observation of a nematic phase displayed by a polysiloxane with trinitrofluorenones as side groups

Makrornol. Chem. 193,2659-2668 (1992) 2659

Observation of a nematic phase displayed by a polysiloxane with trinitrofluorenones as side groups

Manfred Moller, Vladimir I.: Tsukruk"), Jiirgen Wendling, Joachim H. Wendorff *

Deutsches Kunststoff-Institut, SchloBgartenstr. 6, 6100 Darmstadt, Germany

Holger Bengs, Helmut Ringsdorf *

Universitat Mainz, Institut fur Organische Chemie, J. J. Becher-Weg 18-20, 6500 Mainz, Germany

(Date of receipt: March 18, 1992)

SUMMARY The synthesis and the results of the structural study of two copolysiloxanes with laterally fixed

trinitrofluorenone (TNF) units is reported. The two copolysiloxanes having 2,4 (1 a) and 5,3 (1 b) dimethylsiloxane comonomer units per TNF side group differ significantly in their phase behaviour as evident from optical microscopy, differential scanning calorimetry and X-ray scattering: 1 b shows a nematic mesophase whereas l a is an amorphous material. The different phase behaviour is discussed in terms of microphase separation between the siloxane backbone and TNF side groups.

Introduction

Amorphous acceptor polymers with nitrofluorenone derivatives (e. g. 2,4,7-trinitro- 9-fluorenone, TNF) as electron-deficient units are known since more than two decades'-Q. They are of interest since they are able to form charge transfer (CT) complexes with donor compounds and therefore induce miscibility even in polymer blends composed of acceptor and donor polymers 5 - 8 ) . Polymer-fixed CT complexes are important from a practical point because of their particular mechanical and processing properties.

Recently it has been reported that mixing of amorphous acceptor main-chain polymers with discotic low-molar-mass liquid crystals (LCs) leads to the induction of liquid crystallinity in the blended compounds which opens novel routes towards a wide spectrum to polymeric discotic LCs '). However, a mesophase formation in pure TNF- acceptor polymers has not been observed so far.

This contribution presents the results of investigations of two copolysiloxanes with TNF acceptor moieties as side groups. Depending on the number of dimethylsiloxane comonomer units per acceptor in the polymer backbone the phase behaviour of both copolysiloxanes differs significantly: the acceptor copolymer 1 b (Scheme I ) with the higher amount of dimethylsiloxane counits displays a thermotropic mesophase while the second one (1 a) does not. The different phase behaviour of the two copolymers will be discussed on the basis of microphase separation.

a) Permanent address: Institute of Bioorganic Chemistry, Academy of Science of Ukraine, 252094, Kiev, Ukraine. Present address: Institute of Polymer Science, Akron University, Akron, Ohio, 44325, USA.

0 1992, Huthig & Wepf Verlag, Basel CCC 0025-1 16X/92/$05.00

2660 M. Moller, V. V. Tsukruk, J. Wendling, J. H. Wendorff, H. Bengs, H. Ringsdorf

t 0 -0 al r /

.d - - w

m L al I

0 '

Results and discussion

lb

Fig. 1. DSC curves of the copolysiloxanes 1 a and 1 b (second heating runs with a rate of 20 K/min)

l a

Synthesis of the polymers

Both acceptor copolysiloxanes 1 a and 1 b were synthesized by means of a polymer- analogous reaction. The esterification reaction between a polymeric polyalcohol 2a and 2 b with acceptor acid 3 was carried out under the conditions of the DCC (dicyclo- hexylcarbodiimide) method (Scheme I ). The number-average molar masses of 1 a and 1 b are in the range of 3 500 g/mol.

Scheme I:

OH

Characterization of the phase behaviour

The copolymers were investigated by means of differential scanning calorimetry (DSC), X-ray investigations and polarizing microscopy. All samples studied were melted and subsequently annealed for 1 - 5 d prior to subjecting them to the analysis.

At first, polysiloxane l a which is characterized by a short length of the dimethylsiloxane units between TNF-containing side chains (ratio 2,4 : 1) will be considered. Thermal measurements show that this polymer possesses an amorphous state. The DSC curve indicates a glass transition around room temperature (Fig. 1, Tab. 1). The absence of birefringence as observed by polarization microscopy and the occurrence

Observation of a nematic phase displayed by a polysiloxane with . . . 2661

Tab. 1. Thermal dataa) for the polymers 1 a and 1 b

Polymer T,,/"C T,,/"C Ti,/"C AH/(kJ emol-')

- - 21 l a l b - 94 8 116 8 2

a) T~ = glass transition temperature; T ~ , = isotropization temperature.

