Relaxation processes in crystalline polymers: experimental behaviour — a review

Derived Coarse-Grained Potentials for Semicrystalline Polymers with a Blended Multistate Iterative Boltzmann Inversion Method

In this article, we employ the multistate iterative Boltzmann inversion (MS-IBI) method to develop coarse-grained potentials capable of representing molecular structure in both the amorphous and crystalline phases of semicrystalline polymers with improved accuracy while allowing for tunable control over the dynamics governing the α-relaxation process. A unique feature of this method is that the potentials are blended using the product of the target structural distributions, for example, the radial density function, for each phase and a weighting factor. To demonstrate this approach, a family of potentials for polyethylene is developed where the weighting factor of the crystalline phase ranges is varied from zero, incorporating information only from the amorphous phase, to unity, where the model is trained from only the crystalline phase. The most accurate representation of structural distributions was obtained when the crystalline phases is weighted at 50%. However, we show that when the crystalline phase is weighted at 90%, the model more accurately represents dynamics of the α-relaxation process, with realistic predicted values of activation energy and diffusion rates, with relatively minor impact on accuracy in structure.

Effects of end groups and entanglements on crystallization and melting behaviors of poly(ε-caprolactone)

The topology including end groups, entanglement loops, and tie molecules has a significant impact on the rheological and crystallization behavior and consequently on the functionality of a polymer. Unentangled, weakly entangled, and strongly entangled poly(ε-caprolactone)s (PCLs) with end groups and various molecular weights were synthesized. POM and DSC were used to observe spherulite growth and characterize thermal properties during crystallization and melting. The viscosity and structure of the samples were probed by rheology and X-ray analysis, respectively. The crossover of the scaling relationship of viscosity vs molecular weight demonstrates that the samples cover a wide range of entanglement density, and the bulky end groups cause deviations from the classical scaling laws. In situ simultaneous SAXS/WAXS investigations showed that the crystal structure of PCLs did not change with end groups and heating. The results of POM and DSC imply that the end groups and entanglements affect the crystallization rate and the spherulite morphology. The melting of PCLs containing end groups was found to be a multi-step process involving various nanoscale crystalline structures. The evolution of nanoscale crystalline structures of isothermally crystallized PCLs during heating was analyzed by fitting 1D SAXS profiles, and the continuous structural evolution was found to be a process influenced by end groups and entanglements. The results show that end groups and entanglements affect the chain dynamics and lead to constrained crystallization behavior and the formation of metastable structures, ultimately affecting the structure evolution during melting.

Catalytic Control of Crystallization in Dynamic Networks

The effect of catalysts with varying nucleophilic strength on thiol-thioester bond exchange dynamics and concomitant crystallization was studied in a model semicrystalline polymer network. It was found that the characteristic time scale of covalent bond exchange, τ, could be tuned over a ∼10¹-10³ s range simply by changing the nucleophilicity of the catalyst. Using isothermal crystallization measurements via differential scanning calorimetry, thermodynamic and kinetic features of crystallization were considered. A depression in melting temperature was observed with increasing bond exchange rate, suggesting a dependence of crystalline organization on network dynamics. Furthermore, a systematic slowing of crystallization kinetics with faster covalent bond exchange rates was observed. Lauritzen-Hoffman analysis showed a near doubling in the barrier for secondary nucleation for dynamic networks, suggesting that that bond exchange slows crystallization by limiting secondary nucleation and further growth. Finally, longitudinal DSC studies reveal a long-term increase in melting temperature for samples held at ambient temperature with bond exchange activated at room temperature, indicating that while bond exchange slows crystallization on short time scales it facilitates isothermal long-term crystal rearrangement and growth on longer time scales.

The Photovoltaic Cell Based on CIGS: Principles and Technologies

Semiconductors used in the manufacture of solar cells are the subject of extensive research. Currently, silicon is the most commonly used material for photovoltaic cells, representing more than 80% of the global production. However, due to its very energy-intensive and costly production method, other materials appear to be preferable over silicon, including the chalcopyrite-structured semiconductors of the CIS-based family (Cu(In, Ga, Al) (Se, S)₂). Indeed, these compounds have bandwidths between 1 eV (CuInSe₂) and 3 eV (CuAlS₂), allowing them to absorb most solar radiation. Moreover, these materials are currently the ones that make it possible to achieve the highest photovoltaic conversion efficiencies from thin-film devices, particularly Cu(In, Ga)Se₂, which is considered the most efficient among all drifts based on CIS. In this review, we focus on the CIGS-based solar cells by exploring the different layers and showing the recent progress and challenges.

