A Mass-Spectrometric Study of the Behavior of Thermotropic Liquid-Crystalline Polymers on Heating O
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Technical Physics Letters, Vol. 27, No. 5, 2001, pp. 351–353. Translated from Pis’ma v Zhurnal Tekhnicheskoœ Fiziki, Vol. 27, No. 9, 2001, pp. 1–7. Original Russian Text Copyright © 2001 by Pozdnyakov, Redkov, Egorov. A Mass-Spectrometric Study of the Behavior of Thermotropic Liquid-Crystalline Polymers on Heating O. F. Pozdnyakov, B. P. Redkov, and E. A. Egorov Ioffe Physicotechnical Institute, Russian Academy of Sciences, St. Petersburg, 194021 Russia e-mail: [email protected] Received October 31, 2000 Abstract—Physicochemical processes accompanying the thermal treatment of a liquid-crystalline aromatic copolyester Ultrax-4002 at temperatures in the vicinity of the thermotropic transition were studied by mass spectrometry in order to reveal possible features of the mechanism of polymer strengthening. It is shown that the thermal treatment leads to additional polycondensation in the material and the degradation of macro- molecules with the accumulation of low-molecular-mass products in the polymer bulk. The energy parameters of the polycondensation, thermal degradation, and diffusion processes were determined. © 2001 MAIK “Nauka/Interperiodica”. The unique combination of the thermal and mechan- many). The molar fractions of the fragments of tere- ical properties of thermotropic liquid-crystalline (LC) and isophtalic acids and p,p-dioxybisphenol in macro- polymers naturally attracts the attention of researchers. molecules of this polymer [4] are 0.25, 0.25, and 0.5, A high thermal stability of these LC polymers, together respectively. The chemical formula of the polymer with good mechanical strength, makes these materials chain unit is as follows: highly promising for various applications [1–5]. A unique feature of the LC polymers is a considerable –(CO–Ph–CO)–(CO–Ph–CO)–(O–Ph–Ph–O)– (severalfold) increase in the tensile strength and elastic 0.25 tere- 0.25 iso- 0.5 modulus upon a thermal treatment involving a long- where Ph is the phenyl group (C6H4). time heating at temperatures close to the thermotropic The Ultrax-4002 samples were studied in the form transition temperature Ttt. However, the reasons for this of granules and oriented fibers. In order to minimize the strengthening are still not completely clear. effect of diffusion upon the gas evolution process, we Some researchers believe that not only large-scale also studied ultrathin (submicrogram) samples obtained structural transformations but certain chemical changes by rubbing granules on the surface of a heated metal in the polymer structure as well may account for a support provided with a thermocouple. Preliminary cal- spontaneous increase in the mechanical properties ibration measurements showed that a ~5-mm-long observed upon the thermal treatment [5]. It should be trace transferred about 0.04 µg of the polymer from the emphasized that the thermal treatment is a necessary granule onto the substrate surface. A total of 60 non- stage of the technological process imparting required overlapping traces were sufficient to prepare an properties to the LC material (fiber or film). The tem- ultrathin sample. For MTA, the samples were placed perature interval employed for the thermal treatment of into a vacuum chamber of the mass spectrometer and LC copolyesters extends up to 350–400°C. These tem- heated at a rate of 7 K/s. peratures are sufficiently high to feature intense diffu- The mass-spectrometric analysis of volatile prod- sion and degradation processes in these polymers. ucts liberated from polymer granules during the first In order to obtain objective information about the heating showed these substances to represent a mixture, chemical processes occurring in LC copolyesters the main components being (in the order of decreasing treated in the vicinity of Ttt, we studied these materials proportion) CO2, CO, phenol, acetic acid, and mole- by mass-spectrometric thermal analysis (MTA). Using cules such as phenyl benzoate and/or dibenzoyl. The MTA, it is possible to determine the detailed composi- latter molecules represent large fragments of the poly- tion of the products liberated from the samples simulta- mer chain. During the repeated heating, the most sig- neously with measuring the kinetics of the product for- nificant qualitative change was observed for acetic mation in the course of a thermal treatment. The MTA acid, which ceased to evolve. was effected using a modified mass spectrometer of the Attempts at measuring the temperature dependence MX-1320 type [6]. of the kinetics of liberation of the volatile products of The experiments were performed on the fully aro- thermal degradation from a massive sample represent- matic random copolyester Ultrax-4002 (BASF, Ger- ing a part (~10 mg) of the polymer granule showed that 1063-7850/01/2705-0351 $21.00 © 2001 MAIK “Nauka/Interperiodica” 352 POZDNYAKOV et al. (dN/dt)Ph, a.u. the degradation products are accumulated in the sample volume upon cooling. Figure 1a shows typical plots of (a) the phenol evolution rate versus temperature measured 150 3 during three sequential heating cycles. As seen, the onset temperature of Ph evolution during the first heat- ing is ~350°C, while during the second and third heat- 100 ing this product is observed at ~250°C. Judging by curve 2, the amount of phenol accumulated in the sam- 2 ple upon the first heating is rather small (the sample 50 was heated only to ~425°C). In curve 3, the accumula- tion effect is much more pronounced because the sec- 1 ond heating was continued up to 480°C. 0 200 250 300 350 400 450 500 The kinetics of the accumulated phenol evolution AA corresponds to the first-order reaction, since the reac- (dN/dt) , a.u. tion rate is directly proportional to the concentration. 40 (b) Ph We have determined the activation energy Edif for the Ph diffusion in copolyester Ultrax-4002 in the 240– 30 340°C temperature range using curve 3 in Fig. 1a. The Ph corresponding Arrhenius plot (Fig. 2a) gives Edif = 20 97 ± 5 kJ/mol, which qualitatively agrees with the usual level of the diffusion activation energies Edif reported 10 for diffusants of this type in crystalline polymers [7]. Subsequent measurements were conducted using 0 submicrogram samples, where the degradation product 150 200 250 300 350 400 450 accumulation effect was not observed and the diffusion activation energy significantly increased to reflect the T, °C chemical kinetics of the thermal degradation process. Ph Fig. 1. Typical plots of the evolution rate of (a) phenol Here, the ETD value determined from the temperature Ph AA (dN/dt) and (b) acetic acid (dN/dt) versus temperature variation of the phenol evolution rate (dN/dt)Ph was measured during the thermal treatment of Ultrax-4002 ± ° ° copolyester. The evolution of phenol was measured during 240 10 kJ/mol for T0 = 350 C and Tmax = 470 C three sequential heating cycles 1–3 with intermediate cool- (Fig. 2b). ing of the sample to room temperature; the formation of ace- It is interesting to compare the latter value to the tic acid was studied during the first heating of the sample. activation energy for mechanical fracture (220 kJ/mol) of the heat-treated fibers of an aromatic copolyester [2] log(dN/dt)Ph with the composition close to that studied in our exper- iments. Note a good agreement between the activation 3.0 3.0 b energies for thermal degradation and the mechanical 2.5 2.5 fracture of the LC polymers, which is evidence that we deal with the rupture of similar chemical bonds deter- 2.0 a 2.0 mining the process kinetics in the polymer chain in both cases. 1.5 1.5 Figure 1b shows the kinetics of acetic acid (AA) 1.0 1.0 evolution during the first heating of the copolyester studied. This kinetic curve shows the following charac- 0.5 0.5 teristic features: a significant increase in the (dN/dt)AA ° 0 0 value begins at a temperature of about 230 C, the max- 1.4 1.5 1.6 1.7 1.8 1.9 imum evolution rate is observed at 320°C, and the acti- 3 vation energy for the AA formation is 36 kJ/mol (close 10 /T to a value typical of the nonequilibrium polycondensa- tion kinetics [8]). Note that, according to the NMR Fig. 2. The Arrhenius plots illustrating determination of the ° activation energies for (a) the phenol diffusion in a massive data, it is this temperature interval (250–300 C) where Ph polymers of the Ultrax type exhibit a transition from sample of Ultrax-4002 (Edif = 97 kJ/mol) and (b) the ther- “solid state” to anisotropic LC melt [9]. mal degradation of the polymer measured by the phenol As is known, the polycondensation of diatomic phe- Ph evolution from submicrogram samples (ETD = 240 kJ/mol). nol diesters (primarily diacetates) with dicarboxylic See the text for explanations. acids is usually conducted on heating to 300°C in the TECHNICAL PHYSICS LETTERS Vol. 27 No. 5 2001 A MASS-SPECTROMETRIC STUDY OF THE BEHAVIOR 353 melt or in the solid state, whereby the AA evolution tak- process of thermal degradation accompanying the poly- ing place beginning with 210°C [10]. A comparison of condensation reaction negatively affects the mechani- these data to our results suggests that, with a high prob- cal properties. Provided that the MTA conditions are ability, the polycondensation process in Ultrax-4002 is properly selected, mass spectrometry can be used for incomplete. Therefore, the thermal treatment may be the quantitative monitoring of chemical processes in accompanied by an additional polycondensation of the materials of the Ultrax-4002 type. material. Evidence for this conclusion is provided by Acknowledgments. This study was supported by the results of quantitative MTA. In particular, we deter- the Russian Foundation for Basic Research, project mined the ratio of the amounts of phenol and AA liber- nos.