Novel Polyimide-block-poly(dimethyl siloxane) copolymers: Effect of time on the synthesis and thermal properties

Abstract

Polyimide-block-poly(dimethyl siloxane) copolymer was synthesized by a two-step process, initiated by coupling anhydride terminated poly(amic acid), AT-PAA with amino terminated poly(dimethyl siloxane), (NH2)2-PDMS to form poly(amic acid)-block-poly(dimethyl siloxane). The resulting copolymer is then thermally treated to produce polyimide-block-poly(dimethyl siloxane), PI-PDMS. Because of the high glass transition temperature, Tg of polyimide, it is usually cured at a high temperature of about 300°C for over 2.5 h. Copolymerization of polyimide with polysiloxane, reduces the imidization temperature while maintaining high thermomechanical properties. A series of instruments were used to monitor the progress of copolymerization. The time-based analysis of the product of copolymerization enables the optimization of the structure and properties of the copolymers. The chemical structure and composition of the copolymer were studied by Fourier Transform Infrared Spectroscopy, (FT-IR). The incorporation of PDMS blocks into the copolymer and the degree of imidization of the polyimide block increased with increasing reaction time. The change in the viscosity of the copolymerizing solution was monitored by simple shear viscometry conducted with the Brookfield Viscometer. The reported increase in solution viscosity with increasing copolymerization time is associated with increasing molecular weight of the copolymer. The intrinsic viscosity of the copolymer solution was measured as a function of copolymerization time and it was found that the intrinsic viscosity of the copolymer solution increased with increasing reaction time. The glass transition temperature (Tg) and the thermal stability of the copolymer were determined by differential scanning calorimetry, DSC and thermogravimetric analysis, and TGA, respectively. Between 25°C and 420°C, the copolymers synthesized in this study show two glass transition temperatures due to the polyimide, PI block at around 380°C and another peak associated with PDMS plasticized polyimide at about 290–300°C. The two Tg peaks observed in the DSC thermogram are believed to be indicative of the structure of a block copolymer. TGA analysis shows that the thermoxidative stability of the copolymers increased with increasing reaction time, due to the incorporation of increased amount of PDMS unit into the copolymer. The combination of increasing molecular weight of copolymer, higher degree of imidization of polyimide blocks and enhanced thermoxidative stability may translate into improved flame retardancy of copolymers. This suggested enhancement in flame retardancy in air atmosphere, is believed to be due the incorporated PDMS blocks, which can be converted into silica, SiO2, a recognized thermally stable material.

A Study of the Degradation Mechanism of Ladder-like Polyhedral Oligomeric Silsesquioxane via Fourier Transform Infrared Spectroscopy

“…The char yield of the ATL-POSS is about 74% in nitrogen and 54% in an air atmosphere. In comparison, a linear amine-terminated PDMS had a char retention of about 4.3 and 10.5% in nitrogen and air atmospheres, respectively [34]. The significant increase in char yield of ATL-POSS over the linear amine-terminated polysiloxane is indicative of the superior thermal stability of the ladder-like conformation.…”

Study of the Mechanism of Degradation of Ladder-Like Polyhedral Oligomeric Silsesquioxanes by Fourier Transform Infrared Spectroscopy

“…The char yield of ATL-POSS is about 74.37% in nitrogen and 53.99% in air atmosphere. In comparison, a linear amino terminated PDMS shows char retention of about 4.3 and 10.48 % in nitrogen and air atmosphere, respectively [34]. The significant advantage in char yield of ATL-POSS over the linear polysiloxane counterpart, symbolizes the superior thermal stability of ladder-like conformation.…”

Polyimide Copolymers and Nanocomposites: A Review of the Synergistic Effects of the Constituents on the Fire-Retardancy Behavior

“…Polyimide is usually synthesized by a two-step process [48][49][50][51][52][53][54][55][56][57]. The first step is the synthesis of the precursor, poly(amic acid).…”

“…It is believed that partially cured polyimide will undergo further imidization when heated to a temperature ≥300 • C, thereby releasing water vapor into gaseous phase. During combustion, the released water vapor will dilute the flammable species [57]. Based on their thermal properties, partially imidized polyimides can serve as excellent alternative to smart flame-retardant materials.…”

“…Kumar et al pointed out that the presence of high percentage of phosphorus in phosphazene ring can provide a reasonable explanation for high char yield. In Figure 11, a comparison of thermal decomposition behavior in air for neat PI, neat phosphazene, PI-PDMS [57] and PI-co-phosphazene [136] is shown. Neat polyimide and hexachlorophosphazene decomposed almost completely in air, like most carbon-based polymers.…”