amphiphilic liquid-crystalline networks — phase behavior and alignment by mechanical fields

Mucromol. Rapid Commun. 16, 435-447 (1995) 435

Amphiphilic liquid-crystalline networks - phase behavior and alignment by mechanical fields

Peter Fischer, Claudia Schmidt, Heino Finkelmann *

Institut fur Makromolekulare Chemie der Universitat Freiburg, Sonnenstr. 5 , D-79104 Freiburg i. Brsg., Germany

(Received: December 21, 1994; revised manuscript of February 6, 1995)

SUMMARY A cross-linked polysiloxane carrying non-ionic amphiphilic side-groups attached with

their hydrophobic end to the polymer backbone is synthesized. The amphiphilic groups are selectively deuterated at the a-position of the hydrophobic alkyl chains. The phase behavior with water is studied by deuterium nuclear magnetic resonance spectroscopy. A lamellar phase (La) is observed on the low-water-concentration side of the liquid-crystalline regime. The domains of the L,-phase can be aligned macroscopically by uniaxial compression of the sample.

Introduction

Thermotropic liquid-crystalline elastomers (LCEs), composed of mesogens attached to a cross-linked polymer backbone via a flexible spacer, combine the properties of the elastomeric state (e. g., form-stability, elasticity) and of the anisotropic liquid- crystalline (LC) state (e. g., optical birefringence) ' -3). The application of mechanical stress to LCEs results in a uniform ordering of the LC domains which leads to macroscopically ordered anisotropic materials. This mechanical director reorientation is caused by the stress-induced anisotropic deformation of the polymer backbone, since polymer coil conformation and LC order are coupled4). With this very effective orientation mechanism not only nematic but even the more viscous smectic') or discotic columnar phases 6, can be oriented.

From these results on thermotropic LCEs the question arises whether the concept of mechanical alignment can be applied similarly to lyotropic LC systems built from cross- linked amphiphilic side-chain polymers swollen with water. In a previous paper, it was shown that such materials can exhibit lyotropic polymorphism7). Furthermore, it was demonstrated that the H , phase composed of hexagonally ordered rodlike micelles becomes macroscopically oriented by the anisotropic deformation of the elastomer due to uniaxially constrained swelling in a cylindrical tube7).

In the following, we will present another synthetic route to lyotropic LCEs which is analogous to that previously established for lyotropic side-chain polymers 8*9) and thermotropic LCEs ' -3, s, 6, l o ) . Polymeric networks with specifically deuterated side- chains are synthesized and the lyotropic phase behavior with water is investigated by deuterium nuclear magnetic resonance ('H NMR) spectroscopy. Finally, we will demonstrate that the director of the Iyotropic LCE in the lamellar phase (La) can be aligned by a mechanical field. The preferred director orientation under uniaxial compression is analyzed both by *H NMR and by X-ray measurements.

0 1995, Huthig & Wepf Verlag, Zug CCC 1022-1 336/95/$02.50

436 P. Fischer, C. Schmidt, H. Finkelmann

Synthesis and characterization

The system studied is a cross-linked polysiloxane with specifically deuterated amphiphilic side chains attached with their hydrophobic ends to the polymer backbone. The corresponding monomeric amphiphile hexaethylene glycol methyl dodecyl-a , a d , ether ' ' I '*) and the linear poly(amphiphi1e) 9, have been investigated in detail previously.

The synthesis of the deuterated amphiphilic side-chain hexaethylene glycol methyl w-undecenyl-a,a-d, ether (5)a) is outlined in Scheme I 'I.

Scheme 1: Synthetic route to the deuterated nonionic amphiphilic side-chain precursor 5

