Elsevier

Thermochimica Acta

Volume 319, Issues 1–2, 5 October 1998, Pages 47-53
Thermochimica Acta

Transition-temperature differences on cooling for a monotropic liquid-crystal polyester observed in DSCs of different design

https://doi.org/10.1016/S0040-6031(98)00405-5Get rights and content

Abstract

Most commercial differential scanning calorimeters (DSC) belong to two types of design, namely the power-compensation DSC and the heat-flux DSC. There have been some reports in the literature indicating differences in response between these two types of instrument. Most of these differences have been observed on heating, and it is often assumed that the response on cooling is symmetrical. In this work, we report on differences observed on cooling a monotropic liquid-crystal polyester in several DSCs of the two designs. The monotropic liquid-crystal polyester used here had two exothermic transitions under moderate cooling rates. It was found that the heat-flux instruments tended to give overlapping exothermic peaks, suggesting that the second transition started before the first was complete. The power-compensation instruments, on the other hand, showed completely separated peaks at the same cooling rates. A monotropic liquid-crystal polymer with appropriate crystallisation kinetics may be a good material to evaluate the DSC response on cooling.

Introduction

The family of polyesters based on the condensation of 4,4′-biphenyl dicarboxylic acid and aliphatic diols HO–(CH2)n–OH (n=2–9, see below) shows mesophase behaviour 1, 2. We have studied the polyester with n=8, namely poly(octamethylene bibenzoate) or BB8.

The BB8 polyester has a monotropic liquid-crystal phase as the mesophase is only observed on cooling 2, 3, 4. The phase sequence is K–I on heating, and I–S–K on cooling (K=crystal phase, S=smectic liquid-crystal phase, I=isotropic melt phase). We were puzzled at first, by the transition temperatures and mesophase windows which we found using the differential scanning calorimeter (DSC) when it was compared with those reported in the literature for BB8. According to Watanabe and Hayashi [2], BB8 has two transitions on cooling, the first at 171°C and second at 139°C (mesophase window of 32°C). According to Perez et al. [4], the I–S and S–K peaks are at 168° and 142°C, respectively, (mesophase window of 26°C). Krigbaum and Watanabe [1]did not show their DSC results explicitly, but listed the peaks for BB8 in table. On cooling of BB8, they reported peaks at 160°, 153° and 95°C, but these were not assigned. It is, however, probable that the DSC peaks at 160° and 153°C correspond to the isotropic mesophase transition and the second to the crystallisation transition; if this is correct, then their mesophase window on cooling was only 7°C wide. The third peak at 95°C, reported by Krigbaum and Watanabe [1], was not mentioned by the other invesigators of BB8 2, 4.

In our own investigation, we were even more puzzled because the mesophase window recorded on two different DSC instruments varied greatly for the same sample of BB8. We found that the heating scans of BB8 were similar on the two different calorimeters, but there were certain intriguing and striking differences in the cooling scans. One instrument showed, on cooling, two overlapping exothermic transitions at 170° and 161°C (i.e. a narrow mesophase window of 9°C). The second instrument showed two non-overlapping transitions with a wider window of (19°C peak-to-peak). Repeated experiments on both instruments confirmed this behaviour. Hence, it forced us to consider the possibility that the observed transitions on cooling are affected by the measuring instrument.

There were differences in the design of the two instruments. The first (Mettler DSC 30) was a `heat-flux' instrument, whereas the second (Perkin–Elmer DSC 7) was a `power-compensation' device. We suspected that the difference in instrumental design could affect the results to quite a significant extent in this particular example of a monotropic liquid-crystal polymer. In this work, we shall report how the instrument itself affects the transitions observed with BB8 and the apparent width of the mesophase window.

Section snippets

Experimental

The polymer was synthesised by transesterification [1]of 4,4′-biphenyl diethyl dicarboxylate ester with 1,8-octane diol using titanium isopropoxide as catalyst. The cream-coloured polymer was mechanically crushed and extracted with acetone and dried at 50°C. The polymer showed good extinction in the polarising microscope on isotropisation. The BB8 was run on a number of power-compensation and heat-flux calorimeters from different manufactures. These were: Mettler DSC 30 with a metal sensor,

Results and discussion

Before discussing the results, we shall briefly mention the salient features of the design of DSC instruments. Most commercial scanning calorimeters can be classified into two general categories: these are the power-compensation and heat-flux DSCs [5]. With the usual twin cell design of the power-compensation DSC, there are separate platinum resistance sensors for temperature measurement and individual platinum resistance heaters for the addition of heat. The sample S and reference R are placed

Conclusions

This work clearly illustrates the point that the DSC performance on heating and cooling may not be symmetrical (i.e. equally good). The response on cooling appears to be in general, better, with the power-compensation instruments. With a monotropic liquid-crystal polymer, such as BB8, the power-compensation instruments generally gave the best performance at −10 and −5°C/min, even with the use of relatively large sample masses (9 mg). That is, they showed complete separation of the two

Acknowledgements

We should like to thank the following instrument manufacturers and their agents for their cooperation in running the BB8 sample: Miss Francoise Peyron and Mr. M. Thimon of Setaram, France, Dr. R. Bottom of Mettler-Toledo, UK, Mr. G. van der Veldt of V.A. Howe, UK (Shimadzu) and Dr. R. Marsh of Rheometrics Scientific, UK (Polyer Labs. DSC).

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