Recent Progress in Understanding Polymer Crystallization Saalwächter, Kay; Thurn‐Albrecht, Thomas; Paul, Wolfgang
Macromolecular chemistry and physics,
April 2023, 2023-04-00, 20230401, Letnik:
224, Številka:
7
Journal Article
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Recent developments concerned with elucidating the semicrystalline structure formation in bulk polymer melts, with a focus on possibly important factors that have received less systematic attention, ...are reviewed. This includes the presence or absence of chain motion through the crystallites, which has a strong impact on the final crystallite thickness, and the effects of entanglements, which limit crystal growth and lower the crystallinity. From the theoretical side, interest is shown in shedding light on the thermodynamic driving forces of polymer crystallization, thus providing a theoretical background against which the kinetic limitations determine the resulting structures in experiments.
Recent insights into important factors that govern the semicrystalline morphology of crystallizable polymers, but have received less systematic attention, are reviewed. This concerns the influence of intracrystalline chain motion and the effect of entanglements in the amorphous phase. New perspectives on the thermodynamic driving forces underlying the structure formation are provided by computer simulations.
Crystallization initiated at interfaces is a ubiquitous phenomenon, covering both simple liquids and polymer melts. Understanding the physical process underlying interface‐induced crystallization in ...polymers is of fundamental interest and is relevant for many applications, especially for films of functional semicrystalline polymers. Interface‐induced crystallization of liquids can proceed either by heterogeneous nucleation or by the less explored equilibrium phenomenon of prefreezing. This study reports recent progress in the theoretical and experimental investigation of the effect of substrate‐material interactions on the thermodynamic properties of prefreezing and on the kinetics of heterogeneous nucleation in model polymers on different flat substrates. Through a combination of various microscopy and scattering techniques, supported by theoretical analysis, a detailed picture of the two different phenomena of interface‐induced crystallization in polymers is achieved.
This perspective discusses recent achievements in the theoretical and experimental investigation of two phenomena of interface‐induced crystallization in polymers – prefreezing and heterogeneous nucleation. Special attention is paid to the influence of substrate‐material interactions on crystallization temperature, crystal orientation, and semicrystalline morphology.
The bulk enthalpy of melting of α‐crystals of poly (L‐lactic acid) (PLLA) is evaluated by fast scanning calorimetry (FSC), by correlating the melting enthalpy of samples of different crystallinity ...with the corresponding heat capacity at 90 °C, that is at a temperature higher than the glass transition temperature of the bulk amorphous phase and lower than the melting temperature. Extrapolation of this relationship for crystals formed at 140 °C towards the heat capacity of fully solid PLLA yields a value of 104.5±6 J g−1 when melting occurs at 180–200 °C. The analysis relies on a two‐phase structure, that is, absence of a vitrified rigid amorphous fraction (RAF) at the temperature of analysis the solid fraction (90 °C). Formation and vitrification of an RAF are suppressed by avoiding continuation of primary crystallization and secondary crystallization during cooling the system from the crystallization temperature of 140 °C to 90 °C, making use of the high cooling capacity of FSC. Small‐angle X‐ray scattering (SAXS) confirmed thickening of initially grown lamellae which only is possible if these lamellae are not surrounded by a glassy RAF. Linear crystallinity values obtained by SAXS and calorimetrically determined enthalpy‐based crystallinities agree close to each other.
A specific route of crystallization of poly (L‐lactic acid) (PLLA) assures absence of a vitrified rigid amorphous fraction, which permits correlating the heat capacity slightly above the glass transition temperature, yielding the crystal fraction, and the enthalpy of melting. This correlation allows a straightforward re‐evaluation of the bulk enthalpy of melting of α‐crystals of PLLA.
