Brill Transition Research Articles

Influence of Crystallization Kinetics and Flow Behavior on Structural Inhomogeneities in 3D-Printed Parts Made from Semi-Crystalline Polymers.

We report the results of a study focusing on the influence of crystallization kinetics and flow behavior on structural inhomogeneities in 3D-printed parts made from polyamide 12 (PA12) and poly(lactic acid) (PLA) by dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), fast scanning calorimetry (FSC), and wide-angle X-ray diffraction (WAXD). Temperature-dependent WAXD measurements on the neat PLA filament reveal that PLA forms a single orthorhombic α phase during slow cooling and subsequent 2nd heating. The PA12 filament shows a well pronounced polymorphism with a reversible solid-solid phase transition between the (pseudo)hexagonal γ phase near room temperature and the monoclinic α' phase above the Brill transition temperature TB = 140 °C. The influence of the print bed temperature Tb on structure formation, polymorphic state, and degree of crystallinity χc of the 3D-printed parts is investigated by height and depth-dependent WAXD scans and compared with that of 3D-printed single layers, used as a reference. It is found that the heat transferred from successive layers has a strong influence on the polymorphic state of PA12 since a superimposed mixture of γ and α phases is present in the 3D-printed parts. In the case of PLA, a single α phase is formed. The print bed temperature has, in comparison to PA12, a major influence on the degree of crystallinity χc and thus the homogeneity of the 3D-printed parts, especially close to the print bed. By comparing the obtained results from WAXD, DMA, DSC, and FSC measurements with relevant printing times, guidelines for 3D-printed parts with a homogeneous structure are derived.

Linearly-Changed Thermal Behavior and Depressed Brill Transition in Long-Chain Polyamides Substituted by Methyl Side Groups.

Side substitution is an effective way of functionalizing and modifying the properties of polyamides. Meanwhile, side substitution would significantly influence the crystallization kinetics and polymorphic phase transition of polyamides, which, however, has not been well elucidated. Herein, we synthesized the side-substituted long-chain polyamides with various content of methyl pendent groups and investigated their crystallization and phase transition behaviors. We find that the thermal parameters of side-substituted polyamides vary linearly with the side group content, analogous to the isomorphic crystallization of random copolymers. All the solution-crystallized polyamides experience the α-γ Brill transition during heating, with the Brill transition temperature linearly decreasing as the side group content increases. Intriguingly, the γ-α transition of polyamides during cooling is suppressed with the presence of side methyl groups due to the difficulty in H-bond reorganization and gauche-trans conformational changes. This work has demonstrated the critical role of side substitution in the polymorphic crystallization and phase transition of long-chain polyamides.

Tailoring the crystalline structures and mechanical properties of polyamide 1012 specimens by material extrusion-based additive manufacturing

In this work, the evolution of the crystalline structures of MEAM-printed polyamide1012 (PA1012) parts with the nozzle temperatures (Tn) and thermal annealing temperatures (Ta) was investigated. It is found that Tn plays an important role in determining the crystalline structure of PA1012 parts. The freshly-printed PA1012 parts crystalize as γ crystals at Tn lower than 230 °C. With Tn increasing to above 230 °C, a small amount of α phase coexists with γ phase in the PA1012 parts. Also, the thermal annealing treatment can tailor the crystalline structure of the MEAM-printed PA1012 parts. Specifically, the crystalline structure exhibits a transformation from γ phase to α crystal, after annealing the PA1012 parts at a Ta higher than 90 °C for 10 min. Due to the presence of a high content of γ phase, the Brill transition occurs at a low temperature for the MEAM-printed PA1012 parts, implying a close relationship between the H-bond sheet structures and the formation of γ phase. Moreover, the crystalline structure exerts a significant influence on the mechanical properties of the PA1012 parts. The samples with only γ phase exhibit much higher tensile strength and Young's modulus, but much lower ductility, compared to those containing only α crystal. Hence, this work provides effective pathways for regulating crystalline structures as well as mechanical properties of MEAM-printed long-chain polyamide materials.

