Calorimetric study of filled linear polyurethanes

Calorimetric study of filled linear polyurethaues

119

3. A. B. ZEZIN, N. F. BAKEYEV, O. A. ALEKSINA and P. V. KOZLOV, Dokl. Akad. N a u k SSSR 172: 889, 1967 4. A . B . ZEZIN, N. F. BAKEYEV, V. M. GUREVICH and P. V. KOZLOV, Abstracts Internat. Maeromol. Chem. Symposium, Toronto, 1968 5. C. H. BAMFORD, A. ELLIOT and W. E. ttANBY, Synlhotic Polypeptides, New York, 1956 t;. G. D. FASMAN, In: Polyami,~o Acids, Polypeptidcs ~md Protei,~s, M. Stahmann (Ed.), p. 221, University of Wiscousin Press, Madison, Wis., 1962 7. Yu. Ye. EIZNER, Vysol¢onw)l. soycd. A l l : 364, 1969 (Translatod in Polymer Sci. U.S.S.R. 11: 2, 409, 1969) S. V. Ye. ESKIN and I. N. SERDYUK, Vysokom()l. soy(~d. A l l : 372, 1969. (Translated in Polymer Sci. U.S.S.R. l l : 2, 417, 1969)

CALORIMETRIC STUDY OF FILLED LINEAR POLYURETHANES* V. P. PRIVALKO, Yu. S. LYPATOV, Yu. Yu. KERC~A and L. V. MOZZHUKI-IINA Institute of High Polymer Chemistry, Ukrain. S.S.R. Academy of Sciences S. V. Lebedev All-Union Synthetic Rubber Research Institute (Rece~:ved 9 October ] 969)

A FILLED, crystallizing oligomer (oligoethyleneglycol adipate) was used as example [1] to show that the fundamental principles governing the polymer-solid phase reaction, established earlier on amorphous polymers [2], are also applicable to crystalline polymers. The earlier results [1] have indicated that moderate filler concentrations will only cause a slight alteration of the crystalline and amorphous phase contents due to the appearance of "boundary" regions of polymer near the filler surface. It can be assumed that the earlier reached conclusions [1] will be correct also for filled linear polyurethanes (PU), where the properties depend on the oligo-ester blocks of the main chain [3-5]. It should be noted that suitable verification objects of this theory are the linear PU based on polyesters capable of crystallizing only after a certain period [3], so that they can be prepared in the amorphous as well as the crystalline state. The study of the thermodynamic and kinetic properties of such systems with a boundary surface present is also the aim of the work reported here. EXPERIMENTAL

The s t u d y objects were linear P U samples based on oligocthyleneglycol adipate of molecular weight' (mol.wt.) 2000 and 2,4-toluylene diisocyanate (2,4-TD) filled with carbon TM-70 having a 70-75 mP/g specific surface, and with aerosil, spec. surface 175 m~/g. The filler content was 1, 5, 10 and 20 parts b y weight (pbw) per 100 pbw of polymer. The filler * Vysokomol. soyed. A13: No. 1, 103-110, 1971.

120

V. P. PRIVALKO '/g.~ ol ~2 A3 v4 v5 0"5

2A L_

l

,I "

-50

I

0 "£,°C

I

50

100

FIO. 1. T h e heat content of: a--crystallized, and b--annealed unfilled P U samples. Filler contents: 1--0, 2--1 pbw aerosil, 3--1 pbw carbon, 4--10 pbw aerosil, 5-- .10 pbw carbon. was introduced by milling it with the polymer for 10-20 min at 20-25°C. The calorimetric measurement method was the same as that used earlier [ 1, 4]. The sample weight was 0.3-0.35 g and the heating gradient did not exceed l°C/min.