\L(CH,),COOH

LiAID, I \L(CH,),CD,OH 3 I NaH

CH,(OCH,CH,),OH

SOCI,/Et,N I I I 0

CH,(OCH,CH,),CI 1

Na (OCH,CH,),OH

CH,(OCH,CH,),OH 2

TosCI/Et,N

4 \~(CH,),CD,(OCH,CH,),OCH, 5

Starting from the chlorinated triethylene glycol monomethyl ether (1) b), the hydrophilic group hexaethylene glycol monomethyl ether (2)c) is obtained by etherification with triethylene glycol (3,6-dioxaoctane-l,8-diol). For the 2H NMR measurements it is necessary to synthesize a specifically deuterated derivative. For the sake of synthetic simplicity and to obtain a large, measurable bond order parameter 1 3 )

a) Systematic IUPAC nomenclature: a-(undecyl-10-ene-1 ,1 -d2)-w-methoxyhexakis(oxy-

b, Systematic IUPAC nomenclature: 2-chloroethyl 2-(2-methoxyethoxy)ethyl ether. c , Systematic IUPAC nomenclature: a-hydro-w-methoxyhexakis(oxyethylene).

ethylene).

Amphiphilic liquid-crystalline networks - phase behavior and alignment . . . 431

this deuteration is carried out at the hydrophobic/hydrophilic interface via o- undecylenyl alcohol-a,a,d, (3) synthesized from undecylenoic acid by reduction with LiAlD,. Etherification of 3 with the tosylated hydrophilic head group 4 yields 5. As illustrated in Scheme 2, cross-linked amphiphilic side-chain polymers are obtained by hydrosilylation of 5 with poly[oxy(methyl)silylene)] (number average degree of polymerization = 70) and bis-l,4-(o-~ndecylenoxy)benzene (6) as cross-linking agent.

Scheme 2: Synthesis of the amphiphilic side-chain elastomers El- E7

A series of elastomers with different amounts of cross-linking agent was synthesized. The effective cross-linking density of the amphiphilic networks was characterized by the sol fraction and the swelling behavior in toluene and H,O. The results are shown in Tab. 1. With increasing amount of cross-linking agent 6, both the sol fraction (ms,,,) and the degree of swelling (4) for both solvents decrease as expected. The sol fraction is large because of the short chains of the prepolymer. The poor swelling in H,O compared to toluene can be explained by the fact that H,O is compatible with the hydrophilic oxyethylene moieties only, whereas toluene is a good solvent for all constituents of the elastomer, namely siloxane backbone, hydrophilic and hydrophobic moieties.

Gelation could not be achieved with concentrations of 6 lower than 3 mol-To. To minimize effects from the non-amphiphilic cross-linking agent, the following investiga- tions of the phase behavior were made with elastomer E5 ( 5 mol-To of 6), which offers a good compromise between low cross-linking density and mechanical stability.

43 8 P. Fischer, C. Schmidt, H. Finkelmann

Tab. 1. Composition and swelling properties of the synthesized elastomers El - E7

d) q H , O

C) Elastomer X a) msol b, qtoluene ~~

El 0,20 0,16 2,7 i 0,3 1,s f 0,4 E2 0,14 0,18 4,O k 0,6 1,9 f 0,3 E3 0,08 0,23 6,3 k 0,4 2,7 f 0,5 E4 0,06 0,29 6,6 ? 1,0 3,2 f 0,4 E5 0,os 0,41 6,6 k 0,7 3,O f 0,6 E6 0,04 0,44 10 k 3 3,6 f I ,1 E7 0,03 0,43 14 f 3 4,6 f 1,2

a) Mole fraction (in relation to Si-H valences, see Scheme 2) x of the cross-linking agent 6. b, Relative loss of mass during sol extraction.

Volume swelling factor in toluene. d, Volume swelling factor in H,O.

*H NMR analysis of lyotropic phase behavior

The analysis of the phase behavior of LC networks by the standard method of polarizing microscopy can fail because of stress-induced birefringence effects and lack of characteristic textures.

For lyotropic systems, the changes of enthalpy at the phase transitions are normally too small to be registered by differential scanning calorimetry (DSC). X-Ray diffraction usually does not yield enough reflections for an unambiguous identification of the phase structure and is not very sensitive to biphasic regions 1 4 ) . Therefore, deuterium NMR spectroscopy was chosen as a useful tool for the characterization of the phase behavior 15). Additionally, the orientational distribution of the director can be determined by this method. However, ,H NMR spectra of mixtures of non-labeled polymeric amphiphiles with D,O as a probe suffer from broadened individual signals combined with narrow total linewidths; hence angular resolution is low. Therefore, to obtain better resolution, we recorded spectra of the specifically deuterated amphiphilic polymer network, which shows much larger total linewidths because of less efficient motional averaging of the quadrupole coupling.

