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, methoxy polyethylene 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 confinement effects are strengthened by the decrease in grafting density, leading to an increase in decomposition temperature and to the disappearance of the self-nucleation Domain (i.e., Domain II), when self-nucleation experiments are performed by DSC. Overall isothermal crystallization kinetics was studied by DSC and the results were analyzed with the Avrami equation. An Avrami index of n≈3 was obtained for neat MPEG (indicating that instantaneous spherulites are formed). However, in the case of MPEG-g-SiO2 with the lowest grafting density, the Avrami index of (n) was less than 1 (first order kinetics or lower), indicating that nucleation is the determining factor of the overall crystallization kinetics, a signature for confined crystallization. At the same time, the crystallization from the melt for this MPEG-g-SiO2 with the lowest grafting density occurs at Tc ≈-30 ºC, a temperature close to the glass transition temperature (Tg) of MPEG, indicating that this confined MPEG crystallizes from homogeneous nuclei.This project was supported by the National Natural Science Foundation of China (21574141) and the Ministry of Science and Technology of China (2017YFE0117800). The authors gratefully acknowledge the funding of project BIODEST, Research and Innovation Staff Exchange (RISE) H2020-MSCA-RISE-2017-778092. The authors thank beamline BL16B1 (Shanghai Synchrotron Radiation Facility) for providing the beam time and helps during experiments

Confined Crystallization of Polymers within Nanopores

Unformatted post-print version of the accepted articleCrystallization of polymeric materials under nanoscopic confinement is highly relevant for nanotechnology applications. When a polymer is confined within rigid nanoporous anodic aluminum oxide (AAO) templates, the crystallization behavior experiences dramatic changes as the pore size is reduced, including nucleation mechanism, crystal orientation, crystallization kinetics, and polymorphic transition, etc. As an experimental prerequisite, exhaustive cleaning procedures after infiltrations of polymers in AAO pores must be performed to ensure producing an ensemble of isolated polymer-filled nanopores. Layers of residual polymers on the AAO surface percolate nanopores and lead to the so-called “fractionated crystallization”, i.e., multiple crystallization peaks during cooling. As the density of isolated nanopores in a typical AAO template exceeds the density of heterogeneities in bulk polymers, the majority of nanopores will be heterogeneity-free. This means that the nucleation will proceed by surface or homogeneous nucleation. As a consequence, a very large supercooling is necessary for crystallization, and its kinetics is reduced to a first-order process that is dominated by nucleation. Self-nucleation is a powerful method to exponentially increase nucleation density. However, when the diameter of the nanopores is lower than a critical value, confinement prevents the possibility to self-nucleate the material. Because of the anisotropic nature of AAO pores, polymer crystals inside AAO also exhibit anisotropy, which is determined by thermodynamic stability and kinetic selection rules. For low molecular weight poly(ethylene oxide) (PEO) with extended chain crystals, the orientation of polymer crystals changes from the “chain perpendicular to” to “chain parallel to” AAO pore axis, when the diameter of AAO decreases to the contour length of the PEO, indicating the effect of thermodynamic stability. When the thermodynamic requirement is satisfied, the orientation is determined by kinetics including crystal growth, nucleation and crystal growth rate. An orientation diagram has been established for PEO/AAO system, considering the cooling condition and pore size. The interfacial polymer layer has different physical properties as compared to the bulk. In poly(L-lactic acid), the relationship between the segmental mobility of the interfacial layer and crystallization rate is established. For the investigation of polymorphic transition of poly(butane-1), the results indicate that a 12 nm interfacial layer hinders the transition of Form II to Form I. Block and random copolymers have also been infiltrated into AAO nanopores, and their crystallization behavior is analogously affected as pore size is reduced.This work was supported by the National Key R&D Program of China (Grant No. 2017YFE0117800) and the National Natural Science Foundation of China (Grant Nos. 21873109, 51820105005, and 21922308). We also acknowledge the financial support from the BIODEST project; this project has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie Grant Agreement No. 778092. A.J.M. acknowledges funding from MINECO, Grant No. MAT2017-83014-C2-1-P, and from the Basque Government through Grant No. IT1309-19. G.L. is grateful to the Youth Innovation Promotion Association of the Chinese Academy of Sciences (Grant No. Y201908)

