[Isothermal stepwise crystallization (ISC) technique], which is able to separate the simultaneous crystallization processes of β- and α-modifications in iPP samples. An evaluation method is also suggested in this study for the description of the
The nucleating efficiency and selectivity of different
β-nucleating agents was characterised and compared by differential scanning
calorimetry, (DSC) and temperature-modulated DSC (TMDSC). The nucleating agents
were the calcium salts of pimelic and suberic acid (Ca-pim and Ca-sub), linear trans-γ-quinacridone (LTQ), a commercial nucleator
NJ Star (NJS) and an experimental product (CGX-220). The efficiency and the
selectivity of Ca-sub and Ca-pim are extremely high. NJS is efficient above
a critical concentration, which is connected with its partial dissolution
in polypropylene melt. LTQ and CGX-220 possess strong overall nucleating ability
and moderate selectivity. Using TMDSC, we found that three consecutive processes
take place during the heating of β-nucleated samples cooled down to room
temperature: reversible partial melting of the β-form, irreversible βα-recrystallisation,
and the melting of the α-modification formed during βα-recrystallisation
or being present in samples prepared with non-selective β-nucleators.
Melting of the α-phase contains both reversible and irreversible components.
The characteristics of crystallization, melting and spherulitic growth of a random propylene copolymer (PRC) containing small amount of ethylene were studied in the presence of a selective Β-nucleating agent (calcium pimelate). It was established that the products of isothermal and non-isothermal crystallization are very rich in Β-modification but have mixed polymorphic composition. The formation of α-modification may be attributed to Βα-transition on the surface of growing Β-spherulites resulting in αΒ-twin-spherulites. During melting of PRC of Β-modification, the characteristics observed with Β-nucleated propylene homopolymers, namely, a Βα-recrystallization of recooled samples and separated melting of non-recooled samples (i.e. the melting memory effect), as well as a ΒΒ-recrystallization leading to a perfection of the structure within the Β-modification, are also demonstrated. The disturbance of regularity of the polymer chain highly reduces the tendency to Β-crystallization. In contrast to the observations with propylene homopolymers, the growth rate of α-modification (Gα) is higher than that of Β-modification (Gβ) and no critical crossover temperature can be found (T(Βα)=413 K) below whichGα>Gβ. The experimental results show that a partial disturbance of chain regularity by incorporation of comonomer units considerably reduces the tendency to Β-crystallization.
In this work the solid-state characterization of anhydrous D-mannitol has been performed: α and β modifications can be distinguished only by XRPD and FTIR as they show melting temperature
and enthalpy that are the same within the standard deviation. The understanding of the thermal behaviour of the δ form (obtained
by re-crystallization in acetone) has required XRPD experiments performed at variable temperature. This form during heating
undergoes a solid phase transition to α modification. By cooling a melted sample, under a wide range of experimental conditions,
a very fast crystallization occurs. Independently of the starting crystal form (β or δ form), the re-crystallization of D-mannitol from melt always leads to α form.
agent of PP was divided into α- and β-nucleating agent. PP nucleated by α-nucleating agent formed α-modification (α-PP) [ 6 – 15 ] with good rigidity, while PP nucleated by β-nucleating agent might form β-modification (β-PP) with special properties [ 16
As one of the most important semicrystalline polymers, isotactic polypropylene (iPP) has been widely studied for its polymorphic characteristic (monoclinic α-modification, trigonal β-modification, orthorhombic γ
into the crystal lattice of the 8-HQ. 8-HQ crystal is orthorhombic with lattice parameter a = 29.18, b = 25.36, c = 3.91 Å [ 13 ]. The cell volume of 8-HQ is 2893.42 Å 3 . Crystal structure of α-modification of 4-NP belongs to monoclinic with
polymorphic transition of the ordered lithium pentaferrite α-modification into the disordered β-modification [ 18 ]. This additionally confirmed the lithium pentaferrite formation during RT annealing.
The DTG dependences obtained by differentiation
αmodifications), and Sb 2 Zn 3 (with α and β modifications). Investigations of the Sb–Zn system and of specific intermetallics (such as the Sb 3 Zn 4 phase) are becoming increasingly interesting in the field of thermoelectric materials [ 6 ]. In
approximately located at 168 °C is characteristic for the melting of the α-form [ 26 ]. These results illustrate that only α-modification is formed in the non-nucleated system. In contrast, the β-nucleated samples crystallize mostly in β-form, no matter how many