Predicated execution is an effective technique for dealing with conditional branches in application programs. However, there are several problems associated with conventional compiler support for predicated execution. First, all paths of control are combined into a single path regardless of their execution frequency and size with conventional if-conversion techniques. Second, speculative execution is difficult to combine with predicated execution. In this paper, we propose the use of a new structure, referred to as the hyperblock, to overcome these problems. The hyperblock is an efficient structure to utilize predicated execution for both compiletime optimization and scheduling. Preliminary experimental results show that the hyperblock is highly effective for a wide range of superscalar and VLIW processors.

Lamellar single crystals grown in dilute solutions can be used as templates for tethered chain analysis. Two series of diblock copolymers, poly(ethylene oxide)-block-polystyrene (PEO-b-PS) and poly(Llactic acid)-block-polystyrene (PLLA-b-PS), were used as model templates to generate tethered PS blocks on the single-crystal basal surfaces. Controlled and tunable reduced tethering density, σ ˜, defined by σπR g 2 (where σ is the tethered chain density and is equal to the reciprocal of the covered area of the chain and R g is the radius of gyration of this tethered chain in its end-free state at the same conditions), could be achieved in a broad range (up to 24) by changing the molecular weights (MW's) of the crystalline and amorphous blocks and by varying the crystallization temperature (T x ) of different PEO-b-PS and PLLA-b-PS solutions. For PEO and PLLA homopolymers crystallized in dilute solutions, the lamellar crystal thicknesses (d CRYST ) were observed to be proportional to the reciprocal undercooling ∆T (where ∆T ) T d -T x and T d is the equilibrium dissolution temperature of the crystals). The σ ˜of the tethered PS chains on the crystal surface increased with decreasing ∆T because at a fixed MW of the PEO or PLLA block, an increase in the d CRYST was evidence of a decrease in the number of folds. When we plotted the relationships between 1/d CRYST and T x for these two series of diblock copolymers, sudden and discontinuous changes of the slopes in some of these were observed at σ ˜) 3.7 (σ ˜*). This was as a result of the drastic interaction change of the neighboring PS tethered chains. An average reduced surface free energy of the tethered PS chains (Γ PS ) was defined and used as a parameter to characterize the PS tethered chain interactions. The relationship between Γ PS and σ ˜showed a discontinuous transition at σ ˜*, which had a close similarity to the hard-sphere-like interaction model. This could be identified as the onset of the tethered PS chain overcrowding in solution. This transition indicates that the extra entropic surface free energy created by the repulsion of tethered PS chains started to affect the nucleation barrier of the PEO or PLLA block crystallization. On the basis of the scaling laws, the onset of highly stretched brush regime could be identified at σ ˜) 14.3 (σ ˜**). In the Γ PS vs σ ˜plot, the transition appears to be continuous. Thus, a crossover regime in the tethered PS chains exists between σ ˜* ) 3.7 and σ ˜** ) 14.3. It is defined as the regime where the interaction of the tethered PS chains undergoes changes from being noninteracting toward penetration to, finally, chain stretching normal to the surface.

We proposed an approach to precisely control the density of tethered chains on solid substrates using PEO-b-PS and PLLA-b-PS. As the crystallization temperature Tx increased, the PEO or PLLA lamellar crystal thickness d(L) increased as well as the reduced tethering density sigma; of the PS chains. The onset of tethered PS chains overcrowding in solution occurs at sigma(*) approximately 3.7-3.8 as evidenced by an abrupt change in the slope between (d(L))(-1) and Tx. This results from the extra surface free energy created by the tethered chain that starts to affect the growth barrier of the crystalline blocks.

A series of poly(ethylene oxide)-b-polystyrene (PEO-b-PS) diblock copolymers were designed and synthesized to study the change in crystal orientation of PEO blocks under different confinement sizes. The volume fraction of PEO blocks (f PEO) in these copolymers was kept almost identical (the f PEO values were between 0.45 and 0.48) but with different number-average molecular weights for the PS and PEO blocks (M̄ n PEO and M̄ n PS). Therefore, the phase morphology of these copolymers was a lamellar structure with different PEO and PS layer thicknesses (d PEO and d PS) detected by synchrotron small-angle X-ray scattering (SAXS) experiments. Since the melting temperature of these PEO crystals is lower than the glass transition temperature of the PS layers, the PEO block crystals could be melted and recrystallized under one-dimensional (1D) confinement at different crystallization temperatures (T c). The PEO block crystal orientation changes were monitored using synchrotron wide-angle X-ray diffraction (WAXD) experiments. It was found that the crystalline PEO chain orientation (the c-axis in the crystals) underwent a change from being perpendicular (homogeneous) to the layer normal direction (n̂) at low T cs to parallel (homeotropic) at high T cs in these 1D confined samples with the d PEO ranging from 8.8 to 23.3 nm. However, with the gradual release of this 1D confinement, i.e., increasing the d PEO, a broad T c region in which the inclined c-axis orientation was originally observed in the PEO-b-PS with the low M̄ n PEO (8.7K g/mol) and M̄ n PS (9.2K g/mol) became increasingly narrowed by pushing the starting T c where the tilting initiates toward higher T c and reducing the ending T c where the parallel orientation of the c-axis with n̂ starts. In the PEO-b-PS sample with the highest M̄ n PEO (57K g/mol) and M̄ n PS (61.3K g/mol) in this study, this T c region was narrowed to less than 5 °C, suggesting the confinement size effect on the crystal orientation of the PEO crystals. The homogeneous to homeotropic orientation change of the c-axis in the PEO crystals with increasing T c was explained to be largely governed by the primary nucleation and crystal growth processes of the PEO blocks for developing the maximum crystallinity. A semiquantitative calculation was attempted to illustrate why the homogeneous orientation of the PEO crystals takes place in the 1D confinement based on the SAXS, WAXD, and differential scanning calorimetric results. It was expected that when the d PEO becomes large enough, the homogeneous orientation of the c-axis in the PEO crystals would disappear.

