Segmental Orientation Studies of Block Polymers. I. Hydrogen-Bonded Polyurethanes

Abstract

Differential infrared dichroism was used to study the orientation behavior of a series of elastomeric polyurethane block copolymers. The orientation of hard and soft segments was studied under uniaxial extension, as a function of time and temperature, and in cyclical deformation experiments. The mode of hard segment orientation was found to be a function primarily of hard segment length. Three distinct mechanisms were found, de-pending on a large scale whether or not the hard domains are interlocking and on the molecular level whether the aromatic urethane segments were amorphous or partially crystalline in internal order. Soft segment orientation is little affected by changes in polymer composition. Advantages of the differential method over conventional dichroism studies are discussed and illustrated.

Electrospinning of aligned fibers with adjustable orientation using auxiliary electrodes

“…Likewise the congruent decrease in X is consistent with established deformation models for polyurethanes with a quasi-amorphous hard block, indicating a higher molecular orientation for the electrospun samples [35,40,41].…”

“…As the target speed was increased from 0.9 to 4.5 m s −1 , the diameter of the deposited fibers decreased from 1.19 ± 0.41 to 0.70 ± 0.23 µm, without the use of auxiliary electrodes. In contrast, fibers electrospun with the auxiliary electrodes had a [34,35]. Since this hydrogen bonding is mainly of intra-urethane origin, [34] hard blocks possessing free hydrogen bonds are thought to reside mixed in the soft phase or at the interface between soft and hard blocks.…”

“…According to Seymour et al [35] this segregation can be quantified by the 'hydrogen-bonding index' R:…”

“…Therefore, to elucidate the effect of the diminished bending instability on the molecular structure FTIR dichroism measurements would be advantageous [30,35]. They should be complicated because for random nonwoven fibers one would need to select individual fibers to avoid orientational averaging.…”

“…Infrared spectroscopy of polyurethanes has been comprehensively applied to study the hydrogen bonding mechanism and thus the interaction of hard and soft segments, indirectly providing also morphological information [32,34,35,37,40]. Seymour et al [34] investigated the morphology change under strain up to 300% and linked the strain to a decrease of phase separation of the hard and soft block, while in a subsequent publication they used dichroism to evidence considerable molecule orientation in the same range of strain [35]. The FTIR-ATR spectral analysis summarized in table 1 reveals a decrease of the phase segregation fraction X from 0.788 ± 0.021 to 0.713 ± 0.021 and 0.723 ± 0.016 during electrospinning of fibers with and without electrodes as compared to the cast sample, respectively.…”