UDK 678-1 Izvirni znanstveni članek ISSN 1580-2949 MATER. TEHNOL. 35(3-4)139(2001) M. ULČNIK-KRUMP ET AL.: PREDICTION OF POLYMER BLENDS STRUCTURE PREDICTION OF POLYMER BLENDS STRUCTURE NAPOVEDOVANJE STRUKTURE MEŠANIC POLIMEROV Manica Ulčnik-Krump1, Lucca Freitas Liane2, Tatjana Malavašič3 1Center for Experimental Mechanics, University of Ljubljana, 1000 Ljubljana, Slovenia 2Instituto de Quimica, Univ. Fed. de Rio Grande do Sul, A. Bento Gongaves 9500, Porto Alegre, Brazil 3National Institute of Chemistry, Hajdrihova 19, 1000 Ljubljana, Slovenia manica.ulcnikŽki.si Prejem rokopisa - received: 2000-11-10; sprejem za objavo - accepted for publication: 2001-03-10 Many everyday engineering applications of polymers depend on their structure-property relationship, which strongly influences polymer processability. In order to obtain a satisfactory understanding of polymers’ behavior their structure should be investigated and correlated with their properties. That is particularly in the case of complicated polymer systems, e.g. block copolymers, polymer blends or compatibilized polymer blends, very difficult and time-consuming procedure. In the present study we have shown how investigations of rheological properties can be used directly for predicting the structure of polymer blends. In spite of the fact that polymer blends are thermo-rheologicaly complex materials, master curves of the mechanical response were constructed. It was shown that mechanical spectroscopy might be used as a supplementary tool for predicting structure, even in such complicated polymer systems as polymer blends. Key words: polymers, structure, rheological properties Vsakodnevna uporaba polimerov je odvisna od medsebojnega vpliva njihove strukture in lastnosti, kar močno vpliva na predelovalne lastnosti polimerov. Da bi razumeli vedenje polimerov je treba poznati njihovo strukturo in jo primerjati z njihovimi lastnostmi. To je posebej v primeru zapletenih polimernih sistemov, kot so blokkopolimeri, mešanice polimerov ali kompatibilizirane mešanice polimerov, zahteven in dolgotrajen proces. V naših raziskavah smo pokazali, kako lahko rezultate reoloških raziskav direktno uporabimo za napovedovanje strukture mešanic polimerov. Kljub dejstvu, da so mešanice polimerov termoreološko kompleksni materiali, smo narisali t. i. sestavljene krivulje mehanskega odziva. Ugotovili smo, da lahko mehansko spektroskopijo uporabimo kot dodatno orodje za napovedovanje strukture celo v tako zapletenih polimernih sistemih, kot so mešanice polimerov. Ključne besede: polimeri, struktura, reološke lastnosti 1 INTRODUCTION Polyurethanes are an important class of polymers that are used in fibbers, elastomers, foams, coatings and plastics. In general, they lack thermal flexibility and stability, what consequently cause poor mechanical properties over a broad temperature range. By blending thermoplastic polyurethane (TPU) with styrene/ acrylonitrile (SAN) many different properties can be achieved because of the different structures of polyurethanes (hard and soft segment character and amount, domain separation, etc.) with different SAN copolymers (weight % of AN, different distributions of styreneand AN). A comparison of theadvantages and disadvantages of TPU and SAN, suggests that TPU/SAN blends could be promising materials with a broad technological potential. They have, sofar, not been used commercially due to their very complex structure and incompatibility. Onepossibleapproach for achieving better compatibility is to use compatibilizers. In spite of numerous research activities in the field of compatibilization of different polymer pairs, the compatibilized TPU/SAN blends were not studied in details until now 1,2. 2 EXPERIMENTAL 2.1 Materials Commercial TPU with polyester soft segments and SAN with 20 wt.