A series of poly(ethylene oxide)-block-polystyrene (PEO-b-PS) diblock copolymers were used to generate nucleation sites for the crystal growth of a homo-PEO fraction in solution. The numberaverage molecular weights of the PEO blocks (M n PEO ) were similar, and the number-average molecular weights of the PS blocks (Mn PS ) ranged from 4.6K to 17K g/mol. In PEO-b-PS/(chlorobenzene/octane) solutions, square-shaped single crystals bounded by four {120} planes were isothermally grown and observed with transmission electron and atomic force microscopy. A "sandwich" lamellar structure, constructed by a PEO single crystal layer covered by two tethered PS block layers on the top and bottom crystal basal surfaces, was found. These diblock copolymer single crystals were used as seeds to grow homo-PEO (M n PEO ) 56K g/mol) single crystals in amyl acetate. When the Mn PS in the block copolymer was 4.6K g/mol, the edges and corners of the {120} bounded PEO-b-PS single crystals served as nucleation sites to initiate the further growth of the homo-PEO single crystal. As the Mn PS of the block copolymers increased, the homo-PEO crystal growth was increasingly hampered along the {120} edges of the PEOb-PS single crystals. When the Mn PS of the block copolymer was 17K g/mol, only the four corners of the PEO-b-PS single crystal could still act as nucleation sites. The four edges were chemically "shielded" by the tethered PS blocks. This indicates that increasing the Mn PS led to a higher reduced tethering density of the PS blocks on both the basal surfaces of the PEO-b-PS single crystals. The repulsion generated among the tethered PS blocks caused the PS blocks located near and at the edges to advance along the [120] direction. Interestingly, this local environment only accepted the PEO-b-PS molecules but rejected the homo-PEO molecules from further growth. As a direct result of this study, novel channel-wire arrays on a submicrometer length scale having chemical and geometric recognitions could be fabricated via alternating crystal growth of PEO-b-PS and homo-PEO. This fabrication provided robustly controlled arrays with spacing down to 50 nm.

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Nanoscale tailored polymer crystalline morphology is studied in a polystyrene-b-poly-(ethylene oxide) (PS-b-PEO) diblock copolymer with number-average molecular weights for the PS and PEO blocks being 17 000 and 11 000 g/mol, respectively. The PEO volume fraction is 0.39. After large amplitude planar reciprocating shear, a hexagonally perforated layer (HPL) structure is obtained. Since the glass transition temperature of the PS blocks (72 °C) is higher than the PEO crystal melting temperature (∼51 °C when the crystallization temperature, T c, is lower than 40 °C), the PEO block crystallization is confined within the HPL structure. The PEO crystal (the c axis) orientation within this complex confined environment is investigated using simultaneous synchrotron two-dimensional smallangle and wide-angle X-ray scattering (SAXS and WAXS) methods. The PEO crystal orientations with respect to the layer plane (or the {000l}) of the HPL structure have been found to be dependent upon T c. At very low Tcs (below -50 °C), the PEO crystals have a random orientation. Between -50 °C e Tc e -10 °C, the PEO crystal c axes preferentially orient parallel to the layer plane. Above Tc ) 0 °C, the crystal c axes orient inclined to the layer plane of the HPL structure, and the tilt angle with respect to the layer plane increases with Tc. Contrary to the confined polymer crystallization in nanolamellar structure, however, the c axis orientation perpendicular to the layer plane is not found at least up to Tc ) 40 °C in this HPL confined environment. Meanwhile, the ribbonlike PEO crystal growth is specifically tailored along the {101 h0} planes of the hexagonal PS perforations. Apparent crystallite size analyses using the Scherrer equation confirm the one-dimensional crystal growth at high Tcs. Using time-resolved WAXS experiments, the crystal orientation is observed to occur in the early stage of crystallization with a crystallinity of ∼7 wt %. Based on the results of specifically designed self-seeding experiment, the crystal orientation is determined by the crystal growth (surface nucleation) in the confined HPL phase rather than the preorientation of primary nuclei.

The hexagonally perforated layer (HPL) phase in a polystyrene-b-poly(ethylene oxide) (PS-b-PEO) diblock copolymer was systematically studied by small-angle X-ray scattering (SAXS) in reciprocal space and transmission electron microscopy (TEM) in real space. Detailed crystallographic analyses revealed that the shear-induced, “single-crystal”-like HPL phase in the PS-b-PEO diblock copolymer sample contained a mixture of trigonal twins (∼80%) and hexagonal (∼20%) structures. Both structures had the same orientation and the same in-plane unit cell parameters (a and α) but different out-of-plane (c-axis) dimensions, because the trigonal structure was constructed by three layers (ABC) and the hexagonal structure had two layers (AB). Computer-simulated diffraction and TEM results implied that the correlation length of the trigonal structure along the [001] direction was relatively large (at least over 20 layers, i.e., ≥∼400 nm) while that of the hexagonal structure was relatively small. The formation mechanism of the HPL phase was investigated by TEM. Various edge dislocations formed by “plastic deformation” under a large-amplitude mechanical shear were the cause of the trigonal twins and the hexagonal structure. Low-frequency rheological studies indicated that the HPL phase was metastable, and it transformed into a more stable double gyroid phase when the temperature exceeded 160 °C. The HPL structure reappeared after the sample was again subjected to the large-amplitude mechanical shear. Furthermore, the transformation from the HPL phase to the DG phase was also studied. It was found that the perforations of the hexagonal structure rearranged themselves first, followed by the perforations in the trigonal structure.