PLA and cellulose based degradable polymer composites

At present there is significant interest for the development of environmentally friendly polymers and polymer based composites from renewable resources. Concerns about greenhouse gas emissions produced by using fossil fuel combined with low degradability of polymers derived from fossil fuel has led to increase in research to develop degradable polymers and composites. Polylactic acid, PLA, and cellulose are two such examples of degradable polymers that come from renewable resources.;In this study, we have investigated the process-structure-property relationship in PLA-cellulose composites. The effects of particle size, filler-matrix compatibility and the processing technique used on the composites mechanical, thermal properties and degradation behavior were studied. Cellulose in the form of microcrystalline cellulose particles and cellulose whiskers derived by hydrolyzing microcrystalline cellulose particles was used for reinforcing PLA. A brief background on PLA and cellulose including synthesis techniques and applications is given in chapter 2. This chapter also summarizes the different surface modification techniques employed and the theories associated with surface modification. Relevant micromechanical theory as developed for fiber reinforced composite is also reviewed.;Chapter 3 provides detailed procedures used for hydrolyzing microcrystalline cellulose, surface modification of cellulose particles and degradation of neat PLA and PLA-cellulose composites. Description of instruments used for characterizing samples in terms of model, method and experimental conditions employed are also given.;In chapter 4 we investigated both the influence of unmodified microcrystalline cellulose particles as reinforcement in PLA and the influence of processing technique on the composite properties. The use of unmodified particles resulted in increased Young's modulus and storage modulus but decrease in toughness of PLA-cellulose composite. Processing PLA by melt mixing resulted in some thermal degradation and reduced the polymer chain length. The decreased chain length increased crystallization rate and resulted in the melt processed samples having higher crystallinity compared to solution processed samples. The higher crystallinity translated into melt processed samples having higher stiffness.;In chapters 5 and 6, the effect of surface modification of microcrystalline cellulose particles, both un-hydrolyzed and hydrolyzed, on composites mechanical and thermal properties was studied. Surface modification by grafting small length lactic acid chains or larger length polylactic acid chains resulted in increased filler-matrix compatibility. The improved compatibility resulted in relative increase in static and dynamic mechanical properties of the composite at lower cellulose content. The cellulose particles also acted as nucleating agents and increased the crystallization rate of the PLA matrix chains. The particles size and surface chemical composition were found to influence its nucleation efficiency.;In chapter 7, the degradation of neat PLA and PLA-cellulose composites are studied. Under the degradation conditions employed and the sample size studied, the degradation was found to occur via surface erosion mechanism. The rate of degradation was found to depend on the initial sample crystallinity, concentration of the alkaline medium employed. The addition of cellulose particles increased the degradation rate of PLA. For all samples the degradation occurred preferentially in the amorphous regions as indicated by the increase in crystalline fraction of degraded samples.;This work provides detailed structure-property analysis of PLA-cellulose composite system. Broad variations in the properties were achieved through appropriate choice of processing technique and by manipulating the PLA-cellulose interfacial interactions. A simple and effective method to modify surface of cellulose particles by grafting lactic acid or polylactic acid chains that required no catalyst was successfully demonstrated. Reinforcing PLA with modified cellulose particles improved the overall mechanical properties and increased the degradability of PLA matrix under alkaline conditions.