The lineshape of 2H NMR spectra is dominated by the quadrupolar interaction of the deuteron (spin Z = 1) with the electric field gradient at the site of the nucleus 1 6 ) .

In lyotropic liquid-crystalline phases the quadrupolar coupling and thus the measured splittings Av of the two ,H transition frequencies are drastically reduced by motional averaging resulting from conformational exchange, orientational fluctuations and molecular diffusion along the curved micellar interface. In uniaxial phases the observed residual splitting is A v = S,, . 6(3cos2 ,8 - I ) , where ,8 is the angle between the director and the static magnetic field, 6 is 3/4 times the quadrupolar coupling constant e 2 . q Q / h (ca. 167 kHz for aliphatic C-D-bonds) and S,, is the order parameter of the C-D bond, which is a measure of the average bond orientation with respect to the director. For isotropic phases S,, vanishes and a singlet is observed. This holds also for cubic lyotropic phases, provided diffusion along the curved interface is not


restricted and fast enough compared to the time scale of the NMR experiment, like for monomeric amphiphiles.

Since Av depends on the director orientation, polydomain samples with an isotropic distribution of p give rise to characteristic powder (Pake) spectra”), whereas the spectrum of a macroscopically aligned sample is a simple doublet of peaks with a splitting given by the fixed value of p.

Fig. 1 shows series of ’H NMR spectra for samples of elastomer E5, swollen with water to 40, 60 and 80 wt.-To elastomer, respectively. The 80 wt.-To sample (see Fig. 1) shows the characteristic line shapes (Pake spectra) of an isotropically disordered LC phase over the whole temperature range up to the transition to the isotropic phase, which starts a t about 328 K. These Pake spectra are attributed to an La phase in analogy to the corresponding non-cross-linked polymer 9). Pake spectra also occur for the 40 and 60 wt.-To samples, but only above ca. 290- 300 K. At lower temperatures, the Pake spectra gradually develop into bell-like lineshapes, indicating a transformation to another phase with a different structure. In the phase diagram depicted in Fig. 2 this

Fig. 1. ’H NMR spectra of water- swollen amphiphilic polymer network E5 at different temperatures. The concentrations are given in wt.-Vo polymer

40 wt.-YO

- -2okHz 2okHz

60 wt.-% 80 wt.-%

274 K

280 K

286 K

292 K

298 K

304 K

310 K

316 K

322 K

328 K

334 K

340 K -- -2okHz 2okHz -2okHz 2okHz


I I I I I I I I I 1 1 ,

- -

L -

360 e

5 350

340

.- * E

E 330

320

310

300

290

280

270

260

250

- - -

Fig. 2. Phase diagram of the aqueous sample E5. (L, = lamellar phase, L = isotropic phase, X =

unidentified LC phase, A, = solid amphiphile, H 2 0 , = solid H,O)

phase is labeled X and the temperature range in which the gradual transition from Pake- like to bell-shaped spectra occurs is marked by vertical bars.

To understand the different line shapes which are observed in the LC region, a comparison of the lyotropic phase behavior of cross-linked elastomer, linear polymer '), and the corresponding monomer is helpful. The monomeric amphiphile hexaethylene glycol methyl dodecyl-a,a-d, ether shows a polymorphism ranging from a hexagonal phase (HI) at low temperatures and concentrations over a bicontinuous viscous cubic phase (V, ) to a lamellar phase (L,) in the high temperature and concentration regime ' I ) 1 2 ) . The three different LC phases can be clearly discerned by polarizing microscopy, for example, when using the water penetration technique. This polymorphism can be explained by the change of the curvature of the hydrophobic- hydrophilic interface as a function of temperature and concentration '*). Lamellar phases occur for flat micelles the interface curvature of which is about zero, whereas the formation of the complex bicontinuous structures of the V , phase and the rod-like micelles of the H I phase, occurring with decreasing temperature and concentration, goes along with an increasing interface curvature. A similar trend in the curvature of the micellar interface is also expected for the linear and the cross-linked polymer.

