Elsevier

Polymer

Volume 41, Issue 3, February 2000, Pages 975-983
Polymer

Interaction, miscibility and phase inversion in PBI/PI blends

https://doi.org/10.1016/S0032-3861(99)00236-0Get rights and content

Abstract

The miscibility of polybenzimidazole {poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole], (PBI)} and a polyimide {poly[2,2′-bis(4-(3,4-dicarboxyphenoxy)phenyl)propane-m-phenylenediimine] (PI)} was studied in the entire composition range. Films were cast from solution by a procedure developed in a preliminary work. The blends were studied by FT-IR spectroscopy, thermal and thermomechanical methods, and by measuring vapour absorption at an ambient temperature. Direct relationships were found between the strength of hydrogen-bonding of the components, the glass transition temperature of the blends, and the solvent diffusion rate (water and 1,2,4-trichlorobenzene). The results of the experiments indicate partial miscibility between the two polymers. Phase inversion occurs between 0.4 and 0.6 PI content.

Introduction

Blends of polybenzimidazole {poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole], (PBI)} and several polyimides (PI) have been widely investigated in the last 12 years because of the theoretical and practical importance of these high performance polymers. From the very initial studies, it was claimed that miscibility occurs over a wide composition range, as the blends displayed single intermediate glass transitions (Tg) in the first DSC heating [2]. Further studies revealed phase separation upon annealing [2], [3], [4], and the phase behaviour of the systems was found to be dependent both on the type of polyimide and thermal history. Thus blends of PBI with the condensation product of 3,3′,4,4′-benzophenone tetracarboxylic dianhydride and 5(6)-amino-1-(4′-aminophenyl)-1,3,3-trimethylindane (XU 218, Ciba-Geigy) showed phase separation only above 400°C, at temperatures determined by the composition [4]. From calorimetric studies of blends of PBI with poly[2,2′-bis(4-(3,4-dicarboxyphenoxy)phenyl)propane-m-phenylenediimine] (Ultem 1000, General Electric) it was inferred that phase separation might occur between ambient temperature and the glass transition temperature, if it were not prohibited by the restricted mobility of the polymer chains in the glassy state [2]. Infrared studies revealed hydrogen bonding between the N–H groups of PBI and the carbonyl groups of PI [5] which relaxed during the thermal treatment resulting phase separation [6], [7].

For the PBI/Ultem 1000 system the actual phase separation could not be detected below Tg either by DSC [2] or by dielectric relaxation [8]. The only direct evidence for the existence of separate phases seems to be the results of mechanical relaxation experiments. The loss tangent measured at 1 Hz shows two or three composition dependent maxima between the glass transition temperatures of the components [8].

Complex relaxation studies drew attention to the importance of the sample preparation technique—especially the amount of residual solvent and water—in the properties of binary blends of PBI and Ultem 1000 [8]. As a consequence, sample preparation conditions were studied in the first phase [1] of an intensive study on the composition/structure/property relationships in PBI/Ultem 1000 blends. The results of the study of interactions, miscibility and phase behaviour of this polymer pair prepared by the selected technique are presented in this paper.

Section snippets

Materials

Poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole] (PBI); Hoechst Celanese Corp; Tg=420°C; density: 1.269 g/cm3.

Poly[2,2′-bis(4-(3,4-dicarboxyphenoxy)phenyl)propane-m-phenylenediimine] (PI); grade Ultem 1000; General Electric Co; Mw=30 000±10 000, Mn=12 000±4000 g/mol; Tg=220°C; density: 1.187 g/cm3.

Sample preparation

The sample preparation technique was developed in a preliminary study carried out to determine the effects of preparation conditions on the stability, residual solvent and water content of the polymer [1].

Results and discussion

The FT-IR spectroscopic study of PBI/PI blends of varying composition confirmed the results of earlier experiments [5], [6]. The N–H stretching vibration of PBI shifts to lower frequencies with increasing PI content (Fig. 1). The differences between the present and earlier results observed at high PI contents are attributed to the different sample preparation techniques. They indicate that the strength of interaction between the two polymers is influenced by the traces of moisture remaining in

Conclusions

Blends of PBI and PI were prepared over the entire composition range by film casting from N,N′-dimethyl acetamide solution using a method developed in a preliminary work. The interaction and miscibility of the components were studied as a function of composition.

FT-IR studies revealed relatively weak hydrogen bonding between the components (compared to that of polyamide chains). Thermal (DSC) and thermomechanical (TMA) experiments yielded single intermediate glass transitions for all

Acknowledgements

One of the authors (FEK) thanks USAFOSR for support. The Hungarian authors are grateful for the financial support of the National Scientific Research Fund of Hungary (Grant No. F007362 and T023421) making possible the experiments at ICCRC.

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