Strain-Induced Crystallization of Segmented Copolymers: Deviation from the Classic Deformation Mechanism

Dual Ratiometric Fluorescence Monitoring of Mechanical Polymer Chain Stretching and Subsequent Strain-Induced Crystallization

Tracking the behavior of mechanochromic molecules provides valuable insights into force transmission and associated microstructural changes in soft materials under load. Herein, we report a dual ratiometric fluorescence (FL) analysis for monitoring both mechanical polymer chain stretching and strain-induced crystallization (SIC) of polymers. SIC has recently attracted renewed attention as an effective mechanism for improving the mechanical properties of polymers. A polyurethane (PU) film incorporating a trace of a dual-emissive flapping force probe (N-FLAP, 0.008 wt %) exhibited a blue-to-green FL spectral change in a low-stress region (<20 MPa), resulting from conformational planarization of the probe in mechanically stretched polymer chains. More importantly, at higher probe concentrations (∼0.65 wt %), the PU film showed a second spectral change from green to yellow during the SIC growth (20-65 MPa) due to self-absorption of scattered FL in a short wavelength region. The reversibility of these spectral changes was demonstrated by load-unload cycles. With these results in hand, the degrees of the polymer chain stretching and the SIC were quantitatively mapped and monitored by dual ratiometric imaging based on different FL ratios (I525/I470 and I525/I600). Simultaneous analysis of these two mappings revealed a spatiotemporal gap in the distribution of the polymer chain stretching and the SIC. The combinational use of the dual-emissive force probe and the ratiometric FL imaging is a universal approach for the development of soft matter physics.

Inherent Flame-Retardant, Humid Environment Stable and Blue Luminescent Polyamide Elastomer Regulated by Siloxane Moiety

The rapid development of the polymeric materials market has created an urgent demand for the thermoplastic polyamide elastomer (TPAE) owing to its greater functionality, and ability to be synthesized via a facile and industrial route. In this work, a series of novel silicone-containing polyamides (PA1212/Si12) were successfully synthesized from 1,12-dodecarboxylic acid (LA), 1,12-dodecarbondiamine (DMDA), and 1,3-bis (amino-propyl) tetramethyldisiloxane (BATS), via a one-pot melt polycondensation method in the absence of a catalyst. FTIR, ¹H-NMR, GPC and inherent viscosity results cohesively prove that the polymerization of monomers was well conducted, and the chemical structure was in high accordance with the design. As expected, the Si12 unit-content of the copolymers regulate the properties of the series. As the feeding ratio of BATS in the diamines increases from 5 mol% to 40 mol%, the thermal transition temperatures, T_g and T_m, decline steadily before finally stabilizing at ~6 °C and 160 °C, respectively, indicating that the co-polyamides possess improved chain flexibility but restricted crystallization ability. The conspicuous evolution in crystalline morphology of the series was observed by XRD and AFM. The increased PA Si12 phase induces the crystallized PA 1212 phase to transit from a thermally-favorable large and rigid crystal structure (α phase) to a kinetically-favorable small and ductile crystal structure (γ phase). Reflected in their stress–strain behavior, PA1212/Si12 copolymers are successfully tailored from rigid plastic to ductile elastomer. The tensile strength mildly drops from above 40 MPa to ~30 MPa while the reversible elongation increases from ~50% to approximately 350%. Accordingly, the moderate surface tension differences in the monomers facilitate the efficient conduction of the co-polymerization process, and the distributed short siloxane unit in the backbone fulfills the copolymer with desirable elasticity. Interestingly, the novel silicone-containing polyamides also display Si12 unit-content dependent flame retardancy, humidity stability, and unconventional solid-state fluorescence properties. The elastomers exhibit a low bibulous rate and anti-fouling characteristics to dye droplets and mud contamination, pass the V–1 rating (UL 94) with a constantly declining PHRR value, and emit blue luminescence under a 365 nm light source. Herein, we propose a new facile strategy for developing a high-performance and multifunctional silicone-modified polyamide, which bears promising industrialization potential. In addition, this first reported silicone-containing thermoplastic polyamide elastomer, which is self-extinguishing, anti-fouling and blue-luminescent, will further broaden the application potential of thermoplastic polyamide elastomers.

