Confined Crystallization Behavior Research Articles

Crystallization and polymorphic behaviour of melt miscible blends of crystalline homopolymers with close melting temperatures under confinement

The confined crystallization behavior of the crystalline/crystalline blends of poly(ethylene oxide) (PEO) and poly(butylene adipate) (PBA) in the electrospun nanofibers fabricated from the ternary blends of polystyrene (PS), PEO, and PBA, where PS was present as the majority component, was investigated. The electrospun nanofibers had PS as the matrix whereas the PEO/PBA mixed phase, with total weight fraction of 0.2 or less, constituted the dispersed domains having sizes of the order of only few hundred nanometers. Hence, the crystallization induced phase separation of PEO and PBA occurred within the confined domains. Despite extremely close melting temperature of PEO and PBA, faster crystallization kinetics of PEO led to entrapment of PBA in the intraspherultic regions of the former. Hence, further crystallization of PBA occurred under hierarchical confinement where the larger and smaller length-scale confinements were imposed by the glassy PS matrix and PEO crystals, respectively. Such a scenario resulted in crystallization occurring both via homogenous as well as heterogeneous crystallization mechanism. Interestingly, the homogenous nucleation of PEO was suppressed whereas the heterogeneous nucleation was enhanced in the ternary blend nanofibers even at very low weight fraction. Most significantly, it was observed, that during melt-recrystallization, memory of chain orientation is retained which favoured the formation of α-crystals in PBA. The present study highlights the role of confinement as well as chain orientation, on the crystallization behaviour, in crystalline/crystalline blend systems for which limited studies have been done so far.

Correlation between molecular weight and confined crystallization behavior of polymers grafted onto a zero-dimensional filler

In this work, polymer chains of different molecular weights are grafted onto a zero-dimensional filler.

Correlation between Grafting Density and Confined Crystallization Behavior of Poly(ethylene glycol) Grafted to Silica

The interfacial interactions of polymer–nanoparticles have dramatical effects on the crystallization behavior of grafted polymers. In this study, methoxypolyethylene glycol (MPEG) (molecular weights 750, 2000, and 4000 g mol–1) was grafted onto amino-modified nanosized silica (SiO2-NH2) by the “grafting to” method. The effects of the grafting density and molecular weight on the confined crystallization of grafted MPEG (MPEG-g-SiO2) were systematically investigated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and wide-angle X-ray scattering (WAXS). It was found that confinement effects are stronger when lower molecular weights of grafted MPEG are employed. These grafted MPEG chains are more difficult to stretch out on SiO2-NH2 surfaces than when they are free in the bulk polymer. Both crystallization temperature (Tc) and crystallinity of grafted MPEG chains decrease with reductions of grafting density. Additionally, covalent bonding effects and interfacial interaction confi...

Isothermally crystallization behavior of poly (L‐lactide) from melt under high pressure

The poly (L‐lactide) (PLLA) samples from melt were isothermally crystallized at 175°C and 190°C under pressures (PC) ranging from 0.1 to 400 MPa. The crystalline structures and thermal properties were investigated by using wide‐angle X‐ray diffraction (WAXD) and differential scanning calorimetry (DSC). On the basis of the DSC and WAXD results, it is confirmed that an α crystal was formed in the low‐pressure region (PC ≤ 50 MPa), and the disordered α crystal (α′) was formed in the medium‐pressure region (100 MPa ≤ PC ≤ 200 MPa) for the PLLA samples isothermally crystallized at 175°C. However, the only α crystal was formed in the pressure range from 0.1 to 200 MPa, and no α′ crystal could be found in the PLLA samples isothermally crystallized at 190°C. Interestingly, the full amorphous PLLA was obtained at 175°C and 190°C, when the PC was above 250 MPa. There seems to exist a cut‐off pressure for the crystallization of PLLA, which divides the pressure into high and low regions. The peculiarities of crystalline structure could be explained by the enhanced melt supercooling and the confined crystallization behaviors of PLLA under high pressure. The effect of the compression rate on the formation of amorphous PLLA was also researched with different compression rates. The results indicate that the molten PLLA could be easy to solidify as full amorphous phase under the higher pressure at a lower critical compression rate.

