Studies On Structure And Mechanical Properties Of Polyurethane And Waterborne Polyurethane-Bascd Nanocomposites

Polyurethane is a class of polymer containing duplicate carbamate (NHCOO), mainlysynthesised by multi-isocyanate (diisocyanate) and polyol (including polyester polyols andpolyether polyols) via the gradual addition polymerization. By adjust the structure andproportion of dissimilar soft segment and hard segment, we could obtain polyurethane withthe optimal mechanical properties, it has a number of exceptional properties, such as a widerange of hardness adjustment, low temperature resistance, good flexibility, adhesion, and so on.Waterborne polyurethane (WPU) is a binary colloidal system using water as the dispersionmedium. It not only preserved the traditional solvent-based polyurethane in a number of goodproperties, such as good wear resistance, flexibility, low temperature resistance and resistanceto fatigue, etc., but also has the advantages of non-toxic, non-flammable, non-polluting,energy saving, safe and reliable, which makes waterborne polyurethane has been appliedsuccessfully in solvent-based polyurethane applications.Nanoparticles modified polyurethane combine stiffness, dimensional stability, thermalstability of nanoparticles with toughness, ease of processing of WPU perfectly, while takingadvantage of the nano-surface effect, quantum size effect and macroscopic quantumtunneling effect, enhance significantly the optical, thermal, electrical, magnetic andmechanical properties of coating.In this paper, polyurethane-based and waterborne polyurethane-based nanocompositewas studied. All the relative studies are outline as follows.(1) Polyurethanes/multi-walled carbon nanotube (PU/CNT) composites were preparedwith a help of ultrasonically dispersing CNT in the traditional procedure of synthesizingpolyurethane. In this case, the various loading levels, sizes and surface-modified groups wereconsidered to regulate the mechanical performances of the PU/CNT nanocomposites.Moreover, the structure and mechanical properties of all the PU/CNT nanocomposites wereinvestigated by attenuated total reflection-Fourier transform infrared spectroscopy, dynamicmechanical analysis, scanning electron microscope, transmission electron microscope, andtensile testing. The results showed that a moderate loading-level of0.1wt%and a diameter of10-15nm for CNT could produce the maximum tensile strength and elongation while it wasworth noting that the surface carboxylation of CNT could further enhance the tensile strengthand elongation of the PU/CNT nanocomposites.(2) The unmodified rectorite (REC), a kind of layered silicate, was incorporated intopolyurethane (PU) as matrix by the process of one-pot synthesizing polyurethane in situ, andhence produced a series of nanocomposite materials with enhanced strength and elongation. It is worth noting that the nanocomposite containing2wt%REC had the maximum elongation(1449%) and strength (32.66MPa) as ca.2.7-and1.4-fold over those of neat PU film,respectively.Meanwhile, the unexfoliated agglomerates and exfoliated nanoplatelets of RECco-existed in PU matrix. By virtue of strong interfacial interaction on the surface of REClamella, the stress facilely transferred to the rigid RECs and hence contributed to theenhancement of strength in spite that the original structure and interaction in the PU matrixwere partly cleaved. Moreover, the intertwisting of polymer chains in PU matrix with REC aswell as the gliding among the REC lamellae might produce greater strain. Nevertheless,excess unexfoliated REC agglomerates under high loading level inhibited the enhancement ofmechanical performances, which verified the key role of exfoliated REC nanoplatelet inimproving mechanical performances. As a result, this work submitted a simple method todevelop a polyurethane-based nanocomposite with high mechanical performances withoutany modification of layered silicates and the complicated treatment for exfoliation anddispersion.(3) Starch nanocrystals (StNs), which possesses a distinct platelet-like structure similar toexfoliated layered silicate, was incorporated into waterborne polyurethane (WPU) matrix athigh loading levels to prepare WPU/StN nanocomposites. As reported, the StN loading levelwas restricted to lower than8wt%because self-aggregation of StN resulted in itssedimentation during preparation. However, in this work, good dispersion of the StNnanophase in the nanocomposites was observed even when the StN loading level reached30wt%. Furthermore, the resultant composites exhibited prominent enhancement in bothstrength and Young’s modulus, and maintained an elongation of greater than ca.300%. A StNloading level of10wt%showed the maximum tensile strength (31.1MPa) and an enhancedYoung’s modulus, respectively ca.1.8-and35.7-fold over those of neatWPU.The activesurface and rigidity of StN facilitated formation of an interface for stress transfer andcontributed to higher stress-endurance. As the StN loading level increased, self-aggregation ofStNs resulted in a decrease in strength; however, the rigidity of StN supported an increase inYoung’s modulus, which was highest in nanocomposites containing30wt%StNs. Highperformance waterborne polyurethane-based “green” bionanocomposites were therebyestablished.(4) New composites of waterborne polyurethane (WPU) as a matrix were prepared byincorporating rigid supramolecular nanoplatelets (SN) as a filler, which were self-assembledby the selective inclusion of β-cyclodextrin (β-CD) onto poly(propylene oxide)(PPO)segment in the poly(ethylene oxide)-block-PPO-block-poly(ethylene oxide)(PEO-b-PPO-b-PEO). It is worth noting that the SN with moderate PEO length resulted in thesimultaneous increase in strength, elongation and Young’s modulus. It was attributed to therigidity of SN, and the essential association between the WPU matrix and rigid crystalline aggregates of β-CD-based polyrotaxane mediated with stretching free PEO chains. If therewas no stretching free PEO chain, both strength and elongation decreased in spite of anincrease in Young’s modulus. However, too long PEO chains tended to crystalline, and henceinhibited the interfacial entanglement with the WPU matrix, resulting in the decrease ofmechanical performances. Furthermore, the relatively higher loading-level of SN induced theincrease in the size and number of the SN phase, and hence inhibited the enhancement ofstrength and elongation.