Atactic PHB

In the case of homopolymers, the tacticity of the chains directly influences the material properties. Atactic PHB has an oily consistency and is of less use for plastic industry, whereas completely isotactic PHB shows comparably interesting... 

Bacterial PHBV (3HV content 12%, Mn = 336 000, Mw/Mn = 2,8) was supplied by Aldrich. Atactic PHB (a-PHB Mn = 13 000, Mw/Mn = 1,3) was synthesized via anionic ring opening polymerization of racemic P-butyrolactone initiated with the 18-crown-6-siq)ramolecular complexes of potassium hydroxide. Natural PHB was supplied by Biomer (Germany) and the fiction soluble in chloroform (Mn = 103 000, Mw/Mn = 5,2) was used for experiments.. The binary PHBV/a-PHB blends (contained 30 and 40 wt% of a-PHB, respectively), PHBV/PHB blend (70/30 wt%) as well as PHBV/PHB/a-PHB blend (50/25/25 wt%) were prepared by casting from chloroform solution on glass plates at room temperature. Films with thickness of 0,2-0,3 mm were obtained. The films of PHB and PHBV were prepared similarly. 

A preliminary investigation on the biodegradation of graft copolymer and blends of PVA with chemically synthesized atactic PHB was carried out in an aqueous medium by using a PVA-degrading selected bacterial culture as inoculum. 

Figure 5. Biodegradation curves of atactic PHB and PVA samples and their graft copolymers and blends in an aqueous medium in the presence of PVA-degrading microorganisms.

The limited bioassimilation observed in the case of the aPHB2 sample can be explained by considering that very likely PVA-degrading microorganisms are able to use only the low molecular weight finctions of atactic PHB. 

GPC analysis of the atactic PHB having the highest molecular weight (aPHB2 sample) in the original status and once retrieved fi-om the degradation treatment, clearly evidenced that a small, but significant. 

Blends of various PHAs or PHAs with other biodegradable polymers are commonly used for adjusting the properties of the final materials, while maintaining their biodegradability. The blends of two different PHAs are usually immiscible [175], at least in the crystalline state [176], even in the case of a blend of isotactic bacterial PHB and synthetic atactic PHB, which is miscible only in a limited concentration range [177]. 

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