Spherulite Size Research Articles

Preparation and characterization of functionalized multiwalled carbon nanotubes filled polyethylene oxide nanocomposites

The functionalized multi-walled carbon nanotubes (F-MWCNTs) filled polyethylene oxide (PEO) nanocomposites were prepared by casting technique and characterized by scanning electron microscopy (SEM), polarized optical microscopy (POM), thermal gravimetric analysis (TGA) and differential thermal analysis (DTA). The MWCNTs were functionalized by Friedel–Crafts acylation, which introduced aromatic amine groups on the side wall of nanotubes. The SEM micrograph of F-MWCNTs/PEO nanocomposites fractured surfaces indicated that the F-MWCNTs were well dispersed within the PEO matrix. The POM study presented that the neat PEO upon crystallization showed considerable size of crystalline spherulites. The sizes of spherulites decreased with incorporation of F-MWCNTs into the polymer matrix, which might be due to the nucleation role of F-MWCNTs in F-MWCNTs/PEO composite. The DSC analysis showed that the Tm of pure PEO was about 66 °C, which slightly decreased by the incorporation of F-MWCNTs into the polymer matrix. The Tm of 1, 3 and 5 wt% F-MWCNTs/PEO nanocomposites was 65.6, 64 and 63.8 °C, respectively. It was also observed that the thermal stabilities of pure PEO were shifted towards higher temperature when F-MWCNTs were incorporated into the polymer matrix. The TGA analysis represents that pure PEO and 1 wt% of F-MWCNTs/PEO nanocomposite decomposed completely at about 460 °C while thermal stability of 3 and 5 wt% of F-MWCNTs/PEO nanocomposites were shifted towards higher temperature.

Enhancing the Crystallization Performance of Poly(L-lactide) by Intramolecular Hybridizing with Tunable Self-assembly-type Oxalamide Segments

In this work, hydroxyl-terminated oxalamide compounds N1,N2-bis(2-hydroxyethyl)oxalamide (OXA1) and N1, N1′-(ethane-1,2-diyl)bis(N2-(2-hydroxyethyl)oxalamide (OXA2) were synthesized to initiate the ring-opening polymerization of L-lactide for preparation of oxalamide-hybridized poly(L-lactide) (PLAOXA), i.e., PLAOXA1 and PLAOXA2. The crystallization properties of PLA were improved by the self-assembly of the oxalamide segments in PLAOXA which served as the initial heterogeneous nuclei. The crystal growth kinetics was studied by Hoffman-Lauritzen theory and it revealed that the nucleation energy barrier of PLAOXA1 and PLAOXA2 was lower than that of PLA. Consequently, PLAOXA could crystallize much faster than PLA, accompanied with a decrease in spherulite size and half-life crystallization time by 74.8% and 86.5% (T = 125 °C), respectively. In addition, the final crystallinity of PLAOXA1 and PLAOXA2 was 6 and 8 times higher, respectively, in comparison with that of neat PLA under a controlled cooling rate of 10 °C/min. The results demonstrate that the hybridization of oxalamide segments in PLA backbone will serve as the self-heteronucleation for promoting the crystallization rate. The higher the content of oxalamide segments (PLAOXA2 compared with PLAOXA1) is, the stronger the promotion effect will be. Therefore, this study may provide a universal approach by hybridizing macromolecular structure to facilitate the crystallization of semi-crystalline polymer materials.

Investigation of residual stresses in polypropylene using hot plate welding

During the cooling process of the molten material, residual stresses appear because the reduced volume of the cooled material cannot fully fill the space formerly occupied by the molten material. The morphology in and around the weld is formed by different factors depending on the material and process parameters. Different morphological structures relate to different mechanical properties. The process parameters and the welding results including morphology and residual stress are linked together. In this article, residual stresses and the mechanical properties of a hot-plate-welded polypropylene specimen with 0.1 wt.-% content of carbon black are investigated in relation to the morphology. Different measurement positions and joining displacements of parts to be joined result in different residual stress states and morphological structures. The higher the joining displacement, the higher the residual stress. Investigations of the morphology show a relation between the size of the alpha spherulites and the joining displacement. Diffractions patterns of wide-angle X-ray scattering (WAXS) are not able to resolve the beta phase of the specimen.

