Crystallization Of Copolymers Research Articles

Quiescent crystallization of poly(lactic acid) studied by optical microscopy and light‐scattering techniques

ABSTRACTThe crystallization behavior of poly(lactic acid) (PLA) has been studied extensively, and this has resulted in different reported values for the nucleation densities (Ns) and crystal growth rates (Gs) for similar grades. These inconsistencies may be magnified when they are used in subsequent modeling studies. Therefore, the quiescent crystallization behaviors of three PLA grades were studied with polarized optical microscopy and small‐angle light‐scattering experiments. The Gs and Ns were determined at several isothermal crystallization temperatures with a device that provided near‐instantaneous cooling to the isothermal crystallization temperature. Two growth rate regimes, which were attributed to α and α′ crystallization with a transition around 120 °C, were observed. Avrami analysis revealed that the poly(l‐lactic acid) homopolymer crystal growth was three‐dimensional and was unaffected by the presence of stereocomplex PLA. The PLA copolymer crystals had a transition from an initial sheaflike conformation to three‐dimensional growth. Furthermore, the lamellar twisting of the homopolymer was observed at the isothermal crystallization temperature around 144 °C. These findings can be used for future modeling studies to predict material behavior in various industrial processes. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44566.

Homo- and Stereocomplex Crystallization of Star-Shaped Four-Armed Stereo Diblock Copolymers of Crystalline and Amorphous Poly(lactide)s: Effects of Incorporation and Position of Amorphous Blocks.

Star-shaped four-armed stereo diblock copolymers with an amorphous poly(dl-lactide) (PDLLA) core and a crystalline poly(l-lactide) (PLLA) or poly(d-lactide) (PDLA) shell (4-DL-L and 4-DL-D) and with a crystalline PLLA or PDLA core and amorphous PDLLA shell (4-L-DL and 4-D-DL) copolymers were synthesized. The effects of incorporation and position of amorphous PDLLA blocks on homo- and stereocomplex (SC) crystallization behavior of unblended copolymers and their blends (4-DL-L/4-DL-D and 4-L-DL/4-D-DL) were investigated during isothermal crystallization from the melt and DSC heating. The incorporated amorphous PDLLA blocks disturbed the SC crystallization and orientation of SC lamellae in the copolymer blends as well as homocrystallization and orientation of homocrystalline lamellae in the unblended copolymers compared to those reported for four-armed PLLA/four-armed PDLA homopolymer blends or four-armed PLLA homopolymers, irrespective of the position of amorphous PDLLA blocks. The disturbance effect was stronger for the amorphous PDLLA shell than for the amorphous PDLLA core. The crystallizability was higher for SC crystallites in the copolymer blends than for homocrystallites in the unblended copolymers, irrespective of the position of amorphous PDLLA blocks. SC crystallization in the copolymer blends disclaimed the positional effects of crystalline PLLA and PDLA blocks and amorphous PDLLA blocks on spherulite growth rate and crystallinity.

PLLA Crystallization in Linear AB and BAB Copolymers ofl-Lactide and 2-Dimethylaminoethyl Methacrylate

Well-defined poly(l-lactide-b-2-dimethylaminoethyl methacrylate) (PLLA-b-PDMAEMA) linear AB block copolymers and BAB block copolymers with PLLA as the central block (PDMAEMA-b-PLLA-b-PDMAEMA) were synthesized by combined ring-opening polymerization and atom transfer radical polymerization. Molar masses of the PLLA block vary from 13K to 19K and the PDMAEMA block from 5K to 35K. Thermal properties, morphology, and crystallization kinetics of the copolymers were analyzed for crystallization conducted in the bulk, in thick and thin films, and in solution as well as for a limited number of stereocomplexes of diblock and triblock copolymers with poly(d-lactide) in thin films and of partially quaternized diblock copolymers. In the bulk, it is shown that crystallization rates are reduced and morphologies disordered when adding a PDMAEMA block to PLLA; these tendencies are more pronounced in the triblock compared to diblock copolymers of the same PDMAEMA content. Usually, G, the local growth rate, and K, the over...

