Viscoelastic Properties of Polyurethanes with Particular Reference to Morphology, Thermal & Mechanical Behavior

Abstract

The thesis describes research study on the mechanical and viscoelastic properties of the polyurethanes (PU). Both natural precursor i.e. castor oil and synthetic HTPB based polyurethane systems respectively were studied. In particular, the influence of microphase segregation and dynamic mechanical loading on the propensity to the temperature rise following viscous dissipation in the polymer has been investigated. Consideration of the mechanical properties and shelf life of the fabricated product leads to the conclusion that the natural precursor „castor oil‟ is undesirable for long term use. It is therefore necessary to employ a synthetic analogue that yields predictable and vastly improved mechanical properties. A new PU system based on synthetic precursor HTPB is developed which possesses much improved mechanical properties as compared to the conventionally used polymer compositions. Study of the viscous heating effect results in some interesting conclusions as outlined below. A wide range of the recipes comprising different cross-linking agents and chain- extenders have been investigated to provide a comparison and analysis of competing factors. Mock composites using KCl particles employing both the polyurethane precursors are fabricated and compared for their mechanical response. Composite with HTPB based polymer showed four times the tensile strength of castor oil based composite. This is attributed to the better wetting and adhesion properties of HTPB as revealed in the SEM analysis of the tensile fractured specimens. In the second part of the research, series of elastomers based on HTPB are synthesized to conduct a systematic study of the mechanical and viscoelastic properties. Trifunctional alcohols like 1,2,6 hexanetriol and trimethylolpropane as cross-linking agents and short chain diols like 1,4 butanediol and 1,6 hexanediols as chain extenders are incorporated in the base polymer to produce two morphologically different polymeric structures. Curing reaction and morphological differences are investigated using IR-spectroscopy. Swelling experiments are conducted to determine the actual levels of cross-linking. Mechanical properties are measured to elucidate the structure property relationship in these polymers.

Viscous heating effect is studied by characterizing the specimens on dynamic mechanical analyzer (DMA) at different frequencies as a function of temperature. It is demonstrated that the incorporation of the short chain diols (chain extenders) yields a polymer with phase segregation due to the interaction between the hard segment domains leading into increase in both the tensile strength and the elongation in comparison to the cross-linked structures. However, diminution in the tendency to viscous dissipation is observed to occur in the cross-linked structures. Though the chain-extended system is susceptible to viscous heating, it shows higher modulus in all operating conditions than that of the cross-linked system. The thesis records the comparison of the two possible systems based on HTPB in regard to the mechanical and viscoelastic properties. The data are surely helpful in designing the optimum systems by keeping into consideration both the ultimate properties and the viscous heating effect. The research is reported in international peer reviewed journals; (i) Nadeem Ahmad, M.B Khan, Noaman Ul-Haq, “Comparison of the polymer/composite based on polyurethane with different -OH backbone, Advanced Material Research 65-72, (2011) 326, (ii) Nadeem Ahmad, Ihtasham-ur-Rehman, M.B Khan, Xiaoyan Ma, Noaman Ul-Haq, “Dynamic mechanical characterization of the cross-linked and chain-extended HTPB based polyurethanes” (accepted in polymer and polymer composites) and (iii) Nadeem Ahmad, M.B Khan, Xiaoyan Ma, Noaman Ul-Haq, “The influence of the cross-linking/chain extension structures on the mechanical properties of HTPB based polyurethane elastomers” (accepted in The Arabian Journal for Science and Engineering)