Gelation Behavior of Polysiloxanes Having Low Molecular Weight Gelators as Gelation-Causing Segment

K. Hanabusa, K. Ando, M. Suzuki, M. Kimura, and H. Shirai Graduate School of Science and Technology, Shinshu University, Ueda, 386-8567, Japan

Tel: 0268-21-5487, Fax; 0268-21-5608, e-mail: hanaken@giptc.shinshu-u.ac.jp

The gelation of organic fluids by low molecular weight gelators1-4 is an attractive phenomenon from the standpoint of both academic interests and practical applications. Up to date, a considerable amount of gelators have been reported and it is known that the fascinating feature responsible for gelation relies on the formation of three-dimensional networks by macromolecule-like aggregates. Gels formed by low molecular weight gelators occasionally turn into crystals because of the metastable state arising from noncovalent bonds like hydrogen bonding between low molecular weight compounds. There has been rising interest in application of gelators,5-8 however, the crystallization meaning the collapse of gels is undesirable for the applications. On the other hand, polymers can rarely be separated from solution as crystals due to the molecular weight distribution and the entanglement of polymer chain. To our knowledge, there are very few reports on synthetic polymers possessing thermally reversible gelation behavior towards organic fluids, a considerable number of biopolymers forming hydrogel are well-known though.9

Our present question is if polymers covalently containing gelation-causing segments would act as thermally reversible gelators reflecting the original gelation-causing segments. It is an important subject to decide what are gelation-causing segments. It is thought that all low molecular weight gelators reported so far are potential candidates for gelation-causing segments.1-4 Now, we describe the successful preparation of polymer type of gelators based on the concept of gelation-causing segments.

We first prepared a compound (1) as gelation-causing segment and introduced it into methylhydrosiloxane-dimethylsiloxane copolymer (molecular weight average; 1200) by hydrosilylation reaction.

NH

HN

O

OC18H37

Si O

CH3

CH3

CH3

Si

CH3

CH3

O Si O Si

CH3

H

CH3

CH3

CH3

1

m n

Pt cat./THF P-1

Si O

CH3

CH3

CH3

Si

CH3

CH3

O Si O Si

CH3

CH3

CH3

CH3m n

NH

HN

C18H37

O

O Compounds 1, having terminal olefinic group, is an excellent gelator capable of gelling a wide

variety of organic fluids. Prepared polysiloxane (P-1) with gelation-causing segments also exhibited strong gelation abilities reflecting gelator 1. This indicates that the polysiloxane backbone rarely disturbs the molecular aggregation of the gelation-causing segments necessary for gelation. Minimum gel concentrations and aspect of gels formed by 1 and P-1 are summarized in Table 1.

The most striking characteristic of polysiloxanes with gelation-causing segments is the formation of stable gels. Namely, the formed gels are so stable that they do not turn into crystals. It

can be deduced that the stability of gels is attributed to the miscibility and mobility of polysiloxane backbone in fluids. It is known the bond angle (Si-O-Si) is meaningfully wider (~143º) and the bond length (Si-O) is longer (0.165 nm) than comparable C-C-C (109º, 0.140 nm). Therefore, the obstacle to rotation is very low and the Si-O bond can freely rotate.10 This is the reason why P-1, P-2, and P-3 can form the extremely stable gels.

Table 1 Minimum gel concentrations (g L-1) and aspect of gels by 1 and P-1 at 25ºC

Solvent mgc of 1 aspect of gel by 1 mgc of P-1 aspect of gel by P-1

Hexane 5 translucent 7 transparent EtOH 12 opaque 10 transparent Acetone 7 opaque 5 transparent Toluene 11 transparent 25 transparent DMA 13 opaque 7 translucent Kerosene 5 opaque 5 transparent Silicone oil 2 transparent 2 transparent

Another characteristic of polysiloxanes with gelation-causing segments is the formation of transparent gels (see Table 1). Despite the gelator 1 has a tendency to form opaque gels for polar solvents, P-1 can form transparent gels towards various organic fluids regardless of the polarity of solvents. Figure 1 shows the aspect of opaque EtOH gel by 1 (left) and transparent EtOH gel by P-1 (right).

