Effects Of Orientation,Enhancement And Surface Treatment Of Nanofibers On Properties Of Transparent Composites

Poly(methyl methacrylate)(PMMA,commonly known as Plexiglass)is one of the most widely used transparent polymers with excellent optical transmittance up to 96%.It has been universally applied in transparent parts for aircraft,automobile,and ship,as well as optical lenses and building decoration,but its low mechanical properties,especially mechanical strength and impact toughness,limit its application efficacy.Previously,intensive efforts have been made to improve the mechanical properties of PMMA through methods of polymer blends,particles reinforcement,and continuous fiber reinforcement.Recently,with the advances in nanofibers technology,it offers an innovative approach to improve the mechanical properties of PMMA with potential of minimal loss in transparency.Many previous studies showed that the dispersibility,fineness,orientation,content,reinforcing capacity and interface effect of the nanofibers in PMMA matrix are still the primary factors that govern the performance of transparent composites.In this thesis,polyamide(PA-6)and PMMA are chosen to be the basic material components for making PA-6 nanofibers reinforced PMMA nanocomposites.With the electrospinning as the basic nanofibers-fabrication technology,we explore the feasibility of developing high-performance transparent nanocomposites(TNCs)via improving the dispersion of nanofibers in PMMA matrix,using aligned nanofibers as reinforcement,strengthening PA-6 nanofibers’ reinforcing capacity by introducing graphene(Gr)into PA-6 nanofiber matrix,and ameliorating the interfacial compatibility between PA-6 nanofibers and PMMA matrix.The main contents and results are briefly summarized as follows:(1)To examine the nanofiber directionality effects on optomechanical properties of PMMA,aligned fiber-hybrid mats consisted of PA-6 nanofibers and PMMA microfibers are prepared using a self-blending co-electrospinning(SBCE)method,followed by hot press molding to fabricate the fiber mats into transparent nanocomposites.Effects of nanofiber orientation degree in two orthogonal directions and loading fraction on the optomechanical behavior of the nanocomposites are assessed.Optical transmittance differences parallel and perpendicular to the nanofibers’ alignment direction are found to vary in a range of 3.9–5.4% at 589 nm,and strong mechanical anisotropy is observed with the 1% PA-6/PMMA nanocomposites.In addition,a maximal of 3% PA-6 nanofiber loading maintains the nanocomposite high transmittance(>75%)with improved strength and toughness along the nanofiber axis.It thus reveals evident anisotropic optomechanical properties of the transparent nanocomposites,and highlights the great designability of transparent nanocomposites by using aligned nanofibers as the designing elements.(2)To improve the reinforcing capacity of PA-6 nanofibers,graphene-incorporated-Nylon 6(Gr/PA-6)nanocomposite nanofibers are proposed as an alternative reinforcement for PMMA.By introducing the concept of electrospun PA-6 nanofibers as the dispersing carrier for graphene nanosheets and by employing the above used SBCE approach for homogeneously hybridizing nanocomposite nanofibers of Gr/PA-6 with PMMA microfibers,aggregation issue of the involved nanofillers(i.e.,the Gr nanosheets and the Gr-incorporated PA-6 nanofibers)within the PMMA matrix could be effectively addressed.Visible light transmittance and tensile mechanical properties of the hot-pressed Gr/PA-6/PMMA nanocomposite are examined in relation to the loading fractions of the Gr nanosheets in the nanocomposite.It is demonstrated that a significant enhancement in tensile mechanical properties of the Gr/PA-6/PMMA nanocomposite is accomplished at a Gr loading of merely 0.01 wt%;that is,a nearly 56%,113% respective improvement of tensile strength,Young’s modulus,and noticeably above 250% increase of fracture toughness are achieved,while transmittance of the nanocomposite is maintained above 70%(in other words,less than 10% loss in transparency in comparison with neat PMMA)in the visible wavelength range of 400–800 nm.