dynamic mechanical and thermal behavior of thermotropic polyesters based on...

Dynamic mechanical and thermal behaviorof thermotropic polyesters based on

4,40-alkane-1-x-diylbis(4-hydroxybenzoic acid) and4,40-(pentane-1,5-diyloxy)dibenzoic acid

Y.A. Demchenko a, A. Razina b,c, Z. Sedl�aakova b, A. Sikora b, J. Baldrian b,M. Ilavsk�yy a,b,*

a Faculty of Mathematics and Physics, Charles University, 180 00 Prague 8, Czech Republicb Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 162 06 Prague 6, Czech Republic

c Institute of Macromolecular Compounds, Russian Academy of Sciences, 199004 St. Petersburg, Russia

Received 22 April 2002; accepted 8 May 2002

Abstract

Thermotropic polyesters based on 4,40-alkane-1-x-diylbis(4-hydroxybenzoic acid) and 4,40-(pentane-1,5-diyloxy)-dibenzoic acid were studied by dynamic mechanical spectroscopy, X-ray scattering, differential scanning calorimetry

and polarizing microscopy. The effect of structure changes in the mesogenic group as well as in the flexible spacer, in

particular the incorporation of Cl atoms into the mesogen, introduction of ether oxygen into spacer and the number of

CH2 groups in spacer was investigated. More complex mechanical and thermal behaviour was found on second heating

scan than on first cooling of the isotropic melt; higher values of mechanical functions were observed in the isotropic

state than in the nematic state of melts. While an even number of CH2 groups and the presence of ether oxygen in spacer

shifts the transition temperatures to higher values, the incorporation of Cl atoms into the mesogen leads to disap-

pearance of ordered structure and the polymers behave as amorphous materials.

� 2002 Elsevier Science Ltd. All rights reserved.

1. Introduction

The formation of the liquid crystalline (LC, nematic,

smectic, etc.) phase, which is an intermediate phase

(mesophase) between the crystalline solid and the iso-

tropic liquid, in thermotropic main-chain polymers

(LCPs) is not straightforward since the crystalline/LC (C/

LC) transition temperature increases substantially with

increasing chain length [1]. From this point of view

semiflexible polyesters, in which hard mesogenic groups

are connected by flexible spacers, are convenient for

preparation of LCPs due to lower C/LC temperatures

and their still relatively high decomposition temperatures

[1–7]. Main-chain and side-chain LC polyesters with

mesogenic units incorporated into the backbone or into

side chains were prepared. The flexible spacers, such

as (CH2)x groups (xP 4), allow easier arrangement of

mesogenic units to obtain both the LC and isotropic

states [1,4,8–10]. Physical properties of LCPs are strongly

dependent on the shape of mesogens as well as on the

length and flexibility of spacers. The flexibility of spacer

can be controlled, e.g., by introduction of ether oxygen

replacing CH2 group in the aliphatic sequence. In most

cases this leads to a change of thermorheological pro-

perties of LC polyesters [11]. The change of the meso-

genic unit shape has also a large influence on the physical

behavior of LCPs; for instance, incorporation of bulky

substituents into the benzene rings of the mesogenic

European Polymer Journal 38 (2002) 2333–2341

www.elsevier.com/locate/europolj

* Corresponding author. Tel.: +420-2-21912363; fax: +420-2-

21912358.

E-mail address: [email protected] (M. Ilavsk�yy).

0014-3057/02/$ - see front matter � 2002 Elsevier Science Ltd. All rights reserved.

PII: S0014-3057 (02 )00146-5

mail to: [email protected]

group leads, in some cases, to a complete disappearance

of LC properties [4]. In recent years also the rheological

behaviour of LCPs has attracted much attention [5,8,11–

15] as in nematic-mesophase polymers exhibit lower

viscosity in comparison with the isotropic state.