Effect of long chain branching on the rheological behavior, crystallization and mechanical properties of polypropylene random copolymer

Long chain branched polypropylene random copolymers (LCB-PPRs) were prepared via reactive extrusion with the addition of dicumyl peroxide (DCP) and various amounts of 1,6-hexanediol diacrylate (HDDA) into PPR. Fourier transform infrared spectrometer (FTIR) was applied to confirm the existence of branching and investigate the grafting degree for the modified PPRs. Melt flow index (MFI) and oscillatory shear rheological properties including complex viscosity, storage modulus, loss tangent and the Cole-Cole plots were studied to differentiate the LCB-PPRs from linear PPR. Differential scanning calorimetry (DSC) and polarized light microscopy (PLM) were used to study the melting and crystallization behavior and the spherulite morphology, respectively. Qualitative and quantitative analyses of rheological curves demonstrated the existence of LCB. The effect of the LCB on crystalline morphology, crystallization behavior and molecular mobility, and, thereby, the mechanical properties were studied and analyzed. Due to the entanglements between molecular chains and the nucleating effect of LCB, LCB-PPRs showed higher crystallization temperature and crystallinity, higher crystallization rate, more uniformly dispersed and much smaller crystallite compared with virgin PPR, thus giving rise to significantly improve impact strength. Moreover, the LCB-PPRs exhibited the improved yield strength. The mobility of the molecular chain segments, as demonstrated by dynamic mechanical analysis (DMA), was improved for the modified PPRs, which also contributed to the improvement of their mechanical properties.

Rheological characterization of polypropylene/poly(ethylene-co-propylene) in-reactor blends synthesized under different polymerization conditions

In-reactor blends of polypropylene/poly(ethylene-co-propylene) with complex microstructure, synthesized through different polymerization procedures; two-step (one homopolymerization and one copolymerization under high ethylene concentration) and three-step (with an additional copolymerization step under low ethylene concentration), were characterized by rheological measurements. The effects of a change in the polymerization process on the types and amounts of block copolymers in the blends were evaluated using small amplitude oscillation rheometry in the linear viscoelastic region. The Palierne model in its complete form was employed to model the rheological behavior of the blends. For this analysis the reactor products were separated into xylene cold insoluble (XCI) and xylene cold soluble fractions. Besides, another two copolymer fractions at 80 and 100 °C, which are crystallizable copolymer fractions and contain block copolymers rich in polyethylene and polypropylene, respectively, were separated from XCI fraction by xylene using temperature gradient elution fractionation method. Considering all copolymer fractions as dispersed and the remained fraction (mostly polypropylene) as matrix phase, it was shown that the rheological properties of the blends could not be predicted by Palierne model. It was found that only by considering part of block copolymer fractions having long polypropylene sequences along with polypropylene homopolymer as one phase, the rheological properties of the blends could be predicted by Palierne model. By rheological modeling, it was confirmed that the amounts of copolymers with long polypropylene sequences which are miscible with the matrix are higher in the case of three-step blends and also the elasticity of three-step polymerized blends is higher than two-step polymerized blends.

Controlled co‐delivery of hydrophilic and hydrophobic drugs from thermosensitive and crystallizable copolymer nanoparticles

ABSTRACTFunctionalized amphiphilic block copolymers poly(N‐isopropyl acrylamide)‐b‐poly(stearyl methacrylate) (PNIPAM‐PSMA) are synthesized. Their self‐assembled core‐shell nanoparticles have the hydrophilic thermosensitive shell and hydrophobic crystallizable core. Nanoparticles exhibit volume phase transition at temperature of 38 °C and its poly(stearyl methacrylate) (PSMA) moiety could form nano size crystals to retain drugs, making them good carriers for drug co‐delivery system. Thermosensitivity and crystallinity of nanoparticles are characterized with dynamic light scattering (DLS), differential scanning calorimetry (DSC), small‐angle X‐ray scattering (SAXS), and atomic force microscopy (AFM). The interactions and relationship between chemical structures of copolymer nanoparticles and loading drugs are discussed. Different loading techniques and combined loading of hydrophobic/hydrophilic drugs are studied. Nanoparticles show a good and controllable drug loading capacity (DL) of hydrophilic/hydrophobic drugs. The drugs release kinetics is analyzed with Fick's law and Weibull model. A general method for analyzing drug release kinetics from nanoparticles is proposed. Weibull model is well fitted and the parameters with definite physical meaning are analyzed. PNIPAM‐PSMA nanoparticles show a quite different thermal response, temporal regulation, and sustained release effect of hydrophilic and hydrophobic drugs, suggesting a promising application in extended and controlled co‐delivery system of multi‐drug. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 44132.

