Controlling Morphologies Of Stereocomplex Crystals In Poly(Lactic Acid)/Cellulose Nanocrystal Composites At High Pressure

Polylactic acid(PLA)is an environmentally friendly material and possibly the most promising one due to its excellent biocompatibility,biodegradability and processability.PLA shows intensive applications in medical,packaging and other fields.Nonetheless,drawbacks to the broad adhibitions of PLA are obvious too,such as poor thermal stability and low mechanical strengh.In recent studies,sc-PLA,a new stereocomplex structure with satisfying thermodynamic and mechanical properties,has emerged through blending poly(L-lactide)(PLLA)with poly(D-lactide)(PDLA).Cellulose nanocrystals(CNCs),mainly deriving from renewable resources including wood and cotton,with good bio-degradability,high aspect ratio,rigidity and density,has been considered as suitable reinforcing agent for the processing of polymer nanocomposites.However,it still needs to be solved for the poor compatibility between the CNCs and hydrophobic polymeric matrixes.Major work of this study inlcudes successful preparation of PLLA/PDLA/ PDLLA and PLLA/PDLA/CNC ternary blends via solution casting,whereafter the composites was subjected to a high pressure crystallization treatment.By carefully adjusting the composition and crystallization pressure,time and temperature,we have effectively controlled the crystallization behaviors of sc-PLA in PLLA/PDLA/CNC composites.We also studied the hydrolytic degradation behaviors of the composites in different aqueous media.The main conclusions are as follows:(1)High pressure crystallization and hydrolytic degradation behaviors of PLLA/PDLA/PDLLA ternary blendsCompared with the reference samples prepared at atmospheric condition,high pressure treatment promoted the formation of stereocomplex crystals in the blends.Based on X-ray diffraction techniques(XRD)and differential scanning calorimetry(DSC)tests,it was concluded that the optimum crystallization condition for the growth of sc-PLA in the blend system was at 200? and 200 MPa,at that point the degree of crystallinity of the stereocomplex crystals was 16.25%.scanning electron microscopy(SEM)images showed the detailed surface morphologies of the stereocomplex crystals with micro/nano scale hierarchical structures.Also,self-organized crystalline structures of sc-PLA were observed with SEM.Furthermore,the wettability test results showed the hydrophilicity of the composite surfaces could be improved through pressure treatment followed by delicate chemical etching,and this character was positively correlated with the content of PDLLA.Finally,the comparison for the surface morphologies of the same sample before and after hydrolysis suggested that in acidic and neutral solutions,the hydrolysis degradation process followed the bulk corrosion mechanism,while in alkaline solution it followed the surface corrosion mechanism.(2)High pressure crystallization and hydrolytic degradation bahaviours of PLLA/ PDLA/CNC nanocompositesCompared with PLLA/PDLA/PDLLA blends,the addition of CNCs greatly promoted the formation and crystallinity of stereocomplex crystals in PLLA/PDLA/CNC composites.Optimum condition for sc-PLA formation in this ternary blend system was at 220? and 200 MPa for 4 h,at that point the degree of crystallinity was 33.27% with the stereocomplex crystals making up 93.11% of the whole crystals.Surface morphologies of the stereocomplex crystals in PLLA/PDLA/CNC blends was slightly different from that of the PLLA/PDLA/PDLLA sample,which should be assigned to the dispersion of CNCs in the PLA matrix.The ternary blend system showed better hydrophilicity,and subsequent hydrolysis experiments proved the positive function of CNC in promoting the hydrolysis degradation of the samples.