Mobile Amorphous Fraction Research Articles

Contribution of the rigid amorphous fraction to physical ageing of semi-crystalline PLLA

The physical ageing of poly(l-lactic acid) (PLLA) samples crystallized in a wide temperature range, is compared, through the analysis of the enthalpy relaxation, with the behavior of a completely amorphous sample, in the same undercooling conditions. A preliminary determination of the percentage of the different phases in the semi-crystalline samples (crystalline, mobile amorphous and rigid amorphous fractions) has been performed. The enthalpy loss (Δha), normalized to the mobile amorphous content, has allowed to differentiate PLLA samples crystallized at different temperatures, because the samples crystallized at lower temperatures, characterized by a slightly constrained mobile amorphous fraction, exhibit a smaller enthalpy loss. In addition, the normalized Δha data relative to the semi-crystalline PLLA samples have been found to deviate from the Δha values of the amorphous sample at high undercooling. This trend has been ascribed to the contribution of the rigid amorphous fraction to the structural relaxation process. The experimental data have been explained by supposing that structural relaxation in amorphous and semi-crystalline PLLA occurs at low undercooling via cooperative segmental motions, whereas, at high undercooling, through small-scale local motions, which take place also in the rigid amorphous fraction.

Electrospun fibers of poly(l‐lactic acid) containing lovastatin with potential applications in drug delivery

ABSTRACTLovastatin (Merck's Mevacor) is a statin drug designed to lower cholesterol, and reduce the risk of heart attack and stroke. We use electrospinning to combine the biomedical properties of lovastatin with the advantages of electrospun fibers to prepare a composite biomaterial for lovastatin delivery. Poly(l‐lactic acid) (PLLA), a biodegradable and biocompatible polymer, was co‐spun with lovastatin. Incorporation of lovastatin at 5 or 10 wt % improved fiber alignment and surface smoothness, and increased fiber diameter. Influence of lovastatin on the phase structure (crystal, mobile amorphous, and rigid amorphous fractions) was investigated using scanning calorimetry and synchrotron X‐ray scattering. Addition of lovastatin resulted in increased crystallinity and reduced mobile amorphous fraction. PLLA fibers were characterized in terms of their drug release kinetics in comparison to PLLA film. High drug entrapment efficiency (ranging from 72% to 82%) and appropriate release profiles were achieved. In vitro drug release studies demonstrated that release occurred in two stages: an initial rapid release over the first day and a slower second stage of release which approached a plateau after 7 days. PLLA fibers have a higher release rate than comparable film. Electrospun biomaterial fibers of PLLA provide a promising new release strategy for delivery of lovastatin. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45287.

The influence of the preparation conditions and filler content on thermal properties of poly-l -lactide and hydroxyapatite/poly-l -lactide nanocomposite

Poly-l-lactide (PLLA) and hydroxyapatite/poly-l-lactide (HAp/PLLA) are two widely used biomaterials for three-dimensional scaffolds, drug release matrices and implantable medical devices for reparation of bone tissue; diversity in the initial preparation and filler content has a significant influence on different properties such as morphology and crystallinity, thus playing a considerable role in most of these applications. For this reason, PLLA and HAp/PLLA samples with a large difference in crystallinity (from below 20% to over 70%) and filler content (up to 86 wt% of HAp nanoparticles with an average diameter of 80 nm) were prepared and consequent dissimilarities in morphology, crystallinity and thermal properties were investigated by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD) measurements and Fourier transform infrared (FTIR) spectroscopy. Special attention was devoted to analyzing data obtained from thermal measurements. A three-phase model was employed in order to describe the heat capacity step decline in the nanocomposite; the evolution in different polymer fractions, the crystalline fraction and the mobile and rigid amorphous fractions, with filler content was determined. © 2017 Society of Chemical Industry

Crystallization of Polymers Investigated by Temperature-Modulated DSC.