Mechanism of polymorph selection during crystallization of random butene-1/ethylene copolymer

Starting from an initial sample of butene-1/ethylene copolymer with stable form I’, we examined the nucleation of different crystalline polymorphs (here metastable form II and stable form I’) at different isothermal crystallization temperatures after being melted at different melt temperature (T melt). When T melt was just above the melting temperature (T m) of the crystallites, self-seeding took place. There, residue crystallites served as nuclei leading to the crystallization of the same crystalline phase. When T melt was a few degrees above the T m, self-seeding was disabled due to complete melting of the initial crystals. Upon crystallization, the selection of the different polymorphs in this random copolymer was found to depend on an interplay between the domain size of segregated long crystallizable sequences and the size and energy barrier of the critical nucleus of the respective crystalline forms. Our results provide a clear understanding of the polymorphs selection during crystallization of a random copolymer as well as homo-polymers under confinement.

Strong memory of strain-induced copolymer crystallization as revealed by Monte Carlo simulations

Abstract We performed dynamic Monte Carlo simulations of strain-induced crystallization of homopolymer and random copolymers under cyclic loading of strains. We found that since the second loading random copolymers shift down the onset strain of crystallization and raise up the crystallinity, in contrast to homopolymer. We attributed the strong memory to the remaining of sequence-length segregation raised by copolymer crystallization during the first loading of strains. The mechanism is consistent with that for the strong memory of copolymer crystallization under cyclic cooling, as revealed by previous experiments and simulations. Our results showed a new effect of chain-sequence defects on the cyclic loading performance of rubbers.

Nonreversible Enhanced Crystallization of Olefin Block Copolymer Induced by Preshearing

In this work, the influence of preshearing on crystallization of olefin block copolymer (OBC) was systematically investigated. It was found that upon an interval of melt preshear the crystallization rate of OBC was prominently elevated which was evidenced by both nonisothermal and isothermal crystallization results. The lamellar thickness of OBC increased and the size of spherulites decreased after shear. Interestingly, annealing and multiple heating and cooling experiments demonstrated that the enhancement effect on crystallization was nonreversible. The morphology observations suggested that preshearing caused an alteration in mesophase structure of OBC. On the basis of our results, we speculated that the increase in the amount of hard blocks that dissolved in the soft-block-rich matrix induced by shear promoted the unique “pass-through” crystallization process of OBC, which should be responsible for the largely enhanced crystallization of sheared OBC.

Crystallization of Polyethylene at Large Undercooling.

Extremely fast crystallization of high-density polyethylene and random copolymers of ethylene with up to 16 mol % 1-octene was observed for the first time by ultrafast scanning calorimetry. In order to account for the inherently high crystallization rate of polyethylenes, in nonisothermal and isothermal crystallization experiments cooling rates up to 1 000 000 K/s and crystallization times as short as 10 μs, respectively, were employed. It was possible to supercool the melt of high-density polyethylene down to 57 °C and the melt of a random ethylene/1-octene copolymer with 16 mol % 1-octene down to -33 °C, without prior crystallization. At these temperatures, the characteristic time of the primary crystallization process is of the order of magnitude of 100 μs. Complete vitrification of the liquid would require cooling even faster than 1 000 000 K/s. Compared to the homopolymer, the cooling-rate dependence of the crystallization temperatures and the temperature dependence of the characteristic time of primary crystallization of random ethylene/1-octene copolymers both are nearly parallel shifted to lower temperatures. Fast crystallization under conditions of reduced linear crystal growth rate is possibly caused by boosting homogeneous nuclei density up to 1027 m-3 and urgently requires further investigation.

