Amorphous Polymers Research Articles

Effect of Polymer Matrix on Inelastic Strain Development in PI- and PEI-Based Composites Reinforced with Short Carbon Fibers under Low-Cyclic Fatigue

Since the inelastic strain development plays an important role in the low-cycle fatigue (LCF) of High-Performance Polymers (HPPs), the goal of the research was to study the effect of an amorphous polymer matrix type on the resistance to cyclic loading for both polyimide (PI)- and polyetherimide (PEI)-based composites, identically loaded with short carbon fibers (SCFs) of various lengths, in the LCF mode. The fracture of the PI and PEI, as well as their particulate composites loaded with SCFs at an aspect ratio (AR) of 10, occurred with a significant role played by cyclic creep processes. Unlike PEI, PI was less prone to the development of creep processes, probably because of the greater rigidity of the polymer molecules. This increased the stage duration of the accumulation of scattered damage in the PI-based composites loaded with SCFs at AR = 20 and AR = 200, causing their greater cyclic durability. In the case of SCFs 2000 µm long, the length of the SCFs was comparable to the specimen thickness, causing the formation of a spatial framework of unattached SCFs at AR = 200. The higher rigidity of the PI polymer matrix provided more effective resistance to the accumulation of scattered damage with the simultaneously higher fatigue creep resistance. Under such conditions, the adhesion factor exerted a lesser effect. As shown, the fatigue life of the composites was determined both by the chemical structure of the polymer matrix and the offset yield stresses. The essential role of the cyclic damage accumulation in both neat PI and PEI, as well as their composites reinforced with SCFs, was confirmed by the results of XRD spectra analysis. The research holds the potential to solve problems related to the fatigue life monitoring of particulate polymer composites.

Impact of Topological Parameters on Melt Rheological Properties and Foamability of PS POM-POMs

Rheological properties highly impact the foaming behavior of polymers, as a high melt strength and low shear viscosity support cell expansion by reducing cell coalescence and rupture. Since rheological properties originate from molecular topology, topology is consequently a governing parameter for foaming. We synthesized well-defined polystyrene (PS) POM-POM-shaped model systems to study the impact of the topology-originated melt rheological parameters on the physical foaming behavior within purely amorphous polymers without any crystallization effects. Therefore, we systematically varied the topological parameters of the POM-POM topology, i.e., the number of arms q, the arm length Mw,a, and the backbone length Mw,b. We investigated the melt rheological properties of the samples concerning their zero-shear viscosity and elongational strain hardening and correlated their resulting foam structure. Within the sample series, η0 can be reduced by up to 104, compared to linear PS with the same total molecular weight, whereas the maximum achieved strain hardening factor is SHF = 141 at T = 160 °C. The foaming experiments revealed that the mean cell size D and the volume expansion VER can be precisely controlled via molecular topology. By controlling the topological parameters, it was possible to increase VER from VER = 2.0 to 15.6, whereas D varied between D = 1.3 and 17.2 μm within 100–150 °C under a pressure of 500 bar. In comparison, the VER of linear PS increased from VER = 2.6 to 5.4, while D increased from D = 2.3 to 4.8 μm. Moreover, the influence of topology is more pronounced at higher temperatures, indicating that the molecular relaxation times and the applied shear rates need to be related. Hence, it is possible to tailor the cellular structure to a specific application by considering and controlling the polymer topology.

Fully Bio-based Poly(ketal-ester)s by Ring-opening Polymerization of a Bicylcic Lactone from Glycerol and Levulinic Acid.