Competition between crystal growth and intracrystalline chain diffusion determines the lamellar thickness in semicrystalline polymers

The non-equilibrium thickness of lamellar crystals in semicrystalline polymers varies significantly between different polymer systems and depends on the crystallization temperature Tc. There is currently no consensus on the mechanism of thickness selection. Previous work has highlighted the decisive role of intracrystalline chain diffusion (ICD) in special cases, but a systematic dependence of lamellar thickness on relevant timescales such as that of ICD and stem attachment has not yet been established. Studying the morphology by small-angle X-ray scattering and the two timescales by NMR methods and polarization microscopy respectively, we here present data on poly(oxymethylene), a case with relatively slow ICD. It fills the gap between previously studied cases of absent and fast ICD, enabling us to establish a quantitative dependence of lamellar thickness on the competition between the noted timescales.

Effect of Thermal Aging on Mechanical Properties and Color Difference of Glass Fiber/Polyetherimide (GF/PEI) Composites

This research study is aimed at evaluating the mechanical characteristics in terms of tensile strength and flexural strength of glass fiber reinforced Polyetherimide (GF/PEI) under different thermal aging. Tensile testing and bending testing were performed on the thermally aged polyetherimide composites. The mechanical properties of the thermally aged samples were also correlated with their color difference. The experimental results showed that both the tensile strength and flexural strength of the GF/PEI composite samples decreased with increasing aging temperature. However, the elastic modulus of the composite samples is nearly independent on the thermal aging. The thermally aged samples exhibited brittle fracture, resulting in low strength and low ductility. The loss in strength after thermal aging could be also linked to the change of their color difference, which can indirectly reflect the change of the strength for the composites after thermal aging.

The Viscoelastic Behaviour of Waterlogged Archaeological Wood Treated with Methyltrimethoxysilane

Waterlogged wood treatment with methyltrimethoxysilane (MTMS) proved effective in stabilising wood dimensions upon drying (anti-shrink efficiency of 76-93%). Before the method can be proposed as a reliable conservation treatment, further research is required that includes the evaluation of the mechanical properties of treated wood. The aim of the study was to characterise the effect of the treatment on the viscoelastic behaviour of archaeological waterlogged elm and oak wood differing in the degree of degradation. Dynamic mechanical analysis in the temperature range from -150 to +150 °C was used for the study. To better understand the viscoelastic behaviour of the treated wood, pore structure and moisture properties were also investigated using Scanning Electron Microscopy, nitrogen sorption, and Dynamic Vapour Sorption. The results clearly show that methyltrimethoxysilane not only prevents collapse and distortions of the degraded cell walls and decreases wood hygroscopicity (by more than half for highly degraded wood), but also reinforces the mechanical strength by increasing stiffness and resistance to deformation for heavily degraded wood (with an increase in storage modulus). However, the MTMS also has a plasticising effect on treated wood, as observed in the increased value of loss modulus and introduction of a new tan δ peak). On the one hand, methyltrimethoxysilane reduces wood hygroscopicity that reflects in lower wood moisture content, thus limiting the plasticising effect of water on wood polymers, but on the other hand, as a polymer itself, it contributes to the viscous behaviour of the treated wood. Interestingly, the effect of silane differs with both the wood species and the degree of wood degradation.

Perm-waved human hair: a thermorheologically complex shape memory composite

A "permanent" bent shape can be imposed on a straight human hair by a two-stage reduction/oxidation (perm-waving) process. The process relies on the molecular level on sulfhydryl/disulfide interchange as bond exchange reaction (BER). We expected a well-documented transition temperature around 60°C to be the trigger for the shape memory (SM) process of perm-waved hair. We confirm the existence of the SM process as such and investigate its time and temperature dependence. The results show a two-stage SM behavior, implying two distinct variations of the BER. The model to fit the data contains two fractional, normalized, elastic bending rigidities, which are strictly compensatory. They show Arrhenius-type temperature dependence and a common activation energy (E_A) of ∼-12 kJ/mol. The characteristic relaxation time for the first SM process shows little, if any, temperature dependence (E_A = -4 ± 2.7 kJ/mol). This is in contrast to the second process (E_A = -58 ± 5.5 kJ/mol) but in line with the expected properties of the suggested BERs. None of the parameters shows any sign of the expected trigger transition (∼60°C). We hypothesize that this specific transition occurs only for large tensile deformations, when specific SS bonds in the intermediate filaments of hair are activated. There is thus no specific "trigger" transition for the SM behavior of bent, perm-waved hair.