Effect of the Trifunctional Chain Extender on Intrinsic Viscosity, Crystallization Behavior, and Mechanical Properties of Poly(Ethylene Terephthalate)

In this work, poly(ethylene terephthalate) (PET) chain-extending products with different molecular weights were prepared by reactive extrusion using isocyanate trimer (C-HK) as the trifunctional chain extender. The effect of the chain extender C-HK on the intrinsic viscosity, melt flow property, crystallization behavior, crystallization morphology, and mechanical property of PET was investigated. The results showed that when the content of the chain extender was increased from 0.6 to 1.4 wt%, the viscosity average molecular weight of PET was effectively increased from 2.36 × 104 to 5.46 × 104 g·mol–1. After the chain extending, the crystallinity and the time of semicrystallization of PET were significantly decreased. After the isothermal crystallization at 220 °C for 5 min, the spherulites formed by pure PET became larger. With the increase in molecular weight of PET after chain extension, its spherulite size was significantly decreased without changing the crystalline structure. The chain-extended PET also exhibited more excellent bending-resistant and impact-resistant properties. While the tensile strength of PET after chain extension was slightly decreased, the bending strength was increased by a maximum value of 56.8%, and the impact strength was increased by a maximum value of five times.

Effect of different particle size of silica on structure, morphology, and properties of thermoplastic cassava starch

Thermoplastic cassava starch (TPS)/silica (SiO2) composites were prepared through melt-mixing, and the effect of different SiO2 particle size (20 nm, 200 nm, and 23 µm) on the structure, morphology, and properties of TPS/SiO2 composites was investigated. TPS/SiO2 composite containing 200 nm SiO2 has the lowest plasticizing time and torque, and the highest tensile and impact strength. The 23 µm SiO2 particle has the strongest effect on promoting retrogradation of starch, while the 200 nm SiO2 particle could inhibit the retrogradation of starch. TPS/SiO2 composite containing 200 nm SiO2 has the highest thermal stability, resistant water absorption, and light transmittance. The 200 nm SiO2 particle could be uniformly and finely dispersed in the starch matrix, however 20 nm SiO2 particle easily aggregated in TPS. The spherulites size was uniform in TPS/SiO2 composite containing 200 nm SiO2. TPS/SiO2 composite indicates the mixture of A + V types of crystalline structure, and TPS/SiO2 composite containing 23 µm SiO2 had the largest crystal size.

Effect of Hexagonal Boron Nitride Nanoplatelet on Crystal Nucleation, Mechanical Behavior, and Thermal Stability of High‐Density Polyethylene‐Based Nanocomposites

AbstractHexagonal boron nitride nanoplatelets (BNNP) significantly enhance the mechanical and thermal properties of high‐density polyethylene. In order to ensure superior dispersibility of BNNP, two different techniques, colloidal solution, and solvent blending, are used for fabricating nanocomposites. Compared to solvent blending, the colloidal solution‐based fabrication technique has superior dispersion capabilities for nanofillers in the PE matrix, which helps in improving the thermal stability and crystallinity in synthesized nanocomposites. A significant increase in the rate of crystallization of polyethylene (PE) is reported at 1% weight fraction of BNNP, which further increases with the increase in weight percentage of BNNP. BNNP acts as a new source of nucleating sites for crystallization in PE that also reduces the size of secondary structures spherulites. Enhanced tensile strength and Young’s modulus of BNNP/PE nanocomposites are reported with the increase in weight contribution of BNNP in the range of 1% to 5%.

Effect of Crystallinity on Water Vapor Sorption, Diffusion, and Permeation of PLA-Based Nanocomposites.

The effects of crystalline morphology and presence of nanoparticles such as cellulose nanofibers (CNFs), organically modified nanoclay (C30B), or a combination of both on water vapor sorption and diffusion in polylactide (PLA) were evaluated by a quartz spring microbalance (QSM). It was found that the large spherulite size induced by high-temperature processing leads to an increase in water sorption and a substantial reduction of diffusion with increasing crystallinity. Contrarily, small-sized spherulites, arising after low-temperature processing during solvent-casting, showed a different behavior with a slight decrease in both water vapor sorption and diffusion with increasing crystallinity. These observations suggest that solvent-casting at low temperatures should not be used to predict the properties a material will show after industrial-scale processing. From the analysis of the nanocomposite materials, it was concluded that nanoparticles affected the material′s properties not only by themselves but also by modifying the crystalline morphology. Interestingly, this led to CNF showing similar performance to C30B, decreasing water diffusivity (21 vs 27%) on isothermally crystallized materials despite its less favorable geometry. Additionally, the incorporation of 1 wt % CNF and C30B decreased water vapor transmission rate (WVTR) by 24% under an amorphous state but by 44% in a crystallized state, which makes hybrid CNF/C30B composites a promising food packaging material.

