Preparation, characterization and crystalline transitions of odd–even polyamides 11,12 and 11,10

Water-Induced Crystal Transition and Accelerated Relaxation Process of Polyamide 4 Chains in Microfibers

Microplastics have recently been identified as one of the major contributors to environmental pollution. To design and control the biodegradability of polymer materials, it is crucial to obtain a better understanding of the aggregation states and thermal molecular motion of polymer chains in aqueous environments. Here, we focus on melt-spun microfibers of a promising biodegradable plastic, polyamide 4 (PA4), with a relatively greater number density of hydrolyzable amide groups, which is regarded as an alternative to polyamide 6. Aggregation states and thermal molecular motion of PA4 microfibers without/with a post-heating drawing treatment under dry and wet conditions were examined by attenuated total reflectance-Fourier transform infrared spectroscopy and wide-angle X-ray diffraction analysis in conjunction with dynamic mechanical analysis. Sorbed water molecules in the microfibers induced the crystal transition from a meta-stable γ-form to a thermodynamically stable α-form via activation of the molecular motion of PA4 chains. Also, the post-drawing treatment caused a partial structural change of PA4 chains, from an amorphous phase to a crystalline phase. These findings should be useful for designing PA4-based structural materials applicable for use in marine environments.

Reprocessing of Covalent Adaptable Polyamide Networks through Internal Catalysis and Ring-Size Effects

Here, we report the introduction of internally catalyzed amide bonds to obtain covalent adaptable polyamide networks that rely on the dissociation equilibrium between dicarboxamides and imides. While amide bonds are usually considered to be robust and thermally stable, the present study shows that their dynamic character can be activated by a smart choice of available building blocks without the addition of any external catalyst or other additives. Hence, a range of polyamide-based dynamic networks with variable mechanical and viscoelastic properties have been obtained in a systematic study, using a straightforward curing process of dibasic ester and amine compounds. Since the dissociation process involves a cyclic imide formation, the correlation between ring size and the thermomechanical viscosity profile was studied for five- to seven-membered ring intermediates, depending on the chosen dibasic ester monomer. This resulted in a marked temperature response with activation energies in the range of 116-197 kJ mol^-1, yielding a sharp transition between elastic and viscous behavior. Moreover, the ease and versatility of this chemistry platform were demonstrated by selecting a variety of amines, resulting in densely cross-linked dynamic networks with T_g values ranging from -20 to 110 °C. With this approach, it is possible to design amorphous polyamide networks with an acute temperature response, allowing for good reprocessability and, simultaneously, high resistance to irreversible deformation at elevated temperatures.

Process Behavior of Short Glass Fiber Filled Systems during Powder Bed Fusion and Its Effect on Part Dimensions

In powder bed fusion of polymers, filled systems can provide a wide range of part properties, which is still a deficit in additive manufacturing, as the material variety is limited. Glass fiber filled polymers provide a higher strength and stiffness in parts; nevertheless, the process behavior differs from neat polymer systems. In this study, the optical properties and their effect on the part dimensions are analyzed. A higher glass fiber content leads to an increased absorption of laser energy, while the specific heat capacity decreases. This results in larger part dimensions due to higher energy input into the powder bed. The aim of the study is to gain process understanding in terms of ongoing mechanisms during processing filled systems on the one hand and to derive strategies for filled polymer systems in powder bed fusion on the other hand.

Synchrotron X-ray Scattering Analysis of Nylon-12 Crystallisation Variation Depending on 3D Printing Conditions

Nylon-12 is an important structural polymer in wide use in the form of fibres and bulk structures. Fused filament fabrication (FFF) is an extrusion-based additive manufacturing (AM) method for rapid prototyping and final product manufacturing of thermoplastic polymer objects. The resultant microstructure of FFF-produced samples is strongly affected by the cooling rates and thermal gradients experienced across the part. The crystallisation behaviour during cooling and solidification influences the micro- and nano-structure, and deserves detailed investigation. A commercial Nylon-12 filament and FFF-produced Nylon-12 parts were studied by differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS) to examine the effect of cooling rates under non-isothermal crystallisation conditions on the microstructure and properties. Slower cooling rates caused more perfect crystallite formation, as well as alteration to the thermal properties.

