| Poly(lactic acid)(PLA)is an eco-friendly,fully biodegradable polyester material with excellent strength and modulus,good processing thermoplasticity,outstanding optical transparency and biocompatibility.Due to its versatility,PLA has been considered to have broad application prospects.However,some inherent defects of PLA must be overcame to further enhance its performances,such as high brittleness,slow crystallization rate,poor heat resistance,and narrow processing temperature window,etc.In this paper,thermoplastic poly(ether-ester)elastomer(TPEE)is used to toughen PLA matrix,and polyepoxy chain extender(ADR)is added to reactively compatibilized the immiscible PLA/TPEE b ends.(1)The quenched PLA/TPEE/ADR specimens were annealed at 90? and 110?respectively to improve the impact toughness and heat resistance.The relationships between annealing temperature,crystal structure,impact toughness and heat resistance have been researched in detail.The chain extension and viscosity tackif ying effect of ADR is confirmcd by using torque rheology and melt mass flow rate(MFR)measurements.According to the non-isothermal DSC measurement,it is found that the PLA/TPEE/ADR melts show a very slow melt-crystallization rate.As a result,the quenched PLA/TPEE/ADR specimens have a very small crystallinity,being roughly amorphous.Although the addition of ADR doesn't make an obvious influence on the melt-crystallization behavior of PLA/TPEE blends,the chain extension/branched reaction between PLA,TPEE and ADR evidently depresses the cold crystallization ability of PLA matrix through the tackifying effect which would restrain the regular arrangement ability of chains and segments.Moreover,isothermal cold crystallization kinetics is researched by using DSC.It is clear that the cold crystallization rate of PLA matrix increases and the crystallization half-time(t1/2)reduces with increasing crystallization temperature(Tc)from 80? to 110?.t1/2 shows a tendency of prolonging with ADR loading at the same Tc.However,as Tc is more than 80?,the hindering effect on cold crystallization resulted from ADR loading becomes less noticeable.Through the calculation of crystallinity,it is demonstrated that the crystallinitics of annealed specimens increase by about 31%to 41%compared with those of quenchedspecimens.Moreover,the crystallinities of specimens annealed at 110? Care slightly higher than those at 90?.Combined with the results of wide-angle X-ray diffraction(WAXD)and differential scanning calorimetry(DSC)measurements,it can be seen that the crystal structure of PLA is not affected by the incorporation of TPEE and ADR,and the a' form forms after annealed at 90? while a form mainly exists at 110?.In addition,with the increase of crystallinity,the impact toughness and heat resistance are improved largely.Above all,it suggests that the PLA/TPEE/ADR samples annealed at 110? for 10 min is a better choice based on consideration of the comprehensive performance and productivity.(2)For PLA/TPEE/ADR(95/5/X)blends prepared by one-step blending method,it is clear that the incorporation of TPEE enhances the tensile toughness of PLA matrix.However,the PLA matrix has a great loss in visible light transmittance resulted from the obscuring effect of TPEE domains with larger size.In order to improve the light transmittance of PLA/TPEE blends,ADR is used as a reactive compatibilizer to refine the TPEE domains.However,It is found that the light transmittance cannot be effectively improved just by increasing the ADR loading due to the weaker compatibilization and refining effect of ADR while the PLA/TPEE/ADR blend is prepared by using the one-step blending method.In this paper,the two-step blending method is used to increase the interfacial reaction compatibility of PLA/TPEE/ADR blending system.Compared with the one-step method,pre-synthesis of high-concentration epoxy prepolymer(TPEE-ADR)in the two-step method increasesthe interfacial reaction compatibility leading to a stronger refinement of TPEE domains.As a result,a PLA/TPEE/ADR(95/5/X)blend with excellent tensile toughness and improved light transmittance can be obtained by using the two-step blending method.The results of torque and MFR reveal that there is a positive dependency of chain extension and melt viscosity onADR loading.Due to the difference between their reaction compatibilization mechanisms,the values of torque and MFR of the PLA/TPEE/ADR blends prepared by the two-step method are less than those prepared by the one-step method.Scanning electron microscopy(SEM)and transmission electron microscopy(TEM)images of PLA/TPEE blends show that the two-step method has a stronger compatibilization and refinement effect than the one-step method.And even the TPEE domain size could be refined to about 70 nm.Correspondingly,the transparency of the blends increases significantly with the dimension refinement of TPEE domains.Here,the one-step blends have a higher transmittance of 3%to 20%than the binary mixtures.Meanwhile the overall transmittance of the two-step method is 26%?31%more than the one-step method.The tensile strengths of the PLA/TPEE/ADRblends prepared by the two-step methodare slightly lower than that of neat PLA.However:it is 6?12 MPa higher than those of the PLA blends prepared by the one-step method.Furthermore,the tensile elongation at break of the blends reaches more than 250%,which is much more brilliant than neat PLA.Based on consideration of the transmissibility and tensile properties of the blends in this study,the PLA/(TPEE/ADR)95/(5/0.5)blend shows the optimal performance,which is an excellent modified PLA materials to fabricate PLA films with high light transmission and tensile performance. |
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