Microencapsulated Comb-Like Polymeric Solid-Solid Phase Change Materials via In-Situ Polymerization.

To enhance the thermal stability and permeability resistance, a comb-like polymer with crystallizable side chains was fabricated as solid-solid phase change materials (PCMs) inside the cores of microcapsules and nanocapsules prepared via in-situ polymerization. In this study, the effects on the surface morphology and microstructure of micro/nanocapsules caused by microencapsulating different types of core materials (i.e., n-hexadecane, ethyl hexadecanoate, hexadecyl acrylate and poly(hexadecyl acrylate)) were systematically studied via field emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM). The confined crystallization behavior of comb-like polymer PCMs cores was investigated via differential scanning calorimeter (DSC). Comparing with low molecular organic PCMs cores, the thermal stability of PCMs microencapsulated comb-like polymer enhanced significantly, and the permeability resistance improved obviously as well. Based on these resultant analysis, the microencapsulated comb-like polymeric PCMs with excellent thermal stability and permeability resistance showed promising foreground in the field of organic solution spun, melt processing and organic coating.

Confined crystallization behaviors in polyethylene/silica nanocomposites: Synergetic effects of interfacial interactions and filler network

ABSTRACTThe confinement effects introduced by nanoparticles have been reported to influence the phase behaviors thus the properties of polymer nanocomposites. In this study, molecular dynamics and crystallization behaviors of polyethylene (PE) composited with three types of silica (SiO2) nanoparticles, namely unmodified SiO2, hydrophobically modified SiO2, SiO2‐APTES (3‐aminopropyltriethoxysilane) and SiO2‐PTES (n‐propyltriethoxysilane), were systematically investigated via a combination of DSC, XRD and 1H solid‐state NMR measurements. The suppressions in crystallization and chain mobilities of PE rank in the order of unmodified SiO2 < SiO2‐APTES < SiO2‐PTES due to the increasing interfacial interactions between PE and SiO2 nanoparticles. Additionally, independent of polymer–nanoparticle interactions, a silica network forms for all three kinds of nanocomposites when SiO2 content reaches 83 wt %. The mobilities of polymer chains are severely restricted by such a percolated network structure, leading to a turning point in the crystallization ability of nanocomposites and a new crystallization peak at 45 °C lower than that of pure PE. The synergetic effects of interfacial interactions and filler network on polymer crystallization have been thoroughly studied in this work, which will provide guidance on modifying and designing nanocomposites with controlled properties. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 498–505

Polymethylene-b-poly(acrylic acid) diblock copolymers: Morphology and crystallization evolution influenced by polyethyene polyamine with dual confinement effects

In this work, we systematically investigated the aggregation and confined crystallization behavior of polymethylene-b-poly acrylic acid diblock copolymers (PM-b-PAA, M93A94) under the influence of polyethyene polyamine (PPA). With the increasing molar ratio of PPA, the morphologies of PM-b-PAA micelles experienced sphere-to-band-to-sheet transformations with regular changes of sizes on cooling from 80 °C mixed solution. Further investigation of the crystalline feature of PM-b-PAA indicated that the crystallization of PM block was profoundly restrained by PPA with the dual confinements, i.e., chemical and physical confinement which exerted through the hierarchical hydrogen bonding and 3D hydrogel network, respectively, in the mixed system. In particular, when the PPA and PM-b-PAA aqueous solution were mixed with equivalent volume, hydrogels was accessible. And rheology property indicated that the BCP/PPA-0.5 hydrogels hold the maximal storage modulus G′, and meanwhile the minimum loss tangent, tanδ.

