Abstract Polymer crystallization under nanoscale confinement is governed by finite-size and interfacial effects, yet its behavior at length scales approaching the lamellar thickness remains poorly understood. Here, we investigate the crystallization of poly(ethylene oxide) (PEO) with varying molecular weights confined within silicon nanopores (d ∼ 7–8 nm). Crystallization is strongly suppressed, with the crystallization temperature approaching the homogeneous nucleation regime while deviating from conventional volume scaling. Furthermore, the melting temperature exhibits a pronounced reduction (∼20 K), indicating that lamellar thickness is directly constrained by the geometry. Interestingly, a transition from an extended-chain crystal to a folded-chain crystal occurs in PEO with a molecular weight of 2000 g/mol (PEO2k), while PEO with a molecular weight of 1000 g/mol (PEO1k) maintains an extended-chain crystal, because the contour length of the PEO2k chain exceeds d, whereas that of the PEO1k is comparable to d. These results demonstrate that, when the confinement dimension approaches the lamellar thickness, geometric confinement dictates nucleation and attainable crystal dimensions, whereas interfacial interactions govern crystal orientation, thereby defining the fundamental limits of polymer crystallization under extreme nanoconfinement.

Abstract Polylactic acid (PLA) components fabricated by fused filament fabrication (FFF) typically suffer from poor interlayer bond strength. Owing to PLA's intrinsically weak crystallization ability, the role of crystallization in interlayer bonding has rarely been investigated, limiting the development of interfacial enhancement strategies. To address this, we designed a PLA printing material with enhanced crystallization ability by incorporating polyethylene glycol (PEG-4K), and then systematically regulated the crystallization behavior in FFF-printed part via varying bed temperatures (Tb) and applying thermal annealing. When printed or annealed at 90 °C, both neat PLA and PLA/PEG-4K exhibited rapid crystallization with half-crystallization times (t1/2) below 9.8 min. Such extensive crystallization restricted chain diffusion across adjacent layers, resulting in interfacial bond strengths below 15 MPa. Conversely, decreasing Tb below 60 °C extended t1/2 to over 90 min, yielding the printed samples with low crystallinity and markedly increased bond strength. Notably, incorporating PEG-4K shortened the segmental relaxation time by 30% and decreased the melt viscosity, synergistically improving the bond strength to 24.7 MPa. These results confirmed that the kinetic competition between crystallization and chain interdiffusion determined the interlayer bonding. This work also established a quantitative correlation between crystallinity and interfacial bond strength, offering a theoretical basis for the mechanical optimization of FFF-printed PLA parts.

Mitigating leakage risk and expanding the application range of phase change materials (PCMs) are essential approaches to improve their practical value. Herein, a multi-functional polyethylene glycol (PEG) cross-linked silica network (PEG-cro-SiO2)/carbon nanotube (CNT)/PEG composite-based form-stable PCM (CSTP) is developed in which the cross-linked silica network and chain entanglement maintain the shape stability, while the synergistic combination of SiO2 and CNT promotes the formation of thermal conduct pathways. Building upon the in-depth exploration of the dynamic balance between the phase change ability and shape stability regulated by the cross-linking and entanglement density, the PEG-cro-SiO2/PEG composite-based PCMs achieve wide range enthalpy regulation (108.0-151.2 J g(-1)) and exhibit remarkable cyclic stability (enthalpy loss <= 0.5 % after 100 heating-cooling cycles). Moreover, the establishment of efficient thermal conduct pathway stimulates the multiscenario applications of CSTP film. Therefore, it displays high solar-to-thermal energy conversion efficiency (eta) of 93.7 %, and demonstrates a maximum output voltage of 468 mV in solar-thermal-electric conversion. Practical simulation demonstrates superior thermal management property: the phase change roof equipped with certain amount of CSTP film could maintain vehicle cabin temperature equilibrium at 62.5 C-degrees for 114 s with efficiency of 228 s g(-1). Besides, the CSTP film challenges the conventional single-mode passive heat transfer provided by thermal grease in smartphones. The dual thermal regulation of CSTP film for power management integrated circuit (PMIC), absorbing heat to maintain the temperature equilibrium during the initial heating stage and facilitating passive transfer of excessive heat in subsequent heating stage, could enhance smartphone operational stability.