We emphasize that the present concept of gelation-causing segments bound in polymers opens up new pathways to develop polymer type of gelators, which can form transparent and semipermanently-stable gels.

1) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133. 2) Abdallah, D. J.; Weiss, R. G. Adv. Mater. 2000, 12, 1233) Esch, J. H.; Feringa, B. L. Angew. Chem. Int. Ed. Engl.4) Gronwald, O.; Shinkai, S. Chem. Eur. J. 2001, 7, 4329.5) Hanabusa, K.; Hiratsuka, K.; Kimura, M.; Shirai, H. Ch6) Mizoshita, N.; Suzuki, Y.; Kishimoto, K.; Hanabusa,

2197. 7) Jung, J. H.; Ono, Y.; Sakurai, K.; Sano, M.; Shinkai, S.8) Kubo, W.; Murakoshi, K.; Kitamura, T.; Yoshida, S.

Wada, Y.; Yanagida, S. J. Phys. Chem. B 2001, 105, 129) Guenet, J-M. Thermoreversible Gelation of Polymers a10) Randal, M. H. Silicone Surfactants, Marcel Dekker, Ne

7. 2000, 39, 2263. em. Mater. 1999, 11, 649.

K.; Kato, T. J. Mater. Chem. 2002, 12,

J. Am. Chem. Soc. 2000, 122, 8648. ; Haruki, M.; Hanabusa, K.; Shirai, H.; 809. nd Biopolymers, Academic Press, 1992. w York, 1998.

Figure 1. Aspect of opaque EtOH gel (12 g L-1)by 1 (left) and transparent EtOH gel (10 g L-1) by P-1 (right).

Gelation Behavior of Polysiloxanes Having Low Molecular Weight Gelators

as Gelation-Causing Segment

Kenji Hanabusa*, Keiichi Ando, Masahiro Suzuki, Mutsumi Kimura, and Hirofusa Shirai

Department of Functional Polymer Science, Shinshu University,

Ueda 386-8567, Japan

Key words; Gelator, Physical gelation, Gelation-causing segment

Mixed and heated

Solution

Cooled to r. t.

Gel

Actual procedure for gelation

NH

NH

C11H23

C11H23

O

O

toluene

cooling

Crystallization

heating

cooling

＝Fibrous aggregate

Gelation

heating

Solution

Behavior of Low Molecular Weight Compounds

Crystalsolvent

Features of “gelators”

1. Good solubility upon heating and inducement of smooth gelation

2. Gelation at low concentration; less than 50 g L-1

3. Thermally reversible sol-to-gel phase transition

4. Cooperating noncovalent interactions as driving forces; hydrogen bonding, hydrophobic interaction, ππππ−−−−ππππ interaction, electrostatic interaction…….

Toluene gel prepared from 1.50 g of gelator and 100 mL of toluene

NH

NH

O

O

Fig. Typical gelators for practical use

Hardener for cooking oil, Cosmetics Cosmetics

Hardener for spilled oil, Cosmetics

For making grease

OHOH

OO

O

O

O

HOOH

NH

HN

O

HNO

O

HN

HN

HN

HN

OO

Crystallization(Rearrangement for crystal)

After standing

Gel

Fibrous aggregates

Defect of low molecular weight gelators

Separation of crystals, i.e., collapse of gels

The purpose of this work; “the development of gelators forming semi-permanent stable gels”

The factor which prevents the crystallization should be included in the molecular design of gelators.

Focus on oligomers and polymersOligomers and polymers never precipitate as crystals from the solutions.

“Gelation-causing segments” are connected to oligomers or polymers.

