Kinetics of ageing and re-embrittlement of mechanically rejuvenated polystyrene

Essential Oils from Citrus reticulata cv. Shatangju Peel: Optimization of Hydrodistillation Extraction by Response Surface Methodology and Evaluation of Their Specific Adhesive Effect to Polystyrene

Essential oil (Eo) from the Citrus reticulata peel has been widely used, and its adhesive effect on polystyrene (PS) was found accidentally. To analyze the essential oils of C. reticulata cv. Shatangju (CrspEos), the extraction of these oils by steam distillation was optimized using the response surface method. The chemical composition of CrspEos was analyzed by gas chromatography-mass spectrometry. Then, the adhesive effect of these essential oils on PS was evaluated. The adhesion area, the influence of adhesion on the thickness of the essential oil on the materials, the effect of adhesion on the transmittance of PS, the strength of adhesion point, and specificity of adhesion were determined. The optimum extraction conditions resulting in the extraction yield of 47.37 μL g^-1 were a ratio of liquid-to-solid of 8.94:1, a soaking time of 199.45 min, and an extraction time of 138.71 min. The major component in the essential oils was d-limonene (56.66%), followed by myrcene (6.62%). CrspEos presented a specific adherence effect on PS without influencing the thickness and transmittance of PS but with stronger tenacity than the parent material. CrspEos can be used as an environmentally friendly specific adhesive for PS.

Durably Ductile, Transparent Polystyrene Based on Extensional Stress-Induced Rejuvenation Stabilized by Styrene-Butadiene Block Copolymer Nanofibrils

The glassy polymer of polystyrene (PS) enjoys a good reputation as a promising optical material; however, the inherent brittleness hinders its further applications. Conventional toughening methods are realized based on the premise of a sacrifice in transparency and stiffness. In this work, we found an unprecedented strategy to address these obstacles by combining extensional stress-induced ductility and suppressing physical aging. PS-based film with a high stiffness, long-term ductility, and excellent transparency is achieved by introducing a styrene-butadiene block copolymer into the PS matrix and subsequently annealing stretched. A nanofibrillar structure of the polybutadiene (PB) phase is formulated surrounded by a PS matrix, and thus, the elongation at break enhances from 3.1% up to 86.8%, accompanying the yield strength enhanced from 25.5 to 62.2 MPa. More significantly, compared with neat PS, these films survive from physical aging and persistent ductility over time. The morphology deformation induced by stress makes an obvious contribution to the improvement of transparency. Investigating the dynamics of chain segments indicates that the incorporation of the copolymer can restrict rearrangement and local relaxation to the PS chain. This work could pave a potential route toward high-performance PS and might be transferable to other glassy polymers with a fragile character.

Physical Ageing of Polystyrene: Does Tacticity Play a Role?

The ageing kinetics of amorphous atactic (a-PS), isotactic (i-PS), and syndiotactic (s-PS) polystyrene were studied by means of flash-differential scanning calorimetry. The specimens were aged for up to 2 h at six different ageing temperatures: the optimum ageing temperature, that is, the temperature at which the enthalpy overshoot at the glass transition is maximal for the given elapsed time, and five ageing temperatures ranging from 20 to 80 K below the optimum ageing temperature. A logarithmic increase of the enthalpy overshoot with ageing time is observed for specimens at their optimum ageing temperatures. For temperatures significantly lower than the optimum, there is a range where the enthalpy overshoot is constant, but for higher temperatures (still below the optimum), a logarithmic increase is also observed. Moreover, the ageing kinetics appear to depend on tacticity, with s-PS and i-PS exhibiting the slowest and fastest ageing kinetics, respectively, and a-PS exhibiting ageing kinetics between these two extremes.

Simultaneous SAXS-WAXS Experiments on Semi-Crystalline Polymers: Example of PA11 and Its Brill Transition

This manuscript of the special issue “Microstructural Evolution and Mechanical Behavior of Semi-Crystalline Polymers” aims to show that Small Angle X-ray Scattering (SAXS) and Wide Angle X-ray Scattering (WAXS) experiments performed simultaneously constitute a unique tool to obtain valuable information on the hierarchical structure of semi-crystalline polymers. These structural quantitative data are needed to model macroscopic properties of polymeric materials, for example their mechanical properties. To illustrate our point, we focus our study on the structure and morphology of polyamide 11. Through a simultaneous SAXS-WAXS experiment, we show that the absence of enthalpic signal in Differential Scanning Calorimetry (DSC) is not synonymous with the absence of structural and morphological evolution with temperature. The case of a thermally activated crystal–crystal transition, the Brill transition, is particularly detailed. Through this SAXS-WAXS study, we show, among other points, and for the first time, that the periodicity of crystalline lamellae (LP) changes at the transition, probably due to a modification of the amorphous phase’s free volume at the Brill transition. We also explain the crucial role of annealing to stabilize polymeric materials that may experience temperature changes over their lifetime. The influence of the annealing on the perfection of crystalline structure, morphology and mechanical behavior is more particularly studied.

Highly Anisotropic Glassy Polystyrenes Are Flexible

We show that stretching polystyrene melts at a rate faster than the inverse Rouse time, followed by rapid quenching below the glass transition temperature, results in a material that is flexible and remains so for at least six months. Oriented micro/nanofibers are observed in the flexible samples after the mechanical tests. The fibers are probably related to the highly aligned molecules in melt stretching. At room temperature, a tensile strength over 300 MPa has been achieved for the flexible polystyrenes.

Time Allowed for Equilibration Quantifies the Preparation Induced Nonequilibrium Behavior of Polymer Films

Performance and properties of materials may strongly depend on processing conditions. This is particularly so for polymers, which often have relaxation times much longer than the processing times and therefore may adopt preparation dependent nonequilibrated molecular conformations that potentially cause novel properties. However, so far it was not possible to predictably and quantitatively relate processing steps and resulting properties of polymer films. Here, we demonstrate that the behavior of polymer films, probed through dewetting, can be tuned by controlling preparation pathways, defined through a dimensionless parameter [Formula: see text], which is the appropriate preparation time normalized with the characteristic relaxation time of the polymer. We revealed scaling relations between [Formula: see text] and the amount of preparation-induced residual stresses, the corresponding relaxation time, and the probability of film rupture. Intriguingly, films of the same thickness exhibited hole nucleation densities and subsequent dewetting kinetics differing by up to an order of magnitude, indicating possibilities to adjust the desired properties of polymer films by preparing them in appropriate ways.

Modeling energy storage and structural evolution during finite viscoplastic deformation of glassy polymers

The enthalpic response of amorphous polymers depends strongly on their thermal and deformation history. Annealing just below the glass transition temperature (T_{g}) causes a large endothermic overshoot of the isobaric heat capacity at T_{g} as measured by differential scanning calorimetry, while plastic deformation (cold work) can erase this overshoot and create an exothermic undershoot. This indicates that a strong coupling exists between the polymer structure, thermal response, and mechanical deformation. In this work, we apply a recently developed thermomechanical model for glassy polymers that couples structural evolution and viscoplastic deformation to investigate the effect of annealing and plastic deformation on the accumulation of stored energy during cold work and calorimetry measurements of heat flow. The thermomechanical model introduces the effective temperature as an additional state variable in a nonequilibrium thermodynamics setting to describe the structural evolution of the material. The results show that the model accurately describes the stress and enthalpy response of quenched and annealed polymers with different plastic predeformations. The model also shows that at 30% strain in uniaxial compression, 45% of the applied work is converted into stored energy, which is consistent with experimental data from literature.