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In recent years superglassy polymers exhibiting intrinsic microporosity established a new perspective for a number of applications, especially for gas separation membranes as These polymers Combine extremely high permeabilities with attractive selectivities. The essential factor governing the structure Formation in the solid film or layer is either a contorted rigid Backbone (polymers of intrinsic microporosity - PIMs) or extremely bulky side groups (polynorbornenes and polytricyclonenenes).
For a deeper understanding of both types of such high-Performance polymers for gas separation membranes and their further development broadband dielectric spectroscopy (BDS) can provide a substantial contribution.
BDS addresses molecular relaxations characterizing the dynamics of the solid polymer as a major factor determining the gas transport properties but also the physical aging behavior which is an essential issue for such polymers.
BDS is applied on PIMs where fluctuations of molecular dipoles connected to the backbone can be directly monitored. Furthermore, also polynorbornenes were investigated which carry no dipole moment in their repeat unit - the high resolution of modern equipment allows for the detailed analysis also for very small dielectric losses originating from partially oxidized moieties or marginal catalyst residues.
Additionally, from interfacial polarization phenomena, such as Maxwell-Wagner-Sillars (MWS) polarization due to blocking of charge carriers at internal interfacial boundaries on a mesoscopic length scale, valuable information on the intrinsic microporosity and its changes induced by physical aging can be obtained.
Finally, also conductivity can be characterized in detail in such polymeric systems revealing contributions of interactions of aromatic moieties (π-π-stacking) or the drift motion of charge carriers. These features also determine the structure formation in the solid state.
Polymers of intrinsic microporosity (PIMs) have recently emerged as novel materials for a broad range of high-performance applications from gas separation to electronic devices. The very rigid, contorted polymer chains show only limited molecular mobility and therefore pack inefficiently giving rise to intrinsic microporosity with pore sizes generally smaller than 1 nm resulting in BET surface areas larger than 700 m2/g. Using conventional thermal analysis techniques, no glass transition temperature (Tg) of PIMs could be unambiguously detected up to now. Employing fast scanning calorimetry (FSC) based on a one chip sensor, decoupling the time scales responsible for the glass transition and the thermal decomposition is a reliable strategy to overcome this limitation. The FSC device is capable to heat and cool a small sample (ng-range) with ultrafast rates of several ten thousand K/s. Evidence of a glass transition is obtained for a series of PIMs with different chain rigidities. Local small-scale fluctuations are held responsible for the glass transition of highly rigid PIMs rather than segmental motions as in conventional polymers.
Slow crack growth (SCG) under the influence of external fluid media, usually termed environmental stress cracking (ESC), is still one of the most frequent origins of severe damage and failure of polymeric materials. For polyethylene (PE) this is relevant for mass products, like bottles for consumer products, but also for high-performance materials for pipes or industrial packaging, including containers for chemicals and other dangerous goods. Especially for high-density polyethylene (PE-HD) the susceptibility to SGC and ESC is depending on the complex interplay between molecular weight and architecture and the resulting semicrystalline morphology, especially the formation of tie-molecules.
A reliable assessment of the resistance against this damage phenomenon is essential for demanding as well as safety-critical applications and has to take into account suitable testing methodologies and conditions in combination with environmental media reflecting the properties representative for typical fillings and relevant components causing ESC. In this context a better understanding of different influencing factors, such as sorption, swelling or surface activity is necessary together with a detailed characterization of different stages of crack propagation and underlying mechanisms. Therefor well-established testing methods, such as the Full Notch Creep Test (FNCT), were complemented by detailed fracture surface analysis using e.g. LSM and SEM in combination with a time-dependent optical monitoring of the progressing crack growth.
For the crack growth also the ratio of fibrillated craze zone to crack opening is important for the deeper understanding of crack propagation and related material parameters which can be additionally addressed by X-ray computed tomography.
Polymers with intrinsic microporosity are promising candidates for the active separation layer in gas separation membranes. These polymers are characterized by a high permeability and reasonable permselectivity. The latter point is somehow surprising because for microporous systems a more Knudson-like diffusion is expected then a size dependent temperature activated sieving process. It was argued in the framework of a random gate model that molecular fluctuations on a time scale from ps to ns are responsible for the permselectivity.
Here series of polymers of intrinsic microporosity (PIMs) as well as microporous polynorbornenes with bulky Si side groups and a rigid backbone are considered. The polymers have different microporosity characterized by high BET surface area values.
First inelastic time-of-flight neutron scattering measurements were carried out to investigate the low frequency density of state (VDOS). The measured data show the characteristic low frequency excess contribution to the VDOS above the Debye sound wave level, generally known as the Boson peak in glass-forming materials. It was found that the frequency of the maximum position of the Boson peak correlates with the BET surface area value.
Secondly elastic scans as well as quasielastic neutron scattering measurements by a combination of neutron time-of-flight and backscattering have been out. A low temperature relaxation process was found for both polymers. This process was assigned to the methyl group rotation. It was analysed in terms of a jump diffusion in a three-fold potential. The analysis of the dependence of the elastic incoherent structure factor on the scattering vector yields the number of methyl groups which might be immobilized.
The neutron scattering experiments were accompanied by fast scanning calorimetry and broadband dielectric investigations as well as atomistic molecular dynamic simulations.