An approach for compatibilization of the starch with poly(lactic acid) and ethylene-vinyl acetate-glycidyl-methacrylate

In this study, we evaluated the possibility of dimethyldidodecylammonium bromide (DDAB)-modified graphene oxide quantum dots (DDAB-GOQDs) as a new compatibilizer to improve the compatibility of hydrophilic starch with hydrophobic polylactic acid (PLA) and ethylene-vinyl acetate-glycidylmethacrylate (EVA-GMA). The successful synthesis of DDAB-GOQDs was verified by Fourier transform infrared spectroscopy (FTIR), Transmission electron microscopy (TEM) and Zeta-Plus assay. The differential scanning calorimetry (DSC), polarized light optical microscopy (POM) and iso-thermal crystallization kinetics analysis showed that 0.25% DDAB-GOQDs could significantly increase the crystallization rate and nucleation density as well as reduce the spherulites size of the nanocomposites. Compared to pure composites, the nanocomposites containing 0.25% DDAB-GOQDs showed obvious enhancement in thermal stability and surface hydrophobicity, with an increase of ~22 °C and ~24.1° in the maximum decomposition temperature and contact angle, coupled with similar improvement in the storage modulus and UV-shielding capacity. Morphological analysis, FTIR and creep behavior experiments revealed that adding 0.25% DDAB-GOQDs enhanced the interface compatibility of the nanocomposites by promoting the formation of the interpenetrated network through strong polar hydrogen bond or electrostatic force within the polymeric components. Overall, (0.25%) DDAB-GOQDs can serve as an effective compatibilizer for the PLA/starch/EVA-GMA system due to improved crystallization, thermal stability, barrier properties and creep-resistance.

Study of the changes in the polar phase and optical properties of poly (vinylidene fluoride) matrix by neodymium compound addition

In this work, a study of the changes in the physical properties of PVDF by neodymium (Nd2O3) and neodymium trans-3,4-(Methylenedioxy)cinnamate Nd(MCA) compound addition was conducted. The Nd2O3 insertion in the polymeric PVDF films increases the spherulite sizes. On the other hand, the spherulite sizes decreases by the insertion of the Nd(MCA) compound. The calculus of the relative percentage of β phase revealed that the Nd(MCA) filler decreases the polar phase and increases the optical absorption increases in the UV region. The fluorescence measurements show a broadening of the emission band in the visible range for all concentrations of the PVDF/Nd(MCA) compound. This effect is related to a transfer energy process from MCA ligand to Nd2O3 metal center and PVDF Matrix. In summary, the present study demonstrates the promising prospect of these composites as optical and photonic devices, as well as a light white emitting device.

Protic ionic liquids/poly(vinylidene fluoride) composite membranes for fuel cell application

Poly(vinylidene fluoride), PVDF, membranes have attracted considerable attention as polymer electrolytes for fuel cells. This study explores the effect of solvent on the spherulite size and the crystallinity of the polymeric membranes. Based on Hansen solubility parameters theory, the mixture of DMC and DMSO was selected among a dozen of solvents for the preparation of PVDF membranes by thermally induced phase separation. The addition of two protic ionic liquids (PILs), bis(2-ethyl hexyl) ammonium hydrogen phosphate [EHNH2][H2PO4], and imidazolium hexanoate [Im][Hex] to PVDF membranes at concentrations (10% < wPIL < 50%) has been investigated by SEM, FTIR, DSC, TGA, EIS, and DMA. The inclusion of ionic liquids into the polymer matrix influences structural parameters (degree of crystallinity and electroactive phases), thermal stability, proton conductivity and mechanical properties of the membranes. The membranes become transparent regardless type of ionic liquid employed. A small amount of ionic liquids increases the degree of crystallinity and facilitates the production of polar β and γ crystals. The proton conductivity mechanism (Grotthuss) is dependent on the ionic liquid structure (due to its self-organization in water) and the content in the PVDF membrane, as well as the membrane water uptake. Different behavior has been observed for the two ionic liquids, which stresses the challenge on selecting an appropriate cation and anion combination. The obtained composite membranes exhibited excellent mechanical performance and reduced elastic modulus, with respect to the pure polymer matrix. These results indicate that PVDF/IL composite membranes have a high potential for PEMFC applications.