Exploring the relationship between interfacial adhesion, molecular dynamics, and the brill transition in fully bio-based polyamide 1010 nanocomposites reinforced by two-dimensional materials

One of the greatest goals of modern polymer science can be considered the production of high-performance polymer nanocomposites from renewable sources due to the growing interest in environmental and sustainability issues. Within that scope, a good strategy is to pair the processability of thermoplastic materials with the outstanding characteristics of 2D nanomaterials. However, the final performance of the produced nanocomposite is mainly attributed to the interphase, i.e., the volume fraction of the bulk polymer that has its molecular dynamics affected by interactions with the surface of the filler. Therefore, we aim to investigate how different 2D nanomaterials, i.e., graphene oxide (GO), hexagonal-boron nitride (h-BN), and molybdenum disulfide (MoS2) affect the molecular dynamics of a fully bio-based polyamide 1010 (PA 1010) by a new correlation between experimental results from differential scanning calorimetry, broadband dielectric spectroscopy, and dynamic mechanical analysis. For that, nanocomposites based on single and hybrid-fillers were produced through melt mixing. It was observed that, although all nanofillers seem to hinder the molecular relaxation of the polymer chains, GO and h-BN were more effective than MoS2. This remained true when the fillers were applied individually and in the hybrid form. This effect has been attributed to the following aspects: (I) higher enthalpy related to the melting of the polyamide crystals after the Brill transition, which indicates lower thermal motion being gained before the transition takes place; (II) higher activation energy of the glass transition, which was estimated from the fitting of the dielectric spectra to the Havriliak-Negami function and then to the Vogel-Fulcher-Tammann model; (III) lower values of the “adhesion factor”, which is calculated from the decrease in intensity of the mechanical tan(δ) in the nanocomposites with respect to the neat polymer, indicating higher adhesion for the composites filled with GO and h-BN. To confirm the trends observed experimentally for each nanomaterial, molecular dynamics simulations, which enabled the investigation of the adhesion force as a function of nanosheet displacement, were also carried out. The simulations corroborated with the experimental observations, in which h-BN leads to a more intense force of separation, followed by GO, and lastly by MoS2. Since the level of interfacial interactions is what mainly dictates the performance of polymer nanocomposites, it is suggested that GO, and especially h-BN, might present greater promise for novel bionanocomposites based on PA 1010.

Nucleation Retardation of Polyamide 6,6 by Tuning Hydrogen Bonding Reorganization via Nigrosine for High-Speed Injection Molding Applications

The mechanism of regulating hydrogen bond organization for polyamide 6,6 (PA 6,6) by nigrosine was investigated by temperature-dependent FTIR and in situ synchrotron X-ray diffraction during cooling/heating processes. The intermolecular interaction between amide groups and nigrosine was verified by the FTIR spectroscopy method. Upon melt cooling, the formation of H-bonds is promoted by nigrosine at the initial stage (regime I) with increased enthalpy change (ΔH1), which results from denser molecular coils as the interaction with nigrosine increases the ability to form H-bonds between adjacent amide groups, while at a lower temperature (regime II), the ΔH1 decreases with increasing nigrosine as nigrosine restrains the mobility of PA 6,6 chains that retards the formation of H-bonds between nonadjacent amide groups. Moreover, the ordering arrangement of H-bonds into 2-D hydrogen bond sheets (H-sheets) is hindered for both the regime I′/II′ stages with decreased ΔH2 due to the decreased mobility of chain segments, which retards the nucleation of PA 6,6 crystallization. The crystal perfection index (CPI) was investigated by WAXD, and it was found to decrease with the increase of nigrosine. Combined with microstructure analysis, we proposed that partial nigrosine can insert between H-sheets, which therefore tunes the Brill transition behavior of PA 6,6. Overall, the regulation mechanism of nigrosine on PA 6,6 crystallization has been established via interaction with amide groups to tune the organization of H-sheets. The retardation effects on crystallization can improve the flowability of PA 6,6 for high-speed injection molding applications.

