Preparation And Properties Of Graft Copolymers Of Rigid Macroactivators And Polyamide 6 By Anionic Polymerization

The usage of polyamide 6 (PA6) is limited by some drawbacks, such as high moisture absorption, low dimensional stability and low thermal degradation temperature. The modification of linear PA6 obtained by hydrolytic polymerizations ofε-caprolactam (CL) can easily be modified by physical blending or reactive compatibilization. Blends between linear polyamide 6 and rigid polymers, such as polystyrene (PS), poly(methyl methacrylate) (PMMA) and polyimide (PI), are now readily available. Reactive and nonreactive compatibilizations, however, are not practicable for the modification of monomer casting polyamide 6 (MCPA6) prepared by anionic polymerization of CL, which commonly results in a crosslinked and structural irregular polymer because of Claisen-type condensations and other side reactions.During the anionic polymerization of CL, an activator is commonly added to speed up the activation step. Thereby functionalized MCPA6 can be prepared by chemically modifying activators, i.e., by functionalized macroactivators. Premade polymers with pendants of esters and imides were used as the functionalized macroactivators. These pendants reacted with CLs to form N-carbamated caprolactam (CCL) moieties, from which PA6 chains grew. However, the activation capacities of esters and imides are relatively low. The time necessary for the completion of polymerization is longer (more than two hours) than the mean residence time of a typical MCPA6 (a few minutes).In this paper, CCL pendant was incorporated into PS, PMMA and poly(styrene-alt-N-phenylmaleimide) (PS-alt-NPMI) by free radical copolymerization for preparing rigid macroactivators. CLs were grafted onto the macroactivator via initiating CCLs along its backbone to form graft copolymers. The modified MCPA6 has lower moisture absorption, higher dimensional stability and thermal degradation temperature. The properties of graft polymers were also investigated. The usage of macroactivators resulted in a polymerization time of less than 30 min.(1) An allyl monomer with CCL moiety (ACCL) was prepared by 2,4-toluene diisocyanate (TDI) reacting with CL and allyl alcohol successively. Three kinds of macroactivators, PS-CCL, PMMA-CCL and (PS-alt-NPMI)-CCL, were prepared by solution copolymerization of ACCL and rigid monomers. ACCL and the macroactivators were characterized by 1H-NMR, FTIR and element analysis. Molar fraction of CCL on rigid macroactivator was calculated by 1H-NMR.(2) CL has been grafted onto rigid macroactivators to form graft copolymers, i.e., PS-g-PA6, PMMA-g-PA6 and (PS-alt-NPMI)-g-PA6, via anionic ring-opening polymerization in the presence of catalyst sodium caprolactam. FTIR, 1H-NMR and the selective solvent extraction consisted of using methanol and chloroform as extracting solvents were used for the evaluation of graft copolymers. The single Tg indicates that there is only one relatively homogeneous structure in the product. The activating mechanism of macroactivator was discussed.(3) Graft copolymers with different content of rigid polymers were synthesized to study the effect of rigid polymer on morphology, crystallinity, dimensional stability and thermal properties, using scanning electron microscopy, X-ray diffraction, water absorption measurement, thermogravimetric analysis and differential scanning calorimetry. For comparison, pure PA6 activated by ACCL were also investigated. Rigid polymers were well dispersed in PA6 matrix. As the rigid macroactivator content increases, the glass transition temperature of graft copolymer decreases, but the percentage crystallinity and the crystalline melting temperature increase. These variations can be attributed to the fine dispersion of rigid polymer in PA6 matrix, which increases the disorder of system and destroys the crystallinity of PA6. The copolymers PS-g-PA6 and PMMA-g-PA6 have lower moisture absorption and higher dimensional stability than pure PA6. Upon the incorporation of 18.3 wt % PS into the polymeric system, the water absorption of PS-g-PA6 decreased from 4.8% of pure PA6 to 1.9%. The PMMA-g-PA6 with 19.9 wt % PMMA decreased to 2.1%. The copolymers PS-g-PA6 and (PS-alt-NPMI)-g-PA6 have improved thermal properties. Upon the incorporation of 18.3 wt % PS into the polymeric system, the degradation temperature (Td) of PS-g-PA6 is about 45oC higher than that of pure PA6. When the sample loses 20 % of its original weight, the temperatures of (PS-alt-NPMI)-g-PA6 copolymers are about 40-60oC higher than that of pure PA6 upon the incorporation of 7.2-28.8 wt % PS-alt-NPMI into PA6. The incorporation of PS and PS-alt-NPMI into PA6 matrix for the graft copolymer slows down the degradation rate of PA6 at low temperature, and contributes to the protection of matrix. However, the Td value of PMMA-g-PA6 decreased to 275℃because of the degradation of PMMA.