A study has been made by differential scanning calorimetry of the phase separation phenomena of solutions of poly(2,6 dimethyl-1,4 phenylene-oxide) (PPO) in toluene. Upon cooling a homogeneous solution, liquid-liquid phase separation always precedes crystallization effects. By assuming the correction parameter g (in the expression for the chemical potential difference of the solution) to be temperature-dependent, the value of the heat of fusion is calculated to be 8·2 cal g−1 for PPO toluene crystals

Emmerik, P.T. van

Smolders, C.A.

English

van Emmerik, P.T.

NARCIS 

Differential scanning calorimetry of poly(2,6 dimethyl-1,4 phenylene-oxide)-toluene solutions

University of Twente Research Information

European Polymer Journal, 1973, Vol. 9, pp. 293-300. Pergamon Press. Printed in England. DIFFERENTIAL SCANNING CALORIMETRY OF POLY- (2,6 DIMETHYL- 1,4 PHENYLENE-OXIDE)-TOLUENE SOLUTIONS P. T. VAN EMMERIK and C. A. SMOLDERS Twente University of Technology, Enschede, The Netherlands (Receired 23 October 1972) Abstraet--A study has been made by differential scanning calorimetry of the phase separation phenomena of solutions of poly(2,6 dimethyl-l,4 phenylene-oxide) (PPO) in toluene. Upon cooling a homogeneous solution, liquid-liquid phase separation always precedes crystallization effects. By assuming the correction parameter g (in the expression for the chemical potential difference of the solution) to be temperature-dependent, the value of the heat of fusion is calculated to be 8"2 cal g- 1 for PPO-toluene crystals. I N T R O D U C T I O N  SEVERAL studies have been performed on the transition properties ofpoly(2,6 dimethyl- 1,4 phenylene-oxide). *~1'2'a) These studies showed that crystallization of PPO occurs if dilute solutions of  PPO-solvent  mixtures are allowed to stand for a rather long period at a phase separation temperature. Even then crystallization is poor  and will be destroyed by thermal treatment. Experimental results indicated the importance of the solvent, since crystallization occurred only in the presence of  certain organic liquids which played an important  role in building up the crystal lattice. Melting points of  the carefully dried material were reported as between 510-540 K, glass transition temperatures between 480 and 500 K. A differential scanning calorimetry study has been made of  PPO-a-chloronaph-  thalene solutions in order to measure the melting point depression temperatures. ~ The curve thus obtained could be explained by the theory of  melting point depression of  polymer solutions with a temperature and concentration independent interaction parameter.  We have studied phase separation phenomena in concentrated PPO-toluene solutions which showed that crystallization of  PPO is always preceded by liquid- liquid phase separation. ~4'5) Computer  calculations with a temperature and con- centration dependent interaction parameter  gave good agreement between the calculated and experimental phase separation curves. (6) I f  melting point depression theory is adapted with such a parameter,  values can be obtained for the heat of  fusion of PPO-- to luene  crystals. The results obtained by differential scanning calorimetry are reported in this paper. E X P E R I M E N T A L  The poly(2,6 dimethyl-l,4 phenylene oxide) used was characterized by 1VIn = 25000 and N-lw = 39000 Tq = 495 K. PPO-toluene samples were weighed in glass tubes, flushed with nitrogen and sealed under vacuum. Complete dissolution occurred in a thermostat bath at an appropriate temperature. Homogeneous solutions in these tubes were brought into liquid N2 which caused such fast cooling that no visible * PPO. 293 294 P.T. VAN EMMERIK and C. A. SMOLDERS phase separation took place and the solution