In this work, the differences of crystallization behavior between polyethylene glycol (PEG) cross-linked SiO2 (PEG-cro-SiO2) nanoparticles and single-end methoxypoly(ethylene glycol) grafted SiO2 nanoparticles (MPEG-g-SiO2) were systematically investigated. It reveals that low-molecular-weight (6K, 11K) PEG-cro-SiO2 nanoparticles exist in a severely confined state due to the synergistic constraints from cross-linking bonds on the surfaces of two nanoparticles, exhibiting only a single low-temperature crystallization peak (LCP, approximately -30 °C) with homogeneous nucleation characteristics. In contrast, high-molecular-weight (20K) PEG-cro-SiO2 nanoparticles exhibit double crystallization peaks, LCP and an additional high-temperature crystallization peak (HCP), indicating that polymer chains far from the nanoparticle surface possess enhanced mobility. The crystallization behavior of MPEG-g-SiO2 nanoparticles is regulated by grafting density and molecular weight. Low-molecular-weight (5K) grafted chains adopt "trains" conformation at any grafting density, showing only a single LCP; As the molecular weight increases to 10K and 20K, elevated grafting density can promote the transition of partial chain segments from "trains" to "loops" and "tails" conformations, thereby inducing fractionated crystallization phenomena. The absence of Domain II in self-nucleation experiments further confirmed the severely confined nature of LCP in the PEG-cro-SiO2 and MPEG-g-SiO2 nanoparticles. Under comparable molecular weight conditions, the confinement effect of PEG-cro-SiO2 is significantly stronger than that of the MPEG-g-SiO2 system. This study provides a theoretical foundation for the proactive design of nanoparticle microstructures based on performance requirements.

Nucleation, which is the initial step of crystallization, critically governs the polymer crystallization behavior, influencing the crystallization temperature, kinetics, and morphology. However, the direct observation of the nucleation process in polymers remains elusive owing to spatial and temporal resolution limitations. This feature article summarizes the recent progress in understanding polymer nucleation within confined and interface-dominated environments, focusing on three representative systems: anodic aluminum oxide templates and nanocomposites containing nanoparticles or nanosheets. The interplay between finite size and interfacial effects has revealed some novel phenomena, such as homogeneous nucleation, surface nucleation, prefreezing, and supernucleation.

Currently, material extrusion‐based additive manufacturing (MEAM) technique, also known as fused filament fabrication (FFF) technique has been widely used to prepare customized porous scaffolds for bone tissue engineering. However, porous scaffolds often lack desirable osteogenic properties due to the poor hydrophilicity of polymer materials used for FFF technique. In this work, biocompatible materials suitable for FFF technique are prepared by blending polycaprolactone (PCL), polylactic acid (PLA), and tricalcium phosphate (TCP) at various compositions. These composite materials are subsequently printed into cylindrical scaffolds with controllable pore sizes ranging from 200–800 µm, by regulating the infill density during the FFF process. The FFF‐printed scaffolds have the highest modulus at a PLA/PCL ratio of 0.7 and a pore size of ≈ 200 µm. Furthermore, surface treatment is applied to these FFF‐printed scaffolds in sodium hydroxide solution. As a result, the surface roughness, hydrophilicity and serum adsorption of the scaffolds are significantly enhanced. More importantly, these surface‐treated scaffolds can promote the osteogenic differentiation of MC3T3‐E1 cells, comparable to commercial Bio‐Oss substitutes. Thus, this study offers a cost‐effective technique for the development of bioactive scaffolds for potential bone tissue engineering applications.

In this work, the melt reinforcement behavior of polyethylene grafted silica (PE-g-SiO2) and polypropylene grafted silica (PP-g-SiO2) filled polymer nanocomposites (PNCs) was investigated by small angle X-ray scattering (SAXS), transmission electron microscopy (TEM), and linear rheology from small-amplitude oscillatory shear (SAOS) measurements. The effects of nanoparticle loading (Phi) and dispersion state on the rheological behavior of PNCs were examined. Compared with unmodified nanoparticles, polymer grafted nanoparticles exhibit better dispersibility in both PE and PP PNCs. The TEM and SAXS results show that the dispersibility of PE-g-SiO2 systems is worse than that of PP-g-SiO2 systems, which originates from the lower grafting molecular weight of PE-g-SiO2 systems. Both the HDPE14K/HDPE1K-g-SiO2 system and the PP370K/PP90K-g-SiO2 system show the deviation from the Guth-Gold model when Phi reaches 5 wt.%. SAOS results reveal that once Phi exceeds the percolation threshold, nanoparticles form interconnected networks, thereby significantly enhancing the mechanical performance of PNCs. Due to the different conformation structures of the grafting chains on the surface of silica, the nanoparticles contact each other by brush chains and bridge into a polymer-mediated nanoparticle network in HDPE14K/HDPE1K-g-SiO2 PNCs, resulting in a more significant modulus reinforcement than that of PP370K/PP90K-g-SiO2 PNCs with a soft interfacial structure of grafted chains.

