Controlling Stereocomplex Crystal Morphologies In Poly (Lactic Acid)/Carbon Quantum Dots Composites At High Pressure

Poly(lactic acid)(PLA)is a synthetic biodegradable thermoplastic aliphatic polyester,but there are still many shortcomings in its mechanical properties,processing performance and degradation performance,which restrict it from wide applications.When blending PLLA and PDLA,the strong interaction between the molecular chains of PLLA and PDLA can lead to the formation of stereocomplex poly(lactic acid)(SC-PLA),which has excellent heat resistance and mechanical properties.Carbon quantum dots are a new type of zero-dimensional nanomaterials with a size of 10 nm or less,and with excellent photoelectric properties and good biocompatibility.In view of this,the main purpose of this study is to control the morphologies of stereocomplex crystals in PLLA/PDLA/CQDs composites by the synergistic action of carbon quantum dots and pressure,and to conduct a preliminary investigation on the mechanisms of hydrolysis degradation of PLLA/PDLA/CQDs composites under various environments.The study presented herein may be helpful in developing a new generation of heat-resistant biopolymer composites with controllable degradation properties.In this work,the high-pressure crystallized samples of PDLA,PLLA/PDLA blends and PLLA/PDLA/CQDs were prepared,and their high-pressure crystallization and hydrolysis degradation behaviors are investigated in detail.The main work and conclusions are listed as follows:(1)High-pressure crystallization and hydrolysis degradation of PDLACompared to their counterparts fabricated at atmospheric pressure,the melting points and crystallinities of the PDLA samples prepared by the high-pressure treatment are greatly improved.SEM results showed that no new crystal structure was formed at high pressure,and the obtained PDLA crystals were still in a form.However,the change of crystallization temperature,pressure and crystallization time affected the size and perfection of PDLA spherulites.Too high pressure and too long time lead to the formation of a lot of defects in the crystals.The hydrolysis experiments showed that the hydrolysis rates of different pressure-crystallized samples in the same medium were as followings:PDLA-200>PDLA>PDLA-500.Particularly,the hydrolysis abilities of samples were improved on the condition that they were prepared at an appropriate pressure.In the process of hydrolysis,the surface morphology observations suggested that the hydrolysis of PDLA should be assigned to a bulk erosion mechanism in acid and neutral solutions and a surface erosion mechanism in alkaline solution,respectively.(2)High-pressure crystallization and hydrolysis degradation of PLLA/PDLA blendsCompared with normal pressure crystallization,high pressure crystallization promoted the transition of PLLA and PDLA from homopolymers into stereocomplex in the binary blends,which made both the production rate and crystallinities of stereocomplex crystals greatly improved.The DSC results showed that the optimum crystallization conditions for the formation of stereocomplex crystals in the mixture system were 220℃,200 MPa and 4h.Under these conditions,the production rate of the stereocomplex crystals was 87.46%,and the crystallinity of the stereocomplex reached 28.25%.SEM results showed that the crystals of poly(lactic acid)stereocomplex belonged to a kind of micro-nano hierarchical topological structure,i.e.unique micrometer-sized three dimensional spherical crystallites constructed with nanometer-sized crystalline fibrils.Although the hydrophilicity of the original PLLA/PDLA blends is poor,the transition from the hydrophobic surface to the hydrophilic surface can be achieved by a delicate chemical etching process.The hydrolysis experiments of the PLLA/PDLA blend in different solutions suggested that the samples had the fastest hydrolysis rate in acid solution.Also,the hydrolysis rates of different samples in the same solution were as followings:PLLA/PDLA-500>PLLA/PDLA-200>PLLA/PDLA,and the largest weight loss of PLLA/PDLA-500 samples reached 34%after 29 days in acid conditions,indicating that the hydrolysis abilities of the PLLA/PDLA blends were improved by high pressure crystallization.The study of the surface morphology during the samples hydrolysis suggested that the hydrolysis of PLLA/PDLA blends should be assigned to a bulk erosion mechanism in acid and neutral solutions and a surface erosion mechanism in alkaline solution,respectively.(3)High-pressure crystallization and hydrolysis degradation of PLLA/PDLA/CQDs compositesThe high-pressure crystallizaion samples of PLLA/PDLA/CQDs composites still had excellent fluorescence properties.The UV absorption peak position is 350nm,and the fluorescence emission wavelength is 440nm.Particularly,they had excellent multi-color photoluminescence effects.Compared with atmospheric pressure crystallization,high pressure crystallization can promote the transition of PLLA and PDLA in the ternary PLLA/PDLA/CQDs blends from the state of homopolymer into stereocomplex,resulting in greatly increased production rate and crystallinity of the stereocomplex crystals.The DSC results showed that the optimum conditions for the formation of stereocomplex crystals in the composites were 200℃,200MPa and 4h.Under these conditions,the production rate of the stereocomplex crystals in the system reached 73.81%,and the crystallinity of the stereocomplex was 27.25%.However,the production rate and crystallinity of the stereocomplex crystals were decreased when the content of the carbon quantum dots was too high(3%).SEM results showed that the stereocomplex crystals in the PLLA/PDLA/CQDs composites were actually with unique three dimensional micro/nano hierarchical topological structures.Under normal pressure,the stereocomplex crystallites in the ternary blends were very small and with uniform sizes.Nevertheless,they self-organized into such size-distributed micro/nano hierarchical structures,driven by multiple interaction forces at high temperature and high pressure.The hydrolysis experiments showed that the hydrolysis rates of the high-pressure crystallized samples with 1%CQDs loading were almost the same as those of the samples without the addition of CQDs.Moreover,too high CQDs loading(3%)inhibited the hydrolysis of the high-pressure crystallized PLLA/PDLA/CQDs samples.