Abstract
The mechanical properties of crystalline polystyrene (IPS) with different tacticities were studied as a function of crystallinity and molecular orientation.
It was found that both tensile strength and Young's modulus of crystalline isotactic polystyrene decrease, but its impact strength slightly increase by annealing sufficiently at 180°C, independent of the degree of crystallinity and molecular weight (_??_v=3.4_??_24.40×105). In this case, the ratio of the tenile strength to the modulus is a constant value of 0.01, regardless of draw ratio. These results may be well interpreted in accordance with Buchdahl's consideration for glassy state of polymer.
The intrinsic birefringence, Δnc° for a model crystal of polystyrene with perfect c-axis orientation was calculated as -0.080 and -0.133 respectively with use of Denbigh's and Bunn's values for the bond principal polarizabilities. The birefringence is generally recognized to increase with draw ratio, approaching an equilibrium value at about 400% of drawing. The contributions of crystalline and amorphous phases to the birefringence are estimated separately under an assumption of additivity of the contribution of both phases. The total birefringence, Δn may be expressed as Δn=XΔnc+(l-X)Δna, and Δnc=fnc°, where Δnc and Δna are birefringences of crystalline and amorphous phases per unit viume, X the volume fraction of crystalline phase, and f the orientation parameter. Using the values of X and f obtained from X-ray study, each term of the equation is discussed. As a result, it was found that XΔnc, increased monotonously with draw ratio, while (1-X)Δna approached a maximum value at about 350% drawing.
It is recognized by studying the dynamic mechanical properties of the samples that in a plot of loss tangent versus temperature, the temperature where the loss tangent through a maximum due to molecular mobility in amorphous phase is independent of either the degree of crystallinity, or the draw ratio.