T A B L E ] . T H E M E L T I N G P A R A M E T E R S OF C R Y S T A L L I N E P U

SAMPLES AS A FU-NCTI01~ OF F I L L E R

CONTENT

Filler content, pbw/100 pbw of polymer

Carbon 0 I

5 10 2O Aerosil 1

5 10 20 * Calculatedfor 1 g polymer,

Samples after 3-day crystallization

Original samples

cal/g. °C

heat of melting, eal/g*

0.352 0'294 0"305 0"330 0.342

7.47 6.49 5.35 5.21 5.12

41 41 40 39 39

0"306 0"322 0"348 0"356

6.38 4.86 5.35 4.9

39 40 40 39

Glo~

m . p.~

°C

heat of melting, eal/g*

m . p.~

oC

2.93 5.91

38 39

3-51

36

4.75

39

2.50

37

Calorimetric study of filled linear polyurethanes

121

Figure l a and b show respectively the heat contents curves of crystalline (original) and amorphous (annealed) samples of filled PU. The curves of all the studied crystalline samples (Fig. la) consist of a linear part in the range from -- 50 to the starting temperature of the glass transition range, and are followed by a section showing a steady increase in this range, and then by a linear change of the heat content up to 20°C, after which the latter steadily increases and changes to the melting peak. The end of melting at about 47-49°C is again followed, after melting, by a linear dependence of the heat content on temperature. Some of the results obtained from these curves during heating and melting of crystalline samples are contained in Table 1 which shows that an increase of the contents of both fillers caused the heat of melting ef the starting material (calculated per 1 g polymer) to decrease steadily, and this TAB$~E

2 . 51'~F~ E F F E C T

OF FILLEftS

ON THE PU

G L A S S TaA]~7,~ITION RARIGE

Original samples Filler content, pbw/100 pbw of polymer

Carbon 0 1 5 l0 2O Aerosil 1 5 l0 2O

glass transition range start

end

--35 --32 --37 --36 --30

--20 --15 --18 --20 --12

--32 --30 --36 --36

--10 --12 --22 --24

OF FILI.ED

I~]NEAR

SAMPLES

ACYv, cal/g. °C*

Annealed samples glass transition range

ziGv, cal/g. °C*

start

end

0.076 0.072 0-070 0.067 0.065

--43 --40 --40 --35 --33

--25 --25 --27 --25 --24

0'112 0'099 0-096 0'086 0'084

0.071 0.068 0.064 0.063

--40 --40 --35 --36

--26 --28 25 27

0'098 0'090 0"083 0"081

* Calculated for 1 g polymer.

agreed with earlier findings [1]. The absolute value of the heat content increased as a function of increasing filler content, but remained below that of the unfilled polymer. The same was also observed in the study of annealed samples (Fig. lb), where the curves again consisted of a linear section in front of and behind the glass transition range; this was evidence of an absence of crystallization during the test. The glass transition range tended to shift towards higher temperatures on increasing the filler contents (Fig. lb, Table 2), b u t it also became narrower in the case of the annealed samples.

RESULTS A f a i r l y l a r g e n u m b e r of r e p o r t s is k n o w n to d e a l w i t h t h e p r o b l e m s o f s t u d y i n g t h e effect o f a solid s u r f a c e o n t h e p r o p e r t i e s of c r y s t a l l i n e p o l y m e r s [6-10], b u t the m a j o r i t y of these m a i n l y i n v e s t i g a t e d the influence of solid particles on the s t r u c t u r a t i o n kinetics a n d the morphology of the polymers, while n o t h i n g was done to clarify the question of t h e r m o d y n a m i c p r o p e r t y changes resulting f r o m t h e a d d i t i o n o f d i s p e r s e fillers. I n o u r e a r l i e r w o r k [1] of a n a l y s i n g t h e t h e r m o -

