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Synthesis and properties of novel polyurethanes containing the mesogenic moiety of α-methylstilbene derivatives
European Polymer Journal 37 (2001) 303±313
Synthesis and properties of novel polyurethanes containing the mesogenic moiety of a-methylstilbene derivatives Chien-Kung Lin, Jen-Feng Kuo *, Chuh-Yung Chen Department of Chemical Engineering, National Chen Kung University, Tainan, 70101 Taiwan, ROC Received 9 August 1999; received in revised form 10 April 2000; accepted 12 May 2000
Abstract 4,40 -Bis(x-hydroxyalkoxy)-a-methylstilbene with various spacer lengths (HMSn, where ``n'' is the carbon number of the hydroxyalkoxy group) and two series of novel thermotropic liquid crystalline polyurethanes containing the mesogenic core of 4,40 -bis(x-hydroxyalkoxy)-a-methylstilbene were synthesized. One series was obtained by polyaddition reaction of 2,4-toluene diisocyanate (2,4-TDI) with HMSn, denoted as TDIn, and the other was obtained by the reaction of 4,40 -diphenylmethane diisocyanate (MDI) with HMSn, denoted as MDIn. The polyurethanes were investigated by IR spectroscopy, DSC, wide angle X-ray diraction, polarizing microscope and thermogravimetric analysis. All the polyurethanes except for TDI11 exhibited enantiotropic smectic phases which are not similar to those of corresponding HMSn. The transition temperatures and the temperature ranges of mesophases changed with the length of alkoxy spacer and the kind of diisocyanate. The thermal degradation behaviours under nitrogen atmosphere depend on the spacer length but not much on the diisocyanate moiety. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Thermotropic liquid crystal; Polyurethane; a-Methylstilbene derivatives
1. Introduction Polyurethanes are industrially important polymers with various structures and wide applications [1]. Since 1981, when the ®rst liquid crystalline polyurethanes (LCPUs) based on mesogenic diisocyanates and commercial diols were synthesized by Iimura [2], the induction of mesomorphic behaviour to polyurethanes have attracted much more attention [2±21]. Two strategies were used to obtain thermotropic polyurethanes: The ®rst method involved polyaddition reactions based on mesogenic diisocyanates with commercially available diols. The other was based on mesogenic diols with
*
Corresponding author. Tel.: +886-6-275-7575, ext.: 62638; fax: +886-6-234-4496. E-mail address: [email protected] (J.-F. Kuo).
commercially available diisocyanates and attracted much more interest. 4,40 -Dihydroxy-a-methylstilbene (HMS) was introduced to polymers by some researchers. HMS could depress the melting point and improve the solubility of the polymer because the a-methyl group presumably increased the cross-dimension of the mesogen [23±28]. Interestingly, 4,40 -bis(x-hydroxyalkoxy)-a-methylstilbene (HMSn) (the mesogen HMS with spacer) exihibited various mesomorphic behaviours [29]. The purpose of this article is to prepare liquid crystalline 4,40 -bis(xhydroxyalkoxy)-a-methylstilbene (HMSn) and then synthesize a new series of polyurethanes by the reaction of diisocyanate monomers, i.e., 2,4-toluene diisocyanate (2,4-TDI) and 4,40 -diphenylmethane diisocyanate (MDI) with HMSn. Two series of polyurethanes TDIn and MDIn were prepared. The microstructure, thermal properties, mesomorphic behaviours and thermal degradation properties were investigated for these polyurethanes.
0014-3057/01/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 0 ) 0 0 1 3 2 - 4
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2.3. Synthesis of 4,40 -bis(x-hydroxyalkoxy)-a-methylstilbene (HMSn: n 3; 6; 11)
2. Experimental 2.1. Materials 2,4-Toluene diisocyanate (2,4-TDI, TCI, >99% purity,) and 4,40 -diphenylmethane diisocyanate (MDI, TCI, >98% purity) were used as received. N ,N-Dimethylacetamide (DMAc, Tedia) was mixed with CaH2 for one day at room temperature and then vacuum distilled at 50°C. The middle portion was kept and stored over 4 molecular sieve before used. Mesogenic diols (HMSn) A were dehydrated at 50°C under vacuum for 12 h prior to use. Stannous octoate (Sigma, 95% purity) was used as catalyst.
2.2. Synthesis of 4,40 -dihydroxy-a-methylstilbene HMS was synthesized by the procedure reported in Refs. [23,25,29]. 0.3 mol chloroacetone and 0.6 mol phenol were charged into the reactor under vigorous stirring and was kept at ÿ10°C throughout the experiment. The concentrated H2 SO4 (0.3 mol) was added dropwise into the above solution. The reaction ended when the viscous slurry could no longer be stirred. The crude product was recrystallized from ethanol/water, extracted with benzene and washed with petroleum ether.
HMSn was prepared by the etheri®cation of HMS with x-halogenated alkanols. The method was also described in Refs. [5,16,21]. A typical one is as follows: sodium hydroxide (6.64 g, 0.166 mol), 4,40 -dihydroxy-amethylstilbene (9.40 g, 0.0416 mol) and 200 ml absolute ethanol were placed in a 500 ml ¯ask equipped with a stirrer, re¯ux condenser and dropping funnel. x-bromoor chloro-1-alkanol (0.17 mol) dissolved in 50 ml absolute ethanol was added into the reactor dropwise. The reaction mixture was re¯uxed for 24 h and then poured into cold water. The resulting precipitate was washed three times with distilled water and recrystallized from dioxane. The characterization of the structures and detailed studies of the mesophases of the products (HMSn) were described in the previous paper [29]. 2.4. Polymer synthesis Scheme 1 shows the synthetic route of mesogen (HMS), mesogenic diols (HMSn) and liquid crystalline polyurethanes. The polyaddition reaction of these polyurethanes by equimolar amounts of diisocyanates and 4,40 -bis(x-hydroxyalkoxy)-a-methylstilbene is given. 7.50 mmol mesogenic diol, 20 ml DMAc with two drops of catalyst were added and then kept at 55°C. Puri®ed nitrogen was made to ¯ow slowly throughout the reac-
Scheme 1.
