Chemistry Reference
In-Depth Information
Monomer
O
O
Condensation Polymerization Reaction
HO
HO
OH
OH
low M
w
CH
3
H
H
H
3
C
L-Lactide acid
D-Lactide acid
increased susceptibility to biodegradation
Dimer
O
O
O
Ring Opening Polymerization (ROP) Reaction
CH
3
CH
3
CH
3
O
O
O
high M
w
O
O
O
H
3
C
H
3
C
H
3
C
decreased susceptibility to biodegradation
O
O
O
Meso Lactide
L-Lactide
D-Lactide
FIGURE 13.5
Monomer and dimer conformations of lactic acid before polymerization reaction.
fermentation of agricultural by products
[6,14,18]
. PLA can be polymerized by either
or a combination of condensation polymerization and ring opening polymerization
(ROP), shown in
Fig. 13.5 [18]
. Condensation polymerization reactions produce
low-molecular-weight PLA versus ROP reactions which yield high-molecular-weight
PLA corresponding to increased mechanical properties as well as a higher glass tran-
sition temperature (T
g
) and temperature of degradation
[18]
. Also, molecular weight
(M
w
) of PLA has an impact on susceptibility to microbial attack. Low-molecular-
weight PLA tends to be more susceptible than high-molecular-weight PLA
[18]
.
Another factor that influences PLA's properties is the degree of crystallinity.
Depending on how PLA is synthesized, three stereochemical forms can exist:
poly(L-lactide) (PLLA), poly(D-lactide) (PDLA), and poly(DL-lactide) (PDLLA).
PLLA and PDLA are semicrystalline materials and are optically pure forms. A
mixture of these stereochemical configurations leads to the resulting material
being more amorphous. The crystallinity of the polymer has a noticeable impact
on degradation rates for both water and biodegradation
[18]
. Amorphous PLA
degrades more easily than the crystalline form
[19,20]
. Crystallinity is controlled
by slow cooling or annealing it above the glass transition temperature, T
g
. PLA is
the most abundant bioplastic on the market, with trademark Ingeo (produced by
NatureWorks LLC) having the largest market share.
13.3.3
Production of Polymers by Microorganisms
Bioplastics in this category are polymers produced within the microorganism,
bacteria. The bacteria produce the polymers when a carbon source is in excess
and at least one other nutrient essential for growth has been depleted
[21]
. The
polymers accumulate in the form of intercellular granules as a carbon reserve for
energy
[22]
. The bacteria use sugar from plant feed stocks to fuel this cellular
process
[6]
. This growth and accumulation of polymers within the bacteria occur
in a bioreactor during the fermentation process
[6]
. The polymer from the micro-
organism is extracted by a sequence of isolating the bacteria by means of