The aim of this review is to summarize studies conducted by temperature-modulated differential scanning calorimetry (TMDSC) on polymer crystallization. This technique can provide several advantages for the analysis of polymers with respect to conventional differential scanning calorimetry. Crystallizations conducted by TMDSC in different experimental conditions are analysed and discussed, in order to illustrate the type of information that can be deduced. Isothermal and non-isothermal crystallizations upon heating and cooling are examined separately, together with the relevant mathematical treatments that allow the evolution of the crystalline, mobile amorphous and rigid amorphous fractions to be determined. The phenomena of ‘reversing’ and ‘reversible‘ melting are explicated through the analysis of the thermal response of various semi-crystalline polymers to temperature modulation.

Effects of interfacial interactions and of crystallization on rigid amorphous fraction and molecular dynamics in polylactide/silica nanocomposites: A methodological approach

We employ a partly new experimental approach to polymer nanocomposites (PNCs) based on a semicrystalline matrix, in order to distinguish between phenomena affiliated, on the one hand, to interactions between polymer and nanoparticles, and, on the other hand, to polymer crystals. Thus, effects of silica nanoparticles and of crystalline fraction (CF) on glass transition and segmental dynamics in poly (l–lactic acid) were investigated by means of differential scanning calorimetry (DSC) and broadband dielectric relaxation spectroscopy (DRS). Analysis of results involves combination of measurements on initially amorphous and on semicrystalline (annealed) samples. No change in the glass transition temperature by the filler is observed by DSC, whereas the heat capacity step decreases in the PNCs. The segmental α relaxation (dynamic glass transition) in DRS becomes, however, faster and weaker in the PNCs. Results are rationalized in terms of the rigid amorphous fraction (RAF) due to filler (interfacial polymer, RAFfiller) and due to polymer crystals (RAFcrystal). RAFfiller and RAFcrystal were disentangled from the total RAF via two assumptions: (a) assuming the same RAFcrystal to CF ratio in the neat matrix and the PNCs, and (b) assuming the same RAFfiller in amorphous and semicrystalline samples. Changes of the various polymer fractions with composition show similar trends in DSC and DRS. RAFfiller was found to increase with filler fraction, with a saturation for the largest loading (20 wt%), assigned to filler aggregation confirmed by morphological characterization. Both RAFcrystal and mobile amorphous fraction, MAF, were found within experimental error almost constant with filler loading. The overall results suggest that interfacial interactions (RAFfiller) in combination with changes in semicrystalline morphology dominate polymer dynamics in semicrystalline PNCs.

Dynamic glass transition of the rigid amorphous fraction in polyurethane-urea/SiO2 nanocomposites.

We report molecular dynamics in the rigid amorphous fraction (RAF) of the polymer bound at the interfaces with nanoparticles in polymer nanocomposites and calculate the glass transition temperature, Tg, for this bound layer of polymer. We follow the '3-phase-model' for semicrystalline polymers where the polymer matrix consists of the crystalline fraction (CF), the mobile amorphous fraction (MAF) and the RAF. While the amorphous polymer bound by crystallites is completely rigid, neither contributing to the glass transition, nor displaying molecular dynamics, the amorphous polymer bound at the interfaces with filler displays decelerated dynamics, as compared to the bulk polymer. Reports in the literature suggest a discrepancy between Tg values obtained by Differential Scanning Calorimetry (DSC) and by Dielectric Relaxation Spectroscopy (DRS). As a plausible explanation we suggest that DRS results in Tg values taking into account the bound polymer, whereas DSC does not. For this investigation we use semicrystalline polyurethane-urea/SiO2 nanocomposites and employ, next to DSC and DRS, SEM, SAXS and WAXS for morphological characterization. It is our intention to use DRS as a tool for investigating the RAF.

