Polymer blending is a simple and effective means to bring desired properties that are demanded by end uses. However, compared to single-polymer systems, the foaming behavior and the cell structure of polymer blends are difficult to be precisely controlled. The reason lies in many influential factors, such as the phase morphology, the heterogeneous nucleation, and the different viscoelasticity.
Phase morphology change via varying compositions or applying interfacial compatibilizers significantly affects the foaming behavior. Whereas, during polymer processing, polymer blends would undergo thermal annealing at melt states, which would induce morphology change, however, with the composition and the interfacial tension remaining unchanged. In such cases, how does the phase morphology change affect the foaming behavior is still unknown.
To address this issue, the Advanced Polymer Processing Group from Ningbo Institute of Materials Technology and Engineering (NIMTE), Chinese Academy of Sciences (CAS) revealed the effect of phase structure evolution on the foaming behavior and the foaming temperature window of polystyrene/poly(methyl methacrylate) (PS/PMMA) blends.
PS/PMMA blends are usually employed for foaming applications, and they are typical immiscible polymer blends as well. Phase separation occurs during thermal annealing at melt states, which induces phase morphology evolution. Consequently, through controlling the phase separation time, the relationship between the structure evolution and the foaming behavior can be built.
The group members found that the cell structure of PS/PMMA blends is closely related to the morphology evolution. For blends with small domains, even after a long annealing, the cell structure still keeps uniform. In comparison, for those with bicontinuous phase structures or big domains, a long annealing significantly affects the cell structure. When phase domains grow big, the heterogeneous cell nucleation is gradually weakened, while the homogeneous cell nucleation is no longer insignificant, thus leading to the different foaming characteristics visible between the dispersed phase and the matrix (Fig. 1).
They also found that the lower foaming temperature is not sensitive to the structure evolution or the saturation pressure. For the blend after a long-time annealing, the upper foaming temperature is not sensitive to the lower saturation pressures. However, it is very vulnerable to the higher ones, thus resulting in a very narrow foaming temperature window (Fig. 2). It is believed that the viscoelasticity difference between the dispersed phase and the matrix is responsible for this phenomenon.
The meanings of the study lie in the following perspectives: first, it has enriched the understandings of the relationship between phase morphology and cell structure; second, the dependence of the foaming window on structure evolution and saturation pressure is proposed for polymer blends; and third, the crucial role of the difference in viscoelasticity in determining the foaming window is clarified. The findings are expected to provide guidance to advance the techniques for polymer foaming.
The study was financially supported by National Natural Science Foundation of China (No. 51603222), Public Welfare Technology Application Research Project of Zhejiang Province (No. LGG19E030003), and Natural Science Foundation of Ningbo (No. 2018A610033).
Fig. 1 Cell structures of samples foamed after 100 min thermal annealing (Image by NIMTE)
Fig. 2 The dependence of foaming temperature window of PS60/PMMA40 blends on saturation pressure after annealing for 2 min and 100 min (Image by NIMTE)
Ningbo Institute of Materials Technology and Engineering