turned completely into the glassy state. The tubes were opened, small pieces were cut off the sample and hermetically sealed in the sample pans normally used for liquids. To determine the Tg of the samples, the sealed pans were heated upon a heating stage to 150 °, after a certain time quenched in liquid nitrogen, and quickly transferred into the calorimeter (DSC-1B) already kept at --100 °. After the temperature equilibrium was restored, Tg could be determined at a heating rate of 8°/min. The contents of the sample holder were between 8 and 15 mg. To determine the heat effects due to phase separation at other temperatures, the sample holders were heated in the DSC to 150 ° and subsequently cooled in the DSC to the appropriate temperature. THEORY OF M E L T I N G  POINT DEPRESSION For an equilibrium between polymer crystals and a polymer solution, the following relation is valid: iT) /~,c _ /~ (o )  __/zul _ / ~  (o). (1) /~,¢ = thermodynamic potential of  one mole of  repeating units for the polymer in the crystalline state. /~(o)___ thermodynamic potential of  one mole of  repeating units for the polymer in the standard state. /~1 = thermodynamic potential of  one mole of  repeating units for the polymer in solution. This relation is equal to - A t , y . .  = + a~,~.,. (2) f s t ands  for melting, M for mixing. By using Eqn. (2), the following relation can be derived for the depression of  the melting temperature :(7) Tm o /Vx = (R V JA H,  Vl) (I --  BVI vt/RTm). (3) :I'm ---- melting temperature of  the polymer crystals in solution. T ° = melting temperature of  the pure crystalline polymer. V, ---- molar volume of  the repeating unit. VI = molar volume of  the diluent. AH, = heat of  fusion. Bt = interaction parameter of  the solvent-solute pair. va ----- volume percentage diluent. R ----- universal gas constant. Using expression (3) a plot of  the quantity (÷m against v~/T., should give a straight line. This has been reported to be the case for the system PPO-a  chloronaphthalene. ° )  For PPO-toluene solutions, this is not the case. The system PPO-toluene can be described using an interaction parameter, $, in the Differential Scanning Calorimetry of PPO Solutions 295 expression for the free energy of mixing (6) which is linearly dependent on concentra- tion and temperature g ----- go(T) + gl(T)wl. Then AtzM.u is given by At~u,u Mo Mo RT -- M, In wl + (1 -- w~) ~ -- (1 -- wl) + (go + 2gawl)(1 -- wx) 2. (4) Mo = molecular weight of the solvent. M, = number average molecular weight of  the polymer. w~ = weight fraction of the polymer. go and g~ = empirical correction parameters (dependent only on T). Taking ~8~ AT.T, ,  A/zs,, = AH. ° . (TmO)------ T. (5) A combination of Eqns. (2), (4) and (5), replacing T by T,~ gives AHO AT. Tm RTm {Mo Mo • (TmO)------ 5  -~ - -  ~ In wl + (1 - -  w~)  ~ - -  (1 - -  wl) + (go + 2g~w~)(1 -- Wl)Zt, (6) o r  AT _ _  R/AHO {Mo Mo = In w~ + (1 -- w,)-M-~, -- (1 --  w,) ( r  o)----~ 2glwl)(1 -- w~)2). (7) + (go + The only term which is a function of temperature in the right-hand side of Eqn. (7) is the term (6) (go -I- 2g~wl), with 566-0 -- 6.7 (T~ -- 273) go = , (8) R T .  1251-8 -- 0.7 (Tin -- 273) g~ = (9) RT,. Plotting AT/(T,,,°): against the term in brackets on the right-hand side of Eqn. (7) and taking account of (8) and (9), a straight line should be obtained with slope equal to -- R/ AH~, °. RESULTS In the system PPO-toluene, three-phase separation temperatures can be detectedJ 4~ When a homogen ,ous solution is cooled in the DSC at a rate of 16 ° min -~, an exotherm is observe,  at a certain temperature T~. Upon rect ating of  such a solution, a very broad endotherm is observed ending ~ temj ~:i:ure/'2. When reheating occurs a certain time after