(28e) Crystallization Dynamics of Thermally Quenched Linear Block Copolymers Comprising the Semicrystalline Block Poly-3-Hexylthiophene and the Amorphous Block Polystyrene

Introduction. Scaling-up the processing of commercial plastic electronics remains a challenge because the fundamental kinetic phenomena that give rise to functional nanostructures are poorly understood. A regular, nanophase separated structure of semicrystalline and amorphous polymer phases is desirable for applications combining conductivity with, for example, mechanical strength. Linear block copolymers self-assemble into regular equilibrium nanostructures, the size and shape of which are well-studied when both blocks are amorphous¹. When one block is semicrystalline, the nanostructures are susceptible to kinetic trapping because “breakout” crystallization occurs more rapidly than the slow, diffusive polymer reptation that leads to equilibirium nanophase separation². This study explores the effects of thermal quench depth on a series of semicrystalline-block-amorphous copolymers to determine how the kinetics of crystallization affect the formation of structure in these materials.

Materials and Methods. The block copolymer chosen for study comprises crystallizable, conductive poly-3-hexylthiophene (P3HT) and amorphous, insulating polystyrene (PS). Three linear block copolymers were purchased from Polymer Source along with a PS homopolymer, and a P3HT homopolymer was purchased from Rieke. The molecular weights and volume fractions are listed in Table 1. These block copolymers enable us to construct two comparisons: the effect of polymer molecular weight for similar block fractions (C12-A31 compared to C15-A45) and the effect of PS block length for similar P3HT block lengths (C10-A15 compared to C12-A31).

For applications outside the thin-film regime, like battery cathodes and transistors, melt processing is attractive because it avoids the use of toxic or costly solvents and requires relatively low energy input to the process on account of the low melting temperatures and glass transitions of most polymers. Notably, melt processing is constrained to bulk applications in which the polymer thickness is >~10 mm. These results will not directly inform thin-film processing, which is largely accomplished via solvent printing methods; however, understanding the effects of thermodynamic driving force on structure formation is likely to benefit both film and bulk processing methods. Motivation for this bulk study was drawn from the observation of nanoscale P3HT crystallites that formed in thin films of the diblock copolymer (Figure 1).

Experimental Results: Samples were quenched from a one-phase melt to temperatures ranging from 100°C-245°C, corresponding respectively to the glass transition of PS and the limit at which kinetic results could be observed within hours rather than days. The kinetics of crystallization were measured in situ using differential scanning calorimetry (DSC) and combined small- and wide-angle x-ray scattering (SAXS/WAXS) at the Australian Synchrotron. The offset melting point observed in DSC was up to 70°C lower than the temperature at which WAXS peaks diminished, indicating complete melting of all nuclei. Prior to this realization, a complete DSC experiment was performed using quenches from 250°C, the ‘apparent’ melting point measured in DSC; a sample set of quenches are shown in Figure 2. These samples contain crystalline nuclei and the measured rates are dominated by crystalline growth, given that the rate of nucleation was later measured by WAXS to be around two orders of magnitude slower. The enthalpy of crystallization was measured for these seeded growth studies and is reported in Figure 3. The molecular weight of the P3HT block appears to control the rate of crystal growth and the temperature at which peak growth occurs. In agreement with expectations, the lower molecular weight blocks crystallize more rapidly³. These growth results will be compared to calorimetry for a full set of quenches from the one-phase region in order to parse the rate of nucleation.

Samples quenched from the one-phase melt exhibited measurable induction times in WAXS for shallow quenches (temperature drop ~50°C). Deeper quenches resulted in the immediate emergence of crystalline diffraction peaks. Attempts to quench the sample into an amorphous state proved unsuccessful. The rapid transfer of molten polymer into liquid nitrogen resulted in a scattering profile that showed both crystalline diffraction and a low-q shoulder in the small-angle scattering. Whilst most quenches resulted in a slight change in the low-q scattering, only the fastest quenches into liquid nitrogen induced a clear shoulder. The SAXS profiles do not exhibit a distinct enough structure factor to rigorously assign a morphology to the growing crystallites; however, the low-q scaling laws and shoulder positions indicate the dimensionality and size of growing crystals.

Prospectus: This work aims to place nucleation and growth of P3HT crystallites that are part of a block copolymer into a thermokinetic framework such that the thermal driving force, block molecular weights, and Flory-Huggins χ parameter can be used to predict dynamic data. The larger goal is to control the structure and properties of functional, nanostructured polymers using conventional controlled parameters in a chemical process; e.g. slot-die temperature, melt flow rate, and cooling rate. Most nanostructured block polymers require enhanced functionality through the loading of nanoparticles, like carbon black for conductivity. Whilst equilibrium models, like self-consistent field theory, are evolving to accommodate nanoparticle fillers⁴, the complexity of competing self-assembly and crystallization pose a vast challenge for the adoption of these advanced materials. Lower cost manufacture of the precursors, low-temperature processing and the value-add of self-assembling structures are compelling motivations to pursue this research direction.

Acknowledgements: This research was undertaken on the SAXS/WAXS beamline at the Australian Synchrotron, part of ANSTO. LBA and AJN acknowledge the Department of Chemical and Materials Engineering at The University of Auckland for an early-career PhD scholarship.

References:

¹FS Bates and GH Fredrickson. Block copolymers–designer soft materials. Physics Today, 1999, 52(2): 32-38.

²SB Myers and RA Register. Crystallization of Defect-Free Polyethylene within Block Copolymer Mesophases. Macromolecules, 2010, 43:393-401.

³F Koch et al. The impact of molecular weight on microstructure and charge transport in semicrystalline polymer semiconductors–poly(3-hexylthiophene), a model study. Progress in Polymer Science, 2013, 38:1978-1989.

⁴ MW Matsen and RB Thompson. Particle distributions in a block copolymer nanocomposite. Macromolecules, 2008, 41:1853-1860.