A fully renewable bio-based bicyclic lactone containing a five-membered cyclic ketal moiety, 7-methyl-3,8,10-trioxabicyclo[5.2.1]decan-4-one (TOD), was synthesized through a two-step acid-catalyzed process from glycerol and levulinic acid. The ring-opening polymerization (ROP) of TOD at 30°C with benzyl alcohol (BnOH) as the initiator and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) as the catalyst can afford high molar mass PTOD with a cis-2.4-disubstitued 2-methyl 1,3-dioxolane moiety in its repeating unit. PTOD is an amorphous polymer with a glass transition temperature (Tg ) of 13°C. It can be hydrolyzed into structurally defined small molecules under acidic or basic conditions by the selective cleavage of either the cyclic ketal or the ester linkage respectively. The TBD-catalyzed copolymerization of L-lactide (L-LA) and TOD at -20°C was investigated. It was confirmed that L-LA polymerized quickly with racemization to form PLA, followed by a slow incorporation of TOD into the formed PLA chains via transesterification. By varying the feed ratios of L-LA to TOD, a series of random copolymers (PLA-co-PTOD) with different TOD incorporation ratios and tunable Tg s were obtained. Under acidic conditions, PLA-co-PTOD degrades much faster than PLA via the selective cleavage of the cyclic ketal linkages. This work provides insights for the development of more sustainable and acid-accelerated degradable alternatives to aliphatic polyesters.

Bio-inspired hydrogel actuator with rapid self-strengthening behavior

Inspired by intelligent biomaterials which often have flexible responsiveness to multiple environmental cues and strengthen their mechanical properties by trainings, emerging soft actuators require programmable manipulation. Currently, the integration of such characteristics as rapid self-strengthening, strain-adaptive stiffening and smart actuation, into a single hydrogel actuator is urgently needed. Here, we report a self-strengthened hydrogel actuator based on the semi-interpenetrating polymer network consisting of polyvinyl alcohol (PVA) and poly(N-isopropylacrylamide) (PNIPAm), which can display diverse programmable actuations by responding to temperature/salt stimuli. In the design, the layer of freeze-thawed PNIPAm/PVA (PPGel-F) is assembled with another original PNIPAm/PVA hydrogel layer (PPGel) into one device. By taking advantage of the PVA crystalline nanofibrils in the PPGel-F matrix, the differentiated swelling degree across the bilayer structure gives rise to asymmetric deformations and the resultant shape transformation. Moreover, upon the mechanical training with less than 100 cycles, the anisotropic arrangement of PVA nanofibrils through strong hydrogen bonding interactions can swiftly immobilize the amorphous polymer chains orientation along the tensile direction. This enhances the strain-induced crystallization, thereby generating the rapid self- strengthening behavior. The proposed work provides potential solution for constructing dynamically adaptive hydrogel systems that can mimic biological tissues for more intelligent soft robotics and bionic research.

Designing a Length‐Modulated Azide Photocrosslinker to Improve the Stretchability of Semiconducting Polymers

AbstractTo impart high stretchability to semiconducting polymers, researchers have used a photocrosslinking approach based on the nitrene chemistry of an azide‐incorporated molecular additive. However, understanding of the molecular design of azide crosslinkers with respect to their effects on the electrical and mechanical properties of semiconducting polymer thin films is lacking. In this study, the effects of an azide photocrosslinker's molecular length and structure on the microstructural, electrical features, and stretchability of photocrosslinked conjugated polymer films is investigated. For a systematic comparison, a series of nitrene‐induced photocrosslinkers (n‐NIPSs) with different numbers of ethylene glycol repeating units (n = 1, 4, 8, 13) that bridge two tetrafluoro‐aryl azide end groups is synthesized. Two semicrystalline conjugated polymers and two nearly amorphous conjugated polymers are co‐processed with n‐NIPSs and crosslinked by brief exposure to UV light. It is found that, among the synthesized n‐NIPSs, the shortest one (1‐NIPS) is the most efficient in improving the stretchability of crosslinked indacenodithiophene‐benzothiadiazole films and that the improvement is achieved only with nearly amorphous polymers, not with semicrystalline conjugated polymers. On the basis of systematic studies, it is suggested that crosslinking density in amorphous regions is important in improving thin film stretchability.

