Fluorescent Visualization of Bond Breaking in Polymer Glasses

Mechanofluorescent polymer probes were used to visualize stresses and bond scission in polystyrene and polycarbonate. Sonication of polystyrene probes with a molar mass of 1.1 × 105 g·mol-1 in solution resulted in 30% activation after 1 h, while shorter probes showed lower activation percentages. Single-asperity sliding friction tests were performed on mechanophore-containing polystyrene and polycarbonate films. Polystyrene showed clearly visible crack formation with a correlated pattern in the friction force, penetration depth, and fluorescent activation of the mechanophore. Significant mechanophore activation in polystyrene was observed for an applied normal load of 100 mN, whereas in polycarbonate, activation only occurred at a normal load higher than 400 mN. The different degrees of activation correlate well with the toughness of polycarbonate compared to polystyrene.</p

Fiber-induced crystallization in elongational flows

Morphology development at the fiber/matrix interphase in fiber-reinforced isotactic polypropylene composites is a widely studied topic. While the application of shear flow may strongly enhance the nucleation density around the fiber, little is known about the influence of fibers on the crystallization of polypropylene subjected to an extensional flow. In this work, the flow around a single glass fiber upon uniaxial elongation of the melt is examined using X-ray scattering and diffraction techniques and compared to the response measured for the neat matrix. A comparison between a neat and compatibilized matrix is made given the strong influence of the addition of an adhesion modifier on the bulk crystallization kinetics of polypropylene. The flow is applied using an in-house-built filament stretching extensional rheometer, which, due to its midfilament control scheme, allows for in situ X-ray experiments. Combined small-angle X-ray scattering/wide-angle X-ray diffraction patterns are acquired during the flow and subsequent crystallization step. Postcrystallization area scans of the filament show that the introduction of a single glass fiber gives rise to the development of β-phase crystals, particularly in the area around the fiber ends, and in contrast to what is observed for the matrix materials alone, where solely α-phase is found. Surprisingly enough, the addition of a single fiber (0.00045 vol %) alters the crystallizing polymorph in almost the entire filament. However, the addition of the adhesion modifier hinders the formation of β-phase crystals around the fiber due to an acceleration of the bulk crystallization kinetics. Finite element simulations provide insight into the flow field around the fiber during stretching and demonstrate that the flow is no longer uniaxial extension, but dominated by shear, even though the volumetric amount of fiber as compared to the matrix is negligible. These findings explain the experimental observation of substantial β-phase formation after the introduction of a single fiber, while this is not observed in the matrix material. Worth noting, the formation of β-phase polypropylene depends not only on the presence and the strength of the flow but predominantly on the type of flow, i.e., shear as opposed to elongation

Numerical analysis of the crystallization kinetics in SLS

In the selective laser sintering of polymers, the most widely used powders are based on polyamide 12 (PA12), which is a semi-crystalline polymer. Because the mechanical properties of the printed parts depend largely on the microstructure, knowledge on the crystalline architecture is important. We developed a numerical model based on the finite element method to solve the flow, temperature and crystallization kinetics of PA12 powder during sintering using two different geometries. Our results show that the temperature plays a crucial role in the crystallization kinetics and that simplified 0D calculations can be used to study the crystallization kinetics if the temperature behavior in time at a certain location is known. With our choice of initial and boundary conditions, we found primarily crystals of the α′-phase

Deformation kinetics of single-fiber polypropylene composites:Adhesion improvement at the expense of toughness

Despite the maturity of the technology, processing of fiber-reinforced thermoplastic materials remains challenging, and difficulties in processability often result in material formulations with high modulus and strength, yet rather poor ductility compared to the pure polymer matrix. To gain fundamental insight into the deformation mechanisms present in such materials, the complexity of the system is step-wise increased; first, the effect of the most commonly applied adhesion enhancement, the addition of MAH-g-PP compatibilizer, on the bulk properties is assessed. The small-strain tensile properties, i.e., modulus and yield stress, appear to be only marginally affected by the addition of such compatibilization agent, however, the strain-at-break is strongly reduced, even before the addition of the fiber reinforcement. Subsequently, using in-situ X-ray characterization methods upon tensile deformation, the time evolution of crystal structure and lamellar morphology is determined, and at first glance the compatibilizer addition appears to better preserve the crystalline structure. The onset of local failure (cavitation) is quantified at the interface of a single glass fiber. By increasing the adhesive interaction between fiber and matrix the stress concentration at the interface is increased, leading to an acceleration in void formation followed by unstable growth, which in turn strongly embrittles the composite. By the addition of various selective nucleating agents, it is demonstrated that the role of local phase composition and morphology on the deformation kinetics and subsequent failure mechanisms is much more pronounced than the increased adhesion between fiber and matrix by compatibilization or sizing effects. These findings may specify a new route towards tougher fiber-reinforced composites with reduced complexity in the material formulation.</p