In our previous paper [16] the synthesis of main-chain

LC polyesters, prepared from 4,40-alkane-1-x-diylbis-(4-hydroxybenzoic acid) and 4,40-(pentane-1,5-diyloxy)-

dibenzoic acid, was described. In this contribution

differential scanning calorimetry, dynamic mechanical

spectroscopy, X-ray scattering and polarizing optical

microscopy have been used for investigation of these

systems. Main attention is devoted to the effect of

structure changes in the mesogenic group as well as in

flexible spacer on physical behaviour.

2. Experimental

2.1. Materials and sample preparation

All polymers and copolymers were prepared [16]

by phase transfer catalyzed condensation reaction of

diacid chlorides (K) (dissolved in 1,2-dichloroethane)

and aqueous solutions of sodium salts of bisphenols (F5,

F6); the benzyl(tributyl)ammonium bromide was used

as a phase transfer agent. The mixture was stirred

for 15 min at T ¼ 25 �C and then poured into metha-nol. The polymers were purified twice by precipita-

tion into methanol from hot chloroform solution.

Properties of three types of polyesters are reported in

this paper:

In the three cases, nmeans the number of CH2 groups in

a flexible aliphatic sequence of Fn. The chemical structures

of the synthesized polymers were verified by NMR [16].

From polymers F5K5 and F6K5 also the copolymer with

the ratio F5K5=F6K5 ¼ 1=1 by weight was prepared.

2.2. Methods of measurements

Dynamical mechanical measurements were performed

with a Rheometrics System IV instrument in the parallel

plate mode with a plate diameter of 1 cm. Storage and

loss shear moduli (G0 and G00, respectively) and loss

tangent, tgd ¼ G00=G0 were determined. The polymer

powder was placed between the plates of the rheometer

and heated until an isotropic melt was achieved (the

sample is transparent and G00 is greater than G0). Two

types of measurements in the linear viscoelastic region

were carried out:

1. The temperature dependence of the loss and storage

moduli at the constant frequency f ¼ 1 Hz in thetemperature range from �150–200 to 25–30 �C. Therate of the decrease and subsequent increase in tem-

perature was 2 �C=min.2. The moduli G0 and G00 were measured at angular rates

ranging from x ¼ 10�1 to 102 rad/s at temperaturesfrom 180 to 40 �C. Using frequency-temperature su-perposition of data [17], the superimposed curves of

the moduli G0p ¼ G0bT and G00

p ¼ G00bT against re-

duced angular rate xaT were obtained for some poly-mers. The horizontal and vertical shift factors (aT andbT , respectively) were determined as well.

Thermal properties were measured using a Perkin–

Elmer differential scanning calorimeter DSC-2. To avoid

any preparation or thermal history, samples were first

heated until the isotropic state was reached and experi-

mental data were collected on cooling and subsequent

heating at a rate of 10 �C=min.

The texture of LC phases was determined with a po-

larizing optical microscope (Photomicroskop III, Zeiss-

Opton, crossed polarizers) equipped with a Mettler FP5/

FP52 heating stage. The heating/cooling rate varied

between 10 and 3 �C=min.

2334 Y.A. Demchenko et al. / European Polymer Journal 38 (2002) 2333–2341

Wide-angle X-ray diffractograms were taken on an

HZG4A diffractometer (Freiberger Pr€aazisionsmechanik,Germany) using Ni-filtered CuKa radiation. For high-temperature measurements a heating chamber with ther-

mal stability of 0.5 �C was attached.

3. Results and discussion

3.1. Thermal and dynamic mechanical behavior of FnK5

polymers and copolymer

Fig. 1 shows an example of measured DSC thermo-

grams for the F5K5 polymer. For the polymer crystal-

lized from methanol (first run), two transitions at �140and 143 �C were observed. On cooling the isotropic meltpolymer forms an ordered structure at �133 �C ac-

companied by an exothermic peak (DH ¼ �4:6 J/g) onDSC trace; this structure freezes into the ordered glassy

state below a glass transition temperature of Tg � 57 �C.WAXS diffraction patterns of polymer F5K5 are pre-

