Pederson et al. (2006) synthesized PHA-containing block copolymers in Cupriavidus
necator (also called R. eutropha) using periodic substrate addition. PHB segments
were formed during fructose utilization. Pulse feeds of pentanoic acid resulted in
the synthesis of (R)-3-hydroxyvalerate (3HV) monomers, forming PHBV random
copolymer. A combination of characterization techniques applied to the polymer
batches strongly suggests the presence of block copolymers. Analysis of thermodynamically
stable polymer samples obtained by fractionation by differential scanning
calorimetry and nuclear magnetic resonance spectroscopy indicates that
approximately 30% of the total polymer sample exhibits melting characteristics and
nearest-neighbor statistics indicative of block copolymers. Rheology experiments
indicate additional mesophase transitions only found in block copolymer materials.
In addition, dynamic mechanical analysis shows extension of the rubbery plateaus
in block copolymer samples, and uniaxial extension tests result in differences in
mechanical properties (modulus and elongation at failure) expected of similarly
prepared block copolymer and single polymer type materials.
McChalicher and Srienc (2007) showed that films consisting of block copolymers
retained more elasticity over time with respect to films of similar random
copolymers of comparable composition. Two PHBV films containing either 8 or
29% 3HV exhibited a quick transition to brittle behavior, decreasing to less than
20% elongation at fracture within a few days after annealing. Conversely, the block
copolymer samples had higher than 100% elongation at fracture a full 3 months
after annealing. Because block copolymers covalently link polymers that would
otherwise form thermodynamically separate phases, the rates and degrees of crystallization
of the block copolymers are less than those of the random copolymer
samples. These differences translate into materials that extend the property space
of biologically synthesized scl PHA.
Wu et al (2008) succeeded in producing PHB–poly(d,l-lactide) (PLA)–poly
(e-caprolactone) triblock copolymers using a low molecular weight methyl-PHB
oligomer precursor as the macroinitiator through ring-opening polymerization with
d,l-lactide and e-caprolactone. The triblock copolymers exhibited flexible properties
with good biocompatibility.
Pederson et al. (2006) synthesized PHA-containing block copolymers in Cupriavidusnecator (also called R. eutropha) using periodic substrate addition. PHB segmentswere formed during fructose utilization. Pulse feeds of pentanoic acid resulted inthe synthesis of (R)-3-hydroxyvalerate (3HV) monomers, forming PHBV randomcopolymer. A combination of characterization techniques applied to the polymerbatches strongly suggests the presence of block copolymers. Analysis of thermodynamicallystable polymer samples obtained by fractionation by differential scanningcalorimetry and nuclear magnetic resonance spectroscopy indicates thatapproximately 30% of the total polymer sample exhibits melting characteristics andnearest-neighbor statistics indicative of block copolymers. Rheology experimentsindicate additional mesophase transitions only found in block copolymer materials.In addition, dynamic mechanical analysis shows extension of the rubbery plateausin block copolymer samples, and uniaxial extension tests result in differences inmechanical properties (modulus and elongation at failure) expected of similarlyprepared block copolymer and single polymer type materials.McChalicher and Srienc (2007) showed that films consisting of block copolymersretained more elasticity over time with respect to films of similar randomcopolymers of comparable composition. Two PHBV films containing either 8 or29% 3HV exhibited a quick transition to brittle behavior, decreasing to less than20% elongation at fracture within a few days after annealing. Conversely, the blockcopolymer samples had higher than 100% elongation at fracture a full 3 monthsafter annealing. Because block copolymers covalently link polymers that wouldotherwise form thermodynamically separate phases, the rates and degrees of crystallizationof the block copolymers are less than those of the random copolymersamples. These differences translate into materials that extend the property spaceof biologically synthesized scl PHA.Wu et al (2008) succeeded in producing PHB–poly(d,l-lactide) (PLA)–poly(e-caprolactone) triblock copolymers using a low molecular weight methyl-PHBoligomer precursor as the macroinitiator through ring-opening polymerization withd,l-lactide and e-caprolactone. The triblock copolymers exhibited flexible propertieswith good biocompatibility.
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