The Temperature Interval of the Liquid–Glass Transition of Amorphous Polymers and Low Molecular Weight Amorphous Substances

We present calculation results of the temperature interval δTg characterizing the liquid–glass transition in amorphous materials obtained on the basis of available data of the empirical parameters C1 and C2 in the Williams–Landel–Ferry (WLF) viscosity equation. We consider the unambiguous dependence of the relative transition temperature interval δTg/Tg on the fraction of the fluctuation volume fg frozen at the glass transition temperature Tg utilizing Sanditov’s model of delocalized atoms. The parameter f = ΔVe/V, which determines the molecular mobility characteristic of delocalized atoms in the liquid–glass transition region, is weakly dependent on the nature of most vitreous substances and can be found as fg = 1/C1. We show that the temperature interval δTg is less than 1% of the Tg for most amorphous substances. This result conforms with Simon’s classical idea of a small temperature range in which the structure freezes. The structural relaxation time τg at Tg of polymers and chalcogenide glasses is also calculated.

Cellulose Acetate polymer spectroscopic study comprised LaFeO3 perovskite and graphene as a UV-to-visible light converter used in several applications

A simple and fast method was used to synthesize cellulose acetate (CA), CA/LaFeO3, CA/graphene (G) and CA/LaFeO3/G nanocomposite films. X-ray diffraction (XRD) illustrates that all films have a broad peak at 2θ = 22.63° corresponding to the amorphous CA polymer. The Raman spectra of nanocomposite films show a strong band at 3075 cm−1 characteristics of asymmetric CH2 vibrations of CA and its intensity decreased in the case of G nanofiller due to the reaction between the CA polymer and nanoparticles. Field emission scanning electron microscope (FESEM) shows the porous nature of the smooth surface morphology of CA film with an average diameter of 2.23 µm. The addition of LaFeO3 and G to CA film has good distribution. The UV spectra of the prepared films revealed a broad absorption band ranging from 208 to 300 nm, corresponding to the carbonyl group in the CA polymer. The optical properties of the films show a direct transition with an optical band gap ranging from 3.9 to 4.2 eV. All the investigated samples converted harmful UV radiation into red light, but the CA/G film sample had the highest conversion efficiency, making it useful in a variety of applications.

A mean-field shear transformation zone theory for amorphous polymers

The complex nonlinear constitutive behavior of amorphous polymers has been extensively investigated in numerous experiments, displaying a strong dependence on the thermal history and loading path. Theoretical models have been proposed to conjoin the underlying mechanisms to certain aspects of mechanical features, such as yielding, strain hardening, physical aging, and the Bauschinger effect. However, an urgent need for theoretical understanding capable of fulfilling the comprehensive predictions of the featured constitutive behaviors of amorphous polymers is to be addressed. In this paper, we develop a micromechanical mean-field shear transformation zone (STZ) model to systematically describe the stress responses of amorphous polymers. The dynamics of STZ is affected by the nonequilibrium structural state described by effective temperature and also related to the anisotropic internal structure induced by the microscopical deformation. Our model exhibits the ability to fully capture the behaviors of amorphous polymers in constant rate/switch rate loading, creep, relaxation, and loading-unloading-reloading cycles. The model is further verified by reproducing the Bauschinger effect in experiments, which remains a challenge to other widely used models. Our results show that the deformation-induced material polarization, in the form of an anisotropic distribution of STZs, is the governing mechanism of the Bauschinger effect. This work establishes a comprehensive relationship between the microscopic mechanisms and the constitutive behaviors of deforming amorphous polymers, which advances the fundamental understanding of complex mechanical behaviors in amorphous glassy polymers.

Light-/steam-driven polymeric crosslinking with porous multistructure pattern for ultrastable and fast-speed memory

Organic memories typically comprise memristive polymer mediums sandwiched between two electrodes, with the advantages of wet manufacturing and modulated function. However, the issues associated with structural instability, low-speed switch, and hard pattern in polymeric memories are the main obstacles towards practical uses. Here, we present an ultrastable and fast-speed memory array that uses amorphous polymer nanofilm with light-/steam-driven crosslinked porous multistructure as memristive materials. The polymer diode shows nonvolatile rewritable flash memory characteristics, with a high ON/OFF ratio, long retention time, and high speeds of set (70 ns) and reset (845 ns) operations. Impressively, the memory cell undergoes harsh conditions in ultraviolet irradiation and extreme temperatures. By rationally integrating the array with target sensors, an artificial sensory memory architecture is constructed to mimic visual/thermal perception and recording, demonstrating great potential for biomimetic neuromorphic electronics. Our results advance a commercial perspective on memristive organics capable of integration patterns and high performance.

