High Performance Polymers

A series of copolyamic acid (PAA) were synthesized from various ratios of two diamines p-phenylene diamine and 4,4′-oxydianiline (ODA) via copolymerization with pyromellitic dianhydride (PMDA) in polar solvent dimethylacetamide. A mixture of acetic anhydride and triethylamine was used as the cyclizing agent and added to the PAA in order to chemically convert it into partly imidized PAA, which then was spun into fibers by dry-jet wet-spinning. Finally, polyimide (PI) fibers were prepared after the as-spun fibers had been heat-treated. The results indicated that the intrinsic viscosity of partly imidized PAA was higher than that of pure PAA. Moreover, when the partly imidized PAA was spun into a suitable composition coagulation bath, the as-spun fibers gained a circular and dense internal morphology. The tensile strength and initial modulus of the heat-treated PI fibers were better than those of the homo-PI PMDA-ODA fibers, respectively, and the thermal stability of the PI fibers was also enhanced.

Thermoset polyimide matrix resins with high melt processability for high-temperature carbon fiber composites were prepared from the diethyl ester of 3,3′,4,4′-benzophenonetetracarboxylic acid (BTDE) and the aromatic diamine mixtures consisting of 4,4′-bis(4-amino-2-trifluoromethylphenoxy)biphenyl (6FBAB) and p-phenylenediamine (p-PDA) with the monoethyl ester of 5-norbornene-2,3-dicarboxylic acid (NE) as the molecular-weight-controlling and reactive endcapping agent. The effects of diamine mixture compositions on the melt processability of the B-staged oligoimides and the thermal and mechanical properties of the thermally cured polyimide resins were systematically investigated. Experimental results indicated that the polyimide matrix resins with p-PDA concentration of ≤ 40% in 6FBAB + p-PDA showed lower melt viscosities, corresponding to a higher melt processability, than the conventional thermoset polyimide (PMR-15). After thermal curing, the thermoset polyimides exhibited a very good combination of thermal and mechanical properties with the glass transition temperature (Tg) of as high as 353 °C determined by differential scanning calorimetry and impact strength of 15.2 kJ m⁻². Carbon fiber-reinforced composite derived from the representative polyimide resin showed good mechanical properties at temperatures of as high as 288 °C.

Distyryl- and nitrodistyrylanthracene-based polymers (P1 and P2, respectively) have been synthesized and characterized. The polymers are fully soluble in common organic solvents and have number-average molecular weights of 18 570 and 23 480 g mol⁻¹ for P1 and P2, respectively. The optical properties of the polymers were investigated by UV-visible absorption and photoluminescence spectroscopies. The optical gaps were estimated from the absorption-onsets of the polymer films; their values were 2.79, 2.75 eV for P1 and P2, respectively. A blue photoluminescence was observed in dilute solution. In solid thin film, Π–Π interactions influence the optical properties, and red-shifted photoluminescence spectra were obtained; a green emission (573 nm) for P1 and an orange one (605 nm) for P2 were observed. The HOMO and LUMO levels were estimated using cyclic voltammetry analysis. Single-layer devices of the indium-tin oxide/polymer/aluminum configuration were fabricated and showed relatively low turn-on voltages (5.14 V for P1 and 5.00 V for P2).

The present investigations were undertaken to find out whether and how often cycling, processing and programming can be repeated, whether repeated programming affects the one way effect and how much irreversible strain the shape memory polymeric material accumulates at a particular temperature. The effect was investigated in dependence of different stress levels, and the effect of both recovery temperature and recovery time was considered. As a model material the commercially and industrially applicable amorphous shape memory polymer Tecoflex® was examined and subjected to 50 programming cycles. Tecoflex® is characterized by a glass transition temperature, Tg, of 74 °C, above which it looses all its strength. During tensile testing at 20 °C (T < Tg), stresses a steady increase to 26 MPa as strains approached the rupture strain of 25%. It is observed that at 60 °C (T < Tg, but near Tg) the material can be strained to more than 2500% before rupture occurs while stresses slowly increase to values less than 0.3 MPa. It turns out that programming, cooling, unloading and heating to trigger the one way effect causes an increase of irreversible strain that is associated with a corresponding decrease of the intensity of the one way effect during the first thermomechanical cycles.

