Hoffman-Weeks Plot Research Articles

Evaluation of the Equilibrium Melting Temperature and Melting Behavior of Poly(Phenylene Sulfide) Crystals.

Focusing on the melting process of poly(phenylene sulfide) (PPS) crystals, one of the high-performance resins with a very wide range of applications, we attempted to clarify the melting process and derive the equilibrium melting temperature with structural analysis using X-ray scattering and thermal measurements. Two melting points were observed in the crystallized PPS films by DSC measurements. We focused on the low-temperature melting point, in situ small-angle and wide-angle X-ray scattering measurements using synchrotron radiation and DSC thermograms indicate the melting of microcrystals because of thickening of the amorphous part from small-angle X-ray scattering results. In addition, the Hoffman-Weeks plot based on the results of fast scanning calorimetry showed that the melting temperature is almost parallel to the straight line where melting temperature and crystallization temperature are equal, indicating that the melting is of a metastable structure. The melting point on the high temperature side was attributed to the melting of lamellar crystals. The equilibrium melting point was derived from the Hoffman-Week and Gibbs-Thomson plots as 318 °C.

Melting and Annealing Peak Temperatures of Poly(butylene succinate) on the Same Hoffman-Weeks Plot Parallel to Tm=Tc Line

The crystallization and melting behavior of polymers is of theoretical importance. In this work, poly(butylene succinate) (PBS) was selected as an example to study such behavior at low supercooling via introduction of the extended-chain crystal (ECC) of the same polymer as nucleating agent. The crystallization of PBS with its ECC as nucleating agent in a wide temperature range (90–127 °C) and the following melting behavior were studied. It is revealed that the melting point (Tm', for Tc≥113 °C) and the annealing peak temperature (Ta', for Tc=90–100 °C) show similar asymptotic behavior. Both Tm and Ta approach to a value of ca. 3.3 °C higher than the corresponding Tc when the crystallization time tc approaches the starting point. That is to say, the Hoffman-Weeks plot is parallel to Tm=Tc line. The crystallization line became parallel to the melting line when PBS was crystallized at Tc higher than 102 °C. Based on these results, we propose that the parallel relationship and the intrinsic similarity between the Ta and the Tm observed at the two ends of the Tc range could be attributed to the metastable crystals formed at the beginning of crystallization.

Reexamination of the equilibrium melting temperature of natural rubber crystals

The equilibrium melting temperature ($$T_{\mathrm{m}}^0$$) of natural rubber crystals was reexamined by the Hoffman–Weeks plot and the Gibbs–Thomson plot using the lamellar thickness obtained from the same sample as the one used for the measurement of melting temperature. The sample was prepared from deproteinized latex. As a result, the $$T_{\mathrm{m}}^0$$ of NR was estimated to be 61 °C. At the same time, the fold-surface free energy was estimated to be 0.046 Jm−2. These values should be more reliable than the previously assumed values. The equilibrium melting temperature ($$T_{\mathrm{m}}^0$$) of natural rubber crystals was reexamined by the Hoffman–Weeks plot and the Gibbs–Thomson plot using the lamellar thickness obtained from the same sample as the one used for the measurement of melting temperature. The sample was prepared from deproteinized latex. As a result, the $$T_{\mathrm{m}}^0$$ of NR was estimated to be 61 °C. At the same time, the fold-surface free energy was estimated to be 0.046 Jm−2. These values should be more reliable than the previously assumed values.

