Synthesis of Polyurethane Based Nanocomposites Using Polymer Alloy Approach for Enhanced Properties

Abstract

Polyurethane (PUs) belongs to the versatile class of polymer materials having tune-able properties. PUs has alternative hard and soft segments domains due to different cyanates, polyols, and amines in their backbone which provides extraordinary flexibility and strength to these materials and can be molded into the desired shape by heating above the glass transition temperature (Tg). In this particular research, we have synthesized series of PUs its nanocomposites and PUs blends with neat-polystyrene (PS), amino-functionalized PS, and nitro-functionalized PS (PS-NO2) and their composites with functionalized multiwall carbon nanotubes (FMWCNTs) as follows. The composition of the first series comprises of PU (pre polymer)/FMWCNTs and second series consist of PU/dual-FMWCNTs nanofiller varying concentrations of FMWCNTs respectively. The third series recipe consist of PU/PS/FMWCNTs, the fourth series was based on PU/PS-NO2/FMWCNTs based nanocomposites. The fifth series was prepared with PUs blend nanocomposites having polyurethane/aminated-PS/FMWCNTs (PU/PU-NH2/FMWCNTs). All the synthesis of PUs, its nanocomposites and blends were performed in a catalyst-free environment to avoid toxicity and enhance bio-compatibility of the polymeric materials to enhance their practical life applications such as robotics skin, moveable joints, robotics body parts and smart auto body parts. The pre-designed structure of all synthesized PUs and its blend nanocomposites was confirmed by FTIR analysis and the degree of crystallinity and amorphous nature was determined with X-Ray diffraction analysis (XRD). The surface morphologies were studied and compared through SEM analysis at different resolutions, the evident change in surface structures with varying ratios of FMWCNTs and by using functionalized-PS filler in all series confirms blends formation and their nanocomposites. A comprehensive comparison for the enhancement in different properties such as thermal stabilities was performed with (TGA), mechanical/tensile test, hardness, flexibilities, thermal conductivities and shape recovery analysis was performed for all the series. First series (PU pre-polymer/FMWCNTs) of PU (pre-polymer) based nanocomposites were prepared with varying concentrations of FMWCNTs with solution mixing approach through one-shot process. An excellent interfacial interaction between polymer and functionalized filler was confirmed as the loading amount of FMWCNTs increases, the evident change of surface morphologies with different loading concentrations and enhancement of other properties confirms the FMWCNTs incorporation in prepolymer matrix. A significant increase in thermal stabilities, tensile strength, and modulus is evident with increasing the loading amount of xxix functionalized filler in the polymer matrix. The ultimate tensile strength was improved from 16.6 to 43.7 MPa for PU nanocomposite to pristine-PU with 83% weight loss at 653oC and 100% recovery in less than 78s. Practically 98-100% shape recovery was observed around 80oC for all the samples with repeatability and without any change in nanocomposite properties and strength. In second series (PU/dual FMWCNTs) thermoplastic-PU and its nanocomposites were synthesized of improved thermal, mechanical, and shape recovery properties by using chain growth process in catalyst free environment. A mixture of dual-FMWCNTs (amino and acid functionalized) was used to enhance chemical and physical interlinking between carbon filler and PU matrix in nanocomposites. A clear change in surface morphologies is evident as filler concentration increases (neat-PU smooth surface to PU- nanocomposites rough and porous surface with very little visible nanotubes at the surface due to embedment up to 5% dual FMWCNTs content). The tensile strength was improved from 32.17 MPa for neat-PU to 62.13 MPa as the loading amount of filler increases in PU matrix. A prompt fast recovery response in less than 10 seconds was recorded for the nanocomposite sample with excellent improved mechanical strength and enhanced thermal stabilities. According to our best knowledge, a fast recovery time of less than 10 seconds is not reported till the time for polymer nanocomposite samples. In the third series (PU/PS/FMWCNTs) based blends and their nanocomposites were synthesized with varying amounts of FMWCNTs. Thermal analysis confirms enhancement in thermal stabilities up to 630 0C with very little char residue. A positive shift was observed in thermal stabilities of synthesized blend nanocomposites with increasing loading amount of nanofiller. Surface studies confirm decreases in pore size as the concentration of filler increases in the matrix due to better interaction of filler with polymer matrix which also causes the uniform distribution of filler in both polymer layers. Approximately 100% shape recovery was observed by triggering the shape memory effect through inductive heating due to better polymer matrix interaction and generating physically interpenetrating networks (IPNs). In fourth series (PU/PS-NO2/FMWCNTs) blends were prepared by using acid-FMWCNTs. A clear change in surface morphologies of sample from neat polymer and its composites was confirmed by SEM analysis. The porous spongy cluster provides efficient shape recovery with excellent flexibility to the composite material with excellent thermal stabilities and mechanical properties. The 50% decomposition of blend with 0.1g loading amount of FMWCNTs was achieved around 430-432 oC with 22.8% of char residue and improvement in tensile strength up to 39.2MPa was achieved. Almost 100 percent shape recovery was observed for all samples xxx with varying shape recovery times due to greater stability and selectivity of one phase was observed. In fifth series (PU/PS-NH2/FMWCNTs) two-step approach was used to synthesize thermo responsive polyurethane and its blends composites. Noticeable enhancement in mechanical properties to 59.3±2.8 MPa from 28.6±1.4 MPa was observed for blends mainly due to strong chemical and physical interaction of an amino group of PS and FMWCNTs filler. Thermal stabilities of the synthesized blends were also improved as filler content was increased in the polymer matrix e to strong interfacial adhesion between filler and polymer. The 70% decomposition of blend with 0.25g filler loading was observed around 569oC. Almost 100% shape recovery was achieved and recovery response time of blend nanocomposite was decreased almost half of the pristine –PU. All the series were synthesized in catalyst free environment and the comparison of thermal stabilities, mechanical strength, flexibilities and shape recovery time was performed. It is evident from results that introduction of second polymers such as PS effect sufficiently on tensile strength of sample and provides flexibilities to the material which help in repeatability and reuse of these non-toxic materials in practical life which make these material novels. Another important aspect of these material is shape recovery time the physical and chemical interaction and FMWCNTs incorporation sufficiently effects on shape recovery time which give them selectivity to this material where control recovery is required. It was also noticeable that PU nanocomposites in series1 &2 have good tensile strength but due to brittleness these films are easy to break and the introduction of second phase provide them more flexibilities and control recoveries in next series.