However, neither the linear polymer nor the network shows a clear distinction of different mesophases when observed in the microscope; the water penetration

Amphiphilic liquid-crystalline networks - phase behavior and alignment . . . 44 1

technique shows only one broad LC region without phase boundaries. 'H NMR spectra, on the other hand, reveal regions of different line shapes for both polymer systems. For the linear polymer, three types of spectra are observed9): wide Pake spectra a t high concentrations, bell-shaped spectra at intermediate concentrations, and narrow Pake spectra a t low concentrations, with gradual changes of the line shapes from one region to the next. These different line shapes indicate that the expected change of interface curvature and thus a reorganization of the phase structure indeed takes place. The network shows a behavior similar to that of the linear polymer, but without the change from bell-shaped lines to narrow Pake spectra. The latter probably are not observed because the capacity of the elastomer to absorb water is limited by the swelling equilibrium; hence a more dilute concentration regime is not accessible.

Overall, we can conclude that the phase behavior of the polymer systems is similar to the polymorphism of the corresponding monomer. The wide bell-shaped spectra and the short quadrupole echo relaxation times observed for the polymers a t intermediate concentrations, where the corresponding monomer shows the narrow quasi-isotropic peaks of the V , phase, can be explained by the restricted mobility of the amphiphilic side-chains. The lateral diffusion of the amphiphilic moieties is strongly hindered because they are covalently bonded to the polymer backbone. The short relaxation times in the X phase proving particular slow motions may indicate high viscosity similar to a V , phase. Based on numerical simulations, we found that bell-shaped spectra like those observed may arise from the superposition of Pake spectra with different residual quadrupole splittings. Such a size distribution of motionally averaged quadrupole splittings could arise from spatially restricted lateral diffusion of the side- chains along the interface. Side-chains in sites with different mobility or with different interface curvature, caused by defects or finite aggregate size, give rise to differently averaged splittings and the superposition of these different contributions to the signal could result in the observed bell-shaped spectra.

Mechanical alignment of LC domains

To investigate the effect of mechanical stress on the director orientation, a sample of E5 in the L, phase (70 wt.-To polymer network) was compressed between two teflon@ half-cylinders to about half of its original thickness. 2H NMR spectra of the compressed sample were recorded for different orientations of the compression axis with respect to the magnetic field (go). In Fig. 3, the lineshapes of the compressed sample are compared with the typical Pake spectrum of the non-compressed sample (Fig. 3a). The fact that the quadrupole splitting after compression remains about the same proves that no significant change of phase structure or concentration occurs. Fig. 3b shows the spectrum for the orientation of the compression axis perpendicular to the magnetic field. Two rather narrow peaks at frequencies corresponding to p = 90" show that most of the directors are oriented perpendicularly to go. When the axis of compression is parallel to Bo (Fig. 3c) the highest peaks of the spectrum (outer doublet) correspond to /? = 0". The smaller inner doublet stems from a superimposed Pake spectrum of non-aligned domains.


Fig. 3. Effect of uniaxial compression of a disordered elastomer in the lamellar phase on the deuterium NMR line shape: (a) disordered sample before compression, (b), (c) compressed sample with the axis of compression (force F ) at different angles 0 to the external magnetic field Bo: (b) 0 = 90°, (c) B = 0"

The NMR experiments thus prove that under uniaxial compression of the La phase, the preferred orientation of the director, and hence of the amphiphilic side-chains, is parallel to the axis of compression. The amphiphilic bilayers become aligned perpendicularly to the axis of compression. This result is confirmed by the small-angle X-ray diagram of a compressed swollen sample, shown in Fig. 4. The reflections of the layers appear along the vertical axis (0 and 180°), which corresponds to the direction of the compression. In contrast, non-compressed samples show a concentric intensity distribution, indicating an isotropic director orientation.