Advanced analysis of poly(3-hydroxybutyrate) phases based on vibrational heat capacity

The new quantitative thermal analysis of semicrystalline poly(3-hydroxybutyrate) (P3HB) phases based on vibrational, solid and liquid heat capacities has been presented. The apparent heat capacity of P3HB was measured using the standard differential scanning calorimetry, and temperature-modulated DSC and quantitative analysis allow for the study of any glass transition, melting/crystallization process and heat capacity of P3HB in the entire range of investigated temperature (183–465 K). The formation and description of phases during thermal processes of P3HB by advanced thermal analysis were examined. The mobile amorphous fraction, degree of crystallinity and rigid amorphous fraction were determined depending on the thermal history of semicrystalline P3HB included after isothermal and non-isothermal crystallization. The experimental, apparent heat capacity of P3HB in the non-equilibrium state was analyzed in reference to the solid and liquid heat capacities and all thermodynamic functions (enthalpy, entropy and Gibbs function) in the equilibrium state.

The Three-Phase Structure of Random Butene-1/Ethylene Copolymers

Abstract The three-phase arrangement of random copolymers of butene-1 with ethylene was investigated and compared with isotactic poly(butene-1) homopolymer (iPB-1). In all the analyzed compositions, isothermal crystallization leads to a three-phase structure, made of one crystal phase and two amorphous fractions that differ in mobility: the mobile amorphous fraction (MAF), made of the polymer chains that relax at the glass transition, and a rigid amorphous fraction (RAF) made of the amorphous segments coupled with the crystal phase. Copolymerization with ethylene leads to a drop in crystal fraction and to a sizable increase of both the RAF, and of the specific RAF, i.e. of the RAF normalized to crystallinity. Analysis of crystal growth rate allowed quantifying the fold surface free energy, which increases of about 50 to 100% in the copolymers, compared to iPB-1 homopolymer. In the butene-1/ethylene random copolymers, ethylene units are mostly excluded from the crystals and accumulate at the crystal/amorphous interphase, thus affecting the rigid amorphous area. The varied composition and higher mobility of the rigid amorphous fraction of the copolymers affects also the Form II to Form I transformation of poly(butene-1) crystals, which occurs with enhanced kinetics in the copolymers, compared to iPB-1 homopolymer.

Molecular Dynamics of Poly(ethylene‐2,5‐furanoate) (PEF) as a Function of the Degree of Crystallinity by Dielectric Spectroscopy and Calorimetry

The segmental and local dynamics of the bio‐based polyester poly(ethylene‐2,5‐furanoate) (PEF) are investigated by dielectric spectroscopy (DS) at atmospheric pressure as a function of temperature and frequency for samples prepared with different degrees of crystallinity. Crystallization significantly alters the segmental process as this is manifested in the dielectric strength and the distribution of relaxation times. Enhanced crystallinity by the different annealing protocols increases the glass temperature of the mobile amorphous fraction. A two‐phase model is adequate in describing the dynamics associated with the β‐process within the glassy state. However, the same model fails in describing the segmental dynamics at and above the glass temperature. The combined differential scanning calorimetry and DS results provide evidence that for samples with a degree of crystallinity of ≈40%, approximately a third of segments are located within the restricted amorphous fraction.image

Time and Temperature Evolution of the Rigid Amorphous Fraction and Differently Constrained Amorphous Fractions in PLLA

In semi‐crystalline polymers, amorphous segments located in different regions and distance from the crystalline domains can exhibit different dynamics. In poly(l‐lactic acid) (PLLA), the existence of (i) two distinct mobile amorphous fractions (a completely mobile amorphous fraction and a slightly constrained mobile amorphous fraction), which vitrify/devitrify in the Tg region, and (ii) a rigid amorphous fraction, which vitrifies/devitrifies at temperatures higher than Tg, has been reported in the literature. The percentage of the three different amorphous fractions, characterized by different segmental mobility, is generally calculated at Tg. In the present study, for the first time, the time and temperature evolution of all the amorphous fractions of PLLA are measured in parallel with the crystalline phase (i) during crystallization at two different temperatures, and (ii) upon the successive cooling step down to room temperature. The kinetics of vitrification/devitrification of the different amorphous fractions in diverse conditions of crystallization and solidification has been determined during the entire thermal history of the material. Rigid amorphous fraction is found to develop not only during crystallization but also upon cooling, after completion of crystallization. image