Long Chain Polyamides: Influence of Methylene Sequence Length and External Forces on Structural Features

AbstractCrystallographic studies on a series of even–even polyamides PA10.n (12, 14, 16, and 18) with variable methylene sequence length (n − 2) are performed. Temperature‐dependent X‐ray diffraction studies show that a triclinic α phase is the preferred polymorph after slow cooling to room temperature like in many other even–even polyamides. For PA10.12 and PA10.14 the pseudo‐hexagonal high temperature phase γ is observed between the Brill transition temperature TB and melting point TM. For both higher members a distinct Brill transition is absent below TM. The influence of temperature on the packing state of the stems during cooling is discussed and compared to related polymers. An analysis of well oriented crystalline states in annealed fibers reveals that the crystalline state achieved under ambient conditions is influenced by external forces due to small energetic differences between different α and β polymorphs. All PA10.n samples show in the fiber‐oriented state a prominent β polymorph which is dominant in PA10.12 and PA10.14 but superimposed with a significant α fraction in both higher members. The consequences of changes in the polymorphic state under the influence of external forces for crystalline states achieved under application relevant processing conditions are considered.

Influence of drawing and annealing on the structure and properties of bio‐based polyamide 56 fibers

AbstractThe influences of drawing and annealing parameters on the structure and mechanical properties of melt spinning polyamide 56 (PA56) fibers were investigated and discussed in details. The as‐spun PA56 fibers have the relatively low crystallinity and rather low orientation. The crystalline phase in as‐spun PA56 fibers is mainly in γ phase. PA56 α‐like crystalline phase develops from the amorphous phase with the increase in chain orientation upon drawing of the as‐spun PA56 fibers. The crystallinity of PA56 fiber increases with rising draw ratio, which accompanies the increase in thickness of the crystalline lamella. The increase in draw ratio leads to the increase in tensile strength and the decrease in elongation at break of the resultant fibers. Tension annealing of the full‐drawn PA56 fibers at specific temperatures has obvious influences on the structure and properties of the fibers. The crystallinity and orientation increase with rising annealing temperature in the range of 150–210°C, during which the content of PA56 α‐like phase increases with the rising annealing temperature. Annealed the fibers at 230°C results in the increase in the content of PA56 γ phase with a tremendous decrease in the content of α‐like phase due to Brill transition. The optimum annealing temperature is around 170°C, at which the tenacity of the PA56 fibers is the highest. These results can provide valuable information for the production of PA56 fibers.

Structure transition of aliphatic m,6-Polyurethane during heating investigated using in-situ WAXS, SAXS, and FTIR

Thermoplastic polyurethanes (TPU) produced in the reaction of hexamethylene diisocyanates (HDI) and diols are widely applied in industry. The phase transition behavior of the HDI-diol hard domain was investigated as a structural analog to conventional polyamides (PA), wherein an oxygen atom substituted a methylene group. This substitution brought about a distinct phase transition behavior; the apparent merging of two diffraction peaks into the intermediate peak was not observed; upon heating, the methylene groups orderly packed in the crystalline domain in a trans conformation, gradually transforming into a disorderly gauche conformation was also absent. Such a transition observed in this work is not in accordance with the Brill transition no matter that it is defined by apparent experimental criterion or essential molecular fundamental. The melting sequences of the phase transition, i) mesophase; ii) crystallites; iii) blocks mixing, were jointly revealed by multiple in-situ techniques, WAXS, FTIR and SAXS.

Hydration, Refinement, and Dissolution of the Crystalline Phase in Polyamide 6 Polymorphs for Ultimate Thermomechanical Properties.

Timescales of polyamide 6 melt-shaping technologies, relative to the dynamics of conformational rearrangements upon crystallization, challenge the formation of the most thermodynamically favorable chain packing and thus optimum performance. In this publication, we make use of the mediation of hydrogen bonding by water molecules in the superheated state of water, i.e., above 100 °C in a closed environment, in the structural refinement of polyamide 6 for enhanced thermomechanical performance. The paper addresses dissolution and (re)crystallization of different polyamide 6 polymorphs in the superheated state of water by time-resolved simultaneous small- and wide-angle X-ray scattering and solid-state 1H NMR spectroscopy and the effect on mechanical properties. The experiments reveal that upon heating in the superheated state of water, the pseudo-hexagonal phase dissolves at relatively low temperature and instantly crystallizes in a defected monoclinic phase that successively refines to a perfected monoclinic structure. The dissolution temperature of the pseudo-hexagonal phase of polyamide 6 is found to be dependent on the degree of crystal perfection originating from conformational disorder and misalignment of hydrogen bonding in the lattice, retrospectively, to the Brill transition temperature. The perfected monoclinic phase below the dissolution temperature can be preserved upon cooling but is plasticized by hydration of the amide moieties in the crystalline phase. The removal of water from the hydrated crystals, in the proximity of Brill transition temperature, strengthening the hydrogen bonding, occurs. Retrospectively, the most thermodynamically stable crystallographic phase is preserved and renders an increase in mechanical properties and dimensional stability of the product. The insight obtained on the influence of superheated water on the structural refinement of imperfected crystallographic states assists in polyamide 6 postprocessing strategies for enhanced performance.