C.-K. Lin et al. / European Polymer Journal 37 (2001) 303±313
tion. Subsequently, diisocyanates (7.50 mmol) and 15 ml DMAc were added slowly within 30 min and then kept at 70°C for another 12 h. Finally, the temperature was raised and kept at 90°C for an additional 6 h. The product was poured into 1000 ml distilled water to arrest the reaction. The precipitated product was ®ltered, washed with 200 ml methanol, and dried at 50°C in vacuo for 24 h (yield 90±96%). 2.5. Characterization A TA instrument 2910 dierential scanning calorimeter and RCS cooling system were used to determine the thermal transitions with a 30 ml/min ¯ow of dry nitrogen as a purge gas and calibrated by the Indium standard. All the measurements were carried out at a heating rate of 10°C/min. The weight of the sample used was about 6±7 mg. The maximum point of the thermograms was taken as the transition temperature. Thermogravimetric analyses were performed on a Perkin±Elmer 7 TGA at a heating rate of 20°C/min up to 600°C under nitrogen atmosphere. 1 H-NMR analysis of the structure of the polymers and their intermediates was performed by a Bruker AM 200 spectrometer operated at 50 MHz. Deuterated dimethylsulphoxide (DMSO-d6 ) was used as a locked solvent and tetramethylsilane (TMS) as an internal standard. Wide-angle X-ray scattering measurements were carried out by use of Inel MPX with CuKa radiation. Fourier transform infrared spectra were recorded at room temperature (25°C) by a Bio-red FTS-40A spectrometer with a resolution of 2 cmÿ1 (64 scans). The specimens were in the form of KBr pellet. Olympus BH-2 polarizing optical microscope equipped with a Linkam hot stage was used to study liquid crystal textures. Fig. 1 illustrates the 1 H-NMR spectra of the mesogenic diol of 4,40 -bis(x-hydroxyalkoxy)-a-methylstilbene, HMS11, and LCPUs of MDI11, TDI11 which were obtained in a deuterated dimethylsulphoxide (DMSO-d6 ) solution at 80°C. 3. Results and discussion The general synthetic route for the preparation of the precursor, HMS, the chain extender, HMSn, and polyurethanes based on TDI and HMSn as well as polyurethanes based on MDI and HMSn are shown in Scheme 1. The series of TDI-based polyurethanes and MDI-based polyurethanes, are hereafter referred to as TDIn and MDIn, respectively (where n is the length of alkoxy group of HMSn). The mesomorphism and stability of HMSn were obtained in the previous paper [29]. The length of alkoxy group aected the mesomorphism and stability of mesophases as well as the transition temperature. HMS3
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showed a monotropic trimorphism in cooling including the nematic phase, the smectic C phase and the crystalline G or H phase. However, HMS6 presented an enantiotropic trimorphism, and the variant of mesomorphism is the same as HMS3. On the other hand, HMS11 exhibited an enantiotropic crystalline G or H phase. 3.1. FTIR analysis Hydrogen bonding behaviour of liquid crystalline polyurethanes by FTIR analysis has drawn a great deal of interest [12,16,19±22]. Figs. 2 and 3 show the FTIR spectra of TDIn and MDIn recorded at 25°C, respectively. All the polyurethanes dier in the length of ¯exible chain and urethane moiety. The characteristic absorption bands of FTIR spectra for the LCPUs are substantially similar to one another. However, there is still a subtle dierence in the bands sensitive to the mode of H-bonding and the conformation of polymer chains. Both series of polyurethanes present the characteristic band of C@O stretching vibration in the region of 1650±1750 cmÿ1 depending on the kind of urethane moiety and spacer length. Figs. 4 and 5 show the enlarged absorption band of carbonyl stretching. Among the TDIn, TDI11 shows the split band while TDI6 and TDI3 show a band with shoulder, whereas, for the MDIn, all the homologues exhibit the split band. The carbonyl absorption band may be split into three components, including hydrogen-bonded carbonyl stretching and free carbonyl stretching. The deconvolution technique based on a Gaussian function was performed for the absorption band. As shown in Figs. 4 and 5, three components are obtained, i.e., at 1700, 1709±1710 and 1734±1737 cmÿ1 . The ®rst two components are attributed to the order and disorder hydrogen-bonded carbonyl stretchings, respectively. The last component is due to ``free'' carbonyl group stretching. Table 1 summarizes the frequency (m), the half-band width (w1=2 ), and the area and area ratio of the three components. The change of peak frequency is insigni®cant in each component with both the factors of spacer length and the urethane moiety structure. However, the area change of each component depends signi®cantly on both the factors. As shown in Fig. 4 and Table 1, it is clear that the fraction of order H-bonded C@O increases with the increase of the spacer length, whereas that of free C@O decreases in TDIn. The MDI series shows a similar result as shown in Fig. 5 and Table 1. The spacer lowers the steric hindrance of H-bond interaction between C@O and N±H group and regularizes the conformation of polyurethane chain. This results in the increase of the order H-bonded urethane domain. The region of N±H stretching vibration suggests the information about the hydrogen-bonded NH groups and free NH groups. For the hydrogen-bonded NH
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Fig. 1. 1 H-NMR spectra of (a) HMS11, (b) MDI11 and (c) TDI11 which were obtained in d-DMSO solution at 80°C.
groups, the peak frequency of the band is at 3310±3324 cmÿ1 , depending on the kind of diisocyanate and the spacer length. The shoulders at 3390 and 3450 cmÿ1 , slightly changing with the spacer length may be assigned to free NH groups [30,31]. The trends in the change of peaks are similar to that obtained by the carbonyl group
analysis that the peak of H-bonded NH absorption grows with increasing spacer lengths, whereas the free N±H absorption decreased. For H-bonded NH groups, the spacer length with n 11 gives the lower frequency, indicating that longer spacer length provides stronger hydrogen bonding.
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Fig. 2. Infrared spectra of polyurethanes TDIn at 25°C: (a) TDI3, (b) TDI6 and (c) TDI11.
Fig. 3. Infrared spectra of polyurethanes MDIn at 25°C: (a) MDI3, (b) MDI6 and (c) MDI11.
For both TDI11 and MDI11, two absorption bands appear at 2921 and 2852 cmÿ1 , which are assigned to the asymmetric and symmetric stretching vibrations of CH2 , respectively. Snyder et al. [32] indicated that if the polymethylene chains are fully in trans conformation, two bands near 2920 and 2855 cmÿ1 are to be expected. Therefore, the undecyl groups in the two polyurethanes are expected to arrange in the trans conformation. They also show a weak band at 721 cmÿ1 for the rocking vibration of CH2 , indicating that a minimum of four consecutive trans sequence exists [33,34]. 3.2. Thermal behaviour and mesomorphisms of polymers Fig. 6 shows the typical second heating and cooling DSC traces of TDIn at a rate of 10°C/min. Thermal properties associated with the DSC analysis are summarized in Table 2. Thermal transition behaviours remarkably vary with the length of the spacer. TDI3 with a glass transition temperature (Tg ) at 83°C and an iso-
Fig. 4. Infrared spectra in the C@O stretching region of TDIn: (a) TDI3, (b) TDI6 and (c) TDI11.
tropization temperature (Ti ) at 125°C is later evidenced by the WAXS analysis. While TDI6 presents a smaller and broad endothermic peak as well as a relatively larger endothermic peak on heating, the same phenomenon takes place on cooling. The smaller endotherm occurs at 65°C, and the larger peak is at 133°C. The enthalpy change of the smaller endothermic peak is 1.2 J/g, and that of the larger endothermic peak is 12.8 J/g. However, TDI11 exhibits completely dierent thermal transition behaviours. On heating, it shows four endothermic peaks at 91°C, 101°C, 110°C, and 122°C. The enthalpy changes of them are 7.1, 4.2, 1.7 and 22.6 J/g, respectively. However, there are only two exothermic peaks at 115°C and 72°C on cooling. Their enthalpy changes are 23.0 and 17.5 J/g, respectively.
