Effect Of One-and Two-Dimensional Carbon Nanomaterials On The High Pressure Crystallization And Hydrolysis Degradation Behavior Of Poly (LACTIC ACID)

Blending two or more materials is one of the most important approaches in preparing high-performance composites, which can not only make up the imperfection of the single material but also complement each others’ advantages. Especially, the combination of polymers with zero-, one- and two-dimensional nano carbon materials offers an attractive route to combine the merits of organic and inorganic materials into novel hybrid nanocomposites, with synergistic improvement observed in mechanical, electrical and thermal properties.Poly (lactic acid) (PLA) is an ever green biodegradable polymer derived from sustainable sources. Depending on the conditions of thermal treatment or processing, PLA exhibits a pronounced polymorphism, transforming between several crystal forms under specified external field. The crystalline morphology of the polymer is crucial in determining its mechanical properties and hydrolysis degradation rate. So it is particularly important to investigate the nano carbon based PLA composites at hydrostatic high pressure in realizing the control of multi-level morphological features and environmental degradation properties.In this work, PLA was blended with different carbon nanomaterials such as carbon nanotubes (CNT), graphene nanosheets (GNP) and graphene (Gr) through a simple and environmentally friendly approach, i.e. pre-mixing at high speed followed by melt compounding. PLA/CNT, PLA/GNP and PLA/Gr composites were prepared respectively, and then their high pressure crystallization and hydrolysis degradation behaviors were invesigated by wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and laser scanning confocal microscope (LSCM). The main work and conclusions are listed as follows:(1) Transmission electron microscopy (TEM) was undertaken to reveal the state of dispersion of the nanofillers in the as fabricated hybrid composites. The results showed that both GNP and Gr were homogeneously dispersed in the polymer matrix, existing a wrinkle and platelike state. For PLA/CNT (95/5, wt/wt) composite, CNT also achieved a generally good dispersion in PLA matrix, though CNT aggregations within smallscale were occasionally observed.(2) The thermal analysis results showed that the three different carbon nanomaterials acted as an efficient nucleating agent in enhancing nucleation density, reducing crystal size and promoting crystallizability. But, for these three carbon nanomaterials, the ability of inducing crystallization was different, GNP was best, and CNT was worst while Gr was just between them. Meanwhile, the most appropriate conditions for the high pressure crystallization were not exactly the same. For PLA/CNT (95/5, wt/wt) and PLA/Gr (99/1, wt/wt) composite, they were 140℃,200 MPa and 4 h, and the highest crystal linity were 49.96% and 51.6%, respectively. For PLA/GNP (99/1, wt/wt) composite, they were 180℃, 200 MPa and 4 h, and the highest crystallinity was 55.17%. Exorbitant pressure or temperature was found to be unfavorable for the high pressure crystallization of the as fabricated nano carbon based composites.(3) The structures of the high pressure crystallized PLA crystals in respective composites were determined by WAXD, DSC and SEM. The results showed that no new crystal structure other than a-form was crystallized in the presence of CNT, GNP or Gr at high pressure. Due to the existence of high pressure and carbon nanomaterials, the obtained polymer crystals in the binary composites was smaller, and had a few more defects than those in neat PLA.(4) The hydrolysis degradation tests showed that the degradation rates of PLA were diversed in different solutions for the three high pressure crystallized composites, fastest in alkaline solution and slowest in neutral solution. According to the obtained weight-loss cruves, the addition of CNT or GNP accelerated the hydrolysis degradation of PLA, while the introduction of Gr will make the hysrolysis degradation rate controllable by adjust the pressure during preparing samples. The weight loss data and LSCM images suggested that the hydrolysis degradation of the nano carbon based PLA composites should be assigned to a bulk erosion mechanism in acid and neutral solutions and a surface erosion mechanism in alkaline solution, respectively.