Conformation-dependent dynamic organic phosphorescence through thermal energy driven molecular rotations

Organic room-temperature phosphorescent (RTP) materials exhibiting reversible changes in optical properties upon exposure to external stimuli have shown great potential in diverse optoelectronic fields. Particularly, dynamic manipulation of response behaviors for such materials is of fundamental significance, but it remains a formidable challenge. Herein, a series of RTP polymers were prepared by incorporating phosphorescent rotors into polymer backbone, and these materials show color-tunable persistent luminescence upon excitation at different wavelengths. Experimental results and theoretical calculations revealed that the various molecular conformations of monomers are responsible for the excitation wavelength-dependent (Ex-De) RTP behavior. Impressively, after gaining insights into the underlying mechanism, dynamic control of Ex-De RTP behavior was achieved through thermal energy driven molecular rotations of monomers. Eventually, we demonstrate the practical applications of these amorphous polymers in anti-counterfeiting areas. These findings open new opportunities for the control of response behaviors of smart-responsive RTP materials through external stimuli rather than conventional covalent modification method.

Modelling strategy for tailorable mechanics and thermochemically driven shape memory effect in amorphous polymers

Thermochemically responsive shape-memory polymers (SMPs) with biocompatibility and variable stiffness have shown great potential in biomedical applications. However, it is difficult to characterize the relaxation behavior and visco-elastoplastic deformation of this kind of SMPs due to the complex interactions between polymer segments and solvent molecules. Herein, a thermo-chemo-mechanical coupling model was proposed to predict the shape-memory behavior and thermomechanical properties of SMPs at different solvent volume fractions and temperatures. By combining the Vrentas–Duta free volume theory and the Flory-Huggins theory, the effect of free volume change on the Gibbs free energy, chemical potential, thermal expansion coefficient, and relaxation time of SMPs was investigated. Subsequently, these functions were substituted into the Maxwell and Boyce models to characterize the visco-elastoplastic mechanical behavior of thermochemically responsive SMPs under large deformation. Based on the phase transition model, the chemical plasticizing effect of solvent molecules on the glass transition temperature, shape-recovery strain, and yield strength of SMPs was further investigated. The numerical results are in good agreement with the experimental results. The proposed model is expected to provide theoretical guidance for designing SMPs with tailorable mechanics.

Characterizing the biaxial compressive deformation behavior of epoxy polymer through cruciform experiment and finite element analysis

The multiaxial deformation behaviors of amorphous epoxy polymers were characterized using planar biaxial compression tests and finite element (FE) analyses of cruciform specimens. The multiaxial limit of elastic deformation was investigated considering buckling instability of the planar biaxial compression tests. An offset-based yield criterion was proposed to determine the initial yield surface considering the nonlinear elastic deformation characteristics observed in the deformation tests. An FE analysis confirmed using the results of the tests revealed that the proposed method captured the initial multiaxial yielding in the central gauge of the specimen, demonstrating that existing pressure-dependent classical yield functions, such as the paraboloidal and conical yield functions, cannot appropriately describe the initial yield surface of the epoxy polymer. The yield function previously customized from molecular dynamics (MD) data exhibited a relatively good agreement with the initial yield surface. The determined shape of the initial yield surface and its agreement with the MD data–driven yield function were not consistently maintained during subsequent yielding. The results indicate that a new customized yield function properly describing the broader initial yield surface than those of the classical yield functions and its shape transition to plastic behavior is necessary to accurately reflect the entire multiaxial deformation behavior.