Random Copolymers from Polyamide 11 and Polyamide 12 by Reactive Extrusion: Synthesis, Eutectic Phase Behavior, and Polymorphism

Random copolymers from polyamide 11 (PA11) and polyamide 12 (PA12) were obtained from the parent homopolymers by transamidation reactions during high-temperature reactive extrusion. Concomitantly, the product molecular weight increased by postcondensation reactions. With both species being crystallizable, eutectic phase behavior was observed and explained in terms of the Flory theory for random copolymer melting. Close to the eutectic composition, both species crystallize in a competitive mode, which induces mesomorphic phase formation. Although the crystal centers most likely remain PA11 or PA12 pure, it is argued that foreign comonomer units might be tolerated at the crystal borders, at least for PA11 crystals. To understand the copolymer crystallization behavior in general, large amounts of rigid amorphous material need to be created alongside the crystallites. Because of their low melting point and since the crystallinity and mechanical performance typical of the parent homopolymers are preserved, ran...

Isothermal Crystallizaiton Kinetic and Melting Behaviors of Nylon 6/66/510

In this work, nylon 6/66 (S0) and nylon 6/66/510 (S1~S3) containing 5 ~ 15 mol % nylon-510 were synthesized by melt condensation polymerization and the isothermal crystallization kinetics and melting behaviors after isothermal crystallization of copolymer were systematically studied by differential scanning calorimetry (DSC). The chemical structures and molecular weights were characterized by means of nuclear magnetic resonance ( 1 H-NMR). The values of t 1/2 and the equilibrium melting temperature for all samples were calculated by Avrami equation and HoffmanWeeks, respectively. The crystal structure of the copolyamides were also investigated by wide-angle X-ray diffraction (WARD). The results shows that: (1) Avrami model can be used to well describe the isothermal crystallization behavior of the copolyamides, and all the values of t 1/2 increase with increasing the Tc or the mole fraction of nylon 510. (2) The crystallization activation energy ( ∆E) and nucleation constant ( Kg) both increase with increasing the mole fraction of nylon-510, which indicating that the addition of nylon nylon-510 decreases the crystallization rate of copolymer.

Short-range ordered photonic structures of lamellae-forming diblock copolymers for excitation-regulated fluorescence enhancement.

Photonic crystals can be represented by periodic nanostructures with alternating refractive indices, which create artificial stop bands with the appearance of colors. In this regard, nanodomains of block copolymers and the corresponding structural colors have been intensively studied in the past. However, the practical application of photonic crystals of block copolymers has been limited to a large degree because of the presence of large defects and grain boundaries in the nanodomains of block copolymers. The present study focuses on the alternative opportunity of short-range ordered nanodomains of block copolymers for fluorescence enhancement, which also has a direct relevance to the development of fluorescence sensors or detectors. The enhancement mechanism was found to be interconnected with the excitation process rather than the alternation of the decay kinetics. In particular, we demonstrate that randomly oriented, but regular grains of lamellae of polystyrene-block-polyisoprene, PS-b-PI, diblock copolymers and their blend with PS homopolymers can behave as Bragg mirrors to induce multiple reflections of the excitation source inside the photonic structures. This process in turn significantly increases the effective absorption of the given fluorophores inside the polymeric photonic structures to amplify the fluorescence signal.

Exploring the crystallization-induced mesophase evolution in an olefin block copolymer through a rationally designed two-step isothermal crystallization strategy

The competition between crystallization and phase separation has always been a major concern for semi-crystalline block copolymers. In this work, in order to examine the mesophase evolution during crystallization in an olefin block copolymer (OBC), a “two-step isothermal crystallization” strategy was designed elaborately: the OBC samples were first isothermally crystallized at different temperatures (Tc) to create diverse crystallization histories; after melting, the second crystallization process was carried out to detect the degree of mesophase separation change during the first crystallization. It was quite interesting that the first crystallization at a lower Tc displayed little influence on the second crystallization process, while the first crystallization at a higher Tc could obviously accelerate the subsequent crystallization. In combination with the observation of the crystalline morphologies and phase morphologies in the melt, we speculated that hard blocks mainly crystallized separately during relatively rapid crystallization with a large quantity of nuclei; while crystallization-induced aggregation of hard blocks occurred during slow crystallization with a limited number of nuclei, changing the distribution of the hard blocks and accelerating the subsequent crystallization as a consequence. This work contributes to the understanding of the phase evolution during crystallization of OBC and such multi-block copolymers under different circumstances.

