thermochimica acta ELSEVIER Thermochimica Acta 276(1996)145-160 Correlation of observations made by DSC and hot-stage optical microscopy of the thermal properties of a monotropic liquid-crystal polyester Z.Bashir*N.Khan Courtaulds,7 Lockhust Lane.Coventry CV65RS.UK Received 22 May 1995:accepted 20 September 1995 Abstract The polyester formed by condensation of 4.4'-bis(6-hydroxyhexoxy)biphenyl(BHHBP)and CSabiscdhmanynteretingandintriguingth properties cocaagatamk mc:oncoonghowever.the material transtormsrom the isotropic melttom and smectic ore crystal g.H phase transition (crystal-isotropic)on heating.then the multiple peaks need to be explained the n histor of the .The multiple peaks during heating.whereas slowly crystallized polymer showed a single melting endotherm.By quench the isotropic melt,it was not possible toobtain the material aks Here.we have peaks(correspondingto-N.N-SandSK)can be resolved by goin to very ling rates (0.. min )It was f ound that even nematic was never complete and was overtaken eventually by crystallization.This kindo incomplete phase transformation appears to be a feature of this monotropic liquid-crystalline *Corresponding autho 5s7680808Yg502756.4
therm0chimica acta ELSEVIER Thermochimica Acta 276 (1996) 145-160 Correlation of observations made by DSC and hot-stage optical microscopy of the thermal properties of a monotropic liquid-crystal polyester Z. Bashir *, N. Khan Courtaulds, 72 Lockhurst Lane, Coventry CV6 5RS, UK Received 22 May 1995; accepted 20 September 1995 Abstract The polyester formed by condensation of 4,4'-bis(6-hydroxyhexoxy)biphenyl (BHHBP) and isophthalic acid has been reported to show many interesting and intriguing thermal properties. By optical microscopy, we established that the polymer shows monotropic liquid-crystalline behaviour. Thus, on heating, no mesophases are formed and the crystal melts directly to the isotropic state; on cooling, however, the material transforms from the isotropic melt to a nematic and smectic A mesophase before crystallizing. However, the DSC heatin 9 scan of this polymer was very complex, showing an exotherm and two endotherms; further, unlike the phenomenon of cold crystallization, the exotherm occurred between the two endotherms. If there is only a single phase transition (crystal-isotropic) on heating, then the multiple peaks need to be explained. In this work, we have shown that the multiple peaks in the heating scan are solely related to the thermal history of the sample. The nascent polymer and quench-cooled material showed multiple peaks during heating, whereas slowly crystallized polymer showed a single melting endotherm. By quench cooling from the isotropic melt, it was not possible to obtain the material in an amorphous form, because this polyester had a propensity for fast ordering. In the previous work, on coolin9, we only found two instead of three DSC peaks. Here, we have attempted to see if three peaks (corresponding to I-N, N-S A and S A K) can be resolved by going to very slow cooling rates (0.1°C min- 1). It was found that even at such low cooling rates, I-N and the N-S A transitions could not be separated into distinct peaks. This is in accord with optical microscope observations which indicated that the transformation to the smectic phase from the nematic was never complete and was overtaken eventually by crystallization. This kind of incomplete phase transformation appears to be a feature of this monotropic liquid-crystalline polymer. * Corresponding author. Elsevier Science B.V. SSDI 0040-6031(95)02756-4
146 Z.Bashir,N.Khan/Thermochimica Acta 276(1996)145-160 Keywords:4,4-Bis(6-Hydroxyhexoxy)biphenyl:Isophthalic acid:Monotropic liquid-crystal: DSC;Optical microscopy;Polyester:Thermal history 1.Introduction The polyeste forme condensation of isophthalic acid and 4,4'-bis(6-hy. droxyhexoxy)biphenyl(BHHBP)has interesting thermal properties.The polyester is shown below(Scheme 1). In the earliest reports on various condensation polymers formed with the diol BHHBP [1,2]it was stated that the above polyester formed a smectic A liquid-crystal phase.Subsequently,a more detailed investigation [3]showed that this polyester formed not only a smectic but also a nematic liquid-crystal phase.Moreover,it was shown that the mesophases were monotropic in character and were observed only on cooling and not on heating [3].The phase sequence on cooling was proposed to be I→N-→S+K and K→I on heating (I=isotropic.N=nematic,.S smectic A K=crystal).In another investigation.it was dem nstrated that deformation of this polyester in the smectic melt state led to an extraordinary chain orientation perpen dicular to the flow direction or