the exotherm F .1.9 296 P.T. VAN EMMERIK and C. A. SMOLDERS | heating 60 rain. after cooling below T 1 I I ,, I ENDOTHERM I heating immediately after cooling below T I ~ ,  cooling 16" C/rain 'l i - -  E I i h 'Ta r3 296 334 363 ~ T K  FIG. 1. Phase separation temperatures and heat effects determined with differential scanning calorimetry for a 30 per cent of weight poly(2,6 dimethyl-l,4-phenylene-oxide)-toluene solution. effect has been observed, then next to the broad endotherm effect a second endotherm heat effect is observed at a temperature higher than T2, ending at the temperature Ta. In Fig. 1 this is shown for a 30 per cent of  weight PPO-toluene solution. I f  a homogeneous solution is cooled to a temperature just above T1 and reheated immediately no effect is observed at all. When reheating occurs after the solution is held at a temperature just above Tt for a certain time, a broad endotherm is observed which becomes larger as a function of ageing time at T~ until a maximum is reached. The second endotherm appears only after the first endotherm is observed. In Fig. 2 this is shown for a 30 per cent of  weight PPO-toluene solution. f I T=300 K i I + I i ENDOTHERM I / / ~ x i  ~ ~ - ~ .  I *~ )l heating 30 min. af ter  cooling at I ~o ', T=~K', ~: I heating 10 rain. a l ter  cooling at  I I T=300 K' ) ' , 1 l heating immediately after cooling at i I T=300 KI II I I t I IT, i'2 i'3 -I 300 334 363 = T K  FIG. 2. Heat effects determined with DSC for a 30 per cent of  weight poly (2,6 dimethyl- l ,4 phenylene-oxide)-toluene solution cooled at T = 300 K for various periods of  time. Differential Scanning Calorimetry of PPO Solutions 297 Samples of  pure PPO, carefully isolated from solutions in a manner described elsewhere, ~9) did not show any crystallinity by DSC measurements or with X-ray diffraction. Only those PPO samples isolated from solutions which were held at a phase separa- tion temperature for a certain time (long enough so that the second endotherm was observed by DSC) gave an exotherm at T = 510 K. Hence these samples were partially crystalline. In all other samples, Tg was 495 K. / / 150- .o~ C / i :B 5 0 -  oc f ' ~ .  // l I / • 11 ©- - 50- /4 r2~/ 0 20 40 60 weight Der'cent polymer" Fie. 3. Phase separation temperatures of poly(2,6 dimethyl-l,4-phenylene-oxide)-toluene solutions. Full lines: A, spinodal curve; B, cloudpoint curve; C, melting point curve, obtained earlier. C4) 0 ,  Spinodal temperatures; V, cloudpoint temperatures; (3, melting point temperatures, determined by DSC. - - - ,  Glass transition temperatures determined earlier; t*) ×, the same, determined by DSC. In Fig. 3 all phase transition temperatures obtained by DSC calorimetric measure- measurements are shown, together with the glass transition temperatures. Since the lower temperature limit of the DSC is --100 °, no glass transition tem- peratures could be determined in solution less than 20 per cent of weight PPO-toluene. Also plotted in Fig. 3 are the phase separation results obtained earlier/4~ Using data from Fig. 3 and taking Tm° = 510 K, in Fig. 4 AT/(T,.°) 2 is plotted against - ~ In w l  + ~ (I - wx) - (1 - w l )  + (go + 2 g , w l ) ( 1  - w , )  2 . A linear dependence is obtained, in accordance with theory. The slope obtained by the least squares method, being equal to R/AHu °, gives for one mole repeating unit AHu ° = 1365.0 cal. If  PPO crystallizes without toluene this would mean A H f  = 11.4 cal g - 1  a value also obtained by Schultz (a~ (11.63 cal g-1). 298 P.T. VAN EMMERIK and C. A. SMOLDERS Z~'T . 