Modified liquid polysulfide polymers : their preparation, characterisation, photocuring and potential photoapplications
Liquid polysulfide polymers (LPs) H(SCH2CH20CH20CH2CH2S)nH are of wide commercial importance in the sealants industry. Chemical oxidation of the mercaptan groups, by inorganic and peroxide catalysts, leads to the formation of durable, flexible and adhesive elastomers. Terminal modification of the prepolymers, by substituting a wide range of carbonylcontaining groups for the thiol protons, has led to the creation of photopolymerisable oligomers with certain of the desirable properties mentioned above. UV irradiation of all the modified polysulfides, which exist as viscous oils, resulted in the development of a solid elastomeric material with considerable powers of adhesion. The acylation process involves the addition of a slight excess of an acyl halide to the prepolymer in solution, followed by the addition of an amine to neutralise the HCI formed. This procedure has been adapted to react di- and polyfunctional termini with LPs to produce block copolymer polysulfides. When exposed to UV irradiation, the oligomers slowly photocrosslink, forming an insoluble elastomer. The increase in degree of polymerisation has been successfully monitored by 1H NMR spectroscopy; this was achieved by irradiating the oligomers in NMR tubes containing a thin capillary tube containing a deuterium-lock solvent. These new materials were characterised by IR, 1H and 13C NMR, GPC, viscometry and, particularly electrospray and FAB-mass spectrometry. FAB spectra have also been obtained for the first time for the parent polysulfide as well as the modified forms; these enable characterisation of the polysulfide backbone and its irregularities. Characterisation of these cured products has been made by hardness testing, solvent resistance, flexibility and tensometry. Lap shear measurements have been made of all of the functionalised cured polysulfides; adhesion is greatly enhanced by the addition of an adhesion promoter (optimum level 2%). All the systems slowly photopolymerise without sensitisers or additives, but the addition of radical photoinitiators shortens the cure time considerably. The modified LPs can be combined with commercially available polyurethane systems to make a fast curing photosystem yielding products with excellent adhesion and flexibility. Each of the modified LPs contains a carbonyl group, which has photoactive potential, thus excitation of the carbonyl group leads to an n->π* transition to form a reactive triplet state which then attacks another LP chain to crosslink the polysulfide. Monitoring of 1H NMR linewidths shows a gradual broadening of lines as 120 Hz polymerisation proceeds, reaching a plateau value of ΔH1/2=120 Hz, illustrating the elastomeric properties of the polysulfide. Addition of plasticisers eases the uniform spreading of the oligomers onto sheets before irradiating, which gives a potential application as a surface coating. Photoresist imaging agents are another potential application as the modified oligomers act as negative resists.