Physicochemical ways of regulating the supermolecular structures and mechanical behaviour of amorphous polyarylates based on phenolphthalein and its derivatives

PHYSICOCHEMICAL WAYS OF REGULATING THE SUPERMOLECULAR STRUCTURES AND MECHANICAL BEHAVIOUR OF AMORPHOUS POLYARYLATES BASED ON PHENOLPHTHALEIN AND ITS DERIVATIVES* G. L. SLO~n~SKII, V. V. KORSWAw, S. V. VIlq'OGRADOVA,A. I. KITAIGORODSKII, A. A. ASKADSK~, S. N. SALAZK1Wand YE. M. BELAVTSEVA Institute of Elementary Organic Compounds, U.S.S.R. Academy of Sciences (Received 13 October 1966)

I~ RECEIqT years a n u m b e r of works has been published on the synthesis and properties of new heat-resisting amorphous polyarylates based on phenolphthalein and its derivatives [1-4]. These polymers have high softening points (frequently a b o v e 300 °) and good di~ectgical and mechanical properties, wh:ch valuable properties are retained in a wide range of temperatures. A typical feature of polyarylates based on phenolphthalein is their excellent solubility in a number of widely accessible organic solvents, which means t h a t articles can be produced from solution (film, fibre) without having to use high temperatures, and therefore without thermal degradation, which occurs when polymers are processed by hot press moulding, pressure casting and so on. Besides these valuable properties, polyarylates based on phenolphthalein and its derivatives are highly brittle, and this makes it difficult to convert t h e m or give articles made of t h e m practical application (their high brittleness is known from [1, 4], in which they were described for the first time). We have noticed t h a t if films of phenolphthalein polyarylates produced from solution contain about 3% residual solvent, quite regular polygonal structures (Fig. la) are formed on a shock temperature change (from 20 to 200-220°), and these are quite large in size (hundreds of microns). I t can be seen from Fig. la t h a t the space inside the polygons is not uniform, b u t consists of several finer formations about 3 microns in size. Electron microscopy analysis of the surface of these polygons showed t h a t each point inside was an irregular-shaped oval consisting of globular formations on a spindle (Fig. lb). These prompted a more detailed investigation of the structure of films and blocks, and such investigations were performed on the polyarylate of isophthalic acid and phenolphthalein (polyarylate P-1). The investigations show (Fig. 2a) thut this glasslike transparent polymer was formed on not completely regular spheroidal particles ( ~ 1000 A in size) consisting of even smaller and also almost spheroidal particles (30-40 A). * Vysokomol. soyed. A9: No. 2, 402-408, 1967.

453

454

G. L. SLONIMSX.UeZ a/.

Approximate calculation showed that the diameters of completely coiled macromolecules of a given molecular weight (M=28,000 determined b y the light-scattering method) should be of these dimensions. This means that polyarylate film consists of peculiar globular bundles in which coiled macromolecules are grouped together.

FIG. 1. Polygonal structures in polyarylate: light-optical (a) and electron microscopic (b) pattern

FIG. 2. Electron microscope patterns of the surface of a polyarylato P-1 film synthesized in ditolylmethano (a) and ~-chloronaphthalein (b). Electron microscope analysis of the supermolecular structure in a block of this polyarylate revealed the same. I t is interesting that a number of fine cracks were observed on the optical pattern of the surface of a P-1 fragment (Fig. 3a) indicating the loose connection between the spherical particles.

Physicochemical ways of regulating the supermolecular structures

455

The above convinced us that one of the reasons for the high brittleness of the glasslike polyarylate P-1 is its globular type of supermolecular structure. On the basis of the new ideas regarding the role of supermolecular structures in developing the mechanical properties of polymers [5-12] it can be shown that a change in the type of supermolecular structure will influence the combined

FIo. 3. Light-optical surface pattern of spinters of polyarylate P-1 synthesized in ditolyl. methane (a) and a-chloronaphthaloin (b).

mechanical properties of polyarylates, and can be used to improve them. Since the chemical structure of the macromolecules of these polyarylates shows them to have high rigidity (which is supported by thermomechanical tests, a "segment" kg/cmz 800

b

400

I

I

:FIo. 4. Stress-strain diagrams of polyarylato P-1 synthesized in diotolylmothane (a) and a-chloronaphthaloin (b).

of such macromolecules being equal to the entire length of the macromolecule), and also since all attempts to change the tape of supermolecular structure by the formation of films from these polyarylates in different solvents has produced no positive results, it is natural to assume that they have stable and rigid coiled conformations which are the reason for the globular supermolecular structures.

