X-ray Diraction Study of the Changes Induced During the Thermal Degradation of Poly (Methyl Methacrylate) and Poly (Methacryloyl Chloride

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X-ray Diraction Study of the Changes Induced During the Thermal Degradation of Poly (Methyl Methacrylate) and Poly (Methacryloyl Chloride
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  Turk J Chem28 (2004) , 725 – 729.c  T¨UB˙ITAK X-ray Diffraction Study of the Changes InducedDuring the Thermal Degradation of Poly (MethylMethacrylate) and Poly (Methacryloyl Chloride) Rizwan HUSSAIN 1 , Din MOHAMMAD 21 P.O. Box 2216, Nescom, Islamabad-PAKISTAN e-mail: lithiumpk@yahoo.com  2 P.O. Box 1356, Pinstech, Islamabad-PAKISTAN  Received 18.11.1998 Poly (methyl methacrylate) – PMMA – and poly (methacryloyl chloride) – PMACl – were synthesizedby free radical solution polymerization. The thermal stabilities of these polymers were determined withthe help of thermogravimetry and differential thermal analysis. The structural changes in these materialsat different temperatures (90, 140, 220 and 410  ◦ C) were studied by X-ray diffraction. These studiesindicate that PMACl is less amorphous than PMMA due to the presence of bulky chloride groups. Introduction The wide ranging applications of poly (methyl methacrylate) - PMMA – and allied polymers are welldocumented [1,2]. These polymers unlike ionic and metallic crystals and low molecular weight substancesconsist of long chain molecules arranged in aggregates, which assume complex shapes and structures. Manyuses of polymers involveelevated temperatures, thus necessitating the determination of the structural changesthat take place at these high temperatures [3]. The structural information gives an insight into the physicaland mechanical properties of polymers [4]. The application of X-rays in material analyses makes it possibleto determine detailed information on the state of order and disorder of the system [4-6]. Recently Saini [7]and Spevacek [8] utilized XRD for the characterization of PMMA.In this paper, the structural changes occurring in PMMA and poly (methacryloyl chloride) - PMACl– when subjected to heat were studied employing X-ray diffraction. Experimental Synthesis of PMMA : PMMA was synthesized by solution polymerization in methanol of freshly purifiedmonomer on a vacuum line (10 − 5 torr) using 0.05% w/v azo bis isobutyronitrile (AIBN) as a free radicalinitiator. The polymerization was carried out in water bath kept at 60  ◦ C. The contents of the dilatometerwere poured in constantly stirred diethyl ether to precipitate PMMA. After drying in a vacuum oven for 24h, the sample was characterized [5,9] by infrared (IR) and nuclear magnetic resonance (NMR) spectroscopies.725  X-ray Diffraction Study of the Changes Induced During the ...,  R. HUSSAIN, D. MOHAMMAD  The IR spectrum of PMMA showed strong peaks for C=O at 1720-1740 cm − 1 and –C-O at 1020-1030 cm − 1 and medium (sharp) for C-H at 2925-2945 cm − 1 . A medium C=C peak at 1630 cm − 1 was absent, indicatingthe formation of polymer. The NMR spectrum showed singlets at  δ  =1.5,  δ  =2.3 and  δ  =3.6 for ethylene,methyl and methoxy protons respectively [5]. Synthesis of PMACl : Freshly purified methacryloyl chloride and toluene was vacuum distilledinto a dilatometer containing 0.05% w/v AIBN. The polymerization under vacuum (10 − 5 torr) at 60  ◦ Cwas stopped after 4 h by pouring the reaction mixture in diethyl ether. The precipitated polymer wascharacterized by IR and NMR after drying in a vacuum oven for 24 h. The IR spectrum showed