Based on a limited set of model samples it has been recently shown that semicrystalline polymers exhibit different morphological characteristics depending on the existence and timescale of ...intracrystalline chain diffusion (ICD) relative to the kinetics of crystal growth. Here, the generality of these reports for the case of so‐called crystal‐fixed polymers without or very slow ICD is tested by providing a detailed nuclear magnetic resonance (NMR) and small‐angle X‐ray scattering analysis of polybutylene succinate (PBS), a biodegradable polyester. By using a combination of NMR techniques covering different timescales it is shown that there is no intracrystalline dynamics on a time scale faster than 1 s, that is, PBS is crystal‐fixed. An expected crystallinity below 50% and the typical morphology consisting of lamellar crystals with a well‐defined crystal thickness and a broad thickness distribution of the amorphous interlayers are confirmed. By combining these results with differential scanning calorimetry measurements, a more precise value could be provided for the enthalpy of melting than previously available. The mechanical properties at room temperature are furthermore influenced by additional insertion crystallization taking place during cooling, which leads to an increase of the mechanical modulus by a factor of ≈2.5 as compared to the state at the end of isothermal crystallization.
Semicrystalline polymers with and without intracrystalline chain dynamics have been suggested to display different morphological characteristics. Combined nuclear magnetic resonance and small angle X‐ray experiments measurements reveal that polybutylene succinate belongs to the class of crystal‐fixed polymers without this dynamics and confirm the existence of the expected morphology.
We present temperature-dependent SAXS measurements on symmetric poly(styrene-block-2-vinylpyridine) and poly(styrene-block-ethylene oxide) with added lithium triflate. Salt doping led to a strong ...increase of the order–disorder transition temperatures and increased domain spacings. Based on a detailed analysis of the scattering data close to the order–disorder transition, three contributions to the structural changes can be distinguished: an increased incompatibility between the different monomers, the additional volume of the added salt, and chain stretching due to coordination between polymer and salt. At the phase transition, i.e. at constant interaction parameter χ, for low concentrations the increase in domain size is quantitatively explained by the volume of the added salt, and at higher concentrations chain stretching sets in. Structural and thermodynamic effects are considerably stronger in PEO than in P2VP.
Phase segregation between different macromolecules and specific weak interactions are the basis of molecular organization in many biological systems, which are held together by attractive hydrogen ...bonds (H‐bonds) and dissociated by phase segregation. We report significant changes in the association behavior of covalent H‐bonds by the phase of attached polymer chains. Depending on the aggregation state, we observed either intact H‐bonds despite segregation of the phases, or macrophase separation with a larger amount of H‐bonding dissociation.
Forces to reckon with: Demixing and coupling through hydrogen bonding are two opposing forces observed in a set of supramolecular diblock copolymers. Hydrogen bonds were torn apart by phase segregation of the attached polymer chains (see picture), as shown by temperature‐dependent small‐angle X‐ray scattering and solid‐state 1H NMR spectroscopy.
Kitset hollow spheres: The combination of twin polymerization with hard templates makes hollow carbon spheres (HCSs) with tailored properties easily accessible. The thickness and pore texture of the ...HCS shells and also the diameter of the spherical cavity can be varied. The application potential of synthesized HCS is substantiated by an excellent cycling stability of lithium–sulfur batteries.
Comb-shaped poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) polymers with quaternary ammonium (QA) groups have been synthesized organizing into well-defined micro-morphology for efficient anion ...(hydroxide) transport. These molecular comb structures show a dramatic enhancement in conductivity and water resistance compared with non-comb-shaped PPOs.
Solid‐state creep, ductility, and drawability are relevant mechanical properties of “crystal‐mobile” polymers, related to a large‐scale chain transport through the crystal, which is in turn mediated ...by intracrystalline monomer jumps. Here, high‐Mw poly(ethylene oxide) is used as a well‐controlled model system, modulating the properties of the amorphous phase by diluting with a non‐crystallized oligomer. Faster intracrystalline motions are found upon oligomer addition, indicating little changes in the fold surface and a dominant influence of the somewhat reduced lamellar thickness.
The amorphous‐phase properties of the crystal‐mobile polymer poly(ethylene oxide) are modified by diluting with a non‐crystallizing oligomer. Upon dilution, the overall crystallinity of the samples decreases significantly, while the thickness of the crystalline lamellae only shows a slight decrease. The faster helical‐jump motions for thinner crystals suggests a marginal influence on or of the defect generation in the fold surface.
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