456

G. L. SLONIMSKHet

al.

To produce a different type of supermoleeu'ar structure it was therefore necessary to interfere with the synthesis of the polyarylate, to lead to the formation of rigid uncoiled macromolecules. In the synthesis of polyarylates P-1 and P-2 described in [1] (polyester of terephthalic acid and phenolphthalein) the polymer was formed in ditolylmethane. But ditolylmethane does not dissolve polyarylates P-1 and P-2, and therefore the free energy of coiled macromolecule formation should be less than of uncoiled macromolecule formation, since the former have a smaller number of contacts with the non-solvent. Since all isobaric-isothermal processes are associated with

FIG. 5. Structure of block specimens of polyarylat6 P-7 synthesized by interracial polycondensation (a) and high-temperature polycondensation in g-chloronaphthalein (b). a reduction in free energy, the difference in the free energies of coiled and uncoiled macromolecule formation must be responsible for the development of globular forms of macromoleeule, with the consequent development of globular types of supermolecular structure. Besides this, we note that a polymer non-solvent (ditolylmethane in particular) is a medium in which it is difficult for a polymer to form from technological considerations also. Allowing for these considerations we synthesized polyarylates P-1 and P-2 in specially chosen media, sovol, which has been described in [2, 4, 13], ~-chloronaphthalein and nitrobenzene, which have been described in the present work

Physicochemical ways of regulating the supermolecular structures

457

a n d in [13, 14]. All these high-boiling organic materials are v e r y good solvents for p o l y a r y l a t e s P - l , P-2 a n d some others. I t was therefore to be e x p e c t e d t h a t m a i n l y uncoiled macromolecules would be synthesized, a n d this would p r o d u c e corresponding bundles of macromolecules a n d o t h e r fibrillar supermolecular structures. Finally, p o l y a r y l a t e s w i t h this k i n d of s t r u c t u r e should be less brittle. EXPERIMENTAL

The polyarylates P-l, P-2 and P-7 were synthesized in ~-chloronaphthalein, sovol and nitrobonzene in condensation test tubes in an argon flow followed by the following temperature treatment: heating from 100 to 180° in 30 rain, soaking 1 hr at 180°, 30 min at 200 ° and 12 hr at 220% During the reaction the stock was a homogeneous thick solution. On completion the polymer was dissolved in chloroform (concentration 5-8 g/100 ml) and precipitated in methanol; then washed in methanol, acetone, ethylether followed by vacuum drying at 120° and 3 mm Hg. The polyarylato P-7-M was synthesized by interracial polycondensation using the procedure described in [3] with benzene as the organic solvent and Nekal as the emulsifier. The replica method was used for the electron microscopy analysis of the surface structure of films and splinters, with single-stage platinum-carbon replicas. These are known to be the best of the replicas which can be produced by thermal sputtering. They have low granularity, do not coagulate on exposure to an electron beam and give a high contrast. The replicas were removed from the surfaces of t31ma and splinters by means of gelatine which was then washed in water. The objects were studied under an UEMV-100 electron microscope. The results o f the syntheses a n d studies o f the s t r u c t u r e a n d mechanical properties of P-1 a n d some of t h e o t h e r p o l y a r y l a t e s showed t h a t in the film of t h e m a t e r i a l synthesized in a-chloronaphthalein there was a definite supermolecular s t r u c t u r e of t h e fibrillar t y p e (Fig. 2b), which is also typical of monolithic specimens. U n d e r an optical microscope (Fig. 3b) the difference between t h e breakd o w n p a t t e r n s of p o l y a r y l a t e s with globular fibrillar structures is quite distinct on the surface of a P-1 splinter. T h e s t r u c t u r e a n d mechanical properties also show corresponding differences (Table 1). At the same molecular weights, the specific i m p a c t toughness of p o l y a r y l a t e P-1 synthesized in good solvents h a d i m p r o v e d 4-10 times as c o m p a r e d with the same material synthesized in ditolylm e t h a n e . The elongation on r u p t u r e h a d also increased several times. I t is evid e n t f r o m Fig. 4 t h a t at the same molecular weight ( M = 2 8 , 0 0 0 d e t e r m i n e d b y light scattering) t h e r u p t u r e elongation of globular P-1 is ~ 1 6 ° , a n d t h a t of fibrillar ~80O/o . I t is interesting t h a t the stress-strain diagram of fibrillar P-1 is reminiscent of t h a t for crystalline polymers. The synthesis o f p o l y a r y l a t e s on a p h e n o l p h t h a l e i n base in a m e d i u m which dissolves the p o l y m e r t h u s favours fibrillar structures with a n i m p r o v e m e n t in the mechanical properties as c o m p a r e d with a p o l y m e r of the same molecular weight p r o d u c e d b y p o l y c o n d e n s a t i o n in a m e d i u m which does not dissolve the p o l y m e r a n d produces a globular structure. I t can therefore be assumed t h a t in t h e case of interfacial polycondensation, where t h e p o l y m e r is f o r m e d at t h e interface between two liquid phases and is not itself soluble in either o f t h e m ,