122

V. 1), PRIVALKO

dynamics of the reaction of the crystalline oligomer with fillers we used parameters which were sensitive to the relative contents of crystalline and amorphous zones in the system (crystallinity), and also to the structure of the amorphous zones (the absolute heat content of crystalline samples). By extending this approach to the linear crystalline PU, we shall assess the effect of fillers on the crystallinity of the samples crystallized by us. To calculate the crystallinity of a polymer, it is usually necessary to know either the density of samples with zero and 100% crystallinity, or the enthalpy of melting of completely crystalline samples [11], or the constant of the K u h n Mark-Houwink equation for the viscosity of a dilute solution of the particular polymer in a 0-solvent [12], etc. As these values were not known in our case, we calculated the crystallinity by a simple method suggested recently by other authors [13, 14], according to which the crystallinity X is determined from the formula:

x = 1-Ac/Jc

,

(1)

in which AC and ACa are heat increments during glass transition of the studied sample and of one with zero crystallinity respectively. These values were in our case those of the crystallized and the annealed samples (Table 2). Insertion into eqn. (1) of the values of AC----0.076 and ACa=O'll2 cal/g.°C for the filled sample gave X=0.32. This value and the heat of melting, which is AH*----7.47 cal/g (Table 1), can be used to determine the heat of melting of a P U sample with 100~o crystallinity from:

(2) The enthalpy of melting of a 100~o crystalline linear P U sample based on oligoethyleneglycol adipate of mol.vt. 2000 and 2,4-TD, is thus AHm----23"3 cal/g : 4 . 0 7 kcal/mole. The entropy of melting was ASm:AHm/Tm----4070/314~12"95 cal/mole. ° C : 1.27 eal/°C.bond (the m.p. of the unfilled sample Tm--41°C-- 314°K). We can now compare here the obtained melting entropy with t h a t of the original oligo-ester. Earlier we had said [1] that the enthalpy of melting oligoethyleneglycol adipate crystals was 30 cal/g=5.1 kcal/mole. As the Tm of the annealed oligomer sample was 53°C [1], ASm----5100/326~15"6 cal/mole. °C----1.56 cal/°C •bond, which agreed well with the melting entropy of some polyesters of similar chemical type [15]. A rough approximation of the entropy of the liquid state (melt) of the P U shows it to be the same as that of the original oligo-ester, and comparison of the melting entropies of these polymers indicated that the P U should have a larger entropy in the crystalline state than the oligo-ester. This conclusion confirms the assumption made earlier [4] and is thermodynamic proof of the fact t h a t the 2,4-TD units are the crystal lattice defects of P U [16, 17]. The crystallinities of the studied objects, calculated from eqns. (1) and (2) (X1 and X~. respectively) are shown as fimetions of the filler content by volume in Fig. 2. It shows t h a t an increase of the filler content produced a regular decrease of the crystalline phase present in the system, and t h a t the calculated crystal-

CMorimetric study of filled linear polyurethanes

123

linities agreed well. An increase of the filler content also produced a steady decrease of the peak of the heat of glass transition of the crystalline and annealed filled samples (Table 2). The transition from the glass-like to the highly elastic state is known to be a cooperative process and the size of the glass transition peak will obviously depend oll the number of molecules, or of their segments, -2 °I

x~A.:~

0.3

0"2 "~ ~ o

z~3 °

,,# o

0,% F~c:. 2. The erysta.llinity of filled PU ealeulat,ed from: 3,4--eqn. (1), 1,2--eqn. (2) as a. function of the ~ v/v content of 1,3--,~erosil, 2,4--carbon.

participating in this transition. As vitrification can take place only in the amorphous zones of "semierystalline" polymers [18], the peak reduction must mean that some parts of the macromolecules are excluded from this process. This result, which is evidence of a structural ehange in the amorphous phase of filled P U compared with the unfilled material, naturally must be explained by the creation of boundary areas by the polymer near the filler surfaee, which greatly suppress maeromolecular movement [1, 2]. A point worth assessing was the amount of polymer actually present in the boundary areas. I f one assumes that the maeromoleeules present near the filler surface do not participate in vitrification, which agrees with the statement made above, the amount of such "excluded" macromoleeules could be determined from formula (1), in which the ACa value of 0.112 cal/g.°C for the annealed, unfilled sample is replaced by that for the annealed, filled samples (Table 2). The approximation was carried out for systems containing 5 pbw of both the fillers. The AC value of 0.096 cal/g. °C was that of the system P U - 5 pbw carbon, and insertion into formula (1) showed the amount of polymer present in the boundary surface, y, to be 0.14. The thickness of the layer, AG, using formula (1), is (G+AG/G) 3 - 1 =y (C/1--C'),

