Thermoplastic polyurethanes with varying hard segment components. Mechanical performance and a filler crosslink conversion of hard domains as monitored by SAXS

When monitoring tensile tests of thermoplastic polyurethanes TPU by small angle X ray scattering SAXS we find a filler to crosslink conversion of hard domain function. Its strength is related to the chemical composition and governs the mechanical performance of the TPUs. Acting as fillers, the domains provide a high modulus of elasticity. Once the domains take load, they lose their filler function and the material gains in extensibility. All the five machine cast TPUs have soft segments from PTHF 1000 and a hard segment content amp; 8776; 45 . The hard segments are built from different diisocyanates DI and diols chain extenders, CE . The base material has hard segments made from 1,4 butanediol BD and methylene diphenyl diisocyanate MDI . Two other TPUs contain as DIs either the hydrogenated, isomeric MDI H12MDI or hexamethylene diisocyanate HDI , respectively. In two other materials the CE is varied. Here the BD is replaced by either the shorter 1,3 propanediol PD or by the longer 1,6 hexanediol HD . A morphological model is fitted to the SAXS data. It returns nanoscopic parameters, e.g. discriminating between the total Vt and the crosslinked Vx volume of hard domains. Fillers are Vf Vt amp; 8722;Vx. Results show that Vf domains can be converted into Vx domains. Hydrogenation of the aromatic base DI does not change Vt, but Vx lags behind. Young s modulus is higher filler function, high Vf , but the material breaks earlier low Vx . Generally, Vt increases for small strains strain induced domains, SIDs and decreases for strain gt;1. SIDs start as fillers. When MDI is replaced by HDI the formation of SIDs is boosted leading to strain induced hardening only at low strain. At higher strain the modulus lowers conversion Vf amp; 8594; Vx . Only in this material are so many domains converted that Vx increases during stretching. The material breaks late. The long CE increases the average distance between crosslink domains and narrows the distribution of the distances. With the medium CE domains appear less stable at low strai

Thermoplastic polyurethanes with varying hard segment content. Morphology evolution mechanism under strain

Poster presented at the 16th International Conference on Small-Angle Scattering, held on 13-18th September, 2015, Berlin (Germany).Preparation and composition of thermoplastic polyurethanes (TPU) are varied systematically. The samples are strained and monitored by SAXS. The analysis comprises longitudinal projections [1], and in real space the chord distribution function (CDF) [2]. We aim to identify straining mechanisms and to retrieve characteristic parameters for the modeling to design customized TPU materials. Part of the work is presently being published [3-5]. 3 groups of TPU materials with different hard-segments content are compared. Figure1 demonstrates a fundamental morphological difference between hard-segment (MDI+BD) for HSC=30%, HSC=43% and HSC=56%. An interesting result concerns the occurrence of relaxing HHS sequences when stretching. The systematic variation of sample parameters has led to clear indications herein. Correspondingly, more domains are sacrificed and less experience relief when the HSC is rising from 30% to 43%. Finally relaxation is absent in the material with HSC = 56wt.-%. Material in HSC=56%, the long period increases linearly with strain. Materials with lower HSC% have CDF long periods that reside in stationary bands. Their positions form a Fibonacci series. This sequence of hard and soft domains relates to the synthesis route (polyaddition). The differences are explained by different homogeneity of the reacting mixture of raw materials. Straining mechanisms are discussed

Machine Prepared Thermoplastic Polyurethanes of Varying Hard Segment Content Morphology and Its Evolution in Tensile Tests

Machine cast thermoplastic polyurethanes are strained and monitored by small angle X ray scattering SAXS . They are prepared from 4,40 methylene diphenyl diisocyanate, 1,4 butane diol, and polytetrahydrofuran. Upon stretching hard domains are destroyed. Most stable are the domains of materials with a hard domain content HSC of 30 . Domain stability decreases with increasing HSC and crosslinking. Most materials show stability up to a strain 0.6. At higher strain, the apparent long period decreases for the materials with HSC530 . Correlated hard domains, the strain probes relax as others are destroyed. The fraction of relaxing probes and their ultimate relaxation decrease with increasing HSC. Chord distribution functions computed from the SAXS exhibit the same sequence of static long period bands. The band positions form a Fibonacci series, related to the underlying polyaddition process. This indicates a nearly quasicrystalline arrangement of stringed hard domains, identified as the strain probes of the discrete SAXS. At strains lt;0.6, the probes experience half of the macroscopic strain, which reflects hard domain rigidity. VC 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B Polym. Phys. 2015, 00, 000 00