Confined crystallization behaviors of cross-linked comb-like copolymers

Cross-linked PEG-based copolymers were obtained via synthesis of polyethylene glycol (PEG) and methoxy polyethylene glycol (MPEG) by the bridging and/or cross-linking agent of 2,4-tolylene diisocyanate (TDI) and/or hexamethylene-1,6-diisocyanate homopolymer (HDI trimer). The effects on the crystallization behaviors of the samples could be found with the changes in molecular weight of MC-PEG and molecular weight of SC-PEG in certain cross-linked density. It is revealed that the samples appear not to crystallize when the molecular weight of MC-PEG and SC-PEG are 1000 g/mol or less than that. The samples begin to crystallize when the molecular weight of either MC-PEG or SC-PEG reaches 2000 g/mol. The crystallinity of crystallized MC-PEG decreases with the increasing molecular weight of uncrystallized SC-PEG and the crystallinity of crystallized SC-PEG declines with the rise of molecular weight of uncrystallizable MC-PEG. The chains of SC-PEG (M n = 2000 g/mol) can fold and align easilier than those of MC-PEG (M n = 2000 g/mol), when the content of PEG is the same.

Spontaneous Formation of an Ideal-Like Field-Effect Channel for Decay-Free Polymeric Thin-Film Transistors by Multiple-Scale Phase Separation.

We demonstrate semiconducting polymer-based thin-film transistors (PTFTs) with fast switching performance and an uncommon nondecaying feature. These PTFTs based on widely studied poly(3-hexylthiophene) are developed by incorporating the insulating polymer into the active channel and subjecting the compound to specific, spontaneous multiple-scale phase separation (MSPS). An in-depth study is conducted on the interfacial and phase-separated microstructure of the semiconducting/insulating blending active layer and its effect on the electrical characteristics of PTFTs. The polyblends exhibit a confined crystallization behavior with continuously semiconducting crystalline domains between scattered insulator-rich domains. The insulator-rich domains can block leakage current and strengthen the gate control of the channel. A small amount of the insulating polymer penetrates the bottom of the active channel, resulting in effective interface modification. We show specific MSPS morphology of the present blending films to reduce charge trapping effects, enhance charge accumulation, and create a high-seed switching channel. The findings enable us to develop the required morphological conceptual model of the ideal-like field-effect-modulated polymer-based active channel. The polyblend-based PTFTs with MSPS morphology also have promising sensing functions. This study offers an effective approach for overcoming the major drawbacks (instability and poor switching) of PTFTs, thus allowing such transistors to have potential applications.

Crystalline structure and morphology of poly(L‐lactide) formed under high pressure

ABSTRACTThe poly(l‐lactide) (PLLA) samples were prepared by the annealing under 100 MPa at 75–145°C and 200 MPa at 105–145°C for 6 h, respectively. The crystalline structures, thermal properties and morphology were investigated using differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction (WAXD), and scanning electron microscopy (SEM). On the basis of the DSC and WAXD results, it can be seen that the α′ form was formed by the annealing under 100 MPa at 85–95°C but not found under 200 MPa at 105–145°C. A phase diagram of PLLA crystal form under high pressure was constructed under the given experimental conditions, which displayed the α′ form was formed at limited temperature and pressure range. Besides, SEM suggested that the PLLA samples annealed under 100 MPa crystallize to form lamellar‐like crystals due to the low growth rate and the confined crystallization behavior under high pressure. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40637.

Thermal performance and crystallization behavior of poly(ethylene glycol) hexadecyl ether in confined environment

A series of poly(ethylene glycol) hexadecyl ether (Brij58)/porous active carbon (AC) composites were fabricated via a physical method, and their crystallization behavior was characterized in detail using thermogravimetric analysis, X-ray diffraction (XRD) and differential scanning calorimetry. The Brij58/AC composites exhibit a good structural-stabilized property, and the decomposition temperature is higher than that of pure components. Obvious crystalline structure and thermal behavior are found when the amount of Brij58 loaded is above 25 wt%. A new peak at 18.9° in XRD patterns proves that the porous AC influences the crystallization and nucleation process of Brij58. From non-isothermal crystallization kinetics, the increased half-time t1/2 and F(T) demonstrate the confined crystallization behavior of Brij58 in AC matrix because of the facile entrapment of hydrophilic poly(ethylene glycol) segments by AC nanopores. Further explanations also arise from the viewpoint of molecular mobility under nanoscale conditions. © 2013 Society of Chemical Industry