Gibbs–Thomson, Thermal Gibbs–Thomson, and Hoffman–Weeks Plots of Polyethylene Crystals Examined by Fast-Scan Calorimetry and Small-Angle X-ray Scattering

Crystallization and melting behaviors of polyethylene (PE) crystals were examined by fast-scan chip sensor calorimetry (FSC) and small-angle X-ray scattering (SAXS). The melting point TM of chain-folded thin lamellar crystals of PE was determined by utilizing FSC and the crystalline lamellar thickness dc by SAXS. The range of crystallization temperature Tc from SAXS as well as from FSC was extended much more broadly than before by examining those samples prepared on a chip sensor of FSC and applying a deep temperature jump for crystallization at large supercooling ΔTc. While the equilibrium melting point TM0 was reconfirmed with linear relationship of the melting and crystallization lines in the Gibbs–Thomson (G-T) plots of TM and Tc against (dc)−1, respectively, and the Hoffman–Weeks (H-W) plot of TM against Tc at relatively small ΔTc, those G-T and H-W plots seriously deviated from linear straight lines at large ΔTc. The origin of the curved G-T plots was ascribed to the Tc-dependent folding surface free energy σe. The determined values of TM0- and Tc-dependent σe were supported by a newly proposed thermal G-T plot of TM against the inverse of specific heat of fusion Δhfs in terms of the increase during the secondary stage of crystallization on long-term isothermal annealing at Tc of chain-folded PE crystals transforming to more stable states by lamellar thickening and crystal perfecting. The deviation from a linear H-W plot at large ΔTc was then ascribed to the crystal reorganization on heating of those less stable crystals formed at large ΔTc, even with fast heating. Application of fast heating and cooling by FSC plays an essential role in the confirmation of those thermodynamic behaviors of metastable polymer crystals with chain folding.

Crystallization and melting behaviors of poly(vinylidene fluoride) examined by fast-scan calorimetry: Hoffman-Weeks, Gibbs-Thomson and thermal Gibbs-Thomson plots

Crystallization and melting behaviors of chain-folded polymer crystals have been examined by fast-scan calorimetry (FSC) combined with small angle X-ray scattering (SAXS) and wide angle X-ray diffraction (WAXD) for poly(vinylidene fluoride). The melting point of lamellar crystals formed isothermally at Tc was measured by FSC in terms of the heating rate dependence and calibrated on the basis of the modeling of melting kinetics for the determination of the melting point at zero heating rate TM. The Hoffman-Weeks (H-W) plot of TM against Tc was on a linear straight line over Tc range broader than 30 K, and suggested the equilibrium melting point TM0 ≅ 200 °C of chain-extended infinite-size crystals of the α form. On the other hand, utilizing crystalline lamellar thickness dc determined by SAXS, both of the Melting and Crystallization lines in the Gibbs-Thomson (G-T) plots of TM and Tc against (dc)−1, respectively, were found to seriously deviate from linear straight lines. By examining the secondary stage of crystallization on long time isothermal annealing at Tc in terms of the changes in TM and the specific heat of fusion Δhfs by FSC, a newly proposed thermal G-T plot of TM against (Δhfs)−1 has confirmed the reliability of TM0 of the α form determined by the H-Wplot and the Tc dependent folding surface free energy σe, which brings the curved Melting and Crystallization lines in the G-T plots. In addition, on long time isothermal annealing, the changes in dc by SAXS and in crystallite size by WAXD have confirmed the molecular origin of the increase in TM and Δhfs in the secondary stage with thickening and perfecting of metastable chain-folded polymer crystals.

Development of poly(ε‐caprolactone)/pine resin blends: Study of thermal, mechanical, and antimicrobial properties

Blends of poly(ε‐caprolactone) (PCL) with pine resin, an extract from the plant Pinus caribaea—Hondurensis was prepared by melt mixing in mass ratios from 90/10 to 50/50. The thermal, crystallization, morphological, and mechanical properties of the blends were studied by differential scanning calorimetry, Fourier transform infrared spectroscopy, polarized optical microscopy, scanning electron microscopy, and tensile test. Enzymatic degradation of the blends was investigated using porcine pancreatic and Candida rugosa lipase. Antimicrobial activity of the blends was tested against four strains of bacteria; Staphylococcus aureas, Escherichia coli, Bacillus subtilis, and Pseudomonas aeruginosa using the zone inhibition method. Miscibility of the blends was confirmed by the depression in the equilibrium melting temperature () of PCL estimated from Hoffman–Weeks plot and the presence of extinction rings in the spherulites of blended PCL. Interactions between the two components involved the carbonyl and the C‐O‐C groups. The tensile strength of the blends with low pine resin content was comparable to PCL but decreased with higher pine gum content. Enzymatic degradation of the blends increased with increasing pine resin content. The blends showed antimicrobial property with all the bacteria except E. coli. The developed biomaterial shows promising candidacy in medical applications. POLYM. ENG. SCI., 59:E32–E41, 2019. © 2018 Society of Plastics Engineers