Theory of low-temperature elasticity of amorphous polymers: Deformation at a constant rate and relaxation of deforming stress

The effect of temperature variations over a wide range on the rheological properties of amorphous polymers with high rubber-like elasticity (elastomers) is discussed. A theoretical study of the transition from the deformation state of a warm elastomer to the state of a frozen one, the effects of structural-strain glass transition and forced elasticity was done. Two types of mechanical testing of polymer samples are considered in detail: slow tensile deformation at a constant rate and relaxation of the deforming stress after deformation stops. The study was carried out on the basis of the previously proposed molecular-kinetic model of the processes of highly elastic deformation of amorphous polymers and the corresponding nonlinear rheological equation (V. D. Natsik and H. V. Rusakova, Fiz. Nizk. Temp. 48, 281 (2022) [Low Temp. Phys. 48, 253 (2022)]).

Possibilities and Limitations in Monomer Combinations for Ternary Two-Dimensional Covalent Organic Frameworks.

The diversity and complexity of covalent organic frameworks (COFs) can be largely increased by incorporating multiple types of monomers with different topologies or sizes. However, an increase in the number of monomer types significantly complicates the COF formation process. Accordingly, much remains unclear regarding the viability of monomer combinations for ternary or higher-arity COFs. Herein, we show that, through an extensive examination of 12 two-nodes-one-linker ([2 + 1]) combinations, monomer-set viability is determined primarily by the conformational strain originating from disordered monomer arrangements, rather than other factors such as the difference in COF formation kinetics between monomers. When monomers cannot accommodate the strain associated with the formation of a locally disordered, yet crystalline framework, the corresponding [2 + 1] condensation yields a mixture of different COFs or an amorphous polymer. We also demonstrate that a node-linker pair that does not form a binary COF can be integrated to generate a single-phase framework upon addition of a small amount of the third component. These results will clarify the factors behind the successful formation of multicomponent COFs and refine their design by enabling accurate differentiation between allowed and disallowed monomer combinations.

Significance of Hydrophobic Polymer in Novel Drug Delivery System

Polymers, due to their diverse topology and chemistry, account for a significant portion of the materials used in controlled release formulations and drug-targeting systems. They have played an integral role in the advancement of drug delivery technology by providing controlled release of therapeutic agents in constant doses over long periods, cyclic dosage, and tunable release of both hydrophilic and hydrophobic drugs. Eudragit is the brand name for a diverse range of polymethacrylate-based copolymers. It includes anionic, cationic, and neutral copolymers based on methacrylic acid and methacrylic/acrylic esters or their derivatives. Eudragits are amorphous polymers having glass transition temperatures between 9 to > 150o C. Eudragits are non-biodegradable, nonabsorbable, and nontoxic. Anionic Eudragit L dissolves at pH > 6 and is used for enteric coating, while Eudragit S, soluble at pH > 7 is used for colon targeting. In this review, the physicochemical characteristics and applications of different grades of Eudragit in colon-specific/enteric-coated/ sustained release drug delivery and taste masking have been addressed.

Antiaromatic Covalent Organic Frameworks Based on Dibenzopentalenes

Despite their inherent instability, 4n π systems have recently received significant attention due to their unique optical and electronic properties. In dibenzopentalene (DBP), benzanellation stabilizes the highly antiaromatic pentalene core, without compromising its amphoteric redox behavior or small HOMO-LUMO energy gap. However, incorporating such molecules in organic devices as discrete small molecules or amorphous polymers can limit the performance (e.g., due to solubility in the battery electrolyte solution or low internal surface area). Covalent organic frameworks (COFs), on the contrary, are highly ordered, porous, and crystalline materials that can provide a platform to align molecules with specific properties in a well-defined, ordered environment. We synthesized the first antiaromatic framework materials and obtained a series of three highly crystalline and porous COFs based on DBP. Potential applications of such antiaromatic bulk materials were explored: COF films show a conductivity of 4 × 10-8 S cm-1 upon doping and exhibit photoconductivity upon irradiation with visible light. Application as positive electrode materials in Li-organic batteries demonstrates a significant enhancement of performance when the antiaromaticity of the DBP unit in the COF is exploited in its redox activity with a discharge capacity of 26 mA h g-1 at a potential of 3.9 V vs. Li/Li+. This work showcases antiaromaticity as a new design principle for functional framework materials.