Rippled Sheets: The Early Polyglycine Days and Recent Developments in Nylons.

The rippled sheet structure is a remarkable insight due to Pauling and Corey, that supplements the pleated sheet structure of homochiral proteins introduced in 1951. Whereas the pleated sheet was immediately adopted by the scientific community, the rippled sheet has remained more confidential since it applies only to blends of poly(L-peptides) and poly(D-peptides). The present account tells the intimate but patchy relationship developed by the author with the rippled sheet. In the 1970s, twenty years after Pauling and Corey's proposal, the rippled sheet was recognized as a valid model for the sheet structure of the achiral polyglycine, polyglycine I, which helped improve the structure of Bombyx mori silk fibroin. Very recently, pleated and rippled sheets were found to account for unsolved crystal structures of a variety of nylons. These structures help to explain a mysterious high temperature "Brill transition" first reported in nylon 6-6 by Brill in 1942.

Thermal induced crystalline transition of bio-based polyamide 56

Thermal induced crystalline transition of polyamide 56 (PA56) was investigated by DSC, in-situ WAXD, and in-situ FTIR spectroscopy. The PA56 α-like phase, crystallized from formic acid solution, showed a Brill transition that completed at 200 °C (TB) and only γ phase crystallites were mainly obtained when cooling the PA56 samples to room temperature from molten state. The characteristic absorbance bands of PA56 α-like and γ phases on FTIR spectra were assigned. The strength of the hydrogen bonds in PA56 γ phase was stronger than those in α-like phase, which could be the main reason for the irreversible Brill transition behavior of the PA56 α-like phase. In-situ FTIR results indicated that the intermolecular hydrogen bond strength of α-like phase reduced during Brill transition before 200 °C upon heating. During the melting process, the intermolecular hydrogen bond strength first enhanced and then reduced. The Brill transition of PA56 upon heating was further confirmed by the decrease and increase in the intensity of the band at 1631 cm−1 and 1637 cm−1, corresponding to the vibration band (amide I) of CO group of α-like and γ phases of PA56, respectively. FTIR results also confirmed the deformation, twisting, and rocking vibrations of CH2 groups were enhanced during the Brill transition procedure.

The Brill Transition in Long-Chain Aliphatic Polyamide 1012: The Role of Hydrogen-Bonding Organization

The Brill Transition in Long-Chain Aliphatic Polyamide 1012: The Role of Hydrogen-Bonding Organization

A Fresh Look at the Structures of Nylons and the Brill Transition

A Fresh Look at the Structures of Nylons and the Brill Transition

In Situ SAXS and WAXD Investigations of Polyamide 66/Reduced Graphene Oxide Nanocomposites During Uniaxial Deformation.

Epitaxial crystallization between Polyamide 66 (PA66) and reduced graphene oxide (RGO) can enhance the interfacial interaction and the mechanical properties of PA66/RGO nanocomposites. In situ two-dimensional synchrotron radiation wide angle X-ray diffraction and small angle X-ray scattering were used to track the structural evolution of the PA66/RGO nanocomposites with an epitaxial crystal during uniaxial deformation. In the PA66/RGO nanocomposites, the structural evolution of non-epitaxial and epitaxial crystals could be clearly analyzed. The non-epitaxial crystal, whose crystal plane can slip, shows the rearrangement and the Brill transition during uniaxial deformation. While the PA66 chains of an epitaxial crystal are held by RGO, the crystal plane could therefore not slip. The epitaxial crystal also constrains the deformation of the amorphous phase and the crystal form transition of non-epitaxial crystals around them. With the content increase of epitaxial crystals, the constraint effect becomes more obvious. Therefore, the rigid epitaxial crystals in the PA66/RGO nanocomposites promote mechanical properties. The present findings can deepen the understanding of structural evolution during the tensile deformation of PA66/RGO nanocomposites and the influence of the epitaxial crystals on the mechanical property in semicrystalline polymers with a H-bond.