30.05 26.38 21.08 26.00 22.21 18.60 21.93 21.61 22.53 26.26 27.73 26.68
Ad =At (%)
4.33 4.98 4.21 6.02 3.78 3.94 15.7 13.5 10.4 15.2 10.2 9.6 1738 1735 1737 1735 1734 1734 3.16 4.08 4.50 6.08 4.72 5.65 22.1 20.7 20.5 25.2 17.4 12.8 1710 1710 1711 1709 1709 1708 6.92 9.82 11.26 11.05 8.52 11.59 28.5 23.9 23.8 28.4 23.0 26.6
Ad W1=2 (cmÿ1 ) m (cmÿ1 ) W1=2 (cmÿ1 )
Ao m (cmÿ1 )
1699 1700 1701 1699 1700 1701 TDI3 TDI6 TDI11 MDI3 MDI6 MDI11
m (cmÿ1 )
W1=2 (cmÿ1 )
Af
48.02 52.01 56.39 47.74 50.06 54.72
Ao =At (%) Free Disordered Ordered
Hydrogen bonded Polymer
WAXS spectra of both series of TDIn and MDIn as a function of temperature were obtained by ®rst heating the sample to isotropization and then cooling it to the desired temperature. As shown in Fig. 7, when TDI3 was cooled to 100°C, it presents a typical amorphous diraction peak in the region of long-range positional order and a distinct sharp peak at the d-spacing of 27.58 which is shorter than the length of repeating unit of A, HMS3 and TDI moiety in the fully extended chain indicating the existence of the conformation (29.63 A), smectic C phase. When TDI3 was further cooled to 25°C, it still remains a diraction spectrum as that at 100°C. Examining the DSC and WAXS spectrum of TDI3, there is no Tm within it. So, the mesophase of
Table 1 Curve-®tting results in the C@O stretching region of polyurethanesa
Fig. 5. Infrared spectra in the C@O stretching region of MDIn: (a) MDI3, (b) MDI6 and (c) MDI11.
a Ao : area (arbitrary units) of C@O sub-band attributed to ordered domains, Ad : area of C@O sub-band attributed to disordered domains, Af : area of C@O sub-band attributed to non H-bonded carbonyl groups and At : total area of carbonyl band Ao Ad Af W1=2 width at half-height of the Gaussian curve.
C.-K. Lin et al. / European Polymer Journal 37 (2001) 303±313 Af =A (%)
308
DHs3±i 22.6 17.5 DHs2±s3 1.7 ± b
a
Transition temperatures were determined by DSC measurement with a heating and cooling rate of 10°C/min. k: crystal, s1: smectic G or H phase, s2: smectic I phase, s3: smectic C phase, i: isotropic phase. c Transition temperature, Ts3±k . d DH DHs±3k . e TDI3 with a Tg at 83°C and no Tm exists.
DHs1±s2 4.2 ± DHk±s1 7.1 23.0e Ts3±i 122 115 Ts2±s3 110 ± Tk±s1 c 91 72d TDI11 Heating Cooling
Ts1±s2 101 ±
1.0 0.7 85 60 6.8 12.5 113 126 7.5 12.8 125 133 83 65
1.7 1.2
309
TDI3 TDI6
Ts3±k (°C) Ti±s3 (°C) DHm (J/g)
Ti (°C)
DHi (J/g)
DHi±s3 (J/g)
Phase transition behaviour in second cooling
Tm (°C)
Polymer
Table 2 Thermal properties of thermotropic polyurethanesa
TDI3 is between 83°C (Tg ) and 125°C (Ti ). Therefore, TDI3 exhibits an enantiotropic smectic C phase. The polarizing micrograph of TDI3 at 100°C on cooling is shown in Fig. 8. At 120°C, TDI6 shows a similar X-ray diraction pattern as that of TDI3 indicating a smectic phase. However, the X-ray spectrum of 25°C presents peaks in the wide angle region indicating the crystalline phase. It shorter than the has the lamella spacing of 28.11 A length of repeating unit of TDI6 in the fully extended Hence, TDI6 shows an chain conformation (36.14 A). enantiotropic smectic C phase between 65 (Tm ) and 133°C (Ti ). The WAXS spectra of TDI11 were obtained when it was cooled to 105°C and further to 25°C. Then, TDI11 was heated to 98°C, 108°C and 120°C as referred in the DSC thermograms. The spectra obtained at 98°C and 108°C on heating are substantially similar to one another. They include one distinct re¯ection peak at the d as well as the peaks at 4.5, 4.2, and spacing of 29.9 A, The d-spacing of 29.9 A is the length of the layer 3.9 A. structure which is shorter than the length of repeating unit of TDI11, whereas peaks of shorter d-spacing indicate long-range positional order. The spectrum at 98°C has more distinct re¯ection peaks in the wide angle region compared to that at 108°C. The DSC analysis mentioned above indicates that the smectic phase at 98°C is dierent from that at 108°C. Hence, the smectic phase shown at 98°C has a more orderly-structure than that at 108°C. It may be the crystalline G or H phase. Whereas, at 120°C, the spectrum includes a distinct and with the broader peak with the d-spacing of 30.02 A broader peak in the long-range order region. Hence, the mesophase is assumed to be the smectic C phase. Referring to the above characteristic WAXS spectra and the variants of trimorphism suggested by Demus [35], it is assumed that TDI11 may exhibit the trimorphism on
Phase transition behaviour in second heating
Fig. 6. The typical 10°C/min second heating: (a) TDI3, (b) TDI6 and (c) TDI11; cooling: (a0 ) TDI3 (b0 ) TDI6 and (c0 ) TDI11 DSC thermograms.
b
DHs3±k (J/g)
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Fig. 7. The X-ray diraction spectra of polyurethanes, TDI11 at (a) 105°C, (b) 25°C, (c) 98°C, (d) 108°C and (e) 120°C; TDI6 at (f) 120°C and (g) 25°C; TDI3 at (h) 100°C and (i) 25°C.
heating as follows: K 91°C Cry; G or H 101°C SmI 110°C SmC 122°C I (based on heating). The spectrum obtained at 105°C on cooling is similar to that at 120°C on heating; hence, it shows the smectic C phase. In conclusion, the spacer length of hydroxyalkoxy groups in¯uences the hydrogen-bonding eects among the urethane moiety, the structure and stability of mesophases. The shorter spacers of TDI3 and TDI6 show a smaller fraction of ordered H-bonding, and a stable single mesophase in the temperature range 42±68°C. However, TDI11 with the longest spacer has a larger fraction of ordered H-bonding and exhibits trimorphism including the cry G (or H) phase, the smectic I and smectic C phases on heating. Their temperature ranges are not beyond 12°C indicating that the mesophase is less stable. It is interesting to note that as compared with the mesomorphism of HMSn, the polyurethanes exhibit the mesomorphisms dierent from that of the corresponding small molecular liquid crystal HMSn. TDI3 shows an enantiotropic monomorphism of smectic C phase, but HMS3 showed a monotropic trimorphism on cooling. TDI6 presents an enantiotropic monomor-
Fig. 8. Polarizing optical micrographs on cooling: (a) TDI3 at 100°C and (b) MDI3 at 165°C.
