“The studies proved that the method is very sensitive to small structural changes and resolves the hierarchical structure of materials. We made the self-organization of chains dependent on the crystallization temperature and, thanks to the high spatial resolution of O-PTIR, we detected the second crystallization and branching in lamellae in spherulites that crystallized under dynamic conditions.”
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Reporting in Analytical Chemistry, researchers at Jagiellonian University’s Solaris National Synchrotron Radiation Centre examined how the addition of a low-molecular-weight poly(ethylene glycol) plasticizer and variable crystallization temperatures alter the three-dimensional macromolecular organization of poly(L-lactic acid) spherulites. Understanding these structure–property relationships is critical for predicting the mechanical and degradation behavior of PLLA-based materials, which are widely used in biomedical and packaging applications. Conventional imaging tools such as polarized optical microscopy and atomic force microscopy provide high spatial resolution but do not give direct access to molecular orientation information — particularly the full 3D chain arrangement, including out-of-plane tilt — leaving a fundamental characterization gap in polymer crystallization research.
PLLA/PEG (70/30) films were crystallized under three distinct thermal protocols — isothermal at 132 °C, nonisothermal (132 °C then 86 °C), and isothermal at 86 °C — producing three morphologically distinct spherulite types. Polarization-controlled O-PTIR measurements were performed on a mIRage instrument using a QCL pump beam and 532 nm probe, with a rotary wire-grid polarizer delivering four IR polarization angles. Images were collected simultaneously at 1094 cm⁻¹ (the νas(O−CCO) + ω(CH₃) mode, parallel to the main chain) and 1044 cm⁻¹ (the ν(C−CH₃) mode, perpendicular to the chain), at a step size of 250 nm in co-propagation mode. These two non-parallel absorption bands, combined with a four-polarization algorithm, yielded per-pixel maps of azimuthal (ψ), axial (θ), and rotational (ϕ) orientation angles alongside the Hermans order parameter ⟨P₂⟩ — providing full 3D chain orientation maps across each spherulite type without requiring IR-compatible substrates or specialized sample preparation.
The 3D orientation maps revealed distinct chain assembly mechanisms across the three spherulite types. Type 1 (isothermal, 132 °C) showed chains aligned nearly perpendicular to the lamella growth axis with ring-like features attributed to crack formation rather than lamella twisting — a conclusion drawn from the discontinuous ψ jumps at ring boundaries rather than the smooth angular transitions expected for twist. Type 2 (nonisothermal) displayed a two-phase growth morphology: an inner nest region with chain alignment closer to parallel relative to the growth axis (ψr oscillating around 20°) and out-of-plane angles exceeding 45°, followed by an outer region with conventional perpendicular chain arrangement. Type 3 (isothermal, 86 °C) exhibited dendritic, multiply branched lamellae with increasingly irregular chain packing and decreasing IR band intensity with spherulite radius, consistent with progressive material depletion during growth. Across all sample types, ⟨P₂⟩ values for the amorphous and crystalline phases were 0.05 ± 0.01 and 0.11 ± 0.01 respectively — consistent with FT-IR results from prior work by the same group, confirming methodological transferability.
The results demonstrate that polarization-controlled O-PTIR, operating below the IR diffraction limit at 250 nm step resolution, can resolve structural details in polymer spherulites — including secondary crystallization events and lamellar branching under dynamic conditions — that are inaccessible to conventional diffraction-limited IR microscopy. The method’s independence from IR-specific substrates and its compatibility with standard sample preparation extend its practical utility for routine polymer morphology studies. The authors indicate that this 3D orientation capability has further potential in biological tissue characterization, where the combination of chemical specificity and sub-micron structural resolution addresses similar challenges in hierarchical material organization.
First author:
Karolina Kosowska, Solaris National Synchrotron Radiation Centre
Jagiellonian University
