Abstract
To develop a facile method for reducing the coefficient of volumetric thermal expansion (CVE) of polymer films, the thermal expansion behaviors of thermally cross-linkable polyimide (PI) films with isomeric diamine structures were investigated via thermal mechanical analyses and optical interferometry measurements. The degree of crosslinking of the PI films containing the diphenylethynylene (Ph–C≡C–Ph) structure in the main chain was characterized by far-infrared (far-IR) spectra and density functional theory (DFT) calculations, and variations in the CVE induced by thermal crosslinking were quantitatively estimated. The crosslinking reactions effectively reduced the CVEs of the PI films by suppressing intermolecular free volume expansion and local molecular motions promoted at elevated temperatures. The lowest CVE value observed for a crosslinked PI cured at 400 °C (+98 ppm/K at 80–280 °C) was one of the smallest values reported to date in polymers. Incorporating interchain crosslinking into the main chain is an effective method for reducing the CVE of aromatic polymers.
Highlights
The dimensional stability of polymeric dielectric layers in electronic devices as well as packaging against repeated thermal cycles during fabrication have become increasingly important with rising demand for higher-density integrated devices
density functional theory (DFT) calculations of model compounds for EBPA predicted that the IR-active absorption modes related to the diphenylethynylene group would appear in the far-IR region
The suppression of local molecular motion is important for lowering the coefficient of volumetric thermal expansion (CVE).TIn a previous study, we study, we reported that the films exhibiting vigorous local motion below g, which is represented study, we reported that the PI films exhibiting vigorous local motion below Tg, which is represented reported that the PI films exhibiting vigorous local motion below
Summary
The dimensional stability of polymeric dielectric layers in electronic devices as well as packaging against repeated thermal cycles during fabrication have become increasingly important with rising demand for higher-density integrated devices. The relationship between the chemical structure of aromatic PIs and their anisotropic linear and volumetric thermal expansion (VTE) behaviors has been recently characterized [4,5,6]. PIs with bulky side groups, such as –CH3 and –CF3 , or those with flexible linkages, such as –O– and –CO–, in the main chain exhibit larger coefficients of volumetric thermal expansion (CVE) compared to PIs containing rigid and linear main chain structures without side groups [4]. Polymers 2018, 10, 761 much smaller CVEs than those with loose molecular packing This indicates that the control of chain packing via preparation conditions is key for reducing the VTE of PIs. Introducing structures withwith interchain crosslinking is effective controlling aggregation polymerFor density.
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