Researches On Stretching And Some Nucleating Agents Inducing Crystallization Of PLA Films

In recent years, the exhaustion of fossil energy, the emission of large amounts of industrial toxic gases and the excessive use of non-degradable materials lead to serious damage to the global environment. Researchers around the world are looking for biodegradable materials to deal with resource and environment crisis. Products of polylactic acid, which are green and environmental friendly, most important, they can degradate by microorganism when lost used value, and own excellent biodegradability, which make it the most promising polymer material. However, polylactic acid product under normal state, is hard and brittle, poor impact resistance, which greatly restrict the scope of its application. While, the crystallization of polymer can improve the mechanical properties of the polymer, simultaneously, causing changes in polymer thermal stability, optical properties, air permeability and heat resistance. So researches on inducing crystallization of PLA film have an important theoretical significance and potential greatly economic value. In addition, tensiling and adding some nucleating agents to induce crystallization of PLA film are simple operation, low energy consumption, convenient and feasible, therefore,it has a very good operability.In this paper, We try to use D,L-PLA as the research object, dichloromethane as the solvent to get the film by tape casting, the films were stretched on the Dynamic mechanical analyzer (DMA) or added into different minute quantity of nucleating agents to induce crystallization. We investigated the crystal morphology and the change of crystallization of D,L-PLA film, and measured its crystallinity by using polarization microscope(POM), fourier transform infrared spectrometer (ATR-FTIR), differential scanning calorimetry (DSC) and x-ray diffraction(XRD). From the point of polymer mesophase, we try to explain the change of crystallinity, the behavior of crystallization and polymorph at the micro state.Results show that the type racemic polylactic acid film changed from amorphous to semi-crystal type after stretched or added into different minute quantity of nucleating agents, the crystallinity and aggregated structure were also changed.The crystallinity and crystal structure of poly (lactic acid) (PDLLA) is not only related with the temperature, but also related to tensile strain. Temperature influences the move ability of molecular chains of poly (lactic acid) (PDLLA), the higher the temperature is,the stronger the movement ability of the molecular chain; and the poly (lactic acid) (PDLLA) polymer mesophase turned into crystalline blocks. Tensile induces the molecular chain of poly (lactic acid) (PDLLA) orientated and makes the structure of poly (lactic acid) (PDLLA) mesophase change into crystallization phase. Accordingly, this increased the crystallinity of polylactic acid membrane. Among these factors, temperature is the biggest one. POM shows that with other conditions remain unchanged, range from the glass transition temperature to the melting temperature, the change of the crystallinity of racemic polylactic acid film is very sharp, crystal increased from 7.41% to 40.96%.DSC tests show that the crystallinity of racemic polylactic acid film increased from 12.1% to 33.18% when the tensile strain range from 0% to 31.16% at 90 ?. DSC and XRD tests show that the tensile strain-induced orientation and nucleating agent can effectively increase the crystallinity of the polylactic acid film, and the crystal form of ?-type.