384 Synthetic Metals, 55-57 (1993) 384-387

LAYERED STRUCTURE WITH SIDE CHAIN CRYSTALLINITY IN UNDOPED POLY(3-

ALKYL THIOPHENES)

K.-S. HO, J. BARTUS, K, I~EVON, J. MAO AND W.-Y. ZHENG Polymer Research Institute, Polytechnic University, Brooklyn, NY (USA) J. LAAKSO and T. TAKA Technology Centre, Neste Oy, P.O. Box 310, SF-06101, Porvoo (Finland)

ABSTRACT

The order formation of alkyl-substituted rigid polymers, like poly(3-alkyl thiophenes), is

dependent on the length of the alkyl side chain. The formation of a layered structure can be

observed by X-ray diffraction experiments. A linear relationship with the d-spacing for (200)

plane and n has been obtained along with the observation of the endotherms for the mesophase

transition by the thermal analysis. The endothermic transition temperatures observed by DSC

decrease with the increasing number of carbons (n) in the alkyl side chain. Side chain

crystallization becomes possible when n is larger than 10, resulting in hexagonal packing of the

alkyl chains between the main chain layers.

KEYWORDS: Poly(3-alkyl thiophene), layered structure

INTRODUCTION

Recent studies have shown that poly(3-alkyl thiophenes) form orthorhombic crystal structures

where the c-axis are separated with each other by the alkyl side chains. The X-ray analysis

shows further that the polymers form a layered structure within this orthorombic cell. ~ Our

recent investigation showed that with an increase in entropy through thermal energy, a liquid-

crystalline phase is formed as observed by thermal analysis. 2 As a conclusion, we have shown

that these alkyl substituted polythiophenes form similar layered structures, as characterized and

defined in detail for several other alkyl-substituted rigid polymers by Ballauff et al.3 In this

study we have related the structure formation to planarity of the thiophene rings in the backbone

structure as analyzed by UV spectroscopy.

Elsevier Sequoia

385

EXPERIMENTAL

Poly(alkyl thiophenes) (P3AT): Poly(hexyl thiophene) (PHT), poly(octyl thiophene) (POT),

poly(dodecyl thiophene) (PDDT), and poly (octadecyl thiophene) (PODT) were all kindly given

by Neste Company. Poly(tetradecyl thiophene) (PTDT) and poly(hexadecyl thiophene) (PHDT)

were synthesized by Mr. Wen-Yu Zheng at the Department of Chemistry, Polytechnic

University. Poly(pentyl thiophene) (PPT) and poly(hexadecyl thiophene) (PHDT) were provided

by Dr. Jan Bartus.

P3AT were purified by dissolving in THF, filtered, precipitated into methanol, and isolated

by filtration. Finally the purified samples were dried under vacuum in room temperature.

POT and PDDT were cast into thin film from THF solutions. Thermal analysis was done

with a Perkin-Elmer DSC-7 differential scanning calorimeter. 10 mg of sample was placed in

a sealed A1 - sample pan and the measurements were done in N2 atmosphere. A Phillips X-ray

generator was used for the X-ray diffraction studies. The samples were cast on the slide or

placed on the slide. The sample was exposed Cu/Ni radiation with 45 V and 35 mA from 2 °

to 40 °, the exposure time, 50 sec in every 0.04 ° was used for this experiment.

A Nikon FII optical microscope was applied for the morphology studies. The temperature

was controlled by a Mettler hot stage. The crystal structures were characterized under crossed

polarization. A DuPont TGA was used to follow the degradation of the polymer. Temperature

range was from 50 ° to 650 ° with a heating rate of 20°C per minute. FT-lnfrared Spectroscopy

was done by Bio-Rad FTS-60 (Digilab Division) Spectrometer. Cary-2300 Spectrophotometer

(Varian Co.) was used to obtain the UV-Vis-spectra of the samples. The wavelength ranged

from 200 nm to 800 nm at a 2nm/sec scanning rate.

RESULTS AND DISCUSSION

The X-ray data as summarized in Figure 1 reveals that the d-spacing between the poly

thiophene axis changes from 16A to 27A as the number of carbons in the alkyl chain is increased

from 5 to 16. This is as expected in the layered structures. A slight deviation of a linear

relation between d(400) spacing and n may be related to the non-equilibrium effects during the

sample preparation. The repeating unit along the c-axis is 7.79A in all of the samples indicating

the existence of two monomer units in a unit cell. The b-axis changes from 3.9A to 4.2A where

the 4.2A, similar to that of paraffin, for PHDT and PODT show that the side chains may

crystallize.

386

d-spacing (A) 3 0 - -

25

20

15

10

5

0

/

/

0 [ ] [ ] [ ] [ ] 13 [ ]

I I I I I I I I I I I I I I

5 6 8 10 12 14 16 18

# of c a r b o n s of s ide chain

Series 1 ~ S e r i e s 2 + S e r i e s 3 [] Series 4

Figure 1. d-spacings from the X-ray diffraction experiments for

thiophenes).

the poly(3-alkyt

The FTIR spectrum of P3DDT with absorptions at 730 cm 4 and 720 cm 1 (CH2 rocking

band for ordered and disordered states) shows that side chains have partially crystallized under

these conditions.

B

I I I i I I I I I I - 5 0 . 0 - 2 S . 0 0 . 0 2 5 . 0 5 0 . 0 7 5 . 0 1 0 0 . 0 1 2 5 . 0 1 5 0 . 0 1 7 5 . 0

r~porokuro ('C)

Figure 2. DSC thermograms of P3OT (A), P3DDT (B) and P3ODT (C).

387

The DSC thermograms (Figure 2) for P3OT, P3DDT and P3ODT show that the

isotropization temperature decreases from 180°C to 135°C when the length of alkyl side chain

increases from 8 to 18. Additionally, a transition appears at 40°C showing the melting of the

side chain crystals. A third transition between the two transitions becomes clear with the

increase of the side chain length. This transition is the crystal-liquid crystal transition. The two

dimensional liquid crystallinity in the layered structures has been explained in detail by Ballaufi s.

It is obvious that the side chains act as additional solvent causing the melting point

depression and the formation of liquid crystalline phase especially when they are long enough

(even to crystallize themselves). The interesting part is the observation of the UV absorption

behavior of these polymer as shown in Figure 3. It is to be noted that the side chains increase

the planarity to a certain extent (up to P3DDT) and then causing disordering (P3TDT, P3HDT).

The UV spectra measured on heating also show that these changes in the conjugated backbone

are related to the first order thermodynamic transitions.

MS× Absorption 5 5 0 | 5 4 0 [ 530 b

J

520 t 5 1 0 [ 500

49ok 480 l 470 [ 450 [ 450 " k i

0 2 4 I I I I t ~ I

6 8 10 12 14 16 18 20

# of Ca rbon Atoms

Figure 3. UV-absorption maxima of the P3ATs

REFERENCES

1. K. Tashiro, K. Ono, Y. Minagawa, M. Kobayashi, T. Kawai, K. Yoshino, J. Pol. Sci.,

Pol. Phys., 29 (1991) 1223.

2. K.-S. Ho, J. Mao, W.-Y. Zheng and K. Levon, to be submitted.

3. M. Ballauff, Angewandte Chemie, 28 (1989) 253.