Synergy of Fiber Surface Chemistry and Flow: Multi-Phase Transcrystallization in Fiber-Reinforced Thermoplastics

Fiber-reinforced polymer composites are largely employed for their improved strength with respect to unfilled matrices. Considering semi-crystalline materials under relevant processing conditions, the applied pressure and flow induce shear stresses at the fiber–polymer interface. These stresses may strongly enhance the nucleation ability of the fiber surface with respect to the quiescent case. It is thus possible to assume that the fiber features are no longer of importance and that crystallization is dominated by the effect of flow. However, by making use of an advanced experimental technique, i.e., polarization-modulated synchrotron infrared microspectroscopy (PM-SIRMS), we are able to show that the opposite is true for the industrially relevant case of isotactic polypropylene (iPP). With PM-SIRMS, the local chain orientation is measured with micron-size spatial resolution. This orientation can be related to the polymer nucleation density along the fiber surface. For various combinations of an iPP matrix and fiber, the degree of orientation in the cylindrical layer that develops during flow correlates well with the differences in nucleation density found in quiescent conditions. This result shows that the morphological development during processing of polymer composites is not solely determined by the flow field, nor by the nucleating ability of the fiber surface alone, but rather by a synergistic combination of the two. In addition, using finite element modeling, it is demonstrated that, under the experimentally applied flow conditions, the interphase structure formation is mostly dominated by the rheological characteristics of the material rather than perturbations in experimental conditions, such as shear rate, layer thickness, and temperature. This once again highlights the importance of matrix–filler interplay during flow and, thus, of material selection in the design of hybrid and lightweight composite technologies

Epitaxy in Polybutene‑1 Form II-on-Form I Cross-Nucleation Revealed by Nanofocused X‑ray Diffraction on Ad Hoc Morphology

The existence of epitaxy in polybutene-1 form II-on-form I cross-nucleation was investigated by nanofocused synchrotron X-ray diffraction. To this aim, form I hedrites, which show a lamellar stacking with a uniform crystal lattice orientation, were adopted as the substrate. The form II crystals develop a spherulitic transcrystalline layer on the lateral surface of the form I substrate, due to cross-nucleation. Through mapping both the intensity and azimuthal orientation of characteristic planes of the two polymorphs, a clearly defined nucleation area between form II (daughter) and form I (parent) could be identified. Comparing the two-dimensional diffraction patterns in that area, a preferred mutual orientation of the two structure is revealed. In particular, the (200)II plane of form II and the (110)I plane of form I are reciprocally oriented at a fixed angle of ∼8.5°. This orientation results in almost parallel (110) planes between the two structures. It is calculated that the mismatch between interchain distances within the common (110) planes is about 4%, while that along the chain axes is less than 10%, both well below the accepted mismatch criterion for epitaxial crystallization. These results provide solid evidence for the existence of an epitaxial relationship in the cross-nucleation between polybutene-1 form II and form I. We note that the (110) contact plane between the two structures in cross-nucleation is the same as the one involved in the well-studied solid-state phase transformation from form II to form I. Moreover, the X-ray nanofocus approach and the proposed data analysis could be effectively applied to other cross-nucleating systems to shed light on the role of epitaxy in this peculiar phenomenon of nucleation between polymorphs

Polarization modulated infrared spectroscopy: A pragmatic tool for polymer science and engineering