From a Three-Phase Model to a Continuous Description of Molecular Mobility in Semicrystalline Poly(hydroxybutyrate-co-hydroxyvalerate)

In most cases, a three-phase model consisting of crystalline domains surrounded by rigid amorphous areas dispersed in the mobile amorphous phase is required and eventually sufficient to accurately describe the microstructure of semicrystalline polymers. This work shows that the microstructure developed by poly(hydroxybutyrate-co-hydroxyvalerate) by cold crystallization is better described by a complex two-phase model in which the boundaries of the crystalline domains and the surrounding amorphous environment form a “continuum of mobility”. This concept can be extended to a wide range of semicrystalline polymers. When the crystalline and the amorphous phases are strongly coupled, the rigid and mobile amorphous fractions are hardly fractionated, and the whole noncrystalline phase should be rather depicted as continuum with a broad distribution of the relaxation times associated with the glass transition. Such a depiction of the amorphous phase allows taking into account any modification of the mobility land...

Mobile amorphous, rigid amorphous and crystalline fractions in isotactic polypropylene during fast cooling

Crystallization of isotactic polypropylene was analyzed by fast scanning calorimetry (Flash DSC) at cooling rates between 1 and 4000 K s−1. By quantitative analysis of the glass transition intensity and the specific transformation enthalpy, the amount of mobile amorphous (MAF), rigid amorphous (RAF) and crystalline fractions (CFs) and its dependency on the cooling rate were determined. During cooling, two different crystalline phases are formed. At slow cooling rates (below 90 K s−1), the α-phase is crystallized. At faster cooling rates mesophase aggregates are formed. The crystalline phase reduces the relaxation time in the MAF independently from the kind of the crystalline phase. The RAF is formed during the crystallization process. In the case of mesophase crystallization, the ratio between CF and RAF is independent of the crystallinity. For α-phase crystals, this ratio depends on the crystallinity. A model for the CF-RAF structure is derived from the reported results.

Highly Enhanced Mesophase Formation in Glassy Poly(l-lactide) at Low Temperatures by Low-Pressure CO2 That Provides Moderately Increased Molecular Mobility

The mesophase structuring in melt-quenched poly(l-lactide) (PLLA) treated in low-pressure CO2 at 2 MPa and 0–35 °C was investigated by using infrared spectroscopy, differential scanning calorimetry (DSC), temperature-modulated DSC, and atomic force microscopy (AFM). It was found that the mesophase formation in glassy PLLA was significantly enhanced, in particular at lower temperature (0 °C), which promoted a distinctly faster formation rate. AFM results revealed that the CO2-enhanced mesophase exhibited nodular morphology with dramatically increased nucleation density. A framework of multistage model in combination with the moderately improved molecular mobility exerted by CO2 was proposed to explain the main findings. Because of the moderate molecular mobility, a tremendous number of metastable mesomorphic layers were formed and stabilized by the accompanying development of the rigid amorphous fraction (RAF), leading to the immobilization of the remaining mobile amorphous fraction (MAF). The mesomorphic ...