Uniaxial and Mixed Orientations of Poly(ethylene oxide) in Nanoporous Alumina Studied by X-ray Pole Figure Analysis

The orientation of polymers under confinement is a basic, yet not fully understood phenomenon. In this work, the texture of poly(ethylene oxide) (PEO) infiltrated in nanoporous anodic alumina oxide (AAO) templates was investigated by X-ray pole figures. The influence of geometry and crystallization conditions, such as pore diameter, aspect ratio, and cooling rates, was systematically examined. All the samples exhibited a single, volume-dependent crystallization temperature (Tc) at temperatures much lower than that exhibited by bulk PEO, indicating “clean” microdomains without detectable heterogeneous nucleation. An “orientation diagram” was established to account for the experimental observations. Under very high cooling rates (quenching), crystallization of PEO within AAO was nucleation-controlled, adopting a random distribution of crystallites. Under low cooling rates, growth kinetics played a decisive role on the crystal orientation. A relatively faster cooling rate (10 °C/min) and/or smaller pores lead to the * ║ pore axis (n⃗) mode (uniaxial orientation). When the cooling rate was lower (1 °C/min), and/or the pores were larger, a mixed orientation, with a coexistence of * ║ n⃗ and * ║ n⃗ , was observed. The results favor the kinetic model where the fastest growth direction tends to align parallel to the pore axis.This work is supported by the National Natural Science Foundation of China (NSFC, 21873109, 51820105005, 21274156). G. L. is grateful to the Youth Innovation Promotion Association of the Chinese Academy of Sciences (2015026). G. L., D. W., and A. J. M. also acknowledge European funding by the RISE BIODEST project (H2020-MSCA-RISE-2017-778092). The authors thank Dr. Zhongkai Yang for assistance with pole figure measurement

Competition between Chain Extension and Crosslinking in Polyamide 1012 during High Temperature Thermal Treatments as Revealed by SSA Fractionation

Unformatted post-print version of the accepted articleSelf-nucleation and annealing (SSA) is an efficient way to thermally fractionate semi-crystalline polymers. The thermal fractions produced by SSA have distinct melting points that correspond to different average lamellar thickness. In this research, SSA was adopted to investigate the in-situ evolution of lamellar thickness of polyamide 1012 (PA1012), which was affected by high temperature thermal treatments. SSA successfully fractionated PA1012 into 4 thermal fractions with different average lamellar thicknesses. The integrated area of the first or second SSA fraction against the total endothermic integrated area was plotted as a function of thermal treatment time to study the kinetics of lamellar thickness changes. Two opposing structural effects, chain growth and crosslinking, occurred during the applied thermal treatment (which consisted in thermally treating the material by holding it isothermally at temperatures in the range of 140-250 ºC) and they were detected as a function of time by SSA, rheology and dissolution behavior. The structural changes increased the viscosity and Tg and decreased the overall crystallization rate. Based on the construction of a master curve of “time-temperature superposition” at a reference temperature (T0) of 190 oC, the mechanism for lamellar thickness evolution was divided into three stages: (a) Stage I: Initially, PA1012 end groups reacted rapidly with active sites to generate chemically crosslinked structures. (b) Stage II: As the number of end groups rapidly increased, amidation reactions between carboxylic end groups and amine end groups resulted in linear chain growth. Linear chain growth and crosslinking occurred simultaneously, and there was no change in lamellar thickness or its distribution. (c) Stage III: Eventually, an increasing number of end groups was formed in the system, most of which led to linear chain growth via chain end-group reactions. These structural changes during the applied thermal treatments enhanced the mechanical properties and the heat resistance of PA1012. This work provides specific guidance for improving the toughness, strength and heat resistance of polyamide materials.We acknowledge generous financial support from the following grants: National Key R&D Program of China (2017YFB0307600) and STS project of Chinese Academy of Sciences (KFJ-STS-QYZX-113). A.J.M. acknowledges funding from the Basque Government through grant IT1309-19. We would like to thank the financial support provided by the BIODEST project; this project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement no. 778092

Promotion of Self-Nucleation with Latent Form i Nuclei in Polybutene-1 and Its Copolymer

