From a unified analysis of the temporal decrease of molar mass
of a polylactide (PLA) withw4 mol% D units subject to isothermal at
220 C, biological and photo degradation, the following conclusions
are reached:
The main step in the three degradation processes is a random
chain excision, with large differences in degradation rates. Due
to the high temperature, chain excision is the fastest under
isothermal degradation at 220 C followed by photo degradation.
Degradation in soil is the slowest of the three processes. At
equivalent degradation time, photodegraded specimens have
about half the molar mass of soil degraded samples.
The kinetics of the decrease of molar mass under biodegradation
obtained from the present data, and data from published
work, follow closely a first order process M ¼ Moekt. The value
of the rate constant, k, changes greatly depending on the actual
biodegradation environment, be either composting, hydrolysis
or in active soil.
The molar mass of specimens tested during thermal and photo
degradation follows a second order law (1/M ¼ (1/Mo) þ kt).
Literature data obtained in similar degradation conditions
were also adequately fitted with this equation. A distinctive
feature of photodegraded specimens is the generation of
anhydride groups in a concentration proportional to the
degradation time. These groups are absent in PLA subject to soil
degradation and confirm a mechanism recently proposed for
the photodegradation of PLA in outdoor conditions [32].
Isothermal spherulitic linear growth rates were analyzed
comparatively to extract the influence of molar mass, and chain
functional groups generated during degradation on crystallization.
The molar mass dependence of the spherulitic linear
growth rates for bio and photodegraded samples follows
a power law. However, chain defects, such as the anhydride
groups formed during photo degradation decrease the crystallization
rate compared to biodegraded specimens of equivalent
molar mass. The reduction of the rate is proportional to
the concentration of anhydride groups.
The temperature coefficient of the growth rate, analyzed
according to secondary nucleation leads to one regime, or
a linear dependence for both bio and photodegraded specimens.
The best fits of the experimental data were obtained
with parameters U* w4200 cal/mol and C2 ¼ 51.6. Assuming
growth to proceed in Regime I, the calculated surface free
energy (se) was found to decrease from w85 to 55 erg/cm2
with decreasing molar mass for both degradation types. The
decreasing se infers an energetically favored folding in the
shorter PLA molecules.