RESEARCH NEWS

November 2004 25

Widely used in a range of industries,

polyurethanes (PU) have seldom been

studied for medical applications, partly

because such devices are often

discarded after use. PUs would,

therefore, present an environmental

problem because of their resistance to

disintegration and biodegradation.

A solution to this problem would be to

include a material in the formulation of

PUs that provides biodegradability. For

example, starches, various

polysaccharides, and soybean oil have

all been used in PU foam formulations

to promote foam disintegration.

However, there has been almost no

work done on using cellulose or

cellulose derivatives for this application.

These additions would be an attractive

biodegradable component in PU

formulations because of the range of

their solubility and thermal properties.

Researchers at Bergische Universität

Wuppertal in Germany and División de

Estudios de Posgrado e Investigación

del Instituto Tecnológico de Cd. Madero

in Mexico have studied the use of

widely available cellulose derivatives

carboxymethyl cellulose, cellulose

sulfate, cellulose acetate, and

trimethylsilyl cellulose in PU

formulations [Rivera-Armenta et al.,

Eur. Polym. J. (2004) doi:10.1016/

j.eurpolymj.2004.07.015].

The cellulose derivatives contain varying

numbers of free hydroxyl groups that

react during the formation process of

PU foams and are believed to

chemically bind into the structure. The

researchers used Fourier transform

infrared (FTIR) and 13C nuclear

magnetic resonance (NMR)

spectroscopy to demonstrate that

cellulose derivatives are indeed

incorporated into the PU foam

structure by chemical bonding.

The storage modulus increases with

increasing content of the cellulose

derivatives. The highest value is

obtained for foams prepared with

cellulose sulfate. This derivative is the

only one that modifies the PU foam

thermal properties as well. Cellulose

sulfate delays thermal decomposition

and provides higher glass transition

temperatures. The capacity of the PU

foam to dissipate energy increases with

the amount of cellulose derivative.

Scanning electron microscopy indicates

that each cellulose derivative provides a

different cell shape in the PU foam,

thereby producing a change in

mechanical properties.

John K. Borchardt

Improving polyurethane biodegradabilityPOLYMERS

Reworkable cross-linked polymersPOLYMERS

Cross-linked polymer networks are excellent materials formany applications. However, while their cross-linked structuregives them many positive attributes, it also makes theminsoluble. As a result, it is very difficult to reprocess orrecycle cross-linked polymers without resorting to such hightemperatures that thermal degradation occurs. Controlleddisassembly of the cross-links could result in soluble polymersthat can be reprocessed or disposed of in an environmentallyresponsible manner. Such ‘reworkable’ resins are ofincreasing interest as a means of reducing environmentalproblems associated with polymer disposal. Masamitsu Shirai and coworkers at Osaka PrefectureUniversity in Japan have synthesized novel photo-cross-linkable copolymers that can be dissolved in water afterbaking [Shirai et al., Polymer (2004) 45 (22), 7519]. Theyfirst synthesized monomers containing both an epoxy groupand a sulfonate ester linkage. These monomers were thencopolymerized with tert-butyl methacrylate (TBMA) indegassed tetrahydrofuran or chloroform solution at 50°Cusing azo-bis-isobutyronitrile (AIBN) as an initiator and 1-dodecanethiol as a chain transfer reagent. Upon irradiationof the resulting copolymer with light (wavelength 254 nm) inthe presence of triphenylsulfonium triflate (TPST) and thecomonomer, the TPST reacts with the epoxide ring shown atthe bottom of the linear polymer structure (a). This ringopening forms cross-links between the linear polymer chainsand the resulting cross-linked polymer becomes insoluble.TPST functions as a photoacid generator. Thermal treatmentof the polymer at 120-200°C breaks the cross-links bycleaving the sulfonate ester group to form two separatelinear polymers (b). This cleavage destroys the cross-linksbetween the polymer chains, restoring polymer solubility.

Thus, films and coatings of these polymers can be removedfrom substrates after use by baking and soaking thesubstrates in water.The insolubilization and redissolution behavior are stronglyaffected by the monomer structure and the conditions of boththe polymer photo-irradiation and cross-linked thermolysis. Ofthe polymers studied, poly(7-oxabicyclo[4.1.0]hept-3-yl)methylp-styrenesulfonate) (OHMSS) appears to have the mostuseful properties, particularly its ease of cross-linking andwide effective temperature range (160-220°C) of the baketreatment for dissolution. High solubility in water resultsfrom the p-styrenesulfonic acid moiety produced by thethermal decomposition of p-styrenesulfonate esters (b).OHMSS has a relatively complex chemical structure but maybe synthesized in a two-step reaction. All the cross-linkedpolymers become insoluble again if baked above 220-260°Cbecause of the formation of carboxylic acid anhydride units. John K. Borchardt

SO2

CH=CH2

O

CH2

O

OHMSS monomer

SO2

(CH-CH2)x

O

CH2

O

(CH2-C-)y

CH3

C=O

O

C(CH3)3

linear polymer

(a) Polymerization reaction and (b) cross-link cleavage with thermal treatment.

SO2

(CH-CH2)x

OH

(CH2-C-)y

CH3

C=O

O

C(CH3)3

crosslinked polymer

heat

( O-)n

SO2

(CH-CH2)x (CH2-C-)y

CH3

C=O

O

C(CH3)3

O

CH2

( O-)n

+

(a) (b)