Crystallization Features of Normal Alkanes in Confined Geometry

How polymers crystallize can greatly affect their thermal and mechanical properties, which influence the practical applications of these materials. Polymeric materials, such as block copolymers, graft polymers, and polymer blends, have complex molecular structures. Due to the multiple hierarchical structures and different size domains in polymer systems, confined hard environments for polymer crystallization exist widely in these materials. The confined geometry is closely related to both the phase metastability and lifetime of polymer. This affects the phase miscibility, microphase separation, and crystallization behaviors and determines both the performance of polymer materials and how easily these materials can be processed. Furthermore, the size effect of metastable states needs to be clarified in polymers. However, scientists find it difficult to propose a quantitative formula to describe the transition dynamics of metastable states in these complex systems. Normal alkanes [CnH2n+2, n-alkanes], especially linear saturated hydrocarbons, can provide a well-defined model system for studying the complex crystallization behaviors of polymer materials, surfactants, and lipids. Therefore, a deeper investigation of normal alkane phase behavior in confinement will help scientists to understand the crystalline phase transition and ultimate properties of many polymeric materials, especially polyolefins. In this Account, we provide an in-depth look at the research concerning the confined crystallization behavior of n-alkanes and binary mixtures in microcapsules by our laboratory and others. Since 2006, our group has developed a technique for synthesizing nearly monodispersed n-alkane containing microcapsules with controllable size and surface porous morphology. We applied an in situ polymerization method, using melamine-formaldehyde resin as shell material and nonionic surfactants as emulsifiers. The solid shell of microcapsules can provide a stable three-dimensional (3-D) confining environment. We have studied multiple parameters of these microencapsulated n-alkanes, including surface freezing, metastability of the rotator phase, and the phase separation behaviors of n-alkane mixtures using differential scanning calorimetry (DSC), temperature-dependent X-ray diffraction (XRD), and variable-temperature solid-state nuclear magnetic resonance (NMR). Our investigations revealed new direct evidence for the existence of surface freezing in microencapsulated n-alkanes. By examining the differences among chain packing and nucleation kinetics between bulk alkane solid solutions and their microencapsulated counterparts, we also discovered a mechanism responsible for the formation of a new metastable bulk phase. In addition, we found that confinement suppresses lamellar ordering and longitudinal diffusion, which play an important role in stabilizing the binary n-alkane solid solution in microcapsules. Our work also provided new insights into the phase separation of other mixed system, such as waxes, lipids, and polymer blends in confined geometry. These works provide a profound understanding of the relationship between molecular structure and material properties in the context of crystallization and therefore advance our ability to improve applications incorporating polymeric and molecular materials.

Formation and Confined Crystallization of Polyethylene Nanophases in Epoxy Thermosets

In this contribution, we reported the investigation of the formation and confined crystallization behavior of polyethylene nanophases in epoxy thermosets. The nanostructured epoxy thermosets were prepared by the use of a poly(ε-caprolactone)-block-polyethylene-block-poly(ε-caprolactone) (PCL-b-PE-b-PCL) triblock copolymer. The crystalline midblock (viz. PE) of the triblock copolymer was prepared from an α,ω-diacetoxy-terminated polycyclooctadiene with the molecular weight as high as Mn = 11,000, which was synthesized via the ring-opening metathesis polymerization (ROMP) of cyclooctadiene catalyzed by Grubbs second generation catalyst. The formation of PE nanophases in epoxy thermosets was evidenced with transmission electronic microscopy (TEM), small-angle X-ray scattering (SAXS) and dynamic mechanical thermal analysis (DMTA). It was found that in the nanostructured thermosets, the spherical nanophases of PE with the size of 20–30 nm in diameter were dispersed into the continuous epoxy matrices. Wide angle X-ray diffraction (XRD) showed that the formation of PE nanophases did not alter the structure of PE crystals. The investigations of isothermal and nonisothermal crystallization kinetics showed that the crystallization of PE in the nanostructured thermosets was in a confined manner and the confinement has been interpreted on the basis of nanoscaled space, interdomain connectivity, and the cross-linked structures of epoxy matrices.