% of acrylonitrile, were used as the blend’s basic components. The materials were supplied by BASF, Ludwigshafen, Germany. The homopolymers and block copolymers with a well-defined structure which were used as possible compatibilizers were synthesized via living anionic polymerization. The compatibilizer’s characteristics, determined by size-exclusion chromatography (SEC) and proton nuclear magnetic resonance (1H-NMR), aresummarized in Table 1. Compatibilized and non-compatibilized TPU/SAN 25/75 blends were prepared from a melt at 210 °C, using a co-rotating twin-screw mixer with conical screws, developed by the TU Eindoven, Netherlands. 2.2 Characterization techniques Ultra-thin sections of blends were cut at -60 °C with a Reichert cryoultramicrotom using a diamond knife. All samples were stained with a 4 wt.% aqueous solution of phosphotangstic acid. Electron micrographs were obtained with a CEM 902 Zeiss transmission electron microscope (TEM) using an accelerating voltage of 80 kV. MATERIALI IN TEHNOLOGIJE 35 (2001) 3-4 139 M. ULČNIK-KRUMP ET AL.: PREDICTION OF POLYMER BLENDS STRUCTURE The Rheometric Dynamic Stress Rheometer (SR-500) is a stress-controlled rheometer, which was used to characterize the rheological properties of the polymer blends. Isothermal frequency sweeps were recorded between 130 and 220 °C in steps of 10 °C, in order to obtain master curves. A further increase in the temperaturewas impossiblebecauseof thesamples’ thermal instability. The frequency was varied from 100 to 10-2 rad/s. The region of linear viscoelastic behavior was determined using stress sweeps. Master curves were constructed using the Rheometric RheCurve software, which determines shift factors empirically. The program calculates theshift factors that givethebest superposition of the data points when they are multiplied by the frequency of each measurement made at a given temperature. Table 1: Molecular properties of synthesized polymers. Tabela 1: Molekularne lastnosti sintetiziranih polimerov Compati-bilizer’s denotation Structure Mw·10-3a) Mw/Mn a) wt.% of A block b) (g/mol) C1a PCl 104 1.31 - C1b PCl 87 1.24 - C1c PCl 49 1.40 C2 PS-b-PCl 65 1.25 30 PS-b-PMMA 78 1.16 24 C3 PB-b-PMMA 105 1.07 29 PI-b-PCl 137 1.18 65 a) ObtainedbySEC for C2 b) Obtained by 1H NMR for C3 A-block = PS A-block = PB or PI o 6 C2 I» J/\ if •¦y ¦> C3 C1b C1c without compatibiliser C1a A\ 0.1 0.2 0.3 0 Particle size (|jm) Figure 1: Normal distribution curves of particle radius Slika 1: Krivulje normalne porazdelitve premera delcev 3 RESULTS AND DISCUSSION The effect of the compatibilizers on the morphology of the TPU/SAN 25/75 blends was investigated on the basis of thedistribution of domain sizes 3. Thearea of each particle obtained from corresponding TEM micrographs was measured and the particle radius calculated assuming that all particle images are circles. The normal distribution curves obtained from the corresponding histograms are shown in Figure 1. Becausethecurves arenot normalized, only thecurve widths arecomparableindicators of dispersed domain sizeand thecurveheights haveto beneglected. The influence of the molecular weight of C1x (PCls with different Mw) on theblends’ morphology can beclearly s een. Theincreaseof thesizeof thedispersed TPU domains indicates that C1a (PCl with the highest Mw) probably causes coalescence, while the addition of C1c (PCl with thelowest Mw) reduces the size of the domains. Moreover, a narrower distribution of domain sizes was obtained with the addition of C1c. The broader distributions show that thesizeof thedispersed TPU domains increases in blends compatibilized with C2 and C3, when compared with the non-compatibilized blend. This indicates that C2 and C3 do not stabilize the morphology against coalescence. The TEM micrograph of the TPU/SAN blend compatibilized with C1c is shown in Figure 2. In spiteof many contradictory opinions about the possibility of using the time-temperature superposition (TTS) principle to generate master curves for thermo-rheologically complex materials 4,5 and thefact that immisciblepolymer blends arenot, strictly speaking, thermo-rheologically simple materials, we applied TTS for non-compatibilized and compatibilized TPU/SAN 25/75 blends. The behavior of these blends Figure 2: TEM micrograph of TPU/SAN 25/75 blend compatibilized with 5 wt.