The observed reorientation under compression can be explained by the interaction between polymer backbone and micelle-forming side-chains in analogy to the model suggested for thermotropic elastomers 19). In this model, the shape of a polymer coil in an LC matrix is not spherical, but either prolate or oblate because of the coupling between the mesogens and the polymer backbone. In the case of the lyotropic La phase, where the hydrophobic polymer backbone is attached to the hydrophobic ends of the amphiphilic side-chains, the backbone is located most likely in the hydrophobic layer and must adopt an oblate coil conformation like in thermotropic smectic systems. Macroscopic compression of the sample with an initially isotropic distribution of domains and hence of the principal axes of polymer coils, flattens the polymer coils

Amphiphilic liquid-crystalline networks - phase behavior and alignment 443

Fig. 4. Azimuthal intensity distribution of the 001 reflection (lamellar period of (5,15 f 0,09) nm) for a compressed ( 0 ) and a non-compressed sample ( o ) of ca. 70 wt.-Yo E5. For the X-ray pattern of the compressed sample the axis of compression is along the vertical direction (O', 180")

. IP . 0 .

:

8 . i i

0" 60" 120" 180" 240' 300" 360" Scan angle

along the axis of compression in an affine fashion. Two alignment mechanisms are conceivable: (i) Oblate coils not yet aligned with their principal axis parallel to the axis of compression are either deformed so much, that their shortest principal axis of the coil finally is approximately along the axis of compression or, (ii), the coils suffer a torque which tends to align them by simple rotation. In both cases, the coupling between polymer backbone and amphiphilic side-chains ensures that the final orientation of the bilayers is perpendicular to the axis of compression.

The behavior of the H , phase can be explained in an analogous fashion. The rod- shaped micelles of this phase are compatible with a prolate polymer coil conformation. Therefore the micelles can be oriented by affine uniaxial elongation of the backbone as has been shown before').

Conclusions

The results above show that lamellar mesophases of cross-linked amphiphilic side- chain polymers can be macroscopically aligned by mechanical compression of the swollen network. For a deeper insight into the interaction of elastic forces and molecular self-assembly, quantitative studies of the relationship between stresshtrain and orientation are presently under way.

It should be possible to synthesize irreversibly aligned lamellar networks by fixing the ordered structure by a second cross-linking process under mechanical stress lo).

Such permanently macroscopically ordered lyotropic elastomers are interesting


because of their structural similarity to natural tissue like skin, which is built of ordered layers of biomembranes. Comparable diffusion properties and permeation selectivities are expected.

Experimental part

Synthesis of the amphiphilic side-chain

As outlined in Scheme 1 the amphiphilic side-chain precursor 5 was synthesized according to standard procedures. Compounds 1, 2 and 3 were prepared as described in ref. 4 and 5 were purified by column chromatography using silicagel and mixtures of ethyl acetate/ petroleum etherhethan01 (volume ratio 5/2/0,4 and 5/2/0,2) as eluant.

Diethylene glycol methyl 2-chloroethyl ether (2-chloroethyl2-(2-methoxyethoxy)ethyl ether, 1)

b.p. (80 Pa): 86°C. Yield: 66%. IR (film): 2930 cm- ' (vs; -CH,-, -CH,), 1440 (s; -CH,-, -CH,), 1340 (s;

' H NMR (CDCI,): 6 = 3,35 (s; 3H, -OCH,), 3,50-3,75 (m; 12H, -OCH,CH,-). -CH,), 1285, 1240, 1 180, 1 100 (br, vs; C-0-C), 840, 735, 655.

I3C NMR (CDCI,): 6 = 42,77 (-cH2C1), 59,08 (--CH3), 70,6-70,8 (-OcH2CH2-), 71,43 (-CH,CH,CI), 72,Ol (-CH,OCH,).

C,H,,O,CI (182,7) Calc. C 46,03 H 8,28 C1 19,41 Found C 45,47 H 8,20 C1 19,60

Hexaethylene glycol monomethyl ether (a-hydro-w-methoxyhexakis(oxyethylene), 2) b.p. (50 Pa): 193-195°C. Yield: 60%. IR (film): 3 500 cm - ' (vs, br; -OH), 2 850 (br, vs; -CH2-, -CH,), 1 440 (s; -CH,-,

'H NMR (CDCI,): 6 = 3,05 (t, 1 H; -OH), 3,40 (s, 3H; -OCH,), 335 (m, 2H;

13C NMR (CDCl,): 6 = 58,99 (-CH,), 61,64 (-CH,OH), 70,31, 703.5

-CH,), 1340 (s; -CH,), 1285, 1240, 1 180, 1 100 (br, vs; C-0-C), 940, 840.

