Molar melting enthalpy in an homologous series of mesophasic polymers

Eur. Polym. J. Vol. 18, pp. 759 to 762, 1982

0014-3057/82/090759-04503.00/0 Copyright © 1982 Pergamon Press Ltd

Printed in Great Britain. All rights reserved

MOLAR MELTING ENTHALPY IN AN H O M O L O G O U S SERIES OF MESOPHASIC POLYMERS PIO IANNELLI, ANTONIO ROVIELLO and AUGUSTO SIRIGU Istituto Chimico dell'Universitfi, Via Mezzocannone 4, 80134 Napoli, Italy (Received 24 N o v e m b e r 1981)

Abstract--The melting point depression of polymer-solvent mixtures as a function of the concentration has been measured by DSC methods for a group of semiflexible linear mesophasic polymers of formula [--OOC(CH 2), - 2 - - C O O - - O - - C ( C H 3 ) = = = C H - - O - - ] x . The molar enthalpic changes associated with the phase transitions have been determined. The values of the ratio between the molar isotropization entropy and the total molar entropic change relative to the sequence of phase transitions leading from the crystal phase to the isotropic liquid have been taken into account as an indication of the order of the mesophase. A particularly high value of this ratio has been found for the even members of the homologous series of polymers.

INTRODUCTION We have evaluated experimentally the m o l a r melting enthalpy of some linear semiflexible mesophasic polymers for two purposes. (i) Polymers exist normally in a biphasic status consisting of a crystalline phase and of a non-crystalline one intermixed in variable relative amounts. The knowledge of the fraction of crystalline phase in a sample is useful; physical and mechanical properties depend on the a m o u n t of crystallinity. Evaluation of this parameter can be easily done by measuring the enthalpy change at the melting transition provided the enthalpy change associated with the melting of one mole of pure crystal phase is known. (2) The degree of order of a mesophase (with respect to the isotropic liquid) is related to the molar entropic change at the clearing transition. The F l o r ~

Pn = [ - O O C

the sample or equivalently of the molar melting entropy of the pure crystal phase. For evaluation of this quantity, we have measured the depression of the melting temperature of polymer diluent mixtures as a function of the concentration.

EXPERIMENTAL The melting points of polymer diluent mixtures have been measured as a function of the concentration. The classical formula [5] l/T,, - l / T ° = ( R / A H , ) ( V f f V I ) ( v I - Zlc 2)

has been utilized to evaluated AH,. The following polymers have been examined:

(CH2)n_2 - C O O - < ~ - C ( C H a ) = C H - ( ~ - ] x

Ronca theory [-1, 2], developed for an assembly of rigid rodlike molecules of arbitrary length, relates the m o l a r isotropization entropy of a nematic phase to the value of the order parameter at the transition temperature. O n the other hand, examination of the thermodynamic data for the phase transitions of low molecular weight mesophasic c o m p o u n d s shows that the ratio A S i / A S r (ASI = molar entropic change at the isotropization transition; A S r = total m o l a r entropic change for the sequence of phase transitions leading from the crystal phase stable at r o o m temperature to the isotropic liquid) is small (normally <0.06) for a nematic phase while m u c h higher values may be found for smectic phases [3, 4]. Evaluation of A S i / A S r for low molecular weight c o m p o u n d s normally offers no problems. With liquid crystalline polymers however, while measurement of AS~ by DSC methods is straightforward, the evaluation of ASr requires knowledge of the crystallinity of

n = 9, 11, 12, 13, 14. Synthesis of polymers of this type has been already described [6]. Polymers were characterized by viscometry of chloroform solutions. The following values of the intrinsic viscosity (at 26.00 ± 0.02 °) have been found: [r/]P9 = 3.15 dl g 1; [~/]Pll = 2.05 dl g 1; [q]P12 = 3.50dig

1;

[r/]P13 = 2.22 dl g 1; [q]P14 = 3.90 dl g ~. ~-chloronaphthalene was utilized as solvent. For the evaluation of the molar volumes (V,, VI) at the appropriate temperature the following equations were used [7]. V~(t) = 1.342.102 + 1.044.10- 1 t cm 3 tool- i t/:.

759

V~(t)(P9) = 314.1 + 0.237t

cm 3 tool-

Vu(t)(Pl 1) = 348.2 + 0.242 t

cm 3 mo] 1

Vu(t)(P12) = 357.0 + 0.299 t

cm 3 mol

Vu(t)(Pl3) = 383.9 + 0.250 t

cm 3 mol L

760

P]o IANNELLIet al.

