Real-time measurements of crystallization processes in viscoelastic polymeric photonic crystals

David R. E. Snoswell, Chris E. Finlayson, Qibin Zhao, and Jeremy J. Baumberg

Phys. Rev. E 92, 052315 – Published 30 November 2015

Abstract

We present a study of the dynamic shear ordering of viscoelastic photonic crystals, based on core-shell polymeric composite particles. Using an adapted shear-cell arrangement, the crystalline ordering of the material under conditions of oscillatory shear is interrogated in real time, through both video imaging and from the optical transmission spectra of the cell. In order to gain a deeper understanding of the macroscopic influences of shear on the crystallization process in this solvent-free system, the development of bulk ordering is studied as a function of the key parameters including duty cycle and shear-strain magnitude. In particular, optimal ordering is observed from a prerandomized sample at shear strains of around 160%, for 1-Hz oscillations. This ordering reaches completion over time scales of order 10 s. These observations suggest significant local strains are needed to drive nanoparticles through energy barriers, and that local creep is needed to break temporal symmetry in such high-viscosity nanoassemblies. Crystal shear-melting effects are also characterized under conditions of constant shear rate. These quantitative experiments aim to stimulate the development of theoretical models which can deal with the strong local particle interactions in this system.

Received 12 September 2014
Revised 17 April 2015

DOI:https://doi.org/10.1103/PhysRevE.92.052315

Authors & Affiliations

David R. E. Snoswell^1,*, Chris E. Finlayson^2,†, Qibin Zhao¹, and Jeremy J. Baumberg^1,‡

¹Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, United Kingdom
²Department of Physics, Prifysgol Aberystwyth University, Aberystwyth, Wales SY23 3BZ, United Kingdom

^*DSnoswell@slb.com
^†cef2@aber.ac.uk
^‡jjb12@cam.ac.uk

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Issue

Vol. 92, Iss. 5 — November 2015

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Images

Figure 1
(a) Experimental geometry used in shear-cell measurements, with opal sample introduced between two plates with optical viewing windows. Shearing of the sample is achieved by the relative rotational motion of the plates; the strain in all subsequent experiments is defined at the point of observation in the (middle of the) viewing window, which is offset from the center. (b) Core-shell architecture of precursor particles, (c) self-assembled into polymer opals.
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Figure 2
Storage and loss moduli, and the viscosity of polymer opal material, obtained using an ARES-G2 rheometer. Data are shown in (a) as a function of strain-shear oscillation frequency (temperature of 100 °C), and in (b) as a function of strain magnitude (temperature of 100 °C and an oscillation frequency of 10 rads/s). The polymer opal sample characterized here has a representative core-shell particle size of $\approx 250 nm$ , and $T_{glass} \approx - 15^{\circ} C$ .
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Figure 3
(a) Optical spectra taken from the preshear phase of an opal sample, showing the shear melting of the crystal over 3 s. (b) Transmittance at $λ = 530 nm$ as a function of shearing time. The points are fitted to a sigmoid function to extract a decay time.
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Figure 4
(a) Optical spectra taken during the crystallization of an opal sample by means of an oscillatory shear, showing the progress of crystallization over 10 s. (b) Corresponding semilog plot reveals detail of the spectral features present. (c) Optical extinction coefficient at $λ = 530 nm$ as a function of shearing time, together with a fit to an offset exponential function, of lifetime $τ = 2.4 s$ . (d) Representative transmission microscope images; on the left is a sample (preoscillatory shear) where significant regions of poor crystalline ordering are evident at the moment of image capture. On the right, a sample with uniform ordering and an intense green structural color, following oscillatory shear, is imaged. In both cases, the magnification was 5× and the scale bar is ≈0.2 mm. The dashed circles on the images indicate the spot size over which spectra are obtained.
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Figure 5
Crystallization of an opal sample, as inferred from the optical extinction coefficient at the low wavelength side of the resonance ( $λ = 530 nm$ ), as a function of strain, given a shear cycle of oscillation frequency 1 Hz for 10 s. The data points are fitted to a Gaussian function to guide the eye, with the minimum value of optical extinction indicating that optimal crystalline ordering occurs at strain magnitudes of 160%. The inset defines the shear strain (δ $l / d$ ) within the cell geometry as indicated.
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Figure 6
(a) Effect of variation of shear oscillation frequency on the end transmission spectrum, retaining other parameters from Table 1, and using a strain magnitude of 75%. (b) Spectra after multiple repetitions of the standard four-step shear sequence.
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