Solution crystallization and annealing of polyarylate

Eur. Polym. J. Vol. 27, No. 9, pp. 965-968, 1991

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SOLUTION CRYSTALLIZATION A N D A N N E A L I N G OF POLYARYLATE J. I. EGUIAZABAL,l* J. J. DEL VAL,2 M. CORTA.ZARl and J. J. IRUIN1 ~Departamento de Ciencia y Tecnologla de Polimeros and 2Departamento de Fisica de Materiales, Facultad de Ciencias Quimicas, P.O. Box 1072, San Sebastian, Espafia (Received 3 December 1990)

Abstract--A polyarylate of bisphenol-A and isophthalic/terephthalic acids has been crystallized from solution in ortho-dichlorobcnzene. The ability of this polymer to crystallize in these conditions is demonstrated by means of differential scanning calorimetry and X-ray analysis. The heats of fusion are low, indicating that small degrees of crystallinity are obtained. Annealing at various temperatures and times leads to variation in the heat of fusion of the polymer while the melting temperature tends towards a value of 530 K at the higher temperatures and times. Annealing affects also the X-ray diagrams of the polymer.

INTRODUCTION

EXPERIMENTAL PROCEDURES

It is known that the ability of polymers to crystallize depends very much on the chain flexibility and therefore flexible polymers crystallize easily whereas rigid molecules are difficult to transfer to a crystalline structure. Thus, bisphenol-A polycarbonate (PC) has a rigid and extended molecule and is obtained from the melt in a completely amorphous state. However, this polymer can be crystallized easily by the action of a liquid or vapour, the molecules of which swell the polymer, so increasing the chain mobility and depressing the glass transition temperature. The topic of solvent-induced crystallization and vapour-induced crystallization of PC has received much attention [1-8]. Various solvents, including methanol, acetone, carbon tetrachloride, have been used to crystallize PC. Poly(2,6-dimethyl phenylene oxide) (PPO) is another polymer which cannot be thermally crystallized but crystallizes fairly readily in the presence of solvents such as ~-pinene, tetralin [9] and methylethyl ketone [10]. In a recent work [11] it was reported that some block copolyarylates can be crystallized by means of solvent treatment but r a n d o m copolyarylates remained uncrystallized with the treatment. The polyarylate of bispbenol-A and an equimolar mixture of isophthalic and terephthalic acids (PAr), is a very rigid polymer that, like PC and PPO, is generally obtained as a totally amorphous product. In this work, we have tried to crystallize polyarylate by the action of a solvent, as stated above. It has been observed that PAr crystallizes from solution in ortho-dichlorobenzene (o-DCB). The effect on the crystalline phase of annealing at various temperatures and times has been also studied.

The polymer used in this work was ARILEF U-100 (Solvay & Cie). Average molecular weights were determined by GPC in THF using the equation [12]:

*To whom all correspondence should be addressed.

[r/] = 4.00 x 10 -4 x ~ . + s o ,

and the universal calibration procedure. The values obtained were Mw = 51,400 and M, = 21,500. A film of PAr was prepared by casting from a 15% solution of the polymer in o -DCB on a glass plate. Evaporation of the solvent was carried out for five days at room temperature in a stream of air. After evaporation, the film was dried/n vacuo at 80° for 48 hr. Films were also obtained from solutions in other solvents, using the same procedure. The thermal treatments of the films were carried out in a METTLER FP52 hot stage, for various periods and at several temperatures. Thermal analysis was carried out in a Perkin-Elmer DSC-2C differential scanning calorimeter, operated at a heating rate of 20 K/rain. Calibration was achieved with an indium standard. The glass transition temperatures were determined at the onset of the transition. The melting points were measured at the maximum of the melting peaks. The heats of fusion were computed from the areas under the peaks. The values are slightly erratic because of difficulties in determining the baseline position because of the width of the melting peaks and also of the low melting heats. X-ray diffraction intensity curves were measured with 1.54 .~ (CuK=) radiation using a Philips PW1729 generator with a PWI820 vertical goniometer with a graphite monochromator. RESULTS AND DISCUSSION

PAr films obtained by casting and using solvents of low boiling point which evaporate rapidly at room temperature, such as methylene chloride and chloroform, appear to be totally transparent, indicating that the polymer is in a totally amorphous state. When the solvent has a higher boiling point (e.g. dioxane or 1,2-dichloroethane), a very slight turbidity is observed in the films, indicating the 965