Ferroelectric performance of nylons 6-12, 10-12, 11-12, and 12-12

Nylons have great potential for electrical applications requiring high polarizability and low dielectric loss. Recently, the narrow single hysteresis loop with relaxor ferroelectricity and the double hysteresis loop due to antiferroelectricity have been reported in nylon random copolymers, terpolymers, and common even-numbered nylons. Although several studies of ferroelectric nylons have been reported, even–even-numbered and odd–even-numbered nylons have not been sufficiently explored. Here, the ferroelectricity of spin-coated even–even-numbered and odd–even-numbered nylons was investigated. A series of even–even-numbered nylons, including nylons 6-12, 10-12, and 12-12, and an odd–even-numbered nylon, nylon 11-12, were polymerized with 1,10-dodecanedicarboxylic acid (12) and four aliphatic diamines with various methylene units, 1,6-hexanediamine (6), 1,10-decanediamine (10), 1,11-undecanediamine (11), and 1,12-dodecanediamine (12). The obtained nylon polymers were spin coated and then subjected to melt-quenching or thermal annealing followed by quenching. From the X-ray diffraction and the electrical hysteresis loop data, the correlation between the ferroelectricity and the crystal parameters of crystallinity and crystallite size of the γ crystal phase was investigated. Furthermore, the free volume of the nylon samples was estimated to correlate with the ferroelectricity. Temperature-dependent ferroelectricity was investigated for nylon 10-12. At a high temperature, the nylon samples showed a narrow polarization–electric field hysteresis loop and a rhombus-shaped polarization current–electric field hysteresis loop due to the relaxor ferroelectricity. This behaviour was caused by electrically rotating the nanodomains with weakened hydrogen bonds at higher temperatures.

Nylons have great potential for electrical applications requiring high polarizability and low dielectric loss.

Crystalline Structures and Structural Transitions of Copolyamides Derived from 1,4-Diaminobutane and Different Ratios of Glutaric and Azelaic Acids

Copolyamides derived from even 1,4-butanediamine and different mixtures of odd dicarboxylic acids with a great difference in the number of methylene groups (i.e., glutaric and azelaic acids with 3 and 7 groups, respectively) have been synthesized, characterized and structurally studied. Calorimetric analyses revealed a complex behavior with multiple melting peaks associated to lamellar reordering and the presence of defective crystals. Equilibrium melting temperatures were evaluated and showed a eutectic behavior with composition. Copolymers were able to crystallize even for samples with comonomer percentages close to 50%. Negative and ringed spherulites from the melt state and small lath-like lamellar crystals from dilute solution crystallizations were attained. Furthermore, calorimetric data pointed out the exclusion of the less abundant monomer from the lattice of the predominant structure. All samples at room temperature showed a similar crystalline structure (form I) defined by two predominant reflections at spacings close to 0.430 and 0.380 nm, which has been related for even-odd nylons with a two-hydrogen bonded structure. Real time synchrotron experiments showed that melt crystallized samples have two polymorphic transitions on heating, which were practically reversible and consequently were also detected during cooling from the melt state. Interestingly, a different behavior was detected among solution crystallized samples and specifically the transition to the intermediate structure (form II) was not detected during heating for samples enriched on the azelate component or more precisely when they were exclusively crystallized in the form I.

Renewable (Bis)pyrrolidone Based Monomers as Components for Thermally Curable and Enzymatically Depolymerizable 2-Oxazoline Thermoset Resins

In this study we describe the synthesis of bis(pyrrolidone) based dicarboxylic acids from itaconic acid and their application in 2-oxazoline resins for fully renewable thermoset materials. The monomers are obtained using a bulk aza-Michael addition of a diamine and two itaconic acid molecules using a catalytic amount of water. The monomers can be isolated in high purity after recrystallization, though their yield proved to be highly dependent on the selected diamine spacer length: In general, only the dicarboxylic acids containing diamines with an even number of methylene spacers are isolated in high yields. Through NMR, GPC, and FTIR analysis we demonstrate that these bis(pyrrolidone) based dicarboxylic acids exhibit significantly enhanced curing rates in 2-oxazoline resins compared to resins containing aliphatic dicarboxylic acids such as sebacic acid. Overall, we demonstrate that the rate of 2-oxazoline ring-opening addition with carboxylic acid functionalities is determined by the used dicarboxylic acid, whereas the ring-opening addition of the 2-oxazoline functionality with amide groups is determined by the used bis(2-oxazoline) compound. The thermosets obtained after curing proved to be readily plasticized by water, opening up possibilities for enzymatic degradation.

