Surface crystallization of polypropylene

Crystallization of compression-molded isotactic polypropylene and polyethylene is invariably spherulitic; generally, nucleation occurs randomly throughout the sample. In a special case where nucleation predominates at the surface, spherulitic growth centers become crowded and are forced to propagate unidirectionally into the bulk (transcrystallinity). Conditions for the formation of transcrystallinity have been investigated by optical and scanning electron microscopy. The occurrence of transcrystallinity is attributed to heterogeneous nucleation induced at the mold surface. To be effective, the mold surface must have a nucleating efficiency equal to or greater than that of adventitious nuclei present in the polymer. As the crystallization temperature approaches the melting point, the activity of mold surfaces is found to increase leading invariably to transcrystalline formation. The degree of activity of various mold surfaces correlates with the known activity of specific dispersed nucleating agents having similar chemical structures. Contrary to claims in the literature, the surface energy of the mold surface and temperature gradients across the melt surface do not play a primary role in transcrystalline formation of polypropylene.     

Effect of annealing on the ferroelectric behavior of nylon - 11 and nylon - 7

We have discovered recently that melt-quenched and cold-drawn Nylon - 11 films exhibit very clear ferroelectric hysteresis behavior. In the present study, a remanent polarization as high as 86 mC/m² has been found in Nylon - 7 samples; this is significantly higher than that usually observed in poly (vinylidene fluoride) films. The effect of annealing on the electric displacement versus electric field characteristics of both Nylon - 11 and Nylon - 7 films have been studied and show an increased coercive field (62 to 115 MV/m for Nylon - 11 and 79 to 97 MV/m for Nylon - 7) and a decreased remanent polarization (51 to 17.3 mC/m² for Nylon - 11 and 86 to 70.5 mC/m² for Nylon - 7) with increasing annealing temperature from 25 to 145°C.     

Effects of annealing on the polarization switching of phase I poly(vinylidene fluoride)

To obtain information about microscopic processes involved in the polarization switching in uniaxially oriented poly (vinylidene fluoride) film, a least-squares estimation of nonlinear parameters was developed to yield parameters of an equation which describes the nucleation and domain growth process. Time domain measurements of polarization reversal revealed that switching times decreased as the annealing temperature T_a increased (67.0, 52.4, and 41.3 μs at –20°C under a 200 MV/m pulse field for the as-stretched samples, the samples being annealed at 120 and 160°C, respectively). The analysis showed that the value of the domain growth speed increased as T_a increased. This is consistent with x-ray diffraction data which indicated that the annealing process brought about better chain packing and increased crystallite perfection. The analysis also showed that the nucleation probability significantly increased as T_a increased. This result was interpreted in terms of a morphological transformation, which was indicated by the decrease in elastic modulus with increasing T_a with no corresponding loss of orientation. It is suggested that the annealing process brought about an increase in the number of nucleation sites as a result of a transformation from a fibrous structure to a crystal-amorphous series structure which has increased boundary zone area.     

Time-related luminescence spectroscopic investigation of electron trapping and recombination in infrared stimulated luminescence

Infrared stimulated luminescence (ISL) occurred in CaS:Eu,Sm due to formation of luminescent centres Eu²⁺ and electron trapping centres Sm³⁺. The electron trapping centres Sm³⁺ became occupied (forming Sm²⁺ by trapping excited electrons) in photoluminescence (PL) excitation (PLX) process causing simultaneous ionization of luminescent centres Eu²⁺ (leaving Eu³⁺ by losing an electron or capturing a hole). In this paper, the electron trapping in PLX and the recombination in ISL were examined by the time-related PL and ISL spectra of CaS:Eu,Sm. The spectroscopic evidence confirmed that the ISL in CaS:Eu,Sm was produced due to recombination of de-trapped electrons and previously ionized luminescent centres (Eu³⁺). It was believed that the electron trapping occurred concurrently as occurrence of the PL of Eu²⁺ in PLX process. However, the recombination of de-trapped electrons and previously ionized luminescent centres took about 10 s or even more to occur after infrared irradiation. PACS 78.55.-m; 78.45.+h     

Role of embedded clustering in dilute magnetic semiconductors: Cr doped GaN

Results of extensive density-functional studies provide direct evidence that Cr atoms in Cr:GaN have a strong tendency to form embedded clusters, occupying Ga sites. Significantly, for larger than 2-Cr-atom clusters, states containing antiferromagnetic coupling with net spin in the range 0.06-1.47 muB/Cr are favored. We propose a picture where various configurations coexist and the statistical distribution and associated magnetism will depend sensitively on the growth details. Such a view may elucidate many puzzling observations related to the structural and magnetic properties of III-N and other dilute semiconductors.     

Interfaces and the question of regional congruence in spin glasses

We present a general theorem restricting properties of interfaces between thermodynamic states and apply it to the spin glass excitations observed numerically by Krzakala and Martin and separately by Palassini and Young in spatial dimensions d = 3,4. We show that such excitations, with interface dimension d(s) < d, cannot yield regionally congruent thermodynamic states. More generally, zero density interfaces of translation-covariant excitations cannot be pinned (by the disorder) in any d but rather must deflect to infinity in the thermodynamic limit. Additional consequences concerning regional congruence in spin glasses and other systems are discussed.     

Piezoelectricity in uniaxially stretched and plasticized nylon 11 films

The combined effect of uniaxial stretching and plasticization of nylon 11 films on the resulting piezoelectric response was studied. Three different kinds of samples were studied for nylon 11 α′-form films: (1) uniaxially stretched at 150°C, (2) samples uniaxially stretched at 150°C and then plasticized by immersion in 2-ethyl 1,3-hexanediol, and (3) samples plasticized and then uniaxially stretched. The largest piezoelectric response was obtained from the samples which were plasticized prior to uniaxial stretching under identical poling conditions. For the case of nylon 11 δ′-form films, two different kinds of experiments were performed: (1) samples uniaxially stretched at room temperature were subsequently plasticized by immersion in the plasticizer and a comparison of their piezoelectric response made with that of the unplasticized samples as a function of poling field; (2) the plasticizer content dependence of the piezoelectric response from these samples was studied. In both cases, d₃₁ was observed to be higher for the plasticized films compared with the unplasticized films. The piezoelectric stress constant e₃₁ showed a small decrease with plasticization. X-ray diffraction studies indicated a small conversion of δ′-phase to α′-phase with plasticization. No significant changes were observed in the x-ray diffraction scans taken before and after poling.