the fibre axis [4]. f the erential e poly alo imetry(DSC).The DSC principally by hot-sta ould on th ole nterpre e pha equence own abo t there re s g aspects wh d no n resolved For were co plex,sh owing multiple e nermic peaks,even th It was argued that there is only one phase transition(K I).At he ating rates o min or lower,it was found that there were two endothermic peaks s with an exothermic peak lying between them [2].This is reproduced in Fig.1a.Although an exothermic peak before and endothermic peak is quite common on heating semi- crystalline polymers,owing to"cold crystallization",an exothermic peak between two endothermic peaks was more unusual and difficult to explain.We postulated that this may be a result of partial melting.followed by recrystallization to a more perfect form, followed by remelting of the perfected form at a higher temperature.Preliminary annealing experiments were cited as supporting this proposal but sufficient experimen- tation had not been conducted at the time to be definitive about the nature of the multiple peaks observed in the heating scan [27.Further,on cooling.two exothermic peaks were found:these started to overlap as the cooling rate was lowered.However,as the phase sequence cited above shows,on cooling there are three transitions and hence (-0-(CH26-0 -oM Scheme 1.The polymer med and 44-bsydroxyhey)biphenyl
146 Z. Bashir, N. Khan/Thermochimica Acta 276 (1996) 145-160 Keywords: 4,4'-Bis(6-Hydroxyhexoxy)biphenyl; Isophthalic acid; Monotropic liquid-crystal; DSC; Optical microscopy; Polyester; Thermal history 1. Introduction The polyester formed by condensation of isophthalic acid and 4,4'-bis(6-hydroxyhexoxy)biphenyl (BHHBP) has interesting thermal properties. The polyester is shown below (Scheme 1). In the earliest reports on various condensation polymers formed with the diol BHHBP [1,2] it was stated that the above polyester formed a smectic A liquid-crystal phase. Subsequently, a more detailed investigation [3] showed that this polyester formed not only a smectic but also a nematic liquid-crystal phase. Moreover, it was shown that the mesophases were monotropic in character and were observed only on cooling and not on heating [3]. The phase sequence on cooling was proposed to be I ~ N ~ S a ~ K and K ~ I on heating (I = isotropic, N = nematic, S A = smectic A, K = crystal). In another investigation, it was demonstrated that deformation of this polyester in the smectic melt state led to an extraordinary chain orientation perpendicular to the flow direction or the fibre axis [4]. These phases of the polyester were established principally by hot-stage microscopy and differential scanning calorimetry (DSC). The DSC results could on the whole be interpreted according to the phase sequences shown above, but there were still certain intriguing aspects which had not been resolved properly. For example, the heating scans of the polyester were complex, showing multiple endothermic peaks, even though it was argued that there is only one phase transition (K--* I). At heating rates of 5°C min 1 or lower, it was found that there were two endothermic peaks with an exothermic peak lying between them [2]. This is reproduced in Fig. la. Although an exothermic peak before and endothermic peak is quite common on heating semicrystalline polymers, owing to "cold crystallization", an exothermic peak between two endothermic peaks was more unusual and difficult to explain. We postulated that this may be a result of partial melting, followed by recrystallization to a more perfect form, followed by remelting of the perfected form at a higher temperature. Preliminary annealing experiments were cited as supporting this proposal but sufficient experimentation had not been conducted at the time to be definitive about the nature of the multiple peaks observed in the heating scan [2]. Further, on cooling, two exothermic peaks were found; these started to overlap as the cooling rate was lowered. However, as the phase sequence cited above shows, on cooling there are three transitions and hence O O (-- O --(CH2)6-- O ~~~- O -- (CH2)6-- O ~ ) n Scheme 1. The polymer formed by condensation of isophthalic acid and 4,4'-bis(6-hydroxyhexoxy) biphenyl
