The structures and properties of segmented polyurethanes(I)

• Main Authors: Chui, Jia Yeong, 邱家永
• Other Authors: Chen Teng Ko
• Format: Others
• Language: zh-TW
• Published: 1997
• Online Access: http://ndltd.ncl.edu.tw/handle/44194309702259589958

博士 === 國立中央大學 === 化學工程學系 === 85 === The morphological effect on the soft segment thermal transition of a series of 4, 4''-diphenylmethane diisocyanate (MDI) based cis-polybutadiene (cis-PB) polyurethanes was studied by differential scanning calorimetry and small-angle X-ray scattering (SAXS). The cis-PB polyurethanes were synthesized by two-step solution polymerization. Number average molecular weight of the soft segments ranged from 1000 to 4200. The hard segment content ranged from 20 to 69%. For the cis-polybutadiene soft segment, thermal transition behaviors of the glass transition temperature (Tg), the breadth of the glass transition temperature (△B), and the specific heat capacity (△Cp) were investigated. Experimental results indicated that increases in Tg and △B that corresponded to a decrease in the soft segment molecular weight are attributed to decrease in free volume and mobility due to bonding of soft segments to the rigid urethane blocks. In each series of polyurethanes with a constant soft segment molecular weight, both Tg and △B remain constant as hard segment content increased from 20 to 69%, indicating the phase separation between the soft and the hard segments is completed even down to the interfacial region. This well-defined two phase system simplifies the study on the soft segment △Cp. By definition, △Cp is the heat capacity change per unit mass of soft segment at the glass transition temperature. Thus, △Cp relates to the entropy increase, △Q/T, for specific amount of soft segment at glass transition temperature. Since a soft segment chain''s conformational freedom can be restricted by the nearby hard segment phase, an entropy restriction model was proposed to account for the effect of morphological changes on △Cp. This effect can easily cause a 50% decrease in △Cp for conventional polyurethanes with a soft segment molecular weight and hard segment content ranging from 1500-2000 and 30 to 55%, respectively. Since △Cp was the most important property in quantitatively accessing the degree of phase separation in segment polyurethane systems, neglecting to consider entropy restriction effect would indicate a serious inadequacy to accurately characterize a phase separated polyurethane system. The lack of complete glass transition thermograms for segment polyurethanes with wide range of composition has given rise to controversies on the thermal property and phase composition for half a century. By using a new series of complete phase separated polyurethanes and under melt-quenched condition, this study indicates that the glass transition temperature (Tg) of the amorphous hard phase is highly dependent on both the soft segment and hard segment molecular weights, whereas the amorphous soft phase Tg is only slightly dependent on its own molecular weight. On the other hand, the specific heat capacity changes (△Cp) in either phase are dependent both on segmental length and morphology. These results not only throw fresh light on the behavior of polyurethane''s glass transition, but also add to fundamental understanding of the thermal properties of amorphous polymeric nanomaterials that have a domain size much smaller than the critical molecular weight between entanglement. The long-term confusing glass transition temperature, Tg, and specific heat capacity, △Cp, of the 4,4''-diphenylmethane diisocyanate (MDI) and 1,4-butanediol (BD) hard segment in segment polyurethanes have been clearly evidenced by using a new series of completely phase mixed polyurethanes while extrapolating to 100% hard segment content. The hard segment Tg and △Cp thus obtained are 108℃ and 0.38 J/g℃, respectively, which are equivalent to the reported Tg and △Cp of a high molecular weight MDI-BD homopolymer. In addition, if a single homogeneous phase is present, the observed △Cp and Tg are found to be given by the linear weighted combination of the pure constituent values. This provide a simple relationship to access the composition of the individual microdomain in segment polyurethanes, providing that the phase separate morphology does not affect the microdomain glass transition behavior.