Brill Transition in Nylons: The Structural Scenario()

Pleated and rippled sheet crystal structures of even–even nylons that feature twisted amide group–aliphatic part links were introduced in the previous contribution (Macromolecules, 2021, 54, DOI: 10.1021/acs.macromol.0c02404). Pleated sheets are made of isochiral conformational isomeric stems, and rippled sheets are racemic stereocomplexes of enantiomeric stem conformations. These models are used to reanalyze the various and puzzling manifestations of the Brill transition first reported in 1942. The transition is generally considered to result from the generation of gauche bonds in the aliphatic parts of the nylon α phase which travel to the junctions with the amide groups. Recognition of the pleated/rippled sheet structures suggests a different scenario that takes into account, but challenges, this analysis. The transition takes place in pleated/rippled sheets with twisted chain conformations. It is a dynamic interconversion between the two possible conformers (mirror images) of the stems in the crystal lattice. The interconversion timescale is in the ≈100 picosecond range, as determined by NMR. During the interconversion, the aliphatic part C–C–C plane flips by ≈120° (from + to −60°). The intrasheet H bonds are preserved, but transitory intersheet H bonds may be formed, which accounts for the frequent but not systematic pseudo-hexagonal cell of the Brill structure. This scenario is fully consistent (and more so than the earlier models) with the wide body of experimental data available on the Brill transition. The interconversion between conformers is generic for all types of nylons (even, even–even, etc.) and is therefore a valid scenario for their Brill transitions as well. On this basis, the Brill transition should be defined by a molecular process (the conformation interconversion) rather than by an experimental criterion that lacks general validity (the merging of equatorial reflections).

Mesomorphic Intermediate Stages During Brill Transition of Nylon 6/6

The Brill transition in nylon 6/6 is generally perceived as a fully reversible solid–solid transformation between a low-temperature triclinic lattice (α-phase) and a high-temperature pseudohexagona...

Stretch-Induced Reverse Brill Transition in Polyamide 46

Structural transitions of polyamide 46 (PA46) films during the tensile stretch at temperatures from 37 to 249 °C were studied using in situ synchrotron radiation wide-angle X-ray diffraction (SR-WAXD) and Fourier transform infrared (FTIR) techniques. The structural evolution during the stretch at different temperatures can be roughly divided into three regions. In region I (37 ≤ T < 180 °C), stretch enlarges the d-spacing gap between the (100) and (010/110) planes of α phase (triclinic), which is opposite to that during heating. In region II (180 ≤ T < 230 °C), stretch induces a reverse Brill transition, which is characterized by the single diffraction of γ phase (pseudohexagonal) splitting into two diffractions of α phase, while further increasing the strain drives the two split diffractions to transform to a single broad diffraction. As in situ FTIR measurements reveal the continuously increasing content of trans conformation, we speculate that stretch may induce a new form, which is an intermediate structure close to both α and γ phases (named α′ phase) at large strain. In region III (230 ≤ T ≤ 249 °C), the γ phase may skip the α phase and directly transform into the α′ phase. The nonequilibrium crystal phase diagrams of PA46 were constructed in the strain–temperature space and true stress–temperature space. Current results demonstrate that stretch plays a countereffect of heating on the Brill transition of PA46.