sented in Fig. 2. A sample was measured during cooling

and subsequent heating, starting from the melt. The

diffraction patterns at 150 and 130 �C correspond to theamorphous structure. The ordered texture formation

starts between 130 and 100 �C. The structure is indicatedby four crystalline reflections. Their positions corre-

spond to the spacings 6.25, 5.19, 4.28 and 3.62 �AA. Thistexture is preserved during cooling and also subsequent

heating. The degree of crystallinity is about 0.21 on

cooling and heating up to 70 �C; for pattern measured at120 �C the crystallinity is about 0.25. On subsequent

heating, the polymer exhibits a more complex DSC be-

havior. The glass transition Tg occurs at almost the sametemperature (Fig. 1, Table 1). Above Tg as mobility inthe system increases, a small minimum (endothermic

transition) recorded around 90 �C is probably associatedwith the increase in the amount of crystalline phase of

same texture (see Fig. 2), which melts at approximately

107 �C. At higher temperatures a new ordered texture isformed, which finally melts during two transitions into

an isotropic state above 140 �C.The dynamic mechanical measurements of polymer

F5K5 are shown in Fig. 3a. Temperature dependences of

the storage, G0 and loss, G00 moduli and loss tangent, tgd,measured at frequency f ¼ 1 Hz confirm the DSC and

X-ray data and the polarizing microscope observations.

At the highest temperatures (T > 150 �C), liquid-likebehavior (G00 > G0) is found. As the cooling curves of G00

and G0 exhibit one sharp minimum (tgd shows the

maximum) at approximately 130 �C, we believe that thecrystallization proceeds first through the nematic meso-

phase formation (nematic structure exhibits lower values

of G0 and G00 than isotropic one [12]). At the lowest

temperatures the glassy state is entered, and the crys-

talline phase is frozen-in below 50 �C. At these tem-peratures the high values of storage modulus G0, typicalFig. 1. DSC Thermograms for polymer F5K5.

Fig. 2. X-ray scattering diffractograms for polymer F5K5 ob-

tained on cooling and subsequent heating.

Y.A. Demchenko et al. / European Polymer Journal 38 (2002) 2333–2341 2335

of the glassy state, are attained (G0 > 108 Pa). The cor-responding tgd maximum at Tg is found at �57 �C. As

expected from DSC traces, mechanical behavior of

polymer during the subsequent heating run is more

Table 1

DSC transitions for measured polyesters

Polyester Composition Tg (�C) Tm1 (�C) Tm2 (�C) Tm3 (�C) Tm4 (�C) Tm5 (�C)

1 F5K5 C 57.5 133.4

H 58.6 88.8 107.2 135.3 140.1

2 F6K5 C 55.1 173.7 226.8

H 55.8 215.7 232.0

3 F5K5=F6K5(1:1) C 50.5 198.0

H 49.8 75.5 152.2 169.2 182.3 197.7

4 F5K(O) C 118.0

H 146.7 155.0

5 F6K(O) C 118.7 170.5

H 135.6 151.2 157.7 164.4 176.4

6 F5K(Cl) C 40.1

H 39.7

7 F6K(Cl) C 40.3

H 41.7

C, H––cooling, heating (second run); Tg––temperature of glass transition; Tm1–Tm5––temperatures of structural transitions above the Tg.

Fig. 3. Dependences of storage G0 and loss G00 moduli and loss tangent tgd on temperature in cooling and heating scans for the in-dicated polymers.


complex. A sharp drop in G0 and G00 found around 60 �Cis associated with the glass transition. However, at

temperatures higher than 70 �C, an increase in G0 is

observed. Apparently, this increase reflects formation of

an additional crystalline structure, detected by DSC and

WAXS (degree of crystallinity increased from 0.21 to

0.25) measurements. A similar sharp increase in the G0

value during structure development was observed earlier

in LC polymers with discotic moieties which organized

into a columnar structure [18] and for LC polyurethanes

with mesogens in the main chain [7]. A second trans-

formation of the ordered structure, associated with the

increase in G0 and G00 can be seen at approximately 120

�C. The final rapid softening (sharp decrease in G0 and

G00) at T � 140 �C can be related to melting of the

mesophase and at T > 140 �C an isotropic state of meltis reached. In this state the mechanical behavior of the

melt is typical of that of amorphous polymers (G00 PG0)

and the lowest values of dynamic functions are ob-

served.

