Phase segregation in diethylene glycol (adipate-terephthalate) co-oligomers

PolymerScienceVol. 33, No. 9, pp. 1856-1861, 1991 Printed in Great Britain.

096.g-545X/91 $15.00+ .00 ~) 1992 Petganm~ Press Ltd

PHASE SEGREGATION IN DIETHYLENE GLYCOL (ADIPATE--TEREPHTHALATE) CO-OLIGOMERS* A. T. GUBAIDULLIN, B. YA. TEITEL'BAUM, N. P. APUKHTINA, S. M. YAKZHINA and T. A. YAGFAROVA A. Ye. Arbuzov Institute of Organic and Physical Chemistry, Kazan Science Centre of the U.S.S.R. Academy of Sciences S. V. Lebedev All-Union Research Institute of Synthetic Rubber

(Received 23 October 1990)

Statistical co-oligoesters containing 36% by weight of terephthalate units were prepared and studied. By small-angle X-ray scattering the presence of a dispersed phase is revealed, formed by association of terephthalate units. The scattering characteristics change nonuniformly with the content of these units-qhis can he explained by the size, shape and internal structure of the particles. The presence of the microphase does not lead to curing of the oligomers. A regular increase in background scattering with growing molecular mass of the oligomers was observed, practically uncorrelated with the content of the terephthalate units.

MANY MACROMOLECULARcompounds built of blocks of various nature show a tendency to segregation, forming microheterogeneous structures [1]. It is customary to assume that such structures arise when the blocks are of sufficient length, so that the generated associates (domains) are large enough to form microphase particles enriched with one of the blocks. (It is kept in mind that these particles are not simply dispersed in the medium-matrix, but rather are chemically bound by the tie chains.) It appears natural that such segregation does not occur in statistical copolymers. From this point of view it is unexpected that, as described below, microphase separation can be observed in co-oligoers which were prepared by procedures assumed to lead to a statistical distribution of monomeric units (condensation of mixtures of diethyleneglycol and b/s(diethyleneglycol)-terephthalate with adipic acid) [2]. We have studied a series of co-oligomesters with various contents of terephthalate (TP) units, and also of various molecular mass (MM) (Table 1). Microheterogeneities in these objects were revealed by small-angle X-ray scattering which is recognized as the most effective method of microphase studies [3]. TABLE 1.

CHARACTERISTICS OF CO-OLIGOMERS AND PARAMETERS OF MICROHETEROGENEOUS STRUCTURE ACCORDING TO SMALL-ANGLE X ' R A Y SCA'ITERING

Content of TP

R

/~,, units, wt% T, *C 1880 1980 1770 3150 2440

9.02 9.02 17.8 17.8 36.3

•

V¢ × 10-6Ve × 10 -6

Q × 10 -3,

-44 -45 -40 -35 -24

A -6

Rse, A

Nfel

0.83 1.22 0.81 1.25 1.12

91.5 60.9 69.7 71.7 51.3

8.0 18.0 19.0 15.3 100

A 140 90 90 110 21

*Vysokomol. soyed. A33: No. 9, 1980-1985, 1991.

1856

Rse, A Ira, A 25.2 20.7 19.8 25.3 126

89.2 57.7 57.6 70.5 57.9

99.8 63.0 59.6 79.4 52.7

1~3 2.07 0.56 0.67 1.28 0.23

E, A 2.45 0.61 0.60 1.14 0.27

15.7 15.8 13.1 8.1 14.8

Phase segregation in co-oligomers

1857

I, re/. u n i t s 500

-

i

300

tO0

-

. . . . .

