Viscoelastic relaxations in thermotropic polybibenzoates

Journal of Non-Crystalline Solids 131-133 (1991) 891-893 North-Holland

891

Viscoelastic relaxations in thermotropic polybibenzoates J.M. Pere~aa, M . M . M a r u g ~ n , A. Bello a n d E. P6rez Instituto de Ciencia y Tecnologla de Polimeros, Juan de la Cieroa 3, 28006-Madrid, Spain

The dynamic mechanical relaxations of two thermotropic polybibenzoates containing the biphenyl group in the main chain have been determined in the temperature range from -150 to 130°C, at several frequencies. The two polyesters are poly(heptamethylene p,p'-bibenzoate) (P7MB) and poly(oxyditrimethylenep,p'-bibenzoate) (PDTMB). These two polymers differ only in the change of the central methylene in the spacer of P7MB by an oxygen atom in PDTMB. Both polymers show three viscoelastic relaxations and the more remarkable difference is found in the relaxation occurring at higher temperature, considered as the glass transition of the mesophase. This transition appears at a temperature 26°C lower in the case of PDTMB, due to the higher flexibility introduced by the central ether group.

1. Introduction The interest in thermotropic mesogenic polyesters has grown steadily in the last years. Flexible oxyalkylene spacers are often used in order to reduce the high melting points of all-aromatic polyesters and to increase the solubility of these polymers. One of the mesogen groups with the ability to produce liquid crystalline structures is the biphenyl group. Several works [1-3] have been devoted to study the influence of the length of the alkylene spacer on the phase behaviour of polybibenzoates, a kind of thermotropic polyesters incorporating the biphenyl structure into the main chain, We have also recently analyzed the effect of the introduction of an oxygen atom in the spacer [4], performing a comparative study of the phase behaviour of poly(heptamethylene p , p ' - b i b e n z o a t e ) (PTMB) and poly(oxyditrimethylene p , p ' - b i b e n zoate) (PDTMB). These two polymers differ only in the change of the central methylene in the spacer of P7MB by an oxygen atom in PDTMB. It was found that while the characteristics of the mesophase are very similar in the two polybibenzoates, however the formation of a three-dimensional crystal structure in P D T M B is inhibited, presumably as a consequence of the increase in flexibility introduced by the ether group. The re-

suit is that the mesophase of P D T M B can be maintained at room-temperature while that of P7MB presents a rapid transformation into a three-dimensional crystal, in such a way that very severe supercooling procedures must be adopted to quench the mesophase. One of the consequences of this mesophase-crystal transformation is the difficulty of studying the glass transition in P7MB, because the presence of crystallinity makes the calorimetric glass transition undetectable. This work presents some preliminary results about the comparative analysis of the dynamic mechanical relaxations of P7MB and PDTMB, in order to obtain further information about the influence of the central ether group on the viscoelastic relaxations.

2. Experimental The two polybibenzoates have been obtained by melt trans-esterification of the diethyl ester of p,p'-bibenzoic acid (4,4'-biphenyldicarboxylic acid) and the corresponding glycol, using isopropyl titanate as catalyst. Commercial heptamethylene glycol was used in the synthesis of P7MB, while 3,3'-oxydipropanol (the dimer of trimethylene glycol) was employed in the case of PDTMB. The details of the preparation and characteriza-

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892

J.M. Pere~a et aL / Viscoelastic relaxations in thermotropic polybibenzoates

tion of the dimer and the polyesters have been previously reported [3,4]. The actual specimens for the dynamic mechanical analysis were obtained by compression molding the two polymers at 200°C and quenching with cooling water. This procedure allows the neat mesophase of PDTMB to be obtained, but a small amount of transformation into the three-dimensional crystal is produced in P7MB, as revealed by the X-ray diffractogram of the sample, compared with the shapes of the pure phases previously reported [3]. The complex modulus, resolved into its storage and loss components, has been measured over the temperature range from - 150 to 130°C, by using a PL-DMTA apparatus working in the tensile mode, at 3, 5, 10 and 30 Hz. The specimens used were 10 mm long, 2.2 mm wide and - 0.6 mm thick. Activation energies were calculated from the shift of the relaxation temperatures with frequency, according to the usual Arrhenius-type equation, The absolute error of the activation energy value is obtained by assuming a maximum variation of + 0.5 °C in the temperatures assigned from plots of loss modulus versus temperature.