As can be seen from Table 1, an even number of CH2

groups in the spacer of the Fn component practically

does not affect the glass transition temperatures Tg incomparison with odd numbers; as in the previous case of

F5K5 also for polymer F6K5 Tg values roughly inde-pendent of cooling or heating scan were found. On the

other hand, an even number of CH2 groups in spacer

shifts the melting temperatures to considerably higher

values; such results are usually found in literature

[1,4,12]. We believe that such differences are due to

better rearrangement of macromolecules with an even

number of groups in aliphatic chains. On cooling from

isotropic melt a nematic texture is formed at �230 �C; asexpected, this transition is accompanied by a decrease in

G0 and G00 and the nematic structure exhibits lower

values of mechanical functions than the isotropic one

(Fig. 3b). The next transition into the crystalline texture

takes place at �175 �C; the transformation is accom-panied, as previously, by a rapid increase in dynamic

function values. The semicrystalline polymer is in glassy

state below 40 �C. Differences in structure developed at225 and 170 �C on cooling can be observed in polarizingmicroscope (Fig. 4). It can be seen that while at higher

temperature the nematic phase exists, at lower temper-

ature crystallization already started. On heating, the first

transition for polymer F6K5 is observed at �216 �C(Table 1); in this case we do not observe an increase in

the crystalline phase content as in case of polymer F5K5.

After melting of the crystalline phase, the nematic phase

is formed. Finally, at �232 �C the mesophase melts andsample reaches an isotropic state at higher temperatures.

All these transitions can be clearly seen in temperature

dependences of dynamic mechanical functions on heat-

ing (Fig. 3b). As was observed on cooling also in this

case the melt viscosity in the isotropic state is higher

than that in the nematic state.

Copolymer F5K5=F6K5 (1=1 by weight) exhibits

thermal behavior between that of homopolymers (Table

1, Fig. 3c); such behaviour is usual for random copoly-

mers. The Tg values, roughly independent of cooling orheating scan, lie at �50 �C. On cooling, the isotropic/nematic transformation, accompanied by a viscosity

decrease, takes place at �198 �C. The highest values ofG0 > 108 Pa, characteristic of LC glass, are reached at 50�C. On heating, an increase in crystallinity takes place at�60 �C. This increase is associated with the increase inboth moduli as in homopolymer F5K5. The texture melts

into a nematic structure in the temperature range from

150 to �170 �C (DSC trace indicates several small

transitions, Table 1). For temperatures higher than 200

�C, the melt is in the isotropic state; as previously, thenematic/isotropic transition is accompanied by an in-

crease in both moduli values.

3.2. Effect of ether oxygen in spacer and Cl substituents in

mesogen on thermal and mechanical behaviour

Polymers F5K(O) and F6K(O) with ether oxygen

incorporated into the aliphatic sequence of K exhibit

relatively simple thermal properties (Table 1, Fig. 5b

and c). On cooling F5K(O) from the isotropic melt, only

one isotropic/crystal transition at 118 �C was observed.No distinct glass transition was detected on the DSC

trace, which means that the degree of crystallization is

high. As expected, the formation of a highly crystalline

phase is associated with a pronounced increase in both

Fig. 4. Optical micrographs of polymer F6K5 at 170 �C (on the left) and 225 �C (on the right).