% of C1c Slika 2: TEM-posnetek mešanice TPU/SAN 25/75, kompatibilizirane s 5 mas.% C1c 140 MATERIALI IN TEHNOLOGIJE 35 (2001) 3-4 M. ULČNIK-KRUMP ET AL.: PREDICTION OF POLYMER BLENDS STRUCTURE 107 L 10 b 10s 10* TPU/SAN 25/75 without compatibiliser C3 P(B-MMA)+P(I-CI) C3 P(S-CI)+P(S-MMA) fV C1c(PCIc) /./ / j jr./ C1b(PCIb) 10 10 10 10 10 10 10 10 10 10 10 10 10 Frequence (0)xaT) (rads'1) --------¦- Figure 3: Master curves of the loss shear modulus (G") of TPU/SAN 25/75 blends as a function of frequency (Tr = 150 °C) Slika 3: Sestavljene krivulje dinamskega strižnega modula izgub (G") mešanic TPU/SAN 25/75 v odvisnosti od frekvence (Tr = 150 °C) 2.5x10' 5.0x10" TPU/SAN 25/75 without compatibiliser C1b(PCIb) 0 50 100 150 200 250 300 350 400 450 500 550 Time (min) ------*¦ Figure 5: Viscosity of non-compatibilized and compatibilized TPU/SAN 25/75 blends during a creep period of 600 min Slika 5: Viskoznost nekompatibiliziranih in kompatibiliziranih mešanic TPU/SAN 25/75 med strižno obremenitvijo 600 min was expected to be complex and dominated by the SAN matrix. Wefound that theTPU/SAN 25/75 blends showed a rheological behavior, that is between the behavior of the purecomponents 3. However, the influence of SAN predominates since it builds the matrix phase 6. At higher frequencies the blends behave similar to SAN, but at the lower frequencies the influence of the dispersed TPU phase was observed, and as a consequence, the dynamics data do not follow well-known patterns such as Arhenius or WLF. A dependence of approximately G ? ?0.5 was found, which is typical for phase-separated polymers 3,7,8. Thetransition from theelastic plateau to theflow region is broad, which is typical for polymers with a broad molecular-weight distribution 9. 10" C1b (PCIb) C2 P(S-CI)+P(S-MMA) C3 P(B-MMA)+P(I-CI) Č(rv C1c(PCIc) TPU/SAN 25/75 without compatibiliser 10'7 IO* IO* 10'" 10a 10* 10'1 10° 101 102 10a 104 10* Frequence (u)x aT) šrads'1) -----------*č Figure 4: Master curves of the loss shear modulus (G") of TPU/SAN 25/75 blends as a function of frequency (Tr = Tg SAN + 50 °C) Slika 4: Sestavljene krivulje dinamskega strižnega modula izgub (G") mešanic TPU/SAN 25/75 v odvisnosti od frekvence (Tr = Tg SAN +50 °C) Figure 3 shows the effect of the compatibilizers on thedynamic moduli of theTPU/SAN 25/75 blends, with 150 °C taken as the reference temperature for all master curves. At high frequencies there are only small differences between non-compatibilized and compati-bilized TPU/SAN 25/75 blends. Larger changes in the modulus values are observed for blends compatibilized with C1x. Furthermore, the addition of the PCl homopolymers shifts the flow region to higher frequencies, in comparison with the non-compatibilized blend and the blends compatibilized with C2 and C3. A shift to higher frequencies corresponds to lower relaxation times and means that for a given temperature, blends compatibilized with the C1x will flow within a shorter period. The shift of the flow region to higher frequencies can be a consequence of changes in the morphology of the blend or of changes in the Tg of the phases, mainly of thephasewith thehighest Tg. In order to eliminatetheinfluenceof theTg of the SAN-rich phaseon theterminal flow region of the blends, master curves with Tr = Tg(SAN-rich phase) + 50 °C were also constructed from the experimental data (Figure 4). In this case, we can clearly see that only the addition of C1c shifts the flow region to higher frequencies, while the addition of the other compati-bilizers shifts the flow region to even lower frequencies, indicating that only the blend compatibilized with C1c should have smaller domains of the dispersed phase. This is in agreement with the