458

G. L. SLOmMSKr~ et al. TABLE 1. PROPERTIES OF POLYAI~YLATE ~:)-1

Medium in which polyarylates were synthesized Properties

Molecular weight S o f t e n i n g p o i n t * , °C

Tensile strength, kg/cm2 Elongation at rupture, % Specific impact toughness, kg.cm/cmz

ditolylmethane

sovol

~-chloronaphthalein

28,000 270 640 10-20

33,000 280 640 85

28,000 48,000 290 300 740 780 80 100

21--3

11

I0

20

nitrobenzeno

26,000 275 650 40-70 14

* The softening point was taken as the temperature at which the stresses in a monolithic specimen relaxed to zero [15].

the supermolecular structure must be globular. This is in very good agreement with the data known on the mechanical properties of interracial polyarylates, which show that they are very brittle. To confirm the above w9 carried out appropriate electron microscopy and mechanical investigations on specially synthesized types of polyarylate P-7 (produced b y polycondensation of terephthalyl chloride and phenolphthalein anilide). The results confirmed the above proposition. As we can see from Fig. 5a, the supermolecular structure of interracial polyarylate P-7-M is definitely globular. With a very high magnification (85,000) it can be seen that globules, which are ~ 1000 A in size, have a complicated structure, and consist mainly of smaller globules ~ 50 A in size, with also the possibility of particles of another shape. In contrast to polyarylate P-7-M produced by interfacial polycondensation, P-7, which was synthesized in a homogeneous polymer-solvent medium, has a completely different supermolecular structure. The mechanical properties of the polyarylates are also different. It can be seen from Table 2 and the thermomechanical curves in Fig. 6 that P-7 has much higher resistance to impact load, greater rupture elongation and a higher softening point than P-7-M. The latter is particularly desirable, because it definitely shows that an amorphous polymer of the same chemical structure m a y have different softening points depending on the supermolecular structure. The reason for the shift of ~ 35 ° in the already high softening point of the polymer seems to occur because in rigid-chain polymers, which includes polyarylates, it is very close to the yield point. It is therefore at the softening point that the secondary formations of the polymer begin to disintegrate into the elements of its structure. I f the structure is globular, and formed as a result of macromolecular coiling, this disintegration will start at lower temperatures because the globular particles are quite loosely linked, both individually and in groups. With the fibrillar structure made up of long bundles in which uncoiled macromolecules are grouped together, this disintegration starts at a higher

Physicochemical ways of regulating the supermolecular structures

459

temperature since the interaction energy between the structural elements is much higher and greater thermal energy is therefore required to start movement. TABLE 2. PROPERTIES OF POLYARYLATES P - 7 A~I) P - 7 - M

Properties

P-7-1Yl

P-7

Softening point, °C Breaking stress, kg/cm' Elongation at rupture, % Specific impact toughness, kg. cm/cm 2

280-285 960 13

315-320 1000 40-50 7-9

~

1

Up until now we have been comparing the ultimate (in strength) properties of polyarylates with different supermolecular structures. No less important, however, are their relaxation properties. Studied in a wide temperature range these would permit conclusions regarding the influence of supermolecular structure on the equilibrium elasticity, and therefore the workability of the polymer. We therefore determined the workability range of these polyarylates using the procedure described in [15]. This method gives us the range of stresses and temperatures in which the polymer definitely remains hard and can be used for rigid components. As an example, Fig. 7 shows the workability range of polyarylate P-2 synthesized in ditolylmethane. To illustrate the influence of super-