Poly(lactides) co-electrospun with carbon nanotubes: thermal and cell culture properties

Multi-walled carbon nanotubes (MWCNTs) often serve as an effective nucleating agent that facilitates the crystallization of semicrystalline polymers. Here we study the influence of MWCNTs on thermal and structural properties of electrospun fibers of Poly-lactide (PLA), a biodegradable and biocompatible thermoplastic polymer. MWCNTs were co-electrospun with either poly(l-lactide) (PLLA, with 100% l-isomer) or poly (d-lactide) (PDLA, containing 4% d-isomer) in weight ratios ranging from 1.0 to 4.0wt.% MWCNT. Electrospun fibers had average diameters below one micron, and addition of MWCNTs reduced the average fiber diameter and narrowed the diameter distribution. Using heat capacity measurements, the crystal, mobile amorphous, and rigid amorphous fractions were evaluated in MWCNT-containing PLA fibers. More rigid amorphous fraction was observed upon increasing the MWCNT concentration to 1.0wt.% due to the restricted polymer chain mobility induced by MWCNTs. However, at the highest concentration of 4.0wt.% MWCNTs, the crystal and rigid amorphous fractions were both reduced due to MWCNT aggregation. Fibers were assessed for their efficacy as substrates for growth of human umbilical vein fibroblasts. Cell culture results of fibroblasts plated on oriented PLLA fibers indicated cells grew preferentially in the same direction as the fiber orientation. Cell metabolic activity was reduced, compared to a control substrate, with greatest reduction occurring in crystalline fibers, or those containing MWCNTs.

Three-Phase Morphology of Semicrystalline Polymer Semiconductors: A Quantitative Analysis.

Quantitative morphological analysis is essential to the fundamental understanding of semiconducting polymers. Temperature modulated differential scanning calorimetry is used to determine the amount of crystalline and noncrystalline phases within regioregular poly(3-hexylthiophene) (rrP3HT). Careful optimization of the experimental conditions shows that the glass transition of rrP3HT consists of three parts corresponding to the devitrification of the side chains, mobile amorphous fraction (MAF), and rigid amorphous fraction (RAF), consecutively. Measurements taken from this, as well as from the melting transition, allows the first calculation of the degree of crystallinity, MAF and RAF, to be achieved in a single experiment for rrP3HT. This technique thus enables the morphological phases to be determined and potentially related to the performance of electronic devices made from semiconducting polymers.

Morphology, thermal properties and crystallization kinetics of ternary blends of the polylactide and starch biopolymers and nanoclay: The role of nanoclay hydrophobicity

Polylactide/butylated-starch/nanoclay (70/25.5/4.5 wt%) composites were prepared by melt blending with nanoclays of varying hydrophobicity. Electron microscopy studies indicated that the interphase boundary interaction was highest in the clay with intermediate hydrophobicity but decreased with clays of higher or lower hydrophobicity. Conventional and modulated differential scanning calorimetry studies showed that, in the case of composites, the change in the ratio of rigid and mobile amorphous fractions depends on the hydrophobicity of clays used for the preparation of blend composites. The mobile amorphous fraction related to the polylactide phase decreases in the case of intermediate hydrophobic clay compared with less and more hydrophobic clays or neat blend samples. Avrami bulk crystallization analysis and polarized optical microscopic observation demonstrated that hydrophobic clays hindered the crystallization of the PLA phase into the formation of disc-shaped spherulites. On the basis of obtained results, we propose a general understanding on how the morphology and thermal properties of the blend composites are related to the hydrophobicity of the nanoclays.

Crystallization kinetics and structure evolution of a polylactic acid during melt and cold crystallization

The isothermal melt and cold crystallization and the evolutions of the mobile and rigid amorphous fractions during the crystallization are fully investigated for a polylactic acid using differential scanning calorimetry. The crystallization kinetics is analyzed through fitting the Avrami equation to two different sets of crystallinity data, one obtained from the crystallization extherms and one from the enthalpies of melting and cold crystallization. Both sets yield similar results, indicating the consistency between these two methodologies. The cold crystallization shows clearly faster crystallization rate and has a higher activation energy of 172.3 ± 8.9 versus 109.6 ± 8.8 kJ mol−1 for the melt crystallization. The Avrami exponent of the cold crystallization seems to decrease with increasing crystallization temperature, whereas the exponent of melt crystallization is relatively constant. In addition to the crystalline phase, properties of the mobile and rigid amorphous fractions are also studied and compared. With the increase in crystallinity, the glass transition temperature (Tg) of the mobile amorphous phase is found to decrease during the cold crystallization, whereas Tg decreases and then increases during the melt crystallization, indicating slightly different environment formed. However, no obvious difference is found regarding the heat capacity change at Tg nor the cooperative length scale associated with Tg. Furthermore, the rigid amorphous fraction is determined and found to be 30 % at the maximum crystallinity for the both cold- and melt-crystallized samples.