Serious shrinkage and warpage are obstacles to the development of ideal isotactic polypropylene (iPP) materials for polymer-based powder bed fusion (PBF) technique. In this work, the variations of the dimensional accuracy of the PBF-printed iPP parts were investigated with various printing parameter and nucleating agent. The iPP parts printed at a scanning speed of 700 mm/s exhibit smaller extents of shrinkage and curling than those at lower speeds, due to a lower degree of crystallinity. Interestingly, iPP blended with the alpha- or beta-nucleating agent demonstrates more serious part deformation with respect to neat iPP. Especially, alpha-nucleating agent tends to trigger the most severe curling under the investigated printing parameters. Based on X-ray diffraction and differential scanning calorimetry (DSC) results, both neat iPP and alpha-iPP parts crystallize into alpha-crystal, while beta-iPP parts display the coexistence of beta- and alpha-crystals. And, the difference of the crystallinity is less than 3% in three specimens. This suggests that both crystallinity and crystalline structure are not the main reasons for the shrinkage and warpage in this case. Instead, the severe part deformation of the alpha-iPP parts is assigned to the narrower sintering window as well as the higher onset crystallization temperature of alpha-iPP, which hinder relaxation of residual stresses. This work provides insights into the part deformation mechanism for PBF-processed polymer materials. image

In this work, the evolution of the crystalline structures of MEAM-printed polyamide1012 (PA1012) parts with the nozzle temperatures (T-n) and thermal annealing temperatures (T-a) was investigated. It is found that T-n plays an important role in determining the crystalline structure of PA1012 parts. The freshly-printed PA1012 parts crystalize as gamma crystals at T-n lower than 230 degrees C. With T-n increasing to above 230 degrees C, a small amount of alpha phase coexists with gamma 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 gamma phase to alpha crystal, after annealing the PA1012 parts at a T-a higher than 90 degrees C for 10 min. Due to the presence of a high content of gamma 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 gamma phase. Moreover, the crystalline structure exerts a significant influence on the mechanical properties of the PA1012 parts. The samples with only gamma phase exhibit much higher tensile strength and Young's modulus, but much lower ductility, compared to those containing only alpha crystal. Hence, this work provides effective pathways for regulating crystalline structures as well as mechanical properties of MEAM-printed long-chain polyamide materials.

In order to investigate the effect of surface properties of nanoparticles (NPs) on the crystallization behavior of nanocomposite, three kinds of silica (SiO2) NPs are dispersed in isotactic polypropylene (iPP) matrix by solution blending method. Specifically, the poor compatibility between inorganic bare SiO2 NPs and organic PP matrix triggers the formation of aggregates and induces the fractionated crystallization with two crystallization peaks at higher particle loading (Phi), i.e., main crystallization peak (MCP) and low temperature crystallization peak (LCP, always located at 124.0 degrees C). Once short PP chains (0.9 x 10(4) g mol(-1)) are grafted onto the surface of SiO2 NPs, the nanocomposites with all Phi only exhibit a single MCP which is attributed to the strong interfacial interaction. With the increase of grafting chain length (9 x 10(4) g mol(-1)), the more stretched conformation of grafting chains can provide numerous heterogeneous nucleation sites and significantly accelerate the crystallization rate of PP chains near the surface of SiO2 NPs, which lead to the formation of an extra high temperature crystallization peak (HCP, almost kept constant at 133.0 degrees C). The occurrence of various crystallization peaks implies that the surface properties of nanoparticles play a vital role in the nucleation ability of nanoparticles.