3. Results The variations of the storage and loss moduli as a function of temperature are plotted in figs. 1

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Fig. 2. Temperature dependence of the storage, E', and loss,

E", moduliof PDTMB at 5 Hz.

and 2 for P7MB and PDTMB, respectively. It can be observed that the two polymers show three viscoelastic relaxations, which are named alpha, beta and gamma, in order of decreasing temperature. The alpha-relaxation takes place at 20°C (5 Hz) in the PDTMB sample (fig. 2), while it appears at 46°C (3 Hz) in the case of P7MB, as can be seen in fig. 1. The beta-relaxation is very broad for the two polymers, and appears around - 6 0 ° C . Finally, the gamma-relaxation is clearly apparent in both polybibenzoates, even though the beta- and gamma-relaxations are partially overlapped. The maximum of the loss modulus for the gamma-relaxation occurs at - 1 2 0 ° C (at 3 Hz for P7MB and 5 Hz for PDTMB).

3Hz

4. Discussion

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Fig. 1. Temperature dependence of the storage, E ' , and loss,

E", moduliof PTMBat the indicated frequencies,

The alpha-relaxation is considered as the glass transition of the two polybibenzoates, due to the high intensity of the relaxation and to the correspondingly strong decrease of the storage modulus at the relaxation maximum. The activation energy of the alpha-relaxation in P7MB is higher than 100 kcal/mol (106 + 20 kcal/mol), reasserting the consideration of the alpha-relaxation as the glass transition of these polymers. Moreover, the glass transition of PDTMB is detected by differential scanning calorimetry (DSC) at 17°C [4], and it is

J.M. Pereha et al. / Viscoelastic relaxations in thermotropic polybibenzoates

found at 41°C for the actual specimen of P7MB used in this work. The DSC temperatures are slightly lower than those determined by dynamic mechanical measurements, as usual. It has to be taken into account that the mesophase formation is a very rapid process in the two polymers and therefore the reported glass transitions correspond to relaxations of the liquid crystal glass. On the other hand, the difference between the glass transition temperatures of P7MB and PDTMB is attributed to the increased flexibility of the PDTMB chain, with a spacer including a central oxygen atom. Concerning the beta-relaxation, it is generally agreed that in the polyester series this relaxation originates from movements of phenyl [5], carboxyl [6] and methylene [7] groups. The very broad peaks observed in figs. 1 and 2 are the consequence of this complex character. We are presently investigating a possible fine structure of this relaxation in the two polyesters by changing the crystallization conditions and widening the frequency range. The temperature location of the gamma-relaxation and its activation energy (10.5 _+ 0.5 kcal/mol) allows one to consider it as a relaxation arising from crankshaft movements of methylenic chains containing at least three consecutive methylenic units. This relaxation has been also found in other polymer series containing trimethylenic sequences in the main chain [8,9] or in side chains [10], but it has not been detected when the number of consecutive methylenes is less than three [10]. The results of the present work confirm that the three consecutive methylene groups of P7MB cause the joint movement which generates the gamma-relaxation. Moreover, this relaxation presents a higher intensity in the case of P7MB than for PDTMB. This fact is attributed to the presence of longer methylenic sequences in P7MB, which leads to a better resolution of the peak corresponding to the gamma-relaxation.

893

A more detailed analysis in a broad range of frequencies (including also dielectric measurements) is in progress in order to have a better understanding of the viscoelastic relaxations of these and other thermotropic polybibenzoates with different spacers.

5. Conclusions (1) The alpha-relaxation of PDTMB, assigned to the glass transition of the mesophase, appears at a temperature 26°C lower than in the case of P7MB, attributed to the higher flexibility of the spacer in PDTMB. (2) The gamma-relaxation, partially overlapped by the beta peak, presents a higher intensity for P7MB, due to the presence of longer methylenic sequences in this polymer. The financial support of the Comisi6n Interministerial de Ciencia y Tecnologia (project no. MAT88-0220) is gratefully acknowledged.

References [1] P. Meurisse, C. Noel, L. Monnerie and B. Fayolle, Br. Polym. J. 13 (1981) 55.

[2] J. Watanabe and M. Hayashi, Macromolecules 21 (1988) 278.

[3] E. P~rez, A. Bello, M.M. Marugfin and J.M. Pereha, Polymer Commun. 31 (1990)386. [4] A. Bello, E. P6rez, M.M. Marugfin and J.M. Pereha, Macromolecules 23 (1990) 905. [5] C. Chung and J.A. Saner, J. Polym. Sci. A2, 9 (1971) 1097. [6] K.H. Illers and H. Breuer, J. Colloid Sci. 18 (1963) 1. [7] G. Farrow, J. McIntosh and I.M. Ward, Makromol. Chem. 38(1960) 147.

[8] J.M. Pereha and C. Marco, Makromol. Chem. 181 (1980) 1525. [9] C.C. Gonz,Mez, J.M. Pereha and A. Bello, J. Polym. Sci. Part B: Polym. Phys. 26 (1988) 1397. [10] P. Saavedra. PhD thesis, Universidad Complutense de Madrid (1988).