Study On Preparation And Properties Of Biodegradable Polymers Nanocomposites

The interest in biodegradable polymers has recently gained exponential momentum,and, within that broad family, poly (ε-caprolactone)(PCL) and poly(lactic acid)(PLA)are promising materials because of its flexibility, good biodegradability andbiocompatibility. However, PCL has low melting point, poor stability, and tendency torack when stressed limited its widely applications. Other properties of PLA, such astoughness, dimensional stability, gas barrier properties and slow biodegradation rate, areoften not sufficient for its further processing and end-use applications. In the presentdissertation, based on the disadvantages of nanometer material and nanocomposites,biodegradable PCL/silica nanocomposites and PLA/silica at various silica loadingsranging from1to9wt%were prepared via direct melt compounding method. Inaddition to this, biodegradable PLA/organic clay were prepared too. Main contents andconclusions are as follows:(1) Biodegradable PCL/silica nanocomposites with different silica loadings wereprepared through direct melt compounding method. When the silica content was <3wt%, the nanoparticles dispersed evenly in the PCL matrix and exhibited onlyaggregates with particle size of less than100nm. At a higher silica content (≥3wt%),the number of large aggregates increased markedly. Some of the aggregates evenexceeded the size of250nm. As the silica loadings reaches up to9wt%, percolatedsilica network structures can be formed. The sample exhibit evident solid-like response in the low frequency region. The long-range motion of PCL chains is highly restrained.The tensile strength, modulus and yield strength values of the nanocomposites wereenhanced by the incorporation of inorganic silica nanoparticles owing to the goodcompatibility of the silica with PCL matrix as well as uniform dispersion of silica; at thesame time, the nanocomposites still retained good ductility. The biodegradation rateshave been enhanced obviously in the PCL/silica nanocomposites than in neat PCL dueto a facile attack of the enzyme molecule toward the ester groups of PCL chains, whichmay be of great use and importance for the wider practical application of PCL.(2) Nonisothermal melt crystallization of neat PCL and the PCL/silicananocomposites was studied with DSC at various cooling rates. The crystallization peaktemperature is higher in the nanocomposites than in neat PCL, which first increases anddecreases with increasing the silica loading. Isothermal melt crystallization kinetics ofneat PCL and the PCL/silica nanocomposites was also investigated with DSC atdifferent crystallization temperatures. The overall crystallization rate is faster in thenanocomposites than in neat PCL, which also first increases and then decreases withincreasing the silica loading. For both nonisothermal and isothermal melt crystallizationstudies, the crystallization of PCL is enhanced by the presence of silica, and theenhancement is influenced by the silica loading in the nanocomposites. The effect ofsilica on the crystallization is twofold: the presence of silica may provide a number ofheterogeneous nucleation sites for the PCL crystallization while the aggregates of silicamay restrict crystal growth of PCL. Therefore, the overall crystallization of PCL firstincreases and then decreases with increasing the silica loading from1to9wt%in the PCL/silica nanocomposites for both nonisothermal and isothermal melt crystallizationof PCL. However, the crystallization mechanism and crystal structure of PCL remainalmost unchanged after nanocomposites preparation.(3) ESEM observation indicates that when the silica content was <5wt%, the silicadispersed evenly in the PLA matrix and exhibited only aggregates with particle size ofless than100nm. However, at a higher silica content (>5wt%), some of the aggregateseven exceeded the size of250nm. As the silica loadings reaches up to5wt%,percolated silica network structures can be formed. Once the percolated silica networkstructures formed, the modulus increases with increase of temperature evidently. Thebiodegradation rates have been enhanced obviously in the PLA/silica nanocompositesthan in neat PLA, which may be of great use and importance for the wider practicalapplication of PLA. The erosion mechanism of neat PLA and its nanocomposites wasfurther discussed, and the biodegradation of neat PLA and its nanocomposites mayproceed via surface erosion.(4) Biodegradable PLA/clay nanocomposites with different clay loadings anddifferent kind of clay were prepared through direct melt compounding method. Thecrystallization of PLA is enhanced by the presence of clay from DSC test result and theenhancement order is: MMT-Alk>MMT-COOH>MMT-OH. As the MMT-ALk loadingsis3-5wt%, and MMT-COOH loading reaches up to5wt%, percolated clay networkstructures can be formed. Once the percolated clay network structures formed, themodulus increases with increase of temperature smally. Moreover, percolated claynetwork structures can not be formed on this condition.