Eur. Polym.J. Vol. 32, No. 11, pp. 1351-1354,1996 Copyright 0 1996Elsevier ScienceLtd
Printed in Great Britain. All rights reserved
MISCIBILITY IN THERMOPLASTIC POLYURETHANE ELASTOMER/POLY(STYRENE-CO-ACRYLONITRILE) BLENDS B. zERJAL,‘a
2. JELCIC2 and T. MALAVASIC’*
‘University of Maribor, EPF, Institute of Technology, Maribor, Slovenia 21NA-OKI, Polymer Materials Development Zagreb, Croatia 3National Institute of Chemistry, Ljubljana, Slovenia (Received 21 December 1994; accepted in final form 2 November 1995) Abstract-Blends of a thermoplastic polyurethane (TPU) with poIy(styrene-co-acrylonitrile) (SAN) were investigated by differential scanning calorimetry (DSC) and dielectric spectroscopy. Blends were prepared by kneading. A shift of the glass transition temperatures (TI) and specific heat increments (AC,) at the Tg have shown that SAN dissolves more in the TPU-rich phase than TPU in the SAN-rich phase. From dielectric measurements it could be concluded that the relaxation processes in the TPU/SAN blends occur at the same temperatures as the relaxation processes in pure components, which suggests that the motions of TPU chains are decoupled from the motions of SAN chains. The addition of TPU to SAN copolymer causes a plasticizing effect at very low concentrations. Copyright @ 1996 Elsevier Science Ltd
polyester from adipic acid and 1,6-hexanediol, i,4-butanediol (BD), and 4,4’-diisocyanato-diphenylmethane (MDI).
Blending of two or more polymers is a well-established strategy to achieve modified physical properties, without the need to synthesize new polymer systems. In polymer blends the interactions between both components are of great importance. Blending of thermoplastic polyurethane elastomer (TPU) with other thermoplastics has been of interest in recent years [l-9]. The structural properties and of the TPU/SAN blends were investimorpholo gated by i!YrJa1 et al. [l, 21. Mechanical properties of these blends have been correlated with the theory derived for the mechanical properties of polymer composites. TPU/SAN blends were investigated on the basis of the analysis of viscoelastic behavior of the components for characterization of the formation of morphology in melt mixing processes . Ratzsch et a/.  showed the influence of additional interactions in blends of TPU with SAN on the phase behavior and macroscopic properties. In the present study the miscibility of TPU/SAN blends was examined by means of experimentally determined glass transition temperature (T& and specific heat increment (AC,,) measured by differential scanning calorimetry and by dielectric spectroscopy.
The molar masses of polymers were determined by gel permeation chromatography (GPC) relative to polystyrene standards in dimethylformamide given in Table 1.
(DMF). The values are
TPU and SAN were dried for 6 hr at 373K. TPU/SAN34 and TPU/SAN24 blends of different weight ratios (100/O, 90/10, 75/25, 60/40, SO/SO,40/60, 25/75, 10/90, O/100) were prepared by melting them in a Brabender kneading chamber (rotation speed SO/min) at 468K for 10 min. All blends were milled at room temperature. The milled samples were molded in a hydraulic press at 493K and 15 MPa and then water-cooled to room temperature. Methods Glass transition temperatures (TI) were measured by differential scanning calorimetry (DSC-7, Perkin Elmer). The samples were put in to the sample holder at 373K, then quickly cooled to 198K and heated to 273K with a heating rate of ZOK/min. TI was taken as the midpoint of the transition. The values were reproducible within f0.5K. Dielectric measurements were made on an apparatus Unirelax, Audrey. In the measuring scheme e’(o), e”(W)and tan (S) were measured at four uniformly spaced frequencies on a logarithmic scale covering the entire frequency range, i.e. 0.1, 1, 10 and 100 kHz, while the temperature was scanned at l.ZK/min. The measurements were made over a temperature range of 83-423K, above this temperature the samples become too soft to make reliable measurements.
EXPERIMENTAL Materials Commercial polyester TPU (Elastollan C 90 A, BASF) and commercial SANs (Luran 388 S, BASF, further on marked as SAN34 and Luran 368R, BASF, further on marked as SAN24) were applied. By “C-NMR, it was found that TPU was a polyester urethane on the basis of a #Deceased. *To whom all correspondence should be addressed.