The properties of polymer nanocomposites (PNCs) greatly depend on the dispersion of nanoparticles within the polymer matrix. In this work, linear high-density polyethylene grafted with SiO2 nanoparticles (HDPE1289-g-SiO2, where 1289 indicates the weight-average molar mass of the grafted polyethylene) was synthesized via the "grafting to" method and added to PE matrices with three different molecular weights and branching degrees (LDPE4k, LDPE34k, and HDPE67k) by a solution blending method. The dispersion and crystallization behaviors of PE/HDPE1289-g-SiO2 PNCs were studied by transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), and polarized light optical microscopy (PLOM). In a low-density polyethylene (LDPE4k) matrix with a small ratio of matrix-to-grafted chain length (P/N), HDPE1289-g-SiO2 nanoparticles are better dispersed when compared with unmodified SiO2, a result related to the low P/N (entropic effect). With the increase of the molecular weight of the matrix (LDPE34k) with high P/N, both the HDPE1289-g-SiO2 and unmodified SiO2 nanoparticles have a poor dispersion with nanoparticles' aggregates in the matrix and similar nucleation effects on the matrix. In the above two systems, we used the P/N parameter commonly used for linear systems as an approximation because the C-13 NMR results showed that LDPE4k and LDPE34k contain only butyl and longer branches (>= 6C). Interestingly, in the high-density polyethylene (HDPE67k) matrix with high P/N, HDPE1289-g-SiO2 nanoparticles are better dispersed than unmodified SiO2 due to the miscibility between the grafted and matrix polyethylene (enthalpic effect). For the HDPE67k matrix, HDPE1289-g-SiO2 nanoparticles exhibit a supernucleation effect on HDPE, with the nucleation efficiency higher than 100% due to their good dispersion. Our results show that for the PE/HDPE1289-g-SiO2 PNCs, the lower the P/N or the more similar the chain structure of the grafted and matrix polymer is, the better the dispersion of HDPE1289-g-SiO2 nanoparticles, thus improving their nucleation ability.

The high crystallinity and fast crystallization rate of polypropylene (PP) often result in severe warpage of final parts prepared by means of material extrusion additive manufacturing (MEAM). In this work, the effect of bed temperature ( T b ) and nozzle temperature ( T n ) on the dimensional accuracy and mechanical property of MEAM‐printed PP copolymer parts was investigated. It is found that raising T b and T n could significantly reduce the warpage of PP parts by regulating the crystallization behavior of PP. Specially, the amount of γ‐crystal grows obviously while the total crystallinity remains unchanged, as T b or T n increases. Moreover, the more the content of γ‐crystal, the lower the warpage height of PP samples is. Hence, a close correlation between the content of γ‐crystal and part deformation of MEAM‐printed PP parts is successfully established. This is mainly related to the large rigidity of γ‐crystal, which significantly reduces the volume shrinkage of PP during MEAM process. Besides, the raise of T b or T n also enhances the tensile strength, modulus and the elongation at break of PP parts. This wok provides an effective and convenient method for the manufacture of PP parts with good dimensional accuracy and excellent mechanical properties by means of MEAM techniques.

As a model system for thermal interface materials (TIMs), the complex rheological behavior of polydimethylsiloxane oligomer (oDMS)/alumina sphere particles (AlS) suspensions (i.e., oDMS/AlS) is investigated systematically in this work. Aluminum particles with 1 and 10 mu m were used as fillers. Here, a surface treatment by using a silane coupling agent as found to be effective in improving the fluidity of the suspensions with small sized AlS particles. Peculiar large amplitude oscillatory shear (LAOS) results were observed for the suspensions that exhibited three or four responses (Hookean regime, shear softening, shear hardening and a second shear softening) depending on the surface properties of AlS particles. For the oDMS/AlS10 neat system, the width of the Hookean regime is determined by the shear stress (tau$$ \tau $$) instead of the shear strain (gamma$$ \gamma $$), whereas it completely disappears in the oDMS/AlS10t system due to the increase potential energy between particles. Both critical stress and strain at the onset point for strain hardening decreased with the increase of particle concentration. Frequency dependent measurements illustrates that it is the critical strain instead of the stress that controls the shear hardening behavior. Average distance of neighboring particles is assumed to play a key role in the formation of hydroclusters.