The formation of form I nuclei of polybutene-1 (PB-1) and its copolymer with polyethylene (PB1-ran-PE) has been studied by means of modified self-nucleation protocols. Even when the self-nucleation temperature was high enough and all form II crystals melt, recrystallization can be accelerated if the melt-crystallized sample was annealed at low temperatures (below 60 \ub0C for PB-1 and 75 \ub0C for PB1-ran-PE) for just 3 min. These results suggest the formation of latent form I nuclei within form II crystals. This hypothesis is consistent with the observed growth of a small amount of form I crystals during heating, after previous annealing at temperature lower than 20 \ub0C. In addition, a peculiar phenomenon was found in PB1-ran-PE, as both form II and form I\u2032 can be induced by the presence of latent form I nuclei, due to cross-nucleation and self-nucleation effects, respectively. The final ratio of the two kinds of crystal forms is a result of the competition between the two nucleation rates, which strongly depend on crystallization temperature. In this work, we have shown that careful design of novel self-nucleation protocols can yield evidence of the early stages of form II to form I transition, even when the degree of transformed crystals is below the limit of detection of conventional techniques sensitive to crystalline order (DSC, WAXD, and FTIR)

Peculiar self-nucleation behavior of a polybutene-1/ethylene random copolymer

Unformatted post-print version of the accepted articleThe self-nucleation behavior of a polybutene-1/ethylene random copolymer, P(B1-ran-E), which undergoes a complex crystal-crystal transition behavior, has been studied in detail. Similar to PE random copolymers, this material shows a strong melt memory effect even above equilibrium melting point of PB-1 homopolymer. Different polymorphic forms can be obtained when P(B1-ran-E) is cooled from different self-nucleation Domains. The trigonal form I' could only be nucleated in the presence of remaining form I crystals via self-seeding, while the melt memory in Domain IIa could only act as self-nuclei for kinetically favored form II. Furthermore, observations from optical microscopy illustrated that melt memory is able to enhance nucleation density but it does not affect the spherulitic growth rate.Financial supports from the National Science Foundation of China (Grant No. U1510207) and the Key Program for Coal-based Science and Technology of Shanxi Province (MH-2014-08) are gratefully acknowledged. We would like to acknowledge the financial support from the BIODEST project, this project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 778092. AJM would also like to acknowledge funding from the Basque Government through grant IT1309-19

Super nucleation and orientation of poly (butylene terephthalate) crystals in nanocomposites containing highly reduced graphene oxide

The ring opening polymerization of cyclic butylene terephthalate into poly (butylene terephthalate) (pCBT) in the presence of reduced graphene oxide (RGO) is an effective method for the preparation of polymer nanocomposites. The inclusion of RGO nanoflakes dramatically affects the crystallization of pCBT, shifting crystallization peak temperature to higher temperatures and, overall, increasing the crystallization rate. This was due to a super nucleating effect caused by RGO, which is maximized by highly reduced graphene oxide. Furthermore, combined analyses by differential scanning calorimetry (DSC) experiments and wide angle X-ray diffraction (WAXS) showed the formation of a thick {\alpha}-crystalline form pCBT lamellae with a melting point of ~250 {\deg}C, close to the equilibrium melting temperature of pCBT. WAXS also demonstrated the pair orientation of pCBT crystals with RGO nanoflakes, indicating a strong interfacial interaction between the aromatic rings of pCBT and RGO planes, especially with highly reduced graphene oxide. Such surface self-organization of the polymer onto the RGO nanoflakes may be exploited for the enhancement of interfacial properties in their polymer nanocomposites

Generating Triple Crystalline Superstructures in Melt Miscible PEO-b-PCL-b-PLLA Triblock Terpolymers by Controlling Thermal History and Sequential Crystallization