phism of smectic C phase, but HMS6 showed an enantiotropic trimorphism. Whereas TDI11 exhibits a trimorphism on heating, it exhibits a monomorphism of smectic C phase on cooling. On the other hand, HMS11 showed simply an enantiotropic monomorphism of smectic G or H phase. 3.3. Thermal behaviour and mesomorphisms of MDIn polymers Fig. 9 shows a typical heating and cooling DSC traces of MDIn. The transition temperatures and thermodynamic properties associated with the DSC analysis are summarized in Table 3. MDI3 exhibits a distinct glass transition and two broad endothermic peaks at 91°C, 179°C and 201°C on heating. The enthalpy changes of two peaks are 6.2 and 13.4 J/g, respectively. However, it presents an exothermic peak at 168°C with a shoulder at 159°C on cooling. The total enthalpy change is 16.6 J/g. Therefore, if there existed any mesophase, it is not stable because it has a narrow temperature range. Whereas MDI6 and MDI11 present a broad endothermic peak with shoulder on heating, which has the overall enthalpy change of 28.9 and 26.0 J/g, respectively. On
159d 141c 133c 168c 150d 154d Transition temperatures were determined by DSC measurement with a heating and cooling rate of 10°C/min. k: crystal, lc: smectic C phase and i: isotropic phase. c Peak maximum. d Shoulder. e Overall enthalpy. b
a
13.4 ± ± 201 168d 154d 6.2 28.9e 26.0e 179 163c 143c 91 ± ±
Ti (°C) DHm (J/g) Tm (°C)
311
16.6e 27.7e 31.0e
Tlc±k (°C) Ti±lc b (°C) Tg (°C)
DHi (J/g)
Phase transition behaviour on second cooling
MDI3 MDI6 MDI11
The decomposition behaviour of TDIn and MDIn was studied by TGA at a heating rate 20°C/min under nitrogen atmosphere as shown in Figs. 11 and 12, respectively. The decomposition temperatures at 5%
Polymer
3.4. Thermal degradation behaviour
Table 3 Thermal properties of thermotropic polyurethanesa
cooling, it shows two exothermic peaks, not well resolved. Therefore, the mesophases are also not stable if they exist. From the DSC thermograms, the longer the spacer length, the lower is the transition temperature. The WAXS spectra as functions of temperature of MDI series are shown in Fig. 10. MDIn present the spectra essentially similar to one another. For MDI11, the spectrum shows a typical crystalline re¯ection patterns at room temperature. On cooling, at 145°C, the spectra present a broad peak in the long-range order region and a distinct peak corresponding to a d-spacing MDI3 exhibits a stronger layer structure, but of 33.7 A. MDI6 shows a weak one than the others. All the dspacings of layer structures are smaller than the length of the repeating unit of the corresponding polymers. Therefore, all the mesophases are assumed to be in the smectic C phase. Referring to the above DSC spectra, it may be assumed that MDI3 presents the smectic C phase between 168°C and 159°C on cooling. MDI6 and MDI11 present the smectic C phase between 150°C and 141°C and between 154°C and 133°C on cooling, respectively. The mesophase is not stable because the temperature breadth is quite narrow. Obviously, unlike the TDIn and HMSn, the MDIn can form only the enantiotropic smectic C phase, which is not stable due to the narrow temperature range of mesophase obtained. Consequently, the diisocyanate unit in the polymer chain backbone remarkably in¯uences the mesomorphism and thermal properties of these liquid crystalline polymers.
Phase transition behaviour on second heating
Fig. 9. The typical 10°C/min second heating: (a) MDI3, (b) MDI6 and (c) MDI11; cooling (a0 ) MDI3, (b0 ) MDI6 and (c0 ) MDI11 DSC thermograms.
DHi±k (J/g)
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Fig. 12. TGA spectra of polyurethanes MDIn at a heating rate of 20°C/min: (a) MDI3, (b) MDI6 and (c) MDI11.
in the polymers. Therefore, the diisocyanate unit does not nearly in¯uence the thermal decomposition behaviour but the spacer length does. However, MDIn show a higher Td than that of TDIn. 4. Conclusions
Fig. 10. The X-ray diraction spectra of polyurethanes, MDI11 at (a) 145°C and (b) 25°C; MDI6 at (c) 143°C and (d) 25°C; MDI3 at (e) 165°C, (f) 25°C and (g) 190°C.
weight losts (Td ) for MDIn is 307±319°C and for TDIn 289±293°C. TDI3 and MDI3 exhibit a higher Td than the others do. The spacer length signi®cantly in¯uences the decomposition behaviours. Polymers with n 3 presents the drastic decomposition at the ®rst stage. The weight loss of polyurethanes at the ®rst stage decreases with increasing spacer lengths. It is similar to our previous paper [21] that the trend is consistent with the weight fraction of the diisocyanate monomers contained
Fig. 11. TGA spectra of polyurethanes TDIn at a heating rate of 20°C/min: (a) TDI3, (b) TDI6 and (c) TDI11.
TDI-and MDI-based polyurethanes containing the mesogenic core of HMSn with various alkoxy length are all liquid crystalline polymers. The mesomorphisms obtained are not the same as that of the corresponding HMSn. MDIn, TDI3 and TDI6 simply exhibit enantiotropic monomorphism, whereas TDI11 showed a trimorphism on heating and a monomorphism on cooling. The length of alkoxy spacer remarkably in¯uences the thermal decomposition behaviour under nitrogen atmosphere but the diisocyanate unit does not. References [1] Hepburn C. Polyurethane elastomers. 2nd ed. London: Elsevier, 1992. [2] Iimura K, Koide N, Tanabe H, Takeda M. Makromol Chem 1981;182:2569. [3] Tanaka M, Nakaya T. Makromol Chem 1986;187:2345. [4] Tanaka M, Nakaya T. J Macromol Sci-Chem 1989; A26(12):1655. [5] Stenhouse MPJ, Valles EM, Kantor SW, MacKnight WJ. Macromolecules 1989;22:1467. [6] Papadimitrakopoulos F, Kantor SW, MacKnight WJ. Polym Preprints 1990;31:486. [7] Mormann W, Baharifar A. Polym Bull 1990;24:413. [8] Mormann W, Brahm M. Macromolecules 1991;24:1096. [9] Mormann W, Benadda S. Polym Preprints 1992;33:1109. [10] Sato M, Ito T, Kobayashi T, Mukaida KI. Eur Polym J 1992;28:1345. [11] Papadimitrakopoulos F, Hsu SL, MacKnight WJ. Macromolecules 1992;25:4671. [12] Papadimitrakopoulos F, Sawa E, MacKnight WJ. Macromolecules 1992;25:4682.