The crystalline structures and their spatial distributions of polyamide 12 (PA12) parts processed by material extrusion additive manufacturing (MEAM) were studied in this work by means of microfocus wide-angle X-ray scattering combined with transmission electron microscopy. There are two types of gradient structures observed in the PA12 parts. One of the gradient structures is found in the topmost layer, where the total crystallinity drops gradually from the free surface to the core region further to the weld region. Moreover, the coexistence of alpha and gamma crystals is found in all regions. The gradient structure in the topmost layer arises from partial melting and chain relaxation occurring during the welding of the free surfaces between adjacent layers. Another gradient structure is that the crystallinity in PA12 parts demonstrates a gradual variation with decreasing the distance from the deposited layer to the platform, especially at a platform temperature above 60 degrees C. Specifically, the bottom layer has much higher crystallinity than the top layer, with the middle layer in between, since the annealing effect of the hot platform gradually weakens from bottom to top due to the low heat conduction of PA12. This work provides a detailed insight into the crystallization mechanism of polyamide materials during MEAM process, which is meaningful for the manipulation of crystal structures in MEAM-printed semicrystalline polymers.

The matter at surfaces or interfaces exhibits physical properties that differ from the bulk. The physiochemical questions at surfaces/interfaces are of fundamental importance for chemistry and material science. In recent years, new crystallization phenomena related to surfaces/interfaces have emerged and have caused attention. Based on the research work of our team, this article summarizes the recent results on this topic, focusing on the physical mechanism behind the different experiment findings in various systems. First, several basic concepts are introduced, especially surface tension and nucleation. Then, several examples are summarized in detail, including surface freezing of long-chain alkanes, prefreezing of polymers on solid substrates, metastable states enhancement induced by surface freezing, and the impact of surface adsorption on crystallization. After that, the relationships among different surface-related crystallization phenomena are discussed. Finally, the unsolved questions in polymer surface crystallization are proposed.

Polymer parts fabricated by fused filament fabrication (FFF) technique usually exhibit weak and anisotropic mechanical properties, compared with their injection-molded (IM) counterparts. Thus, the evolution of FFF from rapid prototyping into manufacture tool requires three-dimensional enhancement ways for FFF-printed parts. To this end, we develop a series of carbon nanotubes (CNTs)-embedded polyamide 12 (PA12) filaments and investigate the effect of CNTs on the mechanical properties of the FFF-printed PA12 parts. In the direction parallel to the deposited strands, PA12/CNT parts show considerable mechanical reinforcement compared with neat PA12 specimens, including the increase of tensile and impact strength by 18% and 125%, respectively. The marked improvement in impact strength arises from the oriented structures induced by the shear field during FFF process as well as the layer-by-layer microstructure, which provide multiple ways to dissipate impact energy. However, these oriented CNTs and inter-filament voids tend to act as stress concentration points under the tensile stress, thus accounting for a slight increase in tensile strength. In the direction perpendicular to the deposited strands, the FFF-printed PA12/CNT parts also exhibit improved tensile strength, i.e. interfacial weld strength, as CNTs allow the rapid transfer of the thermal energy from the newly-deposited filament to the weld interface and thus provide longer time for molecular interdiffusion. Altogether, the anisotropy in tensile strength of the FFF-printed parts is reduced from 0.36 for neat PA12 parts to 0.28 for PA12/CNT specimens. This work is believed to provide a facile route to overcome mechanical limitations in FFF technique.

The synthesis of polymer-grafted nanoparticles is crucial for the development of polymer nanocomposites with excellent properties, but the synthesis of polyethylene (PE)-grafted SiO2 nanoparticles (PE-g-SiO2 NPs) remains a significant challenge due to the chemical inertia of PE. In this study, PE-g-SiO2 NPs with different grafting densities and molecular weights were successfully synthesized by a simple "grafting to" method, and differential scanning calorimetry was used to study the crystallization behavior of grafted PE. Some PE-g-SiO2 samples exhibited three crystallization peaks in the cooling curve (i.e., fractionated crystallization phenomena). Self-nucleation (SN) and successive SN and annealing (SSA) results reveal that the nanoparticles in PE-g-SiO2 samples provoke both confinement and nucleation effects on the grafted PE chains. For PE-g-SiO2 NPs with higher grafting density and molecular weight, the PE chains are more stretched and the interfacial chain segmental dynamics are less suppressed, which enhance the nucleation effect and weaken the confinement effect.

