Thermal properties of bulk polyimides: insights from computer modeling versus experiment

Abstract

Due to the great importance for many industrial applications it is crucial from the point of view of theoretical description to reproduce thermal properties of thermoplastic polyimides as accurate as possible in order to establish "chemical structure-physical properties" relationships of new materials. In this paper we employ differential scanning calorimetry, dilatometry, and atomistic molecular dynamics (MD) simulations to explore whether the state-of-the-art computer modeling can serve as a precise tool for probing thermal properties of polyimides with highly polar groups. For this purpose the polyimide R-BAPS based on dianhydride 1,3-bis(3',4-dicarboxyphenoxy)benzene (dianhydride R) and diamine 4,4'-bis(4''-aminophenoxy)biphenyl sulphone) (diamine BAPS) was synthesized and extensively studied. Overall, our findings show that the widely used glass-transition temperature Tg evaluated from MD simulations should be employed with great caution for verification of the polyimide computational models against experimental data: in addition to the well-known impact of the cooling rate on the glass-transition temperature, correct definition of Tg requires cooling that starts from very high temperatures (no less than 800 K for considered polyimides) and accurate evaluation of the appropriate cooling rate, otherwise the errors in the measured values of Tg become undefined. In contrast to the glass-transition temperature, the volumetric coefficient of thermal expansion (CTE) does not depend on the cooling rate in the low-temperature domain (T < Tg) so that comparison of computational and experimental values of CTE provides a much safer way for proper validation of the theoretical model when electrostatic interactions are taken into account explicitly. Remarkably, this conclusion is most likely of generic nature: we show that it also holds for the commercial polyimide EXTEM, another polyimide with a similar chemical structure.

Influence of the carbon nanotube surface modification on the microstructure of thermoplastic binders

“…Two main types of polyimides for matrices are considered which are synthesized and actively explored in the Institute of Macromolecular Compounds of the Russian Academy of Sciences: 11,13,[15][16][17][18][19]58,59 crystallisable R-BAPB based on 1,3-bis-(3',4-dicarboxyphenoxy)-benzene (dianhydride R) and 4,4' bis-(4''-aminophenoxy)-diphenyl (diamine BAPB) and amorphous R-BAPS based on dianhidride R and 4,4'-bis-(4''-aminophenoxy)-diphenylsulfone (diamine BAPS), whose chemical structures are given in Fig. Two main types of polyimides for matrices are considered which are synthesized and actively explored in the Institute of Macromolecular Compounds of the Russian Academy of Sciences: 11,13,[15][16][17][18][19]58,59 crystallisable R-BAPB based on 1,3-bis-(3',4-dicarboxyphenoxy)-benzene (dianhydride R) and 4,4' bis-(4''-aminophenoxy)-diphenyl (diamine BAPB) and amorphous R-BAPS based on dianhidride R and 4,4'-bis-(4''-aminophenoxy)-diphenylsulfone (diamine BAPS), whose chemical structures are given in Fig.…”

“…The pressure and temperature were maintained at the required level using the Berendsen thermostat and barostat; 73 the time constants for which were taken as τ T = 0.1 ps for the thermostat, and τ P = 0.5 ps for the barostat. 58,62,65 The calculated partial charges were published in our previous studies. To maintain the set lengths of the bonds between the atoms, the LINCS algorithm was used.…”

Influence of the carbon nanofiller surface curvature on the initiation of crystallization in thermoplastic polymers

“…Thus, we can conclude that the cell size is large enough and the effects observed are due to interaction of the polymer with the nanoller surface but not due to steric hindrance between parallel at surfaces. As shown in our recent studies, [42][43][44][45][46] in some cases the atomic partial charges could be taken equal to zero in order to make the long microsecond-scale equilibration possible with the available supercomputer resources. The accounting of the atomic partial charges in computer simulation may, in principle, inuence signicantly the nal results, especially when the compounds contain highly-polarized chemical groups.…”

“…Constant temperature (600 K, that is 5K higher than the experimentally-determined value of the R-BAPB melting temperature) and pressure P ¼ 1 atm were maintained using a Berendsen thermostat and barostat, 48,49 with time constants of s T ¼ 0.1 ps and s P ¼ 0.5 ps, respectively. These parameters were used in our previous papers [42][43][44][45][46] and allowed us to reproduce successfully thermophysical properties of bulk PIs. All bond lengths were kept constant with the LINCS algorithm 50 .…”

“…41 This approach was used in our previous studies to investigate the thermophysical properties, such as glass-transition temperature and coefficient of thermal expansion, of polyimides without ller [42][43][44][45][46] and lled with CNT. 32 The MD simulations allowed understanding the relationship between these properties and the polyimide chemical structure.…”