In the area of polymer crystallization, the most widely used techniques to quantify structure, morphology and molecular orientation are fundamentally based on light or X-ray scattering and absorption. In particular, synchrotron X-rays are used for detailed studies on the semicrystalline structure in polymeric materials. The technical requirements for such techniques, especially when high spatial resolution is essential, make the application of X-ray diffraction not straightforward. Direct information on the chain orientation in different semicrystalline morphologies requires rather complex sampling and analysis procedures. Surprisingly, a simple yet versatile technique based on infrared spectroscopy is hardly applied in the field of polymer crystallization. By modulating the polarization of the incident light, local anisotropy can be studied in real time on a submolecular length scale. In this article, we provide the relevant details of the polarization modulated infrared microspectroscopy technique for the study of semicrystalline materials from an engineering perspective. We demonstrate the essence of the method using as model systems spherulitic and transcrystalline morphologies and present its applicability to polymer/fiber composite technology and the study of injection-molded parts. The results provided in the present work serve to illustrate the applicability of this informative technique in the field of semicrystalline polymer science

An experimentally validated model for quiescent multiphase primary and secondary crystallization phenomena in PP with low content of ethylene comonomer

While crystallization behavior of isotactic polypropylene homopolymers had been subject to a wide range of experimental and modeling studies, this is not the case for propylene-ethylene random copolymers (PPR). This class of polymers offers up to now significant challenges, both from an experimental as well as a modeling perspective. The ethylene incorporation in the propylene chains, as well as the distribution of this comonomer, has a marked effect on the crystallization kinetics. Moreover, the presence of these defects causes a clear separation between primary crystallization (i.e. space filling) and subsequent secondary crystallization (increase of crystallinity in filled space) within the spherulitic skeletons, particularly subsequent at high primary crystallization temperatures. In this work, the underlying mechanism is first quantified by means of a combination of in-situ WAXD and SAXS experiments, as well as ex-situ WAXD experiments and calorimetric measurements. Based on these experiments an extended model framework is presented, capable of predicting multiphase non-isothermal crystallization kinetics as well as the final crystallinity as a function of the applied thermal conditions relevant for processing. The chemical composition distribution (CCD) of the ethylene comonomer serves as critical input to parameterize the model. Optical microscopy- and DSC experiments are used for parameterization of the primary crystallization model. The model developed in this study is, in principle, applicable to all polypropylenes, ranging from homo-polymers to random copolymers with variable comonomer content and/or CCD but, so far, only applied and validated on one PPR. To validate the model and the parameters for a given PPR, several non-isothermal and isothermal experiments (the latter followed by subsequent cooling) are conducted over a wide range of crystallization temperatures and cooling rates. The good match between experiments and model predictions demonstrates the power of the newly developed framework. The final crystallinity, the amount of α- and γ-phase, and the ratio between primary and secondary crystallization can be predicted as a function of the time-temperature history. To the best knowledge of the authors, it is the first time that such a direct connection with the CCD is incorporated in a crystallization model. Consequently, the model offers a new tool to bridge the gap between chemical structure and resulting product properties, which now has come one step closer for PPR systems

Shaping and properties of thermoplastic scaffolds in tissue regeneration: The effect of thermal history on polymer crystallization, surface characteristics and cell fate

Abstract: Thermoplastic semi-crystalline polymers are excellent candidates for tissue engineering scaffolds thanks to facile processing and tunable properties, employed in melt-based additive manufacturing. Control of crystallization and ultimate crystallinity during processing affect properties like surface stiffness and roughness. These in turn influence cell attachment, proliferation and differentiation. Surface stiffness and roughness are intertwined via crystallinity, but never studied independently. The targeted stiffness range is besides difficult to realize for a single thermoplastic. Via correlation of thermal history, crystallization and ultimate crystallinity of vitamin E plasticized poly(lactide), surface stiffness and roughness are decoupled, disclosing a range of surface mechanics of biological interest. In osteogenic environment, human mesenchymal stromal cells were more responsive to surface roughness than to surface stiffness. Cells were particularly influenced by overall crystal size distribution, not by average roughness. Absence of mold-imposed boundary constrains makes additive manufacturing ideal to spatially control crystallization and henceforward surface roughness of semi-crystalline thermoplastics. Graphic abstract: [Figure not available: see fulltext.