Blends of Polybutadiene with Different Vinyl Contents and Polystyrene Studied with Small-Angle Neutron Scattering in Varying Temperature and Pressure Fields Henrich Frielinghaus, †,‡ Dietmar Schwahn,* ,† and Lutz Willner † Forschungszentrum Ju ¨ lich GmbH, Institut fu ¨ r Festko ¨ rperforschung, D-52425 Ju ¨ lich, Germany, and Risø National Laboratory, DK-4000 Roskilde, Denmark Received June 12, 2000; Revised Manuscript Received December 6, 2000 ABSTRACT: Blends of polybutadiene with three different vinyl contents and polystyrene were studied by small-angle neutron scattering as a function of temperature and pressure. The data were analyzed with a crossover function combining the universality classes of the 3D-Ising and the mean field behavior and thus yield the spinodal temperature T S, the Ginzburg number Gi, and the critical amplitude CMF. From these parameters the mean field Flory-Huggins interaction parameter is calculated in terms of entropic and enthalpic contributions. The application of the theory of random copolymers delivers large enthalpic and entropic contributions of the intramolecular interaction between the 1,2- and 1,4-butadiene units. These numbers could not be confirmed from measurements of the corresponding intermolecular interaction contributions in an h-PB(1,2)/d-PB(1,4) blend, which thus leads to speculations about the equivalence of inter- and intramolecular interactions in random copolymers. The Ginzburg number proved to be specific to the vinyl content and thus deviated from the universal behavior Gi ∝ V -1 predicted by deGennes. By our pressure experiments we find that the Clausius-Clapeyron equation suggests the phase boundary to be influenced by the Flory-Huggins parameter and the Ginzburg number. I. Introduction The exploration of the thermodynamic properties of binary polymer melts is of considerable interest in fundamental and applied research. 1,2 On the fundamen- tal side, the experimental technique of small-angle neutron scattering (SANS) plays a prominent role as it is a very sensitive and direct tool; the scattering contrast between the two polymers can be made very large by the use of fully hydrogenated and deuterated compo- nents so that the scattering of neutrons from thermal composition fluctuations become strong and the domi- nant contribution. By the method of contrast variation cooperative phenomena as well as single-polymer prop- erties can be studied in a blend. 3,4 In this paper we present SANS scattering experi- ments on binary polymer blends to explore thermal composition fluctuation in different pressure and tem- perature fields. The basic samples are three binary polymer blends of nearly critical composition of poly- styrene (PS) and three polybutadienes (PB) of different microstructure. The polybutadienes were fully deuter- ated (therefore d-PB) for reasons of strong scattering contrast. The d-PB polymers were synthesized by an- ionic polymerization from perdeuterated butadiene mono- mers. Three different solvent/cosolvent mixtures were used for the polymerization, leading to polymer chains with different ratios of 1,4 and 1,2 addition: d-PB(1,4) with 7%, d-PB(1,2;1,4) with 54%, and d-PB(1,2) with 91% 1,2 (vinyl) repeat units. The d-PB polymers are effectively considered as “statistical copolymers” of 1,4 and 1,2 repeat units. The molar volumes of the polymers were about 2000 cm 3 /mol. All samples show a phase boundary at temperatures between 60 and 90 °C and decompose into macroscopic large phases at low tem- peratures. The phase boundary is elevated by the vinyl content of the PB chain, and accordingly different values for the Flory-Huggins (FH) interaction parameters are evaluated from the SANS experiments. Because the phase boundary and the FH parameter of the d-PB/PS blend changes with vinyl content, a finite FH parameter must exist between the 1,4 and 1,2 repeat units. The theoretical approach for a blend consisting of statistical copolymers predicts a relatively strong repulsive in- tramolecular interaction between the 1,4 and 1,2 repeat units. However, an additional SANS experiment on a d-PB(1,4)/h-PB(1,2) blend showed a much smaller FH parameter, in contradiction with the former experi- ments. The conclusion is that the existing theoretical description of statistical copolymers does not consis- tently describe our experimental results. A further aspect of this work is related to the effect of thermal fluctuations on the thermodynamic proper- ties as already discussed in earlier studies. 5 The FH theory is based on a mean field approach, neglecting the effect of thermal composition fluctuations, and the FH parameter Γ therefore represents a mean field or bare interaction parameter. From SANS experiments one usually gets Γ ) Γ h /T - Γ σ with the corresponding phenomenological enthalpic and entropic terms Γ h and Γ σ , respectively. For symmetrical binary blends with polymer components of equal molar volume V the critical composition is Φ C ) 0.5 and the FH parameter at the critical temperature T C is according to T C ) 2/V inversely proportional to the molar volume of the two components. However, the experiments are usually not performed in the range of mean field approximation. This is true for most binary polymer blends of upper critical solution type (UCST), whose molar volume has to be sufficiently small to show a moderate critical temperature. We therefore cannot directly analyze the experimental data with the FH model but need a more sophisticated model, which includes the effects of ther- mal fluctuations. We applied a crossover model derived by Belyakov et al., 6,7 which sufficiently describes well * To whom correspondence should be addressed. † Institut fu ¨ r Festko ¨rperforschung. ‡ Risø National Laboratory. 1751 Macromolecules 2001, 34, 1751-1763 10.1021/ma001015k CCC: $20.00 © 2001 American Chemical Society Published on Web 02/10/2001