moduli (G0 and G00 values increased by more than five

orders of magnitude, Fig. 5b). As in previous cases of

heating of polymer F5K(O), the semicrystalline structure

melts at higher temperature than on cooling. In this case

the DSC shows two transitions at �146 and 155 �C. Themechanical data obtained on heating (Fig. 5b) are in

accord with DSC measurements. As in the previous case

of an aliphatic spacer in K (Table 1, Fig. 3a and b), an

even number of CH2 groups in the F6K(O) polymer

leads to an additional thermal transition in comparison

with F5K(O) on both, cooling and subsequent heat-

ing. On cooling, the isotropic/nematic transition occurs

at �170 �C. The mesophase/crystalline transition takesplace at �119 �C. The heating scan reveals a trans-formation of crystalline phase at 135 �C (probably to

another crystalline modification), its melting to a meso-

phase in the range from 151 to �175 �C (several smalltransitions were found, Table 1) and the final transition

to the isotropic state at �176 �C. Dynamic mechanicaldata correspond to DSC measurements. From data

shown in Table 1 and Fig. 5b and c, we can conclude

that the presence of ether oxygen in the aliphatic se-

quence of K leads generally to a lowering of structure

transition temperatures of polyesters in comparison with

the presence of CH2 group in K.

As can be expected, the simplest thermal behaviour

exhibited by polymers with bulky Cl atoms on benzene

rings of K. On cooling, the F5K(Cl) and F6K(Cl) poly-

mers shows behaviour typical of amorphous polymers

(Table 1, Fig. 6b and c). Only the glass transition, lo-

cated at �40 �C, can be observed on DSC scans for thesepolymers; the cooling and subsequent heating scans are

virtually identical. It is interesting to note that in chlo-

rinated polymers the odd and even number of CH2

groups in F has no effect on Tg. The change in specificheat Dcp found for these transitions changes ranges from0.35 to 0.40 J/g; these values are typical of amorphous

polymers [17].

In accordance with DSC measurements thermo-

rheologically simple mechanical behaviour of FnK(Cl)

polymers (n ¼ 5, 6) was observed and a frequency–temperature superposition could be applied to the dy-

Fig. 5. Dependences of storage G0 and loss G00 moduli and loss tangent tgd on temperature in cooling and heating scans for indicatedpolymers with ether oxygen atoms in the K5 component.


namic mechanical data. In Fig. 7b the superimposed

curves of reduced moduli G0p, G

00p and loss tangent tgdp

are shown for polymer F5K(Cl); similar dependences

were obtained for a sample of F6K(Cl). Corresponding

horizontal aT and vertical bT shift factors for polymerF5K(Cl) are shown in Fig. 8b. The horizontal shift

factor satisfied the Williams–Landel–Ferry (WLF)

equation in the form [17]

log aT ¼ ð1=2:3f0ÞðT � T0Þ=ðf0=af þ T � T0Þ ð1Þ

where f0 is the fractional free volume (f0 ¼ 0:058) and afis the temperature expansion coefficient of the free vol-

ume (af ¼ 9:2 10�4 K�1) at the reference temperature

T0 ¼ 60 �C for polymer F5K(Cl). These values are withinthe range found for amorphous polymers [17].

For comparison, superimposed curves of mechanical

functions obtained on cooling of neat polymer F5K5 are

shown in Figs. 7a and 8a. In this case the shift factors

aT and bT were obtained from the requirement that

values of tgd, G0 and G00 coincide at highest x for various

temperatures. As follows from Figs. 7a and 8a, a more

complex frequency and temperature behaviour was

found and superposition cannot be properly applied due

to structural changes on cooling; this polymer exhibits a

thermorheologically complex behavior. Large deviations

from smooth superimposed curves of reduced moduli

and loss tangent can be seen and complex temperature

dependences for both shift factors were detected (e.g., aTcannot be described by one WLF equation).

Such strong influence of Cl substitution on thermo-

rheological properties may be explained by shielding of

mesogenic groups and aliphatic sequences of compo-

nent K by relatively big Cl atoms in the benzene rings.

In such case no planar arrangement between meso-

genic groups is possible, probably due to high distances

between them. Also the fact that in the case of Cl-sub-

stituted polyesters there is much less differences in

thermal properties of polymers with even and odd

number of CH2 groups in aliphatic spacer of Fn than inunsubstituted polyesters is in accord with this expla-

nation.

