Effect of modified filler surfaces and filler-tethered polymer chains on morphology and physical properties of poly(azo-pyridyl-urethane)/multi-walled carbon nanotube nanocomposites

Abstract

Poly(azo-pyridyl-urethane)/multi-walled carbon nanotube (PAPU/MWCNT) nanocomposites were prepared by first synthesizing polyurethane followed by solution dispersing MWCNT in the matrix. We have used two different MWCNTs: the first set of MWCNT contained carboxylic acid group (MWCNT-COOH) and the second set contained acid chloride group (MWCNT-COCl). Afterward, these MWCNT particles were used in the preparation of PAPU/MWCNT nanocomposites. In the first set of nanocomposites, MWCNT-COOH and PAPU were just physically mixed and were designated as PAPU/MWCNT-A. In the second set of nanocomposites, PAPU chains were tethered onto the surfaces of MWCNT-COCl particles and designated as PAPU/MWCNT-AC. Two types of nanocomposites were thus fabricated, including acid functionalized nanotube-based non-tethered PAPU/MWCNT and acid chloride functionalized MWCNT-based tethered system. We have incorporated hydroxyl end-terminated polyurethane via “grafting to” approach to acid chloride functional MWCNT through esterification. Fourier transform infrared spectra confirmed that the matrix was covalently attached to sidewalls of nanotube. Various nanotube loading levels and surface-modified groups were considered to regulate mechanical, thermal and electrical performance of PAPU/MWCNT. The experimental results showed that a moderate loading level of 5 wt. % MWCNT produced the maximum tensile strength (63.1 MPa) in tethered nanocomposites, while the presence of surface carboxylation of MWCNT relatively decreased the tensile strength (49.7 MPa). Comparative studies based on scanning and transmission electron microscopy of the chemically bonded samples revealed unique nano-fibriller morphology. Dynamic mechanical analysis of nanocomposite films showed an increased segmental rigidity with T_g of 144–153℃ in the tethered system relative to pristine PAPU (133℃). Addition of acid chloride functional MWCNT also contributed to an enhancement in the conductivity (2.9–4.7 S cm⁻¹), relative to PAPU/carboxylated MWCNT (2.0–2.7 S cm⁻¹).