CH,OCH,-), 3,60-3,75 (m, 22 H; -OC€3,CH20-).

(-OCH&H,-), 71,92 (CH,OCH,-), 72,61 (-cH,CH,OH).

c ,,H,,O, (29694) Calc. C 52,69 H 9,52 Found C 51,86 H 937

10-Undecylenyl alcohol-a,a-d, (3) b. p. (20 Pa): 73°C. Yield: 76%. IR (film): 3500 cm- ' (vs, br; -OH), 3050 (m; =C-H), 2980-2800 (vs; -CH2-,

-CH,), 2 140, 2040 (w; -CD,-), 1625 (m; C=C), 1 440 (m; -CH,--, -CH,), 1 340 (s;

' H NMR (CDCI,): 6 = 1,20- 1,45 (m, 12H; -CH,-), 1,55 (m, 2H; -OCD,CH,-), 2,05 (m, 2H; =CHCH,-), 2,40 (br, s, 1 H; -OH), 4,95 (d, J = 10 Hz; 1 H, E-HC=), 5,05 (d, J = 18 Hz, 1 H; Z-HC=), 5,90 (m, 1 H; =CH(-CH,CH,-)).

-CH,), 1285.

I3C NMR (CDCI,): 6 = 25,81 (-~H,CH,CD,O-), 29,O-29,6 (-CH,-), 32,76 (-CH,CD,), 33,84 (=CHCH,), 58,96 (-CH,), 114,14 (H&=), 139,20 (H,C=C-).

C1 lH@,O (17293) Calc. C 76,74 H 12,79 Found C 76,65 H 12,76


Hexaethylene glycol monomethyl ether tosylate (a-methyl-(o-4-methyltoluenesulfonato)- hexakis(oxyethylene), 4)

Yield: 68%. IR (film): 3050 crn-' (vw; -ArH), 2915 (vs; -CH,), 2850 (br, vs; -CH,-, -CH,),

1 580 (m; aryl C=C), 1480 (vw; aryl C=C), 1450 (m; -CH,-), 1 350 (vs; -SO,O-), 1 170 (vs; -SO,O-), 820 (aryl -CH).

' H NMR (CDCl,): 6 = 2,45 (s, 3H; Ar-CH,), 3,40 (s; 3H, -OCH,), 3,50-3,70 (m; 22H, -OCH,CH,), 4,15 (t, J = 5 Hz, 2H; -SO,OCH,-), 7,35 (d, J = 8 Hz; 2H, aryl (--CH),CCH,)), 7,80 (d, J = 8 Hz; 2H, aryl -SO,C(CH),-).

I3C NMR (CDCI,): 6 = 21,62 (Ar-CH,), 59,Ol (-CH3), 68,69, 69,25

(aryl -0SO2C(CH),), 12933 (aryl (-CH),CCH,), 133,09 (aryl --SO,C--), 144,78 (aryl -CCH,).

(-SO,OCH&H2-), 70,53, 70,58, 70,76 (-OCH&H,-), 7 1,95 (-CH,OCH,), 127,98

Hexaethylene glycol methyl o-undecenyl-a,a-d, ether (5 ) Yield: 76%. IR (film): 3050 cm-' (m; =C-H), 2980-2800 (vs; -CH,--, -CH,), 2 140,2040 (w;

-CD,-), 1625 (m; C=C), 1440 (m; -CH,-, -CH,), 1 340 (s; -CH3), 1285, 1240, 1 180, 1 100 (br, vs; C-0-C), 900, 850.

' H NMR (CDCI,): 6 = 1,20-1,45 (m, 12H; -CH,), 1,55 (m, 2H; -OCD,CH,-), 2,05 (m, 2H; =CHCH,), 3,40 (s, 3H; -OCH,), 3,50-3,70 (m, 24H; -OCH,CH,-), 4,95 (d, J = 10 Hz, 1 H; E-HC=), 5,05 (d, J = 18 Hz, 1 H; Z-EC=), 5,80 (m, I H; =C€I(CH,-)).