Flexible polymer to hinder crystallization

Gelation-causing segment

Gelation-causing segmentH-siloxane

NH

HN

O

O

NH

HN

O

O

NHHN

O

OCHC6H5H

H

O O

Hydrosilylation

H-siloxane Gelation-causing segment Siloxane-containing gelators

Si

CH3

CH3

H3C O Si

CH3

CH3

O Si

CH3

H

O Si

CH3

CH3

CH3

Si

CH3

CH3

H O Si

CH3

CH3

O Si

CH3

CH3

O Si

CH3

CH3

H

Si Si

CH3

CH3

O Si

CH3

CH3

H

CH3

CH3

H O

m n

m = ca.8, n = ca.8

n

n = ca.80

O Si

CH3

R"

HR' CH2 CH R O Si

CH3

R"

CH2CH2R'Pt cat.

R+

50-55% Methylhydrosiloxane-dimethylsiloxane copolymerMw ≒ 1200m≒8, n≒8

Si

CH3

CH3

CH3

O Si

CH3

CH3

O Si

CH3

H

O Si

CH3

CH3

CH3

m nH2PtCl6 6H20

/THF

NH

HN

O

O

Hydrosilylation reaction

Polymer segment to hinder crystallization

NH

HN

O

O

Si

CH3

CH3

CH3

O Si

CH3

CH3

O Si

CH3

O Si

CH3

CH3

CH3

m nSegment to cause gelation

NH

HN

R

O

O

HN

NH

R

O

O

NH

HN

R

O

O

HN

NH

R

O

O

P

P

P

P

Solvent A B C Methanol 6 8 P Ethanol 12 11 37 1-Propanol 19 18 36 Ethyl acetate 9 9 15 2-Butanone 7 10 18 Cyclohexanone 9 13 soln. Acetone 7 7 25 1,4-Dioxane 7 11 P Nitrobenzene 9 13 14 Toluene 11 16 6 DMF 13 17 soln. DMSO 7 16 soln. Chloroform soln. soln. soln. Kerosene 5 7 6 Light oil 3 7 3 Silicone oil 2 8 3 Octamethylcyclotetrasiloxane g-like g-like 6 Decamethylcyclopentasiloxane g-like g-like 5

Table. Gelation test and minimum gel concentration at 25℃℃℃℃

soln. = solution, g-like = gel like, P = precipitation, Values mean minimum gel concentrations ( g / l )

HN

ONH

O

CH2C6H5

H

OO

NH

HN

O

O

NH

HN

O

O

H

A

B

C

Gelation-causing segments

Table. Gelation test and minimum gel concentration at 25℃℃℃℃

A

1

Solvent A 1Decane 10 36Methanol 6 8Ethanol 12 101-Propanol 19 13Ethyl acetate 9 52-Butanone 7 8Cyclohexanone 9 10THF soln. soln.1,4-Dioxane 7 11Benzene 15 31Nitrobenzene 9 5Toluene 11 25DMF 13 9DMSO 7 7Chloroform soln. soln.CCl4 35* 30*Kerosene 5 5Light oil 3 12Silicone oil 2 2

soln. = solution, ＊＊＊＊= gelation test was carried out at 20℃℃℃℃Values mean minimum gel concentrations ( g / l )

NH

HN

O

O

NH

HN

O

OC18H37

Si

CH3

CH3

CH3

O Si

CH3

CH3

O Si

CH3

O Si

CH3

CH3

CH3

m n

Question; whether the polymers covalently containing gelation-causing segments would act as gelators?

