Makromol. Chetii., Rapid Cotnmun. 9,309 -316 (1988) 309

Synthesis, structure, and phase behaviour of laterally n-alkyl-substituted polyesters with cyclohexylene moieties in the main chain

Dietrich Braun *, Harald Hirschrnann, Otto Herrrnann-Schonherr, Matthias Lienert, Joachirn H. Wendorff

Deutsches Kunststoff-lnsti tut , Schlofigartenstrafie 6R, D-6100 Darmstadt, FRG

(Date of receipt: December 28, 1987)

Introduction

In recent years investigations on rigid chain polymers have become an interesting field of research, because fibers of high strength and stiffness can be formed by melt spinning”. Many polymers of this kind have been synthesized from aromatic units linked by ester or amide groups in such a way that a linear rod like shape of the macromolecule arises. A property of these polymers is that they are able to form an- isotropic melts with a nematic structure. Because of the small entropy change at the phase transition crystalline-nematic the melting points are too high for processing.

There are several ways known to reduce the melting temperature such as for example the insert of kinked, crankshaft, halogen-substituted or flexible units into the rigid chain2,-”. Replacing some of the phenylene groups in the aromatic chain by cyclohexylene units can also lower the melting temperatures4- I ” ) . This modification preserves the linearity of the macromolecule but increases flexibility in a way that a quasi-rigid chain arises. Another approach in order to reduce melting temperatures by preserving linearity consists in synthesizing rigid chain polymers with flexible side chains. Investigations on laterally alkyl-substituted polymers show that the melting temperatures decrease with an increase of the length of the side chains” I ” .

A surprising result was the occurrence of novel layered mesophases. The X-ray analysis of a poly(imin0- I ,4-phenyleneiminoterephthaloyl) substituted with six side chains per repeating unit exhibits a boardlike packing of the macromolecules, which has been termed as a “sanidic” phaseIs). A similar phase was recently described for poly(oxy- I ,4-phenyleneoxyterephthaloyl)s substituted with two flexible side chains per repeating unitth1. It is not clear, what types of phases arise, i f the number of the side chains per repeating unit is further decreased. One would expect that those polymers are no longer able to form boardlike structures, because the space cannot be filled up by a simple side by side packing of the macromolecules.

To the authors’ knowledge investigations on laterally n-alkyl-substituted rigid polymers containing cyclohexylene moieties are not yet described in literature. In this communication the synthesis and characterization of poly[oxy(2-alkyl-l,4-pheny- 1ene)oxycarbonyl-trans- 1,4-cyclohexylenecarbonyl]s (1) are presented.

0173-2803/88/$01 .00

310 D. Braun, H. Hirschmann, 0. Herrmann-Schonherr, M. Lienert, J. H. Wendorff

l a l b

1

qinh a) Tg /"Cb) Ti /"CC)

d1.g-l

0,30 61 204 0,19 63 181

Results and discussion

Synthesis and phase behauiour

Polyesters l a and 1 b were synthesized by solution polycondensation of 2-hexyl- and 2-dodecylhydroquinones, respectively, with trans- 1,4cyclohexanedicarbonyl di- chloride. For the synthesis of the 2-alkylhydroquinones the reactions reported by Brown et al.I9) and by KabalkaZ0) were applied; they use the alkylation of benzoqui- none with trialkylboranes. trans- 1,4Cyclohexanedicarbonyl dichloride was prepared from the corresponding acid and thionyl chloride.

The inherent viscosity values and transition temperatures of polymers 1 are collected in Tab. 1. The transition temperatures were determined by differential scanning calorimetry (DSC). The cis/trans ratio of the cyclohexylene units in the polyesters 1 changes from 5/95 to 33/66 in the isotropic melt. This behaviour of rigid polyesters containing cyclohexylene moieties was already described by Kricheldorf and Schwarzzl). Therefore, the transition temperatures were taken from the first heating run.

Both polymers 1 a and 1 b form liquid-crystalline melts. The DSC-measurements were verified by optical polarizing microscopy. No characteristic textures could be observed. Polyesters 1 show a glass transition at rather low temperatures and further- more a first order transition LC -+ isotropic (see Fig. 1). The transition temperatures LC --* isotropic decrease from l a to l b . This corresponds to the usually observed thermal behaviour of rigid polymers with varying side chain length. Polyesters similar to 1, synthesized by condensation of 2-alkylhydroquinones with terephthalic di- chloride, have already been investigated by Maijnusz, Catala and LenzI2). The transi-

Synthesis, structure, and phase behaviour of laterally. . , 31 I

tion temperatures LC + isotropic of these fully aromatic polyesters are about 100°C higher than those of the comparable polyesters 1 with cyclohexylene moieties in the main chain. Polyesters 1 are well soluble in usual organic solvents such as chloroform, tetrahydrofuran, or toluene.

