MECHANISM OF SET OF DISULFIDE-CROSSLINKED POLYCAPROLACTAM FIBERS IN VARIOUS AQUEOUS MEDIA

Abstract

Two kinds of disulfide-crosslinked polycaprolactam (DSPC) fibers with different disulfide (SS) contents (M2 and M4) were prepared from N-mercaptomethylated polycaprolactams and the set of them in various aqueous media was investigated.When M2 (sulfur content, 1.2%) was treated at 100% extension in cold water for 5 min, temporary set (measured after relaxation in the air) of 60% was obtained. The set did not increase significantly with prolonged setting treatment. When M2 was kept at 100% extension in cold water, the initial rapid decay of the stress followed by levelling-off was observed. It is considered that the mechanism of set of M2 in cold water is rearrangement of hydrogen-bonded network stabilizing the set structure. The temporarily set M2 supercontracted when relaxed in boiling water, since the hydrogen-bonded network formed during setting was broken in boiling water. M4 (sulfur content, 3.3%) was rubbery in water and could not be set by treatment with cold water.When M4 was treated in boiling water for 60 min, temporary set of 50% was obtained. Permanent set of 22% was retained when the set fiber was relaxed in boiling water for 60 min. A smaller permanent set of 9% was obtained by the treatment of M2 in boiling water for 60 min. Both M2 and M4 contained a small amount of mercapto (SH) groups. M2 modified with 2, 4-dinitrofluoro-benzene, which blocks SH, could not be set with boiling water. The controlling mechanism of set and its release in boiling water appears to be rearrangement of crosslinked structure via SH/SS interchange reaction.When M2 and M4 were treated in aqueous ammonium thioglycolate at 25°C, large amounts of temporary sets were obtained. The stress relaxation and X-ray diffraction studies revealed that SS crosslinks were ruptured in a short time and DSPC fibers underwent orientation and crystallization. The set was lost when the fibers were relaxed in boiling water.When M4 was treated in 0.5% KCN at 50°C for 1hr, the highest amound of permanent set (67%) was obtained. It is considered that the set with KCN is due to the breakdown of SS crosslinks and subsequent formation of monosulfide crosslinks which are stable in boiling water. M4 pretreated with KCN solution could not be set with boiling water. This is another supporting evidence for SH/SS interchange mechanism to the setting in boiling water.The mechanism of set of wool keratin fibers was discussed in relation to the above results.