Dynamic Monte Carlo simulations of strain-induced crystallization in multiblock copolymers: effects of dilution

Multiblock copolymers containing alternating semicrystalline and molten blocks are good thermoplastic elastomers. Their crystallization in the stretching process is however complicated by the dilution effects, prior microphase separation and contrast chain rigidity of the molten blocks. We designed our systematic investigation with three integrated steps, and herein, as the first step, we considered only the dilution effects without prior microphase separation and contrast chain rigidity. We compared two extreme situations of local dilution separately corresponding to parallel-posited and antiparallel-posited block copolymers upon strain-induced crystallization. Our dynamic Monte Carlo simulations of diblock and tetrablock copolymers demonstrated that the stretching introduces a constraint on the diffusion of locally posited crystallizable blocks along the stretching direction for crystallization and thus enhances the dilution effects to result in a higher diversity in crystal stabilities. We observed that the strain-induced crystallization of parallel-posited copolymers behaved like the melt crystallization of homopolymers; in contrast, the strain-induced crystallization of antiparallel-posited copolymers yielded crystallites near the block junction, which are relatively small and less stable due to their local dilution suppressing their melting points. Similar to the case of spider dragline silks, two contrasting stabilities of crystallites in semicrystalline multiblock copolymers explain their good toughness. Our modeling approach paves the way toward a better understanding of the structure-property relationship in the semicrystalline thermoplastic elastomers.

Structures and properties of newly synthesized semi-aromatic polyamide thermoplastic elastomers

Novel semi-aromatic polyamide based thermoplastic elastomers containing both strong and weak H-bond units were fabricated via a facile “two-step” melt polycondensation method. The structures and properties of a series of TPAEs are discussed.

Strain-Induced Form Transition and Crystallization Behavior of the Transparent Polyamide

A transparent polyamide, poly(4,4'-aminocyclohexyl methylene dodecanedicarboxylamide) (PAPACM12), was studied and characterized by in situ wide-angle X-ray diffraction (WAXD) to establish the relationship between its crystallization behavior, crystalline form transition under external fields, and macroscopic properties. During the heating process, cold crystallization occurred and increased, and there was no form transition below the melting point. During the isothermal process, PAPACM12 exhibited the same crystalline structure as that during the heating process. The crystalline structure of PAPACM12 was attributed to α-form crystal, which is the stable form, according to the WAXD diffraction peaks of the conventional AABB-type polyamides. During stretching deformation, the crystal transition from α-form to γ-form and strain-induced crystallization were observed to contribute to the PAPACM12 with higher breaking strength and elongation. This study firstly determine the crystalline structure of transparent polyamides, and then the controlled strain-induced crystallization and transformation are demonstrated to be effective preparation methods for polyamides with high properties.

Microphase separation/crosslinking competition-based ternary microstructure evolution of poly(ether-b-amide)

The temperature dependence of the rheological properties of poly(ether-b-amide) (PEBA) segmented copolymer under oscillatory shear flow has been investigated. The magnitude of the dynamic storage modulus is affected by the physical microphase separation and irreversible crosslinking network, with the latter spontaneously forming between the polyamide segments and becoming the dominant factor in determining the microstructural evolution at temperatures well above the melting point of PEBA. From the rheological results, the initial temperature of the rheological properties dominated by the microphase separation and crosslinking (T cross) structures were determined, respectively. Based on the two obtained temperatures, the microstructure evolution upon the heating can be separated into the ternary microstructure domains: homogenous (temperature below ), microphase separation dominating (between and T cross), and crosslinking dominating domains (above T cross). When the PEBA is heated to above T cross, the content of crosslinking network increases with time and temperature, leading to an irreversible and non-negligible influence on the rheological, crystallization, and mechanical properties. A more pronounced strain-hardening phenomenon during the uniaxial stretching is observed for the sample with a higher content of crosslinking network.

Multicolor Organometallic Mechanophores for Polymer Imaging Driven by Exciplex Level Interactions

Photoluminescent compounds can undergo various structural changes upon interaction with light. When these changes manifest themselves in the excited state, the resulting emitters can obtain a sensory function. In this work, we designed coordination compounds that can vary their emission color in response to thermal and mechanical stimuli. When embedded in a polymer matrix, Cu-NHC sensors act as mechanophores, and their color-based response can readily describe mechanical stress and phase transition phenomena. A strong practical advantage of new mechanophores over previous generations of organometallic stress sensors stems from their reliance on emission color variations that are easy to detect. In a broad context, our work implies that emission color variations that we often view as thermally governed can also be triggered mechanically and used to generate sensory information.

Dielectric Behavior and Phase Behavior of Block Copolymer PEO13-PPO30-PEO13 Aqueous Solution

Dielectric spectroscopy can be applied to study the structure and dynamics of block polymer. In this work, dielectric measurements of block copolymer Pluronic L64 solution are carried out in the frequency range between 40 Hz and 110 MHz with variable temperatures and concentrations. We analyze the phase behavior of the PEO₁₃-PPO₃₀-PEO₁₃ (Pluronic L64) aqueous system according to the concentration/temperature-dependence of direct current conductivity. The result indicates the sensitivity of the phase behavior and conductivity of the Pluronic L64 solution to temperature. Besides, two relaxations were observed: relaxation 1 (0.5 MHz) is related to the gelation process, while relaxation 2 (5 MHz) is caused by the interface polarization. On the basis of relaxation 2, the volume fraction and permittivity of the particle were calculated. The formations of the block copolymer micelle and gel are monitored successfully by the temperature/concentration-dependence of the dielectric parameters and the volume fraction.

