Solid-state modification of polyamide-6,6

Polyamide-6,6 (PA-6,6) is well known and used in many applications because of the high dimensional stability under dry conditions, good mechanical and thermal properties, and excellent solvent resistance. Also polyamide copolymers have gained attention, since additional properties can be introduced while retaining the advantageous properties. PA-6,6 is mainly synthesized by using a condensation method with hexamethylenediamine and adipicacid to a medium molecular weight (Mn ca. 25 kDa) and later the Mn is increased via a solid-state post-condensation (SSPc) by subjecting the prepolymer to higher temperatures (ca. 180-200 °C) for 8-24 hours. Copolyamides are conventionally prepared via melt- or solution-polymerization and have a random chemical microstructure and, as a result, have a lower crystallinity and lower crystallization (Tc) and melting (Tm) temperature. The SSM process of a semi crystalline polycondensate, viz. poly(butylene terephthalate) (PBT) was already reported by Cor Koning et al. The SSM modification resulted in an non-random incorporation of significant amounts of comonomer (e.g. 2,2-bis[4-(2-hydroxyethoxy)phenyl]propane, bis(2-hydroxyethyl)terephthalate and 2,2’-biphenyldimethanol) into PBT without completely losing the crystallinity of the resulting block-like copolyester. This is due to the fact that the reaction was limited only to the mobile fraction of the amorphous phase, leaving long crystallizable sequences of pure PBT in the main chain. As SSM exclusively occurs in the mobile amorphous phase of the polymer, the glass transition temperature is enhanced depending on the rigidity of the aromatic comonomer and the Tc, Tm is nearly retained from the starting material. The main objective of this research was to modify the semi-crystalline PA-6,6 with an aromatic comonomer in the solid state with the enhancement of Tg and retention of the crystallization behavior. For the first time for polyamides, a solid-state modification of the amorphous phase of a polyamide was performed below the melting temperature. Semi-aromatic para- and meta-xylylenediamine:adipic acid ‘nylon salts’ were incorporated into the main chain of PA-6,6. Since the PA-6,6 chain segments present in the crystals during the solid-state modification (SSM) process do not participate in the transamidation reactions, the SSM reaction results in relatively long blocks of pure PA-6,6 segments and blocks of copolyamide segments. As expected, the kinetics of the transamidation is much faster at 230 than at 200 °C and the SSM-modified copolyamide initially shows a significant molecular weight reduction by chain scission by transamidation with the incorporated salt, followed by a built-up of the molecular weight to a value close to the starting value after 8-12 h of reaction time. At higher reaction temperatures branching and cross-linking reactions, resulting in gel formation, are observed after 8 h of reaction. In order to understand the kinetics of the SSM reactions, copolyamides, (PA66xMxAdy) with different mol% of MxAd were synthesized via solid-state modification (SSM) and melt polymerization (MP). The progress of the transamidation reaction as revealed by the initial decrease in molecular weight during the incorporation of the aromatic comonomer salt was monitored by using SEC. The molecular weight Mn,SEC decreases at a higher rate during the first transamidation step when higher molar percentages of MxAd salt were premixed with the PA-6,6 homopolymer. This molecular weight decrease, caused by chain scission, is followed by a built-up of Mn,SEC due to post-condensation of the free amine and acid end groups resulting from the first transamidation step. The SSM reaction mechanism was investigated and it was found that the transamidation or chain scission was initiated by protonated amine species (-NH3+). It was also observed that neither an excess of diamine nor an excess of dicarboxylic acid in the salt resulted in a significant change in the kinetics of the transamidation reaction with respect to the equimolar salt system, since for both cases no significant change was observed in the number-average molecular weight (Mn) development as a function of time. An in-depth study on the chemical microstructure was performed on the (PA66xMxAdy)SSM and (PA66xMxAdy)MP samples with similar overall compositions. The sequence analysis based on solution 13C NMR experiments confirmed that the copolyamides prepared via SSM and containing 20-30 mol% of MxAd salt in the feed exhibited a more blocky structure with overall degrees of randomness Rtotal of ca. 0.4. On the other hand, the melt-polymerized copolyamides having a similar overall composition exhibited Rtotal values close to unity, typical for fully random copolymers. The earlier mentioned presumption that during SSM the