_L_._ ~ll

I 70

I10 28, m i n

FIG. 1. Small-angle X-ray scattering diagrams of oligomers, obtained with slit collimation. Here and in Fig. 2, content of TP units: 9.02 (1,2); 17.8(3, 4); 36.3 (5) and 0 wt% (6)..~/, = 1880(1), 1980(2), 1770(3), 3150 (4), 2440(5) and 1820(6). Small-angle X-ray scattering of CuK~ radiation monochromatized with a Ni filter was measured on a KRM-1 apparatus (collimator of Kratky type), in automatic operation controlled with a DZ-28 computer [4]; this computer was also used for primary data treatment. For further mathematical treatment (normalization, correction for collimation errors, smoothing, mathematical modelling and calculation of structural parameters), a PC/XT computer was used together with the programs developed by one of the authors [4]. All the main calculated data are shown in Table 1. The wide-angle measurements were made on the URS-50I apparatus with CuK~ radiation (Ni filter, amplitude discrimination). The studied samples, having the character of viscous liquids, were placed in aluminium cells 3 mm thick, with lavsan windows. The scattering intensities were determined as the difference between parallel measurements with a filled and empty cell; in this way the parasitic scattering of the camera was excluded. The experimental curves of small-angle X-ray scattering obtained with slit collimation are shown in Fig. 1. The considerable scattering (curves 1-5) reveals the presence of inhomogeneity domains in the oligomers. This indicates intermolecular association of TP units; for the homopolymer which does not contain such units (curve 6), the scattering intensity does not exceed the background limit. However, proportionality between the scattering intensity and content of TP units is not observed. The reasons for this can be found by studying the structure of the oligomer molecules and the morphology of the particles constituting the disperse phase. The wide-angle diffractograms of all oligomers exhibit one diffuse maximum in the range 20 ~ 20°, the intensity and shape of which depends on the composition of the samples. This indicates that the occurrence of microdomains does not lead to the appearance of long range order, while the character of the mutual placement of TP units in the microdomains is similar for all co-oligomers. It must be assumed that in interactions of aromatic TP units, parallel orientation of their axes predominates, similarly as in liquid-crystalline nematic formations. Orientation effects are therefore assumed to be the reason for the deviations from spherical shape in the microphase particles.

A . T . GUBAIDULLINet al.

1858

tntZ) q q I

:

Z3 2

0

-2

I -3.0

q,O

I

-2,0

L.(s) FIo. 2. Small-angle X-ray scattering diagrams of co-oligomers, reduced to point collimation after subtraction of background (logarithmic scale).

Application of the method of standard curves of small-angle X-ray scattering (Fig. 2) enabled us to approximate, in terms of the theory of reference [5], the shape of the particles as rotational ellipsoids with the axes r and R. The correctness of the approximation is confirmed by the similarity of the values of the mean radius of gyration found from small-angle scattering data, Rs e, and those calculated in terms of the model, R8c. Also the values of the mean chord l,,, independently determined from the integral scattering characteristics [6]

fsl(s)ds lm= fs21 (s) ds' are in good agreement with the values calculated for the model particles. (Here and in the following, s = 47r. sin 0/A, where 20 is the scattering angle and it is the radiation wavelength.) The presented data indicate a considerable anisometry of the particles. As previously shown [7, 9], for extremely oblate and elongated particles, in the expression for the scattering intensity it is possible to separate members corresponding to cross-section and thickness of the particles. Thus for a strongly oblate particle approximated by a lamella, the thickness T and the surface Sc of the lamella can be determined as T = iim ( ~ ' s 2 1 ( s ) ) / Q $-.-*0

Sc = 2~rl(0)/lim (~rs21(s)), s--,0

while for an elongated (cylindrical) particle, the length L and the cross-section A can be determined as

L = ¢rl (0)/lim

(sl (s))

a~-*0

A = lim (sl(s))/Q. s--.0

Phase segregation in co-oligomers

1859

Specifically for the oblate particles of the co-oligomer with Mn = 1880, the thickness T = 51.9/~ and the mean lamella radius from Sc equal to 135.3 ,~, were obtained; for the elongated particles of the co-oligomer with .~n = 2440, the length L = 197/~, and the mean cross-section radius R, = 20.9/~, were found. The similarity of these values to those in Table 1 appears as a further support of the selected models. By extrapolation of the logarithmic curves logl-s 2 it was possible to determine the value of zero intensity I(0) (for the angle 20 = 0) and the value of the invariant Q connected with the difference of electron densities in the particles and in the medium, p~-~

a = fs21(s)ds. Thus it was also possible to determine the corresponding values for the volume Vc of one "mean" scattering particle