Transport of nanoparticulate material in self-assembled block copolymer micelle solutions and crystals.

Water soluble poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) [PEO-PPO-PEO] triblock copolymers self-assemble into thermoreversible micellar crystals comprised of periodically spaced micelles. The micelles have PPO cores surrounded by hydrated PEO coronas and the dimensions of the unit cell of the organized micelles is on the order of several to tens of nanometers. Fluorescence recovery after photobleaching (FRAP) is used to quantify nanoparticle transport in these nanostructured polymer micelle systems. Diffusivity of bovine serum albumin (BSA, Dh ∼ 7 nm) is quantified across a wide range of polymer, or micelle, concentrations covering both the disordered fluid as well as the structured micellar crystal to understand the effects of nanoscale structure on particle transport. Measured particle diffusivity in these micellar systems is reduced by as much as four orders of magnitude when compared to diffusivity in free solution. Diffusivity in the disordered micellar fluid is best understood in terms of diffusion through a polymeric solution, while transport in the structured micellar phase is possibly due to hopping between interstitial sites. These results not only show that the nanoscale structures of the micelles have a measureable impact on particle diffusivity, but also demonstrate the ability to tune nanoscale transport in self-assembled materials.

Interplay between poly(ethylene oxide) and poly(l-lactide) blocks during diblock copolymer crystallization

The influence of composition and crystallization conditions on the behavior of double crystalline poly (ethylene oxide-b-L-lactide) (PEO-b-PLLA) diblock copolymers is investigated. Poly(L-lactide) contents in the synthesized copolymers vary from 50 to 91%, and the molecular weight of the PLLA block ranges from 2 to 20 kg mol−1, while that of the PEO block is kept constant at 2 kg mol−1. In bulk samples, DSC results show a synergistic interaction between the crystallization processes of the two blocks. The PEO block provides heterogeneities and exerts a plasticizing action which favors the crystallization of the PLLA block with a nucleation efficiency of 30%. In contrast, the subsequent crystallization of the PEO block is subject to two opposing effects: (a) the nucleating action of PLLA crystals and (b) the topological and geometrical constraints imposed by PLLA crystals, especially when the PEO content is 20 wt% or less. In the case of ultra thin films, block copolymers with PEO contents equal or smaller than 20 wt% form distorted PLLA single crystals when crystallized from the melt. However, upon increasing the PEO content in the system to 33 wt% (by blending or copolymerization), the distortions disappear and the angle between the {110} growth faces changes from 140° to 121°, since the PEO block acts as a solvent or plasticizer for the PLLA block during the crystallization process. PEO incorporation can therefore tailor the rate and morphology of PLLA block crystallization. TEM and AFM studies allowed direct observation of the PEO block dendritic crystals on the surface of lozenge-shaped PLLA crystals previously formed during cooling from the melt.

Arrangement of Conductive Rod Nanobrushes via Conductive–Dielectric–Conductive Sandwiched Single Crystals of Poly(ethylene glycol) and Polyaniline Block Copolymers

Conductive rod nanobrushes of polyaniline (PANI) were developed via the growth of conductive–dielectric–conductive sandwiched single crystals obtained from poly(ethylene glycol) (PEG5000)-b-PANIn, PANIn-b-PEG6000-b-PANIn, and PANIn-b-PEG35000-b-PANIn block copolymers synthesized by interfacial polymerization fostering two different oxidants (ammonium peroxydisulfate (APS) as a weaker and potassium hydrogen biiodate (PHD) as a stronger oxidant). Based on the dispersity of the diameter of the PANI nanofibers and the various molecular weights of PEG substrates, two distinct morphologies were detected, i.e., matrix–dispersed morphology for PANIn-b-PEG35000-b-PANIn and dispersed–dispersed morphology for PANIn-b-PEG6000-b-PANIn and PEG5000-b-PANIn single crystals. In matrix–dispersed single crystals, through an elevated crystallization temperature (Tc), a convergence occurred between the heights of matrix (partly stretched PANIs) and disperses (fully stretched PANIs). Because of their higher conductivity (e.g.,...