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J.I. EGUIAZ~.BALet al.

existence of some crystallinity. Finally, solvents of high boiling point, (such as chlorobenzene, o-DCB and cyclohexanone) give rise to completely opaque films, which appear to be crystalline. The different degrees of crystallinity suggested by the different opacity levels are confirmed by the X-ray diagrams shown in Fig. 1. Thus, the diagrams corresponding to samples obtained from methylene chloride or chloroform solutions are indicative of the amorphous nature of PAr, whereas some crystaUinity appears in the sample from 1,2-dichloroethane. Finally, the samples obtained from chlorobenzene or o-DCB show the existence of crystalline material in the PAr samples. Such different characteristics are due to the different rates of evaporation of the solvents. The higher the rate, the shorter is the time during which th.e solvent acts as a plasticizer, and the PAr chains have sufficient mobility to give rise to the ordered structures of the polymer crystals. The PAr film obtained by evaporation from the 15% o-DCB solution was opaque, as previously mentioned. The calorimetric scan for the as-cast product is shown in Fig. 2. A glass transition appears at 418 K, a temperature which is lower than the Ts of pure PAr, (460 K). This result indicates that some solvent is present in the product. The impossibility of removing all the solvent from the crystallized polymer by drying has been reported [8]. The o-DCB content in the film is ca 8%. The presence of this residual solvent may affect some of the results obtained in this work. The DSC scan in Fig. 2 also shows a broad and small endothermic peak, which corresponds to the melting of the PAr crystallites formed during casting. The melting point, as calculated at the maximum of the peak is 519 K, and the heat of fusion is ca 10.5 J/g. This value is lower than that obtained for the unanhealed PC, AHm = 20.6 J/g [13], and also somewhat lower than that obtained by Karasz et al. [14] for PPO, AHm = 15.1 J/g. The crystallinity of PAr cannot

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Fig. 1. X-ray diagrams for PAr samples obtained by casting from methylene chloride (A), choloroform (B), 1,2dichloroethane (C), chlorobenzene (D) and o-DCB (E).

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Fig. 2. DSC scan for PAr obtained by casting from o-DCB. be calculated because the melting heat for 100% crystalline polymer is unknown. However, the very low melting heat obtained indicates a small degree of crystallinity for PAr, possibly because this product is a copolymer. As already mentioned, the melting endotherm is rather broad, extending from 480 to 540 K, indicating that a wide distribution of crystal sizes is obtained from the solution. On the other hand, the T s / T m ratio calculated for PAr using its glass transition temperature (Ts = 460 K) and the melting temperature (Tin = 519 K) is ca 0.9 which is much higher than the usual values of 0.5-0.67 found for most other polymers. The same ratio for PPO [14] and unannealed PC [7] is also 0.9. It has been suggested that such high T s / T m values can be due to the existence of very thin lamellae in the crystalline polymer [15]. The X-ray diagram for the PAr sample cast from o-DCB [Fig. I(E)] shows, as mentioned, the partially crystalline nature of the polymer. In this diagram, two main diffraction peaks are observed at 20 = 17° and 20 °, which correspond to spacings of 5.21 and 4.44 ,/~, respectively. These peaks are coincident with those obtained by Lee and Tsai for a polyarylate of bisphenoI-A and isophthalic acid and also for other copolyarylates [11]. The diffraction diagram also shows other minor diffraction peaks. It might be interesting to pay special attention to the shoulder which appears in the as-cast sample at 20 = 14°. As observed in Fig. 1 this shoulder is not present in amorphous samples while it is clearly observed in samples obtained from chlorobenzene or o-DCB solutions. In the 1,2-dichloroethane-cast sample, the shoulder is much less clear. Although the shoulder could be attributed to the crystalline phase of PAr, it is interesting to note that, in spite of the different treatments applied, its position is coincident with that observed by Cebe et al. [16] for heat treated poly(ether ether ketone) and which is attributed to the presence of a metastable disordered phase. No clear conclusion can be drawn about the exact nature of this shoulder. Figure 3 shows the effect of the annealing temperature on the thermal transitions of PAr, for annealing times of 30 and 360 min as an example. Intermediate behaviours are obtained for intermediate times. It is clear that Ts increases as a consequence of the annealing and, the longer the annealing time, the sharper the increase. At the higher temperatures, Ts