A study was made of how aspects of the mechanical response of polyurethane elastomers (PUs) based on poly(ethylene adipate) (PEA) vary with the number of CH₂ methylene groups when using chain extenders of the type HO–(CH₂)_p–OH. The effect of varying the odd–even number of CH₂ groups in regard to the chain extender was followed. The best mechanical behavior as elastomers was revealed for polyurethanes with CH₂ even numbers on the chain extender, when the different parts of the PUs macromolecular chains can adopt extended linear forms. The effect of different concentrations of the hard segment content on a set of PUs based on PEA and 1,4-butanediol as chain extender was also studied. A hitherto overlooked isocyanate of variable geometry, 4,4′-dibenzyl diisocyanate was used in the PUs synthesis, based on a –CH₂CH₂– spacer group between the two aromatic rings.

Shape memory polyurethanes (SMPUs) have been extensively investigated but shape memory polyurethane-ureas (SMPUUs) with aromatic diamines as chain extenders have been rarely reported. In the present study, a series of SMPUs and SMPUUs having crystalline reversible phase were prepared. The shape memory properties including shape recovery, shape fixity and recovery stress of the SMPUs and the SMPUUs were compared systematically. When hard-segment content (HSC) was 30% the polyurethane-urea exhibited higher shape recovery than its polyurethane analogue. With the increase of HSC the shape recoveries of both the SMPUs and the SMPUUs decreased but those of the SMPUUs decreased more apparently. When HSC was 40%, the shape recovery of the polyurethane-urea was lower than that of the polyurethane. This is because the great increase of physical cross-linking density of SMPUUs resulted in significant increase of irreversible deformation. In comparison with those of the SMPUs, the shape recovery curves of SMPUUs shift to lower temperatures. This is because the higher physical cross-link density of the SMPUUs results in more decrease of the melting temperature of soft phase. Moreover, the shape fixities of the SMPUUs were lower than those of the SMPUs with identical HSC also because of their higher physical cross-link density. The recovery stresses of the SMPUs and the SMPUUs increased with the increase of HSC. The recovery stress of a polyurethane-urea was higher than that of its polyurethane analogue by over 100%. Hence it is possible to produce SMPUUs with much larger recovery stress and higher shape recovery without sacrificing much shape fixity by controlling appropriate HSC.

The present research was focused on investigating the ability of different siloxane-based polymeric coatings to provide environmental protection to monumental limestone (Repedea from Romania) used in the north-eastern part of Romania for the construction of many stone monuments. Three siloxane-based polymers used throughout the world in protecting stone monuments, namely Lotexan-N, Silres^® BS 290 and Tegosivin HL 100 and a newly designed hybrid nanocomposite containing silsesquioxane units (TMSPMA) were selected as polymeric coatings. The ability of the coatings to protect the stone from dry SO₂ deposition in the presence of humidity in a standardized accelerated ageing test was determined. A comparative study of the obtained results indicates that the stone samples treated with the new hybrid siloxane-based nanocomposite exhibited improved resistance to degradation after being exposed to a SO₂ saturated atmosphere.

Through the use of thermogravimetry (TG), this work explores the thermal degradation mechanism and kinetics of Kapton (polyimide) film. Isoconversional kinetic methods, which provide activation energy as a function of degradation, present insight into the development of a five-step competitive and consecutive kinetic model. To evaluate the accuracy and validity of the model, a statistical analysis of the implemented kinetic model is also presented. Coupled TG-mass spectroscopy and TG-Fourier transform infrared analysis were used to investigate the decomposition mechanism and to more fully explain the complex, heating-rate-dependent, decomposition pathway of Kapton.