Design of novel high modulus TPUs for nanocomposite applications

• Main Author: Albozahid, Muayad
• Other Authors: Saiani, Alberto ; Wilkinson, Arthur
• Published: University of Manchester 2018
• Subjects: 620; TPUs nanocomposites
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.740372

This thesis focuses on designing thermoplastic nanocomposites with good mechanical, thermal and electrical properties. Thermoplastic nanocomposites, which are used in this work, are composed of thermoplastic polyurethane (TPU) matrices and graphene nanofillers (GNFs). TPUs were synthesised with large ratios of hard segments (HS), including 60, 70 and 80 Wt. % HS. The influences of HS content and annealing treatment at 80oC on the thermal, electrical and mechanical properties of TPUs and TPUs/GNFs samples have been investigated. The crystallinity, Tg, tensile strength, yield strength, and tensile modulus of all pure TPU samples are seen to increase after annealing treatment due to microphase separation. This also depends on the HS content in order to achieve better properties with various annealing times. The nanofillers include graphene nanoplatelets (GNP), graphene oxide (GO), and reduced graphene oxide (rGO). Various dispersion routes have been utilised to achieve better dispersion and distribution of the nanofillers. In-situ polymerisation, melt-compounding and solution-mixing techniques have been used to study dispersion effects and further moulding by injection moulding to prepare new TPU nanocomposites. Filler-matrix and filler-filler interfaces significantly influence the final performance of TPU nanocomposites. There is therefore a balance to be struck in the design of graphene-based TPU nanocomposites between the ability to achieve higher loadings of reinforcement and the reduction in effective Young’s modulus of the reinforcement as the number of layers in the nanofillers is increased. Effective stress transfer is achieved as a result of both dispersion and interfacial interaction. It has been demonstrated that graphene plays a reinforcing role in nanocomposites through its effective modulus as well as bonding with TPU phases. The first nanofiller studied is GNP, which shows good mechanical, thermal and electrical properties. The in-situ polymerisation approach was found to be the best dispersion method compared to melt compounding and solution mixing, as the optimum properties of GNP nanofillers can be achieved in the TPU/70 HS matrix. The TPU/70 HS had higher values compared to both TPU/60 HS and TPU/80 HS. However, the preparation of GO and rGO incorporating TPU/70 HS was not successfully synthesised due to the suppression of chain growth during polymerisation. Thus, the melt-compounding process was used instead for both GO- and rGO-based TPU matrices. TPU/70/GO nanocomposites displayed better mechanical performance compared to TPU/70/rGO nanocomposites, as a result of greater interaction between TPUs chains and fillers surfaces. It was found that the percolation thresholds for GO- and rGO-filled nanocomposites were significantly lower than that of GNP-filled nanocomposites due to their higher aspect ratio. Annealing treatment showed lower mechanical properties of TPU nanocomposite samples compared to non-annealed TPU nanocomposite samples, resulting from disruption of phase separation and restacking of nanofillers. The tensile moduli of nanocomposites were predicted using modified Halpin-Tsai models. Results showed good agreement at low loadings of GNP (≤1, 3 and 5 Wt. %), which depend on the effect of TPU phase interaction. However, poor agreement was observed at higher loadings of nanofillers, where the TPU nanocomposites displayed reduced reinforcement efficiency. This is due to the fact that the model assumes perfect adhesion between the nanofillers and the matrix, uniform particle dispersion and distribution, and complete exfoliation and total orientation in the direction of applied stress. This also correlates with results from SEM, TEM, POM and diffusivity mapping images, which showed aggregation, agglomeration and poorer distribution of nanofillers in the TPU matrix at higher loadings.