Fig. I . DSC curve of 1 a I l l

20 60 100 1LO 180 220 T / 'C

Structure analysis

The X-ray scattering diagram of an unoriented polymer l a is displayed in Fig. 2. It is characterized by the occurrence of several sharp reflections, denoted by 001, 002, 003, and 004 which obviously belong to the same one dimensional lattice as well as by the occurrence of the sharp reflection denoted by 100. These reflections seem to be superimposed on a broad halo, the maximum value of which is located at a scattering angle 2 0 of about 22". A similar diagram is observed for polymer l b . The lattice dimensions which can be calculated from the scattering diagram are given below in Tab. 2.

It is an unusual feature of the one-dimensional layer structure that the third order reflection displays the highest intensity for polymer l a and that the second order reflection is missing in the case of polymer 1 b (see Tab. 2).

Fig. 2. Wide angle X-ray diagram of 1 a at room tempera- ture

x m C W C

c .- c

I

m C W C

c .- c

I I I I I I

0 5' 10' 15' 20' 25" 30°

I j 002

I I I I I I

0 5' 10' 15' 20' 25" 30°

312 D. Braun, H. Hirschmann, 0. Herrmann-Schbnherr, M. Lienert, J . H. Wendorff

Tab. 2. Layer distances (d) of polymers 1

Reflex l a d/nm

l b d/nm

1,699 0,842 0,564 0,425

0,407 0,45

-

2,286

0,762 0,571 0,455 0,417 0,45

-

In order to obtain additional information on details of the structure, oriented samples were prepared by inducing a cold flow along a given direction (2), due to a pressure acting along direction 3 (see Fig. 3).

i.- Fig. 3. Assignment of axes for the uniaxial - '

- pressed sample; 2: direction of flow, 3: direction of applied pressure

2

Scattering diagrams were then taken for the cases that the propagation direction of the X-ray primary beams was parallel either to axes 1, 2 or 3. It was observed in all cases that the azimuthal intensity distribution of the halo was constant. This is, however, not the case for the sharp reflections 001 and 100. Figs. 4 and 5 represent the results for the cases that the primary beam propagates along 1-direction (Fig. 4) and along the 2-direction i.e. the flow direction (Fig. 5 ) .

It is obvious that only the reflections A corresponding to the one dimensional layer structure (001) are observed if the primary beam propagates along the 1-direction and that both of these reflections as well as the 100 reflection (B) are visible, if the primary beam propagates along the 2-direction. It should be mentioned that only the 100 reflection is observed, if the primary beam is parallel to the 3-direction.

Based on these results as well as on results of structure factor calculations to be discussed below, the following structural model evolves [Figs. 6(A) and (B)]. The backbones of the chain molecules are to the first approximation extended and parallel to each other. The fact that no reflections are observed which correspond to a regular arrangement of monomeric units along the chain backbone show that some devia- tions from the fully extended state have to exist.

The arrangement of the chain molecules along the c axis (which is parallel to the pressure direction in the macroscopic sample) is controlled by the aliphatic side

Synthesis, structure, and phase behaviour of laterally

Fig. 4. Scattering diagram of an oriented sample 1 b; primary beam along I-direction

Fig. 5. Scattering diagram of an oriented sample 1 b; primary beam along 2-direction

Fig. 6.(A),(B). Structural model for the main chain polymers 1 in different planes

313

t o

(Bl

314 D. Braun, H. Hirschmann, 0. Herrmann-Schenherr, M. Lienert, J. H. Wendorff

chains. Both the magnitude of the lattice dimension in this direction as well as its variation with the length of the side chain suggest that the side chains interpenetrate each other, as depicted in Fig. 6(A). The experimental finding is that the layer distance decreases by an amount of 0,58 nm as the length of the side chain is decreased from n = 12 to n = 6 (see Tab. 2). A linear extrapolation to n = 0 yields a value of 1 , l l nm, which corresponds exactly to twice the width of the rigid backbone of the chain, in agreement with the structural model. Additional evidence for the interpenetration comes from structure factor calculations. Tab. 3 represents the results of such calculations for the proposed structure. It is obvious that the structural model is able to account for the very peculiar intensity variations of the 001 reflec- tions.