Table. Gelation test and minimum gel concentration at 25℃℃℃℃

C

3

Solvent C 3Cyclohexane insol. insol.Methanol P insol.Ethanol 37 91-Propanol 36 8Ethyl acetate 24 insol.2-Butanone 15 10Cyclohexanone 18 soln.THF soln. 71,4-Dioxane 50 PBenzene 5 soln.Nitrobenzene 14 10Toluene 6 25DMF soln. 14DMSO soln. 8Chloroform soln. g-likeCCl4 6 insol.Kerosene 6 insol.Light oil 13 insol.Silicone oil 3 insol.

soln. = solution, g-like = gel like, insol. = almost insoluble, P = precipitationValues mean minimum gel concentrations ( g / l )

HN

ONH

O

CH2C6H5H

H

OO

Si

CH3

CH3

CH3

O Si

CH3

CH3

O Si

CH3

O Si

CH3

CH3

CH3

NH

OHN

O

C6H5CH2 H

H

O O

m n

Table. Gelation test and minimum gel concentration at 25℃℃℃℃

Solvent 2 5 8 Methanol 20 P 30 Ethanol 27 P 20 1-Propanol 45 soln. 20 Ethyl acetate 17 soln. 21 2-Butanone 15 soln. 13 Cyclohexanone 25 soln. 10 Acetone 10 P 17 1,4-Dioxane 18 soln. 13 Nitrobenzene 15 soln. 19 Toluene g-like soln. 11 DMF 9 soln. 21 DMSO 7 soln. 37 Chloroform soln. soln. soln. Kerosene 20 soln. 11 Light oil 12 soln. 11 Silicone oil 18 v.f. g-likeOctamethylcyclotetrasiloxane g-like 33 g-likeDecamethylcyclopentasiloxane g-like 36 g-like

soln. = solution, g-like = gel like, v.f. = viscous fluid, P = precipitationValues mean minimum gel concentrations ( g / l )

Si

CH3

H3C

CH3

O Si

CH3

CH3

O Si

CH3

O Si

CH3

CH3

CH3

m n

NH

HN

O

OC18H37

Si

CH3

CH3

O Si

CH3

CH3

O Si

CH3

CH3n

HN

NH O

OC18H37 (CH2)4

NH

HN

O

OC18H37(CH2)4

Si

CH3

CH3

O Si

CH3

CH3

O Si

CH3

CH3

HN

NH O

OC18H37 (CH2)4

NH

HN

O

OC18H37(CH2)4Si

CH3

CH3

O

m = ca.8, n = ca.82

n = ca.805

8

NH

HN

O

OC18H37

Si

CH3

CH3

CH3

O Si

CH3

CH3

O Si

CH3

O Si

CH3

CH3

CH3

m n

A 1

lightlight

Transparent gel of EtOH

Opaque gel　　　　of EtOH

NH

HN

O

OC18H37

Solvent A 1 Ethanol OP TP 1-Propanol TL TP 2-Propanol TL TP 1-Butanol TL TP Ethyl acetate OP TP 2-Butanone OP TP Acetone OP TP Cyclohexaneone TP TP 1,4-Dioxane TL TP DMA OP TP DMF OP TL DMSO TL TP Toluene TP TP Hexane TL TP Kerosene OP TP

TP = transparent gelTL = translucent gelOP = opaque gel

Table. Aspects of gels

A 1

Si

CH3

CH3

CH3

O Si

CH3

CH3

O Si

CH3

O Si

CH3

CH3

CH3

m nNH

OHN

O

C6H5CH2 H

H

O O

Fig. Aspects of gels formed by 3

1-PrOH EtOH

3

Stability of gelsStability of gels

HN

ONH

O

CH2C6H5H

H

OO

Fig. Aspects of gels formed by C

1-PrOH EtOH

C

24 hourlater

1-PrOH EtOH

one yearlater

1-PrOH EtOH

Sol-gel transition (65 ℃℃℃℃)

H-bondingν N-H (3289 cm-1)

Sol-gel transition (65 ℃℃℃℃)

H-bondingν C=O (1635 cm-1)

Fig. FT-IR spectra of decane gel (10 wt-%) by 1 at various temperatureTemperature range; 40 ℃℃℃℃ to 100 ℃℃℃℃

