| Polylactide (PLA) has been widely used in the fields of package, agriculture, clothing andmedical materials due to its good biocompatibility, mechanical properties, processing propertiesand non-toxicity. However, the widespread application of PLA is still limited to a large extentbecause of its low crystallization rate, brittleness and poor heat-resistance. In recent years, muchattention has been paid to the high heat resistant stereocomplex PLA (sc-PLA), which shows amelting temperature at230oC that is approximately50oC higher than homocrystallizedpolylactides (PLLA or PDLA). In general, sc-PLA can be prepared by melt or solution blendingPLA’s two optical isomers, i.e. poly(L-lactic acid)(PLLA) and poly(D-lactic acid)(PDLA), andthe best blending ratio is1:1to form stereo-complex(sc) crystallites. However, sc-PLA preparedby melt blending equivalent proportion of PLLA/PDLA usually contains small amountshomo-crystallites (hc), and the mechanical properties of sc-PLA usually show a decreasing trend.In this thesis, flax fibers were used as reinforcing fibers and equivalent proportion of PLLA/PDLAwere melt compounded and then followed by injection molding to prepare the flax/sc-PLAcomposites with improved mechanical properties and heat-resistance property, which couldbroaden the applications of PLA composite in the automotive parts, decoration and package fields.Since the basic characteristics of raw material PLLA and PDLA such as optical purity,molecular weight, racemization degree and melt point Tmplay important role in the formation ofsc, three kinds of PLLA and two kinds of PDLA were analyzed and compared. Finally, PLLA-3and PDLA-1, which showed lowest racemization degree, similar molecular weight and easyprocessing, were choosen as the raw material for manufacturing stereocomplexed PLA.On the basis of above materials,10%flax/sc-PLA composite was fabricated by meltcompounding equivalent proportion of PLLA/PDLA and followed by injection molding, andcompared with sc-PLA and PLLA. The results showed that sc-PLA and10%flax/sc-PLAcomposite exhibited both homo-crystallites(hc) and stereocomplex crystallites (sc). The degree ofcrystallinity of composite was increased by reinforcement of flax fibres, and the sc yield (fsc) of10%flax/sc-PLA composite could reach92.7%, higher than that of sc-PLA (66.4%). Comparedwith PLLA, the heat-resistance property of sc-PLA increased greatly, while its mechanicalproperties decreased somewhat. By the addition of10%of flax fibres, the mechanical propertiesof sc-PLA were improved significantly, the tensile strength, Young’s modulus and Izod notchedimpact strength of10%flax/sc-PLA composite were increased by34.1%,49.8%and289.1%,respectively. Moreover,10%flax/sc-PLA composite showed a good heat-resistance property, itsVicat softening temperature (VST) could achieve155.6℃, increased nearly92℃than PLLA.To further improve the interfacial strength between sc-PLA and flax fibres, and increase themechanical properties of the10%flax/sc-PLA composite, the effect of different surface modification methods, alkali treatment and silane coupling agent treatment, on the structural andproperties of the composite were investigated. The results showed that hemicellulose sticking tothe flax fiber surface was almost removed after alkali treatment, while silane coupling agenttreatment had little effect on flax fibres. The surface modification had no obvious impact oncrystallinity of the composites. Moreover, the UN-flax/sc-PLA composite exhibited both thehighest mechanical and heat-resistant properties, followed by the ALK-flax/sc-PLA, and thenSIL-flax/sc-PLA. The above results suggest that the performance of flax/sc-PLA compositecouldn’t be improved by the surface modification of flax fibre.Considering that the mechanical performance of10%flax/sc-PLA composite still could beincreased, the effect of flax fibre content on the structural and properties of the flax/sc-PLAcomposite was explored. The results revealed that with the increase of the flax fibre content, boththe degree of crystallinity and the sc yield fscof flax/sc-PLA composite increased gradually. Whenthe fibre content was added to30%, fsccould reach98.4%and simultaneously crystallinity of thecomposite increased from sc-PLA’s17.2%to35.6%. With the increase of the flax fibre content,the Young’s modulus and Izod notched impact strength of flax/sc-PLA composite improvedsignificantly, whereas the tensile strength of composite increased firstly and then decreased withthe further addition of flax fiber. When the flax fibre content was30%, flax/sc-PLA compositeshowed the best comprehensive mechanical properties. Conpared with sc-PLA, the tensile strength,Young’s modulus and Izod notched impact strength of30%flax/sc-PLA composite were increasedby44.5%,132%and343%, respectively. Additionally, the Vicat softening temperature ofcomposite improved with the increase of fiber content.30%flax/sc-PLA composite showed ahighest VST(162.5℃), nearly100℃higher than that of PLLA(63.7℃).At last, the Mo method and Ozawa method were used to analyze and compare thenon-siothermal crystallization behaviors of sc-PLA and10%flax/sc-PLA composite. The resultsshowed that10%flax/sc-PLA composite showed a multiple cystallizaiton behavior during thenon-siothermal cooling process, in which sc forms at the higher temperature and hc forms at thelower temperature. In contrast, sc formed during the cooling process of sc-PLA show hysteresisand overlap with hc. Furthermore, the Ozawa index m of10%flax/sc-PLA and sc-PLA are2.1~2.5and2.2~2.9, respectively, calculated according to the Mo method.10%flax/sc-PLA compositeforms sc firstly during its melt crystallization process, then sc forms crystals by a way ofheterogeneous nucleation and two dimensional growth, and the grains become smaller and smaller.The crystallization activation energy of sc calculated by Kissinger method is-151.8kJ/mol.Moreover, sc-PLA forms both sc and hc during its melt crystallization process, and sc crystallizesby a way of heterogeneous nucleation and tridimensional growth, and its melt crystallizationbehavior arises hysteresis phenomenon, while hc forms two dimensional grains because of limitedspace. |
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