Fig. 6. Dependences of storage G0 and loss G00 moduli and loss tangent tgd on temperature in cooling and heating scans for theindicated polymers with Cl substituents in the benzene rings.


4. Conclusions

From DSC, X-ray, polarizing microscopy and dy-

namic mechanical results obtained on LC polyesters

which differ in the structure of mesogenic groups as well

as in flexible spacer length and its structure, the fol-

lowing conclusions can be made:

1. In general, a more complex thermal and mechanical

behavior was observed in the second heating run in

comparison with the cooling regime; for instance,

the increase in mechanical functions above the Tg at�75 and �125 �C, associated with increasing degreeof crystallization and transformation of ordered

phases was found for polymer F5K5.

2. The dynamic mechanical and thermal behavior is

strongly dependent on the structural changes in meso-

genic group as well as on the structure of flexible

spacer. In most cases a complex thermorheological

behavior with several structural transitions above

the glass transition was found. Differently organized

structures were observed.

3. In all cases lower values of G0 and G00 moduli in the

isotropic state of polymers in comparison with those

in the nematic state were observed. Formation of

crystalline phase was accompanied by a rapid in-

crease in both moduli.

4. Introduction of Cl atoms into benzene rings of meso-

genic group leads to disappearance of the order in

structure and polymers with these substituents ex-

hibit behaviour typical of amorphous materials.

Acknowledgements

The authors thank the Grant Agency of the Czech

Republic for support (grant no. 203/00/1314) and the

Ministry of Education of the Czech Republic (project

MSM 113200001).

References

[1] Aharoni SM, Edwards SF. Adv Polym Sci 1994;118:1.

[2] Asrar J, Toriumi H, Watanabe J, KrigbaumWR, Cifferi A.

J Polym Sci, Polym Phys Ed 1983;21:1119.

[3] Galli G, Chiellini E, Laus M, Biugnozzi MC, Angeloni

AS, Francescandeli O. Macromol Chem Phys 1994;195:

2247.

Fig. 7. Frequency dependences of superimposed reduced stor-

age G0p and loss G

00p moduli and loss tangent tgd for the indicated

polymers.Fig. 8. Dependence of the horizontal log aT and the verticallog bT shift factors on temperature for the indicated polymers.


[4] Langelaan HC. Rheological studies of thermotropic liquid

crystalline polymers. PhD thesis, TU Delft, November

1995.

[5] Kim SS, Han ChD. Macromolecules 1993;26:6633.

[6] Green DI, Collins TLD, Davies GR, Ward IM. Polymer

1997;38:5355.

[7] Ilavsk�yy M, Bouchal K, Valentov�aa H, Lednick�yy F, SikoraA, Baldrian J. J Macromol Sci––Phys 1998;B37:645.

[8] Tanaka M, Nakaya T. Makromol Chem 1989;190:3067.

[9] Mormann W, Brahm M. Makromol Chem 1989;190:

631.

[10] Damman SB, Buijs JAHM. Macromol Chem Phys 1994;

195:2261.

[11] Salashoor-Kordestani S, Hanna S, Windle AH. Polymer

2000;41:6619.

[12] Kim SO, Kim TK, Chung IJ. Polymer 2000;41:4709.

[13] Romo-Uribe A, Lemmon TJ, Windle AH. J Rheol 1997;

41:1117.

[14] Gao P, Lu SH, Chai CK. Polym Eng Sci 1996;36:2771.

[15] Lin YG, Winter HH. Macromolecules 1991;24:2877.

[16] Razina AG, Sedl�aakov�aa Z, Bouchal K, Ilavsk�yy M. Vysoko-

mol Soedin, in press.

[17] Ferry JD. Viscoelastic properties of polymers. 3rd ed. New

York: Wiley; 1980.

[18] Hsu T-Ch, Huser B, Pakula T, Spiess H, Stamm M.

Makromol Chem 1990;191:1597.


dynamic mechanical and thermal behavior of thermotropic polyesters based on...

Documents