Effect of Dendritic Side Groups on the Mobility of Modified Poly(epichlorohydrin) Copolymers

The macromolecular dynamics of dendronized copolymer membranes (PECHs), obtained by chemical modification of poly(epichlorohydrin) with the dendron 3,4,5-tris[4-(n-dodecan-1-yloxy)benzyloxy] benzoate, was investigated. In response to a thermal treatment during membrane preparation, these copolymers show an ability to change their shape, achieve orientation, and slightly crystallize, which was also observed by CP-MAS NMR, XRD, and DSC. The phenomenon was deeply analyzed by dielectric thermal analysis. The dielectric spectra show the influence of several factors such as the number of dendritic side groups, the orientation, their self-assembling dendrons, and the molecular mobility. The dielectric spectra present a sub-Tg dielectric relaxation, labelled as γ, associated with the mobility of the benzyloxy substituent of the dendritic group. This mobility is not related to the percentage of these lateral chains but is somewhat hindered by the orientation of the dendritic groups. Unlike other less complex polymers, the crystallization was dismantled before the appearance of the glass transition (α_Tg). Only after that, clearing transition (α_Clear) can be observed. The PECHs were flexible and offered a high free volume, despite presenting a high degree of modifications. However, the molecular mobility is not independent in each phase and the self-assembling dendrons can be eventually fine-tuned according to the percentage of grafted groups.

An explorative study of polymers for 3D printing of bioanalytical test systems

BACKGROUND

The 3D printing is relevant as a manufacturing technology of functional models for forensic, pharmaceutical and bioanalytical applications such as drug delivery systems, sample preparation and point-of-care tests.

OBJECTIVE

Melting behavior and autofluorescence of materials are decisive for optimal printing and applicability of the product which are influenced by varying unknown additives.

METHODS

We have produced devices for bioanalytical applications from commercially available thermoplastic polymers using a melt-layer process. We characterized them by differential scanning calorimetry, fluorescence spectroscopy and functional assays (DNA capture assay, model for cell adhesion, bacterial adhesion and biofilm formation test).

RESULTS

From 14 tested colored, transparent and black materials we found only deep black acrylonitrile-butadiene-styrene (ABS) and some black polylactic acid (PLA) useable for fluorescence-based assays, with low autofluorescence only in the short-wave range of 300-400 nm. PLA was suitable for standard bioanalytical purposes due to a glass transition temperature of approximately 60°C, resistance to common laboratory chemicals and easy print processing. For temperature-critical methods, such as hybridization reactions up to 90°C, ABS was better suited.

CONCLUSIONS

Autofluorescence was not a disadvantage per se but can also be used as a reference signal in assays. The rapid development of individual protocols for sample processing and analysis required the availability of a material with consistent quality over time. For fluorescence-based assays, the use of commercial standard materials did not seem to meet this requirement.

Reversible Actuation via Photoisomerization-Induced Melting of a Semicrystalline Poly(Azobenzene)

Photoisomerization of azobenzene in polymer matrices is a powerful method to convert photon energy into mechanical work. While most previous studies have focused on incorporating azobenzene within amorphous or liquid crystalline materials, the limited extents of molecular ordering and correspondingly modest enthalpy changes upon switching in such systems has limited the achievable energy densities. In this work, we introduce a semicrystalline main-chain poly(azobenzene), where photoisomerization is capable of reversibly triggering melting and recrystallization under essentially isothermal conditions. These materials can be drawn into aligned fibers, yielding optically driven two-way shape memory actuators capable of reversible bending.

Nanoscale Heat Conduction in CNT-POLYMER Nanocomposites at Fast Thermal Perturbations

Nanometer scale heat conduction in a polymer/carbon nanotube (CNT) composite under fast thermal perturbations is described by linear integrodifferential equations with dynamic heat capacity. The heat transfer problem for local fast thermal perturbations around CNT is considered. An analytical solution for the nonequilibrium thermal response of the polymer matrix around CNT under local pulse heating is obtained. The dynamics of the temperature distribution around CNT depends significantly on the CNT parameters and the thermal contact conductance of the polymer/CNT interface. The effect of dynamic heat capacity on the local overheating of the polymer matrix around CNT is considered. This local overheating can be enhanced by very fast (about 1 ns) components of the dynamic heat capacity of the polymer matrix. The results can be used to analyze the heat transfer process at the early stages of “shish-kebab” crystal structure formation in CNT/polymer composites.

Synthesis, microstructure and mechanical properties of partially biobased biodegradable poly(ethylene brassylate-co-ε-caprolactone) copolyesters

High-molecular-weight poly(ethylene brassylate-co-ε-caprolactone) copolyesters within a wide composition range were prepared via triphenyl bismuth catalyzed copolymerization of ethylene brassylate (EB) and ε-caprolactone (ε-CL) in bulk. Microstructural analysis of the resulting copolyesters demonstrated that the comonomer units were completely random distribution. DSC and WAXD recognized that the copolyesters cocrystallize within the lattices analogous to either of the parent homopolymers. It confirmed the isodimorphism behavior with a pseudo-eutectic point of melting temperatures as well as lattice spacings at 75 mol% ε-CL units. The crystal cell would be stretched in one dimension rather than expanding in both dimensions with the incorporation of comonomer units according to the result of WAXD. The mechanical properties of the copolyesters are well tunable by the composition, and its trend is consistent with the isodimorphism behavior, in particular, the maximum elongation at break over 2000% is located at the pseudo-eutectic point. The intralamellar shear occurred at the low tensile rate while both intralamellar shear and interlamellar shear occurred at high tensile rate. The copolymers exhibit excellent hydrolytic stability.