(3)

in which G--filler particle radius, C--polymer content by volume in the system, C~0.975 in a system contain 5 pbw carbon. The insertion of this value into eqn. (3) gives AG/G=0.86, Assuming that the average particle dimension of carbon is about 400 A, the thickness of the boundary layer of the system will be about 170 J~. Where the system contains aerosil (5 pbw), AC=0.090 cal/g. °C, so that y=0.20. By using C-~0.975, we get AG/G= 1.04 from eqn. (3). As the aerosil particles have an average diameter of about 250 A, the system P U - 5 pbw aerosil will have a 130 A depth of boundary layer. The depth of boundary layers obtained for filled P U correlated well with a value of 100 A cMculated earlier for the initial oligo-ester containing 5 pbw aerosil [1],

124

V . P . PRIVA~KO

The thermodynamic analysis of the effect of the solid surface on the properties of linear P U in the crystalline and amorphous states thus showed that the introduction of fillers will reduce the crystallinity (the proportion of crystalline zones) and also cause the amorphous phase structure to change due to the transfer of some maeromolecular parts to the boundary layer near the solid surface [1]. One can gather from Table l that the absolute heat capacity of the polymer phase of filled systems will be lower than of the unfilled PU. This can be interpreted, on the basis of earlier work [1], as the consequence of a reduction in chemical potential which must characterize an improved distribution order of the macromolecules relative to each other in this layer, compared with the remaining bulk of amorphous polymer. One can therefore expect an improvement of thermodynamic and kinetic conditions for the polymerization of filled systems containing small filler quantities, which will act as crystallization centres. Confirmation of this theory was sought in studies of the rate of crystallization of filled P U as a function of filler content and the nature of the surface. We investigated samples which were stored for 3 days under identical conditions at room temperature after heating to 100°C. The measure of the rate of crystallization was the melting heat of the crystallized samples, which is proportional to the amount of produced crystalline phase. These experimental results are contained in the last two columns of Table 1, which shows that a 1 pbw addition of both fillers actually increased the rate of crystallization; both the melting heats of these filled systems were higher than that of the unfilled polymer, This behaviour can be explained by the small filler quantities having acted as centres of crystallization. A larger filler content produced some decrease of the rate of crystallization; this was evident on comparing the heats of melting of samples containing 20 or 1 pbw of each filler. The reason seems to be that a larger polymer quantity transfers into the boundary layer, which results in a general viscosity increase and this hinders crystallization, with formation of smaller size crystals, and also creates defects. According to the isothermal crystallization theory of polymers, which states that the maximal melting temperature of crystals depends on their perfectness [9], one can assume that the dominant factor in the crystallization of highly filled systems will be the increase in the number of defects in the crystals, because the m.p. of systems containing 20 pbw filler was lower than that of systems with 1 pbw filler content, as Table 1 shows. This also seems to explain the regular crystallinity decrease resulting from increasing the filler content (Fig. 2); crystallization here took several months. I t should also be noted that the rate of crystallization of aerosil-containing systems was smaller than that of systems containing the same carbon quantities (Table 1). It can be suggested that this is explained by the increased surface of the solid phase, as well as a greater affinity of P U for the polar surface of aerosil. The different types of polymer reaction with the fillers must also influence the glass transition temperature of filled systems, but Fig. lb and Table 2 show these to be similar where the systems contained similar filler quantities. The slight

Calorimetric study of filled linear polyurethanes

125

glass temperature increase on increasing the filler content is known to be explained b y an increase of the amount of polymer present in the boundary layer, which is less mobile [2]. Cp,i ~[/,.q.°O

0.5

2 "2

1 0

50

~

~

%

'~

'

or-"

