eicosanediol sample with the monoclinic and
orthorhombic phases
1
.5. CP/MAS C NMR spectra of the 1,20-
3
3
To investigate the mobility for each CH carbon of
2
eicosanediol sample with the monoclinic and
orthorhombic phases
the 1,20-eicosanediol sample crystallized from the 1%
1
3
toluene solution by quenching at 48C, C spin-lattice
relaxation times ꢀT ) have been measured by the
CPT1 pulse sequence [45] at different temperatures.
1
3
Fig. 7 shows the CP/MAS C NMR spectra of the
,20-eicosanediol crystallized from the 1% toluene
solution by quenching at 48C. At 388C, the spectrum
1
C
1
Fig. 8 shows the plot of T1C values thus obtained
0
is found to include both monoclinic and orthorhombic
against the temperature. As for the monoclinic ꢀM )

62
K. Kuwabara et al. / Journal of Molecular Structure 569 ꢀ2001) 55±64
1
3
C spin-lattice relaxation behavior is concerned, the
odd±even effect for the number of CH carbon are
2
found to be negligibly small in the orthorhombic
phase.
4. Conclusions
The structure and molecular motion for the a,v-
alkanediols, which form orthorhombic crystals at
room temperature, have been elucidated by high-reso-
1
3
1
lution solid-state C and HNMR spectroscopies and
the following conclusions have been obtained:
1
. The C chemical shift values for the inner CH2
3
1
2
Fig. 8. T1C values for the respective CH carbons of the 1,20-
sequence of orthorhombic 1,20-eicosanediol and
1,23-tricosanediol give the same values with
those for the inner CH sequence of orthorhombic
n-alkane and the orthorhombic polyethylene crys-
tals. These results reveal that n-alkane is useful as
the standard material for the chemical shift value of
eicosanediol sample with the monoclinic and orthorhombic phases.
phase at 908C and the hexagonal phase at 988C, T1C
values are almost equal to those for the previous 1,20-
eicosanediol sample crystallized from the 1% toluene
solutionbyslowcooling [9]. Inotherwords, the effectof
the crystallization condition is discernible above the
orthorhombic-to-monoclinic transition temperature.
2
At 388C, the T1C value of the inner CH carbons is
2
shorter in the orthorhombic phase than in the mono-
clinic phase. This result indicates that the ortho-
rhombic crystals are much mobile compared to the
monoclinic crystals. Because of the dif®culty in
obtaining spectra with the suf®cient signal/noise
ratio accurate T1C values for the chain end carbons
ꢀa-, b-, g-, and d-CH carbons) would not be deter-
mined at 388C.
2
To evaluate qualitatively the mobility of the chain
ends at 388C, partially relaxed spectra obtained by the
CPT1 pulse sequence with different T1C relaxation
delays ꢀt) are shown in Fig. 9. Almost the same
decays are observed for a-CH and g-CH carbons
2
2
as well as the inner CH carbons; the orthorhombic
2
component decays faster than the monoclinic compo-
nent, indicating the higher mobility of the orthor-
hombic component compared to the monoclinic
component.
It should be also noted here that the resonance lines
of the a-CH and g-CH carbons at the trans end
almost disappear at t ˆ 4 s whereas those at the
gauche end still remain even at t ˆ 40 s. These results
are in accord with the results for the 1,23-eicosanediol
samples shown in Fig. 5. As a consequence, as far as
2
2
1
3
Fig. 9. Partially relaxed MAS C NMR spectra of the 1,20-eicosa-
nediol sample with the monoclinic and orthorhombic phases. These
spectra were obtained by the CPT1 pulse sequence at 388C. To
increase the signal/noise ratio, appropriate line broadening was
performed for each spectrum.

K. Kuwabara et al. / Journal of Molecular Structure 569 ꢀ2001) 55±64
63
the trans-zig±zag methylene sequence in the crys-
talline phase.
. The resonance lines for the a-CH and g-CH
[12] B. Ewen, E.W. Fischer, W. Piesczek, G.R. Strobl, J. Chem.
Phys. 61 ꢀ1974) 5265.
[
13] B. Ewen, G.R. Strobl, D. Richter, J. Chem. Soc. Faraday
Discuss. 69 ꢀ1980) 19.
2
2
2
1
3
carbons appear as doublet in the CP/MAS
C
[
14] B. Wunderlich, J. Grebowicz, Liquid Crystal Polymers II/III,
Springer, Berlin, 1984.
[15] M. Hikosaka, Polymer 28 ꢀ1987) 1257.
NMR spectra. Such doublets re¯ect different
conformations at chain ends: One chain end
consists of the all-trans conformation whereas
another end contains the gauche conformation.
[
[
[
[
16] D.C. Bassett, Developments in Crystalline Polymers 2,
Applied Science, London, 1988.
17] D.C. Bassett, B.A. Khalifa, B. Turner, Nat. ꢀPhys. Sci.) 239
1
3
3
. In the orthorhombic phase, C spin-lattice relaxa-
tion time measurements reveal that the chain end
consisting of the all-trans conformation is much
more mobile compared to another chain end
including the gauche conformation. This result
indicates that the gauche-included chain end is
more tightly stabilized by hydrogen bonding
compared to the all-trans chain end.
ꢀ
1972) 106.
18] K. Takamizawa, Y. Ogawa, T.-O. Oyama, Nat. ꢀPhys. Sci.) 14
ꢀ1982) 441.
19] S. Tsubakihara, A. Nakamura, M. Yasuniwa, Polym. J. 28
ꢀ
1996) 489.
[
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