The synthesis, morphology, and properties of segmented poly(ether amide)s based on flexible PTHF segments (M n = 1.1 × 103 g mol–1) and uniform rigid oxalamide segments were investigated. The amount of oxalamide groups in the hard segment and the spacer length of bisoxalamide-based hydrogen bonded arrays were varied systematically. The segmented poly(ether amide) with single oxalamide groups connecting the polyether blocks was a sticky solid with a melting temperature of ∼25 °C. Incorporation of uniform hard segments consisting of two interconnected oxalamide units provided highly phase-separated thermoplastic elastomers with a broad temperature-independent rubber plateau. By decreasing the aliphatic spacer length separating two oxalamide units from 10 to 2 methylene groups, the melting transitions increased from 140 to 200 °C. FT-IR evidenced strongly hydrogen bonded and highly ordered bisoxalamide hard segments with degrees of ordering between 66 and 90%. AFM revealed the presence of fiber-like nanocrystals with lengths up to several hundreds of nanometers randomly dispersed in the soft PTHF matrix. The long dimension of the crystals was found parallel to the direction of the hydrogen bonds. One of the other small dimensions of the crystal approximately equals the length of the oxalamide segment, whereas the other one may correspond to the height of a stack containing ca. 10–20 hydrogen-bonded sheets. Upon heating, the crystalline phase melts over a broad temperature range to give a homogeneous melt according to temperature-dependent FT-IR, SAXS, and rheology data. Increasing the number of oxalamide groups in the hard segment to three afforded a highly phase-separated material with a melting transition above 200 °C. The segmented copolymers with two or three oxalamide groups in the hard segment show a distinct yield point and have an elastic modulus between 121 and 210 MPa, a stress at break ranging from 15 to 27 MPa, and strain at break of 150 up to 900%. The results demonstrate that alternating block copolymers with soft PTHF segments and uniform hard segments containing two or three oxalamide groups are TPEs with good thermal and mechanical properties.
Thermoplastic elastomers (TPEs) are known to exhibit a phase-separated morphology which depends on their chemical structure and processing. The design of novel TPEs with predefined properties which are also independent of the material thermal history has so far remained a challenge. The focus of this work is on the semicrystalline morphology of allaliphatic thermoplastic elastomers consisting of alternating polytetrahydrofuran (PTHF) segments and uniform glycine or β-alanine bisoxalamide units. The thickness of the hard segment crystals was found to be highly monodisperse and independent of the sample thermal history. Using Nanocalorimetry, we observed that at cooling rates as high as 12 000 °C s −1 the bisoxalamide segments can still crystallize although the crystallization temperature decreases by ca. 26 °C. The surface free energy of the hard block crystals is found to be extremely low (∼18 mJ•m −2 ), which is likely due to the entropic contribution of soft segments forming tie chains bridging the neighboring crystals. To investigate the combined effect of crystal orientation and phase transitions, simultaneous time-resolved X-ray scattering and mechanical tensile tests were performed. Upon stretching, elastomeric PTHF segments with lengths above 1000 g mol −1 crystallize at ambient temperatures. Under these conditions two main morphologies were observed: at low strains the long axes of the fibril-like crystals were oriented parallel to the flow direction, whereas higher strains caused bisoxalamide crystal fragmentation and changed their preferential direction to the one perpendicular to the drawing direction. The chain tilts in the bisoxalamide crystals were calculated from the characteristic fourspot SAXS patterns and were ∼5°−16°in the case of glycine end-groups and 24°for alanine and propyl terminal groups. To our knowledge, this is the first attempt to determine the chain tilt for the nonlamellar crystals in block copolymers.
Abstract13C nuclear magnetic resonance (NMR) is traditionally considered an insensitive technique, requiring long acquisition times to measure dilute functionalities on large polymers. With the introduction of cryoprobes and better electronics, sensitivity has improved in a way that allows measurements to take less than 1/20th the time that they previously did. Unfortunately, a high Q‐factor with cryoprobes creates baseline curvature related to acoustic ringing that affects quantitative NMR analyses. Manual baseline correction is commonly used to compensate for the baseline roll, but it is a time‐intensive process. The outcome of manual baseline correction can vary depending on processing parameters, especially for complicated spectra. Additionally, it can be challenging to distinguish between broad peaks and baseline rolls. A new anti‐ring pulse sequence (zgig_pisp) was previously reported to improve on the incumbent single pulse experiment (zgig). The original report presented limited comparison data with 13C NMR, but a thorough validation is needed before broader implementation can be considered. In this work, we report the round‐robin testing and comparison of zgig_pisp and zgig pulse sequences. During the testing phase, we found that zgig_pisp is practically equivalent to zgig to ±2% for the majority of integrals examined. Additionally, a short broadband inversion pulse (BIP) was demonstrated as an alternative to the originally reported adiabatic CHIRP shaped pulse. The zgig_pisp pulse sequence code for Bruker spectrometers is also simplified.
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