Segmental dynamics of polymer by rotational fluorescence correlation microscopy

The segmental dynamics of polymers is known to be closely related to the glass transition where the glass transition is the single most important parameter in its application. In this study, we designed an efficient and reliable experimental method to study the ensemble segmental dynamics of polymers by probing rotation of fluorescent molecules in the polymer matrix using a home-built microscope setup. The rotational dynamics of fluorescent molecules was analyzed using a fluorescence correlation method that extracts information through orthogonally polarized fluorescence images. From fluorescence intensities, autocorrelation functions (ACFs) were obtained in many areas simultaneously and by averaging several ACFs, well-defined ACF and precise experimental values were obtained from a single measurement movie. The robustness of the method and optimal experimental conditions were investigated by performing experiments with various probe concentrations, frame rates, and measurement lengths. By employing a home-built vacuum chamber, a wide temperature range was achieved, and we demonstrate the versatility and efficiency of imaging rotational FCM (fluorescence correlation microscopy) by probing segmental dynamics of different polymeric systems with glass transition temperature that differ by ≈100 K and with fragility ranging from 49 to 131. The imaging rotational FCM covers dynamics up to 4 orders of magnitude near the glass transition, and it was found that the rapidity of the stretching exponent β variation with temperature correlates with the fragility of polymers.

Nanometer scale thermal response of polymers to fast thermal perturbations

Nanometer scale thermal response of polymers to fast thermal perturbations is described by linear integro-differential equations with dynamic heat capacity. The exact analytical solution for the non-equilibrium thermal response of polymers in plane and spherical geometry is obtained in the absence of numerical (finite element) calculations. The solution is different from the iterative method presented in a previous publication. The solution provides analytical relationships for fast thermal response of polymers even at the limit t → 0, when the application of the iterative process is very problematic. However, both methods give the same result. It was found that even fast (ca. 1 ns) components of dynamic heat capacity greatly enhance the thermal response to local thermal perturbations. Non-equilibrium and non-linear thermal response of typical polymers under pulse heating with relaxation parameters corresponding to polystyrene and poly(methyl methacrylate) is determined. The obtained results can be used to analyze the heat transfer process at the early stages of crystallization with fast formation of nanometer scale crystals.

Evaluating models for polycaprolactone crystallization via simultaneous rheology and Raman spectroscopy

The crystallization of a polymer melt is characterized by dramatic structural and mechanical changes that significantly impact the processing conditions used to generate industrially-relevant products. Relationships between crystallinity and rheology are necessary to simulate and monitor the effect of processing conditions on the properties of the final product. However, separate measurements of crystallinity and rheology are difficult to correlate due to differences in sample history, geometry, and temperature. Recently, we have developed a rheo-Raman microscope for simultaneous rheology, Raman spectroscopy, and polarized reflection-mode optical measurements of soft materials, which allows for quantitative crystallinity measurements through features in the Raman spectrum. In this work, we apply this technique to monitor the isothermal crystallization of polycaprolactone to probe the relationship between structure, crystallinity, and rheology. Both crystallinity and the shear modulus vary over comparable timescales, but the birefringence increases much earlier in the crystallization process. We directly plot rheological parameters as a function of crystallinity to probe a range of suspension-based and empirical models relating the complex modulus to crystallinity, and we find that the previously developed models cannot describe the crystallinity-modulus relationship over the crystallization process. By developing a suspension-based model we can fit the complex modulus over the crystallization range. The crystallization process is characterized by a critical percolation fraction and a single scaling exponent.

Dipolar filtered magic-sandwich-echoes as a tool for probing molecular motions using time domain NMR

We present a simple ¹H NMR approach for characterizing intermediate to fast regime molecular motions using ¹H time-domain NMR at low magnetic field. The method is based on a Goldmann Shen dipolar filter (DF) followed by a Mixed Magic Sandwich Echo (MSE). The dipolar filter suppresses the signals arising from molecular segments presenting sub kHz mobility, so only signals from mobile segments are detected. Thus, the temperature dependence of the signal intensities directly evidences the onset of molecular motions with rates higher than kHz. The DF-MSE signal intensity is described by an analytical function based on the Anderson Weiss theory, from where parameters related to the molecular motion (e.g. correlation times and activation energy) can be estimated when performing experiments as function of the temperature. Furthermore, we propose the use of the Tikhonov regularization for estimating the width of the distribution of correlation times.