Miscibility Study of Poly(Butylene Succinate) and Pine-Gum Blends

Miscibility between poly (butylene succinate) [PBS], a semi-crystalline polymer with a natural gum extracted from the pine tree (Pinus Caribaea – Hondurensis), was investigated in solution cast blends using Differential Scanning Calorimetry [DSC] and Fourier Transform Infrared Spectroscopy [FTIR]. The spherulite morphology of PBS in the blends was observed with polarized optical microscopy [POM]. Depression in the equilibrium melting temperature of PBS in the blends was determined using the Hoffman-Weeks plot method. The depression in the crystallization temperature of the blends with increasing pine gum ratio and the emergence of extinction rings in the spherulites of the blends confirmed the blends to be miscible at the molecular level. Infrared spectroscopy indicated that interactions occurred between the hydroxyl groups of the pine-gum and the carbonyl group of PBS.

Stabilization of Nuclei of Lamellar Polymer Crystals: Insights from a Comparison of the Hoffman–Weeks Line with the Crystallization Line

We have studied melting of poly(butylene succinate), isothermally crystallized over a wide temperature range, employing a combination of the Hoffman–Weeks plot and the Gibbs–Thomson crystallization line, determined by small-angle X-ray scattering measurements. A change in the slope α of the Hoffman–Weeks (H–W) line, accompanied by a change of the slope of the crystallization line, was observed for crystallization temperatures higher than 110 °C. α was reaching a value of 1, implying that no intersection point between the H–W line and the Tm = Tc line could be obtained. (Tm is the measured melting temperature and Tc is the temperature at which the sample was crystallized). This observation was corroborated by the crystallization line, which was found to be parallel to the melting line for Tc > 110 °C. We relate these changes in slope to different stabilization mechanisms of the secondary nuclei at the growth front of polymer lamellar crystals. For Tc > 110 °C, secondary nuclei are proposed to be stabilized...

Melting behaviors of polyethylene crystals: An application of fast-scan DSC

Melting behaviors of linear polyethylene (PE) have been experimentally examined by fast-scan and conventional differential scanning calorimetry (DSC). By fast-scan heating up to 104 K s−1, the melting peak temperature, Tpeak, is free from the influences of re-organization and re-crystallization and follows the shift only with superheating, which has been evaluated by a power-law behavior. The melting point, TM, of the chain-folded crystals has been determined as Tpeak extrapolated to zero heating rate without the influences of non-equilibrium effects. The Hoffman–Weeks plot of TM against crystallization temperature, Tc, suggests the equilibrium melting point TM0=141.1°C of the linear PE having Mw = 52.0 × 103 and the doubling of lamellar thickness concluded from the linear relationship between TM and Tc with the slope close to 1/2. While the lamellar doubling is a well-known behavior of PE crystallized from the melt, the present results with Tc down to ∼100 °C suggest the doubling completed in a quite short time interval. The value of TM0=140.4°C has also been obtained by the Gibbs–Thomson plot of TM against the inverse of lamellar thickness, (dc)−1, determined by small angle X-ray scattering. The agreement of the results within ±1 °C suggests the Hoffman–Weeks plot as an appropriate method for the melting point determination with fast-scan heating, only with which the determination procedure is free from re-organization and re-crystallization. The consistent behavior has also been confirmed in terms of the end-surface free energy, σe, determined by the Gibbs–Thomson plot and by the growth kinetics of crystallization.