Thermally Induced Structural Transitions of Nylon 4 9 as a New Example of Even⁻Odd Polyamides

Crystalline morphology and structure of nylon 4 9 have been studied by means of optical and transmission electron microscopies, and X-ray diffraction. Rhombic crystals were characteristic of crystallization from glycerin dilute solutions, although the final morphology was dependent on the crystallization temperature. In any case, a single electron diffraction pattern was always obtained, being characteristic a 2 mm symmetry and reflections at spacings that were indicative of a projected rectangular unit cell with hydrogen bonds established along two planar directions (i.e., the diagonals of the unit cell), as it was determined from related polyamides. Crystallization from the melt gave rise to negative birefringent spherulites with a morphology (axialitic, speckled or ringed) that was dependent on the crystallization temperature. Kinetic analysis indicated that melt crystallization took place according to two growth mechanisms (Regimes II and III), which reflect distinct secondary nucleation rates. A complex polymorphic behavior on heating and cooling processes was evidenced by real time synchrotron experiments, being determined an intermediate crystalline structure as well as the typical pseudohexagonal arrangement associated to the Brill transition. Polymorphic transitions were highly dependent on the initial crystalline structure, being enhanced the structural transition from the low temperature structure to the intermediate one when traces of the latter were initially present. Calorimetric and infrared studies supported also the detected thermal transitions of nylon 4 9.

Improvement of color value of bio-based polyamide 56 fibers

This paper introduces a feasible method to improve the color value of bio-based polyamide 56 (PA56) fiber. Three types of whitening modifiers were introduced into the bio-based PA56 fibers by the in-situ polymerization method and melt blending method in order to improve the fiber color. The color values and mechanical properties of PA56 fibers were tested and analyzed and the optimum additive ratio and process conditions were discussed. The results show that the improved color value of bio-based PA56 fibers can be achieved by the melt blending method of fluorescent whitening masterbatch and reasonable spinning condition control. When the additive amount of fluorescent whitening agent is 0.3%, the index of lightness (L) increased from 84 to 90, while the yellow index (YI) reduced from 16.8 to 12.6. Moreover, PA56 fibers show high breaking strength about 4.27 cN/dtex and good yellowing resistance and durability.

Synthesis of nylon 1 in supercritical carbon dioxide and its crystallization behavior effect on nylon 11

Nylon-1, with the highest density of dipoles among odd-numbered nylons, was synthesized through urea in supercritical CO₂.

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.

Synthesis of copolyesters with bio-based lauric diacid: structure and physico-mechanical studies

Copolyesters developed from bio-based long-chain dicarboxylic acid show promising mechanical properties that are helpful for their application as biodegradable materials.

Novel Polyamide 6,10 Variants Synthesized by Modified Interfacial Polymerization for Application as a Rate-Modulated Monolithic Drug Delivery System