I

loo T,°g

FIt~. 3. The heat eonten~ of a crystallized PU stunp|e containing 20 pbw aerosii which had been heated to (°C): 1--200, 2--100. In conclusion we shall examine the mechanism of the reaction of the polymers with fillers. Various groups of investigators emphasized the chemical [20, 21] and physical [22, 23] reaction mechanisms. An analysis of our results for the purpose of clarifying the nature of the reaction of P U macromolecules with the solid phase, having obtained the samples by mixing oll rollers (milling), which could have resulted in the formation of polymeric macro-radicals [24], showed that a chemical reaction could take place with the filler surface. We shall assume that such a chemical reaction is irreversible, while the physical reaction is reversible (adsorption). An important criterion of the nature of this reaction is the sign ill front of the thermal adsorption coefficient of the macromolecules on the solid surface [26]. It is quite clear that a reversible (physical) type of reaction in the boundary layers would diminish on raising the temperature, while the reverse would be true for a chemical reaction. We studied the effect of the pre-heating temperature on the crystallization kilmties and on vitrification of systems containing 20 pbw aerosil. The results are reproduced in Fig. 3, which shows that prior heating to 200°C greatly increased the amount of crystalline phase in the system after a 3-day crystallization period (the heat of sample melting was 8.65 cal/g), as compared with a sample pre-heated to 100°C (heat of melting=2.50 cal/g). Also, a sample tempered after heating to 200°C showed the same position and width of the glass transition range. Samples containing 10 pbw carbon gave the same results, and the findings can thus be explained by the amount of polymer fix the boundary layer becoming smaller on increasing the temperature from 100 to 200°C as a result of greater macromolecular mobility and macromolecular desorption from the filler surface. As the peak height of the glass transition heat of the annealed sample remained smaller than that of the unfilled polymer (0.098 and 0.122 cal/g.°C respectively)

126

V.P. PIClVALKO

a decrease of the boundary layer depth can occur only b y desorption of that number of the macromolecules which present in the periphery of this layer. The main polymer mass present in the boundary layer must thus be irreversibly bound to the filler sm'face under our experimental conditions. The results of studying the effect of the boundary phase on the properties of a crystallizing oligo-ester and of linear P U based on it thus showed that the properties of filled polymer systems will depend on the amount of polymer present in the boundary layer near the solid surface, and on the changes in properties which occur in it, compared with those of the polymer in the rest of the space; this will be true regardless of the physical state or phase state of the polymer. CONCLUSIONS

(1) The heat capacity of polyurethane containing finely disperse fillers (aerosil and carbon) was studied in the range --50 to +100°C as a function of temperature. (2) The addition of fillers was found to cause some decrease in the crystallinity of the filled polyurethane and greatly reduced the absolute heat content of the polymer; this is explained by the appearance of macromolecules with reduced mobility in the amorphous zones near the contact surface with the solid particles.' (3) The rate of crystalline phase formation in the systems containing aerosil was found to be smaller than in those with carbon. (4) Confirmation was obtained that the thermodynamic and kinetic properties of filled polymer systems depend on the amount of polymer present in the boundary layer near the solid phase surface, and on changes of its properties when compared with the bulk of the polymer. Translated by K. A. ALLE~

REFERENCES

1. V. P. PRIVALKO, Yu. S. LIPATOV and Yu. Yu. KERCHA, Vysokomol. soyed. All: 237, 1969 (Translated in Polymer Sei. U.S.S.R. 11: 1, 266, 1969) 2. Yu. S. LIPATOV, Fiziko-khimiya napolnemwkh polimerov (The Physical Chemistry of Filled Polymers). Izd. "Naukova Dumka", 1967 3. V. P. PRIVALKO, Yu. S. LIPATOV and Yu. Yu. KEIgCHA, Dokl. Akad. N~uk Ukrain. SSR, Seriya B, No. 3, 255, 1969 4. Yu. S. LIPATOV, V. P. PRIVALKO, Yu. Yu. KERCHA and B. Ye. MYULLER, Sb.: Sintez i fiziko-khimiya poliuretanov (In: The Synthesis and Physical Chemistry of Polyurethanes). Izd. "Naukova Dumka", 1969 5. A. Ye. NESTEROV, Yu. S. LIPATOV, B. Ye. MYULLER and L. V. MOZZHUKHINA, Vysokomol. soyed. B10: 900, 1968 (Not translated in Polymer Sci. U.S.S.R.)

