Authors:Ion Sava, Ştefan Chişcă, Maria Brumă, and Gabriela Lisa
Aromatic polyimides generally have excellent thermal, mechanical, and electrical properties primarily because of their heterocyclic structure [ 1 – 6 ]. However, even higher thermal stability and mechanical
Authors:M. Saraf, Sarfaraz Alam, R. Gupta, K. Surekha, and G. Mathur
Polyimides have aromatic moieties in the backbone structure which are responsible for their increased thermal stability. If
phosphorus is introduced in the main chain structure of polyimides, there is further improvement in the thermal stability.
This has been proved by the work carried out in our group. The polyimide having amine termination can be used for crosslinking
of epoxy resins.
In the present study amine terminated phosphorus containing nadicimide were taken as curing agent for DGEBA resins. The curing
characteristics of DGEBA resin were studied by DSC using different amounts of nadic endcapped phosphorylated amines. DSC thermogram
showed the heat of polymerization was lower as compared to system cured with aromatic amines.
Three different thermoanalytical methods are introduced as significant time-saving techniques in conventional life tests of some organic electrical insulating materials. DTA in cyclically alternated atmospheres of nitrogen and oxygen according to Randino and Andreotti, and TG analysis methods at different heating rates according to Flynn and Wall and to Broido were applied to two electrical insulations based on aromatic poly-imide and epoxy resin. The activation energies obtained are compared with those derived from the slopes of the life-lines produced via conventional life tests. The assumptions necessary for applying the thermal analyses in these cases are discussed.
Authors:J. Mitchell, S. Yegnasubramanian, and L. Shepherd
Bulk and surface nitrogen levels of compression-molded samples of polyacrylonitrile, poly (styrene-co-acrylonitrile), poly(methacrylonitrile), polycaprolactam, polyimide, and a cured epoxy resin (EPON 828 — JEFFAMINE T-403, 100:50) were determined, respectively, by 14 MeV NAA (14N(n,2n)13N) and by analysis of proton tracks registered in cellulose nitrate detectors (CN85) as a result of their exposure to protons from the termal neutron induced reaction,14N(n,p)14C. Except for a few polymers, agreement of results obtained for nitrogen levels in the bulk and surface regions is within expected limits. Intrinsic limitations of the method for practical determinations of nitrogen are covered. The spatial homogeneity of materials consisting of nitrogen and non-nitrogen containing segments is easily determined by image analysis of proton track densities. These data are then used to construct nitrogen distribution plots and topographical maps. The potential ability of the proton track image analysis technique for distinguishing phase separation or immiscibility of nitrogen containing polymers is discussed.
Authors:Y. Ito, H. Mohamed, K. Tanaka, K. Okamoto, and K. Lee
Positron annihilation lifetimes were measured for several polymers in the atmosphere of high pressure CO2 gas. At low CO2 pressured both
3 andI3 decreased due to the Langmuir-type sorption, and at higher pressures their values recovered because the Henry-type sorption takes over. The amount of sorbed CO2 and dilation of the bulk volume were measured simultaneously, and the free volume fraction was determined at each CO2 pressure. The free volume fraction became smaller (for polyimide and polycarbonate) or slightly larger (for polyethylene) with the progress of sorption. However, the size of the o-Ps hole estimated from the
3 value increased regardless of the change of the free volume fraction. It appears that o-Ps is selectively looking at larger holes or expanding the holes in which it is accommodated. For polycarbonate, which remains to be glassy even at the largest CO2 sorption attained in the experiment, the o-Ps hole size became larger than that before sorption. This implies that, even if the polymer is glassy as bulk, the sorption site is strongly prone to molecular displacement by the pressure of the penetrating Ps. Cautious consideration is evoked about directly correlating the o-Ps lifetime and intensity with the free volume in general.
Authors:Z. Wu, F. Li, L. Huang, Y. Shi, X. Jin, S. Fang, K. Chuang, R. Lyon, F. Harris, and S. Cheng
The thermal mechanical properties and degradation behaviors were studied on fibers prepared from two high-performance, heterocyclic polymers, poly(p-phenylenebenzobisthiazole) (PBZT) and poly(p-phenylenebenzobisoxazole) (PBZO). Our research demonstrated that these two fibers exhibited excellent mechanical properties and outstanding thermal and thermo-oxidative stability. Their long-term mechanical tensile performance at high temperatures was found to be critically associated with the stability of the C—O or C—S linkage at the heterocyclic rings on these polymers' backbones. PBZO fibers with the C—O linkages displayed substantially higher thermal stability compared to PBZT containing C—S linkages. High resolution pyrolysis-gas chromatography/mass spectrometry provided the information of the pyrolyzates' compositions and distributions as well as their relationships with the structures of PBZT and PBZO. Based on the analysis of the compositions and distributions of all pyrolyzates at different temperatures, it was found that the thermal degradation mechanisms for both of these heterocyclic polymers were identical. Kevlar®-49 fibers were also studied under the same experimental conditions in order to make a comparison of thermo-oxidative stability and long-term mechanical performance at high temperatures with PBZO and PBZT fibers. The data of two high-performance aromatic polyimide fibers were also included as references.
Authors:Chinnaswamy Thangavel Vijayakumar, Rajendran Surender, Kumaraswamy Rajakumar, and Sarfaraz Alam
Recently, polyimides especially aromatic polyimides have more and more applications in microelectronics, automotive, and aerospace sector because of their high thermal stability, mechanical strength, and excellent
Authors:Hongyan Wang, Chunshan Li, Zhijian Peng, and Suojiang Zhang
. Magaraphan et al. [ 6 ] studied the preparation, structure, properties, and thermal behavior of rigid-rod polyimide/montmorillonite nanocomposites. Castelein et al. [ 7 ] focused on the influence of heating rate on the thermal behavior and mullite formation