Counits Content and Stretching Temperature-Dependent Critical Stress for Destruction of γ Crystals in Propylene-Ethylene Random Copolymers

Tensile deformation behavior of three random propylene-ethylene copolymers with the same molecular weight and different contents of counit was investigated at different temperatures from room temperature to close to melting point via tensile tests, step-cycle tests, and in situ wide-angle X-ray diffraction techniques. Upon stretching, the original crystalline lamellae must be destroyed, generating new highly oriented ones. A critical stress has been suggested, under which the original crystallites can be destructed. The propylene-ethylene copolymer samples in pure γ-form transformed gradually into α-form during tensile stretching. This crystalline transition proceeded via a destruction (melting) of the original γ-form crystals followed by recrystallization of the freed polymeric chain segments into α-form along stretching direction. This result provides a marker for investigating the critical stress mentioned above. Such critical stresses triggering the destruction of γ-form crystals for the propylene-ethylene copolymers of different ethylene counit contents were successfully calculated. It turned out that this critical stress depended on the ethylene counit content and stretching temperature. Samples with less ethylene counits show higher critical stress because of a lower degree of insertion of the ethylene counits into the crystalline unit cell than samples with higher ethylene counit content. The critical stresses remained constant when the samples were stretched at low-temperature region, whereas they decreased significantly at high stretching temperatures close to the melting points due to strong thermal distortion of the crystalline lattices, making the crystallites less stable.

Double Crystalline Multiblock Copolymers with Controlling Microstructure for High Shape Memory Fixity and Recovery

The shape memory performance of double crystalline poly(butylene succinate)-co-poly(ε-caprolactone) (PBS-co-PCL) multiblock copolymers with controlling microstructure was studied, and the corresponding microstructure origin was further quantitatively analyzed by wide and small-angle X-ray scattering (WAXS and SAXS) experiments. It was found that the multiblock copolymer with higher PCL content, proper deformation strain, and inhibited crystallization of PBS (lower crystallinity and smaller crystal size, which could be realized by quenching from the melt) would exhibit better shape memory fixity and recovery performance. WAXS and SAXS results revealed that the shape fixity ratio (R_f) was closely related with the relative crystallinity of the PCL component, while the shape recovery ratio (R_r) strongly relied on the deformation and recovery behavior of the PBS and PCL components that changed along with compositions and deformation strains. For the copolymer with higher PCL content (BS₃₀CL₇₀), at the lower deformation strain (0% ∼ 90%), both the PBS and PCL components after recovery had no orientation (labeled as stage I), resulting in almost complete recovery; with the deformation strain increasing (90% ∼ 200%), it was the irreversible deformation of the PCL component that largely took responsibility for the decreased R_r (stage II). On the contrary, when the PCL content decreased to 50 wt % (BS₅₀CL₅₀), stage I (0% ∼ 50%) and stage II (50% ∼ 100%) appeared in much lower strains; with the deformation strain increasing (100% ∼ 200%), the irreversible deformation of both PBS and PCL components was mainly responsible for the further reduction of R_r (stage III). It could exhibit excellent shape memory performance for biodegradable double crystalline multiblock copolymers by controlling the composition, deformation strain, and crystallization, which might have wide application prospects in biomedical areas.

Self-Associated Polyamide Alloys with Tailored Polymorphism Transition and Lamellar Thickening for Advanced Mechanical Application

Long chain polyamides with various number of methylene units in recurring amide groups, PA1012 and PA612, were blended to combine their unique advantages. The Brill transition and accompanied lamellar thickening were investigated by in situ wide angle X-ray scattering (WAXS) and small angle X-ray scattering. From WAXS patterns, the transformation from the α- to γ-crystalline phase, known as "Brill transition", can be independently observed in the constituent phases of the long chain polyamide alloys (LCPAs) during heating. A constant T_b (ca., 100 °C) irrespective of the blend composition and proportional variations of the phase content was obtained. Additionally, with elevated temperature, a gradual increase in both the crystalline layer (L_c) and amorphous layer (L_a) was detected in constituent polyamides. The compositional independence of the Brill transition in LCPAs and similar lamellar thickening originate from the complete immiscibility of both polyamides, which share stronger intramolecular rather than intermolecular hydrogen-bonding interaction and hence exhibit self-association. Contributed by the γ phase, with less extended structure and increased lamellar thickness with compact stacking, LCPAs with controlled strength and flexible features can be achieved, which can be utilized in advanced mechanical applications, particularly for hoses of automobiles. The unusually linear compositional dependence of mechanical parameters makes it possible to tailor the polymorphic and tensile properties.