Effect Of Inorganic Nanoparticles On The Crystallization Behaviors Of Polylactide

Polylactic (PLA) is a novel biodegradable polymer material which is entirely derived fromrenewable resources, and it is a kind of linear aliphatic polyester. PLA has good biocompatibility,thermal plasticity, mechanical properties, and is easily manufactured; it is thus a potentialpolymer to replace petroleum-based plastics in various applications. That is why PLA attractedmuch attention of both academic and industry perspectives over the last two decades. PLA is akind of semicrystalline polymer, the mechanical, thermal properties, and even biodegradabilityare strongly dependent on the crystal morphology and structure. Therefore, studying thecrystallization kinetics of PLA can enrich the basic theory of crystallization, it also can help tocontrol the microscopic structure of PLA. Meanwhile, it can ensure the stability of product andexpand the scope of application.In this article, Polylactic (PLA) with various nanosilica and nano-titanium dioxide loadingwere prepared by the method of solution blending. Differential scanning calorimeter (DSC) wasused to investigate the thermal transition temperature and crystallinity, isothermal andnon-isothermal crystallization process of the samples and polarized optical microscopy (POM)was used to study the crystalline morphologies. By analysing of relevant formulas and methods,get the following conclusions:Compared with PLA, the influence of SiO2and TiO2on the glass transition temperature (Tg)is not obvious, but the values of the cold crystallization temperature (Tcc) and the meltingtemperature (Tm) for SiO2/PLA and TiO2/PLA is slightly higher than the neat PLA. There issmall exothermic peak just before the melting temperature, it is attributed that the crystallizationis not complete in the low cold crystallization temperature. As the temperature increases, theimperfect crystals became perfect. To some extent, the appropriate amount of nanosilica canincrease the crystallinity (Xc) of PLA, while the values of Xcdecreased for the samples containing TiO2.The isothermal crystallization process was analyzed by Avrami equation andLauritizen-Hoffman theory. The Avrami exponent (n) of SiO2/PLA and TiO2/PLA were largerthan those of the neat PLA for the same isothermal crystallization temperature. One possiblereason is that the incorporation of SiO2and TiO2have affected the nucleation and growthmechanisms of the PLA crystals. Comparing the values of crystallization half time (t0.5),nucleation constant (Kg) and the fold surface free energy (σe), we found that the presence of SiO2and TiO2in PLA promote crystallization with the small content of nanoparticles. In contrast,SiO2and TiO2inhibited crystallization with the larger content of nanoparticles. For all thesamples, the overall crystallization rate were first increased and then decreased with theincreasing of the isothermal crystallization temperature. Comparing the POM images, it is foundthat the crystal size became larger with the increasing of isothermal crystallization temperature.The number of crystals in the early crystallization for SiO2/PLA and TiO2/PLA is slightly morethan PLA, which limited the growth space for each crystal and lead to form smaller crystals.With the growth of crystal, the edge of each crystal was blurred and it’s difficult to distinguishthe crystals one by one.In the non-isothermal crystallization process, the value of crystallization peak temperature (Tp)decreased with the increasing of the cooling rates. Jeziorny equation was used to analyse thepre-process of non-isothermal crystallization, and the n values indicated that adding the SiO2andTiO2into PLA changed the nucleation and growth mechanisms. The rate of crystallizationincreased with the increasing of the cooling rate by comparing kcvalues. Then using the Moequation to further analyze the non-isothermal crystallization process, the values of F(T)increased which means the crystallization rate of SiO2/PLA and TiO2/PLA decreased. The valuesof effective activation energy (ΔEc) which were calculated by Kissinger equation indicated thatPLA was easier to crystallize than other samples.