1. SUMMARY
This PHD thesis deals with various innovative biodegradable materials systems, as
part of broader efforts to replace non-biodegradable petroleum-based polymers, with
bio-materials derived from natural sources, or fully biodegradable environmentally
friendly polymers, in order to resolve issues associated with common oil based plastics.
Nevertheless, in several cases, such materials have limited possibilities in terms of their
mechanical properties, their thermal stability and their barrier properties, a fact that
results to restrictions on their application possibilities. Reinforcement of these
materials, with various sized inclusions, offers new possibilities for them, as completely
biodegradable composites/nanocomposites are formed, with enhanced properties and,
as a result, new perspectives.
The first part of the study on biodegradable materials, involves the preparation, by
melt-mixing and compression molding, of nanocomposite materials based on polylactic
acid (PLA) and a PLA-PBAT blend (trade-named Ecovio®), with nanoclays (MMT) and
silica nanoparticles at low content (2-5 wt%). The second nanocomposite material
system had not been prepared and studied, regarding the existing literature. The
materials were studied by various experimental techniques, such as scanning electron
microscopy (SEM), tensile test, dynamic mechanical analysis (DMA), creep, differential
scanning calorimetry (DSC), thermogravimetric analysis (TGA), Wide Angle X-ray
scattering (WAXS). The influence of different type nano-inclusions in each of the two
polymeric matrices was evaluated by the above techniques and useful results came out,
about the degree of reinforcement, the thermo-mechanical behavior, the time-
dependent properties in combination with the quality of dispersion of the nanoparticles
in the polymer, the aggregation and the quality of the interphase between the matrix
and the inclusions, which depends on their adhesion to the polymer. Differences about
the interaction between the polymer and the inclusions were observed, by comparing
the two polymers used as matrices for the nanocomposites. Moreover, a parametric
analysis via a micromechanics model for the calculation of nanocomposite materials’
stiffness was performed. The model considers an "effective particle" with an interface
between the polymer and the particle and our analysis was based on fitting the
experimental results regarding the PLA and Ecovio® nanocomposites. In addition to that,
a finite element model was generated, considering a representative volume of the
material, including the "effective particle" with its two different phases. A comparison of
the results of the two models and a study of their difference gap were performed.
Following the study of biodegradable nanocomposite materials, the effect of
nanoparticles on PLA aging, when subjected to degradation conditions, was

2. investigated. Specifically, examination of the deterioration of the thermo-mechanical
properties was performed, on two different aging conditions: (a) 80% RH, 40 ° C. (b)
immersion in buffer solution, pH 7.4, 37 °C. The study was conducted using the pre-
mentioned experimental techniques and additionally by Size Exclusion Chromatography
(SEC) for the control of the evolution of the molecular weight of PLA and by cytotoxicity
assessment, in order to test the biocompatibility of nanocomposite materials concluding
silica nanoparticles. The acceleration of the degradation process due to the presence of
nanoparticles, is a general conclusion of this study. It is regarded that the results are
based on the variations in crystallinity content versus time, due to aging, and the
damage due to hydrolysis, on the matrix-nanoparticle interphase. Moreover, a
hydrolytic damage model and a model for the evaluation of filler-matrix interaction
were used. By means of these models, the hydrolytic damage of the materials was
quantitated and the evolution of the filler-matrix interaction versus hydrolysis time was
studied.
The second part of the study on biodegradable polymeric systems concerns the
preparation of bio-composite materials. These materials include various sized fibers,
derived from natural sources, and in combination with the biodegradable polymer
matrix as a carrier, a fully biodegradable composite material ("green composite") is
formed. Specifically, two systems were studied: (a) Composite with various types of
micro-sized wood fibers (WPC) based on the polymeric matrix Ecovio®, and long fiber
thermoplastic composites, reinforced with oriented flax fibers, based on three polymer
matrices, Ecovio®, PLA and Bionolle® (PBS trade name). The comparative study of the
effect of various types of natural fibers on the macroscopic properties of the polymers is
a new contribution on bio-composites research field. The performance of these systems
depends mainly on the quality of the adhesion of the fibers into the matrix in order to
bear the receiving load effectively, which is greatly difficult due to the hydrophilic
nature of the fibers. The study was conducted through the experimental techniques
mentioned before, plus flexural testing for the long fiber composites and water
uptake/thickness swelling tests for the WPC. Following the WPC study, their short-term
creep behavior was investigated through Findley’s and Burger’s viscoelastic models. The
effect of the applied stress, the temperature and the fiber type on the creep response of
the material, were studied through the interpretation of the parameters of the models.
The applied stress-temperature conditions seem to affect the fitting of the models to
the experimental data, in a different way for each model.