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)