-

of just diffuse halos in X-ray scattering (see Fig. 2) point out the amorphous nature of 1 a. The diffuse halos around 23" in the WAXS curve correspond to a distance in the range of 0,36-0,43 nm resulting from a short-range ordering of side-groups. The diffuse halo near 12", on the other hand, reflects the short-range packing of the siloxane backbone units with an average intermolecular distance of 0,72 nm as observed in siloxane side-group LC polymers10-'2). An interesting feature is that the SAXS curve displays a weak shoulder corresponding to a repeat distance of d = 5,3 nm (Fig. 3a) which will be reconsidered further below.

In contrast to 1 a polysiloxane 1 b with a higher content of dimethylsiloxane units per TNF acceptor (ratio is 5,3 : 1) exhibits a mesomorphic behaviour. The increase of the length of the siloxane backbone between the TNF-containing side chains in 1 b (Scheme I ) gives rise to a rather complex self-organization process. The induction of order is

ln 7o \ m C 3

c

60

2; c ._

50 e c -

LO

30

20

Fig. 2. WAXS curves for polymers 1 a, 1 b and mixture 1 b/4 (1 : 1) at different temperatures

' 0

- l a 25OC w lb 25OC M lb 85OC - lb/4 11: 1) 25OC

2" 6 O 10" 1L" lao 22" 26O 30°

Scattering angle 28


25000 Lo

C 0

- V

>; 21000 - .- UI C a, C c

- 17000

13000

9000

5000

1000

2 12000 C 3 0 u

5 10000 Lo C W

C c -

8000

6000

LOO0

2000

0

(a)

w lb 25OC w lb 85OC A-+ lb 15OOC - l a 2 5 0 c

I ' " " " ' ' I ~ I , . '

10 20 30 10 50

Scattering angle 2 8

Fig. 3. SA ;S curves for l a an 1 b at different temperatures (a) and for polymer 1 b and mixtures 1 b/4 (1 : 1) and 1 b/4 (1 : 4) at room temperature (b) , . . I I I

Scattering angle 28

Observation of a nematic phase displayed by a polysiloxane with . . . . 2663

apparent first of all from the appearance of an endothermic peak at a temperature of about 116°C in the corresponding DSC curve (Fig. 1 , Tab. 1) as well as from the observation of two glass transitions located at about -94°C and at 8 "C, respectively. Note that the higher temperature glass transition is followed by an exotherm related to structure formation.

The presence of two glass transition temperatures indicates that a phase separation has taken place, evidently between backbone and side-chain units. The low-temperature glass transition corresponds to an onset of the segmental mobility of siloxane backbone units and the second one reflects the onset of mobility of the TNF-containing side chains as was shown for LC polymers with siloxane backbones ''9 1 3 ) .

A bright optical texture of unspecific type with disclination lines is observed for 1 b in the temperature range from room temperature to 116°C (Fig. 4).

Fig. 4. Optical texture of copolysiloxane 1 b in the liquid-crystalline state

Thus, an anisotropic, i.e. LC state seems to exist in the polymer unless the birefringence results from traces of TNF side groups able to crystallize (see below). The phase transformation at 116°C is connected in any case with the transition to an isotropic state (Tis) as judged from optical microscopy.

The wide-angle X-ray diagrams display just diffuse halos as expected for phases possessing a short-range ordering. The reflections are located at scattering angles of about 12 and 23 ". The corresponding d spacings deduced from the fits of the scattering maxima by Gaussians (R = 3 - 5%) are summarized in Tab. 2. Peaks characteristic for crystalline TNF 14) are absent so that in any case the degree of crystallinity has to be below 0,5%. This small concentration is not able to account for the observed texture under the polarizing microscope. In addition, no indication of a smectic-like layer ordering is present.

The conclusion based on the DSC and microscopic investigations as well as on the structural analysis thus is that the side-chain polymer 1 b displays a nematic phase. The diffuse character of all scattering maxima observed for well annealed samples (at 105 "C for 5 d) is in agreement with this interpretation.


Tab. 2. angle X-ray scattering (WAXS) data

Polymer T/"C Dl/nma) D2/nm a) D3/nma) L,/nmb)

Structural parameters of polymers 1 a, 1 b and of mixture 1 b/4 (1 : 1) deduced from wide-

l a 25 0,72 0,430 0,356 l b 25 0,74 0,435 0,357 1,8 l b 85 0,76 0,446 0,358 292 lb/4(1:1) 25 0,82 0,421 0,356 1 3

a) D spacings are calculated from theafits of the maxima by Gaussians with R = 3-5%. b, L , is the expansion of correlations in the packing of the backbones calculated from the

widths of the first WAXS maxima.