The morphology, crystallization behavior and properties of multi-crystalline polymer systems based on triple crystalline biodegradable PEO-b-PCL-b-PLLA triblock terpolymers are reviewed. The triblock terpolymers, with increasing PLLA content, exhibited a triple crystalline nature. Upon cooling from melt, the PLLA block crystallizes first and templates the spherulitic morphology of the terpolymer. Then, the PCL block crystalizes and, lastly, the PEO block. These triblock terpolymers are probably melt miscible, as SAXS experiments confirmed. Thus, the crystallization of PCL and PEO blocks takes place within the interlamellar zones of the PLLA spherulites that were formed previously. Therefore, the lamellae of PLLA, PCL and PEO exist side-by-side within a unique spherulite, constituting a novel triple crystalline superstructure. The theoretical analysis of SAXS curves implies that only one lamella of either PCL or PEO can occupy the interlamellar space in between two contiguous lamellae of PLLA. Several complex competitive effects such as plasticizing, nucleation, anti-plasticizing and confinement take place during the isothermal crystallization of each block in the terpolymers. New results on how Successive Self-nucleation and Annealing (SSA) thermal treatment can be used as an additional suitable technique to properly separate the three crystalline phases in these triple crystalline triblock terpolymers are also included in this contribution.The POLYMAT/UPV/EHU team would like to acknowledge funding from MINECO through project: MAT2017-83014-C2-1-P, and from ALBA synchrotron facility. We also acknowledge funding by the European Union´s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 778092. The support of the National Key R&D Program of China (2017YFE0117800) is also gratefully acknowledged

Interfacial nucleation in iPP/PB-1 blends promotes the formation of polybutene-1 trigonal crystals

The formation of trigonal Form I\ub4 crystals of polybutene-1 (PB-1) directly from melt has drawn much attention in past decades. In this study, we investigate the fractionated crystallization behavior of PB-1 within droplets formed by blending PB-1 with an excess of isotactic polypropylene (iPP) employing DSC, SEM, in situ synchrotron WAXD and FT-IR. When PB-1 is dispersed into a large number of small size droplets, the heterogeneous nucleation of Form II crystals can be inhibited because the number of droplets is larger than that of active nucleation sites for Form II (i.e., active heterogeneities originally present in bulk PB-1). The nucleation of the finely dispersed PB-1 droplets does not occur homogenously, but at the interface with the iPP matrix, which induces the crystallization of the droplets into Form I\ub4. The crystallization rate of Form I\ub4 at different temperatures was determined by Fourier transform infrared spectroscopy. It was found that trigonal Form I\ub4 crystallizes faster when the content of PB-1 in the blend is lower, and the specific interfacial surface area is larger. The opposite effect has been observed for the kinetics of the metastable Form II formation. It is therefore suggested that Form I\ub4 crystallization is driven by the nucleation of PB-1 at the crystalline iPP surface, which competes with the crystallization of Form II induced by nucleating heterogeneities present in PB-1 droplets

The origin of memory effects in the crystallization of polyamides: Role of hydrogen bonding

The effect of hydrogen bonding stability on the memory effects in the crystallization of long chain polyamides have been investigated by the self-nucleation calorimetric technique. Self-nucleation is characterized by three domains in decreasing temperature order: complete melting or Domain I, exclusive self-nucleation or Domain II and, self-nucleation and annealing or Domain III. The memory effect is observed in the high temperature range of Domain II (when all crystals are molten, or in Domain IIa). In the low temperature range of Domain II, crystal remnants act as self-seeds (i.e., Domain IIb). The hydrogen bonds between amide groups were detected with FTIR, and a ratio of the content of hydrogen bonded vs. free amide groups could be calculated. The energy needed to break the hydrogen bonds decreases as the self-nucleation temperature (Ts) increases. This means that hydrogen bonds become weaker (and their amount decrease), while the crystalline memory disappears upon crossing from Domain IIa to Domain I. Comparing the widths of Domain IIa in different polyamides, we found for the first time a clear correlation with the relative content of amide groups with respect to methylene groups in the repeat units. In conclusion, we have demonstrated that memory in polyamides is a strong function of hydrogen bonding between chain segments.This work was financially supported by the National Natural Science Foundation of China (No. 21574140) and the National Key R&D Program of China (No. 2017YFB0307600). The SSRF beamlines BL16B1 are acknowledged for kindly providing the beam time and assistance. We thank Dr. François Bouéfrom CEA UMR12 Lab Léon Brillouin-Orphée Neutron Reactor for the good discussion and help on this work. We also acknowledge funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 778092