Study on crystal structure and phase transitions of polyamide 12 via wide-angle X-ray diffraction with variable temperature

Polyamide 12 (PA12) is a long carbon chain polyamide with plenty of excellent properties. In this work, PA12 samples were prepared in three ways: solution casting (α-crystal form at 30 °C and α + γ-crystal form at 40, 50, 60, and 80 °C, respectively), melt slow cooling (γ-crystal form), and melt quenching (γ-crystal form), which undergoes isothermal crystallization in the range from room temperature to the temperature close to the melting point. The crystallization behaviors, including the crystal transition, d-spacing, and crystallinity, upon heating and cooling were measured by the wide-angle X-ray diffraction (WAXD) from 30 to 235 °C. The results show that the α-crystal form gradually transforms into γ-crystal form during heating, indicating the occurrence of Brill transition. The γ-crystal form is stable below Tm, whereas the γ-crystal form can transform into γ-crystal form above 110 °C. Only γ-crystal form appears when cooling, and most of the crystallization has been completed before cooling to 170 °C. The investigation provides more theoretical supports of phase transitions in PA12 for deep understanding of the modification, optimization, and application.

Temperature and deformation dependence of structural evolution in polyamide 1010

Polyamide 1010 (PA1010) is a representative of long-chain polyamides, used in applications requiring good mechanical, thermal and chemical properties. It has a lower melting point and slightly higher flexible properties compared to nylon 66. Very few reports have been focused on the structures and properties of long-chain polyamides, which brings some troubles on their processing and applications. For the purpose of understanding mechanical changes within different temperature interval, dynamic structure transition of PA1010 was investigated by wide-angle X-ray diffractometer (WAXD) equipped with a hot stretching stage. During the process of the simultaneous thermal stretched, in addition to the Brill transition phenomenon, it was also accompanied by changes in crystallinity, orientation and tensile force. Both annealing and stretching can promote crystallization, in addition, stretching can also induce and destroy crystallization. With stretch ratios increasing, the crystallinity increases firstly, reaches the maximum when stretch ratio is 2.25 times and then decreases. The degree of orientation increase firstly but increase poorly after 2.50 times. The stretch force increase gradually too. The stretch force has a stronger ability than temperature to induce the PA1010 undergo the Brill transition. The presented results show that Brill transition is completed and the α-crystal structure completely transformed into γ-crystal structure when the stretch ratio is 2.50 times.

Low-Frequency Vibrational Modes of Nylon 6 Studied by Using Infrared and Raman Spectroscopies and Density Functional Theory Calculations.

Far-infrared (FIR) and low-frequency Raman bands in the 90?400 cm?1 region of crystalline nylon 6 in ? form were assigned based on comparisons of experimental spectra and quantum mechanical calculations. A fragment methodology was applied in the calculations for explicit consideration of interchain interactions and crystal symmetry. The main features in both Raman and FIR spectra were reproduced well, which enabled the band assignments based on density functional theory and the significant improvement of the conventional assignments for which there had been a big dispute. Temperature dependence of the experimental FIR spectra has revealed that both bands at 222 and 111 cm?1 are characteristic of the ?-form structure. Their intensities linearly decreased with increasing temperature with marked two transition points, which correspond to glass and Brill transitions. Both bands can be indicators of the lattice length of ?-form nylon 6. On the basis of the calculations, the FIR and Raman bands at ?100 cm?1 were successfully assigned to methylene torsion and transverse motion of amide groups in which NH and O atoms move out of the amide plane. Decomposition of the calculated spectra revealed that the intensities at ?100 cm?1 in both spectra mainly originate from the amide groups and only secondarily from the methylene groups. Moreover, the FIR intensities at ?100 cm?1 were nearly perfectly governed by the amide groups, which could be a reason why this FIR band is particularly sensitive to hydrogen bonds among the low-frequency bands. The FIR band at 222 cm?1 was assigned to methylene torsion and transverse motion of NH groups. Both FIR bands at 222 and 111 cm?1 contain perpendicular motions of methylene and amide groups. This will be a reason for their sensitivity to interchain interactions in ?-form nylon 6. Contrarily, the FIR band at 294 cm?1 is in parallel polarization to the chain direction and assigned to a deformation of C?CH2?CH2 and bending motion of C?O in the amide plane. This is the reason why this band is not sensitive to the structural transitions of nylon 6. Our previous works revealed that in regions of 125 and 70 cm?1, there are specific vibrational peaks of crystalline polyesters primarily arising from out-of-plane motion of ester groups. We can find a similarity in 125 and 70 cm?1 regions between crystalline polyesters and nylon 6 that both polymers show specific out-of-plane vibrational peaks around 100 cm?1, which are sensitive to the lattice length among polymer chains.