transamidation reactions selectively occur in the amorphous phase, leaving the homopolymer of crystalline PA-6,6 intact, was further supported by the observation that the (PA66xMxAdy)SSM products exhibited only slightly lower degrees of crystallinity and enthalpies of melting as compared to pure PA-6,6, which can only be explained by the presence of long homo PA-6,6 blocks in the SSM products. The thermal analysis of the copolyamides prepared via SSM and MP showed that SSM copolyamides have a higher melting temperature compared to copolyamides with similar composition prepared via MP. The relatively high crystallization temperature of the SSM copolyamides results in significantly lower super coolings in comparison with MP copolyamides with similar chemical composition. Accordingly, the SSM copolyamides are more suitable for injection-molding applications. The higher degree of crystallinity of the SSM copolyamides compared to the MP samples, results a higher glass transition temperature of the SSM copolyamide because of its relatively higher content of the rigid MXDA monomer residues in the amorphous phase. Finally, it turned out that the block-like microstructure of the SSM copolyamides, with all its advantages with respect to thermal transitions and crystallizability, is quite robust with respect to melt processing: even after a residence time in the melt for 15 min at 290 °C the crystallization and melting temperatures remain significantly higher than those of the corresponding random MP counterparts, suggesting that full randomization of the structure does not occur. Next, the structure of the copolyamides using WAXD, temperature variable ss-NMR and DMTA was analyzed. The SSM copolyamides prepared using PA-6,6 and isomeric aromatic comonomers, viz. (PA66xMxAdy) and (PA66xPxAdy), were investigated by using WAXD. It was found that the aromatic PXDA moieties can co-crystallize with the PA-6,6 repeat units. On the other hand, the MXDA-modified SSM copolyamide had a very similar WAXD pattern as the PA-6,6, implying that the MXDA residues do not co-crystallize with the 6,6 repeat units. The temperature-variable solid-state 13C NMR study on (PA6680MxAd20)SSM confirmed that the aromatic moieties (MXDA) are indeed only present in the amorphous phase, as the peaks corresponding to the phenyl group (ca. 128-140 ppm) became broader during heating and upon further heating beyond the glass transition temperature the peaks disappeared due to the increased dynamics of the chain segments. An alternative approach to investigate the transamidation reaction and to study whether it can also yield a copolyamide containing comparable block length was performed in the melt extrusion process. The (PA6680MXD620)Ext product obtained by melt-mixing of the two homopolymers, i.e. PA-6,6 and MXD6, was analyzed in detail. Interestingly, the sequence analysis revealed that the melt-mixed homopolyamides (PA6680MxAd20)Ext exhibited longer aromatic blocks with lower R values (ca. 0.25) than that of the SSM copolyamide (R ca. 0.4) with a similar overall composition. The crystallization kinetics of the melt-mix product was examined using fast-scanning calorimetry (FSC) and it was found that the melt-mixed product exhibited a higher rate of crystallization compared to copolyamides prepared via SSM and MP route with a similar composition. Furthermore, the moisture absorpotion studies revealed that the (PA6680MxAd20)Ext product has the lowest moisture absorption (ca. 4.1 %) due to the presence of longer sequences of the aromatic MXD6 moieties in the amorphous phase compared to the shorter sequence lengths obtained via SSM using MxAd comonomers. Although the degree of crystallinity of the (PA6680MxAd20)SSM copolyamide is comparable (ca. 35%) to PA-6,6 (ca. 40%), it exhibited a lower moisture absorption (ca. 4.6 %) compared to pure PA-6,6 (ca. 6.3 %), which is due the presence the hydrophobic aromatic moiety present in the amorphous phase of SSM copolyamide. From the overall properties analysis, it can be concluded that the melt-mixed copolyamides exhibit the best thermal and mechanical properties of all studied materials, which had a comparable molecular weight. In comparison to the starting PA-6,6, the SSM-product showed some improvement of the properties, while the MP copolyamides demonstrated a deterioration of the properties due to the random distribution of the rigid aromatic moieties. Although the melt-mixed copolyamides exhibited better properties than the SSM copolyamides, the SSM process can be most preferred industrially for any tailor-made product and have flexibility of different comonomers to impart various properties, such as flame retardancy, flow improvement, toughening etc. The SSM route can be used to prepare copolyamides with thermally less stable monomers or polymers that cannot be processed via melt-mixing route.