C.-K. Lin et al. / European Polymer Journal 37 (2001) 303±313 [13] Mormann W, Brahm M. Polymer 1993;34:187. [14] Cidade MT, Bordado JCM, Figueirinhas J, Martins AF. Liq Cryst 1993;13:295. [15] Sugiyama K, Shiraishi K, Kato K. Polym J 1993;25:103. [16] Lee JB, Kato T, Yoshida T, Uryu T. Macromolecules 1993;26:4989. [17] Tang JC, Chang TC. Eur Polym J 1994;30:1059. [18] Lee JB, Kato T, Uryu T. Polym J 1995;27:664. [19] Lee DJ, Lee JB, Koide N, Akiyama E, Uryu T. Macromolecules 1998;31:975. [20] Yen FS, Lin LL, Hong JL. Macromolecules 1999;32:3068. [21] Lin CK, Kuo JF, Chen CY. Eur Polym J 2000;36:1183. [22] Pollack SK, Shen DY, Hsu SL, Wang Q, Stidham HD. Macromolecules 1989;22:551. [23] Roviello A, Sirigu A. Makromol Chem 1980;181:1799. [24] Roviello A, Sirigu A. Makromol Chem 1982;183:409. [25] Percec V, Shaer TD, Nava H. J Polym Sci Polym Lett Ed 1984;2:637.
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[26] Percec V, Asami K, Tomazos D. Mol Crysts Liq Cryst 1991;205:67. [27] Cebe P, Carbeck J, Schreuder-libson H, Bluhm A, Yeomans W. Polym Preprints 1992;33:331. [28] Cheng YY, Cebe P. Macromolecules 1994;87:5440. [29] Wan CH, Kuo JF, Chen CY. Liq Cryst 2000;27:523. [30] Seymour RW, Cooper SL. Macromolecules 1973;6:48. [31] Christenson CP, Harthcock MA, Meadows MD, Spell HL, Howard WL, Creswick MW, Guerra RE, Turner RB. J Polym Sci Polym Phys Ed 1986;24:1401. [32] Snyder RG, Hsu SL, Krimm S. Spectrochim Acta 1978; 34(A):395. [33] Schachtschneider JH, Snyder RG. Spectrochim Acta 1963; 19:117. [34] Snyder RG. J Chem Phys 1967;47:1316. [35] Demus D, Diele S, Grande S, Sackmann H. In: Brown GH, editor. Advances in liquid crystals. vol. 6. New York: Academic Press, 1983. p. 2±20.
Synthesis and properties of novel polyurethanes containing the mesogenic moiety of a-methylstilbene derivatives Chien-Kung Lin, Jen-Feng Kuo *, Chuh-Yung Chen Department of Chemical Engineering, National Chen Kung University, Tainan, 70101 Taiwan, ROC Received 9 August 1999; received in revised form 10 April 2000; accepted 12 May 2000
Abstract 4,40 -Bis(x-hydroxyalkoxy)-a-methylstilbene with various spacer lengths (HMSn, where ``n'' is the carbon number of the hydroxyalkoxy group) and two series of novel thermotropic liquid crystalline polyurethanes containing the mesogenic core of 4,40 -bis(x-hydroxyalkoxy)-a-methylstilbene were synthesized. One series was obtained by polyaddition reaction of 2,4-toluene diisocyanate (2,4-TDI) with HMSn, denoted as TDIn, and the other was obtained by the reaction of 4,40 -diphenylmethane diisocyanate (MDI) with HMSn, denoted as MDIn. The polyurethanes were investigated by IR spectroscopy, DSC, wide angle X-ray diraction, polarizing microscope and thermogravimetric analysis. All the polyurethanes except for TDI11 exhibited enantiotropic smectic phases which are not similar to those of corresponding HMSn. The transition temperatures and the temperature ranges of mesophases changed with the length of alkoxy spacer and the kind of diisocyanate. The thermal degradation behaviours under nitrogen atmosphere depend on the spacer length but not much on the diisocyanate moiety. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Thermotropic liquid crystal; Polyurethane; a-Methylstilbene derivatives
1. Introduction Polyurethanes are industrially important polymers with various structures and wide applications [1]. Since 1981, when the ®rst liquid crystalline polyurethanes (LCPUs) based on mesogenic diisocyanates and commercial diols were synthesized by Iimura [2], the induction of mesomorphic behaviour to polyurethanes have attracted much more attention [2±21]. Two strategies were used to obtain thermotropic polyurethanes: The ®rst method involved polyaddition reactions based on mesogenic diisocyanates with commercially available diols. The other was based on mesogenic diols with
*
Corresponding author. Tel.: +886-6-275-7575, ext.: 62638; fax: +886-6-234-4496. E-mail address: [email protected] (J.-F. Kuo).
commercially available diisocyanates and attracted much more interest. 4,40 -Dihydroxy-a-methylstilbene (HMS) was introduced to polymers by some researchers. HMS could depress the melting point and improve the solubility of the polymer because the a-methyl group presumably increased the cross-dimension of the mesogen [23±28]. Interestingly, 4,40 -bis(x-hydroxyalkoxy)-a-methylstilbene (HMSn) (the mesogen HMS with spacer) exihibited various mesomorphic behaviours [29]. The purpose of this article is to prepare liquid crystalline 4,40 -bis(xhydroxyalkoxy)-a-methylstilbene (HMSn) and then synthesize a new series of polyurethanes by the reaction of diisocyanate monomers, i.e., 2,4-toluene diisocyanate (2,4-TDI) and 4,40 -diphenylmethane diisocyanate (MDI) with HMSn. Two series of polyurethanes TDIn and MDIn were prepared. The microstructure, thermal properties, mesomorphic behaviours and thermal degradation properties were investigated for these polyurethanes.
0014-3057/01/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 0 ) 0 0 1 3 2 - 4
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2.3. Synthesis of 4,40 -bis(x-hydroxyalkoxy)-a-methylstilbene (HMSn: n 3; 6; 11)
2. Experimental 2.1. Materials 2,4-Toluene diisocyanate (2,4-TDI, TCI, >99% purity,) and 4,40 -diphenylmethane diisocyanate (MDI, TCI, >98% purity) were used as received. N ,N-Dimethylacetamide (DMAc, Tedia) was mixed with CaH2 for one day at room temperature and then vacuum distilled at 50°C. The middle portion was kept and stored over 4 molecular sieve before used. Mesogenic diols (HMSn) A were dehydrated at 50°C under vacuum for 12 h prior to use. Stannous octoate (Sigma, 95% purity) was used as catalyst.
2.2. Synthesis of 4,40 -dihydroxy-a-methylstilbene HMS was synthesized by the procedure reported in Refs. [23,25,29]. 0.3 mol chloroacetone and 0.6 mol phenol were charged into the reactor under vigorous stirring and was kept at ÿ10°C throughout the experiment. The concentrated H2 SO4 (0.3 mol) was added dropwise into the above solution. The reaction ended when the viscous slurry could no longer be stirred. The crude product was recrystallized from ethanol/water, extracted with benzene and washed with petroleum ether.