The aim of this study was to explore and elucidate the possibility of employing an aliphatic polyamide 6,10 (PA 6,10) synthesized by a modified interfacial polymerization process as a novel rate-modulated monolithic matrix drug delivery system. A Plackett—Burman experimental design was used to synthesize 14 different PA 6,10 polymers using the interfacial polymerization process of synthesis comprising the monomers namely hexamethylenediamine and sebacoyl chloride and the solvents namely hexane (nonpolar phase) and deionized water (polar phase). This process was modified by variations in stoichiometry of monomers, volume ratios of solvents as well as solvent phase modification using cyclohexane and sodium hydroxide for the nonpolar and polar solvent phases, respectively. The micromechanical parameters of the newly synthesized PA 6,10 variants were elucidated in terms of the matrix resilience (MR), matrix hardness and deformation energy, in which case matrix hardness and deformation energy were expressed as second-order polynomial hydration rate constants. The effect of changes in pH of the hydration media on these parameters was also explored as part of the characterization process. Scanning electron microscopy and Fourier transform infrared spectroscopy were used to correlate the effect of synthesis variables on the micromechanical behavior PA 6,10 and its subsequent ability to impact drug release. The micromechanical values revealed that all independent formulation variables had a significant influence on the responses. Furthermore, the applied statistical model was utilized in selecting a combination of reaction variables to produce optional physicomechanical properties. The MR was selected for optimization among other parameters since it had a prominent effect on matrix integrity as well as drug release. The one-way analysis of variance, comparison of experimental versus fitted data, the R2 and P-values as well as the Durbin—Watson statistic indices were used in ascertaining the accuracy of the model. The rate-modulating drug release ability of synthetic aliphatic PA 6,10 was explored to direct the optimization using a higher resolution Box—Behnken statistical design. Constraints were set to obtain levels of independent variables that optimized the physicomechanical properties and the mean dissolution time fixed at eight hours.

Comparative Variable Temperature Studies of Polyamide II with a Benchtop Fourier Transform and a Miniature Handheld Near-Infrared Spectrometer Using 2D-COS and PCMW-2D Analysis

The main objective of this communication is to compare the performance of a miniaturized handheld near-infrared (NIR) spectrometer with a benchtop Fourier transform near-infrared (FT-NIR) spectrometer. Generally, NIR spectroscopy is an extremely powerful analytical tool to study hydrogen-bonding changes of amide functionalities in solid and liquid materials and therefore variable temperature NIR measurements of polyamide II (PAII) have been selected as a case study. The information content of the measurement data has been further enhanced by exploiting the potential of two-dimensional correlation spectroscopy (2D-COS) and the perturbation correlation moving window two-dimensional (PCMW2D) evaluation technique. The data provide valuable insights not only into the changes of the hydrogen-bonding structure and the recrystallization of the hydrocarbon segments of the investigated PAII but also in their sequential order. Furthermore, it has been demonstrated that the 2D-COS and PCMW2D results derived from the spectra measured with the miniaturized NIR instrument are equivalent to the information extracted from the data obtained with the high-performance FT-NIR instrument.

Evaluation of the impacts of formulation variables and excipients on the drug release dynamics of a polyamide 6,10-based monolithic matrix using mathematical tools

Drug release from hydrophilic matrices is regulated mainly by polymeric erosion, disentanglement, dissolution, swelling front movement, drug dissolution and diffusion through the polymeric matrix. These processes depend upon the interaction between the dissolution media, polymeric matrix and drug molecules, which can be significantly influenced by formulation variables and excipients. This study utilized mathematical parameters to evaluate the impacts of selected formulation variables and various excipients on the release performance of hydrophilic polyamide 6,10 (PA 6,10) monolithic matrix. Amitriptyline HCl and theophylline were employed as the high and low solubility model drugs, respectively. The incorporation of different excipient concentrations and changes in formulation components influenced the drug release dynamics as evidenced by computed mathematical quantities (t x%, MDT x%, f 1, f 2, k 1, k 2, and К F). The effects of excipients on drug release from the PA 6,10 monolithic matrix was further elucidated using static lattice atomistic simulations wherein the component energy refinements corroborates the in vitro and in silico experimental data. Consequently, the feasibility of modulating release kinetics of drug molecules from the novel PA 6,10 monolithic matrix was well suggested.

Spectroscopic characterization of the oligomeric surface structures on polyamide materials formed during accelerated aging

Crystalline surface species were observed at the surface of polyamide 12 materials upon accelerated aging. After detection of the depositions with scanning electron microscopy (SEM), the crystalline surface precipitations were analyzed with Fourier transform infrared (FT-IR) and Raman imaging microscopy. These surface species were supposed to be cyclic oligomers (dimer and trimer) of the PA12 monomer laurolactam, which are usually present in polyamide materials and tend to migrate to the surface when the material is subjected to accelerated aging. The evidence for the chemical identity of the crystalline surface structures to be mainly the cyclic dimer and trimer of laurolactam was given by melting-point identification and mass spectroscopic analysis of the methanol eluate of the surface. The Raman and FT-IR spectra of the mixture were extracted from the recorded images.