strongpeaks at 1800, 1500, 1440 and 840 cm − 1 resulting from C=O and C-Cl groups. Olefinic peaks at 1630 cm − 1 were absent [10]. Thermal analysis : TG and DTA curves of samples were recorded on a DT 30B (Shimadzu, Japan)thermal analyzer. The polymers were heated at 10  ◦ C/min from ambient temperature to 500  ◦ C in a dynamicnitrogen atmosphere with a flow rate of 50 Ml/min. X-ray diffraction studies : A Philips PW1450/70 X-ray diffractometer equipped with a PW1390channel control goniometer and an argon – filled proportional counter was used for recording the diffrac-tograms. Radiation was generated from a copper anode tube (Cu K α  1.5418 ◦ A) using a Philips PW1730X-ray generator operated at 40 kV and 30 mA. Pyrolysis of polymers : Samples were heated at a heating rate of 10  ◦ C/min in a tube furnacemodel RO7/50 supplied by Heraeus (Germany) up to temperatures of 90, 140, 220 and 410  ◦ C. Each samplewas kept at the required temperature for 15 min. Results and Discussion Before studying the thermal effects on these polymers, thermal stabilities and degradation patterns weredetermined by employing TG and DTA. It is evident from Figure 1 that PMMA decomposes in 2 stages,i.e. degradation of allylic chain ends followed by main chain scission [11]. The polymer starts to degrade at200  ◦ C, followed by a second stage commencing at 340  ◦ C. The DTA curve also shows peaks in the sametemperature regions. The DTA peak appearing at 130  ◦ C, where no weight loss occurs, can be attributed tophase transitions. A similar peak is observed in DTA curves of PMACl reproduced in Figure 2 at 150  ◦ C.The polymer is stable up to 270  ◦ C beyond which it degrades via 2 stages. The endotherms with maximaat 292  ◦ C and 425  ◦ C represent the decomposition of PMACl.In light of the results obtained by TG and DTA experiments, these samples were subjected to heatingin a tube furnace at 90  ◦ C, 140  ◦ C, 220  ◦ C and 410  ◦ C. The samples were kept at limit temperatures for 15min. XRD patterns were recorded at the end of each thermal cycle (heating stage) to assess the changes inthese materials.The XRD patterns of PMMA and PMACl samples are reproduced in Figures 3 and 4 respectively.These figures show diffuse halos at lower 2 θ  values, which is typical of amorphous polymers. The ‘degree of amorphousness’, i.e. disorder in polymer chains, is obtained by an arbitrary measure of the radial intensitydistribution assuming that the more random and amorphous it is, the broader will be the halo. The radialwidth at one half maximum intensity might be used as a parameter for comparison. It must be kept in mindthat it is only for comparison and is of no theoretical significance. It is evident from the data presentedin Table that PMMA is more amorphous than PMACl. This may be explained based on the assumption726  X-ray Diffraction Study of the Changes Induced During the ...,  R. HUSSAIN, D. MOHAMMAD  that the pendant groups in PMACl are better aligned around the polymer backbone than the methoxygroups (having greater stearic hindrance) in PMMA. The degree of amorphousness decreases up to 140  ◦ Cfor PMMA and PMACl, beyond which it starts to increase. This is indicative of the improved alignment of macro molecular chains upon heating only up to 140  ◦ C. 1.00.80.60.40.20EXOENDO0 100 200 300 400 500Temperature (C ° )    W  e   i  g   h   t   F  r  a  c   t   i  o  n Figure 1.  TG and DTA curves of PMMA. 1.00.80.60.40.20EXOENDO0 100 200 300 400 500Temperature (C ° )    W  e   i  g   h   t   F  r  a  c   t   i  o  n DTATG Figure 2.  TG and DTA curves of PMACl. 