Thermal and optical behavior of poly(ethylene-b-ethylene oxide) block copolymer dispersed liquid crystals blends

Different block copolymer dispersed liquid crystals blends based on poly(ethylene-b-ethylene oxide) (PE-b-PEO) and modified with low molecular weight nematic liquid crystals, 4′-(hexyloxy)-4-biphenylcarbonitrile (HOBC) and N-(4-ethoxybenzylidene)-4-butylaniline (EBBA) were prepared and characterized. The miscibility between each block of PE-b-PEO block copolymer and nematic liquid crystals was studied using differential scanning calorimetry (DSC). DSC results were in good agreement with the miscibility prediction supported by the solubility parameters and miscibility effect in polymer dispersed liquid crystals blends based on polyethylene oxide (PEO) homopolymer. Thermal stability of PE-b-PEO/HOBC and PE-b-PEO/EBBA blends was also studied. Block copolymer dispersed liquid crystals blends maintained the nematic/isotropic transition in function of the ratio between PE-b-PEO block copolymer and nematic liquid crystals indicating the potential application of the designed materials.

Tuning the instrument resolution using chopper and time of flight at the small-angle neutron scattering diffractometer KWS-2.

Following demand from the user community regarding the possibility of improving the experimental resolution, the dedicated high-intensity/extended Q-range SANS diffractometer KWS-2 of the Jülich Centre for Neutron Science at the Heinz Maier-Leibnitz Center in Garching was equipped with a double-disc chopper with a variable opening slit window and time-of-flight (TOF) data acquisition option. The chopper used in concert with a dedicated high-intensity velocity selector enables the tuning at will of the wavelength resolution Δλ/λ within a broad range, from 20% (standard) down to 2%, in a convenient and safe manner following pre-planned or spontaneous decisions during the experiment. The new working mode is described in detail, and its efficiency is demonstrated on several standard samples with known properties and on a completely new crystallizable copolymer system, which were investigated using both the conventional (static) and TOF modes.

Spherulitic morphologies of the triblock Poly(GL)-b-poly(GL-co-TMC-co-CL)-b-poly(GL) copolymer: Isothermal and non-isothermal crystallization studies

Crystallization of a biodegradable segmented copolymer constituted by polyglycolide hard segments and a middle soft segment constituted by a random disposition of glycolyl, ε-caproyl and trimethylene carbonyl units has been studied by means of optical microscopy, atomic force microscopy and time resolved X-ray diffraction techniques. This Poly(GL)-b-poly(GL-co-TMC-co-CL)-b-poly(GL) copolymer is widely employed as surgical suture and has similar characteristics than previously studied copolymers having a middle soft segment constituted by only two monomers (i.e. glycolide and trimethylene carbonate).FTIR and NMR spectroscopies demonstrated that the middle segment had an amorphous character and a random microstructure as consequence of transesterification reactions that took place during synthesis. Nevertheless, polyglycolide segments were able to crystallize giving rise to peculiar positive birefringent spherulites with a morphology, which depends on crystallization temperature (i.e. flat-on and edge-on crystals) as verified by AFM and electron diffraction patterns.Complete bell shaped curves that defined the temperature dependence of the crystal growth rate could be experimentally obtained from both, isothermal and non-isothermal crystallizations. Data from both analyses were in close agreement and pointed out a secondary nucleation constant (2.42–2.88×105K2) which was clearly higher than that determined for the related system with two components. Lamellar morphologic parameters were similar for samples crystallized from the melt state and after the reordering process that took place on heating. Comparing to the bicomponent system, significant differences were again observed highlighting the influence of the soft segment on the crystallization behavior.