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http://www.che.vt.edu/Faculty/Wilkes/GLW/matts_website_stuff/matt_webpage.htm

Characterization of Spandex Elastomers

    The role of elastomers within the field of polymer science has been and is one of key importance. Elastomeric materials, and particularly synthetic elastomers, are virtually indispensable with regard to their myriad industrial, medical and consumer applications and continue to make up a considerable portion of annual polymer production and sales.Elastomeric materials based upon poly(urethane) and/or poly(urethane-urea) chemistry have a rich history, both in terms of practical application and research. Spandex is a linear, segmented poly(urethane-urea) elastomer, generally used in fiber form, first developed in the late 1950s.The spandex elastomers consist of short, alternating, chemically incompatible hard and soft segments which attempt to phase separate, but are limited in their ability to do so due to being covalently bonded to one another. Rather than forming two distinct phases, these materials exhibit microphase separation in which domains rich in hard segments form within a matrix rich in soft segments. In addition, many poly(urethane-urea)s, such as spandex, exhibit bidentate hydrogen bonding (red dashed lines) in which the oxygen of the urea carbonyl hydrogen bonds with two groups on a neighboring chain. The level of hydrogen bonding is substantial enough that the elastomers will degrade before melt processing becomes possible. For this reason, spandex must be processed from solution.

    The performance characteristics of the spandex elastomer is influenced by such variables as segment size, hard segment content, choice of hard and soft segment chemistry, degree of microphase separation, and a variety of processing variables.Throughout the history of spandex, it has been found that poly(tetramethylene ether glycol) has been the most advantageous choice of soft segment. Recent advances in catalyst technology has led to the development of ultra-low monol content poly(propylene glycol), which shows great promise for use as a spandex soft segment. Ultra-low monol content PPG offers substantial advantages over PTMEG including lower cost of materials, improved process economics, enhanced environmental aspects, and elastomer performance characteristics.

    The goal of this study is to conduct structure-property characterization of these PPG-based spandex elastomers in an attempt to gain insight into how chemical composition and processing influence the morphological structure of the elastomers. Coupling this knowledge with determination of the macroscopic properties of the elastomers will enable the development of improved PPG-based spandex elastomers which may challenge conventional PTMEG-based elastomers. A variety of rheo-optic, microscopy, thermal, and mechanical characterization techniques were used in this study.

    Small angle x-ray scattering (SAXS) is useful technique which can reveal the average distance between the hard domain islands in the matrix of soft segments, as well as provide an approximation of the thickness of the interphase region between the hard and soft domains.Additionally, SAXS can provide an approximation of the degree of mixing of the soft and hard segments within the hard and soft domains. The figure below shows how SAXS may be used to determine the average spacing between the hard domains in spandex systems using either 2000 g/mol ultra-low monol PPG and 2000 g/mol PTMEG for soft segments. It was seen that for identical molecular weights PTMEG had a slightly greater interdomain spacing, which is consistent with the fact that PTMEG has about 35% more backbone bonds than PPG for comparable molecular weights.

    Where small angle x-ray scattering provided essentially an numeric representation of the microphase separation characteristics, use of atomic force microscopy (AFM) allows for the presentation of visual images of the microphase separation characteristics of the spandex elastomers at the surface.Using tapping mode atomic force microscopy and a technique known as phase imaging, it is possible to resolve a surface into regions of hard and soft material.Shown below are a pair of AFM phase image micrographs, where the lighter regions corresponds to hard domain material.The left image is that of a PTMEG-based spandex material, and the image on the right is that of a PPG-based spandex material, with comparable soft segment molecular weights.

    As can be seen, the PPG-based material has a finer grain structure of the light colored hard domains within the darker soft domain matrix. Atomic force microscopy dramatically revealed changes in the surface morphology of thin spandex films as the level of a low molecular soft segment component, tri(propylene glycol), was added to the formulation. From left to right in the three AFM images shown below, the amount of TPG is increasing, lowering the average soft segment molecular weight. What is seen, is that as the amount of TPG increases, the size and amount of the hard domains also increases, implying that TPG is acting more as a hard segment component than as a soft segment component as originally suspected.

  A variety of thermo-mechanical characterization methods have also been applied to the spandex materials, but will not be discussed in detail in this brief report. These include dynamic mechanical analysis, differential scanning calorimetry, and conventional Instron tensile strength measurements.