HMSn was prepared by the etheri®cation of HMS with x-halogenated alkanols. The method was also described in Refs. [5,16,21]. A typical one is as follows: sodium hydroxide (6.64 g, 0.166 mol), 4,40 -dihydroxy-amethylstilbene (9.40 g, 0.0416 mol) and 200 ml absolute ethanol were placed in a 500 ml ¯ask equipped with a stirrer, re¯ux condenser and dropping funnel. x-bromoor chloro-1-alkanol (0.17 mol) dissolved in 50 ml absolute ethanol was added into the reactor dropwise. The reaction mixture was re¯uxed for 24 h and then poured into cold water. The resulting precipitate was washed three times with distilled water and recrystallized from dioxane. The characterization of the structures and detailed studies of the mesophases of the products (HMSn) were described in the previous paper [29]. 2.4. Polymer synthesis Scheme 1 shows the synthetic route of mesogen (HMS), mesogenic diols (HMSn) and liquid crystalline polyurethanes. The polyaddition reaction of these polyurethanes by equimolar amounts of diisocyanates and 4,40 -bis(x-hydroxyalkoxy)-a-methylstilbene is given. 7.50 mmol mesogenic diol, 20 ml DMAc with two drops of catalyst were added and then kept at 55°C. Puri®ed nitrogen was made to ¯ow slowly throughout the reac-
Scheme 1.
C.-K. Lin et al. / European Polymer Journal 37 (2001) 303±313
tion. Subsequently, diisocyanates (7.50 mmol) and 15 ml DMAc were added slowly within 30 min and then kept at 70°C for another 12 h. Finally, the temperature was raised and kept at 90°C for an additional 6 h. The product was poured into 1000 ml distilled water to arrest the reaction. The precipitated product was ®ltered, washed with 200 ml methanol, and dried at 50°C in vacuo for 24 h (yield 90±96%). 2.5. Characterization A TA instrument 2910 dierential scanning calorimeter and RCS cooling system were used to determine the thermal transitions with a 30 ml/min ¯ow of dry nitrogen as a purge gas and calibrated by the Indium standard. All the measurements were carried out at a heating rate of 10°C/min. The weight of the sample used was about 6±7 mg. The maximum point of the thermograms was taken as the transition temperature. Thermogravimetric analyses were performed on a Perkin±Elmer 7 TGA at a heating rate of 20°C/min up to 600°C under nitrogen atmosphere. 1 H-NMR analysis of the structure of the polymers and their intermediates was performed by a Bruker AM 200 spectrometer operated at 50 MHz. Deuterated dimethylsulphoxide (DMSO-d6 ) was used as a locked solvent and tetramethylsilane (TMS) as an internal standard. Wide-angle X-ray scattering measurements were carried out by use of Inel MPX with CuKa radiation. Fourier transform infrared spectra were recorded at room temperature (25°C) by a Bio-red FTS-40A spectrometer with a resolution of 2 cmÿ1 (64 scans). The specimens were in the form of KBr pellet. Olympus BH-2 polarizing optical microscope equipped with a Linkam hot stage was used to study liquid crystal textures. Fig. 1 illustrates the 1 H-NMR spectra of the mesogenic diol of 4,40 -bis(x-hydroxyalkoxy)-a-methylstilbene, HMS11, and LCPUs of MDI11, TDI11 which were obtained in a deuterated dimethylsulphoxide (DMSO-d6 ) solution at 80°C. 3. Results and discussion The general synthetic route for the preparation of the precursor, HMS, the chain extender, HMSn, and polyurethanes based on TDI and HMSn as well as polyurethanes based on MDI and HMSn are shown in Scheme 1. The series of TDI-based polyurethanes and MDI-based polyurethanes, are hereafter referred to as TDIn and MDIn, respectively (where n is the length of alkoxy group of HMSn). The mesomorphism and stability of HMSn were obtained in the previous paper [29]. The length of alkoxy group aected the mesomorphism and stability of mesophases as well as the transition temperature. HMS3
305
showed a monotropic trimorphism in cooling including the nematic phase, the smectic C phase and the crystalline G or H phase. However, HMS6 presented an enantiotropic trimorphism, and the variant of mesomorphism is the same as HMS3. On the other hand, HMS11 exhibited an enantiotropic crystalline G or H phase. 3.1. FTIR analysis Hydrogen bonding behaviour of liquid crystalline polyurethanes by FTIR analysis has drawn a great deal of interest [12,16,19±22]. Figs. 2 and 3 show the FTIR spectra of TDIn and MDIn recorded at 25°C, respectively. All the polyurethanes dier in the length of ¯exible chain and urethane moiety. The characteristic absorption bands of FTIR spectra for the LCPUs are substantially similar to one another. However, there is still a subtle dierence in the bands sensitive to the mode of H-bonding and the conformation of polymer chains. Both series of polyurethanes present the characteristic band of C@O stretching vibration in the region of 1650±1750 cmÿ1 depending on the kind of urethane moiety and spacer length. Figs. 4 and 5 show the enlarged absorption band of carbonyl stretching. Among the TDIn, TDI11 shows the split band while TDI6 and TDI3 show a band with shoulder, whereas, for the MDIn, all the homologues exhibit the split band. The carbonyl absorption band may be split into three components, including hydrogen-bonded carbonyl stretching and free carbonyl stretching. The deconvolution technique based on a Gaussian function was performed for the absorption band. As shown in Figs. 4 and 5, three components are obtained, i.e., at 1700, 1709±1710 and 1734±1737 cmÿ1 . The ®rst two components are attributed to the order and disorder hydrogen-bonded carbonyl stretchings, respectively. The last component is due to ``free'' carbonyl group stretching. Table 1 summarizes the frequency (m), the half-band width (w1=2 ), and the area and area ratio of the three components. The change of peak frequency is insigni®cant in each component with both the factors of spacer length and the urethane moiety structure. However, the area change of each component depends signi®cantly on both the factors. As shown in Fig. 4 and Table 1, it is clear that the fraction of order H-bonded C@O increases with the increase of the spacer length, whereas that of free C@O decreases in TDIn. The MDI series shows a similar result as shown in Fig. 5 and Table 1. The spacer lowers the steric hindrance of H-bond interaction between C@O and N±H group and regularizes the conformation of polyurethane chain. This results in the increase of the order H-bonded urethane domain. The region of N±H stretching vibration suggests the information about the hydrogen-bonded NH groups and free NH groups. For the hydrogen-bonded NH
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Fig. 1. 1 H-NMR spectra of (a) HMS11, (b) MDI11 and (c) TDI11 which were obtained in d-DMSO solution at 80°C.
groups, the peak frequency of the band is at 3310±3324 cmÿ1 , depending on the kind of diisocyanate and the spacer length. The shoulders at 3390 and 3450 cmÿ1 , slightly changing with the spacer length may be assigned to free NH groups [30,31]. The trends in the change of peaks are similar to that obtained by the carbonyl group
analysis that the peak of H-bonded NH absorption grows with increasing spacer lengths, whereas the free N±H absorption decreased. For H-bonded NH groups, the spacer length with n 11 gives the lower frequency, indicating that longer spacer length provides stronger hydrogen bonding.
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Fig. 2. Infrared spectra of polyurethanes TDIn at 25°C: (a) TDI3, (b) TDI6 and (c) TDI11.
