The role of the chain extender on the phase behaviour and morphology of high hard block content thermoplastic polyurethanes: Thermodynamics-Structures-Properties

Thermoplastic polyurethanes (TPUs) have attracted a lot of attention over the last decades in both academic and industrial research. They are multi-block co-polymers with statistically alternating hard (HS) and soft segments (SS) along their structure. Polyurethanes owe their great versatility to the micro-phase separation that takes place between their thermodynamically incompatible HS and SS. The objective of this study was to examine how minor alternations in a specific part of the hard segment (HS) -the chain extender (CE)- affects the micro-phase separation behaviour as well as the morphology and the physical properties of linear TPUs with a high HS% w/w in their structures (?65). For this purpose, the HS of TPUs synthesised in solution were chain extended by four different alcohols: 1,3-propanediol (13PD), 2-methyl-1,3-propanediol (2M13PD), 1,5-pentanediol (15PD) and 3-methyl-1,5-pentanediol (3M15PD). TPU samples containing the same SS and di-isocyanate were found to exhibit the highest crystallisation and phase separation degrees when extended by 13PD. TPUs containing 2M13PD and 15PD demonstrated lower degrees of phase separation. The use of a linear CE led to a higher degree of order, becoming even higher for shorter CEs. Samples incorporating 3M15PD were observed to be totally phase mixed. The reason was proposed to be an enhanced compatibility between the HS and SS resulting from the elongation of the backbone chain of the CE (better mobility) along with the introduction of the CH3 (hindering close packing and therefore crystallisation of the hard domains). This behaviour was also noticeable in SAXS, DMTA and DSC measurements through absence of scattering peaks, softening of the polymers at ambient temperatures and the presence of a single, broad glass transition at room temperature respectively. TGA of as-cast TPUs showed that severe degradation took place at temperatures higher than the observed endotherms in the DSC experiments with the exception of the 13PD. In those samples, simultaneous mixing-degradation was believed to occur during their elevated endothermic transitions. SAXS measurements revealed that the d-spacing was affected when the CE and/or the HS% w/w was altered. Higher values were observed for the linear CEs whereas introduction of chemical branches in the CE seemed to decrease the inter-lamellar spacing through an increase in the thickness of the ?inter-penetration? region. FTIR provided evidence that the phase separation of the different series was directly connected to the amount of the inter-chain hydrogen bonds in the TPU. Mechanical characterisation of the lowest available HS% w/w through tension mode DMTA and melt-rheology, confirmed that the samples with a better phase separation had higher storage moduli at room temperature as well as a higher viscosity at elevated temperatures respectively. Comparisons of the DSC data concerning the phase separation of the TPUs, showed that samples extended by 2M13PD separated upon post-melting heating, whereas the ones containing 15PD segregated upon cooling from the melt. Higher mobility for the HS and SS in the case of 15PD was suggested to be the reason. Complementary characterisation of samples with different thermal histories (melt-quenched, isothermally annealed) came to support those suggestions with additional WAXS, SAXS, and FTIR experiments. Annealing studies were only applied to samples containing 2M13PD and 15PD, since the TPUs prepared using other CEs proved unsuitable for such studies mainly due to extensive thermal degradation.