Miscibility with thermal and mechanical properties for poly(pentamethylene 2,6-naphthalate) and poly(heptamethylene 2,6-naphthalate) blends

We have prepared the blends of poly(pentamethylene 2,6-naphthalate) (PPN) with poly(heptamethylene 2,6-naphthalate) (PHepN) by solution blending method and investigated their glass transition behaviour, melting behaviour, and tensile properties. It was observed that the blends of PPN/PHepN(9/1) and PPN/PHepN(1/9) have a single glass transition, reflecting a homogeneous phase, whereas those of PPN/PHepN(7/3), PPN/PHepN(5/5), and PPN/PHepN(3/7) exhibit double glass transitions, representing the existence of two phases. The PPN homopolymer annealed below 90 °C shows triple melting peaks (Tm1, Tm2, and Tm3). It was proved that Tm1 is attributed to melting of thin lamellar formed during secondary crystallization process, Tm2 to melting of thick lamellar created during primary crystallization, and Tm3 to melting of crystals recrystallized after melting the primary crystals at Tm2. For the annealed PHepN homopolymer, double melting endotherms (Tm1 and Tm2) were observed, caused by dual lamellar population with different thickness, i.e. Tm1 corresponding to the melting of secondary crystal and Tm2 to primary one. The Hoffman-Weeks plots, applied to the melting of primary crystals (Tm2s), indicate that the equilibrium melting temperatures of PPN homopolymer, PPN/PHepN(9/1), and PPN/PHepN(7/3) blends are same to be 147 °C, and those of PHepN homopolymer, PPN/PHepN(1/9), and PPN/PHepN(3/7) blends to be 145 °C. Both the glass transition and melting behaviours demonstrate that the PPN/PHepN blend system is partially miscible. In addition, both the modulus and strength for the blends almost follow additive rule against blend composition, indicating that the PPN/PHepN blends are mechanically compatible over all blend compositions.

Isothermal crystallization and melting behaviors of bionanocomposites from poly(lactic acid) and TiO2 nanowires

AbstractTwo representative poly(lactic acid) (PLA) nanocomposites with 1% TiO2 nanowires were prepared through in situ melt polycondensation and easy solution‐mixing approaches, respectively. The former was denoted as ISPLANC, and the latter as SMPLANC. The isothermal crystallization kinetics and melting behaviors of pure PLA, ISPLANC, and SMPLANC were comparatively investigated by differential scanning calorimetry in the temperature range of 80–115°C. Maximum crystallization growth rate (Gexp) was observed at 100°C for all three samples. The well dispersed TiO2 nanowires acted as effective nucleation agents in ISPLANC, which exhibited much higher Gexp in compared to pure PLA and SMPLANC below 110°C. However, much smaller crystallization enthalpy of ISPLANC was obtained because of its restricted chain mobility in forming crystalline lamellar. The crystallization behavior of all three samples fit the Avrami equation quite well, with most of the R2 values larger than 0.9990. Double‐melting behaviors were observed after heating the samples after isothermal crystallization at various temperatures, which was explained by the melt recrystallization of the smaller and imperfect crystals formed at lower isothermal crystallization temperatures. We also obtained the equilibrium melting temperatures of the three samples by carrying out Hoffman–Weeks plots. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Crystallization Behavior of Vetiver Grass Fiber-Polylactic Acid Composite

this work, vetiver fiber was used as a filler for poly (lactic acid) (PLA). The thermal properties of neat PLA and vetiver fiber-PLA composites were investigated. Talc as a nucleating agent was used to compare the nucleating effect on crystallization of the composites from vetiver fiber. It was found that crystallization rate was highest in the case of talc-PLA composites. Beside, the crystallization rate of PLA with 20%vetiver fiber content was higher than that of neat PLA and 1-10% (w/w) vetiver fiber-PLA composites. The equilibrium melting temperature (T0m) of neat PLA and PLA composites was obtained from Hoffman-Weeks plot. It was found that the presence of vetiver fiber and talc caused T0m values of PLA decreased compared to that of neat PLA. In addition, T0m values of PLA decreased with increasing vetiver fiber content. For non-isothermal crystallization, degree of crystallinity ( %XC )increased with the presence of vetiver fiber compared to that of neat PLA. As vetiver fiber content increased, %XC increased. Similarly, the incorporation of talc led to an increase in %XC compared to that of neat PLA. PLA with 1%talc content showed the highest %XCcompared to neat PLA and other PLA composites. Moreover, %XC decreased with increasing talc content.

