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Introduction.The self-assembly of block copolymers below the order-disorder transition (ODT) temperature results in interesting nanostructures. An understanding of the methods of manipulating the orientation of the nanostructures in bulk sample is important both as a means of producing oriented materials of practical use and understanding the ordering process of these materials. Shear is an established method of controlling orientation, as is the use of flow. 1,2 Recently, there have been many reports concerning the effects of shear on the orientation of diblock copolymers forming lamellar morphology. [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] In addition, electric fields 20 and surfaces 21-23 have been shown to cause alignment of block copolymer nanoscale patterns. However there are few (if any) experimental reports of using temperature gradient to align diblock copolymers. There are a few recent theoretical efforts concerning the effect of a temperature gradient, in the form of a propogating front, during the ordering process. A recent effort along these lines is Zhang et al.'s use of simulations to examine the effect of a shifting quench boundary on the orientation of lamella. 24 Previously, front propagation in a diblock copolymer system has been investigated numerically by Liu and Goldenfield. 25 Dynamics of front propagation were studied by Paquette. 26 Finally, Chen et al. 27 discussed front propagation rate selection. Since the effects of flow fields, surfaces, and electric fields have been shown to be important in aligning diblock copolymer, one naturally asks if the use of a temperature gradient is also important in orienting a diblock copolymer sample.In this communication, we report on the use of an applied temperature gradient to orient the lamellar structure formed by a diblock copolymer. To show the effect of temperature gradient (∇T effect), we need to clearly identify the effect of the sample surfaces on the lamellar orientation. This was achieved by placing a glass surface at an approximately 45°angle with respect to the temperature gradient. In this way, the surface effect will be dramatically tilted compared to the ∇T

The order-disorder transition (ODT), microdomain morphology, and phase behavior in mixtures of polystyrene-block-polyisoprene (SI) diblock blended with homopolystyrene (HPS) were investigated. SI with a total molecular weight of 2.0 × 10 4 and volume fraction of polystyrene (PS) of 0.51 (designated SI-11/9) was blended with a homopolystyrene of molecular weight 6.1 × 10 3 (designated S-6). Binary mixtures of diblock copolymer and homopolymer were prepared by solvent casting. The ODT was quantitatively identified using the discontinuity observed in a plot of the reciprocal of the peak smallangle X-ray scattering (SAXS) intensity, Im -1 , as a function of the reciprocal of the absolute temperature, 1/T, except for the mixtures showing the disordered sphere morphology for which we determined the temperature of the demicellization/micellization transition (DMT) instead of the ODT by the disappearance of the form factor peak with increasing temperature. We systematically measured the ODT or DMT temperature as a function of the volume fraction of homopolymer. SAXS data were also used to investigate the microdomain structure of the blends. Furthermore, for two blends of SI-11/9 and S-6 with volume fractions of SI of 0.77 and 0.71, we observed an order-order transition (OOT) from a cylindrical structure to a gyroid structure on heating above 110 °C for the 0.77 volume fraction blend and 100 °C for the 0.71 volume fraction blend. However, the reverse transition from gyroid to cylinder on cooling the 0.77 volume fraction blend to below 110 °C was not observed even after annealing at temperatures below 110 °C for more than 10 h, possibly due to kinetic effects. Slow cooling (2-3 h) of the blend from the disordered state led to the gyroid structure even below 110 °C, while the low-temperature cylindrical phase could only be accessed by fast cooling (1.5 h) from the disordered state. Experimentally determined ODTs or DMTs are compared with predictions based on mean field theory. The predicted effect of homopolymer concentration on the ODT or DMT temperature was quantitatively consistent with that found experimentally. The phase diagram of the diblock copolymer/homopolymer blend was found to show the same complexity as and similar features to phase diagrams of pure diblock copolymers.