Reaction mechanism and mechanical properties of geopolymer based on kaolinitic coal tailings

Valorization of coal tailings as supplementary cementitious material is vital for green development, including their application as aluminosilicate in geopolymer materials. Coal tailings from southern Brazil were activated mechanically and thermally. This work analyzed the effect of calcium addition via Portland cement and flue gas desulfurization on the reaction mechanism of geopolymer using setting times and semi-adiabatic calorimetry. Additionally, un-calcined coal gangue was added to geopolymer cement to evaluate the effect of sulfur in the microstructure of geopolymer. The experimental results showed kaolinitic coal sludge was transformed into a metastable phase with higher reactivity, while X-ray pyrite reflection was observed for un-calcined coal gangue. The findings showed that calcium modified the reaction mechanism by accelerating the monomers' condensation and increasing the energy released, promoting the hardening of the geopolymer at 24 h. In addition, a solid matrix of geopolymer was developed compounded mainly by an amorphous inorganic polymer and C-A-S-H gel as a secondary phase. The thermodynamic simulation confirmed the addition of coal gangue reduced the pH of the pore solution and increased the content of SO42− and Fe2+. These results evidenced that partial oxidation of pyrite in an alkaline medium releases iron and sulfate ions and this process can lead to acidification of the pore solution resulting in internal degradation.

Application of Fractional Viscoelastic Models to Amorphous Polymers

Application of Fractional Viscoelastic Models to Amorphous Polymers

Comparison between the Mechanical Recycling Behaviour of Amorphous and Semicrystalline Polymers: A Case Study

The increase in waste has motivated the adoption of the circular economy concept, which assumes particular relevance in the case of plastic materials. This has led to research of new possibilities for recycling plastics after their end-of-life. To achieve this goal, it is fundamental to understand how the materials’ properties change after recycling. This study aims to evaluate the thermal and mechanical properties of recycled plastics, namely polycarbonate (PC), polystyrene (PS), glass fibre-reinforced polyamide 6 (PA6-GF30), and polyethylene terephthalate (PET). With this purpose, injected samples were mechanically recycled twice and compared through thermal and mechanical tests, such as differential scanning calorimetry, hardness, tensile strength, and the melt flow rate. The results show that the amorphous materials used do not suffer significant changes in their properties but exhibit changes in their optical characteristics. The semicrystalline ones present some modifications. PET is the material that suffers the biggest changes, both in its flowability and mechanical properties. This work demonstrates that the mechanical recycling process may be an interesting possibility for recycling depending on the desired quality of final products, allowing for some materials to maintain comparable thermal and mechanical properties after going through the recycling process.

Evolution of Statistical Strength during the Contact of Amorphous Polymer Specimens below the Glass Transition Temperature: Influence of Chain Length.

A comprehensive study of the statistical distribution of the auto-adhesion lap-shear strength (σ) of amorphous polymer-polymer interfaces using various types of statistical tests and models is a useful approach aimed at a better understanding of the mechanisms of the self-healing interface. In the present work, this approach has been applied, for the first time, to a temperature (T) range below the bulk glass transition temperature (Tgbulk). The interest of this T range consists in a very limited or even frozen translational segmental motion giving little or no chance for adhesion to occur. To clarify this issue, the two identical samples of entangled amorphous polystyrene (PS) with a molecular weight (M) of 105 g/mol or 106 g/mol were kept in contact at T = Tgbulk - 33 °C for one day. The as-self-bonded PS-PS auto-adhesive joints (AJ) of PSs differing in M by an order of magnitude were fractured at ambient temperature, and their σ distributions were analyzed using the Weibull model, the quantile-quantile plots, the normality tests, and the Gaussian distribution. It has been shown that the Weibull model most correctly describes the σ statistical distributions of the two self-bonded PS-PS AJs with different M due to the joints' brittleness. The values of the Weibull modulus (a statistical parameter) m = 2.40 and 1.89 calculated for PSs with M = 105 and 106 g/mol, respectively, were rather close, indicating that the chain length has a minor effect on the σ data scatter. The Gaussian distribution has been found to be less appropriate for this purpose, though all the normality tests performed have predicted the correctness of the normal distribution for these PS-PS interfaces.