Miscibility Determination in Poly(ϵ-caprolactone)/Poly(Vinyl Formal) Blend by Equilibrium Melting Temperature and Spherulite Morphology

Miscibility of poly(ϵ-caprolactone), (PCL), containing 1, 5, and 10 wt.% poly(vinyl formal) (PVF) blends was investigated by polarized optical microscopy (POM), atomic force microscopy (AFM), and differential scanning calorimetry (DSC) for spherulitic morphology and equilibrium melting temperature (T°m, via Hoffman-Weeks plot). The T°m of PCL in the blends was similar to that of pure PCL, indicating immiscibility. Isothermally, melt crystallized virgin PCL between 30°C and 50°C showed spherulitic morphology with negative birefringence, Maltese cross, and without extinction rings. The nucleation and growth rates of PCL spherulites were found to be dramatically reduced with the addition of PVF. Extinction rings and a change in the sign of the birefringence of the PCL spherulites were observed and were found to be dependent on blend composition and crystallization temperature. The presence of a ring pattern in spherulites was an indication of miscibility between the two polymers that had failed to be detected by thermal methods. The formation of a ring pattern is discussed in terms of lamella twisting originating from a change in the crystallization mechanism.

Multiple melting and partial miscibility of ethylene‐vinyl acetate copolymer/low density polyethylene blends

AbstractMultiple melting behaviors and partial miscibility of ethylene‐vinyl acetate (EVA) copolymer/low density polyethylene (LDPE) binary blend via isothermal crystallization are investigated by differential scanning calorimetry (DSC) and wide angle X‐ray diffraction (WAXD). Crystallization temperature T (°C) is designed as 30, 50, 70, 80°C with different crystallization times t (min) of 10, 30, 60, 300, 600 min. The increase of crystallization temperature and time can facilitate the growth in lateral crystal size, and also the shift of melting peak, which means the completion of defective secondary crystallization. For blends of various fractions, sequence distribution of ethylene segments results in complex multiple melting behaviors during isothermal crystallization process. Overlapping endothermic peaks and drops of equilibrium melting points of LDPE component extrapolated from Hoffman–Weeks plots clarify the existence of partial miscibility in crystalline region between EVA and LDPE. WAXD results show that variables have no perceptible influence on the predominant existence of orthorhombic crystalline phase structure. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Polyamide-based composite membranes: Part 2. Interaction, crystallization and morphology

In this work, the interaction, crystallization and morphology of membranes derived from polyamides/poly (vinylalcohol) (PA/PVA, PA: PA66, PA69, PA610 and PA612) blend materials are studied at various weight fractions and various crystallization temperatures. The experimental work includes differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), polarized optical microscopy (POM) and scanning clectron microscopy (SEM). The equilibrium melting temperatures of polyamides in the blends are obtained using Hoffman-Weeks plots, and the interaction parameters are calculated using the Nishi-Wang equation, which is based on the Flory-Huggins theory. The values of the Flory-Huggins interaction parameters χ 12 are negative. Significant upward shifts of ν NH are observed with increasing volume fraction of PVA. The morphology of membranes is impressively complex, diverse, and irregular. All compositions that show nodular morphology are in the μm-scale and the size of the nodules increases with increasing PA6 content. Large void cavities are observed in the substructure for the lower polymer concentrations.