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460

G.L. SLONIMSKIIet al.

the globular structure, as we have already shown, is subject to rapid partial disintegration under stress. Thus, on going from low to high temperatures there is a change in the ratio of the mechanisms of relaxation processes due to a dif~,,kg/em 2

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Physicochemical ways of regulating the supermolecular structures

461

v e n t s are usually p r o d u c e d w i t h a higher molecular weight t h a n those synthesized in d i t o l y l m e t h a n e w i t h e x a c t l y the same concentrations of initial m o n o m e r . The possibility of synthesizing p h e n o l p h t h a l e i n - b a s e d p o l y a r y l a t e s w i t h high molecular weights in good solvents is definitely interesting w i t h r e g a r d to improving the mechanical properties. F o r instance, specimens h a v e a l r e a d y been p r o d u c e d on a base of p o l y a r y l a t e 1)-2, with a specific i m p a c t toughness of 30-40 k g . c m / c m 9. Besides this, synthesis in good solvents makes the technological process easier [13]. CONCLUSIONS

(1) T h e conditions of synthesis e x e r t an influence on the f o r m a t i o n of supermolecular s t r u c t u r e s in p o l y a r y l a t e s based on phenolphthalein, a n d on t h e c o m b i n a t i o n of mechanical properties of a m o r p h o u s glasslike polymers of this type. (2) T h e solvent c a p a c i t y o f the m e d i u m in which the synthesis is p e r f o r m e d is the decisive f a c t o r in the f o r m a t i o n o f rigid macromolecules w i t h either coiled or uncoiled conformations, which result respectively in globular or fibrillar t y p e s o f supermolecular structure. (3) B y altering t h e conditions of synthesis a n d t h u s affecting the t y p e o f supermolecular structure, it is possible to a d j u s t t h e properties of a p o l y m e r , in p a r t i c u l a r to r e d u c e its brittleness. Translated by V. ALFORD REFERENCES

1. V. V. KORSHAK, S. V. VINOGRADOVA and S. N. SALAZKIN, Vysokomol. soyed. 4: 339, 1962 (Not translated in Polymer Sci. U.S.S.R.) 2. S. V. VINOGRADOVA, S. N. SALAZKIN and V. V. KORSHAK, Izv. Akad. l~auk. SSSR, set khim., 308, 1966 3. S. V. VINOGRADOVA, V. V. KORSHAK, S. N. SALAZKIN and S. V. BEREZA, Izv. Akad. Nauk. SSSR, ser khim., 6, 1555, 1964 4. S. V. VINOGRADOVA, V. V. KORSHAK, S. N. SALAZKIN and S. V. BEREZA, Izv. Akad. l~auk. SSSR, ser khim., 6, 1403, 1964 5. V. A. KARGIN, A. I. KITAIGORODSKH and G. L. SLONIMSKII, Kolloidn. zh., 19, 131, 1957 6. T. I. SOGOLOVA, Dissertation, 1963 7. V. A. KARGIN, T. I. SOGOLOVA and G. Sh. TALIPOV, Vysokomol. soyed. 5: 1809, 1963 (Translated in Polymer Sci. U.S.S.R. 5: 6, 937, 1964) 8. V. A. KARGIN, T. L SOGOLOVA and L. I. NADAREISHVILI, Vysokomol. soyed. 6: 165, 169, 1964 (Translated in Polymer Sci. U.S.S.R. 6: 1, 191; 197, 1964) 9. T. I. SOGOLOVA, Mekh. polimerov, 1, 5, 1965 10. T. I. SO{~OLOVA, Mekh. polimerov, 5, 643, 1966 11. G. L. SLONIMSglI, T. I. SOGOLOVA and V. A. KARGIN, Report on the 22nd Ann. Conf. of the SPE, Canada, Montreal, March 1966, Tech. papers Vol. XII, paper X-2 12. L I. KURBANOVA, Dissertation, 1966 13. S. N. SALAZKIN, Dissertation, 1965 14. G. L. SLONIMSgr[~ V. V. KORSHAK, S. V. VINOGRADOVA, A. I. KITAIGORODSgr[,

A. A. ASKADS]KII: S. N. SALAZKIN and E. M. BELAVTSEVA. Dokl Akad. Nauk. SSSR, 156, 924, ll}64; 164, 1323, 1965 15. G. L. SLONIMSKII and A. A. ASKADSKII, Mekh. polimerov, 1, 36, 1965