Cryo-FIB and Synchrotron SAXS/WAXS Studies of Confined Crystallization of PDMS in Tubular Network Block Copolymer Morphologies

Crystallization within confinements has been of interest to a broad community of researchers in soft polymers, hard materials, and life sciences [1–7]. A nanoscale microphase separated but crystallizable block in the presence of a non-crystallizable block provides a unique environment for study of confinement effects on crystallization. Typically, depending on whether the glass transition temperature (Tg) of the amorphous block is higher or lower than the crystallization temperature (Tc) of the crystallizable block, the crystallization is confined or there is breakout [3]. To date, such studies have focused mostly on spherical, lamellar, or cylindrical microdomains [8–10] with little focus on complex tubular network structures, e.g. the bi-continuous Double Gyroid (DG), Double Diamond (DD), and Double Primitive (DP). We have explored the DG (Ia3d space group) cubic microdomain structure in a diblock copolymer (BCP). An added advantage of studying crystallization of relatively optically isotropic BCP morphologies (due to their cubic symmetries) is the enhanced birefringence contrast generated from the crystallization events which enables characterization at optical length scales using polarized light. Being non-destructive and fast, optical microscopy also allows industrial-scale high-throughput screening. Polydimethylsiloxane (PDMS) typically undergoes rapid crystallization below temperatures of ∼ - 75 C into a tetragonal unit cell and melts at a (melting temperature) Tm ∼ - 42 C. From differential scanning calorimetry (DSC) and simultaneous small angle and wide-angle scattering (SAXS/WAXS) at Brookhaven National Laboratory beamlines (NSLS-II 12-ID SMI and 11-BM CMS) with a microfocus beam, we observed that PDMS crystallizes within these complex geometries and exhibits about 50–60% crystallinity. For these experiments, films (∼ 100 um thickness) of the BCP (Polystyrene-b-PDMS) (PS-PDMS, 50 kg/mol (PS)-33 kg/mol (PDMS) are fabricated using solution casting from toluene. From the SAXS data, the unit cell parameter of the DG structure is ∼ 150 nm. Further, we saw that as we cool the sample to ∼ -30oC (still above Tm), there is a surprising ∼100X increase in the diffuse low q small angle scattering. This is presumably due to large density fluctuations and voids from within the much greater thermally contracting PDMS domains due to the mechanical resistance of the surrounding 3D glassy PS matrix and/or due to the emergence of more densely packed PDMS crystallization precursors (figures 2(a) and 2(b)). Next, by using 3D volume imaging under cryogenic temperature with DualBeam (Thermo Scientific™ Helios™ 5 CX DualBeam), we investigated the influence of the intermaterial dividing surface (IMDS) on the 3D size, shape, and continuity of the organic electron beam-sensitive PDMS crystals and voids. The sample is mounted on an aluminum sample stub using silver paste (Ted Pella) to enhance electronic and thermal conductivity. The stage can be cooled either post loading the sample or precooled to ∼ (-90 C to (-100 C)) using a stage temperature controller before loading the sample using a Leica Vacuum Cryo Transfer system (Leica EM VCT500). For preparing 3D volume imaging under room temperature, a protective Pt coating is deposited on the surface of the region of interest that will undergo milling, then the specimen is milled using Ga+ focused ion beam at 30 keV, 80 pA. For site preparation under cryogenic temperature, condensation of the organic Pt precursor was performed and then cured with ion beam irradiation to form the protective layer. The clean ion-milled surface cross section is imaged with immersion mode using through lens detector (TLD) at 1 keV, 43 pA current. Further, the serial milling and image acquisitions are performed using Thermo Scientific Auto Slice & View 4 Software, which allows for automated acquisition of high-resolution 3D images. Finally, the 3D reconstructions are obtained after segmentation and using Avizo 3.0. The presence of Si atoms in the backbone of PDMS chains results in higher secondary electron emission from the PDMS domains, since the PS block only contains C,H atoms. Thus, in the SEM micrographs acquired, the PDMS domains appear bright, while the PS domains are dark (figures 1(a) and 1(b)). Further, PDMS chains being above their Tg have much more conformational freedom compared to PS and as a result can densify up to 10–15 % upon cooling [11]. This densification of PDMS creates severe mechanical stresses between the PS and the PDMS microdomains thus resulting in the generation of nano-voids/cavities. Typical diameters of the voids (or the long axis of the elliptical shaped voids) are ∼ 30–70 nm which can be 100 – 200 nm long/deep (figures 1(b) and figure 3(b)). The SEM micrograph acquired (figure 2) at ∼ -95 C illustrates the presence of such voids exclusively within the DG microdomain channels. In conclusion, we saw that the PDMS densifies, hence creating voids and undergoes confined crystallization with approximate sizes of crystals residing within the DG nanoconfinements ranging between 20–30 nm. Thus, by combining real space cryoFIB/SEM and reciprocal space synchrotron SAXS/WAXS studies we were able to characterize the phase separation, crystallization, and densification of PDMS within the confining DG channels [12]. (a): SAXS/WAXS at 25 C of PS-PDMS 50-33 kg/mol sample exhibiting the DG morphology. (b): SAXS/WAXS post-crystallization at -95 C of PS-PDMS 50-33 kg/mol sample illustrating the formation of voids (low q scattering in SAXS) and crystallization peaks (WAXS) corresponding to the tetragonal unit cell of PDMS within the DG microdomain PDMS channels. (a): 3D reconstruction at 25 C of PS-PDMS 50-33 kg/mol sample exhibiting the DG morphology. Blue - Left-handed PDMS network; Red - Right-handed PDMS network. The matrix PS is rendered transparent. (b): 3D reconstruction of a void residing within the PDMS channel post-crystallization at -95 C of PS-PDMS 50-33 kg/mol sample. Purple - The shape and size of a void likely caused due to densification and associated mechanical stresses is shown.