Ve = 27r21(O)/Q. These values are sufficiently near to those calculated for model ellipsoids. Table 1 also shows the value of the thickness E of the transition zone [8] in which the transition from p~ to Pz occurs, as well as the values of the mean number of particles Nrel in unit volume. Together with the data on the mean volume of the individual particles, the value of Nrc~ enables us to compare the overall values of the mass of the scattering material which, as explained, are not directly proportional to the mass fraction of TP units. An orientation calculation indicates that at a concentration of ~9% TP units per oligomer molecule (M~ = 1880 or 1980), there occurs on the average one single TP unit to which diethyleneglycol adipate sequences are attached at one or both sides. Such structure is suitable for the formation of associaties of the fringed micelle type. But even in this favourable case, such a "mean" oligomer with statistical chain structure may contain molecules with no TP units, or with several such units. The former will evidently be found outside the micelles, in the matrix, while the latter, entering the micelles, will reduce their order, and this effect will be enhanced by the molecular mass distribution. For both oligomers studied containing ~9% of TP units, the ellipsoids modelling the micelles are strongly oblate, with similar values of r (size along the rotation axis), but differ by the equatorial radii R, and consequently by volume. The differences in the mean particle dimensions in the two samples, and in the number of particles in unit volume are the cause of their different integral characteristics of small angle X-ray scattering. At --18% TP units per one molecule with .~/,, -- 1770, the chain contains already more than two, and with M, = 3150, four TP units on the average, randomly distributed over the chain length. Together with molecular mass distribution, such structure makes interactions of TP units and segregation of the microphase more difficult, as the generated associates are connected by tie chains of various length. A part of these tie chains is necessarily drawn inside the associates, thus lowering their electronic contrast with respect to the medium. Because of this, irrespective of the considerably higher content of TP the value of Q remains near to that for the oligomers with ~9% TP units. The scattering pattern corresponds to particles of oblate ellipsoidal shape, with a rotational axis differing but little in size from that discussed above. It should be noted that the oligomer with M,, = 3150 is unique by its low value of E. In our opinion this is caused by the circumstances that this MM is near to the limit where chain folding begins to be possible, so that the packing of molecules at the particle surface can be more compact. Transition to the oligomer containing ~36% TP units marks a profound change in the character

1860

A . T . GUBAIDULLINel aL

of small-angle X-ray scattering. The scattering particles no longer have the shape of an oblate ellipsoid (of "lens" type), but rather are elongated along the rotation axis ("cigar" type). Such change in particle morphology with composition is certainly not unusual in disperse systems, where in extreme cases it can even lead to phase inversion [10, 11]. In our case the shape of elongated ellipsoid (or approximately cylinder) may be connected with the need of packing within the domain of one molecule, or its large part, owing to the higher number of aromatic groups in each molecule. These results indicate that the phase structure of the oligomers is cross-determined both by concentrational, and by constitutional factors; both these factors control the size, shape, internal structure and number of particles of the disperse phase. The manifestations of phase segregation cannot in due extent increase with a growing content of comonomer (as would be the case with block-copolymers), owing to the increasing role of randomization in chain structure. In any case, segregation does not reach the level where correlation with particle placement could occur which would be manifested in the small-angle X-ray scattering pattern. The accumulation of the rigid-chain TP fragments leads to a lowering of the flexibility of the macromolecules, as indicated by the increase in the glass temperature Tg following from thermomechanical analysis (TMA). TMA curves reveal only one glass transition range. The occurrence of associates does not endow the oligomers with solid-like properties; they exhibit the properties of viscous liquids. And thus it may be concluded that by microphase segregation, a disperse system of liquid-liquid type is generated. In the course of our studies of small-angle X-ray scattering, we have made an interesting observation: the intensity of the background scattering Ib from the oligomers studied shows a steep dependence on MM. Independently of the content of TP units, the experimentally determined (according to the procedure of reference [12]) value of lb increases up to some value of M,, and then remains constant. (In Fig. 3, besides the data for the previously characterized co-oligomers, the data for homo-oligomers are also presented, without TP units and with M, = 800 and 1820.) It is well known that the value of lb is controlled by thermal density fluctuations, connected with compressibility and free volume. It may be assumed that the observed effect reflects the increase in /b. teL units

o /00

60

I

I

15

2~

.J-. ;5 IVI,, X l O - a

FiG. 3. Dependenceof backgroundscatteringintensitylh on number-averagemolecularmassof oligomers.