RESULTS AND DISCUSSION
DSqPrential scanning calorimetry The miscibility of TPU/SAN blends was estimated by shifts in T,s of both components. TPU/SAN blends have two T,s: Tt of TPU soft segments and T, of SAN. Although the changes in Tg are small, it can be concluded that Tg of TPU soft segments in
masses of components GPC
Zerjal et al
Table 2. r,s and apparent TPU-rich
TPU SAN34 SAN24
249,145 293,603 321,898
136.557 164.325 173,927
I .83 1.79 I .85
467,876 498,010 58 1,699
of TPU and SAN in the
phase for TPU/SAN34
measurements) TPU-rich Blend
TPUjSAN blends increases linearly with increasing weight fraction of SAN to the composition SO/SO (Fig. 1). T8 of SAN in the blends decreases with the weight fraction of TPU (Fig. 2). At least five measurements were made for each T,. The estimated error at the determination was +0.5K. From the T, of TPU and SAN component in TPU/SAN blends the apparent weight fractions of TPU and SAN dissolved in the TPU-rich phase and the SAN-rich phase, respectively, can be estimated. The apparent weight fractions of TPU were determined in the TPU-rich phase and the SAN-rich phase by the empirical equation [lO-121, which is often used to describe the dependence of T8 on composition in random copolymers and plasticized systems: T* =
phase and SAN-rich
loo/o 229 9OjlO 230 75125 231 60/40 233 so/so 234 40160 233 25175 234 IO/90 No observed O/loo -
where TB was the observed T8 of the blend of the miscible or a partially miscible system, w, is the weight fraction of TPU having r,, and M’~is the weight fraction of SAN having Tez. Equation (1) may be rearranged and transformed into
where w; is the apparent weight fraction of TPU in the TPU-rich phase, Z”gl.bis the observed TB of TPU-rich phase in blends, and T,, and Tp are the T,s of TPU and SAN.
r,: W) 370 371 372 373 374 375 376 379
1.00000.0000 0.9933 0.0067 0.9866 0.0134 0.9733 0.0267 0.9666 0.0334 0.9733 0.0267 0.9666 0.0334 ~ -
0.0600 0.0534 0.0467 0.0400 0.0334 0.0267 0.0200 o.OOOo
0.9400 0.9466 0.9533 0.9600 0.9666 0.9733 0.9800 1.0000
Weight fractions of both polymers in the TPU-rich and SAN-rich phase were calculated and are given in Tables 2 and 3. AC, at the glass transition for TPU and SAN in TPU/SAN blends are presented in Figs 3 and 4. AC, of TPU decreases with an increased amount of SAN in TPU/SAN blends (Fig. 3), and AC, of SAN decreases with an increased amount of TPU in TPU/SAN blends (Fig. 4). Dielectric
T,, + M.:T,z
Dielectric spectra of TPU are shown in Figs 5 and 6 (denoted by zero). The glass transition of soft segment phase of TPU (a-transition) is located around 250K. The B-transition (very low intensity) of TPU occurs in the region from 170 to 180K. As the temperature rises above the T8 of the TPU soft segment phase, molecular mobility increases. The glass transition temperature of the hard segment phase is not observed in the TPU dielectric spectra due to difficulties in handling samples at temperatures higher than the glass transition temperature of soft segment. The flexibility of the blends increases with increasing the concentration of TPU (acting as a stress concentrator). The higher flexibility should result in a higher dielectric dissipation (E”) in the blends. Consequently there is a high level of conduction, which causes interfacial charge build-up. However, the observed E’ values are not much larger than previously observed in polyurethane dielectric spectra [13-211. The spectra of SAN (denoted by 100) in Figs 5 and 6 have Tg around 390K due to the large scale motions
Table 3. r,s and apparent weight fractions of TPU and SAN in the TPU-rich phase and SAN-rich phase for TPU/SAN24 blends (DSC measurements) TPU-rich Blend
Fig. 1. Influence of blend composition on the r, (TPU) in TPU/SAN blends.