10 4 (Try)2 7.0 l • • 6% 5.C 4.C + (go+2g,wl)('- Wl)2 J C).1 0-,~ 0[3 C).4 0'.5 Fro. 4. A plot of AT/(Tm*) 2 against {Mo Mo } -- ~ l n w l + ~ ( 1 - - w l ) - -  ( l - - w l )  + (go + 2g twl ) ( l - -wl )  2 . Later work indicates °°) that the unit cell of PPO, crystallizing with low molecular weight solvent, contains eight polymer segments and four solvent molecules. As a consequence AH, ° = 8.2 cal g-1 DISCUSSION PPO belongs to the class of slowly crystallizing polymers3 l°) Crystallization occurs only after very careful treatment of very dilute solutions. It is to be expected then that upon cooling concentrated solutions, crystallization will hardly occur. A general description of the phase separation phenomena of such solutions has been given by Papkov and YefimovaJ 11) Upon cooling of a concentrated solution, liquid-liquid phase separation will take place, leading to a concentrated and a dilute phase. Two mechanisms of liquid- liquid phase separation are possible, viz. nucleation and growth of one of the phases or spinodal decomposition. Both mechanisms occur in PPO-toluene solutions, t4) The nucleation process starts after induction periods which depend on temperature312) If a homogeneous solution of PPO-toluene is cooled at a rate of 32 ° rain- 1, nuclea- tion will not take place in a detectable amount and the spinodal area is entered. The solution in that ease is not stable even to the smallest fluctuations in concentra- tion and demixes instantaneously, tla) This is shown as an exotherm at the spinodal temperature/'1 in Fig. 1, measured by DSC. Direct reheating of the demixed solution, preventing crystallization, will cause a continuous change of the phase composition. This is observed on the DSC thermo- gram as a very broad endotherm. The temperature T2 represents the cloudpoint of that solution. At this temperature the solution becomes homogeneous again and upon further heating no thermal effect is observed. Differential Scanning Calorimetry of PPO Solutions 299 W h e n  a demixed  so lu t ion  is kep t  a t  the t empera tu re  o f  phase  separa t ion  for  a cer ta in  t ime,  the concen t ra ted  phase  will pa r t i a l ly  crystall ize.  I f  the po lyme r  is i so la ted  careful ly  f rom this solut ion,  the  t h e r m o g r a m  shows a mel t ing peak  at  510 K.  The  hea t  effect has such values tha t  the percentage  o f  crys ta l l in i ty  o f  the pure  PPO can  be es t imated  at  a b o u t  2 pe r  cent  in solu t ions  o f  5 or  10 weight  pe r  cent  P P O -  to luene  and  even less in more  concen t ra ted  solut ions.  W h e n  demixed  solut ions  in which crys ta l l iza t ion  has occurred  are  hea ted  in the ca lor imeter ,  two b r o a d  endo the rm peaks  are observed.  The first one is the hea t  effect caused by  dissolut ion  o f  the non-crys ta l l ized  po lymer ,  the second endo the rm is the  hea t  effect caused by  the mel t ing o f  the crystal l ine mater ia l .  The  t empera tu re  where  the so lu t ion  becomes  comple te ly  homogeneous  again  is t aken  as the mel t ing po in t  o f  tha t  pa r t i cu la r  solut ion.  The  mel t ing po in t  curve and  the c loudpo in t  curve thus  ob ta ined  are in agreement  with those  ob ta ined  previously34) The  sp inoda l  curve, t aken  before  as a five minutes  t ime l imi t  o f  phase  separa t ion  af ter  cool ing,  is several  degrees higher.  The  glass t rans i t ion  t empera tu res  measured  by  D S C  are much  lower  tha t  those  previous ly  obta ined .  In  ear l ier  work ,  glass t rans i t ion  t empera tu res  were ob ta ined  by microscopic  observa t ions  for  lower,  and  D .T .A.  measurements  for  higher  concent ra -  t ions which for  concen t ra t ions  above  40 per  cent  P P O - t o l u e n e  solut ions,  gave too  high values.  