Polycaprolactone hydrolysis

Thermoplastic polyurethane elastomers have now been available for many years (and were described in the first edition of this book). The adipate polyester-based materials have outstanding abrasion and tear resistance as well as very good resistance to oils and oxidative degradation. The polyether-based materials are more noted for their resistance to hydrolysis and fungal attack. Rather specialised polymers based on polycaprolactone (Section 25.11) may be considered as premium grade materials with good all round properties. 

Hillshafer et al. reported that aromatic polyester urethanes based on orthoph-thalic anhydride had better hydrolysis resistance than polycaprolactone urethanes, despite high acid numbers [91]. 

Polybutadienes, polycaprolactones, polycarbonates, and amine-terminated polyethers (ATPEs) are shown in Scheme 4.4 as examples of other commercially available polyols. They are all specialty materials, used in situations where specific property profiles are required. For example, ATPEs are utilized in spray-applied elastomers where fast-reacting, high-molecular-weight polyamines give quick gel times and rapid viscosity buildup. Polycarbonates are used for implantation devices because polyuredtanes based on them perform best in this very demanding environment. Polycaprolactones and polybutadienes may be chosen for applications which require exceptional light stability, hydrolysis resistance, and/or low-temperature flexibility. 

When polyester-hydrolyzing activity was isolated using synthetic polyesters such as polycaprolactone, and the enzyme was examined in detail, it was found that it was a cutinase that was responsible for the hydrolysis [113]. Similarly, the polyester domains of suberin were found to be degraded by cutinase. Cutinase is a polyesterase, and similar enzymes may be widely distributed and can degrade a variety of natural and synthetic polyesters. Microbial polyhydroxy-alkanoic acids that are attracting increasing attention as biodegradable polyesters can be hydrolyzed by bacterial polyesterases that share some common features with cutinases [114] and this area is covered in another chapter [115]. 

Figure 7.7 SAXS profiles for two hydroxyl-terminated oligomers crosslinked by alkoxysilane sol-gel chemistry. First, 1 mole of macrodiol, SS (hydrogenated polybutadiene, HPBD or polycaprolactone, PCL, Mn= 2 kg mol-1), was reacted at 80°C with 2 mole of dicyclohexylmethane diisocyanate, H12 MDI. After complete reaction, the prepolymer was dissolved in tetrahydro-furan and the y-aminosilane, yAPS was added dropwise at room temperature. After 1 h of reaction, the solvent was removed under pressure. The final network was obtained in the absence of a solvent by hydrolysis and condensation of the ethoxysilane groups by the addition of 0.1 mol% TFA, trifluor-oacetic acid. After stirring at room temperature, the mixture was cast into a mold and cured for 24 h at 100°C under pressure, and then postcured at 150°C for 12 h. (Cuney et al., 1997 - Copyright 2001, Reprinted by permission of John Wiley Sons, Inc.)...

Polycaprolactone is relatively stable against abiotic hydrolysis, but it was shown early that it is degraded by microorganisms [69]. The molecular weight of PCL remained unchanged during the degradation and it was stated that the biotic... 

As pointed out by Heller (2), polymer erosion can be controlled by the following three types of mechanisms (1) water-soluble polymers insolubilized by hydrolytically unstable cross-links (2) water-insoluble polymers solubilized by hydrolysis, ionization, or protonation of pendant groups (3) hydrophobic polymers solubilized by backbone cleavage to small water soluble molecules. These mechanisms represent extreme cases the actual erosion may occur by a combination of mechanisms. In addition to poly (lactic acid), poly (glycolic acid), and lactic/glycolic acid copolymers, other commonly used bioerodible/biodegradable polymers include polyorthoesters, polycaprolactone, polyaminoacids, polyanhydrides, and half esters of methyl vinyl ether-maleic anhydride copolymers (3). 

Hakkarainen [19,36] Poly(vinyl chloride)/ polycaprolactone-carbonate blend Thermo-oxidation, hydrolysis 6-Hydroxyhexanoic acid, caprolactone PDMS-DVB... 

Fig. 4 Migration of 6-hydroxyhexanoic acid from different polycaprolactone homopolymers during hydrolysis in phosphate buffer (A) porous structure, ( ) linear disc and (x) network. Reprinted from [73] with permission of Taylor Francis. Taylor Francis (2007)...

Polycaprolactone (PCL) polyols, due to the presence of a relatively long repeated hydrophobic segment -(CH2)5- are recognised as polyesters which lead to polyurethanes with good resistance to hydrolysis [7, 16]. 

The type of enzyme to be used, and quantification of degradation, will depend on the polymer being screened. For example, Mochizukiet studied the effects of draw ratio of polycaprolactone (PCL) fibres on enzymatic hydrolysis by lipase. Degradability of PCL fibres was monitored... 

Mochizuki, M., Hirano, M., Kanmuri, Y., Kudo, K., and Tokiwa, Y., 1995, Hydrolysis of polycaprolactone fibers by lipase - Effects of draw ratio on enzymatic degradation./ Appl. Polym. Sci. 55 289-296. 

Some other specialised polyester polyols, like polycarbonate and polycaprolactone, possess superior hydrolytic resistance. Aliphatic polyhydrocarbon polyol such as hydroxyl terminated polybutadiene (HTPB), is advantageous in many respects. These substances are resistant to acidic or basic hydrolysis, possess good adhesion and can be used where low polarity and good electrical insulation are required. 

Polycaprolactone TPUs have the inherent toughness and resistance of polyester-based TPUs combined with low temperature performance and a relatively high resistance to hydrolysis. They are an ideal raw material for hydraulic and pneumatic seals. 

Similarly, the chitin whiskers are made by partial hydrolysis, and incorporated biopolymers such as polycaprolactone (Morin and Dufresne 2002), natural rubber (Nair and Dufresne 2003), and SPI (Lu et al. 2004). Nowadays, different sources of chitin are treated to obtain nanocrystals (Goodrich and Winter 2007). 

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