Taking into account the known results for siloxane side-group LC polymers lo* 1 1 , j 5 )

and crystalline TNF14) all observed maxima can be interpreted in the terms of mdlecular packing. The two overlapping halos in the scattering range of 20-25" are connected with the side-by-side packing of the rigid cores of TNF units with a typical distance of 0,36 nm and the side chains with an average distance of 0,43 nm. The diffuse halo about 20" is typical for any polymers with liquid-like, short-range local packing of the fragments Is). The diffuse halo around 12" reflects as was mentioned above for 1 a the liquid-like packing of the siloxane units with a mean periodicity of 0,74 nm and an extension of the short-range correlations up to 1,8 nm (see Tab. 2).

One important clue to explain the LC behaviour for 1 b comes from the small-angle X-ray region where a broad halo is located at about 1,5" (d spacing: about 5,6 nm) (Fig. 3 a). This small-angle X-ray maximum has to be attributed to the occurrence of a phase separation between the dimethylsiloxane fragments and the TNF units, in agreement with the observation of the corresponding glass transitions. Heating of the sample above the clearing temperature ( Tis) leads to a complete disappearance of this halo (Fig. 3a).

The density distribution profile for 1 b was estimated from the correlation function G(x) (Fig. 5) as derived by Fourier-transformationi6) of the X-ray data using the approach described by StrobllS- l 7 , I 8 ) (Fig. 6) . The presence of strongly damped broad maxima on G(x) reflects the existence of short-range correlations in a local double- layer packing of side chains (see Fig. 6a) (correlation lenght: 15 nm).

The density distribution is evidently characterized by the presence of a broad region with a higher density (of about 1,6 nm), in good agreement with the presence of a local double-layer packing of the side-groups as proposed for LC polysiloxanes lo, l ' ) . The d spacing is controlled by the lengths of the TNF-containing side chains (about 2,O) and by the thickness of the siloxane-containing regions (of about 1,6 nm) (Fig. 6a, b).

In order to check the presence of such structural organization with separated packings of the backbones and side groups miscibility investigations of both samples with decamethyltetrasiloxane (4) representing the backbone units in the polysiloxanes 1 were carried out.

Observation of a nematic phase displayed by a polysiloxane with . . .

Ln

c c ._ = 1.2 r' 0

C ._

0.1

0

-0.1 Fig. 5. One-dimensional

correlation functions for the pure polymer 1 b and for the mixture 1 b/4 (1 : 4) -0.8

f

2665

H lb - lb/4

I " " " " ' I " " " " ' I " " " " ' I

0 5 10 15 20

Distance in nrn

Fig. 6. Proposed structural model based on the X-ray evidence: a local double-layered packing of the macromolecular units in the nematic state for polysiloxane 1 b (a) and corresponding density distribution profiles for the pure polymer 1 b (b) and for the mixture 1 b/4 (1 : 4) (c)

(b)

( c )


Tab. 3. Structural parameters of polymer 1 b and mixture 1 b/4 (1 : 4) deduced from small-angle X-ray scattering (SAXS) data

Polymer d,/nma) I/nmb) L2/nm

l b 1b/4(1:4)

a) d , is calculated from desmeared SAXS maxima using the Bragg's law. b, I is the width of the siloxane backbone packing region. ') L, is the expansion of the local double-layer packing estimated from correlation functions.

The addition of 4 to 1 b (1 b /4 mole ratio 1 : 1 and 1 : 4) leads to an increase of the d spacings, to the decrease of the maxima heights on correlation function and to a certain smearing of the density distribution in the range of packing of the backbones (Tab. 3, Fig. 3 b, 5,6c). It does not prevent, however, the formation of a homogeneous optical texture which disappears at a temperature of about 110 "C. This points out that 4 is inserted mainly into the backbone-containing regions.

Based on these results it seems that the weak small-angle X-ray shoulder observed for l a (Fig. 3a) is related to some weak tendency for phase separation occurring even in this system having fewer dimethylsiloxane units in the chain backbone. The addition of 4 to 1 a leads, however, to a macrophase separation of the components in the mixture as observed under the microscope. No traces of anisotropic regions are observable.

Conclusions

The side-group polysiloxane 1 b is the first example of a side-group acceptor polymer containing laterally fixed TNF derivative as a non-classical mesogen to display an LC phase, a nematic one as apparent from DSC, microscope and X-ray investigations. One possibility to be checked is that the optical anisotropy and the endothermic transition observed are related to a partial crystallization of the TNF side groups. The fact that the transition from the isotropic into the low-temperature ordered state can be supercooled significantly - which is unusual for a nematic transition - seems to strengthen this point of view. However, by recording the X-ray pattern with high accuracy we can state that the degree of crystallinity has to be below 0,5%. So crystallinity can be ruled out as a relevant factor.