Fig. 3. Infrared spectra of polyurethanes MDIn at 25°C: (a) MDI3, (b) MDI6 and (c) MDI11.
For both TDI11 and MDI11, two absorption bands appear at 2921 and 2852 cmÿ1 , which are assigned to the asymmetric and symmetric stretching vibrations of CH2 , respectively. Snyder et al. [32] indicated that if the polymethylene chains are fully in trans conformation, two bands near 2920 and 2855 cmÿ1 are to be expected. Therefore, the undecyl groups in the two polyurethanes are expected to arrange in the trans conformation. They also show a weak band at 721 cmÿ1 for the rocking vibration of CH2 , indicating that a minimum of four consecutive trans sequence exists [33,34]. 3.2. Thermal behaviour and mesomorphisms of polymers Fig. 6 shows the typical second heating and cooling DSC traces of TDIn at a rate of 10°C/min. Thermal properties associated with the DSC analysis are summarized in Table 2. Thermal transition behaviours remarkably vary with the length of the spacer. TDI3 with a glass transition temperature (Tg ) at 83°C and an iso-
Fig. 4. Infrared spectra in the C@O stretching region of TDIn: (a) TDI3, (b) TDI6 and (c) TDI11.
tropization temperature (Ti ) at 125°C is later evidenced by the WAXS analysis. While TDI6 presents a smaller and broad endothermic peak as well as a relatively larger endothermic peak on heating, the same phenomenon takes place on cooling. The smaller endotherm occurs at 65°C, and the larger peak is at 133°C. The enthalpy change of the smaller endothermic peak is 1.2 J/g, and that of the larger endothermic peak is 12.8 J/g. However, TDI11 exhibits completely dierent thermal transition behaviours. On heating, it shows four endothermic peaks at 91°C, 101°C, 110°C, and 122°C. The enthalpy changes of them are 7.1, 4.2, 1.7 and 22.6 J/g, respectively. However, there are only two exothermic peaks at 115°C and 72°C on cooling. Their enthalpy changes are 23.0 and 17.5 J/g, respectively.
30.05 26.38 21.08 26.00 22.21 18.60 21.93 21.61 22.53 26.26 27.73 26.68
Ad =At (%)
4.33 4.98 4.21 6.02 3.78 3.94 15.7 13.5 10.4 15.2 10.2 9.6 1738 1735 1737 1735 1734 1734 3.16 4.08 4.50 6.08 4.72 5.65 22.1 20.7 20.5 25.2 17.4 12.8 1710 1710 1711 1709 1709 1708 6.92 9.82 11.26 11.05 8.52 11.59 28.5 23.9 23.8 28.4 23.0 26.6
Ad W1=2 (cmÿ1 ) m (cmÿ1 ) W1=2 (cmÿ1 )
Ao m (cmÿ1 )
1699 1700 1701 1699 1700 1701 TDI3 TDI6 TDI11 MDI3 MDI6 MDI11
m (cmÿ1 )
W1=2 (cmÿ1 )
Af
48.02 52.01 56.39 47.74 50.06 54.72
Ao =At (%) Free Disordered Ordered
Hydrogen bonded Polymer
WAXS spectra of both series of TDIn and MDIn as a function of temperature were obtained by ®rst heating the sample to isotropization and then cooling it to the desired temperature. As shown in Fig. 7, when TDI3 was cooled to 100°C, it presents a typical amorphous diraction peak in the region of long-range positional order and a distinct sharp peak at the d-spacing of 27.58 which is shorter than the length of repeating unit of A, HMS3 and TDI moiety in the fully extended chain indicating the existence of the conformation (29.63 A), smectic C phase. When TDI3 was further cooled to 25°C, it still remains a diraction spectrum as that at 100°C. Examining the DSC and WAXS spectrum of TDI3, there is no Tm within it. So, the mesophase of
Table 1 Curve-®tting results in the C@O stretching region of polyurethanesa
Fig. 5. Infrared spectra in the C@O stretching region of MDIn: (a) MDI3, (b) MDI6 and (c) MDI11.
a Ao : area (arbitrary units) of C@O sub-band attributed to ordered domains, Ad : area of C@O sub-band attributed to disordered domains, Af : area of C@O sub-band attributed to non H-bonded carbonyl groups and At : total area of carbonyl band Ao Ad Af W1=2 width at half-height of the Gaussian curve.
C.-K. Lin et al. / European Polymer Journal 37 (2001) 303±313 Af =A (%)
308
DHs3±i 22.6 17.5 DHs2±s3 1.7 ± b
a
Transition temperatures were determined by DSC measurement with a heating and cooling rate of 10°C/min. k: crystal, s1: smectic G or H phase, s2: smectic I phase, s3: smectic C phase, i: isotropic phase. c Transition temperature, Ts3±k . d DH DHs±3k . e TDI3 with a Tg at 83°C and no Tm exists.
DHs1±s2 4.2 ± DHk±s1 7.1 23.0e Ts3±i 122 115 Ts2±s3 110 ± Tk±s1 c 91 72d TDI11 Heating Cooling
Ts1±s2 101 ±
1.0 0.7 85 60 6.8 12.5 113 126 7.5 12.8 125 133 83 65
1.7 1.2
309
TDI3 TDI6
Ts3±k (°C) Ti±s3 (°C) DHm (J/g)
Ti (°C)
DHi (J/g)
DHi±s3 (J/g)
Phase transition behaviour in second cooling
Tm (°C)
Polymer
Table 2 Thermal properties of thermotropic polyurethanesa
TDI3 is between 83°C (Tg ) and 125°C (Ti ). Therefore, TDI3 exhibits an enantiotropic smectic C phase. The polarizing micrograph of TDI3 at 100°C on cooling is shown in Fig. 8. At 120°C, TDI6 shows a similar X-ray diraction pattern as that of TDI3 indicating a smectic phase. However, the X-ray spectrum of 25°C presents peaks in the wide angle region indicating the crystalline phase. It shorter than the has the lamella spacing of 28.11 A length of repeating unit of TDI6 in the fully extended Hence, TDI6 shows an chain conformation (36.14 A). enantiotropic smectic C phase between 65 (Tm ) and 133°C (Ti ). The WAXS spectra of TDI11 were obtained when it was cooled to 105°C and further to 25°C. Then, TDI11 was heated to 98°C, 108°C and 120°C as referred in the DSC thermograms. The spectra obtained at 98°C and 108°C on heating are substantially similar to one another. They include one distinct re¯ection peak at the d as well as the peaks at 4.5, 4.2, and spacing of 29.9 A, The d-spacing of 29.9 A is the length of the layer 3.9 A. structure which is shorter than the length of repeating unit of TDI11, whereas peaks of shorter d-spacing indicate long-range positional order. The spectrum at 98°C has more distinct re¯ection peaks in the wide angle region compared to that at 108°C. The DSC analysis mentioned above indicates that the smectic phase at 98°C is dierent from that at 108°C. Hence, the smectic phase shown at 98°C has a more orderly-structure than that at 108°C. It may be the crystalline G or H phase. Whereas, at 120°C, the spectrum includes a distinct and with the broader peak with the d-spacing of 30.02 A broader peak in the long-range order region. Hence, the mesophase is assumed to be the smectic C phase. Referring to the above characteristic WAXS spectra and the variants of trimorphism suggested by Demus [35], it is assumed that TDI11 may exhibit the trimorphism on
Phase transition behaviour in second heating
Fig. 6. The typical 10°C/min second heating: (a) TDI3, (b) TDI6 and (c) TDI11; cooling: (a0 ) TDI3 (b0 ) TDI6 and (c0 ) TDI11 DSC thermograms.