Well-defined polypropylene grafted silica nanoparticles (PP-g-SiO2) were prepared through the reaction of maleic anhydride grafted polypropylene (PP-g-MAH) with amino-functionalized silica (SiO2-NH2) by the 'grafting-to' method. The grafting density of PP-g-SiO2 is found to be controlled by the concentration of silane coupling agent 3-[2-(2-aminoethylamino) ethyl amino] propyl trimethoxy silane (TAMS). The maximum grafting density of grafted PP-g-MAH chains with molecular weight of 9100 g/mol could reach 0.34 chains/nm(2), when the critical concentration of TAMS was 0.0194 mol/L. The critical concentration of TAMS can be explained by the maximum amounts of primary amino groups, which can totally react with PP-g-MAH on the surface of SiO2-NH2, when the silane monolayer is formed. The synthesized PP-g-SiO2 with different molecular weights was mixed with PP by solution mixing to form a series of nanocomposites. The crystallization temperature (T-c) of nanocomposites increased significantly with the particle loading. The PP-g-SiO2 with high molecular weight of grafted chains exhibits a high nucleation ability at 1 wt% nanoparticle loading in PP/PP-g-SiO2 nanocomposites. In summary, we provide an effective method to synthesize the well-defined PP-g-SiO2 with controlled grafting density, which shows excellent nucleation ability.

There has been considerable interest in the nucleation and crystallization of polymers in the presence of nanoparticles (NPs, or nanofillers in general, NFs). Most of the extensive work in this area has focused on anisotropic, non-Brownian NFs (e.g., clay sheets, carbon nanotubes) whose spatial dispersion state in these nanocomposites is controlled by the process by which they are formed. Hence, NF spatial dispersion is generally limited and often remains poorly characterized. Thermodynamic handles that can be used to control NF dispersion state in the polymer melt include (a) favorable interactions between the polymer chains and the bare NP surfaces, or (b) the density and length of the chains, with the same chemistry as the matrix, grafted to the NP surface. These relatively large NFs merely act as stationary objects that affect the kinetics of nucleation by providing heterogeneous sites, and the crystallization rate by confining the polymer in the melt state. The dispersion state of the NFs can dramatically affect the nucleation and crystallization of the matrix, but in most cases reported, the NFs increase nucleation efficiency relative to the neat polymer. At higher NF loadings, the effect of polymer confinement by the NFs dominates, leading to a decrease in crystal growth rates. This review describes the most important lessons learned from these commonly studied systems and then extends to polymer composite systems containing small, mobile spherical NPs (typically smaller than 100 nm in size). The role of NP mobility, which provides for dynamic confinement of the polymer melt, on the kinetics of polymer crystallization (nucleation, growth, and overall crystallization) and how this behavior is mostly consistent with the case of immobile NF is a second important focus of this review. In addition to the role of NFs on crystallization kinetics, recently reported nanoparticle ordering phenomena such as the effect of matrix crystallization on the organization of small spherical NPs within the amorphous regions of the semicrystalline morphology are discussed. Such phenomena are clearly not observed for large NFs and hence provide a point of departure from past works in this area.(c) 2022 Elsevier B.V. All rights reserved.

Polylactide (PLA) materials manufactured by fused filament fabrication (FFF) technique are usually in amorphous form and thereby exhibit poor mechanical properties and thermal resistance. Overcoming this issue requires a good knowledge of the crystallization behavior of PLA materials during the FFF process. In this work, a plasticizer (polyethylene glycol, PEG) and a nucleating agent (tetramethylene-dicarboxylic dibenzoyl-hydrazide, TMC-306), were added in PLA matrix, separately and synergistically, to tailor crystallization behavior of FFF-printed PLA parts. At a bed temperature of 60 degrees C, PEG played a stronger effect on crystallization behavior than the nucleating agent TMC and even the combination of TMC and PEG. This resulted in an increase of the crystallinity from 8% for both neat PLA and PLA/TMC samples to 18% for the samples containing PEG. With the increase of the bed temperature to 90 degrees C, TMC and PEG separately or synergistically had prominent effects on enhancing the crystallization ability of PLA during the FFF process, leading to the highly crystalline PLA parts with the crystallinities in the range of 30-40%. By means of wide angle X-ray diffraction and scanning electron microscopy measurements, shear-induced crystal structures were indentified at the filament surface as well as the weld interface. Nevertheless, the shear-induced effect had negligible influence on the final crystallinity of PLA parts. Instead, PLA parts with a high crystallinity can be attained, only when the material characteristics and the printing conditions are in favor of cold crystallization of PLA. The unique crystallization behavior of FFF-printed PLA materials offers guidelines for the fabrication of products with controlled crystallinity and hierarchical structures for specific applications.