20 ( ° )25 20 15 10 7abcde    I  n   t  e  n  s   i   t  y   A  r   b   i   t  r  a  r  y   U  n   i   t  s Figure 3.  X-ray diffractograms of PMMA heated atA = 25  ◦ C, b = 90  ◦ C, c = 140  ◦ C, d = 220  ◦ C & e = 410 ◦ C. 25 20 15 10 7abcde    I  n   t  e  n  s   i   t  y   A  r   b   i   t  r  a  r  y   U  n   i   t  s 20 ( ° ) Figure 4.  X-ray diffractograms of PMACl heated atA = 25  ◦ C, b = 90  ◦ C, c = 140  ◦ C, d = 220  ◦ C & e = 410 ◦ C. 727  X-ray Diffraction Study of the Changes Induced During the ...,  R. HUSSAIN, D. MOHAMMAD  Table Parameter Poly (methyl methacrylate) Poly (methacryloyl chloride)25  ◦ C 90  ◦ C 140  ◦ C 220  ◦ C 410  ◦ C 25  ◦ C 90  ◦ C 140  ◦ C 220  ◦ C 410  ◦ CD.O.A. 9.65 6.65 6.80 9.00 9.35 7.00 4.45 4.85 6.45 7.15R ( ◦ A) 8.05 8.05 7.38 7.91 8.24 7.95 8.05 8.05 7.51 7.77Lc ( ◦ A) 8.02 11.64 11.39 8.61 8.28 11.07 14.40 13.64 12.02 10.91 where D.O.A. = Degree of amorphousnessR = Interchain separationLc = Degree of order or mean crystallite size The interchain separation (R) can be measured by the value of 2 θ  at which the intensity of the diffusehalo is maximum using the equationR = 5/8 ( λ /sin θ )The results presented in Table show that the interchain separations do not differ much.The X-ray diffractograms of polymers seldom show the sharpness associated with inorganicand organiccrystals, because the polymer crystals are small or imperfect. Hence, the descriptive term used for these is“crystallite”. The degree of order in polymers is determined from the measurement of the crystallite size.The mean crystallite size (Lc) was calculated by the method of Short and Walker [13]Lc = 57.3 K λ / β   cos θ where K = Scherrer’s constant (0.87),  λ  = wavelength of X-rays (1.5418 ◦ A),  β   = peak width at half height(2 θ ). The data in Table show that the Lc values for PMMA increase up to 140  ◦ C, after which theydecrease. It is an established fact that in polymers the randomness of polymeric coils is increased at meltingtemperatures, beyond which degradation starts [14]. Similar phenomena are discernible for PMACl samples,when the polymer starts to degrade, as beyond the melting temperatures the degree of order decreases. References 1. A.W. Birley and M.J. Scott, Plastic Materials – properties and applications, Hill, UK, 1982.2. J.W. Nicholson, The Chemistry of Polymers, The Royal Society of Chemistry, UK, 1991.3. E.F. Kaelble, Handbook of X-rays – for diffractions, emission, absorption and microscopy, McGraw – Hill BookCompany, USA, p 21-9, 1967.4. M. Kakudo and N. Kasai, X-Ray Diffraction by Polymers, Elsevier Publishing Company, Holland, 1972.5. R. Hussain and D. Mohammad,  Mater. Lett., 20 , 375, 1994.6. R. Hussain and D. Mohammad, J. Mater,  Sci. Technol. 11 , 310, 1995.7. A. Saiani, J. Spevacek and J.M - Guenet,  Macromolecules 31 , 703, 1998.8. J. Spevacek and J. Brus,  Macromolecular Symposia 138 , 117 1999.9. R. Hussain, Degradation of Some Polymethacrylates, Ph. D thesis, Quaid-I-Azam University, Islamabad,Pakistan, p. 36, 1992. 728  X-ray Diffraction Study of the Changes Induced During the ...,  R. HUSSAIN, D. MOHAMMAD  10. M.H. Chohan, F. Shah, R. Hussain and J. Mater,  Sci. Letts 14 , 1352, 1995.11. N. Grassie, Chemistry of High Polymer Degradation Processes, Butterworths, UK, p. 32,1956.12. J.E. Johnson,  J. Appl. Polym. Sci. 2 , 205, 1959.13. M.A. Short and P.L. Walker,  Carbon 3 , 1, 1965.14. E.F. Kaelble, Handbook of X-rays – for diffractions, emission, absorption and microscopy, McGraw – Hill BookCompany, USA, p. 17-13, 1967. 729
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