Manipulating Crystal Orientation of Poly(ethylene oxide) by Nanopores

The confined crystallization behavior of a low molecular weight monodisperse polyethylene oxide (PEO) in anodic alumina oxide (AAO) templates was investigated. Homogeneous nucleation of polymer in AAO templates was confirmed. Within AAO with diameter larger than the contour length of PEO chains, the "kinetics selective growth" crystallization mechanism was confirmed based on the observation that the chain axis preferentially aligned perpendicular to the pore axis. However, when AAO diameter further decreases to a value smaller than the contour length of PEO, unique orientation with chain axis aligned parallel to the pore axis was observed for the first time. The results were discussed based on the competition between thermodynamics and kinetics during the crystallization process.

Phase transition behavior of PLLA ultrathin film studied by grazing angle reflection absorption infrared spectroscopy

With a home-made heating cell, the structural evolutions of PLLA ultrathin film during the heating and cooling process were investigated in real time by grazing angle IR reflection absorption spectroscopy (IRRAS). It is found that the phase transition behaviors, including the crystallization, melting and even the glass transition of PLLA ultrathin film, can be clearly revealed by detailed analysis on the temperature-dependent IR spectra collected with IRRAS technique. The quantitative analysis shows that temperature-dependent intensity changes calculated by vibrational modes associated with the CO and COC groups are suitable to reflect the subtle glass transition of PLLA ultrathin film. The qualitative spectral analysis on the CC backbone vibrational modes suggest that the 10/7 helical chain of PLLA formed during the heating process (i.e. cold crystallization) is preferably flat on the Au substrate, whereas it is perpendicular to substrate during the cooling process (i.e. melt crystallization). Our studies show that temperature dependent IRRAS technique is not only valid to study the glass transition but also the confined crystallization behavior of PLLA ultrathin film deposited on a substrate, which is hard to test by the conventional DSC technique. Besides, it can provide more structural information to understand the various phase transition behaviors of polymer ultrathin films on a molecular level.

Carbon nanotubes suppressed crystallization kinetics in PLLA thin film as revealed by in situ RAIR technique

The bulk and thin film crystallization behaviors of PLLA/CNTs nanocomposites were investigated by in situ transmittance IR and reflection absorption IR spectroscopy, respectively. The results show that CNTs can enhance both the cold and melt crystallization rates of bulk PLLA, which is consistent with the common observation in the literature. However, it is interesting to find that the crystallization rate of PLLA thin film is suppressed by the addition of CNTs. Moreover, it is found that the existence of CNTs could not alter the preferred orientation of PLLA helical chain in the thin film. The different crystallization kinetics of PLLA induced by the addition of CNTs in bulk and thin film were explained by the confined crystallization behavior of polymer thin film.

Studies on confined crystallization behavior of nanobiocomposites consisting of acetylated bacterial cellulose and poly (lactic acid)

This article deals with the confined crystallization behavior of nanobiocomposites consisting of acetylated bacterial cellulose (BC) and poly(lactic acid) (PLA). PLA/acetylated BC nanobiocomposites were prepared by compressed molding. Nonisothermal crystallization, melting behavior and isothermal crystallization kinetics within confined spaces were investigated by differential scanning calorimetry (DSC). The results indicated that acetylated BC was favorable to the crystallization of PLA at higher temperatures. On one hand, due to better compatibility between acetylated BC and PLA, crystal nucleation and growth of PLA took place while crystallization process of PLA was restricted. Acetylated BC had an effect of heterogeneous nucleation on PLA crystallization. On the other hand, crystal growth of PLA molecular chain was confined within the nanometer gaps between the microfibrils of acetylated BC. The interaction between microfibrils of acetylated BC and PLA also had an effect on the crystal growth of PLA, which resulted in the growth of confined crystallization. As the time course of acetylation became longer and longer, the compatibility of acetylated BC and PLA was enhanced and the crystallization of PLA became more difficult. This result was in accordance with the observation of polarized optical microscope (POM) pictures. Activation energy of crystallization and free energy surface of PLA were reduced in PLA and acetylated BC composites.

