Origin of Multiple Melting Endotherms in a High Hard Block Content Polyurethane. 2. Structural Investigation

Abstract

The structure and morphology of a set of high hard block content (50% to 100% hard segment by weight) linear thermoplastic polyurethanes has been investigated mainly via small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). All the results obtained in this investigation have been correlated to our previous work on the thermodynamic properties of the same set of samples (Saiani et al. Macromolecules 2001, 34, 9059). The soft segment was based on a poly(propylene oxide) polyol end-capped with ethylene oxide and the hard segment on a 4,4‘-methylenediphenyleneisocyanate (MDI) chain extended by 2-methyl-1,3-propanediol (MP−Diol). Samples with a hard segment volume fraction higher than 65% are shown to have a morphology consisting in soft phase domains embedded in a hard phase matrix. Alignment of the soft phase domains could be observed under specific preparation conditions. From our SAXS results, the same average interdomain distance was found for all the samples (d i ≈ 15 nm). These results confirm our DSC results and suggest a two-phase structure for the melt-quenched samples: “pure” hard segment phase + mixed phase (soft + hard segments) with a hard segment content of 65 wt %. The mixed phase then undergoes phase separation during the annealing at 120 °C. The dynamics of the phase separation was also investigated showing a strong correlation between the peak observed in the scattering curves and the so-called T MMT melting endotherm. The results confirm the assignment of this endotherm to the microphase mixing of the soft and hard segments. The rate of phase separation was found to be a function of the hard segment content of the samples, and a delay time was found before the start of the phase separation process for the high hard block content samples. The degree of phase separation was calculated and found to be the same for all the samples except the PU-50%HS sample, which showed a higher degree of phase separation. The interface thickness is found to increase with increasing hard segment content.

Linking the morphology of a high hard segment content polyurethane to its thermal behaviour and mechanical properties

“…Spherical or cylindrical morphologies [1] have been reported in MDI extended with ethylene diamine, while a morphology without a specific shape was reported for MDI extended with 2-methyl-1,3-propane diol [2] or with butane diol [3]. The TPU morphology affects the mechanical properties [4,5].…”

“…The TPU nanomorphology is commonly visualised by Atomic Force Microscopy (AFM) [8][9][10] or Transmission Electron Microscopy (TEM) [2,5]. However, as these techniques are 1…”

Specific interactions, structure and properties in segmented polyurethane elastomers

“…The results showed that structural units of various forms and sizes develop during polymerization [13][14][15]. Besides a crystalline or at least highly ordered phase, soft and hard segments form corresponding phases which are partially soluble in each other [16][17][18][19][20][21]. Solubility depends very much on the interaction of the components, the urethane groups are capable of forming specific interactions, H-bonds with each other, but also with the soft segments [22,23].…”

Solid particle erosion and viscoelastic properties of thermoplastic polyurethanes

“…Both drop-matrix (Figure 1a and 1d) and co-continuous (Figure 1b and 1c) morphologies can be recognized: in the former, isolated domains (dark) of the soft phase are suspended in the hard matrix (bright); in the latter, the phases are both continuous and appear interpenetrated. Further quantitative information can be collected through SAXS analysis, which is often employed when dealing with polyurethane systems [8,18]. The one-dimensional SAXS patterns of intensity I(q) vs. q of the various TPUs are shown in Figure 2.…”

“…Less intensive and broader peaks are instead noticed for the other samples, suggesting more homogeneous structures in which ordered crystalline and/or para-crystalline hard domains appear randomly dispersed in a continuous soft matrix. An approximate estimate of the average inter-domain distance can be inferred from the position of the intensity peak [18]. The data, computed by means of the Bragg's law, are listed in Table 2.…”