R E F E R E N C E S  (1) J. M. Barrales-Rienda and J. M. G. Fatou, KolloidZ.u.Z. Polymere 244, 317 (1971). (2) F. E. Karasz, J. M. O'Reilly, H. E. Bair and R. A. Kluge, Analytical Calorimetry, p. 59. New York (1968). (3) A. R. Schultz and C. R. McCullough, J. Polym. Sci. A2, 7, 1577 (1969). (4) P. T. van Emmerik and C. A. Smolders, J. Polym. Sci. C38, 73, (1972). (5) P. T. van Emmerik and C. A. Smolders, J. Polym. Sci. C39, 311, (1972). (6) P. T. van Emmerik and C. A. Smolders, Europ. Polym. J. 9, 157 (1973). (7) P. J. Flory, Principles of  Polymer Chemistry, p. 568. Cornell Univ. Press (1953). (8) J. D. Hoffman, J. chem. Phys. 29, 1192 (1958). (9) F. C. Franck, A. Keller and A. O'Connor, Phil. Mag. 4, 200 (1959). (10) E. P. Magre and J. Boon; Proceeding of  the IUPAC International Symposium, Leiden 1970, p. 835. (11) S. P. Papkov and S. G. Yefimova, Faserf. Textiltech. 4, 118 (1967). (12) P. T. van Emmerik, C. A. Smolders and W. Geymayer, Europ. Polym. J. 9, 309 (1973). (13) C. A. Smolders, J. J. van Aartsen and A. Steenbergen, KolloidZ.u.Z. Polymere 243, 14 (1971). R6sum6--On a 6tudi6 par calorim6trie diff6rentielle les ph6nom~nes de s6paration de phase de solu- tions de poly(dim6thyl 2,6 oxyde de ph6nyl~ne 1,4) (PPO) dans le toluene. Pendant le refroidissement d'une solution homog~ne, la s6paration de phase liquide-liquide pr6c~de toujours les effets de cristalli- sation. En supposant que le parametre de correction g (dans l'expression donnant la diff6rence de potentiel chimique de la solution) d6pend de la temp6rature, on a calcul6 que pour des cristaux de PPO-tolubne la chaleur de fusion 6tait de 8,2 cal-x. Sommario--Mediante calorimetria differenziale esploratrice, si 6 eseguito uno studio sui fenomeni di separazione di fase di soluzioni di poli(2,6 dimetil 1-1,4 fenilene-ossido) (PPO) in toluene. In seguito a raffreddamento di una soluzione omogenea, la separazione della fase liquido-liquido precede sempre gli effettiddi cristallizzazione. Supponendo che il parametro correttivo g (che esprime la differenza di potenziale chimico della soluzione) dipenda dalla temperatura, si 6 calcolato che il valore del calore di fusione per cristalli di PPO-toluene 6 di 8,2 cal g-1. 300 P.T.  VAN EMMERIK and C. A. SMOLDERS Zusammenfassung--In L~Ssungen von Poly(2,6-dimethyl-l,4 phenylen) oxid (PPO) in Toluol wurde die Erscheinung der Phasentrennung durch differentielle Scanning Calorimetrie untersucht. Beim Abktihlen einer homogenen L(Ssung geht den Kristallisationseffekten immer eine Trermung in zwei fliissige Phasen voraus. Unter der Annahme, dab der Korrekturparameter g (in der Gleichung f'tir die chemische Potentialditferenz der L6sung) vonder Temperatur abhangig ist, wird f'tir die PPO-Toluol Kristalle ein Wert von 8.2 cal g-1 ftir die Schmelzw/irme berechnet. 

Abstraet--A study has been made by differential scanning calorimetry of the phase separation phenomena of solutions of poly(2,6 dimethyl-l,4 phenylene-oxide) (PPO) in toluene. Upon cooling a homogeneous solution, liquid-liquid phase separation always precedes crystallization effects. By assuming the correction parameter g (in the expression for the chemical potential difference of the solution) to be temperature-dependent, the value of the heat of fusion is calculated to be 8&quot;2 cal g- 1 for PPO-toluene crystals

P. T. Van Emmerik

C. A. Smolders

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DIFFERENTIAL SCANNING CALORIMETRY OF POLY- (2,6 DIMETHYL- 1,4 PHENYLENE-OXIDE)-TOLUENE SOLUTIONS

Analytical Calorimetry,

Principles of Polymer Chemistry,

https://doi.org/10.1016/0014-3057(73)90089-X

Differential scanning calorimetry of poly(2,6 dimethyl-1,4 phenylene-oxide)-toluene solutions

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University of Twente Research Information

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