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Fig. 3. Effect of annealing temperature on the thermal transitions of PAr. O, T~; I-q, Tm. Annealing times of 30 min (open symbols) and 360 min (full symbols). of c a 460 K, corresponding to pure PAr, is obtained. Somewhat higher values are obtained in some cases for the higher annealing times, caused by an increase in the slope of the transition region. This coincides with the appearance of small hysteresis peaks after the glass transition in these samples. Study of these hysteresis peaks is outside the scope of this work. All these results concerning Ts can be explained on the basis of the evaporation of o - D C B during the annealing. Clearly, more complete evaporation is obtained at the higher temperatures and times. The second feature which appears clearly in Fig. 3 is the increase of Tm as the temperature of annealing increases. The greater the annealing time, the more pronounced is the melting point elevation; this effect can be explained on the basis of a reorganization of the PAr crystals during annealing, leading to more perfect crystals with a higher melting point, and also taking into account that the evaporation of the solvent during annealing should give rise to an increase of the melting point. Possibly both factors are responsible for the behavior observed. It can also be seen that, at the higher temperatures of annealing, the melting temperature tends towards a value of 530 K, irrespective of the annealing time. In Fig. 4, the effect of annealing on the shape of the melting endotherms of PAr is shown. The endotherms correspond to an annealing time of 30 min but the effect is similar for other annealing times. It can be observed that, as mentioned before, the maxima of the peaks are shifted to higher temperatures when the annealing temperature is raised. On the other hand, narrowing of the peaks is clearly observed when the annealing temperature increases, indicating the homogenization in the perfection of the crystals obtained after thermal treatment. As far as the effect of the annealing on the heat of fusion is concerned, the exact values of AHm are difficult to determine, as already mentioned. However, the general tendency is an increase of AHm at low annealing temperatures and a decrease

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Fig. 4. Effect of annealing temperature on the DSC scans of PAr. Annealing time: 30 min. as temperature increases. This tendency seems to be maintained irrespective of the time. Thus, it appears that, at low temperatures of annealing, additional crystallization of PAr takes place, whereas at high temperatures some of the crystallized polymer melts during the annealing (it should be remembered that the melting of the as-cast PAr crystals begins at 480 K). The maximum value obtained for AHm is c a

17 J/g. The effect of thermal treatment on the crystalline nature of PAr can also be seen in the diffraction diagrams obtained after thermal treatment. Figure 5

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Fig. 5. Effect of annealing temperature on the X-ray diagrams of PAr. Annealing time: 30 min.

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is similar to the equilibrium melting obtained for PPO, 533 K [14]. No clear the different treatments on the melting deduced as a function of the annealing

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temperature influence of heat can be time.

CONCLUSIONS

The results show that PAr can be crystallized from solution in a high boiling point solvent. It has been also observed that a very low degree of crystallinity is obtained by this method. The melting point is increased by means of annealing and reaches a maximum value of 530 K. These results suggest the possibility of obtaining PAr with a higher degree of crystallinity by modifying the crystallization conditions and by using nucleating agents. Acknowledgement-We acknowledge the financial support of the Gipuzkoako Foru Aldundia.

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REFERENCES

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Fig. 6. Effect of annealing time on the melting temperature of PAr.

shows X-ray diagrams corresponding to PAr annealed for 30min at various temperatures. The main feature in this figure is the fact that, the higher the thermal treatment temperature, the lower the intensity of the peak at 20 = 20”. The effect is more prominent at annealing temperatures equal to or higher than 480 K, in good agreement with the aforementioned fact that the melting of the as-cast polyarylate begins at 480 K and it can be related to the destruction of the less perfect crystals during annealing. Figure 6 shows the variation of the melting temperature with the time of annealing for the various temperatures. It is observed that T,,, increases as the time of annealing increases, and tends towards 530 K. It may also be observed that, the higher the temperature of annealing, the faster T,,, increase. The tendency of the melting temperature towards the value 530 K at high temperatures and times of annealing seems to indicate that it is not possible to obtain crystals that are more perfect than those which melt at 530K. Thus, 530K seems to be the maximum melting temperature possible as a consequence of the crystallizable segment length in PAr. This value

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10. 11.

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