01/08/2018
Chemical structure of PIM-1, heat flow curves for the determination of Tg and calorimetric sensor chip with deposited sample.

Chemical structure of PIM-1, heat flow curves for the determination of Tg and calorimetric sensor chip with deposited sample.

Source: BAM, Division Nanotribology and Nanostructuring of Surfaces and Division Technical Properties of Polymeric Materials

In recent years the so-called “polymers of intrinsic microporosity” (PIMs) established as a new class of high-performance polymers, especially in the field of gas separation membranes but also for a range of other applications such as microelectronics or sensor technology. The outstanding feature of these glassy polymers is the very high amount of fractional free volume giving rise to microporosity characterized by BET surface areas > 700 m2/g. At the same time this results in very attractive gas transport properties, i.e. very high permeability in combination with good selectivity for technical relevant gas mixtures, allowing for highly energy-efficient membrane-based separations – for example in natural gas treatment or biogas upgrading.

The reason for that is the extremely rigid and contorted molecular structure leading to inefficient packing of the polymer chains in the solid state but also constricting the molecular mobility and completely preventing segmental rotations. A key characteristic of a polymer, which is directly related to the molecular mobility and strongly determines its practical applications, is the glass transition temperature Tg.

PIM-1, investigated in this paper, is the archetypal polymer of intrinsic microporosity and so far, it was impossible to detect a glass transition temperature with conventional thermal analysis like DSC or DMTA, instead the decomposition of the polymer was observed above 400 °C. Taking into account that due to the rigid, ladder-like structure the motional degrees of freedom are extremely limited, the fundamental question is raised, whether this kind of polymers can undergo a glass transition at all.

In the present work, for the first time, fast scanning calorimetry (FSC) was employed to investigate the thermal behavior of PIM-1 decoupling the time scales responsible for glass transition and decomposition by applying heating/cooling rates of tens of thousands K/s. The obtained results provide the first definitive evidence of a glass transition of PIM-1 at about 440 °C at a heating rate of 30000 K/s. Furthermore, small-scale bend and flex fluctuations must be considered the origin of the glass transition of PIM-1 as any conformational changes and motions on a segmental length-scale - usually related to the glass transition - are clearly prevented by the rigid molecular architecture of the polymer.

The measurements were carried out together with the Polymer Physics group at the University of Rostock, a Center of Excellence for calorimetry. PIM-1 was obtained from one of its inventors at the University of Manchester.

First clear cut experimental evidence for a glass transition in a polymer with intrinsic microporosity: PIM-1
Huajie Yin, Y. Z. Chua, B. Yang, C. Schick, W. Harrison, P. Budd, Martin Böhning, Andreas Schönhals
Journal Physical Chemistry Letters, 2018, 9 (8), pp 2003–2008
BAM Department Materials Protection and Surface Technolo, Division Nanotribology and Nanostructuring of Surfaces and Department Safety of Structures, Division Technical Properties of Polymeric Materials