I3C NMR (CDCI,): 6 = 26,03 (-~H,CH,CH,O--), 28,89-29,49 (-CH2-), 33,77 (=CH(-CH,)), 58,96 (-CH3), 69,98 (-(CH,),OCH,), 70,57 (-OCH,CH,-), 71,95 (-CH,OCH,), 1 14,08 (H&=), 139,ll (H,C=CJ.

Mass spectrometry (Chemical ionization, isobutane, 170 eVa)): 451 ((M + H ) + , too), 452 ((M + 1 + H ) + , 26,4, Calc.: 28).

C,&@,O, (45097) Calc. C 63,95 H 10,66 Found C 63,51 H 10,65

Synthesis of polymer networks

1,02 ( 1 - 2x) mmol5, 1,OO mmol (as equivalent amounts of Si-H) poly[oxy(rnethyl)silylene)] and 1,02x rnmol6 were degassed and dried at 20"C/80 Pa for about 2 h. Under N,, to avoid side reactions with H,O, 1,2 mL benzene and 6 bL Pt-catalyst SLM86005 (Wacker Chernie, Burghausen) were added and the mixture was filled into a 1,5 cm x 8 cm rectangular teflono mold placed inside a self-constructed glass vessel. The mixture was kept at 60 "C for 30 h. By slow evaporation of benzene the mixture got more and more concentrated, thus favoring intermolecular reaction of the cross-linker molecules as the tendency for phase separation decreased because of covalent bonding of the incompatible components. The networks were freed from the sol fraction by swelling 3 times with new

a) In SI units: l e v z. 1,60218. J.


toluene and subsequent deswelling with toluene/methanol (volume ratios 3 : 1, 1 : 1, 1 : 3), replacing each solvent after one day. Thin layer chromatography of the solvent used last proved the absence of soluble components. After deswelling for 3 days the elastomer films were dried in vacuum (80 Pa) for about 100 h.

Sample preparations

For the 'H N M R measurements, sample pieces of ca 20 mg and an appropriate amount of bidistilled water were weighed into the NMR tube (5 mm outer diameter), which was closed with a tightly fitting silicon cap and additionally sealed with epoxy glue. For the compression experiment, a preswollen elastomer piece was placed between two half- cylindrical teflon@ pieces, compressed and put into the sample tube together with a rubber strip to keep up compression during the N M R experiment. For the X-ray measurement, appropriate amounts of the components were weighed into a capillary (2 mm outer diameter, 0,Ol mm wall thickness). After 1 h, the sample was carefully compressed with an iron stick and sealed. All samples were equilibrated during several temperature cycles 20"C-60"C-20"C and kept at room temperature for more than one week before the measurements.

Swelling measurements

Films with an area of about 2 mm x 2 mm were observed in a microscope (Leitz Ortholux Pol I1 BK) with a magnification of 50 before and after swelling in H,O or toluene. From the average elongation of the edges the volume swelling factor q was determined. The swelling equilibrium was reached within 1 d.

2H NMR measurements

Spectra were recorded at a resonance frequency of 46,07 MHz with a Bruker CXP 300-spectrometer equipped with a Bruker BVT 100-thermostatic unit. A home-built gonio- meter probe head allowed rotation of the sample tube around its axis which is perpendicular to the external magnetic field.

The quadrupole echo pulse sequence go", - T - go", with full phase cycling was applied. Usually 2048 scans were accumulated. Spectra were recorded in 2 K steps after the sample had been kept for 15 min at any given temperature. Lineshapes and splittings from heating runs into the isotropic regime and cooling runs into the mesophase were identical for corresponding temperatures.

X-ray measurements

X-ray diffractograms were obtained with a Kiessig camera using CuK, radiation and a nickel monochromator. An image plate system (512 x 5 12 pixels) was used as a detector. The distance between sample and detector was 160 mm; the total exposure time was 4000 s.

Helpful discussions with W Schnepp are gratefully appreciated. We thank A. Hasenhindl for technical assistance with the N M R measurements. Financial support by Deutsche Forschungsgemeinschaft (SFB 60) and Fonds der Chemischen Industrie is acknowledged.


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amphiphilic liquid-crystalline networks — phase behavior and alignment by mechanical fields

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