(3)Since PA-6 is hydrophilic whereas PMMA hydrophobic,the weak interaction between PA-6 and PMMA is anticipated,which also leads to limited efficiency of PA-6 nanofibers for reinforcing the PMMA matrix.To address this issue,styrene-maleic anhydride copolymer(SMA)as a compatibilizer is proposed to be coated onto the surface of PA-6 nanofibers,denoted as(SMA/PA-6)c-s via coaxial electrospinning,from which ternary nanocomposites of(SMA/PA-6)c-s/PMMA are fabricated via impregnation process.In the meantime,electrospun SMA/PA-6 blend nanofibers,denoted as(SMA/PA-6)b,and electrospun PA-6 nanofibers reinforced PMMA nanocomposites are prepared by the same way as the controls.The results showed that the ternary nanocomposites(SMA/PA-6)c-s/PMMA are of superior mechanical properties without significant compromising transparency compared to that of the(SMA/PA-6)b /PMMA nanocomposites.That is,when the content of SMA is 5 wt%(relative to the PA-6 mass),tensile strength of the(SMA/PA-6)c-s/PMMA film with 2% of(SMA/PA-6)c-s nanofibers incorporation can be increased more than 28%,while its transmittance of the nanocomposite maintained above 70% at the wavelength of 589 nm.Fractographic studies demonstrate that the length of the SMA-modified fibers pulled out of the PMMA matrix is obviously shorter than that of the PA-6 nanofibers within the PMMA.Dynamic mechanical analysis(DMA)results show that the loss factor of PA-6/PMMA composites can be decreased with the introduction of SMA.These results conclude that incorporation of SMA improves the PA-6/PMMA interface compatibility.(4)To further improve the interfacial compatibility between PA-6 and PMMA,as an alternative approach acrylic acid(AA)is grafted onto the surface of plasma-treated PA-6 nanofibers(AA-PA-6),followed by impregnation in PMMA solution for making ternary nanocomposites AA-PA-6/PMMA.Likewise,PMMA-based nanocomposites reinforced by plasma-only-treated PA-6 nanofibers and pristine PA-6 nanofibers are prepared in the same manner.Effect of AA-grafting on the morphology,chemical composition,wettability,thermal properties,crystallinity and mechanical properties of the PA-6 nanofibers are evaluated,together with its impact on the opto-mechanical properties and interface compatibility of the fabricated nanocomposites.The results show that the transmittance of the 2%AA-PA-6/PMMA nanocomposites is improved by 3% compared to that of the PA-6/PMMA nanocomposites,and can be maintained above 76.5% at the wavelength of 589 nm,although the diameter of AA-PA-6 nanofibers is larger.In addition,there are significant enhancement in tensile mechanical properties of the 2%PA-6/PMMA,2%Pla-PA-6/PMMA and 2%AA-PA-6/PMMA nanocomposites;that is,a nearly 38% and 103%,53% and 136%,and 93% and 45% improvement for tensile strength and toughness with respect to the 3 types of nanocomposites are achieved,respectively.DMA results show that the loss factor of PA-6/PMMA composites is significantly decreased with the use of the AA-grafted PA-6 nanofibers as reinforcement.Fractographic observation indicates that the amount of the plasma-only-treated PA-6 nanofibers drawn from the PMMA matrix is obviously decreased,while the AA-PA-6 nanofibers integrated well with the PMMA matrix.These results demonstrate that AA-grafting of PA-6 nanofibers can effectively improve the interface compatibility between the PA-6 nanofibers and PMMA matrix.In conclusion,it is shown that dispersion of nanofibers in PMMA matrix can be achieved via SBCE method,aligned nanofibers reinforced PMMA show anisotropic optomechanical properties,incorporation of a small amount of Gr can effectively enhance the mechanical properties of PA-6 nanofibers and PMMA matrix,and interfacial compatibility between PA-6 nanofibers and PMMA matrix can be improved via using SMA compatibilizer and AA-grafting.Results elaborated in this thesis pave the way for the design and fabrication of high-performance TNCs with controlled structures for practical applications in the near future.