Thermoplastic polymer nanocomposites based on polydopamine-coated clay : preparation, structures and properties
Phua, Si Lei
Date of Issue2014
School of Materials Science and Engineering
Polymer/clay nanocomposites have been widely investigated over past three decades due to the dramatic boost in properties at low filler content. Although organoclay (clay modified with organic surfactants) is commonly used to reinforce the polymer, yet the reinforcing extent has yet reached the optimum performance. Besides, both organic surfactants (organic compounds with long hydrophobic tails and hydrophilic heads) and polymers are susceptible to photo-induced degradation especially in outdoor environments, making polymer/clay nanocomposites vulnerable in practical applications. In order to overcome the aforementioned problems, in this research, D-clay (polydopamine-coated clay) was studied as multifunctional filler to improve not only interfacial interactions with a wide range of polymer matrices but also stabilities of the nanocomposites. D-clay was incorporated into both elastomer (polyurethane) and semi-crystalline thermoplastic (polypropylene) systems. The structure-property relationships of the resultant nanocomposites were investigated using TEM, XRD, DMA, tensile testing, FTIR, TGA and DSC. In particular, the reinforcing mechanism of D-clay in polyurethane (PU) nanocomposites was studied with respect to surface chemistry, filler loading and filler size. On the other hand, the stabilizing function of D-clay was verified using polypropylene (PP) as the polymer matrix since PP is well-known for its poor UV stability. Firstly, D-clay was incorporated into polyether-based PU via solvent mixing and good filler dispersion was obtained. The results showed pronounced improvement in mechanical properties, such as stiffness, tensile strength and strain at break, at 3wt% clay loading. The remarkable improvement can be attributed to the excessive hydrogen bonds between D-clay and the hard segments (hard segments are made of diisocyanate and the short-chain diol) of PU. This strong interfacial interaction between D-clay and hard segments not only facilitates the stress transfer across the filler and polymer matrix, but also acts as nucleating agent for hard segment crystallization, leading to higher hard segment crystallinity. Furthermore, the impact of high D-clay loading on mechanical properties and hard segment crystallization was investigated using polyester-based PU as matrix since severe phase separation was observed in the polyether-based PU. The results showed polyester-based PU nanocomposites with D-clay concentration above 5 wt% formed percolated clay network structure, this hindered the movement of both hard and soft segments to a certain extent. Consequently, polyester-based PU/D-clay nanocomposites showed drastic enhancement in tensile modulus. On the other hand, the effect of particle size was studied using polycaprolactone (PCL)-based PU as matrix. In this case, polydopamine-modified layered double hydroxides (D-LDHs) of different sizes were used as the fillers and the shape memory performance of the nanocomposites was evaluated. It was found that D-LDH interacted strongly with hard segments, enhancing phase separation and promoting crystallization of both hard and soft segments profoundly. The nanocomposite with 2 wt% of small D-LDH exhibited good shape memory properties since most small D-LDH interacted with hard domains at low filler loading. Hence, the incorporation of small D-LDH can reinforce hard domains without sacrificing the elasticity of the system. In order to verify the stabilizing capability of D-clay, D-clay was also introduced into the PP system. This is because PP is vulnerable to degradation owing to the presence of volatile tertiary hydrogens in the polymer backbone. The results showed drastic improvement in UV resistance and thermal stability of PP/D-clay owing to the effective radical scavenging ability of melanin-like PDA layer on clays. Meanwhile, the excellent UV resistance of PP/D-clay nanocomposites can be attributed to the masking effect imposed by PDA coating. Besides, the mechanical properties of PP/D-clay were better than organoclay at similar clay loading on account of the stronger interfacial interactions.