Masamichi Hikosaka

Superheating of the melting kinetics in polymer crystals: a possible nucleation mechanism

Abstract Melting kinetics of polymer crystals has been examined experimentally by calorimetric methods utilizing the combination of a conventional differential scanning calorimetry of heat flux type (CDSC-HF) and a temperature-modulated DSC (TMDSC). The superheating effect in the kinetics has been discussed based on a modeling of the melting kinetics. For low-density polyethylene and linear polyethylene, the melting rate showed nearly linear dependence on the degree of superheating, which indicates the kinetics controlled by heat diffusion or by surface kinetics on rough interface. For isotactic polypropylene, poly(ethylene terephthalate) and poly(ϵ-caprolactone), the dependence is non-linear and close to the limiting case of exponential dependence, which indicates nucleation-controlled kinetics of melting. A possible mechanism of the activation process in the melting kinetics has been discussed in consideration of the specific feature of polymer crystals far from its most stable state. The consistency of the results of CDSC-HF and TMDSC has been confirmed by this analysis with a calibration of peak temperature for the instrumental thermal delay in CDSC-HF.

Three-dimensional morphology of PVDF single crystals forming banded spherulites

Abstract We have examined the morphology of poly(vinylidene fluoride) (PVDF) single crystals grown from melt and from blends with poly(ethyl acrylate) (PEA), PVDF/PEA=0.5/99.5 and 30/70 by weight. The single crystals, of relatively higher molecular weight, were grown isothermally in the temperature range where banded spherulites are formed with sufficient crystallization time. The crystals were extracted by dissolving amorphous PEA and PVDF crystals formed on quenching. The three-dimensional morphology of the single crystals was examined by transmission electron microscopy (bright field, dark field and diffraction) with a tilting stage. For all cases, the tilting of chains (∼25–27°) to the fold surface has been confirmed. The three-dimensional shape of all the crystals was chair type for the 30/70 blend and pure PVDF. In chair crystals, spiral terraces keep the handedness in each growth direction. From these evidence, it is proposed that the chair crystals with consecutive creation of spiral terraces of the same sense are responsible for the twisting relationship between crystallites in the radial direction of the banded spherulites.

Role of entanglement in nucleation and 'melt relaxation' of polyethylene

Abstract An experimental formula of the nucleation rate I of polyethylene as a function of number density of entanglement νe within the melt was obtained as I(νe)∝exp(−γνe), where γ is a constant. In order to obtain a functional form of I(νe), I is determined by changing νe within the melt. The νe within the melt can be changed when crystals with different lamellar thickness l are melted. It is shown that the νe within the melt just after melting is related to l before melting. The νe of folded chain crystals (FCCs) is large, while that of extended chain single crystals (ECSCs) is very small. Therefore, strictly speaking, the experimental formula is a kind of ‘semi-experimental’ one. Because it is obtained by combining an experimental formula of I as a function of l before melting I(l) and a formula between l and νe based on the most probable model. It was found that the νe dependence of I is mainly controlled by the topological diffusion process within the interface between the melt and a nucleus and/or within the nucleus not by the forming process of a critical nucleus. The slope of the plots of logxa0I against ΔT−2 was constant, irrespective of morphologies, FCCs and ECSCs, where ΔT is the degree of supercooling. From this fact, it was concluded that the fold type nucleus are formed from the melt of ECSCs as well as from the melt of FCCs. In our previous study, we found that I decreases exponentially with increase of annealing time Δt at a temperature above the melting temperature. From these results, we proposed a ‘two-stage melt relaxation’, i.e. fast conformational and slow topological relaxations. When the ECSCs are melted, extended chains within ECSCs are rapidly changed to random coiled chain conformation and then chains gradually entangle each other. We also proposed a formula, ν e ( Δ t)∝− ln { const. +A exp (− Δ t/τ m )}, where A is a constant and τm is the ‘melt relaxation’ time.

