The effect of poly(propylene glycol) on the formation of lyotropic liquid
crystalline phases of amphiphiles containing glycerol head groups
Shin-Hong Ou, Sung-Chuan Hung, and Guo-Jhan Wong
Institute of Biotechnology and Chemical Engineering, I-Shou University, Kaohsiung city, Taiwan
Tzong-Ming Wu
Department of Materials Science and Engineering, National Chung Hsing University, Taichung city, Taiwan
Abstract:
In order to prepare a nonlamellar lyotropic liquid crystalline structure, amphiphiles (PPG-Gly 340 and 1000) containing
poly(propylene glycol)s of different molecular weights, 340 and 1000 respectively, as hydrophobic chains and glycerol as
head groups were synthesized and investigated by optical microscopy and small angle X-ray scattering. The effect of
poly(propylene glycol) on the developments of reverse bicontinuous cubic phase (V2) and reverse hexagonal phase (H2) was
examined by the mixing of PPG-Gly with glyceryl monooleate (GMO). The addition of PPG-Gly to GMO expedites the
lamellar-Ia3d (V2)-Pn3m (V2) transformation of the hydrated GMO. Poly(propylene glycol)s of different chain lengths show
rather different effects on both the phase behavior and the dimension of the structure. The Pn3m lattice parameter can be
reduced by more than 20% on the addition of PPG-Gly1000. In contrast, the addition of PPG-Gly 340 increases the
dimension by less than 5%. This result is explained by the hydrophobic chain splay. From polarizing optical microscopy, the
addition of PPG-Gly 1000 to GMO enhances the formation of H2 phase substantially. The appearance of the H2 phase lasts
for a temperature range of ca. 23℃.
Keywords: poly(propylene glycol); glyceryl monooleate; lyotropic liquid crystal; small angle X-ray scattering.
Introduction:
In this study, we exploit the possibility of preparing lyotropic liquid crystalline amphiphiles with reverse phases by employing
poly(propylene glycol) as hydrophobic chains. First of all, the methyl branching structure of PPG appears to be suitable for the
formation of reverse phases. Secondly, lyotropic liquid crystalline phases are affected by hydrophobic chain length. Longer
chain length may increase the bulkiness of hydrophobic region and thus favors the formation of reverse phases.
Poly(propylene glycol)s of different molecular weights are commercially available. By adjusting the molecular weight, one might
be able to manipulate the liquid crystalline phase behavior and the dimension of the structure. Thirdly, the formation of H 2
phase is restricted by the packing constraint in the hydrophobic region. The employment of flexible PPG with suitable chain
length may be able to effectively fill the energetically unfavorable space of reverse hexagonal structure with less energy cost
and facilitate the formation of H2 phase.
Results and discussion
Figure 3
Figure 1
Figure 4
85
90
130
85
80
HO
O
75
70
65
60
)
75
115
℃
Lattice parameter (A)
120
)
℃
O
80
125
Temperature (
GMO 17.5 Å
Temperature (
HO
Figure 5
110
105
100
55
O
90
O
HO
O
55
50
O
O
65
60
95
HO
70
O
n
85
1
2
3
4
PPG-Gly 1000 (mole %)
5
0
1
2
3
4
5
6
7
PPG-Gly (mole%)
20
30
40
50
60
Water (wt%)
PPG-Gly 18.7 Å
Figure 1. Molecular structure and dimensions of
Figure 2
GMO and PPG-Glys. PPGGly 340 was approximated by n = 4 and PPG-Gly 1000 by n = 15.
Figure 2.
SAXS profiles of the hydrated samples with various hydration
values measured at 23℃. (A) GMO (B) PPG-Gly340/GMO= 3/97 (mole/mole) (C)
PPG-Gly 1000/GMO = 3/97 ● indicate The Lα peaks. ▲ indicate the Ia3d peaks.
■ indicate the Pn3m peaks. At 20 wt% H2O, a single peak shows up for all of
the threecompositions. The diffraction peak wasdetermined to be (001) plane
from the Lα phases. New peaks appear for all samples at 30 wt% H2O.
Figure 3.
Lattice parameters of the Pn3m phases of the hydrated samples
with various compositions of PPG-Gly/GMO at 60 wt% H2O as determined by
SAXS. Symbols: PPG-Gly 340/GMO (■), PPG-Gly 1000/GMO (▲), and GMO (●).
Figure 4.
Temperature ranges of the H2 phases of the hydrated samples
with various compositions of PPG-Gly 1000/GMO at 50 wt% H2O as determined
by polarizing optical microscopy. The triangles indicate the starting temperatures
at which the H2 phases were observed. The squares indicate the
finaltemperature at which the H2 phases could be observed.
Figure 5.
Figure 6
Temperature ranges of the H2 phases of the hydrated samples
with the mole composition of PPG-Gly 1000/GMO = 3/97 at various weight
percentages of water as determined by polarizing optical microscopy. The
triangles indicate the starting temperatures at which the H2 phases were
observed. The squares indicate the final temperature at which the H2 phases
could be observed.
Figure 6.
Polarizing optical micrograph of PPG-Gly 1000/GMO = 3/97
(mole/mole) with 50 wt% H2O at 68℃
70