Polyester chain scissions

Although polymers in-service are required to be resistant toward hydrolysis and solar degradation, for polymer deformulation purposes hydrolysis is an asset. Highly crystalline materials such as compounded polyamides are difficult to extract. For such materials hydrolysis or other forms of chemolysis render additives accessible for analysis. Polymers, which may profitably be depolymerised into their monomers by hydrolysis include PET, PBT, PC, PU, PES, POM, PA and others. Hydrolysis occurs when moisture causes chain scissions to occur within the molecule. In polyesters, chain scissions take place at the ester linkages (R-CO-O-R ), which causes a reduction in molecular weight as well as in mechanical properties. Polyesters show their susceptibility to hydrolysis with dramatic shifts in molecular weight distribution. Apart from access to the additives fraction, hydrolysis also facilitates molecular characterisation of the polymer. In this context, it is noticed that condensation polymers (polyesters, -amides, -ethers, -carbonates, -urethanes) have also been studied much... 

Mahapatro, A., Kumar, A., and Gross, R.A. (2000) Control of polyester chain scission by lipase-catalysis. Polym. Preprint, 41 (2), 1826-1827. 

The thermal degradation behaviour of copolymers containing terephthalaic acid along with other acids, while dependent to some extent on the type of co-acid, is largely controlled by diol content, following the now familiar pathway for polyester chain scission [94-96]. 

An implication of the kinetic analysis presented in Sec. IV.A is that the rate of chain scission of polyesters can be retarded by endcapping to reduce the initial carboxylic acid end-group concentration. Alternatively, the rate may be increased by acidic additives that supplement the effect of the carboxy end groups. The first expectation was confirmed by partial ethanolysis of high molecular weight... 

The rate of hydrolysis of the partially ethoxylated polymer was retarded, although not to the extent calculated from theory (Fig. 25), suggesting some contribution to the rate of chain scission by an uncatalyzed process. End-capping poly (glycolic acid-co-lactic acid) has a similar effect on the rate of hydrolysis of this polyester (100). 

A particular polyester network may be considered as a model of degraded ideal polyester based on a prepolymer having an infinite molar mass with a number of chain scissions equal to Ave = b/2. Thus, polyester samples differing by the initial prepolymer molar mass can be used to calibrate rubber-elasticity measurements. 

Aging does not modify the value of E significantly (except in the close vicinity of Tg), so that aging effects on fracture properties are relatively low in the brittle regime, except if defects are created. This is the case in polyesters (osmotic cracks), where durability is controlled by this process rather than by chain scission or any other structural change at the molecular or macromolecular scale. 

It is believed that chain scission occurs through simple hydrolysis, but the kinetics of this hydrolysis are influenced by anions, cations, and enzymes [190]. The process is autocatalytic and the products of hydrolysis such as carboxylic groups participate in the transition state. Water preferentially enters the amorphous parts but crystalline domains are also affected [125]. The degradation of aliphatic polyesters is believed to be dominated by a hydrolytic mechanism but it is also promoted by enzymatic activities [4,7,191-193]. 

Aliphatic polyesters degrade chemically by hydrolytic cleavage of the backbone ester bonds [38,92,93,143-145] and by enzymatic promotion [35,146]. Hydrolysis can be catalyzed by either acids or bases [38]. Polyester hydrolysis is schematically illustrated and exemplified for PLA in Fig. 5. Carboxylic end groups are formed during chain scission, and this may enhance the rate of further hydrolysis. This mechanism is denoted autocatalysis [147] and makes polyester matrices truly bulk eroding [38,43]. Degradation products are resorbed by the body with a minimal reaction of the tissues [8,15,95,148]. 

SCHEME 2.5 Initial chain scissions in polyester.53 (From Levchik, S.V., Thermosetting polymers, in Plastics Flammability Handbook, Troitzsch, J. (ed.), Hanser, Munich, 2004, pp. 83-98.)... 

The methods of increasing molecular weights of polyesters and poly-amids below the melting point by transesterification and transamidation are worth mentioning as a still only little studied tool to improve crystal perfection by tempering through chain scission. Although this t5q>e of reaction is restricted to few polymer classes it should by proper use allow considerable increase in crystal perfection. 

Polyesters - Aliphatic crosslinked polyesters undergo chain scission and crosslink breakage during photooxidation while using ESCA phenolic hydroxyl groups have been identified in photooxidised poly(ethylene terephtha-late). ° The presence of an optical brightener has been found to enhance the surface stability of poly(ethylene terephthalate). ... 

In general, results from NMR analysis In all media Indicate that the polyester to MDI molar ratio remained roughly constant In aging media which contained water since the degraded polyester Is not extracted. In addition, more change occurred In the molar ratio of polyester to MDI In polymer A than In polymer B during aging. In Table II, the amount of methyl ester (detectable by NMR analysis) Is shown to Increase with methanol content In Isooctane solutions. This provides evidence that the extent of chain scission Is proportional to methanol content. 

Our polymers were crossllnked. On molding, sol fractions Increased and on aging Increased still further, indicating that chain scissions occurred. FU 9 and the commercial polyester type were soluble and showed substantial drops in intrinsic viscosity on aging. The last two polymers in Table 1 are polyether based for these less scission is indicated by the changes in sol fractions and intrinsic viscosity. 

Hydrolytic chain scission of polyesters, polyamides and polycarbonates... 

Radiation-induced crosslinking of acetylene impregnated polymers — Enhanced crosslinking and reduction in chain scission are found in the amorphous regions of polycrystalline polyesters, when they are irradiated in the presence of acetylene [7], Similar effects have been observed in the crosslinking of some biopolymers which are otherwise radiation degradable. 

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