Analysis of non-isothermal crystallization during cooling and reorganization during heating of isotactic polypropylene by fast scanning DSC

Crystallization, reorganization and melting of isotactic polypropylene (iPP) were analyzed by DSC and fast scanning DSC. The cooling and subsequent heating DSC curves are measured at scanning rates between 0.1K/min and 30,000K/s. We study the formation of the mesophase and the α-phase during cooling. Using subsequent heating measurements the formation of the mesophase during cold crystallization and the kinetics of the reorganization are analyzed. From the cooling rate dependence of the glass transition and the melting behavior we discuss relations between the crystalline fraction (CF), the mobile amorphous fraction (MAF), the rigid amorphous fraction (RAF) and the influence of these fractions on the reorganization processes. Furthermore, we use the heating rate dependence of the melting behavior for stability analyses of the related crystals and propose the Illers-plot as a simple analysis tool. Using this technique the melting temperature of the mesophase is determined to about 86°C.

Formation of a Rigid Amorphous Fraction in Poly(3-(2'-ethyl)hexylthiophene).

Herein, we detail the formation of a rigid amorphous fraction in poly(3-(2'-ethyl)hexylthiophene) (P3EHT) at high relative crystallinity, yielding a more complete picture of the solid-state structure. In the differential scanning calorimetry (DSC) heating scans of isothermally crystallized P3EHT a distinct endothermic peak appears slightly above the crystallization temperature. This previously undescribed endothermic feature of P3EHT's thermal behavior is observed consistently ∼20 °C above the crystallization temperature-shifting to higher temperatures with increasing crystallization temperature-and increases in magnitude with both time and crystallization temperature. Here, we determine the origins of this endothermic peak with DSC and temperature-modulated DSC (TMDSC). TMDSC reveals that the annealing peak observed in the total heat flow (THF)-heat flow equivalent to that of conventional DSC-is a consequence of an enthalpic relaxation observable as an endothermic peak in the nonreversible heat flow (NHF) and a glass transition evident as a step increase in the reversible heat flow (RHF). In conjunction with conventional DSC observations, these results indicate that the observed annealing peak is a consequence of the formation of distinct amorphous regions-a mobile amorphous fraction (MAF) and a rigid amorphous fraction (RAF)-during the isothermal crystallization process and not the melting of a distinct crystallite population or melt recrystallization.

Rigid amorphous fraction versus oriented amorphous fraction in uniaxially drawn polyesters

Structural changes in poly(ethylene terephthalate) PET and the copolyester poly(ethylene glycol-co-cyclohexane-1,4-dimethanol terephthalate) PETg occurring during uniaxially drawing at 100°C have been investigated as a function of draw ratio. This work compares the “microstructural” and the “thermodynamic” three phase description of drawn polyester films. Using Wide angle X-ray Scattering, both materials were characterized considering crystals, isotropic and oriented amorphous phases. Temperature Modulated Differential Scanning Calorimetry allows to evaluate the fraction of crystals, mobile and rigid amorphous fractions (MAP and RAF respectively). Above a critical draw ratio, RAF appears as soon as a strain induced crystalline (SIC) phase is developed for both materials. The degree of crystalline phase is lower and takes place at higher draw ratio for PETg than for PET. For PET, the oriented amorphous phase is composed by the RAF and a part of the MAP. For PETg, the SIC takes place in PET-rich regions and the quantity of RAF per crystal is high.