The Effect of Different Acrylic-Based Rubbers on the Crystallization Behavior of PLA/PDLA Stereocomplex

This study investigated the performance of two acrylic rubbers on the improvement of mechanical and thermal properties of polylactic acid (PLA)/ poly(D-lactide) (PDLA) stereocomplex. The selected acrylic-based rubbers consisted of a commercial acrylic rubber (ACM) and acrylic core–shell rubber (CSR). In this work, 5 wt% PDLA was melt-blended with PLA to form PLA/PDLA stereocomplex (ST) in the presence of a 15 wt% acrylic-based rubber in an internal mixer at 180 °C and 50 rpm. It was found that the small amount of PDLA could successfully form stereocomplex crystals with PLA during melt mixing. The addition of acrylic-based rubbers not only increased the impact strength of the blends but also assisted PLA in forming more perfect crystals. This is because the rubbers could act as nucleation sites for PLA crystallization leading to smaller spherulite size than that of neat unmodified PLA. Comparatively, CSR could provide much higher %crystallinity with quicker nucleation onset than ACM in both PLA and ST systems, even though the crystallization rate was lower. In terms of mechanical properties, CSR also contributed to the highest impact strength due to the smaller size of CSR than ACM in both PLA and ST systems.

Non-isothermal crystallization behavior of isotactic polypropylene/copper nanocomposites

The Non-isothermal crystallization behavior of isotactic polypropylene (iPP)/copper nanocomposites with four different mass percentages (0.5, 1.0, 2.0 and 4.0 mass%) of copper nanoparticles (nCu) were study by means of differential scanning calorimeter (DSC) at 2.5, 5, 10 and 20 °C min−1. The nanostructure of PP/nCu nanocomposites was also studied by WAXD, SEM and optical microscopy, while their hierarchical crystalline morphologies (e.g., spherulites, lamellas and unit cell) were studied during the crystallization progress through polarized optical microscopy and in situ X-ray scattering at small and wide angle, observing that the spherulite size was not influenced by the nCu’s, although the lamellas size and X-ray diffraction intensity increased with the incorporation of nanoparticles. The PP/nCu nanocomposites presented a mixture morphology with well-dispersed oxidized nanoparticles and some agglomerates, which were larger at higher concentration of particles. The crystallization results obtained by DSC indicated a displacement in the peak crystallization temperature at higher values. These results in combination with the kinetic of crystallization analyzed by the Jeziorny method clearly indicated an acceleration in the crystallization process. Additionally, the crystallization activation energy decreases for all nanoparticle’s concentration, indicating a possible nucleating effect. However, the nucleation activity analyzed by the Dobreva and Gutzow’s method showed that nCu could not be considered such as conventional heterogeneous nucleating agent of iPP. Conversely, the incorporation of nCu in the iPP matrix increased significantly the thermal conductivity of nanocomposites helping to the thermal dissipation from the melted macromolecules to the nCu’s, accelerating the non-isothermal crystallization process.

Nucleation effect of aliphatic polycarbonate in its blends with poly(ethylene terephthalate)

Aliphatic polycarbonate (PDCM) was synthesized from dimethyl carbonate and 1,4-cyclohexanedimethanol via melting polymerization in this study. The molecular weight of PDCM was determined by gel permeation chromatography (GPC) to be ca. 49,900. The crystallization behavior and crystallinity of polyethylene terephthalate (PET) were significantly modified by adding 0.3–5.0 wt% PDCM, which was confirmed by differential scanning calorimetry (DSC), photomicrography with a heating stage and SAXS analysis. In nonisothermal crystallization kinetics analysis, the reciprocal of half-time crystallization (i.e., G parameter) for the 1.0 wt% PDCM/PET blends is 3.5 times greater than that of pristine PET at a cooling rate of 10 °C/min. Observation via polarized optical microscopy showed that the spherulites size of PET was condensed by PDCM. In addition, the fold surface energy (σe) decreased with the presence of PDCM. According to the results of dynamic mechanical analysis (DMA) examination, the mechanical strength of 1.0 wt% PDCM/PET thin film was enhanced as the transparency still maintained above 85%, which illustrated the transparent thin film has better thermal stability and has high potential to be used such as in the electronic appliances.