As is known the TNF moieties fail to form mesophase by themselves. Thus, there is an additional driving mechanism for the formation of a nematic phase in sample 1 b: this obviously is the phase separation of backbone units and the side-chain units (the siloxane backbones themselves are probably able to show a mesomorphic behaviour 19920)). The observation that the transition into the isotropic state is connected with only minor changes in the WAXS curves while the SAXS halo disappears completely at temperatures higher than Ti, has to be taken as an indication that microphase separation and LC structure formation are coupled processes. The interpretation for such a coupled process is that the induction of an LC phase in the

Observation of a nematic phase displayed by a polysiloxane with . . . 2667

side groups promotes phase separation (which fits lattice calculations21s22)) while the spatial restriction of the side-chain units imposed by the phase separation enhances the formation of an LC state. This evidently constitutes a synergistic effect. This coupling also explains the large supercooling of the isotropic nematic transition: phase separation effects can be easily supercooled in polymer blends 23).

The fact that a decrease of the number of dimethylsiloxane units in the backbone between the acceptor side groups leads to the disappearence of the phase separation and the LC phase points again to such a coupling of effects. The tendency towards phase separation is reduced. It thus seems that the coupling of phase separation and induction of LC states constitutes a novel mode of structure formation control in LC systems.

Experimental part

Preparation of the samples

Copolysiloxanes 2 a and 2 b: These polymers were synthesized in the laboratories of RhBne-Poulenc, France. Acceptor copolysiloxanes 1 a and 1 b: As a typical procedure the preparation of acceptor polysiloxane 1 b is described. 0,5 g (1 mmol

of hydroxy groups) of copolymer 2 b, 1 g (2,5 mmol) of P-(2,4,7-trinitro-9-fluorenylideneamino- 0xy)propionic acid 324) and a trace of DMAP (4-dimethylaminopyridin) were dissolved in a mixture of 7 mL dioxane p. a. and 3 mL CH,CI, p. a. by heating. The reaction vessel was kept under nitrogen atmosphere, sealed with a septum and cooled to 0 "C. Hereafter, a solution of 0,51 g (2,5 mmol) DCC in 2 mL CH,Cl, was injected with a syringe during a period of 30 min. The reaction mixture was allowed to warm up over night and stirred at room temperature for additional 2 d. The suspension was poured into methanol and stirred several minutes. The polymer was isolated by means of a centrifuge and reprecipitated from CH,Cl, twice into methanol and twice into acetone.

Yield: 260 mg (25%) of a yellow-brownish product. Analytical data, e. g. 1 b: 'H NMR (200 MHz, CDCI,): 6 = 9,lO (m; arom. H), 8,80 (m; arom. H), 8,50- 8,lO (m; arom.

H), 4,95 (m; =N-0-CH,), 4,14 (m; CO,CH,), 3,56 (m; CH,-OH), 2,92 (m; CH2-C0,), 1,68 (m; Si-CH,--CH,), 0,50 (m; Si-CH,), 0,00 (m; Si-CH, incl. endgroups). Intensity ratio: 0,8:1,6:1,6:1,6:1,6:0,4:1,6:2:2:37.

The amount of non-converted hydroxy groups is approximately 20% for both acceptor copolysiloxanes 1 a and 1 b as estimated from NMR measurements (signal at 3,56 ppm).

( C 4 , * ~ H , , ~ 2 N ~ , ~ 0 2 , ~ S i ) ~ (142,67), Calc. C 41,08 H 539 N 6,28 Found C 40,32 H 5,82 N 6,76

GPC (CHCl,, polystyrene): an = 3 500 g/mol, M,,,/M,, = 1,3. The mixtures la/4 and 1 b/4 were prepared by combining two solutions of 1 and 4 (Lancaster,

98%) in dry tetrahydrofuran. The solvent was evaporated under mild temperatures and ambient pressure.

Characterization of the samples

The optical textures were characterized with a Leitz Orthoplan polarizing microscope. Calorimetric investigations were performed using a Perkin Elmer DSC-4. X-ray diagrams were recorded using a flat plate camera and a Siemens D-500 diffractometer (WAXS). Small-angle X- ray scattering (SAXS) curves were obtained employing a compact Kratky camera equipped with


a one-dimensional position sensitive detector. Ni-filtered CuK, radiation was used. For the processing of SAXS and WAXS data programs FFSAXS and FIT were To estimate interatomic distances computer models were constructed by means of the program INSIGHT (Biosym) on a Silicon Graphics work station26).

V. V. Tsukruk would like to thank the Alexander von Humboldt Foundation for the fellowship provided. The siloxanes 2 were provided by RhBne-Poulenc. The authors would like to thank Dr. S. Diele (Halle) and Prof. C. f! Lillya (Amherst) for helpful discussions.

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observation of a nematic phase displayed by a polysiloxane with trinitrofluorenones as side groups

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