results of the morphological investigations, which clearly show that the size of TPU-rich-phasedomains for theblend compatibilized with C1c is much smaller than thesizeof thedomains of the other blends. Theviscosity (?) versus time curves (Figure 5) of the molten non-compatibilized and compatibilized TPU/SAN blends were obtained from strain versus time curves during a creep period of 600 min at 140 °C. The MATERIALI IN TEHNOLOGIJE 35 (2001) 3-4 141 M. ULČNIK-KRUMP ET AL.: PREDICTION OF POLYMER BLENDS STRUCTURE decrease of ? was ascribed to theshareof PCl in the compatibilizers, which can be related to the decrease of theTg of theSAN-rich phasein theseblends, as found in the investigations of thermal and thermo-mechanical properties 6,10. To obtain someadditional information about behavior in the steady state, creep measurements should also be performed at the temperatures with a constant distanceto thehigher Tg of theblend. 4 FINALREMARKS The master curves of the TPU/SAN 25/75 blends show a rheological behavior between that of their pure components. However, the SAN, which builds the matrix phase, has a stronger influence. When a constant temperature is chosen as the reference temperature, the flow region of all the compatibilized blends is shifted to higher frequencies. When a temperature with a constant distanceto thehighest Tg of the blend is chosen as a reference temperature, only the flow region of the blend compatibilized with C1c is shifted to higher frequencies. Taking into account that a shift to higher frequencies is an indication of a finer morphology and that only with C1c were smaller domain sizes found by TEM, it can be concluded that the differences in Tg haveto betaken into account when constructing master curves and interpreting results. The TPU/SAN blends compatibilized with C2 and C3 (a mixtureof two block copolymers) aretoo complicated to be adequately understood. Nevertheless, our results show that poly(?-caprolactone)s, with an appropriateMw, improvethecompatibility of TPU/SAN blends (smaller domains). A particular advantage of using PCl as a compatibilizer for TPU/SAN blends is that this homopolymer can be more easily prepared on a commercial scale, principally if its synthesis is compared to themuch moretroublesomesynthesis of high-molecular-weight block copolymers, that are normally used as compatibilizers. It was also shown that mechanical spectroscopy might be used as a supplementary tool for the structure prediction of complex polymer systems. However, rheological investigations can be used for the identification of compatibilzation efficiency. ACKNOWLEDGEMENTS This contribution is dedicated to Prof. Dr. Reimund Stadler. The work was performed in his group at the University of Mainz and at the University of Bayreuth. M.U.Krump thanks theMinistry of Scienceand Technology of the Republic of Slovenia for financial support. Additional financial support was provided by Graduiertenkolleg "Physik und Chemie supramolekularer System" in Mainz, the "BMBF/BASF research project" and the DFG research project "Benetzung und Strukturbildung an Grenzflächen". 5 REFERENCES 1 Van Cleve R., Armstrong G. H., Simroth D. W., U.S.Pat. 4 (1982) 350, 780, Union Carbide 2 Jaisankar S. N., Radhakrishnan G., Poly.Eng. Sci. 40 (2000) 621-626 3 M. Ulčnik-Krump, Ph.D. Thesis, University of Ljubljana. Ljubljana, 1999 4 Fresko D. G., Tschoegl. N. W., J.Polym.Sci. 35C (1971) 51-69. 5 Wisniewski. C., Marin G., Monge Ph., Eur.Polym.J., 21 (1985) 479-484. 6 Ulčnik-Krump M., de Lucca Freitas L., Stadler R., Malavasic T. (submitted for publication to Polymer) 7 Bates F. S., Macromolecules 13 (1984) 1602-1613. 8 RosdaleJ. H., Bates F. S., Macromolecules 23 (1990) 2329-2335. 9 Ferry J. D., Viscoelastic Properties of Polymers, 3rd Ed., Marcel Decker, Inc. New York, 1994 ISBN 0-8247-8964-4. 10 Ulčnik-Krump M., Stadler R., Malavašič T., Macromol. Symp. 149 (2000) 131-136. 142 MATERIALI IN TEHNOLOGIJE 35 (2001) 3-4