b
DHs3±k (J/g)
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Fig. 7. The X-ray diraction spectra of polyurethanes, TDI11 at (a) 105°C, (b) 25°C, (c) 98°C, (d) 108°C and (e) 120°C; TDI6 at (f) 120°C and (g) 25°C; TDI3 at (h) 100°C and (i) 25°C.
heating as follows: K 91°C Cry; G or H 101°C SmI 110°C SmC 122°C I (based on heating). The spectrum obtained at 105°C on cooling is similar to that at 120°C on heating; hence, it shows the smectic C phase. In conclusion, the spacer length of hydroxyalkoxy groups in¯uences the hydrogen-bonding eects among the urethane moiety, the structure and stability of mesophases. The shorter spacers of TDI3 and TDI6 show a smaller fraction of ordered H-bonding, and a stable single mesophase in the temperature range 42±68°C. However, TDI11 with the longest spacer has a larger fraction of ordered H-bonding and exhibits trimorphism including the cry G (or H) phase, the smectic I and smectic C phases on heating. Their temperature ranges are not beyond 12°C indicating that the mesophase is less stable. It is interesting to note that as compared with the mesomorphism of HMSn, the polyurethanes exhibit the mesomorphisms dierent from that of the corresponding small molecular liquid crystal HMSn. TDI3 shows an enantiotropic monomorphism of smectic C phase, but HMS3 showed a monotropic trimorphism on cooling. TDI6 presents an enantiotropic monomor-
Fig. 8. Polarizing optical micrographs on cooling: (a) TDI3 at 100°C and (b) MDI3 at 165°C.
phism of smectic C phase, but HMS6 showed an enantiotropic trimorphism. Whereas TDI11 exhibits a trimorphism on heating, it exhibits a monomorphism of smectic C phase on cooling. On the other hand, HMS11 showed simply an enantiotropic monomorphism of smectic G or H phase. 3.3. Thermal behaviour and mesomorphisms of MDIn polymers Fig. 9 shows a typical heating and cooling DSC traces of MDIn. The transition temperatures and thermodynamic properties associated with the DSC analysis are summarized in Table 3. MDI3 exhibits a distinct glass transition and two broad endothermic peaks at 91°C, 179°C and 201°C on heating. The enthalpy changes of two peaks are 6.2 and 13.4 J/g, respectively. However, it presents an exothermic peak at 168°C with a shoulder at 159°C on cooling. The total enthalpy change is 16.6 J/g. Therefore, if there existed any mesophase, it is not stable because it has a narrow temperature range. Whereas MDI6 and MDI11 present a broad endothermic peak with shoulder on heating, which has the overall enthalpy change of 28.9 and 26.0 J/g, respectively. On
159d 141c 133c 168c 150d 154d Transition temperatures were determined by DSC measurement with a heating and cooling rate of 10°C/min. k: crystal, lc: smectic C phase and i: isotropic phase. c Peak maximum. d Shoulder. e Overall enthalpy. b
a
13.4 ± ± 201 168d 154d 6.2 28.9e 26.0e 179 163c 143c 91 ± ±
Ti (°C) DHm (J/g) Tm (°C)
311
16.6e 27.7e 31.0e
Tlc±k (°C) Ti±lc b (°C) Tg (°C)
DHi (J/g)
Phase transition behaviour on second cooling
MDI3 MDI6 MDI11
The decomposition behaviour of TDIn and MDIn was studied by TGA at a heating rate 20°C/min under nitrogen atmosphere as shown in Figs. 11 and 12, respectively. The decomposition temperatures at 5%
Polymer
3.4. Thermal degradation behaviour
Table 3 Thermal properties of thermotropic polyurethanesa
cooling, it shows two exothermic peaks, not well resolved. Therefore, the mesophases are also not stable if they exist. From the DSC thermograms, the longer the spacer length, the lower is the transition temperature. The WAXS spectra as functions of temperature of MDI series are shown in Fig. 10. MDIn present the spectra essentially similar to one another. For MDI11, the spectrum shows a typical crystalline re¯ection patterns at room temperature. On cooling, at 145°C, the spectra present a broad peak in the long-range order region and a distinct peak corresponding to a d-spacing MDI3 exhibits a stronger layer structure, but of 33.7 A. MDI6 shows a weak one than the others. All the dspacings of layer structures are smaller than the length of the repeating unit of the corresponding polymers. Therefore, all the mesophases are assumed to be in the smectic C phase. Referring to the above DSC spectra, it may be assumed that MDI3 presents the smectic C phase between 168°C and 159°C on cooling. MDI6 and MDI11 present the smectic C phase between 150°C and 141°C and between 154°C and 133°C on cooling, respectively. The mesophase is not stable because the temperature breadth is quite narrow. Obviously, unlike the TDIn and HMSn, the MDIn can form only the enantiotropic smectic C phase, which is not stable due to the narrow temperature range of mesophase obtained. Consequently, the diisocyanate unit in the polymer chain backbone remarkably in¯uences the mesomorphism and thermal properties of these liquid crystalline polymers.
Phase transition behaviour on second heating
Fig. 9. The typical 10°C/min second heating: (a) MDI3, (b) MDI6 and (c) MDI11; cooling (a0 ) MDI3, (b0 ) MDI6 and (c0 ) MDI11 DSC thermograms.
DHi±k (J/g)
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Fig. 12. TGA spectra of polyurethanes MDIn at a heating rate of 20°C/min: (a) MDI3, (b) MDI6 and (c) MDI11.
in the polymers. Therefore, the diisocyanate unit does not nearly in¯uence the thermal decomposition behaviour but the spacer length does. However, MDIn show a higher Td than that of TDIn. 4. Conclusions
Fig. 10. The X-ray diraction spectra of polyurethanes, MDI11 at (a) 145°C and (b) 25°C; MDI6 at (c) 143°C and (d) 25°C; MDI3 at (e) 165°C, (f) 25°C and (g) 190°C.
weight losts (Td ) for MDIn is 307±319°C and for TDIn 289±293°C. TDI3 and MDI3 exhibit a higher Td than the others do. The spacer length signi®cantly in¯uences the decomposition behaviours. Polymers with n 3 presents the drastic decomposition at the ®rst stage. The weight loss of polyurethanes at the ®rst stage decreases with increasing spacer lengths. It is similar to our previous paper [21] that the trend is consistent with the weight fraction of the diisocyanate monomers contained
Fig. 11. TGA spectra of polyurethanes TDIn at a heating rate of 20°C/min: (a) TDI3, (b) TDI6 and (c) TDI11.
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