Tab. 3. Relative intensities of the 001 reflections as obtained from structure factor calculations

1 9 0 0 0,84 3,76 1,17

1 ,Ooo 0,002 0,855 1,145

The chain molecules are furthermore reguarly arranged one on top of the other along the a-direction (corresponding to the 1 -direction within the macroscopial sample), as depicted in Fig. 6(B), giving thus rise to the observed 100 reflection. The halo obviously arises from disordered state characteristic of the aliphatic side chains.

So far, a rather ideal nearly crystalline structure has been proposed. The X-ray diagram obtained for the highly oriented samples is, however, characterized by the fact that no mixed reflections h01 occur. This has to be taken as an indication that the regular structures existing along the a- and c-axis are not correlated with each other as one would expect for a truly crystalline state. This is a feature which is characteristic for highly ordered mesophases in general.

Experimental part

2-ANEylhydroquinones: A 250 ml three necked flask fitted with a septum inlet, a magnetic stirring bar, and a condensor was flushed with nitrogen. 0,15 mol of the alkene (1-hexene or I-dodecene) in tetrahydrofuran were placed into the flask. The flask was immersed into an ice bath and 50 ml of a 1 M solution of borane-tetrahydrofuran-complex in tetrahydrofuran (from Fa. Aldrich) was added slowly. The reaction mixture was kept at 50°C for 3 h to complete the hydroboration. Then, 0,04 mol of p-benzoquinone, dissolved in 30 ml of tetrahydrofuran, were added to the trialkylborane solution at room temperature. After stirring for 12 h the reaction mixture was steam-distilled to remove solvent and side products. On cooling, the alkylhydro- quinone separated as a crystalline mass in the steam distillation flask. The product was separated by filtration and air-dried. The yields of the crude alkylhydroquinones were about 9OVo. The purification was completed by recrystallization from toluene for several times. The

Synthesis, structure, and phase behaviour of laterally. . . 315

purity was checked by thin layer chromatography using a mixture of chloroform/ I -propano1 (vol. ratio 9: I ) as eluent.

2-Hexylhydroquinone: m. p. 85 “ C (Lit.22): m. p. 85 “C). 2-Dodecylhydroquinone: m . p . 103°C (Lit.”): m.p. 104°C). trans-l,4-Cyclohexanedicarbonyl dichloride: 0,03 mol of trans- I ,4-cyclohexanedicarboxylic

acid (from Fa. Aldrich, 95% trans isomer) were heated with thionyl chloride for 4 h. The excess thionyl chloride was removed by distillation and the residue recrystallized from benzene. The product showed 95% trans configuration, which was determined by I3C NMR. Yield: 92%; m.p . 67°C (Lit.23): m.p. 67°C).

Polymers: I mmol of the 2-alkylhydroquinone was dissolved in 5 ml of I , I ,2,2-tetrachloro- ethane and 0,5 ml of pyridine. To this mixture at room temperature a solution of I mmol of trans- I ,4-cyclohexanedicarbonyl dichloride in 5 ml of tetrachloroethane was added dropwise with stirring. The mixture was stirred at 80°C for 24 h, cooled, and then poured into 300 ml of methanol. The precipitated polymer was washed with methanol and dried at 40°C i.vac. 95% trans configuration of the cyclohexylene units was observed by I3C NMR.

Measurements: Inherent viscosities of solutions of the polyesters in p-chlorophenol(O,5 g/dl) were determined with an Ubbelohde viscosimeter at 45 “C. The phase transitions were verified by examination on a Leitz polarizing microscope equipped with a hot stage (Mettler FP20). Thermal analysis was performed with a Perkin-Elmer DSC-4-differential scanning calorimeter calibrated with indium. A heating rate of 20°C/min was used. A Bruker WM 300 FT-NMR- spectrometer was used for I3C-NMR measurements. Flat camera and diffractometer X-ray dia- grams were taken using CuK, radiation with a wavelength of 0,154 nm. Oriented samples were prepared by inducing a cold flow due to a pressure acting perpendicular to the flow.

The authors thank the “Fonds der Chernischen Industrie” and the “Oflo Rohm Gedachrnk sriftung” for supporting this work.

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