100/O 229 90/10 230 75125 231 60140 233 so/so 234 40/60 233 25175 234 IO/90 No observed 01100 -
(2; 370 371 372 373 374 375 376 319
11’1 1.0000 0.9934 0.9867 0.9801 0.9735 0.9668 0.9668 -
\I’> o.wOO 0.0066 0.0133 0.0199 0.0265 0.0332 0.0332 -
SAN-rich M‘I 0.0596 0.0529 0.0529 0.0463 0.0463 0.0463 0.0264 O.OlWO
I(‘: 0.9404 0.9471 0.9471 0.9537 0.9537 0.9537 0.9736 1.0000
Miscibility of thermoplastic blends
$ 376 .
5 0,200 ..
3 374 ..
fi 372 .. 370 ‘.
Fig. 2. Influence of blend composition on the r, (SAN) in TPU/SAN blends.
Fig. 4. Specificheat increment at the glass transition of SAN for TPU/SAN blends.
of dipoles. The lower temperature and low intensity process at 190K is caused by the limited motions of dipoles in the glassy state of the copolymer. The values of transition temperatures for TPU/ SAN34 and TPU/SAN24 blends, digitally estimated from the sn maxima at different frequencies, are given in Tables 4 and 5. The values at different frequencies are different because different parts of the chains are excited. The accurateness of the determinations is f3K. Dielectric measurements showed that the relaxation processes in TPU/SAN blends occurred at the same temperatures as the relaxation processes in pure components. It was presumed that the motions of TPU chains were decoupled from the motions of SAN chains. The shoulder at the temperature lower than Tg of SAN in blends clearly indicated a secondary relaxation in blends. The addition of TPU to SAN caused the main relaxation process in SAN to shift slightly toward T, of TPU. On the basis of this shift
a partial miscibility
of TPU/SAN blends could be predicted. In the same way as from DSC data [equations (1) and (2)], from the data in Tables 4 and 5 the apparent weight fractions in blends were calculated (Tables 6 and 7). Although the large loss of the TPU phase above its TE may distort the position of the SAN T, (making it higher, broader and shifting it to higher temperatures), the calculation from dielectric results confirmed shift in T,s observed by DSC. The apparent miscibility determined by the criterion of Tg shift may depend on the applied frequency , which was indeed observed in TPU/SAN blends. As evidenced from the slight but significant changes in transition temperatures as a function of the blend composition, it can be concluded that there exists some mechanical interlocking between otherwise immiscible phases. CONCLUSIONS
In the study of TPU/SAN blends, two values of T, were found at T, of TPU and T, of SAN, using DSC
Fig. 3. Specific heat increment at the glass transition of TPU for TPU/SAN blends.
T(K) Fig. 5. Dielectric spectra of TPU/SAN34 blends.
B. Zerjal er a/. Table 6. Apparent weight fractions in TPU/SANW blends (dielectric measurements) W; in TPU-rich
TPU/ SAN34 (kHa)
75/25 so/so 25175
0.048 0.559 -
0.039 0.083 -
0.018 0.129 -
0.008 0.134 -
W; in SAN-rich
Table 7. Apparent weight fractions in TPU/SAN24 blends (dielectric measurements)
TPU/ W; in TPU-rich SAN24 0.1 1 10 (kHa) 75125 0.066 0.020 0.104 so/so 0.022 0.004 0.011 25175
W; in SAN-rich 100 0.099 0.027
0.1 0.077 -
1 0.083 -
10 0.066 -
T(K) Fig. 6. Dielectric
and dielectric spectroscopy. The values of T8 of TPU in TPU/SAN blends are shown to increase linearly with increasing SAN weight fraction, while values of r, of SAN decrease with TPU weight fraction. The specific heat increment at the glass transition of TPU decreases with an increased amount of SAN in the blend, while the specific heat increment at the glass transition of SAN decreases with an increased
Table 4. Transition temperatures for TPU/SAN34 blends estimated from E” maxima at different freauencies Transition temperature (K) at Sample
251 201 174
252 208 175
252 210 176
254 215 179
394 260 192
408 265 196
amount of TPU in the blend. This slight shifting of the value of r, of SAN towards the value of T, of TPU may indicate the partial miscibility of the blends.
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