Perforated layers

For nearly symmetric compositions the unlike blocks form domains composed of alternating layers, known as lamellar phase (L). Slightly off-symmetry composition results in the formation of a different layered structure. The structure is known as perforated layers (PI) or catenoid phase. Despite an earlier assignment as an equilibrium phase, it is now known to be in a long-lived metastable state that facilitates the transition from I to G phases [9-14], The PL structure consists of alternating minority and majority component layers in which hexagonally packed channels of the majority component extend through the minority component. 

Fig. 2 Sketch of perforated layers (PL) or catenoid structure, a Projection direction perpendicular to layers of perforated lamellar structure appears as a hexagonal honeycomb mesh, b Projection parallel to layers appears as rows of dark spots resulting from cross section of parts. Sketch according to [15]. Copyright 2001 American Chemical Society...

The kinetics and mechanisms of the C —> G transition in a concentrated solution of PS-fr-PI in the PS-selective solvent di-n-butyl phthalate was studied [137,149]. An epitaxially transformation of the shear-oriented C phase to G, as previously established in melts [13,50,150], was observed. For shallow quenches into G, the transition proceeds directly by a nucleation and growth process. For deeper quenches, a metastable intermediate structure appears, with scattering and rheological features consistent with the hexag-onally perforated layer (PL) state. The C -> G transition follows the same pathways, and at approximately the same rates, even when the initial C phase is not shear-oriented. 

Fig. 2.3 Typical isothermal frequency scans for PE-PEE diblocks with indicated compositions in different ordered phases (Zhao ei at. 1996). Qualitative differences between the low frequency rheological response for distinct ordered structures similar to these are observed for other diblocks. S = BCC spheres, C = hex cylinders, G = Ia3d gyroid, HPL = hexagonal perforated layer, L = lamellae. (A) G (x) G . Structural assignments of the ordered phases were made using TEM and SAXS.

In recent years a number of so-called complex phases, such as the bicontinuous gyroid and perforated layer structures, have been identified. The former has been established as an equilibrium structure, whereas the latter seem to be metastable structures observed during transformations to and from the gyroid structure. 

The existence of a second class of complex phases, the modulated and perforated layer structures, has largely been explored by Bates and co-workers (Forster et al. 1994 Hamley et al. 1993, 1994 Khandpur et al. 1995 Schulz et al. 1996), who used SANS and TEM to investigate shear oriented structures. The thermally-induced phase transition from the lam to the hex phase in polyolefin diblocks was studied in detail by Hamley et al. (1993, 1994) using SANS, TEM and rheology. Intermediate hexagonal modulated lamellar (HML) and hexagonal perforated layer (HPL) structures were observed on heating PEP-PEE, PE-PEP and PE-PEE diblocks, where PEP is poly(ethylene-propylene), PEE is... 

Bicontinuous cubic phases have not, to date, been accounted for using SSL theory. The OBDD phase has been shown to be unstable with respect to lam and hex phases (Likhtman and Semenov 1994 Olmsted and Milner 1994a,b). As discussed above, it now appears that the OBDD was a misidentified gyroid phase however, SSL calculations for the gyroid structure have not been performed as yet. A perforated layer structure was found to be unstable by Fredrickson (1991), using SSL theory following Semenov s method. 

The effect of harmonics in the composition profile has been considered in Landau Brazovskii theory, as well as mean field theory. Olvera de la Cruz (1991) found a hexagonal perforated layer (HPL) structure to be stable for symmetric or nearly symmetric diblocks in addition to the classical phases. Recent work has... 

Fig. 2.47 Pseudostable perforated layer structure, observed following a quench from the lam to hex phase using a multimode analysis of the time-dependent Ginzburg-Landau equation, within the single-wavenumber approximation (Qi and Wang 1997). This structure results from the superposition of six BCC-type wavevectors.

Fig. 6.6 TEM images of a catenoid lamellar hexagonal perforated layer (HPL) structure in a blend of a PS-PB diblock (M = 49.9kgmor1, 51wt% PS) and PS homopolymer M = 26 kg mol 1) and a total PS volume fraction fFS = 0.67 (Disko etal. 1993). The sample was annealed at 130 °C prior to microtoming (a) view parallel to the lamellae (b) view normal to the lamellae, showing hexagonal perforations in a PI lamella.

Park I, Lee B et al (2005) Epitaxial phase transition of polystyrene-b-polyisoprene from hexagonally perforated layer to gyroid phase in thin film. Macromolecules 38 10532-10536... 

In addition to these morphologies, perforated layers, bicontinuous layers and other unstable morphologies have been described (Sperling, 2001). It has also been noted that, unlike polymer blends that become less miscible at higher temperatures, block copolymers become more miscible due to the temperature dependence of the Flory-Huggins parameter, x-... 

Also in bulk block copolymers microphase-separate into ordered liquid crystalline phases. A variety of phase morphologies such as lamellae (LAM), hexagonally ordered cylinders (HEX), arrays of spherical microdomains (BCC, FCC), modulated (MLAM) and perforated layers (FLAM), ordered bicontinuous structures such as the gyroid, as well as the related inverse structures have been documented. The morphology mainly depends on the relative block length. If, for instance, both blocks are of identical length, lamellar structures are preferred. 

Figure 28 Phase diagram of ABC star polymers with arm-length ratio 1 1with symmetric interaction parameters. Morphologies are lamella+sphere (L+S), five cylindrical structures in sectional view, perforated layer (PL), lamella + cylinder (L + C), columnar piled disk (CPD), and lamella in sphere (L-in-S). Reprinted from Gemma, T. Hatano, A. Dotera, T. MacromoleculesZOOZ, 35,3225. ...

The classical picture of spheres, cylinders and lamellae was first challenged by the discovery of a new morphology in styrene/isoprene diblocks (Hasegawa et al. 1987) and styrene/isoprene star diblocks (Thomas et al. 1986). In the latter case the morphology has subsequently been identified as a so-called gyroid phase (Hajduk et al. 1995) this is a bicontinous nework with cubic symmetry that has now been identified in a number of different diblock systems (Schulz et al. 1994, Hajduk et al. 1995). Other non-classical phases have also been identified these include a perforated layer phase and a modulated lamellar phase (Bates et al. 1994). 

Firing number Speed (m/s) Stop Depth (mm) Diameter (mm) Number of perforated layers Distance to edge (mm)... 

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