Separation of ¹³C spectra of polyurethane soft and hard segments by ROSY

The ROSY (Relaxation Ordered SpectroscopY) is a method in which the ¹³C CPMAS spectrum of a mixture is classified by a longitudinal relaxation time of ¹H, and the ¹³C CPMAS spectrum is displayed separately for each substance. In solution NMR, each peak in the ¹H spectrum has its own longitudinal relaxation time. In solid-state NMR, however, spin diffusion occurs due to the dipolor interaction between ¹H, and all ¹H have the same longitudinal relaxation in the domain within a certain distance. The ¹³C spectrum can be separated for each domain by using this difference in relaxation time of ¹H. The longitudinal relaxation time (T₁^H) obtained by the saturation recovery method as shown in Fig.1a is usually used to separate the ¹³C spectrum of the mixture. The size of the domain that can be separated by this method is about 100 nm. To separate domains smaller than this, a measurement using the relaxation time at rotational flame (T_1ρ^H) obtained by the spinlock method as shown in Fig.1b is effective. The domain size that can be separated by T_1ρ^H is about several nm, and it is possible to determine the phase separation structure of block copolymers and the molecular compatibility.

Fig.1 Pulse sequence diagrams of saturation recovery (T₁^H) based ROSY(a) and spinlock (T_1ρ^H) based ROSY(b).

T_1ρ^H based ROSY – Separation of soft segment and hard segments of polyurethane –

As a measurement example of the T_1ρ^H based ROSY, we will introduce the separation of soft and hard segments of the ¹³C spectrum of polyurethane (a smart phone cover case). Polyurethane has soft and hard segments in the molecule, each of which aggregates between the molecules to form their respective domains. A typical method for distinguishing between soft and hard segments is measuring CPMAS which tends to observe hard segments, and the LD (Low power Decoupling) MAS which tends to observe soft segments (Fig.2). Although it is possible to see the difference in segments to some extent in the both measurements, it is often difficult to assign domains because CPMAS observes soft segments as well as hard segments, and LDMAS observes hard segments as well as soft segments in small increments. On the other hand, the ROSY separates the spectrum from spatial differences through ¹H spin diffusion, allowing the two segments to be separated more clearly. Fig.3 shows the T_1ρ^H based ROSY spectrum. In the T₁^H-ROSY spectrum, both segments cannot be separated (not shown). On the other hand, in the T_1ρ^H-ROSY spectrum, the soft segment and the hard segment can be clearly separated. In addition, this indicates that the domain size of the segment ranges from a few nm to a few tens of nm.

JNM-ECZ500R
3.2mm HXMAS probe
MAS=5kHz

Fig.2 ¹³C CPMAS and LDMAS spectra of poly urethane.

Fig.3 A T_1ρ^H -ROSY spectrum of poly urethane.