Confined Growth of Poly(butylene succinate) in Its Miscible Blends with Poly(vinylidene fluoride): Morphology and Growth Kinetics

The morphology and confined crystallization behavior of poly(butylene succinate) (PBS) in miscible poly(vinylidene fluoride) (PVDF)/PBS blends has been studied using differential scanning calorimetry (DSC) and optical and atomic force microscopy (OM and AFM). It was found that PBS crystal lamellae nucleated and grew confined inside the matrix of PVDF spherulites. Crystallized PBS domains grow with an ellipsoidal outline within PVDF spherulites formed at a relatively high PVDF crystallization temperature (T(c,PVDF)), while circular domains, engulfing several PVDF spherulites, are seen when growing in the PVDF spherulites created at lower T(c,PVDF). The growth kinetics of PBS confined in the PVDF matrix was investigated under various conditions. The growth rate of PBS (G(PBS)) increases with decreasing crystallization temperature and increasing PBS content under a given PVDF crystallization temperature (T(c,VDF)). For T(c,PVDF) above 145 °C, G(PBS) decreases with T(c,PVDF) for both 50:50 and 30:70 PVDF/PBS blends. However, for T(c,PVDF) below 145 °C, 50:50 and 30:70 PVDF/PBS blends exhibit the opposite G(PBS) trend; that is, G(PBS) for the 50:50 blend decreases with decreasing T(c,PVDF), while for the 30:70 PVDF/PBS blend G(PBS) increases with decreasing T(c,PVDF). It is shown that this behavior cannot be associated with the effect of crossing the boundary of smaller PVDF spherulites formed at a lower temperature. Rather, the behavior appears to be related to the interleaving growth of PBS lamellae among PVDF lamellae or between bundles of PVDF lamellae (fibrils), as in situ AFM observation shows. It is found that the interconnectedness of the molten pockets within the PVDF spherulites, which depends on the PVDF crystallization temperature, is an important factor determining the growth kinetics of PBS confined within the PVDF scaffold.

STUDIES ON CONFINED CRYSTALLIZATION BEHAVIOR OF POLYCAPROLACTONE THIN FILMS

The confined crystallization behavior of polycaprolactone(PCL) in thin and ultrathin films was studied by AFM(atomic force microscopy).It was found that the crystalline morphology of PCL depended on the film thickness.When the thickness d2R_g(radius of gyration),the polymer can crystallize into spherulites;when R_gd2R_g,the dense-branch morphology and dendrites could be found;when dR_g,the "islands" structure could be obtained.Moreover,the effects of the crystallization temperature,substrate and the molecular weight on the crystalline morphology were discussed.It was shown that the crystallization of PCL in thin film is a diffusion-controlled process,and it can be explained by diffusion-limited aggregation.

Studies on confined crystallization behavior of polycaprolactone thin films

The confined crystallization behavior of polycaprolactone (PCL) in thin and ultrathin films was studied by AFM (atomic force microscopy). It was found that the crystalline morphology of PCL depended on the film’s thickness. When the thickness is d > 2 Rg (radius of gyration), the polymer can crystallize into spherulites; when Rg < d < 2 Rg, a dense-branch morphology and dendrites could be found; when d < Rg, an “islands” structure could be obtained. Moreover, the effects of the crystallization temperature and the substrate and the molecular weight on the crystalline morphology were discussed. It was shown that the crystallization of PCL in thin films is a diffusion-controlled process, and it can be explained by diffusion-limited aggregation.