Effects of sepiolite on crystallization behaviors and properties of sepiolite/polyoxymethylene composites

For filler-polymer systems, the ultimate importance of interfacial interactions between fillers and polymers lies in improving morphological developments and properties of composites. In this work, sepiolite fiber (SEP)/polyoxymethylene (POM) is formulated through melt extrusion processing, with the aim of promoting the crystallization and overall performance of POM. Results show that hydrogen bonding prevails between SEP hydroxyls and POM C-O-C groups. Thus SEP fibers are dispersed uniformly in the POM matrix due to interfacial interactions between these two components. During the isothermal crystallization process, POM nuclei originate and grow on the SEP surface, resulting in decreasing spherulite sizes and facilitating the POM crystallization. Meanwhile, the crystallinity of POM is increased from 48.3% to 54.1% by the addition of 5 wt% SEP. Noteworthily, mechanical and thermostability performances of SEP/POM composites have been improved considerably by means of sufficient interfacial interactions, enhanced crystallization, and favorable physical properties of SEP. Maximum values of the tensile strength and modulus at 3.0 wt% SEP loading can reach 66 MPa and 2022 MPa, respectively higher than those of neat POM.

Effect of Silver Nanoparticles on the Microstructure, Non-Isothermal Crystallization Behavior and Antibacterial Activity of Polyoxymethylene.

Silver (Ag) nanoparticles were synthesized by a facile route in the presence of oleic acid and n-propylamine. It was shown that the average primary size of the as-synthesized Ag nanoparticles was approximately 10 nm and the surface of as-synthesized Ag nanoparticles was capped with monolayer surfactants with the content of 19.6%. Based on as-synthesized Ag nanoparticles, polyoxymethylene (POM)/Ag nanocomposites were prepared. The influence of Ag nanoparticles on non-isothermal crystallization behavior of POM was investigated by differential scanning calorimetry (DSC). The Jeziorny, Jeziorny-modified Avrami, Ozawa, Liu and Mo, Ziabicki and Kissinger models were applied to analyze the non-isothermal melt crystallization data of POM/Ag nanocomposites. Results of half time (t1/2), crystallization rate parameter (CRP), crystallization rate function (K(T)), kinetic parameter (F(T)), the kinetic crystallizability at unit cooling rate (GZ) and the crystallization activation energy (∆E) were determined. Small amounts of Ag nanoparticles dispersed into POM matrix were shown to act as heterogeneous nuclei, which could enhance the crystallization rate of POM, increase the number of POM spherulites and reduce POM spherulites size. However, the higher loading of Ag nanoparticles were easily aggregated, which restrained POM crystallization to some degree. Furthermore, the POM/Ag nanocomposites showed robust antibacterial activity against Escherichia coli and Staphylococcus aureus.

НАХОДКА МАРКЕРОВ МЕЖЛЕДНИКОВИЙ В ПЕЩЕРЕ УСЬВИНСКАЯ-1 (СРЕДНИЙ УРАЛ)1

Mineral formation due to water freezing in caves causes supersaturation of the unfrozen part of the solution and deposition of some dissolved compounds in the form of minerals. Evaporation of water and degassing of the solution are associated processes in deposition of minerals. Cryogenic minerals of caves differ in morphology and isotopic composition from similar mineral cave formations not subject to glaciation. The morphology and mineralogy of cryogenic cave minerals depend on the initial chemical composition of karst water, the thickness of the freezing layer of water, and also on the freezing rate. Slow freezing of cave waters at 0°C in permafrost conditions leads to the formation of coarse cryogenic calcite. These carbonates can be used as indicators of permafrost thawing during interglacial periods. Morphological and isotopic study of samples from Usvinskaya-1 Cave (Middle Urals), as well as their 230Th / 234U dating, made it possible to conclude that the calcite is cryogenic. The calcite is represented by aggregates (up to 5 cm in size) of split crystals and spherulites from milky brown to black in color. This calcite, formed during the interglacial periods, indicates the existence of several periods of degrading permafrost in the Middle Urals, namely in the periods MIS9 (295.7 thousand years ago), MIS7 (205.1 thousand years ago), GI23 (approx. 102-104 thousand years ago) and GI22 (89.4-90.0 thousand years ago).

Effect of Crystalline Morphology on Electrical Tree Growth Characteristics of High-Density and Low-Density Polyethylene Blend Insulation