NH

HN

O

OC18H37

Si

CH3

CH3

CH3

O Si

CH3

CH3

O Si

CH3

O Si

CH3

CH3

CH3

m n

1

Molecular interaction on sol-gel phase transition

Β Β Β Β Β Β Β Β Β Β Β Β Β

0 10 20 30 40 50 60 70 80 90 100 110 120

Fig. DSC heating curve(a); Decane gel by 1 (10 wt-%)(b); neat 1

Endo

ther

mic

(a)

(b)

Temperature （℃）

0

0.2

0.4

0.6

0.8

1

40 50 60 70 80 90 100

Temperature（℃）

ν C=O1635 cm-1

ν N-H3289 cm-1

ν Si-C1261 cm-1

Gel-sol transition

Fig. Temperature dependence of peak intensity for decane sample of 1 (10wt-%)

62.3 ℃

91.0 ℃

Abs

orba

nce

FE-SEM image of DMSO gel formed by 1

FE-SEM image of DMSO gel formed by A

TEM image of DMSO gel formed by A

TEM image of DMSO gel formed by 1

Images of fibrous aggregateImages of fibrous aggregate

1.51.51.51.5 µµµµmmmm

1111 µµµµmmmm

500 500 500 500 nmnmnmnm

500 500 500 500 nmnmnmnm

1

NH

HN

O

O

A

NH

HN

O

OC18H37

Si

CH3

CH3

CH3

O Si

CH3

CH3

O Si

CH3

O Si

CH3

CH3

CH3

m n

Helical aggregates with the smallest width of 30 nm

Compounds capable of gelling cyclic Compounds capable of gelling cyclic siloxanessiloxanes

Polysiloxane spacer, which gives the miscibility toward D4 and D5

n = ca.804

n = ca.806

n = ca.805

Octamethylcyclotetrasiloxane (D4)

Decamethylcyclopentasiloxane (D5)

O

Si

O Si

O

Si

OSi

CH3

CH3

H3C

H3C

H3C

CH3

CH3

H3C

OSiSi

O

SiOSi

O

Si

O

CH3

CH3

H3CCH3

CH3

CH3

CH3H3C

H3C

H3C

Table. Minimum gel concentration (g L-1) for D4 and D5 at 25℃℃℃℃

4 5 6D4 23 33 43D5 23 36 33

Si

CH3

CH3

O Si

CH3

CH3

O Si

CH3

CH3n

HN

NH O

OC18H37 (CH2)4

NH

HN

O

OC18H37(CH2)4

Si

CH3

CH3

O Si

CH3

CH3

O Si

CH3

CH3n

HN

NH O

OC18H37 (CH2)4

NH

HN

O

OC18H37(CH2)4

NH

OHN

O

C6H5CH2 H

H

O OSi

CH3

CH3

O Si

CH3

CH3

O Si

CH3

CH3n

(CH2)11

HN

ONH

O

H2CC6H5H

H

OO(CH2)11

Cooling

Heating

Sol phaseSol phase

≡≡≡≡

Gelation-causing segment

Si

CH3

CH3

CH3

O Si

CH3

CH3

O Si

CH3

O Si

CH3

CH3

CH3

CH2CH2Rm n

GelationGelation process of polymers having process of polymers having gelationgelation--causing segmentcausing segment

Fibrous aggregate　　　　

Solvent molecule　　　　

Gel phaseGel phase

1) We proposed a new concept of gelation-causing segmentsfor development of polymer type of gelators.

2) Poly(siloxane)s having gelation-causing segments exhibit strong gelation ability, reflecting the gelation ability of the original gelation-causing segments.

3) Poly(siloxane)s having gelation-causing segments make transparent gels, which are so stable that they do not precipitate crystals. Even after one year, the gels are stable and keeping its transparency.

4) The stability of gels will be attributed to the miscibility and mobility of polysiloxane backbone in fluids.

ConclusionConclusion