Crystal Polymorphism and Crystal Transformations of Isotactic Poly(5-methylhexene-1)

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• 作者：Annamaria Buono ; Giovanni Talarico ; Claudio de Rosa ; Annette Thierry ; Bernard Lotz
• 刊名：Macromolecules
• 出版年：2013
• 出版时间：June 25, 2013
• 年：2013
• 卷：46
• 期：12
• 页码：4872-4881
• 全文大小：531K
• 年卷期：v.46,no.12(June 25, 2013)
• ISSN：1520-5835

文摘

The crystal polymorphism of isotactic poly(5-methyl-hexene-1) (P5MHex1) is reexamined on the basis of new data: electron diffraction of solution- and thin film-grown single crystals, fiber patterns from stroked films. Preliminary crystal structures are determined by conformational analysis and packing energy minimization. A crystal structure initially described as orthorhombic (Corradini, P.; Eur. Polym. J. 1970, 6, 1201) is actually a frustrated, chiral phase with a trigonal unit-cell (a = b = 17.62 脜, c = 6.33 脜, 纬 = 120掳, space group P3₁ or P3₂). Analogies between the structures of isotactic P5MHex1 and poly(5-methyl-heptene-1) reported by Corradini et al. suggest that the latter polymer has also a frustrated structure. The frustrated character is lost upon heating. The high temperature form is best described as a one chain, trigonal unit-cell with probably significant side chain mobility (a=b = 10.17 脜, c = 6.33 脜, 纬 = 120掳, space group P3₁ or P3₂). Interconversion on heating and cooling between the frustrated and the one-chain unit-cells is probable. An original crystal modification formed during synthesis of P5MHex1 but not previously analyzed is obtained as single crystals in dilute solution. The monoclinic cell (a = 20.48 脜, b = 16.94 脜, c = 6.33 脜, but 伪 = 尾 = 纬 = 90掳) houses four 3-fold helices (two pairs of enantiomorphous chains). This metastable form converts by melting-recrystallization to the chiral, frustrated phase. On standing at room temperature, a structural rearrangement takes place in the frustrated form, and a packing of layers similar to the monoclinic phase develops. The rich polymorphism of P5MHex1 illustrates some of the many packing modes available to 3-fold helices of polymers with long, flexible side chains.