Lamellar thickening growth of an extended chain single crystal of polyethylene (II) : ΔT dependence of lamellar thickening growth rate and comparison with lamellar thickening

The degree of supercooling (ΔT0) dependence of lamellar thickening growth rate (U) of an isolated extended chain single crystal (ECSC) of polyethylene is studied. The experimental formula,U = C exp(−D/ΔT0), where C = 130 nm/s and D = 20.0 K is obtained for the first time. The formula is the same as that of lateral growth rate (V). The reason why U and V obey the same formula is well explained by a model named “sliding diffusion model of the lamellar thickening growth”. The model proposed that the lamellar thickening growth is controlled by both chain sliding diffusion within the ECSC and the nucleation on the side surface. The observed fact that the U increases with increase of ΔT0 is opposite to the well known fact that lamellar thickening rate W decreases with increase of ΔT0. This siginificant difference was well explained by the difference between the “primary crystallization” and the “secondary crystallization”, which is a kind of “Ostwalds ripening process”. The origin of the “tapered shape” is well explained by coupling of lamellar thickening and lateral growths.

Second-order phase transition of high isotactic polypropylene at high temperature

Abstract A second-order phase transition of α2 form isotactic polypropylene (iPP) is found at high annealing temperature (T a =159.3 ° C ) by means of X-ray diffraction method. Although the lattice shape and the space group keep the same as those of the α2 form, i.e. monoclinic and P 2 1 / c , with increase of T a , it has been revealed that there are discontinuous increases in the slopes of the lattice constants a and b against T a plots, while the c and the β keep almost constant. As a result, the slope of the unit cell volume V versus T a plot also shows a discontinuous increase at T a =159.3 ° C , indicating the occurrence of the second-order phase transition. In order to distinguish the two phases, the phase above the transition temperature is named α2′ phase and the transition temperature is denoted T α2–α2′ . These facts suggested that the α2′ form is a mobile phase where the molecular chains would become loosely packed and mobile, promoting the better chain sliding diffusion. A fast lamellar thickening process has been confirmed in the higher temperature region than T α2–α2′ , which was reported in the precedent paper. General significance is proposed that mobile phases possibly exist at high temperature, close to the melting temperature and accelerate lamellar thickening, which improves physical properties of polymers.

An unusual behavior in the melting region of isotactic polypropylene crystals revealed by temperature-modulated DSC

We present a new method to analyze irreversible transformation kinetics of melting in polymer crystals with temperature modulated differential scanning calorimetry (TMDSC). In the melting region of several polymers, the apparent heat capacity obtained with TMDSC can be expressed as Cs + (|Fmelt|/β)/(1 + i ω τ (β)), with the true heat capacity,Cs, the endothermic heat flow of melting,Fmelt, the angular frequency of temperature modulation, ω, and the mean time of melting of each crystallite, τ, depending on the underlying linear heating rate, β. In the case of isotactic polypropylene, the frequency dependence cannot be approximated by this formula. The dependence suggests the possibility of the retardation in the melting kinetics to follow temperature modulation.

Periodically modulated driving force applied with tmdsc to the crystallization and melting kinetics of ice crystals confined in a porous silica gel

The application of a periodically modulated driving force has been examined in the melting and crystallization kinetics of ice crystals confined in a porous media. The kinetic response of transformation gives the real and imaginary parts of the ‘apparent’ heat capacity obtained with a temperature modulated differential scanning calorimetry (TMDSC). Based on a modelling of the kinetics, the detailed examination of the frequency dispersion and its dependence on underlying heating/cooling rate enables us to evaluate the transformation rate and the dependence of the rate coefficient on the driving force, i.e. the degree of supercooling or superheating. The experimental results indicate that the transformation processes are limited by heat diffusion from the growth interface of each crystallite to surroundings.

Spherulite Growth of cis-1,4-Polyisoprene Isolated from Natural Rubber

The crystallization behavior of cis-1,4-polyisoprene, isolated from natural rubber, was investigated by polarized light microscopy. Natural rubber was purified by deproteinization followed by transesterification to remove protein, phospholipid, and fatty acids present in the rubber. The purified rubber was fractionated into seven fractions. For the fractionated rubbers, spherulite growth was observed during the course of crystallization. The size of the spherulites increased linearly with crystallization time, and thus, growth rate was estimated. The growth rate was proportional to TΔT, where ΔT is supercooling. Because the slope of the line was independent of the molecular weight, the growth rate was confirmed to be a function of diffusion of the rubber.