The electrical tree initiation and propagation behaviors are a key issue for the polyethylene-based cable insulation. This paper focuses on the effect of crystalline morphology on electrical tree growth characteristics of high-density and low-density polyethylene (HDPE/LDPE) blend insulation. In this paper, the electrical tree growth characteristics of HDPE/LDPE blends with HDPE mass fractions of 0, 10, 15, 20 wt% are investigated under repetitive impulse voltage at 40, 60, 80 °C. It is found that with the rise of HDPE content from 0 to 15 wt%, the growth rate and accumulated damage of electrical tree decreases, with the morphology of electrical tree tending to bush tree. The increasing of voltage amplitude improves the energy of injected charge while the temperature rise leads to the relaxation of molecular chain, which both result in the promotion of collision ionization, thus increasing the density of electrical tree. Compared with branch tree, bush tree illustrates better inhibition of discharge tracks due to uniform electric field at the end of branches. The crystalline characteristics of HDPE/LDPE blends indicate that the crystallinity increases with the addition of HDPE, and the blend comprising 15 wt% HDPE in an LDPE matrix apparently reduces the average size of spherulites and improves the distribution of spherulites evenly. The upsurge of crystal-amorphous interface leads to the increase of deep trap energy level density and the decrease of carrier mobility. Carrier injection and migration at the interface between crystalline and amorphous regions are restrained, thus inhibiting the growth of electrical tree. It is concluded that the crystalline morphology modified by polymer blending has a significant effect on the electrical tree growth characteristics.

Effects of divinylbenzene‐maleic anhydride copolymer hollow microspheres on crystallization behaviors, mechanical properties and heat resistance of poly(l‐lactide acid)

Divinylbenzene‐maleic anhydride copolymer hollow microspheres (DMs) were used as novel organic nucleating agents to promote crystallization of poly(l‐lactide acid) (PLLA). The effects of these DMs on crystal behaviors of the PLLA were investigated by differential scanning calorimeter (DSC), polarizing optical microscopy (POM), and wide angle X‐ray diffraction (WAXD). Both isothermal and non‐isothermal processes in DSC demonstrated that the DMs significantly altered the crystal behaviors of PLLA as both crystallization velocity and degree of crystallinity increased with increasing DM loadings from 0 to 3%. Our POM results also indicated that as nucleating agents, the DMs promoted nucleating densities and decreased spherulitic sizes. In addition, WAXD suggeted that the addition of DMs did not induce new types of crystals. Finally, our results showed that the ductility of the PLLA was enhanced by a small amount of DMs during the PLLA crystallization process since 0.5% DMs added to the PLLA resulted in 1.4‐fold increase in the elongation at break in comparision with the neat PLLA.

Strategy To Improve Printability of Renewable Resource-Based Engineering Plastic Tailored for FDM Applications.

This work features the first-time use of poly(trimethylene terephthalate) (PTT), a biobased engineering thermoplastic, for fused deposition modeling (FDM) applications. Additives such as chain extenders (CEs) and impact modifiers are traditionally used to improve the processability of polymers for injection molding; as a proof of concept for their use in FDM, the same strategies were applied to PTT to improve its printability. The filament processing conditions and printing parameters were optimized to generate complete, warpage-free samples. The blends were characterized through physical, thermal, viscoelastic, and morphological analyses. In the optimal blend (90 wt % PTT, 10 wt % impact modifier, and 0.5 phr CE), the filament diameter was improved by ∼150%, the size of the spherulites significantly decreased to 5% of the ∼26 μm spherulite size found in neat PTT, and the melt flow index decreased to ∼4.7 g/10 min. From this blend, FDM samples with a high impact performance of ∼61 J/m were obtained, which are comparable to other conventional FDM thermoplastics. The ability to print complete and warpage-free samples from this blend suggests a new filament feedstock material for industrial and home-use FDM applications. This paper discusses methods to improve hard-to-print polymers and presents the improved printability of PTT as proof of these methods’ effectiveness.

Rheology of crystallizing polymers: The role of spherulitic superstructures, gap height, and nucleation densities

A longstanding goal in polymer rheology is to develop a physical picture that relates the growth of mechanical moduli during polymer crystallization to that of a structure. Here, we utilize simultaneous mechanical rheology and optical microscopy, with augmentation by deterministic reconstruction and stochastic simulations, to study isothermal crystallization in isotactic polypropylene. We observe the nucleation and growth of the surface and bulk spherulites, which are initially isolated and then impinge to form clusters and superstructures that eventually span the gap. We find that spherulitic superstructures play a critical role in the rheology, especially in the characteristic sharp upturn in moduli. Both the rheology and the spherulitic superstructures show pronounced gap dependencies, which we explain via finite-size effects in percolation phenomena and via surface-induced nucleation. The modulus-crystallinity relationship can be described through a general effective medium theory. It indicates that for thicker gaps, the viscoelastic liquid to solid transition can be described via percolation, whereas for our thinnest gap, it is best described by the linear mixing rule. We describe our results in terms of dimensionless nucleation rates and spherulite size, which enable the estimation of when gap-dependent superstructure effects can be anticipated.