The molar volume of P14 at any temperature was calculated, adding to the corresponding molar volume of P12 the contribution of two moles of methylene groups calculated from literature data [8] based on density measurements of liquid polyethylene. The molar volumes calculated from the above equations refer to the isotropic liquid phase. In fact, as we shall discuss later, in the examined range of composition the mixtures melt to isotropic phases. The melting temperatures were determined using a Perkin-Elmer DSC-2 apparatus. Sealed stainless steel containers for liquid samples were used. The value of T,, for any composition was obtained by extrapolating the melting endotherm on the high temperature side. Most of the measurements were made on thermograms obtained with a 5 K/min heating rate. A final correction was applied by extrapolating to zero heating rate. The optical isotropy of the liquid phase in equilibrium with the solid polymer was recognized by optical polarizing microscopy. The samples, contained in sealed glass capillaries, were heated by means of a Mettler FP5 microfurnace. The nature of the crystal phase at the melting temperature was monitored by X-ray diffraction methods.

of AH, in Table 1 include, therefore, the contribution AHi due to the isotropization of the liquid crystal phase. This is a known quantity [9] and is measurable independently with the pure polymer. The extrapolated values of T ° refer to an equilibrium between crystal phase and isotropic liquid that is not attainable experimentally for those pure polymers for which melting produces an anisotropic liquid phase. Among the odd members of the group, P9 has higher values of AH. and ASr than PI1. This apparent inconsistency is caused by a difference between the crystal structures. On the other hand, the crystal phases of P l l and P13 at the melting transition are isomorphic. The behaviour of the even members of the group needs further discussion. Both P12 and P I 4 exibit a pattern of solid state polymorphism that has already been elucidated [6, 10, 11]. Two crystal forms are observed at r o o m temperature. One of them (k~) is obtained by solution crystallization, the other (kl) by melt crystallization. Two crystal phases (k2, ka) are formed at higher temperatures with the sequence kl(k~)----~k2----~ka. The k3 phase melts to produce a liquid crystal phase in the case of P12 and an isotropic liquid in the case of P14. X-ray analysis shows that, for mixtures with composition 0.1 ~< v~ <~ 0.4, the crystal phase in equilibrium with the isotropic liquid is k2 for both P12 and P14. The values of T ° in Table 1 are referred, therefore, to a melting transition k2 ----, isotropic liquid that is impossible to observe with the pure polymers. In consequence, AH, is the sum of three contributions.

RESULTS AND DISCUSSION

Plots of 1 / T , vs v~ are shown in Figure 1. Their linearity within the range 0 < va < 0.4 is sufficiently good to allow a satisfactory extrapolation to vl = 0 and to assume [O(l/T,,)/OVl]v,_o = c~(1/T.,)/dvl. The results of this operation to obtain T~ and AH, are reported in Table I. Observation with the polarizing microscope of melting mixtures shows that the liquid phase in equilibrium with the solid polymer is isotropic. The values

2o[

°5>

i

2,o-

/ - / / •

,."

.. n=9

AI"

/# /I

11

/e

'/,~//

/

-"

n=13

2.20'

AH. = AH(k2--~ k3) + A H ( k a ~ LC) + A H i

•

jJ

J 2.10

I

I

I

I

I

.10

.20

.30

.40

.so

Fig. 1. Reciprocal of the melting temperatures (1/T,,/K -1) as a function of the composition (v I = volume fraction of the diluent.)

Molar melting enthalpy in mesophasic polymers

761

Table 1. Thermodynamic data relating to the phase transitions n

T°

T,,(exp)

Ti

AH,

AH,.

AHi

ASI

ASr

ASi/AS.~

9 11 12 13 14

458 445 468 438 461

454 434 474 433 473

503 479 498 452 464

34.45 31.7 27.25 37.2 28.5

31.7 28.55 31.8 33.8 36.96

2.72 3.18 7.37 3.39 6.57

5.44 6.61 14.8 7.535 14.15

75.35 72.4 87.0 85.8 97.1

0.07 0.09 0.17 0.09 0.15

(a) Units: T/K; AH/kJ mol 1; A S / J mol- ~ K ~; (b) Symbols: T ° = crystal-isotropic liquid equilibrium temperatures obtained by extrapolation to vt = 0 of 1/T,, plots (Fig. 1). T,, (exp) = experimental values of the crystal-nematic liquid transition temperatures. For n - 14 the melting transition leads to the isotropic liquid. T, = experimental values of thc isotropization temperatures. AH, = molar melting enthalpies from plots of Fig. l (see text for detailed explanation). AH,~ = molar melting enthalpy. For n odd AH,. = AH~ - AH~: for even n AH,, = AH(kl ~ k2) + AH(k2---* k3) + AH(k3---* LC). AH i = molar isotropization enthalpy. AS~ = molar isotropization entropy. A S 7, = total molar isotropization entropy relative to the sequence of phase transitions: crystal phase stable at room temperature---, isotropic liquid. For P14 is ASI = AS(k~ --o k2) + A S ( k 2 - ~ ' k 3) + AS(k~-~I).