6. M. INOUE, J. Polymer Sei. Ah 2013, 1963 7. V. A. KARGIN, T. I. SOGOLOVAand N. Ya. RAPOPORT-MOLODTSEVA, Dokl. Akad. Nauk SSSR 157: 1406, 1964

Oriclltation study on polyformaldehyde fibres

127

8. V. A. KARGIN, T. I. SOGOLOVA and T. K. SItAPOSHNIKOVA, Vysokomol. soyed. 7: 385, 1965 (Translated in Polymer Sei. U.S.S.R. 7: 3, 423, 1965) 9. V. A. KARGIN, T. I. SOGOLOVA and N. Ya. RAPOPORT-MOLODTSEVA, Dokl. Akad. Nauk SSSR 163: 1194, 1965 10. T. O. ZItARINOVA, Dissertation, 1968 11. M. DOLE, J. Polymer Sci. C18: 57, 1967 12. V. P. PRIVALKO, Vysokomol. soycd. B l l : 325, 1968 (Not tr~u~sl~ted m Polymer Sci.

U.S.S.R.) 13. F. E. KARASZ, It. E. BAIR and J. M. O'REILLY, J. Phys. Chem. 69: 2657, 1965 14. G. GIANOTTI and A. CAPIZZI, Europ. Polymer J. 4: 677, 1968 15. L. MANDEL'KEttN, Kristallizatsiya polimcrov (The Crystallization of Polymers) Izd. " K h i m i y a " , 1966 16. B. V. VASIL'EV and O. G. TARAKANOV, Vysokomol. soycd. 6: 2189, 2193, 1964 (Translated in 1Jolymer Sei. U.S.S.R. 6: 12, 2423, 2427, 1964) 17. B. V. VASIL'EV, Dissertation, 1965 18. A. J. KOVACS, Fortsehr. Hochpolymcr Forsch. 3: 394, 1964 19. J. D. HOFFMAN, SPE Trans. 4: 315, 1964 20. P. B. STICKNEY and R. D. FALB, Rubber Chem. ~nd Tcchuol. 37: 1299, 1964 21. A. S. KUZ'MIN~rKII, L. I. LYUBCHANSKAYA al~d K. S. RAKOVSKII, Dokl. Akad. Nauk SSSR 181: 144, 1968 22. Z. RIGBI, Kolloid-Z. und Z. ffir Polymer(~ 223: 127, 1968 23. A. M. RASULOV, M. B. GRANOVSKII ~md V. L. MAIZEL, Mekhanika polimerov, No. 1, 1, 1967 24. R. TSEREZA, Blok- i privityc sopolimcry (Block- and Graft Copolymers). Izd. "Khimiya", 1964

ORIENTATION STUDY ON POLYFORMALDEHYDE FIBRES* M. Y u . KUCHINKA, A. S. GRZHIMALOVSKII, ~B. A. YEGOROV, V. S. TVERDOKHLEB, O. A. BOYAR a n d V. V. ANOKHIN Light I n d u s t r y Technological Institute, Kicv

(Received 10 October 1969)

TI{E drawing of polymer fibres causes considerable stIaxctural changes [1-6], and these affect the mechanical properties ill their turn. The establishment of connection between the molecular orientation and structural changes, and also between the degree of orientation-drawing and the mechanical properties will therefore be of interest, as this will enable us to find out what physical processes are responsible for any of the mechanical property changes of the fibres. * Vysokomol. soyed. A13: No. 1, 111-117, 1971.