Melt crystallized polyamide 6.6 and its copolymers, Part I. Melting point – Lamellar thickness relations in the homopolymer

The effect of isothermal crystallization temperature and time on the lamellar thickness and the melting behavior of polyamide 66 has been studied. Measurements were made of the melting temperature, crystallinity, and the long period. When calculated in the conventional direct manner, for samples crystallized isothermally, the calculated lamellar thickness was found to vary only from 2.4 to 3.2 nm over the entire range of conditions considered. When viewed in a non-critical fashion the polymer appears to conform to normal behavior including typical T c vs. T m behavior on a Hoffman–Weeks plot and apparent linearity in a Gibbs–Thompson plot. SAXS data indicates that there are only small changes in the lamellar thickness occurring over the entire crystallization range despite major changes in the melting point. Accordingly the Gibbs–Thompson plot shows major amounts of scatter, which are well beyond the experimental errors involved. The changes in melting temperature appear to be a result of changes in the structure of the fold surface on the basis of the conventional lamellar thickness analysis. In particular they appear to be due to changes in the character of the surface related to the hydrogen bonding and to the relative amounts of acid and amine segments present in the folds. When a more thorough analysis of the SAXS data are conducted, using a one dimensional correlation function approach, calculation of the crystal core thicknesses and “interfacial layer” thicknesses, a different picture emerges. In this case, the total lamellar thickness remains approximately constant at 2 repeat units in length with isothermal crystallization temperature, however, the core thickness increases with increasing crystallization temperature and time, from 1.5 to 2 repeat units in length, whereas the “interfacial layer” thickness is substantial at lower temperatures and times. When the core thickness is used in a Gibbs–Thompson plot the equilibrium melting temperature is found to be 303.7 °C (cf. 301 °C from solution grown crystals). However, the fold surface free energy is found to be 23.7 erg/cm 2 much lower than the value of 74.6 erg/cm 2 characteristic of solution grown crystals. Such a large discrepancy is believed to be a result of the highly polar solvents used in solution based studies generating the widely accepted “acid folds”. The difference may be because of a switch to folds containing six methylene groups from the diamine mer in the bulk case. Since the polymer is known to crystallize in the hexagonal state and reorganize during cooling to the regularly reported structure it is possible that the “interfacial thickness” is indeed a disordered surface layer within the crystalline lamella that originates from the precursor hexagonal phase during its formation, rather than the conventional disordered surface interpretation, applicable to polymers such as polyethylene. It is also possible that it is reflective of disorder induced in surface layers within the crystal due to the conformational changes occurring during this crystal–crystal transition. For these reasons, we prefer to refer to the “interfacial layer” obtained from SAXS calculations as an innerlayer.

Crystallization kinetics of poly(glycolic acid- alt-6-aminohexanoic acid)

The crystallization behavior of a new regular poly(ester amide) constituted by glycolic acid and 6-aminohexanoic acid units under both isothermal and non-isothermal conditions is studied. Differential scanning calorimetry (DSC) is used to monitor bulk crystallization, and subsequently Avrami and Ozawa analyses are applied. A three-dimensional spherulitic growth from heterogeneous nuclei is deduced for isothermal crystallization, whereas higher exponents are obtained for non-isothermal crystallization when an Avrami equation is applied. However, modifications of the Ozawa methodology indicate a crystallization mechanism similar to that of the isothermal process. The maximum crystallization rate is deduced to take place at a temperature close to 91 °C by considering experimental data and theoretical equations with adjusted parameters. The equilibrium melting temperature is determined to be 168 °C by the characteristic Hoffman–Weeks plot. One crystallization regime is detected by using the Lauritzen–Hoffman kinetic theory for isothermal crystallization and also with an isoconversional method applied for non-isothermal crystallization. Activation energy of molecular transport and nucleation constant are close to 1500 cal/mol and 1.81 × 10 5 K 2, respectively. Crystal morphology, nucleation, and spherulitic growth rates are also investigated with hot-stage optical microscopy (HSOM).

