Nanostructures and properties of blends of homopolymer and elastomeric block copolymer nanoparticles

This thesis is on the topic of nanostructures and properties of blends of homopolymer and elastomeric block copolymer nanoparticles. The objectives of this thesis are: (1) to synthesize elastomeric nanofibers and nanosheets morphology utilizing the self-assembly of PS-b-PI copolymer and cold vulcanization process (2) to study the viscoelastic properties of blends of polystyrene and the elastomeric nano-objects (3) to investigate the morphology effect in elastomeric nano-object blends (4) to study the control of crosslinking density of nanofibers and viscoelastic behavior using dynamic mechanical test (5) to synthesize nanofibers having different PI core size using lamellar PS-b-PI copolymer by adding neat polystyrene (6) to understand the effect of the core PI size and morphology in blends.Chapter 1 presents the motivation and objectives as well as scope of the thesis. Overall introduction and brief review of literature of block copolymer, blends, nanocomposites, and viscoelastic properties are given in chapter 2. Synthesis and characterization of elastomeric nanofibers and nanosheets as well as PCM (partially crosslinked multi-junction) samples are introduced in chapter 3. Nanofibers and nanosheets were synthesized by cold vulcanization process using S2Cl2 crosslinking reagent resulting from self-assembly property of PS-PI block copolymer. The crosslinking reaction was confirmed by FT-IR using characterization peak of double bond of isoprene and by DSC using Tg point of polyisoprene. The blend samples for DMA were prepared by solvent casting method with varying weight percent of the nanoparticles.Chapters 4 and 5 present results of rheological behavior and the effect of morphology between the elastomeric nanofiber and nanosheet blends. The critical volume concentration was investigated by percolation threshold theory for rod and disk. The characterization of nanofibers and nanosheets was carried out by SEM images. Nanofiber and nanosheet blends showed nanofiller effect and their structural change between 5 and 10 wt%, as fitted using Cross-Williamson three parameter model and calculated by percolation threshold theory.Thermo-mechanical properties of two shapes of elastomeric nanoparticles, nanofibers and nanosheets, were measured using dynamic mechanical analysis to study the morphology effects in blends the results are presented in chapter 6. The morphology of the nanoparticles was imaged by SEM and the nanoparticle morphologies in the blends were also characterized from fracture surface of DMA sample by SEM. In the nanofiber blends, storage and loss modulus increased with increasing filler loading in the terminal region. Tan delta results also showed that the value decreased with increasing nanofiber loading because the nanofiber morphology prevented the motion of neat PS. The moduli in the nanosheet blends indicated the values were similar to nanofiber blends in the terminal region but the value between 90°C and 125°C (Tg of neat PS) showed opposite result compared to nanofiber blends. Normalized tan delta values are plotted in terms of T-Tg in order to understand the effect of crosslinking filler and morphology. The study of activation energy using frequency sweep has been explored as well.In chapter 7, the crosslinking density and morphology of nanofillers were investigated resulting in three elastomeric block copolymer nanofillers: fully crosslinked nanofiber (FCF), fully crosslinked multi-junction nanofiber (FCM) and partially crosslinked multi-junction sample (PCM) using dynamic mechanical analysis. For comparison with these nanofillers, uncrosslinked PS-PI block copolymer (UBC) have been studied as well. The crosslinking density is calculated by measuring the change in intensity of the double bond peaks using FT-IR spectroscopy. The blends are prepared by solvent casting by mixing neat polystyrene and four nanofillers: FCF, FCM, PCM, UBC. The thermo-mechanical properties and morphology of the blends were characterized by dynamic mechanical analysis (DMA) and scanning electron microscope (SEM). DMA results show that the modulus increase with increasing filler loading in the terminal region in case of both PS/FCM and PS/FCF systems and the rate of increase is related to the crosslinking density. These results are interpreted as the effect of crosslinking density and the free volumes of fillers in blends.In chapter 8, rheological properties of two nanofibers with the same PS block length but different core PI size are discussed. One type of nanofibers was synthesized and blended with polystyrene by the method described in chapter 3. Binary blending method, which is to blend a homopolymer and a diblock copolymer, was used in order to generate the other type of nanofibers morphology from lamellar PS-b-PI copolymer. The storage and loss moduli increased with increasing nanofiber loading. In order to study the core PI size and morphology effect, zero-shear viscosity and relaxation time are investigated. The values are obtained from three parameter Cross-Williamson model.In chapter 9, conclusions and recommendations for future works are discussed.