The DSC analysis of the phase transitions of polymer samples of unknown crystallinity does not furnish the absolute values of the associated enthalpic changes (but for AHi). The ratios among these quantities are however easily measurable provided that the phase transitions that sequentially take place involve a fixed amount of crystal phase. From the examination of the DSC behaviour of several samples subjected to various degrees of annealing, we have obtained the following results:

AH(kl

---* k 2 ) / A H ( k 2

----* k 3 ) / A H ( k 3

~

LC)

= 1.71/1.00/1.85 for PI2; 2.25/1.00/2.34 for PI4. By use of these coefficients, the value of the molar enthalpic change for each phase transition can be calculated. It is found: for P12 AH(k,---*k2) = 11.93 kJ tool- 1; AH(k2___~k3)= 6.99 kJ mol 1; AH(k3---*LC) = 12.9 kJ tool- ~; the corresponding temperatures [6] for the solid phase transitions are 410 K and 440 K; for P14 A H ( k a - - , k z ) = 14.8kJmol 1; AH(ka---*k3)= 6 , 5 7 k J m o l - 1 ; AH(k3---*LC)= 1 5 . 4 k J m o l - ~ ; the corresponding solid state transition temperatures are [11] 414 K and 438 K respectively. The case of P14 needs additional explanation. The mesophasic behaviour of this polymer is monotropic. Direct measurement of AH~ by the DSC method can be made firstly melting the polymer to the isotropic liquid then cooling to the anisotropic phase and finally heating the latter again to the isotropic phase. The value of AH(k3---*LC) is not measurable directly but must be calculated by subtracting AHi from AH(k3-~I). This last quantity (depending on the crystallinity of the sample) can be measured directly. The set of thermodynamic quantities obtained as mentioned above is shown in Table 1. A measure of the fair consistency of the experimental data is given by the results obtained when the mixtures with vt > 0.4 are considered for P12 and P14 (Fig, 1). If a linear correlation of the experimental I.r.J. 18/9

B

points is assumed, a value of AH, of 40.9 kJ mol 1 is calculated for P12 and a corresponding value of 38.5 kJ mol 1 for P14. The X-ray diffraction analysis of melting mixtures shows that the crystal phase in equilibrium with the isotropic liquid is ks for P12 and kl for P14. AHu is therefore comprehensive of AH(k~--~ k2) for P12 and of dill(k1 ~ k2) for P14. On the other hand the DSC behaviour of P12 appears to indicate that the molar enthalpic contents of phases ks and kl are very similar [6], i.e. AH(kl --*I) ~-, AH(k,--, I). These values of AHu are to be compared with 39.2 kJ m o l - 1 and 43.5 kJ tool 1 respectively for P12 and P14. The discrepancy between the two data for the latter polymer is not bad if we take into account that the uncertainty of a direct measure on a DSC thermogram of the transition enthalpy for a polymer may approach ~ 10°~o. Very few polymers having some stereochemical analogy with those discussed in this paper are available from the literature for a comparison [12]. Among these polydecamethyleneterepht halate (AST ~ 112 J tool- 1 K 1), polyhexamethyleneterephthalate (ASr ~ 8 0 J m o l I K 1) and possibly polypiperazinesebacamide (ASr = 57 J mol 1 K ~) show that the values of AS T listed in Table 1 are of a reasonable order of magnitude. The values of ASi/AS,r are significantly greater than those normally found for low molecular weight nematics. This is true particularly for the even members of the group P12 and PI4. As far as we know, comparable or even higher values of ASi/AST have been measured with low molecular weight compounds only for smectic phases. A tentative conclusion from the present study could be the statement that the nematic phase of a polymer is more ordered than the homologous phase given by the monomeric compound. This conclusion would be supported by theory in the case of rod-like polymers. For semiflexible molecules, however, more investigation on polymers of different stereochemical structures is needed before a conclusion of any generality may be drawn.

762

PIo IANNELLI et al.

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7. P. lannelli, A. Roviello and A. Sirigu, Eur. Polym. J. 18, 745 (1982). 8. E. Hunter and W. G. Oakes, Trans. Faraday. Soc. 41, 49 (1945). 9. A. Roviel[o and A. Sirigu, Makromolek. Chem. 183, 895 (1982). 10. V. Petraccone, A. Roviello, A. Sirigu, A. Tuzi, E. Martuscelli and M. Pracella, Eur. Polym. J. 16, 261 (1980). 11. A. Roviello and A. Sirigu, Makromolek. Chem. 180, 2543 (1979). 12. J. Brandrup and E. H. Immergut, Polymer Handbook (Edited by E. H. Immergut), 2nd Ed. Wiley (1975).

Riassunto--E' stata misurata la depressione del punto di fusione di alcuni polimeri mesofasici di formula generale [--OOC(CH 2). - 2--COO--(/)--C(CH 3)-m--~CH--O--]~ per aggiunta di un diluente. Sono stati ricavati valori asso]uti delle entalpie molari di transizione.