Polyamide-based composite membranes: Part 1. Preparation and characterization

The miscibility, crystallinity and morphology of membranes from polyamides/poly(vinylalcohol) (PAs/PVA) blend materials were studied at various weight fractions and various crystallization temperatures. The blend systems of PA6/PVA, PA66/PVA, PA69/PVA and PA610/PVA can be written as: A xB 1− n / A yC 1− y , where A, B and C are the hydrocarbon chain, amide and methyl hydroxide segments, respectively, and x and y are the molar fractions of the corresponding segments in the repeating units of the polymers. The experimental approaches utilized differential scanning calorimetry, Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy. The Flory-Huggins interaction parameter X 12 and the segmental interaction parameters X′ AB , X′ AC and X′ BC were calculated following a developed Ellis approach. The equilibrium melting temperatures of polyamides in the blends were obtained using Hoffman-Weeks plots, and the interaction parameters were calculated using the Nishi-Wang equation, which is based on the Flory-Huggins theory. The values of the Flory-Huggins interaction parameters X 12 were negative, and the values of the segmental interaction parameters X′ AB and X′ AC were positive, whereas the X′ BC value was negative. FTIR was used to study the hydrogen bonding that enhances the miscibility of the blends. Significant upward shifts of v NH were observed with increasing the volume fraction of the second polymer (Ф 2). The negative values of the interaction parameters and the shifts in the NH frequency reflect the miscibility of the blend systems. The solubility parameters of the polymeric materials can be employed to choose polymeric materials for the RO membranes. Both high-pressure mercury porosity and scanning electron microscopy results are presented.

Melting parameters of poly(glycolic acid)

Equilibrium melting temperature T m 0, heat of fusion Δ H f, and entropy of fusion Δ S f of poly(glycolic acid) (PGA) was determined by using Clapeyron–Clausius equation. Equilibrium melting temperature T m 0 was 504.6 K which was determined by Hoffman–Weeks plots. The pressure dependence of T m 0 was determined by high pressure DTA up to 150 MPa. Volume change Δ V f at melting was determined by using dilatometer. Heat of fusion in PGA was 183.2 (J g −1), which is very close to the value reported by Chujo et al. who determined it by using T m depression in copolymer with poly(lactic acid). Δ S f of PGA was 0.363 (J g −1 K −1), which is about twice that of PLA, and the reason was discussed on the basis of the elastic modulus below T m.

Conditions for Melt Crystallization Without Thermal Degradation and Equilibrium Melting Temperature of Atactic Poly(Vinyl Alcohol)

Optimum conditions for melt crystallization without thermal degradation for atactic poly(vinyl alcohol) (PVA) were found for the first time. Thermal degradation of PVA in the melt has been too serious to permit crystallization from the melt. However, it was found that thermal degradation can be suppressed by selecting an appropriate solvent for film preparation, such as water without dimethyl sulfoxide (DMSO), by careful degassing before melting and by melting under high vacuum. As a result, the sample could be held in the melt at 250°C for 30 min and in the supercooled melt at 220°C for 480 min without thermal degradation. Therefore we could carry out the melt crystallization at a high crystallization temperature (T c) in the range of 208 to 221°C and study the structures and morphologies of melt-crystallized PVA samples for the first time. The irregularly stacked lamellae were observed irrespective of T c. The thick lamellae with high T m, 24 nm and 236.1°C, respectively, were obtained from the melt crystallization at T c=220°C, which indicates that the chain sliding diffusion is accelerated a little with increase of T c. However, the distribution of lamellar thickness was sharp, even for the sample crystallized at high T c, which indicated that the chain sliding diffusion is relatively suppressed by the intermolecular hydrogen bonds. Hence, the lamellar thickening was suppressed during the crystallization, as compared with polyethylene (PE) and isotactic polypropylene (iPP). The equilibrium melting temperature of PVA was determined to be 249°C, applying an improved Gibbs–Thomson plot that was recently proposed by Yamada et al. The Hoffman–Weeks plot accidentally gave a similar It was confirmed that the lamellar thickness is inversely proportional to the degree of supercooling in the case of PVA crystals, which is the reason why both plots gave similar