Z.Bashir.N.Khan/Thermochimica Acta 276(1996)145-160 147 one might have expected three exothermic peaks.Because of the closeness of the transitions,we assumed that the first exothermic peak at 105Cin the DSC represented, in fact,the formation of both the mesophases,whereas the second exothermic peak at ~70C was a result of crystallization(see Fig.la). In this work,we have concentrated on the DSC results and tried to resolve the two complexities mentioned above.Firstly,in order to establish whether the multiple peaks in the heating scan arise purely as a result of thermal history(rather than multiple phase transitions),the heating scans of the nascent polymer were compared with quench- cooled material and also with polymer that had been slowly or isothermally crystal- lized.Secondly,on cooling, an atte see if thre othe ransition ould be mpt has beer made to d b rates thar those tried ower cooling rates sho 60 100 120 140℃ (b) 406080 100 120140℃ polymer (heated quench-cooed heated at scmin
Z. Bashir, N. Khan/ Thermochimica A cta 276 (1996) 145-160 147 one might have expected three exothermic peaks. Because of the closeness of the transitions, we assumed that the first exothermic peak at 105°C in the DSC represented, in fact, the formation of both the mesophases, whereas the second exothermic peak at 70°C was a result of crystallization (see Fig. la). In this work, we have concentrated on the DSC results and tried to resolve the two complexities mentioned above. Firstly, in order to establish whether the multiple peaks in the heating scan arise purely as a result of thermal history (rather than multiple phase transitions), the heating scans of the nascent polymer were compared with quenchcooled material and also with polymer that had been slowly or isothermally crystallized. Secondly, on cooling, an attempt has been made to see if three exothermic transitions could be resolved by using lower cooling rates than those tried previously. Lower cooling rates should enhance peak resolution, albeit at the expense of sensitivity. E (a) 40 60 80 100 120 140°C (b) 40 60 80 100 120 140 °C Fig. 1. DSC heating scans of polymers with different thermal histories: (a) nascent reactor polymer (heated at 5°C min ~); (b) heating scan at 5°C min 1 of slowly crystallized polymer (formed by melting the nascent polymer and cooling from 150 to 30°C at -5°C min 1); (c) heating scan at 5°C min -1 of polymer isothermally crystallized at 100°C for 5 h; (d) heating scan at 5°C min ~ or polymer isothermally crystallized at 105°C for 5 h; (e) heating scan at 5~C min ~ of polymer isothermally crystallized at 110°C for 5 h; (f) quench-cooled polymer heated at 5~'C min 1
1 Z.Bashir,N.Khan/Thermochimica Acta 276(1996)145-160 @50 0 50 100 Fig.(Continued) Hot-stage microscopy experiments were conducted in parallel using the same condi- tions(crystallization temperatures and times,heating and/or cooling rates)to provide a visual backup in the interpretation. 2.Experimental 2.1.Synthesis and characterization of monomer and polymer 2.1.1.4.4'-bis(6-Hydroxyhexoxy)biphenyl(BHHBP) The caction scheme for the thesis of the onomer BHHBP is shown below work[2]
148 Z. Bashir, N. Khan/Thermochimica Acta 276 (1996) 145-160 (c) 't l I' -,50 0 50 100 " *6 t e-xr" (d) -50 0 ,~0 .... 1(i0 " °e Fig. 1. (Continued). Hot-stage microscopy experiments were conducted in parallel using the same conditions (crystallization temperatures and times, heating and/or cooling rates) to provide a visual backup in the interpretation. 2. Experimental 2.1. Synthesis and characterization of monomer and polymer 2.1.1. 4,4'-bis(6-Hydroxyhexoxy)biphenyl (BHHBP) The reaction scheme for the synthesis of the monomer BHHBP is shown below (Scheme 2). The details of preparation of the monomer may be found in the previous work [2]
Z.Bashir.N.Khan/Thermochimica Acta 276(1996)145-160 (e) 50 50 10 0150-10-50050100 Fig.1.(Continued). HoCC-oH+H0入入人a HO-(CH2k-0-(CH2ke-om BHHBP smectic monomer Scheme2.The synthesisof BHHBP 2.1.2.Polyester of BHHBP and IA The same polyester sample that was employed in the previous investigations [2-4] was used in this work.The reaction scheme for the synthesis of the polyester of BHHBP and isophthalic acid(IA)is shown below(Scheme 3).The details of preparation of the polyester are given in a previous paper [2].The polymer was purified by repeated dissolution in dichloromethane followed by reprecipitation in methanol.The purified
Z. Bashir, N. K han/ Thermochimica Acta 276 (1996) 145-160 149 ? l (e) -do 0 50 100 .... *C (f)-150 '100 -5() 6 50 100 " ' °C Fig. 1. (Continued). OH + HO ~'~/~/~/CI NaOH , EtOH HO--(CH2)6-- O ~~- O-- (CH2)6-- OH BHHBP smectic monomer Scheme 2. The synthesis of BHHBP. 2.1.2. Polyester of BHHBP and IA The same polyester sample that was employed in the previous investigations [2-4] was used in this work. The reaction scheme for the synthesis of the polyester of BHHBP and isophthalic acid (IA) is shown below (Scheme 3). The details of preparation of the polyester are given in a previous paper [2]. The polymer was purified by repeated dissolution in dichloromethane followed by reprecipitation in methanol. The purified